Hyblean Plateau Research Articles

METASOMATISM IN THE HYBLEAN UPPER MANTLE: THE OS ISOTOPE CONTRIBUTION

Abundant upper mantle (peridotite and pyroxenite) xenoliths occur in Miocene basaltic diatreme pipes and Quaternary lava flows on the Hyblean Plateau (Sicily, Southern Italy). Peridotites are spinel-facies protogranular-textured harzburgites and lherzolites (Fo90, En89, Cr-diopside, Cr-rich spinel), that equilibrated at temperatures between 950 and 1050°C. Even the freshest Hyblean peridotites exhibit some serpentine content. Major-element distribution and abundance reflect depletion (high MgO, low Al2O3) related to one or more melt extraction events, but evidence of modal and cryptic metasomatism does exist. For instance, phlogopite of metasomatic origin occurs rarely in these peridotites. Geochemical evidences of metasomatism includes Light Rare Earth Element (LREE) (e.g. LaN/YbN = 9-19). Sr-Nd and He isotopic signatures are also consistent with the refertilization of the lithospheric peridotite matrix (full descriptions in Sapienza and Scribano, 2000, Sapienza et al., 2005 and references therein). To better define the metasomatic reservoir, we performed in situ Os isotope analyses on sulphides from mantle peridotite collected from Valle Guffari Miocene diatreme. Eight peridotite xenoliths were cut in blocks, stuck on thin section glasses and polished on one side. Sulfides were imaged using a Cameca SX-100 electron microprobe. Then, Os isotope analyses were performed using a Merchantek LUV266 nm laser ablation microprobe (LAM) attached to a Nu Plasma multi-collector ICP (MC-ICP). The laser spot size was ~60 mm. Only 4 peridotite samples contain measurable sulfides, resulting in 11 analyses. Analytical details are in Pearson et al. (2002). Sulfides in the Hyblean peridotites are Ni-rich, often Ferich, and enclose a Cu-rich phase (Fig. 1). Their shape is variable, from spheroidal to irregularly shaped, usually displaying curvilinear margins (Fig. 1). The grainsize ranges from tens to ~250 mm. Sulfides are mainly related to the serpentine network. A few sulfides are olivine-enclosed but are too small to be analysed. 187O/188Os ratios range from 0.110408 to 0.124398, which correspond to sub-chondritic values. 187Re/188Os ratios range from 0.0296463 to 1.41346, i.e. supra- to subchondritic values (Fig. 2). Five out of 6 sulfides from one sample (GE12) show similar 187Os/188Os but different 187Re/188Os, while the remaining sulfide and the other samples all align along an oblique array showing a negative correlation between 187Os/188Os and 187Re/188Os. Assuming the 187Re decay constant after Smoliar et al. (1996), 187Re/188Os- CHUR and 187Os/188OsCHUR after Walker and Morgan. (1989), we calculate Re-depletion ages (TRD) and model age (TMA). TRD represents the minimum melt-depletion age of the lithospheric mantle, since the calculation minimizes the effect of metasomatism-driven Re addition, while TMA assumes that the considered mantle portion remains in closedsystem conditions through time (Walker et al., 1989). Low- 187Os/188Os sulfides from peridotite GE12 show Paleoprotezoic to Archean TRD ages, while the other sulfides are Neoand Mesoproterozoic. TMA yields meaningless age (future or even older than the Earth’s formation), reflecting recent disturbance of the Re/Os ratio. GE12 sulfides show the same unradiogenic 187Os/188Os composition (except for one sulfide) but 187Re/188Os ranging from 0.8-1.4 (Fig. 2). This indicates that Os isotope composition does not depend on time-integrated in situ 187Re, but rather suggests the metasomatic effects with Re addition probably occurring shortly prior the eruption. The negative correlation between 187Os/188Os and 187Re/188Os in the other samples also supports the metasomatic origin of Re, and rather suggests mixing between two reservoirs with different Re-Os signatures (Wang et al., 2003; Fig. 2). Thus, the Hyblean peridotites we have studied experienced melt extraction event(s) – which would remove Re from the system - and underwent later metasomatic Re-enriching event(s). Although these preliminary Re-Os data - and the large associated uncertainties - on peridotite sulfides do not allow the calculation of a very detailed ages, TRD ages are realistic and require a Proterozoic-Archean minimum age for these portions of the Hyblean lithospheric mantle. Most sulfides in peridotite GE12 testify to recent Re addition, while the other peridotite sulfides and one GE12 sulfide seem to be the results of mixing between two metasomatic end-members. The low-187Os/188Os sulfides in sample GE12 may be representative of the low-187Os/188Os endmember. The occurrence of sulfides with different Os isotope signatures in sample GE12 confirms the importance of using in situ technique in such petrological study.

The Mesozoic and Cenozoic motion of Adria (central Mediterranean): a review of constraints and limitations

This paper presents kinematic analysis on the motion of Adria, which is the continental mass that bridges Africa and Europe in the central Mediterranean. Palaeomagnetic data show a general coherence between the motion of Adria and Africa since the Late Paleozoic. This mutual motion, for the period from 120 Ma and the present, is verified by comparing inferred palaeolatitudes from relatively stable parts of Adria (Apulia, Gargano, Istria, and the Southern Alps) and the Hyblean Plateau, with latitudinal changes that are calculated from the motion of Africa with respect to hotspots. Additional constraints on the motion of Adria are provided from the Late Paleozoic-Early Mesozoic passive margin of Adria in the Ionian Sea. The seismic structure of the floor of the Ionian Sea resembles the structure of the oceanic crust in marginal back-arc basins, suggesting that it formed as a small ocean basin. Furthermore, the Ionian lithosphere in the Calabrian arc has been subjected to rapid rollback, which commonly occurs only when the subducting slab is made of oceanic lithosphere. This oceanic domain marks the Permian-Triassic to Jurassic plate boundary between Adria and Africa, suggesting that a small amount of independent motion between Adria and Africa took place at that time. Since the Jurassic, Adria and Africa have shared a relatively coherent motion path.

Lateral flow of African mantle below the nearby Tyrrhenian plate: geochemical evidence

AbstractThe Quaternary magmatism of the Southern Tyrrhenian basin represents a rare example of an active volcanic arc system where ocean island basalt (OIB) and island arc basalt (IAB) magmas coexist. Although there is general agreement on the importance of the Ionian oceanic lithosphere subduction in the genesis of the IAB magmatism, the tectono‐magmatic processes producing the coexisting OIB magmas are still poorly understood. Here we show that the geochemistry of the Quaternary Southern Tyrrhenian OIB‐type magmatism (i.e. Ustica island and Prometeo, a previously unknown submarine lava field) is very similar to that of OIB‐type volcanoes situated on the nearby African plate (i.e. Etna and Hyblean Plateau). Among the possible geodynamic scenarios proposed to explain the coexistence of OIB and IAB magmas in arc settings, we consider the development of a tear at the edge of the Ionian plate as the more likely mechanism to favour the flow of African asthenospheric mantle below the Tyrrhenian plate.

Evidence of deep magma degassing and ascent by geochemistry of peripheral gas emissions at Mount Etna (Italy): Assessment of the magmatic reservoir pressure

Five gas discharges in the area of Mount Etna volcano (Italy) and in the near Hyblean plateau have been monitored since 1996. All the emissions displayed low contributions from crustal fluids, whereas magmatic gases were the main component. Selective dissolution of these gases into hydrothermal aquifers has been recognized and modeled, allowing us to calculate the original composition of the magma‐released gases. The inferred composition of the magmatic gases exhibits synchronous variations of He/Ne and He/CO2 ratios, which are coherent with the magma degassing process. On the basis of numerical simulations of volatile degassing from Etnean basalts we have computed the initial and final pressures of the magma batches feeding the emissions. We thus can define the levels of the Etna plumbing system where magmas are stored. Pressure values were around 360 and 160 MPa for initial and final stages, respectively, meaning related depths of about 10 and 3 km below sea level, matching those obtained by geophysical investigations for the deep and shallow magma reservoirs. In addition, we have been able to recognize episodes of magma migration from the deeper reservoir toward the shallow one. An important magma injection into the shallow storage volume was detected during the onset of the 2001 eruption (17 July). No further injection had taken place during this period until September 2001, providing a possible reason for the quick exhaustion of the eruption. In view of this we suggest that the sampled emissions are a powerful geochemical tool to investigate the Etna's plumbing system and its magma dynamics, as well as the development of eruptive events.

Deep‐sea ostracods as indicators of palaeoceanographic changes: a case history from the middle–late Miocene of southern Italy (Central Mediterranean)

ABSTRACT The middle–late Miocene palaeoceanography of the Central Mediterranean was investigated using bio‐chronostratigraphically constrained (estimated ages are based on plancktonic foraminifers) ostracod faunas from the Tremiti Islands and the Hyblean Plateau (southern Italy). Specifically, differences and similarities between the hydrographical evolution of the two study areas, inferred from the ostracod record, are herein used to provide new insights into the evolution of the Mediterranean deep‐water circulation. Ostracod assemblage compositions suggest a simultaneous progressive trend from deep thermospheric towards psychrospheric conditions in both the areas during the Langhian. However, this trend seems to have developed fully only in the Hyblean Plateau. Notably, the first sign of an oceanic deep water‐mass supply is shown by the almost synchronous entry of the upper psychrospheric genus Agrenocythere in the middle Langhian. The youngest reliable evidence of deep oceanic influences (common occurrence of Agrenocythere) in the Central Mediterranean comes from the Tremiti Islands and dates back to the earliest Serravallian. Similarities between the two study areas suggest large‐scale modifications in the Central Mediterranean deep basins. This hypothesis is discussed in the context of the global middle Miocene oceanographic scenario.

Miocene ostracods from the Tremiti Islands and Hyblean Plateau: biostratigraphy and description of new and poorly known species

The main taxonomic and stratigraphical results of a study of ostracods from the middle–late Miocene of the Tremiti Islands and the Hyblean Plateau are presented. The faunas are mostly typical of deep thermospheric or psychrospheric bathyal environments. Excluding a few, mostly juvenile shallow water contaminants, a total of 127 species were recorded. Twenty-four taxa are discussed herein; of these, 13 species ( Cytherella dissimilis nov. sp., Cytherella parvula nov. sp., Cytherella subtilis nov. sp., Argilloecia triangularis nov. sp., Retibythere ( Retibythere)? claudii nov. sp., Retibythere ( Retibythere)? gibba nov. sp., Cytheropteron ( Cytheropteron) joachinoi nov. sp., Cytheropteron ( Aversovalva) consuetum nov. sp., Cytheropteron ( Aversovalva) trimerumense nov. sp., Cytheropteron ( Aversovalva) turgidum nov. sp., Loxoconcha julianii nov. sp., Buntonia whatleyi nov. sp., Pterygocythere nuda nov. sp.) and a subspecies ( Costa tricostata dorsoarcuata nov. subsp.) are described as new, while six are left in open nomenclature. The stratigraphical distribution of ostracods is expressed in terms of planktonic foraminiferal zones. Information obtained from this study enriches biostratigraphical data on Mediterranean Miocene deep-sea ostracods, especially with respect to the Langhian.

Integrated micropalaeontological study (ostracods and calcareous plankton) of the Langhian western Hyblean successions (Sicily, Italy)

Four sections cropping out in the Hyblean Plateau (SE Sicily) were studied biostratigraphically and palaeoecologically. The investigated succession was referred to the early–late Langhian on the basis of both the planktonic foraminifers (lowermost part of the Praeorbulina glomerosa sicana Subzone–lowermost part of the Orbulina universa Subzone) and the calcareous nannofossils (uppermost part of the MNN4a Zone–lowermost part of the MNN5b Subzone). All the biozonal markers were recorded and further significant events were identified. The most remarkable ones are two paracmes of Paragloborotalia siakensis within the P. glomerosa sicana Subzone and the distribution of Helicosphaera waltrans, which characterises the upper part of the MNN5a Subzone. The ostracod palaeoecological analysis indicated a progressive and gradual trend from thermospheric to psychrospheric conditions and probably a basin deepening (from upper to lower epibathyal). The flowing of deep Atlantic waters into the basin is documented by the entry of the psychrospheric species Agrenocythere hazelae within the upper part of the P. glomerosa sicana Subzone. Only in the late Langhian, however, were typical psychrospheric conditions probably reached. These data provide evidence that oceanic deep water masses reached the Central Mediterranean, passing through both the western connections to the Atlantic Ocean and the African–Sicilian threshold, during a great part of the Langhian.

Test of Source-Parameter Inversion of the Intensities of a 54,000-Death Shock of the Seventeenth Century in Southeast Sicily

We inverted the macroseismic intensity data set (felt reports) of the M 5.4 earthquake of 1990 in SE Sicily and retrieved source parameters that were compatible with those obtained independently by other authors who treated instrumental observations. Our technique uses Voronoi polygons and our kinematic function KF. After this validation, we attempted the inversion of the macroseismic intensity data set of one of the strongest earthquakes that has ever struck the Mediterranean basin (in 1693 in southeast Sicily; 54,000 casualties). It was possible to invert the regional intensity patterns of these two earthquakes because their data sets are sufficiently uncontaminated by local site responses and retain regional traces of source effects. The 1693 source, constrained by our inversion, is a NNE-oriented fault, approximately 60 km long, dipping from 90° to 54° (80° with positive error of 10°, and negative error of 26°) toward ESE, 50% strike-slip and 50% dip-slip, which crosses SE Sicily from the Hyblean Plateau to the coast of the Ionian Sea, south of the city of Catania; the nucleation is 37.08° N 14.93° E. This source best fits the regional distribution of damage observed in the field. At the limit of the negative error of the dip angle, a value of 54° makes our line source, at depth, compatible with the trace of the Scicli active transcurrent fault, which is 12-14 km to the west. The retrieved source is also compatible with the orientation of the maximum horizontal geodynamical compressive stress, confidently measured in the area, as well as with an authoritative tectonic interpretation, independent from this study. The combined use of tessellation and of our KF model is promising for inverting intensity data sets of preinstrumental earthquakes.

Why Mt Etna?

The Etna volcano is located in an apparently anomalous position on the hinge zone of the Apennines subduction and its Na‐alkaline geochemistry does not favour a magma source from the deep slab as indicated for the Aeolian K‐alkaline magmatism. The steeper dip of the regional foreland monocline at the front of the Apennines in the Ionian Sea than in Sicily, implies a larger rollback of the subduction hinge in the Ionian Sea. Moreover, the lengthening of the Apennines arc needs extension parallel to the arc. Therefore, the larger southeastward subduction rollback of the Ionian lithosphere with respect to the Hyblean plateau in Sicily, should kinematically produce right‐lateral transtension and a sort of vertical ‘slab window’ which might explain (i) the Plio‐Pleistocene alkaline magmatism of eastern Sicily (e.g. the Etna volcano) and (ii) the late Pliocene to present right lateral transtensional tectonics and seismicity of eastern Sicily. The area of transfer of different dip and rollback occurs along the inherited Mesozoic passive continental margin between Sicily and the oceanic Ionian Sea, i.e. the Malta escarpment.

Active fault kinematics and crustal stresses along the Ionian margin of southeastern Sicily

Since the late Cretaceous onset of plate convergence between Africa and Europe, the Malta Escarpment has been converted from a Mesozoic passive margin into a mega-hinge fault system with an additional sinistral strike-slip component. The modern tectonic stress regime with NW–SE-directed maximum horizontal stresses has been established since Late Messinian times. Since the Pleistocene, sinistral strike-slip deformation and contemporaneous normal faulting along the Malta Escarpment have induced the opening of oblique trending onshore grabens at the eastern margin of the Hyblean Plateau. In this study, we focus on the kinematics, the controlling state of stress, and the temporal variation of the neotectonic to active strike-slip and normal fault structures. The stress-tensor calculations reveals that the widespread map-scaled to meso-scaled normal fault structures are governed by the long-term extensional state of stress during the Quaternary. This long-term stress tensor is predominantly controlled by gravitational induced stresses due to vertical load ( σ 1= S V) and lateral extension due to the topographic gradient of the Malta Escarpment ( σ 3= S h=NE–SW). In this case, the average tectonic stresses ( σ 2= S H=NW–SE) transmitted by the regional to plate-tectonic stress field are significantly smaller than the gravitational induced stresses. In contrast, the clear localization of conjugate sets of meso-scaled strike-slip fault structures and shear zones without accompanying normal fault structures give strong indications for episodic seismotectonic strike-slip faulting under critical stress conditions. In this state, tectonically induced maximum horizontal stresses are successively increased by ongoing plate convergence from low-level stress magnitudes ( σ 1= S V, σ 2= S H=NW–SE) up to critical stress magnitudes ( σ 1= S H=NW–SE, σ 2= S V), which are significantly larger than gravitational stresses. At the critical state, seismotectonic stress release occurs by active strike-slip faulting, as recently indicated in mid-crustal levels by the moderate 1990 Augusta earthquake, and re-establishes the long-term extensional state of stress. Because the strike-slip faults and shear zones in the study area were formed as surface ruptures, they additionally give indications of episodic large earthquakes in this region with magnitudes greater than 6.0, as reported by several large earthquakes devastating E and SE Sicily in historical times.

Active faulting in the Calabrian arc and eastern Sicily

The Calabrian arc and eastern Sicily are areas of the central Mediterranean where the effects of Quaternary tectonics are well preserved. The most impressive tectonic feature in this region is represented by a major normal fault belt that runs more or less continuously along the inner side of the Calabrian arc, extending through the Strait of Messina along the Ionian coast of Sicily as far as the Hyblean Plateau. The distinct normal fault segments within the belt, which during Pleistocene times have controlled the evolution of major marine sedimentary basins, have lengths ranging from 10 to 45 km. They exhibit huge fault scarps which defines the fronts of the main mountain ranges of the region (Catena Costiera, Aspromonte, Serre, Peloritani and Hyblean). Morphological features of fault scarps, and the age of the faulted rocks, suggest slip rates of 0.5–1.2 mm/year for the last 700 kyear (Middle Pleistocene-Holocene), reaching values of about 2.0 mm/year in the areas of active volcanism. From a seismological point a view, the Calabrian arc and eastern Sicily represent a very active area which is characterized by crustal earthquakes, the largest of which reached in the last nine centuries an intensity of X–XI (6 < M ≤ 7.4). The occurrence of intermediate and deep focus earthquakes located along the inner side of the arc, beneath the southern Tyrrhenian Sea, is associated to the existence of a slab of Ionian lithosphere. The distribution of crustal seismicity shows that most of the events which have occurred in the area, are located in the hangingwalls of the main Quaternary normal faults hence suggesting a strong relationship between seismic activity and the growth of extensional structures. Geological observations, together with seismological data, indicate that normal faulting in the area results from the development of a main rift zone, related to an overall ESE–WNW extension, which also controls the evolutionary history of the magmatism in this sector of southern Italy.

Quaternary normal faulting in southeastern Sicily (Italy): a seismic source for the 1693 large earthquake

SUMMARY We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian oVshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is aVected by WNW‐ESE regional extension producing normal faulting of the southern edge of the Siculo‐Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by diVerent ages and related to distinct tectonic processes. The older NW‐SE-trending normal fault segments developed up to #400 kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW‐SSE-trending normal fault segments and extends along the Ionian oVshore zone following the NE‐SW-trending Avola and Rosolini‐Ispica normal faults. These faults are characterized by vertical slip rates of 0.7‐3.3 mm yr’1 and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes (M~7) could be related to a 45 km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7 mm yr’1 is calculated, and a recurrence interval of about 550±50 yr is proposed for large events similar to that of January 1693.

Patterns of tectonic stress in Sicily from borehole breakout observations and finite element modeling

The orientation of in situ tectonic stress was deduced from borehole breakout analysis of 22 wells from onshore Sicily. The results allow us to distinguish the stress field of different geological units: (1) A nearly NNW (148°) orientation is detected in the Hyblean Plateau. (2) A NNE SHmax direction characterizes the Gela area. To the north, within the thrust belt (around Mount Judica), the SHmax direction swings to NE. In the northeastern segment of the foredeep, the Catania Plain, the direction of SHmax is roughly parallel to the NE trending grabens that mark the northern margin of the Hyblean Plateau. (3) The southwestern segment of the foredeep has no preferential SHmax orientation and may act as a transition zone with isotropic horizontal stresses. (4) In western Sicily a SHmax orientation from N–S to NW–SE is observed. This fits well the kinematics of the SE migrating Egadi and Adventure thrust belts and the direction of shortening inferred from the modern seismicity of the area. A three‐dimensional finite element modeling, including the most important tectonic features, was performed. Modeling results of the stress field indicate that the NNW trending SHmax is locally influenced by the variation of crustal thickness and local zones of weakness. In addition, the Pantelleria Rift causes a rotation of the regional NW–SE stress orientation to NNE along the northern rim of the rift system, including large on‐shore areas in the adjacent central south Sicily.

Structural styles and regional tectonic setting of the “Gela Nappe” and frontal part of the Maghrebian thrust belt in Sicily

The Gela Nappe of south central Sicily provides an example of a curved segment of an orogenic front that can be examined both onshore and offshore for deformational style and amount of shortening. Synorogenic sediments allow the deformation to be dated. Two distinct structural styles are observed in the Gela Nappe: The central salient part of the nappe (Caltanissetta basin) consists of a single thrust sheet containing a train of continuously tightening folds and the reentrant margins of the nappe (Sciacca and Monte Judica) consist of a stack of several thrust sheets. These different structural styles correspond to the pretectonic Mesozoic stratigraphy of the foreland plate. Carbonate platforms exist on the Adventure bank and Hyblean Plateau ahead of Sciacca and Monte Judica, respectively, while the Caltanissetta basin region appears to have accumulated basinal clay facies. Where the resistant carbonate stratigraphy provides a buttress to the propagation of the thrust front, deformation is taken up by imbrication on‐steep ramps through the carbonates generating a relatively thick orogenic wedge. In the basinal setting, where no strong rheology exists, the low angle of friction on the clay detachment levels requires the growing thrust wedge to be much thinner with a very low foreland dip. Hence the thrust front propagates much farther forward into the basin than it does in the adjacent platformal areas, producing a nonlinear thrust front. In the basinal region, accretion of foreland material to the nappe by imbrication was only prominent during the Messinian when subaerial exposure prevented low‐friction transport of the nappe across the highest levels of the stratigraphy. A steady thickening of the nappe by internal folding suggests an increase in friction along the basal detachment, possibly due to progressive compaction of the clays.

Garnet-spinel-pyroxenite xenoliths from hyblean plateau (South-eastern Sicily, Italy)

Rare garnet-spinel pyroxenite xenoliths occur in some basaltic tuff-breccia levels of Miocene age from the Valle Guffari (Hyblean Plateau, Sicily), together with a number of spinel-bearing mantle xenoliths. The garnet-bearing pyroxenites may be divided into two groups (“a” and “b”) on textural and mineralogical bases. Garnet-bearing spinel websterites with a fully recrystallized texture represent the first group (a). Here the garnet (Py54.5 A1m32 Gr13.5), with a diffuse kelyphitic alteration, forms a reaction corona between coarse spinel grains and the in contact pyroxenes. The transition from the spinel-pyroxenite to the garnet-pyroxenite field may depend on isobaric cooling from higher (magmatic?) temperatures. Garnet-pyroxene geothermometry indicates that the last equilibration most probably occurred at P = 1.0 GPa (ca.), T = 750 °C (ca). The second lithotype (b) is an orthopyroxene-bearing garnet-spinel clinopyroxenite, exhibiting a complex texture. It consists of zones of coarse clinopyroxene grains enclosing euhedral spinel passing to zones where tiny rounded crystals of the same pyroxene and spinel are enclosed in relatively large patches of extensively kelyphitisized garnet (Py64.8 Alm25.6 Gr9.6). Garnet also occurs as inclusion-free grains up to 4 mm in diameter. P-T calculations give significantly higher values than for the former case (a). The origin of the b-type garnet may also depend on subsolidus reaction of spinel and pyroxenes after an isobaric cooling from still higher temperatures, but a primary magrnatic origin might also be possible, especially for the “granular” garnets. P-T estimates for both the pyroxenite types closely match a steady geotherm for 100 mW/m2 surface heat flow. Such a relatively intense heat flow may suggest the occurrence of huge masses of hot magma intruding the Hyblean lithospheric mantle and lower crust at different levels.

Plio–Quaternary tectonic evolution and structure of the Catania foredeep, the northern Hyblean Plateau and the Ionian shelf (SE Sicily)

Available multi- and single-channel seismic reflection profiles, calibrated by onshore borehole data, have been used for defining the structural styles in the shelf and slope of the Ionian Sea between Catania and Augusta (SE Sicily). The geological and geophysical data suggest that this area represents a segment of the foredeep–foreland system which collapsed after Late Pliocene times. The foundering was controlled by normal faults trending NE–SW, which flank the southern margin of the Catania foredeep. Onland, in outcrop, these faults appear largely to be post-dated by Lower Pleistocene sediments, nearshore carbonates passing laterally into basinal clays, which lie unconformably upon older substrata. Offshore, close to the southern edge of the foredeep, seismic lines allow recognition of two distinct units: a syn-rift wedge (Upper Pliocene submarine tholeiites and sediments), and a post-rift sequence which can be correlated with Lower Pleistocene carbonates, sands and clays recognisable on land, both in outcrop and by borehole data. The true frontal part of the thrust belt, as detected by the seismic lines, occupies the inner part of the area investigated and is buried by Upper Pliocene and Lower Pleistocene sediments. However, the compressive deformation seems to propagate toward the south-southeast by means of growing detachment levels developing at depth within Pleistocene marine clays, for a length of about 10 km, ahead of the present-day thrust front. Offshore, the faults trending NE–SW are dissected towards the east by faults trending NNW–SSE, subparallel to the Malta Escarpment, which flank the edge of the submerged Messina Rise. These faults, originating in a steep scarp which drops eastwards to the deep Ionian basin, have triggered submarine slides and affected the present-day seafloor sediments. As shown by seismic lines and as stressed by the modern seismicity of the area, some of the faults along the Malta Escarpment could be still active.

Clinopyroxene geobarometry of pyroxenitic xenoliths from Hyblean Plateau (SE Sicily, Italy)

Clinopyroxene geobarometry of pyroxenitic xenoliths from Hyblean Plateau (SE Sicily, Italy)

The evolution of the lithospheric mantle along the N. African Plate: geochemical and isotopic evidence from the tholeiitic and alkaline volcanic rocks of the Hyblean plateau, Italy

The geochemical characteristics of primitive tholeiitic and alkaline volcanic rocks that erupted in the Plio-Pleistocene along fissures on the Hyblean plateau in Sicily (Italy) were used to constrain the mantle sources involved in the volcanic activity of this area of the Mediterranean. It is shown that some of the Hyblean plateau magmas with the most extreme isotopic compositions have combined radiogenic Nd, Sr and Pb, a feature that is distinct from the mixing end-members of the oceanic array. In addition, alkalinity in the basalts is found to be positively correlated with an increase in some HFSE ratios (e.g. Nb/Ta, Zr/Hf) and negatively correlated with ratios between HFSE and MREE (e.g. Ti/Eu), a characteristic that is attributed to a mantle source that has been modified by a carbonatitic metasomatic agent. This metasomatic enrichment had the effect of increasing the U/Pb of portions of the lower lithosphere, possibly by adding phases such as sodic pyroxene and apatite to the basalt-depleted lithosphere. It is suggested that rock types that formed by melting metasomatized portions of lithosphere-asthenosphere boundary affected by this recent enrichment in U/Pb fall along a trend with a shallow slope in a plot of 206Pb/204Pb versus 207Pb/204Pb, and have Nd isotopic compositions that are as radiogenic as present day MORB. The isotopic compositions and trace-element concentrations of the Hyblean plateau basalts are, therefore, mostly consistent with the interaction of a MORB-type mantle source with a young lithosphere that was probably formed in the Phanerozoic and metasomatized by CO2-rich fluids, possibly during the Jurassic. The absence of a geochemical component indicative of involvement of older Proterozoic lithosphere and continental crust in the evolution of these magmas distinguishes them from those erupted along the margins of the Tyrrhenian sea, and supports the suggestion that at least portions of the lithosphere underneath Sicily have oceanic affinity and may be genetically related to the adjacent Ionian abyssal plain.

The subsidence history of the northern Hyblean plateau margin, southeastern Sicily

The southeastern corner of Sicily is dominated by the Hyblean plateau which is composed of carbonate rocks of Triassic to Miocene age. The plateau forms part of the African foreland which collided during Tertiary times with the Calabrian Arc giving rise to the Maghrebian thrust belt. The present study attempts to reconstruct the subsidence history of the northern Hyblean plateau. During the Neogene climax of the plate collision this area has been a centre for widespread basaltic volcanic activity (8-1 Ma) and uplift. At the close of the collisional events (Pliocene-Early Pleistocene) the area under consideration has been affected by subsidence and the progressive consumption of the northern margin of the Hyblean plateau led to the development of the modern foredeep. A quantitative analysis was done by one-dimensional Airy backstripping of seven deep wells located on this crucial part of the Hyblean plateau. The results of the analysis show that the subsidence history of the northern Hyblean plateau area is characterized by four main phases, similar to other domains in the central and eastern Mediterranean basins. These phases are: (a) Neotethyan rifting during Late Triassic-Early Jurassic time; (b) a slow thermal subsidence from Early Jurassic until Late Cretaceous time, leading to the formation of a widespread Mesozoic carbonate platform; (c) compressional phase during Late Cretaceous-Palaeogene time, which resulted in the formation of fold systems; and (d) uplift and subsidence events during the middle Tertiary which are probably the result of the continued collision of the European-African plates. After the Early Pleistocene, unlike other terranes within the Mediterranean, an uplift occurred in the northern Hyblean plateau following the Upper Miocene-Lower Pleistocene massive volcanic activity. This uplift is probably associated with the latest stages of thrusting along the frontal part of the Maghrebian thrust belt.

Late Quaternary slip rates on the Acireale-Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily)

Mt. Etna is located along the east coast of Sicily, near the boundary between the continental crust of the Hyblean Plateau and the Mesozoic oceanic crust of the Ionian basin. The main active faults near Mt. Etna cut the base of its eastern flank, forming a 30 km long system of NNE- and NNW-trending, ene´chelon fault segments (the Acireale-Piedimonte system), showing dip-slip and oblique (right-lateral) motion. Most segments are associated with shallow-depth seismicity and all have Late Pleistocene to Holocene vertical slip rates ranging between 1 and 2 mm/yr, typical of major active normal faults worldwide. Eruptive fissures, arranged in NNE- to NE-trending zones, cut the highest slopes of the volcano, on the footwall of the normal fault system. Structural analysis suggests that current motions along both the active faults and eruptive fissures are kinematically compatible and simply linked with ongoing, WNW-ESE-directed regional extension. Such extension characterizes, at a greater scale, the active tectonics of southern Italy, where all large, shallow, historical earthquakes have remained confined within a narrow normal fault belt or rift zone stretching from the northern Calabrian Arc to southeastern Sicily. The southernmost, west-dipping, faults of that rift zone in Calabria (Aspromonte) cut across the Calabro-Peloritan thrust belt to join the mostly east-dipping, Acireale-Piedimonte faults, along the western boundary of Ionian oceanic lithosphere, which continues southwards as the Malta escarpment. We thus relate magmatism at Etna to dilational strain on the footwall of an east-facing, crustal-scale normal fault at the bend where the Siculo-Calabrian rift zone veers ≈ 60° eastwards as it begins to cross the Calabrian Arc, leaving the east-facing margin of the Ionian Sea, to catch up with the west-facing margin of the Tyrrhenian Sea farther north.