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

rapid conversion of an oceanic spreading center to a subduction zone inferred from high precision geochronology where and how subduction zones initiate is a fundamental tectonic problem yet there are few well constrained geologic tests that address the tectonic settings and dynamics of the process numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system gurnis m hall c lavier l 2004 geochem geophys geosys 5 q07001 but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere instead other weak zones such as fracture zones have been invoked because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself in the western philippines we find that oceanic crust was less than 1 my old when it was underthrust and metamorphosed at the onset of subduction in palawan philippines implying forced subduction initiation at a spreading center this result shows that young and positively buoyant but weak lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older denser lithosphere and that plate motion rapidly changed from divergence to convergence

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

generation and obduction of ophiolites constraints from the bay of islands complex western newfoundland constraints provided by the bay of islands ophiolite complex boic western newfoundland suggest that its generation as well as its obduction is related to collision of the dunnage zone island arc system with the irregular continental margin of eastern north america the constraints include the supra subduction zone setting of the boic its extensional environment of generation the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the north american continental margin and the restriction of the boic as well as other appalachian ophiolites and associated taconian allochthons to reentrants in the north american margin during the early phases of collision of the dunnage arc with the north american margin subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation the first develops through trench rollback leading to ophiolite generation in the overriding arc plate outboard of the reentrant whereas the second develops in a transtensional environment with spreading centers offset by strike slip faults which propagate through the overriding plate away from the indenter promontory the configuration of the boic spreading ridge normal to the continental margin and the direction of initial obduction parallel to the margin favor generation of the ophiolite in an overall strike slip regime with complete subduction of the oceanic remnant extension in the overriding plate ceases and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the dunnage zone and for termination of arc volcanism along the north american margin

p t paths from high temperature shear zones beneath ophiolites mctamorphic rocks of the st anthony complex of north western newfoundland are best interpreted in terms of a high temperature shear zone formed between down going continental margin rocks and overriding oceanic lithosphere in a subduction zone high grade rocks immediately beneath the oceanic lithosphere peridotite display retrograde meta morphism in high strain zones whereas lower grade rocks near the base of the metamorphic complex display prograde metamorphism in high strain zones mylonite zones in meta basitcs at all levels in the complex contain the assemblage epidote hornblende albite sodic oligoclase these observations suggest that the inverted metamorphic gradient within the st anthony complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone the degree of re equilibration is strongly dependent on the degree of strain and is best achieved in synmetamorphic mylonite zones this interpretation of the st anthony complex can be extended to other sub ophiolite metamorphic sheets which show very similar relationships it is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones the p t paths of which preserve records of burial and exhumation in subduction zones

albite deformation within a basal ophiolite shear zone abstract albite mylonites at the base of the white hills peridotite ophiolite fragment at st anthony newfoundland formed during obduction at lower crustal p t conditions optical microstructures preserve a history of intracrystalline distortion recovery and cyclic dynamic recrystallization associated with hot working the crystallographic fabric is consistent with shear of the polycrystalline aggregate dominated by glide on 010 planes as is also indicated by tem observations slip systems implied by trace analysis and invisibility experiments using tem are b 101 010 b 001 010 and b 1 2 112 201 dissociated dislocations have separations ranging from 8 3 15 nm these separations are notably smaller than in more calcic plagioclases and indicate a relatively higher ductility for albite in the case of cross slip controlled creep the observed dislocation densities relate to a high stress strain rate pulse imposed on the dominant high temperature creep textures

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

constraints on the p t conditions and age of emplacement of the lizard ophiolite cornwall amphibole plagioclase thermobarometry and 40ar 39ar geochronology of basal amphibolites the basal slice of the lizard complex incorporates strongly foliated amphibolitic metabasites lower landewednack schists interpreted as the remnants of a dynamothermal aureole formed beneath the ophiolite during intraoceanic thrusting structurally overlying the basal slice and occurring predominantly within the overlying lizard peridotite is the kennack gneiss a controversial banded and gneissose unit comprising intimately mixed amphibolitic and granitic components interpreted to represent commingled melts thermobarometry of hornblende plagioclase pairs indicates that both the landewednack schists and the mafic component of the kennack gneiss recrystallized synkinematically at ca 600 c and 300 400 mpa three amphiboles two from the kennack gneiss and one from the landewednack schists yield disturbed 40ar 39ar age spectra characterized by low temperature overprinting but with good high temperature plateaux with ages of 366 1 4 2 364 2 4 8 and 359 8 7 4 ma respectively theoretical dif

major role of shear heating in intracontinental inverted metamorphism inference from a thermo kinematic parametric study abstract inverted metamorphism corresponds to the stacking of high temperature metamorphic units structurally on top of lower temperature units and is commonly observed along main thrusts in major orogens yet in spite of many existing models the origin and preservation of the metamorphic inversion in intracontinental collision belts are still debated in this study we use a crustal scale 2d thermo kinematic model in order to investigate the key parameters controlling the inversion of the geothermal gradient at crustal scale our results confirm that the kinematic framework strongly impacts the thermal evolution around the thrust erosion velocity and thermal conductivity of rocks are two parameters that control the spatial location of the thermal perturbation and the intensity of inversion respectively however even in extreme kinematic configurations i e convergence velocities 3 cm yr 1 and relatively high thrust dip angles 30 the thermal inversion is fleeting and thrust temperatures cannot reach the high temperature peak values characteristic of natural occurrences 600 c if shear heating is not taken into account conversion of mechanical energy into heat represents a main contribution to the thermal budget along main crustal shear zones it leads to high temperature conditions in the thrust zone and our results attest that it is the only process that allows the preservation through time of an intense thermal inversion our quantification shows that shear heating is much more efficient than other processes such as accretion and surface denudation and is compatible with the observations of inverted metamorphism in the himalayan or variscan belts for example this comparison with natural occurrences suggests that the formation and preservation of intracontinental inverted metamorphism require shear zone viscosity values of the order of 10 20 10 21 pa s for convergence velocities between 1 and 3 cm yr 1

the sole of an ophiolite the ordovician bay of islands complex newfoundland the early ordovician c 485 ma bay of islands ophiolite complex was obducted onto the laurentian rifted margin as the fore arc of an oceanic arc that collided with the margin during the mid ordovician c 470 ma the subduction zone was nucleated on an oceanic transform fracture zone part of whose remnants occur as the polyphase deformed and intruded mafic and ultramafic rocks of the coastal complex the ophiolite formed as a suprasubduction zone fore arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform fracture zone subduction zone system and ne of a trench trench ridge triple junction a two pyroxene garnet granulite garnet amphibolite epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated roughly synchronously with the ophiolite by the metamorphism of mid ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening and not by subduction zone extrusion the metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite plate slip vector triangles around the triple junction before collision and during obduction are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high to lower temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite linear structures in the sole amphibolites trend nnw the subduction direction those in the greenschist facies obducted oceanic and continental margin rocks trend wsw the obduction direction

40ar 39ar age constraints on taconian and acadian events in the quebec appalachians 40ar 39ar dating of hornblende muscovite and orthoclase from southeastern quebec provides new constraints on the timing grade and regional extent of taconian and acadian metamorphism in the canadian appalachians muscovite ages reveal that the taconic orogeny occurred at ca 463 ma throughout the region lack of reset amphibole indicates that the metamorphic grade did not exceed greenschist facies acadian metamorphism with a peak of 350 c is recorded by an orthoclase cooling age of 377 4 ma this later metamorphism partially reset finer grained muscovite toward the vermont border the calculated 300 340 c closure temperature of orthoclase indicates that this age currently provides the closest estimate of the timing of peak acadian metamorphism north of new england despite the lower grade of taconian and acadian metamorphism in quebec compared to new england peak temperatures appear to have been achieved synchronously in addition investigation of the timing of pre taconic decoupling of the quebec oceanic fragments suggests a protracted obduction history obduction may have displaced slivers of both the subducting and overthrusting oceanic plates

structural history of the metamorphic sole of the bay of islands complex western newfoundland displacement of the early ordovician ophiolitic bay of islands complex was accompanied by the development of a metamorphic sole in underlying accreted gabbro mafic pillow lava pelite and psammite the metamorphic sole contains up to 130 m of amphibolite underlain by up to 80 m of mafic and pelitic psammitic schist which in turn overlies chlorite bearing rocks that overlie mudstone matrix melange a major foliation locally mylonitic and or isoclinally folded is pervasively developed in the greenschists and amphibolites an extension lineation is widely developed and plunges north in amphibolite and trends east west in greenschist consistent with a changing direction of thrust transport from northerly to westerly as the mantle wedge cooled over time shear sense criteria in greenschist facies pelitic psammitic schist indicate thrusting towards the west in the southwestern blow me down massif two sets of folds postdate development of accretion related s l fabrics and resulted in a substantial w

counterclockwise p t t trajectory from the metamorphic sole of a neo tethyan ophiolite turkey abstract the kiziltepe ophiolitic thrust sheet in the bolkar mountains of turkey occurs between two subparallel ophiolite belts bounding the tauride carbonate platform and represents a remnant of the cretaceous neo tethyan oceanic lithosphere it is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra oceanic subduction zone in the neo tethys 92 90 ma blueschist facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing p t with cooling regional tectonic constraints suggest a maastrichtian age for the timing of this blueschist facies metamorphism sodic amphibole rich veins and crossite mg riebeckite rims on hornblende suggest that growth of blueschist facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries we infer a counterclockwise p t t trajectory during which metamorphism was accompanied succeeded by rapid uplift along the northern edge of the tauride belt in late cretaceous early tertiary time

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

stability and composition relations of calcic amphiboles in ultramafic rocks calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions experiments were performed on the stability of tremolite plus forsterite in the system h2o cao mgo sio2 from 5 to 20 kbar a univariant curve was fitted to the experimental brackets using volume water fugacity and heat capacity data the results indicate that the maximum stability of tremolite in the presence of forsterite is about 825 c at 5 kbar addition of al2o3 to this system increases the stability of tremolitic amphibole by only 20 40 c and induces solubility of 5 7 wt al2o3 in the amphibole as determined from quantitative sem analyses of individual amphibole crystals thus substitution of the tschermakite component ca2 mg3al2 si6al2 o22 oh 2 alone cannot lead to the greatly enhanced al2o3 contents or thermal stability of natural calcic amphiboles

early cretaceous migmatitic mafic granulites from the sabzevar range ne iran implications for the closure of the mesozoic peri tethyan oceans in central iran terra nova 22 26 34 2010 abstract the ophiolitic melange of the sabzevar range northern iran is a remnant of the mesozoic oceanic basins on the northern margin of the neotethys that were consumed during the arabia eurasia convergence history occurrence of km scale dismembered mafic hp granulitic slices is reported in this study granulites record an episode of amphibole dehydratation melting and felsic tonalite throndhjemite melt segregation at c 1 1 gpa and 800 c in situ u th pb geochronology of zircon and titanite grains hosted in melt segregations points to an early cretaceous albian age for the metamorphic climax results of this study i impose reconsideration of the current palaeotectonic models of the neothetyan convergent margin during the early cretaceous and ii argue that punctuated events of subduction of short lived back arc oceanic basins accompanied the long lasting history of the neotethyan subduction in the region

thermo mechanical modeling of the obduction process based on the oman ophiolite case obduction emplaces regional scale fragments of oceanic lithosphere ophiolites over continental lithosphere margins of much lower density for this reason the mechanisms responsible for obduction remain enigmatic in the framework of plate tectonics we present two dimensional 2d thermo mechanical models of obduction and investigate possible dynamics and physical controls of this process model geometry and boundary conditions are based on available geological and geochronological data and numerical modeling results are validated against petrological and structural observations of the oman semail ophiolite our model reproduces the stages of oceanic subduction initiation away from the arabian margin the emplacement of the oman ophiolite on top of it and the domal exhumation of the metamorphosed margin through the ophiolitic nappe a systematic study indicates that 350 400 km of bulk shortening provides the best fit for both the maximum pressure temperature conditions of the metamorphosed margin 1 5 2 5 gpa 450 600 c and the dimension of the ophiolitic nappe 170 km width our results confirm that a thermal anomaly located close to the arabian margin 100 km is needed to initiate obduction we further suggest that a strong continental basement rheology is a prerequisite for ophiolite emplacement

obduction processes in ancient modern and future ophiolites geologic and tectonic processes of ophiolite obduction deduced from structural mapping timing of deformation and metamorphism and palinspastic reconstruction of ancient ophiolite complexes compare closely with modern processes operative in the western pacific many ongoing arguments about the origin of ophiolite complexes oceanic island arc or marginal basin and their emplacement subduction related or not are largely semantic and depend on exactly what individuals regard as island arcs marginal basins or subduction zones a review of tectonic processes currently active in the western pacific region shows a wide range in size shape and duration of arcbasin complexes and some of these show uniformitarian models similar to those proposed for emplacement of ancient ophiolites the tethyan ophiolites of the semail oman mountains and spontang western himalaya of ladakh zanskar and the ordovician western newfoundland ophiolites are compared with recent examples of the mariana basin yap trench banda arc luzon arc and papua new guinea it is concluded that they are all connected in some way with marginal basins and island arcs in varying stages of development and maturity emplacement of these ophiolites is related to continental underthrusting of a trapped marginal basin and accretionary prism comparable to present day examples of the n australian margin underthrusting timor and the banda sea and the se china margin underthrusting the luzon arc

great slave lake shear zone canadian shield reconstructed vertical profile of a crustal scale fault zone abstract the great slave lake shear zone gslsz of the northwestern canadian shield a 25 km wide early proterozoic transcurrent dextral mylonite zone is a type example of a crustal scale fault zone profile with time 1 1 the metamorphic grade decreased from granulite to greenschist facies 2 2 the locus of high strain narrowed and jumped laterally abandoning relatively older mylonites progressively younger mylonites represent both cooler temperatures and lower pressures thus the sequence of narrowing mylonite belts represents a series of progressively shallower seated sections through the gslsz 3 3 deformation evolved through the brittle ductile transition initially producing non dilational penetrative breccias followed first by discrete non dilational faulting and subsequently by dilational quartz stockworks developing crustal scale shear zones grow to an optimum width dictated by the requirement of minimum geologically reasonable strain rates for given conditions of wall rock rheology and displacement rate independent of variations in relative plate motion mature shear zones change width and migrate laterally as a function of inferred thermally activated rheological changes within the wall rocks

the emplacement of peridotites and associated oceanic rocks from the lizard complex southwest england upper mantle peridotites and associated oceanic rocks from the lizard complex south west england preserve evidence for a multistage geological history steeply dipping pre emplacement fabrics record high temperature 900 1100 c shearing and exhumation of the mantle peridotites apparently formed during localized ne sw rifting in a pull apart basin setting c 400 390 ma associated oceanic rocks landewednack amphibolites preserve a pre emplacement prograde brown amphibole bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during ne sw rifting in both the peridotites and landewednack amphibolites steep pre emplacement structures are cross cut by low angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra oceanic setting c 390 375 ma the fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages c 500 800 c with the amphibolites preserving two superimposed assemblages green amphibole titanite and colourless magnesio hornblende respectively that are thought to record progressive down temperature deforma tion during thrusting emplacement related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles nw with down dip lineations and kinematic indicators showing consistent top to the nw senses of shear syn emplacement magmatism is recorded by intrusions of foliated kennack gneiss anastomosing serpentine filled faults mark many existing low angle contacts between the peridotites and landewednack amphibolites and appear to represent the final lowest temperature 250 c stages of emplacement c 370 ma this study shows that dynamothermal aureoles in ophiolites may preserve evidence for tectonothermal events that pre date thrust emplacement

forced subduction initiation recorded in the sole and crust of the semail ophiolite of oman subduction zones are unique to earth and fundamental in its evolution yet we still know little about the causes and mechanisms of their initiation numerical models show that far field forcing may cause subduction initiation at weak pre existing structures while inferences from modern subduction zones suggest initiation through spontaneous lithospheric gravitational collapse for both endmembers the timing of subduction inception corresponds with initial lower plate burial whereas coeval or delayed extension in the upper plate are diagnostic of spontaneous or forced subduction initiation respectively in modern systems the earliest extension related upper plate rocks are found in forearcs but lower plate rocks that recorded initial burial have been subducted and are inaccessible here we investigate a fossil system the archetypal semail ophiolite of oman which exposes both lower and upper plate relics of incipient subduction stages we show with lu hf and u pb geochronology of the lower and upper plate material that initial burial of the lower plate occurred before 104 million years ago predating upper plate extension and the formation of semail oceanic crust by at least 8 myr such a time lag reveals far field forced subduction initiation and provides unequivocal direct evidence for a subduction initiation mechanism in the geological record

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

dynamics of intraoceanic subduction initiation 2 suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle here we focus on suprasubduction zone ssz ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame ssz ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation structural petrological geochemical and plate kinematic constraints on their kinematic evolution show that ssz crust forms at fore arc spreading centers at the expense of a mantle wedge thereby flattening the nascent slab this leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during ssz crust crystallization former spreading centers are preserved in forearcs when subduction initiates along transform faults or off ridge oceanic detachments we show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone upon inception of slab pull due to e g eclogitization the sole is separated from the slab remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge we propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust our study highlights how far field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle

hot and deep rock record of subduction initiation and exhumation of high temperature high pressure metamorphic rocks feather river ultramafic belt california abstract studies of a 10 to 300 m thick unit of high grade metamorphic rock external schists that crops out along the western border of the feather river ultramafic belt frb northern california yield new insights into subduction initiation and ophiolite emplacement processes the high temperature t foliation of the external schists dip moderately to steeply eastward beneath the ultramafic rocks of the frb a 150 km long slab of suboceanic upper mantle and the high t fabric shows a tops to the west frb side up sense of shear the structurally highest external schists record peak metamorphic conditions of 650 760 c at 1 3 2 2 gpa in contrast sheeted dikes of the devil s gate ophiolite that overlie the ultramafic rocks yield metamorphic conditions of 710 730 c at about 0 3 0 7 gpa a km scale lens of amphibolite within ultramafic rocks yields somewhat lower pressures than the structurally highest external schist as do the structurally lower rocks within the external schists significant exhumation of the external schists relative to the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant exhumation was accommodated at least 1 5 km structurally above the ultramafic external schist contact based on available geochronology intraoceanic subduction may have initiated at approximately 240 ma and exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the calaveras complex minimum 177 ma a subduction complex that structurally underlies the external schists high t metamorphism of the devil s gate ophiolite may have resulted from partial failed ridge subduction

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating 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 field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

p t conditions during emplacement of the bay of islands ophiolite complex a high temperature contact is described between the basal pargasite bearing spinemherzolites of the bay of islands ophiolite complex and underlying garnet granulite facies metagabbros of its dynamothermal aureole three distinct high temperature hydrous assemblages occur in the basal mylonites of the peridotite and spinel and garnet bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole on the basis of garnet clinopyroxene geothermometry and garnet forming reactions in metabasic rocks p t conditions of 7 11 kbar 750 850 c are estimated for rocks on both sides of the contact steep inverted gradients in both temperature and pressure of equilibration occur in the aureole which most likely represents a thinned overturned and metamorphosed section through an ophiolite sequence it is proposed that the aureole formed in a low angle shear zone cutting the oceanic crust and upper mantle age data shows that the bay of islands complex was 30 40 ma old and therefore relatively cold at the time of formation of the aureole prolonged 1 ma shear heating must therefore have occurred at high shear stresses and movement rates 1 kbar 10 cm yr to produce the high contact temperatures the displacement surface probably initiated as a discrete fault evolving into a viscous shear zone with time downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole the observed pressures at the ophiolite aureole contact are 3 7 kbar in excess of the expected load pressure from the present thickness of the ophiolite the cause of the pressure excess was removed before formation of lower grade parts of the aureole possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole a model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures rapid uplift and erosion during displacement and the coincidence of k ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite

structural evolution of a section along the base of the st anthony complex northwest newfoundland the basal portion of the st anthony complex is made up of structural slices 10 30 m thick with unknown amounts of relative movement between them the lowest rocks are variably deformed pillow lavas and related rocks with one pervasive foliation these rocks are overlain by greenschists phyllonites and mylonites with up to three planar structural elements developed at least one of these latter surfaces the earliest had formed before the schists were thrust over the underlying pillow lavas the overthrusting event resulted in deformation of underlying pillow lavas all deformation occurred under lower greenschist facies conditions fold axes in the least deformed rocks have variable orientations and this variation decreases up section until axes are parallel and plunge gently eastwards parallel to the stretching lineation parallelism between fold axes and stretching lineation implies rotation of fold axes during progressive deformation and is consistent with the rocks being part of a high strain z

development of inverted metamorphic gradient in the internal domain of the taconian belt gaspe peninsula the mont logan nappe is part of the taconian internal domain of the quebec appalachians and is entirely made up of synrift to passive margin elastics and volcanics of the shickshock group rocks o

tectonic structures related to thrusting of ophiolitic complexes the white hills peridotite newfoundland the white hills peridotite and its underlying metamorphic rocks represent the basal portion of a partly eroded ophiolitic complex at the contact with the metamorphic rocks the peridotites display low temperature high stress mylonitic textures which grade upwards to fine grained porphyroclastic textures indicative of higher temperatures and lower stresses petrological and structural evidence indicates that the mylonitic peridotites and underlying metamorphic rocks have been deformed during the oceanic thrusting of the lithosphere the analysis of the main tectonic structures observed in the peridotites shows that the complex has been thrust northwestward along a southeast dipping thrust plane the data are compared with those obtained from the bay of islands ophiolite complex

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

diffuse co2 flux emission in two maar crater lakes from sao miguel island azores portugal abstract santiago and congro are two maar crater lakes located at sao miguel the largest island of the azores archipelago santiago lake is located on the sete cidades volcano at an altitude of 364 m presenting a surface area of 0 25 km2 and a maximum depth of 33 m congro lake is located on the congro fissural volcanic system at an altitude of 420 m and has a surface area of 0 04 km2 and a maximum depth of 22 m both lakes are monomictic in character with low mineralized waters of meteoric origin being the temperature in the range between 12 4 c winter period and 23 8 c summer period the main water type is na hco3 and the relative major ion composition is hco3 cl so42 f for anions and na k mg2 ca2 for cations four sampling surveys were carried out between 2013 and 2016 in each lake in order to estimate surface diffuse co2 degassing a total of 1612 and 713 co2 flux measurements were made with an accumulation chamber at santiago and congro lakes respectively the higher co2 fluxes were measured during the winter surveys 0 2 t d 1 in congro and 5 6 t d 1 in santiago while the lowest values 0 1 t d 1 and 0 2 t d 1 respectively in congro and santiago lakes were recorded in the summer these seasonal differences observed in both lakes are associated with the monomictic character of the lakes as the co2 is not able to ascend to the surface when the water column is stratified during the warmer period due to the physical processes that occur in both lakes the co2 emission is mostly associated to a biogenic origin but the higher co2 emissions measured in santiago lake suggest that a volcanic influence cannot be excluded as also shown through the heavier 13c isotopic measured in this water body the tectonic structures that cross santiago lake probably enable the observed degassing

evidence for an inverted metamorphic gradient associated with a precambrian suture southern wyoming an inverted metamorphic gradient associated with the northern mylonite zone of the cheyenne belt a deeply eroded precambrian suture in southern wyoming has been documented within metasedimentary rocks of the early proterozoic snowy pass supergroup metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite garnet and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone a minimum temperature increase of approximately 100 c over a km wide zone is required for this transition parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the snowy pass supergroup field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite grade rocks over rocks of the snowy pass supergroup along the northern mylonite zone mineral equilibria and garnet biotite geothermometry on synkinematic mineral assemblages within the snowy pass metasedimentary rocks indicate deformation at minimum temperatures of 480 c and pressures of 350 400 mpa 3 5 4 0 kbar this implies tectonic burial or upper plate thickness of 13 15 km the narrow character of metamorphic zonation and microtextures within the snowy pass supergroup which indicate late synkine matic growth of garnet and staurolite preclude rotation of pre existing isograds by folding as a mechanism for development of the inverted gradient conductive transport of heat from the upper into the lower plate across the originally low angle thrust is insufficient to produce the necessary temperatures in the lower plate shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated up dip advection of metamorphic fluids is a feasible but unproven mechanism for heat transport the possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present

fluid evolution history of brittle ductile shear zones on the hanging wall of yellow spring thrust valley and ridge province pennsylvania u s a abstract this work presents the results of structural and fluid inclusion analyses of brittle ductile shear zones bdsz cutting lower paleozoic carbonates in the appalachian fold and thrust belt strike parallel extension of the beds riding up the ramp of yellow spring thrust resulted in development of conjugate bdsz by progressively non coaxial deformation during the shearing syntectonic extension veins were formed by successive episodes of mode i cracking and sealing by the precipitation of quartz saddle dolomite and calcite in paragenetic order geobarometry on co existing methane rich and water rich fluid inclusions reveals the trapping of fluids at large range of fluid pressures the values of maximum and minimum fluid pressures correspond to the events of crack propagation and complete sealing respectively contrary to ductile shear zones in high grade metamorphic rocks this study in unmetamorphosed rocks shows that bdsz evolve by influx of locally derived methane bearing brines of connate or evolved formation water origin at pressures of 80 40 mpa temperatures of 155 165 c and a depth of 6 km it is shown that the salinity and composition of the mineralizing fluids change from one crack seal episode to another in spite of a broad consistency in the pressure temperature conditions

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

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

the formation of garnet in olivine bearing metagabbros from the adirondacks a regional study of olivine bearing metagabbros in the adirondacks has permitted testing of the p pressure t temperature x composition dependence of garnet forming reactions as well as providing additional regional metamorphic pressure data six phases olivine orthopyroxene clinopyroxene garnet plagioclase and spinel which can be related by the reactions orthopyroxene clinopyroxene spinel anorthite garnet and forsterite anorthite garnet occur together both in coronal and in equant textures indicative of equilibrium compositions of the respective minerals are typically fo25 72 en44 75 en30 44fs9 23wo47 49 pp13 42alm39 63gr16 20 an29 49 and sp16 58 when they occur in the same rock equant and coronal garnets are homogeneous and compositionally identical suggesting that chemical equilibrium may have been attained despite coronal textures extrapolating reactions in the simple cmas system to granulite temperatures and making thermodynamic corrections for solid solutions gives equilibration pressures using the thermometry of bohlen et al 1980b ranging from about 6 5 kb in the lowlands and southern adirondacks to 7 0 8 0 kb in the highlands for the assemblage olivine plagioclase garnet these results are consistent with inferred peak metamorphic conditions in the adirondacks valley and bohlen 1979 bohlen and boettcher 1981 thus the isobaric retrograde path suggested by whitney and mclelland 1973 and whitney 1978 for the formation of coronal garnet in olivine metagabbros may not be required application of the same equilibria gives 8 7 kb for south harris scotland and 0 9 kb for the nain complex disagreement of the latter value with orthopyroxeneolivine quartz barometry bohlen and boettcher 1981 suggests that the use of iron rich rocks olivines fa50 results in errors in calculated pressures

parameter estimation using multifrequency range dependent acoustic data in shallow water the estimation of all forward model parameters geometric geoacoustic and ocean sound speed by the inversion of acoustic field observations is considered the data was taken at a mildly range dependent shallow water site in the mediterranean sea the inversion is based on data from a vertical array and carried out using information at multiple frequencies global optimization using a directed monte carlo search based on genetic algorithms and the bartlett objective function is used all geometric parameters are well determined a range dependent geoacoustic model is determined and the ocean sound speed is estimated comparisons of the observed pressure field as a function of depth and the predicted field show good agreement the use of observations at multiple frequencies provides considerable stability for the estimated parameters optimization of only geometric and geoacoustic parameters in a range independent environment is found to be satisfactory at the lower frequencies 165 175 hz but for the h

geochemistry and tectonic significance of metamorphic sole rocks beneath the beysehir hoyran ophiolite sw turkey abstract the beysehir hoyran ophiolite is situated in the western part of the tauride belt sw turkey and crops out at two localities north of the lake beysehir it mainly comprises harzburgitic peridotites that were tectonically emplaced to their present position during the late eocene the ophiolites themselves are tectonically overlain by either slope basin deposits with lava blocks egirler formation or massive triassic limestone blocks deliktas formation high grade sub ophiolitic metamorphic rocks i e epidote amphibolite amphibolite and pyroxene amphibolite together with minor quartzite and calcschist are observed at the base of the ophiolite sequence where they occur as thin tectonic slices with an inverted metamorphic gradient average p t conditions of 630 770 c and c 6 1 5 kbar are calculated for the metamorphism of the amphibolites by conventional geothermobarometry corresponding to a burial depth of 18 20 km both the sub ophiolitic metamorphic rocks and the overlying mantle tectonites were intruded by isolated tholeiitic nb y 0 041 0 108 diabase dikes which do not transect the tectonic contact between the two units geochemical investigations of the amphibolites of the sub ophiolitic rock suite show two different geochemical affinities with the first group being alkaline in character nb y 1 3 86 and the second one being tholeiitic nb y 0 064 0 13 ree patterns trace element plots and tectonomagmatic discrimination diagrams indicate that the most probable protoliths for alkaline amphibolites are within plate type alkali basalts whereas those of the tholeiitic group resemble tholeiitic island arc basalts similarities between the geochemical characteristics of the amphibolites and those of the volcanic rocks of the egirler formation strongly suggest that the latter are the protoliths of the amphibolites

a technique for analyzing the thermal and uplift histories of eroding orogenic belts a scandinavian example geothermometric and geobarometric data from pelitic schists in the upper plate of the forsa thrust in the northern norwegian caledonides are interpreted as recording part of the uplift trajectory of this area during erosion and slow cooling a method developed in this paper for reconstructing the thermal history of orogenic belts from such data is based on a general mathematical description of thermal conduction in a lithospheric slab with an arbitrary initial temperature distribution and uplifted at a constant rate with certain restrictions the uplift trajectory tz path of any rock parcel can be calculated from only a few temperature depth points along that uplift trajectory by applying this method to geothermometry and geobarometry data from near the forsa thrust we can 1 extrapolate tz paths measured over a limited depth range upward to the surface 2 recover geotherms present at various times during the uplift and erosion history of the mountain belt 3 obtain tz paths for rock parcels above and below those for which pt data are available and 4 use pt data in combination with geochronological data to obtain accurate estimates of uplift rates our analysis suggests that shortly after nappe emplacement the nappe stack was hot near 800 c in the lower part of the nappe stack while the underlying autochthon was cold near thermal equilibrium prior to nappe emplacement the maximum burial depth of rocks near the forsa thrust was probably between 35 and 60 km

the peristerite gap in low grade metamorphic rocks coexisting na plagioclases from greenschists both in the thermal aureole of the kasugamura granite japan and in the low p metamorphic zone of yap island western pacific were analyzed in great detail the peristerite solvus was determined for each suite the asymmetric solvus has steep albite rich and gentle oligoclase rich limbs that are similar to those for higher pressure series the present results together with those from vermont new zealand and the sanbagawa belt indicate that the peristerite solvus shifts toward the albite component and higher temperature with increasing pressure with increasing pressure albite co existing with oligoclase an 100 ca ca na 20 varies in composition from an 8 9 in kasugamura through an 3 in yap island and vermont to an 1 in new zealand and an less than 0 5 in the sanbagawa belt the consolute temperatures for the peristerite solvus estimated from available geothermometry are 420 c in kasugamura 450 550 c in vermont and 550 600 c in the sanbagawa belt the variation of plagioclase composition in progressive metamorphic zones is explained by intersection of a plagioclase forming reaction and the peristerite immiscibility gap in an isobaric t xan diagram the greenschist zone is characterized by albite the transition zone by occurrence of peristerite pairs and the amphibolite zone by plagioclase of an 20 50

petrochemistry and origin of sub ophiolitic metamorphic and related rocks in the oman mountains the semail ophiolite complex of the northern oman mountains was obducted during cenomanian to campanian upper cretaceous times onto the arabian continental margin during this time span schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice whole rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid ocean ridges a comparison of trace element geochemistries however suggests that the protoliths were not equivalent to the haybi alkaline and transitional tholeiites found in the underlying structural slice or to the semail ophiolite volcanic rocks which lie structurally higher in the sequence it is therefore suggested that the amphibolites may have been derived from jurassic or lower cretaceous oceanic basalts not recognized elsewhere in the oman mountains structurally beneath the amphibolites within the metamorphic sheet greenschist facies metasedimentary rocks including quartzites and marbles are interbanded with metabasite horizons containing actinolite and less commonly blue crossitic amphiboles it is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement and were accreted to the base of the ophiolite during progressive underthrusting