CRUSTAL DEVELOPMENT WITHIN A RETREATING SUBDUCTION SYSTEM: THE HELLENIDES

In retreating subduction systems, where the subduction rate is faster than the convergence rate of the upper and lower plates, the processes by which the upper plate crust is constructed are unclear. From our studies in the Hellenides, which formed above a retreating slab, we conclude that the external part of the Cenozoic Hellenide orogen was constructed at at least two crustal levels. The upper crustal level within the external Hellenides consists of west-vergent thrust sheets emplaced progressively from east to west along a regional decollement from ~ 35 Ma to present. These thrust sheets consist of Mesozoic and Cenozoic strata, sometimes together with a thin slice of their Paleozoic basement that have been stripped from their underlying basement to form the upper crust of the Hellenides. Most of the stripped off units remain unmetamorphosed whereas in some case they are detached at deeper levels and they obtain a tectonometamorphic signature before their exhumation through extensional detachments. The middle and lower crustal layer consists of slices of continental crust detached from the down going slab at depth and accreted below the upper crustal thrust sheets. These accreted slices represent ~ 35% (or less) of the crust belonging to the subducting lithosphere, the remainder of the continental crust appears to be subducted with the slab. As slices are detached from the slab and accreted to the upper plate, the top of the subducting plate steps from above to below the detached crustal slices. Therefore, while the process of slab rollback may be continuous at depth, the episodic detachment of crustal slices guarantees that roll back is step-like in time at the crustal level. As the subducted lithosphere rolled back beneath the Hellenides, it passed progressively from east to west through the region occupied by present-day lower crust and mantle, where there is a well-defined Moho. Any irregularities that may have been present at the base of the accreted slabs have been smoothed by processes that form the Moho and remain unclear. However, within retreating subduction systems lower crustal levels remain only partly unexposed, so that the actual geometry and thus the detailed processes of detachment, accretion and subsequent changes forming the middle and lower crust are unclear.