THE INFLUENCE OF PSEUDOTACHYLYTE FORMATION AND DEFORMATION ON BULK ROCK STRENGTH WITHIN A SHEAR ZONE FROM THE BASE OF THE SEISMOGENIC ZONE

The positive identification of structures indicative of the variability in deformation mechanisms with the seismic cycle is an important step in the investigation of how these structures affect the behavior and evolution of shear zones below seismogenic faults. On the Sandhill Corner strand of the long-lived, subvertical, transpressional to strike-slip Norumbega fault system in Maine, we document changes associated with the formation and deformation of pseudotachylyte and suggest that these changes led to a weaker bulk strength that enhanced the spatial stability of the shear zone core.

The Sandhill Corner shear zone was exhumed from ~10-15km depths and contains mutually overprinting pseudotachylyte and mylonite. All stages of the pseudotachylyte to ultramylonite 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 in the mylonite exhibit features that, when present together, are distinctive of a pseudotachylyte origin. Using these characteristics, a significant proportion of ultramylonite layers are suspected of originating as pseudotachylyte.

Within the Sandhill Corner shear zone, initial localization occurred along the boundary between two rheologically-contrasting rock units. Field observations show that mylonite fabric intensity and the occurrence of pseudotachylyte both increase with proximity to the lithologic contact and shear-zone core, indicating that seismic rupture also localized along the contact. The transformation from pseudotachylyte to ultramylonite leads to an increase in mica, a decrease in grain size, and a change in texture to an ultra fine-grained rock with interconnected mica-rich layers, all of which would result in bulk weakening. Although grain size reduction in quartz may have led to grain size sensitive creep, viscous creep within the progressively developed network of interconnected mica would ultimately control the bulk strength. Given that a significant volume of pseudotachylyte was created over the lifetime of the shear zone, such weakening effects may have helped to stabilize and maintain the location of the shear zone core at the lithologic contact.