Patricia H. Cashman

Late Paleozoic tectonism in Nevada: Timing, kinematics, and tectonic significance

Three late Paleozoic, angular unconformities, each tightly constrained in age by biostratigraphy, are exposed in Carlin Canyon, Nevada. These record deformation as well as erosion. Folding associated with these deformation events is roughly coaxial; all three sets of fold axes trend northeast. Each unconformity represents tectonic disruption of the middle part of the western North American margin between the times of the initiation of the Antler orogeny (Late Devonian–Early Mississippian) and the Permian–Triassic Sonoma orogeny. This paper focuses on one of these unconformities in the Middle Pennsylvanian—the C6 unconformity—and the deformation and age constraints associated with it. Our data from Carlin Canyon yield detailed glimpses of how the Antler foreland evolved tectonically in Mississippian and Pennsylvanian time. Middle Pennsylvanian (Desmoinesian) northwest-southeast contraction resulted in thin-skinned folding and faulting, uplift, and erosion. These data require reinterpretation of the tectonic setting at the time of the Ancestral Rocky Mountains orogeny and suggest that plate convergence on the west side of the continent played a significant role in late Paleozoic tectonics of the North American continent.

Fault interaction may generate multiple slip vectors on a single fault surface

Slip vectors of multiple orientations on a single fault surface may be explained by the interaction of crustal-scale faults in response to nearby earthquakes. Field observations of at least five sets of striae on a single planar fault surface can be reproduced in numerical experiments. These experimental results suggest that slip vectors having variable orientations may reflect a highly transient local stress state in an otherwise constant regional stress field. Our results suggest that stress-inversion techniques should be applied with caution to areas with complex regional-scale fault patterns.

Paleoseismology of an active reverse fault in a forearc setting: The Poukawa fault zone, Hikurangi forearc, New Zealand

The Poukawa fault zone, on the North Island of New Zealand within the forearc of the Hikurangi subduction zone, consists of a series of en echelon reverse faults and companion hanging-wall anticlines. The geomorphically expressed length of the fault zone is 34 km. However, on the basis of coseismic deformation associated with an M s 7.8 earthquake in 1931 and the presence of blind faults north of the geomorphically expressed fault zone, it appears that the seismogenic length of the fault zone may be as much as 130 km. On the basis of chronostratigraphic horizons identified in each of three trenches evenly distributed along the exposed fault zone, from which a paleoseismological record for the past ∼25 k.y. can be determined, there is not a characteristic rupture length for earthquakes. Some slip events are confined to the ∼10–20-km-long southern part of the fault zone, whereas other slip events may have ruptured the entire 34 km length of the geomorphically expressed fault zone. At least two slip events that occurred in the northern part of the fault zone did not occur in the southern part of the zone. The largest earthquake recorded in the trenches had a maximum reverse slip in excess of 10 m. We infer that this prehistoric earthquake, similar to the 1931 earthquake, entailed slip on faults along the geomorphically expressed fault zone and on blind faults to the north. This prehistoric earthquake may have had a rupture length (surface plus subsurface) in excess of 100 km. Average earthquake repeat times on the fault zone range from 3–7.5 k.y. for the southern and middle part of the zone to 7–12 k.y. for the northern part of the fault zone. Average single-event slip ranges from 3 m to as much as 6 m. Slip was initially accommodated at the surface primarily by folding. With successive slip events, however, coseismic displacements propagated to the surface and surface deformation became increasingly dominated by reverse slip on fault planes. The Poukawa fault zone is part of a foreland-propagating fold and thrust belt in the forearc of the Hikurangi subduction zone. Older, actively eroding hanging-wall anticlines are present to the west of the fault zone toward the volcanic arc, whereas younger folds are developing above blind reverse faults east of the main fault trace. In addition to propagating to the east, the fault zone is propagating northward beneath the Heretaunga Plains. This active propagation testifies to ongoing and evolving contractional forearc deformation in response to oblique plate convergence.

Thermal models of thrust faulting: Constraints from fluid-inclusion observations, Willard thrust sheet, Idaho-Utah-Wyoming thrust belt

The thermal effects associated with emplacement of the Willard thrust sheet within the Idaho-Utah-Wyoming thrust belt have been numerically modeled. Fluid-inclusion studies and mineralogy, including illite crystallinity, narrowly limit choice of permissible thermal models. Permissible models have initial thermal gradients between about 30 °C/km and 35 °C/km and fluid pressures less than lithostatic pressure. Fluid inclusions were trapped in syntectonic veins within the basal part of the Willard thrust sheet and in cataclastically deformed basement within the footwall. Homogenization temperatures have a bimodal distribution in the Willard sheet with maxima at 180 °C and 260 °C and a unimodal distribution in the footwall with a maximum at 200 °C. These temperatures, along with fluid composition, determine fluid density and define isochores along which the fluids were trapped. Illite crystallinity and mineralogy record maximum temperatures in the range of 300 °C to 500 °C with somewhat lower maximum temperatures in the footwall. Thermal models for emplacement of the Willard sheet indicate that the hanging wall undergoes initial rapid cooling, but the footwall undergoes initial warming due to tectonic burial. Later both areas undergo approximately isothermal decompression as erosion proceeds. P-T paths for reasonable parameter values intersect the modal isochores and are consistent with mineralogy, as required for a permissible model. The thermal model provides a reasonable framework for understanding the thermal history of the Willard sheet. The models are sensitive to thrust-sheet thickness, initial heat flow, and thermal conductivity. The thermal models are relatively insensitive to erosion history for reasonable erosion rates and to redistribution of heat-producing elements for the low values typical of sedimentary rocks.

Widespread effects of middle Mississippian deformation in the Great Basin of western North America

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland—created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada—shed sediments eastward across the Antler foreland, burying the unconformity. Post oro genic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America. We propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations.

Mississippian Through Permian Orogenesis in Eastern Nevada: Post-Antler, Pre-Sonoma Tectonics of the Western Cordillera

Mississippian through Permian strata in eastern Nevada, and related strata in southern Nevada and southern Idaho, document a series of tectonic episodes that are either generally unrecognized, or assigned to the Antler or Sonoma orogenies. Some of these were local and others were regional in scale, and none fit either the Antler or Sonoma orogenies as normally defined. They are listed below, along with the last phase of the Antler and the first phase of the Somoma orogenies:

Structural relationships of pre-Tertiary rocks in the Nevada Test Site region, southern Nevada

This report summarizes the evidence for a revised interpretation of major structural features in the pre-Tertiary rocks of the region including and surrounding the Nevada Test Site. The thick miogeoclinal section of Late Proterozoic through Lower Permian sedimentary strata records major foreland-vergent thrust faulting, younger hinterland-vergent folding and thrusting, and local extension on low-angle normal faults. In addition, structural discontinuities in the northeastern part of the Nevada Test Site strongly suggest a broad, north-trending zone of sinistral strike-slip faulting that may have had a cumulative offset of many kilometers.

Middle Devonian-Mississippian Stratigraphy On and Near the Nevada Test Site: Implications for Hydrocarbon Potential

Paleozoic strata on the Nevada Test Site and surrounding area are affected by the intersection of several important geologic trends, including the early Paleozoic rift margin and the Late Devonian-Early Mississippian Antler orogenic foredeep. Upper Paleozoic strata are lithologically diverse and include siliciclastic sediments (carbonaceous shale, clean quartzose sands, bedded chert, and chert-lithic conglomerate and sand) and various carbonates. These are interpreted to have been deposited in a range of environments and water depths. The resulting complex stratigraphy with its dramatic vertical and lateral changes is difficult to correlate, even between nearby surface and subsurface sections. Our detailed stratigraphic studies on and near the Nevada Test Site show that at least three distinct, and largely coeval, Mississippian facies assemblages can be recognized. When dated, correlated, and restored to their original relative geographic positions, these sections allow a new reconstruction of the Devonian and Mississippian geologic history that includes (1) delineation of the Antler foredeep basin and its clastic fill, (2) description of the muddy marginal shelf that lay inboard of the foredeep, and (3) correlation from that shelf to the outer fringes of the carbonate platform that characterizes the edge of the Mississippian epicontinental sea. The position and delineation of the mud-dominated marginal shelf environment is particularly important because shales deposited under such conditions elsewhere in the Great Basin are the hydrocarbon source rocks for the Pine Valley and Railroad Valley oil fields. On the Nevada Test Site, Mississippian shale probably generated hydrocarbons in the past, and still has an organic content and a thermal history appropriate for generating hydrocarbons.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America

Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny.

Stratigraphic trends in detrital zircon geochronology of upper Neoproterozoic and Cambrian strata, Osgood Mountains, Nevada, and elsewhere in the Cordilleran miogeocline: Evidence for early Cambrian uplift of the Transcontinental Arch

U-Pb detrital zircon geochronology provides insight into the provenance of the upper Neoproterozoic–lower Cambrian Osgood Mountain Quartzite and the upper Cambrian–lower Ordovician Preble Formation in the Osgood Mountains of northern Nevada (USA). We analyzed 535 detrital zircon grains from six samples of quartz arenite by laser ablation–multicollector–inductively coupled plasma–mass spectrometry. The detrital zircon age data of these Neoproterozoic–lower Paleozoic passive margin units record a provenance change within the Osgood Mountain Quartzite. Comparison of these data with the work of others reveals that this change in provenance occurred in correlative strata throughout an east-west transect of the Great Basin. From latest Neoproterozoic through earliest Cambrian time, most grains were shed from the 1.0–1.2 Ga Grenville orogen. After that time, drainage patterns changed and most grains were derived from the 1.6–1.8 Ga Yavapai and Mazatzal provinces; very few grains from the Grenville orogen were found in the younger strata. We suggest that this shift records the uplift, in early Cambrian time, of the Transcontinental Arch. Our data also support our interpretation that the Osgood Mountain Quartzite and the Preble Formation are correlative to other contemporaneous passive margin strata in western Laurentia.