Earthquake generated pseudotachylyte in Central Otago, New Zealand

Barker, Shaun L. L. (Shaun Laurence Leslie)

Fault hosted pseudotachylytes have been attributed to friction melting during seismic slip episodes. Near Alexandra, Central Otago, New Zealand, a series of subparallel, NNE-SSW trending, shallow dipping (10-30° E) brittle faults are hosted in TZ Ill quartzofeldspathic schist. These faults contain numerous (n > 100) black, aphanitic pseudotachylyte veins, which are generally 1-2 cm thick and lying subparallel to foliation. Chill margins, crystallites, embayed lithic clasts, flow banding, and the creation of mineral phases that are absent in the host rock provide unequivocal evidence for melting during formation of the pseudotachylyte. Quartz and lesser plagioclase are present in pseudotachylyte veins as lithic clasts, while potassium feldspar microlites, ultra-fine grained chlorite (probable devitrified glass), and amygdules containing a high-temperature mineral assemblage (quartz, titanite, potassium feldspar, graphite, chlorite) form the bulk of newly crystallised material. Geochemical studies suggest that muscovite was preferentially incorporated in pseudotachylyte, while plagioclase was excluded. Melt temperatures of 900- 1100 oc are inferred. The absence of primary hydrothermal phases in pseudotachylyte veins suggests that pseudotachylyte formation occurred in a 'dry' environment, with no pore fluid present. Dilational jogs in fault veins and injection veins, and fault drag indicate a general topto- the-north sense of shear. Fault veins are traceable along strike for distances of up to 200 m. Fault separations inferred from offset pseudotachylyte veins, dilational jogs and a piercing point analysis suggest the pseudotachylyte was generated by slip increments of< 1 cm to 20 cm, comparable to those believed to occur during small to moderate earthquakes. Heat and work calculations suggest that the faulting occurred in 'strong', 'dry' crust, at depths of 6-12 km, with shear stress at slip initiation exceeding 200 MPa on pseudotachylyte generating faults. Furthermore, displacement/rupture length ratios on faults suggest that relatively high shear stress drops (- 30 MPa) accompanied pseudotachylyte formation. A preliminary U-Pb titanite date yields an age of 99 ± 23 Ma for pseudotachylyte formation. It is suggested that the pseudotachylyte-bearing faults were subhorizontal faults, which formed in association with regional extension during the mid-late Cretaceous, and are possibly related to the exhumation and uplift of the Otago Schist Belt.

ix, 190 leaves : ill. (some col.), maps (1 folded) ; 30 cm.

2003Barker