Non-steady behavior in shear zones and its implication for slip transience in the lithosphere
Shear zones operate at the roots of every major fault system on Earth, and their dynamics exert a significant control on the evolution of these systems. Most exhumed shear zones are complex, heterogeneous systems with a hard-to-unravel deformation history. Despite extensive documentation of transient behavior, almost all structural models – conceptual and numerical – of shear zones implicitly assume that deformation is steady state (constant strain rate) through time. This talk examines processes that might be hiding behind this assumption using 1) and analytical model of deformation that is constantly narrowing; and 2) microstructural analysis of a shear zone in the Proterozoic basement of the Grand Canyon. Both these examples highlight the important role that evolving geometry (in this case, shear zone narrowing) can play on the dynamic state of shear zones. The analytical model shows that even small changes to the width of highly localized shear zones can trigger brittle failure in nominally ductile crust. In the Grand Canyon example, electron backscatter diffraction of micro-scale quartz mylonite zones reveal a grain architecture the developed under an increasing stress regime over century to millennium timescales.
Bio: Nick Roberts is a structural geologist who investigates the rheological evolution of the crust and lithosphere. His research focuses in two broad areas: the interaction between magmatism and tectonic deformation and the rheological evolution of shear zone systems. Of particular interest are the ways in which the changing geometry of deforming systems along strike and through time drive transient processes. His field areas include the Pilbara Craton of Western Australia and the Proterozoic basement of the Grand Canyon. Nick got his B.A. in geology at Carleton College and his Ph.D. at University of Wisconsin-Madison. He is currently assistant professor of geosciences at Hamilton College.