区分晚第四纪断层作用驱动的和气候引起的流水下切——以桑干河大同盆地河段为例

DISTINCTION BETWEEN LATE QUATERNARY FLUVIAL INCISION INDUCED BY FAULTING AND BY CLIMATE: A CASE STUDY OF THE SANGGAN RIVER

针对晚第四纪期间 ,华北大同盆地内桑干河河道相继受到玄武岩涌入、沿河谷断层作用和气候波动等外来变量的扰动 ,我们提出一个概念模型以区分断层作用驱动的和气候引起的流水下切。如果由玄武岩阻塞作用所造成的基准面抬升河段有足够的河流功率进行垂直下切 ,并等于基准面抬升量的话 ,那么 ,该河段河床高度减小 ,其比降相对于邻近上游河段会降低。研究区盛行的温湿气候条件有利于长期提供充足水流 ,维持基准面抬升河段的垂直下切。我们利用平衡剖面分析 ,从量值上把沿河谷断层作用驱动的流水下切从气候引起的流水下切中区分出来 ,并通过对河流阶地记录的流水作用历史的复原 ,进一步区分由断层作用驱动的和由气候引起的流水下切事件。同时 ,提出掘蚀作用。

In consideration of the disturbances of the Datong Basin reach of the Sanggan River during late Qua-ternary time caused by external variables,such as the emplacement of basalts,along-valley faulting pro-cesses and climate changes,a conceptual model is developed for distinguishing fluvial incision induced by faulting from that induced by climate.Provided that lava blocking causes the rise of base-level of a river reach,so that stream flow of the reach has sufficient power to incise vertically at an amount equal to the base-level rise,then the bed altitude of the reach will decrease and consequently the gradient of the reach relative to the next upstream reach will also be reduced.Therefore,a supply of flowwith sufficient energy over a long period of time is of great importance to maintain the vertical incision of the reach with rising base-level.Under the prevailing warmer and wetter climate condition during the late Quaternary period in the study area,the establishment of backwater condition in the next upstream reach due to basalt blocking resulted in quick fluvial incision in the lava dam reach;on the other hand,fluvial incision induced by a-long-valley faulting led the lava dam upstream reach to attaining and maintaining over a long period of time a graded profile,which was able to provide the flowwith sufficient discharges but relatively less sediments for sustaining vertical incision in the lava dam downstream reach.The distinction between the fluvial inci-sion induced by faulting and by climate was made both in quantity and in event.Equilibrium profile analy-ses prove that displacement produced by along-valley faulting is equal to fluvial incision induced by fault-ing,allowing us to distinguish fluvial incision induced by faulting from that induced by climate in quantity.Meanwhile,the fluvial incision events caused by faulting and by climate were further distinguished in terms of the reconstruction of the stream operation history recorded by stream terraces.The results indicate that the post-basalt fluvial incision induced by faulting is slightly predominant in the lava dam upstream reach;in contrary,the post-basalt fluvial incision induced by climate plays a leading role in the lava dam reach.It is suggested that the synchronous landforms eventually produced by long-term effects of the adjustment of stream operation in response to the disturbances caused by external variables,can be apparently regarded as the basis for correlating the fluvial incision with aggradation events and for reconstructing the evolution-ary history of the stream longitudinal profile in the studied reach.The principal erosion processes in the la-va dam reach might have been implemented by knickpoint migration as well as quarrying and abrasion of basalt controlled by structural properties,such as the frequency and orientation of joint and the thickness of basalt bed.