Prograde Metamorphism and Upper-Crustal Cooling in the Northern Panamint Range, California: New Insights into the Jurassic and Mid-Cretaceous Evolution of the Southern Sevier Fold-Thrust Belt

The Jurassic-Cretaceous southern Sevier fold-thrust belt, which experiences changes in orientation and structural style as it approaches intersection with the Mesozoic continental arc, has been pervasively overprinted by Neogene-Quaternary faulting, rendering its evolution enigmatic. Raman spectroscopy of carbonaceous material (RSCM) thermometry data from 14 samples and zircon (U-Th)/He (ZHe) thermochronology data from 23 samples collected in the northern Panamint Range illuminate the history of metamorphism and upper-crustal cooling associated with this belt of shortening structures. Best estimate peak temperatures derived from RSCM thermometry increase from 166 ± 19°C at 2.5 km structural depth (measured from the top of the exposed Pennsylvanian-Permian Bird Spring Formation) to 470 ± 84°C at 10.5 km structural depth, defining a prograde metamorphic field gradient of 39 ± 2°C/km that likely developed during the Middle-Late Jurassic period. ZHe data preserve a Mesozoic fossil He partial retention zone with a base at ~6.2 km structural depth, an interval of enhanced cooling between structural levels of 6.2-7.4 km between ca. 110-100 Ma, and an interval of slow cooling between structural levels of 7.4-11.1 km from ca. 100-32 Ma. The interval of slow cooling is extended to 7.4-12.3 km from ca. 100-24 Ma including previously published data from the central Panamint Range. The transient, mid Cretaceous cooling event, previously undocumented in the southern Sevier belt, is interpreted to reflect hanging-wall exhumation during slip along the Panamint thrust, which dips to the west and roots beneath the Panamint Range. The protracted, mid-Cretaceous through Oligo-Miocene event is interpreted to reflect the erosional denudation and isostatic uplift of thickened orogenic crust. I suggest that much of the exposed bedrock in the northern Panamint Range is within the Lee Canyon thrust sheet, rather than the Wheeler Pass thrust sheet, and that the northern Panamint Range may exhibit a structural style intermediate between classic foreland structures in the Spring Mountains and ductile shearing near Clark Mountain, with footwall collapse and underthrusting resulting in subdued hanging-wall uplift, erosion, and cooling