DEEP ELECTRICAL CONDUCTIVITY STRUCTURE OF THE RIO GRANDE RIFT IN COLORADO AND NEW MEXICO: EARLY RESULTS FROM A TWO-YEAR MAGNETOTELLURIC STUDY

A wideband and long-period magnetotelluric experiment is underway across the Rio Grande Rift in Colorado and New Mexico in order to provide constraints on the thermal and rheological state of the lithosphere beneath this region of intra-continental extension. Magnetotellurics is a passive source electromagnetic technique that at long periods has depth penetration into the deep crust and upper mantle. Important questions about continental rifting remain unresolved, including the role of magmatism, volatiles and inherited lithospheric structure in the initiation and development of rifting. Recent seismic imaging studies show thinned crust and low seismic wavespeeds in the crust and upper mantle beneath the Rio Grande Rift. New and ongoing geodetic work confirms the low strain-rate environment of the region yet shows surprisingly uniform deformation over an area far wider than the rift’s physiographic expression. Electrical conductivity models from this experiment will provide information complementary to these studies and may be used to determine the relative contributions of thermal and compositional heterogeneity in the crust and upper mantle to processes of continental extension.

Over the past two years, magnetotelluric data has been collected at ~100 site locations along three 450 km long east-west transects of the rift axis. These three profiles extend across the northern, central, and southern portions of the rift and include sites in the High Plains, Colorado Front Range, southern Rocky Mountains, San Juan Basin, Sangre de Cristo Mountains, and southern Basin and Range along the New Mexico/Mexico border. A comparison of results from these segments will be used to examine along-strike variation in the spatial extent of rifting and associated modification of the lithosphere. Data assessment shows high-quality signal to periods in excess of 10 000 s, which corresponds to upper-mantle depths in this region of high upper-crustal conductivity and low crustal thickness. Resistivity cross sections of the central and southern segments of the rift, stitched together from preliminary one-dimensional modeling, will be presented, as well as a comparison to existing seismic models of the study area and an overview of the potential tectonic implications of this new data set.