Active Tectonics In The Southern Gobi-alashan Block And Its Response To The Interactions Of The Adjacent Crustal Blocks

The Gobi Alashan block is located to the north of Tibetan Plateau, which was dominated by compressional deformation in late Cenozoic time, and to the west of the Ordos block, which is bounded by extensional fault systems. It was considered to be a relatively stable block in the foreland of the Tibetan Plateau because of the low topographic relief, limited seismicity, and few active faults in the Gobi Alashan region. Recently, workers have begun to recognize that fault systems within the Gobi Alashan block may indeed be active, albeit at low strain rates. The information about the geometry and kinematics of active deformation within the Gobi Alashan block provide insight into the information of whether or not the northeastern Tibetan Plateau, a block experiencing northeastward extrusion during the late Cenozoic, has extruded into the Gobi Alashan block.In this study, combined interpretation of high-resolution satellite imagery, geomorphic mapping along the faults, and shallow trench investigations to determine fault geometry, we present geometry, kinematics, slip rates, and linkage relations of the active faults in the southern Gobi Alashan block, such as the Taohualashan fault, Ayouqi fault, Yabrai fault, and the eastern Bayanwulashan fault. Combining our data and previous studies on the Qilian Shan, the Hexi Corridor, and the western Ordos block, we discuss the significance of the deformation pattern in the Gobi Alashan block and what kind of dynamics drives the deformation in the Alashan block, by the Tibetan Plateau or the Ordos Block.The Taohualashan fault lying at the northwestern corner of the Longshou Shan is about 30 km. This fault was identified to be an active left-lateral strike-slip fault in the Late Quaternary and consists of two segments. The western segment of the fault stretches 20 km in west-eastern direction and characterized by left-lateral displacement. In contrast, the eastern one is about 9 km in direction of NWW-SEE along the strike and appears to be sinistral strike-slip with reverse component. The reverse component increased toward the eastern tip of the fault. Dating ages(OSL and 10Be) of displaced alluvial fans yield a average left-lateral strike-slip rate to be 0.14 ± 0.01~0.93 ± 0.11 mm/yr,and a average reverse slip rate to be 0.1~0.3 mm/yr during the late Quaternary.The Ayouqi fault zone consists of 7 secondary faults with different strikes, lengths, senses of motion. Among of them, the major fault is lying in the inner-mountain valley to the east of the Ayouqi county town. This fault is about 30 km in east-western direction, which is primarily a left-lateral strike-slip fault. The most recent earthquake occurred between 11.05 ± 0.52 ka and 4.06 ± 0.29 ka at the major fault. By the geological and geomorphic mapping, we conclude that the Ayouqi fault zone has not connected to the Taohualashan fault and the Yabrai fault in the surface. The dips of folded deposited layers and geometries of active faults suggest that the tectonic strain field is in NE-SW direction in the Ayouqi region, which is compatible with the northeastward extrusion of the Tibetan Plateau.Geomorphic mapping of the active fault trace and trench investigations reveal that the Yabrai fault is composed of tree segments of varying fault strike, but for which the sense of motion, scarp height, and slip history appear to be kinematically compatible along the fault. Displaced Holocene and late Pleistocene alluvial deposits indicate that the southwestern segment is characterized by oblique-normal displacement with a minor sinistral component, whereas the middle segment appears to exhibit nearly dip-slip normal displacement. In contrast, slip along the northeastern segment appears to be primarily sinistral strike-slip with a minor reverse component. Geomorphically fresh fault scarps are developed within late Pleistocene-Holocene alluvial fans and terraces along the southwestern and northeastern segments, whereas the middle segment of the fault defines the bedrock-alluvial contact along the range front. The 10 Be exposure ages of displaced alluvial fans along the southwestern segment yield a throw rate of ~0.1 mm/yr over late Pleistocene time. Lateral slip rates along the northeastern fault segment range between 0.23 ± 0.02 mm/yr and 0.78 ± 0.12 mm/yr. Regionally, the orientation and sense of motion along the Yabrai fault are consistent with NE-SW shortening.In the east of the southern Gobi-Alashan block, few active faults were identified, including Alatanaobao fault, the northern Kebuerhai fault, Bayannuorigong fault, and the eastern Bayanwulashan fault. These faults are not connected systematically with active evidence at several points. By recognition of the dikes at both side of the Kebuerhai-Bayannuorigong tectonic zone, we suggest that it appears no large left-lateral strike-slip motion occurred along this zone previously. The eastern Bawulashan fault strikes NNE-SSW and extends for ~40 km along the eastern range front, which was inferred to be a normal fault during late Quaternary by Song and Cao(1994). However, we identified some mostly pure later-strike slip evidence along the southern segment of the eastern Bayanwulashan fault.Combined with the study on active faults in the Gobi-Alashan block and adjacent regions including the Hexi Corridor, Qilian Shan, and western Ordos block, we concluded that the deformation in the Gobi-Alashan block is incompatible with the extension around the Ordos block, and we infer that the geometry of distributed faults and different sensed of motion at each faults could be driven from the northeastward oblique extrusion of the Tibetan Plateau.