Present-day seismicity,stress field and crustal deformation of Egypt

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.     

Lower Crustal Magma Genesis and Preservation: a Stochastic Framework for the Evaluation of Basalt-Crust Interaction

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 10^–4–1·0in 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     

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

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     

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

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 (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),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     

中国大陆主要构造块体现今运动的基本特征

为了研究中国大陆现代地壳运动和地球动力学,在９０年代初布设了由２２个站组成的全国ＧＰＳ监测网,测站均匀布设在中国大陆不同块体上,１９９２、１９９４和１９９６年对该网进行了３次观测,本文主要评述了３次观测的资料处理和精度,分析了３次测获得的中国大陆现今块体运动的基本特征。研究结果表明,ＧＰＳ网３次观测获得的资料精度均达到１０＾－８～１０＾－９,能满足地壳运动监测的需要,利用３次复测资料结果所建立的我国大陆块体运动模型,有效地检验了地质学和地球物理学的研究结果,在全球框架中,我国大陆整体存在着明显的向东运动,相对于西伯亚的区域性运动特征是：我国西部地区主要受印度板向北的俯冲和推挤的影响,青藏高原在南北向压缩的同时,存在着明显的向东的侧向滑移,这对大陆逃逸模型是较为有利的支持;东部地区的块体运动,同时受到印度板以及太     

Basement Characteristics and Crustal Evolution of the Copper-Gold Metallogenic Belt in the Middle and Lower Reaches of the Yangtze River: Some Isotope Constraints

Studies of the Pb, Sr and Nd isotopic composition of Mesozoic intrusive rocks indicate that the basement of the copper-gold metallogenic belt of the middle and lower reaches of the Yangtze River has "two-layer structure" and partly has "multi-layered structure", and is inhomogeneous and shows the distinct feature of E-W provincialism. The calculated model lead ages (t1) are mostly greater than 2600 Ma, and the model neodymium ages (TDM) vary from 953 to 2276 Ma and concentrate in two time intervals: 1800-2000 Ma and 1200-1600 Ma. It is concluded that the basement of the MBYR is composed of the Late Archaeozoic to Middle Proterozoic metamorphic series and that the crust was initiated in the Archaean and continued to grow in the Early and Middle Proterozoic, and the proportion of new crust formed by mantle differentiation during the Late Proterozoic is low.     

利用Ground\|Truth资料和Messy GA方法反演渤海湾盆地及其邻区地壳P波速度模型

收集了1978～2003年位于渤海湾盆地及其邻近地区（大体位于东经112°～122°，北纬34°～44°之间）所记录到的的总计10534个地震或人工爆破事件的Pn、Pg以及P波到时数据，运用GT5准则，从中挑选出相对精确和可信度较高，且具有代表性的的10个地震事件，使用Messy GA算法，分别反演了这10个地震事件所对应的一维地壳P波速度模型.反演的局部地壳模型与已知结果对应较好，基本反映了震中区的地壳速度结构特征.该方法原理简单，计算方便，可用于确定局部和区域的地壳速度模型.