Red Sea Trough Research Articles

High-Ti continental tholeiites from the Aznam trough, northwestern Saudi Arabia: evidence of “abortive” rifting in the “embryonic” stage of Red Sea opening

The Aznam trough is a NW-trending, asymmetric graben on the northern Saudi Arabian coast of the Red Sea. Miocene or younger, high-Ti, tholeiitic basalts are exposed In the northern part of the trough. These (Aznam) basalts contain microphenocrysts of altered olivine, calcic plagioclase and clinopyroxene with subordinate amounts of ilmenite and orthopyroxene in a cryptocrystalline groundmass. A minor xenocryst/xenolith assemblage consists of quartz, sieve-textured plagioclase and a variety of crustal rock fragments. The Aznam basalts have TiO 2 contents > 3.0%, elevated 87Sr/ 86Sr ratios (0.70486–0.70546), FeO∗/MgO ratios > 2.0, and Mg-numbers between 47 and 50. They are compositionally and isotopically more evolved than tholeiites (MORB) from the Red Sea trough. Minor crustal contamination of the initial basaltic magmas is shown by the elevated Sr isotopic composition and enrichment in some LIL components. Major-element modeling indicates that the Aznam basalts can be generated by fractionation of the major phenocryst phases out of typical Red Sea MORB. Trace-element distributions are mostly consistent with this conclusion. With the exception of minor crustal contamination at Aznam, the Aznam basalts are compositionally almost identical with high-Ti basalts formed in “abortive,” volcanically dormant, rifts in Iceland. Apparently the same process of magmatism and rift jumping that occurred in Iceland also occurred (and may still be occurring) in the northern Red Sea region as a short-lived embryonic rifting that is a precursor to sea-floor spreading in the Red Sea trough.

Vanadiferous minerals from the Nereus Deep, Red Sea

ABSTRACTThe chemical analysis by EMPEDS of 140 samples in cores from six Red Sea troughs and basins has led to the discovery in the Nereus Deep of high vanadium concentrations (up to 1.3% V2O3) in several bulk samples, and to the isolation of a major magnetite phase. This vanado‐magnetite, remarkable for its high content of vanadium (mean = 1.45% V2O3) and its lack of titanium, is frequently zoned, the V‐content decreasing toward the outer zones. Oxides with up to 42% V2O3, have also been found. Since the origin of the magnetite is clearly authigenic, its high vanadium and low titanium concentrations are traced back to the differential hydrothermal leaching of Fe—Ti‐oxides which occur profusely in basic eruptive rock clasts, actually present in some of the overlying seams.

Subsidence and origin of the Northern Red Sea and Gulf of Suez

Sedimentary accumulation and subsidence rates in the northern Red Sea and Gulf of Suez can be explained by extreme separation in central areas of rift basins and only minor thinning and tilting of fault blocks around the margins. During the Oligocene, local relief permitted red clastic sediments to be shed from the present area of the Red Sea. Very high rates of crustal thinning in Late Oligocene or earliest Miocene in the central part of the Gulf of Suez and part of the Red Sea trough permitted rapid subsidence of an early Miocene erosion surface. Miocene and younger sediments are thick only in these central areas. Movement on the Dead Sea shear zone began at the same time as subsidence, effectively isolating the Gulf of Suez from further tectonism and allowing thermal subsidence to occur to the present. Extension in the northern Red Sea, south of the shear zone, has either been so extreme as to leave a crust of less than 10 km thickness or has been accomplished by injections of large quantities of mafic magma, forming a crust intermediate between continental and oceanic. All observations are consistent with models of rifting by pure stretching or by detachment faulting.

Afar Mantle Plume: Rare Earth Evidence

Rare earth pattern zonation about the Afar Triangle, in tholeiitic basalts erupting along the Red Sea Trough and the Gulf of Aden, suggests the presence of a primordial mantle plume rising beneath the Afar and overflowing in a star-like fashion into the soft asthenosphere.

Sea-Floor Spreading and Structural Evolution of Southern Red Sea

The Red Sea has entered the early stages of continental dispersal, and its structural evolution fundamentally has been the rifting and breaching of continental lithosphere by normal faulting attendant on the process of sea-floor spreading. In Oligocene time, the continental lithosphere of the Red Sea area was bowed into a large regional arch with normal faults across the crest. Subsequently, rifting by normal faults that propagated upward through the brittle part of the lithosphere caused strong subsidence on horst and graben and tilted blocks; this rifting led to an extensive marine incursion, and a thick evaporite sequence was deposited in the restricted, hot, arid, low-latitude setting of the Miocene Red Sea trough. A second rift west of the earlier central or axial Mi cene rift originated in the southern Red Sea area in Pliocene time, and the two features now are evolving concurrently over a distance of at least 400 km. An evaporite section of very shallow marine origin accumulated in the western rift during the Quaternary; it constitutes a modern example of salt accumulating in a narrow, restricted, rifted trough which is forming as a consequence of continental breakup. Although considerable separation has occurred in the Red Sea, the present opposing coastlines were never in contact, because the fragmentation of continental lithosphere was very largely attained by normal faulting. Only if a vertical fault cuts the entire thickness of lithosphere could points on opposite coastlines ever have been contiguous.

Sea-floor evolution: rare-earth evidence

A systematic survey of rare-earth (r.e.) abundances in submarine tholeiitic basalts along mid-oceanic ridges has been made by neutron activation analysis. The r.e. fractionation patterns are remarkably uniform along each mid-oceanic ridge and from one ridge to another (Juan de Fuca Ridge, East Pacific and Chile Rise, Pacific-Antarctic, Mid-Indian and Carlsberg Ridge, Gulf of Aden, Red Sea Trough and Reykjanes Ridge). The patterns are all depleted in light r.e. except for three samples (Gulf of Aden and Mid-Indian Ridge) which are unfractionated relative to chondrites. They contrast markedly with tholeiitic plateau basalt which are shown to be related to the early volcanic phases associated with continental drift. Tholeiitic plateau basalts are light r.e. enriched as are most continental rocks. Mid-ocean ridge basalts are also distinguishable from spatially related oceanic shield volcanoes of tholeiitic composition (Red Sea Trough-Jebel Teir Is., East Pacific Rise-Culpepper Island). Thus on a r.e. basis there are tholeiites within tholeiites. The r.e. difference between mid-ocean ridge tholeiites and tholeiitic plateau basalts can be related to distinct thermal and tectonic régimes and consequently magmatic modes and rates of intrusions from the low velocity layer in the upper mantle. The difference between continental and oceanic volcanism appears to be triggered by: (1) presence or absence of a moving continental lithosphere over the low velocity layer, and (2) whether or not major rifts tap the low velocity layer through the lithosphere. Fractional crystallization during ascent of melts before eruption at the ridge crest does not affect appreciably the relative r.e. patterns. R.e. in mid-ocean ridge basalts appear to intrinsically reflect their distribution in the upper mantle source, i.e. the low velocity layer. Based on secondary order r.e. variation of mid-ocean ridge basalts: (1) If fractional crystallization is invoked for the small r.e. variations, up to approximately 50 % extraction of olivine and Ca-poor orthopyroxene in various combinations can be tolerated. However, only limited amount of plagioclase or Ca-rich clinopyroxene can be extracted, the former because of its effect on the abundance of Eu abundance and the latter because of its effect on the [La/Sm] e.f. ratio, alternatively. (2) If partial melting during ascent is invoked, and a minimum of 10% melting is assumed, the permissible degree of melting of originally a lherzolite upper mantle may vary between 10 and 30% . It is not possible to establish readily to what extent these two processes have been operative as they cannot be distinguished on the basis of r.e. data only. However, there is evidence indicating that both have been operative and are responsible for the small r.e. variations observed in mid-ocean ridge basalts. An attempt to correlate second order r.e. variations along or across mid-oceanic ridges with spreading rate, age, or distance from ridge crests has been made but the results are inconclusive. No r.e. secular variation of the oceanic crust is apparent. R.e. average ridge to ridge variations are attributed to small lateral inhomogeneities of the source of basalts in the low velocity layer, and to a certain extent, to its past history. The remarkable r.e. uniformity of mid-oceanic ridge tholeiites requires a unique and simple volcanic process to be operative. It calls for upward migration of melt or slush from a relatively homogeneous source in the mantle—the low velocity layer, followed by further partial melting during ascent. The model, although consistent with geophysics, may have to be reconciled with some evidence from experimental petrology. Models for r.e. composition of the upper mantle source of ridge basalt, formation of layers 2 and 3, and the moho-discontinuity, are also presented.

Sea-Floor Spreading and Structural Evolution of Southern Red Sea: ABSTRACT

The Red Sea is in the early to intermediate stages of continental breakup, and its structural evolution has been fundamentally the rifting and breaching of continental lithosphere by normal faulting attendant to the process of sea-floor spreading. In Oligocene time, the continental lithosphere of the Red Sea area was bowed into a large regional arch with normal faults across the crest. Subsequently, rifting by normal faults that propagated upward from the base of the lithosphere caused strong subsidence on horst and graben and tilted blocks; this rifting led to an extensive marine incursion, and a thick evaporite sequence was deposited in the restricted, hot, arid, low-latitude setting of the Miocene Red Sea trough. A second rift westward of the earlier central or axial M ocene rift originated in the southern Red Sea area in Pliocene time, and the 2 features are now evolving concurrently over a distance of at least 400 km. An evaporite section of very shallow marine origin accumulated in the western rift during the Quaternary; it constitutes a modern example of salt accumulating in a narrow, restricted, rifted trough which is forming as a consequence of continental breakup. The present opposing coastlines of the Red Sea were never in contact, because the fragmentation of continental lithosphere was attained very largely by normal faulting. Only if a vertical fault surface cut the entire thickness of lithosphere could points on opposite coastlines ever have been contiguous. End_of_Article - Last_Page 349------------

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Eastern margin of the Red Sea and the coastal structures in Saudi Arabia

Results of many investigations since 1950 show that the eastern margin of the Red Sea and associated coastal structures in Saudi Arabia have a long geologic history, starting with the deposition of Precambrian eugeosynclinal sedimentary and volcanic rocks before 1000 Ma ago and extending to recent geologic time. The northeastern flank of the Red Sea rift valley is in a shield area affected by possibly four plutonic events at 1000, 720 to 735 (?), 660 to 670, and about 570 Ma. Cratonization of the shield occurred during all or part of the span 520 to 590 Ma. Nubian-type sandstone of Cambrian and Ordovician age laps up on the shield from Jordan southeastward around the rim of the Great Nafud basin, and along the eastern edge of the shield southeastward to 45° E longitude where it is overlapped by Permian limestone. The sandstone reappears to the south and extends southward and westward to the Asir Mountains at the Yemen border. Isolated sandstone outliers are present in the central shield, proving that lower Paleozoic sandstone covered most, if not all, of the basement as now exposed. The Mesozoic era was almost entirely a time of uplift and non-deposition except a middle to late Jurassic fringe marine invasion in the south and a possible narrow invasion from the Gulf of Suez at the end of the era. Marine and non-marine sedimentary deposits of middle and late Tertiary age are found along the Red Sea coast, and Oligocene basaltic flows are present at both low and high altitudes in the coastal ranges. Evidence for important volcanism during Oligocene and earliest Miocene time is widespread, and within the eastern rift fault zone early Miocene hypabyssal intrusives cut the shear zones. Major rifting occurred just before or during early Miocene when the flanks of the rift valley were ramped upward. Shortly after this volcanism a thickness on the order of 3500 m of middle Miocene marl and evaporite beds filled the Red Sea trough. Evidence also exists for widespread subaerial erosion in the Pliocene. Younger lava flows are Pliocene in age but the youngest, near Al Medinah, came as late as A. D. 1250. Lake-bed deposits are very probably in large part Pliocene throughout the shield. The Red Sea coastal plain in Saudi Arabia rises gently eastward from a 3 m littoral surface, generally underlain by dead reef from the Yemen border northward to Al Wajd, a distance of 1400 km. At Jizan, in the south, a salt dome has pushed the 3 m surface up to an elevation of about 50 m. From Al Wajd northward, Pleistocene terraces have been faulted, culminating in several surfaces as high as 520 m above the Red Sea at Tiran Island. Ramping of major fault-bounded blocks along the eastern side of the Red Sea trough—the Midian block in the north, a poorly defined central block, and the Asir block in the south—is connected with renewed movement on regional Precambrian faults. Drainage patterns of wadis in these blocks are characteristically affected by the ramping, and stream capture is common in the Midian and Asir blocks.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Seismic and gravity data from afar in relation to surrounding areas

The Afar triangle is bordered, to the west, by a seismic belt running along and on top of the escarpment. Seventy-five percent of the seismic energy of the area is released along this belt. The epicentre distribution along the western escarpment coincides either with major north-south marginal tectonic features or with cross-rift faulting. A second epicentre lineation runs at N 15° E through central Afar. To the south-east, in the region of the Gulf of Tadjura, epicentre locations offer no distinct lineation. The sum of the free-air gravity anomalies over Afar is almost zero; Bouguer values are generally negative and strictly proportional to elevation. Absolute Bouguer positive values are found only over volcanic centres and along the northeastern coast; their maximum does not compare with the positive values found over the nearby Red Sea trough. Evidence based on attenuation and dispersion of seismic surface waves and on gravity profiles suggests a continental crustal structure of relatively ‘standard’ thickness under the Afar triangle.

THE RELATIONSHIP OF THE RED SEA TO THE EAST AFRICAN RIFT SYSTEM

The Red Sea is characterized by positive gravity anomalies along its centre, whereas the East African Rift Valleys have negative anomalies. The Red Sea trough is more than 200 miles wide but the gravity maximum appears to be associated with a deep trough (about 30 to 40 miles wide), the anomalies on each side being small. A table is given of gravity data for the Red Sea area, and the values have been revised so that they are directly comparable. To assist in the interpretation of these data a brief review is given of the dimensions of the Rift Valleys, and the general geological structure and history of the area. The interpretation of negative anomalies over the East African Rift Valleys is discussed and the Red Sea anomalies are described and contrasted with those of the East African Rifts. The gravity maximum is interpreted as being due to a large basic mass intruded through the down-faulted basement rocks. It is suggested that the formation of the fault trough which the Red Sea occupies and the deep, relatively narrow axial trough is connected with the intrusion and a possible mechanism is given.