南秦岭构造带中段晚中生代陆内变形特征与侧向挤出构造

摘要

南秦岭构造带位于秦岭造山带南部，在中生代时期经历了复杂的陆内变形过程。在晚中生代阶段，南秦岭构造带内发育一系列平行造山带的走滑断裂带。目前对于这些走滑断裂带不同构造位置的变形特征、变形叠加关系以及动力学机制等方面的认识并不充分。构造研究显示，南秦岭构造带内的宁陕断裂和安康断裂均发育大量平行于断裂带的 A 型褶皱群和近水平的拉伸线理，表明南秦岭构造带经历了以走滑剪切变形为主的构造阶段。运动学特征表明，宁陕断裂以左行剪切变形为主，而安康断裂则以右行变形为特征。选择典型岩石样品进行同位素测年来限定断裂活动的时代，其中：宁陕断裂带内同构造花岗岩脉的锆石 SHRIMP U Pb 定年结果为(186.0±4.0)Ma；安康断裂带内云母矿物的40 Ar 39 Ar 定年结果为161.2~173.5 Ma。虽然宁陕断裂和安康断裂的运动方向不同，但是同位素年代学研究限定了两条断裂发生走滑变形的时间都是早中侏罗世，从而共同构成了南秦岭构造带中间块体整体向东挤出的构造特征。同时也表明，相互碰撞的大陆在碰撞之后将很快转变为以平行造山带侧向挤出和走滑位移为主的陆内变形演化阶段。%Southern Qinling belt is located in the south part of the Qinling orogen,which experienced the complex intra-continental deformation in Mesozoic.In the Late Mesozoic period,there were a series of strike-slip faults developed along the orogen in the southern Qinling belt. At present, the understanding of the deformation features,superposition relationship and dynamic mechanism for these strike-slip faults on different location is not sufficient.Structural studies show that the Ningshan fault and the Ankang fault are characterized by the A type fold and sub-horizontal stretching lineation,which indicated the southern Qinling belt underwent a tectonic stage dominated with the strike-slip shear deformation.Kinematic studies revealed the Ningshan fault was characterized by a sinistral shear deformation,while the Ankang fault by dextral characteristics.We chose the typical rock samples for isotopic dating to limit the fault activity time.The zircon SHRIMP U Pb age of the syn-tectonic granite vein in the Ningshan fault was (186.0±4.0)Ma,and the mica 40 Ar 3 9 Ar dating results in the Ankang fault are 1 61.2 173.5 Ma.Although the kinematic features are opposite between the Ningshan fault and the Ankang fault,the geochronological results defined that the deformation time of these two faults is the same,e.g.Early Middle Jurassic,both of which constitute a eastward extrusion for the central block of southern Qinling belt.This tectonic characteristic also indicated that the deformation of the orogen was soon transformed into the intra-continental deformation stage,which was dominant with the lateral extrusion and strike-slip displacement along the orogen.