The Grease River shear zone (GRsz) is a ~7 km-wide, >400 km-long shear zone that cuts the Athabasca granulite terrane, one of Earth's largest exposures of continental lower crust. The GRsz is characterized by penetrative NE-striking, steeply NW-dipping foliations with gently SW-plunging stretching lineations dominated by dextral SW-over-NE kinematics. Neoarchean flow of weak lower crust (sub-horizontal fabrication at ca. 2.62-2.55 Ga) in the Athabasca granulite terrane exposed SE of the GRsz was followed by >650 m.y. of near-isobaric cooling and strengthening of continental lithosphere. In contrast, melt-weakened flow and SW-dipping fabric development in lower crust exposed NW of the GRsz occurred at ca. 1.93-1.90 Ga during culmination of the Taltson orogen.
Paleoproterozoic melt-weakened flow coincided with dextral transpression along the GRsz and development of discrete 10s of m-scale steeply-dipping shear zones in Neoarchean isobarically-cooled rocks exposed SE of the GRsz. This illustrates the dramatic effects of strain-partitioning and strain-hardening in an anisotropic medium, analogous to flow of viscous (seismically slow) crust around the rigid (seismically fast) block of Precambrian crust that floors the Sichuan basin in China.
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