Fluid-enhanced grain boundary sliding in pseudotachylyte survivor clasts: does creep cavitation lead to earthquake rupture?

Elena Miranda, & Alberto Perez-Huerta

Published August 15, 2018, SCEC Contribution #8822, 2018 SCEC Annual Meeting Poster #172

Viscous shearing in the middle and lower crust is observed to localize within pseudotachylyte veins hosted in coeval mylonite, but it is unclear what role hydrous fluids play in enhancing strain localization in pseudotachylyte. We investigate coeval pseudotachylytes and granodiorite mylonites from the footwall of the South Mountains core complex, Arizona, where strain localization in pseudotachylyte was achieved by linear viscous flow accommodated by grain boundary sliding (GBS). We use both electron backscatter diffraction (EBSD) and atom probe tomography analyses to evaluate the role of hydrous fluids during linear viscous flow by targeting polycrystalline quartz survivor clasts for analysis. EBSD analyses reveal the presence of ubiquitous four-grain junctions, small grain diameters (<5 um), and weak CPO patterns, confirming deformation by GBS. We selected a transect across an EBSD-analyzed survivor clast for subsequent atom probe tomography analysis to characterize the presence and spatial distribution of hydrous fluids. Atom probe tomography analyses on 6 prepared tips extracted from grains along the transect reveal the presence of structural water (OH) within the interiors of all grains. The OH content appears spatially uniform within the volumes of the analyzed tips. In addition, atomic density profiles of Si, OH, SiO and O2 in one of the tips reveal a tube-like structure that bears resemblance to a partially-healed fracture. The apparent partially-healed fracture may be indirect evidence for creep cavitation during GBS, where creep cavities coalesce along grain boundaries to make macroscopic cracks. The partially-healed fracture may indicate self-healing of creep cavitation damage, which may help explain how deformation was sustained within zones of GBS. This implies that in the absence of this self-healing mechanism, creep cavitation damage may go unmitigated, leading to macroscopic failure and possibly earthquake rupture, consistent with the presence of these survivor clast microstructures within pseudotachylyte.

Key Words
pseudotachylyte, grain boundary sliding, EBSD, atom probe tomography

Citation
Miranda, E., & Perez-Huerta, A. (2018, 08). Fluid-enhanced grain boundary sliding in pseudotachylyte survivor clasts: does creep cavitation lead to earthquake rupture?. Poster Presentation at 2018 SCEC Annual Meeting.

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