克拉通对比与地球动力学数值模拟对克拉通岩石圈减薄的启示

发布时间：2018-06-21 01:23

  本文选题：克拉通破坏 + 岩石圈减薄　； 参考：《中国地质大学》2017年博士论文

【摘要】：克拉通是地球自形成以来较先固结的浅部地质单元,占现今全球陆地面积的50%以上。由于其面积大、厚度大、地温梯度低、缺乏易熔组份、密度低、粘度大等因素的影响,使得克拉通岩石圈不易受到后期地质作用的破坏而失稳。然而,最近研究表明某些克拉通(如华北克拉通东部、北大西洋克拉通、坦桑尼亚克拉通局部)岩石圈并不像过去所认为的那样稳定,而是也会发生再活化和破坏。但是对于何种驱动力和何种破坏机制导致了再活化和破坏,依然存在较大争议。华北克拉通,作为世界上研究程度最高的克拉通之一,其岩石圈破坏现象为举世所瞩目。其岩石圈破坏期间经历了复杂的地质作用,所涉及的驱动力包括俯冲作用、碰撞作用和地幔上涌或地幔柱作用,相关的减薄机制包括伸展减薄、拆沉减薄、对流侵蚀减薄、水弱化、熔体-橄榄岩反应等,使得华北克拉通岩石圈减薄与破坏的主要驱动力和机制存在诸多争议。但相比之下,北大西洋、坦桑尼亚克拉通破坏期间所经历的地质作用则较为简单,主要为地幔上涌或地幔柱作用相关的热机械侵蚀和伸展减薄。本文通过将这三个克拉通以及全球范围内其它克拉通的地质演化历史、岩石圈改造历史、岩石圈减薄地球物理特征进行对比,定性分析了克拉通边缘和内部减薄的异同点,以及大陆单元不同位置处岩石圈减薄的异同点。定性对比分析显示:(1)边缘减薄主要与俯冲、碰撞、重复伸展相关的交代作用、岩浆作用、拆沉作用、变克拉通化、熔体-橄榄岩反应、流体弱化等作用相关有关,减薄的尺度可达下地壳深度。(2)而内部减薄则与大规模的伸展作用、大规模的脱水和水弱化和对流侵蚀作用、大规模的岩石圈拆沉作用相关。大尺度的岩石圈减薄还可能受到岩石圈本身稳定性的影响。例如岩石圈地幔中层深度(MLD,80-120 km)弱耦合层会使得岩石圈底部发生水平方向的迟滞和错位,并产生减薄现象。(3)活动大陆边缘受俯冲相关的熔体、流体以及深部地幔对流侵蚀作用影响。此外还受(2)中所述的MLD之下岩石圈水平错动的影响。(4)被动大陆边缘则受岩石圈由厚到薄转换或地幔柱产生的次生对流侵蚀的破坏。此外还受(2)中所述的MLD之下岩石圈水平错动的影响。基于以上对比工作和定性分析,笔者选取克拉通内部大规模减薄模型中的几个可能机制(大规模的脱水和水弱化和对流侵蚀作用、大规模的岩石圈拆沉作用和大规模岩石圈水平方向的迟滞和错位)进行地球动力学数值模拟定量分析研究。笔者研究发现:(1)深俯冲的平卧板片大规模扰动富水地幔转换带(≥0.6 wt%),引起软流圈和克拉通岩石圈水弱化和次生对流加强可以导致克拉通岩石圈的大规模减薄。(2)其中富水地幔转换带中的水主要来自于之前多方向深俯冲板片在地幔转换带中弯曲、堆积甚至崩塌、穿透到下地幔时所产生的板片脱水作用和地幔转换带吸水作用。(3)下地壳尺度的拆沉主要发生于克拉通边缘和加厚的大陆高原区域。对于克拉通内部的岩石圈拆沉,MLD深度(80-120km)比下地壳深度(20-40 km)要更容易发生。(4)对于岩石圈密度较小的克拉通,在大陆漂移过程中会发生岩石圈底部沿MLD(80-120km)弱耦合层的解耦,使得岩石圈底部被水平迟滞和错位到大陆漂移尾端的大洋岩石圈之下,并使得大陆漂移前端的岩石圈发生减薄。在某些克拉通区域,MLD的性质与岩石圈-软流圈界面性质相似,可以作为板块运动的底界面。以上数值模拟研究内容和结果均可以与克拉通对比中所发现的关键地质和地球物理实例进行较好的对比,用地球动力学和地质、地球物理资料相结合的思路支持了笔者观点在克拉通演化过程中的可适用性。
[Abstract]:Craton is the first consolidation of the earth's shallow geological unit since its formation, accounting for more than 50% of the global land area today. Due to its large area, large thickness, low geothermal gradient, lack of infusible components, low density, large viscosity and other factors, the cratonic lithosphere is not easily damaged by the failure of later geological action. However, recent research has been studied. It shows that some cratonic rocks (such as the eastern North China Craton, the North Atlantic craton, and the local kcraton of Tanzania) are not as stable as they were thought in the past, but also reactivation and destruction. But there is still a big controversy over what driving force and the destruction mechanism has led to reactivation and destruction. As one of the highest degree of cratonic research in the world, its lithospheric destruction has attracted the attention of the world. Its lithosphere has undergone complicated geological processes during the lithosphere destruction, and the driving forces involved include subduction, collision and mantle upwelling or mantle plume, and the thinning mechanism of the lithosphere includes extensional thinning, sedimentation reduction and convection. There are many disputes over the main driving forces and mechanisms of the thinning and destruction of the North China Craton lithosphere, but in contrast, the geological effects of the North Atlantic and Tanzania craton are relatively simple, mainly due to the thermal machinery related to the mantle upwelling or the mantle plume. By comparing the geological evolution history of the three cratonic cratonic and other cratonic cratats worldwide, the history of the lithosphere transformation and the lithosphere thinning of the geophysical characteristics, this paper qualitatively analyzed the similarities and differences between the cratonic edge and the internal thinning, as well as the similarities and differences between the lithosphere thinning at different locations in the continental unit. The qualitative contrast analysis shows: (1) the thinning of the edges is mainly related to subduction, collisions, repeated extension related metasomatism, magmatism, delamination, carat Tonghua, melts peridotite reaction, fluid weakening and so on, and the thinning scale can reach the lower crust depth. (2) the internal thinning is extended with large-scale extension and large-scale removal. The weakening of water and water and convective erosion are related to the large-scale lithosphere delamination. Large scale lithosphere thinning may also be affected by the stability of the lithosphere itself. For example, the weak coupling layer of the middle layer of the lithosphere mantle (MLD, 80-120 km) will cause the sluggish and dislocation of the square direction of water in the bottom of the lithosphere and the thinning phenomenon. (3) The active continental margin is affected by subduction related melts, fluids and convective erosion in the deep mantle. In addition, the influence of the lithospheric horizontal dislocation below MLD is also affected. (4) the passive continental margin is damaged by the secondary opposite flow erosion caused by the lithosphere from thick to thin or from the mantle plume. In addition, under the (2) of the (2) MLD Based on the above comparison work and qualitative analysis, the author selects several possible mechanisms in the mass thinning model within the craton (large scale dehydration and water weakening and convective erosion, large-scale lithospheric dehydration and large scale hysteresis and dislocation in the horizontal direction of the rock circle). The study found that: (1) the deep subducted flat plate with large scale disturbed water rich mantle transition zone (> 0.6 wt%) caused the weakening of the water and the secondary convection in the asthenosphere and cratonic lithosphere. (2) the water in the water rich mantle transition zone is mainly derived from the water. The previous multi directional deep subduction plates bend in the mantle transition zone, accumulate even collapse, and penetrate into the lower mantle. (3) the reduction of the lower crustal scale mainly occurs at the cratonic edge and the thickened continental plateau region. The lithosphere disintegration within the craton, MLD depth (80) -120km) is more likely to occur than the lower crust depth (20-40 km). (4) for the craton with smaller lithosphere density, the bottom of the lithosphere will be decoupled from the weak coupling layer of the MLD (80-120km) in the course of the continental drift, causing the bottom of the lithosphere to be stagnant and misplaced to the oceanic lithosphere at the end of the continental drift, and before the drift of the continent. In some cratonic regions, the properties of MLD are similar to the lithosphere and asthenosphere interface, which can be used as the bottom interface of the plate movement. The above numerical simulation research and results can be compared with the key geological and geophysical examples found in the craton comparison, using geodynamics. The combination of geological and geophysical data supports the applicability of the author's viewpoint in the evolution of craton.
【学位授予单位】：中国地质大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P541;P31
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本文编号：2046540