50-3 Lithologic heterogeneity and strain rate controls on stress amplification and seismic failure in a regional-scale, lower crustal shear zone; Western Churchill Province, Canada

Session: Latest Research Advances in Structural Geology and Tectonics

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Abstract:

Seismic activity below the frictional viscous transition in continental crust is a poorly understood phenomenon. Typical models of lower crustal strength assume uniform stress for a given depth, compositionally homogeneous layers, and steady state creep by crystal-plastic deformation mechanisms. However, some localized high strain regions in former lower crust such as the km-scale Cora Lake shear zone (1.89-1.88 Ga; 35-25 km paleodepth) exposed in the Athabasca granulite terrane of the Western Churchill Province in the Canadian shield contain evidence of both steady-state creep (mylonites) and episodic seismic activity (pseudotachylyte). Variably mylonitized pseudotachylyte veins contain neoblastic grains of garnet and pyroxene suggesting they were subjected to deformation while at lower crustal levels. Previous workers have interpreted differential stress in the high strain core of the shear zone to be greater than 176 MPa and strain rates of at least 10^-10 s^-1 during viscous deformation, but constraining the conditions that led to brittle failure is more difficult. The shear zone occurs at a domain boundary that juxtaposes the mafic roots of a tonalite batholith against a suite of more quartz-rich granitoid gneisses suggesting that lithologic heterogeneity could be correlated with pseudotachylyte occurrence. Fourier transform infrared spectroscopy on feldspar grains from anorthosite within the shear zone indicate water contents as low as 80 H/10⁶ Si in the high strain core and 2500 H/10⁶ Si in the tonalitic margin suggesting that the dry nature of the rocks could have contributed to the accumulation of differential stresses exceeding failure strength at the depth of the shear zone. We test the hypothesis that progressive deformation along the Cora Lake shear zone led to stress amplification and subsequent brittle failure in the shear zone through an integrated approach of field-based observations and finite element numerical modeling. Modeling results show that stress is concentrated in the quartz-poor side of the shear zone and lithologic variation across strike contributes over 1 GPa of differential stress under the range of P-T-water content conditions that are suspected for the shear zone, consistent with field observations. The bulk strain rate appears to be the most important variable in producing high differential stresses, suggesting a complex interplay of strain localization in inter-seismic and seismic periods during the evolution of the shear zone.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7929

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