David S. Harwood

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.

Detrital zircon geochronology of the Shoo Fly Complex, northern Sierra terrane, northeastern California

U-Pb analyses have been conducted on 92 individual detrital zircon grains from 4 of the main thrust sheets of the Shoo Fly Complex. Samples from the Culbertson Lake allochthon, Duncan Peak allochthon, and Lang sequence yield mainly 1.80-2.10, 2.20-2.45, and 2.55-2.70 Ga ages, which suggests that sediments in these units originated in a cratonal region containing Paleoproterozoic and Archean igneous rocks. These ages match those of basement provinces from the northwestern Canadian shield, suggesting a provenance link with northwestern North America during early Paleozoic time. The Sierra City melange, however, has significantly different zircon ages of 551-635 and 1170-1319 Ma, with only a subordinate population of >1.8 Ga grains. These grains apparently were derived originally from an outboard Neoproterozoic-Cambrian(?) volcanic arc and from 1.0-1.7 Ga basement rocks of southwestern North America. The occurrence of all three sets of ages in a sandstone that accumulated outboard of the Lang, Culbertson Lake, and Duncan Peak thrust sheets indicates that most rocks of the Shoo Fly Complex formed inboard of a volcanic arc located in proximity to the southern portion of the Cordilleran margin.

Detrital zircon geochronology of upper Paleozoic and lower Mesozoic strata of the northern Sierra Terrane, northeastern California

U-Pb analyses of 56 individual detrital zircon grains from mid-Paleozoic through Lower Jurassic clastic strata of the northern Sierra terrane yield two distinct sets of ages: 1.7-2.8 Ga grains mainly in Upper Devonian-Mississippian strata of the Picayune Valley Formation, and ca. 370-185 Ma grains in strata of Permian through Jurassic age. The older ages are most similar to the ages of grains in the underlying Shoo Fly Complex and in the Roberts Mountains allochthon in Nevada. This age similarity, combined with stratigraphic relations that record a provenance link with the Roberts Mountains allochthon, are consistent with models in which the northern Sierra terrane was located in proximity to the Nevada continental margin during Late Devonian-Mississippian time. The ca. 370-185 Ma ages of detrital grains in Permian, Triassic, and Jurassic strata are an excellent match for the ages of volcanic rocks within the northern Sierra terrane. The clastic detritus in these units was presumably derived from these intraterrane volcanic rocks.

Stratigraphy of upper Paleozoic volcanic rocks and regional unconformities in part of the northern Sierra terrane, California

Upper Paleozoic volcanic rocks in the southern part of the Taylorsville strike belt are unconformably truncated at the North Fork of the American River by a lens of Upper Triassic conodont-bearing limestone and Lower and Middle Jurassic graywacke-slate flysch deposits of the Sailor Canyon Formation. The volcanic sequence consists of submarine felsic pyroclastic debris flows and tuffaceous slate of the Late Devonian Sierra Buttes Formation, andesitic turbidites of the Taylor Formation, a lower tuff-siltstone member and an upper chert member of the Peale Formation, and fine-grained tuff of the Reeve Formation. Newly discovered radiolarians date the chert member of the Peale as latest Late Mississippian and(or) Early Pennsylvanian. Shelly fossil fragments found 50 m above the chert member date the lower part of the Reeve in this area as late Early Permian. These rocks formed in the distal part of the submarine volcanic apron, presumably in an island-arc setting, and were deposited unconformably on quartz-rich clastic rocks and tectonically interlayered chert of the pre–Late Devonian Shoo Fly Complex. The Peale marks a period of quiescence in the late Paleozoic volcanism that culminated in deep-water chert deposition during latest Mississippian to the Middle Pennsylvanian. The Peale-Reeve contact is a regional hiatus, probably nondepositional in the south and becoming erosional to the north, that spans most or all of the Late Pennsylvanian and Early Permian. Post-late Early Permian to pre-Late Triassic deformation produced northeast-trending folds and a southeast-trending fault in the late Paleozoic volcanic rocks that are truncated nearly orthogonally by the Late Triassic unconformity and northwest-trending folds in the Jurassic Sailor Canyon Formation.

Configuration of Precambrian Rocks in Southeastern New York and Adjacent New England from Aeromagnetic Data

Two aeromagnetic anomalies of regional extent outline two previously unknown buried masses of highly magnetic, probably Precambrian, rocks in southeastern New York and adjacent Vermont, Massachusetts, and Connecticut. The northern mass extends northeastward from Albany, New York, to Bennington, Vermont, where it appears to be buried beneath weakly magnetic Precambrian rocks of the Green Mountains. The southern mass extends north-northeastward from Beacon, New York, through Stissing Mountain, to Copake, New York, and appears to be the buried northeast extension of the Reading Prong. The shape of the Beacon-Copake magnetic anomaly indicates that the source is near the surface and has a sharp boundary, probably a fault, on its northwestern side; the shape also indicates that the source becomes deeply buried to the southeast and thus supports a parautochthonous interpretation for the northern part of the Reading Prong. In southwestern Massachusetts, the highly magnetic Beacon-Copake mass appears to be overstepped on the east by a buried slice of weakly magnetic Precambrian rocks which, in turn, is overstepped on the east by imbricate slices of weakly magnetic Precambrian rocks exposed along the western front of the Berkshire Highlands. Precambrian rocks exposed in the Green Mountains, the Berkshire and Housatonic Highlands, the eastern part of the Hudson Highlands, and the Manhattan Prong have a remarkably lower amplitude magnetic pattern than those in the Adirondack Mountains and the Reading Prong. This difference in magnetic character appears to represent more than different thicknesses of Precambrian rocks and may reflect a different sequence of Precambrian rocks to the east, a lower grade of Precambrian metamorphism to the east, or possibly a reduction in magnetite content in the eastern Precambrian rocks because of Paleozoic metamorphism.

Continuous Magnetic Profiles near Ground Level as a Means of Discriminating and Correlating Rock Units

Continuous magnetic profiles were recorded by a truck-mounted magnetometer along road traverses over stratified metamorphic rocks and plutonic igneous rocks of the New England Appalachians. The records show a series of distinctive, highly detailed magnetic anomalies which closely reflect the nature and distribution of near-surface bedrock units. The method provides a rapid and versatile means of discriminating bedrock units, which should be especially useful in regions where bedrock exposures are scarce. Under some conditions, analysis of the anomalies may yield critical data on the shape and magnetization of individual rock units. These data should prove useful for deducing geologic structure and for general studies of the magnetization of rocks.