sedimentary basins
Rift basins - The sediments deposited on passive margins are trapped into normal-fault bounded depressions called rift basins. Rift basins are a fundamental manifestation of continental extension at an early stage of continental break-up.
As stretching proceeds, seafloor spreading accommodates divergence of landmasses, extensional strain focusses around mid-oceanic ridges and continental margins become tectonically passive.
Sleep (1971) suggested that the subsidence was due to the relaxation of an earlier perturbation that resulted in doming, uplift, erosion and therefore thinning of the continental crust. Once the cause for doming would wane, subsidence would lead to a depression and sediment deposition. However, the problem with these models is that the massive eroded dome necessary to provide space for the sediments is not observed. Therefore it was realised that the observed passive margin subsidence requires another mechanism.
The sedimentary sequence deposited on passive margins is divided by a breakup unconformity that separates the syn-rift from the post-rift sedimentary sequence. On this basis it was first suggested by Hsü (1965) and Vogt and Ostenso (1967) that the post-rift subsidence of continental shelves could be related to thermal contraction beneath the crust. Sleep (1971) was the first to observe that continental basins subsided exponentially to a constant elevation with a time constant similar to ocean floor.
Animation on the right © Brun, Univ. Rennes FR