FOLD-AND-THRUST BELT DEFORMATION OF THE HONGLIUHE FORMATION: A PERMIAN TECTONIC CLOSURE RECORD OF THE CENTRAL ASIAN OROGENIC BELT, NW CHINA
Well-preserved fold-and-thrust belt style deformation mapped in the upper stratigraphy of the Hongliuhe Formation exhibits dominantly south-southeast verging structure, including shear folding, low-angle thrust ramping, and duplexing. The amount of shortening is unknown due to the limited amount of exposure of the total stratigraphy. In the lower stratigraphy of the Hongliuhe formation, also of Permian age, are large-scale brittle-ductile eye fold structures that are postulated to be sheath folds created through steep sub-vertical faulting. The most examined and mapped structure, 16km wide and visible in profile on satellite imagery, is a synclinal structure with axes plunging steeply towards its center. It is creased along the lengthwise axial surface with a sub-schistose cleavage and slightly ductile deformation. Excepting large-scale folding, the bulk of its strata remain relatively undeformed and have preserved primary soft-sediment deformation structures indicating younging towards the center on both limbs of the synclinal structure. All ductile lineations are sub-vertical, as are both axes of the fold. It is bounded on both sides by a moderate to steeply northward dipping brittle-ductile fault system.
The importance of all such structures in the Permian strata of the Hongliuhe Formation is that they show the transition of deformation styles from brittle-ductile sub-vertical faulting, through to thin skinned fold-and-thrust style deformation in the youngest strata. These events support interpretation that the Permian Hongliuhe Formation was deposited syn-orogenically and deformed in the latest tectonic event resultant of a north dipping subduction system that amalgamated the northward accreting, southward younging archipelago that formed the Central Asian Orogenic Belt.