Alpine fault pseudotachylytes

Abstract

Pseudotachylytes produced during coseismic slip are ubiquitous within the rocks of the Alpine Fault zone, New Zealand. Pseudotachylytes from three principal host rocks are described and characterised from field and petrographic observations. The pseudotachylytes range from ~ 1 mm thick in Alpine Schist-derived mylonites to ~1 cm thick in Western Province-derived augen mylonites and cataclasites. Depth estimates based on field and petrographic relationships divide the differently hosted pseudotachylytes into two broad groups that formed during seismic rupture in the upper crust of the Alpine Fault zone: 1) relatively low volume pseudotachylytes forn1ed in Alpine Schist-derived mylonites at- 8 km depth, where shear stress was relatively low, during low magnitude events just below the brittle-ductile transition; and 2) voluminous pseudotachylytes formed at shallower levels of- 2-5 km, where shear stresses were high, in Western Province granitoid-derived augen mylonites and cataclasites during higher magnitude events.

Bulk-rock powder X-ray Fluorescence (XRF) of host rocks and Electron Microprobe Energy Dispersive Spectroscopy (EMP-EDS) spot analysis on pseudotachylyte matrices reveal that relative to their host rocks, the Alpine Fault pseudotachylytes are depleted in SiO₂ and enriched in Al₂O₃, alkalis and metallic oxides. This is interpreted to be the result of preferential inclusion of hydrous, ferromagnesian mineral phases and to a lesser extent feldspar into the melt, and the exclusion of quartz from the melt. The preferential selection relationship in part controls the voluminous nature of pseudotachylytes within retrogressed host rocks containing high proportions of hydrous, ferromagnesian minerals.

Size analysis of the clasts within the pseudotachylytes reveals power law size-frequency distributions that are modified in the finer fraction of the clasts, suggesting pervasion of friction melt depleted the fine fraction of precursor ultracataclasites by incorporation into the melt phase by uniform rim melting.