PETROGRAPHIC INTERPRETATION OF A COMPLEX SEISMIC CRUST-MANTLE TRANSITION IN THE CENTRAL-EASTERN ALPS

A recent reinterpretation of the existing DSS profiles [Scarascia and Cassinis, Tectonophysics 271 (1997) 157-188] led to a reconstruction o f the Moho boundary and of the lower crust, in the central-eastern Alp ine sector, where the European and the Adriatic domains are deeply ind ented. In that work a complex crust-mantle transition, composed of alt ernating low- and high-velocity layers, was observed in the deepest bo ttom of the European crust. In the same work it was confirmed that the European lower crust beneath the Alpine Range is characterised by a v ery low seismic velocity, while the velocity of the Adriatic lower cru st reaches the average values found in collisional belts. The interpre tation of these features has been performed employing a data-base of u ltrasonic seismic measurements. The Ivrea-Verbano Zone has been assume d as an analogue of crust-mantle transition. A statistical approach wa s used to associate the bodies of the interpreted seismic models to th e lithotypes. A petrological model is attempted in order to suggest an explanation of the observed phenomena, assuming physical and composit ional constraints appropriate to the Alpine collisional belt. Cumuliti c peridotites and pyroxenites lenses in a gabbroic host rock fit the o bserved seismic structure of the crust-mantle transition. The upper ma ntle lies below a transition zone and is characterised by a velocity o f 8.2 km/s, detected by P-n phases. In our study it is interpreted as a spinel-to-garnet peridotite. The anomalous low velocity of the Europ ean lower crust is discussed but still remains an open subject. (C) 19 98 Elsevier Science B.V. All rights reserved.