EXTENSIONAL FAULTING IN AN INTRAOCEANIC SUBDUCTION COMPLEX - WORKING HYPOTHESIS FOR THE PALEOGENE OF THE ALPS-APENNINE SYSTEM

Within the ophiolite complexes exposed in the Ligurian Alps and Apenni nes (NW Italy) several extensional detachment faults are identified wh ich overprint Late Cretaceous high-pressure metamorphic fabrics develo ped in an intraoceanic subduction system. One of these detachment faul ts is the Sestri-Voltaggio Line, which marks an 8 kbar pressure gap be tween blueschist facies ophiolites and calcareous sediments of the Ses tri-Voltaggio Zone, and underlying eclogite facies ophiolites and meta sediments of the Voltri Massif. Radiometric and stratigraphic constrai nts suggest that extension has occurred during the Late Paleocene to p ossibly Early Eocene. From the Middle Eocene onward compressional tect onics prevailed again, resulting in westerly directed imbrications in the Ligurian Alps and easterly directed folding and thrusting in the L igurian Apennines. A strong correlation is identified between the Afri ca-Europe convergence rate and the stability of the accretionary wedge : i.e. the Paleocene extension coincides with a period of strongly red uced convergence velocities, whilst the Middle Eocene transition to co mpressional tectonics concurs with an acceleration of the convergence rate. This inferred relation, between convergence rate and style of in ternal deformation, matches very closely the results from mechanical m odels addressing the stability of orogenic wedges with a strain-rate-d ependent theology, and provides the basis for a working hypothesis on the Palaeogene evolution of the Alps-Apennine orogenic system.