Effects of Temperature on the Frictional Behavior of Material from the Alpine Fault Zone, New Zealand

Abstract:

Temperature is widely believed to be one major factor controlling the distribution of seismicity along plate-boundary faults. However, there are few detailed measurements of the strength and frictional properties of natural fault gouges at elevated temperatures, which are necessary to quantify the role of temperature on friction and to understand underlying mechanisms. Here, we report on a suite of shearing experiments designed to investigate this question using fault rocks from the Alpine Fault in New Zealand, sampled by coring in Deep Fault Drilling Project (DFDP) borehole 1B. The DFDP-1B borehole penetrated 151 m, through – from top to bottom - hanging wall mylonites and ultramylonites, damage zone cataclasite, a ~30 cm-thick clay-rich ultra fine-grained principal slip zone (PSZ) gouge, and granitic footwall cataclasites.

 We tested five samples from DFDP-1B: two chloritic cataclasites from 111.5 m and 127.1 m, PSZ gouge from 128.1 m, and two granitic cataclasites from 130.3 m and 142.9 m. We powdered and sieved samples to <106 µm, and conducted shearing experiments on gouge layers in a triaxial saw-cut geometry. In our experiments, we sheared 1-mm thick gouge layers at an effective normal stress of 80 MPa, and at temperatures ranging from 23-500°C. In each test, we sheared at an initial velocity of 1 µm/s for ~0.5 mm and then conducted velocity-steps over a range of load-point velocities from 0.1-100 µm/s.

Both the hanging wall and footwall cataclasites exhibit an increase in friction coefficient (µ) with increasing temperature from 0.60-0.64 at 23°C to 0.87 at 500°C, and from 0.45-0.62 at 23°C to 0.70-0.84 at 350°C, respectively. The PSZ material friction coefficient (µ = 0.46) at 23°C decreases to 0.20-0.40 at 180°C, and then increases to 0.87-0.90 at 500°C. All of the samples exhibit velocity-strengthening at 23°C, transition to velocity-weakening at 180-350°C, and remain velocity-weakening once at 500°C. The chloritic cataclasites, granitic cataclasites, and PSZ also become frictionally unstable, producing stick-slip behavior at 350-500°C. We suggest that both the increased frictional strength and the transition to velocity-weakening and unstable behavior with increasing temperature may result from enhanced gouge consolidation and lithification associated with shearing at elevated temperatures.