Calcite twinning stress inversion using OIM (EBSD) data
| Authors | |
|---|---|
| Year of publication | 2005 |
| Type | Article in Periodical |
| Magazine / Source | Geolines |
| MU Faculty or unit | |
| Citation | |
| Field | Geology and mineralogy |
| Keywords | calcite; twinning; stress inversion; EBSD; OIM |
| Description | Deformation origin of calcite twin lamellae (e-twins) and their crystallographic laws have been recognized in the end of the 19th century (e.g., Mügge, 1883). During the last 60 years it has been found that twinning is an important intracrystalline deformation mechanism with low critical resolved shear stress (e.g. Turner, 1963; DeBresser, Spiers, 1996) and therefore it is the main deformation feature for low temperatures, low confining pressures and low finite strains (15%). Since the fifties of the 20th century, when Turner (1953) developed a method for determination of stress axes from a set of e-twins (TDA), it became a useful tool for paleostress analysis in deformed calcitic rocks (or rocks containing calcite veins). Several methods of differential stresses estimations (Jamison and Spang, 1976; Rowe and Rutter, 1990) and stress tensor calculations (e.g. Lacombe and Laurent, 1996) have been developed during last 60 years based on experimental and natural (?field) data. Orientation of calcite twin lamellae as well as the c-axis orientation can be measured directly on an universal stage. This cheap method does not require any special samples but it is subjective and inaccurate, especially if c-axis orientation is measured and very thin lamellae may cause problems as well (difficulty of differentiation between cleavage planes and e-twins). However, Orientation Imaging Microscopy (OIM) using Electron Backscatter Diffraction (EBSD) provides precise data without subjective factors. A chosen area within a thin section is investigated using a hexagonal grid of lattice orientation measurements. Such data set can be presented as a bitmap, where each pixel represents one measurement coded by color. One can then directly observe misorientation of grains, subgrains and e-twins. The greatest disadvantage of OIM is that it represents a time-consuming method. One orientation map covers a tiny area (0,03 mm2, 82 000 measurements), so investigating a sample of 1 x 2 cm would take weeks. We propose a grid of linescans to compensate this disadvantage. These linescans with measurement step 0,6 microns arranged in an orthogonal grid with 1 mm interval would cover a much larger area (1,3 x 0,7 cm) using the same number of measurements. This method is able to provide appropriate data from a relative large area in an acceptable time. A new computer program has been developed for stress analysis of calcite twin lamellae, including most of the methods mentioned above, and processing EBSD data files as well. In our view, the optimal method of paleostress orientation and magnitude determination in carbonate complexes is the total search method of Lacombe and Laurent (1996). A modification of this method has been used in the Moravian Karst (Bohemian Massif) model area. ( because recent computers have the capability to process big amounts of data to je obecný údaj, který bych vypustil). Instead of using a set of random reduced stress tensors and then penalisation function to choose the most probable stress tensor, a systematical searching in all possible stress tensors generated from input limits was preferred. Combination of precise calcite lattice orientation measurements (EBSD) and numerical methods of paleostress analysis makes calcite a very useful tool for evaluating deformation pathways in sedimentary complexes. |
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