PETROLOGY OF PSEUDOTACHYLYTES FROM THE ALPINE FAULT OF NEW-ZEALAND

The petrology of pseudotachylytes derived from contrasting host rocks within the Alpine Fault of New Zealand, a well known dextral shear zone, is presented. The structural observations indicate that parallel synthetic shears formed first in a non-dilational system, at low angles to the Alpine Fault. Then, between these shears, fracturing and frictional heating occured according to a new, dilational, Riedel system, accompanied by listric faults. The observations suggest that pseudotachylyte melt is mainly formed in the listric faults and channelled into neighbouring areas of extension. Pseudotachylyte results from in situ melting and its nature depend strictly on the parent rock mineralogy. The microlitic nature of the pseudotachylites is confirmed, and the variability of the textures explained in terms of degrees of undercooling. The morphology and composition of the minerals in the pseudotachylyte (plagioclase, biotite and amphibole) are documented. They crystallized at a higher temperature and pressure than their parent rock minerals. The melting process is believed to be governed by the mechanical properties of mafic minerals, in a fluid-free environment at the beginning of the melting. Mafic minerals provide water which allows the development of the pseudotachylyte.