Velocity Distribution of Upper Crust, Undulation of Sedimentary Formation and Crystalline Basement Beneath the Ordos Basin in North China

Abstract

AbstractAlong the line Yanchuan–Baotou–Mandula in the Ordos basin and adjacent Yinshan mountainous area in the north in North China, a wide‐angle reflection and refraction seismic sounding has been carried out yielding Pg wave data of high resolution. By the traveltime differential tomographic imaging, traveltime inversion of the Pg wave field is performed to determine velocity distribution of the upper crust, undulation of sedimentary formation and crystalline basement along the profile. And geological interpretation to these features is made. The result shows that the upper crust below the Ordos basin is of a double‐layer structure, with a distinct interface between the upper and lower layers. The upper layer is characterized by low velocity and big gradients in vertical direction, while the lower layer has low velocity and changes gently. In the light of the velocity distribution in the upper crust of the study area, it is suggested that the sedimentary formation and crystalline basement undulate in NS direction, generating a series of rising and falling structures. From south to north, they are the depression south to Yulin, Yulin‐Daotu swell, listric depression from northern Daotu to the fault on the northern edge of Ordos and secondary structure in it, and Hohhot‐Baotou depression and its interior faults. In the north, seismic wave velocity in the upper crust beneath the Yinshan Mountains (Mt.) is obviously higher than that below the areas on the both sides, exhibiting a pattern of longitudinal belt. It reflects the feature of the crystalline basement of this area, which is associated with collage and accretion of several blocks such as the Wula'er Shan Mt., Se'erteng Mt., Hejiao batholith, and Bayan Obo group. Beneath the Yinshan Mt. the upper crust is also of a double‐layer structure, though its refraction interface of the basement is not so clear as that below the Ordos basin. Based on the structures of sedimentary formation and undulation of the crystalline basement of the study area, we have discussed the sedimentary features and hydrocarbon prospects of every depression along the profile.