Regional development of an extensional fault-fracture network in the mid-crust : the Glenorchy vein swarm, NW Otago, New Zealand

Begbie, Michael J. (Michael James)

The Otago Schist belt, derived from the amalgamation of at least two terranes accreted at a Triassic convergent continental margin, experienced a major episode of hydrofracturing and fluid redistribution during early Cretaceous exhumation. As a result, presently exposed pumpellyite-actinolite to lower greenschist quartzofeldspathic schists in NW Otago are per~vaded by abundant quartz-filled hydraulic extension and extensional-shear fractures with ass6ciated normal faults hosting quartz ± scheelite ± gold mineralisation. This fault-fracture network is named the Glenorchy Vein Swarm (GVS) and provides evidence of past hydrothermal flow systems leading to fault initiation and reactivation in the mid-crust. Normal faults and extensional hydrofractures form, respectively at angles between 40-60° and subperpendicular to the schist foliation. Once the effects of younger Cenozoic folding have been removed, schist with subhorizontal foliation on a regional scale contains an aligned array of subvertical hydrofractures striking NNW parallel to a set of low-slip normal faults. The array extends over an area > 1500 km2 and over a depth interval >5 km, representing stresscontrolled structural permeability developed within the low permeability schist assemblage, where flat-lying foliation impeded vertical migration of fluid. The fault-fracture system is inferred to have developed in an extensional regime with o 1 subvertical and o3 oriented WSW-ENE (present coordinates). This depth section with its contained structures likely records the progressive passage of the schist assemblage through the base of the seismogenic zone during extensional exhumation. The hydrofracture array comprises a network of interlinked extensional-shear and pure extension veins leading to bulk extensional strains up to 5%. Vein frequency across strike is typically~ 10 m-1 , with most veins 5- 10 mm thick. Veins hosted by extensional-shear and pure extension fractures show mutually cross-cutting relations and incremental growth textures. Extensional hydrofracturing apparently preceded the development of major throughgoing normal faults. Low-displacement normal faults are infilled with laminated quartz(± scheelite ±gold) veins(< 2 m thick), which typically formed at local dilational sites around fault irregularities. Total displacement across individual faults is on the order of a few tens of metres, with vein textures developed incrementally. The inference is that the faultfracture network developed through distributed seismic activity in the mid-crust, towards the base of the seismogenic zone. Quartz 0180 values from normal faults range from 12.8 to 17.4 per mil and between 13.0 and 14.7 per mil for the fracture network. Oxygen isotopic equilibrium conditions were approached between quartz in the schist (13.0 to 14.8 per mil), fracture network and some of the normal faults. Gradients in 0180 values preserved in the normal faults record changes in the time-integrated fluid flux and reflect the length scale of flow paths. Rapid passage of down-temperature fluid flow from the source to the site of mineral deposition via a hydraulically linked network of faults and fractures resulted in isotopic disequilibrium and quartz veins enriched in 0180 relative to quartz in the host rock. Rare earth element analyses of hydrothermal scheelite precipitated in fault-veins indicates that not all the scheelite was derived from the same source, possibly reflecting local host rock variation. The fault-fracture network defines a regionally distributed paleoflow system that developed toward the bottom half of a brittle c.arapace capping a prograding metamorphic belt. Early development of the GVS was characterised by incremental formation of a hydrofracture array throughout a substantial volume of the schist (> 5000 km3 ). Initiation and reactivation of these structures required a low differential stress and suprahydrostatic fluid pressure with Pr > o 3 • V :I This stage of deformation is inferred to have been the precursor to the development of throughgoing normal faults. Normal faults initiated as optimally oriented structures and remained favourably oriented for continued frictional reactivation during deformation inhibiting other modes of brittle failure. As the faults developed, the distributed hydrofracture array became inactive because the Pr > o3 condition could no longer be attained. Focused flow developed when active components of the hydrofracture array coalesced to form throughgoing normal faults and backbone percolation networks ( ~0.1 - 1 km length scale). Rate of network growth and connectivity must have continually changed as the locus and intensity of deformation changed. Repeated pulses of fluid flow only occurred in active structures where permeability was repeatedly renewed. Incremental vein textures are controlled by repeated fluctuations in either fluid pressure and/or shear stress associated with valving action on hydrofractures and faults. Comparison of fault-vein characteristics with "standard:' earthquake rupture parameters suggests that each vein lamination may represent an episode of fluid discharge and hydrothermal sealing following rupture involving a slip increment of up to a few centimetres (i.e. M < 4). Analogies may be drawn between the formation and subsequent exhumation of the Otago Schist belt, and present-day extensional exhumation and earthquake swarm activity in the Aegean and Tyrrhenian back-arc regions of the Mediterranean. The regional vein swarm thus represents a zone of distributed brittle failure induced by migration of overpressured hydrothermal fluids generated by metamorphism and devolatisation at deeper crusta! levels. The fault-fracture network is interpreted as representing a succession of fluid-driven paleoearthquake swarms.

xvi, 207 p. : ill. (some col.), maps ; 30 cm.

2003Begbie

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