Timing and mechanism of formation and exhumation of the Northern Qaidam ultrahigh-pressure metamorphic belt

The Qilian Caledonian orogenic belt on the north margin of the Qinghai-Tibet Plateau were formed by convergence and collision of the Alxa terrain, Qilian terrain and Qaidam terrain. The 350-km-long, WNW–ESE-trending North Qaidam ultrahigh-pressure (UHP) metamorphic belt, lying between the Qilian and Qaidam terrains, was formed between 495 and 440 Ma by deep subduction of the South Qilian Sea and the Qaidam continental crust beneath the Qilian terrain. The UHP belt was exhumed by a process of ‘oblique extrusion’ during transformation from ‘normal’ to ‘oblique’ intracontinental subduction between the Qilian and Qaidam terrains. Exhumation began at 470–460 Ma and was completed by 406–400 Ma. Exhumation structures are well-preserved in the UHP rocks and record extensive retrograde metamorphism.