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Accueil > Recherche > Thèses / HDR > Thèses en cours

AHADI Floriane

Supervisors : Cécile Gautheron
Guillaume Delpech
begenning : october 2015

Exhumation processes and their causes in convergent margin settings (e.g subduction-related orogens) are commonly thought to result from the interplay between tectonics and climate. This interplay is invoked to explain the formation and evolution of reliefs during orogens (Valla et al., 2011 ; Herman et al., 2013). Recently, the role of mantle dynamics has also been suggested to create uplift (Guillaume et al., 2013). However, a clear discrimination between internal and external processes that cause the rock exhumation is not straightforward, moreover, they may act on different timescales and may be responsible of uplift at only local or more regional scale.
One way to improve our knowledge of processes (and their causes ; isostasy, dynamic topography ; Braun et al., 2010 ; 2014) governing the creation and evolution of mountain reliefs is to study the processes of rock exhumation in a geodynamic context where one of these processes only plays a minor role on geological timescales.
The aim of this project is to characterize and quantify for the first time the interplay between the external processes (tectonics, climate) and internal (mantle dynamics) that are responsible for the exhumation of rocks in oceanic islands where climate variations are known and where tectonics only plays a minor role.

The proposed methodology in this project will allow constraining rigorous temperature-time paths by using plutonic rocks (syenites, granites, gabbro) as a marker of rock exhumation. The methodology will require (1) to constrain the conditions for emplacement of plutonic rocks in the oceanic crust by using detailed petrography and thermo-barometric methods (temperature, depth of emplacement) ; (2) the use of low to moderate temperature thermochronology to quantitatively determine the exhumation paths over time, which will allow quantifying in turn exhumation rates.
The data will enable to establish exhumation models according to the role of each internal/external processes. By using numerical modeling (PECBUE, Braun, 2003), the quantitative data will permit to reconstruct the morphological evolution of the islands through time.
To characterize the impact of climate variations and of mantle dynamics on the erosion pattern of oceanic islands during the Cenozoïc, it is proposed to use plutonic rocks from oceanic islands from the subantarctic area (Kerguelen ; Ahadi et al., 2014, Crozet) where climate variations are known during the Cenozoïc. Rocks from different plutonic complexes emplaced from ≈25 Ma to few Ma from Kerguelen Islands will permit to constrain the role of each process through time.
The exhumation models produced using the data acquired on the oceanic islands will be used to better understand the Cenozoïc uplift in a convergent margin, precisely in Japan where the Earth’s most recent plutonic rocks (0.8 Ma ; Ito et al., 2013) outcrop at high heights (up to 3000 m).
The low and moderate temperature thermochronology methods will include (U-Th/He) thermochronology on apatite and zircon, apatite fission track and Ar/Ar thermochronology on various minerals (biotite, amphibole). The crystallization age of plutonic rocks being essential for the definition of temperature-time paths, the use of U/Pb method might be required when the crystallisation age is unknown.
It is required that the applicant owns strong skills in magmatic petrology as well in low and moderate temperature thermochronology. Additional skills in numerical modeling are also desirable but not mandatory. This project is a collaborative project between GEOPS (Paris Sud University), LSCE (CEA), ISTerre (Joseph Fourier University) and the University of Tokyo (Japan).