DISTINCTIVE WEST VERGENCE IN THE LATE PALEOZOIC OF NORTH-CENTRAL NEVADA: KINEMATICS AND TIMING

In northern and central Nevada, east of the Roberts Mountain thrust, a series of structures and related unconformities record late Paleozoic tectonism in southwest Laurentia. New field data from Buck Mountain and the Central Pequop Mountains document precisely constrained, mid-Pennsylvanian structures. The structures include west-vergent folds at Buck Mountain and west-directed imbricate thrust faults in the central Pequop Mountains. These structures occur in the Middle Pennsylvanian (Morrowan-Atokan) Ely Limestone and are overlain by gently E-dipping Middle Pennsylvanian (Desmoinesian) Hogan Formation. The Atokan-Desmoinesian age bracket corresponds to the biostratigraphically constrained C5 unconformity of Trexler and others (2004, GSA Bulletin). Karst features developed at the top of the Ely Limestone at Buck Mountain and central Pequop Mountains document broad sub-aerial exposure of the C5 erosion surface prior to deposition of the overlying Hogan Formation. At Buck Mountain, the axial surface of an open fold in Ely Limestone restores to 010°/64°E when post-Desmoinesian (Hogan) tilt is removed. This fold is therefore west-vergent. In the central Pequop Mountains, imbricated thrust faults have a pre-Desmoinesian (pre-Hogan) average orientation of 016°/20°E. Another small thrust restores to 060°/25°E. These thrusts step up-section to the west across the Ely footwall, which together with the restored dip directions indicates top-to-the-west motion.

These results are regionally significant for several reasons. (1) They add to the record of west-and northwest-vergent structures, anomalous among the typically southeast-vergent structures of late Paleozoic southwest Laurentia. (2) They provide a tighter age bracket on Middle Pennsylvanian west-vergent structures than is known elsewhere in Nevada. (3) They are farther east than other west-vergent structures of this age in Nevada. (4) They record significant shortening, so do not appear to support a west-to-east strain gradient in Middle Pennsylvanian time. Together, these factors suggest a temporary mid-Pennsylvanian change in the tectonic regime along the southwestern margin of Laurentia.