The Cenozoic evolution of the North China Basin

Abstract

The Cenozoic evolution of the North China basin consists of early Tertiary rifting and differential subsidence, late Tertiary regional subsidence and Quaternary rifting and differential subsidence. Present Cenozoic sediment fill shows that Tertiary subsidence was greatest where the present thickness of pre‐Cenozoic crust is least. In contrast, Quaternary subsidence has been greatest where the present thickness of pre‐Cenozoic crust is greatest. The present crustal structure, geologic history and Cenozoic sediment fill of the basin suggest a nearly constant crustal thickness of ∼40 km at the onset of rifting (middle Eocene). If the present thickness of pre‐Cenozoic crust is due solely to crustal thinning from early Tertiary mechanical extension then the central North China basin has been extended at least 30% of its initial width. This is greater than estimates of total Mesozoic plus Cenozoic extension derived from basement faulting, ∼20%. At the present time any possible discrepancies cannot be resolved due to uncertainties in the data. Tertiary subsidence of the basin, derived from backs tripping the present sedimentary sequence, is not satisfactorily explained by either a simple uniform or two‐layer extensional model. No single extension parameter (uniform) or pair of extension parameters (two‐layer) can account for the variable sediment thicknesses or the alternating horst and graben configuration of the basin. We can, however, use the parameters derived from simple one‐ or two‐layer extensional modeling to infer lower bounds for both the average regional extension of the Bohai and the local extension of the depressions within the basin. For example, we find that the Neogene subsidence of a major part of the basin, the central Bozhong depression, has been nearly as rapid as that of young oceanic crust. Furthermore the present heat flow in this area is consistent with that for oceanic crust of 16–24 million years of age (post rift time). This indicates that the subsidence may be due to decay of a lithospheric thermal anomaly that is nearly as large as that in young oceanic crust. This is difficult to understand with a simple extensional framework because reported estimates of early Tertiary extension from basement faulting are only 10–30%. Possible sources of this excess heat are Mesozoic tectonics, effects of plate subduction at the Pacific margin, or energy input from regional tectonics during the early Tertiary. Our conclusions cast doubt upon the adequacy of simple extensional models to explain the Cenozoic evolution of the North China basin; however these conclusions can only be tentative due to inadequacies in the basic data set.