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1.
Christoffer Nielsen  H. Thybo   《Tectonophysics》2009,470(3-4):298-318
The Cenozoic Baikal Rift Zone (BRZ) is situated in south-central Siberia in the suture between the Precambrian Siberian Platform and the Amurian plate. This more than 2000-km long rift zone is composed of several individual basement depressions and half-grabens with the deep Lake Baikal at its centre. The BEST (Baikal Explosion Seismic Transect) project acquired a 360-km long, deep seismic, refraction/wide-angle reflection profile in 2002 across southern Lake Baikal. The data from this project is used for identification of large-scale crustal structures and modelling of the seismic velocities of the crust and uppermost mantle. Previous interpretation and velocity modelling of P-wave arrivals in the BEST data has revealed a multi layered crust with smooth variation in Moho depth between the Siberian Platform (41 km) and the Sayan-Baikal fold belt (46 km). The lower crust exhibits normal seismic velocities around the rift structure, except for beneath the rift axis where a distinct 50–80-km wide high-velocity anomaly (7.4–7.6 ± 0.2 km/s) is observed. Reverberant or “ringing” reflections with strong amplitude and low frequency originate from this zone, whereas the lower crust is non-reflective outside the rift zone. Synthetic full-waveform reflectivity modelling of the high-velocity anomaly suggests the presence of a layered sequence with a typical layer thickness of 300–500 m coinciding with the velocity anomaly. The P-wave velocity of the individual layers is modelled to range between 7.4 km/s and 7.9 km/s. We interpret this feature as resulting from mafic to ultra-mafic intrusions in the form of sills. Petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase-rich low-velocity layers and pyroxene- and olivine-rich high-velocity layers. The mafic intrusions were probably intruded into the ductile lower crust during the main rift phase in the Late Pliocene. As such, the intrusive material has thickened the lower crust during rifting, which may explain the lack of Moho uplift across southern BRZ.  相似文献   

2.
Devana Chasma is a rift system on Venus formed in association with the Beta Regio and Phoebe Regio volcanic highlands, which are interpreted as mantle plumes. We present a new analysis of a 2500-km-long segment of Devana. Based on the rift topography, the horizontal extension across the rift boundary faults is 3–9 km. This is a lower bound on the total rift extension because the altimetry does not resolve the topographic relief across the numerous faults that are visible in radar images of the rift floor. The total extension across Devana is approximately 20 km, similar in magnitude to continental rift systems on Earth. Rift flank elevations are up to 3.1 km in the regions nearest the mantle plumes and decay strongly with increasing distance from the plumes, indicating a strong thermal component to the rift flank topography, unlike the situation usually reported for terrestrial rifts. As on Earth, there is also a flexural uplift component to the flank topography. Rift depths are up to 2.5 km below the surrounding plains, with considerable along-strike variability. There is a 600 km lateral offset along Devana Chasma near the mid-point between the two mantle plumes. Devana most likely formed as two distinct rifts due to the horizontal stresses created by outflow from the upwelling plumes. The offset zone formed as a result of the interaction between the two rift tips, which requires that upwelling at the two mantle plumes overlapped in time.  相似文献   

3.
The investigation data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system obtained with a seismic coda envelope and sliding window are considered. Eleven local districts were described by one-dimensional attenuation models characterized by alternation of high and low attenuation layers, which are consistent with the results obtained previously by Yu.F. Kopnichev for the southwestern flank of the Baikal rift system [9]. The subcrust of the lithosphere contains a thin layer with high attenuation of seismic waves likely related to higher heterogeneity (fragmentation) and occurrence of fluids. The lithosphere basement depth varies from 100–120 km in the west within the Baikal folded area to 120–140 km in the east within the Siberian Platform. It is concluded that there are two asthenosphere layers. Based on specific features of the lithosphere and upper mantle structure, it can be assumed that they were subject to gradual modification involving fluidization processes and partial melting in the Late Cenozoic extension under the influence of distant tectogenesis sources.  相似文献   

4.
Abyssal variations beneath the Baikal rift zone are revealed in an irregular seismic stratification of the crust, the presence of an intracrust waveguide and by the vast (> 200,000 km2) underlying area of anomalously low velocity (Pn = 7.6−7.8 km/sec) uppermost mantle. In its abyssal structure the Baikal rift is heterogeneous along the strike, with sharp changes in crustal thickness (35–50 km).Comparison of first-arrival seismic-velocity curves and also the respective velocity columns reveals the essential similarity of upper-mantle seismic cross-sections for all continental rift zones. The anomalous upper layer of the mantle (ca. 7.7 km/sec) is relatively thin (15-13 km) and can be linked with the mantle waveguide only locally.  相似文献   

5.
The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P–T- composition lithospheric cross-sections in the region for the depth range of 35–80 km.Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of 143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle.Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P–T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.  相似文献   

6.
Petit 《地学学报》1998,10(3):160-169
To better understand how active deformation localizes within a continental plate in response to extensional and transtensional tectonics, a combined analysis of high-quality gravity (Bouguer anomaly) and seismicity data is presented consisting of about 35000 earthquakes recorded in the Baikal Rift Zone. This approach allows imaging of deformation patterns from the surface down to the Moho. A comparison is made with heat flow variations in order to assess the importance of lithospheric rheology in the style of extensional deformation. Three different rift sectors can be identified. The southwestern rift sector is characterized by strong gravity and topography contrasts marked by two major crustal faults and diffuse seismicity. Heat flow shows locally elevated values, correlated with recent volcanism and negative seismic P-velocity anomalies. Based on earthquake fault plane solutions and on previous stress field inversions, it is proposed that strain decoupling may occur in this area in response to wrench-compressional stress regime imposed by the India–Asia collision. The central sector is characterized by two major seismic belts; the southernmost one corresponds to a single, steeply dipping fault accommodating oblique extension; in the centre of lake Baikal, a second seismic belt is associated with several dip-slip faults and subcrustal thinning at the rift axis in response to orthogonal extension. The northern rift sector is characterized by a wide, low Bouguer anomaly which corresponds to a broad, high topographic dome and seismic belts and swarms. This topography can be explained by lithospheric buoyancy forces possibly linked to anomalous upper mantle. At a more detailed scale, no clear correlation appears between the surficial fault pattern and the gravity signal. As in other continental rifts, it appears that the lithospheric rheology influences extensional basins morphology. However, in the Baikal rift, the inherited structural fabric combined with stress field variations results in oblique rifting tectonics which seem to control the geometry of southern and northeastern rift basins.  相似文献   

7.
An analysis of continental sedimentogenesis in Mesozoic rifts of the Transbaikal region and Mongolia, as well as in the Cenozoic Baikal rift zone, revealed a succession of volcanic-sedimentary formations, which constitute the sedimentary cover. The peculiar sedimentation features reflect exogenic processes that are characteristic of rift structures and the influence of deep-seated (exogenic and catagenic) sources. It is proposed to define a specific, rift, type of sedimentogenesis.  相似文献   

8.
Anhydrous and amphibole-bearing peridotite xenoliths occur in roughly equal quantitites in the Bartoy volcanic field about 100 km south of the southern tip of Lake Baikal in Siberia (Russia). Whole-rock samples and pure mineral separates from nine xenoliths have been analyzed for Sr and Nd isotopes in order to characterize the upper mantle beneath the southern Baikal rift zone. In an Sr-Nd isotope diagram both dry and hydrous xenoliths from Bartoy plot at the junction between the fields of MORB and ocean island basalts. This contrasts with data available on two other localities around Lake Baikal (Tariat and Vitim) where peridotites typically have Sr–Nd isotope compositions indicative of strong long-term depletion in incompatible elements. Our data indicate significant chemical and isotopic heterogeneity in the mantle beneath Bartoy that may be attributed to its position close to an ancient suture zone separating the Siberian Platform from the Mongol-Okhotsk mobile belt and occupied now by the Baikal rift. Two peridotites have clinopyroxenes depleted in light rare earth elements (LREE) with Sr and Nd model ages of about 2 Ga and seem to retain the trace element and isotopic signatures of old depleted lithospheric mantle, while all other xenoliths show different degrees of LREE-enrichment. Amphiboles and clinopyroxenes in the hydrous peridotites are in Sr–Nd isotopic disequilibrium. If this reflects in situ decay of 147Sm and 87Rb rather than heterogeneities produced by recent metasomatic formation of amphiboles then 300–400 Ma have passed since the minerals were last in equilibrium. This age range then indicates an old enrichment episode or repeated events during the Paleozoic in the lithospheric mantle initially depleted maybe 2 Ga ago. The Bartoy hydrous and enriched dry peridotites, therefore, are unlikely to represent fragments of a young asthenospheric bulge which, according to seismic reflection studies, reached the Moho at the axis of the Baikal rift zone a few Ma ago. By contrast, hydrous veins in peridotites may be associated with rift formation processes.  相似文献   

9.
The Indo–Asian continental collision is known to have had a great impact on crustal deformation in south-central Asia, but its effects on the sublithospheric mantle remain uncertain. Studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the Asian continent, similar to the observed lateral extrusion of Asian crustal blocks. Here we present supporting evidence from P-wave travel time seismic tomography and numerical modeling. The tomography shows continuous low-velocity asthenospheric mantle structures extending from the Tibetan plateau to eastern China, consistent with the notion of a collision-driven lateral mantle extrusion. Numerical simulations suggest that, at the presence of a low-viscosity asthenosphere, continued mass injection under the Indo–Asian collision zone over the past 50 My could have driven significant lateral extrusion of the asthenospheric mantle, leading to diffuse asthenospheric upwelling, rifting, and widespread Cenozoic volcanism in eastern China.  相似文献   

10.
The recent acquisition of high-quality seismic refraction data in the Jordan—Dead Sea rift and adjacent areas has made possible the investigation of the dynamic properties of seismic P-waves refracted and reflected at the crust—upper mantle boundary.

These waves cause high-amplitude arrivals near the outer cusp of the travel-time curve which are followed by an abrupt decrease in amplitudes at increasing distances beyond the cusp.

It has been shown that such amplitude distributions can only be the result of a smooth rapid increase of velocity with depth. In the case of the Jordan—Dead Sea rift the amplitude distribution indicates the presence of a transition zone between the lower crust and upper mantle in which the velocity increases smoothly. The interpretation of seismic refraction data in the Rhinegraben indicates the existence of a similar transition zone. In both rifts the crust—mantle boundary outside the rift is represented by sharp velocity discontinuity.

The comparison of the velocity structure of the crust—upper mantle boundary suggests that a smooth transition zone at the base of the lower crust is a characteristic property of continental rifts which could be interpreted in terms of crust—mantle interaction.  相似文献   


11.
杨文采 《地质论评》2014,60(5):945-961
本篇讨论大陆岩石圈拆沉、伸展与裂解作用过程。由于大陆岩石圈厚度大而且很不均匀,产生裂谷的机制比较复杂。大陆碰撞远程效应的触发,岩石圈拆沉,以及板块运动的不规则性和地球应力场方向转折,都可能产生岩石圈断裂和大陆裂谷。岩石圈拆沉为在重力作用下"去陆根"的作用过程,演化过程可分为大陆根拆离、地壳伸展和岩石圈地幔整体破裂三个阶段。大陆碰撞带、俯冲的大陆和大洋板块、克拉通区域岩石圈,都可能产生岩石圈拆沉。大陆岩石圈调查表明,拉张区可见地壳伸展、岩石圈拆离、软流圈上拱和热沉降;它们是大陆岩石圈伸展与裂解早期的主要表现。从初始拉张的盆岭省到成熟的张裂省,拆离后地壳伸展成复式地堑,下地壳幔源玄武岩浆侵位,断裂带贯通并切穿整个岩石圈,表明地壳伸展进入成熟阶段。中国东北松辽盆地和西欧北海盆地曾处于成熟的张裂省。岩石圈破裂为岩浆侵位提供了阻力很小的通道网。岩浆侵位作用伴随岩石圈破裂和热流体上涌,成熟的张裂省可发展成大陆裂谷。多数的大陆裂谷带并没有发展成威尔逊裂谷带和洋中脊,普通的大陆裂谷要演化为威尔逊裂谷带,必须有来自软流圈的长期和持续的热流和玄武质岩浆的供应。威尔逊裂谷带岩石圈地幔和软流圈为地震低速带,其根源可能与来自地幔底部的地幔热羽流有关。  相似文献   

12.
J.D. Fairhead 《Tectonophysics》1976,30(3-4):269-298
A compilation of all published and unpublished gravity data for the Eastern rift between latitudes 1°N and 5°S is presented. The Bouguer anomaly map reveals that the shape of the negative regional anomaly associated with the rift is approximately two-dimensional, striking east of north, of width 350 ± 50 km and amplitude500 ± 100 g.u. relative to the background value of−1300 ± 100 g.u. to the west. The regional anomaly is interpreted in terms of an upward thinning of the lithosphere and replacement by low-density asthenosphere. This model is different from previous interpretations in that major lithospheric thinning is restricted to the region of the Eastern rift affected by the domal uplift and does not extend beneath the Lake Victoria region to the west. The gravity and seismic models are compatible if the anomalous upper mantle (asthenospheric part), beneath the rift, is in a state of partial melt. A consequence of the revised regional anomaly is that it reduces previous amplitude estimates of the axial positive residual anomaly within the rift by at least 50% and generates negative anomalies over the rift shoulders in areas covered by Cenozoic volcanics. These negative anomalies are considered to be caused by the low density of the surface volcanics. Within the rift, elongated negative anomalies of amplitude 100–350 g.u. are associated with sedimentary basins and are attributed to low-density sediments up to 3 km thick. The positive residual anomaly along the axis of the rift can be interpreted in terms of either a dyke injection zone less than 15 km wide or by a dense infill body about 2.5 km thick. The positive anomaly is shown to be confined to the volcanic province of the Eastern rift and has its southern termination in the Magadi—Natron area, just north of where the Kenya rift valley changes to block faulting in N. Tanzania. This termination coincides with a change in the spatial distribution of the seismic and geothermal activity.  相似文献   

13.
The largest rift zone of Europe and Asia is located in the region of Lake Baikal. In 1968–1970 deep seismic measurements were carried out along a number of profiles with a total length of about 2000 km within the rift zone and in the adjacent parts of the Siberian platform and the region of the Baikal Mountains. These investigations were of a reconnaissance nature, and therefore the point sounding method was used.A low-velocity region for compressional waves (7.6–7.8 km/sec) has been found and could be traced over a large area in the upper parts of the mantle. The width of this anomalous zone is 200–400 km. The Baikal rift lies in its northwestern part. Within the studied part of the Siberian platform the thickness of the earth's crust is 37–39 km, while in the rift zone it is 36 km, and further to the southeast the crust-mantle boundary lies at a depth of 45–46 km. The Baikal rift proper is bounded in the northwest by a deep fracture zone and does not seem to be associated with any significant “root” or “antiroot” in the relief of the Mohorovi?i? discontinuity.The reduced compressional velocity in the upper parts of the mantle beneath the Baikal zone is considered to correspond to the same phenomena found under the mid-oceanic ridges and the extended rift system in the Basin and Range province of North America. The Baikal rift in the narrow sense of the word lies over the northwestern edge of the anomalous mantle region. This asymmetric position seems to be its main peculiarity.  相似文献   

14.
蒙古—贝加尔裂谷的演化及其形成的动力学机制一直是地学界争论的焦点。至今,对其地热学的相关研究一直比较匮乏。本文根据前人对蒙古—贝加尔及邻区的独特地貌、构造和玄武岩火山岩浆作用的研究,并结合现今地表大地热流特征共同探讨了其地球动力学机制。根据最新大地热流分布特征表明:蒙古地区的高热流区(120 mW/m~2)主要集中在蒙古Hangay穹窿北部Hovsgol裂谷及其周围裂谷内;贝加尔裂谷整体热流都较高,且贝加尔东北部热流达160 mW/m~2以上(比前人报道的更高),其中部热流也高(120 mW/m~2)。综合地热、地质与地球物理成果,本文认为晚新生代的地幔柱对蒙古—贝加尔地区的形成起着重要作用。  相似文献   

15.
We present seismic images of the mantle beneath East Russia and adjacent regions and discuss geodynamic implications. Our mantle tomography shows that the subducting Pacific slab becomes stagnant in the mantle transition zone under Western Alaska, Bering Sea, Sea of Okhotsk, Japan Sea, and Northeast Asia. Many intraplate volcanoes exist in these areas, which are located above the low-velocity zones in the upper mantle above the stagnant slab, suggesting that the intraplate volcanoes are related to the dynamic processes in the big mantle wedge above the stagnant slab and the deep slab dehydration. Teleseismic tomography revealed a low-velocity zone extending down to 660 km depth beneath the Baikal rift zone, which may represent a mantle plume. The bottom depths of the Wadati–Benioff deep seismic zone and the Pacific slab itself become shallower toward the north under Kamchatka Peninsula, and the slab disappears under the northernmost Kamchatka. The slab loss is considered to be caused by the friction between the slab and the surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then the slab loss was enlarged by the slab-edge pinch-off by the hot asthenospheric flow and the presence of Meiji seamounts.  相似文献   

16.
The three-dimensional (3D) lithospheric density structure of the Eastern Alps was investigated by integrating results from reflection seismics, receiver function analyses and tomography. The modelling was carried out with respect to the Bouguer gravity and the geoid undulations and emphasis were laid on the investigations of the importance of deep lithospheric features. Although the influence of inhomogeneities at the lithosphere–asthenosphere boundary on the potential field is not neglectable, they are overprinted by the response of the density contrast at the crust–mantle boundary and intra-crustal density anomalies. The uncertainties in the interpretations are in the same order of magnitude as the gravity field generated by the deep lithosphere.After including the deep lithospheric geometry from the tomographic model it is shown that full isostatic equilibrium is not achieved below the Eastern Alps. However, calculation of the isostatic lithospheric thickness shows two areas of lithospheric thickening along the central axis of the Eastern Alps with a transition zone below the area of the TRANSALP profile. This is in agreement with the tomographic model, which features a change in lithospheric subduction direction.  相似文献   

17.
Based on rheological interpretation of formalized gravity models, earlier known deep-seated structures in the Earth’s crust and mantle of Transbaikalia have been detailed and new ones discovered. The structures are asymmetric and transverse relative to the Baikal rift zone. Their presence explains the peculiar features of the Baikal rift, including the one-way southeasterly direction of horizontal displacement of tectonic masses and northwestern migration of the Earth’s crust extension processes. The prolonged history (more than 250 Ma) of the Baikal rift zone and Transbaikalia mountainous country involved gravity or rotational detachments of rigid tectonic slabs from the craton and their sliding along intracrustal and subcrustal decollement zones into the above-dome area of the Transbaikalia asthenolith.  相似文献   

18.
Interpretation of reprocessed seismic reflection profiles reveals three highly coherent, layered, unconformity-bounded sequences that overlie (or are incorporated within) the Proterozoic “granite–rhyolite province” beneath the Paleozoic Illinois basin and extend down into middle crustal depths. The sequences, which are situated in east–central Illinois and west–central Indiana, are bounded by strong, laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad “basinal” packages that become areally more restricted with depth. Normal-fault reflector offsets progressively disrupt the sequences with depth along their outer margins. We interpret these sequences as being remnants of a Proterozoic rhyolitic caldera complex and/or rift episode related to the original thermal event that produced the granite–rhyolite province. The overall thickness and distribution of the sequences mimic closely those of the overlying Mt. Simon (Late Cambrian) clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post-Cambrian Illinois basin stratigraphic succession, which is centered farther south over the “New Madrid rift system” (i.e., Reelfoot rift and Rough Creek graben). The present configuration of the Illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions, the Proterozoic caldera complex and/or rift in east–central Illinois and west–central Indiana and the New Madrid rift system to the south. Filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any Proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite–rhyolite.  相似文献   

19.
洋-陆过渡带是理解大陆岩石圈破裂和海底初始扩张的关键位置,但是在南海北部地区仍然存在关于相关地质过程的诸多疑问.通过近年开展的国际大洋发现计划航次以及深部地质地球物理探测,取得以下4个方面的认识.（1）南海北部的洋-陆边界一般与自由空间重力异常的正-负值过渡位置对应,而更加准确地限定需要结合反射、折射地震资料.稳定大洋岩石圈生成与大陆岩石圈最终破裂之间的洋-陆过渡边界的位置比以往认为的还应往深海盆方向移动.（2）洋-陆过渡带代表了远端带构造作用减弱和岩浆作用逐渐增强的区域.陆坡地壳发育扩张后岩浆底侵、洋-陆过渡带发育同破裂期岩浆喷出结构和侵入反射体.（3）在中生代的古俯冲带弧前区域,新生代的断裂沿着早期的构造开始活动,岩石圈多处发生强烈的共轭韧性剪切作用.随着大陆岩石圈的进一步拉伸减薄,部分靠陆一侧的裂谷中心停止张裂,成为夭折裂谷,以台西南盆地南部凹陷、白云凹陷、西沙海槽为代表,而南海陆缘异常伸展和最终破裂的地方集中在南侧裂谷中心.夭折裂谷下亦发现地幔蛇纹石化,进一步反映了较弱的同破裂岩浆活动.（4）南海初始洋壳的增生沿着大陆边缘走向具有显著的变化,南海东北部洋-陆过渡带下伏地幔明显抬升和部分蛇纹石化,地震纵、横波速度以及折射波衰减特征都支持此观点,反映南海东北部是一个贫岩浆型大陆边缘.未来,南海北部洋-陆过渡带有望成为南海“莫霍钻”的理想备选钻探区.   相似文献   

20.
The source of hotspot volcanism lies in metasomatized regions of the continental mantle proximal to ancient sutures and failed rifts. Such regions are prone to melting under hotcell conditions on continental rifting, and to erosion into the deeper mantle by asthenospheric flow. In opening basins, rifting parallel to such sutures or failed rifts delaminates and cycles continental mantle into the MORB source. Rifting at some angle to a suture or failed rift generates a hotspot track by preferential melting of the metasomatized mantle as it is cycled toward the rift axis. Continental mantle eroded into the asthenosphere becomes displaced from the continent by net westward drift of the lithosphere relative to the deep mantle to give rise to hotspot volcanism in long-lived ocean basins.  相似文献   

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