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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.

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Unroofing of the Black Mountains, Death Valley, California, has resulted in the exposure of 1.7 Ga crystalline basement, late Precambrian amphibolite facies metasedimentary rocks, and a Tertiary magmatic complex. The 40Ar/39Ar cooling ages, obtained from samples collected across the entire length of the range (>55 km), combined with geobarometric results from synextensional intrusions, provide time-depth constraints on the Miocene intrusive history and extensional unroofing of the Black Mountains. Data from the southeastern Black Mountains and adjacent Greenwater Range suggest unroofing from shallow depths between 9 and 10 Ma. To the northwest in the crystalline core of the range, biotite plateau ages from ~ 13 to 6.8 Ma from rocks making up the Death Valley turtlebacks indicate a midcrustal residence (with temperatures >300øC) prior to extensional unroofing. Biotite 40Ar/39Ar ages from both Precambrian basement and Tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest, subparallel to the regional extension direction. Diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest. The cooling age pattern and geobarometric constraints in crystalline rocks of the Black Mountains suggest denudation of 10-15 km along a northwest directed detachment system, consistent with regional reconstructions of Tertiary extension and with unroofing of a northwest deepening crustal section. Mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment. The well-known Amargosa chaos and perhaps the Badwater turtleback are examples of this "splaying" process. Considering the current distance of the stmcturally deepest samples away from moderately to steeply east tilted Tertiary strata in the southeastern Black Mountains, these data indicate an average initial dip of the detachment system of the order of 20 ø , similar to that determined for detachment faults in west Paper number 92TC00211 0278-7407/92/92TC-0021510.00 central Arizona and southeastern California. Beginning with an initially listtic geometry, a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward. This pattern may be explained by a model where migration of footwall flexures occur below a scoop-shaped hanging wall block. One consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading. This process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust. INTRODUCTION Determining the timing, pattern, and amount of unroofing during orogeny is fundamental to understanding processes of lithospheric deformation. In the Basin and Range province, geologic studies using fluid inclusions [Parry and Bruhn, 1987] and balanced and reconst...