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1.
The tectonics, structure-forming processes, and magmatism in rift zones of ultraslow spreading ridges are exemplified in the Reykjanes, Kolbeinsey, Mohns, Knipovich, Gakkel, and Southwest Indian ridges. The thermal state of the mantle, the thickness of the brittle lithospheric layer, and spreading obliquety are the most important factors that control the structural pattern of rift zones. For the Reykjanes and Kolbeinsey ridges, the following are crucial factors: variations in the crust thickness; relationships between the thicknesses of its brittle and ductile layers; width of the rift zone; increase in intensity of magma supply approaching the Iceland thermal anomaly; and spreading obliquety. For the Knipovich Ridge, these are its localization in the transitional zone between the Gakkel and Mohns ridges under conditions of shear and tensile stresses and multiple rearrangements of spreading; nonorthogonal spreading; and structural and compositional barrier of thick continental lithosphere at the Barents Sea shelf and Spitsbergen. The Mohns Ridge is characterized by oblique spreading under conditions of a thick cold lithosphere and narrow stable rift zone. The Gakkel and the Southwest Indian ridges are distinguished by the lowest spreading rate under the settings of the along-strike variations in heating of the mantle and of a variable spreading geometry. The intensity of endogenic structure-forming varies along the strike of the ridges. In addition to the prevalence of tectonic factors in the formation of the topography, magmatism and metamorphism locally play an important role. 相似文献
2.
The interaction of the Mid-Atlantic Ridge with the North Atlantic Mantle Plume has produced a magmatic plateau centred about Iceland. The crust of this plateau is 30 km thick on average. This abnormal thickness implies that, unlike other slow-spreading ridges, addition of magmatic material to the crust is not balanced by crustal stretching. The thermal effect of the plume also reduces the strength of the lithosphere. Both mechanisms affect the rifting process in Iceland. A structural review, including new field observations, demonstrates that the structure of the Iceland plateau differs from that of other slow-spreading oceanic ridges. Lithospheric spreading is currently accommodated in a 200 km wide deformation strip, by the development of a system of half-grabens controlled by growth faults. Similar extinct structures, with various polarities, are preserved in the lava pile of the Iceland plateau. These structures are identified as lithospheric rollover anticlines that developed in hanging walls of listric faults. We introduce a new tectonic model of accretion, whereby the development of the magmatic plateau involved activation, growth and decay of a system of growth fault/rollover systems underlain by shallow magma chambers. Deactivation of a given extensional system, after a lifetime of a few My, was at the expense of the activation of a new, laterally offset, one. Correspondingly, such systems formed successively at different places within a 200 km wide diffuse plate boundary. Unlike previous ones, this new model explains the lack of an axial valley in Iceland, the dip pattern of the lava pile, the complex geographical distribution of ages of extinct volcanic systems and the outcrops of extinct magma chambers. 相似文献
3.
Major hypotheses on the formation of the Iceland region are considered. It is noted that plate- and plume-tectonic genesis
is the most substantiated hypothesis for this region. Model estimations of the effect of hot plume on the formation of genetically
different oceanic ridges are obtained. Computer calculations are performed for the thermal subsidence rate of aseismic ridges
(Ninetyeast and Hawaiian-Emperor) in the asthenosphere of the Indian and Pacific oceans. Comparative analysis of the calculated
subsidence rates of these ridges with those in the Iceland region (Reykjanes and Kolbeinsey ridges) is performed. The results
suggest that the thermophysical processes of formation of the spreading Reykjanes and Kolbeinsey ridges were similar to those
of the aseismic Ninetyeast and Hawaiian-Emperor ridges: the genesis of all these ridges is related to the functioning of a
hotspot. Analysis of the heat flux distribution in the Iceland Island and Hawaiian Rise areas is carried out. Analysis and
numerical calculations indicate that the genesis of Iceland was initially characterized by the plume-tectonic transformation
of a continental rather than oceanic lithosphere. The level of geothermal regime near Iceland was two times higher (100 mW/m2) relative to the Hawaiian Rise area (50 mW/m2) because the average lithosphere thickness of the Reykjanes and Kolbeinsey ridges near the Iceland was approximately two
times less (40 km) relative to the thickness of the Pacific Plate (80 km) in the Hawaiian area. The main stages of evolution
of the Iceland region are based on geological and geothermal data and numerical thermophysical modeling. The Cenozoic tectonic
evolution of the region is considered. Paleogeodynamic reconstructions of the North Atlantic in the hotspot system at 60,
50, and 20 Ma are obtained. 相似文献
4.
地幔活动在南海扩张中的作用数值模拟与讨论 总被引：8，自引：5，他引：3
对于地幔活动对南海扩张有无影响及影响大小这一问题,目前仍存在不同观点。本文采用数值模拟方法对地幔活动在南海扩张中的作用和影响进行了考查,结果表明:地幔上涌作用能引起南海岩石圈很大程度的减薄,但对南海地壳减薄影响很小;地幔上涌作用是南海扩张的重要动力学因素之一,但只在地幔上涌作用下要很长的时间才能打开南海。 相似文献
5.
The subduction of spreading ridges creates a special geodynamic setting distinguished by the interference of convergent and
divergent boundaries between lithospheric plates and their long-term interaction accompanied by the formation of characteristic
geological complexes and structures. The available data on subduction of the contemporary Chile Ridge make it possible to
reconstruct such settings in the geological past. The subduction of the spreading ridge leads to uplift of the continental
margin, cut off the accretionary wedge by means of tectonic erosion, emplacement of a fold-thrust structure and longitudinal
strike-slip faults, and creates settings favorable for obduction of the young oceanic lithosphere. A lithospheric window expressed
in geological and geophysical features opens beneath the continental margin at the continuation of the ridge axis. The subduction-related
volcanic activity ceases above this window, giving way to specific proximal magmatism close to the boundary with the ocean and distal magmatism at a distance from this boundary. The proximal bimodal magmatism was related to the sources of tholeiitic basalts
characteristic of the ridge involved in subduction and to the partial melting of its oceanic crust and sediments. The distal
basaltic magmatism is a product of melting of the fertile oceanic asthenosphere ascending through the lithospheric window
with subsequent transformation of magma in the mantle wedge and the continental crust. The use of the Chilean tectonotype
for paleoreconstructions is limited by the diverse settings of ridge subduction. The Paleogene magmatism at the Pacific margin
of Alaska, where the kinematics of subduction was close to the Chilean subduction, is similar to the proximal igneous rocks
of Chile in composition and zoning, retaining some geological differences. Another aspect of the paleoreconstruction is discussed
on the basis of Jurassic and Cretaceous granitoids of the Ekonai Terrane of the Anadyr-Koryak System and terranes of southern
Alaska. These localities are known for a special, accretionary type of granitoids in the forearc region related to anatectic
magma formation without participation of the plunging ridge. Proceeding from comparison with the Chilean tectonotype, the
criteria for the identification of granitoids varying in their origin are considered. The effect of subducting ridges on continental
margins changed over geologic time and was subject to the rhythm of supercontinental cycles. 相似文献
6.
华北东部中生代晚期-新生代软流圈上涌与岩石圈减薄 总被引：37，自引：3，他引：34
现今的地幔是由软流圈地幔(热的,主元素饱满、微量元素亏损的,塑性流变性质的)、古老岩石圈地幔(地幔1,冷的,主元素贫瘠、微量元素富集的,刚性的,以方辉橄榄岩为代表)以及现今的岩石圈地幔(地幔2,主元素饱满、微量元素亏损,以二辉橄榄岩为代表,可能包括多时期形成的)组成。古老岩石圈地幔与地幔2样品的共存、100~4·3Ma在地幔内部持续发生的古老岩石圈与软流圈的相互作用以及上述作用的时空不均一性,都表明了岩石圈减薄是软流圈呈“蘑菇云状”大规模上涌的结果。上述事件发生于100Ma以后。软流圈来源的玄武岩大范围喷发并伴随了岩石圈的强烈拉伸是事件发生的主要标志,岩石圈减薄是一个深部地质过程,不像是突发事件。 相似文献
7.
Propagation of brittle failure triggered by magma in Iceland 总被引：1，自引：1，他引：1
The architectures of normal faults at a divergent plate boundary in Iceland are examined by combining surface fault observations with cross-sectional studies at different structural levels to constrain a model of failure propagation. The structures of Holocene faults defining graben are analyzed to characterize the upper-most parts of ruptures. The shapes of faults resulting from growth and interaction of separate segments are used to better understand failure propagation inferred to occur in the intervening stages of displacement accumulation and lateral propagation. Pleistocene faults in volcanic sequences exhumed from 800 to 1000 m are analyzed to characterize deeper portions of failure that occurred beneath the central rift zone. Tertiary dikes exhumed from depths of 1300–1500 m are studied to infer how magma controls the failure initiation. Field studies in combination with a literature review indicate that the planar ruptures are likely to initiate at depth as magma-filled vertical fractures and lengthen upward and laterally. As failures propagate to higher crustal levels, they are likely to change into inclined normal faults. At near-surface levels, faults link with cooling joints and dilational fractures propagating downward from the surface. It is suggested that the inferred stages of fault propagation are characteristic for normal faults developed at spreading ridges. 相似文献
8.
The results of analysis of the anomalous magnetic field of the Reykjanes Ridge and the adjacent basins are presented, including a new series of detailed reconstructions for magnetic anomalies 1–6 in combination with a summary of the previous geological and geophysical investigations. We furnish evidence for three stages of evolution of the Reykjanes Ridge, each characterized by a special regime of crustal accretion related to the effect of the Iceland hotspot. The time interval of each stage and the causes of the variation in the accretion regime are considered. During the first, Eocene stage (54–40 Ma) and the third, Miocene-Holocene stage (24 Ma-present time at the northern Reykjanes Ridge north of 59° N and 17–11 Ma-present time at the southern Reykjanes Ridge south of 59° N), the spreading axis of the Reykjanes Ridge resembled the present-day configuration, without segmentation, with oblique orientation relative to the direction of ocean floor opening (at the third stage), and directed toward the hotspot. These attributes are consistent with a model that assumes asthenospheric flow from the hotspot toward the ridge axis. Decompression beneath the spreading axis facilitates this flow. Thus, the crustal accretion during the first and the third stages was markedly affected by interaction of the spreading axis with the hotspot. During the second, late Eocene-Oligocene to early Miocene stage (40–24 Ma at the northern Reykjanes Ridge and 40 to 17–11 Ma at the southern Reykjanes Ridge), the ridge axis was broken by numerous transform fracture zones and nontransform offsets into segments 30–80 km long, which were oriented orthogonal to the direction of ocean floor opening, as is typical of many slow-spreading ridges. The plate-tectonic reconstructions of the oceanic floor accommodating magnetic anomalies of the second stage testify to recurrent rearrangements of the ridge axis geometry related to changing kinematics of the adjacent plates. The obvious contrast in the mode of crustal accretion during the second stage in comparison with the first and the third stages is interpreted as evidence for the decreasing effect of the Iceland hotspot on the Reykjanes Ridge, or the complete cessation of this effect. The detailed geochronology of magnetic anomalies 1–6 (from 20 Ma to present) has allowed us to depict with a high accuracy the isochrons of the oceanic bottom spaced at 1 Ma. The variable effect of the hotspot on the accretion of oceanic crust along the axes of the Reykjanes Ridge and the Kolbeinsey and Mid-Atlantic ridges adjoining the former in the north and the south was estimated from the changing obliquity of spreading. The spreading rate tends to increase with reinforcing of the effect of the Iceland hotspot on the Reykjanes Ridge. 相似文献
9.
多波束测深技术在国际上是海洋科学研究、海底资源开发和海洋工程建设中的重要技术手段。基于对国内外多波束
地形数据的广泛调研,对洋中脊附近洋底构造地貌形态进行分析研究。文中利用不同扩张速率洋中脊附近的50 m分辨率的多
波束地形数据,基于数字地形空间分析方法,利用不同滑动窗口和阈值自动识别来提取洋中脊附近地形面的最大、最小曲率
以及坡度,并以此对洋中脊进行构造解译。对中大西洋洋中脊和东太平洋洋隆两个实验结果的定量分析表明,基于地形曲面
曲率和坡度的洋中脊构造解译方法是有效且可行的,其结果为洋中脊构造样式解译提供重要参考。但是相比之下, 相似文献
10.
GiovanniB.Piccardo 《《幕》》2003,26(3):193-199
Mantle peridotites were early exposed at the sea-floor of the Jurassic Tethys derived from the subcontinental mantle of the Europe-Adria system. During continental rifting and oceanic spreading, these lithospheric peri-dotites were percolated via diffuse reactive porous flowby melt fractions produced by near-fractional melting of the upwelling asthenosphere. Ascending melts inter-acted with the lower lithosphere, dissolving pyroxenes and precipitating olivine, and crystallized at shallower levels in the mantle column causing melt impregnation.Subsequent focused porous flow formed replacive dunitechannels, cutting the impregnated oeridotites, which were conduits for upward migration of MORB-type liq-uids. Melt migration produced depletionlrefertilization and significant heating of the percolatedlimpregnated mantle, i.e the thermochemical erosion of the litho-sphere. Impregnated and thermally modified lithos-pheric mantle was cooled by conductive heat loss dur-ing progressive lithosphere thinning and was intrudeaby MORB magmas, which formed Mg-rich and Fe-richgabbroic dykes and bodies. Alpine-Apennine ophiolitic peridotites record the deep-seated migration of melts which changed their compositions and dynamics during the rift evolution. The thermochemical erosion of the lithospheric mantle by the ascending asthenospheric melts, which induces significant compositional and rhe-ological changes in the lower lithosphere, is a major process in the evolution of the continent-ocean transi-tion towards a slow spreading oceanic system. 相似文献
11.
The propagation and segmentation of mid-ocean ridges is studied using centrifuged analogue models built with non-linear materials. The deformation of the brittle-ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force. This linear upwelling laterally stretches the model mantle that, in turn, induces failure in the upper layer simulating the brittle crust. Arrays of fractures initiate within zones of high stress concentration above the diapir. Fractures propagate laterally in a direction perpendicular to the maximum tensile (the minimum principal) stress. Secondary tension cracks initiate in the vicinity of parent fracture tips. Through-going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns. The overlap, overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence. Non-overlapping sub-parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture. If overlapping fractures are parallel, they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material. Fracture curvature in this case results from crack–crack interaction and is similar to that of overlapping spreading centers (OSCs) observed along mid-ocean ridges. Overlapping non-parallel fractures tend to coalesce by one of their tips propagating sub-parallel to the spreading direction toward the other fracture. Such offsets can serve as models for the development of the orthogonal ridge-transform fault patterns common along mid-ocean ridges. 相似文献
12.
This paper presents an updated review of recent field/structural and petrologic/geochemical studies on orogenic peridotites from the Alpine–Apennine ophiolites (NW Italy). Results provide determinant constraints to the evolution of the lithospheric mantle during passive rifting of the fossil Ligurian Tethys oceanic basin.The pre-rift, spinel lherzolites precursors, preserved in the mantle section of the Ligurian ophiolites, were resident in the lithosphere along an intermediate geothermal gradient (T about 1000 °C, P compatible with spinel-peridotite facies). Passive rifting by far-field tectonic forces induced whole-lithosphere extension and thinning (the a-magmatic stage). After significant thinning of the lithosphere, the passively upwelling asthenosphere underwent decompression melting along the axial zone of extension. Silica-undersaturated melt fractions infiltrated via diffuse/focused porous-flow through the lithospheric mantle under extension (the magmatic stage) and underwent pyroxenes-dissolving/olivine-crystallizing interaction with the percolated host peridotite.Pyroxenes assimilation and olivine deposition modified the melt compositions into silica-saturated. These derivative liquids migrated to shallower, plagioclase-peridotite facies levels, where they stagnated and impregnated/refertilized the lithospheric mantle. Melt thermal advection by melt infiltration heated to temperatures higher than 1200 °C the lithospheric mantle column above the melting asthenosphere.The syn-rift magmatic and tectonic processes induced significant rheological softening/weakening that destabilized the lithospheric mantle of the Europe–Adria plate along the axial zone of extension. The presence of destabilized lithospheric mantle between the future continental margins played a determinant role in promoting the geodynamic evolution from pre-oceanic rifting to oceanic spreading.The active upwelling of hotter/deeper asthenosphere inside the destabilized axial zone promoted transition to active rifting, enhancing continent break-up. Asthenosphere underwent partial melting and formed aggregated MORB liquids that migrated inside high-porosity dunite channels. The MORB liquids formed olivine-gabbro intrusions and pillowed lava flows (the oceanic crustal rocks).This paper evidences the primary role of mantle destabilization by melt infiltration in the geodynamic evolution of the Ligurian Tethys rifting. 相似文献
13.
Zheng Jianping Yu Chunmei Yuan Xiaoping Yin Li Jiao Shujuan Tang Huayun Zhang Zhihai Lu Fengxiang 《Frontiers of Earth Science》2007,1(1):37-43
A petrochemical analysis was undertaken of peridotitic xenoliths in volcanic rocks that erupted from the Paleozoic to the
Cenozoic within the eastern part of the North China craton, and the peridotites as tectonic intrusion in the Early Mesozoic
from the Sulu orogen. The results show that the cratonic mantle, which was refractory and existed when the kimberlites intruded
in the Paleozoic, had almost been replaced by the newly accreted fertile lithospheric mantle during the Mesozoic-Cenozoic.
The erosion, metasomatism, and intermingling caused by the accreted asthenospheric material acting on the craton mantle along
the weak zone and deep fault (such as the Tanlu fault) in the existing lithosphere resulted in the lithospheric thinning at
a larger scale 100 Ma ago (but later than 178 Ma). The largest thinning would be in the Eogene. The upwelling asthenospheric
material transformed into accreted lithospheric mantle due to the asthenospheric temperature falling in the Neogene (leading
to relatively slight lithospheric incrassation), and finally accomplished mantle replacement. The peridotitic body in the
Sulu orogen represents the products spreading from the modified cratonic lithospheric mantle.
Translated from Earth Science—Journal of China University of Geosciences, 2006, 31(1): 49–56 [译自: 地球科学—中国地质大学学报] 相似文献
14.
Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise 总被引：1，自引：0，他引：1 
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下载免费PDF全文 A comprehensive model for the activity of the elementary accretion segment at fast‐spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis. 相似文献
15.
Fracture-fissure systems found at mid-ocean ridges are dominating conduits for the circulation of metallogenic fluid. Ascertaining the distribution area of active faults on both sides of mid-ocean ridges will provide a useful tool in the search for potential hydrothermal vents, thus guiding the exploration of modern seafloor sulfides. Considering the Mid-Atlantic Ridge 20°N–24°N (NMAR) and North Chile Rise (NCR) as examples, fault elements such as Fault Spacing (?S) and Fault Heave (?X) can be identified and quantitatively measured. The methods used include Fourier filtering of the multi-beam bathymetry data, in combination with measurements of the topographic slope, curvature, and slope aspect patterns. According to the Sequential Faulting Model of mid-ocean ridges, the maximal migration distance of an active fault on either side of mid-ocean ridges—that is, the distribution range of active faults—can be measured. Results show that the maximal migration distance of active faults at the NMAR is 0.76–1.01 km (the distance is larger at the center than at the ends of this segment), and at the NCR, the distribution range of active faults is 0.38–1.6 km. The migration distance of active faults on the two study areas is positively related to the axial variation of magma supply. In the NCR study area, where there is an abundant magma input, the number of faults within a certain distance is mainly affected by the variation of lithospheric thickness. Here a large range of faulting clearly corresponds to a high proportion of magmatism to seafloor spreading near mid-ocean ridges (M) value, and in the study area of the NMAR, there is insufficient magmatism, and the number of faults may be controlled by both lithospheric thickness and magma supply, leading to a less obvious positive correlation between the distribution range of active faults and M. 相似文献
16.
The tectonical setting in Iceland is quite complex due to the interaction of the Iceland hot spot and the Mid Atlantic Ridge. While in the north of the island one active spreading zone exists, the divergent motion in the centre and the south is distributed over at least two volcanic rift zones. The spreading rate increases linearly along the Western Volcanic Zone from north to south up to 8 mm/yr at the Hengill triple junction. On the contrary, the spreading rate of the parallel Eastern Volcanic Zone decreases from 16 mm/yr down to 6 mm/yr at the island's southern coast. The Hreppar microplate between the two predominant rift zones has an independent motion, which is distinct from that of the Eurasian and North American plates. A new detected feature is the spreading activity around the Hofsjökull volcanic zone located in the centre of Iceland with a significant rate of 6 mm/yr. During this investigation the coordinate sets of nearly 20 years of GPS data acquisition on Iceland were combined to get a velocity field for the surface of Iceland. This velocity field is based on a linear kinematic model with the consideration of local non-linear effects like volcano up-doming and displacements due to major earthquakes. 相似文献
17.
《Gondwana Research》2016,29(4):1329-1343
Continental rifting to seafloor spreading is a continuous process, and rifting history influences the following spreading process. However, the complete process is scarcely simulated. Using 3D thermo-mechanical coupled visco-plastic numerical models, we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading. Our modeling results show that the initial continental lithosphere rheological coupling/decoupling at the Moho affects oceanic spreading in two manners: (1) coupled model (a strong lower crust mechanically couples upper crust and upper mantle lithosphere) generates large lithospheric shear zones and fast rifting, which promotes symmetric oceanic accretion (i.e. oceanic crust growth) and leads to a relatively straight oceanic ridge, while (2) decoupled model (a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere) generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3D features. Complex ridge geometries (e.g. overlapping ridge segments and curved ridges) are generated in the decoupled models. Two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models, respectively. Continental detachment faults are generated through rotation of high angle normal faults during rifting, and terminated by magmatism during continental breakup. Oceanic detachment faults form in oceanic crust in the late rifting–early spreading stage, and dominates asymmetric oceanic accretion. The life cycle of oceanic detachment faults has been revealed in this study. 相似文献
18.
华北东部橄榄岩与岩石圈减薄中的地幔伸展和侵蚀置换作用 总被引：24，自引：3，他引：24
对比分析了华北不同时代捕虏体橄榄岩及其南部超高压地质体橄榄岩的矿物化学。具古老难熔岩石圈地幔特征的橄榄岩是古生代金伯利岩捕虏体和早中生代苏鲁变质带地质体的主要岩石类型。具这一性质的橄榄岩也构成了河南鹤壁上新世玄武岩捕虏体的主体部分,并可以在辽宁阜新晚中生代玄武岩中被发现。具饱满岩石圈地幔性质的橄榄岩则是阜新晚中生代火山岩、特别是郯庐断裂带(山旺)及其附近地区(栖霞)中新世玄武岩捕虏体的主要类型。从华北东部已有的捕虏体橄榄岩及地质体橄榄岩所表现出的新生饱满与古老难熔地幔的时、空分布特点,即有些地区捕虏体橄榄岩表现出不同性质地幔共存现象(如阜新、鹤壁)或橄榄石Mg#呈渐变关系看:克拉通岩石圈地幔因扬子板块俯冲所引起的早期(如早中生代)地幔伸展、和晚中生代—渐新世因太平洋俯冲所引起的热扰动的软流圈物质上涌对古老地幔产生强烈的侵蚀作用(引起岩石圈的巨大减薄);中新世以来的软流圈热沉降作用出现新生岩石圈地幔并表现为岩石圈的小幅增厚,从而实现地幔置换过程和华北东部岩石圈的整体减薄过程。岩石圈幔内薄弱带及岩石圈深断裂(如郯庐断裂带)起了软流圈物质侵蚀古老岩石圈地幔的通道作用并导引着深部物质运移和不规则减薄作用等。个别地区(如阜新)强烈的软流圈上涌于晚中生代就已经开始,显示地幔置换作用的强烈不均一性。 相似文献
19.
The evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, is discussed on the basis of a crustal transect obtained by seismic experiment from the Kolbeinsey Ridge to the Jan Mayen Basin. The crustal model indicates a relatively uniform structure; no significant lateral velocity variations are observed, especially in the lower crust. The uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the Kolbeinsey Ridge, but supports a model of continuous spreading along the ridge after oceanic spreading started west of the Jan Mayen Basin. The oceanic crust formed at Kolbeinsey Ridge is 1–2.5 km thicker than normal oceanic crust due to hotter-than-normal mantle from the Iceland Mantle Plume. The observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno-spheric mantle ascending along the Kolbeinsey Ridge have not changed significantly since the age of magnetic anomaly 6B. 相似文献
20.
Ophiolites worldwide show striking diversities in their rock assemblage and structure (i.e., ophiolite diversity), raising a question whether ophiolites are originally similar before intense tectonic dismemberment. Comparison between ophiolites and oceanic lithospheres at modern mid-ocean ridges may provide key constraints on the origin of ophiolite diversity, because oceanic lithospheric structures are inherently controlled by spreading rates. Here, we present a case study of the Xigaze ophiolite in southern Tibet focusing on its gabbroic intrusions outcropping in three localities, i.e., Dazhuqu, Baigang and Jiding. Compared to the Jiding sequence, the Dazhuqu and Baigang gabbroic rocks are less evolved, characterized by higher Cr2O3 contents but lower contents of TiO2 and rare earth element in both clinopyroxene and bulk compositions. It is evident, hence, that the Xigaze ophiolite is characterized by variably evolved and discontinuously distributed gabbroic intrusions, rather by a continuous lower oceanic crust between the mantle and sheeted dike complex as the Penrose-type ophiolites. Our study, along with the identification in previous studies of oceanic detachment faults within the Xigaze ophiolite, demonstrates that the Xigaze ophiolite shows close similarities to oceanic lithospheres at modern slow- and ultraslow-spreading ridges. Hence, the significant structural distinctions between the Xigaze ophiolite and the Penrose-type ophiolites (e.g., the Oman ophiolite) may be inherently associated with different spreading rates of paleo-ridges. Considering the limited scale of the Xigaze gabbroic rocks, here we suggest the Xigaze ophiolite as a typical representative of fossil ultraslow-spreading ridges. 相似文献