Compressional origin of the Aegean Orogeny,Greece

The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ?Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ?Ma), to orogenic collapse (<14 ?Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ?Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ?Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ?km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ?km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ?km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening.     

Kinematics of the Southern Rhodope Core Complex (North Greece)

The Southern Rhodope Core Complex is a wide metamorphic dome exhumed in the northern Aegean as a result of large-scale extension from mid-Eocene to mid-Miocene times. Its roughly triangular shape is bordered on the SW by the Jurassic and Cretaceous metamorphic units of the Serbo-Macedonian in the Chalkidiki peninsula and on the N by the eclogite bearing gneisses of the Sideroneron massif. The main foliation of metamorphic rocks is flat lying up to 100 km core complex width. Most rocks display a stretching lineation trending NE–SW. The Kerdylion detachment zone located at the SW controlled the exhumation of the core complex from middle Eocene to mid-Oligocene. From late Oligocene to mid-Miocene exhumation is located inside the dome and is accompanied by the emplacement of the synkinematic plutons of Vrondou and Symvolon. Since late Miocene times, extensional basin sediments are deposited on top of the exhumed metamorphic and plutonic rocks and controlled by steep normal faults and flat-ramp-type structures. Evidence from Thassos Island is used to illustrate the sequence of deformation from stacking by thrusting of the metamorphic pile to ductile extension and finally to development of extensional Plio-Pleistocene sedimentary basin. Paleomagnetic data indicate that the core complex exhumation is controlled by a 30° dextral rotation of the Chalkidiki block. Extensional displacements are restored using a pole of rotation deduced from the curvature of stretching lineation trends at core complex scale. It is argued that the Rhodope Core Complex has recorded at least 120 km of extension in the North Aegean, since the last 40 My.     

Cenozoic geodynamic evolution of the Aegean

The Aegean region is a concentrate of the main geodynamic processes that shaped the Mediterranean region: oceanic and continental subduction, mountain building, high-pressure and low-temperature metamorphism, backarc extension, post-orogenic collapse, metamorphic core complexes, gneiss domes are the ingredients of a complex evolution that started at the end of the Cretaceous with the closure of the Tethyan ocean along the Vardar suture zone. Using available plate kinematic, geophysical, petrological and structural data, we present a synthetic tectonic map of the whole region encompassing the Balkans, Western Turkey, the Aegean Sea, the Hellenic Arc, the Mediterranean Ridge and continental Greece and we build a lithospheric-scale N-S cross-section from Crete to the Rhodope massif. We then describe the tectonic evolution of this cross-section with a series of reconstructions from ~70 Ma to the Present. We follow on the hypothesis that a single subduction has been active throughout most of the Mesozoic and the entire Cenozoic, and we show that the geological record is compatible with this hypothesis. The reconstructions show that continental subduction (Apulian and Pelagonian continental blocks) did not induce slab break-off in this case. Using this evolution, we discuss the mechanisms leading to the exhumation of metamorphic rocks and the subsequent formation of extensional metamorphic domes in the backarc region during slab retreat. The tectonic histories of the two regions showing large-scale extension, the Rhodope and the Cyclades are then compared. The respective contributions to slab retreat, post-orogenic extension and lower crust partial melting of changes in kinematic boundary conditions and in nature of subducting material, from continental to oceanic, are discussed.     

Cenozoic metallogeny of Greece and potential for precious,critical and rare metals exploration

The Cenozoic metallogeny in Greece includes numerous major and minor hydrothermal mineral deposits, associated with the closure of the Western Tethyan Ocean and the collision with the Eurasian continental plate in the Aegean Sea, which started in the Cretaceous and is still ongoing. Mineral deposits formed in four main periods: Oligocene (33–25 Ma), early Miocene (22–19 Ma), middle to late Miocene (14–7 Ma), and Pliocene-Pleistocene (3–1.5 Ma). These metallogenic periods occurred in response to slab-rollback and migration of post-collisional calc-alkaline to shoshonitic magmatism in a back-arc extensional regime from the Rhodopes through the Cyclades, and to arc-related magmatism along the active south Aegean volcanic arc. Invasion of asthenospheric melts into the lower crust occurred due to slab retreat, and were responsible for partial melting of metasomatized lithosphere and lower crustal cumulates. These geodynamic events took place during the collapse of the Hellenic orogen along large detachment faults, which exhumed extensive metamorphic core complexes in mainly two regions, the Rhodopes and the Cyclades. The detachment faults and supra-detachment basins controlled magma emplacement, fluid circulation, and mineralization.The most significant mineralization styles comprise porphyry, epithermal, carbonate-replacement, reduced intrusion-related gold, intrusion-related Mo-W and polymetallic veins. Porphyry and epithermal deposits are commonly associated with extensive hydrothermal alteration halos, whereas in other cases alteration is of restricted development and mainly structurally controlled. Porphyry deposits include Cu-Au-, Cu-Mo-Au-Re, Mo-Re, and Mo-W variants. Epithermal deposits include mostly high- and intermediate-sulfidation (HS and IS) types hosted in volcanic rocks, although sedimentary and metamorphic rock hosted mineralized veins, breccias, and disseminations are also present. The main metal associations are Cu-Au-Ag-Te and Pb-Zn-Au-Ag-Te in HS and IS epithermal deposits, respectively. Major carbonate-replacement deposits in the Kassandra and Lavrion mining districts are rich in Au and Ag, and together with reduced intrusion-related gold systems played a critical role in ancient economies. Finally hundreds of polymetallic veins hosted by metamorphic rocks in the Rhodopes and Cyclades significantly add to the metal endowment of Greece.     

A first coincident normal-incidence and wide-angle approach to studying the extending Aegean crust

The Aegean Sea is a broad area of submerged continental crust undergoing active extension to varying degrees. A combined near-normal incidence and wide-angle seismic recording programme was conducted in the western Aegean Sea in 1993, with the principal objective of testing the popular hypothesis that lower crustal deformation (particularily extension) is expressed as a seismically “layered lower crust” (LLC). Across the southern margin of the Cretan trough (i.e. North Cretan offshore margin), a LLC was indicated by wide-angle arrivals that was not apparent on either the coincident near-normal-in-cidence profile or on older low-frequency refraction records. North of the northern margin of the Cretan Trough, beneath the Cyclades, a domain of strong reflectivity is recorded from the middle to lower crust. Here, the near-normal incidence sections also show this typical LLC reflectivity. On the wide-angle sections, a distinct interface is suggested in addition, at a larger depth than that previously assumed for the Moho discontinuity. The structural images and interpretations derived from the new seismic data so far do not clearly support either a pure-shear crustal stretching or an asymmetric simple-shear extension model for the Aegean Sea. Our results appear to be consistent with a tectonic model, where middle crust mobilised by flow coincides spatially with upper crust that has been thinned by active extension of an orogenically thickened crust and expressed near the surface as an exhumed metamorphic core complex.     

Preserved organic matter in a fossil Ocean Continent Transition in the Alps: the example of Totalp,SE Switzerland

Evidence from ultraslow spreading mid-ocean ridges and both fossil and present-day Ocean–Continent Transitions (OCT) demonstrates that mantle serpentinization resulting from the interaction of mantle rock and water during tectonic exhumation is widespread. Observations at white smokers in modern ocean settings suggest that methane produced by serpentinization can support methanotrophic bio-systems, which use methane as the only source of carbon. An important question is whether such bio-systems are more generally pervasive in their association with serpentinized mantle in the subsurface. In this study, we examined whether there is evidence for such a methanotrophic system in exhumed serpentinized mantle at a magma-poor rifted continental margin, by probing for characteristic biological markers in these and associated sedimentary rocks in the Totalp unit of SE Switzerland. This unit represents a remnant of the former OCT of the southern Alpine Tethyan margin and was chosen because of its mild Alpine tectonic and low-grade metamorphic overprint during Alpine orogeny, hence giving potential for the preservation of indigenous organic matter (OM). Totalp samples are characterized by low organic carbon contents of 11–647 ppm. The majority of the samples contain hydrocarbons in the form of n-alkanes in the range C₁₇–C₃₆. Some sediments contain isoprenoids, for example pristane and phytane and a suite of steranes that are consistent with a marine origin for the OM preserved in the rocks. Traces of marine planktonic and bacterial OM are preserved in the serpentinized mantle and overlying sediments of this ancient Tethyan OCT, but there is no evidence that the OM has been generated from methanotrophic bio-systems.     

Origin and metamorphism of ultrabasic rocks associated with a subducted continental margin, Naxos (Cyclades, Greece)

Meta-peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian-type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta-peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730–760 °C were calculated for the upper-amphibolite facies meta-peridotites (Fo–En–Hbl–Chl–Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre-M2 histories: whereas the cover-associated horizons have been affected by low-grade serpentinization prior to metamorphism, the basement- associated meta-peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement-associated meta-peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca-loss in the second group. The cover-associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement-associated meta-peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050 °C within the spinel lherzolite field to eclogite facies temperatures of c. 600 °C.     

Blueschist facies metamorphism of peralkaline rhyolites from the Tenda crystalline massif (northern Corsica): evidence for involvement in the Alpine subduction event?

The Tenda crystalline massif (northern Corsica) is a fragment of the western Corsica basement involved in the Alpine orogeny. Rhyolite dykes crosscutting the gabbroic complex of Bocca di Tenda (southern sector of the Tenda crystalline massif) show an unusual metamorphic mineral assemblage, defined by jadeite‐bearing (up to 46 mol percentage) aegirine, riebeckite, celadonite‐rich phengite (Si=3.50–3.65 apfu), quartz, albite and K‐feldspar. Jadeite‐bearing aegirine and riebeckite mostly occur as coronas around jadeite‐free aegirine and arfvedsonite, respectively, which both are relics of igneous origin. This metamorphic assemblage reflects the peralkaline compositions, which are characterised by anomalously high contents of SiO₂ and Na₂O, and negligible CaO and MgO. The evolved rocks of the gabbroic sequence (quartz‐diorites to tonalites) and the surrounding granitoids are characterised by the development of riebeckite/ferroglaucophane, epidote, celadonite‐rich phengite and albite, thus pointing to a metamorphic crystallization in the epidote‐blueschist facies. In all the studied rocks, metamorphic reactions were controlled by fluid‐assisted mass‐transfer through grain boundaries and microfractures. The different mineral assemblages allow the peak P–T metamorphic conditions to be constrained to between 0.8 GPa/300 °C and 1.1 GPa/500 °C. These estimates attest to a geothermal gradient (dT/dP) of 10–13 °C km^?1 and indicate that the Tenda crystalline massif was buried to a minimum depth of 27 km during the Alpine orogeny. The blueschist facies recrystallization in the Tenda crystalline massif has been related to the cessation of an eastward‐dipping subduction event.     

Zircon in amphibolites from Naxos,Aegean Sea,Greece: origin,significance and tectonic setting

We report U–Pb zircon ages of c. 700–550 Ma, 262–220 Ma, 47–38 Ma and 15–14 Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700–550 Ma and 262–220 Ma are assigned to inherited zircon. Available geochemical data suggest that the 15–14 Ma metamorphic rims grew in situ in the amphibolites, corresponding to a high‐grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40 Ma zircon rims also grew in situ, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late‐Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40 Ma zircon rims would have grown in situ, and (ii) emplacement in the Miocene as a result mafic underplating during large‐scale extension. In this case, only the 15–14 Ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks.     

Late Alpine evolution of the central Menderes Massif, western Turkey

The central Menderes Massif (western Turkey) is characterized by an overall dome-shaped Alpine foliation pattern and a N-NNE-trending stretching lineation. A section through the southern flank of the central submassif along the northern margin of Büyük Menderes graben has been studied. There, asymmetric non-coaxial fabrics indicate that the submassif has experienced two distinct phases of Alpine deformation: a top-to-the N-NNE contractional phase and a top-to-the S-SSW extensional event. The former fabrics are coeval with a regional prograde Barrovian-type metamorphism at greenschist to upper-amphibolite facies conditions. This event, known as the main Menderes metamorphism, is thought to be the result of internal imbrication of the Menderes Massif rocks along south-verging thrust sheets during the collision of the Sakarya continent in the north and the Anatolide-Tauride platform in the south across the Gzmir-Ankara suture during the (?)Palaeocene-Eocene. Top-to-the S-SSW fabrics, represented by a well-developed ductile shear band foliation associated with inclined and/or curved foliation, asymmetric boudins, and cataclasites, were clearly superimposed on earlier contractional fabrics. These fabrics are interpreted to be related to a low-grade (greenschist?) retrogressive metamorphism and a continuum of deformation from ductile to brittle in the footwall rocks of a south-dipping, presently low-angle normal fault that accompanied Early Miocene orogenic collapse and continental extension in western Turkey. A similar tectono-metamorphic history has been documented for the northern flank of the dome along the southern margin of the Gediz graben with top-to-the N-NNE extensional fabrics. The exhumation of the central Menderes Massif can therefore be attributed to a model of symmetric gravity collapse of the previously thickened crust in the submassif area. The central submassif is thus interpreted as a piece of ductile lower-middle crust that was exhumed along two normal-sense shear zones with opposing vergence and may be regarded as a typical symmetrical metamorphic core complex. These relationships are consistent with previous models that the Miocene exhumation of the Menderes Massif and Cycladic Massif in the Aegean Sea was a result of bivergent extension.     

Metamorphism of the deepest exposed arc rocks in the Cretaceous to Paleogene Cascades belt, Washington: evidence for large-scale vertical motion in a continental arc

The Swakane Gneiss and the overlying Napeequa Complex in the North Cascade range, Washington, were metamorphosed and deformed during development of a Cretaceous‐Paleogene continental arc, and are among the structurally deepest exposed rocks within the Cordilleran arcs of North America. Peak metamorphic conditions in both the Swakane Gneiss and Napeequa Complex were c. 640–750 °C, 9–12 kbar. Clockwise paths and widespread evidence for high‐P metamorphism in meta‐supracrustal rocks (burial to >40 km) document major vertical tectonic motion during arc construction and unroofing. These and other moderately high‐pressure rocks in the North Cascades‐Coast Mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other Cordilleran arcs. The exhumed arc complexes of the Sierra Nevada and Peninsular Ranges are dominated by relatively low‐P metamorphic and plutonic rocks (typically <6 kbar). There is no evidence that the northern Cordillera was thickened to a greater degree than these other belts, suggesting that the greater magnitude of vertical motion in the Cascades may have been related to exhumation mechanisms: Eocene extension in the northern Cordillera vs. erosional unroofing in the central and southern Cordillera.     

藏南萨迦拉轨岗日变质核杂岩的厘定及其成因

藏南拉轨岗日带出露一系列穹状隆起，具有变质核杂岩体典型的3层结构型式。变质核由拉轨岗日岩群变质杂岩及侵入其中的花岗岩组成，围绕变质核发育多层顺层拆离断层，盖层主要为晚古生代和中生代浅变质岩石。拉轨岗日变质核杂岩与高喜马拉雅变质核杂岩之间存在密切的时空联系，是喜马拉雅造山作用及相关隆升作用过程中发生热隆伸展的结果。     

A strike‐slip core complex from the Najd fault system,Arabian shield

Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal‐scale strike‐slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal‐scale vertical strike‐slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike‐slip fault zone, and the segments were separated by another 30 km of lateral motion. Strike‐slip core complexes like the Qazaz Dome may be common and may have an important local effect on crustal strength.     

Ductile crust exhumation and extensional detachments in the central Aegean (Cyclades and Evvia Islands).

Abstract

The Aegean continental domain is known to be the site of widespread “back-arc” extension since at least 13 Ma, on the basis of seismotectonic, stratigraphic and fault analysis studies. This extension is documented to overprint structures related to the Mesozoic-Cenozoic Hellenic orogeny. Features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered HP/LT metamorphism across the Aegean. This study presents a structural analysis of the central Aegean area (Cyclades and Evvia Islands), examining in particular the relationship between ductile and brittle deformation, both in the field and on a regional scale. Extension appears to be responsible for most of the ductile deformation within HP rock units that have experienced penetrative greenschist facies and higher grade metamorphic over-printing. On each studied island, progressive extensional deformation has occurred through the development of a major normal-sense detachment zone down to depths of about 18-25 km. Large displacement along the detachment zone accounts for rapid cooling and exhumation of ductile lower crust to form a local metamorphic dome or core complex. Structural and stratigraphic features support a progressive migration of normal faulting away from the dome axis, and a rotation of previously active faults toward low dips, as in kinematic models recently suggested for the development of extensional detachment systems. All the studied domes, except that seen on los Island, show a dominant top-to-the north or north-east sense of shear, while on the southern flank of many of them, an opposite sense of shear is observed, displaying the same progressive evolution from ductile to brittle rock behaviour. This opposite sense of shear is thought not to result from shearing along a major conjugate detachment zone, as in some recent models, but from the accommodation in the ductile crust of upward bending of the brittle upper crust in the footwall of the north-dipping detachment. Available radiometric and stratigraphie data indicate an early minimum age (22-19 Ma) for the onset of extension. The relationship between early metamorphic domes and shallow-dipping detachments, on one hand, and Messinian-Quaternary steep normal faults and grabens, on the other hand, is best explained with the progressive and continuous development of new normal faults away from the domes axes, rather than with a two-stage evolutionary model (core-complex stage, then Basin-and-Range stage) of the type invoked for the North American Cordillera.     

Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire,French Massif Central)

In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension-driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high-pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well-preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T  ~  700 ± 20°C, but differ from each other in whole-rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome-margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome-core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low-P/high-T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Episodic,two-stage Neogene extension and short-term intervening compression in Western Turkey: field evidence from the Kiraz Basin and Bozdağ Horst

Western Anatolia (Turkey) is a region of widespread active N-S continental extension that forms the eastern part of the Aegean extensional province. The extension in the region is expressed by two distinct/different structural styles, separated by a short-term gap: (1) rapid exhumation of metamorphic core complexes along presently low-angle ductile-brittle normal faults commenced by the latest Oligocene-Early Miocene period, and; (2) late stretching of crust and, consequent graben evolution along Plio-Quaternary high-angle normal faults, cross-cutting the pre-existing low-angle normal faults. However, current understanding of the processes (tectonic quiescence vs N-S continental compression) operating during the short-time interval is incomplete. This paper therefore reports the results of recent field mapping and structural analysis from the NE of Küçük Menderes Graben—Kiraz Basin—that shed lights on the processes operating during this short-time interval. The data includes the thrusting of metamorphic rocks of the Menderes Massif over the Mio-Pliocene sediments along WNW-ESE-trending high-angle reverse fault and the development of compressional fabrics in the metamorphic rocks of the Menderes Massif. There, the metamorphic rocks display evidence for four distinct phases of deformation: (1) southfacing top-N ductile fabrics developed at relatively high-grade metamorphic conditions, possibly during the Eocene main Menderes metamorphism (amphibolite facies) associated with top-N thrust tectonics (D₁); (2) top-S and top-N ductile gentle-moderatley south-dipping extensional fabrics formed at relatively lower-grade metamorphic (possibly greenschist facies) conditions associated with the exhumation of Menderes Massif along presently low-angle normal fault plane that accompanied the first phase of extension (D₂); (3) moderately north-dipping top-S ductile-brittle fabrics, present configuration of which suggest a thrust-related compression (D₃); and (4) south-facing approximately E-W-trending brittle high-angle normal faults (D₄) that form the youngest structures in the region. It is interpreted that D₄ faults are time equivalent of graben-bounding major high-angle normal faults and they correspond to the second phase of extension in western Anatolia. The presence of thrust-related D₃ compressional fabrics suggests N-S compression during the time interval between the two phases of extension (D₂ and D₄). The results of the present study therefore support the episodic, two-stage extension model in western Anatolia and confirm that a short-time, intervening N-S compression separated the two distinct phases.     

Low-temperature thermochronological record of exhumation of the Bitterroot metamorphic core complex, northern Cordilleran Orogen

The Bitterroot metamorphic core complex is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the North American Cordilleran Orogen, in the northern Idaho batholith region. Extension was accommodated mainly on the Bitterroot mylonite zone, a 500–1500-m-thick shear zone that deforms granitic intrusive rocks as young as 53–52 Ma, as well as older high-grade metamorphic rocks and plutons. Exhumation of the core complex, in Eocene time, is marked in the shear zone by the transition from amphibolite-facies mylonitization, to greenschist-facies mylonitization, chloritic brecciation, to brittle faulting that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 53 –30 Ma based on U–Pb, Ar–Ar, and fission-track data. Apatite and zircon fission-track data record the lower-temperature part of the exhumation history and help define when the shear zone became inactive, as well as the transition from rapid, core complex-style extension to slower basin-and-range-style extension. They indicate that the western part of the complex was exhumed to within 1–2 km of the surface by 48–45 Ma, while the eastern part of the complex was still at amphibolite-facies conditions and that the eastern part of the complex was not exhumed below 60 °C until after 30 Ma. Younger apatite fission-track ages (≤26 Ma) on the eastern range front of the Bitterroot Mountains suggest that the present topographic expression of the mylonite front was due to Miocene high-angle faulting and widening of the Bitterroot Valley.     

HP/LT metamorphism in the Volparone Breccia (Northern Corsica, France): evidence for involvement of the Europe/Corsica continental margin in the Alpine subdution zone

The Alpine belt in Corsica (France) is characterized by the occurrence of stacked tectonic slices derived from the Corsica/Europe continental margin, which outcrop between two weakly or non‐metamorphic tectonic domains: the ‘autochthonous’ domain of the Hercynian basement to the west and the Balagne Nappe (ophiolitic unit belonging to the ‘Nappes supérieures’) to the east. These slices, including basement rocks (Permian granitoids and their Palaeozoic host rocks), Late Carboniferous–Permian volcano‐sedimentary deposits, coarse‐grained polymict breccias (Volparone Breccia) and Middle Eocene siliciclastic turbidite deposits, were affected by a polyphase deformation history of Alpine age, associated with a well‐developed metamorphic recrystallization. This study provides new quantitative data about the peak of metamorphism and the retrograde P–T path in the Alpine Corsica: the tectonic slices of Volparone Breccia from the Balagne region (previously regarded as unmetamorphosed) were affected by peak metamorphism characterized by the phengite + chlorite + quartz ± albite assemblage. Using the chlorite‐phengite local equilibria method, peak metamorphic P–T conditions coherent with the low‐grade blueschist facies are estimated as 0.60 ± 0.15 GPa and 325 ± 20 °C. Moreover, the retrograde P–T path, characterized by a decrease of pressure and temperature, is evidence of the first stage of the exhumation path from the peak metamorphic conditions to greenschist facies conditions (0.35 ± 0.06 GPa and 315 ± 20 °C). The occurrence of metamorphic peak at high‐pressure/low‐temperature (HP/LT) conditions is evidence of the fact that these tectonic slices, derived from the Corsica/Europe continental margin, were deformed and metamorphosed in the Alpine subduction zone during their underplating at ～20 km of depth into the accretionary wedge and were subsequently juxtaposed against the metamorphic and non‐metamorphic oceanic units during a complex exhumation history.     

Geochronological and isotopical review of pre-Devonian crustal basement of the Colombian Andes

Under the flatlands east of the Andes, the crustal basement is exposed in a few places, composed mainly of the Mitú migmatitic complex and the Parguaza granite, whose ages range between 1.78 and 1.45 Ga. Extensive outcrops of high-grade metamorphic rocks are found in several places. Two metamorphisms are dated between 1.2–1.1 and 1.0–0.9 Ga. They are considered blocks that formed during the Grenville orogeny and have Sm–Nd T_DM model ages of 1.87–1.47. The Andaquí terrane is formed mainly by the Garzón Massif, composed of granulites, migmatites, and granites, and the metamorphic rocks of the Sierra de la Macarena, which are covered by undeformed Cambrian sediments. It is believed that after the Grenville orogeny, this unit remained attached to the Amazonic Craton. All the other areas grouped in the Chibcha terrane, though they formed during the Grenville orogeny, are believed to have remained either as part of another continental block or dispersed islands to be amalgamated to the Amazonic Craton during the Lower Paleozoic orogeny, which in the Quetame Massif is dated between the Silurian and Devonian and is named the Quetame orogenic event.