[1] Although evidence for weak detachments underlying foreland thrust belts exists, very little is known about the lateral variations in effective strength, as well as the geological nature of such variations. Using critical taper analysis, we show that a detailed and systematic measurement of surface slope of the Central European Alps reveals variations in strength parameter F along the detachment, based on the argument that the Alps are close to the critical state. We show that the basal detachment is very weak near the deformation front but strengthens toward the hinterland. Very low F (effective coefficient of friction plus normalized cohesion) values of <0.1 and even 0.05 occur within evaporites and within shales in Triassic (west) or Upper Cretaceous/Lower Tertiary sequences (east) used by the Alpine sole detachment. These very low values in shales-comparably low values are reported from other orogens-are caused partly by slightly elevated pore pressures (k > 0.54) but may also require additional mechanisms of dynamic weakening.

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