The Gales Creek Fault – Active Northward Migration of an Oregon Forearc Sliver

Date: 4/24/2019

Time: 11:45 AM

Room: Puget Sound

New geologic mapping, geophysical surveys, and paleoseismic trenching show the Gales Creek fault (GCF) west of Portland to be part of an active, right lateral fault system that accommodates northward motion and uplift of the Oregon Coast Range. Aeromagnetic and geologic mapping of the GCF document about 12 km of dextral offset of an anticline in Eocene Siletz River Volcanics basement (Siletzia) since about 35 Ma, and about 6 km of right lateral separation of tilted flows of the Columbia River Basalt Group at Newberg, Oregon, since 15 Ma (~0.4 mm/yr, average long-term rate). Vertical offset across the GCF is at least 5 km based on depth to Siletzia basement under the Tualatin basin from a recent inversion of gravity data. Crustal models based on gravity and magnetic anomalies suggest a shallow popup structure along the fault and Siletzia basaltic basement on both sides. Five km west of the city of Forest Grove, the fault consists of two, subparallel, NW-striking strands, spaced ~3 km apart. Seismic-reflection data suggest the eastern Gales Creek strand may link southward to the E-W-striking Beaverton fault in the Tualatin basin. The Beaverton fault, in turn, links eastward to the right lateral Canby fault, thus forming a 30 km-wide compressive stepover that uplifts the Chehalem Mountains. Aeromagnetic data indicate the western strand links southward to the Mount Angel fault, the apparent source of the 1993 M 5.7 Scotts Mills earthquake. Recent deformation is indicated by LiDAR data which reveal 6 major (up to 1.5 km) right-lateral stream offsets, scarps, and other youthful geomorphic features for 60 km along the trace of the GCF north of Newberg, Oregon. Near Yamhill, Oregon, paleoseismic trenches document Eocene bedrock thrust over 250 ka surficial deposits along a splay of the fault system. Multiple late Pleistocene and Holocene earthquakes have been documented by the Bureau of Reclamation on the GCF south and west of Forest Grove, Oregon. This is the first direct geologic evidence in Oregon of large scale, northward motion of the Coast Range.

Presenting Author: Ray E. Wells

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