CALCITE TWIN WIDTHS AND INTENSITIES AS METAMORPHIC INDICATORS IN NATURAL LOW-TEMPERATURE DEFORMATION OF LIMESTONE

Twins in naturally deformed coarse-grained limestones of the northern Subalpine Chain (France) show a direct relationship of twin width and intensity (number of twins per mm) to the peak metamorphism under diagenetic and anchimetamorphic conditions. The transition from dominant thin twinning to dominant thick twinning occurs between a mean vitrinite reflectance (Ro) of 1.60 and 2.60, or temperatures of between 150 and 200-degrees-C. Below the transition temperature, increasing strain is apparently accommodated by adding new twins, and above the transition temperature, increasing strain is accommodated by twin widening. Similar transitions from thin to thick twinning with increasing metamorphism are also present in the Helvetic Alps, the Prealps, and the Central Appalachian Valley and Ridge Province. These data for naturally deformed limestones support the interpretation from experimental studies of a temperature effect on twin width and intensity and indicate that the twinning in the northern Subalpine Chain occurred at or near peak metamorphism.