日本南海海槽俯冲增生楔前缘的构造变形特征
摘要
对增生楔不同压力—温度条件下的构造变形、流体活动、沉积特征、岩石物性和化学组成等多方面的直接观测,可以帮助分析俯冲带地震的蕴育和发生的环境与机理。通过参加IODP的日本南海海槽发震带研究项目(NanTroSEIZE)第一阶段316航次所收集到的大量第一手数据和资料,分别在4个站位上(C0004,C0006,C0007,C0008)对日本南海海槽增生楔前缘岩芯尺度上的构造变形进行了详细分析,并且讨论了岩芯尺度上的构造变形与增生楔中大尺度的非序列分支逆冲断层和前缘逆冲断层的构造变形之间的关系。发现逆冲变形不是只在大尺度的逆冲断层面上进行,而是弥散分布在主逆冲断层面、次级逆冲断层面以及断层面之间的更小的尺度上。小尺度构造的倾向与大尺度断层的倾向有较好的一致性,表明它们是在相同的应力场下所形成的。在增生楔浅部高角度的正断层比较发育,显示张性应力场特征,同时所获得的岩芯尺度上的地层倾角较大并倾向与反射地震以及区域地质分析结果非常吻合,而在深部,特别是在大尺度逆冲断层发育带附近,各种类型的断层、滑移变形带、节理等非常普遍,同时层理与劈理的产状的复杂变化更多地受控于复杂的逆冲断层带的作用。
Direct observations and measurements on deformational and sedimentary structures,fluid activities,physical properties,and chemical compositions of core samples from accretionary prisms at different pressure and temperature settings are critical to understanding seismogenic mechanisms of subduction zone earthquakes.Based on core measurements from IODP expedition 316 of the first stage of the NanTroSEIZE program,we analyze deformational and sedimentary structures at the core scale at four sites,C0004,C0006,C0007 and C0008,at the toe the accretionary prism along the Nankai Trough.We also discuss the relationships between structures at the core scale and large scale structures like splay fault and frontal thrust fault.It is found that thrusting deformation in the accretionary prism is accommodated not only by deformation within large-scale thrusting zones,but also by deformation along secondary thrusts and potentially more importantly along core-scale structures such as micro-faults and deformation bands.The consistencies in dipping angles between large scale thrusts and core-scale structures indicate that structures at different scales are formed under a unified stress field within the accretionary prism.At the shallow part of the accretionary prism,high angle normal faults dominate,showing extensional stress field,and beddings and fissilities from core samples show trends and high dips consistent with those estimated from seismic stratigraphy and regional structures.At deeper parts,particularly near the large scale thrusts,faults of all types,joints and deformation bands are common features,and beddings and fissilities show vertiginous attitudes controlled preferably more by large scale thrusting deformation.
Direct observations and measurements on deformational and sedimentary structures,fluid activities,physical properties,and chemical compositions of core samples from accretionary prisms at different pressure and temperature settings are critical to understanding seismogenic mechanisms of subduction zone earthquakes.Based on core measurements from IODP expedition 316 of the first stage of the NanTroSEIZE program,we analyze deformational and sedimentary structures at the core scale at four sites,C0004,C0006,C0007 and C0008,at the toe the accretionary prism along the Nankai Trough.We also discuss the relationships between structures at the core scale and large scale structures like splay fault and frontal thrust fault.It is found that thrusting deformation in the accretionary prism is accommodated not only by deformation within large-scale thrusting zones,but also by deformation along secondary thrusts and potentially more importantly along core-scale structures such as micro-faults and deformation bands.The consistencies in dipping angles between large scale thrusts and core-scale structures indicate that structures at different scales are formed under a unified stress field within the accretionary prism.At the shallow part of the accretionary prism,high angle normal faults dominate,showing extensional stress field,and beddings and fissilities from core samples show trends and high dips consistent with those estimated from seismic stratigraphy and regional structures.At deeper parts,particularly near the large scale thrusts,faults of all types,joints and deformation bands are common features,and beddings and fissilities show vertiginous attitudes controlled preferably more by large scale thrusting deformation.
引文
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[3]Tobin HJ,Kinoshita M.Investigations of seismogenesis at theNankai Trough,Japan[Z].IODP Scientific Prospectus,NanTro-SEIZE Stage 1.doi:10.2204/iodp.sp.nantroseize1.2006,2006.
[4]Kimura G,Screaton E J,Curewitz D,et al.NanTroSEIZE Stage1A:NanTroSEIZE shallowmegasplay and frontal thrusts[R].IO-DP Preliminary Reptort,316.doi:10.2204/iodp.pr.316.2008,2008.
[5]Park J O,Tsuru T,Kodaira S,et al.Splay fault branching alongthe Nankai subduction zone[J].Science,2002,297(5 584):1 157-1 160.
[6]Li Sanzhong,Guo Xiaoyu,Hou Fanghui,et al.Interactions ofridge-plume[J].Plume-trench and Trench-ridge,2004,20(11):6-18.[李三忠,郭晓玉,侯方辉,等.活动大陆边缘的板片窗构造[J].海洋地质动态,2004,20(11):6-18.]
[7]Ruff L,Kanamori H.Seismic coupling and uncoupling at subduc-tion zones[J].Tectonophysics,1983,99(2/4):99-117.
[8]Honza E.Spreading mode of backarc basins in the western Pacific[J].Tectonophysics,1995,251:139-152.
[9]Yao Bochu,Zeng Weijun,Hayes D E,et al.The GeologicalMemoir of South China Sea Surveyed Jointly by China and USA[M].Wuhan:China University of Geosciences Press,1994:1-204.[姚伯初,曾维军,Hayes D E,等.中美合作调研南海地质专报[M].武汉:中国地质大学出版社,1994:1-204.]
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