Vertical axis rotations in the Las Vegas Valley Shear Zone, southern Nevada: Paleomagnetic constraints on kinematics and dynamics of block rotations

Abstract

Paleomagnetic study of vertical axis rotations at 23 localities along the right‐lateral Las Vegas Valley Shear Zone (LVVSZ) in southern Nevada indicates that clockwise rotations generally increase with proximity to the LVVSZ, reaching 100° at the closest localities. Rates of rotation determined at four localities range from l°/m.y. to 12°/m.y. The maximum characteristic size of rotating blocks is 2–4 km, considerably smaller than the dimensions of the zone of deformation associated with the shear zone (∼100 km in length, ∼15 km in half width). Thus the deformation does not accord with kinematic models of block rotations that assume uniform rotations within domains having dimensions of the order of the width of the deforming zone. Instead, the deformation appears to be quasi‐continuous when considered on a scale of > ∼10 km. We suggest that it is consistent with the deformation of a highly plastic layer of upper crustal material, mechanically decoupled from deeper parts of the crust, and that the across‐strike distribution of rotations is controlled by the rheological properties of the upper crust, the length of the shear zone, and the total amount of offset across the shear zone.