LARGE-MAGNITUDE, SYNEXTENSIONAL, SHALLOW STRAINS CAUSED BY SOUTHERLY DIRECTED CRUSTAL FLOW BETWEEN CEDAR CITY, UT AND LAS VEGAS, NV

Asthenospheric-style lateral flow of mid-crustal or deeper rocks has long been recognized as an integral part of the extensional process in highly extended parts of the Basin and Range. This flow is commonly viewed as a consequence of extension. Neogene synextensional north-south shortening ranging to tens of kilometers is common in the region between Cedar City and Las Vegas, as revealed in a spectacular array of structures that include: (1) north-south occlusion in the wake of large-magnitude westerly escape of a wedge-shaped crustal block that includes Las Vegas Valley; (2) crustal-scale east-west folds that are coupled to strike-slip faults in the area around Mesquite, NV; and (3) large-magnitude counterclockwise block rotation in a huge area between Cedar City and Caliente, NV. Each of these large-magnitude strains, and a host of smaller ones, is best interpreted as reflecting near-surface response to south- or southwest-directed crustal flow as a primary tectonic component superposed on the province-scale extensional strain. Lateral flow gradients (velocity gradients) and associated laterally variable tractional forces applied to the base of brittle blocks across decoupling zones are probably important in shaping the details of the deformed areas. Major depressions and uplifts that are transverse to the extension direction may reflect local lateral material withdrawal and influx respectively, imparting a waveform pattern to the upper surface of the flowing medium.