SEISMIC CONSTRAINTS ON LITHOSPHERIC REMOVAL ASSOCIATED WITH THE TRANSITION FROM THE BASIN AND RANGE TO THE GREAT PLAINS AT THE SOUTHERN RIO GRANDE RIFT

East of the Rio Grande Rift, in southeastern New Mexico and west Texas, the lithosphere transitions from the Basin and Range to the Great Plains. Recent analyses of EarthScope seismic data, including body wave tomography and receiver function imaging, indicate that this transition is accompanied by significant increases in depth of the Moho (which ranges from 36-54 km depth) and the seismically-inferred Lithosphere-Asthenosphere Boundary (LAB) (75-112 km depth). Both boundaries occur at generally shallower depths in the vicinity of the Rio Grande Rift, in the west, and at deeper depths beneath the Great Plains craton, in the east.

While the observed lithospheric thinning is broadly consistent with the lateral extension that has occurred in this region since the Miocene, a “gap” in the LAB above a large, seismically fast anomaly in the upper mantle suggests that a more dramatic process of lithospheric removal has occurred, or is perhaps underway. This gap also corresponds to high surface topography, which may be due to thermal uplift after a detached lithospheric block has sunk into the mantle. We note that the timing of the uplift is not well-constrained but is likely to have occurred as a component of the development of the Rio Grande Rift, which initiated at 35 Ma.

The southern portion of the RGR has undergone significantly more extension than the northern portion and the rift itself appears to be propagating northward. Progressively more northerly cross-sections, may therefore represent a time progression, and the LAB gap is confined to the region between 32°N and 33°N (the region subjected to the greatest extension) and does not appear at 34°N. (Our deployment configuration does not allow us to image south of 32°N or north of 34°N.) We therefore propose that the LAB gap and seismically-fast mantle anomaly in the upper mantle are indications that the lower lithosphere has been removed by a process that is associated with east-west extension, and thus the northward propagation, of the Rio Grande Rift. Whether this process has also modified the Great Plains craton is unclear.