Origin of the Torlesse terrane and coeval rocks, South Island, New Zealand

Abstract

The Carboniferous to Lower Cretaceous Torlesse terrane and Haast Schist derivatives constitute the major part of the complexly deformed facies of the Eastern Province of New Zealand. Strata consist mainly of quartzofeldspathic graywacke and mudstone, intercalated with minor but widely distributed conglomerate, and volcanics with associated chert and limestone. Clastic rocks were deposited largely by sediment gravity-flow mechanisms in a deep-marine environment. Also present are a few highly fossiliferous shallow-marine and terrestrial deposits of limited areal extent, which rest unconformably on or in fault contact with Torlesse flysch. Several periods of deformation are recognized and melange is present on both local and regional scales. Metamorphism ranges from zeolite to greenschist facies. The bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages: Permian ( Atomodesma ), Middle Triassic, early Late Triassic (?) ( Torlessia ), Late Triassic ( Monotis ), and Late Jurassic-Early Cretaceous. Contacts between major fossil zones are mainly tectonic. Petrographic analysis permits subdivision of Torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones. The sandstone petrofacies (mainly arkosic), together with the composition of conglomerate clasts (mainly indurated Torlesse rocks), indicate that the source terrane was a continental volcano-plutonic arc, probably part of Gondwanaland, coupled with autocannibalistic reworking of older uplifted Torlesse rocks. In contrast to the quartzofeldspathic nature of the Torlesse, coeval sedimentary rocks of the Eastern Province are volcanogenic. They are thought to represent related forearc-basin (Maitai-Murihiku terranes) and trench-complex (Caples terrane) deposits derived from a volcanic island arc (Brook Street terrane). Three petrofacies are established for Maitai-Murihiku and Caples sandstones. The petrofacies indicate a common, evolving, immature to submature volcanic island arc source for these terranes. A reconstruction of New Zealand's Eastern and Western Provinces is proposed. In Permian and Triassic times, the Torlesse was deposited in trench, slope, or borderland basins along a trench-transform margin fronting a continental volcano-plutonic arc source (Western Province-Gondwanaland). Deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of Torlesse rock younging outward from the Gondwanaland margin. At the same time, the Brook Street terrane volcanic arc and associated terranes were forming to the west of the Torlesse site, separated from Gondwanaland by a marginal sea. In latest Triassic or Early Jurassic times, the Torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the Gondwana margin. The collision event resulted in tectonic thickening of Torlesse and Caples rocks at the plate interface and metamorphism to Haast Schist. The source was then dominated by older, partly metamorphosed Torlesse terrane, newly uplifted along the collision front. Closing of the marginal sea behind the Brook Street terrane in Late Jurassic-Early Cretaceous times resulted in juxtapositioning with the Western Province (Gondwanaland) along the Median Tectonic Line.