Deep Electrical Structure And Dynamic Background Of Central West Qin Ling And Blocks On Its Either Side

The West Qin Ling orogenic belt and blocks on its either side is a special tectonic setting, whereseveral distinct tectonic zones or regions meet each other, i.e. the east-west striking central orogenicsystem of mainland China, the north-south Sichuan-Yunnan and Helan Shen tectonic belts and theQinghai-Tibet Plateau, as well as the north-south seismic zone and northeastern margin of theTibetan plateau. This region is also part of the major gradient zone along which crustal structure,crustal thickness and geophysical fields change remarkably. Study of the deep structure beneath theWest Qin Ling orogenic belt and massive on its both sides is of great importance for understandingthe geological evolution and dynamic background of mainland China.Magnetotelluric(MT) measurements have been carried out at87sites on two profiles that runthrough the Songpan-Ganzi block, central West Qin Ling, and Longxi basin; one alongAba-Ruoergai-Lintan-Lanzhou (WQL-L1), and the other along Ruoergai-Minxian-Zhangxian-Dingxi(WQL-L6). Using remote reference and "Robust" and phase tensor decompositiontechnologies, the MT data, were processed. Then data inversion was performed in terms of NLCGtwo-dimensional method to image the deep electrical structure. In order to probe the deeperelectrical structure, along the Aba-Ruoergai-Lintan-Lanzhou profile, long-period MTobservations were made at12sites, and the one-dimensional regularized inversion was conducted tothis data set to yield the electrical structure in vertical direction.Combined with other geophysical and geological data, interpretations of MT data above led thisthesis to have the following conclusions.（1）Downward extension of faults. The images of deep electrical structure indicate the EastKunlun fault, Diebu-Bailongjiang fault, and Guanggaishan-Dieshan fault jointly form the EastKunlun fault system, which separates the Songpan-Ganzi block and central West Qinl Ling orogenicbelt. The West Qin Ling north-edge fault is a major steep electrical boundary zone that penetrates theMoho. The Lintan-Tanchang fault also shows a feature of the electrical boundary, which extends onthe east and west in different manners. The Maxianshan fault lies in the place of mixture of high-andlow-resistivity values. And the lower crust below the Aba fault hosts a prominent low-resistivitybody and a discontinuity in electricity.（2）Deep electrical structures of blocks. In the upper20km beneath central West Qin Ling,electrical structure overall presents a high-resistivity characteristic like an inverted ""trapezoid",which is deeper in northeast and southwest, and shallower in the middle. Here the high-resistivitybody is underlain by a low-resistivity layer; both are in conjunction each other. In contrast, thelow-resistivity layer north of the Songpan-Ganzi block is deeper in southwest and shallower innortheast. Meanwhile stable layering structure characterizes the Longxi basin to the northeast. These features of deep structures may imply that central West Qin Ling is in a compression that resultsfrom northeastward motion of the Songpan-Ganzi block and southward obstruction of the Longxibasin.（3）The deep seismogenic environment of2013Minxian-Zhangxian Ms6.6earthquakeand the segmentation of seismicity on the Lintan-Tangchang fault. The source of the2013Minxian-Zhangxian Ms6.6earthquake is located in the core of inverted "trapezoid " of thelow-resistivity layer in central West Qin Ling, which is contact between the upper crust ofhigh-resistivity layer and lower crust of low-resistivity layer. It is also situated nearby theLintan-Tanchang fault that showing a low-resistivity broken zone cutting in the crust. Likely, that theSongpan-Ganzi block pushes from southwest to northeast and Longxi basin block impedes fromnortheast to southwest is the external dynamic mechanism for the Minxian-Zhangxian Ms6.6earthquake. While the special material property of the source zone is the internal factor for thisevent.The inverted "trapezoid" body of high resistivity in the middle and upper crust, which is thin inthe west and thick in the east, may account for the lateral variation of earthquakes along theLintan-Tangchang fault.（4）Deep electrical structure and crustal deformation characteristics. The MT resultsreveal that the central West Qinling orogenic belt now is suffering from the extrusion and squeezingof the Songpan-Ganzi block and Longxi Basin. Such an environment of compression and its internalspecial structure might have caused the three-dimensional crustal field in central West Qin Ling to beshortened in the horizontal direction and to rotate clockwise as well as to uplift strongly in themeantime.（5）Deep structure and slip rates of the East Kunlun fault. As revealed by the electricstructure, the East Kunlun fault, Diebu-Bailongjiang fault and Guanggaishan-Dieshan fault are allaffected by the northeastward expansion of the Tibetan plateau. These three faults exhibit wideranges of low-resistivity broken structures, which merge into one at depth, jointly forming the EastKunlun fault system. This work speculates that the end of the East Kunlun fault experiences astructural transformation, of which the Tazang segment, as well as the Diebu-Bailongjiang fault andGuanggaishan-Dieshan fault may jointly form a broom-like structure at the end of the East Kunlunfault system. This system accommodates the slip rates of the east-trending Kunlun fault, which maybe the deep reason why the strike slip rate along the East Kunlun fault reduces to the least from westto east.（6）Lateral structural differences in West Qin Ling. Combining previous MT work and othergeophysical data, this work suggests that the distinct structural characters are present in the east andwest sections of central West Qin Ling, showing evident lateral changes. Roughly west of102oE, islarge-scale high-velocity and nearly complete high-resistivity structure; east of105oE, the crust is ofhigh and low-resistivity combination. And the two MT profiles, that cross West Qin Ling102o to105o, reveal high-resistivity from the surface to20km depth, and a continuous low-resistivity layer inthe lower crust, between which is an visible electric boundary. It means that the structure of central West Qin Ling is different from that in the east and west, which may also be related with earthquakedistribution along West Qin Ling.