LONG-TERM EVOLUTION OF THE EASTERN CALIFORNIA SHEAR ZONE, 10 MA TO PRESENT

Recently published U-Pb ages for sheared opal in eastern California shear zone (ECSZ) faults of the central Mojave Desert indicate fault inception ~12-10 Ma. These ages are in accord with previous estimates based on block rotation rates of 4.0 ± 1.3 degrees/m.y. and total rotations of sinistral fault blocks of ~56 ± 6 degrees in the Fort Irwin area. Cenozoic rock units older than ~13 Ma are offset along faults as much as Mesozoic rock units, further supporting a middle Miocene inception of significant ECSZ deformation. Units such as the Barstow Formation (20 to 14 Ma) accumulated in broad shallow lake basins, topography unlike the rugged, fault-controlled topography associated with the modern ECSZ.

Average slip rates over 10-12 Myr, calculated from observed total offsets of 68 km across the 120-km-wide active ECSZ, compare well with those determined from offsets of dated Quaternary deposits, indicating that the Quaternary average rate of ~6.2 ± 1.3 mm/yr of dextral shear is representative of the long-term Mojave ECSZ. Periods of greater and lesser shear rates may have occurred, however. Simple geometric arguments for kinematic connections between sinistral and dextral domains of the ECSZ demonstrate that the entire zone has been active since 12-10 Ma. Furthermore, early Quaternary and Pliocene strike-slip faults, presently inactive, in the western and eastern Mojave Desert demonstrate that the shear zone was much wider in its early manifestation. It is likely that the shear zone has narrowed with maturity of the plate boundary, much as the broad transtensional belt in northwest Mexico narrowed into the Gulf of California shear zone in late Miocene time (ca. 8-6 Ma). Such a narrowing requires an increase in shear with time on the faults of the ECSZ unless other elements of the plate boundary, such as the ancestral San Andreas fault, accommodated more plate boundary shear with time.