The electrical structure beneath the Yarlung Zangbo suture zone in southern Tibet
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摘要: 为研究日喀则市活动断裂深浅部构造关系及深部孕震机制，跨雅鲁藏布江谢通门—日喀则段部署了48个宽频大地电磁测深点，剖面长度为108 km。在二维反演的基础上对壳幔200 km深度范围内的电性结构进行了探测研究。剖面自南向北依次经过喜马拉雅地块、雅鲁藏布江缝合带和拉萨—冈底斯地块。喜马拉雅地块地壳表现为高阻特性，其北侧的仲巴—郎杰学陆缘移置混杂地体发育了深达上地幔盖层的巨厚的北倾低阻体；雅鲁藏布江主缝合带表现为喇叭状低阻通道，宽约10 km，存在深浅部两处低阻体，浅部南倾深部北倾，低阻通道南部发育近似直立或南倾的高阻日喀则蛇绿岩，北部发育近直立的高阻冈底斯花岗岩体，整体表现为两个高阻异常体中间夹一个连通壳幔的带状低阻通道；拉萨—冈底斯地块以高阻为主，中下地壳普遍发育低阻体。缝合带附近因板块俯冲作用导致壳幔局部增厚或减薄，表现为电性的梯度变化，表现为低阻特性的部分则是壳幔物质的运移通道。Abstract: In order to study the tectonic relationship between the shallow and deep active faults as well as deep seismogenic machanism around Xigaze city in South Tibet, totally 48 wide-band magnetotelluric sounding points were deployed on a 108 km long line across the Xietongmen-Xigaze section of the Yarlung Zangbo suture zone （YZSZ） , hence 2D inversion technique was employed to reveal the deep electrical structure whose depth range is less than 200 km. This south-north trending profile passes through the Himalayan block, YZSZ and Lhasa-Gangdise block in turn. Most of the crust beneath the Himalayan block is of high resistance, and at the north side of the Zhongba-Langjie xue continental margin displaced mixed terrane, it develops a huge north-dipping high conductive body that stretches downward into the upper mantle. There exists a 10 km wide trumpet-shaped low resistance tunnel and two typical high conductive bodies beneath the YZSZ, of which the shallow one is south-dipping whereas the deep one is north-dipping. Besides, in the crust and upper mantle beneath the YZSZ, there also develops two high resistance bodies, the southern one is nearly vertical or south-dipping ophiolite, and the northern one is upright Gangdise granite. Hence it expresses like a low-resistance channel clamped by those two bodies stretching downward into the upper mantle. Lhasa-Gangdise block is dominated by high resistance and the high conductive bodies are generally developed within the middle and lower crust. The subduction of Indian plate leads to the local thicken and thinning of the crust and mantle, therefore there exhibits an obvious gradient change nearby themajor suture belt （MSB） , and the YZSZ control the migration channel of crust-mantle material.
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图 1 青藏高原及邻区构造背景
MBT：主边界逆冲断层；IYS：雅鲁藏布缝合带；BNS：班公—怒江缝合带；JS：金沙江缝合带；AKMS：阿尼玛卿—昆仑—木孜塔格缝合带；SQS：南祁连缝合带；NQS：北祁连缝合带；HB：喜马拉雅地块；LB：拉萨地块；QB：羌塘地块；SB：松潘—甘孜地块；KB：昆仑—柴达木地块
Figure 1. The tectonic background of the Qinghai−Tibet Plateau
MBT：Main boundary thrust；IYS：Indus-Yarlung suture；BNS：Bangong-Nujiang suture；JS：Jinshajiang suture；AKMS：Anyimaqen-Kunlun-Mutztagh suture；SQS：South Qilian suture；NQS：North Qilian suture；HB：Himalayan block； LB：Lhasa block；QB：Qiangtang block；SB：Songpan-Garze block；KB：Kunlun-Qaidam block
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图 3 静态校正前后频率－视电阻率和频率－相位断面图
（a） 校正前TE频率－电阻率断面图； （b） 校正前TE频率－相位断面图；（c） 校正后TM频率－电阻率断面图； （d） 校正后TM频率－相位断面图
Figure 3. Frequency-apparent resistivity profiles and frequency-phase profiles before and after static correction
（a） TE frequency-apparent resistivity profile before correction；（b） TE frequency-phase profile before correction； （c） TM frequency-apparent resistivity profile before correction； （d）TM frequency-phase proile before correction
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图 4 模型响应与实测数据拟合误差
（a） 电阻率拟合误差 ；（b） 相位拟合误差
Figure 4. Fitting errors between model response data and measured data
（a） Fitting error of resistivity；（b） Fitting error of phase
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图 6 主要构造单元视电阻率典型曲线
Figure 6. Typical curves of apparent resistivity for the main structural units
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