西秦岭北缘一个新生代伸展型角度不整合及其地质意义

通过对西秦岭北缘漳县地区渐新统—中新统含盐红层地层与下伏造山带地层之间的角度不整合及其之上的砾岩、地层序列、沉积旋回等特征研究,提出了该角度不整合为伸展型角度不整合的认识。该伸展型角度不整合的存在指示了西秦岭北缘漳县渐新统—中新统含盐红层盆地具有伸展断陷盆地的属性,意味着在渐新世—中新世漳县含盐盆地形成和沉积充填时期,青藏高原东北缘(至少西秦岭北缘)一直处于伸展构造环境。这与印度板块与欧亚板块碰撞汇聚动力学作用向东北缘扩展形成的以挤压缩短和隆升为主的构造环境不协调,也就是说,青藏高原东北缘在渐新世—中新世可能尚未卷入现今青藏高原构造系统。

A Cenozoic extensional angular unconformity on the northern margin of West Qinling Mountain and its geological significance

Based upon detailed field observations and study for the angular unconformity between the Oligocene-Miocene salt-bearing red bed strata and its underlying orogenic strata in Zhangxian area of the northern margin of West Qinling Mountain and the characteristics of the conglomerates, stratigraphic sequence and sedimentary cycle of strata which cover the unconformity, the authors hold that this angular unconformity is an extensional angular unconformity developed under the crustal tensile tectonic setting. This extensional angular unconformity could provide the constraint on the tectonic attributes of the Oligocene-Miocene salt-bearing redbed basins in Zhangxian area of the northern margin of the West Qinling Mountain, that is, Zhangxian Oligocene-Miocene saltbearing red-bed basins is an extensional rift basin, which implies that the northeastern margin of the Tibetan plateau (at least the northern margin of West Qinling) might have been in an extensional tectonic environment in the Oligocene-Miocene period during the development and filling of the Zhangxian salt-bearing red-bed basin. This extensional tectonic environment was inconsistent with the crustal compression shortening and uplifting tectonic environment caused by spreading and extending of Cenozoic IndianEurasian collision to the northeast margin. It is therefore inferred that the northeastern margin of the Tibetan Plateau should not be involved in the present Tibetan Plateau tectonic system during Oligocene-Miocene.