Relative timing of seismicity, lithologic host and fluctuating strain rates in an exhumed lower crustal shear zone; Western Churchill Province, Canada

Corey Flynn, University of Colorado
Kevin Mahan, University of Colorado
Caleb Holyoke, University of Akron
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Abstract

The presence of paleoseismic indicators in continental crust significantly below the brittle-ductile transition calls into question traditional views on crustal strength, rheologic stratification, and assumptions of steady-state behavior in high strain zones. The 1.89-1.87 Ga Cora Lake shear zone in the Western Churchill Province, Canada, displays evidence of paleoseismicity in the form of variably mylonitized veins of pseudotachylyte (PST). Previous work suggests shear zone activity on a retrograde path from 850 to 700°C while decompressing from 1.1 to 0.8 GPa (35-25 km). Ductile overprint in the PST veins ranges from undeformed to moderately and pervasively deformed. Undeformed veins preserve quench textures and other diagnostic features such as high-angle injection veins and entrained wall rock clasts. Moderately deformed veins commonly display an internal foliation that is oblique to the host fabric as well as sheared injection veins and wall rock clasts. Pervasively deformed veins are jet black or dark gray ultramylonite layers with extremely reduced grain size relative to host rock and often contain visible to microscopic garnet and pyroxene neoblasts. Field work reveals an apparent relationship between rock type and the degree of ductile overprint on PST veins. Whereas the full spectrum of PST strain states occurs in mafic granulite and anorthosite, moderately deformed to undeformed PST is observed in tonalite, and only a few undeformed veins have been observed in quartz-rich granitoid and felsic granulite. We use field observations, laboratory analysis, and numerical modeling to constrain the rheological evolution of competing brittle and ductile behavior in the Cora Lake shear zone. Our results support an interpretation that the variation in PST occurrence by rock type and degree of subsequent deformation and metamorphism reflects a relative timing sequence. Numerical modeling shows that rupture producing events were possible in mafic rocks at around 800°C, which suggests that the most strongly mylonitized PST veins likely initiated early in the shear zone's history and is consistent with their common garnet and pyroxene neoblasts. As the shear zone cooled below 800°C, potential brittle failure was expanded to include intermediate rock types such as the tonalite, and as cooling neared 700°C, transient brittle failure criteria was achievable in all major rock types. The ability of the Cora Lake shear zone to sustain seismic related stresses is likely due to the "dry" nature of the lithologies, which could partly stem from deformation-induced water loss and may be a key player in producing stable cratonic interiors.

Session

Large-scale tectonics