Ophiolite Nappe Research Articles

NEW EVIDENCES FOR TRIASSIC MORB MAGMATISM IN THE NORTHERN MIRDITA ZONE OPHIOLITES (ALBANIA)

A succession of mid-ocean ridge basalts (linked at their base to serpentinite slivers) with intercalations of chert levels of Middle and Late Triassic, as well as Early and Middle Jurassic ages have recently been found in the Argolis Peninsula, Greece (Bortolotti et al., 2001; 2002a; 2003; Saccani et al., 2004a). This succession shows the same lithostratigraphic and tectonic setting of the Triassic basalts of the “Volcano- sedimentary Formation” at the top of the “Rubik Complex” of the Mirdita Ophiolitic Nappe in Albania (Bortolotti et al., 1996; 2002b; 2004). Moreover, Argolis and Mirdita ophiolites belongs to the same ophiolitic belt, which extends from Mirdita to Pindos, Vourinos, Othrys and Argolis. The “Volcano-sedimentary Formation” was previously interpreted by Kodra et al. (1993) as an Upper Jurassic sequence, which “precede de peu ou accompagne le debut de l’ouverture (oceanisation) proprement dite du domaine ophiolitique de Mirdita” (p. 66). Bortolotti et al. (1996) confirmed this interpretation (“the sequence can be interpreted as a product of syn-rift, subalkaline magmatism associated with thinning of the continental margin during the Middle Triassic”; p. 5). Thus, on the bases of the radiolarian stratigraphy, this sequence has been retro-dated to the Middle and Late Triassic times (Bortolotti et al., 1996; 2002b; Chiari et al., 1996). The aim of this paper is to assess the petrology and tectono-magmatic significance, with respect to the age of formation, of basalts from this complex in the northern Albania Mirdita zone, to provide new data for a more detailed reconstruction of the Triassic tectono-magmatic events related to the formation of the Tethys oceanic basin in the Dinaric- Hellenic area.

COMPARISON AMONG THE ALBANIAN AND GREEK OPHIOLITES: IN SEARCH OF CONSTRAINTS FOR THE EVOLUTION OF THE MESOZOIC TETHYS OCEAN

In this paper the stratigraphical, structural, geochemical and petrological features of the Mirdita (Albania) and Pindos, Vourinos, Koziakas, Othrys and Argolis (Greece) ophiolitic nappes are summarised and then compared. These ophiolitic nappes occur as a 700 km long belt running from Albania to Greece. These ophiolitic nappes are located between the west-verging imbricate stack of thrust sheets derived from the Adria plate continental margin to the west and the Pelagonian zone to the east. Each ophiolitic nappe is represented by several end-members represented by the sub-ophiolite melange, the ophiolite sequence( s) with their sedimentary oceanic cover and the supra-ophiolite deposits. The latter can be divided in syn- and post-emplacement deposits, the first ones are recognised only in Albania. All the described ophiolite sequences are characterised at their base by a well-developed metamorphic sole that represents a further end-member of the ophiolitic nappe. The comparison among the features of all the end-members recognised in the studied ophiolitic nappes allows providing further constraints for the geodynamic reconstructions of the Mesozoic Tethyan oceanic basin located eastwards of the Adria plate.

Stratigraphical evidence for the depression of the northern margin of the Menderes–Tauride Block (Turkey) during the Late Cretaceous

Upper Cretaceous units on the Menderes–Tauride Block show a transition from a passive continental margin to a pelagic basin. This transition is related to the emplacement of ophiolitic nappes. The pelagic sediments were studied in the Aydıncık and Arslanköy (Mersin) areas in the central Tauride region, and in the Munzur Mountains in eastern Tauride region. In the Aydıncık area Upper Cretaceous units consist of Upper Campanian carbonate breccias with rudist fragments; Upper Campanian–Lower Maastrichtian slope-to-basinal sediments overlie these carbonate breccias disconformably. In the Arslanköy area the Upper Campanian–Maastrichtian Yavca Formation consists of Globotruncana-bearing biomicrites, siliciclastic turbidites, calciturbidites and debris flows and overlies Jurassic–Cretaceous platform carbonates disconformably. The uppermost levels of the Liassic–Cenomanian platform carbonates in the Munzur Mountains are composed of Cenomanian Hippurites/Orbitolina-bearing reefal limestones and then Turonian–Campanian Globotruncanidae-bearing biomicrites.The stratigraphic sequence in the Menderes–Tauride Block shows that rifting to form a northern Neotethyan basin took place in the Triassic and gave rise to the Inner Tauride Ocean. From Jurassic to Cenomanian times, the northern part of the Menderes–Tauride Platform formed a passive continental margin. In the Late Cretaceous the Inner Tauride Ocean is believed to have commenced intra-oceanic, northward subduction. Inner Tauride oceanic crust-derived nappes were emplaced southward onto the Menderes–Tauride Block. Flexural foreland basins in which pelagic sediments were deposited were formed in the Taurus Mountains related to southward thrusting and the emplacement of the ophiolite nappes.

The Bi'r Umq-Nakasib Suture Zone in the Arabian-Nubian Shield: A Key to Understanding Crustal Growth in the East African Orogen

The Bi'r Umq-Nakasib suture zone, 5-65 km wide and over 600 km long, consists of highly deformed ophiolite nappes and metavolcanic, metasedimentary, and intrusive rocks contained in one of the longest ophiolite-decorated shear zones in the Arabian-Nubian Shield. The rocks originated in a variety of juvenile oceanic environments and include assemblages formed in mid-ocean-ridge, subduction-zone, passive-margin, and continental-slope settings. Dating of the ophiolites, volcanic rocks, and pre- and syntectonic plutons indicates that oceanic magmatism in the region was active ∼870-830 Ma whereas suturing occurred ∼780-760 Ma. This chronology suggests that suturing involved the closure of a relatively long-lived oceanic basin and makes the Bi'r Umq-Nakasib shear zone the oldest accretionary structure known among the juvenile Neoproterozoic rocks of the northern East African Orogen. Creation of the shear zone dates the onset of arc-arc convergence in what eventually became the Arabian-Nubian shield, and marks the beginning of the complex, heterogeneous process of terrane amalgamation and continental accretion that led to the eventual accretion of East and West Gondwana.

Tectonic evolution and significance of Silurian – Early Devonian hinterland-directed deformation in the internal Humber zone of the southern Quebec Appalachians

In the southern Quebec Appalachians, the early tectonic history of the Laurentian margin (Humber zone) comprises foreland-propagating, northwest-directed thrust faulting, nappe emplacement, and regional prograde metamorphism in response to the obduction of large ophiolitic nappes during the Taconian orogeny. In the internal Humber zone, this event is dated at 462 ± 3 Ma (late Middle Ordovician), which is interpreted to represent the timing of near-peak Taconian metamorphism. Superimposed hinterland-directed structures are accompanied by retrograde metamorphism and consist of back thrusts and normal faults, which respectively delimit the northwestern and southeastern limbs of the Sutton and Notre-Dame mountains anticlinoria, both salient structures of the internal Humber zone of southern Quebec. Geochronologic data on the timing of hinterland-directed deformation vary from 431 to 411 Ma. Two tectonic models are presented and discussed, which may account for the Silurian – Early Devonian evolution of the Laurentian margin: (1) back thrusting and syn- to post-compressional crustal extension in response to the tectonic wedging of basement-cored duplexes inducing delamination of supracrustal rocks; (2) tectonic exhumation of the internal Humber zone by extensional collapse. Evidence for Silurian – Early Devonian extensional tectonism in the Humber zone provides the basement infrastructures necessary for the creation and the onset of sedimentation in the Gaspé Belt basins (e.g., Connecticut Valley – Gaspé synclinorium). Several structural, metamorphic features in the internal Humber zone of the northwestern New England Appalachians yield analogous characteristics with those of southern Quebec and may have shared a similar Silurian – Early Devonian tectonic evolution.

Ophiolite obduction and the Samail Ophiolite: the behaviour of the underlying margin

Abstract Samail ophiolite emplacement has become a type-example for ophiolite obduction. Despite this, problems and controversy remain with respect to the age of high- P metamorphism, the vergence of structures in deformed rocks beneath the ophiolite, the suprasubduction character of the ophiolite and the P-T conditions of the metamorphic sole. The presence of major, regional-scale NE-facing isoclinal folds and SW- and west-dipping shear zones with top-to-the-NE shear sense in Arabian margin rocks beneath the Samail Ophiolite nappe requires that the margin was not simply passively overridden during obduction of the ophiolite. The development of these folds at 72–76 Ma is at the time the ophiolite is emplaced finally onto the margin (80–70 Ma), accompanied by development of a major shear zone in Saih Hatat (the upper plate-lower plate discontinuity described by earlier workers) at c. 82–80 Ma. Structural scenarios that incorporate these folds and shear zones include lateral escape from a rising buoyant crustal slice along the former subduction interface, back-folding (‘retrocharriage’) associated with major oceanwards-directed underthrusting, or simple underthrusting of the margin by the oceanic realm. Previous models involving craton-directed overthrusting with domal culminations related to deep-seated footwall and lateral ramps are more applicable to the Tertiary structure and Tertiary evolution of the mountain range. Oman ophiolite obduction clearly involves ocean-vergent thrusting within the continental margin platform to slope facies sequences.

Tectonostratigraphic subdivision and petrological characterisation of the deepest complexes of the Betic zone: a review

The Veleta and Mulhacén Complexes, previously considered together as the Nevado-Filábride Complex, constitute the deepest units of the Betic Cordilleras. The Veleta Complex (VC) is mainly formed of several thousand metres of graphite-bearing micaschists and crops out as a series of tectonic windows below the Mulhacén Complex (MC). This latter complex comprises a pile of thrust nappes of crustal origin, composed of a Palaeozoic basement and Mesozoic cover series, between which a Jurassic-Cretaceous ophiolitic nappe is tectonically intercalated. The Alpine metamorphism developed in eclogite facies followed by Ab-Ep amphibolite facies in the MC, and in Ab-Ep amphibolite facies followed by greenschist facies in the VC. Pre-Alpine metamorphism in high gradient amphibolite facies is locally well preserved in some basement levels of the MC. In contrast, only scarce relics of pre-Alpine metamorphism, in greenchists facies, have been preserved in the VC. Within the MC nappes of crustal origin there appear metagranitic rocks, which were generated during two stages of late Hercynian magmatic activity: one of syncollisional acidic magmatism and the other of postcollisional intraplate acidic-to-intermediate volcanism. The ophiolitic nappe is composed of basic, ultramafic and sedimentary rocks affected by ocean-floor and orogenic metamorphism. The basic rocks partly retain their original E-MORB geochemical character. The ultramafic rocks are serpentinites and secondary harzburgites containing partly rodingitized basaltic dykes. The Soportújar Formation of the MC is made up of continental and evaporitic metasediments and meta-andesitic rocks that originated during an intraorogenic relaxation period between the eo-Alpine and meso-Alpine metamorphic events. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Tectonostratigraphic subdivision and petrological characterisation of the deepest complexes of the Betic zone: a review

The Veleta and Mulhacén Complexes, previously considered together as the Nevado-Filábride Complex, constitute the deepest units of the Betic Cordilleras. The Veleta Complex (VC) is mainly formed of several thousand metres of graphite-bearing micaschists and crops out as a series of tectonic windows below the Mulhacén Complex (MC). This latter complex comprises a pile of thrust nappes of crustal origin, composed of a Palaeozoic basement and Mesozoic cover series, between which a Jurassic–Cretaceous ophiolitic nappe is tectonically intercalated. The Alpine metamorphism developed in eclogite facies followed by Ab-Ep amphibolite facies in the MC, and in Ab-Ep amphibolite facies followed by greenschist facies in the VC. Pre-Alpine metamorphism in high gradient amphibolite facies is locally well preserved in some basement levels of the MC. In contrast, only scarce relics of pre-Alpine metamorphism, in greenchists facies, have been preserved in the VC. Within the MC nappes of crustal origin there appear metagranitic rocks, which were generated during two stages of late Hercynian magmatic activity: one of syncollisional acidic magmatism and the other of postcollisional intraplate acidic-to-intermediate volcanism. The ophiolitic nappe is composed of basic, ultramafic and sedimentary rocks affected by ocean-floor and orogenic metamorphism. The basic rocks partly retain their original E-MORB geochemical character. The ultramafic rocks are serpentinites and secondary harzburgites containing partly rodingitized basaltic dykes. The Soportújar Formation of the MC is made up of continental and evaporitic metasediments and meta-andesitic rocks that originated during an intraorogenic relaxation period between the eo-Alpine and meso-Alpine metamorphic events.

Origin of the Upper Cretaceous–Tertiary sedimentary basins within the Tauride–Anatolide platform in Turkey

A number of sedimentary basins formed within the Tauride–Anatolide Platform of Anatolia during the Late Cretaceous–Tertiary period. Previous studies have proposed different tectonic and evolutionary models for each basin. Geological characteristics of the basins, however, suggest that all these basins are of the same origin and that they followed a similar evolutionary model to one another. Basin development within the Tauride–Anatolide Platform took place in a post-collisional environment following the northward subduction of the northern Neotethys ocean beneath the Pontides. The closure of the northern Neotethys ocean ended with collision of the Tauride–Anatolide Platform with the Pontide volcanic arc and resulted in large bodies of oceanic remnants thrust over the Tauride–Anatolide Platform as ophiolite nappes. Formation of the sedimentary basins followed the emplacement of the ophiolite nappes as they formed as piggy-back basins on top of the underlying thrust ophiolite basement.

The basic rocks of the Shetland Ophiolite Complex and their bearing on its genesis

Synopsis The paper presents some of the results of a recent survey of the whole ophiolite complex and the adjacent basement. It combines previously published work with new field, petrographic and chemical results obtained principally from the mafic components of the ophiolite and offers an outline account of the evolution of the complex. The Shetland Ophiolite Complex occupies the largest part of the islands of Unst and Fetlar, part of the Shetland Islands lying north of Scotland. The complex is composed of two ophiolitic nappes, a lower nappe and an overlying upper nappe represented by erosional remnant klippen. The nappes are separated by and underlain by, imbricate zones of basement gneisses, ophiolitic rocks, metavolcanic and ophiolite-debris-bearing metasedimentary rocks. The lower nappe presents a 7 km continuous section through the ophiolite succession from a 2 km thick layer of mantle to the base of the sheeted dyke zone. The mantle layer has been uniformly and pervasively lizardite serpentinized (metaharzburgite), but has later been recrystallized to antigorite in a 100 m zone immediately above the basal obduction thrust. It is considered that obduction took place cold (<500°). Immediately below the obduction thrust are hornblende schists in a relationship to the nappe usually interpreted as a dynamothermal aureole in underlying volcanic rocks caused by obduction over them of hot nappe. However the Shetland hornblende schist is seen to be of MORB composition and to have been intruded up the obduction thrust into the serpentinized nappe, where it became hydrated and crystallized as hornblende schist. Above the metaharzburgite layer are successive layers of metadunite and metagabbro in igneous contact and each up to 2.5 km thick. The y are uniformly and pervasively hydrated to lizardite serpentinite and to amphibole and saussurite respectively. The absence of an intervening cumulate layered unit implies the lack of a magma chamber so that sheeted dykes and metamicrogabbros forming the uppermost levels in the lower nappe cannot have been derived from within the nappe. Also they are of compositions distinctly different to that of the metagabbro. Some of the dykes are of boninitic composition and more primitive than the metagabbro. Hornblende from the hornblende schists in the obduction thrust has given an age of c. 500 Ma for the obduction. The uniformity and complete nature of the hydration of the nappe and the absence of any evidence that hydration was based on fractures needed to allow sea-water penetration is considered to preclude hydration when the ophiolitic slab formed sea-floor. It is suggested that the slab was first subducted to the east and hydrated by water driven upwards within the slab from lower levels and that the magmas forming the sheeted dykes and metamicrogabbro were created by water escaping from the slab into the overlying mantle wedge, followed by their intrusion into the slab from above. Closure of Baltica against Laurentia reversed the subduction and obducted the slab westwards onto Laurentia at about the same time that the older western Norwegian ophiolites were obducted eastwards onto Baltica. A period of subaerial erosion and deposition followed, during which sediments containing ophiolitic debris and volcanic rocks were deposited on the surface of the obducted nappe. Then further pressure from the east thrust a section of the already obducted slab westwards over the lower nappe and its cover of sediments to form the upper nappe, dragging with it slices of underlying ophiolite and basement to form the imbricate zone between the two nappes. The nappe pile was thrust further west-wards to form an imbricate zone beneath the pile and fold, deform and metamorphose the rocks of the imbricate zones. It is possible that the emplacement of the upper nappe coincided with the final closure of Baltica against Laurentia and the eastwards obduction of the younger west Norwegian ophiolites during the Scandian orogeny.

Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications

This paper deals with the tectonic events that result in the accretion of mafic terranes in the fore-arc region and a close juxtaposition of ultramafic rocks, low grade and high-grade mafic terranes in many collisional orogens. The example is taken from New Caledonia where tectonic accretion, subduction, underplating and obduction of mafic terranes took place during the late Eocene in an intra-oceanic forearc setting. The late Eocene tectonic complex comprised three major terranes: an overlying ultramafic, mainly harzburgitic allochthon named the Ophiolitic Nappe, an intermediate mafic, mainly basaltic off-scraped melange, composed of kilometre-scale slices of oceanic upper crust, called the Poya Terrane, parts of which have been metamorphosed into an eclogite/blueschist facies complex, the Pouebo Terrane; and a lower, continental basement formed by the Norkolk Ridge terranes. Based upon exhaustive sampling of the mafic terranes and field surveys, our tectonic, micropaleontologic and geochemical data reveal that Poya and Pouebo terranes rocks originally formed within one single Campanian to late Paleocene oceanic basin, floored by tholeiitic basalt associated with some minor seamount-related intraplate alkali basalt. The tholeiitic basalt displays a continuous range of compositions spanning between “undepleted” and “depleted” end-members; the former being volumetrically predominant. The overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin, the South Loyalty Basin, which almost completely disappeared during Eocene convergence. The opening of this basin originally located to the east of the Norfolk Ridge was synchronous with that of Tasman Sea basin as a consequence of oceanward migration of the west-dipping Pacific subduction zone. Establishing the origin of the ultramafic Ophiolitic Nappe is beyond the scope of this paper; however, it appears to be genetically unrelated to the mafic Poya and Pouebo terranes. Although it was located in the Late Eocene fore-arc, the Ophiolitic Nappe and the corresponding oceanic lithosphere originated before the Late Cretaceous, to the east of the South Loyalty Basin in a back-arc setting; or alternatively in a much older, trapped basin. For reasons that remain unclear, a new east-dipping subduction started in the Eocene and consumed most of the South Loyalty Basin, forming the intra-oceanic Loyalty Arc. Due to a changing subduction regime (underplating of the Diahot Terrane?), the mafic slices that now form the Poya Terrane were tectonically accreted in the Loyalty fore-arc region and remained under low pressure-low temperature conditions (possibly at the subsurface) until the Norfolk Ridge reached the subduction zone diachronously. This resulted in the final obduction of the fore-arc area. The two-step obduction involved first the mafic complex forming the Poya Terrane and thereafter the lithospheric mantle that now forms the Ophiolitic Nappe. In contrast, pieces of the accretionnary complex were dragged down into the subduction zone, underplated at depth ca. 70 km and metamorphosed into high-temperature eclogite to form the Pouebo Terrane metamorphics that display the same geochemical features as the Poya Terrane basalt. A mid-to-late Eocene syntectonic piggy-back sedimentary basin (the Nepoui flysch basin) mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore-arc region due to the accretion and underplating of mafic ocean-related and other material. In contrast, a slightly younger foreland basin located upon the Norfolk Ridge (the Priabonian Bourail Flysch basin) received a massive input of detrital material derived from the Norfolk Ridge itself and a time-increasing amount of mafic, Poya-derived material that recorded the first step of obduction. Thereafter, the Bourail Flysch was overthrust by the Poya Terrane and finally by the Ophiolitic Nappe. At the same time, buoyancy-driven uplift and exhumation of the high-pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone. Finally, for a short time a new subduction started along the west coast of New Caledonia and generated small amounts of active margin-related magmas. These events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre-collision analogue of mafic/ultramafic belts found in many collisional mountain ranges.

Geology of the central Sulawesi belt (eastern Indonesia): constraints for geodynamic models

Sulawesi island, located at the junction of the Asian, Australian and Pacific plates, is geologically important for unraveling the Cenozoic tectonics of South-East Asia. This island, situated on the active margin of the Asian plate suffered at least four major tectonic events in Mid-Cretaceous, Oligo-Miocene, Middle Miocene, and Early Pliocene times. The present paper is focused on the Oligo-Miocene event, which formed the main orogenic belt of this island. Several geodynamic models have been proposed for this belt. Our structural and stratigraphic investigations in the central area of Sulawesi lead us to favor an eastward obduction of an ophiolitic nappe of Asian origin onto a continental Gondwana Block, while a west-dipping slab was subducting underneath the western Asiatic margin. Collision between this fragment of Gondwana and the active Asiatic margin in Late Oligocene or Early Miocene time produced the central metamorphic belt of Sulawesi.

Extrusive Members of Postcollisional A-Type Magmatism in Central Anatolia: Karahidir Volcanics, Idis Dagi-Avanos Area, Turkey

The Idis Dagi Igneous Complex is one of a number of late-stage plutonic bodies within the Central Anatolian Crystalline Complex. It intrudes Paleozoic-Mesozoic metamorphic basement rocks and Late Cretaceous granitoids. The complex comprises mainly quartz syenites and alkali-feldspar quartz syenites known as the Idis Dagi Syenitoids. Trachydacitic, trachyandesitic, and dacitic rocks (Karahidir Volcanics) have been recently found as dikes cutting the Idis Dagi Syenitoids or as huge blocks within an olistostromal unit of latest Cretaceous(?)-Early Paleocene age that immediately overlies the Idis Dagi Igneous Complex. Petrographic and geochemical data indicate that the Idis Dagi Syenitoids and the Karahidir Volcanics display similar humped patterns on chondrite- and MORB-normalized spider diagrams, with peaks at Rb, Th, and Ce, and also negative Nb anomalies. These features are similar to patterns considered typical of postcollisional, A-type igneous rocks. A postcollisional setting is also suggested by the distribution of data on tectonic discrimination diagrams involving Nb, Y, and Rb. As a whole, the geochemical data suggest that the Idis Dagi Syenitoids and Karahidir Volcanics are cogenetic. Thus, the Karahidir Volcanics represent the shallow intrusive and volcanic equivalents of the deeper-level Idis Dagi Syenitoids. The syenitoid rocks are considered the final phase of the magmatism in the Central Anatolian Crystalline Complex and are classified as post-orogenic alkaline rocks. Their chemical features suggest a largely mantle-derived magma contribution, together with a noticeable crustal component in their genesis. They were formed during postcollisional uplift that followed crustal thickening related to the southward emplacement of ophiolitic nappes during closure of the Izmir-Ankara-Erzincan Oceanic strand of Neotethys. The Karahidir Volcanics were exposed during regional extension that resulted in the formation of latest Cretaceous(?)-Early Paleocene extensional basins, and were emplaced as huge blocks into the continental clastic rocks by gravity sliding. This extension phase is characteristic of the development of all the other latest Cretaceous-Early Tertiary intracratonic basins in central Anatolia.

Neoproterozoic (∼800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia): island arc–continent collision at the northeast Rodinia margin

The Tuva-Mongolia Massif is a composite Precambrian terrane incorporated into the Palaeozoic Sayany-Baikalian belt. Its Neoproterozoic amalgamation history involves early (∼800 Ma) and late Baikalian (600–550 Ma) orogenic phases. Two palaeogeographic elements are identified in the early Baikalian stage — the Gargan microcontinent and the Dunzhugur oceanic arc. They are represented by the Gargan Glyba (Block) and the island-arc ophiolites overthrusting it. The Gargan Glyba is a two-layer platform comprising an Early Precambrian crystalline basement and a Neoproterozoic passive-margin sedimentary cover. The upper part comprises olistostromes deposited in a foreland basin during the early Baikalian orogeny. The Dunzhugur arc ophiolite form klippen fringing the Gargan Glyba, and show a comprehensive oceanic-arc ophiolite succession. The Dunzhugur arc faced the microcontinent, as shown by the occurrence of forearc complexes. The arc–continent collision followed a pattern similar to Phanerozoic collisions. When the marginal basin lithosphere had been completely subducted, the microcontinental edge partially underthrust the arc, and the forearc ophiolite overrode it. Continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin. Owing to the lithospheric break, a new subduction zone, inclined beneath the Gargan microcontinent, emerged. Initial melts of the newly-formed continental arc are represented by tonalites intruded into the Gargan microcontinent basement and its cover, and into the ophiolite nappe. The tonalite Rb–Sr mineral isochron age is 812±18 Ma, which is similar to a U–Pb zircon age of 785±11 Ma. A period of tonalite magmatism in Meso–Cenozoic orogenic belts is recognized some 1–10 m.y. after the collision. Accordingly, the Dunzhugur island arc–Gargan microcontinent collision is conventionally dated at around 800 Ma. It is highly probable that in the early Neoproterozoic, the Gargan continental block was part of the southern (in modern coordinates) margin of the Siberia craton. It is suggested that a chain of Precambrian massifs represents an elongate block separated from Siberia in the late Neoproterozoic. The Tuva-Mongolia Massif is situated in the northwest part of this chain. These events occurred on the NE Neoproterozoic margin of Rodinia, facing the World Ocean.

Post-collisional shortening in the late Pan-African Hamisana high strain zone, SE Egypt: field and magnetic fabric evidence

The Hamisana zone (HZ) is one of the major high strain zones of the Pan-African (Neoproterozoic) Arabian–Nubian shield (ANS). It trends broadly N–S from northern Sudan into southeastern Egypt and meets the present Red Sea coast at ≈23°N. The HZ has been the subject of controversy with regard to its importance for the Pan-African structural evolution. Interpretations range from a suture zone, a regional shear zone, or a large-scale transpressional wrench fault system. In this study, we characterize the nature of the high strain deformation by applying the anisotropy of magnetic susceptibility method along with field and microstructural investigations. These investigations demonstrate that deformation in the HZ is dominated by pure shear under upper greenschist/amphibolite grade metamorphic conditions, producing E–W shortening, but with a strong N–S-extensional component. This deformation also led to folding of regional-scale thrusts (including the base of ophiolite nappes such as Gabal Gerf and Onib). Consequently, the high strain deformation is younger than ophiolite emplacement and suturing of terranes. A weak subsequent overprint was mostly non-coaxial. It took place under considerably lower temperature and led to a minor NE–SW-trending, dextral wrench fault. Although it is of only local importance this fault may be itself a conjugate relative to the prominent NW–SE-trending sinistral Najd faults in the northern ANS. Therefore, the HZ is dominated by late orogenic compressional deformation and cannot be related to either large-scale transpressional orogeny or major escape tectonics.

MAFIC ROCKS FROM THE SEDIMENTARY BRECCIAS ASSOCIATED TO THE BALAGNE OPHIOLITIC NAPPE (NORTHERN CORSICA): GEOCHEMICAL FEATURES AND GEOLOGICAL IMPLICATIONS

In the Balagne area (Northern Corsica), two main coarse-grained deposits characterized by the presence of mafic rocks are recognized. The first is represented by the Toccone breccia (Late Cenomanian) belonging to the succession of the Balagne ophiolitic nappe, whereas the second, referred to as Volparone breccia (post-Jurassic), occurs as a slice at the base of the same nappe and displays no relationships with others lithologies. Both breccias can be regarded as deposits located close to their source area, as suggested by their sedimentological/stratigraphical features. Even if not coeval, these breccias were probably supplied by the same source area consisting of a metamorphic basement intruded by granitoids and covered by carbonate deposits. The recognized lithologies suggest that its source area was represented by a domain belonging to the Corsica continental margin. The Toccone and Volparone breccias are both characterized by mafic rocks, wich can be classified as transitional to tholeiitic within-plate basalts. It is noteworthy that clasts of ophiolites are completely lacking. The mafic rocks preserved as clasts in the Toccone and Volparone breccias are the remnants of a magmatism connected with the crustal thinning processes. This magmatism developed during the Late Permian-Jurassic before the opening of the Ligure-Piemontese oceanic basin. This magmatism probably developed in an area of the Hercynian basement that in the Late Jurassic became located in the oceanward side of the Corsica continental margin, i.e. the source area of the studied breccias.

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey. The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange. These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence. Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients. Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit. Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates. The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB). These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids. Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth. This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere. The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway. These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean.

Detachment faults in the Aegean core complex of Ios, Cyclades, Greece

Abstract Several generations of detachment fault systems have been recognized in the Aegean metamorphic core complex of Ios, Cyclades, Greece. Multiple strands occur in each fault system, with fault-bounded slices often characterized by distinctive lithologies. These faults accomplished significant exhumation, and telescoped the crustal section. Different slices have been deformed at different crustal levels, and then juxtaposed, so that adjacent fault slices display different deformation styles. The Aegean detachment systems differ from equivalent faults observed in the US core complexes in that they separate relatively cohesive tectonic slices. However, the Aegean detachments are dissected by the multiple systems of later high-angle normal faults that define the current geomorphology, and this raises significant difficulties in correlating detachment systems between the different islands of the Cycladic archipelago. A number of distinct extensional events can be defined on Ios. The Cycladic blueschist belt was exhumed relatively early during the history of Alpine orogeny, after it had been thrust over a Hercynian ‘basement’ terrane, with the oldest recognized detachment faults associated with Miocene N-S extension. This led to the exhumation of the ‘basement’ terrane from beneath the blueschist ‘series’ rocks. The Ios Detachment Fault system now separates the metasediments of the Cycladic blueschist belt from the mylonitized ‘basement’ complex. It was associated with the operation of the south-directed, crustal-scale, middle to upper greenschist facies, South Cyclades Shear Zone. The youngest detachment faults formed during late-stage W-directed extension, and formed a prominent sequence of low-angle normal faults termed the Coastal Fault system. These were associated with spectacular breccias, and relatively narrow ductile shear zones which formed during and subsequent to the formation of the Ios dome, under significantly lower grade conditions (lower-greenschist facies assemblages). Final exhumation of the Cycladic blueschist belt appears to have been associated with the detachment faults that led to its exposure beneath non-metamorphic sediments, and remnants of the Cyclades ‘ophiolite nappe’ as observed on adjacent islands.

Mise en Évidence du Jurassique inférieur et moyen dans la ceinture ophiolitique de Sulawesi (Indonésie). Conséquences géodynamiques

Calcareous nannofossils, one ammonite and some benthonic foraminifera have been discovered in deposits of Upper Sinemurian (?) to Bathonian age under the ophiolite nappe in East Sulawesi. These biostratigraphical results allow to propose for the first time a general lithostratigraphic succession of the Mesozoic tectonically-dismembered sedimentary rocks under the ophiolite. This succession indicates that the Late Triassic reefal platform of East Sulawesi suffered a major subsidence event during Early Liassic times. Prior to Neogene tectonics, which strongly dismembered East Indonesia, the Ophiolitic tectonic Zone of East Sulawesi was probably part of the widest palaeogeographic block which included some of the Banda Sea continental fragments: Buru, Seram, Buton and the Sinta Ridge.

Evolutionary model for Alpine Corsica: mechanism for ophiolite emplacement and exhumation of high‐pressure rocks

A tectonic model of Alpine Corsica is proposed based on geological studies. Its evolution starts after the Jurassic with intraoceanic subduction, followed by mid‐Cretaceous subduction of the European continental margin under the oceanic segment of the Adriatic plate. After subduction of the continental crust to a depth of ≈ 150 km, slices of crustal material are buoyantly uplifted together with high‐pressure oceanic rocks (ophiolites and ‘schistes lustrés’). High pressure–low temperature continental gneiss units overthrust the outer segments of the European crust, while producing a normal sense motion along the upper surface of the rising crustal body. During the Eocene, the closure of the remnant Ligurian oceanic basin separating the proto Corsican belt and Adria, resulted in a second orogenic phase with the emplacement of unmetamorphosed ophiolitic nappes which overthrust the previously exhumed and eroded HP belt. This Corsican model suggests an original evolutionary path for orogenic belts when continental collision is preceded by intraoceanic subduction.