Very long timescale of grain size and shear zone evolution in a continental strike-slip setting

Kali L. Allison, & Laurent G. Montesi

Published August 2, 2021, SCEC Contribution #11139, 2021 SCEC Annual Meeting Poster #128

Understanding the mechanical interaction between brittle faults and deeper ductile shear zones provides insights into the long-term behavior of active fault systems. We explore this interaction in a 2D model of a continental strike-slip fault zone. The fault, governed by rate-and-state friction, is embedded in a power-law viscoelastic material that uses rheological parameters for feldspar in the crust and olivine in the mantle. Grain size evolves according to the paleowattmeter model, in which grain size increases through static grain growth and is reduced by work done through dislocation creep. This model makes it possible to self-consistently simulate the variations of strain rate, grain size, and stress in the vicinity of a strike-slip fault.

We calculate that the characteristic timescale of grain size evolution in the lower crust grain size is so long that grain size remains approximately constant for interseismic timescales. Starting with a uniform 1 mm grain size, we follow the evolution of the fault and shear zone system over >10,000 years. Earthquakes are neglected in these time-dependent simulations in which the fault slips aseismically instead. Initially, dislocation creep is the dominant deformation mechanism and stress is high. Within the first year, the grain size drops rapidly in the upper mantle and in the lower crust, where the work done by dislocation creep is largest. This causes the dominant deformation mechanism to switch to diffusion creep within the shear zone and the stress to decrease. Following this initial adjustment period, grain size evolution and ductile shear zone development progress more slowly. The brittle-ductile transition (BDT), defined as the depth at which 90% of the tectonic loading is accommodated by slip on the fault, gradually shallows as the system evolves. The depth of the BDT is well-fit by a power law in time. According to this relation, it would take more than 100 Ma to reach steady-state. As this is much longer than most tectonic episodes, is it unlikely that natural shear zones are close to being in steady-state. This is especially true in the mid-crust, where cooler temperatures lead to a very long timescale of grain size evolution. This region’s proximity to the base of the seismogenic zone means that it may have significant implications for earthquake mechanics.

Key Words
grain size, ductile shear zone, strike-slip, numerical model

Citation
Allison, K. L., & Montesi, L. G. (2021, 08). Very long timescale of grain size and shear zone evolution in a continental strike-slip setting. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)