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Solid Earth An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Mar 2019

Research article | 27 Mar 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

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

Matthew S. Tarling1, Steven A. F. Smith1, James M. Scott1, Jeremy S. Rooney2, Cecilia Viti3, and Keith C. Gordon2 Matthew S. Tarling et al.
  • 1Department of Geology, University of Otago, 360 Leith Street, 9016 Dunedin, New Zealand
  • 2Department of Chemistry, University of Otago, Union Place West, 9016 Dunedin, New Zealand
  • 3Dipartimento di Scienze Fisiche, della Terra e dell'Ambiente, Università degli Studi di Siena, Siena, Italy

Abstract. Deciphering the internal structural 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 > 1000 km long terrane-bounding structure that separates the basal portions (peridotite; serpentinised peridotite; metagabbros) of the Dun Mountain Ophiolite Belt from quartzofeldspathic schists of the Caples or Aspiring Terranes. Field and microstructural observations from eleven localities along a strike length of c. 140 km show that the Livingstone Fault is a steeply-dipping, serpentinite-dominated shear zone tens to several hundreds of 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 Caples-side-up (i.e. 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 (< 2 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 multi-generational networks of nephritic tremolite veins that are interpreted to have caused reaction-hardening within metasomatised portions of the shear zone. A general conceptual model is proposed for the internal structure and composition of plate boundary-scale serpentinite shear zones deforming at greenschist-facies conditions. The model 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 localized brittle deformation within creeping shear zone segments.

Matthew S. Tarling et al.
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Matthew S. Tarling et al.
Matthew S. Tarling et al.
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Short summary
Shear zones dominated by hydrated mantle rocks (serpentinites) occur in many tectonic settings around the world. To better understand the internal structure, composition and possible mechanical behaviour of these shear zones, we performed a detailed field, petrological and microanalytical study of the Livingstone Fault in New Zealand. We propose a conceptual model to account for the main physical and chemical processes that control deformation in large serpentinite shear zones.
Shear zones dominated by hydrated mantle rocks (serpentinites) occur in many tectonic settings...