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1.
Terrestrialheatflowdensityistheamountofheatperunitareatransferringfromtheearthinteriortosurface,whichisanexteriorcharacteristicofthethermalandgeodynamicprocessesoccurringindeepmantle[1].Heatflow,asthemostdirectmannerofthermalprocessindeepearthandembodyingabundantinformationofgeology,geophysicsandgeodynamics,isasyntheticallythermalindexandplaysaveryimportantroleinstudyingnotonlytheactivityofcrust,thermalstructureofcrustanduppermantle,aswellasrheologicalstructureoflithosphere,butalsotheevaluation…  相似文献   

2.
The objective of this study is to understand the nature and extent of the regional geothermal system at depth beneath the area of Greece by constructing the Curie isotherms.Spectral analysis of aeromagnetic data in conjunction with heat flow information revealed an almost inverse linear relation between heat flow and Curie depths and was used to construct the Curie isotherms from the existing heat flow data.The results showed that Curie depths in the area range from about 20 km in western Greece, up to 1 km beneath the Hellenic volcanic arc. These results are consistent with the existing geothermal and geotectonic regime in the area.  相似文献   

3.
Melt generation and extraction along the Hawaiian volcanic chain should be largely controlled by the thermal structure of the Hawaiian swell and the heat source underneath it. We simulate numerically the time- and space-dependent evolution of Hawaiian volcanism in the framework of thermal evolution of the Hawaiian swell, constrained by residual topography, geoid anomalies, and anomalous heat flow along the Hawaiian volcanic chain. The transient heat transfer problem with melting relationships and variable boundary conditions is solved in cylindrical coordinates using a finite difference method. The model requires the lithosphere to be thinned mechanically by mantle plume flow. Melting starts quickly near the base of the plate when the hotspot is encountered. Thermal perturbation and partial melting are largely concentrated in the region where the original lithosphere is thinned and replaced by the mantle flow. The pre-shield Loihi alkalic and tholeiitic basalts are from similar sources, which are a mixture of at least three mantle components: the mantle plume, asthenosphere, and the lower lithosphere. The degree of partial melting averages 10–20%, with a peak value of 30% near the plume center. As a result of continuous compaction, melts are extracted from an active partial melting zone of about 10–20 km thickness, which moves upwards and laterally as the heating and compaction proceed. The rate of melt extraction from the swell increases rapidly to a maximum value of 1 × 105 km3/m.y. over the center of the heat source, corresponding to eruption of large amounts of tholeiitic lavas during the shield-building stage. This volume rate is adequate to account for the observed thickness of the Hawaiian volcanic ridge. Melts from direct partial melting of the mantle plume at depth may be important or even dominant at this stage, although the amount is uncertain. At the waning stage, mixing of melts from the mantle flow pattern with those from low-degree partial melting of the lithosphere may produce postshield alkalic basalts. After the plate moves off the heat source, continuous conductive heating can cause very low degree partial melting (less than 1%) of the lithosphere at shallow depths for about one million years. This process may be responsible for producing post-erosional alkalic basalts. The extraction time for removing such small amount of melts is about 0.4–2 m.y., similar to the time gap between the eruption of post-erosional alkalic lavas and the shield-building stage. Our results show that multi-stage Hawaiian volcanism and the general geochemical characteristics of Hawaiian basalts can be explained by a model of plume-plate interaction.  相似文献   

4.
The Bjøirnøya West Basin lies between latitudes 73° and 74°, longitudes 16°E and 18°E, contains at least 8 km of sediments deposited from the Late Jurassic, and is of considerable interest for hydrocarbon exploration. The Cenozoic extensional tectonics in the basin can be clearly seen from seismic data with normal faulting and from subsidence curves with rapid subsidence. The extension occurred during the Late Palaeocene with active extension lasting about 6 million years (m.y.) followed by thermal cooling. The tectonic subsidence within the study area shows a three-phase development: phase 1, synrift (58–52 Ma (million years before the present day)), is characterized by rapid subsidence; phase 2, postrift (52–5 Ma), by slow subsidence with occasional uplift; and phase 3 (5–0 Ma), by rapid subsidence. An adaptive finite-element model, with consideration of the radiogenic heat production in the lithosphere, has been used to model the subsidence and heat flow. The modelling of subsidence shows the β-factor distribution varying from 1.9 to 3.5 with an average of 2.4 for the uniform lithospheric extension. The heat-flow modelling predicts a rapid increase of heat flow during the Early Palaeocene. The maximum heat flow at about 52 Ma, which could be as much as 3.0 hfu (10?6 cal/cm2/s), was followed by a decrease in heat flow. A plate-weakening model has been proposed to explain the rapid subsidence for the last 5 m.y. by flexure of the elastic lithosphere which is weakened by a decrease in elastic thickness caused by an increase of the temperature gradient in the lithosphere. The plate-weakening model predicts a heat-flow increase at 5 Ma of up to 2.0 hfu. Our study, using quantitative modelling of the tectonic subsidence, provides a partial (if not a full) understanding of the tectonic development and thermal evolution of the Bjønøya West Basin.  相似文献   

5.
Paleocene volcanic rocks in West Greenland and Baffin Island were among the first products of the Iceland mantle plume, forming part of a larger igneous province that is now submerged beneath the northern Labrador Sea. A 40Ar/39Ar dating study shows that volcanism commenced in West Greenland between 60.9 and 61.3 Ma and that 80% of the Paleocene lava pile was erupted in 1 million years or less (weighted mean age of 60.5±0.4 Ma). Minimum estimates of magma production rates (1.3×10−4 km3 year−1 km−1) are similar to the present Iceland rift, except for the uppermost part of the Paleocene volcanic succession where the rate decreases to <0.7×10−4 km3 year−1 km−1 (rift). The timing of onset of volcanism in West Greenland coincides with the opening of the northern Labrador Sea and is also strikingly similar to the age of the oldest Tertiary volcanic rocks from offshore SE Greenland and the British–Irish province. This is interpreted as manifesting the impact and rapid (>1 m/year) lateral spreading of the Iceland plume head at the base of the Greenland lithosphere at 62 Ma. We suggest that the arrival, or at least a major increase in the flux, of the Iceland mantle plume beneath Greenland was a contributing factor in the initiation of seafloor spreading in the northern Labrador Sea. Our study has also revealed a previously unrecognised Early Eocene volcanic episode in West Greenland. This magmatism may be related to movement on the transform Ungava Fault System which transferred drifting from the Labrador Sea to Baffin Bay. A regional change in plate kinematics at 55 Ma, associated with the opening of the North Atlantic, would have caused net extension along parts of this fault. This would have resulted in decompression and partial melting of the underlying asthenosphere. The source of the melts for the Eocene magmatism may have been remnants of still anomalously hot Iceland plume mantle which were left stranded beneath the West Greenland lithosphere in the Early Paleocene.  相似文献   

6.
Thermally, the lithosphere may be defined as that outer portion of the earth in which heat is transferred primarily by conduction. It generally includes the crust and part of the mantle. The thermal regime of continental lithosphere is determined by many factors including heat flow from the asthenosphere, the vertical and lateral variation of both thermal conductivity and radiogenic heat production, tectonic history, and such superficial processes as climatic history and the shallow hydrothermal regime. From studies of the global heat flow data set, two generalizations regarding continental lithosphere have arisen, namely that: 1) there is a negative correlation between heat flow and tectonic age of continental lithosphere; and 2) the thermal evolution of continental lithosphere is similar to that of ocean basins with the result that the “stable geotherm” is similar in both environments. When continental heat-flow data are studied from a regional rather than a global point of view, considerable doubt arises as to the general applicability of either statement. R. U. M. Rao and his associates have demonstrated that while Precambrian terranes do have demonstrably lower heat flows than, say, Tertiary terranes, the data are not normally distributed and it is not possible to establish a negative correlation between heat flow and age in any rigorous statistical way. The scatter in the relation may be explained in terms of the variations in the duration, intensity and even the sign of continental thermotectonic events in contrast to the simple situation (creation of new oceanic lithosphere at mid-ocean ridges) which prevails in the oceans. The scatter also is partially attributable to the large and laterally variable radiogenic component of heat flow on continents. For a province for which a heat flow-heat production relation has been established, much of the scatter in surface heat flow due to crustal radiogenic heat production versus age is eliminated by determining reduced heat flow (surface heat flow minus radiogenic component) as a function of tectonic age, but much scatter remains, and it is still not possible to establish a heat flux-age relation in a rigorous way. Primarily because of the spatial variability in radiogenic heat production, no single geotherm can be used to characterize the thermal regime of a stable continental terrane. Thus, while some sites on stable continental blocks may have a geotherm fortuitously similar to that for old ocean basins, there is no reason to expect that this will be true generally, and many stable continental terranes will be characterized by geotherms markedly different from the geotherm for old ocean basins.  相似文献   

7.
沉积盆地现今热流特征是岩石圈构造-热演化过程的综合反映和盆地热史恢复的必要约束条件,其总体变化趋势与热岩石圈厚度密切相关.本文根据新收集的珠江口盆地19口钻井温度数据,新增计算了19个大地热流数据,其中12个数据位于深水区（水深大于300 m）,丰富了该盆地深水区钻井地热数据.结合前人研究成果,绘制了该盆地的大地热流图,并分析了其热流分布特征.在此基础上通过求解一维热传导方程,计算得到36口井位处的热岩石圈厚度,量化了盆地大地热流与热岩石圈厚度间的关系.结果显示,珠江口盆地大地热流值介于24.2~121.0 mW·m-2,平均71.8±13.6 mW·m-2,新增盆地深水区钻井平均热流值高达84.5±4.4 mW·m-2.大地热流分布整体上从陆架区到陆坡区升高,而热岩石圈厚度整体分布趋势与大地热流相反.大地热流与热岩石圈厚度间存在良好的指数相关性.  相似文献   

8.
The heat flow map of Europe was derived from 2605 existing observations, which for this purpose were supplemented by numerous results of deep borehole temperatures, gradients and local heat flow patterns. In areas without data the heat flow field was extrapolated on the basis of the regional tectonic structure and the observed correlation of heat flow and the age of the last tectono-thermal event. The heat flow pattern as obtained in the map may be described by two components: (i) regional part and (ii) local part of the measured surface geothermal activity. The regional part of the heat flow field in Europe is dominated on the whole by a general north-east to south-west increase of the geothermal activity, which is an obvious consequence of the tectonic evolution, the major heat flow provinces corresponding thus to the principal tectonic units. The geothermal fine structure (local part) superimposing the former is mainly controlled by local tectonics, especially by the distribution of the deep reaching fracture zones and by the hydrogeological parameters. The correlation between the heat flow pattern and the crustal structure allows some preliminary geophysical implications: (a) areas of the increased seismicity may be connected with the zones of high horizontal temperature gradient, (b) increased surface heat flow may be generally observed in the zones of weakened crustal thickness, (c) there are considerable regional variations in the calculated temperature on the Moho-discontinuity, as well as in the upper mantle heat flow contribution.  相似文献   

9.
The Curie depth map of India compiled from MAGSAT data has been used for preparing the lithosphere thickness and the surface heat flow density maps of the Indian Craton, utilizing the concept of magnetothermometry. The lithosphere thicknesses of the major Indian geological units/provinces, as obtained from the prepared map, are found to be in reasonably good agreement with the previously published values for these regions. Also, the surface heat flow density values obtained from the prepared maps closely follow the previously published results. The maps are useful in providing first order estimates of lithosphere thickness and surface heat flow density of the important geological units/provinces of India.  相似文献   

10.
从天然气水合物稳定区底界的地震似海底反射BSR(Bottom Simulating Reflector)深度计算得到的BSR热流包含了海底地貌(热流在凹地型会聚,在凸地形发散)和增生楔内部流体活动的影响。从BSR热流中移除地貌效应的贡献就能揭示出流体是否发生了汇聚。在难以使用解析方法计算地貌效应的复杂海底区域,三维有限元方法可以高精度的模拟地貌对背景热流的影响,从而可以对BSR热流进行地貌效应校正,得到平坦地形条件下的BSR热流,并进一步通过与背景热流值的对比,识别目前仪器所不能探测的流体汇聚区。在北卡斯卡底(Cascadia)俯冲边缘陆坡中部的研究区应用该方法,显示黄瓜岭(Cucumber Ridge)高地及其周围的海底热流正异常显著(高出背景热流值10-20%),同时这些区域在地震成像上与海底的裂隙系统相对应,指示了流体沿着这些高渗透率通道进行汇聚,并且很可能导致较高的水合物富集度。  相似文献   

11.
下扬子区地温场和大地热流密度分布   总被引：38，自引：10，他引：38       下载免费PDF全文
根据下扬子区５０余口井温测量资料，从中选取能代表地层温度的井温数据计算地温梯度，在实验室中测量中、新生界的１１０余块砂岩和泥岩热导率及古生界碳酸岩和砂岩１００余块露头样品的热导率．在此基础上，确定了４１口井的大地热流密度值．地温梯度和大地热流密度分布表明，下扬子大部分地区中生代构造热事件伴随的热异常已不明显，而新生代时期岩石层拉张地区─－苏北盆地和郯庐断裂带是高热流密度和高地温梯度区．  相似文献   

12.
The models suggested for the oceanic lithosphere which best predict oceanic heat flow and depth profiles are the constant thickness model and a model in which the lithosphere thickens away from the ridge with a heat source at its base. The latter is considered to be more physically realistic. Such a model, constrained by the observed oceanic heat flow and depth profiles and a temperature at the ridge crest of between 1100°C and 1300°C, requires a heat source at the base of the lithosphere of between 0.5 and 0.9 h.f.u., thermal conductivities for the mantle between 0.005 and 0.0095 cal cm−1 °C−1 s−1 and a coefficient of thermal expansion at 840°C between 4.1 × 10−5 and 5.1 × 10−5 °C−1. Plate creation and subduction are calculated to dissipate about 45% of the total earth heat loss for this model. The efficiency of this mechanism of heat loss is shown to be strongly dependent on the magnitude of the basal heat source. A relation is derived for total earth heat loss as a function of the rate of plate creation and the amount of heat transported to the base of plates. The estimated heat transport to the base of the oceanic lithosphere is similar to estimates of mantle heat flow into the base of the continental lithosphere. If this relation existed in the past and if metamorphic conditions in late Archaean high-grade terrains can be used to provide a maximum constraint on equilibrium Archaean continental thermal gradients, heat flow into the base of the lithosphere in the late Archaean must have been less than about 1.2–1.5 h.f.u. The relation between earth heat loss, the rate of plate creation and the rate of heat transport to the base of the lithosphere suggests that a significant proportion of the heat loss in the Archaean must have taken place by the processes of plate creation and subduction. The Archaean plate processes may have involved much more rapid production of plates only slightly thinner than at present.  相似文献   

13.
Effective elastic thickness of island arc lithosphere under Japan   总被引：1，自引：0，他引：1  
Abstract Using topography and observed gravity anomalies, we have estimated the effective elastic thickness as a measure of strength of Japanese island arc lithosphere. The thickness is found to range from about 3 km to >20 km. The thickness seems to be controlled primarily by the thermal state of the lithosphere. The higher the heat flow, the thinner is the elastic plate. However, several areas show significant deviations. The smaller effective elastic thickness in the northern Ryukyu arc than that inferred from heat flow may be attributed to the stress regime. In Japan, extensional tectonics are going on only in the Ryukyu arc region. Shallow subducting slab under the south-western Japan frontal arc probably increases the effective thickness by several kilometers. The determined effective elastic thickness suggests that when we consider vertical movements in the volcanic arc, we should take account of topographic and subsurface loading over a few hundred kilometers. However, if the dip of the slab is shallow, the flexural responses of the underlying slab, not only that of the island arc lithosphere, should be taken into account for the compensation, as is the case of the south-western Japan frontal arc.  相似文献   

14.
Abstract The Mariana Trough is an active back-arc basin, with the rift propagating northward ahead of spreading. The northern part of the Trough is now rifting, with extension accommodated by combined stretching and igneous intrusion. Deep structural graben are found in a region of low heat flow, and we interpret these to manifest a low-angle normal fault system that defines the extension axis between 19°45' and 21°10'N. A single dredge haul from the deepest (∼5.5 km deep) of these graben recovered a heterogeneous suite of volcanic and plutonic crustal rocks and upper mantle peridotites, providing the first report of the deeper levels of back-arc basin lithosphere. Several lines of evidence indicate that these rocks are similar to typical back-arc basin lithosphere and are not fragments of rifted older arc lithosphere. Hornblende yielded an 40Ar/39Ar age of 1.8 ± 0.6 Ma, which is interpreted to approximate the time of crust formation. Harzburgite spinels have moderate Cr# (<40) and coexisting compositions of clinopyroxene (CPX) and plagioclase (PLAB) fall in the field of mid-ocean ridge basalt (MORB) gabbros. Crustal rocks include felsic rocks (70-80% SiO2) and plutonic rocks that are rich in amphibole. Chemical compositions of crustal rocks show little evidence for a 'subduction component', and radiogenic isotopic compositions correspond to that expected for back-arc basin crust of the Mariana Trough. These data indicate that mechanical extension in this part of the Mariana Trough involves lithosphere that originally formed magmatically. These unique exposures of back-arc basin lithosphere call for careful study using ROVs and manned submersibles, and consideration as an ocean drilling program (ODP) drilling site.  相似文献   

15.

There are six distinct classes of gold deposits, each represented by metallogenic provinces, having 100's to >1000 tonne gold production. The deposit classes are: (1) orogenic gold; (2) Carlin and Carlin-like gold deposits; (3) epithermal gold-silver deposits; (4) copper-gold porphyry deposits; (5) iron-oxide copper-gold deposits; and (6) gold-rich volcanic hosted massive sulfide (VMS) to sedimentary exhalative (SEDEX) deposits. This classification is based on ore and alteration mineral assemblages; ore and alteration metal budgets; ore fluid pressure(s) and compositions; crustal depth or depth ranges of formation; relationship to structures and/or magmatic intrusions at a variety of scales; and relationship to the P-T-t evolution of the host terrane. These classes reflect distinct geodynamic settings. Orogenic gold deposits are generated at mid-crustal (4–16 km) levels proximal to terrane boundaries, in transpressional subduction-accretion complexes of Cordilleran style orogenic belts; other orogenic gold provinces form inboard, by delamination of mantle lithosphere, or plume impingement. Carlin and Carlin-like gold deposits develop at shallow crustal levels (<4 km) in extensional convergent margin continental arcs or back arcs; some provinces may involve asthenosphere plume impingement on the base of the lithosphere. Epithermal gold and copper-gold porphyry deposits are sited at shallow crustal levels in continental margin or intraoceanic arcs. Iron oxide copper-gold deposits form at mid to shallow crustal levels; they are associated with extensional intracratonic anorogenic magmatism. Proterozoic examples are sited at the transition from thick refractory Archean mantle lithosphere to thinner Proterozoic mantle lithosphere. Gold-rich VMS deposits are hydrothermal accumulations on or near the seafloor in continental or intraoceanic back arcs.

The compressional tectonics of orogenic gold deposits is generated by terrane accretion; high heat flow stems from crustal thickening, delamination of overthickened mantle lithosphere inducing advection of hot asthenosphere, or asthenosphere plume impingement. Ore fluids advect at lithostatic pressures. The extensional settings of Carlin, epithermal, and copper-gold porphyry deposits result from slab rollback driven by negative buoyancy of the subducting plate, and associated induced convection in asthenosphere below the over-riding lithospheric plate. Extension thins the lithosphere, advecting asthenosphere heat, promotes advection of mantle lithosphere and crustal magmas to shallow crustal levels, and enhances hydraulic conductivity. Siting of some copper-gold porphyry deposits is controlled by arc parallel or orthogonal structures that in turn reflect deflections or windows in the slab. Ore fluids in Carlin and epithermal deposits were at near hydrostatic pressures, with unconstrained magmatic fluid input, whereas ore fluids generating porphyry copper-gold deposits were initially magmatic and lithostatic, evolving to hydrostatic pressures. Fertilization of previously depleted sub-arc mantle lithosphere by fluids or melts from the subducting plate, or incompatible element enriched asthenosphere plumes, is likely a factor in generation of these gold deposits. Iron oxide copper-gold deposits involve prior fertilization of Archean mantle lithosphere by incompatible element enriched asthenospheric plume liquids, and subsequent intracontinental anorogenic magmatism driven by decompressional extension from far-field plate forces. Halogen rich mantle lithosphere and crustal magmas likely are the causative intrusions for the deposits, with a deep crustal proximal to shallow crustal distal association. Gold-rich VMS deposits develop in extensional geodynamic settings, where thinned lithosphere extension drives high heat flow and enhanced hydraulic conductivity, as for epithermal deposits. Ore fluids induced hydrostatic convection of modified seawater, with unconstrained magmatic input. Some gold-rich VMS deposits with an epithermal metal budget may be submarine counterparts of terrestrial epithermal gold deposits. Real time analogs for all of these gold deposit classes are known in the geodynamic settings described, excepting iron oxide copper-gold deposits.

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16.
渭河盆地岩石圈热结构与地热田热源机理   总被引：7，自引：2，他引：5       下载免费PDF全文
岩石圈热结构是盆地现今地温场研究的重要延伸和扩展,是了解大陆岩石圈构造变形及演化等大陆动力学问题的重要窗口,更是地热田热源机理研究的核心问题.本次工作,在系统分析渭河盆地现今地温场和水动力系统基础上,编制了渭河盆地大地热流分布等值线图;通过实测生热率等热物性参数,利用一维稳态热传导方程计算了研究区岩石圈热结构,并分析了渭河盆地岩石圈热结构特征和地热田热源机理.结果表明,渭河盆地现今大地热流值分布范围为62.5~80.2mW·m-2,平均为70.8±4.8mW·m-2,西部明显高于东部,西安坳陷最高,咸礼凸起次之;渭河断裂并不是控热断裂,其沟通作用引起的水热循环一定程度上影响了浅部热量再分配,对渭河盆地地温场并没有起到明显的控制作用.西安坳陷—咸礼凸起地壳热流介于32.2~37.5mW·m-2之间,平均为34.6mW·m-2;地幔热流分布范围为33.8~38.9mW·m-2,平均为36.0mW·m-2;地壳热流和地幔热流的总体变化趋势一致,西安坳陷高于咸礼凸起,分析认为西安坳陷沉积层厚度大于后者,且沉积层放射性生热率更大,是造成西安坳陷地壳热流高于咸礼凸起的原因,而西安坳陷相比咸礼凸起更高的地幔热流,表明西安坳陷深部活动性强于咸礼凸起.西安坳陷和咸礼凸起地壳/地幔热流比值相近,介于0.93~1.01之间,平均为0.96,"热"岩石圈厚度约为95~101km.渭河盆地岩石圈热结构特征与鄂尔多斯盆地在很大程度上具有相似性,暗示着二者具备相似的深部稳定性,这与渤海湾盆地为代表的中国东部中—新生代主动裂谷盆地岩石圈热结构特征截然不同,表明渭河盆地为被动伸展裂陷.从鄂尔多斯盆地、渭河盆地、山西裂谷到华北盆地,"热"岩石圈厚度的有序变化表明太平洋板块俯冲引起的地幔对流对华北地块深部动力学行为的影响主要发生在太行山以东,而太行山以西的鄂尔多斯盆地和渭河盆地则影响甚微,这种空间差异影响从侧面暗示着华北克拉通破坏过程的有序性.综合分析渭河盆地地质—地球物理资料认为,岩石圈表层伸展破裂、深部重力均衡调整进而引起软流圈被动上涌,其产生的相对高地幔热流的热传导和深大断裂沟通的水体热对流相互叠加作用,共同构成了渭河盆地中—低温地热田的热源机理.  相似文献   

17.
A longitudinal seismic reflection profile of the Reykjanes Ridge, together with earthquake seismicity patterns, is interpreted in terms of the mantle plume hypothesis. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing.The 57°N transition from fractured to unfractured basement occurs about 900 km southwest of the postulated Iceland mantle plume. The fractured province exhibits higher seismicity and rougher basement, on transverse profiles, than does the unfractured province. A similar transition to rougher, more seismic ridge crest also occurs 900 km northeast of Iceland. We propose that flowage of hot, basalt-rich asthenosphere away from the Iceland hot spot keeps the axial lithosphere hot, thin, sparsely fractured, and relatively aseismic out to 900 km from the plume. Similar effects are evident in the vicinity of some other plumes located near spreading axes. Some plumes also exhibit a greater number of earthquakes at some distance from the spreading axis — possibly a reflection of non-axial igneous activity or fracturing due to local, plume-generated stresses.The regional basement slope along the longitudinal profile is about 8 × 10?4. If this slope represents a balance between viscous and gravity forces in the flow, a viscosity of the order 1019 poises can be estimated from the Poiseuille equation.A peculiarly flat, opaque reflector was discovered near the Reykjanes axis, about 300 km southwest of Iceland. Several hypotheses are advanced to account for such reflectors by the exceptional volcanic activity associated with high plume discharge.  相似文献   

18.
Summary The surface thermal flux of the continental margins of the northwestern Mediterranean Sea is interpreted on the basis of a 1-D instantaneous pure shear stretching model of the lithosphere in terms of three components: the background heat flowing out from the asthenosphere (38 mW m–2), the transient contribution depending on the rift age and extension amount (35 mW m–2 at the most), and the contribution due to the radiogenic elements of the lithosphere. The radiogenic component is estimated at the continental margins of the Ligurian-Provençal basin and Valencia trough, and in the surrounding mainland areas by means of available data of surface heat generation from Variscan Corsica, Maures-Estérel and the Central Massif along with a geophysical-petrological relationship between heat production and seismic velocity. The lithosphere radiogenic heat contribution ql decreases with the thinning factor according to the exponential law: ql() = a exp(-b), in which factor b is greater for that part of the lithosphere below the uppermost 10 km. Considering also the heat generated by radioactive isotopes in sediments, the stable Variscan lithosphere produces an average thermal flux of 30 mW m–2 which decreases by about one half where the lithosphere is thinned by one third. Although the surface heat generation is 2·1 – 3·3 µW m–3 in the Maures-Estérel massif — excepting small outcrops of dioritic rocks with lower heat production — and 1·8 µW m–3 for most of Corsica, the radiogenic heating within the lithosphere for such areas is nearly the same and does not explain the higher heat flux of the Corsica margin. This asymmetric thermal pattern with surface heat flux which is 10 – 15 mW m–2 higher than predictions is probably of upper mantle origin, or can be ascribed to penetrative magmatism.  相似文献   

19.
The deformation of the lithosphere due to temperature anomalies caused by a heat source located below or within the layer is usually modelled as one of Pratt local isostasy. A more appropriate model is one of rheologically layered lithosphere comprising of a stiff viscoelastic or elastic layer overlying a weaker viscoelastic layer. The surface deformations are a result of not only the perturbations in body forces due to density changes, but thermal bending moments. In geophysically realistic situations the former contribution dominates. Pratt isostasy is attained if the stresses in the entire lithosphere are allowed to relax and this end state is not contingent upon the lithosphere being confined against horizontal deformation. In a rheologically layered lithosphere, even though the non-isostatic thermal stresses persist in the upper layer, the surface deformations are indistinguishable from that of local isostasy if the horizontal dimension of the heat source exceeds about three times the effective elastic thickness of the lithosphere.  相似文献   

20.
Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO2 and 3He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Teplá-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.  相似文献   

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