Extension in the Aegean nappe-stacks: Numerical Model and their Geological Validation
Résumé
After mountain building, the crust exhibits complex structure. Especially, thickening achieved by nappe-stacking induces rheological heterogenities at all scales: from the fault scale up to the crustal scale. This process is likely to influence post-orogenic evolution. However, it is generally not considered in numerical models. In this study, we consider the impact of pre-existing thrusts and nappes structures on the mode of post-orogenic extension. We focus on thermomechanical modeling of reactivation of convergent structures inherited from compression. The Aegean domain that experienced extension after the formation of the Hellenides is considered as a natural laboratory. Natural data are used to constrain a priori the geometry and rheology of the models and to validate them a posteriori. Three problems at different scales are considered. Firstly, we model the reactivation of a thrust as a low angle normal fault. Recent studies show that Aegean detachments were active in the brittle field with very shallow dips. These observations are in contradiction with the classical fault mechanics theory. In order to reconcile both point of view, we propose a new model by introducing an elasto-plastic frictional fault gouge that is able to compact. Our models show that plastic strain on badly oriented faults is favored by compaction of the fault gouge. Secondly, we model the formation of the Corinth rift. The Phyllite-Quartizte nappe is introduced in the upper crust as a weak shallow-dipping layer between the Pindos and Tripolitza massive carbonate nappes. The competence contrast between this nappe and its surrounding controls the dynamics of rifting. High competence contrast leads to the formation of crustal-scale planar faults rooting on the brittle ductile transition of the crust and thin-skinned listric faults rooting on the nappe itself. This model is consistent with the observed microseismicity patterns, the asymmetry of the Corinth Gulf, and the kinematics of fault migration within the basin. Thirdly, we model the formation of the Cycladic metamorphic core complexes. The initial setup takes into account the wedge structure of the Hellenic nappe-pile before extension and the rheology of the different nappes. A metamorphic core complex form with detachements rooting in the former thrust with the same dip. Our model recover the main characteristics of the Cycladic metamorphic core complexes: geometry, finite deformation field, mean exhumation velocities. Moreover it allows to reproduce the complex distribution of P-T paths in the metamorphic units. As a result, structures inherited from compression have a major impact on the evolution of post-orogenic extension. Particularly, the former thrusts can act as decollement layers at all levels in the crust. This process is likely to take part in the weakness of orogens when boundary conditions become divergent.