Calcite twin morphology: a low-temperature deformation geothermometer

Abstract

Twinning of the e-plane is the dominant crystal -plastic deformation mechanism in calcite deformed below about 400 8C. Calcite in a twin domain has a different crystallographic orientation from the host calcite grain. So-called thin twins appear as thin black lines when viewed parallel to the twin plane at 200-320 £ magnification under a petrographic microscope. Thick twins viewed in the same way have a microscopically visible width of twinned material between black lines. Calcite e-twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse-grained calcite. In this paper, we present a compilation and analysis of data from limestones of the frontal Alps (France and Switzerland) and the Appalachian Valley and Ridge and Plateau provinces (eastern United States) to document this temperature dependence. Mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 8C and thick twins dominate above 200 8C. Above 250 8C dynamic recrystallization is an important deformation mechanism in calcite. Mean twin intensity (twin planes/mm) correlates negatively with temperature, and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation. These relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data.

Different styles of faulting deformation along the Dead Sea Transform and possible consequences for the recurrence of major earthquakes

“…We use the fact that twin width and twin type correlate directly with temperatures of deformation (e.g. Ferrill, 19991;Burkhard, 1993;Ferrill et al, 2004). Thin twins (< 1 !m) dominate below 170° C and thick twins dominate above 200° C, whereas above 250° C dynamic recrystallization becomes more important (Ferrill et al, 2004).…”

How hard were the Jura mountains pushed?

“…However, the linear stress gradient models appear to confirm both the lower values of stress from the samples and the most likely burial depths as those giving the most physically realistic stress values. twinning in all samples, including the borehole, is of type 1 or 2 (mostly type 1) (burkhard 1993;Ferrill et al 2004) except for clearly reworked grains from the Helvetic nappes, suggesting low temperature (<150 °c) deformation and hence burial likely < 5 km.…”

Volcanic Initiation of the Eocene Heart Mountain Slide, Wyoming, USA

“…The stripping procedure was used in cases of high proportions of NEVs and a large number of measured grains. An experimental reevaluation of the CSGT has shown that measurements of about 50 grains on one thin section or 25 grains on two mutually perpendicular thin sections yield the best results (Groshong et al 1984;Evans and Groshong 1994;Ferrill et al 2004). The chance to extract the records of more than two deformations from one data set is limited when dealing with natural rocks (Burkhard 1993).…”