Shifeng Wang

The Chenghai Fault Zone is a major active intrablock structure within the Sichuan–Yunnan Block at the southeastern margin of the Tibetan Plateau and exhibits pronounced along-strike kinematic segmentation. However, the Quaternary left-lateral slip rate of its central Qina segment remains poorly constrained. Here, we integrate field investigations, remote-sensing geomorphic mapping, UAV photogrammetry, and U-series dating to investigate the fault-offset Qiaonan paleolandslide along the Jinjiang Fault. The Qina segment consists mainly of the NNE-striking Qina and Jinjiang faults, which together form an overall left-stepping fault system dominated by left-lateral strike-slip motion. Geomorphic restoration of well-preserved landslide margins using high-resolution UAV-derived orthophotos and digital surface models indicates a cumulative left-lateral displacement of approximately 100 m. Independent DEM-based restoration yields a horizontal displacement of ~97 m, providing a consistent estimate of the fault offset. U-series dating of secondary carbonate cement within the landslide deposit yields a ^230Th–U age of 257.7 ± 3.2 ka. Because the dated carbonate precipitated after landslide emplacement, this age provides only a minimum-age constraint on the paleolandslide. Consequently, the calculated apparent left-lateral slip rate of ~0.39–0.47 mm/yr should be regarded as an upper-bound estimate rather than a precisely determined slip rate for the Jinjiang Fault. The Jinjiang Fault also left-laterally offsets the Jinsha River by ~3.04–3.25 km. Assuming that this displacement began to accumulate with the initiation of the Dali faulted-basin system at ca. 7.6 Ma, the corresponding long-term average slip rate is ~0.40–0.43 mm/yr. However, this estimate depends on an assumed fault-initiation age and therefore represents an end-member long-term average. When considered together with previous interpretations of possible fault-activity migration within the Chenghai Fault Zone, the Quaternary upper-bound rate is compatible with, but does not demonstrate, a reduction in the left-lateral strike-slip component during the Late Cenozoic. Such temporal variations may reflect changes in deformation partitioning among intrablock faults within the Dali faulted-basin system, although this interpretation remains tentative and requires additional chronological and slip-rate constraints. Our results demonstrate the potential of fault-offset paleolandslides to provide valuable geomorphic and chronological constraints on the Quaternary behavior of slowly slipping intrablock faults, while highlighting the importance of independent chronological control for resolving long-term variations in fault slip.

The role of the Karakoram fault (KKF) in evolution of the Tibetan–Himalayan orogenic belt is controversial. Some consider the KKF to be a stable, long-lived feature with several hundred kilometres of offset along its entire current trace, whereas others interpret it as having propagated along its NW–SE trend since initiation at ∼16 Ma with small-scale slip being gradually absorbed by transfer structures along the fault trace. Here we report new zircon U–Pb and mica ⁴⁰Ar/³⁹Ar ages related to the Ayi Shan detachment to better constrain the activity of the KKF in southwestern Tibet. The zircon U–Pb data show migmatite ages of 489, 478, and 435 Ma from the footwall of the Ayi Shan detachment involved in the KKF ductile shear zone. Mylonitized migmatite in the South Ayilari did not record any KKF activity. Similarly aged magmatic and metamorphic information recorded in mylonites and undeformed rocks of the Animaqing Group around the North Ayilari also rules out the effect of KKF movement on zircon growth. Cenozoic information recorded in North Ayilari zircons evidently resulted from Trans-Himalayan magmatic belt (THB) magmatism during 45–50 Ma and 32–25 Ma. Four mica dates from the same mylonitized samples all cluster around 12 Ma. Combined zircon U–Pb and mica ⁴⁰Ar/³⁹Ar ages from the mylonites and undeformed rocks support the hypothesis that the KKF imposed a structural fabric on the rocks of the Animaqing Group and the THB granites at around 12 Ma in the Ayilari Range. Chronologic, kinematic, and geometric studies demonstrate that the fault propagated southeastward into SW Tibet at 12 Ma.