Strain determinations using inoceramid shells as strain markers: a comparison of the calcite strain gauge technique and the Fry method

To test the accuracy and reliability of the strain gauge technique for calcite e-twin analysis, calcite e-twin strain and finite strain were determined using a new tectonic marker, inoceramid shells (macrofossils composed of aggregates of pseudo-hexagonal calcite prisms with a honeycomb microstructure). Although the calcite prisms in the inoceramid shells were not randomly distributed and showed strong crystallographical orientation, the strain gauge technique yields principal strain orientations that correlate precisely with those found by the Fry method. Thus, this technique can also be used to establish the ellipsoid strain orientation in rocks where calcite grains show strong crystallographical orientation.