Abstract

Early twentieth-century geologists ob-served extensive areas of subdued topography in the otherwise strongly dissected landscape of the southern Sierra Nevada that they ascribed to periods of stability following episodes of orogenic uplift. From this evidence, they developed an uplift history for the Sierra Nevada. This conceptual model for landscape evolution of the Sierra has largely been rejected or ignored since the middle of the twentieth century, based in considerable part on observations indicating that these surfaces can be explained as dynamic landforms actively developing in response to conditions of the relatively recent geological past and that long-term erosion rates are much too high to permit pre-Quaternary surfaces to be preserved. We investigated the chronology of basaltic flows and ejecta on several such apparent relict surfaces located west of Bishop, California. Most of the basalt cones and flows that were sampled are nearly concordant with the present topography, generally showing no more than 50 m of landscape inversion. The 40Ar/39Ar ages of the basalts fall into two groups: 11.76 ± 0.05 Ma and 3.40 ± 0.06 Ma. These ages demonstrate that paleosurfaces dating back to at least the Miocene can be preserved close to the summits of the Sierra Nevada with minimal erosion. The elevations and relative sequences of these surfaces do indeed appear to preserve a topographic history of the Sierra Nevada, encouraging continued investigation of their significance within the tectonic history of the Sierra Nevada.

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