Magnetostratigraphy and AMS study of a late Cretaceous-Paleogene succession in the eastern Xining basin: constraint on K-Pg boundary tectonic event in NE Tibet Plateau

Abstract

Cretaceous-Cenozoic basins developed in the NE Tibetan Plateau contain key archives to unravel the growth history of the plateau in response to the India-Eurasia collision. Here we present magnetostratigraphic results of a Late Cretaceous to Paleogene succession of the Zhongba section outcropping at the southern margin of the eastern Xining basin. This succession consists of three lithological units punctuated by two stratigraphic unconformities, which best recorded the deformation history of this foreland basin. Detailed magnetostratigraphic investigation show that the lower terrestrial sedimentary rock unit, the Minhe Group, was deposited in latest Cretaceous in the time span of ~74.5-69.2Ma; the middle unit was deposited in Paleogene in the time span of ~49.3-22Ma; and the upper conglomeratic unit, not dated, possibly was deposited in early Miocene. Accordingly, the K-Pg unconformity, widely observed in the foreland basins of NE Tibet, represents a sedimentary hiatus of ~19.9Ma duration from ~69.2 Ma to ~49.3 Ma, which possibly recorded the far-field response to the tectonic transition from Neo-Tethys oceanic plate subduction to the India-Eurasia collision in southern Tibet. AMS results show a clockwise rotation of the shortening direction from NEN-SWS in Latest-Cretaceous to NE-SW in Paleogene, which support the view that the compressional environment may dominate the NE Tibet in Early-Cenozoic period.