helium and carbon fluxes in lake nyos cameroon constraint on next gas burst on 21 august 1986 a lethal gas burst issued from lake nyos in cameroon western africa killing at least 1700 people although the consensus is that the victims died of asphyxiation by co2 of magmatic origin the frequency of such catastrophic degassing events is still unknown the co2 flux at the bottom of lake nyos is estimated based on the measurement of3he4he and4he20ne ratios and the chemical composition of gases exsolved from the lake water the calculated co2 flux at the bottom of lake nyos is 4 4 2 3 1013cm3stp year combined with the estimated release in the august 1986 event of 8 2 1014 cm3 stp the co2 flux suggests that the gas burst may happen about once in18 10 years although the significant uncertainty should be taken into account for the frequency resulting from assumptions such as steady state fluxes in the lake and small fractionization of the c he ratio during the degassing event several yearly measurements of hypolimnetic fluxes and seasonal measurements of epilimnetic fluxes are needed to constrain better the recurrence interval in addition the results of a regional3he4he survey of carbonated mineral springs in northwestern cameroon are discussed in the context of the regional geotectonics

deep seismic structure of the kaapvaal craton the kaapvaal craton is remarkable for the intensive jurassic cretaceous kimberlite magmatism there are indications that the hot spot heating of the upper mantle in southern africa continued in the cenozoic however surface wave data in spite of their sensitivity to temperature failed to provide any evidence of thermal agitation at depths less than 350 km beneath the kaapvaal craton bloch et al 1969 we investigated the s velocity structure in the depth range of the mantle transition zone with the aid of teleseismic p to s converted phases our analysis suggests that a layer of anomalously low s velocity is present in the 370 470 km depth interval beneath the kaapvaal craton the anomalous layer is most likely of thermal origin its upper boundary may separate the deep mantle root of the craton from the underlying mantle the anomaly could arise beneath the craton already in the mesozoic and melting within this layer could contribute to the kimberlite magmatism

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

hot spot activity and the break up of pangea abstract the mesozoic and cenozoic positions of the continents that formed pangea in the triassic jurassic were derived from paleomagnetic and intraplate volcanic data paleoclimatic observations such as reef and fossil flora distribution and geological observations major hot spots helped to determine the longitudinal position of pangea and to construct a model of plate motion during the pangean break up the position of the northern part of pangea was constrained using iceland and jan mayen hot spots the iceland hot spot was traced from its present day position to greenland in the paleocene to baffin bay in the late cretaceous to the alpha ridge in the early cretaceous to the chukchi borderland in the middle late jurassic to franz joseph land in the early jurassic the yenisei khatanga trough in the middle triassic and finally to west siberia in the late permian early triassic the hot spot activity is expressed by eastern and western greenland volcanics the siberian trap basalts and perhaps by alpha ridge and chukchi borderland volcanics the chukchi borderland volcanics are related to the early stage of the opening of the canadian basin the position of the southern part of pangea was constrained using the bouvet hot spot this hot spot was tracked from its present day position to western antarctica in the early cretaceous late jurassic and to south africa in the early jurassic late triassic this hot spot activity produced the ferrar and karoo volcanics the model of plate motion obtained agrees with other data on intraplate volcanics which are also related to hot spots at the time of the opening of the central atlantic the cape verde and canary island hot spots were located along the ocean s spreading axis the long lasting location of hot spot and associated mantle upwelling plumes could help to explain forces driving the tethys transit plates opening of the ligurian ocean and eastern mediterranean the european hot spots are related to the several mesozoic phases of rifting for example the rhine graben hot spot may have been located at the rifting axis in the north sea several authors have already discussed the contribution of hot spots to the opening of the south atlantic and indian oceans mantle plumes associated with hot spots play an active role in rifting and initial phases of spreading the lithospheric displacement caused by upwelling combined with sometimes remote collisional forces on the other side of continental plate may result in compression and basin inversion

dating of sediments using accelerator mass spectrometry abstract an assessment is given of the current status and future prospects for the dating of recent sediments by the direct counting of radionuclides cosmogenic natural terrestrial and artificial using accelerator based mass spectrometers relevant isotopes and equipment are discussed the latter with particular reference to the 3 mv electrostatic accelerator system of the university of toronto inherent difficulties of the new method are outlined and possible applications are identified the ability to obtain radiocarbon dates from very small samples e g 100 g c illustrates the promise of the method

seismic activity at cadamosto seamount near fogo island cape verdes formation of a new ocean island summary in global catalogues only a handful of earthquakes occurred at or near the cape verde islands the two most recent events occurring in 1998 and 2004 were observed in the southwest of the cape verde archipelago away from aerial volcanic centres event relocation suggests that both earthquakes may occur at the cadamosto seamount a 3 km tall submarine volcanic edifice to the southwest of the active volcanic islands of fogo swath mapping efforts revealed numerous small volcanic cones between the volcanic island of brava and the seamount and between fogo and brava the distribution of volcanic vents may support two plumbing systems one feeding fogo and volcanic cones between fogo and brava and another system feeding cadamosto seamount using ocean bottom seismic stations of opportunity deployed roughly a month after a mb 4 3 earthquake occurred in 2004 august near or at the cadamosto seamount we were able to locate local seismic activity clustering at the seamount we propose that these events reflect brittle rock failure due to magma redistribution in or near a central magma reservoir or more likely dyke intrusion the observations reported here suggest that cadamosto seamount is the seismically most active feature in the archipelago it might be reasonable to hypothesis that ongoing activity causes the cadamosto seamount to grow forming a new oceanic island in the future

long term landscape evolution and post rift reactivation in the southeastern brazilian passive continental margin taubate basin zircon zft and apatite aft fission track low temperature thermochronology was applied at the brazilian passive continental margin in order to understand and reconstruct the post rift evolution since the breakup of southwestern gondwana the thermochronological data obtained from samples of both the precambrian basement and the paleogene to neogene sedimentary rocks from the continental rift of southeastern brazil provided zft ages between 148 15 and 64 6 ma and aft ages of 81 8 29 3 ma these data clearly indicate syn and post rift reactivations during the early cretaceous with great emphasis on paleogene to neogene times integrating the results of older thermochronological studies the reactivation of the southeastern brazilian margin can be described in three main phases related to the rift to post rift evolution of se brazil in general zft and aft data yield spread values that become younger as samples are closer to the reactivated neoproterozoic shear zones and might reflect source area exhumation the analysis of zft and aft data allowed interpretations regarding the main phases that occurred in the study area related to the thermotectonic and tectono stratigraphic evolution in southeastern brazil

shear zones as a result of intraplate tectonics in oceanic crust the example of the basal complex of fuerteventura canary islands ductile shear zones affecting igneous rocks pyroxenites syenites gabbros and carbonatites belonging to the basal complex of the island of fuerteventura are described here they develop typical mylonites with their cortege of structures and crystallographic fabrics an evolution from brittle to ductile conditions has been recognized and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks carbonatites syenites along the shear zones local metamorphic changes favour the ductile behaviour via a reaction enhanced softening mechanism the shear zones are arranged in a nearly orthorhombic pattern kinematic criteria suggest an extensional tectonic scenario with a bulk irrotational nonplane deformation the long axis of the finite deformation ellipsoid is e w and horizontal the available data allow us to establish this deformation as being from the late oligocene to the early miocene this is tentatively related to the plate tectonics evolution and mantle anomalies in the northwestern corner of the african plate

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

root mean square velocities of the continents with respect to the hot spots since the early jurassic the root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m y by combining previously published plate reconstructions these velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data from 169 m y b p to l44 m y b p the hot spot data indicate that laurasia moved rapidly 70 mm yr but the reconstructions permit a velocity as low as 50 mm yr we also estimate that from 63 m y b p to 48 m y b p india moved rapidly 140 20 30 mm yr faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates those attached to downgoing slabs along a substantial fraction of their margin the data require no other continent to have exceeded a velocity of 45 mm yr during the last 180 m y in agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no nettorque analyses we conclude that the continents have moved faster occasionally much faster than at present from this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere instead oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents on the basis of the very rapid early tertiary motion of india we conclude that the velocities of subducting plates can vary by a factor of 2 therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid

cenozoic extension in coastal dhofar southern oman implications on the oblique rifting of the gulf of aden a detailed analysis of fault populations that affect the paleocene to early miocene sedimentary succession of southern dhofar allows the definition of a time sequence of paleostresses and the reconstruction of the rifting history of the eastern gulf of aden two distinct phases of n20j and n160j trending extensions took place during the deposition of the synrift dhofar group from late priabonian 35 ma to middle burdigalian 18 ma a period of tectonic instability preceded these events marked by normal and strike slip faulting as early as the mid eocene the two main directions of extension also recorded in southern yemen developed respectively oblique and normal to the n75e trend of the gulf of aden an anticlockwise succession of the two episodes from n20 to n160 only established in yemen accounts for the observed synrift fault pattern which is consistent with the type and geometry of faults predicted in analogue models of oblique rifting a latest phase of ns compression marked by strike slip faulting may represent a far echo of the arabia eurasia collision d 2002 elsevier science b v all rights reserved

petromagnetic characteristics of cameroon line volcanic rocks abstract volcanic rock samples collected from 154 sites on the cameroon line cl have been analysed to study their petromagnetic characteristics thermomagnetic experiments and electron microscope observations on the samples indicate that the principal magnetic mineral and carrier of natural remanent magnetisation nrm in these rocks is titanomagnetite rich in titanium these also indicate that the level and stability of magnetisation are dependent on the quantity and composition of the magnetic minerals which are controlled by the composition of the original liquid magma their oxidation states and grain sizes the contribution of high and low temperature oxidation observed in some of the magnetic minerals was that of increasing their magnetic resistance rocks with abundant ilmenite lamellae in their titanomagnetite showed increased magnetic resistance during demagnetisation experiments in the analysis the samples exhibited a considerable magnetisation spectrum indicating that most of them are strongly magnetic the general tendency in experimental magnetism of an increase in magnetic resistance with decrease in grain sizes of discrete magnetic minerals is not respected in this natural system because of the effects of the presence of many ilmenite lamellae and mini fractures in some of the titanomagnetite grains in this way the conditions of low crystallisation temperature high fo2 and high ph2o which were the conditions for the crystallisation of the most stable rocks the hawaiites on the line were the conditions favourable for the acquisition of a stable magnetisation in the region the low curie temperatures 74 250 c found for a majority of the rocks indicate that the lower crust in the region and the upper mantle could be nonmagnetic the high regional negative magnetic anomaly over the cameroon line that is not consistent with the small depth previewed by the curie system and by the paramagnetic effect of the acid volcanic rocks in the region is probably derived principally from the differentiated basalts

style and timing of salt tectonics in the dniepr donets basin ukraine implications for triggering and driving mechanisms of salt movement in sedimentary basins abstract the ukrainian dniepr donets basin ddb is a late palaeozoic intracratonic rift basin with sedimentary thicknesses up to 19 km displaying the effects of salt tectonics during its entire history of formation from late devonian rifting to the tertiary hundreds of concordant and discordant salt structures formed during this time it is demonstrated in this paper that the variety of styles of salt structure formation in the ddb provide important constraints on understanding the triggering and driving mechanisms of salt kinematics in sedimentary basins in general salt movement in the ddb began during the devonian syn rift phase of basin development and exerted controls on the later distribution of salt structures though the geometry of basement faults is not directly responsible for the regular spacing of salt structures post rift salt movements in the ddb occurred episodically episodes of salt movement were triggered by tectonic events specifically two extensional events during the carboniferous an extensional reactivation at the end of carboniferous earliest permian and a compressional event at the end of the cretaceous extensional events that induced salt movement were thick skinned i e basement involved in deformation rather than thin skinned most overburden deformation related to salt movements is ductile regardless of sedimentary bulk lithology and degree of diagenesis while the deformation of sedimentary cover in areas where salt is absent is mainly brittle this implies that the presence of salt changes the predominant mode of deformation of overlying sedimentary rocks episodes of salt movement lasted longer than the periods of active tectonics that initiated them buoyancy erosion and differential loading all played a role in driving halokinesis once tectonic forces had pushed the salt overburden system into disequilibrium among these factors erosion of overburden above growing salt structures acted as a key self renewing force for development of salt diapirs very high sedimentation rates related to high post rift tectonic subsidence rates particularly during the carboniferous were able to bury diapirs and to load salt bodies such that buoyancy erosion and differential loading forces eventually became insufficient to continue driving diapirism until the system was perturbed by an ensuing tectonic event in contrast some salt anticlines and diapirs developed continuously during the entire mesozoic because of much reduced tectonic subsidence rates and sedimentation supply during this time however a lower permian salt series and overhangs of buried diapirs played an important role in preventing overburden piercing and fracturing during the mesozoic and specifically during the late cretaceous salt diapirism phase

heat flow and depth versus age for the mesozoic northwest atlantic ocean results from the sohm abyssal plain and implications for the bermuda rise abstract heat flow in the sohm abyssal plain is measured to be 53 mw m 2 at an age of 163 ma this is 25 higher than predicted by conductive cooling models even though the sediment corrected basement depth of 6 5 km at this location is normal for its age an analysis of existing heat flow depth and geoid anomalies in the northwest atlantic shows that there is little correlation between heat flow and depth throughout the entire region depth and geoid are clearly related to the bermuda swell while the associated heat flow anomaly once adjusted for variations with age is limited to 5 mw m 2 and only decays to the south this means that the bermuda swell is probably not caused by extensive thermal reheating within the lithosphere but instead by dynamic uplift at its lower boundary due to the convective upwelling of a mantle plume the regionally high heat flow in the northwest atlantic may be a thermal remanent of previous plumes which passed beneath this region early in its history therefore depth and heat flow anomalies from this region cannot be used to provide constraints on steady state parameters of the lithosphere such as the presence or absence of a long term boundary layer at its base

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

orientation of absolute african plate motion revealed by tomomorphometric analysis of the ethiopian dome the ethiopian dome comprising the ethiopian and yemeni plateaus formed as a result of the uplift of red sea gulf of aden and ethiopian rift shoulders the uplift was essentially due to a mantle plume activity a digital elevation model of this region before rifting constructed by using the rotation of its constituent plates and pulling down the rift shoulders reveals that important high areas existed before rifting these highs are interpreted to be the result of mantle plume activity a fast fourier transform was used to smooth the digital elevation model prerift dome before a tomomorphometric analysis was applied to quantify the orientation of each digital elevation model horizontal slice this procedure allowed us to determine a bearing of n30 e for absolute african plate motion since the paleogene

plate motions in the north pacific the 43 ma nonevent the hawaiian emperor seamount chain in the north pacific ocean is commonly considered to have been produced by motion of the pacific plate over a hotspot if the hotspot is assumed to have remained fixed in a mantle frame of reference the 60 change in trend between the hawaiian and emperor portions of the chain results from a change in motion direction of the pacific plate relative to the mantle at 43 ma during middle eocene time such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the pacific and surrounding plates but no evidence for these events can be found in addition in this paper it is shown that there were no changes in relative plate motions between pacific area plates that can be associated with the hawaiian emperor bend this is done by calculating motion of the pacific farallon and kula plates relative to asia and north america using the plate circuit farallon pacific antarctica africa north america asia with no major regional tectonic event at the time of the bend it is suggested that there was no plate reorganization at 43 ma and that the emperor portion of the seamount chain was formed by a nonstationary hotspot

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

crustal radial anisotropy beneath cameroon from ambient noise tomography abstract to increase the understanding of crustal deformation and crustal flow patterns due to tectonic processes in cameroon we study the lateral variability of the crustal isotropic velocity and radial anisotropy estimated using ambient noise tomography ant rayleigh and love wave noise correlation functions ncfs were retrieved from the cross correlation of seismic ambient noise data recorded in cameroon and phase velocities at periods of 8 to 30 s were measured to perform surface wave tomography joint inversion of rayleigh and love wave data for isotropic velocity models could not fit the observed dispersions simultaneously we attribute the love rayleigh discrepancy to the presence of radial anisotropy in the crust and estimated its magnitude our 3 d radial anisotropic model reveals the spatial variation of strong to weak positive vsh vsv and negative vsv vsh radial anisotropy in the crust we observe negative radial anisotropy in the upper crust that is associated mainly with the location of a previously reported mantle plume the anisotropy could be attributed to the vertical alignment of fossil microcracks or metamorphic foliations due to the upwelling of plume material a strong positive radial anisotropy is centered at the location of an inferred boundary between the congo craton and the oubanguides belt that might be related to the preferred orientation of crustal anisotropic minerals associated with shearing in this fault zone the middle crust is characterized by a widespread negative radial anisotropy that is likely caused by the flow induced alignment of anisotropic minerals that crystallized during magma intrusion the magnitude of the radial anisotropy varies systematically from predominantly negative in the middle crust to positive in the lower crust the imaged patterns of the isotropic velocity and radial anisotropy are consistent with previous studies and agree with regional tectonics

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

crust mantle coupling mechanism in cameroon west africa revealed by 3d s wave velocity and azimuthal anisotropy abstract to understand the depth variation of deformation beneath cameroon west africa we developed a new 3d model of s wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion we found that the cameroon volcanic line cvl is well delineated by slow phase velocities in contrast with the neighboring congo craton in agreement with previous studies apart from the congo craton and the oubanguides belt the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere the direction of fast axis in the upper crust is mostly ne sw but trending approximately n s around mt oku and the southern cvl the observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities a widespread zone of weak to zero azimuthal anisotropy in the mid lower crust shows evidence for vertical mantle flow or isotropic mid lower crust in the uppermost mantle the fast axis direction changed from ne sw to nw se around mt oku and northern cameroon this suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed ne sw fast azimuths are observed beneath the congo craton and are consistent with the absolute motion of the african plate suggesting a mantle origin for the observed azimuthal anisotropy our tomographically derived fast directions are consistent with the local sks splitting results in some locations and depths enabling us to constrain the origin of the observed splitting the different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in cameroon

the origin of platinum group minerals from the freetown intrusion sierra leone inferred from osmium isotope systematics the origin of certain alluvial platinum group mineral pgm grains is controversial with two major proposed models one involves primary formation within ultramafic intrusions followed by weathering into laterites with no further alteration prior to alluvial concentration the other involves in addition solution of the platinum group elements pge during weathering of the ultramafic source rocks probably by organic reactions followed by the supergene growth of the macroscopic grains within laterites in order to provide further evidence concerning the formation of macroscopic m in size platinum group mineral grains the authors have studied the osmium isotope ratios in pgm grains from the freetown peninsula in sierra leone using an ion microprobe indirect isotopic evidence and modeling are required to infer their origin as no pgm grains larger than 10 mm have ever been discovered in situ within the source rocks unlike virtually all other alluvial pgm grains that have been studied the freetown ones are distinguished by 187 os 188 os ratios that are much higher than canonical mantle values and vary considerably between different grains 0 12 187 os 188 os 0 28 the 187 os 188 os ratios are with one exception the same for individual osmium rich inclusions within the platinum iron alloy host of each grain although different grains have significantly different 187 os 188 os ratios in the one significant exception there are two os ru ir alloy inclusions within a single platinum iron alloy host which have 187 os 188 os ratios that are different from each other various models are considered to explain these observations 1 the elevated osmium isotope ratios could be primary and derived from a mantle inhomogeneity such as a mantle plume 2 the intruded material with a canonical mantle signature may have reacted with the host rocks at the time of the intrusion or at a later stage during postulated metamorphic or hydrothermal activity or 3 the supergene remobilization could have modified the osmium isotope signature of the pgm interpretation of the osmium isotopes is by no means unambiguous the range of 187 os 188 os ratios is far higher than known mantle sources which combined with the grain to grain variation seems to rule out the primary formation model sr nd and o isotope evidence indicates no reaction with the host rocks during intrusion or by metamorphic or hydrothermal activity and mixing calculations show that enhancement of the radiogenic os during intrusion could only have been small the evidence points to some extremely localized process which could however have acted in two ways either a preexisting high concentration of rhenium and pge a xenolith was incorporated into the magma chamber remained essentially unmixed with the majority of the material in the chamber and after extrusion of the gabbro pge nuggets were weathered out into laterites or alternatively concentration processes within the magma chamber produced a localized concentration of rhenium possibly in copper sulfides or late magmatic iron oxides in acid intrusions which when chemically dissolved in the supergene environment precipitated pge nuggets with extremely radiogenic 187 os 188 os signatures the data presented in this paper in conjunction with work elsewhere have been used to demonstrate that the processes producing highly radiogenic 187 os 188 os signatures cannot have resulted from large scale processes and have been used to dismiss a number of previously proposed models however a resolution between the two alternative models proposed is not possible using the present data

mesozoic hotspot epeirogeny in eastern north america geologic data especially conodont paleotemperatures imply that southeastern canada and new england have been elevated at least 4 km relative to the central appalachian region apatite fission track ages suggest that the uplift and consequent erosion occurred in cretaceous early tertiary time consistent with the formation of a broad swell as north america moved northwestward over the great meteor hotspot

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

apparent polar wander paths for the major continents 200 ma to the present day a palaeomagnetic reference frame for global plate tectonic reconstructions summary synthetic apparent polar wander apw paths for north america south america eurasia india central africa australia and antarctica for the last 200 myr are proposed computation of these apw paths is based upon the latest version 4 5a of the global paleomagnetic database gpmdb a revised global plate tectonic model since the early jurassic and a new technique for generating smoothed apw paths the smoothing technique includes the following steps 1 pre selection of palaeopoles including pre filtering parameters number of sites number of samples per site 95 per cent confidence circle about mean direction cleaning procedure and time uncertainty 2 generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit 3 independent spline regression analyses of the palaeolatitude and declination plots 4 removal of palaeolatitude or declination data that deviate by more than 10 from the regression curves post filtering process 5 generation of synthetic apw paths from the resulting palaeolatitude and declination plots these synthetic apw paths are then rotated into african coordinates to determine the best fit apw path and a global palaeomagnetic reference frame four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

a compilation of igneous rock volumes at volcanic passive continental margins from interpreted seismic profiles abstract the rifting and breakup of continents and subsequent seafloor spreading give rise to passive continental margins many of which are underlain by enormous volumes of igneous rocks and termed volcanic passive margins the relationships between the igneous rocks rifting and mantle plumes remain unresolved despite extensive studies and proposed alternative models to support such studies we use published previously interpreted seismic reflection and refraction data to compile a dataset of igneous rock volumes and geometries at volcanic passive continental margins the volmir volcanic passive margin igneous rocks dataset is based on margin crossing profiles on which the volumes and geometries of both shallow seaward dipping reflector sdr and deeper high velocity lower crustal hvlc units can be measured it includes information about the ages of continental breakup and profile distances from the associated euler pole and from hotspots that may have played roles in the breakup process despite local variations the dataset shows general patterns the average ratio of sdr to hvlc volumes 0 3 0 4 is relatively consistent between profiles implying that formation of these units is related during continental breakup the volumes of igneous rocks display a moderate positive correlation with distances from the euler pole but only a weak negative correlation with distances from the nearest hotspot at the time of margin formation suggesting that in continental breakup lithospheric processes passive rifting have greater effects than hotspots active rifting these results suggest that this dataset will be useful for exploring aspects of the rifting process relationships between volcanic rifted margins and potential explanations for their similarities and differences

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

reconstructing eroded paleovolcanoes on gran canaria canary islands using advanced geomorphometry abstract original volcanic edifices of two successive stages of gran canaria are reconstructed using a geomorphometric analysis of existent or restored paleosurfaces in the reconstruction surface fitting was applied preferably to planezes i e triangular facets of original volcano flanks and quasi planar surfaces qps those occurring on planezes or scattered slightly eroded portions derived from original cone surfaces with the help of red relief image map rrim analysis out of the long lasting mid miocene to holocene subaerial evolution of the island the late miocene fataga volcano and the subsequent pliocene roque nublo volcanoes were the largest and highest the eruptive center of fataga a composite edifice 12 2 8 8 ma that may have grown up excentrically with respect to the previous tejeda caldera is well defined by both two planezes named veneguera mogan and fataga tirajana and qps remnants its calculated original volume 1000 km3 is close to the largest stratovolcanoes on earth however its 3300 m elevation obtained by exponential fit may have been significantly lower due to the complex architecture of the summit region e g a caldera responsible for ignimbrite eruptions roque nublo a 3 7 2 9 ma stratovolcanic cone which was superimposed upon the fataga rocks 3 km west of the fataga center has left no considerable paleosurfaces behind due to heavy postvolcanic erosion yet its remnant formations preserved in a radial pattern unambiguously define its center moreover surface fitting of the outcropping rocks can be corrected taking the erosion rate for the past 3 ma into account such a corrected surface fit points to a regular shaped 3000 m high cone with a 25 km radius and ca 940 km3 original volume also comparable with the dimensions of the largest terrestrial stratovolcanoes

true polar wander an analysis of cenozoic and mesozoic paleomagnetic poles a true polar wander path is presented for the past 180 ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data a relative displacement of 22 10 is observed the motion between the two reference frames has not been smooth episodes of rapid motion on the order of 80 100 mm yr include the present 65 50 ma 115 85 ma and 180 160 ma periods of slow or little polar motion less than 20 mm yr are observed at 50 5 ma and 160 115 ma the effect of plate motions is removed using morgan s plate motion model a19 this model is tied to a hotspot reference frame assumed to be fixed in the mantle the rotated paleopoles are grouped by ages and then averaged they are found to cluster about points distinct from the north pole as defined by the hotspots these points trace out a path of motion between the rotation axis and the hotspots or mantle this is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle paleomagnetic poles of the past 7 m y are examined independently this eliminates a dependence on the fixed hotspot hypothesis since plate motions are essentially negligible for this time span there is a deviation of 5 2 between the present geographic north pole and the average of the paleopoles at 5 ma this is too great to attribute to plate motion interpreted as true polar wander the rate of this polar motion is about 1 m y astronomical measurements for the past 80 years have recorded actual motion of the earth s rotational axis up to 1 m y in the direction of eastern canada this agrees in rate with paleomagnetic polar motion of the past 7 m y and in both rate and direction for the past 2 m y

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

jurassic extension estimates for the north sea triple junction from flexural backstripping implications for decompression melting models abstract the rattray volcanics lie at the triple junction between the three arms of the north sea rift system the mildly undersaturated volcanics in this province were erupted during the early syn rift phase of a middle late jurassic rifting event and are thought to be the products of decompression induced partial melting of the mantle resulting from lithospheric attenuation during rifting situated as they are in one of the most intensively studied rift basins in the world the rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by mckenzie and bickle 1 this study uses a modified version of the backstripping technique which takes into account the flexural strength of the lithosphere to constrain the stretching factor for the jurassic rift phase two dimensional interpreted sections are flexurally backstripped to the base post rift to give an initial estimate of is then further constrained by forward modelling to the base post rift basin geometry generated by backstripping forward modelling uses the flexural cantilever model a combined simple shear pure shear model for continental extension which takes into account the thermal rheological and flexural isostatic consequences of extension this is an approach that is different from that of previous studies 2 5 which assume local rather than regional isostatic compensation to allow the backstripping of individual wells the results obtained in this new study give maximum jurassic factors on each of the three rift arms close to the triple junction of 1 2 1 35 this suggests a maximum jurassic factor in the triple junction itself of 1 35 1 75 this is considerably lower than previous estimates and is due to the breakdown of the assumption used in these previous studies that airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis whilst such a factor is insufficient to generate melt by decompression alone an anomalously high mantle potential temperature and the presence of volatiles as well as a complex mesozoic and late palaeozoic rifting history are likely to have played a role in the generation of melt

monteregian hotspot track a long lived mantle plume the monteregian hills the younger white mountains and the new england seamounts constitute a well defined hotspot track the evolution of the plume through time is obtained from the marine track the buoyancy flux of the plume when it made the new england seamounts was about 2 mg s 1 which would make it the seventh or eighth most active plume today the hotspot declined in activity after it made nashville seamount the buoyancy flux when it made corner seamounts and today is less than 0 5 mg s 1 the changes in the flux of the marine hotspot limit how far the marine flux can be extrapolated to the west onto the continental track and what can be learned in general about vulnerability of the continental lithosphere to hotspots the lack of an obvious track west of montreal may be due either to failure of the plume to penetrate the canadian shield to the lack of recognizable intrusions or to strengthening of the plume when it approached montreal

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

southern ocean hotspot tracks and the cenozoic absolute motion of the african antarctic and south american plates abstract a detailed analysis based on an antarctica africa finite reconstruction at chron c29 64 ma an assumption of no relative wander between the marion prince edward and tristan hotspots and on recently revised bathymetric maps of the southern ocean region shows that the fixed hotspot reference frame is tenable for absolute plate motions bouvet hotspot and probably trinidade as well also shows little or no cenozoic relative motion contrary to previous models bouvet hotspot is unrelated to the meteor rise cape rise seamount chain instead the bathymetric data when compared with the predicted hotspot tracks indicate another hotspot exists near the southernmost south atlantic spreading ridge segment new geochemical evidence from the latter region supports this hypothesis in showing the effects of plume enrichment from a source that is compositionally distinct from bouvet the peculiar zig zag shape of the cape rise meteor rise lineament is the result of this hotspot crossing the active transform segment of the falkland agulhas fracture zone in late mesozoic times followed by an early cenozoic ridge jump to the pre weakened trace on the then south american plate from the averaged cenozoic absolute motions of the african antarctic and south american plates it is evident that antarctica has been most nearly stationary in an absolute motion sense

seismic study of the transform rifted margin in davis strait between baffin island canada and greenland what happens when a plume meets a transform 1 the davis strait transform margin was studied using a 630 km long wide angle reflection refraction seismic transect extending from se baffin island to greenland dense airgun shots were recorded by 28 ocean bottom seismometers deployed along the line a p wave velocity model was developed from forward and inverse modeling of the wide angle data and incorporation of coincident deep multichannel reflection seismic data off baffin island in the saglek basin 7 to 11 km thick two layered continental crust 5 8 6 6 km s is observed off greenland continental crust is divided into three layers 5 4 6 8 km s with a maximum thickness of 20 km farther offshore greenland the crust thins to 7 12 km and the lower crust disappears between the continental blocks a 140 km wide zone with oceanic crust layer 2 is 5 4 6 2 km s and layer 3 is 6 7 7 0 km s is located the western half of this zone is interpreted to be part of a volcanic margin with seaward dipping reflectors the eastern part is associated with the ungava fault zone ufz the major transform fault in davis strait the ufz thus acted as leaky transform fault during phases of transtension southward flow of material from the iceland plume created a 4 to 8 km thick underplated layer 7 4 km s beneath the thinned portions of the continental crust and beneath previously emplaced oceanic crust plume related paleogene volcanism is indicated by an up to 4 km thick layer 4 3 5 8 km s with basalts and interbedded sediments that can be traced from se baffin island 400 km toward the east

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

rigidity of the atlantic oceanic lithosphere beneath new england seamounts abstract investigation of the flexural response of the oceanic plate subject to loading of new england seamounts provides an insight into the rigidity of the lithosphere fourier domain analysis of the bathymetry and gravity anomalies over new england seamounts suggests that internal density perturbations exist in the crust and or uppermost mantle the loads due to the topography of the seamounts and density perturbations must have been supported by the lithosphere we group the seamounts into three representative ages and adopt a 3 d thin elastic plate model over an invicid mantle to assess the rigidity of the lithosphere under these loads the rigidity values are about 3 7 10 23 n m for lithospheric ages ranging from 30 to 60 my the base of the elastic part of the lithosphere follows an isotherm in the range of 750 850 c suggesting a strong lithosphere that makes the north atlantic ocean difficult to initiate subduction however the thermal blanketing by the thick sediments at scotian basin reduces the rigidity by one to two orders of magnitude and may facilitate initiation of subduction of the north atlantic oceanic lithosphere at scotian margin

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

role of continental break up mantle plume development and fault reactivation in the evolution of the gamtoos basin south africa abstract in response to the break up of gondwana the gamtoos basin rifted in the oxfordian kimmeridgian with reactivation of cape fold belt thrusts as extensional structures rifting was rapid with an initial divergent syn rift sequence passing into a parallel passive infill sequence as rifting waned dextral movement on the agulhas falkland fracture zone during the valanginian resulted in reactivation of oxfordian kimmeridgian structures with extensional compressional and strike slip senses of movement post rift thermal subsidence phase of basin evolution began during latest valanginian times with passive infill and onlap of the topography created during the valanginian subsequently the basin began to be uplifted due to the westward migration of the falkland plateau and natal valley spreading centre past the basin resulting in a non depositional hiatus and local channelling before erosion during the cenomanian following renewed subsidence the basin experienced uplift during the palaeocene and eocene the cause of which remains problematic but is contemporaneous with igneous intrusion in southern africa

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

offshore continuation of meguma terrane southwestern nova scotia eight short drill cores have been examined from the continental shelf southwest of nova scotia four cores recovered granitoid rocks of two types ilmenite bearing granitoid rocks petrographically and geochemically resemble granodiorites of the south mountain batholith and granites of the seal island pluton magnetite bearing granitoid rocks are also peraluminous but have no exact analogues onshore in nova scotia two cores recovered metamorphic rocks in a small area 50 km south of seal island one consits of chlorite muscovite quartz schist geochemically similar to rocks of the halifax formation the second sampled epidote chlorite quartz schist similar to metavolcanic rocks of the white rock formation one further core sampled quartzite and another sampled a metavolcanic rock possibly erratic the regional extent of these lithotypes can be inferred from gravity and aeromagnetic data regional gravity data suggest the presence of a large granite body off southwestern nova scotia in this area magneti

geoid roughness and long wavelength segmentation of the south atlantic spreading ridge the configuration of accreting plate boundaries tends to reflect the geometry of the original continental break up yet there are also many indications that the detailed pattern of spreading ridge and transform fault segments may be an inherent part of the spreading process itself1 2 the mid atlantic ridge for example is segmented with a typical spacing of 40 km between transform faults3 4 here we use the short wavelength 25 110 km part of the seasat altimetry data set to derive a map of the geoid rough ness in the south atlantic between 1 n and 39 s this map reflects changes in the local amplitude envelope of the geoid signal primarily associated with topography zones of high roughness correlate with major fracture zones aseismic ridges and the mid atlantic ridge the most remarkable feature is that the roughness is not uniform along the ridge but displays quasi periodic variations with a wavelength of 400 km a segmentation an order of magnitude larger than described so far which has persisted for at least 60 myr this segmentation may reflect either fabric generated at the ridge by normal plate tectonic processes1 5 or small scale convective instabilities rising from 80 km deep region of the upper mantle3 6 7

seismicity of the cameroon volcanic line 1982 1990 abstract this paper is dedicated to soba djallo director of irgm who died in 1991 results of seismic monitoring of the continental segment of the cameroon volcanic line for the period 1982 to 1990 are reported mount cameroon continues to be seismically the most active area with a well defined seismicity pattern characterized by single and swarm events with duration magnitudes between 2 and 3 occurring at depths down to 55 km on average the earthquakes here occur at the rate of about 2 events every 3 days further north the seismicity is low except for isolated felt events located along the foumban shear zone which probably controls the location of the earthquakes as well as the volcanic centres making up the volcanic line during this eight year period two fatal carbon dioxide gas emissions from crater lakes occurred in 1984 l monoun and 1986 l nyos causing 37 and 1700 deaths respectively seismic monitoring indicates that no seismic activity was associated with these lakes thus favouring an aseismic cause to the disasters these gas emissions the increased number of felt earthquakes and a recent natural explosion on mount cameroon suggest either a more active phase of the cameroon volcanic line and or a greater awareness by the local population of the phenomena associated with this volcanic province

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

magmatic underplating in the canary archipelago abstract the location of the canary archipelago at the east edge of the north atlantic a few hundred kilometres from the west african coast has for years posed a serious difficulty in understanding the complex interaction between long lasting volcanic activity and one of the oldest seafloor basements 150 ma on earth many different hypotheses have been proposed in the last two decades to understand the genesis of the canary archipelago there is an increasing acceptance to explain the archipelago as the product of the slow passage of the african plate over a mantle hotspot in rival tectonic models stretching and thinning of the lithosphere would determine the islands as independent volcanic blocks we review recent geophysical and geological evidence supporting a mantle plume origin for the canary islands based on the recognition of crustal thickening by mafic intrusions produced by the interaction between the mantle plume and the old oceanic lithosphere beneath the canary archipelago

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

u th he dating of kimberlites a case study from north eastern kansas abstract dating kimberlite intrusive rocks by radiogenic isotope geochronology often is a difficult task complicated by both the lack of dateable minerals within kimberlite as well as significant sample alteration that can degrade samples and alter parent daughter ratios this study presents a new geochronologic tool for timing the emplacement of kimberlites using the u th he system to date the cooling of common kimberlite phenocrystic and xenocrystic minerals to demonstrate the use of this technique new apatite titanite zircon magnetite and garnet u th he ages constrain the timing of emplacement for the stockdale tuttle baldwin creek bala and leonardville kimberlite pipes located in riley county kansas zircon from the tuttle pipe and titanite from the stockdale pipe yield u th he ages of 108 6 9 6 ma and 106 4 3 1 ma respectively these data are consistent with new tuttle kimberlite rb sr analyses of phlogopite megacrysts that give a five point isochron age of 106 6 1 0 ma similarly an apatite u th he age of 85 3 2 3 ma from the baldwin creek kimberlite is in agreement with a rb sr phlogopite age of 88 4 2 7 ma these dates demonstrate that u th he thermochronometry provides reliable timing constraints on the cooling of common kimberlite xenocrystic phases thereby timing kimberlite emplacement in addition to the use of more commonly used apatite and zircon u th he thermochronometers we produced reliable emplacement ages of 103 0 7 5 ma for the bala kimberlite using u th he dating of phenocrystic magnetite and an age of 98 8 8 9 ma for the tuttle kimberlite using u th he dating of megacrystic garnet in contrast kimberlitic apatite u th he ages from the stockdale bala tuttle and leonardville kimberlites yield ages ranging from 67 3 4 4 ma to 64 3 5 6 ma suggesting a local possibly hydrothermal reheating event resulting in resetting of the apatite u th he clock in latest cretaceous to earliest tertiary time additional u th he analyses of apatite from nearby sandstone and basement rocks suggest regional cooling below 70 c at 165 ma these u th he and rb sr age data imply that the kimberlites were emplaced over a period of time from 85 110 ma with several pipes subjected to local reheating at 65 ma

crustal structure of ascension island from wide angle seismic data implications for the formation of near ridge volcanic islands abstract the study of the internal structure of volcanic islands is important for understanding how such islands form and how the lithosphere deforms beneath them studies to date have focused on very large volcanic edifices e g hawaiian islands marquesas but less attention has been paid to smaller islands which are more common ascension island a 4 km high volcanic edifice with a basal diameter of 60 km is located in the equatorial atlantic 8 s 90 km west of the mid atlantic ridge on 7 ma oceanic lithosphere we present results of a wide angle seismic profile crossing the island revealing a crustal thickness of 12 13 km an overthickened layer 3 7 km thick and little evidence of lithospheric flexure together these results suggest ascension island may be older than previously assumed and may have begun forming at an on axis position around 6 7 ma this hypothesis is further supported by the presence of a young 1 4 km high edifice directly adjacent to the mid atlantic ridge with a volume about 1 7 that of ascension island possibly representing the earliest stages of seamount formation excess magmatism appears to be related to the tectonic setting at the ridge fracture zone intersection

constraints on north american plate velocity from the yellowstone hotspot deformation field oceanic hotspot tracks generally indicate that the mantle plumes that give rise to them are fixed in position relative to a lower mantle reference frame for timespans of 100 myr1 3 it has not been clear whether continental hotspots are equally stable for example there is evidence that the velocity of the yellowstone hotspot has been quite irregular with respect to the north ameri can plate over the past 16 myr4 7 but measurements of its velocity based on caldera locations have been too uncertain and irregular to be definitive here i describe a new method of obtaining the plume velocity which uses the broader lithospheric effects of the hotspot rather than the locations of individual calderas for the past 10 myr the results indicate a constant north american plate velocity of 2 2 cm yr 1 which agrees with independent estimates from global plate motion inversions8 9 this velocity is considerably lower than previous estimates obtained from caldera locations and clearly differs from the much higher but less well constrained velocities of the previous 6 myr refs 5 7