Geothermal systems related to rapid uplift on the Alpine Fault, New Zealand

Although the largest earthquakes (e.g., 2004 Sumatra) occur where tectonic plates collide, large earthquakes (Mag. 7-8+) also occur on strike-slip faults where plates are moving horizontally past each other. Strike-slip faults such as the San Andreas or the North Anatolian Fault (Turkey) occur in highly populated areas where earthquakes can have devastating human consequences. Although faults are seismic monitored, our knowledge of why earthquakes occur remains poor. This is because we have no samples of rocks that ruptured during a modern earthquake because failure typically occurs deep in the crust (>5-10 km). Nor do we have in situ measurements of the thermal and fluids conditions that determine how materials respond to the relative motion of the plates. Ancient fault rocks do occur but these rocks are commonly altered and have unknown tectonic context. The Alpine Fault is major strike-slip fault, that runs along the western range front of the Southern Alps, New Zealand. The fault is the boundary between the Australian and Pacific plates with the Australian crust moving to the northeast at ~27 mm/year. Because plate motions are not parallel to the Alpine Fault, collision is occurring at an oblique angle. This has resulted in the recent (~5 million years) rapid (>6-8 mm/yr) uplift of the Pacific plate over the Australian plate forming the >3000 m-high Southern Alps. Rocks, that until a few million years ago where more than 25 km deep in the crust, now crop out at the surface along the fault. Importantly, rocks that as recently as a few 10s of thousands of years ago, were fracturing and deforming within the Alpine Fault zone itself, now occur at the surface. This well known tectonic geometry and one-sided uplift along a major strike-slip fault is unique, and provides an excellent natural laboratory to understand earthquake processes. It is surprising that there have been no large earthquakes on the Alpine Fault in European times. However, paleo-seismic evidence indicates a major earthquake in ~1717, and that large earthquakes occurr every 200-400 years. These quakes were very large with up to 8 m horizontal movement in each event. The Alpine Fault is late in its seismic cycle and overdue for a large, devastating earthquake. This has lead an international group of scientists to propose drilling a series of shallow and deep (~4 km) bore holes into the Alpine Fault Zone to sample the fault rocks in situ, and to install instruments (seismicity, strain, temperature, fluid pressure) to monitor a major fault during the final build up to a large earthquake. Data from the Alpine fault can be used to understand other fault zones. Before we can decide where to drill a deep hole, we need to know how hot it is at the target depth. Our proposed work will make estimates of the temperature of rocks at depth by investigating geothermal warm springs (up to 60 deg C) that occur along the Alpine Fault. These warm springs occur because rapid uplift has brought deep hot rocks near to the surface. Geologists commonly use fluids from geysers or seafloor black-smoker vents, as windows into conditions deep within the crust. The chemistry of fluids and gases emitted can tell us where the fluids come from and how they have reacted. Unfortunately, there is very little known about the Alpine Fault geothermal systems because many of the springs are in very remote locations, and the scientists didn't have access to modern techniques. From investigating fluid-rock exchange in other hydrothermal environments, we have developed new methods to understand reactions between fluids and minerals. We will match warm spring fluids to minerals that formed within the Alpine Fault zone, during different stages of the uplift of these rocks to the surface. When matches can be made, we will be know that the reactions and conditions producing modern fluids must be occurring within the Alpine Fault today.