DETERMINING OLIGOCENE TOPOGRAPHY AND TECTONISM ACROSS THE NORTHERN SIERRA NEVADA AND WESTERN BASIN AND RANGE USING STABLE ISOTOPE PALEOALTIMETRY IN VOLCANIC GLASS

Reconstructions of past elevations can improve our understanding of the timing and drivers of mountain building and landscape evolution. Although the topographic and tectonic history of the Sierra Nevada is controversial, recent research suggests that the northern Sierra may have acted as the steep western flank of an early Cenozoic high elevation plateau – the ‘Nevadaplano’ that covered much of what is now Nevada and western Utah. To determine the Oligocene topography and hydrology across western-central Nevada, 31-24 Ma ignimbrites were sampled for analysis of hydrogen isotope compositions (δD) of hydration water in volcanic glass, and these data were compared to coeval samples across the northern Sierra. Volcanic glass incorporates and preserves meteoric waters soon after emplacement, reflecting the influence of ancient topography and hydrology on precipitation. Samples from the CA-NV border east to Carlin, NV, show a gradual decrease in δD from values ranging from -139‰ to -146‰ to values from -153‰ to -167‰ (± 3‰); a significantly lower gradient than that documented to the west of the crest. This may reflect a gradual increase in mean elevation from west to east across that area in the Oligocene, or no change in elevation in a partially closed hydrologic system. The majority of δD values of samples from western Nevada display an initial increase of 6-14‰ from those collected in the Sierra Nevada, which may reflect the introduction of another moisture source or evaporation. This is not present in modern meteoric waters, which supports the conclusion that volcanic glass preserves the δD of initial hydration waters. In addition to providing verification of the paleoaltimetry method in volcanic glass, these results provide further insights into the tectonic evolution of the area. We favor the interpretation that the Sierra Nevada acted as the steep western flank of a gradually sloping high-elevation plateau in the Oligocene, and that Miocene to Recent extension lowered elevations across what is now the Basin and Range, associated with gravitational spreading of over-thickened, magmatically and radiogenically heated crust. Although Sierra arc volcanism stopped in the Late Cretaceous, Laramide compression and rock uplift may have continued across the region, sustaining a high-elevation plateau through the Oligocene.