Compression of the entire continental lithosphere is considered using two-dimensional numerical models to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The numerical scheme incorporates brittle-elastic-ductile rheology, heat transfer, surface processes, and fault localization. Models are based on the central section of the New Zealand Southern Alps, where continental collision has occurred along the Alpine Fault since about 7 Ma. The results are compared to the surface relief, the GPS convergence velocity, the measured electrical conductivity, and the geometry of the crustal root imaged from seismic velocity measurements. The crustal deformation is characterized by localized uplift at the plate boundary (Alpine Fault) and by two secondary zones of faulting. One is located ~60--80 km east of the Alpine Fault, at the start of upper crust bending (or tilting), and the other is located ~100--130 km east of the Alpine Fault as a result of shear strain propagating to the surface through the ductile lower crust. The observed asymmetric shape of the crustal root is best reproduced for mantle lithosphere strength of the order of 200 MPa and an intermediate rate of strain softening. A lower strength of the mantle lithosphere can produce symmetric thickening, but the amplitude of the crustal root is too small when compared to observations. The observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced. This offset is most likely due to the three dimensionality of oblique collision in the Southern Alps.