The Brittle‐Plastic Transition in Quartz‐Albite Mixtures: New Insights From Shear Deformation Experiments at Mid‐to‐Lower Crustal Depth Conditions

Abstract Crustal strength is often characterized using a strength‐depth profile where laboratory‐derived friction and flow laws are connected at depth (i.e., the so‐called “Christmas Tree” diagram). Large, destructive earthquakes frequently nucleate within the transition zone from a frictional‐to‐viscous deformation regime, which represents the strongest part of the crust. However, microscale deformation mechanisms controlling bulk frictional‐to‐viscous transitional behavior remain unclear. To investigate the deformation mechanisms, we conducted shear experiments on room‐dry, powdered quartz‐albite mixtures under upper‐ to mid‐crustal pressure‐temperature conditions using a Griggs‐type deformation apparatus. We simulated depth conditions in the range 7–30 km, by varying temperatures and confining pressures (210–900°C and 185–870 MPa, respectively, by assuming 30°C/km and 2,700 kg/m 3 ). To assess the rate dependence and stability of shear deformation, we sequentially stepped shear strain rates between ∼10 −3 /s and ∼10 −4 /s. At shallower depth conditions, friction coefficients follow Byerlee's law, while at greater depth conditions they deviate from it and strain weakening is observed. Post‐mortem microstructures indicate changing deformation mechanisms with increasing simulated depths. The samples deformed at shallower depth conditions (<18 km) show a predominance of cataclastic grain comminution. At greater depth conditions (>24 km), nano‐grains are observed, as well as polygonal quartz grains at the greatest depth condition (30 km). These results indicate that the controlling deformation mechanisms at the frictional‐viscous transition zone are grain boundary sliding and dynamic recrystallization. We conclude that nano‐scale deformation mechanisms govern the frictional‐viscous transitional deformation in the upper crust, and propose their importance for understanding seismic rupture processes there.