Sedimentary Responses To The Cenozoic Tectonic Evolution Of The Northeastern Corner Of The Tibetan Plateau

The collision of the India-Eurasian not only created the Himalayan orogenic belt, but also caused a ~2000 km wide intra-continental deformation north of the collision zone that has subsequently uplifted to form the Tibetan Plateau referred as to"the roof of the world". The uplift of the Tibetan Plateau posed significantly impacts on the tectonic framework of Eurasian continent, environments of the central Asia and global climate change. In addition, the Tibetan Plateau also provides a natural laboratory for studying the dynamics of collision between continent and continent, intra-continental deformation, and interaction between tectonics and climate change. The northeastern margin of the Tibetan Plateau is defined by the Kunlun fault in the south, the Altyn Tagh fault to the west and the Qilianshan Haiyuan faults to the north. This region is the frontier of the northeastward expanding Tibetan Plateau characterized by left-lateral strike slip faulting and northeastward crustal shortening. As the newest portion of the plateau, the region has been undergone wide-spread Cenozoic tectonic deformation. As a result, a number of arcuate basin and range pairs are formed with west northwest trending in the west and gradually becoming north-sjouth orientation eastward. The arcuate ranges are tectonically controlled by high-angle reverse faults with various amounts of left-lateral strike-slip components. The materials eroded off the arcuate mountains are deposited within the adjacent basins. Thus sediments preserved in the basin documented valuable information of the tectonic deformation, mountain uplift and environmental changes during the process of basin formation. Therefore, according to the basin and range coupling relationship, the studies of sedimentary evolution and tectonic deformation in the basins adjacent to the ranges are important approaches to understand processes of uplifting and outward growing of the Tibetan Plateau .The Cenozoic sediments well crop out in the south part of Ningxia Hui Autonomous Region which is referred as to southern Ningxia basin, which is located in easternmost part of the Qilian Shan-Haiyuan fault, the present-day boundary fault of northeastern margin of Tibetan Plateau. The southern Ningxia basin is a triangular basin with an area of ~10,000 km2 situated in the transition zone between a region of the northeastern margin of the Tibetan Plateau characterized by Cenozoic shortening and a region of Cenozoic extension surrounding the Ordos plateau. The elevation of the southern Ningxia basin drops irregularly from the 2400 m in the southwest near the Liupan Shan to 1600 m along the Huang He (Yellow river) in the north. As the southwest boundary fault, the Haiyuan-Liupan Shan arcuate fault separated the southern Ningxia basin from the Longzhong basin in the south. Within the basin, a series of the arcuate fault zones (Tianjing Shan fault zone, Yanton Shan fault zone and Luo Shan-Niushou Shan fault zone) cut the southern Ningxia basin from the basement to the surface and consist of, together with the Haiyuan-Liupan Shan fault zone, the arcuate basin and range system in the northeastern margin of the Tibetan Plateau. In the process of the activation of these fault zones during the Cenozoic time, thick sedimentary sequence was accumulated to record information related to the geometry and kinematics of the faults in the southern Ningxia basin. Though analysis of the geometry, tectonic setting and sedimentary evolution of the basin, we would be able to reveal the characters, processes and kinematics of the above mentioned fault zones, and to reconstruct the tectonic deformation and uplift history of the northeastern margin of Tibetan Plateau.This study focuses on the southern Ningxia basin. Based on the previous works, including field works from 2007 to 2010 and measurements in the laboratory, we analyzed the paleomagnetostratigraphy, depositional characteristics, sedimentary facies distribution, sedimentary basin evolution, and tectonic deformation. We summarized the primary conclusions and drew them as follows:Along the best exposed Cenozoic section (Sikouzi section in the southern basin margin) paleo-magnetic samples are collected and analyzed to build the temporal series of the Cenozoic sediments in the basin. Constrained by newly mammalian fossils directly collected in the Sikouzi section at a depth of 2010 m, the age of the section is estimated from ~29 Myr to 0.5 Myr. From the base to the top, the section is subdivided into five lithostratigraphic units: the lowest Sikouzi Formation is dated from ~29 to 25.3 Myr, and Qingshuiying Formation is estimated at 23.8-16.7 Myr. Almost a 1.5 Myr hiatus existed between Sikouzi Formation and Qingshuiying Formation. The Honliugou Formation is deposited between 16.7 Myr and 5.4 Myr and the Ganhegou Formation is estimated at 5.4-2.58 Myr. The upper-most Quaternary deposits maybe continue to 0.5 Ma or even younger.The south Ningxia basin initiated as an extensional subsidence which controlled by the arcuate Tianjin Shan fault and Yanton Shan fault in the early Tertiary. The isopach pattern of early Tertiary E3s is restored based on the measured stratigraphic sections, boreholes, and seismic-reflection profiles suggesting that the thickest Sikouzi Formation is more than 800 m near the TJSF and gradually thins southwestward. Another similar southwestward thinning depocenter is bounded by the Yantonshan fault (YTSF) in the north basin margin. Considering the southwest-dipping of TJSF and YTSF (Zhang et al., 1990; 1991), the southwestward thinning Sikouzi Formation in the hanging wall of Tianjing Shan fault and Yanton Shan fault indicate that they are probably the extensional faults when they reactivated in Oligocene time. The data that suggest initiation of basin subsidence and early basin filling was linked to extensional slip on basin-bounding faults directly contradict models for early Tertiary contractional deformation in the region. Thus, the kinematics of this period of deformation may closely relate to the motivation of Ordos block and suggest no link to deformation driven by the Indo-Asian collision. This reference is also supported by the changes of grain-size, sedimentary facies and paleocurrents from clast imbrication and cross-stratification in the Sikouzi Formation.The southern Ningxia basin expanded coeval with the deposition of Qingshuiying Formation. The basin's depositional area extended southwestward across the Haiyun-Liupan Shan fault connecting with Longzhong basin and northeastward overlying Tianjin shan fault and Yantong Shan fault to joint with Yinchuan graben at this stage (23.8-16.7 Myr). Fine-grained deposits comprise the late Oligocene-early Miocene Qingshuiying Formation and no basin-marginal facies association, such as alluvial fan system, exposed in the present-day basin margins. Depositional environments are dominated by playa lake and shallow lacustrine from the northern to the southern basin margins suggesting a broad basin appearance in the Liupan Shan area and its adjacent region. Considering the sedimentary facies distribution and inactivation of border faults coeval with the Qingshuiying formation, we propose that the main reason caused expansion of Sikouiz basin in the late Oligocene-Miocene is climate change, which maybe also trigger the pseudo-unconformity contact between the Sikouzi Formation and Qingshuiying Formation.In a manner similar to the Qingshuiying Formation, the lower and middle subunits of Honliugou Formation (16.7-10.5 Myr) also are dominated by fine-grained sediments. The obvious change of sedimentary environment occurred at about 10.5 Myr. The sedimentary facies evolved from earlier shallow lacustrine into delta-fan and braid river environments suggesting that ranges around the basin initiated to uplift and the depositional area of the southern Ningxia basin gradually reduced after 10.5 Myr. At the same time, accumulation rates pulsed at ~10.5 Myr from ~9 cm/kyr to ~13 cm/kyr. Collectively, the sedimentary environment change and increase in accumulation rate at ~10.5 Myr probably respond to northeastward thrust of the Haiyuan fault and Liupan Shan fault.The isopach pattern of the Honliugou Formation show that more than 800 m thick Honliugou Formaiton is preserved adjacent to the Haiyuan-Liupan Shan fault, and the thickness of the Honliugou Formation gradually thins to the northeastward with less than 200 m near the Tongxi county. This phenomenon implies a possible textural subsidence due to thrusting of Haiyuan-Liupan Shan fault driven by the outward growth of the Tibetan Plateau. Therefore, the reactivation of northeastward thrusting of Haiyuan fault and Liupan Shan fault and associated uplift of Liupan Shan, Nanhua Shan and Xihua Shan probably suggest that the deformation related to the collision of Indo-Eurasia has reached the northeast corner of Tibetan Plateau at about 10.5 Myr. And this region started to became the newest component of the Tibetan Plateau in late Miocene.Depositional area of the southern Ningxia basin distinctly shrank in Pliocene time, when the Ganhegou formation was deposited in a narrow area paralleling the Haiyuan fault and the Liupan Shan fault. The maximum observable thickness of the Ganhegou Formation is > 600 m in the southwest part of the Haiyuan-Liupan Shan fault. To the north, the Ganhegou Formation is absent in the region near the Tianjin Shan fault and Yanton Shan fault. Further to the north, the Ganhegou Formation corps out in the west flank of Niushou Shan. This isopach pattern suggests that the southern Ningxia basin is a typical foreland basin with foredeep, forebulge and back-bulge. The Ganhegou Formation is characterized by molasse deposits indicating that the Ningxia basin was gradually dying out coeval with the deposition of Ganhegou Formation.Based on the studies of the basin evolution and tectonic deformation, the southern Ningxia basin initiated as extensional subsidence in the late Oligocene and became a widespread depression from early to late Miocene. Flexural subsidence associated with northeastward thrusting of Haiyuan-Liupan Shan fault has been developed since about ~10.5 Myr. Tectonic studies show that Tianjin Shan fault displaces the pre-Tertiary sediments over the Quaternary conglomerate suggesting that the reactivation of Tianjing Shan fault started in Quaternary. Although the onset of northeastward thrust along the Yanton Shan fault is unknown, it is clear that the reactivation of Yantong Shan is later than that on the Tianjin Shan fault. All of the data imply that the deformation progressively propagated from the southwest to the northern in the northeastern corner of the Tibetan Plateau.Growth strata are syntectonic sediments which are closely linked to folding and faulting in the foreland basin, and, thus, they can provide precise information on tectonic and depositional interaction. In the Sikouzi section, the age of inception of growth strata is at ~5.4 Ma, deformation apparently continued into the Pleistocene. The implication of this set of growth strata is that the sediments exposed along the Sikouzi section were subjected to syn-sedimentary deformation, which is closely related with the emergence of Madong Shan folds at ~5.4 Ma. Considering that the Madong Shan is located at the east tip of Haiyuan fault, we propose that the deformation in the Madong Shan is caused by the transfer of the left-slip displacement along the Haiyuan fault to shortening in the Madong Shan area. Thus, the left-slip along the Haiyuan fault may initiate at ~5.4 Ma and continue to the present-day.Following the respond of sediments to tectonic activation in the southern Ningxia basin, all of fault zones within the basin have been undergone two major deformational phases since late Miocene. The northeastward thrusting seems to be the first phase of the deformation, which is followed by left-lateral strike-slip on the east-west parts of the arcuate fault zones and the displacement on the left-lateral strike-slip fault zones were transferred to crustal shortening in the north-south trending parts of fault zones. According to the kinematics and ages of fault activations, a northeastward propagated model is propose to describe the deformational style in the northeastern corner of the Tibetan Plateau.