paleolatitudes determined from paleomagnetic data from vertical cores most of the paleomagnetic data available from oceanic plates consist of measurements of inclination but not declination from azimuthally unoriented vertical cores similarly the basic paleomagnetic data to be analyzed from crustal blocks that have rotated by varying amounts about vertical axes are a set of paleoinclinations a new method is proposed for determining the paleocolatitude of a sampling site and the associated confidence limits paleomagnetic inclinations are first converted to paleocolatitudes and averaged to produce a mean colatitude after showing that this mean is biased we derive a paleocolatitude correction to remove the bias using a model for geomagnetic secular variation based on the latitude dependence of secular variation during the past 5 million years after testing the validity of the model by applying it to paleomagnetic data from quaternary lavas for which both inclination and declination are known we apply the method to inclination only paleomagnetic data from suiko seamount the most complete data set from a drilling site the method is then applied to paleomagnetic data from all deep sea drilling project sites for the pacific plate to obtain for each site an unbiased mean paleocolatitude and its confidence limits these paleocolatitudes and confidence limits are then used as input for a least squares analysis to obtain early and late cretaceous poles for the pacific plate and 95 confidence limits for these poles it is shown that the scatter of individual site paleocolatitudes about the best fit pole is consistent with the confidence limits at each site as determined by the new method of analysis

40ar 39ar ages of intrusions in east greenland rift to drift transition over the iceland hotspot abstract sixteen 40 ar 39 ar ages are presented for alkaline intrusions to appraise prolonged post breakup magmatism of the central east greenland rifted margin the chronology of rift to drift transition and the asymmetry of magmatic activity in the northeast atlantic igneous province the alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland the area south of the kangerlussuaq fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56 54 ma 50 47 ma and 37 35 ma in the kangerlussuaq fjord which coincides with a major tectonic lineament possibly the failed arm of a triple junction the alkaline plutons span from 56 to 40 ma to the north and within the continental flood basalt succession alkaline intrusions of the north south trending wiedemann fjord kronborg gletscher lineament range from 52 to 36 ma we show that post breakup magmatism of the east greenland rifted margin can be linked to reconfiguration of spreading ridges in the northeast atlantic northwards propagation of the proto kolbeinsey ridge rifted the jan mayen micro continent away from central east greenland and resulted in protracted rift to drift transition the intrusions of the wiedemann fjord kronborg gletscher lineament are interpreted as a failed continental rift system and the intrusions of the kangerlussuaq fjord as off axis magmatism the post breakup intrusions south of kangerlussuaq fjord occur landward of the greenland iceland rise and are explained by mantle melting caused first by the crossing of the central east greenland rifted margin over the axis of the iceland mantle plume 50 47 ma and later by uplift associated with regional plate tectonic reorganization 37 35 ma the iceland mantle plume was instrumental in causing protracted rift to drift transition and post breakup tholeiitic and alkaline magmatism on the east greenland rifted margin and asymmetry in the magmatic history of the conjugate margins of the central northeast atlantic

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

refined spreading history at the southwest indian ridge for the last 96 ma with the aid of satellite gravity data summary the spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry the high quality satellite derived gravity data that became available in recent years reveal the details of fracture zones which can be used as flow lines to control spreading models we have applied this approach to the southwest indian ridge swir in order to refine its spreading history this is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23 where the magnetic anomalies alone cannot resolve the detailed spreading history we find four main stages in the spreading history of the swir since 96 ma including two that were not noted previously between 96 ma and anomaly 33 76 3 ma and between anomalies 23o 51 7 ma and 18o 40 1 ma o denotes old boundaries of normal magnetization period we also find that the start of the period of complex spreading was at anomaly 33 somewhat earlier than previously proposed we discuss the characteristics of the extension that the old transform faults underwent during the complex spreading phase in response to the counterclockwise rotation of spreading new transform faults appeared at that time considerably widening the transform zones

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

absolute plate motions relative to deep mantle plumes abstract advances in whole waveform seismic tomography have revealed the presence of broad mantle plumes rooted at the base of the earth s mantle beneath major hotspots hotspot tracks associated with these deep mantle plumes provide ideal constraints for inverting absolute plate motions as well as testing the fixed hotspot hypothesis in this paper 27 observed hotspot trends associated with 24 deep mantle plumes are used together with the morvel model for relative plate motions to determine an absolute plate motion model in terms of a maximum likelihood optimization for angular data fitting combined with an outlier data detection procedure based on statistical tests the obtained t25m model fits 25 observed trends of globally distributed hotspot tracks to the statistically required level while the other two hotspot trend data comores on somalia and iceland on eurasia are identified as outliers which are significantly incompatible with other data for most hotspots with rate data available t25m predicts plate velocities significantly lower than the observed rates of hotspot volcanic migration which cannot be fully explained by biased errors in observed rate data instead the apparent hotspot motions derived by subtracting the observed hotspot migration velocities from the t25m plate velocities exhibit a combined pattern of being opposite to plate velocities and moving towards mid ocean ridges the newly estimated net rotation of the lithosphere is statistically compatible with three recent estimates but differs significantly from 30 of 33 prior estimates

new 40ar 39ar age progression for the louisville hot spot trail and implications for inter hot spot motion 1 in this study we present 42 new 40ar 39ar incremental heating age determinations that contribute to an updated age progression for the louisville seamount trail louisville is the south pacific counterpart to the hawaiian emperor seamount trail both trails representing intraplate volcanism over the same time interval 80 ma to present and being examples of primary hot spot lineaments our data provide evidence for an age progressive trend from 71 to 21 ma assuming fixed hot spots this makes possible a direct comparison to the hawaiian emperor age progression and the most recent absolute plate motion apm model wk08g of wessel and kroenke 2008 we observe that for the louisville seamount trail the measured ages are systematically older relative to both the wk08g model predictions and hawaiian seamount ages with offsets ranging up to 6 myr taking into account the uncertainty about the duration of eruption and magmatic succession at individual louisville volcanoes these age offsets should be considered minimum estimates as our sampling probably tended to recover the youngest lava flows these large deviations point to either a contribution of inter hot spot motion between the louisville and hawaiian hot spots or to a more easterly location of the louisville hot spot than the one inferred in the wk08g model both scenarios are investigated in this paper whereby the more eastern hot spot location 52 0 s 134 5 w versus 52 4 s 137 2 w reduces the average age offset but still results in a relatively large maximum offset of 3 7 myr when comparing the new ages to the apm models s04p s04g by steinberger et al 2004 that attempt to compensate for the motion of hot spots in the pacific hawaii or globally hawaii louisville reunion and walvis the measured and predicted ages are more in agreement showing only a maximum offset of 2 3 myr with respect to the s04g model at face value these more advanced apm models which consider both plate and hot spot motions therefore provide a better fit to the new louisville age data the fit is particularly good for seamounts younger than 50 ma a period for which there is little predicted motion for the louisville hot spot and little inter hot spot motion with hawaii however discrepancies in the louisville age distance record prior to 50 ma indicate there is an extra source of inter hot spot motion between louisville and the other pacific hot spots that was not corrected for in the global s04g model finally based on six new 40ar 39ar age dates the 169 w bend in the louisville seamount trail seems to have formed at least 3 myr before the formation of the hawaiian emperor bend the timing of the most acute parts of both bends thus appears to be asynchronous which would require other processes e g plume motions than a global plate motion change between 50 and 47 ma to explain these two observations

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

revised pacific antarctic plate motions and geophysics of the menard fracture zone a reconnaissance survey of multibeam bathymetry and magnetic anomaly data of the menard fracture zone allows for significant refinement of plate motion history of the south pacific over the last 44 million years the right stepping menard fracture zone developed at the northern end of the pacific antarctic ridge within a propagating rift system that generated the hudson microplate and formed the conjugate henry and hudson troughs as a response to a major plate reorganization 45 million years ago two splays originally about 30 to 35 km apart narrowed gradually to a corridor of 5 to 10 km width while lineation azimuths experienced an 8 counterclockwise reorientation owing to changes in spreading direction between chrons c13o and c6c 33 to 24 million years ago we use the improved pacific antarctic plate motions to analyze the development of the southwest end of the pacific antarctic ridge owing to a 45 counterclockwise reorientation between chrons c27 and c20 61 to 44 million years ago this section of the ridge became a long transform fault connected to the macquarie triple junction following a clockwise change starting around chron c13o 33 million years ago the transform fault opened a counterclockwise change starting around chron c10y 28 millions years ago again led to a long transform fault between chrons c6c and c5y 24 to 10 million years ago a second period of clockwise reorientation starting around chron c5y 10 million years ago put the transform fault into extension forming an array of 15 en echelon transform faults and short linking spreading centers

bounds on geologically current rates of motion of groups of hot spots it is widely believed that groups of hot spots in different regions of the world are in relative motion at rates of 10 to 30 mm a 1 or more here we present a new method for analyzing geologically current motion between groups of hot spots beneath different plates in an inversion of 56 globally distributed equally weighted trends of hot spot tracks the dispersion is dominated by differences in trend between different plates rather than differences within plates nonetheless the rate of hot spot motion perpendicular to the direction of absolute plate motion vperp differs significantly from zero for only 3 of 10 plates and then by merely 0 3 to 1 4 mm a 1 the global mean upper bound on vperp is 3 2 2 7 mm a 1 therefore hot spots move slowly and can be used to define a global reference frame for plate motions

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

uncertainties in plate reconstructions relative to the hotspots pacific hotspot rotations and uncertainties for the past 68 million years summary a method previously used for estimating the uncertainty in rotation of a plate relative to the hotspots is shown to be insufficient because of the neglect of the uncertainty in one of the three degrees of freedom of a rotation two new methods for estimating best fitting rotations and associated uncertainties for the rotation of a plate relative to the hotspots are presented both methods require a priori estimates of the uncertainties in the locations of individual hotspots and their ancient tracks for a particular age the use of a priori uncertainties permits the hypothesis of hotspot fixity to be tested the first method the two hotspot method leads to simple geometrical interpretations of best fitting rotations and of the eigenvectors and eigenvalues of the appropriate covariance matrix this simplicity comes at the cost of using only two hotspots and their associated tracks with assumed equal circular uncertainties the second method the n hotspot method allows use of an arbitrary number of hotspots 2 with unequal elliptical uncertainties this generality and power comes at the cost of losing the simple geometrical interpretation of the first method elliptical uncertainties are generally more appropriate because hotspot tracks are less well known along strike than across strike because of gaps and uncertainties in ages along the tracks when both methods are applied to the estimate of the pacific hotspot rotation since the age of the hawaiian emperor elbow using the tracks of both the hawaiian and louisville hotspots they produce similar rotations and similar uncertainties the n hotspot method is further applied to estimate pacific hotspot motion at six additional ages from 10 9 ma to 67 7 ma corresponding to the ages of key magnetic anomalies used in global plate reconstructions the goodness of the resultant fit shows that the assigned uncertainties and assumption of hotspot fixity are mutually consistent the pre elbow i e pre 47 9 ma or emperor pole of rotation is shown to differ significantly from the post elbow i e post 47 9 ma or hawaiian pole of rotation moreover rotation over the past 47 9 ma is shown to have occurred in at least three statistically distinct stages from 47 9 to 20 1 ma from 20 1 ma to 6 ma and from 6 ma to the present the change in pole position at 20 1 ma is accompanied by a highly significant 50 per cent increase in the rate of rotation of the pacific plate relative to the hotspots these encouraging results suggest that the new methods can be used to improve and to quantify plate motions relative to the hotspots for a wide variety of applications

location of the nubia somalia boundary along the southwest indian ridge the northern boundary between the nubian west african and somalian east african plates is marked by the existence of the east african rift which is the locus of roughly east west stretching due to the separation of these plates south of 20 s any expression of deformation or seismicity due to the relative motion of these two presumably distinct plates vanishes or is extremely subtle although the boundary must continue until it intersects another plate boundary until recently there were no observational limits on the location of that intersection and the recent limits were very broad being thousands of kilometers wide here from an analysis of the locations of the old edge of magnetic anomaly 5 which is over 11 ma seafloor we show that the main locus of deformation between nubia and somalia near the southwest indian ridge over the past 11 m y can be no wider than a few hundred kilometers from 100 km west of the du toit fracture zone to 50 km east of the andrew bain fracture zone complex deformation is most likely concentrated along a closely spaced set of several fracture zones known collectively as the andrew bain fracture zone complex which is only 100 km wide the displacement along the nubia somalia boundary over the past 11 m y is 23 km 6 km 95 confidence limits indicating a displacement rate of 2 mm yr much slower than typical rates of seafloor spreading and subduction but comparable to that along the owen fracture zone and dalrymple trough which separate the arabian plate from the indian plate and along the azores gibraltar line which separates the eurasian plate from the nubian plate

morphotectonic evolution of the new caledonia ridge pacific southwest from post obduction tectonosedimentary record abstract the tectonostratigraphic and geomorphic study of two post obduction fluvial sedimentary systems on mainland new caledonia and imaged offshore on seismic reflection lines provides a new perspective on the post orogenic evolution of the new caledonia ridge relations between sedimentary sequence boundaries erosion surfaces and faults both on land and on offshore seismic reflection profiles indicate that an episode of extensional tectonics initiated in the early neogene led to the disruption and collapse of the island landsurface previously shaped during a latest oligocene phase of planation microtectonic analysis further suggests early slip on the normal faults was associated with ridge normal extension a later set of faults accompanied ridge parallel to ridge oblique extension that is interpreted to result from a shift toward a transtensional regime driven by the initiation of east verging subduction of the australian plate beneath the pacific plate starting at least in the late mid miocene

long term tropical morphogenesis of new caledonia southwest pacific importance of positive epeirogeny and climate change extensive mapping of relict lateritic landsurfaces over the 1600 m of relief of mainland new caledonia is used to evaluate long term morphogenesis of the island since its emergence in the oligocene the results support the interpretation of the lateritic levels as abandoned stepped remnant landsurfaces that may be grouped into eight island scale erosion levels recording episodic lowering of base level the separation of the erosion levels results from the combination of positive epeirogenic movements of the new caledonia continental ridge and long term neogene climate variability which is recorded by the contrasted erosion and weathering patterns of the three youngest levels and tentatively correlated with known global and regional paleoclimatic events

revision of paleogene plate motions in the pacific and implications for the hawaiian emperor bend comment according to literature summarized by wright et al 2015 it is generally accepted that hotspots have moved relative to each other since 80 ma and that the hawaiian hotspot moved south relative to the spin axis during formation of the emperor seamounts prior to 48 ma koivisto et al 2014 advanced the alternative view that the minimum relative motion between hotspots is much smaller than generally accepted implying errors in the global plate circuit are larger than generally accepted in particular they suggest that the global circuits are incorrect for motion prior to 48 ma between east and west antarctica in this view southward motion of the hawaiian hotspot is evidence for motion of the entire lower mantle relative to the spin axis wright et al defend conventional wisdom using freshly calculated relative motion solutions for plates in the pacific basin they conclude that the hawaiian emperor bend heb formed at ca 50 42 ma and sharpest at 47 5 ma is a result of the cessation of southward motion of the hawaiian hotspot and is not a result of change of absolute motion of the pacific plate the purpose of this comment is to point out that their results are at least as consistent with a large change in motion of the pacific plate that was responsible for a significant part of the heb wright et al calculate pacific farallon finite half rotations from the pacific plate record at more frequent intervals than previous workers they recognize an acceleration from 75 mm yr full rate prior to 60 ma to 180 mm yr after 40 ma according to their preferred cande and kent 1995 time scale this acceleration is spread out over 57 40 ma but according to the ogg 2012 time scale the acceleration is contained within the interval of 50 43 ma a time interval entirely within the age span of the heb their statement that their results are not strongly dependent on the time scale used is hard to understand in the discussion associated with wright et al s figure 3 they suggest that this acceleration is dominated by eastward acceleration of the farallon plate in an absolute reference frame any conclusion that pacific absolute motion didn t change much is circular because they assume that one of three published models for pacific plate absolute motion is correct and for all of these published models changes in pacific plate absolute motion are small this discussion does not address the possibility that all three of these models for absolute reference frames share common errors in the global plate circuit wright et al briefly mention their analysis of pacific antarctic motion presented in their figure 2 in terms of rate and direction of motion at the pitman fracture zone in the far south pacific in discussion of northern hemisphere events possibly related to pacific farallon acceleration they suggest that westward acceleration of the pacific plate would correspond to increased pacific antarctic spreading rates because they observe a rate decrease at the pitman fracture zone they suggest that change in pacific absolute motion was relatively minor here they make a serious error in failing to consider spherical geometry the pitman transform fault active at 48 ma was 90 100 of arc away from the molokai and murray fracture zones and the heb their discussion frames the most relevant issue as how the motion of the pacific plate changes at the location of the hawaiian hotpsot the velocity of the pacific plate relative to antarctica when calculated at relevant northern hemisphere locations does increase sharply near 48 ma calculated at the location of hawaii the velocity of the pacific plate changes from slow 35 mm yr northwest motion to fast 83 mm yr west northwest motion

large scale structure of the eclogite blueschist belt of new caledonia abstract this paper considers the large scale geometry of the northern part of the eclogite blueschist belt of new caledonia based largely on classical style structural analysis but in consideration of modern concepts regarding crustal extension exhumation of high pressure rocks and uplift shear zones played an important role in the evolution of the region early shear zones and high pressure metamorphism appear to be associated with overthrusting of an ultramafic sheet middle stage shear zones are associated with large scale continental extension during which the high pressure rocks were exhumed the extended crust was subsequently folded during renewed compression producing an orogen scale antiform throughout the high pressure belt that folds all previously formed structures late stage shear zones formed when the orogen was once again thrown into extension relatively youthful normal faults caused late block faulting uplifting a regional peneplain and producing the present geomorphology previously published interpretations of the structure and geometry of the belt have suggested that the high pressure rocks are found in the core of a regionally developed antiform interpreted as a metamorphic core complex our structural mapping shows that allochthonous slices of high pressure rocks are draped over a younger unrelated foliation antiform lower grade retrogressed equivalent rocks are found within the core of the antiform thus the metamorphic core complex model is rejected

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

pacific absolute plate motion since 145 ma an assessment of the fixed hot spot hypothesis 1 using the geometry and ages from 12 pacific seamount chains we have determined two new absolute plate motion models that now extend our self consistent and high resolution models with covariance estimates back to 145 ma the wk08 a model maps the full uncertainty in the age progressions into uncertainties in rotation opening angles yielding a relatively smooth plate motion model the wk08 g model relaxes the mapping of age uncertainties in order to better isolate secondary geometry changes seen along many coregistered chains both models have been used to assess the viability of the fixed hot spot hypothesis in the pacific in determining the models we found that only a small group of age samples had to be discarded on the grounds that they were discordant with the dominant trends we were able to connect plate motions for pre and post emperor age intervals by including the ratak gilbert ellice and musicians trails in our analysis however as no active hot spot locations exist for the older chains their inclusion adds additional model parameters both age and geometry misfits increase with age reflecting the observed increase in age uncertainties and the general widening of trails secondary and short lived changes in absolute plate motion mapped in wk08 g appear to correlate with the timing and sense of motion of known pacific rim tectonic events analysis of interchain distances between coeval samples from the hawaii and louisville chains suggests possible discrepancies during the older emperor stage that are compatible with predictions of hot spot drift we computed a new apparent polar wander path for the pacific and found a high degree of correspondence with paleomagnetically derived paths as long as solutions allowing for anomalous skewness were included in the latter our polar wander path suggests that there might have been some true polar wander during the emperor stage complemented by a smaller amount of hot spot drift than otherwise required we show that chain geometries and ages combined with future paleolatitude determinations from additional sites and chains could enable an observation based description of both hot spot and plate motions without relying on predictions of hot spot drift derived from mantle flow calculations

continental and marine paleoenvironments reflected by west pacific clay sedimentation various stratigraphic and paleoenvironmental results arise from mineralogical geochemical and micromorpho logical investigations on late cretaceous and cenozoic sediments recovered by the deep sea drilling project in the west pacific stratigraphic correlations the deep pelagic red clays deposited on either side of the shatsky rise contain a variety of clay minerals which point to the importance of eolian detrital supply and allow rough stratigraphic correlations climate and paleocirculation neogene sediments of the lord howe rise area reflect closely the relationship between the evolution of the antarctic ice sheet and related aridification stages and the atmospheric and oceanic circulation pattern since 14 my regional and global tectonics the post eocene deposits around new zealand express a sequence of tectonic events and of relaxation stages associated with changes in the circulation regime along the inner wall of the japan trench clay mineral successions document the existence of an outer continental landmass until the late oligocene its collapse associated with volcanism close to the paleogene neogene boundary and an increased detrital influence from japan since the early middle miocene volcanism diagenesis and detrital supply the aptian to maastrichtian sediments in the mariana basin document a complex sequence of geochemical and geodynamical events proximal volcanism and local supply from subaerial archipelagoes early marine diagenesis during lithospheric subsidence and plate migration and finally the occurrence and evolution of terrigenous input from distant sources distributed by marine currents

plate kinematic evidence for the existence of a distinct plate between the nubian and somalian plates along the southwest indian ridge 1 previously we estimated the angular velocity of the nubian plate relative to the somalian plate from an updated set of spreading rates and transform fault azimuths we found that the nubia somalia plate boundary intersects the southwest indian ridge swir between 26 e and 32 e if both the nubian and somalian plates are rigid and if the boundary between them is narrow these prior results are not completely satisfactory mainly for two reasons 1 the four plate circuit somalia antarctica nubia arabia does not close 2 the largest mw 6 8 recorded african earthquakes that are near but not along the swir occur near 48 e well east of the intersection with the nubia somalia boundary here we investigate these problems through detailed analysis of plate motion data especially those along the swir east of the andrew bain transform fault complex we find an improved fit to the data and improved plate circuit closure if a region of the african lithosphere is interpreted as a new component plate this new plate lies between the nubian and somalian plates along the swir and is separated from the latter by a diffuse boundary that includes the locations of the largest off ridge earthquakes following c j h hartnady we call this new plate lwandle use of this new plate geometry shifts the nubia somalia pole of rotation northeastward to just south of south africa and thus alters estimates of current india eurasia plate motion

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

limited latitudinal mantle plume motion for the louisville hotspot hotspots that form above upwelling plumes of hot material from the deep mantle typically leave narrow trails of volcanic seamounts as a tectonic plate moves over their location these seamount trails are excellent recorders of earth s deep processes and allow us to untangle ancient mantle plume motions during ascent it is likely that mantle plumes are pushed away from their vertical upwelling trajectories by mantle convection forces it has been proposed that a large scale lateral displacement termed the mantle wind existed in the pacific between about 80 and 50 million years ago and shifted the hawaiian mantle plume southwards by about 15 of latitude here we use 40ar 39ar age dating and palaeomagnetic inclination data from four seamounts associated with the louisville hotspot in the south pacific ocean to show that this hotspot has been relatively stable in terms of its location specifically the louisville hotspot the southern hemisphere counterpart of hawai i has remained within 3 5 of its present day latitude of about 51 s between 70 and 50 million years ago although we cannot exclude a more significant southward motion before that time we suggest that the louisville and hawaiian hotspots are moving independently and not as part of a large scale mantle wind in the pacific

a maastrichtian palaeomagnetic pole for the pacific plate from a skewness analysis of marine magnetic anomaly 32 summary the asymmetry skewness of marine magnetic anomaly 32 72 1 73 3 ma on the pacific plate has been analysed in order to estimate a new palaeomagnetic pole apparent evective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire pacific plate and spanning a great circle distance of 12 000 km the data were inverted to obtain a palaeomagnetic pole at 72 1 n 26 8 e with a 95 per cent confidence ellipse having a 4 0 major semi axis oriented 98 clockwise of north and a 1 8 minor semi axis the anomalous skewness is 14 2 3 7 the possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible evect on our palaeopole analysis over the range of relevant spreading half rates 25 to 90 mm yr 1 the new pole is consistent with the northward motion for the pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data but divers significantly from and lies to the northeast of coeval seamount poles we attribute the diverence to unmodelled errors in the seamount poles mainly in the declinations comparison with the northward motion inferred from dated volcanoes along the hawaiian emperor seamount chain indicates 13 of southward motion of the hawaiian hotspot since 73 ma when the pole is reconstructed with the pacific plate relative to the pacific hotspots it divers by 14 18 from the position of the pole relative to the indo atlantic hotspots this has several possible explanations including bias in one or more of the palaeomagnetic poles motion between the pacific and indo atlantic hotspots and errors in plate reconstructions relative to the hotspots

radiometric ages for basement rocks from the emperor seamounts odp leg 197 1 the hawaiian emperor seamount chain is the type example of an age progressive hot spot generated intraplate volcanic lineament however our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago improvements in instrumentation sample preparation methods and new material obtained by recent drilling warrant a reexamination of the age relations among the older hawaiian volcanoes we report new age determinations 40ar 39ar incremental heating method on whole rocks and feldspar separates from detroit sites 1203 and 1204 nintoku site 1205 and koko site 1206 seamounts ocean drilling program odp leg 197 and meiji seamount deep sea drilling project dsdp leg 19 site 192 plateaus in incremental heating age spectra for site 1203 lava flows give a mean age of 75 8 0 6 2 ma which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments site 1204 lavas produced discordant spectra indicating ar loss by reheating and k mobilization six plateau ages from lava flows at site 1205 give a mean age of 55 6 0 2 ma corresponding to chron 24r drilling at site 1206 intersected a n r n magnetic polarity sequence of lava flows from which six plateau ages give a mean age of 49 1 0 2 ma corresponding to the chron 21n 22r 22n sequence plateau ages from two feldspar separates and one lava from dsdp site 192 range from 34 to 41 ma significantly younger than the cretaceous age of overlying sediments which we relate to postcrystallization k mobilization combined with new dating results from suiko seamount dsdp site 433 and volcanoes near the prominent bend in the lineament sharp and clague 2002 the overall trend is increasing volcano age from south to north along the emperor seamounts consistent with the hot spot model however there appear to be important departures from the earlier modeled simple linear age progression which we relate to changes in pacific plate motion and the rate of southward motion of the hawaiian hot spot

petrology and k ar ages of dredged samples from laysan island and northampton bank volcanoes hawaiian ridge and evolution of the hawaiian emperor chain summary the hawaiian emperor volcanic chain is made up of at least 107 indentifiable shield volcanoes chase and others 1970 bargar and jackson 1974 clague and others 1980 and is the longest linear island and seamount chain in the pacific fig 1 beginning at the active volcanoes kilauea and mauna loa on the island of hawaii this chain of volcanoes extends west northwestward along the hawaiian ridge for a distance of 3 500 km where it turns northward and continues another 2 300 km as the emperor seamounts ending near the intersection of the aleutian and kurile trenches it is now thought that the hawaiian emperor chain was formed as the pacific lithospheric plate moved first northward and then northwestward relative to the hawaiian hot spot a more or less stable melting anomaly in the asthenosphere and that the bend in the chain reflects a major change in pacific plate motion several mechanisms have been proposed to explain the hawaiian and other hot spots including a propagating fracture betz and hess 1942 jackson and wright 1970 diapiric upwelling along a line of structural weakness green 1971 mcdougall 1971 thermal morgan 1972a 1972b or chemical anderson 1975 plumes of gravitationally unstable material rising from the deep mantle shear melting coupled with thermal feedback shaw 1973 and geographically stabilized by gravitational sinking of refractory residua shaw and jackson 1973 and extrusion induced by lateral motion along plate boundaries handschumacher 1973 despite the variety of dynamic models proposed there has yet to be devised a definitive experiment to test any of them all these mechanisms are however encompassed by the general or kinematic hot spot hypothesis which when used in the broadest sense implies neither mechanism nor excess heat

a high resolution model for eurasia north america plate kinematics since 20 ma summary we derive the first chronologically detailed model of eurasia north america plate motion since 20 ma from ship and airplane surveys of the well expressed magnetic lineations along this slowly spreading plate boundary including previously unavailable dense russian magnetic data from the southern reykjanes ridge and northern mid atlantic ridge near the charlie gibbs fracture zone from more than 7000 crossings of 21 magnetic anomalies from anomaly 1n 0 78 ma to anomaly 6n 19 7 ma we estimate best fitting finite rotations and realistic uncertainties linear regressions of total opening distances versus their reversal ages at different locations along the plate boundary show that reversal boundaries are shifted systematically outwards from the spreading axis with respect to their idealized locations with the outward shift ranging from more than 5 km between iceland and the charlie gibbs fracture zone to 2 km elsewhere this outward displacement which is a consequence of the finite zone of seafloor accretion degrades estimates of the underlying plate motion and is thus removed for the ensuing kinematic analysis the corrected plate motion rotations reveal surprising previously unrecognized features in the relative motions of these two plates within the uncertainties motion was steady from 20 to 8 ma around a pole that was located 600 km north of the present pole with seafloor spreading rates that changed by no more than 5 per cent 1 mm yr 1 along the reykjanes ridge during this period seafloor spreading rates decreased abruptly by 20 2 per cent at 7 5 6 5 ma coinciding with rapid southward migration of the pole of rotation and a5 10 counter clockwise change in the plate slip direction eurasia north america plate motion since 6 7 ma has remained remarkably steady with an apparently stationary axis of rotation and upper limit of 2 per cent on any variations in the rate of angular rotation during this period based on the good agreement between seismotectonic constraints on present deformation in northeast asia and directions of motion that are predicted by our 6 7 ma to present pole we hypothesize that motion has remained steady to the present and attempt to test this hypothesis with published gps estimates for eurasia north america motion we find however that gps estimates that are tied to recent versions of the international geodetic reference frame and rely principally on station velocities from europe give implausible estimates of recent motion with the most recently published gps model predicting convergence along the southern gakkel ridge and in the laptev sea where seafloor spreading occurs an alternative gps estimate that is not tied to the international terrestrial reference frame and employs gps station velocities from northeastern asia is marginally consistent with our 6 7 0 ma motion estimate

motion between the indian capricorn and somalian plates since 20 ma implications for the timing and magnitude of distributed lithospheric deformation in the equatorial indian ocean summary approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the carlsberg ridge and central indian ridge are used to estimate the rotations of the indian and capricorn plates relative to the somalian plate for 20 distinct points in time since 20 ma the data are further used to place limits on the locations of the northern edge of the rigid capricorn plate and of the southern edge of the rigid indian plate along the central indian ridge data south of and including fracture zone n the fracture zone immediately south of the vema fracture zone which intersects the central indian ridge near 10 s are well fit assuming rigid capricorn and somalian plates while data north of fracture zone n are not in agreement with prior results data north of fracture zone h which intersects the central indian ridge near 3 2 s are well fit assuming rigid indian and somalian plates while data south of and including fracture zone h are not resulting in a smaller rigid indian plate and a wider diffuse oceanic plate boundary than found before the data are consistent with capricorn somalia motion about a fixed pole since 8 ma but require rotation about a pole 15 farther away from the central indian ridge from 20 to 8 ma the post 8 ma pole also indicates capricorn somalia displacement directions that are 7 clockwise of those indicated by the pre 8 ma stage pole in contrast india somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 ma india somalia motion has changed little during the past 20 myr nonetheless astronomically calibrated ages for reversals younger than 12 9 ma allow resolution of the following small but significant changes in spreading rate india somalia spreading slowed from 31 to 28 mm yr 1 near 7 9 ma and later sped up to 31 mm yr 1 near 3 6 ma capricorn somalia spreading slowed from 40 to 36 mm yr 1 near 11 0 ma later sped up to 38 mm yr 1 near 5 1 ma and further sped up to 40 mm yr 1 near 2 6 ma the motion between the indian and capricorn plates is estimated by differencing india somalia and capricorn somalia rotations which differ significantly for all 20 pairs of reconstructions india has rotated relative to the capricorn plate since at least 20 ma if about a pole located near 4 s 75 e the rate of rotation was slow 0 11 0 01 myr 1 95 per cent confidence limits from 20 to 8 ma but increased to 0 28 0 01 myr 1 95 per cent confidence limits at 8 ma the onset of more rapid rotation coincides within uncertainty with the inferred onset at 7 8 ma of widespread thrust faulting in the central indian basin and with the hypothesized attainment of maximum elevation and initiation of collapse of the tibetan plateau at 8 ma the plate kinematic data are consistent with steady india capricorn motion since 8 ma and provide no evidence for previously hypothesized episodic motions during that interval the convergence since 8 ma between the indian and capricorn plates significantly exceeds by 13 to 20 km the convergence estimated from three north south marine seismic profiles in the central indian basin where and how the additional convergence was accommodated is unclear

paleolatitude of the hawaiian hot spot since 48 ma evidence for a mid cenozoic true polar stillstand followed by late cenozoic true polar wander coincident with northern hemisphere glaciation

cenozoic latitudinal shift of the hawaiian hotspot and its implications for true polar wander abstract pacific plate equatorial sediment facies provide estimates of the northward motion of the pacific plate that are independent of paleomagnetic data and hotspot tracks analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the hawaiian emperor chain the discrepancy is largest 60 70 ma b p the 60 to 70 ma equatorial sediment facies data agree with recent paleomagnetic results from deep sea drilling on suiko seamount 1 and from a northern pacific piston core 2 equatorial sediment facies data and paleomagnetic data combined with k ar age dates along the emperor chain 3 indicate a position of the spin axis at 65 ma b p of 82 n 205 e in the reference frame in which the pacific ocean hotspots are fixed this pole agrees well with the position of the spin axis in the reference frame in which the atlantic ocean hotspots and the indian ocean hotspots are fixed 4 5 supporting the joint hypotheses that 1 the pacific ocean hotspots are fixed with respect to the hotspots in other oceans 2 the hotspots have shifted coherently with respect to the spin axis and 3 the time average of the earth s magnetic field 65 ma b p was an axial geocentric dipole global neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the neogene in the same direction as the shift between 65 ma b p and the present all data are consistent with a model in which the hotspots and by inference the mantle have shifted with respect to the spin axis about a fixed euler pole at a constant rate of rotation for the last 65 ma

implications of a nonlinear 40ar 39ar age progression along the louisville seamount trail for models of fixed and moving hot spots 1 the louisville seamount trail has been recognized as one of the key examples of hot spot volcanism comparable to the classic volcanic hawaiian emperor lineaments the published total fusion 40ar 39ar data of watts et al 1988 showed an astonishing linear age progression firmly establishing louisville as a fixed hot spot in the south pacific mantle we report new 40ar 39ar ages based on high resolution incremental heating 40ar 39ar dating for the same group of samples showing a marked increase in both precision and accuracy one of the key findings in our reexamination is that the age progression is not linear after all the new data show a significantly decreased apparent plate velocity for the louisville seamount trail older than 62 ma but confirm the linear trend between 47 ma and the present day although based on only three samples over 2150 km the most recent volcanic activity in the louisville seamount trail has now been dated at 1 11 0 04 ma for the most southeastern seamount located at 50 26 s and 139 09 w these results indicate that the louisville age progression should be interpreted on the basis of both plate and hot spot motion in this paper we examine our new results in conjunction with the numerical mantle flow models of steinberger et al 2004 that also predict marked deviations from simple linear age progressions with these models we can achieve a good fit to the geometry of both the hawaiian and louisville seamount trails and their age progressions as well as the 15 paleolatitudinal shift observed by tarduno et al 2003 for the hawaiian hot spot between 80 and 47 ma if the model is restricted to pacific hot spots only we can improve the fit to the nonlinear age trend for the louisville seamount trail by allowing an additional rotation change of the pacific plate around 62 ma and by decreasing the initiation age of the louisville plume from 120 to 90 ma this improved model features a significant eastward hot spot motion of 5 between 80 and 30 ma for the louisville hot spot which is quite dissimilar to the southward motion of the hawaiian hot spot during the same time interval followed by a minor 2 latitudinal shift over the last 30 myr if hot spot tracks are considered globally the age trend observed for the oldest part of the louisville seamount trail does not entirely follow the numerical model predictions this may indicate some remaining inaccuracies in the global plate circuit but it may also indicate that the louisville hot spot experienced a motion somewhat different than in the numerical model faster in the interval between 62 and 47 ma but slower before that

pacific plate reconstructions and uncertainties abstract the pacific plate is positioned into the hotspot reference frame first directly relative to the pacific hotspots and then indirectly relative to the african hotspots by using seafloor spreading data the reconstructions are repeated for 10 15 37 48 and 61 ma the errors associated with each link for both approaches are expressed by covariance matrices which can be combined to yield the overall error in the case of the relative plate motion circuit assuming the hotspots are fixed and that the reconstructions record the true relative positions of the plates the reconstructions should be equivalent within their uncertainties in fact however there is agreement within the uncertainties only for 10 ma with the misfit increasing with age for the earlier reconstructions these misfits can he accounted for by three alternative explanations large hotspot location uncertainties a shift in time of the pacific group of hotspots relative to the african group or an additional plate boundary in the relative plate motion circuit which we assume to be in antarctica by examining each of these possibilities in turn we conclude that the uncertainties in individual hotspot locations required to bring the reconstructions into agreement are unreasonably large and that the data available at present do not allow us to distinguish which of the remaining two possibilities may be the primary source of reconstruction errors

pacific plate apparent polar wander hot spot fixity and true polar wander during the formation of the hawaiian island and seamount chain from an analysis of the skewness of magnetic anomaly 20r 44 ma

ridge spotting a new test for pacific absolute plate motion models relative plate motions provide high resolution descriptions of motions of plates relative to other plates yet geodynamically motions of plates relative to the mantle are required since such motions can be attributed to forces e g slab pull and ridge push acting upon the plates various reference frames have been proposed such as the hot spot reference frame to link plate motions to a mantle framework unfortunately both accuracy and precision of absolute plate motion models lag behind those of relative plate motion models consequently it is paramount to use relative plate motions in improving our understanding of absolute plate motions a new technique called ridge spotting combines absolute and relative plate motions and examines the viability of proposed absolute plate motion models we test the method on six published pacific absolute plate motions models including fixed and moving hot spot models as well as a geodynamically derived model ridge spotting reconstructs the pacific farallon and pacific antarctica ridge systems over the last 80 myr all six absolute plate motion models predict large amounts of northward migration and monotonic clockwise rotation for the pacific farallon ridge a geodynamic implication of our ridge migration predictions is that the suggestion that the pacific farallon ridge may have been pinned by a large mantle upwelling is not supported unexpected or erratic ridge behaviors may be tied to limitations in the models themselves or for indo atlantic models discrepancies in the plate circuits used to project models into the pacific realm ridge spotting is promising and will be extended to include more plates and other ocean basins

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

revision of paleogene plate motions in the pacific and implications for the hawaiian emperor bend reply understanding the relative motion between the pacific plate and its neighboring plates in the paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the pacific ocean basin the history of circum pacific subduction and the cause of the hawaiian emperor bend heb we quantitatively model the farallon vancouver pacific antarctic seafloor spreading history from 67 to 33 ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications we find a well constrained increase from 75 5 mm yr to 101 5 mm yr in pacific farallon full spreading rates between 57 6 ma and 55 9 ma followed by a stepwise increase to 182 2 mm yr from 49 7 to 40 1 ma the increases in pacific farallon spreading rates are not accompanied by any statistically significant change in spreading direction the 57 6 55 9 ma surge of pacific farallon spreading reflects an eastward acceleration in farallon plate motion as it precedes west pacific subduction initiation and is not associated with any significant change in pacific antarctic spreading we interpret the increase in pacific farallon spreading rates after ca 50 ma as a consequence of further acceleration in farallon plate motion we find no indication of a major change in pacific plate absolute motion at this time our model suggests that changes in relative motion direction between the pacific and farallon and pacific and antarctic plates were insignificant around the formation time of the heb ca 47 5 ma and the bend is largely a consequence of hawaiian hotspot motion which ceased rapid motion after 47 ma

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

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated

oblique seafloor spreading across intermediate and superfast spreading centers abstract we show that oblique seafloor spreading occurs in several regions where obliquity was not recognized before these include the slow spreading centers of the red sea 20 intermediate spreading centers of the cocos nazca plate boundary between 91 w and 94 w 9 and superfast spreading centers of the east pacific rise at the nazca pacific plate boundary between 29 s and 32 s 10 and perhaps between 16 s and 22 s 4 thus oblique spreading occurs across slow intermediate and superfast spreading centers but not across fast spreading centers across intermediate and superfast spreading centers oblique spreading does not occur in staircase mode plate geometries but does occur in oblique mode plate geometries across slow and intermediate spreading centers obliquity tends to decrease with increasing spreading rate while across fast and superfast spreading centers it tends to increase with increasing spreading rate oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these using prior methods we estimate the ratio of overpressure to remote tectonic stress for several spreading centers in particular we show along a segment of the cocos pacific plate boundary that magma overpressure is only one fourth as large as remote tectonic stress consistent with a prior inference from other observations the highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume but not all ridge segments near plumes spread obliquely for one set of estimates of plume fluxes the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate

oxygen isotope study of cayenne duricrust paleosurfaces implications for past climate and laterization processes over french guiana abstract oxygen isotopic compositions of weathering goethite hematite kaolinite and gibbsite in duricrust paleosurfaces of the cayenne region northeastern french guiana were investigated samples were microdrilled from different horizons and different pedogenetic microfacies of three well characterized duricrust units of eocene unit 1 and miocene units 2 and 3 age the results provided valuable information on duricrust formation processes and on climate change over french guiana the pristine original isotopic signal suited for reconstruction of past climates appears to be best preserved in the massive duricrust facies middle part of the duricrust in the overlying pisolotic duricrust and in the underlying mottled zone the isotopic signal is affected by subsequent re activation of weathering processes oxygen isotope data from massive duricrust indicate that cayenne duricrusts formed from meteoric waters increasingly depleted in 18o with age the youngest duricrust paleosurface unit 3 formed under humid climate similar to present day while the two older paleosurfaces formed from meteoric paleo water distinctly depleted in 18o reflecting a moonsonal contrasted ancient climate for unit 2 added to the 18o depleted nature of the pre miocene ocean for unit 1 the results further suggest that the major climatic change from tropical wet and dry to present day humid regime recorded in amazonia most likely occurred in the late miocene and that miocene laterization events may be responsible for the secondary bauxitization of the eocene duricrust paleosurface the oxygen isotopic composition of weathering minerals occurring in massive duricrust horizons and in particular in hematite nodules and matrix may be used as a tool to discriminate among the different duricrust paleosurfaces encountered in amazonia and constrain the evolution of past climates

from slow to ultraslow a previously undetected event at the southwest indian ridge at ca 24 ma changes in plate motion are thought to be recorded in the trend of fracture zones even though fracture zones provide no information about the spreading rate using newly compiled published and unpublished magnetic data from the southwest indian ridge we calculated fi nite rotation poles for a13 a8 and a6 from which we determined a 50 decrease in spreading rate from slow to ultraslow at ca 24 ma not accompanied by a signifi cant change in spreading direction this spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries finally we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca 24 ma at the global scale

constraining absolute plate motions since the triassic the absolute motion of tectonic plates since pangea can be derived from observations of hotspot trails paleomagnetism or seismic tomography however fitting observations is typically carried out in isolation without consideration for the fit to unused data or whether the resulting plate motions are geodynamically plausible through the joint evaluation of global hotspot track observations for times 80 ma first order estimates of net lithospheric rotation nlr and parameter estimation for paleo trench migration tm we present a suite of geodynamically consistent data optimized global absolute reference frames from 220 ma to the present each absolute plate motion apm model was evaluated against six published apm models together incorporating the full range of primary data constraints model performance for published and new models was quantified through a standard statistical analyses using three key diagnostic global metrics root mean square plate velocities nlr characteristics and tm behavior additionally models were assessed for consistency with published global paleomagnetic data and for ages 80 ma for predicted relative hotspot motion track geometry and time dependence optimized apm models demonstrated significantly improved global fit with geological and geophysical observations while performing consistently with geodynamic constraints critically apm models derived by limiting average rates of nlr to 0 05 myr and absolute tm velocities to 27 mm year fit geological observations including hotspot tracks this suggests that this range of nlr and tm estimates may be appropriate for earth over the last 220 myr providing a key step toward the practical integration of numerical geodynamics into plate tectonic reconstructions

40ar39ar geochronology of tertiary mafic intrusions along the east greenland rifted margin relation to flood basalts and the iceland hotspot track abstract the east greeland tertiary igneous province includes the largest exposed continental flood basalt sequence within the north atlantic borderlands more than ten layered gabbro complexes including the 55 ma skaergaard intrusion and a large dolerite sill complex are the plutonic equivalents of flood basalts both lavas and intrusions have been regarded as synchronous with continental breakup at 57 54 ma we report ten new ages of the mafic intrusions determined by40ar 39ar incremental heating experiments demonstrating that the mafic intrusions formed in two distinct time windows only intrusion ii of the imilik gabbro complex the skaergaard intrusion and the sorgenfri glestcher sill complex formed at 57 55 ma coeval with the eruption of regional flood basalts and continental breakup other layered gabbro intrusions at imilik intrusion iii kruuse fjord igtutarajik nordre aputite q kap edvard holm and lilloise are distinctly younger and formed between 50 and 47 ma plate kinematic models indicate the axis of the ancestral iceland mantle plume was located under central greenland at 60 ma and subsequently crossed the east greenland rifted continental margin we propose that tholeiitic magmatism along the east greenland rifted margin largely occurred in three distinct pulses at 62 59 ma lavas and dykes 57 54 ma lavas dykes sills and some gabbros and 50 47 ma gabbros dykes and rare lavas related to discrete mantle melting episodes triggered by plume impact continental breakup and passage of the plume axis respectively this model implies northwestward continental drift of greenland relative to the plume axis by 3 9 5 0 cm yr between 60 and 49 ma consistent with estimates from seismic studies of submerged flood basalts

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

semi analytical solution of groundwater flow in a leaky aquifer system subject to bending effect summary the bending of aquitard like a plate due to aquifer pumping and compression is often encountered in many practical problems of subsurface flow this reaction will have large influence on the release of the volume of water from the aquifer which is essential for the planning and management of groundwater resources in aquifers however the groundwater flow induced by pumping in a leaky aquifer system is often assumed that the total stress of aquifer maintains constant all the time and the mechanical behavior of the aquitard formation is negligible therefore this paper devotes to the investigation of the effect of aquitard bending on the drawdown distribution in a leaky aquifer system which is obviously of interest in groundwater hydrology based on the work of wang et al 2004 this study develops a mathematical model for investigating the impacts of aquitard bending and leakage rate on the drawdown of the confined aquifer due to a constant rate pumping in the leaky aquifer system this model contains three equations two flow equations delineate the transient drawdown distributions in the aquitard and the confined aquifer while the other describes the vertical displacement in response to the aquitard bending for the case of no aquitard bending this new solution can reduce to the hantush laplace domain solution hantush 1960 on the other hand this solution without the leakage effect can reduce to the time domain solution of wang et al 2004 the results show that the aquifer drawdown is influenced by the bending effect at early time and by the leakage effect at late time the results of sensitivity analysis indicate that the aquifer compaction is sensitive only at early time causing less amount of water released from the pumped aquifer than that predicted by the traditional groundwater theory the dimensionless drawdown is rather sensitive to aquitard s hydraulic conductivity at late time additionally both the hydraulic conductivity and thickness of the aquifer are the most sensitive parameters in influencing the predicted dimensionless drawdown

the use of duricrusts and topographic relationships in geomorphological correlation conclusions based in australian experience though there are many possible pitfalls duricrusted surfaces provide morphostratigraphic markers which are useful in preliminary extrapolation and correlation at the local and regional scales especially if other factors such as landscape evolution and topographic relationships are also considered particular reference is made to ferruginous and siliceous cappings close stratigraphic dating of the duricrusts allows regional differentiation but correlations are preferably intra rather than inter regional and are at best general and involve age ranges rather than narrow time frames problems broached include type of landscape development the relation of surface and duricrusts and the temporal equivalence of duricrust facies i e duricrusts of similar chemical tendencies but contrasted morphology the roles of duricrusts in relief inversion surface preservation and climatic interpretation are also mentioned

the miocene palaeoenvironmental and palaeoceanographic evolution of the gippsland basin southeast australia a record of southern ocean change abstract the gippsland basin in southeastern australia preserves up to 2 5 km thick tertiary to recent marine carbonate sediments included in the seaspray group during the neogene the seaspray group carbonates of the mackerel and flounder petroleum fields lay at the shelfal to bathyal palaeoenvironmental transition an ideal palaeoceanographic setting to study the neogene history of events in the evolving southern ocean seismic foraminiferal and facies analyses of the miocene seaspray group sediments at the mackerel 1 and flounder 1 wells have revealed the following palaeoenvironmental history of the area relatively cool to temperate upwelling conditions prevailed in the area during the earliest miocene during this time low energy upper slope globigerina ooze calcareous mudstone was deposited in the oxygen minimum zone during the early to lower middle miocene upper slope marl was deposited in oligotrophic conditions the presence of neritic subtropical foraminifera and abundant shallow and deep dwelling warm to cool temperate plankton in the upper slope suggests that warm poorly stratified oceanic conditions prevailed in the region this upper early miocene warm phase and mid miocene climatic optimum in gippsland corresponds to a time when global ocean conditions were much warmer and less vertically and latitudinally stratified than at present these oceanic conditions were associated with a reduced east antarctic ice sheet a higher sea level a relatively weak antarctic circumpolar current and weak atmospheric and wind driven circulation during the upper middle miocene to late miocene cool phase after c 14 5 ma strong upwelling and cooler oceanic conditions prevailed in the outer shelf to upper slope canyon facies in gippsland the occurrence of common deep dwelling plankton taxa in this interval suggests that regional oceanic conditions became more vertically stratified during this time from 14 5 and 5 million years ago the middle to late miocene palaeoenvironmental change recorded in seaspray group sediments corresponds to global cooling and major antarctic ice sheet expansion when the world s oceans became more vertically and latitudinally stratified from around 14 5 million years the transition from a warm miocene optimum to cooler oceanic conditions was associated with a strengthened antarctic circumpolar current and more intense wind driven atmospheric circulation in the southern ocean

a revised kinematic model for the relative motion between pacific oceanic plates and north america since the late cretaceous 1 the convergence between the pacific farallon and kula oceanic plates and the north american plate has provided a driving force for tectonic processes that have shaped the western margin of north america yet reconstructions of plate motion in the pacific basin have traditionally relied on the assumption of fixed hot spots in the last decade substantial cretaceous to paleogene motion between the pacific and atlantic hot spots has been recognized providing motivation to reevaluate the history of relative motion between pacific oceanic plates and north america by means of plate circuit reconstructions this paper presents new kinematic models based on two alternative plate circuits linking the pacific plate to north america since late cretaceous time when compared to models relying on fixity of pacific and indo atlantic hot spots our reconstructions suggest that the pacific kula and pacific farallon ridges were 600 1000 km closer to the western margin of north america and hence that the farallon and kula plates were smaller in late cretaceous to middle eocene time these findings cast significant doubt on the viability of a configuration of the farallon kula ridge that juxtaposes the kula plate with the southern part of the paleo north american margin during this time interval the results of our new reconstructions also suggest more oblique convergence between the farallon or kula plate and north america from the late cretaceous to the middle eocene related to the combination of dominantly northwest motion of the pacific plate and steady southwest motion of north america this provides an efficient mechanism for the coast parallel translation of some accreted terranes of north america it may also explain the structure of geological units in western north america such as the central belt of the franciscan complex which show a dominant coast parallel fabric formed during an interval coeval with the episode of oblique convergence

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

tests of fixity of the indo atlantic hot spots relative to pacific hot spots rates of inter hot spot motion have been debated for decades herein we present updated predictions for the tracks of the tristan da cunha reunion and iceland hot spots assuming them to be fixed relative to pacific hot spots uncertainties in pacific hot spot rotations which include uncertainties in the current locations of hot spots of 100 200 km are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions including uncertainties of 150 200 km in the current locations of the indo atlantic hot spots improvements to reconstruction methods to relative plate reconstructions to age dates along the tracks and to the geomagnetic reversal timescale lead to significant changes from prior results when compared with the observed tracks the predicted tracks indicate nominal rates of motion of only 2 6 mm a 1 of these indo atlantic hot spots relative to pacific hot spots over the past 48 ma within the uncertainties the rates range from no motion to rates as high as 8 13 mm a 1 for reconstructions prior to 48 ma b p however the apparent rates of inter hot spot motion are much larger 46 55 20 mm a 1 if the motion occurred entirely between 68 ma b p and 48 ma b p either hot spots moved rapidly before 48 ma b p and slowed drastically at 48 ma b p or global plate circuits through antarctica become less reliable as one goes increasingly further into the past most paleomagnetic data favor the latter explanation

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

geologic and palaeomagnetic constraints on the formation of weathered profiles near inverell eastern australia abstract we use geologic and palaeomagnetic observations to constrain the timing and duration of weathering of the oakwood bauxite which outcrops in the new england region of new south wales australia the bauxite is developed on and overlain by mafic volcanic flows prior geochronologic results from surrounding regions indicate that age of these flows and thus the age of the bauxite is most likely between 18 and 26 ma though volcanics elsewhere in the central volcanic province have a much greater range of between 6 and 56 ma similarly comparison of a new palaeomagnetic pole computed from the bauxite pole latitude 75 4 n longitude 311 3 e n 58 a95 4 4 with the australian apparent polar wander path gives estimated ages of 21 27 ma individual samples of the bauxite have both normal and reversed polarity components of magnetization which beside providing a positive palaeomagnetic reversal test suggests that the duration of weathering exceeded 10 000 years and more likely lasted several million years our results suggest that the oakwood region had a palaeoclimate and palaeoenvironment that was conducive for deep weathering over a significant period in a the time interval from the mid oligocene to the early miocene

on the increasing size of the orogens moving from the alps to the himalayas in the frame of the net rotation of the lithosphere abstract the tectonic equator represents the great circle of the non random mainstream of plate motions and it is inclined about 30 relative to the geographic equator divergence or convergence rates among plates are in average faster along the tectonic equator and they tend to decrease toward the polar areas moving from western europe to eastern asia the mainstream is roughly oriented southwest northeast here we show how this pattern may have played a role in determining the dimension of the alpine himalayas orogenic belt which is increasing in size and thickness moving from west northwest to east southeast i e moving from high latitude to low latitude of the tectonic mainstream of plates the alps are in average 200 250 km wide whereas the himalayas are regularly 1000 km wide moreover due to the westerly polarization of the lithospheric mainstream relative to the mantle either the net rotation or the westward drift of the lithosphere the subduction zones can be differentiated into two types 1 increasing or 2 decreasing the lithospheric thickness the alpine himalayas system pertains to type 1 and it may represent a prototype of the continental lithosphere growth since the archean the increasing size of the orogens moving from the alps to the himalayas is presently concentrated in the northern hemisphere of the tectonic mainstream because subduction type 2 dominated the western margin of the pacific ocean hence preventing continental growth in the southern hemisphere in that longitude range therefore the largest growth of continental crust and mantle lithosphere should have occurred along the tectonic equator but only where type 1 subduction was generated