THE LATE EOCENE TO MIOCENE COOLING AND EXHUMATION OF THE RUBY MOUNTAINS-EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX, NEVADA: NEW 40AR/39AR THERMOCHRONOLOGIC CONSTRAINTS

The Ruby Mountains-East Humboldt Range-Wood Hills metamorphic core complex (REHW) of northeastern Nevada records a polyphase Mesozoic to Recent tectonic history exposed in a natural crustal cross-section extending to paleodepths >35 km. Though the final exhumation of this terrain indisputably records Neogene extension, the timing of onset, rate, evolving geometry and total magnitude of extension remain issues of debate. New ⁴⁰Ar/³⁹Ar and (U-Th)/He thermochronology (see companion abstract by Metcalf et al.) help to constrain and resolve some of these problems.

Our most densely sampled transect from the central REHW spans ~22.5 km parallel to the extensional transport direction, and records two distinct extensional pulses – one in the Late Eocene (~40 – 35 Ma) and another beginning by ~22-25 Ma and continuing to ~11 Ma. Between these two events, extension likely paused or at least slowed to < 0.5 km/m.y. The earlier extensional pulse is recorded at distances >19 km up-dip from the western flank of the core complex by cooling through ⁴⁰Ar/³⁹Ar hornblende, mica, and (U-Th)/He zircon (ZHe) closure. In addition, bounding isochrons on the high temperature steps of ⁴⁰Ar/³⁹Ar K-feldspar age spectra (Ar-Kfs) and the higher temperature components of Kfs multidiffusion domain models (MDD) also record the Eocene-to-Oligocene episode, marked by cooling rates of 15 – 50 ^oC/m.y. The later extensional pulse is recorded by northwestward younging of ⁴⁰Ar/³⁹Ar mica dates from ~24 Ma to 20.5 Ma over ~15 km up-dip from the western flank of the core complex. (U-Th)/He apatite (AHe) dates, high eU ZHe dates, and lower temperature Ar-Kfs isochrons and MDD models across the full transect also record rapid early to mid-Miocene cooling and WNW-younging suggesting mean extension rates of 2 – 3 km/m.y. The Late Eocene pulse may have accommodated 15 to 25 km extension, whereas the Miocene event likely accommodated ~25 km. Results presently available from farther south suggest that the Miocene event likely extends the full length of the REHW. However, our data support the interpretation of Colgan et al. (2010) that the Late Eocene event likely did not extend so far south. Rather, dates from the eastern part of the 36 Ma Harrison Pass pluton likely record conductive cooling. Nevertheless, the ~22 Ma onset of extension predates their suggested onset at ~17 Ma.