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

Crustal-scale deformation is calculated for models in which the drivin g mechanism corresponds to the asymmetric detachment and underthrustin g of the underlying mantle lithosphere. The plane-strain finite-elemen t model results provide indications of the styles of deformation to be expected in small compressional orogens. In particular these styles o ccur where shortening of the mantle lithosphere is achieved by the nea rly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other. The crust is modelled using Coulomb p lastic (frictional) and thermally-activated power-law viscous rheologi es and the effects of compositional layering and variable geothermal g radients are included. Results are presented for a range of models in which the strength of the coupling between the model crust and its bas al boundary, surface denudation, partial and total subduction of the c rust, and compositional layering are examined. The results show the de velopment of inclined step-up shear zones, which are a consequence of conjugate thrusting in regions of Coulomb-controlled rheology. These z ones link to sub-horizontal shear zones which occur where composition and temperature render the crust viscously weak. The model strain fiel ds 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 fabr ic can be recognized in observations from small compressional orogens.