Polyphase deformation in the Badu complex: Insights into Triassic intraplate orogeny in South China

Abstract

The occurrence of Triassic high-pressure eclogite- and high-temperature granulite-facies metamorphism in the Badu complex contrasts with regional greenschist-to amphibolite-facies metamorphism in most of South China, hence calling for unraveling related structural and geodynamic processes within a regional intraplate-orogenic setting. We analyzed the geometry, kinematics, and overprinting relationships of structures in the central Badu complex, and dated the structural evolution via geo-thermochronology. We identified deformation episodes D1 to D4: D1 features rootless, tight-isoclinal folds (F1), and an initially likely gentle-dipping foliation (S1) with unclear kinematics, with the majority of structures modified/erased by later deformations; D2 comprises a gentle-dipping foliation (S2), and a ∼N-trending lineation (L2), with dominant top-to-the-south shear subparallel or slightly oblique to the strike of the regional Triassic orogen; D3 includes open to tight, upright folds with subhorizontal axes reflecting orogen-normal, ∼E-W shortening; D4 features ∼ N-trending reverse faults. D1 is bracketed between ∼245 and ∼235 Ma and occurred at temperatures >600 °C, following regional high-pressure metamorphism (251‒245 Ma). D2 and D3 occurred between ∼235 and ∼221 Ma under retrograde, amphibolite-to greenschist-facies metamorphic conditions, with cooling rates of ∼17–21 °C/Myr. By combining our new data with published work on the Badu complex, we trace a >120-km-long, ∼25-km-wide, N- to NNE-striking, high-grade metamorphic and intense deformation zone that represents a localized but distinct anomaly in the vast (∼1300-km-wide) Triassic structural belt in South China. The high-pressure metamorphism, the orogen-subparallel shearing, and the orogen-normal shortening suggest localized (oblique) underthrusting of the Coastal terrane beneath the Wuyishan terrane during the Triassic intraplate orogeny within South China.