Fluid flow in an active orogenic mountain belt; the Southern Alps, New Zealand

Abstract

Groundwater can influence processes in mountain belts and associated fault zones through heat and mass transfer and the influence of fluid pressure on deformation. Modelling at a range of scales, utilising new data from the DFDP-2B borehole, is employed to investigate groundwater and heat flow in the Southern Alps mountain belt, New Zealand, which experiences active, rapid uplift on the plate-bounding Alpine Fault. Models indicate bulk permeability of the brittle crust in the orogen in the range 10-16 m2 to 10-13 m2. Estimated permeability for basement rocks obtained using DFDP-2B data (10-16 m2) falls at the lower end of this range. Models of the region’s heavy rainfall, highly elevated water tables, and extensive seeps, indicate that >20 % of precipitation may infiltrate into the groundwater system. The majority of infiltrating water penetrates to <500 m below ground level. DFDP-2B temperature data indicates Darcy velocities of ~10-9 m·s-1. These are sufficiently high that groundwater transports significant amounts of heat, more than three times that transported by rock advection at DFDP-2B. High heat fluxes of ~720 mW·m-2 result from the advection of heat from ridges perpendicular to the Alpine Fault into the intervening valleys by groundwater flow. High bulk permeabilities and less significant permeability reductions with depth increase shallow temperatures beneath valleys relative to ridges and decrease temperatures at the base of the groundwater system in regional models. Groundwater convergence produces steep increases in hydraulic head with depth beneath valleys, with DFDP-2B data indicating >60 m head at 818 m depth. However, thermal decreases in water density restrict increases in head to the near-surface (~1 km below sea level). Thermal buoyancy provides the major driving contribution to vertical flow in parts of the groundwater system and thermally decreased water viscosity may facilitate penetration of greater fluxes of meteoric water to the brittle-ductile transition in the Southern Alps than in settings with lower geothermal gradients.

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ORCID for Damon Teagle: orcid.org/0000-0002-4416-8409

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