Fetheddine Melki

We present a characterization of a poorly studied karstic aquifer, documenting the geometry, resistivity properties and the control of structural discontinuities that affect the Ypresian limestone cropping out in the Mateur imbricated thrust zone in northern Tunisia. The karst system was produced by meteoric diagenesis. Geophysical methods and field investigations were useful in extracting geological features. First, our findings suggest that N110–N140 directions are the preferred dissolution paths orientated parallel to strike-slip and extensional faults. Second, the gravity data analysis (regional–residual separation, horizontal gravity gradient and Euler deconvolution) show a particular tectonic node as a junction of the NE–SW and NW–SE directions. Third, 2D electrical resistivity models display various karstification typologies with high (>1000 Ω m), medium (between 200 and 1000 Ω m) and low ( Ω m) electrical resistivity zones. These electrical resistivity models have contributed to the establishment of a conceptual structural model of the global karst features. Finally, 2D electrical resistivity tomography (ERT) and 1D time-domain electromagnetic analysis highlighted the presence of potential water zones integrated into the aquifer. Thematic collection: This article is part of the Karst: Characterization, Hazards & Hydrogeology collection available at: https://www.lyellcollection.org/topic/collections/karst

Cenozoic extension in the Western Mediterranean has been related to the dynamics of back‐arc domains. Although, in most of its orogenic belts extension propagated into the fore‐arc nappe domains. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell, where HP/LT rocks (350°C at 0.8 GPa), were exhumed by the sequential activity of extensional detachments after heating and decompression (410°C–440°C at 0.6–0.3 GPa) in a plate convergent setting. Normal faults thinning the Tunisian Tell detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving Tellian Triassic evaporites in the hanging‐wall forming halokinetic structures in the Mejerda basin late Miocene. The deeper‐detachment bounds metamorphic domes formed by marbles and metapsammites from the Atlas domain. Illite crystallinity on Triassic rocks shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell‐Atlas nappe belt, respectively. New U‐Pb 49.78 ± 1.28 Ma rutile ages from Tellian metabasites, together with existing phlogopite 23–17 Ma K‐Ar ages in Atlas marbles from the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell rocks underthrusted the Kabylian flysch in the early Eocene. Further, early Miocene shortening thrusted the metabasites over lower‐grade sediments, producing HP/LT metamorphism and ductile stretching at the base of the Atlas belt. The exhumation of midcrustal roots of Western Mediterranean nappe belts after tectonic shortening is a common feature related to tearing at the edges of the subduction systems and inboard delamination of their subcontinental lithospheric mantle.

Cenozoic extension in the Western Mediterranean is related to the dynamics of back-arc domains. However, extension propagated into the external Foreland Thrust Belts (FTB) of the region. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell FTB, where HP/LT blastomylonitic rocks (300-370ºC at 0.9-1.0 GPa), were exhumed by the sequential activity of extensional detachments. Normal faults thinning the Tunisian Tell FTB detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving allochthonous Triassic evaporites at the base of the hanging-wall that form halokynetic structures intruding the Mejerda basin Late Miocene infilling. Meanwhile, the deeper-detachment level bounds metamorphic domes formed by marbles and metapsamnites. Illite crystallinity of Triassic rocks in the region shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell-Atlas FTB, respectively. New U-Pb 49.78 ± 1.28 Ma rutile ages together with existing K-Ar ages in marbles at the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell Triassic rocks underwent Cretaceous extensional metamorphism, followed by crustal thickening and rutile growth in the Early Eocene. Further, Early Miocene thickening thrusted the metadolerites over lower-grade sediments, producing HP/LT metamorphism overprinting the base of the FTB. The exhumation of midcrustal roots of western Mediterranean FTBs after the tectonic shortening phase is a common feature of other FTB´s, like the Betics and Rif, which underwent a late-stage tearing at the edges of the subduction system together with delamination of their subcontinental lithospheric mantle.

This work analyses the tectonic evolution of Northern Tunisia from the Late Miocene to Present. Two orthogonal extensional systems with ENE- and SE-directed transport produced the extensional collapse of the Tell and Atlas Foreland Thrust Belts (FTBs) in northern Tunisia during the Late Miocene to Pliocene in a context of NW-SE plate convergence between Africa and Eurasia. These systems produced the extensional denudation of the Tunisian Atlas and Tell foreland thrust belts, which we related to deep mantle tectonic mechanisms, known as a common feature in other FTB´s in the western Mediterranean, i.e. Betics, Rif, Calabria and Apennines. Low-angle normal faults have extended and reworked the Tunisian Tell external foreland thrust belt, exhuming midcrustal lower-greenschist metapelites and marbles with Triassic protholiths, and forming Late Miocene basins. This extension was followed by later Pliocene to Present tectonic inversion, developing the active shortening structures in Northern Tunisia. The main shortening structure is formed by different reverse and strike-slip fault segments, linked forming the 130 km long Alia-Thibar fault zone. Restored Plio-Quaternary deformation observed on reflection seismic lines indicates deformation rates around 0.6-0.8 mm/yr in the studied segments and larger amounts of shortening to the West of Northern Tunisia (16%) than to the East (7%), which suggests that tectonic inversion started earlier to the West and later propagated eastwards, reaching Northeastern Tunisia in the Late Pliocene. Due to the young age of this tectonic inversion, the present relief of Northern Tunisia is characteristic of a young thrust and fold belt, with dominating axial valleys along synforms and an incipient transverse drainage development propagating from West to East.

The region of Metlaoui (South Atlas of Tunisia) is listed as one of the major seismotectonic zones of the national territory. About 10 major seismic events with moderate magnitude have occurred from the beginning of last century. The event of 7/11/1989 (Ms = 4.4) was undoubtedly the most important. It caused very significant material damage estimated at 224,525,000 Tunisian Dinars. This study uses a mapping tool to define areas with relatively high degrees of hazard and vulnerability. It also seeks to understand the active fault of the seismic events recorded in this region.

Exploration of territories not previously analyzed by landslide experts provides interesting findings. The Chgega landslide, in northern Tunisia, represents a paradigmatic mass movement. It can be classified as a complex landslide, or more specifically as vast rock spreading that evolved into a block slide. It involves a great block of limestone—about 900 m long and 400 m wide—sliding over ductile clays and marls. The viscoplastic creep of the clays drives the landslide and creates, in its crown, a graben ~800 m long and ~120 m wide that breaks the summit of Chgega Mountain. Using Interferometric Synthetic Aperture Radar (InSAR) technologies, we demonstrate that this complex landslide is currently active and moreover shows progressive movement without clear episodic accelerations. The velocity of the limestone block is just above 2 mm/yr. The occurrence of gravity-induced joints indicates that the movement has an orientation towards 333° of azimuth on average, conditioned by the landscape around Chgega. These results were obtained through the analysis of a 3D model and a high-resolution orthoimage created from photographs acquired by an Uncrewed Aerial Vehicle (UAV). We may conclude that the landslide movement is determined by normal faults with directions N060°E and N140–150°E. This characterization of the Chgega landslide can serve as the basis for future studies about the origin of this slope movement. Furthermore, the data provided here may support the recognition of Chgega as a singular geological point that deserves to be declared a geosite.