Description: A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high-velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg-leg multiples generated from with a low-velocity zone between the second and third upper crustal high-velocity layers. The third high-velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low-velocity zone that produces a seismic shadow wherein zone peg-leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high- and low-velocity layers. Synthetic seismogram modeling indicates that a low-velocity zone with transitional upper and lower boundaries generates peg-leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long-offset (90-225 km) data. This may be due to a lower crustal waveguide whose top is the high-velocity (7.6 km/s) layer and whose base is the Moho. A deep (~54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg-leg multiples have been interpreted on some long-range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg-leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide-angle reflection profiles than previously recognized.

Description: During a seismic reflection survey conducted by the California Consortium for Crustal Studies in the Basin and Range Province west of the Whipple Mountains, SE California, a piggyback experiment was carried out to collect intermediate offset data (12–31 km). These data were obtained by recording the Vibroseis energy with a second, passive recording array, deployed twice at fixed positions at opposite ends of the reflection lines. The reflection midpoints fall into a 3-km-wide and 15-km-long region in Vidal Valley, roughly parallel to a segment of one of the near-vertical reflection profiles. This data set makes three unique contributions to the geophysical study of this region. (1) From forward modeling of the observed travel times using ray-tracing techniques, a shallow layer with velocities ranging from 6.0 to 6.5 km/s was found. This layer dips to the south from 2-km depth near the Whipple Mountains to a depth of 5-km in Rice Valley. These depths correspond closely to the westward projection of the Whipple detachment fault, which is exposed 1 km east of the near-vertical profiles in the Whipple Mountains. (2) On the near-vertical profile, the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in Vidal Valley. However, the piggyback data, which undershoot this basin, show that these reflections are continuous beneath the basin. Thus near-surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near-vertical reflection profile. (3) The areal distribution of the midpoints allows us to construct a quasi-three-dimensional image on perpendicular profiles; at the cross points we determined the true strike and dip of reflecting horizons. This analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the Whipple Mountains and dip to the south southward of the Turtle Mountains. The results of this study support the interpretation of crustal reflectivity in the near-vertical reflection profiles to be related to the mid-Tertiary episode of extension which produced the Whipple metamorphic core complex. This association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared with single detachment fault models.

Description: Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore‐arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide‐angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9–6.7 km/s overlying the lower crust with Vp 〉 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore‐arc basin is 〉10 km deep, deposited on 5–10 km thick Australian crust. The fore‐arc mantle of Vp 〉 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore‐arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high‐velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low‐seismic‐velocity, high‐attenuation, low‐density fore‐arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two‐sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25–100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore‐arc highs have not properly been accounted for.

Description: Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33°–46°S), we study the processes of sediment accretion and subduction and their implications on megathrust seismicity. In terms of the frontal accretionary prism (FAP) size, the marine south central Chile fore arc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernández Ridge and north of the Mocha block) characterized by a relative large FAP (20–40 km wide) and (2) the Chiloé segment (south of the Mocha block and north of the Nazca-Antarctic-South America plates junction) characterized by a small FAP (≤10 km wide). In addition, the Maule and Chiloé segments correlate with a thin (〈1 km thick) and thick (∼1.5 km thick) subduction channel, respectively. The Mocha block lies between ∼37.5° and 40°S and is configured by the Chile trench, Mocha and Valdivia fracture zones. This region separates young (0–25 Ma) oceanic lithosphere in the south from old (30–35 Ma) oceanic lithosphere in the north, and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction, respectively. A process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down-deflected oceanic lithosphere under trench sediment loading. On the other hand, a small FAP along the Chiloé segment is coincident with the rupture area of the trans-Pacific tsunamigenic 1960 earthquake (Mw = 9.5), while a relatively large FAP along the Maule segment is coincident with the rupture area of the 2010 earthquake (Mw = 8.8). Differences in earthquake and tsunami magnitudes between these events can be explained in terms of the FAP size along the Chiloé and Maule segments that control the location of the updip limit of the seismogenic zone. The rupture area of the 1960 event also correlates with a thick subduction channel (Chiloé segment) that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation.

Description: We present the first detailed 2D seismic tomographic image of the trench-outer rise, fore- and back-arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo-Australian plate where ~80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending-related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of ~7.3 km/s, ~10% lower than typical for mantle peridotite (~30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc-type igneous basement with uppermost and lowermost crustal velocities of ~3.5 and ~7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0–5.5 km/s) suggests that the outer fore-arc is probably fluid-saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore-arc. Extension in the outer fore-arc is evidenced by (1) trenchward-dipping normal faults and (2) the presence of a giant scarp (~2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore-arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of ~7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle.

Description: Seismic investigations across the convergent Sunda margin off Indonesia provide a detailed image of the crustal architecture of the Sunda plate boundary. The combined analysis and interpretation of wide-angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain, which yield some three-dimensional (3-D) constraints on the velocity-depth structure across the margin. A detailed cross section of the subduction zone is presented, which is confirmed by supplementary gravity modeling. The Sunda convergence zone is a prime example of an accretionary margin, where sediment accretion has led to the formation of a massive accretionary prism, with a total width of 〉110 km between the trench and the forearc basin. It is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high. Moderate seismic velocities derived from wide-angle modeling indicate a sedimentary composition of the outer high. The subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism. The adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate Moho at 16 km depth. We speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic-type crust found at the leading edge of the upper plate beneath the forearc basin.

Description: The deep structure of the south-central Costa Rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible. We present a receiver function study along a transect of broadband seismometers through the northern flank of the Cordillera de Talamanca (south Costa Rica). Below Moho depths, the receiver functions image a dipping positive conversion signal. This is interpreted as the subducting Cocos Plate slab, compatible with the conversions in the individual receiver functions. In finite difference modeling, a dipping signal such as the one imaged can only be reproduced by a steeply (80°) dipping structure present at least until a depth of about 70–100 km; below this depth, the length of the slab cannot be determined because of possible scattering effects. The proposed position of the slab agrees with previous results from local seismicity, local earthquake tomography, and active seismic studies, while extending the slab location to greater depths and steeper dip angle. Along the trench, no marked change is observed in the receiver functions, suggesting that the steeply dipping slab continues until the northern flank of the Cordillera de Talamanca, in the transition region between the incoming seamount segment and Cocos Ridge. Considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision, the southern Costa Rican subduction zone may at present be undergoing a reconfiguration of subduction style, where the transition to shallow underthrusting may be underway but still incomplete.

Description: A joint interpretation of swath bathymetric, seismic refraction, wide-angle reflection, and multichannel seismic data was used to derive a detailed tomographic image of the Nazca-South America subduction zone system offshore southern Arauco peninsula, Chile at similar to 38 degrees S. Here, the trench basin is filled with up to 2.2 km of sediments, and the Mocha Fracture Zone (FZ) is obliquely subducting underneath the South American plate. The velocity model derived from the tomographic inversion consists of a similar to 7-km-thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities >8.4 km s(-1). In the trench-outer rise area, the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults, horst-and-graben structures, and a reduction of both crustal and mantle velocities. These slow velocities are interpreted in terms of extensional bending-related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides with an area of even slower velocities and thinning of the oceanic crust (10-15% thinning), suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone. Slow mantle velocities occur down to a maximum depth of 6-8 km into the upper mantle, where mantle temperatures are estimated to be 400-430 degrees C. In the overriding plate, the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The margin is composed of three main domains: (1) a similar to 20 km wide frontal prism below the continental slope with Vp 〈= 3.5 km s(-1), (2) a similar to 50 km area with Vp = 4.5-5.5 km s(-1), interpreted as a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework with Vp >= 6.0 km s(-1). Frontal prism velocities are noticeably lower than those found in the northern erosional Chile margin, confirming recent accretionary processes in south central Chile.

Description: A seismic wide‐angle and refraction experiment was conducted offshore of Nicaragua in the Middle American Trench to investigate the impact of bending‐related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction. On the basis of the reflectivity pattern of multichannel seismic reflection (MCS) data it has been suggested that bending‐related faulting facilitates hydration and serpentinization of the incoming oceanic lithosphere. Seismic wide‐angle and refraction data were collected along a transect which extends from the outer rise region not yet affected by subduction into the trench northwest of the Nicoya Peninsula, where multibeam bathymetric data show prominent normal faults on the seaward trench slope. A tomographic joint inversion of seismic refraction and wide‐angle reflection data yield anomalously low seismic P wave velocities in the crust and uppermost mantle seaward of the trench axis. Crustal velocities are reduced by 0.2–0.5 km s−1 compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6–7.8 km s−1 and hence 5–7% lower than the typical velocity of mantle peridotite. These systematic changes in P wave velocity from the outer rise toward the trench axis indicate an evolutionary process in the subducting slab consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. If hydration is indeed affecting the seismic properties of the mantle, serpentinization might be reaching 12–17% in the uppermost 3–4 km of the mantle, depending on the unknown degree of fracturing and its impact on the elastic properties of the subducting lithosphere.

Description: A seismic wide-angle section offshore Costa Rica is presented across the boundary between oceanic crust generated at the East Pacific rise (EPR) and at the Galápagos spreading center (GSC) as indicated by magnetic anomalies. This suture, where the Farallon plate broke up ∼23 Ma ago, is marked by pronounced velocity variations throughout the crust including a low-velocity body in the lower crust. This body is well constrained by refracted waves above the inversion zone and by strong PmP reflections from its lower boundary. The distinctness of this body and the local gravity field point to an igneous intrusion rather than serpentinized rock. Typical oceanic crust is found adjacent to the suture zone.