Electrical Conductivity Structure Beneath The Lanping Simao Basin In The Three-river Area Of West Yunan

Lanping-Simao Basin is located in the southeast margin of the Tibetan Plateau,which lies in the convergence belt between the colliding India and Eurasian plates,and of great important geological significance to the structural evolution,geological progress and mineralization regularity of the Three-river area.In order to better understand the lithosphere crust mantle electrical structure of the region,2-D modelings with nonlinear conjugate gradient inversion and 3D modelings with WSINV3 DMT and ModEM were carried out,using the magnetotelluric data which were collected across of the Lanping-simao basin in three districts.The two kinds of inversion model were much comparable and they both complement and constraint each other.The final result shows that,Lanping-simao basin was bounded by Wuliang-yingpan mountain,as to say longitudinal axial fracture zone,both of which had different electrical characteristics.On the west side of the basin,upper crust all present as high-resistivity zones,which can be linked with generally exposed volcanic and metamorphic rocks in this region.layers of nearly north-south strip of high conductivity were found to be widely spread in the middle and lower crust,and the resistivity is less than 10 ohm m,which is in very good agreement with the seismic data showing low-velocity layer.we consider this may be as a large scale structural decollement surface,which is closely related to the eastward subduction of the Indian plate.And on the east side of the basin,the electrical resistivity 2D model shows that layers of high conductivity are less than the west on the transverse span,but there is a tendency to extend to upper mantle in vertical.we think that the high-conductivity layers may be related to the ascending progress of mantle magma,and meanwhile creates conditions for the frequently occurred earthquake in this region.Longitudinal axial fracture zone corresponds with Wuliang-yingpan mountain uplift zone,showing high-resistivity zones in the upper and middle crust range,and it should be reflected in the metamorphic and magmatic rocks in the electrical structure.2D and 3D inversion model both indicate that longitudinal axial fracture zone is roughly NNW trending,eastward or steep dipping,and the cutting depth can reach at upper and middle crust,even the upper mantle.We believe this may be a upward channel of crust mantle and plays an important role in the development of the basin and metallogenic progress.The models indicate that the Lancang River fault zone is a large scale deep fracture,which is eastward and vertically steep dipping from north to south,and the suture belt around is found to develop high-resistivity layers.The Jinsha-Red River fault zone is northeast dipping and shows high-resistivity layers all around.On the whole,the Lancang River and the Jinsha River suture zone show a number of approximateelectrical gradient zones parallel to the subduction zone,and the suture zones have no obvious traces of tectonic motions of plates underthrusting and overthrusting,probably due to the late tectonic motions.