韧性剪切带中片麻岩和超高压榴辉岩变形特征及其与地震波速各向异性的关系:来自中国大陆科学钻探(CCSD)680～1200米岩心的证据
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摘要
中国大陆科学钻探(CCSD)680-1200米区段发育了多个韧性剪切带,带中主要岩石类型包括片麻岩和超高压榴辉岩。片麻岩中的变形石英、面理化榴辉岩中的拉长石榴石和绿辉石的应变轴比都表现为X>Y>Z,Flinn系数分别为0.11-0.27、0.22-0.23和0.23-0.24。随着糜棱岩化作用的增强,变形石英的C轴组构由Z轴极密逐渐向Y轴极密和叶理面上的大圆环带转变。在常温常压下测试了样品的波速,计算出片麻岩Vp和Vs的各向异性分别为30.17％-60.97％和11.52％-35.79％,榴辉岩Vp和Vs的各向异性分别为0.17％-11.19％和2.41％-6.70％。影响各向异性的主要因素有岩石的结构构造、矿物的晶格优选方位(LPO)、形态优选方位(SPO)和定向微裂隙。随着糜棱岩化作用的增强,岩石的P波各向异性逐 渐升高。变形岩石中的黑云母、石英、绿辉石的LPO和SPO是地震波各向异性的主要控制因素。饱水后的片麻岩样品的P波各向异性明显低于干燥片麻岩样品。在东海钻井中的强反射带主要是由于不同岩层之间的波阻抗差异而造成的,榴辉岩/强退变榴辉岩和黑云斜长片麻岩之间的接触界面会产生较强的地震深反射。此外,与LPO相关的地震波各向异性会增强地震波的反射,所以韧性剪切带中的糜棱岩化片麻岩可能是地震反射的良好载体。韧性剪切带中岩石弹性波速度的强各向
Several ductile shear zones developed at depth of 680 - 1200m of CCSD. The main rock types in the shear zone include gneisses and UHP eclogites. The strain axes of deformed quartz in gneisses and stretch garnets and omphacites in eclogites are expressed as X > Y > Z ratios. Their flinn coefficients vary from 0. 11 to 0. 27 , 0. 22 to 0. 23 and 0. 23 to 0. 24, respectively. In the gneiss sample the C - axis fabric of deformed quartz shows a great circle normal to the foliation and a maximum near Z. With the strengthening of mylonitization, the fabrics become a great circle near the foliation and a maximum near Y. P- and S-wave velocities were measured at room temperature and pressure. Calculated P- and S-wave velocity anisotropies of gneisses vary from 30. 17% to 60.97% and 11.52% to 35.79%, while those of eclogite vary from 0. 17% toll.19% and 2.41% to 6.70%, respectively. Seismic velocity anisotropies are mainly caused by lattice preferred orientation( LPO) , shape preferred orientation( SPO) of the major minerals, and oriented microcracks. P-wave velocity anisotropies of gneiss increase with strengthening mylonitization. LPO and SPO of biotite, quartz and omphacite are responsible for the velocity anisotropy of deformed rocks. P-wave velocity anisotropies of water-saturated gneiss are lower than those of dry gneiss. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/retrograde eclogite and biotite-plagioclase gneiss may give rise to strong seismic reflections. In addition, lattice preferred orientation-related seismic anisotropy can increase reflectivity, thus the mylonitized gneiss in ductile shear zone may be good reflectors. The strong anisotropy of ductile shear zone provides important constraints on interpretations of seismic deep reflection results.
Several ductile shear zones developed at depth of 680 - 1200m of CCSD. The main rock types in the shear zone include gneisses and UHP eclogites. The strain axes of deformed quartz in gneisses and stretch garnets and omphacites in eclogites are expressed as X > Y > Z ratios. Their flinn coefficients vary from 0. 11 to 0. 27 , 0. 22 to 0. 23 and 0. 23 to 0. 24, respectively. In the gneiss sample the C - axis fabric of deformed quartz shows a great circle normal to the foliation and a maximum near Z. With the strengthening of mylonitization, the fabrics become a great circle near the foliation and a maximum near Y. P- and S-wave velocities were measured at room temperature and pressure. Calculated P- and S-wave velocity anisotropies of gneisses vary from 30. 17% to 60.97% and 11.52% to 35.79%, while those of eclogite vary from 0. 17% toll.19% and 2.41% to 6.70%, respectively. Seismic velocity anisotropies are mainly caused by lattice preferred orientation( LPO) , shape preferred orientation( SPO) of the major minerals, and oriented microcracks. P-wave velocity anisotropies of gneiss increase with strengthening mylonitization. LPO and SPO of biotite, quartz and omphacite are responsible for the velocity anisotropy of deformed rocks. P-wave velocity anisotropies of water-saturated gneiss are lower than those of dry gneiss. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/retrograde eclogite and biotite-plagioclase gneiss may give rise to strong seismic reflections. In addition, lattice preferred orientation-related seismic anisotropy can increase reflectivity, thus the mylonitized gneiss in ductile shear zone may be good reflectors. The strong anisotropy of ductile shear zone provides important constraints on interpretations of seismic deep reflection results.
引文
Brich F. 1961． The velocity of compressional waves in rocks to lOkbar.J. Geophys. Res. , 66:2199-2224
Brogia A, Lazzarottoa A, Liottab D and Ranallic G. 2003 . Extensionalshear zones as imaged by reflection seismic lines: the Larderellogeothermal field (central Italy). Teetonophysics, 363:127-139．
Chen Jing, Wang Qingchen, Zhai Mingguo, Ye Kai. 1995． Deformationfeatures of garnet in eclogite. Science in China( Series B). 25 (10) :1116-1120(in Chinese).
Christensen N L and Szymanski D L. 1988． Origin of reflection from the Brevard Fault zone. J. Geophys. Res. , 93:1087-1102
Christensen N L. 1989． Relectivity and seismic properties of the deep continental crust. J. Geophys. Res. , 94:17795-17804．
Foutain D M, Hurich C A and Smithson S B. 1984． Seismic reflectivity of mylonite zones in the crust. Geology, 12:195-198
Foutain D M, Boundy T M, Austrheim H. 1994． Eclogite facies shear zone-deep crustal reflectors? Tectonophysics, 232:411-424
Ji S C, Salisbury H. 1993． Shear-wave velocies, anisotropy and splitting in high-grad mylonites. Tectonophysics, 221:453-473．
Jin Shuyan. 1997． Anisotropy of continental lithosphere and its dynamical implications, eded: Modern Methods of Lithosphere study. Beijing: Nuclear Energy Publishing House, 37-44 (in Chinese with English abstract)
Jin Shuyan, Jiao Shuqiang. 1998． Fabric measurements of omphacite from UHP eclogite and its rheological significations. Earth Science-Journal Of China University of Geosciences, 23 (1) :37-40． ( in Chinese with English abstract)
Jin Zhenmin, Zhang Junfeng et al. 2001． Rheological strength of UHP eclogite from Dabie Mountain: evidences from high P-T experiments. Earth Science-Journal Of China University of Geosciences, 26:574-580． (in Chinese with English abstract)
Jones K A, Warner M R, Morgan R P LI et al. 1996． Coincident normal-incidence and wide-angle reflections from the Moho: evidence for crustal seismic anisotropy. Tectonophysics, 264:205-217．
Jones T and Nur A. 1982． Seismic velocity and anisotropy in mylonites and reflectivity of deep crustal fault zones. Geology, 10:260-263．
Kern H, Wenk H R. 1990． Fabric-related velocity anisolropy and shear-wave splitting in rocks from the Santo Rosa Mylonite zone, California. J. Geophys. Res. 95:11213-11223．
Kem H, Wenk H R. 1993． Preferred orientation in deformed metals and rocks. China University of Geosciences Publishing House, 284-324 (in Chinese with English abstract).
Kern H, Liu B, and Popp T. 1997． Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite. J. Geophys. Res. , 102(B2) :3051-3065．
Kern H, Gao S, Jin Z M et al. 1999． Petrophysical studies on rocks from the Dabie ultrahigh-pressure ( UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust. Tectonophysics, 301:191-215．
Kern H, Popp T, Gorbatsevich F et al. 2001． Pressure and temperature dependence of Vp and Vs in rocks from superdeep well and from surface analogues ay Kola and the nature of velocity anisotropy. Tectonophysics, 338:113-134．
Kern H, Jin Z M, Gao S et al. 2002． Physical properties of ultrahigh-pressure metamorphic rocks from the SuLu terrain, eastern central China: implications for the seismic structure at the Donghai( CCSD) drilling site. Tectonophysics, 354:315-330．
Kleinschrodt R, Duyster J P. 2002． HT-deformation of garnet: an EBSD study on granulites from Sri Lanka, India and Ivrea zone. Journal of structural Geology, 24:1829-1844．
Liu Fulai, Xu Zhiqin, Yang Jingsui. 2001． Mineral inclusions in zircon of gneiss from the pre-pilot holes ( CCSD-PP2 ) in northern Jiangsu province and its evidences of UHP metamorphism. Chinese Science Bulletin, 46(3) :241-246(in Chinese with English abstract).
Mazzoli C, Sassi R, and Burlini L. 2002． Experimental study of the seismic properties of the eastern Alps( Italy) along the Aurina-Tures-Badia Valleys transect. Tectonophysics, 354:179-194．
Mu Runchang, Gao Ping, Liu Ruoxin et al. 1995． The experimental stu
Brogia A, Lazzarottoa A, Liottab D and Ranallic G. 2003 . Extensionalshear zones as imaged by reflection seismic lines: the Larderellogeothermal field (central Italy). Teetonophysics, 363:127-139．
Chen Jing, Wang Qingchen, Zhai Mingguo, Ye Kai. 1995． Deformationfeatures of garnet in eclogite. Science in China( Series B). 25 (10) :1116-1120(in Chinese).
Christensen N L and Szymanski D L. 1988． Origin of reflection from the Brevard Fault zone. J. Geophys. Res. , 93:1087-1102
Christensen N L. 1989． Relectivity and seismic properties of the deep continental crust. J. Geophys. Res. , 94:17795-17804．
Foutain D M, Hurich C A and Smithson S B. 1984． Seismic reflectivity of mylonite zones in the crust. Geology, 12:195-198
Foutain D M, Boundy T M, Austrheim H. 1994． Eclogite facies shear zone-deep crustal reflectors? Tectonophysics, 232:411-424
Ji S C, Salisbury H. 1993． Shear-wave velocies, anisotropy and splitting in high-grad mylonites. Tectonophysics, 221:453-473．
Jin Shuyan. 1997． Anisotropy of continental lithosphere and its dynamical implications, eded: Modern Methods of Lithosphere study. Beijing: Nuclear Energy Publishing House, 37-44 (in Chinese with English abstract)
Jin Shuyan, Jiao Shuqiang. 1998． Fabric measurements of omphacite from UHP eclogite and its rheological significations. Earth Science-Journal Of China University of Geosciences, 23 (1) :37-40． ( in Chinese with English abstract)
Jin Zhenmin, Zhang Junfeng et al. 2001． Rheological strength of UHP eclogite from Dabie Mountain: evidences from high P-T experiments. Earth Science-Journal Of China University of Geosciences, 26:574-580． (in Chinese with English abstract)
Jones K A, Warner M R, Morgan R P LI et al. 1996． Coincident normal-incidence and wide-angle reflections from the Moho: evidence for crustal seismic anisotropy. Tectonophysics, 264:205-217．
Jones T and Nur A. 1982． Seismic velocity and anisotropy in mylonites and reflectivity of deep crustal fault zones. Geology, 10:260-263．
Kern H, Wenk H R. 1990． Fabric-related velocity anisolropy and shear-wave splitting in rocks from the Santo Rosa Mylonite zone, California. J. Geophys. Res. 95:11213-11223．
Kem H, Wenk H R. 1993． Preferred orientation in deformed metals and rocks. China University of Geosciences Publishing House, 284-324 (in Chinese with English abstract).
Kern H, Liu B, and Popp T. 1997． Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite. J. Geophys. Res. , 102(B2) :3051-3065．
Kern H, Gao S, Jin Z M et al. 1999． Petrophysical studies on rocks from the Dabie ultrahigh-pressure ( UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust. Tectonophysics, 301:191-215．
Kern H, Popp T, Gorbatsevich F et al. 2001． Pressure and temperature dependence of Vp and Vs in rocks from superdeep well and from surface analogues ay Kola and the nature of velocity anisotropy. Tectonophysics, 338:113-134．
Kern H, Jin Z M, Gao S et al. 2002． Physical properties of ultrahigh-pressure metamorphic rocks from the SuLu terrain, eastern central China: implications for the seismic structure at the Donghai( CCSD) drilling site. Tectonophysics, 354:315-330．
Kleinschrodt R, Duyster J P. 2002． HT-deformation of garnet: an EBSD study on granulites from Sri Lanka, India and Ivrea zone. Journal of structural Geology, 24:1829-1844．
Liu Fulai, Xu Zhiqin, Yang Jingsui. 2001． Mineral inclusions in zircon of gneiss from the pre-pilot holes ( CCSD-PP2 ) in northern Jiangsu province and its evidences of UHP metamorphism. Chinese Science Bulletin, 46(3) :241-246(in Chinese with English abstract).
Mazzoli C, Sassi R, and Burlini L. 2002． Experimental study of the seismic properties of the eastern Alps( Italy) along the Aurina-Tures-Badia Valleys transect. Tectonophysics, 354:179-194．
Mu Runchang, Gao Ping, Liu Ruoxin et al. 1995． The experimental stu
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