Description: Marine geophysical surveys employing Seabeam, multi- and single-channel seismic reflection, gravity and magnetic instruments were conducted at two locations along the continental slope of the Peru Trench during the Seaperc cruise of the R/V “Jean Charcot” in July 1986. These areas are centered around 5°30′S and 9°30′S off the coastal towns of Paita and Chimbote respectively. These data indicate that (1) the continental slope off Peru consists of three distinct morpho-structural domains (from west to east are the lower, middle and upper slopes) instead of just two as previously reported; (2) the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope; (3) the upper half of the lower slope appears to represent the product of mass wasting; (4) thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass-wasting are overprinted by a compressional tectonic fabric; (5) a change in the tectonic regime from tensional to compressional occurs at the mid-slope-lower slope boundary, the accretionary prism being restricted to the very base of the lower slope in the Paita area. The Andean margin off Peru is an “extensional active margin” or a “collapsing active margin” developing a subordinated accretionary complex induced by massive collapse of the middle slope area.

Description: Using older and in part fl awed data, Ruff (1989) suggested that thick sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling. This circumstance was conjectured to favor rupture continuation and the generation of high-magnitude (≥Mw8.0) interplate thrust (IPT) earthquakes. Using larger and more accurate compilations of sediment thickness and instrumental (1899 to January 2013) and pre-instrumental era (1700–1898) IPTs (n = 176 and 12, respectively), we tested if a compelling relation existed between where IPT earthquakes ≥Mw7.5 occurred and where thick (≥1.0 km) versus thin (≤1.0 km) sedimentary sections entered the SZ. Based on the new compilations, a statistically supported statement (see Summary and Conclusions) can be made that high-magnitude earthquakes are most prone to nucleate at well-sedimented SZs. For example, despite the 7500 km shorter global length of thicksediment trenches, they account for ~53% of instrumental era IPTs ≥Mw8.0, ~75% ≥Mw8.5, and 100% ≥Mw9.1. No megathrusts 〉Mw9.0 ruptured at thin-sediment trenches, whereas three occurred at thick-sediment trenches (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2). However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of highmagnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation

Description: Large intraplate earthquakes in oceanic lithosphere are rare and usually related to regions of diffuse deformation within the oceanic plate. The 23 January 2018 MW 7.9 strike-slip Gulf of Alaska earthquake ruptured an oceanic fracture zone system offshore Kodiak Island. Bathymetric compilations show a muted topographic expression of the fracture zone due to the thick sediment that covers oceanic basement but the fracture zone system can be identified by offset N-S magnetic anomalies and E-W linear zones in the vertical gravity gradient. Back-projection from global seismic stations reveals that the initial rupture at first propagated from the epicenter to the north, likely rupturing along a weak zone parallel to the ocean crustal fabric. The rupture then changed direction to eastward directed with most energy emitted on Aka fracture zone resulting in an unusual multi-fault earthquake. Similarly, the aftershocks show complex behavior and are related to two different tectonic structures: (1) events along N-S trending oceanic fabric, which ruptured mainly strike-slip and additionally, in normal and oblique slip mechanisms and (2) strike-slip events along E-W oriented fracture zones. To explain the complex faulting behavior we adopt the classical stress and strain partitioning concept and propose a generalized model for large intra-oceanic strike-slip earthquakes of trench-oblique oriented fracture zones/ocean plate fabric near subduction zones. Taking the Kodiak asperity position of 1964 maximum afterslip and outer-rise Coulomb stress distribution into account, we propose that the unusual 2018 Gulf of Alaska moment release was stress transferred to the incoming oceanic plate from co- and post-processes of the nearby great 1964 MW 9.2 megathrust earthquake.

Description: The geological evolution of the western margin of South America has long been a challenge to geologists interested in convergent plate tectonics. Late in 1986, scientists on the ODP drillship JOIDES Resolution confirmed that the upper slope of the Peruvian margin consists of continental crust whereas the lower slope comprises an accretionary complex. An intricate history of horizontal and vertical movements can be detected, and the locations of ancient centers of upwelling appear to have varied, partly due to tectonic movements of the margin. In this review of Leg 112, the three scientific leaders on this cruise discuss their results.

Description: We examined multichannel seismic records CDP-1, CDP-2, CDP-3, 1017, and records obtained during the site survey for Leg 112 to evaluate stratigraphy, tectonic evolution, and the structural character of the active margin offshore of Peru. From the reprocessed records we learned that the regionally uniform structure of the margin between 4°S and 14°S is modified by local tectonism. Common elements are crust of continental affinity beneath the middle and upper slope and an accretionary complex below the lower slope. The forearc basins have a varied tectonic history, which led to considerable differences in subsidence history and deformational style. Compressional tectonics dominate the front of the margin, whereas extension accounts for deformation landward of the midslope area.

Description: Studies of convergent margin tectonics along the Japan Trench Transect are based on geophysical as well as on drill and conventional sampling data. The large number of geophysical and geological studies in the 1960's made this area an attractive target for convergent margin drill core sampling. Once the transect was scheduled for drilling by the Glomar Challenger, the earth science community in Japan intensified its efforts to collect geophysical and geological data within the transect area. Many of these data are published in Japanese and international journals. This chapter describes both the seismic reflection records along which Legs 56 and 57 drilling took place and the reflection records made after drilling. The seismic data are interpreted in light of the drilling results.

Description: A geophysical and geological survey conducted over the landward slope of the Middle America Trench offshore Guatemala, together with published well information from the outer shelf and Leg 67 drilling results from the toe of the slope indicate that imbricate slices of oceanic crust were emplaced in the landward slope offshore Guatemala in the Paleocene or early Eocene. Since that time, sediment apparently has accumulated on the landward slope primarily as a sediment apron blanketing an older, tectonically deformed prism of sediments and crustal slices. There is little or no evidence for continued tectonic accretion seaward of the volcanic arc during the late Tertiary. Seismic reflection and refraction surveys have revealed landward-dipping reflections that are associated with high compressional wave velocities, large magnetic anomalies, and basic-ultrabasic rock. Multifold seismic reflection data reveal that the edge of the continental shelf is a structural high of Cretaceous and Paleocene rock against which Eocene and younger sediments of the shelf basin onlap and pinch out. The upper part of the continental slope is covered in most places by a 0.5- to 1.0-km-thick sediment apron with seismic velocities of 1.8 to 2.6 km/s. The base of the sediment apron commonly coincides with the base of a gas hydrate zone where water is 1500 to 2300 meters deep. Immediately beneath the sediment apron an irregular surface is the top of an interval with velocities greater than 4 km/s. Within this interval, landward-dipping reflections are traced to about 6 km below sea level. These reflections coincide with the top of seismic units having oceanic crust velocities and thicknesses. The sediment apron pinches out on the lower continental slope where refraction results indicate only a few hundred meters of 2.5-km/s material lying over about a kilometer of 3.0-km/s sediment. Between the 3.0-km/s sediment and a landward continuation of ocean crust, an interval of 4.1- to 4.7-km/s material occurs that thins seaward. Near the interface between the 4 +-km/s material and oceanic crust with velocities of 6.5 to 6.8 km/s, reflection records indicate a landward-dipping horizon that can be followed about 30 km landward from the Trench axis. Coring on the continental slope returned gravels of unweathered metamorphosed basalt, serpentine, and chert, unlike rock generally found onshore in Guatemalan drainage basins feeding the Pacific coast. These gravels, which were probably derived from local subsea outcrops, are similar to lithologies found on the Nicoya Peninsula farther south. A canyon cut in the outer continental shelf and upper continental slope may be associated with faulting, as indicated by an offset of linear magnetic anomalies at the shelf edge. In a general way our observations are consistent with previous suggestions that slices of rock, some of which may have oceanic crustal lithologies, are imbedded in the upper slope. However, the reflection data collected for the Deep Sea Drilling Project site survey do not show the many concave upward landward-dipping reflections that have been reported from other areas offshore Guatemala. The lower slope is probably a tectonically deformed and consolidated sediment wedge overlying oceanic crust, but it is not clear that it is organized into a series of landward thinning wedges. The structures within the landward slope may have originated during the late Paleocene to early Eocene tectonic event and may not be the result of an ongoing steady-state process of sediment accretion by sediment offscraping at the toe of the slope or by underplating of sediment at the base of the sediment wedge beneath the continental slope and shelf.

Description: Diatomaceous mudstones at depth under the deep sea terrace and the trench inner slope off Japan have been variably affected by tectonic stress. Veins, healed fractures, and microfaults occur at all sites except the shallow Site 435 on the upper trench inner slope and Site 436 on the Pacific Plate. Veins, fractures, and faults occur in cores from below 620 meters (lower to middle Miocene) in the landward sites (438 and 439) on the deep sea terrace, and are probably related to normal faulting seen in seismic records. The depth to "consolidated" sediment and to the first occurrence of veins and healed fractures shallows progressively toward the trench. The intensity of deformation also appears to increase seaward. However, no sediments younger than upper Pliocene are deformed. Open fractures may exist in situ at Sites 434 and 441 at levels between about 150 and 500 meters sub-bottom. The Japan Transect sediments—in contrast to deposits in the zone of initial deformation at other convergent margins though highly deformed, are not highly overconsolidated. However, sediment at depth in the trench inner slope sites is overconsolidated relative to that at the same depth in the landward reference site. None of the deformed Japan margin sediments recovered at Legs 56 and 57 sites originated by accretion of oceanic plate material—also in contrast to sediments at some of the margins previously studied. We suggest that tectonic stress related to convergence has been communicated to the slope sediments on the trench inner slope, either continuously or periodically, causing rapid tectonic dewatering and inducing fracturing and faulting. If episodic, the latest of these deformational periods may have occurred during the late Pliocene. The faults and fractures are either rehealed by continued overburden pressure (sediment loading) or may remain open at shallower levels. Fracturing and dewatering of semiconsolidated sediment beneath an unconsolidated but impermeable mud veneer may cause overpressured zones at depths of 200 to 500 meters. These overpressured zones possibly locally reduce shear strength and cause downslope mass movement of sediment, even on low-angle slopes on the trench inner slope.

Description: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1-D and a 3-D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth-migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

Description: Volcanic rocks were dredged from the Cocos and Fisher ridges and seamounts along a 250 km profile parallel to the Pacific coast of Costa Rica. The composition and laser 40Ar/39Ar ages of the Cocos Ridge and Seamounts are consistent with their formation above the Galápagos hotspot 13.0–14.5 Ma. The reconstructed paleoenvironment and chemistry of the Fisher Ridge are consistent with it having originated at a mid-oceanic ridge system. Laser 40Ar/39Ar dating of fresh basalt glass from the Fisher Ridge yielded isochron ages of 19.2 ± 0.3 Ma and 30.0 ± 0.5 Ma. The Fisher Ridge is along a lithospheric fault that may represent an extensional fracture formed when the oceanic floor rode over the Galápagos hotspot. Even though the younger structures are currently at water depths of 〉1000 m, volcanological, geochemical, and geophysical observations indicate that they once formed an emerged archipelago very similar in morphology to the Galápagos islands. The diversity of the biota on the isolated Galápagos islands, as first described by Charles Darwin, has had an important influence on the development of the theory of evolution. The existence of a now-drowned Galápagos archipelago 14.5 Ma considerably increases speciation times for the Galápagos biota and provides a complete solution to a long-standing controversy concerning the divergence of the Galápagos marine and land iguanas from a single ancestral species.