Newsletter_Final2020_021921_red

Geo years PRISMS Newsletter - Issue No. 43 - Final Volume the people | the science 424 km

contents Kristina Walowski Thamer Aldajaani 1 6 foreword science reviews years Ten years of accomplishments across primary sites

Gene Yogodzinski on Alvarez Aramberi the cover 74 100 Illustrating the breath of science program science supported by or closely related to highlights nuggets GeoPRISMS GeoPRISMS unique Portfolio of research elements enabling projects supported by Twilight monitoring of Santiaguito fundamental advances volcano, Guatemala at Workshops on GeoPRISMS Volcanoes 2016. Kari Cooper Photo credit: Dan Rasmussen Participants of the 2015 GeoPRISMS Theoretical & Experimental Institute for the Subduction Cycle & Deformation Initiative, Redondo Beach, CA. Photo credit: Anaïs Férot Mount Tulik Volcano located on the southeastern flank of Okmok Caldera, as viewed from a helicopter during the 2015 GeoPRISMS-NSF-funded geophysical survey to study arc magmatism beneath the caldera. Photo credit: Kerry Key Preparation of an ocean-bottom seismometer during the 2018 NSF- GeoPRISMS-funded South Island Subduction Initiation Experiment (SISIE). Map of the active source deployment from the 2014 GeoPRISMS-NSF-funded collaborative iMUSH Field Campaign to illustrate the magmatic system beneath Mount St. Helens Shear velocity in continental Africa modified from GeoPRISMS-NSF-funded research project Emry et al. (2019) Multichannel seismic profile from the 2014 GeoPRISMS-NSF-funded Eastern North American Margin (ENAM) Community Seismic Experiment, courtesy of PI Anne Bécel (GeoPRISMS Newsletter Spring 2017) Newsletter Production: Anaïs Férot [email protected] www.geoprisms.org The production of the retrospective newsletter suffered major delays due to Coronavirus pandemic. The GeoPRISMS Office thanks all the authors for contributing to this issue. 3Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

foreword 4 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

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100 200 300 4200 100 300 20010 1 5 400 300 00 100 0 science reviews Ten years of accomplishments across primary sites 8 Subduc tion initiation 18 New insights into influences on rift magmatism from research in 28 the East Africa Rift System and Eastern North American Margin 40 Subduc tion megathrust locking and slip 52 behavior: Insights from geodesy 64 Volatile fluxes at rifting and subduc tion margins: Review of results from the NSF MARGINS and GeoPRISMS programs Unraveling subduction zone processes: Evidence from exhumed rocks Under the volcano: Tracing the path of eruptible arc magmas 7Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Science Reviews Subduction initiation Michael Gurnis (California Institute of Technology), Harm Van Avendonk (University of Texas Institute for Geophysics), and Mark K. Reagan (University of Iowa) The initiation of new subduction zones is a key component The growth and stability of island arcs is a factor in the global mass of the plate tectonic cycle and evolution of continental crust balance of continental crust. When a young slab descends into the (Stern and Scholl, 2010). Subducted slabs are the primary mantle, magmatism may be enhanced, especially if subduction was force driving plate motions and formation of new subduction zones initiated by a phase of extension and mantle decompression (Stern and demise of existing ones is associated with the largest changes and Bloomer, 1992). More mature subduction systems may continue in the force balance on tectonic plates (Gurnis et al., 2004). Since to add igneous material, but an equal amount of lower arc crust subduction zones are also a primary location for volatile cycling, may be lost by gravitational instability (Behn and Kelemen, 2006). major temporal changes in subduction are expected to lead to The continental crust mass balance may therefore depend on the changes in outgassing and reinjection of volatiles into the mantle difference between magmatic additions at arcs versus forearc loss (van Keken et al., 2011). Despite the importance of subduction due to tectonic erosion. Subduction initiation may be a mechanism initiation, our understanding of the associated geological record, to maintain a positive or net balance over time. geophysical and geochemical character, and geodynamical causes Major episodes of subduction could play an important role in and consequences remain poorly understood. Addressing the cycling CO₂ and other volatiles between the atmosphere and initiation of subduction was a major focus of the MARGINS and ocean and mantle. Paleogeographic reconstructions have most GeoPRISMS programs through collection of key observations at volcanism occurring along continental-arcs, where subduction primary sites and in exhumed terranes from ancient subduction zone magmatism can react with carbonate-rich crust and generate zones. high concentrations of atmospheric CO₂. Initiation of ocean-ocean subduction zones around the Pacific during the early Eocene likely Although slab pull is the primary driving force for plate motions, impacted CO₂ fluxes to the atmosphere (Lee et al., 2013; Reagan et key features of this force balance remain a research focus. How al., 2013), although the direction of this shift and degree it impacted much of the negative buoyancy from cold slabs is transmitted to the global climate remains uncertain (Kirtland-Turner et al., 2014). converging oceanic plate depends on several factors, notably on plate Wilson (1966) recognized that the Atlantic Ocean opened near bending at oceanic trenches, interplate resistance, and drag around continental sutures in Paleozoic orogenic belts, with older ocean the edges of slabs and base of plates (Conrad and Lithgow-Bertelloni, basins opening and closing there in the past. This Wilson cycle 2002; Stadler et al., 2010). Given the concentration of driving and requires failure of passive margins, but theoretical studies suggest resisting forces within the slab, the initiation and termination of the forces required to fail thick lithosphere of a passive margin are subduction likely determine changes in the rates and direction of too great, even with a sediment load (Mueller and Phillips, 1991). plate motions. Figure 1. Ages for subduction initiation events centered on the Pacific determined from isotopic ages of arc initiation and stratigraphic, structural, plate tectonic and seismic indicators of subduction initiation. The colors of the trenches represent the latest initiation (reinitiation) age, while those of the underlying thick line represent the long-term age. Plate boundaries and absolute plate motions from Müller et al. (2016). Reprint from Hu and Gurnis (2020) by permission of John Wiley and Sons. 10 cm/yr 0 100 200 Age (Ma) 8 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

35° ASB Bonin Trench 0B U1439- A DR U1438 U1442 10 Spreading in ODR Shikoku Shikoku & Parece 30° Basin Bonin Ridge 20 Arc Tholeites & Vela Basins 25° Calc alkaline vol. 30 Spreading Mariana Trench 20° Guagua Ridge Kyushu Palau 40 155˚° Central Basin Ridge Age (Ma) 1100˚° Spreading Center Parece Vela forearc 120° Basin forearc 50 Boninites Guam U1438 Gabbros & 125° 130° 135° 140° 145° 150° Basalts KPR Basalts -5000 -2500 0 60 SPV Bonin Mar CBSC ASB Topo (m) 2500 5000 Figure 2. A. Location Map and sites for ages for key events discussed in the text associated with initiation of the IBM system. Uncertainties on the ages of forearc basalts, gabbros and boninites denoted with lighter color shading. Abbreviations are: DR for Daito Ridge; ODR for Oki-Daito Ridge, ASB for Amami Sankaku Basin, CBSC for Central Basin Spreading Center, KPR for Kyushu Palau Ridge, SPV for the Shikoku and Parece Vela Basins, and Mar for the Marianas. Recent plate reconstructions have continent-ocean plate boundaries Izu-Bonin-Mariana (IBM) accommodating thousands of kilometers of strike-slip during opening and closing of oceans (Dalziel and Dewey, 2019). Actively The Izu-Bonin-Mariana system with its present-day high shearing plate boundaries are weaker than passive margins, so strike- convergence rate, simple geometry, and intact geological record slip motion may precede subduction initiation. has been recognized as an outstanding region for studying the “subduction factory” (Fig. 2). Decades of geophysical research on The key reason that consensus on a unified description of IBM illustrated its structure and state (Kodaira et al., 2007). The subduction initiation has been slow to develop is that initiation is a inputs through plate convergence (Kelley et al., 2003; Takahashi transient process whose record is generally obscured by subsequent et al., 2008) and outputs across the arc and back-arc have been subduction zone processes, most notably burial, overprinting, assessed through on-land field work and recovery of samples from uplift, and compression and over-thrusting. In contract to rifting, the sea floor by diving, dredging, and drilling (Pearce et al., 2005; subduction initiation is the seed for the destruction of its own record. Kelley, et al., 2010; Heywood et al., 2020). Subduction initiation has Despite these limitations, there is a growing acceptance of a diverse been well-studied at IBM because the earliest record of volcanism record of Cenozoic subduction initiation events (Fig. 1). Much of the related to this process is preserved (Stern and Bloomer, 1992; Pearce recent work on subduction initiation has exploited the geological, et al. 1992; Reagan et al., 2010; Li et al., 2019), and the tectonic geochemical and geophysical record of these subduction initiation configuration during the time of initiation has been determined events which represent over half of subduction zones existing today through plate tectonic reconstruction (Hall, 2002; Leng and Gurnis, (Gurnis, et al., 2004). 2015; Wu et al., 2016). The IBM subduction zone is on the eastern edge of the Philippine Sea The GeoPRISMS and MARGINS programs targeted primary sites for Plate (Fig. 2) and based on the age of igneous rocks in the forearc, interdisciplinary research as a means to make accelerated progress. the subduction zone initiated at about 52.5 Ma (Reagan et al., 2019). Following the success of MARGINS and the Ocean Drilling Program To the west of the present trench, there are several active and rifted (ODP), the GeoPRISMS and International Ocean Discovery Program arcs, active and relict back arc basins, extinct spreading centers (IODP) 2013-2023 Science Plans highlighted subduction initiation as and other tectonic features. The active arc and forearc reconstruct a key question that would benefit from this concerted, site-focused to the position of the Kyushu-Palau Ridge (KPR) at about 33 Ma approach. Four IODP Expeditions and a major seismic experiment based on the magnetic lineations and fracture zones within the all in the western Pacific followed to address these questions. At the Shikoku and Parece Vela Basins. KPR, a long N-S feature, is a relict same time, these programs have been complemented by expanded arc of IBM subduction (Ishizuka, et al., 2011a). West of KPR is the focus on other sites, geological synthesis, geochemical analysis, and West Philippine Sea Basin with features predating- and post-dating modelling, which are reviewed here subduction initiation. 9Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Most spreading of the generally E-W Central Basin Spreading Center (Yogodzinsky et al., 2018). With rare exception, fresh pillow-rind postdates subduction initiation, but initially it was thought to predate basalt glasses sampled during Expedition 352 lack evidence for the it, leading to the hypothesis that subduction initiated along an old involvement of subducted fluids in their genesis (Coulthard et al., fracture zone (Uyeda and Ben-Avraham, 1972). 2017). The high SiO2 concentrations of the forearc suggest generation IODP Expeditions 351 and 352 were mounted to constrain patterns from mantle with a relatively steep geothermal gradient (Shervais in volcanism and lithosphere generation diagnostic of subduction et al., 2019). After less than 1.2 million years and while seafloor initiation. Expedition 352 drilled four sites (U1439-U1442) in the spreading was ongoing, water-rich melts became involved in magma forearc to obtain a volcanic record of subduction initiation and with genesis, resulting in a transition to low-Si boninite (see Pearce and earlier diving showed that earliest volcanism produced distinctive Reagan, 2019). About the time that seafloor spreading ceased (c. basaltic to boninitic crust with identical ages at locations that are 51.3 Ma) and the arc began to be established, high-Si boninites currently 1,700 km apart (Cosca et al., 1998; Reagan, et al., 2010; began erupting (Reagan et al., 2019). Eruption of these boninites Ishizuka, et al., 2011b), but may have been about 500 km when they migrated away from the trench and continued to about 44Ma, when formed (Leng and Gurnis, 2015). Forearc basalts with Ar-Ar and lava compositions transitioned toward those of a normal volcanic U-Pb zircon ages of 51.9-51.3 Ma are trench-ward (Sites U1440, arc (Ishizuku et al., 2011b). U1441) of boninites (Sites U1439, U1442) aged 51.3-46 Ma (Reagan et al., 2019). The Puysegur GeoPRISMS site Expedition 351 Site U1438 just to the west of the KPR cored basaltic crust, finding it younger than Bonin forearc basalts with a Ar-Ar age The Puysegur subduction zone (Fig. 3) at the southern tip and of 49.9+/- 0.5 Ma (Ishizuka, et al., 2018), overlapping with boninite offshore South Island, New Zealand was selected as a GeoPRISMS volcanism in the Bonin forearc. These boninites are thought to focus site for studies of subduction initiation. The region is globally represent the first establishment of the volcanic arc (Reagan et al., unique as subduction is now initiating with well-constrained relative 2017), and suggests that the basaltic crust drilled at Site U1438 might motions between over-riding and subducting plates and antecedent represent the first IBM backarc basin. tectonics that are not extensively over-printed. The IBM forearc has an overall structure akin to those found in ophiolites (Stern et al., 2012), including depleted peridotites at The Puysegur Subduction zone accommodates oblique convergence depth followed progressively by gabbroic, doleritic, and basaltic with under-thrusting of the Australian below the Pacific Plate along units. This sequence has been interpreted to represent lithosphere the northern segment of the Macquarie Ridge Complex (Fig. 3A). produced during near-trench seafloor spreading immediately after Cenozoic plate kinematics are well constrained by seafloor magnetic subduction began (Reagan et al., 2010, 2013). The first basalts have anomalies (Cande and Stock, 2004). After Eocene rifting and seafloor compositions rivaling those of the most depleted mid-ocean ridge spreading south of New Zealand, a few hundred kilometers of right- basalts (Li et al., 2019; Shervais et al., 2019). These compositional lateral strike slip motion juxtaposed continental crust of Fiordland traits are shared with Site U1438 basalts and reflect a several hundred and Miocene age oceanic crust. The seismic Benioff zone, reaching million year or longer regional depletion of these mantle sources about 150 km depth, can be used to estimate that under-thrusting began in Fiordland at 16 -10 Ma, and at 11-8 Ma near the Puysegur Trench (Sutherland et al., 2006). 162˚ 164˚ 166˚ 168˚ 170˚ Figure 3. A. Puysegur study region (black rectangle) in B Fiordland the south west Pacific. The Macquarie Ridge Complex RR is the long narrow gravity high/low feature between −46˚ the Hjort Trench (HT) and the Puysegur Trench (PT). SoI 6000 Bathymetry (m) Base map is free-air gravity. B. MCS lines (blue lines), −48˚ 4000 OBS locations (filled triangles and stars) and swath PB 2000 bathymetry (in color) of Puysegur Ridge (PR) and Trench −50˚ region acquired as part of the South Island Subduction PT SZ 0 Initiation Experiment (SISIE) is shown. The combined 204 Stewart Is. OBS/MCS lines SISIE-1 and SISIE-2 are labelled. The 219 200 Snares Zone (SZ) is outlined with a black dashed line, 100 Resolution Ridge is labeled as RR, Solander Island as SoI SISIE−2 and Puysegur Bank as PB. Red arrows are the modern 0 relative plate motion with respect to PAC. Reprinted Campbell −100 from Gurnis et al. (2019) with permission from Elsevier. Plateau −200 PR A BasiSnolander SISIE−1 eTR Gravity (mGal) 117 PT AUS Auck. Is. PAC HT 10 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

There have been large vertical motions of the Puysegur Ridge along CMP Number E strike as the boundary evolved from transpression to incipient subduction. The southern Puysegur Ridge experienced uplift, Two-way Travel-time (s) W 34000 35000 36000 37000 38000 39000 40000 whereas the northern Puysegur Ridge first uplifted and then subsided by about 1.8 km within the Snares Zone, a region with 2 several strike faults and a negative free air-air gravity anomaly as strong as an oceanic trench (Fig. 3). The uplift migrated to the north Solander Basin during the Miocene consistent with about 2 km of dynamic support within Fiordland (House et al., 2002). South of Fiordland, adakitic 3 volcanism on Solander Island commenced within the last 1 Myr (Mortimer et al., 2013). 4 As part of GeoPRISMS, the South Island Subduction Initiation Experiment (SISIE) was successfully completed in February and 5 March 2018 using the R/V Marcus G. Langseth. Seismic reflection and refraction data were acquired across the Puysegur Trench 6 and Ridge and the Solander Basin (Fig. 3B). For the multichannel 5 km seismic (MCS) imaging, the Langseth used a 12.6 km long streamer for much of the 1,300 km of lines acquired. A group of 28 ocean- Figure 4. Details for the Solander Basin just east of the Puysegur bottom seismometers (OBSs) from the University of Texas Institute Ridge from SISIE-2 seismic line (see Fig. 3 for location of SISIE-2). of Geophysics (UTIG) were used at 43 sites on two refraction lines. One refraction/MCS line targeted the more juvenile part of the The seismic images clearly showed that the over-riding plate Puysegur Ridge (SISIE 1) while the other targeted the more evolved during the initiation of subduction was stretched continental crust part (SISIE-2). Two lines of onshore seismic receivers and several broadband seismometers on islands were deployed by New Zealand and not oceanic crust as expected for SISIE. The top panel shows collaborators. interpretation of time-migrated multichannel seismic line. Lines With the new seismic data, much of the crust immediately to the indicate: top of sediments (blue), basement (black), sedimentary east of the Puysegur Trench was found to be rifted continental crust horizons (green), and faults (red). For upper part of section V.E. ~4:1 (Gurnis et al., 2019; Fig. 4) and not oceanic crust as interpreted along @ 2km/s. For depths for basement with faults V.E. ~2:1 @ 4 km/s. the entirety of the MRC. This is an important new observation, Reprinted from Gurnis et al. (2019) with permission from Elsevier. because the density difference across an ocean-continental margin is substantially larger than that across an ocean-ocean margin. motion between 18 and 15 Ma, just before subduction initiation, A plate tectonic reconstruction shows that the density difference and supporting a role for compositional differences in the initiation across the plate boundary rapidly increased during strike-slip of Puysegur subduction. During initiation, a large fault (Tauru Fault, Fig. 3B) within the northern Solander Basin, inverted from normal to reverse. Using sequence stratigraphy with the N-S MCS lines and an existing petroleum exploration well, Patel et al. (2020) constrained the compressional event between 12 and 8 Ma. Using a seismic tomographic model with OBS-inferred velocities mapped to density, Hightower et al. (2019) showed that the crust below the Snares zone was thicker compared to the Puysegur Ridge to the south even though it was topographically depressed. This likely reflects a strong change in the vertical force balance along strike. -10° Vitiaz Trench 0 Forearc Volcanism A B Vanuatu Tonga Trench 10 SZ -20° New Caledonia Norfolk Ridge MHautnthteerwSZ& 20 U1508 Uplift SFB -30° SFB -40° U1507 Kermadec Trench Spreading U1506 NCT Age (Ma) LHR 30 U1508 Folding U1509 U1508 U1511 U1510 U1511U1510 UF1o50ld8 iUnpglift Folding 40 TAS Obduction TrPeunycshegur 50 TAS U1506 Uplift Boninites Spreading -50°150° 160° 170° 180° -170° 60 Trough Formation & Subsidence Topo (m) TAS 0 2500 5000 LHR NCT NC/NR SFB Tonga Forearc -5000 -2500 Figure 5. A. Location map for features associated with Tonga-Kermadec subduction initiation. B. Ages for key events associated with initiation of the Tonga-Kermadec subduction zone. Uncertainties on folding, uplift and subsidence denoted with lighter color shading. Recent IODP Expedition 371 drilling shown with red stars. The small black line at U1508 is the trace of the seismic line shown in Figure 6. Abbreviations are: TAS for Tasman Sea oceanic crust, SFB for South Fiji Basin, LHR for Lord Howe Rise, NCT for New Caledonia Trough, and NR for Norfolk Ridge. 11Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Other sites and observational programs ophiolitic nappes in New Caledonia, and compression throughout northern Zealandia points to an inception of around 50 Ma. The Complementing the IBM and Puysegur work has been research longer-term history of eastern Australia and Zealandia has long-term undertaken on Tonga-Kermadec subduction initiation using three subduction from early Mesozoic but without subduction between sources of information from: the Tonga forearc, ophiolitic terrains about 110 Ma and 50 Ma (Mortimer et al., 2017). The modern of New Caledonia, and the sediments and structure of northern Tonga-Kermadec formed at the boundary of this older Mesozoic Zealandia (that is, Norfolk Ridge, New Caledonia Trough and Lord subduction zone and the initiation was associated with broad-scale Howe Rise) (Fig. 5). compression (Sutherland et al., 2017), local eruption of boninites Initiation of Tonga-Kermadec, like IBM, dates to about 50 Ma as and formation of a trough in-board of the new trench. revealed through analysis of samples recovered from the Tonga Vanuatu also provides two examples of subduction initiation. One is forearc. Dredge samples, primarily tholeiitic basalts, have ages from the nucleation ~15-12 million years ago through a polarity reversal 51 Ma into the Miocene, with one late Mesozoic age (Meffre et al., along the Vitiaz Trench and is significant because of the large 2013). An IBM-type forearc volcanic stratigraphy, with Boninites amount of rollback and back arc opening which transpire right after upslope from basalts, has not been found and the volcanism reflects initiation (Pelletier et al., 1993). In addition, the < 2 Ma Matthew- a longer period and not just a brief burst as at IBM. Hunter subduction zone formed east of the south end of the Vanuatu New Caledonia, at the northern end of the narrow Norfolk Ridge Trench, potentially as it collided with the Loyalty Ridge (Patriat, et (Fig. 5), has several obducted terrains, including a Poya terrain with al., 2019). The trench is migrating southwestward and southward, dolerites, basalts and abyssal sediments dating from 83 to 55 Ma opening a series of sinistral trans-tensional basins proximal to the (Cluzel et al., 2001). On top of Poya lies an ultramafic Peridotite trench (Patriat et al., 2015). A diverse suite of magmas, including Nappe with boninite and felsic dykes intruding it (Cluzel et al., 2016). boninites, adakites and basalts, have erupted within this extensional Within the serpentinite sole of the ophiolite are boninite-series felsic environment and Patriat et al. (2019) have argued that the resulting dikes with ages of ca. 54 Ma, while two boninitic dykes have 40Ar/39Ar terrane is a modern analogue for supra-subduction zone ophiolites. ages of 50.4 +/1 1.3 Ma and 47.4 +/- 0.9 Ma. A possible candidate for intra-oceanic subduction initiation may be Throughout northern Zealandia (Fig. 5A), as evident in multichannel the N-S Gagua Ridge east of Taiwan (Fig. 2A), separating Cretaceous seismic profiling (Fig. 6), is broad-scale folding, thrusting and oceanic crust to the west with the crust of the West Philippine Sea vertical motions broadly dating to the Eocene and Paleocene Basin. Reconstructions (Hall, 2002) suggest that Paleogene relative (Sutherland et al., 2017). Refining the timing of this compression plate motion was strike-slip, and that the Gagua Ridge was likely a and vertical motion was the objective of IODP Expedition 371 long-offset transform fault (Deschamps et al., 1998) until initiation (Sutherland et al., 2020; Fig. 5A). Analysis of the recovered cores of compression. Marine seismic data show that the younger crust showed that Lord Howe Rise rose from about 1 km water depth to is deformed at the Gagua Ridge, where it thickens to 15-18 km sea level and then subsided back, with peak uplift at 50 Ma in the (Eakin et al., 2015) with younger lithosphere underthrusting the north (west of New Caledonia) and between 41 and 32 Ma in the neighboring plate by ~10 km, without leading to subduction. The south (Fig. 5B). The New Caledonia Trough, between Lord Howe failed subduction along the Gagua Ridge suggests that subduction Rise and Norfolk Ridge, subsided 2–3 km between 55 and 45 Ma, may not easily initiate between oceanic plates with normal crustal but whether the trough resulted from rifting or crustal delamination thicknesses. The Laxmi Basin, west of India, may also be an example remains unclear. of failed subduction initiation based on similarities between the The evidence for Tonga-Kermadec initiation from the Tonga forearc, compositions of basement lavas recovered during IODP Expedition 355 and IBM forearc lavas (Pandey et al., 2019). SW REI09-009 REI09-011 NE REI09-012 South Maria Ridge 1.0 Reinga Basin Quaternary-Miocene drape U1508 Figure 6. Seismic line for U1508 from Two-way traveltime (s) 2.0 Seabed Miocene-Eocene Folded IODP Expedition 371 showing the intense 3.0 fanning re ectors Eocene-Cretaceous compression and subsequent tectonic 4.0 subsidence interpreted as part of Tonga- Kermadec subduction initiation. Regional 5.0 seismic reflection Line REI09-012 near Site Jurassic Cretaceous U1508 with interpretation of stratal age Murihiku half graben and structure Location of seismic line and U1508 Site shown in Figure 5A. VE = vertical exaggeration. From Sutherland, et al. (2019) VE=7.6:1 @ 2km/s 0 5 10 km 12 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

0A 1 Myr B 11 Myr -100 Ec Free water -200Depth (km) 6 Myr D 0 - 1 Myr 1 - 3 Myr Topography (km) 6 - 11 Myr 0C Free 2 200 400 600 -100 Ec water 0 -2 -4 -200 3 - 6 Myr 100 200 300 400 500 600 2 Density (g/cm3) 0 -2 2.6 2.8 3.0 3.2 3.4 -4 0 200 400 600 0 Figure 7. Development of a new subduction zone in transition from forced to a self-sustaining state. A-C Total density with temperature contours (black). Eclogite (Ec in B and C) formation starts at ~ 6 Myr. Free water release shown in insets as white contours. D Evolution of surface topography (blue lines at start of interval, red at end) in which ridge rapidly forms within ~ 1 Myr but then subsides when the system becomes self-sustaining through a combination of thermal buoyancy and transformation of basalt to eclogite. From the models in Mao et al. (2017). Some ophiolites, subaerial slivers of oceanic lithosphere found between plates and bending of the high effective viscosity plate. within continental suture zones, have geochemical signatures of During the nucleation of a new trench, oceanic lithosphere should a subduction-related setting (Shervais, 2001). A key geochemical act as an elastic plate (McKenzie, 1977; Toth and Gurnis, 1998); marker of ophiolite formation in a subduction initiation setting is this means that older plates might be less favorable to subduction the presence of low-Ca or high-Si boninites (Pearce and Robinson, initiation (Cloetingh et al., 1989). If the plate boundary does not 2010). Type section boninites from the IBM forearc have these weaken sufficiently fast, the oceanic plate will not slide into the compositions, and they have been interpreted to reflect extreme mantle to allow the negative thermal buoyancy to grow (Gurnis prior mantle depletion caused by genesis of forearc basalts (Stern et al., 2004). Constitutive models with strain-weakening, due to and Bloomer, 1992). Ophiolites bearing high-Si boninites are no grain size reduction, grain damage, or volatile ingestion can lead to older than 2 Ga, suggesting that rigid plate subduction initiation plate instability (Bercovici and Ricard, 2014). During the incipient accompanied by near trench sea-floor spreading may have started phase, as crust on the newly underthrust plate is subjected to greater about this time (Pearce and Reagan, 2019). The most studied pressures, it will transition to eclogite. Eclogite will not only have a ophiolite belts are Tethyan from the Alps to the Himalaya and in age higher density which enhances subduction initiation, but the phase range from Jurassic to Cretaceous. Some have compositional and/ transition releases water which decreases the creep strength (Fig. 7) or stratigraphic affinities with the IBM forearc, suggesting that they leading to more favorable conditions to overcome resisting forces may represent IBM-like subduction initiation events (e.g. Pearce and (Mao et al., 2017). Alternative very large vertical forces at the edges Robinson, 2010; Reagan et al., 2010). Others have more complex of a plate, such as from an adjacent slab, can induce large vertical compositional patterns and age ranges, suggesting formation in velocities and near field extension (Maunder et al., 2020). more complex tectonic settings, such as Hunter-Matthew, where subduction is occurring beneath the northern end of the Lau Basin If the forces resisting subduction at the plate boundary can be (Embley and Rubin, 2018), or south of the Mariana Trough where overcome, one outcome is an oceanic plate that starts to founder and forearc rifts propagate close to the trench (e.g. Ribeiro et al., 2013). move vertically downward. The downward motion will lead to back- arc opening and trench rollback (Hall et al., 2003; Zhou, et al., 2018). Computational studies The initial evolution during initiation leads to both local tectonic changes and changes in the forces which drive plate motions. If the Theoretical and computational approaches have been used to plate founders, the energy released by the descending slab goes into investigate the mechanics of subduction initiation, the expected bending and deforming the plate with no change in absolute velocity geochemical expression as subduction starts, and the role of volatiles of the subducting plate (Leng and Gurnis, 2011). However, without among many other factors. The thermal age of oceanic plates, foundering and trench rollback, the energy released by subduction compositional variations across the nascent boundary, convergence goes into pulling the plate forward. Consequently, the combination rate and in–plane stress (Gurnis, et al., 2004; Nikolaeva, et al., 2010) of local tectonics (structural style, vertical motions and volcanism) and fault strength, fault weakening and plate bending (McKenzie, of new plate boundaries and changes in plate kinematics can be 1977; Toth and Gurnis, 1998) have been shown to be the key diagnostic of how plate forces changed during subduction initiation. factors that drive and retard initiation. Independent of initiation scenarios, driving forces must overcome frictional resistance 13Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Synthesis et al., 2016; Torsvik et al., 2017). In other words, HEB may represent a simultaneous combination of changes in absolute plate motion and Our understanding of the kinematics and dynamics of subduction plume motion at depth, raising the question of whether these two initiation has progressed enormously during MARGINS and processes could be coupled. GeoPRISMS. Observations of the IBM and Tonga-Kermadec systems have helped refine age constraints on the timing of initiation, while Future prospects new data from Puysegur have dramatically improved knowledge of subsurface structure. Subduction initiation is a frequent process The prospects for refining our understanding of subduction initiation with nearly all ocean-ocean subduction zones initiating in the remains bright through continued observationally campaigns linked Cenozoic and ocean-continent subduction zones reinitiating at with commensurate improvements in understanding kinematics plate boundaries that experienced subduction in the past (Fig. 1). A and dynamics. The mechanical details of how oceanic lithosphere key message is that there is no single tectonic pathway which gives transitions to a new subduction zone and how faults nucleate, grow rise to a new subduction zone, and either extension or compression and evolve during transition to a fully self-sustaining system (driven occurring in the near field during initiation. IBM is an example of entirely by local slab pull) remain unresolved and will require rapid and near-field extension, while Tonga-Kermadec and Puysegur evidence from multiple subduction initiation events at different are examples of near- and far-field compression. Hunter-Matthew is stages of initiation or which have particularly well-resolved features. an example of near-field transtension. Progress has also been made Targeting different subduction zones remains essential and there are on the antecedent tectonic conditions that must exist regionally. A important targets that could be accessed with offshore and onshore history of previous subduction likely must have existed in the region work. The Puysegur subduction zone remains a clear target for future of the nucleating boundary. Nucleation of the IBM system occurred work given the globally unique aspects of being in the process of adjacent to relic Mesozoic arcs, Tonga-Kermadec and Puysegur initiation while having both interpretable antecedent tectonics as Trenches formed along old Cretaceous subduction zones, and well as high precision plate kinematic constraints. The GeoPRISMS Hunter-Matthew propagated south- and east-ward from a Miocene Puysegur work acquired high quality seismic data that can now subduction zone. Different factors contributing to subduction be used to target sample collection, especially through deep sea initiation may include the added buoyancy of the relict arc, hydration drilling. Important targets could place precise constraints on the and weakening of lithospheric mantle, and the development of shear timing of in-plane compression and growth of the Puysegur Ridge zones. Ancient shear zones may have reduced grain sizes, fabrics as the new subduction interface nucleated. The evolution of volatiles generated in low-friction phyllosilicates, or produced significant likely plays an essential role during initiation as it is associated with degrees of micro-cracking and dislocations. These factors contribute both the change in forces, by the reaction of basalt to eclogite, and to low strength, which could lead to reactivation as the tectonic reduction in the effective viscosity of the mantle wedge (Fig. 7). conditions or state of stress change. With Puysegur showing different stages of initiation along strike, The connection between Pacific Plate motion and the initiation of we predict an along strike variation in volatile injection and hence subduction during the Eocene remains a tantalizing window into electrical conductivity which could be tested with a focused magnetic how plate tectonics works. The mechanics are being addressed tellurics experiment combined with deployment of broadband as the observational record has improved. On the surface, the OBS for deeper imaging and earthquake locations. The forearcs of connection between plate motion and subduction initiation seems the Aleutian, Tonga-Kermadec, and Vanuatu subduction zones all straightforward: The Hawaiian-Emperor seamount Bend (HEB) and remain essentially unsampled compared to the IBM record. All of initiation of the two largest, fully-oceanic subduction zones, IBM and these subduction zones evolved in different ways compared to IBM Tonga-Kermadec, all date to ~50 Ma. Questions raised are whether and so the expansion of this vital observational record is essential there was a synchronous change in absolute Pacific Plate motion, and to develop a comprehensive understanding of the geodynamics of if that change was a cause or consequence of subduction initiation. subduction initiation. Traditionally, HEB represents a change in absolute Pacific Plate Synthesizing these recently acquired and future observations and motion with respect to a Hawaiian mantle plume fixed in the linking them to the geodynamics of plate motions will remain a lower mantle with no motion between plumes globally (Clague substantial challenge but which becomes more tractable with the and Dalrymple, 1987; Sharp and Clague, 2006). However, there ongoing explosion in computational and information sciences. is no strong readjustment in relative plate motions along Pacific A key direction will be three-dimensional studies that track spreading centers at this time (Norton, 1995) but there is a substantial viscoelastoplastic materials incorporating volatile transport and change of the paleo latitudes of Emperor Seamounts from about magmatic processes from nucleation to self-sustaining subduction as 81 to 47 Ma consistent with southward motion of the Hawaiian plate motions change. The domain of inverse geodynamics is starting hot-spot (Tarduno, 2003). Global geodynamic models with plate to unfold, which will allow direct incorporation of observational motions show that the Hawaiian plume can move rapidly southward constraints into process-based models. during this interval, slowing down at 50 Ma (Hassan, et al., 2016). Nevertheless, global plate reconstructions still show a substantial Acknowledgements component in the change in absolute motion at about 50 Ma (Müller ■Supported by the National Science Foundation through OCE-1654766 and EAR-1645775 to Caltech and OCE-1654689 to UT Austin. 14 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

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2020 GeoPRISMS NSF Awards Photo by Taryn Lopez All GeoPRISMS NSF Awards are available on the GeoPRISMS website Visit p.51 to learn more about the GeoPRISMS Synthesis Workshops NSF Awards 1949148, 1949160 Collaborative Research: Evaluating The exhumation history of the Aleutians with zircon and apatite thermochronology Emily Cooperdock ([email protected]), Claire Bucholz ([email protected]) NSF Awards 1947758, 1948087 Collaborative Research: Incoming plate and forearc structure of the Semidi and SW Kodiak Segments offshore Alaska Peninsula from 3-D active-source and local earthquake tomography Anne Bécel ([email protected]), Juan Pablo Canales ([email protected]) NSF Awards 1948834, 1948862, 1949173 Collaborative Research: Synthesizing arc-scale geochemical, petrologic, and geophysical datasets to investigate causes of volcanic diversity in the Cascade Arc Geoffrey Abers ([email protected]), Adam Kent ([email protected]). Christy Till ([email protected]) NSF Award 1949171 Study of the impact of seamount subduction on the outer wedge of the Hikurangi margin from combined lab analyses of rock properties and marine seismic data Harm Van Avendonk ([email protected]), Nathan Bangs, Nicola Tisato NSF Award 1948902 Linking surface deformation to slab-mantle flow in the Cascadia subduction zone through 3D dynamic models Menno Fraters ([email protected]) NSF Award 1949073 Contrasting active magma- and fault-dominated segments of the East African Rift through the synthesis of InSAR and GPS time series: Implications for rifting dynamics and hazards Bridget Smith-Konter ([email protected]) NSF Awards 1947713, 1948504, 1949130 Collaborative Research: Behavior and structure on and around the megathrust revealed by the Alaska Amphibious Seismic Community Experiment Emily Roland ([email protected]), Susan Schwartz ([email protected]), Geoffrey Abers ([email protected]) NSF Award 1948961 Linking mantle structure and dynamics to the landscape evolution of the Cascadia forearc Joshua Roering ([email protected]) NSF Award 1949219 Constraining properties of pyroclastic density currents with remote infrasound and seismic observations Josef Dufek ([email protected]) NSF Award 1949210 Anisotropic imaging of the Alaska-Aleutian subduction zone from shear wave splitting analyses Colton Lynner ([email protected]) NSF Award 1949208 Quantifying carbon in the mantle lithosphere: Concentration, sources, and forms of carbon stored in carbonates and fluid inclusions Meghan Guild ([email protected])

Science Reviews New insights into influences on rift magmatism from research in the East Africa Rift System and Eastern North American Margin Tyrone O. Rooney (Michigan State University) and Donna J. Shillington (Northern Arizona University) Patterns of magmatism in the GeoPRISMS Rift Initiation and Evolution (RIE) primary sites (the East Africa Rift System - EARS, and the Eastern North American Margin - ENAM) can be used to advance our understanding of controls on rift magmatism and its relationship to extension. Multidisciplinary studies from these and other rifts have illuminated complex temporal and spatial relationships between magmatism and extension that deviate from the classic decompression melting model. A common theme of recent results in both of these rift systems is that the chemical and mechanical evolution of the continental lithosphere before, during and after rifting may account for some of this complexity. Introduction East African Rift System The relationship between magmatism and extension in East Africa The canonical model of rift magmatism involves decompression is not immediately apparent when considering the history of basin melting in response to lithospheric thinning. This model makes development and the timing of eruption of significant volumes of predictions for the timing, composition and volume of magmatism, igneous rocks. Extensional activity during the Mesozoic resulted in but the observed spatial and temporal patterns of magmatism the formation of interconnected rifts (Fig. 1), but the region lacked any diverge from the predictions of the decompression melting model associated wide-scale igneous events (e.g., Purcell, 2018). Deposition hypothesis. Recent studies in GeoPRISMS and MARGINS focus of Cretaceous sandstones persisted in these rift basins until the onset sites have facilitated the development of new constraints on some of magmatic activity during the Cenozoic (e.g., Tiercelin et al., 2012). of these potential controlling influences on rift magmatism. Here we briefly review some examples from recent results that show how 16 the depletion or enrichment of the mantle lithosphere and variations in lithospheric thickness, preceding, during, or after rifting, may account for the complex relationship between magmatism and extension. Temporal evolution of magmatism and extension 12 Observations of magmatism from rifts worldwide demonstrate Afar highly varied temporal relationships between deformation and magmatism, with implications for interplay between the two. Despite 8 Main the clear utility in linking magmatic events with extensional episodes, Ethiopian challenges remain in comparing the magmatic and structural records Rift preserved within rifts. 4 Turkana Figure 1. Generalized location diagram for the East African Eastern Rift after Rooney (2020d). The figure shows the extent of Toro Branch Ankole Neoproterozoic rocks (pink) and Mesoproterozoic and older rocks Kivu & (yellow). The Tanzania craton is outlined in a white overlay (Foley 0 Virunga Kenya Rift et al., 2012). Mesozoic rifting (northwest/southeast) and Cenozoic NTD rifting (north/south) is outlined by stippled patterns (Purcell 2018). Modern East African Rift Creataceous Rifting Eastern Branch regions are shown in yellow; Western Branch locations are shown in black. NTD - Northern Tanzania Divergence. -4 Cenozoic Lavas of East Africa 18 • GeoPRISMS Newsletter Issue No. 43 Fall 2020 Western Precambrian Features Branch NeoPprreoCtaemrobzroiaicn F eatures MesoprNoetoeprrootezrooziocicand older -8 Rungwe Mes oproterozoic and older Extent oE xfteTnatnofzTaannziaanCia rCartaotonn 0 400 800 km 28 32 36 40 44

The earliest manifestations of the Cenozoic Large Igneous Province magmatism and faulting (e.g., Mohr 1967) are evidence for the occurred during the Eocene (~45 Ma; Davidson & Rex, 1980; Ebinger migration of strain away from rift border faults towards rift-central et al., 1993; George et al., 1998), resulting in the eruption of flood zones of faulting and magmatic intrusion (Hayward & Ebinger basalts in southern Ethiopia and northern Kenya (George & Rogers 1996; Ebinger & Casey 2001). Modern magmatism within the 2002). Evidence of contemporaneous extension is ambiguous and less developed sectors of the EARS is broadly centered on discrete best expressed in the Turkana Depression of northern Kenya (e.g., volcanic centers erupting relatively alkaline compositions (e.g., Purcell, 2018). Flood basalt magmatism continued through the Mana et al., 2015; Barette et al., 2017). However, even in the less Oligocene, expanding into northern Ethiopia and Yemen (e.g., mature southern part of the Eastern Rift, strain migration also Baker et al., 1996; Pik et al., 1999; Furman et al., 2016). Flood basalt appears connected to magmatism and magmatic fluids (Muirhead magmatism eventually transitioned to more silicic activity along et al., 2016). the nascent rift (Ukstins et al., 2002), with some basaltic volcanism In aggregate, the existing evidence on the timing of magmatic events persisting in central Ethiopia (Nelson et al., 2019). in East Africa shows that extension and magmatism have become Beginning ca. 26.9 Ma, a new dominantly basaltic phase of tightly linked. The pulsed nature of these magmatic and extensional magmatism was recognized throughout the northern EARS (Rooney events is an important addition to our understanding of what may 2017). These flows typically overlie paleosols, suggesting that this typically be considered a continuous process (Rooney 2020a). The event followed a period of relative quiescence. The origin of this incorporation of such temporal variability into the next generation of event is unclear – rifting continued along the Afar rift and in Turkana rifting models provides potentially new insights into the mechanisms (Purcell 2018), but there is no widespread surface manifestation underpinning rift evolution. of rifting. During this time period, lithosphere-derived alkaline Eastern North American Margin volcanism also began as far south as Kivu-Virunga and Rungwe - the ENAM exhibits a similarly complex apparent relationship between first manifestations of volcanism in the Western Branch of the EARS the timing of magmatic and extensional phases (Figs 2, 3). Wide- (Roberts et al., 2012; Rooney et al., 2014b; Pouclet et al., 2016). This spread extension leading to the formation of the rift basins along phase of magmatism continued to ~16 Ma, extending magmatic ENAM and conjugate margins began at ~235 Ma based on the ages activity into the nascent Kenya Rift (Rooney, 2020a). There then of synrift sediments in rift basins (e.g., Withjack et al., 2012 and followed a largely silicic volcanic event dominated by the eruption references therein). The timing and duration of extension varies of flood phonolites in the southern EARS, while less alkaline activity along strike. In the northern ENAM, extension continued to younger is evident farther north (Smith 1994, Rooney 2020a, b). This silicic ages (~200-195 Ma) in comparison to the south, where extension magmatic event exhibits linkage between the spatial distribution of may have largely ceased by 215-205 Ma (Withjack et al., 2012) magmatism and faulting (Ebinger et al., 2000). (Fig. 3). The earliest stages of extension appear largely amagmatic Beginning ~12 Ma, the Mid Miocene Resurgence Phase is a period of - there is an absence of contemporaneous sills and lavas within the dominantly basaltic activity recorded throughout the EARS (Rooney rift basins, though synrift magmas may have intruded the crust at 2020a), which is associated with a pronounced period of extension depth during extension (Marzen et al., 2020). in Afar and Turkana (e.g., MacGregor 2015) and more widespread volcanism in the Western Branch. This phase was followed -85˚ -80˚ -75˚ -70˚ -65˚ -60˚ -55˚ by silicic volcanism in the now developing rifts of the Eastern Branch (Early Rift Development Phase: Rooney 2020a,b), and Magnetic anomalies cycles of volcanism in the Western Branch (e.g., Fontijin et al., 45˚ Rift basins 2012; Mesko, 2020). The pulsed nature of this magmatism and Select Paleozoic sutures extension becomes ever more apparent with another widespread CAMP dikes basaltic event in the Eastern Branch beginning ~4 Ma (The Stratoid Phase: Rooney 2020a,b,c) that is also linked with a 40˚ East Coast Magnetic Anomaly North period of extension within the Turkana Depression and in Afar. EuropeAmerica In the more developed sectors, zones of focused basaltic 35˚ Figure 2. Eastern North American Margin. Rift Africa basins (blue) and Paleozoic structures (purple) after Withjack et al (2012). Sills and dikes from 30˚ CAMP South 25˚ Extent America Ragland et al (1983) and McHone (2000). Magnetic anomalies from Müller et al (1997). The East Coast Magnetic Anomaly is plotted from EMAG2, with darker colors indicating larger magnetic anomalies. Inset shows the estimated aerial extent of CAMP over a reconstruction of Pangea after Marzoli et al (2018) 19Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Figure 3. Estimated timing of extension in onshore rift basins with respect to CAMP and offshore extension and seafloor spreading (Withjack et al, 2012). Basins are: SG, South Georgia; DR, Deep River; D, Danville/Dan River; FM, Farmville and Briery Creek; NO, Norfolk; R, Richmond; T, Taylorsville; C, Culpeper; G, Gettysburg; N, Newark; P, Pomperaug; H, Hartford; DF, Deerfield; CV, Connecticut Valley (Hartford and Deerfield combined); A, Argana; F,Fundy; M, Mohican; O, Orpheus ; SJ, southern Jeanne d’Arc; NJ, northern Jeanne d’Arc. At ~201 Ma and lasting < 1 million years, the Central Atlantic known due to a lack of deep drilling and uncertainties associated Magmatic Province (CAMP) formed over a 10 million km2 region, with the interpretation of magnetic and seismic data (e.g., Oh et al., including the ENAM (e.g., Hames et al, 2000; Blackburn et al 2013; 1991, 1995; Labails et al., 2010; Heffner et al, 2013; Greene et al., Marzoli et al., 2018; Fig. 2). The timing of extension of onshore basins 2017). The correlation of a dated sill onshore with offshore seismic varies along the margin, leading to varied temporal relationships reflection data was interpreted to indicate that offshore magmatism between extension and CAMP magmatism. CAMP magmatism was considerably younger than CAMP (Lansphere, 1983; Oh et al., may have occurred in the northern regions of ENAM before the 1991), though both the age of the sill and the correlation have been southern regions (Blackburn et al., 2013). In rift basins in the questioned by recent work (Olsen et al., 2003; Hames et al., 2010; northern ENAM, CAMP dikes are parallel to rift basins, and were Heffner et al., 2013). Recent modeling of the magnetic signature thus likely emplaced during rift development (Olsen, 1997; Schlische of SDRs implies they were emplaced over at least 6 million years et al., 2003). In contrast, the extension necessary to form basins in and possibly up to 31 million years (Davis et al, 2018), in contrast the southern ENAM appears to have preceded CAMP (e.g., Schlische to the rapid apparent emplacement of CAMP onshore (Blackburn et al., 2003) as the orientation and distribution of sills and dikes at et al., 2013). the surface bear little relationship to rift basins here (McHone, 2000; Voluminous magmatism during crustal thinning on the US margin Schlische et al, 2003). However, recent studies imply there may be was followed by the emplacement of a ~150-km-wide zone of thin a correlation between some rift basins and magmatic intrusions at and highly faulted crust with anomalously high seismic velocities depth (Marzen et al, 2020). (Shuck et al., 2019; Bécel et al, 2020). This zone could have either Continued extension culminated in the rupture of Pangea and was been emplaced by asymmetric seafloor spreading or by an unstable accompanied by significant magmatism on the ENAM rifted margin early ridge system that later jumped east (Labails et al., 2010; Kneller based on seismic imaging of seaward dipping reflectors - SDRs et al, 2011; Greene et al, 2017). At the Blake Spur Magnetic Anomaly, (e.g., Austin et al.,1990; Oh et al., 1991; Bécel et al, 2020), elevated an abrupt thickening of crust and reduction in faulting is observed lower crustal seismic velocities interpreted to represent mafic (Shuck et al., 2019; Bécel et al., 2020), implying a relatively rapid intrusions and/or underplating (LASE Study Group, 1986; Trehu transition to much more magmatically robust spreading. Farther et al., 1989; Holbrook & Kelemen, 1993, Shuck et al., 2019), and the north, the Canadian part of the ENAM experienced a very different prominent East Coast Magnetic Anomaly (e.g., Alsop & Talwani, history of magmatism. Offshore Nova Scotia, geophysical data 1984). However, the timing of extension and magmatism are poorly suggest a rapid transition from magma-rich to magma-poor rifting 20 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

(Lau et al, 2019), and the Canadian margins farther north are type alkaline eruptions, but the volume of material sampled by such examples of magma-poor rifting followed by the emplacement of events is extremely limited. Alternatively, continental rifts provide highly faulted slow spreading oceanic crust (e.g., Hopper et al., 2004; another avenue by which the continental lithospheric mantle can be Tucholke et al., 2004; Van Avendonk et al, 2006; Shillington et al., studied through destabilization and incorporation into rift magmas. 2006, Lau et al., 2006). Intriguingly, spatially limited magmatism persisted on the US East African Rift System Margin long after rifting, with volcanics as young as ~47 Ma observed in Virginia (Furman and Gittings 2003; Mazza et al., 2017). Both Within the EARS, the type of lithosphere through which a magma Virginia and New England are underlain by low-velocity anomalies erupts is the single most important control on compositional (Wagner et al, 2016; Schmandt & Lin, 2014; Porter et al, 2016; Biryol heterogeneity within the rift (Rooney 2020d), supporting the et al., 2016), implying warmer mantle compared with that expected strong linkage between the existing composition of the continental for a ~200 Ma old passive margin. In conclusion, the evolution of the lithospheric mantle and erupted magmatic products. Prior studies Eastern North American Margin includes apparently magma-poor have shown that lavas erupted in the southern EARS record a early extension, the rapid emplacement of a large igneous province, composition requiring interaction with a relatively thick continental possibly prolonged magmatic rifting leading to rupture, followed by lithospheric mantle that had an extensive history of enrichment and slow, magma-starved early seafloor spreading and then magma-rich overprint (e.g., Furman & Graham 1999; Rogers et al., 1992; 1998). spreading. Spatially limited postrift magmatism continued for over Magmas erupting through the younger lithosphere located in the ~120 million years after the onset of seafloor spreading. northern EARS exhibit evidence of contributions from a continental lithospheric mantle that was enriched during the Pan-African Influence of pre-existing lithospheric composition subduction/orogenic events and during recent plume interaction (Rooney et al., 2014b; 2017; Nelson et al., 2019). Enriched domains The continental lithospheric mantle is dominantly composed of within the lithospheric mantle – termed ‘metasomes’ – are considered peridotite but is compositionally more complex and may play a more the source of highly alkaline eruptions in the Western Branch of the important role in the initiation and development of a continental EARS (Roberts et al., 2012), and in the northern EARS during the rift than initially understood. There is a growing awareness that early Miocene (Rooney et al., 2014b; 2017; Nelson et al., 2019). These the continental lithospheric mantle records interaction with enriched domains of the lithospheric mantle have diverse origins sub-lithospheric reservoirs over the life of the plate. Depletion of the that are formed through the interaction of sub-lithospheric melts/ lithospheric mantle is commonly associated with its initial formation, fluids with the continental lithospheric mantle and contain phases and results from melt extraction. However, the interaction between that will readily melt upon minor thermo-baric perturbation of the the continental lithospheric mantle and fluids/melts that percolate continental lithosphere. These enrichment events, while important from sub-lithospheric reservoirs have the potential to: (A) enrich the in generating signatures of prior instances of mass exchange between continental lithospheric mantle in incompatible elements including lithospheric and sub-lithospheric geochemical reservoirs, have volatiles; (B) create heterogenous lithologic domains; and (C) potentially profound implications for the terrestrial mass distribution generate unusual isotopic signatures. Resulting variations in mantle of important geochemical species such as CO2. Rifting of a continent lithosphere composition may control the location and composition may liberate vast quantities of such species with attendant impacts of magmatism, and the rheology of the plate and its response to on atmospheric CO2 levels (Lee et al., 2016; Brune et al., 2017). extension. Probing the continental lithospheric mantle is commonly achieved through the study of mantle xenoliths carried within crust Figure 4. Cartoon representing generalized melt lithospheric mantle generation processes within the Eastern Branch of the East African Rift System (after Rooney 2020d). amphibole metasome pyroxenite metasome The existing Pan-African aged lithosphere has been enriched by chromatographic metasomatism astenosphere as fluids/melts passed through the lithospheric mantle. The asthenosphere in this area has been hybridized and homogenized by interaction with lithospheric materials. Melts from this hybridized asthenosphere interact with the Afar plume and melt by decompression forming the majority of lavas within the rift. Other magmatic events may be the result of the thermo-baric destabilization of amphibole-bearing metasomes within the lithospheric mantle or from delamination of the lithosphere. 21Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Eastern North American Margin farther north (e.g., Fishwick, 2010). The resulting variations in Prior to the Mesozoic phase of extension, ENAM experienced lithospheric composition and thickness control compositional multiple cycles of collision and extension (e.g., Hatcher et al, 2010) variations in the magmatic products erupted within the EARS, as that exerted a strong control on many aspects of magmatism and we have described earlier, and influence the localization of extension extension (e.g., Puffer, 2003; Thomas, 2006; Withjack et al., 2012; and magmatism (e.g., Corti et al, 2007). For example, recent work Benoit et al., 2014; Whalen et al, 2015; Marzen et al., 2019). Despite in the Tanzanian divergence and northern Malawi Rift shows that the continued debate on the origin of CAMP, an emerging point of lithospheric modification is localized at the boundaries between agreement is that the source of CAMP magmas was enriched with pre-existing lithospheric terranes (Tiberi et al., 2019; Hopper subduction components from prior collisional events (e.g., Puffer, et al., 2020). Superimposed upon this lithospheric arrangement 2003; Whalen et al, 2015). Subduction influences may have included are Mesozoic rifts that have also impacted the development of sediments and sediment melts, pyroxenitic source arising from magmatism in the Cenozoic EARS (Purcell, 2018). Lithospheric the reaction of peridotite with silicious material from subducting attenuation during the Mesozoic may have controlled the subsequent sediments or crust, metasomatism from subduction derived fluids, distribution of flood basalt magmatism throughout East Africa or some combination thereof (e.g., Puffer, 2003; Callegro et al, 2013; (Ebinger & Sleep, 1998), and facilitated the early development of Whalen et al., 2015; Elkins et al., 2020). The mantle source appears magmatism in some regions (e.g., Tepp et al., 2018; Grijalva et al., to vary considerably across CAMP, including along the ENAM, 2018; Rooney 2020b). While pre-existing rifting episodes clearly suggesting local variations in subducted materials and modification impart a significant influence on rift magmatism, ancient reactivated of the mantle lithosphere (Whalen et al., 2015; Elkins et al., 2020). shear zones located throughout the rift have an equally prominent A more significant subduction component is observed in the north role and may help explain the distribution of some off-axis volcanism than the south, possibly due to the different prior accretionary (Abebe et al. 1998;2014; Corti et al., 2018; Le Gall et al., 2008; Smets histories (Whalen et al., 2015). et al., 2016). CAMP was associated with major environmental change and a mass Unsurprisingly, the distribution of magmatism within the EARS is extinction event at the Permian-Triassic Boundary (Blackburn et al., also closely intertwined with modern rifting events. The mechanisms 2013) - increased CO2 was an important component of this event by which thinning of the continental lithosphere proceeds are (e.g., McElwain et al., 1999). Some of this CO2 likely originated from among the most intensely studied in the rifting community and degassing of intruded sediments (Heimdal et al., 2018). However, include plate dilation, thinning, and removal (e.g., Ayele et al., 2007; geochemical analysis of melt inclusions in CAMP lavas demonstrates Mazzarini et al., 2013; Muirhead and Kattenhorn., 2018; Rooney that at least some of the CO2 released in this event must originate et al., 2011). Within developed sectors of the EAR, such as Afar, in the middle/lower crust or mantle (Capriolo et al., 2020), which there exists a clear relationship between the age of magmatism may have been sourced in the subducted components in the mantle and extension, suggesting almost oceanic-like characteristics – the source of CAMP. Degassing from intrusive components may have most recent basalts occur within the axial grabens with evidence also occurred before their extrusive counterparts (e.g., Davies et of symmetric magnetic lineations (Ferguson et al., 2013; Bridges al, 2017). et al., 2012). However, the processes by which such localization occurs remains unclear – recent work has shown zones of focused Lithospheric thickness, removal, and magmatism intrusion occurring outside of the rift border faults (Rooney et al., 2014a; Chiasera et al., 2018). In both ENAM and the EARS, evolving lithospheric thickness While dilation of the lithosphere may be associated with focused and magmatism are broadly linked. The surface expression of zones of magmatic intrusion, thinning of the continental lithosphere magmatism is strongly modified by pre-existing and synrift changes may be revealed in other magmatic events. Lithospheric thinning in lithospheric thickness (e.g., Ebinger & Sleep, 1998; Burov & Gerya, by stretching persists even in regions of advanced rift development 2014; Koptev et al., 2015). Magmatism can also infiltrate and erode such as Afar, and results in significant volumes of basalt erupted the mantle lithosphere (e.g., Holtzman & Kendall, 2010; Havlin et at the surface (Bastow & Keir 2011). Thinning of the plate may al., 2013). Feedbacks between focusing of magmatism in regions also influence and be influenced by early rift magmatism. Seismic of thinned lithosphere and thermochemical erosion can result in imaging and gravity data reveal higher degrees of thinning of the dramatic variations in the thickness and/or velocity structure of lithospheric mantle compared with the crust in the Albertine rift the mantle lithosphere (e.g., Bastow et al., 2010; Tiberi et al, 2019) (Wölbern et al., 2012) and in the northern Malawi Rift (Njinju et and may exert a strong control on the spatiotemporal evolution of al., 2019; Hopper et al., 2020). Although magmatism appears to be magmatism. limited below the Western Rift at present (O’Donnell et al., 2013; East African Rift System Accardo et al, 2020), at least small degrees of magmatism may have The lithosphere within which the EARS formed exhibits significant enabled weakening and thinning the lithosphere by thermochemical diversity of lithospheric structure. The thick Tanzania craton and erosion (Wölbern et al., 2012; Hopper et al, 2020). Another possible surrounding Proterozoic mobile belts dominate the southern portion mechanism for lithospheric destruction is by ‘delamination’ or ‘drip’ of the Eastern Branch and most of the Western Branch of the EARS; wherein gravitational instabilities in the continental lithospheric the thinner Pan-African Mobile Belt lithosphere is most pronounced mantle result in the removal of portions of the continental 22 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

lithosphere, accompanied by a magmatic pulse (Furman et al., 2016). et al., 2016; Biryol et al., 2016; Wagner et al., 2016) that could be Metasomatism of the lithosphere by prior events has been proposed explained by delamination. to make it more susceptible to both foundering (Furman et al., 2016) or to small degrees of melting that could promote thermochemical Discussion and future questions erosion of the lithosphere (Hopper et al., 2020). Eastern North America The factors described in previous sections represent only a few Inherited and evolving variations in lithospheric thickness likely examples of the important controls on rift magmatism probed strongly influenced the evolution of rifting and magmatism along by recent research at GeoPRISMS primary sites. Although there ENAM at different stages of development, though the manner has been substantial progress towards understanding the causes of that influence is still debated. Competing models for CAMP and consequences of rift magmatism over the last decade, many magmatism during the early stages of rifting invoke changes in questions remain, in part due to critical data gaps. One key gap is lithospheric thickness. The edge-driven convection models require a timing. In ENAM, the absence of deep drilling data means that the pre-existing step in lithospheric thickness (King & Anderson, 1995). timing and rates of extension and magmatism, and their relationship Some models require delamination to explain temporal and spatial to onshore events, are very poorly known. In the EARS, there is patterns of CAMP magmatism (e.g., Whalen et al., 2015). During a growing, yet inadequate record of the ages of magmatism, but the late stages of continental rifting, magma-rich rifting and crustal corresponding temporal constraints on rift basin development are rupture was followed by the emplacement of a thin, highly faulted comparatively limited, leaving uncertainties in the relationships early oceanic crust with relatively fast lower crustal seismic velocities between the evolution of magmatism and extension. Another that may be explained by magma generation resulting from elevated important unknown for understanding both ancient and active mantle potential temperatures beneath a ~15- to 20-km-thick rifts is the chemical and rheological evolution of the continental remnant lithospheric lid (Shuck et al. 2019; Bécel et al. 2020). The lithosphere before, during, and after extension, which clearly has a abrupt transition to a thick, smooth oceanic crust at the Blake Spur controlling influence on rift evolution. The final stages of continental Magnetic Anomaly could be the consequence of lithospheric rupture. rupture and transition to seafloor spreading continues to be poorly Finally, ongoing post-rift magmatism along ENAM as recently as understood despite decades of research. As studies of rifted margins 47 Ma may be caused by lithospheric delamination (Mazza et al., continue farther offshore, we learn that this transition may continue 2014; 2017; Meyer & van Wijk, 2015). This is supported by numerical longer and be more complex than previously recognized. Close models that predict instabilities can develop due to changes in collaboration between the rift and ridge communities is required lithospheric thickness resulting from rifting (Meyer & Van Wijk, to address this question. For all of these questions, future progress 2015). 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Development of the asymmetric nonvolcanic rifting and slow incipient oceanic accretion passive margin of Eastern North America: Mesozoic rifting, igneous from seismic reflection data on the Newfoundland margin. J Geophys activity and breakup, Phanerozoic rift systems and sedimentary Res, 111, doi.org/10.1029/2005JB003981 basins. Elsevier, 301-335 Shuck, B.D., H.J.A. Van Avendonk, A. Bécel (2019). The role of mantle Wölbern, I., G. Rümpker, K. Link, F. Sodoudi (2012). Melt infiltration of melts in the transition from rifting to seafloor spreading offshore the lower lithosphere beneath the Tanzania craton and the Albertine eastern North America. Earth Planet Sci Lett, 525, doi.org/10.1016/j. rift inferred from S receiver functions. Geochem Geophys, 13, 8, doi. epsl.2019.115756 org/10.1029/2012GC004167 Smets, B., D. Delvaux, K.A. Ross, S. Poppe, M. Kervyn, N. d’Oreye, F. Kervyn (2016). The role of inherited crustal structures and magmatism in the development of rift segments: Insights from the Kivu basin, western branch of the East African Rift. Tectonophysics, 683, 62-76 Smith, M. (1994). Stratigraphic and structural constraints on mechanisms 26 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

COMING SOON GeoPRISMS Legacy Website . BennettPhoto by S The GeoPRISMS content will continue to be available to the Community after the office closes in April 2021 The website will include an interactive map featuring research projects funded by GeoPRISMS Stay tuned for the official launch of the website! Photos by Liz Cottrell (top) and Dan Rasmussen (bottom) 27Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Science Reviews Subduction megathrust locking and slip behavior: Insights from geodesy Laura M. Wallace (University of Texas Institute for Geophysics, GNS Science, New Zealand), Noel Bartlow (Kansas University), Julie Elliott (Purdue University), Susan Schwartz (UC Santa Cruz) Our understanding of subduction zone slip has been transformed over the last two decades, thanks in large part to the advent of high-precision geodetic techniques. These have illuminated crustal deformation at all stages of the earthquake cycle, as well as a rich variety of transient, aseismic slip processes. These observations have provoked new questions regarding fault mechanics and earthquake occurrence, helping to guide multi-disciplinary investigations at MARGINS and GeoPRISMS focus sites to reveal the processes behind subduction plate boundary dynamics. MARGINS and GeoPRISMS have especially played an important role in expanding geodetic investigations of subduction megathrust slip at many of its focus sites, which include Costa Rica, Nankai, Cascadia, Alaska, and New Zealand. Together, these locales exhibit virtually every known flavor of subduction slip behavior, megathrust locking characteristics, and subduction margin physical properties. Together this diversity has provided an outstanding opportunity to resolve the physical processes leading to episodic slow slip, as well as those causing some subduction zones to lock-up and slip in Great (Mw >8.0) earthquakes versus being dominated by aseismic creep processes. Introduction nucleate) at many subduction zones worldwide, greatly improving our understanding of earthquake and tsunami hazard posed by The advent of space geodetic techniques to monitor crustal subduction plate boundaries. These locked regions are often located deformation has revolutionized our ability to resolve subduction offshore, and wider use of seafloor geodetic techniques is needed to megathrust slip processes at all stages of the earthquake cycle. better define them. During the time between large subduction thrust earthquakes (the More recently, scientists have discovered the existence of episodic interseismic period), the subducting and overriding tectonic plates slow slip events (SSEs) that occur repeatedly on subduction can become locked together, accumulating stresses for hundreds of megathrust faults (Dragert et al., 2001; Schwartz and Rokosky, 2007 years or more that will ultimately be relieved in future subduction and references therein). SSEs involve a few to tens of centimeters of earthquakes. This “locking” or “coupling” (we use the two terms slip along faults over days to years, and can be likened to earthquakes interchangeably here) creates accumulation of elastic strain in the in slow motion. Because SSEs occur so slowly, detecting them surrounding crust, which can be measured as small changes in relies on monitoring millimeter- to centimeter-level changes in ground movement at the Earth’s surface above subduction zones the position of the Earth’s surface above the SSEs using a range of (Fig. 1). Data from satellites that are part of the Global Positioning geodetic methods. Integrating temporally diverse observations from System (GPS) and more recently, other Global Navigation Satellite geodesy (hours to years) and seismology (seconds to minutes) has Systems (GNSS) captured by antennas mounted on geodetic provided further insight into SSEs and related fault slip processes. monuments on the Earth’s surface enable measurement of changes The discovery of SSEs on many subduction zones and other types of of the position of these monuments, at a millimeter-level or better faults around the world over the last fifteen years has revealed that (hereafter, we use GPS/GNSS to refer to technologies related to faults undergo slip in a broad spectrum of slip behaviors, and that GPS and GNSS). We can use these changes in surface movements these slow events play a major role in the earthquake cycle and the to determine where and how fast the plates are locking together and accommodation of plate motion. These discoveries have sparked accumulating stress that may be relieved in future earthquakes. In the exciting new fields of inquiry in geodesy, seismology, and fault case of some recent, major subduction earthquakes, the portions of mechanics. Improved geodetic techniques applied to subduction the subduction plate interface that ruptured generally coincide with zones have also revealed the role that other deformation processes regions where the plate interface was locked prior to the earthquake, play through the seismic cycle, including coseismic (e.g., during the as determined by geodetic measurements (e.g., Loveless and Meade, earthquake) deformation, and afterslip and viscoelastic deformation 2011; Protti et al., 2014; Métois et al., 2016). Delineation of the in the years and decades following major earthquakes (Johnson and locked plate interface using geodetic studies has revealed the likely Tebo, 2018; Li et al., 2018a; Sun et al., 2014). seismogenic zone (e.g., the region on a fault where earthquakes 28 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

APGs csoLAituaecnpscdlibnrweegattwroidonemenmnoaeevrgaeyarmttwhheqrneuutdsaotkgfegesedoudeettoic SAR Satellite GPS/GNSS satellite GNSS-A GPS/GNSS site SubTdruenccthinPcgorsepseiSlbpau,lteboedsrulsochtwianlsglolippwSceloa,eatacriuesotspmheuliqepsoumdlgianeibkcgneeitscCSweuozvebnoeeddnnnuiettcis,toCibonOneaRlliolnKywtesrtcfaoaSbcuteleUpe/aM:pledspedlyogecCwrazrtopoehsanlerlasiueptptset SSEs Strainmeter, tiltmeter Example of continuous GNSS timeseries impacted by SSEs 20 Figure 1. Schematic diagram illustrating the types of geodetic measurements Position (mm) 10 that can be made at subduction zones to discern megathrust coupling and slip 0 behavior. Lower right panel shows an example of a continuous GPS/GNSS site −10 2010 2013 2016 2019 timeseries impacted by slow slip event (SSE) occurrence (SSE timing illustrated −20 Time with shaded blue bars). This schematic is highly generalized, and there are many examples globally where the nature of slip behavior is far more 2007 heterogeneous in the various regions than shown. Methods used to evaluate subduction locking slip behavior (Araki et al., 2017; Davis et al., 2015; Hawthorne and and slip behavior Rubin, 2013; Obara et al., 2004), with much greater sensitivity than either GPS/GNSS or InSAR methods. On the seafloor, GPS/ GPS/GNSS measurements are taken at survey points permanently GNSS-Acoustic methods, which involve acoustic ranging using a attached to the ground either by intermittent (campaign- or survey- ship or a Wave Glider on the sea surface that is precisely located style) or continuous (daily) collection of phase and pseudorange by GPS/GNSS satellites, are capable of detecting centimeter-level data from the constellation of GPS/GNSS satellites that orbit the horizontal deformation rates (Bürgmann and Chadwell, 2014). Earth. Campaign-style measurements lack temporal resolution as Absolute Pressure Gauges, which measure vertical deformation of measurements are typically undertaken months to years apart for the seafloor by continuously recording changes in pressure due to several days at a time, so are largely suitable for investigating long- the overlying water column are becoming widely used to resolve term deformation rates in a region over many years. Continuously centimeter-level vertical deformation offshore during earthquakes operating onshore GPS/GNSS networks have become increasingly and slow slip events (Ito et al., 2013; Wallace et al., 2016). common at subduction zones and other plate boundaries worldwide, Many modelling techniques that connect geodetic displacements enabling extraction of daily or sub-daily positions of these sites to locking and/or slip on a megathrust plate boundary assume that with millimeter-level accuracy. High-rate, real-time positioning the Earth’s crust largely behaves as an elastic medium. The majority of continuous GPS/GNSS sites has also contributed to rapid of fault slip models developed to fit surface geodetic data assume seismological characterization of earthquakes (Crowell et al., 2012), that this behavior can be captured with dislocations in an elastic, and is currently being used to develop geodetic earthquake early half-space, for which widely used analytical equations have been warning systems (Ruhl et al., 2017). High-rate data can also help developed (e.g., Okada, 1992). To address locking at subduction characterize the earthquake rupture process (Miyazaki et al., 2004) zones, a “backslip” approach (Savage, 1983) is the most widely and strong ground motion characteristics (Grapenthin et al., 2018). used, which assumes slip in a direction opposite to that of plate Continuously operating GPS/GNSS networks positioned above motion to determine the elastic component of deformation due to subduction zones in Canada and Japan led to the first discoveries locking. To capture the complex kinematics and active tectonics of episodic slow slip events, lasting weeks to years (Dragert et of some subduction settings, many have turned to elastic block al., 2001; Hirose et al., 1999). Interferometric Synthetic Aperture modelling to discern interseismic coupling, where the velocity Radar (InSAR) techniques have also become widely used to resolve field is fit by rotation of elastic crustal blocks, and backslip along coseismic and postseismic deformation processes at subduction those block boundaries (e.g., McCaffrey, 2002). This approach has zones (Beavan et al., 2011; Lin et al., 2013) as well as some slow been implemented at a number of subduction margins worldwide slip events (Bekeart et al., 2015; Hamling and Wallace, 2015). As (La Femina et al., 2009; Loveless and Meade, 2010; McCaffrey et InSAR relies on repeated satellite images of the Earth’s surface, al., 2000; Nishimura et al., 2018; Schmazle et al., 2014; Wallace et it provides much greater spatial coverage (albeit with reduced al., 2004). Although such elastic models have proven very useful, temporal coverage) than GPS/GNSS, or other techniques that utilize they are limited in their ability to address the influence of other instruments that are typically widely spaced (>20 km). Borehole rheologies on crustal deformation at subduction zones such as instrumentation, such as strainmeters, tiltmeters, and pore pressure viscoelasticity, recognized as an important aspect of deformation sensors as a proxy for volumetric strain at onshore and offshore at during the earthquake cycle (Wang et al., 2012). They can also have a subduction zones are proving increasingly useful to reveal transient large impact on interseismic coupling results (Li et al., 2015; 2018a). 29Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

To investigate transient slip events (episodic slow slip events, and Southwest Japan’s Nankai Trough afterslip following earthquakes), elastic dislocation methods are also commonly used, by inverting surface displacements to estimate The Philippine Sea Plate is subducting westward beneath southwest slip on faults embedded in an elastic half-space. Time-dependent Japan along the Nankai Trough at rates of ~6 cm/yr (DeMets et al., inversions fitting continuous GPS/GNSS timeseries - as opposed 2010). This subduction zone has a long, well-established history to models fitting static displacement fields - enable more thorough of producing Great earthquakes (Mw > 8.0), approximately every exploration of the evolution of these slip events. A number of codes 90-260 years, with the latest of these being the 1944 M ~8.0 Tonankai with a variety of different approaches and assumptions have been earthquake and the 1946 M ~8.2 Nankaido earthquake (Ando, developed to undertake time-dependent inversions for transient 1975). Numerous geodetic studies indicate that the subduction deformation (e.g., Segall and Matthews, 1997; Miyazaki et al., 2006; interface at the Nankai Trough is currently interseismically coupled McCaffrey, 2009; Kositsky and Avouac, 2010). in the source region of these great earthquakes (Mazzotti et al., 2000; Nishimura et al., 2018; Yokota et al., 2016, among others; Geodetic insights into megathrust slip processes Fig. 2), suggesting that the currently accruing elastic strain will at MARGINS and GeoPRISMS sites ultimately be relieved in future megathrust earthquakes. Although most interseismic deformation models are based on data from an Geodetic investigations of crustal deformation at subduction extensive network of land-based continuous GPS/GNSS stations margins provide important context for efforts that use other (Sagiya et al., 2000), Japan has also led the world in acquiring data techniques (seismic imaging, earthquake seismology, heat flow, from a network of about 15 GPS/GNSS-Acoustic arrays overlying electromagnetics, geochemistry, rock deformation experimental the offshore Nankai Trough, providing the first-ever detailed view studies, among others) to constrain the physical controls on of horizontal deformation related to interseismic coupling on an subduction megathrust slip behavior. In particular, previous and plate boundary offshore (Yokota et al., 2016). Together, the onshore ongoing geodetic studies at MARGINS and GeoPRISMS focus sites and offshore data suggest that the down-dip limit of interseismic have provided important underpinning datasets to inform strategies coupling occurs at ~30 km depth beneath Shikoku Island and the Kii for scientific targets and data acquisition efforts, to answer key Peninsula, and locking persists up to 0-10 km depth (Nishimura et questions such as: What governs the size, location and frequency al., 2018; Yokota et al., 2016; Fig. 2). Moreover, repeated levelling and of great subduction zone earthquakes and how is this related to tide gauge datasets acquired since 1947 have revealed variations in the spatial and temporal variation of slip behaviors observed along rates of vertical deformation through different stages of the seismic subduction faults? How does deformation across the subduction cycle, highlighting the influence of viscoelastic mantle flow on the plate boundary evolve in space and time, through the seismic cycle deformation field for several decades following major earthquakes and beyond? http://geoprisms.org/initiatives-sites/scd/. Although there (Johnson and Tebo, 2018). These rich and long-lived onshore some of the geodetic studies summarized here were undertaken and offshore geodetic datasets has made the Nankai Trough one of by international partners, or through funding from other NSF the best-instrumented subduction zones on the planet, enabling programs (or other federal agencies), they have all contributed characterization of crustal deformation processes throughout the greatly to broader outcomes of MARGINS and GeoPRISMS at each megathrust seismic cycle. of the focus sites. 36˚ 0.0 Coupling ratio 1.0 0.5 35˚ Figure 2. Geodetic coupling ratio, SSEs, and past subduction 34˚ 40 20 earthquakes at the Nankai Trough subduction zone, southwest Japan (from Nishimura et al., 2018). Solid blue 33˚ 1944 Mw8.1 lines represent source regions of 1946 Nankai and 1944 2004 Mw7.4 Tonankai earthquakes. Dashed blue lines represent the 2016 Mw6.0 suggested source region for Tokai earthquake. Solid and 1946 Mw8.3 dotted green lines represent source regions of long-term slow 1968 Mw7.5 slip events (SSEs) and short-term SSEs (see Nishimura et al., 32˚ 2018 for source of SSE and earthquake locations). Dotted yellow lines represent source regions of very low frequency Source areas of past large earthquakes earthquakes (VLFEs) determined by National Research Institute for Earth Science and Disaster Resilience. Stars Source areas of anticipated Tokai earthquake represent epicenters of notable earthquakes. Contours on subduction interfaces are isodepths at 10 km intervals. 31˚ Epicenters of past notable earthquakes 30 • GeoPRISMS Newsletter Issue No. 43 Fall 2020 Source areas of long-term SSEs Source areas of short-term SSEs 30˚ Source areas of shallow VLFEs Source areas of shallow SSEs & VLFEs 130˚ 131˚ 132˚ 133˚ 134˚ 135˚ 136˚ 137˚ 138˚

The Nankai Trough is also the site of a diverse range of SSEs and Figure 3. Summary map of the slip behavior of the northern related seismic phenomena (tremor, low-frequency earthquakes), Costa Rica margin. The focal mechanism marks the epicenter of that shed further light on seismic cycle processes on the megathrust the 9/5/2012 Mw 7.6 earthquake with the black solid contour (Obara and Kato, 2016). The most well-known of these are Episodic indicating the region with mainshock slip greater than 1 m. This Tremor and Slip (ETS) events, which involve abundant seismic contour corresponds well to the area of maximum interseismic tremor accompanied by small SSEs (~1-3  cm of inferred slip) strain accumulation. The major slow slip patches are shown in the detected by borehole tiltmeters (Obara et al., 2004) and continuous salmon color and occur up and down-dip of the locked seismogenic GPS/GNSS networks (Nishimura et al., 2013), largely occurring zone. Current GPS/GNSS stations (yellow triangles) and the below the locked seismogenic zone in the down-dip transition from CORK borehole with the pressure sensor (cyan triangle) used brittle to ductile deformation (Fig.  2). There are also long-term to determine the slow slip distribution are indicated. The Cocos SSEs lasting a few years in the Tokai region, down-dip of inferred and Caribbean Plates are labeled along with their convergence interseismic coupling (Ohta et al., 2004; Miyazaki et al., 2006; Ozawa direction, slip rate (red arrow), and seafloor et al., 2012), as well as approximately one-year-long SSEs in the origin: East Pacific Rise (EPR) or Cocos- Bungo Channel (Hirose et al., 1999), in the along-strike transition Nazca (CNS). Plate interface contours from deep coupling at the Nankai Trough to an aseismic creep- are labelled with dashed blue lines. dominated margin offshore Kyushu (Fig. 2). These long-term SSEs appear to recur less frequently, approximately every five to six years 11˚ in the case of the Bungo Channel SSEs (Kobayashi and Yamamoto, 2011). More recently, pore pressure changes detected in borehole 10˚ Mw 7.6 km observatories offshore in the Nankai Trough have revealed episodic 9/5/2012 0 50 SSEs (that often coincide with very low frequency earthquakes and EPR tremor) near the trench, up-dip of the seismogenic zone (Araki et 9˚ CNS -85˚ Caribbean al., 2017). These offshore SSEs may accommodate 30-55% of the overall plate motion near the trench (Araki et al., 2017), consistent Cocos -84˚ with interseismic coupling coefficients on the megathrust of less than 50% from offshore GPS/GNSS-A arrays (Nishimura et al., -86˚ 2018; Fig. 2).The offshore Nankai Trough SSEs are the first that have been clearly observed up-dip of a deeply locked seismogenic co-seismic slip (Yue et al., 2013; Protti et al., 2014; Liu et al., 2015; zone known to produce Great (Mw >8.0) subduction earthquakes. Kyriakopoulos et al., 2016) and afterslip (Malservisi et al., 2015), Episodic SSEs are thought to occur on faults where the frictional and the timing and location of slow slip and tremor events (Walter properties straddle the boundary from seismic (velocity weakening) et al., 2011, 2013; Dixon et al., 2014). to aseismic (velocity strengthening) behavior, and the location of these shallow, offshore SSEs could signify the up-dip limit of the Geodetic observations during the late interseismic phase identified seismogenic zone at Nankai. a region of slip deficit that tightly encompassed the subsequent Costa Rica’s Middle America Trench rupture area of the 2012 Nicoya earthquake (e.g., Feng et al., 2012; Along Costa Rica’s Middle American Trench the oceanic Cocos Protti et al., 2014). This highlights the importance of making GPS/ plate subducts beneath the continental Caribbean plate at a rate of GNSS observations during the intersiesmic phase to identify the ~8.5-9 cm/yr (DeMets et al., 2010). This rapid rate of convergence likely location of asperities in future earthquakes. In addition to is responsible for generating magnitude 7+ earthquakes about hosting large earthquakes every 50 to 60 years, large slow-slip events every fifty years (1853, 1900, 1950 and 2012) beneath the Nicoya (~Mw 7.0) and seismic tremor activity have been observed every Peninsula, the northwestern margin of the Costa Rica subduction 2-3 years. These large regular SSEs have slip both up and down-dip zone. Due to the advantageous location of the Nicoya Peninsula of the locked seismogenic zone, while smaller SSEs occur more extending seaward over the seismogenic zone, the regularity of large frequently with most of their slip constrained to shallow (<15 km) earthquakes, and its timing late in the earthquake cycle, Costa Rica depths (Dixon et al., 2014). Areas that experienced significant slow was one of the first regions chosen as a focus site for the MARGINS slip prior to the 2012 earthquake did not experience seismic rupture Program. With MARGINS’ support, dense campaign and continuous in the 2012 mainshock. GPS/GNSS and regional seismic observations covering the Nicoya Peninsula began in 1999 and continue today. These two decades of instrumental coverage captured the most recent Mw 7.6 earthquake on September 5, 2012, allowing the late interseismic, co-seismic and postseismic phases of the earthquake cycle to be well-recorded (Fig.  3). These data have been used to construct models of the interseismic strain accumulation on the plate interface (Feng et al., 2012; Xue et al., 2015; Kyriakopoulos et al., 2016), the distributions of 31Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

If this behavior is characteristic of other subduction zones, it suggests Onshore GPS/GNSS data in southern Cascadia was also recently that better monitoring of SSEs could provide useful information for used to identify the new phenomenon of dynamically triggered earthquake and tsunami forecasting. Due to the close proximity of changes in plate interface coupling caused by offshore earthquakes the Nicoya Peninsula to the trench (~70-90 km), shallow SSEs are (Materna et al., 2019). well-recorded on near-shore GPS/GNSS stations, but the addition The shallow extent of offshore coupling is very poorly constrained of deformation signals recorded on borehole pressure sensors (Davis by onshore geodetic instrumentation (Schmazle et al., 2014), which et al., 2011, 2015) and fluid flow meters (Brown et al., 2005) near the includes GPS/GNSS sites and borehole strainmeters. Measuring trench has allowed more detailed models of offshore slow slip to be the degree of coupling near the trench is extremely important for constructed. These models reveal shallow slow slip that propagates constraining both earthquake and tsunami hazards, with a higher all the way to the trench and that may trigger second subevents at degree of shallow coupling leading to greater potential for near- depth (Davis et al., 2015; Jiang et al., 2017). Exactly how slow slip trench coseismic rupture and tsunamigenesis. This requires seafloor cycles evolve with time around the locked seismogenic zone is not geodetic techniques, most importantly through installation of GPS/ known and awaits the accumulation of data from future SSEs. GNSS-Acoustic sites. GPS/GNSS-Acoustic sites are also important Cascadia subduction zone for studying offshore displacement in future earthquakes in Cascadia In the Cascadia subduction zone, offshore the Pacific Northwest once they occur (Saunders and Haas, 2018). Four GPS/GNSS- region of the U.S. and Canada, the Juan de Fuca plate subducts Acoustic sites now exist along the offshore portion of the Cascadia obliquely under North America at a rate of approximately 3-4 cm/ subduction zone, two of which were installed with GeoPRISMS year, generally increasing in rate from south to north (Fig. 4). This funding (Fig. 4). Measuring shallow coupling with this technique subduction zone features characteristic large earthquakes every few takes a number of years, and full results are not yet available. hundred years (Goldfinger et al., 2012) with very few earthquakes Preliminary data from the two GeoPRISMS funded sites appear on the plate interface between these large events. The last such event consistent with a moderate-to-large degree of coupling extending was a full margin rupture that occurred on January 16, 1700 with a to the shallow part of the subduction interface, implying significant magnitude of approximately 9.0 (Atwater et al., 2015). Goldfinger tsunami hazard (Chadwell et al., 2018; Fig. 4). et al. (2012) estimate a roughly four in ten chance of an earthquake Similar to the Nankai subduction zone, the Cascadia subduction rupturing the southern part of the Cascadia subduction zone over zone hosts abundant ETS events (Rogers and Dragert, 2003), in the next fifty years, with a likely magnitude of approximately 8.0. which tectonic tremor and geodetically observed slow slip migrate The same study also estimates a one in ten chance of a full margin together along the subduction zone (Bartlow et al., 2011; Wech and rupture similar to the 1700 event over the same time frame. This Bartlow, 2014). The ETS events appear to occur not on the down-dip represents a significant risk to the cities of Portland, OR and Seattle, edge of the strongly coupled zone as might be expected by simple WA and surrounding areas. A number of studies over the past two frictional models, but rather are located deeper, with a gap of little decades, many funded by the MARGINS and GeoPRISMS programs, to no coupling between the strongly coupled zone and the ETS zone have furthered our understanding of the mechanics of the Cascadia (Fig. 4; Hyndman et al., 2015; Bartlow, 2020). subduction zone and what future earthquakes may look like here. Geodetic studies in Cascadia are complicated by other tectonic 50˚N 0 5 10 15 20 25 30 35 40 40 signals, such as the rotation of the forearc blocks relative to North 48˚N mm/year time-averaged ETS slip rate 35 America. Multiple interseismic coupling models created from onshore GPS/GNSS data exist with some variation between them Explorer (e.g. Schmalzle et al., 2014; Pollitz and Evans, 2017; Li et al., 2018a). sub-plate Figure 4 features one of the coupling models from Schmalzle et al., 2014. Most models broadly agree on a strongly coupled zone located ? mainly offshore. An offshore rupture of the region of high coupling inferred from geodesy is consistent with paleoseismic observations 30 of land subsidence during the 1700 earthquake (Wang et al., 2013). 46˚N 40 mm/year 25 20 NGH1 Juan de Fuca plate NCL1 Figure 4. Summary of Cascadia interseismic coupling and episodic 44˚N NNP1 tremor and slip (ETS). Red to yellow colors indicate the degree of coupling from onshore GPS/GNSS data, assuming strong coupling at Paci c plate NCB1 15 the trench (Schmalzle et al., 2014). Rainbow colors indicate the time- 10 averaged slip rate on the plate interface attributable to ETS events 42˚N ? 5 (Bartlow et al., 2020). Brown lines are contours of tremor density 120˚W 0 from the Pacific Northwest Seismic Network catalog (Wech, 2010). Gorda Blue triangles indicate the location of GPS/GNSS-Acoustic sites. NNP1 sub-plate and NGH1 are the original GeoPRISMS funded sites, with preliminary velocities and uncertainties shown (Chadwell et al., 2018). 50 +/- 5 mm/year 31 mm/year 32 • GeoPRISMS Newsletter Issue No. 43 Fall 2020 40˚N 0 0.2 0.4 0.6 0.8 1.0 Coupling fraction 132˚W 130˚W 128˚W 126˚W 124˚W 122˚W

This indicates that ETS behavior is controlled not only by a simple including the 1964 event, and SSEs. Elastic block modeling of GPS/ transition from a coupled plate interface to a freely sliding interface, GNSS data that takes these effects into account reveals a highly but also by other physical property changes. The most likely variable pattern of coupling along the interface and complicated candidate is the presence of high pore fluid pressure in the ETS upper plate motion (Fig. 5). The upper plate rotates counterclockwise zone, leading to very low effective normal stress, thereby altering the throughout south central Alaska before transitioning to increasingly frictional behavior of the interface in this region (Audet et al., 2009 arc-parallel motion through the Alaska Peninsula and the Aleutians Hyndman et al., 2015; Gao and Wang, 2017). The influence of the as crust is extruded westward into the Bering Sea region (Cross and ETS events on the timing of great earthquakes in Cascadia is still a Freymueller, 2008; Li et al., 2016; Elliott and Freymueller, 2020). topic of scientific debate and warrants further study (e.g., Mazzotti Strong coupling occurs beneath Prince William Sound and outboard and Adams, 2004; Beeler et al., 2014). of Kodiak Island (Li et al., 2016; Elliott and Freymueller, 2020). These The Alaska/Aleutian subduction zone areas experienced very high slip during the 1964 earthquake (e.g. The Alaska-Aleutian subduction zone stretches from the Bering Holdahl and Sauber, 1994; Ichinoise et al, 2007). Areas that slipped Glacier in the Gulf of Alaska west to the Kamchatka Peninsula. less during the 1964 event appear to be partially coupled or creeping Over the majority of its 4000-km length, the boundary accomodates in the present day. This suggests that asperities may persist through subduction of the Pacific plate. At its eastern end, however, flat-slab multiple earthquake cycles. West of the 1964 rupture, the Shumagin subduction of the Yakutat Block, an oceanic plateau, occurs. This section of the interface, a GeoPRISMS study area, is partially flat-slab region generated the second-largest earthquake recorded, coupled while regions further to the west are creeping (Fournier the 1964 Mw 9.2 Prince William Sound earthquake. The earthquake and Freymueller, 2007; Li et al., 2018; Elliott and Freymueller, 2020). caused extensive regional damage and generated a tsunami that As mentioned above, SSEs occur along the Alaska subduction zone. resulted in casualities in Oregon and California and damage as far Two events have been geodetically documented to the northwest of away as Hawaii. Other sections of the interface have generated five Prince William Sound (Ohta et al., 2006; Fu et al., 2015) while three Mw7.9+ earthquakes over the past century along with two Mw7+ have been observed at the western end of the Kenai Peninsula (Wei intraslab events (Fig. 5). et al., 2012; Li et al., 2016; Fig. 5). All of these SSEs were multi-year Geodetic evaluation of interseismic coupling along the Alaska- events, with one lasting at least nine years (Li et al., 2016) and occur Aleutian subduction zone is complicated by a number of tectonic and in relatively weakly coupled sections of the interface down-dip of non-tectonic transient signals, including glacial isostatic adjustment, more strongly coupled areas. The observed SSEs are located near ongoing postseismic deformation from several earthquakes and on either side of the transition between the Yakutat flat slab and more “normal” Pacific crust. Figure 5. Summary map of slip behavior along the Alaska subduction zone. PWS is Prince William Sound. A coupling coefficient of 0 indicates a fully creeping fault segment while a coefficient of 1 indicates a fully coupled fault accumulating strain at the full relative plate motion rate. Fault geometries and coupling distribution from Elliott and Freymueller (2020). SSE locations from Li et al. (2016), Ohta et al. (2006), Fu et al. (2015), and Wei et al. (2012). Earthquake locations and focal mechanisms from Estabrook et al. (2000), Estabrook et al. (1994), Lopez and Okal (2006), Kanamori (1970), and the Alaska Earthquake Center database, and the U.S. Geological Survey/Earthquake Hazards Program catalog. 72˚N 72˚N 70˚N 70˚N 68˚N Kobuk Yakutat-Alaska 68˚N 66˚N Interface 66˚N 64˚N Kaltag Paci c-Alaska 64˚N 62˚N Interface 62˚N 60˚N Denali SSEs 60˚N 58˚N Epicenters of M7+ 58˚N 56˚N 2018 M7.1Castle Mtn. earthquakes over 56˚N 2016 M7.1 past century SegKmeennait SegPmWeSnt 1979 Eastern Denali 1964 M9.2 Fairweather 54˚N 1999 M7 1965 54˚N 52˚N 1923 Segment PACIFIC PLATE Queen Charlotte 172˚W 1169865˚W1951791149S664a44Mn1˚a8W9k.69S1eMg7m16en01t9˚W4S8Sheugmm191ieg35na8tn6M˚18SW9.Se28em9gimMdi7e11.n195t52K7o˚dWiak .8 .9 1 52˚N 0 .1 .2 .3 .4 .5 .6 .7 cient 132˚W 128˚W Coupling Coe 140˚W 148˚W 144˚W 136˚W 33Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

All of the geodetic observations discussed above were from land- Hikurangi margin in mid-2019. SSEs at southern Hikurangi are based GPS/GNSS sites at least 100 km from the trench, leading deep (50-20 km), less frequent (4-5 year recurrence intervals), and to poor resolution of coupling along the shallowest regions of the typically last a year or more. In general, the spatial pattern of SSE subduction zone. As part of a recently funded GeoPRISMS project, occurrence tracks along the edges of interseismic coupling at the three seafloor GPS/GNSS-Acoustic sites were established in the southern and central Hikurangi margin, while SSEs offshore the east weakly coupled Shumagin segment and the adjacent strongly coupled coast occupy the apparently mostly creeping portion of the central Semidi segment (Chadwell et al., 2018). The new geodetic data will and northern plate boundary (Fig. 6). help resolve the transition between strong and weak coupling and New Zealand’s historical record is relatively short (~170 years), and determine if and how coupling varies between the up-dip and down- no great (Mw>8.0) earthquakes on the Hikurangi subduction zone dip sections of the segment. have been recorded. The largest recorded subduction thrust events New Zealand’s Hikurangi subduction zone were two Mw ~7.2 earthquakes in 1947, that ruptured the shallow, The Hikurangi subduction zone accommodates westward subduction mostly creeping plate boundary offshore Gisborne, and generated of the Pacific Plate beneath the East Coast of the North Island of large tsunami (8-10 m) (Doser and Webb, 2003; Downes et al., 2000). New Zealand, along the Hikurangi Trough. It continues north of However, paleoseismic evidence suggests that the currently locked New Zealand to link up to the Kermadec and Tonga subduction southern Hikurangi margin (Fig. 6) ruptures every ~300-800 years zones; the southern termination of the subduction zone is not well- (Clark et al., 2019), and there is also evidence for coastal subsidence defined, but likely occurs somewhere in the northern South Island. consistent with great subduction earthquakes at the offshore central Extensive campaign GPS/GNSS datasets have been acquired at the Hikurangi margin (which currently appears to creep and undergo onshore portion of this margin since the mid-1990’s (Beavan et al., episodic slow slip) (Hayward et al., 2016). Despite the limitations of 2016) and have revealed the distribution of interseismic coupling the historical subduction earthquake record, intriguing interplays on the plate interface below (Darby and Beavan, 2001; Wallace et between SSEs and recent earthquakes in New Zealand have been al., 2004, 2012). Elastic block modeling of interseismic GPS/GNSS widely observed. The most spectacular of these interactions was velocities show that the southern Hikurangi subduction interface widespread triggering of slow slip in most of New Zealand’s slow slip undergoes deep (25-40 km) interseismic coupling, while further regions following the 2016 Mw 7.9 Kaikoura earthquake, including north this transitions to a mostly creeping plate boundary (Fig. 6). distant (~600 km) dynamically triggered SSEs at the northern Numerous SSEs have been observed on the Hikurangi subduction Hikurangi subduction zone (Wallace et al., 2017, 2018). Investigation zone by continuous GPS/GNSS sites in the region, operated by of these triggered SSEs was partially supported under a GeoPRISMS GeoNet (Gale et al., 2015; www.geonet.org.nz). They occur at a funded project. Triggering of slow slip events have been observed large range of depths with widely varying duration, recurrence, in other Hikurangi margin earthquakes (Francois-Holden et al., and magnitude characteristics (Bartlow et al., 2014; Wallace and 2008; Koulali et al., 2017), and some New Zealand earthquakes of Beavan, 2010; Wallace, 2020). SSEs at the northern and central Mw 6.0-7.1 may also have been triggered by SSEs (Koulali et al., Hikurangi margin largely occur off the east coast and tend to be 2017; Wallace et al., 2014, 2017). shallow (<15  km depth), relatively frequent (every 1-2 years), and are short in duration--lasting a few to several weeks. A deployment -37˚ of seafloor Absolute Pressure Gauges (APGs) at the offshore northern Hikurangi margin suggest 1-5 cm of uplift of the seafloor during -38˚ Interseismic coupling 40 20 400 100 200 500100 a 2014 SSE indicating that these shallow SSEs propagate close to (based on campaign the trench (Wallace et al., 2016). More recently, GeoPRISMS has GPS velocities Trough supported additional APG and GPS/GNSS-A deployments offshore from 1995-2008) New Zealand, which captured a recent, large SSE at the central -39˚ 300 Hikurangi North Island Figure 6. Interseismic coupling based on campaign GPS -40˚ 200 300 Paci c velocities (1995-2008), shown in terms of interseismic -41˚ 200 100 Plate coupling coefficient (φ ic ; Wallace et al., 2012). Where φ ic = -42˚ 5100 0 then this region of the fault is creeping at the full long-term 300 cumulative slip slip rate and if φ ic = 1 there is no creep in the interseismic in deep 2006 and period. In the case where φ ic is neither 0 nor 1, one could 300 2008 SSEs interpret it as a spatial and/or temporal average of creeping 400 400 10 2002-2014 and non-creeping patches. Contours represent different 300 total SSE slip periods of cumulative SSE slip (green, red, white) or afterslip 01 slip in small (yellow) on the subduction interface (see key for explanation). 200 100 Coupling coe cient 2009 SSE updip Figure modified from Wallace (2020). 500 of locked zone afterslip following 100 the 2016 Kaikoura earthquake 173˚ 174˚ 175˚ 176˚ 177˚ 178˚ 179˚ 180˚ 34 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

Discussion and Hikurangi coincide with prehistoric earthquake ruptures inferred from paleoseismological investigations. These suggest that Together, the GeoPRISMS and MARGINS focus sites have contemporary geodetic coupling estimates are a useful guide to encompassed a wide variety of subduction zones with a range of locations of future subduction megathrust ruptures, with important physical characteristics and megathrust slip behavior. A number of implications for seismic and tsunami hazard. However, we cannot factors have been suggested to influence slip behavior at subduction rule out seismic rupture (and tsunamigenesis) in regions that zones, including thermal state/incoming plate age (Hyndman et appear to be dominated by aseismic creep and SSE processes, as al., 1997), geometric and/or lithological heterogeneity of the plate was observed during a pair of Mw 7.2 earthquakes near the trench boundary fault (Wang and Bilek, 2014; Barnes et al., 2020), sediment offshore the northern Hikurangi in 1947. The 1947 earthquakes thickness on the subducting plate (Ruff, 1989), upper plate crustal produced tsunamis of 8-10 m, and are widely considered to be properties (Bassett and Watts, 2015), metamorphic phase changes “tsunami earthquakes”, where the tsunami that was generated was (Peacock and Hyndman, 1999; Moore and Saffer, 2001), and the much larger than expected based on the earthquake magnitude. influence of fluid pressure on effective stress (Kitajima and Saffer, The conditional frictional stability thought to be present in shallow 2012; Saffer and Tobin, 2011). Cascadia and Nankai are two excellent SSE zones may also make these portions of the subduction interface examples of warm, thickly sedimented (>1 km thick) subduction favorable for hosting tsunami earthquakes (Bilek and Lay, 2002). zones, and indeed, they exhibit many similar slip characteristics It is also possible that regions of strong geodetic locking may vary including deep interseismic coupling, evidence for past Great over time as the physical properties that control it change. However, subduction earthquakes, and some of the strongest associations resolving locking variations over a single to multiple seismic cycles, between tectonic tremor and SSEs observed anywhere. Alaska or determining if it is persistent over many seismic cycles in most is the site of strong along-strike variations in plate age, dip, and locations requires sustained geodetic and seismological monitoring incoming sediment thickness/plate roughness, as well as large along- at subduction margins globally. strike variations in megathrust slip behavior and locking, making Shallow (<15 km depth) SSEs are observed in Costa Rica, north it an excellent locale to investigate the physical controls on slip Hikurangi, and the Nankai Trough. In all three cases, these are behavior. Likewise, the Hikurangi margin (a cold subduction zone relatively short in duration (less than one month), and occur endmember), is the site of a large along-strike variation in sediment relatively frequently (every 1-2 years). Long-term (>1 year), deep thickness and incoming plate roughness, and upper plate properties (>25 km depth), less frequent (every 5 years or more) SSEs have that change in tandem with variations in interseismic coupling been observed at Nankai (Bungo Channel and Tokai SSEs), Alaska, and slow slip distributions. The Middle America Trench in Costa Costa Rica, and southern Hikurangi. This contrast in some shallow Rica is an excellent example of a thinly sedimented, geometrically vs. deep SSE characteristics suggest fundamentally different physical rough, and a moderately young incoming plate age (~15-25 Ma) that conditions or deformation mechanisms may be at play in deep exhibits highly heterogeneous seismic and aseismic slip processes. versus shallow SSE regions. Numerical models based on rate and Taken as a whole, regions with low coupling or highly heterogeneous state friction are able to produce deep, long SSES and shallow, short coupling (e.g., Costa Rica, north Hikurangi, the Shumagins, SSEs with higher effective normal stress in the deep SSE regions and offshore Kyushu southwest of the Nankai Trough; Figs. 2-6) compared to the shallow regions (Shibazaki et al., 2019). Higher coincide with areas of rough crust subduction, suggesting that a effective normal stress in deep SSE regions is not unexpected geometrically and lithologically heterogeneous plate interface may given the greater overburden in the deep SSE regions, and may promote creep, slow slip events, and/or heterogeneous coupling provide one explanation for these differences in shallow vs. deep (Wang and Bilek 2014; Barnes et al., 2020). In contrast, the deeply SSE characteristics. However, deep ETS events observed at Nankai and more uniformly locked Cascadia, Nankai, southern Hikurangi, and Cascadia are shorter (weeks to months), frequent, and more and the Prince William Sound/Kenai Peninsula area of Alaska are strongly associated with seismic signatures (tremor), suggesting all the site of thick incoming sediment packages. These parallels some fundamental differences in physical conditions exist in regions suggest that incoming plate properties may play an important role with ETS vs. deep, long-term SSE regions. It is also worth noting in determining geodetically observed slip behavior. Many other that Cascadia and Nankai have a young incoming plate, which may changes in characteristics of these margins exist, such as upper influence metamorphic phase transitions and subsequent fluid plate structure/properties, the location of thermally controlled release that may play a role in the ETS process (e.g., Fagereng and metamorphic phase transitions, inferred variations in fluid pressure, Diener, 2011). and many other properties that also show associations with different For the most part, deep SSEs at all of the subduction margins types of slip behavior. The challenge in the near-term will be to better discussed here occur down-dip of the interseismically coupled integrate and evaluate these observations (and those from other seismogenic zone. In Cascadia and Nankai, there is thought to subduction zones) to discern the primary controls on megathrust be a gap between the down-dip end of coupling and slow slip/ slip behavior and earthquakes. tremor regions (Figs. 2 and 4). The reasons for this, and the nature Geodetically inferred interseismic coupling in Costa Rica, Alaska of deformation within this gap are still not well-understood. SSEs and Nankai coincide with the rupture areas of well-documented in Costa Rica and Hikurangi closely track the edges of the locked historical earthquakes, while regions of strong coupling in Cascadia seismogenic zone (Figs. 3 and 6). Shallow SSEs at Nankai and Costa 35Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Rica appear to occur up-dip of the seismogenic zone, and may Japan (Mavrommatis et al., 2014) and elsewhere (Bedford et al., represent the up-dip transition from stick-slip (velocity weakening) 2020) suggest the possibility several years to months-long changes in behavior to aseismic (velocity strengthening) behavior near the deformation rates at subduction zones may presage major ruptures, trench. At Hikurangi, Costa Rica, and Cascadia SSE regions appear with important societal implications. However, determining what to be largely creeping over multiple SSE cycles, which suggests all of constitutes a precursor (or not) requires building-up a large number the elastic strain accumulated between SSEs is relieved during slow of such observations in many subduction environments. Long slip. It is important to consider the implications of this for seismic geodetic records spanning all phases of the seismic cycle will help hazard: is all of the plate motion in SSE regions accommodated to address the role of viscoelastic deformation at different stages in aseismically, leaving little to occur during a large earthquake? the seismic cycle, and the influence of this on the megathrust strain accumulation and release processes. Advancing our understanding Future challenges and the way ahead of deformation at all stages of the earthquake cycle also requires us to move beyond widely used elastic half-space models, and account Our understanding of contemporary slip processes at subduction for the influence of both inelastic and anelastic rheologies and spatial plate boundaries has been transformed in the last two decades, variability in crustal elastic properties. largely due to the development and maturation of a range of geodetic techniques, and widespread installation of permanent The discovery of episodic slow slip events has opened our eyes geodetic monitoring networks to monitor crustal deformation. to the highly transient nature of slip and locking on subduction However, scientists are only beginning to piece together the ways megathrusts. However, significant gaps exist in our ability to detect in which plate motion is accommodated at subduction zones, at smaller, shorter transient events below the reoslution limits of more timescales ranging from seconds to hundreds of years, at all stages commonly used geodetic techniques. More widespread use of highly of the earthquake cycle. Significant gaps in our knowledge remain, sensitive tiltmeters, strainmeters, and borehole pressure sensors will particularly with regards to the behavior of offshore subduction help to reveal the full spectrum of deformation processes, and bridge megathrusts, spatiotemporal variation in slip processes throughout the gap between seismologically observed deformation phenomena the earthquake cycle (and the influence of this on future earthquake (e.g., seconds to minutes) and those observed geodetically. Offshore occurrence), and the role of inelastic rheologies on deformation subduction zones are a particularly attractive target for these throughout the earthquake cycle. activities as they offer opportunities for very near-field monitoring, Offshore portions of subduction zones pose the greatest tsunami within a few kilometers of the fault. Evidence is also mounting that hazard and also represent our largest observational gap—widespread significant transience in interseismic coupling processes may exist, application of robust techniques to measure seafloor and subseafloor including spatiotemporal variations in coupling within the locked crustal deformation represents our most challenging frontier. seismogenic zone on short (weeks to months) timescales (e.g., Haines GeoPRISMS has helped to embark on this new frontier by facilitating et al., 2019), increased coupling induced by nearby earthquakes (e.g., new seafloor geodetic experiments in Alaska, Cascadia, and New Materna et al., 2019), and pre-seismic unlocking in the years prior to Zealand, utilizing both GPS/GNSS-Acoustic arrays and Absolute large megathrust events (Mavrommatis et al., 2014). Making more Pressure Gauges for horizontal and vertical (respectively) seafloor headway on detecting these “coupling transients” (and resolving deformation. Offshore scientific drilling initiatives (through the processes that produce them) requires improvements in data the International Ocean Discovery Program) have installed analysis and modelling techniques, sustaining and building on subseafloor observatories at Nankai, Costa Rica, New Zealand, and existing GPS/GNSS infrastructure, and development of low-noise Cascadia, enabling high-fidelity detection of near-trench transient instrumentation capable of detecting longer-term changes (months deformation, while scientists in Japan have amassed an impressive to years) in crustal deformation. Development of efficient modelling array of seafloor geodetic data along their subduction zones. approaches to more robustly address uncertainties in coupling and Although the MARGINS and GeoPRISMS focus sites are more slip distribution on megathrusts is also needed. advanced in in the acquisition of offshore geodetic data than most other subduction margins, more offshore geodetic infrastructure is Although geodetic observations can provide critical insights into the needed in these locations and at subduction zones elsewhere if we varied modes of slip behavior on megathrusts, integration of these are to unravel the nature of deformation near the trench and the observations with a range of geophysical, geological, laboratory, and implications of this for tsunami hazard. modelling studies are required to resolve the underlying physical Major gaps also exist in our knowledge of deformation throughout processes. Programs like MARGINS and GeoPRISMS have enabled the earthquake cycle and how this evolves through time. focused, multi-disciplinary efforts at several subduction zones, Addressing this requires a concerted effort to obtain long (decades producing great advances in our ability to bridge the gap between and beyond), uninterrupted timeseries of crustal deformation observation and process. Future programs, such as the SZ4D at numerous subduction zones, both onshore and offshore, and initiative, will help to ensure continued progress in this societally underscores the importance of continuing to operate continuous important area of research, and generate new discoveries regarding GPS/GNSS networks and other existing geodetic and seismological infrastructure. Tantalizing evidence for pre-seismic transients ■the physical processes underpinning our planet’s largest earthquake leading-up to the Mw 9.0 Tohoku-Oki earthquake in northern and tsunami factories. 36 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

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zone: U.S. Geological Survey Professional Paper 1661–F, 170p. Ridge collision in Central America. Geochem Geophys, 10, Q05S14, https://pubs.usgs.gov/pp/pp1661f/ doi:10.1029/2008GC002181 Grapenthin, R., M. West, C. Tape, M. Gardine, J. Freymueller (2018). Single‐ Li, S., J. Freymueller, R. McCaffrey (2016). Slow slip events and time‐ frequency instantaneous GNSS velocities resolve dynamic ground dependent variations in locking beneath Lower Cook Inlet of the motion of the 2016 Mw 7.1 Iniskin, Alaska, earthquake. Seismol Res Alaska‐Aleutian subduction zone. J Geophys Res Solid Earth, 121, Lett, 89, 3, 1040-1048, doi.org/10.1785/0220170235 1060-1079, doi:10.1002/2015JB012491 Haines, J., L.M. Wallace, L. Dimitrova (2019). Slow slip event detection Li, S., M. Moreno, J. Bedford, M. Rosenau, O. Oncken (2015). Revisiting in Cascadia using vertical derivatives of horizontal stress rates. 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Miyazaki, S., P. Segall, J.J. McGuire, T. Kato, Y. Hatanaka (2006). Spatial and data. J Geophys Res, 102, 22,391–22,409 temporal evolution of stress and slip rate during the 2000 Tokai slow Shibazaki, B., L.M. Wallace, Y. Kaneko, I. Hamling, Y. Ito, T. Matsuzawa earthquake. J Geophys Res, 111, B03409, doi:10.1029/2004JB003426 (2019). Three‐dimensional modeling of spontaneous and triggered Moore, J.C., D. Saffer (2001). Updip limit of the seismogenic zone beneath slow‐slip events at the Hikurangi subduction zone, New Zealand. J the accretionary prism of southwest Japan: An effect of diagenetic Geophys Res Solid Earth, 124, doi.org/10.1029/2019JB018190 to low-grade metamorphic processes and increasing effective stress. Sun, T., et al. (2014). Prevalence of viscoelastic relaxation after the 2011 Geology, 29, 2, 183-186, doi:10.1130/0091-7613 Tohoku-oki earthquake. Nature, 514, 84–87 Wallace, L.M., J. Beavan, R. McCaffrey, D. Darby (2004). Subduction zone Nishimura, T., T. Matsuzawa, K. 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Science Reviews Volatile fluxes at rifting and subduction margins: Review of results from the NSF MARGINS and GeoPRISMS programs Tobias P. Fischer (University of New Mexico), James D. Muirhead (University of Auckland), Donna J. Shillington (Northern Arizona University), Samer Naif (Columbia University) The NSF MARGINS and GeoPRISMS programs have generated and applied new knowledge through community-driven interdisciplinary research on focus sites. In both programs, the role of volatiles in many aspects of the Earth system was recognized, and science plans developed through community meetings and workshops highlighted volatiles and their exchange between the Earth’s interior and exterior. As a result, not only were numerous key discoveries made, but also known processes were quantified and allowed for consideration within the broader Earth science framework. Results related to volatile fluxes at subduction zones and rifted continental margins are briefly reviewed here. Research advances catalyzed through these two programs include better quantification of volatiles stored in the oceanic and continental lithosphere, the processes of hydration of subducted lithosphere and their effect on volatile transport into the mantle, quantification of carbon fluxes from continental rifts, quantification of volatile fluxes into and out of subduction zones, and the role of the forearc for volatile storage. Here we provide a short summary of some of the accomplishments regarding volatiles, with a focus on MARGINS and GeoPRISMS sites. Introduction Volatile storage in the lithosphere The NSF MARGINS and its successor GeoPRISMS programs are Mantle volatile fluxes are strongly dependant on magma production type examples of a community-based approach to shoreline-crossing rates and original mantle volatile concentrations (e.g., Fischer, 2008). science. From the beginning, both of these programs highlighted A recent synthesis of these concentrations is provided by Gibson et the importance of volatiles with regards to magmatic processes, the al. (2020) and shown in Figure 1. Broadly, the volatile content of geochemical evolution of the Earth’s interior, and influencing the convecting mantle is relatively well-constrained from mantle melts rheology that affects rates and processes on Earth. The MARGINS and experimental studies (Jambon and Zimmerman, 1990; Marty, program drew broad attention to the notion that subduction zones 1995; Salters and Stracke, 2004; Hauri et al., 2006; Palme and O’Neill, are efficient recycling machines of volatiles from the subducted slab to 2007; Gibson et al., 2020), whereas volatile contents in the continental the surface via volcanism and past the zones of arc magma generation lithospheric mantle are comparatively less constrained, especially in to the Earth’s interior. The site-focused and interdisciplinary nature regions of long-lived metasomatism and volatile sequestration (Foley of these programs allowed for the acceleration of discoveries and and Fischer, 2017; Malusà et al., 2018). For example, carbon contents interpretations by providing geochemists with the geophysical in cratonic mantle could theoretically be enriched up to 100 times frameworks critical for deciphering processes. The Central America, their original values (Foley and Fischer, 2017), due to infiltration Izu-Bonin-Mariana, New Zealand, and Nankai Subduction Factory of plume and carbon-rich silicate melts generated during mantle sites enabled individual Principal Investigators or small groups of convection and subduction (Kelemen and Manning, 2015; Foley and investigators working on volatiles to leverage their data by taking Fischer, 2017; Malusà et al., 2018). Studies in the East African Rift advantage of discoveries made by groups from around the world System (EARS) reveal that carbonatite and potassium-enriched melts that were attracted to the same sites because of the questions with high CO2 contents occur in and around the Tanzania craton, posed and the excitement of doing research in such a dynamic which exhibits geophysical and petrological evidence for previous environment. We note that although many projects on these focus metasomatism (Rudnick et al., 1993; Koornneef et al., 2009; Mana et sites were not directly funded by MARGINS or GeoPRISMS, they al., 2015; Selway, 2015). Volatiles stored within this lithosphere can likely also benefitted from these programs through community be mobilized during heating and rifting (Bailey, 1980; Lindenfeld building workshops, meetings and accessible data. This synergy and et al., 2012; Lee et al., 2016, 2017; Foley and Fischer, 2017; Hunt et leveraging has reached an even higher level during the GeoPRISMS al., 2017; Malusà et al., 2018). program, in particular with the work on volatiles in the East African Rift and the New Zealand and Aleutian-Alaskan Subduction Zones. 40 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

O craton mantle smalml feratactsioomn avtoiclamtileel-trsich Peridotite Pyroxenite Sub-cratonic mantle FCHO2=O21==3?7p0pmppm 268 ppm Primitive mantle Cl = ? ? HFCO2=O23==1?4p1pmppm 87 ppm HFCO2=O22==525p60p30mppppmm Cl = ? ? Cl = 30 ppm Li = 2.3 ppm Li = 1.6 ppm 2.4 ppm Li = 1.39 ppm B= ? B = 0.187 ppm Figure 1. Block diagram slightly Depleted mantle modified from Gibson et al. HCF O2=O21==171p00p0pmppmpm (2020) showing mantle volatile Cl = 0.51 ppm concentrations in various Li = 1.2 ppm B = 0.077 ppm on- and off-craton localities. References are provided in Figure 9 of Gibson et al. (2020). The volatile content of the oceanic crust and lithosphere evolves by subducting oceanic plates (Rüpke et al., 2004; Hacker, 2008). in response to magmatism and hydrothermal circulation near the Different scales of imaging this hydration process in the incoming mid-ocean ridge, off-axis circulation of seawater, and by faulting and plate are shown in Figure 2. hydration at the outer rises of subduction zones. The upper oceanic Studies at MARGINS and GeoPRISMS sites also revealed significant mantle is generally thought to be relatively depleted in volatiles due to along-strike variations in hydration that often correlated with melt extraction at the mid-ocean ridge, with important implications processes occurring downdip within the subduction zone (e.g., for viscosity (e.g., Hirth and Kohlstedt, 1996). Hydrothermal intraplate seismicity rates and arc chemistry) where the slab volatiles circulation near the ridge axis leads to the formation of hydrous are eventually released via dehydration reactions (e.g., Van Avendonk minerals in the upper crust (e.g., Alt, 1995) at all spreading rates. et al., 2011; Shillington et al., 2015; Canales et al., 2017; Fujie et al, Deeper circulation in the lower crust at fast-spreading ridges 2018). Because active-source seismic methods are often limited has been suggested (e.g., Hasenclever et al., 2014) but remains in their depth-sensitivity and typically image only the uppermost controversial. At slow-spreading ridges, faulting and exposure of the mantle, the availability of new offshore broadband seismic data along lower crust and upper mantle enables more pervasive hydration of subduction margins provides critical constraints on the deeper parts the crust and upper mantle than at fast-spreading ridges. The extent of subducting oceanic plate, as in the case of the Marianas where of off-axis circulation is proposed to be controlled by the sediment extensive hydration is observed ~25 km into the mantle (Cai et cover, spreading rate and thermal structure of the oceanic crust al., 2018, Fig. 2C). This has global implications on the magnitude (e.g., Alt, 1995; Gillis et al., 2015). Increasing seismic velocity of the of water and carbon that are exchanged and recycled between the upper crust with age is likely caused by the precipitation of hydrous surface and deep mantle. minerals in pore-space; the most rapid reduction is observed in the Forearcs may also be important stores of volatiles delivered first ~10-16 million years (e.g., Nedimovic et al., 2008), but could into the subduction zone by subducting sediments and oceanic continue for much longer (e.g., ~60 million years, Kardell et al., 2019; lithosphere. Many studies point to higher volatile fluxes and storage Estep et al., 2019). On- and off-axis hydration also vary along-strike in the shallow forearc than previously thought (e.g., Kastner et al., (e.g., across transform faults, Roland et al., 2012). 2014; and references therein). Hydrocarbons are also generated Just prior to subduction, the incoming oceanic plate is flexed as it and transported with fluids expelled from subducted sediments approaches the trench axis and begins its descent. The associated and are often stored as seafloor gas hydrates beneath forearcs bending stresses are often large enough to generate a network of (e.g., Barnes et al, 2010). In addition to being a potential energy densely spaced normal faults at the outer rise (e.g., Naliboff et al., resource, constraining the formation and distribution of hydrates 2013). Peacock (2001) was one of the first to hypothesize that these has broad applications for forearc hydrogeology and associated faults may act as fluid conduits for seawater to infiltrate and hydrate regional hazards to global climate predictions (e.g, Torres et al., the oceanic mantle, sparking wide interest in this topic during the 2004; Collett et al., 2010; Johnson et al., 2019). The forearc mantle initiation of the MARGINS program. Active-source seismic and wedge is often thought to be a major store of volatiles dehydrated electromagnetic imaging studies at the Central America focus site from the subducting plate trenchward of the arc (e.g., Hyndman and have since confirmed that bending faults do indeed penetrate into Peacock, 2003), although recent studies have questioned the extent the lithosphere (Ranero et al., 2003), drive fluids into the crust (Naif of hydration in all but the warmest subduction zones (Abers et al., et al., 2015), and likely hydrate the uppermost mantle (Ivandic et al., 2017). Such revisions are critical since they change the overall slab 2008, Van Avendonk et al., 2011). This final stage of hydration has the volatile budget transported beneath the arc, back-arc and beyond. potential to significantly increase the amount of volatiles transported 41Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

Sea oor depth (km) 1A EM receivers Another potentially important store of volatiles are magma-poor Fluid seeps rifted margins, where seawater can penetrate to the upper mantle 2 once the crust is thinned enough to be entirely brittle (e.g., Pérez- 2 Gussinye and Reston, 2001). At these settings, reduced upper 3 4 mantle velocities interpreted to result from serpentinization are 6 widely observed (e.g., Dean et al., 2000; Funck et al., 2003), and 4 8 the inferred degree of mantle hydration correlates with faulting 10 (Bayrakci et al., 2016). 5 North motion 20 Depth below seasurface (km) plate 4 6 8 Subduction zone volatile fluxes 10 -60 -40 -20 0 Given the complexities of volatile storage that were constrained -80 Distance from trench (km) through recent studies, particularly at MARGINS and GeoPRISMS 100 101 102 103 sites, new and more realistic volatile inventories and recycling budgets are still emerging to date. It has been long recognized that B Resistivity (Ωm) subduction zones are efficient recycling machines of volatiles from Two-way travel time (s) {~0.5 km the Earth’s surface to its interior. Allard (1983) was one of the first to propose this, based on the stable isotope composition of volatiles bending faulting sediments measured in volcanic gas emissions, a significant proportion of trench top of crust Moho these elements (aHl.2O(1,9C93, )S,aNnd) may come from the subducted slab. Work by Alt et Alt and Teagle (1999) showed how 8 much sulfur and carbon can be stored in the subducted crust (i.e., Line 5 - Shumagin Gap 10 10 km sediments and altered igneous oceanic crust). The global compilation of subducted sediment compositions by Plank and Langmuir (1998) provided firm trace element evidence that subducted slab 53 components are recycled in to arc magmas (Plank and Langmuir, Depth (km) 10 56 1993), allowing researchers to place constraints on the amount and composition of materials delivered to zones of arc magma generation 15 8 and beyond. This framework provided the basis for an early global 20 reduced upper Velocity (km/s) and arc-by-arc estimate of volatiles delivered to zones of arc magma mantle velocities 468 25 2 60 40 20 generation (Hilton et al., 2002), utilizing experimental work that 160 140 120 assessed volatile retention and release during subduction (Schmidt 100 80 B’ 0 Distance (km) and Poli, 1998). These early investigations have been improved BC upon with new volatile data from oceanic drilling (Li et al., 2007), Bathymetry (km) the assessment of additional volatile components (Barnes et al., 2019), the quantification of depth-dependant release of water from 4.2 0 10 the slab (van Keken et al., 2011), the realization that mass transfer Depth (km) 10 3.6 3.9 4.2 Vsv (km/s) from the slab directly leads to the oxidation of the mantle wedge 20 30 4.8 (Kelly and Cottrell, 2009) and new insights on how volatiles are transported from the slab to the surface (Kelemen and Manning, 40 4.6 2015). MARGINS- and GeoPRISMS-supported work led to key 4.4 50 4.2 insights and global quantifications of volatile cycling and transport 60 4.0 into the deep mantle, summarized in Wallace (2005), utilizing 3.8 160 120 80 40 0 −40 −80 −120 Distance from the trench (km) 3.6 work from melt inclusions and later by Dasgupta (2013) showing 3.4 how volatile cycles evolved through Earth’s geologic history. More recent summaries provide new and critical insights on the storage, Figure 2. Bending of the incoming plate has been recognized release and transport of carbon from the subducted slab (Kelemen to result in faults that drive fluids into the crust and even and Manning, 2015; Plank and Manning, 2019). the mantle below. Panels shows geophysical imaging of this process at different scales. Panel A shows imaging of conductive In particular for carbon, it was recognized early on that a large zones inferred to arise from fluids along faults in the upper portion of carbon emitted by arc volcanoes was sourced from the crust for the Central American subduction zone (Naif et al., subducted slab (Marty and Jambon, 1987; Marty et al., 1989; Sano 2016). Panel B shows seismic reflection imaging of bending and Marty, 1995). However, it was work by Sano and Williams faults (upper panel) and reduced P-wave velocities in the (1996) that first utilized the actual volcanic arc outputs (Marty upper mantle interpreted to arise from hydration outboard of et al., 1989; Williams et al., 1992), in combination with volatile the Alaska subduction zone (Shillington et al., 2015). Panel C provenance, to discriminate between the volumetric contributions shows reduced shear wave velocities continuing >20 km into the mantle for the Mariana subduction zone, interpreted as evidence for deep hydration (Cai et al., 2018). 42 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

of the mantle wedge and subducted slab. They also compared these al., 1993; Gill and Williams, 1990; Lin et al., 1990), and benefitted values to mid-ocean ridge and plume emissions. Around the same greatly from off-shore drilling data of subducted slab compositions time Giggenbach (1992) and Taran (1992) argued that much of the (Li et al., 2007; Sadofsky and Bebout, 2004).The Izu-Bonin-Mariana water coming out of volcanoes is magmatic, not meteoric water, focus site led to new insights into the redox budget of the mantle and sourced from the subducting slab, coining the term “andesitic wedge and showed that oxidizing slab fluids significantly elevate water”. This caused an uproar in the geochemistry community, the oxygen fugacity of the mantle wedge (Brounce et al., 2019). This who mostly believed the gospel of Harmon Craig’s meteoric water- work furthermore showed that much of those oxidizing phases are dominated steam discharges, based only on low temperature and transported into the deeper mantle where they may contribute to mostly continental gases that were indeed mainly surface water the oxidation of Ocean Island Basalts. (Craig, 1963). For nitrogen, it was Matsuo et al. (1978), Kiyosu (1986) and then Kita et al. (1993) who first showed, again with gases The framework of investigating volatile recycling through from Japan, that N2 is sourced primarily from the subducting slab, subduction zones, that was to a large extent set during the MARGINS contrasting with the notion that all nitrogen in volcanic gases is program, was taken to a new level of collaboration by targeted essentially air-derived. The next logical step was to combine what we expeditions to the Aleutians Volcanic Arc in 2015, where joint knew about subduction inputs, volcanic gas outputs and the sources USGS-NSF-DCO-funded scientists sampled rocks and volcanic of these gases to test whether the amount supplied by subduction fluids on volcanoes along the entire chain. New air-borne sample explained the volumetric outputs of a single arc volcano (Fischer collection and ship-based isotope analyses were utilized (Fischer et al., 1998). Around this time, researchers expressed the need for and Lopez, 2016). As these results emerge, volatile emissions and more rigorous and systematic approaches to address hypotheses sources of Alaska-Aleutian volcanoes are being constrained (Lopez surrounding subduction zones and geochemical cycles, which were et al., 2017) and better quantitative constraints on the sources of tackled through community-based research as part of the NSF carbon in volcanic arcs are becoming apparent (Lopez et al., 2019). MARGINS program. Within the framework of the role of water for generating Aleutian magmas (Zimmer et al., 2010), it is now recognized that high oxygen Research in the Central American Arc led to major advances in fugacities can drive Fe-depletion and calc-alkaline differentiation our understanding of transport of slab-derived carbon beyond trends (Cottrell et al., 2020). The New Zealand focus site, which zones of arc magma generation (de Leeuw et al., 2007; Shaw et al., includes the Hikurangi Trench and the Taupo Volcanic Zone, is 2003; Snyder et al., 2001), the first quantification of emissions of providing details on volatile cycling through the forearc. With the CO2 in the forearc (Furi et al., 2010), recycling of nitrogen from application of a combination of isotopic tracers (Cl, Li, B), this the slab back to the surface (Elkins et al., 2006; Fischer et al., 2002; site is providing unprecedented constraints on the role of the fluid Snyder et al., 2003; Zimmer et al., 2001) and chlorine isotopes as permeability of the upper plate (Barnes et al., 2019). Likewise, in the slab tracers for serpentine-derived fluids (Barnes et al., 2009). Some Cascadia subduction zone, the role of subduction fluids in affecting of these early studies were complemented and expanded through the major and trace element compositions and oxygen fugacities later interdisciplinary studies that highlighted the importance of of erupted arc magmas has been elucidated in detail by Rowe et al. biological processes for CO2 uptake in the forearc (Barry and al., (2009), with global implications for the significance of subducted 2019) and new insights into the role of the incoming plate for the fluids for arc magma genesis. The most recent compilation of global nitrogen budget (Lee et al., 2017). Mass balance approaches relied volcanic volatile fluxes that encompasses much of the work started heavily on off-shore studies constraining subduction inputs through during MARGINS and continued through GeoPRISMS and the Deep ocean drilling (Li and Bebout, 2005) and stood in contrast to work Carbon Observatory is from Fischer et al. (2019) and summarized utilizing volatile elements measured in metamorphic rocks and in Table 1. phase relations that provided a transport process-based approach to understanding subduction zones (Busigny et al., 2003). These studies As volatiles are subducted, they have a profound influence on were on-going whilst other groups utilized trace elements (Patino processes throughout the subduction zone, from slip behavior on et al., 2000; Walker et al., 2003) and melt inclusions (Sadofsky et the megathrust to arc magmatism. For example, data from several al., 2007) to constrain sources and recycling efficiencies that built MARGINS and GeoPRISMS focus sites suggest that fluid pressure on early work that unambiguously identified a young subduction conditions may influence slip behavior along the megathrust at a component in arc magmas (Morris et al., 1990). range of depths (e.g., Bangs et al., 2015; Naif et al., 2016; Han et al., 2017, Li et al., 2018, Barnes et al., 2019; Audet et al., 2009), again Like in Central America, the Izu-Bonin-Mariana focus site offered highlighting the value of targeted cross-disciplinary studies. Recent the opportunity to study volatile cycles. Here, gas studies (Mitchell geophysical imaging and numerical modeling studies, together et al., 2010) were performed in tandem with petrological studies with constraints from petrology, are also beginning to elucidate focusing on melt inclusions and radiogenic isotopes (Kelley et al., the pathways of volatiles from the slab to the arc (e.g., Wilson et al., 2002; Kent and Elliott, 2002; Shaw et al., 2008), which built on early 2014; McGary et al., 2014; see Fig. 3), which strongly depends on work on fluid transport processes constrained from radiogenic the permeability and viscosity of the mantle wedge. isotopes and trace elements in the region and elsewhere (Gill et 43Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

CO2 SO2 CO2 SO2 H2O HCl (Tg/y) (Tg/y) (109 mol/yr) (109 mol/yr) (109 mol/yr) (109 mol/yr) ARCS South America 6.44 7.81 146 122 1680 7 Central Am. + Mex 4.14 2.05 94 32 3759 14 Alaska + Aleutians 1.66 0.72 38 11 1407 17 Kamchatka+Kuriles 4.28 2.18 97 34 5117 25 Japan 2.79 1.53 63 24 18243 72 IBM 1.07 1.07 24 17 PNG 5.40 3.01 123 47 1958 13 Indonesia 7.55 2.56 172 40 2739 19 Philippines 1.10 0.27 25 4 400 3 Lesser Antilles 1.43 0.47 33 7 New Zealand 0.87 0.15 20 2 N and S Vanuatu 7.77 4.5 177 70 Scotia 0.79 0.15 18 2 Italy 3.76 0.81 86 13 619 3 Total ARC 49.06 27.28 1115 426 35923 173 Continental RIFT Congo 1.00 1.29 22.66 20.16 60.56 0.22 Tanzania 0.29 6.64 Yemen 0.16 0.04 3.64 0.59 Ethiopia 0.02 Antarctica 0.02 Total RIFT 1.45 1.37 32.93 20.74 60.56 0.22 PLUMES Iceland Galapagos 0.14 0.01 3.27 Hawaii 1.16 1.83 26.33 28.62 17.09 0.23 Reunion 0.04 0.09 0.80 1.33 Total PLUME 4.24 4.67 96.27 71.45 17.09 0.23 Table 1. Global fluxes of major volatiles from subaerial volcanoes from Fischer et al. (2019). Continental rifts and mid-ocean ridges volatile fluxes volcanoes (Kagoshima et al., 2015). Deep carbon released from the global mid-ocean ridge system (MOR) is arguably the most Mid-ocean ridges critical volatile component, as it has important implications for The global mid-ocean ridge system represents the largest plate understanding climate in deep time. The solubility of CO2 in basaltic boundary on Earth, with an estimated total length of ~57,000 km magma is low compared to other volatiles (e.g., Dixon and Stolper, (Gale et al., 2013; Burley and Katz, 2015; Wong et al., 2019). Globally, 1995; Jendrzejewski et al., 1997; Shishkina et al., 2010), and thus it is the dominant region of magma production and associated heat nearly all magmas erupted at mid-ocean ridges have degassed the loss. Recent estimates of magma production rates are based on majority of their original carbon. Deep carbon flux estimates are measured crustal thicknesses and observed spreading rates (e.g., thus based on known CO2/3He combined with MOR 3He fluxes Le Voyer et al., 2019), or numerical model simulations of mantle or trace elements ratios (e.g., CO2/Ba) combined with numerical melting during sea-floor spreading (e.g., Keller et al., 2017), with modelling of sea-floor spreading (e.g., Marty and Tolstikhin, 1998; estimates varying from 16.5 km3 yr-1 to 22 km3 yr-1. Resing et al., 2004; Shaw et al., 2010; Burley and Katz, 2015; Keller Direct measurements of volatile outgassing at mid-ocean ridges are et al., 2017). Estimates for deep carbon fluxes from mid-ocean largely prohibited, and thus global estimates of volatile discharges ridges developed in the last decade typically vary between 10 and consider magma production rates within the context of estimated 40 Mt yr-1, with an average value of ~21 Mt C yr-1 (see compilation mantle volatile contents and dissolved magmatic gases (e.g., Marty by Wong et al., 2019, Plank and Manning 2019). All other volatile and Tolstikhin, 1998; Fischer, 2008; Keller et al., 2017; Le Voyer et fluxes from MOR are usually linked to MORB magma production al., 2019). Volatile fluxes commonly examined at mid-ocean ridges rates through ratios with trace elements (Le Voyer et al., 2019) or iEnsctliumdaeteHd2vOo,laStOile2,flHux2Se,s CfrOom2, CarHc 2v,oNlc2a,nCole,satnydpicFal(lFyisecxhceere,d2t0h0o8s)e. other volatiles, usually 3He (Marty and Tolstikhin, 1998). Therefore, of mid-ocean ridges; for example, recent estimates of sulphur fluxes accurate magma productions rates remain a key parameter for from mid-ocean ridges are seven times less than those from arc quantification of volatiles fluxes at MOR as well as continental rifts, discussed in the next section. 44 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

Volcanic front Depth (km) 0 400 Continental crust Mt. Rainier Trench-close anomaly Decompression 400 Forearc anomaly 800 melting tBraasnaslfto-ermcFoloraetg-iaiortcnesl+aHb d2Oeh, yCdOsrae2triopn amph 10 800 40 Deep anomaly 10 512t0o010 voFl%luimd-ealstsisted cld 10 Electrical conductivity (S/m) <5 vol% 1200 80 0.1 melt melting law serp, chl 1 120 Figure 3. Comparison of a schematic representation of a subduction zone on the left with a geophysical image of electrical conductivity from Cascadia on the right (McGary et al., 2014), from Pommier and Evans (2017). When combined with petrology, the geometry of the high conductivity anomaly constrains slab fluid sources and the flow path of mantle melts and fluids to the arc and forearc. Continental rifts the Eastern Branch, when extrapolated ~2000 km from Erta Ale Continental rifts represent the other common divergent plate (Ethiopia) to the northern end of the Manyara basin (Tanzania). boundary on Earth, which, in some cases, can evolve to the point of Although magma fluxes in the EARS are lower than mid-ocean complete lithospheric rupture and initiation of sea-floor spreading ridges, volatile fluxes (particularly mantle CO2) are comparatively (Dunbar and Sawyer, 1989; Whitmarsh et al., 2001; Ebinger, 2005; high. Active rift volcanoes, such as Nyiragongo, exhibit estimated Corti, 2009). The cumulative length of continental rifts globally is SO2 and CO2 fluxes of 1.2 and 3.4 Mt yr-1, respectively (Sawyer et al., a maximum of ~15,000 km (Brune et al., 2017; Wong et al., 2019). 2008), and Oldoinyo Lengai volcano emits an estimated 2.42 Mt yr-1 Estimating magma production rates from this global rift system is of CO2 (Brantley and Koepenick, 1995), although this high number problematic, given that a significant volume of magma is intruded has recently been revised to 0.3 Mt/yr to reflect more recent estimates within the lower crust (Coffin and Eldholm et al., 1994; Thybo et (Fischer et al., 2019). Geophysical studies in the Eastern Branch of al., 2000, 2009; Keir et al., 2009; White et al., 2008), particularly the East African Rift also provide evidence for significant magma during early rift stages (Ebinger et al., 2017; Roecker et al., 2017; volumes trapped in the crust and upper mantle away from these Weinstein et al., 2017). Furthermore, crustal magma additions likely volcanic centers (Mechie et al., 1994; Keranen et al., 2004; Kendall vary over many orders of magnitude between different continental et al., 2005; Roecker et al., 2017; Plasman et al., 2017). These magma rift systems, and are shown to vary between basins along individual bodies, which likely tap an enriched subcontinental lithospheric rift systems, including in the Gulf of California, East Africa Rift mantle source in Kenya and Tanzania (Halldórsson et al., 2014; System and Eastern North American Margin (e.g., Lizarralde et al, Mana et al., 2015; Lee et al., 2017), provide a potential source for 2007; Shillington et al., 2009; Franke, 2013; O’Donnell et al., 2015; massive CO2 emissions (Foley and Fischer, 2017; Malusà et al., 2018). Ebinger et al., 2017; Accardo et al., 2017, 2020; Keranen et al., 2004; Recent diffuse degassing and geophysical observation studies in the Shuck et al., 2019; Lau et al., 2019; Marzen et al., 2020). EARS suggest this exsolved CO2 is transported along the pervasive Despite these variations, recent geophysical studies in the some extensional fault systems (Lindenfeld et al., 2012; Hutchison et parts of the apparently magma-poor Western Branch of the EARS al., 2015; Lee et al., 2016; Hunt et al., 2017; Roecker et al., 2017; support the presence of minor magmatic additions in the lower Weinstein et al., 2017), resulting in an estimated mantle CO2 release crust (e.g., Lake Tanganyika; Hodgson et al., 2017), and a number of of ~20 Mt yr-1 (Hunt et al., 2017) or even up to ~ 70 Mt/yr (Lee et al., magmatic provinces are situated within transfer zones between major 2016). Similar geophysical and geochemical observations from the rift segments (e.g., the Virunga, South Kivu and Rungwe Provinces; Eger and Rio Grande Rifts support significant CO2 discharges along Furman, 2007; Corti et al., 2004; Wauthier et al., 2013). A recent rift faults (Geissler et al., 2005; Smith, 2016; Tamburello et al., 2018; compilation of erupted products of the Kenya Rift of the EARS reveal Kämpf et al., 2019). Global estimates of deep carbon fluxes from eruption rates of 7.2 km3 kyr-1 since 5 Ma (Guth, 2015). Modelled continental rifts are 2.5 ± 2.0 kt C km-1 yr-1, compared to an estimated cumulate proportions of erupted products in Ethiopia suggest that 0.37 ± 0.23 kt C km-1 yr-1 for mid-ocean ridges (summarized and intrusive volumes in the Eastern Branch are 3-5 times greater than compiled in Wong et al., 2019). CO2 from extrusive and intrusive erupted volumes (Hutchison et al., 2018), suggesting magma fluxes magmatism associated with continental rupture may contribute between 28.8-43.2 km3 kyr-1 when applied to the Kenyan example. to environmental changes and biotic crises, such as the Triassic These data crudely support magma flux rates ranging 48-72 km3 km-1 extension following the Central Atlantic Magmatic Province (e.g., Myr-1 across the ~600 km-long Kenya Rift and 0.1-0.14 km3 yr-1 for Marzoli et al., 2018 and references therein; Capriolo et al., 2020). 45Fall 2020 Issue No. 43 GeoPRISMS Newsletter •   

volcanic degassing tectonic degassing Tg CO2/yr 110 - 183 165 - 231 mid-ocean ridge continental rifts 59 volcanic plume di use emission: 92 (48 - 172) East Africa: 70 ± 33 51.7 ± 5.9 Global: 110 - 183 soil di use hydrothermal soil di use CwOa2treircsh CwOa2treircsh soil di use emissions plume emissions metamorphism emissions magma soil di use + + degassing emissions gas vent gas vent mantle CwOa2treircsh degassing geothermal systems mantle mantle degassing degassing subduction (282 - 348) Figure 4. Summary of the flux of CO2 from various tectonic settings in Tg CO2/yr Fischer and Aiuppa (2020). The subduction input fluxes of CO2 AOC 66 - 92 are from Plank and Manning (2019). Sed 209 - 220 Peridotite 4.8 - 37 Despite significant advances in the last few decades constraining Finally, controls on variability between and within subduction zones volatile fluxes from volcanic arcs and mid-ocean ridges, global remain a major uncertainty; for example, recent studies come to volatile fluxes from continental rifts remain comparatively uncertain. conflicting conclusions on the role of pre-existing features in the Given the propensity for widespread diffuse degassing along oceanic lithosphere (Shillington et al., 2015; Fujie et al., 2018). extensional fault systems at rift settings, constraining volatile fluxes Following the onset of subduction, there are myriad questions from continental rifts remains an ongoing challenge (e.g., Werner concerning the flow of volatiles and their interactions with the et al., 2019), with recent examinations into rift-wide diffuse CO2 surrounding lithosphere at every stage, from slab release to surface degassing highlighting continental rifts as critical sites of mantle escape and from trench to arc and back-arc. The MARGINS and volatile discharge. Much of the recent work in the volatiles fluxes GeoPRISMS programs have demonstrated that tackling these critical community has focussed on CO2, where we now have significantly questions requires focused, shoreline crossing, cross-disciplinary better constraints on emission from the various tectonic settings studies. summarized in Figure 4. Although much progress has been made toward constraining the global CO2 flux from volcanoes (Aiuppa et al., 2019; Fischer et Future directions al., 2019; Werner et al., 2019), built on decades of research, much uarnecleikrtealiynetqyuraelmlyasiingsnoifnictaenctt,oansiwc ealnl dasdtihffeursoeleCoOf 2theemoivsseirolynisn,gwchriucsht MARGINS- and GeoPRISMS-related research over the last twenty as a CO2 contributor and the extrapolation of CO2 fluxes throughout years has yielded fundamental new constraints on the storage and geologic history (Fischer and Aiuppa, 2020). Furthermore, our transport of volatiles and their significance for the suite of processes knowledge of other major volatile species currently lags behind. operating at plate tectonic boundaries. Many key questions have While the volcanic emissions of sulfur are now well constrained emerged whilst others remain. Late-stage hydration of oceanic plates thanks to satellite data (Carn et al., 2017) and continuous global at the trench-outer rise is now a widely recognized phenomenon, yet ground-based networks (Galle et al., 2010), subduction zone input the volume and distribution of volatiles is still poorly constrained data remains sparse and the effect of sulfur as an oxidizing agent of (Grevemeyer et al., 2018), particularly the extent of lower crustal and the mantle wedge is potentially significant (de Moor et al., 2013). upper mantle hydration, and the relative contributions of hydrous The volatile budgets of the reduced species (H2, CO, H2S, CH4), minerals and cracks in explaining reduced seismic velocities in the critical for understanding the rise of oxygen on the planet (Holland, subducting plate (Korenaga, 2017). Coupled numerical models and 2002), also remain poorly constrained. The rates of volatile release new observations are needed to delineate whether this hydration during early Earth are being revised through novel applications of is focused along narrow fault zones or more broadly distributed, noble and radiogenic gases (Marty et al., 2019), while the relative which in the former case would reduce the hydration estimates by recycling efficiencies of H2O, C, and N continue to be investigated up to an order of magnitude (Miller and Lizarralde, 2016). Volatile (Hirschmann, 2018). These remaining questions and research distribution will also impact the release of volatiles at depth (Wada et al., 2012). Recent results suggesting hydration of the subducting ■avenues necessitate further data acquisition utilizing sophisticated mantle up to ~25 km below the Moho in the Marianas raise questions about the balance of volatile input and output (Cai et al., 2018). field, laboratory, experimental, and modeling approaches. 46 • GeoPRISMS Newsletter Issue No. 43 Fall 2020

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