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Theses

La déformation de la plaque supérieure dans les zones de subduction en retrait

Abstract : The Earth’s surface is constantly reshaped by the tectonic plate motion, which is mainly driven by subduction of plates into the deeper mantle. Subduction trenches are also mobile plate boundaries, and are observed to retreat towards the subducting plate or advance towards the upper plate over geological time. Trench retreat has been historically thought to cause extension in the upper plate above the subducting slab. However, natural subduction systems show several examples of retreating trenches that are associated with upper-plate compression. This thesis explores upper plate (back-arc) deformation in retreating subduction systems. Three techniques are used: large-scale numerical models addressing physical processes; seismic profiles in the Central Aegean addressing basin-scale fault patterns; and field-scale observations clarifying fault kinematics in the Central Aegean. The large-scale thermo-mechanical models deal with viscous deformation of the upper plate, and investigate the relationship between slab pull, slab rollback, trench retreat and upper plate deformation at scales of 100 to 1000 km. They show that asthenosphere flows below the plates (100-200 km depth) can control both trench retreat and upper plate deformation. The type of deformation in the upper plate also depends on the plate’s far-field conditions: if the plate is free to move, deformation tends to be compressive, but a fixed upper plate shows extension. The latter is comparable to the Aegean region, an upper plate exhibiting extension above a narrow, retreating subduction zone. Related extensional structures in the central Aegean have been analysed from seismic and field data, revealing co-existing normal, oblique and strike slip faults. These features reflect a combination of rollback-related extension and extrusion-related strike slip activity. Resulting block rotation and trench retreat re-activate inherited normal faults in oblique-normal slip, while new pure-normal faults are created. We also infer a recent change in stress state possibly related to the slab tear on the western side of the Hellenic slab. Additionally, accelerated trench retreat and upper plate extension are the cause of the Aegean’s high surface heat flow, which makes it potentially suitable for geothermal energy production. As a final perspective on the application of geodynamic research, an assessment of the role of tectonic modelling in predicting geothermal energy potential is presented, using the stretched Aegean upper plate as an example.
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Manar Alsaif. La déformation de la plaque supérieure dans les zones de subduction en retrait. Other. Université Montpellier, 2019. English. ⟨NNT : 2019MONTG026⟩. ⟨tel-02468158⟩

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