A RECORD OF FLUCTUATIONS IN FLUID PRESSURE AND DEFORMATION AT THE FRICTIONAL-VISCOUS TRANSITION

The Baraboo syncline, in south-central Wisconsin, is an asymmetric, doubly plunging fold formed by buckling followed by further shortening and top-to-the-south simple shearing. Peak metamorphic temperatures reached ~340-390°C. We present data from slickenfibers that record bed-parallel, top-to-the-south slip consistent with the latest stage of syncline formation. Slip surfaces are preferentially located at boundaries between quartzite and thin interlayers and laminae of phyllite. In contrast to the bulk of the Baraboo Quartzite, phyllosilicates associated with quartz slickenfibers are stably coexisting pyrophyllite + kaolinite, which, assuming the fluid phase to be water, constrain temperature to ~280-330°C. Healed microcracks decorated by fluid inclusions formed at right angles to quartz slickenfibers. Similar microstructures have elsewhere been interpreted to record numerous microearthquakes that progressively opened space for quartz precipitation. Baraboo slickenfibers, however, also exhibit subgrains and recrystallized grains at slickenfiber margins; undulose extinction; and small folds. Subgrains and new grains locally are cut by, and locally overprint, healed microcracks. Fluid inclusions are aqueous with ~12% NaCl. Using the previously mentioned temperature constraints, fluid inclusion analysis restricts pressures to 280-360 MPa, equivalent to entrapment at ~8-12 km depth. Because the fluid is not pure water, these PT estimates are considered maxima. They are, however, consistent with microstructures, which record deformation at the frictional-viscous transition in quartz. We interpret data as indicating episodic seismogenesis, inferred to be driven by elevated pore fluid pressure, separated by periods of aseismic creep when pore fluid pressure was low. If slickenfibers with healed microcracks are, as previous workers have suggested, a record of low frequency earthquakes, these observations are consistent with slow slip at the frictional-viscous transition, suggesting variations in fluid pressure control deformation mechanisms and seismicity under these conditions. Grain size determined by EBSD constrains strain rates for periods of viscous, aseismic creep.