Subduction and deformation of the continental lithosphere in response to plate and crust-mantle coupling

Abstract

Physical analogue experiments are used to investigate the effect of plate and intra-lithospheric coupling on the efficiency of continental lithosphere subduction and the style of collision. Key parameters investigated in this study are: the degree of plate coupling, regulated by the viscosity ratio of the decoupling zone and the surrounding crust and/or mantle lithosphere; and the depth of decoupling. The experimental results show that subduction of the slab is deepest in cases with strong decoupling at the plate interface and at the level of the lower crust of the downgoing plate, with upper-plate deformation restricted to the area close to the plate contact. In these cases, the strongly asymmetric orogenic wedge is widest, consists of a series of imbricated upper-crustal slices derived from the lower plate, and lacks a retro-wedge. In contrast, a reduced strength contrast across the plate interface, at the depth of either the lithospheric mantle or the ductile crust, leads to a combination of subduction and thickening of the mantle lithosphere in both the upper and the lower plates. The degree of plate coupling determines the efficiency of subduction of continental lithosphere under conditions of collision of neutrally buoyant lithospheres, whereas the vertical position of decoupling horizons within the subducting plate controls the amount of subducted lower crust. Transfer of strain to the upper plate depends critically on (1) the degree of plate coupling, with stronger coupling leading to more deformation, and (2) the presence of decoupling horizons within the upper plate, which act as strain guides to propagate deformation into the upper plate. The experimental results explain the geometry and the sequence of deformation in subduction dominated orogens, such as the Carpathians or the Dinarides, and provide a mechanical basis for the transfer of strain to the upper plate.