strain state and kinematic evolution of a fold nappe beneath the samail ophiolite oman abstract the saih hatat domal culmination of the oman mountains arabian peninsula contains a major refolded ne facing recumbent anticlinal fold nappe within pre ordovician sedimentary cover autochthon and mesozoic platform carbonates that underlie the samail and hawasina nappes tracts of overturned stratigraphy covering over a 1000 km 2 are associated with a marked strain gradient towards sheath like a type folds that define the fold nappe lower limb transitional into an elongated sw facing recumbent syncline and the underlying upper plate lower plate shear zone directions of stretching recorded by fibrous pressure shadow fringes on pyrite show a 20 30 counterclockwise rotation in the foliation plane strain and kinematic constraints require the fold nappe to form progressively during top to the ne shearing of the arabian margin with 1 development of a high strain zone and 2 subsequent folding of the strongly deformed to transposed stratigraphy in the hanging wall to the shear zone rolling of the autochthonous stratigraphic layering of the upper limb through the hinge accompanied by differential shear strain produced the regional scale highly attenuated a type folds at the structurally lowest levels this occurred by simple underthrusting of the footwall lower plate as the upper limb of the fold nappe has to be pinned to the autochthon

age and stratigraphic relationships of structurally deepest level rocks oman mountains u pb shrimp evidence for late carboniferous neotethys rifting abstract igneous zircons in felsic schist or metatuff infolded with mafic schist calcschist and quartz mica schist of the as sifah lower plate window ne oman have yielded a u pb shrimp crystallization age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 298 pm 3 end document ma the metatuff was previously considered to represent permian bimodal volcanism but these data indicate that the magmatism is older and suggest that neotethys intracontinental rifting on the oman part of the gondwana northern margin was initiat

late cretaceous exhumation of the oman blueschists and eclogites a two stage extensional mechanism the continental material of the saih hatat window has been affected by a late cretaceous obduction related hp lt metamorphism below the oman ophiolite a high rate exhumation process is witnessed by the maastrichtian palaeocene onlap onto the blueschist facies rocks drastic metamorphic omissions are documented between the lowest eclogitic units and the overlying blueschist facies ones widespread late metamorphic shear structures point to a top to the nne detachment in opposition to the sense of the late cretaceous obduction the inversion of the shearing sense occurred under similar low temperature conditions in both the blueschist and eclogite facies units admitting that these hp lt metamorphic rocks formed progressively at various depth in the subducting arabian margin a two stage extensional mechanism of exhumation is suggested i early uplift of the eclogitic rocks up to the blueschists depth by ductile thinning or squeezing of a blind extensional allochthon ii exhumation of the whole hp lt metamorphic core complex by inversion of the obduction sole thrust and isostatic rebound of the lower plate

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

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

petrology of fe mg carpholite bearing metasediments from ne oman abstract fe mg carpholite occurs in metasediments of tectonically disrupted basement shelf and foreland basin units that structurally underlie the semail ophiolite in ne oman in the lower grade structurally higher units fe rich carpholite coexists with paragonite quartz illite kaolinite and chlorite whereas in deeper units fe mg carpholite occurs with pyrophyllite sudoite phengite and or chloritoid mineral compositions in these units indicate that chlorite is more magnesian than coexisting fe mg carpholite at low temperatures and pressures but at higher metamorphic grades xmg decreases in the order sudoite carpholite chlorite chloritoid this suggests a reversal in fe mg partitioning between fe mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite quartz to pyrophyllite and at xmg 0 35 phase relations and mineral equilibria indicate that the p t conditions of formation of the fe mg carpholite bearing rocks of ne oman range from 280 315 c 3 6 kbar for the structurally highest units to 325 440 c 6 9 5 kbar for the deepest units indicating a systematic down section increase in metamorphic grade textural relations in these rocks interpreted in the context of pertinent equilibria are consistent with the clockwise p t paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks

sm nd and zircon u pb ages from garnet bearing eclogites ne oman constraints on high p metamorphism poly deformed and poly metamorphosed glaucophane eclogite mega boudins beneath the samail ophiolite oman record an early subduction related high p metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation previously published rb sr ages of 78 ma and 40ar 39ar ages of 82 79 ma record the major ne directed shearing event that partially exhumed the eclogites to a shallower crustal level new sm nd garnet garnet leachate whole rock isochron data from garnet bearing eclogite assemblages in the as sifah subwindow in ne oman are 110 9 ma dg02 87d 5 point isochron and 109 13 ma dg02 86e 3 point isochron on the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde peak high p metamorphism in the rocks structurally beneath the samail ophiolite this metamorphism clearly predates the age of formation of the obducted samail oceanic lithosphere 97 94 ma as well as the subsequent obduction onto the margin 80 70 ma a u pb shrimp zircon age from small 200 m in length zircons with herring bone textured zoning from dg02 87d indicate that rapid zircon growth associated with high si phengites occurred at 82 1 ma zircon growth is possibly related to liberation of zr on garnet breakdown during decompression metamorphism under high p conditions with exhumation these data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the oman high p metamorphism

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

change of direction of overthrust shear in the helvetic nappes of western switzerland abstract the lowest of the helvetic nappes in western switzerland the morcles and the doldenhorn nappes are large scale recumbent folds in equivalent tectonic positions in the alpine chain a comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend n s whereas those in the two stratigraphically inverted sequences trend nw se a similar geometrical relationship has been found from an analysis of the calcite c axis directions of these limestones the two normal sequences show nw se directed c axes whereas the two inverted sequences show w e directed c axes these observations lead to the following conclusions 1 regionally consistent shear movements occurred over a strike distance of more than 60 km and these gave rise to similar deformation patterns in both nappes 2 a progressive rotation of these shear movements from a northerly direction in the earlier formed limbs to northwesterly in the later formed inverted limbs can be deduced the finite strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history

ophiolite emplacement in ne oman constraints from magnetotelluric sounding summary magnetotelluric mt data have been acquired across the samail ophiolite oman mountains along a 115 km transect in january 2005 twenty five mt stations were deployed approximately every 4 km along a profile perpendicular to the coastline near muscat the profile extends from the huw l meeh subwindow in the northwestern part of the profile and crosses the saih hatat window a tertiary domal culmination that folds a major ne facing recumbent fold nappe and ends along the southern boundary between the dasir and ibra ophiolite blocks the survey aimed to investigate the tectonic evolution of the emplacement of the samail ophiolite by delineating major faults and geological boundaries on a crustal scale the data shows complex behaviour with the influence of static shift effects and anomalously high phases exceeding 90 phase tensor analysis and 3 d forward modelling hint that the upper plate lower plate up lp shear zone has a 3 d effect on the data in the northern part of the profile rotated mt impedances were inverted using a 2 d code major resistivity interfaces coincide with the location of the up lp shear zone near the surface and dip towards the arabian margin suggesting a southwest orientated underthrusting prior to the ophiolite emplacement the para autochthonous and less deformed upper plate is a resistor whereas the strongly deformed lower plate is more conductive

structural and stratigraphic controls on the origin and tectonic history of a subducted continental margin oman eclogites and blueschists exposed in saih hatat oman record the subduction and exhumation of continental crustal material beneath the cretaceous semail ophiolite during ophiolite obduction the eclogite bearing lower plate originally part of oman s distal continental margin is exposed in two tectonic windows through the less metamorphosed upper plate by a previously mapped low angle high strain decollement structure a major tectonic break currently poorly exposed records the juxtaposition of the highest pressure eclogites and garnet blueschists against lower pressure epidote blueschists the subsequent exhumation of the entire lower plate to mid crustal levels is marked by a pervasive shearing event associated with a regional greenschist facies overprint the decollement truncates structures and the metamorphic field gradient in the lower plate but does not significantly truncate structures or stratigraphy in the upper plate it is not responsible for the exhumation of the high pressure rocks to mid crustal levels most of the displacement across this structure was accommodated during continuing convergence after the subduction system had ceased to be active and post ophiolite emplacement onto the platform carbonate sequences a revised tectonic model is presented which accounts for the structural geochronological and metamorphic observations

petrology of lawsonite pumpellyite and sodic amphibole bearing metabasites from north east oman the assemblages chlorite pumpellyite lawsonite albite quartz chlorite lawsonite quartz epidote and chlorite epidote albite quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic melange in the structurally highest unit of the autochthonous and parautochthonous section underlying the semail ophiolite in saih hatat north east oman the pre permian basement of this section contains mafic units characterized by the assemblage crossite epidote chlorite quartz albite el actinolite these assemblages indicate a down section increase in metamorphic grade from lawsonite albite facies conditions in the melange to epidote blueschist facies conditions in the basement application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the p t conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d c for the melange and p 6 8 kbar t 310d c for the basement units textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise p t paths of evolution the estimated p t conditions and down section increase in metamorphic grade in central western and northern saih hatat are consistent with the hypothesis relating metamorphism to the late cretaceous tectonic loading of the continental margin by an ophiolite slab 18 km in thickness these results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of saih hatat which may have formed at an earlier stage in response to an early cretaceous collisional event

exhumation of high pressure rocks in northeastern oman the exhumation of high p rocks in the northeast saih hatat window oman mountains formed regional nappes associated with greenschist facies metamorphism exhumation resulted in the juxtaposition of two plates with different structural style stratigraphy and metamorphic history a major crustal discontinuity separates the two plates with lower metamorphic grade rocks in the hanging wall lower plate units were exhumed during strong top to the northeast shearing resulting in the formation of regional closures that evolved by folding and transposition of the high pressure fabrics while the lithostatic pressure was decreasing upper plate units were later exhumed with the juxtaposition of the two plates truncation of lower plate closures and the formation of upper plate nappes that are attenuated and sheathlike toward the upper and lower plate boundary in some high p terranes apparent increases in metamorphic grade down structural section and low grade c type shear bands are often attributed to extensional exhumation in oman however these exhumation features formed during crustal scale contraction at a convergent margin the low geothermal gradient regimes afforded by convergent margins are critical for the generation and preservation of high p low t rocks

obduction related planar and linear fabrics in oman under the oman ophiolites three different types of alpine synmetamorphic fabrics are seen each of them is taken as the signature of a particular stage of the obduction process the complex high grade fabric in the basal metamorphic sheet is linked to the intraoceanic detachment stage pre obduction sensu stricto the l s fabric of the muscat nappes and hatat internal border associated with low temperature high pressure metamorphism is regarded as the effect of an early obduction that is the epicontinental thrusting of the matrah peridotites the much simpler fabric of the anchizonal hawasina and autochthonous cover resulted from the gravity sliding of the semail ophiolites during the final obduction phase

chlorine stable isotope composition of granulites from lofoten norway implications for the cl isotopic composition and for the source of cl enrichment in the lower crust abstract the cl isotopic composition of lower crustal rocks as revealed from granulites from the lofoten islands northern norway centres around the value of standard mean ocean chloride smoc the investigated rocks and mineral separates of cl amphibole and cl biotite have 37 cl values 37 cl 35 cl normalized to smoc of 1 12 to 0 79 with most values concentrated in the range of 0 3 to 0 11 and an average of 0 15 samples from gabbroic and from dry granitic rocks magmatic mangerites and charnockites with highly cl enriched hydrosilicates and from a later amphibolite with low cl amphiboles show no distinct differences in their 37 cl values the results indicate that there was only minor involvement of mantle derived cl in fluid involving processes in the middle and lower crust assuming a 37 cl of 4 7 for the undegassed mantle 1 cl isotopic values of the granulites do not support the influx of significant amounts of mantle derived cl the maximum possible mantle contribution to the sample with the highest 37 cl is about 33 thus the common occurrence of cl enriched minerals in granulite facies terrains is regarded as a product of remobilized crustal cl either during magmatism or during later fluid activity the results support the hypothesis that the crust can be regarded as a reservoir with cl isotopic values very similar to smoc

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

role of shear in development of the helvetic fold thrust belt of switzerland the geometric features of the helvetic nappes of the alps show intense internal deformation set up predominantly by simple shear during nappe transport the nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport stratal layering is oblique to nappe boundaries thrust surfaces formed after ductile shear zones developed in underlying pre triassic gneissic basement the broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults contraction along competent layers obliquely inclined to shear surfaces produced buckle folds the geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear producing normal faults

tectonic setting origin and obduction of the oman ophiolite the semail ophiolite in the oman mountains is the world9s largest and best preserved thrust sheet of oceanic crust and upper mantle 10 000 km 2 550 km long 150 km wide it was emplaced onto the arabian continental margin during late cretaceous time the ophiolite originated 96 94 ma at a spreading center above a northeast dipping subduction zone associated with initiation of immature island arc tholeiitic lavas lasail arc at the highest levels of the ophiolite simultaneous underthrusting of triassic and jurassic mid oceanic ridge basalt and alkalic volcanic rocks beneath 12 km of upper mantle depleted harzburgites produced garnet clinopyroxene amphibolites formed at temperatures of 850 c dated as 95 93 ma subduction cannot have been initiated at a mid oceanic ridge otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above in the northern part of the oman mountains in the bani hamid area united arab emirates 870 m of granulite facies rocks enstatite spinel diopside quartzites garnet diopside wollastonite calc silicate marbles clinopyroxene bearing amphibolites were formed at temperatures similar to those of the garnet diopside amphibolites of the oman sole 800 850 c but at slightly higher pressures as much as 9 kbar they are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out of sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction subduction of the arabian continental crust beneath the obducting semail ophiolite to 78 90 km depth has been proven by thermobarometry of the as sifah eclogites to 20 23 kbar in the eastern sector in the united arab emirates the subducted continental crust began to partially melt producing unusual biotite muscovite garnet tourmaline cordierite andalusite bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite we suggest that the entire leading northeast edge of the arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks higher temperatures and pressures in the united arab emirates sector possibly due to a thicker or double thickness ophiolite section led to blueschist amphibolite and granulite facies conditions in the metamorphic sole and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite a model for ophiolite obduction is presented which accounts for all the structural and metamorphic conditions reported from the oman mountains

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

odd partnerships among middle eocene bartonian nummulites examples from the transylvanian romania and dorog hungary basins abstract nummulites assemblages recovered from the middle eocene of the transylvanian basin romania and dorog basin hungary were studied in order to infer the paleoecological structuring of the assemblages the studied material consists mostly of a and b generation tests of nummulites perforatus in both basins however specimens of nummulites beaumonti and of nummulites striatus were sporadically recorded in the transylvanian and dorog basin respectively the co occurrence of two different nummulites taxa which show a striking size difference in protoconch and adult test suggests that the small sized nummulites beaumonti and nummulites striatus were the odd partners of the large sized nummulites perforatus during the bartonian middle eocene the co occurrence of these closely related species might reflect their different life strategies growth rate and or the use of different reproduction seasons the association of the same large partner with two distinct small sized partners in two different but connected basins is due to the wider paleogeographic distribution and more abundant occurrence of the larger partner n perforatus as compared to n beaumonti and n striatus the smaller sized partners

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

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

structure and metamorphism of blueschist eclogite facies rocks from the northeastern oman mountains the northern part of the saih hatat window oman shows high pressure metamorphic rocks derived from shelf sediments and pre premian continental basement and is atypical of sub ophiolite metamorphism elsewhere the high pressure rocks are divided into structural units originally bounded by foreland propagating thrusts formed during ophiolite obduction although now many contacts are backthrusts normal faults or extensional shear zones metamorphic breaks exist across many unit boundaries the deepest unit as sifah has eclogite facies assemblages in metabasites and metapelites which record evolution of p t conditions along a clockwise path culminating al 23 2 5kbar 540 75 c contrasting markedly with overlying units 5 10 kbar 200 500 c although separated by 40 ar 39 ar apparent ages our tectonic model relates all the high p units to a single convergent event in the late cretaceous the as sifah eclogites were exhumed in two stages i tectonic emplacement against other units at c 20 25 km depth and ii exhumation of the entire high p zone by culmination collapse after obduction

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

geometry and significance of internal windows and regional isoclinal folds in northeast saih hatat sultanate of oman saih hatat contains tiered sets of low angle structural breaks that reflect a complex stacking of sheets which requires multiple upward movement of units within an unstable orogenic wedge in a convergent setting early recumbent folds within the saih hatat window are subparallel to but cut by a major low angle contractional fault decollement that separates lower plate high pressure rocks some eclogites exposed in two windows hulw and as sifah windows from high pressure upper plate rocks containing carpholite and lawsonite parts of this decollement had been mapped or speculated upon to explain the variation in metamorphic grade across saih hatat upper plate fold nappes are controlled by the mapped decollement surface below the decollement sheath like lower plate closures formed independently in an intense zone of top to the northeast shearing eclogites occur within mafic boudins at the structurally lowest level of the retrograde lower plate shear zone this zone was responsible for the partial exhumation of the lower plate very early in the observed structural history and has a normal movement sense the two lower plate windows are essentially foliation domes reflecting fold interference between regional scale sheath like recumbent isoclines and more open upright northwest north and northeast trending fold sets the early upper plate fold nappes reflect a major top to the north or northeast movement of the rocks away from the oman margin at the same time the ophiolite was being emplaced over the top in the opposite direction top to the southwest or south south directed faulting and shearing is superimposed on the early recumbent folds in the mountain range mapped relationships and structural observations agree with bureau de recherches geologiques et minieres mapping that recognised fold nappes in this part of the oman mountains all of the mapped structures are associated with exhumation of the high pressure rocks making reconstruction of the margin during high pressure metamorphism problematic