STYLES OF CRUSTAL DEFORMATION IN COMPRESSIONAL OROGENS CAUSED BY SUBDUCTION OF THE UNDERLYING LITHOSPHERE

Crustal-scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere. The plane-strain finite-element model results provide indications of the styles of deformation to be expected in small compressional orogens. In particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other. The crust is modelled using Coulomb plastic (frictional) and thermally-activated power-law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included. Results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary, surface denudation, partial and total subduction of the crust, and compositional layering are examined. The results show the development of inclined step-up shear zones, which are a consequence of conjugate thrusting in regions of Coulomb-controlled rheology. These zones link to sub-horizontal shear zones which occur where composition and temperature render the crust viscously weak. The model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation. Finally, we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens.