西秦岭北缘北西向断层特征和形成的构造机制及地质意义

西秦岭北缘构造带不仅发育一系列继承性多期活动或新生的近东西向断层,而且新生代地层中还发育与近东西向断层走向不一致且具有独特构造特征的北西向左旋走滑断层。这种北西向左旋走滑断层带不发育断层角砾岩、磨砾岩、碎粉岩、断层泥、摩擦镜面、擦痕线理、断层阶步等脆性断层中常见的构造现象,仅表现为地层旋转和剪切拉断形成的一定宽度的透镜化带,两条断层之间地层产状发生旋转形成了约1 km宽,平面上类似膝折构造几何形态地层扭折带。该北西向断层横切了渐新统—中新统地层,并被上新统砾岩覆盖和第四纪以来的近东西向左旋走滑断层斜切,指示了其形成于渐新世—中新世沉积地层形成之后,上新世砾岩沉积之前,即上新世早期。北西向断层带不发育脆性断层典型构造现象和断层左旋走滑作用在渐新统—中新统沉积地层中形成了类似膝折构造几何形态地层扭折带,说明其变形具有韧脆性过渡和缓慢剪切变形的特征,是西秦岭北缘一种新的断层类型。其形成机制为基底或中下地壳中大型左旋走滑韧性或韧脆性剪切带向上扩展延伸到上部沉积盖层中之结果,也就是说,新生代沉积盖层中这种北西向断层和地层扭折带是下部韧性剪切带的左旋走滑剪切在盖层中被动构造响应。这种基底或中下地壳北西向左旋韧性剪切带可能指示了上新世初期西秦岭北缘构造带深部韧性地壳物质向南东流变蠕动的构造标志,代表深部地壳缩短增厚向地壳韧性物质侧向扩展流动的转换过程。这种特殊的断层类型对理解青藏高原东北缘新生代构造变形体制转换和地壳隆升具有重要的科学意义。

Characteristics and formation mechanism of NW trending faults in the northern margin of West Qinling Mountains and its geological significance

In the northern margin of West Qinling, there not only developed a series of inherited multiple active or new-born near EW trending faults, but also a series of NW-trending sinistral strike-slip faults in the Oligocene-Miocene strata which are inconsistent with the archly near EW-trending regional faults and have unique structural characteristics. These NW-trending sinistral strike-slip faults do not develop the typical tectonic phenomena that generally developed in brittle faults, such as fault breccia, grinding conglomerate, crushed powder rock, gouge, slickenside, striation, fault step and so on, and only shows a certain width of lenticular zone and the stratigraphical twisted zone by shear pull-apart or strata rotation. Furthermore, between two NW-trending sinistral strike-slip faults developed 1 km wide stratigraphic twist zone which is similar geometrically to kink structural configuration on horonzontal plane. This faults transected the Oligocene-Miocene strata, and was covered by Pliocene conglomerates and cut by near EW-tending sinistral faults since the Quaternary, which indicate that the faults formed after the Oligocene-Miocene and before late Pliocene, i.e., the Early Pliocene. The NW-trending sinistral strike-slip faults, a new type of fault in the north margin of West Qinling, do not have the characteristcs of typical brittle faults and its sinistral strike-slipping formed stratigraphic twist zones which is similar to kink-fold geometric configuration, indicating that the deformation has the characteristics of ductile-brittle transition and slow shear deformation. Therefore, the formation mechanism of this kind of fault is the upperward propagation of the large scale sinistral strike-slip ductile or ductile brittle shear zone in the basement or middle-lower crust into the upper sedimentary cover strata, which imply that the NW-trending faults and stratigraphic twist zones formed by fault’s sinistral strike-slipping in the Upper Cenozoic sedimentary cover are just the passive structural responses to NW-trending sinistral ductile shear zone in the basement or middle-lower crust. The NW-trending ductile shear zone in the basement or middle-lower crust may indicate the tectonic indicator of SE-directed flowing of the deep ductile crustal material to along the northern margin of the West Qinling during the Early Pliocene , representing the transformation of the deep crust shortening-thickening to the lateral expansion of the crustal ductile matter. This special type of fault is of great scientific significance to the understanding of Cenozoic tectonic deformation regime transformation and crustal uplift in the northeastern margin of Qinghai-Xizang Plateau.