Is extension in Death Valley accommodated by thinning of the mantle lithosphere beneath the Sierra Nevada, California?

Abstract

While relatively rapid and large‐scale extension occurred in the Death Valley region of California during Neogene time, little or no shallow extension occurred in the adjacent Sierra Nevada. This contrast in tectonic history has often been extrapolated to include the entire lithosphere, but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the Sierra than under the Death Valley region. Upper mantle seismic velocities observed beneath the High Sierra are lower than those observed in other regions with comparable surface heat flow. This discrepancy could be resolved if the mantle lithosphere beneath the High Sierra had become warmer, presumably by tectonic thinning, in the last 10 m.y. Upper mantle seismic velocities, averaged topography, and Bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the High Sierra than beneath the California portion of the Basin and Range Province to the east. This suggests that extension of the crust near Death Valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the Sierra Nevada.The extension in the crust of the California Basin and Range Province and the thinning of the mantle lithosphere under the High Sierra appear to share the same bounds in time and space. The uplift of the High Sierra occurred over the past 9 m.y., which coincides with most of the extension that occurred in the California Basin and Range Province. Because the orientation of extension in the California Basin and Range Province is inferred to be approximately N60°W from geologic, geodetic, and in situ stress measurements, the northern and southern edges of the Death Valley extensional subprovince may extend N60°W from the inferred northern and southern limits of west dipping low‐angle normal faults of the Death Valley region. Pronounced changes in the averaged topography and Bouguer gravity anomaly across these two bounds both in the Basin and Range Province and in the Sierra Nevada support a connection between the tectonics of both regions. The geomorphic history of the southern Sierra suggests an up‐to‐the‐north warp of the Sierra across this southern bound during latest Cenozoic time. Hence extension near Death Valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the Sierra Nevada. This geometry requires.