AuthorsA. Said
TitleTectonique Active De L'Atlas Sud Tunisien: Approche Structurale Et Morphotectonique
AfilliationScientific Computing, Scientific Computing, Scientific Computing
StatusPublished
Publication TypePhD Thesis
Year of Publication2011
PublisherUniversité de Toulouse
Thesis Typephd
ISBN Number00000
Abstract

This thesis is a contribution to the determination of the crustal geometry, kinematic history, active tectonics and seismic hazard in the Southern Tunisian Atlas (STA). It is based on the acquisition and analysis of field data (geology, geomorphology and paleoseismology), the interpretation of satellite images, the dating of morphological markers offset by active faults, the seismic interpretation and the construction of balanced cross-sections. The STA is characterized by thin skinned and thick skinned thrust structures and a lateral variation of regional deformation (structural geometry and shortening rate) controlled by NW-SE oblique ramps inherited from borders of a Late Triassic-Early Jurassic rift. The first increments of compressive deformation were recorded in the Turonian-Maastrichtian by a pro parte tectonic inversion of NW-SE and EW normal faults. Compression then has spread back from Serravalian-Tortonian into the STA, where it was manifested by fault propagation folds in the post Triassic sedimentary cover. The major Eocene “Atlas event” described in hinterland domains and in the Eastern Tunisia did not deform significantly the STA, which corresponded probably in this period to the backbulge depozone of the Maghrebides. The Gafsa fault is the longest oblique ramp of the STA structure inherited from the rift. It has a complex geometry with a decoupling between the post-Paleozoic sedimentary cover and the basement above thanks to a Triassic evaporite layer. We show that despite a moderate instrumental and historical seismicity, this fault has produced M \geq 6 earthquakes with a return period of ca. 500 - 5000 years during the Late Quaternary. The latest large event having produced a surface rupture on the fault occurred around 8000 years BP, suggesting a M \geq 6 earthquake is overdue on the fault. The fault has a minimum reverse component of slip rate of 0.21 - 0.34 mm / yr over the past 50 ka and paleoseismological analysis prove that rare M \geq 7 paleoearthquakes may be suspected. Such strong earthquakes would require the activation of the buried basement fault and its transient coseismic linkage with the overlying listric fault ramping off the décollement layer. A microseismic study is essential to support this hypothesis. The STA folds are worn by ramps which are expressed by the emerging of active thrusts. These thrusts, whose slip rates are comprised between 0.05 and 0.15 mm/year, have the potential to generate M \geq 6 earthquakes. Two significant thrusts by their size, and therefore by the magnitude that they can produce (Chotts and Métlaoui) should be the subject for detailed complementary studies.