Origin of the Torlesse Terrane and Coeval Rocks,North Island,New Zealand

On the basis of differing areal extent, age, petrographic modes, and bulk chemical composition, the sandstones of the northern quarter of the Torlesse terrane are subdivided into four new petrofacies. A comparison of these petrofacies with existing South Island Torlesse classifications indicates continuation of the Triassic Rakaia subterrane and Late Jurassic–to–early Cretaceous Pahau subterrane into the central part of the North Island (as Axial-A and Axial-B petrofacies, respectively). The Waioeka petrofacies defines a new and provisional Late Jurassic-to–early Cretaceous Waioeka subterrane that is not present in the South Island. The Omaio petrofacies is common to deformed Albian basement sequences in the Torlesse of both islands, and in the Houhora Complex of Northland.

The composite Torlesse terrane evolved by Early Jurassic accretion of allochthonous Rakaia rocks followed by parautochthonous deposition of Pahau and Waioeka sandstones. Waioeka sandstones are compositionally similar to sandstones in the coeval eastern Waipapa terrane, but may have been dextrally displaced from their original depositional site by up to 300 km since the middle Cretaceous.     

Provenance comparisons between the Nambucca Block,Eastern Australia and the Torlesse Composite Terrane,New Zealand: connections and implications from detrital zircon age patterns

Detrital zircon U–Pb LAM-ICPMS age patterns for sandstones from the mid-Permian –Triassic part (Rakaia Terrane) of the accretionary wedge forming the Torlesse Composite Terrane in Otago, New Zealand, and from the early Permian Nambucca Block of the New England Orogen, eastern Australia, constrain the development of the early Gondwana margin. In Otago, the Triassic Torlesse samples have a major (64%), younger group of Permian–Early Triassic age components at ca 280, 255 and 240 Ma, and a minor (30%) older age group with a Precambrian–early Paleozoic range (ca 1000, 600 and 500 Ma). In Permian sandstones nearby, the younger, Late Permian age components are diminished (30%) with respect to the older Precambrian–early Paleozoic age group, which now also contains major (50%) and unusual Carboniferous age components at ca 350–330 Ma. Sandstones from the Nambucca Block, an early Permian extensional basin in the southern New England Orogen, follow the Torlesse pattern: the youngest. Early Permian age components are minor (<20%) and the overall age patterns are dominated (40%) by Carboniferous age components (ca 350–320 Ma). These latter zircons are inherited from either the adjacent Devonian–Carboniferous accretionary wedge (e.g. Texas-Woolomin and Coffs Harbour Blocks) or the forearc basin (Tamworth Belt) farther to the west, in which volcaniclastic-dominated sandstone units have very similar pre-Permian (principally Carboniferous) age components. This gradual variation in age patterns from Devonian–late Carboniferous time in Australia to Late Permian–mid-Cretaceous time in New Zealand suggests an evolutionary model for the Eastern Gondwanaland plate margin and the repositioning of its subduction zone. (1) A Devonian to Carboniferous accretionary wedge in the New England Orogen developing at a (present-day) Queensland position until late in the Carboniferous. (2) Early Permian outboard repositioning of the primary, magmatic arc allowing formation of extensional basins throughout the New England Orogen. (3) Early to mid-Permian translocation of the accretionary wedge and more inboard active-margin elements, southwards to their present position. This was accompanied by oroclinal bending which allowed the initiation of a new, late Permian to Early Triassic accretionary wedge (eventually the Torlesse Composite Terrane of New Zealand) in an offshore Queensland position. (4) Jurassic–Cretaceous development of this accretionary wedge offshore, in northern Zealandia, with southwards translation of the various constituent terranes of the Torlesse Composite Terrane to their present New Zealand position.     

Composition of Monocrystalline Detrital and Authigenic Minerals,Metamorphic Grade,and Provenance of Torlesse and Waipapa Graywacke,Central North Island,New Zealand

Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.     

An Australian provenance for the eastern Otago Schist protolith,South Island,New Zealand: evidence from detrital zircon age patterns and implications for the origin of its gold

Uranium–lead age patterns of detrital zircons in Otago Schist meta-sandstones from eastern Otago, including areas of orogenic gold mineralisation, are mostly consistent with a Rakaia Terrane (Torlesse Composite Terrane) accretionary wedge protolith. Southwest of the Hyde-Macraes and Rise & Shine shear zones the depositional age is regarded as Middle–Late Triassic. At the south and west margins, there are two areas in the Late Triassic Waipapa Terrane protolith. Northeast of the Hyde-Macraes Shear Zone, the schist protolith has Middle to Late Triassic and middle to late Permian depositional ages of Rakaia Terrane affinity. At the northeastern margin of the Hyde-Macraes Shear Zone, there is a narrow strip with a mid-Carboniferous protolith, which may be a counterpart of the Carboniferous accretionary wedge in the New England Orogen, eastern Australia. Ordovician–Silurian zircons are a minor but distinctive feature in many of the protolith age patterns and form significant age components at hard-rock gold locations. These constrain the provenance of Rakaia Terrane protolith sediments to Late Triassic time and within the Permian–Triassic magmatic arcs at the northeastern Australian continental margin and partly within the Ordovician–Silurian granitoids of the Charters Towers Province hinterland and environs. The latter have extensive gold mineralisation and thus upon exhumation might be the origin of Otago gold.     

Australian provenance for Upper Permian to Cretaceous rocks forming accretionary complexes on the New Zealand sector of the Gondwanaland margin

U–Pb (SHRIMP) detrital zircon age patterns are reported for 12 samples of Permian to Cretaceous turbiditic quartzo‐feldspathic sandstone from the Torlesse and Waipapa suspect terranes of New Zealand. Their major Permian to Triassic, and minor Early Palaeozoic and Mesoproterozoic, age components indicate that most sediment was probably derived from the Carboniferous to Triassic New England Orogen in northeastern Australia. Rapid deposition of voluminous Torlesse/Waipapa turbidite fans during the Late Permian to Late Triassic appears to have been directly linked to uplift and exhumation of the magmatically active orogen during the 265–230 Ma Hunter‐Bowen event. This period of cordilleran‐type orogeny allowed transport of large volumes of quartzo‐feldspathic sediment across the convergent Gondwanaland margin. Post‐Triassic depocentres also received (recycled?) sediment from the relict orogen as well as from Jurassic and Cretaceous volcanic provinces now offshore from southern Queensland and northern New South Wales. The detailed provenance‐age fingerprints provided by the detrital zircon data are also consistent with progressive southward derivation of sediment: from northeastern Queensland during the Permian, southeastern Queensland during the Triassic, and northeastern New South Wales — Lord Howe Rise — Norfolk Ridge during the Jurassic to Cretaceous. Although the dextral sense of displacement is consistent with the tectonic regime during this period, detailed characterisation of source terranes at this scale is hindered by the scarcity of published zircon age data for igneous and sedimentary rocks in Queensland and northern New South Wales. Mesoproterozoic and Neoproterozoic age components cannot be adequately matched with likely source terranes in the Australian‐Antarctic Precambrian craton, and it is possible they originated in the Proterozoic cores of the Cathaysia and Yangtze Blocks of southeast China.     

青藏高原地体的初步划分及构造特征简述

本文在分析了青藏高原不同地体的区域构造的基础上，讨论了青藏高原的构造演化史。青藏高原的板块构造经历了两个开合旋回。第一个开合旋回发育的时间为泥盆纪到二叠纪，其中泥盆纪到石炭纪为洋盆扩张阶段，二叠纪为洋盆俯冲缩小阶段。第二个开合旋回发育的时间为三叠纪到白垩纪，其中早中三叠世和晚三叠世早期为洋盆扩张阶段，晚三叠世晚期到白垩纪对于整个青藏高原来说，为洋盆俯冲缩小阶段。洋盆扩张阶段，地体朝南运动，洋盆俯冲缩小阶段，地体朝北运动。     

Evidence from U–Pb zircon and 40Ar/39Ar muscovite detrital mineral ages in metasandstones for movement of the Torlesse suspect terrane around the eastern margin of Gondwanaland

New U–Pb detrital zircon ages from Triassic metasandstones of the Torlesse Terrane in New Zealand are compared with ⁴⁰Ar/³⁹Ar muscovite data and together, reveal four main source components: (i) major, Triassic–Permian (210–270 Myr old) and (ii) minor, Permian–Carboniferous (280–350 Myr old) granitoids (recorded in zircon and muscovite data); (iii) minor, early middle Palaeozoic, metamorphic rocks, recorded mainly by muscovite, 420–460 Myr old, and (iv) minor, Late Precambrian–Cambrian igneous and metamorphic complexes, 480–570 Myr old, recorded by zircon only. There are also Proterozoic zircon ages with no clear grouping (580–1270 Myr). The relative absence of late Palaeozoic (350–420 Myr old) components excludes granitoid terranes in the southern Lachlan Fold Belt (Australia) and its continuation into North Victoria Land (East Antarctica) and Marie Byrd Land (West Antarctica) as a potential source for the Torlesse. The age data are compatible with derivation from granitoid terranes of the northern New England Orogen (and hinterland) in NE Australia. This confirms that the Torlesse Terrane of New Zealand is a suspect terrane, that probably originated at the NE Australian, Permian–Triassic, Gondwanaland margin and then (200–120 Ma) moved 2500 km southwards to its present New Zealand position by the Late Cretaceous (90 Ma). This sense of movement is analogous to that suggested for Palaeozoic Mesozoic terranes at the North American Pacific margin.     

The Otago schist megaculmination: Its possible origins and tectonic significance in the Rangitata orogen of New Zealand

In South Island, New Zealand, the Otago schist, 30,000 square km in extent, consists mainly of greenschist facies quartzo-feldspathic metagreywacke and meta-argillite with minor metavolcanics and metacherts. Before metamorphism the sediments were probably Carboniferous to Jurassic; the flanking, steeply dipping greywackes are Triassic in the northeast and southeast, and Permian in the west and southwest.Regional metamorphism culminating in the Late Jurassic was accompanied by pervasive deformation generating a variety of interrelated folds on all scales. The scarcity of distinctive reference units makes recognition of macroscopic structures difficult, and much progress has depended on observations of vergence of mesoscopic folds interpreted as defining macroscopic folds having axial plane separation of between 2 km and 6 km.At least two phases of synmetamorphic deformation are distinguishable locally, but regionally have an overlapping multiphase relationship. The regional schistosity structure is an irregular flat-crested antiform trending S and SE. The internal megascopic structure defined by the mesoscopic folds, appears to consist of a stack of nappe-folds facing east and northeast, which pass into reclining isoclinal folds in the west, southwest and north-east, and is interpreted to be a megaculmination. Mineral and textural metamorphic zones were developed during deformation, and a relatively simple regional pattern established at a late stage by continuing recrystallisation.The Otago schist originated in a complex sequence of Paleozoic—Mesozoic plate interactions near the southwest Pacific margin of Gondwanaland. It included part of a volcaniclastic frontal arc-basin assemblage (Murihiku and Caples Terranes) lying north or northeast of an older crystalline foreland, and a quartzo-feldspathic assemblage of plutonic-metamorphic provenance lying further to the northeast (Torlesse Terrane). Parts of these terranes underwent mainly greenschist facies metamorphism during Late Jurassic subduction-collision to form the Haast Schist Terrane of which the Otago schist is a major part.The earliest Torlesse sediments are thought to have prograded as a vast fan complex onto oceanic crust from the southwesterly crystalline foreland in the Carboniferous, then in the Permian were separated from their source by a spreading zone which thereafter isolated them from the sedimentary province of the newly developing arc system. Tectonic recycling of these sediments at a Permian to Jurassic oceanic subduction zone is considered to have developed the westward progradation features and the products of limited vulcanism found in the present Torlesse Terrane. The New Zealand Geosyncline appears to have consisted of a spreading zone between two inwardly facing convergent zones, one flanked by a foreland to the southwest, the other wholly oceanic.The metamorphic climax of the Rangitata Orogeny was the result of the medial spreading zone passing into the westerly subduction zone, so permitting the convergent zones to collide, with the Torlesse sediments caught between them.The mantle system driving the spreading zone appears to have continued to function, and soon after the collisional climax caused Late Jurassic—Cretaceous rifting of the sialic edge of Gondwanaland, igneous activity, differential shear of the New Zealand region, and initiation of the Alpine Fault. It subsequently commenced sea-floor spreading in the Tasman Sea, and later in the southwest Pacific Ocean.     

Detrital zircon age constraints on depositional history and provenance of the Murihiku Supergroup,Murihiku Terrane,North Island,New Zealand

In the Murihiku Terrane of New Zealand, U-Pb detrital zircon ages in Murihiku Supergroup sandstones of Late Triassic, Jurassic and possibly earliest Cretaceous age have a marked youngest age component that is close to, and sometimes coincident with, established biostratigraphic ages, thus reflecting contemporary volcanism. However, youngest Huriwai Group samples yield 137–142 Ma zircon age components (earliest Early Cretaceous) in conflict with palynofloras that suggest only a latest Jurassic age. This is resolved if the age of the Jurassic/Cretaceous boundary is lowered to ca. 140 Ma. Older, reworked zircons are mainly Early Jurassic, Late Triassic and Late Permian reflecting an enduring exhumed magmatic arc source nearby. This might be in the adjacent Median Batholith but as a Murihiku sediment source its Jurassic, Triassic and Permian elements are not well-matched in terms of extent, age and bulk compositions. A connection between the Murihiku (proximal forearc) and Waipapa Composite (distal accretionary wedge) terranes is probable, with a common magmatic arc, speculatively situated in the New England Orogen, eastern Australia.     

Pb-Nd Isotopes Indicate the Origin of Island Arc Terranes in the Early Paleozoic Pacific

The Takaka Terrane in New Zealand is one of the best exposed arc fragments of the early Paleozoic Australian-Antarctic convergent margin and constitutes one of the most outboard terranes of this margin in paleogeographic reconstructions. Pb-Nd isotope compositions of clinopyroxenes from the Cambrian Devil River Volcanics of the Takaka Terrane enable identification of the location of the terrane in the Paleo-Pacific Ocean. The Devil River Volcanics, a suite of primitive arc and back-arc rocks, are interbedded with the partly continent-derived Haupiri Group sediments. Extremely radiogenic Pb and unradiogenic Nd compositions in the arc rocks cannot be explained by assimilation of the Haupiri Group sediments or a continental basement of such a composition. Pb isotope compositions of the Takaka Terrane sediments are much less radiogenic and overlap with crustal compositions of the Lachlan Fold Belt in Australia, suggesting that both units are derived from one source, the Australian-Antarctic Pacific margin. Pb-Nd isotope compositions in the Devil River Volcanics reflect contamination of their mantle sources by subducted sediments derived from Archean provinces in either Antarctica or Laurentia. Both provinces show characteristically high 207Pb/204Pb500 and were located at the Pacific rim in the Cambrian. Mixing between mantle and Proterozoic continental material from present western South America or eastern Laurentia cannot explain the high 207Pb/204Pb500 in the New Zealand rocks. As in New Zealand, extreme spreads in Pb-Nd isotope compositions in other Cambrian volcano-sedimentary sequences in southeast Australia and Tasmania can be explained by the same model, suggesting that all these fragments originated along the Australian-Antarctic Gondwana margin. Pb isotope compositions of arc rocks, therefore, provide a new tool for terrane analysis in the early Paleozoic Pacific ocean.     

Reconnaissance Feasibility Study of Heavy-Mineral Suites in the Fine-Grained Matrix of Several Lithostratigraphic Terranes,Southern New Zealand

Nineteen samples of mélange matrix and volcanogenic sandstone matrix were collected from the Maitai (including the Hawtel), Caples, and Torlesse terranes, South Island, and southernmost North Island, New Zealand, in an attempt to identify contrasting provenances for these lithostratigraphic units. Heavy minerals were concentrated employing water settling columns followed by high-speed centrifugation utilizing bromoform and methyl iodide. Semiquantitative scanning-electron-microscope and quantitative-microprobe investigations of heavy-mineral concentrates support conclusions of previous workers. (1) The Torlesse Complex is a richly quartzofeldspathic, at least partially multicycle, unit derived from an evolved, well-dissected continental margin or mature island arc. It contains a diverse suite of igneous, metamorphic, and sedimentary detritus including widespread traces of clastic zircon, rutile, biotite, barite, almandine, clinopyroxene, Ca-amphibole, staurolite, chromite, and epidote. (2) Mélanges belonging to the Maitai and Caples terranes possess heavy-mineral suites suggestive of somewhat more restricted, first-cycle volcanogenic arc-like provenances, being richer in unstable rock fragments, aluminosilicates, grossular-andradite garnets, and chlorites as well as amphiboles and pyroxenes. Neoblastic hydrous, calcic aluminosilicates and layer silicates suggest that the Hawtel mélange recrystallized at lithostatic pressures of less than 3 kbar, whereas the other investigated terranes were metamorphosed at slightly higher pressures.     

Fluid evolution during metamorphism of the Otago Schist, New Zealand: (I) Evidence from fluid inclusions

Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350  °C, with modal values for individual samples ranging from 163 to 229  °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting show a narrower range of T  _h from 86 to 170  °C with modes from 116 to 141  °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400  °C and 4.1–6.0  kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades.     

Characterisation and origin of New Zealand nephrite jade using its strontium isotopic signature

Nephrite jade occurs in three terranes (Dun Mountain–Maitai, Caples and Torlesse) in New Zealand, where it is associated with ultramafic and ophiolitic rocks in narrow metasomatic reaction zones at the margins of serpentinite (having harzburgite/gabbro/dolerite precursors) with silicic metasediments and metavolcanics. True nephrite fabrics are developed only locally where marginal shearing is intense, and late in the metamorphic history. ⁸⁷Sr/⁸⁶Sr values of these nephrites do not display the primitive values of their gabbro/dolerite precursor component i.e. 0.7030–0.7035, as expected if formed during serpentinisation. Rather, the nephrites have more evolved ⁸⁷Sr/⁸⁶Sr values inherited from the metasediment component at a later stage, and which fall within particular terrane groups: Dun Mountain–Maitai 0.7045–0.7060, Caples 0.7058–0.7075 and Torlesse 0.7085–0.7110. Rb–Sr ages and initial ⁸⁷Sr/⁸⁶Sr ratios of the metasediment component from in situ nephrite localities, when compared with their counterparts throughout the host terrane, show that nephrite Sr isotopic compositions are characteristic of the host terrane.     

Gondwana breakup via double-saloon-door rifting and seafloor spreading in a backarc basin during subduction rollback

A model has been developed where two arc-parallel rifts propagate in opposite directions from an initial central location during backarc seafloor spreading and subduction rollback. The resultant geometry causes pairs of terranes to simultaneously rotate clockwise and counterclockwise like the motion of double-saloon-doors about their hinges. As movement proceeds and the two terranes rotate, a gap begins to extend between them, where a third rift initiates and propagates in the opposite direction to subduction rollback. Observations from the Oligocene to Recent Western Mediterranean, the Miocene to Recent Carpathians, the Miocene to Recent Aegean and the Oligocene to Recent Caribbean point to a two-stage process. Initially, pairs of terranes comprising a pre-existing retro-arc fold thrust belt and magmatic arc rotate about poles and accrete to adjacent continents. Terrane docking reduces the width of the subduction zone, leading to a second phase during which subduction to strike-slip transitions initiate. The clockwise rotated terrane is caught up in a dextral strike-slip zone, whereas the counterclockwise rotated terrane is entrained in a sinistral strike-slip fault system. The likely driving force is a pair of rotational torques caused by slab sinking and rollback of a curved subduction hingeline.By analogy with the above model, a revised five-stage Early Jurassic to Early Cretaceous Gondwana dispersal model is proposed in which three plates always separate about a single triple rift or triple junction in the Weddell Sea area. Seven features are considered diagnostic of double-saloon-door rifting and seafloor spreading:

i) earliest movement involves clockwise and counterclockwise rotations of the Falkland Islands Block and the Ellsworth Whitmore Terrane respectively;

ii) terranes comprise areas of a pre-existing retro-arc fold thrust belt (the Permo-Triassic Gondwanide Orogeny) attached to an accretionary wedge/magmatic arc; the Falklands Islands Block is initially attached to Southern Patagonia/West Antarctic Peninsula, while the Ellsworth Whitmore Terrane is combined with the Thurston Island Block;

iii) paleogeographies demonstrate rifting and extension in a backarc environment relative to a Pacific margin subduction zone/accretionary wedge where simultaneous crustal shortening occurs;

iv) a ridge jump towards the subduction zone from east of the Falkland Islands to the Rocas Verdes Basin evinces subduction rollback;

v) this ridge jump combined with backarc extension isolated an area of thicker continental crust — The Falkland Islands Block;

vi) well-documented EW oriented seafloor spreading anomalies in the Weddell Sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback;

vii) the dextral strike-slip Gastre and sub-parallel faults form one boundary of the Gondwana subduction rollback, whereas the other boundary may be formed by inferred sinistral strike-slip motion between a combined Thurston Island/Ellsworth Whitmore Terrane and Marie Byrd Land/East Antarctica.

Cretaceous termination of subduction at the Zealandia margin of Gondwana: The view from the paleo-trench

During the Permian to Cretaceous, Zealandia occupied a position on the proto-Pacific-facing, convergent margin of Gondwana. Subduction on this margin ceased somewhere between ~105 Ma and perhaps 70 Ma, but the timing of this tectonic transition remains controversial. Resolution of this uncertainty is important for tectonic reconstructions of the southwest Pacific and for global plate-tectonic models. Here, we revisit the problem by reference to new stratigraphic and geochemical data from the East Coast Basin of New Zealand, which occupied a position adjacent to and possibly superimposed on the relict Cretaceous subduction trench at the time subduction ceased; this basin is expected to preserve direct structural and stratigraphic evidence for or against Late Cretaceous subduction.In western parts of the East Coast Basin – the “Western Sub-belt” – a conspicuous unconformity separates undoubted accretionary prism rocks of the Torlesse Composite Terrane from younger Cretaceous “cover” units. Strata beneath and overlying this unconformity vary in age from place to place, but abundant paleontological data and published ages of detrital zircons (some reinterpreted herein), indicate that exposed Torlesse rocks are nowhere younger than ~100 Ma. Overlying Zealandia Megasequence “cover” strata are mostly younger than ~110 Ma. Between ~110 and ~85 Ma, the entire length of the Western Sub-belt reveals complex stratigraphic architecture of relatively small-scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion. There is widespread evidence for compression on the Western Sub-belt over this period, although the overall amount of shortening is likely to be relatively modest. In contrast, the Eastern Sub-belt preserves a record of near-continuous and apparently simple deposition over the same interval of time. We see little and somewhat equivocal evidence for extension in either sub-belt through the Late Cretaceous. Superimposed on this general pattern, we observe discrete, widespread or basin-wide, tectonic events at 95 Ma, and within the intervals ~86–83 Ma and ~83–81 Ma, indicated by the presence of unconformities and sedimentary facies changes. Importantly, all these events can be correlated with coeval events recorded elsewhere in Zealandia, suggesting that the East Coast Basin was subject to the same tectonic regime as the rest of Zealandia and shared a common history during the mid- to Late Cretaceous.Integrating these findings with data from elsewhere in Zealandia, we argue that there is diverse evidence to indicate that subduction finished along the New Zealand segment of the Gondwana margin by 100 Ma. That said, the causes of on-going, Late Cretaceous compression in the East Coast Basin and in other parts of Zealandia are not well constrained, although several possible explanations are plausible. There are few analogue, abandoned subduction margins described in the literature; however, the situation in the South Shetland Trench shows strong similarities with patterns and complexities observed in the East Coast Basin.     

The origin and pre-Cenozoic evolution of the Tibetan Plateau

Different hypotheses have been proposed for the origin and pre-Cenozoic evolution of the Tibetan Plateau as a result of several collision events between a series of Gondwana-derived terranes (e.g., Qiangtang, Lhasa and India) and Asian continent since the early Paleozoic. This paper reviews and reevaluates these hypotheses in light of new data from Tibet including (1) the distribution of major tectonic boundaries and suture zones, (2) basement rocks and their sedimentary covers, (3) magmatic suites, and (4) detrital zircon constraints from Paleozoic metasedimentary rocks. The Western Qiangtang, Amdo, and Tethyan Himalaya terranes have the Indian Gondwana origin, whereas the Lhasa Terrane shows an Australian Gondwana affinity. The Cambrian magmatic record in the Lhasa Terrane resulted from the subduction of the proto-Tethyan Ocean lithosphere beneath the Australian Gondwana. The newly identified late Devonian granitoids in the southern margin of the Lhasa Terrane may represent an extensional magmatic event associated with its rifting, which ultimately resulted in the opening of the Songdo Tethyan Ocean. The Lhasa−northern Australia collision at ~ 263 Ma was likely responsible for the initiation of a southward-dipping subduction of the Bangong-Nujiang Tethyan Oceanic lithosphere. The Yarlung-Zangbo Tethyan Ocean opened as a back-arc basin in the late Triassic, leading to the separation of the Lhasa Terrane from northern Australia. The subsequent northward subduction of the Yarlung-Zangbo Tethyan Ocean lithosphere beneath the Lhasa Terrane may have been triggered by the Qiangtang–Lhasa collision in the earliest Cretaceous. The mafic dike swarms (ca. 284 Ma) in the Western Qiangtang originated from the Panjal plume activity that resulted in continental rifting and its separation from the northern Indian continent. The subsequent collision of the Western Qiangtang with the Eastern Qiangtang in the middle Triassic was followed by slab breakoff that led to the exhumation of the Qiangtang metamorphic rocks. This collision may have caused the northward subduction initiation of the Bangong-Nujiang Ocean lithosphere beneath the Western Qiangtang. Collision-related coeval igneous rocks occurring on both sides of the suture zone and the within-plate basalt affinity of associated mafic lithologies suggest slab breakoff-induced magmatism in a continent−continent collision zone. This zone may be the site of net continental crust growth, as exemplified by the Tibetan Plateau.     

Gondwana dispersion and Asian accretion: Tectonic and palaeogeographic evolution of eastern Tethys

Present-day Asia comprises a heterogeneous collage of continental blocks, derived from the Indian–west Australian margin of eastern Gondwana, and subduction related volcanic arcs assembled by the closure of multiple Tethyan and back-arc ocean basins now represented by suture zones containing ophiolites, accretionary complexes and remnants of ocean island arcs. The Phanerozoic evolution of the region is the result of more than 400 million years of continental dispersion from Gondwana and plate tectonic convergence, collision and accretion. This involved successive dispersion of continental blocks, the northwards translation of these, and their amalgamation and accretion to form present-day Asia. Separation and northwards migration of the various continental terranes/blocks from Gondwana occurred in three phases linked with the successive opening and closure of three intervening Tethyan oceans, the Palaeo-Tethys (Devonian–Triassic), Meso-Tethys (late Early Permian–Late Cretaceous) and Ceno-Tethys (Late Triassic–Late Cretaceous). The first group of continental blocks dispersed from Gondwana in the Devonian, opening the Palaeo-Tethys behind them, and included the North China, Tarim, South China and Indochina blocks (including West Sumatra and West Burma). Remnants of the main Palaeo-Tethys ocean are now preserved within the Longmu Co-Shuanghu, Changning–Menglian, Chiang Mai/Inthanon and Bentong–Raub Suture Zones. During northwards subduction of the Palaeo-Tethys, the Sukhothai Arc was constructed on the margin of South China–Indochina and separated from those terranes by a short-lived back-arc basin now represented by the Jinghong, Nan–Uttaradit and Sra Kaeo Sutures. Concurrently, a second continental sliver or collage of blocks (Cimmerian continent) rifted and separated from northern Gondwana and the Meso-Tethys opened in the late Early Permian between these separating blocks and Gondwana. The eastern Cimmerian continent, including the South Qiangtang block and Sibumasu Terrane (including the Baoshan and Tengchong blocks of Yunnan) collided with the Sukhothai Arc and South China/Indochina in the Triassic, closing the Palaeo-Tethys. A third collage of continental blocks, including the Lhasa block, South West Borneo and East Java–West Sulawesi (now identified as the missing “Banda” and “Argoland” blocks) separated from NW Australia in the Late Triassic–Late Jurassic by opening of the Ceno-Tethys and accreted to SE Sundaland by subduction of the Meso-Tethys in the Cretaceous.     

The basement of the Punta del Este Terrane (Uruguay): an African Mesoproterozoic fragment at the eastern border of the South American R��o de La Plata craton

The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino?CSierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U?CPb ages of ca. 1,000?Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U?CPb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000?Ma inheritance formed at ca. 750?Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640?Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570?Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535?Ma of post-tectonic granitoids (Santa Teresa and José Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Río de La Plata Craton were in their present positions by ca. 535?Ma.     

ZEDONG TERRANE, A MID CRETACEOUS INTRA-OCEANIC ARC, SOUTH TIBET

ZEDONG TERRANE, A MID CRETACEOUS INTRA-OCEANIC ARC, SOUTH TIBET     

Geodynamic evolution of the European Variscan fold belt: palaeomagnetic and geological constraints

The Variscan fold belt of Europe resulted from the collision of Africa, Baltica, Laurentia and the intervening microplates in early Paleozoic times. Over the past few years, many geological, palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt. Whereas it is now fairly well established that Avalonia drifted from the northern margin of Gondwana in Early Ordovician times and collided with Baltica in the late Ordovician/early Silurian, the nature of the Gondwana derived Armorican microplate is more enigmatic. Geological and new palaeomagnetic data suggest Armorica comprises an assemblage of terranes or microblocks. Palaeobiogeographic data indicate that these terranes had similar drift histories, and the Rheic Ocean separating Avalonia from the Armorican Terrane Assemblage closed in late Silurian/early Devonian times. An early to mid Devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow Rhenohercynian basin which closed progressively between the late Devonian and early Carboniferous. In this contribution, we review the constraints provided by palaeomagnetic data, compare these with geological and palaeobiogeographic evidence, and present a sequence of palaeogeographic reconstructions for these circum-Atlantic plates and microplates from Ordovician through to Devonian times.