MICROSTRUCTURAL AND PETROLOGICAL OBSERVATIONS OF MULTIPLE GENERATIONS OF DEEP CRUSTAL SEISMIC SLIP

Deep crustal pseudotachylyte, (PST) solidified frictional melt generated during seismic slip, is potentially a key to understanding the nature of crustal scale fault system behavior at depths significantly below the frictional-plastic transition. The Cora Lake shear zone in the Western Canadian Shield is a world-class exposure of such a deep fault system root: a 1.88 Ga granulite to upper amphibolite facies (1.0-0.8 GPa, 800-700 °C) strike-slip shear zone, equivalent to ~15 km below the paleo-schizosphere. The Cora Lake shear zone consists of sinistral mylonite to ultramylonite 4-6 km across strike and >90 km along strike, containing multiple foliation-parallel zones of progressively ductiley sheared and garnet-bearing PST veins.

In one thin section, optically at least three PST generations can be readily distinguished by relative darkness and cross-cutting relationships in plane polarized light, with the darker veins cross-cutting the next lightest vein. All three generations have localized along the same fault plane, possibly indicating that PST-generating rupture preferentially followed the contact of previous PST generations with the hosting mylonite. Degree of transparency in transmitted light is proportional to both the relative age of the PST generation and in ductile deformation of the PST vein material. Wall rock breccia within the deformed PST generations have been ductiley smeared into a foliation plane parallel to that of the granulite-facies mylonite, and commonly form sinistral sigma clasts matching the sense of granulite-facies shear. Under electron microprobe investigation, a fourth generation of PST can be identified by mineralogy and mineral morphology. The relative youngest PST generation is absent of garnet, but the other three generations contain up to 25% modal garnet increasing with relative age, and average garnet size increasing with degree of plastic shear and age.

The high temperature plastic and metamorphic overprint of these PST veins suggests that seismic slip occurred contemporaneous with the Cora Lake shear zone under deep crustal conditions.