Article

Rift propagation at craton margin: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

CEA–CNRS, UMR 1572, LSCE, Domaine du CNRS Bat. 12, Avenue de la Terrasse, 91198 Gif sur Yvette Cedex, France
Tectonophysics (Impact Factor: 2.87). 06/2013; DOI: 10.1016/j.tecto.2007.11.005

ABSTRACT A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro–Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi–Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi–Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further North.

Download full-text

Full-text

Available from: Jacques Déverchère, Sep 04, 2015
3 Followers
 · 
465 Views
 · 
216 Downloads
  • Source
    • "(Figure2,boxC).FollowingLe Galletal.[2008],thisareaextendsfromthe60kmwideNatron-Magadibasinsouthwardintoa300km wide,faultedzonecontainingtheEyasi,Manyara,andPanganiriftbasins(Figure3).Locatedinthemiddle ofthisregionisa200kmwide,magmatictransferzone,knownastheNgorongoro-KilimanjaroVolcanic Belt[LeGalletal.,2008;Delcampetal.,inpress](Figure3).AdditionalvolcanismoccursoffaxisoftheNorth TanzanianDivergenceintheChyuluHillsregion[HaugandStrecker,1995;Isolaetal.,2014](Figure3). 2.1.2.TheNaivasha-NakuruBasinandMainEthiopianRift:Evolved,MagmaticRifting Examplesofevolved,magmaticriftsincludetheBosetmagmaticsegmentintheAdamabasinoftheMain EthiopianRift(Figure2,boxA)andNaivasha-NakurubasinoftheKenyaRift(Figure2,boxB).Synriftgrowth Figure1.AnnotatedDEMimage(90mSRTM)oftheeasternportionoftheAfricanconti- nent.Faults(blacklines)arefromEbinger[1989]andChorowicz[2005].Majorlakesare coloredblueintheDEM,andcommonlyoccurindeepriftbasins.Insetglobesimplifies theoverallextentoftheeasternandwesternbranchesoftheEastAfricanRiftandthe inferredextentoftheTanzanianCratonfromCorti[2009]andMorley[2010]. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1,500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along-axis variations in the distribution and geometries of dike intrusions as a result of changing tectono-magmatic conditions. In younger (<10 Ma) basins of the North Tanzanian Divergence, dikes are largely restricted to zones of rift-oblique faulting between major rift segments, referred to here as transfer zones. Cone lineament trends are highly variable, resulting from the interplay between (1) the regional stress field, (2) local magma-induced stress fields, and (3) stress rotations related to mechanical interactions between rift segments. We find similar cone lineament trends in transfer zones in the western branch of the EAR, such as the Virunga Province, Democratic Republic of Congo. The distributions and orientations of upper crustal dikes in the eastern branch of the EAR vary during continental rift evolution. In early-stage rifts (<10 Ma), upper crustal dikes play a limited role in accommodating extension, as they are confined to areas in and around transfer zones. In evolved rift basins (>10 Ma) in Ethiopia and the Kenya Rift, rift-parallel dikes accommodate upper crustal extension along the full length of the basin.
    Geochemistry Geophysics Geosystems 07/2015; DOI:10.1002/2015GC005918 · 3.05 Impact Factor
  • Source
    • "The eruption age of Olmani and Lashaine is poorly constrained. However, neighboring volcanic centers with similar morphologies have eruption ages between 2.5 and 1.56 Ma (Evans et al., 1971; Le Gall et al., 2008; MacIntyre et al., 1974; Wilkinson et al., 1986). Hypothesis (1) is therefore improbable. "
    [Show abstract] [Hide abstract]
    ABSTRACT: We have analyzed the microstructures and crystal preferred orientations (CPO), and calculated the seismic properties of 53 mantle xenoliths from four localities within the North Tanzanian Divergence of the East African rift: two within the rift axis and two in the transverse volcanic belt. Olivine OH concentrations were measured in 15 xenoliths. Most samples have harzburgitic to dunitic compositions and high olivine Mg#. Microstructures and olivine CPO patterns vary strongly depending on the location. In-axis peridotites display mylonitic to porphyroclastic microstructures, which record recent deformation by dislocation creep. Highly stretched orthopyroxenes inmylonites indicate that the deformation was initiated under high stress and probably low temperature. Orthopyroxene replacement by olivine in mylonitic and porphyroclastic peridotites suggest syn-kinematic melt–rock reactions and further deformation under near-solidus conditions. Exsolutions in orthopyroxene imply significant cooling between melt-assisted deformation and xenolith extraction. Late metasomatismis evidenced by the occurrence of veins crosscutting the microstructure and interstitial clinopyroxene and phlogopite. Axial-[100] olivine CPOs predominate, suggesting activation of the high temperature, lowpressure [100] {0kl} slip systems and, probably, transtensional deformation. In the volcanic belt, Lashaine peridotites display very coarse-granular textures, indicating deformation by dislocation creep under low deviatoric stress conditions followed by annealing. Axial-[010] olivine CPOs are consistent with transpressional deformation or simultaneous activation of the [100](010) and [001](010) slip systems. Intermediate microstructures and CPOs in Olmani suggests heterogeneous deformation within the volcanic belt. Olivine OH concentrations range between 2 and 12 ppm wt. H2O. No systematic variations are observed between in- and off-axis samples.MaximumP wave azimuthal anisotropy (AVp) ranges between 3.3 and 18.4%, and the maximum S wave polarization anisotropy (AVs) between 2.3 and 13.2%. Comparison between seismic properties of in-axis peridotites and SKS splitting data suggests transtensional deformation in the lithospheric mantle beneath the rift.
    Tectonophysics 01/2015; 639:34-55. DOI:10.1016/j.tecto.2014.11.011 · 2.87 Impact Factor
  • Source
    • "These K–Ar dates are commonly used to establish Essimingor as the oldest volcano of the NTD (e.g. Dawson, 2008; Le Gall et al., 2008). Whole rock, matrix, and when possible, nepheline separates were "
    [Show abstract] [Hide abstract]
    ABSTRACT: The North Tanzanian Divergence zone (NTD), at the southern end of the eastern branch of the East African Rift, is part of one of Earth's few currently active intra-continental rift systems. The NTD preserves a complex tectono-magmatic evolution of a rift in its early stage of activity. The oldest magmatism reported in the NTD is associated with the centrally located Essimingor volcano. Although major element oxides show narrow compositional variations suggesting fractional crystallization, trace element abundances and Sr–Nd–Pb isotopic data have complex distributions that require open-system processes. The more primitive samples (MgO>9 wt.%) likely reflect partial melting of a metasomatized lithospheric mantle characterized by residual garnet, phlogopite and minor amphibole. The range of radiogenic Pb isotopic compositions indicates the presence of mixing between this source and the lithosphere of the western branch of the East African Rift (Toro Ankole and Virunga). Laser-incremental heating of selected samples gives 40Ar/39Ar ages that range from 5.76±0.02 Ma to 5.91±0.01 Ma, suggesting an age roughly 2 myr younger than previously reported.
    Lithos 12/2012; 155. DOI:10.1016/j.lithos.2012.09.009 · 3.65 Impact Factor
Show more