LITHOSPHERIC EROSION AT LAURENTIA'S SOUTHWESTERN EDGE

The Rio Grande Rift (RGR) marks the western edge of the stable North American craton in the southern United States. Much of the tectonic activity in the southwestern United States can be attributed to the foundering of the Farallon slab at ~40 Ma. However, a tectonic lull in the region followed the slab's foundering and active rifting of the Rio Grande Rift and the Southern Rockies dates to just ~10 Ma. Small-scale “edge-driven” convection may explain the pattern and timing of recent tectonic activity. and such convection would be expected to produce both mantle downwelling and crustal thinning. A fast seismic velocity anomaly can be seen in the mantle beneath the eastern flank of the Rio Grande Rift in the results from the linear La Ristra seismic deployment (Gao et al., 2004) and the crust appears to thin above this anomaly (Wilson et al., 2003). These results suggest that a closer look at the 3D geometry of the crust and upper mantle beneath the eastern flank of the Rio Grande Rift is warranted.

In 2008, as part of a Seismic Investigation of Edge Driven Convection Associated with the Rio Grande Rift (SIEDCAR), investigators deployed a two-dimensional array of broadband seismic stations interspersed between stations of EarthScope's Transportable Array (TA) in southeastern New Mexico and west Texas. A total of 164 seismographs distributed with an average lateral spacing of 35 km recorded earthquakes from around the world between August 2008 and July 2010. A combination of seismic analyses, including body wave and ambient noise tomography, shear wave splitting measurements, and receiver functions reveal a fast anomaly in the upper mantle beneath west Texas that is distinct from, but comparable in wavespeed to, the Great Plains craton. We will summarize this evidence and discuss its implications for the evolution of both rifts and cratons.