Late Quaternary Slip Rates Of The Fodongmiao-Hongyazi Fault, Northern Margin Of Qilian Shan

The Hexi corridor foreland basin is located in the leading edge of northeastern Tibet plateau. Research of active Tectonics in this basin is crucial for current tectonic deformation of the plateau. Many active faults exist within the basin and surrounding, such as the Qilian Shan north-edge fault. Previous studies have suggested different vertical slip rates in this fault since late Pleistocene, eg. less than 1mm/a and 2.8mm/a. What on earth is the long-time slip rate of this fault? Whether such a rate is consistent with the short-term geological slip rate or geodetic rate? How the deformation of the plateau is absorbed by thrust fault zones on the northern margin of Qilian Shan and in the Hexi Corridor basin? This thesis attempted to address these issues, and it chose the Fodongmiao-Hongyazi(FH) fault at the central Qilian Shan as the target. Combining geomorphic mapping, topographic surveys of the deformed terraces, optically stimulated luminescence(OSL) dating, radiocarbon(14C) dating, and terrestrial cosmogenic nuclides dating, the fault slip rates at intermediate time scales were obtained.Firstly, the alluvial fans in the study area were divided into four stages based on the surface characteristics, altitudes, development features of alluvial fans, and drainage. Dating results in comparison with climate curve or events indicate the late middle-Pleistocene alluvial fans with ages of >190ka, 38±7ka and 15±3ka, formed during the transition from glacial to interglacial or interglacial periods. In addition, the FH fault is divided into three segments, roughly bounded by Hongshancun and Mayinghe based on fault geometry and structures. In the west and middle sections, some sites with developed offset landforms were chosen of the study on long-term fault slip rates.At the estuary of Hongshuiba river, ten terraces were identified, of which T5 terrace and those above were offset by the FH fault. Terrace T6 to T10 are confined to the hanging wall of the fault due to differential tectonic uplift. Using the SPOT-DEM data with 3~6m vertical precision and buroal depth correction, scarp heights of T9,T7 and T5 terraces were estimated to be ~236m, 91~100m and 17~20.5m, respectively. Combining the exposure age of terraces derived from 10 Be dating, which are 190 ka, 40±5ka and 15±3ka, fault vertical slip rates were determined of 1.3±0.1mm/a, 2.2±0.1mm/a and 1.3±0.1mm/a, respectively, with an average 1.6±0.3mm/a. In addition, a3 c fan was displaced at 3.2±0.5 m height, which was estimated of the age of 11.43±0.185 ka based on 14 C dating. These data yielded a vertical slip rate of 0.28±0.04mm/a.Terrace T1 to T3 terrace were all displaced by the fault at Shiyanjuan site. And scarp height of T3 is 4.8±0.5m. In the west wall of the gully, brick-red mudstone and sandstone of Baiyanghe group thrusts onto the Late Pleistocene gravels. The throw of the top surface of the bedrock is about 5.7m. The 10 Be and OSL dating provided constraints on the age of T3 terrace to be 14.83±0.45 ka, and the vertical slip rate of the fault was 0.3±0.1mm/a since T3 terraces was abandoned. The scarp heights of both T1 and T2 terraces are ~1m, indicating they experienced same seismic events after T1 was abandoned.Five terraces developed at the mouth of the Fengle river(T4-T0', old to young). They are mostly faulted except the lowest two terraces. Due to material of a3 fan accumulated in the footwalls of T3 and T4, fault scarp height of T4 and T3 in the east bank are higher than and those of west bank. So, the fault scarp heights in the east bank was used to represent the minimum scarp height of T3 and T4. The 10 Be depth profile in west T4 terrace yielded an exposure age of ~41.44 ka. With a displacement of 53~62m derived from SPOT-DEM data, this yielded a vertical slip rate of 1.4±0.2mm/a. Combining the exposing ages of 16.33±0.3ka and 18.24±0.3ka of western T3 terrace, exposure ages of 22.2±2.2ka of eastern T3 and scarp height of 18.5~21m, the vertical slip rate was estimated as 1.1±0.2mm/a. T2 terraces scarp height is ~5.4m. OSL and 14 C methods gave the age of 7.3±0.4ka and 6200~5995a BP, which led to a vertical slip rate of 0.77±0.08mm/a.Integrating research results above, this work obtained the vertical average slip rate of 1.6±0.3mm/a and 1.3±0.2mm/a for the western and middle sections of the FH fault. Since Holocene the vertical slip rate of this fault was ~0.8mm/a. Combining fault slip rates around Jiudong basin, the overall crustal shortening rate of the basin accounts for about 20% of the Qilian block shortening rate.The topographic profile along the Qilian Shan displays elevation in the middle section is the highest, and eastern Lenglongling has a slightly lower elevation, possibly indicating tectonic uplift in the middle section is the fastest. The average vertical slip rates of the faults in Northern Qilian Shan since the late Pleistocene show that a characteristic of larger in the middle and smaller at either end, consistent with that derived from geomorphology. That long-term vertical slip rates are larger than rock exhumation rate and denudation rates in catchments, more likely indicates the central part of Qilian Shan appears to be in a pre-steady state at present.