Deep resistivity structure along the Longmen Mountain fault zone in the eastern Tibetan plateau of China

1.Introduction Many researchers (e.g.,Wang et al.,2009) have proposed the relevant knowledge of tectonic evolution and dynamic characteristics of the Longmen Mountain belt as well as the Songpan-Ganzi and Yangtze blocks in the past few decades, the knowledge of shallow thrust nappe tectonic along the belt has then been generally recognized. It's, however, still difficult to image the deep crust and mantle structures and reveal the dynamic mechanism of the crustal formation under the Longmen Mountain. In this study, we carried out the MT experiments along and across the Longmen mountain region and investigated the relationships between the crust structure and seismic activity basing on the latest MT geological results. 2. Field observations We conducted three MT experiment profiles in the eastern Tibetan Plateau. One is along the Mingshan-Guangyuan profile parallel with the structural direction, and another two profiles (Maqu-Gaoliangzhen and Luqu-Hechuan) perpendicular to the Longmen Mountain fault zone. In this study, we use the conventional magnetotelluric (MT) data combine with the long-period magnetotelluric (LMT) data to observe electromagnetic response. The MT and LMT data was observed by using the V8 instrument and LEMI-417, respectively. 3. Conclusion (1) According to the results of MT inversion, we find that the high concentration of stress process along the Songpan-Ganzi block and the Yangtze block colliding zone might result from the deep crust-mantle tough shear Zone of Longmen Mountain expanded to mid-upper crust, and finally leads to a new rupture. This could be one of the focal mechanisms of the Wenchuan earthquake (Ms 8.0) generating. The deep resistivity structure along the Longmen Mountain fault zone can be divided into southern,middle and northern segments from southwest to northeast. The total resistivity of southern segment is lower than the middle and northern portions. We suggest that the upper crust of the Longmen Mountain, south of Dayi, might be of lower rock strength. We then propose that it's difficult to accumulate the stress to result in upper crust crack under the south segment compared with that of the northern segment. This may be one of the reasons that the southern segment has fewer aftershocks after Wenchuan earthquake. (2) Due to the thick rigid plate and the western Qinghai-Tibet Yangtze plate extrusion from the southeast direction, the Longmen Mountain and Songpan-Ganzi region have dynamic properties of the shallow abduction to the Yangtze plate, and the deep subduction to the Yangtze plate. In addition, the GPS study indicates that there is a relative movement between Yangtze plate and Songpan-Ganzi region, and the displacement rate of Yangtze plate is faster than Songpan-Ganzi region. Therefore, the Yangtze plate forms a wedge insert into the Longmen Mountain, i.e., the thrust nappe structure of upper crust and the development of the lower crust "subduction zone", leading to a "Crocodile mouth"-shape structure across the Longmen fault zone.