New Mexico Geological Society Annual Spring Meeting — Abstracts

The LA RISTRA seismic experiment extends northwestward 1400 km from west Texas, on the North American craton, across the Rio Grande rift (RGR) and Colorado Plateau (CP) in New Mexico, Arizona, and Utah, to the Basin and Range province of western Utah. The experiment consists of two deployments (the second of which is still in progress) of IRIS PASSCAL broadband seismometers spaced at intervals of approximately 20 km. The line is parallel to the Laramide-age (70-40 Ma) subduction zone at the western North American plate margin, entirely crossing a region of postulated Laramide flat-slab subduction (Saleeby, 2003, GSA Bull., 115, 655). Studies include shear-wave splitting, travel-time tomography, and analysis of surface-waves and receiver functions.

Although still in progress, preliminary conclusions arising from the full 1400 km LA RISTRA include: (1) Crustal thickness is 45-50 km beneath the CP, 35 km beneath the RGR, and 44 km beneath the Great Plains (GP). Thus, the RGR is coincident with upwarp of asthenosphere mantle. The broadly symmetric crustal thinning beneath the Rio Grande rift is indicative of a pure-shear mechanism of lithospheric thinning and suggests that extension may be gravity-driven. (2) Subcrustal lithospheric mantle beneath the Colorado Plateau is preserved to a depth of 125-150 km (i.e., was not removed by Laramide subduction). The presence of ancient lithospheric mantle is compatible with geochemical and isotopic attributes of Miocene to Quaternary magmatism in the region. (3) Northwest-dipping mantle discontinuities are imaged beneath the CP, and other subcrustal discontinuities beneath the GP. These may represent preserved ancient structures, remaining from the original formation of Proterozoic lithosphere. Deeper (>300 km) discontinuities may represent the foundering Farallon slab and part of an upper mantle convection cell. Finally, (4) flat discontinuities at depths of 410 and 600 km do not support a large-scale thermal anomaly beneath the RGR region at these depths, precluding a deep-seated “active” rifting mechanism.

LA RISTRA is supported by the National Science Foundation and by the Los Alamos National Laboratory Institute of Geophysics and Planetary Physics.