New Mexico Geological Society Annual Spring Meeting — Abstracts

152 new apatite (U-Th)/He dates are presented from 34 sample locations along the flanks of the Rio Grande rift in New Mexico and Colorado. These data are combined with existing apatite fission track analyses of the same rocks and modeled together to create well constrained cooling histories from ~120-30 °C for the rift flanks. When combined with existing apatite (U-Th)/He dates from northern Colorado, these data together encompass >850 km of the length of the Rio Grande rift, and provide the time-space constraints needed to test geodynamic models for initiation and evolution of continental rifting. The new data and cooling models indicate that the rift opened simultaneously and abruptly from northern Colorado to southern New Mexico, with large magnitude fault slip and exhumation of rift flanks between ~20-10 Ma.

We propose a new geodynamic model of near-synchronous extension involving foundering of the Farallon slab that was controlled by thick cratonic keels beneath Wyoming, the Great Plains, and western Texas/southeastern New Mexico. As the shallowly-subducting Farallon slab encountered these thicker regions it was forced to tear and decouple from the base of the North American lithosphere. Incoherent foundering of the slab in the Oligocene is evident in the migration of silicic calderas through time. The San Juan volcanic field in Colorado displays a southwest sweep in volcanism, the Mogollon-Datil volcanic field in New Mexico displays a west sweep in volcanism, and the Trans-Pecos volcanic field in Texas displays a southwest sweep in volcanism. In our model, ~35-25 Ma ignimbrite events and associated lithosphere delaminations led to near-simultaneous foundering near the bend in the Farallon slab from northern Colorado to southern New Mexico in the region underlying the present-day Rio Grande rift. This event triggered small-scale mantle convection and a change in crustal stress regimes that ultimately drove near-synchronous surface extension ~20-10 Ma. Our model accounts for the observed temporal and spatial patterns of extension along >850 km of the length of the Rio Grande rift while placing these results within the Cenozoic tectonic and magmatic history of western North America.