Sampling of the active alpine fault in New Zealand reveals extreme hydrothermal conditions
- Date:
- August 30, 2017
- Source:
- Osaka University
- Summary:
- An international research team has identified significant pore fluid pressures and temperatures at relatively shallow depths in the active alpine fault of New Zealand, which play an important role in seismic stability. This is because frictional and mechanical properties are primarily controlled by temperature and fluids in fault zones, and thus ultimately influence earthquake behavior.
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A recent study published in Nature has demonstrated unusual heat generation and fluid movement in the Alpine Fault of New Zealand that has implications for understanding earthquakes in the region. Large plate-boundary faults, such as the Alpine Fault, are important areas of stress build-up and release, which can lead to earthquakes. There is increasing evidence that faults in such regions have lower than predicted frictional shear strength, and are subject to very limited heat generation during fault slippage.
To investigate fault strength and heat generation in the Alpine Fault, the Deep Fault Drilling Project (DFDP) was commissioned. "The primary motivation of the DFDP was to provide an understanding of the ambient conditions, rock properties and geophysical phenomena that occur immediately before a large earthquake, because we just don't know enough about active faults before they rupture" says Naoki Kato of the Department of Earth and Space Science, Osaka University who co-authored the work.
The research team drilled to a depth of 893 m directly into the active Alpine Fault in Whataroa, New Zealand. The fault moves at about 26 mm per year and has, over time, brought rocks to the near-surface from depths of 30 km. Various geophysical techniques, including fiber optics, were used to obtain very precise temperature measurements, and revealed a pressure gradient almost 10% greater than expected, and temperature gradients (>80 °C.km-1) more typical of active volcanic regions.
The temperature structure of the fault was modeled in terms of heat conduction, rock advection and fluid advection related to topography. "Our models show that rock advection and thermal diffusion are the primary heat transport mechanisms in the principal slip zone, and it is fault slip itself that brings both rock and heat up from depth" Naoki Kato says. The models and drilling data both show that lateral fluid movement transports significant amounts of heat and fluids from depth, both of which concentrate into valleys.
Heat generation and fluid migration is important in active faults because both directly affect the stability of phyllosilicate minerals (clays), thermal rock expansion and the formation of physical and chemical reaction products in the fault slip zone. These in turn control the frictional and mechanical behavior of faults, and therefore the behavior of earthquakes that may occur during slippage. This study sheds new light on earthquake development in active fault regions because shallow temperature and hydrothermal anomalies, and their lateral variation, affect dynamic strength along the length of the fault.
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Journal Reference:
- Rupert Sutherland, John Townend, Virginia Toy, Phaedra Upton, Jamie Coussens, Michael Allen, Laura-May Baratin, Nicolas Barth, Leeza Becroft, Carolin Boese, Austin Boles, Carolyn Boulton, Neil G. R. Broderick, Lucie Janku-Capova, Brett M. Carpenter, Bernard Célérier, Calum Chamberlain, Alan Cooper, Ashley Coutts, Simon Cox, Lisa Craw, Mai-Linh Doan, Jennifer Eccles, Dan Faulkner, Jason Grieve, Julia Grochowski, Anton Gulley, Arthur Hartog, Jamie Howarth, Katrina Jacobs, Tamara Jeppson, Naoki Kato, Steven Keys, Martina Kirilova, Yusuke Kometani, Rob Langridge, Weiren Lin, Timothy Little, Adrienn Lukacs, Deirdre Mallyon, Elisabetta Mariani, Cécile Massiot, Loren Mathewson, Ben Melosh, Catriona Menzies, Jo Moore, Luiz Morales, Chance Morgan, Hiroshi Mori, Andre Niemeijer, Osamu Nishikawa, David Prior, Katrina Sauer, Martha Savage, Anja Schleicher, Douglas R. Schmitt, Norio Shigematsu, Sam Taylor-Offord, Damon Teagle, Harold Tobin, Robert Valdez, Konrad Weaver, Thomas Wiersberg, Jack Williams, Nick Woodman, Martin Zimmer. Extreme hydrothermal conditions at an active plate-bounding fault. Nature, 2017; 546 (7656): 137 DOI: 10.1038/nature22355
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