What Causes the Development of the Eastern California Shear Zone?

The San Andreas Fault (SAF) system is the transform plate boundary between the Pacific and the North American plates, yet up to 25% of the relative plate motion is now accommodated by the Eastern California Shear Zone (ECSZ), which includes both the NW-trending fault zones in the Mojave Desert and the Walker Lane belt. Some workers have suspected that the ECSZ may eventually replace the SAF as the main plate boundary. The causes that have been suggested for the initiation of the ECSZ include the opening of the Gulf of California between 12 to 5 Ma, a kinematic change of the Great Valley-Sierra Nevada block around ~8 Ma, the development of the Big Bend of the SAF, and westward encroaching of the Basin and Range province. Here we present results of geodynamic exploration of these factors using a regional-scale 3D visco-elasto- plastic finite element model. The model includes an elastoplastic schizosphere on top of a viscoplastic plastosphere. Tectonic loading is simulated by imposing the relative plate motion as a far-field boundary condition; long-term strain partitioning due to both fault slips and plastic deformation outside fault zones are considered. Our preliminary results suggest that development of the Big Bend is likely the primary cause for the formation of the ECSZ. The Big Bend hampers the relative plate motion on the SAF and results in strain concentration along the ECSZ, which is further enhanced by rheologic contrast between the hot and weak Basin and Range and the stiff Great Valley-Sierra Nevada block. Development of the restraining bend over the San Bernardino Mountain segment of the SAF may also contributed to strain localization along the ECSZ. Development of the ECSZ is shown to significantly reduce long-term slip rates on the SAF and alter the stress field in California. The modeling results provide useful insights into long-term fault evolution in the SAF plate boundary zone as it continuously seeks for the most efficient way to accommodate the relative plate motion.