Experimental and Natural Deformation of Magnesian Carbonates

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Shallow marine carbonate rocks that are deposited at continental margins are subjected to intense deformation at convergent plate boundaries, and a growing body of evidence indicates that these carbonate rocks are entrained in subduction zones to significant mantle depths. By comparison with rocks composed of silicate minerals, rocks composed of carbonates are weak, so that their deformation exerts a strong influence on the structures that develop in mountain belts resulting from continental collisions, and they may also influence deformation in the mantle during subduction. Carbonate rocks are composed largely of the minerals calcite, dolomite, and magnesite. These minerals have similar crystal structures, but are compositionally and mechanically different. While the rheology of calcite has been well established, the rheology of dolomite is less well constrained by experiments, and almost nothing is known about the rheology of magnesite, which is stable to greater depths in the mantle than calcite or dolomite. In order to accurately model deformation processes within the crust and mantle during subduction and mountain building processes, the rheology of dolomite needs to be refined, the rheology of magnesite determined, and then compared to the rheology of calcite. This project will use both experimental and observational approaches to achieve the following: (1) determine the rheology for dislocation creep of dolomite through experiments, in a triaxial rock deformation apparatus, to establish the temperature and strain rate dependence of this deformation mechanism in dolomite; (2) determine the mechanical properties of magnesite through experiments, in a triaxial rock deformation apparatus, over a wide range of pressures, temperatures and strain rates to establish the ranges of temperature and strain rate over which different deformation mechanisms operate; (3) investigate the microstructures and fabrics formed in experimentally shortened and sheared dolomite; and (4) compare the microstructures and fabrics formed during experimental deformation with those observed in natural dolomite shear zones to provide the foundation to apply laboratory determined rheologies to nature.This research will investigate, through deformation experiments and analyses of rocks deformed in nature, the strength and deformation processes of the carbonate minerals, dolomite and magnesite. Carbonate-bearing rocks make up a significant portion of the rocks that are deformed during mountain building at convergent plate boundaries. Therefore, the strength and deformation behavior of these rocks will strongly influence the structures that develop in mountains, which are responsible for the accumulation of natural resources. The data generated through this study will be used by theoretical modelers and field geologists to determine how and where resources accumulate. Carbonate rocks are also carried to significant depths in the earth?s interior within subduction zones, and their strengths and deformation behavior will thus exert a strong influence on processes such as subduction and mantle convection. Data on the deformation behavior of these rocks will enable theoretical modelers to more accurately predict the dynamics of these long-term geologic processes, including a better understanding of the dynamics of deep earthquakes.