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
The mechanical and seismological properties of serpentinite-bearing shear zones are controlled by shear zone structure and composition. Because serpentinite influences the rheology of the lithosphere in a range of tectonic settings, substantial effort has been aimed at collecting experimental
Examples of important serpentinite-bearing shear zones include oceanic detachment faults and fracture zones
While specific processes and microstructures related to serpentinisation and deformation in serpentinite-bearing shear zones have been reported in a wide range of studies
Field-based studies of large serpentinite shear zones typically document a pervasive anastomosing foliation in the bulk of the shear zone
The purpose of this paper is to present field and microstructural observations of the internal structure and composition of a plate-boundary-scale serpentinite shear zone that is well exposed in the South Island of New Zealand. The Livingstone Fault represents an important opportunity to document the characteristics of a large serpentinite-dominated shear zone from the submicron scale up to the crustal scale. We present structural and petrological data on the geometry, kinematics and composition of the shear zone. These data are used to propose a general conceptual model that could be used as a framework to help interpret the mechanical behaviour and seismological and physical properties of active serpentinite-bearing shear zones.
Regional geological setting.
The continent of Zealandia is composed of a series of Cambrian to Cretaceous tectonostratigraphic terranes that were amalgamated on the paleo-Pacific Gondwana margin (Fig.
Simplified geological map of the Livingstone Fault with 11 study sites marked, from Cosy Gully in the north to West Burn in the south. The approximate thickness of the serpentinite shear zone based on field observations is shown at each locality. Stereonets (all lower hemisphere, equal area) represent measurements of the main shear zone fabrics, including undifferentiated scaly foliations, S–C fabrics where present and lineations. At Cow Saddle, additional data are provided for the orientations of brittle faults and associated lineations that cut through pods of massive serpentinite. Stereonets produced using Stereonet v10.1.6
The Livingstone Fault is the terrane boundary that defines the eastern margin of the DMOB (Figs.
The DMOB and Livingstone Fault are well exposed in intermittent locations through a
Detailed field mapping, sampling and structural data collection were performed at 12 locations along a strike length of ca. 140
Drone imagery and photogrammetry were used to produce a high-resolution orthorectified aerial photo of the Livingstone Fault at Serpentine Saddle (Fig.
Thin sections of fault rocks were cut parallel to lineation and perpendicular to foliation. Standard 30
Thin sections prepared for Raman spectroscopy were glued using a low-fluorescence epoxy (Epofix cold-setting embedding resin, Struers)
Typical field exposures of the Livingstone Fault where it crosses high passes. Red dashed lines approximate the boundaries of the serpentinite shear zone.
The Livingstone Fault consists of a serpentinite-dominated shear zone that separates the mainly quartzofeldspathic schists of the Caples and Aspiring Terrane from mafic or ultramafic portions of the DMOB (Fig.
Boundaries between the serpentinite shear zone and the wall rocks are commonly steeply dipping and well defined (Fig.
Deformation in the Caples–Aspiring Terrane wall rocks at Fiery Col and Cow Saddle.
Photo showing well-defined S–C fabrics at Cow Saddle, wrapping around a fractured pod of massive serpentinite. Structural data from Cow Saddle are shown in Fig.
All exposures of the serpentinite shear zone are characterised by a strongly foliated matrix with a scaly fabric, defined by sub-centimetre asymmetric phacoids of serpentinite (Fig.
At Cow Saddle, pods of massive serpentinite are cut by en echelon brittle faults that have a similar orientation to the C shear bands in the surrounding matrix (Figs.
Tremolite vein networks and associated discrete slip surfaces.
Detailed geological map and cross section of the Livingstone Fault at Serpentine Saddle derived from field mapping onto high-resolution, drone-acquired orthophotos. The map highlights the internal structure and composition of the serpentinite shear zone at this locality, including the traces of the scaly fabrics, pods of rodingite and massive serpentinite, and larger domains of massive serpentinite containing gently dipping rodingite dykes. The locations of images shown in Fig.
The Livingstone Fault contains metasomatic reaction zones in a number of structural locations, and the reaction zones are ubiquitous at all the investigated localities shown in Fig.
Shear zone structures at Serpentine Saddle. Location of the images shown on the map in Fig.
The shear zone at Serpentine Saddle is up to 420
The shear zone in the mapped area consists of the following (Fig.
Large domains of massive serpentinite tens to hundreds of metres long (Fig.
Pods of serpentinite contain a central core of massive serpentinite surrounded by an outer cladding that transitions towards scaly serpentinite (Fig.
Another type of resistant pod observed in the shear zone consists of moderately foliated serpentinite containing embedded fragments of partially rodingitised gabbroic to doleritic dykes (up to tens of centimetres wide by tens of centimetres to metres in length). Networks of tremolite veins radiate out from the dyke fragments and cross-cut the surrounding serpentinite foliation.
The S–C fabrics and associated lineations at Serpentine Saddle indicate an overall Caples–Aspiring-Terrane-up shear sense (east-side up), consistent with other localities (Figs.
Massive serpentinite within pods contains relatively undeformed pseudomorphic bastite and mesh textures, suggesting that these regions represent largely intact serpentinised peridotite
Serpentinite textures in pods and massive serpentinite domains.
Evolution of serpentinite texture and mineralogy in scaly shear zone serpentinites. Each figure shows an optical microscope image in crossed-polarised light (left-hand side) and plane-polarised light (right-hand side).
The most common scaly serpentinite in the shear zone consists of phacoids of serpentinite that have rounded edges and sigmoidal shapes that contribute to the asymmetry of the S–C fabric (Fig.
The shape of individual phacoids and the spacing of the foliation surfaces can vary substantially throughout the shear zone. In samples of scaly serpentinite that have relatively widely spaced foliation surfaces, deformed mesh and ribbon-textured serpentinite are preserved inside phacoids, and the phacoids are coated by continuous and interconnected seams of magnetite (Fig.
Raman and TEM characterisation of the scaly serpentinite fabric.
A combination of TEM and Raman spectroscopy mapping reveals that the scaly serpentinite is composed of fibrous chrysotile (70 wt %–80 wt %; Fig.
Aggregates of antigorite are present within small (10–400
The thermodynamic stability of the serpentine group minerals is poorly constrained
Observations from 11 localities along a strike length of 140
A conceptual model of the structure and composition of large serpentinite shear zones based on the most important structural elements and deformation processes observed in the Livingstone Fault.
On a regional scale, the thickness of the shear varies between 5 and 480
The most important structural elements and processes within the Livingstone Fault are as follows.
The scaly matrix serpentinite contains a pervasive foliation that wraps around pods (Fig. Large domains of massive serpentinite preserve relatively intact and flat-lying rodingite dykes (Fig. Competent pods of massive serpentinite, veined serpentinite, rodingite and schist are entrained within the shear zone and cut by networks of brittle faults and fractures (Fig. Metasomatic reactions occurred wherever serpentinites and schist are in contact, which includes the eastern boundary of the shear zone and the margins of schist pods (Fig. Fault surfaces are coated by layers of magnetite, which cut across the scaly fabric (Fig.
The overall structure and composition of the Livingstone Fault may be representative of other large serpentinite-dominated shear zones. In particular, the scale and composition of the shear zone, the “block-in-matrix” style of deformation, and the juxtaposition of ultramafic or mafic wall rocks against quartzofeldspathic wall rocks are similar to the likely characteristics of the plate-boundary-scale shear zone thought to be present along the slab–mantle interface in the shallow forearc region of subduction zones
Concentrations of magnetite along scaly foliation surfaces are interpreted to reflect the preferential dissolution of serpentine during deformation and subsequent enrichment of magnetite along dissolution surfaces
Fracture networks radiating away from the contact areas between large pods suggest elevated stresses during interactions and collisions between pods (Fig.
Metasomatic reaction zones are found at the contact between schist or rodingite pods and the surrounding serpentinite. Such reactions between serpentinite and silicic–calcic lithologies that form tremolite, talc, diopside and chlorite are extensively documented in the literature
Finally, we speculate that progressively concentrating magnetite along foliation surfaces and shear bands via a pressure–solution mechanism (Figs.
The Livingstone Fault is a plate-boundary-scale serpentinite shear zone that is tens of metres to several hundred metres wide. The bulk of the shear zone consists of a pervasive scaly fabric dominated by chrysotile and lizardite–polygonal serpentine. The scaly fabric wraps around fractured and faulted pods of massive serpentinite, rodingite and partially metasomatised quartzofeldspathic schist up to hundreds of metres long. Well-developed S–C fabrics in the scaly serpentinite indicate an east-side-up shear sense and preserve textural evidence to suggest that pressure–solution was an important deformation mechanism during shearing. Metasomatic reactions were ubiquitous wherever serpentinite contacted schist or rodingite, forming multigenerational vein networks filled by nephritic tremolite. Based on field and microstructural observations, we present a conceptual model of the structure and composition of large serpentinite shear zones deforming at greenschist-facies conditions. The model involves bulk-distributed deformation by pressure–solution creep accompanied by localised brittle deformation within pods or along magnetite-coated fault surfaces. Metasomatic reactions can generate vein networks, leading to reaction hardening and embrittlement within metasomatised portions of the shear zone. The scale, internal structure and composition of the Livingstone Fault and its wall rocks suggest that it could provide a suitable analogue for other plate-boundary-scale serpentinite shear zones, including the serpentinite-bearing shear zone expected to occur along the slab–mantle interface in subduction zones.
All data produced and used to write the paper are available in numerical or graphical form in the figures and/or within the paper itself.
MST, SAFS and JMS carried out fieldwork and performed microstructural analysis of fault rocks. MST and CV performed transmission electron microscopy. MST and JSR performed Raman analysis with input from KCG. MST wrote the paper with discussion and input from all authors. SAFS and JMS supervised the project.
The authors declare that they have no conflict of interest.
This work was supported by the Marsden Fund Council (project UOO1417 to Steven A. F. Smith) administered by the Royal Society Te Apārangi, with additional funding from a University of Otago research grant. Luke Easterbrook provided valuable assistance in drone imagery and photogrammetry. Chris Tulley provided field assistance on many of the field expeditions and contributed data on the Mount Raddle section collected during his BSc Hons project. Jordan Crase contributed data on the Serpentine Saddle, Cow Saddle, Fiery Col and Cosy Gully sections collected during his MSc project. We thank Marianne Negrini, Brent Pooley, Claudia Magrini and Giovanna Giorgetti for technical support. Finally, we thank Telemaco Tesei and an anonymous reviewer for their insightful reviews of the paper.
This research has been supported by the Marsden Fund Council (grant no. UOO1417).
This paper was edited by Cristiano Collettini and reviewed by Telemaco Tesei and one anonymous referee.