Abstract

In a recent paper authors have examined the central sector of the Ligurian Alps and proposed a kinematic model for the inner Brianconnais units, simply labelling as "Upper Block" all the superposed nappes. Within the same sector, the present work takes into consideration these nappes and, on the basis of their stratigraphy, present position, internal tectono-metamorphic features and after having discussed the arguments that support their assumed original location, proposes a model for their superposition on to the Brianconnais domain. The "Upper Block" is formed by two main groups of units, the presently lower ones, coming from the Paleoeuropean continental margin (Pre-Piedmont domain) and from the adjacent Piedmont-Ligurian oceanic sector (ophiolitic and Helminthoid Flysch nappes). Concerning the Piedmont-Ligurian units, the metamorphic alpine parageneses of the only ophiolitic slab cropping out in the central sector of Ligurian Alps suggest that it was dragged into the subduction zone. At the same time, Flysch--the uppermost, non-metamorphic ensemble--was decolled and transported toward the foreland. The Pre-Piedmont units form, on the whole, a great duplex, bound at its base by a shallow nearly flat surface represented by the boundary with the underlying Brianconnais ensemble, and at the top by the Flysch units. Within the duplex a lower slice of pre-Namurian basement is confined below a major thrust; its metamorphic alpine parageneses indicate that it also entered the subduction zone, though stopping at a relatively shallow depth. The upper slightly metamorphic nappes are mainly composed of post-Namurian successions. Thrusts, progressively activated from the inner sectors and displaying a smoothed flat-ramp geometry, separate the different units. Besides the uppermost Cretaceous-lower Eocene intra-oceanic first step of the evolution, the main phase that produced the nappe stack--not severely affected by later deformations--must have ended before the Lower Oligocene. This, in fact, is the age of the base of the unconformable post-nappe deposits.

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