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Dynamic of intra-continental extension in magmatic rift context: Turkana Rift (Northern Kenya) from Eocene to Present

Abstract : Continental extension and rift geometry are controlled by the interaction of various parameters (thermal, mechanical, kinematical) that interplay at different spatial and temporal scales. In order to provide some new insights about the dynamics of rift segment evolution, this study focuses on the Turkana rift (Northern Kenya) which belongs to the eastern branch of the East African Rift System (EARS). This NS Cenozoïc rift portion developed since 45 Ma. and is unusual insofar as it is developed between the termination of two previous oblique cretaceous basins (N140°E Anza and Soudan). Its deep geometry is dominated by a set of syn-rift Oligocene-Miocene half-grabens, but since its present-day surface arrangement is marked by a subdued topography and it is characterised by weak seismicity, the definition of recent/active movements is not straightforward.

This work, based on interpretation of Landsat ETM+ satellite imagery correlated with digital topography (SRTM) and seismic reflection dataset, leads us to propose and discuss a tectono-magmatic reconstruction of the poly-phased evolution of the Turkana rift from five successive ‘restored' maps (45-23 Ma., 23-15 Ma., 15-6 Ma., 6-2.6 Ma., 2.6 Ma.-Actual), combined with two additional maps dealing with pre-existing structures (Precambrian basement, previous Cretaceous rift). Our model fully demonstrates the influence of: 1) two regional-scale transverse corridors (NKFZ: N'Doto-Karisia N140°E, 100x600 km; KBFZ: Kataboi-Buluk N50°E, 30x250 km) and 2) magmatic domains on the nucleation and propagation of extensional structures during different stages of rifting. Both the locations of newly-formed basins and their final geometry is directly controlled by transverse fault zones. Syn-rift basins firstly develop within the N140°E NKFZ transverse corridor while they are locked by N50°E KBFZ structures that could also act as transfer zones. Relationships between magmatic domains and extension highlight the key-role played by inner volcanic domes structures (extrado faults/fractures, syn-magmatic faults) on the propagation of basin s across the paleo-domes after the cessation of volcanic activity. At a greater scale, considering plume/lithosphere interactions associated with the migration of the African plate (~1000 km to the NE) since 50 Ma. above two Cenozoïc mantle plumes, we propose two hypotheses to explain the lack of thermal uplift along the persistent Turkana depression: 1) the lateral migration of plume head material beneath the pre-stretched cretaceous lithosphere which is characterised by associated thermal subsidence or 2) the strengthening of the lithosphere in response to the cooling of the cretaceous underplating that prevent any bending of the lithosphere at a large wavelength. [William – its is not clear to me what you mean here and how it causes the depression – I have therefore modified part of it so that it makes sense to me = change it back if this is not what you mean; I am not sure what the 2nd point means]

Concerning the recent/active deformations (<5 Ma.) of the Turkana rift, the kinematic analysis of recent inversions tectonics (<3.7 Ma.) developed within some of the Oligocene-Pliocene basins located above the NKFZ N140°E transverse fault zone, confirms the key-role played by this first-scale discontinuity on the evolution of the Turkana rift from Eocene to present. We explain the formation of these compressive structures using a three-stage kinematic model characterised by: 1) a pure extensional regime before 5 Ma. (σ3 horizontal oriented at EW, σ1 vertical), 2) a slight clockwise rotation (~20°) of σ3 axis between 5 and 3.7 Ma. that induced the dextral reactivation of the NKFZ and the formation of a N20° T-type fault network and 3) a clockwise rotation of σ3 (~20°) after 3.7 Ma. coeval to a permutation of σ1/σ2 axes that induced the reversed reactivation of N20° fault network.

In the western part of the Turkana rift, the exceptionally well-exposed recent Kino Sogo fault network (<3 Ma.; 150x40 km) comprises a series of horsts and grabens within an arcuate 40 km-wide zone that dissects Miocene-Pliocene lavas overlying an earlier asymmetric fault block and bounded by N140°E, N50°E and NS structures. Quantitative geometric analysis of the scaling properties of this fault system reveals several unusual features of the Kino Sogo fault network: 1) the system accommodates very low strains (<1%) and is probably characterised by a low rate of extension and deformation (~0.1 mm/yr and 10-16 s-1, respectively), 2) fault length distributions subscribe to a negative exponential scaling law, as opposed to the power-law scaling typical of other fault systems, 3) the long faults (length up to 40 km) are characterised by maximum throws of no more than 100 m, hence providing displacement/length ratios which are significantly below those of other fault systems. The under-displaced nature of the Kino Sogo fault system is attributed to early stage rapid fault propagation possibly arising from reactivation of earlier underlying basement structures/fabrics and/or magmatic-related fractures.
Finally, a detailed analysis of the well-defined Turkana drainage network in this EARS sector, confirms the general structural and kinematic framework of the Turkana rift segment. Three drainage anomalies are studied and highlight: 1) the rapid swing of the Turkwell River (from NS to EW) along an EW second-order faulted corridor (Turkwell-Mount Porr; 20x100 km), 2) the locking of a dense river pattern by the recent rejuvenation (<5 Ma.) of the West Napedet Oligocene-Miocene border fault plane and 3) the formation of the Kalabata circular drainage anomaly surrounding an antiformal structure associated with the recent phase of structural inversions (<3.7 Ma.).

More generally, the new insights fully discussed in this work have direct implications for the study of the processes associated with the nucleation and propagation of a magmatic rift segment under a low deformation rate and controlled by an important structural inheritance (basement, previous rifted system).
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https://tel.archives-ouvertes.fr/tel-00009294
Contributor : William Vetel <>
Submitted on : Monday, May 23, 2005 - 3:28:05 PM
Last modification on : Tuesday, February 2, 2021 - 9:52:02 AM
Long-term archiving on: : Friday, April 2, 2010 - 10:18:43 PM

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  • HAL Id : tel-00009294, version 1

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William Vetel. Dynamic of intra-continental extension in magmatic rift context: Turkana Rift (Northern Kenya) from Eocene to Present. Applied geology. Université de Bretagne occidentale - Brest, 2005. English. ⟨tel-00009294⟩

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