Papers by Franco Marco Elter
geosciences, 2024
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Special Issue Advanced Studies in Structural Geology: The Role of Tectonics on Applied Geology Αspects, 2024
Sardinia (Italy) represents one of the most comprehensive cross-sections of the Variscan orogen. ... more Sardinia (Italy) represents one of the most comprehensive cross-sections of the Variscan orogen. The metamorphic and structural complexity characterizing its axial zone still presents many unresolved issues in the current state of knowledge. The data presented from the structural study of the entire axial zone of this area have allowed the authors to propose a subdivision into two new structural complexes. In particular, a younger complex is identified as the New Gneiss Complex, containing remnants of an older and higher-grade metamorphic complex defined as the Old Gneiss Complex. The structural and geometric relationships between the two complexes suggest the redefinition of the axial zone of Sardinia as part of the intracontinental East Variscan Shear Zone/medium-temperature Regional Mylonitic Complex. Comparable relationships are also highlighted in many other areas of the Variscan chain (e.g., Morocco, Corsica, Maures Massif, and Argentera Massif). Extending this new structural interpretation to other axial zones of the South European Variscan orogen could provide new hints for reconstructing the collision boundaries between Gondwana and Laurussia in the late Carboniferous to the early Permian periods.
MDPI GEOSCIENCES, 2023
The Portofino Conglomerate (PC) cropping out in the Eastern Liguria is an approximately
500 m thi... more The Portofino Conglomerate (PC) cropping out in the Eastern Liguria is an approximately
500 m thick, very gently folded succession mainly composed of poorly bedded and mostly matrixsupported conglomerates. It stratigraphically rests on the Helminthoid Flysch (UA3) thrusted onto
the Antola Unit. We vertically distinguished three mostly ruditic litho/petrofacies: (i) Paraggi (fP)
with carbonate clasts from an Helminthoid Flysch succession; (ii) Monte Pallone (fMP) with prevailing
carbonate and meta-carbonate clasts and minor quartz-rich (meta)siliciclastic and high-pressure–
low-pressure (HP-LP) metabasite clasts; and (iii) Monte Bocche (fMB) with dominant quartz-rich
(meta)siliciclastic, meta-carbonate clasts, and minor granitoid elements and medium-temperature–
high-temperature (MT-HT) regional metamorphic rocks. The middle-upper Eocen age of Paraggi
litho/petrofacies is constrained by well-preserved microforaminifers (e.g., Globigerinatheka) recovered
in the matrix. During its sedimentation, the directions of the paleocurrents would indicate that the
PC underwent a counterclockwise rotation coeval with the first Cenozoic rotational phase of the
Sardinia–Corsica system (50–30 Ma) and then stopped before the sedimentation of the Monte Pallone
and Monte Bocche litho/petrofacies. The vertical compositional variation in the sedimentary inputs
suggested that the PC is the result of a progressive deepening of the erosional level of a tectonic pile
that can be located in the Ligurian Alps Chain. We considered the PC as the likely apical part of
a submarine fan deposited in a piggy-back/thrust-top basin within the Alpine nappe stack. This
sedimentary body was later tectonically transported eastward with its UA3 Helminthoid Flysch
substrate (similarly to Epiligurian Units of the Northern Apennines) onto the Apenninic orogenic
system (i.e., the Antola Unit).
Ofioliti, Jan 7, 2002
... The ophiolitiferous Cala Grande Unit represents, together with similar successions of the Tus... more ... The ophiolitiferous Cala Grande Unit represents, together with similar successions of the Tuscan Archipelago (eg Franco Promontory in the Giglio Island and Gorgona Island: Jolivet et al., 1998; Capponi et al., 1990; 1994; 1997; Pandeli et al., 2001; Orti et al., this volume), the ...
Ofioliti, Jan 7, 2005
The Erro-Tobbio (ET) ophiolitic peridotite (Voltri Massif – Liguria, Italy) represents a sector o... more The Erro-Tobbio (ET) ophiolitic peridotite (Voltri Massif – Liguria, Italy) represents a sector of subcontinental lithospheric mantle that has been emplaced at crustal, sub-oceanic levels during rifting and opening of the Jurassic Ligurian Tethys (Ernst and Piccardo, 1979). Structural and petrologic works (e.g. Drury et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al., 1993) have demonstrated that the Erro-Tobbio peridotites were uplifted along a subsolidus P-T trajectory, characterized by progressively decreasing temperature. Pristine granular mantle protoliths, completely equilibrated at subcontinental lithospheric mantle depths (i.e. 1000-1100°C and spinel-facies conditions), were deformed along km-scale extensional shear zones, where they were transformed to spinel peridotite tectonites, spinel- and plagioclase-bearing mylonites, hornblende/ chlorite peridotite mylonites and, finally, serpentine mylonites. This composite P-T evolution has been interpreted as the exhumation trajectory of mantle sections evolving as the footwall of an asymmetric extensional system dominated by simple shear mechanisms. Recent contributions have revealed the presence of: i) reactive spinel peridotites, that were formed by melt-peridotite interaction under spinel-facies conditions (Piccardo et al., 2004; Rampone et al., 2004); ii) large areas of melt impregnated, plagioclase-rich peridotites, that are cut by a network of replacive spinel dunite channels and gabbroic dikelets (Piccardo et al., 2004). Our ongoing field, microstructural and geochemical investigations allow: i) to evidence and document some main steps in the composite evolution of the Erro-Tobbio peridotite, which are characterized by significant melt-peridotite interaction, and ii) to reconstruct the time-space relationships between these melt-related events and the subsolidus tectonicmetamorphic evolution of the Erro-Tobbio peridotite. Pristine mantle protoliths are sporadically preserved in the Erro-Tobbio massif: they are moderately depleted lherzolites, showing complete recrystallization under spinel-facies conditions (T below 1100°C; P below 2.5 GPa, according to Hoogerduijn Strating et al., 1993), which has been related to the accretion of pristine asthenospheric masses to the mantle lithosphere. They have relatively Na-Al-rich clinopyroxenes and preserve structural relics of their previous evolution, i.e. rounded opx+sp clusters, suggesting spinel-facies breakdown of a precursor garnet, i.e. a pristine garnet-bearing protolith. These spinel-facies lithospheric peridotites were subsequently affected by extensional deformation which formed km-scale shear zones, consisting of spinel peridotite tectonites and mylonites (Hoogerduijn Strating et al., 1993). In the field, spinel tectonite peridotites are replaced by coarse granular spinel peridotites, which: i) show microstructural (i.e. pyroxene dissolution and olivine precipitation) and compositional features indicating their reactive origin, due to interaction with percolating pyroxene-undersaturated melts; ii) preserve microstructural features (i.e. the opx+sp clusters), which indicate that they were formed by almost complete recovering of pristine lithospheric peridotites. Available clinopyroxene trace element compositions suggest complete trace element equilibration with the percolating melts, consisting of depleted melt increments formed by 6% of fractional melting of a DMM asthenospheric mantle source. Both tectonite and coarse granular spinel peridotites are replaced by plagioclase-rich peridotites, showing rather sharp contacts with the spinel peridotites. Plagioclase peridotites show microtextural and compositional characteristics [i.e.: i) opx replacement on mantle olivine, ii) mm-size noritic pods and veins, iii) opx+plg coronas replacing mantle cpx] which indicate melt/peridotite interaction and interstitial crystallization of pervasively percolating melts, having orthopyroxene(-silica)-saturated, clinopyroxene-undersaturated characteristics. Plagioclase peridotites frequently preserve microstructural features (i.e. olivine coronas replacing pyroxenes) which indicate that the pre-impregnation spinel peridotites frequently were represented by reactive peridotites. Geochemical modeling indicates that melts which percolated and impregnated the Erro-Tobbio spinel peridotites had a strongly depleted signature: they, most probably, were formed as depleted melt increments by fractional melting and attained orthopyroxene(-silica)-saturation during reactive migration in the lithospheric mantle column. Presence of channels of replacive spinel dunites, cutting both spinel and plagioclase peridotites, indicates that: i) further upwelling melts were forced to migrate within focused channels where both ortho- and clinopyroxenes were completely dissolved by reaction with pyroxene-undersaturated melts, and ii) these high permeability channels allowed more “rapid” migration of melts. Frequently coarse…
Bollettino Della Societa Geologica Italiana, 1998
Geodinamica Acta, Aug 1, 2006
New structural data pointed out the presence of an older scattered migmatization event (Devonian?... more New structural data pointed out the presence of an older scattered migmatization event (Devonian?, M1) overcome by the well known Variscan migmatization event (Lower-Middle Carboniferous, M2) related to the Late extensional tectonic that affected the High Grade Metamorphic Complex (HGMC) in the Variscan Belt of Sardinia (Italy). The M1 event is only recognizable in the kyanite-amphibole bearing migmatitic gneiss. Both migmatization events (M1 and M2) are characterized by a syn-tectonic non coaxial deformations (D1 and D2 deformational events). D1 shows a top to NW sense of shear while the D2 event a top to NE/SE sense of shear (the shear senses are considered at the present Sardinia-Corsica block position in the Mediterranean sea). The M2+D2 is characterized by a complicate, composite normal shear network; the M1+D1 by inverse shear zones. The M2+D2 is transposed by the late D3 strike slip shear event: the D3 is characterized by strike slip shear zones syn-kinematic to the emplacement of Late Carboniferous granitoids (320 Ma-300 Ma). Despite the absence of geochronological data about the M1+D1 event, the field relationships suggest, for first time, an older migmatization process (Devonian?) syn-tectonic with the late stage of thickness of the Sardinia Variscan Belt. Similar evolutions are recognized in different segments of the Variscan Belt such as the Massif Central (France) or in the eastern mid-European Variscides.
Ofioliti, 2001
The metaophiolites and the metasediments of the Cala Grande Unit ("Schistes Lustrés"). The trip m... more The metaophiolites and the metasediments of the Cala Grande Unit ("Schistes Lustrés"). The trip moves from Albinia and reaches Porto Santo Stefano. We take the Panoramica road on the northwestern side of the Mt. Argentario Promontory. Here the "Calcare
Ofioliti, 2001
The complex framework of the Elba tectonic pile has always been a matter of debate within the geo... more The complex framework of the Elba tectonic pile has always been a matter of debate within the geology of the Northern Apennines and of the Tyrrhenian area. The structural data collected on the epi-and anchimetamorphic units of central and eastern Elba (Ortano Unit, Acquadolce Unit, Monticiano-Roccastrada Unit and Gràssera Unit) allow more detailed reconstructions of the complicate tectonic evolution of the Elba stack. Because of their similar structural framework, the Ortano Unit and the Monticiano-Roccastrada Unit acquired their main tectono-metamorphic imprints together as part of a single Paleozoic to Tertiary Tuscan-type succession. The probable Ligurian Piedmontese Acquadolce Unit, which occurs tectonically intercalated between the Tuscan epimetamorphic units, is characterized by a different tectono-metamorphic framework (e.g. shears show opposite sense relative to the Tuscan units). Moreover, the anchimetamorphic Gràssera Unit, that is another possible Ligurian Piedmontese unit, is located in between the non-metamorphic Ophiolitic Unit and the Tuscan Nappe. The present, complex piling up of Tuscan and Piedmontese units occurred within the time interval of 19 Ma (according to the radiometric age of the main foliation of the Acquadolce Unit) and
Geodinamica Acta, Feb 1, 2004
The post-collisional Variscan evolution in Sardinia (Italy)-Corsica is characterized by Late Carb... more The post-collisional Variscan evolution in Sardinia (Italy)-Corsica is characterized by Late Carboniferous-Permian extensional tectonics that affected the whole belt and was due to gravitational collapse of the previously thickened crust. Evidence of this late Hercynian extensional tectonism is widespread throughout the basement and its development is chronologically constrained by radiometric, sedimentary and paleontological data. Extensional tectonism in Sardinia may be related to a composite shear framework characterized by two events: an Early Shear Event (ESE) and a Late Shear Event (LSE). The ESE is characterized by shear zones in amphibolite facies conditions with a top-to S/SE shearing in relation to a dome structure. The LSE consists of strike slip ductile shear zones (mainly dextral shear zones) associated with retrograde metamorphism, commonly coeval with syntectonic plutons. The ESE accommodated the exhumation of the Variscan belt in Sardinia, while the LSE belongs to the Late Variscan strike-slip faulting common in SW Europe. The Late Variscan geodynamic evolution of the Maures Massif (south-eastern France) is characterized by extensional tectonism related to noncoaxial deformation in HT to LT metamorphic conditions: the close relationships between these two areas suggests that they belong to the same segment of the Variscan belt.
AGUFM, Dec 1, 2010
ABSTRACT The geodynamic evolution of the central Mediterranean area is linked to the interaction ... more ABSTRACT The geodynamic evolution of the central Mediterranean area is linked to the interaction between Gondwana and Laurussia/Laurasia plates. The interaction between these plates led to the development of Variscan, Alpine and Apennine Orogenic belts. In spite of the different ages of the orogenic systems, it is possible to hypothesize that their geodynamic evolution was linked to the complex interactions between the eastern boundary of Gondwana and the western boundary of Laurussia/Laurasia. This irregular boundary could have played the role of a pre-existing tectonic barrier which started to develop during the Upper Carboniferous. The kinematic along the boundary was related to a transpressive regime which evolved through a shear zones system (``snake'' strike-slip and oblique shear zones). The transpressive environment led to the formation of restraining/releasing bends along the boundary between the two plates. The exhumation of middle/deep crustal rocks by telescoping processes, coeval with the transcurrent movement, occurred in the core of this narrow shear zone. The first occurrences of restraining/releasing bends are related to the Early Visean-Bashkirian (Variscan orogeny). The Variscan eastern boundary between the two colliding plates is characterized by a long-narrow band of HT rocks (East Variscan Shear Zone, EVSZ). From South to North they are: Calabria-Peloritani Terrane (Calabria and Sicily), deep basement of northern Apennines, Sardinia, Corsica, Maures-Tanneron Massif and Alpine Massifs. The same boundary was later reworked by the opening of the Alpine Tethys, dated at the Middle-Upper Jurassic. The opening of the Alpine Tethys led to the separation of the Variscan crust which will represent the future Alpine Massifs (External and Internal Massifs). The closure of the Alpine Tethys and the beginning of the Alpine Orogeny (Late Cretaceous to Eocene) re-assembled the system. The roto-translation of the Sardinia-Corsica and Calabrian-Peloritan blocks during the Upper Eocene-Early Miocene partially shifted the EVSZ, as well as the southern sector of the Alpine Belt, to the East and led to the beginning of the Apenninic orogenic events. The shifted portion has been again reworked by the opening of the northern Tyrrhenian Sea in the Late Miocene simoultaneously to the migration toward SE of the Calabrian-Peloritan sector. The evolution of the northern Apennines orogenic sector developed through transpressive and transtensive structures such as restraining and releasing bends which were active during the Upper Oligocene-Pliocene time interval. The aim of this work, in spite of the well knowledge of the geodynamic frame of the central Mediterranean area, is to emphasize the role played by the eastern boundary between Gondwana and Laurussia/Laurasia plates (EVSZ) as a persistent tectonic barrier from the Late Carboniferous to Late Miocene.
The EGU General Assembly, Apr 1, 2019
Bollettino Della Societa Geologica Italiana, 1995
Geoheritage, Nov 10, 2021
The wollastonite–garnet–diopside-bearing marbles cropping out in the Migmatite Complex west to th... more The wollastonite–garnet–diopside-bearing marbles cropping out in the Migmatite Complex west to the Tamarispa and San Lorenzo villages share a common metamorphic and deformational history with the surrounding migmatite, with metamorphic peak conditions between 650 and 850 °C. Within the marble, there is an interesting and rare garnet mineralization. The peculiar characteristic and geological–cultural and touristic attraction of this geosite is the presence of large garnet crystals (up to 20 cm). The whitish rock matrix is characterized by coarse-grained rock-forming minerals (mainly wollastonite, calcite, diopside and subordinately pectolite, quartz, plagioclase, epidote, apatite, titanite) with a compositional layering and a weak foliation (S2 schistosity), parallel to that of the surrounding gneiss and migmatites. At the outcrop scale, the giant garnet crystals often show a brown core in high relief surrounded by a darker rim with less relief. Under conservation state, the wollastonite–garnet–diopside-bearing marbles show an evident differential alteration with dissolution processes of the matrix and an increasingly pronounced enucleation of the garnet crystals. The Tamarispa outcrop with spectacular giant garnet crystals is here proposed as a new, potential geosite relevant for didactic, cultural, and touristic purposes. Conservation and valorization aspects are discussed within the more general framework of the geological, natural, and environmental resources of the local territory.
Transactions, Aug 1, 2000
ABSTRACT An attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain o... more ABSTRACT An attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain of southwest Sardinia and the southern Cévennes-Montagne Noire Variscan domain of France, with particular regard to the Cambrian-hosted economic lead-zinc concentrations and the spatially associated precious-metal (Au) occurrences. Geological, lithogeochemical and lead isotope investigations led to the following conclusions. (1) The major lead-zinc deposits of Iglesiente and southern Montagne Noire have a Mississippi Valley-type origin, which involved remobilization of pre-existing Cambrian syngenetic mineralization during the Ordovician distensive 'Sardic phase'. Lead isotope evidence suggests mixed crustal sources for the Iglesiente lead. (2) The southern Cévennes stratiform Minerai Zéro of the Malines district is not syngenetic with the Cambrian host rocks. For this ore type a synto late-tectonic Variscan formation model is proposed, similar to the model indicated for the genesis of the Salsigne gold deposit in the Montagne Noire. A reevaluation of the Vigan gold occurrences, regionally associated with the Minerai Zéro economic lead-zinc bodies, is therefore suggested. (3) The newly discovered Tertiary epithermal gold province of Sardinia is genetically quite distinct from the Palaeozoic lead-zinc province of Iglesiente.
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Papers by Franco Marco Elter
500 m thick, very gently folded succession mainly composed of poorly bedded and mostly matrixsupported conglomerates. It stratigraphically rests on the Helminthoid Flysch (UA3) thrusted onto
the Antola Unit. We vertically distinguished three mostly ruditic litho/petrofacies: (i) Paraggi (fP)
with carbonate clasts from an Helminthoid Flysch succession; (ii) Monte Pallone (fMP) with prevailing
carbonate and meta-carbonate clasts and minor quartz-rich (meta)siliciclastic and high-pressure–
low-pressure (HP-LP) metabasite clasts; and (iii) Monte Bocche (fMB) with dominant quartz-rich
(meta)siliciclastic, meta-carbonate clasts, and minor granitoid elements and medium-temperature–
high-temperature (MT-HT) regional metamorphic rocks. The middle-upper Eocen age of Paraggi
litho/petrofacies is constrained by well-preserved microforaminifers (e.g., Globigerinatheka) recovered
in the matrix. During its sedimentation, the directions of the paleocurrents would indicate that the
PC underwent a counterclockwise rotation coeval with the first Cenozoic rotational phase of the
Sardinia–Corsica system (50–30 Ma) and then stopped before the sedimentation of the Monte Pallone
and Monte Bocche litho/petrofacies. The vertical compositional variation in the sedimentary inputs
suggested that the PC is the result of a progressive deepening of the erosional level of a tectonic pile
that can be located in the Ligurian Alps Chain. We considered the PC as the likely apical part of
a submarine fan deposited in a piggy-back/thrust-top basin within the Alpine nappe stack. This
sedimentary body was later tectonically transported eastward with its UA3 Helminthoid Flysch
substrate (similarly to Epiligurian Units of the Northern Apennines) onto the Apenninic orogenic
system (i.e., the Antola Unit).
500 m thick, very gently folded succession mainly composed of poorly bedded and mostly matrixsupported conglomerates. It stratigraphically rests on the Helminthoid Flysch (UA3) thrusted onto
the Antola Unit. We vertically distinguished three mostly ruditic litho/petrofacies: (i) Paraggi (fP)
with carbonate clasts from an Helminthoid Flysch succession; (ii) Monte Pallone (fMP) with prevailing
carbonate and meta-carbonate clasts and minor quartz-rich (meta)siliciclastic and high-pressure–
low-pressure (HP-LP) metabasite clasts; and (iii) Monte Bocche (fMB) with dominant quartz-rich
(meta)siliciclastic, meta-carbonate clasts, and minor granitoid elements and medium-temperature–
high-temperature (MT-HT) regional metamorphic rocks. The middle-upper Eocen age of Paraggi
litho/petrofacies is constrained by well-preserved microforaminifers (e.g., Globigerinatheka) recovered
in the matrix. During its sedimentation, the directions of the paleocurrents would indicate that the
PC underwent a counterclockwise rotation coeval with the first Cenozoic rotational phase of the
Sardinia–Corsica system (50–30 Ma) and then stopped before the sedimentation of the Monte Pallone
and Monte Bocche litho/petrofacies. The vertical compositional variation in the sedimentary inputs
suggested that the PC is the result of a progressive deepening of the erosional level of a tectonic pile
that can be located in the Ligurian Alps Chain. We considered the PC as the likely apical part of
a submarine fan deposited in a piggy-back/thrust-top basin within the Alpine nappe stack. This
sedimentary body was later tectonically transported eastward with its UA3 Helminthoid Flysch
substrate (similarly to Epiligurian Units of the Northern Apennines) onto the Apenninic orogenic
system (i.e., the Antola Unit).