Thermochronométrie basse température (U-Th) et géodynamique : application aux marges passives et aux domaines actifs (Maghreb et Sud-Est de l’Afrique)
Directrice : Cécile GAUTHERON
Co-encadrant : Yves MISSENARD
Financement : Allocation MESR
Understanding tectonic processes at plate boundaries is a major geological issue, because of their strong geophysical and geological implication. Plate convergence results in mountain belt building at, or close to, the plate borders. The apatite (U,Th)/He thermochronometer is a key geochemical tool to study such ranges, improving the characterisation of exhumation rate and date. This PhD project proposes to combine geodynamics and low-temperature thermochronologyin order to study the African Plate. The investigation will bear on several mountain belts in Morocco, Mozambique, and Zimbabwe, which represent the products of the different african orogenies.
Part of the work will consist in pursuing the research efforts towards a better understanding of the (U-Th)/He thermochronometer on apatite. It belongs to a wider research program, the ANR HeDiff. Apatite (U-Th)/He is now a well-constrained and extensively used dating tool, but several questions remain to be investigated. Despite adequately calibrated helium diffusion models, other processes impacting helium ages are unresolved. To this aim, we intend to : (i) study the apatite (U-Th)/He data in selected locations where the thermal history is known to cross-calibratethe diffusion model on geological time scale, and (ii) perform experimental geochemistry in order to study the impact of crystal defect density on He diffusion and their annealing as a function of apatite chemistry.
Another part of thesis will be to combine structural geology and apatite (U-Th)/He data, to determine the history of mountain exhumation. The study will bear on the evolution of two complimentary African sites : the Moroccan High-Atlas and South-East African belts. The work will be in collaboration with Omar Saddiqi (Univ. of Casablanca) and Roderick Brown (Univ. of Glasgow). Regarding Morocco, two field investigations will take place : one in the High-Atlas (Marrakech), and another in the south. The exhumation of both areas will be characterizedand dated, enabling to precise the intraplate deformation history. Other Maghrebian samples will be analyzed (North Algeria), in order to also study deformation at the plate boundary. South-East Africa attended several ancient orogenies, with major deformation phases leading to Gwondana opening, followed by erosion phases. We plan to investigate : (i) the chronology and motors of vertical movements, and (ii) the processes that occurred during the Cenozoïc erosion phase. The study will bear on samples from Mozambique and Zimbabwe, and a field investigation. Finally, a thermochronological database and geodynamical models will be performed at the scale of the African plate.
Mots clefs : thermochronologie, orogenèse, Afrique, hélium, apatite.