devonian tetrapod trackways and trackmakers a review of the fossils and footprints abstract the earliest tetrapods are known from the upper devonian their remains are becoming better known from increasing numbers of specimens localities environments and ichnofossils each of the eight or possibly nine genera now represented by skeletal fossils is reviewed in its sedimentological faunal and stratigraphic context with an assessment of what might be inferred about the habitus and locomotory capabilities of each fossil trackways and their interpretations are then re examined in the context of the known body forms and consideration given to the degree of fit between the skeletal fossils the trackways and their interpretations the currently known devonian tetrapods are unlikely to have made any of the known tracks unless they were produced under water neither the skeletal fossils nor the trackways show good evidence of terrestrial locomotion among devonian tetrapods when the fossil material and recent phylogenetic analyses are taken in combination it appears that neither tetrapods nor limbs with digits are likely to have arisen before the frasnian this should be borne in mind in palaeoecological studies of these animals

paleomagnetism of early paleogene marine sediments in southern tibet china implications to onset of the india asia collision and size of greater india article i nfo we report a paleomagnetic study of paleocene marine sediments in the gamba area of the tethyan himalayan terrane southern tibet which aims to accurately locate the position of the northern margin of greater india and further constrain timing of initial contact between india and asia following detailed rock magnetic and paleomagnetic experiments on a total of 675 drill core samples collected from the zongpu and upper jidula formations characteristic remanent magnetizations chrms were successfully isolated from most samples following alternating field af or integrated thermal and af demagnetization the chrms are of dual polarity and pass fold and reversal tests indicating a pre folding origin together with detailed biostratigraphic investigation of the sampling section a magnetic polarity sequence is constructed from data at 167 sampled horizons and satisfactorily correlates with polarity chrons c24r to c27r dating the zongpu formation between 56 2 and 61 8 ma on the geomagnetic polarity time scale the chrm directions from the zongpu formation are grouped stratigraphically into 33 sites and yield two paleopoles of 71 6 n 277 8 e a95 2 5 and 67 3 n 266 3 e a95 3 5 for the time intervals comprising 56 to 59 ma and 59 to 62 ma respectively these new paleopoles imply that the tethyan himalayan terrane was sited at low latitudes in the northern hemisphere during the interval 62 56 ma suggestingthatinitial contact between thetethyanhimalayaand lhasa terranes was established before60 5 1 5 ma and verylikelyoccurred nearthecretaceous tertiary boundary at least in the central partof the suturezone the results alsoindicate that atleast 1500 480 kmof post collisionalcrustal shortening occurred within the himalayas from the analysis of available paleomagnetic data obtained on both sides of the suture zone we propose a conceptional collision model for the india asia collision

the coupling of indian subduction and asian continental tectonics abstract in order to understand the potential controls on asian tectonics during the subduction of the tethys and indian lithospheres we reconstruct the coupled subduction continent deformation history using tomographic imaging kinematics constraints and numerical modeling the global p waves tomographic images of the mantle below the india asia collision zone provide constraints on the deep structure of continents and subduction history linking the slab positions in the mantle to the asian tectonics reconstructions and the indian plate kinematics we reconstruct the timing and location of successive subduction and breakoff events showing one major breakoff occurred between india and the tethys ocean 45 ma in the western syntax a vertical slab continuous to the continent is shown to override the deeper detached tethys slab in the central region similar structure is found with a detached slab yet closer to the tethys slab in the eastern syntax no slab is imaged it is inferred that after tethys slab had broke off subduction only resumed in the center of the margin while underthrusting took place at both extremities of the convergent margin during following convergence a second breakoff event detached the central indian slab from the margin 15 ma ago which renewed indian lithosphere underthrusting below asia this most probably occurred when the tibetan plateau was already uplifted implying that uplift is not a direct result of underthrusting numerical models of breakoff during subduction illustrate the controls of slab detachment on the complexities of the indian margin in these models the subduction of continental lithosphere resumes after breakoff only where this is entrained by the mantle flow associated with the long lasting oceanic slab sinking that is in the center of the margin while converging continent edges underthrusts the upper plate furthermore the breakoff during subduction has profound implications on the asian intra plate tectonics in the models the breakoff is rapidly followed by large stresses in the upper plate interiors propagating at large distance from the margin along a belt oriented at 45 from the trench the long term evolution of the asian continental tectonics shows drastic changes in the fault pattern with successive strike slip faulting across the asian continent which are in agreement with the mechanisms illustrated by the models transient large coupling at the trench caused by the breakoff events during india asia convergence offers an explanation for episodic nucleation of lithospheric faults within the asian continent and their link to deep processes

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

structural evolution of the yushu nangqian region and its relationship to syncollisional igneous activity east central tibet field mapping geochronological analyses and cross section construction reveal a protracted deformation history and a minimum of 61 km of cenozoic ne sw shortening in present coordinates across the yushu nangqian thrust belt in northern tibet cenozoic contraction started prior to 51 ma and was followed first by northwest striking right slip faulting and later by northwest striking left slip faulting renewed ne sw contraction is expressed by folding of neogene strata and thrusting which again was followed by northwest striking left slip faults late neogene deformation is expressed by local north striking normal faults shortening across the yushu nangqian belt appears to be accommodated by thin skinned thrusting which raises the question of how the deformation was accommodated in the lower crustal levels to resolve this problem we perform geochemical analysis of igneous rocks dated as 51 49 and 38 37 ma the rocks exhibit geochemical signatures characteristic of subduction which implies that coeval crustal thickening in northeastern tibet was most likely induced by continental subduction

miocene volcanism in the lhasa block tibet spatial trends and geodynamic implications miocene ultrapotassic up shoshonitic p and calc alkaline ca volcanism is recognized widely over the tibetan plateau this volcanism occurred after the end of india s oceanic lithosphere subduction beneath the plateau ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data we have sampled two volcanic fields in the lhasa block 1 the eastern edge of the zabuye salt lake central part of the block 2 the southwestern edge of the yangbajin graben major and trace elements display high k2o 6 06 6 54 over a wide range of sio2 75 03 55 30 na2o 1 32 3 30 and lree hree enrichment with a significant rare earth element fractionation 266 la yb n 6 42 as well as negative nb ta and ti anomalies these features indicate that our samples are chemically identical to the post collision miocene up to p volcanic rocks already identified all over the tibetan plateau and the southern plateau specifically but distinct from the ca rocks found in the lhasa block the miocene volcanics found in the lhasa block present greater chemical variation than the rest of the tibetan plateau 40 ar 39 ar ages on both sanidine and biotite gave ages ranging from 16 16 g 0 12 ma to 16 01 g 0 12 ma and 10 60 g 0 14 ma to 10 88 g 0 17 ma 2 c excluding systematic errors relative to fish canyon sanidine at 28 02 ma for zabuye salt lake and yangying geothermal field volcanics respectively these results are consistent with the range of previously published ages from this part of the tibetan plateau the presence of n s trending dikes and the location of the volcanic fields suggest that local e w extension in the lhasa block was coeval we propose that the synchronous up p and ca volcanism in the lhasa block between 23 and 8 ma is the result of progressively deepening northward subduction of indian continental lithosphere producing asthenospheric upwelling convective thinning delamination of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub continental mantle lithosphere as well as eclogitic lower crust this mechanism could also explain the local e w extension observed in the same period 18 10 ma we also propose that lateral migration of the deepening indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the lhasa terrane

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently 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 owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

u pb zircon geochronology of silicic tuffs and chronostratigraphy of the earliest old red sandstone in the munster basin sw ireland

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

reconstructing greater india paleogeographic kinematic and geodynamic perspectives abstract key in understanding the geodynamics governing subduction and orogeny is reconstructing the paleogeography of greater india the indian plate lithosphere that subducted since tethyan himalayan continental collision with asia here we discuss this reconstruction from paleogeographic kinematic and geodynamic perspectives and isolate the evolution scenario that is consistent with all three we follow recent constraints advocating a 58 ma initial collision and update a previous kinematic restoration of intra asian shortening with a recently proposed model that reconciles long debated large and small estimates of indochina extrusion our new reconstruction is tested against paleomagnetic data and against seismic tomographic constraints on paleo subduction zone locations the resulting restoration shows 1000 1200 km of post collisional intra asian shortening leaving a 2600 3400 km wide greater india from a paleogeographic sediment provenance perspective eocene sediments in the lesser himalaya and on undeformed india may be derived from tibet suggesting that all greater indian lithosphere was continental but may alternatively be sourced from the contemporaneous western indian orogen unrelated to india asia collision a quantitative kinematic paleomagnetic perspective prefers major cretaceous extension and a greater india basin opening within greater india but data uncertainty may speculatively allow for minimal extension finally from a geodynamic perspective assuming a fully continental greater india would require that subduction rates close to 20 cm yr was driven by a down going lithosphere crust assemblage more buoyant than the mantle which seems physically improbable we conclude that the greater india basin scenario is the only sustainable one from all three perspectives we infer that old pre collisional lithosphere rapidly entered the lower mantle sustaining high subduction rates whilst post collisional continental and young greater india basin lithosphere did not inciting the rapid india asia convergence deceleration 8 myr after collision subsequent absolute northward slab migration and overturning caused flat slab subduction tibetan shortening arc migration and arc volume decrease

slab breakoff a critical appraisal of a geological theory as applied in space and time abstract the idea that prominent geological phenomena observed in the crust and at the surface may be caused by the detachment of subducting dense lithosphere has been anticipated by tectonic models in the sixties and seventies and imaged with improved geophysical techniques in the eighties in the mid nineties the model of slab breakoff was defined formally by davies and von blanckenburg initially proposed as a thought provoking working hypothesis the theory rapidly received wide a priori acceptance and was applied to virtually every mountain range around the world and even in orogens as old as paleoproterozoic and archean to explain a range of different phenomena these include magmatic flare ups rapid topographic uplift increasing sediment supply fast exhumation of metamorphic rocks ore mineralizations anomalous distribution of seismicity changes in stress regimes and inversion of subduction polarity even multiple breakoff events were assumed to have occurred at different times in the same orogenic belt or subduction zone in the last 20 years slab breakoff has been invoked in so many settings and time frames that it could have hardly taken place in each and every case in which it was called upon this article does not critically examine the theory but critically examines its use the extensive review of the vast literature presented here on the subject reveals how the model has been often applied to provide ad hoc explanations for a range of poorly understood observations based on incomplete evidence of deep seated processes our aim is to illustrate a paradigmatic example of how earth scientists in the face of evidence that challenges our capacity of understanding often recur to hypotheses based on other hypotheses such an approach may induce researchers to look for confirmation in the absence of compelling constraints or even in the face of conflicting evidence the faith in models should not lead us to confuse speculative theories with axiomatic truths and to build upon them theoretical edifices that are vulnerable and exposed to the risk of circular reasoning

direct stratigraphic dating of india asia collision onset at the selandian middle paleocene 59 1 ma the collision of india with asia had a profound influence on cenozoic topography oceanography climate and faunal turnover however estimates of the time of the initial collision when indian continental crust arrived at the transhimalayan trench remain highly controversial here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when asian derived detritus was first deposited onto india in the classical sangdanlin section of the central himalaya which preserves the best paleocene stratigraphic record of the distal edge of the indian continental rise deep sea turbidites of quartzarenite composition and indian provenance are replaced upsection by turbidites of volcano plutoniclastic composition and asian provenance this sharp transition occurs above abyssal cherts yielding radiolaria of paleogene radiolarian zones rp 4 6 and below abyssal cherts containing radiolaria of zone rp6 and calcareous shales with nannofossils of the paleocene calcareous nannofossil zone cnp 7 constraining the age of collision onset to within the middle paleocene selandian the youngest u pb ages yielded by detrital zircons in the oldest asia derived turbidites indicate a maximum depositional age of 58 1 0 9 ma collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 1 ma this age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern himalaya and suggests that within the resolution power of current methods the india asia initial collision took place quasi synchronously in the western and central himalaya

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

ichnology and depositional environment of the middle devonian valentia island tetrapod trackways south west ireland abstract nine tetrapod trackways are described from the middle devonian continental valentia slate formation on the north eastern coast of valentia island in county kerry ireland the trackways occur at three sites dohilla coosadillisk and culoo head the latter two being recorded for the first time morphological and taphonomic analyses of the trackways suggest they were made by populations of similar tetrapods but of varying size body length 0 5 to 1 m width 0 15 to 0 30 m that moved in some cases by terrestrial locomotion and in others possibly by a slow paddling gait while the substrate was submerged by very shallow floodwater sedimentological facies analysis of the trackway bearing sequences has been carried out for the first time and shows that the preserved trackways are associated with the late stages of 1 a principal river channel margin coosadillisk 2 a minor floodplain drainage channel culoo head and 3 a sand rich crevasse splay medial alluvial ridge environment dohilla the valentia island tetrapods inhabited marginal environments of proximal river channels 170 km northwards into the continent from postulated shallow marine habitats as they evolved this capability over a potential 4 5 million year period during the middle devonian

an eifelian u pb zircon date for the enagh tuff bed from the old red sandstone of the munster basin in nw iveragh sw ireland the first geochronological data from the minister basin sw ireland contradict the accepted biostratigraphical maximum age late givetian early frasnian of the oldest alluvium in the basin depocentre magmatic zircons from the enagh tuff bed a sub plinian air fall tuff within fluviatile old red sandstone of nw iveragh give a pb pb isotope age of 384 9 0 7 ma this confirms middle devonian probably eifelian sedimentation and allows possible early devonian basin initiation associated second cycle gravels suggest a pre 385 ma minimum age for the caherbla group of the dingle basin correlation with alluvium containing tetrapod trackways indicates a pre 385 ma age for the trace maker

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

integrated ichnofacies models for deserts recurrent patterns and megatrends abstract although it is commonly assumed that the trace fossil record of eolian dunes and associated environments is invariably poor a systematic review of the available information indicates that this is not necessarily the case a model involving five main phases of colonization of desert environments through the phanerozoic is proposed in this paper the first phase cambrian silurian involved animal incursions into coastal dune fields directly from the sea although it is unlikely that these animals would have remained for long periods of time in coastal deserts the second phase devonian reflects the activities of dune pioneers that left their fluvial habitat to enter temporary or permanently into inland deserts the third phase carboniferous permian involved the colonization of deserts by tetrapods the fourth phase triassic cretaceous involved a major exploitation of the infaunal ecospace as reflected by the appearance of more varied behavioral patterns in sub superficial structures the fifth phase paleogene recent reflects the appearance of the ecological dynamic that characterizes modern desert communities the invertebrate ichnofacies for eolian dunes is re named herein as the octopodichnus entradichnus ichnofacies honoring the seminal work of previous workers who simultaneously tackled the issue of eolian dune ichnofacies the chelichnus ichnofacies is retained for vertebrate trace fossil assemblages in eolian settings both ichnofacies occur in mobile and temporary stabilized sandy substrates subject to frequent erosion and deposition and to strong seasonality desert settings consist of complex mosaics of habitats or physical units associated with organism activity trace fossil distribution can be understood as reflecting the partitioning of desert settings in a mosaic of landscape units characterized by water content and its temporal fluctuations nutrient availability nature of the substrate and the dominant organisms present in turn desert systems are dynamic entities that change as a response to regional climate landscape units such as eolian sand seas salt flat and playa lake systems ephemeral rivers and alluvial fans interact in response to regional scale climate variations in hyper arid arid and semiarid climatic settings ancient deserts completely developed under hyper arid climatic conditions rarely preserve trace fossils due to the absence of moisture near the surface however the alternation of wet periods may represent windows for life development and thus preservation of biogenic structures arid deserts display complex patterns of dunes combined with dry wet and flooded interdunes dry desert elements e g dunes interdunes sand sheets typically record the entradichnus octopodichnus and chelichnus ichnofacies slight rises in regional precipitation produce elevation of the water table and increase of fluvial discharges that provide water and sediment to the system these processes may result in the local concentration of trace fossils in wet interdunes and ephemeral fluvial systems illustrating the scoyenia and chelichnus ichnofacies in semiarid systems playa lakes expand by the addition of freshwater evolving into freshwater lakes and fluvial systems may become more common lake margins and fluvial overbanks typically contain trace fossil assemblages that may be ascribed to the scoyenia ichnofacies

india s hidden inputs to tibetan orogeny revealed by hf isotopes of transhimalayan zircons and host rocks abstract to better understand tibetan orogenesis which involves complex tectonic processes here we report the first integrated analysis of hf isotopes of zircon separates and their host rocks from the transhimalayan batholiths the rocks studied include 19 granitoids from different parts of the gangdese batholith the largest transhimalayan intrusive complex and five postcollisional adakites that occurred as plugs or dikes cutting the gangdese batholith in the southern lhasa terrane southern tibet published zircon hf isotope data from other transhimalayan granitoids and the linzizong volcanic rocks are also synthesized the magmatic zircons crystallizing between ca 200 and 15 ma show depleted mantle type hf isotopic characteristics throughout the mesozoic but shift markedly in the paleogene the hf isotopic shift is interpreted as tracking the evolving progress of himalayan sediment subduction driven by the approaching indian continent and thus signals the initiation of the india asia collision that we infer to have occurred by 55 ma our data furthermore indicate that southern tibet underwent significant crustal thickening during ca 45 and 30 ma before emplacement of the postcollisional adakites that exhibit hf nd isotopic systematics suggesting binary mixing of melts from the juvenile gangdese mafic crust and incorporated himalayan sediments in the petrogenesis

non tetrapod trace fossils from the middle devonian tetrapod tracksite at zachelmie quarry holy cross mountains poland abstract the middle devonian eifelian dolomites exposed in the zachelmie quarry holy cross mountains southern poland contain the earliest fossil record of tetrapods low diversity but unique assemblages of trace fossils characterize the track bearing interval within the lower complex of the wojciechowice formation the reconstructed environment of the trace fossil rich beds includes ephemeral lakes and or shallow water lagoons separated from an open marine area by sparsely vegetated islands and spits the most abundant non tetrapod trace fossils from these beds are large horizontal to vertical burrows and dish circular to oval shaped shallow depressions the burrows are abundant on the upper surfaces of thick dolomicritic beds especially in some palaeosol levels but they are also associated with tetrapod trackways large semi aquatic or terrestrial animals e g arthropods or vertebrates most likely produced these burrows the circular to oval shaped depressions observed on a large surface and other smaller isolated blocks from the same stratigraphical level are interpreted as fish feeding nesting resting traces piscichnus sanctacrucensis isp nov is proposed as a new ichnospecies of the oval shaped fish trace fossil from the middle devonian of poland a single medium sized undichnia specimen a fish swimming trace is associated with the piscichnus traces the described trace fossils provide new insight into the palaeoecology and taphonomy of the middle devonian tetrapod tracksite from poland and clarify remaining controversies regarding the interpretation of some poorly preserved trace fossils from the zachelmie quarry

contemporaneous faulting and the eruption and preservation of the lough guitane volcanic complex co kerry interbedded with the clastic fluvial sediments of the munster basin the lough guitane volcanic complex represents a rare volcanic episode in the upper devonian of sw ireland from three distinctly separate volcanic centres an exclusively rhyolitic suite of thick lava flows and tuffs were erupted the largest of the centres possessed a system of contemporaneous faults producing a subcircular depressed area itself enclosed within a graben 1 5 km across these faults lead to rapid thickness variations in many volcanic units and a pooled lava flow reaches a thickness of 300 m the site of a major elongate vent lies along one such fault and rapid erosion of its poorly lithified walls generated coarse sandstone boulder agglomerates and is related to the absence throughout the complex of normal air fall tuffs unmixed with terrigenous sand the containment of the bulk of the volcanics by contemporaneous faults and subsequent rapid burial by terrigenous sediments has led to preservation of the volcanic succession virtually intact

the discovery of a new devonian tetrapod trackway in sw ireland a tetrapod trackway has been discovered in devonian sedimentary rocks of sw ireland it is the first discovery of its kind in europe details of the footprints are not preserved probably because of strong variscan pressure solution cleavage however after removing the effect of variscan deformation the large number of footprints more than 150 makes it possible to calculate the original dimensions of the footprint pattern stride approximately 34 cm and pace approximately 35 cm these values are higher than those for other traces of comparable age the total length of the tetrapod is estimated to be 1 m lithostratigraphical and biostratigraphical arguments indicate an age of mid to late devonian

tetrapod trackways from the early middle devonian period of poland the fossil record of the earliest tetrapods vertebrates with limbs rather than paired fins consists of body fossils and trackways the earliest body fossils of tetrapods date to the late devonian period late frasnian stage and are preceded by transitional elpistostegids such as panderichthys and tiktaalik that still have paired fins claims of tetrapod trackways predating these body fossils have remained controversial with regard to both age and the identity of the track makers here we present well preserved and securely dated tetrapod tracks from polish marine tidal flat sediments of early middle devonian eifelian stage age that are approximately 18 million years older than the earliest tetrapod body fossils and 10 million years earlier than the oldest elpistostegids they force a radical reassessment of the timing ecology and environmental setting of the fish tetrapod transition as well as the completeness of the body fossil record

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

structural evolution of the irish variscides the effects of the variscan orogeny on the upper palaeozoic succession are recognizable throughout central and southern ireland the deformation can be zoned using structural styles and trends which illustrate a northward decrease in deformation intensity coupled with increasing control by pre variscan structures in the basement the most intense deformation occurs to the south of the dingle dungarvan line where shortening approaches 50 as a result of layer parallel shortening followed by buckling and finally thrusting the deformation is considered to have developed above a shallow dipping sole thrust which has been located offshore by the swat seismic reflection profiles cross faults which are common throughout the orogen resulted from n s shear couples this is consistent with their origin as accommodation structures due to differential shortening along strike the original shape of the munster basin has exerted a control on the distribution and overall facing of the major structures the sole thrust extends to the north of the dingle dungarvan line and the structural zonation of the orogen is due to variations in the stratigraphic level of this thrust the leinster massif is considered to be essentially autochthonous slip on the sole thrust decays towards the massif which acted as a rigid passive block and was partially responsible for the arcuate trend of the orogen

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

tibetan garnet records early eocene initiation of thickening in the himalaya tectonic reconstructions of the himalayan orogeny depend on the age at which crustal thickening commenced to investigate this age we analyzed garnet from middle crustal rocks exposed in the north himalayan mabja and kangmar gneiss domes of tibet using lu hf geochronology garnet yielded lu hf ages of 54 52 ma in mabja and 51 49 ma in kangmar samples on the basis of microstructural and major element and rare earth element zoning observations the lu hf ages are interpreted as recording garnet growth during contractional deformation in the middle crust at 54 3 0 6 ma followed by variable recrystallization during subsequent high temperature ductile extension the new lu hf ages are the first to confirm that crustal thickening and contraction in the tibetan himalaya was broadly synchronous with the early eocene collision between greater india and the eurasian plate

paleomagnetism of eocene red beds in the eastern part of the qiangtang terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the tibetan plateau abstract a primary magnetic component with the acquisition time of 56 0 43 2 ma was isolated between 580 c and 685 c from the eocene gonjo and ranmugou formations in the eastern part of the qiangtang terrane tibetan plateau the tilt corrected site mean direction is d s 35 5 i s 29 3 k 45 9 95 3 2 the site mean inclination increased from 29 3 to 41 6 after multiple inclination shallowing corrections giving a paleopole of 57 9 n 192 1 e a 95 2 9 comparison of the paleogene paleomagnetic data for the qiangtang terrane and lhasa terrane reveals that both terranes experienced latitudinal crustal shortening before 54 43 ma indicating the uplift of southern and central tibet in the early eocene subsequently since 35 4 2 4 ma the north of the qiangtang terrane experienced 1300 410 km of crustal shortening indicating the uplift of the northern tibetan plateau the lhasa terrane and qiangtang terrane have not experienced further crustal shortening since the late eocene and the southeastern part of tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the tibetan plateau the crust of the tengchong terrane and shan thai block did not experience significant southward extrusion since the late oligocene early miocene the indochina block was situated in the north of the qiangtang terrane before the oligocene and since the early oligocene the indochina block began to experience southward extrusion from the north of the qiangtang terrane which absorbed part of the crustal shortening in the north of the qiangtang terrane

eocene to late oligocene history of crustal shortening within the hoh xil basin and implications for the uplift history of the northern tibetan plateau the timing and magnitude of deformation across the northern tibetan plateau are poorly constrained but feature prominently in geodynamic models of the plateau s evolution the fenghuoshan fold and thrust belt located in the hoh xil basin provides a valuable record of the cenozoic deformation history of the northern tibetan plateau here we integrate fault gouge geochronology low temperature thermochronology geologic mapping and a balanced cross section to resolve the deformation history of hoh xil basin chronologic data suggest that deformation initiated in the mid eocene continued until at least 34 ma and ceased by 27 ma the balanced cross section resolves 34 12 km upper crustal shortening 24 9 we explore whether the observed cenozoic shortening can account for the modern elevation and lithospheric thickness in the northern tibetan plateau for a range of reasonable preshortening conditions we conclude that the observed shortening alone cannot achieve modern crustal and mantle lithospheric thicknesses or modern elevation without either the removal of lithospheric mantle the influx of lower crustal material or some combination of these processes our results along with previous studies suggest that crustal shortening propagated into the northern tibetan plateau shortly after the onset of the indo asian collision the small magnitude of shortening and the late oligocene cessation of deformation in the northern tibetan plateau raise questions of how and where the remaining indo asian convergence was accommodated between eocene to mid miocene time prior to the approximately late miocene establishment of the deformation patterns observed in the present day

the vertebrate track record a renaissance in the study of fossil footprints has been driven by a multitude of discoveries and the realization that vertebrate ichnology makes important contributions to our understanding of terrestrial vertebrates more striking is the insight the track record gives us into bias and incompleteness in the vertebrate fossil record

a diverse pleistocene marsupial trackway assemblage from the victorian volcanic plains australia abstract a diverse assemblage of late pleistocene marsupial trackways on a lake bed in south western victoria provides the first information relating to the gaits and morphology of several megafaunal species and represents the most speciose and best preserved megafaunal footprint site in australia the 60 110 ka volcaniclastic lacustrine sedimentary rocks preserve trackways of the diprotodontid diprotodon optatum a macropodid probably protemnodon sp and a large vombatid perhaps ramsayia magna or phascolomys medius and possible prints of the marsupial lion thylacoleo carnifex the footprints were imprinted within a short time period demonstrating the association of the taxa present rather than the time averaged accumulations usually observed in skeletal fossil deposits individual manus and pes prints are distinguishable in some trackways and in many cases some digital pad morphology is also present several parameters traditionally used to differentiate ichnotaxa including trackway gauge and the degree of print in turning relative to the midline are shown to be subject to significant intraspecific variation in marsupials sexual dimorphism in the trackway proportions of diprotodon and its potential for occurrence in all large bodied quadrupedal marsupials is identified here for the first time

the palaeoenvironment and deformation of beaconites like burrows in the old red sandstone at gortnabinna sw ireland the gortnabinna area contains a number of well preserved deformed burrows within the fluviatile old red sandstone of s ireland detailed logging of the local succession enables subdivision into four facies facies 1 represents sheet flood deposits facies 2 ephemeral stream channels facies 3 marginal sheet flood deposits and facies 4 low energy settling of fines the burrows occur in facies 1 and 4 the sediments are now folded into a large upright westward plunging anticline the fold has a well developed axial planar cleavage and is extended along strike by listric normal faults the burrows in facies 1 were originally circular and orthogonal to bedding and reveal a n s layer parallel shortening strain of 2 1 followed by a shear strain of 1 48 during folding these burrows have a bimodal distribution of diameters with peaks in the ranges 50 80 and 110 170 mm the burrows of facies 4 are sub parallel to the bedding plane and are generally curved with a maximum width of 180 mm and lengths of up to 2 3 m these burrows display passive infill following abandonment the burrows are morphologically similar though distinct from beaconites barratti which has been described from elsewhere in the old red sandstone and are tentatively interpreted as lungfish burrows

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

flexural shear in a periclinal fold from the irish variscides abstract periclinal folds occur in a structural culmination in the centre of the irish variscan fold belt the culmination marks a zone of enhanced shortening which is associated with a locally attenuated stratigraphy and an anomalous deformation sequence with respect to the remainder of the orogen originally vertical burrows have been used along with cleavage fold relations to study both the three dimensional evolution of the periclinal folding and the temporal relationship between folding and cleavage as well as cleavage forming flattening strains the burrows record along strike shear related to the propagation of the periclinal folds a comparison with strain measurements made outside the culmination zone shows that the galley head area suffered higher strains integration of field data with published analyses of plasticine modelling of periclinal folds yields models for the propagation of a pericline and for the geometry of its lateral terminations

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

petrochronology of himalayan ultrahigh pressure eclogite the timing and nature of the india asia collision earth s largest ongoing continent continent collisional orogen are unclear ultrahigh pressure metamorphism of indian continental margin rocks is used as a proxy for initial collision because it indicates subduction of india records of this metamorphism are preserved only at kaghan valley pakistan and tso morari ladakh india separated by 500 km and having published ages of peak pressure of 46 2 0 7 ma and 53 51 ma respectively the apparent 6 m y age difference may refl ect multiple subduction events a large promontory along the former indian margin or inadequate constraints on the time of peak pressure recrystallization at tso morari we present 108 coupled in situ u th pb and rare earth element ree analyses of zircons in two tso morari eclogites to obtain age and petrologic information the ages range from ca 53 ma to 37 ma and peak at ca 47 43 ma flat heavy ree slopes and the absence of an eu anomaly are compatible with eclogite facies zircon re crystallization this re crystallization probably occurred at ultrahigh pressure because 64 of the analyses are from zircon included in ultrahigh pressure garnet and omphacite these results are consistent with those from kaghan valley and suggest that a single protracted ultrahigh pressure metamorphic event occurred contemporaneously across much of the orogen following initial contact of the indian and asian continents at ca 51 ma or later

a cretaceous eocene depositional age for the fenghuoshan group hoh xil basin implications for the tectonic evolution of the northern tibet plateau the fenghuoshan group marks the initiation of terrestrial deposition in the hoh xil basin and preserves the first evidence of uplift above sea level of northern tibet the depositional age of the fenghuoshan group is debated as are the stratigraphic relationships between the fenghuoshan group and other terrestrial sedimentary units in the hoh xil basin we present new radiometric dates and a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the fenghuoshan group from these data we infer an 85 51 ma depositional age range for the fenghuoshan group u pb detrital zircon age spectra from this unit are compared to age spectra from tibetan terranes and mesozoic sedimentary sequences to determine a possible source terrane for fenghuoshan group strata we propose that these strata were sourced from the qiangtang terrane and may share a common sediment source with cretaceous sedimentary rocks in nima basin field relationships and compiled biostratigraphic data indicate that the fenghuoshan and tuotuohe groups are temporally distinct units we report late oligocene ages for undeformed basalt flows that cap tilted fenghuoshan group strata together our age constraints and field relationships imply exhumation of the central qiangtang terrane from the late cretaceous to earliest eocene followed by eocene oligocene deformation and shortening of the northern qiangtang and southern songpan ganzi terranes crustal shortening within the hoh xil basin ceased by late oligocene time as is evident from flat lying basaltic rocks which cap older deformed strata

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

large scale subduction of continental crust implied by india asia mass balance calculation continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle when two continents collide the mass balance for the continental crust is therefore assumed to be maintained here we use estimates of pre collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the india asia collisional system using the current best estimates for the timing of the diachronous onset of collision between india and eurasia we find that about 50 of the pre collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at earth s surface today represented by the mass preserved in the thickened crust that makes up the himalaya tibet and much of adjacent asia as well as southeast asian tectonic escape and exported eroded sediments this implies large scale subduction of continental crust during the collision with a mass equivalent to about 15 of the total oceanic crustal subduction flux since 56 million years ago we suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent continent collisions is reflected in some ocean crust and ocean island basalt geochemistry the subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry buoyant continental crust is thought to resist subduction calculation of the crustal mass balance during the collision between india and eurasia indicates that about 50 of pre collisional continental crust has been subducted into the mantle

the tethyan himalayan detrital record shows that india asia terminal collision occurred by 54 ma in the western himalaya the himalayan orogen is a type example of continent continent collision knowledge of the timing of india asia collision is critical to the calculation of the amount of convergence that must have been accommodated and thus to models of crustal deformation sedimentary rocks on the indian plate near the suture zone can be used to constrain the time of collision by determining first evidence of asian derived material deposited on the indian plate however in the himalaya for this approach to be applied successfully it is necessary to be able to distinguish between asian detritus and detritus from oceanic island arcs that may have collided with india prior to india asia collision zircons from the indian plate asian plate and kohistan ladakh island arc can be distinguished based on their u pb ages combined with hf signatures we undertook a provenance study of the youngest detrital sedimentary rocks of the tethyan himalaya of the indian plate in the western himalaya we show that zircons of asian affinity were deposited on the indian plate at 54 ma we thus constrain terminal india asia collision when both sutures north and south of the kohistan ladakh island arc were closed to have occurred in the western himalaya by 54 ma

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

eocene neo tethyan slab breakoff constrained by 45 ma oceanic island basalt type magmatism in southern tibet slab breakoff is one of the primary processes in the evolution of many collisional orogens in the tibet himalaya orogen the timing of breakoff of the neo tethyan slab remains controversial because of a scarcity of solid evidence this study reports the discovery of eocene gabbros dated at 45 0 1 4 ma in situ u pb age of titanite using secondary ion mass spectrometry from the eastern segment of tethyan himalaya in southern tibet these rocks show geochemical characteristics similar to those of himu high type oceanic island basalt and have depleted sr nd isotopes 87sr 86sr t 0 70312 0 70317 end t 4 9 to 5 0 it is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination this event combined with results from relevant studies along the indus yarlung suture zone is best explained by a sudden and full scale detachment of subducted neo tethyan slab at great depth the breakoff model may account for coeval tectonomagmatic activities development of small scale short lived magmatism and subsequent termination of the gangdese arc magmatism in southern tibet and for the abrupt slowdown ca 45 ma of indo asia convergence

variable shortening rates in the eastern himalayan thrust belt bhutan insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions 1 we present data on the burial displacement and exhumation history of the himalayan fold thrust belt in eastern bhutan these data document the magnitude and timing of displacement of large discrete structures and highlight temporal variability in shortening rates eight new40ar 39ar ages from white mica 32 new zircon u th he ages 7 new apatite fission track ages and 1 new u pb zircon la mc icp ms metamorphic rim growth age are combined with published cooling ages and deformation temperatures and incremental shortening magnitudes from restorations of two published balanced cross sections to illustrate the kinematic and temporal development of the bhutan thrust belt integrating these data from 23 ma to the present illustrates rapid horizontal shortening rates 28 35 mm yr between 23 20 ma and 15 10 ma separated by more moderate rates 10 23 mm yr shortening rates decrease significantly to 7 10 mm yr and possibly as low as 3 4 mm yr from 10 to 0 ma this decrease is interpreted to represent the onset of strain partitioning in the eastern part of the himalayan tibetan orogenic system between shortening in the bhutan thrust belt uplift of the shillong plateau and deformation and outward growth of the northern and eastern tibetan plateau within estimated error horizontal shortening rates during emplacement of the main central thrust sheet and during construction of the upper lesser himalayan duplex approached india asia tectonic velocities thus for periods of time between 23 20 ma and 15 10 ma the bhutan thrust belt may have absorbed nearly all india asian convergence at this longitude

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

the 2000 nemuro hanto oki earthquake off eastern hokkaido japan and the high intraslab seismic activity in the southwestern kuril trench 1 the 2000 nemuro hanto oki earthquake mw6 8 occurred in the southwestern part of the kuril trench the hypocenter was located close to the aftershock region of the great 1994 kuril earthquake mw8 3 named the 1994 hokkaido toho oki earthquake by the japan meteorological agency for which the fault plane is still in debate analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event the hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage they clearly show that the 2000 event was an intraslab event occurring on a shallow dipping fault plane between 55 and 65 km in depth the well focused aftershock distribution of the 2000 event the relative location of the 1994 event with respect to the 2000 event and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow dipping fault plane in the subducting slab the recent hypocenter distribution around the 1994 aftershock region also supports this result large intraslab earthquakes occurring to the southeast of hokkaido may occur due to a strong coupling on the plate boundary which generates relatively large stress field within the subducting pacific plate

potassic volcanic rocks and adakitic intrusions in southern tibet insights into mantle crust interaction and mass transfer from indian plate abstract elucidating geodynamic processes at depth relies on a correct interpretation of petrological and geochemical features in magmatic records in southern tibet both potassic volcanic rocks and adakitic intrusions exhibit high sr y and la yb and low y and yb concentrations but these two rock types have contrasting temporal spatial distributions and isotopic variations here we present a systematic study on the postcollisional potassic and adakitic rocks in order to investigate their petrogenetic links with the coeval mantle derived ultrapotassic rocks and shed light on the potential input from underthrusted indian continental crust we found that adakitic intrusions with higher k2o na2o tend to display lower y and higher sio2 suggesting that the mantle derived ultrapotassic melts showing relatively high y and yb concentrations only played a minor role in adakitic magmatism therefore the unradiogenic 143nd 144nd and the dramatic decrease of zircon ehf t values since 35 ma shown by postcollisional adakites should be interpreted as reflecting the crustal input from indian plate unlike adakitic intrusions in southern lhasa subterrane potassic volcanic rocks share similar spatial distributions with ultrapotassic rocks and their isotopic discrepancy is diminishing with volcanic activity becomes younger and migrates eastward evidence from whole rock pb and zircon hf isotopes further indicates that potassic volcanic rocks are more likely to originate from partial melting of the overthickened and isotopically heterogeneous lhasa terrane crust rather than the underthrusted indian continental crust the elevated rb sr and varying sr cao in potassic volcanic rocks provide an argument for sanidine plagioclase clinopyroxene as the major fractionating phases during magmatic differentiation these findings not only highlight the significance of potassic and adakitic rocks in providing constraints on the geodynamic processes beneath southern tibet but also imply that special caution is needed if we attempt to probe into the nature of mantle lithosphere using isotopic tracers of the tibetan ultrapotassic rocks

mid eocene high sr y granites in the northern himalayan gneiss domes melting thickened lower continental crust abstract within the himalayan collisional belt granotoids occur along two sub parallel belts the northern himalayan gneiss domes nhgd and the high himalayan crystalline series hhcs in the yardoi area of nhgd two mica granite a new type granite occurs in the core of the yardoi gneiss dome ygd dala and quedang from north to south and extends at least 50 km long these granites have similar mineral composition elemental and radiogenic isotope geochemistry and age of formation shrimp zircon u pb dating indicates that the yardoi and the quedang two mica granites formed at 42 6 1 1 ma and 42 8 0 6 ma respectively similar to the dala pluton these two mica granites have 1 high sio2 68 wt al2o3 15 wt and a cnk 1 0 2 relatively high sr and lree but low y 40 and up to 250 and la yb 30 4 very weak or no eu anomalies and 5 as compared with those in the himalayan leucogranites low initial sr 87sr 86sr i

amount of asian lithospheric mantle subducted during the india asia collision abstract body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle focusing on the india asia collision zone we postulate the existence of several asian continental slabs based on seismic global tomography we observe a lower mantle positive anomaly between 1100 and 900 km depths that we interpret as the signature of a past subduction process of asian lithosphere based on the anomaly position relative to positive anomalies related to indian continental slab we propose that this anomaly provides evidence for south dipping subduction of north tibet lithospheric mantle occurring along 3000 km parallel to the southern asian margin and beginning soon after the 45 ma break off that detached the tethys oceanic slab from the indian continent we estimate the maximum length of the slab related to the anomaly to be 400 km adding 200 km of presently asian subducting slab beneath central tibet the amount of asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km using global seismic tomography to resolve the geometry of asian continent at the onset of collision we estimate that the convergence absorbed by asia during the indentation process is 1300 km we conclude that asian continental subduction could accommodate at most 45 of the asian convergence the rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction underthrusting processes continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like eurasia

upper devonian tetrapod palaeoecology in the light of new discoveries in east greenland for the first time new material of one of the world s oldest known tetrapods acanthostega has been studied in its sedimentological context the reported evidence suggests that acanthostega was primarily aquatic and inhabited active fluvial channels in fluvial dominated environments

crustal structure a key constraint on the mechanism of ultra high pressure rock exhumation abstract the distribution of ultra high pressure uhp metamorphic rocks demonstrates that burial to 100 km and rapid exhumation 1 cm a 1 of continental crust is a normal part of early 10 ma continental collision currently there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper crustal structures resulting from early collisional uhp rock exhumation characteristic features requiring explanation include structural domes that are cored by uhp nappes associated medium to high pressure nappes displaying a distinct pressure gap overlying lower grade rocks including suture zone ophiolites and coeval foreland directed thrust faults and syn exhumation normal faults we present a geodynamical model involving crustal burial and exhumation in a subduction channel below an accretionary wedge competition between down channel shear traction and up channel buoyancy forces expressed as the exhumation number e controls burial and exhumation leading to rapid up channel flow when e 1 exhuming uhp material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge driving thrusting and extension this solution is compelling because it explains both the geology and the petrology of the tso morari and other uhp complexes and because it demonstrates that pulse like buoyant exhumation from deep in the subduction channel creates observed upper crustal structures this places constraints on the exhumation mechanism and provides a test of alternative models other proposed mechanisms such as continuous circulation in a lithospheric scale wedge or overpressured subduction channel predict different types of upper crustal structures and are therefore unsatisfactory explanations for early collisional exhumation of uhp terranes

tetrapod or near tetrapod fossils from the upper devonian of scotland since 1932 the earliest known undisputed tetrapods have been of uppermost famennian late upper devonian age1 3 although a probable tetrapod jaw has been described from the lower famennian4 and tetrapod tracks of supposedly frasnian5 and possibly earlier6 age are known no fossil limb material older than the latest famennian has been discovered the panderichthyids panderichthys7 and elpistostege8 from the lower frasnian early upper devonian are regarded by some8 10 as the closest known sister group of tetrapods but panderichthyshas paired fins rather than limbs11 here i describe a hitherto unrecognized tibia from the upper frasnian middle upper devonian site of scat craig near elgin scotland12 14 collected during the nineteenth century which extends the fossil record of the tetrapod type hind limb by roughly seven million years15 other isolated bones from the same locality also show tetrapod characteristics the tibia a humerus and some incomplete jaws are discussed below but a complete description of the material is in preparation

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

geochemistry of eocene high mg adakitic rocks in the northern qiangtang terrane central tibet implications for early uplift of the plateau it is generally believed that the tibetan plateau is the result of crustal thickening in response to the collision of the asian and indian plates however the specific timing and uplift mechanism remain controversial the widespread occurrence of cenozoic lavas in the northern qiangtang terrane provides a unique opportunity to constrain the dynamic processes that resulted in uplift of the northern tibetan plateau eocene lavas from the northern qiangtang terrane display adakitic geochemical characteristics such as high sio 2 and al 2 o 3 contents low y and yb contents positive sr anomalies and high sr y and la yb ratios in combination with high mg 43 69 and negative anomalies for nb and ta which suggest a garnet rutile in and plagioclase free source residue the same samples also have high k 2 o and th contents high th ce ratios and low nb u ce pb ti eu and nd sm ratios as well as high 87 sr 86 sr i 0 7062 0 7075 and low e nd t 6 3 to 2 9 which show a clear continental crust affinity these high mg adakitic rocks combined with other characteristics of tibetan cenozoic lavas indicate that they were derived from partial melting of delaminated lower continental crust which subsequently reacted with surrounding mantle peridotites during ascent to crustal depths the eocene high mg adakitic rocks 46 38 ma north south trending shoshonitic dikes 47 38 ma and contemporaneous mantle derived mg rich potassic and shoshonitic lavas indicate that the thickness of the crust was at least 50 km before ca 46 ma at which time rapid uplift and extension occurred most likely caused by small scale delamination of the lithospheric mantle at 46 38 ma eocene in central tibet