Eo-alpine evolution of the central southern alps. Insights from structural analysis and new geochronological constraints

The timing of the Alpine deformation in the Central Southern Alps (CSA or Orobic Alps) has always been a debated topic, since the scarcity of reliable absolute age constraints has prevented an accurate chronological reconstruction of the evolution of this sector of the European Alps. In this work, detailed structural analyses performed in different areas of the CSA allowed us to distinguish different compressive features within both the crystalline basement and the sedimentary cover. The integration of these field data with new isotopic data provides time constraints for the reconstruction of the evolution of the CSA during the Alpine orogeny. In the northern sector of the belt a Variscan polymetamorphic basement is stacked southward on the Permian to Mesozoic cover along two main regional faults (Orobic and Porcile thrusts). Fault zones, slightly postdating a first folding event of Alpine age (D3), experienced a complex evolution through the ductile and brittle deformation regime, showing greenschists facies mylonites overprinted by a penetrative cataclastic deformation (D4). Generation of fault-related pseudotachylytes marks the onset of brittle conditions, lasting up to the youngest episodes of fault activity. Thrusting along this structures also produced thrusting within the Permian-Triassic cover with the formation of different south-verging thrust stacks. This first thrusting event was followed by the activation of new deeper thrust surfaces leading to the emplacement of three regional anticlines (Orobic Anticlines) which tilted to the south the previously stacked units. During this long compressive stage (Orobic-Porcile thrusts and Orobic Anticlines) the sedimentary cover of the CSA was also involved in thrusting and different stacks of Mesozoic units were emplaced to the south. 40Ar/39Ar dating of the pseudotachylyte matrix of 9 samples from both the Orobic and Porcile thrusts give two separated age clusters: Late Cretaceous (80-68 Ma) and Early to Middle Eocene (55-43 Ma). These new data provide evidence that the pre-Adamello evolution of the CSA was characterized by the superposition of different tectonic events accompanying the exhumation of the deepest part of the belt through the brittle-ductile transition. The oldest pseudotachylyte ages demonstrate that south-verging regional thrusting in the CSA was already active during the Late Cretaceous, concurrently with both the HP metamorphism that affected the Austroalpine units of the eastern Alps, and the development of a syn-orogenic foredeep basin where the Upper Cretaceous Lombardian Flysch was deposited. In the Early to Middle Eocene a minor reactivation of the Orobic and Porcile thrusts occurred, as testified by the youngest pseudotachylyte ages obtained by 40Ar/39Ar dating. This event was probably related with the closure of the Ligurian-Piedmont and the ongoing of the Europe-Adria collision. South of the Orobic Anticlines system the Triassic sedimentary succession is stacked into several units bounded by south-verging low-angle thrust faults, which are related to different steps of crustal shortening. Different thrust stacks occur within the Triassic cover between the Como Lake to the west and the Adamello batholith to the east. They usually have an antiformal arrangement and are separated by each other by different N-S trending transverse zones, such as the poorly known Grem-Vedra Transverse Zone (GVTZ), formed during complex deformational phenomena in a transpressional regime coeval with thrust emplacement. The GVTZ formed during the southward imbrication of the older thrust sheets of the Menna-Arera group, strongly interacting with syn-thrust ductile structures, and was reactivated during the growth of the Orobic Anticlines belt. The GVTZ and other transverse zones of the CSA probably reflect the occurrence of pre-existing fault systems that characterize the Norian to Jurassic rifting history of the Lombardian basin, and were reactivated as strike-slip features during Alpine tectonics. In the Gandino and Presolana areas thrust surfaces are cut by high-angle extensional and strike-slip faults, which controlled the emplacement of hypabissal magmatic intrusions that post-date thrusts motions. Intrusion ages based on SHRIMP U-Th-Pb zircon dating span between 42±1 and 39±1 Ma, suggesting close time relationships with the earliest Adamello intrusion stages and, more in general, with the widespread calc-alkaline magmatism described in the Southern Alps. Fission track ages of magmatic apatites are indistinguishable from U-Pb crystallization ages of zircons, suggesting that the intrusion occurred in country rocks already exhumed above the partial annealing zone of apatite (depth < 2-4 km). These data indicate that the northern and central sectors of the CSA were already structured and largely exhumed in the Middle Eocene and no major internal deformations has occurred in these areas after the Bartonian. Neogene deformations were instead concentrated further south, along the frontal part of the belt (Milano Belt). These new data provide a direct evidence that thrusting and nappe stacking were active during Late Cretaceous times not only in the Eastern Alps, but also in the CSA, significantly extending southward the sector of the Alpine belt affected by the Cretaceous orogenic event. In this view, the Late Cretaceous Southern Alps can be interpreted as the south-verging retrobelt of a pre-collisional orogenic wedge, which formed during the subduction of the Alpine Tethys beneath the attenuated northern Adria margin.