PROBING THE CORE OF THE NORTH AMERICAN CORDILLERA -- INSIGHTS FROM GEOLOGIC MAPPING OF THE PEQUOP MOUNTAINS, NE NEVADA

The geology in NE Nevada provides clues about the North American Cordillera, including Mesozoic contraction and crustal thickening as part of an inferred orogenic plateau, and subsequent Cenozoic extension. Various tectonic models dispute the timing, magnitude, and style of crustal thickening and subsequent extension and crustal thinning. The Ruby Mountain-East Humboldt Range (REHR) core complex of northeast Nevada records this complex history, but different investigative approaches yield numerous disparate interpretations. For example, classic geobarometry studies suggest deep Mesozoic peak burial (>20-30 km) for the Paleozoic stratigraphy, which is more than double estimates from palinspastic reconstructions based on mapping and field relationships. These conflicting interpretations require different magnitudes of Cenozoic extension and denudation, and dissimilar peak geotherms that vary by a factor of two: 20-25°C/km or 40-50°C/km. To resolve these discrepancies, we have been conducting detailed geologic mapping in the least deformed eastern part of the REHR system in the Pequop Mountains. Here we present new 1:24k mapping across three quadrangles from the Pequop Mountains with our growing temperature (i.e., conodont alteration index, raman spectroscopy on carbonaceous material, and garnet-biotite exchange) and time (i.e., Ar-Ar and U-Pb dating) datasets to address these issues. Important observations include: (1) cross-cutting relationships suggest dominantly Jurassic contractional deformation with negligible Cretaceous shortening or related crustal thickening; (2) temperature-depth correlations are inconsistent with deep burial of Paleozoic stratigraphy, (3) numerous small Jurassic, Cretaceous, and Eocene intrusions locally metamorphosed the Paleozoic section and led to the relatively high observed geothermal gradient (>40°C/km); and (4) normal-fault observations demonstrate >3 km of stratigraphic section omission with an unclear temporal relationship to an Eocene paleovalley and volcanic rocks. Similar enigmatic relationships have been documented in other Cordilleran core complexes, such as the Snake Range to the south. This study highlights the importance of geologic mapping to document critical map relationships to address long-standing tectonic issues.