Stefano Zanchetta

Transverse zones (TZs) in fold‐and‐thrust belts typically comprise strike‐slip to transpressional faults and related folds, oriented sub‐parallel to the regional shortening direction. These zones are typically associated with inherited faults or lateral stratigraphic changes, which promote strain partitioning, stress rotation, and kinematic decoupling between adjacent blocks. In the Central Southern Alps (CSA), transverse zones sub‐parallel to the N–S shortening direction originated as pre‐orogenic rift‐related faults, later reactivated during the Alpine orogeny under compressional regimes. Their structural complexity has long been recognized, but no systematic absolute dating has constrained their long‐term evolution and reactivation history. To address this gap, we combine geological mapping, multi‐scale structural analysis, and U–Pb geochronology on syn‐tectonic carbonates from TZs and major thrust faults in the Lecco area to reconstruct their structural evolution within the Alpine thrust system. We infer a pre‐orogenic origin for N–S TZs and some E–W striking faults linked to rift‐related extensional tectonics active during the Late Norian and Early–Middle Jurassic times. U–Pb ages provide new insights into the Early Cretaceous‐Miocene syn‐orogenic evolution of the Central Southern Alps. During this interval, rift‐inherited N–S faults were episodically reactivated in strike‐slip mode under N–S shortening for nearly 100 Ma. We discuss how the TZs influence the build‐up of the belt, promoting along‐strike strain partitioning, acting as lateral connectors of thrusts and contributing to shortening accommodation when oblique to the maximum horizontal compression.

In the Central Alps, the Adula unit exposes high‐ and ultra‐high‐pressure (HP‐UHP) metamorphic rocks, providing key insights into the subduction and exhumation dynamics of the former distal European margin. We investigate the San Bernardino Shear Zone (SBSZ), a top‐to‐the‐east extensional shear zone developed within orthogneiss and paragneiss at the top of the Adula unit, containing eclogite boudins. Using detailed structural analysis, petrography, thermodynamic modeling, and ⁴⁰ Ar/ ³⁹ Ar geochronology, we constrain the Pressure‐Temperature‐deformation‐time ( P ‐ T ‐ d ‐ t ) path of these high‐pressure (HP) rocks. Phengite in eclogites yields minimum age of peak metamorphism of 38–39 Ma, making them the northernmost Alpine HP record in the Adula unit. Eclogite facies conditions reached ∼2.0–2.35 GPa and 580°–655°C. Progressive deformation in the eclogite host rocks led to the overprinting of earlier NW‐directed fabrics by top‐to‐the‐east shearing, recorded in synkinematic mica and associated with resetting of ⁴⁰ Ar/ ³⁹ Ar ages to 36–29 Ma. This temporal‐spatial pattern indicates a short interval between subduction, nappe stacking, and extensional exhumation. Our results demonstrate that the SBSZ played a key role in the exhumation of the eastern Lepontine Dome, supporting a model of distributed orogen‐parallel extension. This mechanism, partially coeval with amphibolite‐facies metamorphism and crustal anatexis, suggests that HP units were exhumed during orogen‐perpendicular shortening and orogen‐parallel extension.

Mylonites and pseudotachylytes play a crucial role in defining the rheology of extending continental crust. In this context, determining the age of brittle‐ductile deformation is fundamental to understanding the rifting evolution stages. Here, we investigate the Premosello Shear Zone, an extensional structure of the Ivrea‐Verbano lower crust, where ultramylonitic granulites are crosscut by pseudotachylytes. These features record the rift‐related evolution of the Adriatic crust. Middle Triassic–Early Jurassic ductile shearing is confirmed by U–Th–Pb dating of syn‐kinematic Th‐depleted rims of monazite. Ar‐Ar dating of a pseudotachylyte along high‐angle extensional faults yields an age of 199.2 ± 1.4 Ma, revealing a ‘semi‐brittle’ crustal behaviour of this lower crustal level during the onset of Early Jurassic rifting. Our study highlights: (1) the importance of coupling different radiometric dating methods on several rock types to constrain deformation history, and (2) the role of major extensional faults in localising crustal deformation during rift evolution.

The Early Permian in the present-day Europe area was a time when a major tectonic shift occurred, leading from the tectonic collapse of the Variscan orogeny to the crustal extension and thinning that characterized the Early Permian times. Crustal extension was associated with extensive magmatism at different crustal levels: from gabbro in the lower crust or at the mantle/crust transition to subaerial high-SiO ₂ volcanic activity. In the whole Southalpine Domain, the Early Permian intrusive bodies occur from the west (e.g. Ivrea-Verbano Complex and “Graniti dei Laghi”) to the east (Ifinger, Brixen and Cima d’Asta intrusive complexes). Among these, in the central Southern Alps (comprised between the Giudicarie Belt and the Lake Como), minor intrusive complexes also occur. The Val Biandino Intrusive Suite consists of two magmatic units: the Val Biandino Quartz-Diorite (VBQD) and the Valle di San Biagio Granite (VSBG). The first of them consists of gabbro-diorite to granodiorite bodies associated with leucocratic cordierite-bearing granitic dikes that intruded the pre-Permian basement. To the west, a W-dipping normal fault of Permian age represents the boundary between this unit and the Valle di San Biagio porphyric granite. All rock varieties of the Val Biandino Intrusive Suite display a high-K calc-alkaline affinity with metaluminous to peraluminous character. Field crosscutting relationships point to a late generation of the cordierite granites of the Val Biandino Quartz-Diorite unit with respect to the more mafic types. SHRIMP U–Pb zircon dating provided an age of 285.2 ± 1.9 Ma for a cordierite granite of the Val Biandino Quartz-Diorite unit and 283.2 ± 1.9 Ma for the porphyric Valle di San Biagio Granite. Geochemical data suggest that gabbro-diorite, quartz-diorite, granodiorite and leucogranite are not co-magmatic. The existing gaps in term of SiO ₂ wt% and the higher HREE contents in mafic and intermediate rocks with respect to granite coupled with similar LREE in all rocks support this hypothesis. The high Rb/Sr ratios (> 1) in leucogranite, together with the occurrence of white mica and cordierite point to a significant contribution of crustal partial melting in their genesis. The Val Biandino Intrusive Suite was likely formed through the interaction of magma genesis at the mantle/crust transition and partial melting of the heterogeneous pre-Permian basement of the Southalpine Domain. This scenario explains the strong heterogeneity displayed by the relatively small intrusive complex and the strong crustal signature exhibited by all the magmatic types of the Val Biandino Intrusive Suite.

Syn‐rift extensional faults play a significant role during the early stage of rifting. Constraining the age of faulting, and how and when deformation shifts from the proximal to the distal margin areas, is crucial for the reconstruction of the rifting process. Previous assessments of the structural and temporal evolution of rift‐related faults of the Adria proximal margin have primarily relied on indirect biostratigraphic evidence. Additionally, the majority of rift‐related faults underwent tectonic inversion during the Alpine orogeny. In this study, in‐situ U‐Pb geochronology was applied on syn‐kinematic calcites to unravel the activity of the Amora Fault, a remarkable example of a Jurassic growth fault unaffected by the Alpine orogeny. The obtained ages, spanning from Hettangian to Callovian, extend the AF activity beyond the previously established Early Jurassic time. This chrono‐structural model has significant implications on the role of major extensional faults in focusing deformation throughout the rift system's evolution.

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper Carboniferous-Permian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

The Cenozoic Adamello batholith in the Southern Alps records solid-state deformation structures including, in order of decreasing relative age, cooling joints, shear zones and faults. In the present study we constrained age and duration of each phase with ⁴⁰ Ar/ ³⁹ Ar geochronology. The host granodiorite, cooling joints, mylonites, pseudotachylytes and cataclasites were characterized through microstructural and mineralogical analysis, micro-computerized tomography and electron probe microanalysis. The dated K-bearing phases are (1) magmatic biotite, (2) biotite and K-feldspar in joints and mylonites, (3) pseudotachylytes and (4) hydrothermal K-feldspar in cataclasites. The wall-rock biotite has an age of 33.2 ± 0.2 Ma, independent of grain size, overlapping with the age of the cooling joints. Bulk biotite-rich mylonites have ages between 32.4 ± 0.5 and 30.8 ± 0.08 Ma. The K-feldspar cementing cataclasite has an age of 25.5 ± 1.1 Ma. Pseudotachylyte ages cluster between 29.8 ± 0.3 and 31.7 ± 3.1 Ma, with one of 25.6 ± 0.3 Ma. The resolvable difference in age between magmatic biotite and mylonites indicates that biotite is not a thermochronometer, as its age is mostly controlled by deformation and fluid–rock interactions. ⁴⁰ Ar/ ³⁹ Ar dates mostly confirm the relative ages determined from field relationships, with mylonites active within a time window of 1.6 myr and subsequent seismic faulting occurring for almost 6 myr. Supplementary material: Full analytical data are available at https://doi.org/10.6084/m9.figshare.c.5838146 Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation