DO SMALL FAULTS IN THE BARABOO SYNCLINE PRESERVE EVIDENCE OF EPISODIC TREMOR AND SLIP (ETS)?

Geophysical data show that plate-bounding faults can exhibit not only seismicity and aseismic creep, but also slow earthquakes that release energy through microseismic events over periods of weeks to months. Geophysical signatures of these events suggest they record elevated pore fluid pressure, prompting investigation of the structural record of microseismicity in exhumed interplate faults. Proposed records of slow slip include slickenfibers that decorate numerous small faults in subduction zone rocks exhumed from depths typical of microseismic behavior, where ambient temperatures average 300°C. Similar, yet shorter, slickenfibers occur on small-displacement thrust faults in a very different tectonic setting: the Baraboo Syncline in south-central Wisconsin, USA. Microstructures in quartz slickenfibers in the Baraboo quartzite include secondary fluid inclusion planes (FIP) that both crosscut and are cut by subgrains, suggesting that deformation within the faults alternated between brittle failure and ductile flow. To constrain conditions under which slickenfiber formation took place, we employed backscattered electron (BSE) imaging and energy dispersive spectroscopy (EDS), fluid inclusion microthermometry, and characterization of syntectonic layer silicates. BSE observations and EDS analyses of layer silicates on slickenfiber-bounding surfaces and host rock show that while some samples host kaolinite or pyrophyllite exclusively, others contain both phases in stable coexistence. In samples containing the latter relationship, ambient temperature is restricted to ~280-330°C along the univariant curve of the reaction kaolinite + 2quartz = pyrophyllite + water. Within this range of temperatures, isochores defined by fluid inclusion microthermometry restrict pressure to 240-340 MPa, or roughly 9-13 km depth, during inclusion entrapment. This temperature range is similar to that of ETS source regions and it corresponds to the brittle-ductile transition for quartz. We interpret fluctuating fluid pressure to have controlled deformation mechanisms at Baraboo, with elevated fluid pressure driving brittle failure and precipitation of slickenfibers, followed by periods of aseismic creep recorded by microstructures indicating subgrain rotation recrystallization.