Bernard Le Gall

Karoo large igneous province: Brevity, origin, and relation to mass extinction questioned by new 40Ar/39Ar age data

The peak activities of continental flood basalts are currently considered as huge and brief (∼1 m.y.) magmatic events, with strong implications for geodynamics and biotic turnover. New 4 0 Ar/ 3 9 Ar dates on the Karoo flood basalts (southern Africa) show a longer duration of magmatism (∼8 m.y., with 6 m.y. for the main volume) with an apparent south-to-north migration, along with briefer distinctive pulses inside the province. This suggests that the Karoo province does not fit the general plume model invoked for most continental flood basalts (including the Karoo) and may explain the absence of a major contemporaneous mass extinction.

Neogene-Holocene rift propagation in central Tanzania: Morphostructural and aeromagnetic evidence from the Kilombero area

Based on field studies supplemented by remote sensing and aeromagnetic data from central Tanzania, a Phanerozoic structural history for the region can be developed and placed in a broader rift context. The major contribution of this work is the recognition of rift morphology over an area lying 400 km beyond the southern termination of the Eastern, or Kenya, Rift. The most prominent rift structures occur in the Kilombero region and consist of a wide range of uplifted basement blocks fringed to the west by an east-facing half-graben that may contain 6–8 km of sedimentary strata. Physiographic features and river drainage anomalies suggest that Holocene/Neogene deformation occurs along both rift-parallel and transverse faults, in agreement with the seismogenic character of a number of oblique faults. The present-day rift pattern of the Kilombero extensional province results from the complete overprinting of an earlier (Karoo) rift basin by Neogene- Holocene faults. The Kilombero rift zone is assumed to connect northward into the central rift arm (Manyara) of the Eastern Rift via an active transverse fault zone. The proposed rift model implies that incipient rifting propagates throughout the cold and strong lithosphere of central Tanzania following Proterozoic basement weakness zones (N140°E) and earlier Karoo rift structures (north-south). An eventual structural connection of the Kilombero rift zone with the Lake Malawi rift further south is also envisaged and should imply the spatial link of the eastern and western branches of the East African Rift System south of the Tanzanian craton.

Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

The Tadjoura rift forms the westernmost edge of the westerly-propagating Sheba ridge, between Arabia a nd Somalia, as it enters into the Afar depression. Fro m structural and remote sensing dataset, the Tadjou ra rift is interpreted as an asymmetrical south-facing half-gr aben, about 40 km-wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1-3 Ma-old Gulf Basalts which onlapped the older S omali Basalts along its shallower southern flexural margin. The m ajor and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basa ltic types, namely the Gulf, Somali, Goumarre and H ayyabley Basalts. These results, together with radiometric a ge data, lead us to propose a revised volcano-strat igraphic sketch of the two exposed Tadjoura rift margins, an d to discriminate and date several distinct fault n etworks of this oblique rift. Morphological and statistical an alyses of onshore extensional fault populations sho w marked changes in structural styles along-strike, in a dir ection parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagati on against pre-existing discontinuities in the NS-o riented Arta transverse zone. According to our model, the sinist ral jump of rifting into the Asal-Ghoubbet rift seg ment results from structural inheritance, in contrast with the en echelon or transform mechanism of propagation that prevail ed along the entire length of the Gulf of Aden extensi onal system.

Dynamics of prolonged continental extension in magmatic rifts: the Turkana Rift case study (North Kenya)

Abstract The Turkana magmatic rift (Northern Kenya) initiated at 45 Ma as one of the nucleation zones of rifting in the East African Rift. It forms an anomalously broad-rifted zone (c. 200 km) striking with a north-south trend and lying within a NW-SE topographic depression, floored on both sides of the Turkana area by Cretaceous rifts in the Sudan and Anza plains. From a compilation of available data, combined with newly acquired remote sensing and DEM dataset, we propose a five-stage tectono-magmatic model for the Turkana rift evolution (45–23 Ma; 23–15 Ma; 15–6 Ma; 6–2.6 Ma and 2.6 Ma-Present). The corresponding ‘restored’ maps clearly show the changing spatial distribution of magmatism and fault/basin network with time, hence supplying some clues about dynamics of continental extension. First-order basement-rooted transverse faults zones are identified and their influence on nucleation and propagation of strain is demonstrated, whereas the role of magmatic ‘soft-spots’ as concentrating strain is minimized. Blocking of deformation as well as rift jump and lateral transfer of strain are discussed in relation to various types of fault interaction (dip direction, strikes and acute/obtuse angle of the intersecting faults). The causal links between rift nucleation ‘cells’ and inherited transverse weakness zones in the Turkana rift might also exist elsewhere along the eastern branch of the East African Rift, hence suggesting a complex and discontinuous mode of rift propagation.

Les Montagnes Noires : cisaillement bordier méridional du bassin carbonifère de Châteaulin (Massif Armoricain, France). Caractéristiques structurales et métamorphiques

ResumeL’evolution structurale et metamorphique de la zone des Montagnes Noires, qui borde vers le sud le bassin carbonifere de Châteaulin, est controlee par le fonctionnement d’un cisaillement trans-current dextre. Le caractere unique, mais progressif et inhomogene, de la deformation regionale d’âge carbonifere superieur conduit a remettre en cause le concept d’une structuration hercynienne polyphasee et celui de « phase bretonne ».

Flip-flop detachment tectonics at nascent passive margins in SE Afar

We propose a two-stage tectonic evolution of SE Afar in Djibouti leading to the complex development of highly asymmetric conjugate margins. From c. 8.5 to c. 2 Ma, an early mafic crust developed, associated in the upper crust with synmagmatic growth faults dipping dominantly to the SW. After an erosional stage, a new detachment fault system developed from c. 2 Ma with an opposite sense of motion (i.e. to the NE), during an amagmatic extensional event. In the Asal area, break-up occurred after c. 0.8 Ma along the footwall of an active secondary detachment fault rooted at depth above the lithospheric necking zone. This evolution suggests that flip-flop detachment tectonics is developed during extension at passive margins, in connection with the dynamics of the melting mantle and the associated magma plumbing of the crust.

The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data: REPLY REPLY

[Riley et al. (2006a)][1] question our interpretations of 40Ar/39Ar data obtained on the Karoo flood basalts ([Jourdan, et al., 2005][2]). We believe their arguments are partially based on (1) poorly reliable 40Ar/39Ar age data (e.g., analyses of whole rocks that might have suffered hardly

The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data: REPLYREPLY

[Riley et al. (2006a)][1] question our interpretations of 40Ar/39Ar data obtained on the Karoo flood basalts ([Jourdan, et al., 2005][2]). We believe their arguments are partially based on (1) poorly reliable 40Ar/39Ar age data (e.g., analyses of whole rocks that might have suffered hardly