SYNCONVERGENT EXTENSION ASSOCIATE WITH A RETREATING SUBDUCTION ZONE, NORTHERN APENNINES, ITALY

The Apennines provide an excellent onland setting for the study of orogen-parallel extension within an active collision zone. The RETREAT project (2002-2007) is a multidisciplinary investigation, including tectonic geomorphology, structural geology, thermochronology, and GPS geodesy to reconstruct the orogenic wedge, and passive-source seismology to image mantle flow beneath the wedge. The Apennines wedge was initiated at ~28 Ma when the Adratic-Apulia-African plate started to subduct northward beneath southern France. The wedge has migrated counterclockwise around an Euler pole in the western Alps, in concert with rollback of the subducting plate. Tomographic images indicate a total of ~450 km of subduction beneath the northern Apennines. At ~15 Ma, the Apennines wedge started to override the western passive margin ramp of the Adratic platform. This event marked the transition from oceanic subduction to continental collision, but continued subduction has managed to consume ~225 km of continental plate.

Many geologists hold that subduction stopped in the northern Apennines at ~1 Ma, and that modern horizontal extension is due to post-orogenic collapse. Our work provides a very different view: 1) Uplifted Quaternary fluvial terraces indicate that subduction remains active today, at rates similar to the past. 2) GPS measurements show clear evidence for active shortening and extension, but at rates less than those indicated by long-term geomorphic evidence. We speculate that the modern surface velocities are reduced due to an overlap in the elastic strain fields associated with the close proximity of active thrust and normal faults. 3) Structural reconstructions indicate accretion has been dominated by underplating, which provides one explanation for the synconvergent extension. 4) Stratigraphic evidence indicates that, following collision, the Apennines wedge was reduced in relief by a factor of four, an observation we have yet to fully account for. 5) Slab rollback should excite a divergent flow field within the asthenosphere that overlies the subducting slab. Our ongoing passive seismic experiment is designed to test for the presence of this flow field and to determine the location of the divergence point relative to the extensional front observed at the surface near the crest of the modern Apennines.