Abstract: Structural Interpretation of Eastern Aleutian Trench and Adjacent Continental Margin Off Kodiak Island, Alaska
Roland Von Huene
The continental margin off Kodiak Island has formed at converging lithospheric plates; 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 Pliocene-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 midslope terrace, and a relatively smooth upper slope. Core samples from the lower slope yielded tectonically deformed and highly compacted Pliocene-Pleistocene sediment, similar to those that would be expected in a subduction zone. Seismic records across this drill site show few coherent reflecting horizons owing to the deformation.
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 terrigenous sediment filling older structural irregularities. The shelf break at the top of the slope is formed by an alignment of short, relatively broad anticlines, and where 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.
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 to 40 km wide adjacent to the trench.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC