Some remarks on the mechanism of nappe movement

Abstract

The movement of thrust sheets in orogens constitutes a mechanical problem which has puzzled geologists since such structures were recognized late in the 19th century. Thrust sheets which are long and thin cannot be moved horizontally by a push from behind, because of the high stress necessary to overcome the friction along the sole. If, however, the rocks constituting the thrust sheet are sufficiently “incompetent” to collapse slowly under their own weight, then the volume-conserving, lateral spreading involves a horizontal displacement which may reach several hundred kilometres, depending on the amount of collapse. The dynamics of this process of gravitational collapse and spreading is discussed in terms of simple models.