J. Kent Snow

Large-magnitude Permian shortening and continental-margin tectonics in the southern Cordillera

The Last Chance allochthon, with displacement estimated to be 55 to 105 km and lateral extent possibly exceeding 300 km, was inferred to be Middle Triassic and thus kinematically distinct from a belt of Permian-Triassic deformation in the western Mojave Desert. Correlations of structures forming the Death Valley thrust belt, which includes the Last Chance allochthon, several subjacent structures, and their inferred lateral equivalents, suggest that prior to Tertiary extension, individual structures were laterally continuous for distances exceeding 150 km along strike, from the Nevada Test Site area southwest to the Darwin Plateau. Their inferred Mesozoic age and the presence of an Early Triassic overlap sequence on the Darwin Plateau precluded projection of these structures farther south, in spite of long-recognized strain compatibility problems associated with large displacement on thrusts that apparently terminated there without suitable accommodation structures, such as tear faults or lateral ramps. These problems may be resolved if the Last Chance allochthon is Permian, consistent with structural correlations, new isotopic data from post-tectonic stocks, and stratigraphic evidence. A large Permian turbidite basin near the Darwin Plateau can be interpreted as a marine foredeep resulting from emplacement of the 5-km-thick allochthon, rather than a local trough caused by transcurrent faulting during inferred Permian-Triassic continental truncation. I propose that anomalous eugeoclinal rocks in the western Mojave Desert have been emplaced against cratonic strata there by the Last Chance allochthon, which foreshortened an originally narrow continental margin.

Thermal and barometric constraints on the intrusive and unroofing history of the Black Mountains: Implications for timing, initial dip, and kinematics of detachment faulting in the Death Valley Region, California

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 >300°C) 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 structurally 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°, similar to that determined for detachment faults in west central Arizona and southeastern California. Beginning with an initially listric 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.

Uniqueness of geological correlations: An example from the Death Valley extended terrain

The northern Death Valley-Furnace Creek fault zone is a major northwest-trending fault system in the Death Valley region, but the magnitude of offset and its significance to extensional tectonism in the region are controversial. Stratigraphic data have provided first-order constraints on offset across the fault zone but offer limited resolution. Previous correlations of Mesozoic thrust faults across the fault zone have relied on regional stratigraphic trends to fingerprint structurally similar thrust plates. We show that a northeast-trending Mesozoic backfold within the predominantly east-directed fold and thrust belt was probably continuous from the southern Panamint Range, California, to the Belted Range, Nevada. The relative position of this antithetic structure allows uniquely compatible correlations of three Mesozoic structures between two range blocks across the northern Death Valley-Furnace Creek fault zone independent of stratigraphic arguments. The offset structures have geometric properties of size, vergence, order, and spacing that indicate a probability less than 10 -4 of the Mesozoic orogen identically repeating this structural suite in two unrelated places. Reconstruction of these correlative structures indicates 68 ± 4 km N48°W ± 6°W of apparent dextral offset between the Cottonwood and Funeral Mountains, in general agreement with previous estimates based on stratigraphic observations but of substantially higher precision. This reconstruction, and the restoration of a 30-km separation between the Funeral and Grapevine Mountains indicated by offset segments of the backfold, suggest a simple pre-extensional geometry for the Mesozoic orogen in the Death Valley region consistent with observations from unextended blocks and analogous to that of the Cordilleran orogen at the latitude of Calgary, Alberta.

Permian-Triassic plutonism and tectonics, Death Valley region, California and Nevada

Significant contractional structures that deform Permian rocks but predate an Early Triassic overlap sequence are recognized within the Cordilleran orogen, western United States. Thrusting in the Death Valley region of the orogen, however, has been regarded as Middle Triassic or younger and thus kinematically distinct. We present new isotopic age limits on two posttectonic stocks that intrude major structures of the Death Valley thrust belt. The stocks are no younger than Middle Triassic, but are likely Late Permian in age, consistent with stratigraphic and structural data suggesting that thrusting predates the overlap sequence. We hypothesize that Permian shortening may have affected more than 700 km of the Cordilleran orogen at the same time arc activity began within cratonic North America but prior to Early Triassic emplacement of the structurally higher Sonomian arc terrane.

Paleogeographic and structural significance of an Upper Mississippian facies boundary in southern Nevada and east-central California: Discussion and reply

A major facies boundary separating Meramecian (lower Upper Mississippian) shallow-water, carbonate-platform rocks to the south-east from coeval to slightly younger relatively deep-water siliciclastic rocks to the northwest can be traced throughout southern Nevada and east-central California. Although the precise depositional relation between the two facies is not entirely clear, the outer margin of the shallow-water carbonate rocks almost certainly marks the edge of the carbonate platform in earliest late Meramecian time. This facies boundary is distinct and unambiguous enough that it and a parallel subfacies boundary within the carbonate-platform rocks can be used to estimate amounts of displacement on Cenozoic right-slip faults in the region. The carbonate-siliciclastic facies boundary is offset by about 170 km in a right-lateral sense parallel to the Death Valley-Furnace Creek fault zone between the west side of that fault zone and the northeast side of the Las Vegas Valley shear zone. This offset is Cenozoic in age and is interpreted to be distributed as follows: 45-50 km on the Las Vegas Valley shear zone, 80 km on the Death Valley-Furnace Creek fault zone, 30 km on the Stewart Valley fault, and an additional 10-15 km that is not specifically accounted for. Restoration of these offsets successfully reconstructs the Meramecian carbonate shelf edge and aligns several thrust faults now exposed on opposite sides of the right-slip faults.