A RECORD OF FLUCTUATIONS IN FLUID PRESSURE AND DEFORMATION AT THE BRITTLE-DUCTILE TRANSITION

The Baraboo syncline of south-central Wisconsin is an asymmetric, doubly plunging fold formed by I) buckling followed by II) further shortening and top-to-the-south simple shearing ca. 1630 Ma. Peak metamorphic temperatures reached ~340-390°C. We present data from slickenfibers that record bed-parallel, top-to-the-south slip consistent with the later stage of formation of the syncline. Slip surfaces are preferentially located at boundaries between quartzite and thin interlayers of phyllite. In contrast to the bulk of the Baraboo Quartzite, phyllosilicates associated with quartz slickenfibers consist of stably coexisting pyrophyllite + kaolinite, which, assuming the fluid phase to be water, constrains temperature to ~280-330°C by the univariant curve quartz + kaolinite = pyrophyllite + water. Quartz slickenfibers exhibit healed microcracks decorated by fluid inclusions, which cut across fault steps at right angles to quartz fibers. Similar microstructures have elsewhere been interpreted to record numerous microearthquakes that progressively opened space for quartz precipitation. The Baraboo slickenfibers, however, also exhibit subgrains, undulose extinction, and, in one sample, small folds, all consistent with crystal plastic deformation of quartz fibers. The magnitude of recrystallization of fibers increases with decreasing phyllosilicate content. 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, but is saline, these PT estimates are considered maxima. They are, however, consistent with microstructures, which record deformation at the brittle-ductile transition in quartz. We therefore interpret these data as indicating episodic seismogenesis, which we infer 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 work has suggested, a record of low frequency earthquakes, these observations are consistent with slow slip at the brittle-ductile transition, and suggest variations in fluid pressure control deformation mechanisms and seismicity under these conditions.