EVOLUTION OF THE WESTERN U.S. WALKER LANE AND EAST CALIFORNIA SHEAR ZONE: INSIGHTS FROM GEODYNAMIC MODELING

Cenozoic deformation in the central western United States Cordillera involves a transition from broadly distributed Miocene extension in the Great Basin to more focused post-Miocene transtensional deformation in the Walker Lane and East California Shear Zone (WLB/ECSZ). Inception of major strike-slip deformation in the late Miocene in the WLB/ECSZ approximately coincided with the onset of rifting in the Gulf of Baja, leading to the suggestion that this represents an eastward shift of the Pacific-North America (PAC/NAM) plate boundary. Geodynamic models suggest that this shift in the plate boundary was precipitated by strain weakening of the lithosphere east of the Sierra Nevada associated with Miocene westward migration of Basin and Range extension, and was strongly influenced by Mesozoic and older lithosphere-scale structural fabrics. The thick crust of the middle Cenozoic Sevier orogenic plateau created a broad weak zone in the Great Basin lithosphere. Widely distributed extension in this region resulted in relatively low rates of crustal thinning during most of the Miocene, promoting cooling and strengthening of the lithosphere that forced deformation to migrate outward from the central Great Basin with time. This “wide rift” mode of deformation was favored until extensional deformation reached the western edge of the North American Paleozoic continental margin. At this time, extension became confined to a relatively narrow region between the newly strengthened Great Basin and the relatively strong Sierra Nevada block. Subsequent extension in this narrow region resulted in rapid crustal thinning that inhibited thermal strengthening of the lithosphere and forced extensional forces to be distributed over an increasingly thinner lithosphere. This favors a “narrow rift” mode of deformation that progressively decreases the net strength of the lithosphere. By the end of the Miocene, the WLB/ECSZ lithosphere was sufficiently weak to capture a portion of the PAC/NAM relative plate motion. Current data are insufficient to determine if this represents incipient formation of a new plate boundary, long-term partitioning of plate boundary deformation between the San Andreas and WLB/ECSZ systems, or a transient deformation state that will continue to evolve.