MAGNETOSTRATIGRAPHIC DATING OF RIVER TERRACES - RAPID AND INTERMITTENT INCISION BY THE YELLOW-RIVER OF THE NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU DURING THE QUATERNARY

Up to 23 paleosols have been identified in loess sequences that overli e fluviatile sediments on seven terraces of Huang He (the Yellow River ) and Daxia He in the Linxia Basin at the northeastern margin of the T ibetan Plateau. The magnetostratigraphic record in the oldest sequence appears to be fairly complete and spans the entire Quaternary. It inc ludes evidence for the Jaramillo and Olduvai normal polarity subchrons within the Matuyama chron, as well as the Cobb Mountain and ''stage 5 4'' (formerly Gilsa) events. Age constraints within the Brunhes are pr ovided by C-14 and thermoluminescence/optical stimulated luminescence dating. The magnetostratigraphic chronology of the Quaternary sediment s indicates that terraces were formed at about 1.66, 1.4, 1.1, 0.6, 0. 12, 0.055, and 0.01 Ma. The terraces may represent climatically induce d pauses in long-term tectonically induced incision by the rivers. On the basis of terrace heights and ages, incision rates were highest fro m 1.66 to 1.4 Ma (similar to 1 m/kyr), decreased from 1.4 to 0.12 Ma ( similar to 0.1 m/kyr), and increased from 0.12 Ma to present (similar to 0.75 m/kyr). If river incision is attributed solely to uplift of th e Tibetan Plateau, then this uplift appears to have been rapid as well as intermittent following the first evidence of the existence of Huan g He in this area at about 1.75 Ma. However, the effects of uplift can not be fully separated from incision caused by changes in climate.