Deformation mechanisms in the hydrated lower crust: Competitive strain softening processes in the Eastern Athabasca Mylonite Triangle.
Abstract
Understanding the rheological behaviour of the lower crust is an important component in the study of large-scale tectonic processes. This work investigates the competitive grain-scale mechanisms that facilitated strain accommodation in polymineralic hydrated lower crustal rocks as expressed through the development of quartz crystallographic preferred orientations (CPOs). The Upper-Lower Deck transition within the Eastern Athabasca Mylonite Triangle in northern Saskatchewan (Canada) is a lower crustal shear zone that operated at conditions of ~725 °C and ~0.8 GPa at the interface between felsic granulite above, relatively poor in hydrated minerals, and more hydrated meta-granitoids below. Microstructures and quartz CPOs in the two main lithotypes are similar at lower strain conditions but differ significantly as strain increases. When affected by high strain, the felsic granulites preserve similar mylonitic microstructures as those at lower strain, but and develop stronger quartz CPOs, whereas the meta-granitoids evolve from mylonites to ultramylonites but preserve weak quartz CPOs. Deformation in both the granulites and meta-granitoids is accommodated in two distinct rheological domains characterized by different dominant deformation mechanisms: (i) pure quartz ribbons that record dislocation creep and grain boundary migration recrystallization and (ii) feldspars-rich aggregates that record grain boundary sliding. In the dry felsic granulites, quartz ribbons are relatively weak and accommodate substantial deformation developing strong quartz CPOs. The rheology of the feldspars-rich aggregates appears dependent on the modal proportion of syn-kinematic hydrated mineral phases such as hornblende and biotite. In the hydrated meta-granitoids, pressure-solution and (re)precipitation processes enhance phase mixing and promote grain boundary sliding in the aggregates resulting in weakly developed or absent quartz CPOs. These results demonstrate the influence mineralogy and fluid availability can have on the dominant strain accommodating process(es) in mid-to-lower crustal shear zones.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43C0449G
- Keywords:
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- 8011 Kinematics of crustal and mantle deformation;
- STRUCTURAL GEOLOGY;
- 8012 High strain deformation zones;
- STRUCTURAL GEOLOGY;
- 8030 Microstructures;
- STRUCTURAL GEOLOGY;
- 8031 Rheology: crust and lithosphere;
- STRUCTURAL GEOLOGY