The internal structure and composition of a plate-boundary-scale serpentinite shear zone: the Livingstone Fault, New Zealand

Abstract

Deciphering the internal structure and composition of large serpentinite-dominated shear zones will lead to an improved understanding of the rheology of the lithosphere in a range of tectonic settings. The Livingstone Fault in New Zealand is a terrane-bounding structure that separates the basal portions (peridotite; serpentinised peridotite; metagabbros) of the Dun Mountain Ophiolite Belt from the quartzofeldspathic schists of the Caples and Aspiring Terrane. Field and microstructural observations from 11 localities along a strike length of ca. 140 km show that the Livingstone Fault is a steeply dipping, serpentinite-dominated shear zone tens of metres to several hundred metres wide. The bulk shear zone has a pervasive scaly fabric that wraps around fractured and faulted pods of massive serpentinite, rodingite and partially metasomatised quartzofeldspathic schist up to a few tens of metres long. S–C fabrics and lineations in the shear zone consistently indicate a steep east-side-up shear sense, with significant local dispersion in kinematics where the shear zone fabrics wrap around pods. The scaly fabric is dominated (>98 % vol) by fine-grained (≪10 µm) fibrous chrysotile and lizardite–polygonal serpentine, but infrequent (<1 % vol) lenticular relicts of antigorite are also preserved. Dissolution seams and foliation surfaces enriched in magnetite, as well as the widespread growth of fibrous chrysotile in veins and around porphyroclasts, suggest that bulk shear zone deformation involved pressure–solution. Syn-kinematic metasomatic reactions occurred along all boundaries between serpentinite, schist and rodingite, forming multigenerational networks of nephritic tremolite veins that are interpreted to have caused reaction hardening within metasomatised portions of the shear zone. We propose a conceptual model for plate-boundary-scale serpentinite shear zones which involves bulk-distributed deformation by pressure–solution creep, accompanied by a range of physical (e.g. faulting in pods and wall rocks; smearing of magnetite along fault surfaces) or chemical (e.g. metasomatism) processes that result in localised brittle deformation within creeping shear zone segments.