Late Cenozoic Activity Of The Qingchuan Fault,the Northeastern Segment Of The Longmenshan Fault Zone:Inferred From Geomorphic Features Of Watersheds And Rivers

The Longmenshan(LMS)fault zone is located in the eastern margin of the Tibetan Plateau.It is an important structural boundary between the Tibetan Plateau and the Sichuan Basin,and the active tectonic characteristics and tectonic-geomorphologic evolution are the key to understanding the uplift of the Tibetan Plateau and assessing regional seismic risk.The Qingchuan fault(QCF),i.e.the northeastern segment of the LMS fault zone,is a dextral strike-slip-dominated fault since the late Cenozoic.Since the 2008 Mw 7.9 Wenchuan earthquake,the tectonic activity of the QCF has been paid more attention.Moreover,the Minjiang fault zone,i.e.located between the central and northeastern segments of the LMS fault zone,is considered to be the eastern tectonic boundary of the eastern Tibetan Plateau,while the tectonic role of the outer region of the boundary during the uplift of the Tibetan Plateau is still unclear.Compared with remarkably steep topography in the central segment,the northeastern Longmen Shan has moderate relief.Its tectonic-geomorphologic evolution may,from another perspective,reflects the uplift of the Tibetan Plateau.Based on geomorphic analyses of along-fault streams,rivers and watersheds,this thesis analyzed the active tectonic characteristics of the QCF since the late Cenozoic,especially during the late Quaternary,and discussed the regional tectonic-geomorphologic evolution.Using DEMs and remote sensing images,combined with field observation and cosmogenic nuclide-derived erosion rate estimates and U-Pb dating method,this thesis analyzed the geomorphic characteristics of the QCF at different time scales.Firstly,the late Quaternary strikeslip rate along the QCF is inferred from geomorphic characteristics of along-fault offset streams.Special attention is paid to the compensation of watershed erosion rate.Meanwhile,the vertical movement along the QCF is also analyzed according to geomorphic indices of river longitudinal profiles.Combined with the strike-slip and vertical slip features along the QCF,the late Quaternary tectonic characteristics of the QCF is better constrained.Furthermore,the average slip rate of the QCF since the late Cenozoic is estimated according to the total displacement and initial time of its dextral strike-slip movement.The displacement is obtained from the recovery of systematically deflected rivers,displaced geomorphic features and basement rocks.Finally,the basic topographic features,and the distribution features as well as formation ages of the stepped surfaces of the Bikou Block are analyzed.Combined with the tectonic characteristics of the QCF,this thesis discussed the tectonic-geomorphologic evolution of the northeastern segment of the LMS fault zone.The major conclusions are as follows:(1)The results of geomorphic analysis based on channel offset show that the late Quaternary strike-slip rate of the QCF,which is ranging from 0.4 ± 0.1 to 2.2 ± 0.3 mm/ yr,increases from southwest to northeast,and starts to decrease near the northeastern segment.(2)The geomorphic indices of longitudinal profiles of along-fault channels show that the vertical slip rate decreases from southwest to northeast along the QCF.The QCF appears a normalfault component at the northeastern end.(3)The dextral strike-slip and vertical slip rates help us to further understand the late Quaternary tectonic characteristics of the QCF.The results show that the movement pattern of the LMSFZ has changed from thrust to strike-slip faulting from southwest to northeast.The strikeslip rate of the QCF increases toward northeast,and the thrust component gradually reduces from southwest to northeast until it disappears.It is estimated that the tectonic stress along the QCF gradually changed from compressional to extensional stress.The lower strike-slip rate in the southwestern segment of the QCF may due to its interaction with the northeastern segment of the Beichuan fault.And the Hanzhong Basin may have absorbed part of the dextral slip rate at the northeastern end of the QCF.(4)The largest cumulative offset of the QCF is likely to be 17 km according to the reconstruction of the systematically deflected rivers,displaced geomorphic features and basement rocks.Combined with the formation time in the Hanzhong Basin(?9 Ma),the average slip rate of the QCF since the late Cenozoic is estimated to be about 1.9 mm/yr.(5)The topography of the Bikou Block gradually decreases eastward along the QCF,and the average uplift rate of the Bikou Block also decreases from west to east,which reflecting the geomorphic response of the Bikou Block to the tectonic characteristics of the QCF.The central and the northeastern segment of the LMS fault zone,located on both sides of the Minjiang fault zone,may play two different roles in the uplift process of the Tibetan Plateau.(6)This thesis shows that the aforementioned method which is based on offset stream channels compensated by watershed erosion rate,is a potential method for estimating average slip rate of strike-slip faults,especially for those without chronological information.Furthermore,based on the method,the acquired along-strike distribution of strike-slip rates can help to improve the understanding of the tectonic characteristic of strike-slip fault.