Grain boundary sliding triggers coeval pseudotachylyte development in brittle-ductile transition mylonites: an Electron Backscatter Diffraction (EBSD) case study of mid-crustal interseismic and coseismic deformation

Elena Miranda, Craig Stewart, & Kelly Lourcey

Published August 14, 2017, SCEC Contribution #7629, 2017 SCEC Annual Meeting Poster #197

Exposures of coeval pseudotachylytes and mylonites are relatively rare, but are crucial for understanding the seismic cycle in the vicinity of the brittle-ductile transition (BDT). We use both field observations and electron backscatter diffraction (EBSD) analysis to investigate the coeval pseudotachylytes and granodiorite mylonites exposed in the footwall of the South Mountains core complex, Arizona, to evaluate how strain is localized both prior to and during pseudotachylyte development at the BDT. In the field, we observe numerous pseudotachylyte veins oriented parallel to mylonitic foliation; the veins have synthetic shear sense with adjacent mylonites, and are < 2 cm thick, laterally discontinuous, and confined to a few m in structural thickness. EBSD analysis reveals that deformation is strongly partitioned into quartz in mylonites, where quartz shows subgrain rotation overprinted by bulging recrystallization microstructures and lattice preferred orientation (LPO) patterns indicative of dislocation creep. Foliation-parallel zones of finely recrystallized, (< 5 μm diameter) bulge-nucleated grains in the mylonites show four-grain junctions and randomized LPO patterns consistent with grain boundary sliding (GBS). Pseudotachylyte veins have elongate polycrystalline quartz survivor clasts that also exhibit GBS traits, suggesting that pseudotachylytes form within GBS zones in mylonites. We interpret the onset of GBS as a triggering mechanism for coeval pseudotachylyte development, where the accompanying decrease in effective viscosity and increase in strain rate initiated seismic slip and pseudotachylyte formation within GBS zones. Strain became localized within the pseudotachylyte until crystallization of melt impeded flow, inducing pseudotachylyte development in other GBS zones. We associate the pseudotachylyte veins and host mylonites with the coseismic and interseismic parts of the seismic cycle, respectively, where the abundance and lateral discontinuity of pseudotachylyte veins suggests repeated events. We speculate that periodic, GBS-initiated pseudotachylyte generation may correlate with intermediate slip rate seismic events in the vicinity of the BDT, suggesting that coeval pseudotachylytes and mylonites are evidence of a unique class of seismic event.

Key Words
pseudotachylyte, EBSD, grain boundary sliding, mylonite, rheology

Citation
Miranda, E., Stewart, C., & Lourcey, K. (2017, 08). Grain boundary sliding triggers coeval pseudotachylyte development in brittle-ductile transition mylonites: an Electron Backscatter Diffraction (EBSD) case study of mid-crustal interseismic and coseismic deformation. Poster Presentation at 2017 SCEC Annual Meeting.

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