A combination of SEABEAM swath bathymetric data, closely spaced single-channel seismic reflection profiles, and multichannel seismic reflection profiles from the Shumagin Islands region indicates that the large-scale deformation of the accretionary prism in this segment of the Aleutian Trench occurs progressively through several stages: (1) folding along axes perpendicular to the plate-convergence direction in the region, (2) thrust faulting in the direction of plate convergence and (3) oblique strike-slip faulting along conjugate west-northwest-trending right-lateral and north-northeast-trending left-lateral faults. Strike-slip faults occur in distinct domains, with right-lateral oblique faults present west of a major transverse fault at long 161°W. and left-lateral oblique faults prevalent east of 161°W.

The structural development of the modern Aleutian Trench fore arc in this region is probably affected by pre-Eocene inherited structures that are still active in the present-day stress regime. A major domain boundary fault located at long 161°W. is probably part of a strike-slip fault system that also bounds Sanak Basin farther landward on the Shumagin shelf. This fault zone is nearly coincident with the mapped position of the western edge of the Shumagin seismic gap. These transverse structures in the Shumagin fore arc probably influence the spatial distribution of stress buildup related to subduction, and hence the locations of rupture zones of great interplate earthquakes, along this segment of the Aleutian Trench.

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