The development and behaviour of low-angle normal faults during Cenozoic asymmetric extension in the Northern Apennines, Italy.

Movements on low-angle normal faults (LANF) are not predicted by traditional Anderson–Byerlee frictional fault mechanics. Our investigations centre on three normal fault systems active at distinct times during the regional extension of the Northern Apennines, with each showing different degrees of crustal exhumation. These are (from E to W): the Altotiberina fault system, Umbria; the Radicofani fault system, Tuscan mainland; and the Zuccale fault system, Isle of Elba. Regional extension has been a continuous process since middle Miocene, migrating progressively from west to east, with deformation accommodated by a set of E to E-dipping LANF and more steeply SW-dipping antithetic structures. The LANF acted as regional detachments, accommodating a majority of the extension in the Northern Apennines, with individual faults exhibiting several kilometres of displacement. Regionally, the stress field has been characterised by a vertical s1 and a NE–SW- (Tuscan mainland and Umbria) to E–W (Elba)-trending s3. Where exposed at the surface, the main LANF detachments possess a well-developed fault core of foliated fault rocks in which fluids have played a key role in weakening due to reaction softening and the onset of stress-induced solution–precipitation mechanisms. We speculate that the E-dipping normal faults initiated with relatively low-angles of dip due to differential drag generated by mantle flow following slab retreat and roll-back beneath the Apennine chain.