Effects of buoyancy and mechanical layering on collisional deformation of continental lithosphere: Results from physical modeling

We use two suites of lithospheric-scale physical experiments to investigate the manner in which deformation of the continental lithosphere is affected by both (1) variations of lithospheric density (quantified by the net buoyant mass per area in the lithospheric mantle layer, M-B), and (2) the degree of coupling between the crust and lithospheric mantle (characterized by a modified Ampferer ratio, Am). The dynamics of the experiments can be characterized with a Rayleigh-Taylor type ratio, phi(CLM). Models with a positively buoyant lithospheric mantle layer (M-B > 0 and phi(CLM)> 0) result in distributed root formation and a wide deformation belt. In contrast, models with a negatively buoyant lithospheric mantle layer strongly coupled to the crust (M-B < 0, 0 > phi(CLM) > approximate to - 0.2, and Am > approximate to 10(-3)) exhibit localized roots and narrow deformation belts. Syncollisional delamination of the model lithospheric mantle layer and a wide deformation belt is exhibited in models with negatively buoyant lithospheric mantle layers weakly coupled to the crust (M-B < 0, phi(CLM) < 0, and Am < approximate to 10-3). Syncollisional delamination of the continental lithosphere may initiate due to buoyancy contrasts within the continental plate, instead of resulting from wedging by the opposing plate. Rayleigh-Taylor instabilities dominate the style of deformation in models with a negatively buoyant lithospheric mantle layer strongly coupled to the crust and a slow convergence rate (M-B < 0 and phi(CLM) > approximate to - 0.2). The degree of coupling (Am) between the model crust and lithospheric mantle plays a lesser role in both the style of lower-lithospheric deformation and the width of the crustal deformed zone with increasing density of the lithospheric mantle layer. (c) 2005 Elsevier B.V. All rights reserved.