Graduate Thesis Or Dissertation
 

Evolution of the Corvallis fault and implications for the Oregon coast range

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  • The Corvallis fault is a 50 km long northeast-trending structure, part of which defines the boundary between the central Willamette Valley and the east-central Coast Range of Oregon. Previously the fault had been mapped as either a high-angle reverse or normal fault, with the east block down. New gravity data suggest that the main structure is a low-angle thrust, with early Eocene Siletz River Volcanics thrust southeastward over middle to late Eocene Tyee and Spencer sandstones. The thrust geometry is similar to that of the Laramide thrusts of the Rocky Mountain foreland. Gravity modeling produces a best-fit geometry with the thrust-plane dipping approximately 100 northwest. The surface geology is consistent with a fault-propagation fold geometry. Consistent dips averaging 20° in the hanging wall block suggest a ramp dipping at the same angle, somewhat steeper than the dip indicated by gravity modeling. Vertical separation is about 6.7 km, and if the ramp dip is the same as bedding dips in the Siletz River Volcanics, horizontal displacement is 13-15 km, assuming no other thrust faults repeat the Siletz River stratigraphy. The Corvallis thrust was active during the late Eocene, and was the eastern boundary of a tectonic highland in the Eocene forearc. The highland was a local source of material for the upper Yamhill and lower Spencer Formations, deposited in a partially restricted shallow shelf to neritic setting. Other late Eocene tectonic and volcanic highlands formed an archipelago in the position of the present Coast Range. In the middle Oligocene, the fault was intruded by gabbroic dikes during an intrusive episode that emplaced massive sheets of gabbro throughout the central Coast Range. A younger normal fault paralleling the original Corvallis thrust is interpreted to be the result of gravitational collapse of the tip of the thrust sheet, and has truncated the older structure. Numerous left offsets of the main fault trace along northwest-trending left-lateral faults are interpreted to be the result of clockwise rotations of western Oregon documented by paleomagnetics. Later reactivation of the Corvallis fault as a left-lateral strike-slip fault, indicated by horizontal slickenlines, is consistent with the present north-south compression in Oregon. The Corvallis fault may have continued minor intermittent activity into the late Quaternary.
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