Early Paleozoic Fold and Thrust Belt on the Southern Margin of the Ordos Basin, China and its Influence on the Cenozoic Rifting

Abstract:

Whilst geological survey, petrological and geochemical analyses all indicate significant collision between the North Qinling Terrane and the southern margin of the North China Craton (NCC), few studies have been conducted to reveal how this tectonic event has impacted the structural deformation of the NCC, which partly results from the thick covering of the Late Paleozoic to Cenozoic sequences. Herein, we identified a fold and thrust belt in the southern margin of the Ordos Basin in the western NCC, based on an integrated investigation of seismic reflection data, surface geology and borehole loggings. Three N-S-striking geological sections were constructed and cross the southern Ordos Basin, the Weihe Graben, the North Qinling Terrane and the Shangdan Suture Zone from north to south. Following conclusions are drawn. (1) The fold and thrust belt is characterized by S-dipping, imbricate, thin-skinned thrusts soling into the Lower Cambrian mudstone in the Weibei area, but likely becomes basement-involved to the south. The fold and thrust belt disappears north of Weibei region. (2) The fold and thrust belt was active in the Late Ordovician – Devonian, according to the zircon U-Pb result (443 Ma) of the tuff sample collected from folded strata in the Qishan section and the angular unconformity between the gently deformed Carboniferous strata and intensely deformed underlying rocks. (3) Some of the basement-involved faults in this fold and thrust belt were reactivated as normal faults in the Cenozoic in response to the crustal stretching in the southern Ordos Basin, and served as the main boundary faults of the Cenozoic Weihe Graben. For the first time, our findings reveal the structural response of the Ordos Basin to the Early Paleozoic collision and its influence on the subsequent deformation in the southern margin of the NCC.