Angola Basin
[6] Typically divergent boundaries are described as having landward extension, seaward contraction, and translation, however the order of events in this area are difficult to distinguish in such a clear-cut manner.
[5] This is due to the fact that areas of the basin are superimposed upon one another, which some interpret to show pulses of deformation and uplift that occur at irregular times and places.
[7] The evaporite layer present within the basin is responsible for many topographic features that developed since its deposition as salt movement deforms the surrounding bedrock.
[6] As gravity spreading acts upon the salt layers it causes upslope extension and downslope contraction, which also explains many of the folds and features of the basin.
[10] Post-rift deformation is predominantly caused by raft tectonics, a term that is associated with salt detachment when normal fault blocks are widely separated so that the footwall and hanging wall are not in contact, creating large grabens.
[6] It is considered one of the most extreme forms of extension, and it highly influenced by gravity spreading and increased sediment loading, major factors which act upon the Angola Basin.
[6] The formation of the Angola Basin can be divided into three phases of rifting which took place from approximately 145-113 million years ago from the Jurassic to the Cretaceous.
[12] The final phase of rifting led to the breakup of the lithosphere, initiated seafloor spreading that still acts today, and resulted in the development of oceanic crust.
[3] Though the salt layer creates some ambiguity most agree that the bedrock is composed of volcanic basalts which are likely a result of rifting or Precambrian crystalline rock.
[4] During this time the basin was hypersaline making it inhospitable for normal marine life, although there may have been a small but stable amount of input of terrestrial fresh water.
[14] The Oligocene is also characterized by an erosional event that lasted 10-20 million years that is thought to be controlled by upheavals or depressions of crust over a broad area that are caused by mantle convection and hotspot activity.
[6] From the beginning of the Quaternary to present day much of the sediment is influenced by the Walvis Ridge, a hotspot trail that extends several hundred kilometers off the coast of Africa into the Atlantic Ocean, in addition to the Congo River.
[15] This is due to the Walvis Ridge preventing cold Antarctic bottom waters from circulating the basin allowing for the sedimentation of carbonate materials, including microorganisms such as foraminifera and other calcareous microfossils.
