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 D2 and D3 deformation. The early D2 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 D3 deformation characterized by a gentle-dipping S-3 foliation, subhorizontally to shallowly plunging L3 lineation, some reactived-D2 folds and low-angle normal faults. The D3 fabrics suggest a sinistral transtensional regime under greenschist-facies metamorphism. The timing of the D2 and D3 events have been constrained to the early to middle Triassic (similar to 248-220 Ma) and late Triassic (similar to 220-200 Ma) respectively on the basis of 40Ar/39Ar geochronology and regional geological relations. The change from oblique thrusting (D2) to sinistral transtension (D3) 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.Publisher PDFPeer reviewe

Detrital zircon U-Pb geochronology, Hf isotopes and geochemistry constraints on crustal growth and Mesozoic tectonics of southeastern China

In situ U–Pb geochronological, Hf isotopic and REE geochemical analyses of detrital zircons from Mesozoic sandstones in central Jiangxi and east Hunan of the South China Block (SCB) are used to provide not only information about crustal evolution process, but more importantly new constraints on sedimentary provenances as well as Mesozoic basin evolutions in central SCB. A total of 417 concordant zircon analyses define five U–Pb age populations at ca. 2.6–2.3 Ga, ca. 2.0–1.7 Ga, ca. 1.0–0.7 Ga, ca. 500–400 Ma and ca. 300–200 Ma. Integrated analyses of zircon U–Pb ages and Hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth: ca. 2.5 Ga, ca. 1.7 Ga, ca. 850 Ma, ca. 440 Ma and ca. 230 Ma, with all but the ca. 2.5 Ga episode likely represent that of the SCB.None of the three samples from T3–J1 strata showed strong ca. 1850 Ma and ca. 230 Ma peaks suggesting that the T3–J1 sediments probably sourced from the Yangtze Block. In contrast, stronger peaks of ca. 1850 Ma and ca. 250–230 Ma in post-J1 strata relative to that of T3–J1 strata suggest a dominantly local Cathaysian provenance. In addition, the distinct low εHf(T) values for zircons of ca. 430 Ma from T3–J1 strata in comparison with higher εHf(T) values for that from post-J1 strata also support aforementioned viewpoint. Such sharp changes between the pre-J1 and post-J1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the Late Triassic–Early Jurassic time to a basin-and-range-style province in the Middle Jurassic. The characteristics and time evolution of detrital zircons from the studied area are consistent with the flat-slab subduction model which involves the development of a broad sag basin during the T3–J1 time, and a rapid regional uplift in the Cathaysia Block caused by the reinitiating of normal subduction along the coastal region at ca. 190 Ma