Deep structure and crustal configuration of the Jeffara basin (Southern Tunisia) based on regional gravity, seismic reflection and borehole data: How to explain a gravity maximum within a large sedimentary basin?

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Abstract

The Jeffara basin of southern Tunisia contains a thick sequence of mainly Triassic and Permian sediments that is characterized by a gravity maximum. To explain the positive gravity signature over the Jeffara sedimentary basin and to obtain a more quantitative representation of the subsurface structure, a regional 2.5D gravity model constrained by seismic reflection and borehole data was constructed along a NE–SW trending profile. The depth to the crust/mantle implies that the Jeffara basin is associated with crustal thinning. The gravity model also implies that subsidence is controlled by a basement stepped down by relatively low-displacement faulting. This sedimentary subsidence, as described by a listric-faulting model, is probably caused by a thinned crust.

Introduction

The Jeffara region of southeast Tunisia, situated in north Africa (Fig. 1A), consists of two geographic and geological provinces with vastly different features (Fig. 1B and C). Topographically these are: (1) a western section containing the large-scale Dahar Plateau associated with the Telemzan and Bounemcha structural highs and (2) an eastern part containing the Jeffara coastal plain where the topography averages near mean sea level. This entire region contains a large area of playa lakes that stretch from the lowlands of eastern Tunisia to northwestern Libya.

The simplified geological map of southern Tunisia and northwest Libya (Lefranc and Guiraud, 1990) (Fig. 1C) shows two principal geological provinces: (1) The Jeffara coastal plain which consists of a NW–SE-trending (Fig. 1B), in Tunisia, to E–W-trending, in Libya collapsed block formed during the Late Cretaceous and Cenozoic by extensional faulting, which is overlain by Tertiary and Quaternary sediments (Benton et al., 2000) and (2) the Dahar Plateau which consists of a N–S-trending, in Tunisia, to E–W-trending, in Libya monocline (Jebel Nafusah). In Tunisia, the Dahar Plateau is mostly composed of shallow W to SW dipping (less than 5°) Triassic and Jurassic sequences that are capped by Late Cretaceous limestones (Gabtni et al., 2005). Within the Dahar Plateau there is almost a complete stratigraphic sequence from Late Permian to Late Cretaceous with the Permian sequence in the Tebaga de Medenine (Fig. 1C) being the oldest outcrop in Tunisia (Castany, 1954, Busson, 1967, Bouaziz, 1995, Zouari, 1995, Bouaziz et al., 1996, Bouaziz et al., 2002). Locally, the Permian sequence is displayed in a 20–30° south-dipping monocline affected by E–W striking faults along the northern edge of the Dahar Plateau. This structure seems to have controlled the deposition of the Late Permian deposits and played a major role in the formation and filling of the Tataouine basin (Busson, 1967, Bouaziz et al., 1994).

The Jeffara basin has undergone a complex and polyphase structural history since Carboniferous time (Burollet and Desfoges, 1982, Ben Ayed, 1986). It has been affected by multiple episodes of tectonism, including a late Paleozoic collision with Laurasia and subsequent early Mesozoic rifting associated with the opening of the Tethyan Ocean (Memmi et al., 1986, Gabtni et al., 2006). The effect of the several tectonic phases (Taconic (Ordovician), Caledonian (Devonian), Hercynian (Carboniferous), Austrian (Early Cretaceous), and Alpine (Late Cretaceous–Early Eocene)) on the Jeffara area has been the production of a large sedimentary basin in the eastern part (Jeffara basin) and a fault-bounded structural high in the western part (Bounemcha high). After the Hercynian orogeny, the Bounemcha high (Precambrian metamorphic basement between 1079 and 1121 m below surface in NMT1 (Fig. 4) was formed by a reactivation of Pan African fault systems (Ben Ayed and Khessibi, 1983, Van de Weerd and Ware, 1994). During Mesozoic extensional tectonics, there was an episode of northwestward tilting of the region, resulting in the superposition of a pre- and post-tilting Mesozoic basin on the eroded remains of the Paleozoic basin (Van de Weerd and Ware, 1994, Echikh, 1998, Gabtni, 2006, Gabtni et al., 2006). The Alpine orogeny affected the basin with less intensity than it did in the Atlas Mountains to the north; however, several Hercynian-aged normal faults were locally inverted (Echikh, 1998).

In the present work, regional gravity, seismic reflection and borehole data within the Jeffara basin and the surrounding region are analyzed in order to map the structural configuration of the basin and the general regional crustal structure. This was accomplished by applying a regional/residual anomaly separation and edge enhancements to the gravity data. A NE–SW trending 2.5-dimensional (2.5D) gravity model that intersected four deep wells was constructed to determine the basin architecture and deeper crustal features.

Section snippets

Regional gravity, seismic reflection and borehole data

The Tunisian gravity data (Fig. 2) were obtained from the Entreprise Tunisienne des Activités Pétrolières (ETAP) and the Libyan gravity data were obtained from the BGI (Bureau Gravimétrique International) (Fig. 2). The data sets were tied to the 1971 International Gravity Standardization Net (Morelli, 1996), merged and reduced using the 1967 International Gravity formula. Free-air and Bouguer gravity corrections were made using sea level as datum and 2.67 g/cm3 as a reduction density. We are

Gravity analysis and modelling

The analysis of Bouguer gravity anomaly maps involves comparison of anomaly patterns with features defined by other geophysical techniques and with geologicalal data, to determine possible sources of the anomalies. This can be augmented by edge enhancement analysis that typically involves derivatives of the gravity field, such as the horizontal gravity gradient (HGG), maxima of which are used to locate the edges of lateral density contrasts. The edges or contacts so derived may be analyzed for

Discussion: geodynamic implications

The final interpreted model (Fig. 8) is consistent with the idea that the subsidence of the Jeffara basin was controlled by a series of low-displacement listric faults that involved the basement. Most of these faults are associated with the Jeffara fault system and were contemporaneous with Mesozoic and Cenozoic sedimentation. The principal influence on this sedimentary subsidence was probably the tensional stresses induced by a possible thermal uplift associated with the ascent of upper mantle

Conclusions

Bouguer gravity anomalies show that the Jeffara basin is associated with a NW–SE-trending gravity maximum separated from the Saharan platform (Bounemcha high) by a large NW–SE-trending gravity gradient. This NW–SE trending gravity gradient is the boundary between the thick sedimentary sequences within the Jeffara basin and the thinner sedimentary sequences of the Saharan platform (Bounemcha high). The trend of horizontal Bouguer gravity gradient maxima supports previous satellite image and

Acknowledgements

We would like to thank ETAP who generously supplied us with the Tunisian gravity data, the seismic reflection profile and the well data. Additionally, we would like to thank BGI for the Libyan gravity data. The constructive criticisms of two anonymous reviewers and Pr. Mourad Bedir from CERTE (Tunisia) greatly improved the manuscript and are appreciated.

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