Deep Fault Drilling Project (DFDP), Alpine Fault boreholes DFDP-2A and DFDP-2B technical completion report

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Sutherland, R.; Townend, J.; Toy, V.; Allen, M.; Baratin, L.-M.; Barth, N.; Beacroft, L.; Benson, A.; Boese, C.; Boles, A.; Boulton, C.; Capova, L.; Carpenter, B.; Celerier, B.; Chamberlain, C.; Conze, R.; Cooper, A.; Coussens, J.; Coutts, A.; Cox, S.C.; Craw, L.; Doan, M.-L.; Eccles, J.; Faulkner, D.; Grieve, J.; Grochowski, J.; Gulley, A.; Henry, G.; Howarth, J.D.; Jacobs, K.; Jeppson, T.; Kato, N.; Keys, S.; Kirilova, M.; Kometani, Y.; Kovacs, A.; Langridge, R.M.; Lin, W.; Little, T.; Mallyon, D.; Mariani, B.; Marx, R.; Massiot, C.; Mathewson, L.; Melosh, B.; Menzies, C. (et al.) 2015 Deep Fault Drilling Project (DFDP), Alpine Fault boreholes DFDP-2A and DFDP-2B technical completion report. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/50. xiii, 269 p.; doi: 10.21420/G2N301

Abstract: New Zealand’s Alpine Fault is a site of global importance for the study of active geological faults. The Alpine Fault provides an opportunity to study a major fault late in the cycle of stress accumulation ahead of a future earthquake, to search for precursors to large earthquakes, and to understand the long-term evolution of faults and the formation of fault rocks. The fault’s self-exhuming geometry, relative geological simplicity, and rapid and precisely-known slip rates, mean its accessible surface exposures have already been the subject of >40 years of intensive research. The goal of the Alpine Fault, Deep Fault Drilling Project (DFDP) is to go beneath the surface, to sample and make observations of the fault rocks and physical conditions at a range of depths, and monitor these over coming decades. To date, no active fault drilling experiment has targeted the mid-crustal roots of a long-lived active fault, or addressed fault zone evolution throughout the seismogenic zone and towards the brittle-ductile transition. DFDP-2A drilling started in August 2014 and DFDP-2B was completed in January 2015, having reached a maximum depth of 893 m. Technical problems, most notably the loss (and recovery) of the bottom-hole assembly on two occasions, caused delays in the drilling schedule, and the failure of a casing string ultimately caused drilling to be terminated at a shallower depth than planned and before the fault had been intersected. A very extensive data set was collected including geological, geophysical, geochemical and hydraulic measurements extending the length of the DFDP-2B borehole. Samples of rocks and fluids were collected, and a long-term observatory installed. Of particular significance are the observations that the geothermal gradient is extremely high, exceeding 140°C/km in the upper part of the borehole, and that the hangingwall is overpressured, as predicted prior to drilling. This report documents the technical planning and execution of DFDP-2 drilling, summarizes the scientific methods and types of data collected, and describes the observatory constructed. (auth)