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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.

The structure of the northern Cottonwood Mountains, located in the Death Valley region of southeastern California, is dominated by a faulted, east-facing, monoclinal flexure developed in Paleozoic strata of the Cordilleran miogeocline. We interpret this flexure as a rollover or fault-bend fold above a listric normal fault, probably the north-ward continuation of the Tucki Mountain normal fault system exposed about 45 km south of the study area. Synorogenic Tertiary sediments in the Ubehebe basin, overlying the rollover at the northern end of the Cottonwood Mountains, record the inception of normal faulting and eastward tilting around a horizontal axis trending about N25°W. Paleozoic strata in the Dry Mountain block to the west are subhorizontal. Racetrack Valley, a north-trending topographic depression between the Dry Mountain block and the northern Cottonwood Mountains, is interpreted as a major graben that accommodates differential stratal rotations within the rollover. The juxtaposition of Mesozoic thrust plates by Tertiary normal faults has obscured the structural simplicity of the rollover. Structural and stratigraphic correlations indicate that strata exposed in the Dry Mountain block and northern Cottonwood Mountains are parts of a single Mesozoic thrust plate. This new evidence suggests that the Racetrack thrust of McAllister (1952) does not root in northern Racetrack Valley, contrary to previous interpretations. A previously unrecognized klippe and other parts of the Ubehebe thrust plate lie structurally above the northern Cottonwood Mountains. We suggest that normal faulting has downdropped a portion of the Ubehebe thrust plate into Racetrack Valley between strata of the Dry Mountain block and northern Cottonwood Mountains. Thus, the development of a major graben within the rollover has produced an apparent thrust relation along both sides of the Racetrack Valley block. We correlate the Racetrack Valley block and Ubehebe thrust plate with the Last Chance thrust plate of Stewart and others (1966). Sequentially deformed, palinspastic reconstructions of the rollover indicate that conjugate faults formed symmetrical grabens during development of the rollover. The grabens accommodated bending within the rollover and probably nucleated above areas of maximum curvature on the basal listric fault. Thus, the nucleation of progressively younger grabens during extension apparently migrated toward the breakaway within the hanging-wall block while remaining fixed relative to the bend in the basal fault plane. The pre-middle Pliocene(?) age and structural position of strata in the Ubehebe basin suggest that they are partially correlative with upper Miocene to lower Pliocene strata in the Nova basin, also located on the eastern margin of the Cottonwood Mountains structural block but adjacent to Tucki Mountain. The location of these basins in the hanging wall of the Tucki Mountain normal fault system suggests that they may be allochthonous parts of an earlier basin. Distinctive cobbles with no known local source are abundant in a relatively thin stratigraphic zone within the Ubehebe basin. We interpret these cobbles as reworked clasts derived from the Oligocene Titus Canyon Formation. This is consistent with structural and stratigraphic correlations that indicate about 70 km northwest-directed transport of the Cottonwood Mountains block from a position adjacent to the Funeral Mountains (Snow and Wernicke, 1988,1989; Snow, 1989; Snow and others, 1989).