Denudation intensity and control in the Chinese Tian Shan: new constraints from mass balance on catchment‐alluvial fan systems

Abstract

Tectonics and climate are usually seen as the main controlling factors of denudation rates, which seem to rise with the tectonic activity and to decrease when the climate becomes drier. However, the low denudation rates observed in semi-arid to arid contexts are generally measured on orogenic plateaus where the respective influence of the flat relief and the dry climate cannot really be unravelled. The Chinese Tian Shan was chosen as a case study. In the northern piedmont of this mountain range, a series of well-preserved Quaternary alluvial fans offer the opportunity to perform a mass balance study at the scale of several catchment areas and several hundreds of thousands of years. Based on a geometrical reconstruction of these fans, the volumes of sediments exported out of 10 drainage basins during the Middle–Late Pleistocene (from ∼300 to ∼12 kyr) and the Holocene (from ∼12 kyr to present) have been estimated. From these volumes, an average denudation rate of ∼135 m/Myr was determined in the Tian Shan Range for the last 300 kyr. In agreement with other mass balances performed in the same area, the typical denudation intensity of the northern Tian Shan is thus of a few hundred meters per million years at most, regardless of the space and time scales considered. From a comparison with denudation rates in other mountain ranges throughout the world, we suggest that a dry climate can dramatically limit the denudation intensity even in active orogenic systems with a high topographic gradient like the Tian Shan. As a result, the time required to reach equilibrium between denudation and rock uplift in these systems could be extremely long (i.e. of more than several million years)

Lag and mixing during sediment transfer across the Tian Shan piedmont caused by climate‐driven aggradation–incision cycles

“…Guerit et al . () also suggested that the dry Central Asian climate of the last few million years is responsible for the imbalance between these very low erosion rates and the comparatively large shortening rates of 10 ± 3 mm year −1 (Reigber et al ., ).…”

“…Guerit et al . () estimated erosion rates of around 0.135 mm year −1 for the last 300 kyr from the mass balance of 10 alluvial fans in the northern piedmont of the Eastern Tian Shan. Guerit et al .…”

“…The gravel previously deposited at the range front is now remobilised by incision and transported farther downstream, feeding the lower fans, located about 30‐km downstream from the range front (Fig. , see also Jolivet et al ., ; Guerit et al ., ).…”

Late Miocene to Quaternary slip history across the Qiulitag anticline in the southern Tianshan piedmont

“…Therefore, this strong Pleistocene increase in slip rates suggests that a significant proportion (40-50%) of the recent deformation of the whole Tianshan is now accommodated across the Qiulitage and Yakeng anticlines only and that the deformation has migrated from inner structures toward the external structures of the piedmonts. This acceleration, also observed in northern Tianshan, may suggest evolution of the orogen center toward a more steady topography (Charreau et al, 2018;Guerit et al, 2016). We determined the vertical offset and its uncertainty by estimating the slopes and intercepts and associated uncertainties of the best-fit lines representing the surface on both side of the axial surface (Thompson et al, 2002).…”

Self-similar growth of a bimodal laboratory fan

“…Indeed, the geometrical reconstruction of a fan provides an estimate of its volume which, through mass balance, yields the average denudation rate of the catchment (Kiefer et al, 1997;Jayko, 2005;Jolivet et al, 2014;Guerit et al, 2016). Furthermore, when the river transports multiple grain sizes, it usually deposits the coarser sediment (gravel) near the fan apex and the finer sediment (sand) at its toe.…”