Modes of Lithospheric Extensional Consumption and Magmatism: Testing for End-member Cases at the Great Basin-Colorado Plateau Transition with a Magnetotelluric Resistivity Transect

University Of Utah, Salt Lake City UT

Investigators

Abstract

EAR-0230027 Wannamaker EAR02-30027: Modes of lithospheric extensional consumption and magmatism: testing for end-member cases at the Great Basin-Colorado Plateau transition, Utah, with a magnetotelluric resistivity transect P.I.: Philip E. Wannamaker (University of Utah, Energy & Geoscience Institute, 423 Wakara Way, Suite 300, Salt Lake City, UT 84108, pewanna@egi.utah.edu) Abstract End-member possibilities for stable lithospheric extension will be tested according to their influence on physical properties of the deep crust and upper mantle across the Great Basin-Colorado Plateau transition zone, south-central Utah. The main geophysical property to be investigated is electrical conductivity, together with its implications for thermal and rheological state, which will be imaged using the magnetotelluric (MT) technique. New MT surveying primarily across the transition zone (TZ) per se will join existing data into an E-W transect nearly 400 km in length, with a depth range of sensitivity from 100's of m to 250-400 km approximately. Data will be acquired using wideband and long-period recorders of the University of Utah and University of Washington, which reside in the NSF-supported EMSOC instrument facility. Existing geophysical indicators show a strong N-S strike orientation at upper crustal to lithospheric scales and should allow accurate cross sections of model conductivity using mainly current 2-D regularized inversion platforms. Substantially reduced electrical resistivity values at lower crustal and upper mantle depths inferred from MT field data most likely reflect melts and exsolved ionic fluids. Previous surveying indicates that the onset of low resistivity in the deep crust occurs for T > ~500-550 o C in high-grade metaigneous host rocks. Depth variation in low resistivity thus will approximate an isotherm which should evolve gradually from the GB to the CP in the case of uniform TZ warming, or show an upwelling under the little-extended TZ in the case of non-uniform, enhanced extension at depth. Exsolved fluids weaken the lithosphere greatly through diffusion creep, so resistivity will provide a clear view of at least relative rheological contrasts. Conductivity, heat flow and constraints on composition and timing of extension, will be used in time-dependent, 2-D thermal modeling for an improved projection of the thermal field into the upper mantle.