Deformation and chemical reactions in an ultramafic terrane boundary: The Livingstone Fault, New Zealand

Abstract

The Livingstone Fault is a >1000 km long terrane boundary that defines the eastern margin of the Dun Mountain Ophiolite Belt in New Zealand. The fault is spectacularly exposed where it juxtaposes ultramafic parts of the ophiolite belt (e.g. peridotite, serpentinite) against schist of the continental Caples Terrane. The Livingstone Fault Zone provides an example of deformation processes and chemical reactions in a system where peridotite, serpentinite and quartzofeldspathic rocks are juxtaposed, a common situation in many plate tectonic settings (e.g. portions of the San Andreas Fault, subduction zones, and central and southern sectors of the Alpine Fault). Results of fieldwork carried out in three localities on the southern portion of the Livingstone Fault (Serpentine Saddle, Fiery Col and Cosy Gully), together with detailed studied of the mineralogy and microstructures of samples, reveal features similar to experimental data and other natural fault zones of this type. In such areas, the fault consists of a 50-4 0 m-wide serpentinite shear zone entraining competent pods of massive serpentinite, Caples Terrane rocks and various volcanic rocks. This shear zone has a steeply dipping, north-south striking S-C fabric which bends into alignment (northeast-southwest) with the Alpine Fault at Cosy Gully. The S-C fabric and steeply plunging slickenlines indicate a Caples Terrane (east side) up movement as observed in the Fiery Col area and Cosy Gully. Study of the mineralogy and microstructures revealed features that suggest processes of serpentinisation and metamorphism peaking at c. 350°C based upon the presence of lizardite, antigorite and chrysotile; metasomatism forming talc and tremolite due to the interaction of fluids from the ultramafic serpentinite and more quartz rich Caples Schist; and changing deformation styles from dissolution-precipitation creep to brittle fracturing induced by change in temperature, water:rock ratio, fluid chemistry and/or slip velocity. Evidence for very low frequency earthquakes is also present in the form of crack-seal veins in dilational jogs that have characteristic opening increments of c. 40 μm. Past chemical and deformational processes that occurred in the LFZ could be analogous to current processes occurring on fault zones of similar rock types. Analysis of the Livingstone Fault may provide a better understanding of these processes. This may be achieved by further analysis of samples from the fault zone through methods such as Raman Spectroscopy and TEM analysis.