New paleoseismic trenching indicates late Quaternary oblique right‐lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south‐to‐north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary oblique right‐lateral slip on west‐northwest‐striking forearc faults north of the Olympic Mountains and in the southern Strait of Georgia are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike‐slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right‐lateral slip and the published regional SHmax">SHmaxSHmax directions. Vertical separation and the formation of scarp‐derived colluvium along one fault also indicate a dip‐slip component. Radiocarbon charcoal dating within offset glacial till and scarp‐derived colluvium suggest a single surface rupturing earthquake at 9.4±3.4 ka">9.4±3.4 ka9.4±3.4 ka. The oblique right‐lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west‐northwest‐striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting.
|Title||Paleoseismic trenching reveals late quaternary kinematics of the Leech River Fault: Implications for forearc strain accumulation in Northern Cascadia|
|Authors||Nicolas Harrichhausen, Kristin D. Morell, Christine Regalla, Scott E. K. Bennett, Lucinda J. Leonard, Emerson M. Lynch, Edwin Nissen|
|Publication Subtype||Journal Article|
|Series Title||Bulletin of the Seismological Society of America|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Geology, Minerals, Energy, and Geophysics Science Center|
Pacific Northwest Geologic Mapping: Northern Pacific Border, Cascades and Columbia
Scott E.K. Bennett, Ph.D.
Pacific Northwest Geologic Mapping: Northern Pacific Border, Cascades and ColumbiaThe Pacific Northwest is an area created by active and complex geological processes. On its path to the Pacific Ocean, the Columbia River slices through a chain of active volcanoes located along the western margin of the U.S. in Washington, Oregon, and northern California. These volcanoes rest above the active Cascadia subduction zone, which is the boundary where the oceanic tectonic plate dives...
Scott E.K. Bennett, Ph.D.