We investigate the pressure distribution with depth in regions
undergoing horizontal shortening and experiencing crustal thickening
both analytically and numerically. Our results show that, in a
convergent tectonic setting, pressure can be considerably higher than
lithostatic (the pressure resulting from the weight of the overburden).
Increases in pressure with respect to lithostatic conditions result from
both the contribution of horizontal stresses and the flexural vertical
loads, the latter generated by the deflection of the upper crust and of
the mantle because of the presence of topographic relief and a root,
respectively. The contribution of horizontal stresses is particularly
relevant to the upper crust and uppermost mantle, where rocks are
thought to deform brittlely. In these domains, pressure gradients twice
lithostatic can be achieved. The contribution of horizontal stresses is
less important in the ductile domains as differential stresses are
progressively relaxed; nevertheless, the effects are still noteworthy
especially close to the brittle-ductile transition. Flexural vertical
loads generated by the deflection of the upper crust and lithospheric
mantle are relevant for rocks of the weaker lower crust. As a result of
the combination of the two mechanisms, the pressure gradient varies
vertically through the lithosphere, ranging from negative (inverted)
gradients to gradients up to several times the lithostatic gradient. The
pressure values range from one to two times the lithostatic values (1
rho gz to 2 rho gz).