Evolution of Fluid and Deformation Regimes in Extensional and Transpressional Tectonism Revealed Through Electrical Resistivity Structure, with Implications for Seismicity, Gold Occurences and Geothermal Resources
Phil Wannamaker
Energy and Geoscience
Institute
University of Utah
Deformation of the continents is heterogeneous and reflects the interplay between force and strength. Variations of crustal strength in both brittle and ductile regimes have fluid variation and thermal contrasts as principal controls. These physical properties and other geochemical fluxes can be mapped according to their electrical conductivity using EM survey methods, mainly magnetotellurics (MT). Such weaknesses in lithospheric deformation can be primary inputs to geodynamic models considering e.g. body forces, and geochemical flows implied from geophysical structure can provide insight to permeability pathways and ore deposition. Conductivity provides a strong complement to seismic velocity as a physical property because, in addition to basic structural geometries, it provides unique information on geochemistry and petrology of the deep crust and upper mantle. These concepts will be exemplified in two large-scale integrative studies. The first is over the actively extensional Great Basin and its transition to the Colorado Plateau through Nevada and Utah. These data are revealing probable mantle upwelling zones with anisotropic melt textures, emplacement of basaltic melts into the lower crust, complex fluid exsolution therefrom, and crustal scale detachment zones overhead some of which feed into recognized geothermal systems. Here we are glimpsing many of the processes operant during the early-middle stages of continental margin development, whether the Great Basin is headed that far or not. The second is across the active transpressional orogen of the New Zealand South Island. In the continuum deformation regime of the central island, MT has imaged a root zone of prograde metamorphism and fluid generation impacting crustal rheology, large-scale deformation, a framework for large earthquakes along the Alpine Fault, and migration of gold-bearing fluids in a globally relevant ore deposits model. In a new companion transect of the Marlborough strike slip fault system to the north, the advancing subduction zone has induced distinct conductivity structures interpreted to reflect deep hydrate breakdown, shear-interconnected fluids in holding zones below the brittle-ductile transition, and fault-fracture meshes above the brittle-ductile due to shallow plate dewatering. MT with its wide data bandwidth has the potential to follow thermal and fluid processes from upper mantle source regions, through deep crustal storage and transformation, to upper crustal deposition and dispersal zones.
5500 Campanile Dr • 237 Geology Mathematics and Computer Science Building • San Diego • CA 92182-1020 • (619) 594-5586
