GSA Connects 2021 in Portland, Oregon

Paper No. 36-8
Presentation Time: 3:35 PM


ANDERSON, Megan1, STEELY, Alexander N.1, HORST, Alison2, VON DASSOW, Wesley1, LAU, Todd1, REEDY, Tabor1, AMARAL, Chelsea1 and STANTON, Kelsay3, (1)Washington Department of Natural Resources, Washington Geological Survey, 1111 Washington St SE, Olympia, WA 98504-7007, (2)Geotechnical Resources, Inc., 9750 SW Nimbus Ave, Beaverton, OR 97008, (3)Earth and Space Sciences, University of Washington, Seattle, WA 98195

New geologic mapping, geophysical data and modeling, and geochronology substantially improve our understanding of the last 50 million years of tectonic activity in southwest Washington, focusing on the Doty fault and related structures. These new analyses better constrain the length, geometry and connectedness of different fault strands, and search for evidence of fault activity throughout the region, including with near-surface geophysical investigations of two fault strands. We use geochemistry and geochronology to better determine the ages of geologic units for developing estimates of slip rate. The Doty fault zone (DFZ) is 72 km long, moderately north-dipping (~60°) and consists of a western portion comprising a consolidated zone with fewer fault strands, and an eastern portion with numerous fault strands in a south-stepping geom­etry. Both portions show similar reverse-sense offset of basement rocks (~2.5 km since 47.6 Ma, of which ~0.6 km was after 16.4 Ma), indicating long-term reverse slip rates at ~0.05 mm/yr. The western end of the DFZ connects with the newly-named, north-striking Brooklyn fault zone (BFZ) that includes shallowly-east-dipping (~35°) reverse faults and it is likely these faults are kinemat­ically linked. From this link, we infer a ~N60°E shortening vector that appears relatively consistent in direction and magnitude since the late Eocene. This analysis suggests the DFZ has more left-lateral strike slip than reverse slip, in agreement with qualitative predictions for fault slip from GPS block models in the area. Our data suggest the eastern end of the DFZ likely dies out near and connects to the poorly-defined northern end of the Saint Helens seismic zone. Other faults in the study area that connect to the DFZ and BFZ show evidence for both strike-slip and reverse components of slip, therefore the Doty fault system as a whole is transpressive. The addition of the strike-slip component increases our estimate of the long-term slip rate on the DFZ to 0.08–0.09 mm/yr. Thorough attempts to characterize Quaternary to Holocene activity on the DFZ led to no conclusive evidence for recent fault rup­tures. We conclude that the DFZ and associated faults are likely active but slipping very slowly, leading to poor preservation potential for fault scarps or offset of Quaternary deposits in the region. Given that the estimated DFZ slip rate is an order of magnitude less than active faults to the north (Seattle fault is 0.9 mm/yr), southwestern Washington may be more resistive to deformation than other areas of western Washington.