By Simon Cox
During the spring months of 2014, international attention will be drawn to the Alpine Fault along the western side of the Southern Alps, as the Deep Fault Drilling Project (DFDP) enters its second phase. Scientists from around the world aim to complete a 1.5 km drill hole near Whataroa, recover fault rocks for testing, and install a downhole laboratory that can measure fluid temperature and pressure, stress and listen for tiny earthquakes within the fault at depth. The first phase (DFDP-1) was completed in February 2011 with the successful construction of two shallow boreholes intersecting the fault at Waitangitaona River (Gaunt Ck). It was found then that the Alpine Fault acts as a low-permeability barrier to fluid flow, has a 0.53 MPa fluid-pressure difference across it, and the local geothermal gradient reaches 62.6°C/km downwards into the Southern Alps (Sutherland et al., 2012). Drilling for the second phase, based at Whataroa River, has now begun (Figure 1).The amount of precipitation that infiltrates mountain bedrock is poorly quantified, yet known to strongly influence alpine stream baseflow, landslide initiation, lowland aquifer recharge and potentially fault-strength. Near the township of Franz Josef, University of Otago MSc student Alex Sims monitored groundwater that flows into and out of a 355m tunnel through schist bedrock (Figure 2) during 2012-2013. By comparing tunnel flow to receipts at a nearby rain gauge, Sims modelled expected infiltration rates to compare with those observed. In a location where annual rainfall reaches peak New Zealand rates, infiltration rates were found to vary considerably during a year that included 21 storms, flooding, drought and snow to low levels. Snowmelt during June/July 2012 produced an anomalously high infiltration/tunnel discharge. The work suggests somewhere between 0.002 and 0.2 % of rainfall manages to infiltrate the bedrock. In a paper just published in Earth and Planetary Science Letters (Menzies et al., 2014), Catriona Menzies and others demonstrated that rainwater that does infiltrate bedrock (Figure 3) can penetrate to great depths – well below the Earth’s fractured upper crust. It had been previously thought that surface water could not penetrate the ductile crust – where temperatures of more than 300°C and high pressures cause rocks to flex and flow rather than fracture. However the research found that fluids that infiltrated fractures to form quartz veins, now uplifted in schist rocks, were derived from rainwater. With the infiltration of rainwater being far deeper than most geologists thought, the article is being now widely reported – even in the Financial Times Magazine!
Thermal springs along the Southern Alps, popular for bathing by trampers, have formed as a result of this penetration of meteoric fluids into warm rocks of the Southern Alps. The rainwater becomes heated by a thermal anomaly caused by rapid uplift and exhumation of rocks out of the earth’s warm interior by the Alpine Fault (Figure 4). The hot spring in Copland Valley is one of the most vigorously flowing, hottest of a these thermal springs, discharging strongly effervescent CO2-rich water at 56-58 °C and 6 ± 1 L/s. I have been monitoring the Copland spring for a number of years, and found to my great surprise that it cooled systematically ~1 °C and changed fluid chemistry in response to large distant earthquakes. Although shaking from the Mw 7.8 Dusky Sound (Fiordland) 2009 and Mw 7.1 Darfield (Canterbury) 2010 earthquakes was of low intensity in Copland valley, 350 and 180 km from the earthquake epicentres respectively, it still affected the spring. In a paper about to be published in the Geofluids journal, we show that the relatively low intensity shaking had induced small permanent strains across the alps – opening fractures which then enhanced mixing of relatively cool near-surface groundwater with upwelling hot water. Active deformation, tectonic and topographic stress in the Alpine Fault hanging wall appears to make the groundwater and fluid circulation in the Southern Alps particularly susceptible to earthquake-induced transience.
Dr Simon Cox is a structural geologist at GNS Science.