Inception of the eastern California shear zone and evolution of the Pacific‐North American plate boundary: From kinematics to geodynamics

Abstract

The San Andreas Fault (SAF) is the transform 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). Here we investigate the inception of the ECSZ and its geodynamic interactions with the SAF using a 3‐D viscoelastoplastic finite element model. For a given fault configuration of the plate boundary zone, the model simulates long‐term slip on the faults and plastic strain outside the faults. Our results show that the formation of the Big Bend of the SAF around 5–12 Ma impeded fault slip and localized strain along the ECSZ, causing its inception. Development of the ECSZ was further enhanced by the activation of the Garlock Fault (GF) and lithospheric weakening caused by the encroachment of the Basin and Range extension. Similarly, the San Jacinto Fault (SJF) in southern California developed along a belt of localized strain, which resulted from the formation of the restraining bend along the San Bernardino Mountains segment of the SAF ∼2 Myr ago. Once activated, the SJF reduced slip on both the southern SAF and the ECSZ across the Mojave Desert. These results indicate causative relationship between the SAF and the ECSZ. The inception of the ECSZ and other young faults is the consequence of the evolving SAF plate boundary zone that continuously adjusts itself to accommodate the relative plate motion.