The problem of the Moho in the Alps

Abstract

The Tertiary Alps are the product of continental collision, and material balance estimates require the subduction of continental crust and the underlying Moho. The present Moho in the Alpine region consists of three parts: that of the European plate (ME), that of the Adriatic (African) plate (MA), and a new Moho (M2), believed to have developed in the subducted continental crust by high pressure metamorphism at 40–60 km depth. The fate of M2, however, is uncertain. Some of it is apparently destroyed subsequently, as some eclogite facies (and even highest- P coesite-bearing) rocks somehow find their way to the surface, where they form exotic bodies from microscopic to nappe size. On the other hand, the continental Moho exposed in the Ivrea body tells a different story. Here continental granulite facies rocks are intruded by ultramafic and mafic masses. This suggests an origin in an extensional environment, possibly in subduction-related gravitational collapse basins. Such successor basins succeed the basins of the original Tethys that had been largely consumed by the Late Cretaceous. They puncture the 1000–2000 km wide belt of Africa-Europa collisional interaction (e.g. the Aegean, the Tyrrhenian, and the Pannonian basins). Although in some ways similar to the back-arc basins of the Pacific, these mostly intracontinental basins differ in important aspects such as their small size. In these basins asthenospheric upwelling replaces M2 by a new Moho (M3) and the eclogitic rocks underlying M2 by the ultramafic-mafic intrusions of the Ivrea type. Material balance considerations suggest that most of the eclogues remain part of the mantle for some time. On the other hand, recent deep reflection seismic surveys in several places indicate delamination of a part of the lower crust and the uppermost mantle with the formation of imbrications and duplexes, some of which are wedged into the middle crust of the opposite plate. They may be the first stage in the formation of granulite and eclogite nappes.