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ABSTRACT: Seismic Reflection Study of the Oregon Accretionary Prism

Mary MacKay, Guy Cochrane, G. F. Moore, J. C. Moore, L. D. Kulm

We collected more than 1500 km of multichannel seismic reflection data over the Oregon accretionary prism in September, 1989. A commercial vessel, Digicon's Geotide, fired a tuned 4650 cu in airgun array at 25 m intervals and received the reflection data with a 3600 m long receiving array (144 channels, 25 m active group length). Representative lines have been processed through 72-fold CMP gather, NMO, inside/outside mute, stack and finite difference migration.

The seismic data image three distinct structural provinces along a 100 km segment of the accretionary prism. The southern province is characterized by seaward-verging thrusts and ramp anticlines. A "protothrust" zone of closely spaced blind thrusts extends more than 3.5 km seaward of the toe of slope. Local landward-verging backthrusts are also imaged. Slumping is prevalent along the steep seaward scarps of the ramp anticlines. A transition zone less than 5 km long extends to the north and exhibits both landward and seaward vergence.

In the central region of the study area the entire incoming sedimentary section (3-4 km) is uplifted and overthrust along landward-verging thrust faults. Fault-bend folds form three regularly spaced (2.5 km) asymmetric ridges that extend across the landward-verging province and increase in relief to the north. In some areas, the active thrusts appear to truncate previously overthrust packets. Minor seaward-verging conjugate thrusts form, breaking the seaward-dipping limb near its apex. Farther landward, additional shortening is accommodated along out-of-sequence seaward-verging thrusts.

An abrupt return to seaward vergence characterizes the northern province. Relief is lower and structures less laterally continuous than in either the southern or central provinces.

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