MULTIPHASE DEFORMATION AND DISPLACEMENT WITHIN A BASEMENT-COMPLEX ON A CONTINENTAL-MARGIN - THE WUDANG COMPLEX IN THE QINLING OROGEN, CHINA

The structural evolution of the Wudang Complex in the foreland of the Southern Qinling has been resolved through detailed field mapping and microstructural analysis. Four major deformational episodes were devel oped within the complex. These involved: (1) accretion of preliminary continental nuclei to form the basement of the South China Block in th e late Mesoproterozoic-early Neoproterozoic; (2) extension and subside nce of the continental margin associated with accumulation of thick se diments from the end of the Neoproterozoic to the mid-Triassic; (3) th rust stacking of basement and cover during collision-detachment orogen esis in the Late Triassic to Early Cretaceous; and (4) development of intermontane basins since the Late Cretaceous. The Wudang Complex (inc luding the Wudang and Yaolinghe Groups) is considered Mesoproterozoic in age, based on (1) recent isotopic data, such as a 1304 Ma zircon ag e, and (2) recognition of new stratigraphic and structural relationshi ps, such as the supposed discontinuous sedimentary contact between the Yaolinghe Group and Sinian rocks which actually is a detachment zone. Wide ranging isotopic ages in the Wudang Complex could be due to (1) emplacement of new material associated with magma during extension of the continental margin, and (2) reworking and overprinting of early fa brics during basement subsidence and burial. The Mesozoic orogeny resu lted in renewed intense deformation of the basement complex, but prese rvation of early fabrics in these rocks allowed analysis of the kinema tics of Proterozoic accretion on the northern margin of the preliminar y continental nuclei. The foliation intersection axes (FIA) preserved in porphyroblasts within the Wudang Complex display a consistent 115-d egrees trend independent of the strike of bedding and late foliations. This suggests that the bulk orientation of material movement may have been about N25-degrees-E to S25-degrees-W during that accretion event .