Meso-Cenozoic Tectonic and Geomorphic Evolution of the Eastern South China Margin: Insights from the Planation Surface of the Siming Mountains, Zhejiang Province

Extensive planation surfaces are preserved on the Siming Mountains of eastern Zhejiang Province and provide a key record of Meso-Cenozoic landscape evolution along the eastern South China margin. Existing age constraints rely on geomorphic relationships, stratigraphic evidence, and the ages of overlying Paleogene basalts, all of which constrain post-planation burial and preservation rather than the timing of planation itself. Here, we present apatite fission-track (AFT) and apatite (U-Th)/He (AHe) data from seven intrusive samples of the Liangnong pluton beneath the Siming planation surface and combine them with QTQt thermal-history modeling and age-exhume denudation-rate estimates to reconstruct the post-Late Cretaceous cooling and exhumation history of the range. The thermal-history modeling suggests: rapid cooling at ~90–80 Ma, prolonged slow cooling between ~80 and ~35 Ma, and renewed accelerated cooling after ~35 Ma. Short mean AFT track lengths, dispersed single-grain AHe ages, and modeled exhumation rates of ~0.02–0.05 km/Myr are consistent with long-lived, slow erosion from the latest Cretaceous through the Oligocene. We infer that the Siming planation surface was established during ~80–35 Ma, when relief was progressively reduced and a regional low-relief landscape developed. The three present-day surface levels at ~380, ~600, and ~750 m are interpreted as relict surfaces produced by post-35 Ma dissection, with local basaltic caprock favoring preservation. Compared with adjacent fault zones and tectonically active belts that record rapid Cenozoic cooling, the Siming Mountains preserve a stable intrablock signature, highlighting strong spatial heterogeneity in the tectonic-geomorphic evolution of the eastern South China margin since the Late Cretaceous.