日月山断裂南段晚第四纪活动特征

CHARACTERISTICS OF LATE QUATERNARY ACTIVITY OF THE SOUTHERN RIYUESHAN FAULT

青藏高原东北缘是青藏高原向NE扩展的最前缘,是理解高原扩张的最佳场所。日月山断裂是青藏高原东北缘一条NNW走向的右旋走滑断裂,对其开展活动性研究对于理解高原扩张有重要意义。目前对该断裂南段的晚第四纪活动性质研究较少,对其晚第四纪的活动特征认识尚且不足。文中基于日月山断裂南段的野外考察资料,通过高精度遥感影像解译并结合典型位错点无人机摄影测量等方法获得其精细的几何展布,根据断裂的展布特征自北向南将日月山断裂南段分为贵德和多禾茂2段。结合年代学研究,初步确定日月山断裂南段存在全新世活动,结合典型位错点多级地貌面定年与蒙特卡洛方法,厘定了贵德段和多禾茂段全新世以来的水平滑动速率分别为(3.37+0.55/-0.68)mm/a和(2.69+0.41/-0.38)mm/a。结合前人的研究资料分析认为,在NE向主应力下,鄂拉山和日月山等断裂发生右旋走滑和NE向压扁,共同吸收青藏高原东北缘块体NE向的地壳缩短。

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 ^(14)C 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,(9350~10700)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(2860±30)a BP and(3460±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.