Strain Localization (and De-localization?) in Deep Continental Crust: Examples from an Exhumed Section and Remote Observations from Still Deep Crust in North America

Kevin H. Mahan, University of Colorado-Boulder
Sean Regan, University of Massachusetts-Amherst
Omero P. Orlandini, University of Colorado-Boulder
Michael L. Williams, University of Massachusetts-Amherst
Vera Schulte-Pelkum, University of Colorado-Boulder

Rock records of strain localization provide insight into changing internal and external conditions during shear zone development. They also provide a means for interpreting the evolution of complex multiphase terranes, both those now exhumed and those still deep. This presentation will 1) review examples of multi-scale interactions between deformation and metamorphism in the exposed Athabasca Granulite Terrane (AGT) in the western Canadian shield, and 2) explore seismic anisotropy as a crustal deformation mapping tool with recently published regional examples and a new image from EarthScope's full Transportable Array.

The AGT records at least 700 m.y. of tectonism, but emphasis here is on structures that developed during the peak and exhumational stages of the youngest event (1.9-1.8 Ga). The 1.88 Ga Cora Lake shear zone is a dominantly sinistral strike-slip structure characterized by multiple strands of ultramylonite superimposed on a ~5 km wide zone of mylonite. Rock types vary but all display evidence for deformation under relatively dry, high-pressure granulite-facies conditions (1.0-0.8 GPa, 850-750°C). The ultramylonite zones also host pseudotachylyte networks and associated kinematically compatible shear fracture systems. Stable granulite-facies mineral assemblages (e.g., grain-size reduced pyroxene) in the immediate vicinity of the shear fractures and local development of new garnet in the pseudotachylyte matrix suggest seismic slip under hot and deep crustal conditions.

A younger set of structures in the AGT include two several km-wide zones of 1.85 Ga amphibolite-facies mylonite. The thrust sense Legs Lake and dextral strike-slip Grease River shear zones are both distinguished from the Cora Lake structure by pervasive synkinematic hydration. Fluid infiltration, perhaps initially into brittle or semibrittle structures in deep crust, like those still preserved in the Cora Lake shear zone, allowed re-equilibration under mid-crustal P-T conditions (0.5 GPa, 600°C). Reaction-weakening via growth of mica and amphibole in previously drier rock facilitated a conversion back to fully plastic and more widely distributed flow. Thus, the AGT may record portions of multiple cycles of distributed and focused deformation even within already localized high strain zones via a range of plastic to brittle modes. Data obtained from terranes such as the AGT also form a basis for the use of seismic anisotropy as a detection and mapping tool for both focused and distributed deformation in modern deep crust. Particularly if augmented with available higher resolution regional data, these tools can allow greater advancement in understanding where and how deformation is focused in orogenic systems.

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