Late Cretaceous to Eocene Sedimentary Source Change and Tectonic Evolution of Nanao Event in Taiwan: Implications From Detrital Zircon U‐Pb Geochronology

The tectonic evolution of the Taiwan orogenic belt from the Mesozoic to the Cenozoic remains unclear because of a lack of deterministic geochronological data for unraveling stratal sequences. The precise timing of the Nanao event is crucial for uncovering this evolution and is the focus of this study. We conducted fieldwork and collected 20 rock samples for U‐Pb dating along two river transects that expose representative stratigraphic sections of the upper parts of the Mesozoic Tailuko belt and its overlying Cenozoic Nansuao Formation in northern Taiwan. Our results suggest that the Nanao event occurred between 97 and 60 Ma and comprised two phases. The first phase (97–78 Ma) is characterized predominantly by subduction‐related granitic intrusions, which likely persisted until 60 Ma. The second phase (78–60 Ma) is primarily characterized by regional metamorphism produced by a proposed collision with a seamount. The Nanao event was followed by crustal rifting along the southeastern margin of the Eurasian continent, which persisted until at least 34 Ma in the Taiwan region. Relative probability distributions and population ternary diagrams of detrital zircon ages reveal similarities among the Nansuao Formation, the basal meta‐conglomerate of the Nansuao Formation, and the underlying Tailuko belt. These data and field observations of clast compositions and fabrics suggest that the Nansuao Formation and its basal meta‐conglomerate were locally derived. This is in contrast to previously published studies that argued for a source in southeastern China.