龙门山构造带WFSD-2钻孔岩心磁化率特征及其对大地震活动的响应

doi:10.6038/cjg20170119

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摘要

岩石磁化率特征可以帮助判断岩石的形成环境，对地震过程中滑动摩擦伴随高温的物理-化学变化具有显著反应.本研究以钻穿龙门山中段构造带的汶川地震断裂科学钻探2号孔（WFSD-2）岩心为研究对象，使用Bartington MS2K磁化率仪对500~2283.56 m深度的岩心进行高分辨率无损磁化率测试，并结合岩性特征和显微结构探讨了龙门山构造带主要岩石单元的磁化率特征及其地震断裂活动的磁学响应.磁化率测试结果表明，由花岗岩和火山碎屑岩组成的彭灌杂岩体的磁化率值（数十到数千个10^-6SI）普遍高于上三叠统须家河组沉积岩的磁化率值（数个到数十个10^-6SI）.从WFSD-2岩性分布来看，彭灌杂岩上下出露四段，其磁化率值特征反映它们属于不同的岩石单元，它们与下伏须家河组地层呈断层接触，构成叠瓦状构造，指示了龙门山构造带具有强烈的地壳缩短作用.断裂带中处于滑动带的断层泥和假玄武玻璃具有高磁化率特征，而断层角砾岩和碎裂岩不具有高磁化率值特征，表明断层岩磁化率增高的原因可能主要与地震断裂滑动摩擦过程中高温作用下发生的磁性矿物转换有关，断层岩中高磁化率异常可作为大地震活动的证据.WFSD-2岩心中的映秀-北川断裂带（600~960 m）可识别出约80条高磁化率异常的断层岩带，揭示映秀-北川断裂带是一条长期活动的断裂带，龙门山构造带形成演化过程中伴随着大地震活动.

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	张蕾
	孙知明
	李海兵
	赵来时
	曹勇
	叶小舟
	王雷振
	王焕
	何祥丽
	韩帅
	白明坤
	葛成隆
	赵越

关键词 ：彭灌杂岩体, 断层岩, 磁化率, 地震活动, 龙门山构造带

Abstract：

Magnetic susceptibility is an indication of diagenetic environment, and responds significantly to the physical and chemical transformation caused by sliding friction and high temperature during seismic events. The surface magnetic susceptibility data were measured in the cores of the Wenchuan Earthquake Fault Scientific Drilling Hole-2 (WFSD-2) from 500 m to 2283.56 m depth with an interval of 1 cm using a Bartington MS2K sensor. The magnetic susceptibility of the Neoproterozoic Pengguan Complex which is composed of granite and volcaniclastic rocks (from tens to thousands of 10^-6SI), is higher than that of the Upper Triassic Xujiahe sedimentary rocks (from a few to tens of 10^-6SI). The characteristics of the lithology and magnetic susceptibility indicate that the four parts of the Neoproterozoic Pengguan Complex do not come from the same rock units. The nature of the fault contact between the Neoproterozoic Pengguan Complex and the underlying Xujiahe formation indicates an imbricated structure and the occurrence of a powerful crustal shortening in the Longmenshan belt.The gouge and pseudotachylyte in the fault slip zone have higher values of magnetic susceptibility than the wall rocks. However, the magnetic susceptibility values of fault breccia and cataclasite is similar to that in the wall rocks. Based on the lithologic properties and microscopic structures, the high magnetic susceptibility of fault rocks, which is caused by frictional heating, shows evidence for strong earthquake activity. About 80 zones with high values of magnetic susceptibility are recognized in the Yingxiu-Beichuan fault zone (600~960 m depth in WFSD-2 cores), suggesting that the Longmenshan belt may have experienced powerful seismic activities repeatedly.

Key words： Pengguan complex Fault rocks Magnetic susceptibility Earthquake activity Longmenshan thrust zone

收稿日期: 2016-05-16

PACS:

P318

基金资助:

国家自然科学基金项目（41520104006，41330211），中国地质调查项目（12120115026901），中国地质科学院基本科研业务费项目（YYWF201601）和GPMR（201505）资助.

通讯作者: 孙知明,男,研究员,主要从事区域构造地质学与古地磁学研究.E-mail:sunzm1209@163.com E-mail: sunzm1209@163.com

作者简介: 张蕾,女,1988年出生,博士研究生,构造地质学专业.E-mail:zhanglei881102@126.com

链接本文:

http://www.geophy.cn/CN/10.6038/cjg20170119 或 http://www.geophy.cn/CN/Y2017/V60/I1/225

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