Shallow fluid movement in the hanging wall of the Alpine Fault

Abstract

New Zealand’s best example of large-scale fault processes is the Alpine Fault, which is currently the focus of national and international investigation. Numerical models show that thermal, stress, and hydraulic anomalies associated with the topology of the Southern Alps are likely to be significant at depths of 1000–1500 m near the Alpine Fault (Allis & Shi, 1995), conditions that influence how the fault evolves and produces earthquakes (McCaig, 1988). However, there is little empirical data on the hydrological conditions and fluid-flow within the rock mass adjacent to the Alpine Fault. The objective of this research was to enhance understanding of how meteoric fluids permeate the fault zone, and the over-arching aim was to quantify rates and patterns in the infiltration of meteoric water into uplifted schist bedrock adjacent to the Alpine Fault. Measurements were made in the Tartare Tunnel, located ~2km from the Franz Josef Township and adjacent to the Alpine Fault. Tunnel Discharge and water temperature were measured between April 2012 and January 2013, while water samples were taken at various time throughout 2012 and analysed for major ions and two stable isotopes: 18O and D. Differences in the total volume of water infiltrated, transmission time of water from the surface to the tunnel, and peak tunnel discharge were observed between the April to August and August to January periods. Snow on the surface was thought to be an important control on infiltration; reducing infiltration in the winter months and increasing it during the spring melt period. A conceptual model involved a log-normal distribution of fractures to explain patterns of groundwater movement in the rock mass surrounding the tunnel. Groundwater flow was predominantly percolation leading to movement of fluid into storage between April and August. Between September and January, however, elevation of the water table was variable, and at times intersected the tunnel, inducing lateral groundwater movement in the hillside. Temperature analysis indicated that the rock surrounding the tunnel was saturated between September and January and that the abundance of fluid lowered rock temperature. Chemical analyses suggested that the eastern portion of the rock mass above the tunnel was more permeable than the western part. Stable isotopes indicated that the recharge signal peaks in summer— a pattern attributed to the role of snowmelt— and that infiltrating water did not isotopically exchange with the host rock.

The results of this research show the role of snow in controlling rates and patterns of infiltration toward the Alpine Fault. Further research into the movement of water from the surface towards deeper parts of the Alpine fault is required, and will provide insight into the triggering of landslides, earthquakes and the behaviour of the rock mass during seismic events.