Low temperature thermochronological luminescence dating study and its application to the Fugong valley of Nujiang River and the Longyangxia gorge of Yellow River in the eastern Tibetan Plateau

Abstract

Luminescence dating method has great potential in the development of low temperature thermochronology, due to its low equivalent closure temperatures of 35-80 °C. It can determine the instantaneous denudation rate and true uplift rate, which is faster than the regional exhumation rate, especially for the last 1 million years.

The luminescence signal is a result of the competing effects between the trapping of electrons induced by irradiation and the decay of trapped electrons by heating. The dimensionless luminescence signals are evaluated by the equivalent dose (De). The De value corresponds to the apparent age, which is the time elapsed from the equivalent closure temperature to the present. Numerical simulation indicates that the equivalent closure temperature is dependent on the activation energy E and frequency factor s of electron traps, and the cooling rate η.

Protocols of De determination were evaluated and improved for this thermochronological study. Single aliquot regenerative dose isothermal thermoluminescence (SAR-ITL) and the multiple aliquots regenerative dose thermoluminescence (MAR-TL) protocols were studied using quartz from rock samples collected in the Nujiang (Salween) River of Tibet, China. In the SAR-ITL protocol, temperatures of 235 and 255 °C were selected for the isothermal thermoluminescence (ITL) measurement. A cutheat (preheat) to 245 and 265 °C were used to remove the thermoluminescence (TL) signal from lower temperature peaks, respectively. The integral of ITL signal between 10-20 seconds was used for De calculation. In the MAR-TL protocol, a cutheat (preheat) of 235 °C was used to remove the low temperature peaks. De values at temperatures of 250-290 °C were used for the thermochronological study. The SAR-ITL De values appeared to be 40-50 °C slide lower from De(T) plots of MAR-TL. It was indicated that the SAR-ITL and MAR-TL protocols were both appropriate for the thermochronological studies, and the MAR-TL was more efficient in the De measurement. This is because the MAR-TL protocol can measure De values of different TL peaks in one run. The TL signals at different heating temperatures have different thermal stabilities, and, hence, multiple thermometers of different closure temperatures can be identified from a single TL signal curve. Thus, a group of apparent ages can be obtained for a single sample in one measurement.

The signals of K-feldspar were also explored in the thermochronological study because they were stronger and had better reproducibility. The TL and ITL signals were studied with different measurement procedures, including the multiple aliquots additive dose protocol (MAA), as well as MAR and SAR protocols. The MAA-TL, MAR-TL, MAA-ITL, MAR-ITL and SAR-ITL protocols were compared by measuring the natural De of the K-feldspar samples. The MAR-ITL protocol was not appropriate for the K-feldspar thermochronology work, because it produced a sizeable underestimation at 295 °C. The MAR-TL and SAR-ITL protocols have potential to be applied in future thermochronological studies of K-feldspar.

The Fugong valley of the Nujiang River and the Longyangxia Gorge of the Yellow River were selected to apply the luminescence dating method to qualify the uplift and river incision processes in the eastern margin of the Tibetan Plateau, China. The MAR-TL protocol was used in the measurement of De values. Quartz grains were extracted from twenty samples collected at different elevations on the valley slope. The apparent age results indicate an accelerated incision from 0.4 mm/yr at 263 ka to 18.8 mm/yr at 8 ka. This acceleration increased dramatically since 25 ka. From 263 ka to the present, the valley had been incised ~1042 m. The equivalent closure temperature based on depth of incision is 41-46 °C. This was consistent with a 46 °C closure temperature calculated from a numerical simulation with a incision rate of 0.4 mm/yr (0.01-0.012 °C/ka). This accelerating incision process indicates that crustal uplift was due to the uplift of the southeastern Tibetan Plateau. Assuming the river channel was at a very low elevation (e.g. 0-1000 m) at 263 ka ago, the estimated average uplift rate is ~7.1-10.9 mm/yr.

In the Longyangxia Gorge of the Yellow River, quartz grains were extracted from eight samples collected at different elevations on the gorge wall. The apparent age results indicate that the incision of Longyangxia Gorge initiated before 50.77 ka. This incision gradually accelerated from 4.87 mm/yr at 41.41 ka to 19.60 mm/yr at 7.70 ka gradually. The estimated extent of incision into the granodiorite pluton at Longyangxia Gorge is ˃ 448.5 m during the last 50.77 ka. This indicates a continuous uplift of the northeastern Tibetan Plateau.