Petrographic interpretation of a complex seismic crust–mantle transition in the central-eastern Alps

Abstract

A recent reinterpretation of the existing DSS profiles [Scarascia and Cassinis, Tectonophysics 271 (1997) 157–188] led to a reconstruction of the Moho boundary and of the lower crust, in the central-eastern Alpine sector, where the European and the Adriatic domains are deeply indented. In that work a complex crust–mantle transition, composed of alternating low- and high-velocity layers, was observed in the deepest bottom of the European crust. In the same work it was confirmed that the European lower crust beneath the Alpine Range is characterised by a very low seismic velocity, while the velocity of the Adriatic lower crust reaches the average values found in collisional belts. The interpretation of these features has been performed employing a data-base of ultrasonic seismic measurements. The Ivrea–Verbano Zone has been assumed as an analogue of crust–mantle transition. A statistical approach was used to associate the bodies of the interpreted seismic models to the lithotypes. A petrological model is attempted in order to suggest an explanation of the observed phenomena, assuming physical and compositional constraints appropriate to the Alpine collisional belt. Cumulitic peridotites and pyroxenites lenses in a gabbroic host rock fit the observed seismic structure of the crust–mantle transition. The upper mantle lies below a transition zone and is characterised by a velocity of 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 European lower crust is discussed but still remains an open subject.