西秦岭东段对青藏高原东向扩展过程构造响应的古地磁制约

Paleomagnetic constraints on the tectonic response of the eastern part of the West Qinling to the eastward spreading process of the Tibetan Plateau

为探究青藏高原东向扩展隆升过程中“十字构造”枢纽处西秦岭各部分的变形特征,在舟曲、武都、成县和凤县等地区下白垩统和新近系中开展了详细的古地磁研究.获得了舟曲和成县地区的早白垩世古地磁极(舟曲85.7°N,279.8°E,A_(95)=9.1°;成县72.3°N,226.3°E,A_(95)=6.8°),以及武都地区的新近纪古地磁极(65.8°N,206.1°E,A_(95)=12.8°).将其分别与同时代的华北地区古磁极进行对比后表明:(1)早白垩世之后,舟曲地区相对于华北发生了14.7±5.8°的逆时针局部构造旋转;(2)成县地区早白垩世古地磁极与华北参考极一致;(3)新近纪之后,武都地区相对于华北参考极发生了25.1±5.6°的顺时针局部构造旋转.结合前人成果,我们认为:青藏高原向东扩展演化过程在西秦岭地区的构造响应,各构造部位展示出不同的活动特征.北西侧区域表现为左行走滑断裂,控制断裂两侧地块呈现原地顺时针旋转,与青藏高原东北部地区的动力机制一致;南西侧区域主体受控于松潘甘孜北东向挤压逆冲,并在北东向次生走滑断裂作用下平移错动,伴随断裂附近的拖曳变形,导致局部逆时针转动;东部区域早期走滑断裂形成拉分盆地,晚期受挤压应力改造,边界断裂发生构造翻转,盆地内弱变形.

In order to investigate the deformation characteristics of various parts of the West Qinling Mountains at the'cross tectonic'pivot during the eastward extensional uplift of the Tibetan Plateau,detailed paleomagnetic studies were carried out in the Lower Cretaceous and Neoproterozoic strata of Zhouqu,Wudu,Chengxian and Fengxian areas.The Early Cretaceous paleomagnetic poles were derived for the Zhouqu and Chengxian areas(85.7°N,279.8°E,A_(95)=9.1°for Zhouqu and 72.3°N,226.3°E,A_(95)=6.8°for Chengxian),and the Neogene paleomagnetic pole for the Wudu area(65.8°N,206.1°E,A_(95)=12.8°).A comparison of each with contemporaneous paleomagnetic poles in northern China shows that:(1)A counterclockwise local tectonic rotation of 14.7±5.8°occurred in the Zhouqu area relative to northern China after the Early Cretaceous.(2)The Early Cretaceous paleomagnetic pole in the Chengxian area is consistent with the North China reference pole.(3)After the Neoproterozoic,the Wudu area underwent a local clockwise tectonic rotation of 25.1±5.6°with respect to the North China reference pole.Based on previous research achievements,it is believed that the tectonic response of the eastward expansion and evolutionary process of the Tibetan Plateau in the Western Qinling region exhibits different activity characteristics in each tectonic site.The north-western area is characterized by left-slip faulting,and the land masses on both sides of the controlling fault show in situ clockwise rotation,which is consistent with the dynamic mechanism in the northeastern part of the Tibetan Plateau.The primary body of the south-western region is controlled by the north-eastward extrusive recoil of Songpan-Garzê.It is accompanied by drag deformation in the vicinity of the fracture in a north-easterly secondary strike-slip fracture that is translational and misaligned,resulting in local counterclockwise rotation.The early strike-slip fractures in the eastern region formed a pull-apart basin,which was modified by extrusive stresses in the late stage,with tectonic overturning of the boundary fractures and weak deformation within the basin.