Lithospheric Electrical Structure In Western Junggar And Its Implications

The Western Junggar located in the western part of Junggar basin,which is a key component of Central Asian Orogenic Belt.This region has undergone complex ocean-continent transformation and multiple tectonic events,resulting in controversial interpretations about the origin and tectonic setting.Recently,voluminous geological,geochemical and limited geophysical data have reached a consensus that the Western Junggar has experienced oceanic-continental or oceanic-oceanic subductions in Late Paleozoic,and there may be some remnant of Paleozoic oceanic lithosphere preserved beneath this area.Since the high-quality geophysical data are seriously insufficient,it is difficult to provide constrains on the tectonic formation and evolution of the lithosphere beneath Western Junggar.In our study,we obtained five broadband magnetotelluric(MT)profiles to image the electrical structure of the crust and lithospheric mantle.Combining the MT results with other geophysical and geological information,it offered important insights to understand the lithospheric dynamic processes during the Late Carboniferous in this region.The induction vector and phase tensor have been calculated using the MT sounding data,the results show that the southern part of the Western Junggar can be seen as a two-dimensional structure at the shallow depth and a three-dimensional structure at the deep part;while the northern part is as a three-dimensional structure.Considering the direction of the survey line,and also the differences of dimensionality in the region caused by the tectonic strike and the resistivity change in different geomorphic cover zones,different strategies of inversion have been adopted for the survey data,where LINE1-LINE4 use the two-dimensional inversion of TE and TM modes,and LINE5 uses the two-dimensional inversion of impedance tensors.In the mountain areas near to Karamay,the three-dimensional inversion is performed on all stations in the LINE1-LINE3 and some stations in the LINE5 of the Back Mountain area of Karamay.After analyzing all the results,we choose the two-dimensional joint inversion results of TE and TM modes for the LINE4,and two-dimensional inversion results of impedance tensors for the LINE5.Moreover,the result of the three-dimensional inversion constrained by spatial structural has been selected for all the stations in the Back Mountain area of Karamay.Based on the electrical conductivity structure of five broadband MT profiles and combined with the geology,geochemistry and satellite gravity data of the study region,we found a preliminary comprehensive analysis of the lithosphere evolution process in this region.The results show that there is an intraoceanic subduction system of the Late Carboniferous in the Western Junggar.The Karamay-Urho fault approximately represents the location of the ancient trench.The area between the Darbut and Karamay-Urho faults represents the Paleozoic forearc basin and the accretionary wedge.The area north to the Darbut fault could be interpreted as the center of an immature intraarc extension.The low resistivity zone represents a confluence of metasomatism,which is resulting from upwelling of the upper mantle due to its partial melting.There is a relative high resistivity layer beneath the Junggar Basin,which is inferred as a residual melange formed in the intraoceanic subduction system.The depth characteristics of the Darbut fault in LINE1-LINE4 are not obvious,implying that its extension depth is limited.The Karamay-Urho fault might be a suture zone after strong deformation during several stages of tectonic evolution,which is significance for the tectonic division.We suggest that the formation of Miaoergou rock is related to a relamination process.During the process,the differentiation of magma and the mechanical mixing in the crust cannot be ignored,which causes they have a complex component and their emplacement times are not precisely same as their crystallization ages.