Brittle precursors of plastic deformation in a granite: an example from the Mont Blanc massif (Helvetic, western Alps)

In the Mont Blanc Helvetic massif, granites record mesoscale Alpine structures, which include joints, veins, cataclastic to mylonitic shear zones and foliated granites. A detailed structural analysis indicates that brittle deformation predates plastic strain. Joints never pass through, and veins are offset by, cataclastic shear zones and mylonites. The mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions. Plastic deformation is first localized in the brittle discontinuities and the fine-grained matrix of cataclasites. Then it involves the granite within brittle shear zones, and this is initially accomplished mainly by flow of reaction-softened aggregates of sericite, widely replacing the strain-supporting magmatic plagioclase. The brittle-to-plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity, and these characteristics are derived from reactivation of, and focusing along, pre-existing brittle discontinuities. In addition, mylonites may inherit high angles of intersection, and may contain granite porphyroclasts. These features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics.