Tectonic evolution of an Early Cambrian foreland basin in the northwest Yangtze Block, South China

The Ediacaran to Cambrian in the northwest Yangtze Block, has long been considered to be formed in a passive margin. Wells and seismic data, however, show that a Lower Cambrian thick siliciclastic rock succession occurs in the northwest Sichuan Basin, the provenance of which has not received attention from previous workers. In this study, we first propose that an Early Cambrian foreland basin was formed in the northwest Yangtze Block. Stratigraphic correlation shows a distinct stratigraphic absence from the Lower Cambrian to Devonian in the Bikou terrane, implying an orogeny might take place from NW to SE. A regional seismic profile shows a wedge stratigraphic geometry of the Lower Cambrian from NW to SE, further indicating a typical structure of a foreland basin. Field outcrops show an overall coarsening-upwards siliciclastic succession of the Lower Cambrian. The petrological analysis of siliciclastic rocks presents an immature feature implying a proximal source. Paleocurrent measurements of siliciclastic rocks point to dominant SE-vergent orientations. The age spectra of detrital zircon U-Pb dating of the Canglangpu Formation show a dominant Early Cambrian age of ca. 530 Ma, together with some positive ɛ_Hf(t) values, indicating that the detrital zircon grains from the Lower Cambrian were derived from a northwest proximal juvenile continental arc and older crust. Therefore, the northwest Yangtze Block experienced a tectonic transition from an Ediacaran passive margin to an Early Cambrian foreland basin. The formation of the Early Cambrian foreland basin appears to have been strongly influenced by an orogenic loading northwestward. Here, this previously-overlooked orogenic event is named as the Motianling orogeny. The origin of the Early Cambrian orogeny may be related to subduction of the Proto-Tethys ocean beneath the northwest Yangtze Block, resulted in continental collision and uplift of northwest microterranes that provided siliciclastic sediments to fill the foreland basin southeastward.