Diverse habitats of pseudotachylytes in the Alpine Fault Zone and relationships to current seismicity

Abstract

Abstract Pseudotachylytes are ubiquitous within New Zealand's Alpine Fault Zone, occurring as: (i) thin fault veins parallel to existing hanging wall mylonitic foliation; (ii) thicker fault and injection veins around and within metabasite lenses in hanging wall fault rocks and on the footwall–hanging wall boundary; (iii) chaotic injected masses within footwall-derived, granitoid mylonites; and (iv) chaotic injected masses into cataclasites within the fault core. Overall, pseudotachylytes are not volumetrically dominant enough to have formed during all increments of earthquake slip on the Alpine Fault. We propose they were mostly generated during regular moderate magnitude events or during foreshock and aftershock sequences to larger earthquakes. The largest volume pseudotachylytes occur in footwall-derived mylonites (type (iii)). This may indicate that high-stress, anhydrous seismic slip is most common in the footwall. Most types (i), (ii) and (iii) pseudotachylytes formed at or near the base of the seismogenic zone, at temperatures up to 350 °C and at depths of 7–10 km or more. Ductilely overprinted pseudotachylytes represent the down-dip termination of large fault ruptures in a zone that would usually fail by aseismic creep. Type (iv) pseudotachylytes were formed at shallower depths (4–7 km) in a damage zone around the fault principal slip surface. Rare amygdules indicate that the fault zone locally contained free fluids. Supplementary material: Table of host, rock and pseudotachylyte major and minor oxide proportions is available at http://www.geolsoc.org.uk/SUP18490 .