Contrasting modes of supercontinent formation and the conundrum of Pangea

Repeated cycles of supercontinent amalgamation and dispersal have occurred since the Late Archean and have had a profound influence on the evolution of the Earth's crust, atmosphere, hydrosphere, and life. When a supercontinent breaks up, two geodynamically distinct tracts of oceanic lithosphere exist: relatively young interior ocean floor that develops between the dispersing continents, and relatively old exterior ocean floor, which surrounded the supercontinent before breakup. The geologic and Sm/Nd isotopic record suggests that supercontinents may form by two end-member mechanisms: introversion (e.g. Pangea), in which interior ocean floor is preferentially subducted, and extroversion (e.g. Pannotia), in which exterior ocean floor is preferentially subducted.The mechanisms responsible remain elusive. Top–down geodynamic models predict that supercontinents form by extroversion, explaining the formation of Pannotia in the latest Neoproterozoic, but not the formation of Pangea. Preliminary analysis indicates that the onset of subduction in the interior (Rheic) ocean in the early Paleozoic, which culminated in the amalgamation of Pangea, was coeval with a major change in the tectonic regime in the exterior (paleo-Pacific) ocean, suggesting a geodynamic linkage between these events. Sea level fall from the Late Ordovician to the Carboniferous suggests that the average elevation of the oceanic crust decreased in this time interval, implying that the average age of the oceanic lithosphere increased as the Rheic Ocean was contracting, and that subduction of relatively new Rheic Ocean lithosphere was favoured over the subduction of relatively old, paleo-Pacific lithosphere. A coeval increase in the rate of sea floor spreading is suggested by the relatively low initial ⁸⁷Sr/⁸⁶Sr in late Paleozoic ocean waters. We speculate that superplumes, perhaps driven by slab avalanche events, can occasionally overwhelm top–down geodynamics, imposing a geoid high over a pre-existing geoid low and causing the dispersing continents to reverse their directions to produce an introverted supercontinent.     

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.     

The state of stress at continental margins

Three sources of stress at active (Andean) continental margins are considered: body forces on the plates which drive their motion, thermal stresses generated within the cooling lithosphereand bending stresses due to the flexure of the lithosphere at an ocean trench. It is argued that the bending stresses dominate. The evolution of passive (Atlantictype) continental margins is also considered. Models for the free and locked flexure of the continental and oceanic lithosphere are given. Based on observed gravity anomalies, it is argued that the continental margin fault system must remain active throughout much of the evolution of the margin. These displacements accommodate both the subsidence of the oceanic lithosphere due to its cooling and thickeningand the sedimentary loading. This loading may be responsible for the seismicity on the eastern continental margin of the United States e.g., the Charleston, South Carolina earthquake of 1884.     

Subduction zone stresses: Constraints from mechanics and from topographic and geoid anomalies

There are, in principle, direct relations between several important phenomena associated with subduction zones: the depth of oceanic trenches, the magnitude of the net force from trenches acting on subducting plates, the distribution and fault plane orientations of earthquakes, the magnitude of stresses on subduction faults, the bathymetry of back-arc regions, and the magnitudes of gravity and geoid anomalies. These phenomena are related through the stresses transmitted through surface and subducted lithosphere, and are associated with the mass anomaly of the subducted lithosphere. Quantitative estimates suggest that observed trench depths imply a trench pull force on subducting plates which is comparable to the ridge push force but much less than the excess weight of the subducted lithospheric slab. It is further suggested that either the mass anomaly of subducted lithosphere is much less than would be expected on the basis of conventional thermal and compositional models or that (a) a large resistance acts on the upper part of slabs due to high-stress corner flow, and (b) the mass anomaly of the slab is 70–90% compensated either by a broad 1 km-deep back-arc depression or a low density mantle wedge above the slab or both.     

俯冲带部分熔融

俯冲带是地幔对流环的下沉翼,是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度,因此,一般认为俯冲板片并不会发生部分熔融,而是脱水导致上覆地幔楔发生部分熔融。但是,也有研究认为,在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下,俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时,洋壳物质可以发生饱和水或脱水熔融,基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构,俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比,俯冲的大陆地壳具有很低的水含量,较难发生部分熔融,但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融,形成S型花岗岩和埃达克质岩石。     

Fluids, tectonics and crustal deformation

In the plate tectonic process, lithosphere creation at ocean ridges and its cooling leads to volatile fixation in the oceanic crust. The outer 10 km or so of all crust contains abundant water in pores and fractures and variable amounts of volatiles in minerals. When surface rocks are buried by tectonic processes, fluids must be released and modify the mechanical properties. In the subduction process hydrated oceanic crust may be decoupled from the remaining oceanic lithosphere. At depth rising aqueous fluids or melts lead to a complex series of mass-energy transfer processes which may decouple continental crust near the Moho. Continental crust if subducted, may also be decoupled from its lithosphere by degassing. Fluid release processes which create gas-solid mixtures beneath impermeable cover create low-strength systems subject to facile deformation, hydraulic fracture processes and diapiric phenomena.     

Lamproites as indicators of accretion and/or shallow subduction in the assembly of south‐western Anatolia,Turkey

The geochemistry and mineralogy of lamproites from south‐western Anatolia can be used as a snapshot of the lithospheric composition beneath the Menderes Massif. High and near‐constant K₂O contents, the presence of mantle xenocrystic phlogopite and olivine, highly magnesian olivine phenocrysts and Cr‐rich spinel inclusions all indicate that the lithospheric mantle was phlogopite‐bearing ultradepleted harzburgite at the time of lamproite eruption (20–4 Ma). This mantle assemblage most probably originated in a complex multistage process, including (intra‐oceanic) supra‐subduction zone depletion during the final stages of southern Neotethyan ocean closure, and accretion of the forearc oceanic lithosphere as shallowly subducted material to the already assembled Anatolia. The data presented here support shallow subduction of the oceanic lithosphere as a cause of the uplift of the Menderes Massif, in contrast to the traditional core‐complex model. Terra Nova, 00, 000–000, 2010     

晚中生代东亚多板块汇聚与大陆构造体系的发展

东亚大陆原型形成于三叠纪印支造山运动旋回,其周邻环绕的三大洋（古太平洋、蒙古-鄂霍茨克洋、中特提斯洋）于早侏罗世初期几乎同时向东亚大陆俯冲,开启了东亚多板块汇聚历史。文章通过总结东亚大陆晚中生代构造变形和构造岩浆事件的新近研究成果,简述了东亚多板块汇聚产生的三个陆缘汇聚构造系统（北部蒙古-鄂霍次克碰撞造山带、东部与俯冲有关的增生造山系统、西南部班公湖-怒江缝合构造带）、陆内汇聚构造变形体系和大陆伸展构造体系。在此基础上,重新构建了东亚多板块汇聚大陆构造-岩浆演化的时间框架,将其划分为三个阶段:早侏罗世（200~170 Ma）周邻大洋板块初始俯冲阶段和陆缘裂解事件,中晚侏罗世-早白垩世早期（170~135 Ma）周邻陆缘碰撞造山或俯冲增生造山作用、陆内再生造山作用和汇聚构造体系的形成;中晚白垩世（135~80 Ma）大陆岩石圈的减薄作用和大陆伸展构造体系的发育。研究认为,晚中生代东亚多板块汇聚在时空上的有序演化和深浅构造的复合叠加,不仅产生了东亚大陆复杂的陆缘和陆内构造体系,同时控制了中国东部燕山期爆发式岩浆-成矿作用,也使东亚构造地貌发生东西翘变,早期陆缘汇聚产生的东部高原因晚期大陆岩石圈的减薄和伸展而垮塌。东亚大陆构造体系的形成和演化与联合古大陆的裂解同步,晚中生代东亚多板块汇聚完成了从东亚到欧亚大陆的演替,以东亚大陆为核心的多板块汇聚格局一直延续至新生代,可能成为未来超大陆形成的起点。      

Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective

Tonga and Mariana fore-arc peridotites, inferred to representtheir respective sub-arc mantle lithospheres, are compositionallyhighly depleted (low Fe/Mg) and thus physically buoyant relativeto abyssal peridotites representing normal oceanic lithosphere(high Fe/Mg) formed at ocean ridges. The observation that thedepletion of these fore-arc lithospheres is unrelated to, andpre-dates, the inception of present-day western Pacific subductionzones demonstrates the pre-existence of compositional buoyancycontrast at the sites of these subduction zones. These observationsallow us to suggest that lateral compositional buoyancy contrastwithin the oceanic lithosphere creates the favoured and necessarycondition for subduction initiation. Edges of buoyant oceanicplateaux, for example, mark a compositional buoyancy contrastwithin the oceanic lithosphere. These edges under deviatoriccompression (e.g. ridge push) could develop reverse faults withcombined forces in excess of the oceanic lithosphere strength,allowing the dense normal oceanic lithosphere to sink into theasthenosphere beneath the buoyant overriding oceanic plateaux,i.e. the initiation of subduction zones. We term this conceptthe ‘oceanic plateau model’. This model explainsmany other observations and offers testable hypotheses on importantgeodynamic problems on a global scale. These include (1) theorigin of the 43 Ma bend along the Hawaii–Emperor SeamountChain in the Pacific, (2) mechanisms of ophiolite emplacement,(3) continental accretion, etc. Subduction initiation is notunique to oceanic plateaux, but the plateau model well illustratesthe importance of the compositional buoyancy contrast withinthe lithosphere for subduction initiation. Most portions ofpassive continental margins, such as in the Atlantic where largecompositional buoyancy contrast exists, are the loci of futuresubduction zones. KEY WORDS: subduction initiation; compositional buoyancy contrast; oceanic lithosphere; plate tectonics; mantle plumes; hotspots; oceanic plateaux; passive continental margins; continental accretion; mantle peridotites; ophiolites     

A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens: Speculations on geodynamic correlations

We discuss the potential geodynamic connections between Paleozoic arc development along the flanks of the interior (e.g. the Iapetus and Rheic) oceans and the exterior Paleopacific Ocean. Paleozoic arcs in the Iapetus and Rheic oceanic realms are preserved in the Appalachian–Caledonide and Variscan orogens, and in the Paleopacific Ocean realm they are preserved in the Terra Australis Orogen. Potential geodynamic connections are suggested by paleocontinental reconstructions showing Cambrian–Early Ordovician contraction of the exterior ocean as the interior oceans expanded, and subsequent Paleozoic expansion of the exterior oceans while the interior oceans contracted. Subduction initiated in the eastern segment of Iapetus at ca. 515 Ma and Early to Middle Ordovician orogenesis along the flanks of this ocean is highlighted by arc–continent collisions and ophiolite obductions. Over a similar time interval, subduction and orogenesis took place in the exterior ocean and included formation of the Macquarie arc in the Tasmanides of Eastern Australia and the Famatina arc and correlatives in the periphery of the proto-Andean margin of Gondwana. Major changes in the style of subduction (from retreating to advancing) in interior oceans occurred during the Silurian, following accretion of the peri-Gondwanan terranes and Baltica, and closure of the northeastern segment of Iapetus. During the same time interval, subduction in the Paleopacific Ocean was predominantly in a retreating mode, although intermittent episodes of contraction closed major marginal basins. In addition, however, there were major disturbances in the Earth tectonic systems during the Ordovician, including an unprecedented rise in marine life diversity, as well as significant fluctuations in sea level, atmospheric CO₂, and ⁸⁷Sr/⁸⁶Sr and ¹³C in marine strata carbonates. Stable and radiogenic isotopic data provide evidence for the addition of abundant mantle-derived magma, fluids and large mineral deposits that have a significant mantle-derived component. When considered together, the coeval, profound changes in the style of tectonic activity and the disturbances recorded in Earth Systems are consistent with the emergence of a superplume during the Ordovician. We speculate that the emergence of a superplume triggered by slab avalanche events within the Iapetus and Paleopacific oceans was associated with the establishment of a new geoid high within the Paleopacific regime, the closure of the interior Rheic Ocean and the amalgamation of Laurussia and Gondwana, which was a key event in the Late Carboniferous amalgamation of Pangea.     

北祁连早古生代洋盆是裂陷槽还是大洋盆——与葛肖虹讨论

北祁连早古生代洋盆是裂陷槽还是大洋盆,是一个有争论的问题。文中讨论了蛇绿岩中保存的代表大洋盆存在的印记。认为被夹持于华北地块和柴达木地块之间的北祁连早古生代造山带属于板块增生杂岩带,是由海洋岩石圈残片、消减杂岩、岛弧增生楔等组成的(或许还包括一部分残留陆块).指出北祁连蛇绿岩属于科迪勒拉型,暗示北祁连在早古生代时可能曾经是一个规模较大的洋盆,而非裂陷槽。华北地块和柴达木地块的规模很小,只不过是浩瀚海洋中散布的微小陆块而已。     

冈底斯弧的岩浆作用：从新特提斯俯冲到印度亚洲碰撞

位于青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯大洋岩石圈的长期俯冲过程中,而且在印度与亚洲大陆碰撞过程中叠加了强烈的新生代岩浆作用,是世界上典型的复合型大陆岩浆弧,已经成为研究汇聚板块边缘岩浆作用和大陆地壳生长与再造的天然实验室。基于对现有研究成果的总结,我们将冈底斯岩浆弧的岩浆构造演化划分为5个阶段：第1阶段发生在晚白垩世之前,以新特提斯洋岩石圈长期正常俯冲和钙碱性弧岩浆岩的发育为特征;第2阶段发生在晚白垩世时期,以活动的新特提斯洋中脊发生俯冲和强烈的岩浆作用与显著的新生地壳生长为特征;第3阶段发生在晚白垩世晚期,以残余的新特提斯大洋岩石圈俯冲和正常弧型岩浆作用为特征;第4阶段发生在古新世至中始新世,以印度与亚洲大陆碰撞、俯冲的新特提斯洋岩石圈回转和断离,及其诱发的幔源岩浆作用、新生和古老地壳的强烈再造为特征;第5阶段为发生在晚渐新世到中中新世的后碰撞阶段,深俯冲印度岩石圈的回转和断离,或加厚岩石圈地幔的对流移去导致了加厚下地壳的部分熔融和埃达克质岩石的广泛发育,同时伴随幔源钾质超钾质岩浆作用。冈底斯弧岩浆作用与岩浆成分的系统时空变化很好地记录了从新特提斯洋俯冲到印度亚洲大陆碰撞的完整构造演化过程。     

The Araçuaí-West-Congo Orogen in Brazil: an overview of a confined orogen formed during Gondwanaland assembly

The Neoproterozoic Adamastor-Brazilide Ocean was generated during the breakup of the Rodinia supercontinent, and remnants of its oceanic lithosphere have been found in the Brasiliano-Pan African orogenic system that includes the Araçuaí, West-Congo, Brasília, Ribeira, Kaoko, Dom Feliciano, Damara and Gariep belts. The Araçuaí and the West-Congo belts are counterparts of the same Neoproterozoic orogen. The first belt comprises two thirds of the Araçuaí-West-Congo Orogen. This orogen is rather unique owing to its confined nature within the embayment outlined by the São Francisco and Congo cratons. In spite of this, the presence of ophiolitic remnants, and a calc-alkaline magmatic arc, indicate that the basin/orogen evolution comprise both oceanic spreading and consumption. It is assumed that coeval Paramirim and Sangha aulacogens played a key role by making room for the Araçuaí-West-Congo Basin. Sedimentary successions record all major stages of a basin that evolved from continental rift, when glaciation-related sedimentation was very significant, to passive margin. Rifting started around 1.0–0.9 Ga. The oceanic stage is constrained by an ophiolitic remnant dated at 0.8 Ga. If the cratonic bridge that once linked the São Francisco and Congo palaeocontinental regions did not hinder the opening of an ocean basin, it certainly limited its width. As a consequence, only a narrow oceanic lithosphere was generated, and it was subducted afterwards. This is also suggested by orogenic calc-alkaline granitoids occuping a small area of the orogen. Geochronological data for pre-, syn- and late-collisional granitoids indicate that the orogenic stage lasted from 625 Ma to 570 Ma. A period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535–500 Ma. The features of the Araçuaí-West-Congo Orogen suggest the development of a complete Wilson Cycle in a branch of the Adamastor Ocean, which can be interpreted as a gulf with limited generation of oceanic lithosphere.     

Architecture and composition of ocean floor subducted beneath northern Gondwana during Neoproterozoic to Cambrian: A palinspastic reconstruction based on Ocean Plate Stratigraphy (OPS)

The Blovice accretionary complex, Bohemian Massif, hosts well-preserved basaltic blocks derived from an oceanic plate subducted beneath the northern active margin of Gondwana during late Neoproterozoic to early Cambrian. The major and trace element and Hf–Nd isotope systematics revealed two different suites, tholeiitic and alkaline, whose composition reflects different sources of melts within a back-arc basin setting. The former suite has composition similar to mid-ocean ridge basalts (MORB), yet with striking enrichment in large-ion lithophile elements (LILE) and Pb paralleled by depletion in Nb, in agreement with its derivation from depleted mantle fluxed by subduction-related fluids. In contrast, the latter suite has composition similar to ocean island basalts (OIB) with variable contribution of ancient, recycled crustal material. We argue that both suites represent volcanic members of Ocean Plate Stratigraphy (OPS) and indicate that the oceanic realm consumed by the Cadomian subduction was a complex mosaic of intra-oceanic subduction zones, volcanic island arcs, and back-arc basins with mantle plume impinging the spreading centre. Hence, the basalt geochemistry implies that two distinct domains of oceanic lithosphere may have existed off the Gondwana’s continental edge: an outboard domain, made up of old and less buoyant oceanic lithosphere (remnants of the Mirovoi Ocean surrounding former Rodinia?) that was steeply subducted and generated the back-arcs, and young, hot, and more buoyant oceanic lithosphere generated in the back-arcs and later involved in accretionary complexes as dismembered OPS. Perhaps the best recent analogy of this setting is the Izu Bonin–Mariana arc–Philippine Sea in the western Pacific.     

沉积型锰矿床的形成及其与古海洋环境的协同演化

海相沉积型锰矿的成矿过程受古海洋沉积环境影响，而古海洋环境又与超大陆聚合与裂解、极端地质事件、生命演化等密切相关，因此，海相富锰地层是岩石圈、水圈、大气圈和生物圈等多圈层耦合关系与物质循环相关信息的重要载体。深层海水缺氧模型、最小氧化带模型和幕式充氧模型都显示海水中氧化还原梯度的变化是导致锰矿形成的最主要原因。全球范围内海相沉积型锰矿主要形成于古元古代、新元古代和显生宙3个地质历史时期。其中，元古宙时期，地球上发育了完善的氧化还原分层的古海洋结构；古元古代早期和新元古代，超大陆裂解引起的海平面升降变化导致古海洋氧化还原结构产生动荡，并促使大规模沉积型锰成矿作用发生；地球沉寂期（1800~800 Ma）涵盖了整个中元古代，这一时期仅在华北地台发育了少量沉积型锰矿床，反映该时期古海洋中锰的迁移受到了抑制；显生宙地球再次进入活跃期，经历了数次海洋缺氧事件，冰室-温室气候交替促使海水的化学性质剧烈变化，并在局部氧化还原分层的沉积盆地中富集形成沉积型锰矿床。总之，古海洋氧化还原环境的变化是沉积型锰矿形成的必要条件，同时，区域性沉积盆地的结构、海平面的升降、火山作用导致的物缘供给等多种因素都会影响沉积型锰矿的形成。与沉积型铁矿相比，沉积型锰矿对局部海水化学性质的变化更加敏感，综合研究铁锰矿床的共生与分异过程，将有助于更加有效的识别不同尺度的沉积过程与古海洋环境变化。     

俯冲带形成机制的可检验假说和检验方案

五十年前板块构造理论的诞生是地球科学领域的一场革命,它为理解地球如何运作构建了基本框架。过去五十年对该理论的进一步研究告诉我们地质过程最终都是地球热损失的结果。例如,大洋岩石圈板块在洋中脊形成,其运动和增生以及最终通过俯冲带进入地幔导致地幔冷却降温,从而导致大规模的地幔对流。亦即,板块构造的直接驱动力是俯冲大洋岩石圈板块的下沉力。因此,没有俯冲带就没有板块构造,但是俯冲带如何开始仍然有争议。对俯冲起始的研究从未中断,有数值模拟也有地质推断。2014年在西太平洋用三个IODP航次(350、351和352)来检验“自发”和“诱发”俯冲开始的想法。所有这些努力都值得肯定,但这些是无法检验的想法。无法检验意味着没有结果。本文介绍至今唯一可用地质学方法检验的假说,亦即“岩石圈内横向物质组成差异导致的浮力差是俯冲带形成的起因”。这种浮力差位于海底高原的边部和被动大陆边缘,因此这些部位是未来俯冲带起始的必然轨迹。在远离这些部位的正常洋盆内因缺乏浮力差而俯冲带不可能起始。换句话说,“所有岛弧一定有大陆(或海底高原)基底”,这可以通过采集和研究岛弧基底岩石来验证。     

Progress in the Study of Deep Profiles of Tibet and the Himalayas (INDEPTH)

This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a "sandwich" structure with a relatively rigid-brittle upper crust, a visco-plastic lower crust and a relatively rigid-ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north-directed collision-compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large-scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g     

Supercontinent tectonics and biogeochemical cycle： A matter of ‘life and death＇

<正>The formation and disruption of supercontinents have significantly impacted mantle dynamics,solid earth processes,surface environments and the biogeochemical cycle.In the early history of the Earth,the collision of parallel intra-oceanic arcs was an important process in building embryonic continents.Superdownwelling along Y-shaped triple junctions might have been one of the important processes that aided in the rapid assembly of continental fragments into closely packed supercontinents. Various models have been proposed for the fragmentation of supercontinents including thermal blanket and superplume hypotheses.The reassembly of supercontinents after breakup and the ocean closure occurs through "introversion","extroversion" or a combination of both,and is characterized by either Pacific-type or Atlantic-type ocean closure.The breakup of supercontinents and development of hydrothermal system in rifts with granitic basement create anomalous chemical environments enriched in nutrients, which serve as the primary building blocks of the skeleton and bone of early modern life forms. A typical example is the rifting of the Rodinia supercontinent,which opened up an N—S oriented sea way along which nutrient enriched upwelling brought about a habitable geochemical environment.The assembly of supercontinents also had significant impact on life evolution.The role played by the Cambrian Gondwana assembly has been emphasized in many models,including the formation of 'Trans-gondwana Mountains' that might have provided an effective source of rich nutrients to the equatorial waters,thus aiding the rapid increase in biodiversity.The planet has witnessed several mass extinction events during its history,mostly connected with major climatic fluctuations including global cooling and warming events,major glaciations,fluctuations in sea level,global anoxia,volcanic eruptions, asteroid impacts and gamma radiation.Some recent models speculate a relationship between superplumes,supercontinent breakup and mass extinction.Upwelling plumes cause continental rifting and formation of large igneous provinces.Subsequent volcanic emissions and resultant plume-induced "winter" have catastrophic effect on the atmosphere that lead to mass extinctions and long term oceanic anoxia.The assembly and dispersal of continents appear to have influenced the biogeochemical cycle,but whether the individual stages of organic evolution and extinction on the planet are closely linked to Solid Earth processes remains to be investigated.     

Subduction of a fossil slow–ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif,Ligurian Alps,Northwest Italy

Slow–ultraslow spreading oceans are mostly floored by mantle peridotites and are typified by rifted continental margins, where subcontinental lithospheric mantle is preserved. Structural and petrologic investigations of the high-pressure (HP) Alpine Voltri Massif ophiolites, which were derived from the Late Jurassic Ligurian Tethys fossil slow–ultraslow spreading ocean, reveal the fate of the oceanic peridotites/serpentinites during subduction to depths involving eclogite-facies conditions, followed by exhumation.

The Ligurian Tethys was formed by continental extension within the Europe–Adria lithosphere and consisted of sea-floor exposed mantle peridotites with an uppermost layer of oceanic serpentinites and of subcontinental lithospheric mantle at the rifted continental margins. Plate convergence caused eastward subduction of the oceanic lithosphere of the Europe plate and the uppermost serpentinite layer of the subducting slab formed an antigorite serpentinite-subduction channel. Sectors of the rather unaltered mantle lithosphere of the Adria extended margin underwent ablative subduction and were detached, embedded, and buried to eclogite-facies conditions within the serpentinite-subduction channel. At such P–T conditions, antigorite serpentinites from the oceanic slab underwent partial HP dehydration (antigorite dewatering and growth of new olivine). Water fluxing from partial dehydration of host serpentinites caused partial HP hydration (growth of Ti-clinohumite and antigorite) of the subducted Adria margin peridotites. The serpentinite-subduction channel (future Beigua serpentinites), acting as a low-viscosity carrier for high-density subducted rocks, allowed rapid exhumation of the almost unaltered Adria peridotites (future Erro–Tobbio peridotites) and their emplacement into the Voltri Massif orogenic edifice. Over in the past 35 years, this unique geologic architecture has allowed us to investigate the pristine structural and compositional mantle features of the subcontinental Erro–Tobbio peridotites and to clarify the main steps of the pre-oceanic extensional, tectonic–magmatic history of the Europe–Adria asthenosphere–lithosphere system, which led to the formation of the Ligurian Tethys.

Our present knowledge of the Voltri Massif provides fundamental information for enhanced understanding, from a mantle perspective, of formation, subduction, and exhumation of oceanic and marginal lithosphere of slow–ultraslow spreading oceans.     

On the tectonics of the Ligurian Apennines (northern Italy)

Four nappes have been recognized in the Ligurian Apennines. In the Lavagna Nappe very low-grade metamorphism is combined with very large, originally W-facing isoclinal folds. In the other nappes, no evidence for metamorphism is found and all eutectonic folding was originally E- to NE-facing. Tectonic transport along the major nappe contacts was in an E- to NE-direction. A tectonic model is presented, which explains the generation of the large, originally W-facing folds as a result of originally E-inclined subduction within a young oceanic basin. Young oceanic lithosphere (maximum age approximately 25 Ma) subducted beneath oceanic lithosphere with a maximum age of approximately 40 Ma, under the influence of horizontally oriented compressional forces. Within the tectonic wedge, associated with the subduction, originally W-facing isoclinal folding and metamorphism occurred. Decrease and/or termination of compression resulted in the cessation of the subduction movements, followed by uplift of the region above the subducted plate by means of buoyancy. This uplift formed a slope from which sequences slid in an E- to NE-direction, causing E- to NE-facing folds. Ultimately, detachment and thrusting of gravitational nappes took place, by which process rock sequences of oceanic origin have been externally transported to attain ensialic (continental) domains. The Triassic-Early Oligocene tectonic events recognized in the Ligurian Apennines correlate quite well with the events that preceded the collision phase of the Alps.