TECTONIC CONTROLS ON BASALTIC VOLCANISM NEAR YUCCA MOUNTAIN, NEVADA

Plio-Pleistocene basalts erupted near Yucca Mountain are slightly sodic, transitional alkaline-olivine basalts or trachybasalts having high LREE contents, low Rb/Sr and relatively high 87Sr/86Sr (~.707). Such basalts, which are typical of coeval basalts of the western Great Basin, are generally thought to originate as partial melts of metasomatized lithospheric mantle, uncontaminated by crustal components. Different volcanoes have different basalt chemistries; either the mantle source is chemically heterogeneous on a scale of about one kilometer, or melts evolve differently within spatially restricted enclaves. Each volcano probably represents a batch of magma gathered from a resident equilibrium melt and provided an intrusive pathway by local tectonism. Given the low rate of post-Miocene extension near Yucca Mountain (>2 mm/yr), melt generation is likely not influenced by decompression, and given the small extrusive volumes, magma accumulation and ascent are likely not influenced by buoyancy. Most of the volcanic centers are located in Crater Flat basin, west of Yucca Mountain, or in the Amargosa Valley to the south. Some of the cones show a NNE-oriented alignment. The NNE-aligned small volume eruptions, relatively high volatile content of the extrusions, and lack of crustal contamination or evidence of fractionation, imply that basalt magmas ascend directly from the upper mantle source along fractures influenced by dextral transtension. Commonly accepted detachment mechanisms fail to account for the localization, ascent, and timing of basaltic volcanism, nor do they provide insight into tectonic mechanisms that could affect magma genesis in the upper mantle melt source zone. A tectonic model that treats Crater Flat basin and the Amargosa trough as a graben/rift feature modified by dextral shear, and accounts for dilational effects and fracturing in the upper mantle melt zone, provides a through-the-crust mechanism for basalt intrusion as well as a structural association with observed tectonic features. Volcanism should decrease over time as local magma reservoirs are depleted and the solidus is pushed to greater mantle depths with a cooling lithosphere. This postulate is consistent with apparent waning volcanism.