USING GEOHISTORY ANALYSIS TO UNRAVEL THE COMPLEX HISTORY OF LATE PALEOZOIC BASINS AND TECTONICS IN SOUTHWESTERN LAURENTIA

Western North America has experienced many episodes of tectonism since Paleozoic passive margin deposition was interrupted by the latest Devonian-earliest Mississippian Antler orogeny. Repeated contractional tectonism followed by Basin-and-Range extension overprinted or obliterated much of the direct evidence of Late Paleozoic tectonism and basin formation. Analysis of geohistory curve shapes (e.g., Xie and Heller, 2009) is useful for piecing together basin evolution and tectonic setting when direct evidence of tectonic drivers is scarce, absent, or ambiguous.

For this study, geohistory curves were calculated and analyzed using a number of published stratigraphic sections from the Mississippian-Permian Bird Spring Formation of southern Nevada and California. The Bird Spring Formation contains up to 3000+ meters of dominantly shallow marine carbonate deposited on an equatorial shelf from latest Mississippian (Serpukhovian) to early Permian (Kungurian) time. Locally, deeper water facies including carbonaceous shale, turbidites, and debrites occur within the early Permian portion of the section. Basin strata were collapsed by shortening during the Cretaceous Sevier orogeny. Since the Miocene, the strata have been pulled apart and translated by the Basin-and-Range and Walker Lane systems respectively, making basin reconstruction difficult. The published sections used for this study were recorded within the Sevier orogenic belt in the Spring Mountains and Las Vegas and Arrow Canyon Ranges in southern Nevada.

The resulting geohistory curves show two distinct phases of basin formation. The Mississippian to middle Pennsylvanian parts of the curves have a convex-upward stair-step pattern, similar to curves from foreland basins. This matches the previous interpretation of basin response to periodic loading events in north-central Nevada. Beginning in the late Pennsylvanian, the curves steepen and become more linear, suggesting more rapid and consistent subsidence within the basin. This change in curve character is coincident with documented formation of a continental borderland and southward propagation of a left-lateral plate boundary to the west of the Bird Spring Shelf, leading to the interpretation that the Bird Spring Basin responded to at least two distinct tectonic drivers through time.