Compression to extension switch during the Middle Triassic orogeny of Eastern China: the case study of the Jiulingshan massif in the southern foreland of the Dabieshan

Abstract : The Jiulingshan massif is an E–W trending anticlinorium in the South China Block (SCB) that forms the southern foreland of the Qinling–Dabie orogen. The Jiulingshan consists of Middle Proterozoic (ca. 1 Ga–800 Ma) metamorphic and plutonic rocks unconformably overlain by Sinian to Triassic rocks. In these cover rocks, two episodes of deformation of pre-Late Triassic age are recognized. Namely, a N–S compressional phase characterized by south verging thrusts, upright folds and vertical cleavage followed by an extensional phase characterized by layer parallel slip, collapse folds and extensional allochthons of Proterozoic slate on top of Carboniferous to Middle Triassic carbonates. The extensional tectonics appears to be a mechanical consequence of the compressional tectonics. On a regional scale, the Jiulingshan massif forms one of the domed crustal scale shear zones of the SCB that accommodated the continuing convergence that immediately followed the Qinling–Dabie collision.