Application of Electron Backscatter Diffraction to Calcite-Twinning Paleopiezometry

Electron backscatter diffraction (EBSD) was used to determine the orientation of mechanically
twinned grains in Carrara marble experimentally deformed to a small strain (4%) at room
temperature and at a moderate confining pressure (225 MPa). The thicknesses of deformation twins
were mostly too small to permit determination of their orientation by EBSD but it proved possible to
measure their orientations by calculating possible twin orientations from host grain orientation, then
comparing calculated traces to the observed twin traces. The validity of the Turner & Weiss method
for principal stress orientations was confirmed, particularly when based on calculation of resolved
shear stress. Methods of paleopiezometry based on twinned volume fraction were rejected but a
practical approach is explored based on twin density. However, although twin density correlates
positively with resolved shear stress, there is intrinsic variability due to unconstrained variables such
as non-uniform availability of twin nucleation sites around grain boundaries that imposes a limit on
the achievable accuracy of this approach.