Paleomagnetism and crustal rotations along a shear zone, Las Vegas Range, southern Nevada

Abstract

In southern Nevada, Paleozoic isopachs, Mesozoic structures, and mountain ranges trend north–south, but as they approach the right‐lateral, west‐northwest‐trending Las Vegas Valley Shear Zone (LVVSZ), they are bent clockwise up to 90° in oroflexes 30 km long. In the Las Vegas Range the declination of the characteristic magnetization of the Cambrian Bonanza King Formation has likewise been rotated clockwise more than 50°. For samples from eight sites (13–23 samples per site) in the Las Vegas Range and in its extension south of the LVVSZ, stepwise thermal demagnetization to 600°C removed recent and Tertiary overprints, revealing a nearly horizontal, late Paleozoic viscous partial thermoremanent magnetization (VPTRM) which predates both the Tertiary oroflexural bending and the Sevier thrusting in the area. The declination of the characteristic magnetization (after correcting for the dip of bedding) varies from 133° ± 2° at a site 24 km north of the shear zone to 196° ± 4° 13 km north of it and 204° ± 4° 11 km south of the shear zone. This parallels a similar change in the structural trend. The paleomagnetic data indicate that the Las Vegas Range is broken into rather small blocks (<5 km) which rotate in response to more continuous deformation at depth. This deformation of the upper crust can be modelled using a thin non‐Newtonian viscous plate model, which when applied to the Las Vegas Valley Shear Zone predicts rotations similar to those seen in the Las Vegas Range.