Abstract

The Otago Schist in the South Island of New Zealand represents an exhumed Mesozoic accretionary prism. Two coastal areas (Akatore Creek and Bruce Rocks) south of Dunedin preserve structural and geochemical evidence for the development of postmetamorphic hydrothermal systems that involved widespread fluid-rock reaction at shallow crustal depths. The Jurassic to Triassic pumpellyite-actinolite (Akatore Creek) to upper greenschist facies (Bruce Rocks) metamorphic fabrics were crosscut by sets of regionally extensive Cretaceous exhumation joints. Many of the joints were subsequently reactivated to form networks of small-displacement (<metres) strike-slip faults containing cemented fault breccias and veins composed of hydrothermal calcite, siderite, and ankerite. Paleostress analysis performed on infrequent fault slickenlines indicates an overall strike-slip paleostress regime and a paleo-σ1 orientation (azimuth 094°) similar to the contemporary σ1 orientation in Otago and Canterbury (azimuth c. 110°-120°). High δ18O values in vein calcite (δ18OVPDB=21 to 28‰), together with the predominance of Type I calcite twins, suggest that vein formation occurred at low temperatures (<200°C) in the shallow crust and was associated with strongly channelized fluid flow along the joint and fault networks. Mass-balance calculations performed on samples from carbonate alteration zones show that significant mobilisation of elements occurred during fluid flow and fluid-rock reaction. Whole-rock and in situ carbonate 87Sr/86Sr data indicate varying degrees of interaction between the hydrothermal fluids and the host rock schists. Fluids were likely derived from the breakdown of metamorphic Ca-rich mineral phases with low 87Rb in the host schists (e.g., epidote or calcite), as well as more radiogenic components such as mica. Overall, the field and geochemical data suggest that shallow fluid flow in the field areas was channelized along foliation surfaces, exhumation joints, and networks of brittle faults, and that these structures controlled the distribution of fluid-rock reactions and hydrothermal veins. The brittle fault networks and associated hydrothermal systems are interpreted to have formed after the onset of Early Miocene compression in the South Island and may represent the manifestation of fracturing and fluid flow associated with reverse reactivation of regional-scale faults such as the nearby Akatore Fault.

Highlights

  • Interactions between brittle faulting, fluid flow, alteration, and mineralization in the upper crust can strongly influence rock physical properties and strength [1,2,3], seismogenic potential and the distribution of earthquake sequences [4,5,6], and the evolution and geometry of mineralized zones

  • Akatore Creek and Bruce Rocks are located on the southern limb of a regional-scale antiform in the dominantly quartzofeldspathic Otago Schist (Figure 1; [28, 49])

  • We interpret the paleohydrothermal systems exposed at Akatore Creek and Bruce Rocks to have developed within the Otago Schist under low-temperature and lowpressure conditions (Figure 10)

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Summary

Introduction

Interactions between brittle faulting, fluid flow, alteration, and mineralization in the upper crust can strongly influence rock physical properties and strength [1,2,3], seismogenic potential and the distribution of earthquake sequences [4,5,6], and the evolution and geometry of mineralized zones. The Otago Schist (Figure 1) contains well-studied examples of paleohydrothermal systems that developed within midcrustal faults and shear zones at various stages of regional metamorphism and exhumation [8,9,10,11,12,13]. The purpose of this study is to determine the main structural and geochemical processes that influenced shallow fluid flow and mineralization within the Otago Schists. This will help to constrain the potential sources of fluids in shallow basement rocks, as well as the role of preexisting structures in controlling patterns of faulting and fluid flow. Whole-rock geochemistry, coupled with in situ mineral 87Sr/86Sr, δ13C, and δ18O analyses, enables the interpretation of fluid pathways and the main fluid-rock reactions that occurred during faulting

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