Collaborative Research: Effects of Structural and Compositional H Eterogencity On Upper Mantle Deformation and Rheology
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abstract
The deformational behavior of the lithospheric mantle exerts a critical effect on the strength of the tectonic plates. Whereas experimental rock deformation studies form the basis of our understanding of mantle deformation and rheology, field-based studies of exposed mantle sections are needed to investigate the rheology and deformation of naturally deformed mantle at larger spatial scales (> 10 cm). Recent field studies have illustrated complex patterns of deformation that challenge a classical homogeneous rheological model for mantle deformation. A multi-scale (cm to km), integrated approach of the processes that result in localization and deformational overprinting in lithospheric mantle, allows an assessment of mantle rheology at larger spatial scales. The Dun Mountain ophiolite belt contains multiple intact blocks of lithospheric mantle - including the Red Hills, Red Mountain, and Dun Mountain - that reveal spatial and temporal patterns of deformation in naturally deformed lithospheric mantle and elucidate fundamental processes of mantle deformation, localization, and rheology. First-order tectonic questions about deformation in the lithospheric mantle addressed by this research include: 1) What is the scale and extent of mantle heterogeneity?; 2) What processes result in mantle heterogeneity at different scales?; 3) Do heterogeneous fabrics result from temporally and/or spatially localized deformation?; and 4) What is the effect of heterogeneous deformation on fabric and texture preservation and magnitude? These questions are answered with primary data on the scale and extent of mantle deformational heterogeneity including structural analyses (fabric analysis, compositional and structural domain characterization, identification of localizing or overprinting phenomena, strain analysis, estimates of effective viscosity), microstructural and textural analyses (relationship between texture and fabric in rocks with heterogeneous fabrics), and analyses of deformation processes by analyzing both intrinsic (e.g., composition, melt fraction, grain size) and extrinsic (e.g., pressure, temperature, stress, oxygen fugacity) rock characteristics. The uppermost (lithospheric) part of the mantle is thought to be the strongest part of the tectonic plates that form the Earth?s surface. As such, the deformational behavior of the lithospheric mantle exerts a first-order control on ongoing and ancient mountain building processes. Portions of the lithospheric mantle are uplifted and exhumed during mountain building events and can now be found on the Earth?s surface. These localities offer the opportunity to study mantle processes at the scales at which deformation occurs in nature. The major question in this research is the extent to which variations in the exact proportion and orientation of different minerals in the mantle control the deformational behavior. This work can only be done in a few places in the world with sufficient exposure; the proposed work will take place in New Zealand. These data are critical in synthesizing results from rock deformation experiments and geophysical observations, to create realistic and predictive models of uppermost mantle deformation. This project is a multidisciplinary collaborative effort between researchers at Texas A&M University and the University of Wisconsin. In addition to the research goals of the project, it is fostering scientific training of graduate and undergraduate students. The project also involves an international collaboration with a faculty member from the University of Otago, New Zealand.
