Soft collision and polyphasic tectonic evolution of Wuxia foreland thrust belt: Evidence from geochemistry and geophysics at the northwestern margin of the Junggar Basin

Abstract

A key unit of the Paleozoic Central Asian Orogenic Belt (CAOB), the northwestern margin of the Junggar Basin is located between the Kazakhstan, Siberian and Tarim plates with widespread late Paleozoic volcanic rocks and molasse deposits. In recent years, there has been intense debate about how the Carboniferous-Permian soft-collision geodynamic setting was developed and how long-distance detachment nappes were formed. In this study, on the basis of seismic reflection data, magnetotelluric profiles, and geochronological and geochemical data, we established the superimposed tectonic framework of the Wuxia thrust belt and Hala’alat Mountain and reconstructed the polyphasic tectonic evolution since the Late Carboniferous period. The petrographic and geochemical data suggest that the basaltic rocks of the Late Carboniferous were derived from a metasomatized depleted mantle source in a subduction-related island arc setting, while the Early Permian magmas derived from subduction-modified lithosphere mantle sources in a post-collision intraplate environment. Analysis of the Rittmann Index, Gottini Index and DI frequency curve suggested that the study area had experienced a key tectonic transition from a Late Carboniferous (i.e. end of Carboniferous) subduction-orogen setting to an Early Permian post-collisional extension setting. The Hala’alat Mountain and Zhayier Mountain exhibited a similar tectonostratigraphic framework and structure in space, such as double-layer nappes, long-distance slab detachment and unilateral transection by the northern Darbut fault. Both of these mountain ranges could be called soft-collision foreland thrust belts. On the basis of described four regional unconformities, the tectonic evolution of the soft-collision Wuxia thrust belt and adjacent regions could be divided into four crucial tectonic stages. As a result, thick autochthonous Lower Permian strata are buried under long-distance Hala’alat nappe and extended to the northern Darbut fault, which showed the huge oil and gas exploration potential in the northwestern margin of the Junggar Basin.