The dominant driving force for supercontinent breakup: Plume push or subduction retreat?

Understanding the dominant force responsible for supercontinent breakup is crucial for establishing Earth's geodynamic evolution that includes supercontinent cycles and plate tectonics. Conventionally,two forces have been considered: the push by mantle plumes from the sub-continental mantle which is called the active force for breakup, and the dragging force from oceanic subduction retreat which is called the passive force for breakup. However, the relative importance of these two forces is unclear. Here we model the supercontinent breakup coupled with global mantle convection in order to address this question. Our global model features a spherical harmonic degree-2 structure, which includes a major subduction girdle and two large upwelling(superplume) systems. Based on this global mantle structure,we examine the distribution of extensional stress applied to the supercontinent by both subsupercontinent mantle upwellings and subduction retreat at the supercontinent peripheral. Our results show that:(1) at the center half of the supercontinent, plume push stress is ～3 times larger than the stress induced by subduction retreat;(2) an average hot anomaly of no higher than 50 K beneath the supercontinent can produce a push force strong enough to cause the initialization of supercontinent breakup;(3) the extensional stress induced by subduction retreat concentrates on a ～600 km wide zone on the boundary of the supercontinent, but has far less impact to the interior of the supercontinent. We therefore conclude that although circum-supercontinent subduction retreat assists supercontinent breakup, sub-supercontinent mantle upwelling is the essential force.     

Spatiotemporal characteristics of Qinghai Lake ice phenology between 2000 and 2016

Journal of Geographical Sciences - Lake ice phenology is considered a sensitive indicator of regional climate change. We utilized time series information of this kind extracted from a series of...     

海勃湾水库用于黄河内蒙古河段防凌作用的研究

黄河内蒙古河段冰期一般在冬季封河期和春季开河期０容易出现严重凌情,造成灾害。故良好的封河形势和开河形势是避免凌灾发生的重要条件,自１９６８年刘家峡水库运用以来,黄河内下河段凌情已大有缓解。但是,由于有峡水库距黄河内下河段较远。对河道凌情变化反应不够灵敏。因此,一些年份仍有较为严重的凌灾发生。在分析黄河内蒙古河段封、开河形势特点的基础上,深入研究了海勃湾水库用于该段春季防凌调度的作用和能力,为黄河内     

藏南康马地区三叠系研究新进展

根据康马地区三叠系吕村组和涅如组新发现的化石及区域地质背景 ,把该区的地层时代分别厘定为中三叠世中晚期至晚三叠世早期和晚三叠世中晚期 ,并认为缺失早三叠世至中三叠世早期的沉积。研究表明康马地区三叠系与二叠系之间为微角度不整合接触 ,是“藏南运动”和“印支伸展运动”共同影响形成的。晚二叠世末至中三叠世早期 ,康马地区露出海面 ,接受剥蚀并形成喀斯特化风化壳。中三叠世末至晚三叠世 ,这一地区发生强烈伸展—裂陷 ,地壳迅速沉降 ,形成被动大陆边缘裂谷盆地 ,发育巨厚的半深海—深海复理石沉积 ,并伴随大量基性岩浆贯入。涅如组下部有两期基性岩侵入 ,早期基性岩床形成于印支晚期 ,晚期穿层侵入形成于燕山早期。     

塔中地区志留系碎屑锆石测年及其地质意义

志留系是塔里木盆地第一套砂岩储层广泛分布的沉积盖层,其沉积来源与成因对志留纪构造演化及周边造山带的研究具有重要意义。塔中地区3个志留系样品的碎屑锆石LA?ICP?MS U?Pb定年研究表明,志留系具有比较集中的二期物源年龄：南华纪中期、古元古代中期。碎屑锆石测定的年龄表明塔中志留系物源均来自前寒武纪,塔中东部源区方向的塔南隆起基底在奥陶纪已隆升成为蚀源区。大量的新元古代中期锆石年龄表明塔里木板块在新元古代时期可能与Rodinia超大陆具有相似的聚合与裂解演化史。     

华北古大陆与哥伦比亚超大陆

由Rogers和Santosh等 (2 0 0 2 )提出的哥伦比亚超大陆 ,是约从 1.9～ 1.5Ga由Nena ,Ur和Atlantic等 3个大陆块体群 ,通过逐步汇聚而形成的一个超级大陆。它是前罗迪尼亚古 -中元古时期的超大陆。从 1.5Ga开始的裂解作用使哥伦比亚超大陆逐步破裂 ,并在 1.0Ga左右这些破裂的大陆块体又重新汇聚形成罗迪尼亚超大陆。文中除介绍Rogers等提出的哥伦比亚超大陆的概念、组成和古大陆重建图外 ,重点阐述了中国华北古大陆 2 .0～ 1.8Ga期间吕梁—中条造山运动和 1.8～1.6Ga时期大规模裂解事件群的性质、特点和同位素年龄数据。文中提出哥伦比亚超大陆汇聚主要峰期与吕梁—中条造山运动的时限相一致 ,华北古大陆属于哥伦比亚超大陆的组成部分 ,并可能为Nena大陆块体群的一员 ,在造山及裂解事件群的性质、特点和时代等特征上 ,华北与北美、西伯利亚和西北欧有更大的相似性。     

早期碰撞造山过程与板块构造：前寒武纪地质研究的机遇和挑战

普遍认为修正后的板块构造模式仍适用于新太古代地质研究，但是早期板块构造过程与后期有明显差异，包括陆块规模、造山带类型、碰撞造山过程等。典型碰撞造山带在地史上的初次形成具有划时代的构造演化意义，涉及典型板块构造初始发生过程、最早超级大陆拼合、威尔逊旋回及板块碰撞造山过程等方面。华北中部保留一条近南北向的碰撞造山带（2 600~2 500 Ma BP），保留特征的巨型复式褶皱、不同层次推覆构造、蛇绿岩混杂带等。围绕华北中部造山带及其25亿年重大构造热事件的研究，对认识华北早期构造演化及其超大陆再造具有重要意义，也是早期板块构造研究的关键突破口之一，开展其不同地壳层次构造变形及其前陆盆地的研究，将深化早期板块边界及其造山过程的科学认识。     

中国与蒙古之地质

按照构造单元和构造阶段讨论中国和蒙古的演化史。中国前寒武纪地壳演化可分3大阶段:陆核的聚结(2·8Ga);原地台在吕梁运动中固结和侧向增生(1·8Ga);地台在晋宁运动中固化拼合成华夏超大陆(830Ma)。晋宁运动后,中国和蒙古以离散大陆和洋盆并存为特征,至早古生代末聚合为中国和北蒙古两个古大陆。晚古生代时,斋桑—南蒙古—兴安和乌拉尔—天山两大海域陆续消减,形成了海西期的主缝合带。中国蒙古各地块大致于印支运动末期(210Ma)重新聚合,成为劳亚超大陆,即二叠纪—三叠纪泛大陆北支的一部分。印支期后大阶段的特征是泛大陆裂解和大西洋扩张导致了环太平洋域的出现,这一新的构造型式使中国由南北部之间的差异转变为东西部之间的差异。中国东部,也包含蒙古在内,在中—新生代基本上处于张性构造状态,发育张裂盆地和大陆内部火山活动;而在中国西部,中—新生代的构造发展过程则表现为亲冈瓦纳诸地块陆续向北增生拼贴到古亚洲大陆之上。这个过程最终导致了青藏高原在中新世至第四纪的迅速上隆。     

Superplume, supercontinent, and post-perovskite: Mantle dynamics and anti-plate tectonics on the Core–Mantle Boundary

The Western Pacific Triangular Zone (WPTZ) is the frontier of a future supercontinent to be formed at 250 Ma after present. The WPTZ is characterized by double-sided subduction zones to the east and south, and is a region dominated by extensive refrigeration and water supply into the mantle wedge since at least 200 Ma. Long stagnant slabs extending over 1200 km are present in the mid-Mantle Boundary Layer (MBL, 410–660 km) under the WPTZ, whereas on the Core–Mantle Boundary (CMB, 2700–2900 km depth), there is a thick high-V anomaly, presumably representing a slab graveyard. To explain the D″ layer cold anomaly, catastrophic collapse of once stagnant slabs in MBL is necessary, which could have occurred at 30–20 Ma, acting as a trigger to open a series of back-arc basins, hot regions, small ocean basins, and presumably formation of a series of microplates in both ocean and continent. These events were the result of replacement of upper mantle by hotter and more fertile materials from the lower mantle.The thermal structure of the solid Earth was estimated by the phase diagrams of Mid Oceanic Ridge Basalt (MORB) and pyrolite combined with seismic discontinuity planes at 410–660 km, thickness of the D″ layers, and distribution of the ultra-low velocity zone (ULVZ). The result clearly shows the presence of two major superplumes and one downwelling. Thermal structure of the Earth seems to be controlled by the subduction history back to 180 Ma, except in the D″ layer. The thermal structure of the D″ layer seems to be controlled by older slab-graveyards, as expected by paleogeographic reconstructions for Laurasia, Gondwana and Rodinia back to 700 Ma.Comparison of mantle tomography between the Pacific superplume and underneath the WPTZ suggests the transformation of a cold slab graveyard to a large-scale mantle upwelling with time. The Pacific superplume was born from the coldest CMB underneath the 1.0–0.75 Ga supercontinent Rodinia where huge amounts of cold slabs had accumulated through collision-amalgamation of more than 12 continents. A high velocity P-wave anomaly on a whole-mantle scale shows stagnant slabs restricted to the MBL of circum-Pacific and Tethyan regions. The high velocity zones can be clearly identified within the Pacific domain, suggesting the presence of slab graveyards formed at geological periods much older than the breakup of Rodinia. We speculate that the predominant subduction occurred through the formation period of Gondwana, presumably very active during 600 to 540 Ma period, and again from 400 to 300 Ma during the formation of the northern half of Pangea (Laurasia). We correlate the three dominant slab graveyards with three major orogenies in earth history, with the emerging picture suggesting that the present-day Pacific superplume is located at the center of the Rodinian slab graveyard.We speculate the mechanism of superplume formation through a comparison of the thermal structure of the mantle combined with seismic tomography under the Western Pacific Triangular Zone (WPTZ), Laurasia (Asia), Gondwana (Africa), and Rodinia (Pacific). The coldest mantle formed by extensive subduction to generate a supercontinent, changes with time of the order of several hundreds of million years to the hottest mantle underneath the supercontinent. The Pacific superplume is tightly defined by a steep velocity gradient on the margin, particularly well documented by S-wave velocity. The outermost region of the superplume is characterized by the Rodinia slab graveyard forming a donut-shape. We develop a petrologic model for the Pacific superplume and show how larger plumes are generated at shallower depths in the mantle. We link the mechanism of formation of the superplume to the presence of the mineral post-perovskite, the phase transformation of which to perovskite is exothermic, and thus aids in transporting core heat to mantle, and finally to planetary space by plumes.We summarize the characteristics of tectonic processes operating at the CMB to propose the existence of an “anti-crust” generated through “anti-plate tectonics” at the bottom of the mantle. The chemistry of the anti-crust markedly contrasts with that of the continental crust overlying the mantle. Both the crust and the anti-crust must have increased in volume through geologic time, in close relation with the geochemical reservoirs of the Earth. The process of formation of a new superplume closely accompanies the process of development of anti-crust at the bottom of mantle, through the production of dense melt from the partial melting of recycled MORB, observed now as the ULVZ. When CMB temperature is recovered to near 4000 K through phase transformation, the recycled MORB is partially melted imparting chemical buoyancy of the andesitic residual solid which rises up from CMB, leaving behind the dense melt to sink to CMB and thus increase the mass of anti-crust. These small-scale plumes develop to a large-scale superplume through collision and amalgamation with time. When all recycled MORBs are consumed, it is the time of demise of superplume. Immediately above the CMB, anti-plate tectonics operates to develop anti-crust through the horizontal movement of accumulated slab and their partial melting. Thus, we speculate that another continent, or even a supercontinent, has developed through geologic time at the bottom of the mantle.We also evaluate the heating vs. cooling models in relation to mantle dynamics. Rising plumes control not only the rifting of supercontinents and continents, but also the Atlantic stage as seen by anchored ridge by hotspots in the last 200 Ma in the Atlantic. Therefore, we propose that the major driving force for the mantle dynamics is the heat supplied from the high-T core, and not the slab pull force by cooling. The best analogy for this is the atmospheric circulation driven by the energy from Sun.