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ABSTRACT: Deep-Crustal Structure of the Continental Margin Adjacent to the Eastern Aleutian Trench

Michael A. Fisher, Thomas M. Brocher, Terry R. Bruns, Eric L. Geist

Onshore and marine seismic-reflection data obtained near the Trans-Alaskan Crustal Transect (TACT), in the region of the eastern Aleutian trench and lower Cook Inlet, reveal highly reflective midcrustal layering that begins at a depth of 10-15 km within the upper plate of the Aleutian subduction zone. Beneath the continental shelf, midcrustal reflections were recorded over broad areas and occur in subhorizontal bands that are 1 to 3 s thick. The reflections extend beneath complexly deformed late Mesozoic and early Cenozoic accreted rocks that are exposed at the surface. Preliminary interpretation of seismic refraction data indicates that under the shelf the top of the reflections corresponds in depth with a sharp increase in rock velocity, from 5.9 km/s to 6.6 km/s. North and northwest of the shelf, beneath the Chugach and Kenai mountains, midcrustal features dip 20°-30° north or northwest, and below the Chugach Mountains, the top of the reflections corresponds to a velocity decrease from 6.5 to 5.7 km/s. The cause of the midcrustal reflections may vary geographically and could include underplated Eocene deep-sea fan deposits, the top of the subducted Yakutat terrane, a midcrustal decollement, or a transition from brittle to ductile deformation regimes.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990