The sedimentary and tectonic framework of the Ordovician foreland basin in the southern margin of the Ordos Block and its evolution

鄂尔多斯地块南缘位于鄂尔多斯地块与北秦岭造山带结合部位，是研究北秦岭造山带奥陶纪弧-陆碰撞事件沉积-构造响应的理想场所。目前对鄂尔多斯地块南缘奥陶纪盆地的沉积充填特征、构造属性、结构以及演化历史仍缺乏清晰的认识。本文通过对露头区典型剖面观测结果和覆盖区地球物理资料的综合分析，发现鄂尔多斯地块南缘奥陶纪盆地沉积充填整体表现为南厚北薄、顶底均被区域性不整合面所限定的楔状沉积体；由北向南可以划分出局限沉积环境的坳陷带、持续抬升的隆起带和局限浅水-海相深水-陆相冲积扇环境的盆地带。结合北秦岭造山带奥陶纪大地构造属性及演化历史分析，得出鄂尔多斯地块南缘奥陶纪盆地是与北秦岭造山带奥陶纪弧-陆碰撞事件相关的周缘前陆盆地，经历了以下四个阶段的演化：①冶里-亮甲山期，前陆盆地初始挠曲阶段；②马一-马五期，前陆盆地结构成型阶段；③马六-平凉期，前陆盆地快速沉降阶段；④背锅山期，前陆盆地快速充填阶段。此外，该周缘前陆盆地具有经典前陆盆地的四元结构：褶皱冲断带位于包括渭河地堑在内的西安-宝鸡一带，前渊带呈近东西向展布于岐山-富平一带，前缘隆起带位于庆阳-运城一带，隆后坳陷带大致位于延安-榆林-绥德一带，呈不规则的椭圆形。该研究不仅有助于查明鄂尔多斯地块南缘奥陶纪盆地性质、结构与演化历史，还为北秦岭造山带奥陶纪弧-陆碰撞事件提供了盆地内部的地质历史证据。;The southern margin of the Ordos Block is located in the transition zone between the block and the North Qinling orogenic belt, and therefore it is an ideal place to study the sedimentary or tectonic response of Ordovician arc-land collision event in the North Qinling orogenic belt. There is still a lack of clear understanding of the sedimentary filling characteristics, tectonic properties, structure and evolutionary history of the Ordovician basin in the southern margin of the Ordos Block. In this paper, through comprehensive analysis of the typical section observation results in the outcrop area and the geophysical data of the overburden area, it is found that the sedimentary filling of the Ordovician basin in the southern margin of the Ordos Block is a wedge-shaped sedimentary body defined by regional unconformities at the top and bottom respectively, which is thick in the south and thin in the north. From north to south, the tectonic framework of this region can be divided into the depression zone confined depositional environment, the continuous uplift zone and the basin zone that have undergone restricted shallow water, marine deep water and terrestrial alluvial environments. Combining the analysis of Ordovician geotectonic properties and evolutionary history of the North Qinling orogenic belt, it can be concluded that the Ordovician basin in the southern margin of the Ordos Block is a peripheral foreland basin associated with the Ordovician arc-land collision event in the North Qinling orogenic belt, and it has undergone the following four stages of evolution: ① Yeli-Liangjiashan stage, the initial flexural stage of the foreland basin; ② Ma1-Ma5 stage, the structure-forming stage of the foreland basin; ③ Ma6-Pingliang stage, the rapid subsidence stage of the foreland basin; ④ Beiguoshan stage, the rapid filling stage of the foreland basin. The Ordovician foreland basin in the southern margin of the Ordos Block has the typical structure of a classic foreland basin, in which the fold-thrust belt is located in the Xi’an-Baoji area including the Weihe graben; the foredeep zone is spread in the Qishan-Fuping area in the near east-west direction; the forebulge zone is located in the Qingyang-Yuncheng area; the back-bulge zone is roughly located in the Yan’an-Yulin-Suide area in an irregular oval shape. This study not only helps to find out the nature, structure and evolution history of the Ordovician basin in the southern margin of the Ordos Block, but also provides geological and historical evidence for the Ordovician arc-land collision event in the North Qinling orogenic belt.