Tectonic evolution of the northern African margin in Tunisia from paleostress data and sedimentary record
Introduction
The present-day structure of the Northern African margin in Tunisia results from a complex tectonic evolution that originated in the Late Permian with the beginning of the break up of Pangea, and ended with the Cenozoic Alpine orogeny of the Maghrebide chain. Tunisia occupies a special position within the northern African margin at the boundary between two major domains (Fig. 1): (1) the passive continental margin that extends eastward in Northern Libya and Egypt at least since the Late Cretaceous, and (2) the Cenozoic Alpine orogenic belt that fringes the northwestern stable Saharan Platform from Tunisia to Morocco. Thus, in Tunisia the tectonic evolution of both the eastern and western domains of the Northern African margin can be appraised. A wide range of well-exposed sedimentary formations, Late Permian to Present in age, allows us to investigate the tectonic evolution over a long period of time covering the whole Tethyan history in the central Mediterranean. In addition, Tunisia provides a N–S cross-section from the Alpine domain to the North, to the stable African platform to the South, through the folded Atlasic domain (Fig. 2).
This paper presents a synthesis of the tectonic evolution of Tunisia from the Late Permian to Present. Letouzey and Trémolières (1980) first proposed a tectonic calendar of Tunisia based on paleostresses in their study of the Mediterranean domain. Extensive field analysis of brittle tectonics aiming at paleostress reconstructions, initiated in Tunisia in the early nineties in the Saharan Platform Barrier et al., 1993, Bouaziz et al., 1994, Bouaziz et al., 1999. Then, they developed in central Tunisia Bouaziz, 1995, Bouaziz et al., 1998, and more recently extended to northern Tunisia Dlala, 1996, El Euchi et al., 1998. The density of paleostress data in Tunisia is one of the highest in the world, covering a period of more than 260 Ma.
Within the framework of the Peri-Tethys Programme, a synthetic reconstruction of the paleostress and tectonic evolutions of Tunisia since Late Permian has been conducted, together with the elaboration of palinspastic maps. In this paper, the paleostress regimes controlling the sedimentation and the activity of the major structures of the Tunisian basins are discussed and considered in the context of the geodynamical evolution of the Southern Tethyan margin and then of the Southern Mediterranean margin. A complete calendar of tectonic evolution is proposed, where several major periods are distinguished, corresponding to consistent stress patterns.
Section snippets
Geological setting of Tunisia
It is generally accepted that the complex structural pattern of Tunisia is mainly a consequence of the polyphased Cenozoic reactivation of inherited faults from various origins (basement faults, major faults of the Paleozoic and Mesozoic basins). The tectonic map of Tunisia (Fig. 2) results from a compiling of previous works Burollet, 1956, Burollet, 1973, Delteil, 1981, Turki, 1985, Zargouni, 1985, Ben Ayed, 1986, Ben Ferjani et al., 1990, Delteil et al., 1991, Bedir, 1995, Zaier et al., 1998,
Methodology
The paleostress regimes are accurately reconstructed based on analyses of fault slip data sets measured in the field. Data include fault plane orientations and directions and senses of slips. The reconstruction of paleostress tensors includes two distinct stages: (1) a detailed field analysis, and (2) the processing of the fault populations selected and measured in the field.
The field investigation is based on the detailed analysis of many sites. Each site generally consists of a population of
The Late Permian to Late Cretaceous extensional tectonics
The long period lasting from Late Permian to Senonian is dominated by extensional tectonics. Several major extensional events succeeded (1) from the Late Permian to the Middle–Late Triassic, (2) from the Jurassic to the Early Cretaceous, and (3) in the Late Cretaceous. In this general extensional context, basins developed in all the Tunisian domains. Between these extensional events, only two short transpressive periods intervened in the Middle Norian and in the Late Aptian–Early Albian.
First compressional events and Cenozoic stages of extension
The convergence between Africa and Eurasia began in the Late Cretaceous, about 90 Ma ago, involving the progressive closure of the remnant Tethyan oceanic domains. The first compressional event in relationship with this convergence occurred in Tunisia in the Late Maastrichtian–Early Paleocene. Since Late Maastrichtian times this compressional context did not last continuously until present (Fig. 4). Compressional and extensional periods alternated the latter generating Cenozoic basins in
The Late Cenozoic compressions
Late Cenozoic is marked in the Maghreb by the major regional orogeny lasting a large part of Miocene. It is a consequence of the north dipping subduction of the remnant Mesozoic oceanic crust beneath the southern Iberian margin. This subduction associated with the back-arc opening of the Algerian Basin, originated the southward migration of the western Mediterranean blocks, which collided with the Northern African margin, generating the south-vergent Maghrebides chain (Fig. 1).
Conclusions
The reconstruction of the tectonic evolution of Tunisia developed in this paper is based on high density of paleostress data (Fig. 4, Table 1), as well as through tectonic and sedimentary approaches Fig. 3, Fig. 11. Such an analysis, including several complementary domains of investigation, led to a well-constrained model of tectonic evolution of a segment of the Northern African margin since Mesozoic times.
Acknowledgements
This study was supported by the Peri-Tethys Programme and the Franco-Tunisian Cooperation Project (CMCU, 99F/1008). We like to thank Mr. J. Delteil, A. M'Rabet and B. Colletta for reviewing the manuscript and for valuable comments.
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