mariana trough basalts mtb trace element and srnd isotopic evidence for mixing between morb like and arc like melts abstract mariana trough basalt mtb glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal morb and basaltic rocks from the active northern mariana island arc miab the chemical variation is observed in trace elemental abundances and ratios such as lil lil and lil hfs mtb glasses with high lil hfs and ba sm ratios and low k rb k ba and sm nd ratios have more enriched sr and nd isotopic compositions comparison of the sr nd isotopic compositions of mtb and miab suggests that the source region within the mantle wedge is heterogeneous the diverse trace element and isotopic compositions of mtb glasses both within and between dredge sites near 18 n imply small scale source heterogeneity correlation between sm nd and143nd 144nd of the mtb glasses is interpreted as due to recent binary mixing rather than closed system evolution of a common homogeneous source mixing of melts at or near the source region between a mantle component with long term lree and lil element depletion morb like and a relatively enriched component with lower integrated143nd 144nd arc like is suggested by trends of the mtb data on ratio ratio ratio element and element element plots

an evaluation of volcanic cloud detection techniques during recent significant eruptions in the western ring of fire we examined recent volcanic cloud events in the western pacific and indonesian area to validate the performance of remote sensing techniques used to support the international airways volcano watch iavw five events were considered during which eruptions from eight volcanoes injected ash into the upper troposphere or lower stratosphere for one of the eruptions at miyakejima japan at least five aircraft encountered volcanic ash clouds and the cost to three operators alone exceeded us 12 000 000 in aircraft repairs diversions and lost operating time we performed reverse absorption and pattern analysis using gms 5 vissr modis and avhrr data and we examined toms so2 and aerosol index data surface based observations pilot reports and dispersion model output our results verify that the introduction of reverse absorption using the geostationary gms 5 platform significantly enhanced our capacity to monitor volcanic ash clouds in the region in one case we tracked an eruption cloud for approximately 80 h the primary impediment to remote monitoring is the presence of overlying cloud or substantial amounts of ice within the volcanic clouds toms data showed success in identifying volcanic clouds during these conditions but was limited by the infrequency of observations more effective future operation of the iavw relies on developing complementary methods of volcanic cloud remote sensing and greatly increasing the amount and quality of available surface and air observations including observations of precursor activity an understanding of the likely future limitations of remote sensing techniques will aid in the refining of iavw procedures

the north fiji basin basalts and their magma sources part ii sr nd isotopic and trace element constraints abstract sr nd isotope and trace element data are reported for basalts from the north fiji basin nfb nfb basalts are characterized by extreme variations in isotopic ratios and trace element abundances which are related to mantle heterogeneities values of 87 sr 86 sr for basalts from the northern segments n160 and triple junction range from 0 7029 to 0 7041 whereas 143 nd 144 nd values vary from 0 51281 to 0 51313 most of the basalts from these segments are characterized by strong relative enrichments in rb ba sr k nb ta la ce and ti that are comparable to oib components the central segments n15 and n s have 143 nd 144 nd ratios between 0 51298 and 0 51363 and 87 sr 86 sr ratios between 0 7029 and 0 7033 and are depleted in large ion lithophile light rare earth and high field strength elements similar to n morb covariation between trace element and isotopic ratios among nfb basalts supports a model in which melts from the nfb rift system are derived by mixing of oib type and depleted n morb mantle components the sr nd isotopic and trace element variability indicates that the nfb basalt source is heterogeneous on the scale of individual melt batches

a 40 50 day oscillation in sea surface temperature along the central california coast abstract previous studies of the tropical troposphere indicate the presence of a 40 50 day quasi periodic oscillation in zonal winds spectral analysis of extensive daily sea surface temperatures ssts reveals a significant increase in variance i e a quasi periodic oscillation between 40 and 50 days along the california coast as far south as santa barbara 34 3 n and at least as far north as port arena 39 n increases in the amplitude of this oscillation at granite canyon 36 n in some cases coincide with local el nino warming episodes the local surface winds which also contain a quasi periodic oscillation between 40 and 50 days are coherent with and lead sea surface temperature by approximately 10 days consistent with local wind forcing it is suggested that the 40 50 day oscillation in sea surface temperature off central california may be related to the 40 50 day oscillation in the tropical troposphere through atmospheric teleconnections between the tropics and mid latitudes

a re evaluation of the stratigraphy and evolution of the kohistan arc sequence pakistan himalaya implications for magmatic and tectonic arc building processes new field mapping and structural data combined with published geochemical data from the kohistan arc in the nw himalaya enable a re evaluation of the arc stratigraphy key lithological units and their relationships are more clearly defined permitting the construction of a revised magmatic tectonic history for the arc the oldest units are transitional oceanic type basalts which form the basement to the subduction related sequence arc type gabbroic sheets and plutons intrude the oceanic basalts together these form the kamila amphibolite belt metasediments and basaltic lavas were deposited within an extensional basin onto the kamila amphibolite belt basement this sequence exposed across the arc forms a distinct stratigraphic unit which is formally defined here as the jaglot group sediment charged turbidity currents transported material into the basin whilst submarine eruptions contributed the basaltic component this period of extension culminated in the eruption of high mg boninites of the chalt volcanic group which overlie the rocks of the jaglot group the earliest granitoids of the kohistan batholith predate suturing and intrude the jaglot and chalt sequences at c 100 ma kohistan sutured to asia suturing being accompanied by thickening of the arc with the development of major intra arc shear zones and a penetrative regionally developed steep cleavage at c 85 ma intra arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the chilas complex which clearly intrudes the kamila amphibolite belt to the south and the jaglot group to the north the chilas complex has been regarded as part of the pre suturing juvenile arc sequence field evidence summarized here show this to be not so heat advection associated with emplacement of the complex caused amphibolite facies regional metamorphism melting of the lower arc and plutonism some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 ma before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 ma at least three phases of extension and rifting each separated by short lived phases of compression charac terized arc evolution much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone

early cretaceous subvolcanic calc alkaline granitoid magmatism in the nubra shyok valley of the shyok suture zone ladakh himalaya india evidence from geochemistry and u pb shrimp zircon geochronology abstract the lithounits constituting the ladakh himalaya are exposed along the indus and shyok sutures zones of northwest himalaya the shyok suture zone ssz in northern ladakh represents a highly tectonized zone of a back arc basin which is mainly composed of volcano sedimentary formations shyok and khardung formations intimately associated with intrusive granitoids in the nubra shyok valley of the ssz calc alkaline granitoids of batholithic dimension are exposed in the tirit region referred herein as the tirit granitoids which are intrusive evidently into the shyok volcanic rhyolite rocks belonging to the shyok formation in this valley the northern margins of granitoids of the ladakh batholith can also be found intrusive into the metasediments shale slate and metavolcanics of the shyok formation the compositions and crystallization pressures 66 to 91 mpa of amphiboles in the intrusive tirit granitoid corroborate a calc alkaline nature and solidification of tirit granitoid melt at subvolcanic level equivalent to a minimum of 2 5 km to a maximum of 3 5 km thick overburden of shyok volcanics u pb shrimp zircons from the tirit granitoids have yielded mean crystallization ages of 109 4 1 1 ma and 105 30 0 80 ma which strengthen the idea of early cretaceous subduction beneath the karakoram terrain inherited older zircon cores 278 393 ma 476 519 713 952 ma and 1933 ma suggest a contribution from heterogeneous palaeozoic and proterozoic sources in the generation of the tirit granitoids similar to those observed elsewhere in the karakoram kohistan region a mean crystallization age 105 30 0 80 of zircons in the tirit granitoid hosting xenoliths of porphyritic volcanics places a minimum eruption age of ca 105 ma for the shyok volcanics the ladakh granitoid tirit granitoids and porphyritic volcanic xenolith belong to a calc alkaline series a mean crystallization age 67 32 0 66 ma for zircon in the ladakh granitoid implies that the shyok metasediments shale slate were deposited and metamorphosed prior to ca 68 ma present and earlier determined ages on tirit granitoids revealed that the subduction related calc alkaline magmatism in the nubra shyok valley of the ssz prevailed between 110 and 68 ma and hence a minimum age of early cretaceous can be suggested for development of the ssz

large scale geometry offset and kinematic evolution of the karakorum fault tibet the total offset lifespan and slip rate of the karakorum fault zone kfz western tibet are debated along the southern fault half ongoing oblique slip has exhumed dextrally sheared gneisses intruded by synkinematic leucogranites whose age v23 ma u pb on zircon indicates that right lateral motion was already in progress in the late oligocene ar ar k feldspar thermochronology confirms that rapid cooling started around 12 ma likely at the onset of the present dextral normal slip regime correlation of suture zones across the fault requires a total offset greater than 250 km along the active northern fault branch an average long term slip rate of 1 0 3 cm yr is inferred assuming that this offset accrued in a time span of 23 34 ma southwest of the ladakh karakorum range the large scale boudinage of ophiolitic units suggests that an offset of several hundreds of kilometers exists along another southern branch of the kfz towards the southeast in the mount kailas region the fault zone does not end at gurla mandatha but continues eastwards as a transpressive flower structure along the indus tsangpo suture our new data thus suggest that the kfz contributed to absorb hundreds of kilometers of india asia convergence

ash from changbaishan millennium eruption recorded in greenland ice implications for determining the eruption s timing and impact major volcanic eruptions can impact on global climate by injecting large quantities of aerosols and ash into the atmosphere that alter the radiative balance and chemical equilibrium of the stratosphere the millennium eruption of tianchi paektu china north korea was one of the largest late holocene eruptions uncertainty about the precise timing of the eruption has hindered the recognition of its climate impact in palaeoclimate and historical records here we report the compelling identification of the eruption s volcanic signal in greenland ice cores through the association of geochemically characterized volcanic glass represented in by bimodal populations that compare with proximal material from the source eruption the eruption most probably occurred in the a d 940s 7 years after the eldgja eruption on iceland we examine the eruption s potential for climate forcing using the sulfate records from the ice cores and conclude that it was unlikely to have had a global or extraregional impact

geological evidence against the shyok palaeo suture ladakh himalaya evolution of the ladakh himalaya is related to the cretaceous tertiary subduction of the indian oceanic plate below the southern margin of the eurasian plate along the indus suture zone1 11 the plate tectonic reconstruction suggests two stages of subduction due to the presence of a pre cretaceous suture along the shyok valley12 13 this hypothesis is based on there being ophiolite belts in the indus and shyok valleys the indus suture zone is well established but there have been insufficient data on the shyok suture to confirm the existence of the palaeo suture i have studied the saltoro hills in the western part of the shyok valley and report here the salient geological features with a few exceptions the igneous and sedimentary rocks indicate that the indus and shyok sutures had the same cretaceous oligocene history however the volcanic eruption during the deposition of post oligocene molasse in the saltoro hills of the shyok valley is the youngest igneous activity in the whole of the ladakh himalaya hence the geological evidence described here does not accord with the theory of palaeo suture

tectonic framework of the indus and shyok suture zones in eastern ladakh northwest himalaya abstract three different phases of plutonic volcanic activity viz 1 1 cretaceous volcanics of the dras and luzarmu formations 2 2 early tertiary granitoids of the ladakh plutonic complex 3 3 oligocene acid volcanics and dykes of the khardung formation have been recognised in the ladakh magmatic arc the dras formation of the indus suture zone and the luzarmu formation of the shyok suture zone were one stratigraphic unit into which granitoids of the ladakh plutonic complex intruded the rocks of the shyok suture zone are interpreted to represent the tectonised remainder of a marginal back arc basin the closure of the marginal basin occurred at the eocene oligocene time boundary and crustal shortening was responsible for generation of neogene 2 mica granites of the karakoram plutonic complex and regional metamorphism of the pangong tso group

satellite data empirical orthogonal functions and the 1997 1998 el nino off california abstract the features of the 1997 1998 el nino event were analyzed by empirical orthogonal functions eof statistical methods applied to the remotely sensed sea surface temperature anomalies ssta measured by avhrr radiometers anomalies of water circulation derived from sea surface height anomalies ssha measured by topex poseidon radar altimeter and meteorological information air temperature upwelling index and wind stress curl eof statistics demonstrated the features of an el nino event during the second half of 1997 and the first half of 1998 with sea level elevated along the coast and with ssha gradients indicating a retarding of both the equatorward california current and the alongshore poleward southern california countercurrent the positive sst anomaly developed first in the southern california bight and then in the zone of upwelling to the north of point conception the anomalies of upwelling index and the wind stress curl pattern also changed during the el nino event but these changes occurred later than hydrographic variations and were too weak to explain the observed changes in ssta and ssha we conclude that off central and southern california oceanic teleconnection i e the consequences of propagation northward of coastally trapped downwelling waves was responsible for the 1997 1998 el nino event

dextral transpression and late eocene magmatism in the trans himalayan ladakh batholith north india implications for tectono magmatic evolution of the indo eurasian collisional arc the trans himalayan ladakh batholith is a result of arc magmatism caused by the northward subduction of the tethyan oceanic lithosphere below the edge of the eurasian plate the batholith dominantly consists of calc alkaline i type granitoids which are ferromagnetic in nature with the presence of magnetite as the principal carrier of magnetic susceptibility the mesoscopic and magnetic fabric are concordant and generally vary from wnw ese to ene wsw for different intrusions of ferromagnetic granites in different parts of the batholith strike of magnetic fabric is roughly parallel with the regional trend of the ladakh batholith in the present study area and is orthogonal to the direction of india eurasia collision in khardungla and changla section the magnetic fabric is distributed in a sigmoidal manner it is inferred that this sigmoidal pattern is caused by shearing due to transpression induced by oblique convergence between the two plates u pb zircon geochronology of a rhyolite from the southern parts of the batholith gives a crystallization age of 71 7 0 6 ma coeval with 68 ma magmatism in the northern parts of the batholith the central part of the batholith is characterized by s type two mica granites which gives much younger age of magmatism at 35 5 0 5 ma the magnetic fabric of these two mica granites is at a high angle to the regional trend of the batholith it is proposed that these two mica granites were emplaced well after the cessation of subduction and arc magmatism along fractures that developed perpendicular to the regional strike of the batholith due to shearing

integrated satellite observations of the 2001 eruption of mt cleveland alaska abstract satellite data were the primary source of information for the eruption of mt cleveland alaska on 19 february and 11 and 19 march 2001 multiple data sets were used pre syn and post eruption to mitigate the hazard and determine an eruption chronology the 19 february eruption was the largest of the three resulting in a volcanic cloud that formed an arc over 1000 km long moved to the ne across alaska and was tracked using satellite data over more than a 50 h period the volcanic cloud was concurrently detected on the goes avhrr and modis data at various times and their respective signals compared all three sensors detected a cloud that had a very similar shape and position but there were differences in their areal extent and internal structural detail goes data showed the largest volcanic cloud in terms of area probably due to its oblique geometry modis bands 31 and 32 which are comparable to goes and avhrr thermal infrared wavelengths were the least effective single channels at detecting the volcanic cloud of those investigated modis bands 28 29 31 and 32 modis bands 28 and 29 detected the largest volcanic clouds that could easily be distinguished from weather clouds of the split window data modis bands 29 minus band 32 detected the largest cloud but the band 31 minus band 32 data showed the volcanic cloud with the most internal structural detail the puff tracking model accurately tracked the movement and predicted the extent and shape of this complex cloud even into areas beyond satellite detection numerous thermal anomalies were also observed during the eruption on the twice daily avhrr data and the high spatial resolution landsat data the high resolution radarsat data showed that the avhrr thermal anomalies were due to lava and debris flow features and a newly formed fan along the west coast of the island field observations and images from a hand held forward looking infrared radiometer flir showed that the flow features were a a lava debris flows and a warm debris fan along the west coast real time satellite data were the primary tool used to monitor the eruption track changes and to mitigate hazards high resolution data even though coverage is infrequent were critical in helping to identify volcanic processes and to compile an eruption chronology

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

the dras arc two successive volcanic events on eroded oceanic crust abstract the dras arc is recognized as a volcanic arc system in the western part of the indus suture zone and it constitutes the link between the ladakh batholith and the kohistan arc this study is based on detailed mapping of the area between dras kargil and sanku which revealed the following 1 1 the ultramafics of dras and thasgam can be followed across the suru dras ridge and are not intrusive into the arc volcanics but instead constitute the most probably oceanic substratum of these volcanics 2 2 successive volcanic events are distinguished 2 1 a dras i is a variable volcaniclastic series rich in slates and carbonates which can probably be assigned to the albo cenomanian as dated by orbitolines this series is intruded by gabbro diorite and granite and is deformed essentially in the northern part it is unconformably overlain by 2 2 b the dras ii pyroclastics which grade southward into volcanic breccia and thus enable the location of the centres of volcanic activity during this younger period

a jurassic cretaceous island arc in the ladakh himalayas abstract the ladakh mesozoic ophiolite belt western himalaya contains a pile of volcanic thrust sheets dras unit which differ significantly in structure and composition from the ophiolitic melange zones the dras unit is composed of pillow lavas doleritic sills very irregular basaltic basaltic andesites and dacitic flows intercalated with pyroclastics volcanoclastic sediments and radiolarian cherts according to fossil evidence this volcanism must have been active between upper jurassic and upper cretaceous the presence of relict primary minerals such as magnesiochromite clinopyroxene hastingsitic hornblende and ti magnetite as well as distinctive bulk chemistries suggests that the volcanics belong to island arc tholeiite and to calc alkaline rock series typical of present island arcs in the caribbean and pacific model calculations incorporating probed phenocryst phases indicate that in addition to olivine clinopyroxene and plagioclase amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt the latter process must have taken place at about 1000 c and at moderate depth of 5 15 km within or underneath the island arc today hornblende bearing mafic cumulates appear in the vicinity of kargil within and close to the dras volcanics in a sr evolution diagram the dras volcanics have yielded a pseudo isochron with a low initial 87 sr 86 sr ratio of 0 7035 0 0003 which is in the same range as the mean of modern island arc volcanics however a geologically unrealistic age of 263 m y is obtained from the slope of this isochron the upper mantle is regarded as the source material for the island arc tholeiitic magmas enrichment in k ba sr and lree supports the involvement of components derived from dehydration or incipient melting of subducted tethyan oceanic crust in the mantle

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

intraseasonal oscillations in sea level along the west coast of the americas abstract hourly observations of coastal sea level at stations from peru to british columbia are analyzed for low frequency content a space time contour plot of sea level from four years of data during the 1971 75 period shows the meridional structure of the seasonal cycle and interannual variability associated with the 1972 73 el nino oscillations with intraseasonal periods of 36 73 days are also evident coherent over alongshore distances of several thousand kilometers further investigation using spectral methods and empirical orthogonal function analysis in the frequency domain reveals in particular that intraseasonal sea level variability has a peak in spectral density along the coasts of south america central america and mexico with high coherence from near the equator tumaco 2 n north to central california 34 n and south to at least callao 12 s phase propagation north of the equator is poleward at 150 200 kilometers day atmospheric pressure alongshore wind stress and wind stress cu

the 1989 1990 eruption of redoubt volcano alaska impacts on aircraft operations abstract the december 1989 june 1990 eruption of redoubt volcano affected commercial and military air operations in the vicinity of anchorage alaska these effects were due to the direct impact of volcanic ash on jet aircraft as well as to the rerouting and cancellations of flight operations owing to eruptive activity between december and february five commercial jetliners were damaged from ash encounters the most serious incident took place on december 15 1989 when a boeing 747 400 aircraft temporarily lost power of all four engines after encountering an ash cloud as the airplane descended for a landing in anchorage while there were no injuries to passengers the damage to engines avionics and aircraft structure from this encounter is estimated at 80 million four additional encounters between jet aircraft and redoubt ash clouds occurred in the anchorage area on december 15 and 16 1989 and february 21 1990 none resulted in engine failure two additional encounters took place on december 17 1989 when jet airliners encountered the redoubt cloud over west texas at the time of these encounters the cloud was up to 55 hours old and had traveled in excess of 2 900 nautical miles 5 300 km following the december 15 encounters anchorage international airport remained open however most airline companies canceled operations for up to several days as communications between federal agencies and airlines improved and as a better understanding of the nature and behavior of ash rich eruption clouds was achieved most airlines resumed normal service by early january 1990 the resulting loss of revenue at anchorage international airport during several months following the eruption is estimated to total 2 6 million the impact on general aviation and military operations consisted mostly of cancellation and rerouting of flights

local and remote forcing of sea surface temperature in the coastal upwelling system off chile an analysis is presented of records of sea surface temperature sst sea level and winds collected during the period 1991 1995 along the coast of northern and central chile sea level also from the peru coast in 1991 1994 and of sst from remote sensing during the austral summers of 1991 and 1992 off northern chile large interannual fluctuations of sst and sea level off northern chile are linked to vertical displacements of the equatorial thermocline during el nino la nina events in the intraseasonal 50 day band sst and sea level are intimately related all along the coast whereby sst anomalies lag sea level perturbations by about 12 days such sea level fluctuations are due to free coastal trapped waves ultimately forced via equatorial kelvin waves by 50 day wind events in the western and central tropical pacific these results and those of the remote sensing analysis suggest significant remote forcing of sst fluctuations in the coastal zone off chile by the action of the coastal trapped waves off central chile sst is also related to local wind stress in the intraseasonal band simulations with a simple local wind driven upwelling entrainment model capture rather well observed synoptic and intraseasonal oscillations of sst there off northern chile however this simple model is not successful in simulating observed sst an attempt to include effects of coastal trapped waves into this simple one dimensional model did not improve model fit to data at either location an explanation for the apparent strong influence of these waves on sst along this coast should therefore be sought in other effects not considered by this model such as cross shelf and alongshore advection

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

the northern suture pakistan margin of a cretaceous island arc the northern suture is a fault separating the cretaceous kohistan island arc terrain northwest himalayas from palaeozoic sediments of the asian plate to the north the kohistan arc includes volcanic and sedimentary rocks andesitic lavas tuffs volcaniclastics slates and limestones metamorphosed to greenschist facies and intruded by the two phase kohistan batholith asian continental margin sediments are mainly of shelf type are variably metamorphosed and intruded by the karakoram batholith the northern suture is a zone of melange from 150 m to 4 km wide and contains blocks of volcanic greenstone limestone red shale conglomerate quartzite and serpentinite in a slate matrix it has a strong planar fabric but in many places bedding is preserved in blocks and matrix and depositional rather than tectonic contacts are seen between the two the melange is inferred to be an olistostrome largely derived from the kohistan arc formed in a small back arc basin between kohistan and asia limestone blocks in the melange are dated as aptian albian post tectonic intrusions yield radiometric ages from 111 to 62 ma the northern suture therefore probably formed in the early late cretaceous during closure of the back arc basin the tethys ocean lay south of kohistan where the main mantle thrust represents the westward continuation of the indus tsangpo suture

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

record of volcanism since 7000 b c from the gisp2 greenland ice core and implications for the volcano climate system sulfate concentrations from continuous biyearly sampling of the gisp2 greenland ice core provide a record of potential climate forcing volcanism since 7000 b c although 85 percent of the events recorded over the last 2000 years were matched to documented volcanic eruptions only about 30 percent of the events from 1 to 7000 b c were matched to such events several historic eruptions may have been greater sulfur producers than previously thought there are three times as many events from 5000 to 7000 b c as over the last two millennia with sulfate deposition equal to or up to five times that of the largest known historical eruptions this increased volcanism in the early holocene may have contributed to climatic cooling

magmatic and tectonic evolution of the ladakh block from field studies abstract the ladakh block is in an intermediate position between the indian plate in the south and the karakorum tibetan plate in the north to the west it is separated from the kohistan arc by the nanga parbat syntaxis to the east it is cut off from the lhasa block by the gartok nubra fault present data together with previously published results show that the ladakh block consists of an island arc in the south and a calc alkaline batholith in the north with remnants of a continental crust migmatitic gneisses and metasedimentary sequences such as quartzites and metapelites interbedded with basaltic volcanics and overlain by thick platform carbonates were found as evidence of a continental crust remnants of megafossils megalodon and lithiotis within the high grade metamorphic marbles indicate a probable age of late triassic to early jurassic these sediments were intruded by a faintly layered hornblende gabbro which preceded the calc alkaline magmatic episode gabbro and gabbronorites are found as roof pendants and large inclusions within diorites and granodiorites the major part of the batholith consists of granodiorite and biotite granite plutons ranging from late cretaceous to tertiary associated with the intrusives are volcanic rocks with trachyandesite to alkalibasalt and basalt andesite to rhyolite compositions garnet bearing leucogranites succeeded the emplacement of the major plutons the magmatic stage ended finally by intense fracturing and injections of ne sw striking andesitic dykes the southernmost unit of the ladakh block is formed by oceanic crust with serpentinized peridotite and hornblende gabbro and is covered by volcanics of an island arc type dras volcanics these units are intruded by gabbronorite as well as middle and upper cretaceous granodiorite and coarse grained biotite granite in a plate tectonic view the ladakh block represents a transitional sector between the pure island arc of kohistan in the west and the andean type margin of the lhasa block in the east

the latitude height structure of 40 50 day variations in atmospheric angular momentum abstract quasi periodic variations in the relative angular momentum of the atmosphere on time scales of around 40 50 days have been observed by langley et al 1981 a description of the two dimensional latitude height structure of the winds responsible for these changes is constructed here from five years of nmc twice daily global analyses using cross spectral and amplitude phase eigenvector techniques we find these variations are associated with wavelike motions in the tropical upper troposphere that propagate and downward in phase within the tropics a coherently connected midlatitude northern hemisphere component is also present whose phase is essentially independent of height we believe the tropical component to be the zonally averaged part of the motions described by madden and julian 1971 1972 the northern hemisphere midlatitude component may be a direct response to the tropical motions or both motions may be the common response to an as yet unidentified tropical forcing

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

k ar and ar ar geochronology of the himalayan collision in nw pakistan constraints on the timing of suturing deformation metamorphism and uplift mica and hornblende k ar and ar ar data are presented from each of the three crustal components of the himalayan collision zone in north pakistan the asian plate the kohistan island arc and the indian plate together with u pb and rb sr data published elsewhere these new data 1 date the age of suturing along the northern suture which separates kohistan from asia at 102 85 ma 2 establish that the basic magmatism in kohistan which postdates collision along the northern suture predates 60 ma and that the later granite magmatism spanned a range of 60 25 ma 3 show that uplift amounts within kohistan are greater toward the nanga parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 ma to a current figure of about 5 5 mm a year 4 show that much of southern kohistan had cooled to below 500 c by 80 ma and that the major deformation which imbricated kohistan probably predated 80 ma and certainly predated 60 ma and was related to the kohistan asia collision rather than the kohistan india one 5 imply that uplift along the hunza shear in the asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 ma and was associated with breakback thrusting in the hanging wall of the main mantle thrust 6 suggest that the indian plate has a lengthy pre himalayan history with an early metamorphism at about 1900 ma major magmatism at 500 550 ma and early jurassic lithospheric extension or inversion and 7 show that the indian plate rocks were metamorphosed shortly after the collision within kohistan which occurred at circa 50 ma and subsequently cooled back through 500 c at circa 38 ma and 300 c at 30 35 ma with ages of cooling through 200 and 100 c as determined by fission track data locally controlled by nanga parbat related uplift tectonics

metamorphic evidence for an inverted crustal section with constraints on the main karakorum thrust baltistan northern pakistan abstract the shyok suture zone separates rocks in the asian plate from rocks in the kohistan ladakh island arc in baltistan this suture has been reactivated by the late break back main karakorum thrust mkt the p t histories of metamorphic rocks both north and south of the mkt have been determined in an effort to place constraints on the tectonic history of this zone the terranes north and south of the mkt have different unrelated metamorphic histories rocks from the kohistan ladakh island arc south of the mkt have undergone a static low p 2 4 kbar c 500 c thermal metamorphism the p t paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc this metamorphism pre dates folding and deformation of these rocks rocks in the karakorum metamorphic complex north of the mkt have experienced a complex deformational and metamorphic history prograde metamorphic isograds have been deformed by subsequent south verging folding and by gneiss dome emplacement however decompression metamorphic reactions occurred during nappe emplacement higher pressure rocks are associated with higher level nappes creating an inverted pressure metamorphic sequence 8 9 kbar rocks over 5 6 kbar rocks there is little variation in temperature with structural level 550 625 c these two different terranes have been juxtaposed after metamorphism by the late south directed mkt

age and origin of post collision baltoro granites south karakoram north pakistan insights from in situ u pb hf and oxygen isotopic record of zircons abstract origin of post collision plutonism is critical to understand the tectonothermal evolution of the over thickened continental crust in collision zones this has proven difficult to reconcile with the conventional whole rock geochemical and field based studies alone we report in situ study of zircon u pb hf and o isotopes from five samples of the baltoro plutonic unit bpu in south karakoram the plutonic unit is the western part of the southern asian margin of the india asia convergent zone baltoro granites and a biotite rich enclave yielded similar and overlapping u pb ages ranging from 26 to 15 ma hafnium isotopic composition ehf 0 is very heterogeneous ranging from 17 1 to 4 4 while the oxygen isotopic composition of the granites is homogeneous with mean 18 o ranging from 7 2 to 9 4 based on u pb geochronology and hf o isotopic composition the involvement of two main sources is suggested 1 cretaceous calc alkaline karakoram crust and 2 karakoram gneisses moreover possible involvement of metasomatized asian lithospheric mantle is supported by elevated oxygen composition of granites and identical hf composition of biotite rich enclave to the mantle derived baltoro lamprophyre however direct contribution from juvenile pristine mantle is unlikely as no juvenile mantle type hf and oxygen values were obtained this also precludes the involvement of southward juvenile arc related component of kohistan ladakh batholith our new u pb and hf data are comparable to the mesozoic karakoram batholith miocene two mica leucogranites in the pangong range and magmatism from the lhasa terrane in south tibet suggesting a genetic link between the karakoram and the rocks to the east this magmatic event is best explained by lower crust partial melting promoted by both thermal equilibration following crustal thickening and heat advection by ultrapotassic magmas associated with the breakoff of the indian continental margin

sea level response to enso along the central california coast how the 1997 1998 event compares with the historic record abstract long term monthly sea level and sea surface temperature sst anomalies from central california show that during winter months positive anomalies are associated with el nino events and the negative ones with la nina events there is no significant impact on monthly mean anomalies associated with pacific decadal oscillations although there is a tendency for more extreme events and greater variance during positive decadal oscillations the very strong 1997 1998 el nino was analyzed with respect to the long term historic record to assess the forcing mechanisms for sea level and sst beginning in the spring of 1997 we observed several long period 30 days fluctuations in daily sea level with amplitudes of over 10 cm at san francisco california fluctuations of poleward long period alongshore wind stress anomalies awsa are coherent with the sea level anomalies however the wind stress cannot entirely account for the observed sea level signals the sea level fluctuations are also correlated with sea level fluctuations observed further south at los angeles and tumaco columbia which showed a poleward phase propagation of the sea level signal we suggest that the sea level fluctuations were to a greater degree forced by the passage of remotely generated and coastally trapped waves that were generated along the equator and propagated to the north along the west coast of north america however both local and remote awsa can significantly modulate the sea level signals the arrival of coastally trapped waves began in the spring of 1997 which is earlier than previous strong el nino events such as the 1982 1983 event

a cretaceous dike swarm provides evidence of a spreading axis in the back arc basin of the kohistan paleo island arc northwestern himalaya pakistan abstract a 100 4000 m wide and 15 km long dike swarm consisting of basalt and dolerite occurs at the base of the thelichi formation in the kohistan paleo island arc terrane north pakistan the dikes contain hornblende altered from diopsidic augite diopsidic augite relics ophitic to subophitic texture chlorite epidote sphene apatite zircon ilmenite titanomagnetite and magnetite the geochemistry reveals two groups of dikes 1 higher tio 2 2 74 3 50 wt na 2 o fe 2 o 3 and lower al 2 o 3 12 65 14 16 wt and mgo 3 73 5 04 wt 2 lower tio 2 1 24 2 05 wt na 2 o fe 2 o 3 and higher al 2 o 3 14 02 16 52 wt and mgo 3 98 7 52 wt the mgo contents 3 73 7 52 wt show a variation in the dikes from relatively primitive to more evolved compositions the dikes contain high amounts of both lile and hfse the major trace and rare earth elements data confirm the morb affinity and the back arc basin origin of the dike swarm the nw se orientation of the dike swarm and its 134 3 ma k ar age suggest the spreading axis of the back arc basin in the early cretaceous

oceanic variability off the central california coast abstract mesoscale variability off the central california coast is strongly influenced by coastal upwelling and related processes off point sur there is significant space time variability in oceanic properties over periods of days and distances of several tens of km however the internal density field averaged over space ca 120km alongshore and time ca 18 days reveals the expected characteristics of a coastal upwelling regime including an equatorward surface jet 25km to 40km offshore and weaker poleward flow below 150m and within 20km of the coast upper ocean circulation is strongly influenced by the bathymetry offshore to water depths of at least 1000m based on repeated horizontal grids of oceanographic stations horizontal correlation scales for temperature are of the order of 30km in the upper 100m hence a horizontal sampling rate of ca 10km is desirable vertical correlation scales for temperature are of the order of 100m hence a vertical sampling rate of ca 10m is desirable similarly there are indications of a temporal correlation scale of a few weeks hence a temporal sampling rate of ca a few days is desirable empirical orthogonal functions eofs in the cross shore vertical plane suggest the influence of coastally trapped motions within 40km of the coast with an intense alongshore near surface transient coastal jet located ca 20km offshore sudden spring transitions to coastal upwelling conditions are often pronounced in coastal temperature records and they may precede the offshore migration of the major upwelling front by at least a month or more the spring transitions occur over a period of about a week in 1980 the spring transition propagated poleward along the california coast at a speed of ca 64km d 1 during spring and summer movement of a coastal upwelling front often located between 15km and 50km offshore is a major contributor to cross shore variability the upwelling front meanders with an alongshore scale of about 80km a time scale of 30 to 40 days and an amplitude scale of 10km rms based on satellite infrared ir imagery the along shore mean position of the major upwelling front migrates offshore during spring and summer offshore displacement of the front may be caused by ekman transport over periods of days to weeks and by rossby wave dispersion over longer periods during summer a time dependent offshore scale based on rossby wave dynamics was more appropriate than the baroclinic rossby radius every three to four years during 1971 through 1984 el nino episodes produced interannual variability in coastal sea surface temperature sst off central california most of the warming occured during fall and winter el nino episodes were often followed by major spring transitions the recent 1982 1983 episode was 2 to 3 c warmer than the three previous episodes over the past 14 years and its influence lasted almost twice as long

petrology of indus river sands a key to interpret erosion history of the western himalayan syntaxis the indus river has been progressively transformed in the last decades into a tightly regulated system of dams and channels to produce food and energy for the rapidly growing population of pakistan nevertheless indus river sands as far as the delta largely retain their distinct feldspar and amphibole rich composition which is unique with respect to all other major rivers draining the alpine himalayan belt except for the brahmaputra both the indus and brahmaputra rivers flow for half of their course along the india asia suture zone and receive major contributions from both asian active margin batholiths and upper amphibolite facies domes rapidly exhumed at the western and eastern himalayan syntaxes compositionofindussandschangesrepeatedlyandmarkedlyinladakhandbaltistan indicatingoverwhelmingsedimentflux from each successive tributary as the syntaxis is approached provenance estimates based on our integrated petrographic mineralogical data set indicate that active margin units karakorum and transhimalayan arcs provide 81 of the 250f50 10 6 t ofsedimentsreachingthetarbelareservoireachyear partitioningofsuchfluxamongtributariesandamongsourceunitsallowsus to tentatively assess sediment yields from major subcatchments extreme yields and erosion rates are calculated for both the karakorum belt up to 12 500f4700 t km 2 year and 4 5f1 7 mm year for the braldu catchment and nanga parbat massif 8100f3500t km 2 yearand3 0f1 3mm year thesevaluesapproachdenudationratescurrentlyestimatedforsouthkarakorum and nanga parbat crustal scale antiforms and highlight the major influence that rapid tectonic uplift and focused glacial and fluvial erosion of young metamorphic massifs around the western himalayan syntaxis have on sediment budgets of the indus system detailed information on bulk petrography and heavy minerals of modern indus sands not only represents an effective independent method to constrain denudation rates obtained from temperature time histories of exposed bedrock but also

the 30 50 day mode at 850 mb during monex abstract the analyzed wind field at 850 mb during the summer monsoon experiment monex is subjected to a time series analysis to confirm the existence of a peak in the time range of 30 50 days having established that this study presents a mapping of the motion field for this time scale in this short paper we illustrate the steady meridional propagation of a train of troughs and ridges that seem to form near the equator and dissipate near the himalayas the meridional scale of this mode is around 300 km and its meridional speed of propagation is 0 75 latitude per day the amplitude of the wind for this mode is around 3 6 m s 1 the salient contributions here are the mapping and the demonstration of a very regular behavior of this mode its existence is here noted from a period well before the onset of monsoon i e from early may to late july three major storms during monex were noted to form within the trough line of this system and the period of major cessation of rains over the indian subconti

from extension to tectonic inversion mid cretaceous onset of andean type orogeny in the lhasa block and early topographic growth of tibet

geology of the onshore makran accretionary wedge synthesis and tectonic interpretation abstract this contribution is a synthesis of studies carried out since 2004 in the makran accretionary wedge exposed in se iran these studies were intended to improve our understanding of the behaviour of accretionary wedges and of the significance of the makran wedge in the frame of tethys tectonics reassessing the stratigraphy and structure of the iranian makran has allowed distinguishing four main litho tectonic units the lithological content of these four units is briefly described to discuss the sedimentary environments which record the general filling of an oceanic basin with clastic sediments derived from the north the petrology of ophiolites exposed in the backstop region to the north of the accretionary wedge itself demonstrates the existence of an oceanic basin formed in the jurassic late cretaceous to the north of continental slivers which had separated from central iran there is petrological evidence for a subduction related magmatic arc during the late cretaceous which sourced much of the clastic material in the cretaceous lower miocene turbidites present throughout the wedge the literature survey shows that the late cretaceous to present evolution of the pakistan part of the makran fold and trust belt is in its major attributes similar to that of the iranian makran these observations and results are discussed in a tectonic model that integrates knowledge on the oman mountains on the other side of the current oman subduction

middle cretaceous back arc formation and arc evolution along the asian margin the shyok suture zone in northern ladakh nw himalaya abstract the shyok suture zone of the ladakh palaeovolcanic arc is made of cretaceous volcanosedimentary formations intruded by granodioritic plutons field observations show a tectonic stacking of litho units from different volcanic arc and back arc environments in the western part skardu area the shyok suture zone can be subdivided into two groups 1 the northern group which consists of olistolith basaltic blocks and tuffs the basalts are lree depleted with a lile enrichment and a slight nb depletion suggesting a back arc basin origin 2 the southern group which consists predominantly of andesites that show lree enrichment a flat hree pattern strong nb ta depletion and lile enrichment the volcanic rocks of the southern group appear to have island arc tholeiite iat to calc alkaline affinities in the eastern part of the suture zone nubra shyok area albian to cenomanian age silicoclastic sediments predominate these sediments correspond to a large detrital platform built on the northern part of the ladakh arc at the top these sediments interlayer with basaltic to andesitic lavas these lavas appear to be more differentiated and calc alkaline in nature than the skardu southern group lavas but show similar volcanic arc affinities no evidence of a back arc basin was found in this area our data from these two areas shows a northwest southeast evolution from back arc to arc formations in northern ladakh opening of this back arc basin occurred on the northwestern side of the ladakh arc this back arc was progressively closing eastward the arc itself was more mature in the east resembling the southern tibetan continental arc and was characterised by more continental sedimentation subsequent himalayan tectonometamorphic evolution closure of the back arc basin and deformation along the shyok suture reflects an early asymmetrical geometry along the asian margin and kohistan ladakh arc

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

field relationships and geochemistry of pre collisional india asia granitoid magmatism in the central karakoram northern pakistan in the central karakoram range northern pakistan seven major granitic units which were emplaced prior to the eocene ca 50 ma india asia collision have been mapped and analysed geochemically the mid cretaceous hunza plutonic complex dominates the karakoram batholith in the west and is composed of quartz diorite granodiorite plutons which have been deformed in the south by later collision related thrusting the sost and khunjerab plutons intrude the palaeozoic early mesozoic sediments north of the batholith in the east pre collision magmatic units occur both north of k2 gneiss broad peak quartz diorite muztagh tower unit and south of hushe gneiss the batholith which is dominated by a miocene monzogranite leucogranite pluton baltoro unit the plutonic units to the north of the batholith are mid to late cretaceous with an age range of ca 115 80 ma whereas the hushe gneiss to the south is dominantly jurassic spanning the age range 200 145 ma all the pre collision magmatic units display similar chemical and isotopic characteristics major trace and ree variations in the hunza plutonic complex are controlled by high level fractionation of clinopyroxene hornblende plagioclase biotite ilmenite and allanite high lile hfse and lree hree ratios together with negative nb p and ti anomalies suggest an ultimate source in the mantle wedge above the subducting slab 87sr 86sr initial ratios 0 7055 0 7157 in the hunza plutonic complex can best be explained by contamination of a mantle derived magma by a crustal component with highly radiogenic sr all the units except the hushe gneiss are probably related to subduction during closure of a back arc basin between the karakoram to the north and the kohistan dras arc to the south during the mid to late cretaceous and correlate with widespread plutonism from the same period in kohistan ladakh and southern tibet along the southern margin of asia the hushe gneiss is probably related to an earlier northward directed subduction phase during the jurassic

changing source regions of magmas and crustal growth in the trans himalayas evidence from the chalt volcanics and kohistan batholith kohistan northern pakistan the kohistan batholith and chalt volcanics form a major part of the kohistan island arc terrane in northern pakistan they record some 70 80 ma of magmatism within the northwestern himalayan orogen the chalt volcanics are albian aptian mid cretaceous and currently available rbsr age data for the batholith indicate an intrusive age span of 102 to 30 ma the volcanics are composed of medium k calc alkaline andesites and low k high mg tholeiites some of which have boninitic and basaltic komatiitic chemistry three intrusive stages are recognised in the batholith 1 110 90 ma comprises low k trondhjemites and medium high k calc alkaline gabbro diorites with associated hornblendite cumulates 2 85 40 ma comprises low high k calc alkaline gabbro diorites with hornblendite cumulates granodiorites and granites and 3 circa 30 ma comprises biotite muscovite garnet leucogranites 87sr 86sr0 for the batholith range between 0 7039 and 0 7052 four magmatic source regions sce 1 sce 4 have been identified sce 1 is a variably metasomatised mantle wedge situated above an active subduction zone during stages 1 and 2 sce 2 is a harzburgite depleted with respect to major elements and incompatibles whilst retaining subduction related trace element ratios sce 2 melted during stage 1 within a fore arc position during the subduction of young hot oceanic crust to provide the necessary extra thermal input for harzburgite anatexis melts from sce 2 produced the high mg tholeiitic volcanics sce 3 melted during stage 1 to produce the low k trondhjemites and it is envisaged as incompatible element depleted primitive arc crust with a similar composition to the depleted chalt volcanics sce 4 melted at 30 ma during stage 3 as india underthrust eurasia frictional heating and dehydration of the indian plate caused crustal anatexis within the deep kohistan arc and these melts formed the stage 3 leucogranites the low 87sr 86sr initial ratios for the great bulk 95 of the batholith indicate that it represents a major juvenile addition to the continental crust this conclusion has important implications for crustal growth theories of island arc accretion and batholith underplating