Detrital Cr-spinel in the Šambron–Kamenica Zone (Slovakia): evidence for an ocean-spreading zone in the Northern Vardar suture?

The Šambron–Kamenica Zone is situated on the northern margin of the Levočské vrchy mountains and Šarišskà vrchovina Highland, where the Central Carpathian Paleogene joins the Pieniny Klippen Belt. Sandstone outcrops in this area. From Cretaceous to Late Oligocene in age, these sediments suggest transport directions from S and SE. The heavy mineral assemblages of this sandstone include Cr-spinel grains, mainly displaying types II and III alpine-peridotite affinities, and are representative of Ocean Island Basalt volcanism. A sample from Upper Eocene sediments at Vit’az shows a clear change in Cr-spinel composition, which turns out to have types I and II peridotite affinities, and to derive from arc and Middle Ocean Ridge Basalt volcanism, with sediment transport directions from SW and WSW. These data indicate major variations in the Upper Eocene tectonic setting, giving constraints to paleogeographic reconstruction of the Slovak Central Carpathians.     

Introducing dolomite seams: hybrid compaction–solution bands in dolomitic limestones

Bedding‐parallel dolomite seams occur in a clay‐poor carbonate succession of the Apennines. The seams are composed of a high concentration of dolomite crystals compared to the hosting dolomitic limestone. SEM images document microcracking, and in many cases even crushing and fragmentation, of dolomite crystals and accumulation of non‐carbonate insoluble material both along micro‐stylolites within the seams and around dolomite crystals of the seams. We interpret the seams as hybrid structures between pressure‐solution seams and compaction bands, which formed during burial. The euhedral dolomite crystals scattered in the micritic matrix represent the insoluble residue produced by the progressive dissolution of calcite. As calcite dissolution proceeds, the concentration of dolomite crystals increases, eventually resulting in a dolomite seam in which locally a dolomite crystal‐supported texture is attained. At this stage, the dolomite crystals within the seam start to collide, crush and fragment, so that the dolomite seam behaves like a compaction band.     

Tectonic evolution of the ‘Liguride’ accretionary wedge in the Cilento area,southern Italy: A record of early Apennine geodynamics

The early stages of southern Apennine development have been unraveled by integrating the available stratigraphic record provided by synorogenic strata (of both foredeep and wedge-top basin environments) with new structural data on the Liguride accretionary wedge cropping out in the Cilento area, southern Italy. Our results indicate that the final oceanic subduction stages and early deformation of the distal part of the Apulian continental margin were controlled by dominant NW–SE shortening. Early Miocene subduction-accretion, subsequent wedge emplacement on top of the Apulian continental margin and onset of footwall imbrication involving detached Apulian continental margin carbonate successions were followed by extensional deformation of the previously ‘obducted’ accretionary wedge. Wedge thinning also enhanced the development of accommodation space, filled by the dominantly siliciclastic Cilento Group deposits. The accretionary wedge units and the unconformably overlying wedge-top basin sediments experienced renewed NW–SE shortening immediately following the deposition of the Cilento Group (reaching the early Tortonian), confirming that the preceding wedge thinning represented an episode of synorogenic extension occurring within the general framework of NW–SE convergence. The documented Early to the Late Miocene steps of southern Apennine development are clearly distinct with respect to the subsequent (late Tortonian-Quaternary) stages of fold and thrust belt evolution coeval with Tyrrhenian back-arc extension, which were characterized by NE-directed thrusting in the southern Apennines.     

The Pliocene‐Quaternary wedge‐top basins of southern Italy: an expression of propagating lateral slab tear beneath the Apennines

In recent years, contrasting seismic tomographic images have given rise to an extensive debate about the occurrence and implications of migrating slab detachment beneath southern Italy. One of the most pertinent aspects of this process is the concentration of the slab pull force, and particularly its surface expression in terms of vertical motions and related basin subsidence/uplift. In this study we focused on shallow‐water to continental, Pliocene‐Quaternary basins that formed on top of the Apennine allochthonous wedge after its emplacement onto a large foreland carbonate platform domain (Apulian Platform). Due to the thick‐skinned style of deformation controlling the Pliocene‐Pleistocene stages of continental shortening, a high degree of coupling with the downgoing plate appears to characterize the late tectonic evolution of the southern Apennines. Therefore, the wedge‐top basins analysed in this study, although occurring on the deformed edge of the overriding plate, are capable of recording deep geodynamic processes affecting the slab. Detailed stratigraphic work on these wedge‐top basins points to a progressive SE‐ward migration of basin subsidence from c. 4 to c. 2.8 Ma over a distance of about 140 km along the strike of the Apennine belt. Such a migration is consistent with a redistribution of slab‐pull forces associated with the progressive lateral migration at a mean rate in the range of 12–14 cm y^–1 of a slab tear within the down‐going Adriatic lithosphere. These results yield fundamental information on the rates of first‐order geodynamic processes affecting the slab, and on related surface response.     

Active tectonics of the Northern Apennines and Adria geodynamics: new data and a discussion

The active geodynamic setting of the Northern Apennines is characterised by extension in the axial zone of the chain, and by a more complex tectonic behaviour in the frontal part of the belt. In the latter sector, moderate seismicity occurs, displaying compressional, strike-slip and extensional focal plane solutions with variably oriented P and T axes. For this area, a review of available geological and geophysical data has been integrated by the analysis of seismic reflection lines calibrated with deep well logs. This study confirms that, as already suggested by some previous workers, thrusting and related folding in the study area ceased in Early Pleistocene times. This feature is in contrast with the hypothesis of active thrusting related to a subducting lithospheric slab beneath the chain—an issue which is largely debated based on available geophysical information. Our analysis shows that the Northern Apennines are characterised by an active tectonic setting which is similar to that of the central and southern portions of the belt. These areas all display a Late Quaternary inactivity of the thrust front. NE–SW oriented extension (perpendicular to the strike of the orogen) is well established in their axial zones, whereas a less homogeneous stress field characterises their external sectors and the adjacent foreland. Within this framework, the seismotectonic behaviour of the Northern Apennines—and probably of the whole Italian peninsula between the Po Plain and the Southern Apennines (north of the Calabrian Arc)—may be interpreted as essentially controlled by two main processes. The first of them involves tectonic uplift, possibly related with slab detachment and associated unbending of the foreland plate. The second process consists of a present-day northwestward motion of the Adria block with respect to stable Europe.     

Syn-orogenic extension in the Peloritani Alpine Thrust Belt (NE Sicily,Italy): Evidence from the Alì Unit

Structural and petrological analyses on the Alì Unit, in the Peloritani Thrust Belt, document the first evidence for Alpine exhumation associated with syn-orogenic extension in this part of the Calabria-Peloritani Arc. The Alì Unit displays ductile structures occurred during three Alpine deformation phases (D_a1, D_a2, D_a3). D_a1 and D_a3 developed in a contractional context, whereas D_a2 was generated in an extensional regime. The present-day tectonic contact between the Alì Unit and the overlying Mandanici Unit is interpreted as a low-angle extensional detachment responsible for the metamorphic break between the two units. Structural overprinting relationships indicate that the development of D_a2 structures and related tectonic exhumation occurred during syn-convergence extension, and were followed by further nappe stacking in the Peloritani Belt. To cite this article: R. Somma et al., C. R. Geoscience 337 (2005).     

Fault zone characteristics and scaling properties of the Val d’Agri Fault System (Southern Apennines, Italy)

Systematic in situ analysis of active fault zones in Val d’Agri (southern Italy) suggests that the acquisition of quantitative data on fault-related structural discontinuities is fundamental for (1) discriminating between the latter and older regional features, (2) defining the architecture and related permeability structure of faults, and (3) performing simple statistics in order to evaluate the validity range within which fault characteristics may be considered to be fractals. This type of information can be integrated with regional seismotectonic analysis in order to asses the present-day conditions of deformation characterising the area, and to constrain the possible kinematics of the seismogenic structures controlling earthquake activity in this sector of the southern Apennines. The results of our study are also of interest for modelling earthquake sources, since a knowledge of the permeability structure and scaling properties of a faulted rock volume is potentially relevant for simulating the time and space dependent behaviour of fault zones during a seismic cycle.     

Complex basin development in a wrench-dominated back-arc area: Tectonic evolution of the Crati Basin,Calabria, Italy

Field data and seismic reflection profiles of various resolutions, calibrated by deep well logs, have been used to unravel the tectonic evolution of the Crati Basin (southern Italy). The study area is located in the northern portion of the Calabrian Arc, a well-developed arc-shaped feature of the circum-Mediterranean belts, consisting of a series of ophiolite-bearing tectonic units and overlying basement nappes. NW–SE oriented left-lateral strike-slip faults exerted a major control on the tectonic evolution of northern-central Calabria, from Middle Miocene to Lower Pleistocene times. Such faults, arranged in an en-échelon geometry and dissecting the pre-existing Late Oligocene–Early Miocene orogenic belt, led to a structural setting including major N-S striking synforms – as the offshore Paola Basin and the Crati Basin are interpreted based on our results – separated by a broad antiformal ridge. Since the Middle Pleistocene, both E- and W-dipping normal faults developed in the southernmost sector of the Crati Basin, probably as a consequence of both uplift of the orogenic edifice and Tyrrhenian back-arc extension. The pre-existing regional strike-slip faults became inactive in this sector of the belt. However, working as persistent barriers, it is envisaged here that they inhibited the southern propagation of the newly formed normal faults, which therefore propagated towards the north. A minimum value of cumulative displacement of ca. 600 m has been unraveled for the central sector of the Crati Basin since Middle Pleistocene times. This yields a vertical strain rate of ca. 0.9 mm/y during the last 700 ka.     

Determination of metamorphic grade in siliceous muscovite-bearing rocks in Madagascar using reflectance spectroscopy

Visible short-wave infrared laboratory reflectance spectra of quartzites and micaschists from the Precambrian basement of central-western Madagascar are dominated by the muscovite spectrum. The spectra were measured from rock samples collected along an E–W traverse across a metamorphic sequence recording progressively higher metamorphic grades. The wavelength of the Al–OH band around 2200 nm decreased westward and was inversely correlated with Al^VI content of muscovite in rocks showing the same nonlimiting assemblage. This shift is correlated also with an increase of band intensity, which is attributed to the progressive increase of metamorphic grade. All of these variations in rock spectral properties can be detected by the high spectral resolution MIVIS and AVIRIS sensors, as well as by the lower resolution TM sensor.