Late Quaternary Activity Of Several Main Faults In East Section Of Eastern Tian Shan And Its Implication On Regional Deformation Mechanism

Central Asia is an excellent geological laboratory for the study of intracontinental deformation, and it has a profound influence on the understanding of the mechanism of intra-continental deformation. The Tianshan Mountains, which were originally the late Paleozoic ancient orogenic belt, were reactivated by the the the long distance effect of the collision between India and Eurasia since Cenozoic. Acting as a typical intra-continental orogenic belt, the deformation of the Tianshan Mountains is mainly concentrated on the front thrust faults and large strike slip faults, accomadating large amout of shortening. From west to East, GPS velocity field and active tectonics research have confirmed that the shortening rate of Tianshan gradually become smaller, and graded from 20mm/a in Kashi area of Southwest Tianshan to little in East Tianshan area, suggesting that deformation mechanism of Eastern Tianshan is different from Western Tianshan Mountains and Southwest Tianshan.The active tectonic research in Western Tianshan Mountains and Southwestern Tianshan is has been the research hotspot of geology circles for a long time, but the study of the eastern Tianshan, especially active tectonic and neo-tectonic, attaracted little attention and the study is sparse. From west to east, the eastern Tianshan mountain chain contains Bogda shan, Barkol tagh and Karlik tagh. It meets the Altai in the north and connects the Gobi Altai in the east, and defines the tectonic intersection zone of the Altai, Gobi Altai and Tianshan. More importantly, the WNW and NNW Altai dextral tectonic and the NE~E Gobi Altai sinistral tectonic convergent on it. The late Quaternary deformation characteristic of the eastern Tianshan, which means the mode of tectonic deformation since the late Quaternary, is not clear at present. As we know, in west of the eastern Tianshan, the Tianshan Mountains mainly accommodated the convergence of Indo-Euroasia plate by shortening, while in the north and east of the eastern Tianshan, the Altai and the Gobi Altai region tend to present a deformation mechanism of lateral strike-slip and rotation of crustal blocks. But what we don’t know is that the deformation mechanism in this tectonic transformation zone, the eastern Tianshan. We don’t know if there developed late Quaternary strike-slip tectonic in the eastern end of East Tianshan, which is widely distributed in Altai and Gobi Altai. To solve these two problems, we must first know the nature of the activity of the several main faults in the study region. Also we need to know the relationship between the fault activity and two M71/2 historical earthquakes in 1842 and 1914 in eastern Tianshan, the cycle of seismic activity and the tectonic dynamic mechanism reflected by the seismic activity. Furthermore, if we confirmed that there is strike slip deformation, and then we should know the relationship between compression deformation and strike slip deformation, answering about the mechanism of how these two deformations collectively adjust the tectonic evolution between Barkol basin and eastern Tianshan. Solving these problems, not only can provide quantitative data for the seismic risk mitigation of the eastern Tianshan, but also help to understand the present-day deformation mechanism in eastern Tianshan, and the tectonic transformation mechanism between eastern Tianshan tectonic domain, Altai and Gobi Altai tectonic domain.Based on the collation and analysis of the previous data, the high resolution satellite image interpretation, we applied detailed field geological survey and investigated several main faults geometry distribution characteristics in the eastern Tian Shan, determining the surface rupture zones of the two M71/2 historical earthquakes in 1842 and 1914. The trench excavation, coseismic displacement measurement and topography measurement were carried on the several important segments of the faults, examining the fault rupture behavior. Combined with the use of 14 C and OSL dating, the author restored the rupture history and paleoseismology of the faults since the late Pleistocene. In the same time, added with 10 Be exposure geomorphic surfaces dating, the author suggested the slip rate of faults since late Pleistocene. Finally, on the basis of fault slip rate and GPS velocity field, this article proposed a comprehensive analysis of the tectonic coupling mechanism between the strike-slip deformation and shorten deformation in east segment eastern Tianshan, suggesting a preliminary regional deformation mode. Through the above researches, the main understanding of this paper is as follows:1) Along the eastern margin of piedmont of Bogda shan, the Jianquanzi – Tulaiquan fault trends EW, showing an acute angle with regional principal compressive stress, and developing the obvious sinistral strike slip movement and producing several pull-apart basins along the fault trace. Topographic measurements show that the fault has a minimum displacement of about ~4m. In the east most of the fault, the fault connects with the Barkol tagh through Tulaiquan pull apart basin. The east-west length of this basin is about 7km long and north-south width is about 4.5km, generating a length to width ratio about 1.55, which is far below the 3 mature pull-apart basin ratio, indicating the basin is still in development. The Qp2 alluvial fan in the basin shows a cumulative offset of 1.2~1.5km which may represents the overall displacement of the whole fault system. Based on the cumulative displacement, the author e suggests a rough estimate of the slip rate of 1.5~2.3mm/a since Qp2.2) After extending into Barkol Shan, the Jianquanzi–Tulaiquan fault divided into South and North branches along the Barkol Shan boundary. The North branch was labeled as south Barkol basin fault. The western segment of it trends EW, showing clearly evidence of sinistral movement. There is about 15 km of the surface rupture zone developed along Xiongkuer, which can be confirmed as the result of the 1842 M71/2 Barkol historical earthquake. Trenching study on Xiongkuer segment revealed the latest four paleoseismic events, of which the oldest event(Event4) occurred in 4663BC~3839BC, showing that the fault at least experienced four plaeoearthquakes since 4663 B.C. and the average recurrence intervals of these events since 4663BC~3839BC is about 1420a~1626a. The fault activities follow quasi-periodic law. The sinistral gully displacement survey and statistics along the Xiongkuer surface rupture showed the movement of the fault follows the characteristic slip model, producing ~4m coseismic offset in each event, and the measured 16 meters of the accumulative displacement is likely represents four events since the gully’s formation. Accordingly, the author suggests a Holocene average sinistral strike slip rate 2.4~2.8mm/a in the western part of south Barkol basin fault, which is very close to the preliminary estimate of value 1.5~2.3mm/a from Tuolaiquan pull basin. Due to a angle change between the fault trend and the axis of regional principal compressive stress, the kinematics of the fault began to change in Luobaoquan, west of Xiongkuer. Thrust component motion makes fault display flower structure, in which north branch fault shows thrust movement, generating ~8m high fault scarps on the surface and the south main fault near the foothills still shows predominantly left-lateral movement.3) The east segment of the south Barkol basin fault strikes NW and perpendicular to regional principal compressive stress, making the fault moves at a low angle thrust near surface. In Hongshan section, trench study revealed the latest 3 events. Combined 14 C dating, this study confirmed the fault at least experienced one paleoseismic events since 9710 BP and the minimum average recurrence period is 3355 ±40a. Through paleo-seismic event analysis and balance profile restoration, the author suggests the average vertical displacement of the fault is 1.6±0.1m, while the average shortening is 1.7±0.4m. The ratio between vertical displacement and shortening is close to 1 and is close to the tangent of the main fault dip in the trench. According to the existing data, the minimum uplift rate of the fault is 0.15~0.17mm/a, and the minimum shortening rate is 0.13~0.22mm/a.4) The east segment of south Barkol basin fault extends into the Karlik tagh, known as the central Karlik fault. Compared with the east segment of south Barkol basin fault, the angle between fault and regional principal compressive stress is become smaller, making left lateral strike slip motion component is obvious. The analysis of landforms in the vicinity of Dangshenggou-Baiyangou shows that the accumulative sinistral displacement on this fault segment is about 1km. On south Yiwu basin fault, about 10 km surface rupture zone was identified. It offsets the latest floodplain deposition, and the author confirmed it as seismogenic tectonic of the 1914 M71/2.5) The south branch of the Jianquanzi-Tuolaiquan fault is north Hami basin fault, distributing along the piedmont of the Barkol tagh and striking NW about 200 km. It mainly shows thrust movement and its activity is most evident in the vicinity of Dewaili-Sidaogou. The trenches excavated on T2 geomorphic surface show movement of a typical low angle thrust fault, and fault dip ~30° near surface. Combined with OSL dating, the author proposes the average recurrence period of large earthquake about 5.6±1.3~9.5±1.1ka. The long-term minimum vertical slip rate is 0.08~0.13mm/a since ~20ka, which was obtained from the analysis of the comprehensive study trough strata analysis and the OSL dating of the paleo-soil layers in the trenches. According to cosmogenic nuclide exposure age and the fault scarp height of 5.3~8.2m on T2, the author suggests a long term vertical slip rate of 0.10~0.18mm/a since the ~50ka. Correspondingly, the shortening rate is 0.17~ 0.31mm/a, which is introduced by the fault angle dip of ~30° revealed in the trenches.6) Specifically, the slip characteristics of the fault in east section of eastern Tianshan correlated with the angle between the fault strike and the regional principal compressive. In the case of the angle is acute, the faults show strike slip movement. While in the case of the angle is high and near-perpendicular, the faults show thrust movement. The synchronous development of the strike slip tectonic and the compression deformation show that the east section of eastern Tianshan has the properties of the transpression. Strike slip rate(2.4~2.8mm/a) is far greater than the shortening rate, suggesting the strike slip tectonic and the compression deformation at least have the same tectonic role in the present-day deformation in east section of eastern Tianshan. The shortening deformation in mountain roots is not the only deformation mechanism in study region and the strike slip tectonic also plays as an important part in lateral deformation adjustment. Eastern Tianshan connected with the Altai-Gobi Altai tectonic domain through Gobi Tian Shan Fault System(GTSFS), and the lateral adjustment of the strike slip tectonic may be transferred to the east of Mongolia by it. The sinistral slip rate in Gobi Altai interior section has a ~1mm/a loss when compared with the rate of sinisitral slip in eastern Tian shan, which may be absorbed by the uplift and the shortening of the moutains in the process of passing along the GTSFS.