tectonic interactions between india and arabia since the jurassic reconstructed from marine geophysics ophiolite geology and seismic tomography gondwana breakup since the jurassic and the northward motion of india toward eurasia were associated with formation of ocean basins and ophiolite obduction between and onto the indian and arabian margins here we reconcile marine geophysical data from preserved oceanic basins with the age and location of ophiolites in nw india and se arabia and seismic tomography of the mantle below the nw indian ocean the north somali and proto owen basins formed due to 160 133 ma n s extension between india and somalia subsequent convergence destroyed part of this crust simultaneous with the uplift of the masirah ophiolites most of the preserved crust in the owen basin may have formed between 84 and 74 ma whereas the mascarene and the amirante basins accommodated motion between india and madagascar east africa between 85 and circa 60 ma and 75 and circa 66 ma respectively between circa 84 and 45 ma oblique arabia india convergence culminated in ophiolite obduction onto se arabia and nw india and formed the carlsberg slab in the lower mantle below the nw indian ocean the nne ssw oriented slab may explain the anomalous bathymetry in the nw indian ocean and may be considered a paleolongitudinal constraint for absolute plate motion nw india asia collision occurred at circa 20 ma deforming the sulaiman ranges or at 30 ma if the hindu kush slab north of the afghan block reflects intra asian subduction our study highlights that the nw india ophiolites have no relationship with india asia motion or collision but result from relative india africa arabia motions instead key points we present a new tectonic model for the evolution of nw indian ocean subducted slab under the carlsberg ridge resulted from arabia india convergence

late palaeozoic to neogene geodynamic evolution of the northeastern oman margin when the highlands of arabia were still covered with an ice shield in the latest carboniferous early permian period separation of gondwana started this led to the creation of the batain basin part of the early indian ocean off the northeastern margin of oman the rifting reactivated an infra cambrian rift shoulder along the northeastern oman margin and detritus from this high was shed into the interior oman basin whereas carbonate platform deposits became widespread along the margin of the neo tethys northern rim of oman drifting and oceanization of the batain basin started only in late jurassic early cretaceous time extensional tectonics was followed in the late cretaceous by contraction caused by the northward drift of greater india and afro arabia this resulted in the collision of afro arabia with an intra oceanic trench and obduction of the semail ophiolite and the hawasina nappes south to southwestward onto the northern oman margin 80 m y ago during the middle cretaceous the oceanic lithosphere including the future eastern ophiolites of oman drifted northwards as part of the indian plate at the cretaceous palaeogene transition 65 ma oblique convergence between greater india and afro arabia caused fragments of the early indian ocean to be thrust onto the batain basin subsequently the lower permian to uppermost maastrichtian sediments and volcanic rocks of the batain basin along with fragments of indian ocean floor eastern ophiolites were obducted northwestward onto the northeastern margin of oman palaeogene neo autochtonous sedimentary rocks subsequently covered the nappe pile tertiary extensional tectonics related to red sea rifting in the late eocene was followed by miocene shortening associated with the collision of arabia and eurasia and the formation of the oman mountains

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

intraplate stresses and dynamical aspects of rifted basins abstract intraplate stresses play a important role during the syn and post rift evolution of sedimentary basins deviations from long term thermal patterns of post rift subsidence as well as the development of unconformities within rifted basins reflect to a large extent the temporal fluctuations of the regional stress fields in the lithosphere it appears that incorporation of intraplate stresses in dynamical models for basin development together with a quantification of the role of petrological changes in the crust lithosphere system can explain some of the observed shortcomings of thermal models of post rift basin subsidence late neogene accelerations in tectonic subsidence in the deeper parts of rifted basins in the northern atlantic and mediterranean regions could reflect a change in the level of intraplate stress over large areas of the globe non thermal contributions to the subsidence record affect estimates of crustal extension and consequently influence the depth at which hydrocarbon source rocks enter the maturity window stress induced changes in basement topography have also important consequences for the budget of fluid flow in the deeper parts of the sediment fill of the rifted basins as well as on the dynamics of diapirism

off ridge alkaline magmatism and seamount volcanoes in the masirah island ophiolite oman abstract the masirah ophiolite offers an unique opportunity to study well preserved small seamount structures obducted seamounts have not been described up to now and from the present day ocean floor they are almost exclusively known from bathymetric studies the thin oceanic crust of the masirah ophiolite was formed at a ridge transform intersect in upper jurassic time it was overprinted and reworked by a major intra oceanic tectono magmatic event at mid cretaceous time that has been well dated owing to the presence of interstratified sedimentary rocks late hauterivian to early barremian c 130 125 ma this mid cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites volcanism occurred in a nw se extensional regime small elongate submarine volcano structures seamounts developed within widespread alkalibasaltic pillow lava and pillow breccia deposits which are interfingered with deep marine pelagic sediments the volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height the seamounts are built up of basic to acid subvolcanic stock or sheet like intrusions several generations of dikes vent agglomerates and pyro to epiclastic deposits the latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment in the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place the alkaline rocks show a pronounced ocean island basalt oib character indicating the considerable contribution of a mantle plume source hotspot as cause of the volcanism we propose a combination of original transform setting followed by drift past the marion hotspot during the major plate tectonic reorganization between greater india madagascar and africa starting in mid cretaceous time

seamount abundances and distributions in the southeast pacific abstract sea beam bathymetry was recorded for 17 277 km of ship track in the southeast pacific and has been analyzed for seamount population characteristics all the ship tracks are over the pacific plate and most fall along 7 lines parallel to the east pacific rise between 7 s and 22 s the lines fall into three age categories one line is over 0 5 2 ma crust three are over 5 10 ma crust and three are over 30 40 ma crust seamount locations were recorded diameters were manually estimated and heights were measured if the swath crossed the seamount center over 382 features were counted along the entire ship track with heights ranging from 50 m to over 2500 m with sampling most consistent at heights between 100 m and 1000 m height to radius ratios vary considerably suggesting that seamount shapes do not scale to a single parameter the observed variety of morphological forms demonstrates that there is a wide range of seamount shapes for small features the distribution of sizes for the population was approximated by exponential dependence and by power law dependence using methods developed by smith and jordan 17 the power law distribution overestimates abundances at the smallest size ranges and both distributions fail to predict abundances of large seamounts determined from wide beam data 17 size distributions were also determined for seamounts in each seafloor age category almost all small seamounts appear to be produced on crust younger than 0 5 2 ma while the number of larger seamounts increases to 5 10 ma for all size ranges observed here the number of seamounts at 5 10 ma and at 30 40 ma is roughly identical these observations suggest that most small seamounts are formed on very young thin lithosphere that permits the passage of small volumes of magma small volumes would cool in older thicker lithosphere before reaching the surface although larger magma bodies might not numerous small sources of melt must exist near the ridge crest to supply the small seamounts probably trapped remnants of the large scale upwelling

mass balance and ice flow law parameters for east antarctica on compare les donnees de vitessses et d epaisseurs obtenues sur des profils de la zone etudiee entre 90 e et 135 e au nord de 80 s aux debits d equilibre calcules spri on determine le rapport entre la vitesse moyenne et la vitesse basale les variations de la vitesse de deformation les debits montrent que cette region n est pas significativement desequilibree

cretaceous sedimentary rocks on the masirah ophiolite sultanate of oman evidence for an unusual bathymetric history autochthonous sedimentary rocks on the masirah ophiolite preserve a record of vertical movements of their oceanic basement the masirah ophiolite was formed in the uppermost jurassic and subsequently underwent thermal subsidence for about 20 ma as documented by pelagic sediments submarine alkali basaltic volcanism in the lower barremian marks the onset of rapid basement uplift to or above sea level carbonate platforms grew in barremian mid aptian time on highs of oceanic basement the uplift is interpreted as the result of transpressive tectonism along a large oceanic fracture zone the platforms submerged in the mid aptian and reached the ccd in the albian ribbon cherts were deposited during the albian to santonian time the first influx of siliciclastic detritus shed from the arabian craton is recorded in coniacian sandstones ophiolitic conglomerates interfinger in the campanian to mid maastrichtian with continent derived sandstones reflecting regional compressive tectonism crystalline exotics deposited in the late maastrichtian are derived most likely from the precambrian basement of arabia whilst the origin of the sedimentary exotics dated triassic to cretaceous remains unclear the masirah ophiolite was overthrust at the cretaceous tertiary boundary by an upper ophiolite nappe and emplaced onto the margin of southeast oman

extremely thin oceanic crust in the proto indian ocean evidence from the masirah ophiolite sultanate of oman the masirah ophiolite is a good example of thin oceanic crust below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1 1 5 km the gabbroic lower crust barely exceeds 500 m in thickness in spite of this reduced thickness the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence the crust was formed during the uppermost jurassic circa 150 ma when the indian madagascar plate separated from the african arabian plate and is therefore related to the opening of the coeval somali basin geological relationships indicate that this portion of oceanic crust was formed at a ridge transform intersect the peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment rather than the consequence of a tectonic thinning the cooling effect due to the vicinity of two large continental lithospheric blocks indian madagascar and african arabian plates during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness

geometric boundary conditions for modelling the velocity field of the antarctic ice sheet a comparison of the two dimensional horizontal velocity field obtained on the basis of conservation of mass balance velocity with the diagnostic velocityfield calculated with an ice sheet model dynamics velocity may yield information on shortcomings in the way the ice sheet model describes the ice flow thispaper presents improved geometric boundary conditions surface elevation and ice thickness required as input in such calculations in particular the surfaceelevation grid is described in detail as it has been generated specifically for such a study and represents a new standard in accuracy and resolution forcalculating surface slopes the digital elevation model was generated on a 10 km grid size from over 20 000 000 height estimates obtained from seven 35 dayrepeat cycles of ers 1 radar altimeter data for surface slopes less than 0 5i the relative elevation accuracy is better than 1 m in areas of high surface slope coastal and mountainous regions the altimeter measurements have been supplemented with data taken from the antarctic digital database south of 81 5i data from the spri folio map have been used the ice thickness grid was produced from a combination of a re digitisation of the spri folio and the originalradio echo sounding flight lines for areas of grounded ice the bedrock was estimated from surface elevation and ice thickness significant differences inexcess of 25 percent of ice thickness were obtained between an earlier digitisation of the folio bedrock map and the dataset derived here furthermore a newvalue of 26 6 x 106 km3 was obtained for the total volume of the ice sheet and shelves which is a reduction of 12 percent compared with the original estimatederived during the compilation of the spri folio these differences will have an important influence on the results obtained by numerical ice sheet models

vertical crustal movements at the vema fracture zone in the atlantic evidence from dredged limestones abstract a crustal block shallower by over 3 km than the level predicted by the thermal contraction model for the ocean crust is located on the southern side of the vema transform in the atlantic lithology magnetic and gravity data suggest the anomalous block is an uplifted piece of crust and upper mantle and not a constructional volcanic feature the crest and upper slopes of the anomalous block are capped by shallow water reef limestones determination of paleofacies and ages of samples of these limestones suggest that 1 a the crest of the anomalous crustal block reached sea level and was exposed subaerially in middle pliocene time about 3 m y ago and subsequently sank to its present depth at an average rate of about 0 3 mm year that is a rate one order of magnitude faster than that predicted by the thermal contraction model for crust of the same age 2 b the anomalous block reached at or above sea level either continuously or intermittently throughout the late miocene from roughly 10 m y ago 3 c paleocene sediments dated at about 55 58 m y have been recovered from the uplifted block where assuming a spreading rate of 1 1 cm year the crust should not be more than about 35 m y old vertical crustal movements at the vema transform provide an example of vertical tectonism in the large oceanic transform fracture zones

identifying areas of low profile ice sheet and outcrop damming in the antarctic ice sheet by ers 1 satellite altimetry a digital elevation model dem of the surface of the antarctic ice sheet is compared with a simple two dimensional ice flow model to illuminate gross distortions 500 m of the ice surface elevation we use a dem derived from ers l satellite altimetry airborne data and twerle balloon data this is compared with an ice sheet elevation model generated by applying theoretical surface elevations calculated for twodimensional ice flow to isolines of distance from the grounding line continentality the model is scaled using only one parameter to match the measured surface elevation at dome argus the model is far from rigorous violating continuity conditions ignoring variations in surface mass balance and temperature and assuming uniform basal conditions however the comparison of model and observed surface elevations is illuminating in terms of the behaviour of the ice sheet at a continental scale across the ice sheet the rms difference between modelled elevation and the dem is around 300 m but much of this results from isolated areas of much greater disagreement we ascribe these gross differences to the effects of basal conditions in four areas the observed surface is more than 500 m higher than the modelled surface most of these are immediately upstream of substantial areas of rock outcrop and are caused by the damming effect of these mountain ranges in nine areas the measured surface is more than 500 m lower than predicted eight of these areas in west antarctica and the l ambert glacier basin are associated with suspected areas of basal sliding the ninth is an area of 250 000 km2 in east antarctica not previously noted as having unusual flow characteristics but for which very few data exist we speculate that this area results from unusual basal conditions resulting in a low profile ice sheet a low profile ice sheet of this size within the east antarctic ice sheet indicates that basal conditions are perhaps more variable than previously thought

successive shifts of the india africa transform plate boundary during the late cretaceous paleogene interval implications for ophiolite emplacement along transforms abstract the arabian sea in the nw indian ocean is a place where two major transform boundaries are currently active the owen fracture zone between india and arabia and the owen transform between india and somalia these transform systems result from the fragmentation of the india africa transform boundary which initiated about 90 myrs ago when the india seychelles block separated from madagascar to move towards eurasia therefore the geological record of the arabian sea makes it possible to investigate the sensitivity of a transform system to several major geodynamic changes here we focus on the evolution of the india africa transform system during the 47 90 ma interval we identify the late cretaceous 90 65 ma transform plate boundary along chain ridge in the north somali basin from 65 to 42 47 ma the india africa transform is identified at the chain fracture zone which crossed both the owen basin and the north east oman margin finally the transform system jumped to its present day location in the vicinity of the owen ridge these shifts of the india africa boundary with time provide a consistent paleogeographic framework for the emplacement of the masirah ophiolitic belt which constitutes a case of ophiolite emplaced along a transform boundary the successive locations of the india africa boundary further highlight the origin of the owen basin lithosphere incoming into the makran subduction zone

spatial variations in heat at the base of the antarctic ice sheet from analysis of the thermal regime above subglacial lakes antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure melting point we have used a one dimensional vertical heat transfer equation to determine theoretical temperature values for the ice sheet base above 77 subglacial lakes identified from airborne radio echo sounding data covering 50 of antarctica variations in our temperature results to below the pressure melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from a internal ice deformation and b basal sliding in the thermal model our results indicate that when the geothermal heat flux is set at 54 mw m 2 the ice sheet base above 70 of the known antarctic subglacial lakes is calculated to be at the pressure melting value these lakes are located mainly around dome c ridge b and vostok station for the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide using the same thermal model loss of heat due to vertical advection is calculated to be relatively high in such regions if the ice sheet base is at the pressure melting point heat lost due to vertical advection must be supplemented by heat from other sources for the three lakes beneath terre adelie and george v land for instance the basal thermal gradient calculated to produce pressure melting at the ice sheet base is equivalent to 1 5 2 times the value obtained when 54 mw m 2 of geothermal heat is used as the sole basal thermal component we suggest that as distance from the ice divide increases so too does the amount of heat due to internal ice deformation and basal sliding moreover by considering the ice sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline we demonstrate empirically that the heat due to these horizontal ice motion terms varies pseudo exponentially with distance from the ice divide the location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large scale basal sliding

u pb zircon ages from the spontang ophiolite ladakh himalaya the spontang ophiolite in the ladakh himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of india during the late cretaceous precise isotopic ages of himalayan ophiolites are presently unavailable u pb dating of zircons from a dioritic segregation in the high level gabbros of the spontang ophiolite shows that the complex formed at 177 1 ma mid jurassic and we suggest that it represents a fragment of neo tethyan oceanic crust this jurassic age contrasts with late cretaceous ages 91 95 ma previously reported for the formation of the troodos and oman ophiolite complexes further west along the neo tethyan suture zone and also predates the early to mid cretaceous masirah ophiolite c 150 ma and its alkaline volcanic seamounts 115 125 ma an andesitic rock from an arc sequence overlying the spontang ophiolite was also dated using as 88 5 ma this late cretaceous age constrains the minimum age of initiation of subduction beneath the spontang ophiolite the complex was emplaced during the turonian maastrictian some 80 ma after its formation when the indian continental margin collided with a cretaceous arc about 25 ma before the india asia collision the lack of a sub ophiolite metamorphic sheet beneath the spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction

formation and evolution of the masirah ophiolite constrained by paleomagnetic study of volcanic rocks abstract the extrusive rocks of the lower ophiolitic nappe of masirah island oman were paleomagnetically studied four mean directions of magnetization were isolated one at low temperature the others at high temperature the low temperature component found in most samples corresponds to a recent viscous and or chemical remagnetization the high temperature component found at three sites in the centre area is interpreted as a possible primary magnetization acquired during extrusion or more probably as an early remagnetization related to subsequent hydrothermal alteration and indicates formation of the masirah ophiolite around a paleolatitude of 40 s close to the present position of the west somali basin a second high temperature component found in three other sample sites in the hakl and in the thumi areas is probably a chemical remagnetization related to the emplacement of the upper ophiolite nappe in late maastrichtian to paleocene times finally a last high temperature component found only in the two sites from naft area could be a post tectonic remagnetization acquired in oligocene or miocene times or represents differential tilting of pillows and sedimentary blocks the high temperature components are usually fairly scattered due to the very complex tectonic history of the area nevertheless the relation between the defined paleopoles and the indian polar wander is unambiguous and the evolution of the masirah ophiolite from formation up to the emplacement of the second ophiolitic nappe is clearly related to the northward movement of the indian plate this new paleomagnetic study rules out the possibility of a common or related origin for the masirah and the semail ophiolites

the northern mid atlantic and reykjanes ridges spreading center morphology between 55 50 n and 63 00 n seamarc ii and hydrosweep acoustic imagery and bathymetry reveal that the northern mid atlantic ridge mar and reykjanes ridge between 55 50 n and 63 00 n are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from iceland the northern 100 km of the mar 55 50 n 56 47 n is oriented 005 perpendicular to the spreading azimuth and contains four third order spreading segments separated along strike by right and left stepping axial discontinuities the boundary between mar and reykjanes ridge occurs at 56 47 n where the bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092 this is the only transform fault between 55 50 n and 63 00 n and is inferred to have generated the bight fracture zone the reykjanes ridge lies between the bight transform and iceland s reykjanes peninsula and is oriented 30 oblique to the spreading orthogonal direction axial faults and volcanic systems on the reykjanes ridge are arranged in a right stepping en echelon pattern and display strikes intermediate between the spreading orthogonal direction and the strike of the ridge which is compatible with experimental models of oblique extension spreading segments on the northern mar and reykjanes ridge are defined structurally and bathymetrically at the smallest scale the volcanic axis is segmented into individual volcanic systems 4 45 km long which are separated along strike by intervolcano gaps or offsets these features are analogous to fourth order spreading segments documented elsewhere on the mar axial volcanic systems are superimposed on intermediate wavelength 13 65 km axial topographic highs that constitute third or second order spreading segments segment boundaries occur at axial depth maxima which correspond to offsets in the volcanic axis four primary characteristics of the spreading axis axial valley depth axial boundary fault throw relief along the neovolcanic axis and degree of inter segment structural discontinuity decrease toward the north this variation is consistent with higher crustal temperatures near iceland due to either thicker crust or elevated mantle temperatures

development of concepts concerning the genesis and emplacement of tethyan ophiolites in the eastern mediterranean and oman regions this article uses evidence mainly from the eastern mediterranean and oman to help understand the origin and emplacement of ophiolites field studies in these regions emphasize the diversity of ophiolite related sedimentary and tectonic processes topics to be discussed here include the sedimentary covers of ophiolites their metamorphic soles related melanges and emplaced continental margin slope rise units modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites the writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones evidence from their intact sedimentary covers however indicates that such subduction was only incipient in most cases as directly associated volcanic arcs or back arc basins are rare metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open ocean to continental slope rise settings underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the eastern mediterranean and oman regions ophiolite emplacement was commonly achieved by trench margin collision although strike slip transpression and large scale accretion along subducting margins played a role in some cases taking account of the mesozoic palaeotectonic setting of the eastern mediterranean region it is widely believed that ophiolites of two main ages mid jurassic and late cretaceous were formed in several small oceanic basins separated by rifted microcontinents the history of tethys in pre mesozoic time remains more open to question lastly giant subduction related ophiolites as in oman cyprus or the balkans may not be forming anywhere in the world ocean today but may instead relate to rare events in earth history when regional to global plate boundaries were drastically reorganised

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

west northwest directed obduction of the batain group on the eastern oman continental margin at the cretaceous tertiary boundary the batain coast area in eastern oman is dominated by allochthonous permian to late maastrichtian sedimentary and volcanic rocks batain group unconformably overlain by neoautochthonous tertiary sediments the allochthonous rocks of the batain coast were previously attributed to the hawasina complex the permian to coniacian santonian sedimentary infill of the neo tethyan hawasina basin off northern oman previous structural interpretations suggested that the batain group along with the hawasina complex and the semail ophiolite was obducted in the coniacian to campanian from ne to sw onto the northern oman continental margin results of our work in the batain area differ from previous interpretations with most significant differences concerning timing and direction of obduction our results show that wnw directed tectonic movements formed a fold and thrust belt and led to the obduction of allochthonous rocks onto the east oman continental margin during latest maastrichtian earliest paleocene times this is coeval with emplacement of ophiolitic fragments along the eastern coast of oman eastern ophiolite belt but is about 15 20 myr later than emplacement of hawasina complex and semail ophiolite in northern oman postemplacement structural evolution during the tertiary involved intraplate extension possibly reflecting the red sea gulf of aden opening and late tertiary shortening related to convergence between arabia and eurasia late tertiary contractional deformation resulted in refolding of the batain nappes and in folding of the overlying tertiary sediments a palinspastic reconstruction of the batain area indicates that the permian to upper cretaceous sediments were formerly deposited in the batain basin a part of the proto indian ocean along the present day eastern oman margin this leads us to propose that permian breakup of gondwanaland created both continental margins of oman and led to the opening of two major basins the neo tethyan hawasina basin in the north and the proto indian ocean batain basin in the east the latter separating arabia from greater india

predictions of vertical uplift caused by changing polar ice volumes on a viscoelastic earth measurements of crustal uplift from bedrock around the edges of antarctica or greenland could help constrain the mass balance of those ice caps present day changes in ice could cause vertical displacement rates of several mm yr around antarctica and up to 10 15 mm yr around greenland horizontal displacement rates are likely to be about 1 3 the vertical rates the viscoelastic response of the earth to past changes in ice could cause uplift rates that are several times larger by measuring both gravity and vertical displacements it is possible to remove the viscoelastic effects so that the observations can be used to constrain present day thickness changes

from rifted margins to foreland basins investigating provenance and sediment dispersal across desert arabia oman u a e arid arabia where chemical weathering and anthropic modifications are negligible is a huge natural laboratory for the study of sediment provenance mafic and ultramafic detritus from ophiolite belts is found along the oman mountains and on masirah island obduction orogen provenance dominant cellular serpentinite grains from residual mantle harzburgites of the sama il ophiolite contrast with gabbroic diabase and basaltic grains from igneous crustal rocks of the masirah ophiolite detritus from autochthonous basement and cover rocks is found along the arabian sea margin rift shoulder provenance transition from carbonate dominated lithic sands undissected stage to hornblende rich plagioclase arkoses dissected stage is best documented at the northern edge of the gulf of aden along the older strike slip owen basin margin only locally are feldspars hornblende and garnet derived from small outcrops of pan african basement modern eolian sands from yemen to the arabian persian gulf display marked enrichment in quartz low plagioclase feldspar ratios and depleted dense mineral assemblages with rounded epidotes hornblende garnet and ultrastables pointing to extensive recycling also fostered by quaternary changes the rub al khali and wahibah sand seas have distinct quartz feldspar ratios reflecting largely separate transport pathways dune fields fed chiefly from rift highlands in the west and south mix with orogenic detritus in the northeast carbonate dominated sands in coastal uae are mostly derived from the zagros mountains blown by northerly shamal winds across the gulf during pleistocene lowstands to reach the backbulge depozone and the foreland beyond contributions from the oman mountains reach only 5 10 of bulk sediment along their southern apron and mostly indirectly through deflation of plio pleistocene fans the sama il ophiolite supplies enstatite and diopside to the eastern uae and wahibah sands the masirah ophiolite supplies amphiboles and clinopyroxenes locally along the batain coast the composition of desert sediments faithfully mirrors tectonic history of source terranes deformation style during orogeny and unroofing level during extension bulk petrography coupled with dense mineral analysis thus represents a powerful tool to investigate not only provenance and sediment transport but also crustal dynamics and geologic evolution at continental scale

early cretaceous intra oceanic rifting in the proto indian ocean recorded in the masirah ophiolite sultanate of oman abstract the masirah ophiolite sultanate of oman was part of an oceanic basin proto indian ocean formed by the break up of gondwana in late jurassic times similar to the somali basin it was obducted onto the arabian continental margin in the early paleocene 100 ma after its formation hence it is possible to investigate the different tectonic and magmatic processes that have affected the oceanic lithosphere during these 100 ma tithonian ridge magmatism tectonism and hydrothermal alteration are responsible for the formation of the oceanic crust of the masirah ophiolite in the early cretaceous hauterivian barremian after 20 ma of normal drift and subsidence the oceanic lithosphere underwent extensional tectonics and renewed magmatism geometry kinematics intrusion mechanisms and related sedimentation during this intra oceanic rifting are widely described and illustrated by field observations exhumation of deep seated oceanic lithosphere alkaline volcanism intrusion of a hornblende gabbro dolerite granite suite and uplift of crustal blocks to sea level with the unconformable deposition of platform carbonates are the processes characterising this intra oceanic rifting the hauterivian barremian age of oceanic rifting coincides with an important reorganisation of the motion of the indian plate relative to africa antarctica and australia we interpret the rifting recorded in the masirah ophiolite as the local response to the motion of the indian plate due to the opening of the south atlantic and the spreading in the eastern indian ocean

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

tracking the paleogene india arabia plate boundary the location of the india arabia plate boundary prior to the formation of the sheba ridge in the gulf of aden is a matter of debate a seismic dataset crossing the owen fracture zone the owen basin and the oman margin was acquired to track the past locations of the india arabia plate boundary we highlight the composite age of the owen basin basement made of paleocene oceanic crust drilled on its eastern part and composed of pre maastrichtian continental and oceanic crust overlaid by ophiolites emplaced in early paleocene on its western side a major fossil transform fault system crossing the owen basin juxtaposed these two slivers of lithosphere of different ages and controlled the uplift of marginal ridges along the oman margin this transform system deactivated 40 myrs ago coeval with the onset of ultra slow spreading at the carlsberg ridge the transform boundary then jumped to the edge of the present day owen ridge during the late eocene oligocene period before seafloor spreading began at the sheba ridge this migration of the plate boundary involved the transfer of a part of the indian oceanic lithosphere formed at the carlsberg ridge to arabia this late eocene oligocene tectonic episode at the india arabia plate boundary is synchronous with a global plate reorganization event corresponding to geological events at the zagros and himalaya belts the owen ridge uplifted later in late miocene times and is unrelated to any major migration of the india arabia boundary

the eismint benchmarks for testing ice sheet models we present a series of benchmark experiments designed for testing and comparing numerical ice sheet models following the outcome of two eismintworkshops organized to intercompare large scale ice sheet models currently in operation model benchmark experiments are described for ice sheets underfixed and moving margin conditions these address both steady state and time dependent behavior under schematic boundary conditions and with prescribedphysics a comparison was made of each model s prediction of basic geophysical variables such as ice thickness velocity and temperature consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice sheet modeling codes can be assessed

modern sand from obducted ophiolite belts sultanate of oman and united arab emirates abstract oman and the united arab emirates host the largest ophiolites on earth and are characterized by arid tropical climate and desert conditions they thus provide an unexcelled natural laboratory in which to study sediments derived from allochthonous oceanic lithosphere and to define an actualistic petrogenetic model for obduction orogen provenance spectacularly exposed in se arabia are two distinct obducted ophiolite sequences sama il and masirah that contrast widely in formation setting spreading rate detachment level thermal state during emplacement and relief masirah beach sand chiefly consists of basaltic diabase to metadiabase plagioclase gabbroic and dunite grains from progressively deeper seated levels of the multilayered oceanic crust dense minerals include diopsidic clinopyroxene and hornblende from gabbros epidote and tremolite actinolite from altered dikes and augite from basaltic lavas in contrast sand from the sama il ophiolite includes cellular serpentinite grains and en

the transition between the sheba ridge and owen basin rifting of old oceanic lithosphere summary magnetic quiet zones are present along the margins of the entire length of the gulf of aden to the owen fracture zone this includes the easternmost 300 km between the eastern edges of arabia and africa to the owen fracture zone where old oceanic lithosphere was rifted to form the sheba ridge within this easternmost region the boundary between the quiet zone and the old oceanic lithosphere is marked by ridge complexes the sharbithat ridge complex to the north and the error ridge complex to the south these ridge complexes which lack a magnetic signature occupy a structural position similar to the hinge zones at the continental margins to the west they appear to have formed early in the opening of the gulf of aden or perhaps to have been pre existing features the boundary between sheba ridge and the northern magnetic quiet zone is often marked by an abrupt end of the sheba ridge seafloor spreading magnetic anomaly pattern and a sharp basement deepening to the north the boundary between the east sheba ridge and northern magnetic quiet zone becomes less distinct near the owen fracture zone this is also accompanied by changes in the east sheba ridge specifically a decrease in magnetic anomaly amplitudes increase in ridge flank depths and a loss of the cooling curve ridge flank shape this may be the result of lower mantle temperatures in the vicinity of the owen fracture zone the sediments within the magnetic quiet zone can be divided into a lower disturbed unit and an upper unit consisting of flat lying reflectors the disturbance of the lower sediments may have resulted from a period of diffuse extension in the magnetic quiet zone prior to the establishment of the sheba ridge spreading centre the similarity of the easternmost quiet zone to quiet zones at rifted continental margins leads to the suggestion that these regions were formed by diffuse extension of old oceanic lithosphere using a two layer lithospheric attenuation model and assuming extension during a period of 15 myr followed by 10 myr of cooling the basement depths and heat flow measurements can be adequately matched the modelling implies 45 per cent crustal extension in the quiet zone the amount of extension calculated is compatible with documented motion between arabia and africa the old oceanic lithosphere thus must have been substantially thinned to a thickness similar to 10 14 myr old ocean before seafloor spreading was initiated at the sheba ridge