【作者】 李建彪；

【导师】 冉勇康； 郭文生；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2006， 博士

【摘要】 本文采取从河套盆地第四纪沉积记录和表面地形中筛选出第四纪气候变化、构造过程定量信息的方法,试图较深入地认识盆地及其边界断裂的长期活动特征,为进一步开展强震预测和盆-山动力学研究提供基础资料。河套断陷盆地是华北地区一条重要的地震构造带,活动构造研究程度较高。前人通过长期艰苦的工作取得了丰硕成果,已经比较清晰地论述了河套断陷盆地活动构造的基本特征。其中,最重要的是明确了盆地边缘断裂带上升盘存在晚更新世湖相地层、湖岸台地和冲洪积台地,获得了这些上升地貌面的高度分布,从而得到了一序列断层垂直位移量和位移速率,对评价该地区构造垂直差异活动奠定了很好的基础。但是,由于河套盆地第四纪地层还未得到全面系统的研究,故而影响到了对河套盆地第四纪构造演化的认识,进一步影响了人们对盆地边缘第四纪沉积物和微地貌形成过程的认识,从而不可避免地导致对山前断裂带晚第四纪不同阶段位移量和滑动速率在认识上的偏差。前人估算断层垂直滑动速率的主要依据是断层陡坎控制的上升洪积扇面高度和山前古湖岸台地与断层下降盘湖相地层顶面位差。显然,在没有其它因素影响的情况下,这种技术途径对正断层而言是可行和被普遍采用的。但由于第四纪地层分布不稳定,岩性变化极大,断裂两侧难以找到明显的“岩性标志层”;盆地边缘是一个陡的斜坡,在断裂两侧一定距离内存在位差;加上难以准确测定各沉积层的绝对年龄,特别是现有的测年技术还难以准确地获得湖岸台地面的形成年龄,从而可能使所得结果存在较大不确定性。为了更合理地认识河套盆地及其边界断裂的活动性,本文以盆地第四纪湖相地层和盆地边缘湖岸台地为研究的切入点,首先正确了解河套盆地及其边界断裂在第四纪时期的总体演化特征、第四纪气候变化特点和古湖水位变化的主要影响因素;然后,重点分析晚更新世古湖水位变化的影响因素、影响程度及变化规律,并力争获得某一时期的古湖水位海拔高程作为参考面;再以古湖水位为主线,通过综合分析盆地边缘晚更新世湖相地层和湖岸台地形成过程和分布规律,筛选出受后期活动正断层垂直运动抬升的相同年代的湖相地层和台地海拔高程,最终获取较为合理的断裂带上升盘的抬升速率及断层滑动速率。论文在充分应用前人工作成果的基础上,以盆地深钻孔岩芯编录、野外地质观察和样品测试结果等作为基本数据,通过构造地质学、第四纪地质学、地貌学、地层学、沉积学、(古)生物学、(古)气候学和年代学等多学科综合研究,取得了如下一些研究成果:1)对深钻孔岩芯综合进行岩石学、年代学和孢粉组合等的研究表明,河套盆地第四纪地层从下而上可划分为:下更新统、中更新统沟子板组、上更新统萨拉乌苏组和城川组、全新统,其中,本文新建了中更新统沟子板组,重新划分了上更新统。同时说明了河套盆地在第四纪的早更新世、中更新世早期、中更新世晚期、晚更新世4个时期存在古湖泊,也意味着黄河外流减少或停止以至成为内流河。2)定量恢复的古气候参数显示,年均温度(T)和年均降水量(R)曲线是同步的,说明古气候是以暖湿、冷干为主要形式的季风气候。早更新世1070~800kaB.P.,T与现代相当,R比现代多约170mm,是一个温和湿润期,也是剖面记录中最润湿的时期。中更新世524~ 192.2kaB.P.,大部分时间比现代稍湿,部分时段与现代相似,存在4次明显的冷干降温事件;晚更新世112.8~10.1kaB.P.,T为6℃, R为518mm, 7月平均温度20.9℃,也存在4次冷干降温事件,但降温幅度已经远不如中更新世强烈。这总共8次降温事件的时间间隔相对较短,强度从早到晚是逐渐减小的。3)河套盆地晚更新世的气候变化特征可以与中国黄土和深海氧同位素反映的气候特征对比,气候的起伏与全新世也较为接近。与现代气候相比,年均降水量相等或稍多,温度相近。综合分析河套盆地沉积史和古气候定量恢复参数,可以认为构造活动决定了河套古湖泊更多还原

【Abstract】 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:更多还原