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 r ocks and granites generally considered as ''basement'' rocks formed du ring an early Paleozoic orogeny. Our structural and radiometric reappr aisal of this massif shows that it results from a Mesozoic doming. In the metamorphic rocks, the foliation defines an E-W elliptical shape a nd bears a N-S stretching lineation. The axial zone of the dome is occ upied by oriented plutons, The outer part of the dome consists of Devo nian 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, respect ively, The same divergent pattern of motion is recorded from brittle s tructures up to the Jurassic sandstone, The late Cretaceous red beds a re undeformed. In the axial zone, coaxial Bow indicated by symmetric p ressure shadows and quartz fabrics predominates. Along the N and S fla nks of the dome, post-foliar recumbent folds are symmetrically overtur ned to the N and S respectively, but at the two extremities of the dom e, the fold axes are parallel to the stretching Lineation. Ar-40/Ar-39 ages of biotite and muscovite from mylonite and granite suggest a lat e 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, s outheastern 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 th ermal softening and gravitational instability that allowed the crust t o extend. The formation of the Wugongshan dome and other metamorphic c ore complexes appears to be a the direct consequence of plutonic activ ity and crust rheology.