THERMOCHRONOLOGIC CONSTRAINTS ON THE COOLING AND EXHUMATION OF THE NORTHERN RUBY MOUNTAINS, NEVADA

The transition from Mesozoic crustal shortening to Cenozoic extension in the western U.S. Cordillera is incompletely constrained. Existing low temperature thermochronometer data and syntectonic stratigraphy from extensional basins suggest mid-Miocene crustal extension while high temperature thermochronometers, structural field and petrologic relationships, and P-T-t paths signal longer-lived and older, late-Cretaceous to Oligocene, extension and exhumation. The cooling and exhumation history of deep crustal rocks through the middle and upper crust can help constrain this transition. This study is part of a larger investigation into the thermal history of the Ruby Mountains – East Humboldt Range – Wood Hills metamorphic core complex in NE Nevada which exposes an almost complete crustal cross section.

New low temperature, (U-Th)/He thermochonometer data from apatites and zircons from the Ruby Mountains suggest cooling through >180°C to <70°C during the mid-Miocene, ~17 Ma. These samples come from a ~6 km east-west transect across an extensional mylonitic shear zone and metamorphic core complex. Samples were taken primarily from the abundant late-Cretaceous to mid-Cenozoic leucogranites found throughout the complex. Our data records a late-Oligocene to early Miocene cooling and exhumationl history with zircon (U-Th)/He dates ranging from 23.5 ± 0.7 to 12.1 ± 0.8 Ma while apatite (U-Th)/He dates range from 18.5 ± 1.3 to 14.2 ± 0.7 Ma. Complimentary to the previous low temperature themochronometer findings farther south in the Ruby Mountains, our data suggests rapid exhumation along a normal fault from 17 – 16 Ma, however our results from the northern Ruby Mountains are consistent with a more shallow fault system than indicated farther south. These results imply complex fault system activity throughout the Rubies during the mid-Miocene. Ongoing work integrating these results with higher temperature thermochronometry, as well as with similar studies throughout the entire core complex, are consistent with this complex picture.