南海西北次海盆的磁条带重追踪及洋中脊分段性
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摘要
由于缺少有效钻孔资料,对于南海扩张的时间一直存在较大的疑问.在南海三大海盆中,西北次海盆面积最小、磁条带特征不明显,因此对其扩张年代的争议最大.最新采集的高密度(小于10km测线间距)船测地磁资料清晰地显示了西北次海盆磁条带的存在.在OBS和多道地震资料的约束下,利用船测地磁资料,本文对西北次海盆的地壳年龄进行了重追踪.根据定量的比较,西北次海盆的主体扩张始于35.8Ma(C16n,2n),在34.7Ma(C15)时其西南部开始扩张,扩张最终同时终止于33.2Ma(C13n),整体的全扩张速率在40～50mm/a之间.这表明南海的扩张可能首先起源于西北次海盆,在其结束扩张后,东部次海盆才开始打开(约30Ma).得益于数据精度和密度的提高,利用化极后的磁力异常以及反演的磁化强度可以对西北次海盆进行二级中脊段的划分.我们共划分出六个中脊段和一个明确的转换断层.中脊的分段性与OBS反演的地壳厚度的变化相一致.转换断层东侧,中脊主体分为四个中脊段,每个中脊段长度均在30km左右.转换断层西侧,存在一个长约50km的中脊段和一个不确切的中脊段.中脊段上磁化强度的变化幅值和中脊段长度在整体上成正比.每个中脊段中央的磁化强度弱于中脊段两端的磁化强度,这与扩张速率相近的大西洋中脊的磁化强度特征一致.
Since there is no valid deep sea drilling core,there are some debates about the opening age of the South China Sea(Nanhai).Owning to the small area,few magnetic polarity reversals and low magnetic anomalies amplitude of the northwestern sub-basin,the controversies about its opening age are very intense.The latest marine magnetic profiles which are closely spaced less than 10 km reveal the obvious magnetic lineation in the northwestern sub-basin.Constrained by newly measured ocean bottom seismic and multi-channel seismic data,we present a new identification to the latest marine magnetic profiles.According to the results of quantitative comparison,we prefer the northwestern sub-basin initially opened at 35.8 Ma(C16n,2n),propagated to southwest at about 34.7 Ma(C15) and all relict ridges ceased at the same time(33.2 Ma,C13n).The full spreading rate is about 40～50 mm/a.The result implies the South China Sea was initially opened in the northwestern sub-basin and the eastern basin began to spread(about 30 Ma) after the cease of the northwestern sub-basin.The accurate and dense data give the great opportunity to study the segmentations with magnetic anomalies reducted to the pole and the inversed magnetization.Six segments and one obvious transform fault are revealed here and the segmentation is consistent with the crustal thickness variation based on OBS data.To the east of the transform fault,the main part of relict ridge includes four segments which are connected by non-transform offsets and each segment is about 30 km.To the west of the transform fault,one segment is about 50 km and another one is uncertain here.The variation of magnetization amplitude along each segment is correlated with corresponding segment length.Magnetization in the center of segments is lower than that of segments ends and its magnetic structure along spreading axis is similar to the slow-spreading mid-Atlantic ridge which also has similar full spreading rate.
Since there is no valid deep sea drilling core,there are some debates about the opening age of the South China Sea(Nanhai).Owning to the small area,few magnetic polarity reversals and low magnetic anomalies amplitude of the northwestern sub-basin,the controversies about its opening age are very intense.The latest marine magnetic profiles which are closely spaced less than 10 km reveal the obvious magnetic lineation in the northwestern sub-basin.Constrained by newly measured ocean bottom seismic and multi-channel seismic data,we present a new identification to the latest marine magnetic profiles.According to the results of quantitative comparison,we prefer the northwestern sub-basin initially opened at 35.8 Ma(C16n,2n),propagated to southwest at about 34.7 Ma(C15) and all relict ridges ceased at the same time(33.2 Ma,C13n).The full spreading rate is about 40～50 mm/a.The result implies the South China Sea was initially opened in the northwestern sub-basin and the eastern basin began to spread(about 30 Ma) after the cease of the northwestern sub-basin.The accurate and dense data give the great opportunity to study the segmentations with magnetic anomalies reducted to the pole and the inversed magnetization.Six segments and one obvious transform fault are revealed here and the segmentation is consistent with the crustal thickness variation based on OBS data.To the east of the transform fault,the main part of relict ridge includes four segments which are connected by non-transform offsets and each segment is about 30 km.To the west of the transform fault,one segment is about 50 km and another one is uncertain here.The variation of magnetization amplitude along each segment is correlated with corresponding segment length.Magnetization in the center of segments is lower than that of segments ends and its magnetic structure along spreading axis is similar to the slow-spreading mid-Atlantic ridge which also has similar full spreading rate.
引文
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[10]姚伯初.南海西北海盆的构造特征及南海新生代的海底扩张.热带海洋,1999,18(1):7-15.Yao B C.Tectonic characteristics of northwest subbasin andseafloor spreading history of South China Sea in cenozoic.Journal of Tropical Oceanography(in Chinese),1999,18(1):7-15.
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[12]吴振利.南海西北次海盆张裂特征及扩张方式—广角地震折射/反射特征与地壳深部结构研究.青岛:中科院海洋研究所,2010.Wu Z L.Rifting characteristic and spreading pattern of theNW subbasin—research on characteristic of wide angleseismic reflection/refraction and deep crust structure(inChinese).Qingdao:Institute of Oceanology,ChineseAcademy of Sciences,2010.
[13]丁巍伟,黎明碧,赵俐红等.南海西北次海盆新生代构造-沉积特征及伸展模式探讨.地学前缘,2009,16(4):147-156.Ding W W,Li M B,Zhao L H,et al.Cenozoic tectono-sedimentary characteristics and extension model of theNorthwest Sub-basin,South China Sea.Earth ScienceFrontiers(in Chinese),2009,16(4):147-156.
[14]Wissmann G,Kudrass H,Hinz K.Continuing rifting on theSouth China margin:results of cooperation by stategeoscience institutes BGR/Hannover and SOA/Hangzhou inSONNE cruises 49and 58.Geowissenschaften,1996,14:310-312.
[15]周蒂,王万银,庞雄等.地球物理资料所揭示的南海东北部中生代俯冲增生带.中国科学D辑,2006,36(3):209-218.Zhou D,Wang W Y,Pang X,et al.Mesozoic subduction-accretion zone in northeastern South China Sea inferred fromgeophysical interpretations.Science in China Series D:EarthSciences(in Chinese),2006,49(5):471-482.
[16]高金耀,张涛,谭勇华等.不规则重磁测线网误差模型的约束最小二乘平差.海洋测绘,2006,26(4):6-10.Gao J Y,Zhang T,Tan Y H,et al.Constraint Least-squareadjustment for marine gravity and magnetic track-line data inirregular survey network with arbitrary error model.HydrographicSurveyingandCharting(in Chinese),2006,26(4):6-10.
[17]曾维军.南海磁异常对比方法的改进.热带海洋,1999,18(2):39-47.Zeng W J.Improvement on correlation method of magneticanomaly lineations in South China Sea.Tropicoceanology(inChinese),1999,18(2):39-47.
[18]Smith W H F,Sandwell D T.Global sea floor topographyfrom satellite altimetry and ship depth soundings.Science,1997,277(5334):1956-1962.
[19]郝天珧,徐亚,赵百民等.南海磁性基底分布特征的地球物理研究.地球物理学报,2009,52(11):2763-2774.Hao T Y,Xu Y,Zhao B M,et al.Geophysical research ondistribution features of magnetic basements in the SouthChina Sea.Chinese J.Geophys.(in Chinese),2009,52(11):2763-2774.
[20]Li X.Magnetic reduction-to-the-pole at low latitudes:Observations and considerations.The Leading Edge,2008,27(8):990.
[21]Parker R L,Huestis S P.The inversion of magneticanomalies in the presence of topography.J.Geophys.Res.,1974,79(11):1587-1593.
[22]Divins D L.NGDC total sediment thickness of the world'soceans&marginal seas.Retrieved date goes here,http:∥www.ngdc.noaa.gov/mgg/sedthick/sedthick.html.
[23]Mendel V,Munschy M,Sauter D.MODMAG,a MATLABprogram to model marine magnetic anomalies.Computers&Geosciences,2005,31(5):589-597.
[24]Lin J,Purdy G M,Schouten H,et al.Evidence from gravitydata for focused magmatic accretion along the Mid-AtlanticRidge.Nature,1990,344(6267):627-632.
[25]Hsu S K,Yeh Y C,Doo W B,et al.New bathymetry andmagnetic lineations identifications in the northernmost SouthChina Sea and their tectonic implications.Mar.Geophys.Res.,2004,25(1-2):29-44.
[26]Ravilly M,Dyment J,Gente P,et al.Axial magneticanomaly amplitude along the Mid-Atlantic Ridge between 20°N and 40°N.J.Geophys.Res.,1998,103(B10):24201.
[27]Pockalny R A,Smith A,Gente P.Spatial and temporalvariability of crustal magnetization of a slowly spreadingridge:Mid-Atlantic Ridge(20°—24°N).Mar.Geophys.Res.,1995,17(3):301-320.
[28]Grindlay N R,Fox P J,Vogt P R.Morphology and tectonicsof the Mid-Atlantic ridge(25°—27°30′S)from Sea Beam andmagnetic data.J.Geophys.Res.,1992,97(B5):6983-7010.
[29]Tivey M A,Johnson H P.The central anomaly magnetichigh:Implications for ocean crust construction and evolution.J.Geophys.Res.,1987,92(B12):12685-12694.
[30]Weiland C M,Macdonald K C,Grindlay N R.Ridgesegmentation and the magnetic structure of the southern Mid-Atlantic Ridge 26°S and 31°—35°S:Implications formagmatic processes at slow spreading centers.J.Geophys.Res.,1996,101(B4):8055-8073.
[31]李家彪,金翔龙,高金耀.南海东部海盆晚期扩张的构造地貌研究.中国科学D辑,2002,32(3):239-248.Li J B,Jin X L,Gao J Y.Morpho-tectonic study on late-stage spreading of the eastern subbasin of South China Sea.Science in China Series D:Earth Science(in Chinese),2002,45(11):978-989.
[32]Wessel P,Smith W M F.Free software helps map anddisplay data.EOS Trans.AGU,1991,72(41):441-446.
[2]Briais A,Patriat P,Tapponnier P.Updated interpretation ofmagnetic anomalies and seafloor spreading stages in the SouthChina Sea:implications for the Tertiary tectonics ofSoutheast Asia.J.Geophys.Res.,1993,98(B4):6299-6328.
[3]Taylor B,Hayes D E.Origin and history of the South ChinaSea basin.∥Hayes D E ed.The Tectonic and GeologicalEvolution of Southeast Asian Seas and Islands.Washington DC:AGU Geophysical Monograph,1980,23:89-104.
[4]姚伯初,曾维军,Hayes D E.中美合作调研南海地质专报.武汉:中国地质大学出版,1994.Yao B C,Zeng W J,Hayes D E.The geological memoir ofSouth China Sea surveyed jointly by China&USA(inChinese).Wuhan:China University of Geosciences Press,1994.
[5]Cande S C,Kent D V.Revised calibration of the geomagneticpolarity timescale for the Late Cretaceous and Cenozoic.J.Geophys.Res.,1995,100(B4):6093-6095.
[6]何廉声,陈邦彦.南海大地构造图.∥何廉声,陈邦彦.南海地质地球物理图集.广州:广东地图出版,1987.He L S,Chen B Y.The tectonic map of the South ChinaSea.∥He L S,Chen B Y eds.Atlas of Geology andGeophysics of the South China Sea(in Chinese).Guangzhou:Guangdong Atlas Press,1987.
[7]吕文正,柯长志,吴声迪等.南海中央海盆条带磁异常特征及构造演化.海洋学报,1987,9(1):69-78.LüZ W,Ke C Z,Wu S D,et al.The character of magneticbelt and tectonic evolution in the center subbasin of SouthChina Sea.Acta Oceanologica Sinica(in Chinese),1987,9(1):69-78.
[8]姚伯初.南海西南海盆的海底扩张及其构造意义.南海地质研究,1997,(9):20-36.Yao B C.The sea floor spreading in the southwest subbasinof South China Sea and its tectonic significance.GeologicalResearch of South China Sea(in Chinese),1999,(9):20-36.
[9]姚伯初.南海西南海盆的岩石圈张裂模式探讨.海洋地质与第四纪地质,1999,19(2):37-48.Yao B C.On the lithospheric rifting model in the southwestsubbasin of south China sea.Marine Geology&QuaternaryGeology(in Chinese),1999,19(2):37-48.
[10]姚伯初.南海西北海盆的构造特征及南海新生代的海底扩张.热带海洋,1999,18(1):7-15.Yao B C.Tectonic characteristics of northwest subbasin andseafloor spreading history of South China Sea in cenozoic.Journal of Tropical Oceanography(in Chinese),1999,18(1):7-15.
[11]Barckhausen U,Roeser H A.Seafloor spreading anomalies inthe South China Sea revisited.Geophysical Monograph,2004,149:121-126.
[12]吴振利.南海西北次海盆张裂特征及扩张方式—广角地震折射/反射特征与地壳深部结构研究.青岛:中科院海洋研究所,2010.Wu Z L.Rifting characteristic and spreading pattern of theNW subbasin—research on characteristic of wide angleseismic reflection/refraction and deep crust structure(inChinese).Qingdao:Institute of Oceanology,ChineseAcademy of Sciences,2010.
[13]丁巍伟,黎明碧,赵俐红等.南海西北次海盆新生代构造-沉积特征及伸展模式探讨.地学前缘,2009,16(4):147-156.Ding W W,Li M B,Zhao L H,et al.Cenozoic tectono-sedimentary characteristics and extension model of theNorthwest Sub-basin,South China Sea.Earth ScienceFrontiers(in Chinese),2009,16(4):147-156.
[14]Wissmann G,Kudrass H,Hinz K.Continuing rifting on theSouth China margin:results of cooperation by stategeoscience institutes BGR/Hannover and SOA/Hangzhou inSONNE cruises 49and 58.Geowissenschaften,1996,14:310-312.
[15]周蒂,王万银,庞雄等.地球物理资料所揭示的南海东北部中生代俯冲增生带.中国科学D辑,2006,36(3):209-218.Zhou D,Wang W Y,Pang X,et al.Mesozoic subduction-accretion zone in northeastern South China Sea inferred fromgeophysical interpretations.Science in China Series D:EarthSciences(in Chinese),2006,49(5):471-482.
[16]高金耀,张涛,谭勇华等.不规则重磁测线网误差模型的约束最小二乘平差.海洋测绘,2006,26(4):6-10.Gao J Y,Zhang T,Tan Y H,et al.Constraint Least-squareadjustment for marine gravity and magnetic track-line data inirregular survey network with arbitrary error model.HydrographicSurveyingandCharting(in Chinese),2006,26(4):6-10.
[17]曾维军.南海磁异常对比方法的改进.热带海洋,1999,18(2):39-47.Zeng W J.Improvement on correlation method of magneticanomaly lineations in South China Sea.Tropicoceanology(inChinese),1999,18(2):39-47.
[18]Smith W H F,Sandwell D T.Global sea floor topographyfrom satellite altimetry and ship depth soundings.Science,1997,277(5334):1956-1962.
[19]郝天珧,徐亚,赵百民等.南海磁性基底分布特征的地球物理研究.地球物理学报,2009,52(11):2763-2774.Hao T Y,Xu Y,Zhao B M,et al.Geophysical research ondistribution features of magnetic basements in the SouthChina Sea.Chinese J.Geophys.(in Chinese),2009,52(11):2763-2774.
[20]Li X.Magnetic reduction-to-the-pole at low latitudes:Observations and considerations.The Leading Edge,2008,27(8):990.
[21]Parker R L,Huestis S P.The inversion of magneticanomalies in the presence of topography.J.Geophys.Res.,1974,79(11):1587-1593.
[22]Divins D L.NGDC total sediment thickness of the world'soceans&marginal seas.Retrieved date goes here,http:∥www.ngdc.noaa.gov/mgg/sedthick/sedthick.html.
[23]Mendel V,Munschy M,Sauter D.MODMAG,a MATLABprogram to model marine magnetic anomalies.Computers&Geosciences,2005,31(5):589-597.
[24]Lin J,Purdy G M,Schouten H,et al.Evidence from gravitydata for focused magmatic accretion along the Mid-AtlanticRidge.Nature,1990,344(6267):627-632.
[25]Hsu S K,Yeh Y C,Doo W B,et al.New bathymetry andmagnetic lineations identifications in the northernmost SouthChina Sea and their tectonic implications.Mar.Geophys.Res.,2004,25(1-2):29-44.
[26]Ravilly M,Dyment J,Gente P,et al.Axial magneticanomaly amplitude along the Mid-Atlantic Ridge between 20°N and 40°N.J.Geophys.Res.,1998,103(B10):24201.
[27]Pockalny R A,Smith A,Gente P.Spatial and temporalvariability of crustal magnetization of a slowly spreadingridge:Mid-Atlantic Ridge(20°—24°N).Mar.Geophys.Res.,1995,17(3):301-320.
[28]Grindlay N R,Fox P J,Vogt P R.Morphology and tectonicsof the Mid-Atlantic ridge(25°—27°30′S)from Sea Beam andmagnetic data.J.Geophys.Res.,1992,97(B5):6983-7010.
[29]Tivey M A,Johnson H P.The central anomaly magnetichigh:Implications for ocean crust construction and evolution.J.Geophys.Res.,1987,92(B12):12685-12694.
[30]Weiland C M,Macdonald K C,Grindlay N R.Ridgesegmentation and the magnetic structure of the southern Mid-Atlantic Ridge 26°S and 31°—35°S:Implications formagmatic processes at slow spreading centers.J.Geophys.Res.,1996,101(B4):8055-8073.
[31]李家彪,金翔龙,高金耀.南海东部海盆晚期扩张的构造地貌研究.中国科学D辑,2002,32(3):239-248.Li J B,Jin X L,Gao J Y.Morpho-tectonic study on late-stage spreading of the eastern subbasin of South China Sea.Science in China Series D:Earth Science(in Chinese),2002,45(11):978-989.
[32]Wessel P,Smith W M F.Free software helps map anddisplay data.EOS Trans.AGU,1991,72(41):441-446.
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