Geologic Controls on Up-Dip and Along-Strike Propagation of Slip During Subduction Zone Earthquakes from a High-Resolution GEOSPHERE, V

Home , Accretionary wedge, Basement high, Chile Triple Junction

Research Paper THEMED ISSUE: Subduction Top to Bottom 2

GEOSPHERE Geologic controls on up-dip and along-strike propagation of slip during subduction zone earthquakes from a high-resolution GEOSPHERE, v. 15, no. 6 seismic reflection survey across the northern limit of slip during https://doi.org/10.1130/GES02099.1

18 figures; 1 supplemental file the 2010 Mw 8.8 Maule earthquake, offshore Chile Anne M. Tréhu1, Bridget Hass1,*, Alexander de Moor1,†, Andrei Maksymowicz2, Eduardo Contreras-Reyes2, Emilio Vera2, and Michael D. Tryon3,§ CORRESPONDENCE: [email protected] 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA 2Departamento de Geofísica, Facultad de Ciencias Físicas y Mathemáticas, Universidad de Chile, Santiago, Chile CITATION: Tréhu, A.M., Hass, B., de Moor, A., Maksy- 3Scripps Institution of Oceanography, La Jolla, California 92093, USA mowicz, A., Contreras-Reyes, E., Vera, E., and Tryon, M.D., 2019, Geologic controls on up-dip and along- strike propagation of slip during subduction zone earthquakes from a high-resolution seismic reflection survey across the northern limit of slip during ABSTRACT ■■ INTRODUCTION

the 2010 Mw 8.8 Maule earthquake, offshore Chile: Geosphere, v. 15, no. 6, p. 1751–1773, https://doi.org A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was The Tohoku earthquake (offshore Japan) in 2011 /10.1130/GES02099.1. acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip generated a devastating tsunami as slip extended to during the 2010 M 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic the subduction trench, and the structural signature Science Editor: Shanaka de Silva w Guest Associate Editor: Laura M. Wallace experiment designed to monitor the post-earthquake response of the outer accretionary prism. of this process was captured in a remarkable pair We describe the MCS data and discuss the implications for the response of the accretionary prism of “before” and “after” seismic reflection images Received 1 December 2018 during the earthquake and for the long-term evolution of the margin. The most notable observa- (Kodaira et al., 2012). Since then, a number of inves- Revision received 17 June 2019 tion from the seismic reflection survey is a rapid north-to-south shift over a short distance from tigators have studied the structural characteristics of Accepted 23 August 2019 nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed other subduction zones to try to infer tsunamigenic

Published online 7 November 2019 trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating potential from the seismic reflection signature of the our structural observations with other geological and geophysical observations, we conclude that deformation front (e.g., Dean et al., 2010; Gulick et al., sediment subduction beneath a shallow décollement is associated with propagation of slip to the 2011; Cubas et al., 2016; Bécel et al., 2017; Han et al., trench during great earthquakes in this region. The lack of resolvable compressive deformation in 2017). As part of an experiment to monitor potential the trench sediment along this segment of the margin indicates that the plate boundary here is very post-seismic deformation of the outer accretionary weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip prism up dip from the patch of greatest slip during

down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the 2010 Mw 8.8 Maule earthquake (offshore Chile) the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strike- (Tréhu and Tryon, 2012), we acquired 1500 km of slip faults. We do not detect any variation along strike in the thickness or reflective character of the high-resolution multichannel seismic reflection trench sediments adjacent to the change in deformation front structure. This change, however, is data using a 600-m-long, 48-channel hydrophone correlated with variations in the morphology and structure of the accretionary prism that extend streamer and two Generator-Injector (GI) guns in as far as 40 km landward of the deformation front. We speculate that forearc structural heterogene- 45/105 mode (Fig. 1), referring to the volume (in cubic ity is the result of subduction of an anomalously shallow or rough portion of plate that interacted inches) of the generator and injector airguns. We also with and deformed the overlying plate and is now deeply buried. This study highlights need for acquired coincident swath bathymetric, 3.5 kHz sub- three-dimensional structural images to understand the interaction between geology and slip during bottom profiling, and gravity data. Although many subduction zone earthquakes. of the early slip models for the Maule earthquake indicated that slip did not extend to the trench (e.g., Moreno et al., 2010; Delouis et al., 2010; Tong et al., 2010; Lorito et al., 2011; Vigny et al., 2011), the up-dip *Now at National Ecological Observatory Network, Boulder, Colorado 80301, USA This paper is published under the terms of the †Now at Canyonlands Field Institute, Moab, Utah 84532, USA extent of slip is poorly constrained in most models, CC‑BY-NC license. §Retired and recent studies suggest that slip may have locally

GEOSPHERE | Volume 15 | Number 6 Tréhu et al. | Geologic controls on up-dip and along-strike propagation of slip during 2010 Maule earthquake Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1751/4876538/1751.pdf 1751 by guest on 30 September 2021 Research Paper

reached the trench (Yue et al., 2014; Maksymowicz 74˚W 73˚W 72˚W LEGEND et al., 2017; Wang et al., 2017). Water depth In this paper, we examine the seismic reflec- (meters) Epicenter of 2010 Maule earthquake N tion data collected over the trench and accretionary 400 32˚S Centroid moment tensor, Maule prism north of the epicenter and near the north- 1300 earthquake ern boundary of slip from the Maule earthquake 2200 Slip model of Moreno et al. (2012), contours labeled in meters to evaluate whether along-strike variations in the 3100 structure of the deformation front can be related to 4100 Multichannel seismic (MCS) data VFB from R/V Melville cruise MV1206 variations in the response of the outer prism to plate Juan Fernández Ridge boundary slip on both short and long time scales. Absolute pressure gauge deployed during MV1206 Most models indicate that the largest slip during the 33˚S Valparaiso Ocean-bottom seismometer earthquake occurred in this region. We document deployed during MV1206 SAC the presence of an along-strike transition over a SPOC seismic lines (Geersen et al., short distance from nearly total sediment accretion 66 mm/yr 2011) to dominantly sediment subduction at the deforma- VG02 seismic lines (Contardo et al., 2008) tion front, and look both seaward and landward of the deformation front for possible mechanisms to aipo MGL1701 seismic lines (Bangs io M R et al., 2017) explain this variability. We do not detect any sys-

ODP Leg 203 drill sites tematic changes in the reflectivity structure or total 34˚S 2 VG02-02 thickness of the trench sediments that can explain 4 Axial channel this observation. Landward of the deformation front, Pichilemu Large-aperture seismic pro les we provide evidence for a transpressive boundary (Contreras-Reyes et al., 2017) VG02-03 6 that separates the active accretionary prism from Continental backstop 8 the metamorphic rocks of the paleo–accretionary 6 (Contreras-Reyes et al., 2017) 16 prism, which acts as a backstop to subduction here 10 VFB Valparaiso forearc basin 12 (Moscoso et al., 2011; Contreras-Reyes et al., 2017) 35˚S 14 SAC San Antonio Canyon Rio and note variations in prism and backstop mor- Mataquito VG02-06 phology and structure that are correlated with the

Figure 4 ConstitucionConstitución along-strike change in deformation front structure. Rio Maule Our ultimate objectives are to explore whether

10 2 Geersen et al.Geersen (2011) segment boundaries structural patterns in the outer prism preserve 4

4 a signature of consistent, long-term patterns of strain release and to investigate the implications 36˚S of such observations for seismic and tsunami haz- 8 ard evaluation. Integrating our observations with Site 1235 Site 1234 prior results from bathymetric, seismic, potential field, geodetic, and coastal uplift studies (e.g., Riet- Rio Itata brock et al., 2012; Lange et al., 2012; Métois et al., 2012; Hayes et al., 2013; Cubas et al., 2013; Lieser et ConcepcionConcepción al., 2014; de Moor, 2015; Maksymowicz et al., 2015, 6 37˚S 6 2017; Bassett and Watts, 2015a, 2015b; Saillard et al., 2017), we conclude that sediment subduction is Rio Bio Bio 10 associated with up-dip propagation of slip during 8 earthquakes in this region and speculate that the

Figure 1. Bathymetry and topography of the central Chile subduction zone. Elevations are from Global Multi-Resolution Topography correlation between the along-strike transition in (Ryan et al., 2009), version 3.6.6. SPOC—Subduction Processes Off Chile; ODP—Ocean Drilling Program. deformation front structure and the accretionary prism morphology results from subduction of a

particularly shallow or rough, and now completely is linked to glaciation-deglaciation and rapid denu- trench is anomalously high in the region of our buried, part of the oceanic Nazca plate. dation of the Andes (e.g., Bangs and Cande, 1997; study compared to segments to the north or south Rauch, 2005; Melnick and Echtler, 2006; Völker et al., and suggests that tectonics influences channel 2006). Sediment is delivered from the continent to dynamics. ■■ BACKGROUND the trench through deep canyons and redistributed within the trench from south to north (Thornburg et Plate Tectonic Setting al., 1990; Völker et al., 2008, 2013). Numerous sub- Tectonics of the Accretionary Prism marine canyons cut across the continental shelf and The south-central Chilean margin (32°–46°S) slope and are offshore prolongations of the main The tectonics of the south-central Chilean is characterized by the subduction of the oceanic rivers of Pleistocene glacial valleys (Gonzalez, 1990). forearc have been discussed by a number of investi- Nazca plate beneath South America at a present Sediment fans are also commonly found where the gators, including Bangs and Cande (1997), Contardo rate of ~66 mm/yr (Angermann et al., 1999) in a canyons enter the trench (Thornburg et al., 1990; et al. (2008), Geersen et al. (2011), Moscoso et al. N78°E direction. This rate, determined from satellite Völker et al., 2006). (2011), and Contreras-Reyes et al. (2017). Based on data, is slower than the global plate model rate of Rauch (2005) estimated that the age of the several seismic lines across the margin between ~85 cm/yr (DeMets et al., 2010), which represents current trench is at most 600 ka by comparing 36°S and 40°S acquired from the R/V Conrad (Fig. 1), an average over several million years, suggesting the progressive onlap of sediments on the west- Bangs and Cande (1997) concluded that sediment a recent decrease in the convergence rate. None- ern flank of the trench to the subduction rate and accretion, nonaccretion, and subduction erosion theless, this rate is fast enough to drive a high level assuming a steady rate of trench fill and subduction. along the margin were episodic and controlled by of historic interplate seismogenic slip. The Maule He documented a repeating pattern of high- and climatic as well as tectonic factors, with sediment earthquake filled a well-documented seismic gap low-amplitude reflections in the trench, which he accretion occurring during glacial periods when the that had not ruptured in a major earthquake since estimated to have been periodically deposited rate of sediment input to the trench is high. A.D. 1835 (Campos et al., 2002; Lomnitz, 2004; every ~120 k.y. by cross-correlating reflection data Contardo et al. (2008) analyzed forearc struc-

Ruegg et al., 2009; Ruiz and Madariaga, 2018). with the temperature and CO2 concentration his- ture between 33.5°S and 36.5°S based on seismic Because of the length of the Peru-Chile trench and tory of Earth’s atmosphere derived from ice cores. reflection lines acquired by the R/VVidal Gormaz the systematic along-strike variation in parameters Völker et al. (2013) argued that during glacial peri- using an airgun source in a region that overlaps such as the age of the subducting plate, sediment ods, along-trench sediment transport from source the region covered by the ChilePEPPER reflection thickness, and incoming plate roughness, many regions in the glaciated south is dominant, whereas survey (Fig. 1). They described the slope basins studies of this region have attempted to isolate during interglacial periods, sediment input is domi- as asymmetric half-grabens of variable size inter- the dominant parameters affecting the subduction nated by input through the rivers and canyons and preted to result locally from subducted seamounts process and associated seismogenesis (e.g., Con- by slope instability, which recycles sediment from passing beneath the forearc and more generally treras-Reyes and Osses, 2010; Contreras-Reyes et the accretionary prism back into the trench. from underplating of sediment packages beneath al., 2010; Contreras-Reyes and Carrizo, 2011). A striking feature of the Chile trench is its axial the margin, which results in uplift and tilting of the channel, which extends continuously for ~1500 km, prism. The R/V Vidal Gormaz profiles stopped at from ~41°S to the northern limit of the filled trench the deformation front and did not cross the trench. Sedimentation in the Trench near 32.75°S. It is interpreted to have initiated in Geersen et al. (2011) focused on the forearc from the Pleistocene (Völker et al., 2006). Within the 35°S to 40°S using an extensive swath bathymetric Our seismic survey, acquired as part of Chile- trench, the channel is narrow and deeply incised data set and seismic reflection profiles acquired as PEPPER (Project Exploring Prism Post-Earthquake in places and broader in others as it meanders part of the SPOC (Subduction Processes Off Chile) Response; Tréhu and Tryon, 2012), was located just between the western and eastern boundaries project and identified four different tectonic seg- south of the intersection of the Juan Fernández of the trench (Fig. 1). In deeply incised areas of ments, each characterized by a distinct morphology. Ridge with the trench at ~33°S; this ridge acts as the trench, a thin layer of sediment is sometimes In the northern and southern Concepción segments, a barrier to northward transport of trench turbid- imaged filling the floor of the channel, suggesting located south of our field area (Fig. 1), they inter- ites (Fig. 1), resulting in a sediment-starved trench episodic sedimentation activity. From 38°S to 40°S, preted several thrust ridges interpreted to be the to the north and a sediment-flooded trench to the a younger channel can be observed in the floor seafloor manifestation of splay faults accommo- south (von Huene et al., 1997). The high sedimenta- of the channel. Where deflected by sedimentary dating some of the Nazca–South America plate tion rate in the trench between the Juan Fernández fans or slump deposits, the channel is commonly motion. Like Contardo et al. (2008), they identified Ridge and the Chile triple junction since the Pliocene wider and more poorly defined. Sinuosity of the normal faults and tilted basins that they attributed

to gravitationally driven extension driven by uplift and high-pressure environment and later uplifted offshore south-central Chile (e.g., Moreno et al., in response to sediment underplating. and is paired with exhumed roots of a magmatic 2010; Lay et al., 2010; Delouis et al., 2010; Tong et arc onshore (Willner, 2005). Near the seafloor, the al., 2010; Lorito et al., 2011; Vigny et al., 2011; Kiser boundary between the middle prism and continen- and Ishii, 2011; Lin et al., 2013; Yue et al., 2014; Wang Deep Structure of the Forearc tal framework rocks is interpreted to coincide with et al., 2017). While most indicate that the patch of the shelf break, which is characterized by a prom- greatest slip occurred northwest of the epicenter The deeper structure of the forearc in our study inent fault scarp up to 2 km high. Contreras-Reyes (Fig. 3), the cross-strike location of greatest slip, the region has been determined through tomographic et al. (2017) pointed out that the patch of greatest relationship between slip during the main shock inversion of travel times from marine sources slip during the Maule earthquake occurred where and aftershock activity, and whether slip propa- recorded on ocean-bottom and onshore seismom- the distance between the deformation front and gated to the trench or was arrested beneath the eters. Prior studies of the forearc crustal structure the shelf edge is greatest (34.0°S and 35.5°S). They outer accretionary prism vary among the models. in our study region (Moscoso et al., 2011; Contre- noted a similar relationship for the slip distribution This is due, at least in part, to different data sets ras-Reyes et al., 2017) indicate the presence of an in the great Chilean earthquake of A.D. 1960, which used to infer slip and the lack of data to resolve outer prism with a P-wave velocity <3 km/s formed overlapped with the southern end of the Maule rup- coseismic seafloor deformation offshore. Resolv- of accreted trench sediments; a middle prism ture and extended from 38°S to 45°S. Saillard et al. ing these details may shed light on the conditions formed of older accreted sediments (P-wave veloc- (2017) made a similar observation and concluded under which coseismic slip can propagate beneath ity ~4 km/s) overlain by an apron of low velocity that regions of coastal uplift were correlated with or through the accretionary prism and the implica- slope sediments; and continental framework rock creeping segments of the plate boundary. tions for tsunamigenesis. with a velocity of >5 km/s (Fig. 2). The middle prism Sladen and Trevisan (2018) recently documented is a region of potential growth by underplating and a global correlation between outer-rise aftershocks internal deformation, resulting in the half-grabens Slip Distribution during the 2010 Maule and apparent slip to the trench during megathrust and thrust ridges discussed in the previous section. Earthquake and Aftershock Distribution earthquakes. The Maule earthquake was followed The continental framework in this region is thought by pronounced outer-rise seismicity from 33.7°S to represent a late Paleozoic subduction complex Many different slip models have been published to 35.0°S (Fig. 3) including a normal-faulting event

that has been metamorphosed in a low-temperature for the 27 February 2010 Mw 8.8 Maule earthquake with Mw 7.4 (Ruiz and Contreras-Reyes, 2015). Very

Figure 2. (A) P-wave velocity (Vp) model for large-aperture seismic profile P03 (Contreras-Reyes et al., 2017) (see Fig. 1 for location). Solid black line is the interpreted boundary between the middle accretionary prism and the Paleozoic metamorphosed accretionary prism rocks thought to form the continental framework. The top and bottom of the crust of the Nazca plate are shown by long and short dashed lines, respectively. (B) Schematic cross-section based on profile P03 showing the major elements of the continental margin in this region identified primarily from large-aperture seismic lines (adapted from Moscoso et al., 2011; Contreras-Reyes et al., 2017). The red and blue lines represent an interpretation of the frictional characteristics of the plate boundary: blue—stable sliding, red-dashed— conditionally stable but can slip seismically during large earthquakes, red—stick-slip. Throughout this study, we use “prism” to refer to the accretionary prism (often called the accretionary wedge) and re- serve “wedge” to describe a local, recent sediment deposit found within the trench (Figs. 6, 7).

(e.g., Rietbrock et al., 2012), or that this region through the water was <5 kt to mitigate tow noise. did not slip during the earthquake because of a The record length was 9 s for most of the survey, velocity-strengthening rheology, implying that it but was increased to 10 s near the end of the sur- should have deformed aseismically after the main vey when the depth of penetration of the seismic shock (e.g., Lange et al., 2012). In the latter case, energy and thickness of the trench sediments one might expect to observe very-low-frequency became apparent. Sample rate was 1 ms. (VLF) earthquakes and non-volcanic tremor (NVT), Marine mammal observers were on deck during as has been reported from the outer prism in the all seismic acquisitions conducted during daylight Nankai Trough (offshore Japan) (e,g., Obara and hours, and reported numerous sightings during the Ito, 2005). However, with the exception of a few cruise that required turning off the airguns (Tréhu possible small slow-slip events and tremor epi- and Tryon, 2012). When the airguns were shut sodes (Tréhu et al., 2019), only limited evidence down for <10 min, we continued along the profile for VLF or NVT activity (compared, e.g., to the Nan- and resumed shooting when the area was clear, kai Trough) was found in broadband ocean-bottom leaving gaps in the data. For longer shutdowns, data acquired during ChilePEPPER (de Moor, 2015), we turned and filled the gap once shooting could a negative result consistent with an interpretation resume. In all, there were 110 separate sightings in which the outermost prism slipped during the of 265 individual animals during the cruise. The earthquake, releasing rather than increasing stress most commonly sighted mammals were fur seals. in the outer prism. Additional evidence that coseis- A number of whales and dolphins were also seen. mic and/or rapid postseismic slip extended to the Data were processed onboard through frequen- Figure 3. Map showing three representative slip models trench is supplied by comparing swath bathymetry cy-wavenumber migration using SIOSEIS software for the 2010 Maule earthquake (offshore south-central Chile): 5, 10, and 15 m slip contours from Lorito et al. (2011) acquired in 2008 and bathymetry along the same (https://sioseis.ucsd.edu). The processing sequence in red; 10 and 15 m slip contours from Vigny et al. (2011) in track acquired in 2011 and during cruise MV1206 included sorting, normal moveout (NMO), stack, dark blue; 10 and 15 m slip contours from Yue et al. (2014) on the R/V Melville, which resolved ~5 m of uplift and frequency-wavenumber migration. A velocity in white. Red star is the epicenter of the main shock. Light blue dots are aftershock epicenters. Source mechanism of the outer prism between 2008 and 2011 (Maksy- of 1485 m/s was used for both NMO and migra- solutions of the largest aftershocks are also shown. White mowicz et al., 2017). Whether slip extended to the tion. In general, each line was processed within a squares are seismic stations deployed onshore for the seafloor at the deformation front or was arrested few hours of acquisition. Preliminary interpreta- international aftershock monitoring array known as iMAD. Black triangles are volcanos. Black lines represent the within 6 km of the deformation front is not con- tions of the data guided subsequent acquisition. Chilean coast to the west and the border between Chile clusively resolved by the bathymetric change data. Seismic sections were input into a Kingdom Suite and Argentina to the east. Adapted from Rietbrock et al. (https://ihsmarkit.com/products/kingdom-seismic (2012) to add the Yue et al. (2014) slip model. -geological-interpretation-software.html) project ■■ MULTICHANNEL SEISMIC DATA: while at sea for interpretation and integration few earthquakes were located within or beneath ACQUISITION AND PROCESSING with bathymetric and potential-field data. Data the outer accretionary prism, with apparent excep- images shown here were generated either with tions up dip of the epicenter near 36°S and near Multichannel seismic (MCS) data were acquired Kingdom Suite or Seismic Unix (https://github.com the northern end of the rupture zone. Although by the Scripps Institution of Oceanography (SIO) /JohnWStockwellJr/SeisUnix). the earthquakes in Figure 3 were located using Shipboard Geophysical Group (SGG) using a 600 Figure 4 shows the location of the ChilePEPPER only onshore seismic stations, lack of aftershock m, 48-channel streamer during cruise MV1206 on seismic lines overlain on swath bathymetry. Because activity within and beneath the outer prism has the R/V Melville. The number of channels, however, of the slow ship speed and good weather conditions been supported by several ocean-bottom seis- was decreased to 40 soon after the beginning of during most of the cruise, data quality is excellent. mometer studies (Lieser et al., 2014; Hicks et al., the cruise because shark bites penetrated two of The data were acquired in a grid of lines that covered 2014; de Moor, 2015). That the outer prism was the eight-channel streamer sections and only one the extent of ocean-bottom seismometers, abso- seismically quiet after the main shock has been spare section was available. The source was two lute-pressure gauges, and seafloor flowmeters variably interpreted to indicate that this was the GI-guns in a 45/105 cm3 configuration. The guns deployed to detect any local earthquakes, NVT, VLF region of greatest slip during the mainshock, with a were mounted on a crossbar that maintained them earthquakes, seafloor uplift or subsidence, and dif- down-dip gradient of increasing seismicity marking at constant separation and were fired at an inter- fuse fluid flow through the seafloor (Tréhu and Tryon, the down-dip edge of slip during the main shock val of 25 m as determined from GPS. Ship speed 2012). Additional profiles were acquired along the

trench and across the deformation front. Although the spatial footprint of this survey is small, the lines are more closely spaced than those of other MCS surveys conducted to date along the Chile margin (Fig. 1), providing a unique opportunity to understand structural relationships in the accretionary prism in three dimensions along the segment of the margin that experienced the largest slip during the 2010 Maule earthquake. Notable bathymetric features shown on Figure 4 include: normal faults nearly perpendicular to the trench, reflecting seafloor spreading fabric formed at the East Pacific Rise; horst-and-graben structure oriented slightly oblique to the trench resulting from plate bending; the Bucalemu seamount, which is cut by normal faults; many smaller seamounts; an axial channel that meanders from side to side of the sediment-filled trench; variations along strike in the development of protothrusts seaward of the primary deformation front; a distinct embayment in the deformation front; a thrust ridge identified by Geersen et al. (2011); and several deeply incised canyons. The Maule and Mataquito Canyons can be linked to major rivers onshore, similar to the Bio Bio Canyon, although high-resolution bathymetry is not available from the nearshore region to con- firm this. In contrast, the Huenchullami Canyon is associated with a much smaller river that originates in the Chilean Coastal Range and ends abruptly ~30 km east of the trench. Additional bathymetric features within the study region will be discussed in the next section in conjunction with their subsurface structure as imaged in the seismic data.

■■ MULTICHANNEL SEISMIC DATA: OBSERVATIONS

Figures 5A–5C show three representative seismic lines located ~15 km apart. Detailed images of Figure 4. Bathymetry of the study area (south-central Chile) from Global Multi-Resolution Topography (Ryan et al., 2009) version portions of these and other lines are discussed in 3.6.6 (including data acquired during cruise MV1206 on the R/V Melville). Bathymetric contours are shown at 500 m intervals. this section, moving from west to east across the Black lines show the tracks along which multichannel seismic reflection data were acquired. Line numbers are indicated, omitting the prefix “CP” used in the text. Overlays on the track lines correspond to data shown in figures as indicated in the legend (see trench, deformation front, and accretionary prism. footnote 1 for Fig. S1). VG02-02—Line 2 from cruise VG02-02 (Contardo et al., 2008). Blue dashed line indicates a forearc structure Raw data, migrated stacks, and images of each seis- south of the study region interpreted as a thrust ridge by Geersen et al. (2011). This structure, which marks the boundary between mic line are available through the Academic Seismic the middle prism and continental framework, appears to continue north of the study region but is not clearly defined within the study region. White dashed rectangle outlines the region shown in Figure 12. AC—axial channel within the trench. Portal maintained by the University of Texas Insti- tute for Geophysics (http://www-udc .ig .utexas .edu

/sdc/). Line CP06 is coincident with large-aperture seismic profile P03, which was modeled and discussed by Moscoso et al. (2011) and is the basis for the simplified cross-section shown in Figure 2. It is also coincident with the bathymetric swath used by Maksymowicz et al. (2017) to infer uplift during or soon after the Maule earthquake. Figure 5D illus- trates the along-strike topographic variability that characterizes this segment of the margin. Note the striking difference in the slope of the outer prism between profile CP19 and the other two profiles. The reflection character and topography of the middle prism also varies significantly along strike, as does the topographic boundary between the middle prism and the continental framework.

Lower Plate Structure and Trench Fill

Figure 6 shows seismic lines along and across the trench. The outer rise seaward of the trench along this segment exhibited a high level of aftershock activity in response to the 2010 Maule earthquake (Fig. 3). Outer-rise faulting here is approximately perpendicular to faulting that reflects the crustal fabric formed at the spreading center and is oblique to the trench (Fig. 4). The incoming plate is also marked by the presence of numerous seamounts of various sizes, including a large seamount west of the trench near 34°28.233′S, 73°52.457′W that is clearly cut by two outer-rise bending faults, forming a graben at the crest of the seamount (Fig. 4). The seafloor appears to be capped by a thin layer of pelagic sediment of variable thickness that follows the basement and is overlain by trench fill (e.g., seismic line CP11, Fig. 6). This layer can be clearly identified only when onlapped by trench sediments. Some of the basement blocks appear to also uplift trench sediments (e.g., line CP24, Fig. 6). This may explain the anomalously narrow width of the trench and apparent decrease in trench sediment volume estimated by Völker et al. (2013) along this segment of the margin because they included in their estimate Figure 5. (A–C) Three seismic lines across the south-central Chilean margin (CP19, CP06, CP10) spaced ~15 km apart (see Fig. 4 for only sediment east of the western boundary of the location), illustrating the structural elements shown in Figure 2. All profiles are oriented from east to west. Considerable variation is observed along strike in the basement structure of the incoming plate and shallow structure of the upper plate. Letters A–G trench as defined by topographic contours. indicate bathymetric features labeled in Figure 12. (D) Topography along each of the profiles shown in A–C.

Figure 6. Six trench crossings (seismic lines CP06, CP08a, CP10, CP11, CP24, CP25) and a composite seismic line along the trench (lines CP17, CP18, CP18a, CP22), south-central Chile (see Fig. 4 for location). Dotted lines show the intersection between the cross-trench and along-trench lines. Orange line shows the basement surface; green line is a sediment horizon that can be traced on all the lines within the trench and is used to define the position and thickness of a shallow wedge of sediment in the trench located immediately south of where the Maule Canyon enters the trench. BR1, BR2—basement ridges discussed in the text; AC—axial channel; UTS—uplifted trench sediment; TSW—thick shallow wedge. PS—pelagic sediment.

Outer-rise normal faults and the resulting horsts, grabens, and tilted blocks show vertical offsets of 500 m or more and extend beneath the trench sediments. Offsets in the trench sediments overlying the basement ridges decrease with decreasing depth beneath the seafloor and have little or no offset at the seafloor, indicating that these faults are active as they continue into the trench but that normal-faulting activity decreases as the faults approach the deformation front (e.g., lines CP10 and CP24, Fig. 6). Although it is difficult to unambigu- ously trace individual ridges between seismic lines, even with this relatively dense line spacing, we identify two en echelon basement ridges, labeled BR1 and BR2 in Figures 4 and 6. On some profiles (e.g., lines CP10 and CP24, Fig. 6), BR1 appears as a tilted block bounded on its northwestern flank by a normal fault, whereas on line CP08a (Fig. 6), it appears as a fault-bounded horst. On line CP06 (Fig. 6), a composite basement high defined by an envelope of diffraction hyperbolae indicative of a Figure 7. (A) Contours showing the thickness of the strata above the green horizon shown in Figure 6 (black rough surface adjacent to a small fault-bounded lines) overlain on a map of the seafloor gradient measured perpendicular to the trench. Red indicates that block is observed were we expect to see BR1. No the seafloor is sloping to the southeast; blue indicates slope to the northwest. Contour interval is 80 ms two-way travel time (equivalent to 80 m assuming a sediment velocity of 2000 m/s for the wedge). BS—Bu- basement high is observed on line CP21 (Fig. 4) at calemu seamount; TSW—thick shallow wedge; PAB—pull-apart basin; UD—uplifted dome; H—bathymetric the corresponding position. We interpret the obser- high; L—bathymetric low. Yellow lines show the locations (from north to south) of the Mataquito, Huenchul- vations on lines CP06 and CP21 to indicate that the lami, and Maule Canyons. Short and long dashed lines outline structural features discussed in the text in conjunction with Figure 12. (B) Sediment thickness at the deformation front (see black dots in A) assuming amplitude of the tectonic ridge identified as BR1 a constant velocity of 2500 m/s for the trench sediment fill. decreases to the northeast and is coincident with a small buried seamount at its northeastern terminus. A larger buried seamount overlain by a thin layer of the axis of the trench and sediment supplied locally the high sinuosity and indistinct cross-section of pelagic sediment unconformably overlain by trench to the trench from the Maule canyon. the axial channel along this segment of the trench. fill sediments is present at the northernmost end of The TSW has a strong effect on the depth and Basement age in the study region is ca. 34 Ma. the composite trench-parallel line (Fig. 6). width of the axial channel. On seismic line CP11 The closest cores available to constrain the age of A distinctive feature of the study region is a thick (Fig. 6), it is ~100 m deep and ~1500 m wide with the trench sediments is Ocean Drilling Program shallow wedge (TSW) of sediment deposited imme- a flat floor and steep sides; no paleo-channel is (ODP) Site 1232 (Mix et al., 2003), located near the diately south of where the Maule canyon enters the observed in the trench sediments. Going north, western edge of trench near 40°S. At ODP Site 1232, trench. The ChilePEPPER data (Fig. 6) reveal the the channel appears to be deflected to the west by ~390 m of turbidites was recovered. Although poros- internal stratigraphy and thickness of this deposit the sediment lobe and is wide and shallow (lines ity decreased rapidly from ~90% to 55% in the upper and the erosional character of its western margin. CP25 and CP24, Fig. 6). The channel then swings 15 m of the core, the porosity was nearly constant The thickness of this deposit is shown in map view abruptly to the east around the northern edge of below 15 m, indicating underconsolidation and in Figure 7A, and the sediment thickness at the the sediment wedge and is located at or near the high pore pressure. Paleomagnetic measurements deformation front is shown in Figure 7B. Similar deformation front (lines CP06 and CP08a, Fig. 6) indicated that this entire section was emplaced deposits have been documented associated with before swinging back to the western edge of the within the Bruhnes Chron (<0.78 Ma), indicating the intersection of other major canyons and the channel, where it is again clearly incised into the a sedimentation rate of at least 475 m/m.y. (Mix Chile trench (Thornburg and Kulm, 1987; Thornburg trench floor (line CP18a, not shown). The compet- et al., 2003). Assuming this rate, age of the oldest et al., 1990; Völker et al., 2006) and reflect complex ing effects of the TSW and tectonic uplift of the trench sediments would be ca. 5 Ma. However, it interactions between sediment transported along western edge of the trench may be responsible for is likely that the sedimentation rate for the TSW is

significantly higher. If one assumes that the rate of 475 m/m.y. applies to sediments north of the TSW, the age of the oldest sediments would be ca. 3.4 Ma.

The Deformation Front

Figures 8–10 show examples of data crossing the deformation front. On line CP19 at the northern end of the survey (Fig. 8), the deformation front is characterized by a seaward-verging blind thrust fault that extends from the seafloor to near the base of the trench sediment. Similar structures are seen in SPOC line SO161-42 at 37°S (Völker et al., 2013). The offset in time on this fault decreases from ~0.050 s at point A to 0.038 s at B to 0.025 s at C (Fig. 8). Assuming a velocity of 1800 m/s near the surface, Figure 8. Detail of the northernmost segment of the trench seismic line (CP22) and the western end of line CP19 increasing to 3600 m/s near the base of the sedi- (see Fig. 4 for location). Although the background noise level was higher when line CP22 was acquired, the two lines match well at their intersection except for out-of-plane reflections. A, B, and C show places where ment pile due to sediment compaction (Moscoso the offset on the frontal blind thrust fault was estimated to be 0.025, 0.038 and 0.050 s two-way travel time, et al., 2011), the observations suggest a constant respectively. This corresponds to an offset of ~45 m assuming a P-wave velocity of 1800 m/s at A and 3600 m/s offset of ~45 m on this fault, in contrast to the at C. The region imaged by line VG02-03 (Contardo et al., 2008) is indicated. decreasing offset with decreasing depth observed for the normal faults in the trench. Whether this fault was developed in a single earthquake, in many 1 Supplement to: Insights into controls on up-dip and along-strike propagation of slip during 2 subduction zone earthquakes from a high-resolution seismic reflection survey across the small events, or through aseismic creep cannot be 3 northern limit of slip during the 2010 Mw8.8 Maule earthquake, by Tréhu et al. 4 resolved from cruise MV1610 on the R/V Roger Rev- 5 This supplement presents several figures that, while not essential to conveying the primary Figure 9. (A) Deformation front on 6 results of this paper, may be of interest to some readers. The reader is referred to the Academic seismic line CP06 (see Fig. 4 for 7 Seismic Portal managed by the University of Texas Institute for Geophysics for images and elle data, although the constant offset along the fault 8 processed segy-format files of all of the seismic reflection data acquired during MV1206. location). Note apparent under- 9 Unprocesses data are also available. and seafloor manifestation of a frontal fold indicate 10 plating of the upper 30%–35% of 11 Figure S1. Seismic line VG02-02 across the deformation front north of our study area. that it was initiated relatively recently. the incoming sediment beneath 12 13 Figure S2. Seismic lines from Figure 9 without the interpretive overly. Landward of the frontal thrust, the outer 6 km the outer prism and subduction 14 of the entire sediment package 15 Figure S3. CP06 across the outer and middle prisms. Letters are keyed to the bathymetric map in of the accretionary prism on line CP19 (Fig. 8) 16 Figure 12. underlying a bright package of re- 17 appears to be formed primarily by folding of the 18 Figure S4. Vertical derivative of the gravity field (from Maksymowicz et al., 2015). The gravity flections in the trench sediments, 19 anomaly interpreted to be a subducted seamount is labeled R1. H1 and PA are gravity anomalies entire sediment layer, and the top of the subducted which appears to form a regional 20 corresponding to the Cobquecura and Pichilemu P-wave velocity anomalies identified by Hicks 21 et al. (2014). oceanic crust is observed to ~2 s two-way travel décollement. The dotted line 22 shows the interpreted position of 23 Figure S5. Comparison of seafloor morphology near 34°S, where we image near total accretion time (TWTT) (~3 km assuming an average velocity 24 of trench sediment, to morphology near 38°S, where near total accretion of sediment has also this bright reflection horizon as 25 been imaged. Between these segments, the margin is characterized by sediment subduction. 26 Coastal uplift rate reported by Saillard et al. (2017) is also shown and is correlated with the of 3 km/s for the outer accretionary prism; Moscoso it merges with the décollement. 27 accretionary segments of the margin. Arrows indicate canyons that do not reach the trench. (B) Deformation front on lines CP21 28 Bathymetry from GMRT version 3.6 (Ryan et al., 2009). et al., 2011). In contrast, the R/V Vidal Gormaz data 29 from line VG02-03 (Contardo et al., 2008), which and CP06 (see Fig. 4 for location) 30 showing continuity of the reflec- is nearly coincident with line CP19 (Fig. 1), did 1 tion from the top of the subducting not extend far enough west to image the frontal oceanic crust of the Nazca plate, thrust fault; because line VG02-03 was processed the décollement, and the interpreted paleo-décollement. On line only to 8.2 s TWTT, it also does not image the top 1 Supplemental Material. Includes additional examples CP21, the paleo-décollement does of seismic data, a gravity map of the study region, and of the subducting oceanic crust beneath the outer not extend to the surface (marked a comparison of the morphology of the deformation accretionary prism (Fig. 7). Similar structures are by “?”). BR1—basement ridge. front in the study region to that of the deformation observed on line CP23 and on line VG02-02, which front near 39°S. Please visit https://doi.org/10.1130 /GES02099.S1 or access the full-text article on www crossed the deformation front ~30 km north of line .gsapubs.org to view the Supplemental Material. CP19 (Fig. S1 in the Supplemental Material1).

In contrast to line CP19, lines CP21 and CP06 (located 10–15 km south of CP19) are characterized by a seaward frontal thrust that reaches the seafloor and subduction of all of the incoming trench sediment (Fig. 9; see Fig. S2 [footnote 1] for an uninterpreted version of this figure). The Figure 10. Deformation front on seismic frontal thrust cuts through the upper 0.5 s TWTT line CP11 (see Fig. 4 for location). The (~500 m) of the trench sediments before flattening boundary between the shallow wedge and apparently following a stratigraphically con- discussed in the text and the underlying trench sediments (dashed line) is at trolled surface, or décollement, located ~0.9 s TWTT ~7.4 s two-way travel time. (~1 km) above the crust of the subducting plate. An unconformity between the uppermost subducted sediment and the décollement suggests that some of the subducted sediment is underplated to the base of the outer prism (Fig. 9A). The décollement reflection and top of the subducted crust can be followed until ~15 km landward of the deformation front. The position of the plate sediments (Fig. 10). Instead, they appear to be con- and the margin segment to the south. The abrupt boundary, ratio of subducted to accreted sediment, fined to the sediments that form the TSW. Most of nature of this transition is evident in the left-lateral and presence or absence of deformation within the the trench sediments below the TSW are under- offset of the deformation front immediately north subducted sediments deeper within the subduction thrust in a manner similar to what is observed from of line CP21 (Fig. 4). zone are not constrained by these data. A possible lines CP21 to CP10. We conclude that the shallow The stratigraphic horizon into which the frontal earlier frontal thrust and décollement are observed protothrusts on the southern lines are distinctly thrust appears to sole on all lines south of line CP19 ~2.5 km landward of the deformation front (Fig. 9), different from those on line CP19 and that there is a is observed as a strong, subhorizontal reflection suggesting a cyclical pattern of overthrusting and fundamental change in deformation front structure at ~7.5 s TWTT within the trench (Fig. 6). Figure 11 accretion at the deformation front as interpreted between the segment of margin crossed by CP19 shows the portion of this horizon between lines by Gutscher et al. (1998) for the Aleutian Islands (Northern Pacific Ocean). A similar décollement beneath which most of the incoming sediment is subducted with no evidence for protothrusts in the trench sediments is observed on lines CP08a and CP10 (Fig. 6). The absence of protothrusts on these profiles indicates a weak and shallow décollement that does not transmit horizontal compressive stress to the trench sediments, in contrast to line CP19 and further north. Analysis of data acquired during cruise MGL1701 on the R/V Marcus Langseth with a large-volume airgun source array and a 15-km-long streamer coincident with line CP06 should provide more detail about the velocity structure and consolidation state of the underthrust sediments and the geometry of the front thrust at greater depth along this segment of the margin (Olsen et al., 2017; Bangs et al., 2017). South of line CP10, protothrusts seaward of the

frontal thrust are observed although, unlike on line Figure 11. Detail of trench sediments imaged on the segment of seismic line CP18a between its intersections CP19, they do not extend to the base of the trench with CP19 and CP21 (see Fig. 4 for location).

CP21 and CP19. Although the event appears to have positive polarity, indicating a velocity increase, it is characterized by a package consisting of several cycles of bright reflections, which suggests interlay- ing of high- and low-velocity layers. No systematic change in the character of this reflection package is observed between line CP21, where the décollement merges with this horizon, and line CP19, where it is offset by the frontal thrust. The surface into which the frontal thrust on line CP19 soles corresponds to a bright reflection of apparently negative polarity. Although this reflection laps onto basement along the segment of profile shown in Figure 11, it is likely continuous with a reflection seen near the trench on line CP06 (Fig. 9). Why the structure of the deformation front changes so dramatically between lines CP19 and CP21 is not apparent from these data. Figure 12 summarizes the observations of the deformation front and accretionary prism in map view. Where all of the trench sediment is being subducted, the seafloor slopes to the east and the axial channel is located at the deformation front. South of this segment, the axial channel is deflected to the west by the TSW; north of this segment, where all trench sediment is accreted, the axial channel swings to west side of the trench. The local east- ward dip of the trench is likely related to plate flexure as sediment is subducted. Note that the uplifted trench sediment perched on top of outer rise horsts along the western margin of the trench is also restricted to this segment of the trench within our study region. Figure 12. Bathymetric map showing structural features discussed in the text (see Fig. 4 for location). Black lines show the tracks along which multichannel seismic reflection data were acquired. Line numbers are indicated for lines shown in subsequent figures, omitting the prefix “CP” used in the text. Overlays on the track lines correspond to data shown in figures as indicated in the legend. Letters A–G correspond to structures shown in Figure 5B and The Active Accretionary Prism Figure 13. Blue line is the axial channel. Pink line outlines the region of disrupted topography discussed in the text. BSR—bottom-simulating reflection; H—bathymetric high; L—bathymetric low. Figure 5 and Figure S3 (footnote 1) show the transition from the deformation front and to the continental framework on line CP06 with features front. The dashed pink line in Figure 12 delimits the the deformation front corresponding to these struc- labeled A–G keyed to bathymetric features labeled region of the forearc characterized by such structures, we tentatively interpret them to be surficial on Figure 12. Feature A corresponds to the dis- tures. This region also includes structures that are erosional gullies, and note that this region does continuous band of landward-dipping reflections perpendicular to the trench-parallel ridges corre- not show the impact of basement topography, tentatively interpreted to be an earlier frontal thrust sponding to A–C. Unfortunately, no seismic lines consistent with formation through underplating of in Figure 9; B and C are similar bands of land- are oriented properly to image the structure under- sediment subducted beneath a shallow décollement. ward-dipping reflections in an otherwise incoherent lying these subtle topographic features—which The feature labeled D on Figure 5 and Figure reflective background that can be correlated with are more clearly seen in the slope map (Fig. 7)—in S3 (footnote 1) represents a transition from the small ridges that are parallel to the deformation cross-section. Given the absence of offsets along highly deformed accreted sediments of the outer

prism to a region of older accreted sediments with multiple episodes of extension, tilting, com- higher resolution than was previously available. that are overlain by distinct slope basins that we pression, and folding, as previously discussed by Line CP26 (Fig. 13) crosses a bathymetric ridge that refer to as the middle prism. The middle prism is Contardo et al. (2008) and shown in Figure 13. The is similar to and in line with the thrust ridges iden- also characterized by a decrease in slope of the new data presented in this study show the complex, tified by Geersen et al. (2011) further south (Figs. 4, seafloor. These basins record a complex history along-strike variation in structure in this region with 12). This topographic ridge is bounded by near-vertical faults and resembles a flower structure formed by strike-slip motion in a transpressive stress field. Further north, lines CP15 and CP19 show faulting and folding in the sedimentary fill of the Huenchul- lami Canyon that extend nearly to the seafloor and also exhibit a flower structure (Fig. 14). Although there are no samples to provide direct age con- straints on these structures, they appear to have formed recently and are likely still active. The two regions where transpressive structures are observed are indicated by solid blue lines on Figure 12; dashed blue lines indicate the extension to the north and south of these structures based on bathymetry (Fig. 4). The offset between these segments is consistent with an extensional stepover. We therefore interpret the structural complexity of the middle prism in our study region to represent a pull-apart basin, outlined by a thin dashed line in Figure 12. The sense of motion implied by this structural analysis is consistent with the oblique plate motion vector (Fig. 1) as well as with bends in the Maule, Huenchullami, and Mataquito Canyons (Figs. 4, 12). At the current plate motion rate (66 mm/yr) and obliquity (30°), the 15-km-long north- south–trending segment of the Huenchullami Canyon that is aligned with an apparent strike-slip fault could have formed over ~0.5 m.y. In contrast, the middle prism in the segment of the margin crossed by line CP19 is generally deeper (Fig. 5D) and exhibits pronounced along- and across-strike bathymetric variability. This portion of the accretionary prism is located northwest of the solid pink line in Figure 12; local highs and lows within this apparently collapsed part of the middle prism are outlined by dotted lines labeled H and L. Unlike the Maule Canyon, which cuts across the entire margin to reach the trench, the Huenchul- lami Canyon has no bathymetric expression in this region. Further north, the Mataquito channel also Figure 13. Three seismic line crossings of the middle prism showing a pull-apart basin located at an extensional stepover does not reach the trench. We infer that this part of in a dominantly strike-slip fault zone located near the boundary between the middle prism and continental framework. See Figure 12 for locations; F and G refer to positions on that map. BSR—bottom-simulating reflection indicative of the the prism accumulated through accretion of trench presence of gas hydrate. sediment along a décollement located near the top

deformation and collapse due to subduction of seafloor topography, opening fluid pathways that allowed methane-rich fluids to escape.

The Inner Prism–Continental Framework

Figure 15 shows examples of data along the western edge of our study region (for location, see Fig. 12). On the basis of a few two-dimensional seismic refraction profiles perpendicular to the Figure 14. (A) Detailed view of the “flower” structure imaged on seismic line CP26 (see margin, Contreras-Reyes et al. (2017) proposed Fig. 13). (B) Strike-slip faulting in the Huen- that the boundary between the middle prism and chullami Canyon imaged on lines CP19 and continental framework corresponds to the location CP15. See Figure 12 for locations. of the shelf break, which is approximately defined by the 1000 m bathymetric depth contour along the central Chile margin (Figs. 1, 4), and noted a correlation between the northern limit of slip in 2010 and an abrupt landward deflection of this contour. The eastern end of our survey approaches this boundary and indicates considerable along-strike variation in the subsurface structure (Fig. 15). The eastern ends of lines CP06 (Figs. 15A, 15D), CP08 (Fig. 15E), and CP20 (not shown) show basement rocks at the seafloor, which form a bathymetric and structural dome (labeled “basement uplift” on Figs. 12, 15). This uplift is overlies a sub- of the oceanic crust, and that the bathymetric het- segment of the margin. The seaward extent of BSR ducted seamount with a radius of ~15 km modeled erogeneity in this region results from the sensitivity observations is indicated as a dashed black line in by Maksymowicz et al. (2015) based on the grav- of this process to basement topography. Figure 12. No BSRs are observed near the defor- ity gradient (Fig. S4 [footnote 1]). The numerous mation front, similar to the central Cascadia margin normal faults that characterize the top of this local (offshore western North America) (Phrampus et uplift and the slump scars along its western margin Evidence for Gas Hydrates al., 2017). As in Cascadia, the observation of a gap (Fig. 12) support an interpretation of rapid local between the deformation front and the onset of a uplift overlying a subducted seamount, although no A bottom-simulating reflection (BSR) of vari- BSR along this portion of the Chile margin is cor- distinctive wake indicative of seamount passage is able amplitude is discontinuous but widespread related with subduction of a significant fraction of observed to the west along the plate motion direc- throughout the accretionary prism in this region the incoming sediment. We speculate that subduction. We speculate that the characteristic signature (Figs. 13, 15; Figs. S3, S5 [footnote 1]), indicating tion of trench sediment delays upward migration of a subducted seamount that has plowed through the presence of free gas underlying gas hydrate. of pore fluids into the gas hydrate stability zone. the accretionary prism (uplift followed by subsid- However, no indications of significant fluid venting In contrast to what is observed in Cascadia, the ence parallel to the plate convergence direction) to the seafloor (e.g., gas chimneys in the subsur- seaward limit of the BSR is deflected even farther has been overprinted by the effects of transpression face, gas plumes in the water column, or seafloor landward where all incoming sediment is currently and transtension in this region. mounds and bright spots) have been observed, in being accreted. No BSR is observed beneath the North and south of the basement uplift, the east- contrast to venting observed further south along morphologically distinct region north and northern ends of lines CP15 (Fig. 15B), CP19 (Fig. 15C), the margin between 35°S and 37°S (e.g., Klaucke west of the solid pink line in Figure 12. The lack of and CP11 (Fig. 15F) show that the subsurface et al., 2012). The distribution of the BSR provides a BSR in this region is consistent with our inter- structure is characterized by faulted, folded, and additional insights into the tectonic history of this pretation that this region experienced widespread tilted sediments similar to those observed in the

Figure 15. Seismic lines crossing the boundary between the middle prism and nominal continental framework showing a localized dome interpreted to be older, highly deformed accretionary complex material uplifted by and overlying a subducted seamount. Line locations are shown in Figure 12. (A,B) Seismic lines showing the regional setting (shown by dashed red lines in Fig. 12). BSR—bottom-simulating reflection. (C–F) Data examples contrasting the localized uplift to structure immediately to the north and south of the dome (shown by solid red lines in Fig. 12).

basement rocks on these lines represent the continental framework, the top of basement on line CP15 is at a depth of ~2400 m, in contrast to 1000 m at the crest of the uplifted dome. These results highlight the three-dimensional structure of the subduction backstop in this region.

■■ DISCUSSION

Summary of New Stratigraphic and Structural Observations

The ChilePEPPER seismic reflection data provide a unique opportunity to examine the relationship between deformation front structure and slip during a major subduction zone earthquake.

For the 2010 Mw 8.8 Maule earthquake, most models place the maximum amplitude of slip offshore near the northern end of an ~500-km-long rupture plane. A primary observation of our study is that the structure of the deformation front changes abruptly from subduction of most of the incoming sediment up dip from the patch of greatest slip during the 2010 Maule earthquake to nearly complete accretion immediately to the north. This transition occurs over a distance of <10 km, as constrained by MCS lines ~10 km apart (Figs. 8, 9), and is likely more localized, as suggested by an abrupt change in seafloor morphology. South of this transition, the frontal thrust appears to sole Figure 16. Seismic lines CP13a and CP15 across the Mataquito basin where it is cut by the Mataquito channel. See Figure 4 for locations. into a detachment surface, resulting in underplating of approximately one-third of the incoming trench sediment beneath the outermost prism and transpressive zone to the west. These sediments the structural complexity of this lithological con- subduction of the remaining two-thirds to greater overlie a highly deformed, acoustically transparent tact. The northeastern extent of lines CP13a and depth. Although the topographic signature of accre- “basement” that has been uplifted relative to the CP15 (Fig. 16) are consistent with this interpretation tion reappears south of seismic line CP10, the MCS basement to the west. The shallowest overlying sed- because the most recent sediments were depos- data indicate that accretion here is restricted to the iments are subhorizontal and relatively undeformed. ited in a broad basin rather than in multiple smaller sediments of a recent surficial thick shallow wedge We tentatively interpret this transition to represent basins bounded by faults. The dominant mecha- (TSW) located upstream from where the Maule Can- the transition from the active accretionary prism nism for stratigraphic disruption in these sediments yon joins the trench (Fig. 10). Such sedimentary to the continental framework, which implies that is the complex interplay between canyon migration deposits are typical of areas where canyons enter this transition occurs west of where it was mapped and instability of the canyon sides. Strata parallel to the trench along the Chile margin (e.g., Thornburg by Contreras-Reyes et al. (2017) based solely on the basement overlain by a pronounced unconfor- and Kulm, 1987; Völker et al., 2006). bathymetric data. Our proposed accretionary prism– mity on line CP13a show evidence for a period of The frontal thrust on all ChilePEPPER lines, continental framework transition is characterized uplift on both sides of the basin followed by rapid including and south of line CP21, appears to sole by uplift and subsidence along strike, reflecting sedimentation within the basin. Assuming that the into the same stratigraphic horizon. In the trench,

this horizon is a multicycle packet of strong reflec- implies that the elevation (and presumably the two possible interpretations for this observation: tivity, suggesting that the décollement surface at thickness) of the continental framework basement (1) the deformation front structure does not influ- depth beneath the outer prism is a smooth, strati- here varies by at least 1.4 km along strike and that ence whether slip in an earthquake reaches the graphically controlled surface where the margin the strong curvature of the topographically defined trench and is therefore a poor proxy for past earth- is dominated by sediment subduction (Fig. 11). edge of the continental framework near 34.5°S quake behavior, or (2) the slip contours are biased Because the amplitude of this reflection does not (Figs. 1, 4) may be due to local crustal thinning ~20 km to the north or to the south. Noting that change to the north, where it is cut by the fron- and subsidence rather than to a landward swing of our results imply an offset in the plate boundary tal thrust in the region of near-complete sediment the contact between the active accretionary prism fault when viewed in cross-section (Fig. 17) and accretion and the décollement follow a deeper and the continental framework. that studies of strike-slip faults indicate that such bright reflection, we do not a find an explanation offsets often act as barriers to slip propagation (e.g., for this along-strike structural change in the trench King and Nabelek, 1985; Oglesby, 2005), we consider sediment structure. Implications for Inferring the Up-Dip Extent interpretation 1 to be unlikely. A distinct change in the seafloor morphology of Seismogenic Slip from Deformation Front If the Yue et al. (2014) contours are not a pre- can be traced from the transition in deformation Structure cise indicator of where slip extended to the trench, front structure across the accretionary prism then which structural segment is the most likely (Fig. 12). Northeast of where nearly all sediment Based on seafloor geodetic evidence for slip culprit? Maksymowicz et al. (2017) differenced is being accreted, the seafloor is deeper and more extending to the trench in the 2012 Tohoku earth- swath bathymetry obtained before and after the irregular than to the south, where sediment is being quake (Fujiwara et al., 2011) and on seismic images of earthquake, and concluded that the accretionary subducted. We interpret the persistence of the mor- the deformation front before and after the earthquake prism along seismic line CP06 experienced ~5 m of phologic transition deep into the accretionary prism (Kodaira et al., 2012), which show a low-angle thrust uplift during or soon after the 2010 earthquake that to indicate that the current along-strike change in fault extending into the trench in the post-earthquake extended to within 6 km of the trench. The results the structure of the deformation front is not an image that was not present prior to the earthquake, reported by Tréhu et al. (2019), who observed few ephemeral characteristic of the margin. other investigators have interpreted similar defor- aftershocks and scant evidence for slow earth- Along the landward edge of the middle prism at mation front and trench structure to be a proxy for quakes or seafloor fluid flow anomalies on a the southern and northern boundaries of the study future tsunamigenic potential (e.g., Bécel et al., 2017). small-aperture ocean bottom seismometer network area (Figs. 13, 14), we observe “flower” structures, We now discuss our results in this context. deployed on the outermost prism from May 2012 which suggest strike-slip motion in a transpressive Figure 3 shows the 10 and 15 m slip contours to March 2013, are consistent with this observation. stress field that is consistent with the oblique plate from Yue et al. (2014), who suggested that slip Slow earthquakes and other indicators of fluid flow motion vector in this region. An apparent pull-apart locally extended to the trench along this segment of would be expected in response to at least 15 m of basin has formed in response to an extensional the Maule rupture. The Yue et al. (2014) slip contours slip farther down dip if the outermost prism were offset in this shear zone. straddle the transition from near-total accretion of characterized by velocity-strengthening behavior The basement rocks east of the shear zone trench sediment to significant sediment subduction during the earthquake. are characterized by an uplifted dome where nor- with apparent deformation of the trench sediments We conclude that the most likely scenario is mal-faulted basement is exposed at the seafloor. approaching the deformation front. We consider that slip extended to the trench along the segment This dome is approximately coincident with the patch of greatest slip and is flanked to the north and south by basins filled by tectonically undeformed sediments (although their stratigraphy has been Figure 17. Schematic cross-sec- disrupted by canyon incision and slumping of the tions of the deformation front canyon walls). Based on an apparent decrease in (left) based on seismic lines CP19 (top) and CP06 (bottom) and a hy- the amount of deformation in the slope basin sed- pothetical cross-section along the iments from west to east across the shear zone, outer prism parallel to the defor- we infer that the shear zone represents the bound- mation front (right) showing how the imaged geometry implies a ary between the active accretionary complex and stepdown in the megathrust near the continental framework, which is thought to be the trench. composed of older accretionary complex overlain by a slope basin in this region. This interpretation

where the frontal thrust soles into a shallow detach- suggested that the subducting Mocha fracture zone during the Maule earthquake may have controlled ment and nearly all of the trench sediment is (Fig. S5) may be responsible for deformation of the both up-dip and along-strike slip behavior. We have subducted. The seismic reflection character of this trench sediments near 38°S; however, no oceanic also noted that this contrast appears to be a long- segment of the accretionary prism, which shows no fracture zone is present in the Nazca plate near 34°S. term feature of the margin that has persisted over resolvable deformation within the trench except for A similar (albeit less abrupt) change in defor- many earthquake cycles. Long-term persistence within the shallow wedge deposited near the inter- mation front structure from partial subduction to of the observed shift from sediment subduction to section with the Maule Canyon, indicates a very near-total accretion occurs along the Cascadia sediment accretion should lead to an increasing weak plate boundary thrust that does not trans- subduction zone (MacKay, 1995). Han et al. (2017) offset in the deformation front over time, which mit stress from the plate boundary to the lower documented a change in the velocity structure is not observed. plate. Contrasting seafloor topography between the of trench sediments along strike that is roughly Two possible mechanisms that can result in a segment of the margin characterized by sediment correlated with this change in structure, with high- small offset in the deformation front even though accretion and that characterized by subduction er-velocity (and presumably stronger) sediments there is a sharp transition from accretion to sub- extends across the outer and middle accretionary associated with sediment accretion offshore Wash- duction of trench sediment are: (1) cyclical behavior prism, indicating that these bathymetric features ington (USA). Based on observations of strong of the deformation front, as elegantly proposed are relatively long-lived features of the margin. sediments near the base of the 4-km-thick incom- and modeled by Gutscher et al. (1998) to explain Undeformed trench sediment and sediment ing sediment pile offshore Sumatra (e.g., Gulick et a similar along-strike contrast in deformation subduction are observed along much of the mar- al., 2011), where the 2004 earthquake generated a front behavior in the Aleutians; or (2) healing of gin to the south in the region that slipped during devastating tsunami, Han et al. (2017) suggested an indentation in the deformation front caused by the great A.D. 1960 earthquake south of our study that rupture may be more likely to propagate to subduction of a topographic high, as proposed by area (Geersen et al., 2013; Olsen et al., 2017). How- the deformation front in northern Cascadia, where Tréhu et al. (2012) to explain an anomalous deep ever, thrust faults extending through the trench all sediment is being accreted. The relationship basin on the Cascadia margin that is seaward of a sediments from the seafloor to basement, simi- between sediment subduction or accretion and tsu- subducted seamount inferred from topographic, lar to those on line CP19, are observed locally on namigenic potential proposed by Han et al. (2017) seismic, and potential-field data. Although the seismic lines that cross the deformation front at is the opposite of what we are proposing here. This cyclical model may locally be consistent with the 37.75°S (line SPOC-43, Geersen et al., 2013) and at may be due to different sediment thickness or com- observations on seismic line CP06 (i.e., apparent 38.0°S (Olsen et al. 2017), adjacent to the Arauco position, different underlying plate age (and thus underplating of the upper 30%–35% of incoming Peninsula where the northern boundary of slip in temperature), different sediment consolidation state, sediment beneath the outer prism, near-constant the 1960 earthquake and the southern boundary of or the ambiguity of relating structure to the up-dip thickness of the sediment subducted to greater slip in 2010 overlap. Like the accreting segment in extent of slip in large earthquakes where the details depth, and the paleo–frontal thrust), it fails to our study area, the segment of the margin between of the slip distribution are not known. Identifying explain why the structural change at the deforma- 37.75°S and 38°S is also characterized by seafloor the geologic signature of plate coupling that can be tion front is coincident with a change in margin topography indicative of fold development and used to anticipate likely slip behavior in future plate morphology that persists across the margin. We outward building of the prism and a submarine boundary earthquakes in subduction zones is chal- now examine the possibility that this margin has canyon that is cut off before it reaches the trench lenging, even in recently active and well-monitored been affected by subduction of a region of anom- (Fig. S5 [footnote 1]). These segments of the margin regions (e.g., von Huene et al., 2019), and additional alously thick crust and rough topography. are correlated with high uplift rates onshore, which high-resolution observations of geologic structure Numerous studies elsewhere have concluded has been interpreted to be a geological proxy for in the source regions of large, well-documented that subducting plate topography interacting with upper plate deformation and long-term creep on subduction zone earthquakes is needed. the upper plate has a profound effect on forearc the plate boundary, which accommodates some of structure and seismicity (e.g., Cloos, 1992; Domin- the convergence (Saillard et al., 2017). We conclude guez et al., 1998; Kodaira et al., 2000; Bilek et al., that underthrusting beneath a shallow, lithologi- Reconciling the Long-Term Contrast in 2003; Bangs et al., 2006; Wang and Bilek, 2011; Tréhu cally controlled décollement with little deformation Margin Structure with the Contrast in et al., 2012; Morgan and Bangs, 2017). As mentioned of trench sediments may be characteristic of slip Deformation Front Behavior earlier, Maksymowicz et al. (2015) inferred the pres- to the trench in large subduction earthquakes in ence of a subducted seamount that underlies the south-central Chile (and by inference, in similar We have documented that an abrupt shift from uplifted dome imaged in the MCS data based on the subduction zones characterized by a moderate sediment accretion to sediment subduction at the gravity gradient (Fig. S4A [footnote 1]). To explain thickness of trench turbidites). Olsen et al. (2017) deformation front at the northern boundary of slip the absence of a subducted seamount wake, they

speculated that it had been sheared off of the subducting plate and obducted to the upper plate, forming an asperity that may have been responsible for the patch of greatest slip in the 2010 earthquake. We propose that the newly identified shear zone and pull-apart basin immediately seaward of the uplifted dome may have contributed to obscuring the seamount wake. Because no significant velocity anomaly is observed in the P-wave tomographic model of Hicks et al. (2014) beneath the offshore continental margin in this region, we conclude that the velocity of the subducted seamount is similar to that of the lower crust of the upper plate. Onshore, Hicks et al. (2014) imaged high-velocity structures in the lower crust beneath the Chilean Coastal Range near 34°S and 36°S. The presence of these velocity anomalies has been correlated with slip segmentation in the Maule earthquakes and interpreted to be due to ultramafic intrusions related to Triassic extension and mantle upwelling (Bishop, 2018). To explain the margin-wide morpho- logical and structural observations reported here, we suggest that a possible alternative origin for the anomaly beneath the coast at 34°S is subduction of a region of anomalously rough, shallow, and thick oceanic crust that has been entirely subducted. Figure 18A shows the locations of these gravity and seismic velocity anomalies along with the locations of the major shallow structural boundaries identified in this study, seismicity before and after the 2010 Maule earthquake, and the region of crit- ical prism topography determined by Cubas et al. (2013), which corresponds well with the transition in deformation front and accretionary prism structure. Not surprisingly, the region that ruptured in 2010 was relatively quiet prior to the earthquake, and many aftershocks were recorded in this region afterwards. As mentioned in the introduction, the outer prism was seismically quiet both before and Figure 18. (A) Epicenters of earthquakes offshore south-central Chile with magnitude >3.5 from the U.S. Geological Survey Compre- after the earthquake. Although outer-rise aftershock hensive Earthquake Catalog (https://earthquake.usgs.gov/earthquakes/search) for 1 January 2000 to 26 February 2010 (before the activity extends along the entire study region, we Maule earthquake of 27 February 2010), and aftershocks in the same magnitude range from 28 February 2010 to 27 February 2011. note that it is centered on the segment where we The 10 and 15 m slip contours from the models shown in Figure 3 (white, red, and blue lines) and several structures interpreted from new data or in other recent studies are overlain on both maps for reference. (B) Schematic illustration of a hypothesized scenario document sediment subduction, uplift of trench that may explain the observations summarized in A. The ages on the panels indicate minimum ages based on the plate motion sediments along the western edge of the trench, vector and could be older if the structures responsible for the anomalies are currently detached from the subducting plate. Arrow and deflection of the axial channel to the east, sug- shows the current plate motion direction. BS—Bucalemu seamount. (C) Schematic cross-section of the postulated structure of the seaward edge of the continental framework. See “present” panel of B for location. Green line is the seafloor; yellow shading is gesting that loading of the subducting plate near slope sediment; brown is upper plate crust; and violet is lower plate crust. (D) Legend for symbols used in A and B. The backstop the trench by the Maule earthquake is greatest here. is defined by Contreras-Reyes et al. (2017) as the contract between the accretionary prism and the continental framework rock.

Just north of the uplifted dome is a swath of to equilibrium through sediment accretion and forearc and the relationship between the crustal the outer and middle prism that is nearly devoid internal prism deformation. We propose that the structure and along-strike and up-dip slip propa- of aftershock activity. The lack of activity in this segment of margin characterized by accretion of gation in plate boundary earthquakes. region was confirmed by the ChilePEPPER OBS nearly all trench sediment and heterogeneous mor- The seismic data demonstrate that the ratio of study (Fig. 18A). This is also where the seaward edge phology of the adjacent accretionary prism reflects sediment accretion to subduction at the deforma- of the continental framework swings sharply west healing of this embayment and restoration of the tion front changes abruptly south of 34.43°S from when approximated by the 1000 m isobath (Fig. 1). approximately linear structure of the Chile trench, subduction of most of the incoming sediment to We propose that the basement in this embayment which has persisted since the Cretaceous. Later accretion of nearly all the trench sediment on a may be continental framework crust that is anoma- subduction of a smaller seamount has resulted in seaward-verging frontal thrust fault to the north. lously thin and fractured because of prior subduction the localized dome. We propose that along-strike Within the segment dominated by sediment sub- of an anomalously thick region of oceanic crust. It transport of the outer and middle accretionary prism duction, which is located up dip from the patch of has been suggested that the fractured upper plate relative to the backstop along a dominantly strike- highest slip, 30%–35% of the subducted sediment left in the wake of subduction of high relief would slip structure has obscured the expected subducted appears to presently be underplated beneath the result in heterogenous stresses on the plate bound- seamount wake. Similar processes may be occur- outermost prism and 60%–65% is subducted to ary, promoting the accommodation of plate motion ring along other accreting segments of the trench, greater depth. Only sediment in a local, shallow through upper plate deformation and creep and such as that near 38°S offshore from the Arauco trench wedge formed near the confluence of the small- to moderate-size earthquakes along the plate Peninsula, where the process of margin disruption Maule Canyon and the trench is frontally accreted. boundary (Wang and Bilek, 2011), consistent with the due to subducted topography may be at an earlier The décollement at depth corresponds to the same observations of pre- and post-earthquake seismicity stage of evolution, with subduction of the Mocha bright, multicycle package of reflections observed and with the lower degree of interseismic locking fracture zone continuing at present. In contrast, we in the trench along the entire imaged segment reported for this segment by Métois et al. (2012). postulate that in our study region, the main topo- south of 34.43°S. The absence of resolvable faulting Such a zone of partial locking adjacent to a strongly graphic “disrupter” has been entirely subducted. in the trench sediments seaward of the deforma- locked patch on the plate boundary can act as a Maksymowicz et al. (2015, p. 276) concluded that: tion front (with the exception of faulting within barrier to rupture propagation (Perfettini et al., 2010). “Details of the relationship between local geologic sediments composing the shallow trench wedge) These observations suggest the plausible, albeit structures in the forearc, inter-seismic coupling, indicates that the plate boundary is weak and does speculative, scenario shown in Figure 18B. Ages slip during plate boundary earthquakes, and after- not transfer stress to the underlying sediments. are estimated based on the plate convergence shock activity, however, remain ambiguous, and North of 34.43°S, a frontal thrust is observed rate assuming that the structures depicted are still a process-based understanding will require that that extends nearly to the top of the subducting oce- attached to the subducting plate and therefore rep- uncertainties in all of these characteristics of a sub- anic crust, and the outer prism is formed of folded resent minimum ages. In this model, a region of duction margin be resolved more accurately.” We and faulted trench sediment. The transition in the rough, anomalously thick oceanic crust that is now hope that the speculations advanced here, which are deformation front structure is not correlated with manifested as the Pichilemu seismic velocity anom- suggested by the analysis and interpretation of new any detectable changes in the thickness or seismic aly (Hicks et al., 2014) was present seaward of the high-resolution seismic reflection data, represent a reflection character of the trench fill. It is, however, subduction zone by at least 2 Ma. Subduction of this step forward toward a process-based understand- coincident with a distinct change in the morphology structure resulted in uplift followed by subsidence ing by presenting a model for the evolution of this of the outer and middle accretionary prism, indicat- and formation of a large embayment in the outer apparent subduction zone segment boundary that ing that the along-strike variation in the mechanical and middle prism at ca. 1 Ma. Continued subduction can be tested through higher-resolution imaging response of the outer prism has been a persistent would have resulted in fracturing and erosion of the of the deep structure of the margin and three-di- feature of the margin. leading edge of the continental basement, resulting mensional modeling of subduction zone dynamics. The abrupt nature of the transition from sedi- in the highly variable basement and basin structure ment subduction to accretion along strike implies observed landward of the active accretionary prism. a downward stepover in the plate boundary fault The cross-section in Figure 18B shows the present ■■ CONCLUSIONS near the deformation front that may have acted as structure along the leading edge of the continental a barrier to northward propagation of slip in the framework implied by this scenario. High-resolution seismic reflection images of the Maule earthquake. We conclude that the appar- The large embayment left in the wake of sub- continental margin near the northern boundary of ently weak décollement overlying thick subducted

duction of a small oceanic plateau would have a slip in the 2010 Mw 8.8 Maule earthquake highlight sediment may have allowed slip to extend to subcritical topographic profile and would return the strongly three-dimensional structure of the the trench along this segment during the Maule

earthquake. This conclusion is consistent with the We speculate that earlier passage of a larger .edu/sdc/). Other underway geophysical data are archived at the Rolling Deck to Repository data facility (https://www.rvdata.us pattern of outer-rise aftershock activity (Sladen and welt of anomalously rough, thick, and shallow oce- /catalog/MV1206). Generic Mapping Tools (Wessel and Smith, Trevisan, 2018) and with a change in elevation of anic crust created a large embayment in the margin 1998) and GeoMapApp (http://www.geomapapp.org) were used up to 5 m in this region that occurred between north of the uplifted basement dome and that the to compile the maps. 2008 and 2011–2012 (Maksymowicz et al., 2015). heterogeneous forearc morphology and trunca- It implies that the signature of slip to the trench tion of the Huenchullami and Mataquito channels REFERENCES CITED in Chile may be quite different from the signature before they reach the trench that characterizes this observed in Tohoku, where deformation within the segment of the margin are relicts of this history. In Angermann, D., Klotz, J., and Reigber, C., 1999, Space-geodetic estimation of the Nazca–South American Euler vector: Earth trench sediments appears to have resulted from this model, a pronounced embayment in the edge and Planetary Science Letters, v. 171, p. 329–334, https://doi the 2011 earthquake, or off Sumatra, where seis- of the continental shelf reflects an along-strike .org/10.1016/S0012-821X(99)00173-9. mogenic slip on a deeply buried décollement is depression of the continental basement rather than Bangs, N.L., and Cande, S.C., 1997, Episodic development of a convergent margin inferred from structures and processes thought to have been enabled by the presence of a landward deflection of the boundary between along the southern Chile margin: Tectonics, v. 16, p. 489–503, unusually thick and stiff trench sediment. A better active accretionary and continental basement rocks. https://doi.org/10.1029/97TC00494. understanding of the impact of trench sediment In summary, our detailed analysis of the forearc Bangs, N.L.B., Gulick, S.P.S., and Shipley, T.H., 2006, Seamount thickness, composition, and temperature is needed subduction erosion in the Nankai Trough and its potential structure at the northern limit of rupture during impact on the seismogenic zone: Geology, v. 34, p. 701–704, before the structure of the deformation front can the 2010 Maule earthquake reveals a strong rela- https://doi.org/10.1130/G22451.1. be used as a reliable proxy for slip to the trench tionship between rupture propagation during the Bangs, N.L., Arnulf, A.F., Tréhu, A.M., and Contreras-Reyes, E., and tsunami potential in the absence of a recent main shock, aftershock seismicity, and long-term 2017, Extensive sediment subduction along the south-central Chile margin and its role in the Maule and Valdivia large earthquake. structural evolution of the forearc. Defining the megathrust earthquakes: Abstract S51G-02 presented at Seismic data within the middle prism indicate structures at subduction zone segment boundaries the American Geophysical Union 2017 Fall Meeting, New a complex pattern of local uplift and subsidence, in three dimensions and reconstructing their his- Orleans, Louisiana, 11–15 December. Bassett, D., and Watts, A.B., 2015a, Gravity anomalies, crustal with several features interpreted to be “flower” tory requires dense grids of seismic reflection data structure, and seismicity at subduction zones: 1. Seafloor structures indicative of a component of strike-slip and integration of these data with a wide variety roughness and subducting relief: Geochemistry Geophys- motion along the eastern edge of this structural of complementary data sets. While it is prema- ics Geosystems, v. 16, p. 1508–1540, https://doi .org /10 .1002 domain, consistent with the oblique plate conver- /2014GC005684. ture to interpret structural patterns as proxies for Bassett, D., and Watts, A.B., 2015b, Gravity anomalies, crustal gence vector. A mid-slope basin in this segment slip in future large subduction zone earthquakes, structure, and seismicity at subduction zones: 2. Interre- may represent a pull-apart basin formed because focused studies of three-dimensional structure lationships between fore-arc structure and seismogenic behavior: Geochemistry Geophysics Geosystems, v. 16, of an extensional stepover in a transpressional tied to well-documented slip patterns in large p. 1541–1576, https://doi.org/10.1002/2014GC005685. fault system. In contrast to the margin to the north earthquakes has promise for anticipating future Bécel, A., Shillington, D.J., Delescluse, M., Nedimović, M.R., and south, the eastern boundary of this marginal earthquake behavior. Abers, G.A., Saffer, D.M., Webb, S.C., Keranen, K.M., Roche, domain is not clearly defined by the bathyme- P.-H., Li, J., and Kuehn, H., 2017, Tsunamigenic structures in a creeping section of the Alaska subduction zone: Nature try, indicating a structurally complex boundary Geoscience, v. 10, p. 609–613, https://doi.org/10.1038 between the accretionary prism and the continen- ACKNOWLEDGMENTS /ngeo2990. tal framework rock, interpreted to be pre-Cenozoic, We thank the captain and crew of the R/V Melville for excellent Bilek, S.L., Schwartz, S.Y., and DeShon, H.R., 2003, Control of metamorphosed accretionary prism. A dome-like ship handling and Lee Ellett and Jay Turnbull of the SIO Ship- seafloor roughness on earthquake rupture behavior: Geol- board Geophysical Group for operating the seismic reflection ogy, v. 31, p. 455–458, https://doi.org/10.1130/0091-7613 structure of uplifted basement rocks is exposed and other geophysical acquisition instrumentation. Vern Kulm (2003)031<0455:COSROE>2.0.CO;2. on the seafloor locally, and the basement surface very kindly spent a morning bringing us up to speed on ideas Bishop, B.T., 2018, Investigation of the effects of Nazca–South drops abruptly to the north and south of the dome about sediment processes in the Chile trench. The paper bene- America plate collision along the Peruvian-Chilean active fitted greatly from thoughtful and probing reviews from guest continental margin through teleseismic receiver function and is buried beneath slightly folded forearc basin editor Laura Wallace and two anonymous reviewers. Funding for analysis [Ph.D. thesis]: Tucson, University of Arizona, 292 p. sediments. The uplifted basement dome is coinci- this work was provided by grants OCE1130013 and OCE1129574 Campos, J., Hatzfeld, D., Madariaga, R., Lopez, G., Kausel, E., dent with an anomaly in the vertical derivative of from the U.S. National Science Foundation (NSF) to Oregon Zollo, A., Iannacone, R., Fromm, R., Barrientos, S., and Lyon- Caen, H., 2002, A seismological study of the 1835 seismic the gravity field that has been interpreted to be State University and the Scripps Institution of Oceanogra- phy, respectively, and by CONICYT/FONDECYT grant number gap in south-central Chile: Physics of the Earth and Planetary due to a subducted seamount (Maksymowicz et 11170047 and CONICYT PIA/Anillo ACT172002 to the Universidad Interiors, v. 132, p. 177–195, https://doi.org/10.1016/S0031 al., 2015). The absence of a “seamount wake” is de Chile. Geophysical data were acquired using the NSF-sup- -9201(02)00051-1. attributed to disruption by the transpressional and ported Shipboard Geophysical Group at the Scripps Institute of Cloos, M., 1992, Thrust-type subduction-zone earthquakes and Oceanography. All seismic data are archived at and are freely seamount asperities: A physical mode for seismic rupture: transtensional deformation affecting the middle available from the U.S. Academic Seismic Portal maintained Geology, v. 20, p. 601–604, https://doi .org /10 .1130 /0091 -7613 prism region. at the University of Texas at Austin (http://www-udc.ig.utexas (1992)020<0601:TTSZEA>2.3.CO;2.

Contardo, X., Cembrano, J., Jensen, A., and Díaz-Naveas, J., Geersen, J., Behrmann, J.H., Völker, D., Krastel, S., Ranero, C.R., Lange, D., Tilmann, F., Barrientos, S.E., Contreras- Reyes, E., 2008, Tectono-sedimentary evolution of marine slope basins Diaz-Naveas, J., and Weinrebe, W., 2011, Active tectonics Methe, P., Moreno, M., Heit, B., Bernard, P., Vilotte, J.-P., and in the Chilean forearc: (33°30′–36°50′S): Insights into their of the south Chilean marine forearc (35°S–40°S): Tectonics, Beck, S.L., 2012, Aftershock seismicity of the 27 February link with the subduction process: Tectonophysics, v. 459, v. 30, TC3006, https://doi.org/10.1029/2010TC002777. 2010 Mw 8.8 Maule earthquake rupture zone: Earth and p. 206–218, https://doi.org/10.1016/j.tecto.2007.12.014. Geersen, J., Völker, D., Behrmann, J.H., Kläschen, D., Weinrebe, Planetary Science Letters, v. 317–318, p. 413–425, https:// Contreras-Reyes, E., and Carrizo, D., 2011, Control of high W., Krastel, S., and Reichert, C., 2013, Seismic rupture during doi.org/10.1016/j.epsl.2011.11.034. oceanic features and subduction channel on earthquake the 1960 Great Chile and the 2010 Maule earthquake limited Lay, T., Ammon, C.J., Kanamori, H., Koper, K.D., Sufri, O., and ruptures along the Chile–Peru subduction zone: Physics of by a giant Pleistocene submarine slope failure: Terra Nova, Hutko, A.R., 2010, Teleseismic inversion for rupture process the Earth and Planetary Interiors, v. 186, p. 49–58, https:// v. 25, p. 472–477, https://doi.org/10.1111/ter.12060. of the 27 February 2010 Chile (M‐w 8.8) earthquake: Geo- doi.org/10.1016/j.pepi.2011.03.002. Gonzalez, E., 1990, Hydrocarbon resources in the coastal zone physical Research Letters, v. 37, L13301, https://doi .org/10 Contreras-Reyes, E., and Osses, A., 2010, Lithospheric flexure of Chile, in Ericksen, G.E., Canas Pinochet, M.T., and Rein- .1029/2010GL043379. modelling seaward of the Chile trench: Implications for emund, J.A., eds., Geology of the Andes and Its Relation to Lieser, K., Grevemeyer, I., Lange, D., Flueh, E., Tilmann, F., and oceanic plate weakening in the trench outer rise region: Hydrocarbon and Mineral Resources: Circum-Pacific Coun- Contreras-Reyes, E., 2014, Splay fault activity revealed by after-

Geophysical Journal International, v. 182, p. 97–112, https:// cil for Energy and Mineral Resources Earth Science Series, shocks of the 2010 Mw 8.8 Maule earthquake, central Chile: doi.org/10.1111/j.1365-246X.2010.04629.x. Volume 11: Houston, Texas, p. 383–404. Geology, v. 42, p. 823–826, https://doi.org/10.1130/G35848.1. Contreras-Reyes, E., Flueh, E.R., and Grevemeyer, I., 2010, Tec- Gulick, S.P.S., Austin, J.A., McNeill, L.C., Bangs, N.L.B., Martin, Lin, Y.N., Sladen, A., Ortega-Culaciati, F., Simons, M., Avouac, tonic control on sediment accretion and subduction off south K.M., Henstock, T.J., Bull, J.M., Dean, S., Djajadijardja, Y.S., J.-P., Fielding, E.J., Brooks, B.A., Bevis, M., Genrich, J., central Chile: Implications for coseismic rupture processes and Permana, H., 2011, Updip rupture of the 2004 Sumatra Rietbrock, A., Vigny, C., Small, R., and Socquet, A., 2013, of the 1960 and 2010 megathrust earthquakes: Tectonics, earthquake extended by thick, indurated sediments: Nature Coseismic and postseismic slip associated with the 2010 v. 29, TC6018, https://doi.org/10.1029/2010TC002734. Geoscience, v. 4, p. 453–456, https://doi .org /10 .1038 /ngeo1176. Maule earthquake, Chile: Characterizing the Arauco Penin- Contreras-Reyes, E., Maksymowicz, A., Lange, D., Grevemeyer, Gutscher, M.-A., Kukowski, N., Malavieille, J., and Lallemand, sula barrier effect: Journal of Geophysical Research. Solid I., Muñoz-Linford, P., and Moscoso, E., 2017, On the relation- S., 1998, Episodic imbricate thrusting and underthrusting: Earth, v. 118, https://doi.org/10.1002/jgrb.50207. ship between structure, morphology and large coseismic Analog experiments and mechanical analysis applied to Lomnitz, C., 2004, Major earthquakes of Chile: A historical survey,

slip: A case study of the Mw 8.8 Maule, Chile 2010 earth- the Alaskan, Accretionary Wedge: Journal of Geophysical 1535–1960: Seismological Research Letters, v. 75, p. 368–378, quake: Earth and Planetary Science Letters, v. 478, p. 27–39, Research, v. 103, p. 10,161–10, 176, https://doi .org/10.1029 https://doi.org/10.1785/gssrl.75.3.368. https://doi.org/10.1016/j.epsl.2017.08.028. /97JB03541. Lorito, S., Romano, F., Atzori, S., Tong, X., Avallone, A., McClos- Cubas, N., Avouac, J.-P., Souloumiac, P., and Leroy, Y.M., 2013, Han, S., Bangs, N.L., Carbotte, S.M., Saffer, D.M., and Gib- key, J., Cocco, M., Boschi, E., and Piatenesi, A., 2011, Limited Megathrust friction determined from mechanical analysis son, J.C., 2017, Links between sediment consolidation and overlap between the seismic gap and coseismic slip of the of the forearc in the Maule earthquake area: Earth and Plan- Cascadia megathrust behavior: Nature Geoscience, v. 10, great 2010 Chile earthquake: Nature Geoscience, v. 4, p. 173– etary Science Letters, v. 381, p. 92–103, https://doi.org/10 p. 954–959, https://doi.org/10.1038/s41561-017-0007-2. 177, https://doi.org/10.1038/ngeo1073. .1016/j.epsl.2013.07.037. Hayes, G.P., Bergman, E., Johnson, K.L., Benz, H.M., Brown, L., MacKay, M.E., 1995, Structural variation and landward vergence Cubas, N., Souloumiac, R., and Singh, S., 2016, Relationship and Meltzer, A.S., 2013, Seismotectonic framework of the at the toe of the Oregon accretionary prism: Tectonics, v. 14,

link between landward vergence in accretionary prisms and February 27, 2010 Mw 8.8 Maule, Chile earthquake sequence: p. 1309–1320, https://doi.org/10.1029/95TC02320. tsunami generation: Geology, v. 44, p. 787–790, https://doi Geophysical Journal International, v. 195, p. 1034–1051, Maksymowicz, A., Tréhu, A.M., Contreras-Reyes, E., and Ruiz, .org/10.1130/G38019.1. https://doi.org/10.1093/gji/ggt238. S., 2015, Density-depth model of the continental wedge at Dean, S.M., McNeill, L.C., Henstock, T.J., Bull, J.M., Gulick, Hicks, S.P., Rietbrock, A., Ryder, I.M.A., Lee, C.-S., and Miller, M., the maximum slip segment of the Maule Mw8.8 megath- S.P.S., Austin, J.A., Jr., Bangs, N.L.B., Djajadijardja, Y.S., 2014, Anatomy of a megathrust: The 2010 M8.8 Maule, Chile rust earthquake: Earth and Planetary Science Letters, v. 409, and Permana, H., 2010, Contrasting décollement and prism earthquake rupture zone imaged using seismic tomogra- p. 265–277, https://doi.org/10.1016/j.epsl.2014.11.005. properties over the Sumatra 2004–2005 earthquake rupture phy: Earth and Planetary Science Letters, v. 405, p. 142–155, Maksymowicz, A., Chadwell, C.D., Ruiz, J., Tréhu, A.M., Con- boundary: Science, v. 329, p. 207–210, https://doi .org /10 .1126 https://doi.org/10.1016/j.epsl.2014.08.028. treras-Reyes, E., Weinrebe, W., Díaz-Naveas, J., Gibson, /science.1189373. King, G.C.P., and Nabelek, J.L., 1985, Role of fault bends in the ini- J.C., Lonsdale, P., and Tryon, M.D., 2017, Coseismic sea- Delouis, B., Nocquet, J.-M., and Vallée, M., 2010, Slip distribution tiation and termination of earthquake rupture: Science, v. 228, floor deformation in the trench region during the Mw8.8 of the February 27, 2010 Mw = 8.8 Maule Earthquake, central p. 984–987, https://doi .org /10 .1126 /science .228 .4702.984. Maule megathrust earthquake: Scientific Reports, v. 7, 45918, Chile, from static and high-rate GPS, InSAR, and broadband Kiser, E., and Ishii, M., 2011, The 2010 Mw 8.8 Chile earthquake: https://doi.org/10.1038/srep45918. seismic data: Geophysical Research Letters, v. 37, L17305, Triggering on multiple segments and frequency-depen- Métois, M., Socquet, A., and Vigny, C., 2012, Interseismic cou- https://doi.org/10.1029/2010GL043899. dent rupture behavior: Geophysical Research Letters, v. 38, pling, segmentation and mechanical behavior of the central DeMets, C., Gordon, R.G., and Argus, D.F., 2010, Geologically cur- L07301, https://doi.org/10.1029/2011GL047140. Chile subduction zone: Journal of Geophysical Research, rent plate motions: Geophysical Journal International, v. 181, Klaucke, I., Weinrebe, W., Linke, P., Kläschen, D., and Bialas, v. 117, B03406, https://doi.org/10.1029/2011JB008736. p. 1–80, https://doi.org/10.1111/j.1365-246X.2009.04491.x. J., 2012, Sidescan sonar imagery of widespread fossil and Melnick, D., and Echtler, H.P., 2006, Inversion of forearc basins de Moor, A., 2015, Local seismicity recorded by ChilePEPPER: active cold seeps along the central Chilean continental mar- in south-central Chile caused by rapid glacial age trench fill: Implications for dynamic accretionary wedge response and gin: Geo-Marine Letters, v. 32, p. 489–499, https://doi .org /10 Geology, v. 34, p. 709–712, https://doi .org /10 .1130 /G22440 .1. long-term prism evolution [M.S. thesis]: Corvallis, Oregon .1007/s00367-012-0283-1. Mix, A.C., Tiedemann, R., Blum, P., et al., 2003, Proceedings of State University, 91 p. Kodaira, S., Takahashi, N., Nakanishi, A., Miura, S., and Kaneda, the Ocean Drilling Program, Initial Reports, Volume 202: Dominguez, S., Lallemand, S.E., Malavieille, J., and von Huene, Y., 2000, Subducted seamount imaged in the rupture zone of College Station, Texas, Ocean Drilling Program, https://doi R., 1998, Upper plate deformation associated with seamount the 1946 Nankaido earthquake: Science, v. 289, p. 104–106, .org/10.2973/odp.proc.ir.202.2003. subduction: Tectonophysics, v. 293, p. 207–224, https://doi https://doi.org/10.1126/science.289.5476.104. Moreno, M., Rosenau, M., and Oncken, O., 2010, 2010 Maule .org/10.1016/S0040-1951(98)00086-9. Kodaira, S., No, T., Nakamura, Y., Fujiwara, T., Kaiho, Y., Miura, earthquake slip correlates with pre-seismic locking of Andean Fujiwara, T., Kodaira, S., No, T., Kaiho, Y., Takahashi, N., and S., Takahashi, N., Kaneda, Y., and Taira, A., 2012, Coseismic subduction zone: Nature, v. 467, p. 198–202, https://doi.org Kaneda, Y., 2011, The 2011 Tohoku-Oki earthquake: Dis- fault rupture at the trench axis during the 2011 Tohoku-oki /10.1038 /nature09349. placement reaching the trench axis: Science, v. 334, p. 1240, earthquake: Nature Geoscience, v. 5, p. 646–650, https://doi Moreno, M., Melnick, D., Rosenau, M., Baez, J., Klotz, J., Oncken, https://doi.org/10.1126/science.1211554. .org/10.1038/ngeo1547. O., Tassara, A., Chen, J., Bataille, K., Bevis, M., Socquet, A.,

Bolte, J., Vigny, C., Brooks, B., Ryder, I., Grund, V., Smalley, Ruiz, J.A., and Contreras-Reyes, E., 2015, Outer rise seismicity R., Aranda, C., Valderas-Bermejo, M.-C., Ortega, I., Bondoux,

B., Carrizo, D., Bartsch, M., and Hase, H., 2012, Toward under- boosted by the Maule 2010 Mw 8.8 megathrust earthquake: F., Baize, S., Lyon-Caen, H., Pavez, A., Vilotte, J.P., Bevis,

standing tectonic control on the Mw 8.8 2010 Maule Chile Tectonophysics, v. 653, p. 127–139, https://doi.org/10.1016 M., Brooks, B., Smalley, R., Parra, H., Baez, J.-C., Blanco,

earthquake: Earth and Planetary Science Letters, v. 321–322, /j.tecto.2015.04.007. M., Cimbaro, S., and Kendrick, E., 2011, The 2010 Mw 8.8 p. 152–165, https://doi .org /10 .1016 /j .epsl .2012 .01 .006. Ruiz, S., and Madariaga, R., 2018, Historical and recent large Maule megathrust earthquake of central Chile, monitored Morgan, J.K., and Bangs, N.L., 2017, Recognizing seamount- megathrust earthquakes in Chile: Tectonophysics, v. 733, by GPS: Science, v. 332, p. 1417–1421, https://doi.org/10 forearc collisions at accretionary margins: Insights from p. 37–56, https://doi.org/10.1016/j.tecto.2018.01.015. .1126/science.1204132. discrete numerical simulations: Geology, v. 45, p. 635–638, Ryan, W.B.F., Carbotte, S.M., Coplan, J.O., O’Hara, S., Melko- Völker, D., Wiedicke, M., Ladage, S., Gaedicke, C., Reichert, C., https://doi .org /10 .1130 /G38923 .1. nian, A., Arko, R., Weissel, R.A., Ferrini, V., Goodwillie, A., Rauch, K., Kramer, W., and Heubeck, C., 2006, Latitudinal Moscoso, E., Grevemeyer, I., Contreras-Reyes, E., Flueh, E.R., Nitsche, F., Bonczkowski, J., and Zemsky, R., 2009, Global variation in sedimentary processes in the Peru-Chile Trench Dzierma, Y., Rabbel, W., and Thorwart, M., 2011, Revealing Multi-Resolution Topography synthesis: Geochemistry Geo- off central Chile, in Oncken, O., Chong, G., Franz, G., Giese, the deep structure and rupture plane of the 2010 Maule, physics Geosystems, v. 10, Q03014, https://doi .org/10.1029 P., Götze, H.-J., Ramos, V.A., Strecker, M.R., and Wigger, Chile earthquake (Mw = 8.8) using wide angle seismic /2008GC002332. P., eds., The Andes: Active Subduction Orogeny: Berlin, data: Earth and Planetary Science Letters, v. 307, p. 147–155, Saillard, M., Audin, L., Rousset, B., Avouac, J.-P., Chlieh, M., Hall, Springer Verlag, p. 193–216, https://doi.org/10.1007/978-3 https://doi.org/10.1016/j.epsl.2011.04.025. S.R., Husson, L., and Farber, D.L., 2017, From the seismic -540-48684-8_9. Obara, K., and Ito, Y., 2005, Very low frequency earthquakes cycle to long-term deformation: Linking seismic coupling Völker, D., Reichel, T., Wiedicke, M., and Heubeck, C., 2008, excited by the 2004 off the Kii peninsula earthquakes: and Quaternary coastal geomorphology along the Andean Turbidites deposited on southern central Chilean sea- A dynamic deformation process in the large accretionary megathrust: Tectonics, v. 36, p. 241–256, https://doi.org/10 mounts: Evidence for energetic turbidity currents: Marine prism: Earth, Planets, and Space, v. 57, p. 321–326, https:// .1002/2016TC004156. Geology, v. 251, p. 15–31, https://doi.org/10.1016/j.margeo doi.org/10.1186/BF03352570. Sladen, A., and Trevisan, J., 2018, Shallow megathrust earth- .2008.01.008. Oglesby, D.D., 2005, The dynamics of strike-slip step-overs with quake ruptures betrayed by their outer-trench aftershocks Völker, D., Geersen, J.M., Contreras-Reyes, E. and Reichert, linking dip-slip faults: Bulletin of the Seismological Soci- signature: Earth and Planetary Science Letters, v. 483, C., 2013, Sedimentary fill of the Chile Trench (32–46°S): ety of America, v. 95, p. 1604–1622, https://doi .org/10.1785 p. 105–113, https://doi.org/10.1016/j.epsl.2017.12.006. Volumetric distribution and causal factors: Journal of the /0120050058. Thornburg, T.M., and Kulm, L.D., 1987, Sedimentation in the Geological Society, v. 170, p. 723–736, https://doi .org/10 Olsen, K., Bangs, N.L., Arnulf, A.F., Tréhu, A.M., and Contre- Chile Trench: Depositional morphologies, lithofacies, and .1144/jgs2012-119. ras-Reyes, E., 2017, Development of a shallow decollement stratigraphy: Geological Society of America Bulletin, v. 98, von Huene, R., Corvalán, J., Flueh, E.R., Hinz, K., Korstgard, J., along the south-central Chile margin from 2D seismic reflec- p. 33–52, https://doi.org/10.1130/0016-7606(1987)98<33: Ranero, C.R., Weinrebe, W., and the CONDOR scientists, tion data: Abstract S53C-2364 presented at the American SITCTD>2.0.CO;2. 1997, Tectonic control of the subducting Juan Fernández Geophysical Union 2017 Fall Meeting, New Orleans, Loui- Thornburg, T.M., Kulm, L.D., and Hussong, D.M., 1990, Sub- Ridge on the Andean margin near Valparaiso, Chile: Tec- siana, 11–15 December. marine-fan development in the southern Chile Trench: tonics, v. 16, p. 474–488, https://doi.org/10.1029/96TC03703. Perfettini, H., Avouac, J.-P., Tavera, H., Kositsky, A., Nocquet, A dynamic interplay of tectonics and sedimentation: Geo- von Huene, R., Miller, J.J., and Krabbenhoeft, A., 2019, The J.-M., Bondoux, F., Chlieh, M., Sladen, A., Audin, L., Farber, logical Society of America Bulletin, v. 102, p. 1658–1680, Shumagin seismic gap structure and associated tsunami D.L., and Soler, P., 2010, Seismic and aseismic slip on the https://doi .org /10 .1130 /0016 -7606 (1990)102 <1658: SFDITS>2 hazards, Alaska convergent margin: Geosphere, v. 15, Central Peru megathrust: Nature, v. 465, p. 78–81, https:// .3.CO;2. p. 324–341, https://doi.org/10.1130/GES01657.1. doi.org/10.1038/nature09062. Tong, X., Sandwell, D., Luttrell, K., Brooks, B., Bevis, M., Shimada, Wang, K., and Bilek, S.L., 2011, Do subducting seamounts gen- Phrampus, B.J., Harris, R.N., and Tréhu, A.M., 2017, Heat flow M., Foster, J., Smalley, R., Jr., Parra, H., Báez Soto, J.C., erate or stop large earthquakes?: Geology, v. 39, p. 819–822, bounds over the Cascadia margin derived from bottom sim- Blanco, M., Kendrick, E., Genrich, J., and Caccamise, D.J., II, https://doi.org/10.1130/G31856.1. ulating reflectors and implications for thermal models of 2010, The 2010 Maule, Chile earthquake: Downdip rupture Wang, L., Hainzl, S., and Mai, P.M., 2017, To which level did the subduction: Geochemistry Geophysics Geosystems, v. 18, limit revealed by space geodesy: Geophysical Research 2010 M8.8 Maule earthquake fill the pre-existing seismic p. 3309–3326, https://doi.org/10.1002/2017GC007077. Letters, v. 37, L24311, https://doi .org/10 .1029/2010GL045805. gap?: Geophysical Journal International, v. 211, p. 498–511, Rauch, K., 2005, Cyclicity of Peru-Chile trench sediments Tréhu, A.M., and Tryon, M., 2012, Cruise report R/V Melville https://doi.org/10.1093/gji/ggx304. between 36°S and 38°S: A footprint of paleoclimatic varia- MV1206—The 2010 Maule, Chile earthquake: Project Eval- Wessel, P., and Smith, W.H.F., 1998, New, improved version of tions?: Geophysical Research Letters, v. 32, L08302, https:// uating Prism Post-Earthquake Response (Chile-PEPPER). the Generic Mapping Tools released: Eos (Transactions, doi.org/10.1029/2004GL022196. http://www.rvdata.us/catalog/MV1206. American Geophysical Union), v. 79, p. 579, https://doi .org Rietbrock, A., Ryder, I., Hayes, G., Haberland, C., Comte, D., Tréhu, A.M., Blakely, R.J., and Williams, M.C., 2012, Subducted /10.1029/98EO00426. Roecker, S., and Lyon-Caen, H., 2012, Aftershock seismicity seamounts and recent earthquakes beneath the central Willner, A.P., 2005, Pressure-temperature evolution of a late of the 2010 Maule Mw = 8.8, Chile, earthquake: Correlation Cascadia forearc: Geology, v. 40, p. 103–106, https://doi .org Paleozoic paired metamorphic belt in north-central Chile between co-seismic slip models and aftershock distribu- /10.1130/G32460 .1. (34°–35°30′S): Journal of Petrology, v. 46, p. 1805–1833, tion?: Geophysical Research Letters, v. 39, L08310, https:// Tréhu, A.M., de Moor, A., Madrid, J.M., Sáez, M., Chadwell, https://doi.org/10.1093/petrology/egi035. doi.org/10.1029/2012GL051308. C.D., Ortega-Culaciati, F., Ruiz, J., Ruiz, S., Tryon, M.D., Yue, H., Lay, T., Rivera, L., An, C., Vigny, C., Tong, X., and Báez Ruegg, J.C., Rudloff, A., Vigny, C., Madariaga, R., de Chabalier, 2019, Post-seismic response of the outer accretionary prism Soto, J.C., 2014, Localized fault slip to the trench in the 2010

J.B., Campos, J., Kausel, E., Barrientos, S., and Dimitrov, D., after the 2010 Maule earthquake: Geosphere, https://doi .org Maule, Chile Mw = 8.8 earthquake from joint inversion of 2009, Interseismic strain accumulation measured by GPS /10.1130/GES02021.1. high‐rate GPS, teleseismic body waves, InSAR, campaign in the seismic gap between Constitución and Concepción Vigny, C., Socquet, A., Peyrat, S., Ruegg, J.-C., Métois, M., GPS, and tsunami observations: Journal of Geophysical in Chile: Physics of the Earth and Planetary Interiors, v. 175, Madariaga, R., Morvan, S., Lancieri, M., Lacassin, R., Cam- Research: Solid Earth, v. 119, p. 7786–7804, https://doi .org p. 78–85, https://doi.org/10.1016/j.pepi.2008.02.015. pos, J., Carrizo, D., Bejar-Pizarro, M., Barrientos, S., Armijo, /10.1002/2014JB011340.