Exhumation-induced residual stress in undeformed, ultra-high-pressure metamorphic rock

The exhumation of high-pressure metamorphic rocks from subduction zones involves dramatic pressure-temperature changes, triggering complex micromechanical responses at the grain-to-sub-grain scale. However, the mechanical aspects of these processes, particularly the origins and persistence of residual stresses within rock microstructures, remain poorly understood. To address this problem, we employed synchrotron-based three-dimensional X-ray diffraction to investigate residual strain, stress, and intra-grain misorientation in a garnet-quartz metamorphic rock from the Lago di Cignana ultra-high-pressure unit in the Western Alps. Our analysis reveals long-range residual stress heterogeneities spanning tens to hundreds of micrometers, with magnitudes reaching several hundred MPa. Significant intra-grain misorientations in both quartz and garnet provide insights into the interplay between plastic and elastic deformation processes.  These stress signatures are preserved in a sample lacking apparent macroscopic deformation, suggesting that subtle mechanisms—such as decompression-induced anisotropic expansion, grain interactions, and garnet compositional gradients—play a key role in stress retention. These findings highlight the potential of synchrotron X-ray diffraction for capturing the stress field within polycrystalline rocks. The ability to resolve three-dimensional strain and stress distributions across scales offers new opportunities to advance our understanding of micromechanical processes associated with rock deformation and metamorphism.