The Red Sea is an ocean basin located between Africa and the Arabian Peninsula. This basin is significant because it is one of the youngest ocean basins on Earth, with the oldest oceanic crust being from 5 to 10 Ma and extension of the basin beginning as early as 30 Ma. The youth of the basin provides a unique opportunity for geoscientists to study the ocean-continent transition and the evolution of a passive margin.
The term ‘passive margin’ is commonly used on geology to refer to a margin within a tectonic plate that represents a transition from continental to oceanic crust. The adjacent oceanic crust contains a spreading axis, or Mid Ocean Ridge, at which new oceanic crust is formed and continues spreading. When these margins develop, much of the geologic structure in the form of normal faults develops during the continental rifting stage, before oceanic crust has formed in the rift basin. Rifts and spreading centers are the surface manifestations of a large scale thinning of the lithosphere, the rigid upper layer of the Earth that contains the mostly brittle ‘Crust’. When the lithosphere thins it indicates that new, generally hotter asthenosphere, or plasticly deformed mantle rock, must rise in response to the decreased local load of the crust over the spreading axis and the relatively higher load on the adjacent asthenosphere from thicker crust. There is much argument over whether this asthenospheric upwelling is passive or active. Passive upwelling meaning that the asthenosphere is responding to thinning of the lithosphere that is being driven by other, far field forces such as the subduction of an oceanic slab attached to one of the diverging plates. Active upwelling is the opposing end-member model where rising asthenosphere is heating the base of the lithosphere, and possibly causing dynamic uplift of the lithosphere by buoyant rise and impact on the base of the lithosphere. In the case of active upwelling, the upwelling itself is seen as a driving force behind the extension of the lithosphere.
Most passive margins see a cycle of uplift and subsidence due to the relatively abrupt change in the thermal state of the lithosphere, and also local thinning on normal faults. Initially the rift flanks, that may eventually become part of the passive margin, are uplifted. The uplift is driven locally by normal faults. As the hanging wall of the normal fault moves down relative to the footwall, pressure is removed from the footwall. The removal of the load in this way causes local uplift of the footwall block. This type of uplift is usually short-wavelength and ubrupt when viewing topography. Uplift can also be caused by heating and removal of lithosphere from the rift zone. As the lithosphere thins, the thickness of the crust or lithosphere becomes much less. In some cases, hotter material is juxtaposed against initially colder lithosphere and the lithosphere begins to heat up. The heating of the lithosphere causes the overall density of the lithosphere to decrease and so the lithosphere rises.