Lithospheric pressure-depth relationship in compressive regions of thickened crust

We investigate the pressure distribution with depth in regions undergoing h orizontal shortening and experiencing crustal thickening both analytically and numerically. Our results show that, in a convergent tectonic setting, p ressure 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 stre sses and the flexural vertical loads, the latter generated by the deflectio n of the upper crust and of the mantle because of the presence of topograph ic relief and a root, respectively. The contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle, where ro cks are thought to deform brittlely. In these domains, pressure gradients t wice lithostatic can be achieved. The contribution of horizontal stresses i s less important in the ductile domains as differential stresses are progre ssively 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 f or rocks of the weaker lower crust. As a result of the combination of the t wo mechanisms, the pressure gradient varies vertically through the lithosph ere, ranging from negative (inverted) gradients to gradients up to several times the lithostatic gradient. The pressure values range from one to two t imes the lithostatic values (1 rho gz to 2 rho gz).