A Study On The Lake Forming Environment And Tectonic Activity During The Late Quaternary In The Hetao Basin, Inner Mongolia Autonomous Region, China

Quantitative information that is used to infer climate changes and tectonic movement was derived from the Quaternary sedimentary records and surface topography of the Hetao basin, to demonstrate a long-term active characteristic of the basin and its boundary faults, which may provide some foundation data for strong earthquake prediction and the study of dynamic relationship between basins and ranges.The elongate Hetao fault-depression basin is a major earthquake belt in North China. Its active tectonics has been intensively studied. Previous studies have clearly revealed its active tectonic features. The most important point is that upper Pleistocene lacustrine strata, lake-bank- mesa, and alluvial-proluvial mesa were identified in the rising walls of the basin boundary faults with precise elevation data of these elevated topography. Such studies were used to deduce the displacements and slip rates of the boundary faults, forming an excellent basis for evaluation of vertical tectonic movement.However, the Quaternary tectonic evolution of the Hetao basin is not understood completely due to the lack of a thorough study of its Quaternary strata. This shortening also prevented us from getting a reasonable interpretation for the forming process of the Quaternary sedimentary material and micro-topography along the basin boundary, resulting in an unavoidable discrepancy in the estimation of the Quaternary displacements and slip rates along the piedmont faults. A fault slip rate is previously estimated according to the elevation difference between the elevated proluvial fan and paleo-lake-bank- mesa in the hanging wall of the fault and the top surface of lacustrine strata in the footwall. Generally, such a method is commonly acceptable for a normal fault. But both distribution and lithology of the Quaternary strata in the Hetao basin are widely variable. It is very hard to identify an obvious lithologic maker unit. Besides, the basin brink itself is a steep slope with an obvious elevation difference along the boundary fault. More important, precise ages for the Quaternary strata, especially for the lake-bank mesa are unavailable due to the limitation of dating techniques so far. Thus, the present estimations of the displacement and slip rate are problematic.In order to reveal the active features of the Hetao basin and its boundary fault system with higher reasonability, this thesis took the Quaternary lacustrine strata and boundary lake-bank mesa as the major research cut-in position. At first, it documents a general evolutionary feature of the Hetao basin and its boundary fault system; analyzed climate changes features and main controlling factors for the changes in the paleo-water surface during the Quaternary time. Then it focuses on the variable degree and pattern and controlling factors of the water-surface changes during the upper Pleistocene period, and tries to identify a paleo-elevation of the water surface as a reference. Based on the reference water surface, the elevations of the upper Pleistocene lacusrine strata and lake-bank mesas, which have the same geologic ages and were displaced by faults, are extracted to estimate a rational uplift rate of the hanging wall and the slip rate of the associated faults.After a thorough evaluation of previous studies, based on the data of drilling core records, field observations, and laboratory sample analyses, this thesis attempts to address much better the above-mentioned questions of the Hetao basin by means of multi-disciplinary methods including structural geology, Quaternary geology, geomorphology, stratigraphy, sedimentology, paleontology, paleo-climatology, and geo-chronology. The following conclusions are derived:...