A Mesozoic orogenic plateau across South China

Abstract

The South China Block (SCB) underwent intense magmatism, deformation and mineralisation in the Mesozoic. These events were the combined result of Triassic collisions along three plate margins, and subduction of the Paleo-Pacific Ocean from the east. Beyond this framework there are gaps in our understanding. We propose that growth of an orogenic plateau controlled the Mesozoic tectonic and magmatic evolution of South China. The plateau was similar to the modern Central Andes, but >1000 km across. Thickening was the result of compressional stresses generated at the Paleo-Pacific convergent margin, and continental collisions at other margins. Retroarc, compressional deformation migrated westwards through the Jurassic and Cretaceous, thickening crust as far as the Sichuan Basin. Structures are convex towards the foreland, consistent with the influence of gravitational flow. Two sets of folds occur in the Eastern Yangtze Fold Zone. One set of folds is roughly concentric, convex towards the plate interior. The other set is radial and contains nine long wavelength folds (~100 km). Folding may be the result of constrictional strain, producing two sets of structures simultaneously. We find little evidence for Triassic or Jurassic extension, which is commonly inferred from pluton geochemistry. Regional extension began in the Early Cretaceous at ~135 Ma and was oriented WNW-ESE. Extension was plausibly triggered by a change in the regional force balance, itself related to plate re-organization. Extension in the interior was contemporary with thrusting to the west, in the East Sichuan fold-and-thrust belt, and to east, in the Southeast Coast Magmatic Belt; this configuration is difficult to explain solely by Paleo-Pacific plate rollback or foundering mechanisms. Permian and Mesozoic magmatism took place within regions affected by progressive crustal thickening from the Permian to the Early Cretaceous. A ~700 km wide magmatic belt was in existence by the Late Triassic, ~60 Myr after initial Paleo-Pacific subduction. Magmatism includes I-, S- and A-type granites. Different compositions were broadly contemporary, with a tendency for A-type intrusions to postdate other types. Magmatism continued through the Jurassic and Cretaceous, with lulls inland at 200-180 Ma and 115-100 Ma. There were lateral shifts in the focus of magmatism over time, with a pronounced move to the southeast in the Early Cretaceous. A-type granites may result from high temperature crustal melting, generated in thickened crust by radiogenic heating. We propose that thick, hot, lower crust of orogenic plateaus is a productive tectonic setting for the generation of A-type granites, without needing extensional strain and crustal thinning, in regions dehydrated by prior melt extraction during orogeny. Within-plate signatures in rare Jurassic mafic rocks may relate to lithospheric drips. Topographic plateau relicts are distributed across South China, including the Wu Shan and Xuefengshan ranges. Scattered surfaces at 1500-2000 m altitude may be remnants of an extensive plateau, and consistent with an original crustal thickness of ~45-50 km. Plateau development took place between the Late Triassic and Early Cretaceous. The minimum age is constrained by Lower Cretaceous syn-rift strata which disrupt the plateau remnant surfaces, and by ~135 Ma extensional core complexes.