tectonic evolution of a part of the tethyside orogenic collage the kargi massif northern turkey the central part of the rhodope pontide fragment one of the major tectonic units in turkey provides critical data for evaluating the cimmeride and alpide evolution of the mediterranean tethysides tectonic events that affected the central part of the rhodope pontide fragment since the end of the paleozoic generated east west trending belts with the event of every episode redeforming and partly obliterating the structures of previous episodes this evolution may be conveniently described in terms of three major episodes 1 two different realms of pre dogger oceanic rocks are present in the area the northern realm coincided with main branch of paleo tethys that was being actively destroyed by south dipping subduction the southern realm the karakaya ocean a back arc basin related to this subduction began opening by rifting of a retroarc carbonate platform during the permo triassic to the west a continental domain with sparse magmatism seperated the two oceanic areas toward the east the two oceans become united by the wedging out of the continental domain these two pre dogger oceans closed during the lias and their remnants were emplaced between the southern margin of laurasia and the fragments of the cimmerian continent 2 the second episode partly overlapped the first with rifting south of the cimmerian continent fragment during the lias this rifting was followed by a transgression which covered the ruins of the cimmeride orogenic belt by the malm this rifting concurrently led to the development of the northern branch of the neo tethys and a south facing atlantic type continental margin a southerly thickening sedimentary prism developed on this margin during the lias to early cretaceous interval 3 the floor of the northern branch of neo tethys began to be consumed along the north dipping subduction zone beneath the previosly constructed continental margin this convergent margin generated a magmatic arc to the north and to the south a subduction accretion complex present width of which approaches 80 km to the south of the subduction accretion complex yet another convergent margin is indicated by the presence of the remnants of an ensimatic arc the northern branch of the neo tethys was totally consumed in the late cretaceous to eocene interval

crust and upper mantle structure in the northwest basin and range province abstract seismic waves from nuclear explosions and from well located local earthquakes have been recorded along several profiles through western nevada southeast oregon and eastern california p n delay times from these data and from additional travel times of nts explosions recorded across the western nevada seismic network have been interpreted in terms of varying crustal thickness of the western great basin our interpretation of these data implies that the crust thins from greater than 32 km thick in the vicinity of mono lake to 20 to 22 km thick over a broad region in northwest nevada the p n headwave propagates to a distance of about 550 km with a velocity of 7 8 km sec at distances greater than 600 km a low velocity 7 44 km sec larger amplitude phase is observed this phase may result from long period energy diffracting into a shadow zone a prominent high velocity second arrival following p n by 1 to 2 sec in the distance range 340 to 475 km is interpreted as a reflection from the base of the lithosphere at 60 to 70 km depth

tectonic stacking of hp lt metamorphic rocks in accretionary wedges and the role of shallowing slab mantle decoupling hp lt chloritoid bearing micaschists crop out widely in the central part of northern turkey and represent deep seated subduction accretionary complexes three peak metamorphic assemblages are identified in the area studied 1 garnet chloritoid glaucophane with pseudomorphs after lawsonite 2 chloritoid with pseudomorphs after glaucophane and 3 chloritoid with pseudomorphs after jadeite in addition to phengite paragonite quartz chlorite rutile and apatite the latter is interpreted as transformation of a chloritoid glaucophane assemblage to chloritoid jadeite with increasing pressure pt modeling indicates 17 and 22 25 kbars for the two peak parageneses the diversity of peak metamorphic assemblages and the pt estimates suggest that basal accretion occurred at different depths within the wedge the depth of the basal accretion is possibly controlled by the slab mantle decoupling depth stretching and thinning of the lithospheric forearc induced by the slab roll back possibly caused shallowing of the slab mantle decoupling depth which limited depth of the basal accretion from 70 80 km to 55 km within the subduction channel a slab mantle coupling depth controlled basal accretion may also explain the scarcity of eclogite and high grade blueschist facies metamorphic rocks in active intra oceanic subduction zones because the overriding plate is young and hot in intra oceanic subductions the slab and mantle are coupled at a relatively shallow depth before eclogitization of the oceanic crust this prevents accretion and exhumation of eclogite along the subduction channel

geological geophysical and thermal characteristics of the salton sea geothermal field california abstract the salton sea geothermal field is the largest water dominated geothermal field in the salton trough in southern california within the trough local zones of extension among active right stepping right lateral strike slip faults allow mantle derived magmas to intrude the sedimentary sequence the intrusions serve as heat sources to drive hydrothermal systems we can characterize the field in detail because we have an extensive geological and geophysical data base the sediments are relatively undeformed and can be divided into three categories as a function of depth 1 low permeability cap rock 2 upper reservoir rocks consisting of sandstones siltstones and shales that were subject to minor alterations and 3 lower reservoir rocks that were extensively altered because of the alteration inter granular porosity and permeability are reduced with depth field permeability is enhanced by renewable fractures i e fractures that can be reactivated by faulting or natural hydraulic fracturing subsequent to being sealed by mineral deposition in the central portion of the field temperature gradients are high near the surface and lower below 700 m surface gradients in this elliptically shaped region are fairly constant and define a thermal cap which does not necessarily correspond to the lithologic cap at the margin of the field a narrow transition region with a low near surface gradient and an increasing gradient at greater depths separates the high temperature resource from areas of normal regional gradient geophysical and geochemical evidence suggest that vertical convective motion in the reservoir beneath the thermal cap is confined to small units and small scale convection is superimposed on large scale lateral flow of pore fluid interpretation of magnetic resistivity and gravity anomalies help to establish the relationship between the inferred heat source the hydrothermal system and the observed alteration patterns a simple hydrothermal model is supported by interpreting the combined geological geophysical and thermal data in the model heat is transferred from an area of intrusion by lateral spreading of hot water in a reservoir beneath an impermeable cap rock

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

petrofabrics of high pressure rocks exhumed at the slab mantle interface from the point of no return in a subduction zone sivrihisar turkey the highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is 2 5 gpa corresponding to the maximum decoupling depth mdd 80 10 km identified in active subduction zones beyond the mdd the point of no return exhumation is unlikely the sivrihisar massif turkey is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record p t fluid deformation conditions during exhumation from 80 to 45 km crystallographic fabrics and other features of high pressure metasedimentary and metabasaltic rocks record transitions during exhumation in quartzite microstructures and crystallographic fabrics record deformation in the dislocation creep regime including dynamic recrystallization during decompression and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression 2 5 to 1 5 gpa phengite lawsonite and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation and omphacite developed an l type crystallographic fabric in marble aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite p 1 2 gpa results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface this large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return

metamorphism of pelites in nkfmash a new petrogenetic grid with implications for the preservation of high pressure mineral assemblages during exhumation a petrogenetic grid for metapelites in the system nkfmash is presented the p t range is investigated in three sections 1 the high and ultrahigh pressure range is discussed in the system nfmash because phengite is the only stable potassic phase 2 the transition region is characterised by four nkfmash invariant points that separate high pressure glaucophane bearing from medium pressure biotite bearing metapelites 3 the medium pressure range contains the fifth nkfmash invariant point the univariant reactions of this point terminate the stability range of paragonite which breaks down to form staurolite or kyanite and plagioclase during decompression and or heating as the growth of albitic plagioclase by decomposition of paragonite via continuous reactions may be conspicuous already before these staurolite or kyanite producing reactions are reached such albite porphyroblast schists are typical indicators of a former high pressure metamorphic history considering the preservation of high pressure metapelitic assemblages those crossing the nkfmash transition region during exhumation commonly dehydrate and preservation is unlikely three types of metapelites have a fairly good survival potential 1 low temperature metapelites up to c 540 c with an exhumation path back into the chlorite albite stability field 2 assemblages with chloritoid glaucophane and 3 the relatively high temperature glaucophane kyanite and jadeite kyanite bearing parageneses that are relatively dry at the onset of exhumation a comparison with data from the literature shows that these rock types are the most abundant in nature

intraoceanic unroofing of eclogite facies rocks in the omachi seamount izu bonin frontal arc electron microscope and electron microprobe observations of amphibolites from the omachi seamount in the izu bonin frontal arc show that they contain micrometer sized inclusions of omphacite kyanite indicating eclogite facies metamorphism the garnet zoisite amphibolite was subjected to metamorphic conditions of 600 700 c and 2 gpa in the early stage of metamorphism these metabasites were subducted to a depth of 70 km beneath the izu bonin arc and were exhumed together with serpentinite schist their intraoceanic unroofing was probably induced by backarc extension of the upper plate whose axes were intersected and cooled by the newly formed izu bonin subduction system this intraoceanic exhumation associated with suprasubduction extension can be a modern analogue for ancient on land eclogite terranes accompanied by ophiolites

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

the importance of blueschist eclogite dehydration reactions in subducting oceanic crust the metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure temperature p t paths with a model of metabasalt phase equilibria in steady state subduction zones with high rates of shear heating the upper parts of the subducting oceanic crust progress through the greenschist amphibolite granulite eclogite facies whereas lower parts of the subducting oceanic crust progress through the blueschist eclogite facies in steady state subduction zones with moderate rates of shear heating most of the subducting oceanic crust passes through the blueschist eclogite transition in steady state subduction zones with low rates of shear heating the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km for oceanic crust containing 1 2 wt h 2 o dehydration will not begin until the onset of eclogite or amphibolite facies metamorphism depending on the p t path for many subduction zones the most important dehydration reactions in the subducting oceanic crust occur at the blueschist eclogite facies transition associated with the breakdown of lawsonite or clinozoisite glaucophane and chlorite large amounts of h 2 o released by blueschist eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

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

shallow drilling in the salton sea region the thermal anomaly during two shallow thermal drilling programs thermal measurements were obtained in 56 shallow 76 2 m and one intermediate 457 3 m depth holes located both onshore and offshore along the southern margin of the salton sea in the imperial valley california these data complete the surficial coverage of the thermal anomaly revealing the shape and lateral extent of the hydrothermal system the thermal data show the region of high thermal gradients to extend only a short distance offshore to the north of the quaternary volcanic domes which are exposed along the southern shore of the salton sea the central thermal anomaly has an arcuate shape about 4 km wide and 12 km long across the center of the anomaly the transition zone between locations exhibiting high thermal gradients and those exhibiting regional thermal gradients is quite narrow thermal gradients rise from near regional 0 09 c m to extreme 0 83 c m in only 2 4 km the heat flow in the central part of the anomaly is greater than 600 mw m2 and in the two local anomalies exceeds 1200 mw m2 the shape of the thermal anomaly is asymmetric with respect to the line of volcanoes previously thought to represent the center of the field with its center line offset south of the volcanic buttes there is no broad thermal anomaly associated with the magnetic high that extends offshore to the northeast from the volcanic domes the salton sea thermal anomaly can be described as three nested thermal features superimposed on the regional salton trough heat flux a broad region of elevated conductive heat flow with constant temperature gradients of about 0 1 c m a central anomaly of high 0 4 c m near surface gradients observed through a conductive thermal cap underlain by a nearly isothermal zone of hydrothermal circulation and two local anomalies with shallow temperature gradients as high as 0 8 c m superimposed on the central geothermal system these observations of the thermal anomaly provide important constraints for models of the circulation of the hydrothermal system thermal budgets based on a simple model for this hydrothermal system indicate that the heat influx rate for local hot spots in the region may be large enough to account for the rate of heat flux from the entire salton trough

chloritoid bearing rocks associated with blueschists and eclogites northern new caledonia chloritoid bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the tertiary high pressure metamorphic belt of northern new caledonia the typical assemblage of chloritoid bearing rocks in the epidote zone is quartzchlorite muscovite garnet chloritoid in the omphacite zone epidote is an additional member of the chloritoid bearing assemblage paragonite is rare plagioclase was not detected and rutile and ilmenite are the fe ti oxide phases chloritoid glaucophane is not a common assemblage chloritoid bearing rocks have relatively low ca k na al ratios and the chloritoids are relatively mg rich with mg mg fe up to about 0 4 a comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560 c in the omphacite zone and a minimum temperature limit near 450 c at 10 kbar an empirical garnet chlorite fe mg exchange thermometer does not yield consistent results for the higher grade rocks suggesting ts ranging from 390 to 535 c in the omphacite zone and 420 465 c in the epidote zone the distribution coefficient kd fe mg ctd fe mg chl for chloritoid and chlorite ranges from 3 9 to 6 4 values which are lower than those 10 from lower greenschist facies rocks but are near those of upper greenschist facies and albite epidote amphibolite facies

dating of the black sea basin new nannoplankton ages from its inverted margin in the central pontides turkey the eocene uplift and inversion of a part of the black sea margin in the central pontides allows to study the stratigraphic sequence of the western black sea basin the revision of this sequence with 164 nannoplankton ages indicates that subsidence and rifting started in the upper barremian and accelerated during the aptian the rifting of the western black sea basin lasted about 40 myr from late barremian to coniacian in the inner inverted black sea margin the syn rift sequence ends up with shallow marine sands the uppermost albian to turonian was a period of erosion or non deposition this regional mid cretaceous stratigraphical gap might result from rift flank uplift as expected in the case of a thick and cold pre rift lithosphere however coeval collision of the kargi block along the north tethyan subduction zone at the southern margin of the pontides might also have contributed to this uplift a rapid thermal post rift subsidence of the margin occurred during the coniacian santonian collision of the kirsehir continental block commenced in early eocene time zone np12 giving rise to compressional deformation and sedimentation in piggyback basins in the central pontides whereas the eastern black sea was still opening

low temperature hydrothermal alteration in near surface sediments salton sea geothermal area as part of the lawrence livermore sandia national laboratories shallow drilling 19 wells into salton sea sediments four 1 m cores were recovered from 76 m subbottom depth the cored wells are along a transect from the margin toward the interior of the mullet island thermal anomaly the thermal gradients range from 0 191 c m near the margin to 0 816 c m near the interior of the thermal anomaly bottom hole temperature in a cored well close to the center of the thermal anomaly is 87 c pore water chemical composition and bulk and clay size sediment mineralogical and chemical compositions are influenced by hydrothermal reactions which intensify with increasing thermal gradient x ray diffraction and petrographic microscopic analyses of bulk and clay size fractions of the sediments indicate that the relative proportions of smectite to kaolinite plus illite and of dolomite to calcite increase from the margin toward the interior of the thermal anomaly region clay mineral recrystallization and dolomitization are indicated by increased k al and decreased ca mg molar ratios in the clay size sediment fraction calcium silica potassium lithium and strontium are released from sediments into pore fluids these data suggest that even at temperatures less than 100 c hydrothermal activity significantly alters the pore water chemistry and moderately alters the sediment mineralogical and chemical composition

late palaeozoic marginal basin and subduction accretion the palaeotethyan kure complex central pontides northern turkey the kure complex located in the central pontides of northern turkey is a c 20 km thick pile of thrust imbricated deep sea sediments intercalated with a dismembered ophiolite the ophiolitic rocks include serpentinized harzburgite massive gabbro cumulate and isotropic sheeted dykes and basic volcanic rocks the extrusive rocks are mainly pillow lavas sheet flows and lava breccias geochemical evidence of immobile major and trace elements and pyroxene chemistry indicate mid ocean ridge mor and volcanic arc basalt vab compositions and suggest that the kure ophiolite was generated above a subduction zone chrome spine1 analysis also supports this interpretation in some thrust sheets the extrusives are depositionally overlain by hemi pelagic shales passing up into terrigenous turbidites and shales which are up to several hundred metres thick in intact successions cyprus type massive sulphides interpreted as precipitates from black smokers are located along the lava sediment contact individual intact sediment units up to several kilometres thick within the kure complex are separated by narrow up to 10 m wide zones of intense shearing layer parallel extension and melange formation asymmetrical folds reverse faults thrusts and duplex structures throughout the kure complex indicate mainly northwards emplacement the kure complex is thought to have opened in the late palaeozoic as a marginal basin above a northward dipping palaeotethyan subduction zone located along the southern margin of eurasia the kure marginal basin later closed in response to inferred southward subduction leading to development of an accretionary prism prior to late jurassic time the kure complex is unlikely to represent the main palaeotethyan suture as has been suggested in some earlier tectonic reconstructions regional comparisons with dobrogea crimea and caucasus suggest the kure basin opened then closed along a south eurasia active margin

late cretaceous to paleocene oroclinal bending in the central pontides turkey 1 the turkish pontide fold and thrust belt formed since the paleozoic and is an important element in the africa eurasia convergence and the resulting closure of the neo tethys ocean it has a peculiar arc shaped geometry in its central part along the black sea coast which may have resulted from oroclinal bending we have determined the vertical axis rotation history of this area using paleomagnetism on cretaceous to eocene rocks from 47 sites and critically analyzed previously published data we applied the same reliability criteria to all data our results show that late cretaceous sites have clockwise and counterclockwise rotations perpendicular to the structural trend in the central pontides in the eastern pontides they show only local rotations paleocene to eocene rocks in the central and eastern pontides show no rotation we conclude that the central pontide northward arc shaped geometry results from oroclinal bending in latest cretaceous to earliest paleocene times the timing and scale of geological processes that occurred in the region make it likely that orocline formation resulted from neo tethys closure between the pontides and the metamorphic promontory of the anatolide tauride block earlier studies on the southerly located cankiri basin reveal that clockwise and counterclockwise rotations occurred in eocene oligocene times this implies that the entire region underwent continuous deformation from late cretaceous to eocene caused by convergence of the pontides and the anatolide tauride block with a southward moving deformation front deformation was first localized in the northern part of the central pontides until the paleocene resulting in oroclinal bending and from at least eocene times it shifted toward the cankiri basin region

kastamonu granitoid belt of northern turkey first arc plutonism product related to the subduction of the paleo tethys many granitoid plutons of middle jurassic age arc plutonic in character form a belt to the north of the paleo tethyan suture zone in northern anatolia these plutons are here called the kastamonu granitoid belt when they are interpreted in a regional geologic setting it can be shown that they may be formed by the magmatism related to the northward subduction of paleo tethyan oceanic crust

evolution of the calabrian accretionary wedge central mediterranean 1 the neogene tectonics of the central mediterranean are related to the subduction and trench rollback of the ionian basin under eurasia causing the opening of the liguro provencal and tyrrhenian back arc basins and the formation of the calabrian accretionary wedge the calabrian accretionary wedge is a partially submerged accretionary complex located in the ionian offshore and laterally bounded by the apulia and malta escarpments while the history of the back arc extension process is fairly well defined the structure and evolution of the wedge are still poorly known we have analyzed and interpreted the available reflection multichannel and single channel seismic profiles in the ionian offshore and integrated them with other geological and geophysical data acquired in the last 40 years here we present unpublished seismic profiles to provide a new map of the tectonic structure of the ionian offshore and to define the structure of the wedge and its evolution during the last 15 myr our reconstruction points out that the messinian salinity crisis represents an important break in the evolution of the wedge as the basal decollement ramps up onto the messinian salt deposits producing a dramatic and fast forward propagation of the frontal thrust and resulting underplating of the underlying crustal ionian sequence during progressive trench rollback our results provide new insight into the style of accretion in a weakly converging setting which is typical for the mediterranean region

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

graphitization in a high pressure low temperature metamorphic gradient a raman microspectroscopy and hrtem study the graphitization of carbonaceous material cm in a high pressure metamorphic gradient is characterized along a cross section in the schistes lustres formation western alps along this 25 km cross section both the cm precursor and the host rock lithology are homogeneous and the prograde evolution of the pressure temperature metamorphic conditions from the lower blueschist facies 13 kbar 330 c to the eclogite facies 20 kbar 500 c is tightly constrained by literature data raman microspectroscopy shows that at the micrometre scale this process is progressive and continuous with increasing metamorphic grade and that the structure of cm is very sensitive to temperature variations at the nanometre scale hrtem the cm is composed of a mixture of a microporous phase and an onion ring like phase both known as non graphitizing under the effect of temperature at ambient pressure the hp lt graphitization produces structurally and microtexturally heterogeneous cm with increasing metamorphic grade the graphitization of the two types of cm proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of cm the microporous material is progressively transformed into graphite through a macroporous transitional stage in this case graphitization mainly occurs on the pore walls as a result of pore growth in the case of concentric onion ring like material graphitization occurs in the regions with the largest radius of curvature i e on the outer part of the ring in comparison with 1 bar experiments pressure seems to induce microtextural changes which allows the subsequent structural modifications of the starting material

eclogite breccias in a subducted ophiolite a record of intermediate depth earthquakes understanding processes acting along the subduction interface is crucial to assess lithospheric scale coupling between tectonic plates and mechanisms causing intermediate depth seismicity despite a wealth of geophysical studies aimed at better characterizing the subduction interface we still lack critical data constraining processes responsible for seismicity within oceanic subduction zones we herein report the finding of eclogite breccias formed at 80 km depth during subduction in an almost intact 10 km scale fragment of exhumed oceanic lithosphere monviso ophiolite western alps these eclogite breccias correspond to meter sized blocks made of 1 10 cm fragments of eclogite mylonite cemented by interclast omphacite lawsonite and garnet and were later embedded in serpentinite in a 30 150 m wide eclogite facies shear zone at the mineral scale omphacite crack seal veins and garnet zoning patterns also show evidence for polyphased fracturing healing events our observations suggest that a possible seismic brecciation occurred in the middle part of the oceanic crust accompanied by the input of externally derived fluids we also conclude that these eclogite breccias likely mark the locus of an ancient fault zone associated with intraslab intermediate depth earthquakes at 80 km depth

evolution from fore arc oceanic crust to island arc crust a seismic study along the izu bonin fore arc 1 petrological studies have suggested that oceanic crust is formed during the initial stage of subduction however there is little geophysical evidence for the formation of oceanic crust in the fore arc region we conducted an active source seismic survey in the fore arc region of the izu bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction we used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction stack migration of picked reflection travel times to derive reflectivity images these data show a remarkably thin crust 10 km thick along the northern half of the bonin ridge and abrupt crustal thickening to 20 km toward the south of the ridge comparison of velocity depth profiles of the thin fore arc crust of the bonin ridge with those of typical oceanic crusts showed them to be seismologically identical boninitic magmatism is evident in the area of thin crust and tholeiitic calcalkaline andesitic volcanism in the area of thick crust high precision dating of the volcanic rocks showed that the thin fore arc oceanic crust was created soon after initiation of subduction 48 45 ma and that the nonoceanic thick crust was created with tholeiitic calcalkaline andesitic magmatism after the boninitic magmatism was ceased our seismological image strongly supports the view that fore arc oceanic crust was formed by fore arc spreading during the initial stage of subduction along the izu bonin intraoceanic arc

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs a review abstract more than a decade after the discovery of deep episodic slow slip and tremor or slow earthquakes at subduction zones much research has been carried out to investigate the structural and seismic properties of the environment in which they occur slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner where three major structural elements are in contact the subducting oceanic crust the overriding forearc crust and the continental mantle in this region thermo petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions and their consumption by hydrating the mantle wedge these fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore fluid pressure and or reducing friction in fault gouges resolving the fine scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging and most advances have been made using teleseismic scattering techniques e g receiver functions in this paper we review the teleseismic structure of six well studied subduction zones three hot i e cascadia southwest japan central mexico and three cool i e costa rica alaska and hikurangi that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior we conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior and that near lithostatic pore fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

raman spectroscopic carbonaceous material thermometry of low grade metamorphic rocks calibration and application to tectonic exhumation in crete greece we present new raman spectra data of carbonaceous material cm to extend the range of the raman spectra of cm thermometer rscm to temperatures as low as 100 8c previous work has demonstrated that raman spectroscopy is an excellent tool to describe the degree of graphitization of cm a process that is independent of pressure but strongly dependent on metamorphic temperature a linear relationship between temperature and the raman parameter r2 derived from the area of the defect band relative to the ordered graphite band forms the basis of a previous thermometer because r2 shows little variability in low temperature samples 330 8c serves as a lower limit on the existing thermometer herein we present raman spectra from a suite of low temperature 100 to 300 8c samples from the olympics mountains and describe other aspects of the raman spectra of cm that vary over this range in particular the raman parameter r1 the ratio of heights of the disordered peak to ordered peak varies regularly between 100 and 350 8c these data together with published results from higher temperature rocks are used to calibrate a modified rscm thermometer applicable from 100 to 700 8c application to low grade metasediments in the otago region in the south island of new zealand gives temperatures consistent with previous estimates demonstrating the reliability of the modified rscm thermometer we apply the modified rscm thermometer to 53 samples from crete to evaluate the role of the cretan detachment fault in exhuming miocene high pressure low temperature metamorphic rocks exposed there the metamorphic rocks below the detachment the plattenkalk and phyllite quartzite units give metamorphic temperatures that range from 250 to 400 8c consistent with previous petrologic estimates we also demonstrate that the tripolitza unit which lies directly above the detachment gives an average metamorphic temperature of about 260 8c the modest break in metamorphic temperature in central crete indicates that the cretan detachment accounts for only 5 to 7 km of exhumation of the underlying hp lt metamorphic rocks which were initially accreted at 35 km we argue that the bulk of the exhumation 28 km out of 35 km total occurred by pervasive brittle stretching and erosion of structural units above the detachment d 2005 elsevier b v all rights reserved

eclogitic metasediments from the tso morari area ladakh himalaya evidence for continental subduction during india asia convergence metasediments in the tso morari area ladakh himalaya provide new insights into the higher himalayan metamorphism in the northwestern part of the himalayan belt whole rock analyses and petrologic observations show that the metasediments correspond to fe rich metapelites mg rich metapelites intermediate metapelites and metagreywackes of the indian continental margin jadeite chloritoid paragonite garnet in the fe rich metapelites indicate pressures of 20 2 kbar at temperatures of 550 50 c according to major element partitioning thermobarometry stability fields of minerals and thermocalc p t estimates these results are consistent with p t estimates on other metasediments and with the occurrence of eclogites subsequent retrogression at the eclogite blueschist facies transition from 18 to 13 kbar and 540 50 c was followed by an increase in temperature to 630 30 c at amphibolite facies conditions the metamorphic evolution is related to subduction of the indian continental margin beneath the southern asian margin at the onset of the indian eurasian collision

depth of the decoupling plate interface and thermal structure under arcs subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate at depth below this mechanically decoupling plate interface the subducting plate couples with the overlying mantle and flow is induced in the mantle wedge this induced flow is the main factor that controls thermal structure under arcs depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern japan arc using a two dimensional numerical simulation of thermal structure the estimated depth is about 70 km the termination depth of thrust earthquakes agrees well with the depth of the coupling transition which indicates that these earthquakes occur at the decoupling plate interface the mechanism of this decoupling plate interface is discussed in this paper it is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2 0 gpa and have little temperature dependence termination depths of thrust earthquakes estimated from seismic studies are nearly the same 60 80 km for various arcs this indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust across arc heat flow profiles observed in various subduction zones show similar features heat flow is very low in the forearc region and increases steeply at the volcanic front considering that the depth of the slab is about 100 km at the volcanic front for various arcs this heat flow feature is closely related to the depth of the slab thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust

the kapanbogazi formation a key unit for understanding late cretaceous evolution of the pontides n turkey abstract the pontides forming the south western continental margin of the black sea consist of two tectonic units the istanbul zone in the west and the sakarya zone in the central and eastern parts the sinop basin in the sakarya zone is filled from base to top by hauterivian to albian turbidites cenomanian turonian red pelagic sediments turonian campanian magmatic arc and related deposits and by the uppermost campanian to middle eocene post magmatic units developed on the southern passive margin of the black sea based on nannofossil dinoflagellate foraminifera and radiolaria data we describe the kapanbogazi formation a cenomanian turonian unit in the sinop basin represented by red calcareous siliceous pelagic shales limestones and cherts passing gradually from the albian black shales these sediments possibly represent deepest depositional conditions of the basin during the cenomanian turonian interval and also reflect the transition from an anoxic to an oxic palaeoenvironmental setting the istanbul zone to the west was emerged during the deposition of the kapanbogazi formation in the sakarya zone in the pontides red pelagic sediments were deposited at different times during the cenomanian maastrichtian interval because the kapanbogazi formation was deposited only in the sakarya zone and because it is present in limited outcrops due to structural reorganization and thick overlying volcanoclastic pile most previous authors assumed cenomanian turonian hiatus herein we describe detailed palaeontological data from this unit and discuss their importance to the interpretation of depositional history and tectonics of the black sea region as well as climatic and eustatic implications

the formation of high temperature clay minerals from basalt alteration during hydrothermal discharge on the east pacific rise axis at 21 n abstract aluminous high temperature clay minerals form from alteration of tholeiitic basaltic glass and calcic plagioclase during hydrothermal venting on the crest of the east pacific rise at 21 n the clay alteration assemblages are layered crusts up to 1 mm thick completely replacing glass and calcic plagioclase adjacent to surfaces exposed to hydrothermal fluids the interiors of the affected basalt samples have unaltered appearances and oxygen isotopic compositions just slightly heavier than that of morb the surficial alteration crusts are mixtures of beidellitic smectite aluminous dioctahedral randomly interstratified mixed layer al rich chlorite smectite minor chlorite an x ray amorphous aluminosilicate material and possible minor serpentine amesite a 18 o value of 4 1 0 2 smow is determined for the beidellitic smectite assuming that this smectite equilibrated with hydrothermal fluid having an oxygen isotope value between that of seawater 0 and 350 c hydrothermal fluid from epr 21 n vents 1 6 an equilibration temperature between 290 c and 360 c is calculated for the beidellitic smectite this is substantially higher than any previously reported temperature for an oceanic smectite the mixed layer al rich chlorite smectite has a 18 o value of 3 5 which corresponds to equilibration at 295 360 c the aluminous composition of the alteration assemblage is uncommon for clay minerals produced by submarine hydrothermal basalt alteration we suggest that this assemblage is largely the product of high temperature interaction between basalt glass plagioclase and mg poor acidic hydrothermal fluids with possibly some contribution of mg from bottom seawater and that the aluminous clays either incorporate al 3 remobilized from basalt by lowph hydrothermal fluids or are residual phases remaining after intense alteration of basaltic glass plagioclase

garnet chloritoid equilibria in eclogitic pelitic rocks from the sesia zone western alps their bearing on phase relations in high pressure metapelites abstract a detailed study of garnet chloritoid micaschists fom the sesia zone western alps is used to constrain phase relations in high pressure hp metapelitic rocks in addition to quartz phengite paragonite and rutile the micaschists display two distinct parageneses namely garnet chloritoid chlorite and garnet chloritoid kyanite talc has never been observed garnet and chloritoid are more magnesian when chlorite is present instead of kyanite the distinction of the two equilibria results from different bulk rock chemistries not from p t conditions or redox state estimated p t conditions for the eclogitic metamorphism are 550 600 c 15 18 kbar the presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible afm topologies for the sesia metapelites the paper describes a kfmash multisystem for hp pelitic rocks which extends the grid of harte hudson 1979 towards higher pressures and adds the phase talc observed parageneses in hp metapelites are consistent with predicted phase relations critical associations are gt ctd chl and gt ctd ky at relatively low temperatures and gl chl ky and gt tc ky at relatively high temperatures

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

jurassic arc volcanism on crimea ukraine implications for the paleo subduction zone configuration of the black sea region article i nfo the early cretaceous and younger opening of the black sea has obliterated much of the older record of tethyan subduction below southeastern europe the earlier mesozoic evolution was dominated by opening and closure of tethyan oceans between gondwana and laurasia with their consumption at least in part accommodated along the southern eurasian margin crimea ukraine a peninsula in the northern black sea represents the northernmost region of southeastern europe that exposes a record of a pre cretaceous tethyan active margin to shed new light on the paleosubduction zone configuration of the southeastern european margin in the jurassic we report 40

slab surface temperature in subduction zones influence of the interplate decoupling depth and upper plate thinning processes abstract the thermal state of the top of the subducting plate is strongly affected by the mantle wedge flow structure we identify three main factors controlling the influence of the corner flow on the slab surface temperature 1 the mantle rheology 2 the interplate decoupling depth and 3 the intensity of thermomechanical ablation of the overriding plate the first two factors are discussed using results from published simulations we perform 2d numerical simulations of mantle wedge flow to assess the role of the third factor the non newtonian rheology depends on temperature pressure yield stress and composition the thermochemical convection code includes a water transfer model and the water weakening effect on mantle rocks slab dehydration triggers water saturation of the overlying mantle wedge and upper plate when weakening of mantle rocks by hydration is included in the simulations the interplate decoupling depth decreases and the upper plate thermal thinning increases a relatively cold blanket develops on top of the slab below the interplate decoupling depth cold materials removed from the thinned upper plate are advected by the corner flow and are accreted to the viscous layer dragged along the slab surface as a result the thickness of the insulating layer covering the slab surface increases with the water weakening effect together with the upper plate thermal thinning rate pressure temperature p t paths followed by crustal rocks during subduction quantify the influence of upper plate thinning processes on the slab surface thermal state the slab surface temperature below 100 km can be lowered by as much as 130 c due to increased thermal convection at the base of the upper plate at shallower depths

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

late cretaceous infant intra oceanic arc volcanism the central pontides turkey petrogenetic and tectonic implications abstract a tectonic slice of an arc sequence consisting of low grade metavolcanic rocks and overlying metasedimentary succession is exposed in the central pontides north of the izmir ankara erzincan suture separating laurasia from gondwana derived terranes the metavolcanic rocks mainly consist of basaltic andesite andesite and mafic cognate xenolith bearing rhyolite with their pyroclastic equivalents which are interbedded with recrystallized pelagic limestone and chert the metasedimentary succession comprises recrystallized micritic limestone with rare volcanogenic metaclastic rocks and stratigraphically overlies the metavolcanic rocks the geochemistry of the metavolcanic rocks indicates an arc setting evidenced by depletion of hfse ti p and nb and enrichment of fluid mobile lile identical trace and rare earth elements compositions of basaltic andesites andesites and rhyolites suggest that they are cogenetic and derived from a common parental magma the arc sequence crops out between an albian turonian subduction accretionary complex representing the laurasian active margin and an ophiolitic melange absence of continent derived detritus in the arc sequence and its tectonic setting in a wide cretaceous accretionary complex suggest that the kosdag arc was intra oceanic zircons from two metarhyolite samples give late cretaceous 93 8 1 9 and 94 4 1 9 ma u pb ages these ages are the same as the age of the supra subduction ophiolites in western turkey which implies that that the kosdag arc may represent part of the incipient arc formed during the generation of the supra subduction ophiolites the low grade regional metamorphism in the kosdag arc is constrained to 69 9 0 4 ma by 40 ar 39 ar muscovite dating indicating that the arc sequence became part of a wide tethyan cretaceous accretionary complex by the latest cretaceous non collisional cessation of the arc volcanism is possibly associated with southward migration of the magmatism as in the izu bonin mariana arc system

lower palaeozoic stratigraphy and palaeontology karadere zirze area pontus mountains northern turkey lower palaeozoic rocks in the karadere zirze area east of safranbolu pontides northern turkey range from early ordovician to silurian overlying the probably tremadoc bakacak formation are aydos formation quartzites followed conformably by the karadere formation dated as early arenig to early llanvirn by means of graptolites which are assigned to seventeen genera and include three new forms eoglyptograptus bouceki prolasiograptus haplus praecursor and undulograptus mui late arenig trilobites from the karadere formation include bergamia cyclopyge dionidella leioshumardia and seleneceme in the limestone member of the overlying ketencikdere formation uncommon trilobites suggest only a mid to late ordovician age but con odonts with colour alteration index 5 6 indicate the amorphognathus tvaerensis biozone early caradoc macrofossils are rare in the siltstone member but conodonts from the middle of the unit suggest the highest subzone of the a tvaerensis biozone the youngest visible strata are on acritarch evidence at least as high as caradoc but the ashgill is not confirmed and the contact with overlying silurian rocks is unexposed the findikli formation comprises a lower member black argillites with llandovery graptolites and acritarchs and an upper member grey shales with late wenlock graptolites overlain unconformably by devonian rocks the succession differs significantly from contemporaneous deposits in southern turkey and its affinities lie with western europe including the welsh basin

geochronology of the karadere basement nw turkey and implications for the geological evolution of the istanbul zone earlier geological work in the istanbul zone western pontide tectonic belt has revealed the presence of extensive basement outcrops exposed underneath palaeozoic and mesozoic to tertiary cover sequences the basement of suspected neoproterozoic age plays an important role in understanding the crustal accretion process in nw turkey we report the first results of a detailed pb pb and u pb zircon study complemented by nd sr whole rock and mineral data from basement rocks exposed in the karadere valley safranbolu area five samples were selected for this study comprising three metagranitoids and two metasediments zircon geochronology indicates that the metagranitoids were formed during late proterozoic pan african magmatic events between 590 and 560 ma the rocks are of tonalitic and granitic composition and have low nb y ratios and ti contents consistent with those of arc rocks a continental arc setting is supported by their sr and nd isotope data that indicate a contribution of a mantle source as well as crustal assimilation during magma genesis the metasediments can clearly be distinguished from the metagranitoids by their higher 87sr 86sr ratios and lower end values at 580 ma which supports the suggestion that the arc was underlain by mature continental crust zircons from the metasediments yield a range of pb pb ages between 1 860 and 710 ma thirty per cent of them fall between 890 and 710 ma possibly suggesting a derivation from gondwana afro arabian regions a sm nd garnet whole rock analysis obtained on a metagranite gives an age of 559 8 ma which either reflects pre metamorphic magmatic growth of garnet in a felsic melt or a syntectonic high temperature metamorphic event uplift and cooling of the basement is further constrained by rb sr biotite ages of 548 545 ma these lower cambrian mineral ages demonstrate that the istanbul zone was not thermally reactivated during the hercynian cimmerian or alpine orogeny in contrast to its neighbouring tectonic zones confirming its role as a suspect terrane in the modern western pontide tectonic belt

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

late jurassic amphibolite facies metamorphism in the intra pontide suture zone turkey an eastward extension of the vardar ocean from the balkans into anatolia the palaeogeography of neotethys during its closure is still a matter of debate this study provides new insights into the neotethys closure by the discovery in the intra pontide suture zone turkey of an accretionary complex that recorded a late jurassic c 163 ma amphibolite facies metamorphism results are discussed in the framework of the accretionary events that occurred at the southern margin of laurasia the resulting picture supports a new model in which the vardar suture zone in the balkans and the intra pontide suture zone in anatolia represent remnants of the same elongate oceanic basin supplementary material analytical procedures representative mineral compositions geothermobarometric estimates step heating and in situ 40 ar 39 ar data and backscattered electron photomicrographs showing the distribution of laser spot ages on rock chips are available at www geolsoc org uk sup18759

modelling the interplate domain in thermo mechanical simulations of subduction critical effects of resolution and rheology and consequences on wet mantle melting abstract the present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate a 2d thermo mechanical model is used to simulate a subduction channel made of oceanic crust free to evolve convergence at constant rate is imposed under a 100 km thick upper plate pseudo brittle and non newtonian behaviours are modelled the influence of the subduction channel strength parameterized by the difference in activation energy between crust and mantle e a is investigated to examine in detail the variations in depth of the subduction plane down dip extent z coup first simulations show that numerical resolution may be responsible for an artificial and significant shallowing of z coup if the weak crustal layer is not correctly resolved second if the age of the subducting plate is 100 myr subduction occurs for any e a the stiffer the crust is that is the lower e a is the shallower z coup is 60 km depth if e a 20 kj mol and the hotter the fore arc base is conversely imposing a very weak subduction channel e a 135 j mol leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions partial kinematic coupling at the fore arc base occurs if e a 145 kj mol if the incoming plate is 20 myr old subduction can occur under the conditions that the crust is either stiff and denser than the mantle or weak and buoyant in the latter condition cold crust plumes rise from the subduction channel and ascend through the upper lithosphere triggering 1 partial kinematic coupling under the fore arc 2 fore arc lithosphere cooling and 3 partial or complete hindrance of wet mantle melting z coup then ranges from 50 to more than 250 km depth and is time dependent if crust plumes form finally subduction plane dynamics is intimately linked to the regime of subduction induced corner flow two different intervals of e a are underlined 80 120 kj mol to reproduce the range of slab surface temperature inferred from geothermometry and 10 40 kj mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near primary arc magmas and seismic tomographies therefore an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations a mantle viscosity reduction by a factor 4 20 caused by metasomatism in the mantle wedge is proposed from these results i conclude that the subduction channel down dip extent z coup should depend on the subduction setting to be consistent with the observed variability of sub arc depths of the subducting plate surface

the wister mud pot lineament southeastward extension or abandoned strand of the san andreas fault we present the results of a survey of mud pots in the wister unit of the imperial wildlife area thirty three mud pots pot clusters or related geothermal vents hundreds of pots in all were identified and most were found to cluster along a northwest trending line that is more or less coincident with the postulated sand hills fault an extrapolation of the trace of the san andreas fault southeastward from its accepted terminus north of bombay beach very nearly coincides with the mud pot lineament and may represent a surface manifestation of the san andreas fault southeast of the salton sea additionally a recent survey of vents near mullet island in the salton sea revealed eight areas along a northwest striking line where gas was bubbling up through the water and in two cases hot mud and water were being violently ejected

triassic limestone turbidites and serpentinite the cimmeride orogeny in the central pontides the basement of the central pontides and by implication that of crimea consists of pre permian low grade metaclastic rocks intruded by latest permian early carboniferous 305 290 ma granitoids further up in the stratigraphic sequence are triassic limestones which are now preserved as olistoliths in the deformed upper triassic turbidites new conodont and foraminifera data indicate an anisian to carnian middle to late triassic age for these hemi pelagic hallstatt type limestones the siliciclastic turbidites surrounding the triassic limestone contain the norian late triassic bivalve monotis salinaria the same species is also found in the tauric series in crimea the upper triassic flysch in the central pontides is locally underlain by basaltic pillow lavas and includes kilometre size tectonic slices of serpentinite both the flysch and the serpentinite are cut by an undeformed acidic intrusion with an ar ar biotite age of 162 4 ma callovian oxfordian this indicates that the serpentinite was emplaced into the turbidites before middle jurassic time most probably during latest triassic or early jurassic time and that the deformation of the triassic sequence pre dates the middle jurassic regional geological data from the circum black sea region including widespread upper triassic flysch upper triassic eclogites and blueschists of oceanic crustal affinity and apparent absence of a cimmerian continent between the cretaceous and triassic accretionary complexes indicate that the latest triassic cimmeride orogeny was accretionary rather than collisional and is probably related to the collision and accretion of an oceanic plateau to the southern active margin of laurasia

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses