An EDGE deep crustal seismic reflection transect of the eastern Aleutian arc-trench traces oceanic crust and Moho more than 200 km beneath the accretionary prism to depths of more than 30 km. These horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the Mesozoic core of the prism. Earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active Augustine volcano. Rapid lateral growth of the prism in Eocene-Oligocene time coincided with uplift of the Mesozoic core of the prism. During lateral growth, maintenance of critical taper requires thickening, either by internal deformation or underplating. Because exposed rocks show only modest postemplacement shortening, thickening most likely occurred by underplating, probably of the layered reflectors. The overall geometry of the layered reflectors is reminiscent of nappe structures, and their emplacement may represent crustal-scale duplexing associated with underplating. The EDGE reflection data and borehole results indicate that the shelf edge is marked by an active out-of-sequence thrust that separates the Paleogene and Neogene prisms. This thrust apparently developed in response to the prism's need to maintain critical taper and demonstrates that contrasts in lithology can result from mechanisms other than terrane emplacement.

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