[1] The tendency of the lower crust to flow is controlled by its rheology.
Ductile flow in the lower crust is assumed to be controlled by the deformation of quartz and/or plagioclase feldspar as its composition is thought to be granodioritic to dioritic.
With normal thickness continental crust and a normal geothermal gradient, the lower crust, below the brittle–ductile transition zone, exhibits ductile flow behaviour under geological strain rates.
Factors that can vary this behaviour include: water content, thickness, heat flow and strain-rate.
In some areas of continental collision, the lower part of the thickened crust that results is interpreted to flow laterally, such as in the Tibetan plateau,[2] and the Altiplano in the Bolivian Andes.