The Hongliuhe fold-and-thrust belt: Evidence of terminal collision and suture-reactivation after the Early Permian in the Beishan orogenic collage, Northwest China

The Early Permian strata of the Hongliuhe Formation are located in the Beishan orogenic collage, a tectonically significant region between the Tianshan orogen and the Southern Mongolian accretionary system, of the Central Asian Orogenic Belt (also called the Altaids). Stratigraphic reconstruction of the formation shows a general fining upwards sequence of rhythmically interbedded sandstones and siltstones with thick basal successions of conglomerates, all unconformably overlying an earlier mid-Paleozoic volcanic assemblage. The clast lithotypes of conglomerates successively change from being polymictic metamorphic tectonites at the base to a nearly monomictic granitoids mid-section, illustrating the unroofing sequence of an arc-related provenance. The intra-oceanic Gongpoquan composite arc that comprises most of the central Beishan fits the clast-lithotype profile of the conglomerates, and is the most probable provenance. The Hongliuhe Formation stratigraphy experienced a thin-skinned fold-and-thrust belt style of deformation that has a consistent southward vergence. Deformation in the lower stratigraphic levels created thrust imbrication of formation-scale blocks, each separated by steep, north-over-south brittle–ductile shear zones. The blocks are internally deformed with large-scale folding, with one significant block containing a sheath fold 15 km in breadth. The upper stratigraphic levels accommodated strain through low-angle thrust ramping, shear folding, imbricate fanning and duplexing. This indicates that the Hongliuhe Formation was syn-orogenically deposited in a progressively deepening terrestrial- to nearshore environment within the Gongpoquan composite arc. Our observations and data indicate that the basin overlies an earlier Paleozoic suture zone. This implies that the northern half of the Gongpoquan composite arc overrode the southern half in a fault system reactivation centered about this zone of weakness, and created a stratigraphic and structural response consistent with a subduction- or collision-driven fold-and-thrust belt. Its southward structural vergence is supporting evidence of a north-dipping subduction-related fault system under the northern Gongpoquan. The timing of both the genesis of the Hongliuhe Formation and this deformation event coincides with the subduction of the Liuyuan Ocean or docking of the Dunhuang block further south, at the southern margin of the Gongpoquan composite arc.