Rifting and arc-related early Paleozoic volcanism along the North Gondwana margin: geochemical and geological evidence from Sardinia (Italy)

Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian–Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (ca. 492–480 Ma), is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative ϵNdi values (−3.03 to −5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician–Early Silurian (ca. 440 Ma) volcano-sedimentary cycle consists of an alkalic mafic suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta and have Zr/Nb ratios in the range 4.20–30.90 (typical of a rift environment) and positive ϵNdi values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination

Canard Cycles and Poincar\'e Index of Non-Smooth Vector Fields on the Plane

This paper is concerned with closed orbits of non-smooth vector fields on the plane. For a subclass of non-smooth vector fields we provide necessary and sufficient conditions for the existence of canard kind solutions. By means of a regularization we prove that the canard cycles are singular orbits of singular perturbation problems which are limit periodic sets of a sequence of limit cycles. Moreover, we generalize the Poincar\'e Index for non-smooth vector fields.Comment: 20 pages, 25 figure

Photoinduced Electron Pairing in a Driven Cavity

We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red (blue) detuned from the cavity yields attractive (repulsive) interactions whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO3 and SrTiO3

Regimes in moist stratified flows over isolated topography: numerical experiments

Moist flows over simply shaped 3D mountains have been studied in numerical simulations made with a mesoscale meteorological model. Our aim is to examine the possible existence of multiple solutions, searching for different solutions depending on the path followed by the system in the parameter space. Results from three different sets of experiments are discussed here. In the first set of simulations, the height of the mountain has been progressively changed in time. In the second group of experiments, the humidity of the air flowing over the obstacle has been increased in time by adding a source term to the equation of evolution of moisture. The case of advection of moist air towards an obstacle, initially embedded in dry air, has been studied as a third type of flow. A dependence on the past history of the flow seems to characterise some types of system evolution, leading to different flow regimes over the obstacle. The experiments indicate that this result is mainly a consequence of changes of state of water, associated with the presence of humidity inthe atmosphere. These effects are emphasised in the case of an elliptical mountain, with its longer axis perpendicular to the main flow. In the three different sets of experiments presented here, evident differences with simulations where flow parameters are kept constant from the beginning persist in the flow regimes, also for periods of time much longer than the characteristic time of evolution towards stationary solutions

Melnikov analysis in nonsmooth differential systems with nonlinear switching manifold

We study the family of piecewise linear differential systems in the plane with two pieces separated by a cubic curve. Our main result is that 7 is a lower bound for the Hilbert number of this family. In order to get our main result, we develop the Melnikov functions for a class of nonsmooth differential systems, which generalizes, up to order 2, some previous results in the literature. Whereas the first order Melnikov function for the nonsmooth case remains the same as for the smooth one (i.e. the first order averaged function) the second order Melnikov function for the nonsmooth case is different from the smooth one (i.e. the second order averaged function). We show that, in this case, a new term depending on the jump of discontinuity and on the geometry of the switching manifold is added to the second order averaged function

A WARM CONVEYOR BELT MECHANISM ACCOMPANYING EXTREME PRECIPITATION EVENTS OVER NORTH-EASTERN ITALY

A Lagrangian methodology for the reconstruction and the analysis of the airstreams governing the transport of water vapour has been applied to analyse three recent extreme precipitation events over the Alps (3-5 November 1966, 16-18 November 2000 and 24-26 November 2002). The analysis outlines that the precipitation over the North-East of Italy is strongly conditioned by the evaporation rate and by properties of airmasses over the Central Mediterranean area. In particular most of the water vapour contributing to the precipitation ending over the Alps originates over Tunisia, the coastal regions in the western Lybia, the Channel of Sicilia, the Gulf of Gabes and the southerly Thyrrenian Sea. A simplified conceptual model is also proposed to visualize the traits of the moist airstreams inside the Mediterranean cyclones. Two patterns of airstreams can be identified, according to the scheme proposed by Browning and Roberts, 1994. The trajectory population of the two portions of WCB is a crucial factor controlling the precipitation production

Numerical study of a banded precipitation event over Italy

Satellite images of 30 October 2008 show the development over north-central Italy of rainbands and multiple waves during a strong south-westerly wind episode associated with a deepening synoptic trough and cold front passage. The event was studied by means of the ISAC model chain constituted of the hydrostatic model BOLAM and the nested non-hydrostatic model MOLOCH at 1.1 km resolution. Diagnostics of model output was performed to reveal the physical origin of the dynamical features and precipitation field as simulated. Based on our results we propose a theoretical framework in which symmetric instability underlies some of the observed precipitation patterns