A SEISMIC ANOMALY IN THE UPPER MANTLE BENEATH THE TRANS PECOS

A recent broadband seismic deployment in southeastern New Mexico and west Texas, called “SIEDCAR”, allows us to image with new clarity the crustal thickness, lithosphere-asthenosphere boundary (LAB), and three-dimensional variations in P- and S-wave speeds beneath the Trans Pecos region. A large anomaly of unusually fast Vp and Vs, distinct from but parallel to the Great Plains “craton”, extends from near Portales, NM southward through the Big Bend region into northeastern Chihuahua, Mexico. Abrupt changes in both the LAB and Moho coincide with boundaries of this anomaly, as do changes in surface topography.

We will discuss the proposition that the anomaly is associated with Trans Pecos volcanism and, specifically, the relative merits of processes that might explain the anomaly’s origin. Among these candidate processes are the possibility that the anomaly results (wholly or in part) from fractionation of delaminated lower lithosphere. Another is that the anomaly represents a downward component of a small-scale convection cell. Convection could be driven locally by the thickening of the lithosphere to the east, as in “edge-driven convection”, or by mantle diapirs, proposed by some investigators to arise from a thermal boundary layer on the 410-km discontinuity, among other possibilities. A third model is that the anomaly represents a fragment of Great Plains lower lithosphere that was cleaved off by the “Delaware Aulacogen”, a mafic intrusion proposed, by some, to be a failed rift associated with the Grenville orogeny at ~1.1 Ma. A fourth model holds that the delamination of the lower lithosphere propagates eastward from the west, where a dramatically slow anomaly coincides with the Rio Grande Rift in our images. None of these latter mechanisms is incompatible with a link to Trans Pecos volcanism.