Palaeomagnetic investigation of Middle Devonian limestones of Algeria and the Gondwana reconstruction

• Published in: Geophysical Journal International
• Main Authors: Smith, Brigitte, Moussine-Pouchkine, Alexis, Ahmed, Ali Aït Kaci
• Format: Text
• Language: English
• Published: Oxford University Press 1994
• Subjects: Articles; South Pole; Daly; South pole
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/119/1/166; https://doi.org/10.1111/j.1365-246X.1994.tb00920.x

We have carried out a palaeomagnetic investigation of the upper middle Devonian marly limestone of the Hazzel Matti Formation outcropping at Meredoua in the northern Ahaggar in the Sahara Desert. The Givetian age of the formation is well constrained by palaeontological arguments. The formation has been deformed later than the Moscovian (upper middle Carboniferous). In the altered yellowish samples and in the heterogeneous samples made of mixed fresh and altered zones, several obviously secondary magnetization components are present. In the freshest bluegrey samples a SE down-dipping component can be isolated, particularly when a combined thermal and AF treatment is applied. This component, observed in 53 samples from three sites, passes the fold test of McFadden & Lowes (1981) and McFadden & Jones (1981) at the 95 per cent probability level. the mean direction for the three sites after tilt correction is: D s = 118.9°, I s ,=34.2°, k = 378, α 95 = 6.3°, which corresponds to a south pole (MER) situated at φ p = 61.7°, λ p = -16.2°, A 95 = 4.2° and places the site area under a palaeolatitude of -18.8°. However, the best grouping of the directions is achieved after 85 per cent of unfolding using a statistical method based on a bivariate extension of the Fisher's statistics (Legoff 1990; Legoff, Henry & Daly 1992). Such a tilt correction changes the pole position by 1.5°. As the α 95 and K curves obtained from stepwise unfolding display a broad minimum and maximum value respectively, and because there was probably slight original syn-sedimentary dips, it is likely that 85 per cent of tilt correction is not significantly different from complete unfolding. The magnetic behaviour of the samples upon thermal and AF demagnetization, the evolution of the initial susceptibility upon thermal treatment, hysteresis cycles and investigation of the magnetic mineralogy through microscope examination and electron microprobe analyses lead to the following conclusion: greigite was probably a primary magnetic mineral ...