ABSTRACT

Understanding the strength and mechanical behavior of the continental lithosphere is a first-order issue in tectonics. Mechanical behavior of the lithosphere depends mostly on the development of weak and strong layers. In the laboratory, insight into this process can be gained through the study of reactions and deformation mechanisms in minerals, fluid flow, and fabric evolution. The wide range of temporal and spatial scales at work in deforming lithosphere presents challenges in extrapolating results from laboratory studies to natural mountain belts. Through a study of deformed diamictites, rocks composed of mainly mud and cobbles, these researchers will evaluate processes that reduced the rock strength and resulted in concentrated deformation. The deformation will be addressed through measurements of the shape changes that the cobbles have undergone. These studies will improve our understanding of how strong and weak parts of the crust evolve during mountain building, and processes that localize deformation into fault zones and partly control nucleation of large earthquakes.

The investigators will integrate field, microtextural, and microchemical analyses of variably deformed and altered granitic gneiss and quartzite to quantify relationships between progressive strain, deformation mechanisms, and fluid-chemical processes. The diamictite displays a km-scale gradient in bulk deformation intensity, providing a record of hydrolytic weakening and reaction softening processes. Strain will be estimated for different clast types and sizes using the Rf/?Ö technique. Volume change will be estimated from geochemical changes and microstructural relations. Deformation mechanisms will be evaluated from petrographic studies of microstructures, combined with electron backscatter diffraction analysis of lattice preferred orientation and subgrain boundaries. Amounts and locations of water and related species, which control hydrolytic weakening, will be evaluated using Fourier transform infrared spectroscopy and synchrotron infrared radiation. Cathodoluminescence will provide data on locations of quartz neocrystallization and healed microcracks. Changes in bulk chemical composition of clasts and matrix will be quantified using X-ray fluorescence, X-ray spectral mapping and scanning electron microscopy. This combination of techniques will provide new information on the interactions between deformation, fluids, and chemical processes during progressive strain.

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