Identifying deformed pseudotachylyte and its influence on the strength and evolution of a crustal shear zone at the base of the seismogenic zone

Abstract

On a strand of the Norumbega fault system, a Paleozoic, subvertical, seismogenic fault system in northeastern New England, USA, we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite/phyllonite layers. We consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone.The Norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of ~10–15km, some of which preserve evidence for mutually-overprinting pseudotachylyte and mylonite. Along one of these, the Sandhill Corner shear zone, all stages of the pseudotachylyte to ultramylonite/phyllonite transformation are preserved, from (1) primary pseudotachylyte structures to (2) initial mineral crystallization, (3) grain coarsening and reactions, and (4) viscous deformation. Our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin. Using these features, we estimate that ~5–50% of the rock volume in the Sandhill Corner shear zone (mean ~30%, locally >50%) is deformed pseudotachylyte, suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone.The Sandhill Corner shear zone localized along the contact between two rheologically-contrasting units. Mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core, indicating that seismic rupture also localized there. A decrease in grain size promoting grain-size-sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers. The formation of multiple generations of weak, deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core.