Passive margin inversion controlled by stability of the mantle lithosphere

Abstract

Contractional deformation of passive continental margins may resemble early stages of induced subduction initiation. Mechanical instabilities are required to permit underthrusting of the oceanic plate. Therefore, the success of developing a new subduction zone will largely depend on the rheology of the mantle lithosphere. In this physical analogue modelling study, a range of mantle viscosities subject to different convergence rates serve as a proxy to simulate contraction of differently aged passive margins with the purpose of describing and quantifying passive margin deformation and mantle stability using the buoyancy number. The experiments illustrate distinct differences in geometry and length-scale of deformation as a function of lithospheric mantle strengths. The results indicate that deformation occurs at passive margins for intermediate strength lithospheric mantle and high strain rate. In contrast, low and high strength mantle lithospheres lead to dominantly intra-oceanic deformation or long wavelength buckling of the entire model, respectively. Our experiments portray an evolution in which early-stage deformation commences at the ocean-continent transition and is controlled by the ductile lower crust of the continent. In the next stage, shear localization through the formation of a decollement within the ductile passive margin crust favors underthrusting of the oceanic lithosphere, leading to a reduction of the area affected by deformation. Prior to underthrusting, the primary response of the lithosphere to compression is by folding at scaled wavelengths of 100–300 km and 500–1000 km, controlled by the strength of the mantle lithosphere.