A new map of the Permo-Carboniferous cover and Variscan metamorphic basement in the Central Orobic Alps, Southern Alps - Italy: structural and stratigraphical data

Structural relationships between metamorphic basement and Permo-Mesozoic cover, in the central sector of the Orobic Alps, facilitate the individuation of Alpine and pre-Alpine tectonic histories. Geometries, belonging to different phases of deformation, referable to Alpine and pre-Alpine tectonic evolution of this portion of the Southalpine belt, are synthesized on a structural map at 1:25.000 scale, covering about 170 square kilometres. This map is located nearby the Orobic lineament which represents the major tectonic discontinuity between the Variscan metamorphic basement, outcropping mainly in the northern sector of the belt, and the Permian sedimentary cover, well exposed in the southern sector. The pre-Alpine tectonic history is polyphase as testified by the occurrence of superposed groups of structures associated with an amphibolite to later greenschist facies metamorphism, followed by post-metamorphic and syn-sedimentary tectonics responsible for the opening of Permian basins. The more ancient tectonic imprints are therefore preserved exclusively in the metamorphic basement that is composed mainly of gneisses and micaschists in which quartzites, minor metagranitoids, metabasics and marbles are interlayered. Two generations (D1, D2) of pre-Alpine syn-metamorphic structures coherently deform all these rocks: i) the D1 folds are small scale rootless folds, with a differentiated axial plane crenulation cleavage and associated with a metamorphic imprint under amphibolite facies conditions, as testified by the occurrence, in the matapelites, of the assemblage: garnet + biotite + muscovite +kyanite+ staurolite + plagioclase + quartz + ilmenite and relict rutile; ii) during the D2 deformation widespread greenschist retrogradation took place. The metamorphic evolution of this basement unit of the Orobic Alps is thermally coherent with a post-thickening uplift, occurring during pre-Permian time. The Upper Paleozoic sedimentary sequence unconformably overlies the Variscan basement and the oldest sediments belong to Late Carboniferous-Early Permian "Basal Conglomerate" Fm. The Basal Conglomerate is a southwards thinning sedimentary wedge consisting of red-quartz-pebble conglomerate, sandstone and minor siltstones deposited in a braided alluvial plain setting. Both Basal Conglomerate facies and sediments distribution were influenced by rejuvenation of pre-existing basement shear zones. The Basal Conclomerate is conformably overlain by the Early Permian "Collio" Fm, which is here divided in two members. The lower one (acid volcanics and minor andesites interbedded with sediments) was deposited both within and out of a large volcano-tectonic depression (cauldron) during two major periods of volcanic activity. U-Pb age determinations on zircon confirm the Early Permian age of the first cycle. The upper member (conglomerates, sandstones, black shales, minor evaporites and volcanics) disconformably lies on the lower one. It was deposited in a half-graben which hosted a large variety of sedimentary environments (alluvial fans, braided river alluvial plains and saline lakes). The upper Collio basin was bounded to the north by steep, fault-controlled escarpments which originated along older basement mylonite-bearing fractures in response to regional strike-slip transtensional tectonics. Towards the end of Early Permian a deformation phase, possibly transpressional, which caused gentle folding, faulting, uplift and erosion occurred. In Late Permian the investigated area became once again an important continental depositional site and fluviatile conglomerates, sandstones and mudstones of the "Verrucano Lombardo" Fm were unconformably laid on the Collio Fm. Continental conditions continued until Early Triassic. Both basement and cover were affected by two Alpine deformation phases (D3, D4). The first one is responsible for the setting of an Alpine system of southverging folds and stacked thrust sheets. D3 is characterized by a regional chevron fold system in the basement rocks (without a differentiated axial plane foliation), while in the cover it is associated with a pervasive pressure-solution cleavage. Moreover these structures are related to the development of cataclastic bands. D4 structures are discontinuous and revealed by a gentle waving of the pre-existing fabric. The occurrence of andesitic dykes crosscutting D3 structures indicates that the earliest phase of Alpine thrusting predated the Tertiary magmatism; eo-Alpine age of convergent tectonics is demonstrated by the Cretaceous (ca. 100 Ma) radiometric age of Uranium remobilization along the Alpine thrusts.