Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting: role of the Pyrenean orogeny

The Gulf of Lion margin results from the Cligo-Aquitanian rifting and Burdigalian crustal separation between continental Europe and Corsica-Sardinia. Immediately before the onset of extension, the area of the Gulf of Lion was affected by the Pyrenean orogeny which controlled the structural style of the evolving margin. During extension, the foreland of the Pyrenean orogen was affected by extensional thin-skinned tectonics. The décollement level ramped down into the basement, in areas where the latter was thickened during orogeny. In this intermediate part, the margin was extended by several crustal-scale low-angle faults, which generated small amounts of syn-rift sedimentation compared with the accumulation of post-rift sediments. However, more than 4 km of syn-rift sediments were deposited in the Camargue basin, which is located at the transition between thin- and thick-skinned extensional systems. Kinematic restorations and stratigraphy suggest a pre-rift surface elevation above sea-level of at least 1 km in the intermediate part of the margin, which is in agreement with reduced syn-rift sedimentation. The slope area extends seaward of the North Pyrenean Fault, a terrane boundary inherited from the Pyrenean collision. This part of the margin was stretched by seaward dipping low-angle block tilting of the upper crust, and antithetic lower crustal and sub-crustal detachment. The lithospheric structures inherited from the Pyrenean orogeny exerted a strong control on the kinematics of the rifting and on the distribution and history of subsidence. Such parameters need to be integrated in the definition of pre-rift initial conditions in future basin-modelling of the Gulf of Lion.     

华南新元古代裂谷盆地演化——Rodinia超大陆解体的前奏

沉积学研究表明，华南新元古代沉积盆地具典型裂谷盆地沉积演化特征。代表裂谷盆地早期形成阶段的成因相组合有：冲洪积相组合、陆相（或海相）火山岩及火山碎屑岩相组合、滨浅海相沉积组合、淹没碳酸盐台地及欠补偿盆地黑色页岩相组合；而代表中、后期形成阶段的成因相组合有：滨岸边缘相至深海相组合，冰期冰积岩相组合、碳酸盐岩及碳硅质细碎岩相组合。华南裂谷盆地岩相古地理演化经历了5个重要的时期，整体上反映了一个由陆变海、由地堑－地垒相间盆地变广海盆地、由浅海变深海、盆地上小变大的演化过程。裂谷盆地的形成经历了裂谷基的形成、地幔柱作用与裂谷体的形成，被动沉降（下坳）与裂谷盖的形成三个阶段。华南裂谷盆地的形成演化与Rodinia超大陆在新元古代时期的裂解作用密切相关，它是超大陆解体过程的一个重要组成部分。     

Sedimentary and Tectonic History of the Terranes in the Corridor of the GolmudEjin Qi Geotransect and Its Adjacent Areas

１．ＩｎｔｒｏｄｕｃｔｉｏｎＡｎｉｍｐｏｒｔａｎｔａｓｐｅｃｔｏｆｔｈｅｓｔｕｄｙｏｆｔｈｅＧＧＴ（ｉ．ｅ．ＧｌｏｂａｌＧｅｏｓｃｉｅｎｃｅＴｒａｎｓｅｃｔ）ｉｓｔｏｒｅｃｏｎｓｔｒｕｃｔｔｈｅｈｉｓｔｏｒｙｏｆｔｈｅｔｅｒｒａｎｅｓｔｈａｔｔｈｅＧＧＴｒｕｎｓａｃｒｏｓｓ（Ｍｏｎｇｅｒｅｔａｌ．,１９８５）．Ｂｅｃａｕｓｅｔｈｅｓｅｄｉｍｅｎｔｓａｒｅｔｈｅｐｒｏｄｕｃｔｓｏｆｖａｒｉｏｕｓｇｅｏｌｏｇｉｃａｌｅｆｆｅｃｔｓｉｎｔｈｅｇｅｏｌｏｇｉｃａｌｈｉｓｔｏｒｙ,ｗｅｃａｎｒｅｃｏｎｓｔ…     

Lithospheric structure of the Southwest South China Sea: implications for rifting and extension

ABSTRACT

The South China Sea (SCS) is an excellent site for studying the process of conjugate margin rifting, and the origin and evolution of oceanic basins. Compared with the well-defined northern margin of the SCS, the western and southern segments of the SCS margin have not been researched in significant detail. To investigate the regional structure of the southwestern SCS, a gravity model is constructed, along with the lithospheric thermal structure along a wide-angle seismic profile. The profile extends across the conjugate margins of the Southwest Sub-Basin (SWSB) of the SCS and is based on the latest multiple geophysical measurements (including heat flow and thermo-physical parameters). The results show that the average thicknesses of the crust and thermal lithosphere along the profile are about 15 km and 57 km, respectively. The overall amount of extension of continental crust and lithosphere is more than 200 km. Thermal structure of the lithosphere shows that the continental margins are in a warm thermal state. The southwest SCS is characterized by ultra-wide, thinned continental crust and lithosphere, high Moho heat flow, early syn-rift faulted basins, undeformed late syn-rifting, and high seismic velocities in the lower crust. These various pieces of evidence suggest that the break-up of the mantle lithosphere occurred before that of the continental crust favouring a depth-dependent extension of the southwestern SCS margin.     

History of the Ordovician continental platform shelf margin of Australia

Much of South Australia, western New South Wales, and Tasmania was affected by the Late Cambrian‐Early Ordovician Delamerian Orogeny. Areas of the former shelf margin exhibit molasse‐type conglomerates overlying a major late Middle to Late Cambrian unconformity (Jukesian Movement in Tasmania or Mootwingee Movement in western N.S.W.). In continental platform areas to the north the effects of the orogeny were less intense with, in the Georgina Basin for instance, only dis‐conformable relationships, and the overlying deposits consisting of fine elastics and carbonates. Regression accompanied this first phase of tectonic upheaval and was followed by a period of ‘late Tremadoc’ transgression of the sea into several embayment areas of the continental platform. This short‐lived transgression was succeeded by ‘early Arenig’ regression which appears to be related to a second, less intense Delamerian orogenic phase. Expression of this phase ranges from unconformity in west Tasmania (Haulage and Lynchford Movements) to disconformity in the Georgina Basin (Kelly Creek Movement).

A second, more extensive and long‐lived transgression of the sea from the ‘middle Arenig’ to about the end of the ‘Llanvirn’ resulted in the development of the epicontinental Larapintine Sea, permitting interchange of warmer and cooler waters from either ends of the seaway. Combined fossil, lithological and palaeomagnetic evidence suggests that, of the fragments of Gondwanaland, Australia alone straddled the Ordovician equator, with its present west coast approximately along the line of Lat. 30°S. Influxes of sand from the areas of mild‐high relief to the south appear to have been deflected in an anticlockwise direction along the open, ocean‐facing Gnalta Shelf of western N.S.W. towards the eastern end of the Larapintine Sea perhaps as a result of a major westward‐flowing equatorial current. The influxes progressively constricted and finally blocked off the eastern end of the seaway by the end of ‘Llanvirn’ time. The closure, and final regression of the sea from all continental platform areas, seems to have been accompanied by a phase of local uplift and erosion (Dullingari Movement of northeastern South Australia). In cratonic areas of central and northern Australia a period of Late Ordovician or Early Silurian uplift and erosion (Rodingan Movement) followed. Dullingari and Rodingan Movements may be correlated with phases of the Benambran Orogeny of the Tas‐man Geosyncline.

Late Ordovician cratonic sedimentation was restricted to the shelf margin. On the Tasmanian Shelf carbonates accumulated during a long period of relative tectonic quiescence and gentle subsidence. First signs of onset of the Benambrian Orogeny are shown by the appearance of clastics in the topmost beds of the Tasmanian Ordovician sequence.     

台湾海峡上地幔流变学状态及其构造意义

本文详细研究了台湾澎湖列岛上地幔幔源包体的变形结构、组构和橄榄石位错构造。根据包体矿物（辉石）化学成分的计算，平衡温度９８６－１１１６℃，压力为１．５０－２．６０ＧＰａ。上地幔变形事件可分为３个序列：（１）高温低应力均匀稳态流动变形；（２）高温较高应力上地幔底辟边缘的剪切流动变形；（３）低温高应变速率．（１１０）滑移条带的淬取变形。本区上地幔变形事件和热结构特征表明，福建东部和台湾澎湖列岛－带具有     

Patterns and Dynamics of Rifting on Passive Continental Margin from Shelf to Slope of the Northern South China Sea:Evidence from 3D Analogue Modeling

Affected by thermal perturbation due to mantle uprising,the rheological structure of the lithosphere could be modified,which could lead to different rifting patterns from shelf to slope in a passive continental margin.From the observed deformation style on the northern South China Sea and analogue modeling experiments,we find that the rift zone located on the shelf is characterized by half grabens or simple grabens controlled mainly by long faults with large vertical offset,supposed to be formed with normal lithasphere extension.On the slope,where the lithosphere is very hot due to mantle upwelling and heating,composite grabens composed of symmetric grabens developed.The boundary and inner faults are all short with small vertical offset.Between the zones with very hot and normal lithosphere,composite half grnbens composed of half grabens or asymmetric grabens formed,whose boundary faults are long with large vertical offset,while the inner faults are relatively short.Along with the thickness decrease of the brittle upper crust due to high temperature,the deformation becomes more sensitive to the shape of a pre-existing weakness zone and shows orientation variation along strike.When there was a bend in the pre-existing weakness zone,and the basal plate was pulled by a clockwise rotating stress,the strongest deformation always occurs along the middle segment and at the transition area from the middle to the eastern segments,which contributes to a hotter lithosphere in the middle segment,where the Baiyun (白云) sag formed.     

藏南特提斯喜马拉雅带晚二叠纪基性岩浆作用及其构造地质意义

野外地质调查和SHRIMP锆石U-Pb地质年代学研究在藏南雅拉香波地区厘定了一规模较大,形成于273.0±2.2Ma的辉绿岩体。该辉绿岩体侵入到由页岩和细砂岩组成的特提斯沉积岩中,表明这些沉积岩形成时间早于晚二叠纪,而不是晚三叠纪地层。该岩体具有以下地球化学特征:(1)富集LREE,亏损HREE;(2)高场强元素含量较高;(3)较高的Sr(87Sr/86Sr(t)=0.7063～0.7078)和Nd(εNd(t)=-1.1～-2.4)同位素组成,与西部同时代溢流玄武岩相当;及(4)较高的εHf(t)(+2.5～+3.9)。本研究及已有数据表明:沿新特提斯带发育一系列晚二叠纪基性岩浆岩,是冈瓦纳大陆北缘裂解和新特提斯洋初始张开时深部岩浆作用的产物。     

Analyse sismo-stratigraphique du bassin d'Abda (Maroc occidental), exemple de structures inverses pendant le rifting atlantiqueSeismo-stratigraphic analysis of the Abda Basin (West Morocco): a case of reverse structures during the Atlantic rifting

The review of the seismic reflection and well data from the coastal Abda Basin (western Morocco) shows that its Triassic and Jurassic sequences were deposited in a submeridean sag basin, whose eastern margin is characterised by progressive truncations and pinching out of these sequences against a prominent Palaeozoic high. The uplift of this latter is interpreted as a response to an Upper Triassic–Middle Jurassic local compressional event that controlled Triassic–Jurassic sedimentation within the Abda Basin. The present day ‘West Meseta Flexure’ is a surface expression of this uplift. To cite this article: H. Echarfaoui et al., C. R. Geoscience 334 (2002) 371–377.     

Characteristics of crustal structures in the Yamato Basin,sea of Japan,deduced from seismic explorations

Crustal structures around the Yamato Basin in the southeastern Sea of Japan, inferred from recent ocean bottom seismography (OBS) and active-source seismological studies, are reviewed to elucidate various stages of crustal modification involved from rifting in the crust of the surrounding continental arc to the production of oceanic crust in the Yamato Basin of the back-arc basin. The northern, central, and southern areas of the Yamato Basin have crustal thicknesses of approximately 12–16 km, and lowermost crusts with P-wave velocities greater than 7.2 km/s. Very few units have P-wave velocities in the range 5.4–6.0 km/s, which corresponds to the continental upper crust. These findings, combined with previous geochemical analysis of basalt samples, are interpreted to indicate that a thick oceanic crust has been formed in these areas of the basin, and that this oceanic crust has been underplated by mantle-derived magma. In the central Yamato Basin, the original continental crust has been fully breached and oceanic crust has been formed. Conversely, the presence of a unit corresponding to the continental upper crust and the absence of a high-velocity part in the lower crust implies that the southwestern edge of the Yamato Basin has a rifted crust without significant intrusion. The Oki Trough has a crust that is 17–19 km thick with a high-velocity lower crust and a unit corresponding to the continental upper crust. The formation of the Oki Trough resulted from rifting with magmatic intrusion and/or underplating. We interpret these variations in the crustal characteristics of the Yamato Basin area as reflecting various instances of crustal modification by thinning and magmatic intrusion due to back-arc extension, resulting in the production of a thick oceanic crust in the basin.