scholarly journals Early syn-rift igneous dike patterns, northern Kenya Rift (Turkana, Kenya): Implications for local and regional stresses, tectonics, and magma-structure interactions

Geosphere ◽  
2020 ◽  
Vol 16 (3) ◽  
pp. 890-918 ◽  
Author(s):  
C.K. Morley
Keyword(s):  
Middle Miocene ◽  
Ethiopian Rift ◽  
East African ◽  
Kenya Rift ◽  
Main Ethiopian Rift ◽  
Northern Kenya ◽  
Half Graben ◽  
African Rift ◽  
A Minor ◽  
Regional Stresses

Abstract Four areas (Loriu, Lojamei, Muranachok-Muruangapoi, Kamutile Hills) of well-developed Miocene-age dikes in the northern Kenya Rift (Turkana, Kenya) have been identified from fieldwork and satellite images; in total, >3500 dikes were mapped. Three areas display NNW-SSE– to N-S–oriented dike swarms, with straight, radial, and concentric patterns in zones <15 km long, and indicate NNW-SSE to N-S regional maximum horizontal principal stress (SHmax) directions in the early to middle Miocene. Individual dikes are typically <2 m wide and tens to hundreds of meters long and have accommodated <2% extension. In places (Loriu, Lojamei, Lokhone high), dikes trend at a high angle to the rift trend, suggesting some local influence (e.g., overpressured magma chamber, cracked lid–style dike intrusions over a sill or laccolith, preexisting fabric in basement) on orientation, in addition to the influence from regional stresses. Only a minor influence by basement fabrics is seen on dike orientation. The early- to middle-Miocene dikes and extrusive activity ended a long phase (up to 25 m.y.) of amagmatic half-graben development in central Kenya and southern Turkana, which lay on the southern edge of the early (Eocene–Oligocene) plume activity. The Miocene dike sets and extension on major border faults in Turkana contrast with larger, more extensive arrays of dikes in evolved systems in the Main Ethiopian Rift that are critical for accommodating crustal extension. By the Pliocene–Holocene, magmatism and intrusion along dikes had become more important for accommodating extension, and the tectonic characteristics began to resemble those of rift basins elsewhere in the eastern branch of the East African Rift.

The Main Ethiopian Rift: Ongoing deformation inferred from earthquake mechanism

2021 ◽  
Author(s):  
Ameha Muluneh
Keyword(s):  
Tectonic Setting ◽  
Ethiopian Rift ◽  
Rift System ◽  
East African Rift System ◽  
East African ◽  
Active Deformation ◽  
Main Ethiopian Rift ◽  
African Rift ◽  
Rheological Modeling ◽  
Deformation Patterns

<div>The northern Main Ethiopian Rift (MER), which forms the northern part of the East African Rift System, offers an excellent tectonic setting to study the transition from continental to oceanic crust and also from tectonic to magmatic rifting. Opening of the rift started at 11 Myr ago. Until about 7 Ma, deformation was mainly accommodated at the rift border faults. Between 7 and 3 Ma, deformation migrated from the border faults to 20-30 km wide, 60 km long  magmatic segments. Earlier geodetic and field geological observations suggest that more than 80% of the present day opening of the rift is accommodated beneath these magmatic segments. On the contrary, recent observations indicate that deformation is more widespread than previously thought, with only 40% of the present day deformation being accommodated at the rift centre. </div><div> </div><div>Detailed understanding on the depth and epicentral distribution of earthquakes provides an important constraint on how strain is partitioned between the rift floor and border faults. Here I use high resolution earthquake catalogue and thermo-rheological modeling to constrain the active deformation patterns in the northern MER by assuming that the long term properties of the lithosphere represent the short term earthquake cycle. The final result of this study has significant implications for the location and magnitude of seismic hazard in the rift. </div>

scholarly journals AMS-14C Chronology of a Lacustrine Sequence from Lake Langano (Main Ethiopian Rift): Correction and Validation Steps in Relation with Volcanism, Lake Water and Carbon Balances

Radiocarbon ◽  
2002 ◽  
Vol 44 (1) ◽  
pp. 75-92 ◽  
Author(s):  
Elisabeth Gibert ◽  
Yves Travi ◽  
Marc Massault ◽  
Jean-Jacques Tiercelin ◽  
Tesfaye Chernet
Keyword(s):  
Dissolved Inorganic Carbon ◽  
Regional Scale ◽  
Ethiopian Rift ◽  
Lake Basin ◽  
Main Ethiopian Rift ◽  
Tectonically Active ◽  
Half Graben ◽  
Rift Lake ◽  
The 1960S ◽  
Radiocarbon Chronology

Located in the Ziway-Shala Basin of the Main Ethiopian Rift, Lake Langano is part of an asymmetric half-graben, defined by a series of north-northeast-trending faults in the tectonically active zone of the rift. A 15-m deep succession of organic homogeneous muds, silts, bioclastic sands, and pyroclastic layers was cored in 1994. The definition of a certified radiocarbon chronology on these deposits required the indispensable establishment of modern hydrological and geochemical balances. The isotopic contents of the total dissolved inorganic carbon (TDIC) of surface water clearly show the influence of a deep CO2 rising along the main fault crossing the lake basin. The 5.8 pMC disequilibrium existing in 1994 with the atmosphere likely produces the aging of authigenic materials developing at the lake surface. However, with a mean residence time of ~15 years, this apparent 14C aging of Lake Langano water still integrates the 14C produced by the nuclear tests in the 1960s. Reconstructing the natural 14C activity of the lake TDIC allows for the quantification of the deep CO2 influence, and for the correction of AMS-14C datings performed along the core. The correction of the AMS-14C chronology defined on Lake Langano allows for a better understanding of paleohydrological changes at a regional scale for at least the last 12,700 cal BP.

Imaging segmentation in early stage rifting (prior to breakup) using a joint inversion of Rayleigh waves from teleseisms and ambient noise tomography in the northern East African Rift

2020 ◽  
Author(s):  
Emma L. Chambers ◽  
Nicholas Harmon ◽  
Derek Keir ◽  
Catherine Rychert ◽  
Ryan Gallacher
Keyword(s):  
Upper Mantle ◽  
Rayleigh Waves ◽  
Ambient Noise ◽  
East African Rift ◽  
Ethiopian Rift ◽  
Lateral Migration ◽  
East African ◽  
Slow Velocity ◽  
Melt Infiltration ◽  
African Rift

<p>Within the melt-rich northern East African Rift system, extension progresses from continental rifting in the Ethiopian rift to near continental breakup in Afar. Multiple models have been proposed to understand the evolution of lithospheric stretching and magmatism, but previous studies do not provide a single absolute seismic velocity model of the crust and upper mantle for all stages of the rift. Here we jointly invert surface waves from ambient noise and teleseismic Rayleigh waves to obtain shear velocity maps from 10 to 210 km depth, enabling us to analyse variations in crustal and upper mantle shear wave velocity structure spatially and in depth. Using one model allows us to interpret and understand the pattern of crustal and lithospheric thinning from the rift flanks into the rift, the depth and locus of melt generation, and how these processes vary as a rift evolves towards incipient seafloor spreading.</p><p>We observe in areas unaffected by rifting, a fast lid (>0.1 km/s faster than surroundings) at lithosphere-asthenosphere-boundary depths (~60 - 80 km). The fast-lid is not visible directly beneath the rift and we instead observe slow velocities (slow enough to contain partial melt (3.95 – 4.10 ± 0.03 km/s)), which we interpret as evidence for melt infiltration into the uppermost mantle beneath the rift. In addition, the fast lid thins into the rift, until it is no longer observed, suggesting the rift is more stretched than the surrounding plate (~18% thinner). The slow velocities in the asthenosphere beneath the rift are segmented, ~110 km wide, ~60 – 120 km deep with ~70 km spacing between segments.  The shallowest and slowest anomalies occur beneath Afar, which is at later stage rifting. At crustal depths we observe a broadening in the slow velocity zones along the length of the Main Ethiopian Rift. Furthermore, the slow crustal velocities laterally spread to beneath areas of the Ethiopian Plateau that were affected by flood basalt volcanism (velocities of 3.30 – 3.80 ± 0.04 km/s). We interpret the broadening of the slow velocity as the Moho acting as a barrier causing lateral migration of melt into areas of pre-existing weakness. Our model provides the first comprehensive seismic model of the northern East African Rift allowing us to interpret rift structure. The segmented slow velocities in the asthenosphere suggest discrete melt-rich upwelling may drive the early the breakup process, with shallowing of the top of the melt-rich zone as the rift evolves and the lithosphere is modified by melt infiltration, with the Moho and lithosphere thinning later in the rifting process.</p>

scholarly journals Aborted propagation of the Ethiopian rift caused by linkage with the Kenyan rift

2020 ◽  
Author(s):  
G Corti ◽  
R Cioni ◽  
Z Franceschini ◽  
F Sani ◽  
S Scaillet ◽  
...  
Keyword(s):  
Rift Zone ◽  
Numerical Models ◽  
Ethiopian Rift ◽  
Magmatic Activity ◽  
Geological Data ◽  
East African ◽  
Ridge Segmentation ◽  
Pure Extension ◽  
African Rift ◽  
Ocean Ridge

© 2019, The Author(s). Continental rift systems form by propagation of isolated rift segments that interact, and eventually evolve into continuous zones of deformation. This process impacts many aspects of rifting including rift morphology at breakup, and eventual ocean-ridge segmentation. Yet, rift segment growth and interaction remain enigmatic. Here we present geological data from the poorly documented Ririba rift (South Ethiopia) that reveals how two major sectors of the East African rift, the Kenyan and Ethiopian rifts, interact. We show that the Ririba rift formed from the southward propagation of the Ethiopian rift during the Pliocene but this propagation was short-lived and aborted close to the Pliocene-Pleistocene boundary. Seismicity data support the abandonment of laterally offset, overlapping tips of the Ethiopian and Kenyan rifts. Integration with new numerical models indicates that rift abandonment resulted from progressive focusing of the tectonic and magmatic activity into an oblique, throughgoing rift zone of near pure extension directly connecting the rift sectors.

The Aquatic Civilization of Middle Africa

1974 ◽  
Vol 15 (4) ◽  
pp. 527-546 ◽  
Author(s):  
J. E. G. Sutton
Keyword(s):  
Lake Victoria ◽  
Eastern Africa ◽  
Cultural Tradition ◽  
Main Stage ◽  
African Culture ◽  
Way Of Life ◽  
East African ◽  
Aquatic Life ◽  
Kenya Rift ◽  
African Rift

Between the ninth and third millennia B.C. wetter conditions prevailed over most of Africa. Lakes and rivers were fuller and some of the internal basins were temporarily linked, especially in the ‘Middle African’ belt. This comprises the southern Sahara and Sahel, stretching from the Upper Niger to the Middle Nile, with a south-easterly extension into the Upper Nile basin and the East African rift valleys. This situation was exploited by people who developed a decidedly aquatic economy and culture. From their waterside camps and settlements archaeologists have recovered bones of fish and aquatic animals which these people ate, as well as the distinctive harpoon-heads carved from bone with which they obtained them, and also pottery, bearing peculiar decoration executed with fish-bones and water-shells, made in imitation of (fishing-) baskets. Boating and other cultural developments are deducible. The harpoons date back to 7,000 b.c. at least; the pottery dates back to more than 6,000 b.c. and was clearly an African invention. It reflects important developments in gastronomy and home life.In the Kenya rift valley the main stage of Leakey's ‘Kenya Capsian’ culture is essentially the local manifestation of this far-flung ‘aquatic civilization’.Its greatest extent was achieved during the wettest times of the seventh millennium b.c., and probably involved the expansion of Negroid peoples across this continent-wide savanna belt. Also explained perhaps is the extensive, though now fragmented, distribution of languages which Greenberg combines in his ‘Nilo-Saharan’ super-family. It is suspected that aspects of this ancient aquatic way of life may be maintained or reflected by latter-day isolated or ‘unclean’ lake or swamp communities. This subject has been largely neglected by African culture-historians.Drier conditions in the late sixth and fifth millennia b.c. signalled a decline of this aquatic civilization and, in particular, broke its geographical continuity. Nevertheless, there was a qualified revival in many parts in the fourth and third millennia. In the Kenya rift this later phase seems to equate with the first stage of the ‘stone bowl cultures’. Around Lake Victoria a devolved relic survived until the eve of Bantu expansion about two thousand years ago. Other late or modified examples are known on the Nile and in the western Sudan. Generally, however, the viability and prestige of an aquatic way of life were undermined by the second millennium b.c. In the Sahara and Sahel as well as in the northerly parts of eastern Africa this decline was paralleled by the spread of pastoralism as a new basis of subsistence and prestige. Those who introduced cattle to Kenya from Ethiopia were Cushitic-speakers maintaining, significantly, a fish-taboo.This subject should prove of considerable historiographical interest. The aquatic way of life flourished through Middle Africa at the very time when grain-agriculture and stock-raising were being pioneered in the Near East; and the slow spread of agriculture in Africa, sometimes considered an indication of ‘backwardness’, may be partly explicable by the very success of the aquatic life and of its distinct cultural tradition which was ascendant for a while across the widest part of the continent.

Timing of volcanism and evolution of the northern Kenya Rift

2008 ◽  
Vol 146 (1) ◽  
pp. 34-47 ◽  
Author(s):  
IAN McDOUGALL ◽  
FRANCIS H. BROWN
Keyword(s):  
Late Eocene ◽  
Basaltic Volcanism ◽  
Kenya Rift ◽  
Northern Kenya ◽  
Sedimentary Sequences ◽  
Basement Rocks ◽  
Half Graben ◽  
Early Palaeozoic ◽  
Rhyolitic Volcanism ◽  
Drill Holes

AbstractThe northern Kenya Rift is bounded on the west by uplands of Turkana which comprise horst-like blocks that include metamorphic basement rocks, locally overlain unconformably by the Cretaceous Lubur Sandstone, in turn overlain by predominantly volcanic sequences in which relatively thin sedimentary packages occur. Amphibolite facies crystalline rocks of the basement yield Early Palaeozoic K–Ar cooling ages reflecting the Pan-African Orogeny. Volcanism in Turkana was initiated through voluminous eruptions of transitional tholeiitic basalts commencing about 36 Ma ago in the Late Eocene, with some evidence for concomitant rhyolitic volcanism. Volcanism became dominantly rhyolitic in the interval from about 27 to 23 Ma ago, but remained bimodal as basaltic lavas are also known from this period. From about 19 to 15 Ma or younger, basaltic volcanism again dominated, often alkaline in nature, with thin but significant sedimentary sequences interleaved that have yielded important vertebrate faunal assemblages. Parallels exist between the volcanic history recorded in Turkana and that found in the Nabwal Hills east of Lake Turkana. In the southern Turkana region, oil exploration by seismic methods and deep drill holes has shown the existence of northerly-trending half-graben with up to 7 km of fill, and that these developed from at least Oligocene and possibly Late Eocene times. This suggests that the widespread basaltic volcanism at about 36 Ma ago (Late Eocene) heralds an earlier initiation of the Kenya Rift in northern Kenya than most workers have previously suggested.

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