Sedimentologic, petrographic, and tectonic aspects of Torlesse and related rocks: South Island, New Zealand.

Abstract

Carboniferous to Lower Cretaceous Torlesse rocks and Haast Schist derivatives constitute the major part of the ''eugeosynclinal" facies of the Eastern Province of New Zealand. Strata consist largely of quartzofeldspathic graywacke and mudstone, with minor but widely distributed conglomerate and associated volcanics, chert and limestone. Structure is complex, several periods of deformation are recognized; isoclinal and steeply plunging folds are widespread; and melange is present on both local and regional scales. Metamorphic grade is zeolite and prehnite-pumpellyite facies in the Torlesse and mainly pumpellyite-actinolite and greenschist facies in the schist. Fossils are sparsely but widely distributed, The bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones: Atomodesma (Permian); Daonella (Middle Triassic); Torlessia (lower Upper Triassic?); Monotis (Upper Triassic) ; and combined Upper Jurassic-Lower Cretaceous zones. Contacts between major fossil zones are indeterminate and for the most part are probably tectonic.

Structural and sedimentologic aspects of a small area (≈ 6 sq km) of Monotisse-bearing Torlesse rocks near Arthurs Pass are described. Strata consist largely of sandstone-dominated sediment gravity flow deposits. Initial deformation probably began shortly after deposition in Late Triassic times. Strata were folded and faulted and locally highly disrupted prior to a prehnite-pumpellyite metamorphic maximum in the Jurassic-·Cretaceous.

Sedimentologic columns from seven well exposed sequences on the South Island are described and interpreted in terms of depositional environments. Previous sedimentological studies are discussed, and a regional summary of Torlesse sedimentation on the South Island is presented.

Strata are predominantly thick to very thick-bedded massive and graded sandstone with subordinate thinner bedded sandstone and mudstone deposited by turbidity currents and other types of sediment gravity flows in a deep marine environment. Overall sandstone : mudstone ratio is 2 : 1 to 3: 1. In terms of submarine fan models, most strata are similar to middle and upper fan deposits. Mudstone-dominated slope, upper fan interchannel and lower fan-basin plain deposits are rare, except for Lower Cretaceous strata where lower fan-basin plain deposits are probably common. The Torlesse was not deposited as one large fan but consisted of a number of smaller sediment gravity flow deposits, not necessarily fan shaped, that were progressively accreted as deposition proceeded.

Highly fossiliferous shallow- and non-marine deposits are present in a few areas of the Torlesse. Some are limestones deposited on local submarine highs. The others are clastic sequences of Middle Triassic and Jurassic age which rest unconformably on or in fault contact with Torlesse flysch. They are of limited areal extent and together occupy less than 1% of total Torlesse exposure on the South Island.

Detrital mineralogy of Torlesse sandstones is largely preserved in zeolite and prehnite-pumpellyite facies sandstones with low secondary matrix content. Potassium feldspar has been removed during metamorphism of some prehnite-pumpellyite and most higher grade sandstones. […]

The major source of Torlesse detritus was a continental volcano-platonic arc and associated siliceous sedimentary and metasedimentary country rock coupled with an autocannibalistically reworked Torlesse source. Variations in detrital composition through time reflect changes in source terrane composition. From Permian—Carboniferous through Late Triassic times a plutonic source was dominant, volcanism was variable and reworking of Torlesse rocks, some of prehnite-purnpellyite facies, was minor but locally extensive. In Late Jurassic and Early Cretaceous times, the major sources were older Torlesse rocks, including some textural grade 2 and 3 Haast Schist, and acid volcanics. The inferred characteristics of the source terrane are compatible with a Western Province New Zealand - Gondwanaland source.

In contrast to the quartzo-feldspathic nature of the Torlesse, coeval sedimentary rocks of the Eastern Province are volcanogenic. They are thought to represent related forearc (Maitai - Murihiku terranes) and trench complex (Caples terrane) deposits derived from a volcanic island arc (Brook Street terrane) . […] A reconstruction of New Zealand's Eastern and Western Provinces is presented. 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 (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. Indurated and in-part metamorphosed Torlesse rocks were locally exposed to erosion in trench-slope-break or borderland settings with deposition of shallow marine and terrestrial deposits in these same areas during the Middle Triassic. 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 roughly 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 in-part metamorphosed Torlesse terrane, newly uplifted along the collision front, on which some Upper Jurassic shallow- and non-marine rocks were deposited. 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.