Mechanical decoupling of high-pressure crustal units during continental subduction

The mechanics of the transition from continental subduction toward upper crustal nappe stacking is still poorly understood and is studied here through a 2D thermal and strength numerical modeling of a subducted passive margin. Geological observations in the core of most mountain belts show the piling up of several HP-LT upper crustal units that are most likely related to the detachment of upper crustal units from the subducted continental margin and to the subsequent stacking of the detached units at depths. The Adula unit (Lepontine Dome, Central Alps, Switzerland) is a long and thin upper crustal unit and is used here as a natural case-study as it provides a well-documented example of these units. 2D thermal modeling shows that two steps, successive in time, characterized the burial history of the passive margin undergoing continental subduction: 1-an increase in the margin strength due to an increase in the confining pressure during the first million years of the margin subduction and 2-the progressive heating of the subducted margin from the overlying lithosphere leads to a decrease in the margin strength due to thermal weakening, which progressively counter-balances the increase in confining pressure. Two strength gradients develop within the subducted lithosphere: 1-along the slab, the strength decreases with increasing burial depth and 2-perpendicular to the slab, the strength increases with depth due to an inverse temperature distribution. The detachment of HP-LT continental units from the subducted margin could occur when the slab strength becomes lower than the applied net stress. This allows the detachment of ductile weakened thin and long upper-crustal units. The thickness and length of the detached crustal units are controlled by the following parameters, in order of their importance: subduction dip angle, crustal rheology, mantle heat flux and subduction velocity. The comparison of our model results with the geometry and PT conditions of the Adula unit yields an estimate of the Alpine subduction dip angle at the time of deformation and metamorphism.