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


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.

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    • "(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]. "
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    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.
    Full-text · Article · Jul 2015 · Geochemistry Geophysics Geosystems
    • "Nonkinematic models based only on structural lineaments suggest oblique opening under ENE-WSW extension with associated strike-slip movements (Chorowicz, 2005; Le Gall et al., 2008). Most specifically, Le Gall et al. (2008) refined the chronology of faulting and volcanism of the North Tanzanian divergence zone and suggested that the Natron rift is affected by orthogonal rifting, whereas the area from Ngorogoro to Kilimanjaro, i.e., where the largest volcanoes of the North Tanzanian divergence zone are located, is affected by oblique rifting with an E-W extension. The Aswa shear zone is of Precambrian origin (Ruotoistenmäki, 2014) and might have been reactivated locally as a normal fault during the Cenozoic (Fig. 1A). "
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    ABSTRACT: The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being—to our knowledge—previously unknown. Our field survey around Mount Meru allowed previous “lahar” deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/ strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.
    No preview · Article · Jun 2015 · Geological Society of America Bulletin
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    • "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. "
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    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.
    Full-text · Article · Jan 2015 · Tectonophysics
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