Reconstruction and visualisation of melt topology in veined microdioritic enclaves

A number of xenocrystic microdioritic enclaves within the Ross of Mull Granite contain narrow veins of monzonitic material that have formed through the selective partial fusion of granitic magma mechanically incorporated into the microdiorite magma. Field evidence suggests that, as melting of the granitic material ensued, a monzonitic melt formed and exfiltrated from the surrounding microdioritic matrix resulting in Saffman–Taylor instabilities and the development of viscous fingering phenomena. The monzonitic melt network is now preserved as monzonitic veins. Mechanical serial sectioning through a representative veined microdioritic enclave shows that: (a) the topology of the veins is complex but crudely polyhedral, (b) there is porosity (monzonitic melt fraction) variation throughout the length of the veined enclave and (c) that the monzonitic veins are interconnected in three dimensions. Using image analysis software, the total porosity of individual veined enclave sections has been estimated, along with the variation in porosity with depth. Three dimensional rendering and animation software has been used to reconstruct and visualise the monzonitic melt network using Apple QuickTime™ software. The above method provides a cheap and powerful way of visualising remnant macroscopic transport properties (such as connectivity and melt topology) in partially molten igneous rocks.Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

The 18.7 ka phreatomagmatic flank eruption on Etna (Italy): relationship between eruptive activity and sedimentary basement setting

This paper documents a phreatomagmatic flank eruption that occurred 18 700 ± 100 a BP , on the lower north-eastern slope of Etna during the Ellittico volcano activity, which produced fall and surge deposits. This type of eruption is connected to a sedimentary basement ridge at Etna. The interaction between the rising magma and the shallow groundwater hosted in the volcanic pile overlying the impermeable sediments resulted in phreatomagmatic instead of strombolian activity. Three eruptive phases are distinguished based on field and analytical data: (i) an explosive phreatomagmatic opening, (ii) a main phase producing coarse lithic-rich fallout and a strombolian deposit, and (iii) the final pulsating surge-forming phase. The discovery of this phreatomagmatic flank eruption, which occurred at lower altitude, raises important issues for previous hazard assessments at Etna.     

Failure mechanisms and pore water pressure conditions: analysis of a riverbank along the Arno River (Central Italy)

Mechanisms of failure occurring in two portions of a riverbank along the Arno River (Central Italy), are investigated in detail starting by a series of periodic field observations and bank profile measurements. Two dominant mechanisms involving the silty sand portion of the bank have been observed: (a) alcove-shaped failure in the middle portion of the bank; (b) slab failure involving the middle–upper bank.

A portion of the riverbank was subject to laboratory (grain size analysis; phase relationship analysis; triaxial tests) and in situ tests (borehole shear tests (BSTs)) to characterise the geotechnical properties of the overbank deposits. Two different procedures of bank stability analysis have been performed: (1) a complete analysis, coupling seepage analysis with the limit equilibrium method; (2) two simplified analyses, through the limit equilibrium method with simple assumptions on pore water pressures distribution.

For the complete analysis, saturated/unsaturated flow within the riverbank was modelled by finite element seepage analysis in transient conditions, using as boundary conditions eight hydrographs with increasing water stage. Riverbank stability analyses have been conducted by the Morgenstern–Price rigorous method, dividing each of the eight hydrographs in 21 time steps and calculating the safety factor for each step. The analysis revealed the occurrence of two possible mechanisms of failure (slab-type and alcove-shaped sliding failures), according to the field observations, related to different river stages and pore water pressures within the riverbank: alcove failures are likely to occur with moderate flow events, while slab failures are favoured by flow events with higher peak river stage.

A first type of simplified analysis, representing critical conditions reached during a rapid flow event, was based on the main hypothesis of the occurrence of a zero pore water pressures zone within the portion of the bank between the low-water stage and the peak stage reached. A second type of simplified analysis was applied in order to represent rapid drawdown conditions following a prolonged flow event (worst case), with the main assumption of total saturation of the material up to the same elevation of the peak river stage. The first simplified analysis has given similar results to the complete seepage/stability analysis, confirming slab-type and alcove-shaped failure as the two main mechanisms of instability, while the second type of simplified analysis has conducted to too conservative results compared to the other previous analyses.

Field observations regarding different characteristic bank geometries in adjacent sub-reaches have been summarised in a conceptual cyclic sketch, that include all the possible paths of bank evolution depending on the succession of river stages reached during flow events and related pore pressure conditions.     

Active tectonics of the Northern Apennines and Adria geodynamics: new data and a discussion

The active geodynamic setting of the Northern Apennines is characterised by extension in the axial zone of the chain, and by a more complex tectonic behaviour in the frontal part of the belt. In the latter sector, moderate seismicity occurs, displaying compressional, strike-slip and extensional focal plane solutions with variably oriented P and T axes. For this area, a review of available geological and geophysical data has been integrated by the analysis of seismic reflection lines calibrated with deep well logs. This study confirms that, as already suggested by some previous workers, thrusting and related folding in the study area ceased in Early Pleistocene times. This feature is in contrast with the hypothesis of active thrusting related to a subducting lithospheric slab beneath the chain—an issue which is largely debated based on available geophysical information. Our analysis shows that the Northern Apennines are characterised by an active tectonic setting which is similar to that of the central and southern portions of the belt. These areas all display a Late Quaternary inactivity of the thrust front. NE–SW oriented extension (perpendicular to the strike of the orogen) is well established in their axial zones, whereas a less homogeneous stress field characterises their external sectors and the adjacent foreland. Within this framework, the seismotectonic behaviour of the Northern Apennines—and probably of the whole Italian peninsula between the Po Plain and the Southern Apennines (north of the Calabrian Arc)—may be interpreted as essentially controlled by two main processes. The first of them involves tectonic uplift, possibly related with slab detachment and associated unbending of the foreland plate. The second process consists of a present-day northwestward motion of the Adria block with respect to stable Europe.     

Southern Ocean deglacial record supports global Younger Dryas

In Northern Hemisphere deglaciation records, the transition from the last glacial to the Holocene indicates a rapid return to near-glacial conditions during the Younger Dryas, whereas their Southern Hemisphere ice core counterparts record two separate cooling events: the Antarctic Cold Reversal and the Oceanic Cold Reversal. Spatial distribution and relative timing of these events in both hemispheres are central for our understanding of causes and mechanisms of abrupt climate change. To date, no marine record from the southern mid-latitudes conclusively demonstrates that the Younger Dryas was a significant event in the Southern Ocean. Here, we present high-resolution oxygen isotope and iron content records of a radiocarbon-dated sedimentary sequence from the Great Australian Bight, which constrains oceanic and atmospheric changes during the last deglaciation. Oxygen isotopes from planktonic foraminifera indicate two rapid cold reversals (between 13.1 and 11.1 kyr BP) separated by a brief warming. The sedimentary iron content, interpreted as a proxy for wind strength, indicates a simultaneous change in atmospheric circulation pattern. Both records demonstrate the existence of cooling events in the Southern Hemisphere, which are synchronous with the Northern Hemisphere Younger Dryas cold reversal (between 12.9 and 11.5 kyr BP). Such evidence for the spatial distribution and timing of abrupt climatic fluctuations is essential data for groundtruthing results derived from global climate models.     

Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis

The carbon and nitrogen isotope composition of organic matter has been widely used to trace biogeochemical processes in marine and lacustrine environments. In order to reconstruct past environmental changes from sedimentary organic matter, it is crucial to consider potential alteration of the primary isotopic signal by bacterial degradation in the water column and during early diagenesis in the sediments.In a series of oxic and anoxic incubation experiments, we examined the fate of organic matter and the alteration of its carbon and nitrogen isotopic composition during microbial degradation. The decomposition rates determined with a double-exponential decay model show that the more reactive fraction of organic matter degrades at similar rates under oxic and anoxic conditions. However, under oxic conditions the proportion of organic matter resistent to degradation is much lower than under anoxic conditions. Within three months of incubation the δ¹³C of bulk organic matter decreased by 1.6‰ with respect to the initial value. The depletion can be attributed to the selective preservation of ¹³C-depleted organic compounds. During anoxic decay, the δ¹⁵N values continuously decreased to about 3‰ below the initial value. The decrease probably results from bacterial growth adding ¹⁵N-depleted biomass to the residual material. In the oxic experiment, δ¹⁵N values increased by more then 3‰ before decreasing to a value indistinguishable from the initial isotopic composition. The dissimilarity between oxic and anoxic conditions may be attributed to differences in the type, timing and degree of microbial activity and preferential degradation. In agreement with the anoxic incubation experiments, sediments from eutrophic Lake Lugano are, on average, depleted in ¹³C (−1.5‰) and ¹⁵N (−1.2‰) with respect to sinking particulate organic matter collected during a long-term sediment trap study.     

Ductile shear zones on carbonates: the calcaires plaquettés of northern Calabria (Italy)

Field, petrographic and quantitative strain analysis have been performed on highly foliated limestones outcropping in northern Calabria and generally attributed to two distinct units (San Donato and Pollino) on the basis of presence/absence of metamorphism. The deformation, however, is comparable in all the studied outcrops indicating that the widely accepted distinction of units in the area should be revised. Deformation is very localized in narrow ductile shear-zones affecting Tertiary conglomerates and Jurassic oncoidal packstones. The former are more strained because of their closeness to siliciclastic beds. The main deformation mechanism was grain boundary sliding with minor intragranular strain and pressure solution. To cite this article: A. Iannace, S. Vitale, C. R. Geoscience 336 (2004).     

Syn-orogenic extension in the Peloritani Alpine Thrust Belt (NE Sicily,Italy): Evidence from the Alì Unit

Structural and petrological analyses on the Alì Unit, in the Peloritani Thrust Belt, document the first evidence for Alpine exhumation associated with syn-orogenic extension in this part of the Calabria-Peloritani Arc. The Alì Unit displays ductile structures occurred during three Alpine deformation phases (D_a1, D_a2, D_a3). D_a1 and D_a3 developed in a contractional context, whereas D_a2 was generated in an extensional regime. The present-day tectonic contact between the Alì Unit and the overlying Mandanici Unit is interpreted as a low-angle extensional detachment responsible for the metamorphic break between the two units. Structural overprinting relationships indicate that the development of D_a2 structures and related tectonic exhumation occurred during syn-convergence extension, and were followed by further nappe stacking in the Peloritani Belt. To cite this article: R. Somma et al., C. R. Geoscience 337 (2005).     

Mass-conservative finite volume methods on 2-D unstructured grids for the Richards’ equation

The solution to the 2-D time-dependent unsaturated flow equation is numerically approximated by a second-order accurate cell-centered finite-volume discretization on unstructured grids. The approximation method is based on a vertex-centered Least Squares linear reconstruction of the solution gradients at mesh edges.A Taylor series development in time of the water content dependent variable in a finite-difference framework guarantees that the proposed finite volume method is mass conservative. A Picard iterative scheme solves at each time step the resulting non-linear algebraic problem. The performance of the method is assessed on five different test cases and implementing four distinct soil constitutive relationships. The first test case deals with a column infiltration problem. It shows the capability of providing a mass-conservative behavior. The second test case verifies the numerical approximation by comparison with an analytical mixed saturated–unsaturated solution. In this case, the water drains from a fully saturated portion of a 1-D column. The third and fourth test cases illustrate the performance of the approximation scheme on sharp soil heterogeneities on 1-D and 2-D multi-layered infiltration problems. The 2-D case shows the passage of an abrupt infiltration front across a curved interface between two layers. Finally, the fifth test case compares the numerical results with an analytical solution that is developed for a 2-D heterogeneous soil with a source term representing plant roots. This last test case illustrates the formal second-order accuracy of the method in the numerical approximation of the pressure head.     

Assessment of the stability of H/V spectral ratios from ambient noise and comparison with earthquake data in the Cologne area (Germany)

Situated in an active tectonic region the highly industrialised Cologne area (Germany) suffers from moderate-sized earthquakes. Our contribution to the mitigation of earthquake risk includes a microzonation study using ambient seismic noise and earthquake recordings from two field campaigns. An analysis of these data using the horizontal-to-vertical (H/V) as well as the classical spectral ratio (CSR) technique led to the following observations and conclusions: (1) The spatial variation in the thickness of the sedimentary cover is reliably retrieved using the fundamental resonance frequency estimated from the peak in the (H/V) ratio of ambient noise. (2) This fundamental resonance frequency is stable in time, but the amplitude of the peak (the amplification factor) is not. (3) The relative amplification variation of the H/V ratio in the area should therefore be checked systematically with repeated measurements before drawing conclusions about its significance. (4) The analysis of the H/V ratio of ambient noise provides the lower frequency bound, above which amplification may occur. (5) The shapes of the spectral ratios obtained by the different methods are generally in good agreement. However, the analysis of earthquake data shows that significant amplification of ground motion may also occur at frequencies higher than the fundamental one even when thick sediments are present.