Linking rapid grain size coarsening in the Neogene Xiyu Conglomerates to gravel–sand transitions in modern northern Tian Shan rivers: Evidence of shared origins from field and provenance investigations

The Cenozoic uplift of the Central Asia Tian Shan Mountains has driven significant subsidence in the foreland basins along its northern and southern flanks, leading to the extensive deposition of the Late Cenozoic alluvial-gravel deposits at its piedmonts known as the Xiyu Conglomerates. At the base of these conglomerates, localized gravel deposition replaces sandstones vertically over tens of meters with a sharp increase in median grain size (D50) by c. 100-fold. However, the origin of the transition remains a subject of controversy, with multiple potential factors intricately linked to regional tectonics and climatic variations. To address this question, we investigated the grain size variations of modern riverbed sediments along six rivers and the Xiyu Conglomerates in two sections within the northern foreland area of Tian Shan Mountains. We observed a rapid gravel–sand transition (GST) along the present-day rivers, 20–50 km downstream from the outlet, as well as a sharp conglomerate–sandstone transition at the base of the Xiyu Conglomerates, both of which exhibit similar fining rates. Furthermore, a provenance investigation of the Jingou River basin, using heavy mineral assemblages and detrital zircon U–Pb ages, indicates consistent sources for both the Xiyu Conglomerates and modern riverbed sediments. The combined results suggest that the striking grain size changes observed in both the Xiyu Conglomerates and along these modern rivers from similar internal hydraulic processes within the piedmont rivers, specifically size-selective sorting controlled by the bimodal grain size distribution of sediments. This implies that the emergence of sharp grain size transitions in the vertical successions was a result of the continuous northward progradation of the GST in the basin, driven by the long-term northward thrusting of the Tian Shan Mountains, independent of sharp and specific changes in climatic or tectonic forcing events. The average northward migration rate of the GSTs is calculated to be 3.9 ± 0.2 mm/yr since c. 7.5 Ma along 85°30′E, and 7.6 ± 2.1 mm/yr since c. 2.1 Ma along 86°30′E. These rates closely reflect the long-term crustal shortening rates across the northern Tian Shan Mountains, and its increase may denote an acceleration of the shortening post-Miocene.