Structure, timing and rates of deformation, and the aspect ratio of the external zones of the Umbria--Marche thrust system (UMTS) have been investigated with the aim of comparing features and processes occurring during the growth of a tectonic wedge with those inferred from mechanically based model predictions. The results of the UMTS test seem to support the idea that most of the characteristic features of a thrust system may be considered as the result of the incremental, transient increase of the pore fluid pressure (Pf) as a consequence of the amount of shortening of a crustal section. The strong dependence of Pf on strain rates and on the thickness of the rock mass involved in the deformation may also help to explain local and/or regional variations in the aspect ratio of the deformed-state profiles of different fold and thrust belts. ¿ American Geophysical Union 1995