Hyblean Plateau Research Articles

Mesozoic and Paleogene megabreccias in Southern Sicily: New data on the Triassic Paleomargin of the Siculo-Tunisian platform

Mesozoic and Paleogene clastic carbonates in deep-water successions outcropping in the Sicani mountains (central southern Sicily) represent debris-flow and turbidite deposits accumulated in slope/base-of-slope sectors of the Sicanian Basin, a Permian to Miocene deep-water sedimentary domain of Sicily. Reef-derived carbonates of late Triassic age are frequently found among the clastic elements of these deposits, in association with other shallow and deep-water Mesozoic carbonates. The provide us with new data on the stratigraphic setting of a platform paleomargin now buried beneath the Sicilian thrust and fold belt. This paleomargin bounded the wide middle and upper Triassic carbonate platform which is now known in the subsurface of the Southern Sicilian thrust and fold belt. This paleomargin bounded the wide middle and upper Triassic carbonate platform which is now known in the subsurface of the Southern Sicilian mainland and offshore in the Pelagian Platform, from the Malta escarpment to the Sciacca and Trapani areas through the Hyblean Plateau. The hinge zones between this platform domain and the Sicanian basin were particularly affected by the paleostresses related to the Mesozoic and Paleogene evolution of the Southwestern Tethys. The sedimentary successions of these areas recorded repeated episodes of progradation, aggradation, backstepping, uplift and erosion of the platform-basin system, under eustatic and tectonic forcing.

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.

Neotectonic deformations in SE Sicily: The Ispica fault, evidence of late miocene-pleistocene decoupled wrenching within the central mediterranean stress regime

The Ispica fault is a NE-SW trending dextral transpression zone on the Hyblean Plateau in SE Sicily. Drag folds in Late Miocene to Pleistocene sediments within a narrow bend along the fault indicate neotectonic activity. The Ispica fault is part of a continental transform zone connecting the recent Pantelleria Rift system between Sicily and Tunisia with the eastern Sicilian active thrust belt (Gela Nappe). On the Hyblean foreland, regional fold- and thrust-planes in Upper Miocene-Pleistocene strata are parallel to the Ispica fault and reflect SE-NW directed maximum horizontal compression corresponding to the central Mediterranean stress regime. The contemporaneity of parallel strike-slip movements along the Ispica fault and the formation of regional parallel folds suggests decoupled wrenching for the Ispica fault.

Palaeoenvironmental, paleontologic and stratigraphic significance of vertebrate remains in pleistocene limnic and alluvial deposits from southeastern Sicily

In southeastern Sicily on the Hyblean Plateau limnic deposits overlie early Pleistocene marine sediments and are covered by early Middle Pleistocene marine sands. In the limnic deposits fossil remains of large mammals have been collected and these represent the oldest found in a well-established Pleistocene stratigraphic succession in Sicily. The faunal association includes: Elephas falconeri Busk, the smallest Sicilian elephant, an elephant less reduced in size than E. falconeri, a canid ( Vulpes sp.?), rodents [ Leithia melitensis (Adams)], Chiroptera, Insectivora, fishes, birds, amphibians ( Bufo sp., Discoglossus sp.), and reptiles ( Lacerta sp., Testudo sp., Emys orbicularis Linnaeus). Alluvial fan deposits overlying the limnic deposits contain Elephas mnaidriensis Adams, Hippopotamus pentlandi Meyer, Cervus siciliae Pohlig and Bovidae. These new findings provide the stratigraphic evidence to definitively discredit previous chronological schemes for the Pleistocene dwarf elephants, which assumed a progressive decrease of the size with time, a hypothesis recently questioned by isoleucine epimerization age determination, which, however were object of discussion. These findings also represent a new step toward a better knowledge of the taphonomic conditions of the Pleistocene mammal-bearing deposits in Mediterranean islands, traditionally linked to karstic environment. Distribution of mammal faunas in the Hyblean Plateau proves fundamental in recognizing and dating Pleistocene mammal faunas in Sicily; the new findings fully confirm the assumption that Sicily experienced two elephant immigrations in Pleistocene times.

Origin and deformation of the Neogene–Recent Maghrebian foredeep at the Gela Nappe, SE Sicily

SE Sicily contains one of the few exposures of orogenic foreland to the Maghrebian-Apennine orogenic system in the central Mediterranean. Here the African foreland is represented by the Hyblean plateau, adjacent to the frontal thrust structures of the Maghrebian chain. These structures include the disrupted foredeeps to earlier parts of the thrust system. One of these, the Gela nappe, is a major structure composed of late Oligocene to early Pliocene sediments emplaced onto Pliocene–Pleistocene rocks of the buried foreland. This paper presents a correlation of late Miocene stratigraphic units between the foreland and the foredeep basin. Abrupt lithological and thickness variations of these units suggest that the foredeep basin was limited towards the foreland by normal faults. In the hinterland these foredeep sediments lap back onto previously emplaced thrust sheets, including the far-travelled units of ‘Numidian flysch’. The precursor Oligocene–Serravallian foredeep may also have been controlled by normal faulting. Compressional tectonics are reflected in folding and thrusting which post-dates deposition of lower Pliocene chalks (Trubi Formation), although minor folding and erosion preceded this unit. Larger thrusts repeat the basin stratigraphy and carry it out onto the adjacent foreland. A minimum of 8 km shortening since Messinian times can be demonstrated although a more realistic figure is c. 25 km, at a time-averaged rate off. 0.5cm/a −1 . The stratigraphic relationships on the foredeep sedimentary sequences may provide analogues for offshore areas of the thrust belt.

Tectonic analysis of a flexed foreland: the Ragusa Platform

Detailed field studies of fracture patterns and paleostress reconstructions based on analyses of fault-slip data sets allow the identification of the Late Cenozoic stress pattern evolution of the Ragusa Platform. During the Late Miocene to Early Pleistocene period, a complex extensional pattern developed. It was dominated in almost the entire Hyblean Plateau by NW-SE to NNW-SSE extensions related to the flexure of the foreland under the load of the allochthonous units of the Sicilian Fold and Thrust Belt. These extensional tectonics were characterized by NE-SW to ENE-WSW trending normal faults and grabens. In eastern Hyblea, NE-SW to ENE-WSW extension have prevailed at least since the Late Cretaceous. They are linked to the effects of the Malta Escarpment. In the western Ragusa Platform, a third, minor, WNW-ESE extension associated with sets of NNE-SSW trending normal faults, may be correlated with the development of the Gela basin. In addition, these major extensions induced complex patterns of perpendicular normal fault systems through permutations between the principal stress axes σ2 and σ3. The Pliocene-Pleistocene extensional paleostress field evolution is marked in almost the entire platform by the alternation of two or three main extensional stress patterns (NE-SW to NNW-SSE, WNW-ESE and NE-SW to ENE-WSW extensions), each of them controlled by particular boundary conditions. Three minor compressional paleostress orientations have been identified. They correspond to three minor compressional events dominated by 020°–030°, 070°–080° and 100°–110° compressional directions. These events occurred in within the general Pliocene-Pleistocene extensive context. The 100°–110° event is of Early-Middle Pliocene age. Its large regional distribution suggests that this event may have resulted from interactions between the major blocks constituting the Africa-Eurasian boundary. The relative chronology shows that the 020°–030° compression predates the 070°–080° one. The origins of these compressional events are still uncertain due to poor regional control.

Late Cretaceous and Recent tectonic stress orientations recorded by basalt dykes at Capo Passero (southeastern Sicily)

At Capo Passero, on the southeasternmost tip of Sicily (Hyblean Plateau) a basaltic sequence is intercalated in Upper Cretaceous limestones. This rock succession is transected by a swarm of feeder dykes (84-71 Ma) trending roughly NE-SW. The basaltic dykes show the same orientation as the direction of maximum horizontal compression which has been inferred for the Late Cretaceous by the analysis of micro- and mesostructures on the coeval deep-marine carbonates (Amerillo Fm.) on the western margin of the Hyblean Plateau. Deformations within the basaltic dykes during their intrusion have been recognized by the analysis of petrofabrics. Microstructures consist of sinistral strike-slip faults trending N80°, which probably formed during the late stage of cooling of the magma. The analysis of the anisotropy of the magnetic susceptibility (AMS) highlights the primary nature of the magmatic fabric. The magnetic foliation is in fact subparallel to the walls of the dykes and corresponds to the magma flow. Locally, the Upper Cretaceous dykes are displaced up to 8 m along dextral strike-slip faults trending N110–120° which originated under a tectonic stress regime that is still active in this region, as confirmed by in situ stress measurements carried out in the neighbouring areas. The analysis of microstructures within the dykes which can be ascribed to this new deformational process shows a predominant pattern of fractures trending N120° and N140° infilled with calcite exhibiting different phases of growth. These fractures cut and displace pre-existing structures. A deeper analysis of the directions of the minimum susceptibility k 3 shows that the k 3 component is parallel to the recent shear planes cutting the dykes and fits their sense of movement. This could imply a tectonic overprinting of the magnetic fabric by the intervening recent stress field.

NEOGENE AND QUATERNARY SEDIMENTATION PATTERNS IN THE NORTHWESTERN HYBLEAN PLATEAU (SE SICILY): THE EFFECTS OF A COLLISIONAL PROCESS ON A FORELAND MARGIN

Miocene carbonates and marls in the northwestern margin of the Hyblean Plateau (SE Sicily) exhibit considerable thickness variations consistent with synsedimentary tectonism. In the Early-Middle Mio­cene carbonates (Ragusa Formation) a series of parallel swells dose to the platform edge encouraged the development of a coarse-grained bioclastic facies. The overlying Middle-Late Miocene marls (Tellaro Formation) thin over these seafloor highs and associated mafie Tortonian volcanics are particularly developed along the hinge zones between swells and local sea-floor depressions. The basic extrusives appear to have followed in­cipient NE-SW fault lines propagated from deeper basement fractures. During the succeeding Messinian these faults were activated to produce horst structures (Vallone Lamia horst, Monte Caratabia horst and Cameme horst). These separate locai NE-SW oriented narrow grabens in which thick Messinian evaporites were deposited, probably connected with the main Centrai Sicilian Basin. Further subacrial deposits and volcanics levelled the Hyblean horst and graben topography prior to the major Early Pliocene marine high-stand. The ensuing transgression covered the study area with a thick pelagic Early Pliocene chalk. Local regressions, the product of Middle Pliocene and Early Pleistocene major tectonic activity along the margins of the plateau, are associated with local unconformities. Final emergence along the margins of the plateau occurred at the close of Early Pleistocene times when the emplacement of the Gela Nappe, from the NE, partly filled the foredeep basin. The withdrawal of the sea from the foredeep occurred shortly afterwards.

The sedimentology and development of Terravecchia Formation carbonates (Upper Miocene) of North Central Sicily: Possible eustatic influence on facies development

On at least two occasions during the Late Tortonian, siliciclastic deposition ceased and carbonates accumulated at the northern margin of the “Caltanissetta Basin” in north-central Sicily. Bioclastics dominated in the shallower proximal areas, while coral reef facies developed where shallow-water sand and cobble substrates existed or where sea-floor highs has created local shallows. The genetic model proposed is that siliciclastic deposition was repeatedly terminated during the Late Tortonian by rises in sea level which transgressed fluviatile and deltaic environments. Carbonate deposition was widespread, but, later erosion and reworking removed much of the sediments except within abandoned delta distributary channels and minor sea-floor depressions. Reef developments, associated with channel mouths and the shelf slope break, have been preserved in situ, whereas channels in more proximal locations are filled with siliciclastic, bioclastic and intraclastic rubble from elsewhere on the delta top. The wide lateral extent of the truncated tops of the carbonate beds argues for eustatic, rather than tectonic controls of erosion and subsequent reworking. These events may be closely related in time to other recorded Late Tortonian eustatic events in the foreland zones, such as the Hyblean Plateau and the Maltese Islands further south. Collectively they give some support to a model involving multiple eustatic fluctuations associated with the onset of the Messinian desiccation of the Mediterranean.

Magnetic interpretation related to geo-magnetic provinces: the Italian case history

A comparison of the results obtained from the geophysical interpretation with the available information regarding the Italian geological situation permitted us to establish our so-called “geo-magnetic provinces”. The relationship between the magnetic characteristics and the tectonic evolution permitted the identification of each of the four following “provinces” that exist inside the global tectonic confines: 1. (1) The “Sardo-Corso” block, which represents the eastern boundary of stable Europe. This province is characterized by medium-wavelength low-amplitude anomalies that originate in an outcropping basement (Hercynian granites s.l.). 2. (2) The “Orogenic belt”, which constitutes the double Alps-Apennines system. From a magnetic standpoint, this system is characterized by short-wavelength, low-amplitude anomalies that are due to a Paleozoic outcropping and/or buried, acid, crystalline basement. These anomalies are found to overlie long-wavelength, medium-amplitude regional anomalies that are related to a deep, Apenninic basement (central-northern Apennines). 3. (3) The “Foreland area”, which occurs from Sicily (Hyblean Plateau) and Apulia northwards to Ancona and the Po Plain. This area is characterized by a crystalline basement which is typically “differentiated” from the standpoint of basicity. 4. (4) The “Rifting zone”, which is represented by the part of the Tyrrhenian Sea lying south of the “41st-Parallel discontinuity”, the Ionian Abyssal Plain and the Pantelleria and Linosa troughs in the Sicily Channel. The main characteristic of this province is its extreme basicity differentiation in the crystalline basement, which is induced by the geochemical processes connected with the deep, geodynamic events. These phenomena do not permit the correlation of the semioceanic basement of the Rifting zone with the continental basement. By means of interactive modeling, a structural-tectonic picture of the basement of the various Italian “geo-magnetic provinces” has been obtained. The main result was found to be that the Orogenic belt basement is undergoing intense tectonic compression and overlies the structural highs of the Foreland area, which mainly show tensive events.

Sedimentary and tectonic evolution of the eastern Hyblean Plateau (southeastern Sicily) during late Cretaceous to Quaternary time

Two palaeogeographic domains have been distinguished in the Cretaceous-Quaternary sequences which crop out in the Hyblean region of southeastern Sicily. In the northeast shallow-water algal-bryozoan calcarenites of Oligocene-Miocene age overlie either Cretaceous rudist limestones or volcanics. Further to the northeast, the stratigraphic gap increases as the Miocene limestone thins, and in the A.G.I.P. well Catania 10, Plio-Pleistocene sediments and volcanics directly overlie Upper Jurassic rocks. In the southwestern part of the Hyblean Plateau pelagic sediments occupy the Cretaceous-Miocene interval. They were deposited in a shallow basin dipping gently towards the southwest. The transition between the two zones is marked by much evidence of synsedimentary tectonic activity, such as widespread slumping and the development of megabreccias composed of Cretaceous and Eocene blocks. The palaeogeographic picture is one of carbonate reefs and volcanic seamounts developing in the Upper Cretaceous. They formed platforms, fault bounded to the west and southwest, from which were derived the detrital sediments found in the basin, and occasional spectacular gravity slides. In present-day terms the structural high probably corresponds to the northeastern margin of the Hyblean Plateau and in part to the (Siracusa) Syracuse-Malta Escarpment. The pattern is little changed in the Upper Miocene with volcanoclastics of phreatomagmatic origin, intercalated with coral bioherms, overlying algal-bryozoan calcarenites. Upper Tortonian-Lower Messinian limestones with a restricted fauna indicate a shallow-water environment which grades up into continental conditions. In the eastern Hyblean Plateau, the Tertiary sequence is capped by Pliocene submarine to subaerial mafic lava flows. Lower Pleistocene sediments consist of biocalcarenites and clays which fill deep graben delimited by Plio-Pleistocene faults, the inshore continuation of the Malta-Syracuse Escarpment system of faults. From a tectonic standpoint the Hyblean region, a gently deformed segment of the African continental margin, acted as a foreland during Tertiary times. A Cretaceous-Eocene tectonic phase was responsible for the palaeogeographic pattern which existed during most of the Cenozoic. The present pattern is the result of three main tectonic phases following the regional Upper Miocene uplift, of Early-Middle Pliocene, Plio-Pleistocene and middle Pleistocene-Recent age which influenced the tectonic evolution of the present Ionian coast and the Malta-Syracuse Escarpment.