Slip dynamics and the effects of metasomatism on fault rheology in ultramafic rocks, the Livingstone Fault, New Zealand

The Livingstone Fault is a >1000 km long terrane boundary in the South and North Islands of New Zealand that mainly juxtaposes ultramafic rocks (primarily peridotite and serpentinite) of the Dun Mountain Ophiolite Belt against quartzofeldspathic lithologies (schist, volcanics, sandstone and siltstone) of the continental Caples Terrane. The fault is characterized by a zone of sheared serpentinite mélange tens to several hundreds of meters wide containing a well-defined scaly fabric entraining pods of massive serpentinite and quartzofeldspathic schist. Talc- and tremolite-forming metasomatic reactions occurred along the margins of the mélange and at the edges of pods as the result of interaction between serpentinite and fluids derived from the siliceous quartzofeldspathic schist. Layers of tremolite up to 1 m thick are characterized by a highly indurated microstructure consisting of networks of tightly interwoven, acicular crystals. In these regions, discrete cataclastic slip zones are associated with well-polished slickenlined surfaces at the interfaces of the serpentinite and Caples Terrane schist. Additionally, `crackle-breccias' with veined stockworks of tremolite provide evidence for the local attainment of fluid overpressure during metasomatism. Tremolite-forming metasomatic reactions, combined with episodic fluid-driven brecciation events, promoted localized and unstable fault slip within a mélange shear zone that was otherwise deforming by creep. In support of this, SEM-EDS and sub-micron Raman mapping of discrete magnetite-bearing slip surfaces indicates the presence of small (10-50 μm) patches of nano-crystalline olivine and enstatite, which may be the result of localized serpentinite dehydration reactions during episodes of slip.