Marco Meccheri

Tertiary extensional tectonics in Tuscany (Northern Apennines, Italy)

Abstract Two extensional events have been recognized in the Tertiary evolution of Tuscany. The first event occurred in the period between the Late Oligocene and Late Tortonian and caused a stretching of 120% or greater; the second event occurred between the Late Tortonian and Quaternary Periods and caused a stretching not greater than 10%. The first event is characterized by the exhumation of middle crust rocks and therefore by the development of core complexes. The most significant structures of the first event are: low-angle normal faults, which characterize the upper plate of the core complexes, and ductile shear zones, which characterize the lower plate. The low-angle normal faults tend to die out in the Late Triassic evaporite level, which constitutes the main detachment of the core complexes. The main structures of the second extensional event are high-angle normal faults which dissect all the previous tectonic features. These faults border tectonic depressions which were the site of clastic sedimentation from Late Tortonian to Quaternary time. These normal faults may tend to flatten in a seismic lozenge-shaped band explained as an active extensional shear zone, which is located at the brittle/ductile boundary. A similar explanation is given for the detachment located in the Late Triassic evaporite level during the first extensional event. While the first extensional event is in the framework of the Northern Apennine post-collisional evolution, the second extensional event, because it affects an already stretched crust, could be attributed to the southern Tyrrhenian opening, which began during Late Tortonian.

Inner Northern Apennines

The Northern Apennines are a fold—thrust belt formed during the Tertiary by the tectonic superposition from W to E of the Ligurides on the Tuscan nappe and on the Tuscan metamorphic complex (Boccaletti et al., 1971; Alvarez et al., 1974; Kligfield, 1979). The ophiolite-bearing Ligurides derived from the southern extension of the Ligurian—Piedmont ocean, from which similar mafic components of the Western Alps also derive (Fig. 14.1). The Tuscan units derived from the continental palaeomargin of the Adria microplate and contain a Hercynian continental basement with its upper Carboniferous Tertiary cover (Vai, this vol., Ch. 10).

Microfabric study on the deformational and thermal history of the Alpi Apuane marbles (Carrara marbles), Italy

Marbles from different geometrical and structural positions within the Alpi Apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure, c-axis orientation and temperature analysis. Statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80–100 μm to 250–300 μm. This is correlated with an equilibration calcite-dolomite temperature from 360–380°C to 420°–430°C. Two kinds of dynamically recrystallized microstructures have been investigated: a first one exhibiting coarse grains (150–200 μm) with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size (40–50 μm) and predominantly straight grain boundaries. These microstructural types, associated with localized post-thermal peak shear zones and meter- to kilometer-scale folds, are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms (dislocation creep) associated with predominant grain boundary migration (type-B1) or subgrain-rotation recrystallization (type-B2). These differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type-B1 (390°C) than in type-B2 (370–340°C). We have been able to relate the development of the different microfabric types to the successive stages of deformation of the Alpi Apuane metamorphic complex.

Structural history and tectonic evolution of the boundary between the Wilson and Bowers terranes, Lanterman Range, northern Victoria Land, Antarctica

Abstract The boundary between the Wilson and Bowers terranes is one of the major tectonic discontinuities of northern Victoria Land (Antarctica) and is referred to as the Lanterman Fault Zone by most authors. Fieldwork carried out in this area indicates that the structural evolution of this boundary is polyphase and four main tectonic phases can be recognized: (1) west over east thrusting of Ross age (i.e. around 500 Ma), coeval with amphibolite and amphibolite–greenschist facies transition metamorphism; (2) sinistral strike-slip shearing, coeval with greenschist facies metamorphism; this event may belong to the Ross Orogeny or may represent evidence of the Borchgrevink Orogeny (i.e. around 360 Ma); (3) large-wavelength folding of late-Ross or Borchgrevink age; (4) Cenozoic brittle tectonics, expressed by small to km-scale structures, with dextral strike-slip displacement. Evidence for this structural evolution is not confined to a single tectonic lineament, but occurs throughout the area close to the boundary between the Wilson and Bowers terranes. It follows that the docking of the Wilson and Bowers terranes was complex and polyphase and it is not possible to define a single tectonic setting for the Lanterman Fault Zone, since it was the site of contrasting tectonic regimes at different times and at different structural levels.

Mineralogical, petrographic and geochemical characterisation of white and coloured Iberian marbles in the context of the provenancing of some artefacts from Thamusida (Kenitra, Morocco)

A multi-analytical study has been carried out on a collection of white and coloured Iberian marbles. A total of 135 marble specimens were collected in Spain and Portugal from the Betic chain (Alhaurin de la Torre, Mijas, Macael), Ossa Morena (Alconera, Almaden de la Plata and Viana do Alentejo), and the Estremoz Anticline (Bencatel, Borba and Estremoz) areas. X-ray diffractometry and carbon and oxygen stable isotope analysis were carried out on these samples; 38 samples were also investigated by optical and scanning electron microscopy. The results provide a set of diagnostic parameters that allow discriminating the sampled marble quarries. The carbonate minerals composition is distinctive for the Mijas and Alhaurin de la Torre marbles; the isotopic analysis allows discriminating also between these two dolomitic marble quarries. The Ossa Morena and Estremoz Anticline marbles share a similar stable isotope composition; the accessory mineral content, the maximum grain size (MGS) and the fabric are particularly useful in the distinction between them. In the framework of archaeometric provenance studies on Thamusida (Kenitra, Morocco) Roman marble artefacts, a specific comparison between this new Iberian database and archaeological findings has been carried out. The hypothesis of commercial exchanges between the Iberian regions and Roman Morocco is supported by the results of the provenance study, which suggested the Almaden de la Plata and Mijas quarries as possible sources of raw materials for the production of archaeological artefacts.

The Pre-Alpine Basement in the Alpi Apuane (Northern Apennines, Italy)

The Alpi Apuane is a well-known tectonic window in the Northern Apennines (Tuscany), in which a pre-Alpine lithostratigraphic sequence crops out widely and which appears to have been deeply affected, together with the Alpine cycle cover, by the Tertiary tectonometamorphic evolution of the Apenninic chain.

The overprint of the Alpine tectono-metamorphic evolution on the Hercynian orogen: an example from the Apuane Alps (Northern Apennines, Italy)

Abstract The Apuane Alps represent a tectonic window in the nappe structure of the Northern Apennines. In its lowermost part the Apuan Metamorphic Complex (i.e. “Autochthonous” Auct. and Massa Unit) reveals a polyphase structural pattern, as the result of two main Alpine events of Tertiary age. The first event is responsible for intense isoclinal folding and the development of a pervasive schistosity in greenschist facies, the second is connected with the subsequent late stage uplift. The pre-Triassic rocks in the core of the main isoclinal anticlines were severely tectonized, completely transposing a pre-existing foliation and a greenschist pre-Alpine synkinematic metamorphism, the traces of which are obvious and widespread. Hercynian structures of megascopic, mesoscopic and microscopic size are outlined here.

An overview on the characteristics of geothermal carbonate reservoirs in southern Tuscany

This paper focuses on brittle deformation and fluid-rock interaction, for enhancing permeability in carbonate geothermal reservoir. The relationships between fractures and fluid flow at different structural levels within a geothermal circuit are described through examples from exhumed geothermal systems cropping out in southern Tuscany, with emphasis on the carbonate reservoirs, located within the late Triassic evaporite level and/or at the base of the Tuscan Nappe. The description is based on the fact that geothermal fluids are mainly made up of meteoric water channelled to depth through structural conduits, affecting regionally hot rocks. In this pathway, the meteoric water is transformed in geothermal fluid, becoming chemically aggressive, thus favouring leaching of hosting rocks, and enhancing and maintaining permeability. The fluid-rock interaction is promoted by existing fractures and/or by unhomogeneities in the rock-textures, as it is the case of the Miocene cataclasite located within the late Triassic evaporite. Travertine deposits can occur if fluids reach the surface after having circulated in carbonate reservoirs. Since permeability is controlled by fluid-rock geochemistry and by the possibility to have fluids continuously renewed, we conclude that the fluid-rock interaction and high temperature of hosting rocks make the geothermal issue a specific case of study and therefore the conclusion on oil reservoirs formation cannot be completely transferred to geothermal exploitation issue.

The Tuscan Nappe structures in the Monte Amiata geothermal area (central Italy): a review

The present knowledge on the structures affecting the Tuscan Nappe in the Monte Amiata area is here presented, highlighting that the tectonic evolution of the area is incorporated in the inner Northern Apennines framework. In fact, field and subsurface data indicate that the Tuscan Nappe is internally characterized by tectonic doublings, occurred during the collisional stage. Subsequently, as well as in the whole southern Tuscany, Miocene extension determined the lateral segmentation of the Tuscan Nappe; in the Mt. Amiata area, it resulted in three isolated geological bodies, partly cropping out. During Pliocene, the Tuscan Nappe was cross-cut by normal to oblique faults which contributed to channel the hydrothermal fluids that gave rise to the Pleistocene Hg-Sb ore deposits and Present geothermal resources. As it regards the deformation ages, since the late Oligocene-Aquitanian Macigno Fm is involved in the thrusts, the internal stacking of the Tuscan Nappe is related to the post-Aquitanian and pre-Langhian time period, on the basis of the Langhian sediments deposited in an extensional setting and presently recognised underneath the sediments of the nearby Pliocene Radicofani extensional Basin. It is therefore concluded that the switch from compression to extension occurred in a time span encompassed between early Burdigalian and early Langhian.

'Dating thrust Events Using 87Sr/86Sr: an example from the Northern Apennines, Italy'

Isotopic and petrographic analyses of the major thrust zone (Calcare Cavernoso) in the Apuane region of the Northern Apennines are used to date the timing of motion on that thrust zone and to document the type and the amount of fluid focused there. The thrust zone consists of a porous cataclasite composed of polylithologic clasts and finely crystalline dolomite included within a recrystallized calcite matrix that shows no affect of subsequent brittle or ductile deformation.