A Revised Cooling and Extensional Exhumation History for the Harrison Pass Pluton, Southern Ruby Mountains Metamorphic Core Complex, Elko County, Nevada

A key question in the tectonic evolution of the Sevier orogenic belt of the western
U.S. Cordillera is when and why the overthickened crust of the hinterland plateau
began to collapse giving rise to the modern extensional tectonic regime. Delineating
the exhumation history of the Ruby Mountains, East Humboldt Range and Wood
Hills metamorphic core complex (REHW) of Elko County, Nevada offers important
evidence bearing on this question. Recent work from the northern REHW records a
three-phase extensional history: (1) ~15–20 km of Late Eocene extension, (2) a second
pulse of extension of similar rate and magnitude beginning in the late Oligocene or
early Miocene (by 21 Ma) and continuing to approximately 11 Ma, and (3) the Basinand-
Range extensional regime continuing at reduced rate to today. In contrast, previous
work from the Harrison Pass area in the southern REHW does not recognize an
imprint from the Late Eocene phase of extension, and places the onset of the second
extensional phase after ~17 Ma. New intermediate closure temperature thermochronology
from the Harrison Pass pluton indicates that it remained at significant depth
until at least ~25 Ma, severely limiting any possible Late Eocene to early Oligocene
extension, consistent with previous interpretations. However, the new results challenge
the previously proposed post-17 Ma onset for extension at Harrison Pass. New,
intermediate closure temperature (U-Th)/He titanite and zircon ages from the eastern
half of the pluton almost entirely predate 17 Ma and instead support an extensional
onset bracketed between the Early Miocene (21 Ma) and the late Oligocene (25 Ma).
Integrating potassium feldspar 40Ar/39Ar multi-diffusion domain modeling with the
lower closure temperature thermochronometric systems reveals an inflection to faster
cooling rates after ~25 Ma and further supports this inference. Nevertheless, all but
the farthest east and structurally shallowest of the samples also show a second inflection
point at ~17 Ma. We argue that previously reported apatite fission track and apatite
(U-Th)/He data captured this post-17.5 Ma reacceleration event but missed the
earlier, late Oligocene-early Miocene extension recorded by the higher temperature
thermochronometers. The latest Oligocene to early Miocene extensional phase correlates
with extensional events reported from southern Nevada and Arizona that may
relate to the relaxation of contractional boundary conditions during the early evolution
of the San Andreas margin. However, the post-17.5 Ma resurgence in extension
probably correlates with large-scale crustal weakening across the northern Basin and
Range province attending the arrival of the Yellowstone thermal plume.
Key Words: Ruby Mountains, Extension, Thermochronology, Core Complex