Abstract
The Sierran microplate is a northwest-translating block entrained in distributed motion east of the Pacific plate. To the north, the Oregon coast block (OCB) moves northward within the hanging wall of the Cascadia subduction zone, above the obliquely converging Juan de Fuca plate. Analysis of GPS velocity data indicates that relative motion between the rigid Sierran and OCB microplates is characterized by several millimeters per year of dextral shear directed ∼N70°W, which is distinct from and counterclockwise to macroscopic dextral shear in the Walker Lane east of the Sierran microplate. We present a new analysis of focal mechanisms from small earthquakes in an 80-km-wide zone that spans the geodetically defined Sierran-OCB boundary to evaluate patterns of distributed deformation. We find that the direction of macroscopic dextral shear in this region is parallel to Sierran-OCB motion derived from GPS data. The seismogenic deformation is consistent with postulated dextral shear within an incompletely studied west-northwest–trending zone of faults and lineaments that traverses the northern Sierra Nevada; the faults and lineaments terminate westward against Quaternary folds in the northern Sacramento Valley. Active deformation at the Sierran-OCB boundary accommodates the relative motion of the bounding microplates and probably does not represent discrete transfer of Walker Lane motion to the Cascadia subduction zone in a restraining left step across the northern Sierra and Sacramento Valley.
Highlights
Large-scale deformation in California and the Pacific Northwest (USA)adjacent to the oceanic plate boundaries is dominated by microplate translation and rotation (Fig. 1)
The motion of the Sierra Nevada–Central Valley (Sierran) microplate relative to stable North America (S-NA) is described by counterclockwise rotation about an Euler pole located off the coast of southwestern California (Argus and Gordon, 1991, 2001), whereas Oregon coast block (OCB)–North American motion (OCB-NA) is characterized by clockwise rotation about an Euler pole near the Oregon-Idaho border at lat ~45°N (McCaffrey et al, 2013) (Fig. 1)
The heterogeneous deformation in the region centered on the northern Sacramento Valley and surrounding parts of the Sierra Nevada and Klamath Mountains generally is characterized by counterclockwise rotation of the principal strains relative to the Walker Lane and backarc region east of Mount Shasta (Fig. 3)
Summary
Large-scale deformation in California and the Pacific Northwest (USA)adjacent to the oceanic plate boundaries is dominated by microplate translation and rotation (Fig. 1). The Sierra Nevada–Central Valley (Sierran) microplate, which comprises much of central California, is bounded on the west by the Pacific plate and moves ~11–13 mm/yr northwest with respect to stable North America (Argus and Gordon, 1991, 2001). The Sierran and OCB microplates move approximately subparallel to tractions across their western boundaries imparted by the oceanic plates (McCaffrey et al, 2013; Fig. 2). The motion of the Sierran microplate relative to stable North America (S-NA) is described by counterclockwise rotation about an Euler pole located off the coast of southwestern California (Argus and Gordon, 1991, 2001), whereas OCB–North American motion (OCB-NA) is characterized by clockwise rotation about an Euler pole near the Oregon-Idaho border at lat ~45°N (McCaffrey et al, 2013) (Fig. 1). The Walker Lane and northern Basin and Range Province are east of these microplates and accommodate additional distributed deformation relative to stable North America (Wells and Simpson, 2001; Hammond and Thatcher, 2007; Fig. 1)
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