Magnetic and structural constraints for the non-cylindrical evolution of a continental forebulge (Hyblea, Italy).

The Hyblean Plateau in southeastern Sicily, Italy, consists of an isolated and elevated forebulge structure formed since the early Miocene by bending the foreland lithosphere below the advancing Maghrebian thrust-fold belt. The Hyblean forebulge is presently located in front of a reentrant of the Maghrebian belt and is partly surrounded by two orogenic salients. We analyzed magnetic (anisotropy of magnetic susceptibility) and structural (joints, faults, slickenside lineations, and bed attitudes) fabric data from Neogene carbonate rocks exposed atop the Hyblean forebulge. Results showed unidirectional fabrics developed in response to a NW-SE extension at the end of early Miocene and beginning of middle Miocene, and duple-to-multiple fabrics developed in response to major NW-SE and NE-SW extensions from the Langhian onward. We interpreted these results as the evidence for the growth of a doubly-plunging forebulge due to the presence of foreland crustal heterogeneities, which enhanced differential retreating processes of the foreland along the subduction zone and the subsequent formation of orogenic salients and reentrants. A semi-analytical model show that the observed brittle deformations potentially involved a significant thickness of the carbonate sedimentary cover, thus reducing the lithosphere flexural rigidity and enhancing its bending aptitude.