西秦岭北缘漳县新生代伸展断陷盆地确定及其地质意义

摘要

西秦岭北缘断裂带新生代以来挤压逆冲变形起始于何时？挤压逆冲变形之前是否经历过伸展拉张过程？北缘断裂带北侧的新生代红层盆地到底是类前陆压陷挠曲盆地还是拉张断陷盆地？上述问题对西秦岭新生代盆-山构造格局重建和印度-欧亚板块碰撞汇聚的远程构造响应的时间与方式等科学问题的认识具有重要的地质约束.本文通过对西秦岭北缘构造带内漳县渐新世—中新世含盐红层盆地沉积序列和沉积旋回特征以及盆地边界断裂之间的几何学-运动学关系分析,认为西秦岭北缘构造带内漳县含盐红层盆地具有拉张伸展动力学背景下形成的断陷盆地的地质特征.西秦岭北缘构造带内渐新世—中新世断陷盆地的确定,指示了印度板块与欧亚板块碰撞汇聚而导致的青藏高原构造挤压缩短作用至少在盆地沉积充填阶段尚未扩展到西秦岭北缘及以北地区.而漳县含盐红层盆地沉积地层褶皱缩短变形以及之后角度不整合在漳县含盐红层盆地之的上新统韩家沟粗砾岩,可能记录了西秦岭北缘由伸展边界向挤压缩短逆冲边界的转换过程.因此,青藏高原东北缘真正成为青藏高原体系组成部分是在上新世的漳县含盐红层盆地封闭-构造反转之后.这一认识对地学界长期以来认为印度板块与欧亚板块碰撞汇聚而导致的高原隆升和构造挤压早在渐新世就已经波及西秦岭北缘的观点提出了挑战.%Whether the northern margin of Western Qinling in the Cenozoic is the extensional bounder or compressive thrusting border? Whether the Cenozoic red bed basins to the north of the northern margin of western Qinling are foreland sag deflection basin or extensional rift basin? These issues are of the key constraints on the reconstruction of the Cenozoic basin mountain tectonic framework in the Western Qinling and the key to better understand the time and the way of the remote tectonic responses to the India-Eurasia collision.Based on the investigation of the sedimentary sequence of Oligocene-Miocene Zhangxian salt red bed basin and the analysis of the relationship between Zhangxian salt red bed basin and northern margin faults in the Western Qinling,it is suggested that the Zhangxian salt red bed basin is a rift basin dominated by regional crustal extension.The existence of Zhangxian extensional rift basin in the north margin of Western Qinling supported the idea that lithosphere compression and shortening triggered by collision between the Indian plate and Eurasian plate since the Cenozoic have not yet reached to the northern margin of Western Qinling before the closure-inversion of the Zhangxian red bed basin,which means that the Western Qinling have not turned into the part of the Tibetan Plateau before the Pliocene.Pliocene Hanjiagou coarse conglomerate which covered in angular unconformity on the Zhangxian salt red bed basin in Zhangxian region may have recorded the tectonic transformation from extensional rifting to compression-shortening.This result presents a challenge to the recognition that the geological academia has tong argued that the uplift of the plateau and tectonic extrusion led by India plate-Eurasia plate collision has spread to Western Qinling since the Late Oligocene.