WHAT DRIVES EXTENSION IN DEATH VALLEY NATIONAL PARK, CA

Tectonic and geochronological data suggest that Neogene to Recent magmatism was coeval with crustal extension in the southern Death Valley region, CA. These geologic processes may be related by (1) cessation and relaxation of compressive stress (ie subduction along the west coast of Laurentia), and (2) the rise of the asthenosphere and associated decompression melting in lithospheric or sublithospheric mantle. Modern plate reconstruction suggests that subduction stopped at the latitude of Death Valley when the East Pacific Rise was subducted at ca 12 Ma; relaxation of compressive stress resulted in subsequent rise of the asthenosphere. The buoyant rise of the asthenosphere inflated and extended the lithosphere, and resulted in decompression melting of uppermost mantle domains beneath Death Valley. Our chemical and isotopic data suggest that synextensional basalts and trachyandesites in southern Death Valley were derived by mixing of asthenospheric and lithospheric mantle melts. The small volume of these mafic rocks, however, is a problem. Low-volume melting may be the result of limited extension (and therefore decompression melting), or the relatively cold and old lithospheric mantle beneath Mojavia allowed only small-fraction melting. Combined, our data support the hypothesis that Neogene extension in southern Death Valley was driven by relaxation of compressive stress followed by decompression melting of sublithospheric and lithospheric mantle domains in response to rising asthenosphere.