The Alpine cycle: Modelling orogenic wedge formation from generation of hyper-extended passive margins and forced subduction to continent-continent collision

International audienceMany orogens, such as the Western Alps, are characterised by the collision of hyper-extended margins. Before theonset of subduction these margins may have undergone several phases of deformation including rifting periodsand periods without significant deformation in which the margins have been thermally relaxed. In consequence,modelling orogenic wedge formation in a self-consistent manner with application to the Western Alps requiresmodelling over large time spans (200 Ma) and sufficiently high resolution of the distinct deformation phases inboth space and time.We perform high resolution long-term (>200 Ma) 2D thermo-mechanical numerical simulations includingthe lithosphere and upper mantle down to 660 km depth to study the dynamics of the Alpine cycle. Modellingof this cycle is subdivided into the following four stages: (I) in a slow spreading rift system of 60 Ma durationa 400 km wide basin is generated which contains exhumed lithospheric mantle and is bounded by regions ofhyper-extended continental crust. (II) the modelled basin and hyper-extended passive margin system is thermallyrelaxed for 70 Ma. During this stage, the lithosphere is neither compressed nor extended. No spontaneoussubduction initiation due to the densification of the cooling exhumed mantle occurs in the models. (III) the evolvedsystem is shortened and a one-sided, forced subduction is initiated by thermal softening within the proximal partof the passive margin without prescribing any weak zone. (IV) after ca. 60 Ma of convergence the basin is closedand an orogenic wedge forms during continent-continent collision.We first quantify the impact of the initial geometric configuration from the onset of rifting on the Alpinecycle by varying the mechanical strength of the crust, being either an alternating sequence of horizontal mechanicallystrong and weak layers, or a homogeneous crust. Second, we investigate the control of lithospheric mantlecharacteristics on the Alpine cycle by (I) varying the rheological flow law of the mantle, (II) computing effectivedensities by either using a linearized equation of state, or a precomputed look-up table using Perple_X and (III)investigating the impact of serpentinisation of the lithospheric mantle exhumed in the basin by replacing theolivine rheology of the topmost 7 km by an antigorite rheology.First results show that rheology and density structure of the mantle lithosphere have a strong impact on theestablishment and intensity of upper mantle convection. When convection cells are established a long-termthermal field is stabilised so that Moho temperatures do not decrease during the 130 Ma of extension and thermalrelaxation. The geothermal profile across the passive margins is a crucial parameter for their effective strength andfor subsequent forced subduction initiation by thermal softening. The flow law of the mantle lithosphere hencecontrols upper mantle convection which controls the pre-convergence thermal state which controls subductioninitiation and the style of deformation within the resulting orogenic wedge