Geodynamic evolution of Loei and Phetchabun regions - Does the discovery of detrital chromian spinels from the Nam Duk formation (Permian, north-central Thailand) provide new constraint?

The discovery of the chromian spinel detrirus in sandstones from the Permia n Nam Duk Formation (Phetchabun region) confirms that the siliciclastic pan of this formation is related to the erosion of a mountain belt caused by c ompressional deformation. The question discussed is whether this detritus i s derived from an older orogen exposed today in the region of Loei east of the Nam Duk Basin, or it is from a mountain belt which evolved during Permi an times in the west (Nan-Uttaradit region). In the first case the Nam Duk Formation would represent the passive contine ntal margin sequence of the "Indochina craton", and in the second case it w ould be part of the sedimentary wedge associated with the compressional def ormation and subsequent uplift to the erosional level during Middle to Uppe r Permian of a rising mountain belt further in the west. In this contributi on, arguments in favour of the second scenario are discussed. The region cl ose to the Nan-Uttaradit suture zone shows evidence of compressional deform ation and subsequent uplift to an erosional level in the Permian when the c hromian spinel detritus was deposited in the Nam Duk Formation.