NON-UNIFORM EXTENSIONAL PROCESSES ACROSS THE GREAT BASIN TO COLORADO PLATEAU TECTONIC TRANSITION, UTAH, REVEALED THROUGH DEEP MT RESISTIVITY SURVEYING

Extensional consumption of stable lithosphere is a fundamental process deforming the continents but it conceivably could exhibit a variety of modes. The Great Basin-Colorado Plateau transition is a unique example in placing a widely-distributed, active extensional domain abruptly against an apparently stable plate province which has seen little deformation during the Phanerozoic. The enigmatic transition zone, although showing a low degree of extension at the surface, may overlie ranges of extensional types as extreme as simple warming and collapse of thickened crust, to highly non-uniform and magmatically active extension where the dominant locus shifts with depth in a variant of whole-lithosphere simple normal shear, according to current ambiguous geophysical data. To establish and describe the extensional processes whereby such a transition may deform, we have collected a 360 km long, detailed transect of 107 wideband and 25 long-period magnetotelluric (MT) soundings across the eastern Great Basin and into the Colorado Plateau interior. The period range of the data spans the range of 0.005 to 10,000 s wave periods, with corresponding imaging depths from about 200 m to 300 km. Two-dimensional regularized inversion of the profile shows a highly conducting lower crust beneath the eastern Great Basin extending eastward under the GB-CP transition at least 100 km beyond the physiographic transition. The most likely cause of the conductor is high temperature saline fluids and possible crustal melts resulting from active basaltic underplating. The most intense lower crustal conductivity zones also overlie low resistivity zones in the upper mantle which may represent melt sources. Also seen is an interesting set of nested, whole-crustal detachment-like structures in the transition zone soling into the lower crustal conductor, although displacement is indeterminate. Colorado Plateau interior has only a slight, Moho-level conductor in keeping with its nearly stable state. Given this family of structures, it appears more likely that a highly non-uniform mode of extension is occurring below this transition than uniform extension from simple lateral heating.