Mantle convection modeling of the supercontinent cycle： Introversion,extroversion, or a combination？

The periodic assembly and dispersal of continental fragments,referred to as the supercontinent cycle,bear close relation to the evolution of mantle convection and plate tectonics.Supercontinent formation involves complex processes of"introversion"(closure of interior oceans),"extroversion"(closure of exterior oceans),or a combination of these processes in uniting dispersed continental fragments.Recent developments in numerical modeling and advancements in computation techniques enable us to simulate Earth’s mantle convection with drifting continents under realistic convection vigor and rheology in Earth-like geometry(i.e.,3D spherical-shell).We report a numerical simulation of 3D mantle convection,incorporating drifting deformable continents,to evaluate supercontinent processes in a realistic mantle convection regime.Our results show that supercontinents are assembled by a combination of introversion and extroversion processes.Small-scale thermal heterogeneity dominates deep mantle convection during the supercontinent cycle,although large-scale upwelling plumes intermittently originate under the drifting continents and/or the supercontinent.     

Supercontinent formation from stochastic collision and mantle convection models

The large-scale tectonics in the last billion years (Ga) are predominated by the assembly and breakup of supercontinents Rodinia and Pangea. The mechanisms controlling the assembly of supercontinents are not clear. Here, we investigate the assembly of a supercontinent with 1) stochastic models of randomly-moving continental blocks and 2) 3-D spherical models of mantle convection with continental blocks. For the stochastic models, we determined the time required for all the blocks to assemble into a single supercontinent on a spherical surface. We found that the assembly time from our stochastic models is significantly longer than inferred for Pangea and Rodinia. However, our study also suggests that the assembly time from stochastic models is sensitive to the rules for randomly assigning continental motion in the models. In our dynamic models of mantle convection, continental blocks are modeled as deformable and compositionally distinct materials from the mantle. We found that mantle convective planform has significant effects on supercontinent assembly. For models with moderately strong lithosphere and the lower mantle relative to the upper mantle that lead to degree-1 mantle convection, continental blocks always assemble to a supercontinent in  250 million years (Ma) and this assembly time is consistent with inferred for Pangea and Rodinia. However, for models with intrinsically small-scale mantle flows, we found that even when continental blocks merge to form a supercontinent, the assembly times are too long and the convective structures outside of supercontinent regions are of too small wavelengths, compared with observed.     

The making and breaking of supercontinents: Some speculations based on superplumes, super downwelling and the role of tectosphere

The mechanisms of formation and disruption of supercontinents have been topics of debate. Based on the Y-shaped topology, we identify two major types of subduction zones on the globe: the Circum-Pacific subduction zone and the Tethyan subduction zone. We propose that the process of formation of supercontinents is controlled by super downwelling that develops through double-sided subduction zones as seen in the present day western Pacific, and also as endorsed by both geologic history and P-wave whole mantle tomography. The super-downwelling swallows all material like a black hole in the outer space, pulling together continents into a tight assembly. The fate of supercontinents is dictated by superplumes (super-upwelling) which break apart the continental assemblies. We evaluate the configuration of major supercontinents through Earth history and propose the tectonic framework leading to the future supercontinent Amasia 250 million years from present, with the present day Western Pacific region as its frontier. We propose that the tectosphere which functions as the buoyant keel of continental crust plays a crucial role in the supercontinental cycle, including continental fragmentation, dispersion and amalgamation. The continental crust is generally very thin, only about one tenth of the thickness of the tectosphere. If the rigidity and buoyancy is derived from the tectosphere, with the granitic upper crust playing only a negligible role, then supercontinent cycle may reflect the dispersion and amalgamation of the tectosphere. Therefore, supercontinent cycle may correspond to super-tectosphere cycle.     

华北克拉通对前寒武纪超大陆旋回的基本制约

全球大陆克拉通在前寒武纪至少记录了3次超大陆聚合－裂解的构造旋回。不同大陆前寒武纪地质的研究证明，板块的构造模式可以前推至新太古代。超大陆的聚合表现为大规模造山带的穿时性发育，而裂解则表现为大陆裂谷系、非造山花岗岩及巨型基性岩浆岩省的同期快速发育。广泛的区域地质研究揭示华北克拉通前寒武纪地质构造演化具有明显的阶段性差异特征，克拉通主体形成于新太古代陆壳增生与碰撞造山过程。华北克拉通在太古宙末期首次经历强烈的裂解作用，在古元古代晚期涉及强烈的陆缘再造作用。在古元古代末期发生第二次大规模的裂解活动，随后以中元古代末期的造山带拼合为Rodinia超大陆的组成部分。详细的区域构造对比证明，华北克拉通长期以来与波罗的地质、东南极克拉通、印度南部克拉通、巴西克拉通等具有构造亲缘关系。     

Annual report of IGCP Project No.440 in 2003--Rodinia assembly and breakup

With the latest information on geology, isotop chronology, geochemistry and aerial geophysics, the structural enviroment, geological event characterists and evolution history of component units of Rodinia Supercontinent on a global scale are discussed. And some neo views and genetic pattern are provided. The East Eurppean Cratorn had a complex evolution history between 1.7 and 0.9 Ga. The arthors propose a new reconstruction of Laurentia acient land and Siberia at ca. 1 050～1 000 Ma. The largest litho-structural record ofthe Meso-Neoproterozoic orogenic collage in South America made up the western border of the South American Platform African Cratons are the result of convergence of Paleoproterozoic/Archaem Cratonic blocks. A part of Eastern Antarctica attached to southern Africa in Mesoproterozoic.Neoproterozoic felsic magmatic events in New India made the western border of Rodinia Pre-Grevoillian Laurentia was established as a major continental block by the end of the Paleoproterozoic. South China is geologically plausible to be between southern Laurentia and eastem Australia. Yangzi-Tarim connection or neighborhood is proposed. According to the abovementionded, the assembly and breakup paattern of Rodinia proposed by Pisarevsky is tested. It telles that primary break up is along the western border of Laurentia ancient land, which is similar to northem Atlantic. Another characteristic is that some continents are not considered as component parts of Rodinia, eg. India, Congo and San-Francisco.     

东昆仑夏日哈木矿区新元古代早期二长花岗岩锆石U-Pb年代学、地球化学及其构造意义

本文报道了东昆仑造山带中部夏日哈木矿区二长花岗岩体的锆石U-Pb年代学和全岩地球化学资料,以确定岩体的形成时代、岩石成因及其构造属性。夏日哈木矿区的二长花岗岩呈岩株状和脉状捕虏体出露于矿区中部。2件二长花岗岩样品中岩浆锆石LA-MC-ICP-MS U-Pb加权平均年龄分别为923.7±2.5 Ma(MSWD=0.27)和920.1±2.8 Ma(MSWD=0.18),属新元古代早期。岩石高硅(SiO_2=73.24%~73.83%)、富碱(Na_2O+K_2O=7.96%~9.79%)、贫钙(CaO=0.31%~1.13%)、贫镁(MgO=0.11%~0.21%),属于高钾钙碱性、过铝质系列花岗岩;稀土配分曲线呈现轻稀土元素相对富集的右倾分布特征,具明显的Eu负异常(δEu=0.30~0.45);原始地幔标准化微量元素蛛网图显示Sr、Ba、Nb、P、Eu和Ti的负异常。岩石学及地球化学特征表明其属于S型花岗岩。岩石具有较低的CaO/Na_2O比值(介于0.13~0.26之间,平均0.19),较高的Rb/Sr比值(介于1.27~12.45之间,平均7.61),显示出上部地壳源区的成分特征,可能由泥质岩石部分熔融形成。结合区域构造演化及构造判别,本文认为该区二长花岗岩形成于同碰撞的构造环境,岩石成因可能与加厚陆壳上部泥质岩石减压熔融并经历了斜长石等矿物的分离结晶作用有关,热源主要来自于陆壳加厚过程中K、Th、U等元素放射性蜕变产生的热量。综合分析认为,东昆中隆起带中部地区存在新元古代早期的岩浆活动,时间上对应于全球Rodinia超大陆的汇聚时间。     

云南东川望厂组熔结凝灰岩锆石SHRIMP U Pb年龄及其意义

本文报道了云南东川地区望厂组中熔结凝灰岩锆石的SHRIMP U-Pb年龄。其中自形结构、环带清晰锆石的不一致曲线与谐和线上交点的207Pb/206Pb年龄分别为2317±13Ma(n=13,MSWD=3.3),13个点的加权平均年龄为2299±14Ma(n=13,MSWD=5.9),表明望厂组地层的沉积时代为古元古代早—中期。东川地区下四组地层与滇中昆阳群(黄草岭组—美党组)和四川会理群(淌塘组—天宝山组)的沉积时代相差近1.0Ga。为了区别于昆阳群和会理群,建议对本区下四组地层(洒海沟组、望厂组、菜园湾组、平顶山组)独立建群,即汤丹群。汤丹群是目前为止康滇地区已发现的最古老的地层建造之一,经历过古元古代中期的挤压过程(东川运动)和古元古代末期的拉张过程,可能是相对于扬子古陆核独立存在、并构成早—中元古宙Columbia超级大陆的一个小的古老陆块。     

华北基性岩墙群的古地磁极及其哥伦比亚超大陆重建意义

从华北克拉通的中西部和东部发育的基性岩墙群获得三个精确定年的实际古地磁极位置(VGPs)。华北克拉通中西部恒山GU岩墙的斜锆石U-Pb年龄为1769±3Ma,该岩墙的古地磁给出古地磁极位置在北纬36°,东经247° (dp=2, dm=4); 华北克拉通东部DY岩墙的斜锆石U-Pb年龄为1620.8±6.9Ma, 获得的古地磁极位置为北纬6.9°东经81.9°(dp=4.31, dm=6.09); 东部另一条岩墙LW4的SHRIMP U-Pb 年龄为1157±18Ma, 获得的古地磁极位置为北纬27.1°东经162°(dp=15.8, dm=18.7)。通过将这三个从基性岩墙获得的华北克拉通古地磁极位置与加拿大地盾的中元古代视极移曲线对比,在中元古代早期（1800~1600Ma）,华北克拉通与加拿大地盾一起同属哥伦比亚超大陆,而在中元古代晚期（1600~1200Ma）,华北克拉通与加拿大地盾分离,响应哥伦比亚超大陆的最终裂解事件。     

http://www.sciencedirect.com/science/article/pii/S1674987113000558

Several stratigraphic breaks and unconformities exist in the Mesoproterozoic successions in the northern margin of the North China Block.Geologic characters and spatial distributions of fve of these unconformities,which have resulted from different geological processes,have been studied.The unconformity beneath the Dahongyu Formation is interpreted as a breakup unconformity,representing the time of transition from continental rift to passive continental margin.The unconformities beneath the Gaoyuzhuang and the Yangzhuang formations are considered to be the consequence of regional eustatic fuctuations,leading to the exposure of highlands in passive margins during low sea-level stands and transgressive deposition on coastal regions during high sea-level stands.The unconformity atop the Tieling Formation might be caused by uplift due to contractional deformation in a back-arc setting,whereas the uplift after the deposition of the Xiamaling Formation might be attributed to a continental collision event.It is assumed that the occurrences of these unconformities in the Mesoproterozoic successions in the northern margin of the North China Block had a close bearing on the assemblage and breakup of the Columbia and Rodinia supercontinents.     

华北克拉通北部长城系底界年龄小于1670Ma:来自北京密云花岗斑岩岩脉锆石LA-MC-ICPMS U-Pb年龄的约束

近期在北京密云沙厂北东的大龙门村附近发现一条花岗斑岩岩脉,该岩脉侵入于新太古代密云群角闪斜长片麻岩当中,其顶部则与片麻岩一起被常州沟组含砾砂岩沉积不整合覆盖.对该岩脉采用LA-MC-ICPMS进行锆石U-Pb同位素年龄测定,获得了(1 673±10)Ma的侵位年龄.这表明华北北部的常州沟组底界(也即长城系的底界)年龄小...