the origin and age of the metamorphic sole from the rogozna mts western vardar belt new evidence for the one ocean model for the balkan ophiolites abstract this study brings new geochronological and petrochemical data from the metamorphic sole beneath the rogozna mts western vardar ophiolite belt the rogozna metamorphic sole is located at the base of a serpentinite nappe and consists of amphibolites and talc chlorite schists the rogozna amphibolites are medium to fine grained rocks with nematoblastic texture and pronounced foliation they consist of green amphibole 70 vol with variable silica contents 6 4 to 7 8 si apfu as well as mg molmg mg fe tot 0 53 to 0 77 and variably albitized plagioclase 30 vol ab 24 ab 98 amphibolites are overprinted by a retrograde assemblage containing actinolite epidote clinoclore sericite chlorite and magnetite the amphibolites formed due to metamorphism of two basaltic suites subalkaline tholeiitic and alkaline subalkaline tholeiitic amphibolites possess low zr nb y th hf tio 2 and p 2 o 5 values and a lree depleted pattern typical for the n morb normal mid ocean ridge basalt to bab back arc basalt origin alkaline amphibolites show elevated concentrations of zr nb y th hf tio 2 and p 2 o 5 with a lree enriched pattern typically displayed by oib ocean island basalt amphibolites were crystallized during intra oceanic thrusting at temperatures between 685 c and 765 c and at a depth of 12 17 km 40 ar 39 ar cooling ages of amphibole ranging from 165 to 170 ma slightly postdate the sole formation talc chlorite schists are related to retrograde greenschist facies metamorphism they consist of fe rich talc and cr rich chlorite peninite diabantite pseudomorphs after amphibole and morb type cr al spinel surrounded by al and mg poor ferrit chromite the occurrence of ferrit chromite is related to earlier amphibolite facies metamorphism chlorite pseudomorphs after amphibole were formed at 300 410 c this study suggests that there is no essential difference in the emplacement age of the dinaric and west vardar ophiolite belts supporting the interpretation involving a single mesozoic ocean in the balkan sector

formation and tectonic evolution of the cretaceous jurassic muslim bagh ophiolitic complex pakistan implications for the composite tectonic setting of ophiolites the muslim bagh ophiolitic complex balochistan pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the indian and eurasian plates these ophiolites were obducted onto the indian continental margin around the late cretaceous prior to the main collision between the indian and eurasian plates the upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes each pluton has a transitional dunite rich zone at its base and new geochemical data suggest a similar mantle source region for both the plutons and dykes in contrast the lower nappe consists of pillow basalts deep marine sediments and a me lange of ophiolitic rocks the rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid ocean ridge the basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the indian plate the two nappes of the muslim bagh ophiolitic complex are thus distinctively different in terms of their age lithology and tectonic setting the recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved

franciscan subduction off to a slow start evidence from high precision lu hf garnet ages on high grade blocks lu hf analyses of garnet from metabasic amphibolite glaucophane schist and eclogite facies blocks from the franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the franciscan subduction zone garnets yield 176 lu 177 hf ratios ranging from 1 5 to 28 with the highest ratios from garnets with high spessartine pyrope ratio sulphuric acid leaching sal of garnets revealed the presence of inclusions with significantly higher lu hf ratios than those of garnet itself most likely apatite their removal by sal brings the 176 lu 177 hf ratios in garnets down by as much as 40 this suggests that 176 lu 177 hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions sm nd garnet analyses were dominated by inclusions mainly sphene and failed to provide precise and accurate age information the oldest lu hf ages are 168 7f0 8 and 162 5f0 5 ma on plagioclase bearing garnet amphibolite from panoche pass and the berkeley hills respectively which suggests initiation of the subduction zone at about 169 ma coeval with the formation of the tectonically overlying coast range ophiolite relatively high temperature conditions persisted for about 14 ma as indicated by 153 4f0 8 ma garnet growth recorded in epidote amphibolite and 157 9f0 7 in eclogite from ring mountain and jenner respectively a 146 7f0 7 ma age was obtained from garnet glacuophane schist metamorphosed at around 400 jc the sequence of ages from central and northern california shows a younging trend with decreasing metamorphic grade which supports previous suggestions that the high grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca 15 jc ma the thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km ma during the initial stages of franciscan subduction such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre the data also suggest slow exhumation rates and significant residence time at depth of the earliest franciscan rocks

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

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

petrological evidence for stepwise accretion of metamorphic soles during subduction infancy semail ophiolite oman and uae metamorphic soles are tectonic slices welded beneath most large scale ophiolites these slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intra oceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate this study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the semail ophiolite oman and united arab emirates based on thermobarometry and thermodynamic modelling it is shown that metamorphic soles do not record a continuous temperature gradient as expected from simple heating by the upper plate or by shear heating as proposed in previous studies the upper high temperature metamorphic sole is subdivided in at least two units testifying to the stepwise formation detachment and accretion of successive slices from the down going slab to the mylonitic base of the ophiolite estimated peak pressure temperature conditions through the metamorphic sole from top to bottom are 850 c and 1 gpa 725 c and 0 8 gpa and 530 c and 0 5 gpa these estimates appear constant within each unit but differing between units by 100 to 200 c and 0 2 gpa despite being separated by hundreds of kilometres below the semail ophiolite and having contrasting locations with respect to the ridge axis position metamorphic soles show no evidence for significant petrological variations along strike these constraints allow us to refine the tectonic petrological model for the genesis of metamorphic soles formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover metamorphic soles result not so much from downward heat transfer ironing effect as from progressive metamorphism during strain localization and cooling of the plate interface the successive thrusts originate from rheological contrasts between the sole initially the top of the subducting slab and the peridotite above as the plate interface progressively cools these findings have implications for the thickness the scale and the coupling state at the plate interface during the early history of subduction obduction systems this article is protected by copyright all rights reserved

metamorphism and deformation along the emplacement thrust of the samail ophiolite oman abstract the base of the samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement the classic exposure of this metamorphic sole at wadi tayin consists of a few meters of garnet clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite overlying 150 m of lower amphibolite facies mafic and sedimentary schist the sole is bounded by faults but is internally coherent peak metamorphic temperatures decrease exponentially from 825 c at the top of the sole to 500 c at its base as documented by changes in grain size hornblende composition the disappearance of chlorite and epidote garnet clinopyroxene element partitioning and garnet hornblende partitioning the inverted peak temperature gradient decreases from 4000 c km in the upper 50 m to 100 c km in the lower 150 m contrary to earlier reports greenschist facies tectonites are not present at wadi tayin although the entire sole underwent greenschist facies retrogression driven by the influx of fluids from the overlying peridotite or underlying carbonate calculations based on an amphibolite constitutive relation reveal that although the section represents a lithologic continuum the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column

assessment of the garnet clinopyroxene fe mg exchange thermometer using new experimental data a detailed high pressure experimental study of two mafic xenoliths in which coexisting garnet and clinopyroxene plagioclase spinel and olivine were crystallized over a p t range of 10 30 kbar and 950 1200 c has revealed significant differences in temperatures from those estimated for coexisting garnets and clinopyroxenes using the ellis green fe mg exchange thermometer the results show perfect matching at 30 kbar 1150 1200 c but increasing deviation at lower pressure and lower temperature with the ellis green calibration reaching a t overestimate of c 145 c at 10 12 kbar and 950 c the grossular content of the garnet increases from c 21 mol at 10 kbar to 26 31 mol at 30 kbar these results confirm other recent experimental studies that show that the pressure correction and possibly to a lesser extent the correction for grossular content applied by ellis green are not appropriate for lower pressure conditions and give estimated temperatures that are significantly high when applied to granulitic terranes formed at c 10 kbar the new reconnaissance results allow a graphical interpolation of a garnet clinopyroxene geothermometer based on the fe mg exchange reaction which should be applicable to assemblages formed under lower crustal conditions

comment on dating the geologic history of oman s semail ophiolite insights from u pb geochronology by c j warren r r parrish d j waters and m p searle

the grain fluid interaction as a self stabilizing mechanism in fluvial bed load transport a grain scale model of fluvial bed load transport is described with particular emphasis on the equilibrium between the saltating grains and the near bed flow and its role in determining transport rate the model calculates explicitly the modification of the velocity profile by the moving grains together with the consequential reduction in surface fluid shear stress as the surface fluid shear stress is reduced by the moving grains so the entrainment rate decreases and the model reaches a steady state the results provide insight into two important questions at a macroscopic level first they show that in the absence of large static roughness the dynamic roughness caused by the moving grains may be a significant contributor to flow resistance secondly the model indicates the manner in which transport may be limited by a combination of the transport capacity of the flow and the availability of sediment for entrainment only in the case of high sediment availability does the fluid shear stress acting at the surface approach the critical entrainment value reproducing the behaviour suggested by bagnold 1956 and owen 1964 this suggests that prediction formulae based on this assumption only describe the bed load transport system under particular conditions

synchronous seafloor spreading and subduction at the paleo convergent margin of semail and arabia the semail ophiolite is the largest and best exposed ophiolite having preserved intact record of a precollision system related to neotethyan ocean closure breaking with the alternative models of ophiolite exhumation and emplacement this paper builds on integrating the joined contribution of tectonic evolution of the continental margin in the premise of the ophiolite obduction connecting the along strike segmentation of the ophiolite recorded in the lithospheric flow structure and recent high precision dating results in a 3 d model of evolution of the ophiolite and its continental margin during jurassic cretaceous time among the relevant characteristics newly accounted 1 the contamination of the ophiolite by fluids and magmas of continental origin 2 the discrimination of different types of metamorphics associated to the ophiolite and conventionally classified metamorphic soles 3 the instability of the arabian margin during the late jurassic cretaceous times 4 the new precise ages relating to overlapping of spreading and detachment and 5 the inferred vergence of neotethyan closure during late mesozoic are stressed the complex stem revealed may relate to processes associated with spreading center subduction at ridge trench transform rtt triple junction as documented along a convergent margin the semail ophiolite emplacement on the arabia continental margin would result from a temporary arabian platform directed subduction zone that consumed the southern tethyan plate during late jurassic cretaceous time

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

hydrocarbon generation and migration under a large overthrust the carbonate platform under the semail ophiolite jebel akhdar oman abstract the natih b source rock of the jebel akhdar dome in the oman mountains hosts at least three different generations of solid bitumen this study presents detailed analyses of thermal maturity based on solid bitumen reflectance rock eval pyrolysis fourier transform infrared spectroscopy and raman spectroscopy of matrix and solid bitumen the natih b source rock was sampled in different valleys wadis around the jebel akhdar dome detailed petrography of organic material was conducted by optical and scanning electron microscopy sem the petrographic data was used to calibrate an integrated thermal basin model of the wadi nakhr area due to ophiolite obduction the natih b source rock was buried to 7 4 to 8 km and a maximum temperature of c 240 c hydrocarbon generation and associated primary migration was burial induced and took place from 78 to 66 ma with solid bitumen precipitation in the source rock a resulting high reflective solid bitumen generation reflects temperatures of deepest burial in the order of 225 c to 240 c based on an elaborated paragenetic sequence two other solid bitumen generations were linked to two oil migration events taking place during uplift 55 50 ma and 48 45 ma after deepest burial around 65 ma

fluid assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone semail ophiolite this study reports on feedback mechanisms between fluid migration ductile deformation and strain localization processes in an incipiently forming mantle wedge the basal banded unit of the semail ophiolite these peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction during this stage they were affected by coeval ductile deformation forming proto mylonites at 900 800 c to ultramylonites at 700 c and interaction with subduction fluids from the petrological and microstructural study of these hydrated peridotites and their protolith preserved lenses of porphyroclastic tectonites we show that peridotite interaction with hydrous fluids triggered dissolution precipitation processes dissolution of coarser grains and precipitation of new and smaller ones mainly pyroxenes spinel and amphibole resulted in a drastic grain size reduction phase mixing and a switch from olivine dislocation to grain size sensitive creep in proto mylonites data also evidence a feedback process of fluid focusing in actively deforming shear zones this rapid switch in olivine deformation mechanism driven by subduction fluid peridotite interaction triggered an intense weakening of the peridotites at 900 800 c and a transition from a mechanically decoupled to coupled plate interface as witnessed by the detachment and underplating of a high temperature metamorphic sole to the base of the ophiolite it also explains the intense strain localization 1 km thick shear zone along this ductile portion of the plate interface considering that similar mechanisms take place in mature subduction zones they may explain plate interface coupling at subarc depths in worldwide subduction zones

significance of transport parallel strain variations in part of the raft river shear zone raft river mountains utah usa this paper examines part of the raft river shear zone rrsz in northwestern utah that exhibits an extreme transport parallel increase in strain intensity coupled with a transition from flattening to constrictional strain detailed geologic mapping and finite strain quartz c axis fabric and kinematic vorticity analyses demonstrate local necking of the shear zone associated with an increase in transport parallel elongation accommodated by a stretching fault at the base of the shear zone the domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak rocks this domain is characterized by strain in the constrictional field caused by transport perpendicular flow into the area of high transport parallel elongation where rocks cut by the stretching fault are rheologically strong the rrsz locally records flattening strain and lower strain intensities limiting the amount of stretching needed at the base of the shear zone in any one direction and or recording transport perpendicular flow into adjacent highly extended domains the rheology of rocks cut by the stretching fault directly controlled the amount and style of zone normal shortening and transport parallel elongation and these observations provide an example of deformation partitioning in a crustal scale structure driven by local rheological transitions

obduction related remagnetization at the base of an ophiolite paleomagnetism of the samail nappe lower sequence and of its continental substratum southeast oman mountains this paper considers the effect of obduction tectonics on the magnetic properties of a fast spreading ridge ophiolite the paleomagnetism of the ultramafics seven sites and layered gabbros two sites of the samail wadi tayin ibra and muscat massifs of oman has been investigated along with that of the permian metabasites from the subophiolitic saih hatat continental window five sites of the 202 studied samples 183 exhibit stable components with a dominant nnw trend and a shallow positive or less frequently negative inclination superposition of normal and reverse polarities was observed in one gabbro three peridotite and two metabasite sites the in situ characteristic directions are similar in both the nappe and metamorphic window except for a lone low grade metabasite with a component d 168 i 35 referred to a preserved permian magnetization the moderate directional scatter can be explained by postnappe tilting along the nne trending axis the high grade metabasites in the window were completely remagnetized during the metamorphism associated with nappe emplacement and subsequent exhumation tectonics between 80 and 70 ma since the overlying ophiolites have similar normal or reverse directions they were remagnetized contemporaneously the magnetic mineralogy of the ultramafic layered gabbro sequence was likely modified extensively at that time by serpentinization from the upward metamorphic fluid advection in the faulted ophiolite therefore the magnetic properties of the lower part of the southern samail ophiolite cannot be used for modeling magnetic anomalies of oceanic lithosphere

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

the death of an accretion zone as evidenced by the magmatic history of the sumail ophiolite oman abstract the ophiolitic crustal sequence of the salahi block in the northern part of oman shows a complex magmatic history the oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates isotropic gabbros a sheeted dike complex and a first tholeiitic lava series v1 depleted in compatible elements the second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite v2 directly linked as can be seen from field evidence to more differentiated high level intrusive plutons this volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements a third volcanic event v3 exhibiting a transitional character enriched in lree s caps the ophiolite these v3 lavas are separated from the v2 unit by a 15 m thick pelagic sedimentary section a geotectonic evolution of the sumail ophiolite is proposed on the basis of field mineralogical and geochemical data after the oceanic accretion phase that gave birth to the main ophiolitic sequence a second stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode such a model accounts well for the depleted character of the v2 lavas the compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second stage melts which are then emplaced in the crust and partly extruded the v3 volcanism postdates the intraoceanic detachment of the ophiolite but precedes its obduction onto the arabian continental margin it is interpreted as an intraplate seamount type magmatism

progress in microboudin method for palaeo stress analysis of metamorphic tectonites application of mathematically refined expression abstract the microboudin method for palaeo stress analysis is mathematically refined by incorporating the recently proposed shear lag model which describes the relationship between stress distribution along a fibre embedded within an elastic matrix and the far field differential stress applied to the matrix the refined probability density function of fractured fibres g with respect to the aspect ratio of the fibre r and the stress parameter is given by g r 1 exp m 1 m r m e f e q m 1 1 e q e f 1 cosh ar m where eq and ef are the elastic constants of the matrix and fibre m is the weibull modulus of the fibre material and a is a constant the stress parameter is defined as 0 s where 0 is the far field differential stress applied to the matrix and s is the modal fracture strength of the fibre material at r 1 this method is applied to tourmaline and piemontite boudins in quartzose rocks from six areas and it is revealed that the probability density function adequately models the proportion of boudinaged tourmaline and piemontite grains the value of is determined for each sample allowing 0 to be determined by this method when s is known

tectonics of the arabian margin associated with the formation and exhumation of high pressure rocks sultanate of oman continental crustal rocks now structurally beneath the allochthonous samail ophiolite underwent blueschist to eclogite facies metamorphism prior to the emplacement of the oman ophiolite onto the arabian margin the recognition of a major low angle fault within this polydeformed and polymeta morphosed sequence of metacarbonates metabasites quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the saih hatat window ne oman carpholite bearing upper plate rocks consist of pre permian to jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate the lower plate rocks have higher peak pressure and temperature assemblages they are exposed in two windows separated by jabal abu daud although more intensely deformed the lower plate rocks are recognizable as metamorphosed continental platform sediments fold axes are parallel to the regional nne ssw lineation sense of shear indicators yield a transport direction of south over north in the lower plate opposite to the sense of motion inferred for the emplacement of the ophiolite lower plate eclogite facies metabasalts are only preserved in kilometer scale megaboudins found in the easternmost window at as sifah these metamorphic assemblages along with their remnant east west fabrics define the existence of a short lived arabian platform directed nascent subduction zone the stretching lineation elsewhere within the upper and lower plates the hatat schist the basement and the metamorphic sole of the ophiolite is consistently nne ssw suggesting that the exhumation of high pressure metamorphic rocks of saih hatat is related to the ophiolite obduction exhumation of the high pressure rocks was accompanied by intense deformation involving regional scale fold nappes in a convergent margin setting the geotherm remained suppressed for a period 30 myr greater than the thermal relaxation time of the crust the geometric and thermal constraints from oman may be applicable to the general problem of the formation and preservation of high pressure low temperature rocks

subduction zone polarity in the oman mountains implications for ophiolite emplacement two end member models have been proposed to account for the structure and metamorphism of rocks beneath the semail ophiolite in the oman mountains model a involves a single continuous ne directed subduction away from the continental margin during the late cretaceous the ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km sw onto the continental margin subduction of triassic jurassic oceanic basalts to c 10 kbar c 39 km depth led to the accretion of amphibolite facies rocks to the base of the ophiolite thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c 20 kbar c 78 km depth choking the subduction zone buoyancy forces caused the rapid exhumation of eclogites blueschists and carpholite grade hp rocks along the ne margin of the continental plate during the later phase of foreland propagating thin skinned thrusting in the sw ne facing backfolding and backthrusting occurred in the hinterland with the final exhumation of the hp rocks model b follows recent suggestions that a nascent sw dipping subduction zone dipping beneath the continental margin existed between 130 and 95 ma prior to formation and emplacement of the ophiolite a major ne facing fold nappe structure in the pre permian basement rocks of saih hatat is interpreted as reflecting subduction beneath the margin two high pressure metamorphic events have been suggested the first predating ophiolite emplacement the second caused by ophiolite loading this model is untenable being based on a misinterpretation of the ne facing structures in northern saih hatat and on some dubious older 40ar 39ar cooling ages from the eclogite facies rocks of as sifah we conclude that all structures in northern oman and all the reliable geochronology point to a single emplacement obduction event lasting from cenomanian turonian time c 95 ma when amphibolites were accreted along the metamorphic sole of the ophiolite to campanian time when the continental margin was subducted to the ne producing blueschists and eclogites to the final thin skinned emplacement of all thrust sheets which ended before the late maastrichtian at c 68 ma

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

kinematic significance of l tectonites in the footwall of a major terrane bounding thrust fault klamath mountains california usa abstract detailed geologic mapping cross section reconstructions strain analyses and kinematic analyses enable the reconstruction of a one kilometer wide domain of l tectonites in the east west striking subhorizontal to gently south dipping pigeon point high strain zone pphsz associated with a major thrust fault separating oceanic and arc affinity terranes in the klamath mountains california l tectonites are associated with 1 a convex upward warp of the upper high strain zone boundary 2 a transition from mafic metavolcaniclastic rocks to micaceous quartzites 3 folds subparallel with mineral lineations 4 emplacement of synkinematic ultramafic mafic intrusive bodies and 5 a local temperature increase from greenschist to amphibolite facies conditions pure shear dominated deformation accommodated zone normal shortening and transport parallel elongation coupled with subordinate top to the west directed thrust style simple shear l tectonite formation was controlled by the shape of the high strain zone boundary driving lateral flow into the apex of the lens shaped zone in response to a favorable kinematic geometry and bulk strain in the constrictional field localized magmatic heating best explains the shape of the high strain zone boundary but l tectonites are not partitioned into a single rheological domain during terrane amalgamation strain path partitioning occurred with localized top to the west directed simple shear partitioned into a structurally overlying thrust zone and pure shear dominated subvertical shortening and transport parallel elongation partitioned into the pphsz

high temperatures and inverted metamorphism in the schist of sierra de salinas california new field and thermobarometric work in the californian salinian block clarifies current and pre tertiary relationships between the schist of sierra de salinas and cretaceous arc related granitic rocks the contact is variably preserved as a brittle fault and hightemperature mylonite zone the salinas shear zone which represents the contact between north america and sediments accreted above the farallon slab between 76 ma and 70 ma near granulite facies prograde replacement of hornblende with clinopyroxene is associated with deformation of plutonic rocks at the base of the upper plate in the lower plate the schist of sierra de salinas garnet biotite thermometry indicates decreasing temperatures down section from at least 714 8c to 575 8c over an exposed thickness of 2 5 km consistent with petrologic evidence of an inverted metamorphic gradient the measured temperatures are significantly higher than observed at shallow levels above subducting slabs or predicted by 2d computational models assuming low shear stresses previous workers have called upon shear heating to explain similar observations in the correlative pelona schist an unlikely scenario given the results of recent rock deformation experiments which predict that feldspar quartz mica aggregates are far too weak to withstand stresses of 70 mpa required by the shear heating hypothesis as an alternative we propose that high temperatures resulted from conductive heating while the leading edge of the schist traveled 150 km beneath the recently active salinian continental arc during the initiation of shallow subduction weakening of the schist due to high temperatures helped facilitate the collapse of the salinian arc as the schist was emplaced schist emplacement coincided with loss of lower mafic portions of the arc and therefore evolution of the southern california crust towards a more felsic composition d 2005 elsevier b v all rights reserved

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

fabric development in a middle devonian intraoceanic subduction regime the careon ophiolite northwest spain abstract a middle devonian suprasubduction zone ophiolite the careon unit northwest spain displays amphibolite facies ductile deformation fabrics related to the onset of the rheic ocean closure two different fabrics an early high t foliation and a subsequent lower t foliation each of which characterized by distinct deformation mechanisms have been identified in two distinct crustal scale shear zones of the same ophiolitic thrust sheet combined quantitative texture analysis by electron backscattered diffraction and time of flight neutron diffraction were carried out on the shear zones and correlated with micro and macrostructural data the results indicate that the regional lineation and shear zone kinematics east west top to the east represent fabrics developed essentially during the intraoceanic subduction of the rheic ocean and their orientation may be considered a reference vector for convergence models in this part of the variscan belt

ecosystem development following deglaciation a new sedimentary record from devils lake wisconsin usa abstract processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences but this approach often requires records that contain the start of primary succession most lakes in the upper midwestern u s were formed by glaciers at the end of the last ice age approximately 11 700 cal yr bp devils lake wisconsin is a rare example of a lake from this region whose sediments extend into the pleistocene and may include the last glacial maximum sediment magnetic geochemical pollen and charcoal records were generated from a 10 m core whose basal sediments may be 28 000 years old together with a previously published pollen record these proxies combine to reveal a history of long term climatic vegetative and geologic change during the late pleistocene to holocene we identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site productivity in the lake was low during the late pleistocene and increased during the holocene as reflected by the sediment lithology which shows a sudden shift from glacial vivianite rich and organic poor clastic dominated sediments to holocene diatomaceous sapropels several important processes initiated around 17 000 cal yr bp including the onset of organic matter accumulation and fire in the terrestrial ecosystem however the post glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation likely a spruce tundra survived near the site a switch to a hardwood forest period during the holocene also led to a change in the fire regime with increased frequency of burning aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record nutrient cycling as recorded by sedimentary 15n was variable but not directional and appeared to be correlated with climate conditions early in the record and terrestrial ecosystem processes later in the record throughout the holocene magnetic mineral concentration decreased as productivity increased and the source of magnetic material shifted from almost exclusively lithogenic to approximately 50 derived from soil or biogenic sources magnetic grain size ambrosia pollen percentages and charcoal concentration increased and 15n decreased in the most recent part of the record due to anthropogenic influence in the region including agricultural activities

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

multiphase structural evolution of a continental margin during obduction orogeny insights from the jebel akhdar dome oman mountains the structural evolution of the carbonate platform in the footwall of the semail ophiolite emplaced onto the passive continental margin of arabia helps to better understand the early stages of obduction related orogens these early stages are rarely observable in other orogens as they are mostly overprinted by later mountain building phases we present an extensive structural analysis of the jebel akhdar anticline the largest tectonic window of the oman mountains and integrate it on different scales outcrop observations can be linked to plate motion data providing an absolute timeframe for structural generations consistent with radiometric dating of veins top to s overthrusting of the semail ophiolite and hawasina nappes onto the carbonate platform during high plate convergence rates between arabia and eurasia caused rapid burial and overpressure generation and migration of hydrocarbons and bedding confined veins but no major deformation in the carbonate platform at reduced convergence rates subsequent tectonic thinning of the ophiolite took place above a top to nne crustal scale ductile shear zone deforming existing veins and forming a cleavage in clay rich layers in early campanian times ongoing extension occurred along normal to oblique slip faults forming horst graben structures and a precursor of the jebel akhdar dome campanian to maastrichtian this was followed by ne sw oriented ductile shortening and the formation of the jebel akhdar dome deforming the earlier structures thereafter exhumation was associated with low angle normal faults on the northern flank of the anticline we correlate the top to nne crustal scale shear zone with a similar structure in the saih hatat window to develop a unified model of the tectonic evolution of the oman mountains

gravity profiles across the samail ophiolite oman four hundred and sixty gravity stations have been established over the samail ophiolite and adjacent units exposed in the oman mountains the bouguer anomaly pattern follows more or less parallel to ophiolite exposures along the coastal areas the bouguer anomaly values range from 20 to 140 mgal and increase northward four gravity profiles across the ophiolite are correlated with known geology and structure the main gravity profile extending 325 km northward from ibra to muscat and into the gulf of oman shows two positive bouguer anomalies 85 over the southern folded ophiolite nappe and 110 mgal near the shoreline at muscat that are associated with two nappes of tabular form the nappes are nearly continuous and there is no evidence that they are rooted in the mantle the gravity high across the southern nappe is centered over the axis of the synform maximum nappe thickness its position over the gabbro peridotite contact rather than over the peridotite suggests extensive serpentinization of the peridotite at the southern end of the profile in the ibra dome area the negative bouguer anomaly 10 to 15 mgal is interpreted as an infracambrian salt piercement similar to those in the zagros fold belt geologic modeling of the main gravity profile provides a good correlation in terms of known thicknesses of the mappable ophiolite units the nature of the gravity anomaly across the continental margin indicates a transition from 42 km thick continental crust to a thinner 16 18 km oceanic crust the two gravity profiles in the north oman region show a doublet of positive residual bouguer anomalies 50 to 90 mgal the anomalies correlate well with imbricated ophiolite exposures a result of low angle thrusting caused by strong compressive forces in the absence of imbricate thrusting the gravity profile in the central region has only one large positive residual bouguer anomaly 130 mgal the anomalies of all the four profiles are further analyzed to elucidate the nappe transport distances

ptt path in metamorphic rocks of the khoy region northwest iran and their tectonic significance for cretaceous tertiary continental collision abstract metamorphic rocks in the khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of precambrian paleozoic age to the northeast the qom formation oligocene miocene with a basal conglomerate transgressively overlies all of these rocks the metamorphic rocks consist of both metasediments and metabasites the metasediments are micaschist garnet staurolite schist and garnet staurolite sillimanite schist with some meta arkose marble and quartzite the metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc alkaline rocks geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 c at 5 5 and 7 5 kb pressure rims of minerals reveal a considerable decrease of pressure ptt path of this metamorphism is normal the mfg line passes above the triple junction of al 2 sio 5 polymorphs and the average geothermal gradient during metamorphism was from 27 to 37 c km which is more concordant with the temperature regime of collision zones we infer that crustal thickening during post cretaceous possibly eocene collision of the arabian plate and the azerbaijan albourz block was the main factor that caused the metamorphism in the studied area

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

from rifted margins to foreland basins investigating provenance and sediment dispersal across desert arabia oman u a e arid arabia where chemical weathering and anthropic modifications are negligible is a huge natural laboratory for the study of sediment provenance mafic and ultramafic detritus from ophiolite belts is found along the oman mountains and on masirah island obduction orogen provenance dominant cellular serpentinite grains from residual mantle harzburgites of the sama il ophiolite contrast with gabbroic diabase and basaltic grains from igneous crustal rocks of the masirah ophiolite detritus from autochthonous basement and cover rocks is found along the arabian sea margin rift shoulder provenance transition from carbonate dominated lithic sands undissected stage to hornblende rich plagioclase arkoses dissected stage is best documented at the northern edge of the gulf of aden along the older strike slip owen basin margin only locally are feldspars hornblende and garnet derived from small outcrops of pan african basement modern eolian sands from yemen to the arabian persian gulf display marked enrichment in quartz low plagioclase feldspar ratios and depleted dense mineral assemblages with rounded epidotes hornblende garnet and ultrastables pointing to extensive recycling also fostered by quaternary changes the rub al khali and wahibah sand seas have distinct quartz feldspar ratios reflecting largely separate transport pathways dune fields fed chiefly from rift highlands in the west and south mix with orogenic detritus in the northeast carbonate dominated sands in coastal uae are mostly derived from the zagros mountains blown by northerly shamal winds across the gulf during pleistocene lowstands to reach the backbulge depozone and the foreland beyond contributions from the oman mountains reach only 5 10 of bulk sediment along their southern apron and mostly indirectly through deflation of plio pleistocene fans the sama il ophiolite supplies enstatite and diopside to the eastern uae and wahibah sands the masirah ophiolite supplies amphiboles and clinopyroxenes locally along the batain coast the composition of desert sediments faithfully mirrors tectonic history of source terranes deformation style during orogeny and unroofing level during extension bulk petrography coupled with dense mineral analysis thus represents a powerful tool to investigate not only provenance and sediment transport but also crustal dynamics and geologic evolution at continental scale

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

controls on the rheological properties of peridotite at a palaeosubduction interface a transect across the base of the oman uae ophiolite abstract studies of experimentally deformed rocks and small scale natural shear zones have demonstrated that volumetrically minor phases can control strain localisation by limiting grain growth and promoting grain size sensitive deformation mechanisms these small scale studies are often used to infer a critical role for minor phases in the development of plate boundaries however the role of minor phases in strain localisation at an actual plate boundary remains to be tested by direct observation in order to test the hypothesis that minor phases control strain localisation at plate boundaries we conducted microstructural analyses of peridotite samples collected along a 1 km transect across the base of the oman united arab emirates uae ophiolite the base of the ophiolite is marked by the semail thrust which represents the now exhumed contact between subducted oceanic crust and the overlying mantle wedge as such the base of the ophiolite provides the opportunity to directly examine a former plate boundary our results demonstrate that the mean olivine grain size is inversely proportional to the abundance of minor phases primarily orthopyroxene as well as clinopyroxene hornblende and spinel consistent with suppression of grain growth by grain boundary pinning our results also reveal that mean olivine grain size is proportional to cpo strength both of which generally decrease towards the metamorphic sole suggesting that the fraction of strain produced by different deformation mechanisms varied spatially experimentally derived flow laws indicate that under the inferred deformation conditions the viscosity of olivine was grain size sensitive as such grain size and thereby the abundance of minor phases influenced viscosity during subduction related deformation along the base of the mantle wedge we calculate an order of magnitude decrease in the viscosity of olivine towards the base of the ophiolite which suggests strain was localised near the subduction interface our data indicate that this rheological weakening was primarily the result of more abundant minor phases near the base of the ophiolite our interpretations are consistent with those of previous studies on experimentally deformed rocks and smaller scale natural shear zones that indicate minor phases can exert the primary control on strain localisation however our study demonstrates for the first time that minor phases can control strain localisation at the scales relevant to a major plate boundary

synchronous formation of the metamorphic sole and igneous crust of the semail ophiolite new constraints on the tectonic evolution during ophiolite formation from high precision u pb zircon geochronology abstract the semail oman united arab emirates and other tethyan type ophiolites are underlain by a sole consisting of greenschist to granulite facies metamorphic rocks as preserved remnants of the underthrust plate sole exposures can be used to better understand the formation and obduction of ophiolites early models envisioned that the metamorphic sole of the semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement however calculated pressures from granulite facies mineral assemblages in the sole suggest the metamorphic rocks formed at 35 km depth and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle up to 15 20 km we have used high precision u pb zircon dating to study the formation and evolution of the metamorphic sole at two well studied localities our previous research and new results show that the ophiolite crust formed from 96 12 95 50 ma our new dates from the sumeini and wadi tayin sole localities indicate peak metamorphism at 96 16 and 94 82 ma 0 022 to 0 035 ma respectively the dates from the sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust and were later juxtaposed with the base of the ophiolite these data combined with existing geochemical constraints are best explained by formation of the ophiolite in a supra subduction zone setting with metamorphism of the sole rocks occurring in a subducted slab the 1 3 ma difference between the wadi tayin and sumeini dates indicates that in contrast to current models the highest grade rocks at different sole localities underwent metamorphism and may have returned up the subduction channel at different times

ultramafic rock carbonation constraints from listvenite core bt1b oman drilling project the occurrence of the quartz carbonate alteration assemblage listvenite in ophiolites indicates that ultramafic rock represents an effective sink for dissolved co2 however the majority of earlier studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships here we present the first observations on ultramafic rock alteration obtained from the 300 m deep bt1b drill hole icdp oman drilling project allowing for a continuous and high resolution investigation hole bt1b recovered continuous drill core intersecting surface alluvium 200 meters of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 m and 180 m depth and 100 meters of the underlying metamorphic sole textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low angle boundaries in the serpentinite carbonate spheroids are composed of alternating magnesite cores and dolomite rims whereas texturally similar carbonate in the listvenite is composed of fe rich magnesite cores and ca fe rich magnesite rims the distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of bt1b the presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near equilibrium conditions during the final stage of carbonation the carbonate clumped isotope thermometry constrains carbonate crystallization between 50 c and 250 c implying repeated infiltration of reactive fluids during ophiolite uplift and cooling

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation 1 the influence of second phases e g pyroxenes on olivine grain size was studied by quantitative microfabric analyses of samples of the hilti massif mantle shear zone semail ophiolite oman the microstructures range from porphyroclastic tectonites to ultramylonites from outside to the center of the shear zone starting at conditions of ridge related flow they formed under continuous cooling leading to progressive strain localization the dependence of the average olivine grain size on the second phase content can be split into a second phase controlled and a dynamic recrystallization controlled field in the former the olivine grain size is related to the ratio between the second phase grain size and volume fraction zener parameter in the latter dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size in both fields the average olivine and second phase grain size decreases with decreasing temperature combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites 1100 c and mylonites 800 c formed under the predominance of dislocation creep since olivine rich layers are intercalated with layer parallel polymineralic bands in the mylonites nearly equiviscous conditions can be assumed in the ultramylonites diffusion creep represents the major deformation mechanism in the polymineralic layers it is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine grained polymineralic domains at low temperatures 700 c underlining the role of the second phases on strain localization in cooling mantle rocks

modern sand from obducted ophiolite belts sultanate of oman and united arab emirates abstract oman and the united arab emirates host the largest ophiolites on earth and are characterized by arid tropical climate and desert conditions they thus provide an unexcelled natural laboratory in which to study sediments derived from allochthonous oceanic lithosphere and to define an actualistic petrogenetic model for obduction orogen provenance spectacularly exposed in se arabia are two distinct obducted ophiolite sequences sama il and masirah that contrast widely in formation setting spreading rate detachment level thermal state during emplacement and relief masirah beach sand chiefly consists of basaltic diabase to metadiabase plagioclase gabbroic and dunite grains from progressively deeper seated levels of the multilayered oceanic crust dense minerals include diopsidic clinopyroxene and hornblende from gabbros epidote and tremolite actinolite from altered dikes and augite from basaltic lavas in contrast sand from the sama il ophiolite includes cellular serpentinite grains and en

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

the central ailaoshan ophiolite and modern analogs abstract the central ailaoshan cal ophiolite represents an important tectonic component of the jinshajiang ailaoshan song ma suture zone separating the south china and indochina blocks in the mainland se asia the cal ophiolite occurs as a complex tectonic melange and preserves the history of the opening and closure of the once vast jinshajiang ailaoshan song ma branch of the paleotethys new and existing geological data indicate that the cal ophiolite contains magmatic rocks generated by 1 l devonian e carboniferous ca 380 330 ma volcanic passive margin breakup development in the nw gondwana margin 2 l permian ca 258 ma emeishan large igneous province related continental rift magmatism together with and intruded by 3 earliest m triassic ca 244 ma continent continent syn collisional s type granitoids the devono carboniferous suites of the cal ophiolite are highly comparable with many continental margin type alpine tethyan ophiolites in addition the various cal magmatic suites have strong south china block affinities with coeval magmatism particularly in the western south china block jinshajiang song ma and song da terranes

a statistical approach to determination of a mineral lineation abstract a statistical model of the bipolar von mises type was applied to the analysis of mineral lineation defined by the preferred dimensional orientation of inequant grains on a foliation surface orientation data of 203 258 grains from each of four rock specimens fit the model well suggesting a useful analytical technique with three advantages 1 confidence in whether a metamorphic tectonite has a mineral lineation can be determined using a 2 test 2 the orientation of the mineral lineation can be represented by the mean direction with a specified confidence interval and 3 the degree of preferred dimensional orientation of grains can be quantified by the concentration parameter

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