Lithospheric structures of the central Solonker-Xar Moron-Changchun-Yanji Suture (Inner Mongolia) revealed by a deep seismic reflection profile

Abstract

The Solonker-Xar Moron-Changchun-Yanji Suture (SXCYS) is regarded as a tectonic boundary between the Central Asian Orogenic Belt and the North China Craton. Formed during the closure of Paleo-Asian Ocean and modified by the subsequent magmatic activities, this suture might have recorded complex processes of accretionary orogeny and crustal recycling. Thus, seismic reflection profiling was conducted along a length of 160 km from Naiman to Ar Horqin Banner, Inner Mongolia, and the lithospheric structures of central SXCYS have been examined. A prominent feature is a group of strong reflections from the middle crust to the topmost mantle beneath the center of the profile, which consists of massive south dipping reflections and a few middle crustal reflections with a opposite vergence. This group of strong reflections is interpreted to be the subduction relict formed during the final closure of the Paleo-Asian Ocean. The estimated lateral extent of the suture zone is ca. 100 km across strike, and its subduction polarity is southward with an apparent dip angle of 15 to 20°. The upper crust along the whole profile is characterized by weak reflectivity, which also extends to the middle crust. This absence of reflective structures may be attributed to the magmatic activities during or after the continental convergence. Two strong reflection clusters extending from the bottom of upper crust to the lower crust are also identified, which may be caused by distinct magmatic intrusions. Overprinted on the dipping reflections, the Moho is identified at a depth of 33 to 36 km, which is inferred to be formed at an extensional background contemporary with the subsurface Mesozoic sediments and extensional faults. A detailed model of the evolution processes of the central SXCYS is proposed, which includes the oceanic subduction, the final collision, and the modification by subsequent magmatic activities.