Structural and kinematic analysis of the Early Paleozoic Ondor Sum-Hongqi mélange belt, eastern part of the Altaids (CAOB) in Inner Mongolia, China

Abstract : We present a stuctural and kinematic study of an Early Paleozoic subduction mélange and a magmatic arc that form the main elements of the Southern Orogen Belt of Inner Mongolia, which lies in the eastearn part of the Altaids or Central Asia Orogenic Belt. The structural analysis of the mélange conducted in the Hongqi and Ondor Sum areas (western Inner Mongolia) shows two phrases of ductile deformation. The D1 event is responsible for the pervasive S1 foliation, NW-SE striking L1 stretching lineation and F1 intrafolial folds. These microstructures are coeval with a greenschist facies metamorphism. During D2, NW-verging F2 folds associated with a S2 axial planar cleavage deformed S1 and L1. The D1 kinematic criteria indicate a top-to-the-NW sense of shear. D1 and D2 developed before the unconformable deposition of the Early Devonian shallow water sandstone. A lithosphere scale geodynamic model involving an Early Paleozoic southeast-directed subduction beneath the North China Craton and late Silurian collision of the North China Craton with an hypothetical microcontinent is proposed to account for the microstructural evolution.