Abstract 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 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 rate 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.

Abstract After the 1964 Alaska earthquake, seismic records were collected across the eastern Aleutian Trench to study subduction-zone tectonics because geologic and seismologic studies indicated a subduction-related cause. Single-channel seismic-reflection records collected during the 1960s penetrated strata in the trench and about 1,000 m (3,281 ft) of rocks under the slope; only rarely was the top of the igneous oceanic crust detected for more than several kilometers beneath the front of the subduction zone (von Huene, 1972). In the first published multichannel seismicreflection records across the eastern Aleutian Trench (Seely, 1977), the reflection from the oceanic crust can be traced about 40 km (25 mi) landward of the trench, and the deformed stratification above this crust was shown in sufficient detail that a general structural style could be outlined. Structural style was also apparent in a series of records collected subsequently (von Huene, 1979; von Huene et al, 1979), but none of these multichannel records are migrated. The record presented here, from the Aleutian subduction zone off southern Kodiak Island (Figure 1), is migrated. A comparison of the unmigrated and migrated sections shows the improved resolution of structural detail that results from the migration process. This record and those from previous publications show a remarkable diversity of structural style in the subduction zone near Kodiak Island. In the record displayed here, the foot of the landward slope of the trench has a relatively simple structure in comparison with that evident in adjacent records from a seismic network off southern Kodiak Island. This record exemplifies one structural style of the subduction zone off southern Kodiak Island and shows the local formation of thrust packets within an accretionary complex.