Dominique Cluzel

Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications

This paper deals with the tectonic events that result in the accretion of mafic terranes in the fore-arc region and a close juxtaposition of ultramafic rocks, low grade and high-grade mafic terranes in many collisional orogens. The example is taken from New Caledonia where tectonic accretion, subduction, underplating and obduction of mafic terranes took place during the late Eocene in an intra-oceanic forearc setting. The late Eocene tectonic complex comprised three major terranes: an overlying ultramafic, mainly harzburgitic allochthon named the Ophiolitic Nappe, an intermediate mafic, mainly basaltic off-scraped melange, composed of kilometre-scale slices of oceanic upper crust, called the Poya Terrane, parts of which have been metamorphosed into an eclogite/blueschist facies complex, the Pouebo Terrane; and a lower, continental basement formed by the Norkolk Ridge terranes. Based upon exhaustive sampling of the mafic terranes and field surveys, our tectonic, micropaleontologic and geochemical data reveal that Poya and Pouebo terranes rocks originally formed within one single Campanian to late Paleocene oceanic basin, floored by tholeiitic basalt associated with some minor seamount-related intraplate alkali basalt. The tholeiitic basalt displays a continuous range of compositions spanning between “undepleted” and “depleted” end-members; the former being volumetrically predominant. The overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin, the South Loyalty Basin, which almost completely disappeared during Eocene convergence. The opening of this basin originally located to the east of the Norfolk Ridge was synchronous with that of Tasman Sea basin as a consequence of oceanward migration of the west-dipping Pacific subduction zone. Establishing the origin of the ultramafic Ophiolitic Nappe is beyond the scope of this paper; however, it appears to be genetically unrelated to the mafic Poya and Pouebo terranes. Although it was located in the Late Eocene fore-arc, the Ophiolitic Nappe and the corresponding oceanic lithosphere originated before the Late Cretaceous, to the east of the South Loyalty Basin in a back-arc setting; or alternatively in a much older, trapped basin. For reasons that remain unclear, a new east-dipping subduction started in the Eocene and consumed most of the South Loyalty Basin, forming the intra-oceanic Loyalty Arc. Due to a changing subduction regime (underplating of the Diahot Terrane?), the mafic slices that now form the Poya Terrane were tectonically accreted in the Loyalty fore-arc region and remained under low pressure-low temperature conditions (possibly at the subsurface) until the Norfolk Ridge reached the subduction zone diachronously. This resulted in the final obduction of the fore-arc area. The two-step obduction involved first the mafic complex forming the Poya Terrane and thereafter the lithospheric mantle that now forms the Ophiolitic Nappe. In contrast, pieces of the accretionnary complex were dragged down into the subduction zone, underplated at depth ca. 70 km and metamorphosed into high-temperature eclogite to form the Pouebo Terrane metamorphics that display the same geochemical features as the Poya Terrane basalt. A mid-to-late Eocene syntectonic piggy-back sedimentary basin (the Nepoui flysch basin) mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore-arc region due to the accretion and underplating of mafic ocean-related and other material. In contrast, a slightly younger foreland basin located upon the Norfolk Ridge (the Priabonian Bourail Flysch basin) received a massive input of detrital material derived from the Norfolk Ridge itself and a time-increasing amount of mafic, Poya-derived material that recorded the first step of obduction. Thereafter, the Bourail Flysch was overthrust by the Poya Terrane and finally by the Ophiolitic Nappe. At the same time, buoyancy-driven uplift and exhumation of the high-pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone. Finally, for a short time a new subduction started along the west coast of New Caledonia and generated small amounts of active margin-related magmas. These events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre-collision analogue of mafic/ultramafic belts found in many collisional mountain ranges.

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

Geodynamics of the Ogcheon Belt (South Korea)

Abstract The Ogcheon Belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early Palaeozoic rift which was affected by polyphase tectonics. Its pre-Cretaceous structure results from three main orogenic events: the “Ogcheon” (late Silurian-Devonian (?)), Indosinian (late Permian-Triassic) and Yenshanian (middle Jurassic-early Cretaceous) tectonic phases. The external, autochthonous or para-autochthonous units represent platform zones with shallow-water deposits whereas rift facies characterize the internal domain. The Yeongweol sequence, forming an outer shelf on the southeastern margin of the rift, displays affinities with the Jiangnan basin of South China. It is in wrench fault contact with the easternmost Duwibong sequence correlated with the North China-North Korea platform. In the internal metamorphic units, post-Cambrian-early Ordovician pelitic and turbiditic rift metasediments overlie a thin Cambrian or Cambrian-Ordovician early platform sequence. Two subdomains are distinguished: the upper units (Chung Ju and Pibaryeong units) representing the deep “axial” part of the basin contain distal flyshoids and large olistoliths derived from the northwestern “narrow” margin; the lower units (Turung San, Poeun and Iwharyeong) are related to the “wide” margin located between the deep rift domain and the stable southeastern platform. The latter transitional domain is characterized by thick submarine debris flows, and volcanics of alkali or transitional tholeiitic affinity. The volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting. Lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the Cambrian-Ordovician platform sequence in one single basin. The post-middle Silurian-pre-mid-Carboniferous “Ogcheon” ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area. It gave rise to an intracontinental fold-thrust belt without ophiolites. This tectonic event may have been contemporaneous with early Palaeozoic tectonism in the Quinling Belt in Central China and in the Southeast China fold belt. The definitive constitution of the Sino-Korean craton took place before the mid-Carboniferous and “molasse” sedimentation was initiated on a single platform located on the western edge of the Yakuno oceanic basin of Southwest Japan. In late Permian and early Triassic times, the closure of the oceanic area and the subsequent westward collision of the Honshu Block are marked in Korea by the intracratonic “Indosinian” tectonism. Since late Triassic times, predominantly transcurrent movements have occurred along NNE-SSW faults associated with syntectonic intermontane troughs and important granitization.

Structural and Geochronological Study of High‐Pressure Metamorphic Rocks in the Kekesu Section (Northwestern China): Implications for the Late Paleozoic Tectonics of the Southern Tianshan

Blueschist‐ and eclogite‐facies high‐ to ultrahigh‐pressure (HP/UHP) metamorphic rocks occur in the southern Tianshan Belt. Their deformation and metamorphic history is important for understanding the Paleozoic tectonics of the Central Asian Orogenic Belt. Our study focuses on the structural analysis and geochronology of the HP metamorphic rocks and the surrounding rocks in the Kekesu Section in the southern Chinese Tianshan. Geometric and kinematic analyses indicate three ductile deformation events: a top‐to‐the‐north thrusting, a top‐to‐the‐south shearing, and a dextral wrenching. New 40Ar/39Ar laser probe plateau ages were obtained on white mica from retrograde blueschist ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Late Paleozoic tectonic evolution of the northern West Chinese Tianshan Belt

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Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia

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Late Paleozoic pre- and syn-kinematic plutons of the Kangguer–Huangshan Shear zone: Inference on the tectonic evolution of the eastern Chinese north Tianshan

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