汾渭地堑的河流阶地对第四纪时期印度-欧亚板块碰撞带的构造响应

RIVER TERRACES IN THE FEN WEI GRABEN, CENTRAL CHINA, AND THE RELATION WITH THE TECTONIC HISTORY OF THE INDIA-ASIA COLLISION SYSTEM DURING THE QUATERNARY

秦岭造山带前缘的汾渭地堑是新构造运动非常活跃的地区,地堑内发育了一系列第四纪河流阶地。通过对河流阶地及其沉积物的沉积学、土壤地层学、年代学及形成机制的研究,得到以下认识:渭河地堑发育了5级河流阶地,这些阶地均由底部的河床相和其上的第四纪风成沉积组成,这5级阶地最底部的黄土地层分别是L33,L15,L9,L6和L2。在磁性地层学和气候地层学基础上,通过与深海氧同位素记录的对比,上述黄土层分别对应于深海氧同位素阶段104,36,22,16和6;阶地年龄分别为2·60Ma,1·20Ma,0·90Ma,0·65Ma和0·15Ma。新构造作用在渭河地堑的5级河流阶地的形成中起了决定性的作用,汾渭地堑及上述由正断层控制的河流阶地的形成与华北板块内部的NW-SE向地壳拉张密切相关,而后者与印度板块向北俯冲导致扬子板块与华北板块沿秦岭走滑断层的相对运动有直接联系,所以上述阶地的形成时期也正是第四纪时期印度板块向北阶段性俯冲加剧的时期。

The Fen Wei Graben, adjacent to the Qinling orogenic belts in Central China, is an active tectonic region, containing a well-preserved flight of Pleistocene fluvial terraces. Our studies on the sedimentology, stratigraphy, ages, and forming mechanism of the terraces allow us to summarize the following general points. There are five river terraces distributed within the Fen Wei Graben, composed of a basal channel gravel layer and the overlying thick loess deposits characterized by the alternating loess （L） and palaeosols （S）. Stratigraphic studies on the aeolian deposits of the five terraces （ from T5 to T1 ） indicate that they contain 33 （ from So to S32 ）, 15 （from S0 to S14 ）, 9 （from S0 to S8 ）, 6 （from S0 to S5 ）, and 2 （ from S0to S1 ） palaeosols, respectively, and the basal layers of the loess deposits of the terraces are L33, L,5 , L9 ,L6, and L2. The combined dating approaches of palaeomagnetism and climatostratigraphy, permit the loess-palaeosol sequences to be compared closely with the marine oxygen isotope time series （Shackleton and Pisias, 1955; Shackleton et al. , 1990; 1995a, 1995b）, and thus, their ages can be determined as closely as an isotopic stage. The records indicate that the basal loess beds of the five loess overlying river terraces can be correlated with marine oxygen isotopic stages 104 （TS）, 36 （T4）, 22 （T3）, 16 （T2）, and 6 （T1）, yielding ages ca. 2.60Ma, 1.20Ma, 0.90Ma, 0.65Ma, and 0.15Ma, respectively. River incision in the Wei He Graben is not caused by climatic changes, but mainly controlled by tectonics. Formation of the Fen Wei Graben is related to the NW - SE crustal extension which is associated with the left lateral movements between the North and South China blocks along the Qingling Orogen, in response to the northward movement of India towards Asia since Cenozoic time. Thus, the stepped terraces, associated closely with the normal faults in the Fen Wei Graben, provide information about the India-Asia collision system in which terrace ages define episodes of accelerated northward movement of India to Asia during the Quaternary.