Abstract

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 (Th) between 189 and 217 °C. The assemblage quartz + prehnite + epidote suggests trapping temperatures (Tt) of ∼280 °C, requiring the addition of an ∼70 °C correction to Th values. Late calcite contains inclusions with noticeably lower salinity (0.0–0.9 wt% NaCl) and Th values (129–175 °C). Studies on quartz + pumpellyite ± calcite veins from nearby Lake Aviemore show similar fluid-inclusion salinity and Th values.

Fluid-inclusion gas analyses show all the vein samples to be dominated by H2O (99.3–99.9 mol%) with few other gases apparent, including CH4 (≤0.5%), N2 (≤0.1%), CO2 (≤0.1%), and C2-C4 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 Tt values, indicate that the water responsible for the deposition of Benmore and Aviemore quartz had δ18O compositions of 9.4‰ and 4.8‰, respectively. Calcite δ13C values between−25.3‰ and−38.0‰ are indicative of oxidation of CH4 to CO2 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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