FLUID PRESSURE VARIATIONS: IMPLICATIONS FOR THE ROCK RECORD OF EPISODIC TREMOR AND SLIP

Episodic tremor and slip (ETS) is characterized by low-frequency tectonic tremor and geodetically resolvable slow slip events with repeat times of 3-14 months. Quartz slickenfibers have been hypothesized to provide a rock record of ETS, critical to understanding the physical mechanisms underlying the geophysical signal. This hypothesis, however, requires that quartz precipitation rates be fast enough to seal slickenfiber opening increments over the time scales of ETS recurrence. We report results of a test of the hypothesis that rapid quartz precipitation during slickenfiber formation is driven by large, coseismic decreases in pore-fluid pressure during slip. Microstructures in quartz slickenfibers from small faults in Baraboo quartzite, WI, USA, reveal abundant fluid inclusion planes with a mean spacing (a proxy for fracture opening/slip increments) of 25 µm. Fluid inclusion planes exhibit a mutually cross-cutting relationship with microstructures recording recrystallization (subgrains, bulging grain boundaries), indicating alternating brittle and ductile deformation. Stably coexisting kaolinite and pyrophyllite record temperatures of ~280 – 330 ºC during deformation, consistent with depths of ~9-11 km and the brittle-ductile transition of quartz. The combination of these temperature constraints with fluid inclusion microthermometry provides insight into variations in pore-fluid pressure during slickenfiber formation. A Monte Carlo sampling of these data allows a probabilistic assessment of possible coseismic decreases in pore-fluid pressure and associated changes in silica supersaturation during slip events. Combined with kinetic calculations of quartz precipitation rates, these data indicate that the probabilities of sealing a 25 µm wide fracture opening over 3 months are 0% and <7% assuming 280 and 330 ºC trapping temperatures, respectively. In one year however, the probabilities increase to 3% and 55%. These results challenge the validity of quartz slickenfibers as a rock record of ETS.