Formation and evolution of an Early Cambrian foreland basin in the NW Yangtze Block, South China

The Ediacaran to Cambrian succession in the NW Yangtze Block has long been considered to have formed in a passive margin. The Ediacaran is dominated by thick carbonate rocks, whereas the Lower Cambrian comprises a thick clastic succession. The transition from carbonate to clastic rocks and the provenance and tectonic setting of the clastic succession are poorly understood. Stratigraphic correlation shows a distinct stratigraphic absence from the Lower Cambrian to Devonian in the Bikou terrane. A regional seismic profile shows a wedge geometry of the Lower Cambrian from NW to SE. Outcrops reveal an overall coarsening-upward Lower Cambrian succession. The petrographic analysis of clastic rocks indicates immaturity, implying a proximal source. Palaeocurrent measurements of clastic rocks point to dominant SE-vergent orientations. The age spectra of detrital zircons imply that they were derived from Early Cambrian continental arcs and older continental crust. Geological, geophysical and geochemical evidence indicates that an Early Cambrian foreland basin was formed in the NW Yangtze Block. This foreland basin appears to have been strongly influenced by orogenic loading northwestward. We propose that this orogenic event should be named the Motianling orogeny, the origin of which may be related to subduction of the Proto-Tethys ocean beneath the NW Yangtze Block. Supplementary material: Supplementary tables A–D and a petrological description of detrital zircons are available at https://doi.org/10.6084/m9.figshare.c.6332980