Abstract
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 the Apennines. There is an overall difference between the dip of the relative monoclines, and there is also a strong lateral variation along both arcs. In the Alps, the regional dip varies between 0° in the remote foreland, to an average of 2–3° at the front of the thrust belt below the foredeep, to about 5° 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° at the front of the thrust belt below the foredeep, to about 10° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. There are areas where the dip exceeds 20°. The Apennines though topographically lower than the Alps present higher monocline dips and a deeper foredeep. Moreover, there are variations in the dip of the monocline moving 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. Within the salients the distance between thrust ramps increases. Therefore, there are two orders of mean values of the dip of the foreland monocline, the first at the orogen scale (more than 1000 km wavelength), the second at the regional scale (100–200 km wavelength) within the single orogen. Lateral variations in the lithospheric buoyancy due to the inherited Mesozoic stretching may explain the second order variations in foreland dip, but not the first order mean values which seem to be more sensitive to the geographic polarity of the subduction rather than to the lithospheric composition which is rather similar in the Alpine and in the central-northern Apennines slabs.
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