Physics and Chemistry of the Earth

Orogenic architecture of the Mediterranean region and kinematic reconstruction of its tectonic evolution since the Triassic

In this paper, a new discrimination diagram using absolute measures of Th and Nb is applied to post-Archean ophiolites to best discriminate a large number of different ophiolitic basalts. This diagram was obtained using >2000 known ophiolitic basalts and was tested using ∼560 modern rocks from known tectonic settings. Ten different basaltic varieties from worldwide ophiolitic complexes have been examined. They include two basaltic types that have never been considered before, which are: (1) medium-Ti basalts (MTB) generated at nascent forearc settings; (2) a type of mid-ocean ridge basalts showing garnet signature (G-MORB) that characterizes Alpine-type (i.e., non volcanic) rifted margins and ocean-continent transition zones (OCTZ). In the Th-Nb diagram, basalts generated in oceanic subduction-unrelated settings, rifted margins, and OCTZ can be distinguished from subduction-related basalts with a misclassification rate <1%. This diagram highlights the chemical variation of oceanic, rifted margin, and OCTZ basalts from depleted compositions to progressively more enriched compositions reflecting, in turn, the variance of source composition and degree of melting within the MORB-OIB array. It also highlights the chemical contributions of enriched (OIB-type) components to mantle sources. Enrichment of Th relative to Nb is particularly effective for highlighting crustal input via subduction or crustal contamination. Basalts formed at continental margin arcs and island arc with a complex polygenetic crust can be distinguished from those generated in intra-oceanic arcs in supra-subduction zones (SSZ) with a misclassification rate <1%. Within the SSZ group, two sub-settings can be recognized with a misclassification rate <0.5%. They are: (1) SSZ influenced by chemical contribution from subduction-derived components (forearc and intra-arc sub-settings) characterized by island arc tholeiitic (IAT) and boninitic basalts; (2) SSZ with no contribution from subduction-derived components (nascent forearc sub-settings) characterized by MTBs and depleted-MORBs. Two additional discrimination diagrams are proposed: (1) a Dy-Yb diagram is used for discriminating boninite and IAT basalts; (2) a Ce/Yb-Dy/Yb diagram is used for discriminating G-MORBs and normal MORBs. The proposed method may effectively assist in recovering the tectonic affinity of ancient ophiolites, which is fundamental for establishing the geodynamic evolution of ancient oceanic and continental domains, as well as orogenic belts.

A new method of discriminating different types of post-Archean ophiolitic basalts and their tectonic significance using Th-Nb and Ce-Dy-Yb systematics

Tectonic units of the Alpine collision zone between Eastern Alps and western Turkey

The Hellenide orogen in Greece is part of the AlpineHimalaya mountian belt, created during the destruction of Tethys by the convergence of Gondwana and Eurasia. Within Greece, there is the record of a complete Wilson tectonic cycle of continental rifting, sea-fl oor spreading, plate subduction, and continental collision during Mesozoic and Cenozoic time. This book presents a new synthesis of the geological history of Greece as revealed by the varied igneous rocks. It is based on more than 30 years of fi eld and laboratory studies by the authors together with a synthesis of the widely scattered published literature that has been written in many different languagues. Basement rocks record Hercynian subduction and plutonism on the northern margin of Gondwana, which in the Permian and Triassic rifted into several microcontinents, thereby creating the eastern Mediterranean Neotethys ocean. Partial closure of strands of the Neotethys ocean resulted in widespread emplacement of Jurassic and Cretaceous ophiolites. Early Tertiary collision produced a Hellenide mountian chain similar to the Alps and Himalayas. Rapid Neogene extension of the Hellenides behind the modern South Aegean arc has formed the Aegean Sea, triggered widespread backarc igneous activity, and unroofed mid-crustal rocks. The geological setting, geochemistry, and tectonic signifi cance of each group of rocks is presented in detail, with numerous original maps and fi gures.

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The igneous rocks of Greece: The anatomy of an orogen. Georgia Pe-Piper and David J.W. Piper.

In this paper, we describe the stratigraphic and structural features of the tectonic units cropping out along the Zlatibor-Maljen geotraverse, located in western Serbia at the boundary with Bosnia-Herzegovina, and we present also a 1:100,000 scale geological map. The study area corresponds to a SSW-NNE geotraverse, where the main oceanic and continental tectonic units of the Dinaric-Hellenic belt are well exposed. Along this geotraverse, the tectonic pile includes at the top the units derived from the European Plate, here represented only by the Ljig Unit, that was thrust over the oceanic units, cropping out in two distinct massifs, the Zlatibor and Maljen ones. In both massifs the oceanic units consist of a sub-ophiolite melange overthrust by an ophiolite unit represented exclusively by mantle peridotites with the metamorphic sole at their base. In turn the oceanic units are overthrust the Adria-derived units, here represented by the East Bosnian-Durmitor and Drina-Ivanjica Units, respectively located westward and eastward of the Zlatibor Massif. The geological data and the tectonic reconstruction suggest that the ophiolites of the two s may have originated in the same composite oceanic basin that experienced oceanic opening, intra-oceanic subduction, development of supra-subduction oceanic basins and finally closure, in a time span ranging from Middle Triassic to Late Jurassic. The stratigraphic and structural dataset presented in this paper allows some insights about the geodynamic history of the northern area of the Dinaric-Hellenic belt, as well as a comparison with the reconstructions proposed for the southernmost area by other authors.

The geology of the zlatibor-maljen area (western serbia): a geotraverse across the ophiolites of the dinaric-hellenic collisional belt

In Etolia (Central Greece), west of the Parnassus/Vardoussia units, the Pindos succession crops out, with its higher formation: the Pindos Flysch. In its upper portion levels of debris flow deposits and slide blocks (olistostromes and olistoliths) contain ophiolitic material with fragments derived from the Parnassus/ Vardussia formations.
This ophiolitic material consists of serpentinites, basalts of WPB, E- and N-MOR affinity, and radiolarian cherts of Middle-Late Triassic and Middle-Late Jurassic age. Petrologic and biostratigraphic analyses confirm that the melange has the same features of the sub-ophiolitic melanges present at the base of the ophiolitic masses in Greece. Linked to the flysch, with contacts of unclear nature, a rhyolites body of Middle Triassic age indicates the continental nature of the Pindos Basin. In fact, here, as all over the Albanian-Greek section of the Dinarides, no record of an oceanic area in the central portion of the Dinarides exists: the Parnassus and Vardoussia units were directly thrust onto the Pindos Basin.
The intercalations of ophiolitic and continent-derived material in the flysch, are interpreted as the forerunners of the Ophiolitie Nappe which, coming from the Vardar Ocean located to the east, reached during the Eocene the Pindos Basin.

The ophiolite-bearing mélange in the early tertiary pindos flysch of etolia (central greece)

The reconstruction of the tectono-stratigraphical framework of the oceanic units (Ligurian Units Auctt.: Helminthoid Flysch Units, Vara Units, Lanciaia Fm.) cropping out in southern Tuscany is difficult because of the complex tectonics which characterize this sector of the Northern Apennines. Moreover, the widespread occurrence of Miocene-Pliocene and Quaternary sedimentary covers interrupt the areal continuity of the outcrops of the structural pile. To reconstruct the stratigraphic-structural evolution of the Ligurian Units (LU) of southern Tuscany and to compare their sedimentary and compositional features in areas crossed by the CROP-18 profile, the authors performed sedimentological, biostratigraphical, petrographical and structural studies on some key outcrops (Gambassi-Montaione, S. Donato in Poggio, Northern Monti della Gherardesca, Micciano-Libbiano and Serrazzano, Lanciaia-Montecastelli Pisano). Data from geothermal wells were also used in the regional correlation of units. The Cretaceous helminthoid flysches have been grouped into a single formation, the Monteverdi Marittimo Fm., subdivided into three members: (i) the Larderello Member; (ii) the Montaione Member; (iii) the San Donato Member. These represent different parts of the oceanic basin. The Larderello Member (Flysch CalcareoMarnoso, Flysch di Monteverdi M.mo Auctt.) represents normal sedimentation in the middle part of the basin; the Montaione Member (Flysch di Montaione, Flysch di Chianni, Flysch di Castelluccio Auctt.) was deposited in the inner (western?) part and close to an ophiolitic ridge because of the occurrence of ophiolitic olistoliths and breccias; while the San Donato Member (Formazione di S. Donato: BORTOLOTT1, 1962) represents the distal eastern part of the turbiditic system. Even though the LU of southern Tuscany were affected by a complex polyphasic evolution (including out-of-sequence thrusting), we can recognize the sequence of five main tectonic units (from bottom to top of the pile): Morello Unit, Castelnuovo Val di Cecina Unit (e.g. Marly-Calcareous Flysch Unit, Auctt.), Vara Unit (Lower Ophiolitiferous Unit Auctt.) and Lanciaia Fm., Castelluccio Unit (Montaione Flysch Unit Auctt.) and Montignoso Unit (Upper Ophiolitiferous Unit Auctt.). The Lupicaia Creek Unit (Montecatini Sandstones Unit Auctt.) is also present locally at the lop of the pile (Montecatini Val di Cecina and south of the Montaione-Gambassi areas). In the Early to Late Paleocene, the thrusting of the Vara Unit onto the Monteverdi M.mo Fm. occurred and was sealed by the sedimentation, in a piggyback basin, of the Lanciaia Fm., which can be considered an earlier Epiligurian unit. From the Late Eocene the Lanciaia Fm. was deformed by the out-of-sequence thrusting of the Castelluccio Unit and, finally, of the Montignoso Unit. The main structural features of the LU have an «anti-Apenninic» trend (N80 to N110) superimposed by a late «Apenninic» (N150 to N180) trend. The former trend involved the LU at least until Middle-Late Eocene, while is not recognizable in the Tuscan Units, and the Neogene deposits which preserve only the Apenninic orientation. Moreover, the regional distribution of the Cretaceous-Eocene ophiolitic debris in the flysch sequences reflects the anti-Apenninic trend.

The ligurian units of southern tuscany

The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure

The Grammos-Arrenes ophiolites crop out between the Pindos Flysch to the East and the Meso-Hellenic basin to the West and represent the northernmost exposure of the Pindos Ophiolite belt in Greece. Field work and sampling on key outcrops of the Sub-ophiolitic Melange provided new data on the oceanization age and on the geochemistry of the effusive products. The basalts samples show a N-MOR affinity and are associated with latest Bajocian-early Bathonian radiolarian cherts.
Emplacement of the ophiolites onto the continental margin is preceded by mass flow deposition of ophiolitic material in the basins facing the advancing ophiolitic nappe. Ophiolite-bearing debris flow deposits and slide blocks are recognized in the Early Cretaceous deep water sediments of the Beotian Unit and in the upper portion of the Tertiary Pindos Flysch. The intercalations of ophiolitic material in the Beotian Flysch first, and in the Pindos Flysch, later, are interpreted as the forerunners of the Ophiolitic Nappe which derived from the Vardar Ocean to the east, and was emplaced westwards onto the Adria continental margin.
The collected data allow consolidating the constraints for the timing and mechanism of ophiolite emplacement in the Pindos-Grammos area. In addition, taking into consideration the geometry of the tectonic stack in the Grammos-Arrenes area and the ages of the involved sedimentary deposits it is possible to reconstruct a geodynamic history comparable to that of the other zones studied along the Dinaric-Hellenic Chain.

G. Nirta

Papers

The geology of the zlatibor-maljen area (western serbia): a geotraverse across the ophiolites of the dinaric-hellenic collisional belt

The ophiolite-bearing mélange in the early tertiary pindos flysch of etolia (central greece)

The ligurian units of southern tuscany

The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure

Ophiolites from the grammos-arrenes area, northern greece: geological, paleontological and geochemical data