Extreme hydrothermal conditions at an active plate-bounding fault.

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300-450 degrees Celsius, usually found at depths greater than 10-15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional-mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

Energy-flux control of the steady-state, creep, and dynamic slip modes of faults.

Faults exhibit a gamut of slip styles from stable sliding and creep events to earthquakes. These slip styles are affected by the fault properties, e.g., weakening or strengthening, and the properties of the loading system. Here, we investigate the poorly understood effect of energy-flux to the fault that should equal or exceed the energy-dissipation-rate along the slipping fault. We explore the relationship between energy-flux and slip style in shear experiments along granite and diorite laboratory faults, during which the faults were subjected to controlled energy-flux, and responded spontaneously to it. The monitored evolution of slip-velocity, shear stress, and slip-distance revealed three slip styles that depend on the applied energy-flux: (1) steady-state slip; (2) spontaneous creep events of small displacement with negligible weakening; and (3) spontaneous, unstable events with slip-velocities up to 0.8 m/s, slip-distances up to 0.5 m, and stress-drops up to 1 MPa, which are comparable to observed values of moderate earthquakes. These slip styles are similar in character to those observed along natural faults. We further propose that the rate of energy flow from crustal blocks can control the slip velocity during earthquakes.