No abstract

No abstract

When considered within a regional, tectono-sequence stratigraphic framework, Cenozoic strata in the Cottonwood Mountains provide a relatively complete record of major extensional tectonism within the Death Valley region, California and Nevada. Using detailed mapping, measured stratigraphic sections, and 40 Ar/ 39 Ar geochronology on intercalated volcanic units, we erect a formal stratigraphy for Oligocene through recent strata in the Cottonwoods that serves as a reference section for much of the northern Death Valley area. We assign Tertiary strata throughout the Cottonwood, Panamint, Grapevine, and Funeral Mountains to time-transgressive tectonosequences of early, syn-, and late extensional age relative to development of the extended terrane. Strata of the early extensional Grapevine Sequence, comprising the Titus Canyon, Ubehebe, and Panuga Formations, were deposited prior to the onset of large-magnitude extension at about 15 Ma in the Death Valley region. This early phase of relative tectonic quiescence was interrupted twice, at around 36 and 25 Ma, by local normal faulting that accommodated limited regional extension. Strata of the synextensional Amargosa Sequence, including the Leadfield, Bat Mountain, Artist Drive, and Navadu Formations, were deposited within the Bullfrog, Furnace Creek, and Nova sedimentary basins that developed as range-scale crustal fragments progressively separated during regional large-magnitude extension. Strata of the late extensional sequence, including the Furnace Creek, Funeral, and Nova Formations, overlapped synextensional strata after the local cessation of significant extensional tectonism. The time-transgressive record of Amargosa deposition indicates an eastto-west migration in the locus of active extension, in accord with previous observations documenting a progressive westward shift in the cessation of major extension. Unconformities within the Amargosa Sequence document stratal tilting within the Cottonwood Mountains rollover system, a large fault-bend fold above a listric detachment. Such tilting, which is coupled to active extension, predates a surprisingly large fraction of deposition in the Nova basin, a graben that resulted from separation of the Cottonwood and Panamint Mountains during formation of the rollover. Thus, a significant time lag is apparent between major extensional events and infilling of concomitant sedimentary basins by locally eroded material. Recognition of such Snow, J. K.

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.