Uniform Permian 40Ar/39Ar detrital mica ages in the eastern Qaidam Basin (NW China): where is the source?

New single-grain 40Ar/39Ar detrital white-mica ages from the Lulehe section at the eastern Qaidam Basin yield uniform Permian ages between 250 +/- 3 and 279 +/- 3 Ma throughout the whole Cenozoic sequence. This is inconsistent with the present hinterland, which is composed of early Palaeozoic metamorphic units with subordinate early Palaeozoic and few Permian granites. The new data indicate that Permian tectonic units are likely more widespread at the north-eastern margin of the Tibetan plateau as known at present, particularly within the Qilian Mountains. The preferred explanation is that the Qaidam block represents a rigid indenter, which indented during late Tertiary times into early Palaeozoic orogenic units. This is consistent with recent findings of a NW-trending sinistral Permian ductile shear zone and a dextral, NW-trending Tertiary fault system close to the north-eastern margin of the Qaidam Basin.