The structure of the Bracco Ophiolite complex (Ligurian Apennines, Italy); a change from Alpine to Apennine polarity

• Published in: Journal of the Geological Society Vol. 146; no. 6; pp. 933 - 944
• Main Authors: Hoogerduijn Strating, E. H, Van Wamel, W. A
• Format: Journal Article
• Language: English
• Published: London Geological Society of London 01-11-1989; The Geological Society of London; Geological Society
• Subjects: Apennines; Bracco Ophiolite; Canetolo Nappe; Cenozoic; Cretaceous; Earth sciences; Earth, ocean, space; Europe; Exact sciences and technology; Italy; Lavagua Nappe; Liguria Italy; Ligurian Apennines; lithofacies; Melange Nappe; Mesozoic; Paleogene; sedimentary rocks; Southern Europe; stratigraphy; structural geology; tectonics; Tectonics. Structural geology. Plate tectonics; tectonostratigraphic units; Tertiary; Upper Cretaceous; Italy, Liguria, Genova, Lavagna; Mesozoic; Paleogene; Alpine Orogeny; Polyphase processes; Apennines; Nappes; Europe; Tertiary; Paleocene; Eocene; Extension faults; Subduction; Thrust fault; Italy; Models; Upper Cretaceous; Gravity sliding; Ligurian Apennines; Folds; Uplifts; Ophiolite
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/gsjgs.146.6.0933

The Bracco ophiolite complex, exposed in the area studied as the Bracco Subunit of the Melange Nappe, is tectonically intercalated between the underlying Canetolo Nappe and the overlying Lavagna Nappe. Detailed structural analysis and mapping in these units reveals a multi-phase deformation history. In the Bracco Subunit and the Lavagna Nappe, this history is characterized by a change, from SW (i.e. ‘Alpine’) directed folding of the ophiolitic basement and its sedimentary cover, to NE (i.e. ‘Apennine’) directed folds and thrusts. The Canetolo Nappe only shows Apennine vergent structures.
The Alpine vergent structures are considered to be developed during the Late Cretaceous to Palaeocene in an accretionary wedge associated with an east-dipping subduction zone. The first Apennine directed structures are low-angle extensional faults and cascading folds, indicating that the change to Apennine polarity resulted from gravitational collapse of the accretionary wedge during the Palaeocene to Eocene. Gravitational instability is considered to have resulted from a drastic decrease of subduction rate during the Palaeocene, which has led to extension and uplift of the wedge. The rapid decompression under isothermal to slightly decreasing temperature conditions of the high-pressure terrains in the Voltri Massif concurs well with this tectonic scenario.