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Red Sea Rift
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The Red Sea Rift is a mid-ocean ridge between two tectonic plates, the African plate and the Arabian plate. It extends from the Dead Sea Transform fault system, and ends at an intersection with the Aden Ridge and the East African Rift, forming the Afar triple junction in the Afar Depression of the Horn of Africa.
The Red Sea Rift was formed by the divergence between the African and Arabian plates. The rift transitioned from a continental rift to an oceanic rift.[1] Magnetic anomalies suggest that the spreading rate on either side of the Red Sea is about 1 cm/year. The African plate has a rotation rate of 0.9270 degrees/Ma (million years), while the Arabian plate has a rotation rate of 1.1616 degrees/Ma.[2]
Spreading model
[edit]A two-stage spreading model explains the tectonic evolution in this region. The first major rift motion was seen in the lower/middle Eocene, followed by major seafloor spreading in the late Eocene and early Oligocene. This was followed by a period of 30 Ma of no motion, during which a large amount of evaporites were deposited. After this quiet period of deposition, a new period of activity started about five million years ago. This new phase of spreading caused considerable disturbance to the sediments that were deposited, which created an unstable situation as the crust and sediments parted and the axial trough evolved. Normal faulting along the rift valley during earthquakes shows that the extensional motion is continuing.[3]
Mechanism of rifting
[edit]A three-step process[4] has been proposed for the mechanism of rifting.
First, a thermal anomaly developed in the mantle in the earliest stages of rifting, causing the rise of the asthenosphere and the thinning of the subcrustal continental lithosphere. There have been several mechanisms proposed to achieve this, such as convective thermal thinning.
This was followed by decompressions, which occurred due to uplift related to the gradual stretching and thinning of the crust as rifting continued. Stretching and thinning can take place either according to a symmetrical, pure shear, extension model, or due to an asymmetric, detachment-delamination model. Basaltic dykes are also injected during the stretching and thinning.
As the basaltic injections become restricted to a narrow axial zone, true seafloor spreading initiates with the Vine-Matthews-type magnetic anomaly stripes. Axial propagation of the oceanic rift occurs, resulting in a continuous axis of spreading. The rift may be intersected by a shear or fracture zone, which act as locked zones and prevent further propagation. Zones of compression may develop.
Composition
[edit]While there is a general agreement that the axial trough of the Red Sea originated by seafloor spreading, and therefore is underlain by oceanic crust, the nature of the crust beneath the main trough and coastal plains of the Red Sea is still controversial, leading to the development of a few theories. One theory suggests that the entire Red Sea basin is underlain by oceanic crust, while another theory claims that the main trough is underlain only in part by oceanic crust. A third theory suggests that outside the axial trough, the crust has a continental composition, with the presence of basaltic dykes, while another hypothesis suggests that the lower crust in the rift consists of rift meta-sediments, in direct contact with the upper mantle. All geophysical data from the axial trough reveal seismic velocities typical for oceanic crust. The main trough, however, is characterized by a high degree of lateral variation, with abrupt changes in basement velocities from typical continental to typical oceanic signatures.[5]
Volcanic activity
[edit]The rift zone includes the island of Jabal al-Tair, formed by the basaltic stratovolcano of the same name, located northwest of the Bab al-Mandab passage at the mouth of the Red Sea, about halfway between Yemen and Eritrea. The volcano erupted on 30 September 2007, after 124 years of dormancy.
Resources
[edit]The axial deep of the rift was the location of the first known hot hydrothermal brines discovered on the sea floor. Workers from 1949 through the 1960s confirmed the presence of hot (60 °C (140 °F)) saline brines and associated metalliferous muds. The hot solutions were emanating from an active subseafloor rift.[6]
See also
[edit]References
[edit]- ^ McGuire, A. V.; Bohannon, R. G. (1989). "Timing of mantle upwelling: Evidence for a passive origin for the Red Sea rift". Journal of Geophysical Research. 94 (B2): 1677–1682. doi:10.1029/jb094ib02p01677.
- ^ Mckenzie, D. P.; Davies, D.; Molnar, P. (1970). "Plate Tectonics of the Red Sea and East Africa". Nature. 226 (5242): 243–248. doi:10.1038/226243a0.
- ^ Girdler, R. W.; Styles, P. (1974). "Two Stage Red Sea Floor Spreading". Nature. 247 (5435): 7–11. doi:10.1038/247007a0.
- ^ Davison, I.; Al-Kadasi, M.; Al-Khirbash, S.; Al-Subbary, A. K.; Baker, J.; Blakey, S.; Bosence, D.; Dart, C.; Heaton, R.; Mcclay, K.; Menzies, M.; Nichols, G.; Owen, L.; Yelland, A. (1994). "Geological evolution of the southeastern Red Sea Rift margin, Republic of Yemen". GSA Bulletin. 106 (11): 1474–1493. doi:10.1130/0016-7606(1994)106<1474:geotsr>2.3.co;2.
- ^ Bonatti, E.; Seyler, M. (1987). "Crustal underplating and evolution in the Red Sea Rift: Uplifted gabbro/gneiss crustal complexes on Zabargad and Brothers Islands". Journal of Geophysical Research. 92 (B12): 12803–12821. doi:10.1029/jb092ib12p12803.
- ^ Degens, E. T.; Ross, D. A., eds. (1969). Hot Brines and Recent Heavy Metal Deposits in the Red Sea: A Geochemical and Geophysical Account. Springer Berlin. doi:10.1007/978-3-662-28603-6. ISBN 978-3-662-28603-6.
Red Sea Rift
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Geography and Overview
Location and Extent
The Red Sea Rift is a major tectonic feature situated between the African and Arabian plates, extending approximately from 12°N to 28°N latitude and 32°E to 43°E longitude. It measures about 2,000 km in length and varies in width from 200 to 350 km, forming an elongated depression that separates the northeastern margin of the African continent from the southwestern edge of the Arabian Peninsula.[8] To the west, the rift is bounded by the Nubian Shield, a Precambrian crystalline basement of the African Plate, while to the east it abuts the Arabian Shield, the equivalent Precambrian terrain of the Arabian Plate.[9][10] This rift forms a key component of the broader Afro-Arabian rift system, which encompasses interconnected features such as the Gulf of Suez and the Gulf of Aqaba to the north.[9] The system reflects the ongoing divergence between the African and Arabian plates, with the Red Sea Rift serving as the primary locus of extension in this region.[11] The rift is commonly divided into three main segments: the northern segment (from approximately 25°N to 28°N); the central segment, characterized by broader axial structures; and the southern segment, extending toward the Bab-el-Mandeb Strait.[12][13] These segments exhibit variations in crustal architecture and extension styles, influenced by the underlying plate boundaries.[14]Morphological Features
The Red Sea Rift forms a narrow, elongated basin approximately 2,000 km long and up to 355 km wide, characterized by a distinctive bathymetric profile that transitions from shallow coastal shelves to a deep central axial trough. The margins exhibit asymmetry, with the western (African) side generally steeper and narrower due to more pronounced faulting, while the eastern (Arabian) margin features broader shelves and greater volcanic influence. This topographic variation reflects the rift's extensional tectonics, where the basin floor deepens abruptly toward the axis, flanked by continental slopes that drop from near sea level to over 2,000 m in depth over short horizontal distances.[15][16] The axial trough, a prominent morphological element, bisects the southern and central portions of the rift south of about 21°N, reaching depths of up to 2,000-2,500 m in its main segments and locally exceeding 3,000 m in features like the Suakin Trough. It is typically 10-60 km wide, with rugged seafloor marked by volcanic constructs and fault-controlled depressions, and is flanked by shallow shelves less than 200 m deep that extend 50-100 km from the coasts. Steep escarpments border these shelves, rising 1,000-2,000 m above sea level on average, though reaching up to 3,000 m in elevated regions of the Arabian and Nubian shields, forming dramatic topographic steps that expose Precambrian basement rocks.[17][8][18] Along the rift margins, key surface features include the Dahlak Archipelago on the Eritrean shelf and the Farasan Islands on the Saudi Arabian side, both comprising volcanic and carbonate platforms that emerge as structural highs amid the rift's extensional fabric. These archipelagos, along with extensive coral reefs fringing the shallow shelves and marking shelf breaks at depths of 50-1,000 m, contribute to the rift's diverse coastal morphology, supporting unique ecosystems while delineating the transition to deeper waters. In the central rift, isolated deeps such as the Atlantis II Deep exemplify hypersaline basins within the axial trough, reaching approximately 2,000 m depth with brines exhibiting salinities up to 26%—over seven times that of typical seawater—due to evaporite dissolution and hydrothermal input.[19][20][21] On land, the rift's surface expressions are dominated by grabens, prominent fault scarps, and alluvial fans that radiate from the coastal highlands into the coastal plains. These grabens, often 10-50 km wide, form elongate depressions filled with Quaternary sediments, while fault scarps up to several hundred meters high define the rift flanks and offset older landforms. Alluvial fans, constructed from eroded escarpment material, prograde into the shallow marine shelves, creating sediment wedges that influence nearshore bathymetry and highlight ongoing tectonic activity. Volcanic features occasionally modify these landforms, adding basaltic flows to the coastal morphology.[15][22]Geological History
Initiation and Early Rifting
The Red Sea Rift initiated during the Late Oligocene, approximately 30–25 million years ago, as part of the broader separation between the Arabian and African plates, coinciding with the early opening of the Gulf of Aden.[23] This onset is evidenced by the nucleation of a small rift basin in the Eritrean segment of the southern Red Sea around 27.5–23.8 Ma, with synchronous extension propagating northward to the central and northern sectors between 24 and 21 Ma.[1] The rifting direction was primarily oriented N65°E, nearly orthogonal to pre-existing structures, and marked the transition from a unified continental block to active divergence.[24] The separation was driven by a combination of mantle plume activity and far-field tectonic forces. A mantle plume impinged beneath the Afar region around 31–29 Ma, triggering widespread flood basalts over more than 600,000 km² and initiating bi-directional rifting that linked the Red Sea, Gulf of Aden, and East African Rift systems.[25] Concurrently, slab-pull forces from the subduction of the Neotethys Ocean generated regional extension, with the Arabian Plate responding to northward-directed drag from the subducting slab remnants.[23] These mechanisms interacted to overcome the resistance of the thick continental lithosphere, setting the stage for localized thinning and magmatism in the proto-rift zones.[1] Early evidence of rifting appears in the form of extensional faulting within the continental crust and the development of initial sedimentary basins. In the northern Red Sea-Gulf of Suez system, normal faulting reactivated inherited WNW-trending Pan-African shear zones, creating segmented half-graben structures up to 100 km long and 50–90 km wide.[24] Basin formation in the Gulf of Suez began with deposition of Upper Oligocene continental clastic sediments, including red beds of the Nakheel and Abu Zenima Formations, accompanied by minor syn-rift basalts.[25] By the Early Miocene (around 24–23 Ma), shallow marine clastics of the Nukhul Formation accumulated in these basins, recording increasing subsidence and marine incursion as extension intensified.[1] Prior to rifting, the region featured a continuous Arabian-Nubian Shield, a Neoproterozoic juvenile crust exposed as a peneplaned surface near sea level across what is now the rift axis.[23] This pre-rift configuration, part of a passive Paleo-Tethyan margin, included deep-seated shear zones that guided early fault propagation without significant pre-rift uplift or doming.[25] The shield's uniform basement facilitated the initial symmetric extension before later tectonic adjustments.[24]Evolutionary Stages
The evolutionary stages of the Red Sea Rift progressed from initial continental extension to the development of oceanic crust, reflecting a classic sequence of rift maturation. Continental rifting commenced around 30 Ma, driven by the separation of the Arabian and African plates, leading to significant thinning of the continental crust through faulting and stretching.[1] This phase persisted until approximately 13 Ma, characterized by syn-rift sedimentation and dispersed magmatism, with the lithosphere undergoing extension factors (β) of 2-3 in the central and northern segments.[13] During this period, the rift basin accumulated clastic sediments derived from adjacent highlands, marking the early depositional record of extension.[1] The transition to seafloor spreading initiated around 13 Ma in the southern and central Red Sea, where localized magmatism and dike intrusions facilitated the rupture of continental lithosphere, giving way to the formation of new oceanic crust.[11] This shift propagated northward, with oceanic crust now dominating south of approximately 20°N, as evidenced by linear magnetic anomalies and axial trough development indicative of organized spreading.[2] In the central Red Sea, transitional lithosphere features dispersed diking and magmatic underplating, bridging the gap between thinned continental domains and mature oceanic spreading centers.[2] A major tectonic reorganization around 14–12 Ma shifted the extension direction to N15°E, aligning with the Aqaba-Levant transform fault and reducing activity in the Gulf of Suez. A key aspect of this evolution involved Miocene desiccation phases, during which evaporite deposition—primarily halite and anhydrite—occurred around 14 Ma and 10 Ma, filling the rift basin as sea levels fluctuated and restricted marine incursions.[1] Stratigraphic records provide robust evidence for these stages, with syn-rift clastics from the late Oligocene (~27.5-23 Ma) overlying pre-rift basement, overlain by thick Miocene salt layers that seal the margins.[1] Post-rift marine sediments, including Pliocene shales and carbonates, blanket the evaporites following the Messinian unconformity, signaling the onset of widespread inundation and spreading.[1] These sequences highlight the rift's progression from terrestrial to hypersaline and then fully marine environments. Recent post-2020 seismic and magnetic studies have refined understanding of the ongoing transitions, revealing asymmetric spreading in the southern Red Sea, where the Arabian margin exhibits thicker oceanic crust and greater extension (β_L ~2.5) compared to the African side (β_L ~1.8-2.0), influenced by sublithospheric plume flow from Afar.[13] High-resolution seismic tomography further shows low-velocity zones along axial "deeps," indicating active asthenospheric upwelling and the northward propagation of spreading, potentially stalled in the north by the Zabargad Fracture Zone.[2] New magnetic anomaly maps confirm this propagation, with seafloor spreading exhibiting increasing obliquity northward, underscoring the rift's dynamic, non-uniform evolution.[8]Tectonic Processes
Plate Separation Model
The plate separation in the Red Sea Rift follows a kinematic model of oblique divergence between the Arabian and Nubian (African) plates, occurring at a full spreading rate of approximately 10–16 mm/year, with rates varying along strike from slower in the north (~10 mm/year at 25.5°N) to faster in the central region (~16 mm/year near 18°N).[26] This divergence exhibits moderate obliquity, typically between 20° and 40° relative to the rift trend, influenced by the regional tectonic framework. The relative motion is described as counterclockwise rotation of the Arabian Plate with respect to the Nubian Plate, centered around an Euler pole located at approximately 31.61°N, 24.22°E, with an angular velocity of about 0.387°/Myr.[27] The direction of separation involves northeastward motion of the Arabian Plate relative to the Nubian Plate, at rates contributing to the broader Afro-Arabian plate boundary system.[28] This motion integrates with the East African Rift System to the south, where extension partitions between the Red Sea and Gulf of Aden branches, accommodating the overall divergence of the Nubian, Somalian, and Arabian plates.[29] GPS measurements from networks spanning the region confirm present-day extension rates of ~15 mm/year across the rift, consistent with geodetic models of plate boundary deformation.[29] Paleomagnetic data from volcanic rocks and sedimentary sequences further validate these rates, indicating ~15 mm/year of extension since the Pliocene (~5 Ma), aligning with magnetic anomaly patterns that record symmetric spreading since chron 2A (~3.2 Ma).[30] The rift displays asymmetry in spreading, with more pronounced magmatism, uplift, and lithospheric thinning on the eastern (Arabian) margin compared to the western (Nubian) side, attributed to the influence of transform faults such as the Zabargad Fracture Zone that offset the spreading axis and promote differential extension.[2][31] This asymmetry underscores the role of inherited crustal structure in modulating plate divergence.Rifting Mechanisms
The rifting of the Red Sea is primarily driven by passive mechanisms, where far-field extensional stresses arise from the tectonic reconfiguration following the India-Eurasia collision, leading to the divergence of the Arabian and African plates.[32] This collision, occurring around 50 million years ago, induced plate boundary adjustments that propagated stresses southward, initiating continental extension without requiring localized mantle thermal anomalies.[33] However, an active component may contribute in the southern sector, linked to mantle upwelling beneath the Afar region, where asthenospheric flow potentially enhances lithospheric weakening and extension.[34] This upwelling is evidenced by elevated mantle temperatures and plume-like structures influencing the Afar triple junction.[35] Deformation within the Red Sea Rift is characterized by listric normal faulting, where faults flatten with depth into a ductile detachment, accommodating asymmetric extension and block rotation along the rift margins.[36] Crustal thinning is pronounced in the axial zone, reducing thickness to less than 10 km through ductile stretching and exhumation of lower crustal rocks.[14] Magmatic underplating further modifies the structure, with basaltic melts accumulating at the base of the thinned crust, compensating for extension and stabilizing the rift axis during early stages.[37] Geophysical data support these mechanisms through gravity anomalies that reveal Moho uplift beneath the rift, with positive Bouguer anomalies indicating thinned crust and uplifted mantle.[38] Seismic tomographic models further depict low-velocity zones in the upper mantle, consistent with asthenospheric upwelling and elevated temperatures facilitating extension.[39] The extension factor, denoted as β, quantifies the degree of lithospheric stretching and is defined as the ratio of the final length (L_final) to the initial length (L_initial) across the rift:
This parameter originates from the uniform stretching model of continental rifting, where the lithosphere thins uniformly under pure-shear extension, preserving crustal volume laterally. To derive it, consider the initial undeformed width of the continental block (L_initial) measured from pre-rift geological markers, such as matching stratigraphic horizons or paleogeographic reconstructions. The final width (L_final) is observed from the current rift geometry, including the axial trough and marginal basins. For the central Red Sea, seismic refraction and gravity modeling estimate crustal stretching factors (β_C) of 5–7, with an initial crustal thickness of 43 km. This aligns with lithospheric stretching (β_L) of ~2.5, confirming significant extension with hyper-thinning in the axial zone.[13]