mid ocean ridge and island arc affinities in ophiolites from iran palaeographic implications complementary reference abstract fifty six samples of ophiolitic rocks from various late cretaceous eocene suture zones in iran have been analyzed for major and trace elements in order to assess their petrogenetic affinity and to determine their original tectonic setting in both southern baluchestan i e between the makran range and the lut block the fanuj maskutan sequence and northeastern baluchestan i e in the east iranian ranges the tchehel kureh sequence the ophiolitic complexes are well developed the rocks display affinities with mid ocean ridge tholeiites the basaltic rocks are comparable to respectively transitional fanuj maskutan and normal tchehel kureh morb in contrast the sequence along the zagros the sahneh sequence appears to have formed in an island arc setting in iranshar i e in southeastern baluchestan ophiolites are dismembered into a melange which includes cumulate intrusives similar to those from fanuj maskutan and lavas with island arc tholeiitic affinity a similar island arc origin may be inferred for the sabzevar ophiolitic sequence north of the lut block studied by other authors the chemical data thus help to elucidate the development of the jigsaw puzzle of microplates of gondwanian origin that during the mesozoic were intermediate in position between afro arabia india and eurasia and that now constitute the middle east ophiolite obduction in iran occurred during arc basin continent or ocean continent collisions in the late cretaceous early paleogene largely predating the final suturing between arabia and india with eurasia to which the intermediate microplates were already attached which did not take place before the neogene

strong biological controls on sr ca ratios in aragonitic marine bivalve shells 1 it is well known that skeletal remains of carbonate secreting organisms can provide a wealth of information about past environments sr ca ratios have been successfully used as a temperature proxy in corals and sclerosponges previous work on aragonitic bivalve shells has not been conclusive but suggests a major control of growth rate on sr ca ratios as many studies have used bivalve growth rates to determine temperature we tested if sr ca ratios could predict temperature through its relationship with growth rate shells from the two species of clams from the same family veneroidea studied here saxidomus giganteus and mercenaria mercenaria show vastly different seasonal sr ca profiles a strong relationship between average annual sr ca ratios and annual growth rate was found in s giganteus shells from both washington r2 0 87 and alaska r2 0 64 usa but not in m mercenaria shells from north carolina usa furthermore the sr ca growth rate relationship was also evident upon a more detailed inspection of subannual growth rates in s giganteus r2 0 73 although there were significant positive correlations between sr ca ratios and temperature in s giganteus shells the correlations were weak 0 09 r2 0 27 and thus sr ca ratios cannot be used as a reliable temperature proxy in these species of aragonitic bivalves it is clear from this study that sr ca ratios are not under thermodynamic control in either clam species since thermodynamics predict a negative correlation between sr ca ratios and temperature in aragonite this points toward dominance of biological processes in the regulation of sr2 this is also reflected by the largely differing sr ca partition coefficients dsr in these shells dsr 0 25 when compared to inorganic coral and sclerosponge studies dsr 1 all of which show a negative dependence of sr ca on temperature we suggest that caution be taken when using sr ca in any biogenic aragonite as a temperature proxy when the dsr greatly deviates from one as this indicates the dominance of biological controls on sr ca ratios

seismotectonic and seismicity of makran a bimodal subduction zone se iran abstract the present study combined spatial distribution and mechanism of seismicity seismicity parameters geomorphology in satellite imagery and tectonic features in makran inland to show that the makran low angle thrust fault is divided into two separate segments depth dispersal of earthquakes especially relocated events shows that the sinking plate is steeper in the west than in the east of makran significant facts were identified from the variation of moho depth between west and east slip rates of the inland faults and crustal shortenings in the north of makran the sistan suture zone plays a significant role in dividing the overriding plate into a non deforming helmand block and a less deforming lut block these blocks have different resistances against the northward subduction large and great events such as the 1989 chabahar mw 5 5 and 1945 gwadur mw 8 1 occurred in the south of subduction fronts of the western and eastern makran respectively they occurred in shallow depth and were the pieces of evidence of weak and strong coupling between the oceanic and continental crusts on a low angle and a very low angle reverse faults respectively spectral analysis and depth determination of the instrumental and pre instrumental earthquakes respectively helped to separate some plate boundary and intraplate events this separation geometry and non characteristic behavior of the subduction zone revealed from spatial and temporal distribution of the earthquakes probably dedicate a mature aleutian type for the subduction zone in the east missing great m 6 1 earthquakes less slip rate of the continental faults and steeper dipping of the sinking plate in the west probably indicate a kurile type for the iranian part of subduction makran is a bimodal subduction because geomorphological and seismic pieces of evidence indicate a nnw surficial and right lateral fracture zone in the continental crust and a nw deep and left lateral fracture zone in the oceanic crust respectively the fractures and geometry of subductions showed that the eastern makran has seismotectonically a wider area for potential coupling of a larger earthquake than the western makran

metamorphism at the base of the samail ophiolite southeastern oman mountains metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies occur beneath the samail ophiolite the metamorphic protolith was a section of marine basalts mn rich cherts and argillites garnet clinopyroxene amphibolites occur within 2 m of the contact hornblende clinopyroxene assemblages occur within 80 m of the contact and lower amphibolite facies rocks persist to 130 m from the contact the higher grade rocks show polyphase deformation cataclastic fabrics and partial retrogression to greenschist facies assemblages however lower grade phyllites show evidence of only one episode of metamorphic crystallization unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation the distribution coefficient kd feo mgo gar feo mgo cpx ranges from 4 4 to 5 3 suggesting a t range of 755 865 c at 200 mpa low jadeite content in clinopyroxene and low glaucophane content of ca amphiboles are consistent with moderate pressures of crystallization the metamorphic sheet is inferred to be of composite origin the highest grade rocks were tectonically transported the greatest distance and the lower grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks residual heat from the ophiolite is the dominant heat source for the metamorphism and this could have been augmented by a limited amount of frictional heat generated during thrusting these thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the samail ophiolite

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

plagiogranite magmatism in the mesozoic island arc structure of the pekulney ridge chukotka peninsula ne russia tectonic setting geochemistry and petrogenesis of low k granitoid magmatism in the fore arc of late jurassic early cretaceous pekulney zolotogorskaya fossil arc are addressed using a case study consisting in two types of neocomian plagiogranite complexes from the eastern pekulney ridge central chukotka these plagiogranites outcrop as small dyke like bodies and are confined to a zone of strong deformation related to an olistostrome development at the boundary between allochthonous middle jurassic early cretaceous oceanic volcanics and cherts and parautochthonous late jurassic early cretaceous island arc complexes the geochemical features of plagiogranites hfse depletion and lile enrichment nb and ta negative anomalies in ocean ridge granite org normalized plots and rb vs nb y and zr vs nb zr n correlations indicate that these rocks are likely to have originated in suprasubduction zone setting the parameters of tectonic structure and the morphology of the pekulney fossil arc segment in neocomian time structural position of plagiogranites and their ree patterns militate in favour of the following models for plagiogranite origin 1 migration of tholeiite basalt melts from mantle wedge into the fore arc of the island arc and their subsequent fractionation 2 partial melting of metabasite crustal source in the fore arc and subsequent migration of felsic melts geochemical modelling shows that one type of plagiogranites could have originated by 10 15 partial melting of metabasite source in a metabasite gabbroic complex of pre late mesozoic basement of the island arc pile and that the other type could be generated by progressive fractional crystallization of low k tholeiite from the late jurassic early cretaceous island arc complex

dating the subduction of the arabian continental margin beneath the semail ophiolite oman the presence of high pressure p eclogites along the northeast margin of the arabian continental plate beneath the semail ophiolite demonstrates that subduction of thinned continental crust to pressures of 20 kbar 80 km depth has occurred current debate centers on whether this high p metamorphism occurred prior to during or following ophiolite emplacement which is known to have started ca 95 93 ma five concordant u pb zircon ages from a garnet clinopyroxene phengite crossite eclogite from as sifah northeast oman precisely constrain the age of high p metamorphism at 79 1 0 3 ma this age demonstrates that high p metamorphism occurred during continental subduction 15 m y after formation of the ocean floor that subsequently became the semail ophiolite

modification of polar low development by orography and sea ice abstractthe life cycles of intense high latitude mesoscale cyclones and polar lows are strongly shaped by their ambient environments this study focuses on the influence of the orography of svalbar

sr isotopic tracer study of the samail ophiolite oman we have measured rb and sr concentrations and sr isotopic compositions in 41 whole rock samples and 12 mineral separates from units of the samail ophiolite including peridotite gabbro plagiogranite diabase dikes and gabbro and websterite dikes within the metamorphic peridotite ten samples of cumulate gabbro from the wadir kadir section and nine samples from the wadi khafifah section have mean 87sr 86sr ratios and standard deviations of 0 70314 0 00030 and 0 70306 0 00034 respectively the dispersion in sr isotopic composition may reflect real heterogeneities in the magma source region the average sr isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern midocean ridge basalt the 87sr 86sr ratios of noncumulate gabbro plagiogranite and diabase dikes range from 0 7034 to 0 7047 0 7038 to 0 7046 and 0 7037 to 0 7061 respectively these higher 87sr 86sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater mineral separates from dikes that cut harzburgite tectonite have sr isotopic compositions which agree with that of cumulate gabbro these data indicate that the cumulate gabbro and the different dikes were derived from partial melting of source regions that had similar long term histories and chemical compositions

the berit transect of the tauride thrust belt s turkey late cretaceous early cenozoic accretionary collisional processes related to closure of the southern neotethys abstract we report on an integrated study of a critical segment of the tauride thrust belt in se turkey berit area units related to two sutures are exposed in the northerly and southerly parts of the area studied in the north the upper cretaceous binboga melange reflects the closure of a northerly mesozoic oceanic basin in the south contrasting units document the closure of the southern neotethys in the south the berit or goksun ophiolite formed as an incipient oceanic arc within the southern neotethys during the late cretaceous c 90 ma intercalated amphibolite granulite facies rocks may record a dismembered metamorphic sole an underlying meta sedimentary meta volcanic unit is interpreted as accreted continental rift and oceanic seamount lithologies the berit ophiolite was accreted to the tauride active continental margin to the north around 85 ma and was then intruded by calc alkaline granites c 70 85 ma northerly margin units were exhumed and covered by shelf carbonate sediments during latest cretaceous early cenozoic time northward subduction during the mid cenozoic c 45 ma resulted in back arc extension along the tauride active margin and subduction influenced volcanism possibly controlled by slab roll back during oligocene early miocene time the southern neotethys closed and the tauride active margin was telescoped to create the present south vergent thrust stack this was accompanied by mass wasting of the advancing allochthon to form sub aqueous frontal olistostromes the arabian platform was over ridden by mid miocene time followed by left lateral strike slip along the east anatolian fault zone as anatolia began to escape westwards towards the aegean sea

southern ocean mesocyclones and polar lows from manually tracked satellite mosaics a new reference dataset of mesocyclone activity over the southern ocean has been developed from the manual analysis of high resolution infrared satellite mosaics for winter 2004 of the total 1735 mesocyclones which were identified and analyzed about three quarters were classified as being polar lows i e intense systems see rasmussen and turner 2003 the dataset includes mesocyclone track size associated cloud vortex type and background synoptic conditions maxima in track density were observed over the bellingshausen sea and around east antarctica and are highly correlated with cyclogenesis regions a comparison against quikscat and reanalyses wind characteristics shows that the reanalyses while capturing mesocyclone events tend to considerably underestimate their wind speed by up to 10 ms 1 this mesocyclone dataset is available as a reference for further analysis of mesocyclones and for the evaluation and development of cyclone tracking algorithms

trondhjemite and metamorphosed quartz keratophyre tuff of the ammonoosuc volcanics ordovician western new hampshire and adjacent vermont and massachusetts the ammonoosuc volcanics and equivalent rocks of ordovician age are exposed in the oliverian domes along the bronson hill anti clinorium bha between northern new hampshire and southern connecticut in western new hampshire and adjacent vermont and massachusetts the ammonoosuc lithology consists of a lower mainly mafic unit of homblende plagioclase amphibolite and an upper mainly felsic metamorphosed quartz keratophyre tuff these lithologies are locally interlayered and both are intruded by sills dikes and plugs of trondhjemite trondhjemite also constitutes the interior gneissic core of several small domes or plutons the trondhjemite is highly siliceous sio 2 73 81 low in a1 2 o 3 11 3 13 5 generally contains 2 o and thus resembles some trondhjemites in island arc or continental margin settings chemical trends of both trondhjemite and ammonoosuc volcanics felsic and mafic are essentially calc alkaline variations in both major and trace elements of trondhjemites in several of the domes suggest several somewhat different sources along the bha overall however the major and minor element chemistry of the trondhjemites is closely similar to that of the ammonoosuc quartz keratophyre tuff these rocks could have been produced either by partial melting or by fractional crystallization of basaltic source rocks the partial melting model is preferred because of the largely bimodal basalt quartz keratophyre ammonoosuc assemblage in which andesitic and other intermediate compositions are virtually lacking the relatively thin ammonoosuc section appears to preclude generation of trondhjemite at the presently exposed base of an island arc as has been postulated for very similar trondhjemite amphibolite assemblages twillingate trondhjemite little port complex in newfoundland instead generation of the felsic ammonoosuc rocks more likely occurred at deeper levels along a subduction zone dipping eastward under the bha as postulated in current plate tectonic models the close juxtaposition in space and time of sialic crust and ammonoosuc volcanics may explain the calc alkaline trends of the latter and suggests a paleotectonic environment of convergent oceanic continental plate margins possibly with significant crustal shortening across the arc

genesis and development mechanisms of a polar mesocyclone over the japan sea abstracta polar mesocyclone pmc observed over the japan sea on 30 december 2010 was studied using a nonhydrostatic mesoscale numerical model with a horizontal resolution of 2 km the numerical simulation successfully reproduced the observed life cycle of the pmc the results of the numerical simulation suggest that the life cycle of the pmc may be divided into three stages an early development stage in which a number of small vortices appear in a shear zone a late development stage which is characterized by the merger of vortices and the formation of a few larger vortices and a mature stage in which only a single pmc is present during the early development stage vortices are generated in the shear zones of strong updrafts in discrete cumulus convection cells in contrast during the late development stage the vortices develop as a result of barotropic instability in the shear zone a cloud free eye and spiral cloud bands accompany the mature stage of a simulated pmc a warm core structure also

the role of vortical hot towers in the formation of tropical cyclone diana 1984 abstract a high resolution 3 km horizontal grid spacing near cloud resolving numerical simulation of the formation of hurricane diana 1984 is used to examine the contribution of deep convective processes to tropical cyclone formation this study is focused on the 3 km horizontal grid spacing simulation because this simulation was previously found to furnish an accurate forecast of the later stages of the observed storm life cycle the numerical simulation reveals the presence of vortical hot towers or cores of deep cumulonimbus convection possessing strong vertical vorticity that arise from buoyancy induced stretching of local absolute vertical vorticity in a vorticity rich prehurricane environment at near cloud resolving scales these vortical hot towers are the preferred mode of convection they are demonstrated to be the most important influence to the formation of the tropical storm via a two stage evolutionary process i preconditioning of the local environment via diabatic production of mul

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

geologic section through the samail ophiolite and associated rocks along a muscat ibra transect southeastern oman mountains regional mapping at a 1 60 000 scale of a 30 km strip from the gulf of oman muscat across the oman mountains 130 km to the south provides the geologic setting for the 95 m y ibra section of the samail ophiolite where best preserved the ibra ophiolite section is an 8 km thick section of oceanic consisting of 0 5 km of pillow lavas 1 2 1 6 km of sheeted diabase dike complex 0 2 1 0 km of high level noncumulate gabbro and 3 0 5 0 km of cumulate gabbro that is underlain by tectonite peridotite 9 12 km thick the ibra section is found on the southward dipping limb of the sayah hatat antiform the tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high temperature low stress asthenospheric flow discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite the structural base of the tectonite peridotite is overprinted by a high stress low temperature deformation that can be related to its oceanic detachment the layered gabbros are predominantly olivine clinopyroxene plagioclase cumulates and orthopyroxene does not occur as a cumulus phase occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber high level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes both the diabase dike complex and pillow lavas are hydrothermally altered and alteration and metamorphism increase downward zeolite to epidote amphibolite facies in spite of pervasive alteration relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros the present day samail thrust surface truncates ophiolite stratigraphy and puts the samail ophiolite on top of unmetamorphosed hawasina melange the last motion on this surface was probably no older than maestrichtian 70 65 m y garnet amphibolites exposed as remnants of earlier thrusting 90 m y record initial ophiolite detachment at a 14 20 km depth within the tethyan oceanic lithosphere the hawasina group which underlays the samail ophiolite is a block melange where exposed near muscat in the north and it grades into an imbricated broken formation at the southern limit of the map area the hawasina group is thrust over permian to late cretaceous shelf carbonates that represent autochthonous arabian continental shelf deposits recent post miocene collapse and dome structures such as the ibra dome have complicated ophiolite stratigraphy south of jabal dimh our geologic studies strongly indicate the samail ophiolite represents a large coherent slab of transported oceanic lithosphere formed at a late cretaceous spreading center in the tethyan sea

k ar ages of metamorphic rocks at the base of the samail ophiolite oman hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the samail ophiolite and phyllites farther from the peridotite contact have weighted mean sup 40 ar sup 39 ar total fusion ages of 90 0 3 0 m y and 79 5 3 0 m y respectively the amphibolities represent the first tectonic slice welded to the base of the samail ophiolite after it was detached from the tethyan oceanic crust formation of the amphiobolities occurred no more than 3 to 7 m y after crystallization of plagiogranite in the ophiolite the phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the tethyan spreading axis the k ar ages suggest assuming a half spreading rate of 2 to 5 cm yr that detachment of the samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center using the same spreading rate one can calculate a minimum half width of 300 to 750 km for the tethyan ocean during the late cretaceous

tectonic evolution of the northern oman mountains part of the strait of hormuz syntaxis new structural and paleothermal analyses and u pb dating of synkinematic calcite

age and petrogenesis of na rich felsic rocks in western iran evidence for closure of the southern branch of the neo tethys in the late cretaceous abstract intermediate to felsic volcanic and granitic rocks with high na2o concentrations 5 2 9 1 wt are widely distributed in the harsin area along the zagros thrust zone in western iran most of these rocks are classified as low potassium tholeiite display affinity with oceanic plagiogranite and contain somewhat high na content and low concentrations of k2o al2o3 rb sr and ca thus we prefer to apply the term na rich felsic rocks to this complex u pb dating yielded ages of 94 6 2 7 ma 2 from baddeleyite and 95 0 2 4 ma 2 from zircon grains indicating that the complex crystallized in the late cretaceous based on the mineral compositions the crystallization occurred at low pressures mostly

age origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the piedmont ligurian ocean basin abstract u pb zircon dating sr nd isotope tracing and major trace re element analyses were performed to constrain the age origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the ligurian alps and the northern apennines in addition a host fe diorite was investigated samples from the ligurian alps were collected from the voltri group and the sestri voltaggio zone whereas the plagiogranites from the northern apennines were taken in the bracco unit all these units have been affected by alpine metamorphism reaching eclogite facies in the voltri group blueschist degree in the sestri voltaggio samples and prehnite pumpellyite facies in the bracco unit which has additionally been affected by rodingitization u pb zircon ages of 150 1 153 1 and 156 ma were obtained respectively for two plagiogranites and the host fe diorite in the ligurian alps and an age of 153 1 ma was determined for the plagiogranite in northern apennines inherited components in zircon and initial pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1 7 2 1 ga old continental crust material ree patterns in both the plagiogranites and the host diorite are characterized by high ree abundance and moderate lree enrichment nd isotopic compositions lie in the range of n morb sources yielding initial epsilon nd values between 8 8 and 9 7 whereas sr is isotopically heterogeneous the geochemical pattern of the plagiogranites and the host fe diorite requires melting of a morb type mantle source that experienced lree enrichment shortly before melting the most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region the basaltic lree enriched parent magmas generated from this enriched domain have probably undergone up to about 72 of low pressure fractional crystallization prior to their emplacement into the gabbro peridotite complex the 156 150 ma magmatism occurred in close relation to normal faulting sedimentation of breccias and detachment of the mantle complex from its overlying continental crust followed by exposure on the ocean floor this tectono magmatic event in the ligurian alps and the northern apennines reflects rifting of the adriatic iberian continental plate segment preceding wider opening of the piedmont ligurian ocean basin and pillow basalt deposition

numerical study of horizontal shear instability waves along narrow cold frontal rainbands abstractthe effects of variations in low level ambient vertical shear and horizontal shear on the alongfront variability of narrow cold frontal rainbands ncfrs that propagate into neutral and slightly unstable environments are investigated through a series of idealized cloud resolving simulations in cases initialized with slightly unstable sounding and weak ambient cross frontal vertical shears core gap structures of precipitation along ncfrs occur that are associated with wavelike disturbances that derive their kinetic energy mainly from the mean local vertical shear and buoyancy however over a wide range of environmental conditions core gap structures of precipitation occur because of the development of a horizontal shear instability hsi wave along the ncfrs the growth rate and amplitude of the hsi wave decrease significantly as the vertical shear of the ambient cross front wind is reduced these decreases are a consequence of the enhancement of the low level local vertical shear immediately beh

climatology of polar mesocyclones over the sea of japan using a new objective tracking method abstractpolar mesocyclones pmcs are mesoscale cyclonic vortices that develop poleward of the main polar front this article reports on a new algorithm for the objective tracking of pmcs including meso scale vortices which will facilitate the study of their climatology the algorithm is based mainly on the vorticity field and consists of three parts the identification of vortices the connection of vortices at consecutive time steps and discrimination between pmcs and synoptic scale disturbances the objective tracking method was applied to mesoscale analysis ma data provided by the japan meteorological agency which has a horizontal resolution of 5 km the detected tracks of pmcs were confirmed by subjective analysis of the ma data and satellite images the method used here to discriminate between pmcs and synoptic scale disturbances differs from that used in previous studies which is based on the difference between the sea surface temperature and the temperature at 500 hpa but gives a consist

tectonic setting for the genesis of oceanic plagiogranites evidence from a paleo spreading structure in the oman ophiolite abstract in the oman ophiolite detailed structural and lithological mapping of the 60 45 km maqsad area has revealed several plagiogranite intrusions ranging from metre scale dykes to hectometre scale bodies these plagiogranites are spatially related to kilometre scale mafic plutons displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick isotope data demonstrate the mantle origin of these pegmatites and plagiogranites nd ranges from 6 3 to 9 9 and has an average value 26 samples of 8 1 within the field of present day morbs and similar to the average nd 7 8 of the oman ophiolite primary igneous sequence these intrusions are not restricted to a given structural level their paleovertical extent exceeds 5 6 km from 2 km below the moho up to the base of the sheeted dyke complex their modal composition evolves up section from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites diorites and quartz diorites at crustal level petrography whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt plagiogranites although more abundant at upper crustal level are not restricted to this horizon plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth from highly calcic 70 95 an in the mantle section intermediate and highly scattered in the lower crust 10 85 an to quite sodic at upper crustal level 5 20 an highly calcic plagioclases in quartz bearing rocks is a puzzling character inconsistent with fractional crystallization even if high degrees of fractional crystallization of a hydrated basaltic melt remain the simplest way to account for the chemistry of the upper crustal plagiogranites processes such as fluid induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites mantle and crustal structures in the maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting when a mantle diapir soaked with basaltic melt intruded a cool hydrothermally altered lithosphere they were eventually transposed in an off axis setting as the ascension of the diapir progressed up to moho level it is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid this is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time

boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the oman ophiolite abstract boron content and the isotopic composition of boron and oxygen were determined for a complete 5 3 km thick oceanic crust in the wadi fizh area of the oman ophiolite a fragment of cretaceous oceanic lithosphere obducted onto the arabian shield the depth profiles of the 18 o values and the secondary mineral assemblages are consistent with successively higher temperature hydrothermal alterations occurring with increasing depth the upper pillow basalts underwent low temperature alteration 18 o 10 the lower pillow basalts and upper sheeted dikes 18 o 6 to 10 the lowermost sheeted dike complex and upper gabbros underwent high temperature alteration at 300 450 c 18 o 450 c 18 o 18 o values compared with the adjacent rocks suggesting the production of 18 o enriched melt the boron content of the rocks in the oceanic crust decreases in both abundance and range with increasing stratigraphic depth 1 4 29 1 g g in pillow lava 7 9 g g on average 1 5 11 6 g g in sheeted dike complex 5 3 g g on average 1 6 5 0 g g in dolerite dike in gabbro 2 9 g g on average 0 25 3 8 g g in gabbro 1 3 g g on average considering an original boron content of 0 72 0 47 g g for basalt and 0 06 0 09 g g for gabbro boron from seawater was incorporated into the rocks through hydrothermal alteration even at temperatures higher than 300 c the 11 b values systematically increase with stratigraphic depth 1 1 to 11 9 in pillow lava 5 5 on average 1 1 to 17 5 in sheeted dike complex 6 3 on average 8 3 to 18 6 in dolerite dike in gabbro 13 9 on average 7 3 to 17 7 in gabbro 12 0 on average the whole rock 11 b values negatively correlate with the 18 o values suggesting that the 11 b values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids and the increase in the 11 b values is caused by a decrease in the rock fluid boron isotopic fractionation factor with increasing alteration temperatures the 11 b values estimated for hydrothermal fluids from rocks completely altered at 300 450 c range from 28 to 33 values indistinguishable from those of vent fluids observed at modern mid ocean ridges the boron content of the bulk oceanic crust of the oman ophiolite is estimated to be 3 6 g g and the 11 b value is estimated at 7 9 in contrast to previous views the hydrothermally altered gabbro section is a large boron sink accounting for 30 of the total boron in the oceanic crust with a high 11 b value of 13 this boron enriched high 11 b lower oceanic crust may impact estimates of the 11 b value of fluid liberated from the subducted oceanic slab which is believed to largely control the 11 b value of arc magma generated in the mantle wedge

investigation of the strength contrast at the moho a case study from the oman ophiolite we investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the oman ophiolite our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase rich lithology typical of the lower crust and an olivine dominated harzburgite we constrain deformation conditions using field relationships geothermometry grain size piezometry and electron backscatter diffraction the viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws these flow laws predict that a significant viscosity contrast exists at the crust mantle boundary where the crustal lithology is dominated by plagioclase or where deformation localizes within plagioclase rich layers

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

synthetic dual doppler analysis of a winter mesoscale vortex abstract this article presents a detailed examination of the kinematic structure and evolution of the 5 december 1997 winter mesoscale vortex in the vicinity of lake michigan using the synthetic dual doppler sdd technique when such a mesoscale event propagates a distance large enough that the viewing angle from a single doppler radar changes by about 30 and the circulation is sufficiently steady during this time period then the sdd method can reveal reliable details about the circulation one such detail of the observed vortex was a pattern of convergence and divergence associated with radial bands where heavier snowfall was located another was the steadiness and vertical coherence of the derived vorticity and convergence patterns within the cyclonic circulation on 5 december 1997 the observed reflectivity field remained remarkably steady for nearly 2 5 h as the vortex moved southeastward allowing for the application of the sdd technique the reflectivity field exhibited a pronounced asymmetric c

biostratigraphic zonation and 40ar 39ar ages for the neotethyan khoy ophiolite of nw iran abstract the khoy ophiolite in northwestern iran represents a piece of oceanic lithosphere formed in the mesozoic neotethys this ophiolite consists from bottom to top of well defined basal metamorphic rocks peridotites gabbros and a variety of upper to lower cretaceous deep marine rocks these include pelagic fossiliferous carbonates and basalts as interlayers or exotic blocks four hornblende separates from gabbros and amphibolites give values from 108 to 111 ma the results from microfossil studies show the presence of late cretaceous e g marginotruncana globotruncana and middle to late eocene e g globorotalia orbulonoides planktonic foraminifera the data suggest that although the initial rifting and crystallization of the oceanic crust of the khoy ophiolite began in the late albian the volcanic activities continued through to the late eocene time another possibility for such variation of lithofacies and biofacies and disturbances in the fossil record may be the result of bioturbation and transportation and rapid changes of depositional conditions during eruption of the basaltic lava flows pelagic sediments alone have been listed and of the foraminiferas the first three are planktonic as would be expected only orbulonoides is a benthic form and aberrant the third possibility is that the khoy ophiolite may represent a classic supra subduction type ophiolite where volcanic members of the different parts of oceanic environment e g morb volcanic arc ocean island are now juxtaposed

a polar low over the japan sea on 21 january 1997 part ii a numerical study abstract a remarkable isolated polar low observed over the japan sea on 21 january 1997 is studied by numerical simulations using a 20 km mesh regional primitive equation model regional spectral model rsm of the japan meteorological agency and a 5 km mesh nonhydrostatic mesoscale model meteorological research institute nonhydrostatic model mri nhm and by sensitivity experiments using the mri nhm the 24 h integration of rsm starting from 0000 utc 21 january reproduces quite well the polar low s movement and development including the initial wrapping of the west part of an east west oriented vorticity zone the vorticity budget analysis indicates that the stretching term plays a dominant role in the polar low development the simulation by mri nhm reproduces surprisingly well the wrapping of the western part of the east west oriented vortcity zone associated with a cloud band the spiral shaped cloud pattern and the eye structure near the vortex center the wind speed and resulting sea surface

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

u pb dating of perovskite by la icp ms an example from the oka carbonatite quebec canada abstract the potential for u pb dating of the mineral perovskite using laser ablation inductively coupled plasma mass spectrometry la icp ms has been evaluated perovskite can have relatively elevated u and th contents making it a potentially useful geochronometer kimberlite lamprophyre and carbonatite intrusions are prone to alteration and typically contain abundant xenocrysts thus standard techniques for dating the magmatic emplacement of these rocks becomes highly problematic perovskite however is commonly present as a primary magmatic mineral in these rocks and has been used to constrain the timing of kimberlite and lamprophyre emplacement the main limitations to the production of accurate la icp ms u pb ages for perovskite are the lack of suitable standards coupled with the large common pb correction required 204 hg can produce large isobaric interference on the non radiogenic 204 pb isotope and methods based on 208 pb 206 pb and u th ratios are hampered by the enhanced th content present in most perovskite crystals after careful calibration using zircon reference materials common pb corrected 206 pb 238 u ages can be determined on each individual la icp ms analysis by using the measured 207 pb 206 pb ratio which is dominated by the common pb proportion in phanerozoic rocks this method assumes that the perovskite crystals analyzed are cogenetic and have the same initial pb ratios the la icp ms method has been applied to data collected on two large single perovskite crystals from the oka carbonatite complex quebec canada the weighted mean 206 pb 238 u age of 131 7 ma n 25 mswd 0 42 is consistent with published geochronology data from the region and suggests that la icp ms dating of perovskite could be a useful low cost alternative to compliment existing techniques

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

rb sr and sm nd isotopic dating of an early precambrian spilite keratophyre sequence in the wutaishan area north china preliminary evidence for nd isotopic homogenization in the mafic and felsic lavas during low grade metamorphism the spilite keratophyre sequence in the wutaishan area north china has been dated by zircon from the quartzkeratophyre and a volcanic crystallization age of 2522 ma liu dunyi et al 1985 was obtained this volcanic rock sequence experienced a low grade metamorphism of greenschist facies during 1800 2000 ma however combined sm nd whole rock data from the volcanic rock sequence if interpreted in terms of an isochron gives a younger age of 2250 182 ma if separately calculated sm nd isochrons for quartz keratophyre and spilite yield two consistently younger ages of 1977 385 ma and 1981 178 ma these sm nd isochron ages are consistent with a rb sr age of 1871 98 ma and with the time of regional greenschist facies metamorphism consequently we interpret the 2250 ma alignment on the sm nd diagram as an errorchron the two sm nd metamorphic ages suggest that partial homogenization of nd isotopic composition in water rich spilite keratophyric meta volcanics can be achieved during low grade metamorphism as a result of ree mobility using the nd values at the time of metamorphism and the average 147sm 144nd ratios of the spilite and the quartzkeratophyre initial nd values of 2 6 3 5 and 3 0 6 1 respectively are estimated at the time of volcanic eruption 2522 ma similar positive initial nd values for the spilites and quartz keratophyres suggest that they might have been derived from the same depleted mantle source or that the quartz keratophyres are derived from a short lived mafic source

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

magmatic cycles and formation of the upper oceanic crust at spreading centers geochemical study of a continuous extrusive section in the oman ophiolite 1 the data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the oman ophiolite in order to provide detailed constraints on the construction of the upper oceanic crust a continuous volcanic transect 300 m thick was sampled at high frequency in the semail ophiolite along wadi shaffan the wadi shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites the sampling performed along wadi shaffan implies temporal variations in the activity of the ridge the section is characterized by chemical compositions consistent with those of v1 geotimes volcanism the wadi shaffan transect was built through two main petrological and geochemical sequences of volcanic activity trace element ratios e g zr nb and la yb allow us to distinguish two main sequences with two different parental magmas differences in the degree of partial melting are required to explain these trace element ratio variations beyond these differences in parent melt composition variations in trace element abundances tio2 zr ree involve differentiation processes prior to emplacement in the lower sequence less differentiated lavas are in the upper part of the cycle magma mixing is proposed to explain this reversed geochemical evolution through time in the upper sequence geochemical analysis suggests a different magma chamber process this sequence consists of multiple events of magma emplacement variations in trace element abundance suggest four magmatic cycles each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time the upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the mtz mantle crust transition zone and with lower gabbros we propose a model in which the upper sequence lavas were directly derived from the mtz and lower gabbro sills and then transported to the surface without interaction with higher crustal levels

atmospheric karman vortex shedding from jeju island east china sea a numerical study abstracta mesoscale atmospheric numerical model is used to simulate two cases of karman vortex shedding in the lee of jeju island south korea in the winter of 2013 observed cloud patterns associated with the karman vortex shedding are successfully reproduced when the winter monsoon flows out from the eurasian continent a convective mixed layer develops through the supply of heat and moisture from the relatively warm yellow sea and encounters jeju island and dynamical conditions favorable for the formation of lee vortices are realized vortices that form behind the island induce updrafts to trigger cloud formation at the top of the convective boundary layer a sensitivity experiment in which surface drag on the island is eliminated demonstrates that the formation mechanism of the atmospheric karman vortex shedding is different from that behind a bluff body in classical fluid mechanics

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

structure of a shear line polar low during march 2013 a series of polar lows originated in a high vorticity 10 3 s 1 shear zone that was associated with a prolonged marine cold air outbreak over the norwegian sea a detailed analysis of one shear line polar low at the leading edge of the outbreak is presented using comprehensive observations from a well instrumented aircraft dropsondes scatterometer and cloudsat data and numerical modelling output from a convection resolving configuration of the met office unified model the maximum low level wind gradient across the shear line was 25 m s 1 over 50 km high windsto the north and west were within the cold air mass and were associated with large surface turbulent heat fluxes and convective clouds low wind speed to the south and east of the shear line were associated with low heat fluxes and a clear eye in the polarlow shear line meso scale instabilities merging into the polar low appeared important to its structure and development the model captured the shear line and the polar low structure very well in particular the strength of the horizontal shear and the mesoscale thermodynamic fields the spatialstructure of convective cloud bands around the polar low wassimulated reasonably well but the modelsignificantly underestimated the liquid water content and height of the cloud layers compared to the observations shear line polar lows are relatively common however this case is arguably the first to be examined with a wide range of in situ and remote observations allied with numerical model output

numerical study of the influence of environmental conditions on lake effect snowstorms over lake michigan abstract numerical simulations are used to examine the influence of environmental parameters on the morphology of lake effect snowstorms over lake michigan a series of model sensitivity studies are performed using the colorado state university mesoscale model to examine the effects of lake land temperature difference surface roughness atmospheric boundary layer stability humidity and wind speed and direction on the morphology of simulated storms four morphological types of lake effect snowstorms have been identified i broad area coverage which may become organized into wind parallel bands or cellular convection ii shoreline bands with a line of convection roughly parallel to the lee shore and a well developed land breeze on the lee shore iii midlake band with low level convergence centered over the lake and iv mesoscale vortices with a well developed cyclonic flow pattern in the boundary layer the model is able to reproduce all four morphological types simulations varying environmenta

impact of arc continent collision on the conditions of burial and exhumation of uhp lt rocks experimental and numerical modelling abstract a 2 d physical and finite element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere the experimental technique allowed us to model the whole subduction collision process from oceanic subduction to deep subduction of the continental crust with the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process the results obtained by both techniques are coherent and mutually complementary they show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction the conditions for such a failure the weakening of this plate are prepared during oceanic subduction the weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates and due to back arc spreading in oceanic subduction zones with a compressional regime no back arc opening thick and strong back arc lithosphere the weakest zone is volcanic arc area when weakening becomes sufficient during subduction the lithosphere fails in this area the failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore arc almost half of the presently active subduction zones are characterised by a tensional subduction regime with back arc spreading in such subduction zones the weakest place is not the volcanic arc but the back arc spreading centre when a subduction regime changes from tensional to compressional failure occurs in the vicinity of the extinct spreading centre this process can occur during oceanic subduction again along either a trench vergent or trenchward dipping fault but the formation of a trench verging fault is most likely in this latter case which is a principal subject of our study the failure is followed by partial subduction of the arc plate complete subduction occurs during arc continent collision subduction of the continental margin when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle the arc itself can be subducted completely or be partially or completely scraped off and accreted a deeply subducted material including continental margin is preserved at relatively low temperatures between the lithospheric mantle and the cold subducted arc plate to about 150 km depth subduction of the arc plate is a major phenomenon which affects all processes associated with continental subduction from deep burial and hp lt metamorphism to exhumation of subducted material does this process occur in nature future investigations will allow us to answer this question in this paper we analyse the conditions of emplacement of a very young oceanic lithosphere samail ophiolite on the continental crust in oman in the late cretaceous and argue that this lithosphere formed in a back arc basin it reached and overthrust the arabian continent after complete subduction of the arc plate

evidence of eocene high temperature high pressure metamorphism of ophiolitic rocks and granitoid intrusion related to neotethyan subduction processes dogansehir area se anatolia

u pb zircon ages from the spontang ophiolite ladakh himalaya the spontang ophiolite in the ladakh himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of india during the late cretaceous precise isotopic ages of himalayan ophiolites are presently unavailable u pb dating of zircons from a dioritic segregation in the high level gabbros of the spontang ophiolite shows that the complex formed at 177 1 ma mid jurassic and we suggest that it represents a fragment of neo tethyan oceanic crust this jurassic age contrasts with late cretaceous ages 91 95 ma previously reported for the formation of the troodos and oman ophiolite complexes further west along the neo tethyan suture zone and also predates the early to mid cretaceous masirah ophiolite c 150 ma and its alkaline volcanic seamounts 115 125 ma an andesitic rock from an arc sequence overlying the spontang ophiolite was also dated using as 88 5 ma this late cretaceous age constrains the minimum age of initiation of subduction beneath the spontang ophiolite the complex was emplaced during the turonian maastrictian some 80 ma after its formation when the indian continental margin collided with a cretaceous arc about 25 ma before the india asia collision the lack of a sub ophiolite metamorphic sheet beneath the spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction

plate tectonic evolution of the mediterranean middle east region abstract an interpretive model for the mesozoic cenozoic plate tectonic evolution of the mediterranean and adjacent areas is illustrated by a series of paleoposition maps at selected intervals between the late triassic and recent this interval witnessed an important period of tensional development during the triassic and jurassic that fragmented pangea after its late paleozoic consolidation a number of oceanic areas evolved through jurassic time all of which have since been consumed during the alpine orogeny during the cretaceous and tertiary sea floor spreading geometry in the north and south atlantic resulted in convergence between africa and eurasia that controlled the evolution of the mediterranean and adjacent middle east areas from the late mesozoic to recent the interpretation departs from several previous ones in the following respects 1 1 the eastern mediterranean and adjacent middle east area are interpreted to have developed as two seaways with an intervening continental sliver derived from africa that now comprises central turkey and west iran these seaways persisted from the late triassic to late cretaceous by which time they were largely consumed by northward subduction 2 2 spreading that produced the present eastern mediterranean sea developed during the late cretaceous driving the tripolitza eratosthenes iskenderun continental fragments northward from africa to collision with central turkey as the preceding southern arm of the mesozoic tethys sea floor was entirely consumed initial block faulting in this zone occurred during the jurassic and early cretaceous but without subsidence reaching clearly bathyal depths until the cenomanian turonian this differs from the timing of events in the ionian sequence of the pindic nappe of crete which shows a change from shallow carbonate shelf in the triassic to deep basin in the liassic to the north of equivalent platform facies of the tripolitza nappe 3 3 moesia and rhodope are concluded to have been a single plate until a rifting event that started during the late jurassic the balkan mountains are interpreted to represent an alpine inversion of this mesozoic rift system that extended west from the black sea basin 4 4 the eastern carpathian microcontinental sliver is concluded as continuing southward as the serbo macedonian massif that separates rhodope from the vardar zone rather than as rhodope itself 5 5 the scutari pec lineament is considered as representing a major discontinuity between the apulia pannonian plate and adjacent greece and turkey ophiolites to the northwest of this lineament originated in a single oceanic area the vardar zone ophiolites to the southeast of this lineament evolved from two oceanic areas those of interior greece and northern anatolia and the associated ankara melange relating to the vardar zone the former north arm of tethys and the pindos southern anatolia zagros oman ophiolites relating to a former south arm of mesozoic tethys 6 6 based on the history of development of various elements of the alpine system there is a strong suggestion that important dextral movement took place along a major strike slip fault system reaching from the middle east to the central alps between late cretaceous and middle eocene times

late ordovician glaciation in southern turkey abstract we present a new survey of several palaeozoic sections in both the taurus range and the border folds that documents typical glacial features including a glacial pavement and striated dropstones halevikdere formation and demonstrates the former presence of an ice sheet in southern and south eastern turkey evidence for the late ashgill hirnantian age of this episode is provided by macro and microfossils found within the glacial formation the extension of ice related deposits into the northernmost part of the arabian platform mardin region implies a much wider distribution of the ordovician ice sheet than was previously believed and strongly suggests that southern turkey lay close to egypt during the lower palaeozoic

seismological evidence for crustal scale thrusting in the zagros mountain belt iran summary crustal receiver functions rfs computed from the records of 45 temporary seismological stations installed on a 620 km long profile across central zagros provide the first direct evidence for crustal thickening in this mountain belt due to a rather short 14 km average station spacing the migrated section computed from radial rfs displays the moho depth variations across the belt with good spatial resolution from the coast of the persian gulf to 25 km southwest of the main zagros thrust mzt the moho is almost horizontal with slight depth variations around 45 km crustal thickness then increases abruptly to a maximum of 70 km beneath the sanandaj sirjan metamorphic zone between 50 and 90 km northeast of the surface exposure of the mzt further northeast the moho depth decreases to 42 km beneath the urumiehdokhtar magmatic assemblage and the southern part of the central iranian microcontinent the region of thickest crust is located 75 km to the northeast of the bouguer anomaly low at 220 mgals gravity modelling shows that the measured moho depth variations can be reconciled with gravity observations by assuming that the crust of zagros underthrusts the crust of central iran along the mzt considered as a crustal scale structure this hypothesis is compatible with shortening estimates by balanced cross sections of the zagros folded belt as well as with structural and petrological studies of the metamorphic sanandaj sirjan zone

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

geochemical bioenergetics during low temperature serpentinization an example from the samail ophiolite sultanate of oman abstract various classes of microbial and biomolecular evidence from global studies in marine and continental settings are used to identify a set of reactions that appear to support microbial metabolism during serpentinization of ultramafic rocks geochemical data from serpentinizing ecosystems in the samail ophiolite of oman are used to evaluate the extent of disequilibria that can support this set of microbial metabolisms and to provide a ranking of potential metabolic energy sources in hyperalkaline fluids that are direct products of serpentinization results are used to construct hypotheses for how microbial metabolism may be supported in the subsurface for two cases ecosystems hosted in rocks that have already undergone significant serpentinization and those hosted by deeper active serpentinization processes

shear instability wave along a snowband instability structure evolution and energetics derived from dual doppler radar data abstract this article presents a detailed analysis of a meso scale 17 km wavelength shear instability wave along a snowband using a series of dual doppler radar data the wave developed along a low level shear line that formed under the strain wind field caused by an adjacent mesoscale vortex the horizontal wind shear across the line was largest at lower levels and the eddy component horizontal winds and the retrieved pressure anomaly showed a bottom intensified structure as well the resultant vertical pressure gradient force was found to be responsible for the enhancement of alternating updrafts and downdrafts that were subsequently related to the formation of the reflectivity core gap structure of the wave eddy kinetic energy eke budgets of the evolving disturbance were investigated using time series of retrieved kinematic and thermodynamic data the wave grew at an approximately constant growth rate for about 40 min from its onset the eke in this quasi linear growth period was primarily gen

spatial and temporal variability in crystallization of celadonites within the troodos ophiolite cyprus implications for low temperature alteration of the oceanic crust celadonite a low temperature 30 c hydrothermal alteration mineral is a predominant fracture and void filling phase within the volcanic rocks of the troodos ophiolite the combined chemical and structural properties of celadonite i e high k2o and ar retentive along with its common occurrence provide a valuable tracer for studying the temporal and spatial variability of low temperature hydrothermal fluid circulation and alteration within this ancient oceanic crust some 54 new k ar age determinations of celadonites from various geographic and stratigraphic locations within the extrusive rocks of troodos yield crystallization ages ranging from 90 9 1 0 to 49 8 0 5 ma with the oldest in close agreement with the estimated 91 92 ma crystallization age of troodos igneous rocks the youngest age indicates that low temperature water rock chemical exchange continued for at least 40 m y after crustal formation this represents a 100 increase over previous estimates based on limited numbers of k ar dates of celadonite of the duration of low temperature mineral precipitation in troodos correlation between celadonite ages and field relationships suggests the following 1 there is no apparent relationship between celadonite crystallization and stratigraphic depth 2 celadonite crystallization occurs homogeneously on an outcrop scale with adjacent samples from a single flow unit yielding equivalent k ar ages 3 flow units with relatively high primary permeabilities e g pillows and breccias yield celadonites with younger k ar ages 4 celadonite precipitation at distinct locations occurred rapidly with little or no age difference between rim and core sections of single large deposits the above correlations suggest that low temperature hydrothermal fluid circulation and secondary mineral precipitation are controlled by the local alteration conditions such as intrinsic permeability degree of fracturing and water rock ratios and are independent of time and space integration of these local heterogeneities over the entire extrusive sequence suggests that the upper oceanic crust undergoes relatively homogeneous closure to hydrothermal fluid circulation no evidence is found to support a progressive upward sealing of the oceanic crust

40ar 39ar hornblende and biotite geochronology of the bulfat igneous complex zagros suture zone ne iraq new insights on complexities of paleogene arc magmatism during closure of the neotethys ocean abstract in ne iraq the eastern edge of the arabian plate is overlain by arc rock allochthons whose genesis and tectonic emplacement were related to the consumption and closure of the neotethys ocean this paper demonstrates the occurrence of unrelated paleogene arc rocks in two adjacent allochthons the bulfat igneous complex at wadi rashid ne iraq is an intrusion within the upper allochthon albian cenomanian gimo qandil sequence suprasubduction zone assemblage a thrust separates this allochthon from the underlying lower allochthon of the eocene oligocene walash naopurdan volcanic sedimentary arc rocks the bulfat igneous complex at wadi rashid consists of gabbro and granitic composite intrusions in which components mingle down to a small scale textural relationships in the bulfat igneous complex rocks indicate emplacement at high crustal levels with rapid cooling which is consistent with amphibole geobarometry indicating crystallisation pressures between 250 and 300 mpa ti rich igneous pargasite and ti rich igneous fe biotite from gabbroic and granitic components yielded 40ar 39ar ages of 39 23 0 21 and 38 87 0 24 ma respectively these ages agree within analytical error and suggest coeval emplacement and rapid cooling of mafic and felsic magmas in the eocene in an event that was distinct and much younger than the host albian cenomanian rocks this igneous event was unrelated to formation of cenozoic rocks in the underlying tectonically separate lower allochthon the trace element signatures of the wadi rashi volcanic rocks show volcanic arc characteristics for the granites and the gabbroic rocks resemble e type morb the presence of eocene arc related rocks in two allochthons suggests complexity in paleogene subduction systems with possibly two subduction zones operating at that time

jurassic accretionary complex and ophiolite from northeast turkey no evidence for the cimmerian continental ribbon permian triassic and late cretaceous accretionary complexes ascribed to the consumption of two distinct oceans the paleo and neo tethys are exposed over extensive areas in the eastern mediterranean region however a separating continental ribbon the so called cimmeride continent between the paleo and neo tethys during early mesozoic time cannot be defined here we report a previously unknown early jurassic metamorphic oceanic accretionary complex and ophiolite from northeast turkey bounded by oceanic accretionary complexes of permian triassic and late cretaceous age to the north and the south respectively without a continental domain in between this special tectonic position and widespread coexistence of permian triassic and late cretaceous accretionary complexes alongside the izmir ankara erzincan suture imply that 1 the southern margin of laurasia in the eastern mediterranean region grew by episodic accretionary processes from late paleozoic to end mesozoic time without involvement of a cimmerian continental ribbon and 2 the paleo tethys and northern branch of the neo tethys were not distinct oceans in the eastern mediterranean region

late stage magmatic evolution of oceanic gabbros as a result of hydrous partial melting evidence from the ocean drilling program odp leg 153 drilling at the mid atlantic ridge petrographic observations within oceanic gabbros drilled by ocean drilling program odp at leg 153 from the mid atlantic ridge kane fracture zone mark area 23 n in combination with experimental findings suggest that the late stage magmatic evolution of these gabbros is characterized by pervasive hydrous partial melting water saturated melting experiments on a variety of natural gabbros between 900 and 1000 c at 200 mpa produced newly formed minerals forming a characteristic paragenesis consisting of plagioclase orthopyroxene and pargasitic amphibole clinopyroxene the an content of the new plagioclases is uniformly higher than that of the protolith olivine and clinopyroxene primocrysts react to form neoblastic orthopyroxene and pargasitic amphibole these features can also be observed in the gabbros from the mark area here we found zones at plagioclase grain boundaries showing a strong enrichment in an component with an contents up to 20 to 25 mol higher than those of the host plagioclase primary olivines and clinopyroxenes in contact with such zones react to orthopyroxene and pargasitic amphibole these phases rim olivine and clinopyroxene and grow interstitially typical petrographic characteristics of a late stage magmatic phase the observed late stage microstructures in the gabbros thus appear to be the results of partial melting processes triggered by water rich fluids and are not crystallization products of a percolating differentiated late melt here we demonstrate that hydrothermal circulation within the gabbroic layer starts at much higher temperatures 900 1000 c than up to now believed water rich fluids propagate on grain boundaries in a ductile regime causing hydrous partial melting on a large scale a cracks system a prerequisite in current models for enabling hydrothermal circulation is not necessary the observed process has the potential for transfer of heat and mass between the upper and lower oceanic crust provided that the water rich fluids triggering the partial melting process are seawater derived this process may have a significant influence on the cooling of the deep oceanic crust which is in concordance with new thermal models implying that high temperature hydrothermal circulation is regarded to play an important role in transport of heat in the deep oceanic crust copyright 2005 by the american geophysical union

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

a numerical modeling study of mesoscale cyclogenesis to the east of the korean peninsula numerical simulations and the analysis of observational data are employed to understand the mesoscale cyclogenesis in a polar airstream that occurred over the sea to the east of the korean peninsula on 28 29 january 1995 the observational analysis shows that a mesoscale low develops over the southeastern east sea japan sea on 29 january 1995 satellite imagery also indicates that a meso b scale vortex forms on the lee side of the northern korean mountain complex kmc which is located in the northern korean peninsula and that a meso a scale cyclone develops over the southeastern east sea at a later time the mesoscale cyclone forms in the lower troposphere with strong baroclinicity and cyclonic circulation under the influence of an upper level synoptic scale cold vortex numerical simulation has captured major features of the observed cyclogenesis very well the cyclogenesis occurs in a progressive manner basically four distinctive stages of the cyclogenesis are identified 1 first a surface pressure trough forms on the lee side of the kmc under a northwesterly synoptic scale flow that is deflected anticyclonically over the kmc 2 second the lee trough deepens further into a strong convergence zone and a meso b scale vortex 3 next the meso b scale vortex develops into a meso a scale vortex as the vortex and the trough begin to move southeastward from the lee of the kmc 4 finally the surface trough deepens into a closed low and the meso a scale vortex becomes collocated with this deepening surface low to form a meso a scale cyclone over the southeastern east sea several sensitivity experiments are performed to isolate the effects of a topography warmer sea surface diurnal thermal forcing and latent heat release during stages 1 and 2 it is found that the kmc and low level baroclinicity are responsible for generating the strong lee trough and vortex during stage 3 the development of the meso a scale vortex is brought on by the tilting of horizontal vorticity and vertical stretching in a synopticscale cyclonic circulation in the final stage the condensational heating plays the key role for the development of the meso a scale cyclone under the influence of an upper level synoptic scale cold vortex the presence of the warm sea surface is found to be a necessary condition for the development of a polar air convergence zone and the mesoscale cyclone it is also found that the low level baroclinicity is essential for the present case of mesoscale cyclogenesis

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

the magmatic feeding system of el reventador volcano sub andean zone ecuador constrained by texture mineralogy and thermobarometry of the 2002 erupted products abstract after a 26 years long quiescence el reventador an active volcano of the rear arc zone of ecuador entered a new eruptive cycle which lasted from 3 november to mid december 2002 the initial sub plinian activity vei 4 with andesite pyroclastic falls and flows shifted on 6 and 21 november to an effusive stage characterized by the emission of two lava flows andesite to low silica andesite lava 1 and basaltic andesite lava 2 containing abundant gabbro cumulates the erupted products are medium to high k calc alkaline and were investigated with respect to major element oxides mineral chemistry texture and thermobarometry inferred pre eruptive magmatic processes are dominated by the intrusion of a high t mafic magma possibly up to 1165 15 c into an andesite reservoir acting as magma mixing and trigger for the eruption before this refilling the andesite magma chamber was characterized by water content of 5 3 1 0 high oxygen fugacity nno 2 and temperatures in the upper and lower part of the reservoir of 850 and 952 65 c respectively accurate amphibole based barometry constrains the magma chamber depth between 8 2 and 11 3 km 2 2 km the 6 october 2002 seismic swarm hypocenters from 10 to 11 km preceding el reventador eruption supports the intrusion of magmas at these depths the widespread occurrence of disequilibrium features in most of the andesites e g complex mineral zoning and phase overgrowths indicates that convective self mixing have been operating together with fractional crystallization inferred from the cognate gabbro cumulates before the injection of the basic magma which then gave rise to basaltic andesite and low silica andesite hybrid layers magma mixing in the shallow chamber is inferred from the anomalous sio 2 al 2 o 3 whole rock pattern and strong olivine disequilibria both lavas show three types of amphibole breakdown rims mainly due to heating mixing processes and or relatively slow syn eruptive ascent rate decompression of the magmas the lack of any disequilibrium textures in the pumices of the 3 november fall deposit suggest that pre eruptive mixing did not occur in the roof zone of the chamber a model of the subvolcanic feeding system of el reventador consistent with the intrusion of a low al 2 o 3 crystal rich basic magma into an already self mixed andesite shallow reservoir is here proposed it is also inferred that before entering the shallow chamber the basaltic magma underwent a polybaric crystallization at deeper crustal levels

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

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

petrology and geochemistry of plagiogranite in the canyon mountain ophiolite oregon plagiogranites in the canyon mountain ophiolite oregon include a wide range of rock types ranging from diorite to trondhjemite the plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates the plagiogranites are typically low in k2o and high in na2o and are enriched 10 20 times chondrites in ree and overlap with abundances in basic rocks from canyon mountain all samples of plagiogranite are relatively depleted in lree with more silicic samples characterized by a slightly lesser degree of lree depletion total ree content is not consistently correlated with contents of major and other trace elements

a u pb age for the shetland islands oceanic fragment scottish caledonides evidence from anatectic plagiogranites in layer 3 shear zones high precision u pb data obtained from zircons extracted from plagiogranite within the gabbro unit of the shetland islands oceanic fragment of northeast scotland yield an age of 492 3 ma field relations indicate that the plagiogranites were generated by the partial melting of amphibolitized gabbros within high temperature shear zones formed due to crustal deformation and fluid infiltration occurring in proximity to a spreading centre the u pb data therefore constrain the crystallization age of the shetland complex this age is similar to u pb ages obtained from the leka 497 2 ma karmoy 493 7 4 ma and gulfjellet 489 3 ma oceanic fragments of the norwegian caledonides and the pipestone pond 494 3 2 ma and betts cove 489 3 2 ma oceanic fragments of the canadian appalachians

rhenium osmium isotope systematics and platinum group element concentrations in oceanic crust knowledge of the 187 os 188 os ratio as well as the inventories of rhenium and platinum group elements pge in oceanic crust allows quantification of the proportion of recycled oceanic crust in oceanic basalt sources our knowledge is limited by the availability of well characterized sections of oceanic crust specifically of the plutonic lower portion that has not been drilled in situ to the moho here we report new data for plutonic rocks that compose the bottom 4680 m of an ocean crust section from the oman ophiolite major and trace element data as well as mineral analyses indicate that oman gabbros are primitive cumulates from melts similar to typical mid oceanic ridge basalt the mean weighted composition of this section re 427 pg g os 55 pg g ir 182 pg g pd 2846 pg g pt 4151 pg g initial 187 os 188 os 0 142 indicates significantly higher os and lower re concentrations than previously analyzed partial sections of ocean crust that lack cumulate lower crust deep sea drilling project ocean drilling program dsdp odp hole 504b odp hole 735b emphasizing that the lower cumulate oceanic crust dominates the os budget of oceanic crust analyses of mineral grain size fractions indicate that rhenium pge and lead are enriched in the sulfur rich fine fraction this corroborates the notion that small accessory phases and the melt migration processes affecting them control these elements budgets distributions and susceptibilities to alteration the re os pge inventories of a hypothetical 6 5 km thick composite section that consists of 1825 m of dsdp hole 504b like upper oceanic crust and 4680 m of oman like lower ocean crust re 736 pg g os 45 pg g ir 133 pg g pd 2122 pg g pt 2072 pg g initial 187 os 188 os 0 146 provide a new comprehensive assessment of oceanic crust composition upon recycling and mixing with reasonable proportions of mantle peridotite this composite requires at least 2 g y to develop sufficiently radiogenic 187 os 188 os to generate high himu 238 u 204 pb basalts

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

structure and evolution of the passive margin of the eastern tethys abstract an extensive passive margin was formed in the triassic along the periphery of arabia including the tauric carbonate platform this event is related to the opening of the mesozoic tethys when a number of microcontinents split off from gondwana triassic extension and continental rifting resulted in the formation of a structural pattern which is uniform from the dinarides to oman it includes the following elements 1 1 shelf 2 2 continental slope 3 3 deep basin probably with a floor of attenuated sialic crust 4 4 inner carbonate platform in the jurassic cretaceous stable conditions prevailed influenced only by eustatic oscillations of the sea level turbidites accumulated on the continental rise while cherts and radiolarites were deposited in the deep basins hawasina pichakun antalya pindus below the ccd level sedimentation on the shelf was controlled by north northeast transverse tectonic elements which also continued across the passive margin dividing it into a number of segments collision with an island arc led to obduction of the oceanic crust deformation of the passive margin and overthrusting of its sedimentary cover onto the arabian shelf obduction and deformation lasted for about 10 m y and created a new tectonic pattern with concentric structural zones surrounding the arabian promontory these zones include 1 1 the flysch basin a remnant of the closing tethys 2 2 an uplift a site of periodical emergence and erosion corresponding to the frontal part of the ophiolitic nappes 3 3 the border furrow a depocenter of low energy calcareous marls 4 4 the arabian shield constantly emerged during the tertiary tectonic deformation of these zones caused by the collision of arabia with eurasia began prior to the early miocene and it is still going on data on afghanistan demonstrate that its central part the gelmend argandab and kabul blocks belonged during the paleozoic and early mesozoic to the continental shelf of india

an island arc origin of plagiogranites at oytag western kunlun orogen northwest china shrimp zircon u pb chronology elemental and sr nd hf isotopic geochemistry and paleozoic tectonic implications abstract this paper presents detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope geochemistry of three intrusive plagiogranite bodies formed in an island arc setting within the oytag ophiolite suite of the oytag kuda suture zone shrimp zircon u pb dating of two of these intrusions shows that they were emplaced into the oytag volcanic rocks at 337 5 4 1 ma and 327 7 4 9 ma and that the younger intrusion contains inherited zircons ranging from 435 3 to 480 8 ma both intrusions are composed of low al trondhjemite and tonalite the rocks have low k 2 o and sr and high y contents with low sr y ratios they have relatively low rare earth element ree concentrations and are depleted in light ree lree and relatively enriched in heavy ree hree with marked negative eu anomalies in the chondrite normalized ree patterns they have initial 87 sr 86 sr ratios of 0 7048 to 0 7068 and e nd t values of 6 2 to 7 6 which are lower than those of typical morb in situ hf isotopic compositions of zircons with early carboniferous age from both intrusions are characterized by positive initial e hf values ranging from 12 5 to 19 5 similar to typical morb the inherited ordovician zircons in the younger intrusion have initial e hf values of 1 9 to 3 1 similar to zircons from ordovician granitoids in the region detailed elemental and isotopic data suggest that the oytag plagiogranites were formed in an oceanic island arc setting by differentiation of arc tholeiitic magmas that formed in response to the intra oceanic subduction the inherited zircons are interpreted as the xenocrysts captured from the incipient oceanic crust that contains relicts of discrete fragments of continental crust with ordovician zircons through which the plagiogranite was emplaced the oytag ophiolite suite might be the remnants of an ancient island arc the basement of which was the incipient oceanic crust the oytag kuda suture zone is suggested to be a multiple suture zone that represents the remnants of both the proto tethyan and paleo tethyan oceans the western kunlun orogen was therefore not a long lived active continental margin during the early and late paleozoic times it most likely underwent at least two orogenic cycles and thus might represent a multiple orogenic belt

multiple polar mesocyclones over the japan sea on 11 february 1997 abstract the characteristics and development mechanism of four meso scale polar mesocyclones m pcs which appeared as a vortex train over the japan sea from 0000 to 1700 utc on 11 february 1997 are studied by means of observational analyses and numerical simulations these four m pcs were first observed at 0000 utc along the southern edge of a meso scale cloud system that looked like an occluded extratropical cyclone in a japanese geostationary meteorological satellite 5 gms 5 infrared satellite image detailed evolutionary processes of these m pcs were described based on satellite and radar images the horizontal scales of these m pcs were on the order of 100 km and the spatial intervals between their centers were about 200 km their lifetimes were around 10 h the occlusion like features of the meso scale cloud system and the synoptic situation prior to the development of these m pcs are examined by using regional objective analysis ranal data of the japan meteorological agency jma in o

crustal scale cross sections across the nw zagros belt implications for the arabian margin reconstruction quantified balanced and restored crustal cross sections across the nw zagros mountains are presented in this work integrating geological and geophysical local and global datasets the balanced crustal cross section reproduces the surficial folding and thrusting of the thick cover succession including the near top of the sarvak formation 90 ma that forms the top of the restored crustal cross section the base of the arabian crust in the balanced cross section is constrained by recently published seismic receiver function results showing a deepening of the moho from 42 2 km in the undeformed foreland basin to 56 2 km beneath the high zagros the internal parts of the deformed crustal cross section are constrained by new seismic tomographic sections imaging a 50 ne dipping sharp contact between the arabian and iranian crusts these surfaces bound an area of 10800 km 2 that should be kept constant during the zagros orogeny the arabian crustal cross section is restored using six different tectonosedimentary domains according to their sedimentary facies and palaeobathymetries and assuming airy isostasy and area conservation while the two southwestern domains were directly determined from well constrained surface data the reconstruction of the distal domains to the ne was made using the recent margin model of wrobel daveau et al 2010 and fitting the total area calculated in the balanced cross section the arabian continental oceanic boundary at the time corresponding to the near top of the sarvak formation is located 169 km to the ne of the trace of the main recent fault shortening is estimated at 180 km for the cover rocks and 149 km for the arabian basement including all compressional events from late cretaceous to recent time with an average shortening rate of 2 mm yr 1 for the last 90 ma