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991.
992.
Variation of crustal thickness in the Philippine Sea deduced from three-dimensional gravity modeling
Abstract Crustal thickness of the northern to central Philippine Sea was gravimetrically determined on the simple assumption of four layers: seawater, sediments, crust and lithospheric mantle, with densities of 1030, 2300, 2800 and 3300 kg/m3 , respectively. As for the correction of the regional gravity variation, a 15 km difference of the lithospheric thickness with a density difference of 50 kg/m3 against the asthenosphere below between both sides of the Kyushu-Palau Ridge was taken into consideration. Mantle Bouguer anomalies were calculated on the assumption of constant crustal thickness of 6 km, and then the crustal thickness was obtained by three-dimensional gravity inversion method. The results show occurrence of thin crust areas with a thickness of approximately 5 km in the southern part and at the western margin of the Shikoku Basin and also of thick crust areas in the northwestern and northeastern parts of the Parece Vela Basin. We suggest that these are because of the variation of magma supply at the time of sea floor spreading in the Shikoku and Parece Vela Basins, which is possibly related to the variation of spreading rate and enhanced magmatism near the past arc volcanic fronts. The results further show the occurrence of crust thinner than 5 km in the northeastern part of the West Philippine Basin, of crust thicker than 15 km in the Amami Plateau, the Daito and Oki-Daito Ridges, and also in the northern part of Kyushu-Palau Ridge, whereas the southern part of the Kyushu-Palau Ridge the crust is thicker than 10 km. It was also inferred that small basins in the Daito Ridge province have the thinnest oceanic crust of less than 5 km in the Kita-Daito Basin. 相似文献
993.
994.
Several aspects of the Liaodong Paleoproterozioc fold belt, including its components, tectonic, metamorphic and magmatic characteristics are discussed in the paper, with emphasis on the asymmetry of the fold belt and differences between the northern lineal - fold belt and the southern dome - fold belt. It is suggested that the fold belt is one magrnatic passive margin, instead of a rift zone or the classical passive continental margin. The formation and evolution of the fold belt is discussed with a simple shear model, which may best explain the co - existence of contemporaneous tectonic facies with different characteristics, the volcanic and plutonic activities, sedimentary formations and their geological attributes etc. 相似文献
995.
A target of our study was the Bohemian Massif in Central Europe that was emplaced during the Variscan orogeny. We used teleseismic records from ten broadband stations lying within and around the massif. Different techniques of receiver function interpretation were applied, including 1-D inversion of R- and Q-components, forward modelling of V
s
velocity, and simultaneous determination of Moho depth and Poissons ratio in the crust. These results provide new, independent information about the distribution of S wave velocity down to about 60 km depth. In the area of Bohemian Massif, the crustal thickness varies from 29 km in the NW to 40 km in the SE. A relatively simple velocity structure with gradually increasing velocities in the crust and uppermost mantle is observed in the eastern part of the Bohemian Massif. The western part of the massif is characterized by more complicated structure with low S wave velocities in the upper crust, as well as in the uppermost mantle. This could be related to tectono-magmatic activity in the Eger rift that started in the uppermost Cretaceous and was active in the West Bohemia-Vogland area till the late Cenozoic. 相似文献
996.
ZHANG Zhongjie BAI Zhiming WANG Chunyong TENG Jiwen Lü Qingtian LI Jiliang LIU Yifeng & LIU Zhenkuan . Institute of Geology Geophysics Chinese Academy of Sciences Beijing China . Institute of Geophysics China Seismological Bureau Beijing China . Institute of Deposition Resource Chinese Academy of Geosciences Beijing China . School of Exploration Information China University of Geosciences Beijing China 《中国科学D辑(英文版)》2005,48(9):1329-1336
The Sanjiang area in southwest China is considered as a tectonic intersection belt between the Tethys-Alps and the western Pacific, and has endured three-phase evolution processes: Proto-Tethys,Paleo-Tethys and Meso-Tethys[1―4]. In this area, its tectonics and struc- ture are extremely complicated, and intensively extru-sive deformation and faults are widely developed[1―3]. For that, the area is considered as the ideal na- ture-laboratory to study the evolution of Paleo-Tethys and also … 相似文献
997.
In this study we investigate present-day seismicity and crustal deformation of Egypt based on a comprehensive earthquake catalog
from 1900 to 2004 by focal mechanism stress inversion and by recent GPS observations. Spatial distribution of earthquake epicenters
indicates that Egypt has been suffered from both interplate and intraplate earthquakes. Most earthquake activity (more than
70%) has been concentrated in northern Egypt along the geologically documented borders of Sinai subplate (northern Red Sea
and its two branches Suez rift and Aqaba–Dead Sea transform). The majority of inland earthquake focal mechanisms in Egypt
are normal with strike-slip component or strike-slip faulting events. Only a small minority, namely four events, exhibits
reverse faulting. The inversion method of Gephart and Forsyth (1984) was applied to calculate the orientation of the principle
stress axes and the shape of the stress tensor. The best fitting tensor in Egypt shows homogeneity stress field. The tension
stress regime is dominant in northern Egypt. The stress directions are well resolved by the 95% confidence limits, the relative
stress magnitude has a value of about 0.3. However, along southern Egypt the strike-slip regime is dominant. The shape factor
(R-value) is 0.5, which means that the deviatoric components of σ1 and σ3 are of the same magnitude, but of opposite signs. The average horizontal velocity of GPS stations in Egypt is 5.15± 1.1 mm/year
in mostly NNW direction. The results of deformation analysis indicate that the northern Egypt is deformed more than the southern
part. Only the Egyptian-Mediterranean coastal–Nile Delta zone dominates as a compression deformation area. However, an extensional
deformation has been observed throughout the rest of country. This means that the relative motion of African plate with respect
to both Eurasian and Arabian has highly controlled the deformation processes in Egypt. 相似文献
998.
Lower Crustal Magma Genesis and Preservation: a Stochastic Framework for the Evaluation of Basalt-Crust Interaction 总被引:7,自引:0,他引:7
We present a quantitative assessment of the thermal and dynamicresponse of an amphibolitic lower crust to the intrusion ofbasaltic dike swarms in an arc setting. We consider the effectof variable intrusion geometry, depth of intrusion, and basaltflux on the production, persistence, and interaction of basalticand crustal melt in a stochastic computational framework. Distinctmelting and mixing environments are predicted as a result ofthe crustal thickness and age of the arc system. Shallow crustal(30 km) environments and arc settings with low fluxes of mantle-derivedbasalt are likely repositories of isolated pods of mantle andcrustal melts in the lower crust, both converging on daciticto rhyodacitic composition. These may be preferentially rejuvenatedin subsequent intrusive episodes. Mature arc systems with thickercrust (50 km) produce higher crustal and residual basaltic meltfractions, reaching 0·4 for geologically reasonable basaltfluxes. The basaltic to basaltic andesite composition of bothcrustal and mantle melts will facilitate mixing as the networkof dikes collapses, and Reynolds numbers reach 1041·0in the interiors of dikes that have been breached by ascendingcrustal melts. This may provide one mechanism for melting, assimilation,storage and homogenization (MASH)-like processes. Residual mineralassemblages of crust thickened by repeated intrusion are predictedto be garnet pyroxenitic, which are denser than mantle peridotiteand also generate convective instabilities where some of thecrustal material is lost to the mantle. This reconciles thethinner than predicted crust in regions that have undergonea large flux of mantle basalt for a prolonged period of time,and helps explain the enrichment of incompatible elements suchas K2O, typical of mature arc settings, without the associatedmass balance problem. KEY WORDS: crustal anatexis; delamination; lower crust; magma mixing; thermal model 相似文献
999.
The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism 相似文献
1000.
We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing 206Pb/204Pb (19·8220·33)and 87Sr/86Sr (0·702900·70310), and decreasingNd (7·06·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing 206Pb/204Pb(19·8219·03), but also increasing 87Sr/86Sr(0·702900·70359), increasing 187Os/188Os(0·1300·245) and decreasing Nd (7·04·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes 相似文献