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
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

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 S_ 0 to S_ 32), 15 (from S_ 0 to S_ 14), 9 (from S_ 0 to S_ 8), 6 (from S_ 0 to S_ 5), and 2 (from S_ 0 to S_ 1) palaeosols, respectively, and the basal layers of the loess deposits of the terraces are L_ 33, L_ 15, L_ 9,L_ 6, and L_ 2.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, 1985; 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 (T5), 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.