The survey on fine lithospheric structure beneath Hohhot-Baotou basin by deep seismic reflection profile
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摘要: 跨呼和浩特-包头盆地(以下简称"呼包盆地")完成的91.8 km长的深地震反射剖面,揭示了呼包盆地的岩石圈精细结构和断裂的深、浅构造特征.结果表明,本区地壳和岩石圈具有清晰的层状反射结构特征,其中,地壳厚度约45~48 km,岩石圈厚度约82~87 km.莫霍面在大青山之下出现约3.5 km的抬升,暗示大青山的隆升不是因为地壳物质增厚所致,即大青山可能不存在"山根".呼包盆地为南浅、北深的"箕状"断陷盆地,盆地沉积层最厚处位于大青山山前,其厚度约为7~8 km.鄂尔多斯北缘断裂和大青山山前断裂作为呼包盆地的南、北边界断裂,在剖面上均表现为由3~4条断裂组成的"Y"字形断裂构造,它们对呼包盆地的形成、地层沉积、基底变形和地震活动都有重要的控制作用.剖面揭示的岩石圈深断裂位于大青山山前断裂的下方,该断裂向上进入上地壳,向下切割中-下地壳、莫霍面,进入上地幔.深断裂的存在为深部热物质的上涌与能量强烈交换提供了通道,而上涌的软流层物质与岩石圈地幔发生交代和侵蚀作用导致岩石圈减薄.Abstract: Hohhot-Baotou basin (referred to as Hu-bao basin) is a Mesozoic-Cenozoic fault basin, locating between Yinshan orogenic belt and Ordos block. To study the fine lithospheric structure and the relation between deep and shallow tectonic structures of faults, we conduct a 91.8 km-long deep seismic reflection profile cross Hu-bao basin in 2010 and get clearer image of the lithospheric structure along the profile.As for the data acquisition, the observing system, asymmetric receiving on both sides, is made up of 30 m group interval, 600 channel receiving and 50 covering. The recording time is 30 s and the sampling interval is 2 ms. Seismic wave explosion applies the drilling blasting to be the hypocenter which the hole depth is 25~30 m,the dose is 24 kg and 180 m shot interval. We give the priority to improving the S/N ratio of seismic data in data processing. Improvements include tomographic statics correction, amplitude anomaly elimination, spherical dispersion compensation and surface consistent amplitude compensation, time-varying band-pass filter and 2-D filter, surface consistent deconvolution, many iterations of velocity analysis and residual static correction, DMO and poststack noise attenuation. The results show that the crust and lithosphere of Hu-bao basin have clearly layered reflection features. The crustal thickness is about 45~48 km, and the lithospheric thickness is about 82~87 km. The Moho uplift occurrs about 3.5 km under the Daqingshan Mountain, which implies the uplift of the Daqingshan Mountain is not because of crust thickening, namely no existence of "the root of a hill". Hu-bao basin is a "half-graben" fault basin with the feature as shallow in the south and deep in the north. The thickest part of its sedimentary layer is located in the Daqingshan Piedmont with the thickness being about 7~8 km. As the south and north boundary faults, the fault on the northern edge of Ordos and the Daqingshan piedmont faults are showed the "Y" shaped fault structure made up of 3~4 faults in the profile. These faults have the important role in controlling the Hu-bao basin formation, sedimentary layers, basement deformation and seismic activities. The profile reveals the lithospheric deep fault is located below the Daqingshan piedmont fault, going upward into the upper crust, cutting down the mid-lower crust, the Moho and then going into the upper mantle.The deep seismic reflection profile of Hu-bao basin reveals the deep tectonic background and deep-shallow tectonic characteristics of faults, which provides seismic evidences for understanding tectonic characteristics of Hu-bao basin and coupling relations between the basin and areas around it. Hu-bao basin is located in the sewing position between Yinshan Mountains and Ordos basin, being formed and developed under the common effect of squeezing by Qinghai-Xizang block to Ordos block in the NE trending and extrusion by North Yanshan block. The existence of deep faults below the Daqingshan piedmont provides a channel for the upwelling and energy exchange of deep hot material, and the metasomatism and erosion between upwelling asthenospheric materials and lithospheric mantle lead to the lithospheric thinning. Strong arc-shape reflection of the lower crust in the deep seismic reflection profile may be the reaction to lower mantle magma swarming into it. Magma upwelling leads to the heating squeezing and lateral material migration of the corresponding position in the lower crust, and furtherly promotes the development of Hu-bao basin and expansion of the edge fault.
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