CRITICAL REEVALUATION OF DIFFERENTIAL STRESS ESTIMATES FROM CALCITE TWINS IN COARSE-GRAINED LIMESTONE

Comparison of two techniques to estimate differential stress magnitude based on calcite twinning reveals significantly different calculated values of differential stress for limestone experimentally and natural ly deformed at low temperature. This inequality may be due to fundamen tal differences between the rock type (marble versus limestone) and de formation temperatures (above and below 200 degrees C) for samples use d in technique calibration versus samples used in testing and applicat ion. A technique that was empirically derived from marble experimental ly deformed at 200-800 degrees C yields values of differential stress that are factors of 2 to 9 times larger than maximum differential stre ss measured during experimental deformation of limestone at low temper ature (<200 degrees C). Differential stress estimates from naturally d eformed rocks differ by factors of 4 to 20, depending on the stress es timation technique used. Calcite tends to develop thin twins when defo rmed at temperatures below 200 degrees C, and thick twins when deforme d at temperatures above 200 degrees C. Because of the temperature depe ndence of calcite-twin morphology, and possible effects of strain, gra in size, and porosity, twin-based differential stress estimates should be used cautiously and particular attention given to deformation temp erature. (C) 1998 Elsevier Science B.V. All rights reserved.