Matteo Maino

U–Pb zircon ages for post-Variscan volcanism in the Ligurian Alps(Northern Italy)

Abstract: Ages of Permian volcanic rocks from the Ligurian Briançonnais domain (Western Italian Alps) have been determined by laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon. Three major volcanic units yielded zircons that were dated: calc-alkaline rhyolites (285.6 ± 2.6 Ma), andesites (with inherited cores yielded ages c. 476 Ma and older) and voluminous rhyodacites–rhyolites (272.7 ± 2.2 Ma). Following an amagmatic, sediment-starved time gap of c. 14 Ma, alkaline volcanic activity is recorded, at the top of the sequence, by K-alkaline rhyolites dated at 258.5 ± 2.8 Ma. The Ligurian segment of the Southern Variscan belt records transtensional and then extensional tectonics associated with the volcanic activity. The switch from calc-alkaline to alkaline activity corresponds to the transition from a post-orogenic to an anorogenic setting in the Southern Variscides; it may represent progressive and increasing delamination of the continental lithosphere, accompanied by partial melting of the lithospheric mantle. Supplementary material: Technical and data acquisition parameters, U–Th–Pb isotope analyses and calculated ages of zircons from samples and standard, and trace element compositions of selected zircons are available at: http://www.geolsoc.org.uk/SUP18322.

Structural map of Variscan northern Sardinia (Italy)

In this paper, we present a geological structural map (1:100,000 scale, ∼2300 km2 surface area) of the Variscan basement of northern Sardinia. The map integrates field structural analysis, extensive gamma-ray spectrometry, and high-resolution ELA-ICP-MS U/Th-Pb zircon and monazite dating. A set of 10 samples of granitic rocks collected from different plutons were characterized for their crystallization age. This provided an accurate timing of magmatic events related to the development of the Corsica-Sardinia Batholith. The structural map, complemented with geochronological results represents a benchmark for future studies on Variscan geodynamics.

The thrust zone of the Ligurian Penninic basal contact (Monte Fronté, Ligurian Alps, Italy)

ABSTRACT This paper presents a geological map and interpretative cross-sections, which illustrate the structure of a major fault zone of the Alps, that is, the Ligurian segment of the Penninic Basal Contact (PBC), which led to the emplacement of the orogenic wedge onto the European crust. Chaotic deposits, whose origin is debated, characterize the footwall of this complex thrust zone. The lower part of the sequence containing chaotic deposits consists of low deformed paraconglomerates and exotic megablocks embedded with turbidites. Conversely, the highly deformed upper part of the sequence englobes fragments of substratum-derived succession and is bounded by thrust planes. The nature of these chaotic deposits suggests an origin by gravitational processes related to the unstable front of the advancing wedge associated with offscraping of tectonic slices during the thrusting of allochtonous nappes along the PBC thrust zone.

Large sheath folds in the Briançonnais of the Ligurian Alps reconstructed by analysis of minor structures and stratigraphic mapping

This study presents a geometric and structural analysis of curvilinear sheath folds exposed in blueschist rocks of the Ligurian Alps. Field data are presented in a geological map of the structural synthesis with related geological sections (at the 1:10,000 scale) that illustrate the relationships and evolution of large-scale, sheath folds within metamorphic rocks. We based our analyses on the geometric parameters of more than 40 minor folds, as shape (hinge-Lm1 angle, main axial plane/S1 angle, interlimb angle and hinge curvature), asymmetry, fold hinge/stretching lineation obliquity and structural facing pattern. The summary of the whole data depicts a coherent 3D structure showing several orders of minor folds. Sense of asymmetry of minor folds and stratigraphic order has been used to reconstruct a reliable large-scale structure, and to define the sheath fold shape.

2D-thermo-mechanical modelling of spatial P-T variations in heterogeneous shear zones

We present a suite of two-dimensional thermo-mechanical simulations of a compositionally heterogeneous shear zone characterized by viscous matrix and inclusions. The experiments have been performed using a visco-elasto-plastic 2D finite difference code that reproduces the stress, strain, temperature and pressure variations occurring within the shear zone at lithospheric scale. From here, we extrapolated and discuss the results of a narrow (