Tectono-sedimentary evolution of the External Liguride units (Northern Apennines,Italy): insights in the pre-collisional history of a fossil ocean-continent transition zone

Abstract

In the Northern Apennines, the External Liguride (EL) units are interpreted as derived from the domain that joined the Ligure–Piemontese oceanic basin to the Adriatic plate continental margin. The EL units can be divided into two different groups according to the lithostratigraphic features of the basal complexes underlying the Upper Cretaceous–Lower Tertiary carbonate flysch (e.g. Helminthoid flysch). The first group includes the western successions characterized by Santonian–Campanian sedimentary melanges where slide blocks of lherzolitic mantle, gabbros, basalts, granulites, continental granitoids are represented. The second group is represented by the eastern successions where the Cenomanian–Campanian basal complexes mainly consist of sandstones and conglomerates where the mafic and ultramafic rocks are scarce or completely lacking. Their original substrate is represented by the Middle Triassic to Lower Cretaceous, mainly platform carbonate deposits, found as slices at the base of the eastern successions.

The stratigraphic features shown by the basal complexes allow the reconstruction of their source area that is assumed to be also representative for the pre-Upper Cretaceous setting. The proposed reconstruction suggests the occurrence in the EL domain of two distinct domains. The eastern domain was characterized by a thinned and faulted continental crust belonging to the Adriatic continental margin. The western domain was instead floored by subcontinental mantle associated with lower and upper continental crust, representing the ocean–continent transition. This setting is interpreted as the result of the opening of the Ligure–Piemontese oceanic basin by passive rifting, mainly developed by simple shear, asymmetric extension of the continental crust. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Contrasting radon background levels in volcanic settings: clues from 220Rn activity concentrations measured during long-term deformation experiments

To better understand the mechanisms leading to different radon background levels in volcanic settings, we have performed two long-term deformation experiments of 16 days using a real-time setup that enables us to monitor any variation of radon activity concentration during rock compression. Our measurements demonstrate that, in the case of highly porous volcanic rocks, the emanating power of the substrate changes as a function of the volcanic stress conditions. Constant magmatic pressures, such as those observed during dike intrusions and hydrothermal fluid injections, can result in pervasive pore collapse that is mirrored by a significant radon decrease until a constant emanation is achieved. Conversely, repeated cycles of stress due to, for example, volcano inflation/deflation cycles, cause a progressive radon increase a few days (but even weeks and months) before rupture. After rock failure, however, the formation of new emanation surfaces leads to a substantial increase of the radon signal. Our results suggest that surface deformation in tectonic and volcanic settings, such as inflation/deflation or constant magmatic pressures, have important repercussions on the emanating power of volcanic substrates.     

Uncertainty and sensitivity analysis: tools for GIS-based model implementation

A novel procedure to analyse the uncertainty associated to the output of GIS-based models is presented. The procedure can handle models of any degree of complexity that accept any kind of input data. Two important aspects of spatial modelling are addressed: the propagation of uncertainty from model inputs and model parameters up to the model output (uncertainty analysis); and the assessment of the relative importance of the sources of uncertainty in the output uncertainty (sensitivity analysis). Two main applications are proposed. The procedure allows implementation of a GIS-based model whose output can reliably support the decision process with an optimized allocation of resources for spatial data acquisition. This is possible in low cost strategy, based on numerical simulations on a small prototype of the GIS-based model. Furthermore, the procedure provides an effective model building tool to choose, from a group of alternative models, the best one in terms of cost-benefit analysis. A comprehensive case study is described. It concerns the implementation of a new GIS-based hydrologic model, whose goal is providing near real-time flood forecasting.     

Physically-based prediction of the maximum corner displacement magnification of one-storey eccentric systems

This paper gives a new insight into the linear dynamic behavior of one-storey eccentric systems, with particular attention devoted to provide a comprehensive physically-based formulation of the maximum corner displacement magnification, which involves three contributions (translational response, torsional response and their combination). It is shown that the largest magnifications, which mainly occur for the class of torsionally-flexible systems, are due to the translational contribution which is caused by the shift of the fundamental period of the eccentric system with respect to that of the equivalent not-eccentric system. A simplified method for the estimation of the maximum corner displacement under seismic excitation, based on the physical properties of the eccentric system, is finally proposed.     

Investigating prediction performance of an artificial neural network and a numerical model of the tidal signal at Puerto Belgrano, Bahia Blanca Estuary (Argentina)

In the present study we compare performances of the prediction of hourly tidal level variations at Puerto Belgrano, a coastal site in the Bahia Blanca Estuary (Argentina), by means of the MOHID model, which is a numerical model designed for coastal and estuarine shallow water applications, and of an artificial neural network (ANN). It was shown that the ANN model is able to predict the hourly tidal levels over long term duration with at least seven days of observations and with a better performance in respect to the numerical model. Our findings can be useful to implement ANN-based tools for future studies of the hydrodynamics of Bahía Blanca estuary.     

Melt inclusions in augite from the nakhlite meteorites: A reassessment of nakhlite parental melt and implications for petrogenesis

Abstract– The nakhlites, a subgroup of eight clinopyroxenites thought to come from a single geological unit at the Martian surface, show melt inclusions in augite and olivine. In contrast to olivine‐hosted melt inclusions, augite‐hosted melt inclusions are not surrounded by fractures, and are thus considered preferential candidates for reconstructing parent liquid compositions. Furthermore, two types of augite‐hosted melt inclusion have been defined and characterized in four different nakhlites (Northwest Africa [NWA] 817, Nakhla, Governador Valadares, and NWA 998): Type‐I isolated inclusions in augite cores that contain euhedral to subhedral augite, Ti‐magnetite, and pigeonite plus silica‐rich glass and a gas bubble; Type‐II microinclusions that form trails crosscutting host augite crystals. Fast‐heating experiments were performed on selected pristine primary augite‐hosted melt inclusions from these four samples. Of these, only data from Nakhla were considered robust for reconstruction of a nakhlite parental magma composition (NPM). Based upon careful petrographic selection and consideration of iron‐magnesium ratios, our data are used to propose an NPM, which is basaltic (49.1 wt% SiO₂), of high Ca/Al (1.95), and K₂O‐poor (0.32 wt%). Thermodynamic modeling at an oxygen fugacity one log unit below the QFM buffer using the MELTS and PETROLOG programs implies that Mg‐rich olivine was not a liquidus phase for this composition. Our analysis is used to suggest that olivine megacrysts found in the nakhlites are unlikely to have coprecipitated with augite, and thus may have been introduced during or subsequent to accumulation in the magma chamber, possibly from more evolved portions of the same chamber.     

Petrological, geochemical and isotopic characteristics of the lithospheric mantle beneath Sardinia (Italy) as indicated by ultramafic xenoliths enclosed in alkaline lavas

Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb)_N ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr–Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (⁸⁷Sr/⁸⁶Sr from 0.70385 to 0.70568 and ¹⁴³Nd/¹⁴⁴Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite.     

The Density Distributions and the Correlations of an Earthquake's Source Parameters

The data analysis of the source parameters of five sets of earthquake sequences, aftershocks and earthquakes scattered in a region shows that the scalar seismic moment is correlated with the linear size of the fault and the static stress drop. We tentatively imply that a correlation also exists between the radius of the faults and the static stress drops and it is suggested that the static stress drop may be decreasing with increasing radius of the source. It is shown that the density distribution of the source radius, calculated through the source rupture duration obtained from the body wave pulse (Boatwright, 1980) using the time T between the P-wave onset and the first zero crossing on the seismogram, may be represented by a power law as the density distribution of the stress drops and of the moment which are also computed. It is also suggested, and tentatively verified, that the density distribution of the areas of the broken barriers on the faults is similar to that of the density distribution of the static stress drops. It is finally suggested that the seismicity of a region may be studied two-dimensionally as a function of the stress drop and the radius of the source instead of the classic b and b₀ values. Concerning the discussion on the range of the values of the static stress drop, whether it is almost constant in a seismic region and varies only from one region to another, it is seen that in the aftershocks of the 1994 Northridge earthquake it covers a range of almost 5 orders of magnitudes. Finally it is ascertained that the density distribution of the source parameters does not give equipartition of seismic moment release.