Deformation history of the Hengshan–Wutai–Fuping Complexes: Implications for the evolution of the Trans-North China Orogen

The Trans-North China Orogen separates the North China Craton into two small continental blocks: the Eastern and Western Blocks. As one of the largest exposure in the central part of the orogen, the Hengshan–Wutai–Fuping Complexes consist of four lithotectonic units: the Wutai, Hengshan and Fuping Complexes and the Hutuo Group. The Hengshan Complex contains high pressure mafic granulites and retrograded eclogites. Structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding (D1 to D3) and two stages of ductile thrust shearing (STZ1 between D1 and D2 and STZ2 after D3). The D1 deformation formed penetrative axial planar foliations (S1), mineral stretching lineations (L1), and rarely-preserved small isoclinal folds (F1) in the Hengshan and Fuping Complexes. In the Wutai Complex, however, large-scale F1 recumbent folds with SW-vergence are displayed by sedimentary compositional layers. Penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones (STZ1). The kinematic indicators of STZ1 in the Hengshan and Wutai Complexes show top-to-the-S230°W thrusting likely related to northeastward, oblique pre-collisional subduction. D1 resulted in crustal thickening with resultant prograde peak metamorphism. The Hutuo Group did not undergo the D1 deformation, either because sedimentation was coeval with the D1 deformation or because it was at a high structural level and was not influenced directly by the early deformation. The D2 deformation produced NW-verging asymmetric and recumbent folds. The D2 deformation is interpreted to have resulted from collision between the Eastern and Western Blocks of the North China Craton. In the Hutuo Group and the Fuping Complex, the development of ESE-verging asymmetric tight folds is associated with D2. The structural pattern resulting from superimposition of D1 and D2 is a composite synform in the Hengshan–Wutai–Fuping Complexes. All four lithotectonic units were superposed during the later D3 deformation. The D3 deformation developed NW-trending open upright folds. Ongoing collision led to development of transpressional ductile shearing (STZ2), forming the transpressional Zhujiafang dextral ductile shear zone between the northern Hengshan Complex and the southern Hengshan Complex, and generating the sinistral Longquanguan ductile shear zone between the Fuping Complex and the Wutai Complex, respectively. The STZ1 and D2 deformation were possibly responsible for fast syn-collisional exhumation of the high pressure mafic granulites and retrograded eclogites. The structural patterns and elucidation of the deformation history of the Hengshan–Wutai–Fuping Complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the Eastern and Western Blocks underwent northeastward-directed oblique subduction beneath the western margin of the Eastern Block, and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the North China Craton.