rare earth element characteristics and k ar ages of the band ziarat ophiolite complex southeastern iran the band ziarat complex of southeastern iran is located on the western boundary c f the jaz murian depression rand is bounded by two ma jar fault systems the principal rock units of this complex are a gabbro sequence that includcs low and high level cumulate gabbros a late intrusive sequence that consists of diorite and plagiogranite and a volcanic sequence that includes diabase dikes and a lesser anaount of basaltic lava mantle rocks are virtually absent because of the presence of the two bounding fault systems but we consider the complex to be an ophiolite in nature rare earth cleislcnt ree whole rock data clearly diffcremtiate the classic ophiolitic lithologies for the crustal rocks in this complex bas4 on the ree data there are two distinct types of basalt presenl at band ziarat i those that formed from an initial basaltic melt with a light rare earth element lree enriched signature similar to intraplate hasalts and ii those that have lree depleted patterns similar to normal mid ocean ridge basalts the data also suggest i that the gabbros are adcumulates and were derived from a source slightly enriched in lree with fractic natioam controlled by removal of clinopyroxene or hornblende and plagioclase and ii that the late intrusive rocks as well as a ma jority of the cliabnse dikes are cogenetic and were derived from thc same lree enriched source k ar ages ranging from 134 4 to 146 5 ma for lowlevel gabbros and from 821 4 to 930 4 ma for high level gabbros were measured on five hornblende and two whole rock samples which suggests that these rocks may have formed early in the cretaceous period

what controlled mid late miocene long term aridification in central asia global cooling or tibetan plateau uplift a review abstract debate continues over whether global cooling or uplift of the tibetan plateau provided the first order driver for the aridification moisture levels of central asia throughout the mid late miocene between about 17 and 5 ma this review attempts to throw new light on the relations between the aridification and these two key factors this paper examines the evolution of miocene climate both moisture and temperature within five separate regions of eurasia to help understand the large scale controls of long term moisture in central asia the five regions include 1 europe 2 high latitude asia 3 the east asian monsoon region 4 the south asian monsoon region and 5 central asia itself because moisture reaching central asia has to firstly cross at least one of the other four regions temperature proxy data compiled from these five regions correlate with ocean temperatures from the global deep sea oxygen isotope records furthermore compiled moisture proxy data from the four regions surrounding central asia co vary and correlate with each other this can be explained by positive feedbacks between drying and cooling and supports the assumption that global cooling provides a dominant driving factor for the drying of eurasia global cooling reduces the amount of water vapor held in the atmosphere and thereby can cause terrestrial drying however in central asia the moisture evolution shows less similarity with its surroundings the uplift of the tibetan plateau including the tianshan mountains could provide a possible explanation for this difference the changing topography resulting from uplift events over time and space strongly influenced the moisture patterns in central asia during miocene times future research on the spatial timing and amplitude of tibetan plateau uplift should be useful to understand the moisture processes in central asia during the miocene

brittle tectonic reconstruction of palaeo extension inherited from mesozoic rifting in west zagros kermanshah iran abstract the present day zagros mountain belt of sw iran corresponds to the former arabian passive continental margin of the southern neo tethyan basin which existed since a permian triassic rifting episode and underwent later collisional deformation in mid to late cenozoic time in west zagros the kermanshah radiolarian trough separated from the neo tethyan basin by the narrow continental platform of the bisotun limestone resulted from the triassic jurassic rifting of the passive margin brittle tectonic analyses of syndepositional normal fault slip data have been undertaken to study the extensional history of the passive margin in terms of palaeostress reconstruction dominant east west trends of the inherited rift related normal faults in west zagros indicate an approximately north south trend of extension that prevailed during the entire mesozoic syn to post rift evolution of the radiolarian trough this extension characterizes a stretched continental margin similar to the present day passive margin of the british isles considering the structural pattern of the inherited basement faults as revealed by the present day earthquake focal mechanisms an oblique crustal stretching model is proposed for the rifting process

tectonostratigraphic evolution of the helleh paleo high nw persian gulf insights from 2d and 3d restoration methods abstract seismic and well data restored cross sections and surfaces as well as non seismic data were utilized to reconstruct the tectonostratigraphic evolution of the helleh paleo high a gentle elongated n ne trending structure in the iranian sector of nw persian gulf based on bouguer gravity anomaly and basement depth maps the southeastern margin of the helleh paleo high is defined by the bushehr fault a ne sw trending deep seated fault which contributed to the deposition of thicker late precambrian early cambrian hormuz salt to the se there is also a significant increase in thickness of the late devonian to latest triassic succession in the southeastern block of this fault a feature which is interpreted as evidence for syn depositional fault activity after a period of relative tectonic quiescence during the jurassic and early cretaceous the helleh paleo high became emerged and eroded from the cenomanian to maastrichtian early paleocene uplift and erosion continued until the early miocene but at lower rates compared to the late cretaceous the repeated reactivations of the bushehr fault together with the remobilization of the hormuz salt in the core area of the helleh paleo high could have contributed to these phases of amplified erosion in the early miocene the peripheral loading related to the southward advance of the zagros fold and thrust belt resulted in the establishment of the zagros foredeep basin this event exerted a ne downward tilting on the helleh paleo high the progressive tilting continued during the middle miocene when the structure attained its maximum extent as a 4 way closure the latest miocene pleistocene was a critical period in the structural history of the helleh paleo high when the final stage of zagros foredeep basin evolution tilted the structure to such a degree that all pre existing structural reliefs in the post upper triassic reservoir units were completely destroyed in fact the helleh paleo high is an example of a damaged hydrocarbon trap due to recent tectonic tilting

metamorphism at the base of the samail ophiolite southeastern oman mountains metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies occur beneath the samail ophiolite the metamorphic protolith was a section of marine basalts mn rich cherts and argillites garnet clinopyroxene amphibolites occur within 2 m of the contact hornblende clinopyroxene assemblages occur within 80 m of the contact and lower amphibolite facies rocks persist to 130 m from the contact the higher grade rocks show polyphase deformation cataclastic fabrics and partial retrogression to greenschist facies assemblages however lower grade phyllites show evidence of only one episode of metamorphic crystallization unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation the distribution coefficient kd feo mgo gar feo mgo cpx ranges from 4 4 to 5 3 suggesting a t range of 755 865 c at 200 mpa low jadeite content in clinopyroxene and low glaucophane content of ca amphiboles are consistent with moderate pressures of crystallization the metamorphic sheet is inferred to be of composite origin the highest grade rocks were tectonically transported the greatest distance and the lower grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks residual heat from the ophiolite is the dominant heat source for the metamorphism and this could have been augmented by a limited amount of frictional heat generated during thrusting these thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the samail ophiolite

onset timing of significant unroofing around qaidam basin northern tibet china constraints from 40ar 39ar and ft thermochronology on granitoids 40ar 39ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of qaidam basin constrain the timing of a tertiary denudation event and their tectonic association in northern tibet the cooling histories based upon 40ar 39ar and fission track ages suggest a rapid cooling event 7 5 10 7 c ma during oligocene early miocene time this cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the north qaidam and kunlun shan mountain ranges with consideration to the field evidence sedimentation rate of the qaidam basin and the propagation rate along the altyn tagh fault the rapid uplifting and denudation are probably due to crustal thickening associated with cenozoic propagation of the altyn tagh fault in accommodating the collision of india with asia

the uplift history of the haiyuan liupan shan region northeast of the present tibetan plateau integrated constraint from stratigraphy and thermochronology abstractthe haiyuan liupan shan delimits the northeastern margin of the tibetan plateau we present the integrated results of new stratigraphic investigations apatite fission track analyses and related thermal history modeling tied to the structural fabric of this region these constraints provide insight into the timing of northward propagation of the plateau with 1 local relief creation and enhanced erosion in the latest jurassic to earliest cretaceous representing the local element of a complex regional far field deformation response to the lhasa qiangtang collision with the south asian margin 2 evidence for likely late cretaceous exhumation but this is uncertain given the regional sedimentary gap that exists across this interval 3 eocene relief generation as a first likely response to far field effects of the india asia collision and related to wider regional events recognized in this part of northwestern china and 4 the latest deformation episode propagating strongly into the haiyuan

structural framework and tectono stratigraphic evolution of the eastern persian gulf offshore iran abstract seismic and well data are utilised to describe the tectono stratigraphic evolution of the eastern persian gulf during the late mesozoic and cenozoic the structural framework of the study area is the result of the late cretaceous to early miocene oman orogeny overprinted by the southwestward advance of the zagros orogeny through the late miocene and pliocene two structural domains can be identified the ne sw trending musandam fold thrust belt mftb and the nw se trending persian gulf foredeep basin the mftb formed as a result of multiple compressional events late cretaceous late paleocene to early eocene and late oligocene to early miocene the first compressional event cenomanian turonian to the maastrichtian is characterized by emplacement obduction of the allochthonous rocks onto the passive continental margin of the arabian plate in the oman mountains this event led to regional deformation and uplift which caused deep erosion of the albian to santonian units during the santonian to maastrichtian in addition to regional deformation a ne sw trending foredeep basin developed in front of the late cretaceous thrust sheets creating accommodation space for turbiditic deposits of the gurpi formation during the late paleocene early eocene initiation of a new compressional deformation led to establishment of the pabdeh foredeep basin in this stage the basin depocenter migrated 30 km to the west the third compressional event began in the late oligocene reaching a climax in the early miocene this event resulted in regional deformation and uplift which caused deep erosion of the pre middle miocene strata the persian gulf foredeep basin has developed in front of the rising zagros mountains since late miocene pliocene folding related to advancement of the zagros orogeny into the persian gulf is mainly restricted to northeastern flank near the coast of iran the effect of folding decreases to the south of the zagros deformational front

cenozoic exhumation in the qilian shan northeastern tibetan plateau evidence from detrital fission track thermochronology in the jiuquan basin the india asia collision resulted in the cenozoic framework of faults ranges and tectonic basins and the high topography of the northeastern tibetan plateau but how and when these features formed remains poorly understood leading to conflicting tectonic models however information on the tectonic evolution of these active orogenic belts is well preserved in synorogenic basin sediments in this study we carefully analyze the detrital apatite fission track aft ages of cenozoic synorogenic sediments from the jiuquan basin to decipher the entire exhumation process of the adjacent qilian shan throughout the cenozoic our data indicate that initially rapid cenozoic exhumation occurred in the qilian shan during the late paleocene early eocene 60 50 ma almost synchronous with the india asia collision the qilian shan subsequently experienced long lived exhumation that continued until at least the middle miocene 45 10 ma during this period of exhumation in the qilian shan tectonic deformation occurred throughout the northeastern tibetan plateau the early cenozoic deformation in the northeastern tibetan plateau may have been caused by the transfer of tectonic stress from the distant india asia collision boundary through the complex lithospheric environment of the tibetan plateau the present tectonic configuration and topography of the qilian shan and the northeastern tibetan plateau likely became established since the middle miocene and after the long lived deformation began in the early cenozoic

south china sea surface water evolution over the last 12 myr a south north comparison from ocean drilling program sites 1143 and 1146 1 planktonic foraminifera pf from ocean drilling program odp sites 1143 and 1146 in the southern and northern south china sea scs respectively were quantitatively analyzed in order to reconstruct the sea surface environment over the last 12 myr the observed decrease in deep dwelling pf species after 10 ma at both sites is interpreted to reflect a depression of the upper water thermocline corresponding to the closure of the indonesian seaway around 11 9 ma this upper water column structure implies the intensification of equatorial pacific warm currents and the initial formation of the western pacific warm pool wpwp during the early late miocene the consistent pattern of south north thermocline evolution and the synchronous disappearance of globoquadrina dehiscens 9 8 ma at both sites 1143 and 1146 together imply that the entire scs was likely under the influence of the newly developed wpwp at 10 ma after 8 ma sea surface temperatures and thermocline variations evolved differently between the southern and northern scs the total deep dwelling pf fauna at site 1143 decreased gradually in abundance from 6 6 to 2 ma indicating a deepening of the thermocline in the southern scs in contrast deep dwelling pf species increased in abundance from 3 1 to 2 ma at site 1146 reflecting a shoaling of the thermocline in the northern scs this south north contrast reflects two major environmental regimes 1 the southern scs which has mainly been under the influence of the wpwp since the late late miocene and 2 the northern scs where effects of the east asian winter monsoon have prevailed especially since the late pliocene estimate of past sea surface temperatures ssts at site 1143 suggests a relatively stable and warm environment in the southern scs since about 2 5 ma with an increased influence of warm subsurface waters after the mid pleistocene transition 1 2 0 9 ma in the northern scs however a gradual decrease in winter sst recorded at site 1146 over the last 4 myr records east asian monsoon evolution especially the enhancement of the east asian winter monsoon between 3 1 and 2 ma

detrital apatite fission track evidence for provenance change in the subei basin and implications for the tectonic uplift of the danghe nan shan nw china since the mid miocene abstract the subei basin is located at the intersection of the northeastern end of the altyn tagh fault atf and the western end of the danghe nan shan in the northeastern tibetan plateau the basin contains up to 4000 m thick cenozoic terrestrial clastic sediments which contain information on the uplift of the danghe nan shan in order to estimate the detrital apatite thermochronology with fission track dating we collected eight sandstone samples from the tiejianggou section of the subei basin which has an established depositional age range from 22 8 to 9 ma we found that a distinct change in slope of the peak age and lag time of the detrital apatite fission track occurred at 14 ma which we interpret as a provenance change caused by the uplift of the western end of the danghe nan shan combined with previous studies in adjacent areas this work implies that the initiation of successive episodes of surface uplift in the northeastern margin of the tibetan plateau took place in the mid miocene

comparison of ikonos and quickbird images for mapping mangrove species on the caribbean coast of panama mangrove stands of differing species composition are hard to distinguish in conventional coarse resolution satellite images the new generation of meter level satellite imagery provides a unique opportunity to achieve this goal in this study an ikonos geo bundle image and a quickbird standard bundle image were acquired for a study area located at punta galeta on the caribbean coast of panama the two images cover the same area and were acquired under equivalent conditions three comparison tests were designed and implemented each with separate objectives first a comparison was conducted band by band by examining their spectral statistics and species by species by inspecting their textural roughness the ikonos image had a higher variance and entropy value in all the compared bands whereas the quickbird image displayed a finer textural roughness in the forest canopy second maximum likelihood classification mlc was executed with two different band selections when examining only multispectral bands the ikonos image had better spectral discrimination than quickbird while the inclusion of panchromatic bands had no effect on the classification accuracy of either the ikonos or quickbird image third first and second order texture features were extracted from the panchromatic images at different window sizes and with different grey level gl quantization levels and were compared through mlc classification results indicate that the consideration of image texture enhances classifications based on the ikonos panchromatic band more than it does classifications based on comparable quickbird imagery an object based classification was also utilized to compare underlying texture in both panchromatic and multispectral bands on the whole both ikonos and quickbird images produced promising results in classifying mangrove species

40ar39ar ages and tectonic setting of ophiolite from the neyriz area southeast zagros range iran abstract an ophiolite considered to be an allochthonous fragment of tethyan oceanic crust and mantle crops out near neyriz in the zagros range iran 40 ar 39 ar ages ranging from 76 8 23 8 ma to 105 23 3 ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite the most precise ages are 85 9 3 8 ma for a diabase and 83 6 8 4 ma for a plagiogranite the weighted mean age of hornblende from the five samples is 87 5 7 2 ma which indicates that the igneous part of the neyriz ophiolite formed during the early part of the late cretaceous pargasite from amphibolite below peridotite of the neyriz ophiolite has a 40 ar 39 ar age of 94 9 7 6 ma the pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite comparable ages have been measured on igneous rocks from the samail ophiolite of oman and on amphibolite below peridotite of the samail ophiolite

a revised chronology for tertiary sedimentation in the sikouzi basin implications for the tectonic evolution of the northeastern corner of the tibetan plateau abstract the space time pattern of development of high topography associated with the tibetan plateau remains incomplete due in large part to sparse proxy data that allow estimates of the growth of individual mountain ranges here we present the results of a stratigraphic investigation of a 3100 m section of tertiary fluvio lacustrine strata in the sikouzi basin located at the northeastern most corner of the tibetan plateau results of magnetostratigraphic analysis coupled with a newly discovered miocene in situ fossil assemblage suggest that the section preserves a nearly complete record extending from initial basin formation at 29 myr until 0 5 myr sediment accumulation rates increase from relatively slow accumulation during the oligocene miocene 9 cm kyr to rapid accumulation in the pliocene 22 cm kyr apparently occurring in two sustained pulses at 10 5 ma and 5 ma respectively the increase in accumulation rates starting at 10 5 ma is interpreted as heralding the initial uplift of the liupan shan consistent with cooling and exhumation of the range zheng et al 2006 the pliocene increase in accumulation rate is accompanied by a distinct change in lithofacies the appearance of coarse detritus with provenance from the madong shan and up section changes in bedding orientation that are interpreted as growth strata collectively our data suggest that the generation of relief in the madong shan began at 5 4 ma moreover analysis of paleomagnetic declinations from the sikouzi section reveals a clockwise rotation of the sikouzi basin during the time period 24 5 4 ma that was followed by a subsequent counterclockwise rotation 5 4 0 5 ma our study suggests that activity along the liupan shan haiyuan fault system probably initiated at 10 5 ma but that the present day configuration of the fault system and associated ranges near the fault tip dates to pliocene time

petrology geochemistry and tectonics of the sabzevar ophiolite north central iran abstract the sabzevar ophiolite is a highly dismembered ophiolite complex located along the northern boundary of the central iranian microcontinent cim and is one of the internal iranian group of ophiolites and colored melanges the igneous rocks of this complex consist of peridotites harzburgite dunite and lherzolite serpentinite minor pyroxenite gabbros and a volcanic sequence that exhibits a wide range of composition from basalts and basaltic andesites to rhyodacite dacites rhyolites and basanites sedimentary rocks include a variety of upper triassic to lower cretaceous deep and shallow marine rocks these include pelagic fossiliferous carbonates that are mixed with the pillow basalt and basaltic andesite as interlayers or exotic blocks ranging in size from 10 to 100 m also present are extensive units of radiolarian chert which are interbedded within the basalts and basaltic andesites a combination of petrographic observations and analyses of incompatible trace elements and rare earth elements indicates the presence of at least four different types of extrusive rocks in the sabzevar ophiolite the geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts i the group 1 basaltic rocks which formed from an initial melt with n morb like lree depleted chemical signatures and are petrogenetically related to the gabbros ii group 2 basaltic rocks which have e morb chemical signatures and iii group 3 basaltic rocks with lree enriched signatures and incompatible trace element patterns that suggest an island arc affinity the data also suggest that the gabbros and the group 1 basaltic rocks are petrogenetically related and produced by fractionation of plagioclase and to a lesser extent clinopyroxene and hornblende similar to other studied iranian ophiolites e g khoy band e zeyarat dar anar neyriz kermanshah and baft the results from the sabzevar ophiolite demonstrate the presence of relics of mid ocean ridge basalt morb like signatures in the extrusive units of neo tethyan ophiolites and suggests that the fossil oceanic lithosphere preserved in these ophiolites developed at an oceanic spreading center in addition the sabzevar ophiolite contains other volcanic rocks that have non morb like geochemical signatures e g within plate island arc the differences between these geochemical signatures could be a result of a differing upper mantle composition or different degrees of partial melting of the same upper mantle the presence of sizable nb anomalies in the extended rare earth element ree patterns for andesites and the group 3 basalts is characteristic of volcanic arc magmas or possibly are products of the ascending magma that was modified by crustal contamination the presences of the basanites and the silica undersaturated rocks could be attributed to extrusive activity following ophiolite emplacement plate reconstructions suggest that rocks of the sabzevar ophiolite were part of the tethyan oceanic crust that was formed during rifting of a narrow but deep seaway that separated the cim from the eurasian plate the ophiolite was emplaced during northeast dipping subduction i e closure of this segment of the tethyan seaway sabzevar ocean

neogene extension and volcanism in the kunlun fault zone northern tibet new constraints on the age of the kunlun fault 1 the geology of the jingyu basin north tibet provides constraints on the relationship between localized e w extension in the qiangtang block and strike slip motion along the kunlun fault eocene to oligocene sw ne compression formed topographic relief in the present western kunlun shan pull apart basins started forming in the western kunlun ranges around 15 ma possibly coincident with the initiation of subduction of the tarim qaidam lithosphere under the kunlun shan extensional tectonism is still active and the jingyu basin is extending as a normal fault connected with the kunlun fault was recently active localized extension allows tertiary and quaternary shoshonitic magmas to reach the surface from crustal depths of 50 to 60 km miocene strike slip faulting indicates that the initiation of the kunlun strike slip fault is early neogene in age and that the total amount of horizontal movement that occurred along the fault has to be reconsidered

kinematic vorticity flow analysis and 40ar 39ar geochronology related to inclined extrusion of the hp lt metamorphic rocks along the zagros accretionary prism iran abstract the zagros accretionary prism in southwestern iran is exposed along the nw se trending of the zagros thrust system and inclined zagros transpression zone this accretionary prism consists of two units the upper sedimentary melange unit on top and the high pressure metamorphic melange unit at the bottom both units show characteristics of a tectonic wedge the upper unit consists of type i ii and iii melanges which display s c and c type shear band cleavages quartz ribbons and rectangular or fish head boudins the lower unit fabrics display and type porphyroclasts and quartz ribbon mylonites these fabrics formed from a combination of 60 5 simple shear and 39 5 pure shear both components were involved in a lateral exhumation of the high pressure low temperature metamorphic rocks in an inclined transpression wedge shaped geometry the estimated kinematic vorticity number wk was calculated from quartz c axis patterns rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries using the mean estimated wm value of 0 84 the inclination angle for the thrust wedge on top of the ne subducting neo tethyan oceanic lithosphere is 18 the 40ar 39ar plateau ages of early generations of biotite from the lower metamorphic melange are 119 95 0 88 ma and 112 58 0 66 ma this late aptian age is related to early thrusting and formation of hp lt metamorphic rocks the dating of two amphibole samples from the amphibolite yields a weighted mean age of 91 23 0 89 ma this turonian cenomanian age suggests a later metamorphic event associated with subduction and obduction of the neyriz ophiolite and later lateral extrusion of hp lt metamorphic rocks along the inclined zagros accretionary prism

continental collision slowing due to viscous mantle lithosphere rather than topography the rate of india s convergence with eurasia has decreased exponentially since their collision which suggests that the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved the forces that control plate motion are fundamental to plate tectonic theory it has generally been thought that deforming and thickening continental crust is responsible for slowing down the convergence between two continental plates marin clark now shows however that the rate of penetration of the indian subcontinent into eurasia has decreased exponentially since the plates collided which suggests that contractional strain across tibet has been constant throughout the collision she infers that this reflects deformation of the mantle lithosphere and that under conditions of constant stress and strength the deforming continental lithosphere creates viscous resistance that affects plate motion irrespective of how topography evolved because the inertia of tectonic plates is negligible plate velocities result from the balance of forces acting at plate margins and along their base1 observations of past plate motion derived from marine magnetic anomalies provide evidence of how continental deformation may contribute to plate driving forces2 3 4 5 6 7 8 a decrease in convergence rate at the inception of continental collision is expected because of the greater buoyancy of continental than oceanic lithosphere2 3 but post collisional rates are less well understood slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion7 8 9 10 however the role of deforming continental mantle lithosphere on plate motions has not previously been considered here i show that the rate of india s penetration into eurasia has decreased exponentially since their collision the exponential decrease in convergence rate suggests that contractional strain across tibet has been constant throughout the collision at a rate of 7 03 10 16 s 1 which matches the current rate a constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress constant force applied to a relatively uniform layer or interface at depth this finding follows new evidence that the mantle lithosphere beneath tibet is intact11 which supports the interpretation that the long term strain history of tibet reflects deformation of the mantle lithosphere under conditions of constant stress and strength the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved

cenozoic tectonic evolution of the elashan range and its surroundings northern tibetan plateau as constrained by paleomagnetism and apatite fission track analyses abstract magnetostratigraphic studies of sediments in the western gonghe and eastern qaidam basins together with apatite fission track aft analyses in the elashan range suggest that the elashan range and its surroundings experienced a two stage evolution a protracted uplift from the cretaceous to the early tertiary which explains the absence of the paleogene strata in the region and an accelerated growth of the elashan range since the latest oligocene caused by extensively distributed thrust faults in the region this later phase of uplift resulted in the subsequent thick sediments within the both easternmost qaidam and gonghe basins basal ages of 21 22 ma in the wulan section of 20 ma in the southern gonghe section and of l2 ma in the chaka section the two phases of thrust motion and range growth mean that the qaidam and gonghe basins were never united during cenozoic times the strike slip is likely to have occurred at l0 ma as shown by the paleomagnetic declination data which show a rapid transition in rotation sense from counterclockwise to clockwise at that time this time is also similar to the 9 ma age of initiation of the wenquan fault determined by backward calculation based on its late quaternary slip rate eventually the qinghai nan shan thrust belt began to develop at 6 ma

k ar ages of metamorphic rocks at the base of the samail ophiolite oman hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the samail ophiolite and phyllites farther from the peridotite contact have weighted mean sup 40 ar sup 39 ar total fusion ages of 90 0 3 0 m y and 79 5 3 0 m y respectively the amphibolities represent the first tectonic slice welded to the base of the samail ophiolite after it was detached from the tethyan oceanic crust formation of the amphiobolities occurred no more than 3 to 7 m y after crystallization of plagiogranite in the ophiolite the phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the tethyan spreading axis the k ar ages suggest assuming a half spreading rate of 2 to 5 cm yr that detachment of the samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center using the same spreading rate one can calculate a minimum half width of 300 to 750 km for the tethyan ocean during the late cretaceous

cenozoic volcanism and tectonic evolution of the tibetan plateau abstract cenozoic volcanism on the tibetan plateau which shows systematic variations in space and time is the volcanic response to the india asia continental collision the volcanism gradually changed from na rich k rich to potassic ultrapotassic adakitic compositions along with the india asia collision shifting from contact collision i e soft collision or syn collision to all sided collision i e hard collision the sodium rich and potasium rich lavas with ages of 65 40 ma distribute mainly in the lhasa terrane of southern tibet and subordinately in the qiangtang terrane of central tibet the widespread potassic ultrapotassic lavas and subordinate adakites were generated from 45 to 26 ma in the qiangtang terrane of central tibet subsequent post collisional volcanism migrated southwards producing ultrapotassic and adakitic lavas coevally between 26 and 8 ma in the lhasa terrane then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since 20 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the tibetan plateau these processes involve roll back and break off of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root and consequently northward underthrusting of the indian lithosphere the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust indian lithosphere

stress state reconstruction of oblique collision and evolution of deformation partitioning in w zagros iran kermanshah summary the w zagros of iran is characterized by a deformation partitioning pattern with a right lateral strike slip motion along the main recent fault and a shortening component across the zagros simple fold belt under the late cenozoic n s oblique collision between the arabian and iranian continental plates in this study we examined the brittle structures that developed during the mountain building process in the high zagros belt of kermanshah province which is structurally located between the two partitioned components of deformation to decipher the history of polyphase deformation and variations in stress fields within the sedimentary cover since the onset of collision the earthquake focal mechanisms of w zagros were analysed to evaluate the recent stress fields within the basement analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of the corresponding stress tensor with reference to folding our stress reconstruction indicates a major change in stress state from a miocene pliocene syn folding compressional regime to a pliocene recent post folding strike slip system with a significant anticlockwise reorientation of the horizontal 1 stress axis from ne sw to n s the results of the recent stress states indicate that the stress trends are similar throughout the basement and sedimentary cover the differences between stress and strain axes suggest different possible degrees in partitioning of seismic and aseismic deformation as revealed by earthquake focal mechanisms in the basement and geodetic surveys at the surface of the sedimentary cover the integrated palaeostress results on the regional tectonic framework suggest that the late cenozoic brittle tectonic evolution of the w zagros was achieved through the transition from thin to thick skinned deformation under the oblique plate convergence highlighting a southwestward shift of the deformation partitioning during the final pliocene collisional reorganization

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

ophiolites as indicators of the geodynamic evolution of the tethyan ocean abstract reviewing the ophiolites of the alpine chain from the western mediterranean to the indian ocean we subdivide them into ten groups of outcrops the geological environment through time the internal composition and history are given for each emphasis is put on the variegated and successive geodynamic processes which can be reconstructed for the different ophiolitic belts since their birth as an ocean like crust to their incorporation through accretion to a continent one can distinguish low versus high tholeiitic partial melting fairly or poorly established ridges simple ridge type crust versus complex ridge ensimatic arc and even more complex type long lived versus short lived oceanic crust stages abrupt large scale pure obduction versus progressive collision related obduction or simple progressive juxtaposition to continent the major events through time in the tethyan ophiolitic belts can be listed as follows spreading ridge activity during the jurassic liguria dinaro hellenides lesser caucasus and the cretaceous peri arabic nain sabzewar with a jump of ridge in between repeated change from ridge to ensimatic arc dinaro hellenic lesser caucasus peri arabic west indian during the latest jurassic and cretaceous upper cretaceous tertiary collision related obductions pontic lesser caucasus liguria squeezing out the elevated portions of these basins lack of oceanic magmatism during the cenozoic but progressive accretion along accretionary wedges of the elevated portions of the remaining mesozoic crust van nain sabzewar zahedan mekran

geochemical characteristics 40ar 39ar ages and original tectonic setting of the band e zeyarat dar anar ophiolite makran accretionary prism s e iran abstract the makran accretionary prism in southeastern iran contains extensive mesozoic zones of melange and large intact ophiolites representing remnants of the tethys oceanic crust that was subducted beneath eurasia to the north of the makran accretionary prism lies the jaz murian depression which is a subduction related back arc basin the band e zeyarat dar anar ophiolite is one of the ophiolite complexes it is located on the west side of the makran accretionary prism and jaz murian depression and is bounded by two major fault systems the principal rock units of this complex are a gabbro sequence which includes low and high level gabbros an extensive sheeted diabase dike sequence late intrusive rocks which consist largely of trondhjemites and diorites and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks including radiolarian chert chondrite and primitive mantle normalized incompatible trace element data and age corrected nd pb and sr isotopic data indicate that the band e zeyarat dar anar ophiolite was derived from a midocean ridge basalt like mantle source the isotopic data also reveal that the source for basalts was indian ocean type mantle based on the rare earth element ree data and small isotopic range all the rocks from the band e zeyarat dar anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average e morb fractionation was controlled by the removal of clinopyroxene hornblende and plagioclase three 40ar 39ar plateau ages of 140 7 2 2 142 9 3 5 and 141 7 1 0 ma and five previously published k ar ages ranging from 121 4 to 146 5 ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the late jurassic early cretaceous plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the masirah ophiolite which has crystallization age of 150 ma like masirah the rocks from the band e zeyarat dar anar ophiolite complex represent southern tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90 100 ma bitlis zagros ophiolites including the samail ophiolite

the sabzevar blueschists of the north central iranian micro continent as remnants of the neotethys related oceanic crust subduction the sabzevar ophiolites mark the neotethys suture in east north central iran the sabzevar metamorphic rocks as part of the cretaceous sabzevar ophiolitic complex consist of blueschist amphibolite and greenschist the sabzevar blueschists contain sodic amphibole epidote phengite calcite omphacite quartz the epidote amphibolite is composed of sodic calcic amphibole epidote albite phengite quartz omphacite ilmenite and titanite the greenschist contains chlorite plagioclase and pyrite as main minerals thermobarometry of a blueschist yields a pressure of 13 15 5 kbar at temperatures of 420 500 c peak metamorphic temperature depth ratios were low 12 c km consistent with metamorphism in a subduction zone the presence of epidote in the blueschist shows that the rocks were metamorphosed entirely within the epidote stability field amphibole schist samples experienced pressures of 5 7 kbar and temperatures between 450 and 550 c the presence of chlorite actinolite biotite and titanite indicate greenschist facies metamorphism chlorite albite and biotite replacing garnet or glaucophane suggests temperatures of 300 c for greenschist facies the formation of high pressure metamorphic rocks is related to north east dipping subduction of the neotethys oceanic crust and subsequent closure during lower eocene between the central iranian micro continent and eurasia north iran

biostratigraphic zonation and 40ar 39ar ages for the neotethyan khoy ophiolite of nw iran abstract the khoy ophiolite in northwestern iran represents a piece of oceanic lithosphere formed in the mesozoic neotethys this ophiolite consists from bottom to top of well defined basal metamorphic rocks peridotites gabbros and a variety of upper to lower cretaceous deep marine rocks these include pelagic fossiliferous carbonates and basalts as interlayers or exotic blocks four hornblende separates from gabbros and amphibolites give values from 108 to 111 ma the results from microfossil studies show the presence of late cretaceous e g marginotruncana globotruncana and middle to late eocene e g globorotalia orbulonoides planktonic foraminifera the data suggest that although the initial rifting and crystallization of the oceanic crust of the khoy ophiolite began in the late albian the volcanic activities continued through to the late eocene time another possibility for such variation of lithofacies and biofacies and disturbances in the fossil record may be the result of bioturbation and transportation and rapid changes of depositional conditions during eruption of the basaltic lava flows pelagic sediments alone have been listed and of the foraminiferas the first three are planktonic as would be expected only orbulonoides is a benthic form and aberrant the third possibility is that the khoy ophiolite may represent a classic supra subduction type ophiolite where volcanic members of the different parts of oceanic environment e g morb volcanic arc ocean island are now juxtaposed

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

magnetostratigraphy of sediments from the yumu shan hexi corridor and its implications regarding the late cenozoic uplift of the ne tibetan plateau abstract the yumu shan located on the northernmost margin of the north qilian shan contributes key information for deciphering the deformation and uplift of the ne tibetan plateau this paper presents magnetostratigraphic results from late cenozoic sediments in the yumu section of the northwestern yumu shan the upper shulehe fm yumen conglomerates fm and jiuquan gravels fm were deposited before 5 23 ma during the periods 3 58 to perhaps 1 77 ma and after 0 8 ma respectively four significant tectonic episodes occurred at 5 89 ma 3 58 ma during 2 88 2 58 ma and 0 9 0 8 ma based on sedimentation magnetic susceptibility and angular unconformities being synchronous with tectonic activity throughout the ne tibetan plateau these tectonic ages do not support the northward growth model for the ne tibetan plateau during the late cenozoic

constraints on palaeodrainage evolution induced by uplift and exhumation on the southern flank of the zagros iranian plateau foreland sedimentary rocks from the northern fars region of iran contain a record of deformation associated with the cenozoic collision between arabia and eurasia that resulted in formation of the zagros orogen the timing of the deformation associated with this event is poorly known to address this we conducted a study of miocene foreland sedimentary rocks 19 7 14 8 ma of the chahar makan syncline using clast composition clay mineralogy and low temperature fission track dating the results showed that most of the sedimentary rocks were sourced from ophiolitic rocks detrital apatite fission track aft age signatures of miocene sedimentary rocks record exhumation in the hanging wall of the main zagros thrust and confirm that the change from underthrusting of the stretched arabian margin to widespread crustal thickening and deformation in the zagros region is no younger than 19 7 ma a transition from late oligocene to mesozoic eocene aft detrital age signatures between 19 7 16 6 ma and 16 6 13 8 ma is interpreted to reflect a possible rearrangement of palaeodrainage distribution that resulted from folding and expansion uplift of the zagros iranian plateau region supplementary material details of the succession of the study area a complete presentation of sample preparation and fission track age measurements of apatite grains and examples of x ray diffractograms are available at http www geolsoc org uk sup18502

subduction high p metamorphism and collision fingerprints in south iran constraints from zircon u pb and mica rb sr geochronology the esfandagheh region of the zagros orogenic belt is an ideal area to address many aspects of continental convergence between arabia and eurasia including incorporation of late neoproterozoic ediacaran basement subduction related magmatism and the formation of hp lt rocks the rock units exposed here represent a presumably jurassic magmatic arc within the sanandaj sirjan zone ssnz remnants of oceanic lithosphere blueschist and greenschist facies rocks and a distinct group of poorly characterized rocks u pb ages define four populations related to paleoproterozoic ediacaran carboniferous and jurassic magmatic events u pb ages of 1 8 1 7 ga for a pegmatite represent the first report of paleoproterozoic rocks in iran zircon u pb ages from the ssnz provide evidence for ediacaran 547 ma carboniferous 326 312 ma and jurassic 194 186 ma magmatic activity zircons from the haji abad ophiolites yielded jurassic ages the new rb sr results from white micas provide indications of a poorly constrained 85 ma high pressure metamorphic history rb sr ages of two chlorite epidote actinolite schists indicate that greenschist facies p t conditions had already been attained around 130 125 ma the new results are consistent with a model in which the closure of the esfandagheh ocean and the subsequent collision between arabia and iran led to incorporation of paleoproterozoic and cadomian rock units and tectonic juxtaposition of all lithotectonic elements from oceanic lithosphere to continental crust along the main zagros suture zone

cenozoic thermo tectonic evolution of the northeastern pamir revealed by zircon and apatite fission track thermochronology abstract the northeastern pamir is a key location to explore asian intracontinental tectonic processes during the cenozoic new zircon fission track zft data show a 20 to 50 km wide region of partially reset ages on the northeastern margin of the pamir salient interpreted as an exhumed and tilted partial annealing zone paz widespread zft age peaks at 50 ma within the zft paz likely date early motion of the kashgar yecheng transfer system kyts but suggest this fault system was narrower in the early cenozoic than it is today apatite fission track aft ages of 10 6 ma combined with field observations across the kyts hint at an episode of strong thrusting related exhumational cooling which indicates that the modern fault system probably formed at this time to the southwest of the kyts the combination of new fission track and existing thermochronology data allows establishing temperature time trajectories that present diachronous rapid cooling from 450 to 120 c in the sares 13 10 ma muztagata 10 7 ma and kongur shan 3 1 ma domes rapid cooling in the eastern sares and southern muztagata massifs is driven by doming as supported by kinematic analyses of the shen ti fault successive rapid cooling of these massifs confirms eastward propagation of doming processes shortly postdating magma emplacement at 11 ma we propose that the synchronicity of regional tectonism magmatism and metamorphism implies that strong crustal thickening and contraction occurred beneath the northeastern pamir during the middle late miocene possibly associated with initial collision between the pamir and tian shan

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

u pb zircon ages field geology and geochemistry of the kermanshah ophiolite iran from continental rifting at 79ma to oceanic core complex at ca 36ma in the southern neo tethys abstract the geodynamic evolution of the zagros mountains of iran remains obscure in particular the time of formation of the zagros ophiolites and the closure of the neo tethys ocean are highly controversial here we present new precise zircon u pb ages that show that the younger part sahneh kamyaran of the kermanshah ophiolite formed at 35 7 0 5 ma and the older part harsin at 79 3 0 9 ma field relations and geochemical evidence show that the younger sahneh kamyaran part is probably a fossil oceanic core complex and the older sahneh part is probably a continental oceanic transition complex both the sahneh kamyaran and sahneh parts were later emplaced into an accretionary complex we conclude and infer that the final closure time of the southern neo tethys ocean was after the late eocene our data and tectonic model have crucial implications for the geodynamic evolution of the zagros region

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

magnetostratigraphy of cenozoic deposits in the western qaidam basin and its implication for the surface uplift of the northeastern margin of the tibetan plateau abstract thick cenozoic deposits in the northwestern qaidam basin record erosion of the altyn tagh and high terrain west of that basin and presumably the concurrent growth of the northeastern tibetan plateau a detailed magnetostratigraphic study of the huatugou section northwestern qaidam basin reveals that this section spans the period from 30 to 11 ma magnetostratigraphic and sedimentological studies indicate that the accumulation rate abruptly increased near 15 ma the acceleration in sedimentation rate suggests enhanced tectonic deformation in the qaidam basin since 15 ma that may have begun simultaneously with accelerated deformation along the altyn tagh kunlun and haiyuan faults which contributed to the growth history of the qaidam basin and its surroundings since 15 ma

paleomagnetism and anisotropy of magnetic susceptibility of the tertiary janggalsay section southeast tarim basin implications for miocene tectonic evolution of the altyn tagh range abstract despite receiving considerable attention the nature and timing of cenozoic tectonic evolution of the altyn tagh fault aft remain highly debated here we conducted detailed magnetostratigraphy and anisotropy of magnetic susceptibility ams studies on the cenozoic janggalsay section southeast tarim basin magnetostratigraphy indicates that the measured janggalsay section spans a time interval of 22 11 ma the magnetic declination data suggest that the janggalsay site has rotated significantly counterclockwise 17 2 8 5 from 22 to 17 15 ma while the rotation has terminated from 17 15 to 11 ma the results are consistent with a two stage tectonic model for the atf with a phase of large magnitude strike slip motion along the atf from the oligocene to middle miocene and a second phase of lower amounts of strike slip motion and more widespread contraction since the middle miocene the ams fabric data and sedimentology of the janggalsay section likely indicate the rapid rock uplift of the altyn tagh range since 16 ma

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

elemental geochemistry and strontium isotope stratigraphy of cenomanian to santonian neritic carbonates in the zagros basin iran abstract a neo tethyan upper cenomanian santonian neritic carbonate ramp succession sarvak and ilam formations drilled in the zagros basin in southwest iran was investigated via detailed sedimentology microfacies analysis elemental geochemistry and sr isotope stratigraphy sis the succession contains two exposure surfaces which are known as the ct es and mt es cenomanian turonian and middle turonian respectively and associated prominent negative carbon isotope excursions that represent important regional stratigraphic marker horizons precise knowledge about the onset of platform exposure and the duration of the exposure related hiatus however is currently lacking due to a rather low resolved shallow water biostratigraphic framework and a bulk carbonate carbon isotope pattern that clearly differs from global late cretaceous reference curves therefore the existing bio chemostratigraphic framework was complemented by bulk carbonate strontium isotope stratigraphy sis as bulk carbonate material is in particular prone to diagenetic alteration a careful selection of least altered samples has been carried out by means of elemental geochemistry and petrography in contrast to what could be expected the meteoric alteration of limestones beneath both exposure surfaces is not clearly expressed by increasing iron and manganese and coeval decreasing strontium contents on the contrary the impact of meteoric diagenesis is well illustrated via pronounced increases in rb concentrations and concomitant prominent positive shifts to radiogenic strontium isotope values an observation that clearly reflects the decay of continentally derived 87 rb into 87 sr rubidium corrected strontium isotope values place the ct es around the cenomanian turonian boundary and point to an exposure duration of less than 0 4 myr this rather short term ct es related hiatus is supported by petrographic evidence which indicates a youth karstification stage of strata beneath the ct es following sis the ilam sarvak transition at the top of nezzazatinella dicyclina interval zone coincides with the mt es carbonates placing above this transition ilam formation are ascribed to the earliest to latest early santonian while carbonates immediately beneath the mt es sarvak formation are dated as late turonian sis thus indicates a long lasting hiatus of 4 5 myr associated with the mt es emergence represented by the ct es is here proposed as harbinger of the mt es in the zagros basin which most likely resulted from stepwise peripheral bulging due to ophiolite obduction in combination with a small scale global sea level fall around 94 ma

tectonic evolution of the orogenic belt in the region located between kerman and neyriz abstract tectonic evolution of the orogenic belt in the region located between kerman and neyriz is explored within the context of two ne dipping neotethyan subduction zones the northern subduction zone was responsible for formation of the mesozoic sanandaj sirjan magmatic arc and the late cretaceous shahr babak baft ensimatic back arc extensional basin the southern subduction zone was responsible for formation of the reverse primitive neyriz island arc the central iranian volcanic belt and its line of porphyry copper deposits as well as the rafsanjan back arc extensional basin proto rafsanjan depression due to the collision of the neyriz island arc with the sanandaj sirjan active continental margin the shahr babak baft back arc basin was tectonically inverted into the shahr babak baft ophiolite belt finally the continental collision between the arabian and the central iranian plates took place during the neogene this led to the tectonic inversion of the rafsanjan ensialic basin into the rafsanjan retro arc basin the present rafsanjan depression and emplacement of the syncollision to late or post orogenic granitoid rocks felsic mass in the rafsanjan region

paleocene early eocene uplift of the altyn tagh mountain evidence from detrital zircon fission track analysis and seismic sections in the northwestern qaidam basin

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

constraints on the collision and the pre collision tectonic configuration between india and asia from detrital geochronology thermochronology and geochemistry studies in the lower indus basin pakistan abstract knowledge of the timing of india asia collision is a fundamental prerequisite for understanding the evolution of the himalayan tibetan orogen and its role in global climate oceanic chemistry and ecological evolution despite much active research the basic pre collision tectonic configuration and the timing of terminal india asia suturing remain debated for example debates regarding when and how the intervening kohistan ladakh arc was sutured with india and asia still remain elusive some models propose the arc collided with asia at about 100 ma with india asia collision at ca 55 ma whilst a newer model proposed the arc s collision with india at 50 ma and subsequently with asia at 40 ma another example is the recent proposition that an oceanic greater india basin separated the tethyan himalaya microcontinent from the remaining indian plate until 20 25 ma with the consumption of this oceanic basin marking the final collision at this time these controversies relate to whether the commonly documented 50 ma contact represents the terminal india asia suturing or the amalgamation between various arcs or microcontinents with india or asia here we present an integrated provenance study of geochronology thermochronology and geochemistry on the late cretaceous pleistocene sediments from the lower indus basin on the indian plate the detrital zircon u pb and fission track data show a reversal in sediment source from a pure indian signature to increasing inputs from the suture zone and the asian plate between the middle paleocene and early oligocene the nd and sr isotopes narrow down this change to 50 ma by revealing input of asian detritus and the establishment of a nd sr isotopic pattern similar to the present day indus fan by 50 ma with no significant variations up section contrary to what might be expected if later major collisions had occurred our isotopic data indicate that greater india was occupied by a fluvial deltaic system analogous to the present day indus and named as the paleo indus which has been transporting asian detritus southward across the suture zone and kohistan ladakh arc since 50 ma suggesting no other ocean basins intervened between india and asia after this time in this region our data require that in the west the india asia collision were accomplished by 50 ma

eastward migration of the qaidam basin and its implications for cenozoic evolution of the altyn tagh fault and associated river systems the qaidam basin is the largest cenozoic intermontane basin within the tibetan plateau bounded by the qilian shan range to the north the east kunlun range to the south the altyn tagh range to the west and the ela shan range to the east its deposits consist of nonmarine sedimentary rocks with a maximum thickness exceeding 13 400 m modern sedimentation mainly takes place along the marginal areas of the basin where alluvial fans are formed along a series of rivers that originate from the surrounding mountains nevertheless drilling sedimentary and seismic data reveal that the depocenter of the basin during much of cenozoic time was not developed as foreland basins of the qilian shan belt to the north and the east kunlun belt to the south but instead the depocenter was in the center of the western part of the basin and had a northwest southeast trend that shifted eastward 380 km during oligocene to quaternary time this indicates that most of the basin sediments were not derived from the surrounding mountain ranges the results of the fi ssion track age dating reveal that the rapid uplift of the yousha shan anticline bounding the depocenter to the southwest initiated at ca 31 ma indicating that the southeastward shift of the depocenter began at that time yielding a migration rate for the depocenter of 10 2 mm yr the northwestern margin of the depocenter of the qaidam basin lies to the east of the westward prolongation of the qaidam basin the tula trough a 300 km long narrow trough fl anked by the altyn tagh range along the altyn tagh strikeslip fault on the north and the east kunlun thrust belt on the south the sedimentary and deformational evidence from this region indicates that the tula trough was formed as a syncline along which a paleoriver fl owed to the east into the qaidam basin to the southwest this river valley merges with the trace of the altyn tagh fault the latter and its westward continuation are interpreted to have extended to the northern margin of the pamir folded belt north of the western syntaxis of the himalaya we propose that most of the sediments within the depocenter of the qaidam basin were transported from the northern margin of the pamir folded belt along a progressively lengthening 2000 kmlong longitudinal river that developed along the left lateral altyn tagh fault and its western continuation as the eastward extrusion of the tibetan plateau took place in oligocene time both the crustal uplift and continuous southeastward aggradations of the sediments carried by the paleo kunlun river are interpreted to be the cause for southeastward migration of the depocenter of the qaidam basin the longitudinal river disappeared in late cenozoic time 2 4 ma due to stream capture and climate change

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

magmatic history of the northeastern tibetan plateau 1 the northeastern margin of the tibetan plateau is underlain by the qaidam and qilian terranes which consist primarily of mid proterozoic through lower paleozoic oceanic and arc type assemblages that have been accreted to the southern margin of the tarim sino korean craton most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the tarim sino korean craton during early paleozoic time the main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins or as a broad expanse of accretionary complexes our geochronologic data support a model suggested by sobel and arnaud 1999 in which the qaidam and qilian terranes are separated from the tarim sino korean craton by a mid paleozoic suture that closed along a southwest dipping subduction zone the basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of 920 930 ma granitoids early paleozoic arc type magmatism occurred between 480 and 425 ma apparently sweeping southwestward across much of the qilian and qaidam terranes accretion related magmatism along the inboard margin of the qilian terrane occurred between 423 ma and 406 ma following silurian devonian accretion the region has experienced late paleozoic and mesozoic uplift and erosion and has been severely overprinted by tertiary thrusting uplift and strike slip motion along the altyn tagh fault correlation of geologic features and magmatic histories between the altun shan and the nan shan suggests that the eastern altyn tagh fault has a total left lateral offset of 375 km

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

geochronology of the initiation and displacement of the altyn strike slip fault western china abstract 40ar 39ar dating studies have been carried out along the dangjin pass transect across the altyn strike slip fault assf the samples gave ages of 445 2 454 3 ma in the northern belt 164 3 178 4 ma in the mesozoic shear zone and 26 3 36 4 ma in the cenozoic shear zone using the piercing point of the bashikaogong fault and the cangma heihe fault an offset of 350 400 km along the assf has been estimated the 40ar 39ar dating of the syntectonic growth or syntectonic resetting minerals from the samples within the assf belt and offset estimations from different age piercing points suggest that the assf should be initiated in the middle jurassic 178 4 160 ma combined with previously reported ages our studies show that the assf is characterized by multi phase re activation during 85 100 25 40 and 8 10 ma following its initiation in the middle jurassic in the regional tectonic setting of convergence between the indian and eurasian continents

late tertiary reorganizations of deformation in northeastern tibet constrained by stratigraphy and provenance data from eastern longzhong basin the deformation of the tibetan plateau is central to unraveling the process and mechanism of continental tectonics although most agree that crust shortening and plateau growth were protracted throughout the cenozoic indo asian collision particular deformation histories relating to tectonic kinematics and dynamics are still incomplete due to sparseness of diagnostic geological information from plateau margin here we present combined investigation of stratigraphy magnetostratigraphy and provenance for the eastern margin of longzhong basin to show two reorganizations of basin formation and tectonic regime during the late tertiary first the depocenter migrated from the dispersed paleogene sequences to the wushan tianshui foreland sequence during the earliest miocene circa 22 ma accompanied by shift of sedimentary provenance from double sources including the eastern qilian block and eastern west qinling terrain to single source within the west qinling it suggests reorganization of deformation from nw se extension to ne sw contraction and initial uplift of the eastern west qinling second massive coarse grained fluvial beds were revived in the wushan basin during the late miocene circa 10 ma associated by eastward depositional expansion and another shift of sedimentary provenance toward northeast it reflects thrusting up of the northern edge of the west qinling and liupan shan mountains linked with relocation of crust shortening from ne sw direction to ene wsw direction and accelerated deformation of northeastern tibet these transitions of deformation regimes imply variation of geodynamic mechanisms during the process of plateau growth

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

a new tectonic scenario for the sanandaj sirjan zone iran abstract recent geochemical studies of volcanic rocks forming part of the ophiolites within the zagros and naien baft orogen indicate that most of them were developed as supra subduction ophiolites in intra oceanic island arc environments intra oceanic island arcs and ophiolites now forming the naien baft zone were emplaced southwestward onto the northeastern margin of the south sanandaj sirjan zone while those now in the high zagros were emplaced southwestward onto the northern margin of arabia thereafter subduction continued on opposite sides of the remnant oceans the floor of neo tethys ocean was subducted at a low angle beneath the entire sanandaj sirjan zone and the floor of the naien baft ocean was subducted beneath the central iranian micro continent the naien baft ocean extended into north west iran only temporarily this failed ocean arm between the urumieh dokhtar magmatic assemblage and the main zagros thrust was filled by thick upper triassic upper jurassic sediments the naien baft ocean finally closed in the paleocene and neo tethys closed in the early to middle eocene after arabia was sutured to iran the urumieh dokhtar magmatic assemblage recorded slab break off in the middle eocene

stable isotopic constraints on the tectonic topographic and climatic evolution of the northern margin of the tibetan plateau abstract this study presents oxygen and carbon isotopic records from lacustrine paleosol alluvial and fluvial carbonate sampled in thirteen cenozoic sedimentary sections that span the northern margin of the tibetan plateau isotopic analyses of 1475 carbonate samples yield 18 o values that range from 13 1 to 38 9 smow and 13 c values range from 11 5 to 3 7 pdb based on these analyses we see two isotopic trends regionally we interpret a paleogene decrease in average oxygen isotope values to result from an increase in the mean elevation of the study region s drainages as they tapped waters from a growing tibetan plateau to the south of the study area we interpret a neogene increase in both carbon and oxygen isotope values to result from the initiation of local mountain building in the study area and the withdrawal of marine waters which increased basin isolation aridity and temperatures the timing of inferred initiation of uplift in this region is consistent with structural thermochronological and sedimentological evidence for a tectonic transition in the early and middle miocene from accommodation of india eurasia convergence through extrusion along the altyn tagh fault to accommodation of convergence through active shortening and mountain building in this region this study also supports tectonic models for the differential uplift of the tibetan plateau in that mountain building in this study region significantly postdates estimates for the attainment of high elevation from isotopic studies of the central tibetan plateau

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

the growth of northeastern tibet and its relevance to large scale continental geodynamics a review of recent studies recent studies of the northeastern part of the tibetan plateau have called attention to two emerging views of how the tibetan plateau has grown first deformation in northern tibet began essentially at the time of collision with india not 10 20 myr later as might be expected if the locus of activity migrated northward as india penetrated the rest of eurasia thus the north south dimensions of the tibetan plateau were set mainly by differences in lithospheric strength with strong lithosphere beneath india and the tarim and qaidam basins steadily encroaching on one another as the region between them the present day tibetan plateau deformed and its north south dimension became narrower second abundant evidence calls for acceleration of deformation including the formation of new faults in northeastern tibet since 15 ma and a less precisely dated change in orientation of crustal shortening since 20 ma this reorientation of crustal shortening and roughly concurrent outward growth of high terrain which swings from nne ssw in northern tibet to more ne sw and even ene wsw in the easternmost part of northeastern tibet are likely to be in part a consequence of crustal thickening within the high tibetan plateau reaching a limit and the locus of continued shortening then migrating to the northeastern and eastern flanks these changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation

tectono metamorphic evolution of the neyriz metamorphic complex quri kor e sefid area sanandaj sirjan zone sw iran abstract the neyriz region includes outcrops of metamorphic rocks that are thrust over the neotethyan ophiolites these rocks are affected by a major deformational event the result of which includes a shearing polyphase foliation present in gneissic core domes overprinted by a crenulation cleavage these fundamental structures developed contemporaneously with a medium pressure metamorphism which is characterized by the syn kinematic crystallization of kyanite and the beginning of anatexis followed by the development of retrometamorphic mineral parageneses the major deformation phase in the area occurred during the early cimmerian orogeny in the late triassic following the orogeny the gneiss domes started to rise into the upper levels of the crust from the geodynamic point of view after the mid permian the studied area was situated at southern passive margin of the iranian plate the central iranian microcontinent at that time was separated by the neotethys ocean from the gondwanian supercontinent after the late triassic the region became an active margin associated with an accretionary prism the margin was finally involved in an orogenic wedge after the closure of the neotethyan oceanic basin in the late mesozoic closure of the basin resulted in a major thrusting of the metamorphic rocks of the southern iranian margin over the neotethyan ophiolites

early eocene sedimentary recycling in the kailas area southwestern tibet implications for the initial india asia collision abstract syncollisional sedimentary rocks in the himalayan orogen record important information about the early history of the india asia collision this article presents a combined stratigraphic sedimentologic micropaleontologic and provenance data for the early eocene clastic strata dajin formation in the kailas area southwestern tibet the dajin formation comprises ungraded and normally graded matrix and clast supported conglomerates ungraded and cross stratified sandstones and massive to poorly laminated mudstones deposition of the rocks is characterized by subaerial and subaqueous debris flows waning gravity flows and suspension fallout the larger benthic foraminifera and the youngest ages of the detrital zircons constrain the depositional age to the earliest eocene 56 54 ma the dajin formation contains abundant sandstone and mudstone clasts indicating significant sedimentary recycling in the source area u pb ages of detrital zircons are mostly clustered at 120 54 ma and have positive e hf t values suggesting a gangdese affinity clasts of the dajin formation are interpreted as having recycled from the forearc strata that were originally derived from the gangdese magmatic arc highly immature texture and recycled provenance lead us to propose that deposition of the dajin formation was a result of the development of fold thrusts in the gangdese forearc our new results and published data from the coeval strata in the himalayan orogen indicate that an underfilled foreland basin was initiated soon after the initial india asia collision and the dajin formation records the wedge top depozone of the basin system

two stage evolution of the altyn tagh fault during the cenozoic new insight from provenance analysis of a geological section in nw qaidam basin nw china introductionthe 1600 km long active altyn taghfault atf de nes the northern edgeof the tibetan plateau links thewestern kunlun eastern kunlun andqilian orogenic belts fig 1a and gures prominently in constrainingthe timing mechanism and stress dis sipation of the tibetan uplift e g yinand harrison 2000 tapponnier etal 2001 cowgill et al 2003 aspectof its cenozoic evolution has beenattracting hot debate zhou and gra ham 1996 yue and liou 1999 yinet al 2002 liu et al 2007 and akey issue is the initiation of left slipmovement on the fault which wasconsidered to occur i during lateeocene early oligocene evidenced bythermochronology in the altynmountain chen et al 2001 andsedimentology adjacent to the fault meng et al 2001 yue et al 2001 ritts et al 2004 or ii since middlemiocene through structural analysisin the qaidam basin yuan et al 2008 wang et al 2010 apatite s sion track result along the atf wanet al 2001 chen et al 2002 andcomprehensive analysis of the north ern tibetan plateau burch el et al 1989 wang 1997 meyer et al 1998 yin et al 2002 argued a much earlierinitiation age 49 ma of the westernand central segments of the atf butwith questionable proofs ritts et al 2004 two factors probably lead tothis uncertainty i the atf is toolarge to allow us to nd an appropri ate proxy deciphering accurately itsevolution and ii most extant studiesshare a bias that the atf has beenalways a left slip fault during thecenozoic which we regard as a mis understanding in this paper provenance analysis has beenproved e ective on revealing theuplifting history of mountains andevolution of major faults e g lindseyet al 2007 miao et al 2008 linet al 2009 marsaglia et al 2010 the qaidam basin is the largest sed imentary basin inside the tibetan pla teau and accumulated thick terrestrialsediments during the cenozoic rieseret al 2006 making itself an idealplace for deciphering the uplift andpropagation of the plateau yin et al 2007 2008 as it is bounded by theatf to the northwest fig 1b itssedimentation has possibly been tiedto the development of this large scaleintracontinentalfault fewworkshavebeen done on this subject due to thetough geographic conditions in thispaper we investigated a geologicalsection spanning eocene pliocene inthe nw qaidam basin and analysedin detail provenance of the sediments we link the result to cenozoic evolu tion of the atf and try to illustrate together with some other evidences that the atf was used to be a basalshear zone before its initiation of left slip since middle miocene which rec onciles the two contrasting viewpointson the cenozoic initiation of the atfabovementioned geological settingthe study section is seated at thesouthern side of the atf and incisesthe western limb of the nw trendingganchaigou anticline bounded to thenorth by a north directed reversefault a branch fault of the atf fig 1c the cenozoic strata are wellexposed in the section including thexiaganchaigou xg shangganchai gou sg xiayoushashan xy shangyoushashan sy and shizigou szg formations in an ascendingorder the xg formation consists of redmudstone siltstone and sandstoneattaining bed thickness of 0 2 1 m interbedded with conglomerate andpebble sandstone layers 10 20 cmthick figs 2 and 3a the stratumeven comprises more than 100 mthick calcareous mudstones in theupper part in the drillhole chaishen 3 about 6 km east of the sectionabstract

far field lithospheric deformation in tibet during continental collision 1 crustal deformation along the present day northern margin of the tibetan plateau has occurred since mid to late eocene time soon after india collided with eurasia assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet we show that far field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins when a strong region analogous to cratonic lithosphere like that of the tarim basin is included in the calculations shear and vertical strain rates concentrate along the southern edge of the strong region resulting in enhanced deformation there in calculations scaled to the dimensions of india and the tibetan plateau shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins crustal thickening accelerates later in the calculations we propose that the apparent increased exhumation and perhaps surface uplift around 10 ma may signal the arrival of the thickening front

transforming the miocene altyn tagh fault slip into shortening of the north western qilian shan insights from the drainage basin geometry although it has long been proposed that active crustal shortening within the qilian shan accommodates slip along the altyn tagh fault questions regarding the temporal and spatial amount of slip transfer remain debated traditional shortening calculations cannot yet be made accurately because the geology is not known well enough following the view that long linear drainage basins in the north western qilian shan developed in response to internal shortening associated with slip transfer along the altyn tagh fault we targeted this special irregular drainage basin geometry to develop an alternative way to estimate shortening we infer a total of 66 km of shortening in the western qilian shan with a north eastward decrease in the magnitude of shortening this 39 shortening strain is comparable to previous estimates estimates of dates of initiation support the view that significant shortening and topographical growth within the north western qilian shan occurred in post middle miocene time

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

the geology of the kuh e dalneshin area of southern iran and its bearing on the evolution of southern tethys the kuh e dalneshin area lay close to the northern margin of the arabian continent during mesozoic and palaeogene times recent mapping has shown that except perhaps in the late cretaceous it was inactive until it collided with central iran in the miocene the collision represented by a neogene melange zone in the ne resulted in the zagros orogeny and is expressed in the shelf sediments by folding parallel faulting and a possible arcuate sinistral shear in the late cretaceous a localized deepening of the shelf dates from the cenomanian a narrow trough in the w subsided to abyssal depths in the turonian following a period of faulting a more wide spread subsidence affected the shelf in the coniacian and santonian autochthonous cherts and associated sediments accumulated basaltic volcanics were erupted in the ne part of this basin and permian to cenomanian neritic exotic blocks were derived presumably from an uplift at the shelf edge the rocks from the outer edge of this basin slid westwards over their autochthonous counterparts probably in the campanian accompanied by further gliding of masses of earlier mesozoic cherts which had previously been emplaced on the continental margin a sheet of mantle peridotite partly serpentinized and enclosing neritic and metamorphic exotics in turn slid into the basin shelf sedimentation was renewed in the late campanian or maestrichtian but the transgression did not reach the area until the eocene the derivation and emplacement of the allochthonous rocks in the late cretaceous are still imperfectly understood alternative models are discussed one involving gliding from an aborted spreading ridge along the continental margin the other more conventionally based on subduction processes a preference is expressed for the former

geochemical 40ar 39ar age and isotopic data for crustal rocks of the neyriz ophiolite iran trace element data including the rare earth elements in the crustal sequence of the neotethyan neyriz ophiolite in southwest iran indicate normal mid ocean ridge basalt n morb or island arc tholeiite chemistry for the tang e hana basalt the data suggest that the tang e hana rhyodacite basalt plagiogranite and gabbro derived from a low k tholeiitic parent magma trace element distributions in amphibolite clasts in the sole detachment of the ophiolite south of lake neyriz correlate well with distributions in basalt clasts in the melange and in the tang e hana basalt these trace elements suggest that the amphibolite originated from metamorphism and deformation of a tholeiitic basalt protolith new 40 ar 39 ar incremental heating plateau ages from two hornblende plagiogranite specimens in the crustal sequence in tang e hana are 92 07 1 69 and 93 19 2 48 ma isotopic data for five tang e hana basalts yield end values of 7 8 and 7 9 and 87 sr 86 sr values of 0 70368 to 0 70476 the isotopic compositions ophiolite tectonostratigraphy and correlation of the 40 ar 39 ar cooling plagiogranite and deformation amphibolite ages suggest emplacement of the neyriz ophiolite either into an accretionary prism through offscraping and subduction erosion and or formation in a supra subduction zone environment around 82 96 ma progressive accretion probably led to the de velopment of a fore arc basin and deposition of upper cretaceous eocene fore arc and arc derived sediments on the ophiolite resume des donnees sur les elements traces incluant les terres rares dans la sequence crustale de l ophiolite neo tethysienne de neyriz dans le sud ouest de l iran indiquent que le basalte de tang e hana est de type basalte oceanique de type n n morb ou une tholeiite d arc insulaire les donnees recueillies suggerent que la rhyodacite le basalte le plagiogranite et le gabbro de tang e hana proviennent d un magma tholeiitique parent a faible teneur potassique la repartition des elements traces dans les clastes amphiboliques dans l unique portion separee de l ophiolite au sud du lac neyriz est en bonne correlation avec celle trouvee dans les clastes basaltiques du melange ophiolitique et du basalte de tang e hana ces elements traces laissent penser que l amphibolite provient du metamorphisme et de la deformation d un protolithe de basalte tholeiitique de nouveaux ges 40 ar 39 ar par paliers de temperature pour deux specimens de plagiogranite a hornblende appartenant a la sequence crustale de tang e hana ont donne des valeurs de 92 07 1 69 ma et de 93 19 2 48 ma des donnees isotopiques de cinq basaltes de tang e hana ont donne des valeurs end de 7 8 et de 7 9 et des valeurs 87 sr 86 sr de 0 70368 a 0 70476 les compositions isotopiques la tectonostratigraphie ophiolitique et la correlation entre les ges 40 ar 39 ar de refroidissement plagiogranite et de deformation amphibolite laissent croire que l ophiolite de neyriz a ete mise en place dans le prisme d accretion sous l effet de l ecaillage et de l erosion par subduction et ou s est formee au sein d un environnement a zone de supra subduction il y environ 82 ma a 96 ma l accretion graduelle a probablement conduit au developpement d un bassin d avant arc et au depot de sediments provenant de l avant arc et de l arc sur l ophiolite au cours du cretace superieur et de l eocene

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

geochemistry and ar ar muscovite ages of the daraban leucogranite mawat ophiolite northeastern iraq implications for arabia eurasia continental collision abstract daraban leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the mawat ophiolite kurdistan region ne iraq these coarse grained muscovite tourmaline leucogranites are the first leucogranite dykes identified within the mawat ophiolite they are mainly composed of quartz k feldspar plagioclase tourmaline muscovite and secondary phologopite while zircon xenotime corundum mangano ilemnite and cassiterite occur as accessories the a cnk value of the granite dyke samples varies from 1 10 to 1 22 indicating a strongly peraluminous composition cao na2o ranges from 0 11 to 0 15 and al2o3 tio2 from 264 to 463 similar to the strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas ar ar muscovite step heating dating yields 37 57 0 25 and 38 02 0 53 ma plateau ages for two samples which are thought to reflect either their magmatic emplacement or resetting during collision related metamorphism mineral chemistry shows evidence of both primary and secondary types of muscovite with cores favouring the magmatic interpretation and slight effects of a late syn serpentinization fluid seen at the rims geochemical features of daraban leucogranite dykes favour a syn collisional tectonic setting they probably formed in response to the continental collision between eurasia and arabia during the initial stage of the opening of the gulf of aden at 37 ma the muscovite ages and geochemical features of daraban leucogranite are strong evidence for the timing of the continental collision between northeastern arabia and eurasia in kurdistan region of iraq

magnetostratigraphy of paleogene sediments from northern qaidam basin china implications for tectonic uplift and block rotation in northern tibetan plateau abstract we have carried out a detailed magnetostratigraphic study of tertiary sediments in the northern margin of the qaidam basin northern tibetan plateau a total of 400 paleomagnetic samples with an interval of about 2 m were collected from the shanggancaigou and xiagancaigou formations in the hongsanhan section stepwise thermal demagnetization was used to isolate the magnetic components in these samples a higher temperature component htc was separated between 300 and 680 c from most of samples positive fold and reversal tests suggest a primary origin for the htc a magnetic polarity sequence is established and then compared with the geomagnetic polarity time scale ostracoda assemblages and also a fission track age from an adjacent section suggest a tertiary age of the shanggancaigou and xiagancaigou formations in the hongsanhan area the magnetozones in the xiagancaigou formation may correspond to subchrons c16n c18 ranging from 40 to 35 5 ma mid eocene and the shanggancaikou formation to subchrons c8 2n c15 age from 35 5 to 26 5 ma late eocene to late mid oligocene the accumulation of sandy conglomerate and conglomerate layers on top of the shanggancaikou formation and an abrupt increase in sedimentation rates are probably resulted from the main uplift of the north qaidam at about 30 ma when compared with coeval directions expected from eurasia the mean declinations indicate a slight vertical axis rotation of the northern qaidam basin since the mid eocene sinistral shear strain may be concentrated on the altyn tagh fault atf