南海西南次海盆与南沙地块的OBS探测和地壳结构
摘要
跨越南海西南次海盆南部陆缘和南沙地块中部的OBS973-1测线是南海南部首次采集的海底地震仪(OBS)广角反射与折射深地震测线,本文通过震相分析和走时正演拟合,获得了沿测线的二维纵波速度结构模型.模拟结果显示表层沉积物速度2.5～4.5 km/s,厚度1000～3000 m,局部基底面起伏较大.结晶基底的速度从顶部的4.5～5.5 km/s增加到地壳底部的6.8～6.9 km/s,中地壳有一个小的速度不连续面(0.1～0.2 km/s),而地壳底部的莫霍面有较大的速度反差(1.2 km/s),上地幔顶部的速度为8.0～8.1 km/s.莫霍面埋深和地壳厚度在测线的北段和南段有很大的不同,在测线北段的海盆区,莫霍面埋深约11 km,结晶地壳的厚度仅为5～6 km,表现为典型洋壳的特征,而在测线南段的陆块区,莫霍面埋深最大达24 km,地壳厚度可达20 km,表现为减薄陆壳的特征,从海盆区到陆块区莫霍面埋深和地壳厚度迅速增加.陆块区上下地壳的厚度和变化趋势相似,下地壳没有看到高速层(HVL),可能说明地壳内部是以纯剪拉张的均匀减薄为主,地壳下部的岩浆底侵不发育.对比OBS973-2和OBS973-3测线的结构模型,可以推测南沙地块的中部和东部具有相似的构造性质,西南次海盆两侧是一对非火山型的不对称共轭陆缘.
OBS973-1 is a wide angle reflection and refraction deep seismic line using Ocean Bottom Seismometers(OBS),deployed for the first time in the southern part of South China Sea.The line crosses the southern margin of the Southwest Sub-basin and the middle part of Nansha Block.By seismic phase analysis and travel time fitting,a 2D P-wave velocity model along the line is constructed.The modeling results show that the surface sedimentary layer has a velocity of 2.5～4.5 km/s and a thickness of 1000～3000m,with locally rough basement interface.The crystalline basement has a velocity of 4.5～5.5 km/s on the top,increasing to 6.8～6.9 km/s near the bottom of the crust.In the middle crust there is a small velocity discontinuity(0.1～0.2 km/s),while the Moho at the bottom of crust has a large velocity contrast(1.2 km/s).On the top mantle the velocity is 8.0～8.1 km/s.The Moho depth and the crustal thickness have much difference between the northern and southern segments along the line.The northern segment is in the deep sea basin,where the Moho is 11 km deep and the crystalline crust is only 5～6 km thick,which are typical characteristics of oceanic crust.In contrast,the southern segment is on the continental block,where the Moho has a maximum depth of 24 km and the crust has a corresponding thickness of 20 km,which are representative characteristics of thinned continental crust.From the deep sea basin to the continental block,the Moho depth and the crustal thickness increase rapidly.In the continental area,the upper crust and lower crust have similar thickness and variation.No High Velocity Layer(HVL) is seen in the lower crust.These may imply that pure-shear extension and uniform thinning are dominating inside the crust,and magma underplating is not developed in the lower crust.Comparing with the structure models of OBS973-2 and OBS973-3,it is inferred that the middle and eastern parts of Nansha Block have similar tectonic characters,and the two sides of Southwest Sub-basin are a pair of non-volcanic asymmetric conjugate continental margins.
OBS973-1 is a wide angle reflection and refraction deep seismic line using Ocean Bottom Seismometers(OBS),deployed for the first time in the southern part of South China Sea.The line crosses the southern margin of the Southwest Sub-basin and the middle part of Nansha Block.By seismic phase analysis and travel time fitting,a 2D P-wave velocity model along the line is constructed.The modeling results show that the surface sedimentary layer has a velocity of 2.5～4.5 km/s and a thickness of 1000～3000m,with locally rough basement interface.The crystalline basement has a velocity of 4.5～5.5 km/s on the top,increasing to 6.8～6.9 km/s near the bottom of the crust.In the middle crust there is a small velocity discontinuity(0.1～0.2 km/s),while the Moho at the bottom of crust has a large velocity contrast(1.2 km/s).On the top mantle the velocity is 8.0～8.1 km/s.The Moho depth and the crustal thickness have much difference between the northern and southern segments along the line.The northern segment is in the deep sea basin,where the Moho is 11 km deep and the crystalline crust is only 5～6 km thick,which are typical characteristics of oceanic crust.In contrast,the southern segment is on the continental block,where the Moho has a maximum depth of 24 km and the crust has a corresponding thickness of 20 km,which are representative characteristics of thinned continental crust.From the deep sea basin to the continental block,the Moho depth and the crustal thickness increase rapidly.In the continental area,the upper crust and lower crust have similar thickness and variation.No High Velocity Layer(HVL) is seen in the lower crust.These may imply that pure-shear extension and uniform thinning are dominating inside the crust,and magma underplating is not developed in the lower crust.Comparing with the structure models of OBS973-2 and OBS973-3,it is inferred that the middle and eastern parts of Nansha Block have similar tectonic characters,and the two sides of Southwest Sub-basin are a pair of non-volcanic asymmetric conjugate continental margins.
引文
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[28]Zelt C A,Smith R B.Seismic traveltime inversion for 2-D crustal velocity structure.Geophys.J.Int.,1992,108(1): 16-34
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[33]夏少红,丘学林,赵明辉等.南海共轭大陆边缘地壳结构及其类型特征.地球科学,2011,36(5):877-885 Xia S H,Qiu X L,Zhao M H,et al.Crustal structure and features in the conjugate margins of South China Sea.Earth Science(in Chinese),2011,36(5):877-885
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[2]Tapponnier P,Peltzer G,Le Dain A Y,et al.Propagating extrusion tectonics in Asia:new insights from simple experiments with plasticine.Geology,1982,10(12):611-616
[3]Taylor B,Hayes D E.Origin and history of the South China Sea basin.In:Hayes D E,ed.The Tectonic and Geologic Evolution of South Eastern Asian Seas and Islands,Ⅱ.AGU Geophysical Monograph,Washington,D.C.,1983,27: 23-56
[4]刘昭蜀等.南海地质构造与陆缘张裂.北京:科学出版社, 1988.398 Liu 2 S,et al.Geological Tectonics and Continental Margin Rifting of the South China Sea(in Chinese).Beijing:Science Press,1988.398
[5]金庆焕.南海地质与油气资源.北京:地质出版社,1989. 350 Jin Q H.The Geology and Oil-Gas Resources of South China Sea(in Chinese).Beijing:Geology Press,1989.350
[6]刘光鼎.中国海区及邻域地质地球物理系列图集.北京:科学出版社,1992 Liu G D.Geophysical Series Maps in China Sea and Adjacent Regions(in Chinese).Beijing:Science Press,1992
[7]李家彪.中国边缘海形成演化与资源效应.北京:海洋出版社,2005.509 Li J B.Evolution of Chinese Marginal Seas and Its Effect of Natural Resources(inChinese).Beijing:OceanPress, 2005.509
[8]汪品先.南海—我国深海研究的突破口.热带海洋学报, 2009,28(3):1-4 Wang PX.Toward scientific breakthrough in the South China Sea.Journal of Tropical Oceanography(in Chinese), 2009,28(3):1-4
[9]姚伯初,曾维军,Hayes D E等.中美合作调研南海地质专报.武汉:中国地质大学出版社,1994.204 Yao B C,Zeng W J,Hayes D E,et al.The Geological Memoir of South China Sea Survey Jointly by China and USA (in Chinese).Beijing:China University of Geosciences Press,1994.204
[10]Nissen S S,Hayes D E,Bochu Y,et al.Gravity,heat flow, and seismic constraints on the processes of crustal extension; Northern margin of the South China Sea.J.Geophys.Res., 1995,100(B11):22447-22483
[11]Yan P,Zhou D,Liu Z S.A crustal structure profile across the northern continental margin of the South China Sea. Tectonophysics,2001,338(1):1-21
[12]Qiu X L,Ye S Y,Wu S M,et al.Crustal structure across the Xisha Trough,northwestern South China Sea. Tectonophysics,2001,341(1-4):179-193
[13]丘学林,施小斌,阎贫等.南海北部地壳结构的深地震探测和研究新进展.自然科学进展,2003,13(3):231-236 Qiu X L,Shi X B,Yan P,et al.Recent progress of deep seismic experiments and studies of crustal structure in northern South China Sea.Progress in Natural Science(in Chinese),2003,13(7):231-236
[14]Wang T K,Chen M K,Le C S,et al.Seismic imaging of the transitional crust across the northeastern margin of the South China Sea.Tectonophysics,2006,412(3-4):237-254
[15]Zhao M H,Qiu X L,Xia S H,et al.Seismic structure in the northeastern South China Sea:S-wave velocity and V_p/V_s ratios derived from three-component OBS data.Tectonophysics, 2010,480(1-4):183-197
[16]吴振利,李家彪,阮爱国等.南海西北次海盆地壳结构:海底广角地震实验结果.中国科学(D辑):地球科学,2011, 41(10):1463-1476 Wu Z L,Li J B,Ruan A G,et al.The crustal structure of the Northwest Sub-basin,South China Sea:results from OBS wide-angle seismic expriment.Science in China(Series D): Earth Sciences(in Chinese),2011,41(10):1463-1476
[17]赵明辉,丘学林,徐辉龙等.南海南部深地震探测及南北共轭陆缘对比.地球科学,2011,36(5):823-830 Zhao M H,Qiu X L,Xu H L,et al.Deep seismic surveys in the southern South China Sea and contrast on its conjugate margins.Earth Science(in Chinese),2011,36(5):823- 830
[18]阮爱国,牛雄伟,丘学林等.穿越南沙礼乐滩的海底地震仪广角地震试验.地球物理学报,2011,54(12):3139-3149 Ruan A G,Niu X W,Qiu X L,et al.A wide angle Ocean Bottom Seismometer profile across Liyue Bank,the southern margin of South China Sea.Chinese J.Geophys.(in Chinese),2011,54(12):3139-3149
[19]吕川川,郝天珧,丘学林等.南海西南次海盆北缘海底地震仪测线深部地壳结构研究.地球物理学报,2011,54(12):3117- 3126 Lu C C,Hao T Y,Qiu X L,et al.A study on the deep structure of Southwest Sub-basin from ocean bottom seismic data,South China Sea.Chinese J.Geophys.(in Chinese), 2011,54(12):3117-3126
[20]姚伯初.中南-礼乐断裂的特征及其构造意义.南海地质研究,1995,7:1-14 Yao B C.Characteristics and tectonic meaning of Zhongnan-Lile fault.Geological Research of South China Sea(in Chinese),1995,7:1-14
[21]Li C F,Zhou Z Y,Li J B,et al.Structures of the northeasternmost South China Sea continental margin and ocean basin:geophysical constraints and tectonic implications. Mar.Geophys.Res.,2007,28(1):59-79.doi:10.1007/ s11001-007-9014-9
[22]Briais A,Patriat P,Tapponnier P.Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea:Implications for the Tertiary tectonics of southeast Asia.J.Geophys.Res.,1993,98(B4):6299-6328
[23]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 Southwest Subbasin of South China Sea.Marine Geology & Quaternary Geology(in Chinese),1999,19(2):37-48
[24]Barckhausen U,Roeser H A.Seafloor spreading anomalies in South China Sea revisited.In:Clift P,Wang P,Kuhnt W, et al,eds.Continent-Ocean Interactions Within East Asian Marginal Seas.Geophys.Monogr.Ser.,AGU,Washington, D.C.2004,149:121-125
[25]金庆焕,李唐根.南沙海域区域地质构造.海洋地质与第四纪地质,2000,20(1):1-8 Jin Q H,Li T G.Regional geologic tectonics of the Nansha Sea area.Marine Geology & Quaternary Geology(in Chinese),2000,20(1):1-8
[26]刘海龄,郭令智,孙岩等.南沙地块断裂构造系统与岩石圈动力学研究.北京:科学出版社,2002.247 Liu H L,Guo L Z,Sun Y,et al.The Fracture Tectonic System and Lithosphere Dynamics Studies of Nansha Block, South China Sea(in Chinese).Beijing:Science Press,2002. 247
[27]吴世敏,周蒂,刘海龄.南沙地块构造格局及其演化特征.大地构造与成矿,2004,28(1):23-28 Wu SM,Zhou D,Liu H L.Tectonic framework and evolutionary characteristics of Nansha Block,South China Sea.Geotectonica et Metallogenia(in Chinese),2004,28 (1):23-28
[28]Zelt C A,Smith R B.Seismic traveltime inversion for 2-D crustal velocity structure.Geophys.J.Int.,1992,108(1): 16-34
[29]丁巍伟,李家彪.南海南部陆缘构造变形特征及伸展作用:来自两条973多道地震测线的证据.地球物理学报,2011, 54(12):3038-3056 Ding WW,Li J B.Seismic stratigraphy,tectonic structure and extension factors across the southern margin of the South China Sea:evidence from two regional multichannel seismic profiles.Chinese J.Geophys.(in Chinese),2011,54(12); 3038-3056
[30]郝天珧,徐亚,孙福利等.南海共轭大陆边缘构造属性的综合地球物理研究.地球物理学报,2011,54(12):3098-3116 Hao T Y,Xu Y,Sun F L,et al.Integrated geophysical research on the tectonic attribute of conjugate continental margin of South China Sea.Chinese Journal of Geophysics (in Chinese),2011,54(12):3098-3116
[31]苏达权,刘元龙,陈雪等.南海的三维莫霍界面.见:张中杰等主编.中国大陆地球深部结构与动力学研究庆贺滕吉文院士从事地球物理研究50周年.北京:科学出版社, 2004.357-365 Su D Q,Liu Y L,Chen X,et al.The 3D Moho interface of South China Sea.In:Zhang Z J,et al.eds.Deep Structure and Dynamics in the Continent of China(in Chineses). Beijing:Science Press,2004.357-365
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