Architecture and tectonic of Paleozoic intracratonic Basins: Impact on the sedimentary record and associated geometries. Example of peri-Hoggar Basins (North Gondwana marge)
Started in november 2015
Defense the 27 june 2019
uB supervisors: Michel Guiraud and Emmanuelle Vennin
ENGIE supervisor: Eric Portier, Isabelle Moretti
UPMC supervisor: Laetitia le Pouhriet
The Paleozoic Saharan platform including the peri-Hoggar Basins (i.e. Murzuq, Illizi, Mouydir, Ahnet, Reggane and Tim Mersoï basins) are defined as intracraonic basins. Their histories have been dominated by slow long-wavelength vertical motions leading to overall low subsidence rate (i.e ca. 10 m/Ma to 50 m/Ma) and accumulation of an extensive cover of platformal sediments (i.e. shallow deposits environments), rhythmed by pulsatile periods of increasing and decreasing rate probably triggered by regional geodynamic events. The vertical motions of the platform produced several arches also called domes, swells, highs, ridges (e.g. the Tihemboka, Amguid El Biod, Arak-Foum Belrem and Azzel Matti Arches) and basins (syncline-shaped) with different wavelengths going from several hundred to more than a thousand kilometres. The persistence of a rather uniform pattern of vertical motions seems to control the architecture of the basins indicating a large-scale control (i.e. lithospheric). This latter controls spatially and temporally the deposition and the erosion dynamics. Several major erosion events significantly truncated the pre-existing sediments over wide areas, producing regional unconformities, especially restricted and amalgamated on arches, which separate the platformal cover into divisions. Through an original multidisciplinary integrated approach going from a geological basin analysis, coupling the substrate and the basin architecture to a numerical thermo-mechanical modelling of the lithosphere, this study has led to decipher the forcing factors of the intracratonic basins of the Saharan platform.
The Arches-Basins architecture is highlighted through the identification of tectono-sedimentary structures (growth strata, truncatures…). This architecture is featured by thickness variation and facies portioning, organized by sub-vertical planar normal faults (sometimes blind faults) forming horst-graben systems associated with forced folding in the cover. Connected and nucleated to major mega-shear zones, horst-graben systems are inverted (positive inversion) or reactivated (forced folds) during successive geodynamic events (e.g. Cambro-Ordovician extension, Ordo-Silurian glacial rebound, Siluro-Devonian “Caledonian” extension/compression, late Devonian extension/compression and “Hercynian” compression).
Formed under a Precambrian lithosphere of accretionary type, inherited during several paleo-orogenies (e.g. Eburnean, Pan-African), a substrates zonation of the Arches-Basins framework is described, where the Archean to Paleoproterozoic terranes are forming the structural highs and the Meso-Neoproterozoic terranes the structural lows.
Based on these geological observations and the hypothesis of conserved differential densities (implying an isostatic potential) between the inherited different accreted terranes in the lithosphere (i.e. archean and proterozoic terranes), a 2D thermo-mechanical numerical model is proposed. The first and second order forcing factors, respectively recorded in the subsidence rate pattern by the low long-lived and by their cyclic deviations, are well constrained reconciling the singular Arches-Basins tectono-stratigraphic architecture. The different simulations have shown the importance of thermal anomaly, tectonics (weak strain rate) and external sediment supply on the dynamic of these intracratonic basins. Where, sediment flux controls the speed and the duration of basin infill until achievement of the isostatic equilibrium. The thermal anomaly and the tectonics compel the tectono-stratigraphic complexification such as the arches framework (intra-arches, boundary secondary arches…) and the stratigraphy architecture (wedges, diachronic unconformities).
Furthermore, by comparing the basins architecture and the signature of the subsidence and the thermal pattern between numerical model and geological data from peri-Hoggars Basins, we see that all the forcing parameters associated can be linked to geodynamic events such as glaciation/deglaciation, global warming, rifting, intra-plate volcanism, and local deformation, probably triggered by far field stresses.
Finally, an original classification is proposed based on the best fit of each peri-Hoggar Basins with different thermomechanical numerical models and their forcing factors dominance (tectonics dominated, thermal anomaly dominated, sediment flux dominated and/or different combination of these latter).
Saharan platform, peri-Hoggar Basins, Arches-Basins, Precambrian structural heritages, lithosphere heterogeneity, terranes, thermo-mechanical, far field stresses, density, potential isostatic equilibrium
Sierd Cloetingh, Université de Utrecht – referee
Rémi Eschard, Total – referee
Isabelle Moretti, Sorbonne université – examiner
Eric Portier, Neptune energy – examiner
Laetitia Le Pourhiet, Sorbonne université – examiner
Michel Guiraud, UBFC – supervisor
Emmanuelle Vennin, UBFC – co-supervisor