THE WALKER LANE – EASTERN CALIFORNIA SHEAR ZONE, POTENTIAL HEIR TO THE SAN ANDREAS FAULT: OUTSTANDING REGIONAL-SCALE RESEARCH QUESTIONS AND LESSONS LEARNED FROM GEODESY, HISTORICAL SEISMICITY, AND LONG-TERM EVOLUTION

The Ridgecrest EQs were a reminder that the Walker Lane-eastern California shear zone (WL) is a fundamental part of the North American-Pacific plate boundary. Since ~30 Ma, western North America has evolved from an Andean margin to a dextral transform as marked by arc retreat, orogenic collapse, and inland steps of the San Andreas fault system. Inception of the WL in the late Miocene coincided with a change in relative plate motions (~N60°W to N37°W), east shift of the south part of the transform to the Gulf of California (GOC), and development of the Big Bend of the San Andreas. Dextral shear was favored in the WL, as it paralleled the new plate motion while aligning with the GOC and avoiding both the convergent bottleneck of the Big Bend and relatively rigid Sierra Nevada block. The WL currently accommodates ~20% of the dextral plate motion (~10 mm/yr). In contrast to the continuous 1,100-km-long San Andreas fault, the WL is marked by shorter discontinuous faults. Progressive NW-younging in the onset of deformation (~10 to <4 Ma) along with a general decrease in length and offset on dextral faults indicate NW propagation of the WL. The WL ends near the south end of the Cascade arc directly inboard of the Mendocino triple junction (MTJ). NW-ward termination of the WL appears to be accommodated by transfer of dextral shear to WNW-directed extension, as evidenced by dextral faults terminating in arrays of NNE normal faults. Continued N-ward migration of the MTJ and NW-propagation of the WL suggest that they will intersect off southern Oregon in ~7-8 m.y. The primary plate boundary may then step inland to the WL, similar to the late Miocene shift to the GOC.

Thus, the WL provides a superb laboratory for analyzing the initiation and progressive development of a major transform fault. However, outstanding questions remain, including whether WL dextral shear is the primary driver of adjacent Basin-Range extension, and whether major WL earthquakes enhance the likelihood for major normal faulting events in the nearby Basin-Range. Addressing these and other questions requires deciphering the kinematic evolution of disparate structural domains, developing 3-D lithospheric models of deformation and stress transfer, and elucidating links between geodetic strain rates, historic seismicity, Quaternary deformation, and longer-term development.