Shear localization from the interplay between fault motion and grain size evolution

Kali L. Allison

Submitted August 15, 2019, SCEC Contribution #9771, 2019 SCEC Annual Meeting Poster #282

The formation of ductile shear zones in the lower crust and upper mantle and their interaction with brittle faults remains an open question in continental tectonics. We explore how grain size evolution, shear zone structure, and fault motion are coupled in a two-dimensional numerical model of a continental strike-slip fault zone. The model considers brittle deformation in the upper crust, characterized by localized slip on a fault, and distributed viscous flow in the lower crust and upper mantle. Two viscous deformation mechanisms are included: grain-size sensitive diffusion creep and grain-size insensitive dislocation creep. Grain size evolves following a physics-based paleowattmeter and depends on stress, strain rate, and time. This model makes it possible to self-consistently simulate the variations of strain rate, grain size, dominant deformation mechanism, and stress in the vicinity of a strike-slip fault and its deep extension in the lower crust.

We consider the effects of varying composition, background geotherm, and pore fluid pressure on the fault, which controls its strength. In general, our models produce a roughly 5 km wide shear zone in the lower crust, with a grain size of 10-100 µm, as much as two orders of magnitude lower than in the surrounding lower crust. In the mantle, the shear zone is less localized. Its width increases to about 20 km and the grain size, which ranges over several orders of magnitude, is not smaller than about 100 µm. While warmer geotherms produce a continuous shear zone through the lower crust and upper mantle, cooler geotherms lead to significant fault motion in the uppermost mantle that interrupts the shear zone and may be expressed as microseismicity in natural conditions. In all models, diffusion creep is the dominant deformation mechanism in the lower crust. Shear zone localization is dominated by the enhanced stress at the base of the fault. In the mantle, diffusion creep dominates within 20-40 km of the fault and its deep extension, whereas dislocation creep dominates in the far field. Shear zone localization corresponds to the change in dominant deformation mechanism. Seismic anisotropy is expected to develop only when dislocation creep is the dominant deformation mechanism and may be absent beneath the fault.

Key Words
shear zone, fault, viscoelastic relaxation

Citation
Allison, K. L. (2019, 08). Shear localization from the interplay between fault motion and grain size evolution. Poster Presentation at 2019 SCEC Annual Meeting.


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