AGE AND SIGNIFICANCE OF THE COPPER BASIN FAULT SYSTEM, ELKO COUNTY, NEVADA

The Copper Mountains of northeastern Nevada expose a low grade metamorphic terrain that was exhumed by >8 km displacement on the Copper Basin normal fault system. Though now at low angle, relationships between the normal faults and hanging wall rocks suggest that they initially cut at steeper angles through the upper crust and rotated to their current dips of <30^o due to syntectonic rotation.

Before Cenozoic unroofing, the Copper Mountains were intruded by a quartz monzonitic pluton, the Coffeepot stock, at ~110 Ma followed by conductive cooling through ⁴⁰Ar/³⁹Ar mica closure temperatures (~ 300-350^oC) between ~105 and 90 Ma. New cooling age data on potassium feldspar from the footwall of the Copper Basin fault system suggest that cooling continued through Late Cretaceous time, with final cooling probably not occurring until the Late Oligocene, possibly synchronous with unroofing due to displacement on the Copper Basin fault system.

The age of the normal fault system is best constrained by relationships with Late Eocene to Miocene volcanogenic sequences. The fault system cuts and rotates two Tertiary formations in its hanging wall, the Deadhorse and Meadowfork Formations. The Deadhorse Formation includes a thick sequence of tuffs, sparse pebble conglomerates lacking footwall clasts and Late Eocene lacustrine strata bracketed between 41.3 and 37.4 Ma. Overlying the Deadhorse Formation, the Meadowfork Formation is dominated by boulder conglomerate containing abundant clasts of characteristic footwall rocks such as the Coffeepot quartz monzonite. ⁴⁰Ar/³⁹Ar biotite and hornblende ages on airfall tuff from the upper part of this sequence yield a preliminary age of ~30 Ma. The fact that both the Deadhorse and Meadowfork formations are equally rotated suggests that most displacement occurred in Oligocene time synchronous with deposition of the Meadowfork formation. A basalt flow yielding an ⁴⁰Ar/³⁹Ar plateau age of 16.5 Ma on plagioclase unconformably overlies the Deadhorse and Meadow Fork formations and is not significantly rotated, placing a younger age bracket on the timing of normal faulting. Exhumation of the Copper Mountains can be correlated with other Late Oligocene extensional systems in the northeastern Basin and Range province.