鄂尔多斯盆地东南南召地区中生代以来的构造演化研究——来自低温热年代学的证据

Tectonic evolution since the Mesozoic of the Nanzhao area in southeast of the Ordos Basin:evidence from low-temperature thermal chronology

有关鄂尔多斯三叠纪原型盆地的东南向展布情况不是非常明确,之前有研究表明其东南缘可能位于南召地区,对该区构造演化过程开展研究,可为鄂尔多斯三叠纪原型盆地研究提供重要信息。因此,为了研究南召地区中生代以来的构造演化史及其与鄂尔多斯盆地之间的关系,本文对研究区3条野外剖面上3件三叠系样品开展锆石、磷灰石裂变径迹研究。其锆石裂变径迹年龄为270±15~181±8Ma,与地层年龄相近或大于地层年龄,不能很好地反映地层经历的构造改造时限,可能更多地代表了物源区的信息。磷灰石裂变径迹年龄为57±3~47±5Ma,结合裂变径迹年龄和热史模拟,本文认为南召地区自三叠纪以来经历了4期较大规模的构造改造,早期是三叠纪末遭受了秦岭造山带强烈逆冲推覆对本区的影响;中期是中晚侏罗世到晚白垩世初;晚期是晚白垩世;末期是喜马拉雅期,4期构造改造均与秦岭造山带的构造演化息息相关。此外,通过与鄂尔多斯盆地周缘地区展开对比,发现二者构造演化过程具有相似的时限性,从构造演化的角度支持南召地区属于鄂尔多斯原型盆地的观点。

Although previous studies indicate that the southeast margin of the prototype Ordos Basin may be located in the Nanzhao area,the southeast extent of the basin remains unknown.Therefore,a study on the tectonic evolution of the area can provide important information for the Triassic prototype Ordos Basin research.In this paper,three Triassic samples from three field sections are investigated with zircon and apatite fission tracks to study the tectonic evolution history since Mesozoic in the Nanzhao area and its relationship with the Ordos Basin.The fission track ages of zircons range from 270±15 Ma to 181±8 Ma,which are close to,or older than the stratigraphic age,and may more represent provenance area information.The apatite fission track ages range from 57±3 Ma to 47±5 Ma.Combining these ages with thermal history simulation,this paper suggests that the Nanzhao area has undergone four large-scale structural transformations since the Triassic.In the early stage,it was affected by the strong overthrust of the Qinling Orogenic Belt at the end of the Triassic;the middle stage was from Middle-Late Jurassic to Early of Late Cretaceous;the late stage was Late Cretaceous;and the last stage was Himalayan.These four stages of structural transformation have close relationship with tectonic evolution of the Qinling Orogenic Belt.In addition,through the comparison with the surrounding areas of the Ordos Basin,it is found that the structural evolution processes of the Nanzhao area and the Ordos Basin have similar timelines,which supports the view that the Nanzhao area belongs to the Ordos prototype basin from the perspective of structural evolution.