岷山隆起带与西秦岭构造带中段位于青藏高原物质东向流动的必经之处,又是南北地震带的组成部分和GPS速度场非连续性衰减和转换的关键部位,其地壳结构及地壳变形机制受到国内外地质地球物理学家的广泛关注,了解研究区深部细结构及主要边界断裂空间展布特征,对青藏高原隆升机制及中强地震孕震构造的研究有重要意义.本文依托分别横跨岷山隆起带及西秦岭构造带中段的两条大地电磁剖面(SG-WQL-L1与SG-WQL-L2)小点距观测数据,采用大地电磁相位张量分解技术对两条剖面上各个测点的电性走向、二维偏离度进行计算分析,根据分析结果对原始数据进行主轴方位角校正处理,进一步采用NLCG(非线性共轭梯度)二维反演方法开展TE与TM模式的相位和电阻率联合反演,获取沿剖面方向30 km以浅的电阻率结构模型,并完成了地质地球物理综合解释.两条大地电磁剖面勘探成果揭示出,马尔康地块中上地壳发育的壳内低阻层与岷山隆起上地壳低阻体在深部交汇,岷江断裂带与虎牙断裂带受控于马尔康地块与岷山隆起带上地壳底部的滑脱面,滑脱面呈现往东角度逐渐变陡峭的趋势且在岷江附近出现“断坡”构造,历史强震震源深度显示虎牙断裂为岷山隆起带新生代强震的发震断裂;西秦岭构造带中段中上地壳沿剖面方向表现为横向分块、纵向分层的电性结构特征,中地壳12～25 km左右发育厚度不等的壳内低阻层,壳内低阻层多与研究区次级地块的边界断裂在深部交汇,次级地块以及区分次级地块的活动断裂带可能是GPS速度场在研究区呈现非连续性的递减并伴随方向转换的构造成因;青藏高原内部的软流圈物质向NE和SSE流动,驱动巴颜喀拉地块东缘上地壳沿中上地壳低阻层东向运移,受到摩天岭高阻地块的阻挡作用,软弱的岷山隆起带发生地壳褶皱变形并向东逆冲推覆从而形成高耸的岷山山脉,岷江断裂与虎牙断裂的左旋运动加速了岷山的隆起.%The Minshan uplift zone and the central West Qinling tectonic belt are located in the eastern margin of the Tibetan plateau,part of the north-south seismic belt,where GPS velocity discontinuity decays and converts,so receiving much attention of domestic and foreign geophysicists.Study of the deep structure and spatial distribution characteristics of boundary fault zones in this region is important for the understanding of the Tibetan plateau's uplift mechanisms and the causes of strong earthquakes there.This study is based on two magnetotelluric (MT) sounding profiles (SG-WQL-L1 and SG-WQL-L2) with small measurement spacing,which pass through the two zones aforementioned.Data processing and analysis include calculation of the electric strike and 2D skewness of the area using phase vector decomposition,and processing the original data according to the electric strike.The NLCG 2D inversion is performed on apparent resistivity and phase data of TE+-TM modes,obtaining resistivity structure models along the profiles.The results of inversion show that low-resistivity layers are present at depth beneath the Barkam block,which contacts with the upper crust low-resistivity body of the Minshan uplift at depth.The Minjiang and Huya faults are controlled by the detachment beneath the Barkam block and Minshan uplift,of which the angles become steeper to the east and dislocation formation occurs at depth.The focal depths of history earthquakes reveal that the Huya fault zone is the seismogenic fault in Cenozoic but the Minjiang fault zone is not.The structure characteristics of the stratified vertically and the blocking laterally mid-upper crust are revealed by the resistivity profiles of the central section of the West Qinling orogenic zone.The boundary faults of subblocks in the study area and the low-resistivity layers which are present at the bottom of upper crust are converging at depth.The subblocks and the active fault zones bounding the subblocks are likely responsible to the decay and conversion of GPS velocity discontinuities.Thermal mantle flow in the Tibetan plateau shifts toward east along the asthenospheric channel,and then induces upper crust of the eastern Bayan Har block to move toward east along the low resistivity layer in the mid-upper crust.Due to the obstruction of the rigid crust of the Motianling block,poly-stage overthrusting and folding happened in the crust of the plastic Minshan block,leading to the uplift of the Minshan Mountains,while sinistral slip of the Minjiang fault and Huya fault accelerated the Minshan uplift.
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