Deformation conditions in the northern Subalpine Chain, France, estimated from deformation modes in coarse-grained limestone

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In low-temperature experiments, the deformation mode and ductility in a single rock type are most directly controlled by the confining pressure and strain magnitude. A graphical technique is developed from published experimental microstructural data that allows for the determination of deformation mode for coarsegrained limestone on the basis of microstructural point-count data. The deformation mode and strain are used to constrain the confining pressure during thin-section scale deformation by comparison with experimental deformation results. The technique is applied to coarse-grained samples of Cretaceous and Tertiary limestone from the northern Subalpine Chain, France, and reveals that the majority of samples deformed mostly by twinning in the uniform flow mode at minimum depths, estimated from deformation mode and strain, of between 0 and 6.0 km and maximum depths, interpreted from peak metamorphic temperatures and geothermal gradient, of about 2.7–9.7 km. In the remaining samples, brittle failure probably occurred at relatively shallow depths (< 2.5 km) or in response to lowered effective confining pressure due to elevated fluid pressures.