藏南裂谷成因机制的大地电磁测深研究

Sonthern Tibet developed rifts. From west to east, there are six rifts located in southern Tibet . These are Yare rift、Takoula-Longgeer rift、Guocou-Wenbu rift、Dingjie-Shenza rift、Yadong-Yangbajing rift and Couna rift. The studying of genetic mechanism is very important to resolve the uplift situation of Tibetan Plateau.It exists some models on the rifts developed in southern Tibet. The main difference of the models is that the rifts resulted from westeast extension of upper crust,or originated from the crust-mantle convections.For this problem related on inter-action between crust and mantle,it is difficult to scienstifically answer from geological investigation. We reconstruct conductivity cross-section across Gucou-Wenbu rift and Dingjie-Shenza rift as well as Yadong-Yangbajing rift by conducing magnetotelluric sounding (MTS) observation from Cuoqin to Dangxiong. According to the distribution of high conductivity layers(bodies),we will trace the tunnels of thermal fluids,and understand the dynamic way of inter-action between crust and mantle.Application of numerical simulation to compute the MTS responses of the models. Comparing the responses with MTS observation result, we can obtain some benefit informations. Thus we can scientifically infer the the genetic mechanism of the rifts on southern Tibet. It will help us to well realize the uplift situation and the evolution process of Tibetan Plateau.

藏南南北向裂谷相当发育，从西到东依次发育有亚热裂谷、塔口啦-隆格尔裂谷、古错-文部裂谷、定结-申扎裂谷、亚东-羊八井裂谷、以及错那裂谷。藏南裂谷成因机制的研究对深刻认识高原隆升状态具有重大意义。目前，国内外关于藏南裂谷的地质成因模式众多。其本质分歧在于裂谷带的演化形成是源于上地壳东西向的伸展活动，还是源于岩石圈地幔对流。对于这一涉及壳-幔层间作用的深层次问题，地表地质观测很难做出科学的判断。通过对藏南三大裂谷（古错-文部裂谷、定结-申扎裂谷、亚东-羊八井裂谷）的大地电磁测深（MTS）的观测研究，重建错勤-当雄MTS剖面（0-200km深度上的）导电率结构，根据壳-幔高导层（体）的发育状况，追踪岩石圈热流体的路径，了解裂谷壳-幔层间作用的动力学方式。并利用MTS正演数值模拟技术，搭建MTS与地质成因模式沟通的平台。从综合（MTS、地质）层面，解释推断藏南裂谷成因机制。

藏南南北向裂谷相当发育，从西到东依次发育有亚热裂谷、塔口啦—隆格尔裂谷、古错—文部裂谷、定结—申扎裂谷、亚东—羊八井裂谷。在藏南裂谷的成因机制上，其地质模型众多，分歧较大。厘清藏南裂谷成因机制对深化认识青藏高原隆升状态具有非常重要的意义。. 该项目研究主要是通过MTS观测，构建藏南裂谷带岩石圈的电性结构，通过透析其壳、幔高导层（体）的发育状况，追踪壳—幔对流物质迁移路径，了解壳—幔层间作用的动力学方式，解释与推断藏南裂谷带的成因机制。. 根据研究目标，我们在藏南冈底斯地区开展了宽频大地电磁（WBMT—Wide Band Magnetotellurics）和长周期大地电磁（Long Period Magnetotellurics）联合观测。针对藏南三条裂谷带（古错—文部裂谷；定结—申扎裂谷；亚东—羊八井裂谷），开展了东西向（EW）措勤—当雄MTS（WBMT+LPMT）剖面、南北向（NS）墨竹工卡—班戈MTS（WBMT+LPMT）剖面的观测，并应用3D数值仿真技术，研究了抑制MTS剖面邻近咸水湖产生的静态效应问题。为更合理地为2-3D MTS反演导入模型（初始模型），开发了基于最速下降方向的随机反演算法。在此基础上，应用2D非线性共轭梯度（NLCG）反演算法，重建了（EW）措勤—当雄MTS剖面的电阻率断面和（NS）墨竹工卡—班戈MTS剖面的电阻率断面。通过解析EW MTS 剖面和 NS MTS 剖面壳、幔高导层（体）的发育状况，探讨了藏南裂谷壳—幔层间作用动力学机制，提出了藏南裂谷带的演化与形成是早期地幔岩浆侵入和后期上地壳EW拉张相继作用的结果。该成果将有助于人们从新的、更宽的视野认识藏南裂谷带的演化、形成与青藏高原隆升状态的耦合关系。