UNDERTHRUSTING, UPPER-CRUSTAL COOLING, AND PROGRADE METAMORPHISM IN THE NORTHERN PANAMINT RANGE, CA: NEW INSIGHTS INTO THE EVOLUTION OF THE SOUTHERNMOST SEVIER FOLD-THRUST BELT

We present a reconsideration of field relations supplemented by new U-Pb geochronology in the northern Panamint Range, California, that demonstrates the existence of a major hinterland or magmatic arc-style reverse fault, as well as new thermometric and thermochronometric data that record peak metamorphic temperatures and the Mesozoic through Cenozoic upper-crustal cooling history of this segment of the southern U.S. Cordillera. A steeply dipping, Miocene (ca. 14 Ma; new U-Pb zircon MDA) alluvial conglomerate in the northern Panamint Range unconformably overlies the hanging wall of the Panamint thrust, which juxtaposes Cambrian units over Permian units and is subhorizontal in outcrop. Rotating this package to restore deformation post-dating the deposition of the Miocene conglomerate implies that the Panamint thrust dipped ~55° to the west-northwest during fault slip, while hanging-wall stratigraphy was subhorizontal and footwall stratigraphy was folded into a syncline with isoclinal limbs parallel to the thrust plane. Hanging-wall prograde metamorphic temperatures derived from Raman spectroscopy of carbonaceous material thermometry are consistent with greenschist facies conditions at the deepest exposed stratigraphic levels and define a metamorphic field gradient of 39 ± 2°C/km that developed during the Middle-Late Jurassic period. Hanging-wall zircon (U-Th/He) thermochronology data preserve a possible interval of enhanced cooling between ca. 110-100 Ma and an interval of slow cooling from ca. 100-32 Ma. The first, transient cooling event may reflect hanging-wall exhumation during slip along the Panamint thrust or reheating of the upper crust during the middle to Late Cretaceous. The protracted, mid-Cretaceous through Oligo-Miocene event is interpreted to reflect the erosional denudation of a thickened orogenic welt. We discuss competing Mesozoic tectonic models that reconcile these data and produce coherent along-strike correlations to other structures within the southern Sevier fold-thrust belt, including ones in which: 1) much of the bedrock in the northern Panamint Range lies within the Lee Canyon thrust sheet, which may have been emplaced at ca. 110-110 Ma; and 2) nearly all of the Panamint Range lies within the Wheeler Pass thrust sheet, which was emplaced during the Late Jurassic-earliest Cretaceous, but upper-crustal ZHe ages were reset by increased heat flux between ca. 110-110 Ma.