Cenozoic active margin and shallow Cascades structure: COCORP results from western Oregon

Abstract

Two deep seismic reflection profiles totaling 98 km were recorded by the Consortium for Continental Reflection Profiling (COCORP) in western Oregon in 1984, providing some of the first onshore deep seismic reflection data from the forearc and arc of an active convergent margin. These data are of lower quality than most COCORP data, but when combined with other geological and geophysical data, they provide some useful insights into the subduction zone beneath Oregon and into extensional structures in the Cascades. Line 1 crossed the Eocene sediments and underlying pillow basalts of the Coast Range and western Willamette Valley. Reflections from the Willamette Valley clearly define the lower and western boundary of flat-lying, rhythmically bedded Eocene sediments. These sediments appear to be underlain by as much as 8 km of seismically transparent Eocene pillow basalts. Layered reflections at depths of 8-16 km may indicate a remnant crust upon which the Eocene pillow basalts were erupted. East-dipping reflections at depths of 35-40 km may represent the decollement above the subducting Juan de Fuca plate. Oregon line 2 crosses the Cenozoic volcanic arc terrains of the Western and High Cascades. Interpretation of COCORP seismic data suggests modification of existing models for normal faulting that postulate a symmetric graben or grabens in the Cascades of Oregon. In the High Cascades along COCORP Oregon line 2, reflections suggest a large, gently west-dipping half-graben with major offset only on the west side of the range. The seismic reflection data along with other geological and geophysical data suggest that the High Cascades were built on a series of blocks, rather than a single graben. The blocks were differentially faulted during the Pliocene. Seismic data from the Western Cascades indicate several normal faults with down-to-the-east offset, including a major, previously unidentified fault of probable Miocene age.