龙门山断裂带晚三叠世大地震的孕震深度和环境的岩石磁学证据  

Rock magnetism reveals the formation depth and environment of Late Triassic pseudotachylytes in the Longmen Shan Fault Zone

作　　者：张蕾 李海兵[1,2] 孙知明 周祐民[4] 曹勇 王焕[1,2] 郑勇[1,2] 司家亮 ZHANG Lei;LI HaiBing;SUN ZhiMing;CHOU YuMin;CAO Yong;WANG Huan;ZHENG Yong;SI JiaLiang(MNR Key Laboratory of Continental Dynamics,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Jiangsu Donghai Continental Deep Borehole Crustal Activity National Observation and Research Station,Donghai 222300,China;MNR Key Laboratory of Paleomagnetism and Tectonic Reconstruction,Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China;Department of Ocean Science and Engineering,South University of Science and Technology,Shenzhen 518055,China)

机构地区：[1]中国地质科学院地质研究所,自然资源部大陆动力学重点实验室,北京100037 [2]江苏东海大陆深孔地壳活动国家野外科学观测研究站,东海222300 [3]中国地质科学院地质力学研究所,自然资源部古地磁与古构造重建重点实验室,北京100081 [4]南方科技大学海洋科学与工程系,深圳518055

出　　处：《岩石学报》2024年第12期3984-3997,共14页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42172262、42072240、41972229);中国地质调查项目(DD20221630);中国地质科学院基本业务费项目(J2403)联合资助。

摘　　要：假玄武玻璃可以形成于深部孕震带到近地表的不同深度,确定其形成深度对揭示断裂带力学性质和大地震孕震环境具有重要意义。本文以龙门山映秀-北川断裂带南段八角庙出露的晚三叠世假玄武玻璃及其围岩(碎裂岩和初碎裂岩)为研究对象,通过岩石磁学、显微结构和地球化学测试,探讨了龙门山断裂带晚三叠世大地震的孕震深度和孕震环境。岩石磁学结果表明,假玄武玻璃和碎裂岩中主要磁性矿物是磁铁矿、单斜磁黄铁矿和顺磁性矿物;初碎裂岩中主要磁性矿物是磁铁矿和顺磁性矿物。假玄武玻璃具有高磁化率值特征,其形成的主要原因是顺磁性矿物在同震摩擦热作用下生成了磁铁矿和单斜磁黄铁矿。磁性矿物的转换过程指示了含有硫化物的弱流体作用的还原性的孕震环境。热磁曲线(χ-T)测试结果揭示了初碎裂岩在自然界中经历的最高温度约为275~300℃,根据地温梯度推算初碎裂岩和假玄武玻璃的形成深度为12.8~14.0km。根据孕震深度、假玄武玻璃构特征和地震机制估算,龙门山断裂带孕震深度的正应力σ≤240.2~264.1MPa,剪应力τ≤144.1~158.5MPa。龙门山断裂带在其漫长的演化过程中发生了多次大地震活动,其中在孕震深度(<14km)固结的碎裂岩带内形成的假玄武玻璃比较容易保存在地震断裂带内,并记录了深部较强的应力特征和弱流体作用的还原性环境。Pseudotachylytes form in seismic fault zones at shallow depths down to typical seismogenic depths.Determining their formation depth is key to reconstruct the stress properties and seismogenic environment of the faults.To obtain the Late Triassic pseudotachylyte formation depth and seismogenic environment of faults in the Longmen Shan Fault Zone(LMFZ),we carried out rock magnetic,microstructural and geochemical analyses of pseudotachylyte,cataclasite and protocataclasite from the Bajiaomiao outcrop along the Yingxiu-Beichuan fault.The magnetic minerals in the pseudotachylyte and cataclasite are newly-formed magnetite,monoclinic pyrrhotite,and paramagnetic minerals,while those in the protocataclasite are paramagnetic minerals as well as primary magnetite.Pseudotachylytes have higher magnetic susceptibility values than wall rocks.The high temperature during frictional heating induces the thermal decomposition of paramagnetic minerals,forming magnetite and monoclinic pyrrhotite,and thus contributing to the higher magnetic susceptibility values of the pseudotachylytes and indicating a reductive seismogenic environment with weak fluid that contains few sulfides.Theχ-T results show that protocataclasite has never been subjected to heating over 275~300℃in nature,which implies that the protocataclasite and pseudotachylyte were formed at a depth of 12.8~14.0km.From the seismogenic depths,the structure of the pseudotachylyte,and the seismic mechanism,we estimate the normal stressσas≤240.2~264.1MPa and the shear stressτas≤144.1~158.5MPa for the LMFZ.Previous powerful earthquakes occurred repeatedly in the region,and the pseudotachylytes formed within the consolidated cataclastic rocks at the seismogenic depth(<14km)are relatively easy to be preserved.They also recorded strong stress and reduced seismogenic environments with weak fluid activity.

关 键 词：假玄武玻璃 岩石磁学 孕震深度和环境 映秀-北川断裂带 

分 类 号：P313[天文地球—固体地球物理学] P542[天文地球—地球物理学]