Abstract
A major intracontinental Mesozoic continental basin in Huabei plate was developed when the Yangzi plate obducted towards Huabei plate.The collision has caused the lithosphere of the former to be depressed and forced the isostatic compensation surface to low down,thus leading to its further development.The compressional stress was exercised on and controlled by both the Tethys and Pacific plates.The eastern margin of this basin had gradually migrated westward under the influence imposed through activities of the Pacific plate.In the Later Trias-sic,this basin involved an extensive area west of Tanlu fracture zone;in the Jurassic,its eastern margin migrated to the west of the Taihang Mountains;whereas in Early Cretaceous,the basin was confined only in Ordos region.Meanwhile,coarse wastes of great thickness were accumulated along the northand south parts of Helan Mountains.The Alxa deltaic massif,being compressed both by northward pushing of the Tethys plate and the southward advancing of the Augaris,produces an eastward component force,and,with the lapse of time,results in a eastern migrating fore-deep type depression along the eastern margin of the massif.Because of the continuous activities of the Tethys and Pacific plates,the Earlier Cretaceous basin continued to remain in the Ordosregion,and has brought about its original configuration ever since.Thus the influence of the tectonic activities can be seen clearly from the regular anti-clock wise migration of the depocentres during the Later Triassic,Jurassic and Earlier Cretaceous.
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