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

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

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

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

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

locating seismic events in the ctbt context the verification of the comprehensive nuclear test ban treaty ctbt requires the determination of accurate location of seismic events from a fixed network of seismic stations across the globe the requirements of possible on site inspections mean that the goal is to place the location estimate in a zone smaller than 1000 km2that includes the true location because a defined set of stations will be used corrections can be refined to represent the influence of departures from the global reference model iaspei91 the primary stations in the international monitoring scheme ims are mostly seismic arrays and therefore the present location scheme is based on minimisation of a misfit function built from arrival time azimuth and array slowness residuals the effective network will change markedly with the magnitude of the event and as a result regional information has to be integrated into the location process

characterizing earthquake location uncertainty in north america using source receiver reciprocity and usarray abstract the comprehensive nuclear test ban treaty community often uses calibration events with well determined origins to improve absolute locations of nearby seismic events by accounting for the biasing effects of unknown velocity structure but the number of these ground truth events is limited to provide additional constraints source receiver reciprocity allows us to use seismic stations as calibration events with known locations the dense and uniform spacing of the usarray transportable array stations makes them ideal to measure the spatial coherence of mislocation vectors across north america and hence to assess how close calibration events or stations need to be to target events to improve locations for a given region we use a grid search approach for the station relocations using both teleseismic earthquakes and simulated regional events our results show that the mislocation vectors are spatially coherent for scales up to 500 km in many regions but that in some places such as regions that can be associated with strong velocity anomalies in the upper mantle mislocation vectors exhibit large changes over short distances online material figures showing station location and dislocations and travel time residuals

seismic location calibration of the european arctic a crustal velocity model has been developed for fennoscandia the baltic shield and adjacent areas this model represents a simplified average of various models developed for parts of this region we show that p wave travel times calculated with this model provide an excellent fit to observations at the fennoscandian krsc and iris station networks for a set of seismic events with known or very well constrained locations the station event paths cover large parts of western russia and the barents sea thus indicating that this model which we denote the barents model is appropriate for this entire region we show by examples that significant improvements in event location precision can be achieved compared to using the 1aspei model we finally use the barents model to calculate locations of recent small seismic events in the novaya zemlya region of interest in a ctbt monitoring context

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

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

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

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

rill erosion in natural and disturbed forests 2 modeling approaches 1 as forest management scenarios become more complex the ability to more accurately predict erosion from those scenarios becomes more important in this second part of a two part study we report model parameters based on 66 simulated runoff experiments in two disturbed forests in the northwestern u s the 5 disturbance classes were natural 10 month old and 2 week old low soil burn severity high soil burn severity and logging skid trails in these environments the erosion rates were clearly detachment limited and the rill erodibility parameters calculated from four hydraulic variables increased by orders of magnitude the soil shear stress based erodibility parameter kr was 1 5 10 6 s m 1in the natural plots 2 0 10 4 s m 1 in the high soil burn severity plots and 1 7 10 3 s m 1 in the skid trail plots kr values for the low soil burn severity plots had negative sign the erodibility value for the skid trail plots fell within ranges reported for tilled agricultural fields and also for forest roads the kr values decreased as erosion occurred in the plots and therefore should not be a constant parameter the stream power produced the largest r2 value 0 41 when hydraulic predictors and the sediment flux were log transformed but none of the four hydraulic variables soil shear stress stream power unit stream power and unit length shear force explained much of the variability in sediment flux rates across the five levels of disturbance when evaluated in the linear form of the erosion models under consideration

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

the effect of ash and needle cover on surface runoff and erosion in the immediate post fire period abstract hillslopes are thought to be most susceptible to enhanced surface runoff and erosion immediately following wildfire due to removal of protective vegetation and litter cover and in many cases a fire induced reduction in soil wettability this enhanced susceptibility declines as vegetation and litter layer recover for logistical reasons however few studies have been able to examine the responses of burnt terrain immediately following burning and little is therefore known about the effect of the wettable ash layer that often covers the ground until it is redistributed or removed by wind or water erosion here we quantify the effects of ash and needle cast ground cover on surface runoff generation and soil erodibility in the immediate post fire period 2 10 days following a severe wildfire in an aleppo pine forest over calcareous soils in mediterranean eastern spain a homogenous vegetation cover and lack of wind during the fire resulted in 100 ash cover 36 mm mean depth across the study area the ground fire did not consume the tree crowns but its high intensity caused complete tree mortality leading to subsequent needle fall hydrological and erosional responses were measured for fifteen 1 hour rainfall simulations 55 mm h 1 on 0 25 m2 circular plots for each of three terrain types i ash and needle covered ii ash only covered and iii bare furthermore ash and needle cast cover were monitored over an eleven week period on otherwise undisturbed terrain to examine its evolution during the first critical months following burning the soils exhibited no or slight post fire water repellency the overlying wettable ash layer had a porosity of 83 providing a water storage capacity of 30 mm a third of the plots with ground cover did not generate any surface runoff ash needle and ash only plots showed 25 and 18 fold reductions in surface runoff respectively compared to bare terrain mean sediment yield from covered plots was more than two orders of magnitudes lower than from bare plots the ash cover remained unchanged during the first two weeks while overlying needle cast more than doubled heavy rainfall 153 mm over a subsequent six day period removed the ash almost entirely where no needle cast was present whereas on ash and needle covered ground only a limited redistribution and accumulation into litter dams occurred the results quantify the potential importance of ash deposits on the susceptibility of hillslopes to surface runoff and erosion immediately following fire the combination of ash and needle cast appears particularly effective in reducing hillslope responses the outcomes demonstrate clearly that the widely held notion of landscapes being most susceptible to surface runoff and erosion directly after burning is not correct where a fire has resulted in substantial ash deposition instead the susceptibility will be greatest following a storm event of sufficient severity to remove this protective layer and before the onset of vegetation recovery

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

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

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

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

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

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

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

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

locating and modeling regional earthquakes with two stations 1 we developed a new technique caploc to retrieve full source parameters of small seismic events from regional seismograms which include origin time epicenter location depth focal mechanism and moment magnitude despite rather complicated propagation effects at short periods a simple localized one dimensional model can well explain signals of periods 3 10 s if we break the three component records into different segments and allow differential time shifts among them these differential time shifts once established from a calibration process or a well determined tomographic map can be used together with p wave travel times to refine an event s location in this study we tested whether our new method could produce satisfactory results with as few as two stations so that we can improve source estimates of poorly monitored events with sparse waveform data we conducted the test on 28 events in the tibetan plateau the focal mechanisms and locations determined from only two stations agree well with those determined from a whole passcal array in particular our new method produces better locations than international seismological centre with the average mislocation error reduced from 16 to 5 km we also tested whether an event s depth and mechanism can be determined separately from its epicenter relocation in a two step approach we find that the two step approach does not always give the correct solution but the reliability of a solution can be evaluated using a reduced chi square value

spatial heterogeneity of soil detachment capacity by overland flow at a hillslope with ephemeral gullies on the loess plateau abstract ephemeral gullies are typical erosional landforms and widespread on the chinese loess plateau to better understand the spatial variability in soil detachment capacity dc by overland flow and its influence at a hillslope with an ephemeral gully this study investigates a hillslope on the loess plateau using classical statistics geostatistical analysis and principal component analysis undisturbed soil samples were collected from 202 sites along nine 90 m transects in two completely developed ephemeral gullies and were scoured in a laboratory flume under consistent hydraulic conditions the results indicate that dc varied widely from 0 0004 to 1 25 kg m 2 s 1 with a mean of 0 22 kg m 2 s 1 the coefficient of variation also shows a high variability in dc the ephemeral gullies were divided into four sections uppermost upper middle and lower slopes and dc differed significantly among the four sections a semivariogram of dc indicated a moderate spatial dependence the sampling interval significantly affects the spatial pattern of dc when the sampling interval decreased from 10 to 2 m the nugget variance decreased whereas structured variance spatial dependence and range increased distribution maps of dc derived from kriging interpolation showed that samples in the lower slope position have greater dc than the other positions dc significantly correlates with clay content sand content median soil grain size bulk density cohesion water stable aggregate and litter mass density principal component analysis pca and a minimum data set mds method identified that the median soil grain size bulk density and litter mass density were the major factors affecting the spatial variability in dc

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

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

hydraulics and erosion in eroding rills rills often act as sediment sources and the dominant sediment and water transport mechanism for hillslopes six experiments were conducted on two soils and a uniform sand using three experimental methodologies the results of this study challenge the assumption often used in hydrologic and erosion models that relationships derived for sheet flow or larger channel flow are applicable to actively eroding rills velocity did not vary with slope and reynolds number was not a consistent predictor of hydraulic friction this result was due to interactions of slope gradient flow rate erosion and the formation of rill roughness bed structures and head cuts a relationship for rill flow velocities was proposed stream power was found to be a consistent and appropriate predictor for unit sediment load for the entire data set while other hydraulic variables were not the data for stream power and sediment load fit the form of a logistic curve r2 0 93 which is promising relative to recently proposed erosion models which are based on probabilistic particle threshold theory

rill erosion in natural and disturbed forests 1 measurements 1 rill erosion can be a large portion of the total erosion in disturbed forests but measurements of the runoff and erosion at the rill scale are uncommon simulated rill erosion experiments were conducted in two forested areas in the northwestern united states on slopes ranging from 18 to 79 we compared runoff rates runoff velocities and sediment flux rates from natural undisturbed forests and in forests either burned at low soil burn severity 10 months or 2 weeks post fire high soil burn severity or subject to skidding of felled logs the runoff rates and velocities in the natural sites 2 7 l min 1 and 0 016 m s 1 respectively were lower than those in all the disturbed sites 12 to 21 l min 1 and 0 19 to 0 31 m s 1 respectively except for the 10 month old low soil burn severity site where the velocity 0 073 m s 1 was indistinguishable from the natural sites the mean sediment flux rate in the natural sites also was very small 1 3 10 5 kg s 1 as compared to the rates in the disturbed areas 2 5 10 4 to 0 011 kg s 1 the hillslope gradient did not affect the runoff or sediment responses the sediment flux rates generally were greater in the initial stage of each inflow period than in the steady state condition but there was no corresponding transient effect in runoff rates rill erosion modeling implications based on these data are presented in part 2 of this study

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

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

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

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

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

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

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

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

effectiveness of needle cast at reducing erosion after forest fires 1 needle cast from partially burnt conifer trees commonly occurs after forest fires the effectiveness of needles in reducing soil erosion was investigated in this study two needle types ponderosa pine and douglas fir needles were used at four different cover amounts 0 15 40 and 70 percent on granitic and volcanic derived soils simulated rainfall was used to examine interrill erosion added inflow was used to determine rill erosion in a laboratory setting after a series of runs data showed that sediment delivery was greater for the granitic soil compared with the volcanic soil douglas fir needles were more effective at reducing interrill erosion compared with the ponderosa pine needles ponderosa pine needles because of their shape and being bundled together often caused minidebris dams to form the minidebris dams formed by ponderosa pine needles reduce flow within the rill resulting in less rill erosion than the douglas fir needles a 50 percent cover of douglas fir needles reduced interrill erosion by 80 percent and rill erosion 20 by percent a 50 percent cover of ponderosa pine needles reduced interrill erosion by 60 percent and rill erosion by 40 percent we also compared the effectiveness of using stream power rather than shear stress to model rill erosion stream power was a better predictor of sediment load than shear stress rill detachment rates based on stream power decreased with increasing cover for both needle types these results challenge the use of shear stress detachment rates in current erosion models and provide insight into the use of stream power detachment rates

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

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

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