EVIDENCE FOR DISTRIBUTED CLOCKWISE ROTATION OF THE CRUST IN THE NORTHWESTERN UNITED STATES FROM FAULT GEOMETRIES AND EARTHQUAKE FOCAL MECHANISMS

Paleomagnetic and GPS data indicate that Washington and Oregon have rotated clockwise for at least 16 Myr. Neogene and Quaternary fault geometries, seismicity lineaments, and earthquake focal mechanisms provide evidence that in the west this rotation is accommodated by N-S shortening along thrust faults and NNW-trending strike-slip faults, and in the east by W- to SW-directed extension accommodated by normal faults and W- to SW-trending strike-slip faults. Curvilinear NW- to NNW-trending high-angle strike-slip faults and seismicity lineaments in western Washington and NW Oregon define an approximate Euler pole of rotation relative to North America near Spokane, Washington (117.7°W, 47.9°N). Many late Cenozoic faults and earthquake focal mechanisms throughout northwestern US and southwestern British Columbia have orientations consistent with this Euler pole. Large active normal faults radial to the Euler pole, that may accommodate crustal rotation via crustal extension, are widespread to the southeast, 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 may help transfer slip between thrust and normal faults throughout the northwestern US, but are most prominent in Washington and Oregon where they transfer slip between convergent structures. Potential drivers of the clockwise crustal rotation include westward rollback of the Juan de Fuca slab and a mega-gravity slide down the Yellowstone geoid high. The current surface velocity field may integrate the motion of rotating blocks and distributed deformation between and within them. Geoid highs to the west, representing the western end of the mega-gravity slide, may explain the transition between regions undergoing contraction and extension in Washington and Oregon.