Characteristics and deformation evolution of fault rocks in the F1 fault on the northern margin of the western Qinling

The West Qinling northern margin fault is a key tectonic boundary in the northeastern region of the Qinghai-Xizang Plateau. Its deformation history is of great scientific significance for understanding the Cenozoic basin-mountain coupling of the northeastern region of the Qinghai-Xizang Plateau, clarifying the tectonic attributes of the Cenozoic basins, revealing the crustal deformation and uplift dynamics of the northeastern margin of the Qinghai-Xizang Plateau, and the remote tectonic response of the Indian-Eurasian collision and convergence. The types of fault rocks and their meso- to micro-scale characteristics of the exposed F1 fault zone with 100 meters width of the northern margin of the West Qinling Mountains in the Zhangxian-Wushan area, have been studied, and the following understanding has been obtained: 1) the presently exposed F1 fault is mainly composed of fault breccia-gouge, cataclasite-ultracataclasite, and fission-foliation-lensing zone. 2) The macro- to micro-scale structural characteristics of different types of fault rocks in the F1 fault indicate deformation mechanisms ranging from brittle to brittle-ductile, including brecciation, cataclasis, and foliation to schistosity. According to the cross-crustal model of brittle faults and the rheological behavior in experimental deformation of carbonate rocks, it is estimated that the formation depth of fission-foliation-lensing zone is about 6~8 km, the formation depth of cataclasite series is about 4~5 km, and the formation depth of fault breccia-fault gouge is about 1.5 km. 3) Based on the deformation mechanism, geometric and kinematic differences of F1 fault rocks, as well as the cross-cutting relationships of different structural elements, it is proposed that F1 fault has experienced three phases deformation evolution, that is, extensional normal faulting, high angle thrusting, and oblique sinistral strike-slip faulting. 4) Based on the analysis of the tectonic coupling relationship between the three-phase deformation events of the F1 fault and the Oligocene-Miocene sedimentary sequences, as well as the Pliocene molasse conglomerate, it is proposed that three-stage geological evolution model of the F1 fault in the northern margin of the western Qinling is established which are the Oligocene-Miocene extensional faulting and fault-bounded basin development; Pliocene compressional thrusting accompanied by the deposition of molasse conglomerates; and Quaternary strike-slip faulting coupled with rapid uplift. The above evolution models of the F1 fault in the northern margin of the West Qinling Mountains provides geological evidence and constrains for further understanding the northeastward growth and spreading of the Qinghai-Xizang Plateau.