Pseudotachylyte-generating faults in Central Otago, New Zealand

Abstract

In Central Otago, New Zealand, a series of subparallel, NNE–SSW trending, gently dipping (10–30° E) brittle faults are hosted in quartzofeldspathic schist. These faults contain numerous ( n>100) pseudotachylyte veins. Pseudotachylyte fault veins are traceable along strike for distances of up to 200 m, and are between <1 mm and 5 cm thick. Fault separations inferred from offset quartz veins, dilational jogs and a piercing point analysis suggest that pseudotachylyte was generated by slip increments of <1 cm to 20 cm, comparable to those believed to occur during small to moderate-sized earthquakes [Sibson, R.H., 1989. Earthquake faulting as a structural process. Journal of Structural Geology, 11 (1–2) 1–14.]. Dilational jogs, fault drag and quartz vein separations imply a top-to-the-north (normal) sense of shear. Evidence for melting is provided by chilled margins, microlites and the formation of mineral phases not present in the host rock. Melting temperatures of 900–1100 °C are inferred from petrological observations. Heat-work calculations, utilising displacements on pseudotachylyte-bearing faults, and inferred melting temperatures suggest that the faulting occurred at depths of 6–12 km, with average frictional shear resistance exceeding 100 MPa on pseudotachylyte-generating faults. These inferred depths and relatively high levels of frictional shear resistance on pseudotachylyte-generating faults are similar to those inferred by some previous studies of pseudotachylyte (e.g. [Sibson, R.H., 1975. Generation of pseudotachylyte by ancient seismic faulting. The Geophysical Journal of the Royal Astronomical Society, 43 (3) 775–794; Killick, A.M. and Roering, C., 1998. An estimate of the physical conditions of pseudotachylite formation in the West Rand Goldfield, Witwatersrand Basin, South Africa. Tectonophysics, 284 (3–4) 247–259.]).