柴北缘逆冲断裂带地壳、上地幔三维电性结构研究

Near Dachaidan which was in the northern Qaidam Basin, two broad band magnetotelluric (MT) profiles including 30 stations and across the North Qaidam Thrust were designed. The two profiles would be parallel in EW and the total length of them was ~300 km. Also MT data from line 5000 & 6000 which were NS and finished in 2009 and 2010 would be reprocessed. Three-Dimensional MT inversion method was preferred because of the complex deep structures beneath the research area which would cause possible electrical distortion. Finally we expected to get the reasonable 3D electrical model of crust and upper mantle beneath the North Qaidam Thrust. It was effective to determine the location, dip direction and depth of faults by analyzing the resistivity gradient, so information of the North Qaidam Thrust would be enriched. It was well known that the crust of Qaidam Basin was thickened but the mechanism was still controversial. It was approved that MT was useful to discuss the deep process by studying the resistivity value and electrical unit. It was very important to research if there existed evidence of “Channel flow” beneath north of Qaidam basin to discuss the reason of the thickened crust.

在柴达木盆地北缘大柴旦一带，穿过柴北缘逆冲断裂带，布置两条平行的、东西向宽频大地电磁测深剖面，两条剖面设计总长约300km，平均点距10km。同时整合已完成大地电磁测深剖面（5000线和6000线，两条剖面为近南北向）的部分测点，利用三维大地电磁测深反演方法，获得研究区内地壳及上地幔的三维电性结构特征。依据所得到的柴北缘逆冲断裂带下的三维电性结构模型，厘定柴北缘逆冲断裂带的位置、倾向、切割深度等信息，为柴达木盆地北缘地体边界的确定提供电性依据。同时利用地下深部物质的电阻率值范围、电性单元构成等信息，探究研究区内地下物质状态及深部过程，分析壳内是否有“管道流”存在的可能性，并进一步研究柴达木盆地地壳增厚作用的机制。

在青藏高原柴达木盆地北部，柴北缘逆冲断裂带区域，完成了39个大地电磁测深（MT）宽频点的野外数据观测任务，整合原有5000线、6000线MT测深剖面的部分数据（共21个宽频测点），在研究区形成三维MT测深数据网。利用MT三维反演算法，得到了研究区壳内三维电性结构模型。电阻率模型水平切片结果表明：柴达木盆地内浅表高导层向下延深超过地下5km，推测为第四纪沉积覆盖层的反映；中、下地壳表现为高阻特征，最大电阻率超过1000Ωm，表明柴达木盆地具有较稳定的刚性结构。柴北缘逆冲断裂带地区，浅部3km以上有零星高导体分布，地下3-10km范围内的上地壳介质主要表现为高阻特征，而地下15km以下的中、下地壳内则分布有高导体，且向下延深超过地下45km，这一结果似乎表明该地区深部可能发生强烈的构造运动，或者壳幔交互作用，而高导体则可能是逆冲构造作用生热或深部岩浆上涌而导致的局部熔融。南北向二维垂向剖面表明，沿剖面呈现数条电性梯度带，推测为柴北缘逆冲断裂带在电性特征中的反映。认为柴北缘逆冲断裂带总体为北倾，伴随一定的南倾次生断裂，断裂切割深度不超过下地壳。依据电性结构模型，我们得到如下认识：（1）由于柴北缘逆冲断裂带的逆冲、推覆作用，南祁连造山带基底物质可能进入柴达木盆地，并形成沉积覆盖层；（2）柴达木盆地具有较强的中、下地壳，而位于盆地北缘和南祁连南缘之间的逆冲断裂带区域，则表现为弱的深部特征。认为研究区内地壳的增厚作用主要为下地壳逆冲重复的结果。（3）柴达木盆地北缘复杂的被动顶板双重逆冲构造体系，有可能形成油气资源的有利构造圈闭。