Evidence for distributed clockwise rotation of the crust in the northwestern United States from fault geometries and focal mechanisms

Abstract

Paleomagnetic and GPS data indicate that Washington and Oregon have rotated clockwise for the past 16 Myr. Late Cenozoic and Quaternary fault geometries, seismicity lineaments, and focal mechanisms provide evidence that this rotation is accommodated by north directed thrusting and right‐lateral strike‐slip faulting in Washington, and SW to W directed normal faulting and right‐lateral strike‐slip faulting to the east. Several curvilinear NW to NNW trending high‐angle strike‐slip faults and seismicity lineaments in Washington and NW Oregon define a geologic pole (117.7°W, 47.9°N) of rotation relative to North America. Many faults and focal mechanisms throughout northwestern U.S. and southwestern British Columbia have orientations consistent with this geologic pole as do GPS surface velocities corrected for elastic Cascadia subduction zone coupling. Large Quaternary normal faults radial to the geologic pole, which appear to accommodate crustal rotation via crustal extension, are widespread and can be found along the Lewis and Clark zone in Montana, within the Centennial fault system north of the Snake River Plain in Idaho and Montana, to the west of the Wasatch Front in Utah, and within the northern Basin and Range in Oregon and Nevada. Distributed strike‐slip faults are most prominent in western Washington and Oregon and may serve to transfer slip between faults throughout the northwestern U.S.

Local Source Vp and Vs Tomography in the Mount St. Helens Region With the iMUSH Broadband Array

“…Many of the earthquakes in the SHZ show focal mechanisms consistent with right‐lateral strike slip motion on faults quasi‐parallel to the epicentral trend (Moran et al, ; Weaver et al, ), whereas the WRSZ focal mechanisms show faults oblique to the epicentral trend (Moran et al, ; Stanley et al, ). The orientation and earthquake focal mechanisms of the SHZ are consistent with the clockwise rotation of the Oregon crustal block caused by the northwest movement of the Pacific plate (Brocher et al, ; Wells et al, ; Wells & Simpson, ). The SHZ could possibly represent a localized terrane boundary related to the accretion of Siletzia and, as a result, could act as a zone of weakness in the crust allowing magma to ascend and erupt at MSH (Moran et al, ; Weaver et al, ).…”

“…The orientation and earthquake focal mechanisms of the SHZ are consistent with the clockwise rotation of the Oregon crustal block caused by the northwest movement of the Pacific plate (Brocher et al, 2017;Wells et al, 1998;Wells & Simpson, 2001). The SHZ could possibly represent a localized terrane boundary related to the accretion of Siletzia and, as a result, could act as a zone of weakness in the crust allowing magma to ascend and erupt at MSH (Moran et al, 1999;Weaver et al, 1987).…”

The Story of a Yakima Fold and How It Informs Late Neogene and Quaternary Backarc Deformation in the Cascadia Subduction Zone, Manastash Anticline, Washington, USA

“…The relatively obvious topographic prominence of the Yakima folds has focused the majority of previous regional work. However, Brocher et al (), in a recent analysis of seismicity and reconciliation with geodetic data, also highlight the importance of less obvious strike‐slip faults and how these faults sets may accommodate the westward increase in rotation rate and transfer strain between dip‐slip structures. Our identification of a more subtle north‐to‐northeast striking fault set near Manastash Ridge supports the argument that multiple fault sets must exist to accommodate the observed strains from geodetic observations as well from geologic mapping.…”

“…Aside from the Wallula fault (Brocher et al, ; Sherrod et al, ), strike‐slip features are not easily identifiable in the Yakima fold province. The subtle nature of the north‐northeast striking fault set in the Manastash area may indicate that strike slip is broadly distributed over the Yakima folds province region.…”

“…As noted above, how strike‐slip versus dip‐slip motion is seismically expressed in central Washington remains unclear. Several studies indicate that large magnitude earthquakes are possible within the region, both on reverse faults (Casale & Pratt, ; West et al, ) and along right‐lateral faults (Brocher et al, ). However, whether earthquakes on the distributed network of small faults is possible and, if so, constitute background seismicity versus on‐fault seismicity is debatable and bears directly on how Probabilistic Seismic Hazard Assessment models are constructed.…”