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

the distribution of intraplate volcanism in the pacific ocean basin a spectral approach summary although the major linear island chains in the pacific ocean basin have been extensively studied relatively little attention has been given to the origin of the numerous seamounts and oceanic islands that are concentrated in the western pacific spectral analysis of shipboard gravity and bathymetry data provides a means for statistically estimating the distribution of this volcanism in space and time estimates of the power spectrum of the bathymetry fb and the average gravitational admittance z cross spectrum correlation of gravity and bathymetry were obtained as functions of age of the oceanic lithosphere by dividing the pacific plate into eight age regions a h the power spectrum of the bathymetry is indicative of the overall bathymetric roughness primarily volcanic in each region this roughness shows three gradations regions a d 140myr with many large volcanic features are very rough the average gravitational admittance is indicative of the degree of isostatic compensation of bathymetric features in each region and can be used to estimate the proportionh of total bathymetric power that was emplaced on relatively old lithosphere and the proportion that formed on young lithosphere at or near a mid ocean ridge crest the highest proportion of bathymetry that formed on old lithosphere occurs in region h fi 0 90 and the lowest proportion occurs in regions g and e fi 0 40 0 60 the power spectrum of the bathymetry and the gravitational admittance allow an estimate of the total bathymetric power pi ipb accounted for by intraplate volcanism intraplate power and the power pr 1 fi pb which is of on ridge origin ridge power at long wavelengths in regions e g the amount of on ridge volcanism exceeds the amount of intraplate volcanism pr pi indicating the presence of several large plateaus and rises which formed at ridge crests this suggests the

nontransform offsets of the pacific cocos plate boundary and their traces on the rise flank a multi beam bathymetric reconnaissance of the fast spreading crest of the east pacific rise between 7 n and 2 6 n found the axial ridge to be interrupted by four 4 13 km non transform offsets they have probably persisted since initiation by a small change in spreading direction 3 5 ma at each offset the volcanic rift zones veer 15 toward each other and overlap for 5 25 km around deep basins intervening lateral displacements of the rift zones are too short 2 5 km to break the linear axial volcanoes but cause local bends in their plan shapes some examples have a morphology that mimics the larger intervolcano offsets interpretation of a thorough bathymetric and magnetic survey around the 9 km 5 5 n offset indicates that the mainly one sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small scale jumps of the eruptive zone the northern rift zone is elongating propagating at the expense of the southern one the resulting along axis migration of this and other examples at speeds comparable to spreading rates causes the rise flank scars of the nontransform offsets equivalent to the fracture zones of transform faults to be oblique to the spreading direction these traces are recognizable within the highly lineated fault block terrain of the rise flanks because of the distinctive morphology of the 5 25 km wide bands that accreted at overlapped sections of the rift zones here lineation is less marked and oblique to the spreading direction and volcanic relief is more important than relief caused by faulting the west flank trace of the 5 5 n offset is occupied by a line of volcanoes and shallow volcanic ridges several other volcanic chains previously identified as hot spot traces seem to have grown in similar tectonic settings

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

theory of melt segregation a review abstract the processes by which melt is extracted from partially molten source regions at depth are collectively known as melt segregation during the past three years a dynamical theory has been developed which provides a unified description of the mechanical thermodynamical and chemical aspects of melt segregation in this theory a partially molten region is regarded as a deformable porous medium or matrix saturated with melt migration of melt relative to the matrix can be driven either by the buoyancy of the melt or by nonhydrostatic pressure gradients associated with the deformation of the matrix this paper reviews the basic governing equations of the theory and the solutions which have been obtained during the past three years

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

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

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

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

south east asia as a part of an ordovician gondwanaland a palaeobiogeographic test of a tectonic hypothesis the hypothesis that thailand and malaysia the sibumasu block were adjacent to australia in the early palaeozoic has been tested by an examination of the ordovician sequences and faunas of sibumasu and australia the relatively stenogeographic nautiloids of the two areas are remarkably similar and have a simpson index of 0 92 at the generic level two new genera of discosorids are restricted to the two blocks andgeorgina andmesaktoceras are found elsewhere only in tibet very close affinities are also evident between the gastropod polyplacophoran and rostroconch molluscs the ordovician brachiopod faunas are also very close including the genusspanodonta other very close similarities are found between the upper cambrian trilobite faunas and the ordovician conodonts and stromatoporoids no ordovician faunas younger than upper whiterockian were found during this study in either northern australia or sibumasu and a stratigraphic gap probably exists from the upper whiterockian to the upper ordovician over most of the two blocks these remarkably close faunal similarities are good evidence in favour of the hypothesis that sibumasu was adjacent to australia during the early palaeozoic similarly close faunal relationships between north china south east asia tibet and australia may also suggest close proximity of those blocks during the early palaeozoic

3 d mapping of permeable structures affecting a deep granite basement using isotropic 3c vsp data summary this paper illustrates the efficiency of vertical seismic profiling vsp for the investigation of dipping and hydraulically conductive structures affecting a granitic basement covered by sediments a three component 3c vsp data set has been acquired in the gpk1 and eps1 wells of the soultz sous forets enhanced geothermal system egs located within the upper rhine graben urg our study focuses on the isotropic processing of profiles acquired with vertical vibrator p and their subsequent interpretation mainly p s converted reflections are identified from the analysis of the 3c records these p s conversions occur on steep permeable faults that are positioned in space by traveltime modelling these faults cut the granite basement in several hectometric scale blocks and represent the main fluid paths between the boreholes these faults are thought to be inherited from late variscan and alpine deformation periods reactivated by the current stress field when properly processed and interpreted vsp allow the scale gap between surface and borehole data to be bridged

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

dynamic variation of middle to late ordovician cephalopod provincialism in the northeastern peri gondwana region and its implications abstract middle to late ordovician cephalopod occurrence data from south china and adjacent terranes in the northeastern peri gondwana region are used to define biogeographic provinces several multivariate methods are adopted for biogeographic analysis including cluster analysis non metric multidimensional scaling and network analysis during the middle ordovician three biogeographic provinces in this region are recognized the australia the north china tibet sibumasu nts and the south china altun sa provinces in the late ordovician three provinces with modified geographic ranges may also be recognized as the cephalopods of the tibetan region and sibumasu terrane changed significantly and accordingly they have been included with south china altun to form a south china tarim tibet sibumasu province stts while the other two late ordovician provinces are the australia and the north china provinces the dynamic variation of cephalopod provincialism in the northeastern peri gondwana region may have been controlled by the changing palaeolatitude and differentiated movement of these plates or terranes from the middle through late ordovician

paleomagnetism of northern cocos seamounts constraints on absolute plate motion paleodeclinations of several small seamounts on the northern cocos plate demonstrate that it rotated approximately 30 counter clockwise about a pole close to its northern boundary during the past 1 to 6 m y paleoinclinations agree with models of absolute plate motion in which the northern component of velocity for the cocos plate is about 50 mm yr the paleomagnetic data dredge samples and seamount morphology either independently or in concert are reasonably reliable for distinguishing fault blocks from central volcanoes and for determining the approximate age of a structure relative to that of the surrounding sea floor

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

bathymetric artifacts in sea beam data how to recognize them and what causes them sea beam multibeam bathymetric data have greatly advanced understanding of the deep seafloor however several types of bathymetric artifacts have been identified in sea beam s contoured output surveys with many overlapping swaths and digital recording on magnetic tape of sea beam s 16 acoustic returns made it possible to evaluate actual system performance the artifacts are not due to the contouring algorithm used rather they result from errors in echo detection and processing these errors are due to internal factors such as side lobe interference bottom tracking gate malfunctions or external interference from other sound sources e g 3 5 khz echo sounders or seismic sound sources although many artifacts are obviously spurious and would be disregarded some particularly the omega effects described in this paper are more subtle and could mislead the unwary observer artifacts observed could be mistaken for volcanic constructs abyssal hill trends hydrothermal mounds slump blocks or channels and could seriously affect volcanic tectonic or sedimentological interpretations misinterpretation of these artifacts may result in positioning errors when seafloor bathymetry is used to navigate the ship considering these possible geological misinterpretations a clear understanding of the sea beam system s capabilities and limitations is deemed essential

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

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

abundances distribution and sizes of volcanoes in the pacific ocean and implications for the origin of non hotspot volcanoes abstract in this paper i present data on the abundances sizes and crustal age for all volcanoes volcanic islands and seamounts which appear on published bathymetric charts of the pacific ocean these new data shed light on the origin of non hotspot volcanoes and are important in combination with data on the chemical compositions of seamounts and volcanic islands for estimates of the bulk composition of ocean crust these data also provide firm constraints on off ridge oceanic volcanism models results of this study show that the size frequency distribution of pacific volcanoes is poisson like and that the smallest volcanoes are much more abundant than large ones this study shows clearly that the most abundant volcanoes on the earth are the submerged oceanic volcanoes which comprise 5 25 of the oceanic volcanic layer on pacific crust of eocene age and younger the abundance of volcanoes number of volcanoes per unit area increases monotonically with increasing age assuming steady state the production rate of new off ridge volcanoes number of volcanoes per unit area per unit time is inversely proportional to the square root of the lithosphere age 1 on crust older than eocene the number of volcanoes per unit area of crust decreases monotonically with increasing age however the total volume of lava represented by these edifices increases with increasing age size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old pacific crust as well as the effect of increased lithosphere thickness on seamount size these general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes 2 and volcanoes built at fossil constructional plate margins 3

vsp in crystalline rocks from downhole velocity profiling to 3 d fracture mapping abstract vsp surveys have been carried out at several potential nuclear waste disposal sites in finland since the mid 1980s to date more than 200 three component profiles have been measured the main purpose of the surveys was to detect fracture zones in the crystalline bedrock and to determine their positions most seismic events could be linked to zones of increased fracturing observed in the borehole logs the more pronounced seismic reflectors could be correlated with hydro geologically significant zones which have been the main targets in the investigations processing and interpretation methods have been developed specifically for vsp surveys in crystalline rocks weak reflections from thin fracture zones are enhanced by multi channel filtering techniques based on the radon transform the position and orientation of the fracture zones are determined by polarisation analysis and by combining data from several shot points the compilation of the results from several boreholes gives a comprehensive image of the fracture zones at the scale of the whole site the discussion of the methodology is based on examples from the olkiluoto site in sw finland

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

shape analysis of pacific seamounts abstract shape statistics have been compiled from 85 profiles of well surveyed pacific seamounts in the height range 140 3800 m a flat topped cone was fit to each seamount s cross sectional profile maintaining the slopes of the sides as closely as possible on each profile a basal width d b a summit width d t and a maximum height h were measured the height to basal radius ratio is h is estimated by the ratio 2hd b and flatness f by the ratio d t d b slope angle arctan e is estimated from e 2h d b d t summit height and basal radius are found to be highly correlated r 0 93 the 85 point sample mean of the height to basal radius ratio is h 0 21 0 08 implying that a seamount s summit height is typically one fifth its basal radius despite the high correlation individual points show some scatter and there may be groupings into different morphological types for example all but one of the seamounts with summit heights above 1000 m have values of h that are larger than the sample mean the 85 point sample mean of flatness is f 0 31 0 18 data points show a large scatter with values of f varying between 0 a pointy cone and 0 69 a flat topped cone a histogram representation of flatness however indicates that certain values of f may be more common than others the histogram shows a bimodal distribution with maxima occurring at values of f in the ranges 0 10 0 20 and 0 35 0 50 moreover there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large sized seamount may be a pointy cone slope angle has an 85 point sample mean of 18 6 individual values of vary between 5 and 36 in addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle 15 to determine which variables account for most of the observed variation in the seamount shapes a multivariate principal component analysis was performed on the data using five shape variables summit height basal radius summit radius flatness and slope the analysis indicates that most of the variation is described by two variables flatness and summit height

comparison of the shapes and sizes of seafloor volcanoes on earth and pancake domes on venus the fact that flat topped pancake domes observed on venus resemble flat topped volcanoes on earth s seafloor suggests that volcanic processes on the seafloor and on venus may operate to construct volcanic forms with similar characteristics to test this the shapes and size distributions of terrestrial submarine volcanoes seamounts from several magmatic and tectonic provinces are quantitatively compared with those of venusian pancake domes seamount heights and diameters are highly correlated suggesting that there may be a universal mechanism for their growth the 2014 point sample mean of the height to diameter ratio is d 0 10 0 04 in contrast heights and diameters of 52 pancake domes exhibit no obvious trend and values of d are as much as three orders of magnitude smaller with a 52 point sample mean of d 0 02 0 03 seamounts with basal diameters db 10 km have flatness values ratio of top to bottom diameter that span the entire range between 0 pointed cone and 1 truncated cone larger sized seamounts tend to have small values of flatness and more pointed tops this is not true for pancake domes which show large flatnesses at all sizes additionally seamounts have distinctive profiles with their flank slopes increasing in gradient toward the summit flank slopes of pancake domes however commonly increase in gradient toward the base with respect to sizes the diameter distributions of the different seamount populations are consistent with a model in which the number of seamounts decreases exponentially with increasing diameter the diameter distribution of 126 pancake domes by contrast is more uniform its mean is db 22 13 km this size is at least an order of magnitude larger than the estimated characteristic diameters of the seamount populations based on the above results i conclude that seamounts have significantly different shapes and size distributions than venusian pancake domes that have been identified and measured to date this suggests that one or more of the variables e g magma composition crystallinity magma volume effusion rate cooling rate pre existing topography governing their formation differs in terms of shapes and size distributions the small db 20 km volcanic edifices first identified from venera data are good morphologic matches to seamounts and should be the focus of future work investigating the formation of comparable volcanic features in these two different planetary environments

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

a review of lower and middle palaeozoic biostratigraphy in west peninsular malaysia and southern thailand in its context within the sibumasu terrane abstract fossils from the cambrian to devonian rocks of southern thailand the langkawi islands mainland kedah perlis north perak and central west peninsular malaysia are listed and reviewed and their stratigraphy and correlation reassessed the hitherto anomalous record of the trilobite dalmanitina from malaysia is reviewed and found to be of latest ordovician hirnantian age rather than lower silurian age as previously reported and is considered a probable synonym of the widespread mucronaspis mucronata a new stratigraphical nomenclature is erected for part of the langkawi mainland kedah and perlis area successions in which the term setul limestone which stretched from the ordovician to the devonian is abandoned and replaced by the middle ordovician kaki bukit limestone the late ordovician and early silurian tanjong dendang formation the silurian mempelam limestone and the early devonian timah tasoh formation all underlying the paraconformity with the late devonian langgun red beds there was a single depositional basin in the generally shallow water and cratonic areas of southern thailand langkawi and mainland kedah and perlis in contrast to the deeper water basin of north perak only silurian rocks are dated with certainty within another basin in central west malaysia near kuala lumpur which were also cratonic and shallow water although to the east in west pahang there are basal devonian deeper water sediments with graptolites the area is reviewed in its position within the sibumasu terrane which in the palaeozoic also included central and northern thailand burma myanmar and southwest china part of yunnan province

on the geometrical form of volcanoes abstract many volcanic edifices have a remarkably symmetric geometrical form an example is mount fuji in japan we model this form assuming that the surface of the volcano is a surface of uniform hydraulic potential that an erupting magma will follow the path of minimum resistance to the surface in order to model the resistance to fluid flow we assume the volcanic edifice is a uniform porous medium the vertical flow of magma is also resisted by the gravitational body force if the volcano becomes too tall flank eruptions will widen it if the volcano becomes too wide summit eruptions will increase its elevation using the dupuit approximation for an unconfined aquifer it is shown that the percolation equation is applicable as magma reaches the surface it is assumed to extend the solid porous matrix a similarity solution is obtained to this moving boundary problem the solution predicts a uniform shape for all volcanoes this shape is shown to be in excellent agreement with the geometrical form of mount fuji

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

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

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

laccoliths and small volcanoes on the flank of the east pacific rise clusters of low circular domes and discs each a few hundred metres high and a few kilometres across have been mapped but not sampled during a seabeam traverse of old 9 18 m y oceanic crust at at 19 20 s in the south pacific where they replace the typical lineated fault block terrain for several hundred square kilometres the landforms are probably produced by laccolith intrusion beneath a sediment cover in an off axis volcanic province their overall shapes are distinctly different from the small volcanic shields that also dot the rise flank and from small satellite cones that are abundant on archipelagic aprons though some of the domes do bear small cratered shields and cones on their peripheries and even shallow summit depressions

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

ordovician nautiloid faunas of central and southern thailand nautiloids from the shallow water ordovician carbonates of central and southern thailand can be grouped into five broad assemblages 1 the middle ibexian fauna of indeterminate endocerids from tarutao island 2 hardmanoceras chrysanthimum kobayashi of the upper ibexian age strata on tarutao island 3 the upper ibexian manchuroceras nakamense sp nov from ron phibum southern thailand 4 wutinoceras sp chaohuceras sp of lower to middle whiterockian age strata from satun province 5 wutinoceras sp armenoceras chediforme kobayashi and georgina sp of lower middle whiterockian age strata from kanchanaburi central thailand all of these genera occur in australia and china

ordovician successions in southern central xizang tibet china refining the stratigraphy of the himalayan and lhasa terranes abstract the ordovician stratigraphy of southern central xizang tibet has been revised based on new conodont data recovered from 43 samples in four stratigraphic units and their integration with existing nautiloid and graptolite data the histiodella holodentata and pygodus serra biozones have been identified respectively in the alai and jiaqu formations of the chiatsun group exposed near alai village in nyalam county within the himalayan terrane and the yangtzeplacognathus foliaceus subbiozone lower part of the pygodus serra biozone in the sangqu formation exposed at the guyu section within zayu county in the lhasa terrane recognition of these biozones has increased the precision of correlation of the middle upper darriwilian strata in the region regional reassessment of the ordovician stratigraphy permitted by new biostratigraphic data has allowed revised definitions for the chiatsun and keerduo groups and the sangqu and xainza formations the chiatsun group is defined herein to include three lithologically distinctive formations in descending order the jiaqu the alai and the adang formations the stratigraphic age for the jiaqu and alai formations in the type area ranges from the middle darriwilian histiodella holodentata biozone to middle katian hamarodus brevirameus biozone but the age of the adang formation remains less certain

on the applicability of thermal conduction models to mid plate volcanism comments on a paper by gass et al the proposition that mid plate hot spot magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate although hot spots perhaps tend to avoid cratonic areas the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle plate thickness or speed is of lesser importance the high level of african mid plate volcanism since 25 m y b p appears to be part of a global pattern controlled by events in the mantle rather than a result of the africa plate coming to rest over sublithospheric thermal sources about 40 m y b p

transition from circular to stellate forms of submarine volcanoes large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones multiple volcanic centers and embayments due to giant flank failures small mid ocean ridge volcanoes in contrast are commonly subcircular in plan view and show only embryonic rift zones in order to characterize the transition between these two end members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice irregularity was characterized by measuring perimeter distance elongation and moment of inertia of the contours assuming an ideal edifice is circular the analysis reveals a general transition over 2 4 km edifice height best transition estimate 3 km while some large edifices 4 5 km high show no major embayments or ridges suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones the various origins of the transition are discussed and the upper limit of magma chambers many of which lie above the basement of the larger edifices is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth the origins of the truncated cone shape of mid ocean ridge volcanoes are also discussed of the eruption mechanisms that have been proposed to explain their flat summits the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low density upper oceanic crust the discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes supporting table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on searching and submitting electronic supplements is found at http www agu org pubs csupp about html

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

global distribution of seamounts from ship track bathymetry data the distribution of submarine volcanoes or seamounts reflects melting within the earth and how the magma generated ascends through the overlying lithosphere globally 60 latitude we use bathymetry data acquired along 39 5 106 km of ship tracks to find 201 055 probable seamounts an order of magnitude more than previous counts across a wider height range 0 1 1 km implying that 24 000 60 remain to be discovered

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

submarine central volcanoes on the nazca plate high resolution sonar observations abstract this paper describes observations of 214 submarine central volcanoes made with long range side scan sonar and narrow beam echosounder between peru and the east pacific rise they are all circular in plan with basal diameters ranging from less than 1 to over 10 km profiles of four volcanoes were obtained with the narrow beam echosounder and those of a further nine were inferred from their acoustic shadows all the profiles show steep mostly concave sides a distinct summit region could be identified on about two thirds of the volcanoes in just under half of these the volcano culminates in a simple crater but the rest have broad slightly convex summit plateaus with or without craters almost half of all the volcanoes have one or more craters summit plateaus are relatively more common on the larger volcanoes so that for central volcanoes with a basal diameter greater than 4 km the commonest morphology is a steep sided truncated cone with a slightly domed summit plateau in most cases this form must represent the mode of formation not the result of subsequent wave erosion it is suggested that this form is the result of relatively fluid lavas travelling outward on low slopes from a central vent but terminating in steeply dipping hyaloclastites and volcanic breccias the spatial distribution of volcanoes is very non random in the area studied the east pacific rise out to 6 5 ma has less than one volcano per 10 4 km 2 whereas a field hundreds of kilometers across with a peak density of over 60 per 10 4 km 2 was seen north of mendana fracture zone on 6 5 26 ma old crust

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

late ordovician molluscs of the central and eastern anti atlas morocco more than 30 species of tergomyan gastropod bivalve and cephalopod molluscs are described fromthe late ordovician of central and eastern anti atlas morocco for the cephalopods this represents t

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

density and size distribution of seamounts in the eastern pacific inferred from wide beam sounding data theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow beam and wide beam echo sounders the shapes of seamounts are approximated as truncated right circular cones of constant slope e height to radius ratio and flatness f i e a seamount of basal radius r has a height h r a flat top of radius fr and a slope angle of arctan e the set of all seamounts in a given physiographic province is assumed to be a poissonian ensemble specified by a function v r the average number of seamounts per unit area for which r r a simple choice for the size distribution of seamounts and one shown to be consistent with data in the eastern pacific is to specify that it decays exponentially with r v r vo exp r in the case of a profile of length l taken in water of depth d with a wide beam echo sounder half beam width the shallow slope deep water approximation e 1 e d yields that the expected number of seamounts with apparent heights h h is n h l 2 1 ed 2 fh lv0e h we apply this sampling theory to apparent height data measured from 44 020 km of wide beam profiles on pacific crust of post mesozoic age although the spatial distribution of seamounts in the eastern pacific is certainly not poissonian the abundance statistics do not involve spatial correlations and we show they are robust with respect to deviations from the poisson model provided the statistics are interpreted as averages over the area actually sampled by the track lines the observed falloff of apparent heights in the eastern pacific is very nearly exponential justifying our use of an exponential model of seamount size distribution the shape parameters are estimated from multibeam surveys of pacific seamounts an analysis of 30 features in the height range 200 4000 m yields 0 21 0 011 and f 0 21 0 028 using these parameters we obtain vo 4 0 1 1 10 9 m 2 and 6 3 0 8 10 4 m 1 these values imply that per 106 km2 in the eastern pacific there are 1600 400 seamounts with summit heights greater than 300 m and 200 60 greater than 1000 m integrated over the entire size distribution seamounts occupy about 6 of the seafloor area and constitute about 0 4 of the oceanic crustal volume our estimates of seamount abundance are appreciably greater than those of most other authors a discrepancy we attribute primarily to the sampling bias of previous studies which generally used uncorrected map counts this hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central pacific from the scripps charts above 3000 m the map counts are consistent with the predictions of the exponential model but below this height they are biased to low values

guyots of the dutton ridge at the bonon mariana trench juncture as shown by multi beam surveys very accurate bathymetric data for five guyots from the newly named dutton ridge are presented along with that of two associated nearby guyots these new data are based on u s naval oceanographic office swath mapping by the sass multi beam sonar system glenn 1970 the dutton ridge is a major east west trending feature that intersects the junction of the bonin and mariana trenches its trend parallels that of the michelson ridge which lies further to the north at the intersection of the izu and bonin trenches as of this writing no samples have yet been dredged from the dutton guyots but the new bathymetric data presented allows some interpretation of the geologic history of the ridge i suggest that the dutton ridge formed as a volcanic plateau at the extinct triple junction of the phoenix farallon and pacific plates this once continuous plateau was then fractured and faulted to produce the present group of guyots and ridges later volcanism may have altered its morphology subsequently the ridge

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

imaging the high temperature geothermal field at krafla using vertical seismic profiling abstract geophysical exploration and in particular active source seismic imaging of geothermal fields is important to assess and optimize the exploitation of natural heat sources for energy production and direct use the first multi offset moving source vertical seismic profiling vsp experiment over the high temperature geothermal field in krafla iceland was carried out in spring 2014 with the aim to test whether vsp is a suitable method to map volcanic stratigraphy fractures dykes steam zones and magmatic bodies at this site and for volcanic environments in general in this study we present a workflow for processing the sparse krafla vsp dataset recorded with receivers in either of two boreholes the analysis involved first arrival traveltime inversion and seismic reflection processing the seismic velocity model obtained by traveltime tomography reveals structural information between the two boreholes and can be linked to an existing geological model showing that the seismic velocities are mainly controlled by lithology the zero offset seismic reflection data were processed into two corridor stacks walk away vsp reflection data were migrated with a novel multicomponent kirchhoff migration algorithm that includes p and s wave isolation to obtain separate pp ps and ss migrated images the reflections imaged in the corridor stacks can be linked to the main lithological units known from borehole logging information migrated images from the walk away data reveal reflectors below and to the sides of the two boreholes considering a priori information such as hypocenter locations from earthquake seismology studies the reflectors can be related to changes in lithology fault zones dykes and possibly the top of the krafla magma chamber we found that vsp is potentially a useful method to image the key lithological boundaries and volcanic stratigraphy in the complex magmatic environment at krafla but a priori information proved to be essential to constrain the processing and interpretation of the sparse array dataset

lithospheric age dependence of off ridge volcano production in the north pacific data for numbers of seamounts on north pacific ocean crust of different age indicate that the production rate of new off ridge seamounts volcanoes is proportional to the inverse of the square root of the age of the lithosphere this observation is consistent with several hypotheses which have been offered to explain the origin of off ridge oceanic volcanism and in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes leads to a new self consistent model for off ridge volcanism in this model the production rate of off ridge volcanoes is controlled primarily by the availability of fracture zone conduit systems in the thickening lithosphere in contrast mantle plume or hotspot volcanoes may punch through the oceanic lithosphere decreasing production rate of off ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous mantle material

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