http://www.sciencedirect.com/science/article/pii/S1674987114001601   总被引：3，自引：2，他引：1  

Tim E.Johnson  Chris Clark  Richard J.M.Taylor  M.Santosh  Alan S.Collins 《地学前缘(英文版)》2015,6(3):373-387

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time[P-T-t) evolution of the Nagercoil Block at the southernmost tip of peninsular India.An inferred peak metamorphic assemblage of garnet,K-feldspar.sillimanite,plagioclase,magnetite,ilmenite,spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900℃.Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 C.SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages.New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation,and the minimum age of the metamorphic peak,to around560 Ma.The data suggest two stages of monazite growth.The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite.Relatively REE-depleted rims,which have a pronounced negative europium anomaly,grew during melt crystallisation along the retrograde path at around 535 Ma.Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer,supporting a long-lived high-grade metamorphic history.The metamorphic conditions,timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.  相似文献   

http://www.sciencedirect.com/science/article/pii/S167498711400108X     

Peter G.Betts  Louis Moresi  Meghan S.Miller  David Willis 《地学前缘(英文版)》2015,6(1):49-59

We present three 3D numerical models of deep subduction where buoyant material from an oceanic plateau and a plume interact with the overriding plate to assess the influence on subduction dynamics,trench geometry,and mechanisms for plateau accretion and continental growth.Transient instabilities of the convergent margin are produced,resulting in:contorted trench geometry;trench migration parallel with the plate margin;folding of the subducting slab and orocline development at the convergent margin;and transfer of the plateau to the overriding plate.The presence of plume material beneath the oceanic plateau causes flat subduction above the plume,resulting in a "bowed" shaped subducting slab.In plateau-only models,plateau accretion at the edge of the overriding plate results in trench migration around the edge of the plateau before subduction is re-established directly behind the trailing edge of the plateau.The plateau shortens and some plateau material subducts.The presence of buoyant plume material beneath the oceanic plateau has a profound influence on the behaviour of the convergent margin.In the plateau + plume model,plateau accretion causes rapid trench advance.Plate convergence is accommodated by shearing at the base of the plateau and shortening in the overriding plate.The trench migrates around the edge of the plateau and subduction is re-established well behind the trailing edge of the plateau,effectively embedding the plateau into the overriding plate.A slab window forms beneath the accreted plateau and plume material is transferred from the subducting plate to the overriding plate through the window.In all of the models,the subduction zone maintains a relatively stable configuration away from the buoyancy anomalies within the downgoing plate.The models provide a dynamic context for plateau and plume accretion in Phanerozoic accretionary orogenic systems such as the East China Orogen and the Central Asian Orogen(Altiads),which are characterised by accreted ophiolite complexes with diverse geochemical affinities,and a protracted evolution of accretion of exotic terranes including oceanic plateau and terranes with plume origins.  相似文献   

http://www.sciencedirect.com/science/article/pii/S1674987114000255     

S.Monna  A.Argnani  G.B.Cimini  F.Frugoni  C.Montuori 《地学前缘(英文版)》2015,6(1):39-48

Geophysical studies point to a complex tectonic and geodynamic evolution of the Alboran Basin and Gulf of Cadiz.Tomographic images show strong seismic waves velocity contrasts in the upper mantle.The high velocity anomaly beneath the Alboran Sea recovered by a number of studies is now a well established feature.Several geodynamic reconstructions have been proposed also on the base of these images.We present and elaborate on results coming from a recent tomography study which concentrates on both the Alboran and the adjacent Atlantic region.These new results,while they confirm the existence of the fast anomaly below the Alboran region,also show interesting features of the lithosphere-asthenosphere system below the Atlantic.A high velocity body is imaged roughly below the Horseshoe Abyssal plain down to sub-lithospheric depths.This feature suggests either a possible initiation or relic subduction.Pronounced low velocity anomalies pervade the upper mantle below the Atlantic region and separate the lithospheres of the two regions.We also notice a strong change of the upper mantle velocity structure going from south to north across the Gorringe Bank.This variation in structure could be related to the different evolution in the opening of the central and northern Atlantic oceans.  相似文献   

区域断裂对韩国东南部地区现今应力状态的影响     

Chang C  Kang T S  李妍 《国际地震动态》2011,(1):98-103

利用朝鲜半岛东南部地区浅层地应力数据,给出区域现今应力状态的分布特征及其与断层分布的关系.地应力数据表明,区域最大主压应力呈ENE-WSW到NE-SW向,这与地震震源机制解及欧亚板块东部构造应力方向的一阶模式一致.或许是受到断层的影响,现今应力张量在大小和方向上表现出不均匀性.区域水平主应力变化较为一致,而垂向则变化较...  相似文献   

余震密度随距离的衰减不代表动态应力触发过程     

K.R.-Dinger  R.S.Stein  S.Toda  魏志恒  冯蔚 《世界地震译丛》2011,(3)

分辨多数余震和后续主震是由静态应力触发(Stein,1999;Todaetal,2005;Parsons,2002)还是动态应力触发(Hilletal,1993;kilb,2003;Brodsky and Prejean,2005;Gomberg and Johnson,2005;Hill and Prejean,2007),对理解地震内在作用和预测地震危险性是有必要的(Freed,2005)。Felzer和Brodsky(2006)分析了2≤M<3和3≤M<4级主震后5min内余震的空间分布情况,发现距主震50km范围内M≥2余震具有特定的幂律衰减关系,且衰减曲线的斜率为-1.35。据此,他们认为余震随距离的衰减只能用动态触发来解释。基于上述假设我们进行了一系列的检验,但没有一个可通过检验,进而,本文中对这种衰减关系提出了另外一种解释。在距离2≤M<3主震300m以外的地方,主震前5min内的地震活动性衰减与主震后5min内的衰减没有显著的区别,表明主震对它静态触发区域以外的地方没有影响。表征余震基本特性的大森随时间衰减关系在距离主震10km以外的地方是不存在的。最后,发现在主震波前未到达之前的余震中发现了...  相似文献   

Secondary Calcification of Planktic Foraminifera from the Indian Sector of Southern Ocean     

Rahul MOHAN  Suhas S. SHETYE  Manish TIWARI  Narayanpillai ANILKUMAR 《《地质学报》英文版》2015,89(1):27-37

This study focused on planktic foraminifera in plankton tows and surface sediments from the western Indian sector of Southern Ocean in order to evaluate the potential foraminiferal secondary calcification and/or dissolution in the sediment. It is found that the symbiotic foraminiferal species are abundant in the subtropical region, whereas non-symbiotic species dominate in the sub-Antarctic and polar frontal regions. The distribution of the symbiotic and non-symbiotic foraminiferal species is controlled by temperature, salinity, light, nutrients and phytoplankton biomass. There is also a lateral southern extent in abundance of planktic foraminifera from surface sediments to plankton tows. The shell weights of the planktic foraminifera N. pachyderma, G. bulloides and G. ruber within the surface sediments are on an average heavier by 27%, 34% and 40% respectively than shells of the same size within the plankton tows, indicative of secondary calcification. The planktic foraminiferal isotopes show the presence of heavier isotopes in the surface sediment foraminifera as compared to plankton tows, thus confirming secondary calcification. Secondary calcification in G. ruber occurs in the euphotic zone, whereas in case of N. pachyderma and G. bulloides it is at deeper depths. We also observed a decrease in the shell spines in surface sediment foraminifera as compared to plankton tows, indicative of the morphological changes that foraminifera underwent during gametogenesis.  相似文献   

Quantitative morphometric analysis of the Jama River profile in a tectonically active margin (Northwestern Ecuador)     

Pedro S.B.REYES  Mauricio E.VALAREZO  Jenry CóRDOVA  Fran?ois A.MICHAUD  Camilo ZAPATA 《山地科学学报》2018,15(5):966-975

The longitudinal profile of a river channel can be described in function of mathematical expressions. The logarithmic fit is the most used method to describe the relative equilibrium of a channel elevation profile. Rivers showing zones of high channel gradient and convex-upward profiles can be evaluated in function of the offset distance with respect to the logarithmic curve. The Jama River profile has been constructed using differential GPS data for the downstream reach and 4-m-grid DEM data base for the remaining headward profile. The resulting longitudinal profile shows a prominent knickpoint of about 80 m in elevation characterized by sharp local convexity. The offset distance in vertical direction from the logarithmic plot is interpreted as the successive accumulation of multiple uplift episodes associated with the Jama Fault System activity. The horizontal offset is suggested to represent the remaining retreated distance for the upstream propagation of the Jama knickpoint. The highly resistant Cretaceous rocks outcropping along the Jama knickpoint reach acts as an inflection point between two subprofiles that show well-fitted logarithmic curves, thereby representing a temporary partitioned equilibrium along the Jama River profile.  相似文献   

A possible precessing nozzle and the Lense-Thirring effect in blazar 3C 454.3     

Shan-Jie Qian  S. Britzen  A. Witzel  T. P. Krichbaum  Heng-Qian Gan  Long Gao 《天文和天体物理学研究(英文版)》2014,(3):249-274

The kinematics of superluminal components in blazar 3C 454.3 are studied.Nine components are included:superluminal knots R1,R2,R3,R4,A,B,C and D(from Britzen et al.2013) and C4(from Pauliny-Toth 1998).We find that their kinematics derived from VLBI observations can be consistently interpreted in terms of a jet precession scenario with a period of about 14.5 yr.We discuss the model fits of their trajectory,distance from the core and apparent velocity.We show that the bulk Lorentz factor(in the range 4 to 15) derived for these components does not have any dependence on the phase of the precession(or position angle for ejection).The LenseThirring effect is assumed to interpret the precession of the jet nozzle.The results obtained for blazar 3C 454.3 are only suggestive.They are not unique and have yet to be tested,but they might be useful for understanding the kinematics of superluminal components in blazars and for disentangling different mechanisms and factors.  相似文献   

日本岛弧和俯冲作用     

Uyeda  S 潘秋霞 《海洋地质译丛》1992,(4):41-46

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THE CANON OF MARINE TRANSGRESSION IN POST-PALAEOZOIC TIMES.     

J.S.Lee 《地质学报》1928,7(1):85-132

Ⅰ. Introduction In attempting to attack the fundamental problem ot geotectonics, the writer has endeavoured to show, along very broad lines, that the more important marine transgressions in the Northern Hemisphere during the Palaeozoic Era  相似文献