Brittle faulting and fluid flow in basement rocks of coastal Otago (South Island, New Zealand)

Abstract

Fault and fracture networks in foliated basement rocks control the strength, fluid flow properties and seismogenic behaviour of the crust in many areas worldwide. Understanding faulting patterns in foliated basement rocks is important because much seismicity (e.g. in the south island of New Zealand) occurs within basement rocks and basement faults are commonly linked to the formation of major ore bodies (e.g. in central Otago). Strongly foliated schists (and greywackes) are well exposed across extensive, clean outcrops along coastal sections in Otago, providing an important opportunity to study the nature of brittle deformation and faulting in the outboard region of the Otago reverse-fault province. Furthermore, it offers an opportunity to evaluate the extent to which pre-existing anisotropy in the basement schist (foliation and joints) has influenced the patterns of brittle deformation along the coastal platform.

Using high-resolution aerial photography, lineaments (n=6625) with lengths of ~3 m to c. 200 m were mapped along the 16.5 km-long coastal platform between Taieri Mouth and Chrystalls Beach, Otago. Significant patterns noted in the lineament data include strong preferred orientations trending 50-70° and 120-140°. Comparison to regional-scale faults in the Otago region (as recognised on GNS QMAP) shows a strong correlation between the coastal lineaments trending 120-140° and a set of NW-SE striking regional faults. However, many faults in the Otago region, including the nearby Akatore Fault, trend NE-SW (30-40 ± 10°), an orientation that is conspicuously absent in our coastal lineament analysis.

Detailed structural mapping has showed that SE-NW (120-140°) lineaments correspond to first-order faults (≤ 2 m wide) hosting breccias and small sinistral strike-slip faults that nucleated on continuous, planar, steeply-dipping joints. The latter are associated with paired quartz-calcite veins and small breccia pods developed in dilational jogs between adjacent joint tips. ENE-WSW (50-70°) lineaments correspond to a second (often dextral) strike-slip fault set hosting thin, continuous breccia layers formed within intact schist. Both fault sets host shallowly plunging lineations and form a conjugate set. Inversion of kinematic indicators, primarily from the conjugate fault set, indicates the paleostress field during faulting was similar to the modern-day stress field in Canterbury and Otago, characterized by subhorizontal σ1 trending c. 114° and subvertical σ2, i.e. a strike-slip stress regime. From this, we infer that the faults are post-Miocene (< 25 Ma) in age and formed in the modern-day stress regime.

From cross-cutting relationships in the field and thin section, relative chronology of formation can be inferred as follows; 1) establishment of a slaty cleavage during Jurassic metamorphism, 2) formation of a joint network during Late Cretaceous exhumation of the schist, 3) reactivation of exhumation joints in the past 25 Ma as conjugate SE-NW (sinistral) and E-W (dextral) striking faults hosting quartz and calcite veins and breccias, 4) opening of extensional fractures parallel to foliation and precipitation of fibrous calcite veins, and 5) deposition of botryoidal calcite in unconfined voids.

Estimated depth-temperature conditions of faulting during stages 3 - 5 above are ≤ 2 km and ≈ 50°C. Calcite veins and calcite-bearing breccia matrix formed during these stages show wide ranging δ18O and δ13C values that are paired with tightly clustered 87Sr/86Sr values of ≈ 0.7062. A decoupled fluid model is proposed where shallow fluids circulating down through the Cenozoic sedimentary sequence interacted primarily with limestone and a smaller volume of fluid interacted with coal to acquire a mixture of δ13C values with relatively heavy δ18O values. 87Sr/86Sr values may reflect the breakdown of Ca-rich (low 87Rb) minerals in the schist.

The orientations and kinematics of the basement fault fabric documented in coastal Otago mimics aspects of the faulting pattern from the 2010-2011 Canterbury earthquake sequence documented from seismological data. In particular, dextral E-W striking strike-slip faults (e.g. the Greendale Fault) ruptured during the Canterbury earthquake sequence and SE-NW striking sinistral strike-slip faults are inferred from aftershock alignments. In addition the Darfield mainshock nucleated on a steeply dipping, NE-SW striking reverse fault similar to several regional Otago faults. Our observations therefore suggest that there may be a common basement fault fabric developed throughout Otago and Canterbury that plays an important role in controlling earthquake sequences developed within the contemporary stress field.