Southern Alps

In the considered wide sector of the West-Mediterranean southern Europe, the collisional phase of the Variscan orogeny during Late Carboniferous and Permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental, alluvial to lacustrine basins originated by wrench- to normal-fault systems. The first volcanic cycle (generally Late Carboniferous-Early Permian in age) is represented by early calc-alkaline andesites and rhyolites, in variable amounts, and by following large volume of rhyolites, and by dacites. Both andesites and rhyolites show K-normal and high-K calc-alkaline character. The origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle–crust interface telescoped within a thickened crust. The melting is considered as the consequence of thermal re-equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt; the ascent of liquids occurred in a (trans-)tensional regime. The second magmatic cycle is represented by alkaline magmatism, and exhibits typical anorogenic features consistent with a rifting regime. This event was no more related with the collapse of the Variscan belt, but rather to the post-Variscan global re-organization of plates that evolved during Late Triassic times to the neo-Tethyan rifting. In both cycles, important differences in timing, areal distribution and outpoured volumes arise.

Thin continental Carboniferous sequences crop out sparsely in the western Southern Alps (Alpe Logone, Mesenzana, Grantola, Bosco Valtravaglia-Fabiasco, Val Tresa, etc.) and are currently the subject of a detailed litho- and biostratigraphic revision, to reconsider their chronological position with respect to previously published age constraints. The age of these Upper Palaeozoic sedimentary successions, scattered over a wide area and strongly tectonized along major structural lineaments, has long been debated between Westphalian and Stephanian. The present work focuses mainly on the palynology of the Brezzo di Bèdero section (Luino, Lake Maggiore). The recovered palynoflora is assigned to 42 spore genera and 76 species of which one genus and 10 species are newly proposed. Fifty-seven spore species and 19 pollen species are described and illustrated. Qualitatively, the Bèdero palynoflora shows strong affinities to those of Western Europe. The most abundant palynomorphs are trilete spores known to be characteristics of the late Westphalian and early Stephanian assemblages. These are characterized by the remarkable presence of Florinites and subordinately Wilsonites species, low numbers of Potonieisporites, rare Limitisporites and Vesicaspora, and very rare Latensina-Cordaitina pollen. The occurrence of diverse monolete spores Laevigatosporites, Punctatosporites, Spinosporites, Thymospora and Torispora, with common Lundbladispora gigantea, L. simonii, Stenozonotriletes rubius n. sp. together with the absence of plicate and taeniate pollen grains suggests a strong resemblance to the late Westphalian–early Stephanian interval assigned to the OT Zone of Western Europe. Variations in the quantitative composition can be likely attributed to variations in the environmental setting with regards to non-forming peat deposits. The palynological suite is indicative of the existence of a well-established lowland Cordaiteans vegetation and well settled pterophytic, pteridosperm and subordinately sphenophytic and lycophytic, hygrophytic plant communities.Des séquences continentales peu épaisses du Carbonifère supérieur qui affleurent de façon discontinue dans la partie occidentale des Alpes méridionales (Alpes de Logone, Mesenzana, Grantola, Bosco Valtraglia-Fabiasco, Val Tresa, etc.) font l’objet d’une révision lithostratigraphique et biostratigraphique détaillée afin de reconsidérer leur position chronologique. L’âge Westphalien ou Stéphanien de ces séquences sédimentaires – qui s’étendent sur une vaste région et montrent d’importantes déformations tectoniques le long de leurs plus grandes lignes structurales – a longtemps été discuté. Le travail présenté est principalement consacré à l’étude palynologique de la très intéressante section de Brezzo di Bedèro (Luino, lac Majeur). Quarante-deux genres et 76 espèces de spores dont un genre et dix espèces nouvelles ont pu être identifiés. Cinquante-sept espèces de spores et 19 espèces de pollen sont décrites et illustrées. D’un point de vue qualitatif, l’association palynologique de Bedèro montre de fortes affinités avec celle d’Europe occidentale. Les palynomorphes les plus abondants sont des spores trilètes typiques des associations du Westphalien supérieur et du Stéphanien inférieur. Ces associations se caractérisent par une présence importante de Florinites et dans une plus faible proportion de Wilsonites, un faible nombre de Potonieisporites, de rares Limitisporites et Vesicaspora, ainsi que de très rares pollens Latensina-Cordaitina. La présence de spores monolètes divers telles que Laevigatosporites, Punctatosporites, Spinosporites, Thymospora et Torispora, avec de fréquents Lundbladispora gigantea, L. simonii, Stenozonotriletes rubius n. sp., ainsi que l’absence de pollens pliqués et ataeniates suggèrent un intervalle de temps attribué à la zone OT de l’Europe occidentale, comprise entre le Westphalien supérieur et le Stéphanien inférieur. Des variations quantitatives de la composition des assemblages peuvent être attribuées à des variations environnementales par rapport aux dépôts de tourbe typiques de l’Europe occidentale. L’association palynologique indique une végétation de plaine alluviale riche en Cordaites, avec une communauté bien établie de plantes hygrophiles comme les Ptéridophytes, les Ptéridospermaphytes et, en moindre proportion, les Sphénophytes et les Lycophytes.