Reconstruction of paleotopography of the Yunkai Mountains in south China: Insights into the onset of the Asian monsoon

The Asian monsoon has a large influence range and a significant impact on the lives of nearly half of the world's population. Yet, the origins and evolutionary processes of the Asian monsoon remain elusive. The rain shadow effect serves as a pivotal indicator of the monsoon's trajectory, while paleotopography reconstruction aids in ascertaining the existence or absence of this effect. The Yunkai Mountains, among the highest mountain ranges in southern China, constitute a natural barrier impeding the northward expansion of the warm and humid Asian monsoon. The topography can provide critical information on the onset and development of the Asian monsoon. We utilized apatite and zircon (U-Th)/He data collected from a Gaozhou-Luoding horizontal cross-section to reconstruct the two-dimensional paleotopography of the Yunkai Mountains and determine the mean exhumation rates. The results show the following: (1) The Yunkai Mountains reached a maximum elevation of ∼3.4 km around 80 Ma, followed by an asymmetrical decrease. (2) The relatively high exhumation rate of the NE segment during 80–40 Ma may be linked to the thermal extensional tectonics, whereas the gradual exhumation rate and significant elevation reduction of the SW segment since 40 Ma were likely associated with the impact of the Asian monsoon or fault activity. (3) No rain shadow effect existed in the Yunkai Mountains in the Paleocene. The climate boundary near the Yunkai Mountains in the Paleocene was most likely the northern boundary of the Intertropical Convergence Zone. Therefore, we conclude that the Asian monsoon formed after the Paleocene.