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941.
南秦岭勉略蛇绿构造混杂带辉长辉绿岩类微量-稀土元素地球化学 总被引:2,自引:0,他引:2
赖绍聪 《地球科学与环境学报》2005,27(3):5-9
应用元素地球化学方法.研究了勉略带中辉长辉绿岩的相关特征.结果表明.勉略蛇绿构造混杂带内的辉长岩属高铝强碱质辉长岩类,是地幔橄榄岩部分熔融的产物。辉绿岩墙群属强碱铝质基性岩类,过渡族元素蛛网图与辉长岩类几乎完全相同,其不相容元素与辉长岩类总体趋势相一致.反映了一种递进岩浆演化趋势。因此,本区辉长岩与辉绿岩墙群具同源岩浆分异演化的地球化学特征.其均为勉略蛇绿岩的重要组成端元。 相似文献
942.
1 IntroductionTomonitorthecurrentcrustalmotionandearthquakehazardsinNorthChinaarea,theFirstCrustalMonitoringandApplicationCenter (FC MAC)oftheChinaEarthquakeAdministration(CEA)hasestablishedaGlobalPositioningSystem(GPS)monitoringnetworkinNorthChinaandtheCapital(Beijing)Circlearea .TheobservedstationsaremainlydistributedinthenorthernpartofNorthChinaandCapitalCirclearea (Fig .1 ) .Theregionisbetween 1 0 7°Eand 1 2 2°E (about 1 40 0km) ,andbetween 33°Nand 42°N (about 80 0km) .… 相似文献
943.
944.
945.
位于青藏高原东南部的横断山区,是我国东部环太平洋区与西部古地中海区间的交接过渡地带,也是我国地质结构最为复杂的一个区域。作者根据1981—1985年在当地考察资料,将本区分出5条具洋壳性质的强变形构造带,2块具陆壳性质的地块,它们相间并列。文章论述了各条块的发育特征及区域演化历史。 相似文献
946.
947.
Summary. Most crustal earthquakes of the world are observed to occur within a seismogenic layer which extends from the Earth's surface to a depth of a few tens of kilometres at most. A model is proposed in which the shear zone along a transcurrent plate margin is represented as a viscoelastic medium with depth-dependent power-law rheology. A frictional resistance linearly increasing with depth is assumed on a vertical transcurrent fault within the shear zone. Such a model is able to reproduce a continuous transition from the brittle behaviour of the upper crust to the ductile behaviour at depth. Assuming that the shear zone is subjected to a constant strain rate from the opposite motions of the two adjacent plates, it is found that there exists a maximum depth H below which tectonic stress can never reach the frictional threshold: this may be identified as the maximum depth of earthquake nucleation. The value of H is consistent with observations for plausible values of the model parameters. The stress evolution in the shear zone is calculated in the linear approximation of the constitutive equation. A change in rigidity with depth, which is also introduced in the model, may reproduce the high vertical gradient of shear stress, which has been measured across the San Andreas fault, and the fact that most earthquakes are nucleated at some depth in the seismogenic layer. A crack which drops the ambient stress to the dynamic frictional level is then introduced in the model. To this aim, a crack solution is employed without a stress singularity at its edges, which is compatible with a frictional stress threshold criterion for fracture. A constraint on the vertical friction gradient is obtained if such cracks are assumed to be entirely confined within the seismogenic layer. 相似文献
948.
青海石炭系分布广泛,沉积类型多样,产有丰富的动植物化石,并具重要特征。石炭系保存有属种众多而又丰富的孢子和花粉,其组合基本可与国内和西欧、苏联、北美同时期孢子花粉带比较。根据孢子花粉材料,讨论了关于时代、古地理及孢子花粉分布问题。 相似文献
949.
K. A. JONES 《Journal of Metamorphic Geology》1994,12(1):69-88
This paper describes the progressive metamorphism and deformation of a series of metasediments, Le Conquet Schists and their higher grade equivalents, which occur as tectonically emplaced screens within a sequence of foliated gneisses, the Gneiss de Brest and Gneiss de Lesneven. The sequence exhibits a steep south to north increase in metamorphic grade from garnet-staurolite schist to sillimanite gneiss and sillimanite-K-feldspar migmatite. The relationship of mineral growth to foliation development has been established for individual screens. At least five phases of deformation (D1-D5) are preserved. Analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations S1-S4. Porphyroblasts continued to grow during the subsequent development of C-S mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high-strain zones (D5). Mineral assemblages, phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions. Microstructure-porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions, these only operated episodically. Phase relations, mineral chemistry and P-T estimates are used to constrain P-T trajectories and these are linked to the deformation histories within individual screens. A comparison between the resulting pressure-temperature-deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade. It is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting. The subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike-slip transtensional shear zone. 相似文献
950.
Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction. 相似文献