Alpine extensional detachment tectonics in the Grande Kabylie metamorphic core complex of the Maghrebides (northem Algeria)
The Maghrebides are part of the peri-Mediterranean Alpine orogen. They expose in their inner zone inliers of high-grade crystalline rocks surrounded by Oligo-Miocene and younger Miocene cover. Detailed mapping coupled with structural and petrological investigations in the Grande Kabylie massif, and the reinterpretation of the available geochronological data, allow us to refute the traditional concept of rigid behaviour of this massif during Alpine events. We show that the dome geometry, the kinematic and metamorphic evolutions and the age pattern are typical of metamorphic core complexes exhumed by extension.
A major low-angle detachment fault defined by mylonites and by younger cataclasites has been traced in the massif. The upper unit encompasses pre-Permian phyllites with Variscan 40Ar/ 39Ar cooling ages, capped by unconformable Mesozoic to Tertiary cover of the Calcareous Range, both mainly affected by extensive Tertiary brittle deformation and normal faulting. The lower unit exposes in two half-domes a continuous tectonic pile, 6-8 km thick, of amphibolite facies rocks and orthogneisses affected by syn-metamorphic ductile deformation, devoid of retrogression.
The regular increase of paleotemperature downward and the 40Ar/ 39Ar plateau ages around 80 Ma suggest that the high-tempera-ture foliation and associated WNW-directed shear under a high geothermal gradient relate to extensional tectonics developed during Mesozoic lithospheric thinning of the Variscan south European margin. To the north, the Sidi Alli Bou Nab massif exposes another crustal section affected throughout by WNW-directed extensional shèar during HP/HT syn-metamorphic thinning and with overall 40Ar/ 39Ar plateau ages of 25 Ma.
The Eocene oblique collisional event responsible for crustal thickening was totally overprinted by this new extensional regime, synchronous with the beginning of the opening of the Western Mediterranean oceanic basin. This was also coeval with south-directed thrusting of foreland nappes to the south. Post-Miocene tectonic events cause significant overprinting.
A. Saadallah a, 1 , R. Caby b,*
a IST, USTHB, El Alia, BP. 32 Algiers, Algeria
b Laboratoire de Tectonophysique, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
Received 13 July 1995; accepted 13 May 1996
Keywords: Alpine tectonics; Maghrebides; Grande Kabylie; Algeria; metamorphism