Effects of Water on the Deformation of Rocks at the Brittle–Plastic Transition: Insight from High-pressure and High-temperature Deformation Experiment on Quartz + Water Aggregates

The rheology of rocks is reviewed in the brittle–plastic transition (BPT), a key depth range in the continental crust and subduction plate boundaries where earthquakes, slow earthquakes, and stable sliding can all occur. Understanding the physical processes governing deformation in this zone is essential for linking laboratory observations to fault-slip behavior at the scale of plate boundaries. Quartz aggregates are a key material because quartz is abundant in the upper crust, as well as in pelitic and psammitic rocks in the subduction zone, and they serve as an analog for other rocks due to their well-studied deformation mechanisms. This focus is on fault zones within the BPT that contain abundant pore water, drawing primarily upon experimental studies on polycrystalline quartz. High-temperature, high-pressure deformation tests with controlled fluid content show that fluid-filled porosity markedly weakens quartz shear zones across the BPT. The rheology of these zones is controlled by pore fluid pressure, porosity, and fluid topology. Within the BPT, strain is partitioned between brittle and plastic deformation, but the degree of partitioning is complex and non-linear. The variability and complexity of deformation and fluid-filled porosity in the BPT may help explain the diverse spectrum of slip behaviors observed along plate boundaries.