Rosanna Maniscalco

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.

Timing and magnitude of rotations in the frontal thrust systems of southwestern Sicily

We report new paleomagnetic and anisotropy of magnetic susceptibility (AMS) results from upper Tortonian to middle Pleistocene sediments which were deposited upon and adjacent to active thrust structures in southwestern Sicily. The data show that the Plio-Pleistocene sediments from the Belice and Menfi basins (covering the Saccense shelf limestones) underwent any internal shortening after the early Pleistocene (Santernian), as well as any net rotation. Sediments around this area (which overlie basinal Meso-Cenozoic successions) record systematic rotations: one upper Tortonian site to the west is ∼30° counterclockwise rotated, while to the east, lower Pliocene to middle lower Pleistocene sites within the Gela Nappe domain show 25° to 56° clockwise (CW) rotations. These data show that the ductile basinal sediments were bent and rotated around the rigid Saccense carbonates during the thin-skinned southward propagation of the orogenic front. We document here that the coastal sediments from the southwestern Gela Nappe underwent both a post middle early Pleistocene ∼30°CW rotation and a post middle Pleistocene E–W to ESE–WNW flattening (revealed by AMS). Our data then constrain to the late Pleistocene-Holocene the age of the last shortening episode occurring in the southwestern Gela Nappe front. Pleistocene rotations of similar amount also characterize the Sicanian domain, implying that it was incorporated in the Gela Nappe wedge during the recentmost episodes of deformation. This evidence allows us to better understand the very large (up to 114°) post Mesozoic rotations reported by Channell et al. [1980, 1990] for the Sicanian limestones, as related to both Miocene (or older?) deformational episodes and the Plio-Pleistocene evolution of the Gela Nappe.

Pattern of orogenic rotations in central–eastern Sicily: implications for the timing of spreading in the Tyrrhenian Sea

New palaeomagnetic data from upper Triassic to Pliocene sediments reveal that in eastern Sicily a major 70° clockwise (CW) rotation took place between Oligocene and late Tortonian time, followed by a further 30° CW rotation. Results from central Sicily are less coherent. They show 44–83° post-Oligocene CW rotation, local 14° post-late Tortonian counterclockwise (CCW) rotation, and 25° post-mid-Pliocene CW rotation. We interpret the larger CW rotation observed in eastern Sicily as related to a more internal palaeogeographical position with respect to central Sicily. Our results complement pre-existing data from the northwestern Sicily carbonates, and indicate that all the internal carbonate nappes coherently rotated by c. 100° CW during tectonic emplacement, implying a west-to-east increase of shortening in the Sicilian Maghrebian belt. In Sicily, compressive deformation started during the Langhian, i.e. just after the deposition of the upper Oligocene–upper Burdigalian Numidian Flysch turbidites. Therefore the age of the older 70° palaeomagnetic rotation (synchronous to the thrusting) is constrained to occur between the Langhian and late Tortonian. Furthermore, by considering a maximum possible rotation rate of 20° Ma−1, we infer that CW rotation started in Sicily in Langhian–Serravallian times, between 15–16 and 11–12 Ma ago. The 100° CW rotation observed in pre-orogenic strata from the whole of Sicily is mirrored by 80° orogen-scale CCW rotations characterizing the internal southern Apennines. Palaeomagnetism therefore shows that during orogenesis, the southern Apennines and the Sicilian Maghrebides rotated in a ‘saloon-door’ fashion, synchronous to back-arc spreading of the southern Tyrrhenian Sea. Consequently, our palaeomagnetic data suggest that the southern Tyrrhenian back-arc basin started to spread during Langhian–Serravallian times (from 15–16 to 11–12 Ma), significantly earlier than the late Tortonian age (8 Ma) suggested so far by oceanic drilling data.

Shallow burial and exhumation of the Peloritani Mountains (NE Sicily, Italy): Insight from paleothermal and structural indicators

We used vitrinite refl ectance and mixedlayered clay minerals to investigate levels of diagenesis of the Oligocene-Miocene basin developed on the nappes of the Alpine orogen exposed in the Peloritani Mountains (NE Sicily). Paleothermal indicators were integrated with stratigraphic and structural analyses and published apatite fi ssion-track and (U-Th-Sm)/He ages to defi ne the late evolutionary stages of the Peloritani Mountains. This multimethod approach allowed us to reconstruct the paleogeothermal gradient of the basin in Oligocene-Miocene times, to constrain its burial evolution, and discriminate between areas where it has been affected by sedimentary and/or tectonic load. In the southern area of the basin, organic and inorganic thermal parameters increase as function of depth, suggesting that their evolution was ruled by sedimentary burial. They record a decrease in paleogeothermal gradient values marking the evolution of the basin from a forearc to a thrust-top setting during the convergence-collision process between the Calabria-Peloritani Arc and the African plate. On the other hand, in the northern edge of the basin, vitrinite refl ectance values (0.46%‐0.58%) indicate that the thermal evolution of this area was controlled by tectonic burial related to late Langhian‐early Serravallian out-of-sequence thrust tec tonics. The tectonic overburden has been totally removed by extensional tectonics and/or erosion since the late Miocene. The short time span at maximum temperature (<2 m.y.) elapsing between thrust stack emplacement and the beginning of tectonic overburden removal has allowed only vitrinite refl ectance and thermochronological indicators to record this compressive reactivation.

Correlation of Diagenetic Data from Organic and Inorganic Studies in the Apenninic‐Maghrebian Fold‐and‐Thrust Belt: A Case Study from Eastern Sicily

Temperature‐dependent clay mineral assemblages and vitrinite reflectance data have been used to investigate levels of diagenesis from the Apenninic‐Maghrebian fold‐and‐thrust belt in eastern Sicily at the footwall of the Peloritani‐Calabride Arc. Data are from units sampled along a regional transect between the Nebrodi Mountains to the north and Mount Judica to the south. These units developed in very different tectonic settings from those of oceanic to passive continental margin domains deformed during the Cenozoic mountain building and related active margin deposits. The integration of organic and inorganic thermal indicators allowed us to distinguish among different tectonic settings, with thermal maturity generally decreasing from hinterland to foreland as a result of progressively less severe thermal evolution and/or tectonic loading during the mountain building. Specifically, the highest vitrinite reflectance (VRo%) values (ca. 0.60%–0.75%) and percentages of illite layers in illite‐smectite (I‐S; 60%–80%) are found in trench‐involved and accreted passive margin units. Lower VRo% values (0.20%–0.47%) and percentages of illite layers in I‐S (30%–60%) are found in thrust‐top and foredeep basin deposits and far‐traveled Sicilide units that have escaped involvement in trench evolution. Furthermore, either sedimentary or long‐lived tectonic burial (at least more than 5 m.yr.) seem to have affected levels of diagenesis of the studied successions. The correlation between organic and inorganic thermal indicators is satisfactory for most of the samples derived from hemipelagic and siliciclastic deposits, whereas it is poor for some proximal siliciclastics. A tentative calculation of paleotemperatures is also proposed for the studied tectonostratigraphic units.

Structural evolution of the sedimentary accretionary wedge of the alpine system in Eastern Sicily: Thermal and thermochronological constraints

Temperature-dependent clay-mineral assemblages, vitrinite reflectance, and apatite fission-track data have been used to investigate levels of diagenesis and time of exhumation of the double-verging Sicilide-Antisicilide accretion-ary wedge in Eastern Sicily. The integration of organic and inorganic thermal indicators allowed us to distinguish parts of the accre-tionary wedge with different thermal signature and evolution. We recognize a warmer core made up of the Mount Soro and Troina units and two colder rims (Antisicilide and far-traveled Sicilide units). The Antisicilide unit was thrust back toward the hinterland, and the far-traveled Sicilide units were gravity-driven toward the Hyblean Plateau. In detail, the highest percentages of vitrinite reflectance (VR o ) values (0.60%–0.96%) and percentages of illite layers in illite-smectite (I-S; 60%–85%) are found in the Mount Soro and Troina units. Apatite fission-track data, together with the paleotemperature estimates from vitrinite-reflectance data and clay-mineral–based geothermometers, indicate that fission tracks were partially to totally annealed during wedge accretion and that the subsequent exhumation occurred mainly in Burdigalian times. Low VR o values (0.35%–0.50%) and percentages of illite layers in I-S (30%–60%) occur in early thrust-top deposits (Reitano Flysch) that unconformably overlie the Sicilide Complex, as well as the far-traveled Sicilide and Antisicilide units. Apatite fission-track data for the Antisicilide unit confirm low paleo-temperature values. Thus the far-traveled Sicilide and Antisicilide units were probably at higher structural levels in the original accretionary prism and were remobilized since late Aquitanian–Burdigalian times.

Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

Abstract The Gran Sasso range is a striking salient formed by two roughly rectilinear E–W and N–S limbs. In the past ∼90° counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west–east increase of rotation-related northward shortening along the E–W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E–W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E–W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N–S front was reactivated during early–middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW–SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas.

Lithofacies and faunal succession (faunal phase analysis) as a tool in unravelling climatic and tectonic signals in marginal basins; Messinian (Miocene), Sicily

Many basins are filled under the influence of tectonics and variable climatically driven processes. Unravelling the relative contributions of these is generally easier in fossiliferous sequences if faunal diversity patterns are considered in conjunction with lithofacies analysis. This approach has been successfully applied to Late Miocene strata of Sicily, where progressive restriction and desiccation within the Sicilian Caltanissetta region occurred under a semi-arid climate during the Messinian Stage. This restriction was partly due to Antarctic cooling which produced Palaeo-Mediterranean sea-level oscillations during the Early Messinian, as a prelude to closure of the Atlantic connections. In addition, regions within this Afro-European plate collision zone were also subjected to syndepositional tectonic deformation associated with progressive perched basin development above foreland (south) verging thrust sheets. The relative impacts of climatic (eustatic) versus tectonic control on sedimentation patterns within these perched basins has been debated at length. However, progress towards resolving this problem by a combined approach involving lithofacies analysis, and faunal phase analysis provides clues for separating the effects of the controlling factors. Faunal phase analysis identifies general faunal associations with minimal emphasis on species diversity. The resulting ‘phase’ plots for Early Messinian strata appear to be transgression skewed when controlled by eustatic cyclicity, and regression skewed when driven by regional tectonics associated with perched basin tectonics.

Stratigraphic variations control deformation patterns in evaporite basins: Messinian examples, onshore and offshore Sicily (Italy)

Three-dimensional seismic data are increasingly resolving original compositional heterogeneity and structural complexity in evaporitic successions within sedimentary basins. The relationship between basin structure, evaporite composition and its influence on subsequent deformation are investigated here using Messinian examples from the Maghrebian thrust system of Sicily and applied to the adjacent Ionian sea-bed. By integrating outcrop and onshore subsurface data, we demonstrate variations in evaporite stratigraphies deposited across thrust-top basins, and how these variations have controlled subsequent deformation of these basins. Gypsum and carbonate units develop broad single-layer buckle fold trains, with wavelengths reflecting layer thickness. The development of deformation appears limited by bending resistance at fold hinges, which can be overcome by syntectonic erosion. In contrast, the thick halite and K-salt accumulations in growth synclines have deformed with short-wavelength folds and distributed strain. These structures can display rapid lateral variations (on a scale of hundreds of metres). Similar structural styles, with buckle fold trains passing laterally into more homogeneously shortened, short-wavelength folding, are evident on seismic data from the buried Messinian interval beneath the Ionian Sea. Using the Sicilian outcrop as analogues, the structural styles for the Ionian may be used to infer evaporite type in these subsurface examples.

Geological map of the Corvillo and Mandre Basins (Caltanissetta Basin, Central Sicily): Explanatory notes

In Sicily, the imbrication of the Apenninic thrust belt above the African foreland determined the progressive southward migration of marine basins that were shortened from the Middle Miocene until the Pleistocene. The area studied in detail is located in the Caltanissetta Basin, which was in fact, during Neogene times, a series of thrust-top basins (e.g. Corvillo and Mandre). The relationships between Neogene sedimentation and the synchronous development of fold and thrust structures are recorded by unconformities, lap relationships of strata, hiatuses. The stratigraphy of the substratum is dominated by varicoloured clays and siliciclastic deposits of Cretaceous-Early Miocene age. The thrust-top basin stratigraphy is represented by Tortonian siliciclastic deposits and evaporitic deposits, which are the product of the Messinian lowstand. The Early Pliocene flooding was responsible of deposition of chalks and marls and a later regional uplift produced a regression, during Late Pliocene times, with deposition of marls and sandstones. Compressional deformation acted continuously from Middle Miocene until, at least, Middle Pliocene, amplifying the pre-existing structures and creating differing styles of deformation depending on the lithologies involved.