茫崖-肃北段阿尔金断裂右旋走滑运动的确定

在茫崖至肃北段阿尔金断裂内,奥陶纪地层中产出的塑性变形构造形态完整而清楚,室内的岩组分析和镜下所见发育良好的显微构造,为进一步确定阿尔金断裂的右旋走滑运动提供了有力证据。右旋走滑发生时间可能介于奥陶纪晚期至泥盆纪之间。

DETERMINATION OF THE DEXTRAL SLIP OF MANGYA-SUBEI SEGMENT OF ALTUN FAULT

This paper deals with the dextral sense of displacement of Mangya-Subei segment of Altun strike-slip fault.This fault,over 1600 km long,strikes northeast-southwest.Within the fault zones,metamorphism,rheology,progressive plastic and brittle deformation are common.The well-preserved rheomorphic and ductile structures outcropping in Ordovician rocks,including flow folds,intrastratal folds,S-C fabrics,shear cleavage and quartz c-fabrics,are representative of the products of tectonic movement occurring in the Altun mountain area.The dextral slip sense of faulting is recognized by the integration of macro-micro structures and petrofabric studies (quartz c-axis fabrics).On plan view,the two flanks of the rheomorphic folds are obviously asymmetrical.The western flanks are long and gentle,the eastern ones short and steep.Their axial planes,dipping steeply NW,strike NE-SW.They assume right-lateral shearing.Under the microscope,the well developed S-C fabrics can be seen.The S-surface cut by the C-surface is composed of oriented biotite,chlorite and quartz.It rotates 20?-40癱lockwise to the C-surface.The relative configuration of the C and S-surfaces permits the dextral shear sense to be determined.In addition,quartz c axes show pronounced asymmetrical scattering at an anticlockwise low angle from the shearing direction.Quartz c-axis fabrics are asymmetric with respect to the motion plane (ab),the sense of asymmetry accords with dextral shearing.From above analysis it can be seen that the microscopic examination and petrofabric study confirm the dextral shear sense interpreted in the field.The dextral offset probably occurred between late Ordovician and Devonian time,and activated strongly at least no later than early Devonian time.