The role of subduction in the evolution of the Apennines foreland basin

Seismic tomographic images of the Mediterranean region lead to the formulation of an hypothesis involving detachment (or alternatively break-off) of subducted lithosphere, propagating horizontally in the along strike direction: lateral migration of slab detachment [Wortel, M.J.R., Spakman, W., (1992). Structure and dynamics of subducted lithosphere in the Mediterranean region. Proc. K. Ned. Akad. Wet. 95, 325-347]. On the basis of this hypothesis variations in vertical loads are expected along strike of the convergent plate margin involved. We investigate whether such variations in vertical loads can be inferred from the Plio-Pleistocene foreland basin along the eastern side of the Apennines. Our aim is to test the hypothesis of a laterally migrating slab detachment beneath Italy. We employ a finite difference method to obtain the flexural downbending of the Adriatic plate resulting from known loads, such as topography and sediments. The unknown slab pull is inferred by constraining the model to fit the horizon that is assumed to reflect flexural downbending in two-dimensional sections perpendicular to the strike of the Apennines. Our method forms a consistent treatment of all loads involved in two-dimensional lithospheric flexure. If we assume that the downgoing Adriatic plate behaves purely elastically, we conclude that there is a trade-off relation between slab pull and elastic plate thickness for sections in north Italy. For these sections gravity anomaly data cannot constrain a best elastic model. If we make the more realistic assumption of a continental lithosphere with a depth-dependent rheology, we obtain a good match for sections in the north. In the south, however, we find that a match cannot be obtained. This may imply that the data reflect more than purely flexural processes - probably thrusting. The conclusion is that a trend in vertical loads along strike of the Apennines cannot be resolved from Plio-Pleistocene deflection data. Therefore, the hypothesis of lateral migration of slab detachment can neither be shown to be consistent nor inconsistent with the observations.