Cessation of plastic deformation during exhumation of metamorphic tectonites revealed by microboudinage structures

Microboudinage structures are analysed to examine how quartzose metamorphic tectonites cease plastic deformation during retrograde metamorphism in exhumation. The microboudinage of columnar mineral grains embedded in the quartz matrix record whether plastic deformation ceased as result of the relaxation of differential stress or the plastic-brittle transition of the matrix. The fundamental strain data for microboudinage structures reveal four possible cases in terms of the timing of the plastic-brittle transition in the differential stress history. The classification is closely related to the degree of post-tectonic annealing, which obliterates the microstructures produced during earlier plastic deformation. The approach is applied to three metamorphic tectonites from high-pressure/temperature metamorphic belts, and all are demonstrated to have ceased plastic deformation due to plastic-brittle transition of the matrix, suggesting that the rocks experienced increasing differential stress during exhumation in retrograde metamorphism.