The rock record of seimic nucleation: a case study from the Whipple Mountains Detachment Fault, eastern California

Daniel Ortega-Arroyo, Whitney M. Behr, & Emilie Gentry

Submitted August 15, 2017, SCEC Contribution #7686, 2017 SCEC Annual Meeting Poster #114

Despite advances in our understanding of the physics of earthquakes, the mechanisms by which dynamic rupture nucleates in the middle crust remains enigmatic. Proposed mechanisms include flash heating of asperities, thermal pressurization of pore fluids, dynamic instabilities, and fracture interactions. We investigate this issue in the rock record using exhumed mid-crustal rocks exposed beneath the Whipple Detachment fault (WDF) in eastern CA. Analysis of pseudotachylites (PS) beneath the WDF, representing paleo-earthquakes, reveal two types: Type 1 PS exhibit little to no precursory cataclasis and are either concentrated along shear bands at the margins of feldspar-rich lenses embedded in more quartz-rich domains or along lithological boundaries between the different mylonitic protoliths. The former appear synkinematic with S-C fabrics in the surrounding mylonites and exhibit finely dynamically recrystallized grains in quartz found at their margins suggesting coeval ductile deformation. By contrast, Type 2 PS occur along the principal slip surface of a brittle shear zone and show evidence for precursory cataclasis, brecciation, and fracturing. Some cataclasites inject into the host rock, forming eddies along the boundary with the PS. Slip appears to localize progressively into a 2 cm thick fault core, with PS concentrated primarily in the interior--the presence of solidified melt and fluidized cataclasite as clasts within the fault core suggests multiple slip events are preserved.

We interpret the two types of pseudotachylites to represent different conditions and mechanisms of earthquake nucleation near the brittle-ductile transition (BDT). Type 1 PS are interpreted to represent nucleation in deeper sections of the BDT by failure along mineralogically-controlled stress concentrations hosted within an otherwise viscously deforming mylonite. Our data suggest that these do not develop into large-magnitude EQ’s because seismic slip is dampened into the surrounding quartz-rich viscous matrix; instead, they may represent deep microseismicity and/or seismic tremor. By contrast, Type 2 PS are interpreted to nucleate when thermally pressurized pore fluids are able to escape into the permeable damage zone, causing a recovery in the fault’s effective friction, and promoting melting. Type 2 PS appear to experience greater weakening, accumulate larger slip, and may represent larger-magnitude seismicity at the base of the seismogenic zone.

Key Words
pseudotachylyte, earthquake,nucleation mechanism, dynamic weakening

Citation
Ortega-Arroyo, D., Behr, W. M., & Gentry, E. (2017, 08). The rock record of seimic nucleation: a case study from the Whipple Mountains Detachment Fault, eastern California. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology