DEEP-STRUCTURE OF THE CENTRAL ALPS IN THE LIGHT OF RECENT SEISMIC DATA

The deep seismic reflection traverses across the Central Alps (NFP 20, ECORS-CROP) contain a new set of data on the lower crust which has be en interpreted in different ways. One currently fashionable model depi cts the European lower crust (ELC) as gently dipping below the Adriati c crust. However, this model requires that an observed sharp terminati on of the ELC under the internal border of the External Massifs is due to the nontransmission of organized seismic energy through the comple x upper crust. This explanation is questioned as other reflections in this and similarly complex areas are recorded, and as the same sharp t ermination of the ELC under the internal border of the External Massif s is observed on all seismic lines for a length of 300 km. A tectonic - metamorphic cause appears to more satisfactorily explain the obeserv ations, and therefore an alternative model combining surface and deep geophysical data is proposed. It consists of three mutually largely de coupled tectonic levels. (1) The shallow obducted part or lid, bounded at its base by the combined Late Miocene Jura and Lombardic basal thr usts. Estimates of shortening based on balanced sections are at least about 100 km. (2) The intermediate level between the brittle - ductile transition and the top of the subducted mantle. It contains a stack o f lower crust imbrications (with a minor admixture of upper mantle) ac commodated by ('inducted into') the ductile middle crust. Estimates of shortening based on area balancing are again of the order of slightly more than 100 km. (3) The subducted upper mantle, for which there are no reflection data. In the Central Alps the Late Miocene phase was de xtrally transpressive, producing flower structures at the shallow leve l (External Massifs); the stacks of lower crust imbrications at the in termediate level may be the equivalent of the External Massifs at that level. Inverted flower structures of the subducted mantle are possibl e, but no detailed data are available.