Question

How to differentiate between twined and un twined part of calcite?

Answer 1

Stress reconstructions were carried out based on inversion of calcite twin data from slightly deformed Quaternary reefal limestones from SW Taiwan. Results demonstrate that twinning reliably recorded the complex Quaternary and present‐day stress fields in a young mountain range undergoing collisional shortening. The stress regimes reconstructed at microscopic scale are predominantly compressional and reflect the main stages of the neotectonic evolution of the collision belt. No twinning has been observed in a sample collected in the Holocene platform of southern Taiwan, indicating that these formations did not undergo sufficient stress to initiate calcite twinning. Three different stress regimes have been recorded in the samples collected in Pleistocene reef limestones that developed in relation to active fold systems of the Western Foothills. These regimes appear to be homogeneous at the site scale and at the scale of SW Taiwan. The reconstructed ENE and NW trending compressions, only present in SW Taiwan, probably reflect local perturbations of the regional 105° trending compression. The perturbation might be due to activation of a set of folded‐faulted blocks in the studied area. A NW‐SE extension, also identified in these samples, is related to extrados stretching due to fold development. The results are consistent with independent stress data, i.e., Quaternary faulting, borehole breakouts, and focal mechanisms of earthquakes. The analysis of mechanical twins in calcite rocks constitutes a powerful tool for tectonic analyses in fold‐and‐thrust belts.