THE NORTHERN CALIFORNIA SHEAR ZONE—MISSING LINK IN THE PACIFIC-NORTH AMERICAN PLATE TRANSFORM MARGIN?

Within the distributed P-NA plate transform margin and translating 10-11 mm/yr to the NW through a CCW arc relative to interior NA, is the Sierra Nevada-Great Valley (SN-GV) microplate. Motion of the microplate is accommodated by dextral shear across the Walker Lane shear zone (WL), extension in the Basin and Range, crustal shortening across the Coast Range boundary zone and, as previously suggested, by transpressional shear along unknown structures north of the microplate (e.g., Unruh et al., 2003; Williams et al., 2006). The northern WL accommodates 6-8 mm/yr of dextral shear at the latitude of Mohawk Valley, but there is little if any shear strain measured further north in the Mt. Shasta-Medicine Lake region (e.g., Hammond and Thatcher, 2007) particularly after removing CW rotation of the Oregon block (Poland et al., 2006). This northern limit of significant shear strain, coupled with microplate motion recorded near the southern end of the Cascadia subduction zone (Williams et al., 2006), seemingly requires a zone of deformation between about latitude 40^o-41^oN to accommodate westward translation of the microplate and transfer ≤6-8 mm/yr of regional shear strain.

More than two decades of seismic hazard research reveals a distributed zone of Quaternary faults and fold-fault structures, the “Northern California shear zone”, that extends >225 km westward from the northern WL and forms a large-scale left stepover across much of northern California. The structurally integrated southern extent of the shear zone splays W-NW from the Mohawk Valley fault zone, delineating the northern edge of the microplate and forming a 90 km-wide restraining stepover that drives crustal shortening in the Inks Creek fold belt. The arcuate northern extent splays from or near the Honey Lake fault zone and cross-cuts the Cascadia volcanic arc as a fold-and-thrust belt, south of Mt. Shasta. The belt widens westward and appears to define the structural boundary between the Oregon Coast block and the northern Central Valley. Thus, the structurally complex Northern California shear zone apparently accommodates westward translation of the northern SN-GV microplate, and thereby transfers ≤6-8 mm/yr of WL shear strain across the Central Valley-Klamath Mountains region to the primary plate margin in northwestern California.