Structural characteristics of the Northburn Vein Swarm, Central Otago, New Zealand

Luketina, Genevieve May.

The Northbum Vein Swarm, Central Otago, New Zealand, formed in an extensional tectonic setting, during Cretaceous exhumation of the Otago Schist associated with New Zealand rifting from Gondwana. The Northbum Vein Swarm is hosted by greenschist facies, TZlV, Otago Schist; and consists of about 20 large quartz veins ranging from 10 to 120 cm in thickness, which are hosted by NW striking, moderately dipping (45°±10°SW) normal faults; and clusters of NW striking, near-vertical veins hosted by extension fractures. Fluid released during metamorphism of the schist was prevented from migrating vertically by the sub-horizontal schistosity. High fluid pressures led to brittle failure in the rock, resulting in faults and extension fractures through which the fluid could flow. Decompression of the fluid caused quartz to precipitate, sealing the fractures, and preserving veins. The veins formed by incremental opening of fractures and multiple episodes of slip along the faults. Vein geothermometry indicates that both extension veining and fault veining occurred at a temperature of -368°C and at -11-12 km depth, with extension vein fluid pressures -195 MPa, and fault vein fluid pressures -240 MPa. This is contrary to expectations, as fluid pressures involved in forming extension fractures should be higher than those in faults. Field relationships indicate that normal faulting post-dates extension veining. Analysis of the orientations of brittle structures indicates a near vertical cr1 and a near horizontal, NE trending cr3 at the time of formation. This NE-SW extension direction is consistent with nearby ductile extension, and regional brittle normal faulting. The fault veins dip at a lower angle than would be expected in a simple Andersonian ex tensional regime. Restoration of the schistosity to horizontal steepens the dip on the fault veins. The low angle of the fault veins may be due to a domino effect, where the faults initiated at more optimal orientations, and rotated towards frictional lock-up. Around 50,000 to 100,000 m3 of quartz is present in the mapped veins, which, when the temperature and pressure of vein formation are taken into account, corresponds to - 3-7x 108 m3 of water needed to precipitate the quartz. Conjugate sets of more optimally oriented NE and SW dipping barren normal faults post-date veined fractures and faults. The lack of vein infilling on these faults suggests less fluid discharge accompanying failure, as the crust tended towards an equilibrium state of hydrostatic fluid pressure, after releasing residual overpressure through repeated failure of faults and fractures.

vi, 72 leaves : ill. (some col.), maps ; 30 cm. + 1 map (59 x 87 cm. folds to 30 x 19 cm.)

2007Luketina

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