shear wave splitting in northeast venezuela trinidad and the eastern caribbean abstract we present new measurements of splitting of core shear wave phases sks skks pks recorded at seven stations in the eastern caribbean region six stations in northeastern venezuela and trinidad constituted the secasa92 temporary broadband array the seventh station sjg is installed on puerto rico we also attempted to measure source side splitting of s waves from earthquakes in the lesser antilles subducted slab but with little success mean splitting parameters fast polarization direction and delay time t at the stations are consistently east west trending and over 1 s respectively delay times at the secasa92 array are amongst the highest yet measured reaching 2 1 s and generally over 1 5 s we note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost the secasa92 array is situated in the wide south america caribbean sa ca plate boundary zone and sjg is within the north america caribbean na ca plate boundary zone in both cases trends are parallel to the plate boundary zones locally we interpret these results to indicate that the sa ca plate boundary deformation extends to depths of at least 200 km and that south american crust lithospheric mantle and asthenosphere are coherently deformed the extreme delay times we measure are probably the result of transpression in the sa ca plate boundary and overriding of the lesser antilles slab by south america compressional flattening results in extremes of foliation development and dextral shearing yields a strong lineation in upper mantle olivine by contrast the delay time at sjg 1 2 s is near the global average and reflects the fact that the na ca plate boundary zone is purely sinistral or includes minor transtension we note that recent counterclockwise rotation of puerto rico does not apparently affect upper mantle deformation beneath the island we suggest that this rotation is crustal driven by far field sinistral shear and partly decoupled from the mantle shear our results are consistent with the trench parallel mantle flow hypothesis russo and silver 1994 science 263 1105 1111 and indicate that asthenospheric mantle flow beneath the caribbean plate is eastward and probably drives the plate s eastward motion

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

mid to late cambrian docking of the rio de la plata craton to southwestern gondwana age constraints from u pb shrimp detrital zircon ages from sierras de ambato and velasco sierras pampeanas argentina abstract the early palaeozoic stratigraphy and tectonic history of the eastern sierras pampeanas of central argentina are complicated by metamorphism and deformation resulting from the pampean 545 510 ma and famatinian 490 440 ma orogenies we report u pb sensitive high resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at quebrada de la cebila c 28 459s 66 259w the ambato and the la cebila metamorphic complexes the ambato zircons record age peaks corresponding to pampean 530 10 ma brasiliano c 570 and c 640 ma grenville c 950 to c 1025 ma and minor neoarchaean ages similar peaks are also apparent in the la cebila sample but it additionally contains palaeoproterozoic zircons c 2 1 ga corresponding to the age of the rio de la plata craton from which they are considered to have been sourced our interpretation is that the protolith of the ambato complex was deposited prior to juxtaposition with the craton and is older than the early ordovician la cebila metamorphic complex we infer that the craton reached its current relative position in the mid to late cambrian after the main pampean tectonothermal event 530 520 ma and before deposition of the la cebila protolith and the achavil formation sierra de famatina which contain comparable detrital zircon populations

subduction influence on magma supply at the east scotia ridge despite a spreading rate of 65 70 km ma 1 the east scotia ridge has along most of its length a form typically associated with slower rates of sea floor spreading this may be a consequence of cooler than normal mantle upwelling which could be a feature of back arc spreading at the northern end of the ridge recently acquired sonar data show a complex rapidly evolving pattern of extension within 100 km of the south sandwich trench new ridge segments appear to be nucleating at or near the boundary between the south american and scotia sea plates and propagating southwards supplanting older segments the most prominent of these north of 56 30 s has been propagating at a rate of approximately 60 km ma 1 for at least 1 ma and displays a morphology unique on this plate boundary a 40 km long axial high exists at the centre of this segment forming one of the shallowest sections of the east scotia ridge beneath it seismic reflection profiles reveal an axial magma chamber or amc reflector similar to those observed beneath the east pacific rise and valu fa ridge simple calculations indicate the existence here of a narrow 1 km wide body of melt at a depth of approximately 3 km beneath the sea floor from the topographic and seismic data we deduce that a localised mantle melting anomaly lies beneath this segment rates of spreading in the east scotia sea show little variation along axis hence the changes in melt supply are related to the unique tectonic setting in which the south american plate is tearing to the east perhaps allowing mantle flow around the end of the subducting slab volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed a southward mantle flow may have existed beneath the axis of the east scotia ridge throughout its history

source side splitting of s waves from hindu kush pamir earthquakes abstract we present eight new measurements of source side s shear wave splitting parameters derived from 20 earthquakes in the hindu kush region for each event station pair we corrected for receiver side splitting in order to isolate the source side contribution to observed splitting the receiver correction splitting parameters were taken from published values but we confirmed these values to ensure their accuracy when possible via our own observations of sks splitting at the stations for two events we found source side splitting parameters that were identical within errors after correction for receiver splitting at two distant stations with different receiver splitting parameters fast polarization directions o vary systematically both geographically along the hindu kush and pamir subducted slabs and with depth at around 100 km depth in the transition region between the two slabs o s trend north at 200 km depth in the hindu kush slab o s trend east northeast approximately parallel to the strike of the slab one measurement at 120 km depth in the pamir slab trends within 20 of the local slab strike delay times t range from 2 3 to 3 7 s indicating strong upper mantle deformation and alignment of olivine and or long source side travel paths through the anisotropic medium we interpret our results in the context of two subducted slabs of opposite dip hindu kush slab dipping north pamir slab dipping southeast and shear and compressional flattening around the two slabs caused by the india eurasia collision thus north trending o s at 100 km depth in the transition zone between the two slabs probably represent shear alignment of upper mantle olivine along the western boundary of the indian continental lithosphere now indenting eurasia along reactivated mesozoic sutures such as the chaman fault ene trending o s from events deeper in the hindu kush slab represent flattening and mantle flow along the slab below 200 km a similar horizontal slab parallel flow or olivine alignment may occur beneath the pamir slab on the basis of our one measurement there

upper mantle seismic anisotropy beneath antarctica and the scotia sea region summary recent investigations on shear wave splitting from recordings of permanent and temporary antarctic seismological stations have lead to a greater understanding of the upper mantle dynamics of the scotia sea region and the continental margin in the eastern weddell sea in terms of their tectonic evolution the analysis of shear wave splitting from teleseismic core sks skks pks and direct s waves reveals the seismic anisotropy and the strain field of the upper mantle similar to the caribbean anisotropy structures in the antarctic peninsula and scotia sea regions are assumed to be influenced by mantle flows in easterly directions around the subducting nazca plate in general anisotropy polarization directions in the scotia sea do not contradict this hypothesis with polarizations oriented nearly e w and therefore aligning with the suggested mantle flow patterns anisotropy strengths decrease from delay times of dt 1 8 s pmsa palmer station in the west towards the east with delay times of dt 0 3 s beneath hope south georgia and cand candlemas south sandwich islands nevertheless a lithospheric and therefore fossil origin cannot be ruled out only the exceptionally high delay times at pmsa probably originate in part from recent asthenospheric flow around the subduction slab of the former phoenix plate beneath the northwestern margin of the antarctic peninsula the continental margin of western dronning maud and coats land plays a crucial role in understanding the early processes during the break up of gondwana upper mantle seismic anisotropy with delay times well over dt 1 s in this region gives new constraints on ancient deformation processes during break up and former episodes two layer modelling reveals archaean anisotropy in the upper layer corresponding well to polarization directions of the south african kaapvaal craton lower layer anisotropy is assumed to have been created during early gondwana rifting stages

a teleseismic shear wave splitting study to investigate mantle flow around south america and implications for plate driving forces summary closure of the pacific ocean basin by the convergence of its surrounding plates some of which have deep continental roots implies that there is net mass flux out of the mantle under the pacific here we report on a shear wave splitting study designed to test the prediction that there should be flow around its southern margin our results show no evidence for present day flow around the tip of southern south america instead the results suggest present day flow directions in the southern atlantic that parallel the south american absolute plate motion direction even under antarctica the results also provide evidence for absolute plate motion driven by the basal drag of ocean basin scale mantle flow and suggest that 200 km thick flow boundary layers exist under south america and antarctica and also demonstrate that mantle flow directions cannot be reliably inferred from present day plate morphology

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

sks splitting and the seismic anisotropy of the mantle beneath the hikurangi subduction zone new zealand abstract the analysis of 12 sks phases recorded on broadband stations above the hikurangi subduction zone in new zealand shows clear evidence of mantle anisotropy with the fast direction 28 5 almost parallel to the strike of subduction and the dominant geology of the region the slow shear wave delay times show a systematic change with the azimuth of the arrivals which if hexagonal symmetry is assumed indicates that either the axis of symmetry of the anisotropic volume beneath the subduction zone is not horizontal or that more than one anisotropic layer is present the magnitude of the delays 1 5 0 4s suggests that the anisotropy is most probably confined to the top 300 km of the mantle

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

sub slab anisotropy beneath the sumatra and circum pacific subduction zones from source side shear wave splitting observations understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid earth system observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs providing a tool for studying subduction dynamics while many observations of seismic anisotropy have been made in subduction systems our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub slab region recently the source side shear wave splitting technique has been refined and applied to several subduction systems worldwide making accurate and direct measurements of sub slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub slab mantle here we present source side shear wave splitting measurements for the central america alaska aleutians sumatra ryukyu and izu bonin japan kurile subduction systems we find that measured fast splitting directions in these regions generally fall into two broad categories aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate trench parallel fast splitting directions dominate beneath the izu bonin japan and southern kurile slabs and part of the sumatra system while fast directions that parallel the motion of the downgoing plate dominate in the ryukyu central america northern kurile western sumatra and alaska aleutian regions we find that plate motion parallel fast splitting directions in the sub slab mantle are more common than previously thought we observe a correlation between fast direction and age of the subducting lithosphere older lithosphere 95 ma is associated with trench parallel splitting while younger lithosphere 95 ma is associated with plate motion parallel fast splitting directions finally we observe source side splitting for deep earthquakes transition zone depths beneath japan and sumatra suggesting the presence of anisotropy at midmantle depths beneath these regions

neoproterozoic ages of the kuluketage diabase dyke swarm in tarim nw china and its relationship to the breakup of rodinia the widely exposed kuluketage diabase dyke swarm tarim block nw china has been considered to have been emplaced in permian times new precise zircon u pb shrimp ages for two samples from the dyke swarm yield neoproterozoic ages of 823 8 8 7 ma and 776 8 8 9 ma correlated with peaks of magmatism in south china and australia at c 825 ma and c 780 ma these two new ages provide significant information for palaeocontinental reconstructions the prolonged duration of the magmatic events combined with regional stratigraphic relationships imply that the tarim block may have been affected by a mantle plume during the breakup of rodinia

development of mantle seismic anisotropy during subduction induced 3 d flow 1 the dynamics of subduction can be indirectly constrained by studying the induced mantle flow however inferring the circulation of the mantle around subducting plates from the interpretation of shear wave splitting patterns remains elusive we calculated the strain induced lattice preferred orientation lpo developed in 3 d models of subduction where retreat motions are maximized and found that in the mantle layer entrained with the downgoing slab the seismic anisotropy is trench perpendicular and becomes trench parallel deeper where the toroidal flow accommodates slab retreat synthetic sks splitting shows that in the fore arc slab rollback favors trench parallel polarization of the fast shear wave component while plate advance enhances trench perpendicular seismic anisotropy the interference between these two competing mechanisms yields subslab delay times of 0 5 to 1 3 sec comparable with those observed at most natural subduction zones the magnitude of the subslab trench parallel splitting is independent of the rate at which the slab migrates instead it is proportional to the amount of retreat

the mesoproterozoic maz terrane in the western sierras pampeanas argentina equivalent to the arequipa antofalla block of southern peru implications for west gondwana margin evolution the rocks of sierra de maz and sierra del espinal western sierras pampeanas represent pre famatinian ordovician basement isotope compositions nd and pb of metasedimentary rocks and shrimp u pb dating of detrital zircons combined with other geological evidence show that three parallel n s domains can be recognized the central maz domain contains pre grenvillian metasedimentary rocks deposited between 1 2 and 1 6 ga that underwent grenvillian granulite facies metamorphism and were intruded by mafic igneous rocks and massif type anorthosites metasedimentary rocks have high nd tdm ages 1 7 2 7 ga and very radiogenic pb 9 8 10 2 suggesting provenance from reworked early proterozoic or archean continental crust the domains to the east and west of the maz domain consist of three metasedimentary sequences with nd tdm ages between 1 2 and 1 6 ga and variably radiogenic pb 9 6 10 0 u pb shrimp dating of detrital zircons nd tdm model ages and comparison with other data suggest that these sequences are post grenvillian i e neoproterozoic and or early paleozoic the maz domain is interpreted as a suspect terrane similar to the northern arequipa antofalla craton that forms the basement of the central andes both underwent grenville age orogeny and were probably once continuous along the western margin of amazonia west gondwana

0 3 byr of drainage stability along the palaeozoic palaeo pacific gondwana margin a detrital zircon study the palaeo pacific margin of gondwana in the present day south central andes is marked by tectonic activity related to subduction and terrane accretion we present detrital zircon u pb data encompassing the palaeozoic era in northern chile and northwestern argentina cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase the main age clusters for these zircon grains are ediacaran to palaeozoic with an additional peak at 1 3 0 9 ga and they can be correlated with grenvillian age and the brasiliano pampean and famatinian orogenies the zircon data reveal main transport from the nearby ordovician famatinian arc and related rocks the silurian sandstone units are more comparable with cambrian units with brasiliano and transamazonian ages 2 2 1 9 ga being more common because the silurian deposits were situated within or east of the extinct famatinian arc hence the arc acted as a transport barrier throughout palaeozoic time the complete suite of zircon ages does not record the accretions of exotic terranes or the palaeozoic glacial periods we conclude that the transport system along the palaeo pacific margin of gondwana remained stable for c 0 3 byr and that provenance data do not necessarily reflect the interior of a continent hence inherited geomorphological features must be taken into account when detrital mineral ages are interpreted supplementary material u pb data cl images and detailed geological maps are available at www geolsoc org uk sup18796

u pb shrimp zircon dating of grenvillian metamorphism in western sierras pampeanas argentina correlation with the arequipa antofalla craton and constraints on the extent of the precordillera terrane abstract metamorphism of grenvillian age ca 1 2 ga u pb zircon dating is recognized for the first time in the western sierras pampeanas sierra de maz conditions reached granulite facies ca 780 c and ca 780 mpa comparing geochronological and petrological characteristics with other outcrops of mesoproterozoic basement particularly in the northern and central arequipa antofalla craton we suggest that these regions were part of a single continental crustal block from mesoproterozoic times and thus autochthonous or parautochthonous to gondwana

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

geological signature and possible position of the north china block in the supercontinent rodinia recent studies suggest that the south china block scb was located at a central position in the neoproterozoic supercontinent rodinia between ca 1000 ma and 800 ma but fewer studies dealt with the north china block ncb in the reconstruction of rodinia the ncb was suggested in general to be in a stable cratonic setting from ca 1650 ma to ca 160 140 ma our recent study suggests that there were two metamorphic magmatic events at 1300 1000 ma and 800 650 ma in the ncb although these events were weak they have been regarded as marking the assembly grenville age orogenic event and breakup neoproterozoic mafic dyke swarm and granitoid intrusive of the supercontinent rodinia a possible grenvillian ophiolite or convergent margin complex in the ncb exists only in the north qinling end mesoproterozoic orogenic belt the major meta volcanic sedimentary rocks in this complex belong to the kuanping group and are associated with gabbros and serpentinized peridotites collisional and post collisional granites have been recognized along the north qinling belt volcanic and sedimentary rocks along northern margin of the ncb indicate possible continent rift or continental margin environment the main rock associations of ca 800 650 ma are mafic dyke swarms with end t values of enriched mantle therefore this paper suggests that the mafic dykes were possibly related to the breakup of the rodinian supercontinent the tectonostratigraphic constraints and palaeomagnetic reconstruction show that the proterozoic ncb was different from the scb but somewhat similar to siberia the above features possibly suggest that the ncb as part of the supercontinent rodinia was possibly located in a marginal location not directly connected to the scb but likely adjacent to the siberian craton

geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics tectonic features at the earth s surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow a model with shallow return flow and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations because of the progressive shrinkage of the pacific averaging 0 5 km2 yr over the last 180 m y this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the pacific caribbean drake passage australian antarctic gap in this model upper mantle return flow streams originating at the western pacific trenches and at the java trench meet south of australia filling in behind this rapidly northward moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the australian antarctic discordance the long continued tectonic movements toward the east that characterize the caribbean and the easternmost scotia sea may be produced by viscous coupling to the predicted pacific outflow through the gaps and the caribbean floor slopes in the predicted direction if mantle outflow does pass through the gaps in the pacific perimeter it must pass beneath three seismic zones central america lesser antilles scotia sea none of these seismic zones shows foci below 200 km mantle material flowing through the caribbean and drake passage gaps would supply the mid atlantic ridge while the java trench supplies the indian ocean ridges so that deep mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations the analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents the second part of the paper suggests a possible driving mechanism for the plate tectonic process which may explain why the continents move this hypothetical driving mechanism has four aspects 1 the lower mantle convects 2 this convection drives the continents by drag on their deep roots 3 return flow in the upper mantle provides a volumetric balance for the motion of the continental masses without passing under them and 4 oceanic plates are effectively decoupled from the asthenosphere and are driven largely by slab pull this mechanism accounts for the opening of the atlantic the ability of spreading ridges to stay on the midlines of oceans and the penetration of india into asia following their collision continental motions strongly imply lower mantle upwelling and divergence beneath the atlantic and southeast indian oceans and convergence and subsidence along the tethyan belt this picture disagrees with the concept derived from hot spot studies of an undeforming absolute reference framework at depth but weaknesses in current hot spot theory would make a rejection of the present model on this ground premature if the present model is generally correct a fairly simple pattern of lower mantle convection with as few as four cells may explain much of the tectonic complexity of the earth

a search for source side mantle anisotropy shear wave splitting due to source side mantle anisotropy is estimated from broadband teleseismic s waves using a suite of 19 subduction zone events more than 50km depth and recorded at several rstn stations we assume that there are at most two anisotropic regions along the ray path one on the source side and one on the receiver side and that receiver side splitting is known from analysis of sks it is found that splitting is not detectable below the slab in most cases there are however two specific regions one beneath peru and one beneath kamchatka that indicate significant source side anisotropy based on two nearby intermediate focus events in each region splitting delay times are found to be 1 0 1 5s with the fast polarization direction perpendicular to the trench assuming 4 intrinsic anisotropy this would correspond to path lengths of 100 170 km below the slab both regions are characterized by abrupt changes in along strike slab properties a change in slab dip in the case of peru and a cessation of deep focus seismicity in the case of kamchatka we speculate that the anisotropy is caused by an associated local disturbance in asthenospheric flow that the shear wave splitting parameters of s and sks are consistent with each other in most cases suggests that large scale anisotropy in the lower mantle is very small probably no more than a fraction of a percent

the petrogenesis of a type magmas from the amram massif southern israel the 550 530ma alkaline magmatic suite of the amram massif southern israel was emplaced during the transition from an orogenic to an intra plate tectonic setting in the northeastern arabian nubian shield ans the suite ranges from 45 6t o 788 wt sio2 and consists of rhyolites alkali quartz syenites quartz syenites monzonites and co magmatic mafic to felsic alkaline dikes these rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content the felsic members of the suite display a type characteristics and are genetically linked through fractionation to the more mafic ones moderately positive initial end values a2 05 low initial 87 sr 86 sr values 07036 2 high mgo and fe2o3 concentrations 410 895 and 100 125 wt respectively and relatively flat rare earth element patterns la yb na 64 09 in the amram mafic dikes 456 495 wt sio2 suggest their derivation from the sub continental lithospheric mantle above the garnet stability zone the melts program was used to quantitatively model the chemical evolution of the suite extensive anhydrous fractionation 490 of plagioclase alkali feldspar clinopyroxene olivine and minor timagnetite and apatite from parental mafic magmas represented by the amram mafic dikes produced the rhyolitic compositions as well as the intermediate members of the suite this suggests the presence of a large unexposed body of cumulate rocks at depth as well as fusion of a large source region equivalent to an 5km layer in the lithospheric mantle regarded as a representative example for similar a type outcrops in this region this petrogenetic model further suggests that neoproterozoic early cambrian a type magmatism in the northeastern ans represents a significant post orogenic addition of mantlederived material to the juvenile crust this magmatic episode was of a similar magnitude to that of the cenozoic extensionrelated alkaline volcanism of the arabian plate

anisotropic loci in the mantle beneath central peru abstract seismic anisotropy of the upper mantle beneath the central part of peru is examined by analyzing shear waves observed at broad band stations and a temporary array of short period stations shear wave splitting is seen on various shear phases such as direct s waves from local intermediate to deep earthquakes scs waves from regional deep earthquakes and sks waves from teleseismic earthquakes it is inferred that the shear wave anisotropy in the uppermost 100 km of the subcontinental mantle overlying the subducted nazca plate is 0 5 at most while the anisotropy in the subslab asthenosphere depth range of about 150 350 km is 2 or greater the fast shear wave polarization direction in this depth range as observed at two broad band stations is 30 40 different from the absolute motion of the nazca plate this does not fit simple two dimensional 2 d models of olivine alignment caused by slab induced mantle flow and implies either the existence of a complex flow pattern in the asthenosphere underneath the nazca plate or the presence of an unknown mechanism for the anisotropy formation the lower mantle beneath central peru is found to be effectively isotropic for nearly vertically propagated shear waves

mantle flow in the south sandwich subduction environment from source side shear wave splitting 1 the occurrence of seismic anisotropy in the earth s upper mantle is a global phenomenon related to subcrustal deformation and flow processes the shear wave splitting analysis method has led to a global set of anisotropy maps mainly derived from receiver side analysis remote places with few seismometers deployed remain unexamined source side splitting analysis allows mapping of mantle fabrics in these regions here we investigate seismic anisotropy in the south sandwich islands subduction environment core reflected scs waves recorded at the neumayer seismograph network are corrected for well constrained receiver anisotropy and then analysed for source anisotropy sub slab mantle minerals are aligned horizontally almost parallel to the trench indicating a westward flow around the subducting slab this is consistent with a model of horizontal mantle flow due to slab rollback that was previously inferred from marine and geochemical studies in the back arc region

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

petrogenesis of the 723 ma coronation sills amundsen basin arctic canada implications for the break up of rodinia the coronation mafic sills and associated natkusiak flood basalts in arctic canada are part of the 723 ma franklin igneous events a flood basalt and dyke swarm province that extends across the southern arctic of north america and greenland two representative sills 70 and 25 m thick respectively outcrop near coronation gulf in nunavut they are composed of clinopyroxene plagioclase olivine and fe ti oxides and have tholeiitic composition similar to the natkusiak basalts their end values range between 4 2 and 5 9 and have la yb n ratios of 1 5 2 these rocks are mantle derived and were affected by minor crustal contamination compositional variations indicate that the coronation sills have experienced extensive fractionation including flow differentiation during emplacement and show increased differentiation from west to east across the province a feature similar to the mackenzie dyke swarm however the mackenzie dyke swarm and the franklin igneous events have distinct chemical composition the franklin rocks are related to a mantle plume located near victoria island at the center of the dyke swarm and could represent an offshoot from the superplume related to the break up of rodinia the absence of such dykes and sills in siberia argues against locating siberia against northern laurentia in a rodinia reconstruction 2003 elsevier b v all rights reserved

distribution of fe2 and mg between coexisting garnet and hornblende in synthetic and natural systems an empirical calibration of the garnet hornblende fe mg geothermometer abstract multiple regression analysis of a compilation of the fe2 mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions n 22 and coexisting garnet clinopyroxene hornblende from natural intermediate to basaltic rocks n 43 has been performed to define ln kd fe2 mg grt hbl as a function of temperature and garnet composition the regression of data covering a large span in pressure 5 16 kbar temperature 515 1025 c and composition yields the ln kd fe2 mg grt hbl p t compositional relationship r2 0 93 t c 1504 1784 x ca grt x mn grt ln k d fe 2 mg grt hbl 0 720 273 where k d fe 2 mg grt hbl fe 2 mg grt fe 2 mg m 1 m 3 hbl x ca grt ca ca mn fe 2 mg in garnet x mn grt mn ca mn fe 2 mg in garnet application of this expression to natural garnet hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods

seismic anisotropy reveals focused mantle flow around the calabrian slab southern italy 1 sks splitting at the calabrian subduction zone with delay times t up to 3s reveals the presence of a strong anisotropic fabric fast directions are oriented nne ssw in the calabrian arc c a and rotate nnw sse to the north following the arcuate shape of the subducting plate we interpret the trench parallel as local scale mantle flow driven by the retrograde motion of the slab the absence of trench perpendicular below the southern apennines s a excludes the presence of a seismically detectable return flow at its ne edge this may be due to the relative youth and limited width of the s a slab tear a possible return flow is identified farther north at the boundary of the s a and central apennines different and weaker anisotropy is present below the apulian platform a p this implies that the influence of the slab rollback in the sub slab mantle is limited to less then 100 km from the slab

motion of the scotia sea plates earthquake data from the scotia arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the scotia sea region events with magnitude greater than or equal to5 which occurred on or near the boundaries of the scotia and sandwich plates and for which centroid moment tensor cmt solutions are available are examined the newer data fill some of the previous sampling gaps along the boundaries of the scotia and sandwich plates and provide tighter constraints on relative motions variations in the width of the brunhes anomaly on evenly spaced marine magnetic profiles over the east scotia ridge provide new estimates of scotia sandwich plate spreading rates since there are no stable fracture zones in the east scotia sea the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading 18 newrate estimates and four azimuths from the east scotia ridge are combined with 68 selected earthquake slip vectors from the boundaries of the scotia sea in a least squares inversion for the best fitting set of euler poles and angular rotation rates describing the present day motions of the scotia and sandwich plates relative to south america and antarctica our preferred model tlp2003 gives poles that are similar to previous estimates except for scotia plate motion with respect to south america which is significantly different from earlier estimates predicted rates of motion also differ slightly our results are much more robust than earlier work we examine the implications of the model for motion and deformation along the various plate boundaries with particular reference to the north and south scotia ridges where rates are obtained by closure

northern ancestry for the goochland terrane as a displaced fragment of laurentia the ancestral position of the goochland terrane an isolated block containing mesoproterozoic crust in the appalachian piedmont of eastern virginia during grenvillian orogenesis has direct bearing on the breakup of rodinia ages lithology geochemistry and pb and nd isotope compositions of the montpelier anorthosite and state farm gneiss of the goochland terrane allow correlation with the grenvillian roseland anorthosite and associated igneous suites of the blue ridge farther west in virginia in contrast extensive metapelites which underwent devonian metamorphism and intrusion and amphibolites of the goochland terrane lack equivalents in the blue ridge dextral slip faults which bound the goochland terrane as well as dextral faults farther north suggest large scale translation prior to late paleozoic reaccretion ages of extension related a type granitoids within the virginia blue ridge 735 680 ma suggest physical separation from compositionally similar granitoids in the goochland terrane 630 588 ma reading prong 602 ma and manhattan prong 563 ma pre iapetan restoration of the goochland terrane northeastward of the blue ridge and outboard of the reading prong accounts for its affinities to the blue ridge unique anorthosites manhattan prong abundant amphibolites and reading prong ca 600 ma magmatism thus similar to the translation of madagascar out of africa the goochland terrane was rifted from laurentia marooned in oceanic crust and then dextrally translated southward 500 km prior to late paleozoic reaccretion documentation of large scale dextral translation of eastern laurentia relative to its fragments and other cratons during the neoproterozoic and paleozoic may assist in locating far traveled pieces of the grenville orogen for reconstruction of rodinia

ellipticity of teleseismic s particle motion summary in the horizontal components of grf records particle motions of the teleseismic s waves in the period range between 5 and 20s are usually elliptic we correct the particle motions for the effect of azimuthal anisotropy at grf and explain the residual ellipticity by assuming that it is produced at the source side of the wave path by interference of two waves with orthogonal polarizations and differing traveltimes the estimates of polarization direction of the fast wave and time delay between the waves can be found by analysing the residual particle motion for groups of closely spaced seismic events the technique was applied to about 40 grf records of events from the northern and north western pacific in the source regions the s wave pulses propagate in subduction zones it is found that the surface projections of polarization of the fast wave of the shallow events coincide approximately with the strikes of the corresponding island arcs this regularity can be explained by anisotropy in the subduction zones provided that the a axis of olivine is parallel to the plane of subduction the fast direction for the deep events in the sea of japan and the sea of okhotsk is close to the strike of the kurile island arc the time delay between the fast and the slow waves for the deep events is smaller than for the shallow kurile events 0 9s versus 1 4s

sediment subduction subduction erosion and strain regime in the northern south sandwich forearc 1 the first swath bathymetry and side scan sonar imagery from the south sandwich forearc reveals detailed seafloor morphology tectonic fabric and sedimentary features in an area where plate convergence is approximately normal to the trench simultaneously collected seismic reflection gravity and magnetic data provide information about the structure and composition of the forearc crust interpretation of these data together with the marine magnetic record of seafloor spreading in the east scotia sea back arc basin since 15 ma has enabled quantitative estimation of sediment subduction and subduction erosion rates any accretionary prism present is constrained to be very small extending 6 km or less from the trench the contrast between this result and a time integrated estimate of potential accretionary prism volume suggests that 95 of the sediment which has entered the trench since 15 ma has been subducted the position of the south sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated 70 km westward relative to the sandwich plate since 15 ma taking account of published observations concerning the typical geometric evolution of arc trench systems the average rate of forearc slope retreat during this interval is inferred to be 3 1 4 7 km myr 1 gravity modeling suggests that the forearc crust averages 10 km in thickness within 110 km of the trench for 10 km thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31 47 km3 km 1 myr 1 at the present convergence rate of 74 km myr 1 this erosion rate requires a 420 635 m thick layer of subducting material derived from the forearc crust a time integrated estimate of the potential volume of forearc derived trench fill sediments subducted since 15 ma implies that frontal erosion can only account for 40 of total subduction erosion therefore basal erosion must significantly exceed frontal erosion the new data also provide insights concerning forearc strain regime and forearc basin evolution in the area studied extensional faulting near the trench slope break is probably caused by gravitational instability of the steep lower forearc slope a lack of extensional strain indicators in the upper forearc and arc may be a consequence of the fact that the east scotia sea is a mature back arc basin contributing significant ridge push in the vicinity of the arc the data reveal no evidence of arc parallel extension or of large serpentinite seamounts as found in the mariana forearc and a causal link between these two observations is proposed seismic profiles across the forearc basin suggest that the balance between sediment accumulation and erosion is sensitive to changes in the elevation of the trench slope break it is hypothesized that the basin is a dynamic feature which goes through repeated cycles of growth and destruction controlled by cyclic uplift and collapse of the trench slope break

testing models of sub slab anisotropy using a global compilation of source side shear wave splitting data recently a number of source side shear wave splitting measurements that directly constrain anisotropy in the upper mantle beneath subducting slabs have been published such measurements have yielded an observational foundation on which to base our understanding of the dynamics of the sub slab mantle here we compile measurements from recent studies of source side splitting beneath slabs that employ identical measurement and processing techniques we use this compilation to test the predictions of a number of recently proposed conceptual models for the dynamics of the sub slab mantle including those that invoke three dimensional return flow beneath slabs strong radial anisotropy in suboceanic asthenosphere that is entrained via subduction and a model based on the correlation between sub slab splitting behavior and the age of the subducting lithosphere we find that a model in which fast splitting directions are determined by slab age matches the observations better than either the 3 d return flow or radial anisotropic models based on this observation we propose that the sub slab mantle is characterized by two distinct anisotropic and mantle flow regimes beneath younger lithosphere 95 ma the entrained layer is thin and effectively serves as decoupling layer the dynamics of the sub slab region beneath old lithosphere is therefore dominated by three dimensional return flow this variation in the amount of mechanical coupling may be facilitated by the onset of small scale convection beneath older lithosphere

sr c and o isotope geochemistry and stratigraphy of precambrian and lower paleozoic carbonate sequences from the western sierras pampeanas of argentina tectonic implications abstract sr c and o isotope data are presented for carbonate rocks from the sierra de pie de palo and other crystalline outcrops of the western sierras pampeanas these are used to distinguish three groups of rocks of quite different ages within the nappe pile that constitutes the sierra de pie de palo carbonates from the grenville age ophiolitic unit at the bottom of the pile probably resulted from the interaction between hot seawater and contemporaneous oceanic crust these carbonates are tentatively classified as ophicalcites carbonates from the difunta correa sedimentary sequence are part of a cover sequence to a grenvillian basement together forming the upper nappes isotope stratigraphy suggests a middle to late neoproterozoic age 580 720 ma for these carbonates this cover might be correlated with cover sequences to the archean and early to middle proterozoic cratons that are well preserved over large areas of brazil this fact together with the apparently absence of an equivalent neoproterozoic carbonate bearing cover in the appalachian margin of laurentia suggests that the western sierras pampeanas which are considered part of the exotic argentine precordillera terrane of allegedly laurentian derivation could be autochthonous or para autochthonous to gondwana other geochemical and geological arguments reinforce this hypothesis the caucete group carbonates underlie the nappe pile and are separated from it by a first order thrust the pirquitas thrust these latter carbonates are cambrian in age and isotopically similar to the carbonate platform of the precordillera we thus conclude that the pirquitas thrust is the boundary between the exotic precordillera terrane and the autochthonous or para autochthonous western sierras pampeanas

enhanced magma supply at the southern east scotia ridge evidence for mantle flow around the subducting slab abstract bathymetric and seismic data show that the southernmost segment of the east scotia ridge segment e9 is anomalous in its curved plan form and in the presence of a large axial volcanic ridge avr spreading commenced only within the past 1 million years on this segment which appears to have propagated both northward and southward the presence of a caldera at the summit of the avr indicates that a shallow magma chamber of limited extent existed recently beneath the ridge eruption from this magma chamber probably within the past 0 1 million years may have led to the formation of the avr itself careful examination of seismic reflection profiles suggests that small pockets of magma may still exist beneath the avr magmatism on segment e9 may be enhanced as a result of the flow of shallow mantle around the southern end of the subducting slab beneath the south sandwich arc in a manner similar to that proposed for the northern end this mantle probably carries a slightly enriched hotspot signature and is affected by volatiles released from the edge of the slab both tending to increase the supply of magma to the back arc spreading centre

peraluminous grenvillian ttg in the sierra de pie de palo western sierras pampeanas argentina petrology geochronology geochemistry and petrogenetic implications abstract combined petrological geochemical isotopic and geochronological data shed light on the origin and evolution of a peraluminous garnet bearing two mica granitoid el tigre granitoid etg cropping out in southwestern sierra de pie de palo 31 31 30 s 68 15 12 w and to constrain the age and petrogenetic conditions of this intrusive event etg experienced amphibolite to greenschist facies metamorphism after igneous crystallization followed by strong deformation restricted to narrow mylonite zones etg shear zone and partial dynamic recrystallization under lower t conditions a dextral shear sense is compatible with kinematic observations registered along the nne striking regional las pirquitas overthrust active at 473 10 ma k ar on 1 1 normative corundum low cao values between 1 72 and 2 41 plotting mostly in the granite trondhjemite fields of the ab an or diagram the trace element contents show a relatively low abundance of rb hfs elements such as y nb ta ga and zr and high concentrations of ba sr and lree the chondrite normalized ree pattern has a high slope with la yb n 9 48 55 32 and a negative or absent europium anomaly relationships between trace elements suggest the classical setting of granitoids produced in a convergent plate setting a u pb shrimp crystallization age on zoned igneous zircon of 1105 5 4 1 ma suggests that the etg could be part of the magmatic complex forming the grenvillian basement of the western sierras pampeanas 87 sr 86 sr 1105 values of 0 70543 and nd of 4 2 indicate a rather immature source for its origin with similar initial isotopic ratios to those found in orthogneisses from elsewhere in the western sierras pampeanas on the other hand sm nd model ages t dm for the etg range from 1 20 to 1 39 ga geochemical and isotopic signatures of the etg could be explained by low pressure partial melting from a basaltic source under high geothermal gradient conditions this thermal anomaly could be associated with the 1 1 ga global period of enhanced mantle plume activity developing widespread global magmatism

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

age constraints on the tectonic evolution and provenance of the pie de palo complex cuyania composite terrane and the famatinian orogeny in the sierra de pie de palo san juan argentina abstract new u pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the pie de palo complex represent a mesoproterozoic fragment of suprasubduction zone oceanic crust a gabbroic pegmatite interpreted to have formed during arc rifting or subsequent back arc spreading yielded a u pb age of 1204 5 3 4 7 ma highly tectonized ultramafic mafic cumulates occurring at the structural base of the pie de palo complex and previously interpreted to represent remnants of a primitive arc phase prior to rifting and back arc spreading could not be dated but should be older than 1204 ma if these inferences are correct tabular sill like bodies of leucogabbro diorite and calc alkaline tonalite granodiorite sills yielded ages of 1174 43 and 1169 8 7 ma respectively they may represent a younger more evolved arc phase established after arc rifting or a younger tectonically unrelated mesoproterozoic arc shrimp analysis of metamorphic zircon rims with low th u ratios in vvl 110 gave a 206pb 238u age of 455 10 ma similar to lower intercept dates determined by discordia lines combined these data indicate that the bulk of the amphibolite facies metamorphism present in the pie de palo complex was generated during the famatinian orogeny analysis of six single detrital zircon grains in a metasedimentary quartzofeldspathic garnet mica schist tectonically interleaved with the igneous rocks of the pie de palo complex and tentatively correlated with the difunta correa metasedimentary sequence of other workers yielded three age populations 1150 1160 ma 1050 1080 ma and 665 ma indicating that these sedimentary rocks were deposited during the late neoproterozoic or early paleozoic in addition they confirm structural evidence that intercalation of rocks of the pie de palo complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications these ages are also consistent with a laurentian provenance and earlier interpretations that these rocks once represented a sedimentary cover to the pie de palo complex the zircon population of 1050 1080 ma could be derived from grenville age felsic plutons identified elsewhere in the pie de palo complex by other workers however no evidence has been found in our samples for a grenville age orogenic event invoked previously to explain accretion of the oceanic pie de palo complex to laurentia prior to the late neoproterozoic early cambrian rifting and drift of cuyania

grenvillian massif type anorthosites in the sierras pampeanas we report the discovery of massif type anorthosites in the andean basement of the western sierras pampeanas of argentina u pb zircon dating by sensitive high resolution ion microprobe of a cogenetic gabbronorite dyke yields ages of 1070 41 ma for igneous emplacement and 431 40 ma for metamorphism these anorthosites are petrologically and geochemically comparable with those of the grenville province of laurentia palaeogeographical reconstructions of rodinia at 1 0 1 1 ga suggest that the sierras pampeanas anorthosites were part of a large anorthosite province in the late mesoproterozoic

sub slab seismic anisotropy and mantle flow beneath the caribbean and scotia subduction zones effects of slab morphology and kinematics subduction systems are vitally important to plate tectonics and mantle convection but questions remain about many aspects of subduction dynamics particularly the nature of sub slab mantle flow observations of seismic anisotropy can shed light on the pattern of mantle flow in subduction systems but major uncertainties remain regarding the interpretation of sub slab anisotropy here we present measurements of shear wave splitting due to anisotropy in the sub slab mantle beneath the caribbean and scotia subduction zones these subduction systems are morphologically similar with high arc curvature and short arc length but there are differences between them in terms of their kinematic and tectonic settings we apply the source side shear wave splitting technique to direct teleseismic s phases originating from slab earthquakes we restrict our analysis to seismic stations at which we have examined the receiver side splitting in detail to ensure accurate corrections for anisotropy in the upper mantle beneath the station we observe a well defined pattern of trench parallel fast directions f i n the northern half of the caribbean subduction zone with a transition to dominantly trenchperpendicular f at the southern end there is more scatter in the measurements for scotia but we observe generally trench parallel f in the northern portion of the subduction system with a mix of trench parallel perpendicular and oblique fast directions to the south our preferred interpretation of these splitting patterns is that they reflect 3 d return flow of the sub slab mantle due to trench rollback in both systems we infer that sub slab flow is being driven from south to north beneath scotia this is likely driven by differential migration of the scotia trench along strike beneath the caribbean we hypothesize that mantle flow around the southern edge of the slab is inhibited by the presence of the south american continental keel enabling trench perpendicular stretching in the subslab mantle and forcing mantle flow to escape to the north the similarities and differences we observe between the two systems yield insight into the relative contributions of slab morphology and plate kinematics in controlling mantle flow beneath subducting slabs

slice of intraoceanic arc insights from the first multichannel seismic reflection profile across the south sandwich island arc we present the first multichannel seismic mcs reflection line that crosses the south sandwich trench south sandwich island arc and east scotia sea backarc basin the line is used in combination with earthquake catalogue data to interpret the strain distribution across the sandwich plate and the relationship of forearc structures to processes operating at the trench the mcs data reveal a 1 2 km high fault scarp associated with a 20 km wide arcward tilted block in the mid forearc these features indicate large scale gravitational collapse and earthquake data are consistent with trench normal extension at shallow depth in this area there is however little evidence of distributed extension within the interior of the sandwich plate the mcs data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench backarc magnetic data mid forearc extension and the small size of the frontal wedge are all consistent with long term and ongoing subduction erosion earthquake data suggest that this erosion is taking place in an environment of low interplate stress

neoproterozoic a type magmatism in the western sierras pampeanas argentina evidence for rodinia break up along a proto iapetus rift a type orthogneisses of mid neoproterozoic age 774 6 ma u pb shrimp zircon age are reported for the first time from the grenvillian basement of the western sierras pampeanas in argentina these anorogenic meta igneous rocks represent the latest event of rodinia break up so far recognized in grenvillian basement exposures across andean south america moreover they compare well with a type granitoids and volcanic rocks along the appalachian margin of laurentia blue ridge thus adding to former evidence that the western sierras pampeanas grenvillian basement was left on the conjugate rifted margin of eastern laurentia during rodinia break up and the consequent opening of the iapetus ocean

recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer the garnet al silicate plagioclase gasp geobarometer has been recalibrated using four recent garnet activity models four analogous garnet biotite temperature models and two recent plagioclase activity models a typical sillimanite bearing sample that formed at about 5 25 kbar 575 c shows a possible p range of 0 7 kbar due to t error 1 kbar due to range of garnet activity model 0 9 kbar due to range of plagioclase activity model and 5 4 kbar due to range of experimental end member reversals extended by one sigma calibrations were further constrained with the kyanite sillimanite k s phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer samples with low grossular or anorthite component were rejected the end member calibrations are constrained to pass through the the best determined portion of the gasp experimental reversals at 1230 c 26 6 kbar these individual end member calibrations provide self consistent models that tend to compensate for error in the garnet and plagioclase activity expressions the models were also tested on a set of 59 samples from the alps the recommended calibration is the average garnet activity model and average garnet biotite t model of holdaway 2000 the fuhrman and lindsley 1988 plagioclase activity model and h grs 6628521 s grs 258 76 to combine with the remaining phases in the berman database to produce the optimum end member gasp curve these thermodynamic data are for the gasp geobarometer only error is about 0 8 kbar absolute and about 0 6 kbar relative geological error is the largest component of error in many of these samples care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet that the peak t portions of garnet and plagioclase are selected that the peak t al silicate is determined and that the t calculated is the most accurate possible these calibrations represent an improvement over previous published gasp calibrations these eight models are available for distribution as three programs t p p t intersection for the dos based personal computer