Evolution of Brittle Structures in Plagioclase‐Rich Rocks at High‐Pressure and High‐Temperature Conditions—Linking Laboratory Results to Field Observations

Plagioclase‐rich granulites exposed on the Lofoten archipelago, Northern Norway, display strain localization in pseudotachylytes as well as ductile shear zones that formed under similar high‐pressure and high‐temperature conditions. Pseudotachylytes or pseudotachylyte networks reveal no or very little hydration, whereas ductile shear zones reveal significant hydration. We combine these observations from the field with experimental results to characterize the structural evolution of brittle faults in plagioclase‐rich rocks at conditions of the lower continental crust. We performed a series of deformation experiments on intact granulite samples prepared from a natural granulite sample at 2.5 GPa confining pressure, a strain rate of 5 × 10−5 s−1, and temperatures of 700°C and 900°C to total strains of ~7–8% and ~33–36%. Samples were either deformed “as‐is” or with ~1 wt.% H2O added. Striking similarities between the experimental and natural microstructures suggest that the transformation of precursory brittle structures into ductile shear zones at eclogite‐facies conditions is most effective in samples deformed with added water triggering reaction and subsequent plastic deformation of the products along the faults and in the adjacent wall‐rock. Plain Language Summary Pseudotachylytes and ductile shear zones, both reflecting the localization of strain, can be found in high‐pressure, high‐temperature rocks of the lower continental crust, for example, in the Lofoten archipelago, Northern Norway. In contrast to the pseudotachylyte mineral assemblage, ductile shear zones show a significant amount of hydrous minerals. To experimentally investigate the influence of water‐rich fluids on the evolution of brittle structures at conditions of the lower continental crust, we conducted deformation experiments on intact granulite samples and compared our laboratory results to natural microstructures of the Lofoten samples. To investigate the microstructural evolution of the experimental samples, we terminated the tests at ~7–8% and 33–36% axial shortening. We studied the importance of water for the microstructural evolution of the samples by either deforming natural samples “as‐is” or by adding approximately 1 wt.% H2O. In addition, we tested the influence of temperature by performing the experiments at 700°C and 900°C. All tests were conducted at a confining pressure of 2.5 GPa and a strain rate of 5 × 10−5 s−1. The experimental microstructures demonstrate that the transfo