The continental margin off Kodiak Island has formed at converging lithospheric plates as indicated by the eastern Aleutian Trench, the Aleutian volcanic chain, and the well-defined Benioff zone; hence, its structure should reflect the tectonic features of a subduction zone. Igneous oceanic crust seaward of the trench is overlain by at least 35 m of lower Miocene pelagic sediment coarsening upward to Miocene and Plio-Pleistocene turbidite sequences. Downbowing of this oceanic crustal sequence has formed the trench, which is filled by a wedge of turbidites 0.8 to 1.0 km thick and less than 1 m.y. old. The structure of the adjacent continental slope is expressed morphologically by a rough lower slope, a poorly developed mid-slope terrace, and a relatively smooth upper slope. Core samples from the lower slope yielded tectonically deformed and highly compacted Plio-Pleistocene sediment, similar to those that would be expected in a subduction zone. However, the original environment of deposition cannot be determined from lithologic and paleontologic data to establish whether these samples are oceanic-basin material accreted to the continent or deformed slope deposits. Seismic records across this drill site show few coherent reflecting horizons owing to the deformation. In contrast, records from an adjacent area show coherent, little-deformed reflections of the oceanic-basin sequence that can be traced beneath the slope, landward from the trench to about the mid-slope area. Such variability in intensity of deformation along the leading edge of a subduction zone

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is surprising and difficult to explain. The upper slope's smooth morphology is due to thick terrigeneous sediment filling older structural irregularities, a circumstance that suggests relatively less intense late Cenozoic tectonism than has affected the lower slope. The shelf break at the top of the slope, is formed by an alignment of short, relatively broad anticlines, and where they are breached by erosion, Miocene and younger sediment has been recovered. Growth of the anticlines has helped to pond sediment landward in a broad, deep shelf basin. The seismic and drilling data allow a rough comparison between the minimum volume of oceanic sediment that was on the subducted plate, and the maximum accreted material at the continental margin of equivalent late Tertiary age. Assuming the ate of subduction was 5 cm/yr, these volumes appear similar suggesting that all subducted sediment may have been accommodated in the margin. The data suggest that rapid underthrusting occurs mainly along the lower continental slope in a zone 30-40 km wide adjacent to the trench. The intensity of tectonism diminishes landward just above this zone so that the shelf basin has been only mildly affected by subduction.

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