landward dipping reflectors in accretionary wedges active dewatering conduits landward dipping seismic reflectors ldrs within the accretionary wedge at convergent plate margins have generally been interpreted as tilted bedding or thrust fault surfaces e g mexico northern japan this interpretation accords with models in which trench axis sediments are scraped off the descending plate to form a series of imbricate thrust sheets however intensely disturbed melanges with a regionally extensive landward dipping foliation are found beneath slope sediments in many ancient and active subduction complexes this observation combined with the fact that large volumes of fluid are released during dewatering of subducting and previously accreted sediments leads to the proposal that some of the seismic reflectors are porous laterally extensive fractureways filled with dewatering fluids derived from deeper underthrust water rich sediments these fractureways trend subparallel to the regional foliation of the accreted melange wedge the contrast in acoustic impedance between the high porosity fluid filled fractureways and the melange wall rocks is enhanced whenever methane and or carbon dioxide exsolve from rising waters ancient ldrs may be recognized where mineralized veins trend subparallel to the regional foliation of melange belts generated during subduction accretion 42 references 2 figures

comparison between the hot spot and geomagnetic field reference frames hot spots are located on the earth s surface and are caused by hot plumes of upwelling mantle material they manifest themselves by volcanic activity which changes its location in response to the movement of the lithosphere over the mantle plume it has been suggested1 3 that hot spots are relatively stationary with respect to each other and also that they represent a frame of reference with which to measure the absolute movement of the lithospheric plates over the surface of the earth another frame of reference can be partially defined by the position of the mean magnetic dipole field of the earth as the dipolar field is believed to be closely related to the spin axis this frame of reference automatically gives information about the palaeolatitude and orientation of the lithospheric plates but since the position of the ancient dipole field of the earth does not give any information about palaeolongitude and since in many cases the definition of the field is relatively poor due to less than adequate rock collections during the time of interest hot spot reference frames have been favoured by many people we now report a comparison of the two frames of reference and show that over the past 200 myr there has been relative movement between the two frames of reference of about 20 and that this motion has not been constant with time

geoforschungszentrum anomaly magnetic map gamma a candidate model for the world digital magnetic anomaly map the world digital magnetic anomaly map wdmam is an ongoing effort toward the mapping of worldwide available aeromagnetic data it is led by a task force of the international association for geomagnetism and aeronomy iaga and aims at distributing a global map in printed and digital forms in this paper we describe in detail our candidate model which has to be evaluated by the iaga task force together with five other candidate maps after discussing the quality of the available data we show a simple but effective method applied to successfully process reduce and merge together individual compilations the near surface data are corrected using global field models and further refined with two dimensional polynomial corrections after the upward continuation to 5 km altitude data are resampled to a 3 minute grid and merged together we then calculate a spherical harmonic model up to degree 199 and analyze the magnetic spectrum of the global map this helps us to confirm that wavelengths larger than 400 km are spurious at a global scale in aeromagnetic compilations therefore we substitute them using a satellite based lithospheric field model mf5 to degree 100 finally our proposed candidate map for wdmam is presented

secondary palaeoremanence of tethyan sediments from the zanskar range nw himalaya summary palaeomagnetic investigations have been carried out on tethyan sediments from the nw zanskar range 33 9 n 76 5 e a total of 455 oriented samples from 41 sites were drilled from four stratigraphic units of middle cretaceous to early eocene age shillakong formation marpo limestone spanboth formation stumpata quartzarenite spanboth formation kong formation a characteristic remanence chrm could be isolated for 27 sites 254 specimens through detailed thermal and alternating field demagnetization and multicomponent analysis all the chrm directions identified are similar irrespective of geological age they fail the fold test and yield an overall in situ mean direction of d 336 6 i 39 6 with 95 5 9 and k 22 9 this corresponds to a palaeopole position at 66 5 n 323 5 e n pole isothermal remanence irm acquisition and thermal demagnetization of a saturation irm identify pyrrhotite as the dominant ferrimagnetic mineral and carrier of the chrm the pyrrhotite remanence is probably a thermoremanent magnetization acquired when the temperature of low grade metamorphism decreased below about 300 c the chrm directions are thus clearly secondary in origin and a negative conglomerate test carried out on the kong formation demonstrates that remanence acquisition post dates the early eocene the mean chrm inclination suggests that the remanence was acquired at about 20 n cooling events in the higher himalayan crystallines indicate an oligocene age for this remanence acquisition the mean chrm declination shows a counterclockwise rotation of 23 4 since remanence acquisition assuming remanence acquisition at about 30 myr ago a slight counterclockwise rotation of about 10 relative to the stable indian plate can be concluded this does not fit with the general pattern of palaeomagnetic results from neighbouring areas within the western syntaxis of the himalaya from which a clockwise rotation relative to the stable indian plate is expected for the zanskar range

formation and tectonic evolution of the cretaceous jurassic muslim bagh ophiolitic complex pakistan implications for the composite tectonic setting of ophiolites the muslim bagh ophiolitic complex balochistan pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the indian and eurasian plates these ophiolites were obducted onto the indian continental margin around the late cretaceous prior to the main collision between the indian and eurasian plates the upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes each pluton has a transitional dunite rich zone at its base and new geochemical data suggest a similar mantle source region for both the plutons and dykes in contrast the lower nappe consists of pillow basalts deep marine sediments and a me lange of ophiolitic rocks the rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid ocean ridge the basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the indian plate the two nappes of the muslim bagh ophiolitic complex are thus distinctively different in terms of their age lithology and tectonic setting the recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved

contact metamorphism and depth of emplacement of the manaslu granite central nepal implications for himalayan orogenesis abstract the manaslu massif central nepal provides a well exposed example of a deeply eroded pluton its contact aureole can be followed from the base to the top along medium to low grade mesozonal to anchizonal tethyan metasediments contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18 21 km for the base and 9 13 km for the roof calculated temperatures within the aureole at 550 40 c are compatible with the intrusion temperature of a leucogranitic magma microstructural evidence shows that the temperature remained high 500 c at the base of the massif during and after granite emplacement whereas towards the top of the granite deformation proceeded rapidly at lower temperature heating of the abundant calcareous rocks in the contact aureole released a local co2 rich fluid whereas a h2o and boron rich fluid seems to have pervaded the whole zone this fluid is probably exsolved from the granitic melt during its crystallization the depth of emplacement of the massif has an important implication for the reconstruction of the himalayan geodynamic evolution implying a burial of the tethyan metasediments by a major refolding of the sedimentary cover or more probably by extensive development of the north himalayan nappes towards the south for more than 70 km before granite emplacement i e before the miocene the young age of the granite and its depth of emplacement suggest a rapid tectonic denudation in the order of 1 mm a 1 probably by normal faulting north of the massif during its cooling

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

palaeomagnetic data about southern tibet xizang i the cretaceous formations of the lhasa block summary a palaeomagnetic study of middle to late cretaceous redbeds from linzhou basin lhasa block north of the yarlung zangbo suture zone gives a stable palaeomagnetic direction of magnetization with a positive fold test six sites 57 samples d 333 i 38 k 78 95 8 pole 64 n 348 e we discuss the problem of a possible remagnetization but consider that this direction of magnetization gives a good approximation for the palaeolatitude of the lhasa block during middle to late cretaceous time results from more recent tibetan formations are also presented late cretaceous to palaeocene sediments and volcanics give a lower palaeolatitude of 10 n and but more recent andesites have emplaced about 30 n close to the present day latitude an interpretation is proposed whereby the lhasa block which was a part of asia in the early cretaceous has undergone first a southward motion accompanied by an anticlockwise rotation and then after the palaeocene a northward motion under the constraint of the colliding india

fluid evolution and graphite genesis in the deep continental crust metamorphism in deep continental crust is often accompanied by carbon rich h2o poor fluids1 5 and is characterized by the development of granulite facies mineral assemblages3 5 graphite a minor phase in many high grade metamorphic rocks6 17 has also been recognized in recent xenoliths of deep crustal material18 demonstrating its presence in modern deep continental crust although graphite is most common in biotite schists it has also been recorded in pyroxene gneisses two feldspar quartz pegmatites marbles and quartzo feldspathic gneisses11 15 19 the presence of supracrustal sequences in granulite facies regions6 19 suggests that the latter developed through progressive metamorphism and dehydration of shallow level crustal material much of this material must have experienced a metamorphic recrystallization in the amphibolite facies accompanied by the development of hornblende although graphite in the high grade rock may develop from oxidation of organic material this may not be the most common means for graphite genesis in much of west greenland14 and in southern norway12 the low grade hornblende bearing material does not contain significant quantities of organic compounds although graphite is found in higher grade metamorphic equivalents this strongly suggests that introducing co2 into deep crustal material leads to fluid evolution and graphite formation i outline here the evolution of fluid composition in the c o h system which is in equilibrium with common crustal mineral assemblages thermodynamic data for co2 h2o mixtures20 and mineral components21 are used to model the chemical changes in fluid composition which accompany introduction of co2 into various types of rock the results demonstrate that graphite genesis is a direct consequence of deep crustal metamorphism in the presence of a co2 rich fluid phase in addition the calculations demonstrate that the deep continental crust will not be a reservoir for large volumes of methane rich gas

paleomagnetic constraints on the late cretaceous and cenozoic tectonics of southeastern asia abstract many features of the cenozoic tectonic history of central and southeastern asia can be understood as direct consequences of the thrust and penetration of india into asia recent indentation experiments with plasticine tapponnier et al 7 have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism the available cretaceous and cenozoic paleomagnetic data from this part of the world have been reviewed and a new apwp for eurasia has been constructed for reference the negligible rotation of south china and large clockwise rotation of indochina are consistent with the model i e with an history of large scale left lateral strike slip motion along the altyn tagh and red river faults data from malaya and borneo can be reconciled with the model although in a less straightforward fashion the large counter clockwise rotation of south tibet implies that it rotated in sympathy with india during the collision and suggests that future indentation experiments should include this feature finally a middle cretaceous reconstruction of the south margin of asia is proposed one interesting result is the restored continuity of geological features in tibet and indochina with active subduction of oceanic indian plate crust taking place to the south at subtropical latitudes

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

new insights on intraplate volcanism in french polynesia from wavelet analysis of grace champ and sea surface data in this paper we discuss the origin of superswell volcanism on the basis of representation and analysis of recent gravity and magnetic satellite data with wavelets in spherical geometry we computed a refined gravity field in the south central pacific based on the grace satellite ggm02s global gravity field and the kms02 altimetric grid and a magnetic anomaly field based on champ data the magnetic anomalies are marked by the magnetic lineation of the seafloor spreading and by a strong anomaly in the tuamotu region which we interpret as evidence for crustal thickening we interpret our gravity field through a continuous wavelet analysis that allows to get a first idea of the internal density distribution we also compute the continuous wavelet analysis of the bathymetric contribution to discriminate between deep and superficial sources according to the gravity signature of the different chains as revealed by our analysis various processes are at the origin of the volcanism in french polynesia as evidence we show a large scale anomaly over the society islands that we interpret as the gravity signature of a deeply anchored mantle plume the gravity signature of the cook austral chain indicates a complex origin which may involve deep processes finally we discuss the particular location of the marquesas chain as suggesting that the origin of the volcanism may interfere with secondary convection rolls or may be controlled by lithospheric weakness due to the regional stress field or else related to the presence of the nearby tuamotu plateau

time dependent crustal temperature modeling central alps abstract the conventional modeling of crustal temperatures is based on the steady state approach which is reliable in tectonically stable regions only the present work focuses on the assessment of the influence of the transient effects on the thermal field at regions of ongoing continental collision in the first part of this study the thermal effects of most evident processes operating in collision zones thickening and flaking of the crust frictional heating on the surfaces of flakes bulk replacement of mantle lithosphere by hotter rocks and also uplift and erosion are estimated on the basis of 1 d time dependent synthetic models the results show that surface heat flow caused by the most of investigated transient processes may differ from the steady state one by quantities of the order of magnitude larger than the uncertainties of the heat flow measurements these differences may exist over tens of millions of years in the second part three 2 d time dependent temperature sections gradually including the above assessed effects have been calculated for the central alps the results show that time dependent models give better agreement of the calculated and measured heat flow patterns than previously developed for the same region steady state model cermak et al 1990 best agreement is achieved when only the transient effects of thickening and flaking and increased heat inflow from the mantle are taken into account a poorer coherence of the other versions with the measured heat flow is probably attributable to uncertain knowledge and consequently great simplification of spacing and time dependence of erosion and or frictional heating

regional modelling of the southern african geomagnetic field using harmonic splines summary over the southern african region the geomagnetic eld is weak and changes rapidly for this area series of geomagnetic eld measurements exist since the 1950s we take advantage of the existing repeat station surveys and observatory annual means and clean these data sets by eliminating jumps and minimising external eld contributions in the original time series this unique data set allows us to obtain a detailed view of the geomagnetic eld behaviour in space and time by computing a regional model for this we use a system of representation similar to harmonic splines initially the technique is systematically tested on synthetic data after systematically testing the method on synthetic data we derive a model for 1961 to 2001 that gives a detailed view of the fast changes of the geomagnetic eld in this region

error distribution in regional inversion of potential field data summary an increasing number of regional inversions of gravity and magnetic field data have recently been computed some based on satellite and ground data some on satellite data only in each case it is important to quantify the errors resulting from the confined data region or the satellite parameters in this study we investigate the error distribution of regional inversions for scalar gravity and vector geomagnetic data from satellite data only we include the effects of satellite altitude size of the data region signal to noise ratio and spherical harmonic degree of an a priori global field model we evaluate our results on the basis of a regional inversion of the earth s magnetic field

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

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

new constraints on the tectonic evolution of the eastern indian ocean abstract from marine magnetic anomaly studies a fossil spreading ridge is identified beneath the nicobar fan in the northwestern wharton basin several north south trending transform faults offset this ridge left laterally east of the 86 e transform fault our findings show that this ridge which was part of the plate boundary between the indian and australian plates ceased its spreading shortly after formation of magnetic anomaly 20 45 6m y b p since the breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p we suggest that the indian australian and antarctic plates were moving relative to one another from about 90 to 45 m y b p a triple junction would have existed in the southeastern indian ocean during that period of time at anomaly 19 time 45m y b p the junction became inactive and australia and india became a single plate the northwest southeast trending southeast indian ridge was formed by connecting the india antarctica spreading center with the australia antarctica spreading center its activity has continued to the present time

india asia convergence driven by the subduction of the greater indian continent the most spectacular example of plate convergence on earth was the motion of the indian plate towards eurasia and the subsequent collision density estimates of the greater indian continent after its upper crust is scraped off at the himalayan front suggest that this continental plate is readily subductable potentially explaining why the convergence did not halt on collision the most spectacular example of a plate convergence event on earth is the motion of the indian plate towards eurasia at speeds in excess of 18 cm yr 1 ref 1 and the subsequent collision continental buoyancy usually stalls subduction shortly after collision as is seen in most sections of the alpine himalayan chain however in the indian section of this chain plate velocities were merely reduced by a factor of about three when the indian continental margin impinged on the eurasian trench about 50 million years ago plate convergence accompanied by eurasian indentation persisted throughout the cenozoic era1 2 3 suggesting that the driving forces of convergence did not vanish on continental collision here we estimate the density of the greater indian continent after its upper crust is scraped off at the himalayan front and find that the continental plate is readily subductable using numerical models we show that subduction of such a dense continent reduces convergence by a factor similar to that observed in addition an imbalance between ridge push and slab pull can develop and cause trench advance and indentation we conclude that the subduction of the dense indian continental slab provides a significant driving force for the current india asia convergence and explains the documented evolution of plate velocities following continental collision

geological interpretation of dekorp 2 s a deep seismic reflection profile across the saxothuringian and possible implications for the late variscan structural evolution of central europe abstract a 250 km long deep seismic section the first of its kind in west germany s dekorp project is interpreted in terms of late variscan collisional tectonics the structural model used is based on the intracontinental thrust tectonics concept of modern collision orogens the crustal profile across the saxothuringian zone seems to indicate north facing ramps antiformal stacks and culminations within the crystalline basement of the internal mid european variscides that have been only partially overprinted by permo mesozoic crustal stretching

observatory crustal magnetic biases during magsat and orsted satellite missions 1 computing main field models derived from magsat and orsted satellite magnetic measurements the crustal influence on satellite data can be treated as random noise satellite data can thus be used in conjunction with magnetic observatory measurements to isolate the non core field at the observatory locations crustal biases for the horizontal northward x eastward y and vertical downward z components are computed for all magnetic observatories where data are available for magsat 1979 1980 and orsted 1999 2000 epochs to study their correlation over twenty years for a set of observatories installed after 1979 new crustal biases are computed

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

lamprophyric lavas in the colima graben sw mexico the colima graben located in sw mexico is one of three grabens which intersect about 50 km ssw of guadalajara forming a triple junction the 90 km long 20 60 km wide colima graben represents a n s rift of the e w trending mexican volcanic belt since the early pliocene the colima graben has served as a locus for the eruption of alkaline lavas the most recent of which are basanites and minettes erupted from late pleistocene cinder cones luhr and carmichael 1981 in this paper we report on older alkaline lavas which crop out in the graben s walls these rocks include phlogopite and hornblende bearing lamprophyres a phlogopite kalsilite ankaratrite and high k andesites these lavas crop out throughout the colima graben area and are intimately associated with calc alkaline lavas in the field compared to these the alkaline rocks are strikingly enriched in the incompatible elements particularly ba sr p and the lree unlike the younger late pleistocene alkaline cinder cone lavas most of the graben wall lamprophyres and the high k andesites represent magmas that appear to have undergone significant evolution since their generation including fractionation crustal contamination and possible magma mixing least squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres the occurrence of these alkaline lavas in an active calc alkaline volcanic arc is unusual and we suggest that they are a manifestation of the rifting processes which produced the colima graben

early tertiary deformation of the zhongba gyangze thrust in central southern tibet abstract as the boundary thrust between india and asia in southern tibet the zhongba gyangze thrust zgt emplaced the yarlung zangbo suture zone yzsz units in the hanging wall southward onto tethyan himalaya sequences ths of the northern indian continental margin in the footwall detailed field investigation electron backscatter diffraction ebsd analysis detrital zircon u pb geochronology and 40 ar 39 ar thermochronology were conducted to understand the evolution of the zgt in sangsang area central southern tibet the shear zone of zgt is located within the sedimentary matrix melange of yzsz that is mainly composed of matrix of blueschist with meta pelagic hemipelagic siliceous and siliciclastic rocks and blocks of basalt limestone and sandstone penetrative f1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope strong lattice preferred orientation lpo fabric initiated by the low temperature 350 450 c 010 001 slip system was detected by ebsd in the sodic amphiboles of the blueschist the 40 ar 39 ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of zgt occurred between 71 and 60 ma in the ths a newly documented younger unit preserving detrital zircons from the southern asian margin lies above the triassic cretaceous sequences that carries only detrital zircons from the indian continent this unit is dated to be 61 ma by the detrital zircon ages similar to the sangdanlin formation and representing the yarlung zangbo foreland basin yzfb system the zgt had probably been active due to the initial india asia collision and acted as the frontal thrust controlling the development of yzfb

high 3 he 4 he ratio in ocean sediments two decades ago merrihue1 reported 3he 4he ratios of 10 4 in ferromagnetic separates from a pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to 1 of the sediment a decade later krylov et al 2 compiled 3he 4he isotopic data on ocean sediments measured in the soviet union and observed that the 3he 4he ratio is generally higher in pelagic sediments where the sedimentation rate is lower they suggested that the high 3he 4he ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor however these important discoveries were almost completely neglected until we re examined the problem we have measured 39 sediments from 12 different sites 10 sites from the western to central pacific and two sites from the atlantic ocean we find 3he 4he ratios 5 10 5 for six sites well above the values generally observed in common terrestrial materials the very high 3he 4he ratio in the sediments is probably due to input of extraterrestrial materials input of stratospheric dust of 1 p p m which corresponds to a fallout rate of 2 000 tons per year can explain the observation

downward continuation of magsat lithospheric anomalies to the earth s surface summary the assumed lithospheric component of magsat data is downward continued to the earth s surface over continental sized areas using the depleted basis method with a regularizing constraint parker shure 1982 to make the problem numerically tractable and stable respectively the depleted basis is arranged to have a high density of points over the area of interest lower elsewhere all three components at satellite altitudes are used in the inversion for three orthogonal components at the earth s surface edge effects are seen if the depleted basis changes rapidly from a high to low spatial density which are eliminated by inserting a region of intermediate density basis points the downward continued field fits the data to within their probable uncertainties regularizing the solution produces a much smoother solution for very little increase in misfit some of the anomalies in the downward continued field over africa correlate with known tectonic features although the relationship between magnetic highs or lows and basins or cratons is not clear cut this magsat derived downward continued field can be used to control the longer wavelength component of the crustal magnetic field that is not defined by aeromagnetic surveys the method is illustrated on an aeromagnetic compilation of about 30 surveys from south africa however there is a mismatch in amplitudes between the aeromagnetic data and downward continued magsat data

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

lidar observations of sporadic sodium layers at mauna kea observatory hawaii the characteristics of very dense narrow upper atmospheric na layers observed above mauna kea hawaii 19 50 n 155 28 w are described these layers were observed on a total of 16 occasions during 30 hours of lidar measurements from january 17 to 22 1987 the most prominent sporadic layer which formed on the night of january 21 exhibited a peak density of 2 8 104 cm 3 near 96 km with a full width of about 2 km the rapid growth and decay of the layer lasted for about 40 min the apparent na production and decay rates of this layer were approximately 60 cm 3 s 1 even after the decay the narrow layer was observed continuously for almost 8 hours during this period the layer moved downward steadily with a mean apparent velocity of 31 cm s 1 the most significant characteristic of the 16 sporadic layers appears to be occurrence times the layers formed either in the late evening between 2100 and 2330 lst or in the early morning between 0300 and 0600 lst the layers of the early morning began forming within a short time span of 15 min from 0253 to 0308 lst on 3 different days the mean time of the maximum peak density of the early morning layers occurred about 6 hours after that of the late evening layers the mechanisms responsible for creating these layers appear to be related to diurnal tides and sporadic e layers

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

grimm the gfz reference internal magnetic model based on vector satellite and observatory data summary in this paper the new gfz reference internal magnetic model grimm is presented the model has been derived from nearly 6 yr of champ satellite data and 5 yr of observatory hourly means at high latitudes full vector satellite data are used at all local times which allows a separation between on one hand the fields generated by ionosphere and field aligned currents and on the other hand the fields generated in the earth s core and lithosphere this selection technique leads to a data set without gaps during the polar summers resulting in a core field model that has an unprecedented time resolution the modelled static core field secular variation and lithospheric field are all in good agreement with previously published magnetic field models order five b splines are used to model the variation in time of the core field the energy in the secular acceleration has therefore a smooth behaviour in time and increases continuously from 2003 5 mapping the field acceleration from 2001 5 to 2005 5 reveals its rapid and complex evolution over this time period at the earth s surface due to the applied regularization technique the acceleration energy in spherical harmonics 6 11 is significantly larger than for other models and we show that such a spectrum is acceptable

uranium enrichment in archaean crustal basement associated with overthrusting uranium deposits cumulatively in the 100 000 tonne u3o8 range occur within ductile shear zones transecting archaean basement gneisses at lagoa real north east of caetite state of bahia brazil1 3 fig 1 the gneisses dated at 2 600 3 000 myr are at amphibolite to granulite facies of meta morphism there is a general concensus that such rocks form near the base of the crust and typically exhibit pronounced depletion of u relative to the upper crust5 6 to the east and west of lagoa real the archaean gneisses overlie proterozoic meta sediments and volcanics of the espinhaco supergroup along well documented thrust surfaces7 8 limited u pb dating of the deposits indicates that u mineralization occured at 820 myr ref 9 involving removal of si and k coupled with na addition in oxidizing conditions v pb and light rare earth elements lree were introduced along with uranium from hypersaline co2 hydrocarbon rich fluids with 18o 4 at temperatures of 540 c depths of 15 km and pfluid pload we show here that the geological constraints are consistent with release through hydrofracturing of 1 000 km3 of over pressurized meteoric water recharged formation brines in the proterozoic sediments up through the hot overriding gneisses

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

lidar observations of mesospheric fe and sporadic fe layers at urbana illinois lidar measurements of mesospheric fe were conducted at urbana il 40 degree n 88 degree w during 4 nights in late october 1989 the average fe abundances layer centroid heights and rms widths varied between 1 0 2 0 times 10 sup 10 cm sup minus 2 89 0 90 5 km and 3 2 4 1 km respectively the peak densities of the layer near 90 km varied between 15 25 times 10 sup 3 cm sup minus 3 considerable gravity wave and tidal activity was observed in the fe profiles on 3 nights the formation and dissipation of 3 large sporadic fe fe sub s layers was observed these are the first observations of this phenomenon the 2 largest fe sub s layers formed at 102 4 km and 91 4 km over a period of about 20 min and their widths were approximately 2 3 km fwhm the peak densities of the fe sub s layers were as high as 31 times 10 sup 3 cm sup minus 3 and the abundances were as high as 5 8 times 10 sup 9 cm sup minus 2 the observations are compared with previous measurements of mesospheric fe and with recent observations of sporadic na na sub s layers

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

paleoseismological evidence of surface faulting along the northeastern himalayan front india timing size and spatial extent of great earthquakes the similar to 2500 km long himalayan arc has experienced three large to great earthquakes of m w 7 8 to 8 4 during the past century but none produced surface rupture paleoseismic studies have been conducted during the last decade to begin understanding the timing size rupture extent return period and mechanics of the faulting associated with the occurrence of large surface rupturing earthquakes along the similar to 2500 km long himalayan frontal thrust hft system of india and nepal the previous studies have been limited to about nine sites along the western two thirds of the hft extending through northwest india and along the southern border of nepal we present here the results of paleoseismic investigations at three additional sites further to the northeast along the hft within the indian states of west bengal and assam the three sites reside between the meizoseismal areas of the 1934 bihar nepal and 1950 assam earthquakes the two westernmost of the sites near the village of chalsa and near the nameri tiger preserve show that offsets during the last surface rupture event were at minimum of about 14 m and 12 m respectively limits on the ages of surface rupture at chalsa site a and nameri site b though broad allow the possibility that the two sites record the same great historical rupture reported in nepal around a d 1100 the correlation between the two sites is supported by the observation that the large displacements as recorded at chalsa and nameri would most likely be associated with rupture lengths of hundreds of kilometers or more and are on the same order as reported for a surface rupture earthquake reported in nepal around a d 1100 assuming the offsets observed at chalsa and nameri occurred synchronously with reported offsets in nepal the rupture length of the event would approach 700 to 800 km the easternmost site is located within harmutty tea estate site c at the edges of the 1950 assam earthquake meizoseismal area here the most recent event offset is relatively much smaller 2 5 m and radiocarbon dating shows it to have occurred after a d 1100 after about a d 1270 the location of the site near the edge of the meizoseismal region of the 1950 assam earthquake and the relatively lesser offset allows speculation that the displacement records the 1950 m w 8 4 assam earthquake scatter in radiocarbon ages on detrital charcoal has not resulted in a firm bracket on the timing of events observed in the trenches nonetheless the observations collected here when taken together suggest that the largest of thrust earthquakes along the himalayan arc have rupture lengths and displacements of similar scale to the largest that have occurred historically along the world s subduction zones

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

laminated veins and hydrothermal breccia as markers of low angle faulting rhenish massif germany abstract the emplacement of bedding veins and vein breccia in a fold and thrust belt setting is discussed we relate vein types to thrusting regimes using the rhenish massif in germany as case example the rhenish massif is a slate belt in a hercynian fold and thrust system the slate belt cleavage generally dips to the southeast and locally has been rotated into low angle shear zones parts of these are parallel to bedding and marked by quartz carbonate veins both shear veins and extensional veins are distinguished on the basis of fibrous crack seal microstructures most shear veins grew by incremental pull apart between en echelon slip surfaces s0 the overlap of which produced a vein stack the stacks have a lineation l0 resulting from a combination of slip fibre growth and dissolution pressure solution slip the attitude of l0 relative to fold axes and the asymmetry of vein microstructures are consistent with vein emplacement during the hercynian thrusting early shear zone veins were thrusted mylonitized and finally boudinaged or folded depending on their setting the shear veins include coeval sheets of vein hosted breccia the breccia contains angular fragments of mylonitized veins and slate the orientation of s0 and s1 in the breccia varies randomly per fragment indicating random fragment rotation unencumbered spin of country rock debris implies rock expansion possibly by rapid pull apart events during the thrusting

geological link between fluid inclusions dilatant microcracks and paleostress field seismic investigations using three component instruments have demonstrated shear wave splitting in widely different geological and tectonic environments shear wave analysis suggests that the inferred anisotropy can best be explained if the rocks contain distributions of subparallel subvertical fluid filled microfractures which are aligned according to the current stress field the crampin extensive dilatancy anisotropy eda model geological evidence for widespread fluid activity in the earth s crust supports this hypothesis and suggests that the myriads of planes of microscopic secondary fluid inclusions which characterise most geological materials represent fossil eda cracks field studies confirm a strong spatial relationship between the orientation of fluid inclusion arrays and the paleodirections of maximum compressive stress at elevated temperatures and pressures the healing of fluid filled cracks in the crust of whatever origin is considered to be a fairly rapid process moreover the inclusions which form as a result of crack healing respond to changes in temperature pressure and stress by changing shape and or volume this indicates a high degree of fracture compliance sufficient perhaps to react to rapid changes in the current stress field and associated shear wave splitting though individual inclusions typically 20 m will have no effect on the propagation of seismic waves with wavelengths of tens to hundreds of meters the rock volume sampled by the waves will be rendered anisotropic if the overall average of the inclusions shows a preferred orientation thus fluid inclusions may constitute one of the principal types of fluid filled microcrack described by the eda hypothesis

rb sr geochronology of granites and gneisses from the mount everest region nepal himalaya nineteen samples of granites orthogneisses and paragneisses from the high himalaya basement nappe s of the mount everest region have been dated by the rb sr method the post metamorphic tourmaline leucogranites of the upper imja drangka nuptse lhotse glacier have high initial sr87 sr86 characteristic of an anatectic origin from crustal material a whole rock isochron age of 52 m y early eocene has been obtained for the samples from the granite body of lhotse glacier apparently sr isotopic homogenization was not reached throughout the much larger nuptse granite

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

multicyclic history of the northern india continental margin northwestern himalaya 1 the geologic evolution of northern india is best recorded in the stratigraphic succession of the zanskar range northwestern himalaya which represents the most complete transect through this ancient continental margin sedimentary history began in the late proterozoic and recorded a late pan african orogenic event around the cambrian ordovician boundary when the gondwana supercontinent was eventually assembled the following long period of epicontinental deposition in shallow seas linked to palaeo tethys lasted until the early permian when a neo tethyan rift began to open between paleo india and the cimmerian microcontinents neo tethyan history can be subdivided into two sedimentary megasequences both recording a major tectonic and magmatic event in the lower part the first one began with breakup in the late permian and lasted until the end of the jurassic the second one started in the early cretaceous with the final detachment of india from gondwana and the opening of the indian ocean and ended with the india eurasia collision in the early eocene the two megasequences can be in turn subdivided into six transgressive regressive supersequences bounded by tectonically enhanced unconformities basal sandstone units of early permian late permian norian callovian early cretaceous and paleocene age are invariably associated with oolitic ironstones or reworked glauco phosphorites and mark the transgre sive part of each supersequence next condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing upward marly units capped by regressive platformal carbonates the six tectono eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of india from the rest of gondwana and document the combination of plate microplate reorganizations and eustatic climatic and oceanographic changes in the tethyan domain after the onset of collision between india and asia close to the paleocene eocene boundary obduction of the remnants of the neo tethys ocean floor onto the indian margin began and the latter underwent multiphase deformation with fold thrust shortening followed by heating and extension after the main metamorphic event ophiolitic nappes were re thrusted and finally emplaced with their sedimentary sole on top of the passive margin succession

plate rupture by hydraulic fracture resulting in overthrusting lithospheric plates bending into subduction zones may become ruptured by hydraulic fracturing initiated by the injection of mobilized sediments thick poorly indurated water laden sediments of the proximal continental rise are the most obvious potential source for the large volume of material required for this process rupture of the descending plate may result in the subducted oceanic part of the lithosphere becoming detached and sinking independently while the more buoyant continental part rises by isostatic rebound we show here that applying this model to eastern indonesia provides an explanation for the banda arc geometry the apparent absence of a normal forearc and for the position of the thrust sheets relative to the benioff zone1 we also report that it can account for the short phase of deformation of the australian continental margin preceding its uplift of 3 km ref 1

impact of arc continent collision on the conditions of burial and exhumation of uhp lt rocks experimental and numerical modelling abstract a 2 d physical and finite element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere the experimental technique allowed us to model the whole subduction collision process from oceanic subduction to deep subduction of the continental crust with the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process the results obtained by both techniques are coherent and mutually complementary they show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction the conditions for such a failure the weakening of this plate are prepared during oceanic subduction the weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates and due to back arc spreading in oceanic subduction zones with a compressional regime no back arc opening thick and strong back arc lithosphere the weakest zone is volcanic arc area when weakening becomes sufficient during subduction the lithosphere fails in this area the failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore arc almost half of the presently active subduction zones are characterised by a tensional subduction regime with back arc spreading in such subduction zones the weakest place is not the volcanic arc but the back arc spreading centre when a subduction regime changes from tensional to compressional failure occurs in the vicinity of the extinct spreading centre this process can occur during oceanic subduction again along either a trench vergent or trenchward dipping fault but the formation of a trench verging fault is most likely in this latter case which is a principal subject of our study the failure is followed by partial subduction of the arc plate complete subduction occurs during arc continent collision subduction of the continental margin when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle the arc itself can be subducted completely or be partially or completely scraped off and accreted a deeply subducted material including continental margin is preserved at relatively low temperatures between the lithospheric mantle and the cold subducted arc plate to about 150 km depth subduction of the arc plate is a major phenomenon which affects all processes associated with continental subduction from deep burial and hp lt metamorphism to exhumation of subducted material does this process occur in nature future investigations will allow us to answer this question in this paper we analyse the conditions of emplacement of a very young oceanic lithosphere samail ophiolite on the continental crust in oman in the late cretaceous and argue that this lithosphere formed in a back arc basin it reached and overthrust the arabian continent after complete subduction of the arc plate

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

the nuclear waste disposal problem an overview from a geological and geochemical perspective abstract in this paper we present a brief review of nuclear waste disposal research from a geological and geochemical perspective examples were chosen to illustrate both laboratory and field aspects of nuclear waste disposal and to show how these could be linked together by geological analogy and observation this strategy of linking geological observations and laboratory experiments and tailoring the disposal concept to simulate the more attractive features of the geology should give greater credence to the overall performance assessment of a nuclear waste disposal system

further palaeomagnetic data from chitral eastern hindukush evidence for an early india asia contact the eastern hindukush forms part of an elongate belt central domain collage of cimmerian microcontinents that encircles the northern part of the indian subcontinent a gondwanan origin is commonly assumed for this belt but a laurasian origin for the chitral region has been argued on palaeontological talent and mawson 1979 and palaeomagnetic klootwijk and conaghan 1979 grounds the laurasian view was based on a pilot study we undertook of upper devonian pisolitic ironstones from a thrust sheet at kur gh spur in chitral preliminary results showed a characteristic magnetization component d 318 i 6 5 n 7 block samples k 14 95 16 5 indicating an equatorial palaeoposition this component was thought to be of primary origin and was interpreted in terms of a late devonian laurasian affinity of the kur gh spur rocks this controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the resh n kur gh b ni region of chitral and the primary origin of the characteristic magnetization component refuted thermal demagnetization of 333 block samples from middle to upper devonian variegated sediments permian quartz flysch permo triassic carbonates and mid cretaceous redbeds showed two interpretable components a softer component of recent origin a and a harder characteristic component b of both normal and reverse polarity whose mean direction d 314 1 i 6 0 n 4 thrust sheets k 198 2 95 6 5 is comparable to the characteristic component observed in our preliminary study however the universal presence of this component throughout the thrust pile proves its overprint origin which we attribute to initial india asia contact palaeomagnetic information pertinent to the controversy of a laurasian versus a gondwanan origin of the chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt hence we retract herewith our original conclusion of a late devonian laurasian affinity of the chitral region on the basis of the palaeomagnetic evidence the secondary component b comprises a suite of secondary magnetizations acquired at equatorial to low northern palaeolatitudes and is attributed to initial contact between greater india and southern asia component b has been observed previously in the himalayan tibetan region both north and south of the indus tsangpo suture zone identification is herein extended to the hindukush region north of the northern kohistan or shyok suture zone which is a western continuation of the indus tsangpo suture comparison of this suite of collision at tributed equatorial palaeolatitude data from the india asia convergence zone with new palaeolatitude constraints from the ninetyeast ridge on the northward movement of the indian plate constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of greater india indicates that initial contact between northwestern greater india and southern asia was established at or before the cretaceous tertiary boundary the overprint origin of component b at about this time is further supported by observations by zeitler 1985 on rocks from the sampled area in chitral of partially reset zircon fission track ages around 68 55 ma the nw se declination axis of component b indicates a 60 70 counterclockwise rotation of the sampled thrust pile with respect to eurasia and a counterclockwise rotation between 10 and 30 with respect to india some of the recent field components a show a comparable rotation and indicate that the tectonic activity that led to the formation of the hindukush pamir karakorum syntaxial zone has continued into recent times

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

modelling the geomagnetic field from syntheses of paleomagnetic data abstract this review examines results from time varying geomagnetic field models that span several thousand years and from variations in dipole moment strength up to million year time scales for the past 400 years twin magnetic flux lobes bordering the inner core tangent cylinder in both northern and southern hemispheres dominate the geomagnetic field and appear more or less fixed in location in contrast the millennial scale view shows that such features are quite mobile and subject to morphological changes on time scales of a few centuries to a thousand years possibly reflecting large scale reorganization of core flow the lobes rarely venture into the pacific hemisphere and average fields over various time scales generally reveal two or three sets of lobes of diminished amplitude thus millennial scale models are suggestive of thermal core mantle coupling generating a weak bias in the average field rather than a strong inhibition of large scale field changes the recovery of variations in dipole moment on million year time scales allows frequency domain analyses to search for characteristic time scales for core dynamics that might be associated with excursion and reversal rate time taken for reversals or any signs of control by earth s orbital parameters the spectrum is characteristically red for the time interval 0 160 ma suggesting non stationarity associated with average reversal rate changes probably reflecting the impact of superchrons and a continually evolving core distinct regimes of power law decay with frequency may reflect different physical processes contributing to the secular variation evidence for non stationarity at shorter time scales is also present in dipole moment variations over 0 2 ma with average growth rate faster than the decay process rates of change of dipole moment and rapid local field variations found in the paleomagnetic record are evaluated in the context of the 400 year historical record and the spectrum of geomagnetic variations for 0 160 ma

paleomagnetism of miocene sediments from the turfan basin northwest china no significant vertical axis rotation during neotectonic compression within the tian shan range central asia abstract paleomagnetic results are reported from 377 samples of miocene red sediments in the turfan basin of east tian shan northwest china thermal demagnetization for most specimens isolates a low temperature component that groups around the present geomagnetic field a high temperature characteristic component is isolated from four localities and interpreted as primary remanent magnetization from apparent pre folding origin and positive reversal test as for previous results obtained from tertiary rocks in central asia the magnetic inclination is significantly shallower than the reference field computed from the eurasian apw path at 10 and 20 ma magnetic susceptibility measured in 324 specimens exhibits low anisotropy of less than 1 05 for 80 of the sample the low anisotropy degrees ams shape and orientations suggest that the studied rocks are unlikely to have undergone significant strain during compaction or tectonism we argue that the shallow inclination is likely to result from an initially shallow detrital remanent magnetization comparison of the paleomagnetic declination from eurasia north china and tarim suggests that the turfan basin has been subjected to little or no vertical axis rotation since the miocene together with paleomagnetic declinations on both sides of the talas ferghana fault we propose that the tian shan to the east of the talas ferghana fault has remained relatively stationary with respect to adjacent blocks since the miocene

on the global distribution of sporadic sodium layers 1 limb scanning measurements of the dayglow radiance at 589 nm made with the osiris spectrometer on the odin satellite are used to retrieve na layer profiles with sufficient vertical resolution 2 km to observe sporadic na layers ssls a two year data base 2003 2004 has been analyzed to determine the global distribution of the ssl occurrence frequency at 0600 and 1800 hrs local time this shows that ssls are much more prevalent in the southern hemisphere with a particularly active region extending from south america at latitudes greater than 40 s to the antarctic peninsula the global average ssl occurrence frequency is about 5 by analyzing the occurrence of ssls in successive limb scans it appears that most ssls have a horizontal extent of less than 300 km however on one occasion during this 2 year study two apparently very large ssls extending about 70 in latitude were observed

a regional model of the geomagnetic field over the pacific ocean for epoch 2002 1 the pacific region has been geomagnetically less surveyed in the sense that there are a very limited number of ground stations that continuously measure the earth s vector magnetic field recently however long term vector geomagnetic observation has started at several stations mostly on oceanic islands but including one seafloor station which presently reinforces monitoring the geomagnetic field by the land based observation network the new geomagnetic data together with those collected by intermagnet magnetic observatories imos being operated in the pacific and its rim were combined to give a regional geomagnetic reference field over the western pacific rgrf p for epoch 2002 using spherical cap harmonics schs the resultant snapshot of the regional geomagnetic field was found to coincide well with existing global models e g the 10th generation of international geomagnetic reference field igrf 10 this implies that the regionally denser geomagnetic network still sees small contribution of non dipole components over the pacific

how far did india drift during the late cretaceous placenticeras kaffrarium etheridge 1904 ammonoidea used as a measuring tape abstract india once a member of the lost supercontinent gondwana broke away from it and made a solitary northward excursion and finally collided with asia during its long voyage india remained isolated for 100 ma and is expected to be characterized by stunning endemic biodiversity but this is not recognized by the terrestrial faunal and floral content and their distribution patterns paint no simple scenario for example the inter trappean vertebrate faunas of india which lived during india in exile do not show any made in india assemblages but rather betray a mixed biota having both gondwanan and laurasian affinities these differential distribution patterns of fauna and flora and their affinities with those of other areas prompted many workers to envisage an array of suggestions regarding the time of india s final separation from gondwana the time of northward drifting and different palaeopositions during its long journey but closer examination of the nature of the vertebrate fossil records reveals that the so called elusive endemicity of indian fauna during its sojourn is in fact a product of taxonomic artefact the majority of the faunas have been described on the basis of poor fossil data and comparisons for biogeographic correlations are made at higher taxonomic levels which perhaps masked india s faunal distinctiveness yet the biological processes that constrain biogeographical distribution operate at the species level in this paper we present our own data to reconstruct the palaeoposition of india in the late cretaceous and to estimate the time of its northward migration the present study is based on a newly recorded ammonite species placenticeras kaffrarium etheridge from the coniacian horizons in bagh central india the species abounds in bagh and represents a complete population structure it resembles significantly the populations described from the coeval horizons of madagascar and zululand south africa p kaffrarium has a stunning display of intraspecific variability represented by at least seven distinct morphotypes reported from zululand and madagascar the indian population significantly matches the latter two populations variant by variant and remarkably also for the sexual dimorphs within each variant geographic range is a biological property of a species and ammonite species have differential distribution patterns depending on the length of larval longevity oceanic circulation patterns and favourable environments placenticeras was a nektobenthic genus living in the shallowest depths of epicontinental seas and could not cross a narrow strait wider than a few hundred kilometers palaeogeographic distribution patterns of all placenticeratid species during the late cretaceous support this view and show the wandering nature of the genus over time however they always exhibit a latitudinally limited distribution and are restricted mainly to the subtropics north and south of the equator thus it appears that the p kaffrarium population of zululand madagascar and bagh constitute a homogenous single breeding population implying geographical proximity of these areas connected by exclusively shallow seas till the coniacian other fossil records of ammonites and geophysical data also support this view shortly after this india started its northward drift by the late maastrichtian india was completely isolated by a wide oceanic gap from the rest of the gondwana fragments and the northern part of india crossed the equator this is evident from strong endemicity of ammonite species assemblages of the northwest and southern indian subcontinent which constitute the most diverse communities known

crust mantle relationships in the french variscan chain the example of the southern monts du lyonnais unit eastern french massif central garnet lherzolite from the lyonnais area eastern french massif central occurs as several lenses elongated within the regional foliation of garnet biotite sillimanite gneisses within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses petrological and thermobarometric studies emphasize a tectonometamorphic re equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution during the burial stage interpreted as lithospheric subduction the peridotites underwent their mylonitic deformation under high pressure conditions 23 30 kbar in contrast the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions 6 8 kbar 700 c our thermobarometric investigations allow us to interpret the granulitic ultramafic association from the monts du lyonnais area as a lithospheric section buried into a palaeozoic subduction zone laminated during continental collision and uplifted by erosion processes

shortening budgets and the role of continental subduction during the india asia collision abstract the india asia collision provides a remarkable example of diffused deformation because the zone of compressive strain extends far into the hinterland of the himalaya molnar and tapponnier s science 189 1975 419 concept of india as a rigid indenter responsible for the diffused deformation has been widely accepted perhaps uncritically this review gives a reappraisal of landmark papers by dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 which discussed the shortening budget in se asia and the evidence for the amount and the rate of the convergence between india and asia taking place after the initial collision of these continents at roughly 55 50 ma evidence for the total convergence comes from magnetic anomaly palaeomagnetic and volumetric balancing studies the results are consistent in showing about 1800 2100 km convergence between india and asia in the western sector 2475 km in the central and 2750 2800 km in the eastern sector the rate of convergence between india and asia over the period since initial collision is roughly 5 cm year however the possible long term rate of convergence within the himalaya is considerably less i e 1 5 2 0 cm year a fraction of the total convergence this shortfall can be made up by adding to the rate for the himalaya alone the convergence rates in other parts of the diffused zone of compressive strain i e tibet tien shan altai this works well for the post oligocene but not for the earlier post collisional history when the areal extent of thrust fold tectonics was much more limited possibly the apparent shortening deficit before the miocene was made up by strike slip faulting but with the notable exception of the red river fault evidence for widespread pre miocene strike slip faults is not convincing summation of the amount of shortening in thrust fold regimes in the himalaya and its hinterland falls short of that required by the total convergence figures quoted above the deficit arises firstly because of the likelihood that pre collisional crustal shortening of uncertain areal extent in tibet reduces tibet s contribution 40 according to dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 to the shortening budget for the cenezoic revised shortening budgets are presented on the assumption that most of the lhasa block southern tibet was already thickened prior to collision this thickening is difficult to quantify secondly balanced cross sections from various parts of the himalaya give 60 70 shortening that is only about 500 km roughly half of the shortening proposed by dewey et al the conclusion of le pichon et al tectonics 11 1992 1085 that only indian mid upper crust is involved in the himalayan thrust sheets is supported but it needs to be reassessed in the light of the recognition of eocene oligocene thickening in the himalaya this early crustal thickening antedates the sequence of thrusts which started with the main central thrust and therefore the shortening estimates based on balanced cross sections probably underestimate the total shortening in the himalaya the prospect of substantial underthrusting of india beneath tibet minimizes the amount of horizontal shortening of tibetan lithosphere during the cenozoic the conclusion is that either the total convergence deduced from geophysical and volumetric work is too large or the deficit must be made up by shortening mechanisms other than thrust fold regimes i e strike slip faulting which serves to move rocks out of the way of india

structural regularities in the lithosphere of continents and plate tectonics abstract two fundamental but competing earth science concepts have been under discussion in russia the first one that of endogenous regimes is based on the assumption that permanent vertical relationships or long term interactions between the crust and upper mantle control crustal evolution significant horizontal movements of the lithosphere as required by the second concept that of global plate tectonics would destroy these crust mantle interactions certain regular features of the crust and upper mantle support the endogenous regime concept and are difficult to explain in terms of conventional plate tectonics in particular the close correlation between near surface features and deep 400 km mantle inhomogeneities suggests that many geological structures are deeply rooted in the mantle moreover geophysical studies have failed to reveal a well defined and continuous asthenosphere at relatively shallow depths 100 km that would allow lithospheric plates to be transported over large distances and the rheology of the lithosphere itself is found to be sufficiently inhomogeneous as to cast doubt on the principle of thin rigid plates in contrast palaeomagnetic and other data require that horizontal movements of many near surface geological structures must have taken place to explain this apparent contradiction it is suggested here that the crust and its connected deep root are capable of gliding along one of the deep mantle phase transition zones with respect to the inner earth

petrogenetic and tectonic significance of young leucogranites from the northwestern himalaya pakistan in the himalaya of northwestern pakistan small discordant bodies of leucogranite intrude metamorphosed basement of the indian crust as the first step in assessing the tectonic significance of these granites we have used the shrimp ion microprobe to obtain u pb dates on zircon zircons of igneous appearance are scarce in the leucogranites and with one exception are associated with xenocrystic components which occur as discrete grains and as cores rimmed by young zircon zircons of igneous appearance are high in u 5000 to 40 000 ppm low in th u 0 01 to 0 05 and frequently colored a distinct blue xenocrystic components give a range of proterozoic ages consistent with ages previously reported for indian crust in sharp contrast zircons of igneous appearance yield clusters of concordant tertiary ages two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 ma swat and about 50 ma naran together with previously determined 40ar 39ar and fission track cooling ages these ages document immediate and rapid metamorphism and denudation following eocene collision three leucogranite dikes from northern localities within the nanga parbat haramosh massif are very young having emplacement ages of about 2 3 ma 5 ma and 7 ma the formation and emplacement of these leucogranites during the rapid late tertiary denudation of the nanga parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis

geophysical controls of chemical disequilibria in europa the ocean in jupiter s moon europa may have redox balance similar to earth s on earth low temperature hydration of crustal olivine produces substantial hydrogen comparable to any potential flux from volcanic activity here we compare hydrogen and oxygen production rates of the earth system with fluxes to europa s ocean even without volcanic hydrothermal activity water rock alteration in europa causes hydrogen fluxes 10 times smaller than earth s europa s ocean may have become reducing for a brief epoch for example after a thermal orbital resonance 2 gyr after accretion estimated oxidant flux to europa s ocean is comparable to estimated hydrogen fluxes europa s ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active as evidence of geologic activity and subduction implies

horizontal structures in sporadic sodium layers at 23 s during 1979 and 1980 the inpe lidar located at sao jose dos campos brazil was operated in a steerable mode measuring the sodium profiles sequentially at three points in the sky twelve sporadic sodium layer events ssls which occurred in this period are studied in the present work the evolution of the sporadic peaks at the three positions shows short time structures generally separated by consistent time lags in almost all events on some occasions the time evolution at one position is very different than at the other two these data indicate that short duration ssls have cloud like structures which are advected over the measuring station by the horizontal winds and the long duration layers show a patchy and wave like structure in no case did we observe rapid growth in the sodium density to occur simultaneously at all 3 measuring points on this basis we believe that there is no evidence for fast production of sodium and consequently that there is no need for theories for the formation of ssls to be consistent with such fast production

occurrences of large magnitude earthquakes in the kachchh region gujarat western india tectonic implications abstract moderate to large damaging earthquakes in the peninsular part of the indian plate do not support the long standing belief of the seismic stability of this region the historical record shows that about 15 damaging earthquakes with magnitudes from 5 5 to 8 0 occurred in the indian peninsula most of these events were associated with the old rift systems our analysis of the 2001 bhuj earthquake and its 12 year aftershock sequence indicates a seismic zone bound by two linear trends nnw and nne that intersect an e w trending graben the bouguer gravity values near the epicentre of the bhuj earthquake are relatively low 2 mgal the gravity anomaly maps the distribution of earthquake epicentres and the crustal strain rate patterns indicate that the 2001 bhuj earthquake occurred along a fault within strain hardened mid crustal rocks the collision resistance between the indian plate and the eurasian plate along the himalayas and anticlockwise rotation of the indian plate provide the far field stresses that concentrate within a fault bounded block close to the western margin of the indian plate and is periodically released during earthquakes such as the 2001 m w 7 7 bhuj earthquake we propose that the moderate to large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain hardened environment

magnetostratigraphy of syntectonic growth strata and implications for the late cenozoic deformation in the baicheng depression southern tian shan abstract the collision between india and eurasia in the cenozoic has caused a series of intracontinental deformation in the foreland basins of tian shan but there are debates about the timing of tectonic deformation and the relationship between tectonic uplift and sediment accumulation in the foreland basins based on the magnetostratigraphy of growth strata in the baicheng depression southern tian shan we suggest that an episode of crustal shortening in the late cenozoic evidenced by syntectonic growth strata in the kelasu yiqikelike structural belt kysb initiated at 5 3 ma since then the sedimentation rate accelerated abruptly and coarse molasse deposits accumulated combined with the results of growth strata on both flanks of tian shan and the fact that the xiyu formation on the southern limb of the kasangtuokai anticline was involved into the growth strata we conclude that the period from 7 5 ma to the early pleistocene was one of the important episodes of intracontinental deformation in the foreland basins of tian shan as a response to the cenozoic collision between india and eurasia

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

continental underthrusting beneath the southern alps of new zealand both the inferred thermal regime and the variation in crustal thickness beneath the southern alps suggest that the mode of crustal shortening changes substantially along the length of the alpine fault north of mt cook crustal shortening is being accommodated predominantly by uplift and erosion of pacific plate continental crust as postulated in the wellman adams tectonic model however south of mt cook the absence of high heat flow and a large crustal root that is not in isostatic equilibrium may have resulted from underthrusting of as much as 100 km of indian plate continental crust

conditions for a crustal block to be sheared off from the subducted continental lithosphere what is an essential factor to cause features associated with collision 1 crustal blocks are detached from the subducting lithosphere at various depths i call such phenomena shearing off and investigate the mechanical conditions for shearing off to occur in the continental lithosphere the buoyancy of the crustal block does not work selectively as a driving force until it has been sheared off and i suppose that shearing off occurs when the shear traction at the thrust whose maximum value is calculated from the strength along the thrust overcomes the strength of the boundary faults of the block and determine the depth of the leading edge of the subducted crustal block when shearing off occurs the temperatures of the slab and at its surface are calculated by the steady state formula of molnar and england 1990 as a function of the surface heat flow dip angle and convergence velocity of the subducted continental plate as the shear stress and temperature at the ductile part of the thrust zone are consistent with the flow law the shear stress at the brittle part is a strong function of the pore fluid pressure ratio i show that shearing off of an upper crustal block is possible only for a small value of such as 0 4 when the surface heat flow value is 70 mw m2 i suggest that the essential factor to cause shallow offscraping of crustal blocks in collision zones is not a buoyancy of the continental crust but a low value of which would result from a little amount of dehydration from the subducted continental plate

39 ar 40 ar dating of the trans himalayan calc alkaline magmatism of southern tibet the age of the trans himalayan calk alkaline magmatism has not been clearly established on the basis of geological control it is possible to infer a pre late cretaceous age for the plutonites at the southern edge of the batholith belt and a late cretaceous or younger age for the lava series on the top and or to the north of the plutons k ar radiometric data previously obtained by chinese groups agree with these geological ages the 39ar 40ar method has been used to analyse more accurately some of the rocks which have been sampled during the joint french chinese field work in southern tibet in the summer of 1980 the incremental degassing of argon gives generally more information about the thermal history of the sample using this method we now show that the earlier basic plutons of the southern edge of the trans himalayan belt have ages of 90 110 myr albian to cenomanian whereas the lavas of the upper unconformable series are clearly younger 60 myr palaeocene the ladakh and kohistan magmatic belts further west are probable equivalents of the trans himalaya north of the indus suture their precise ages are also poorly known orbitolinas of middle cretaceous aptian age have been found in the drosh yasin series which lie unconformably on dioritic plutons in the northern edge of the belt the few radiometric data that are available give ages in fairly good agreement with those we have obtained in the trans himalaya so we conclude that a subduction zone has been active during the cretaceous between india and asia the only outstanding question is whether this active margin was at that time close to india or to asia or somewhere in between

the tectonic history of kohistan and its implications for himalayan structure the tectonic history of kohistan northern pakistan involves two collisional events cleavage and folding developed at 90 100 ma along the northern suture between the kohistan island arc and the asian plate at the same time there was major folding and shearing of the lower part of the kohistan arc approximately 100 km south of the suture this deformation was followed by ocean subduction south of the kohistan arc generating the kohistan calc alkaline batholith with subsequent ocean closure during the eocene and obduction of the kohistan arc together with the adjacent part of the asian plate over the indian continental crust the construction of balanced cross sections through the imbricated upper part of the indian continental crust in the footwall to this southern suture indicates a minimum displacement of 470 km requiring the western himalayan hinterland to be underlain by a large wedge of indian middle to lower crust there is some shortening of the overriding kohistan and asian plates by thrusts and shear zones but it is insufficient to satisfy the palaeomagnetic data there must be major crustal shortening involving thrusts in the hindu kush and pamirs north of kohistan the post eocene thrust direction which for most of pakistan is towards 160 is almost perpendicular to that immediately to the east in the himalayan belt generating complex refolded thrust patterns in the hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the nanga parbat syntaxis these different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics cannot always be related to plate movement vectors but are modified by or develop from complex rotations during place collision or from the gravitational spreading of a thickened crust a regional approach is required to recognize and correctly attribute the various components in thrust displacements

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

fluids tectonics and crustal deformation abstract in the plate tectonic process lithosphere creation at ocean ridges and its cooling leads to volatile fixation in the oceanic crust the outer 10 km or so of all crust contains abundant water in pores and fractures and variable amounts of volatiles in minerals when surface rocks are buried by tectonic processes fluids must be released and modify the mechanical properties in the subduction process hydrated oceanic crust may be decoupled from the remaining oceanic lithosphere at depth rising aqueous fluids or melts lead to a complex series of mass energy transfer processes which may decouple continental crust near the moho continental crust if subducted may also be decoupled from its lithosphere by degassing fluid release processes which create gas solid mixtures beneath impermeable cover create low strength systems subject to facile deformation hydraulic fracture processes and diapiric phenomena

the eastern iranian orocline abstract the eastern iranian range is seen as a large scale india eurasia collisional event and the subsequent amalgamation of the afghan and lut micro continents including the neo tethyan accretionary complexes and associated island arcs revised examination of the nature of tectono stratigraphic units distributed in both sides of the n s elongated eastern iranian orogen reveals two remarkable facts first from a general perspective it is structurally a giant parallel antiform in profile view second lithologically the most time equivalent comparable tectonic units from both northern and southern domains comprising the eastern iranian orogen are connectable follow the strike of hinge and limbs zones of the antiform and a m shaped disharmonic pattern in its core these characters are thought to be formed in response to two principal deformational phases resulting from two perpendicular stress directions the first was n s associated with the north dipping subduction of the tethyan oceanic lithosphere beneath the roughly ribbon shaped cimmerian supercontinent including the afghan and lut blocks before late eocene afterward the second was e w shortening originated by the westward escape of the afghan block as a consequence of the indian plate indentation into eurasia this orogen parallel stress was correspondingly accommodated by tens of degrees counter clockwise rotation of the lut block around a vertical axis the result of such shortening was oroclinal buckling and concomitant retreating of the subduction zone and closure of the sistan ocean a remaining wedge shaped part of the neotethys ocean formed by the ribbon curvature the development of the eastern iranian orogen already included the sistan suture zone an enormous mass of folded eocene flysch deposits with a general n s axial surface and some cretaceous ophiolitic remnants the plate scale buckling which was formed by this late stage orogenic event is hereby considered as an orocline call eastern iranian orocline lithosphere buckling around a vertical axis caused a large scale crustal folding coeval with the bending of its mantle part increasing the lithospheric thickness of the orocline compared to adjacent regions the syn orocline deformation associated delamination and resulting magmatic events took place in eastern iran during a short period of about 10 to 20 ma in the late paleogene the orocline hinge zone is occupied by the khorasan outer arc and the birjand core while its limbs are composed of the baluch inner arc and the southwestern and southeastern terminations of the lut and afghan blocks the eastern iranian orocline seems to be the middle segment of a triple bending belt with n s two opposite facing bend including afghan lut and sanandaj sirjan blocks this large curvature is possibly an example of secondary oroclines which was formed by collision related escape tectonics although the limited available kinematic data show a kind of progressive orocline

contemporaneous east west extension and north south compression at 43 ma in the himalayan orogen abstract it was previously proposed that the onset of east west extension in the himalayan orogen occurred after ca 19 ma here we present a re os isochron age of ca 43 ma for sulfides from the zhaxikang zn pb sb ag polymetallic deposits which occur within n s striking normal faults that reflect e w extension in the tethyan himalayan orogen this age is consistent with previously reported ages for regional metamorphism ca 48 42 ma granitoids ca 47 40 ma and gabbros ca 45 ma occurring along the indus yarlung suture in addition this type of structure also broadly occurs in the lhasa terrane and plays a critical role in the eocene porphyry skarn copper polymetallic mineralization there by ca 45 43 ma the north south directed horizontal deviatoric stresses had peaked and subducted neo tethyan slab breakoff had occurred at great depth leading to abrupt slowdown of indo asia convergence ca 45 ma e w extension initially developed at shallow depths resulting in formation of the north south trending normal faults in the plateau this type of extension would lead to collapse of the tibetan plateau and decrease in altitude subsequently resulted in central asian and the global climate changes our study places new constraints on the timing and nature of early cenozoic tectonics and metallogenesis in the himalayan orogen and climate changes in central asia

topo oz insights into the various modes of intraplate deformation in the australian continent abstract as the fastest lowest flattest and amongst the most arid of continents australia preserves a unique geomorphic record of intraplate tectonic activity evidencing at least three distinct modes of surface deformation since its rapid northward drift commenced around 43 million years ago at long wavelengths several 1000s km systematic variations in the extent of neogene marine inundation imply the continent has tilted north down southwest up at intermediate wavelengths several 100s km several undulations of 100 200 m amplitude have developed on the 1 10 myr timescale at still shorter wavelengths several 10s km fault related motion has produced local relief at rates of up to 100 m myr over several million years the long wavelength north down tilting can be related to a dynamic topographic effect associated with australia s northward drift from the geoid low dynamic topography low now south of the continent to the geoid high dynamic topography low centred above the south east asian and melanesian subduction zones the short wavelength fault related deformation is attributed in time to plate wide increases in compressional stress levels as the result of distant plate boundary interactions and in space in part to variations in the thermal structure of the australian lithosphere at the intermediate wavelengths transient low amplitude undulations can be ascribed to either lithospheric buckling or the development of instabilities in the thermal boundary layer beneath the lithosphere in the latter case topographic asymmetries suggest the australian lithosphere is moving north with respect to the mantle beneath providing a unique attribution to the progressive alignment of seismic anisotropy and absolute plate motion observed near the base of the australian lithosphere

sodium lidar observed gravity wave breaking followed by an upward propagation of sporadic sodium layer over hefei china the university of science and technology of china ustc sodium temperature wind lidar observed a strong zonal wind shear 60 m s km near 98 km between 1315 and 1330 ut on 29 july 2013 and a cooling near 96 100 km above a warming near 90 95 km between 1330 and 1430 ut this suggests a possible gravity wave gw breaking event comparison of the lidar results with observations from a nearby meteor radar and with satellite results indicated that the gw likely broke down over a large horizontal area in addition the sodium number density suddenly increased by 10 12 times 13 000 atmos cm3 near 95 km at 1400 ut immediately following the gw breaking this sporadic sodium layer ssl which is different from most previously observed ssl events which propagate downward tended to propagate upward and also appeared 2 minutes earlier in the west channel than in the east channel the horizontal propagation direction of the ssl was aligned with the horizontal wind direction which was likely due to the propagation of a high density sodium layer from the northwest of the lidar site and was possibly related to the earlier gw breaking event