August 2009-July 2013
Level of Access
The primary intellectual impact of this project will be in improving our understanding of the mechanics that shape the Earth's crust. In recent years, earth scientists have used the increasing body of geodetic data towards that end, but the mechanical properties of the middle and lower crust remain only loosely constrained. This project focuses on the magnitude of strain weakening in shear zone networks. In detail, the research will explore the grain-scale and outcrop-scale deformation mechanisms in minerals that lead to this weakening, followed by modeling of the results to understand the weakening process on the larger scale. These conceptual and numerical models will allow better prediction of where and how fast the continental crust will deform and in turn this will benefit society. In the future, locations that have recently been deglaciated, and those in tectonically active areas where earthquakes are likely, may be interesting targets for further application of this research. This project also serves as a vehicle to continue and enhance ongoing educational and outreach initiatives at the K-16 and graduate levels. Support for students enrolled in the University of Maine's Master of Science in Teaching program will allow pre-service K-12 teachers to become involved in an active research project and to participate in creating an environment where science is more accessible. The PI and colleagues will develop new content based around supercomputer, visualization, and field video library projects, the combined results of which will enhance professional development, graduate and undergraduate courses, and outreach to K-12 students in Maine's rural areas.
Throughout the lithosphere, strain localization plays a fundamental role in tectonic processes affecting, for example, seismicity, exhumation, fluid migration and mineralization, magma transport, and topographic and plate boundary evolution. Previous research involving field observations and numerical modeling has produced many constraints on the causes and consequences of strain localization, but researchers lack a thorough understanding of the magnitude of strength variation in the deep crust in particular. Due to exceptional exposure and deep exhumation this research group will use the Parry Sound domain of the Grenville Province, southern Ontario, as a natural laboratory. Along one margin of the domain, granulite facies mineral assemblages have been transformed under upper amphibolite facies conditions along meter-scale shear zones. After field-based mapping, the PI will quantify the change in strength associated with the development of the interconnected meter-scale shear zones through model calculations using the natural geometric framework. In addition, to develop better tools to predict weakening of this magnitude elsewhere, he will determine the mechanisms by which the shear zones developed and the weakening occurred. The PI has hypothesized that one such mechanism involved pegmatite-derived fluids that infiltrated adjacent zones, allowing mineralogical changes along which shear zones could nucleate. Establishing the crustal-scale significance of strength changes requires determining if meter-scale processes produced, for example, the regional-scale, domain-bounding Twelve Mile Bay shear zone. Preliminary observations indicate that fractures with no lateral offset assisted full transposition of the fabric as they evolved into interconnected meter-scale shear zones during progressive deformation. This research group will evaluate whether these preliminary interpretations are valid through the mapping of thermobarometric and geochronologic patterns along the inferred length of the Twelve Mile Bay shear zone.
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Gerbi, Christopher, "Quantifying Syntectonic Weakening in Deep Orogenic Crust" (2013). University of Maine Office of Research and Sponsored Programs: Grant Reports. 432.