EVOLUTION OF CENOZOIC VOLCANISM AND EXTENSION IN THE COPPER MOUNTAINS, NORTHEASTERN NEVADA

The Copper Mountains of northeastern Nevada provide a record of coevolving Late Eocene to Miocene volcanism and extension in a greenschist to amphibolite facies terrain bounded on the east by two normal faults that juxtapose the footwall against a 1.5 km thick sequence of volcanic and sedimentary rocks. New ⁴⁰Ar/³⁹Ar biotite data on volcanic rocks document ages ranging from 47.3 to 37.4 Ma for the Dead Horse Formation, bracketing the age of lacustrine deposits that may record the onset of basin subsidence associated with the Copper Creek Fault. However, conspicuous footwall-derived detritus does not appear until the deposition of the overlying Meadow Fork Formation beginning at 32.5 Ma, suggesting a ~5 m.y. hiatus in basin-filling and presumably in normal fault activity. Deposition of the Meadow Fork Formation continued at least to 29.5 Ma, followed by 25^o westward tilting and capping by flat-lying volcanic rocks at 16.5 Ma.

New ⁴⁰Ar/³⁹Ar K-feldspar and U-Th/He cooling age data from lower plate rocks further constrain the progression of extensional unroofing. A boulder of quartz monzonite collected from the Meadow Fork Formation documents that the footwall clast had cooled through U-Th/He apatite closure (nominally 70^oC) by 43 Ma followed by uplift to the surface, erosion and redeposition in the adjoining basin by 29.5 Ma. Similarly, granitoid samples from the structurally highest, western part of the lower plate yield U-Th/He apatite cooling ages of 40.9 to 42.1 Ma. In addition, the structurally deeper eastern part of the range yields U-Th/He zircon ages of 46.1 and 53.7 Ma, and K-feldspar MDD modeling suggests that final cooling began at 40 to 45 Ma. Finally, an intermediate fault slice between the two bounding faults yields an U-Th/He apatite age of 32.6 ± 2 Ma, synchronous with the second pulse of normal faulting identified above. Taken together, these results document one of the the earliest and most sustained records of extension in northeastern Nevada.