The structure, petrology and mechanics of a plate-boundary-scale serpentinite shear zone: The Livingstone fault, New Zealand

Abstract

Serpentinite-bearing shear zones are important in a range of tectonic settings, including the slab-mantle interface, oceanic detachment faults, and large-displacement transform faults in the oceanic and continental lithosphere. Despite abundant information regarding the mechanical, mineralogical and geochemical characteristics of serpentinites, important questions remain regarding the physical and chemical processes that operate in large serpentinite shear zones, and how these processes can influence rheology and slip behaviour during the seismic cycle. This thesis presents a multidisciplinary study of the internal structure and composition of a plate boundary-scale serpentinite shear zone in the South Island of New Zealand. The Livingstone Fault is a >1000 km-long terrane boundary in the North and South Islands of New Zealand. The fault juxtaposes ultramafic and mafic portions of the Dun Mountain Ophiolite Belt against mainly quartzofeldspathic schists of the Caples or Aspiring Terranes. In South Westland, the fault is well-exposed where it outcrops on high alpine passes, presenting a unique opportunity to examine the internal structure of large serpentinite shear zones over sub-micron to kilometre scales. Field observations show that the Livingstone Fault is characterised by a steeply-dipping, serpentinite-dominated shear zone up to 420 metres wide. The shear zone has a pervasive scaly fabric dominated by fibrous chrysotile and lizardite, which wraps around pods of massive serpentinite, rodingite and quartzofeldspathic schist. Kinematic indicators indicate a steep east-side-up shear sense. Dissolution seams enriched in magnetite, combined with growth of fibrous chrysotile, suggest that bulk deformation occurred mainly by pressure-solution-mediated creep. In places, the scaly fabric is cut by polished fault surfaces coated by a thin layer (<300 um) of magnetite. Transmission Electron Microscope observations of serpentinite inclusions within a magnetite layer document the preservation of serpentine dehydration products (amorphous serpentine, talc, olivine, enstatite). Finite-element modelling suggests that the dehydration products could have formed by coseismic frictional heating during an earthquake of magnitude 2.7 – 4. These observations represent the first record of a fossil earthquake in a natural serpentinite shear zone. Wall rocks juxtaposed by the Livingstone Fault have contrasting chemistries, which promoted metasomatic reactions along the shear zone boundaries and around the margins of pods. Scaly serpentinite was progressively converted in to nephritic tremolite + H2O by the addition of calcium and silica. The reactions produced networks of multi-generational tremolite veins, causing a transition from distributed ductile creep to localised brittle failure. These observations are used to argue that chemical reactions in serpentinite-bearing shear zones can generate fluid overpressures, triggering hydrofracturing and a transition to brittle deformation. This has important implications for the possible role of chemical reactions in the source region of deep episodic tremor and slip (ETS) along the slab-mantle interface in subduction zones. Finally, serpentinite-bearing fault rocks are notoriously difficult to study, because conventional analytical techniques (e.g. optical microscopy, most geochemical techniques, XRD) fail to unambiguously distinguish between the common serpentine varieties. To overcome this, novel methodologies of Raman spectroscopy mapping were developed that allow for in-situ identification and high-resolution (370 nm) mapping of texturally complex serpentinite fault rocks using standard polished thin sections.