青藏高原东北缘何时卷入现今青藏高原构造系统?——来自西秦岭北缘漳县盆地新生代沉积记录的约束

When the northeastern margin of the Qinghai-Tibet Plateau was in-volved in its present tectonic system:Constraints from the Cenozoic sedimentary sequences

新生代以来印度欧亚板块持续碰撞汇聚形成号称世界第三极的青藏高原。青藏高原的扩展生长和构造变形系统形成的动力学过程是地球科学研究的重大科学问题。青藏高原东北缘新生代以来构造演化过程及其与印度欧亚板块碰撞汇聚的动力学耦合关系研究对于揭示青藏高原扩展生长过程具有重要地质意义。尽管前人已经开展了大量研究探索,提出各种构造隆升模型,但青藏高原东北缘何时卷入印度欧亚碰撞汇聚的青藏高原构造系统尚未达成共识。作为青藏高原东北缘组成部分的西秦岭北缘构造带漳县地区不仅新生代地层记录齐全,而且断裂构造发育,构造变形现象丰富,是研究青藏高原东北缘新生代构造演化及印度欧亚碰撞汇聚远程构造响应的良好区域。通过对西秦岭北缘构造带漳县地区新生代沉积盆地地层构造格架、沉积地层序列和沉积旋回等详细野外观测研究,结合区域断裂带几何学运动学及变形历史分析,取得如下认识:(1)西秦岭北缘漳县地区新生代沉积地层主要由为不整合分隔的两套构造性质完全不同的构造地层单元组成,即渐新世—中新世伸展断陷盆地沉积和上新世再生前陆磨拉石盆地沉积;(2)渐新世—中新世时期的地壳伸展拉张构造环境与印度欧亚碰撞汇聚的挤压环境相悖,指示了西秦岭北缘在渐新世—中新世尚未卷入现今的印度欧亚碰撞汇聚构造系统;(3)上新世磨拉石盆地的发育标志着西秦岭北缘构造带从伸展到挤压的构造体制转换,可能指示了印度欧亚碰撞汇聚的挤压构造作用这时才波及西秦岭北缘;(4)上新世粗砾岩、西秦岭造山带地层和中生代沉积地层共同经历了抬升剥蚀作用,形成了西秦岭北缘广泛发育的夷平面。第四纪以来夷平面的抬升和解体、现代河流侵蚀系统和多级河流阶地的出现,指示了青藏高原东北缘整体的不均匀大规模抬升而进入现今青藏高原构造系统。

Since the Cenozoic era,the continuous collisional convergence between the Indian-Eurasian Plates led to the formation of the Qinghai-Tibet Plateau that is often called the third pole in the world.The geodynamic process of the spreading,growth of the Qinghai-Tibet Plateau and the formation of the tectonic system are key issues in Earth sciences.The Cenozoic tectonic evolution of the northeastern margin of the Qinghai-Tibet Plateau and its dynamic coupling relationship with the Indian-Eurasian collisional convergence have important geological constraints on the spreading and growth process of the Qinghai-Tibet Plateau.Although various tectonic uplift models have been proposed in previous studies,there is no consensus on when the northeastern margin of the Qinghai-Tibet Plateau was involved in the Indian-Eurasian collisional convergence tectonic system.The Zhangxian area on the northern margin of West Qinling,a typical region of the northeastern margin of the Qinghai-Tibet Plateau,has not only complete Cenozoic stratigraphic records,but also well-developed faults and abundant tectonic deformation phenomena.It provides a good opportunity for studying the Cenozoic tectonic evolution of the area as well as revealing the formation of the plateau tectonic system and the spatial-temporal spreading and growth model of the Qinghai-Tibet Plateau under the dynamic background of India-Eurasian collisional convergence.Based on a detailed field study of the stratigraphic framework,sedimentary stratigraphic sequence and sedimentary cycle of the Zhangxian Cenozoic sedimentary basin,combined with the geometry-kinematics and deformation history analyses of the regional faults,the following conclusions were reached:(1)The Zhangxian Cenozoic sedimentary stratigraphic framework is mainly composed of two tectonic stratigraphic units:Oligocene-Miocene extensional rift basin deposits and Pliocene regenerated foreland molasse basin deposits,separated by an angular unconformity.(2)The extensional tectonic setting during the Oligocene-Miocene is contrary to the compressional setting resulting from India-Eurasia collisional convergence,implying the northern margin of West Qinling was not involved in the present India-Eurasia collisional convergence system during the Oligocene-Miocene.(3)Pliocene molasse basin sedimentation marked the tectonic transition from extension to compression on the northern margin of West Qinling,which may imply the response to compression setting led by India-Eurasia collisional convergence in the area.(4)In the above area,Pliocene coarse conglomerate,strata of the western Qinling orogenic belt and Mesozoic sedimentary strata experienced uplift and denudation,finally resulting in the wide development of planation plane.Since Quaternary,the uplift and disintegration of this planation surface,and afterward the appearance of modern river system and multi-stage river terraces,marked the northeast margin of the Qinghai-Tibet Plateau unevenly and massively uplifted and then entered into its present tectonic system.