Transpressional deformation, strain partitioning and fold superimposition in the southern Chinese Altai, Central Asian Orogenic Belt

Abstract Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S 1 ), is locally recognized in low strain areas, and is commonly isoclinally folded by F 2 folds that have an axial plane orientation parallel to the dominant fabric (S 2 ). S 2 is associated with a shallowly plunging stretching lineation (L 2 ), and defines ∼NW-SE tight-close upright macroscopic folds (F 3 ) with the doubly plunging geometry. F 3 folds are superimposed by ∼NNW-SSE gentle F 4 folds. The F 3 and F 4 folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F 3 fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F 3 ) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F 4 folds.