The Role of Crustal Fluids in Strike-slip Tectonics: New Insights from Magnetotelluric Studies

Authors: MARTYN UNSWORTH

Abstract: The presence of fluids in the Earth's crust can dramatically change the rheology and may control a wide range of tectonic processes, especially in regions characterized by strike-slip deformation. Fluids such as water and partial melt change the electrical resistivity of the subsurface and may be detected through geophysical techniques that remotely sense electrical resistivity. For imaging to crustal and upper mantle depths, the most useful technique is magnetotellurics (MT) that uses natural electromagnetic waves as an energy source. Magnetotelluric studies of the Tibetan Plateau have detected a widespread mid-crustal layer of partial melting across almost the entire north-south extent of the plateau. This provides a locus for deformation that decouples the upper and lower parts of the lithosphere and allows the continued convergence of India and Asia to extrude the Asian lithosphere to the east. The melt layer terminates at the North Kunlun fault, one of the major strike-slip faults that accommodate the eastward extrusion. A detailed magnetotelluric study of the San Andreas Fault in Central California has imaged a wedge of fractured, fluid-saturated rock in the upper 3-5 km of the fault zone. The micro-earthquake distribution shows that seismicity begins at the base of this zone. Fault segments with a higher fluid budget exhibit creep, while the relatively dry fault segments are generally locked.

Keywords: crustal fluids, magnetotelluric, strike-slip tectonics, transform plate boundaries, crustal deformation

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