Structural evolution of the Ziyuan detachment of the Yuechengling dome and its tectonic implications to the Late Mesozoic-Cenozoic extension in South China 

Faults are lithospheric fracture zones that undergo substantial displacement driven by tectonic stress, serving as direct indicators of crustal kinematics. Compared to other fault varieties, normal faults are typically less influenced by multi-stage tectonic superposition. Their relatively straightforward structural styles make them ideal candidates for modeling fault evolution. Modes of fault evolution have long been disputed between the constant-length and tip-propagation models, but questions remain: are traditional frameworks overly reductive, or does the hybrid model offer a more accurate representation of geological reality?South China underwent two distinct extensional episodes during the Late Mesozoic, resulting in extensional structures and detachment faults. The Yuechengling area preserves a comprehensive record of these events; specifically, the Tianhu Fault and the Ziyuan Detachment Fault correlate closely with these episodes, providing an ideal laboratory for studying detachment fault evolution. Low-temperature geochronology and thermal history inversions reveal that the Tianhu Fault initiated southward propagation at 140 Ma, accompanied by rapid cooling. While the fault's tips transitioned to a slow-cooling phase at 40 Ma, the central segment reached this stage as early as 70 Ma. Conversely, the Ziyuan Detachment Fault initiated at approximately 100 Ma and did not enter a slow-cooling regime until 40 Ma. The evolution of the Tianhu Fault concurs with the hybrid model, whereas the Ziyuan Detachment Fault initiated synchronously across its strike at 100 Ma, arguing for the constant-length model. We attribute this differential evolution to variations in rock mechanical properties, extension rates, and fluid activity.Although Cenozoic extensional structures in South China are primarily concentrated in southeastern offshore regions—leaving few visible deformation markers—our data suggest that Cenozoic extension was superimposed onto Late Mesozoic faults, driving the continuous uplift and cooling of their hanging walls. This process is consistent with the Late Mesozoic–Cenozoic tectonic migration from the northwest of the South China Block toward the South China Sea. The transition to slow cooling at 40 Ma likely reflects a regional stress field shift: the opening of the South China Sea absorbed major extensional stress, effectively terminating far-field effects within the continental interior.