DOES QUARTZ PRESERVE A RECORD OF THE EARTHQUAKE CYCLE AT THE FRICTIONAL-TO-VISCOUS TRANSITION? A STUDY OF A SEISMOGENIC BIMATERIAL SHEAR ZONE

The frictional-to-viscous transition in the vicinity of seismogenic faults experiences coseismic fracturing/frictional sliding followed by post- and interseismic viscous/plastic deformation. A question central to our work is whether quartz, which flowed viscously in the interseismic period, preserves a record of transient coseismic loading. Microstructural analysis of monomineralic quartz aggregates is performed on the Sandhill Corner shear zone in a seismogenic strike-slip fault system, Maine. The shear-zone core with abundant pseudotachylyte occurs at the contact between quartzofeldspathic (QF) and micaceous schist units. Trends in recrystallized grain size, crystallographic preferred orientation (CPO), and misorientation data of the QF unit show quartz in the proximity of the core has a fine average grain size (down to ~9 μm), a weak CPO pattern, and randomization in the misorientation of randomly selected pixel pairs (“random-pair”), indicating grain-size-sensitive creep. Within the inner shear zone (~40 m wide) containing fully recrystallized quartz domains, the microstructural parameters are unusual, with near-random random-pair misorientations but a CPO pattern clearly indicative of basal <a> slip. The data confirm and build new evidence for the model that during the seismic cycle, quartz grains within the inner shear zone experienced cycles of (a) extensive fracturing (or pulverization) and internal lattice strain by coseismic energy, (b) grain-size reduction by neocrystallization followed by postseismic viscous creep dominantly through grain-size-sensitive processes, and (c) further recrystallization with progressive grain growth, transitioning to basal <a> slip. This model is supported by analyses of quartz fluid inclusion abundance and garnet fragment size distribution in the QF unit, showing brittle damage (~90 m wide) and dynamic fragmentation zones (~63 m wide) during rupture propagation, respectively. In contrast, the schist unit neither has fully recrystallized quartz nor the microstructural characteristics of the inner shear zone. The asymmetrical spatial extent of the unusual quartz microstructures around the shear-zone core is consistent with a preferred rupture propagation direction, commonly observed in strike-slip faults with bimaterial interfaces.