Structure Evolution And Chronology Constrains Of Lang Shan In Neimeng Autonomous Region

Lang Shan is located in the mid-west of Neimeng Autonomous Region. It connects with the Yinshan-Yanshan tectonic belt to the east, the Alashan Block to the west, and also links with the western marginal thrust system of the Ordos Basin to the south. Thus, the NE-trending Lang Shan structural belt is a very important tectonic terrane between the western and eastern China.During Mesozoic time, the Lang Shan structural belt has experienced a complex intraplate deformation and sedimentation, mainly developed with NE-trending strike-slip motion and SE-vergent thrusting, where that the thrusting deformation transformed the earlier strike-slip faults. In the ductile shear zones, granitic intrusion was extensively suffered by NE-, NNE-trending cleavage deformation. During these deformation stages, muscovite and biotite developed as synkinematic minerals elongated parallel to the stretching lineations and deformational surfaces. U-Pb zircon LA-ICP-MS dating and muscovite, biotite 40Ar/39Ar dating yielded sequenced age constraints: 1) ~242-234Ma, Middle Triassic: with the closure of Paleo-Asian Oceanic Sea, the Siberian plate was coming into collision with the North China Craton, and the Lang Shan was intruded with a series of intermediateacidity granite. Syn- or post these granitic intrusions, the Lang Shan occurred ductile shear and muscovite's Ar-Ar isotope ages are yielded as at ~237-213Ma. 2) Middle to Late Jurassic: NE-trending strike-slip motion happened in the mid-upper in the Lang Shan area, the biotite's Ar-Ar isotope ages are yielded as at 173-178Ma. This NE-trending strike-slipping may be related to the far-field stress-release of the west Pacific plate subduction, which resulted in the Ordos block's counter-clockwise rotation at the same time. 3) The most prominent structure on the cross-sections is a moderate to low-angle, SE-vergent and NW-dip thrust faults that juxtaposes crystalline units against a variety of footwall strata. We dated biotite and got another group of the Ar-Ar isotope ages at ~121-116Ma. The SE-vergent compression maybe represent the final closure of the Mongol-Okhotsk Ocean, and the southward moving of the Siberian Block. This deformation stage can be compared to the ESE-ward crustal thinning of eastern China during the Early Cretaceous.