| About 55 to 45 Ma,India-Eurasian plate convergence collision constantly promote the Tibet Plateau uplifting and expanding towards the north and the east.The nearly 900-km-long Qilian-Haiyuan Fault System(QHFS)along the Qilian mountain is the most important boundary tectonic belt at the Tibet Plateau NE frontier,and also the tectonic boundary zone between the Tibetan,Ordos and Gobi Alashan blocks.The fault system can be divided into eastern and western sections.The western is a narrow fault(the Lenglongling fault,LLLF)along the Qilian Shan,and the eastern splays apart into two arc fault zones,of which the northern one mainly consists of the Tianqiaogou-Huangyangchuan Fault(THF,also known as the Gulang Fault)and the Xiangshan-Tianjingshan Fault(XTF)In the past,due to the relatively low strike-slip rate along the THF(~4 mm/a)dwarfed by nearly one order of magnitude by the slip rates of the other faults like the LLL and the Haiyuan fault in the QHFS(1~2 cm/a),its tectonic role receives less attention and even is neglected.In recent years,several more detailed and plausible work constrained the slip rates along the LLLF and the Haiyuan fault to be 3-7 mm/a,showing a comparable activity to that of the THF.Accordingly,a series of new scientific problems arise:how to adjust the distribution of structural deformation in the east and west sections of fault system?What role does the THF play in the tectonic movement of this region?In addition,the QHFS was successively hit by two huge earthquakes with M>8 in the1920 at Haiyuan and 1927 at Gulang.Contrast to Haiyuan earthquake,the Gulang earthquake is still lack of much study about its seismogenic structure,which is debated a lot for a long time.The major point of debate is whether the THF was involved in the earthquake rupture.At the same time,the dispute is of great importance to the seismic hazard assessment of the region.To solve the above scientific questions,I took the THF as the research object,based on high resolution remote sensing image interpretation,carried out the detailed field geological surveies,measurements and dating.Based on the above,the late Quartery activity of the THF and its implications on the NE Tibet Plateau were discussed.The THF mainly distributes along the southern boundary of Wuwei Basin,connected to the Lenglongling Fault on the east and adjacent to the Xiangshan-Tianjingshan Fault on the west.The active tectonic landform along the fault is very remarkable with typical left-lateral strike-slip features.The THF starts from the east bank of the Honggou River,extends eastward roughly along a series of negative terrain including the Tianqiaogou River,the Zhangjiahe River and the Huangyangchuan River,and gradually disappears at the north piedmont of Qinjiadashan Mountain,with a total length of~120 km.Based on our detailed active fault mapping,the THF is apparently divided into two segments by a~15 km gap.The segment at the east of the gap extends from Hongyaoxian eastwards to Jiapigou with the length of~92 km long,and is very continuous and straight,striking west to east with the central part protrudes slightly to the south.According to the faulted geomorphology,the maximum cumulative dislocation is about 5 km,and the minimum offset is about 5 m.With the help of UAV to obtain the high precision geomorphic dislocation data,combined with the C14 and OSL dating methods,we analysed the offset of the river terrices at Huoshaotai,Haxi river,Liutiao river,Songshuwan,Gulang river,and Dalashuixiagou,and computed the sinistral strike-slip rate since the Holocene as 3.5±0.4 mm/a.If the slip rates of the THF and the HF are decomposed to the direction as the strike of the LLLF,sum their components is roughly consistent with the slip rate of the LLLF.Meanwhile,the slip rates of the northern arc faults gradually decreases from west to east,showing that THF plays an important adjusting transformation role between the crust shortening along the Qilianshan thrust belt to the Tianjingshan and Yantongshan thrust belt,respectively.On the other hand,we carried out the paleoseismic study at Hongyaoxian,Motaizi,east of the Liutiao River,Kuangou,Pandaogou,Guodiwan,Jiucaigou,and Hujiajing village.All the trenches revealed a total of 5 paleoseismic events since the Holocene,including:1.8±0.2ka?3.8±0.1 ka?5.9±0.3 ka?8.2±0.5 ka?10.3±0.5 ka.The elapsed time is about 1.8 ka,and the mean recurrence internal(RI)was about 2.1 ka.The activities of the faults in the QHFS illustrated by their RIs are similar to those derived from their slip rates.Therefore,the THF plays an important role accommodating of the northeastward extruding of the northeastern Tibet relative to the Gobi Alashan block.Moreover,our work concludes that the THF was not involved during the 1927earthquake.Accordingly,surface-ruptured earthquakes have been absent along the THF for a long time despite its strong activity in the Holocene.Hence,the THF may be another seismic gap with a potential of M?7 earthquakes like the Tianzhu gap near the triple junction at the west end of the THF.In this case,it is in a high seismic risk along the THF and in the neighboring area.What's more,the Tianjingshan block is confined within the QHFS.Its southern boundary fault(the HF)seems to have a decreasing left-lateral slip rate since it started strike-slipping,while the slip rates of the northern boundary including the THF and the XTF seems to be in a trend of decrease.We infer the difference between the movement trends is due to the Tianjingshan block is gradually separated from the Longzhong basin block at the different tectonic affections from the surrounding Tibet,Ordos,and Gobi Alashan blocks.Besides,both the 10~6-scale strike-slips since the onset of the left-lateral strike-slip and the the 10~4-scale strike-slips since the late Quaternary on the QHFS don't support the rigid block model which suggests high slip rates along the boundary faults.And different movements between the secondary blocks around the QHFS and the shear deformation of the blocks itself also cannot be well explained with the simple rigid block model. |
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