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The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)

Published online by Cambridge University Press:  20 August 2012

PAOLA MANZOTTI  and
MICHELE ZUCALI
PAOLA MANZOTTI*
Affiliation:
Institut für Geologie, Universität Bern, Baltzerstrasse 1+3, 3012 CH-Bern, Switzerland
MICHELE ZUCALI
Affiliation:
Dipartimento di Scienze della Terra ‘Ardito Desio’, Università degli Studi di Milano, Via Mangiagalli, 34 20133 Milano, Italy CNR-IDPA – Sezione di Milano, Via Mangiagalli 34 20133, Milano, Italy
*
†Author for correspondence: manzotti@geo.unibe.ch
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Abstract

The Valpelline unit is a large slice of continental crust constituting the Austroalpine Dent Blanche nappe (NW Italy). The pre-Alpine evolution of this unit holds important clues about the Palaeozoic crustal structure at the northern margin of the Adria continent, about the history of rifting in the Alpine region, and thus about the thermomechanical conditions that preceded the Alpine convergent evolution. Several stages of the deformation history and of partial re-equilibration were identified, combining meso- and micro-structural analyses with thermobarometry. Reconstructed pre-Alpine P–T–t–d paths demonstrate that the Valpelline unit experienced an early stage at pressures between 4.5 and 6.5 kbar followed by migmatite formation. A subsequent stage reached amphibolite to granulite facies conditions. This stage was associated with the development of the most penetrative fabrics affecting all of the Valpelline lithotypes. The pre-Alpine evolution ended with a weak deformation associated with a local mineral-chemical re-equilibration under greenschist facies conditions at ≈ 4 kbar and T < 450°C. A Permo-Mesozoic lithospheric extension is thought to be responsible for asthenosphere upwelling, thereby causing high temperature metamorphism at medium pressure and widespread partial melting, which led to upper crustal magmatic activity.

Keywords

Valpelline unitpre-Alpine metamorphismAustroalpinePermiangeodynamic evolution
Type
Original Articles
Information
Geological Magazine , Volume 150 , Issue 1 , January 2013 , pp. 153 - 172
Copyright
Copyright © Cambridge University Press 2012

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