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ABSTRACT: Deep Seismic Surveys of a Dormant Subduction Zone in the Pacific Northwest, United States

William J. Gwilliam, W. D. Stanley, G. V. Latham, J. K. Westhusing

Seismic reflection data have been obtained in western Washington along a 110 km (68 mi), east-west profile from the Chehalis basin to the east flank of the Cascadese. A 1000-channel sign-bit recording system was used with multiple vibrator sources and relatively innovative processing techniques to achieve penetration of 4-6 sec in this area of extensive surface volcanics. The seismic data support a model developed using magnetotelluric interpretations that hypothesized the existence of a marine sedimentary complex in a dormant, early Eocene subduction zone. An eastward-dipping reflector on the west of the profile is interpreted to be related to trapped oceanic crust of the fossil subduction zone. This reflector occurs at the base of low-resistivity units from the magnetotelluric surve s hypothesized to be the marine sedimentary rocks. Near the town of Morton, the magnetotelluric and seismic data also outline a prominent anticline that is cored with the low-resistivity rocks. However, much (up to 10 km) of the low-resistivity section mapped with magnetotelluric surveys and interpreted to be marine sedimentary rocks is seismically transparent, suggesting a complicated internal structure. If these interpreted marine sedimentary rocks are found to have source rock potential, a new hydrocarbon exploration rationale is suggested for the Pacific Northwest.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990