Post-seismic ductile flow beneath the brittle-plastic transition: constraints on rheology from microstructural and electron backscatter diffraction (EBSD) analyses of mylonitized pseudotachylite, South Mountains metamorphic core complex, Arizona

Craig Stewart, & Elena Miranda

Published August 22, 2016, SCEC Contribution #7006, 2016 SCEC Annual Meeting Poster #078

Post-seismic ductile flow promotes the upward rebound of the brittle-plastic transition (BPT) after an earthquake, but it is difficult to determine the rheologic controls on post-seismic flow due to compositional and structural heterogeneities in the ductile middle crust. We use microstructural and electron backscatter diffraction (EBSD) analyses to evaluate the deformation mechanisms of constituent minerals in pseudotachylyte-bearing granodiorite mylonites from the footwall of the South Mountains core complex, Arizona, to interpret the rheologic controls on post-seismic flow. We focus on three pseudotachylyte-bearing samples developed within an extensional shear zone associated with the core complex detachment fault system, and they both entrain and are variably overprinted by mylonitic fabrics, demonstrating multiple episodes of seismic slip within ductilely flowing crust. All samples show pseudotachylyte veins that are coplanar with or oriented at low angles to mylonitic fabric, where the pseudotachylyte matrix contains glass and ultrafine-grained mica, and clasts are dominated by polycrystalline quartz and a paucity of feldspars. The host granodiorite mylonite in all samples shows extensive subgrain rotation recrystallization of quartz, and large, microfaulted feldspar porphyroclasts with local bulging recrystallization along fractures. The samples differ in that there are variable amounts of polycrystalline quartz clasts in the pseudotachylyte, and that some pseudotachylyte veins have clasts with higher aspect ratios compared to other veins. Recrystallized quartz microstructures in host mylonite are similar to those in the polycrystalline clasts in pseudotachylyte, indicating that pseudotachylyte preferentially formed in recrystallized, quartz-rich foliation layers. However, some polycrystalline quartz clasts exhibit 4-grain junctions, indicating modification of microstructures during overprinting of pseudotachylyte. These microstructures are consistent with a quartz-controlled rheology of the crust below the BPT, where dislocation creep in quartz is accompanied by the onset of grain boundary sliding during post-seismic flow in mylonitized pseudotachylyte. Our results suggest that post-seismic flow is partitioned into the weaker, quartz-rich layers in the mylonites, and that localized flow in weak layers could increase the rate at which the BPT rebounds, promoting faster reloading of the brittle fault and more frequent earthquakes.

Key Words
rheology, BPT, brittle-plastic transition, pseudotachylyte, pseudotachylite, mylonite

Citation
Stewart, C., & Miranda, E. (2016, 08). Post-seismic ductile flow beneath the brittle-plastic transition: constraints on rheology from microstructural and electron backscatter diffraction (EBSD) analyses of mylonitized pseudotachylite, South Mountains metamorphic core complex, Arizona. Poster Presentation at 2016 SCEC Annual Meeting.

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