Indosinian high-strain deformation for the Yunkaidashan tectonic belt, south China: Kinematics and ⁴⁰Ar/ ³⁹Ar geochronological constraints

Structural and ⁴⁰Ar/ ³⁹Ar data from the Yunkaidashan Belt document kinematic and tectonothermal characteristics of early Mesozoic Indosinian orogenesis in the southern part of the South China Block. The Yunkaidashan Belt is tectonically divided from east to west into the Wuchuang-Sihui shear zone, Xinyi-Gaozhou block, and the Fengshan-Qinxi shear zone. Indosinian structural elements ascribed to the Indosinian orogeny include D ₂ and D ₃ deformation. The early D ₂ phase is characterized by folding and thrusting with associated foliation and lineation development, related to NW-SE transpression under amphibolite- to greenschist-facies conditions. This event is heterogeneously overprinted by D ₃ deformation characterized by a gentle-dipping S ₃ foliation, subhorizontally to shallowly plunging L ₃ lineation, some reactived-D ₂ folds and low-angle normal faults. The D ₃ fabrics suggest a sinistral transtensional regime under greenschist-facies metamorphism. The timing of the D ₂ and D ₃ events have been constrained to the early to middle Triassic (∼248-220 Ma) and late Triassic (∼220-200 Ma) respectively on the basis of ⁴⁰Ar/ ³⁹Ar geochronology and regional geological relations. The change from oblique thrusting: (D ₂) to sinistral transtension (D ₃) may reflect oblique convergence and crustal thickening followed by relaxation of the overthickened crust. In combination with the regional relations from Xuefengshan to Yunkaidashan and on to Wuyishan, the early phase of the Indosinian orogeny constituted a large-scale positive flower structure and is related to the intracontinental convergence during the assembly of Pangea in which the less competent South China Orogen was squeezed between the more competent North China and Indosinian Blocks.