Middle Proterozoic–early Palaeozoic evolution of central Baltoscandian intracratonic basins: evidence for asthenospheric diapirs

The three intracratonic sedimentary basins located in central Baltoscandinavia, namely the Bothnian Gulf basin, the Bothnian Sea basin and the Baltic basin, developed in response to Middle Proterozoic and Late Proterozoic tectonic events, separated in time by about 800 Ma. Only the Baltic basin was subsequently affected by Caledonian orogenesis and Mesozoic rifting. Crustal extension was minor or did not take place during the Proterozoic basin evolution phases. However, according to the Moho topography, crustal thinning did take place. This was probably a result of subcrustal magmatism. On a craton-wide scale, the ages of granitoids, which intruded during the Middle Proterozoic basin formation, generally decrease from east to west. This fact, combined with the evidence provided by mantle-derived flood basalt magmatism, points to a moving asthenospheric diapir as the cause for basin development. Asthenospheric upwelling was probably also responsible for the second, Late Proterozoic, basin evolution phase, as evidenced by the lack of crustal thinning and extension, and the occurrence of tholeiitic intrusions. In addition, a Late Proterozoic thermally induced palaeo-high, located at about the position of the intracratonic basins, is compatible with indications from glaciations. As the ages of Late Proterozoic intracratonic basins also decrease from east to west across the craton, the location of asthenospheric diapirism during this time interval was also moving. For the Fennoscandian lithosphere, the presence of fundamental lithospheric weakness zones (e.g. terrane boundaries) might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the Fennoscandian Shield. The spacing and size of the Proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated. Therefore, sublithospheric convective processes might be the cause for the asthenospheric upwellings. Such processes are related to Rayleigh–Taylor instabilities in the sublithospheric mantle. Emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere. Modelling results demonstrate that erosion of the surficial high, succeeded by cooling of the lithosphere, can explain the accumulation of early Palaeozoic sediments in the Bothnian Sea basin, taking into account post-Ordovician vertical and lateral erosion of the basin fill.     

克拉通内盆地研究中的几个重要问题

克拉通内盆地不仅记录了克拉通在显生宙的演化历史，而且是油气富集的重要场所。本文提出了在克拉通内盆地研究中的几个重要问题：（１）克拉通内盆地基底的非均一性；（２）克拉通内盆地地层层序及其边界不整合分析；（３）克拉通内盆地沉降史分析；（４）克拉通内盆地成因探讨；（５）克拉通内盆地演化与板块构造作用的关系。通过对这些问题的研究，可以深入理解克拉通内盆地形成、充填、改造作用及其对油气分布的控制。     

四川乐山震旦系灯影组火山碎屑岩锆石LA-ICP-MSU-Pb定年及盆地裂陷演化讨论

四川盆地中部晚震旦(埃迪卡拉)世—早寒武世克拉通内裂陷与安岳震旦(埃迪卡拉)系—寒武系特大气田的形成关系密切,年代学研究对认识克拉通内裂陷构造意义重大。对四川盆地乐山先锋剖面灯影组三段火山碎屑岩进行锆石LA-ICP-MS U-Pb定年,研究结果表明四川盆地灯影组三段火山碎屑岩的锆石U-Pb年龄为(539.6±1.4)Ma,年代学结果表明四川盆地灯三段与灯四段属于寒武系。基于灯影组底界(551.1 Ma)、含火山碎屑岩的灯三段(539.6 Ma)、含小壳化石的麦地坪组(535.2 Ma)和下寒武统Ni—Mo多金属硫化物富集层(521.0 Ma)四个等时面构建了克拉通内裂陷演化的时间框架。在此基础上认为四川盆地中部晚震旦(埃迪卡拉)世—早寒武世隆坳构造的形成主要受边界断裂控制,断层活动使裂陷内水体加深,少量灯四段含硅质白云岩沉积后,麦地坪组与筇竹寺组开始沉积,麦地坪组与灯四段的界线在裂陷内与裂陷两侧并不等时,裂陷内麦地坪组、筇竹寺组与裂陷两侧灯影组四段为同期异相沉积。     

新疆塔中石炭系层序地层学研究——一个克拉通内坳陷盆地的层序地层框架模式

塔中地区石炭纪原型沉积盆地属挤压性克拉通内坳陷盆地。在石炭系中可识别出3个Ⅰ型层序界面和3个Ⅱ型层序界面,据此,划分出5个沉积层序。它们具有标准层序所具有的地层特征,相当于三级海平面升降旋回的沉积单元。在区域上或全球范围内这些旋回基本上具可对比性。与被动大陆边缘型盆地相比,其层序地层框架具有明显的差异,主要表现为缺失低水位早、中期的沉积体系,深切河谷不发育,代表最大海泛期的凝缩层不发育;Ⅱ型层序界面较普遍,海岸平原沉积体系发育,在潮湿气候条件下常与陆架型扇三角洲体系共生,组成海陆交互的岩相框架。     

On the Carboniferous Sequence Stratigraphy in the Tazhong Area, Xinjiang——A Model of the Sequence Stratigraphy Framework of Intracratonic Depressional Basins

The Carboniferous prototype sedimentary basin in the Tazhong (Central Tarimbasin) area is recognized as a compressive intracratonic depressional one. Three type Ⅰ sequenceboundaries and three type Ⅱ sequence boundaries can be identified in the CarboniferousSystem, which can accordingly be divided into five sedimentary sequences. These sequencespossess stratigraphic characters of the standard sequence and correspond to the depositionalstratigraphic unit of a third-order eustatic cycle. They can be regionally or globally correlatedwith each other. The framework of sequence stratigraphy of the intracratonict basin in thestudy area distinctly differs from that of the passive continental-margin basin in the lack ofdepositional systems of early-middle lowstand, poor development of the deeply incised valleyand condensed section of the maximum sea-flood, good development of type Ⅱ sequenceboundaries and coastal plain depositional systems coexisting with shelf-type fan deltas underwet climatic conditions, Which consequently led to the formation of a paralic lithofacies frame-work.     

Regional high-pressure metamorphism during intracratonic deformation: the Petermann Orogeny, central Australia

The Petermann Orogeny is a late Neoproterozoic to Cambrian ( c . 560–520  Ma) intracratonic event that affected the Musgrave Block and south-western Amadeus Basin in central Australia. In the Mann Ranges, within the central Musgrave Block, Mesoproterozoic granulite facies gneisses, granites and mafic dykes have been substantially reworked by deep crustal non-coaxial strain of late Neoproterozoic to early Cambrian age. Dolerite dykes have recrystallized to garnet granulite facies assemblages, associated with the development of a mylonitic fabric at P =12–13  kbar and T  =700–750 °C. Migmatization is restricted to discrete shear zones, which represent conduits for hydrous fluids during metamorphism. Peak metamorphism was followed by decompression to c . 7  kbar, reflecting exhumation of the terrane along the south-dipping Woodroffe Thrust. In scattered outcrops north of the Mann Ranges, peak metamorphism occurred at P =9–10  kbar and T  = c . 700 °C. The Woodroffe Thrust separates these deep crustal mylonites from granites that were metamorphosed during the Petermann Orogeny at P = c . 6–7  kbar and T  = c . 650 °C. The similarity in peak temperatures at different crustal levels implies an unusual thermal regime during this event. The existence of a relatively elevated geotherm corresponding with Th- and K-enriched granites that were in the mid-crust during the Petermann Orogeny suggests that radiogenic heat production may have substantially contributed to the thermal regime during metamorphism. This potentially has implications for the mechanisms by which intra-plate strain was localized during this event.     

中国西北地区早—中侏罗世盆地原型分析

文中进行了中国西北地区早—中侏罗世盆地的原型分析 ,着重阐述 3点 :( 1)根据盆地构造特征 ,将中国西北地区早—中侏罗世盆地划分为 :缓断面伸展断陷盆地、陡断面伸展断陷盆地、克拉通内盆地和克拉通周边盆地 4种类型。 ( 2 )这 4类盆地分布于不同的前侏罗纪构造背景之上。第Ⅰ类和第Ⅱ类伸展断陷盆地位于印支晚期造山带和印支晚期复活的古生代造山带 ,它们系造山带伸展垮塌作用的产物。第Ⅲ类克拉通内盆地分布于克拉通块体内部 ,它们具有冷的刚性岩石圈 ,其古地温梯度达不到使岩石圈弱化和产生伸展断陷盆地的临界值。第Ⅳ类克拉通周边盆地位于克拉通和造山褶皱带之间 ,这类盆地的古地温梯度刚达到形成伸展断陷盆地临界值 ,在应变速率低的情况下造成不对称的宽阔凹陷。 ( 3)由于这 4类盆地的地温梯度、演化和改造程度不同 ,它们的含油气远景差别很大 ,其中以克拉通周边盆地最有希望。     

Reviews

The origin of granite

The Granite Controversy, by H. H. Read, xix + 430 pp. Thomas Murby & Co., London, 1957.

Geology of the Pleistocene

Glacial and Pleistocene Geology by R. F. Flint, xiii‐553 pp., 4 pis., figs., J. Wiley & Sons Inc. New York; Chapman & Hall Ltd., London, 1957, 4 plates and numerous figures. $12.50.     

Isotopic and sedimentological clues to productivity change in Late Riphean Sea: A case study from two intracratonic basins of India

Enriched¹³C/¹²C ratios with δ¹³C ∼3%0 (w.r.t PDB) of two Late Riphean (∼ 700-610 Ma) intracratonic carbonate successions viz., Bhander Limestone of Vindhyan Basin and Raipur Limestone of Chattisgarh Basin suggest higher organic productivity during this period. This view is supported by sedimentological evidence of higher biohermal growth and consequent increase in depositional relief in the low gradient ramp settings inferred for these basins. Oxygen isotope analysis of these carbonates show distinct segregation between enriched deeper water carbonate mudstone and depleted shallow water stromatolite facies that received fresh water influx. This shows that facies-specific analyses can be useful in understanding the depositional setting of these sediments.