DEFORMATION CONDITIONS IN THE NORTHERN SUB-ALPINE CHAIN, FRANCE, ESTIMATED FROM DEFORMATION MODES IN COARSE-GRAINED LIMESTONE

In low-temperature experiments, the deformation mode and ductility in a single rock type are most directly controlled by the confining press ure and strain magnitude. A graphical technique is developed from publ ished experimental microstructural data that allows for the determinat ion of deformation mode for coarse-grained limestone on the basis of m icrostructural point-count data. The deformation mode and strain are u sed to constrain the confining pressure during thin-section scale defo rmation by comparison with experimental deformation results. The techn ique is applied to coarse-grained samples of Cretaceous and Tertiary l imestone from the northern Subalpine Chain, France, and reveals that t he majority of samples deformed mostly by twinning in the uniform flow mode at minimum depths, estimated from deformation mode and strain, o f between 0 and 6.0 km and maximum depths, interpreted from peak metam orphic temperatures and geothermal gradient, of about 2.7-9.7 km. In t he 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.