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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.

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