SE Sicily Research Articles

Analysis of the seismicity of Southeastern Sicily: a proposed tectonic interpretation

Southeastern Sicily is one of the Italian regions with high seismic risk and is characterised by the occurrence in the past of large destructive events (Ms = 6.4-7.3) over a territory which is densely urbanised today. The main earthquakes were analysed and some minor damaging shocks reviewed to investigate the main seismogenic features of the region. The comparison between the pattern of seismicity and evidence of Quaternary tectonics allowed us to propose a first tentative, tectonic interpretation of the earthquakes. On the whole, the seismicity of SE Sicily seems distributed along regional fault systems which have had a role in the recent geodynamic evolution of the area. The Malta escarpment, the only structure whose late Quaternary-recent activity is currently known, appears the most probable source for earthquakes with about 7 magnitude. Although no evidence of tectonics subsequent to the middle Pleistocene is available for them, the Scicli line and the NE-SW fault system delimiting the northern sector of the Hyblean plateau seem seismically active with events with maximum magnitude of 5.2 and 6.4, respectively.

Indoor and soil radon measurements in the Hyblean Foreland (South-East Sicily)

Indoor radon behavior in two sites of SE Sicily was studied as a function of the soil radon concentration. The chosen locations were Ragusa and Modica towns, placed in the Hyblean Plateau (northern margin of the African Plate). Soil samples were analysed by gamma spectrometry to determine the amount of radionuclides. Indoor air and soil gas radon measurements were simultaneously performed in both sites using active detectors. Radon in soil was measured one meter deep. A positive correlation was obtained between indoor radon concentration and the soil gas concentration.

Comparative study of ultramafic xenoliths and associated lavas from South-Eastern Sicily: nature of the lithospheric mantle and insights on magma genesis

Isotope analyses of lavas from the Hyblean Plateau (SE Sicily) provided first order constraints for the characterization of the relative magma sources, contributing to a better understanding of the tectono-magmatic evolution of the northernmost part of the African plate. To integrate this research, we are currently studying ultramafic xenoliths of mantle provenance exhumed by the Hyblean alkaline volcanics. These mainly consist of anhydrous spinel-facies peridotites and subordinate pyroxenites/websterites. The paragenesis of the latter is extremely variable in terms of modal proportions and mineral composition, with clinopyroxene composition ranging from Cr-diopside to Al-augite, and variable amount of spinel ± garnet. New Sr-Nd isotopic analyses carried out on hand-picked (and leached) clinopyroxenes indicate that peridotites have 87Sr/86Sr ranging from 0.70288 to 0.70309, and 143Nd/144Nd from 0.51287 to 0.51292. This Sr-Nd isotopic fingerprint approaches that of the HIMU mantle end-member, regionally referred as EAR (European Astenospheric Reservoir) to emphasize a connection with a sub-lithospheric metasomatic component ubiquitous throughout Europe, the Mediterranean area and North Africa. The Sr-Nd analyses of pyroxenites also reveal an EAR affinity, but are slightly distinct from those of peridotites. They display 87Sr/86Sr ranging between 0.70305–0.70326 and 143Nd/144Nd between 0.51292–0.51299, overlapping the composition typical of the Hyblean alkaline lavas. This possibly implies that pyroxene-rich domain significantly contributed to the genesis of the Hyblean magmas. Therefore, the presented Sr-Nd isotopic ratios suggest that the role of the pyroxene-rich mantle portions was more important than commonly considered in the petrogenetic models of Hyblean magmas. This scenario would be coherent with recent evidences highlighting the importance of pyroxenites (together with peridotites) as contributing sources to basalt generation.

Impact of human activities on the central Mediterranean offshore: Evidence from Hg distribution in box-core sediments from the Ionian Sea

Total Hg concentrations have been measured for five box-core sediments collected seawards of the Augusta industrial area (SE Sicily). In more coastal sediments, upcore increasing Hg concentrations, exceeding the Hg background concentration estimated for the Strait of Sicily, indicate Hg contamination over time due to the industrial area development. Strong correlation between total organic C (TOC) and Hg concentrations was found only for core BX2, that displays organic C to total N (C/N) ratios indicative of autochthonous organic matter. For other sediments, high Hg enrichment factors with respect to TOC indicate, in addition to Hg trapping by TOC, other factors as responsible for Hg accumulation. In the presence of some contribution of detrital organic matter, Hg is mainly adsorbed onto the mineral component of the bottom sediments probably because TOC is saturated by Hg excess. Contaminant impact affected also the open sea environment. Main drivers of Hg flux towards the offshore were dredged materials, which repeatedly discharged sediment, resulting in substantial increases in TOC contents and high C/N ratios. Consistent with the geochemistry of recent turbidites, these anomalous sedimentary inputs induced sediment redox environment modifications, constrained by Mn peaks, which affected Hg distribution.

The migration of plate boundaries in SE Sicily: Influence on the large-scale kinematic model of the African promontory in southern Italy

A detailed field mapping, coupled with structural analyses and morphological investigation, has been carried out along the northern and western borders of the Hyblean Plateau (SE Sicily), in order to define the nature and the kinematics of a major Quaternary fault belt. This, here designed as the Scicli Line Fault Belt, is composed of two N50 oriented extensional basins that, linked by a regional N10 trending transfer zone, originated during the Early Pleistocene and experienced, since the Late Quaternary, a positive tectonic inversion. In both the two stages of deformation, the Scicli Line Fault Belt has been characterised by displacement-rate comparable with the relative velocities measured between the distinct plates composing the central Mediterranean region. In the period going from 1.5–1.2 to 0.85 Ma, the fault belt accommodated the entire divergence between Adria and Nubia. At present, the Scicli Line Fault Belt absorbs most of the Nubia–Eurasia convergence, while the western divergent margin of the Adria microplate has jumped to the eastern and the southern margins of the Hyblean Plateau, along the Late Quaternary Siculo–Calabrian Rift Zone. The off-shore prolongation of the two tectonic boundaries of the Hyblean Plateau has been recognised in the Sicily Channel, where they are both interrupted by a WNW-ESE oriented dextral fault. According to our reconstruction, the Hyblean Plateau represents an isolated lithospheric block, whose evolution can be related to the propagation of the western divergent margin of the Adria microplate, accompanied with the southward migration of the triple junction between Eurasia, Nubia and Adria. In this new large-scale kinematic picture, the GPS velocity measured in the Hyblean region, at the permanent site of NOTO, is actually representative of the local kinematics, rather than of the entire African promontory of southern Italy. This implies a correction of previous regional kinematic models based on combination of GPS vectors. In particular, our data constrain a new interpretation both for the kinematics along the E–W oriented Nubia–Eurasia margin, dominated by prevalent dextral deformation rather than reverse motions, and for the intraplate deformation in the Sicily Channel, within the Africa promontory, which would be dominated by a roughly N110° oriented extension. This conclusion has implication also on the mechanism and the origin of the Pantelleria–Linosa–Malta Rift that is here interpreted as a transtensive feature developed along a major transform fault, rather than the result of passive rifting induced by the Nubia–Eurasia collision, as it is currently interpreted.

Metasomatism of the upper mantle beneath the Hyblean Plateau (Sicily): evidence from pyroxenes and glass in peridotite xenoliths

Abstract Coarse-grained, protogranular spinel peridotite xenoliths in the Upper Miocene Valle Guffari diatreme display a complex history for the shallow mantle underneath the Hyblean Plateau (SE Sicily). The mineral assemblage and composition (olivine Fo 89–91 , orthopyroxene En 88–91 , Cr-diopside En 48–49 Fs 4–6 Wo 45–48 , Cr-rich spinel with cr-number=25–39) record at least one depletion event caused by melt extraction, followed by metasomatic enrichment. One of these samples (HYB40) hosts a fresh glass vein. Rare earth elements (REE) in clinopyroxenes from these peridotites show three patterns: (1) light REE-enriched (La n /Yb n =7–17); (2) spoon-shaped (La n /Yb n =18–20; La n /Sm n =21–34; Sm n /Yb n <1); (3) nearly flat (La n /Yb n ∼3). Whole-rock and clinopyroxene trace elements indicate that these patterns are associated with more or less complete equilibration with at least two distinct metasomatic melts: an alkaline silicate melt resembling the host basalt and a hawaiitic melt (for peridotite HYB40). P – T estimates yield 0.9–1.2 GPa and 870–1050 °C, suggesting that refertilization by metasomatizing melts occurred at the Crust–Mantle boundary or just below. In addition, the P – T data coincide with the palaeogeotherm reported by an earlier worker that is consistent with a high geothermal gradient. However, this thermal regime does not fit with the occurrence of an active mantle plume beneath the Hyblean area because of the deduced mantle potential temperatures, which are almost 200°C lower than those typical for a mantle plume. f O 2 calculation gives a redox state above the fayalite–magnetite–quartz buffer FMQ (up to +1.7 Δlog units) related to melt-driven metasomatism.

Mercury levels in sediments of central Mediterranean Sea: A 150+ year record from box-cores recovered in the Strait of Sicily

To evaluate the degree of anthropogenic mercury pollution, Hg contents have been measured for box-core sediments sampled along three nearshore–offshore transects in the Strait of Sicily and well constrained for their mineralogy, bulk geochemistry and TOC%. Hg values are generally low (from 15 to 70 μg kg −1); however, depth profiles clearly display upcore rising concentrations (up to 202 μg kg −1 near the SE Sicily coast) that are attributed to anthropogenic load. Based on 210Pb chronology, these trends are more dramatic across the last 20–30 years. Geogenic influence is thought to explain some anomalies related to volcanic emission and geothermal activity. Combined effects due to eastward flowing of the Modified Atlantic Water (MAW) and sea bottom morphology drove major anthropogenic accumulation.

Late Pliocene volcanic island growth and flood basalt‐like lava emplacement in the Hyblean Mountains (SE Sicily)

Since ∼200 Ma, the Hyblean Mountains (southeastern Sicily) have been the site of recurrent, mainly submarine, mafic volcanism. In the upper Pliocene, large volumes of tholeiitic products were erupted in a shallow marine environment, leading to the growth of volcanic islands. The deposits of this tholeiitic episode can be grouped into two main facies associations: (1) pillow lavas and hyaloclastite tuffs erupted and deposited entirely under water, and (2) submarine pillow breccias, fed from subaerial lava flows entering the ocean and subaerial lavas, which are distinguished into a coastal lava platform facies and an inland facies. Lava flows into the ocean led to the growth of lava deltas consisting of a basal flow foot breccia and an overlying sequence of pāhoehoe flow units, the latter rarely exceeding 0.5 m in thickness. In outcrop, this facies association shows evidence for significant instability of the lava deltas during growth, indicating very similar processes to those observed during the current eruption of Kīlauea volcano, Hawaii. Away from the coast of the growing island, the inland facies is characterized by thicker (up to several meters) pāhoehoe flow units showing evidence for sheet flow inflation and limited outcrops of scoria and/or bomb deposits. Evidence for lava transport through tubes exists in the form of tumuli and multiple vesicle layers; drained tubes are absent. All tholeiitic products were emitted in a single, instantaneous, voluminous event (≥120 km3), consistent with very high effusion rates similar to those of flood basalts or the 1783 Skaftár Fires (Iceland) eruption.

Multi-disciplinary study of the heavy-oil reservoirs in the Armatella Field, Sicily

The Armatella Field in SE Sicily is structurally complex with two fractured, carbonate reservoirs containing heavy-oils, with the heavier hydrocarbon overlying the lighter one. One well has been drilled in the field – a vertical pilot completed with two opposite, stacking, open-hole horizontal drains. A complex compositional evolution of the fluids produced has been observed. A reservoir study integrating geophysics, geochemistry, petrophysics and reservoir engineering assessed the reservoir geometry and the field production behaviour. This allowed the identification of two different phases of hydrocarbon migration that were geologically constrained according to the structural evolution of SE Sicily. The 3D structural reconstruction of the field, built with a Shared Earth Model, was utilized as input to a 3D dual porosity numerical model. The dynamic simulation allowed identification of the drilling location of an additional non-conventional well aimed at draining undeveloped reserves. The study used advanced laboratory methodologies for fluid characterization, state-of-the-art methods for analysing the reservoir fracture network and properly designed modules for simulating non-conventional completions in a 3D dynamic model.

Structural and taphonomic analysis of a columnar coralline algal build-up from SE Sicily

Unusual “coralligenous” build-ups were found at the infra–circalittoral boundary from SE Sicily. They are columnar, rising up perpendicularly from the bottom, and consist of an inter-growth of encrusting algae and invertebrates. A selected build-up and its longitudinal section, together with some thin sections of selected parts, were studied to detail the growth structures, composition and fabric of the trapped sediments. The coralline algae show a succession of “concentric”, “fruticulose” and “foliaceous” structures. The framework delineates cavities bounded by algal thalli, sometimes filled by sediments. The organisms constituting the framework were grouped into functional guilds (Fagerström, 1991): “primary frame builders” (mainly coralline algae); “secondary frame builders” (vermetids, serpulids and some bryozoans); “bafflers” (erect, flexible bryozoans, some sponges and soft algae); “binders” (encrusting bryozoans, some crustose coralline algae and serpulids); “destroyers” (a few endolithic algae, rare sponges and bivalves); “dwellers” (brachiopods, foraminifers, bivalves, small bryozoans and some motile organisms). The distribution pattern of taphonomic features (composition, grain size and distribution of sediments filling cavities; presence of cement and its location; mineralogical composition and crystal size), together with interactions between growth structures and functional guilds, allowed one to distinguish three taphofacies, each recording different environmental conditions. They have been tentatively related with cyclic sequences testifying to local hydrodynamic regimes, with subsequent slighter and stronger intensity phases, during the last 2200 years.

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.

Cooling evidence from Pleistocene shelf assemblages in SE Sicily

Abstract A short Lower Pleistocene section, cropping out in SE Sicily, is studied. The sequence mainly consists of richly fossiliferous sandy layers with Arctica islandica , deposited in mid-shelf environments. Molluscs, bryozoans, serpuloideans and calcareous algae are investigated, to identify species with palaeoclimatological implications. Apart from some well-known North Atlantic molluscs, other species are identified as palaeoclimatological tools. Cooling evidence is provided by such palaeoclimatological indicators and by the increasing shell size in some species.

Mantle sources of the Cenozoic Iblean volcanism (SE Sicily, Italy): Sr-Nd-Pb isotopic constraints

A wide range of relatively undifferentiated lavas, ranging in composition from quartz-tholeiites to ankaratrites, were erupted in the Iblean area (SE Sicily) during the Upper Miocene-Lower Pleistocene. New Sr-Nd-Pb isotope analyses, presented in this paper, have been used to constrain the petrogenesis of the Iblean magmas, and to characterise their sources. The results (87Sr/86Sr: 0.70271–0.70328;143Nd/144Nd: 0.51325–0.51291;206pb/204Pb: 19.25–20.00) indicate that the Iblean lavas are mantle-derived anorogenic melts, involving DM+HIMU mantle components. Isotope and trace element data suggest that magmas formed from a heterogeneous mantle source that equilibrated in the spinel-peridotite field; the Iblean melts were thus formed within the subcontinental lithosphere. The mantle source region, was originally characterised by a depleted isotopic signature (DM), and was repeatedly and variably impregnated (refertilized) by metasomatic agents of sublithospheric origin characterised by a HIMU signature.

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)

Strontium geochemistry of Miocene primary gypsum; Messinian of southeastern Spain and Sicily and Badenian of Poland

ABSTRACT The strontium contents of primary marine gypsum facies were studied in Messinian (Upper Miocene) basins of SE Spain and Sicily, and in the Badenian (Middle Miocene) Carpathian foreland basin of Poland. The results obtained were interpreted on the basis of the findings reported for the gypsum facies of recent salinas in the western Mediterranean coast. The strontium profile obtained from the lower part of the gypsum deposit of the Sorbas basin (SE Spain) has homogeneous strontium contents, reflecting deposition under conditions of hydrological stability and relatively deep waters, but the upper part has a more variable strontium content related, in accordance with other sedimentological criteria, to a shallowing of the basin. This is consistent with data reported from other Messinian basins of SE Spain (San Miguel de Salinas and Palma de Mallorca). In the Caltanissetta basin (Sicily), the section studied in Eraclea Minoa (Upper Evaporite) is made up of 6-8 gypsum cycles that have a facies evolution from fine-grained laminar gypsum at the base of each cycle to selenites at the top, accompanied by increasing strontium values. This evolution is attributed to increasing salinity conditions and is consistent with geochemical data reported in the literature. In the Badenian basin the six gypsum sections studied are from different subbasins. The gypsum deposits comprise two members. In the lower member, which is mainly selenitic, two parts can be differentiated and a significant strontium increase (attributed to a rise in salinity) was recorded in the transition from the lower to the upper part. In the upper member, which is mainly clastic gypsum, a general strontium decrease indicates a fall in salinity linked to a deepening or freshening of the platform. In the Badenian selenitic layers a lateral variation in the strontium contents exists: higher and less homogeneous strontium contents were recorded in the marginal zones, suggesting variable salinity conditions, whereas in the deeper areas the gypsum layers show more homogeneous strontium values, expected from more stable and less concentrated brines. This distribution is in agreement with data reported from the Messinian basin of San Miguel de Salinas (SE Spain). The strontium contents in both the Messinian and Badenian selenitic samples are lower than those in the selenitic crusts of the recent salinas. These lower contents may be a result of a lower salinity of the mother brines at the steady state achieved during the precipitation of the thick selenitic sequences, with respect to the highly concentrated (with no reflux) brines of the recent salinas. Our findings highlight the relevance of studying strontium contents in primary gypsum facies of marine formations as a tool for interpreting the paleogeography and the paleosalinity of the evaporitic enviroents and for correlating cyclic selenite formations. This method could be especially helpful when combined with other geochemical indicators (i.e., isotopic and/or fluid inclusions composition).

First data on the coastal dynamics and the sedimentary characteristics of the area influenced by the River Irminio basin (SE Sicily)

The various lithological outcrops in the drainage basin of the River Irminio are described for their contribution to the riverborne material and therefore to the sedimentary balance of the coastal strip. The main contribution to the present sedimentary balance of the subaqueous delta cannot be attributed to the River Irminio, owing to the extreme difference between fluvial and marine sediments. The river's hydraulic characteristics have not always been constant but have tended to decrease in three stages: the first, with a strong competence, shown by the Quaternary alluvial sediments with pebbles; the second, up to 1985, of medium competence, shown by sandy-gravelly deposits in the coastal strip and by finer sediments in the distal areas; the third, at the present day, with low competence, shown by a prevalently suspended load sediment with practically no fluvial influence and with a sedimentation dominated by the littoral currents; this is because between the 1960s and 1970s, owing to interventions along the river channel, the sediments from the basin were reduced to the point of non-existence and a new sedimentary cycle was set up on the delta with allochthonous sediments, exclusively made up of quartz clasts, whereas in the distal zones the mineralogical and compositional characteristics of the ancient river mouth were maintained.

An ion microprobe study of clinopyroxenes in websteritic and megacrystic xenoliths from Hyblean Plateau (SE Sicily, Italy): constraints on HFSE/REE/Sr fractionation at mantle depth

Eleven clinopyroxenes from websteritic (Cr-diopside- and Al-diopside-bearing) and Al-augite±orthopyroxene±spinel-bearing megacrystic xenoliths occurring in alkaline volcanics of the Hyblean Plateau (SE Sicily) were analysed for rare-earth and other trace elements (Sc, Ti, V, Cr, Sr, Y, Zr) with an ion microprobe. Trace-element analysis shows that Hyblean pyroxenites are crystal/liquid segregates of basaltic magmas characterized by different composition (from tholeiitic to alkaline) and variable LILE enrichment. Megacrystic clinopyroxenes are, most probably, high-pressure liquidus phases of OIB-type alkaline basalts characterized by smooth chondrite-normalized trace-element abundance plots, except for a slight negative Sr anomaly. Clinopyroxenes from websterites crystallized from either alkaline or tholeiitic melts and later experienced metamorphic (orthopyroxene exsolution) and/or metasomatic (interaction with fluids or melts) processes which modified to variable extent their original major-and trace-element signatures. In Hyblean high-pressure alkaline basalts, compositional factors (i.e. major-element activities in the melt and clinopyroxene crystal chemical configuration) caused a significant D Ti increase, mild D REE and D Zr increases, and virtually no D Sr variations with increasing degree of fractionation. In particular, clinopyroxene crystal chemical constraints, namely electrostatic balance and M1-site geometry, appear to play a major role in the creation of HFSE/REE and Sr/REE decoupling in clinopyroxene-bearing mantle rocks. A sharp decrease of D Zr with increasing pressure is predicted from crystal chemical considerations.

The Pelagian Block (Central Mediterranean): Exploration and New Opportunities: ABSTRACT

The Pelagian Block, extending from eastern Tunisia to offshore western Libya and northward to Malta and Sicily, is a complex foreland area structurally active since the Carboniferous, that separates western North Africa from southern Italy. The north (Sicily) and central parts of the Pelagian Block contain Triassic-Lower Jurassic peritidal carbonates with rare evaporites; Middle Jurassic-Eocene pelagic carbonates, and paralic to marine clastics of Oligocene to Pleistocene age, with Messinian evaporites. The Middle Jurassic/Eocene platform to pelagic carbonates of Sicily are gradually replaced southward by basin margin, marine to paralic carbonates and clastics. In eastern Tunisia, these rocks overlie Triassic-Lower Jurasic evaporites. Principal petroleum resources occur offshore SE Sicily, offshore Tunisia and in adjacent Libyan waters. The largest oil fields are Bouri (800 MMB recoverable reserves) and Ashtart (recoverable reserves of 250 MMB). The largest gas field is Miskar (recoverable reserves of 800 BCF). Smaller accumulations are distributed throughout the region. New oil discoveries, ranging in size from 8 MMB to 50 MMB, have recently been made onshore and in the shallow offshore. Proven and potential source beds for hydrocarbons occur in various paleotectonic settings: anoxic lagoons formed in the early continental rifting stages (Triassic/Liassic organic rich units of SE Sicily);more » subcontinental/paralic coals and shales (Lower Cretaceous of Tunisia); and deeper water anoxic basins (late Mesozoic and Tertiary organic sequences). Additional exploration opportunities are expected in undrilled or sparsely drilled acreage, with traditional plays similar to those tested in the past; or in new plays directed to the exploration of new reservoir objectives.« less