The use of anisotropy of magnetic susceptibility and brittle mesostructures analysis to investigate the tectonic evolution of the oriental Atlas. Case study of the southern edge of the Tunisian trough, NW Tunisian Atlas

Abstract

This study used a combination of the Anisotropy of Magnetic Susceptibility (AMS) and the structural method to assess the tectonic evolution of the northwestern Tunisian Atlas. A total of 370 oriented specimens were sampled at 18 sites from fine grained Cretaceous sediments ranging from Aptian to Campanian. In order to identify the main magnetic carriers of the AMS signal, representative samples were analysed using thermomagnetic techniques. The results revealed that ferromagnetic (s.l) minerals play a trivial role, favouring the conclusion that paramagnetic minerals are the main contributors. The AMS results showed 12 sites exhibiting normal AMS fabrics, typical for weakly deformed sediments, with a well-defined magnetic lineation and a magnetic foliation sub-parallel to the bedding plane. Only two main orientations of the magnetic lineation were defined and were greatly controlled by the tectonic deformation recorded in the area. Both AMS and structural analyses illustrate a transition from a generally N-S extensional direction for the Aptian-Albian formations to a NW-SE shortening direction for Cenomanian- Campanian series. Except for the Albian-Cenomanian Formation, where the AMS results disaccord with the microtectonic analysis, the roughly N-S magnetic lineation was attributed to the lower Cretaceous syn-deposit rifting phase and was not overprinted by the later NW-SE compressive phases. Thus, it can be concluded that the AMS texture was acquired during early stages of the deformation and once lithified, it was hardly altered by subsequent tectonic movements. Besides, the AMS results also revealed two undefined fabrics: one sedimentary and two inverse. The integrated magnetic and mineralogical investigations confirmed the influence of the magnetic mineralogy on the resulting AMS fabric.