New Mesozoic paleomagnetic results from the Qilian Orogenic Belt and their geological significance

The Qilian Orogenic Belt, located on the northern Tibetan Plateau, has preserved farfield tectonic evidence reflecting the reactions to the closure of the Paleo-Tethys and Neo-Tethys oceans. In this study, we conducted meticulous paleomagnetic and rock magnetic investigations on Middle Triassic strata in Tianjun County within the South Qilian tectonic belt, as well as Early Cretaceous strata in Qilian County within the North Qilian tectonic belt. We aimed to determine the deformation characteristics of the primary body of the Qilian Orogenic Belt during the Mesozoic era' s multistage tectonic response. Our experiments revealed that the Triassic samples exhibited stable High-Temperature Component (HTC) and successfully passed the reversal test at the sample level. Similarly, the Cretaceous samples displayed stable HTC and passed the fold test, indicative of the primary magnetization acquired during rock formation. Subsequent calculations yielded the Middle Triassic and Early Cretaceous poles (T-2 :82. 3 degrees N, 223. 6 degrees E, A(95) = 4. 8 degrees; K-1 :65. 9 degrees N, 188. 3 degrees E, A(95) = 6. 2 degrees), which were compared with contemporaneous poles from the Alxa Block and North China Block (NCB) regions. The Middle Triassic pole exhibited noticeable clockwise rotations concerning the Alxa Block (20. 2 +/- 6. 8), while the Early Cretaceous pole displayed negligible rotation relative to the NCB (0. 3 +/- 8. 6 degrees). By integrating these findings with previous research, we infer that the Qilian Orogenic Belt's tectonic response to the Late Jurassic-Early Cretaceous Lhasa/Eurasian collision event involved asymmetric tectonic shortening on the northern and southern slides, leading to the overall clockwise rotation of the orogenic belt. Subsequent to the Early Cretaceous, regional tectonic rotations occurred proximate to the Altyn Tagh Fault in the western Qilian Mountains and the Xining Lanzhou Basin in the eastern sector. The primary body of the Qilian Orogenic Belt experienced predominant NE-oriented tectonic shortening.