关键词: 青藏高原东北缘, 日月山断裂, 晚第四纪活动, 滑动速率

Abstract:

Due to the collision between the Indian plate and the Eurasian plate, the Tibetan plateau has experienced violent uplift and strong intraplate deformation inside the plateau, which has a great impact on the tectonic evolution of the surrounding areas. The northeastern edge of the Tibetan plateau is the forefront of the northeastward expansion of the Tibetan plateau, which is the ideal place to study the deformation of the plateau as well as the far-field deformation associated with continental collision between the Eurasia and India plates. In recent years, scholars have gained a certain understanding of the characteristics of late Quaternary tectonic activity in the northeast margin of Tibetan plateau. Within the northeastern margin of Tibetan plateau, there are two major fault systems: One is the near EW-trending left-lateral strike-slip fault system, including the Kunlun, Haiyuan and western Qinling faults, the other one is the NNW-trending right-lateral strike-slip fault system, including the Elashan and Riyueshan faults. They are sub-parallel to each other. Since the Riyueshan Fault is one of the major right-lateral strike-slip faults in the northeastern margin of Tibetan plateau, its activity is of great significance for understanding the plateau expansion. Previous studies mainly focused on its northern part which is believed to be active during Holocene. However, its southern part is believed to be active during late Pleistocene, but not active since Holocene. Therefore, there are little studies focusing on the late Quaternary activities of the southern part of the Riyueshan Fault. Hence, our understanding about the characteristics of the late Quaternary activity is insufficient. During our preliminary field survey along the southern Riyueshan Fault, we found distinct deformation of Holocene landforms, such as the young alluvial fan, terrace risers and channels, which indicate its late Quaternary activity. In this study, we firstly analyze the fault geometry of the southern Riyueshan Fault based on high-resolution Superview-1 remote sensing images and carry out field verification. Based on fault geometry characteristics, fault strike orientation etc., we divided the southern Riyueshan Fault into two segments from north to south. One is the Guide segment(generally trending in NW 20°)and the other is the Duohelmao segment(generally striking in NS). During our field investigation, we found two typical sites for slip rate studies, the Rixiaolongwa site on the Guide segment and the Niemari site on the Duohemao segment, respectively. We collected high-resolution images using UAV, and then generated high-resolution DEM of these two sites. By measuring the offsets and corresponding dating results of multi-level terrace risers, we obtained the displacements of the three-level and two-level terraces at Rixiaolongwa and Niemari site, respectively. Then we collected the OSL and ¹⁴C samples on different terrace risers to constrain the age of each terrace. In the Rixiaolongwa area, the corresponding offsets of T1, T2 and T3 terraces are(26.3±3.1)m, (32.7±7.1)m and(38.6±8)m, and the age sequence is(7840±30)a BP, (9 350~10 700)a BP and(11.9±1.3)ka BP, respectively. In the Nimari area, the corresponding offsets of T1 and T2 terraces are(6.3±0.7)m and(9.7±1.7)m, and the ages are(2 860±30)a BP and(3 460±30)a BP, respectively. By applying Monte Carlo method, we obtained the corresponding slip rates of(3.37+0.55/-0.68)mm/a and(2.69+0.41/-0.38)mm/a for the Guide and Duohemao segment, which is comparable to the previously suggested slip rate of northern Riyueshan Fault. Finally, we discussed the role of the Riyueshan Fault in the tectonic deformation of northeastern Tibetan plateau.

Key words: southern Riyueshan Fault, northeastern margin of Tibetan plateau, Late Quaternary activity, slip rate