Abstract

The Basin and Range province at the latitude of Las Vegas, Nevada (approximately 36°N), is ideally suited for reconstructing Neogene extension owing to an abundance of structural markers, primarily Mesozoic thrust faults, developed within the generally conformable Cordilleran miogeocline. In map view, extension is heterogeneous and is divisible into two major extensional domains, the Las Vegas and Death Valley normal fault systems, that lie east and west (respectively) of a relatively unextended median block. We determined horizontal relative-motion vectors between pairs of reference points across the province, chosen so as to best allow geologic markers to constrain the relative motion of the pair during extension. We recognize three sequences of pairs, two in the Las Vegas system and one in the Death Valley system, that define an unbroken path across the entire province. The vectors along these paths sum to give 247 ± 56 km of net extension oriented N73° ± 12°W.

Timing considerations indicate that extension occurred principally during the past 15 m.y. Westward motion of the Sierra Nevada away from the Colorado Plateau occurred at a rate of 20-30 mm/yr in the interval 10-15 m.y. ago, but was no greater than 10 mm/yr over the past 5 m.y. Strike-slip faulting was an important component in the extending system and absorbed perhaps 40-50 km of north-south shortening of the region during extension, indicating a constrictional strain field for the crust as a whole. If one assumes no major rotations of the Sierra Nevada during Cenozoic extension, and about 100 km of pre-15-m.y.-ago extension in the central portion of the northern Great Basin, the crust in the Las Vegas region extended by a factor of 3-4, whereas the wider Great Basin region extended by only a factor of 2. This difference may explain the contrast in regional elevation between the two areas (the northern Great Basin is on average about 1,000 m higher) and the constrictional strain in the Las Vegas region. The more widely distributed extension to the north may not have kept pace with the larger extension to the south, such that the south lost gravitational potential more rapidly. Thus, comparatively buoyant northern Great Basin lithosphere was (and continues to be) forced down the potential gradient into the Las Vegas region. Resolved parallel to the northern San Andreas fault, our reconstruction accounts for 214 ± 48 km of right-lateral shear along the Pacific-North America transform plate boundary.

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