Geometric Pattern And Active Tectonics Of The Hexi Corridor And Its Adjacent Regions

Collision between Indian and Eurasian plates not only created Tibetan Plateau, "the roof of the world", but also posed significant impacts on climate and environments of western China and central Asia. Hexi Corridor consists of a series of western northwest-trending Cenozoic basins along range front of the Qilian Shan. Tectonic setting of the Hexi Corridor marks the northern margin of Tibetan Plateau where abundant active faults and historical earthquakes attests significant on-going tectonic deformation due to outward growth of the Tibetan Plateau. Tectonic deformation of the region is dominated by two major strike-slip faults bounding different deforming units, the Altyn Tagh fault and the Haiyuan fault. Other structures with different styles and senses of motion form complex pattern of active tectonic deformation. It is thus an ideal place to study active tectonic deformation to gain understandings of the processes of Tibetan Plateau’s northward growth. Key scientific questions regarding tectonic processes associated with outward growth of Tibetan Plateau includes: what is major process governing tectonic deformation of the northern margin of the Tibetan Plateau; what are relationships between strike-slip faulting and crustal shortening; how the slips on strike-slip fault are transferred into crustal shortening in other structures; what are slip rates along major faults in the region. Answers to these and other similar questions bear important implications for understanding tectonic processes associated with the outward growth of the Tibetan Plateau.Based on systematic studies on the late Quaternary slip rates along major active faults in Hexi Corridor and adjacent regions in northern Tibet, this paper provides an integrated information to show geometric pattern and kinematics features of active tectonic deformation in northern margin of the Tibetan Plateau. In combining active faulting studies with GPS measurements, we also discuss models of the late Cenozoic deformation and its dynamic mechanism of the northern margin of Tibetan Plateau especially the Qilian Shan and Hexi Corridor. Main conclusions are drawn as follows:The South Heli Shan Fault is the north boundary of the Hexi Corridor basin, and it is typical of transpression feature with predominant reverse faulting and minor left-slip near its ends. The topographic features of the Heli Shan correlate to slips on the fault suggesting active fault controls on mountainous geomorphology. The South Heli Shan Fault can be divided into three segments based on its geometry. Slip of the individual segment is of arch shape distribution, and the slip distribution of the whole fault zone depicts an asymmetric arch shape as eastern side with higher rates and west side with lower rates. Average slip rates are (0.34±0.10)mm/a for the Houertou segment, (0.14±0.10)mm/a for the Wutongjing region, (0.24±0.06)mm/a for central segment, and (0.18±0.07)mm/a for western segment. Left-lateral movements are only observed at local sites near two terminals of the South Heli Shan Fault.Trenching and field survey along the surface ruptures, together historic earthquake investigation indicate that there might be three strong earthquakes occurred along the South Heli Shan Fault. The first event ruptured along the whole fault about 5000a ago, and the other two occurred during the documented history, in 180 A.D and 756 A.D. respectively. More than 60km long surface ruptures was associated with the M71/2 event in 180 A.D. and 20-30km ruptures might have formed during the M 7 event in 756 A.D. The behavior of paleoearthquake activity does not show quasi-periodic pattern.Tectonic deformation and kinematics pattern in the northern Hexi Corridor is characterized by thrust fault related uplift with southward movement and westward tilting, that resulted from the outward growth of the Tibet Plateau toward the relative tectonic stable Alashan block. Topographic profiles constructed from DEM perpendicular to the Heli Shan show that different fault segments are associated with different geomorphology. The primary geomorphic feature is the tilting towards Hexi Corridor basin along the fault. Along the strike of Heli Shan mountain, topographic profile shows westward tilting of geomorphology. This kind of east to west tilting corroborates larger displacements in eastern part of the fault and less offset in western part of the fault. Thus, the topographic expression is representative of the activity of the Heli Shan fault. Slip distribution along the fault and strong earthquake activity suggest the recurrent pattern of strong earthquake may be a combination of characteristic and temporal clustering.Active fault studies in the northern and interior of the Hexi Corridor basin indicate that the Jiayuguan-Heishan fault is a high angle reverse fault with late Pleistocene activity. The slip rate is (0.26±0.06) mm/a since late Pleistocene. The Jinta Nan Shan fault is a Holocene active fault is with a slip rate of (0.22±0.05) mm/a since late Pleistocene. The Jiayuguan-Wenshu Shan fault, a transverse fault obliquely cuts the Hexi Corridor to separate the Jiuquan basin in the east and the Yumen basin in the west. The fault appears to be high-angle reverse fault with hangingwall tilting to the west, and observed slip rate is (0.30±0.05)mm/a. As for the Mushaoliang fault and Dachechang-Ayouqi fault along the north Hexi Corridor, geomorphological pattern and remote sensing interpretation suggest that these faults are of high-angle reverse slipping from north to south.Kinematics of tectonic deformation along the northern Qilian Shan fault shows that the western segment of the Fodongmiao-Hongaizi fault is a premary reverse fault, and its eastern segment is associated with left-lateral components. This fault presents obvious evidence of Holocene activity, and remnant of surface rupture of historical earthquake can still be found in many places. The slip rate of reverse faulting is (0.41±0.09) mm/a and left lateral slip rate is (1.20±0.15) mm/a. Early Holocene geomorphic surface is offset by the northern Yumu Shan fault with slip rate (0.55±0.15) mm/a of reverse faulting and the left lateral slip rate (0.95±0.11) mm/a. Out investigation show that the so called "Luotuocheng fault scarp " is in fact remnants of ancient irrigation canal rather than an active fault. Tectonic deformation of the Yumu Shan is a consequence of the northward growth of the Qilian Shan, and the symmetric topographic shape of Yumu Shan suggests that slip distribution along the Yumu Shan fault is also symmetric. Additional study on slip rate of the Gulang fault shows that Holocene dip-slip rate of the Huangcheng-Shuangta fault is (0.39±0.04) mm/a and left-slip rate of the Tianqiaogou-Huangyangchuan fault is (2.66±0.38) mm/a. Survey on west segment of the Changma fault near west Qilian Shan indicates that late Quaternary throw rate is (0.14±0.02) mm/a and left lateral slip rate is (1.17±0.04) mm/a, which is much less than previous reported 3.3-4.3mm/a.GPS measurements and late Quaternary fault slip rates reveal the kinematic pattern of outward growth of the northern margin of the Tibetan Plateau. Base on the geologic data and GPS velocity slip rates of major boundary faults are less than 10 mm/a including the Altyn-Tagh fault and Haiyuan- Qilianshan fault. The distributions of slip rates along the two major faults show constant slip rate along their middle portion and decrease toward their ends. For example, the left-lateral slip rates on the central segment of Altyn-Tagh Fault appear to be in the range of 8-12 mm/a, but decreases eastward to only 1-2 mm/a near 97°E. On the Haiyuan-Qilian Shan fault, slip rates are 1-2 mm/a in its western segment, and increase to 4-5mm/a in its eastern and middle segments, and then decrease to 1-3 mm/a near its eastern end near the Liupan Shan. This kind of slip distribution suggests that almost all motion along a strike-slip fault is accommodated by crustal shortening or convergence near the ends of the strike-slip fault. The crustal shortening across the Qilian Shan absorbs strike-slip along the Altyn Tagh fault. The convergence in Liupan Shan accommodates left-lateral slip along the Qilian-Haiyuan fault.Northern Tibetan Plateau includes the NNE-teending Altyn Tagh Fault, NWW-striking Haiyuan-Qilian Shan Fault and uplifted Qilian Shan mountain belt, Hexi Corridor basin and Alashan block. Tectonic processes in the region can be described as combination of three structural styles. First, outward growth of the northern Tibetan Plateau cause west northwest trending reverse faulting and folding in the Qilian Shan and Hexi Corridor, and the deformation might have migrated to north of the Hexi Corridor caused reverse faulting and earthquake rupturing in Heli Shan and Longshou Shan. Second, left-lateral strike slip on the main boundary faults (The Altyn Tagh fault and the Qilian-Haiyuan fault) cause reverse faulting, folding and uplifting of mountains to accommodate the lateral motions. And third, internal deformation within the Qilian Shan, Hexi Corridor, and even with the interior of tectonically stable Alashan Block. These three tectonic deformations contribute to outward and upward growth of the northern margin of Tibetan Plateau. Our studies show that tectonic deformation occurred mainly within the northern Tibetan Plateau, and that the rule of strike slip faulting has been reconciling differences of crustal shortening or crustal thickening rather than extruding crustal blocks out of the plateau interior.