Intense faulting and downwarping of Nanpu Sag in the Bohai Bay Basin, eastern China: Response to the Cenozoic Stagnant Pacific Slab

The Nanpu Sag (NPS), located in northwestern Bohai Bay Basin (BBB), East China, has significant oil discoveries in recent years. The effect of intense faulting and downwraping which generated the intense subsidence of NPS in the Dongying Formation (Fm.) (Ed) (28.5-23.8 Ma) of the Late Oligocene is presented in this paper. We analyzed the geochemical characteristics of the widespread Cenozoic basalts in NPS from drill core samples. Our results show that the magma experienced negligible crustal contamination and slight fractional crystallization, and the features of remarkable positive U, Pb, Sr and Ti anomaly, low Rb/Ba and Rb/Sr values of the NPS basalts were inherited from the source region. The NPS basalts in Ed have distinctive characteristics as sub-alkaline basalts with rare earth elements (REE) patterns parallel to enriched mid-ocean ridge basalts (E-MORB), and were generated from a mixture origin of garnet-lherzolite zone with relatively high degree of partial melting (30%-50%) and garnet + spinel-lherzolite transition zone with partial melting degree ranging from 3% to 15%. In contrast, NPS basalts of other Cenozoic periods or Ed basalts of other areas in eastern China are alkaline with light REE enrichment and oceanic island basalts (OIB)-like-pattern, their magmas originated from garnet + spinel-lherzolite transition zone with partial melting degree lower than 5%. Combined with previous studies of the mantle wedge in the mantle transition zone (MTZ) beneath northeastern Asia, forming around 30 Ma, we infer that the stagnating of the Pacific slab triggered the asthenosphere upwelling and increased the degree of partial melting in mantle source, and relatively weak areas of the North China Craton (NCC) like the Tanlu Fault (TLF) where NPS located on were reactivate and susceptible to be modified. Therefore, the intense faulting and downwarping of NPS in Ed, along with the igneous activities, was a response to the Cenozoic stagnant Pacific slab more than the result of tectonic effects from the Pacific subduction like other areas in northeastern China in Paleogene.