The Karoo triple junction questioned : evidence from Jurassic and Proterozoc 40Ar/39Ar ages and geochemistry of the giant Okavango dyke swarm (Botswana).

The lower Jurassic Karoo-Ferrar magmatism represents one of the most important Phanerozoic continental flood basalt (CFB) provinces. The Karoo CFB province is dominated by tholeiitic traps and apparently radiating giant dyke swarms covering altogether ca. 3 106 km2. This study focuses on the giant N110j-trending Okavango dyke swarm (ODS) stretching over 1500 km across Botswana. This dyke swarm represents the main (failed) arm of the so-called Karoo triple junction that is generally considered as a key marker of the impingement of the Karoo starting mantle plume head. ODS dolerites yield six new plagioclase 40Ar/39Ar plateau (and miniplateau) ages ranging from 178.7F0.7 and 180.9F1.3 Ma. The distribution of the ages along a narrow Gaussian curve suggests a short period of magmatic activity centered around 179 Ma, i.e., f5 Ma younger than the emplacement age of Karoo mafic magmas in the southern part of the Karoo CFB province (f184). This age difference indicates that Karoo magmatism does not represent a short-lived event as is generally the case for most CFB but lasted at least 5Ma over the whole province. In addition, small clusters of plagioclase separated from 28 other dykes and measured by "speedy" step-heating experiments (with mostly two to three steps), gave either "Karoo" or Proterozoic ages.Integrated ages of the Proterozoic rocks range from 851 6 to 1672 7 Ma, and one plateau age (959.1 4.6 Ma) and one possibly geologically significant weighted mean age (982.7 4.0 Ma) were obtained. Proterozoic and Karoo mafic rocks are petrographically similar, but Proterozoic dykes display clear geochemical differences (e.g., TiO 22.1%). Geochemical data combined with available Ar/Ar dates allow the identification of the two groups within a total set of 77 dykes investigated: f10% of the bulk ODS dykes are Proterozoic. Thus, the Jurassic Karoo ODS dykes were emplaced along reactivated Proterozoic structures and there is no pristine Jurassic Nuanetsi triple junction as commonly proposed. This throws into doubt the validity of the "active plume head" Karoo CFB rift models as being responsible for the observed "triple junction" dyke geometr

The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province

The structural organization of a giant mafic dyke swarm, the Okavango complex, in the northern Karoo Large Igneous Province (LIP) of NE Botswana is detailed. This N110E-oriented dyke swarm extends for 1500 km with a maximum width of 100 km through Archaean basement terranes and Permo-Jurassic sedimentary sequences. The cornerstone of the study is the quantitative analysis of N>170 (exposed) and N>420 (detected by ground magnetics) dykes evidenced on a ca. 80-km-long section lying in crystalline host-rocks, at high-angle to the densest zone of the swarm (Shashe area). Individual dykes are generally sub-vertical and parallel to the entire swarm. Statistical analysis of width data indicates anomalous dyke frequency (few data <5.0 m) and mean dyke thickness (high value of 17 m) with respect to values classically obtained from other giant swarms. Variations of mean dyke thicknesses from 17 (N110E swarm) to 27 m (adjoining and coeval N70E giant swarm) are assigned to the conditions hosting fracture networks dilated as either shear or pure extensional structures, respectively, in response to an inferred NNW?SSE extension. Both fracture patterns are regarded as inherited brittle basement fabrics associated with a previous (Proterozoic) dyking event. The Okavango N110E dyke swarm is thus a polyphase intrusive system in which total dilation caused by Karoo dykes (estimated frequency of 87%) is 12.2% (6315 m of cumulative dyke width) throughout the 52-km-long projected Shashe section. Assuming that Karoo mafic dyke swarms in NE Botswana follow inherited Proterozoic fractures, as similarly applied for most of the nearly synchronous giant dyke complexes converging towards the Nuanetsi area, leads us to consider that the resulting triple junction-like dyke/fracture pattern is not a definitive proof for a deep mantle plume in the Karoo LIP

Joint inversions of three types of electromagnetic data explicitly constrained by seismic observations: results from the central Okavango Delta, Botswana

The Okavango Delta of northern Botswana is one of the world's largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audiomagnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt-Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow∼40 m thick freshwater sandy aquifer with 85-100 Ωm resistivity, 10-32 per cent interconnected porosity and <13 per cent clay content overlies a 105-115m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15-20 Ωm total resistivity, 1−27 per cent interconnected porosity and 15-60 per cent clay content. A third∼60 m thick sandy layer with 40-50 Ωm resistivity, 10-33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200-4000 Ωm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectivel

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

Riley et al. (2006a) question our interpretations of 40Ar/39Ar data obtained on the Karoo fl ood basalts (Jourdan, et al., 2005). 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 detectable alteration, recoil, etc.), and (2) mis-interpretation of the reliable geochronological data available on Karoo and other continential flood basalts (CFBs)

Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana

International audienceTo determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the wes

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

Earth and Planetary Science Letters, v. 202, n. 3-4, p. 595-606 [433], 2002. http://dx.doi.org/10.1016/S0012-821X(02)00763-XInternational audienc