Pleistocene drainage reorganization of the upper reaches of the Han River and its tectonic significance

Drainage systems are highly sensitive to landform changes, and their formation and evolution history are of great significance for understanding regional tectonic-climatic processes and ecological environmental changes. Since the Cenozoic, the uplift and expansion of the Tibetan Plateau have profoundly influenced the landforms and drainage patterns in its surrounding area. The Qinling-Daba Mountains are located on the northeastern margin of the Tibetan Plateau, and the Han River, as the largest tributary of the Yangtze River, originates from the southern flank of the Qinling Mountains and flows from west to east between the Qinling and Daba Mountains, whose evolution history may document abundant clues of the expansion of the Tibetan Plateau and regional tectonic-climatic responses. Previous studies suggested that a significant river reorganization event may have occurred in the upper reaches of the Han River. However, the timing and mechanism are still vague. In this study, the evolution history of the upper reaches of the Han River is reconstructed through terrace mapping, paleocurrent measurements, K-feldspar Pb isotope provenance analysis, and quartz electron spin resonance (ESR) dating. Combined with the fault kinematic analyses, it is believed that before 0.4 Ma, the Paleo-Han River flowed directly eastward along the Ankang Basin. Between 0.4-0.15 Ma, the continuous left-lateral strike-slip movement along the Ankang Fault resulted in vertical uplift at its compressional bend and caused the Han River to flow southward and bypass into the Daba Mountains. This river evolution event within the Qinling region reflects the adjustment process of the peripheral water systems and landforms under the influence of the expansion of the Tibetan Plateau.