Deciphering the collisional dynamics of the Bangong-Nujiang Tethyan Ocean: multi-regional anisotropy of magnetic susceptibility (AMS) constraints from the Central Tibetan Plateau

The Middle Jurassic to Late Cretaceous interval marks a pivotal geological timeframe, encompassing both the progressive amalgamation of the Lhasa and Qiangtang Blocks and the terminal closure of the Bangong-Nujiang Tethyan Ocean. Investigating the closure evolution of the Bangong-Nujiang Tethyan Ocean during the Middle Jurassic to Late Cretaceous period will significantly enhance our understanding of the broader tectonic dynamics within the Global Tethys Domain. To elucidate the subduction polarity and timing of the closure of the Bangong-Nujiang Tethyan Ocean during this critical period, this study builds upon previous investigations of magmatic activity, stratigraphic sedimentary features, and paleomagnetic data. By conducting zircon U-Pb geochronology and systematic anisotropy of magnetic susceptibility (AMS) analyses across the Lhasa Block, Qiangtang Block, and Bangong-Nujiang suture zone, the following insights were obtained: (1) The northward compressional movement of the Southern Qiangtang Block during the Middle-Late Jurassic is attributed to the northward subduction of the Bangong-Nujiang Tethyan Ocean. While the southward compressional movement of the Southern Lhasa Block during the Cretaceous is associated to the southward subduction of the same oceanic domain. (2) The volcanic breccia and detrital sandstone examined in this study are hosted within the lower part of the Shamuluo Fm., yielding a depositional age of 112-95 Ma. This temporal window corresponds to a critical transition period during which the Bangong-Nujiang Tethyan Ocean underwent a fundamental shift in subduction polarity from northward to southward. (3) The Bangong-Nujiang Tethyan Ocean initially underwent northward subduction during the Mid-Late Jurassic, followed by a tectonic polarity reversal (~ 112-102.9 Ma) spanning the Late Jurassic to Early Cretaceous transition, ultimately establishing southward subduction patterns by the Early Cretaceous. The southward subduction persisted until the Early Shexing Fm. (~ 103.8-92 Ma), with its cessation marking the complete tectonic amalgamation between the Lhasa and Qiangtang Blocks.