Comparison of River Terraces in the Middle Reach Valleys of the Yellow River and Analysis on the Multi-Gradational Features of Tectonism in the Formation of Terrace Series

Where the Yellow River flows through the Haiyuan-Tongxin arc-form tectonic region on the northeastern side of the Qinghai-Xizang (Tibet) Plateau, as many as 10～21 basis and erosion terraces have been produced, among which the biggest altitude above river level is 401m and the formation age of the highest terrace is 1.57 Ma B.P. Based on comparative analysis of the Yellow River terraces located separately in the Mijiashan mountain, the Chemuxia gorge, the Heishanxia gorge and the other river terraces in the vast extent of the northern part of China, it has been found that the tectonic processes resulting in the formation of the terrace series is one of multi-gradational features, i.e., a terrace series can include the various terraces produced by tectonic uplifts of different scopes or scales and different ranks. The Yellow River terrace series in the study region can be divided into three grades. Among them, in the first grade there are 6 terraces which were formed separately at the same time in the vast extent of the northern part of China and represent the number and magnitude of uplift of the Qinghai-Xizang Plateau since 1.6 Ma B. P. ; in the second grade there are 5 terraces which were separately and simultaneously developed within the Haiyuan-Tianjingshan tectonic region and represent the number and magnitude of uplift of this tectonic region itself since 1.6Ma B. P.; in the third grade there are 10 terraces which developed on the eastern slope of the Mijiashan mountain and represent the number and amplitude of uplift of the Haiyuan tectonic belt itself since 1.6Ma B.P. Comparison of the terrace ages with loess-paleosoil sequence has also showed that the first grade terraces reflecting the vast scope uplifts of the Qinghai-Xizang Plateau are very comparable with climatic changes and their formation ages all correspond to the interglacial epochs during which paleosoils were formed. This implies that the vast extent tectonic uplifts resulting in river down-cutting are closely related to the warm-humid climatic periods which can also resnit in river downward erosion after strong dry and cold climatic periods, and they have jointly formed the tectonic-climatic cycles. There exists no unanimous and specific relationship between the formation ages of the second and third grade terraces and climatic changes and it is shown that the formation of those terraces was most mainly controlled by tectonic uplifts of the Tianjingshan block and the Haiyuan belt. The river terraces in the study region, therefore, may belong to 2 kinds of formation cause. One is a tectonic-climatic cyclical terrace produced jointly by vast extent tectonic uplifts and climatic changes, and the terraces of this kind are extensively distributed and can be well compared with each other among regions. Another is a pulse-tectonic cyclical terrace produced by local tectonic uplifts as dominant elements, and their distribution is restricted within an active belt and can not be compared with among regions.

Comparison of River Terraces in the Middle Reach Valleys of the Yellow River and Analysis on the Multi-Gradational Features of Tectonism in the Formation of Terrace Series

Where the Yellow River flows through the Haiyuan-Tongxin arc-form tectonic region on thenortheastern side of the Qinghai-Xizang (Tibet) Plateau, as many as 10 ～ 21 basis and erosionterraces have been produced, among which the biggest altitude above river level is 401m andthe formation age of the highest terrace is 1.57 Ma B.P. Based on comparative analysis of theYellow River terraces located separately in the Mijiashan mountain, the Chemuxia gorge, theHeishanxia gorge and the other river terraces in the vast extent of the northern part of China,it has been found that the tectonic processes resulting in the formation of the terrace series isone of multi-gradational features, i.e., a terrace series can include the various terracesproduced by tectonic uplifts of different scopes or scales and different ranks. The Yellow Riverterrace series in the study region can be divided into three grades. Among them, in the firstgrade there are 6 terraces which were formed separately at the same time in the vast extent ofthe northern part of China and represent the number and magnitude of uplift of the Qinghai-Xizang Plateau since 1.6 Ma B. P.; in the second grade there are 5 terraces which wereseparately and simultaneously developed within the Haiyuan-Tianjingshan tectonic region andrepresent the number and magnitude of uplift of this tectonic region itself since 1.6Ma B.P.;in the third grade there are 10 terraces which developed on the eastern slope of the Mijiashanmountain and represent the number and amplitude of uplift of the Haiyuan tectonic belt itselfsince 1. 6Ma B.P. Comparison of the terrace ages with loess-paleosoil sequence has also showedthat the first grade terraces reflecting the vast scope uplifts of the Qinghai-Xizang Plateau arevery comparable with climatic changes and their formation ages all correspond to theinterglacial epochs during which paleosoils were formed. This implies that the vast extenttectonic uplifts resulting in river down-cutting are closely related to the warm-humid climaticperiods which can also result in river downward erosion after strong dry and cold climaticperiods, and they have jointly formed the tectonic-climatic cycles. There exists no unanimousand specific relationship between the formation ages of the second and third grade terraces andclimatic changes and it is shown that the formation of those terraces was most mainly controlledby tectonic uplifts of the Tianjingshan block and the Haiyuan belt. The river terraces in thestudy region, therefore, may belong to 2 kinds of formation cause. One is a tectonic-climaticcyclical terrace produced jointly by vast extent tectonic uplifts and climatic changes, and theterraces of this kind are extensively distributed and can be well compared with each otheramong regions. Another is a pulse-tectonic cyclical terrace produced by local tectonic uplifts asdominant elements, and their distribution is restricted within an active belt and can not becompared with among regions.