THE GALES CREEK FAULT ACCOMMODATES LARGE DEXTRAL OFFSET IN THE OREGON FOREARC

The northwest-trending Gales Creek fault (GCF), about 40 km west of Portland, forms the structural boundary between the Oregon Coast Range and the Tualatin and northern Willamette basins. Geophysically, it is the largest structure in northwest Oregon; a steep gravity gradient up to 110 mGal extends along the fault for more than 50 km, and sharp magnetic boundaries suggest 10-15 km dextral offset of magnetic Eocene basement. Vertical offset is also substantial–our gravity inversions indicate the Tualatin Basin is 5 km deep. New geologic mapping shows the fault to consist of en echelon strands south of Forest Grove, which merge northward into two subparallel strands striking 340° and separated by 3 km. The eastern strand, previously mapped as the GCF by Schlicker and Deacon, follows Gales Creek north to the Highway 6 bridge but has no obvious physiographic expression on the valley floor. In contrast, the western strand offsets a series of large, east-flowing Gales Creek tributaries several kilometers. An anticline, cored by rare subaerial Siletz River Volcanics (49 Ma) and intruded by Eocene diabase, strikes obliquely into the fault and provides a piercing point across the western strand. It indicates 12 km of post-Eocene right lateral offset northeast of Hagg Lake, consistent with the offset of aeromagnetic anomalies. On LIDAR imagery of the western strand, uphill-facing scarps, offset streams, and shutter ridges can be traced discontinuously along the mapped bedrock fault for 18 km between Hagg Lake and Highway 6. A plan to trench the fault is underway. South of Gaston, the fault may project beneath landslides and Missoula flood deposits through a gap in the Columbia River Basalt (CRB) at Newberg. Although there is no geomorphic evidence of faulting in Missoula flood deposits there, the GCF may be responsible for 6 km of dextral separation of the southeast-dipping CRB monocline between the Dundee Hills and Parrett Mountain.