Earthquake-Induced Soft-Sediment Deformation Structures in Middle Holocene of Xuru Co Area in Tibet and Its Geological Significance
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摘要: 湖相沉积古地震研究是对地表破裂古地震研究的重要补充.通过详细的野外地质调查，在西藏许如错地区全新统湖相地层内新发现大量地震触发软沉积物变形构造(震积岩)，层内发育液化脉、液化曲卷变形、液化角砾岩、液化水压构造、滴状体与锥状体、砾石丘、负载构造和火焰构造等软沉积变形标志，还发育同震断层、震裂缝和同震褶皱等同震构造标志.根据软沉积变形标志与震级之间的关系，结合历史地震统计液化颗粒范围，通过C14和光释光年龄测定，推测古地震事件发生在±7.5 ka，MS>7.5级；填补了该区历史地震的空缺，为恢复青藏高原南北向地堑地震活动历史及迁移规律提供了素材.震积岩中见大量砾石液化现象，这对现阶段以砂土-粉砂土研究为主的砂土液化调查工作提出了新挑战.Abstract: The lacustrine paleo-seismic study is an important supplement to the research of a complete paleo-seismic sequence. A large number of earthquake-induced soft-sediment deformation structures (seismites) were newly discovered in the Holocene lacustrine deposits in Xuru Co area by detailed field geological survey. Soft-sediment deformation markers mainly include liquefied vein,liquefied convolute,liquefied breccia,hydraulic structure,liqufied droplets and cusps,conglomerate mound,load structure and flance structure. The syn-seismic structure deformation markers were also developed including syn-seismic fault,seismic fissure,and syn-fold. Based on the empirical statistical relation between the earthquake magnitude and soft-sediment deformation markers,combining historical earthquake statistics with the range of liquefied particles,it is suggested that the maximum magnitude may exceed 7.5. The C14 and OSL dating results for samples suggest that the age of the paleoearthquake is around 7.5 ka. The work fills the gap of historical earthquakes in the area,and provides material for restoring the history and migration law of earthquakes in the NS-trending grabens of the Tibetan Plateau. A large amount of gravel liquefaction was found in seismite,which poses a new challenge to the investigation of sand liquefaction,which is dominated by sand and silt.
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Key words:
- seismite /
- paleoearthquake /
- Holocene /
- gravel liquefaction /
- Xuru Co graben /
- stratigraphy
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图 1 研究区构造背景图(a、b)和地质简图(c)
图b据曹圣华等(2009).图a:ALT.阿尔金断裂；SKS.昆仑断裂；JSR.金沙江缝合带；JLF.鲜水河断裂；BNS.班公湖-怒江缝合带；YZS.雅鲁藏布江缝合带；MCT.主中央逆冲断裂；MBT.主边界逆冲断裂.图b:Q.第四系；N.中新世碱性火山岩；E3.渐新世日贡拉组；E1-2.林子宗群火山岩；K-J.侏罗-白垩纪地层；C-P.石炭-二叠纪地层；γK1.早白垩世花岗岩；γK2.晚白垩世花岗岩；γE.古近纪花岗岩.图c:1.全新世冲积物；2.全新世湖积物；3.全新世洪冲积物；4.全新世沼积物；5.晚更新世湖积物；6.晚更新世洪冲积物；7.上新世洁居纳卓组；8.中新世布嘎寺组；9.古新世年波组；10.古新世典中组；11.中新世花岗斑岩；12.古新世流纹斑岩；13.古新世二长花岗岩；14.地质界线；15.断层；16.震积岩出露点及编号
Fig. 1. Tectonic setting (a, b) and geological map of the study area (c)
图 2 许如错地区震积岩出露点沉积层序及同位素年龄
1.腐质层；2.粘土层；3.钙质粘土层；4.含生物碎片粘土层；5.砂土层；6.细砾层
Fig. 2. Sedimentary sequences and isotope ages of seismite outcrops in Xuru Co area
图 3 许如错地区全新世震积岩软沉积物液化变形标志
Fig. 3. Soft-sediment liquefied deformations of the Holocene seismites in Xuru Co area
图 4 许如错地区震积岩中液化角砾岩(a)和液化水压构造(b)
Fig. 4. Liquefied breccia (a) and hydraulic structure (b) of seismites in Xuru Co area
图 5 许如错地区全新世震积岩中液化底辟变形(a~d)、负载构造(e)和火焰构造(f)
Fig. 5. Liquefied diapir deformations (a-d), load structure (e) and flance structure (f) of the Holocene seismites in Xuru Co area
图 6 许如错地区全新世震积岩软沉积物同震构造标志
Fig. 6. Soft-sediment co-response deformations of the Holocene seismites in Xuru Co area
表 1 地震引发的软沉积变形构造特征与震级
Table 1. Possible range of the earthquake magnitude related to the observed soft-sediment deformation structures
软沉积变形构造 主要触发因素 里氏震级 本区识别 液化变形 微褶皱、碟状构造、混积层 微震 3~5 微褶皱 液化脉 负载作用或地震活动 5~6 液化脉 球-枕构造 负载作用或地震活动 6~8 负载构造 包卷构造、火焰状构造 负载作用或地震活动 6~8 火焰构造 板刺构造和直立紧闭褶皱 地震活动 6~8 脆性变形 砂(砾岩)墙 地震活动 5~8 砾岩墙 震裂角砾岩 地震活动 7~8 同震断层 注：据Berra and Felletti (2011)修改. 
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