Upper crustal fluid flow in the outboard region of the Southern Alps, New Zealand

The currently active fluid regime within the outboard region of the Southern Alps, New Zealand was investigated using a combination of field observations, carbon- and oxygen-stable isotopes from fault-hosted calcites and interpretation of magnetotelluric (MT) data. Active faulting in the region is dominated by NE striking and N striking, oppositely dipping thrust fault pairs. Stable isotopic analyses of calcites hosted within these fault zones range from 10 to 25‰δ18O and from −33 to 0‰δ13C. These values reflect mixing of three parent fluids: meteoric water, carbon-exchanged groundwater and minor deeper rock-exchanged fluids, at temperatures of 10–90°C in the upper 3–4 km of the crust. A broad, ‘U-shaped’ zone of high electrical conductivity (maximum depth c. 28 km) underlies the central Southern Alps. In the ductile region of the crust, the high-conductivity zone is subhorizontal. Near-vertical zones of high conductivity extend upward to the surface on both sides of the conductive zone. On the outboard side of the orogen, the conductive zone reaches the surface coincident with the trace of the active Forest Creek Faults. Near-surface flow is shown to dominate the outboard region. Topographically driven meteoric water interacts, on a kilometre scale, with either carbon-exchanged groundwater or directly with organic material within Pliocene gravels, resulting in a distinctive low 13C signal within fault-hosted calcites of the outboard region. The high-strain zone in the lower crust focuses the migration of deeply sourced fluids upward to the base of the brittle–ductile transition. Interconnected fluid is imaged as a narrow vertical zone of high conductivity in the upper crust, implying continuous permeability and possibly buoyancy-driven flow of deeply sourced fluids to higher levels of the crust where they are detected by the isotopic analysis of the fault-hosted calcites.

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