40ar 39ar and oxygen isotope studies of polymetamorphism from tinos island cycladic blueschist belt greece petrological oxygen isotope and 40ar 39ar studies were used to constrain the tertiary metamorphic evolution of the lower tectonic unit of the cyclades on tinos polyphase high pressure metamorphism reached pressures in excess of 15 kbar based on measurements of the si content in potassic white mica temperatures of 450 500 c at the thermal peak of high pressure metamorphism were estimated from critical metamorphic assemblages the validity of which is confirmed by a quartz magnetite oxygen isotope temperature of 470 c some 40ar 39ar spectra of white mica give plateau ages of 44 40 ma that are considered to represent dynamic recrystallization under peak or slightly post peak high pressure metamorphic conditions early stages in the prograde high pressure evolution may be documented by older apparent ages in the high temperature steps of some spectra eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation oxygen isotope thermometry of four quartz magnetite pairs from greenschist samples gives temperatures of 440 470 c which cannot be distinguished from those deduced for the high pressure event the exhumation and overprint is documented by decreasing ages of 32 28 ma in some greenschists and late stage blueschist rocks and ages of 30 20 ma in the lower temperature steps of the ar release patterns of blueschist micas almost flat parts of ar ar release spectra of some greenschist micas gave ages of 23 21 ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a 18o enriched fluid rather the compositions indicate that fluid to rock ratios were very low the isotopic compositions being primarily controlled by those of the protolith rocks we assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid a quartz magnetite sample from a contact metamorphic skarn taken near the miocene monzogranite of tinos gave an oxygen isotope temperature of 555 c and calculated water composition of 9 1 the value of 18o obtained from this water is consistent with a primary magmatic fluid but is lower than that of fluids associated with the greenschist overprint which indicates that the latter event cannot be directly related to the monozogranite intrusion

strain rate dependent calcite microfabric evolution an experiment carried out by nature abstract a flanking structure developed along a secondary shear zone in calcite marbles on syros cyclades greece provides a natural laboratory for directly studying the effects of strain rate variations on calcite deformation at identical pressure and temperature conditions the presence and rotation of a fracture during progressive deformation caused extreme variations in finite strain and strain rate forming a localized ductile shear zone that shows different microstructures and textures textures and the degree of intracrystalline deformation were measured by electron backscattered diffraction marbles from the host rocks and the shear zone which deformed at various strain rates display crystal preferred orientation suggesting that the calcite preferentially deformed by intracrystalline plastic deformation increasing strain rate results in a switch from subgrain rotation to bulging recrystallization in the dislocation creep regime with increasing strain rate we observe in fine grained 3 m ultramylonitic zones a change in deformation regime from grain size insensitive to grain size sensitive paleowattmeter and the paleopiezometer suggest strain rates for the localized shear zone around 10 10 s 1 and for the marble host rock around 10 12 s 1 we conclude that varying natural strain rates can have a first order effect on the microstructures and textures that developed under the same metamorphic conditions

thermochronology of the high pressure metamorphic rocks of crete greece implications for the speed of tectonic processes new fission track thermochronologic data from the high pressure p low temperature t rocks of crete greece combined with pressure temperature and stratigraphic constraints reveal that their subduction began between 36 and 29 ma metamorphism took place in western crete at peak conditions of 10 2 kbar and 400 50 c between 24 and 19 ma and rapid exhumation to 10 km and 300 c at a minimum rate of 4 km m y was completed before 19 ma constraints from the thermal history of the plate above the inferred extensional detachment reveal that tectonic unroofing contributed 85 to 90 of the overall exhumation of the high p low t rocks of crete we propose that the hellenic subduction zone has acted as a retreating plate boundary since at least the early oligocene and collision and extension during this time were driven by roll back associated with slab pull rather than by gravitational collapse as a consequence of crustal thickening the speed of subduction and exhumation of the high p low t rocks of crete within 10 m y has important implications for other orogenic belts showing that rocks can be subducted metamorphosed at high pressure and exhumed despite slow overall plate convergence within the uncertainties of many paleontologic and isotopic age data

forced return flow deep in the subduction channel syros greece we present the results of a detailed structural study in the cycladic blueschist unit at fabrika on syros island greece and discuss their significance for tectonic processes at the subduction int

partitioning and budget of li be and b in high pressure metamorphic rocks abstract partitioning and budget of li be and b in high pressure metamorphic rocks from the island of syros greece were studied using secondary ion mass spectrometry inductively coupled plasma optical emission spectrometry and prompt gamma neutron activation analysis partitioning between coexisting mineral phases was found to be rather constant and independent of element concentrations for several mineral pairs apparent partition coefficients vary in a narrow range while concentrations vary by more than an order of magnitude hence it was possible to establish sets of inter mineral partition coefficients for li be and b among 15 different high pressure minerals this data set provides important information on the behaviour of the light elements in different lithologies within subducting slabs from the onset of metamorphism to the eclogite stage it is essential for modelling trace element and isotope fractionation during subduction and dehydration of oceanic crust

extraneous argon in high pressure metamorphic rocks distribution origin and transport in the cycladic blueschist unit greece the 40ar 39ar geochronological technique has significantly contributed to the study of crustal deformation however the frequent occurrence of extraneous argon in high pressure terrains can make the interpretation of 40ar 39ar ages problematic this contribution attempts to i characterize the distribution and origin of extraneous argon in the cycladic blueschist unit cbu greece by applying the 40ar 39ar dating technique on three cycladic islands syros tinos and ios which span a complete transect of the cbu and ii discuss regional implications for the tectonometamorphic evolution of the cbu our results combined with ages available in the literature show that periods of peak burial conditions 55 49 ma and syn 45 38 ma to post orogenic exhumation 37 21 ma are well constrained in the cbu by the 40ar 39ar method moreover variscan apparent ages are preserved in the cycladic basement of ios indicating that the 40ar 39ar system was not completely reset during the eocene metamorphic episode additionally our 40ar 39ar data show that the occurrence of extraneous argon is heterogeneously distributed in the cbu while high pressure rocks of syros and tinos are only locally affected extraneous argon is clearly evidenced in the cbu of ios yielding geologically meaningless ages up to 900 ma on each island the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets we suggest that the cycladic basement a polymetamorphic continental unit behaved as an argon reservoir that has been mobilized by fluid circulation at the eocene these argon enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the cbu finally we claim that a minimum closure temperature of 550 c must be accepted for the phengites of the cbu and conclude that volume diffusion is not the main process resetting the argon isotopic system in high pressure rocks compare to recrystallization enhanced by fluid circulation and deformation

fission track analysis unravels the denudation history of the bonar range in the footwall of the alpine fault south island new zealand we apply fission track thermochronology to shed new light on the tectonic history of zealandia during late cretaceous continental extension and the onset of late tertiary mountain building in the southern alps of new zealand the southern alps are one of the fastest erosionally exhuming mountain belts on earth exhumation of the bonar range in westland just to the northwest of the alpine fault is orders of magnitude slower we report apatite and zircon fission track ages from samples that were collected along an ene wsw profile across the central bonar range parallel to the tectonic transport direction of a prominent ductile fabric in the basement gneiss zircon fission track zft ages show a large spread from 121 9 12 1 ma to 74 9 7 2 ma 1 errors the youngest zft ages of 78 to 75 ma occur at low elevations on either side of the bonar range and become older towards the top of the range thereby showing a symmetric pattern parallel to the ene trending profile across the range age elevation relationships suggest an exhumation rate of 50 100 m ma 1 we relate the zft ages to slow erosion of a tectonically inactive spot in the late cretaceous magmatic arc of zealandia therefore the first main significance of the paper is that it demonstrates that not all of 110 90 ma zealandia was necessarily participating in extreme core complex related extension but that there were enclaves of lithosphere that underwent slow erosion the apatite fission track aft ages range from 11 1 1 9 ma to 5 3 1 0 ma and age elevation relationships suggest an exhumation rate of c 200 m ma 1 we relate the aft ages to the inception of transpressive motion across the alpine fault and modest exhumation in its footwall in late miocene times if so the second significant point of this paper is that transpressive motion across the alpine fault was already under way by c 11 ma

3 d p wave velocity structure in western greece determined from tomography using earthquake data recorded at the university of patras seismic network patnet the 3 d p wave velocity structure of the upper crust in the region of western greece is investigated by inversion of about 1500 residuals of p wave arrival times from local earthquake data recorded in the year 1996 by the newly established university of patras seismic network patnet the resulting velocity structure shows strong horizontal variations due to the complicated structure and the variation of crustal thickness relatively low velocity contours are observed in the area defined by cephallonia zakynthos islands and northwestern peloponnesos this is in addition to some well localized peaks of relatively higher values of p wave velocity may be related to the zone of triassic evaporites in the region and correspond to diapirism that breaks through to the uppermost layer finally a low p velocity deeping zone extending from zakynthos to the gulf of patras is correlated with bouguer anomaly map and onshore and offshore borehole drillings which indicate that thick sediments overly the evaporites which exist there at depth greater than 2 km

tectonics of the syros blueschists cyclades greece from subduction to aegean extension 1 on syros high pressure metamorphism affects a lithological pile that is composed of from base to top 1 the komito vari granitic basement 2 a margin sedimentary sequence that is predominantly made of marbles and schists the pyrgos and kastri units and 3 the kambos metaophiolitic melange the tectonic history occurred in three main stages during the first stage in the mid eocene the kambos oceanic unit was thrust southward on top of the sedimentary pile top to the south southwest ductile senses of shear are synchronous with prograde high pressure metamorphism and associated with this thrusting event the second stage corresponds to a top to the northeast ductile shear that affects the whole metamorphic pile and is synchronous with the metamorphic retrogression from eclogite to greenschist facies however the kambos oceanic unit remained partly undeformed as shown by significant volumes containing undeformed lawsonite pseudomorphs no major extensional detachment related to this exhumation event outcrops on the island the localized semibrittle to brittle deformation of the third stage is associated with the postmetamorphic development of 1 a ramp flat extensional system at the island scale whose southward minimum displacement is estimated at approximately 7 km and 2 two sets of steeply dipping strike slip faults with a normal component trending either east west or around north south indicating that the mean stretching and shortening directions are trending nne ssw and ese wnw respectively this sequence of major tectonic events and their relationship to metamorphism are interpreted within the framework of the subduction of the pindos ocean and then of the adria continental passive margin

vertical ductile thinning and its contribution to the exhumation of high pressure rocks the cycladic blueschist unit in the aegean the contribution of vertical ductile thinning to the exhumation of high pressure rocks is evaluated by estimating finite strain in 75 exhumed high pressure rocks of the cycladic blueschist unit in the aegean sea greece and western turkey strain data indicate heterogeneous deformation principal stretches are 1 24 5 03 for s x 0 63 2 53 for s y and 0 10 0 81 for s z with a tensor average of s x s y s z 1 52 1 28 0 51 a 1d numerical model which integrates velocity gradients along a vertical flow path with a steady state orogen is used to estimate the contribution of ductile thinning of the overburden of the high pressure rocks to exhumation using a strain rate law that is proportional to depth averaged results show that ductile flow contributed c 20 to exhumation a major implication is that the vertical strain in the exhumed rocks is an overestimate of the contribution that ductile flow makes to the total exhumation a proportional strain rate law that scales linearly with depth implies that material points rising towards the surface move quickly out of the more rapidly deforming part of the orogen therefore very large vertical strains 90 in deeply exhumed rocks are needed for vertical ductile thinning to be a major exhumation process

understanding phengite argon closure using single grain fusion age distributions in the cycladic blueschist unit on syros greece abstract the preservation of 40ar 39ar ages of high pressure hp metamorphic white mica reflects an interplay of processes that mobilise 40ar either through mica recrystallisation or by diffusive 40ar loss the applicability of resulting ages for dating tectonic processes is critically dependent on whether either of these processes can be proven to be efficient and exclusively active in removing 40ar from mica if not preservation of an inherited or mixed age signal in a sample must be considered for interpretation the cycladic blueschist unit on syros has become a new focal area in the discussion of the geological significance of argon age results from multi grain step heating experiments while some argue that age results can directly be linked to deformation or metamorphic growth events others interpret age results to reflect the interplay of protracted recrystallisation and partial resetting preserving a mixed age signal here we demonstrate the potential of a new approach of multiple single grain fusion dating using the distribution of ages at the sample section and regional scale we show that in northern syros mica ages display systematic trends that can be understood as the result of three competing processes 1 crystallisation along the prograde to peak metamorphic path 2 a southward trend of increasing 40ar loss by diffusion and 3 localised and rock type dependent deformation or metamorphic reactions leading to an observed age spread typically limited to 10 myr at the section scale none of the sections yielded the anomalously old age results that would be diagnostic for significant excess 40ar the recorded trends in ages for each of the studied sections reflect a range of p t conditions and duration of metamorphism diffusion modelling shows that in a typical subduction metamorphic loop subtle variations in p t t history can explain that age contrasts occur on a regional scale but are limited on the outcrop scale our new approach provides a comprehensive inventory of the range of ages present in different rocks and at different scales which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages we verify the added value of our new approach by comparison with multi grain step heating experiments on selected samples from the same sections

u pb sims dating of synkinematic granites timing of core complex formation in the northern anatolide belt of western turkey secondary ion mass spectrometry sims u th pb dating of magmatic zircon from the synkinematic egrigoz and koyunoba granites and a leucogranite dyke dates core complex formation in the northern anatolide belt of western turkey at 24 19 ma the granites intrude into the footwall of the simav detachment and are strongly elongated in the nne direction parallel to tectonic transport on the detachment although large parts of the granites are undeformed localized mylonitic to ultramylonitic deformation occurs directly beneath the simav detachment and preserves evidence of progressive deformation from ductile to brittle conditions oscillatory zoned rims of long prismatic zircon from the egrigoz and koyunoba granites yield identical and well constrained intrusion ages of 20 7 0 6 ma and 21 0 0 2 ma whereas inherited grains range from palaeoproterozoic 2972 13 ma to neoproterozoic 653 6 ma to 500 5 ma in age a leucogranite dyke yields an intrusion age of 24 4 0 3 ma with inherited neoproterozoic 640 7 ma to 511 6 ma grains our data in conjunction with published 40 ar 39 ar biotite ages indicate very rapid cooling greater than c 200 c ma 1 for the granites during and after synkinematic emplacement

an oligocene extrusion wedge of blueschist facies nappes on evia aegean sea greece implications for the early exhumation of high pressure rocks we show that the styra nappe of the cycladic blueschist unit on evia constitutes a wedge of high pressure rocks extruded during early stages of orogeny the nappe pile on evia was assembled during d 2 top to the ssw directed thrusting in restored oligocene coordinates which emplaced the styra and ochi nappes of the cycladic blueschist unit above the almyropotamos nappe between c 33 and 21 ma peak metamorphism of the almyropotamos nappe at c 23 ma occurred at lower metamorphic pressure showing exhumation of the styra nappe during underthrusting and burial of the almyropotamos nappe this exhumation was largely accomplished by the d 2 top to the nne displacing mt ochi normal sense shear zone normal shearing commenced at c 33 ma under peak high pressure metamorphism in the styra and ochi nappes fission track dating indicates slow cooling after d 2 in the styra nappe subsequently the former thrust contact between the almyropotamos nappe and the styra nappe was reactivated as a d 3 top to the ese extensional shear zone and this extensional phase led to the formation of a number of middle to late miocene graben our main conclusion is that there is strong evidence for an oligocene extrusion wedge accomplishing the early exhumation of the styra nappe which demonstrates the importance of extrusion wedges for the initial exhumation of the cycladic blueschist unit

structural analysis of a complex nappe sequence and late orogenic basins from the aegean island of samos greece the island of samos in the aegean sea exposes high pressure metamorphic rocks of the cycladic blueschist unit which are sandwiched between the mildly blueschist facies kerketas nappe below and the overlying non metamorphic kallithea nappe structural and metamorphic analysis shows that deformation can generally be divided into four main stages 1 eocene and earliest oligocene 0ese wnw oriented nappe stacking d1 and d2 associated with blueschist and transitional blueschist greenschist facies metamorphism m1 and m2 d2 caused emplacement of the blueschist unit onto the kerketas nappe indicating that thrusting occurred during decompression 2 a subsequent history of oligocene and miocene horizontal crustal extension d3 before and after greenschist facies metamorphism m3 ductile flow during d3 was characterized by a high degree of coaxial deformation but in general caused displacement of upper units towards the ene nonetheless the late stage d3 emplacement of the kallithea nappe between 9 and 10 ma had a top to the nw nnw sense of shear 3 a short period of brittle e w crustal contraction d4 occurred between 8 6 and 09 ma 4 a phase of n s directed normal faulting d5 8 6 ma to recent ese wnw directed tectonic transport during d1 through d3 is in contrast to uniform nne sswdirected tectonic transport in the adjacent cyclades greece as well as in the neighbouring menderes massif of western turkey published paleomagnetic data reveal sinistral rotation between the cyclades and western turkey we interpret this rotation as a consequence of diaerential extension between the severely extended aegean and the moderately extended menderes massif during d3 the onset of d3 crustal extension is coeval with a marked change in the thermal structure we propose that the thermal reorganization was associated with the retreat of the subduction zone towards the external hellenides in the early oligocene and a subsequent increase in magmatic activity 1999 elsevier science ltd all rights reserved

unraveling alteration histories in serpentinites and associated ultramafic rocks with magnetite u th he geochronology serpentinization is a widespread process that affects large scale geodynamic processes along plate boundaries including continental breakup seafloor spreading and subduction documenting the timing of serpentinization is critical for our understanding of these processes but direct dating of serpentinites has been challenging or impossible we present the first application of magnetite u th he chronometry to date stages of alteration and cooling in ultramafic rocks in order to demonstrate the viability of magnetite he dating in these lithologies magnetite ages were obtained from two ultramafic lithologies of the kampos melange belt a high pressure low temperature subduction complex on the island of syros greece magnetite u th he measurements of internal fragments from large grains within a chlorite schist and a serpentinite record miocene exhumation related cooling ages whereas smaller grains from the serpentinite record mineral growth associated with hydrothermal fluid flow along pliocene normal faults these age results with magnetite trace element geochemistry reveal evidence for multiple episodes of fluid rock alteration which has implications for the cooling history and local geochemical exchanges of this high pressure low temperature terrane this method provides a new tool that may be expanded to investigate the processes and time scales of serpentinization from a variety of tectonic settings

the north cycladic detachment system low angle normal faults accommodate a large part of continental post orogenic extension besides the intrinsic rheological characteristics of the continental crust that may lead to the formation of shallow dipping shear zones at the brittle ductile transition the role of pre existing low angle structures such as large thrusts has been proposed by several authors we explore this question with the example of the north cycladic detachment system ncds that is composed of a series of distinct detachments cropping out on the islands of andros tinos and mykonos separating the cycladic blueschists in the footwall from the upper cycladic nappe in the hanging wall we show that these extensional structures are part of a single large scale structure more than 200 km along strike that reactivates the vardar suture zone it extends eastward on ikaria and westward offshore evia and thessalia where it probably connects to recent shallow dipping normal faults evidenced on published seismic reflection profiles the ncds started its activity in the oligocene concommitantly with the aegean extension and was still active in the late miocene it has exhumed a series of metamorphic domes from southern evia to mykonos below low angle detachment systems made of low angle normal faults and low angle ductile shear zones the ductile shear zones and the faults were created with a low dip and they kept the same attitude throughout their exhumation we identify three main detachments that are part of a continuum of extension on the ncds tinos detachment livada detachment and mykonos detachment a fourth detachment vari detachment is the reactivation of an eocene exhumation related structure deformation in the footwall is characterized by intense stretching and flattening using the spatial evolution observed along strike from andros to mykonos we construct a history of formation of the ncds starting with the reactivation of former thrusts leading to the exhumation of high temperature metamorphic domes the aegean example shows that reactivation of earlier shallow dipping discontinuities can play a fundamental role in continental post orogenic extension

local high resolution passive seismic tomography and kohonen neural networks application at the rio antirio strait central greece a high resolution passive seismic investigation was performed in a 150 km 2 area around the rio antirio strait in centralgreeceusingnaturalmicroearthquakesrecordedduringthreemonthsbyadense temporaryseismicnetworkconsisting of 70 three component surface stations this work was part of the investigation for a planned underwater rail tunnel and it gives us the opportunity to investigate the potential of this methodology first 150 well located earthquake events were selected to compute a minimum 1d velocity model for the region next the 1d model served as the initialmodelfornonlinearinversionfora3dp ands velocity crustal structure by iteratively solving the coupled hypocenter velocity problem using a least squares method the retrieved vp and vp vs images were used as an input to kohonen self organizing maps soms to identify systematicallyandobjectively theprominentlithologiesintheregion soms are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data this analysis revealed the existence of five major clusters one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes the survey results provide for the first time a 3d model of the subsurface in and around the rio antirio strait it is thefirst time that passive seismic tomography is used together with som methodologies at this scale thus revealing themethod spotential

underthrusting and exhumation a comparison between the external hellenides and the hot cycladic and cold south aegean core complexes greece 1 after their emplacement in the course of the late mesozoic and the cenozoic the hellenic nappes became fragmented during late orogenic extension since the late eocene here we focus on the transition of underthrusting during nappe emplacement to exhumation during late orogenic extension to this end we compared previously published data on the structural geological and metamorphic history of the underthrusted parts of the tripolitza and ionian nappes which were exhumed in the cycladic and south aegean windows with newly obtained data on the sedimentary stratigraphic and structural development of the part of these nappes in the foreland in front of the subduction thrust the results allow the identification of two major events event 1 took place around the eocene oligocene transition and marks the onset of underthrusting of the tripolitza nappe below the pindos nappe and the ionian nappe below the tripolitza nappe respectively this led to the uplift and erosion of the pindos unit and the onset of deposition of the tripolitza and ionian flysch in front of the pindos thrust together with the formation of mylonites at the base of the metamorphosed portions of the pindos unit related to the underthrusting of the tripolitza unit event 2 occurred in the latest oligocene to earliest miocene and marks the decoupling of the ionian unit from the underthrusting plate the accretion of the tripolitza and ionian units to the overriding plate and the onset of late orogenic extension and exhumation in the overriding plate this led to the formation of the south aegean and cycladic core complexes and the subsidence of the klematia paramythia half graben throughout the early miocene

estimation of effective anisotropy simultaneously with locations of microseismic events passive seismic tomography in which the event locations and the velocity model are inferred simultaneously is seldom used to process microseismic surveys acquired in the oil and gas industry we discuss advantages of applying tomographic ideas to typical microseismic data recorded in a single nearly vertical well to monitor hydraulic stimulation of a shale gas reservoir microseismic events are conventionally located in the energy industry applications using a velocity model derived from sonic logs and perforation shots instead of fixing the model as is normally done we alter it while locating the events this added flexibility not only makes it possible to accurately predict traveltimes of the recorded p and s waves but also provides a convincing evidence for anisotropy of the examined shale formation while we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing the obtained models are suggestive of possible time laps

exhumation patterns along shallow low angle normal faults an example from the altotiberina active fault system northern apennines italy a multi method approach palaeothermal and thermochronological analyses thermal modelling is applied to reconstruct the exhumation history of the altotiberina fault atf a representative example of crustal scale active low angle normal faulting in the northern apennines italy thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the atf since 3 8 ma the atf footwall has exhumed at rates of 0 90 mm a 1 exhumation led to bending and deactivation of the atf uppermost portion as a result of tectonic unloading and isostatic adjustment followed by migration of extension and the development of a set of domino like east dipping normal faults rooting on the buried portion of the atf atf activity and isostatic rebound exhumed triassic rock units from depths of about 4 km we suggest that isostatic instability is accommodated at shallow crustal levels in a similar way to what is observed on larger structures at mid low crustal levels

low angle faults above and below a blueschist belt tinos island cyclades greece new observations from the island of tinos greece allow a better definition of the structural position of the alpine eocene blueschist belt exposed in the islands of the aegean sea these blueschists over a significant part of the aegean sea are delimited from below by a low angle thrust fault while from above they are delimited by a low angle normal type fault which omits a substantial crustal interval both underlying and overlying rocks were not affected by the high metamorphism the rapid uplift and exhumation of the high rocks was therefore mainly the result of fault movements rather than erosion and whole crust uplifting the low angle normal fault apparently had a major role in the uplift of the blueschists

miocene south directed low angle normal fault evolution on kea island west cycladic detachment system greece 1 new structural petrologic and thermochronologic data from kea west cyclades define a crustal scale ductile shear zone and ductile brittle low angle normal fault lanf system both the greenschist facies shear zone forming the footwall and the overlying lanf zone formed during constrictional strains with a consistent top to sw s shear sense with increasing finite strains toward higher structural levels but decreasing temperatures from footwall to hanging wall the tectonostratigraphy comprises a 450 m thick footwall of shallowly dipping schists and calcite marbles representing the intermediate unit of the attic cycladic crystalline acc above the footwall is a 60 m thick highly strained lanf zone consisting of phyllonites cataclastic schists ultramylonitic calcite marbles proto mylonitic calcite marbles and cohesive cataclasites that arch over the whole island these fault rocks exhibit multistage lanfs evolving from ductile to brittle conditions an up to 50 m thick brecciated limestone and dolostone sequence forms the unmetamorphosed hanging wall which is most probably part of the upper unit of the acc multiequilibrium p t estimates on chlorite white mica pairs in the footwall yield 7 5 5 kbar 360 c 450 c for inclusions in albite and epidote 5 5 3 kbar 400 c 350 c for the main foliation and 3 2 kbar 350 c 280 c in localized shear bands c and c foliations the 40ar 39ar white mica footwall cooling ages demonstrate that greenschist facies retrogression occurred between 21 and 17 ma localized late decimeter thick shear zones were active and dynamically recrystallized before 15 to 13 ma the lanf on kea together with similar structures in south attica and the west cyclades define the west cycladic detachment system characterized by ductile to brittle top sw s shear sense

pressure temperature evolution of blueschist facies rocks from sifnos greece and implications for the exhumation of high pressure rocks in the central aegean the blueschist and greenschist units on the island of sifnos cyclades were affected by eocene high pressure hp metamorphism using conventional geothermobarometry the hp peak metamorphic stage was determined at 550 600 c and 20 kbar close to the blueschist and the eclogite facies transition the retrograde p t paths are inferred with phase diagrams pseudosections based on a quantitative petrogenetic grid in the model system na2o cao feo mgo al2o3 sio2 h2o reveal coeval decompression and cooling for both the blueschist and the greenschist unit the conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10 12 c km 1 for both units the retrograde overprint can be assigned to low pressure blueschist to hp greenschist facies conditions this result cannot be reconciled with the prograde barrovian type event which affected parts of the cyclades during the oligocene to miocene instead the retrograde overprint is interpreted in terms of exhumation directly after the hp stage without a separate metamorphic event constraints on the exhumation mechanism are given by decompression cooling paths which can be explained by exhumation in a fore arc setting during on going subduction and associated crustal shortening back arc extension is only responsible for the final stage of exhumation of the hp units

timing slip rate displacement and cooling history of the mykonos detachment footwall cyclades greece and implications for the opening of the aegean sea basin we constrain the slip and cooling history of the mykonos detachment footwall using thermochronometry a u pb zircon age of 13 5 0 3 ma dates intrusion of the mykonos monzogranite 40 ar 39 ar hornblende and biotite ages from the monzogranite are 12 7 0 6 ma and 10 9 0 6 ma whereas zircon and apatite fission track ages range from 13 0 8 ma to 10 7 0 8 ma and 12 5 2 2 ma to 10 5 1 8 ma u th he ages range from 13 6 0 6 ma to 9 0 0 7 ma for zircon and 11 1 0 5 ma to 8 9 0 4 ma for apatite the ages in part overlap within 2 errors and together with the long apatite fission track lengths 14 m support rapid cooling at rates 100 c ma 1 the low temperature thermochronometric ages decrease east northeastwards in the direction of hanging wall transport on the mykonos detachment age distance relationships show that the mykonos detachment slipped at an average rate of 6 0 9 2 2 4 km ma 1 causing c 30 km of offset and c 12 km of exhumation this result indicates that miocene low angle normal faulting was not important for the exhumation of the cycladic blueschist unit the opening of the aegean sea basin in the miocene was controlled by a few large magnitude low angle normal faults

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

high resolution passive seismic tomography for 3d velocity poisson s ratio ν and p wave quality qp in the delvina hydrocarbon field southern albania we have studied using traveltimes of p and s waves and initial seismic pulse rise time measurements from natural microearthquakes to derive 3d p wave velocity vp information mostly structural as well as p and s wave velocity vp vs and p wave quality factor qp information mostly lithologic in a known hydrocarbon field in southern albania during a 12 month monitoring period 1860 microearthquakes were located at a 50 station seismic network and were used to obtain the above parameters the data set included earthquakes with magnitudes ranging from 0 1 to 3 0 r richter scale and focal depths typically occurring between 2 and 10 km kohonen neural networks were implemented to facilitate the lithological classification of the passive seismic tomography pst results the obtained results which agreed with data from nearby wells helped delineate the structure of the reservoir two subregions of the investigated area one corresponding to an oil field and one to a gas field were correlated with the

geometry and kinematics of mykonos detachment cyclades greece evidence for slip at shallow dip 1 the eastern part of mykonos island cyclades greece shows the detailed internal structure of a two branch shallow dipping extensional detachment system of miocene age this paper shows that the last stage of slip along the detachment during the deposition of syn rift sediments occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension extensional deformation during and after a 13 ma old granite intrusion is taken up by two main shallow dipping shear zones 1 the lower livada detachment which extends laterally across tinos island is a ductile structure located at the interface between the granite and the upper cycladic nappe metabasites a pervasive top to the ne shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact later brittle faults shallowly and steeply dipping rework the ductile deformation with the same overall shear sense 2 the upper mykonos detachment is brittle and separates the metabasites from late miocene sandstones and conglomerates the detachment fault dips 12 15 toward the nne and its sense of shear is consistent with that of the livada detachment soft sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone indicating that the overall direction of extension is nne ssw with a sense of shear toward the nne and the dip of the fault is throughout very low the two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the aegean sea which emphasizes the importance of low angle extensional faults and shear zones in extensional tectonics

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

post plutonic unroofing and morphogenesis of the attic cycladic complex aegea greece abstract the attic cycladic complex of greece comprises an eocene high p unit with blueschist occurrences unroofing of this unit took place in oligocene miocene times and was accompanied by a regional low p medium t overprint and miocene granitic plutonism apatite fission track ages of 14 crystalline samples from the islands of tinos mikonos and serifos range between 13 1 and 5 3 ma corresponding to the middle and late miocene the frequency distributions of confined track lengths are characterised by high arithmetic means of 14 2 15 1 m and by standard deviations from 0 9 to 1 6 m thermochronological modelling of the data indicates rapid cooling between 10 and 6 ma and subsequent deceleration of the cooling rates for a short time the miocene plutons of tinos mikonos and serifos experienced maximum cooling rates above 50 c ma these exceptionally high cooling rates cannot be explained by strong vertical uplift and fast regional erosion such a process is disproved by preserved remnants of a former peneplain including inselbergs and kaolinized tropical subsoil on mikonos fast post plutonic cooling of the lower plate was apparently accompanied by simultaneous sedimentation on the upper plate we propose a post plutonic cooling model which assumes strong periplutonic heat flow into much cooler host rocks and fast extensional unroofing

the extensional messaria shear zone and associated brittle detachment faults aegean sea greece structural thermochronological and metamorphic data are used to elucidate the tectonic nature and evolution of the ductile extensional messaria shear zone and the associated brittle messaria and fanari detachment faults which exhumed their footwall from mid crustal depths on the island of ikaria in the aegean thermobarometric data indicate that the messaria shear zone formed at 350 400 c and 3 4 kbar i e at a depth of c 15 km normal faulting was accompanied by the intrusion of two granites which together with the thermobarometric data indicate a relatively high thermal field gradient of 25 35 c km 1 zircon and apatite fission track and apatite u th he ages demonstrate rapid cooling in the footwall of the messaria detachment from c 400 c to c 40 c between 11 and 3 ma age distance relationships of the data suggest that the messaria shear zone and the messaria detachment slipped at apparent rates of c 6 9 km ma 1 kinematic indicators show a consistent top to the nne shear sense for the extensional faults however at the southern part of the messaria detachment some late stage shear sense indicators are top to the ssw and are assumed to be associated with updoming of the footwall numerous deformed pegmatite veins in the messaria shear zone allow the reconstruction of deformation and flow parameters the mean kinematic vorticity number ranges from 0 13 to 0 80 indicating that shearing deviated significantly from simple shear that is extensional shearing was associated with vertical ductile thinning which contributed to tectonic exhumation finite strain shows oblate geometries and axial ratios of the finite strain ellipse in sections parallel to tectonic transport and normal to the mylonitic foliation range from 1 8 to 19 9 we calculate using a 1d numerical model that vertical ductile thinning contributed c 20 to exhumation during extensional shearing normal faulting was the major agent exhuming the footwall from c 15 km depth

3d geometrical modelling of post foliation deformations in metamorphic terrains syros cyclades greece superposed to ductile syn metamorphic deformations post foliation deformations affect metamorphic units during their exhumation understanding the role of such deformations in the structuration of metamorphic units is key for understanding the tectonic evolution of convergence zones we characterize post foliations deformations using 3d modelling which is a first order tool to describe complex geological structures but a challenging task where based only on surface data we propose a modelling procedure that combines fast draft models interpolation of orientation data with more complex ones where the structural context is better understood implicit modelling allowing us to build a 3d geometrical model of syros island blueschists cyclades based on field data with our approach the 3d model is able to capture the complex present day geometry of the island mainly controlled by the su perposition of three types of post metamorphic deformations affecting the original metamorphic pile i a top to south ramp flat extensional system that dominates the overall island structure ii large scale folding of the metamorphic units associated with ramp flat extensional system and iii steeply dipping normal faults trending dominantly nnw sse and ew the 3d surfaces produced by this method match outcrop data are geologically consistent and provide reasonable estimates of geological structures in poorly constrained areas

constraining the long term evolution of the slip rate for a major extensional fault system in the central aegean greece using thermochronology the brittle ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary in this study we examine the slip history of the large scale naxos paros extensional fault system npefs cyclades greece by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime based on apatite and zircon fission track aft and zft and u th he dating we observe variable slip rates across the brittle ductile transition on naxos zft and aft ages range from 11 8 0 8 to 9 7 0 8 ma and 11 2 1 6 to 8 2 1 2 ma and u th he zircon and apatite ages are between 10 4 0 4 to 9 2 0 3 ma and 10 7 1 0 to 8 9 0 6 ma respectively on paros zft and aft ages range from 13 1 1 4 ma to 11 1 1 0 ma and 12 7 2 8 ma to 10 5 2 0 ma while the u th he zircon ages are slightly younger between 8 3 0 4 ma and 9 8 0 3 ma all ages consistently decrease northwards in the direction of hanging wall transport most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of 6 8 km myr 1 on the npefs across the brittle ductile transition even the intrusion of a large granodiorite body into the narrowing fault zone at 12 ma on naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate the data also show that the npefs accomplished a minimum total offset of 50 km between 16 and 8 ma

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

intrinsic qp seismic attenuation from the rise time of microearthquakes a local scale application at rio antirrio western greece we determine the attenuation structure of a three dimensional medium based on first pulse width measurements from microearthquake data ninety five microearthquakes from a seventy stations local network were considered in this study measurements of the first half cycle of the wave the so called rise time r were carried out on high quality velocity seismograms and inverted to estimate the p waves intrinsic quality factor q p the results of this investigation indicate that first pulse width data from a local microearthquake network permit retrieval with sufficient accuracy of the heterogeneous q p structure the inferred attenuation variability corresponds to the known geological formations in the region

thermochronometric constraints on the tectonic evolution of the serifos detachment aegean sea greece we constrain the timing and kinematics of the serifos detachment in the southwestern cyclades greece using low temperature thermochronometry fission track dating shows that the serifos detachment was active between 13 and 6 ma and that the serifos granodiorite in its footwall intruded at or before 12 11 ma into the extensional shear zone and initially cooled very rapidly at rates 180 c per million year the mylonite zone at the top of the granodiorite and mylonitic structures in its country rocks record a consistent top ssw shear sense in the ductile crust in the brittle regime top nne shear sense indicators occur as well conjugate top ssw and top nne high angle normal faults are the youngest deformational features and cut across the detachment age distance relationships for the fission track data display a relatively flat pattern we discuss a model advocating initial top ssw movement on the serifos detachment before and during emplacement of the granodiorite updoming of the detachment during exhumation and cooling caused subsequent bivergent extension in the brittle crust

miocene high pressure metamorphism in the cyclades and crete aegean sea greece evidence for large magnitude displacement on the cretan detachment the cyclades in the backarc region of the present hellenic subduction zone are known for widespread late cretaceous to eocene high pressure metamorphism in the cycladic blueschist unit we report 40 ar 39 ar and rb sr phengite ages of 24 21 ma for high pressure metamorphism 8 10 kbar 350 400 c in the lowest tectonic unit in the cyclades the basal unit which structurally underlies the cycladic blueschist unit the basal unit is correlated with the tripolitza unit of the external hellenides in the forearc region of the hellenic subduction zone the tripolitza unit is unmetamorphosed on crete where it is separated from the underlying high pressure 8 10 kbar 300 400 c plattenkalk and phyllite quartzite units by the extensional cretan detachment the age for high pressure metamorphism in the latter units is similar to our age for the basal unit in the cyclades because pressure temperature conditions in the plattenkalk and phyllite quartzite units on crete and the basal unit in the cyclades are also similar they must have been in close proximity in the early miocene hellenic subduction zone a palinspastic reconstruction suggests a subsequent displacement of 100 km on the cretan detachment this is one of the greatest displacement magnitudes ever reported from detachment faults because of this large offset the cretan detachment was an efficient agent for exhuming high pressure rocks

the weak and superfast cretan detachment greece exhumation at subduction rates in extruding wedges low angle normal faults detachments are only efficient agents for bringing rocks from 40 100 km depth back to the earth s surface if they operate with extreme slip rates exceeding 20 km ma 1 here we propose a slip rate of 20 30 km ma 1 for the cretan detachment in the aegean greece the cretan detachment and the subjacent subduction thrust bounded an extruding wedge above the miocene hellenic subduction zone during exhumation the high pressure rocks in the wedge were not significantly deformed very low shear coupling at the bounding faults which are therefore weak is needed to prevent significant deformation in the extruding wedge the proposed slip rate of 20 30 km ma 1 is similar to the subduction rate of 35 45 km ma 1 at the hellenic subduction zone we argue that extruding wedges can only slip at rates similar to subduction rates if there is very low shear coupling at the bounding faults which may also explain the paradox of almost non deformed but very rapidly exhumed rocks

strategy for automated analysis of passive microseismic data based on s transform otsu s thresholding and higher order statistics small magnitude seismic events either natural or induced microearthquakes have increasingly been used in exploration seismology with applications ranging from hydrocarbon and geothermal reservoir exploration to high resolution passive seismic tomography surveys we developed an automated methodology for processing and analyzing continuously recorded single channel seismic data this method comprised a chi squared based statistical test for microseismic event detection and denoising filtering in the stransform domain based on the otsu thresholding method an automatic p phase picker based on higher order statistics criteria was used the method was used with data from a surface seismic station the performance of the method was tested and evaluated on synthetic and real data from a microseismic network used in a high resolution pst survey and revealed a high level of consistency

structural and thermochronometric evidence for multi stage exhumation of southern syros cycladic islands greece abstract the attic cycladic metamorphic core complex province was formed during miocene to recent regional back arc extension in response to slab rollback and gravitational collapse the development of several metamorphic core complexes along extensional low angle detachment faults resulted in the exhumation of lower plate rocks and the tectonic denudation of the upper plate that is only sparsely exposed throughout the aegean sea one of these areas is in southern syros where the tectonic configuration includes three tectonometamorphic units separated by two detachment faults the lower plate cycladic blueschist unit underwent late cretaceous eocene hp lt metamorphism and subsequent greenschist facies overprint during oligo miocene times the tectonically overlying greenschist facies upper unit is a tectonic sliver between the cycladic blueschist unit and the structurally highest unit the vari unit which is dominated by quartzofeldspathic gneisses that are affected by four distinct deformation events d1 d4 the late cretaceous upper greenschist facies d1 episode is related to late cretaceous eocene subduction while d2 d4 fabrics were formed in response to late cenozoic extension and exhumation field relations and structural data suggest that the late miocene brittle vari detachment juxtaposes both vari unit and upper unit against the cycladic blueschist unit mylonitic rocks observed at the base of vari unit along the contact with upper unit and zircon and apatite u th he thermochronometry corroborate this scenario with lower plate cycladic blueschist unit rocks recording rapid cooling at 8 10 ma in contrast cooling ages from the vari unit cluster at 13 15 ma indicating rapid exhumation coeval with activity along the livadi detachment on the neighboring island of tinos these data document new evidence that vari unit was affected by a major middle miocene phase of exhumation and an older semi brittle episode both unrelated to late miocene detachment faulting revealing that crustal extension occurred in multiple stages and in a protracted episodic fashion similar to other parts of the cyclades

rb sr isotope studies on tinos island cyclades greece additional time constraints for metamorphism extent of infiltration controlled overprinting and deformational activity this study presents new rb sr age data concerning the metamorphic evolution of the attic cycladic crystalline belt which represents a complex polymetamorphic terrane within the alpidic orogenic belt of the hellenides two major groups of tectonic units can be distinguished metamorphism in parts of the upper units is commonly considered as a cretaceous event in contrast the group of lower units experi enced tertiary high pressure metamorphism which was followed by a medium pressure overprint we focus on the island of tinos where a representative spectrum of the rock units found in the cyclades is exposed in three tectonic units the upper unit the intermediate unit and the basal unit the complete range of tectono metamorphic and magmatic events affecting the attic cycladic crystalline belt is documented by numerous petrological and tectonic studies phyllites and phyllonites from the ophiolitic upper unit yielded rb sr apparent ages phengite whole rock between c 92 and 21 ma the older age differs from the cretaceous dates reported for upper unit rocks elsewhere in the cyclades it is suggested that the sequence studied belongs to the jurassic ophiolites of the hellenides rather than to cretaceous occurrences the spread to younger ages is related to non pervasive rejuvenation and resetting of the rb sr system during tectonic juxtaposition of the upper unit over the intermediate unit the youngest age obtained so far for a sample from the upper unit 21 ma is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist facies episode producing a pervasive overprint in the struc turally lower tectonic unit the major phyllite meta gabbro serpentinite sequence of the upper unit is inter preted as an emplacement related ductile shear zone which experienced reworking under brittle conditions in the intermediate unit a gradient in rb sr ages from top c 40 ma to the bottom c 22 ma was recog nized which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit possibly controlled by variable intensity of deformation which might be related to tectonic jux taposition onto the basal unit we suggest that synmetamorphic stacking of all three tectonic units took place during an oligocene miocene greenschist event extensional deformation continued after tectonic stacking and after intrusion of the main granite as is indicated by a rb sr whole rock isochron 15 1 0 6 ma for a ductilely deformed garnet bearing leucogranite from the marginal parts of the main undeformed pluton application of the rb sr dating technique provided no unequivocal evidence that previously pub lished eocene k ar and 40 ar 39 ar dates for high pressure phengites from the lower units are significantly contaminated with excess argon

evolution of a passive crustal scale detachment syros aegean region insights from structural and petrofabric analyses in the hanging wall abstract new detailed micro structural investigations quartz petrofabric analyses and geological structural mapping in southeast syros cycladic massif aegean region allow us to place new constraints on the tectonic evolution of the uppermost unit which occupies the immediate hanging wall of the crustal scale vari detachment we show that the eocene oligocene deformation history in the hanging wall of this detachment is associated with sw directed ductile shearing this history includes an early distributed constrictional deformation expressed by transport parallel upright folds l tectonites and cleft girdles quartz c axis fabrics that were formed at temperatures 500 c ductile deformation progressively localized at the bottom of the uppermost unit leading to the formation of a greenschist facies mylonitic zone under plane strain conditions inferred from type i cross girdles quartz c axis fabrics the ongoing mylonitization was also associated with temporally increasing pure shear component of deformation coupled with cooling from 500 c to 400 c we suggest that the vari detachment represents a passive normal sense roof fault resulted from the ne directed ductile extrusion of the blueschist unit footwall at middle eocene oligocene times in the middle miocene the activation of the brittle ssw directed late vari detachment enhanced the brittle exhumation of both the blueschist unit and the vari detachment

assessing the nature of tectonic contacts using fission track thermochronology an example from the calabrian arc southern italy fission track thermochronology applied to the nappe pile of the calabrian arc of southern italy particularly within the continental basement rocks has provided important new constraints on the nature of some of the tectonic contacts in southern calabria an important phase of lower miocene crustal extension is indicated in northern calabria no oligocene or younger extension is seen here the emplacement of continental basement rocks with alpine metamorphism over ophiolitic rocks with little or no metamorphism is constrained as a thrust of lower to middle miocene age related to collision of the calabrian arc with the adria plate margin it is proposed that reduction in the plate convergence velocity during collision of a retreating subduction zone with a continental margin is at least partly an explanation for the onset of extension in southern calabria during the miocene

fast extension but little exhumation the vari detachment in the cyclades greece markedly different cooling histories for the hanging and footwall of the vari detachment on syros and tinos islands greece are revealed by zircon and apatite fission track data the vari akrotiri unit in the hangingwall cooled slowly at rates of 5 15 cm yr 1 since late cretaceous times samples from the cycladic blueschist unit in the footwall of the detachment on tinos island have a mean zircon fission track age of 10 0 1 0 ma which together with a published mean apatite fission track age of 9 4 0 5 ma indicates rapid cooling at rates of at least 60 cm yr 1 w e derive am inimum slip rate of 6 5 km myr 1 and a displacement of 20 km and propose that the development of the detachment in the thermally softened magmatic arc aided fast displacement intra arc extension accomplished the final 6 9 km of exhumation of the cycladic blueschists from 60 km depth the fast slipping intra arc detachments did not cause much exhumation but were important for regional scale extension and the formation of the aegean sea

middle miocene graben development in crete and its possible relation to large scale detachment faults in the southern aegean the linkage between the development of south facing cretan graben and large scale detachment faulting in the southern aegean is unknown widespread serravallian deposits in the ierapetra graben of crete supply constraints to middle miocene graben development in the southern aegean the ierapetra graben and by inference the cretan graben in general were hitherto believed to have formed as a result of sinistral transpression during n s shortening we argue that the formation of the cretan graben is due to n s extension the south dipping n s extending kritsa normal fault served as the master fault controlling graben development in the ierapetra graben the kritsa normal fault is either an antithetic fault related to the top n cretan detachment or a synthetic fault associated with the top s ios detachment the overall geometry and timing relationships lead us to favour a connection with the ios detachment which in turn implies a large scale displacement on the ios detachment

textures of eclogites and blueschists from syros island greece inferences for elastic anisotropy of subducted oceanic crust many blueschists and eclogites are inferred to have formed from oceanic basalts in subducted slabs knowledge of their elastic behaviour is essential for reconstructing the internal structure of subduction zones the cycladic blueschist unit exposed on syros island greece contains rocks belonging to an exhumed tertiary subduction complex they were possibly part of a subduction channel a shear zone above the subducting slab in which exhumation is possible during subduction intense plastic deformation forming crystallographic preferred orientations cpo accompanied blueschist and eclogite metamorphism cpo of the constituent minerals in the collected samples was determined by time of flight neutron diffraction two samples are foliated fine grained blueschists with strong cpo rich in glaucophane zoisite and phengite two coarser grained eclogite samples rich in omphacite and clinozoisite or glaucophane have weaker cpo vp and vs anisotropies were computed from the orientation distribution function and single crystal elastic constants all samples show velocity maxima parallel to the mineral lineation and minima normal to the foliation providing important constraints on orientations of seismic anisotropy in subduction channels vp anisotropies are up to three times higher 6 5 12 in the blueschists than in the eclogites 3 4 pointing to a potentially important lithological control of elastic anisotropy in subducted oceanic crust

high pressure metamorphism in the aegean eastern mediterranean underplating and exhumation from the late cretaceous until the miocene to recent above the retreating hellenic subduction zone 1 we report 40ar 39ar ages from various tectonic units in the aegean and westernmost turkey on the basis of published geochronologic data and our 40ar 39ar ages we propose that the aegean is made up of several high pressure units which were successively underplated from the late cretaceous until the miocene ages for high pressure metamorphism range from 80 83 ma in parts of the vardar izmir ankara suture zone in the north to 21 24 ma for the basal unit in the cyclades and the external high pressure belt on crete in the south published seismic data suggest that high pressure metamorphism is currently occurring underneath crete younging of high pressure metamorphism in a southerly direction mimics the southward retreat of the hellenic subduction zone we propose that distinct stages of high pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat by far most of the exhumation of the high pressure units occurred early during the orogenic history in a forearc position

deciphering subduction from exhumation in the segmented cycladic blueschist unit central aegean greece the cycladic blueschist unit cbu represents the northern passive margin of the adria continental block and ophiolites that are the remnants of the pindos ocean which were affected by high pressure low temperature metamorphism in the blueschist and eclogite facies during the eocene prior to the lutetian the ophiolitic and margin units were thrust towards the sw inside a ne dipping subduction zone two subsequent exhumation stages are characterized by top to the ne senses of shear i from mantle depths to lower crustal levels before 37 ma which was accommodated by an extensional inversion of the vardar suture zone and ii from deep crustal levels to the near surface between 30 and 20 ma which was accommodated by the north cycladic detachment it is during this late stage of ductile exhumation that the central cyclades core complex was exhumed segmentation of the cbu by normal faults started at 12 ma as recorded by the low temperature thermochronometers and was coeval with block rotation and folding with ns trending axes which were predominantly controlled by the myrthes ikaria strike slip fault in the centre of the cyclades a map restoration of the cbu geometry prior to segmentation has been carried out using available structural and paleomagnetic data the restoration shows that following the cbu exhumation from mantle depths to crustal levels a single core complex was exhumed along a single ne dipping detachment

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

the geological signature of a slab tear below the aegean in this paper we explore the geology of the aegean region in the miocene to identify the geological signature of a first order slab tear observed in all tomographic models from 15 to 8 ma the tear is associated spatially and timely with a fast clockwise rotation of the external hellenides alkaline volcanism high temperature metamorphic domes with a predominance of north dipping detachments and south westward migration of granitoid intrusions these features suggest a warmer geodynamic environment during the rotation and the impact of a hot mantle flow associated with the tear the 8 ma duration between the first high temperature metamorphic domes in the centre of the archipelago and the beginning of the fast rotation may correspond to the time needed for the slab to bend stretch and finally tear

what caused the denudation of the menderes massif review of crustal evolution lithosphere structure and dynamic topography in southwest turkey the deformation of earth s lithosphere in orogenic belts is largely forced externally by the sinking slab but can also be driven by internal delamination processes caused by mechanical instabiliti

the geodynamic evolution of the alpine orogen in the cyclades aegean sea greece insights from diverse origins and modes of emplacement of ultramafic rocks

tectonic significance of synextensional ductile shear zones within the early miocene alacamdag granites northwestern turkey synextensional granitoids may have significant structural features leading to the understanding of the evolution of extended orogenic belts one of the highly extended regions the aegean region includes a number of metamorphic core complexes and synextensional granitoids that developed following the alpine collisional events the alacamdag area in northwestern turkey is one of the key areas where miocene granites crop out along the boundary of various tectonic units structural data from the early miocene alacamdag granites demonstrated two different deformation patterns that may provide insights into the development of granitic intrusions and metamorphic core complexes 1 steeply dipping ductile shear zones caused emplacement of syn tectonic granite stocks they include kinematic indicators of a sinistral top to the sw displacement this zone has also juxtaposed the izmir ankara zone and the menderes massif in the west and east respectively 2 gently dipping ductile shear zones have developed within the granitic stocks that intruded the schists of the menderes massif on the structurally lower parts kinematic data from the foliated granites indicate a top to the ne displacement which can be correlated with the direction of the hanging wall movement documented from the simav and kazdag metamorphic core complexes the gently dipping shear zones indicate the presence of a detachment fault between the menderes massif and the structurally overlying izmir ankara zone mesoscopic to map scale folds in the shallow dipping shear zones of the alacamdag area were interpreted to have been caused by coupling between ne sw stretching and the accompanying nw se shortening of ductilely deformed crust during early miocene times one of the ne trending shear zones fed by granitic magmas was interpreted to form the northeastern part of a sinistral wrench corridor which caused differential stretching between the cycladic and the menderes massifs this crustal scale wrench corridor the izmir balikesir transfer zone may have controlled the asymmetrical and symmetrical extensions in the orogenic domains the combination of the retreat of the aegean subduction zone and the lateral slab segmentation leading to the sinistral oblique slip tearing within the eurasian upper plate appears to be a plausible mechanism for the development of such extensive ne trending shear zones in the aegean region

exhumation of eclogite and blueschist cyclades greece pressure temperature evolution determined by thermobarometry and garnet equilibrium modelling high pressure rocks such as eclogite and blueschist are metamorphic markers of paleo subduction zones and their formation at high pressure and low temperature conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years the tectonic mechanisms controlling their exhumation back to the surface are however diverse complex and still actively debated although the cycladic blueschist unit cbu greece is among the best worldwide examples for the preservation of eclogite and blueschist the proposed p t evolution followed by this unit within the hellenic subduction zone is quite different from one study to another hindering our comprehension of exhumation processes in this study we present an extensive petrological dataset that permits refinement of the shape of the p t trajectory for different subunits of the cbu on syros high resolution quantitative compositional mapping has been applied to support the thermobarometric investigations which involve semi empirical thermobarometry garnet equilibrium modelling and p t isochemical phase diagrams the thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions the results were also

thermochronologic evaluation of postcollision extension in the anatolide orogen western turkey 1 to better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen data from multiple low temperature thermochronometers were obtained from the simav detachment fault sdf the earliest developed major extensional structure recognized in the western anatolide orogen of western turkey twenty two zircon fission track ft 26 apatite ft 12 apatite u th he ages and 26 apatite ft track length analyses are presented the data establish that the sdf was a major extensional fault active between 25 and 19 ma the coincidence in timing of magmatism with cessation of sdf activity implies the detachment became locked owing to doming induced by magma emplacement zircon ft ages away from the influence of miocene magmatism record rapid footwall cooling between 25 and 21 ma at a slip rate of up to 15 km myr and demonstrate that active ductile deformation along the sdf migrated northward with time apatite ft ages from the footwall of the sdf are spatially invariant over 100 km and consistently 2 3 myr younger than the zircon fission track ages from the same samples these data are consistent with a regional but relatively rapid erosion linked cooling phase that removed 2 3 km of overburden following cessation of sdf activity but before deposition of sedimentary rocks on the detachment surface at 16 4 ma postcollision extension of the anatolide orogen with development of the sdf can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out of sequence thrust

u pb zircon geochronology of unusual eclogite facies rocks from syros and tinos cyclades greece low temperature eclogite facies rocks from syros and tinos cyclades greece include meta igneous blocks of unclear origin meta olistostrome or tectonic melange with very high trace element concentrations e g zr up to 4850 ppm y up to 475 ppm the unusual geochemistry is considered to be the result of metasomatic alteration in a subduction zone environment locally metasomatic exchange with an ultramafic matrix further enhanced compositional anomalies a concordant u pb zircon age of 78 1 ma recorded for an omphacitite from syros is interpreted as the time of pre eocene high pressure metamorphism in the cyclades zircon dates of 61 and 63 ma for a jadeitite from tinos possibly indicate an additional discrete event deformation these results are a first geochronological indication that high pressure metamorphism in the cyclades commenced signif icantly earlier than indicated by previous white mica chronology which provided ages between c 50 and 40 ma

late miocene ductile extension and detachment faulting mykonos greece the geometry and style of structural relations at map to microscopic scale on mykonos cyclades aegean sea greece suggests ductile extension and detachment faulting during the late miocene this island is dominated by granitic rocks that have a well developed shallowly dipping mylonitic foliation and a northeast southwest trending mineral elongation lineation kinematic indicators in these rocks exhibit a top to the northeast down dip sense of shear textural relations suggest that conditions during mylonitic deformation were greenschist to amphibolite facies and published k ar geochronology suggests a late miocene age for this deformation event mylonitic rocks lie in the footwall of a low angle fault and fault striae parallel to the elongation lineation suggest kinematic coordination between mylonitic deformation and movement along the fault translation along the fault juxtaposes younger tilted sedimentary rocks of low metamorphic grade upon the once deeper mylonitic rocks the style and geometry of deformational fabrics and map relations on this island are strikingly similar to those observed in metamorphic core complexes of the western united states

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

the oxidation state of manganese in ferromanganese nodules and deep sea sediments from the central indian ocean abstract ferromanganese nodules from different sediment facies including siliceous ooze pelagic clay sediments from transition zone of siliceous and pelagic clay and seamount crusts from the central indian ocean were analysed for chemical composition mineralogy and bulk oxidation state of mn by the iodometric method despite considerable variation in chemical composition mn fe ratio times from 1 35 to 7 42 and mineralogy the o mn ratio or mno x solid hydrous manganese oxide does not vary much from 1 73 to 1 81 suggesting most of the mn in the manganese nodules in mn iv the oxidised upper zone in the sediment cores decreases in thickness from 15 cm in the north 5 s to 11 cm in the southern part 14 s of the central indian ocean iodometric titration of deep sea sediments yields o mn ratios of 1 29 1 95 sediments from the manganese nodule field have maximum solid mn concentrations and lower o mn ratios in the core top 0 3 cm with minimum mn and o mn ratios at 11 cm depth and thus may reflect the mechanism controlling the supply of diagenetic mn to the nodules in contrast sediments from the non manganese nodule field have comparatively low solid phase mn concentrations in the core top 0 3 cm and almost uniform o mn ratios with depth indicating that mn is not preserved in the sediments but may be remobilised and deposited somewhere else

extensional tectonics in mt parnon peloponnesus greece peloponnesus in the south western part of the aegean is formed by a heterogeneous pile of alpine thrust sheets that was reworked by normal faulting from upper miocene to recent times upper miocene lower pliocene extension in mt parnon was accommodated by several mappable brittle detachment faults that exhibit a top to the ne ene sense of shear the hanging wall of the detachments comprises a number of highly tilted fault blocks containing abundant evidence of intense internal deformation by normal faulting and layer parallel shearing contemporaneous with faulting these fault blocks are remnants of a cohesive extensional block that slipped to the ne ene and broke up along high angle normal faults that sole into or are cut by the detachments the largest part of this block is located at the eastern edge of the metamorphic core forming the hanging wall of east parnon high angle normal fault that excised part of the aforementioned detachments the lowermost metamorphic unit of the nappe pile does not seem to be affected by the previous extensional episode upper plate reconstruction shows that various units of the nappe pile were affected by high angle normal faults that linked to detachment faults in the weaker layers since the middle upper pliocene further exhumation of the metamorphic rocks has resulted in the formation of high angle normal faults overprinting neogene extensional structures and cut the entire nappe pile this new fault system tilted the earlier extensional structures and produced a ne sw coaxial deformation of mt parnon