Do incrementally developed slickenfibers record episodic tremor and slow slip? Observations from the Baraboo Syncline, WI, USA.

Incrementally developed slickenfibers decorating numerous faults in subduction zone rocks have previously been inferred to record episodic tremor and slow slip (ETS). This hypothesis, however, assumes that crack-seal quartz increments in slickenfibers can form over the time scales typical of ETS recurrence (weeks to a few months), contrary to well established kinetic estimates. We present microstructural, geochemical, and thermobarometric data that document the conditions of quartz growth during slickenfiber development in the Baraboo Syncline, WI, USA. Slip increments in quartz slickenfibers from small-displacement faults in Baraboo quartzite and phyllite are recorded by secondary fluid-inclusion planes that both cut, and are cut by, subgrains. This observation suggests that deformation processes alternated between brittle failure and ductile flow, consistent with faulting near the quartz brittle-ductile transition. Back-scattered electron imaging and energy dispersive spectroscopy of phyllosilicates on slickenfiber-bounding surfaces and in host rock show that some samples contain stably coexisting kaolinite and pyrophyllite. This assemblage constrains ambient temperature to ~280-330°C. Collectively, these observations are consistent with slickenfiber formation at depths typically inferred for ETS occurrence. By combining paleothermometry results with isochore data from fluid inclusion analysis, we find that entrapment pressures were likely variable during quartz growth (between 3 and 30 MPa). This inference is consistent with hypotheses of previous workers who suggested that syn-slip pressure drops may facilitate high degrees of silica supersaturation in pore fluids, leading to quartz growth and slickenfiber formation over ETS time scales.