Apatite-fission-track geochronology and its tectonic correlation in the Dabieshan orogen,central China

AFT data of granitoid and metamorphic samples from the Dabieshan orogen have an age range between 96.4 Ma and 41.9 Ma coupled with confined track lengths of 11.5—14.0 μm, reflecting the cooling time of rocks differentially through the 100℃± isotherm. The Juras- sic-Eogene sediments from the North Huaiyang and Hefei basin, however, yield their AFT ages of 128.8—62.0 Ma with mean track lengths of 8.6—11.9 μm, recording a complicated thermal de- velopment of the basin. These AFT data structurally allowing the coupling between the orogen and the basin are to a great extent considered as the result from the control of NNE-trending regional tectonics. It took place approximately at 95—90 Ma that the orogen developed with an abrupt shift from the doming extension to the differential transtension, which is assumed to be related with the changes of the convergence between the Western Pacific and Eurasia plates. The differentially sinistral strike-slipping from the transtension between Tanlu and Shang- cheng-Macheng faults brought the doming extension of the orogen shrunk sharply in Late Cre- taceous. In Eogene, the full-grown transtension that dominated in East Asia was caused mainly by a rapid decrease of the rates of the Western Pacific convergence, which can be therefore re- garded as a major mechanism creating the zonal anomalies of 60—40 Ma in the orogen. The thermal anomaly of 70—40 Ma zonally along the Tanlu fault cooled more slowly than the anomaly of 60—55 Ma zoning along the Shangcheng-Macheng fault, but the 60—40 Ma anomaly trending E-W in hinterlands of the orogen seems to be relicts of the doming extension as a result of inten- sive rebuilding by the transtension. In addition, the orogen was also reactivated by the far-field effect from India-Asia collision in Eogene, where major NWW-trending faults played an important role in facilitating such far-field tectonic transferring. In Oligocene to Miocene, the Dabieshan orogen remained a few thermal disturbances with their AFT ages particularly in coincidence with the time of opening and closing of the South China Sea.

Late Cretaceous-Cenozoic exhumation history of Tiantangzhai region of Dabieshan Orogen: Constraints from (U-Th)/He and fission track analysis

Integrated fission track and (U-Th)/He analysis is carried out on 6 apatite and 6 zircon samples from a near vertical section in The Tiantangzhai region at the core of the present Dabieshan orogen. The result shows that the region experienced cooling/exhumation during the Late Cretaceous and Early Tertiary period. Age-elevation relationships for different dating systems and different minerals suggest a pulse of rapid exhumation at ～110 Ma before present, preserved in the structurally highest samples. At lower elevations, ages begin to decrease with decreasing elevation, suggesting lower exhumation rates since 90 Ma. Two periods of different exhumation rates are identified since 90 Ma. The average apparent exhumation rate for the period of 43.4-22.5 is 0.062 km/Ma, whereas that for the period of 76.4-47.4 Ma is 0.039 km/Ma.