Active tectonics in Northern Africa: The Nubia-Eurasia boundary in Tunisia

Abstract

This PhD thesis presents a geological and geophysical study that characterizes the Quaternary stratigraphy, geomorphology, recent tectonics, and discusses their implications to understand the geodynamic evolution of North Tunisia, including the emerged land and continental margin. The study area spans much of the region containing the boundary between the African and Eurasian tectonic plates in this sector. I have carried out specific analyses for an onshore and an offshore contiguous regions, where present-day tectonics has been previously poorly studied. Onshore Northern Tunisia we applied a morphometric relief analysis of digital elevation models based in drainage network metrics to characterize active tectonics and its influence into the drainage evolution of the area. This analysis was ground-truthed with a field campaign. Our analysis indicates that recent river captures have been the main factor driving the fluvial network reorganization in Northern Tunisia and are probably driven by the gradual uplift of adjacent axial valleys by reverse/oblique slip faults or associated folding. Several active faults with estimated maximum magnitudes up to (Mw) 6.7 have been identified. To study the offshore North Tunisian continental margin we have integrated a comprehensive dataset of about 12,000 km of 2D parametric echosounder profiles and about 15,600 km2 of high- resolution bathymetry and back-scatter mosaics from two cruises. Additionally to the geophysical dataset, we integrated available age data from a giant piston core collected by the R/V Marion Dufresne in 1995 during the MAST II PALAEOFLUX Program, which provides the calibration of horizons mapped with TOPAS profiles. This large dataset permitted a detailed analysis of the offshore Tunisian plateau for the first time. The geomorphologic and morpho-structural study of the offshore North Tunisian margin has revealed a complex seafloor topography. The coexistence of a wide range of geomorphic features interplay to shape the seafloor and shallow strata structure. This first high-resolution study has been key to later identify and characterize active faulting and related structures. The seismo-stratigraphic analysis of deposits supports that individual Quaternary basins develop syn-tectonic to faulting. This analysis found a change in Sediment Accumulation Rates (SAR) coeval to a change in the strata geometry of the units next to faults at 402 ± 5 ky (mid Middle Pleistocene) supporting a change in active faulting intensity caused by enhanced

contractional tectonics. This change may relate to the NW-SE convergence between Nubia and Eurasia, and probably marks the end of the Tyrrhenian back-arc extension and of subduction trench migration in the Ionian Sea. The structural analysis of the offshore North Tunisian continental margin has revealed a series of active faults, mapped for the first time in the region, with a general transpressional to compressional regime and estimated maximum magnitudes of (Mw) 7.54. Most active faults occur within the north-eastern sector of the mapped area and possibly represent the current reactivation of previous structures and inverted in compression by the present-day NW-SE trending convergence. Most of them are possibly pre-existing faults formed during the opening of the Tyrrhenian Sea and some fewer faults are possibly related to the fold-and-thrust belt system associated to the opening of the Algero-Balearic basin. Overall, the distribution of active faults corresponds to a sparse seismicity band that expands further south than in neighbouring regions. The broad area of deformation is probably related to the geometry and dynamics of the slab underlying Tunisia, and to the collision of the North African lithosphere with relatively thick continental lithosphere of the Sardinia-Corsica. The results of this thesis provide information to improve the regional seismic hazard assessment.

Aquesta tesi presenta un estudi geològic i geofísic que caracteritza l’estratigrafia quaternària, la geomorfologia i l’activitat tectònica recent per entendre l’evolució geodinàmica del Nord de Tunísia, incloent la zona emergida i el marge continental. L’àrea d’estudi inclou la major part de la regió que conté el límit de plaques entre Núbia i Euràsia en aquest sector. A la zona emergida s’ha dut a terme una anàlisi morfomètrica del relleu mitjançant models digitals del terreny per caracteritzar la tectònica recent i la seva influència en l’evolució de la xarxa de drenatge. Aquest estudi es va complementar amb una campanya de camp i ha permès comprovar que les recents captures dels rius provocades pel moviment de falles actives són el principal factor que està causant una reorganització de la xarxa. A més, s’han identificat noves falles actives amb magnituds de terratrèmols estimades de fins a 6.7 Mw. A la zona marina es van adquirir una gran quantitat de dades en dues campanyes, incloent 12.000 km de perfils d’ecosonda i un mapa batimètric d’uns 15.600 km2. Aquestes dades van permetre dur a terme un estudi geomorfològic que ha descobert un fons marí molt complex. També es va dur a terme un estudi sismoestratigràfic dels sediments quaternaris que ha demostrat que les conques sedimentàries recents es desenvolupen sintectònicament a falles. Amb aquesta anàlisi es va identificar un canvi en les taxes de sedimentació a 402 ± 5 ka que demostra un augment en l’activitat de falles degut a la convergencia de plaques actual i que probablement marca la fi de la subducció a la conca del Tirrè. Per altra banda, es va dur a terme una anàlisi estructural del marge continental amb el que es van identificar nombroses falles actives desconegudes anteriorment i de les que s’han obtingut magnituds potencials de terratrèmols de fins a 7.54 Mw. Aquestes falles són falles reactivades de l’obertura del Tirrè i en menor mesura de l’obertura de la conca Algero-Balear. Finalment, es proposa que la gran banda de deformació transpressiva-compressiva de Tunísia està relacionada amb la geometria i dinàmica dels slabs i al fet que el marge de Tunísia està col·lisionant amb l’escorça continental del bloc de Sardenya- Còrsega.