Late Quaternary Activity Of The East-Northeastern Trending Faults In The Southwestern Tianshan And Their Role In The Tectonic Deformation Of The Tianshan Mountains

The Tianshan Mountains are the continental orogenic belt far away from the plate boundary,which is characterized by serious deformation and numerous strong earthquakes. The crustal shortening rate is approximately 20 mm/yr across the western Tianshan, about half of the convergence rate between the India and the Eurasian plates. How the deformation is obsorbed by the Tianshan Mountains, how the Tianshan deformed and what is the deformation pattern within Tianshan region?These critical issues are still in controversal. Three main tectonic belts develop in the Tianshan mountains. The most significant is the reverse faults which are located in the northern and southern area of the piedmont and parallel with the strike of the mountains. In addition, a series of NW trending right-lateral strike-slip faults and NEE trending left-lateral strike-slip faults are developed in the interior of the mountain. These two groups of faults control the Cenozoic tectonic deformation of the Tianshan Mountains.At present, the late Quaternary deformation characteristics and slip rate of the piedmont reverse fault system were well studied. There are also some quantitative data on the late Quaternary active features of the NW trending right-lateral strike-slip faults. But of the late Quaternary deforamation of the NEE trending fault are rarely reported. In this paper, we focus on the NEE trending faults to quantitatively study its late Quaternary activity by obtaining the kinematic character, slip rate and paleoearthquake features of the faults.. We also complement the geometry and kinematics of the faults within Tianshan mountains, by reviewing and re-investigating the deformation features of the other faults.. Then combined with previous research data, seismic activity and GPS data, we discuss the role of these faults in the deformation of Tianshan. Finally, we study the tectonic deformation patterns of Tianshan Mountains and their relationship with the occurrence of strong earthquakes.The main results of this paper are as follows:There are multistage late Quaternary geomorphic surfaces in the Aheqi valley along the Maidan Fault. By OSL, 10 Be exposure age and 14 C dating, we get the terrace ages of the Yushanguxi river. We suggest that the terraces formed during the transition stage of the glacial and interglacial period. The average incision rate is consistent with the fault slip rate before ~20ka,which indicated that the river incision was due to the tectonic uplifting by the Mandan Fault. After~20ka, the incision rate of the river increased. At the middle Holocene, the river incision rate even reached ~12mm/a, which was far greater than the fault slip rate. This phenomenon indicates that the climate plays major role in the river incision since the late Pleistocene. The rapid increase since the Holocene is likely to be caused by the rapid climatic fluctuations.Maidan fault is a thrust with left-lateral component, which is active during Holocene. By measurement the deformation and dating of the late Quaternary geomorphic surfaces, we obatinedthe thrust slip rate of the fault is 1.24 ± 0.20mm/a, and the sinistral strike slip rate is 1.74 ± 0.61mm/a. There are several paleoearthquake events occurred on the Maidan fault during the late Quaternary period. The Maidan fault is the root fault of the Kalpin nappe structure.Paleoearthquake studies show that serveal paleoseismic events occurred since late Quaternary on the frontal fault of the Kalpin nappe. Our studies revealed that on the Maidan Fault, the latest events occurred after 1.76 ka, and the average recurrence interval of paleoearthquakes is3370~4265a. The timing of paleoearthquakes on the root fault and frontal fault was close to each other. Although we can not determine whether the latest paleoearthquake both ruptured the Maidan and Kalpintag faults, but there are two possibilities. First, the 1.76 ka paleoearthquake is one event both ruptured the Maidan and Kalpintag faults. Second, there are two events rupture the Maidan and Kalpintag faults, respectively, which both occurred after ~1.7 ka, suggesting that the strong earthquake activity is clustering in the Kalpin area.The Holocene activity of Maidan fault indicates that the deformation characteristics of the Kalpin nappe structure is different with the other nappe structures. The root and frontal faults of the Kalpin nappe are all still active, indicating that the new activity of the fault is not completely migrated to the frontal belt, which may be an out-of-sequence, or a non-sequence thrust system. In the Southwestern Tianshan area, the sinistral strike slip mainly occurred in the root of the Kalpin nappe, and the thrust movement mainly occurred on the frontal belt. GPS data show that in the western regions of the Kalpin nappe, the Maidan fault did not completely obsorbed the strain, part of strain transferred to the frontal reverse fault- anticline belt through the detachment. In the eastern regions of the Kalpin nappe, strain are mainly obsorbed by the Maidan fault. The frontal tectonic belts have no obvious deformation during interseismic period, which is the reason that the microearthquake activity is significant differences between the west and east part of the Kalpin nappe structure.There are two other NEE trending faults in the southwestern Tianshan Mountains: the Nalati and Kemin faults. The thurst rate of the Nalati fault is about 2.1mm/a, and the left-lateral strike-slip rate is 2.5mm/a. The Kemin fault is a left-lateralstrike-slip fault, the slip rate is about1.5 mm/a since late Quaternary. The total sinistral slip rate of the Maidan, Nalati and Kemin fault is about 6 mm/a, which is consistent with the left lateral strike slip movement caused by the oblique subduction between the Tarim and Tianshan block. In Southwest Tianshan area, the shortening deformation distributes both in the interior mountain and piedmont areas, but the lateral shear only occurs in the interior mountain. The Southwest Tianshan shows a positive flower structure. The deformation is characterized by the shortening and thickening of the crust on the north and south sides of the Southwest Tianshan, and strike slip shearing in the interior mountain.There are two groups of strike-slip faults in the Tianshan region, one is left-lateral strike-slip faults trending NEE direction, the other is right-lateral strike-slip faults trending NWW – NWdirection. These two groups of faults jointly regulate the shear movement in the internal mountain.The high-angule strike-slip faults and the low-angle thrust faults shows the deformation features of the Tianshan region. The compressional deformation is mainly distributed in the piedmont area,and strike-slip component occurs in the interior mountain. Although the deformation of the Tianshan is characterized by eastward decrteasing, the structural deformation styles of the eastern and western sides are basically symmetrical. In the section, the entire Tianshan formed a large scale positive flowe rstructure.Due tothe northward extrusion of the Tarim block, Southwestern Tianshan area trends NEE,however, the eastern segment of Southern Tianshan is trending NWW, and the boundary faults of the Tarim Basin and the South Tianshan Mountains are strikeing EW direction. The three gourps of tectonic belts constitute a "triangular wedge ", which is penetrating northward. The Southwestern Tianshan is a large-scale sinistral shear zone, the eastern segment of Southern Tianshan is a dextral shear zone, and the boundary fault of the Tarim and Southern Tianshan is a thrust fault zone. In the western segment of Northern Tianshan the tectonic line is overall NW –NWW direction, and the tectonic line is NEE direction in the eastern segment. The two groups of tectonic belts constitute a inverted "triangular wedge" with the boundary fault of the Junggar and Nouthern Tianshan. The Western Tianshan is a large- scale dextral shear zone, and the eastern segment of Nouthern Tianshan is a sinistral shear zone. The thrust fault of the Junggar and Nouthern Tianshan consists the bottom margin of the "triangular wedge", which penertrading southward.Under NS direction comression, the Tianshan Mountains is dominated by compressive deformation. A series of strike-slip faults in the interior mountain show that there is a certain lateral extrusion in the direction of the EW. These strike slip faults regulate the difference of compression deformation in different regions of the Tianshan Mountains. Geological and GPS data confirmed that the amount of thrust movement is greater than the strike slip component. The tectonic deformation of the Tianshan Mountains is dominated by the crustal shortening and thickening, and the lateral extrusion is much smaller than the NS shortening rate of 18 mm/yr.