Tectonic and magmatic transitions in the Western Great Basin, USA

The isotope and trace-element geochemistry of late Cenozoic basalts from the Western Great Basin of California and Nevada demonstrates contributions from two distinct mantle source regions. High concentrations of large-ion lithophile relative to high-field strength elements (50 < Ba/Nb < 250) are often taken to reflect a contribution from contemporaneous subduction processes. But in this area they are associated with a marked change in ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd across a major lithospheric boundary, which is normal to the orientation of the palaeo-subduction zone and implies that these samples derived their isotope and trace element signatures from the mantle lithosphere. The second component is similar to that seen in ocean island basalts, and is attributed to asthenospheric source regions within the convecting upper mantle. Significantly, the onset of asthenosphere dominated magmatism migrated northwards during the Plio-Pleistocene about 2-3 Myr behind the trailing edge of the subducted slab. During this period it is proposed that asthenospheric diapirs rose from the level of the subducted slab into the zone of melt generation, implying ascent rates of between 5 and 8 cm per annum.