Extensional faulting in an intraoceanic subduction complex—working hypothesis for the Palaeogene of the Alps-Apennine system

Abstract

Within the ophiolite complexes exposed in the Ligurian Alps and Apennines (NW Italy) several extensional detachment faults are identified which overprint Late Cretaceous high-pressure metamorphic fabrics developed in an intraoceanic subduction system. One of these detachment faults is the Sestri-Voltaggio Line, which marks an 8 kbar pressure gap between blueschist facies ophiolites and calcareous sediments of the Sestri-Voltaggio Zone, and underlying eclogite fades ophiolites and metasediments of the Voltri Massif. Radiometrie and stratigraphie constraints suggest that extension has occurred during the Late Paleocene to possibly Early Eocene. From the Middle Eocene onward compressional tectonics prevailed again, resulting in westerly directed imbrications in the Ligurian Alps and easterly directed folding and thrusting in the Ligurian Apennines. A strong correlation is identified between the Africa-Europe convergence rate and the stability of the accretionary wedge: i.e. the Paleocene extension coincides with a period of strongly reduced convergence velocities, whilst the Middle Eocene transition to compressional tectonics concurs with an acceleration of the convergence rate. This inferred relation, between convergence rate and style of internal deformation, matches very closely the results from mechanical models addressing the stability of orogenic wedges with a strain-rate-dependent rheology, and provides the basis for a working hypothesis on the Palaeogene evolution of the Alps-Apennine orogenic system.