Decoupling along plate boundaries: key variable controlling the mode of deformation and the geometry of collisional mountain belts

The consequences of decoupling between weak orogenic wedges and strong adjacent foreland plates are investigated by means of lithospheric-scale analogue modeling. Decoupling is implemented in the three-layer models by lubrication of the inclined boundary between a strong foreland and a weak orogenic wedge. Plate boundaries are orthogonal to the convergence direction. Experimental results show that strong decoupling between the foreland and the orogenic wedge leads to underthrusting of the former underneath the orogenic wedge and deformation of the orogenic wedge by folding, shearing, and minor backthrusting. Shortening is mainly taken up along the main overthrust, the decoupled boundary, and within the orogenic wedge, leaving the indenter devoid of deformation. In contrast, strong coupling between the foreland and the orogenic wedge favors buckling, involving both the weak zone and the strong plates. The results of these end-member models have implications for collision zones, for example, the Eastern Alps in Europe, such that the switch from localized deformation within the orogenic wedge during the Oligocene-middle Miocene to orogen-scale uplift and deformation during the late Miocene-Pliocene involving the foreland and indenter plates, respectively, is interpreted as reflecting a change from a decoupled to a coupled system.