Superposed deformation in the Helanshan Structural Belt: Implications for Mesozoic intracontinental deformation of the North China Plate

• Syn-folding deformation shows a two-stage Mesozoic compressive field in the HSB.
• The first tectonic event is presented by NNE–SSW shortening in T3–J1.
• The WNW–ESE shortening then led to the formation of the superposed folds in J3–K1.
• The HSB experienced the two-phase contractional deformation in the Mesozoic.

The North China Plate (NCP) underwent multi-phase intracontinental deformation during the Mesozoic, which is a much-debated research subject. Here we present a case study of this deformation inferred from regional-scale Helanshan superposed folds in the west of the NCP. New structural analyses and fault kinematic analyses in the Helanshan Structural Belt (HSB) indicate a two-stage syn-folding paleo-stress field, comprising early NNE–SSW compression, with mean orientations 190°/01° (plunge direction/plunge) for σ1, 283°/26° for σ2, and 100°/63° for σ3, and later WNW–ESE compression with mean orientations of σ1, σ2, and σ3 at 124°/07°, 213°/04°, and 001°/75°, respectively. These results, combined with geochronological analysis, indicate two episodes of shortening in this belt during the Mesozoic. Late Triassic–Early Jurassic NNE–SSW shortening, which produced ∼8.1 km of horizontal shortening, led to the development of open WNW-trending fold structures. Subsequent Late Jurassic–Early Cretaceous WNW–ESE shortening, which produced ∼18.7 km of horizontal shortening, reshaped the early WNW-trending structures and generated close, overprinting NNE-trending folds that resulted in the development of the regional-scale superposed folds in this area. In terms of geodynamics, the early NNE–SSW shortening is likely to be related to Indosinian intercontinental convergence between the North China Plate and the Yangtze Plate, and the later WNW–ESE shortening is likely to have been related to subduction of the Paleo-Pacific Plate. The regional structural analysis indicates that the whole of the western North China Plate experienced two-phase contractional deformation during the Mesozoic, as manifest by the early NNE–SSW and the late WNW–ESE shortening that characterized Mesozoic intracontinental deformation in the western NCP.

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