- journal_title：Bulletin de la Societe Geologique de France
- Contributor：Farhat Rekhiss ; Selma Mguidich ; Jamel Oualiet ; Mohamed Moncef Turki
- Publisher：Societe Geologique de France
- Date：2010-
- Format：text/html
- Language：en
- Identifier：10.2113/gssgfbull.181.3.259
- journal_abbrev：Bulletin de la Societe Geologique de France
- issn：0037-9409
- volume：181
- issue：3
- firstpage：259
- section：Articles
Situated in the northern Tunisian “Diapir and dome zone” (Tunisian Atlas), the Djedeida hills consist of a south-dipping monoclinal series globally trending E-W. This pelitic-carbonate series latest Jurassic (Maiana Formation) to Early Cretaceous (Hamada and Seroula Formations) in age, exhibits clear mineralogical and structural transformations related to significant changes in pressure and/or temperature conditions. Structural and microstructural analysis allowed us to characterize the metamorphic pattern, and to discuss about the geodynamical context of its occurrence connected to the regional evolution.
Deformation patterns mostly consist of two sets of ubiquitous fracture schistosities well-developped over the entire structure, as well as centimetric to metric folds systematically displayed by thin ferruginized layers. These field-observations, which are consistent with microscopic analysis, show evidences for the occurrence of two distinct deformational events successively affecting the area.
Superposition of the two fabrics led to the development of a pervasive parallelepipedic chipping in the rock mass that is particularly obvious in the pelitic unit. Layer-parallel S1 and strongly oblique S2 schistosities are respectivelly associated with chlorite and illite cristallization. Neoformations of illite into chlorite former phases occur in relation with a simple shearing, which broadly characterizes the second event with evidences such as rotated phenoblasts (quartz and pyrite), chlorite sigmoidal deformation, and mica, calcite, feldspar growth into pressure shadow.
These syn-tectonic mineralogical phases record a low-grade metamorphism evolving into the sequence from the anchizone to the epi-metamorphic facies. Based on structural and mineralogical criterions, this analysis allows us to conclude that the first event is consistent with an early stage metamorphism occurring within crustal-thinning conditions, thus likely to express the lower Mesozoic transtension which affected northern Africa as a consequence of the Te-thyan opening. The second event, suggesting a compressive context, is more probably associated to the Alpine convergence responsible for the Atlasic tectonics affecting the area at the Neogene time.