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Fault Zone Structure and Rheology from the Surface to the Base of the Lithosphere

Whitney Behr, UT Austin

The large-scale mechanical behavior of continental lithosphere in response to plate tectonic forces depends critically on the structure and rheology of planes of weakness within it (i.e. faults and associated shear zones). Because continental lithosphere is highly heterogeneous in composition, fault zone structure and rheology can be very complex and can show strong variations with depth. As a result, two questions related to continental deformation have remained unresolved for several decades:

1) What is the magnitude of the peak strength in the continental lithosphere and at what depth does the peak strength reside— the upper crust, lower crust or lithospheric mantle?

2) Although faults are narrow in the upper crust, what is their fate below the seismogenic layer where rocks transition from brittle deformation to ductile flow? Do they persist as narrow ductile shear zones, or at some depth do they sole into broadly distributed zones of ductile shear?

Experimental rock mechanics predicts specific answers to each of these questions, and these predictions can usefully be tested using a wide range of observations from different earth science fields, especially structural geology. In this talk I will briefly review the predictions about fault strength and structure that come from rock mechanics— then I’ll discuss past and ongoing research within my research group that aims to test these predictions through a range of observations of naturally deformed rocks derived from a wide range of depths and tectonic settings.



Whitney Behr is an Assistant Professor in the Department of Geological Sciences at JSG. She completed her Bachelor’s degree at California State University Northridge in 2006 and her Ph.D. at the University of Southern California in Los Angeles in 2011. She then spent 11 months at Brown University in Rhode Island as a Postdoctoral Fellow before joining the Jackson School in August, 2012. Whitney is a structural geologist whose research incorporates a variety of field, analytical and experimental techniques all aimed toward understanding continental deformation in both active and ancient orogenic systems. Whitney presently teaches several courses at UT, including undergraduate Structural Geology and Field Camp, as well as graduate courses Microstructures and Rock Rheology, Active Tectonics, and Tectonic Problems.