Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2,3. 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 decades4,5. 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.
We thank the Friend family for land access and the Westland community for support; Schlumberger for assistance with optical fibre technology; A. Benson, R. Conze, R. Marx, B. Pooley, A. Pyne and S. Yeo for engineering and site support; the CNRS University of Montpellier wireline logging group of P. Pezard, G. Henry, O. Nitsch and J. Paris; Arnold Contracting; Eco Drilling; and Webster Drilling. Funding was provided by the International Continental Scientific Drilling Program (ICDP), the NZ Marsden Fund, GNS Science, Victoria University of Wellington, University of Otago, the NZ Ministry for Business Innovation and Employment, NERC grants NE/J022128/1 and NE/J024449/1, the Netherlands Organization for Scientific Research VIDI grant 854.12.011 and the ERC starting grant SEISMIC 335915. ICDP provided expert review, staff training and technical guidance.