THE REAL EASTERN CALIFORNIA SHEAR ZONE
Recent U-Pb dating of fault-zone opal in the active western ECSZ, U-Pb dating of fault-zone calcite in the mostly inactive eastern ECSZ, and stratigraphic studies in the southern Blythe basin all show that faults related to NW-directed shear were active across a broader ECSZ during late Miocene time. This suggests that distributed dextral plate boundary strain has localized into a narrower zone through time, now mostly focused into the western half of the ECSZ. This contrasts with a model that relates inactivity of strike-slip faults in the east to a conveyor belt that moves brittle faults eastward and out from above a fixed-width ductile shear zone.
Although the active ECSZ is a ~100 km-wide zone of active deformation, the real ECSZ initiated in late Miocene time (ca. 10 Ma) and was originally a >200 km-wide zone of distributed deformation from the San Andreas Fault to the CA-NV border and southwestern AZ. Late Miocene development of the ECSZ occurred in unison with that of the Walker Lane to the north and Gulf of California shear zone to the south, forming a >2,000 km-long, kinematically-linked belt of distributed dextral shear related to the evolving Pacific-North America plate boundary.