The Triassic tectonic transition of the Northwestern margin of Ordos Basin, North China Craton

The Northwestern margin of Ordos Basin (NWOB), which contains the Helanshan Tectonic Belt (HTB) and Northwestern margin of Ordos Basin Thrust Belt (NWOBTB), is situated in the northwest of the North China Craton (NCC). The Triassic sedimentary-tectonic transition of the NWOB provides crucial insights for revealing the tectonic evolution of the NCC. In this study, we have utilized a comprehensive dataset including the palaeocurrent orientations, sandstone modal composition, U-Pb age analysis of detrital zircons, and balanced cross-sections restorations, to clarify the Triassic sedimentary-tectonic evolution of NWOB. The southeast-directed palaeocurrent indicates the sediments were primarily derived from northwest. The composition of clastic material suggests diverse sources, including magmatic arc and continental blocks. Detrital zircons extracted from four sandstones yield five major LA-ICP-MS U-Pb age groups: 2600–2500 Ma, 2300–2200 Ma, 2000–1800 Ma, 500–400 Ma and 390–230 Ma. A comparison of these U-Pb age groups from the Triassic strata in the NWOBTB with the magmatic records in the Alxa Massif, the Qilian Orogenic Belt, and detrital zircons from pre-Triassic strata in the HTB reveals that the Triassic detritus in the NWOBTB was predominantly originated from the Alxa Massif. Furthermore, the absence of detrital zircons in NWOBTB that originated from the HTB suggests the HTB had not been uplifted until the Mid-Late Triassic and remained in a steady sedimentary environment with NWOBTB during this period. In addition, the E-W treading shortening structures, along with the angular unconformity between the Jurassic and the Upper Triassic in the HTB, indicate a phase of extensive N-S shortening occurred in the HTB in the end of the Triassic. Balanced cross-section restorations demonstrate the southern HTB experienced 27% shortening, while the middle HTB underwent 15% shortening. In contrast, the Jurassic strata are in parallel-unconformity with the Triassic strata in the NWOBTB. This suggests the NWOBTB was not significantly effected by the end Triassic N-S shortening deformation. These distinct deformations between the HTB and NWOBTB imply that there was a phase of dextral strike-slip displacement between the two regions in the end of the Triassic, which is related to the closure of Paleo Tethys Ocean.