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-grabe ...

system located in the

Gulf of Aden The Gulf of Aden ( ar, خليج عدن, so, Gacanka Cadmeed 𐒅𐒖𐒐𐒕𐒌 𐒋𐒖𐒆𐒗𐒒) 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 Chann ...

, between

Somalia Somalia, , Osmanya script: 𐒈𐒝𐒑𐒛𐒐𐒘𐒕𐒖; ar, الصومال, aṣ-Ṣūmāl officially the Federal Republic of SomaliaThe ''Federal Republic of Somalia'' is the country's name per Article 1 of thProvisional Constitut ...

and the

Arabian Peninsula The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plat ...

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 tectonic plates that are moving away from each other. Divergent b ...

between the

Somali Somali may refer to: Horn of Africa * Somalis, an inhabitant or ethnicity associated with Greater Somali Region ** Proto-Somali, the ancestors of modern Somalis ** Somali culture ** Somali cuisine ** Somali language, a Cushitic language ** Somali ...

and

Arabian The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plat ...

tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

s, extending from the Owen Transform Fault in the

Arabian Sea The Arabian Sea ( ar, اَلْبَحرْ ٱلْعَرَبِيُّ, Al-Bahr al-ˁArabī) is a region of the northern Indian Ocean bounded on the north by Pakistan, Iran and the Gulf of Oman, on the west by the Gulf of Aden, Guardafui Channe ...

to the

Afar Triple Junction The Afar Triple Junction (also called the Afro-Arabian Rift System) is located along a divergent plate boundary dividing the Nubian, Somali, and Arabian plates. This area is considered a present-day example of continental rifting leading to s ...

or Afar Plume beneath the Gulf of Tadjoura in

Djibouti Djibouti, ar, جيبوتي ', french: link=no, Djibouti, so, Jabuuti 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 ...

. The Gulf of Aden is divided east to west into three distinct regions by large-scale discontinuities, the Socotra, Alula Fartak, and Shukra-El Shiek transform faults. Located in the central region, bounded by the

Alula Fartak fault The alula , or bastard wing, (plural ''alulae'') is a small projection on the anterior edge of the wing of modern birds and a few non-avian dinosaurs. The word is Latin and means "winglet"; it is the diminutive of ''ala'', meaning "wing". The a ...

and Shukra-El Shiek fault, is the Aden spreading ridge. The Aden Ridge connects to the

Sheba Ridge The Aden Ridge is a part of an active oblique rift system located in the Gulf of Aden, between Somalia and the Arabian Peninsula to the north. The rift system marks the divergent boundary between the Somali and Arabian tectonic plates, extending ...

in the eastern region and to the

Tadjoura Ridge Tadjoura ( aa, Tagórri; ar, تاجوراء ''Tağūrah''; so, Tajuura) is one of the oldest towns in Djibouti and the capital of the Tadjourah Region. The town evolved into an early Islamic center with the arrival of Muslims shortly after the ...

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 along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subduct ...

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

- early

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

(~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 Shiek fault at a rate of ~14 cm/yr ~6 Ma ago rifting propagated west of the Shukra-El Shiek 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 A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

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 portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...

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