Steeply dipping strata in the vicinity of Benmore Dam, Otago, New Zealand, are complexly deformed metasedimentary rocks of the Torlesse Supergroup (Middle Triassic age). Over an exposed area ∼100 m wide × 25 m high, these strata are disrupted by a fault-fracture mesh comprising conjugate Coulomb shears interlinked by extensional and extensional-shear fractures, all formed in a common stress field and hosting quartz + prehnite ± epidote ± calcite veining. The combined effect of these structures is shortening perpendicular to beddings and subvertical extension so that in their present attitude, they correspond to a set of conjugate thrust faults with associated extension fractures. On the evidence of incremental vein textures, the development of this distributed fault-fracture mesh is interpreted as resulting from a fluid-driven microearthquake swarm, which postdated regional low-grade metamorphism. Mechanical considerations suggest that the migrating hydrothermal fluids were significantly overpressured, possibly to approximately lithostatic values, if the mesh structure developed in its present attitude. Fluid-inclusion microthermometric studies show that Benmore vein quartz contains two-phase aqueous inclusions with salinities between 1.4 and 2.9 wt% NaCl equivalent and homogenization temperatures ( T h ) between 189 and 217 °C. The assemblage quartz + prehnite + epidote suggests trapping temperatures ( T t ) of ∼280 °C, requiring the addition of an ∼70 °C correction to T h values. Late calcite contains inclusions with noticeably lower salinity (0.0–0.9 wt% NaCl) and T h values (129–175 °C). Studies on quartz + pumpellyite ± calcite veins from nearby Lake Aviemore show similar fluid-inclusion salinity and T h values. Fluid-inclusion gas analyses show all the vein samples to be dominated by H 2 O (99.3–99.9 mol%) with few other gases apparent, including CH 4 (≤0.5%), N 2 (≤0.1%), CO 2 (≤0.1%), and C 2 -C 4 hydrocarbons. Cation and anion analyses, when combined with the gas data, show that NaCl dominates the fluid-inclusion salinities. Oxygen isotope results, when combined with calculated T t values, indicate that the water responsible for the deposition of Benmore and Aviemore quartz had δ 18 O compositions of 9.4‰ and 4.8‰, respectively. Calcite δ 13 C values between−25.3‰ and−38.0‰ are indicative of oxidation of CH 4 to CO 2 as a result of hydrothermal fluids interacting with organic- rich sediments. Fluid-inclusion \({\delta}D_{H_{2}O}\) values for Benmore range between −73‰ and −89‰ compared to−109‰ for the one Aviemore sample. This research has demonstrated that (1) water of meteoric origin, probably from subantarctic latitudes, penetrated to ≥6 km depth and underwent an oxygen isotope shift before depositing the Benmore-Aviemore veins; (2) the migrating hydrothermal fluids were likely overpressured well above hydrostatic to near lithostatic values if the mesh structure was active in its present orientation; and, (3) fluid migration was coupled to distributed brittle failure in the prevailing stress field, “self-generating” a permeable fault-fracture mesh.
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