Kinematics and Age of Syn-Intrusive Detachment Faulting in the Southern Alps: Evidence for Early Permian Crustal Extension and Implications for the Pangea A Versus B Controversy

Permian basin formation and magmatism in the Southern Alps of Italy have been interpreted as expressions of a WSW‐ENE‐trending, dextral megashear zone transforming Early Permian Pangea B into Late Permian Pangea A between ~285 and 265 Ma. In an alternative model, basin formation and magmatism resulted from N‐S crustal extension. To characterize Permian tectonics, we studied the Grassi Detachment Fault, a low‐angle extensional fault in the central Southern Alps. The footwall forms a metamorphic core complex affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, the synmylonitic Val Biandino Quartz Diorite and the postmylonitic Valle San Biagio Granite. U‐Pb zircon dating yielded crystallization ages of 289.1 ± 4.5 Ma for the former and 286.8 ± 4.9 Ma for the latter. Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S. Even though a dextral continental wrench system has long been regarded as a viable model at regional scale, the local kinematic evidence is inconsistent with this and, rather, supports N‐S extensional tectonics. Based on a compilation of >200 U‐Pb zircon ages, we discuss the evolution and tectonic framework of Late Carboniferous to Permian magmatism in the Alps

Petrogenesis of late-Variscan high-K alkali-calcic granitoids and calc-alkalic lamprophyres: The Aber-Ildut/North-Ouessant complex, Armorican massif, France

International audienceThe Aber-Ildut/North-Ouessant Variscan granitoid complex in the Armorican Massif is anexample of high-K alkali-calcic zoned pluton, c. 304 Ma in age. A first magmatic batchintruded through a northern EW-trending sinistral transcurrent shear zone, before injectingsouthwards as a huge horizontal zoned sill, with moderately peraluminous muscovite-freegranitoids in the north and strongly peraluminous muscovite-bearing leucogranites to thesouth. The second magmatic stage resulted in the intrusion of a large two-mica leucograniticbody from a root zone along the same shear zone, prior to the end of crystallization of the firstinjection. Finally, ultrapotassic dykes, including calc-alkalic lamprophyres (leucominettes),intrude the complex. The strongly peraluminous granites are interpreted as pure meltingproducts of crustal clay-rich pelitic material. All the other petrographic types, includingleucominettes, are thought to result from mixing of crustal melts and mantle-derived maficliquids. Highly silicic tourmaline-bearing leucogranites are significantly affected by a tetradeffect, in relation with REE complexing behavior. Aber-Ildut/North-Ouessant granitoidsprobably formed during the Variscan late-orogenic stage associated with exhumation andlithospheric thinning, at low pressure and by advective heating of a ≤ 50 km-thick crust fromhot metasomatized asthenosphere

Relations between basalts and adakitic-felsic intrusive bodies in a soft-substrate environment: the South Ouessant Visean basin in the Variscan belt, Armorican Massif, France

The metasedimentary and magmatic terranes in the southern part of the Ouessant Island (Western Brittany, France) are the offshore prolongation of the Leon Variscan metamorphic domain. They mainly consist of micaschists and subordinate amphibolitic lenses (meta-pillow lavas and volcaniclastic successions) cut by a swarm of trondhjemite sills, together with a large porphyritic monzogranite body, newly dated at 336 Ma, and later syeno-leucogranitic intrusions. A large spectrum of fluidal peperites, including spectacular "fiamme"-bearing breccias, is observable at the contact between metasediments and most of the intrusives. The coexistence of amphibolitized basalts, adakitic trondhjemites, and peraluminous granites in the inferred South Ouessant basin is assigned to a variety of deep subcontemporaneous processes, including asthenospheric partial melting, high-pressure fractionation in lithospheric reservoirs (or partial remelting of deep crystallized mafic intrusions), and continental crust melting. Implications of these new results are discussed in the Visean basinal framework of the Armorican Massif, formed at an early stage of the Variscan orogeny

Wrench-Related Dome Formation and Subsequent Orogenic Syntax Bending in a Hot Orogen (Variscan Ibero-Armorican Arc, the Ouessant Island, France)

During the Carboniferous collision stage, the West European Variscan orogen was affected by oblique convergence, wrenching, plate-scale oroclinal bending, and widespread exhumation of the deep crust. One of these exhumed units forms the Leon dome located on the northern flank of the Ibero-Armorican Arc in the western part of the Armorican massif. Structural field data from the Ouessant Island reveal kinematic changes between 330 and 300 Ma that affected the northwestern margin of the Leon dome. This margin underwent two ductile deformation phases with opposite strike-slip shear senses. Dextral strike-slip wrenching combined with orogen-normal shortening, orogen-parallel stretching, and local top-to-the-NE shearing occurred during the Leon dome formation and exhumation of its gneissic core at 330-310 Ma. During tightening of the Ibero-Armorican Arc at similar to 300 Ma, more localized sinistral wrenching reactivated the northwestern boundary of the Leon dome. The resulting N70 degrees E trending sinistral Porspoder-Ouessant shear zone together with its conjugate dextral South Armorican shear zone controlled the lateral escape of the Leon-North-Central Armorican rigid tectonic wedge triggered by indentation of the Cantabrian orocline at the core of the Ibero-Armorican Arc

Pre-Alpine (Variscan) inheritance: a key for the location of the future Valaisan Basin (Western Alps).

International audienceThe boundary between the Helvetic and the Penninic (= Briançonnais) Zones has long been recognized as a major fault (“Penninic Front”) in the Western Alps. A narrow oceanic domain has been postulated at least along part of this boundary (the Valaisan ocean). However, the information provided by the pre-Triassic basement has not been fully exploited, and will be discussed here in detail. The igneous and metamorphic history of the pre-Triassic basement shows significant differences between the External Massifs from the Helvetic Zone, with abundant late Carboniferous granites, and the basement of the Briançonnais Zone, including the Internal Massifs (Dora-Maira, Gran Paradiso, Monte Rosa), devoid of Carboniferous granites. A major coal-bearing basin, the “Zone Houillère”, opened along this boundary. This limnic intramontane basin has never been properly investigated. The Zone Houillère is not comparable with the external, paralic, flexural, basins on both sides of the Variscan belt, but shows similarities with the Saar-Saale basin. Like the latter, we interpret the Zone Houillère as a transtensional basin opened along a major, crustal-scale, fault zone, namely the East Variscan Shear Zone. The Permian magmatism and sedimentation displays contrasting distributions, being absent or very localized in the Helvetic Zone, and widespread in the Penninic Zone. The above data indicate that the structural inheritance from the Variscan belt plays a major role in defining the future location of the Valaisan basin, i.e. the boundary between the European palaeomargin and the Briançonnais microcontinent