Frictional properties of Alpine Fault rocks of DFDP-1 under hydrothermal conditions and high shear strain

Abstract:

The Alpine Fault, New Zealand, is a major plate-bounding fault that accommodates 65-75% of the total relative motion between the Australian and Pacific plates. The absence of measurable contemporary surface deformation has been interpreted to indicate that the fault slips mostly in quasi-periodic large earthquakes (< M_w 8.0). In order to understand the mechanics of earthquakes, it is important to study the evolution of frictional properties of the fault rocks under conditions representative of the potential hypocentral depth. Here, we present preliminary data obtained on drill core samples of rocks that surround the principal slip zone(s) (PSZ) of the Alpine Fault and the PSZ itself. The drill core samples were obtained during phase 1 of the Deep Fault Drilling Project (DFDP-1) in 2011 at relatively shallow depths (down to ~150 m). Simulated fault gouges were sheared under elevated pressure and temperature conditions in a hydrothermal ring shear apparatus. We performed experiments at temperatures of 25, 150, 300 and 450 °C. Using the shallow geothermal gradient of 63 °C/km determined in phase 1 of DFDP, our highest temperature corresponds to a depth of ~7 km or 10 km using a more moderate geotherm of 45 ºC/km as suggested by Toy et al. (2010). To explore temperature constraints on the limit of the brittle faulting processes, additional experiments at 600 ˚C (or ~10-15 km depth) are planned. Preliminary results on two hanging wall (foliated) cataclasite samples show a transition from velocity-strengthening behavior, i.e. a positive value of (a-b), to velocity-weakening behavior, i.e. a negative value of (a-b) at a temperature of 150 °C. The transition depends on the absolute value of sliding velocity, with velocity-weakening being more dominant at lower sliding velocities. Moreover, shear stress depends linearly on effective normal stress, indicating that shearing is essentially frictional and no transition to ductile (normal stress independent) flow has been observed. Thus, the AF should be able to generate earthquakes at temperatures up to 450 °C, which is consistent with the fact that microseismicity is recorded to depths of 8-12 km. We speculate that in our porous gouges, thermally activated processes operate simultaneously with granular flow, postponing ductile flow to higher temperatures or lower rates.