| Ningxia Arc-shaped Structural belt located in the junction of the Ordos block, theAlxa block and the Qilian Mountains collision orogenic zone, is the forward regionwith the extension to northeast of Qinghai-Tibet Plateau. Since the late Cenozoic, theregions are subjected to crustal shortening and shear, with the function of verticaldifferences in uplifting, which formed a typical extrusion basin-range tectonicframework[11], coupled with not too much transformation of the later geological events,therefore Ningxia arc-shaped belt becomes the ideal place for researching the dynamicsmechanism with the extension to northeast in Qinghai-Tibet Plateau. Meanwhile,moderate-strong earthquakes occurred frequently in this region. As recordedhistorically, nearly30times strong earthquakes (M≥7.0) occurred, especially theHaiyuan earthquake in1920and the Gulang earthquake in1927in this arc structure belt,the regions also formed an excellent historic heritage for researching the seismogenicenvironment of moderate-strong earthquake. For the above reasons, Ningxiaarc-shaped belt has become a hotspots area for the research of the deep geodynamicsand the mechanism of earthquake preparation by geology and geophysics scholars.Under the support of the work project Ningxia magnetotelluric sounding profilemeasurement (1212011120928) in the programs metallogenic belt regionalgeophysical survey of China Geological Survey Bureau, two magnetotelluricsounding profiles are disposed, trending nearly from south to north, and north to eastin Ningxia Arc-shaped Structural belt. The upper crust fine electric structure from themagnetotelluric sounding as the main spindle, the deep electrical structure and thespatial variation of the main boundary fault and the sub-tectonic units in the arcuatestructure belt are studied, combined with the gravity and magnetic data, seismic data, surface geological data and the previous magnetotelluric sounding data in the studyarea. furthermore, the geological meaning is discussed; meanwhile, the earthquakepreparation mechanism is discussed by analysising the seismogenic structure of thetypical earthquake zone. Based on the above studies, some new recognition of deepelectrical structure, seismogenic structure and dynamics characteristics in the studyarea are proposed:（1） Processing the gravity and magnetic data of the Ningxia region by upwardcontinuation and gravity anomaly separation, we found that the crust low density layerof Ningxia Arc-shaped Structural belt distributed mainly in the Ningxia southwest andthe region around Zhongwei City, showing the features that deep in southwest andshallow in northeast; By calculating the diretional derivative of gnetravity&maganeticdata,and enhance the boundary information use Gradient and Laplace operator, we laysout a more detailed fault system in Ningxia region,which is similar to the tectonic unitsmap re-determined by the geological data.(2) Electrical structure profile of mid-upper crust in the Ningxia Arc-shapedStructural belt constituted by high impetance block and low resistivity vulk,withelectrical gradient belts between them, showing the structural styles of chunky totallyand intermittent exist low-impedance&high-conductivity layers in the mid-upper crust,which is the basic electrical structure that is different from the layered electricalstructure and block electrical structure in the mid-upper crust of the study area. It islikely to be the deep tectonic background that strong earthquakes or devastatingearthquakes occurred frequently in the study area. The above understandings offer animportant basis for structural analysis in middle-upper crust.corridor structural belt andthe thrust fold belt of Erdos western region have the similar electrical structure,thatshow a lower resistivity compared the Qilian Orogenic belt,we infer that corridorstructural belt and the thrust fold belt of Erdos western region are all belonged to thewest margin of North China Block.(3) The electrical structure in mid-upper crust in study area indicates that there areclear electrical boundaries between blocks, which offers new evidences for the divisionof local structure and the confirmation of blocks boundaries. Take the westernHaiyuan-Haiyuan-Liupan Mountains fault belt and Qingtongxia-Guyuan fault belt asthe boundary, Ningxia arc-shaped belt could be divided into Qilian Mountains collisionorogenic zone, Corridor structural belt and western thrusting folded zone ofOrdos.Further analysis of the electrical structure of the upper crust,found that corridorstructure belt and Edors western margin have the similar electrical structure,compared to the collisional orogenic belt of Qilian Mountains,performance for a relatively lowerresistance characteristics. corridor structure belt may belong to the western of northchina block.Above conclusions have yet to have electrical section of greater depth andother geological dataconfirm that.(4) The low resistance layer in the crust of Qilian mountains collision orogeniczone is between20to30km under the surface of the earth, which terminate at theHaiyuan boundary belt and intersect at the detachment layer under Xiangshan thrustnappe in north direction. The low resistance layer in the crust of the Xingren-Haiyuandepression zone reduces to10to15km under the surface of the earth, determinate atQingshui River depression zone in northeast direction, and intersect at the earlyProterozoic, middle Proterozoic detachment layer under Ordos thrust nappe. All in all,the low resistance layer in the crust of Ningxia arc-shaped belt, showing thecharacteristics of the deep southwest and the low northeast, and unconnected.(5) A high resistance block is discovered in Qingshui River depression basin.According to the high velocity and high density characteristic that seismic section andgravity data express in high resistance block, the high resistance block is likely to be anIsland chain paleouplift which develop on the basin basement in Late Paleozoic andEarly Mesozoic ear. This discovery offers data for the estimation of earthquake hazardand the dynamics in the northeast margin of Qinghai-Tibet Plateau. Zircon thatcaptured from the diorite porphyrite at North Mountain of Weining has the same age asdating results of the zircon in the rock of Longshou mountain Group from Jinchuanmineralization concentration area, combined with analysising magnetotelluric profilesand surface geology. The paper maintains that Zhongwei-Tongxin depression belt islikely to extend to Gulang earthquake zone. It means Gulang earthquake didn`t occur inHaiyuan fault but in Zhongwei-Tongxin depression belt.（6）The paper confirmed the complete high impedance rigid block in the QilianFolded-Uplift belt,Jingyuan-Xiji Depression-Folded belt, Haiyuan boundary zone,andthe thrust fold belt of Ordos western margin, A high resistance block is found inZhongwei-Tongxin depression basin for the first time,and think that the high resistanceblock is likely to be an Island chain paleouplift which develop on the basin basementin Late Paleozoic and Early Mesozoic ear. Combined with analysis of the distributionof the crust low resistivity layer of Mid-upper crust in Ningxia Arc-shaped Structuralbelt, we infer that the region and boundary zone which have high impedance rigid blockmay be the earthquake potential danger zone,which is very similar to the place whereoccared earthquakes in history. (7) According to the fine electrical structure profiles in middle-upper crustdepend on measured profiles, combined with geology and seismic data in study area, aseries of thrust nappe structure exist in middle-upper crust, which include Xiangshannappe structure and the thrust nappe structure in west margin of Ordos with imbricatedcombination. The depth of their main detachment surface is gradually deep from thenortheast to the southwest, finally disappear in the low resistance layer in crust. ThePaleozoic carboniferous detachment surface, the early Proterozoic or MiddleProterozoic detachment surface and the low velocity and high conductive layerconstitute the main dynamics boundary conditions in study area. Relying on the aboveboundary conditions, Ningxia arc-shaped belt shows dynamic trend of north QilianMountains collision orogenic zone with eastward obduction (thrust nappe), Alxa blockwith southward subduction and Ordos block with westward subduction.(8) It is found that the size of earthquake magnitude is positive proportional to thesize of apparent resistivity by comparative analysis of earthquake magnitude and theapparent resistivity of high resistance rigid block near seismic source. This discoveryhas important significance for earthquake hazard assessment.Confined to thelimitations of the statistics,will be further carried out magnetotelluric in the Ningxiaregion, to get more representative data to verify the point of view. |
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