minimum age of initiation of collision between india and asia north of everest based on the subsidence history of the zhepure mountain section the collision of india with asia one of the most profound tectonic events to have occurred in past 100 m y is thought to have had geological geochemical and climatological consequences of global extent surprisingly the age of initiation of this collision remains poorly constrained estimates range from the late cretaceous 65 ma to latest eocene 40 ma with little consensus in between this paper reviews the stratigraphic section preserved on zhepure mountain on the north flank of everest mount qomolongma and its implied subsidence history zhepure mountain lies about 65 km south of the indus yarlung zangbo suture and contains the most complete and youngest passive margin shelf sediments in the tethyan himalayas on the basis of the subsidence history of the preserved section there is no evidence of acceleration of the subsidence up to the youngest rocks therefore collision related loading and accelerated subsidence must post date the youngest sediments preserved which date from the earl

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

relicts of the early cretaceous seamounts in the central western yarlung zangbo suture zone southern tibet abstract despite more than 30 years studies in the yarlung zangbo suture zone yzsz southern tibet is still a region of discovery and the geological evolution of the neo tethys here remains controversial in this paper we present the new field observation petrography zircon u pb dating and geochemistry of the zhongba mafic rocks in the western segment of the yzsz zircon u pb analyses from a diabase using sims secondary ion mass spectrometry yielded a concordant age of 125 7 0 9 ma all the zhongba mafic rocks exhibit lree enrichment lan ybn 4 7 15 8 without eu anomalies eu 0 97 1 01 their primitive mantle normalized trace element patterns are similar to those of the average oib and hawaii alkaline basalts as well as other oib type rocks already reported in the central western yzsz these observations lead us to suggest the existence of a series of seamounts within the neo tethys during the early cretaceous these oib type rocks could be originated from 1 hotspot mantle 2 near ridge seamounts by melting induced mixing of two distinct mantle sources 3 late stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination combining literature data from the entire yzsz which include studies of the morb and iab type rocks and mantle peridotites we prefer the third explanation that the oib type rocks represent a late stage magmatic activity above an intra oceanic supra subduction zone within the neo tethys

acid and basic late neogene volcanism in central aegean sea its nature and geotectonic significance local eruptions of acid volcanic rocks occurred in the central aegean region around the miocene pliocene boundary rhyolites outcrop on the island of antiparos located in the central part of the attic cycladic massif the age of these volcanic rocks ranges from 4 0 to 5 4 m y and chemical and sr isotopic data suggest they were generated by partial melting of the continental crust at the same time along the border of the attic cycladic massif i e on the island of patmos and caloyeri local eruptions of naalkaline basalts occurred the whole of the eruptive activity is interpreted as an expression of the marked tensional tectonic phase which has affected the central aegean area since middle miocene the contrasting nature of the erupted volcanic rocks crustal and sub crustal is attributed to the different thermal state of the lithosphere beneath the two areas as emphasized also by the presence of a wide granitic belt of mainly miocene age which developed in the median sector of the attic cycladic massif

neogene ignimbrites of the nevsehir plateau central turkey stratigraphy distribution and source constraints abstract in anatolia turkey extensive calc alkaline volcanism has developed along discontinuous provinces from neogene to quaternary times as a consequence of plate convergence and continental collision in the nevsehir plateau which is located in the central anatolian volcanic province volcanism consists of numerous monogenetic centres several large stratovolcanoes and an extensive mainly neogene rhyolitic ignimbrite field vent and caldera locations for the neogene ignimbrites were not well known based on previous studies in the neogene ignimbrite sequence of the nevsehir plateau we have identified an old group of ignimbrites kavak ignimbrites followed by five major ignimbrite units zelve sarimaden tepe cemilkoy gordeles kizilkaya and two smaller less extensive ones tahar sofular other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as from bottom to top kavak zelve sarimaden tepe cemilkoy tahar gordeles sofular kizilkaya our stratigraphic scheme refines previous ones by the identification of the zelve ignimbrite and the correlation of the previously defined akkoy ignimbrite with the sarimaden tepe ignimbrite correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria 1 sedimentological characterisitics 2 phenocryst assemblage 3 pumice vesiculation texture 4 presence and characteristics of associated plinian fallout deposits and 5 lithic types the correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites volumes range between 80 km 3 and 300 km 3 for the major ignimbrites corresponding to 2500 10 000 km 3 in areal extent the major ignimbrites of the nevsehir plateau have an inferred source area in the derinkuyu tectonic basin which extends mainly between nevsehir and the melendiz dag volcanic complex the kavak ignimbrites and the zelve ignimbrite have inferred sources located between nevsehir and derinkuyu coincident with a negative gravity anomaly the younger ignimbrites sarimaden tepe cemilkoy gordeles kizilkaya have inferred sources clustered to the south between the erdas dag and the melendiz dag volcanic complex we found evidence of collapse structures on the northern and southern flanks of the erdas dag volcanic massif and of a large updoming structure in the sahinkalesi tepe massif the present day derinkuyu tectonic basin is mostly covered with quaternary sediments and volcanics the fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred probably coalesced or nested calderas within it

structural elements of the southern tethyan himalaya crust from wide angle seismic data a deep seismic common midpoint cmp profile shot across the southern margin of the tethyan himalaya as the first stage of project international deep profiling of tibet and the himalaya indepth imaged the top main himalayan thrust or mht and bottom moho of the indian continental crust underthrusting the himalaya we used portable seismographs to record the cmp profile shots at a wide range of offsets up to 155 km our short offset data corroborate the cmp profile data while our large offset data are dominated by a band of reflectivity we interpret the bright onset of this reflective band as the basal detachment of the south tibetan detachment system and a phase at the end of the reflective band as the mht we used the cmp profile first break data to model the uppermost 2 km in detail the depth of young extensional basins of the yadong rift system along the cmp profile was constrained to a maximum of 2 km yielding a throw of 4 6 km across the eastern flank of the rift and a rough estimate of approximately 1 5 east west extension by normal faulting in the tethyan himalaya the wide angle data were used to construct a crustal seismic velocity model down to the mht the south tibetan detachment system stds basal detachment reflector is observed dipping 12 5 nne from a depth of about 6 km beneath the surface under the south end of the cmp profile to a depth of 22 km then flattening to a dip of only 2 5 nne thus our observations suggest that the stds basal detachment is a deep rooted basement fault for the mht we observe a dip of 7 5 nne from 20 km depth below sea level at the crest of the high himalaya crystalline sheet to 36 km below sea level 40 km beneath the surface at a distance of about 70 km south of the indus yarlung suture iys from geometrical arguments we suggest that indian crust underthrusts the surface expression of the iys within the crust but does not form the lower crust of central and northern tibet

temperature depth curves and heat flow in central part of anatolia turkey abstract the objective of this study is to determine the crustal thermal regime of central anatolia province and its relationship with regional tectonics investigations include new temperature depth measurements in wells calculations of geothermal gradients from new and previously measured temperature logs and determinations of heat flow with terrain corrections if necessary the results provide us with a better understanding of the regional distribution of subsurface temperatures and heat flow within the study area although central anatolia province is bordered with tectonically active areas moderate heat flow values 64 16 mw m 2 are generally observed within the region in accordance with the relatively less tectonic activity two thermally anomalous zones are indicated in the vicinity of kirsehir and kizilcahamam with heat flow values of higher than 100 mw m 2 anomalous heat flow in these areas can best be explained by high radioactivity of basement rocks although this is not directly confirmed for the kizilcahamam anomaly the high elevation and recent volcanic activity in the region indicate that thermal activity may be present at lithospheric levels but have not yet reached up to crustal levels also no surface heat flow anomaly due to fault activity was observed at the section of the north anatolia fault within the study area

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

the yarlung zangbo paleo ophiolite southern tibet implications for the dynamic evolution of the yarlung zangbo suture zone the yarlung zangbo suture zone a major geological structure in tibet is well known as the locus of tectonic emplacement of the tethyan ophiolites current models propose that most of the east tethyan oceanic lithosphere was subducted within a single subduction zone active during the middle or late cretaceous which was completed during the paleogene collision between india and asia the early cretaceous sedimentary giabulin formation in southern tibet includes conglomeratic members that contain ultramafic and mafic plutonic pebbles as well as radiolarian chert clasts that record the erosion of oceanic lithosphere involved in a subduction event which occurred earlier than previously believed geochemical analyses mineral chemistry stratigraphic chronology and sedimentary analysis including source provenance suggest that the pebbly conglomerate was formed through erosion of an unknown ophiolitic source that was geochemically distinguishable from the xigaze ophiolites within the yarlung zangbo suture zone southern tibet we infer the existence of an older ophiolitic source termed the yarlung zangbo paleo ophiolite that was dismembered and eroded during an earlier subduction stage not taken into account in current models q 2000 elsevier science ltd all rights reserved

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

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

hidden calderas evidenced by multisource geophysical data example of cappadocian calderas central anatolia abstract the cappadocian volcanic field in central anatolia turkey is characterised by a sequence of 10 neogene ignimbrites the associated calderas have been partly dismantled and buried by subsequent tectonic and sedimentary processes and therefore cannot be readily recognized in the field recent progress in the understanding of the stratigraphic correlations and flow patterns has identified two main probable source areas for the ignimbrites detailed study of these areas based on gravity surveys remote sensing data spot and ers1 images and digital elevation models dem has provided evidence for two major caldera complexes and their relationship to old stratovolcanoes and neogene tectonics the older nevsehir acigol caldera complex located between the towns of acigol nevsehir and cardak is inferred to be the source of the kavak and zelve ignimbrites the nevsehir acigol caldera complex is defined mainly by a 35 mgal circular gravimetry anomaly about 15 km in diameter the boundaries of this now buried caldera complex are shown by high gradients on the bouguer gravity anomaly map the younger derinkuyu caldera complex located between the erdas stratovolcano and the ciftlik basin is inferred to be the source of the sarimaden cemilkoy gordeles and kizilkaya ignimbrites it is well defined by a rectangular 35 23 km gravity low 30 mgal with a positive high 20 mgal in the center gravity remote sensing data and the dem provide evidence that the erdas stratovolcano on the northern margin of the derinkuyu caldera complex represents the remnants of a large stratovolcano partly cut by one or more caldera collapses the positive anomaly within the derinkuyu caldera complex is centered on the 15 km wide sahin kalesi volcanic massif field evidence and structural features inferred from the dem and remote sensing data strongly suggest that this massif is a resurgent doming associated with the gordeles ignimbrite eruption high resolution ers1 spot and dem images reveal that the transtensive regime active at least since the miocene influenced the location of eruptive centers and caldera complexes in cappadocia the two caldera complexes are located in transtensive grabens the subsidence of these grabens continuing after the caldera collapse events most likely resulted in the burying of the calderas and could explain the difficulties in identifying them in the field

late cretaceous neo tethyan slab roll back evidence from zircon u pb o and whole rock geochemical and sr nd fe isotopic data of adakitic plutons in the himalaya tibetan plateau

on two dimensional interpretation of magnetotelluric soundings summary the paper presents the views of the 2 d interpretation of magnetotelluric mt data that are characteristic of the russian magnetotelluric school discussing the strategy of 2 d interpretation of mt data we have to answer three questions in decreasing order of importance these questions are as follows 1 which field mode is more sensitive to the near surface and deep structures that are the targets of mt surveys 2 which field mode is more robust to the 3 d evects caused by real geological bodies 3 which field mode is more susceptible to the static shift induced by nearsurface inhomogeneities we examine the transverse magnetic tm and transverse electric te modes of the 2 d magnetotelluric field and show that they satisfy the principle of information complementarity 1 while the tm mode is more sensitive to the near surface structures the te mode may be more sensitive to the deep structures 2 while the tm mode is more robust to the 3 d evects caused by conductive structures the te mode may be more robust to the 3 d evects caused by resistive structures and 3 while the tm mode is more susceptible to the static shift the te mode may be almost undistorted thus the gaps left by one mode can be filled by another mode if so the most comprehensive and reliable information on the conductivity of the earth s interior can be obtained using both modes i e the transverse and longitudinal mt curves the general scheme of this bimodal mt inversion is rather simple the transverse curves provide details of near surface structures e g in the sediments and allow one to evaluate the lithosphere resistance and outline the deep conductive faults while the longitudinal curves help one to detect the conductive zones in deep layers of the lithosphere and in the asthenosphere an eycient two level algorithm for the bimodal mt inversion realizing this scheme is suggested as an illustration the paper presents the geoelectrical model of the kirghiz tien shan constructed by means of the bimodal mt inversion

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

revision of the cretaceous paleogene stratigraphic framework facies architecture and provenance of the xigaze forearc basin along the yarlung zangbo suture zone abstract new field investigation structural analysis review of biostratigraphy stratigraphic framework facies architecture provenance and flow direction of the cretaceous paleogene sedimentary strata in southern tibet resulted in reinterpretation of the evolution of the xigaze forearc basin xfb the xfb is comprised by flysch dominant xigaze group consisting of ngamring chongdoi and sangzugang formations and of the shallow marine cuojiangding group padana qubeiya quxia and gyalaze formations with their ages herein revised as the albian coniacian and the santonian ypresian respectively the deep sea flysch represented by the ngamring formation constitutes the early sedimentary fill of the xfb it was subdivided into five megasequences ms1 ms5 in order the early submarine fan facies stage channel submarine fan stage the mature submarine fan stage the highstand submarine fan stage and outer fan pelagic stage the paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage indicating typical forearc basin sedimentation with longitudinal supply and lateral filling provenance studies point to multiple sources with andesitic debris sourced from the jurassic lower cretaceous yeba formation and sangri group formed in the volcanic island arc chains in the southern gangdese the four periods of the xfb development are 1 the formation of the xfb basement by the xigaze ophiolite and completed before the aptian the coeval deposits on the shelf are the sangzugang formation carbonate 2 the earliest xfb deposits of the earliest aptian age ca 125 120 ma represented by the chongdoi formation related to the residual forearc 3 composite forearc developed represented by the late albian late cenomanian flysch megasequences ms1 ms2 and the turonian coniacian flysch megasequences ms3 ms5 respectively and 4 the final stage of the xfb development started the terminal forearc stage represented by the late late cretaceous early paleogene shallow marine cuojiangding group and terminated during middle ypresian when the gyalaze formation was deposited

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

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

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

upper oligocene lower miocene gangrinboche conglomerate in the xigaze area southern tibet implications for himalayan uplift and paleo yarlung zangbo initiation abstract the gangrinboche conglomerate exposed along the yarlung zangbo suture zone records a crucial stage of the himalayan orogeny the type section of these strata in the xigaze area southern tibet including the qiuwu and dazhuka formations was studied by integrated stratigraphic sedimentologic petrographic and geochemical techniques palynological data and detrital zircon u pb ages indicate that the qiuwu formation was deposited during the latest oligocene to the earliest miocene most probably 23 ma while the overlying dazhuka formation was deposited during 23 18 ma the qiuwu formation was deposited in a deltaic setting and detritus was entirely derived from the gangdese magmatic arc in the north the dazhuka formation in contrast was deposited in mainly braided river environments and contains clasts derived from both the gangdese arc in the north and the himalayan orogen in the south clasts derived from the south first occur at the base of the dazhuka formation and increase in abunda

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

assessment of geothermal energy potential by geophysical methods nevsehir region central anatolia abstract in this study geothermal potential of the nevsehir region central anatolia was assessed by using vertical electrical sounding ves self potential sp magnetotelluric mt gravity and gravity 3d euler deconvolution structure analysis methods extensive volcanic activity occurred in this region from upper miocene to holocene time due to the young volcanic activity nevsehir region can be viewed as a potential geothermal area we collected data from 54 ves points along 5 profiles from 28 mt measurement points along 2 profiles at frequency range between 320 and 0 0001 hz and from 4 sp profiles total 19 km long the obtained results based on different geophysical methods are consistent with each other joint interpretation of all geological and geophysical data suggests that this region has geothermal potential and an exploration well validated this assessment beyond doubt

curie point depth geothermal gradient and heat flow estimation and geothermal anomaly exploration from integrated analysis of aeromagnetic and gravity data on the sabalan area nw iran prospecting the geothermal resources in northwest of iran conducted in 1975 revealed several promising areas and introduced the sabalan geothermal field as a priority for further studies the sabalan mt representing the sabalan geothermal field is a large stratovolcano which consists of an extensive central edifice built on a probable tectonic horst of underlying intrusive and effusive volcanic rocks in this study curie point depth cpd geothermal gradient and heat flow map were constituted from spectral analysis of the aeromagnetic data for the nw of iran the top of the geothermal resource i e the thickness of the overburden was evaluated by applying the euler deconvolution method on the residual gravity data the thickness of the geothermal resource was calculated by subtracting the euler depths obtained from the cpds in the geothermal anomalous region the geothermal anomalous region was defined by the heat flow value greater than 150 mw m2 cpds in the investigated area are found between 8 8 km in the sabalan geothermal field and 14 1 in the northeast the results showed that the geothermal gradient is higher than 62 c km and the heat flow is higher than 152 mw m2 for the geothermal manifestation region the thickness of the geothermal resource was also estimated to vary between 5 4 and 9 1 km these results are consistent with the drilling and other geological information findings indicate that the cdps agree with earthquake distribution and the type of thermal spring is related to the depth of the top of the geothermal resource

the evolution of yarlung tsangpo river constraints from the age and provenance of the gangdese conglomerates southern tibet abstract cenozoic conglomerates are exposed discontinuously along the length of the yarlung tsangpo suture zone on the southern margin of the gangdese arc these conglomerates the gangdese conglomerates herein record a crucial stage in the uplift and erosion histories of the southern tibet after the initial india asia collision in the mt kailas area the gangdese conglomerates strata consist of multiple sedimentary cycles and each cycle is a fining upward sequence that was deposited by alluvial fan braided river and delta systems whereas in the xigaze area the gangdese conglomerates strata comprise a coarsening upward sequence that was deposited by delta braided river and alluvial fan systems based on the detrital and igneous zircon u pb ages the depositional ages of the gangdese conglomerates are late oligocene to early pliocene ca 26 5 ma in the mt kailas area late oligocene to middle miocene ca 26 15 ma in the xigaze area and late oligocene to early miocene ca 26 19 ma in the zedong area paleocurrent measurements and provenance data i e conglomerate clast composition sandstone petrology and detrital zircon age indicate that the initial detritus of the gangdese conglomerates were entirely derived from the north mainly from the gangdese arc sediment resulting from denudation to the south the xigaze forearc basin the yarlung tsangpo suture zone and the northern margin of the indian plate first appeared by the early miocene ca 19 ma and subsequently increased in abundance gradually our new results together with previous data from the xigaze area reveal 3 major stages in the evolution of the yarlung tsangpo river system 1 the southward flowing stage ca 26 19 ma featured southward draining transverse rivers that transported materials from the gangdese arc southward southward paleocurrents in the gangdese conglomerates indicate a northern source 2 the westward flowing stage ca 19 15 ma developed due to the uplift of the suture zone and tethys himalaya to the south northward draining rivers began to develop and lakes resembling a string of beads formed and finally connected together initiating the westward flowing paleo yarlung tsangpo river westward paleoflows were recorded in the gangdese conglomerates 3 the eastward flowing stage ca 15 ma present resulted from differential uplift and denudation of the southern tibet which reversed the direction of the young yarlung tsangpo river by ca 15 ma the deposition of the gangdese conglomerates was controlled by eastward paleoflows at this point the modern eastward flowing yarlung tsangpo river system was established

curie point depth investigation of central anatolia turkey the residual aeromagnetic total field intensity anomalies in central anatolia were calculated from the regional aeromagnetic anomalies surveyed by the mineral research and exploration mta of turkey the residual aeromagnetic data were analyzed to produce curie point estimates by the method of okubo et al 1985 the curie point depth of central anatolia varies from 7 9 km and 22 6 km the shallowest curie point depths were observed around the cappadocia and erciyes volcanic complexes in central anatolia a good correlation was deduced between the curie point depths and the heat flow data measured previously which is most certainly important for the geothermal resources of the region the shallow curie point depths also correlate well with the hot spring locations in central anatolia

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

detecting the limit of slab break off in central turkey new high resolution pn tomography results summary inversion of pn traveltime residuals from a 39 station broad band array provides a highresolution image of the velocity structure in the uppermost mantle beneath central turkey individually picked pn phase arrivals from events recorded by the north anatolian fault passive seismic experiment and the kandilli observatory were combined with additional events associated with the eastern turkey seismic experiment tomography results show no change in pn velocity across the north anatolian fault although longitudinal variations are evident a region of very low pn velocities 8 1 km s 1 w est of the fault the sharp transition along the cafz which follows the palaeotectonic inner tauride suture may represent the location of the edge of the slab window created when the oceanic slab broke off along the bitlis zagros suture around 11 ma as the arabian plate collided with the eurasian plate

paleomagnetism of the xigaze ophiolite and flysch yarlung zangbo suture zone southern tibet latitude and direction of spreading abstract the xigaze ophiolite 29 2 n 89 5 e which outcrops in the yarlung zangbo suture zone represents the remnants of an oceanic lithosphere formed in middle cretaceous times between the lhasa block to the north and the indian plate to the south in an attempt to define the paleo orientation and latitude of the spreading center at which it has been created a paleomagnetic study has been done on three sites in volcanics and overlying or interbedded radiolarites forming the upper part of the ophiolite sequence and also on seven sites in the xigaze group flysch which stratigraphically overlies the volcanics to the north in each site hand blocks carefully oriented both with sun and magnetic compass have been sampled the paleomagnetism data combined with structural data on the ophiolite dolerite intrusives allow a partial reconstruction of south eurasia at the time of formation of the xigaze ophiolite the paleolatitude of accretion and deposition of the xigaze ophiolite and overlying sediments is found to be 10 20 n both ophiolite and basin have encountered a 85 20 anti clockwise rotation the corresponding ridge was close to the southern margin of the lhasa block and was oriented n175 25

structure of the xigaze ophiolite yarlung zangbo suture zone southern tibet china genetic implications three massifs in the xigaze area southern tibet china have been preserved from the intense deformation observed everywhere else in the yarlung zangbo suture zone these massifs have similar lithologies and display a remarkable homogeneity in the attitude of most magmatic and tectonic structures such as lava flow planes dolerite dikes and sills and foliation planes in peridotites this indicates that these ophiolites are only slightly dismembered inasmuch as they locally display complete sequences from pillow lavas at the top to fresh cr diopside bearing harzburgites at the base gabbroic rocks are rare and are present as screens between dolerite intrusives or in small sized pockets at the limit between mafic and ultramafic rocks one of the most important feature of this ophiolite is the occurrence of dolerite sills and dikes throughout the whole sequence indicating that this ophiolite results from to two successive magmatic events with 1 creation of the crustal part of the oceanic lithosphere with deposition of minor cumulates in small magma chambers over mantle residual rocks and 2 intrusion of dolerite sills and dikes throughout the whole sequence

apatite fission track evidence for miocene denudation history in the gangdese conglomerate belt and yarlung tsangpo river implications for the evolution of southern tibet abstract low temperature thermochronology is used widely in the tibet plateau uplift some researches however have defined the time of rapid denudation as simply rock uplift and have neglected the fact that the rock denudation recorded by fission track ft data was controlled by both surface incision and rock uplift the incision of the yarlung zangbo river had a significant influence on uplift history inversion in southern tibet this paper simulated the bedrock denudation and river incision histories using apatite fission track aft data sampled from the gangdese conglomerate belt which is located in the middle of southern tibet and analyzed the geological meaning of the aft age of each sample the results showed the following 1 in the early miocene 22 16 ma both the value of the denudation rate and the incision rate were high 0 56 mm yr and 0 24 mm yr 2 in the middle late miocene the incision rate 0 12 mm yr was similar to the denudation rate 0 09 0 11 mm yr 3 the historical model between river incision and bedrock denudation revealed a significant difference in the denudation rate during the period ca 8 6 ma combining these data with previously published thermochronological ages and synthesizing these ages with regional geological we arrived at the following conclusions 1 in the early miocene the denudation event probably was caused by a combined result of indian plate rollback and the incision of the yarlung zangbo river 2 in the middle late miocene the denudation rate was consistent with the incision rate which suggested that the denudation episode was caused by climate change associated with asian monsoon intensification 3 after 8 ma the stable and slow incision rate indicated that regional drastic uplift had ceased the paleo elevation of the research area had approached and even exceeded the present day elevation in the late miocene

geochemistry of sedimentary rocks from melange and flysch units south of the yarlung zangbo suture zone southern tibet abstract the mesozoic yamdrock melange and the triassic flysch lie immediately south of the yarlung zangbo suture zone yzsz ophiolitic belt in southern tibet the whole rock chemical signature of sandstones mostly greywackes red shales and black shales of the yamdrock melange and flysch units do not present significant variations between the different rock types and geological units and is concordant with a continental passive margin source despite fairly important chemical weathering the geochemical signature was not affected by significant sedimentary recycling nor significant heavy mineral accumulation ree patterns show an enrichment in lree typical of shales and indicate that the neo tethyan turbidites mostly derive from a felsic old upper continental crust source slight enrichments in ti and sc indicate a mafic contribution that could originate from mafic blocks of enriched intraplate geochemical affinity found in the sedimentary units sedimentary rocks from both units were thus derived from the old indian continental crust including granitic and intraplate mafic blocks but were deposited in different tectonic settings in the neo tethys ocean the homogeneity of the yzsz data which cover the whole neo tethyan basin and correlation with himalayan data support the formation of all himalayan lithotectonic units along a continuous passive margin of northern india

conglomerates record the tectonic evolution of the yarlung tsangpo suture zone in southern tibet the histories of individual conglomeratic units along the yarlung tsangpo river suture zone in southern tibet reflect significant phases in the mesozoic to cenozoic tectonic evolution of this area several temporally distinct conglomerate units are recognized along the suture and their detailed examination permits analysis of the collision between india and asia upper jurassic to lower cretaceous conglomerates crop out within the sangri group along the southern lhasa terrane they are dominated by limestone and andesitic volcanic cobbles derived entirely from the lhasa terrane these rocks have experienced amphibolite facies metamorphism and exhibit a strong penetrative regional foliation thick successions of the palaeocene liuqu conglomerate crop out within the suture from xigaze to lhaze they contain detritus sourced from intra oceanic terranes associated with the suture zone as well as clasts of indian affinity while lhasa and xigaze terrane derived material is notably absent these conglomerates record an early suture zone event prior to india asia collision uppermost oligocene to lower miocene gangrinboche facies conglomerates crop out on the southern edge of the lhasa terrane along the length of the suture several correlative units within this facies exhibit broadly similar stratigraphic histories a basal depositional contact upon an eroded lhasa terrane surface is ubiquitous with initial clast derivation from the north up section the first arrival of coarse grained suture zone and india derived clasts is abrupt these southerly derived clasts predominate by the top of most sections an a really restricted succession of gently dipping late neogene ultramafic breccias unconformably overlies folded liuqu conglomerate near quanggong other neogene sediments are extensive west of mount kailas deposition of coarse clastic sediments is presently continuing along the length of the yarlung tsangpo discrimination and detailed investigation of each of these units will improve our understanding of the evolution of the india asia collision

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

new constraints on the india asia collision the lower miocene gangrinboche conglomerates yarlung tsangpo suture zone se tibet abstract lower miocene conglomerates crop out along the length of the yarlung tsangpo suture zone on the southern margin of the lhasa terrane these conglomerates known by various local names are correlated herein as the gangrinboche conglomerates all units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the lhasa terrane is ubiquitous at most localities the tops of sections are either removed by erosion or truncated by north directed thrusts these conglomeratic molasse units developed in response to the india asia collision and record aspects of its development in all units initial clast derivation was from the lhasa terrane on the northern margin of the yarlung tsangpo suture zone up section the first appearance of clasts derived from terranes within the suture zone and the northern margin of india all of which lie to the south of any outcrops of gangrinboche conglomerates is observed although these units were previously thought to be eocene analysis of fossil and structural constraints indicates early miocene deposition as development of the gangrinboche conglomerates records a significant phase in the evolution of the india asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

discovery and significance of cretaceous fossils from the xigaze forearc basin tibet abstract the xigaze cretaceous forearc basin located between the lhasa block and indus yarlung suture was filled with flysch deposits during the mid cretaceous as the tethys oceanic crust was being subducted underneath eurasia the flysch comprises the xigaze group dominated by the angren formation well preserved newly discovered foraminifera within the angren formation constrain its age to late albian to late coniacian radiometric dating of the ophiolite belt impinging on the southern edge of the forearc basin yielded an age of 120 10 ma which implies that the deposition in the forearc basin occurred shortly after initiation of the subduction of the neotethys

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

diagenetic alteration affecting δ18o δ13c and 87sr 86sr signatures of carbonates a case study on cretaceous seep deposits from yarlung zangbo suture zone tibet china abstract seep carbonates provide excellent records of past seepage activities and have been commonly considered to preserve primary unaltered stable carbon isotope signatures however late diagenetic reactions may overprint original isotopic compositions but the mode and effect of such alterations are poorly understood in particular there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis this study reports recently discovered cretaceous hydrocarbon seep deposits from the yarlung zangbo suture zone tibet china that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins unitary linear recursive analysis was applied to 13 c values and 87 sr 86 sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids the 18 o values range from 11 8 to 2 2 13 c values from 34 1 to 12 9 and 87 sr 86 sr ratios from 0 706221 to 0 706808 the heterogeneity in isotopic compositions and the observation that the most negative 18 o values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis the linear correlations between 13 c values and 18 o values for matrix micrites r 2 0 54 and between bulk carbonate 87 sr 86 sr ratios and 18 o values r 2 0 85 are best explained by burial diagenetic overprinting of oxygen strontium and even carbon isotopic compositions rather than by meteoric water hypergenesis extrapolated values of 13 c and ratios of 87 sr 86 sr against a 18 o value of 2 average value of calcite precipitated in isotopic equilibrium with coeval cretaceous seawater that would characterize the primary carbonate give an end member 13 c value of 34 and an end member 87 sr 86 sr ratio of 0 7072 the end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low 13 c values and high 87 sr 86 sr ratios were formed by anaerobic oxidation of methane near the seafloor in contrast the measured 18 o values and 87 sr 86 sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87 sr 86 sr ratio and high temperature our findings reveal that 13 c values can only be moderately and 87 sr 86 sr ratios can be significantly altered during late diagenesis and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

testing the validity of nd isotopes as a provenance tool in southern tibet for constraining the initial india asia collision abstract sm nd isotopic systematics has been proven to be a powerful tool for investigating the source provenance and tectonic significance of sedimentary rocks here i report the nd isotopic compositions of mesozoic to paleogene siliciclastic sediments from southern tibet in an attempt to determine the temporal variations in their provenance the data reveal a shift in end 0 values of mesozoic strata in the tethyan himalaya varying from 16 3 to 12 7 in the jurassic to 8 1 to 3 7 in the cretaceous the less negative end 0 values of the cretaceous siliciclastic rocks indicate increasing input of a juvenile mantle component from the source region such a shift can be attributed to the early cretaceous mantle derived magmatism that was predominant in northern greater india this study indicates that there is a significant overlap in the end 0 values of rocks from the tethyan himalaya and lhasa block implying that nd isotopes may not be a good provenance proxy to constrain the initiation of the india asia collision such an inference is also evidenced by similar nd isotopic compositions between rocks from the bhainskati formation in the nepalese foreland basin sourced from the tethyan himalaya and the enba and zhaguo formations in southern tibet sourced from the lhasa block

provenance of the upper cretaceous eocene deep water sandstones in sangdanlin southern tibet constraints on the timing of initial india asia collision abstractthe first arrival of sedimentary material from asia onto the indian continental margin provides a minimum constraint on the timing of initial india asia collision a combination of petrology detrital cr spinel geochemistry and zircon u pb dating of the upper cretaceous to eocene deep water succession at the sangdanlin section southern tibet provides evidence for major provenance change within the strata the upper cretaceous paleocene denggang formation quartzarenites contain zircons with dominant proterozoic ordovician u pb ages with an additional age peak of early cretaceous which we interpret to be derived from the northern indian margin by contrast the lithic sandstones of the early to middle eocene sangdanlin and zheya formations are dominated by zircons younger than 200 ma showing one major peak at 80 125 ma and two subdominant peaks at 54 70 and 180 196 ma comparable to those from the gangdese magmatic arc cr spinels in the sangdanlin and zheya formations are abundant and char

the dynamothermal aureole of the donqiao ophiolite northern tibet metamorphic rocks found at the base of the jurassic donqiao ophiolite of northern tibet are interpreted as a basal dynamothermal aureole produced during obduction of the massif the rocks form a sequence some 8 m thick varying from high grade amphibolites at the contact with overlying harzburgites to greenschist facies metasedimentary rocks lower down the mineral paragenesis is similar to other such aureoles and indicates that temperatures in excess of 750 c may have been reached during metamorphism the lack of high pressure minerals suggests that the rocks were produced by subcretion in a relatively shallow dipping subduction zone ar ar geochronology on amphibole separates provides dates of 175 180 ma for the displacement of the ophiolite significantly older than the age of emplacement estimated from stratigraphie relationships the ophiolite was clearly obducted very soon after its formation in a suprasubduction zone environment

geochemical and geochronological constraints on the origin and emplacement of the yarlung zangbo ophiolites southern tibet

late devonian oib alkaline gabbro in the yarlung zangbo suture zone remnants of the paleo tethys article i nfo the yarlung zangbo suture zone yzsz is believed to be composed of material largely derived from the destruction of the neo tethys that occurred from early mesozoic to early cenozoic we report here geochronological and petrological data obtained for newly discovered alkaline gabbro blocks embedded in a melange zone of the western yzsz single zircon u pb analyses from one representative gabbro sample by sims secondary ion mass spectrometry yielded a combined crystallization age of about 363 7 1 7 ma 1 in situ hf isotopic analyses yielded ehf t values of 2 6 to 5 5 suggesting an enriched mantle source all of the gabbro samples show typical ocean island basalt oib affinity with little or no continental crust contamination they also display strong geochemical similarities with the hawaii basalts and the xigaze seamount basalts suggestive of their intra oceanic setting these observations in combination with the early carboniferous layered gabbros reported at luobusa indicate that these rocks could represent remnants of the paleo tethys we propose that a branch ocean separating the western qiangtang terrane and the lhasa terrane from the gondwana continent might have been present during the late devonian and the early carboniferous providing new constrains on the configuration of paleo tethys in tibetan plateau during early late paleozoic

continental arc volcanism and tectonic setting in central anatolia turkey abstract the neogene and quaternary volcanism of central anatolia represents the central sector of the anatolian volcanic arc related to continental collision between the afro arabian and eurasian plates it is closely associated with a complex system of tectonic depressions related to brittle deformations of transtentional type and which commenced in the late miocene the volcanism here considered can be divided into three main periods of activity separated by important deformative and erosive events the first period is represented by a mostly andesitic effusive activity the second period is characterized by the emplacement of a thick ignimbritic sequence and shows an areal distribution up to 11 000 km 2 seven ignimbrite units have been recognized the three main units were found at a distance of more than 100 km from the presumed source area geological and sedimentological data lead us to recognize the melendiz dag volcanic complex and the ciftlik caldera as the probable ignimbrites source vent during the third period great andesitic basaltic stratovolcanoes and a number of prevalently acid monogenic centres developed the relationship between the volcanic activity is clearly conditioned by the main transcurrent fault systems present in central anatolia the neogene quaternary volcanic activity prevalently developed along the ene wsw karaman sivas lineament most of the great central volcanoes developed at the intersection between the ene wsw trends and the ecemis and tuz golu transcurrent faults the structural interpretation of the quaternary monogenic centres is more difficult probably they are related to the very recent n s fault swarms which cross the anatolides and the taurus range