黄河晋陕峡谷河流阶地和鄂尔多斯高原第四纪构造运动

本文通过黄河晋陕峡谷河段21个地点的阶地横剖面观察和阶地对比研究,确定出6级宽谷阶地序列,同时,依据17个放射性测年数据对阶地定年。6级宽谷阶地序列揭示了鄂尔多斯高原第四纪区域造陆隆起和局部构造变形,区域造陆隆起的发生时间比之青藏高原隆起要滞后0．113～0．25Ma,而且,平均区域造陆隆起量比之青藏高原的内部和边缘的隆起量小得多。晋陕峡谷河段发生过3期加积作用幕,指示着造陆隆起景观中的气候变化时间线。渭河盆地北缘的韩城断裂悬崖带记录着构造基准面下降,影响的河流长度仅为21～127km。

YELLOW RIVER AND QUATERNARY TECTONIC MOVEMENTS OF THE ORDOS PLATEAU

On the base of the observation of terrace cross-sections at 21 sites and 17radiometric data this paper deals with the terrace corrolation , sequences and dating forthe fluvial terraces of the Shanxi-Shaanxi Canyon reach of the Yellow River ,demonstrates the Quaternary regional epeirogenic uplifts and local tectonic deformationfor the Ordos Plateau and further discusses the relations between the regionalepeirogenic uplift and the Qinghai-Xizang Plateau uplift the climatic change bine linesin the epeirogenic uplift landscape, and the effect of the base-level change.River terraces are landforms that were at one bine constrUcted and maintained asthe active floor of a river but are now abandoned, and their formation are envolvedin the changes of fluvial systems. The graded process of rivers provides a theoretic base for the genesis of river terrace and the reconstruction of evolutionary history ofancient channels.Increase in fluvial landscape relief emanates from rising geologic structures throughthe process of tectonically induced down cutting by streams. Strath terraces are thefundamental tectonic river terrace landform and indicate the regional epeirogenic upliftand its local tectonic deformation. The morphogenic implication in climatic change isto trigger aggradation that forms fill terrace landforms and records brief reversals oflong-term tectonically induced downcutting. The fill terraces represent a bine sequencein the regional epeirogenic uplift landscape.Six strath terraces develop along the Shanxi-Shaanxi Canyon reach of the Yellowhiver. Their ages are: 1.409 8MaB. P., 0.196 7 Ma B. P., 76 400aB. P., 44 000aB. P.,17 600aB. P. and 5 400aB. P. for T6k, T5y, T4t, T3m. T2w. and T1q respectively . Thelongitudinal profiles of the strath terraces reveal the Quaternary tectonic movementsinvolved in the Ordos Plateau, including two components: the regional epeirogenicuplift and the local tectonic deformation. The regional epeirogenic uplift divides intofive episode, with an average elevation of 25- 46m each episode. The local tectonicdeformation can be expressed by tWo patterns: the local differential uplift induced byfaulting and the nonuniform uplift including the relative strong uplift and the relative weak uplift even subsidence. The local tectonic deformation superimposes on theregional epeirogenic uplift.The downcutting induced by the regional epeirogenic uplift for the Shanxi-ShaanxiCanyon reach started after 1.409 8MaB. P., which lags behind the C phase of theQinghai-Xizang Movement for 0.25Ma ago. Increase of the regional epeirogenic upliftrate in Ordos initiated at 0.196 7Ma ago, which also lags behind for 0.113Ma sincethe Qinghai-Xizang Plateau uplift achieved the fastest rate at 0.31Ma ago.There are three episodes of the aggradation induced by climatic changes in theShanxi-Shaanxi Canyon reach. They occurred before 95 900aB.P., 44 300～36 800aB.P.and 17 300～9 300aB.P. respectively . The regional epeirogenic uplift episodes and theaggradational episodes occurred alternately, and the aggradational events become thebine sequence in the regional epeirogenic uplift landscape. Moreover, the aggradationoccurred in the regions where present relative weak uplipt even subsidence.The Weihe fault basin is the relative base-level of the Shanxi-Shaanxi Canyonreach of the Yellow River . The fault-scarps in the Hancheng Fault escarpment zone atthe northern margin of the basin record the falls of the tectonic base-level. The riverlength is limited and influenced by the base-level falling. The geomorphic effects ofboth the tectonic base-level fall and the regional epeirogenic uplift are commonly tosculpture a new graded longitudinal profile.