Cretaceous to Paleogene polyphase extension and geodynamics of the Hong’an–Dabie orogenic belts, central China

Abstract Intense extension generally dismembers and reworks preexisting orogenic belts. During the Cretaceous to Paleogene, polyphase extension occurred in the Permian to Triassic Hong’an–Dabie orogenic belts, modifying their structural configuration. However, extensional orientations, formation ages, and the underlying geodynamics of these polyphase extension episodes remain unclear, thus hindering the accurate restoration of the Hong’an–Dabie orogenic belts. In this study, structural analysis and paleostress inversions, zircon and apatite U-Pb geochronology, and whole-rock element and Sr-Nd isotope geochemistry were carried out for the Hong’an–Dabie orogenic belts. The crosscutting relationships and paleostress inversions revealed three distinct extensional orientations in chronological order: (1) NW-SE, (2) WNW-ESE, and (3) NNE-SSW. Zircon and apatite U-Pb geochronology also pinpointed three stages of magmatic and hydrothermal activity: (1) 121–117 Ma granitic emplacement, (2) ca. 112 Ma basalt eruption, and (3) 101–95 Ma hydrothermal activity. Whole-rock element analysis and Sr-Nd isotopic compositions indicate that the ca. 117 Ma Baiyashan biotite granite belongs to the A2-type granitoid, indicative of a postcollisional extensional setting, while the ca. 112 Ma Fanjiahe basalt belongs to the continental rift–type basalt formed in a within-plate extensional environment. Combining these results with structural analysis and paleostress results, three stages of intracontinental extension can be established for the Hong’an–Dabie orogenic belts during the Cretaceous to Paleogene interval, including (1) early to middle Early Cretaceous (140–117 Ma) NW-SE extension, (2) latest Early Cretaceous to early Paleogene (112–53 Ma) WNW-ESE extension, and (3) middle Paleogene (51–36 Ma) NNE-SSW extension. Notably, the first phase of extension, marked by low-angle ductile shearing, played a significant role in reshaping the original pattern of the Hong’an–Dabie orogenic belts. The Cretaceous to early Paleogene extension and the middle Paleogene extension in the Hong’an–Dabie orogenic belts were driven by slab rollback of the paleo-Pacific plate and the far-field effects of the India-Asia collision, respectively.