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

Gaia Data Release 2: Calibration and mitigation of electronic offset effects in the data

The European Space Agency Gaia satellite was launched into orbit around L2 in December 2013. This ambitious mission has strict requirements on residual systematic errors resulting from instrumental corrections in order to meet a design goal of sub-10 microarcsecond astrometry. During the design and build phase of the science instruments, various critical calibrations were studied in detail to ensure that this goal could be met in orbit. In particular, it was determined that the video-chain offsets on the analogue side of the analogue-to-digital conversion electronics exhibited instabilities that could not be mitigated fully by modifications to the flight hardware. We provide a detailed description of the behaviour of the electronic offset levels on microsecond timescales, identifying various systematic effects that are known collectively as offset non-uniformities. The effects manifest themselves as transient perturbations on the gross zero-point electronic offset level that is routinely monitored as part of the overall calibration process. Using in-orbit special calibration sequences along with simple parametric models, we show how the effects can be calibrated, and how these calibrations are applied to the science data. While the calibration part of the process is relatively straightforward, the application of the calibrations during science data processing requires a detailed on-ground reconstruction of the readout timing of each charge-coupled device (CCD) sample on each device in order to predict correctly the highly time-dependent nature of the corrections. We demonstrate the effectiveness of our offset non-uniformity models in mitigating the effects in Gaia data. We demonstrate for all CCDs and operating instrument and modes on board Gaia that the video-chain noise-limited performance is recovered in the vast majority of science samples

Geometry and growth of an inner rift fault pattern: the Kino Sogo Fault Belt, Turkana Rift (North Kenya)

A quantitative analysis is presented of the scaling properties of faults within the exceptionally well-exposed Kino Sogo Fault Belt (KSFB) from the eastern part of the 200-km-wide Turkana rift, Northern Kenya. The KSFB comprises a series of horsts and grabens within an arcuate 40-km-wide zone that dissects Miocene–Pliocene lavas overlying an earlier asymmetric fault block. The fault belt is 150 km long and is bounded to the north and south by transverse (N50°E and N140°E) fault zones. An unusual feature of the fault system is that it accommodates very low strains (<1%) and since it is no older than 3 Ma, it could be characterised by extension rates and strain rates that are as low as 0.1 mm/yr and 10−16 s−1, respectively. Despite its immaturity, the fault system comprises segmented fault arrays with lengths of up to 40 km, with individual fault segments ranging up to 9 km in length. Fault length distributions subscribe to a negative exponential scaling law, as opposed to the power law scaling typical of other fault systems. The relatively long faults and segments are, however, characterised by maximum throws of no more than 100 m, providing displacement/length ratios that are significantly below those of other fault systems. The under-displaced nature of the fault system is attributed to early stage rapid fault propagation possibly arising from reactivation of earlier underlying basement fabrics/faults or magmatic-related fractures. Combined with the structural control exercised by pre-existing transverse structures, the KSFB demonstrates the strong influence of older structures on rift fault system growth and the relatively rapid development of under-displaced fault geometries at low strains

Static and dynamic evidence for a transition at Tc = 0 in the Ising spin glass Fe0.3Mg0.7Cl2

The critical behaviour of the Ising spin glass Fe0.3Mg 0.7Cl2 has been studied by d.c. magnetisation, a.c. susceptibility and specific heat measurements.We have extended the range of our previous a.c. susceptibility measurements down to 0.02 Hz by using a new set up which utilises the possibilities of a 16 bit analog to digital converter. The relaxation time r and the coefficients a3 and a5 of the non linear susceptibilities are powers of each other and diverge like exponential functions of θ/T. This confirms our former statement that Fe0.3Mg0.7Cl2 is of a class different from that of traditional 3d systems and appears as an example of a real 2d Ising spin glass with Tc = 0. We claim that the essential singularities are obtained as the natural Tc = 0 limit of the usual assumption of the scaling theory. We differ at this point from recent analyses in other systems, where similar evidence is interpreted in the framework of a different theory ad hoc for the Tc = 0 case.L'étude du comportement critique du verre de spin Ising Fe0.3Mg 0.7Cl2 a été réalisée à l'aide de mesures d'aimantation, de susceptibilitd alternative et de chaleur specifique. Les mesures de susceptibilitd alternative ont été effectuées jusqu'à de très basses fréquences (0.02 Hz < f < 3000 Hz) grdce 21 la mise au point d'un dispositif expdrimental utilisant un convertisseur analogique numérique 16 bits rapide. Le temps de relaxation τ comme les coefficients a3 et a5 de la susceptibilité non lindaire sont puissances l'un de l'autre et divergent comme des exponentielles de θ/ T. L'analyse des résultats confirme donc les conclusions d'une première étude du comportement critique dynamique : Fe0.30Mg0.70Cl 2 appartient à une classe d'universalité différente des systèmes 3d et apparaît comme exemple de verre de spin 2d Ising caractérisé par une transition de phase à Tc = 0. Nous affirmons que les singularites observées apparaissent comme la limite naturelle à Tc = 0 des lois habituelles de "scaling". Nous nous différencions sur ce point des analyses présentées récemment pour d'autres systèmes, où un comportement semblable est interprété par une théorie ad hoc pour T c = 0