Rapid denudation of the Himalayan orogen in the Nyalam area, southern Tibet, since the Pliocene and implications for tectonics–climate coupling

Abstract

The Himalayan orogen characterized by very high variability in tectonic and climatic processes, and is thus regarded as a natural laboratory for investigating the coupling of tectonics and climate, as well as the influence of this coupling on geomorphological processes. This study uses apatite fission track (AFT) dating of samples from a 45-km-long section crossing the Great Himalaya Crystalline Complex (GHC) in the Nyalam area, southern Tibet, to constrain the timing and rate of late Cenozoic denudation. The AFT ages can be divided into two groups: (1) 15–6 Ma, to the north of Nyalam town, for which the best-fit line of elevation-age has a gentle slope of 0.05, and for which a denudation rate of 0.27 mm/a is calculated; and (2) 3–1 Ma, south of Nyalam town, for which the best-fit line has a steep slope of 0.64, and for which a denudation rate of 1.32 mm/a is calculated. The whole AFT ages has a positive correlation with sample elevation (i.e., older ages are found at higher elevations), and the geographical location of the point of inflexion of the two fitted lines corresponds closely to the junction of Poqu River near Nyalam town. By integrating the AFT data with thermo-tectonic modeling, it can be inferred that the GHC has experienced two different periods of denudation: (1) slow denudation during middle to late Miocene (15–6 Ma) is recorded in the northern part of the GHC; and (2) rapid denudation from the Pliocene to the Pleistocene (3–1 Ma) is recorded in the southern part of the GHC. An abrupt change in denudation rate occurred between the two periods, with the Pliocene–Pleistocene denudation rate being five times higher than that during the Miocene. This abrupt change in denudation rate during Pliocene pervaded the Himalayan orogen, and was roughly synchronous with a marked change in global climate at 4–3 Ma, and intensification of the Asian monsoon. Importantly, the later period of rapid denudation in the study region closely coupled to the mean annual precipitation, while there is no clear evidence for large-scale faulting activity and associated uplift during this period. Therefore, climate (precipitation) is inferred to be the main cause of the rapid denudation of the Himalayan orogen since the Pliocene.