Preservation of the necking domain in orogens: a case study of the Mont-Blanc massif (Western Alps, France)

Rift-related inheritance plays a key role in orogenic building, by controlling the thermal state and the position of major sedimentary and crustal decollement levels. As recognized by various authors, a switch from thin- to thick-skinned style of deformation in reactivated rifted-margin during convergence occurs where the necking domain of the margin is involved in the subduction. This is observed in the External Crystalline Massifs (Aar, Mont-Blanc, Belledonne, Pelvoux, Argentera) located at the transition between the external and the internal domains of the western Alps corresponding also to the proximal-distal transition (necking domain) of the former Jurassic margin. Necking reactivation during Alpine convergence is accommodated by shear zones, rooted in the ductile middle crust, propagating  the deformation toward the external domain.  This Alpine overprint, which led to a lower greenschist metamorphism (ca. 330°C) in the External Crystalline Massifs, raise the question of the preservation of the rift-related, pre-alpine structures in the western Alps, and their use as fossil-analogues of present-day necking domains.

A case study is the internal Mont-Blanc massif, where preserved pre-rift to syn-rift (Triassic to Mid-Jurassic) cover is observed below the internal nappes, and on top the crustal basement (Mont-Blanc granite). The contact between these deposits and the underlying basement is a fault zone, made of a cataclastic basement overlaid by a black gouge. Above the contact, remnants of allochthonous pre-rift deposits and delaminated carbonates are observed. The syn-rift sandstones (Grès Singuliers Fm), which are either in contact with the fault or located above the pre-rift deposits, contain reworked clasts of cataclasite. Above the contact, in the cataclastic basement, some crinoid-rich sediments of likely Pliensbachian age fill open cracks. Taken together, these observations strongly point to the preservation of a pre-alpine, rift-related detachment fault of Jurassic age.

The petrographical and geochemical analysis of the exhumed fault indicates strong hydration-assisted deformation. In the cataclasite, feldspars breakdown and important element transfer (especially Ba, F, Si, Pb, Zn and REE) suggest fluid circulation in an open system. The black gouge matrix is mostly made of illite, likely recrystallized during the Alpine overprint. In addition, different generations of syn-kinematic veins are observed in the detachment. The first type, composed of graphite precipitated at ~400°C in the cataclasite. Syn-kinematic quartz and quartz hyalophane (Ba-rich feldspars) in the cataclasite and gouge were formed from a fluid above 170°C a salinity of ~9 wt.% NaCl-equivalent. The mobilized elements are the same as those involved in pre-alpine Pb-Zn (Ba-F) ore-deposits of the internal Mont-Blanc (Amône, Mont-Chemin, Catogne), suggesting a genetic link between rift-related faults and mineralisations.

Despite partial Alpine metamorphic overprint, the early tectonic, sedimentary and geochemical records of this rift-related detachment fault are very well preserved, making a good analogue of present-day necking domains. The example of Mont-Blanc massif gives an opportunity to study all these aspects in detail, in particular to understand fluid-mediated element mobility during rifting. Finally, it can be used to better understand the final stages of reactivation of the necking domain in a mature orogenic system.