Rheological Heterogeneity and the Role of Strain Partitioning in the Development of a Continental Transform Fault: A Lower Crustal Record from the Western Canadian Shield

The roles of distributed versus partitioned strain remain a matter of debate regarding orogenic plateau development, particularly in Tibet where through-going crustal-scale faults are required to explain large magnitudes of displacement and strain-partitioning. Recent attempts at reconciling the observed pattern of seismicity in the Himalayan-Tibetan system appeal to linkage between coseismic surface displacements and variable lower crustal rheology at depth. We summarize results from an exhumed lower crustal shear zone that reveal a rheological dichotomy during Paleoproterozoic transpressive strain in the western Churchill cratonic province of the Canadian Shield. This dichotomy provides insight into surface topography across western Tibet, i.e. the Longmen Shan fault zone and Sichuan basin, which is demonstrably related to strain-partitioning influenced by differences in lower crustal strength at depth.

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.