Index Geophysics

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

<p>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&thinsp;<span class="inline-formula">km</span> 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 (<span class="inline-formula">&gt;98</span>&thinsp;%&thinsp;vol) by fine-grained (<span class="inline-formula">≪10</span>&thinsp;<span class="inline-formula">µm</span>) fibrous chrysotile and lizardite–polygonal serpentine, but infrequent (<span class="inline-formula">&lt;1</span>&thinsp;%&thinsp;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.</p>

M. S. Tarling

S. A. F. Smith

J. M. Scott

J. S. Rooney

C. Viti

K. C. Gordon

Geology

QE1-996.5

Stratigraphy

QE640-699

Copernicus Publications

2019-07-01T00:00:00Z

article

10.5194/se-10-1025-2019

1869-9510

1869-9529

https://doaj.org/article/82a2f4aa7dbc40d0893afded455146df

Solid Earth, Vol 10, Pp 1025-1047 (2019)

EN

https://www.solid-earth.net/10/1025/2019/se-10-1025-2019.pdf

https://doaj.org/toc/1869-9510

https://doaj.org/toc/1869-9529

Journal Licence: CC BY