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Episodes 2015; 38(4): 266-282

Published online December 1, 2015

https://doi.org/10.18814/epiiugs/2015/v38i4/82421

Copyright © International Union of Geological Sciences.

Pre-Alpine extensional tectonics of a peridotitelocalized oceanic core complex in the Late Jurassic, high-pressure Monviso ophiolite (Western Alps)

Gianni Balestro1, Andrea Festa1*, Yildirim Dilek2, and Paola Tartarotti3

1Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso, 35, 10125 - Torino, Italy
2Department of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056, USA
3Dipartimento di Scienze della Terra, Università di Milano, Via Mangiagalli, 34, 20133 - Milano, Italy
*Corresponding author: Andrea Festa. E-mail: andrea.festa@unito.it

Correspondence to:* E-mail: andrea.festa@unito.it

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed, eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere. The recent recognition of a pre- Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex, which developed during the initial stages of the tectonic evolution of the Alpine Tethys. The NNW-striking, 20–25-km-long shear zone (Baracun Shear Zone) contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni–Cr concentrations and high Cl contents. Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds, providing a critical age constraint for the timing of its formation. Metabasaltic–metasedimentary rocks in the hanging wall increase in thickness away from the shear zone, characteristic of syn-extensional rock sequences in supradetachment basins. A Lower Cretaceous post-extensional sedimentary sequence unconformably cover the synextensional strata, the detachment shear zone, and the ophiolitic footwall, establishing a strong structural evidence for the intraoceanic, seafloor spreading origin of the tectonic fabric of the Monviso ophiolite, prior to the onset of subduction zone tectonics in the Alpine Tethys. The Monviso ophiolite and the Baracun Shear Zone represent a peridotite-localized oceanic core complex, which survived both the subduction and continental collision tectonic stages of the Alpine orogeny. Intraoceanic detachment faults and oceanic core complexes may play a significant role in subduction initiation, and hence their recognition in orogenic belts is an important step in reconstructing the record of ocean basin collapse and closure.