Effects of temperature on the frictional behavior of material from the Alpine Fault Zone, New Zealand

Temperature is widely believed to act as a primary control on fault rheology, and therefore on the distribution of seismicity along plate boundary faults. However, there are few detailed measurements of the frictional strength and stability of natural fault gouges at elevated temperatures. Here, we report on a suite of shearing experiments designed to investigate the frictional behavior of fault rocks sampled from depths of 111.5–142.9 m along the Alpine Fault in New Zealand obtained by the International Continental Scientific Drilling Program (ICDP) Deep Fault Drilling Project (DFDP). We tested five samples from the DFDP-1B pilot hole: two hanging wall chloritic cataclasites, two footwall granitic cataclasites, and a fault gouge from the principal slip zone (PSZ-1). Each sample was sheared at a range of temperatures from 23 to 500 °C and at an effective normal stress of 80 MPa. The wall rock cataclasites exhibit an increase in the friction coefficient (μ) with temperature, from μ = 0.45–0.64 at 23 °C to μ = 0.87 at 500 °C. The PSZ-1 gouge exhibits lower friction coefficient values than the wall rock at temperatures ≤180 °C (μ = 0.35–0.46 vs 0.45–0.65), but comparable values (μ = 0.87–0.90) at 500 °C. The variation in frictional strength is accompanied by a transition from velocity-strengthening to velocity-weakening behavior at temperatures ≥180 °C for all materials. Extrapolation of the experimentally defined rheological critical stiffness of the fault material and the estimated in situ stiffness of the surrounding crust suggests upper and lower stability boundaries at ~1.8–2.5 km and ~8.5–8.8 km depth, respectively. The upper stability boundary is also consistent with the observed depth-frequency distribution of earthquakes.