,第三讲 新构造学研究的地貌学方法,第一节 水系变迁,先成河的形成过程,后成河的形成过程,Formation of subsequent river.,纵谷,横谷：连江支流清莲江,River Capture,At the same time as formation of broad valley in east Ordos, to further east series of NE grabens formed in late Miocene.,Offset of fluvial valleys, by strikeslip faults, provides some of the most important geomorphic evidence for determination of fault slip.,Deflection of drainage along San Andreas fault,Deflection of drainage along San Andreas fault,Located in eastern margin of the Tibetan Plateau,Tibetan Plateau in eastern Asia,Marginal area along the Tibetan Plateau is tectonically active with many large earthquakes, including the most recent Wenchuan 8.0 earthquake of 2008.,Period of peneplain from late Cretaceous to Eocene.,About mid-Eocene, affected by collision between two continental plates of Indian and Eurasia, the Hetao and Yinchuan basins formed in the north and west Ordos areas, respectively.,By the Oligocene, deposits in the Yinchuan Basin expanded and overlapped eastward onto the north Ordos peneplain due to west warping of the Ordos block.,The Yinchuan Basin also expanded southward at the same time.,Profile of Yinchuan Basin,Intensive subside but northward retreat of Yinchuan Basin since Miocene.,Oligocene,Sinistral deflection of channel along the Haiyuan Fault Zone.,Sinistral offset of hill slopes along the Haiyuan Fault Zone.,even sinistral offset of tree truck.,However, the amount of offset is not always similar even along the same segment of the fault.,Older rivers should exhibit greater amounts of offset than younger ones because a larger total cumulative movement with time.,Former research demonstrates that the offset rivers across the Haiyuan Fault Zone can be divided into three orders, A, B and C, from large to small and from old to young. The maximum offset of rivers of order A is up to 627m; The maximum offset of rivers of order B is up to 60m; The maximum offset of rivers of order C is up to 18m. It is obvious that older and larger rivers exhibit greater offset than younger and smaller rivers.,Why?,However, in the rivers that cross the fault zone, there are still some that are much larger and older than rivers of order A. The problem here is why do the rivers that are larger and older than the rivers of order A exhibit no apparent offset?,Asymmetrical valley,If the amount of each displacement on a fault is larger than the width of the river across the fault, it is possible to form an beheaded channel.,If the amount of each displacement on a fault is smaller than the width of a river across the fault, it is possible to form a deflected channel.,If the amount of each displacement on a fault continues to become smaller or the erosional capability of a river across the fault continues to become larger, the channel is not easily bent or deflected, much less beheaded.,The right bank of the river is subject to erosion, resulting in a steep profile. On the left side of the left-lateral fault block, deposition to form a floodplain develops. Thus, the channel remains in its original position and the valley widens asymmetrically.,The rives may not only erode away the protruding valley slope completely so that the valley slope will retreat some distance back but not to its original position. In this situation the increase of the valley width is smaller than the displacement of the fault, and at the same time, the river channel is deflected.,These two situations, beheaded channel and asymmetrical valley may only be the two end-members of a comprehensive model.,If the river cuts down during this episode of faulting, then, a terrace series, distributed asymmetrically only on one side of the river, will result.,Total cumulative horizontal and vertical displacement on faults (i.e. net displacement of faults) can be estimated since the formation of river by our asymmetrical valley model. For example, by measuring the differential relieves between two equivalent level terraces on either side of a fault, vertical component of slip can be measured. On the other hand, by measuring the differential distances between risers fronting coeval terraces and the modern river, horizontal component of slip can be measured.,Asymmetrical valley of the Yellow River caused by the Haiyuan Fault zone, northeast margin of Tibetan Plateau.,There has been 5km of total strike-slip displacement on this part of the fault has been recorded by the river. It can be seen that if the amount of asymmetrical valley widening of 2km is neglected ( in Fig.), the true amount of cumulative sinistral displacement (i.e. + in Fig) would be underestimated.,Tectonic landform of the Heishanxia Gorge mouth,Another example of asymmetrical valley caused by active strike-slip fault, northeast margin of Tibetan Plateau.,Two very active sinistral-reverse fault zones along northeastern margin of the Tibetan Plateau.,There is no apparent offset or deflection of the large rivers, the Yuanhe, Heb