Valerio Bortolotti

Olistostromes and olistoliths

Abstract The Northern Apennines are a typical area of slide deposits. Sliding phenomena gave rise to various products ranging from gravity nappes to olistostromes and olistoliths. The latter differ from gravity nappes with regard to size and internal structure. Current research in the Northern Apennines suggests using the terms “olistostrome” and “olistolith” with a somewhat different meaning from that originally proposed by Flores (1955). The main differences concern the size limits and the relative position in the sedimentary sequences. Olistostromes occur in Jurassic to Pliocene deposits pertaining to eu-, mio-, late and postgeosynclinal sequences. They are particularly common in Upper Cretaceous to Miocene formations. The material of the Cretaceous and Eocene olistostromes generally comes from the ophiolites and the rocks overlying the ophiolitic suite. The olistostromes occur as thick layers or breccias or paraconglomerates, and, like the olistoliths, are intercalated in the eugeosynclinal flysch formations. The Oligo-Miocene olistostromes are also made of material from the eugeosynclinal sequences, but they are interbedded in the miogeosynclinal flysch or in the late geosynclinal formations. They generally appear as argillaceous bodies with scattered rock fragments (mostly limestones). The genesis of olistostromes and olistoliths is strictly related to the migration of the flysch basins from west to east. Slumping was caused by the eastward orogenic wave: olistostromes were discharged from the uplifted areas and/or from the front of the advancing nappes.

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

Introduction to the geology of the Northern Apennines

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Ophiolites, Ligurides and the tectonic evolution from spreading to convergence of a Mesozoic Western Tethys segment

\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.

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

Abstract The Tuscan and Umbrian sequences represent miogeosynclinal deposition in the Northern Apennines. They are composed of five major rock groups, which are here described in detail and interpreted. The sequences start with a terrigenous basal section, Upper Carboniferous to Carnian, composed of clastic rocks with shallow-marine, lagoonal and continental facies, and of acid volcanics. This group probably represents the Hercynian postgeosynclinal stage. A carbonate-evaporitic section follows, Noric to Rhaetic, overlain by Hettangian neritic limestones. These facies are of wide extent; a geosynclinal basin in the Northern Apennines had probably not yet differentiated. A progressive deepening of the sea began in the Sinemurian; the carbonate-siliceous section (Lower Jurassic-Lower Cretaceous) is made of frequently siliceous micritic limestones and marlstones, and contains bedded cherts, but locally there are biostromal and oolitic limestones. In parts of the geosyncline, submarine dissolution on top of tectonic rises caused the development of paraconformities. The pre-flysch section includes pelitic and marly formations, with some bedded chert, and graded limestones (prototurbidites) in the Tuscan sequence (Lower Cretaceous to Lower Oligocene); limestones and marlstones in the Umbrian sequence (Upper Cretaceous to Middle Miocene). The pre-flysch stage indicates a further deepening of the sea and a considerable slowing of the rate of deposition, but the bottom physiography was in parts irregular and paraconformities were formed. The topmost flysch section consists of thick turbidite formations, migrating in age from west to east, Early-Middle Oligocene to Early Miocene in the Tuscan sequence, Early to Late Miocene in the Umbrian sequence. Each flysch unit starts abruptly and is typically arenaceous in the lower part (ortotubidites), but it becomes progressively marly-argillaceous in the upper part (kataturbidites).

Late Triassic, Early and Middle Jurassic Radiolaria from ferromanganese-chert ‘nodules’ (Angelokastron, Argolis, Greece): evidence for prolonged radiolarite sedimentation in the Maliac-Vardar Ocean

According to Aubouin (1965) the sedimentary evolution of the Northern Apennines geosyncline is divided into a geosynclinal stage proper, represented by eu- and miogeosynclinal sequences, a late geosynclinal and a postgeosynclinal stage. In the Apennines the eugeosynclinal rocks are almost entirely allochthonous. Their interpretation as autochthonous is held to be unrealistic on structural and paleogeographic grounds. The late geosynclinal stage is defined here mainly on the base of tectonic criteria: sediments deposited over folded eugeosynclinal rocks, later subjected to lateral tectonic transport in the same manner as their allochthonous substratum. Owing to the eastward progression of tectonic movements in the Northern Apennines, the tecto-sedimentary stages tend to overlap and coexist (e.g., Oligocene-Miocene miogeosynclinal flysch and late geosynclinal sediments). The eugeosynclinal stage is characterized by the presence of ophiolites and the early development of flysch. Four main groups of sequences are distinguished: (1) Upper Cretaceous to Eocene Helmynthoid Flysch sequences; (2) Jurassic to Eocene Vara Supergroup; (3) Upper Cretaceous to Middle Eocene Calvana Supergroup; and (4) Paleogene Canetolo Complex. The largely autochthonous miogeosynclinal rocks are represented by the Tuscan (Lower Triassic to Lower Miocene) and Umbrian (Carnian to Upper Miocene) sequences. The late geosynclinal sequences are Middle Eocene to Messinian in the Emilian Apennines, Lower and Middle Miocene in Tuscany and Romagna. The postgeosynclinal sediments are Upper Miocene to Pleistocene in the southwest (Tuscany-Latium), Pliocene and Pleistocene in the northeast and east (Emilia and Marche). Four major structural areas are distinguished: 1. (1) The Tyrrhenian area, a zone of intense crustal shortening, containing the axis of symmetry between the structure of the Apennines and that of the Western Alps and “Alpine Corsica”. 2. (2) Southwestern Tuscany, characterized by a fault block structure and an incomplete Tuscan sequence. 3. (3) The main fold range, with reverse faults, overturned folds and overthrusts, all directed eastward and northeastward. There are two major northwest-southeast lines of thrusting and overturned folding, and two major tectonic windows (Alpi Apuane and Monte Pisano), in which the Tuscan sequence is doubled. 4. (4) The outer foothills and Po Valley, where the asymmetric folding and reverse faulting become gradually attenuated. Metamorphism is generally of low grade (greenschist facies), and affects the lower part of the Tuscan sequence in small areas of western Tuscany (Alpi apuane, Monte Pisano, Montagnola Senese, Elba Island). Metamorphism tends to be associated with the tectonic doubling of the sequence. Theories on the tectonic interpretation of the Northern Apennines are summarized. The authors are inclined to accept features of both the “decollement” nappe model of Trevisan et al. (1965) and the orogenetic landslide model of Migliorini (1948) and Merla (1951). The emplacement of the allochthon was essentially through gravitational gliding; detachment and gliding affected also, in parts, rocks of the Tuscan sequence (Tuscan nappe of the Alpi Apuane). Uplift and differential movements (block faulting?) in the miogeosyncline from the Triassic to the Cretaceous are indicated by slumping and unconformities. Much slumping occurred during the Cretaceous in the eugeosyncline. The first recorded eastward gliding movements of nappes are Upper Cretaceous to Paleocene in the southern part of the eugeosyncline, Eocene in the northern part of it. Parts of the eugeosynclinal sequences were folded in the Lower-Middle Eocene and at the Eocene-Oligocene transition. The advancement of nappes onto the miogeosynclinal rocks, accompanied by folding, began at the Oligocene-Miocene transition. It continued, gradually moving eastward, until the Lower Pliocene. The detachment of rocks of the Tuscan sequence in the Alpi Apuane and southern Tuscany (Tuscan nappe) seemingly occurred in the Lower and Middle Miocene.

The geosyncline concept and the northern apennines

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.

The northern apennines geosyncline and continental drift

The Ligurian formations, a stack of sediments deposited over an oceanic (containing ophiolites) basement in the Ligurian—Piedmont oceanic basin, provide unique records for reconstructing the opening, evolution and closure of the portion of Western Tethys that separated the European-Iberia plates to the NW from the Africa-Adria plates to the SE (Abbate et al., 1970, 1980, 1986; Principi and Treves, 1984). They are present all along the Alps and the Apennines down to Calabria, except for central Italy. This chapter deals with the Northern Apennines, where these successions are well exposed and studied.