Compound intracontinental piedmont thrust structure transformation and its formation mechanisms

The transformation types from thick-skinned to thin-skinned structures in compound intracontinental piedmont controls the development of complicated thrust structures in central and western China. No single dynamic model can generalize and explain the causes of the multi-type thrust structure. Joint analysis of deep and shallow structures shows that there are four types of transformation from thick-skinned to thin-skinned structures in the piedmont of central and western China. These are named transition zones Ⅰ, Ⅱ, Ⅰ+Ⅱ and the non-transition zone, according to the form of forward transfer and expansion of tectonic displacement and the geological structure it produces. The four transformation types have specific structural characteristics. The first three types are mainly converted by basement detachment and slippage. Due to the development of a deep slippage layer and different boundary conditions, different structural styles such as imbricate thrusting, triangular zone and double structure are formed. As to the non-transition type, overthrust faults accommodate the abrupt transformation between thick-skinned and thin-skinned structures, without obvious basement detachment or slippage. Large-scale nappe structure is the main structural style of this transformation type. The difference of transition type and thrust structure is mainly due to the different intracontinental deformation responses of different deep geological structures and boundary conditions in the compound continents with multi-continental blocks splicing together under the action of plate margin subduction or collision. The formation of transitional zone Ⅱ and transitional zone Ⅰ+Ⅱ piedmont thrust structures follows the structural model of "intracrustal subduction and reverse detachment thrust", while the non-transitional zone and transitional zone I conform to the structural model of "intracrustal bidirectional subduction and balanced uplift thrust" and "multi-level basement detachment and detachment thrust" respectively.