A geodynamic framework for interpreting crustal‐scale seismic‐reflectivity patterns in compressional orogens

SUMMARY 2‐D, plane‐strain finite‐element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples. The fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere, with deformation being rooted at the stress singularity where the mantle detaches. Deformation steps up into the overlying crust and spreads laterally according to the rheological properties assumed for the crust. The crust is modelled using plastic (frictional) and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients. Overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen. The amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence. Nevertheless, larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space. The continuum deformation calculated by the finite‐element models is qualitatively converted to equivalent patterns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides. Significant styles of deformation include: (1) coupling between step‐up shears and subhorizontal detachments at one or more levels; (2) localization of upper crustal deformation by subduction of the lower crust; and (3) pinning of deformation by surface denudation. A partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle, the number of rheological layers within the crust, the thickness of subducted lower crust, and the extent of surface denudation are varied systematically. Compressional orogens with a wide range of ages are characterized by crustal‐scale seismic reflectivity similar to patterns of fault/shear zones represented in this catalogue of styles. This similarity suggests that crustalscale seismic‐reflection data can be used to constrain the dynamic processes involved in compressional orogenesis.