PRELIMINARY RESULTS FROM A MAGNETOTELLURIC PILOT STUDY OF THE RIO GRANDE RIFT AND SOUTHERN ROCKY MOUNTAINS, SOUTHWEST USA

The tectonic forces and rheologic properties controlling the initiation and development of continental rifting are poorly understood. Within the Rio Grande Rift, it is difficult to reconcile the surface expression of rifting, consisting of volcanism and asymmetrical basin structures confined to a narrow band of deformation, with results from seismology and GPS geodesy that indicate a much broader zone of lithospheric modification. To address these questions, a magnetotelluric pilot study is underway in and around the Rio Grande Rift and southern Rocky Mountains. The magnetotelluric (MT) method is a passive electromagnetic imaging technique used to map subsurface variations in electrical resistivity, and is in particular sensitive to partial melt, aqueous fluids, and mineral alteration. Magnetotelluric methods provide information complementary to that of seismic tomography, and can be used to distinguish between thermal and compositional variations in the crust and mantle. Magnetotelluric data may also be used to constrain geodynamic modeling on a lithospheric scale. During the summer of 2012, broadband (100 Hz to 1000 s) and long-period (up to 10 000 s) MT data were collected at 30 sites distributed along two profiles oriented perpendicular to the rift axis at the latitude of Taos, New Mexico and Denver, Colorado. Station coverage in CO includes sites in the High Plains, the Front Range, and the western slope of the Rocky Mountains near Rifle. Coverage in NM consists of stations spanning the central rift axis, including sites in the Sangre de Cristo Mountains and the Taos plateau. Data analysis shows good signal strength to 10 000 s; initial 1D and 2D modeling demonstrates the ability to image upper mantle structure, despite generally high upper-crustal conductivity. This study represents the first phase of an ongoing project that will be continued in summer 2013.