Off-fault damage at the base of the seismogenic zone and implications for crustal rheology

Won Joon Song, University of Maine
Bo Ra Song, University of Maine
Erik Anderson, University of Maine
Scott Johnson, University of Maine
Christopher Gerbi, University of Maine


The base of the seismogenic zone, approximately coincident with the frictional-to-viscous transition (FVT), experiences cycles of coseismic brittle damage followed by longer-term viscous flow. These cycles are accompanied by cycles of grain-size reduction, permeability evolution and mass transfer/reaction, exerting important influence on crustal rheology. Thus, exploring the vertical and lateral extent of coseismic damage is critical for understanding crustal rheology.

Studies of earthquake-related cycles in the deeper reaches of the seismogenic zone are limited by sparse exposures of ancient earthquake faults exhumed from FVT depths. The Sandhill Corner shear zone, a strand of the Norumbega fault system (an ancient seismogenic strike-slip fault), is characterized by a ~200 m-wide protomylonite to mylonite in the quartzofeldspathic (QF) unit, a ~5 m-wide ultramylonite/phyllonite in the shear zone core, and a ~25 m-wide highly sheared mica-rich schist unit. We analyze several microstructural features that provide quantitative insights into the earthquake damage cycle: (a) fragment size distributions in highly fractured garnet, (b) orientation distributions of healed microcracks and cleavage planes in feldspar, (c) fluid inclusion abundance in quartz, and (d) asymmetry of kink bands in muscovite. Our results indicate that coseismic damage extends >~80 m in the QF unit from the shear-zone core, and dynamic pulverization (extreme brittle damage) exists ~60 m and ~5 m from the shear-zone core in the QF and schist units, respectively. This asymmetric distribution of dynamic pulverization around the shear-zone core is consistent with asymmetric pseudotachylyte distribution and shear-zone development. To estimate post- or interseismic viscous flow stress and deformation mechanisms in quartz, we also collect quantitative data of quartz microstructures across the shear zone including grain size, crystallographic orientation, misorientation, and fabric intensity through electron backscatter diffraction. Based on the quartz data, the inner part (~40 m wide from the shear-zone core) of the QF unit, containing fully recrystallized quartz domains, shows unusual microstructure parameters with near-random random-pair misorientations but a CPO pattern clearly indicative of basal slip and indicates grain-size-sensitive processes. On the other hand, the other part of the shear zone including the schist was deformed dominantly by grain-size-insensitive processes. This implies that intense coseismic damage affects long-term strength of the fault/shear zone system, thus facilitating strain localization.



Session 2: Rheology of the Lithosphere