SEISMOGENIC DEFORMATION BETWEEN THE SIERRAN MICROPLATE AND OREGON COAST BLOCK, CALIFORNIA, AND THE NORTHERN TERMINATION OF THE WALKER LANE BELT

The Sierran microplate, which comprises much of central California, is a northwest-translating block entrained in distributed motion east of the Pacific plate. To the north, the Oregon Coast block (OCB) moves northward in the hanging wall of the Cascadia subduction zone, above the obliquely converging Juan de Fuca plate. The motions of the Sierran and OCB microplates relative to North America (NA) are described by distinct Euler poles, and the western US velocity field reveals that that these motions extend well east into the Walker Lane belt (WLB) and northern Basin and Range of California, Nevada and Oregon. Geodetic data locate the boundary between the Sierran microplate and OCB in northern California near the southern end of the Cascadia subduction zone. Analysis of regional seismicity indicates that distributed deformation occurs in a NE-trending transition zone between the nominally rigid microplates and is characterized by approximately east-west-directed dextral shear, consistent with differential Sierran-OCB motion. In addition, dextral motion likely steps west from the northern WLB to the subduction zone, driving oblique shortening in the northern Sierran microplate. Because the Sierran-NA and OCB-NA motions are locally identical at the transition, small circles about their respective Euler poles must be tangent there as well. This geometric requirement accurately predicts the location and orientation of the Sierran-OCB transition zone, as well as the northern boundary of the Walker Lane belt and the transition from distributed NW dextral shear east of the Pacific plate to CW microplate rotation above the Juan de Fuca plate. These boundaries also are tied to the position of the Mendocino triple junction on the west, which lies at the northwestern corner of the Sierran microplate. The triple junction is migrating to the northwest with the velocity of the Pacific plate, which is about four times that of the Sierran microplate. Thus the Sierran-OCB transition and the northern end of the Walker Lane belt are moving to the north relative to these two microplates. These relationships imply rapidly migrating and changing patterns of late Cenozoic deformation in northern California as the plate boundary to the west evolves.