Note I added the abstract, sorry for being late. The paper is still below 8,000 but all points on the Aden water depth anomaly were discussed. Sorry about this, but adding extra contents will only make the paper redundant and repetitive.
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This paper was completed as a partial requirement for subject here presented to name here of institution here
This is a comprehensive compilation of data from online journals and libraries (EBSCO, SAONASA Astrophysics Data System, JSTOR and other online journals), textbooks, and web sources. It should be noted that some original text from the sources were preserve by the author to maintain the integrity of the paper.
This paper was completed for educational and research purpose only. All cited articlesjournals remains the intellectual properties of each author and publisher. Publication or any means of distribution will need permission from the publishers of the cited articles.
The scope of study is focus in the Water Depth Anomaly in the Gulf of Aden, any similarities or contradictions from previous documented studies are coincidental and not intended by the author.
Abstract
The objective of this paper is to illustrate all the factors that contribute on the water depth anomaly in the Gulf of Aden. Information and data were based on previous studies conducted on the subject and were acquired through extensive research. Bathymetry, magnetic and gravity data were acquired on the Encens-Sheba cruise tracks. The main reasons that cause the water depth anomaly in the Gulf of Aden are the continuous lithosphere rifting influenced by the fracture fault present in the area, the continuous tectonic movement in the area promotes the formations of dykes on the seafloor. The other reason is the magmatic processes that contribute on magmatic sedimentation on Adens seafloor. The details of these processes and how they contribute on the water depth anomaly are discussed on the following sections of the paper.
The Gulf of Aden is young narrow oceanic basin resulting from the lithosphere rifting of Africa and Arabia in the Oligo-Miocene time. Continental lithosphere rifting followed by seafloor spreading creates a characteristics set of features known as passive margins. The result of these passive margins is magnetic quite zone characterized by thinned and faulted continental crust. In cases like the Gulf Aden, the rifting process on its western conjugate margins is associated with large-scale tabular intrusive igneous bodies and volcanic activities.
The Aden is located in the southern limits of the Arabian plate, the Gulf of Aden extends from the Owen fracture zone (58E) to the Gulf of Tadjoura where the Aden Ridge converge with Afar (43E). It is a rectangular basin, with a 300km wide by 900 km long dimensions. Water depth is about 2000-2500 m, except on slices of ridges where it is generally 1000 m.
The objective of this paper is to illustrate through data and discussions the continental lithosphere rifting which resulted in the creation of the Aden and its continental margins, the continuous seafloor spreading, the Ocean-continent transition, magmatic sedimentation, and other factors that contributes on the water depth anomaly in the area.
Geological and Geophysical Setting
Located in the southern limits of the Arabian plate, the Gulf of Aden extends from the Owen fracture zone (58E) to the Gulf of Tadjoura where the Aden Ridge converge with Afar (43E). The Gulf of Aden is the result of the continental lithosphere rifting between Africa an Arabia, which dates back 35 Ma ago in the late Oligence to early Mioscene. The N75E mean orientation of the Aden is oblique to the mean N25E present opening direction. The spreading rate of 1.6 cm yr -1 increases from west (N37E) to cast (N26E) at a rate of 2.34 cm yr -1. . At the longitude of the Sheba Ridge (14.4N and 53 E) the oceanic spreading rate is 2.2 cm yr -1 along a N25E direction (Jestin et al 1994 Fourier et al 2001). The Aden is a rectangular basin, with a 300km wide by 900 km long dimensions. Water depth is about 2000-2500 m, except on slices of ridges where it is generally 1000 m.
The inception of the oceanic spreading started in the eastern part of the Aden at 17.6 Ma (A5d) (Sahota 1990 Leroy et al 2004). The early stage of the Aden is interpreted as WSW propagation of the Carlsberg Ridge to the Afar hotspot ( 30 Ma) (Schilling 1973 Coutillot 1980 Ebinger Hayward 1996 Menzies et al 1997). This is evident on the formation of volcanic margins in the western part of the Aden.
The eastern margins of the Gulf of Aden are non-volcanic, sedimentary straved, and the crop out on land (dAcremont et al 2005). The Aden is divided into three parts by two major fracture zones (Manighetti et al 1997). On the western part is the Shukra-El Sheik fracture zone, which corresponds to the limit of influence of the Afar hotspot, while the Alula-Fartak fracture zone divides the central part from the eastern part of the Socotra fracture zone (Tamsett Searle 1990).
Geological History of Land Margins
Northern Margin
The Northern margin located in 1630N is defined by southern Oman and eastern Yemen in the Dhofar area. The formation is dated in the late Cretaceous to early Eocene platform cut by the Salalah Plain and the Ashawq Graben basins of mean direction N100E. The basins are partly covered with syn-rift Oligo-Miocene sedimentary formations. The fault segments on the northern margin are oriented from N70E to N110E (see figure 3) (Lepvrier et al 2002 Bellahsen et al nd).
The sedimentary deposits is formed in three succession (pre-, syn-, post-rifting) by the Hahramaut, Dhofar and Fars respectively (see figure 4) (Roger et al 1989). The pre-rift sedimentary deposits in Hahramaut is dated Palaeocene to Eocene. The sediment formations in the Dhofar syn-rift which are about 1400 m in thickness, dates between 35 to 18 Myr (Roger et al 1989). The post-rift sediments of the Fars are generally believed to be formed in the Miocene period.
Southern Margin
The southern margin located in 13N is defined by the boundaries of Somalia in the east and the island of Socotra. The pre-rift sediment formations on the Northeastern Somalia are unconformably covered by a series of syn-rift sediments. Syn-rift formation dates from Oligo-Miocence. The sediments are continental deposits, followed by platform and neritic deposits. Post rift deposits are 27 to 22 Myr old (Ali-Kassim 1993). The Socotra Island on the furthest east forms a bathymetric high, it is affected by the Darror fault (N45E) which separates Socotra in two parts (see figure 4). The crystalline Cambrian composition of the eastern part of the island is unconformably covered by pre-rift calcareous sediments. (Beydoun Bichan 1969). The sediments formation on the western part of the island records the separation of Arabia, Africa, and India. (Birse et al 1997). The sediment formations on the N45E are evidence on the Indian and Somalian plate separation in the Jurassic. Sediments in the N140E trend are evidence on the separation of the Afro-Arabian and Laurasian plates in the Neo-Tethys age.
Data Gathering and Processing
A proton magnetometer Geometrics G816, was used in the Encens-Sheba cruise to collect Magnetic-Data. Adjustment was made based on the 1945-2000 International Geomagnetic Reference Field (IGRF) (Olsen et al 2000). Magnetic Anomalies were identified on 19 profiles between the Alula-Fartak and Socotra fracture zones. Data was gathered by comparing the magnetic profiles with a 2-D block model (see Figure 6). The sequence of magnetic anomalies starting from the central anomaly (A1) to anomaly A5 (10 MA) was recognized on both sides of the profiles (Fleury 2001)
SHAPE Figure 6. Magnetic Anomalies profiles K and J, identification of the anomalies was made by comparing them with a 2-D block model (Cande Kent 1995). The 2-D model were derived with a half-spreading rate, in the northern flank, of 11 mm yr-1 starting from the axis to A5 and 13 mm yr-1 starting from A5 to A6. The spreading rate in the southern is 8 mm yr-1, with a 400-m thick magnetized layer and a contamination factor of 0.7 (Tisseau Patriat 1981).
Gravity data were collected using a Lacoste and Romberg S77 marine gravity meter at a 5 s sampling rate. Data processing was based on Etvs standards, standard error is 0.75 mGal. Spatial resolution for the free-air anomaly is 20km.
Onset of Seafloor Spreading
Data collection covers the margin to margin magnetic profiles across the whole basin. This includes the incipient accretion at the foot of the margins and the continuous accretion in the oceanic zone. The purpose of this margin to margin coverage is to localize the boundaries of the continental crust, to identify and differentiate the magnetic signature of the OCT, to identify the oldest magnetic anomaly in the area, to determine the rate and symmetry of accretion, and to study the evolution of the segmentation in the oceanic zone.
The study on the onset of seafloor spreading on the area is focus on the survey older than 10.95 Ma (A5). A5d is identified to be the oldest oceanic magnetic anomaly on the onset of seafloor spreading which dates at 17.6 Ma (see figure 7). Based on the magnetic anomaly map, three domains were identified (1) the oceanic basin between the northern and southern OCT zones displays high-amplitude seafloor spreading anomalies (2) the first kilometers of the OCT contains parallel high-amplitude magnetic anomalies (3) magnetic anomalies near the continental margins vary significantly characterized by variable trends and low amplitudes. These variations on the magnetic anomalies and the low amplitudes are evidence on the existence of stronger magnetic bodies in the transitional crust. It is possible that the upper seismic crust contains magnetic material, moving oceanwards (Whitmarsh Miles 1995).
Figure 7. Calculated magnetic model and all available profiles of the area on the northern and southern domains. The magnetic profiles have been brought into line with respect to the anomaly A5c locations in order to highlight the variations of spreading velocities from one flank to the other. Vertical dashed lines indicated the positions of the anomaly identifications. Horizontal grey lines show the location of the main transfer and fracture zones. Different grey levels map the three domains, oceanic, continental and OCT zone, evidenced from seismic data (profiles A, D, G, J,M corresponding names of seismic profiles indicated as ES d Acremont et al. (2005). On the southern domain (left part), the model is calculated with a spreading rate of 9 mm yr1 and a magnetized layer of 400 m. On the northern domain (right part), the model is calculated with a spreading rate of 13 mm yr1 and a magnetized layer of 400 m. The contamination factor is 0.7. Note the variable width of the OCT zone. The (1), (2) and (3) used to enumerate the domains are explained in the text
Crustal Structure
The data from the gravity profiles allows the identification of structure of the oceanic basin and the conjugate margins. The data collected allows the determination of the gravity signature of the oceancontinent transition (OCT), the identification of the relative continent crustal thickness, oceanic domains, and the identification of discontinuities in the evolution of segmentation from the conjugate margins to the Sheba Ridge.
Free-Air Anomaly
The Free-air anomaly map (see figure 8) was correlated with bathymetry and was contoured at intervals of 10 mGal. Negative gradients outline the oceancontinent transition from the oceanic to the continental domains shown by the joint analysis of seismic data and gravity profiles (dAcremont et al 2005)
Mantle Bouger Anomaly
The Oceancontinent transition of conjugate flanks were derived through the calculation of the mantle Bouger anomaly (MBA) in accordance with procedures currently used in the study of mid-oceanic ridges. The data for the crust and mantle anomalies were derived by subtracting the theoretical gravity effects of water-sediments, sediment-crust, and crust-mantle to the Free-Air Anomaly. Data on the thickness of sediments were based on existing seismic reflection, multibeam bathymetric and magnetic data of the Alula-Fartak and Socotra fracture zones. Depth conversion are in accordance to the velocities measured in drill cores around the area (Fisher et al 1974).
The crustal thickness (6 km), the density of the oceanic crust (2.8 g cm-3), and of the mantle (3.3 g cm-3) were all assumed to be constant. The gravity effect was computed with a multilayer method using a fast Fourier transform technique that is fully 3-D (Maia Arkani-Hamed 2002). The mantle Bouger anomaly is influenced by long wavelength signal produced by the cooling of the lithosphere away from the ridge axis. To eliminate this factor, the infinite half-space model (Davis Lister 1974) was used to calculate the gravity effect of a cooling lithosphere. The depth of the lithosphere which is about 1300C isotherm, depends on the thickness of the thermal lithosphere. The lithosphere thickness was computed based on the thermal age, with a thermal diffusivity of 10-6 m2 s-1. The age of the oceanic domain outlining the continental margins was assumed to be 20 Ma.
Residual mantle Bouguer anomalies (RMBA) are the result of the crustal thickness and the variations in the crust and mantle density. The deviation values are higher in the oceanic domain compared to deviations on the continental margins (see figure 9)
The RMBA amplitude is between -150 to -25 mGal along the fracture zones and 50 mGal in the oceanic domain. The largest amplitudes are observed in the eastern part of the basin in the direction of the Socotra fracture zone, in the conjugate OCT. The negative gradient in the map delimits the transition of crustal density and thickness that signifies the edge effect of the oceanic crust. A 2.8 g cm-3 homogeneous crustal density was assumed in the 3-D model, on the ocean, continent and OCT, and a constant 6km crustal thickness. The gradient marks a mass deficit that reflects the abrupt thickening of the crust and the deepening of the Mohorovicic Discontinuity towards the continent.
Crust Thickness
The residual anomaly can be inverted to arrived at the value of the relative thickness of the crust, by continuing the signal along the average depth of the Mohorovicic Discontinuity (average seafloor depth 6 km). The result is a grid of crustal thickness variations that corresponds to the departure from a constant 6-km-thick crustal model. The result is added to the 6 km constant crust to arrived at a crustal thickness map. (see figure 10)
The crustal thickness is between 0 to 20 km, over the conjugate margins. The Oceancontinent Transition is represented by the thinning of the crust near the margins, while the thickening in the continental domains represents the syn-rift basins. The transfer zones define by seismic data (dAcremont et al 2005) correlates with area of thinner crust.
2-D Gravity Modelling
The 2-D gravity modeling allows the use of definite values for density, structural geometry and deposit thickness compared to 3-D modeling. 2-D modeling data were used to clearly illustrate the geometry of the Mohorovicic Discontinuity and the crustal thickness with a lateral change of density towards the continental crust.
Previous seismic studies was the basis for the location of the OCT (dAcremont et al. 2005). (b) 2-D gravity modelling of the crustal section with a similar density for the both OCT (2.8 g cm3 for theoceanic and the OCT crusts 2.7 g cm3 for the continental crust). (c) 3-D gravity inversion of the corresponding crustal section derived from the RMBA (Section 5.3) with no lateral variation of the density (2.8 g cm3 for the oceanic crust, the OCT and the continental crust)
Gravity computation method for the 2-D model was based on equations derived for the vertical and horizontal components of the gravitational attraction due to a 2-D body of arbitrary shape by approximating it to a n-sided polygon of constant density (Talwani et al. 1959). To remove edge effects, the polygon model was extended for a further 150 km on both sides of the section and the lithosphere thickening effect is assumed to be at 20 Ma. Modification was made on the depth of the Mohorovicic Discontinuity to reduce the difference between the observed and calculated data. The value of the crustal thickness was increased to match the FAAs on the computation. It was assumed that the density of the mantle is 3.3 g cm-3, the continental crust is 2.7 g cm-3 and the oceanic crust is 2.8 g cm-3. The overlying sediments are a 2.1 g cm-3 and 1.03 g cm-3 for the sea water. The model was computed with a density of 2.8 g cm-3 in north and south OCT. The results were compared on the 3-D inversion profile. (see figure 9)
The main difference between the 2-D and 3-D models is in the thickness values of the crust. This difference is caused by the lateral change of density between oceanic and continental crust. The crustal thickness decreases towards the OCT from about 4-5 km to 1km on the northern and southern flank. Oceanic crust thickens to from 1 to 4 km on the southern OCT moving to the continental domain, were there is a significant thinning on the deepest block of the continental crust. This thinning on the continental crust is more evident on the northern margin were the crustal thickness is 1km between the boundaries of the oceanic and transitional domain. This is caused by underplating of high-density material and the presence of serpentinized mantle. The average oceanic crustal thickness is about 4-5km. The two flanks of the Sheba ridge at A5 time show a 1 km crustal thickness were the southern flank is thicker. At A5c time, the southern flank is thicker by 1.5 km that the northern flank. The thickness on the continental crust is about 15-16 km in the north and south.
Based on the model, the oceanic crust is thinner in the eastern domain (3-5 km thick), and that there is strong thinning on the OCT and the deepest continental block.
Sedimentary Evolution
The syn-rift (35-18 Ma) sediments in the offshore basins of the Encens-Sheba area are less that 1200 m thick. This syn-rift sequence is in agreement with the 1400 m syn-rift series in the Dhofar area (Roger et al 1989). The margins are formed by conjugate faults that creates slope deposits that are sometimes followed by horizontal series. The syn-rift series does not show evidences of syn-depositional faulting which is characterized by the presence of fan-shaped geometries. Though there were fan-shaped geometries discovered on both margins, with pattern of two superimposed fans with opposite polarities. The fan geometries are created by successive movements on the antithetic dipping border faults this creates successive uplifts on the rift shoulders.
The post-rift sediments formed in 18-0 Ma are non-evident on the onshore conjugate margins. Seismic data shows that the post-rift sediments deposit marks one complete regressive and transgressive cycle. The regressive cycle is the maximum shoulder lift, synchronous with the end of rifting and the breakup of the continental crust. The regression on the post-rift sediments results in rapid erosion which allows the offshore formation of 1000 m of post-rift sediments. The transgressive cycle is associated with thermal subsidence and the formation of the continental slope. The post-rift sediments onshore are composed of reef deposits and conglomeratic sandstones (Platel Roger 1989 Watchorn et al 1998).
Margins Asymmetry and Inheritance
The oceanic seafloor of the eastern Aden was the result of slow full spreading rates between 24 mm yr-1 to 28 mm yr-1 in 17.6 Ma. The crust thickens from the OCT to A5 on either sides of the ridge. The gravity modelling shows that the southern flank of the ridge is thicker than the continental crust of the north. The northern flank (160 km) is wider compared to the southern flank (80 km). This difference in thickness and spreading rates on the ridge are clear indications of an asymmetric spreading. The variations on the spreading rates along the ridge axis can result on permanent change in geometry (Carboote MacDonald 1992 Sloan Patriat 1992 Cormier MacDonald 1994 Maia Gente 1998).
The comparative structural study of the conjugate margins shows difference in OCT and the margins geometry, and in post-rift sedimentation thickness (dAcremont et al 2005). The difference of 140km in north and 300 km in south in width between the conjugate rifted margins is caused by structural inheritance. Early rifting may have caused the difference in this part of the Gulf of Aden, this is highlighted in the southern margin where a deep Mezosic basin exist. The Alula-Fartak fracture zone is located in the centre of the Mesozic Jeza-Qamar-Gardafui basin. Bothe the proto-Aden and proto-Sheba ridge originates on the southern and northern edges of the Jeza-Qamar-Gardafui basin. The shift between these two ridges resulted on the development of a transfer zone which developed into a transform fault at the start of the seafloor spreading.
Oblique Rifting and Inheritance
The Alula-Fartak fracture zone did not exist before the Oligo-Miocene rifting this is evident on the lack of strike faults on the length of the N26E fracture zone. Before the continental rifting the Alula-Fartak transform fault zone is in the centre of the pre-existing Jeza-Qamar-Gardafui rift basin (see figure 13). On the northern border of the basin is the Sheba Ridge, and the Aden ridge on the southern fault border. The reconstruction is consistent with a dextral transform fault formed in during the OCT and the seafloor spreading phases, in the centre of the Mesozoic Jeza-Qamar-Gardafui basin.
This is in agreement with the offshore structural map which shows a normal network of faults with en enchelon basins with short overlapping zones (see figure 14). The deformation on the conjugate basins is transferred along two fracture fault zones. These further evolve as dextral transform fractured zones initiated in the lengthening of the transfer zones at the onset of seafloor spreading.
The northern external rift zones display a complex fault geometry directions varying from N70E to N120E (Lepvrier et al 2002 Bellahsen et al nd). In contrast, the offshore structural pattern because the density of the seismic profiles doest not show sigmoid basins, faults are systematically trending N110E. In the Gulf of Aden, a major fault population has a trend intermediary between the direction perpendicular to the divergence and the rift axis. Analogue models of oblique rifting do not predict more frequent N110E trending faults, perpendicular to the divergence (Bellahsen et al nd).
Seismic data and analogue models suggest that reactivation is an important process in the Gulf of Aden rifting (Bellahsen et al nd). The N100-120E normal faults on the offshore margins are subparallel to the principal extensional structures seen in the Marib-Al-Jawf-Balhaf grabens, the Masilah basin and the easthern Jeza-Qamar basin onshore Yemen, which are Mesozoic basins. The Oligo-Miocene rifting in the Aden reactivates older Mesozoic basins (Ellis et al 1996 Huchon Khanbari 2003).
The intermediate trends between the perpendicular to the extension (N100E) and the rift axis (N75E) are the result of rifting obliquity (Lepvrier et al 2002). Taking in consideration pre-existing faults perpendicular to the divergence (Bellahsen et al. in press), the data are consistent with the oblique rifting analogue models where the normal fault pattern is from N70E to N120E (Tron Brun 1991 McClay Dooley 1995) and with a major occurrence of N110E trending faults (Bellahsen et al nd).
OCT to A5 model
A5d is the oldest known magnetic anomaly in the area it marks the east-west-trending along the northern and southern margin between the OCT and the Oceanic crust. The seafloor spreading that followed started in each OCT segments of the incipient boundary and started to move from that point resulting on the rifting followed by the plate boundary. The fundamental control of second order axis segmentation is the geometry and dynamics of mantle upwelling beneath the ridge axis (Lin et al 1990b Sempr et al 1990 Gente et al 1995 MaiaGente 1998). Based on residual analysis, the crust is relatively thin below the discontinuities and rather thick below the segment centres. Compared to the Alula-Fartak, the Socotra transform fault and fracture continues to evolve and change. From A5 time to present, a mismatch is evident on the north and south Socotra fracture zones. The transform fault on the Socotra continues to evolve in two discontinuities and the development of two short ridge segments within the 15 km transtensional basin. Between the two transform faults, the second order ridge segmentation is controlled by the initial margins segmentation, which determines the location of the mantle rises at oceanic spreading inception. Afterwards the ridge segmentation is governed by local magmatic (Lin et al. 1990a Gente et al. 1995 Maia Gente 1998 Cannat et al. 1999), or tectonic (Mutter Karson 1992) processes, or more probably by both (Grindlay et al. 1992)
Ocean-Continent Transition
The collected seismic data suggest that the OCT basement is composed of thinned continental crust on the southern margin, where syn-rift sediments are evident, and continental crust or exhumed mantle along the northern margin. (d Acremont et al 2005) In non-volcanic margins, the OCT is poorly constrained, and is mainly formed with restricted magmatism caused by mantle exhumation in the continental break-up similar in the Iberia margins (Boillot et al 1998 Beslier et al 1996 Whitmarsh et al 2001)
In the eastern Aden, the gravity and magnetic studies shows evidence that the south OCT has a thicker crust, and lower amplitude magnetic anomalies compared to the north OCT. The south is made of very thin continental crust with magmatic bodies on the oceanwards boundary. The north where syn-rift sediments are not known, the seismic data is not enough to differentiate the ultra-thinned continental crust, exhumed serpentinized mantle, and the thin oceanic crust. (dAcremont et al 2005)
Gravity data on the northeastern OCT shows a null crust thickness. In this non-volcanic margin, the mass excess can be the result of thin oceanic or very thin continental crust, with the presence of high-density rocks. It is also possible that there is no continental crust in this area, instead its the serpentinized mantle cropping out on the seafloor.
Seismic data gathered through gravity and magnetic data defines the eastern part of the Gulf of Aden continental margins as non-volcanic. The asymmetry of the continental margins in terms of its crustal thickness and Moho geometry was also clearly illustrated in the gravity data. The rifted domains in the northern margins are twice as narrow and 2 km thinner compared to the southern margins. While the western conjugate margins of the Gulf of Aden is associated with large-scale tabular intrusive igneous bodies and continuous volcanic activities.
The continuous seafloor spreading as a result of the movements on the fractures zones on the Aden and the magmatic processes greatly contributes on the different oceanic crust thickness and segmentation in the area. These factors and the evolution of oceanic sediments in the Aden contribute on the water depth anomaly in the area.
Signed Statement
This paper was completed as a partial requirement for subject here presented to name here of institution here
This is a comprehensive compilation of data from online journals and libraries (EBSCO, SAONASA Astrophysics Data System, JSTOR and other online journals), textbooks, and web sources. It should be noted that some original text from the sources were preserve by the author to maintain the integrity of the paper.
This paper was completed for educational and research purpose only. All cited articlesjournals remains the intellectual properties of each author and publisher. Publication or any means of distribution will need permission from the publishers of the cited articles.
The scope of study is focus in the Water Depth Anomaly in the Gulf of Aden, any similarities or contradictions from previous documented studies are coincidental and not intended by the author.
Abstract
The objective of this paper is to illustrate all the factors that contribute on the water depth anomaly in the Gulf of Aden. Information and data were based on previous studies conducted on the subject and were acquired through extensive research. Bathymetry, magnetic and gravity data were acquired on the Encens-Sheba cruise tracks. The main reasons that cause the water depth anomaly in the Gulf of Aden are the continuous lithosphere rifting influenced by the fracture fault present in the area, the continuous tectonic movement in the area promotes the formations of dykes on the seafloor. The other reason is the magmatic processes that contribute on magmatic sedimentation on Adens seafloor. The details of these processes and how they contribute on the water depth anomaly are discussed on the following sections of the paper.
The Gulf of Aden is young narrow oceanic basin resulting from the lithosphere rifting of Africa and Arabia in the Oligo-Miocene time. Continental lithosphere rifting followed by seafloor spreading creates a characteristics set of features known as passive margins. The result of these passive margins is magnetic quite zone characterized by thinned and faulted continental crust. In cases like the Gulf Aden, the rifting process on its western conjugate margins is associated with large-scale tabular intrusive igneous bodies and volcanic activities.
The Aden is located in the southern limits of the Arabian plate, the Gulf of Aden extends from the Owen fracture zone (58E) to the Gulf of Tadjoura where the Aden Ridge converge with Afar (43E). It is a rectangular basin, with a 300km wide by 900 km long dimensions. Water depth is about 2000-2500 m, except on slices of ridges where it is generally 1000 m.
The objective of this paper is to illustrate through data and discussions the continental lithosphere rifting which resulted in the creation of the Aden and its continental margins, the continuous seafloor spreading, the Ocean-continent transition, magmatic sedimentation, and other factors that contributes on the water depth anomaly in the area.
Geological and Geophysical Setting
Located in the southern limits of the Arabian plate, the Gulf of Aden extends from the Owen fracture zone (58E) to the Gulf of Tadjoura where the Aden Ridge converge with Afar (43E). The Gulf of Aden is the result of the continental lithosphere rifting between Africa an Arabia, which dates back 35 Ma ago in the late Oligence to early Mioscene. The N75E mean orientation of the Aden is oblique to the mean N25E present opening direction. The spreading rate of 1.6 cm yr -1 increases from west (N37E) to cast (N26E) at a rate of 2.34 cm yr -1. . At the longitude of the Sheba Ridge (14.4N and 53 E) the oceanic spreading rate is 2.2 cm yr -1 along a N25E direction (Jestin et al 1994 Fourier et al 2001). The Aden is a rectangular basin, with a 300km wide by 900 km long dimensions. Water depth is about 2000-2500 m, except on slices of ridges where it is generally 1000 m.
The inception of the oceanic spreading started in the eastern part of the Aden at 17.6 Ma (A5d) (Sahota 1990 Leroy et al 2004). The early stage of the Aden is interpreted as WSW propagation of the Carlsberg Ridge to the Afar hotspot ( 30 Ma) (Schilling 1973 Coutillot 1980 Ebinger Hayward 1996 Menzies et al 1997). This is evident on the formation of volcanic margins in the western part of the Aden.
The eastern margins of the Gulf of Aden are non-volcanic, sedimentary straved, and the crop out on land (dAcremont et al 2005). The Aden is divided into three parts by two major fracture zones (Manighetti et al 1997). On the western part is the Shukra-El Sheik fracture zone, which corresponds to the limit of influence of the Afar hotspot, while the Alula-Fartak fracture zone divides the central part from the eastern part of the Socotra fracture zone (Tamsett Searle 1990).
Geological History of Land Margins
Northern Margin
The Northern margin located in 1630N is defined by southern Oman and eastern Yemen in the Dhofar area. The formation is dated in the late Cretaceous to early Eocene platform cut by the Salalah Plain and the Ashawq Graben basins of mean direction N100E. The basins are partly covered with syn-rift Oligo-Miocene sedimentary formations. The fault segments on the northern margin are oriented from N70E to N110E (see figure 3) (Lepvrier et al 2002 Bellahsen et al nd).
The sedimentary deposits is formed in three succession (pre-, syn-, post-rifting) by the Hahramaut, Dhofar and Fars respectively (see figure 4) (Roger et al 1989). The pre-rift sedimentary deposits in Hahramaut is dated Palaeocene to Eocene. The sediment formations in the Dhofar syn-rift which are about 1400 m in thickness, dates between 35 to 18 Myr (Roger et al 1989). The post-rift sediments of the Fars are generally believed to be formed in the Miocene period.
Southern Margin
The southern margin located in 13N is defined by the boundaries of Somalia in the east and the island of Socotra. The pre-rift sediment formations on the Northeastern Somalia are unconformably covered by a series of syn-rift sediments. Syn-rift formation dates from Oligo-Miocence. The sediments are continental deposits, followed by platform and neritic deposits. Post rift deposits are 27 to 22 Myr old (Ali-Kassim 1993). The Socotra Island on the furthest east forms a bathymetric high, it is affected by the Darror fault (N45E) which separates Socotra in two parts (see figure 4). The crystalline Cambrian composition of the eastern part of the island is unconformably covered by pre-rift calcareous sediments. (Beydoun Bichan 1969). The sediments formation on the western part of the island records the separation of Arabia, Africa, and India. (Birse et al 1997). The sediment formations on the N45E are evidence on the Indian and Somalian plate separation in the Jurassic. Sediments in the N140E trend are evidence on the separation of the Afro-Arabian and Laurasian plates in the Neo-Tethys age.
Data Gathering and Processing
A proton magnetometer Geometrics G816, was used in the Encens-Sheba cruise to collect Magnetic-Data. Adjustment was made based on the 1945-2000 International Geomagnetic Reference Field (IGRF) (Olsen et al 2000). Magnetic Anomalies were identified on 19 profiles between the Alula-Fartak and Socotra fracture zones. Data was gathered by comparing the magnetic profiles with a 2-D block model (see Figure 6). The sequence of magnetic anomalies starting from the central anomaly (A1) to anomaly A5 (10 MA) was recognized on both sides of the profiles (Fleury 2001)
SHAPE Figure 6. Magnetic Anomalies profiles K and J, identification of the anomalies was made by comparing them with a 2-D block model (Cande Kent 1995). The 2-D model were derived with a half-spreading rate, in the northern flank, of 11 mm yr-1 starting from the axis to A5 and 13 mm yr-1 starting from A5 to A6. The spreading rate in the southern is 8 mm yr-1, with a 400-m thick magnetized layer and a contamination factor of 0.7 (Tisseau Patriat 1981).
Gravity data were collected using a Lacoste and Romberg S77 marine gravity meter at a 5 s sampling rate. Data processing was based on Etvs standards, standard error is 0.75 mGal. Spatial resolution for the free-air anomaly is 20km.
Onset of Seafloor Spreading
Data collection covers the margin to margin magnetic profiles across the whole basin. This includes the incipient accretion at the foot of the margins and the continuous accretion in the oceanic zone. The purpose of this margin to margin coverage is to localize the boundaries of the continental crust, to identify and differentiate the magnetic signature of the OCT, to identify the oldest magnetic anomaly in the area, to determine the rate and symmetry of accretion, and to study the evolution of the segmentation in the oceanic zone.
The study on the onset of seafloor spreading on the area is focus on the survey older than 10.95 Ma (A5). A5d is identified to be the oldest oceanic magnetic anomaly on the onset of seafloor spreading which dates at 17.6 Ma (see figure 7). Based on the magnetic anomaly map, three domains were identified (1) the oceanic basin between the northern and southern OCT zones displays high-amplitude seafloor spreading anomalies (2) the first kilometers of the OCT contains parallel high-amplitude magnetic anomalies (3) magnetic anomalies near the continental margins vary significantly characterized by variable trends and low amplitudes. These variations on the magnetic anomalies and the low amplitudes are evidence on the existence of stronger magnetic bodies in the transitional crust. It is possible that the upper seismic crust contains magnetic material, moving oceanwards (Whitmarsh Miles 1995).
Figure 7. Calculated magnetic model and all available profiles of the area on the northern and southern domains. The magnetic profiles have been brought into line with respect to the anomaly A5c locations in order to highlight the variations of spreading velocities from one flank to the other. Vertical dashed lines indicated the positions of the anomaly identifications. Horizontal grey lines show the location of the main transfer and fracture zones. Different grey levels map the three domains, oceanic, continental and OCT zone, evidenced from seismic data (profiles A, D, G, J,M corresponding names of seismic profiles indicated as ES d Acremont et al. (2005). On the southern domain (left part), the model is calculated with a spreading rate of 9 mm yr1 and a magnetized layer of 400 m. On the northern domain (right part), the model is calculated with a spreading rate of 13 mm yr1 and a magnetized layer of 400 m. The contamination factor is 0.7. Note the variable width of the OCT zone. The (1), (2) and (3) used to enumerate the domains are explained in the text
Crustal Structure
The data from the gravity profiles allows the identification of structure of the oceanic basin and the conjugate margins. The data collected allows the determination of the gravity signature of the oceancontinent transition (OCT), the identification of the relative continent crustal thickness, oceanic domains, and the identification of discontinuities in the evolution of segmentation from the conjugate margins to the Sheba Ridge.
Free-Air Anomaly
The Free-air anomaly map (see figure 8) was correlated with bathymetry and was contoured at intervals of 10 mGal. Negative gradients outline the oceancontinent transition from the oceanic to the continental domains shown by the joint analysis of seismic data and gravity profiles (dAcremont et al 2005)
Mantle Bouger Anomaly
The Oceancontinent transition of conjugate flanks were derived through the calculation of the mantle Bouger anomaly (MBA) in accordance with procedures currently used in the study of mid-oceanic ridges. The data for the crust and mantle anomalies were derived by subtracting the theoretical gravity effects of water-sediments, sediment-crust, and crust-mantle to the Free-Air Anomaly. Data on the thickness of sediments were based on existing seismic reflection, multibeam bathymetric and magnetic data of the Alula-Fartak and Socotra fracture zones. Depth conversion are in accordance to the velocities measured in drill cores around the area (Fisher et al 1974).
The crustal thickness (6 km), the density of the oceanic crust (2.8 g cm-3), and of the mantle (3.3 g cm-3) were all assumed to be constant. The gravity effect was computed with a multilayer method using a fast Fourier transform technique that is fully 3-D (Maia Arkani-Hamed 2002). The mantle Bouger anomaly is influenced by long wavelength signal produced by the cooling of the lithosphere away from the ridge axis. To eliminate this factor, the infinite half-space model (Davis Lister 1974) was used to calculate the gravity effect of a cooling lithosphere. The depth of the lithosphere which is about 1300C isotherm, depends on the thickness of the thermal lithosphere. The lithosphere thickness was computed based on the thermal age, with a thermal diffusivity of 10-6 m2 s-1. The age of the oceanic domain outlining the continental margins was assumed to be 20 Ma.
Residual mantle Bouguer anomalies (RMBA) are the result of the crustal thickness and the variations in the crust and mantle density. The deviation values are higher in the oceanic domain compared to deviations on the continental margins (see figure 9)
The RMBA amplitude is between -150 to -25 mGal along the fracture zones and 50 mGal in the oceanic domain. The largest amplitudes are observed in the eastern part of the basin in the direction of the Socotra fracture zone, in the conjugate OCT. The negative gradient in the map delimits the transition of crustal density and thickness that signifies the edge effect of the oceanic crust. A 2.8 g cm-3 homogeneous crustal density was assumed in the 3-D model, on the ocean, continent and OCT, and a constant 6km crustal thickness. The gradient marks a mass deficit that reflects the abrupt thickening of the crust and the deepening of the Mohorovicic Discontinuity towards the continent.
Crust Thickness
The residual anomaly can be inverted to arrived at the value of the relative thickness of the crust, by continuing the signal along the average depth of the Mohorovicic Discontinuity (average seafloor depth 6 km). The result is a grid of crustal thickness variations that corresponds to the departure from a constant 6-km-thick crustal model. The result is added to the 6 km constant crust to arrived at a crustal thickness map. (see figure 10)
The crustal thickness is between 0 to 20 km, over the conjugate margins. The Oceancontinent Transition is represented by the thinning of the crust near the margins, while the thickening in the continental domains represents the syn-rift basins. The transfer zones define by seismic data (dAcremont et al 2005) correlates with area of thinner crust.
2-D Gravity Modelling
The 2-D gravity modeling allows the use of definite values for density, structural geometry and deposit thickness compared to 3-D modeling. 2-D modeling data were used to clearly illustrate the geometry of the Mohorovicic Discontinuity and the crustal thickness with a lateral change of density towards the continental crust.
Previous seismic studies was the basis for the location of the OCT (dAcremont et al. 2005). (b) 2-D gravity modelling of the crustal section with a similar density for the both OCT (2.8 g cm3 for theoceanic and the OCT crusts 2.7 g cm3 for the continental crust). (c) 3-D gravity inversion of the corresponding crustal section derived from the RMBA (Section 5.3) with no lateral variation of the density (2.8 g cm3 for the oceanic crust, the OCT and the continental crust)
Gravity computation method for the 2-D model was based on equations derived for the vertical and horizontal components of the gravitational attraction due to a 2-D body of arbitrary shape by approximating it to a n-sided polygon of constant density (Talwani et al. 1959). To remove edge effects, the polygon model was extended for a further 150 km on both sides of the section and the lithosphere thickening effect is assumed to be at 20 Ma. Modification was made on the depth of the Mohorovicic Discontinuity to reduce the difference between the observed and calculated data. The value of the crustal thickness was increased to match the FAAs on the computation. It was assumed that the density of the mantle is 3.3 g cm-3, the continental crust is 2.7 g cm-3 and the oceanic crust is 2.8 g cm-3. The overlying sediments are a 2.1 g cm-3 and 1.03 g cm-3 for the sea water. The model was computed with a density of 2.8 g cm-3 in north and south OCT. The results were compared on the 3-D inversion profile. (see figure 9)
The main difference between the 2-D and 3-D models is in the thickness values of the crust. This difference is caused by the lateral change of density between oceanic and continental crust. The crustal thickness decreases towards the OCT from about 4-5 km to 1km on the northern and southern flank. Oceanic crust thickens to from 1 to 4 km on the southern OCT moving to the continental domain, were there is a significant thinning on the deepest block of the continental crust. This thinning on the continental crust is more evident on the northern margin were the crustal thickness is 1km between the boundaries of the oceanic and transitional domain. This is caused by underplating of high-density material and the presence of serpentinized mantle. The average oceanic crustal thickness is about 4-5km. The two flanks of the Sheba ridge at A5 time show a 1 km crustal thickness were the southern flank is thicker. At A5c time, the southern flank is thicker by 1.5 km that the northern flank. The thickness on the continental crust is about 15-16 km in the north and south.
Based on the model, the oceanic crust is thinner in the eastern domain (3-5 km thick), and that there is strong thinning on the OCT and the deepest continental block.
Sedimentary Evolution
The syn-rift (35-18 Ma) sediments in the offshore basins of the Encens-Sheba area are less that 1200 m thick. This syn-rift sequence is in agreement with the 1400 m syn-rift series in the Dhofar area (Roger et al 1989). The margins are formed by conjugate faults that creates slope deposits that are sometimes followed by horizontal series. The syn-rift series does not show evidences of syn-depositional faulting which is characterized by the presence of fan-shaped geometries. Though there were fan-shaped geometries discovered on both margins, with pattern of two superimposed fans with opposite polarities. The fan geometries are created by successive movements on the antithetic dipping border faults this creates successive uplifts on the rift shoulders.
The post-rift sediments formed in 18-0 Ma are non-evident on the onshore conjugate margins. Seismic data shows that the post-rift sediments deposit marks one complete regressive and transgressive cycle. The regressive cycle is the maximum shoulder lift, synchronous with the end of rifting and the breakup of the continental crust. The regression on the post-rift sediments results in rapid erosion which allows the offshore formation of 1000 m of post-rift sediments. The transgressive cycle is associated with thermal subsidence and the formation of the continental slope. The post-rift sediments onshore are composed of reef deposits and conglomeratic sandstones (Platel Roger 1989 Watchorn et al 1998).
Margins Asymmetry and Inheritance
The oceanic seafloor of the eastern Aden was the result of slow full spreading rates between 24 mm yr-1 to 28 mm yr-1 in 17.6 Ma. The crust thickens from the OCT to A5 on either sides of the ridge. The gravity modelling shows that the southern flank of the ridge is thicker than the continental crust of the north. The northern flank (160 km) is wider compared to the southern flank (80 km). This difference in thickness and spreading rates on the ridge are clear indications of an asymmetric spreading. The variations on the spreading rates along the ridge axis can result on permanent change in geometry (Carboote MacDonald 1992 Sloan Patriat 1992 Cormier MacDonald 1994 Maia Gente 1998).
The comparative structural study of the conjugate margins shows difference in OCT and the margins geometry, and in post-rift sedimentation thickness (dAcremont et al 2005). The difference of 140km in north and 300 km in south in width between the conjugate rifted margins is caused by structural inheritance. Early rifting may have caused the difference in this part of the Gulf of Aden, this is highlighted in the southern margin where a deep Mezosic basin exist. The Alula-Fartak fracture zone is located in the centre of the Mesozic Jeza-Qamar-Gardafui basin. Bothe the proto-Aden and proto-Sheba ridge originates on the southern and northern edges of the Jeza-Qamar-Gardafui basin. The shift between these two ridges resulted on the development of a transfer zone which developed into a transform fault at the start of the seafloor spreading.
Oblique Rifting and Inheritance
The Alula-Fartak fracture zone did not exist before the Oligo-Miocene rifting this is evident on the lack of strike faults on the length of the N26E fracture zone. Before the continental rifting the Alula-Fartak transform fault zone is in the centre of the pre-existing Jeza-Qamar-Gardafui rift basin (see figure 13). On the northern border of the basin is the Sheba Ridge, and the Aden ridge on the southern fault border. The reconstruction is consistent with a dextral transform fault formed in during the OCT and the seafloor spreading phases, in the centre of the Mesozoic Jeza-Qamar-Gardafui basin.
This is in agreement with the offshore structural map which shows a normal network of faults with en enchelon basins with short overlapping zones (see figure 14). The deformation on the conjugate basins is transferred along two fracture fault zones. These further evolve as dextral transform fractured zones initiated in the lengthening of the transfer zones at the onset of seafloor spreading.
The northern external rift zones display a complex fault geometry directions varying from N70E to N120E (Lepvrier et al 2002 Bellahsen et al nd). In contrast, the offshore structural pattern because the density of the seismic profiles doest not show sigmoid basins, faults are systematically trending N110E. In the Gulf of Aden, a major fault population has a trend intermediary between the direction perpendicular to the divergence and the rift axis. Analogue models of oblique rifting do not predict more frequent N110E trending faults, perpendicular to the divergence (Bellahsen et al nd).
Seismic data and analogue models suggest that reactivation is an important process in the Gulf of Aden rifting (Bellahsen et al nd). The N100-120E normal faults on the offshore margins are subparallel to the principal extensional structures seen in the Marib-Al-Jawf-Balhaf grabens, the Masilah basin and the easthern Jeza-Qamar basin onshore Yemen, which are Mesozoic basins. The Oligo-Miocene rifting in the Aden reactivates older Mesozoic basins (Ellis et al 1996 Huchon Khanbari 2003).
The intermediate trends between the perpendicular to the extension (N100E) and the rift axis (N75E) are the result of rifting obliquity (Lepvrier et al 2002). Taking in consideration pre-existing faults perpendicular to the divergence (Bellahsen et al. in press), the data are consistent with the oblique rifting analogue models where the normal fault pattern is from N70E to N120E (Tron Brun 1991 McClay Dooley 1995) and with a major occurrence of N110E trending faults (Bellahsen et al nd).
OCT to A5 model
A5d is the oldest known magnetic anomaly in the area it marks the east-west-trending along the northern and southern margin between the OCT and the Oceanic crust. The seafloor spreading that followed started in each OCT segments of the incipient boundary and started to move from that point resulting on the rifting followed by the plate boundary. The fundamental control of second order axis segmentation is the geometry and dynamics of mantle upwelling beneath the ridge axis (Lin et al 1990b Sempr et al 1990 Gente et al 1995 MaiaGente 1998). Based on residual analysis, the crust is relatively thin below the discontinuities and rather thick below the segment centres. Compared to the Alula-Fartak, the Socotra transform fault and fracture continues to evolve and change. From A5 time to present, a mismatch is evident on the north and south Socotra fracture zones. The transform fault on the Socotra continues to evolve in two discontinuities and the development of two short ridge segments within the 15 km transtensional basin. Between the two transform faults, the second order ridge segmentation is controlled by the initial margins segmentation, which determines the location of the mantle rises at oceanic spreading inception. Afterwards the ridge segmentation is governed by local magmatic (Lin et al. 1990a Gente et al. 1995 Maia Gente 1998 Cannat et al. 1999), or tectonic (Mutter Karson 1992) processes, or more probably by both (Grindlay et al. 1992)
Ocean-Continent Transition
The collected seismic data suggest that the OCT basement is composed of thinned continental crust on the southern margin, where syn-rift sediments are evident, and continental crust or exhumed mantle along the northern margin. (d Acremont et al 2005) In non-volcanic margins, the OCT is poorly constrained, and is mainly formed with restricted magmatism caused by mantle exhumation in the continental break-up similar in the Iberia margins (Boillot et al 1998 Beslier et al 1996 Whitmarsh et al 2001)
In the eastern Aden, the gravity and magnetic studies shows evidence that the south OCT has a thicker crust, and lower amplitude magnetic anomalies compared to the north OCT. The south is made of very thin continental crust with magmatic bodies on the oceanwards boundary. The north where syn-rift sediments are not known, the seismic data is not enough to differentiate the ultra-thinned continental crust, exhumed serpentinized mantle, and the thin oceanic crust. (dAcremont et al 2005)
Gravity data on the northeastern OCT shows a null crust thickness. In this non-volcanic margin, the mass excess can be the result of thin oceanic or very thin continental crust, with the presence of high-density rocks. It is also possible that there is no continental crust in this area, instead its the serpentinized mantle cropping out on the seafloor.
Seismic data gathered through gravity and magnetic data defines the eastern part of the Gulf of Aden continental margins as non-volcanic. The asymmetry of the continental margins in terms of its crustal thickness and Moho geometry was also clearly illustrated in the gravity data. The rifted domains in the northern margins are twice as narrow and 2 km thinner compared to the southern margins. While the western conjugate margins of the Gulf of Aden is associated with large-scale tabular intrusive igneous bodies and continuous volcanic activities.
The continuous seafloor spreading as a result of the movements on the fractures zones on the Aden and the magmatic processes greatly contributes on the different oceanic crust thickness and segmentation in the area. These factors and the evolution of oceanic sediments in the Aden contribute on the water depth anomaly in the area.
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