THE 10-MA INCEPTION OF THE EASTERN CALIFORNIA SHEAR ZONE—NEW AGE CONTROL AND IMPLICATIONS

Recent dating of opal that fills secondary strands of faults of the eastern California shear zone (ECSZ) yielded several ages older than 5 Ma. Faults of the east-striking sinistral Cave Mountain fault zone are older than ~10 Ma as indicated by sheared opal that yielded U-Pb ages, using ion microprobe methods, of 11.3 ± 1.2, 10.5 ± 0.5, and 10.0 ± 0.3 Ma (uncertainties are 2 S.E.). Southwest of the Cave Mountain fault, somewhat younger fault-filling opal near the northern termination of the northwest-striking dextral Camp Rock fault was dated at 7.3 ± 0.4, 6.8 ± 0.4, and 6.0 ± 0.6 Ma. These ages support previous studies that argued for inception of the ECSZ during the Late Miocene. This age is further supported by pre-7 Ma initiation of faults of the Eastern Transverse Ranges inferred from the ages of associated basaltic lavas and by paleomagnetically determined rotation rates of 4°/M.y. for playa sediment in fault-bounded blocks of the northeast Mojave sinistral domain of the ECSZ. Geometric relations among rotated blocks and adjacent panels of dextral faults require that movement on all faults of the active ECSZ began in the Miocene, with the possible exception of the Helendale and Lenwood faults. This Miocene-Pliocene ancestral ECSZ probably spanned the entire width of California east of the San Andreas fault (SAF) based on the distribution of inactive strike-slip faults from Antelope Valley to Ivanpah Valley that have histories of early Pleistocene and older deformation. This longer-term view of strike-slip fault activity inboard of the SAF provides a template for evaluating primary questions such as: 1) its coevolution with precursor extension and opening of the Gulf of California and development of the SAF as part of Pacific-North American plate boundary deformation; 2) whether faults thought to have initiated after the Miocene represent reorganization events and, if so, what were the drivers of fault reorganization; 3) long-term slip rates and their variation with time; 4) kinematic compatibility with the apparently younger faults of the Walker Lane; and 5) the mechanisms of deep lithospheric forces in driving deformation observed across a wide expanse.