深地震反射剖面揭示的渭河盆地西安坳陷的地壳精细结构
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摘要
2005年,跨西安坳陷完成一条NW方向,69km长的深地震反射剖面,首次获得该区域的地壳精细结构、主要断裂展布和深、浅构造关系图像.地震反射CMP叠加时间剖面显示,以反射事件C为界,地壳被分成上下两部分.上地壳由多组近水平反射带组成,具有分段连续性好、局部存在反射带明显错断或形态突变等特征,清晰地刻画出西安坳陷新生代沉积分层、坳陷底界、渭河断裂带、临潼—长安断裂带和秦岭山前断裂带的几何形态和关系.反射事件C是结晶地壳内宽度约0.5s的反射带,最深处位于桩号30km,底界约6.5s,向西北缓慢抬升至5.5s,向南东迅速抬升至5.5s.下地壳有两个明显的反射事件RA、RB:RB是位于桩号40～47km之间的局部反射团,而RA为宽度约2s、向坳陷倾斜的反射带.以桩号38km为界,反射Moho形态截然不同,而且出现了显著的错断:大桩号方向,反射Moho为位于双程走时11～14s水平的反射分段连续的过渡带,宽度约3s;小桩号方向,反射Moho为一宽度约2s、并向大桩号倾斜的反射分段连续的过渡带,其形态和反射事件RA相同.根据地震波速度资料,求得这几个反射带顶界的深度分别为:10.5～13.5km(反射带C)、20.3～21.5km(反射带RB)、16.8～34.3km(反射带RA)和32～36.7km(反射带Moho)左右.作者认为形态一致的反射事件RA和反射Moho很可能是古秦岭洋向华北地台俯冲的遗迹.此外,西安坳陷内错断新生界深达反射事件C的渭河和临潼—长安断裂带和莫霍错断的存在,表明该地区地壳现今活动性很强,是未来强震发生值得注意的地区.
In 2005,a cross the Xi′an sag,a NW-direction and 69 km-long deep seismic reflection profile was completed.It is first time to get the fine structures of the crust,the distribution of main faults and image of tectonics relation between deep and shallow structure in this region.CMP time-stack section of seismic reflection shows the crust is divided into two parts by a reflection event C as the boundary.Upper crust consists of multiple nearly horizontal reflection belts,with good segmental continuity,local existence the fault offset reflection belts or the characteristics of morphological mutations that clearly depict Xi′an sag Cenozoic sedimentary layers,the bottom boundary of sag,and the geometry and relationships of the Weihe Fault,Lintong-Chang′an fault zone and piedmont of Qinling fault zone.C is the reflection event in the crystallization crust,with the reflection band width of about 0.5s,the deepest of the bottom boundary of about 6.5s at stake No.30 km,slow uplift to the northwest to the 5.5 s,rapid uplift to the south east to the 5.5 s.There are two distinct lower crustal reflection events RA and RB: RB is the local reflect group at stake No.40-47 km;RA is the band width of about 2 s,which inclines to sag.RA is a strong reflection under C and above Moho south of the stake 34 km.Arguably,the upper crust is above the C interface in the whole profile,but from south of stake 34 km,crust under C interface is divided into middle crust and lower crust by RA interface.From north of stake 34 km,RB is weak,unclear reflection group,so it cannot be a distinct interface to divide middle and lower crust.On both sides of stake No.38 km.Moho reflection is of different forms,but there is also a significant fault to offset in big stake number direction.Moho reflection in the two-way travel time(TWT) for the 11-14 s is a level piecewise continuous reflection transition zone,with width of about 3 s in small stake number direction.Moho reflection is a about 2 s wide,piecewise continuous transition zone and tilts to the big stake number direction.Its shape is almost same as reflection event RA.According to seismic velocity data,these depth of top boundaries of reflective belts are:10.5-13.5 km(reflective belt C),20.3-21.5 km(reflective belt RB),16.8-34.3 km(reflective belt RA) and 32-36.7 km(reflective belt Moho) or so.The authors think the same shape of reflection events of RA and Moho is likely the relic of ancient Qinling ocean to dive to the North China platform.The fracture tectonic in this district developed,according to the reflection wave group characteristics of CMP stack-time section of deep reflection profile and total 11 faults(F1-F11) explained by the obvious characteristics of fracture.In addition,the depth Weihe fault and Lintong-Chang′an fault to the reflective belt C in Xi′an Cenozoic sag and the presence of Moho fault indicates that the region has strong present crustal activity,being worth noted as the future strong earthquake area.
In 2005,a cross the Xi′an sag,a NW-direction and 69 km-long deep seismic reflection profile was completed.It is first time to get the fine structures of the crust,the distribution of main faults and image of tectonics relation between deep and shallow structure in this region.CMP time-stack section of seismic reflection shows the crust is divided into two parts by a reflection event C as the boundary.Upper crust consists of multiple nearly horizontal reflection belts,with good segmental continuity,local existence the fault offset reflection belts or the characteristics of morphological mutations that clearly depict Xi′an sag Cenozoic sedimentary layers,the bottom boundary of sag,and the geometry and relationships of the Weihe Fault,Lintong-Chang′an fault zone and piedmont of Qinling fault zone.C is the reflection event in the crystallization crust,with the reflection band width of about 0.5s,the deepest of the bottom boundary of about 6.5s at stake No.30 km,slow uplift to the northwest to the 5.5 s,rapid uplift to the south east to the 5.5 s.There are two distinct lower crustal reflection events RA and RB: RB is the local reflect group at stake No.40-47 km;RA is the band width of about 2 s,which inclines to sag.RA is a strong reflection under C and above Moho south of the stake 34 km.Arguably,the upper crust is above the C interface in the whole profile,but from south of stake 34 km,crust under C interface is divided into middle crust and lower crust by RA interface.From north of stake 34 km,RB is weak,unclear reflection group,so it cannot be a distinct interface to divide middle and lower crust.On both sides of stake No.38 km.Moho reflection is of different forms,but there is also a significant fault to offset in big stake number direction.Moho reflection in the two-way travel time(TWT) for the 11-14 s is a level piecewise continuous reflection transition zone,with width of about 3 s in small stake number direction.Moho reflection is a about 2 s wide,piecewise continuous transition zone and tilts to the big stake number direction.Its shape is almost same as reflection event RA.According to seismic velocity data,these depth of top boundaries of reflective belts are:10.5-13.5 km(reflective belt C),20.3-21.5 km(reflective belt RB),16.8-34.3 km(reflective belt RA) and 32-36.7 km(reflective belt Moho) or so.The authors think the same shape of reflection events of RA and Moho is likely the relic of ancient Qinling ocean to dive to the North China platform.The fracture tectonic in this district developed,according to the reflection wave group characteristics of CMP stack-time section of deep reflection profile and total 11 faults(F1-F11) explained by the obvious characteristics of fracture.In addition,the depth Weihe fault and Lintong-Chang′an fault to the reflective belt C in Xi′an Cenozoic sag and the presence of Moho fault indicates that the region has strong present crustal activity,being worth noted as the future strong earthquake area.
引文
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[2]国家地震局“鄂尔多斯周缘活动断裂系”课题组.鄂尔多斯周缘活动断裂系.北京:地震出版社,1988:114-136.The Research Group on Active Fault System around OrdosMassif,State Seismological Bureau.Active Fault SystemAround Ordos Massif(in Chinese).Beijing:SeismologicalPress,1988:114-136.
[3]Mats V D.The structure and development of the Baikal riftdepression.Earth Science Reviews,1993,34(2):81-118.
[4]杨巍然,孙继源,Mats V D等.大陆裂谷对比.武汉:中国地质大学出版社,1995.Yang W R,Sun J Y,Mats V D,et al.Comparison forContinental Rifts:Analysis of the Fenwei and Baikal RiftSystems(in Chinese).Wuhan:China University of GeosciencesPress,1995.
[5]陕西省地质矿产局.陕西省区域地质志.北京:地质出版社,1989.Bureau of Geology and Mineral Resources of ShaanxiProvince.Regional Geology of Shaanxi Province(in Chinese).Beijing:Geological Publishing House,1989.
[6]李永善.西安地裂及渭河盆地活断层研究.北京:地震出版社,1992:152-168.Li Y S.Research on Ground Fissures in Xi′an Region andActive Faults in Weihe Basin(in Chinese).Beijing:SeismologicalPress,1992:152-168.
[7]彭建兵,张骏,苏生瑞等.渭河盆地活动断裂与地质灾害.西安:西北大学出版社,1992.Peng J B,Zhang J,Su S R,et al.Active Fault andGeological Disaster of Weihe Basin(in Chinese).Xi′an:Northwest University Press,1992.
[8]杨巍然,隋志龙,Mats V D.俄罗斯贝加尔湖区伸展构造及与中国东部伸展构造对比.地球科学进展,2003,18(1):45-49.
[9]Zeng R S,Wang C Y,Zhang D N.On the dynamics ofextensional basin.Pure and Applied Geophysics,1995,145(3-4):579-603.
[10]张培震,邓起东,张国民等.中国大陆的强震活动与活动地块.中国科学(D辑:地球科学),2003,33(增刊):12-20.Zhang P Z,Deng Q D,Zhang G M,et al.Strong earthquakeactivity and active block in China′s mainland.Science inChina(Series D)(in Chinese),2003,33(Suppl):12-20.
[11]冯希杰.渭河断裂古水断层剖面.地震地质,2000,22(2):209-230.Feng X J.Gushui fault profile on the Weihe fault.Seismology and Geology(in Chinese),2000,22(2):209-230.
[12]张少泉,武利均,郭建明等.中国西部地区门源—平凉—渭南地震测深剖面资料的分析解释.地球物理学报,1985,28(5):460-472.Zhang S Q,Wu L J,Guo J M et al.An interpretation of thedss data on Menyuan-Pingliang-Weinan profile in west China.Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese),1985,28(5):460-472.
[13]李鹏,周仕勇,陈永顺等.利用双平面波干涉面波层析成像方法研究山西断陷盆地及鄂尔多斯地台三维速度结构.CT理论与应用研究,2010,19(3):47-60.Li P,Zhou SY,Chen YS,et al.3Dvelocity structure inShanxi graben and Ordos from two plane waves method.Computerized Tomography Theory and Applications(inChinese),2010,19(3):47-60.
[14]渥伊尔马兹著.地震数据处理.黄旭德,袁明德译.北京:石油工业出版社,1994.Yilmaz O.Seismic Data Processing(in Chinese),Translatedby Huang X D,Yuan M D.Beijing:Petroleum IndustryPress,1994.
[15]任隽,朱元清,秦浩文.上海地震台阵标定及结果分析.西北地震学报,2002,24(4):325-329.Ren J,Zhu Y Q,Qin H W.Result analysis and calibrationfor Shanghai array.Northwestern Seismological Journal(inChinese),2002,24(4):325-329.
[16]Sandmeier K J,Wenzel F.Synthetic seismograms for acomplex crustal model.Geophysics Research Letters,1986,13(1):22-25.
[17]张国伟,张本仁,袁学诚等.秦岭造山带与大陆动力学.北京:科学出版社,2001.Zhang G W,Zhang B R,Yuan X C,et al.Qin-Ling OrogenicBelt and Continental Dynamics(in Chinese).Beijing:SciencePress,2001.
[18]滕吉文,熊绍柏,尹周勋等.喜马拉雅山北部地区的地壳结构模型和速度分布特征.地球物理学报,1983,26(6):525-540.Teng J W,Xiong S B,Yin Z X,et al.Structure of the crustand upper mantle pattern and velocity distributionalcharacteristics at northern region of the Himalayan Mountains.Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese),1983,26(6):525-540.
[19]王椿镛,楼海,吕智勇等.青藏高原东部地壳上地幔S波速度结构——下地壳流的深部环境.中国科学(D辑:地球科学),2008,38(1):22-32.Wang C Y,Lou H,LüZ Y,et al.S wave velocity structureof crust and upper mantle in eastern Qinghai-Tibet plateau:the deep environment of lower crust flow.Science in China(Series D:Earth Sciences)(in Chinese),2008,38(1):22-32.
[20]张先康,嘉世旭,赵金仁等.西秦岭—东昆仑及邻近地区地壳结构——深地震宽角反射/折射剖面结果.地球物理学报,2008,51(2):439-450.Zhang X K,Jia S X,Zhao J R,et al.Crustal structuresbeneath west Qinling-east Kunlun orogen and its adjacentarea——results of wide-angle seismic feflection and fefractionexperiment.Chinese J.Geophys.(in Chinese),2008,51(2):439-450.
[21]张中杰,白志明,王椿慵等.三江地区地壳结构及动力学意义:云南遮放—宾川地震反射/折射剖面的启示.中国科学D辑,2005,35(4):314-319.Zhang Z J,Bai Z M,Wang C Y,et al.Crustal structure andgeodynamics of the Sanjiang region:Yunnan Zhefang-Bingchuan seismic reflection/refraction profile revelatio.Science in China(Series D)(in Chinese),2005,35(4):314-319.
[22]王夫运,张先康,陈棋福等.北京地区上地壳三维细结构层析成像.地球物理学报,2005,48(2):359-366.Wang F Y,Zhang X K,Chen Q F,et al.Fine tomographicinversion of the upper crust 3-D structure around Beijing.Chinese J.Geophys.(in Chinese),2005,48(2):359-366.
[23]Zeng R S,Ding Z F,Wu Q J.The crustal structures fromHimalaya to Qilian and its implications for continent-continent collision process.Chinese J.Geophys.,1998,41(1):49-60.
[24]Zhang Z J,Yang L Q,Teng J W,et al.An overview of theearth crust under China.Earth-Science Reviews,2011,104(1-3):144-166.Yang W R,Sui Z L,Mats V D.Extensional tectonics ofBaikal lake district,Russia:contrast with East China.Advance in Earth Sciences(in Chinese),2003,18(1):45-49.
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