Abstract

We have used migrated 24-fold seismic records to study mechanisms of subduction accretion along the central Aleutian Trench. Two major structural units underlie the landward trench slope and are separated by an acoustically defined décollement surface that can be traced at least 20 km landward of the trench axis. The upper unit consists of three to four structural blocks of deformed trench fill. Each block possesses a consistent internal structure and is bound by major landward-dipping thrust faults. The structures within the blocks include monoclinal sequences, antiforms, synforms, and faulted folds. Beneath the décollement, nearly undeformed trench fill ∼ 1 km thick is being under-thrust along with the underlying oceanic lithosphere. This basal section is an intervening unit between the offscraped sediment and the rough igneous oceanic basement and may prevent the oceanic crust from being incorporated into the subduction complex, at least within 20 km of the front of the subduction zone.

Owing to bending stresses, the oceanic crust fails along normal faults beneath, and at least 15 km landward of, the trench axis. These normal faults, although poorly constrained, are believed to propagate upwards, producing offsets within both the subducting and the accreting sediment. In addition, normal faulting enhances the irregular surface of the oceanic crust. Structural associations imaged on all of the seismic reflection profiles— although possibly fortuitous—suggest that the subduction of these irregular basement highs results in the temporary displacement of the overlying fault-bound sediment packets and promotes the development of the structures found within the accretionary prism. Tectonic riffling of the accreted sediment by the passage of the basement relief may also provide a mechanism by which underthrusting sediments are decoupled from the subducting lithosphere and underplated to the base of the Aleutian accretionary complex.

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