zircon geochronology of basement rocks from the pelagonian zone greece constraints on the pre alpine evolution of the westernmost internal hellenides the pelagonian zone of greece is the westernmost segment of the internal hellenides comprising widespread crystalline basement exposures of granites and orthogneisses we dated these basement rocks in order to identify the major crust forming episodes and to understand the evolutionary history of the area in our study we investigated granites gneisses meta rhyolites and mylonites from the major occurrences of the pelagonian zone we applied single zircon dating techniques such as pb pb evaporation conventional u pb and shrimp the majority of the basement rocks gave permo carboniferous intrusion ages thus emphasizing the importance of this crust forming event for the internal hellenides of greece triassic intrusion ages were obtained however for a meta rhyolite from the western pelagonian zone and two mylonites from the eastern pelagonian zone these ages are interpreted to reflect magmatism accompanying early rifting that led to the subsequent opening of the pindos ocean to the west and the meliata ocean to the east of the pelagonian zone the geochronological results demonstrate that the magmatic episodes during which most of the pelagonian zone crystalline basement formed are predominantly pre alpine in age

mechanisms of blueschist creation and preservation in an a type subduction zone mount olympos region greece deformation and metamorphism in the mount olympos region greece are the result of subduction of continental crust a type subduction during the alpine orogeny new structural and petrologic data from this region provide insight into the creation of low geothermal gradients in a type subduction zones and the preservation of blueschist facies assemblages early convergent deformation was accompanied by greenschist amphibolite facies metamorphism blueschist facies mineral assemblages overprinted higher grade assemblages during large scale episodic shortening that occurred during and after continental collision prehnite pumpellyite facies assemblages are postkinematic with respect to contractional fabrics but predate a prolonged period of extension that was not accompanied by metamorphism under plating and extension permitted unroofing and preservation of the blueschists with no over printing by greenschist facies assemblages

timing kinematics and cause of aegean extension a scenario based on a comparison with simple analogue experiments abstract since the early 1970s most geodynamic models proposed for the eastern mediterranean consider that the initiation of extension in the aegean during the late middle miocene or later was a direct consequence of the westward extrusion of anatolia away from the arabia eurasia collision front in the late 1990s a few models have taken into account the increasing evidence for an earlier initiation of aegean extension arising mostly from the analysis of deformation within the various alpine metamorphic complexes of the area based largely on our own investigations we first present a review of the data that reliably constrain the minimum age for the initiation of extension in these complexes it shows that regional scale extension with a pattern of stretching orientations similar to that of the pliocene pleistocene was already strongly active in the aegean before the onset of arabian indentation into eurasia lower miocene vs middle or upper miocene this implies that the initiation of aegean extension did not result from the lateral extrusion of anatolia instead extension may have started owing to a process of gravity spreading of the continental lithosphere that had previously been thickened during alpine collision the main arguments favouring this interpretation are presented and a preliminary scenario is proposed for the neogene evolution of the eastern mediterranean in which lateral extrusion of anatolia occurred lately during southward spreading of the aegean lithosphere in order to test this hypothesis further physical experiments simulating the horizontal spreading of a continental lithosphere toward a free boundary have been carried out the analysis of various elements of the deformation field displacements strain rotations fault pattern and kinematics both in the experiments and in the aegean domain reveals an excellent agreement between the two not only do early stages of aegean extension appear to fit the gravity spreading hypothesis but so does the recent to present internal deformation of the aegean transverse e w active shortening observed in the northern aegean is commonly taken as one of the most striking pieces of evidence that aegean extension results from the lateral extrusion of anatolia the experiments indicate that such transverse shortening may equally develop as a result of pure gravity spreading associated with a pattern of faults and rotations that compares well with that of the northern aegean pure dextral shearing along some strands of the north aegean trough fault system and the related development of an asymmetric pattern of faults in the northern aegean are eventually the only features that evidently arise from the extrusion of anatolia since less than 3 ma finally we argue that during the early stages of aegean extension since at least 21 ma and until ca 11 ma the distribution of extensional strain on a regional scale has been controlled by the distribution of earlier thickening post thickening thermal weakening through partial melting of deep levels of the crust probably accounts for the localization of extensional strain within the areas of thick crust restoring the aegean in its most likely configuration at the onset of bulk crustal extension suggests that 1 the extensional strain rate remained roughly constant or has eventually decreased since the onset of aegean extension and 2 the amounts of extension before and after 16 ma have been roughly equal

40ar 39ar laserprobe direct dating of discrete deformational events a continuous record of early alpine tectonics in the pelagonian zone nw aegean area greece abstract the pelagonian zone of the hellenides which occurs in the northwestern aegean region of the eastern mediterranean contains remnants of a high pressure metamorphic belt which subsequently underwent localized reworking during low temperature deformational events formation of alpine deformational structures in the ossa massif northern pelagonian zone occurred at metamorphic temperatures which failed to reset most pre alpine 40ar 39ar cooling ages in muscovite porphyroclasts the low metamorphic temperatures at alpine times have preserved the argon isotopic signature in white mica bearing deformational structures which allows direct dating of these structures and refinement of the alpine tectonic history in the region after separation of carefully selected and characterized mica generations application of sensitive 40ar 39ar laserprobe dating has resulted in a continuous record of the early alpine tectonic history of the ossa massif from the early cretaceous well into the eocene the early alpine history of the ossa massif records cooling of basement thrust slices which are mostly hercynian granites and metamorphics below ca 350 c by 100 ma while lower tectonostratigraphic levels of the basement had already cooled below these temperatures at around ca 285 ma blueschist facies mylonite fabrics related to a top to ene direction of tectonic transport yield ages as old as 84 5 3 3 ma exhumation of the blueschist facies sequences was initiated at ca 54 ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones final wsw ward transport of the metamorphic sequence across the structurally lowest and supposed autochthonous series occurred at around 45 ma and resulted in termination of the ductile deformation history in the studied area this study confines the onset of high pressure metamorphism in the pelagonian zone to an interval between 100 ma and 85 ma and has shown that high pressure metamorphism had terminated by ca 54 ma the recognition of an early to middle alpine cycle lasting over 30 million years and involving crustal shortening high pressure metamorphism exhumation and subsequent crustal shortening is a key contribution to a better understanding of the early alpine tectonic history of the aegean region and eastern mediterranean

inferences on the mesozoic evolution of the north aegean from the isotopic record of the chalkidiki block the chalkidiki block is a major domain in the north aegean that contrary to other domains in the region largely escaped thermal perturbations during tertiary extension as a result the chalkidiki block is an ideal candidate to glean information related to the timing of mesozoic thermal events using appropriate geochronological techniques we have undertaken a laser ablation inductively coupled plasma mass spectrometry la icp ms study u th pb on monazites and u pb on zircons coupled with 40ar 39ar dating on nine samples from various structural levels within the thrust system of the chalkidiki block the eastern and structurally lower part of the system revealed a complete isotopic reset of carboniferous early triassic monazites coeval with partial monazite destruction ree mobilisation and formation of apatite allanite epidote coronas at 132 ma a reaction that is commonly observed in amphibolite facies rocks these coronas formed after crystallisation of garnet i e at t 580 c and in all probability either close to the peak temperature conditions 620 c on a prograde path or during retrogression between the peak temperature and the low temperature boundary of the amphibolite facies cooling of these rocks and arrival at mid crustal levels occurred at 95 100 ma by contrast the western and structurally uppermost part of the system went through the same event by 120 125 ma further structural considerations with respect to medium temperature geochronology data imply that syn metamorphic thrusting must have ceased by early late cretaceous we emphasize that with the sole exception of the chalkidiki block no pre 45 ma medium temperature geochronology data are preserved in other north aegean domains a feature that is clearly related to the extension induced thermal perturbation of the region during the tertiary

cretaceous tertiary structures and kinematics of the serbomacedonian metamorphic rocks and their relation to the exhumation of the hellenic hinterland macedonia greece the kinematic pattern and associated metamorphism of the predominant ductile deformation and the subsequent deformational stages of the serbomacedonian metamorphic rocks and granitoids are presented in terms of peri tethyan tectonics a systematic record of structural and metamorphic data gives evidence of a main top to ene to ese ductile flow of cretaceous age 120 90 ma associated with a crustal stretching and unroofing a subordinate wsw to wnw antithetic sense of movement of the tectonic top is observed in places the associated metamorphic conditions are estimated at 4 5 7 5 kbar and 510 580 c during eocene to miocene times these fabrics were successively deformed by low angle extensional de ductile shear zones with top to ne and sw sense of movement and brittle shear zones of similar kinematic pattern suggesting a transition from ductile to brittle deformation de deformation was accompanied during its later stages by nw se directed shortening we also discuss the relation of this cretaceous tertiary deformation of the serbomacedonian metamorphic rocks with the eocene to miocene ductile top to southwestward crustal shear of the adjacent rhodope crystalline rocks we regard the serbomacedonian and the rhodope metamorphic rocks to represent related metamorphic provinces the most recent exhumation and cooling history of which is bracketed between the eocene and neogene

alpine high pressure metamorphism at the almyropotamos window southern evia greece the alpine orogenic belt of the hellenides has been strongly reworked by ductile and brittle extensional tectonics extensional structures have affected the central aegean region and obliterated much of the original orogenic architecture since at least early miocene times in the area of almyropotamos on the island of evia flanking the western part of the aegean a unique remnant compressional nappe stack involving tertiary metamorphic rocks has been preserved this nappe sequence comprises a high pressure rock unit on top of a lower grade unit the upper unit south evia blueschist belt is thought to be the westward continuation of the cycladic blueschist belt metamor phosed at high pressure conditions during late cretaceous eocene times the underlying unit the almyropotamos unit is a continental margin sequence covered by a flysch and containing lutetian nummulites indicating that this unit accumulated sediments until at least late eocene times in the present study we analyse the petrology of the almyropotamos nappe stack and define the p t conditions of each of the different rock units exposed there the presence of glaucophane law sonite rimmed by epidote and jadeite 70 mol suggest that peak p t conditions in the south evia blueschist belt reached approximately 10 12 kbar and 350 450 c unlike previous studies which estimated that the underlying almyropotamos unit reached only greenschist facies conditions glau cophane relics and si rich phengites were found by us in this unit these indicate that high pressure metamorphism and crustal thickening in this part of the aegean lasted until at least the late eocene or early oligocene we note that in this respect the architecture of southern evia resembles that of north ern greece olympos ossa our structural data indicate that rock units in the almyropotamos area record different folding phases with the south evia blueschist belt having a more complex fold his tory than the underlying almyropotamos unit the entire nappe stack shares large scale folds which are e w trending and locally overturned to the south and which may represent at present coordi nates n s contraction and nappe transport

new insights from 40ar 39ar laserprobe dating of white mica fabrics from the pelion massif pelagonian zone internal hellenides greece implications for the timing of metamorphic episodes and tectonic events in the aegean region

neogene recent extension on the eastern flank of mount olympus greece abstract the olympus massif of ne greece comprises metamorphosed and deformed triassic and cretaceous eocene carbonates considered to represent part of the passive southwestern margin of neotethys which separated the apulia plate of mainland greece from southern europe in the mesozoic ocean closure in the early tertiary led to subduction of this passive margin as a result of which the olympus carbonates were tectonically overridden in the eocene by a series of thrust sheets consisting of locally high p t blueschist metamorphosed continental margin sediments basement granitoid gneisses and ophiolitic rocks following collision uplift of the olympus carbonates by 6 8 km was accomplished by early neogene ductile and late neogene recent brittle normal faulting which exposed the carbonates in the form of a structural window through the overriding thrust stack the focus of this exhumation occurred on the eastern flank of the massif where a thick calc mylonite with down to the east sense of shear indicators and a prominent zone of nnw to nw trending brittle normal faults separate the carbonates from telescoped structurally overlying units published 40 ar 39 ar microcline ages that indicate a thermal perturbation close to the contact zone in excess of 100 150 c at 16 23 ma early miocene are taken to date the onset of ductile extension and imply long term uplift rates of 0 4 to 1 km m y degradation of pleistocene glacial features on the olympus massif and the presence on its eastern piedmont of deeply incised streams erosional terraces and fault scarps that offset the sedimentary deposits of three cycles of pleistocene glacial deposition units 1 3 testify to its continuing uplift a prominent frontal fault offsets unit 2 by 150 m and several subsidiary nw trending normal faults with a cumulative displacement of 130 m offset a paleosol developed on unit 1 kinematic indicators indicate normal dip slip movement cumulative fault displacement for unit 1 exceeds 277 m yielding a minimum uplift rate of 1 3 mm yr given the oxygen isotope stage 7 mindel riss age of 210 ka proposed for the unit 1 paleosol cumulative offset of 196 m for the paleosol developed on unit 2 yields an uplift rate of 1 6 mm yr given its proposed isotope stage 5e riss wurm age of 125 ka this estimate is supported by longitudinal valley profile data that document 200 m of stream incision since unit 2 deposition and show the offset as reflecting uplift of the olympus massif rather than subsidence of the adjacent gulf of thermaikos assuming 1 8 ma for the base of the pleistocene these rates yield a total uplift range for the quaternary of 2 3 4 1 km consistent with the present 2 6 km height of olympus above the exposed frontal fault neogene uplift of olympus was likely facilitated by extension accompanying the opening of the aegean back arc basin during the miocene as a result of the southward retreat of the hellenic subduction zone for the pliocene recent uplift was likely facilitated by extension linked to the termination of dextral motion at the western end of the north anatolian fault associated with the westward tectonic escape of anatolia

petrology phase equilibria modelling and zircon u pb geochronology of garnet bearing charnockites from the miyun area implications for microblock amalgamation of the north china craton abstract the north china craton ncc was amalgamated through late neoarchean amalgamation of seven microblocks along zones of ocean closure represented by granite greenstone belts ggb typical basement rocks of the zunhua ggb between the qianhuai and jiaoliao microblocks are exposed in the miyun area of the ncc here we report petrology mineral chemistry metamorphic p t estimation and zircon u pb geochronology of newly identified garnet bearing charnockites from the miyun area the prograde mineral assemblages are composed of quartz biotite clinopyroxene plagioclase occurring as inclusions in garnet the peak metamorphic mineral assemblages are identified as garnet clinopyroxene orthopyroxene quartz plagioclase k feldspar ilmenite hornblende whereas the retrograde assemblages are characterized by the breakdown of garnet and formation of quartz biotite association together with k feldspar moats around garnet and magnetite in the matrix the prograde metamorphic p t condition is defined at

the interpretation of inverted metamorphic isograds using simple physical calculations several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade this configuration may indicate but does not require that temperature gradients were temporarily inverted near the fault we examine the physical conditions under which such a temperature inversion could occur the overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth z on the fault is less than times the characteristic time for diffusion of heat through the block above the fault where is thermal diffusivity an inverted gradient cannot form without heat generation in the fault zone unless v z sin 100 where v is the rate of underthrusting in millimeters per year z is in kilometers and is the dip of the fault this simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone but whereas the overthrusting of hot upon cold rocks cools the fault zone dissipation heats it thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently we show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating discrepancy between these estimates implies that some additional process must have occurred such as the post thrusting disruption of the isograds if there is such a discrepancy the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault we illustrate this analysis by applying it to reported inverted metamorphic zonation in the pelona schist the st anthony complex the mt everest region of the main central thrust and the olympos thrust petrological inferences of maximum temperatures depths of metamorphism and magnitudes of apparent inverted gradients imply that shear stresses of about 100 mpa accompanied thrust faulting in some of these regions and that some zones of inverted metamorphism have been tectonically thinned after the metamorphism occured

neogene upper crustal cooling of the olympus range northern aegean major role of hellenic back arc extension over propagation of the north anatolia fault zone the north anatolian fault zone nafz is one of the most hazardous active faults on earth yet its pliocene space time propagation across the north aegean domain remains poorly constrained we use low temperature multi thermochronology and inverse thermal modelling to quantify the cooling history of the upper crust across the olympus range this range is located in the footwall of a system of normal faults traditionally interpreted as resulting from superposed middle late miocene n s stretching related to the back arc extension of the hellenic subduction zone and a pliocene quaternary transtensional field attributed to the south westward propagation of the nafz we find that accelerated exhumational cooling occurred between 12 and 6 ma at rates of 15 35 c ma 1 and decreased to 3 c ma 1 by 8 6 ma the absence of significant plio pleistocene cooling across olympus suggests that crustal exhumation there is driven by late miocene back arc extension while the impact of the nafz remains limited

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

geometry and kinematics of continental basement deformation during the alpine orogeny mt olympos region greece abstract in the hellenic alps continental crust of the leading edge of the apulian plate was subducted beneath the european plate during late cretaceous early tertiary closure of tethyan ocean basin s carbonate rocks of the external hellenides are exposed in the mt olympos tectonic window overthrust by several thin 0 2 4 km thick thrust sheets of granitic basement derived from the internal hellenides field mapping and structural data from the mt olympos region reported here establish the geometry kinematics and strain history of the continental basement before during and after continental collision and subduction seven deformational events document the change from ductile conditions obtained at depth in the early stages of deformation to lower temperature ductile and semi ductile shortening related fabrics to brittle extensional features related to exhumation of the metamorphic rocks d 1 d 4 occurred during subduction of basement nappes and are marked by thrust faulting and folding under blueschist facies conditions d 5 d 7 are related to unroofing of blueschist facies metamorphic rocks and are associated with extension each of the major shortening phases d 1 d 3 verges toward the foreland southwest oppositely directed d 5 d 7 extension occurred behind the active thrust belt during continued regional shortening normal faults have strongly modified the original thrust related geometry in the mt olympos region and elsewhere in the internal hellenides

comparative evolution of the iapetus and rheic oceans a north america perspective abstract differences in the origin and evolution of the iapetan and rheic oceans profoundly affected the development of the ouachitan appalachian variscan orogen the iapetus ocean was initiated in the late neoproterozoic early cambrian by the separation of large continental landmasses laurentia baltica gondwana whereas the rheic ocean originated in the late cambrian early ordovician as the result of the separation of several ribbon continents collectively termed peri gondwanan terranes from the northern margin of gondwana compared to the classic passive margin developed along the laurentian margin of iapetus passive margin successions to the rheic ocean are more limited in extent during convergence both margins of the iapetus ocean developed western pacific type arc back arc systems whereas subduction of the rheic ocean was primarily directed beneath laurentia and was andean in style closure of iapetus involved a number of arc back arc accretionary events and had occurred by the middle silurian as a result of laurentia baltica collision to form laurussia and the accretion of the peri gondwanan terranes rheic ocean closure occurred in the carboniferous by way of continent continent collision between laurussia and gondwana with gondwana on the lower plate within iapetus the formation of ophiolites occurred early in the orogenic cycle and was followed soon after by obduction hp lt metamorphism and foreland directed thrusting that is intimately related to accretionary events these features dominate the northern appalachian orogen where ht lp metamorphism is attributed to post accretionary slab break off events in contrast hp lt metamorphism within the rheic ocean reflects a type subduction during laurussia gondwana collision and is rapidly succeeded by ht lp metamorphism attributed to orogenic collapse ophiolites are rare and were formed either very early early ordovician or very late devonian carboniferous in the evolution of the ocean the early ordovician ophiolites were not obducted until the carboniferous at least 100 million years after their protoliths were formed rheic ocean closure resulted in the formation of large scale bivergent structures within the orogen with symmetry changing on either side of the rheic suture comparison of the evolution of the iapetus and rheic oceans suggests that differences in rift configuration passive margin development and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the ouachita appalachian variscan orogen

comment on back arc extension and denudation of mediterranean eclogites by dov avigad zvi garfunkel laurent jolivet and jose m azanon

aegean tectonics strain localisation slab tearing and trench retreat abstract we review the geodynamic evolution of the aegean anatolia region and discuss strain localisation there over geological times from late eocene to present crustal deformation in the aegean backarc has localised progressively during slab retreat extension started with the formation of the rhodope metamorphic core complex eocene and migrated to the cyclades and the northern menderes massif oligocene and miocene accommodated by crustal scale detachments and a first series of core complexes mccs extension then localised in western turkey the corinth rift and the external hellenic arc after messinian times while the north anatolian fault penetrated the aegean sea through time the direction and style of extension have not changed significantly except in terms of localisation the contributions of progressive slab retreat and tearing basal drag extrusion tectonics and tectonic inheritance are discussed and we favour a model 1 where slab retreat is the main driving engine 2 successive slab tearing episodes are the main causes of this stepwise strain localisation and 3 the inherited heterogeneity of the crust is a major factor for localising detachments the continental crust has an inherited strong heterogeneity and crustal scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation the dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate successive slab ruptures from the middle miocene to the late miocene have isolated a narrow strip of lithosphere still attached to the african lithosphere below crete the formation of the north anatolian fault is partly a consequence of this evolution the extrusion of anatolia and the aegean extension are partly driven from below asthenospheric flow and from above extrusion of a lid of rigid crust