西秦岭北缘构造带新生代盆地南部边界断层带结构与构造变形演化

The structural characteristics of the southern boundary fault zone of the Cenozoic basin and its tectonic deformation evolution in the northern margin of the western Qinling

西秦岭北缘构造带是青藏高原东北缘的主要构造边界之一,北缘断层及其所控制的新生代沉积盆地是青藏高原东北缘新生代盆—山格局演化、高原扩展隆升与变形的地质记录。因此,西秦岭北缘构造带的断裂构造和断裂控制的沉积盆地研究对于理解青藏高原构造系统形成和高原隆升过程都具有重要的科学意义。本文通过对西秦岭北缘新生代盆地的南部边界断层F1断层结构分带、断层岩类型、几何学—运动学特征分析,获得如下认识:1)F1断层总体走向为290°~300°,倾向北北东,倾角60°~80°,发育近百米宽的由韧性、韧脆性和脆性断层岩等组成的结构复杂的断层带;2)构造分析揭示了F1断层至少经历了3期构造变形事件,第一期为韧性—韧脆性伸展正断层作用,第二期为脆性高角度挤压逆冲断层作用,第三期为近直立的脆性斜向左旋走滑作用;3)该断层近百米宽的断层带内形成于不同构造层次的韧性、韧脆性、脆性等变形现象叠加交织出现在现今地壳浅表层次,说明该断层带经历了从早期较深层次韧性变形域逐渐抬升而进入晚期较浅层次的脆韧性变形域到现今的脆性变形域的韧—脆性变形机制转换;4)根据F1断层对西秦岭北缘渐新统—中新统漳县含盐红层盆地的空间构造配置、控制和改造以及新生代区域构造变形演化历史分析,认为第一期韧性—韧脆性伸展正断层作用与渐新世—中新世断陷盆地形成相匹配,活动时代为晚渐新世—晚中新世;第二期脆性高角度挤压逆冲作用与渐新世—中新世地层翘起、褶皱和底部抬升剥蚀及上新世磨拉石盆地充填相对应,活动时代应该始于中新世末期或上新世早期,持续至第四纪早期;第三期斜向左旋走滑则与西秦岭北缘断层带第四纪以来广泛发育的左旋走滑作用相对应。综上所述,西秦岭北缘新生代漳县盆地南部边界断层F1,虽然仅是北缘构造带中一条断层,但作为构造敏感带,其多期变形历史应该代表了青藏高原东北缘新生代以来的构造变形演化及构造体制转换过程。如果这一新生代沉积盆地边界断层F1在渐新世—中新世一直处于伸展正断作用,那么西秦岭北缘在这个阶段应该处于地壳伸展拉张状态,渐新世—中新世漳县盆地只能是伸展断陷盆地而不可能是挤压挠曲前陆盆地或压陷盆地。因此,我们认为印度—欧亚板块碰撞汇聚产生的构造挤压缩短和地壳隆升效应在中新世尚未波及到西秦岭北缘区域。F1断层在中新世末—上新世初的构造反转挤压冲断和上新世具有再生前陆磨拉石堆积出现才标志着西秦岭北缘卷入青藏高原挤压构造动力学系统。

The northern margin tectonic belt of the western Qinling is one of the major tectonic boundaries on the northeastern margin of the Qinghai-Tibet Plateau.The northern margin faults with its adjacent Cenozoic sedimentary basins are geological records of the Cenozoic basin-mountain evolution,uplift and deformation of the northeastern margin of the Qinghai-Tibet Plateau.Both the study for this faults and sedimentary basins are of great scientific significance for understanding the formation of the tectonic system of the Qinghai-Tibet Plateau and its northward-northeastward growth process.Based on the observation for the types and characteristics of fault rocks and geometric-kinematic analysis for various structural elements in the Cenozoic basin'southern boundary fault F1 in the northern margin of western Qinling,the following understanding is obtained:1)This F1 fault has relative stable strike with 290°~300°and 70°~80°dip to NNE and develops 100 meters wide fault zone with spatially intertwined ductile,ductile-brittle and brittle deformed fault rocks;2)Structural analysis reveals that F1 has experienced at least three phases of tectonic deformation,that is,the first is ductile or brittle-ductile extensional normal faulting,the second phase is brittle thrusting,and the third phase is nearly vertical brittle oblique sinistral strike-slipping.3)The superimposition and interweaving of various deformation records developed in different structural levels in the nearly 100 meters fault zone,such as cataclasite,mylonite,tectonic schist and various foliation,lineation,etc.indicates its complex tectonic deformation history with the ductile-brittle deformation mechanism transformation from the early deeper ductile deformation domain to the late shallower brittle-ductile deformation domain.4)From the analysis for the spatial tectonic configuration and superimposed transformation relationship between the Oligocene-Miocene Zhangxiansalt-bearing red bed basin and F1 in the northern margin of West Qinling,it is suggested that the first phase of ductile or ductile-brittle extensional normal faulting is coupled with the formation of Oligocene-Miocene fault depression basin,which imply that first phase extensional normal faulting occurred during Late Oligocene-Miocene;the second phase of brittle thrusting should corresponds to ending of Oligocene-Miocene red bed basin filling and its late tilting,uplift and denudation,and this tectonic process may continue until the Pliocene molasse basin appeared,which implied that the second phase of brittle thrusting should begin in the end of Miocene or beginning of Pliocene and continue to the early Quaternary.The third phase of oblique sinistral strike-slipping corresponds to the sinistral strike-slipping faulting in the northern margin of the western Qinling since Quaternary.Although F1 fault is just only one of the northern margin faults of the western Qinling,its multiple phase deformation history,as a tectonic sensitive indictors,should represent the tectonic deformation kinematic evolution and tectonic transformation process since Cenozoic in the northeastern margin of Qinghai-Tibet Plateau.If the boundary fault F1 of the Cenozoic sedimentary basin has been in the state of extensional normal faulting from Oligocene to Miocene,then the northern margin of the West Qinling should be in the state of crustal extensional tension at this stage,and the Oligocene-Miocene Zhangxian basin can only be an extensional faulted basin but not an compressional flexed foreland or compressional sag basin.Therefore,we believe that the tectonic compression and shortening and crustal uplift caused by the collision and convergence of the India-Eurasia plate have not spread to the northern margin of the West Qinling Mountains in Miocene.Furthermore,the F1 fault tectonic reversion and thrusting from the end of Miocene to the beginning of Pliocene followed by the formation of Pliocene regenerated foreland molasse basin,indicate that the northern margin of the West Qinling Mountains was involved in the tectonic compressive dynamic system of the Qinghai-Tibet Plateau.