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New aeromagnetic data reveal large strike-slip (?) faults in the northern Willamette Valley, Oregon

¹ U.S. Geological Survey, 345 Middlefield Road, M.S. 989, Menlo Park, California 94025, USA
² Department of Geology, Portland State University, Portland, Oregon 97207, USA
³ College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
⁴ Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise, Idaho 83725, USA

High-resolution aeromagnetic data from the northern Willamette Valley, Oregon, reveal large, northwest-striking faults buried beneath Quaternary basin sediments. Several faults known from geologic mapping are well defined by the data and appear to extend far beyond their mapped surface traces. The Mount Angel fault, the likely source of the Richter magnitude (M_L) 5.6 earthquake in 1993, is at least 55 km long and may be connected in the subsurface with the Gales Creek fault 25 km farther northwest. Northeast of the Mount Angel fault, a 60-km-long, northwest-striking anomaly may represent a previously unrecognized dextral-slip fault beneath the towns of Canby and Molalla. Vertical offsets along the Mount Angel fault increase with depth, indicating a long history of movement for the fault. Dominantly northwest-trending, relatively straight faults, consistent stepover geometries, offset magnetic anomalies, and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip. The 1993 earthquake may have occurred on a left-stepping restraining bend along the Mount Angel–Gales Creek fault zone.

Key Words: aeromagnetic surveys • Cascadia subduction zone • earthquakes • fault zones • tectonics • Willamette Valley

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