Crustal structure of the Ordos block and adjacent regions along an N-S profile of 107. 6 degrees E

The Ordos Block is located in the west of the North China Craton, close to the northeastern margin of the Tibetan Plateau. It has played an important role in the tectonic activities of eastern China mainland since Mesozoic and Cenozoic, especially the evolution and destruction of the North China Craton. To reveal the fine crustal structure with higher resolution beneath the Ordos and adjacent regions, this work analyzed seismic data along an N-S profile of 107. 6 degrees E including 27 temporary broadband stations from the newly deployed ChinArray-Himalaya II that stretches across the Yinshan oregon, Hetao graben and Ordos block. Also data of 17 stations from SOS-Array and permanent CEA stations at the south of this profile, crossing the Weihe graben, Qinling zone and Sichuan basin, were added to enhance the analysis. We derived receiver functions and used the Kirchhoff migration method to create a preliminary image of the Moho under this profile. Detailed structures in the crust and the Moho variation were then obtained with a joint inversion of the receiver functions and surface wave dispersion curves from previous study beneath each station. The results reveal that: (1) the Moho is generally flat at about 45 km depth in the northern part of Ordos, and becomes deeper to about 50 km in the southern part. To the north of Ordos, Moho is also 50 km deep under the Hetao graben. To the south of Ordos, crust is thinned to about 40 km beneath the Weihe graben and then thickened under the Qinling belt and Sichuan basin to about 47 similar to 50 km. (2) A large-scale slow velocity anomaly exists beneath the Hetao basin and goes to a maximum depth of 25 km, which may reflect its prominent extensional and sedimentary history. (3) Beneath the Qinling oregon, the slow velocity anomaly indicates predominantly felsic crustal composition, which could be caused by removal of lower mafic crust by delamination during the Mesozoic. (4) The migration image and joint-inverted velocities under the Ordos block display different crustal structures bounded by 38 degrees N, which may be associated with different tectonic histories of the two parts on either side.

鄂尔多斯地块紧邻青藏高原东北缘,位于华北克拉通的西部,在我国中生代、新生代以来东部地区的构造活动中起到了重要作用.对鄂尔多斯及其周缘地区的研究可以提供有关华北克拉通的形成、演化和破坏过程的重要信息.本文选取了纵贯鄂尔多斯的107.6°E附近南北剖面上的44个流动地震台站进行分析,采用接收函数方法,进行Kirchhoff偏移成像,并且结合在该区域内前人的地震面波频散进行联合反演,获得剖面下方的地壳内部精细结构.研究结果显示:(1)莫霍面在鄂尔多斯北部较平缓,约45 km深;在鄂尔多斯南部有所加深,达到50 km;其北边的河套盆地的地壳厚度约为50 km;南边的渭河盆地到秦岭地区及四川盆地的地壳厚度从约为40 km增厚到47～50 km.(2)河套盆地下方存在大规模的低速异常,最深可达25 km,反映了其显著的拉张构造和沉积历史.(3)秦岭造山带下方的低速异常对应于其主要为长英质的地壳组分,可能是由于中生代的拆沉作用导致的地壳下部基性岩石层的缺失.(4)以38°N为界的鄂尔多斯地块,南北部地壳速度结构存在差异,可能表明了这两部分经历的构造历史不同.