West et al., Structure of the Uppermost Mantle Beneath the RISTRA Array From Surface Waves

The RISTRA array extends from the Colorado Plateau across the Rio Grande Rift into the Great Plains. We explore the state of the uppermost mantle beneath these three distinct geologic provinces to understand how the lithosphere and asthenosphere are related to the tectonic evolution of these regions. 57 three-component broadband seismometers of the RISTRA array provided teleseismic and regional waveforms sampling the crust and upper mantle beneath the array. Rayleigh wave phase velocities for each region are obtained by stacking fourier spectra for many stations and events to find the optimum slant stack velocity. Groups of stations are used to calculate lateral variations in phase velocity for periods of 7 to 150 seconds. The surface wave velocities are inverted for shear velocity structure using crustal constraints from receiver functions and mantle constraints from teleseismic tomography. The velocity structure beneath the Great Plains is similar to what is observed in cratonic shields. The interior of the Colorado Plateau has anomalously low shear wave velocities below 100 km. The velocities are up to 5% lower than at comparable depths beneath the Great Plains, suggesting that a warm mantle may be responsible for the observed uplift across the plateau. Near the Rio Grande Rift in the central part of the array we find a broad region of slow and presumably hot mantle immediately beneath the Moho extending to depths of at least 200 km. Shear velocities in the uppermost mantle are decreased by 8% under the rift. High temperatures are the most likely explanation for the low shear velocities. To explain these velocities requires temperatures that are elevated by several hundred degrees relative to adjacent regions. A strong gravity low across the rift, despite thin crust, is further evidence for hot mantle. The decrease in viscosity that accompanies high temperatures is incompatible with a thick lithosphere. We suggest that the mantle lithosphere beneath the Rio Grande Rift is thinned to a few tens of kilometers. The assertion of thin lithosphere is consistent with independent petrologic and shear wave splitting results.

For more information on LA RISTRA surface waves contact:

Mike West west@nmsu.edu,
New Mexico State University, Department of Physics (MSC 3D), Las Cruces, NM 88003