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Water Depth Anomaly In Gulf Of Aden

This project seeks to study the water depth anomalies in the Gulf of Aden and its rifted continental Margins. The oceanic crust adjacent to continental rifted margins often shows anomalous water depth (bathymetry) and subsidence history compared to the predictions of ocean lithosphere plate theory. These water depth anomalies are important both for their implications for geodynamic theories of lithosphere temperature structure and also for deep-water oil and gas exploration at rifted continental margins. A broad region of positive anomalous bathymetry is observed on the young oceanic crust against the Yemen rifted margin in the Gulf of Aden.

One proposed hypothesis for explaining this anomalous bathymetry is that the young oceanic lithosphere is underline at depth by relict thicker continental lithosphere with a deep temperature structure different to that of oceanic lithosphere. More mundane explanations for these bathymetric anomalies are that the oceanic crust could simply be thicker than usual or could be underlain by very thin continental crust so giving shallower bathymetry though isostasy. This study therefore uses satellite gravity inversion to map crustal thickness and ocean-continent transition location in the Gulf of Aden and its rifted continental margins.

Bathymetric anomalies corrected for sediment loading and lithosphere age is compared with crustal thickness determined from gravity inversion. 1. 0 Introduction The Gulf of Aden within the Red Sea divides the gulf and the horn of Africa. It presents a unique ecosystem and resources that deserves scientific attention. It is characterized by dense, salty water formed by net evaporation with rates up to 1. 4 – 2. 0 m yr-1 (Hastenrath & Lamb 1979). Approximately three million barrels of oil is being transported daily through the Gulf of Aden.

In the olden days, and even now, this Gulf used and still provide a considerable amount of sea food for the inhabitants of the surrounding arid lands (Al Saafani 2008). This Gulf is on the other hand important in transport and livelihoods of the people in the region. It serves as a highway for international trade between east and west. Its importance is fundamental to the fact that the present and future generations of peoples dependence on fishing is at stake. The Gulf of Aden is a small oceanic basin bounded by young conjugate passive margins well preserved beneath a thin postrift sedimentary cover (Leroy et al 2004).

It is an important characterized by the processes of rifting, and break-up of continental lithosphere and evolution of young oceanic basin. According to Marcos (1970), the Gulf of Aden is characterized by a deep convection in the northern section that leads to the formation of a deep water mass flowing out into the Gulf of Aden underneath a layer of less saline inflowing water. The existence of the Arabian monsoon is a phenomenon that dominates the region and affects the general oceanography and meteorology of the region.

Heileman and Mistafa (2008) confirm that a northeast monsoon winds extend well into the Gulf of Aden and the southern Red Sea during winters, causing a seasonal reversal in the winds over this entire region. The prevalent seasonal monsoon reversal and the local coastal configuration combined during summer season forces a radically different circulation pattern composed of a thin surface outflow and an intermediate inflowing layer of Gulf of Aden thermocline water and a drastically reduced outflowing deep layer (Patzert 1974). The general surface circulation within the basin is cyclonic.

2. 0 Geological Background of the area The Gulf of Aden comprises the southern limit of the Arabian plate that started to drift away from Africa around 17. 6 million years ago. It is a young ocean basin formed by the rifting of Asia from Africa. Its opening is usually considered as the westward propagation of a lithosphere crack from the Carlsberg ridge to the Afar volcanic area, which now represents the centre of a broad anomalous region with low S-wave mantle velocity, high elevation and much more evidence of mantle plume activity Lucazeau et al (2008).

It has a well-defined continental margin, small oceanic basin, and an active mid-ocean ridge (Sheba) in the center characterized by a rift valley and fracture zone. The geophysical survey of the basin, reveals continental, oceanic domains and ocean continent transition (OCT) domains with distinct morphological and sedimentological characteristics (Leroy et al 2004) According to Al Saafani (2008), the Gulf of Aden is approximately 900 km in length and varies in width from 26 km at Bab el Mandab to about 320 km at Ras Asir.

The middle region is the deepest at approximately between 2000–2500m and the shallow area is less than 1000 m. It has an average depth of 1800m, which increases from west to east. The Gulf of Aden is therefore a modern analogue for the early stages of mature margins (Lucazeau et al 2008), such as the Atlantic margins. The conjugate margins of the Encens– Sheba survey area were formed during the last period of rifting. The thickness of the oceanic crust of the Gulf of Aden varies from 4. 8 to 8.

4 km, as interpreted from a seismic refraction survey (Cochran, 1982). In these margins, the actual thermal regimes can be analyzed directly by means of surface heat flow, seismic tomography or regional isostasy, and compared with the observed tectonic and magmatic styles. Through the Arabia–Somalia Plate, successive Jurassic and Cretaceous rifting episodes have created major basins (Leroy et al 2004: Cochran, 1982). These basins have a predominant east–west to northwest–southeast orientation.

Seafloor spreading is more recent in the western part of the Gulf of Aden than in its eastern part according to previous magnetic anomaly studies, and that a magnetic quiet zone corresponds to an area of thinned crust (Leroy et al 2004). The structures and evolution of the Gulf of Aden margins are related to successive development of the formation of the Gulf of Aden during the Oligo-Miocene and the continental margin of the Indian Ocean during the Early Cretaceous up to the Paleocene (Leroy et al 2004; Besse and Courtillot, 1988).

According to d’Acremont et al (2004), during the Oligocene times, the Afro Arabian Plate began to separate due to the creation of two divergent basins, which both evolved in the development of two narrow oceanic basins that comprised of the Gulf of Aden to the south, between Arabia and Somalia and the Red Sea to the west, between Arabia and Africa (Nubia).

The East African rift forms the third branch of the Afar ridge–ridge–ridge type (RRR) triple junction (d’Acremont et al 2004; Wolfenden et al. 2004) that is still in the rifting stage. This later Oligo-Miocene stretching episode of the Arabia–Somalia Plate reactivated inherited structures of these Mesozoic basins (d’Acremont et al 2004; Ellis et al. 1996; Granath 2001; Bellahsen 2002).

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