Partial melting and flow of orogens

A comparison of large orogenic belts formed at convergent plate boundaries indicates that orogenic evolution comprises a period of crustal thickening followed by the formation of a continental plateau and commonly syn- to post-convergence extension and thinning of the previously thickened crust. The instantaneous and finite strain patterns recorded in orogenic belts do not appear to be linked in a simple way to plate kinematics. These patterns suggest that deformation of continental crust is not solely controlled by horizontal compression due to plate convergence and that gravity-driven flow is likely to play a significant role on the deformation of thickened crust. Geophysical data in southern Tibet, an actively convergent orogen, and geologic data of the exhumed mid-crustal core of eroded orogens reveals that orogenic crust is affected by widespread partial melting. Partial melting of thermally mature zones of thickened crust results in the generation of a layer of low-viscosity rocks that may affect the behaviour of the orogenic belt. In particular, the presence of this layer favours mechanical decoupling between the subducting plate and the overlying thickened orogenic crust, resulting in flow of the crust driven by contrasts in gravitational potential energy.