The Aden Ridge is a part of an active oblique

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

system located in the

Gulf of Aden The Gulf of Aden (; ) is a deepwater gulf of the Indian Ocean between Yemen to the north, the Arabian Sea to the east, Djibouti to the west, and the Guardafui Channel, the Socotra Archipelago, Puntland in Somalia and Somaliland to the south. ...

, between

Somalia Somalia, officially the Federal Republic of Somalia, is the easternmost country in continental Africa. The country is located in the Horn of Africa and is bordered by Ethiopia to the west, Djibouti to the northwest, Kenya to the southwest, th ...

and the

Arabian Peninsula The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the ...

to the north. The rift system marks the

divergent boundary In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two List of tectonic plates, tectonic plates that are moving away fr ...

between the Somali and

Arabian The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the ...

tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

s, extending from the Owen transform fault in the

Arabian Sea The Arabian Sea () is a region of sea in the northern Indian Ocean, bounded on the west by the Arabian Peninsula, Gulf of Aden and Guardafui Channel, on the northwest by Gulf of Oman and Iran, on the north by Pakistan, on the east by India, and ...

to the Afar triple junction or Afar Plume beneath the Gulf of Tadjoura in

Djibouti Djibouti, officially the Republic of Djibouti, is a country in the Horn of Africa, bordered by Somalia to the south, Ethiopia to the southwest, Eritrea in the north, and the Red Sea and the Gulf of Aden to the east. The country has an area ...

. The Gulf of Aden is divided east to west into three distinct regions by large-scale discontinuities, the Socotra, Alula Fartak, and Shukra-El Sheik transform faults. Located in the central region, bounded by the Alula Fartak fault and Shukra-El Sheik fault, is the Aden spreading ridge. The Aden Ridge connects to the Sheba Ridge in the eastern region and to the Tadjoura Ridge in the western region. Due to oblique nature of the Aden Ridge, it is highly segmented. Along the ridge there are seven

transform fault A transform fault or transform boundary, is a fault (geology), fault along a plate boundary where the motion (physics), motion is predominantly Horizontal plane, horizontal. It ends abruptly where it connects to another plate boundary, either an ...

s that offset it to the north.

Initiation of rifting

Extension of the Gulf of Aden rift system began in the late

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

– early

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

(~35 Ma ago), caused by the northeast escape of the Arabian plate from the African plate at a rate of ~2 cm/yr, and the development of the Afar plume. Extension eventually gave way to seafloor spreading, first initiated near the Owen transform fault ~18 Ma ago. Seafloor spreading then propagated as far west as the Shukra-El Sheik fault at a rate of ~14 cm/yr ~6 Ma ago rifting propagated west of the Shukra-El Sheik fault until terminating at the Afar plume. The Afar plume is believed to have contributed to the initiation of the Aden ridge, due to the flow of hot mantle material being channeled along the thin

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

beneath the Gulf of Aden. Currently, the Aden Ridge is undergoing extension at a rate of ~15 mm/yr.

Segmentation of the Aden Ridge

Compared to its neighboring ridges, the Aden ridge is much more segmented. The Aden Ridge is broken up by seven transform faults with ridge segments of 10 – 40 km. In contrast, the Sheba Ridge is broken by only three transform faults and the Tadjoura Ridge continues essentially uninterrupted to the Afar Plume. Sauter et al. (2001) proposed that variations in the spacing of spreading cells along ridges is a result of spreading rate; i.e., larger spacing results from slower spreading rates. However, the variation in spreading rates across the Gulf of Aden, 18 mm/yr in the east and 13 mm/yr in the west, is not great enough to explain the significant variation in spreading cell length between the Aden ridge and its neighboring ridges. One likely cause for the segmentation of the Aden ridge is its distance from the Afar plume. The westernmost region of the Gulf, where the Tadjoura Ridge is located, has an anomalously high mantle temperature due to its proximity to the Afar plume. The result of this is higher degrees of melting and magmatism below the ridge, which allows for longer spreading segments without transform faults. The difference in segmentation between the Aden and Sheba ridges can be explained by varying degrees of obliquity. The ocean-continent transition (OCT) of the Sheba ridge formed parallel to the syn-rift structure, whereas the OCT of the Aden ridge formed oblique to the syn-rift structure. The former scenario is more accommodating to oblique spreading and does not require as many transform faults for stability.

References

{{coord, 14, N, 52, E, display=title Underwater ridges of the Indian Ocean