Triassic contractional deformation and sedimentary response in the Helanshan fold–thrust belt, Northwest China: evidence from growth strata

The Helanshan fold – thrust belt is located between the Alxa and Ordos blocks and has been a deformation area of tectonic activity since the Mesozoic. During the Mesozoic tectonic deformation, the Jurassic WNW – ESE-trending contraction transformed the Triassic NNE – SSW shortening. Field mapping and structural analysis reveal that the Middle Triassic Zhifang Formation in the northern wing of the Rujigou overturned anticline is characteristic of a growth strata, which is in response to SSW-NNE contraction. Detrital zircon U – Pb age spectra of the Middle Triassic Zhifang Formation show that the Triassic sedimentary materials originated from the North China Craton to the north, the Alxa block to the north, the Qilian orogenic belt to the south, and the Qinling orogenic belt to the south. The youngest age at the bottom of the growth strata is 282 ± 11 Ma, and the youngest age at the top is 291 ± 12 Ma, indicating that the strata maximum depositional age was later than the Permian, revealing that the youngest detrital zircon of the Middle Triassic Zhifang Formation is derived from the Rujigou overturned anticline, reflecting the characteristics of the reverse sedimentary sequence of detrital zircon and suggesting an interaction between structure and deposition. We propose that with the closure of the Paleo-Tethys Ocean, under the far-field response to the collision between the North China Craton and South China Craton, the Helanshan fold – thrust belt expanded northward and formed a northward protruding tectonic node that controlled the sedimentary process of the Triassic foreland basin.