一种雁行石英脉体的形成及构造意义

FORMATION OF ONE TYPE OF AN EN-ECHELON QUARTZ VEIN ARRAY AND ITS IMPLICATION

对发育于牛首山奥陶系砂岩中的一组雁行石英脉体进行了分析。牛首山石英脉体的形成既与Ⅰ型主断裂末端分解的次级雁行脉有相似点,也具有自身的一些特点。它们形成于不同的机制,但区别于张性脉体,单个脉体也属于Ⅱ型破裂。脉体之间的岩桥区变形并不是简单的横弯作用,而且还有一定的韧性变形,发生一定程度的构造减薄,纵弯也起到一定作用。早期岩石中的不均一构造(断裂、节理以及层理面等)对后期雁行脉的生长起到了控制作用。单个脉体均是在左行走滑控制下形成。这组石英脉体是走滑断层(Ⅱ型)端点的断层破碎带内由R'次级走滑断裂控制形成的,而不是由Ⅰ型主断裂末端分解的次级雁行脉。走滑断层端点前的楔型断层破碎带是由主断层控制下形成的次级R和P剪切面所围限的区域,在楔型体内则发育同向或反向剪切作用控制下的破裂或脉体。这种断层端部的变形是一种非常普遍的断层扩展方式。

A set of en-echelon quartz veins in the Ordovician sandstone in the Noushoushan was analyzed. The quart veins in the Niushoushan show both characteristics of those formed at the tip of mode-I par- ent faults and the ones in the tip zone of mode-II faults,which means that they took place by different mechanism from those extensional veins. The bridges between the veins were deformed not only by bending but also by some extent of ductile deformation. And to those early fabrics such as faults ,joints and bedding surfaces in the bridges, the buckling may take place. The early fabric in the rocks was one of the important factors which control the late development of the en-echelon veins. These results show that the development of these veins may be controlled by the R＇ shear zones at the fault-tip dam- age zone of a strike-slip fault （ Mode- II ） , instead of minor en-echelon ones formed at the tip of a mode-I parent fault. The wedge-type fault damage zone at the tip of a major strike-slip fault was bounded by R and P minor shear zones derived from the major fault,and in the wedge region antithetic and synthetic shear veins or fractures will take place. This type of deformation is pervasive at the ter- mination of the strike-slip fault, and it is an important way for the propagation of faults.