Metamorphic Domes in Northern Tunisia: Exhuming the Roots of Nappe Belts by Widespread Post-Subduction Delamination in the Western Mediterranean

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.Ministerio de Ciencia e innovación PID2019- 107138RB-I00P18-RT-3632 of the Junta de Andalucia,Erasmus Mundus External Cooperation Window and by Scientific Cooperation Agreement 0534 between the Office National des Mines (ONM)The Tunis el Manar University and the Group for Relief and Active Processes Analysis (ARPA) from the University of GranadaTunisian Company of Petroleum Activities (ETAP)Universidad de Granada / CBU

Geomorphological, seismic and geological interpretation of Neogene to recent deformations in northern Tunisia

This Thesis analyses the tectonic evolution of Northern Tunisia from the Late Miocene to Present, using multiscale and multisource data analyses involving new technologies and approaches. Two orthogonal extensional systems with ENE- and SE-directed transport produced the extensional collapse of the Tell and Atlas thrust belts in northern Tunisia during the Late Miocene to Pliocene in a context of NW-SE plate convergence between Africa and Eurasia. This new hypothesis suggests for the first time the importance of crustal extension in the 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 epizonal Triassic metapelites 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 shear 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 tectonic this 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. New topographic development is favouring slope instabilities. Thus, in addition, we used a interdisciplinary approach, including Interferometric Synthetic Aperture Radar (InSAR) technologies to analyse the Chgega active landslide, or more specifically as vast rock spreading that evolved into a block slide, characterizing a progressive movement without clear episodic accelerations.En la presente Tesis doctoral analizamos la evolución tectónica del Norte de Túnez desde el Mioceno Superior hasta el presente, usando una metodología multidisciplinar que incluye el análisis tectónico y morfométrico de la región. El cinturón de pliegues y cabalgamientos del Tell tunecino fue adelgazado por dos sistemas extensionales ortogonales con transporte hacia el ENE y ESE, respectivamente, entre el Mioceneo Superior y el Plioceno en un contexto de convergencia NW-SE entre las placas Africana y Euroasiática. Esta nueva hipótesis sugiere por primera vez la importancia de la denudación extensional en el cinturón orogénico del Tell tunecino, en relación con mecanismos tectónicos profundos, como delaminación del manto litosférico subcontinental, descritos en otros cinturones de pliegues y cabalgamientos del Mediterráneo occidental como las Béticas, Rif, Calabria o los Apeninos. Fallas normales de bajo ángulo han extendido el cinturón de cabalgamientos exhumando rocas metamórficas situadas a la base del prisma orogénico a profundidades de corteza media, concomitántemente al desarrollo de cuencas sedimentarias del Mioceno Superior. Esta extensión fue seguida de una inversión tectónica contractiva desde el Plioceno, formándose las presentes estructuras transcurrentes y fallas inversas activas en el norte de Túnez. El principal zona de falla transcurrente de la region es la zona de falla de Alia-Thibar, con una longitud total de 130 km y compuesta por hasta 5 segmentos unidos, con diferente cinemática. La restauración del acortamiento observado en diversas lineas de sísmica de reflexión paralelas a la dirección de acortamiento indica tasas de acortamiento de 0,6-0,8 mm/año en los segmentos estudados y valores de acortamiento mayores al oeste de Tunez (16%) que hacia el este (7%). Esto podría indicar que la inversión tectónica se inició antes hacia el oeste y se ha propagado posteriormente hacia el este, afectando al NE de Túnez a partir del Plioceno superior. El relieve del norte de Túnez es característico de un cinturón de pliegues y cabalgamientos joven, con dominio de sistemas fluviales axiales a lo largo de sinformes y el desarrollo de un sistema de drenaje transverso incipiente, que se propaga de oeste a este. Este nuevo crecimiento topográfico está favoreciendo la inestabilidad de laderas. Por ello, hemos usado una metodología interdisciplinar, incluyendo el análisis interferométrico de radar (InSAR) para estudiar el deslizamiento activo de Chgega, caracterizado por un movimiento progresivo sin aceleraciones episódicas claras.La thèse présente et discute l'activité tectonique du nord de la Tunisie du Néogène, en se basant sur des analyses de données multi-échelles et multi-sources impliquant de nouvelles technologies et approches. Le présent manuscrit met en évidence deux systèmes d'extension orthogonaux avec un transport ENE et SE, produisantt l'effondrement des structures du Tell et de l'Atlas dans le nord de la Tunisie du Miocène supérieur au Pliocène dans un contexte de convergence des plaques NW-SE entre l'Afrique et l'Eurasie. Cette nouvelle hypothèse suggère pour la première fois l'importance de l'extension crustale dans la dénudation des limites de chevauchement de l'Atlas tunisien et de l'avant-pays du Tell qui est liée aux mécanismes tectoniques au niveau du manteau profond, connue comme une caractéristique commune pour d'autres FTB en Méditerranée occidentale, la Bétique et Rif. L'interprétation de la cristallinité de l'illite effectuée sur des métapélites du Trias correspond au modèle d'effondrement polyphasé orogénique tardif, mettant en cause l'évolution diapirique à longue durée de vie proposée précédemment dans des travaux antérieurs. Les structures compressives actives dans le nord de la Tunisie se sont développées par inversion tectonique depuis le Pliocène. La déformation plio-quaternaire est mise en évidence à travers des lignes sismiques réflexions qui caractérisent, via restoration de coupes, des taux de déformation autour de 0,6-0,8 mm/an dans les segments étudiés et des raccourcissements plus importants du Nord-Ouest (16 %) vers le Nord-Est (7 %) de la Tunisie. De ce fait, l’inversion tectonique a commencé vers l'Ouest et s'est ensuite propagée vers l'Est, atteignant le Nord-Est de la Tunisie à la fin du Pliocène. En raison du jeune âge de cette inversion tectonique, le relief actuel du nord de la Tunisie est caractéristique d'un jeune ensemble de chevauchement et de plis, avec des vallées axiales dominantes le long des synformes et un début de développement de drainage transversal se propageant d'Ouest en Est. De plus, les technologies du radar interférométrique à synthèse d'ouverture (InSAR) ont révélé que la structure de Chgega est un glissement de terrain complexe actuellement actif, ou plus précisément comme un vaste étalement de roches qui a évolué en un glissement de blocs, caractérisant un mouvement progressif sans accélérations épisodiques claires identifiées.Tesis Univ. Granada.CGL2015-67130- C2-1-RErasmus Mundus External CooperationOffice National des Mines (ONM) Tunis el Manar University Group for Relief and Active Processes Analysis (ARPA) from the University of GranadaMinisterio de Ciencia e innovación PID2019-107138RB-I00 and P18-RT- 3632Junta de Andaluci

Analysis of the Geological Controls and Kinematics of the Chgega Landslide (Mateur, Tunisia) Exploiting Photogrammetry and InSAR Technologies

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

Analysis of the Geological Controls and Kinematics of the Chgega Landslide (Mateur, Tunisia) Exploiting Photogrammetry and InSAR Technologies

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.The work of J.P.G., P.R., J.V.P.-P., J.M.A., and G.B.-R. was supported by the “Ramón y Cajal” Programme (RYC-2017-23335) of the Spanish Ministry of Science, the project “MORPHOMED”-PID2019-107138RB-I00/SRA (State Research Agency/10.13039/501100011033) and the project “RADANDALUS” (P18-RT-3632), and B-RNM-305- UGR1818 of the FEDER/Junta de Andalucía-Consejería de Transformación Económica, Industria, Conocimiento y Universidades. The access to the Geohazard Exploitation Platform (GEP) of the European Space Agency (ESA) was provided by the Early Adopters Programme. The UAV campaign was carried out with the financial support of the company “Falck Group” (Italy). This study was supported by research projects CGL2015-67130-C2-1-R, Erasmus Mundus External Cooperation Window, and by Scientific Cooperation Agreement 0534 between the Office National des Mines (ONM), the Tunis el Manar University, and the Group for Relief and Active Processes Analysis (ARPA) from the University of Granad