Structural overprinting of Mesozoic thrust systems in eastern California and its importance to reconstruction of Neogene extension in the Southern Basin and Range

Geologic observations from the Resting Spring and Nopah Ranges (California, USA) together with a synthesis of regional data indicate that previous reconstructions of the Death Valley extensional terrane need to be revised because they do not account for three-dimensional, preextensional structures that complicate structural markers used in these reconstructions. This conclusion arises from detailed mapping that indicates structural overprinting of early northeast-trending fold-thrust systems of the Sevier orogenic belt by younger northwest-trending structures. The northwest-trending fold-thrust system includes a series of folds and a major thrust that includes the Nopah Peak and the Gerstley faults. This thrust fully involved crystalline basement, and there are extensive basement exposures in the hanging wall of the thrust in the southern Nopah Range. Basement involvement presumably produced a large ramp anticline to the southwest of the Nopah Range in what is now the Death Valley extensional terrane. The existence of this ramp anticline is supported by occurrences of successively older rocks to the southwest, beneath the Tertiary unconformity. The northwest-trending fold-thrust system is also recognizable to the north and east of the Gerstley–Nopah Peak fault as a series of west-northwest– to northwest-trending folds that have been known for more than 30 years, but were overlooked in earlier reconstructions. We use our observations together with recently published data on the State Line fault and extensional structure in Pahrump Valley to consider two preliminary map-view restorations of the Resting Spring and Nopah Ranges relative to the Spring Mountains, a relatively unextended block to the east. These restorations place the northwest-trending thrust system along strike from the southern Spring Mountains, where a similar overprinting is observed, supporting the basic restoration. One restoration that strictly adheres to published estimates for motion across the Pahrump Valley suggests, however, that the widely cited correlation of the Wheeler Pass and Chicago Pass thrust system is highly suspect. Our preferred interpretation concludes that the Chicago Pass–Shaw thrust system of the Resting Spring∼Nopah Ranges is an along-strike equivalent of the Lee Canyon thrust; the Wheeler Pass thrust correlates to the Montgomery Mountains thrust; and both thrust systems have an uncertain continuation into the Death Valley extensional terrane. The Resting Spring–Nopah–Spring Mountains restoration provides a template for future restorations. We emphasize that the three-dimensional preextensional geometry, together with other markers like Mesozoic magmatic belts and older sedimentary facies trends, provides an opportunity for using modern visualization and database systems to develop high-precision reconstructions using the abundance of crosscutting markers. Thus, although this study, along with other recent studies, indicates that previous reconstructions are not workable, future studies that include the full three-dimensional data could lead to a nearly unique solution to the preextensional paleogeography.