The Quaternary small-scale faults at the Benchi Road section in Jiuhua, Xiangtan and study of tectonic stress fields

A 130 m-long EW-trending artificial inclined section is exposed on the north side of the Benchi Road in the Jiuhua Development Zone of Xiangtan, Hunan Province. Lower part of the section is burgundy fine-grained conglomerate of the Cretaceous Daijiaping Formation while upper part is gravels and vermicular red earth of the Quaternary Baishajing Formation. On the basis of detailed field geological investigation of the section, we found 24 Quaternary small-scale faults along the section. The faults are steep with dip angles of 70°～88°. Based on striking and natures, these faults can be divided into four groups: NW-trending sinistral strike-slip faults, NE-trending dextral strike-slip faults, EW-trending sinistral strike-slip faults and NNE-trending normal faults. Among the four groups, the former two groups are conjugated shear faults. According to their intersection relations, faults can be divided into three stages: the first-stage NNE-trending normal faults, the second-stage NW-trending sinistral strike-slip faults and NE-trending dextral strike-slip faults, and the third-stage EW-trending sinistral strike-slip faults. According to the regional geological settings and the era of cutting layers, the authors suggest that the first stage NNE-trending normal faults were formed in Middle Pleistocene under regional extensional tectonic setting, and their forming mechanism was probably related to the extension and subsidence of Dongting Basin and its adjacent areas. The second stage NW-trending sinistral strike-slip faults and NE-trending dextral strike-slip faults are controlled by EW-trending major pressure stress during Middle Pleistocene-Late Pleistocene, and their forming mechanism is probably related to the eastward compression of Qinghai-Tibet block. The third stage EW-trending sinistral strike-slip faults are controlled by NE-trending major pressure stress during Late Pleistocene-Holocene, and their forming mechanism is probably related to the north-eastward compression of India-Australia plate. The study of the small-sized faults at the Benchi Road section reveals the deformation features of Quaternary regional structure, tectonic regimes and dynamic mechanisms.