Abstract

Recent geophysical data are combined with older information for an updated picture of Alpine kinematics, using semiquantitative considerations of material balance. In the Alps, a conventional pile of thin peel nappes is disrupted by a central longitudinal system of steep faults and steeply limbed folds, both affecting disharmoniously the entire edifice of otherwise flat-lying nappes. It is particularly from late Tertiary phases of deformation that structures of the second type, the Insubric system with the Insubric line and the belt of late Alpine windows, acquired prominence for the present aspect of the Alps. These late structures formed as a dextrally transpressive intracontinental branch of the Africa-Europa plate boundary, mostly between areas of extension. Early motions in the Late Cretaceous to the late Eocene were probably responsible for at least half of the 300-km dextral displacement required for the palinspastic restoration of the inner West Carpathians and the Dinarides. During Oligocene extension, the late Alpine batholiths intruded in a belt roughly along the Insubric line while deep crustal and lithospheric roots were destroyed. In the latest Oligocene to middle Miocene, the Adriatic plate moved dextrally to the west along the Insubric transfer fault. From its frontal Insubric indenter, a lower crust-upper mantle flake, probably first obducted in the Cretaceous, was detached and wedged into the Penninic nappes of the western Alps ("bird's head" of the Ivrea body). Both the northern and the southern transfer faults of the Insubric indenter were disrupted and inactivated by late Miocene to later events, the Giudicarie and Neo-North Apennine events. Instead of the inactivated Insubric fault, the Windows belt of en echelon folds assumed the role of dextral transpression. All of these successive zones of motion are entangled in the "Ligurian knot," largely hidden under the sea or young sediments. A large part of the pre-Alpine crust and practically the whole mantle were subducted; only in some cases were high-pressure rocks of shallow continental origin re-obducted. The lithospheric root of the Neo-Alps is largely contained in a vertical slab under the Alps, but some parts seem to have been removed and others disharmonically displaced with respect to the surface structures.

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