SHEAR ZONES IN CONTINENTAL LOWER CRUST AS MANIFESTATIONS OF STRAIN LOCALIZATION AND STRAIN HARDENING: GREASE RIVER SHEAR ZONE, WESTERN CANADIAN SHIELD

Many Paleoproterozoic intracontinental shear zones in the western Canadian Shield are analogous to the crustal-scale faults/shear zones that transect the Tibetan Plateau. Multi-disciplinary observations of the Tibetan system conclude that these structures must penetrate down to the base of the crust. Results from an exhumed shear zone in the western Canadian Shield reveal an intense episode of high-T dextral strain (650-700°C) near the base (30-40 km) of continental lower crust at 1.93-1.89 Ga.

The Grease River shear zone (GRsz) is a 5-7 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. The structure separates two blocks of lower crust that were rheologically distinct at ca. 1.9 Ga. The southeastern boundary of the GRsz is marked by steep fabrics and a zone of intense strain localization and hardening along isobarically-cooled and rheologically strong orthogneisses of the (2.6 Ga) Mary batholith. The northwestern boundary of the GRsz, in contrast, is defined by moderately SW-dipping fabrics in heterogeneous migmatitic paragneisses and tonalitic orthogneisses, representing penetrative lower crustal flow of weak crust. The geometry of map-scale structures, kinematics, metamorphic petrogenesis, and timing of deformation are most consistent with dextral, transpressive shear strain during sub-horizontal NW-SE shortening and top-to-the-NE transport of rheologically heterogeneous lower crust.

Results from the western Canadian Shield bear directly upon the role of strain localization, hardening, and lower crustal flow in the vicinity of rheologically strong basins bounding the Tibetan plateau. Rocks NW of the GRsz provide an exhumed record of low viscosity flow of lower crust around the rheologically stronger Mary batholith exposed SE of the GRsz. The GRsz represents the region of strain localization (and eventual hardening) between weak and strong blocks of lower crust, a physical record of what has been inferred for the Longmen Shan and Altyn Tagh regions of Tibet and China.