cryptic variation within the cumulate sequence of the oman ophiolite magma chamber depth and petrological implications the cumulate sequence of the oman ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic long lived crustal magma chamber that underlay a late cretaceous spreading ridge regional variations in cumulate sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions microprobe data are presented from a 600 m section of modally layered olivine gabbros coherent cryptic cyclic variation is shown with both normal and reversed geochemical gradients present these are explained in terms of fractionation mixing and eruption in an open system magma chamber the height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m the implications of this result are discussed a contrast between the fo an co variation between olivine and plagioclase in the oman cumulate sequence and that observed for morb votcanics is noted this may reflect compositional stratification in spreading ridge magma chambers or signifcant differences between the petrogenesis of the oman ophiolite and morb

seismic tomography of the north anatolian fault new insights into structural heterogeneity along a continental strike slip fault major funding was provided by the uk natural environment research council nerc under grant ne i028017 1 equipment was provided and supported by the nerc geophysical equipment facility seis uk this project is also supported by bogazici university scientific research projects bap under grant 6922 and turkish state planning organization dpt under the tam project number 2007k120610

the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

structure of the musandam culmination sultanate of oman and united arab emirates and the straits of hormuz syntaxis the musandam mountains form a link both in time and space between the late cretaceous ophiolite obduction tectonics of the oman mountains and the late tertiary continental collision tectonics of the zagros fold belt of iran culmination of the musandam shelf carbonates occurred after the turonian maastrichtian thin skinned thrusting of the oman mountains was completed the basal hagab thrust cuts up section to the west from at least mid permian level up to the late cretaceous aruma group and breached up into the overlying previously allochthonous cretaceous hawasina cherts in the hagil window ras al khaimah the shelf carbonates show from 4 to 15 km of westward translation extrapolation of foreland sequences suggests that upper cretaceous aruma group and hawasina complex sediments could occur in the subsurface along the footwall east of the mountain front the hagab thrust and musandam structures plunge south southwest at increasingly greater depth and with decreasing amount of translation and carry all overlying late cretaceous thrust sheets of the oman mountains in a piggy back fashion the eocene lr oligocene pabdeh foredeep developed as a result of loading in front of the musandam thrust sheets wedging out to the south the trailing south eastern edge of the musandam culmination is a hinged listric normal fault downthrowing to the south east up to 2 5 km at bayah and less than 500 m at the western end of batha mahani and is interpreted as a dorsal culmination collapse feature

metalliferous sediments and the scavenging residence time of nd near hydrothermal vents the isotopic composition of nd is uniform in metalliferous sediments formed at distances varying from 1000 km to within 10 km of the east pacific rise epr palaeoridge these data indicate that hydrothermal vent fluids despite having concentrations more than 500 times greater have no effect on the nd isotopic composition of seawater this implicates efficient scavenging of hydrothermal nd by particulates resulting in extremely short residence times 1 year close to the hydrothermal vents therefore nd isotopic studies of ancient seawater precipitates particularly metalliferous sediments cannot be used to delimit the magnitude of past hydrothermal circulation without independent constraints on local scavenging rates or residence times relative to those of modern oceans

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

duality of magmatism in the plutonic sequence of the sumail nappe oman abstract detailed field mapping of the crustal sequence in several areas of the sumail ophiolite nappe mainly in the fizh the salahi and the haylayn blocks and comparisons with other well studied areas have shown that the crustal sequence was constructed by two distinct although nearly contemporaneous magmatic series the first a gabbroic series was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit the high level isotropic gabbro unit and their by products the sheeted dyke complex and the lower extrusives fine grained homogeneous planar laminated gabbros commonly occur at the top of the layered gabbro sequence in some specific areas wadi haymiliyah and wadi rajmi these are strongly differentiated fe ti rich gabbronorites and norites which coincide with large scale discontinuities within the plutonic sequence interpreted as axial discontinuities of the spreading magma chambers the second series an ultramafic wehrlitic series is locally rooted in the upper mantle i e the impregnated dunitic transition zone this has intruded the whole crustal sequence up to the pillow lavas countless discordant and concordant wehrlitic bodies intruded the plutonic sequence making up 20 40 of it in volume we describe the various settings and forms of these intrusions which were largely underestimated in previous studies we also describe their mineralogy and textures and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems independently from each other we infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion at the base of the crust however indications of immiscibility between gabbros and wehrlites are common suggesting that both magmatic series were quite contemporaneous the wehrlites are commonly isotropic and may exhibit an internal layering of the dunite wehrlite clinopyroxenite type sometimes appearing at the upper or lower margins of the intrusions in this case the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros these plagioclase free ultramafic layered cumulates therefore are not interbedded within the gabbro cumulates but actually belong to the wehrlitic intrusive series picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives which are for the main part cogenetic with the wehrlites the time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre we believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust

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

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

simultaneous inversion of active and passive source datasets for 3 d seismic structure with application to tasmania 1 refraction wide angle reflection and teleseismic arrival time data are combined in a simultaneous inversion for 3 d lithospheric p wavespeed and moho geometry beneath tasmania southeast australia a new iterative non linear tomography scheme which incorporates a novel grid based method of traveltime prediction is used to obtain the solution model synthetic resolution tests demonstrate that the crucial issue of trade off between interface depth and layer velocity is satisfactorily resolved in contrast to previous results from separate inversions of the active and passive source datasets the new images reveal a zone of elevated wavespeed beneath the cambrian mt read volcanics and indicate that both crustal thinning and elevated wavespeeds occur beneath northeast tasmania which supports the case for the existence of a prior passive margin otherwise most major inferences from the previous studies including evidence for remnant subduction and crustal shortening are supported by the new results

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 volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

magma chambers at oceanic ridges how large seismic models of magma chambers at fast spreading oceanic ridges predict small perched magma chambers above a wide low velocity zone seismic attenuation in a low velocity zone allows for 10 melt in hot gabbros in order to reconcile these data with a structural model of magma chambers that is supported by compelling field data in the oman ophiolite we propose that the low velocity zone belongs to the magma chamber in such a dynamic magma chamber gabbros develop strong shape fabrics during magmatic flow in a concentrated suspension this process results in a drastic reduction of the melt fraction that is compatible with magmatic flow as confirmed by preliminary experiments we propose here that dynamic magma chambers below ridges may contain as little as 10 melt

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

interactions between magma and hydrothermal system in oman ophiolite and in iodp hole 1256d fossilization of a dynamic melt lens at fast spreading ridges the transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system it is therefore critical to our understanding of fast spreading ridge accretion processes we present maps of two areas of the oman ophiolite where this transition zone is observed as continuous outcrops our observations which include the base of the sheeted dike being crosscut by gabbros are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens dike assimilation is a possible mechanism for incorporating hydrated phases which result from hydrothermal alteration to the melt lens during upward migrations of its upper boundary upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths this granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity clinopyroxenes present in these granoblastic lithologies are typically low in ti and al content thus strongly contrasting with corresponding magmatic clinopyroxene this may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof which represent the partly fossilized melt lens melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler melt lens migration recrystallization of hydrothermally altered sheeted dikes during reheating stages and assimilation processes observed in the oman ophiolite are consistent with the observations made in iodp hole 1256d we propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption replenishment stages or variations in the hydrothermal magmatic fluxes

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

microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers abstract flow laws for high temperature creep of olivine plagioclase and diabase are used to place constraints on the rheology of partially molten lower oceanic crust this analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene extrapolation of experimental flow laws indicates that at temperatures above 1100 c and stresses less than 10 mpa olivine may be the weakest phase in rocks with gabbroic composition by accounting for variations in the melt fraction 0 10 and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep calculated effective viscosities range from 10 15 to 10 19 pa s based on observations of the geometry of the partially molten zone beneath the east pacific rise and the microstructural and experimental constraints we favor a value of 10 18 pa s this value approaches estimates for the viscosity of the upper mantle beneath ridge axes but is significantly higher than previously suggested for the partially molten lower crust such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution however the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2d sheet like fashion

segmentation at a fossil spreading axis the plutonic sequence of the wadi haymiliyah area haylayn block sumail nappe oman abstract wadi haymiliyah in the central part of the haylayn block offers a splendid continuous section across the crustal sequence of the sumail ophiolite the plutonic sequence of this area presents several specific features of great interest for the understanding of the structure and evolution of this ophiolite the petrological moho and the whole plutonic sequence display a subvertical attitude parallel to the mean dyke complex orientation high temperature folding occurred in the mantle sequence and in the lower crust whereas the upper crust remained unfolded and behaved as a rigid slab during this deformation resulting in a tectonic decoupling between lower and upper crust in the gabbro cumulate sequence a remarkable large scale curvature of the magmatic structures layering and lamination planes whose axis is totally discordant with the regional folds axes is interpreted as a primary magmatic feature indicating the southern ending of a magma chamber the lower crust about 2 km thick consists of a primary layered gabbroic sequence consisting of rhythmic mineral graded layers from several metres to several hundred metres thick syndepositional structures such as oblique stratification microslumps eroding mini channels and small normal faults are widespread the mineralogical composition modal variations and cryptic evolution of the whole layered sequence are described both at the megascale and on the simple cyclic scale the data support a model of crystallization of the layered sequence by open system fractionation at the floor and sides of a periodically replenished steady state magma chamber a unique feature of this cumulate sequence is the ubiquitous presence of orthopyroxene throughout the whole cumulate pile this is very unusual in the sumail crust with the exception of the wadi rajmi section in the fizh block two huge plagioclase free dunite wehrlite laccoliths about 300 m thick and numerous ultramafic sills and mushroom like bodies intrude the base of the layered sequence these ultramafic intrusions exhibit in places an internal layering and dunitic differentiation products they have sharp sinuous and lobate contacts with the gabbros which were still hot and probably not entirely consolidated at the moment of the intrusion the mineral compositions of these intrusions are much more primitive than those of the adjacent gabbros higher in the plutonic section the layered gabbros are overlain by 1 km of fine grained non layered planar laminated gabbronorites characterized by the abundance of pleochroic cumulus hypersthene the disappearance of olivine and the development of fe ti oxides and amphibole as post cumulus phases numerous occurrences of concordant coarse grained layered gabbros occur in this unit which is characterized also by a dense net of highly residual dioritic to tonalitic veins resulting in a huge magmatic breccia in the uppermost few hundred metres where the gabbronorites are brecciated by the acidic rocks hydrothermal veins are also extremely abundant in this unit which coincides with the core of the large scale curvature this unit comprising unusually fractionated rocks is interpreted as resulting from the evolution of the haymiliyah magma chamber towards a closed system fractionation due to the cutting off of source supply our interpretation of both structural and petrological data may be summarized by the following major points 1 1 the haymailiyah magma chamber was located at the southern tip of a spreading cell and was cut off from the magma supply at the end of its evolution either vertically when accretion slowed down or horizontally when it became isolated from the central part of the spreading segment we suggest that orthopyroxene bearing cumulates should be looked for all along the sumail nappe and used as markers of segmentation 2 2 early high temperature compressional deformation took place in both the mantle and crust before the intraoceanic detachment of the ophiolite the development of this compression regime activated filter pressing processes and in particular helped with the extraction of wehrlitic magmas from the upper mantle and their ascent to the uppermost levels of the crust

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

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

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

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

upper mantle structure of the northeastern arabian platform from teleseismic body wave tomography abstract a thick infra cambrian to phanerozoic sedimentary sequence in eastern arabia conceals a poorly known neoproterozoic basement that once separated the currently exposed arabian shield western arabia from nw india in gondwana in this study the arabian platform eastern arabia is targeted by relatively high resolution passive source seismic tomography for the first time which aims to illuminate crust and upper mantle structure in order to make inferences on their nature and origin this is facilitated by a new broadband seismic network of 55 stations in the united arab emirates uae which has recorded sufficient teleseismic body wave data to allow for the determination of three dimensional seismic structure using a grid based eikonal solver and a subspace inversion technique implemented in fmtomo relative arrival time residuals from teleseismic earthquakes are mapped as 3 d p wave velocity perturbations in the upper mantle beneath the seismic network the resultant tomographic images reveal a marked transition from higher velocities in the gulf of oman to lower velocities in the west that we interpret to indicate the lithospheric boundary between cretaceous tethyan oceanic lithosphere and the arabian passive continental margin the northern extent of tethyan oceanic lithosphere appears to terminate against a possible offshore continuation of the dibba fault until now only identified onshore further inland the tomographic model exhibits a number of low and high velocity anomalies with a predominant nw orientation significantly one well defined high velocity anomaly is situated along strike of the neoproterozoic exposure of volcanic arc granodiorites found in northern oman we relate this anomaly to an intra oceanic arc created during the tonian subduction of the mozambique ocean which strongly support the idea of continental growth in the arabian platform via magmatic arc accretion

geochemical variability of the oman ophiolite lavas relationship with spatial distribution and paleomagnetic directions 1 the multielemental composition of 31 lavas sampled in the extrusive section of the oman ophiolite was determined by inductively coupled plasma source mass spectrometry icp ms this study allowed us to define clear geochemical criteria to characterize the different lavas types in oman most of the oman ophiolite extrusive sequence is composed of lavas of composition similar to present day morb with the exception of slight nb ta negative anomalies v1 magmatism v1 lavas display ree patterns and zr hf ratios in the range of morb zr hf 33 15 38 7 in contrast the overlaying v2 lavas which outcrop only in the northern part of the ophiolite are ree depleted relative to v1 and display low zr hf ratios 23 6 30 5 v2 lavas may be further classified into two sub groups v2 type i lavas display a continuous decrease from hree to lree the upper v2 type ii lavas are more depleted in hree and mree but they are enriched in lree they are also distinguished by their low nb ta ratios 10 53 11 65 relative to other oman basalts v1 12 5 14 v2 type i 12 35 14 4 as for n morb melting of an upwelling mantle beneath an oceanic spreading centre formed v1 lavas v2 resulted from fluid enhanced melting of previously depleted mantle residual after v1 extraction this process probably occurred at different melt rock ratios thus resulting in the two v2 sub groups and was enhanced in the northern part of the ophiolite v3 lavas overlie the v1 v2 sequence in the salahi area they display incompatible element rich patterns lree mree hree and a high nb ta ratio typical of within plate basalts in the different studied areas the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data

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

geology and petrology of the plutonic complexes in the wadi fizh area multiple magmatic events and segment structure in the northern oman ophiolite 1 multiple magmatic events are recorded in the gabbroic unit in the fizh area of the northern oman ophiolite gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro wehrlitic intrusions wehrlite i mark the third event of magmatism these three magmatic events occurred at the retreating dying ridge axis because all these rocks are intruded by dolerite dike swarm which is generally regarded as a precursor of advancing ridge axis the next stage of magmatism is a main phase of oceanic crust generation in this area wehrlite ii and then gabbronorite dikes intrude the still hot main gabbro unit all of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit after considerable cooling of the main gabbro unit primitive basalt dikes intrude the main gabbro unit which may correspond to the lasail unit finally the fizh south complex intrudes into considerably cooled crustal sequence being below the brittle plastic transition temperatures the fizh south complex which was regarded as a common wehrlitic intrusion is significantly different from all of the above mentioned rocks with respect to the covariation between plagioclase and associating mafic minerals crystallization order and clinopyroxene compositions the clinopyroxenes are characterized by extremely low ti and na contents comparable with those of the v2 unit alley volcanics suggesting that the fizh south complex correlates with the plutonic facies of the v2 unit during arc stage layered gabbros in the wadi zabin area about 10 km north of the fizh area may be a northern extension of the gabbro blocks of the fizh area because they are intruded by numerous dolerite dikes on the other hand basin like structure of the main gabbroic unit in the northern end of the fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion on the basis of these lines of evidence we propose that the fizh area indicates the northward propagation tip of the ridge axis

moist morb axial magmatism in the oman ophiolite the evidence against a mid ocean ridge origin the oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid oceanic ridge basalt morb its internal structure is regarded by many as being directly comparable to ocean lithosphere from the east pacific rise however there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn because the morb like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites to some this implies that the entire ophiolite formed above a subduction zone others maintain that it formed at a true open ocean mor and that the water required to generate the arc like magmas derived from descending near axis hydrothermal fluids or from ancient subduction a popular compromise posits that the axial suite formed at a true mor and the later magmatism documents the initiation of obduction we test these models by reexamining the morb like character of the early axial lavas and dikes we show that fractionation trends require the presence of water at concentrations significantly higher than any open ocean morb instead trends are identical to those of backarc basin and intraoceanic forearc volcanics by showing that the entire ophiolite formed in a hydrous system we rule out all models in which the oman ophiolite was generated at an open ocean mor instead it formed at a submarine spreading center above a probably newly initiated subduction zone

the petrogenesis of leucogranitic dykes intruding the northern semail ophiolite united arab emirates field relationships geochemistry and sr nd isotope systematics the khawr fakkan block of the semail ophiolite united arab emirates exhibits a suite of 10 100 m scale metaluminous to peraluminous granitic intrusions ranging from cordierite andalusite biotite monzogranites to garnet tourmaline leucogranites which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto ophiolite in an intra oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite moho compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a h2o and pressures of 3 to 5 kbar the lile sr rb and ba covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments initial 87sr 86sr ratios of analysed dykes vary between 0 710 and 0 706 at initial nd values of between 6 3 and 0 5 cogenetic units of a composite sill from ra s dadnah yield a sm nd isochron age of 98 8 9 5 ma mswd 1 18 geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill haybi complex sediments preserved as greenschist asimah area to granulite facies bani hamid area ophiolitic metamorphic sole terranes the sr nd isotope systematics suggest that hybrid granitic melts were derived from pre magmatic mixing of two contrasting subduction zone sources

kar and 40ar39ar study of metamorphic rocks associated with the oman ophiolite tectonic implications abstract k ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the sumail ophiolite amphibolites lying at the sole of the ophiolite yield ages of 95 100 ma which are viewed as reflecting times of crystallization high pressure metamorphics of the saih hatat reveal complex results white micas range in age from 80 to 131 ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas investigation of a few white micas by the 40 ar 39 ar step heating method yields rather intricate age spectra featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between two explanations can be equally envisaged for these convex upward age spectra the first is the mixing of two generations of micas corresponding to two main metamorphisms the first one m1 is a low to medium temperature high pressure event which conceivably occurred between 130 and 114 m y ago the second m2 overprints m1 and has produced rocks typical of the greenschist facies it took place 80 m y ago and also affected the sole of the ophiolites the second is the presence of excess argon in mica mixtures with complex degassing properties thus the two metamorphic phases identified by microscopic inspection are not significantly different in age they occurred in the 70 80 ma interval moreover k ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites assuming that metamorphism is a tracer of tectonic events we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center nevertheless the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable based on metamorphic ages for the motion shift of africa relative to eurasia during the late cretaceous a tentative model of the obduction of the sumail ophiolite is proposed assuming an age of about 130 ma for m1 we stress differences with respect to earlier reconstructions which advocated an 80 m y old blueschist metamorphism

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

basement structure of the united arab emirates derived from an analysis of regional gravity and aeromagnetic database abstract gravity and aeromagnetic data covering the whole territory of the united arab emirates uae have been used to evaluate both shallow and deep geological structures in particular the depth to basement since it is not imaged by seismic data anywhere within the uae thus the aim has been to map the basement so that its structure can help to assess its control on the distribution of hydrocarbons within the uae power spectrum analysis reveals gravity and magnetic signatures to have some similarities in having two main density susceptibility interfaces widely separated in depth such that regional residual anomaly separation could effectively be undertaken the upper density susceptibility interface occurs at a depth of about 1 0 km while the deeper interface varies in depth throughout the uae for gravity this deeper interface is assumed to be due to the combined effect of lateral changes in density structures within the sediments and in depth of basement while for magnetics it is assumed the sediments have negligible susceptibility and the anomalies unrelated to the volcanic magmatic bodies result from only changes in depth to basement the power spectrum analysis over the suspect volcanic magmatic bodies indicates they occur at 5 km depth the finite tilt depth and finite local wavenumber methods were used to estimate depth to source and only depths that agree to within 10 of each other were used to generate the depth to basement map this depth to basement map to the west of the uae oman mountains varies in depth from 5 km to in excess of 15 km depth and is able to structurally account for the location of the shear structures seen in the residual magnetic data and the location of the volcanic magmatic centres relative to a set of elongate n s to ne sw trending basement highs the majority of oilfields in the uae are located within these basement highs therefore the hydrocarbon distribution in the uae basin appears to be controlled by the location of the basement ridges

sedimentological and structural evolution of the arabian continental margin in the musandam mountains and dibba zone united arab emirates the northernmost extremity of the oman mountains the musandam peninsula is composed of an allochthonous sequence of permian to middle cretaceous shelf carbonates these are separated from ophiolitic rocks to the south by a northeast southwest trending belt known as the dibba zone this structurally complex belt is composed of allochthonous slope and basin facies sediments haybi volcanic rocks oman exotic limestones sub ophiolitic metamorphic rocks and ultra mafic slices lithofacies correlation confirms that fragmentation and rifting of a vast east facing carbonate platform occurred in middle to late triassic time resulting in the establishment of a shelf edge and small ocean basin this margin was typified from northwest to southeast by ooid skeletal lime sand shoals or small bioherms on the shelf edge a bypass foreslope of well laminated periplatform ooze and basin margin accumulations of carbonate turbidites or debris flows the shelf edge now coincides with the northern boundary of the dibba zone and appears to have remained stationary from middle triassic to late jurassic time periods of arrested carbonate sedimentation on the platform either because of exposure late triassic and middle cretaceous or because of drowning late jurassic are represented in the basin by starved sedimentation and deposition of radiolarian cherts a spectacular platform margin collapse occurred in the middle and late cretaceous as represented by massive conglomerates with clasts as young as albian above an unconformity that progressively removed all of the lower cretaceous the tethyan basinal and ophiolitic rocks of the dibba zone were emplaced from the east southeast during the turonian to lower maastrichtian after emplacement of these allochthonous units compressional deformation in the mid tertiary resulted in large scale open whaleback folds with wavelengths as much as 15 km generally with north south axes in places these folds which affect the complete shelf and allochthonous sequences are overturned toward the west and thrusting has caused previously lower tectonic units of the late cretaceous stacking order to be thrust over previously higher tectonic units thus reversing the late cretaceous tectonostratigraphy the maximum amount of translation on the later thrusts is in excess of 5 km on the hagab thrust where the complete shelf carbonate sequence of the musandam mountains has been thrust west northwest over the previously higher hawasina and haybi thrust sheets the tertiary folding and thrusting can be correlated in time and space with the zagros fold belt of southwestern iran

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

metalliferous sediments within lava sequences of the sumail ophiolite oman mineralogical and geochemical characterization origin and evolution abstract lens shaped siliceous pelagic sediments and metalliferous deposits sampled in the wadi salahi block of the sumail ophiolite are interbedded within the successive lava flows of the extrusive sequence petrological mineralogical and geochemical characteristics of these sediments allow their oceanic environment of deposition and their early diagenetic and later hydrothermal transformations to be defined the sediments are primary radiolarian oozes deposited in a highly productive and open equatorial pelagic realm the biogenic components are mixed with hydrothermal metallic compounds similar to those found in the axial zone of modern active ridges the deposits are subsequently transformed by various oceanic hydrothermal events which are expressed by specific secondary parageneses the sedimentary lenses from the lower lava sequence are epidotized whereas the uppermost lenses from the upper lava unit contain pumpellyite some peculiar lenses are not affected by these oceanic metamorphisms for instance the deposits closely associated with the zuhan orebody which are fe and silica rich with scarce sulfates or the manganiferous sediments located at the top of the lower lava sequences which were earlier lithified by the deposition of the overlying lava flows nowhere in the whole sedimentary section described were detrital or volcaniclastic remains found disproving the possibility that these sediments may have been deposited in a seamount or island arc environment

structure and dynamics of ridge axial melt lenses in the oman ophiolite 1 the oman ophiolite is regarded as derived from a fast spreading oceanic ridge as in its present day marine analogs expectedly the gabbro unit crystallized in a large magma chamber underneath a small melt lens in ophiolites this melt lens is reduced to a horizon mapped in the field where the floor and the roof of the melt lens joined the magmatic activity of the former melt lens has been studied in gabbros located below the melt lens horizon beneath a stable melt lens gabbros subside and drift through the steep walls of the magma chamber as steeply foliated gabbros in contrast in unstable melt lenses more specifically considered here flat lying gabbros are exposed beneath the lens these gabbros were settling on the floor while the melt lens was retreating their strong magmatic foliations and lineations point to a dynamic deposition on the floor and lineation trends record the structure of convection rolls within the lens time constraints suggest a 100 year periodicity of melt recharge by short and powerful melt pulses followed by a shorter time of convection activity after recharge and a longer period of static magma settling as recently proposed for the east pacific rise at each new melt pulse the lens nearly doubles its volume by surface expansion within a short time lapse before slowly and progressively retreating to feed crustal accretion large horizontal surface inflation of the lens better explains the seawater hydration of the accreting crust than melt lens vertical motion through the lid

numerical models of magma chambers in the oman ophiolite long standing studies conducted in the oman ophiolite have led to models of fast oceanic spreading centers which in some aspects contradict the results of geophysical studies carried out along the east pacific rise epr this is true in particular for the shape size and internal magmatic flow of magma chambers thus justifying numerical modeling our modeling founded on the equations of fluid mechanics is based on a two dimensional finite difference method that takes into account high viscosity contrasts at the walls of the magma chamber boundary conditions deduced from the geophysical data of the epr and geological observations from the oman ophiolite the model predicts a magma chamber shape consistent with field studies whether the magma feeding is from the top by the melt lens or from the bottom by basaltic sills it has only a weak influence on the shape of the chamber the viscosity of the magma plays a crucial role for the setting of convection in the chamber according to geological observations which argue against the existence of convective flow patterns the viscosity of the gabbroic mush within the chamber must exceed 1014 1015 pa s depending on the oceanic plate velocity the flow pattern in the magma chamber and the associated foliation pattern rely critically on whether the underlying upwelling mantle is passive foliations oriented inward or active foliations oriented outward the active structural analysis in the oman model is supported by ophiolite data

complexity of the crustal sequence in the northern oman ophiolite fizh and southern aswad blocks the effect of early slicing abstract on the basis of detailed mapping of the fizh and southern aswad blocks in northern oman the complexity of the crustal sequence is shown lateral facies variations are at least as important as vertical ones and no section can be reconstructed as a stratigraphic section sharp and transitional moho boundaries alternate over a distance of 10 km the important residual mantle crust transition zones are root zones for intrusive wehrlites and cause doming in the moho fractures facilitating the penetration of seawater in the upper crustal parts dikes and lavas show intense hydrothermal alteration and can be related to shear zones in the lower parts of the crustal sequence and occasionally be followed into the peridotites intrusive wehrlites reach as high into the crust as the base of the dike complex and occasionally even the lavas they follow fault zones or n s to nw se directions recrystallization of isotropic gabbro occurs in the vicinity of the wehrlites the geometry observed is not compatible with an oceanic transform zone it may indicate the location of an overlapping spreading centre although too little is known of the deeper structures to confirm this there was certainly an important reaction to the early compression at the oceanic spreading ridge prior to final consolidation confirming an earlier model of the setting

tectonic development of the samail ophiolite high precision u pb zircon geochronology and sm nd isotopic constraints on crustal growth and emplacement 1 new high precision single grain u pb zircon geochronology and whole rock nd isotopic data provide insight into the magmatic and tectonic development of the samail ophiolite the analyzed rocks can be broadly divided into two groups based on their structural position dates and isotopic composition an older group related to on axis magmatism and a younger group of post ridge dikes sills and stocks on axis gabbros tonalites and trondhjemites yielded th corrected 206pb 238u dates from 96 441 0 062 to 95 478 0 056 ma these dates combined with dates from rioux et al 2012 suggest that most of the ophiolite crust formed at an oceanic spreading center in 1 ma the post ridge intrusions come from all depths in the crust the upper mantle and the metamorphic sole post ridge gabbros tonalites and trondhjemites from the crust and mantle yielded th corrected 206pb 238u dates of 95 405 0 062 to 95 077 0 062 ma a small trondhjemitic pod from the metamorphic sole yielded younger th corrected 206pb 238u dates of 94 90 0 38 to 94 69 0 12 ma isotopic data suggest two distinct sources for the post ridge magmas five of the gabbros and tonalites from the crust have end 96 ma 6 90 0 12 to 7 88 0 16 and two trondhjemites from the upper mantle and metamorphic sole have end 96 ma 7 77 0 08 and 7 01 0 16 the negative end t and u pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite 0 25 0 5 ma after formation of the crust the bimodal isotopic composition of post ridge magmas may reflect coeval decompression and or fluid fluxed melting of the mantle and melting dehydration or assimilation of sediment in the down going plate at this time the new data place temporal constraints on mid ocean ridge and supra subduction zone models for ophiolite formation

k ar ages of the metamorphic sole of the semail ophiolite implications for ophiolite cooling history k ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the semail ophiolite the hornblende ages of 99 0 5 and 102 0 8 ma and the 90 ma ages of coexisting micas from the high grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism t 800 850 c p 6 5 9 kbar the new pressure estimates are based on findings of kyanite in garnet amphibolite and cordierite in quartzitic rocks these data indicate a cooling rate of 10 30 c ma the oldest mica ages of 95 1 ma are observed in the lowest grade greenschists these also largely represent cooling ages but might in part also include formation ages the pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low grade metamorphic zone through the high grade rocks and into the base of the overlying ophiolite radiometric ages of hornblendes 92 3 0 5 and 94 8 0 6 ma indicate that the crustal gabbro sequence cooled below 500 c later than the base of the ophiolite sequence metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate the peak of metamorphism must have been contemporaneous with the main magmatism in the semail ophiolite one of the dated muscovites yields an age of 81 3 0 8 ma but this is related to discrete deformation zones that were active during late stage emplacement of the ophiolite

attenuation of surface waves across the arabian peninsula abstract amplitude spectra of fundamental mode rayleigh and love waves are used to determine shear wave q q models for the upper crust of the arabian peninsula the observed spectra are compared to theoretical spectra computed for earthquakes with known fault plane solutions shear wave q in the upper crust can be obtained by trial and error modeling to fit the the slope of the amplitude spectra at short periods 5 30 s if the source time function is less than about 6 s and if the depth of the earthquake is known to within about 3 km shear velocity q values obtained in this way vary from about 60 along the margin of the red sea to 100 150 in the central part of the peninsula to 65 80 in the eastern folded region these values are unusually low for a stable region and suggest that observed anelasticity there is affected by tectonic and epeirogenic activity which has occurred over the past 30 m y the shear wave q models derived from surface waves at periods of 5 30 s closely predict reported values of lg q at 1 hz shear wave q therefore appears to be independent of frequency in this region at periods between about 1 and 30 s

inherited crustal deformation along the east gondwana margin revealed by seismic anisotropy tomography the mechanisms of continental growth are a crucial part of plate tectonic theory yet a clear understanding of the processes involved remains elusive here we determine seismic rayleigh wave phase anisotropy variations in the crust beneath the southern tasmanides of australia a paleozoic accretionary margin our results reveal a complex thick skinned pervasive deformation that was driven by the tectonic interaction between the proto pacific ocean and the ancient eastern margin of gondwana stress induced effects triggered by the collision and entrainment of a microcontinent into the active subduction zone are evident in the anisotropy signature the paleofracturing trend of failed rifting between australia and antarctica is also recorded in the anisotropy pattern as well as a tightly curved feature in central tasmania the observed patterns of anisotropy correlate well with recent geodynamic and kinematic models of the tasmanides and provide a platform from which the spatial extent of deformational domains can be refined

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

kinetic processes for plastic deformation of olivine in the poyi ultramafic intrusion nw china insights from the textural analysis of a 1700 m fully cored succession abstract the ubiquitous presence of undulose extinction and subgrain boundaries in olivine crystals is commonly perceived as originating in the mantle however these plastic deformation features are also well developed in the poyi ultramafic intrusion nw china in this case olivine was deformed through kinetic processes in a crustal magma chamber rather than by deformation processes in the upper mantle moreover accumulation and textural coarsening were critical to the characteristics of crystal size distributions csds of olivines in the poyi intrusion the axial deformational compaction of crystal mush was revealed by virtue of other quantitative textural analyses e g spatial distribution patter alignment factor and aspect ratio additionally based on the contrast of density between crystal matrix and interstitial melt adequate stress was generated by the km scale crystal framework in poyi body 2 11 mpa which triggered the distortion of grain lattice in olivine the deformation mechanisms of olivine primarily are dislocation creep and dislocation accommodated grain boundary sliding disgbs while diffusion creep is subsidiary this study has revealed various kinetic processes in a magmatic system by first demonstrating the genetic relationship between mineral deformation and axial compaction of crystal mush while highlighting the uncertainty of employing the deformation features of olivine in peridotite xenoliths as an indicator for a mantle origin in contrast to the olivine populations of xenocrysts that underwent fragmentation during ascent the deformed primitive olivines in compaction exhibit a distinct shortage of small grains which is conducive to delimiting these two types of deformed grains

crustal and uppermost mantle shear wave velocity structure beneath the middle east from surface wave tomography we have constructed a 3 d shear wave velocity vs model for the crust and uppermost mantle beneath the middle east using rayleigh wave records obtained from ambient noise cross correlations and regional earthquakes we combined one decade of data collected from 852 permanent and temporary broadband stations in the region to calculate group velocity dispersion curves a compilation of 54000 ray paths provides reliable group velocity measurements for periods between 2 and 150 s path averaged group velocities calculated at different periods were inverted for 2 d group velocity maps to overcome the problem of heterogeneous ray coverage we used an adaptive grid parametrization for the group velocity tomographic inversion we then sample the period dependent group velocity field at each cell of a predefined grid to generate 1 d group velocity dispersion curves which are subsequently inverted for 1 d vs models beneath each cell and combined to approximate the 3 d vs structure of the area the vs model shows low velocities at shallow depths 5 10 km beneath the mesopotamian foredeep south caspian basin and the black sea possibly indicating a relatively thin crust in contrast the zagros mountain range the sanandaj sirjan metamorphic zone in western central iran the easternmost anatolian plateau and lesser caucasus are characterized by low velocities at these depths some of these anomalies may be related to thick crustal roots that support the high topography of these regions in the upper mantle depth range high velocity anomalies are obtained beneath the arabian platform southern zagros persian gulf and the eastern mediterranean in contrast to low velocities beneath the red sea arabian shield afar depression eastern turkey and lut block in eastern iran our vs model may be used as a new reference crustal model for the middle east in a broad range of future studies

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

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

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

the structure of the upper mantle beneath the delamerian and lachlan orogens from simultaneous inversion of multiple teleseismic datasets abstract distant earthquake data recorded by seven sub arrays of the ongoing wombat rolling seismic array deployment in southeast australia are combined for the first time to constrain 3 d variations in upper mantle p wavespeed via teleseismic tomography the seven arrays comprise a total of 276 short period recorders spaced at intervals of approximately 50 km thus allowing unprecedented resolution of the upper mantle over a large region in the mantle lithosphere immediately below the crust 50 km depth dominant variations in velocity tend to strike east west and share little resemblance to palaeozoic boundaries in the shallow crust inferred from surface geology and potential field data a broad region of elevated wavespeed beneath northern victoria may represent the signature of underplated igneous rocks associated with detachment faulting during the break up of australia and antarctica a distinct low velocity anomaly in southern victoria appears to correlate well with the quaternary newer volcanic provinces towards the base of the mantle lithosphere the dominant structural trend becomes north south and five distinct velocity zones become apparent of particular note is a transition from higher wavespeed in the west to lower wavespeed in the east beneath the stawell zone implying that the proterozoic lithosphere of the delamerian orogen protrudes eastward beneath the western subprovince of the lachlan orogen this transition zone extends northwards from southern victoria into central new south wales the northward limit of the arrays and is one of the dominant features of the model further east there is a transition from lower to higher wavespeeds in the vicinity of the boundary between the western and central subprovinces of the lachlan orogen which has several plausible explanations including the existence of a proterozoic continental fragment beneath the wagga omeo zone the presence of elevated wavespeeds beneath the melbourne zone in victoria although not well constrained due to limited data coverage provides some support to the selwyn block model which proposes a northward extension beneath bass strait of the proterozoic core of tasmania

high temperature creep of partially molten plagioclase aggregates we have investigated the high temperature creep of synthetic labradorite an60ab40 between 1323 k and 1523 k at atmospheric pressure and low stresses 2 65 mpa average grain size varies from 12 to 16 m samples contained up to 12 vol melt an60ab40 glass fourier transform infrared measurements indicate that trace amounts of water 0 1 0 15 wt h2o were incorporated during crystallization this water was only partly released at high temperature and atmospheric pressure at low stresses and temperatures deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n 1 and an activation energy of 365 25 kj mol regime i with increasing stress and temperature mechanical data and microstructural observations indicate a transition to power law creep with n 3 regime ii this transition occurs in both partially molten and melt free aggregates in regime i the strain rate of samples containing as much as 12 vol melt is between 3 and 5 times higher than in fully crystalline specimens the activation energy is only slightly affected by the melt content between 0 and 10 2 vol transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets grain boundaries are not wetted except surrounding large melt pockets the melt distribution of undeformed and deformed samples is similar the strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water both plagioclase compositions are slightly weaker than synthetic diopside

saudi arabian seismic refraction profile a traveltime interpretation of crustal and upper mantle structure abstract the crustal and upper mantle compressional wave velocity structure across the southwestern arabian shield has been investigated by a 1000 km long seismic refraction profile the profile begins in mesozoic cover rocks near riyadh on the arabian platform trends southwesterly across three major precambrian tectonic provinces traverses cenozoic rocks of the coastal plain near jizan and terminates at the outer edge of the farasan bank in the southern red sea more than 500 surveyed recording sites were occupied and six shot points were used including one in the red sea two dimensional ray tracing techniques used to analyze amplitude normalized record sections indicate that the arabian shield is composed to first order of two layers each about 20 km thick with average velocities of about 6 3 km s and 7 0 km s respectively west of the shield red sea margin the crust thins to a total thickness of less than 20 km beyond which the red sea shelf and coastal plain are interpreted to be underlain by oceanic crust a major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition elsewhere along the profile several high velocity anomalies in the upper crust correlate with mapped gneiss domes the most prominent of which is the khamis mushayt gneiss based on their velocities these domes may constitute areas where lower crustal rocks have been raised some 20 km two intracrustal reflectors in the center of the shield at 13 km depth probably represent the tops of mafic intrusives the mohorovicic discontinuity beneath the shield varies from a depth of 43 km and mantle velocity of 8 2 km s in the northeast to a depth of 38 km and mantle velocity of 8 0 km s depth in the southwest near the shield red sea transition two velocity discontinuities occur in the upper mantle at 59 and 70 km depth the crustal and upper mantle velocity structure of the arabian shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the precarnbrian the shield is currently flanked by the active spreading boundary in the red sea

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

microbursts a hazard for aircraft microbursts are believed to have been responsible for several aircraft accidents in recent years we report here laboratory experiments which show that when a descending column of dense fluid reaches the ground and begins to spread out horizontally an intense vortex with a horizontal axis forms at the leading edge of the outflowing air the intensity of this vortex results from the increase in vorticity due to the rapid stretching of the length of the leading edge the properties of such a horizontal vortex with its associated updrafts and downdrafts are consistent with those found in microbursts which are a severe hazard to aircraft when taking off or landing

can high temperature high heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast spreading mid ocean ridges we present numerical models to explore possible couplings along the axis of fast spreading ridges between hydrothermal convection in the upper crust and magmatic flow in the lower crust in an end member category of models corresponding to effective viscosities m lower than 1013 pa s in a melt rich lower crustal along axis corridor and permeability k not exceeding 10 16 m2 in the upper crust the hot melt rich gabbroic lower crust convects as a viscous fluid with convection rolls parallel to the ridge axis in these models we show that the magmatic hydrothermal interface settles at realistic depths for fast ridges i e 1 2 km below seafloor convection cells in both horizons are strongly coupled and kilometer wide hydrothermal upflows plumes spaced by 8 10 km arise on top of the magmatic upflows such magmatic hydrothermal convective couplings may explain the distribution of vent fields along the east epr and south east pacific rise sepr the lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt rich regions pockets in the axial melt lenses of epr and sepr crystallization of this melt provides the necessary latent heat to sustain permanent 100 mw vents fields our models also contribute to current discussions on how the lower crust forms at fast ridges they provide a possible mechanism for focused transport of melt rich crystal mushes from moho level to the axial melt lens where they further crystallize feed eruptions and are transported both along and off axis to produce the lower crust

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains abstract 170 m y of relatively stable passive margin sedimentation along the northern continental margin of arabia was abruptly terminated during the cenomanian turonian 95 ma when the oman continental margin collapsed and subsided rapidly aruma basin to accommodate obduction of the semail ophiolite complex and underlying thrust sheets haybi and hawasina complexes in the oman mountains the ophiolite was emplaced at least 200 km over the passive margin probably over 450 km in total from ne to sw during the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite bearing rocks 6 8 kbar blueschist 12 15 kbar and eclogite ca 20 kbar facies metamorphism formed in a ductile deforming ne dipping subduction zone along the northeastern margin of the saih hatat culmination in ne oman the mesozoic shelf carbonates are deformed by recumbent folding on all scales the present day structure is dominated by four sw vergent thrust slices in carpholite grade rocks at higher structural levels overlying lower units of crossite bearing blueschist hulw unit garnet glaucophane chloritoid blueschists and garnet clinopyroxene phengite glaucophane eclogites 15 20 kbar the largest shear zone separates 7 8 kbar hulw unit blueschists above from 12 15 kbar retrogressed eclogites and ca 20 kbar eclogites of the as sifah unit below further inboard sw a major shear zone separates the lower plate hulw blueschist facies rocks from the upper plate sedimentary rocks upper plate lower plate discontinuity which also show carpholite grade hp metamorphism towards the ne restoration of the entire saih hatat continental margin shows that the upper plate lower plate discontinuity cuts down section to the sw as far as the proterozoic hatat schists ne directed extensional crenulation schistosity and nne oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with sw directed exhumation of footwall hp rocks ne facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds with shortening in the upper plate balanced by the subduction of the lower plate consistent with a ne directed subduction of the continental margin rocks beneath the sw obducting ophiolite haybi and hawasina thrust sheets recent suggestions of a nascent sw directed subduction beneath the oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of saih hatat two main phases of tectonic uplift of saih hatat and jebel akhdar are firstly the latest cretaceous ramp culmination associated with foreland propagating thrusts in the sw and backfolds in the ne and secondly post late eocene oligocene uplift by ca 2000 m

tornadoes and downbursts in the context of generalized planetary scales abstract in order to cover a wide range of horizontal dimensions of airflow the author proposes a series of five scales maso meso miso to be read as my so moso and muso arranged in the order of the vowels a e 1 o u the dimensions decrease by two orders of magnitude per scale beginning with the planet s equator length chosen to be the maximum dimension of masoscale for each planet mesoscale highs and lows were described on the basis of mesoanalyses while sub mesoscale disturbances were depicted by cataloging over 20 000 photographs of wind effects taken from low flying aircraft during the past 15 years various motion thus classified into these scales led to a conclusion that extreme winds induced by thunderstorms are associated with misoscale and mososcale airflow spawned by the parent mesoscale disturbances

structural geology of the rub al khali basin saudi arabia the rub al khali basin lies below a quaternary sand sea and the structural evolution from the late precambrian to neogene is known only from reflection seismic gravity and magnetic data and wells gravity and magnetic data show north south and northwest southeast trends matching mapped precambrian faults the deepest structures imaged on reflection seismic data are undrilled precambrian rifts filled with layered strata at depths up to 13 km the distribution of ediacaran cambrian ara hormuz mobile salt is restricted to an embayment in the eastern rub al khali the precambrian rifts show local inversion and were peneplained at base phanerozoic a broad crustal scale fold qatar arch developed in the carboniferous and amplified in the late triassic separating subbasins in the west and east rub al khali a phase of kilometer scale folding occurred in the late cretaceous coeval with thrusting and ophiolite obduction in eastern oman these folds trend predominantly north south oblique to the northwesterly shortening direction and occasionally have steep fault zones close to their axial surfaces the trend and location of these folds closely matches the precambrian lineaments identified in this study demonstrating preferential reactivation of basement structures compression along the zagros suture reactivated these folds in the neogene this time the result of highly oblique north northeast to south southwest shortening cretaceous tertiary fold style is interpreted as transpression with minor strain partitioning permian jurassic and eocene evaporite horizons played no role in the structural evolution of the basin but the eocene evaporites caused widespread kilometer scale dissolution collapse structures in the basin center

controls on the sr and nd isotopic compositions of hydrothermally altered rocks from the pindos ophiolite greece abstract the sr and nd isotopic behaviour of hydrothermally altered basaltic rocks from the pindos ophiolite nw greece has been investigated samples were obtained mainly from rocks affected by intense hydrothermal alteration in field areas believed to represent hydrothermal discharge zones formed at a mesozoic seafloor spreading centre 87 sr 86 sr ratios in the altered rocks show a narrow range of values intermediate between those of seawater and fresh rock regardless of the degree of alteration this has been interpreted as the result of high overall water rock ratios which were sufficient to produce a homogeneous overprint throughout most of the hydrothermal cell a sr shift index which uses the sr isotopic composition and sr concentration of both rock and fluid has been derived to express the degree of sr exchange between rock and seawater it has been used to investigate the variations in water rock interaction between known hydrothermal systems and to compare ophiolite hosted with modern oceanic systems the results indicate that even when compared on such a uniform basis all the known present day hydrothermal systems are generally more rock dominated than those of pindos and other ophiolites 143 nd 14 nd ratios in the altered rocks are much less markedly affected but may show a shift from original magmatic values the results presented here suggest that this effect cannot simply be attributed to exchange with seawater nd these shifts must result from seawater transporting basaltic nd of a different isotopic composition from elsewhere in the volcanic pile into the zones of intense alteration this study has shown that sr and nd isotopes vary independently as a result of their different original distribution as well as their contrasting geochemical mobility

the southern zagros collisional orogen new insights from transdimensional trees inversion of seismic noise the natural environmental research council nerc grant ne r013500 1 and from the european union s horizon 2020 research and innovation program under marie sklodowska curie grant agreement 790203

oxygen isotope and chemical studies on the origin of large plagiogranite bodies in northern oman and their relationship to the overlying massive sulphide deposits the extraordinarily well preserved and well exposed semail ophiolite of northern oman hosts several large plagiogranite intrusions in proximity to economic copper sulphide deposits of the lasail mining district a progression of isotopic chemical and mineralogical transformations observed within the plagiogranites and high level gabbros hlg and a comparison of these effects with those in the lowermost dykes of the immediately overlying sheeted dyke complex sdc tracks the evolution of hydrothermal fluids and the alteration of overlying dykes and pillow lavas during discharge of these fluids on the sea floor the largest hydrothermal alteration aureoles and the greatest extent of metamorphic veins and metasomatic replacement features are found adjacent to the largest high level plagiogranite bodies beneath and adjacent to the major ore bodies in northern oman the ubiquitous presence of metamorphic actinolitic hornblende sodic plagioclase epidote and titanite in metabasalts within the high temperature alteration zones points to the most likely mineralogical and structural controls on the development and evolution of the hydrothermal fluids depleted cu contents of the adjacent crustal rocks and cu enrichments above the plagiogranite intrusions demonstrate the redistribution of heavy metals adjacent to the complexes field relationships implicate the formation of both the epidosites and plagiogranites in the genesis of the ore deposits an important process inferred from the field and geochemical data is the assimilation of previously hydrothermally altered basaltic and gabbroic country rocks by stoping into the magma chambers developed near the sdc gabbro horizon in the ophiolite we suggest that this process of combined assimilation fractional crystallization together with replenishment and recharge by injection and quenching of basaltic magma pillows into these plagiogranite magma chambers i e rafc plays a major role in the development of these composite intrusions

imaging ruptured lithosphere beneath the red sea and arabian peninsula abstract the red sea rift an archetype of a newly formed ocean basin is an ideal environment in which to study the controversial processes associated with continental rifting different models have been proposed to explain how rifting in the red sea evolved however accurate constraints on lithospheric structure have not been available to discriminate rifting models we use the s wave receiver function technique to produce the first images of the lithosphere asthenosphere boundary lab structure along the red sea and throughout the arabian peninsula lithospheric thickness varies considerably with thin lithosphere centered on the rift axis thickening toward the arabian interior gravity data are well fit by our structural model and indicate that high surface topography along the rift flank is not in isostatic equilibrium requiring dynamic compensation for its support while our derived structure is consistent with active rifting processes previous studies demonstrated that the red sea initiated as a passive rift therefore our results suggest a two stage rifting history where extension and erosion by flow in the underlying asthenosphere are responsible for variations in lab depth lab topography guides asthenospheric flow beneath western arabia and the red sea demonstrating the important role lithospheric variations play in the thermal modification of tectonic environments

rapid emplacement of young oceanic lithosphere argon geochronology of the oman ophiolite 40 ar 39 ar dates of emplacement related metamorphic rocks beneath the samail ophiolite in oman show that cooling to

hypersolidus deformation in the lower crust of the josephine ophiolite evidence for kinematic decoupling between the upper and lower oceanic crust new mapping and structural observations in the lower crust of the josephine ophiolite provide insights into the geometry and kinematics of hypersolidus flow beneath an oceanic spreading center the lower crust defined here as the sequence of rocks overlying mantle peridotite and beneath exposure of 100 sheeted dikes can be divided into lower wehrlite dunite and upper gabbroic sections the contact between the two is mutually intrusive where exposed hypersolidus fabrics are the dominant structures observed no pervasive crystal plastic deformation is observed restoration of the hypersolidus foliations and igneous modal layers to their on axis orientation indicates that they strike approximately perpendicular to the strike of the inferred paleo spreading center as defined by the orientation of sheeted dikes and onaxis oceanic faults hypersolidus lineations define a dispersed 3 d flow pattern in the lower crust whereas extension directions in the upper crust i e poles to sheeted dikes oceanic normal faults are unidirectional and perpendicular to the paleo ridge axis collectively these observations are consistent with partitioning of deformation between the upper and lower crust and local ridge parallel extension in the partially molten lower crust due to possible subsidence of the thickened axial upper crust however some component of kinematic coupling between the lower crust and mantle peridotite driven by asthenospheric flow cannot be ruled out q 2003 elsevier ltd all rights reserved

geochemistry and palaeo oceanography of metalliferous and pelagic sediments from the late cretaceous oman ophiolite abstract metalliferous and pelagic sediments are exposed within and above the extrusive successions of the upper cretaceous oman ophiolite which on the basis of mostly geochemical evidence is believed to have formed in an incipient marginal basin setting located above a ne dipping subduction zone the ophiolitic extrusives document various volcano tectonic settings which include the axial zones of a spreading ridge fault controlled seamounts and off axis volcanic edifices most of the fe mn and trace metal enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high temperature sulphide precipitating vents the trace element content e g ree and sr was largely scavenged from seawater the sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges and the basal sediments of dsdp wells and of other ophiolite complexes e g troodos cyprus distinctive mound structures located low in the lavas are attributed to percolation of sulphide rich solutions into already deposited metalliferous oxide sediments the resulting iron silica rock was probably originally precipitated as ferruginous silicates major massive sulphides formed off axis at the base of intermediate basic edifices of volcanic arc affinities fe mn and trace metal enrichment in the sediment cover of a flat topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away lasail small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones the metalliferous sediments locally precipitated near these vents are ferromanganiferous but trace metal depleted the metalliferous and pelagic sediment cover of the extrusive successions generally documents waning hydrothermal input after volcanism ended in the area a model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the arabian continental margin with progressive subduction this crust approached the margin initially calcareous sediment accumulated above the calcite compensation depth ccd but then non calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the ccd was higher and marginal upwelling possibly enhanced productivity as the edge of the arabian continental margin entered the trench the over riding ophiolite was regionally uplifted allowing short lived chalk accumulation above the ccd this was followed by volcaniclastic deposition related to the tectonic emplacement

microstructures of olivine and stresses in the upper mantle beneath eastern china abstract we have examined the microstructure of spinel and garnet peridotite xenoliths from eleven basalt localities in eastern china the dislocation substructures of olivine in all of the specimens are similar to those reported previously from other areas of the world and are strongly reminiscent of the substructures produced in laboratory deformation of polycrystals at high temperature and pressure extensive measurements of 100 and 001 subgrain dimensions demonstrate that in almost all cases the frequency of subgrain spacing follows a lognormal distribution with geometric mean spacings of 30 60 m in one locality mingxi two xenoliths show significantly narrower spacings of 15 20 m indicating a much higher stress while others from the same locality have the normal wider spacing and one specimen has a mixed distribution indicating that the strain produced at higher stress was not sufficient to re equilibrate the microstructure even higher stresses but probably trivial strains are suggested by 110 slip bands sporadically superposed on the normal microstructure which presumably were produced during extraction of the xenoliths by the magma the general correspondence between these results and those of previous studies conducted elsewhere in the world suggests that the general level of lithospheric stress of c 10 50 mpa has been essentially constant since the middle tertiary or that the dynamic process of magma generation and or extraction always leads to similar microstructures

the transition from density driven to wave dominated isolated flows an isolated fluid mass travelling horizontally in a stratified layer is a phenomenon described alternatively as a detached gravity current head or a strongly nonlinear solitary wave a key feature of this flow is the transport of mass laboratory experiments examine the transition in time from a regime in which the flow is density driven to one in which it is wave dominated a simple means of creating this transitional regime an isolated flow that exhibits both density and wave effects is achieved by dropping a thermal into a linearly stratified layer this transitional regime is called an isolated propagating flow parameters for which the transitional regime occurs are identified particle tracking studies reveal the vertical flow structure there is an upper zone that is wave dynamical and a lower zone in which transport of mass occurs the transported mass slowly leaks out until the phenomenon resembles a weakly nonlinear solitary wave the experiments mimic a thunderstorm microburst impacting a temperature inversion which has aviation safety implications in the ocean cracks in the ice cap polar leads cause similar flows impacting the thermocline

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

gravity anomalies crustal thickness and the pattern of mantle flow at the fast spreading east pacific rise 9 10 n evidence for three dimensional upwelling gravity bathymetry and crustal structure data from the 9 to 10 n segment of the fast spreading northern east pacific rise are used to investigate density variations in the mantle in order to determine the pattern of mantle upwelling and melt distribution beneath the segment mantle bouguer gravity anomaly contours are nearly parallel to the ridge axis and there is only 3 4 mgal variation in axial mantle bouguer anomalies along the entire segment however inclusion of the gravity effects of variations in crustal thickness and structure results in a significant change in the pattern of gravity anomalies the subcrustal gravity anomalies show a very distinct gravity low centered over the segment axial depth minimum near 9 50 n axial subcrustal gravity anomalies increase by 10 mgal between 9 50 n and 9 23 n a gradient of 0 2 mgal km since the ridge axis has a uniform cross sectional shape throughout the entire segment with abundant evidence of magmatic activity along its entire length it is unlikely that there are significant variations in crustal thermal structure other than at the ridge tips we thus attribute the subcrustal gravity anomalies to density variations within the mantle the distinct gravity low centered over the depth minimum provides qualitative evidence that mantle upwelling is focused or enhanced at that location the effects of lithospheric cooling and subsidence away from the axis were removed from the bathymetry and free air gravity anomalies to isolate residual gravity and gravity anomalies associated with the axis the residual anomalies were modeled by considering the axial bathymetric high to be a flexural feature resulting from the upward buoyant load of a region of low density material beneath the axis after accounting for the isostatic effects of variations in crustal thickness the remaining mass deficiency necessary to support the axial high and satisfy the gravity anomalies is not evenly distributed along the axis but is 35 greater beneath the axial depth minimum at 9 50 n than between 9 17 and 9 27 n mantle upwelling in this segment thus appears focused or enhanced in the area of the depth minimum

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

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com