A comprehensive data set of more than 200 profiles across the Peru-Chile Trench between 4° and 45°S is used to describe the morphology and shallow structure of the trench axis and the downbending oceanic plate just prior to subduction. Five morphotectonic provinces (4°–12°, 12°–17°, 17°–28°, 28°–45°S) show distinct changes in trench depth, axial sediment thickness, oceanic plate fault structures, and dip of the seaward trench slope. In general, the northern and southern regions are characterized by relatively shallow axial depths, moderate to thick trench axis turbidites, and a gently dipping seaward trench slope that exhibits minor normal faults. The deeper central area is almost barren of axial sediments and bends downward more steeply prior to subduction; bending has developed an extensive network of major faults with up to 1,000 m vertical offset on the seaward slope. Two systems of faulting occur in conjunction with subduction. Bending of the oceanic plate causes extensional stress and brittle failure of the upper oceanic crust, resulting in step faults, grabens, and tilted fault blocks on the seaward trench slope. Extensional faulting begins near the outer edge of the trench and develops progressively toward the trench axis. Basaltic ridges and tilted, uplifted trench fill at several locales along the trench can both be explained by thrust faulting. Compressional stress due to plate convergence occasionally can be transmitted seaward from beneath the continental margin through the oceanic plate, emerging as thrust faults within the oceanic crust near the trench axis. Axial turbidites are commonly tilted landward as they are uplifted, probably as a result of downward curving of the underlying thrust fault. Faulting of the oceanic crust prior to and during subduction may have important implications for evolution of convergent continental margins.

The crustal structure and tectonic framework of the central Peru Margin, between lat 7° and 10°S were interpreted using mainly a 102 km long, multichannel seismic section and existing geologic and geophysical data in the region. Thrust faulting occurs in upper layer 2 basalts as the Nazca plate descends beneath the continental slope. This produces basaltic ridges (slabs) within the trench axis and at least 26 km landward beneath the overriding continental plate. Broken low-frequency reflectors within this diffracting subduction complex suggest that ophiolitic slivers of basalt are being incorporated into portions of it, forming a sediment-basalt melange. Three prominent forearc basins, Salaverry, Trujillo, and Yaquina Basins, occupy the central margin from east to west, respectively. Drill holes penetrated Tertiary sediments on the outer shelf and the nearby eastern flank of the Trujillo Basin and bottomed in a metamorphic arc massif. The massif is correlated with seismic refraction velocities greater than 5.7 km/sec and a density of 2.72 to 2.80 g/cm 3 which underlie the continental shelf. Our interpretation of the seaward limit of the massif is uncertain and depends upon the geophysical cirteria used. Each of three forearc models position the arc massif at about 26, 61, or 115 km landward of the trench, with the subduction complex occupying the region between the trench and the massif. The massif-subduction complex interface should be located through drilling to test the proposed models. The intramassif basins, Salaverry and Trujillo, have subsided during Tertiary time to allow the accumulation of 2 to 4 km of marine sediment. The Trujullo Basin apparently has not experienced much vertical movement since the late Miocene, based upon microfossil paleodepth indicators found in dolomicrites and glauconitic micrites dredged from the basin. However, abundant brecciated dolomicrites in the same dredges and disturbed strata in reflection records from both the Yaquina and Trujillo Basins suggest deformation of the basins during the Pliocene-Pleistocene by a compressional regime.