壳幔过渡层及其大陆动力学意义
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摘要
根据地震测量结果,造山带常见一个P波传播速度介于典型地壳和典型地幔之间的圈层,称为壳幔过渡层。这个圈层在地球物质科学领域常常被解释为壳幔混合层,形成机制不十分明确。提供了另一种选择,认为壳幔过渡层实际上可能是幔源岩浆底侵作用和地壳分异作用的产物,其密度随压力的逐渐变化是其波速特征的主要原因,是区域岩石圈重力不稳定的标志;也可能是软流圈物质岩石圈化的结果,是区域岩石圈逐渐冷却的象征。这个模型可以更好地解释造山带岩石圈拆沉作用、壳幔物质交换和岩浆活动,因而壳幔过渡层的查明具有重要的大陆动力学意义。
According to the seismological observations,there is usually a layer called as a transition layer which generally has an intermediate speed between P-wave speeds in typical crust and mantle rocks.This layer is often called as the crust mantle mixing layer in the Earth material science fields.This is not a term with a definite mechanism.In this paper,an other alternative choice is provided.In fact,the geophysical transition layer may be a product of mantle-derived magma underplating and crustal differentiation.The transitional P-wave velocity in this layer is caused by gradual variation of its density with pressure.Therefore,the P-wave transition layer is the mark of lithosphere instability.The layer may also be a product of asthenosphere transformation to lithosphere.In this case,the existence of the layer implies regional lithosphere cooling.Our model could better explain lithosphere delamination,crust-mantle material exchange and magmatism in orogens,and hence suggests important implications for the continental geodynamics.
According to the seismological observations,there is usually a layer called as a transition layer which generally has an intermediate speed between P-wave speeds in typical crust and mantle rocks.This layer is often called as the crust mantle mixing layer in the Earth material science fields.This is not a term with a definite mechanism.In this paper,an other alternative choice is provided.In fact,the geophysical transition layer may be a product of mantle-derived magma underplating and crustal differentiation.The transitional P-wave velocity in this layer is caused by gradual variation of its density with pressure.Therefore,the P-wave transition layer is the mark of lithosphere instability.The layer may also be a product of asthenosphere transformation to lithosphere.In this case,the existence of the layer implies regional lithosphere cooling.Our model could better explain lithosphere delamination,crust-mantle material exchange and magmatism in orogens,and hence suggests important implications for the continental geodynamics.
引文
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[2]Christensen N I,Mooney W D.Seismic velocity structure andcomposition of the continental crust:a global view[J].Journalof Geophysical Research,1995,100(B5):9761-9788.
[3]邓晋福,吴宗絮,杨建军,等.格尔木—额济纳旗地学断面走廊域地壳-上地幔岩石学结构与深部过程[J].地球物理学报,1995,38(增刊2):130-144.
[4]滕吉文.固体地球物理学概论[M].北京:地震出版社,2003:1-896.
[5]林舸,范蔚茗,郭锋,等.壳-幔过渡层特征与大地构造演化[J].大地构造与成矿学,1998,22(增刊):11-18.
[6]Leeman WP,Harry D L.Abinary source model for extension re-lated magmatism in the Great Basin,Western North America[J].Science,1993,262:1550-1554.
[7]Yang J H,Chung S L,Zhai MG,et al.Geochemical and Sr-Nd-Pb isotopic compositions of mafic dikes from the Jiaodong Penin-sula,China:evidence for vein-plus-peridotite melting in thelithospheric mantle[J].Lithos,2004,73:145-160.
[8]Zandt G,Gilbert Hersh,Owens TJ,et al.Active foundering of acontinental arc root beneath the southern Sierra Nevada in Califor-nia[J].Nature,2004,431:41-46.
[9]罗照华,魏阳,辛后田,等.造山后脉岩组合的岩石成因——对岩石圈拆沉作用的约束[J].岩石学报,2006,22(6):1672-1684.
[10]Deng J F,Mo X X,Luo Z H,et al.Inhomogeneity of the litho-sphere of the Tibetan Plateau and implications for geodynamics[J].Sciences in China(Series D),2001,44(supp):56-63.
[11]Mainprice D,Bascou J,Cordier P,et al.Crystal preferred orien-tations of garnet:comparison between numerical simulations andelectron back-scattered diffraction(EBSD)measurements innaturally deformed eclogites[J].Journal of Structural Geology,2004,26:2089-2102.
[12]Zorin Yu A,Mordvinov V V,Turutanov E Kh,et al.Low seis-mic velocity layers in the Earth s crust beneath Eastern Siberia(Russia)and Central Mongolia:receiver function data and theirpossible geological implication[J].Tectonophysics,2002,359:307-327.
[13]赵志丹,谢鸿森,周文戈,等.大别山榴辉岩的密度和波速及其对壳-幔循环的启示[J].矿物岩石地球化学通报,2001,20(1):6-10.
[14]Ghent E D,Dipple G M,Russell J K.Thermodynamic models foreclogitic mantle lithosphere[J].Earth and Planetary ScienceLetters,2004,218:451-462.
[15]Gao S,Zhang B R,Jin Z M,et al.Howmafic is the lower conti-nental crust?[J].Earth and Planetary Science Letters,1998,161:101-117.
[16]罗照华,莫宣学,侯增谦,等.青藏高原新生代形成演化的整合模型——来自火成岩的约束[J].地学前缘,2006,13(4):196-211.
[17]Rudnick R L,Gao S.Composition of the continental crust[J].Treatise on Geochemistry,2003,3:1-64.
[18]邓晋福,吴宗絮,赵海玲,等.下地壳性质与壳幔交换[M]//单文琅.岩石圈构造和深部作用.北京:地质出版社,1999:1-7.
[19]Deng J F,Mo X X,Zhao H L,et al.A new model for the dy-namic evolution of Chinese lithosphere:‘continental roots?plume tectonics’[J].Earth-Science Reviews,2004,65:223-275.
[20]谌宏伟,罗照华,莫宣学,等.东昆仑造山带三叠纪岩浆混合成因花岗岩的岩浆底侵作用机制[J].中国地质,2005,32(3):386-395.
[21]罗照华,柯珊,曹永清,等.东昆仑印支晚期幔源岩浆活动[J].地质通报,2002,21(6):292-297.
[22]王岳军,Zhang Y H,范蔚茗,等.湖南印支期过铝质花岗岩的形成:岩浆底侵作用与地壳加厚热效应的数值模拟[J].中国科学(D辑),2002,32(6):491-499.
[23]Bird P.Initiation of intracontinental subduction in the Himalaya[J].Journal of Geophysical Research,1978,83(B10):4975-4987.
[24]高山,金振民.拆沉作用(delamination)及其壳-幔演化动力学意义[J].地质科技情报,1997,16(1):1-9.
[25]Fyfe W S,Leonardos O H.Ancient metamorphic-magmatite beltsof the Brazilian African coasts[J].Nature,1973,244(5417):501-502.
[26]刘勉.造山带的重力滑塌.[M]//张友学,尹安.地球的结构、演化和动力学.北京:高等教育出版社,2002:77-205.
[27]McKenzie D,Jackson J.Conditions for flow in the continentalcrust[J].Tectonics,2002,21(6):1055-1061.
[28]Jackson J.Strength of the continental lithosphere:Time to aban-don the jelly sandwich?[J].GSA Today,2002,September:4-9.
[29]Morse S A.Basalts and Phase Diagrams[M].New York:Springer-Verlag Inc.,1980:493.
[30]Snelson C M,Henstock TJ,Keller G R,et al.Crustal and up-permost mantle structure along the deep probe seismic profile,Rocky Mountain[J].Geology,1998,33(2):181-198.
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