SCEC Award Number 20191 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Interaction between Earthquake Cycles and Grain Size Evolution
Name Organization
Kali Allison University of Maryland Laurent Montesi University of Maryland
Other Participants
SCEC Priorities 3b, 3e, 2a SCEC Groups FARM, SDOT, Geology
Report Due Date 03/15/2021 Date Report Submitted 03/15/2021
Project Abstract
This project explored grain size evolution and grain size sensitive creep in the ductile shear zone beneath a vertical strike-slip fault over a 10,000-year timescale. Grain size evolves using the paleowattmeter, in which grain size increases through static grain growth and is reduced by work done through dislocation creep (Austin and Evans, 2007). Motivation for this project comes from observations that shear zones are characterized by reduced grain size, and may deform in part through diffusion creep.

We performed time-dependent simulations in which earthquakes are neglected and the fault slips aseismically, with the initial grain size of 1 mm. Under these conditions, dislocation creep is initially the dominant deformation mechanism, and appreciable viscous strain occurs only in the mantle. In the first year, the grain size near the fault drops rapidly below 25 km depth, where the work done by dislocation creep is largest, causing a switch to diffusion creep as the dominant deformation mechanisms in the shear zone. Over the next 10,000 years, a layer of reduced grain size and elevated strain rate forms in the lower crust and upper mantle. Also, the depth of the BDT gradually shallows in a way which is well fit by a power-law (linear on a log scale). Extrapolating this power-law indicates that the system would take more than 100 Ma to reach the steady-state BDT depth, suggesting that the stresses, viscous strains, and grain size in the vicinity of a fault may not be well-represented by the assumption of steady-state.
Intellectual Merit We simulate grain size evolution in a 2D model of a strike-slip environment and associated ductile shear zone. We quantified shear zone development over a 10,000-year time scale. We found that initial grain size reduction within the shear zone is rapid, and as a result the dominant deformation mechanism within the shear zone switches from dislocation creep to diffusion creep within a year. Overall, the timescale of shear zone development is much longer than the lifespan of active strike-slip fault systems, implying that shear zones are never at a steady-state.
Broader Impacts This project supported a junior female scientist and provided her the opportunity to independently manage a research project.
Exemplary Figure Figure 2. Evolution of grain size (top row), deviatoric viscous strain rate (2nd row), and the ratio of diffusion to dislocation creep (bottom row) over 104 years from constant 1 mm grain size initial conditions. The rightmost column shows the final steady-state conditions to which the system is evolving. The transition from crust to mantle is shown in pink at 30 km. In the bottom row, the region above the BDT has been grayed out.