[Eoas-seminar] Geophysics Job talk today @ 2:30

eoas-seminar at lists.fsu.edu eoas-seminar at lists.fsu.edu
Thu Feb 18 10:05:48 EST 2021

Zoom link for the talk: https://fsu.zoom.us/j/95854702538
Title: Grain Expectations: Microphysics of Plate Boundaries from Geological to Human Timescales
Abstract: The evolution and generation of plate tectonics on Earth involves processes that range from planetary-scale mantle convection, to grain scale microphysics of rock creep. Plate motion is enabled by deformation at plate boundaries, which are narrow weak zones in the crust and lithosphere often associated with grain-size reduction, as evident in mylonites.  Plate boundaries are also where most earthquakes occur, and the processes leading to their weakness may also play a role in seismic cycles. Deformation at tectonic plate boundaries is governed by the mechanical properties of crustal and lithospheric rock, which evolve through changes in the microstructure of its constituent minerals. Geological observations of microstructure can, therefore, be used to tease out the rock's deformation conditions, for example through the use of various piezometers (e.g., the relationship between stress and grain size or dislocation density). In this talk, I will present a new theoretical model coupling the evolution of grain boundaries and dislocations to rock deformation and tectonic activity. The model makes a novel prediction that the equilibrium dislocation density at a given stress can be non-unique, with co-existing deformation states or piezometric branches. Grain size plays the determining role as to which piezometric branch is manifested in a deforming rock. Therefore, in order to infer stress from observed microstructure, both measurements of grain size and dislocation density are necessary. Furthermore, the model predicts that the competition between microphysical processes, such as the motion of dislocations in the crystal lattice, and the growth of mineral grains, causes transitions in the deformational response of rocks to changes in stress. Thus, the new theory provides a microphysical model for transient rheological behavior, which is particularly relevant to tectonic activity in which deformation is relatively rapid and where the steady state flow laws are a poor approximation, such as during postseismic relaxation or postglacial rebound.

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