32-11 Stress estimates in a lower crustal, pseudotachylyte bearing shear zone using dynamically recrystallized quartz and plagioclase paleo-piezometry

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

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Abstract:

The most common method of inferring differential stress in the middle and lower crust is dynamically recrystallized grain size paleo-piezometry. The 1.89-1.87 Ga Cora Lake shear zone in the Western Churchill province of the Canadian shield is a several km wide zone of strain localization preserved from 35-25 km paleo-depths. Deformation was primarily accommodated by crystal plastic creep, punctuated by periods of seismic activity indicated by the presence of pseudotachylyte veins. The shear zone is centered on a domain boundary juxtaposing quartz-rich granitoids against the mafic roots of a tonalite batholith. The tonalite domain has a higher plagioclase to quartz ratio which may have influenced the rheological contrast between domains and potentially amplified stresses that locally led to episodic seismic activity. To test this hypothesis, we evaluate new and previously published data on recrystallized grain sizes to assess flow stress patterns across the shear zone. Viscous deformation in the shear zone was dominated by dislocation creep as evidenced by recrystallization microstructures in quartz and plagioclase. Quartz recrystallization in the shear zone margins was accommodated by high temperature grain boundary migration while in the core, recrystallization was driven by subgrain rotation. Plagioclase recrystallization across the shear zone was accomplished by subgrain rotation and bulging grain recrystallization. Recrystallized grain size ranges from 150-50 µm in the margins of the shear zone to <10 µm in the high strain core. Previous researchers estimated that flow stresses in the shear zone core exceeded 176 MPa, but fewer constraints exist in the lower strain regions of the structure. Electron backscatter diffraction was used to collect maps of recrystallized quartz and plagioclase from additional samples. Our new data provide paleo-piezometric estimates from the margins of the shear zone that indicate flow stresses ranging from 15-42 MPa for quartz and plagioclase. When combined with the previous data, the results show progressively increasing flow stress across the shear zone from the outer margins of less than 50 MPa to the inner margin of between 50 and 100 MPa to greater than 175 MPa in the core. This pattern supports an interpretation that localized differential stress enhancement during viscous flow, potentially due to strength contrasts from lithologic heterogeneity, contributed to the nucleation of deep crustal earthquakes in the Cora Lake shear zone.

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

doi: 10.1130/abs/2025AM-9602

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