| The collision between the Indian and Eurasian plates has led to the uplift of the Tibetan Plateau and eastward movement of active blocks within it. Due to the hampering of the relatively rigid Sichuan basin, the velocity of the movement of the Bayan Har block decreases by approximate 12 mm/yr at its eastern boundary. The compaction between the Bayan Har block and Sichuan basin has led to the rapid uplift of the Longmenshan Mountains and the reactivation of the Mesozoic Longmenshan fault as evidenced by remarkable thrust. Although the Longmenshan Thrust Zone (LTZ) is slipping with a very low rate of<1 mm/yr, it generated the 2008 Wenchuan Ms 8.0 earthquake on the central segment and the 2013 Lushan Ms 7.0 earthquake on the southwestern segment of the LTZ, illustrating its capability to produce strong earthquakes. These two strong earthquakes have transferred the focus of the geoscience communities to the northeastern segment of the LTZ on which no strong earthquake has occurred yet. Based on the increased Coulomb failure stress on the LTZ associated with the Wenchuan earthquake, previous study proposed that the recurring time is advanced by approximately 230 years for its northeastern segment implying a high seismic risk for this segment of the LTZ.The sparse seismicity along the northeastern segment of the LTZ relative to the Minshan Uplift has suggested that the slip on the central segment of the LTZ is transferred northeastward to the Minshan Uplift and its boundary faults, so that the northeastern segment of the LTZ is inactive. However, the surface rupture along the Yingxiu-Beichuan fault generated by the Wenchuan earthquake extended beyond the Huya fault, which is the eastern boundary of the Minshan Uplift. Furthermore, the distribution of aftershocks of the Wenchuan earthquake demonstrates that the break below the ground surface may have reached the Qingchuan fault but did not follow the proposed boundary faults between the Bayan Har Block and the Minshan Uplift. These facts raised a question that whether the northeastern segment of the LTZ is active or not. To date, it remains debatable. A few studies suggest that the northeastern segment of the LTZ was active before Late Pleistocene and became inactive since then, whereas other studies propose it is still active in Holocene. Besides, previous studies on the northeastern segment of the LTZ had paid little attention to the slip rate, paleoearthquake history, and rupture behavior, which are of great significance in assessing regional seismic risk, learning the crustal deformation mechanism and tectonic evolution.Moreover, the coseismic displacements vary along a fault which was ruptured during a large earthquake so that parameters at a few sites could not represent the entire activity of a fault. Additionally, the deformation within a tectonic block was neglected in traditionally geological surveys, and thus it brings about some uncertainties in understanding the crustal movement and deformation mechanism.To solve thses problems, to determine the parameters of activity of the northeastern segment of the LTZ, such as late Quaternary activity, movement pattern, slip rate, paleoearthquake history, rupture behavior, and elapsed time since the latest faulting event, this thesis presents the studies on tectonic landforms and geological features based on field surveys and DEM computation on the Qingchuan fault of the northeastern segment of the LTZ and its neighboring areas. The primary content and conclusions are stated below:(1) Through the detailed interpretation on high-resolution aerial photographs and satellite images and field surveys on tectonic landforms and geological outcrops, it is determined that the Qingchuan fault, dipping to the northwest, extends northeastward from Pingwu, through Qingchuan, and terminates at the western corner of the Hanzhong basin with a total length of ～200 km. This fault has been slipping for a very long time so that its surface trace is continuous without any marked step-overs or bends and became very matured in geometry.(2) The Qingchuan fault shows clear linear landforms, such as fault valleys, and compressive ridges along the fault. These notable linear features suggest its movement pattern of right-laterally slipping. Along the fault, a large amount of drainages with different sizes are systematically dextrally offset. Among them, the Jialingjiang River is displaced by approximately 18 km, implying a rough slip rate of ～1.5 mm/yr along the Qingchuan fault since the late Miocene based on the regional thermal history and tectonics. Additionally, the slip rate of the Qingchuan fault in the late Quaternary is estimated to be approximately 0.7 mm/yr using the displacement of a dextrally offset river terrace combined with its estimated age based on regional studies.(3) On the basis of field survey, it is illustrated that compressive ridges are widely present along the segment to the south of the Jialingjiang River, whereas several triangular facets could be found on the northeastern segment of the Qingchuan fault. Such features demonstrate that the tectonic stress field and movement pattern on the southwestern segment are different from those on the northeastern segment of the Qingchuan fault. Furthermore, the scratch traces observed on the slip plane change along the fault in pitch angles, showing that the dip-slip component gets larger northeastward.(4) Based on detailed field survey, this work chose two sites in fault valleys to excavate paleoseismic trenches. Through the analysis on the stratigraphy and structures revealed by the trench walls and evidence for paleoseismic event, this work identified a respective faulting event in each trench and determined the Qingchuan fault is active in Holocene. The paleoseismic event in the Dujiaba trench is constrained by radiocarbon dating to occur between 5120-3820 B.C., while that in the Tangjiaba trench occurred in a time span of 4115-3000 B.C. Due to their overlapping of age ranges and the matured geometry of the Qingchuan fault, these events are considered to be the same faulting event and also the latest one that ruptured both trench sites along the fault. Thus, the latest earthquake on the Qingchuan fault should have occurred in a combined age range of 4115-3820 B.C., implying that the elapsed time of the faulting event is-6000 years, which is in accordance with the slow rate of strain accumulation revealed by GPS observations in the Longmenshan region. Because of the matured geometry of the fault, it is very likely that the paleoearthquake ruptured the full length of the fault (-200 km). Thus, the magnitude of this earthquake can be estimated to be Mw 7.6-7.9 by using empirical scaling laws between magnitude and rupture length. The magnitude of the paleoearthquake identified and the recurrence interval of faulting suggest that the Qingchuan fault is characterized to produce major earthquakes with long recurrence intervals.(5) According to the GPS and geological observations, the Qingchuan fault is slipping dextrally at ～0.7-1.5 mm/yr. Combined with the elapsed time of ～6000 years since the latest earthquake, a slip of ～4.2-9 m is needed to release the strain accumulated on the fault. Namely, it has accumulated a seismic moment equivalent to Mw ～7.3-7.5 on the Qingchuan fault. Considering the increased static Coulomb failure stress triggered by the 2008 Wenchuan earthquake and consequent advancing of the timing of earthquake recurrence by ～230 years, it is reasonable to propose a high seismic risk along the Qingchuan fault and its neighboring areas in the near future.(6) This work calculated the steepness indices of major rivers in the northern Longmenshan region using 90-m-resolution SRTM-3 DEM, the steepness indices of small-size drainage basins on the north wall and hypsometric integrals of small-size drainage on both walls of the Qingchuan fault using 6-m-resolution DEMs extracted from ALOS-PRISM image pairs. The steepness indices of major rivers in the northern Longmenshan region decrease from the west to the east, and those of small-size drainage basins on the north wall of the Qingchuan fault show a similar decreasing gradient. Although the hypsometric integrals of small-size drainage on the north wall of the fault decreases from the southwest to the northeast which is similar to that of steepness indices, those on the south wall show uniform values. The numerical parameters of drainages indicate that the uplift rates of the Bikou block on the north wall of the Qingchuan fault show a feature of eastward decreasing, implying that it is the compressional stress on the west of the block from the Tibetan Plateau that has led to its deformation and uplift as well as slip along the Qingchuan fault. Yet, the block on the south wall of the Qingchuan fault is characterized by uniform uplift rates. Such a differential movement pattern along the Qingchuan fault could be resulted from the differential uplift of the hanging wall caused by the shortening deformation.(7) The calculation based on the geometry and known slip rates of faults in the northern Longmenshan region demonstrates that the shortening structures in the eastern terminus of the Kunlun fault is not extreme enough to accommodate all the crustal movement of the Bayan Har block, and the participation of the Qingchuan fault would match all the slip along the Kunlun fault well. Thus, it is suggested that the northeastern segment of the LTZ is playing a significant role accommodating the eastward movement of the Bayan Har block along the Kunlun fault which drives the right-laterally strike slip on the Qingchuan fault. The very well matched slip rates between the Kunlun fault and the faults in the northern Longmenshan imply that the eastward growth of the Bayan Har block as well as the Tibetan Plateau does not transfer beyond the northeastern segment of the LTZ. Namely, the northeastern segment of the LTZ is the terminal structure of the eastward movement of the Bayan Har block on the eastern margin of the Tibetan Plateau. |
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