LUMINESCENCE DATING OF THE MINJIANG HEADWATER TERRACES AND ITS GEOMORPHOLOGICAL IMPLICATION

The Minjiang River is located at the northeast margin of the Tibetan Plateau, and the upper reaches of the river stretches almost in a north-south direction along the Minjiang Fault. It has been widely investigated to understand the tectonic history of the region during the late Quaternary. Many scholars have studied the ages of the upstream of the Minjiang river by using several different dating methods including ESR, ¹⁴C, TL and IRSL and so on. However, the ages of the terraces obtained are not consistent, implying that the reliability of these ages needs to further confirm. We investigated the river geomorphological characteristic of this region including the type and the height above the present river channel of terraces, and then took samples for luminescence dating in the field. The aim is to establish the chronology of the terrace sequence in the area and assess the reliability of previous ages obtained using other dating methods.

Five terraces were identified in the field, and 24 OSL samples were collected and dated. The lower terraces (T1 and T2) are generally fill terraces, and the higher terraces (T3-T5) are strath terraces. The results show that some samples were not well bleached at the time of deposition. Based on the equivalent dose (De) distribution and the value of the overdispersion (OD), a minimum age model (MAM) was used to calculate the De values for age calculation. The minimum ages obtained are considered to represent the true burial ages of the samples, that is the formation ages of the river terraces. The five river terraces were dated to ~3.3ka, 5.7~8.9ka, 11.8~16.5ka, 22.9~40.9ka and 55.4~58.3ka, respectively. Based on the terrace ages and their elevations above the modern river channel, the river incision rates were calculated to be 0.90-1.49 mm/yr, which reflect the differential uplift rates during the past ~55.4 ka. In addition, the terrace ages are also compared with the ages of moraine deposits located in the Xuebaoding Mountain close to the Minjiang River. The factors affecting the formation of the terraces were discussed, including the uplift of the Minshan and Longmenshan mountains.