Chemical and isotopic evidence for lithospheric thinning beneath the Rio Grande rift

Abstract

Continental rifting disrupts the lithosphere, thermally and chemically modifying the upper mantle and crust beneath a rift. The isotopic and chemical compositions of basaltic rocks associated with continental rifting reflect the physiochemical conditions of their mantle source reservoirs and can be used as probes to trace processes associated with modification of the upper mantle. Basalts of different ages can potentially record the evolution of mantle reservoirs beneath rifts. Late Cenozoic alkali basalts from the Rio Grande rift and adjacent tectonic provinces display a wide range of isotopic values that correlate with tectonic setting and upper-mantle geophysical properties. This correlation indicates that the upper mantle beneath the rift region is composed of two distinct geochemical reservoirs1 that, before lithospheric extension, corresponded to the ancient lithospheric mantle and asthenosphere, part of the convecting upper mantle. Here we discuss major-element and isotopic data for basalts in a small region of the southeastern Colorado Plateau transition zone adjacent to the central Rio Grande rift. Basalts from this area record an episode of lithospheric thinning that occurred between 8 and 4 Myr ago, concurrent with regional uplift and reactivation of rifting.