EXAMINING THE PALEOSTRESSES RESPONSIBLE FOR THE BLACK HILLS UPLIFT THROUGH ANALYSIS OF CALCITE DEFORMATION TWINS USING ELECTRON BACKSCATTER DIFFRACTION

To study the nature of the Laramide uplift in the Black Hills, South Dakota, a method is being developed to determine the orientations and magnitude of paleostresses from calcite deformation twins using electron backscatter diffraction. Compared to traditional methods for calcite deformation twin paleostress analyses, this method has the potential to produce more accurate results in less time. Under low pressure and temperature, calcite deformation twins form intracrystalline twin lamella on the {01-12} planes. Twinning occurs when the resolved shear stress acting along each {01-12} plane exceeds the yield stress value for twinning (~10 MPa). Combination of the relationships of crystallographic orientations and the yield stress value can be used to calculate a deviatoric stress tensor from deformed calcite crystals. Preliminary analyses from orientation image maps of deformed calcite crystals show misorientation angles of ~78 degrees between the host and twin lamella crystal lattices about the {01-12} planes. This angle is consistent with calcite deformation twinning and will be the foundation for development of the method. In order to place the project in a meaningful structural context, a detailed study was conducted in Wind Cave National Park, South Dakota. Fieldwork was carried out though the National Park Service and South Dakota Geological Survey during summer 2008 and summer 2009 and included 1:12,000 scale mapping of surface geology, recording structural data for analyses, and collecting samples for laboratory analyses. By integrating structural data with deformation twin data, the goal to constrain the orientation and magnitude of paleostresses that caused the Black Hills uplift.