Surface motions at convergent plate margins: the Apennines

The Adriatic Sea lies between Italy and former Yugoslavia. Lithosphere of the plate beneath this sea has been subducting in a westward direction beneath Italy. This subduction has occurred during several millions of years and has resulted in the presence of subducted material to a depth of at least a few hundreds of kilometers. Eastward roll-back of the subduction zone opened the Tyrrhenian back-arc basin between Italy and Corsica/Sardinia. Some regional tomographic images of the subducted Adriatic lithosphere suggest that it is detached at depth. Wortel and Spakman (1992, Proc. KNAW 95) hypothesize that this is the result of propagation of a horizontal tear in the subducted lithosphere southwards along strike of the convergent plate margin. The resulting re-distribution of slab pull loads is expected to change vertical and horizontal motions at the surface.

topographic map of italy

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

On the basis of the hypothesis of lateral migration of slab detachment variations in vertical loads are expected along strike of the Italian convergent plate margin. We investigated whether such variations in vertical loads could be resolved from the Plio-Pleistocene foreland basin along the eastern side of the Apennines. We employ a finite difference method to obtain the flexural downbending of the Adriatic plate resulting from know loads, such as topography and sediments. The unknown slab pull is inferred by constraining the model to fit the base of Pliocene level in two-dimensional sections perpendicular to the strike of the Apennines. Both a purely elastic and a depth-dependent rheology are used. For the case of a purely elastic rheology, we conclude that there is a trade-off relation between slab pull and elastic thickness for sections in North Italy. For these sections gravity anomaly data cannot constrain a best elastic model. If we take into account the continental character of the Adriatic lithosphere through a depth-dependent rheology, we obtain a good match for sections in the north (see figure for an example). 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 inferred from Plio-Pleistocene deflection data.

section north italy

The figure shows a section through north Italy, approximately on a line from Corsica to Istria. The red dots are the base of Pliocene level. The green line is the calculated deflection for a depth-dependent rheology. The lithosphere is loaded by the topography of the Apennines, the sediments in the foreland basin and a deep load associated with the subducted plate.

Can slab detachment account for Late Miocene uplift of Northern Italy?

When slab detachment occurs, it is expected to lead to uplift of the Earth's surface. For Northern Italy, observations have been reported of a Late Miocene uplift of possibly a few kilometres. We simulate the evolution of Northern Italy with a two-dimensional finite element model (TECTON) as close as geological constraints allow. We find that slab detachment in this region would lead to significant surface uplifts which are within the range of reported observations.

slab detachment

This figure shows an example of surface uplift resulting from slab detachment. Below is the finite element model, above the (exaggerated) vertical surface uplift (Buiter, 1999).