ISOCHRON 26AL/10BE BURIAL DATING OF YELLOW RIVER GRAVEL TERRACES AT SHAPOTOU, CHINA

The northeastern Tibetan Plateau is a key region to study the tectonic movement of the Tibetan Plateau after the collision of the Indian subcontinent with the Eurasian plate. The Xiangshan-Tianjingshan (XT) fault is one of the most active faults in the area. The Yellow River crosses the XT fault from west to east in the Shapotou area, Ningxia Province, and formed nine terraces on the left bank. To better understand the tectonic activity of the XT fault, the ²⁶Al/¹⁰Be burial dating method was applied to four gravel terraces (T5, T6, T8 and T9). Isochron burial dates for the two highest terraces (T8 and T9) using three different grain size fractions of gravels. Terrace T8 gives an isochron age of 1.23±0.25 Ma, but the dating failed on T9 because of insufficient spread in the ²⁶Al and ¹⁰Be concentrations. An isochron constructed from modern bank gravels shows insignificant pre-burial history, as expected.

Based on the terrace height above current river level, we calculate an incision rate of ~ 90 m/Ma over the last ~ 1.2 Ma. Considered together with previous studies of younger terraces (T5 and T4) by ²¹Ne and OSL methods, we found that incision rates were not constant over this time, but increased by a factor of four during the past million years. Since there was no dramatic climate variation during this period, this accelerated river incision is probably related to changes in the rates of tectonic activity of the XT fault, which likely also promoted the formation of the splendid sigmoid bend of Yellow River at Shapotou today.