Walter S. Snyder

Geometry of Subducted Slabs Related to San Andreas Transform

Development of the San Andreas transform by rise-trench encounter in coastal California influenced the structural evolution of a large region within the adjacent continent. Continuation of arc magmatism and tectonism depends upon the presence of a subducted slab of lithosphere at depth beneath an arc-trench system. The lack of subduction at the transform plate boundary along the California continental margin led to the growth of a slab-free region beneath the part of the continental block adjacent to the San Andreas transform. Geometric analysis based on ideal assumptions predicts that generation of a lengthening transform by rise-trench encounter will also generate an expanding triangular hole or window in the slab of lithosphere subducted beneath the continent. One leg of the slab-window is the adjacent transform, but the orientations and lengths of the other two legs depend upon the relative motions of the three plates involved. By inference, arc volcanism and tectonism cannot persist across the no-slab area at the surface above the slab-window. The actual configuration of the slab-free region adjacent to the transform will depart from ideal predictions where adjustments to the conditions of rise-trench encounter involve changes in the motions of surface plates or subterranean ruptures in subducted slabs. The extent of the expanding slab-free region adjacent to the San Andreas transform can be reconstructed through time from detailed knowledge of oceanic plate boundaries and motions offshore. The progressive switchoff of Neogene arc volcanism conformed to expected patterns in time and space when the age of oceanic lithosphere being consumed near the coast is taken into account. The extent of the growing no-slab area at the surface above the widening slab-window at depth has been largely coextensive with the gradually expanding Basin and Range province of extensional tectonism and bimodal volcanism. Diapiric upwelling of asthenosphere through the evolving slab-window in the subducted lithosphere probably influenced magma genesis and geodynamic behavior within the slab-free region. Bulk uplift of the adjacent Sierra Nevada and the nearby Colorado Plateau, as well as opening of the Rio Grande Rift, were possibly related to the same mantle processes.

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.

Structure of the Havallah sequence, Golconda allochthon, Nevada: Evidence for prolonged evolution in an accretionary prism

The Golconda allochthon of northern and central Nevada contains the Havallah sequence and correlative units of latest Devonian to early Late Permian age. The Havallah sequence is dominated by radiolarian ribbon chert and argillite associated with variable, but generally subordinate, siliciclastic, cal-carenitic, and volcaniclastic turbidites and slump deposits. Cherts of all ages (except Pennsylvanian?) locally rest depositionally on tholeiitic basalts. Some were altered and mineralized by ridge-type hydrothermal systems, suggesting deposition in an ocean basin containing active spreading center(s) for much of the upper Paleozoic. The Havallah sequence was thrust eastward along the basal Golconda thrust onto coeval sediments of the western North American shelf during the latest Paleozoic-earliest Mesozoic Sonoma orogeny. Detailed studies of the Havallah sequence in the Sonoma and Tobin Ranges and in Battle Mountain indicate the presence of complex diagenetic and structural fabrics that developed prior to the obduction of the allochthon. A large number of thrusts, composite thrusts, and shear zones slice the Havallah into numerous tectonic packets of contrasting lithology and/or internal structural style. Internal structures in chert packets include bedding-parallel and bedding-normal solution cleavage, solution boudins, three or more sets of east-verging folds of variable geometry, and features associated with high pore-fluid pressures such as crack-seal fractures, dilation breccias, clastic dikes and sills, and clastic intrusions along thrust surfaces. The fabrics suggest a general progressive evolution from deformation characterized by bedding-normal compression and slight east-west extension (D1) to deformation dominated by bedding-parallel, east-directed shear (D2). Local chert packets may have suffered alternating episodes of high and low pore-fluid pressure. The degree of development and the styles and orientations of various structural elements vary, sometimes radically, from packet to packet. The pre-Golconda thrust fabrics can be modeled as the result of imbrication and deformation of successive batches of ocean-floor sediments into the toe of an accretionary prism in front of an east-facing arc. Relatively undeformed Permian calcarenite units, interpreted as being trench-slope deposits, suggest that this prism was well developed by Permian time. Coarse slump deposits containing chert clasts with pre-depositional structural fabrics suggest a prism that was active long enough to recycle previously tectonized cherts. If we are correct, the classical Sonoma orogeny (D3) marked the culmination of a protracted structural evolution that may have spanned much of the upper Paleozoic.

Tectonic implications of detrital zircon data from Paleozoic and Triassic strata in western Nevada and Northern California

U-Pb analyses of detrital zircons from various allochthonous assemblages of Paleozoic and early Mesozoic age in western Nevada and northern California yield new constraints on the sediment dispersal patterns and tectonic evolution of western North America. During early Paleozoic time, a large submarine fan system formed in slope, rise, basinal, and perhaps trench settings near the continental margin, west of continental shelf deposits of the Cordilleran miogeocline. Our detrital zircon data suggest that most of the detritus in this fan system along the western U.S. segment of the margin was derived from the Peace River Arch region of northwestern Canada, and some detritus was shed from basement rocks of the southwestern United States or western Mexico. In most cases, the detritus in the allochthonous assemblages was recycled through platformal and/or miogeoclinal sedimentary units prior to accumulating in offshelf environments. Lower Paleozoic rocks of the Roberts Mountains allochthon, Shoo Fly Complex, and Yreka terrane are interpreted to have been parts of this fan complex that accumulated along the central U.S. segment of the continental margin, probably within 1000 km of the miogeocline. During the mid-Paleozoic Antler orogeny, parts of the lower Paleozoic fan complex were deformed and uplifted, and strata of the Roberts Mountains allochthon were tectonically emplaced onto the continental margin. This orogeny was apparently driven at least in part by convergence of the Sierra-Klamath arc with the continental margin, as has been proposed by many previous workers, because these arc terranes are overlain by Mississippian clastic strata derived from the Roberts Mountains allochthon. Our data are not sufficient, however, to determine the polarity of the arc, or whether the arc formed along the continental margin or was exotic to western North America. Detrital zircon data indicate that following the Antler orogeny, clastic sediments derived from the Roberts Mountains allochthon were deposited both on the continental margin to the east and within intra-arc and backarc basins to the west. The occurrence of this detritus in terranes of western Nevada and northern California indicates that they were proximal to each other and to the continental margin during late Paleozoic time. The presence of upper Paleozoic volcanic and plutonic rocks and arc-derived detrital zircons in strata of the northern Sierra, eastern Klamath, and Black Rock terranes records the existence of a west-facing magmatic arc near the continental margin during late Paleozoic time. Our data are not supportive of scenarios in which these arc terranes were located farther north or thousands of kilometers offshore of the Nevada continental margin during late Paleozoic time. Following a second phase of uplift, erosion, and allochthon emplacement during the Permian-Early Triassic Sonoma orogeny, Middle and Upper Triassic strata now preserved in west-central Nevada accumulated in a backarc basin. Our data indicate that the basinal assemblages contain detritus from arc terranes to the west as well as the craton to the east.

Chemical and Sr-isotopic variations during diagenesis of Miocene siliceous sediments of the Monterey Formation, California

The Monterey Formation of California preserves Miocene siliceous sediments at progressive (generally with depth) diagenetic stages: diatomite (opal-A) --> CT-chert (opaI-CT) --> quartz chert for initially clay-poor sediments and diatomaceous shale --> CT-porcelanite (porous opal-CT) --> quartz porcelanite for sediments that were initially clay-rich. The chemistry of most diatomite is controlled by three primary phases (biogenic silica, carbonate, and detrital clay) and several subsidiary phases (organic material, apatite, pyrite, and detrital minerals including quartz and plagioclase). The elemental abundances of the clay-poor diagenetic sequence decrease sharply with advancing diagenetic state for those elements controlled by clay (Al, Ti, K, Rb, Cs, Fe, some Group B metals), carbonate (Ca, Mg, Sr, Ba), apatite (P, Ca, Sr, Ba, V), and organics (V, Ni), while silica shows a congruent increase. These first-order variations reflect the initial compositions of the siliceous sediments modified by the addition of silica, the removal of carbonate and apatite, and the migration of petroleum during silica diagenesis. Anomalous abundances of Mn, Ni, Cr, Mo, and some other Group B metals are associated with trapped refractory organic material and suggest that petroleum catagenesis and migration occurred during silica diagenesis: migration may have been facilitated by the release of bound water during silica-dehydration reactions. Second-order variations (in K/Rb, K/Cs, Rb/Sr, 87 Sr/ 86 Sr) with advancing diagenetic state suggest ion-exchange and ion-expulsion by reaction of detrital phases with pore fluids. Diagenetic processes partially equilibrated, but did not homogenize, depositional 87 Rb/ 86 Sr- 87 Sr/ 86 Sr mixing lines generated by detrital clays and authigenic carbonate and silica in siliceous sediments from the Santa Maria region. However, similar mixing lines in diatomites from the Santa Lucia area are reset to nearly horizontal slopes in associated CT-porcelanites. The degree of equilibration may be a function of the rate of formation of relatively impermeable quartz chert which isolated the siliceous system from further interaction with pore fluids. The rapid formation of quartz chert in the Santa Maria region is believed to have prevented isotopic homogenization processes from reaching completion. Initial chemistry of the siliceous systems controlled the rates and pathways of silica diagenesis. Silica diagenesis, in turn, altered the chemistry of the siliceous sediment.

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.

Diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon, Nevada, U.S.A.

Abstract A model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper Paleozoic Golconda allochthon of Nevada, U.S.A. Numerous thrust faults slice the cherts into packets, each with a unique set of internal structures. Fold geometries, boudin profiles, pressure-solution features, etc. vary from packet-to-packet and layer-to-layer. Analogous variations in the diagenetically mixed siliceous sediments of the Miocene Monterey Formation. California, suggest the cherts of the Golconda allochthon were similarly mixed when they were deformed. Radiolarian sediments composed of biogenic silica (opal-A) developed ductile structures and pressure-solution features because of their high porosity, weak lithification and the high solubility of disordered silica. Lowporosity, strongly-lithified, relatively-insoluble quartz cherts, the final product of silica diagenesis, deformed in a brittle fashion and developed fewer pressure-solution features. Diagenetically intermediate CT-cherts and CT-porcelanites exhibited transitional behavior. Mixed diagenetic zones produced structures with layer-by-layer changes in structural style. The dehydration of opal-A and opal-CT helped create the excess pore fluid pressures responsible for thrust faults, hydraulic fractures, dilation breccias and elastic intrusions. The presence of ductile structures and numerous pressure solution features in the oldest cherts of the Havallah sequence suggests that these cherts were deformed while diagenetically immature, possibly within a long-lived upper Paleozoic accretionary prism.

Permian of the Western United States

The former Antler orogenic belt divides Permian of displaced terranes to the west from Permian continental margin miogeoelinal deposits to the east. We will refer to this important structural feature as the Antler belt. The structural setting of the Permian of the displaced terranes accreted to the North American continent is complex and a subject of much active research. The structural setting of the Permian rocks of the miogeocline is becoming well known and will be briefly elucidated here.

U-Pb detrital zircon geochronology of the Golconda allochthon, Nevada

The Golconda allochthon consists of deformed and imbricated deep-marine strata and volcanic rocks of late Paleozoic age that structurally overlie lower Paleozoic rocks of the Roberts Mountains allochthon and overlying upper Paleozoic strata. We analyzed 86 detrital zircon grains from 4 sandstone units of the Golconda allochthon in an effort to help reconstruct the paleogeographic and tectonic setting of the allochthon prior to Permian(?)-Triassic thrusting onto the continental margin. Of these, 81 grains yielded concordant to moderately discordant ages that define three main groups: 338-358 Ma (n = 7), 1770-1922 Ma (n = 38), and 2474-2729 Ma (n = 19). Comparison of these ages with detrital zircon age spectra from adjacent terranes indicates that most of the detritus in Golconda sandstones was probably shed from rocks of both the Roberts Mountains allochthon to the east and the northern Sierra terrane to the west. Some grains may also have been shed from basement rocks of the southwestern United States. These relations support previously proposed tectonic models in which strata in the Golconda allochthon were deposited between the Sierra-Klamath magmatic arc to the west and the previously emplaced Roberts Mountains allochthon to the east.

Petroleum Geology of the Southern Pre-Uralian Foredeep with Reference to the Northeastern Pre-Caspian Basin

The southern Pre-Uralian Foredeep and the northeastern Pre-Caspian Basin of southern Russia and Kazakhstan are at the juncture of two major oil-producing regions, the Volga-Ural Basin and the new fields of the Northern Caspian Basin (e.g., Tengiz). The southern Pre-Uralian Foredeep has produced little oil; nevertheless, the Permian-Carboniferous stratigraphy and the general fold-thrust structure of the Pre- Uralian Foredeep, and adjacent Pre-Caspian Basin, afford the possibility for classic and largely untested sub-salt and sub-thrust plays. Prior to the onset of Uralian orogenic activity, Late Devonian-Early Carboniferous rifting disrupted the East European continent, forming a series of rift basins including the Kama-Kinel troughs and the Pre- Caspian Basin. The Middle Carboniferous to Early-Middle Triassic Uralian Orogenic Belt consists of a complicated series of lower Paleozoic continental margin sequences, basement nappes, and accreted terranes, structurally interleaved via large-scale folding and th...