STRUCTURAL STYLE AND DEFORMATIONAL MECHANISM OF SOUTHERN DABASHAN FORELAND FOLD-THRUST BELT IN CENTRAL CHINA

Based on field investigation, reflecting seismic profiles and other data, the Meso-Cenozoic structural style in southern Daba Mountains is discussed in this paper, and the deformation mechanism and tectonic model are outlined as well. The Daba Mountains lies between the Qinling orogenic belt and the Sichuan cratonic basin. Tectonically, from orogenic belt towards craton, the mountain ranges and their adjacent regions can be divided by Chengkou fault and Tiexi-Wuxi concealed fault into three tectono-stratigraphic units, namely, the northern Dabashan thrust-nappe belt, the southern Dabashan foreland fold-thrust belt, and the Sichuan basin. The southern Dabashan fold-thrust belt is a southwestward-bulging thin-skinned thrust wedge superimposed by dextral transpressive deformation, shows obvious zonation in deformation style and deformation intensity in the direction of dip, and can be divided further into two subzones, i.e., the imbricate thrust zone characterized by imbricate stepped-thrust sheets, and the detachment-fold zone characterized by box fold and closed overturned-isoclinal fold on the outcrops. Several types of structures have been recognized or inferred, including imbricate thrust system, passive-roof duplex (triangle zone), fault-related folds, back-thrust system and pop-up structure, and so on. NE-SW compressive stress from Qinling orogenic belt and detachment layers in the overlying strata are the two most important determinants of deformation style. After the collision between North China plate and Yangtze plate at the end of middle Triassic, the northward intracontinental subduction along the south edge of the Qinling orogenic belt was initiated, which led to the corresponding southward thrusting in the upper crust. At about the end of early Cretaceous, the thrusting was propagated southwestwards and extended to the southmost part of southern Daba Mountains, and caused thrusting deformation to be preferentially developed along major detachment layers and to progress toward foreland from lower Sinian through lower Cambrian and Silurian to middle and lower Triassic.