秦岭南缘青川断裂新生代变形特征及其走滑运动学转换

Cenozoic Structural Deformation and Strike-slip Kinematic Transformation of the Qingchuan Fault, in the Southern Margin of Qinling

青川断裂作为秦岭构造带南部边界断层,新生代以来受到印度-欧亚大陆碰撞产生的远场效应,发生了强烈的走滑复活,调节了青藏高原隆升和向东扩展。本文基于错断地貌测量与断裂带脆性变形的野外调查,建立了该断裂新生代2期走滑运动历史,并讨论了走滑运动学转换的大地构造意义。沿断裂带河流水系偏移地貌分析发现,主要河流的Ⅳ级支流沿断裂发生一致的右旋偏移,指示断裂右旋位错量在200~800 m;河流阶地的右旋位错量在49~62 m。野外调查发现,青川断裂发育5~100 m宽的断裂破裂带,主要由断层泥、磨砾岩、断层透镜体等组成,S-C组构发育,磨砾石旋转定向排列。断裂破碎带运动学指向记录了青川断裂2期脆性走滑变形：早期为左旋走滑活动、晚期为右旋走滑活动。结合断裂带东端汉中盆地地层时代和秦岭山地隆升时代,我们推断晚期右旋走滑运动主要发生在上新世以来,调节了碧口地块的向东挤出;而早期左旋走滑运动则很可能是对古近纪晚期青藏高原隆升和扩展的响应。

The Qingchuan fault, which was caused by the far-field effect from the India-Asia collision since Cenozoic, as the southern boundary of the Qinling Tectonic Belt, strongly reactivates and its strike-slip motions adjust the Tibetan Plateau uplift and eastward expansion. Based on the terraces offset measurement along fault trace and field brittle deformation investigation in the fractured zone, this paper proposes a 2-stage strike-slip development history, and discusses the tectonic significance of this strike-slip kinematic transformation. River channels consistently bend when they meet the fault trace, especially, the Ⅳ-level tributaries of the main rivers commonly occur right-lateral offset, which indicate a right-lateral strike-slip displacement of 200~800 m. In addition, the terrace right-lateral offset of 49~62 m provides estimation for strike-slip displacement in the late Pleistocene. In the field, the Qingchuan fault develops a 5~100 m wide fractured zone, which is composed of fault gouge, fault-grinding breccia and tectonic lens etc. In particular, S-C fabric well develops, and fault-grinding breccia rotate and orientational arrange in the brittle deformation zone. The kinematic characteristics in the fault fractured zone well record two stages of brittle strike-slip motions, which are including the early sinistral strike-slipping and the late dextral strike-slipping movements. Combined with the stratigraphic age in the Hanzhong basin and uplift time on the Qinling mountains, we infer that the late dextral strike-slip motion of the Qingchuan fault mainly occurred since the Pliocene and adjusted the eastward extrusion of the Bikou block, and the early sinistral strike-slip is, instead, likely as a response to the Tibet uplift and expansion in the late Paleocene.