Constraining uplift and denudation of west African continental margin by inversion of stacking velocity data

摘要

An inverse model has been developed to determine the magnitude of denudation at seabed and subsurface unconformities by using root-mean-square (RMS) stacking velocity data derived from processing a set of seismic reflection profiles. This approach provides superior spatial coverage in comparison to other methods, such as vitrinite reflectance, apatite fission track, and sonic velocity modeling, which are restricted to borehole locations. The model assumes exponential porosity decay with depth and a standard velocity-porosity relationship in order to compute a synthetic RMS velocity profile. Denudation values at two levels in the stratigraphy are then adjusted until the fit between the model and the data is optimized. Successful modeling is dependent upon independent estimates of the initial porosity of sediment since significant trade-off occurs between initial porosity and denudation. Application to the west African shelf shows that 0.5–1 km of denudation occurred along the entire margin, probably during late Neogene times. The amount of denudation decreases oceanward and was probably triggered by regional tilting associated with initiation and/or regeneration of continent-wide mantle convective upwelling, which is thought to have affected much of subequatorial Africa. A subsurface Oligocene unconformity represents as much as 2.5 km of denudation and was probably produced by initiation of an oceanic current.