THE COUPLING MECHANISM OF BASIN AND OROGEN IN THE WESTERN ORDOS BASINAND ADJACENT REGIONS OF CHINA

During the Late Triassic and Jurassic, three structural belts develope d in the western Ordos Basin and adjacent regions of China: the Alxa c ompressive wedge, the Helanshan structural belt and the Liupanshan fol d and thrust belt. Three types of basin also developed: a Late Triassi c composite basin with a northwestern Ordos rift sub-basin and a south western Ordos foreland sub-basin; an Early-Middle Jurassic intracraton ic basin; and a Late Jurassic foreland-type molasse wedge. It is sugge sted that these basins resulted from Late Triassic unconstrained later al extrusion and Jurassic constrained lateral extrusion. The Alxa regi on is interpreted as an extrusional wedge bounded by strike-slip defor mation which escaped laterally from the indentation of the Liupanshan thrust belt from the south. During the Late Triassic the Sino-Korean B lock to the east of Helanshan was controlled by a stress state with ne arly E-W trending extension. At this stage extrusion was unconstrained . The Helanshan developed as a rift sub-basin due to transtensional te ctonics at the lateral margin of the wedge. At this time, a foreland s ub-basin was formed in front of the Liupanshan thrust belt. During the Jurassic, the Sino-Korean Block to the east of the Helanshan was grad ually compressed westwards acid the extrusion structure became constra ined. The Early-Middle Jurassic shows a transitional stage from uncons trained to constrained extrusion, and the Ordos Basin developed as an intracratonic basin. From the latest Middle Jurassic to the Late Juras sic, intense lateral escape and opposite compression led to the format ion of the Helanshan fold and thrust belt and its frontal foreland mol asse wedge. At the same time another foreland molasse wedge was develo ped in front of the Liupanshan thrust belt. The switch from rifting to thrusting in the Helanshan is a result of the changing stress field a t the Circum-Pacific plate boundary, the clockwise rotation of the Sin o-Korean Block to the east of the Helanshan and the anticlockwise rota tion of the Alxa Block. (C) 1998 Elsevier Science Ltd. All rights rese rved.