关键词: 晚第四纪, 玄武岩, 断层作用, 气候, 流水下切, 桑干河

Abstract: In consideration of the disturbances of the Datong Basin reach of the Sanggan River during late Quaternary time caused by external variables,such as the emplacement of basalts,along-valley faulting processes 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 along-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 incision induced by faulting and by climate was made both in quantity and in event.Equilibrium profile analyses prove that displacement produced by along-valley faulting is equal to fluvial incision induced by faulting,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 evolutionary history of the stream longitudinal profile in the studied reach.The principal erosion processes in the lava 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.

Key words: late Quaternary, basalt, faulting, climate, fluvial incision, Sanggan River