Tibetan dichotomy exposed in the Canadian Shield: A lower crustal perspective

Abstract

It has been hypothesized that much of central and eastern Tibet coincides with gravitationally driven flow of weak lower continental crust. Magnetotelluric profiles throughout Tibet reveal several deep crustal zones of high conductivity inferred to represent either melt-weakened channelized flow of deep crust or alternatively, migration of fluids or melts along shear zone-fault systems. In eastern Tibet, some workers suggest that deep crustal flow is diverted around cratonic lithosphere beneath the Sichuan basin. This behavior requires a weak versus strong rheological dichotomy in the lower crust. Evidence for this behavior is presented from a deeply exhumed granulite terrane (>0.6–1.6 GPa) in the western Canadian Shield. Data support the hypothesis that at 1.95–1.90 Ga, lower crust northwest of the >400 km-long Grease River shear zone was melt-weakened while lower crust southeast of the Grease River shear zone was dehydrated and strong. Strain localization and dextral transpressive shearing occurred at 1.91–1.90 Ga along the Grease River shear zone – the locus of a rheological dichotomy that represents a field-based analog for lower crust beneath the eastern Tibetan plateau. This work demonstrates that rheology of the lower continental crust can be spatially heterogeneous at <1 m2 to 100s of km2 scales. Deep crustal rheology also depends on fertility: melt weakening and flow during fluid-absent melting (>750°C) and granulite-grade metamorphism is favored in “first-cycle” metasedimentary biotite-rich protoliths, though melt extraction can decrease this fertility. Older and previously dehydrated poly-metamorphic granulites tend to be stronger and less prone to pervasive lower crustal flow.