East-west-trending ductile detachment in the Burang area, Greater Himalaya, and its tectonic significance

Abstract：All the rocks in the east-west planar ductile detachment zone in the Burang area in the western part of the Greater Himalaya terrane have suffered from plastic deformation to different degrees and mylonitization. The foliation of the mylonites dips gently to the east and the stretching lineation plunges to the east at 5°－18°. Asymmetrical rotational strain structure indicates that the ductile detachment had the nature of east-directed movement in the early stage and the features of west-directed brittle-ductile deformation. Electron backscatter diffraction (EBSD) fabric analysis shows that the preferred orientation of deformed quartz in mylonites has two slip systems：the east-directed high-temperature prism1010＜a＞ and intermediate-temperature rhomb glide101 1＜a＞ system and the west-directed intermediate-temperature rhomb glide 101 1＜a＞and low-temperature basal glide0001＜a＞ system. This, combined with the characteristics of the asymmetric rotational structure prevalent in mylonites, suggests that the east-directed ductile detachment in the early stage formed in an intermediate- to high-temperature environment and that the west-directed brittle-ductile in the late stage formed in an intermediate- to low-temperature environment. Zircon SHRIMP U-Pb dating shows that the protolith of the metamorphic basement in the Burang area of the Greater Himalaya terrane formed at 1863 Ma. The isotopic dating of mylonitic granite and undeformed leucogranite show that the east-west-trending Burang planar ductile detachment took place in the early Miocene, which coincides with the ages of the MCT and STDS on the northern sides of the Greater Himalaya terrane. When the compressional stress produced by northward subduction of the Indian plate acted on the arcuate Himalayan orogenic belt, it was resolved into two components, one being orogen-parallel and the other orogen-perpendicular. The former caused plastic flow of the material of the middle and lower crustal below the orogenic belt, thus forming the Burang planar ductile detachment zone, while the latter resulted in the formation of the MCT and STDS. They are different tectonic styles formed in the same tectonic stress field and same tectonic event. The detachment zｏne is the product of large-scale north-south shortening and uplift and eastward escape of a part of material.