Extensional tectonics within a subduction-type orogen. The case study of the Wugongshan dome (Jiangxi Province, southeastern China)

In southeastern China, the Wugongshan massif consists of metamorphic rocks and granites generally considered as "basement" rocks formed during an early Paleozoic orogeny. Our structural and radiometric reappraisal of this massif shows that it results from a Mesozoic doming. In the metamorphic rocks, the foliation defines an E-W elliptical shape and bears a N-S stretching lineation. The axial zone of the dome is occupied by oriented plutons. The outer part of the dome consists of Devonian quartzite, Carboniferous-Permian limestone and Triassic-Jurassic sandstone. Ductile deformation observed in the Permian carbonates and older units develops contrasting kinematics. The northern and southern flanks present top-to-the-north and top-to-the-south motions, respectively. The same divergent pattern of motion is recorded from brittle structures up to the Jurassic sandstone. The late Cretaceous red beds are undeformed. In the axial zone, coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates. Along the N and S flanks of the dome, post-folial recumbent folds are symmetrically overturned to the N and S respectively, but at the two extremities of the dome, the fold axes are parallel to the stretching lineation. {⁴⁰Ar}/{³⁹Ar} ages of biotite and muscovite from mylonite and granite suggest a late Triassic age for the ductile deformation and an early Cretaceous age for the final doming. The Wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime. A model which emphasizes plutonism is discussed. From Permian to early Cretaceous, southeastern China was a subduction-type orogen, the crust of which was thickened by accretion of mantle derived magmas. The emplacement of a large amount of Mesozoic granitoids in southeastern China promoted thermal softening and gravitational instability that allowed the crust to extend. The formation of the Wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology.