Stable isotopic evidence for mixing between metamorphic fluids and surface-derived waters during recent uplift of the Southern Alps, New Zealand

Recent studies have shown that the Southern Alps of New Zealand have active hydrothermal systems driven by tectonic uplift. These studies have concentrated on the rapidly exhumed rocks immediately adjacent to the Alpine Fault. We present new stable isotopic evidence that shows that fluid flow and fluid mixing processes thought to be restricted to rocks near the Alpine Fault also occurred in the low-uplift rate region of the Southern Alps orogen during the Kaikouran orogeny (10 Ma to present). Low δ18Ocalcite values of post-metamorphic veins in the eastern Southern Alps indicate that meteoric waters have penetrated to hot, midcrustal levels (350–300°C, >5 km depth) and mixed with metamorphic fluids in areas far removed from the Alpine Fault. In addition, the isotopic values of calcites precipitated in active faults in the MacKenzie basin define a trend of increasing δ18O, decreasing δ13C and decreasing temperature and form an isotopic mixing line between fault and vein calcites crosscutting metamorphic rocks and authigenic calcites precipitated in MacKenzie basin sediments. The isotope data indicate a second phase of mixing between a modified metamorphic fluid and a surficial diagenetic fluid at shallow depths. We suggest that mixed metamorphic–meteoric hydrothermal systems have developed in the east side of the Southern Alps during uplift and that active faults have played a critical role in transporting metamorphic fluids outward from the mountain front and into the adjacent intermontane basins.