THE EVOLUTION OF DEFORMATION AND TOPOGRAPHY OF HIGH ELEVATED PLATEAUS.1. MODEL, NUMERICAL-ANALYSIS, AND GENERAL RESULTS

A temperature dependent viscoplastic flow model of continental lithosp here is used to investigate the evolution of deformation and topograph y of high elevated plateaus. Such plateaus are products of both contin ent-continent collision (Tibetan Plateau) and ocean-continent collisio n (the Altiplano) and develop in the overriding continental plate. In this study we emphasize the mechanically simpler case of oceanic colli sion, because it does not involve mass transfer between the two plates . The lithosphere is deformed in response to tectonic and buoyancy for ces. The tectonic forces arise from subduction of an oceanic plate (or underthrusting of continental lithosphere) that horizontally indents and vertically shears the overriding lithosphere. The buoyancy forces arise in response to horizontal density variations and tend to relax e xisting topography or thick crust. The time evolution of the deformati on and topography is investigated using a finite element technique tha t solves for the flow field in the overriding lithosphere. The model p roduces dynamically supported near-trench topography and inland mounta in topography that is isostatically supported by a thick crust. A fini te region of localized deformation, thick crust, and high topography d evelops only if the model includes a horizontal thermal perturbation o r an initially thick crust; however, only thermally perturbed lithosph ere generates a plateau topography. The shape and size of the calculat ed plateau depend on the wavelength of the thermal perturbation, Grash of number, and density contrast between the crust and mantle. The time evolution of the deformation shows a significant change in the deform ation pattern as the high elevated plateau evolves. During early stage s, compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone. At later stages, as the crust thick ens, buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland. This migration does not affect the location of the mantle defo rmation, which remains in the thermally weak region, but it is accompa nied by a significant shear deformation in the weak lower crust. The s eparation of the crustal locus from the mantle locus of deformation em phasizes the importance of vertically dependent deformation in the for mation of high elevated plateaus. This demonstrates the limitations of models that ignore changes of deformation with depth, such as plane s tress or thin sheet models.