OBSCURATION AND OBLITERATION OF PSEUDOTACHYLYTE, ALPINE FAULT ZONE, NEW ZEALAND

Friction melts should be generated during seismic slip under relatively dry conditions throughout the continental crust, but it has been observed that pseudotachylytes (PST) are rare relative to typical levels of seismicity. This could be explained three ways: PSTs are rarely generated during coseismic faulting; they are obscured or obliterated after formation; or they are often not recognized even when present.

To investigate this question, we are examining cryptic veins within the hangingwall of the Alpine Fault Zone (AFZ) at the Gaunt Creek exposure. Here, the fault displays a thick damage zone with abundant retrograde assemblages, brittle fractures, cataclasites, veins, and products of metasomatism.

The veins studied are commonly one cm thick and extend a few dm to a few m sub-parallel to the dominant AFZ slip surface. They are highly fractured, discontinuous, and fragments occur as clasts within cataclasite and gouge zones. In detail, they are ultrafine-grained, and commonly thinly banded with narrow, sharply separated pale blue, gray, and green zones, and while vein-like, do not resemble typically black, flinty, aphanitic pseudotachylyte. At the petrographic and SEM scales, the veins consist principally of a mixture of ultracataclasite and PST. PST is diagnosed primarily by the presence of flow-banding, quartz amygdules, and micron-scale microlites in larger fragments. It is fragmented into and along contacts with cataclasite, internally microbrecciated, microfractured in multiple orientations and commonly altered along micofractures, and cut by carbonate-filled veins. Numerous low-temperature alteration minerals are present. Originally thinner PST veins would be unlikely to retain any diagnostic characteristics below TEM-scale.

We suggest coseismic frictional melting in similar settings is much more common than currently believed. Pseudotachylytes are most likely to be recognized where relatively thick veins produced by major earthquakes form in dispersed fault zones that are neither reactivated nor overprinted by thermal metamorphism or metasomatism. In major, long lived fault zones with extensive damage zones and accompanying metasomatism, pseudotachylytes will rarely be preserved in recognizable form in similar proportions to their original volumetric fraction of the fault rock.