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961.
Luc-Emmanuel Ricou 《Tectonophysics》2004,384(1-4):285-300
Episodic plate reorganisations abruptly change plate boundary configurations. To illustrate their role, we review the plate reorganisations that appear in the present-day oceans and in the reconstructed Tethys ocean. These time periods cover the dispersal of the Pangea super-continent and the collisions with Eurasia that foreshadow a new super-continent. Plate reorganisations have played a fundamental role in the tectonic history of the Earth, being responsible for continental break-up and, after oceanic spreading, for continental collisions. As a result, they governed the formation and dispersal of super-continents. We observe a bulk polarity in plate motion that governs continental collision and the opposite bulk polarity in plate reorganisation that governs continental break-up. Such opposite polarities show in the tectonic history that we follow since the 550 Ma formation of the Gondwana super-continent.In order to decipher the rules that govern plate reorganisation, we investigate the distribution of spreading and subduction that derives from the current plate motion. We observe a mismatch between the evolution tendency of the plate boundary network and convection in the deep mantle. The actual network of plate boundaries illustrates a compromise between the two. Based on the opposite polarities in plate motion and plate reorganisation, we propose that this compromise is maintained by plate reorganisations that counterbalance free evolution of the network in abruptly changing its boundaries. We propose that plate reorganisations are basically caused by the mismatch between the free evolution of the plate boundary network and the current convection pattern in the deep mantle.Evidence on Proterozoic rifting and continent collisions allows dating the oldest known plate reorganisation around 2 Ga, which is the age of the oldest known super-continent. Based on the geology of the Archean before 3 Ga, mantle convection appears limited under a greenstone cover and different from the current mantle convection. The distribution of the diapiric granitoids that intrude this cover points to a honeycomb convection centred on downwelling sites separated by diffuse upwelling, which fits the theory on the early Earth mantle convection when plates did not cover the globe. We propose that the plate reorganisation regime appeared sometime between 3 and 2 Ga. 相似文献
962.
Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale (x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations. 相似文献
963.
由于青藏高原东部地区记录了高原约50 Ma演化历史中物质东流的构造史,因此受到地学界的广泛重视. 现代大地测量与地质研究结果给出了该区现代地壳运动的图像,为地球动力学数值模拟提供了重要的边界约束条件. 利用重力异常计算的高原及邻区地幔对流应力场与地表地壳运动格局的明显差异表征了高原东部地壳与地幔物质的运动解耦. 基于随深度变化地壳蠕变率的动力学模拟结果显示,高原东部地壳增厚与高原内部存在很大差异,高原东部地壳增厚主要表现为下地壳的增厚,并且地幔形变过程与地表变化也不一致,同样显示出地壳、地幔运动的解耦. 研究表明,下地壳低强度分布可能是导致这种解耦的重要原因,而了解高原东部地壳及上地幔物理力学性质对我们认识高原物质东流至关重要. 相似文献
964.
上海“0185”特大暴雨过程天气形势分析 总被引:10,自引:2,他引:10
对上海2001年8月5-6日特大暴雨的天气形势做了详细分析指出,在副高内由东风系统发展起来的热带低压由于结构深厚得以在强大副高内稳定维持,而副热带高压断裂的形势变化至关重要,由于西环副高北侧的西北偏西气流渗入,对热带低压内强对流迅速、持久发展起了重要作用,同时东环副高减退,又成为热带低压路径突然东折的主要因素。 相似文献
965.
966.
利用原始方程模式讨论了不同大气层结对冷锋环流的影响及其在激发锋区中尺度强对流系统中的作用,结果表明:层结对锋区环流有非常大的影响,随层结稳定度的减小,锋区非地转越锋环流和上升运动迅速增强,锋前暖区有可能产生中尺度强对流和多重中尺度上升运动带,并有利于对称不稳定的激发;层结对冷锋环流的影响和锋区中尺度强对流系统形成的作用主要由中低层的大气层结状态决定 相似文献
967.
968.
969.
970.
Cooling the core and mantle by plume and plate flows 总被引:2,自引:0,他引:2
Geoffrey F. Davies 《Geophysical Journal International》1993,115(1):132-146