LATE MIOCENE TO RECENT EVOLUTION OF THE WALKER LANE: AN INCIPIENT TRANSFORM FAULT WITH KINEMATIC GROWING PAINS

In the late Miocene, the southern part of the North American-Pacific transform shifted east into the Gulf of California (~13-6 Ma), the Big Bend of the San Andreas developed, and plate motions changed from ~N60^oW to N37^oW (~8 Ma). Coincidentally (~11-6 Ma), dextral shear initiated in the Walker Lane and eastern California shear zone (WLSZ) from southern California to west-central Nevada and has since propagated NW-ward to NE California in concert with the N-ward propagation of the Mendocino triple junction. Plate-boundary dextral shear was favored in the western Great Basin because it paralleled the new plate motion, aligned more directly with the Gulf of California, and avoided the Big Bend bottleneck. Analogous to the Gulf of California, the WLSZ developed directly east of large coherent Mesozoic batholiths (Sierra Nevada vs. Baja), approximately along the axis of a retreating magmatic arc and cuts across the batholiths where weakened by the Cascade arc. Today, the WLSZ accommodates ~20% of the plate motion (~1 cm/yr).

In contrast to the Gulf of California, the WLSZ has had kinematic growing pains. It consists of many disparate domains, some composed of discontinuous NW-striking dextral fault arrays with negligible vertical-axis rotation (e.g. Pyramid Lake and Walker Lake domains) and others consisting of NE- to E-striking sinistral fault systems that accommodate significant clockwise rotation (e.g. Mina deflection and Carson domain). Within the broadly transtensional setting, individual strike-slip faults in both domain types commonly end in normal fault systems rather than link with other strike-slip faults. The length of individual strike-slip faults is generally <100 km and does not exceed ~250 km. Thus, the WLSZ lacks through-going strike-slip faults that cross and link domains. A resultant conundrum is how strain kinematically transfers between disparate domains. For example, the system of ENE- to E-striking sinistral faults accommodating up to 90^o of clockwise rotation in the Carson domain gives way abruptly to dextral faults of the Pyramid Lake and Walker Lake domains. Understanding these strain transfer mechanisms and how such boundaries are eventually breached by through-going dextral faults may be key to understanding how incipient strike-slip fault systems ultimately achieve transform status.