Structural and petrological characteristics of a Jurassic detachment fault from the Mont-Blanc massif (Col du Bonhomme area, France)

• Published in: Journal of structural geology Vol. 159; p. 104593
• Main Authors: Dall’Asta, Nicolas, Hoareau, Guilhem, Manatschal, Gianreto, Centrella, Stephen, Denèle, Yoann, Ribes, Charlotte, Kalifi, Amir
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2022
• Subjects: Alpine tethys; Detachment fault; Grès Singuliers; Mont-Blanc massif; Necking domain; Mont-Blanc massif; Grès Singuliers; Necking domain; Detachment fault; Alpine tethys
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2022.104593

Necking zones, located between the proximal and distal domains of passive margins, are characterized by important crustal thinning and the occurrence of extensional detachment faults. Here we provide a petrographic, microstructural and geochemical study of an extensional detachment fault discovered in the Mont-Blanc massif, which was located in the necking zone of the European margin during Early Jurassic rifting. The footwall of the fault consists of basement gneisses, while its hanging wall presents pre-to syn-rift deposits. A careful observation of the basement/cover contact also reveals the presence of a strain gradient affecting the Variscan gneisses, which are progressively deformed to cataclasite and to black gouges at the top basement, and sealed by undeformed Jurassic syn-rift sandstones, the Grès Singuliers, which rework fault rock pebbles. Microscale observations also indicate fluid-assisted deformation, characterized by the total breakdown of feldspars and the coeval formation of white mica, leading to strain softening along the fault plane. The overall observations point toward a rift-related detachment fault responsible for the exhumation of the internal Mont-Blanc basement, an interpretation which challenges previous models of crustal exhumation in the External massifs of the Alps. Finally, the preservation of an undeformed syn-rift sedimentary cover sealing a tectonized top basement makes this area a fossil analogue for detachment faults in necking domains. It demonstrates that preservation of pre-Alpine, rift related structures is possible even in highly deformed areas, and emphasizes the role played by pre-Alpine inheritance on the building of the Alpine Mountain belt. •Description of a Jurassic rift-related normal fault in the internal Mont-Blanc massif.•Syn-rift deposits lying conformably above the exhumed fault plane indicate that the fault was only slightly dipping at the surface, consistent with a detachment fault.•Deformation occurred in the sub-greenschist facies, and led to major crustal exhumation up to the surface during the lower Jurassic.•This pre-alpine structure was transported passively during alpine orogenesis, and thus preserved from significant alpine deformation.•The studied fault provides one of the first fossil analogue of the necking domain of a rifted margin, where both the syn-rift deposits and the basement are preserved.