Abstract
Abstract The Western Province is a fragment of the c. 500 Ma SE Gondwana active continental margin. The Eastern Province is a terrane assemblage, which is partly stitched by the Median Batholith. Fragments of the batholith are preserved in the adjacent Drumduan and Brook Street terranes. Permian arc magmatism of the Brook Street Terrane involved both oceanic and continental margin settings. The Permian ( c. 285–275 Ma) supra-subduction zone Dun Mountain ophiolite records subduction initiation and subsequent oceanic-arc magmatism. The Permian Patuki and Croisilles melanges represent detachment of the ophiolitic forearc and trench–seamount accretion. The Murihiku Terrane, a proximal continental margin forearc basin, received detritus from the Median Batholith (or equivalent). The south coast, Early–Late Triassic Willsher Group is another proximal forearc basin unit. The sediments of the Dun Mountain–Maitai Terrane (Maitai basin) represent a distal segment of a continental margin forearc basin. The Caples Terrane is a mainly Triassic trench accretionary complex, dominantly sourced from a continental margin arc, similar to the Median Batholith. The outboard (older) Torlesse and Waipapa terranes are composite subduction complexes. Successively more outboard terranes may restore farther north along the SE Gondwana continental margin. Subduction and terrane assembly were terminated by collision (at c. 100 Ma), followed by rifting of the Tasman Sea Basin.
Highlights
Application of tectonostratigraphic terrane nomenclature to New ZealandSeveral of the late Paleozoic–early Mesozoic terranes recognized in New Zealand (Figs 15.2 & 15.3) conform well to the concept of a tectonostratigraphic terrane, arguably as well as any other comparable rock assemblages worldwide (Coombs et al 1976)
The Permian-aged Brook Street Terrane outcrop north of the Alpine Fault, the later Permian arc volcanism of the Gympie Terrane (Rammutt Formation and above) and the Téremba Terrane are all interpreted as successor arc magmatism along, or adjacent to, the SE Gondwana active continental margin
The Dun Mountain ophiolite can be compared with Cordilleran-type ophiolites (Moores 1982) that have accreted along an active continental margin: for example, the Jurassic Coast Range Ophiolite, western USA (Metzger et al 2002; Shervais et al 2004), or the late Precambrian ophiolites of SE Australia (Cawood & Buchan 2007; Cawood et al 2009)
Summary
Several of the late Paleozoic–early Mesozoic terranes recognized in New Zealand (Figs 15.2 & 15.3) conform well to the concept of a tectonostratigraphic terrane, arguably as well as any other comparable rock assemblages worldwide (Coombs et al 1976). The nature and character of several of the individual terranes differ in significant respects including scale, nature of tectonic boundary and internal heterogeneity: (1) terranes such as the Brook Street Terrane, the Dun Mountain–Maitai Terrane and the Torlesse Composite Terrane (Figs 15.2 & 15.3) represent fundamentally different crustal units which are likely to have undergone major tectonic displacement relative to each other (Mortimer et al 2002; Mortimer 2004); in contrast, the Buller and Takaka terranes of the Western Province represent contrasting, mainly Paleozoic, successions that, tectonically assembled, may represent less fundamentally different tectonic units (Cooper & Tulloch 1992; Bradshaw 1993; Roser et al 1996; Adams 2004); (2) the key boundary between the Eastern and Western provinces is difficult to define because of the rarity or absence of host rocks between intrusions (Tuhua Intrusives); (3) the Brook Street Terrane (Eastern Province) includes several different volcanic arc-related units of possibly different age and uncertain relative tectonic displacement (Robertson & Palamakumbura 2019a); (4) the Drumduan Terrane (assigned to the Eastern Province) includes sedimentary and volcanic assemblages that underwent either high pressure–low temperature (HP–LT) or low temperature–high pressure (LT–HP) metamorphism in different outcrops (see below), suggesting that it is a composite unit; and (5) the more outboard (easterly) terranes, notably the Torlesse Composite Terrane, include sandstone turbidites, polygenetic melanges and volcanic rocks of different origins that are generally interpreted as subduction complexes but which are difficult to define as discrete fault-bounded tectonostratigraphic terranes. Sedimentation and metamorphism are to be expected along this active margin but are impossible to evaluate because most of this area is submerged, including beneath the Lord Howe Rise
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