Sheba Ridge Research Articles

Palaeomagnetism and K-Ar and40Ar/39Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr

SUMMARY A new detailed palaeomagnetic study of Tertiary volcanics, including extensive K-Ar and 40 Ar/ 39 Ar dating, helps constrain the deformation mechanisms related to the opening processes of the Afar depression (Ethiopia and Djibouti). Much of the Afar depression is bounded by 30 Myr old flood basalts and floored by the ca 2 Myr old Stratoid basalts, and evidence for pre-2 Ma deformation processes is accessible only on its borders. K-Ar and 40 Ar/ 39 Ar dating of several mineral phases from rhyolitic samples from the Ali Sabieh block shows indistinguishable ages around 20 Myr. These ages can be linked to separation of this block in relation to continental breakup. Different amounts of rotation are found to the north and south of the Holhol fault zone, which cuts across the northern part of the Ali Sabieh block. The southern domain did not record any rotation for the last 8 Myr, whereas the northern domain experienced approximately 12 ± 9 ◦ of clockwise rotation. We propose to link this rotation to the counter-clockwise rotation observed in the Danakil block since 7 Ma. This provides new constraints on the early phases of rifting and opening of the southern Afar depression in connection with the propagation of the Aden ridge. A kinematic model of propagation and transfer of extension within southern Afar is proposed, with particular emphasis on the previously poorly-known period from 10 to 4 Ma.

Long-term elasticity in the continental lithosphere; modelling the Aden Ridge propagation and the Anatolian extrusion process

SUMMARY The evolution of the Gulf of Aden and the Anatolian Fault systems are modelled using the principles of elastic fracture mechanics usually applied to smaller scale cracks or faults. The lithosphere is treated as a plate, and simple boundary conditions are applied that correspond to the known plate boundary geometry and slip vectors. The models provide a simple explanation for many observed geological features. For the Gulf of Aden the model predicts why the ridge propagated from east to west from the Owen Fracture Zone towards the Afar and the overall form of its path. The smaller en echelon offsets can be explained by upward propagation from the initially created mantle dyke while the larger ones may be attributed to the propagating rupture interacting with pre-existing structures. For Anatolia the modelling suggests that the East Anatolian Fault was created before the North Anatolian Fault could form. Once both faults were formed however, activity could switch between them. The time scales over which this should take place are not known, but evidence for switching can be found in the historical seismicity. For Aden and Anatolia pre-existing structures or inhomogeneous stress fields left from earlier orogenic events have modified the processes of propagation and without an understanding of the existence of such features the propagation processes cannot be fully understood. Furthermore a propagating fault can extend into an active region where it would not have initiated. The North Anatolian Fault encountered slow but active extension when it entered the Aegean about 5 Ma and the stress field associated with the extending fault has progressively modified Aegean extension. In the central Aegean activity has been reduced while to the north-west on features such as the Gulfs of Evvia and Corinth activity has been increased. The field observation that major structures propagate and the success of simple elastic models suggest that the continental crust behaves in an elastic-brittle or elastic-plastic fashion even though laboratory tests may be interpreted to suggest viscous behaviour. There are major problems in scaling from the behaviour of small homogeneous samples to the large heterogeneous mantle and large-scale observations should be treated more seriously than extrapolations of the behaviour of laboratory experiments over many orders of magnitude in space and time. The retention of long-term elasticity and localised failure suggests a similar gross rheology for the oceanic and continental lithospheres. Even though it is incorrect to attribute differences in behaviour to the former being rigid (i.e. elastic) and the latter viscous, oceanic and continental lithosphere behave in different ways. Unlike oceanic crust, continental crust is buoyant and cannot be simply created or destroyed. The process of thickening or thinning works against gravity preventing large displacements on extensional or contractional features in the upper mantle. The equivalents of ridge or subduction systems are suppressed before they can accommodate large displacements and activity must shift elsewhere. On the other hand, strike-slip boundaries and extrusion processes are favoured.

New age constraints on the timing of volcanism in central Afar, in the presence of propagating rifts

We investigate the relationship between rift propagation and volcanism in the Afar Depression in the last 4 Myr. Potassium‐argon and thermoluminescence dating allow detailed reconstruction of the temporal evolution of volcanism. Volcanic activity is almost continuous since 3.5 Ma, with intervals characterized by more intense activity, especially around 2 Ma. Spatial distribution of ages reveals that Stratoid Series volcanism migrated northward along a 200‐km trend between 3 and 1 Ma, at about 10 cm/yr, linked to northward propagation of the Gulf of Aden Ridge, after it had cut across the Danakil horst at 4 Ma. Our work underlines the role of rhyolitic volcanism in initiation of rifting. Acid volcanoes, initially formed near the axes of extensional zones, have been subsequently dissected and are presently located on both sides of active rift segments. These lavas were the first to be erupted in areas of low extensional strain and were followed by basaltic lavas as extension increased. Differentiated volcanoes acted as zones of local weakness and guided localization of fractures, then leading to fissural magmatism. This regional‐scale, composite style of rifting, including volcanic and tectonic components, can be compared to the large‐scale continental breakup process itself. Deformation occurs through propagation of faults and fissures under a regional stress field. These become localized because of weakening of the crust (or lithosphere) due to emplacement of magmas, under the influence of a plume in the large‐scale case, or of silicic centers linked to magma chambers in the regional‐scale case.

Reappraisal of the Arabia–India–Somalia triple junction kinematics

We propose alternative kinematics for the Arabia–India–Somalia triple junction based on a re-interpretation of seismological and magnetic data. The new triple junction of the ridge–ridge–ridge type is located at the bend of the Sheba Ridge in the eastern gulf of Aden at 14.5°N and 56.4°E. The Owen fracture zone (Arabia–India boundary) is connected to the Sheba Ridge by an ultra-slow divergent boundary trending N80°E±10° marked by diffuse seismicity. The location of the Arabia–India rotation pole is constrained at 14.1°N and 71.2°E by fitting the active part of the Owen fracture zone with a small circle. The finite kinematics of the triple junction is inferred from the present-day kinematics. Since the inception of the accretion 15–18 Ma ago, the Sheba Ridge has probably receded ∼300 km at the expense of the Carlsberg Ridge which propagated northwestward in the gulf of Aden, while an ultra-slow divergent plate boundary developed between the Arabian and Indian plates. The overall geometry of the new triple junction is very similar to that of the Azores triple junction.

Fault propagation and climatic control of sedimentation on the Ghoubbet Rift Floor: insights from the Tadjouraden cruise in the western Gulf of Aden

A detailed geophysical survey of the Ghoubbet Al Kharab (Djibouti) clarifies the small‐scale morphology of the last submerged rift segment of the propagating Aden ridge before it enters the Afar depression. The bathymetry reveals a system of antithetic normal faults striking N130°E, roughly aligned with those active along the Asal rift. The 3.5 kHz sub‐bottom profiler shows how the faults cut distinct layers within the recent, up to 60 m thick, sediment cover on the floor of the basin. A large volcanic structure, in the centre of the basin, the ‘Ghoubbet’ volcano, separates two sedimentary flats. The organization of volcanism and the planform of faulting, with en echelon subrifts along the entire Asal–Ghoubbet rift, appear to confirm the westward propagation of this segment of the plate boundary. Faults throughout the rift have been active continuously for the last 8400 yr, but certain sediment layers show different offsets. The varying offsets of these layers, dated from cores previously retrieved in the southern basin, imply Holocene vertical slip rates of 0.3–1.4 mm yr−1 and indicate a major decrease in sedimentation rate after about 6000 yr BP, and a redistribution of sediments in the deepest troughs during the period that preceded that change.

Propagation of rifting along the Arabia‐Somalia Plate Boundary: Into Afar

It is generally accepted that the Aden ridge has propagated westward from ∼58°E to the western tip of the Gulf of Aden/Tadjoura, at the edge of Afar. Here, we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the Republic of Djibouti. We show that it veers northward, forming a narrow zone of dense faulting along the northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal‐Ghoubbet and Manda Inakir), connected to one another and to the submarine part of the ridge by transfer zones. Both rifts are composite, divided into two or three disconnected, parallel, NW‐SE striking subrifts, all of which appear to have propagated northwestward. In Asal‐Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (Manda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia‐Somalia plate boundary in Afar. We account for most observations by a mechanical model similar to that previously inferred for the Gulf of Aden, in which propagation is governed by the intensity and direction of the minimum horizontal principal stress, σ3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Central Afar lithosphere where it is thinnest. Such necking may be a consequence of differential magmatic thickening, greater in the center of the Afar depression where the Ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression. The model explains why the Aden ridge foregoes its WSW propagation direction, constant from ∼58°E to Asal‐Ghoubbet. At a smaller scale, individual rifts and subrifts keep opening perpendicular to the Arabia‐Somalia (or Danakil‐Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to jump northward, such that the plate boundary remains inside the regional N‐S necking zone. Changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns, a small angle promoting long, en échelon subrifts, and a high‐angle, smaller, nested, “subrifts within subrifts.” The propagation mechanism is thus similar, whether in oceanic or continental lithosphere, the principal change being the overall propagation path, here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the Aden ridge. Finally, because the same mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia‐Somalia and Arabia‐Nubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large‐scale bookshelf faulting.

Propagation of rifting along the Arabia‐Somalia Plate Boundary: The Gulfs of Aden and Tadjoura

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large‐scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia‐Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates >10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia‐Somalia plate. The longest pause was at the Shukra‐El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia‐Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra‐El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en échelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large‐scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small‐scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia‐Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea‐Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self‐sustaining, plate‐scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.

A detailed geochemical and rock magnetic study of dredged basalt from the Sheba Ridge, Gulf of Aden

We report the results of petrological, geochemical and rock magnetic studies of basalt dredged from the eastern end of the west Sheba Ridge during cruise 11/1979 of R. R. S. Shackleton to the Western Gulf of Aden. The ridge is part of the Red Sea-Gulf of Aden spreading axis and the basalts are olivine tholeiites. The abundances of some elements are characteristic of normal MORB (mid-ocean ridge basalt) but other elemental abundances suggest affinites with transitional-type MORB.

Nd‐Sr‐Pb isotopic variations along the Gulf of Aden: Evidence for Afar Mantle Plume‐Continental Lithosphere Interaction

We report on the rare earth and Nd‐Sr‐Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800‐km‐long Nd‐Sr‐Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil‐Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46°E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48°E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd‐Sr‐Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end‐members which can be interpreted as Pan‐African continental lithosphere along the Gulf of Tadjoura (a hybrid EM‐l‐EM‐2), a mantle plume (relatively young HIMU‐like) which dominates the 46°E anomaly, and the depleted asthenosphere east of 48°E (DUPAL‐like). Combined data from the Gulf of Aden‐Red Sea‐Afar‐Ethiopian rifted zones suggest a radial pattern of geochemical and isotopic variation about the Afar. A working dynamical‐thermal model is presented for the past 30–40 m.y. history of the Horn of Africa. It invokes both passive rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan‐African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800–1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450±150 km in radius, composed of original HIMU‐like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46°E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13°–16°N southern Red Sea segment, which represents a rift zone at a transient stage of either development or abandonment (overlapping with the Afar NW neovolcanic zone), where ocean island alkali volcanism dominates and diffuse lithosphere extension may still operate; (3) the high alkaline field of the Aden Volcanic Series; and (4) the Ethiopian Rift around 8°N in a purely continental setting. The NW Afar neovolcanic zone, which is essentially at a nascent stage of seafloor spreading and is overlapping the ring and the center of the pancakelike flattened mantle plume, is dominated by tholeiites derived from depleted asthenospheric material entrained by the plume during its original ascent. Plate reconstructions further suggest that the original center of the flattened mantle plume head has moved with the lithosphere some 900 km northeastward. The stem feeder of the plume has now been drawn or tilted toward the Afar as a result of the migration of the Gulf of Aden/Red Sea spreading centers which act as sinks of asthenospheric material and the likelihood that the feeder of the mantle plume is encountering with time an African lithosphere increasing in age, thickness, and rigidity.

The gravity field of the Red Sea and East Africa

Abstract Reevaluation of all gravity data from the Red Sea, the Gulf of Aden and East Africa permitted the compilation of a new Bouguer anomaly map. The intensity of the gravity field and its regional pattern correlate closely with the topographic features of the region. The maximum Bouguer values (> + 100 mGal) are located over the median troughs of the Red Sea and Gulf of Aden. Dense juvenile oceanic crust in these rifts and intruding magmas in stretched continental areas produce excess mass responsible for the anomaly highs. In the Red Sea the orientation of the gravity highs is NW-SE in the south, turning to NE-SW in the north, almost parallel to the Aqaba-Dead Sea strike. This pattern reveals that the present basin axis is not identical with that which formed the Tertiary coastal margins and the pre-Red Sea zones of crustal weakness. In the Gulf of Aden, new oceanic crust along the Tadjura Trench and its eastward extension is also expressed in the Bouguer anomaly map by gravity highs and a sharp bending of the isolines. A maximum of approx. +150 mGal is located over the central section of the Sheba Ridge. Bouguer gravity values over the East African and Yemen Plateaus are of the order of −180 to −240 mGal, indicating significant crustal thickening. On the Somali Plateau, the Marda Fault also has a strong gravity signature that can be traced towards Somalia. By constraining crustal thickness and structure with seismic data and density values from the velocity distribution by means of the Nafe-Drake and Birch relationships, we computed density models for the crust and upper mantle. The crustal thickness is of the order of 40 km beneath the plateaus and only 5 to 6 km at the oceanized parts in the central and southern portions of the Red Sea median trough. The flanks of the southern Red Sea and the corresponding Arabian side are underlain by 12 to 16 km thick stretched continental type crust. Oceanization offshore Sudan and Egypt is asymmetrical. The continental crust terminates abruptly at about 20 km off the coastline, followed by an oceanic crust of early Miocene age that was produced in pull-apart basins. By contrast, the eastern side of the Red Sea trough offshore Saudi Arabia is floored by stretched continental crust that extends far into the sea. Seafloor spreading and the generation of oceanic crust in organized spreading centres are limited to the median troughs off Sudan and the northern part of Ethiopia and commenced approx. 5 m.y. BP. They are absent in the northern Red Sea, where crustal fracturing occurs only in pull-apart basins of Dead Sea-Aqaba orientation distributed in en-echelon pattern.

3He and methane in the Gulf of Aden

Abstract During the OCEAT cruise (July, 1987), the vertical and spatial distributions of 3He and methane were measured at six stations over the West Sheba Ridge (Gulf of Aden). The results show significant δ3He anomalies (up to 49%). We conclude that the origin of this signal is independent from the well-known Red Sea hydrothermal 3He (of Red Sea Brines). Thus, active hydrothermalism occurs in this extensional basin associated with spreading along an incipient mid-ocean ridge. The 3He input from the Gulf of Aden accounts for the S-N positive gradient in 3He concentration observed in the western part of the Indian Ocean. Several methane anomalies are also present (up to 664 nl/l, i.e., 25 times the regional methane background), but the CH4 and 3He signals are not systematically correlated, suggesting complex production and consumption mechanisms of methane in these areas. Our results confirm previous observations in the South West Pacific Ocean.

Present‐day motion along the Owen Fracture Zone and Dalrymple Trough in the Arabian Sea

All plate motion data available for describing the present‐day motion between the Arabian and Indian plates are analyzed to understand the present‐day tectonic role of the presumed plate boundary: the Owen fracture zone and Dalrymple trough in the northern Arabian Sea. All prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary. In particular, published global plate motion models predict some convergence across the Owen fracture zone and Dalrymple trough. Convergence along the Owen fracture zone is contradicted by earthquake focal mechanisms, which show right‐lateral strike slip. Convergence along the Dalrymple trough is contradicted by observed normal faulting in the trough, and by normal and strike‐slip focal mechanisms for earthquakes occurring along or near the trough. The sense and rate of motion is further constrained by spreading rates along the Sheba Ridge (northwest of the Owen fracture zone) and along the Carlsberg Ridge (southeast of the Owen fracture zone). To estimate the present‐day motion, we determine 43 3‐m.y.‐average spreading rates from Sheba Ridge, Carlsberg Ridge, and Central Indian Ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the Arabia‐India boundary. We determine an Arabia‐India Euler vector that predicts 2 mm/yr of right‐lateral strike‐slip motion along the Owen fracture zone, with a 95% confidence interval of 0–7 mm/yr, which excludes the faster rates of slips predicted by several prior models. The direction of motion is 35°–50° counterclockwise from that predicted by published global plate motion models. If we omit the data along the Arabia‐India boundary, the motion differs insignificantly from zero, suggesting that magnetic profiles from the Carlsberg and Sheba ridges give spreading rates too imprecise to detect the slow Arabia‐India motion. If the Owen fracture zone and Dalrymple trough are regarded as a plate boundary, it is the slowest slipping plate boundary known on Earth, with motion about 100 times slower than the fastest slipping faults along the East Pacific Rise.

Somali Basin, Chain Ridge, and origin of the Northern Somali Basin gravity and geoid low

The Northern Somali Basin, located between Chain Ridge and the Horn of Africa north of 4°N, is characterized by a distinct 5‐m geoid low and by large negative gravity anomalies. The boundaries of the basin are marked by steep gradients in both gravity and geoid. Basement in Northern Somali Basin is 1–2 km deeper than on the Carlsberg Ridge flank to the southeast or on the Sheba Ridge flank to the north with a sharp discontinuity across the boundary of the basin. The Western Somali Basin, to the south, was created in the Late Jurassic and Early Cretaceous by the movement of Madagascar away from Africa. Reinterpretation of the magnetic anomalies in the Western Somali Basin shows that they record both limbs of a mid‐ocean ridge that was active by M22 time (Kimmeridgian) and died soon after MO (Aptian). Magnetic and gravity data allow the relict ridge crest to be traced from Davie Ridge near the African coast to the Dhow‐VLCC‐ARS fracture zone complex at 50°E. Davie Ridge is a transform fault connecting the Western Somali Basin spreading center with a similar age spreading center in the Mozambique Basin. The Dhow‐VLCC‐ARS complex can be shown to continue north of the 4°N bathymetric high separating the Northern and Western Somali Basins and to intersect the African margin near 7°N, The Northern Somali Basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between East and West Gondwanaland. The original Northern Somali Basin was split apart by the northward motion of India in the Late Cretaceous and Chain Ridge formed along the new boundary. Thermal and gravity modeling shows that the flexure resulting from differential subsidence across Chain Ridge combined with the difference in lithospheric thermal structure (Late Jurassic vs. Early Tertiary) on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across Chain Ridge. The geoid step up from the basin up to the African coast can be modeled as an edge effect between the juxtaposed Jurassic oceanic and African continental crust. Thus, the geoid and gravity low over the Northern Somali Basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly.

Tectonics of the westernmost Gulf of Aden and the Gulf of Tadjoura from submersible observations

The Gulf of Tadjoura, separating the Danakil and Somalia plates1, represents the western extension of the Sheba Ridge between Arabia and Somalia in the Gulf of Aden (Fig. 1). It has been assumed that a simple plate-tectonic scheme could be applied to the area with NE–SW transform faults offsetting roughly east–west zones of accretion represented by the major bathymetrie troughs1–8 (Fig. 1c). More recently this boundary has been interpreted as a westward-propagating rift with its present-day tip on land in the region of the Asal Lake9,10. Here we present data from dives with the submersible Cyana which show that there have been two episodes of extensional faulting, an older inactive phase parallel to the general ENE–WSW direction of the gulf and a younger fault set along discontinuous zones striking 140°. There is no surface evidence for any transform fault zone in the area covered and the incipient spreading segments whose geometry implies extension on 050° do not define a plate boundary. The faulting observed is at odds with that predicted by orthodox plate tectonics.

The transition between the Sheba Ridge and Owen Basin: rifting of old oceanic lithosphere

Summary Magnetic quiet zones are present along the margins of the entire length of the Gulf of Aden to the Owen fracture zone. This includes the easternmost 300 km between the eastern edges of Arabia and Africa to the Owen fracture zone where old oceanic lithosphere was rifted to form the Sheba Ridge. Within this easternmost region the boundary between the quiet zone and the old oceanic lithosphere is marked by ridge complexes, the Sharbithat Ridge Complex to the north and the Error Ridge Complex to the south. These ridge complexes, which lack a magnetic signature, occupy a structural position similar to the hinge zones at the continental margins to the west. They appear to have formed early in the opening of the Gulf of Aden or perhaps to have been pre-existing features. The boundary between Sheba Ridge and the northern magnetic quiet zone is often marked by an abrupt end of the Sheba Ridge seafloor spreading magnetic anomaly pattern and a sharp basement deepening to the north. The boundary between the East Sheba Ridge and northern magnetic quiet zone becomes less distinct near the Owen fracture zone. This is also accompanied by changes in the East Sheba Ridge, specifically a decrease in magnetic anomaly amplitudes, increase in ridge flank depths and a loss of the ’cooling-curve’ ridge flank shape. This may be the result of lower mantle temperatures in the vicinity of the Owen fracture zone. The sediments within the magnetic quiet zone can be divided into a lower disturbed unit and an upper unit consisting of flat-lying reflectors. The disturbance of the lower sediments may have resulted from a period of diffuse extension in the magnetic quiet zone prior to the establishment of the Sheba Ridge spreading centre. The similarity of the easternmost quiet zone to quiet zones at rifted continental margins leads to the suggestion that these regions were formed by diffuse extension of old oceanic lithosphere. Using a two-layer lithospheric attenuation model and assuming extension during a period of 15 Myr followed by 10 Myr of cooling, the basement depths and heat flow measurements can be adequately matched. The modelling implies 45 per cent crustal extension in the quiet zone. The amount of extension calculated is compatible with documented motion between Arabia and Africa. The old oceanic lithosphere thus must have been substantially thinned to a thickness similar to 10-14 Myr old ocean before seafloor spreading was initiated at the Sheba Ridge.

Strain accumulation across the Asal‐Ghoubbet Rift, Djibouti, East Africa

The Asal Ghoubbet rift in the Republic of Djibouti is a subaerial continuation of the Gulf of Aden ridge system. Many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system. Geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm/yr perpendicular to the normal faults that bound the rift. Periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm/yr. Although the observed deformations could be contamined by postseismic relaxation after a rifting episode in November 1978, the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley. The deformation is uniformly distributed in time and in space inside the inner floor of the rift. The necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process.

Late Quaternary calcareous clayey-silty muds in the Obock Trough (Gulf of Aden): hemipelagites or fine-grained turbidites?

Summary Three cores have allowed us to survey Holocene deposits in a trough of the Gulf of Aden ridge. Within this particular environment (steep slopes, tectonic and volcanic activity) sediments are fine-grained, apparently homogeneous calcareous muds, rich in clay and planktonic microfossils: these features suggest that they are hemipelagic deposits. However, high sedimentation rates (20 to 50 cm/1000 yrs), lateral variation, and the occurrence of dynamic sedimentary structures (visible on X-radiographs and in thin sections), suggest that they have been emplaced mainly by gravity currents. These reworked hemipelagic materials are deposited on the shelf edge and on the trough slopes. This is confirmed by the sequential arrangement of sedimentary characteristics, as shown by analysis of microstructures, microfacies, carbonates, clay minerals, grain-size and other components. Two types of sequences have been observed; the first we call the ‘type sequence’. This commonly overlies an erosional surface and comprises three divisions from bottom to the top: Division I (equivalent to Piper’s E1 division); mud with 8–10% sand fraction, rich in biogenic tests which occur as well-sorted horizontal laminae. Division II (equivalent to Piper’s E2 and E3 divisions); mud with low sand fraction (1%), very poor in biogenic material, no lamination, but horizontal orientation of silt-sized bioclasts. Division III (equivalent to Piper’s E2 and E3 divisions); mud with low sand fraction (1%), very poor in biogenic material, no lamination, but horizontal orientation of silt-sized bioclasts. Division III (equivalent to Piper’s F division); mud with low sandy fraction, poorly-sorted biogenic material, no dynamic sedimentary structures. The second type of sequence is less common and comprises two divisions: Division I’a ; very thin layer (millimetric), rich in terrigenous silt, which scours underlying sediment and shows horizontal, oblique and cross lamination. Division I’b ; mud with low biogenic sand content, showing horizontal more or less clear and irregularly-distributed lamination. These two sequences correspond to divisions E and F defined by Bouma (1962) and Piper (1978) in terrigenous turbidite sequences. The ‘type sequence’ is comparable to turbiditic sequences described in pelagic or hemipelagic material, deposited in similar morpho-tectonic environments (e.g. the eastern Mediterranean Sea or Mid Atlantic Ridge fracture zone).

The magnetic quiet zone in the eastern Gulf of Aden: implications for the early development of the continental margin

Summary. The ‘magnetic quiet zone’ in the eastern Gulf of Aden is located between the oceanic crust of Sheba Ridge and the continental crust of Arabia and Somalia, and is separated from both by important structural boundaries. The seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity. The landward boundary is marked by escarpments made up of a series of normal faults. These escarpments extend from 2–3 km below sea-level to 1500 m above sea-level and are equivalent of the ‘hinge zone’ found at mature continental margins. The magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography. The basement lacks the topographic slope characteristic of mid-ocean ridge flanks and is characterized by moderately rough relief. The crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined, they are slightly greater than those of oceanic crust. New heat flow measurements show high values (95.7–123.3 mW m−2) in the quiet zone with values decreasing from Sheba Ridge toward the coast. The unusual structure of the quiet zone and the observations that more opening appears to have occurred between Arabia and Somalia than can be accounted for by the oceanic crust of Sheba Ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational (listric) faulting and dyke injection. This possibility is investigated using both a ‘stretching’ or ‘lithospheric attenuation’ model and a model in which a portion of the extension occurs through dyke injection. It is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension (β= 4–6) are required in the deep seaward portion of the quiet zone. This results in more extension than is compatible with the documented motion between Arabia and Africa. However, formation of the magnetic quiet zone occurred over a period of 10–15 Myr rather than instantaneously as assumed in the simple models. When the effects of a finite length rifting episode are considered, less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone.

The Gulf of Aden: Structure and evolution of a young ocean basin and continental margin

New marine geophysical data are used to describe the structure and history of the Gulf of Aden. Magnetic anomaly data shows seafloor spreading magnetic anomalies on Sheba Ridge from the axial anomaly to anomaly 5 (10 m.y. B.P.) between the Owen fracture zone and 45°E and to anomaly 2′ (3 m.y. B.P.) or anomaly 3 (4 m.y. B.P.) west of 45°E. The data does not support the two episodes of seafloor spreading recently proposed. Landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies. The magnetic quiet zone boundary is also a structural boundary effectively marking the edge of Sheba Ridge, with deeper basement lacking a significant topographic gradient found on the landward side. A magnetic quiet zone is found not only where Sheba Ridge splits continental lithosphere but also on East Sheba Ridge where the ridge splits the old oceanic lithosphere of the Owen and Somali basins. There the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the Owen fracture zone. The magnetic quiet zone boundary is not an isochron in either the Gulf of Aden or the Red Sea, suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading. The offset on the Dead Sea Rift is used to estimate that from 80 to 160 km of opening, amounting to between 65% and 200% extension of the initial rift valley, occurred in the Gulf of Aden and Red Sea prior to the establishment of a mid‐ocean ridge. It is suggested that the development of a new ocean basin occurs in two stages. The first involves diffuse extension over an area perhaps 100 km wide in a rift valley environment without an organized spreading center. This is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading.

Gravity study of the Djibouti area

In March and April 1972, 380 gravity stations were established in the T.F.A.I. The data were reduced to Bouguer anomalies of 5 mGal isolines. Qualitative and quantitative interpretation based on the gravity map, on seismic data and on the magnetics and physiography of the area revealed that: The crust of the Gulf of Tadjura and the central part of the T.F.A.I. is strongly oceanized and is the direct continuation of the Sheba Ridge. To the north of the Gulf, at the Dankali Mountains, the crust increases in thickness and most probably contains sialic fragments, indicating the continuation of the Danakil Chains in the T.F.A.I. To the south, the structure is bordered by a continental block, the Aysha Horst. The oceanization is concentrated only in the area of deep injections, marked by gravity maxima, whereas the rest of the area is to be described as sub-continental. The crust is attenuated from south (Aysha Horst) to north (Red Sea), from 30 to 15 km in thickness. The pattern of the gravity anomalies shows clearly liniations only along the coasts of the Gulf of Tadjura and the Straits of Bab el Mandeb. Inland the field breaks up into relative minima and maxima, indicating the fragmentation of the crust and the ‘triple junction’ nature of the area. The tectonic process is that of extension, with normal faults having maximum displacements at the northern border of the Gulf of Tadjura. The uppermost mantle has low velocity and density values due to thermal processes in the expanding zone. The state of the upper-mantle material must be that of partial melting due to high temperatures of the order of 800° to 1,000° C at about 15 km depth.