LITHOSPHERIC EROSION AT THE SOUTHERN EDGE OF LAURENTIA?

While much of the tectonic activity in the southwestern United States can be attributed to the foundering of the Farallon slab at ~40 Ma, a tectonic lull in the region followed the slab's foundering. 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; such convection would be expected to produce both mantle downwelling and crustal thinning. The linear La Ristra seismic deployment (1999-2004) imaged a fast seismic velocity anomaly in the mantle beneath the eastern flank of the Rio Grande Rift and a thinned crust above this anomaly. 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, in conjunction with EarthScope, is warranted.

In 2008, as part of a Seismic Investigation of Edge Driven Convection Associated with the Rio Grande Rift (SIEDCAR), we 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.

Body wave tomography using SIEDCAR and TA data reveals fast P- & S-wave anomalies in the upper mantle beneath the Trans-Pecos region that are spatially distinct from the Great Plains craton. Receiver function imaging reveals dramatic changes in crustal thickness over short spatial scales. At least one of these dips in the Moho correlates with a mantle anomaly directly beneath it in the mantle. Studies of seismic anisotropy, characterized with SKS splitting measurements, show a fast polarization direction that aligns broadly with the average plate motion of North America but with several coherent trends that correlate with both surface topography and the anomalies imaged tomographically in the mantle.

The accumulated seismic evidence suggests that edge-driven convection is not responsible for these anomalies. Chemical heterogeneity is likely to contribute to the anomalies in addition to thermal heterogeneity; processes and events that could produce such anomalies will be discussed.