sumeini group oman evolution of a mesozoic carbonate slope on a south tethyan continental margin abstract deposition of the mostly mesozoic sumeini group occurred on the slope between the shallow marine arabian carbonate platform and the deep oceanic hawasina basin or south tethys sea these strata record the evolution of the northeast arabian continental margin from permian and triassic rifting to ocean basin closing with the late cretaceous obduction of the semail ophiolite platform margin type and its evolution can be inferred from exposures of these slope sediments at jebel sumeini in the western oman mountains in the lower permian base of the sequence bedded spicular limestones suggest deep marine conditions this is followed by an enigmatic thick sequence of bedded dolomite in the early to middle triassic small carbonate submarine fans formed along a distally steepened slope a subsequent interval of terrigenous clastic sedimentation was followed by development of a steep escarpment margin during ladinian to norian time with coralgal reefs at the shelf break and a debris apron at the base of the scarp the late triassic is marked by another influx of terrigenous sediment which accompanied widespread emergence of the platform in the jurassic a thick apron of thin bedded limestone formed on the slope this apron was cut by channelized gullies that now include blocks of older reefal material and oolitic sands a well documented submergence of the platform during the tithonian and early cretaceous led to the deposition of radiolarian cherts on the slope and basin upper lower cretaceous megabreccias up to 200 m thick suggest an interval of localized slope instability and or tectonism continued deposition of bedded limestone and marl with lesser calcirudite marks the top of stratigraphic sequence at jebel sumeini the early to middle triassic carbonate submarine fan deposits occur as discrete lenticular sequences of bedded calciturbidite modified grain flow and debris flow deposits which were derived from mostly slope sources the ladinian to norian base of slope debris apron deposits were derived from both slope and shallow marine sources including reefs and appear to form a laterally extensive but narrow belt of thick lenticular to sheet like beds of debris which accumulated over a long interval of time coarse limestones in the jurassic gullied slope deposits are distinct in that they occur randomly as individual channelized beds in a sequence dominated by thin bedded lime mudstones thick lower cretaceous megabreccias are distinguished by their great lateral distribution and general lack of matrix

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

origin of the hafit structure implications for timing the tertiary deformation in the northern oman mountains abstract the hafit structure belongs to a group of large scale post obduction periclines that deform the late cretaceous tertiary sediments of the oman mountains foredeep due to one sided compression from the ene the hafit structure grew as a detachment fold above a basal decollement which is probably located in the upper cretaceous lower fiqa shale its reversed vergence back thrust and the folds superposed on its limbs are explained as being caused by simple shear modification of the original structure the interpretation that the hafit structure and other foreland folds in the northern oman mountains have resulted from a zagros related late eocene miocene compressional event is at variance with the stratigraphical and sedimentological evidence from the hafit area this evidence shows that the hafit structure grew synchronously with sedimentation from just before the middle eocene until the end of the miocene whilst the related structures to the east of it were initiated at the beginning of the palaeocene with further evidence of a similar early start to the tertiary deformation emerging from other parts of the northern oman mountains it becomes necessary to push back the date of initiation of the tertiary compressional event to the palaeocene and to set it apart from the plio pleistocene zagros deformation

structure and sequence of thrusting in deep water sediments during ophiolite emplacement in the south central oman mountains abstract sedimentological and stratigraphical investigations of the deep water passive margin sediments of the oman neo tethys the hawasina ocean when allied with structural relationships between duplexes of these units emplaced on to the oman margin during late cretaceous ophiolite obduction allow the reconstruction of sediment distribution in the hawasina ocean from this starting point and the observed stacking order of duplexes the sequence of thrusting of the oceanic sediments can be deduced imbrication proceeded through foreland propagating thrusts whilst the assembled thrust stack was modified at a late stage of emplacement by out of sequence thrusting related to sequential locking of thrust planes towards the hinterland and gravity sliding off the flanks of a major anticline that developed in the underlying autochthonous shelf and basement two main factors governed duplex and imbricate fan formation and distribution firstly two contrasting sedimentary basins were present along the central oman margin the duru basin distal to the al ayn basin secondly competent sedimentary successions were bulldozed ahead of the semail ophiolite whilst less competent shale and chert units were overridden by the ophiolite or imbricated along its leading edge

the maastrichtian transgression onto the northwestern flank of the proto oman mountains sequences of rudist bearing beach to open shelf facies

maastrichtian to early tertiary stratigraphy and palaeogeography of the central and northern oman mountains abstract maastrichtian to tertiary sedimentary rocks outcrop extensively around the periphery of the oman mountains they form part of a geologically distinct suite of strata deposited after obduction of the semail ophiolite and thrusting of the hawasina and sumeini imbricates onto the eastern edge of the arabian plate the lithostratigraphy of maastrichtian and early tertiary rocks of the central and northern oman mountains is revised and a number of new formations are erected the proto oman mountains were emergent in the early maastrichtian and were subject to subaerial erosion and weathering nw and w of the mountains gradual subsidence followed during the maastrichtian allowing for formation of a transgressive onlapping sequence represented by the qahlah fluviatile to shallow marine terrigenous clastics and simsima shallow shelf carbonate formations more rapid subsidence ne of the mountains produced fan delta and submarine debris flow turbidite apron sequences comprising the al khawd and thaqab formations respectively a low angle unconformity separates tertiary strata from older units during the palaeogene basinal and slope facies with debris flows and turbidite sequences were deposited directly upon the unconformity surface in rapidly subsiding basins nw muthaymimah formation and nne ruwaydah formation of the oman mountains elsewhere shallow carbonate shelf facies were extensively developed in less rapidly subsiding areas during the palaeogene the palaeocene and early eocene is represented by the jafnayn limestone formation this is succeeded and overlapped by a variety of later eocene units ene and e of the mountains early eocene regression is marked by restricted facies of the rusayl formation overlying the rusayl formation are shallow open shelf limestones of the seeb formation which is rich in alveolinid and nummulitic foraminifera on the arabian side of the mountains the rusayl formation is not developed and the jafnayn formation is succeeded either by the seeb formation or fine grained faunally restricted limestones and marls of the fahud beds which represent intra shelf facies

a longitudinal carbonate fan from the jurassic of the oman mountains the guweyza limestone formation of the hamrat ad duru abstract when sedimentological stratigraphical and structural data are considered the continental rise sediments of oman neo tethys the hamrat duru group can be restored to their original basin configuration prior to their southwest directed emplacement with the semail ophiolite onto the oman continental margin during the late cretaceous the hamrat duru group sediments are preserved in well exposed imbricate fans the largest of which forms the hamrat ad duru range in the south central oman mountains in the south central hamrat ad duru middle and upper jurassic ooid rich carbonate sediment was transported from the adjacent oman carbonate platform during periods of tectonic instability into the oman neo tethys hawasina basin by high and low density turbidity currents to form sediment sheets with a margin parallel axis of deposition palaeocurrents are northwest directed parallel to the oman margin along the southwest of the hamrat ad duru and northeast directed away from the margin in structurally higher more distal imbricates of the northeastern hamrat ad duru this implies sediment input at the southern end of the hamrat ad duru these trends are reflected in the distribution of sedimentary facies with most proximal facies developed in the southern hamrat ad duru passing into progressively more distal facies to the northwest and northeast sediment thinning and fining is more accentuated away from the margin towards the northeast and sedimentation was concentrated parallel to the oman margin the hamrat ad duru represents a longitudinal fan that was at least 120 km long and extended at least 60 km away from the margin

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

high grade metamorphism dehydration and crustal melting a reinvestigation based on new experiments in the silica saturated portion of the system kalo 2 mgo sio 2 h 2 o co 2 at p 1 5 gpa the system kalo2 mgo sio2 h2o co2 has long been used as a model for the processes of granulite facies metamorphism and the development of orthopyroxene bearing mineral assemblages through the breakdown of biotite bearing assemblages there has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting we performed new experiments in this system at pressures of 342 to 1500 mpa with t between 710 and 1045 c and xflh2o between 0 05 and 1 00 accurately locating most of the dehydration and melting equilibria in p t xflh2o space the most important primary result is that the univariant reaction phl qtz fl en sa melt must be almost coincident with the fluid absent reaction phl qtz en sa melt in the co2 free subsystem in conjunction with the results of previous measurements of co2 solubility in silicate melts and phase equilibrium experiments our theoretical analysis and experiments suggest that co2 cannot act as a flux for partial melting crustal melting in the presence of h2o co2 mixed fluids will always occur at temperatures higher than with pure h2o fluid present magmas produced by such melting will be granitic s l in composition with relatively high sio2 and low mgo contents irrespective of the h2o co2 ratio in any coexisting fluid phase we find no evidence that lamprophyric magmas could be generated by partial fusion of quartz saturated crustal rocks the granitic melts formed will not contain appreciable dissolved co2 the channelled passage of hot co2 rich fluids can cause local dehydration of the rocks through which they pass in rock dominated as opposed to fluid dominated systems minor partial melting can also occur in veins initially filled with co2 rich fluid as dehydration and local disequilibrium drive the fluid towards h2o rich compositions however co2 is unlikely to be a significant agent in promoting regional granulite grade metamorphism melting magma generation metasomatism or long range silicate mass transfer in earth s crust the most viable model for the development of granulite facies rocks involves the processes of fluid absent partial melting and withdrawal of the melt phase to higher crustal levels

transition from subduction to exhumation related fabrics in glaucophane bearing eclogites oman evidence from relative fabric chronology and 40ar 39ar ages abstract controversy over the age of peak metamorphism and therefore the tectonic evolution of the arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane bearing eclogites from the saih hatat window beneath the allochthonous samail ophiolite in ne oman these eclogites contain relicts of earlier fabrics structures and metamorphic assemblages and provide a record of change from subduction to exhumation the eclogites are part of a mafic layer that was disrupted into boudins up to 0 5 km in length within a lower plate shear zone as sifah shear zone the megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ene trending lineation and sheathlike isoclines enveloped by the flat lying schistosity the boudin bearing layer is isoclinally folded with calc schist mafic schist and quartz mica schist where the regional folds have axes parallel to the ne trending stretching lineation a type folds textural evidence suggests multiple growth events for garnet and clinopyroxene requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins the surrounding calc schist and quartz mica schist are both intensely deformed with transposition foliation containing an ne trending lineation in phengite and asymmetric shear indicators such as c type shear bands and asymmetric pressure shadows around garnets that give top to the ne sense of shear although consistent ene trending lineations in all the boudins suggest that they have largely acted as passive nonrotating rigid bodies the presence of ne vergent asymmetric mesofolds extensive dynamic recrystallisation multiple generations of phengites and a range of 40ar 39ar apparent ages within the megaboudins suggest however that they have not acted entirely passively during the later deformation phengites isolated from the high p low t fabrics show groupings in 40ar 39ar apparent ages interpreted as distinct metamorphic cooling intervals at 140 135 120 98 and 92 80 ma microstructural relations suggest that age groupings younger than 100 ma reflect phengite growth during exhumation with the top to the ne shearing the older ages 120 110 ma from fabrics that give top to the s shear sense may reflect growth during the subduction phase the combination of groupings of apparent argon ages older than the crystallisation age of the samail ophiolite the suggestion of different geothermal gradients and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite the glaucophane eclogites are interpreted as having formed at different times under varying pressure temperature conditions during underthrusting with variations in the rate of underthrusting allowing thermal equilibration and or rapid cooling at different crustal levels

sequence of thrusting and origin of culminations in the northern and central oman mountains abstract detailed mapping and structural analysis of three large scale culminations sumeini and asjudi half windows and haybi hawasina window in the oman mountains shows a considerably more complex history of deformation than a simple foreland or downward sequence of thrust development early thrusting processes tended to create a regular stacking order of imbricate slices and major thrust sheets complying with the rules of thrust propagation assembled progressively downwards and forwards in the direction of translation out of sequence thrusts can also be demonstrated in places by truncation of footwall structures folds imbricate slices etc gross strain differences between thrust sheets downward facing structures in footwall units and elimination of thrust sheets beneath late stage thrusts frequently cut up section through the previously assembled stack putting previously younger lower thrust sheets over previously older higher ones many of the culminations in the northern and central oman mountains were formed by ramping associated with this late stage leap frog rethrusting event

determining cyclone frequencies using equal area circles abstract this paper examines recent cyclone frequencies with a methodology that incorporates the use of 5 latitude equal area circles located in a grid for a region centered from 25 to 70 n and 60 to 140 w cyclones were counted in 82 equal area circles for the period 1950 93 over north america the grid of equal area circles eliminated two related problems associated with conventional grid systems area inequality and area normalization and allows for comparison of frequency counts among circles an analysis of winter cyclones among four north to south circles on a meridian indicated latitudinal variability in year to year raw cyclone counts correlation coefficients developed from relating winter cyclone counts from one circle to another were less than 0 29 implying that the location of the axis of maximum winter cyclone frequency varies annually results revealed that winter and spring had the greatest number of cyclones and the southernmost position of the axis of maximum frequency summer and

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

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