CLOCKWISE ROTATIONAL DEFORMATION ALONG THE EASTERN LAS VEGAS VALLEY SHEAR ZONE, LAKE MEAD, NEVADA

The Las Vegas Valley shear zone is a regional dextral strike-slip fault active during Miocene extension; its eastern end runs through the western Lake Mead domain of the central Basin and Range. This study focuses on the style and timing of deformation in the Gale Hills through detailed structure and lithofacies mapping (1:5000) north of the Las Vegas Valley shear zone, coupled with paleomagnetic analysis of the Miocene-age Thumb Member of the Horse Spring Formation. This area has experienced as much as 100° of clockwise, vertical-axis block rotation (Sonder et al., 1994). This study views the structural evolution of the Gale Hills not solely as discrete cross-cutting events but as a temporal and spatial continuum of rotational deformation related to strike-slip faulting. An analysis of structures in the Gale Hills results in a new model of progressive clockwise rotation and faulting along the Las Vegas Valley shear zone. This model is consistent with paleomagnetically-derived estimates of rotation and accounts for all major faulting north of the shear zone. Deformation related to the Las Vegas Valley shear zone reactivated older normal faults into left-lateral strike-slip faults as Riedel prime (R’) shears. The model is primarily based on clockwise rotation of domains between oblique left-lateral faults spaced every 1 – 1.5 km. The faults curve progressively west toward the Las Vegas Valley shear zone and terminate into major folds or areas of complex deformation. The block model style of rotational deformation dominates in the central to western Gale Hills. In the eastern Gale Hills, there is a more complex interplay of modest block rotations and complex folding related to the termination of the Las Vegas Valley shear zone. The magnitude and style of structural accommodation of vertical-axis rotations in the Gale Hills changes along, and towards, the Las Vegas Valley shear zone. The magnitude of clockwise vertical-axis rotations increases to the west along the shear zone from 20° to 90°, and decreases to the north. Vertical-axis rotations are accommodated via mechanisms of block rotations similar to those proposed by Nelson and Jones (1986); a combination of large-block rotations away from the shear zone and small-block continuum rotations (10’s m to a few 100 m scale) near the shear zone is suggested for the Gale Hills.