Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines

In this paper we investigate the forces possibly active in the area of the northern Apennines by means of two-dimensional finite element modelling ass uming a viscoelastic rheology. The forces included in the models are relate d to continental convergence between Africa and Eurasia, to negative buoyan cy of the subducted Adriatic lithosphere and to positive buoyancy of anomal ously hot mantle material underneath Tuscany. Model-predicted stress distri bution is compared with seismotectonic data available for the area of the n orthern Apennines and with earthquakes distribution. Our results indicate t hat continental convergence cannot be the only mechanism acting in the stud y area since it causes insignificant subcrustal stresses, in disagreement w ith intermediate seismicity observed under the Apennines, Asthenospheric up welling under Tuscany (back-arc region) is shown to play a crucial role in the present day dynamics of the Tyrrhenian-Apennines. Positive buoyancy cau ses an upwards and eastwards flow that generates extensional bending stress es in Tuscany and at the top of the slab under the Apennines. Horizontal pu shing of the slab is proposed as an alternative mechanism to slab pull to g enerate subcrustal stresses. Activation of slab pull does not result in a s ignificant change in orientation of the principal stress axes at shallow de pths with respect to the model in which only asthenospheric diapirism is ac tive. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observa tions. Both models successfully reproduce extension in Tuscany and compress ion in the outer margin of the Apenninic belt, but fail to reproduce extens ion along the inner portion of the chain. Our modelling highlights the impo rtance of better constraining the lateral extent of the asthenospheric diap ir in order to find out whether the extension and tectonic uplift in the Ap ennines is caused by asthenospheric upwelling as in the area of Tuscany. (C ) 1999 Elsevier Science B.V. All rights reserved.