The transect of the Basin and Range Province encompassing Death Valley, California has proven influential in the development of concepts in crustal extension. While most now agree that the magnitude of Miocene and younger extension in the region was large (Snow and Wernicke, 2000), both the amount of extension and the manner in which it was accommodated are still strongly debated. A particular challenge to regional syntheses is that constraints on timing are loosely defined, in some cases relying on decades-old K/Ar ages of volcanic rocks quoted in field guides but never properly published. Our objective is to develop a well-defined timing of fault activity in the Basin and Range Province in order to both test proposed mechanisms of extension and to quantify the amount of extension in the region more accurately.
Mechanisms of Extension
The Sheephead fault, a right-lateral southern boundary fault to the Central Death Valley Extended Zone, is part of a proposed rolling hinge model of extension for Death Valley. The rolling hinge model accommodates extension via a low-angle detachment fault system (Miller and Prave, 2002) whereby the upper crustal hinge of a master high-angle normal fault migrates, causing active segments of the fault to become inactive due to isostatic footwall response to uplift (Hayman et al., 2003). The model requires fault activity along the rolling hinge path to be laterally sequential in the direction of hinge migration; therefore the model can be tested by constraining the timing of faulting.
At Sheephead Mountain just north and in the hanging wall of the Sheephead fault, a kilometer-thick of Miocene volcanics are highly tilted from Sheephead fault activity, indicating that Sheephead fault was primarily active after these volcanics were deposited. Using 40Ar/39Ar geochronology to date the volcanic section at Sheephead Mountain, we determined that the upper age limit of the Shoshone Volcanics, the youngest unit, is 9.48 ± 0.1 Ma. Therefore Sheephead fault activity largely postdates 9.48 Ma. Faulting in the western Black Mountains, based on U-Pb dating of several key intrusive dikes, predates 10 Ma (Miller and Friedman, 2003) and therefore predates the timing of Sheephead fault. This conflicts with the laterally sequential timing required by the rolling hinge model.
Amount of Extension
The Eastern California shear zone (ECSZ) of the Mojave Desert and Death Valley regions has accommodated 9-23% of the relative plate motion between the Pacific and North American tectonic plates since the mid to late Miocene by dextral shear (Dokka and Travis, 1990). Traditionally, the Death Valley fault system has been recognized as the boundary of the ECSZ (Guest et al. 2007), leaving major right lateral faults east of the Death Valley fault system out of ECSZ slip budgets. Guest et al. (2007) argues for the extension of the ECSZ ~50 km to the east-northeast to encompass the Stateline fault system.
The right-lateral Sheephead fault, which lies between the Death Valley fault system and the Stateline fault system, has previously been left out of ECSZ slip budgets because the timing of fault activity was poorly constrained. Using the 40Ar/39Ar ages of the Sheephead Mountain volcanics and the lack of deformation in nearby Lake Tecopa sediments (Wright et al., 1991), we have been able to constrain the timing of activity on the Sheephead fault between 10.5-10.3 Ma and ~3 Ma or earlier. Bookshelf-style deformation and tentative piercing points, along with these new constraints on fault timing, suggests an average long-term slip rate of ~ 1.6-2.5 mm/yr for the Sheephead fault (Tremblay et al., 2009). This rate is comparable to that of other right-lateral faults in the region and supports the conclusion of Guest et al. (2007) that the ECSZ extends farther east than previously thought.
Although our 40Ar/39Ar geochronology at Sheephead Mountain calls the rolling hinge model of extension into question, further reconstruction of the regional timing of faulting is necessary to unravel the mechanism of Basin and Range extension. The Resting Spring Range, an east-tilted fault block of Neoproterozoic to Miocene age, located 35 km east of Death Valley, California, is another prime example of poorly constrained timing of faulting. We plan to map the volcanic rocks in the vicinity of Resting Spring pass and on the east flank of the southern Resting Spring Range in order to gauge the pattern of tilting and use 40Ar/39Ar geochronology to establish the timing and rate of tilting in the southern half of the range.
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