Paper No. 16
Presentation Time: 12:45 PM


NOURSE, Jonathan A., Geological Sciences Department, California State Polytechnic University, 3801 West Temple Ave, Pomona, CA 91768, SILVER, Leon T., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 and MCLARTY, Andrew, Geological Sciences Department, California State Polytechnic University, 3801 W. Temple Avenue, Pomona, CA 91768,

The Rand Mountains preserve a key record of Cordilleran orogenesis south of the Garlock fault zone where once coherent lithologic-geochemical zones in the Cretaceous Peninsular Ranges-Sierra Nevada batholith are profoundly disturbed. A regional antiform reveals a window through the Rand thrust complex in which three distinct units of the batholith (Plate 2 garnet-amphibolite facies diorite to granite orthogneiss and marble, Plate 3 Atolia Granodiorite and Plate 4 biotite alkali granite) are stacked on Plate 1 oceanic Rand Schist via low angle faults 1, 2 and 3). Initial juxtaposition of Plates 1, 2 and 3 involved 30+ km crustal shortening after ~80 Ma. Subsequent extension accompanied normal displacement of Plate 4 along Fault 3 and local reactivation of deeper Faults 1 and 2. A system of NE striking oblique sinistral normal faults records an important phase of late Cenozoic transtension.

We present a map compilation of geologic and structural data acquired since 1983; including observations from the western Rand Mountains, inaccessible for the past two decades due to tortoise preserve status. Interpretation of the movement history is complicated by kinematic indicators with different lineation directions and contradictory shear senses at various structural levels. Another factor is likelihood that the eastern California shear zone has rotated all or parts of the area up to 90 degrees CW. Macro and microstructural analyses of greenschist to lower amphibolite grade mylonites yield the following pattern of transport directions (present day geography), from deep to shallow: 1) NW and N-vergent shear along Fault 1 and uppermost Rand Schist, 2) penetrative top-to-SW shear fabrics within thicker portions of Plate 2, overprinted by asymmetric folds with northerly vergence, 3) localized NE-vergent shear zones within Plate 3, and 4) NNE directed transport of Plate 4 across Plates 1-3 along Fault 3. These movement directions require adjustment for vertical and horizontal axis rotations to fully understand true tectonic transport geometry. Overall structural patterns and age relations are consistent with east over west imbrication of the batholith with Rand Schist during Laramide convergence, followed by Miocene extensional detachment, then left-lateral displacement accompanied by CW rotation that may be ongoing.