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

Abstract

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 (

Structural controls on the location, geometry and longevity of an intraplate volcanic system: the Tuatara Volcanic Field, Great South Basin, New Zealand

“…The Alpine Fault offset the basement terranes across Zealandia, such that those beneath the South Island are also present beneath parts of the North Island (e.g. Cassidy and Locke, 2010;Collanega et al, 2018;Cooper and Norris, 1994;Howell, 1980;Lamb et al, 2016;Muir et al, 2000;Tarling et al, 2019). In particular, the Dun Mountain-Maitai and Caples terranes, which underlie our study area, are present beneath the Auckland Volcanic Field in the North Island (Figure 1) (Cassidy and Locke, 2010;McGee et al, 2013;Spörli et al, 2015).…”

“…The Tuatara Volcanic Field occupies a relatively localised (~270 km 2 ) setting above the Livingstone Fault, which marks the boundary between the Dun Mountain-Maitai Terrane to the south, and the Caples Terrane in the north (Figure 10) (Mortimer et al, 2002;Tarling et al, 2019). Here, we examine the controls on the localisation, geometry, and longevity of intraplate volcanic activity at the Tuatara Volcanic Field.…”

“…Projecting onshore terrane boundaries offshore along-strike suggests the study area resides within the Eastern Province and spans the faulted boundary between the Caples Terrane and the Dun Mountain-Maitai Terrane (Figure 1). This terrane boundary corresponds to the Livingstone Fault (Figure 1), which is a serpentinite-dominated shear zone that ranges from 10's to 100's of metres wide, dips steeply to the northeast, and extends down to, at least, the base of the crust (Mortimer et al, 2002;Tarling et al, 2019). The Caples Terrane comprises weakly metamorphosed volcaniclastic greywackes, which accreted to the southern margin of Gondwana during the Permian-to-Triassic (Johnston, 2019;Mortimer, 2004;Robertson et al, 2019), whilst the Dun Mountain-Maitai Terrane includes the Early Permian Dun Mountain, mafic-to-ultramafic ophiolite belt and an overlying, 6 km thick sequence of Late Permian-Middle Triassic volcaniclastic sedimentary rocks (i.e.…”

“…However, we also note that the Tuatara Volcanic Field overlies the Livingstone Fault, which marks the boundary between the Caples and Dun Matai terranes. The Auckland Volcanic Field and volcanoes on the Banks Peninsula are also co-located along prominent basement terrane boundaries, namely the same boundary as the Tuatara Volcanic Field, between the Dun Mountain-Maitai and Caples terranes, and the Rakaia and Pahau terranes respectively (Figure 1a) (Eccles et al, 2005;Mortimer et al, 2002;Tarling et al, 2019). This co-location of volcanic fields with major basement terrane boundaries questions whether the localisation of intraplate volcanic systems may, at least partially, be controlled by pre-existing structures.…”

“…We name this newly discovered province the 'Tuatara Volcanic Field' after the endemic New Zealand reptile, the name of which is derived from the Māori language and fittingly means 'peaks on the back'. The well exposed and studied basement geology onshore New Zealand (e.g., Mortimer, 2004;Tarling et al, 2019), combined with a detailed and comprehensive record of intraplate volcanism throughout the Cenozoic (e.g., Adams, 1983;Cooper et al, 1987;Hoernle et al, 2006;Rout et al, 1993;Speight, 1943;Stipp and McDougall, 1968;Waight et al, 1998) and high-resolution marine geological and geophysical data available in the offshore domain (e.g., Mortimer et al, 2002;Phillips and McCaffrey, 2019;Tulloch et al, 2019;Uruski, 2015;Uruski et al, 2007), allow us to fully characterise and constrain the internal architecture of the Tuatara Volcanic Field and assess how it relates to the surrounding crustal structure. By identifying seismic-stratigraphic onlap and downlap relationships, we show the volcanoes and sills were emplaced between ~85 and ~45 Ma, recording ~40 Myr of punctuated igneous activity spanning the Late Cretaceous-to-Early Eocene.…”