H. Bertrand

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.

40Ar/39Ar geochronology and structural data from the giant Okavango and related mafic dyke swarms, Karoo igneous province, northern Botswana

Abstract In NE Botswana, the Karoo dykes include a major N110° dyke swarm known as the Okavango giant dyke swarm (ODS/N110°) and a second smaller set of N70° dykes belonging to the Sabi-Limpopo dyke swarm (SLDS/N70°). New 40Ar/39Ar plagioclase dating of Karoo dolerites of the giant ODS/N110° and the SLDS/N70° in NE Botswana yield plateau ages between 179.6±1.2 and 178.4±1.1 Ma. Our data are concordant with previous 40Ar/39Ar ages for Northern Karoo dykes and lava flows exposed in western Zimbabwe. The data are tightly clustered, indicating a short-lived (179–181 Ma) flood basalt magmatism in this region. The new radiometric dates allow the definition of a diachronous Jurassic flood basalt activity in southern Africa. A significant south to north younging at the scale of the Karoo igneous province correlates with a chemical zonation from low-Ti (south) to high-Ti (north) mafic rocks. Structural measurements on the ODS/N110° and SLDS/N70° Karoo dykes of NE Botswana suggest that: (1) most of the host fractures are inherited Precambrian structures; (2) dyke emplacement occurred under unidirectional tensional stresses; (3) significant syn- and post-volcanic extensional tectonics are lacking. Combined with regional geology, these geochronological and structural data do not confirm unambiguously the triple-junction hypothesis usually put forward to support a mantle plume model for the evolution of the Karoo igneous province, prior to Gondwana breakup.

The Karoo Large Igneous Province: brevity, origin and relation with mass extinction in question from new 40Ar/39Ar age data.

[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

Enregistrement sedimentaire d'un episode d'approfondissement brutal de la marge sud-guineenne au Quaternaire Recent

Abstract During the equamarge II cruise (1988) along the Guinean continental margin (central Equatorial Atlantic) a 2.17 m long core has been sampled at 840 m water depth on the top of one of the volcanic seamounts cropping out in this area. Planktonic foraminiferal assemblages observed within the 2.13 m unconsolidated sediments indicate an age of 0.20 to 0.15 Ma, corresponding to the transition from interglacial to glacial events. Depth of deposition of the calcareous sand ranges between 150–200 m and progressively decreased during the glacial stage onset. Detailed sedimentological and micropaleontological studies, carried out on the 4 cm thick indurated core cap, indicate a deposition depth of 50–100 m, or even less. Taking into account the Pleistocene sea level fluctuations, these data suggest a rapid subsidence of the magmatic seamount since the upper most Pleistocene.