http://www.sciencedirect.com/science/article/pii/S1674987115001280

The East African Rift system(EARS)provides a unique system with the juxtaposition of two contrasting yet simultaneously formed rift branches,the eastern,magma-rich,and the western,magma-poor,on either sides of the old thick Tanzanian craton embedded in a younger lithosphere.Data on the pre-rift,syn-rift and post-rift far-field volcanic and tectonic activity show that the EARS formed in the context of the interaction between a deep mantle plume and a horizontally and vertically heterogeneous lithosphere under far-field tectonic extension.We bring quantitative insights into this evolution by implementing high-resolution 3D thermo-mechanical numerical deformation models of a lithosphere of realistic rheology.The models focus on the central part of the EARS.We explore scenarios of plumelithosphere interaction with plumes of various size and initial position rising beneath a tectonically pre-stretched lithosphere.We test the impact of the inherited rheological discontinuities(suture zones)along the craton borders,of the rheological structure,of lithosphere plate thickness variations,and of physical and mechanical contrasts between the craton and the embedding lithosphere.Our experiments indicate that the ascending plume material is deflected by the cratonic keel and preferentially channeled along one of its sides,leading to the formation of a large rift zone along the eastern side of the craton,with significant magmatic activity and substantial melt amount derived from the mantle plume material.We show that the observed asymmetry of the central EARS,with coeval amagmatic(western)and magmatic(eastern)branches,can be explained by the splitting of warm material rising from a broad plume head whose initial position is slightly shifted to the eastern side of the craton.In that case,neither a mechanical weakness of the contact between the craton and the embedding lithosphere nor the presence of second plume are required to produce simulations that match observations.This result reconciles the passive and active rift models and demonstrates the possibility of development of both magmatic and amagmatic rifts in identical geotectonic environments.     

Numerical Modeling of the Effect of Mantle Plume on Continental Drift: Revelation of the Rapid Drift of the Indian Tectonic Plate in Late CretaceousEI北大核心CSCD

Numerical modeling is used to investigate the interaction between mantle plume and continental lithosphere, especially the effect of the continental lithosphere structure, the scale and position of mantle plume on the rate of continental drift. Numerical results show that, under the effect of mantle plume, the existence of a thick root of the continental lithosphere affects the rate of continental drift. Moreover, under identical scale of mantle plume, the drift rate decreases with increasing thickness of the root. Besides, the velocity and distance of a continental plate drift are negatively correlated in scale of the continental lithosphere, but correlated with the scale of the mantle plume. For the model without lithospheric root, the mantle plume has a more appreciable impact on the plate drift rate when it comes closer to edge of the continental plate. Our models show that mantle plume can accelerate the continental drift by more than 10 cm/a. The modelling results can provide significant dynamical constraint and geological enlightenment. © 2018, Science Press. All right reserved.     

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of >5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thick-skinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U–Pb age determinations (2637–2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from >140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.     

Experimental modeling of structure-forming deformations in rift zones of mid-ocean ridges

The modern methods of physical modeling of structure-forming deformations in extension zones of oceanic lithosphere are discussed; the methods differ in their experimental equipment, model material, and experimental techniques. The simulation performed with an elastic-ductile model has demonstrated that extension of a brittle lithospheric layer results in disruption of its continuity and in formation of a rift valley according to the mechanism of running fracture propagation. The modeling results provide insights into qualitative pattern of faulting and fracturing within a rift zone, specific features of rift segmentation, and development of various structural elements (axis bends, echelons of fractures, nontransform offsets, small and large overlaps, etc.) under various geodynamic conditions of spreading. The modeling has shown that origination and evolution of structures of various types depend on the lithosphere’s thickness beneath the rift axis; the width of the lithosphere’s heating zone; the spreading orientation; and, to a lesser degree, on the spreading rate. A relatively rectilinear rift broken into particular segments bounded by small-amplitude offsets with or without minor overlaps arises in the case of both a small width of the heating zone, closely related to the axial magma chamber, and a small thickness of the lithosphere (fast-spreading conditions). In the case of a wide heating zone caused by ascent of an asthenospheric wedge or a mantle plume, offsets of rift are more pronounced and deformations embrace a wider region. If, as a result, the thickness of the lithosphere increases, the rift will be less linear and the structural heterogeneity will become more contrasting. In addition to the thickness of the lithosphere, the angle between the rift zone and the extension axis also controls the rift configuration: the greater the angle, the more conspicuous the en echelon arrangement of fractures. For any spreading type, the propagating front of linear microfractures that disrupt the upper brittle layer of the lithosphere predates the origin of mesoscopic fractures and predetermines a general trend of the rift zone. This indicates that the fractures of various sizes propagate simultaneously.     

Geochemistry of the mafic dykes in the Prakasam Alkaline Province of Eastern Ghats Belt, India: Implications for the genesis of continental rift-zone magmatism

Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.     

Permian komatiites and associated basalts from the marine sediments of Chhongtash Formation, southeast Karakoram, Ladakh, India

Summary The Karakoram micro-plate is the southern most sector of the Central Asian micro-plate mosaic which was separated by a narrow rift basin. A major rifting phase started during Permian time, which lead to drift of not only Karakoram but of the entire Eurasian (Asian) Plate from Gondwana land. This was at a time when a prominent sequence of black argillites occupied most part of the Karakoram Tethys basin. The geodynamic setting for this sequence may be interpreted as the evolution of a passive margin affected by extensional tectonics. The extensional activity is evident from the extrusion of basalts and komatiitic rocks in the region. In this paper the geochemical relations between komatiites and basalts of the Chhongtash, southeast Karakoram are investigated. The basaltic and komatiitic (ultrabasic) flows are petrologically and geochemically distinct, yet they display a close spatial and temporal association, and they are related to each other through olivine and clinopyroxene fractionation. The chemical characteristics of the ultrabasic to basic magmatism in the region is consistent with formation above a mantle plume that impinged on the continental lithosphere. Hence, a model of partial melting in a mantle plume and fractional crystallization in a deep-seated magma chamber is envisaged to explain the evolution of these volcanic rocks. The komatiite melts are interpreted to have been derived by high degree partial melting of mantle plumes in the tail region, while the basalts were interpreted to be the result of interaction of source plume with cool mantle through which the plume head passed. This study is the first of its kind, to suggest a rift related nature in the Chhongtash, southeast Karakoram, that represent the initial stage of Mesozoic rifting along the southern margin of Eurasia when Gondwana started to drift away from Eurasia.     

沉积盆地形成的地球动力学机制及其分类

沉积盆地的形成主要与地球内部的热和物质的对流密切相关,它是沉积盆地形成和演化的原始动力。软流圈上涌高谋划在面或地幔羽的位置是地幔对 具体的表现。     

Plume-Lithosphere Interaction and the Origin of Continental Rift-related Alkaline Volcanism--the Chyulu Hills Volcanic Province, Southern Kenya

Geochemical data are presented for primitive alkaline lavasfrom the Chyulu Hills Volcanic Province of southern Kenya, situatedsome 100 km east of the Kenya Rift Valley. In addition to theirprimitive compositions, a striking and ubiquitous feature isa strong but variable depletion in K relative to other highlyincompatible elements when normalized to primitive mantle values.Semi-quantitative models are developed that best explain thepetrogenesis of these lavas in terms of partial melting of asource that contained residual amphibole (but not phlogopite).The presence of amphibole implies a source in the subcontinentallithosphere rather than the asthenosphere. It is suggested thatthe amphibole is of metasomatic origin and was precipitatedin the lithospheric mantle by infiltrating fluids and/or meltsderived from rising mantle plume material. A raised geothermas a consequence of the continued ascent of the plume materialled to dehydration melting of the metasomatized mantle and generationof the Chyulu Hills lavas. It is proposed that the Chyulu HillsVolcanic Province represents an analogue for the earliest stagesof continental rift initiation, during which interaction betweena plume and initially refractory lithosphere may lead to thegeneration of lithospheric melts. KEY WORDS: rift-related alkaline volcanism; residual amphibole; subcontinental lithosphere     

Plume head–lithosphere interactions near intra-continental plate boundaries

Plume–lithosphere interactions (PLI) have important consequences both for tectonic and mineralogical evolution of the lithosphere: for example, Archean metallogenic crises at the boundaries of the West African and Australian cratons coincide with postulated plume events. In continents, PLI are often located near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical, thermal and rheological barriers that interact with the emplacement of the plume head (e.g., Archean West Africa, East Africa, Pannonian–Carpathian system). The observable PLI signatures are conditioned by plume dynamics but also by lithosphere rheology and structure. We address the latter problem by considering a free-surface numerical model of PLI with two stratified elasto-viscous–plastic (EVP) lithospheric plates, one of which is older and thicker than another. The results show that: (1) plume head flattening is asymmetric, it is blocked from one side by the cold vertical boundary of the older plate, which leads to the mechanical decoupling of the crust from the mantle lithosphere, and to localized faulting at the cratonic margin; (2) the return flow from the plume head results in sub-vertical down-thrusting (delamination) of the lithosphere at the margin, producing sharp vertical cold boundary down to the 400 km depth; (3) plume head flattening and migration towards the younger plate results in concurrent surface extension above the centre of the plume and in compression (pushing), down-thrusting and magmatic events at the cratonic margin (down-thrusting is also produced at the opposite border of the younger plate); these processes may result in continental growth at the “craton side”; (4) topographic signatures of PLI show basin-scale uplifts and subsidences preferentially located at cratonic margins. Negative Rayleigh–Taylor instabilities in the lithosphere above the plume head provide a mechanism for crustal delamination. Inferred consequences of PLI near intra-continental plate boundaries, such as faulting at cratonic edges and enhanced magmatic activity, could explain plume-related metallogenic crises, as suggested for West Africa and Australia.     

地幔柱数值模拟研究进展

国外大量的数值模拟研究表明地幔柱由热浮力驱动,流体的黏度对地幔柱的形状和物质组成有着很大的影响,地幔柱可造成岩石圈的熔蚀减薄和地表隆升,数值模拟可以定量地得出地幔柱运动的速度、岩石圈的熔融量和熔融温度等要素,但这些模拟描述的仅是物理过程,缺乏与化学动力学相耦合.国内开发的可对峨眉地幔柱进行数值模拟的软件MantlePlume1.0也存在许多需要进一步研究解决的问题,如峨眉山玄武岩的物质来源、活动中心、喷发时限、喷发规模、岩石圈的隆升程度,以及峨眉地幔柱的温压条件和断裂构造形成机理等.     

Constraints on the thermal evolution of continental lithosphere from U-Pb accessory mineral thermochronometry of lower crustal xenoliths,southern Africa

U-Pb isotopic thermochronometry of rutile, apatite and titanite from kimberlite-borne lower crustal granulite xenoliths has been used to constrain the thermal evolution of Archean cratonic and Proterozoic off-craton continental lithosphere beneath southern Africa. The relatively low closure temperature of the U-Pb rutile thermochronometer (~400-450 °C) allows its use as a particularly sensitive recorder of the establishment of "cratonic" lithospheric geotherms, as well as subsequent thermal perturbations to the lithosphere. Contrasting lower crustal thermal histories are revealed between intracratonic and craton margin regions. Discordant Proterozoic (1.8 to 1.0 Ga) rutile ages in Archean (2.9 to 2.7 Ga) granulites from within the craton are indicative of isotopic resetting by marginal orogenic thermal perturbations influencing the deep crust of the cratonic nucleus. In Proterozoic (1.1 to 1.0 Ga) granulite xenoliths from the craton-bounding orogenic belts, rutiles define discordia arrays with Neoproterozoic (0.8 to 0.6 Ga) upper intercepts and lower intercepts equivalent to Mesozoic exhumation upon kimberlite entrainment. In combination with coexisting titanite and apatite dates, these results are interpreted as a record of postorogenic cooling at an integrated rate of approximately 1 °C/Ma, and subsequent variable Pb loss in the apatite and rutile systems during a Mesozoic thermal perturbation to the deep lithosphere. Closure of the rutile thermochronometer signals temperatures of 𙠂 °C in the lower crust during attainment of cratonic lithospheric conductive geotherms, and such closure in the examined portions of the "off-craton" Proterozoic domains of southern Africa indicates that their lithospheric thermal profiles were essentially cratonic from the Neoproterozoic through to the Late Jurassic. These results suggest similar lithospheric thickness and potential for diamond stability beneath both Proterozoic and Archean domains of southern Africa. Subsequent partial resetting of U-Pb rutile and apatite systematics in the cratonic margin lower crust records a transient Mesozoic thermal modification of the lithosphere, and modeling of the diffusive Pb loss from lower crustal rutile constrains the temperature and duration of Mesozoic heating to 𙡦 °C for ₞ ka. This result indicates that the thermal perturbation is not simply a kimberlite-related magmatic phenomenon, but is rather a more protracted manifestation of lithospheric heating, likely related to mantle upwelling and rifting of Gondwana during the Late Jurassic to Cretaceous. The manifestation of this thermal pulse in the lower crust is spatially and temporally correlated with anomalously elevated and/or kinked Cretaceous mantle paleogeotherms, and evidence for metasomatic modification in cratonic mantle peridotite suites. It is argued that most of the geographic differences in lithospheric thermal structure inferred from mantle xenolith thermobarometry are likewise due to the heterogeneous propagation of this broad upper mantle thermal anomaly. The differential manifestation of heating between cratonic margin and cratonic interior indicates the importance of advective heat transport along pre-existing lithosphere-scale discontinuities. Within this model, kimberlite magmatism was a similarly complex, space- and time-dependent response to Late Mesozoic lithospheric thermal perturbation.     

攀西裂谷的多期活动性及其深部地质意义

攀西裂谷具有明显的长期、继承和多期活动性特点,为一“上叠裂谷”,其基底在晋宁期就已存在裂谷系。是海西—印支期时,随着攀西由板内—低纬度被动陆缘—中纬度活动陆缘—板内的变化,裂谷活动和地幔涌流特点也有所不同。攀西裂谷的演化提供了一个深断裂诱发地幔涌流,并随板块飘移而移动的典型例证。这种壳、幔关系可能正是地槽多旋回活动的主因。自第三纪以来,攀西浅、深部地壳应力场明显不一致,这种现象有可能是不同构造学说结合的纽带。     

Craton destruction links to the interaction between subduction and mid-lithospheric discontinuity: Implications for the eastern North China Craton

The continental craton is generally considered to be stable, due to its low-density and high viscosity; however, the thinning and destruction of cratonic lithosphere have been observed at various parts of the globe, for example, the eastern North China Craton (NCC). Although a large number of geological and geophysical data have been collected to study the NCC, the mechanisms and dynamic processes are still widely debated. In this study, using 2-D high-resolution thermo-mechanical models, we systematically explore the key constraints on the destruction of cratonic lithosphere. The model results indicate that the craton destruction processes can be strongly influenced by the presence of the so-called mid-lithosphere discontinuity (MLD), and its interaction with subduction. The properties of the MLD layer and the density contrast between the lithospheric mantle and asthenosphere play significant roles in the destruction processes. Specifically, the presence of a deep and low-viscosity MLD layer within the cratonic lithosphere tends to enhance instability of the craton, making it easier for lithosphere destruction. In addition, a relatively thick oceanic crust, high convergence rate, and large initial subduction angles favor the craton destruction. Finally, we compare the model results with the observations of NCC, which indicate that the interaction between the Paleo-Pacific subduction and the MLD layer in the cratonic lithosphere has played an important role in the observed large-scale lithospheric removal of the eastern North China Craton.     

Thermo-mechanical controls on intra-plate deformation and the role of plume-folding interactions in continental topography

Thermo-tectonic age and inherited structure exert the main controls on the bulk strength of the lithosphere in intraplate settings. Mechanical decoupling within the lithosphere strongly affects the interaction between deep Earth and surface processes. Thermo-mechanical models demonstrate the particular importance of the rheological stratification of the lithosphere in the preservation of ancient cratonic blocks, in the surface expression of plume- and mantle lithosphere interactions and their impact on the “dynamic” topography in general. The same is true for the effect of large-scale lithospheric folding on intraplate basin formation and associated differential vertical motions. Initiation of continental lithosphere subduction, crucial for linking orogenic deformation to intraplate deformation, appears to be facilitated by plume–lithosphere interactions. We present a discussion of these aspects, focusing on better process- understanding of continental deformation, in the context of a number of well-documented cases of intraplate deformation.     

东亚西太平洋岩石圈三维结构及其地幔动力学

欧亚大陆及其边缘海地区是由约30多块尺度不同、形成时代和性质各异的板块或地块拼合而成。这些岩石圈板块或地块经过长时间的漂移,多次聚合与分离,碰撞与增生,在新生代最后形成现代的拼合欧亚大陆。欧亚大陆及其边缘海的板块或地块可以分为以下六类:(1)前寒武纪巨型克拉通地块及地盾;(2)前寒武纪小型克拉通地块及板块;(3)显生宙造山带及汇聚地块;(4)陆陆碰撞型地块及造山带;(5)新生代边缘海海盆;(6)大陆裂谷盆地及增生地块。高分辨率地震面波层析成像,显示同一类型的板块或地块的岩石圈和软流圈的速度结构十分相似,呈现出其独有的速度分布特征。不同类型板块或地块的速度结构有重大差异。直到400km深度,各个板块和地块的横向差异才逐渐减小。一般而言,前寒武纪克拉通板块及地块的岩石圈巨厚具有高速性质、软流圈很薄或不存在;边缘海、造山带等区域岩石圈较薄和速度较低,软流圈发育。根据欧亚大陆及边缘海地区天然地震层析成像,人工地震剖面数据及其他有关资料,建立了欧亚大陆及其边缘海岩石圈模型。     

Li-Sr-Nd isotope signatures of the plume and cratonic lithospheric mantle beneath the margin of the rifted Tanzanian craton (Labait)

Lithium concentrations and isotopic compositions of olivine and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both δ⁷Li and Li concentrations. Olivines in iron-rich peridotites (Fo_85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ⁷Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively light δ⁷Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas. A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has a ⁸⁷Sr/⁸⁶Sr of 0.7029 and ¹⁴³Nd/¹⁴⁴Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared to previous estimates of the plume that were based on rift basalts. Although elevated δ⁷Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics in Sr, Nd and Li isotope composition. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro–Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi–Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi–Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.     

Control of rheological stratification on rifting geometry: a symmetric model resolving the upper plate paradox

Numerical experiments reproduce the fundamental architecture of magma-poor rifted margins such as the Iberian or Alpine margins if the lithosphere has a weak mid-crustal channel on top of strong lower crust and a horizontal thermal weakness in the rift center. During model extension, the upper crust undergoes distributed collapse into the rift center where the thermally weakened portion of the model tears. Among the features reproduced by the modeling, we observe: (1) an array of tilted upper-crustal blocks resting directly on exhumed mantle at the distal margin, (2) consistently oceanward-dipping normal faults, (3) a mid-crustal high strain zone at the base of the crustal blocks (S-reflector), (4) new ocean floor up against a low angle normal fault at the tip of the continent, (5) shear zones consistent with continentward-dipping reflectors in the mantle lithosphere, (6) the mismatch frequently observed between stretching values inferred from surface extension and bulk crustal thinning at distal margins (upper plate paradox). Rifting in the experiment is symmetric at a lithospheric scale and the above features develop on both sides of the rift center. We discuss three controversial points in more detail: (1) weak versus strong lower crust, (2) the deformation pattern in the mantle, and (3) the significance of detachment faults during continental breakup. We argue that the transition from wide rifting towards narrow rifting with a pronounced polarity towards the rift center is associated with the advective growth of a thermal perturbation in the mantle lithosphere.     

Bottom to top lithosphere structure and evolution of western Eger Rift (Central Europe)

We present model of the structure and development of the entire lithosphere beneath the western Eger Rift (ER). Its crustal architecture and paths of volcanic products are closely related to sutures/boundaries of uppermost mantle domains distinguished by different orientations of olivine fabric, derived from 3-D analysis of seismic anisotropy. Three different fabrics of the mantle lithosphere belong to the Saxothuringian (ST), Teplá-Barrandian (TB) and Moldanubian (MD) microplates assembled during the Variscan orogeny. Dipping fossil (pre-assembly) olivine orientations, consistent within each unit, do not support any voluminous mantle delamination. The variable rift structure and morphology depend on the character of the pre-rift suture between the northern ST unit and the TB/MD units in the southern rift flank. The proper rift with typical graben morphology has developed above the steep lithosphere-scale suture between the ST and TB units. This subduction-related boundary originated from the closure of the ST Ocean. Parts of the crust and mantle lithosphere were dragged there into asthenospheric depths and then rapidly uplifted. The suture is marked by abrupt change in the mantle fabric and sharp gradients in regional gravity field and in metamorphic grade. The secular TB-side-down normal movement is reflected in deep sedimentary basins, which developed since the Carboniferous to Cenozoic and in topography. The graben morphology of the ER terminates above the “triple junction” of the ST, TB and MD mantle lithospheres. The junction is characterized by offsets of surface boundaries of the tectonic units from their mantle counterparts indicating a detachment of the rigid upper crust from the mantle lithosphere. The southwest continuation of the rift features in Bavaria is expressed in occurrences of Cenozoic sediments and volcanics above an inclined broad transition zone between the ST and MD lithospheres. Schematic scenario of evolution of the region consists mainly of a subduction of the ST lithosphere to depths around 140 km, exhumation of HP-HT rocks and the post-tectonic granitoid plutonism.     

Accretion of crust in the axial zone of the Mid-Atlantic Ridge south of the Martin Vas Fracture Zone,South Atlantic

New data are obtained on the structure, evolution, and origin of zones of nontransform offsets of adjacent segments in the Mid-Atlantic Ridge (MAR), which, in contrast to transform fracture zones, so far are studied insufficiently. The effects of deep mantle plumes developing off the crest of the MAR on the processes occurring in the spreading zone are revealed. These results are obtained from the geological investigation of the crest of the MAR between 19.8 ° and 21° S, where bottom sampling, bathymetric survey, and magnetic measurements have been carried out previously. Two segments of the rift valley displaced by 10 km relative to each other along a nontransform offset are revealed. A volcanic center of a spreading cell, which has been active over the last 2 Ma, is located in the northern part of the southern segment and distinguished by a decreased depth of the rift valley and increased thickness of the crust. Magnesian, slightly evolved basalts of the N-MORB type are detected in this center, whereas evolved and high-Fe basalts are found beyond it. The variation in the composition of the basalts indicates that the volcanic center is related to the upwelling of the asthenospheric mantle, which spread along and across the spreading ridge. In the lithosphere, the melt migrated off the volcanic center along the rift valley. In the northern segment, a vigorous volcanic center arose 2.5 Ma ago near its southern end; at present, the volcanic activity has ceased. As a result of the volcanic activity, an oval rise composed of enriched T-MORB-type basalts was formed at the western flank of the crest zone. The isotopic signatures show that the primary melts are derivatives of the chemically heterogeneous mantle. The mixing of material of the depleted mantle with the mantle material pertaining either to the Saint Helena or the Tristan da Cunha plumes is suggested; the mixture of all three sources cannot be ruled out. The conclusion is drawn that the mantle material of the Saint Helena plume was supplied to the melting zone beneath the axial rift near the oval rise along a linear permeable zone in the mantle extending at an azimuth of 225° SW. The blocks of mantle material that got to the convecting mantle from the Tristan da Cunha plume at the stage of supercontinent breakup were involved in melting as well. The nontransform offset between the two segments arose on the place of a previously existing transform fracture zone about 5 Ma ago. The nontransform offset developed in the regime of oblique spreading at the progressive propagation of the southern segment to the north. The zone of nontransform offset is characterized by recent volcanic activity. Over the last 2 Ma, spreading of the studied MAR segment was asymmetric, faster in the western direction. The rates of westward and eastward half-spreading in the northern segment are estimated at 1.88 and 1.60 cm/yr, respectively.