Michele Marroni

The association of continental crust rocks with ophiolites in the Northern Apennines (Italy): implications for the continent-ocean transition in the Western Tethys

Abstract The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide-blocks of ophiolites and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain and the thinned continental margin of the Adria plate. The slide-blocks preserve a record of the long-lived history of rifting which led to opening of the Jurassic Western Tethys Basin. The External Liguride ophiolites consist of: (1) undepleted spinel-peridoties, partly re-equilibrated under plagioclase-facies conditions, which were interpreted as unroofed subcontinental mantle; (2) rare gabbroic rocks (mainly troctolite to olivine-bearing gabbro) probably crystallised from N-MORB magmas; and (3) basalts with N- to T-MORB affinity covered by late Callovian-early Oxfordian radiolarian cherts. Both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re-equilibration to plagioclase-facies conditions. The slide-blocks of lower continental crust are composed of gabbro-derived mafic granulites and felsic granulites. The latter include quartzo-feldspathic granulites and rare quartz-poor to quartz-free charnockitic rocks. In both mafic and felsic granulites, granulite-facies re-equilibration was followed by a retrograde metamorphic evolution to amphibolite-, greenschist- and subgreenschist-facies conditions. Retrogression is commonly accompanied by deformations progressively changing from plastic to brittle. The upper crustal rocks occurring as slide-blocks consist of Hercynian granitoids with orogenic affinity, mainly biotite-bearing granodiorites and peraluminous two-mica leucogranites. Locally, the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts. The granitoids underwent polyphase brittle deformations under subgreenschist-facies conditions which predated the basalt emplacement. The tectono-metamorphic evolution recorded by the slide-blocks of the External Liguride Units started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites. The associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro-derived granulites. Mafic and felsic granulites subsequently underwent tectonic exhumation in Permo-Triassic times, as testified by the development of granulite- to amphibolite-facies ductile shear zones. The granulites were finally exhumed to shallow levels, probably in association with the subcontinental mantle, in Late Triassic-Middle Jurassic times. The latter period was most likely characterized by extensive brittle faulting at shallow crustal levels, thus giving rise to extensional allochthons formed by stretched slices of granitoids. The Western Tethys opening is finally testified by the basalt intrusion and effusion in the Late Jurassic, followed by deep-sea pelagic sedimentation. The External Liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent-ocean boundary. This reconstruction fits well with the available data on the present-day continental margins derived from passive lithosphere stretching.

Tectono-sedimentary evolution of the External Liguride units (Northern Apennines, Italy): insights in the pre-collisional history of a fossil ocean-continent transition zone

Abstract In the Northern Apennines, the External Liguride (EL) units are interpreted as derived from the domain that joined the Ligure–Piemontese oceanic basin to the Adriatic plate continental margin. The EL units can be divided into two different groups according to the lithostratigraphic features of the basal complexes underlying the Upper Cretaceous–Lower Tertiary carbonate flysch (e.g. Helminthoid flysch). The first group includes the western successions characterized by Santonian–Campanian sedimentary melanges where slide blocks of lherzolitic mantle, gabbros, basalts, granulites, continental granitoids are represented. The second group is represented by the eastern successions where the Cenomanian–Campanian basal complexes mainly consist of sandstones and conglomerates where the mafic and ultramafic rocks are scarce or completely lacking. Their original substrate is represented by the Middle Triassic to Lower Cretaceous, mainly platform carbonate deposits, found as slices at the base of the eastern successions. The stratigraphic features shown by the basal complexes allow the reconstruction of their source area that is assumed to be also representative for the pre-Upper Cretaceous setting. The proposed reconstruction suggests the occurrence in the EL domain of two distinct domains. The eastern domain was characterized by a thinned and faulted continental crust belonging to the Adriatic continental margin. The western domain was instead floored by subcontinental mantle associated with lower and upper continental crust, representing the ocean–continent transition. This setting is interpreted as the result of the opening of the Ligure–Piemontese oceanic basin by passive rifting, mainly developed by simple shear, asymmetric extension of the continental crust.

Interaction between Mid‐Ocean Ridge and Subduction Magmatism in Albanian Ophiolites

Albanian ophiolites are represented by two different coeval belts, each displaying well‐exposed, complete ophiolitic sequences that originated in the same oceanic basin and each showing distinct geochemical characteristics. The eastern belt is characterized by suprasubduction zone (SSZ) ophiolitic sequences, including island arc tholeiitic and boninitic volcanic series. The western belt, although composed mainly of mid‐ocean ridge‐type (MOR‐type) ophiolites with high‐Ti geochemical affinity, also exhibits alternating sequences showing distinct geochemical affinities referable to MOR‐ and SSZ‐type volcanics. These volcanics can be geochemically subdivided into four groups: (1) group 1 basalts show high field strength element (HFSE) and rare earth element (REE) concentrations similar to those of ocean‐floor basalts; (2) group 2 basalts, basaltic andesites, dacites, and rhyolites, characterized by HFSE and light REE depletion similar to those in many low‐Ti volcanics from SSZ settings; (3) group 3 basalts exhibit geochemical features intermediate between groups 1 and 2 but also bear SSZ features, being characterized by HFSE depletion with respect to the N‐MORBs; (4) group 4 boninitic dikes display very low‐Ti contents and typically depleted, \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Hidden Terranes in the Northern Apennines, Italy: A Record of Late Cretaceous‐Oligocene Transpressional Tectonics

\textsf{U}

Geodynamic Implications of Jurassic Ophiolites Associated with Island-Arc Volcanics, South Apuseni Mountains, Western Romania

\end{document} ‐shaped REE patterns. These different magmatic groups are interpreted as having originated from fractional crystallization from different primary basalts that were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Consequently, group 1 basalts may derive from partial melting of a fertile MORB source, while group 3 basalts may derive from 10% partial melting of a mantle that previously experienced MORB extraction. Finally, the group 2 basalts and group 4 boninites may be derived from about 10% partial melting of a mantle peridotite previously depleted by primary melt extraction of group 1 and group 3 primary melts. To explain the coexistence of these geochemically different magma groups, we present a model based on the complexity of the magmatic processes that may take place during the initiation of subduction in proximity to an active MOR. This model implies that the initiation of subduction processes close to such a ridge leads to contemporaneous eruptions in a fore‐arc setting of MORBs (group 1) generated from the extinguishing MOR and the initiation of group 3 basalts generated in the SSZ mantle wedge from a moderately depleted mantle source. The development of the subduction in a young, hot lithosphere caused the generation of island arc tholeiitic basalts (group 2) and boninites (group 4) from strongly depleted mantle peridotites in the early stages of subduction, soon after the generation of group 1 and group 3 basaltic rocks.

The deformation history of an accreted ophiolite sequence: the Internal Liguride units (Northern apennines, Italy)

All the geological constraints for an exhaustive reconstruction of the Triassic to Tertiary tectonic history of the southern Dinaric-Hellenic belt can be found in Albania and Greece. This article aims to schematically reconstruct this long tectonic evolution primarily based on a detailed analysis of the tectonic setting, the stratigraphy, the geochemistry, and the age of the ophiolites. In contrast to what was previously reported in the literature, we propose a new subdivision on a regional scale of the ophiolite complexes cropping out in Albania and Greece. This new subdivision includes six types of ophiolite occurrences, each corresponding to different tectonic units derived from a single obducted sheet. These units are represented by: (1) sub-ophiolite mélange, (2) Triassic ocean-floor ophiolites, (3) metamorphic soles, (4) Jurassic fore-arc ophiolites, (5) Jurassic intra-oceanic-arc ophiolites, and (6) Jurassic back-arc basin ophiolites. The overall features of these ophiolites are coherent with the existence of a single, though composite, oceanic basin located east of the Adria/Pelagonian continental margin. This oceanic basin was originated during the Middle Triassic and was subsequently (Early Jurassic) affected by an east-dipping intra-oceanic subduction. This subduction was responsible for the birth of intra-oceanic-arc and back-arc oceanic basins separated by a continental volcanic arc during the Early to Middle Jurassic. From the uppermost Middle Jurassic to the Early Cretaceous, an obduction developed, during which the ophiolites were thrust westwards firstly onto the neighboring oceanic lithosphere and then onto the Adria margin.

Late Cretaceous flysch deposits of the Northern Apennines, Italy: age of inception of orogenesis-controlled sedimentation

We propose that a very oblique or transpressional tectonic regime was dominant during the early (pre‐collisional) orogenic evolution of the Northern Apennines (Late Cretaceous to Early Oligocene). This hypothesis resolves many inconsistencies in the previous reconstructions of this orogenic belt, which were based on a classic model of orthogonal convergence between the European and Adriatic plates. The crucial lines of evidence that point to a major role of oblique tectonics in the structuring of the Northern Apennines are: (1) the plate tectonics framework, that indicates left‐lateral oblique convergence along the Europe/Adria plate margin; (2) the lack of a magmatic arc during the entire pre‐collisional convergent history of the chain (a time span >45 m.y., from Late Cretaceous to Early Oligocene); (3) the long (20 m.y.) residence time of turbidite sequences in the trench (the “dormant” trench); (4) the multiple source areas of turbidites from both sides of the basin, and the associated coarse gravity deposits; (5) the opposite vergence of deformations in some oceanic units; (6) the unmatching stratigraphic features, distinct deformation and metamorphic histories between adjacent overthrust oceanic units (Ligurids), here interpreted as tectonostratigraphic terranes. Specific aspects of Apennine stratigraphy and tectonics and the geometry and structure of the contacts between the Ligurid Units suggest the existence of a number of terranes juxtaposed by transpression during the early (Late Cretaceous to Early Oligocene) orogenic evolution of the chain.

From accretion to exhumation in a fossil accretionary wedge: a case history from Gottero unit (Northern Apennines, Italy)

In the Northern Apennines, in contrast to the Western Alps and Alpine Corsica, upper structural levels of the Late Cretaceous–middle Eocene subduction complex are still preserved and well exposed. This subduction complex developed in the Ligure-Piemontese basin since the Late Cretaceous time as a consequence of convergence between the Eurasia and Adria plates. Representative successions of this ancient subduction complex are well preserved in the Ligurian units of the Northern Apennines, where turbidite and mass-gravity deposits showing pristine stratigraphic features are present. Three main domains, represented by different groups of tectonic units, can be identified, each delineating a different domain of the subduction zone. In this article, we first present a brief history of geological research in the Northern Apennines during the last half of the twentieth century and then a comprehensive picture of the stratigraphy and tectonics of the Ligurian units. A new interpretation of the related tectonostratigraphic units is proposed within the conceptual modern geodynamic framework of convergent margins.