南海西南次海盆反射莫霍面成像及其地质意义
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摘要
深部地质结构是研究海盆动力成因的重要基础。南海西南次海盆以往多道地震资料中莫霍面的成像普遍不清,选取NH973-1测线长排列多道地震数据对西南次海盆的莫霍面反射成像进行研究。该地震资料中层间多次波非常发育,严重掩盖或干扰了莫霍面有效反射信号。针对地震资料特征,首先采用抛物线型Radon变换滤波对部分层间多次波进行压制以拾取一个相对准确的初始速度,在此基础上进一步采用速度滤波和内切除组合方法对层间多次波进行压制。从资料处理效果看,层间多次波得到有效压制,莫霍面成像清晰,呈现出断断续续的特征。由此解释的海盆区地壳(除沉积层外)厚度整体较薄,约为2.3~3.9km,有别于正常洋壳结构,更接近于构造拉伸主导型的地壳。
Deep crust structure is important to understand the dynamic origin of a marginal sea basin.However,reflection Moho has been poorly imaged in the Southwest Sub-basin of the South China Sea so far.So we reprocessed a portion of the seismic line NH973-1,which was collected with a 6km streamer,to improve the Moho imaging.This data is rich of peg-leg multiples,thus the Moho is masked.For this,Parabolic Radon transform filtering is firstly applied to suppress the peg-leg multiples to get a initial velocity of the deep reflections.Then,velocity filtering and inner muting are carried out to further attenuate the multiples.Consequently,peg-leg multiples are attenuated effectively,and the discontinuous strong reflections,likely Moho,appear clearly.The interpreted crust(except sediments)is about 2.3to 3.9km thick,which is different from the normal oceanic crust,but more like the crust created by tectonic dominated seafloor spreading.
Deep crust structure is important to understand the dynamic origin of a marginal sea basin.However,reflection Moho has been poorly imaged in the Southwest Sub-basin of the South China Sea so far.So we reprocessed a portion of the seismic line NH973-1,which was collected with a 6km streamer,to improve the Moho imaging.This data is rich of peg-leg multiples,thus the Moho is masked.For this,Parabolic Radon transform filtering is firstly applied to suppress the peg-leg multiples to get a initial velocity of the deep reflections.Then,velocity filtering and inner muting are carried out to further attenuate the multiples.Consequently,peg-leg multiples are attenuated effectively,and the discontinuous strong reflections,likely Moho,appear clearly.The interpreted crust(except sediments)is about 2.3to 3.9km thick,which is different from the normal oceanic crust,but more like the crust created by tectonic dominated seafloor spreading.
引文
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[3]Li J B,Ding W W,Wu Z Y,et al.The propagation of seafloor spreading in the SW subbasin,South China Sea[J].Chinese Science Bulletin,2012,57(24):3182-3191.
[4]Pichot T,Delescluse M,Chamot-Rooke N,et al.Deep crustal structure of the conjugate margins of the SW South China Sea from wide-angle refraction seismic data[J].Marine and Petroleum Geology,2014,58:627-643.
[5]Song T R,Li C F.Rifting to drifting transition of the Southwest Subbasin of the South China Sea[J].Mar.Geophys Res.,2015,36:167-185.
[6]Ding W W,Li J B,Clift D,et al.Spreading dynamics and sedimentary process of the Southwest Sub-basin,South China Sea:Constraints from multi-channel seismic data and IODP Expedition 349[J].Journal of Asian Earth Sciences,2016,115:97-113.
[7]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[J].地球物理学报,2011,54(12):3129-3138.[LüChuanchuan,HAO Tianyao,QIU Xuelin,et al.A study on the deep structure of the northern part of Southwest sub-basin from ocean bottom seismic data,South China Sea[J].Chinese Journal of Geophysics,2011,54(12):3129-3138.]
[8]王彦林,阎贫.深地震探测的分辨率分析—以南海北部OBS数据为例[J].地球物理学报,2009,52(9):2282-2290.[WANG Yanlin,YAN Pin.Lateral resolution analysis of deep crustal sounding:A case study on the data from ocean bottom seismometers in the northern South China Sea[J].Chinese Journal of Geophysics,2009,52(9):2282-2290.]
[9]Wessel P,Smith W H F.New,improved version of generic mapping tools released[C].EOS Transactions AGU,1998,79(47):579.
[10]Braitenberg C,Wienecke S,and Wang Y.Basement structures from satellite-derived gravity field:South China Sea ridge[J].J.Geophys.Res.,2006,111,B05407,doi:10.1029/2005JB003938.
[11]Tsuji T,Nakamura Y,Tokuyama H,et al.Oceanic crust and Moho of the Pacific Plate in the eastern Ogasawara Plateau region[J].Island Arc,2007,16:361-373.
[12]Aghaei O,Nedimovic M R,Carton H,et al.Crustal thickness and Moho character of the fast-spreading East Pacific Rise from 9°42′N to 9°57′N from poststack-migrated 3-D MCS data[J].Geochemistry Geophysics Geosystems,2014,15:634-657,doi:10.1002/2013GC005069.
[13]Schroeder T,Cheadle M,Dick H.Nonvolcanic seafloor spreading and corner-flow rotation accommodated by extensional faulting at 15°N on the Mid-Atlantic Ridge:A structural synthesis of ODP Leg 209[J].Geochemistry Geophysics Geosystems,2007,8(6):1-16.
[14]Cannat M,Sauter D,Mendel V,et al.Models of seafloor generation at a melt-poor ultraslow-spreading ridge[J].Geology,2006,34(7):605-608.
[15]Hopper J R,Funck T,Tucholke B E,et al.Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cao on the Newfoundland rifted margin[J].Geology,2004,32(1):93-96.