geophysical geological transect and tectonic evolution of the swiss italian alps a complete alpine cross section integrates numerous seismic reflection and refraction profiles across and along strike with published and new field data the deepest parts of the profile are constrained by geophysical data only while structural features at intermediate levels are largely depicted according to the results of three dimensional models making use of seismic and field geological data the geometry of the highest structural levels is constrained by classical along strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units because the transect is placed close to the western erosional margin of the austroalpine nappes of the eastern alps it contains all the major tectonic units of the alps a model for the tectonic evolution along the transect is proposed in the form of scaled and area balanced profile sketches shortening within the austroalpine nappes is testimony of a separate cretaceous age orogenic event west directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the meliata hallstatt ocean further to the east considerable amounts of oceanic and continental crustal material were subducted during tertiary orogeny which involved some 500 km of n s convergence between europe and apulia consequently only a very small percentage of this crustal material is preserved within the nappes depicted in the transect postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments both detachments nucleating at the interface between lower and upper crust large scale wedging of the adriatic or apulian lower crust into a gap opening between the subduced european lower crust and the pile of thin upper crustal flakes alpine nappes indicates a relatively strong lower crust and detachment between upper and lower crust

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

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

two high pressure metamorphic events in ne oman evidence from 40 ar 39 ar dating and petrological data the saih hatat tectonic window in ne oman exposes basement and shelf units that structurally underlie the semail ophiolite these units were metamorphosed under high pressure conditions as evidenced by the occurrence of lawsonite schists carpholite bearing metasediments blueschists and eclogites conventional k ar and 40 ar 39 ar dating of whole rocks and white mica separates from the structurally highest region i of saih hatat and the lowest grade blueschists of the northern part of the structurally lowest saih hatat region iii yield ages of 72 80 ma white micas from the high grade blueschists and eclogites of region iii which formed at t 340 c yielded variably discordant age spectra with weighted mean plateau ages of 106 111 ma the age data combined with structural and petrological criteria suggest that many units exposed in saih hatat experienced two high pressure low temperature high p t metamorphic events the first in the early cretaceous was possibly a result of partial su

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

thrust tectonics of the dibba zone and the structural evolution of the arabian continental margin along the musandam mountains oman and united arab emirates in the northern oman mountains several major thrust sheets of tethyan oceanic rocks dibba zone and a 12km thick complete ophiolite sequence have been thrust westward onto the depressed arabian continental margin musandam zone during the turonian to campanian late cretaceous early thrusting was dominantly a downward foreland propagating sequence as the higher earlier thrust sheets are folded around structurally lower and later culminations breached roof thrusts where imbricate thrusts do not always converge with a major roof thrust but in places actually penetrate up into the overlying duplex are common at outcrop scale affecting adjacent thrust sheets late stage breakback or leap frog thrusts cut through the whole pile of duplexes on to previously higher earlier dulexes late thrusting in the musandam zone affects the deeper previously autochthonous permian cenomanian shelf carbonate sequence and probably also the pre permian basement thrusting these rocks at least 15 km over previously allochthonous hawasina rocks late stage oblique dorsal culmination collapse along the se margin of the musandam mountains has downfaulted dibba zone rocks up to 2 5 km to the se along a hinged listric normal fault strain is transferred from the ne sw trending late cretaceous dibba zone to the n s striking mid tertiary musandam culmination at a major pivot point or transfer zone sw of the shelf carbonate culmination

the rhenium osmium isotope system applications to geochronological and palaeoenvironmental problems the rhenium osmium re os isotope system occupies a unique position amongst commonly used radiogenic parent daughter isotope pairs on account of its siderophilic geochemical characteristics re and os are also concentrated from seawater in organic rich sediments and have been used to obtain absolute ages for the deposition of marine mudrock successions directly additionally re os analyses of ancient mudrocks have provided the best means so far of determining temporal changes in the os isotope composition of pre cenozoic seawater the seawater os isotope record has yielded valuable information about several major episodes of past environmental change and is particularly powerful when used in conjunction with other marine isotopic tracers such as carbon and strontium the present contribution reviews some recent developments in the application of the re os isotope system to geochronological and palaeoenvironmental problems in the mesozoic and earlier new results show that the os isotope composition of callovian seawater c 160 ma ago was very unradiogenic 187 os 188 os 0 26 at the same time that the seawater sr isotope curve recorded its lowest value for the phanerozoic indicating that global continental weathering rates were very low at c 10 15 of today9s levels

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

formation and exhumation of blueschists and eclogites from ne oman new perspectives from rb sr and 40ar 39ar dating seven eclogite facies samples from lithologically different units which structurally underlie the semail ophiolite were dated by the 40 ar 39 ar and rb sr methods despite extensive efforts phengite dated by the 40 ar 39 ar method yielded saddle hump or irregularly shaped spectra with uninterpretable isochrons the total gas ages for the phengite ranged from 136 to 85 ma clinopyroxene phengite epidote phengite and whole rock phengite rb sr isochrons for the same samples yielded ages of 78i2 ma we therefore conclude that the eclogite facies rocks cooled through 500 u ca tc 7 8i2 ma and that the 40 ar 39 ar dates can only constrain maximum ages due to the occurrence of excess ar inhomogeneously distributed in different sites our new results lead us to conclude that high pressure metamorphism of the oman margin took place in the late cretaceous contemporaneous with ophiolite emplacement previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a ne dipping zone choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c 50 km to 10 15 km within c 10 ma and led to their juxtaposition against overlying lower grade rocks final exhumation of all high pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate

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

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

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

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

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

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

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

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

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

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

discharge predictions of a rainfall driven theoretical hydrograph compared to common models and observed data 1 a theoretical hydrograph based on darcy s law and the diffusion equation has been extended into a predictive model for the discharge of diffuse springs and small rivers from a detailed rainfall record using only a single free parameter the basin time constant b the model accurately predicts the flow variations of natural hydrographs including the magnitude and timing of peak flow total flow volume inflection points and recession rate all as a function of effective rainfall amounts basin area and the time constant b predicted hydrographs for a sharp storm event in an unimpounded basin are compared to results for two common transfer functions and our more parsimonious model is statistically superior modeling of a long term record in proximal basins illustrates the effect of variations in b potential applications of the model include flood prediction comparative studies of basin response in a regional context and estimation of actual evapotranspiration

collision tectonics in the swiss alps insight from geodynamic modeling this paper compares results from two dimensional finite element dynamic modeling with the kinematic evolution of the swiss alps during the collision phase in particular we investigate the role of inherited lateral strength heterogeneities on orogenesis a number of first order characteristics are directly comparable at crustal scales in the models the entry of continental crust into the convergent margin marks the end of near perfect subduction accretion of material of the subducting plate to the upper plate creates an orogenic wedge on the incoming pro side and initiates a retroshear zone or model backthrust the addition of material to the upper plate builds a bivergent orogen heterogeneities in the pro crust focus shear and lead to the development of nappe structures the combined action of pro shear nappe stacking and retroshear backthrusting uplifts a plug between the two shear zones subsequent focusing of shear along the retroshear zone results in rotation of the plug and overlying units leading to crustal scale backfolds as observed in the swiss alps the model experiments predict features relevant to alpine dynamics including 1 similar crustal thicknesses and exhumation patterns to those observed in the swiss alps today for erosion rates comparable to natural ones 1 mm yr 1 2 continued accretion and subduction of upper crustal fragments allowing high pressure metamorphic conditions 3 tilting and exhumation of lower crust when a midcrustal weak zone is present and 4 shunting of material across the strong lower crustal wedge of the upper plate

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

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

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

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

petrochemistry and origin of sub ophiolitic metamorphic and related rocks in the oman mountains the semail ophiolite complex of the northern oman mountains was obducted during cenomanian to campanian upper cretaceous times onto the arabian continental margin during this time span schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice whole rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid ocean ridges a comparison of trace element geochemistries however suggests that the protoliths were not equivalent to the haybi alkaline and transitional tholeiites found in the underlying structural slice or to the semail ophiolite volcanic rocks which lie structurally higher in the sequence it is therefore suggested that the amphibolites may have been derived from jurassic or lower cretaceous oceanic basalts not recognized elsewhere in the oman mountains structurally beneath the amphibolites within the metamorphic sheet greenschist facies metasedimentary rocks including quartzites and marbles are interbanded with metabasite horizons containing actinolite and less commonly blue crossitic amphiboles it is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement and were accreted to the base of the ophiolite during progressive underthrusting