The dip of the foreland monocline in the Alps and Apennines

The foreland monocline dips underneath thrust belts and accretionary wedges , both in oceanic and continental subduction zones. We present new data on the dip of the monocline in the frontal part of two orogens, the Alps and t he Apennines. There is an overall difference between the dip of the relativ e monoclines, and there is also a strong lateral variation along both arcs. In the Alps, the regional dip varies between 0 degrees in the remote forel and, to an average of 2-3 degrees at the front of the thrust belt below the foredeep, to about 5 degrees beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. The regional dip of the monocline in the Apennines has an average of 4-5 degrees at the front of t he thrust belt below the foredeep, to about 10 degrees beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge . There are areas where the dip exceeds 20 degrees. The Apennines though to pographically lower than the Alps present higher monocline dips and a deepe r foredeep. Moreover, there are variations in the dip of the monocline movi ng along the strike of the two belts: the low values coincide with Permian- Mesozoic inherited horsts, whereas the steeper values correspond to basinal areas, and they usually match the salients of the thrust belt front. Withi n the salients the distance between thrust ramps increases. Therefore, ther e are two orders of mean values of the dip of the foreland monocline, the f irst at the orogen scale (more than 1000 km wavelength), the second at the regional scale (100-200 km wavelength) within the single orogen. Lateral va riations in the lithospheric buoyancy due to the inherited Mesozoic stretch ing may explain the second order variations in foreland dip, but not the fi rst order mean values which seem to be more sensitive to the geographic pol arity of the subduction rather than to the lithospheric composition which i s rather similar in the Alpine and in the central-northern Apennines slabs. (C) 2000 Elsevier Science B.V. All rights reserved.