Compression Front Research Articles

Long-lasting transcurrent tectonics in SW Alps evidenced by Neogene to present-day stress fields

The SW Alps are an active orogen undergoing intra-mountainous extension and peripheral compression. We discuss the significance of syn-orogenic extension based on a comparison of paleo-stress derived from fault-slip data inversion reflecting the long-term (< 12 Ma) evolution of SW Alps and the present-day stress state obtained by the inversion of the focal mechanisms of the last 30-years seismicity. The resulting stress states of long-term and active tectonic regimes are in good agreement, showing that extension accompanies strike-slip and reverse faulting in the southern part of the belt. The extensional deformation regime is limited to specific tectonic domains that can be interpreted as ‘transitional’ between pure strike-slip segments where the deformation concentrates on inherited ductile shear zones that were formed between 32° and 20 Ma ago. We thus propose that the extensional deformation in the SW Alps can be defined as a local deformation in a pull-apart type domain (High Durance - Jausiers area) or above slowly exhuming internal massifs (Dora Maira - Ivrea Body) along a curved boundary between the slowly rotating Apulian block and the relatively immobile Western Europe. The transcurrent fault system merges into a compressional front along the Mediterranean – Ligurian coast mainly to the east of San Remo. • SW Alps undergo neogene-present intra-mountain extension and peripheral compression. • Comparison of Paleo-stress (fault-slip data) and current stress (focal mechanisms) • Results from both techniques are in good agreement. • Extension is limited to specific tectonic domains between pure strike-slip segments. • The transcurrent system merges into a compressional front along the Ligurian coast.

Plio-Quaternary thin-skinned tectonics along the crustal front flexure of the southern Central Andes: a record of the regional stress regime or of local tectonic-driven gravitational processes?

We present here a record of Plio-Pleistocene deformations above the flexural front of the southern Central Andes of Argentina. We combine a seismic profile with structural and geomorphological observations to show that thin-skinned extension located on top of the crustal front flexure is coeval with thin-skinned shortening at the toe of the topographic bulge. The seismic line shows that a flat zone with no internal deformation separates the stretched and shortened domains. Such features are usually interpreted as the result of strike-slip faulting along basement faults, or tangential longitudinal strain folding in the soft sedimentary cover above crustal bending. We propose an alternative linking extension at the apex of the crustal anticline, to basal contraction by the downslope translation of a rigid thin nappe of sediments (30 × 30 km2 in area) above evaporites at a depth of 700–900 m. The size of such a process is unusually large onshore (630–810 km3) but mimics the gravity gliding observed in deltas and passive margins. Since this process disconnects zones with a shallow stress field from deeper crustal levels, it could allow extension above a compressive deformation front and should not be interpreted merely as a record of the crustal stress regime. Large-scale gravity gliding of the cover down the slope of a structural high could also explain some of the extension observed in mountain hinterlands.

Correction to “Interplanetary shock‐induced current sheet disturbances leading to auroral activations: THEMIS observations”

[1] During the production of our original paper “Interplanetary shock-induced current sheet disturbances leading to auroral activations: THEMIS observations” by Xiaoyan Zhou, Xu-Zhi Zhou, Vassilis Angelopoulos, Quanqi Shi, Chih-Ping Wang, and Harald Frey (Journal of Geophysical Research, 118, 1–15, doi:10.1002/jgra.50175, 2013), Supporting Information that shows an animation was omitted during publication. The Supporting Information is included in this update. [2] In the original paper, the complete data set and our elaboration have provided a comprehensive description for the interplanetary shock effect on the magnetotail current sheet (CS)/plasma sheet and nightside ionosphere. The animation in this update illustrates the difference between the internal (due to the compressional waves) and external (due to the shock in the solar wind) compressions from the spatiotemporal point of view and presents a straightforward and dynamic depiction of the CS thinning as a result of those compressions. This Supporting Information is composed of seven frames in which magnetopause and THEMIS locations are drawn to scale. The magnetopause locations before and after the shock arrival (the time at the sudden commencement) are calculated using Shue’s model [Shue et al., 1998]. Each frame is in the X-Z GSM meridional plane and corresponds to a moment given in Table 2 except for the first frame. Locations of the internal and external compression fronts are marked to scale, which are calculated based on the observed compressional waves speed inside the magnetosphere and the shock speed in the solar wind. The tail current sheet thickness (i.e., 2 h) at the inner probes is calculated using formula (4). The parameter h is given in Table 2 and is drawn to scale as well. [3] In addition to Figures 3–5 in Zhou et al. [2013], this animation shows how much the external compression is behind the internal in distance and time and quantitatively depicts the current sheet thickness variation. Accordingly, the cross-tail current density at the CS center was abruptly enhanced as given in Table 2. Note that while not intense, the shock on 3 March 2009 caused a current sheet thickness reduction of 80% and current density increase of ~6 times in only ~7min.

Analysis of a benchmark solution for non-Newtonian radial displacement in porous media

We present an analytical formulation useful to interpret the key phenomena involved in non-Newtonian displacement in porous media and an analysis of the results obtained by considering the uncertainty associated with relevant problem parameters. To derive a benchmark solution, we consider the radial dynamics of a moving stable interface in a porous domain saturated by two fluids, displacing and displaced, both non-Newtonian of shear-thinning power-law behavior, assuming the pressure and velocity to be continuous at the interface, and constant initial pressure. The flow law for both fluids is a modified Darcy’s law. Coupling the nonlinear flow law with the continuity equation, and taking into account compressibility effects, yields a set of nonlinear second-order partial differential equations. Considering two fluids with the same flow behavior index n allows transformation of the latter equations via a self-similar variable; further transformation of the equations incorporating the conditions at the interface shows for n<1 the existence of a compression front ahead of the moving interface. Solving the resulting set of nonlinear equations yields the positions of the moving interface and compression front, and the pressure distributions; the latter are derived in closed form for any value of n. A sensitivity analysis of the model responses is conducted both in a deterministic and a stochastic framework. In the latter case, Global Sensitivity Analysis (GSA) of the benchmark analytical model is adopted to study how the effects of uncertainty affecting selected parameters: (a) the fluids flow behavior index, (b) the relative total compressibility and mobility in the displaced and displacing fluid domains, and (c) the domain permeability and porosity, propagate to state variables. The relative influence of input parameters on model outputs is evaluated by means of associated Sobol indices, calculated via the Polynomial Chaos Expansion (PCE) technique. The goodness of the results obtained by the PCE is assessed by comparison against a traditional Monte Carlo (MC) approach.

Analytical modeling of spherical displacement for power-law fluids in porous media

We analyze the pressure transient due to the point injection of a non-Newtonian power-law fluid within an infinite homogeneous porous domain initially saturated with another non-Newtonian power-law fluid. Taking into account fluid and domain compressibility yields a displacement front propagating with finite velocity in spherical or semi-spherical geometry; considering two shear-thinning fluids with the same flow behavior index n allows derivation of a self-similar solution in closed form describing the propagation of the displacement front and demonstrating the existence of a compression front ahead of the interface between fluids. The positions of the displacement and compression fronts, and the pressure distributions in the two fluids, are derived in closed form for any value of n < 1=2. Sensitivity analysis of model responses indicates a strong direct dependence on mobility and compressibility ratios and on permeability; an inverse moderate dependence on porosity and a mixed, weaker dependence on flow behavior index.

Gravity driven deformation controlled by the migration of the delta front: The Plio-Pleistocene of the Eastern Niger Delta

The occurrence, in large shelf edge deltas, of gravity driven deformation related to overpressured shales is now well documented in many natural examples. In this paper, we explored the implications of the migration of the delta front on the kinematics of gravity driven deformation, in a case where the stratigraphic framework is detailed enough to discuss deformation kinematics and sedimentary supply at high temporal resolution (×0.1 Myr). We determined the deformation for the last 4 Myr at the scale of the whole Eastern Niger Delta. The upslope extensional domain showed EW asymmetric grabens. The transitional domain displayed (i) deformation restricted to folding to the east and (ii) a thicker sedimentary wedge and a larger deformation accommodated by thrust related folds to the west. The compressional front was a fold and thrust belt. Extensional deformation decreased in amount and rate over the Plio-Pleistocene. It also evolved significantly from distributed over the whole area to localized either to the western or eastern domains over that period. The compressional deformation followed the same evolution (i.e. the greater the extension, the greater the compression in either time or space), demonstrating the strong coupling between the extensional and compressional domains. Gravity deformation during the Plio-Pleistocene was driven by the continental slope spanning the coastal plain to distal deposits over the oceanic crust (about 2800 m of gravity potential). The superimposition of the delta front migration area and the extensional domain show that the associated pressure gradient localized the extensional domain. Deformation was closely linked to migrations of the delta front and the sedimentary supply reaching the delta: (i) the decrease in extension rate over the Plio-Pleistocene is related to a decrease in sedimentary supply over the Plio-Pleistocene, and, (ii) the spatial variations in extension intensity and rate are related to spatial variations of sedimentary supply.

Study of magnetic contrasts applied to hydrocarbon exploration in the Maturín Sub-Basin (eastern Venezuela)

We have studied near-surface magnetic contrasts in nine oil wells from an oil prospective area in eastern Venezuela (compressive deformation front of the northeastern Maturin Sub-Basin). Samples are drill cuttings taken at intervals of about 15 m within approximately the first 1200 m of most of these wells. Rock magnetic experiments, electron paramagnetic resonance (EPR), extractable organic matter (EOM), X-ray diffraction and scanning electron microscopy (SEM) are used to discriminate between two anomalies in magnetic susceptibility (MS), related to either a reducing environment caused by the underlying reservoir (Type A), or to primary lithological contrasts (Type B). Contour maps of MS, S-ratio and organic-matter-free radical concentrations (OMFRC), for the A-like depth levels, show a major central zone of anomalous high values. This area is probably associated with either maximum accumulation of hydrocarbons in the reservoir or with their southerly migration from the northern petroleum source towards the deformation front. We argue that such a result could be used, for future exploration and production ventures in the region, as a preliminary characterization of the reservoir. We also show in a Day plot, that saturation remanence — saturation magnetization ratio (Mrs/Ms) and coercivity of remanence — coercive force ratio (Hcr/Hc) stand as additional criteria to discriminate between these two types of magnetic contrasts. In fact, for the area of study, hysteresis ratios show three distinct trends corresponding to samples from A-like, B-like and non-anomalous depth levels.

STUDYING EXTREME ULTRAVIOLET WAVE TRANSIENTS WITH A DIGITAL LABORATORY: DIRECT COMPARISON OF EXTREME ULTRAVIOLET WAVE OBSERVATIONS TO GLOBAL MAGNETOHYDRODYNAMIC SIMULATIONS

In this work, we describe our effort to explore the signatures of large-scale extreme ultraviolet (EUV) transients in the solar corona (EUV waves) using a three-dimensional thermodynamic magnetohydrodynamic model. We conduct multiple simulations of the 2008 March 25 EUV wave (~18:40 UT), observed both on and off of the solar disk by the STEREO-A and B spacecraft. By independently varying fundamental parameters thought to govern the physical mechanisms behind EUV waves in each model, such as the ambient magneto-sonic speed, eruption free energy, and eruption handedness, we are able to assess their respective contributions to the transient signature. A key feature of this work is the ability to synthesize the multi-filter response of the STEREO Extreme UltraViolet Imagers directly from model data, which gives a means for direct interpretation of EUV observations with full knowledge of the three-dimensional magnetic and thermodynamic structures in the simulations. We discuss the implications of our results with respect to some commonly held interpretations of EUV waves (e.g., fast-mode magnetosonic wave, plasma compression, reconnection front, etc.) and present a unified scenario which includes both a wave-like component moving at the fast magnetosonic speed and a coherent driven compression front related to the eruptive event itself.

Seismic strain and stress field studies in Italy before and after the Umbria-Marche seismic sequence: a review

The seismic sequence that hit Umbria and Marche during 1997 and 1998 was particularly significant because it dramatically marked the evolution of analysis tools and the development of our geophysical knowledge of the region. Since September 1997, we have had a growing, coherent catalogue of source moment tensors that provides reliable information on earthquake sources in Italy and the surrounding regions. Together with borehole and other tectonic data, this has conspicuously improved our knowledge of the regional strain and stress fields. The main impact of these new data that include local information consists of the possibility to change the scale of the regional geodynamic frame. The simple description of extension tectonics that dominate the Apennines belt has evolved into the present-day maps of the strain and stress fields, where the active tectonics involve compression in the eastern Alps, extension and compression fronts in the northern Apennines, extension and strike-slip structures in the southern Apennines, and a compressional front along the southern Tyrrhenian Sea. This recent geodynamic evolution and the present-day seismotectonic sketch of the Italian peninsula are here described on the basis of the recognition of these tectonic features.

Rise of a Dark Bubble through a Quiescent Prominence

We report on a dynamical event observed in a quiescent prominence on 2007 November 8: a well-formed dark with a bright core rose vertically through the prominence without causing it to erupt. This event was observed in Hα and He I 1083 nm with the instruments of the Mauna Loa Solar Observatory. The dark bubble had a size of over 40'' and rose from the prominence base, at an average speed of ~12 km s−1, forming a bright compression front as it traversed the prominence. It finally assumed a keyhole shape before fading. The bright core embedded in the dark bubble was observed to rise from the solar limb, accelerating from ~12 to ~20 km s−1, leaving a thin trail of material behind. Subsequent observations indicate that this was not an exceptional event, but rather that similar disturbances do occur occasionally in prominences without disrupting them. In this Letter we present the November 8 observations, and propose a possible interpretation of the physical mechanism behind these dynamic events.

Test Results from Round Robin on Precracking and CTOD Testing of Welds

Abstract A weld fracture toughness round robin test program was initiated by ASTM to evaluate a proposed annex to E1290-93 on fracture toughness testing of ferritic steel weldments. The focus of the round robin was on specimen preparation procedures and their effect on crack tip opening displacement (CTOD). The local compression procedure was used to reduce the influence of residual stress on precrack curvature. Specimens were tested at −196 °C, which is on the lower shelf for this metal, to minimize variability in fracture toughness and maximize influence of residual stress. Seven labs participated in the round robin and among them conducted 51 tests per E1290-93 to measure CTOD. Statistical analysis of the data showed that crack curvature decreased by 5.2 % ±2.1 % for each 1 % increase in local compression, over the range of compressions tested (0–2.9 % with a nominal target value of 1 %). However, mean values of curvature varied significantly between labs, and when results from all labs are considered, local compression of about 1 % did not reliably reduce crack front curvature. It appears that the procedure used is more important than the amount of local compression, as evidenced by the straight cracks obtained by two labs that exercised tight control on local compression. The statistical analysis revealed that the mean value of measured CTOD for all data with less than 10 % curvature was not statistically different than the mean for less than 20 %, as it is in BS7448 (1997). Perhaps the most surprising observation from this round robin is that there does not appear to be a correlation between CTOD measured at −196 °C and amount of local compression or crack front curvature.

Finite-amplitude standing acoustic waves in a cubically nonlinear medium

The acoustic field in a resonator filled with a cubically nonlinear medium is investigated. The field is represented as a linear superposition of two strongly distorted counterpropagating waves. Unlike the case of a quadratically nonlinear medium, the counterpropagating waves in a cubically nonlinear medium are coupled through their mean (over a period) intensities. Free and forced standing waves are considered. Profiles of discontinuous oscillations containing compression and expansion shock fronts are constructed. Resonance curves, which represent the dependences of the mean field intensity on the difference between the boundary oscillation frequency and the frequency of one of the resonator modes, are calculated. The structure of the profiles of strongly distorted “forced” waves is analyzed. It is shown that discontinuities are formed only when the difference between the mean intensity and the detuning takes certain negative values. The discontinuities correspond to the jumps between different solutions to a nonlinear integro-differential equation, which, in the case of small dissipation, degenerates into a third-degree algebraic equation with an undetermined coefficient. The dependence of the intensity of discontinuous standing waves on the frequency of oscillations of the resonator boundary is determined. A nonlinear saturation is revealed: at a very large amplitude of the resonator wall oscillations, the field intensity in the resonator ceases depending on the amplitude and cannot exceed a certain limiting value, which is determined by the nonlinear attenuation at the shock fronts. This intensity maximum is reached when the frequency smoothly increases above the linear resonance. A hysteresis arises, and a bistability takes place, as in the case of a concentrated system at a nonlinear resonance.

Rubber band recoil

When an initially stretched rubber band is suddenly released at one end, an axial-stress front propagating at the celerity of sound separates a free and a stretched domain of the elastic material. As soon as it reaches the clamped end, the front rebounds and a compression front propagates backward. When the length of the compressed area exceeds Euler critical length, a dynamic buckling instability develops. The rebound is analysed using Saint-Venant's theory of impacts and we use a dynamical extension of the Euler–Bernoulli beam equation to obtain a relation between the buckled wavelength, the initial stretching and the rubber band thickness. The influence of an external fluid medium is also considered: owing to added mass and viscosity, the instability growth rate decreases. With a high viscosity, the axial-stress front spreads owing to viscous frictional forces during the release phase. As a result, the selected wavelength increases significantly.

TEXTURE STUDIES AND COMPRESSION BEHAVIOUR OF APPLE FLESH

Compressive behavior of fruit flesh has been studied using mechanical tests and microstructural analysis. Apple flesh from two cultivars (Braeburn and Cox's Orange Pippin) was investigated to represent the extremes in a spectrum of fruit flesh types, hard and juicy (Braeburn) and soft and mealy (Cox's). Force-deformation curves produced during compression of unconstrained discs of apple flesh followed trends predicted from the literature for each of the "juicy" and "mealy" types. The curves display the rupture point and, in some cases, a point of inflection that may be related to the point of incipient juice release. During compression these discs of flesh generally failed along the centre line, perpendicular to the direction of loading, through a barrelling mechanism. Cryo-Scanning Electron Microscopy (cryo-SEM) was used to examine the behavior of the parenchyma cells during fracture and compression using a purpose designed sample holder and compression tester. Fracture behavior reinforced the difference in mechanical properties between crisp and mealy fruit flesh. During compression testing prior to cryo-SEM imaging the apple flesh was constrained perpendicular to the direction of loading. Microstructural analysis suggests that, in this arrangement, the material fails along a compression front ahead of the compressing plate. Failure progresses by whole lines of parenchyma cells collapsing, or rupturing, with juice filling intercellular spaces, before the compression force is transferred to the next row of cells.

Tracking Large‐Scale Propagating Coronal Wave Fronts (EIT Waves) using Automated Methods

Recently developed mapping algorithms allow automated tracking of a propagating coronal wave, enabling the finding of reproducible fronts and propagation trajectories. Coronal observations taken by the Transition Region and Coronal Explorer (TRACE) on 1998 June 13 show a large-scale bright wave front, comparable to seen with the Extreme Ultraviolet Imaging Telescope aboard the Solar and Heliospheric Observatory ( SOHO EIT). Cross sections measuring density perturbations show roughly Gaussian wave structure, suggesting a single propagating compression front. The wave fronts are also found to propagate nonuniformly, unlike the circular fronts often seen with SOHO EIT. Any perceived dissimilarity between EIT waves and the bright propagating fronts seen by TRACE, however, can be explained by differences in the typical observing sequences of the two instruments.

The shock response of float-glass laminates

Interfaces within glass targets give rise to variations in the mode of failure of material components. The wide use of such laminates merits further investigation of the failure mechanism. It is already known that when shocked above a threshold of 4GPa, glass fails under compression behind a propagating front following the compression front. Work is presented which indicates how this failure process is altered by bonding together two plates to introduce an interface, rather than leaving a monolithic target. After crossing an internal interface, the failure wave propagates only after a delay in soda-lime glass and the failed strength of the material is increased at the inner interface compared with that at the impact face. Addition of a second interface illustrates these effects. Recent work has shown that failure of more than two plates bonded together during impact shapes the pulse transmitted through materials. Indeed it has been suggested that glass sheets bonded together show some of the features of polycrystalline brittle materials. In this work, the stress has been monitored at different stations in the laminate to ascertain the effect of varying the number of tiles within the laminate. It is found that the pulse rises to ca. 4GPa quickly and then is ramped more gradually as the number of glass sheets is increased.

FRACTAL PROPERTIES OF A NEOGENE FAULT SYSTEM IN NORTHWESTERN SICILY, ITALY

A Neogene transcurrent fault system affecting the Apenninic-Maghrebian thrust belt, located in northwestern Sicily, has been investigated using fractal analysis. The present-day structural setting of the sector between the Palermo and Madonie Mountains is the result of the superimposition, in space and time, of two distinct deformational events, a Miocene southeast-verging thrusting followed by Messinian to Pliocene strike-slip faulting, which cuts obliquely through the compressive fronts. The spatial distribution properties of the fault array were investigated by means of a fractal analysis. Fractal dimension was computed by adopting the correlation integral method. Fractal ranges have been evidenced between 350 and 3000 m. The fractal dimension obtained for the whole array is D=1.66. The scaling spatial distribution property of the fault array has been analyzed by calculating the fractal dimension with a moving window.

Early–Middle Pleistocene eastward migration of the Abruzzi Apennine (central Italy) extensional domain

The evolution of the Apennine arc is related to the flexural retreat of the Adriatic lithosphere plate, the NE migration of the compressive front and the contemporaneous rifting of the Tyrrhenian basin. Evidence of the NE migration of the orogen can also be inferred from the analysis of the intra-Apennine Quaternary extensional tectonics. Stratigraphic and structural data collected along NE–SW transects in the Abruzzi Apennines (central Italy), indeed, indicate that the westernmost normal faults (activated during the Pliocene) have to be considered inactive since the Early Pleistocene (Turano valley fault) or the beginning of the Middle Pleistocene (Salto valley and Liri valley faults). In contrast, the easternmost faults (Campo Imperatore and Mt. Morrone faults) were activated during the Early Pleistocene and are still active, as demonstrated by the displacement of Late Pleistocene–Holocene deposits and landforms. Within the innermost portion of the extensional domain, the Fucino and upper Aterno valley fault systems show persistent activity since the Pliocene. Besides the evidence of a progressively NE-shifted intra-Apennine extension, available data suggest that a period of co-existing activity of the presently inactive and newly activated faults (e.g. Liri valley and Mt. Morrone faults, respectively) occurred during the Early Pleistocene. This indicates that for a few hundred thousand years active faults in the central Apennines were more numerous than in the period of the present tectonic regime. This may have resulted in a reduced activity per fault (lower slip rates) or an increased intra-Apennine extension rate. Quaternary faults in the Abruzzi Apennines generally coincide with pre-existing discontinuities such as old thrust ramps or syn-orogenic normal faults. Based on a simple geometric criteria, i.e. direction, dip, length of the pre-existing discontinuities located east of the investigated area, and assuming a still active NE-migration of the extensional domain, it is possible to hypothesize that the old and presently inactive normal faults of the Maiella and Montagna dei Fiori anticlines may accomodate active extension of the future. The NE-ward shifting of the extensional domain would be, instead, locked if the process of lithospheric flexural retreat which drove the Neogene-Quaternary p.p. structural evolution of the Apennine chain has ended during the Early Pleistocene.

Thick‐skinned tectonics in the external Apennines, Italy: New evidence from magnetic anomaly analysis

We discuss the tectonic implications of a new residual magnetic map of the Apennine belt/Adriatic‐Apulian foreland obtained by integrating ground and offshore data sets [Chiappini et al., 2000a]. Negative anomalies are documented over the Adriatic‐Apulian foreland areas, whereas the external Apennine belt is characterized by a ubiquitous low‐amplitude (&lt;30 nT), long‐wavelength positive anomaly. In the central northern Apennines, three ∼100 km wide more intense (100–200 nT) round‐shaped anomalies are superimposed to the long‐wavelength feature. Finally, in the Tyrrhenian Sea and margins, high‐intensity, short‐wavelength positive‐negative couplets coincide with magmatic outcrops or bodies at shallow depth. The low‐amplitude anomaly pattern over Italy suggests that the magnetic basement beneath the Triassic evaporites is ubiquitously incorporated in the external belt compressive fronts, implying a thick‐skinned tectonic style for the external Apennines. The new residual magnetic map resolves the inconsistency between previous aeromagnetic data [AGIP SpA. Italia, 1981], which suggested a lack of basement involvement in the Apennine belt, and recent seismic data, which imaged deep reflectors penetrating the basement. Two magnetic models along NE‐SW transects in the northern and southern Apennines suggest consistent structural styles. In the northern Apennines, positive anomalies roughly coincide with the external compressive fronts, although there are local second‐order differences between the belt front and the edges of the anomaly. Here the magnetic data show that the basement rises southwestward along the thrust fronts from 6–7 km depth in the Adriatic foreland to 2–3 km depth in the axial belt, where some exploration wells have penetrated basement. Within the belt front, basement exhumation is inferred to occur along high‐angle, low‐displacement thrust faults inverting preexisting normal faults. In the southern Apennines, a remarkable positive magnetic anomaly is parallel with and tens of kilometers southwest of the belt front. Seismic data and oil wells show that the basement surface cannot be shallower in the belt than in the foreland. Therefore the observed magnetic anomaly is produced by strongly magnetic basement beneath the belt, likely an internal crustal wedge tectonically interposed between the Apulian carbonate sequences and basement.

Petrogenesis and evolution of Mt. Vulture alkaline volcanism (Southern Italy)

The Late Pleistocene Mt. Vulture strato-volcano developed at the intersection of NE-SW and NW-SE lithospheric fault systems, on the easternmost border of the Apennine compressional front overthrust onto the Apulian foreland. The initial phase of the volcanic activity is represented by pyroclastic deposits, including lava blocks, and subordinate eccentric domes, mostly phonolitic in composition. The later stages of activity formed the bulk of the strato-volcano (pyroclastic products and subordinate lavas), mostly tephritic in composition, with minor intercalations of basanite, mela-foidite and melilitite lavas and dikes. Variations in rock and mineral composition suggest that the volumetrically predominant basanite-tephrite (foidite)-phonotephrite-phonolite series can be accounted for by fractional crystallization processes starting from basanitic parental magmas, in agreement with the remarkably constant 87Sr/86Sr isotopes (0.70586–0.70581). Mass-balance calculations indicate that the variably differentiated magmas may have been produced by removal of wehrlite, clinopyroxenite and syenite cumulates, some of which are occasionally found as cognate xenoliths in the volcanics. Fractionation processes probably developed in multiple-zoned magma chambers, at depths of 3–5 km, corresponding to the tectonic discontinuity between the allochthonous Apennine formations and the underlying Apulian platform. Highly differentiated phonolitic magmas capping the magma chambers and their conduits thus appear to have fed the initial volcanic activity, whereas dominantly tephritic products were erupted in later stages. The least evolved mafic magmas, namely basanites, mela-foidites and melilitites, are characterized by diverse Na/K ratios and critical SiO2-undersaturation, which indicate their derivation as independent melts generated from distinct, heterogeneously enriched mantle sources and by variable partial melting degrees. Primitive mantle-normalized incompatible element patterns of Vulture mafic lavas invariably share analogies with both orogenic subduction-related magmas (high Low Field Strength Elements/High Field Strength Elements ratios, K, Rb and Th contents and marked Ti and Nb negative anomalies) and alkaline lavas from within-plate and rift settings (high Light Rare Earth Elements, P, Zr, Nb and Na). These geochemical features may be accounted for by magma generation from deep lithospheric mantle sources, enriched in Na-alkali silicate/carbonatite anorogenic components, subsequently affected by orogenic subduction-related K-metasomatism, analogous to that which modified magma sources of the Roman Magmatic Province along the internal Apennine Chain.