Abstract: Extensional Tectonics in a Convergent Margin--Pacific Northwest Offshore, Washington and Oregon

Kenneth A. Piper

The Neogene section offshore Washington and northern Oregon is dominated by extensional features representing the collapse of an unconstrained and overloaded sedimentary pile. The top of the melange apparently acts as a glide boundary so that the overlying section may spread laterally, unconstrained horizontally to the west, and detached from the rocks below.

Where Neogene sediments directly overlie melange, or where Paleogene section is thin, as offshore much of Washington, major structural features are growth faults and shale diapirs. The growth faults are more abundant near the shelf-slope boundary; however, several are present well up onto the shelf and may incorporate a thin section of Paleogene rocks overlying the melange boundary. Faults often occur in groups, possibly in imbricate fashion, with younger faults cutting older. The growth faults typically involve rotation of the down-thrown block. Shale diapirs occur over much the same area and are sometimes related to the growth faulting.

Farther south, where significant Paleogene section underlies the Neogene, gravity gliding of Neogene and Paleogene sediments has evidenced areas where the sedimentation rate has been greatest. Deformation is characterized by folding and reverse faulting at the leading edge and normal faulting up-slope.

These extensional features are consistent with a model in which the fore-arc basin is decoupled from the downgoing lithosphere and significant compression is confined to the leading edge of the overriding plate.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994