propagation of rifting along the arabia somalia plate boundary into afar it is generally accepted that the aden ridge has propagated westward from 58 e to the western tip of the gulf of aden tadjoura at the edge of afar here we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the republic of djibouti we show that it veers northward forming a narrow zone of dense faulting along the northeastern edge of the afar depression the zone includes two volcanic rifts asal ghoubbet and manda inakir connected to one another and to the submarine part of the ridge by transfer zones both rifts are composite divided into two or three disconnected parallel nw se striking subrifts all of which appear to have propagated northwestward in asal ghoubbet as in manda inakir the subrifts appear to have formed in succession through north directed jumps from subrifts more farther south at present the northernmost subrifts manda and dirko koma of the manda inakir rift form the current tip of the northward propagating arabia somalia plate boundary in afar we account for most observations by a mechanical model similar to that previously inferred for the gulf of aden in which propagation is governed by the intensity and direction of the minimum horizontal principal stress 3 we interpret the northward propagation on land almost orthogonal to that in the gulf to be related to necking of the central afar lithosphere where it is thinnest such necking may be a consequence of differential magmatic thickening greater in the center of the afar depression where the ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression the model explains why the aden ridge foregoes its wsw propagation direction constant from 58 e to asal ghoubbet at a smaller scale individual rifts and subrifts keep opening perpendicular to the arabia somalia or danakil somalia motion vector and propagate northwestward concurrently such lithospheric cracks are forced to jump northward such that the plate boundary remains inside the regional n s necking zone changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns a small angle promoting long en echelon subrifts and a high angle smaller nested subrifts within subrifts the propagation mechanism is thus similar whether in oceanic or continental lithosphere the principal change being the overall propagation path here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the aden ridge finally because the same mechanism has led rifting along the red sea to propagate southward and jump to the western edge of afar the arabia somalia and arabia nubia plate boundaries tips have missed each other and keep overlapping further leading to strain transfer by large scale bookshelf faulting

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

pericontinental crustal growth of the southwestern abitibi subprovince canada u pb hf and nd isotope evidence the abitibi subprovince is widely regarded as a classic example of neoarchean continental crust formed in an ensimatic setting however sialic crust as old as 2 93 ga occurs in the adjacent wawa subprovince and the two subprovinces are thought to have formed a contiguous block since the initiation of abitibi magmatic activity at 2 75 ga a detailed geochemical u pb geochronological and nd hf isotope study of plutonic and volcanic rocks from the southwestern abitibi provides evidence for the involvement of older crust during magma genesis near the abitibi wawa boundary kapuskasing structural zone inherited zircons dated at ca 2 85 ga occur both in 2747 ma tonalite and 2700 ma granodiorite of the rice lake batholith part of the larger kenogamissi plutonic complex igneous zircons from this batholith display large intrasample variations in 176 hf 177 hf which we attribute to mixing of magmas derived from mantle juvenile crust and older crustal sources nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source however two older felsic volcanic units 2739 and 2729 ma show nd isotope evidence of significant crustal contamination post tectonic plutonic units that were emplaced at 2676 and 2662 ma also contain inherited zircons but none are older than 2 76 ga in contrast tonalitic units dated at 2744 and 2715 ma from the round lake plutonic complex located 150 km east of the kapuskasing structural zone lack zircon inheritance and display less intrasample hf isotope variability in zircons our data suggest that the influence of older sialic crust during abitibi volcanic and plutonic activity is restricted both in space and time similar evidence for an older crustal influence occurs elsewhere in the southwestern abitibi near the kapuskasing structural zone the data collectively suggest that an approximately 75 km wide belt of abitibi crust adjacent to this structure was underlain by older wawa crust from 2 75 ga onward this crustal geometry is consistent with development of the abitibi sub province at the edge of a wawa protocraton perhaps as the result of rifting of this older continental block

the significance of u pb zircon dates in lower crustal xenoliths from the southwestern margin of the kaapvaal craton southern africa the combination of high precision id tims u pb geochronology with cathodoluminescence cl imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust a suite of xenoliths exhumed in cretaceous kimberlites at the southwestern margin of the kaapvaal craton including eclogite granulite amphibolite and greenschist facies plutonic rocks are the subject of these detailed zircon studies cl imaging demonstrates the complexity of chemical and growth structure in single zircons and highlights the importance of sampling on an intragrain scale to extract end member ages for zircon growth events for these diverse metamorphic rocks linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism using an empirical approach we attempt to relate zircon crystallization dissolution and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns mineral inclusions and petrography these lines of evidence indicate the crystallization of zircon under p t conditions ranging from eclogite through greenschist facies we discuss four examples of this approach 1 at the lowest grade of metamorphism examined primary igneous zircon 1900 ma in a syenite xenolith from the klipfontein 08 kimberlite underwent two episodes 1015 and 964 ma of probable fluid mediated resorption and reprecipitation in the greenschist facies 2 an upper amphibolite facies xenolith from the markt kimberlite contains two distinct populations of blocky and fir tree sector zoned zircon which are suggested to date prograde hornblende breakdown reactions 1107 ma and melt infiltration under retrograde metamorphic conditions 1092 ma respectively 3 at granulite facies metamorphic zircon crystallized at 1114 ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the markt kimberlite later 960 ma zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions possibly enhanced by fluids 4 in a much older episode of eclogite facies metamorphism recorded in a xenolith from the lovedale kimberlite zircon crystallized with the peak metamorphic mineral assemblage or possibly early on the cooling path in the late archean 2630 ma each of these examples provides unique information regarding the evolution of the kaapvaal craton while the links to metamorphic reactions and p t conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

archean protolith and accretion of crust in kamchatka shrimp dating of zircons from sredinny and ganal massifs abstract we used an ion microprobe shrimp rg for u pb dating of individual zircons in high grade metamorphic basements of the felsic sredinny and mafic ganal massifs in kamchatka thirty percent of zircons from paragneisses of the sredinny massif contain archean 2800 2500 ma early proterozoic 1700 2100 ma and late proterozoic 1000 ma detrital cores in addition a broad and almost uninterrupted range of phanerozoic detrital cores 460 175 ma is present with jurassic to lower cretaceous 150 96 ma zircons forming the youngest detrital age group ubiquitous documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

circulation hydrography and transport over the summit of axial seamount a deep volcano in the northeast pacific a numerical model of ocean flow hydrography and transport is used to extrapolate observations of currents and hydrography and infer patterns of material flux in the deep ocean around axial seamount a destination node of nsf s ocean observatories initiative s cabled array using an inverse method the model is made to approximate measured deep ocean flow around this site during a 35 day time period in the year 2002 the model is then used to extract month long mean patterns and examine smaller scale spatial and temporal variability around axial like prior observations model month long mean currents flow anti cyclonically around the seamount s summit in toroidal form with maximum speeds at 1500m depth of 10 11 cm s as a time mean the temperature salinity anomaly distribution takes the form of a cold briny dome above the summit passive tracer material continually released at the location of the ashes vent field exits the caldera primarily through its southern open end before filling the caldera once outside the caldera the tracer circles the summit in clockwise fashion fractionally re entering the caldera over lower walls at its north end while gradually bleeding southwestward during the modeled time period into the ambient ocean a second tracer release experiment using a source of only two day duration inside and near the casm vent field at the northern end of the caldera suggests a residence time of the fluid at that locale of 8 9 days

petrography and implications of continental rocks from the agulhas plateau southwest indian ocean seismic refraction and seismic reflection profiles reported elsewhere indicate that the agulhas plateau is a continental fragment that was rifted from the falkland plateau and intruded by basaltic magmas during the early opening of the south atlantic dredge hauls recovering a characteristic suite of low to high grade continental metamorphic rocks indurated sedimentary rocks and basaltic igneous samples support this origin and evolutionary history both the petrography of the gneiss specimens and k ar ages of 1 074 36 m y and 478 17 m y on fresh biotites from two of the rocks are compatible with petrography and ages of the formerly adjacent continental rocks of southern africa the falkland islands falkland plateau and antarctica

new continental margin magnetic anomalies of east antarctica over the past decade australian norwegian and russian marine surveys have collected integrated seismic gravity and magnetic data in the southern indian ocean the more than 350 000 line km of new airborne and marine magnetic observations for the east antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns this compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of east antarctica the eastern sector of the map from bruce rise in the west to the d urville sea in the east is largely dominated by seafloor spreading magnetic anomalies the adelie rift block of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric abrupt magnetic anomaly changes along the oceanic continent transition in the cooperation sea including the enderby basin anomaly extend for more than 1680 km from the kerguelen plateau towards the cosmonaut sea three sectors of the east antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation strong positive seafloor magnetic anomalies mark the southern margin of the kerguelen plateau the maud rise and adjacent areas in the riiser larsen sea the new compilation suggests that at least 300 km of the enderby basin and shackleton basin may be part of the cretaceous kerguelen volcanic province and possibly maps an abandoned fossil spreading center in the central enderby basin the majority of the published age models for the enderby basin and australian sector of the east antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map

proterozoic events in the eastern ghats granulite belt india evidence from rb sr sm nd systematics and shrimp dating metamorphic and protolith ages of five rock types mafic granulite orthopyroxene granulite leptynite sillimanite granite and metapelite from rayagada in the north central part of the eastern ghats granulite belt eggb india were determined from rb sr and sm nd whole rock and mineral isochrons in combination with shrimp u pb zircon data most of the whole rock isochron ages in both sm nd and rb sr systems point to either 1450 or 1000 ma and the mineral isochron ages are 1000 800 and 550 ma shrimp u pb zircon ages of 940 ma were obtained from metapelite which are in close agreement with the sm nd and rb sr isochron ages from all these data four age clusters 1450 1000 800 and 550 ma have been noted the 1450 ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite the 1000 ma age cluster is regarded as the intrusion ages of sillimanite granite and charnockite and associated granulite facies metamorphism two other age clusters 800 a

the kerguelen plateau records from a long living composite microcontinent abstract the kerguelen plateau lies in the southern indian ocean and is classically considered as a large igneous magmatic province lip emplaced above an oceanic crust the interpretation of the gravimetric bathymetric anomalies shows that the plateau is structured by nnw and nw trending en echelon fractures along its north eastern border and by wnw trending faults along its western side which separate very differentiated zones i e elan bank and enderby basin a geochemical contamination by continental material has been evidenced for all the studied magmatic rocks of the southern and central part of the plateau only the basalts of the northern zone recording the signature of ridge emitted magmas the facies evolution of the overlying sediments also shows that the deposition began in a subaerial environment for the southern part of the plateau and that the subsidence notably increased after 60 ma all these evidences suggest that the southern part of the kerguelen plateau is underlain by stretched continental crust as it is the case for elan bank and that the central kerguelen plateau is most probably also underlain by attenuated continental crust in reconstructions the south and central kerguelen plateaus resulted from the interaction between these remnants of continental crust with the kerguelen plume while the latter interacted with the south east indian ridge to form the north kerguelen plateau we thus propose a cinematic scenario characterised by the formation of a composite microcontinent composed of fragments of stretched continental crust separated by ridge propagators which were emplaced successively or contemporaneously between india and antarctica during the rifting process being triggered by the kerguelen plume emplacement

nd and pb isotope evidence from the seychelles granites and their xenoliths mantle origin with slight upper crust interaction for alkaline anorogenic complexes abstract due to different initial pb incorporation the pb pb whole rock data of the seychelles anorogenic complex of late precambrian age form linear arrays on 24 samples with no time significance the existence of two groups of rocks in this complex with different initial pb and nd isotopic characteristics has been evidenced 1 1 the mahe group comprising the biotite hornblende granite the weakly alkaline granite and their melanocratic dioritic to gabbroic xenoliths yields nd be 705 ma 3 1 their pb isotopic compositions in k feldspar are only slightly more radiogenic than those of mantle derived rocks of the same age 2 2 the praslin la digue felicite group comprising the biotite granite and its leucocratic xenoliths yields nd be 705 ma 1 9 k feldspar pb ratios are significantly more radiogenic than those of mahe rocks thus the parental basic magma of this alkaline anorogenic complex appears to originate from a slightly depleted mantle source underlying the african continental crust this magma has been contaminated by the high level part of this crust to different degrees in the two groups the praslin one having been more affected particularly by late magmatic fluids

unsteady late pleistocene incision of streams bounding the colorado front range from measurements of meteoric and in situ 10be 1 dating of gravel capped strath terraces in basins adjacent to western u s laramide ranges is one approach to document the history of late cenozoic fluvial exhumation we use in situ 10be measurements to date the broad surfaces adjacent to the eastern edge of the rocky mountains in colorado and compare these calculated ages with results from meteoric 10be measurements we analyze three sites near boulder colorado gunbarrel hill table mountain and pioneer that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the plio pleistocene transition our in situ 10be results reveal abandonment ages of 95 129 ka at table mountain 175 27 ka at pioneer and ages of 251 10 ka and 307 15 ka at gunbarrel hill all are far younger than previously thought inventories of meteoric 10be support this interpretation yielding ages that are comparable to table mountain and 20 lower than pioneer in situ ages we argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate and that vertical incision rates of several mm yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions in contrast to the traditional age chronology in the area our ages suggest that the deep exhumation of the western edge the high plains occurred relatively recently and at an unsteady pace

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

trace of the kerguelen mantle plume evidence from seamounts between the kerguelen archipelago and heard island indian ocean 1 the gravity and bathymetric highs on the kerguelen plateau in the southern indian ocean between the kerguelen archipelago and heard island are seamounts formed of miocene alkalic basalts similar to those found on the islands dredging during the kerimis survey cruise recovered 1 ton of mostly basaltic rocks one of the dredges d6 yielded a large volume of in situ alkalic picritic pillow basalts the first picritic lavas recovered that are related to the kerguelen plume k ar and 40ar 39ar ages are between 18 and 21 ma for all but one sample and these ages are only slightly younger than the main phase of volcanism on the archipelago the dredged lavas form three distinct groups based on chemical and isotopic compositions incompatible element abundance ratios overlap with compositional groups defined by lavas from both the kerguelen archipelago and heard island indicating that alkalic volcanism in this region of the kerguelen plateau has been spatially diverse olivine and picritic basalts have sr and nd isotopic characteristics similar to most of the lavas exposed on the archipelago and those proposed for the kerguelen plume however compared to kerguelen archipelago lavas the picritic basalts have relatively low 206pb 204pb which is a characteristic of cretaceous basalt forming some parts of the kerguelen plateau we propose that the apparent age trend of the lavas from 34 ma in the northern kerguelen plateau odp leg 183 site 1140 to 24 30 ma on the kerguelen archipelago to 18 21 ma on the dredged submarine volcanoes and even possibly to recent volcanism on heard and mcdonald islands may correspond to the tertiary hot spot track of the kerguelen plume

signals of barremian 116 ma or younger oceanic crust beneath the bay of bengal along 14 n latitude between 81 e and 93 e abstract the magnetic profile along 14 n in the bay of bengal collected on board r v samudra manthan of the geological survey of india was processed in the frequency domain from the energy spectrum the computed average thickness of sediment cover comes to 6 km and the average geothermal gradient to 20 c km analysis of the vertical magnetic structure by a singular point method yields a pseudosection marked by four clear domains the eighty five east ridge appears as a pair of strong magnetic features between 84 e and 86 e whereas the ninety east ridge shows up as a narrow well defined vertical magnetic feature at 92 e by match filtering technique we extracted a magnetic profile of the basement which displays a number of polarity reversals the reversals between 81 e and 84 e appear to be records of barremian or younger seafloor spreading instead of m series reversals between 132 and 119 ma this is based on the fact that neither the basic lavas nor the marine sediments so far obtained from the basin margin are older than barremian 116 ma given the uncertainty in apwp of the indian landmass between 200 and 117 ma the rajmahal hot spot might have powered the rifting of india from antartica from 117 ma onwards initiation of rifting at 132 ma as reconstructed by earlier workers is not proved

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

dry climate near the western pacific warm pool pleistocene caliches of the nansha islands south china sea abstract the nansha spratly islands are located in the middle of the south china sea at about 10 n near the northwestern margin of the western pacific warm pool wpwp a borehole was drilled at an atoll at nansha and cores were taken the upper 165 4 m of the borehole consists entirely of limestone of reef facies and lagoon facies which had been influenced by extensive meteoric diagenesis petrographic and geochemical studies have identified at least four subaerial exposure surfaces ses in the pleistocene carbonate sequence these ses are thought to have resulted from global sea level changes and characterized by caliche formation when developed in limestone caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm compared with 1800 2200 mm of annual rainfall of the present day nansha islands the nansha caliche therefore indicates the existence of several dry climate episodes during pleistocene sea level lowstands lower rainfall and higher evaporation during such dry conditions may explain the higher sea surface salinities reported elsewhere in the south china sea the nansha caliche may also indicate reduced extent of the wpwp and southerly shift of the intertropical convergence zone during major sea level lowstands

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

a revised late eocene age for porphyry cu magmatism in the escondida area northern chile new u pb zircon and 40 ar 39 ar single crystal incremental heating biotite dates are reported for porphyritic rocks from three mineralized centers in the escondida region of northern chile all dates are reported with 2sigma errors at the chimborazo porphyry cu prospect a weighted mean 40 ar 39 ar plateau age of 38 09 or 0 30 ma has been obtained for igneous biotite from a syn or postmineralization feldspar biotite quartz porphyry intrusion at the zaldivar porphyry cu mine zircon and biotite have been analyzed from a similar synmineralization feldspar biotite quartz porphyry the llamo porphyry the zircon u pb age is 38 7 or 1 3 ma whereas the weighted mean 40 ar 39 ar plateau age of igneous biotite is slightly younger at 37 40 or 0 18 ma u pb dating of two other rhyolitic quartz feldspar porphyry bodies that host much of the ore at zaldivar shows that these rocks are paleozoic in age antigua porphyry 290 or 4 ma zaldivar porphyry broadly coeval with antigua but age poorly defined at the escondida deposit the reportedly synmineralization escondida porphyry is dated by u pb at 37 9 or 1 1 ma whereas the reportedly postmineralization rhyolitic porphyry returned a u pb age of 34 7 or 1 7 ma these results indicate a single pulse of synmineralization magmatism in the escondida district at approximately 38 ma followed by minor largely nonmineralizing intrusive events the timing of main stage porphyry mineralization at escondida is apparently bracketed by the ages of the escondida 37 9 or 1 1 ma and rhyolitic porphyries 34 7 or 1 7 ma previously published k ar dates for intrusive rocks and alteration minerals at escondida which range from 39 to 31 ma are interpreted to reflect variable degrees of disturbance of the k ar system by later intrusive events and or by supergene weathering processes an age of approximately 38 ma for ore forming porphyry magmatism in the escondida region is significantly older than previously accepted best estimates based on k ar and u pb dating 32 34 ma but still falls within the age range of other large porphyry systems within the north south trending west fissure zone of northern chile 31 42 ma a common feature of the porphyry intrusions at zaldivar and escondida and also at chuquicamata and el salvador is the presence of zircons with inherited upper paleozoic cores these cores are interpreted to reflect interaction between arc magmas and crustal rocks during melting assimilation storage and homogenization mash and assimilation and fractional crystallization afc processes at various crustal levels

structure and development of a microcontinent elan bank in the southern indian ocean microcontinents appear to commonly form on young continental margins close to hot spots but difficulties in understanding their geology and evolution have inhibited assessment of their global distribution and significance thick volcanic accumulations in areas affected by hot spot magmatism only complicate the issue elan bank a large western salient of the kerguelen plateau is a microcontinent that originally lay between india and antarctica in gondwana recent regional plate tectonic reconstructions suggest that during gondwana breakup elan bank and india initially separated from antarctica and elan bank became isolated in the southern ocean via a ridge jump to the north between elan bank and india in albian time 108 ma voluminous magmatism attributed to the kerguelen hot spot overprinted and radically altered the original microcontinent and its surroundings recent odp investigations deep seismic reflection data and a wide angle seismic line on elan bank allow us to gain the first insight into the feature s integrated crustal structure and geological evolution and the adjacent continent ocean transition zone our analysis shows that elan bank s crust is at least 16 km thick the upper igneous crust consists of a 2 3 km thick layer with seismic velocities ranging from 4 4 to 5 9 km s that can be interpreted as the result of accumulation of lava flows originating from the kerguelen hot spot seismic velocities at the base of the crust are as low as 6 6 km s which is consistent with a fragment of thinned continental crust 14 km thick a high velocity body located at depths of 5 to 10 km could be interpreted as plutonic rocks emplaced during the major regional magmatic episode on the basis of deep seismic reflection data we interpret extensional structures beneath the volcanic flows in albian time when the area was affected by the kerguelen hot spot volcaniclastic material and lava flows accumulated in faulted grabens and basins both on the bank and within the continent ocean transition zone to the south creating the appearance of flat unstructured basement the seismic structure and inferred composition of elan bank revealed by this study contribute to our understanding of microcontinent formation as well as provide a template for identifying microcontinents in accreted terranes and mountain belts

open system processes in the genesis of silica oversaturated alkaline rocks of the rallier du baty peninsula kerguelen archipelago indian ocean abstract the rallier du baty peninsula forms the southwestern part of the kerguelen archipelago indian ocean whose magmatic activity is related to the long lived 115 ma kerguelen plume the peninsula is mostly made of alkaline rocks constituting two well defined ring complexes this paper focuses on the northern ring complex which is not yet known recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6 2 to 4 9 ma and two later volcanic systems 40ar 39ar dating of a trachytic ignimbrite linked to the dome carva volcano complex yields an age of 26 3 ka this represents the last major eruptive event on the kerguelen archipelago the volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas the volume of erupted felsic magma is by far the larger and is represented by abundant pyroclastic deposits and lava flows boulders of plutonic rocks are found to the northwest of dome carva and represent intermediate rocks i e monzogabbros and monzonites that are not present at the surface basic rocks are mostly trachybasalts and trachyandesites while true basalts are scarce their mineralogy consists chiefly of plagioclase olivine diopside and oxides sieve textured plagioclase is common as well as corroded olivine and diopside phenocrysts peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks they consist of abundant sanidine augite and magnetite phenocrysts and interstitial quartz aegerinic pyroxenes and na amphiboles ring dykes of quartz poor alkali feldspar syenites display the same mineralogy except hornblende is common and replaces diopside hornblende is particularly abundant in intermediate monzogabbros major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of mgo cao p2o5 tio2 feo ba sr and ni from basic to felsic rocks however the scattering of the data from the basic rocks indicates that other processes have operated the overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control an increase of incompatible elements from trachyte to rhyolite is observed the felsic lavas display an increase of 87sr 86sr i without any significant variations in the nd isotopic composition the genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation mixing between different basic magmas and probable assimilation of ba rich oceanic crust major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution the overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation high 87sr 86sr i of the trachytes is interpreted to reflect interaction with an ocean derived component probably during assimilation of hydrothermally altered oceanic crust boulders of amphibole bearing monzonites and monzogabbros found to the northwest of dome carva are thought to represent intermediate magma composition that formed at depths but did not erupt

is the ordos basin floored by a trapped oceanic plateau abstract the ordos basin in china has about 10 km of neoarchean to quaternary sediments covering an enigmatic basement of uncertain origin the basement is tectonically stable has a thick mantle root low heat flow few earthquakes and has been slowly subsiding for billions of years the basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition and is similar to some other cratons world wide and also to several major oceanic plateaus it was accreted to the amalgamated eastern block and central orogenic belt of the north china craton ncc in the paleoproterozoic then involved in several proterozoic tectonic events including being over thrust by an accretionary orogen and intruded by andean arc related magmas and then involved in a continent continent collision during amalgamation with the columbia supercontinent thus the basement rocks are deformed metamorphosed to granulite facies and determining their initial origin is difficult we suggest that the data is consistent with an origin as an oceanic plateau that accreted to the ncc and later experienced different episodes of differentiation associated with later subduction and collisions formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases but also re activated and de cratonized in some cases in asia where they have been affected by younger subduction hydration slab roll back and melt peridotite reactions this suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons and de cratonization is not only influenced by younger tectonic activity

climate induced formation of a closed basin great divide basin wyoming large closed basins are often associated with regions of active tectonics in contrast we provide the example of the great divide basin in southern wyoming where climate change through its impact on erosion and fl exure provides the primary mechanism of basin closure two and three dimensional fl exural models that incorporate the effects of local basin abandonment due to aridity as well as minor extrabasinal extension demon strate that basin closure with 40 m of base ment relief tilt could have been achieved in the great divide basin under reasonable assumptions of sediment rock density and fl exural rigidity the primary drive for ba sin abandonment was insuffi cient discharge that retarded river downcutting continued erosion in the surrounding drainage led to fl exural rebound against which erosion by the outlet from the great divide basin was unable to compete eventually the basin became detached and isolated from the sur rounding drainage and a few tens of meters of differential tilt developed between the ba sin fl oor and spillover point an extra amount of tilt up to a few meters could have been at tained by rift shoulder effects associated with minor nearby extension outside of the basin to the north hence closure of the great di vide basin took place with no internal fault ing nor did it require any extrabasinal tec tonic activity to force basin closure hence basin closure and or drainage reorganization need not record tectonic activity in all cases differential erosion provides an alternative hypothesis that does not refl ect local tectonic timing our model of the evolution of the great divide basin illustrates a mechanism by which basins can become closed by cli matic effects alone

the dynamics of kerguelen plateau magma evolution new insights from major element trace element and sr isotope microanalysis of plagioclase hosted in elan bank basalts abstract the kerguelen plateau and broken ridge in the southern indian ocean together represent one of the most voluminous large igneous provinces lips ever emplaced on earth a scientific objective of ocean drilling program odp leg 183 was to constrain the post melting magma evolution of kerguelen plateau magmas in an effort to better understand this evolution isotopic and trace element analysis of individual plagioclase crystals hosted within two kerguelen plateau basalts recovered from elan bank were undertaken previous whole rock studies established that the two host basalts investigated in this study are samples of crustally contaminated lower group and relatively uncontaminated upper group basalt plagioclase phenocrysts from the uncontaminated basalt are dominantly normal zoned and exhibit a 87sr 86sri range of 0 704845 0 704985 which overlaps uncontaminated group whole rock values previously reported plagioclase crystals from the contaminated basalt are dominantly reverse zoned and exhibit a 87sr 86sri range of 0 705510 0 705735 which all lie within contaminated group whole rock values previously reported there are no systematic within crystal core to rim variations in 87sr 86sri from either group with the exception that contaminated group crystal rims have overall less radiogenic 87sr 86sri than other zones these observations indicate that crustal assimilation occurred before the formation of unit 10 plagioclase phenocrysts which is supported by parent magma trace element abundance data inverted using carefully calculated partition coefficients trace element diffusion modeling indicates that the upper group basalt unit 4 experienced a more vigorous eruptive flux than the lower group basalt unit 10 we suggest that plagioclase phenocrysts in both the upper and lower group basalts originated from the shallowest section of what was likely a complex magma chamber system we contend that the magmatic system contained regions of extensive plagioclase dominated crystal mush crustal assimilation was not a significant ongoing process in this portion of the elan bank magmatic system both basalts exhibit compelling evidence for remobilization and partial resorption of crystalline debris e g reverse zoned crystals glomerocrysts we suggest unit 4 and 10 magmas ascended different sections of the elan bank magma system where the unit 10 magmas ascended a section of the magma system that penetrated a stranded fragment of continental crust

provenance of proterozoic garnet biotite gneiss recovered from elan bank kerguelen plateau southern indian ocean at elan bank of the kerguelen plateau in the southeast indian ocean leg 183 of the ocean drilling program recovered clasts of garnet biotite gneiss in a fluvial conglomerate intercalated with basalt flows u pb and pb pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 ma which is much older than the surrounding indian ocean seafloor these dates show that old continental crust resides in the shallow crust of the oceanic kerguelen plateau and that the breakup of gondwana dispersed continental fragments into the nascent indian ocean lithosphere

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka