VERTICAL AXIS ROTATIONS IN THE LAS-VEGAS VALLEY SHEAR ZONE, SOUTHERN NEVADA - PALEOMAGNETIC CONSTRAINTS ON KINEMATICS AND DYNAMICS OF BLOCKROTATIONS

Paleomagnetic study of vertical axis rotations at 23 localities along the right-lateral Las Vegas Valley Shear Zone (LVVSZ) in southern Neva da indicates that clockwise rotations generally increase with proximit y to the LVVSZ, reaching 100-degrees at the closest localities. Rates of rotation determined at four localities range from 1-degrees/m.y. to 12-degrees/m.y. The maximum characteristic size of rotating blocks is 2-4 km, considerably smaller than the dimensions of the zone of defor mation associated with the shear zone (approximately 100 km in length, approximately 15 km in half width). Thus the deformation does not acc ord with kinematic models of block rotations that assume uniform rotat ions within domains having dimensions of the order of the width of the deforming zone. Instead, the deformation appears to be quasi-continuo us when considered on a scale of > approximately 10 km. We suggest tha t it is consistent with the deformation of a highly plastic layer of u pper crustal material, mechanically decoupled from deeper parts of the crust, and that the across-strike distribution of rotations is contro lled by the rheological properties of the upper crust, the length of t he shear zone, and the total amount of offset across the shear zone.