Rift Branches Research Articles

The seismicity in Kenya (East Africa) for the period 1906–2010: A review

Kenya has had a seismic station since 1963 as part of the World Wide Standardized Seismograph Network (WWSSN). In 1990, the University of Nairobi in collaboration with GeoForschungsZentrum (GFZ) started to build up a local seismological network, the Kenya National Seismic Network (KNSN), which operated for about ten years between 1993–2002. This, however, experienced a myriad of problems ranging from equipment breakdown, vandalism and lack of spares. Kenya is seismically active since the Kenya rift valley traverses through the country from north to south bisecting the country into eastern and western regions. In the central part, the Kenya rift branches to form the NW-SE trending Kavirondo (Nyanza) rift. The Kenya rift valley and the Kavirondo (Nyanza) rift are the most seismically active where earthquakes of local magnitude (Ml) in the order of ⩽2.0–5.0 occur. Furthermore, historical records show that earthquakes of magnitudes of the order of Ml⩾6.0 have occurred in Kenya. Such large magnitude earthquakes include the January 6, 1928 Subukia earthquake (Ml 7.1) and an aftershock (Ml 6.2) four days later, as well as the 1913 Turkana region earthquake (Ml 6.2). Since early 1970’s, numerous seismic investigations have been undertaken in Kenya in order to understand the formation and structure of the Kenyan part of the East African rift valley. Earthquake data from these studies is, however, rather disorganized and individual datasets, including that acquired during the period 1993–2002, cannot furnish us with comprehensive information on the seismicity of Kenya for the past ∼100years. The purpose of this paper is, therefore, to review the seismicity in Kenya for the period 1906–2010 by utilizing data and results from different sources. The general seismicity of Kenya has been evaluated using historical data, data recorded by local seismic networks, the United States Geological Survey catalogue as well as earthquake data from the numerous seismic investigations by different individuals and research groups. On the basis of earthquake data from these sources, the entire N–S trending Kenya rift valley and the NW-SE trending Nyanza (Kavirondo) rift are characterized by a high rate of seismicity, and the USGS network has been effective in detecting local M>3.0 earthquakes. A peculiar trend is exhibited by earthquakes of Ml⩾5.1 in that these occur along the N-S and NW-SE trending Kenya rift valley and the Kavirondo (Nyanza) rift zone respectively. Earthquake data from the various sources for the period 1906–2010 is complete for Ml⩾4.4 earthquakes with a b-value of 0.79 which is characteristic of tectonic active regions like rifts. There is need to revive and extend the KNSN for a greater coverage and effective seismic monitoring in Kenya.

Simple shear detachment fault system and marginal grabens in the southernmost Red Sea rift

Abstract The NNW-SSE oriented Red Sea rift, which separates the African and Arabian plates, bifurcates southwards into two parallel branches, southeastern and southern, collectively referred to as the southernmost Red Sea rift. The southern branch forms the magmatically and seismo-tectonically active Afar rift, while the less active southeastern branch connects the Red Sea to the Gulf of Aden through the strait of Bab el Mandeb. The Afar rift is characterized by lateral heterogeneities in crustal thickness, and along-strike variation in extension. The Danakil horst, a counterclockwise rotating, narrow sliver of coherent continental relic, stands between the two rift branches. The western margin of the Afar rift is marked by a series of N-S aligned right-lateral-stepping and seismo-tectonically active marginal grabens. The tectonic configuration of the parallel rift branches, the alignment of the marginal grabens, and the Danakil horst are linked to the initial mode of stretching of the continental crust and its progressive deformation that led to the breakup of the once contiguous African–Arabian plates. We attribute the initial stretching of the continental crust to a simple shear ramp-flat detachment fault geometry where the marginal grabens mark the breakaway zone. The rift basins represent the ramps and the Danakil horst corresponds to the flat in the detachment fault system. As extension progressed, pure shear deformation dominated and overprinted the initial low-angle detachment fault system. Magmatic activity continues to play an integral part in extensional deformation in the southernmost Red Sea rift.

The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150–200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

Gravity analysis of Parga and Hecate chasmata: Implications for rift and corona formation

The two largest rift systems on Venus, Parga, and Hecate chasmata contain one third of all coronae. We map variations in elastic thickness and apparent depth of compensation (ADC) in these two regions using the admittance function for gravity and topography. We examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism, topographic shape, fracture pattern, diameter, and stratigraphic age. At Hecate chasmata, both ADC and elastic thickness correlate with two main rift branches, the fracture style and corona characteristics. Hecate chasma thus appears to be dominated by extensional processes. Parga chasma shows little correlation between lithospheric properties and the location of coronae or rift segments. The ADC at Hecate and Parga chasmata is mostly less than 75–100 km, implying that there is not large‐scale upwelling underlying the rift systems. The variations in the corona population sizes between Parga chasma, which has 131 coronae, and Hecate chasma, with 50 coronae, suggest differences in the evolution of the two rift systems. The much larger population of coronae at Parga contains more small coronae, many of which have relatively low volcanism and post date the rift. Further, many of the small coronae occur in chains and have similar diameters, as predicted by Rayleigh‐Taylor type instabilities. These characteristics suggest that the smaller coronae may form via a later, secondary decompression melting phase. Thus, Hecate chasma may be younger than Parga chasma and could experience a secondary stage of corona formation in the future.

Kinematics of the southern Red Sea–Afar Triple Junction and implications for plate dynamics

GPS measurements adjacent to the southern Red Sea and Afar Triple Junction, indicate that the Red Sea Rift bifurcates south of 17° N latitude with one branch following a continuation of the main Red Sea Rift (∼150° Az.) and the other oriented more N‐S, traversing the Danakil Depression. These two rift branches account for the full Arabia–Nubia relative motion. The partitioning of extension between rift branches varies approximately linearly along strike; north of ∼16°N latitude, extension (∼15 mm/yr) is all on the main Red Sea Rift while at ∼13°N, extension (∼20 mm/yr) has transferred completely to the Danakil Depression. The Danakil Block separates the two rifts and rotates in a counterclockwise sense with respect to Nubia at a present‐day rate of 1.9 ± 0.1°/Myr around a pole located at 17.0 ± 0.2°N, 39.7 ± 0.2°E, accommodating extension along the rifts and developing the roughly triangular geometry of the Danakil Depression. Rotating the Danakil Block back in time to close the Danakil Depression, and assuming that the rotation rate with respect to Nubia has been roughly constant, the present width of the Danakil Depression is consistent with initiation of block rotation at 9.3 ± 4 Ma, approximately coincident with the initiation of ocean spreading in the Gulf of Aden, and a concomitant ∼70% increase in the rate of Nubia–Arabia relative motion.

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

With heights of 4–5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite, damtjernite) and subordinate dolomite-bearing carbonatite with U–Pb perovskite emplacement ages of 590–555Ma occur in the vicinity of Aillik Bay, coastal Labrador. The ultramafic lamprophyres principally consist of olivine and phlogopite phenocrysts in a carbonateor clinopyroxene-dominated groundmass. Ti-rich primary garnet (kimzeyite and Ti-andradite) typically occurs at the aillikite type locality and is considered diagnostic for ultramafic lamprophyre–carbonatite suites. Titanian aluminous phlogopite and clinopyroxene, as well as comparatively Al-enriched but Cr–Mg-poor spinel (Cr-number < 0.85), are compositionally distinct from analogous minerals in kimberlites, orangeites and olivine lamproites, indicating different magma geneses. The Aillik Bay ultramafic lamprophyres and carbonatites have variable but overlapping Sr/Sri ratios (0 70369–0 70662) and show a narrow range in initial «Nd (þ0 1 to þ1 9) implying that they are related to a common type of parental magma with variable isotopic characteristics. Aillikite is closest to this primary magma composition in terms of MgO ( 15–20wt %) and Ni ( 200–574 ppm) content; the abundant groundmass carbonate has dCPDB between 5 7 and 5‰, similar to primary mantle-derived carbonates, and dOSMOW from 9 4 to 11 6‰. Extensive melting of a garnet peridotite source region containing carbonateand phlogopite-rich veins at 4–7 GPa triggered by enhanced lithospheric extension can account for the volatile-bearing, potassic, incompatible element enriched and MgO-rich nature of the proto-aillikite magma. It is argued that low-degree potassic silicate to carbonatitic melts from upwelling asthenosphere infiltrated the cold base of the stretched lithosphere and solidified as veins, thereby crystallizing calcite and phlogopite that were not in equilibrium with peridotite. Continued Late Neoproterozoic lithospheric thinning, with progressive upwelling of the asthenosphere beneath a developing rift branch in this part of the North Atlantic craton, caused further veining and successive remelting of veins plus volatile-fluxed melting of the host fertile garnet peridotite, giving rise to long-lasting hybrid ultramafic lamprophyre magma production in conjunction with the break-up of the Rodinia supercontinent. Proto-aillikite magma reached the surface only after coating the uppermost mantle conduits with glimmeritic material, which caused minor alkali loss. At intrusion level, carbonate separation from this aillikite magma resulted in

Geophysical and morpho-tectonic study of the transition between seafloor spreading and continental rifting, western Woodlark Basin, Papua New Guinea

Two major morpho-tectonic domains, separated by a major transfer zone, are described at the transition between seafloor spreading and continental rifting in the western Woodlark Basin, off-shore eastern Papua New Guinea. The oceanic domain comprises new oceanic crust formed during the Bruhnes Epoch, older transitional crust and the rifted continental margins. Two rift branches are recognized within the continental domain. The southern rift branch has failed while the northern branch is a locus of maximum extension with initial development of oceanic crustal accretion. Based on magnetic data, seafloor spreading in the western part of the Woodlark Basin commenced between 3.5 and 2.5 Ma along the northern margin of the basin and maintained its position adjacent to this margin up to 0.8 Ma (Jaramillo event). Just prior to the Bruhnes Epoch, the seafloor spreading axis jumped southward to its current position. Frequent jumps of seafloor spreading centers and relocation of paleoaxes indicate the instability of the Woodlark extensional system. The transition between the two domains is characterized by differential localization of extensional strain. Variations in the localization can result from progressive change of extensional mode when the initial rifting evolves into seafloor spreading and/or from differences in lithospheric rheology as the rift propagates into the continental margin. A consequence of such variations is the massive production of new oceanic crust during seafloor spreading which is being balanced across the transfer zone by broadly distributed deformation within continental lithosphere.

Tectonic development of the northern Taiizaiiian sector of the East African Rift System

The Eastern Branch of the East African Rift System diverges from a single, c. 50 km wide rift in southern Kenya to a c. 200 km wide zone in northern Tanzania, where it is comprised of three distinct rifts with different orientations. The western part of this zone contains two rift branches: the Natron-Man yara-Balangida and Eyasi-Wembere rifts. Each rift contains individual basins that are defined here on the basis of structural and geophysical interpretations. These basins are shallow (&lt;3km) and total extension across the bounding faults is small. New K/Ar age determinations on basalts from the western rift basins show that volcanism and sedimentation began in the area at c. 5 Ma. Major fault escarpments were present by c. 3 Ma and the present-day rift escarpments developed later than c. 1.2 Ma. Pre-rift volcanism produced large shield volcanoes of a basalt-trachyte-phonolite association that now lie on the rift flanks. Volcanism after the main phase of rift faulting produced volatile- and alkali-rich explosive centres which are active today, and have no equivalent in southern Kenya. The change in morphology of the Eastern Branch of the East African Rift System, and the style of volcanism in northern Tanzania, may be the result of the transition from the rifting of Proterozoic Mozambique Belt lithosphere to the rifting of cratonic Archaean lithosphere.

Tectonic and Deposition Model of Late Precambrian-Cambrian Arabian and Adjoining Plates

During the late Precambrian, the terranes of the Arabian and adjoining plates were fused along the northeastern flank of the African plate in Gondwanaland. This phase, which ended approximately 640 to 620 Ma, was followed by continental failure (620 to 580 Ma) and intracontinental extension (600 to approximately 550 Ma). During the Infracambrian extensional phase, a triple junction may have evolved near the Sinai Peninsula and may have consisted of the (1) Jordan Valley and Dead Sea rift branch, (2) Sinai and North Egypt rift branch, and (3) the Najd wrench-rift branch. The Najd, Hawasina, and Zagros fault systems may have been transverse faults that accompanied rifting in the Arabian Gulf and Zagros Mountains, southern Oman, Pakistan, and Kerman in central Iran. While the area was extending and subsiding, the Tethys Ocean flooded the eastern side of the Arabian plate and Iran and deposited calcareous clastics, carbonates, and evaporites (including the Hormuz and Ara halites). This transgression extended into the western part of the Arabian plate via the Najd rift system. The termination of the extensional phase during the late Early Cambrian was accompanied by a major regression and terrestrial conditions on the Arabian Peninsula. However, by the Early Ordovician, as sea level peaked to a highstand, the Arabian plate was blanketed with marginal marine sediments.

The tectonic style of the atlantic mesozoic rift system

The Atlantic Mesozoic rift system consists of four branches separated from one another by fracture zones. Morphologically it resembles the ridge transform-ridge topography of mid-ocean ridges. The branches include the northern North Atlantic between the New foundland/Gibraltar and Bahamas/Guinea fracture zones, the Southern North Atlantic between the Bahama/Guinea and Equatorial fracture zones, the Equatorial Atlantic between the Equatorial Fracture zone and the Torres/Walvis Ridge, and the South Atlantic between the Torres/Walvis Ridge, and the Falkland/Agulhas Fracture Zone. The fundamental structure along the shear zones is the graben, and within the rift branches the half graben. Rifting began in Late Triassic at the northern end of the Northern North Atlantic Branch in the Early Jurassic at the South Atlantic Branch. The rifts propagated toward one another cutting through the equatorial region sometime in latest Middle Jurassic. Seafloor spreading at the northern end of the Northern North Atlantic Branch began in Middle Jurassic reaching the Bahama/Guinea Fracture Zone toward the end of Middle Jurassic. Seafloor spreading in the South Atlantic Branch in the early Aptian and in the Equatorial Atlantic Branch in the Albian. A continuous North/South Atlantic oceanic basin was formed with the separation of the African and South American continents in the Turonian.