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: A marine geophysical survey that used Seabeam, multi- and single-channel seismic reflection, gravity, and magnetic profiling was conducted in two locations along the landward slope of the Peru Trench. This survey was conducted during the SeaPERC cruise of the Jean Charcot in July 1986 (Bourgois et al., 1986a; Bourgois et al., 1986b; Bourgois et al., 1987; Bourgois et al., in press). These areas were two possible drilling locations for ODP Leg 112, which was scheduled to begin in November 1986. The drilling transects center around latitudes of 5°30'S and 9°30'S, along multichannel seismic lines CDP-3 and CDP-2, respectively. The northern transect is located in the Paita Zone (Fig. 1, Box 2), and the southern transect is in the Chimbote Zone (Fig. 1, Box 6). During Leg 112, we drilled two sites (683 and 685) in the Chimbote area. This became the northern transect of Leg 112. Here, we report the Seabeam data acquired during the SeaPERC cruise. Drilling results from Sites 683 and 685, the CDP-2 multichannel seismic record, and the Seabeam data provide a threedimensional view of this region.

Description: Slope deposits drilled during Leg 67 were later detailed in redisplayed seismic-reflection records. These deposits are of significantly lower seismic velocity and probably lower density than the underlying basement. This relationship indicates a contact between rocks of differing consolidation rather than a continuous sedimentary sequence. The slope deposits cover basement terranes of three different topographies. The shelf edge is an arch whose seaward flank forms a steep (up to 15°) upper slope. The midslope area has a rugged topography covered by thick slope deposits. The lower slope is relatively smooth except where broken locally by benches. The upper and middle slope areas are associated with strong magnetic anomalies and rare, landward-dipping reflections truncated by the rough surface. The rough midslope topography may reflect erosion succeeding the Paleocene uplift of this area and at least local subsidence in the early Miocene. Slope deposits covered the Trench landward slope, contemporaneous with the increased arc volcanism indicated by ash layers and with the present period of subduction. The subducting ocean crust and sediment of the Cocos Plate has a linear horst and graben topography of hundreds of meters relief that disappears beneath the landward slope of the Trench without disturbing its topography. This passive assimilation of oceanic material without significant accretion in the late Neogene argues for significant decoupling at the front of the subduction zone. A base of gas-hydrate reflection is evident in many of the redisplayed seismic records off Guatemala. These reflections are most common where slope deposits are thick; the reflections have not been identified in the underlying acoustic basement. This observation is consistent with the geochemical evidence that gas hydrate has its source in the organic- rich slope sediment. The hydrate depth and the temperature measurements in drill holes indicate a temperature gradient of 30°C/km.

Description: The evolution of Neogene and Quaternary sedimentation in the fore-arc region off northern Honshu is evaluated using multichannel and single-channel seismic records in conjunction with the drill holes of the Japan Trench Transect (DSDP/IPOD Legs 56-57). The outer forearc region, which consisted of older sedimentary rocks and some calc-alkaline volcanic rocks, was subaerially exposed and eroded during the Paleogene and part of the Neogene. The deep sea terrace (fore-arc basin) region subsided below sea level in the early Miocene; most rapid subsidence occurred during the early to middle Miocene. Submergence progressed seaward so that the last vestige of the Oyashio landmass, which is now under the upper trench slope, was below sea level in the latest Miocene. Sediment sources to the outer fore-arc basin changed progressively from lithic, predominantly nonvolcanic material derived from the uplifted landmass during the late Paleogene-early Neogene to volcanic, arc-derived sediment rich in volcanic glass, Plagioclase, and volcanic lithic fragments. The volcaniclastic sediment was probably derived both from Honshu to the west and Hokkaido to the northwest. In response to subsidence the sedimentary depocenters in the fore-arc basin migrated generally seaward through time; the greatest relative seaward migration occurred between the late Miocene and Pliocene. Thick sediment sequences accumulated in slope basins on the trench inner slope. Sediment from the arc moved seaward to spill over the slope via large channels. An abrupt change in morphology and patterns of sedimentation apparently took place in the late Pliocene, coincident with a peak in explosive volcanism recorded in the form of ash layers and increased glass contents in sediment. The deep sea terrace was uplifted several hundreds of meters and a major channel crossing the fore-arc region was tilted landward and filled. At about the same time the midslope terrace basin was created and began rapidly accumulating sediment. The older basins, lower on the trench inner slope, were destroyed, possibly by steep seaward tilting, or filled. Large slump masses were sloughed-off downslope to the trench. Little sediment now accumulates on the trench inner slope in the vicinity of the sites, and older strata crop out on the slope. The locus of deposition has shifted northward off Hokkaido where a large channel feeds sediment to the slope. Large slump masses now fill the trench and are being accreted, creating a "toe" to the slope in this region. The evolution of the fore-arc region off northern Honshu has not been steady state. Tectonic accretion has been discontinuous, and tectonic erosion of the continental margin edge may have occurred periodically. Slope basins have been both created and abruptly destroyed at different points on the trench inner slope. There appears to be little possibility of distinguishing most sediment "scraped off" the oceanic plate from hemipelagic sediment deposited in the fore-arc region of Japan.

Description: In 1946, megathrust seismicity along the Unimak segment of the Alaska subduction zone generated the largest ever recorded Alaska/Aleutian tsunami. The tsunami severely damaged Pacific islands and coastal areas from Alaska to Antarctica. It is the charter member of “tsunami” earthquakes that produce outsized far-field tsunamis for the recorded magnitude. Its source mechanisms were unconstrained by observations because geophysical data for the Unimak segment were sparse and of low resolution. Reprocessing of legacy geophysical data reveals a deep water, high-angle reverse or splay thrust fault zone that leads megathrust slip upward to the mid-slope terrace seafloor rather than along the plate boundary toward the trench axis. Splay fault uplift elevates the outer mid-slope terrace and its inner area subsides. Multibeam bathymetry along the splay fault zone shows recent but undated seafloor disruption. The structural configuration of the nearby Semidi segment is similar to that of the Unimak segment, portending generation of a future large tsunami directed toward the US West coast.

Description: Deep-tow data across the Middle America Trench and the lower part of the landward Trench slope off Guatemala provide high-resolution information on the geological setting of four Leg 67 drill sites. Our 6-kHz profiler data resolve stratified sediments seaward of the Trench floor, which we interpret as deposits of Trench turbidites that flowed part way up the seaward slope of the Trench. Reflectors in the Trench fill are laterally continuous and relatively undisturbed. The Trench is segmented into diamond-shaped basins by ridges that intersect it at oblique angles. The acoustic stratigraphy in the Trench fill in these sub-basins is different on either side of an oblique ridge. A mound of variable height and width occurs at the base of the lower Trench slope; the mound could be either deformed Trench strata or an accumulation of debris that slumped from the Trench slope. The lower part of the landward Trench slope is steep and is linear along strike. There is no evidence on our 6-kHz or side-scan sonar data of large gravity slides in this area. Slope sediments are ponded on a bench along the landward Trench slope. They are acoustically stratified, but are not as continuous laterally as are the Trench strata. The structure parallel to the strike of the Trench slope is variable, with many small-scale folds and faults.