Report | A scientific drilling project at the Alpine Fault – The discovery of an extreme geothermal activity in the New Zealand underground

A scientific drilling project at the Alpine Fault in New Zealand (photo: T. Wiersberg, GFZ).

The Alpine Fault – a transform fault in the western South Island of New Zealand – is highly interesting to geoscientists. Here, alongside the fault between the Australian and the Pacific plate, plates move with an average speed of about three centimetres per year. In 1717 the fault ruptured last. Since it is presumed that major earthquakes occur every 300 to 400 years at this fault the next major quake is thought to happen any time soon.

The Alpine Fault is unique in the fact that rapid uplift and mountain building has exhumed fault rocks from seismically active depths”, says Thomas Wiersberg from the GFZ Centre for Scientific Drilling. “Compared to other faults such as the San Andreas Fault in California or the North Anatolian Fault in Turkey, the Alpine Fault provides the unique opportunity to study earthquakes just before they happen in a comparable shallow depth.”

Hence, the International Continental Drilling Program ICDP funded the Deep Fault Drilling Project. It comprises a total of three boreholes. Two shallow pilot holes that reach down to depths of 100 and 152 metres, respectively, were completed in February 2011, intersecting the Alpine Fault close to Gaunt Creek on the western coast of New Zealand. The main hole aimed at intersecting the fault at a target depth of 1,500 metres but after technical challenges during the drilling the premature end was decided in December 2014 at a borehole depth of 892 metres without reaching down to the fault. Nevertheless, important and unexpected insights were gained.

A recent publication in Nature reports on selected data from the Deep Fault Drilling Project in a multi-author study led by Rupert Sutherland from the Victoria University of Wellington, New Zealand. Accordingly, surprisingly high temperatures are detected in the main hole. The geothermal gradient there is up to 125 degrees Celsius per kilometre depth. In general, the geothermal gradient in the subsurface is about 35 degrees per kilometre depth. In volcanic areas it may be a bit higher, though. Apparently, slippage during earthquakes has uplifted hot rocks from great depth, and the rocks are coming up so fast that they only cool down comparably slowly. Circulating fluids are thereby transporting heat up to shallow depths.

Rocks from the underground help to study quakes

The GFZ scientists Anja Schleicher, Martin Zimmer, Thomas Wiersberg, Samuel Niedermann, and Ronald Conze from the GFZ section Inorganic and Isotope Geochemistry and the Centre for Scientific Drilling supported the drill project on-site. They for example took rock samples and did gas measurements to gain insights in the chemical and mineralogical composition of the subsurface. Moreover, an ICDP Training Course took place in October 2014, organized by the GFZ, where 30 young scientists from 13 countries were able to gain practical experiences in the cause of the Fault Zone Drilling Program. Anja Schleicher, Thomas Wiersberg, and Ronald Conze participated as lecturers.

Within their research the GFZ scientists address questions such as how minerals from the depth add to a weakening of the fault. A transformation of the minerals within the fault may have a large impact on the tension within the rocks which then would react with bending and thereby increase the probability of the occurrence of an earthquake.

The scientists are furthermore interested in the role of fluids and gas. In preparation of the scientific drilling into the Alpine Fault a regional investigation of the gas composition in hot spring waters on both sides of the fault, the Pacific and the Australian side, was performed by Martin Zimmer and Samuel Niedermann. They found that the gas composition in the northwest of the fault, on the Australian plate, was different from that on the pacific plate. Martin Zimmer “Obviously, mantle fluids can penetrate the crust directly at the fault zone and may be diverted to different degrees to both plate sides”. The scientists would like to understand how fluids of different origin are circulating within the fault zone. Of major interest are fluids coming from great depths. These fluids have a slip effect on the rocks which increases the probability of an earthquake. Anja Schleicher: “A future drilling that completely crosses the fault would greatly add to our understanding of the impact of minerals and fluids on the formation of earthquakes.”

31.05.2017, Ariane Kujau

Original study: Sutherland, R., Towned, J., Toy, V. et. al., 2017. Extreme hydrothermal conditions at an active plate-bounding fault. Nature. doi:10.1038/nature22355