The coupling mechanism of basin and orogen in the western Ordos Basin and adjacent regions of China

Abstract

During the Late Triassic and Jurassic, three structural belts developed in the western Ordos Basin and adjacent regions of China: the Alxa compressive wedge, the Helanshan structural belt and the Liupanshan fold and thrust belt. Three types of basin also developed: a Late Triassic composite basin with a northwestern Ordos rift sub-basin and a southwestern Ordos foreland sub-basin; an Early–Middle Jurassic intracratonic basin; and a Late Jurassic foreland-type molasse wedge. It is suggested that these basins resulted from Late Triassic unconstrained lateral extrusion and Jurassic constrained lateral extrusion. The Alxa region is interpreted as an extrusional wedge bounded by strike-slip deformation which escaped laterally from the indentation of the Liupanshan thrust belt from the south. During the Late Triassic the Sino-Korean Block to the east of Helanshan was controlled by a stress state with nearly E–W trending extension. At this stage extrusion was unconstrained. The Helanshan developed as a rift sub-basin due to transtensional tectonics at the lateral margin of the wedge. At this time, a foreland sub-basin was formed in front of the Liupanshan thrust belt. During the Jurassic, the Sino-Korean Block to the east of the Helanshan was gradually compressed westwards and the extrusion structure became constrained. The Early–Middle Jurassic shows a transitional stage from unconstrained to constrained extrusion, and the Ordos Basin developed as an intracratonic basin. From the latest Middle Jurassic to the Late Jurassic, intense lateral escape and opposite compression led to the formation of the Helanshan fold and thrust belt and its frontal foreland molasse wedge. At the same time another foreland molasse wedge was developed in front of the Liupanshan thrust belt. The switch from rifting to thrusting in the Helanshan is a result of the changing stress field at the Circum–Pacific plate boundary, the clockwise rotation of the Sino-Korean Block to the east of the Helanshan and the anticlockwise rotation of the Alxa Block.