geochemistry and u pb detrital zircon dating of paleozoic graywackes in east junggar nw china insights into subduction accretion processes in the southern central asian orogenic belt abstract the southern central asian orogenic belt caob is characterized by multiple and linear accretionary orogenic collages including paleozoic arcs ophiolites and accretionay wedges a complex history of subduction accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern caob in order to provide constraints on the accretionary history we analyzed major and trace element compositions of paleozoic graywackes from the huangcaopo group hg and kubusu group kg in east junggar the hg graywackes have relatively low chemical index of alteration cia values 50 to 66 suggesting a source that underwent relatively weak chemical weathering the identical average index of compositional variability icv values 1 1 for both the kg and hg samples point to an immature source for the paleozoic graywackes in east junggar which is consistent with an andesitic felsic igneous source characterized by low la th ratios and relatively high hf contents these graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin u pb dating of detrital zircons from the lower subgroup of the hg yielded a young age peak at 440 ma indicating a post early silurian depositional age however the youngest populations of detrital zircons from the kg graywackes and the upper subgroup of the hg yielded 206 pb 238 u ages of 346 ma and 355 ma respectively which suggest a post early carboniferous depositional age because of similarities of rock assemblages these two units should be incorporated into the early carboniferous nanmingshui formation the detrital zircon age spectrum of the early paleozoic hg graywackes resembles that of the habahe sediments in the chinese altai which suggests that the ocean between east junggar and the chinese altai was closed before the deposition of the sediments and that the armantai ophiolite was emplaced prior to the early devonian the differences in age spectra for detrital zircons from the post early carboniferous graywackes in east junggar and the harlik arc indicate that the emplacement of the kalamaili ophiolite postdates the early carboniferous therefore a long lasting northward subduction accretion process is suggested for the formation of east junggar and the reconstruction of the early paleozoic evolution of the southern caob

precursory seismic quiescence before the 1994 kurile earthquake mw 8 3 revealed by three independent seismic catalogs we have found that the m w 8 3 kurile earthquake on october 4 1994 followed an outstanding seismic quiescence starting 5 6 years before the mainshock near the ruptured area we have analyzed three independent seismic catalogs institute of seismology and volcanology hokkaido university isv japan meteorological agency jma and international seismology center isc in spite of selecting different magnitude bands and time windows all three catalogs presented the common feature of the seismic quiescence this fact strongly suggests that the seismic quiescence should not be a man made change but actually occurred moreover we have confirmed that the seismic quiescence was the most significant and the earthquake was the largest in the past twenty five years in this region therefore we confidently interpret this seismic quiescence as an indication of a preparation process for the m w 8 3 kurile earthquake

high latitude lower thermospheric neutral winds at eiscat and sondrestrom during ltcs 1 the incoherent scatter radar located at sondre stromfjord greenland 67 n 51 w 74 5 and the eiscat incoherent scatter facility located in northern scandinavia 69 5 n 19 e 66 3 both obtained e and f region measurements during the first campaign of the lower thermosphere coupling study ltcs 1 september 21 25 1987 neutral winds deduced from these measurements have been analyzed for their mean flow and tidal components a number of the altitude profiles for the mean winds and the diurnal and semidiurnal wave components at the two radar locations show similar variations with height indicating that latitudinal rather than longitudinal effects are dominant in determining the observed wind field diurnal tidal amplitudes and phases are reasonably well represented by theoretical model results forbes 1982 the semidiurnal amplitudes and phases although somewhat consistent between the two radars are not well represented in equinox tidal model results forbes and vial this issue results from both radars indicate a vertical wavelength for the zonal semidiurnal oscillation of approximately 60 km during a period of impulsive magnetospheric forcing september 22 23 winds deduced from measurements at both radars show enhanced eastward flows near midnight accompanied by equatorward winds at sondrestrom comparison with the results of a national center for atmospheric research thermosphere ionosphere general circulation model tigcm simulation of the ltcs 1 interval shows generally better agreement with the observations at eiscat than at sondrestrom during the period of activity on september 22 23 the tigcm is reasonably successful at simulating the eastward surge near midnight in the eiscat but not in the sondrestrom observations and the equatorward flow after midnight in the sondrestrom results the observed winds in magnetic latitude and magnetic local time coordinates indicate the presence of anticyclonic divergent flow near dusk and cyclonic converging flow near dawn

formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

vertical distribution of heat generation in the nissho granitic pluton hokkaido japan vertical distribution of radiogenic heat generation is estimated for the nissho granitic pluton emplaced in the upper crust of the kuril arc the estimated profile shows that heat generation steeply decreases with depth using the exponential model the thickness of the layer with relatively high heat generation is estimated to be 7 2 km this thickness is in the range of those estimated in heat flow provinces on continents it is considered that this upward concentration of heat generation is caused by magmatic processes such as the gravitational differentiation in the magma chamber in the upper crust of evolving arcs where the continental type crust is growing magmatic processes in chambers of granitic magmas could be important for the upward concentration of heat generation

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

fragments of oceanic islands in accretion collision areas of gorny altai and salair southern siberia russia early stages of continental crustal growth of the siberian continent in vendian early cambrian time abstract the altai salair area in southern siberia is a caledonian folded area containing fragments of vendian early cambrian island arcs in the vendian early cambrian an extended system of island arcs existed near the paleo asian ocean siberian continent boundary and was located in an open ocean realm in the present day structural pattern of southern siberia the fragments of vendian early cambrian ophiolites island arcs and paleo oceanic islands occur in the accretion collision zones we recognized that the accretion collision zones were mainly composed of the rock units which were formed within an island arc system or were incorporated in it during the subduction of the paleo asian ocean under the island arc or the siberian continent this system consists of accretionary wedge fore arc basin primitive island arc and normal island arc the accretionary wedges contain the oceanic island fragments which consist of oib basalts and siliceous carbonate cover including top and slope facies sediments oceanic islands submerged into the subduction zone and later were incorporated into an accretionary wedge collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high pressure rocks our studies of the gorny altai and salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites therefore it is important to recognize paleo islands in folded areas the study of paleo islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high pressure rocks

subduction of seamounts at the java trench a view with long range sidescan sonar abstract we describe here a 1300 km by 45 km gloria long range sidescan sonar swath along the eastern java trench the swath images the trench axis a narrow strip of oceanic crust to the south and the toe of the accretionary wedge to the north between 108 and 120 e sonar images of the ocean crust show a pattern of normal faults typical of the outer wall of trenches these result from tension related to the bending of the oceanic lithosphere into the subduction zone a number of sub circular seamounts are also seen some of which are currently being subducted isolated ponds of flat lying sediments occur in the trench axis although along much of its length the trench is devoid of such sediment accumulations on the inner trench wall the accretionary wedge is recognised by its distinctive grainy texture with the grain aligned sub parallel to the deformation front where subducting seamounts are colliding with the wedge large crescentic areas of very high backscattering correlate with re entrants in the deformation front and large indentations in the wedge the high backscattering within these collision scars is interpreted to arise from outcropping strata and talus covered slopes resulting from erosion of an inner trench wall shortened and oversteepened by collision with a seamount all of the sediment ponds in the trench axis occur in close association with collision scars strongly suggesting localised erosion of the accretionary wedge in the scar areas however the volume of the indentations appears to be an order of magnitude greater than the volume of eroded sediments deposited in the trench indicating that erosion cannot be the primary mechanism by which indentations are created most of the missing material must be displaced landward by thrusting and folding ahead of the seamount our observations at the java trench are broadly comparable with those made at the japan trench by french and japanese workers however some differences in the detail of the collision process are evident in particular oversteepening and erosion occur from the earliest stage of collision at the java trench while uplift and understeepening initially dominate at the japan trench differences in the effects of subducting seamounts in the two areas are attributed to contrasts between the relatively unlithified strata of the java trench inner wall and the more lithified strata of the non accreting japan trench inner wall

reconstructing multiple arc basin systems in the altai junggar area nw china implications for the architecture and evolution of the western central asian orogenic belt abstract the altai junggar area in northwestern china is a critical region to gain insights on the tectonic framework and geological evolution of the western central asian orogenic belt caob in this study we report results from integrated geological geochemical and geophysical investigations on the wulungu depression of the junggar basin to determine the basement nature of the basin and understand its amalgamation history with the chinese altai within the broad tectonic evolution of the altai junggar area based on borehole and seismic data the wulungu depression is subdivided into two nw trending tectonic units suosuoquan sag and hongyan high by southward vergent thrust faults the suosuoquan sag consists of the middle late devonian basaltic andesite andesite dacite tuff tuffaceous sandstone and tuffite and the overlying early carboniferous volcano sedimentary sequence with lava flows and shallow marine sediments from a proximal juvenile provenance zircon e hf t 6 0 14 9 compared to the late carboniferous andesite and rhyolite in the hongyan high zircon sims u pb ages for dacites and andesites indicate that these volcanics in the suosuoquan sag and hongyan high erupted at 376 3 ma and 313 4 ma respectively the middle late devonian basaltic andesites from well lc1 are calc alkaline and exhibit primitive magma like mgo contents 7 9 8 6 and mg values 66 68 with low initial 87 sr 86 sr 0 703269 0 704808 and positive e nd t values 6 6 7 6 and relatively high zr abundance 98 2 116 0 ppm and zr y ratios 5 1 5 4 enrichment in lrees and liles e g th and u and depletion in nb ta and ti suggesting that they were probably derived from a metasomatized depleted mantle in a retro arc extensional setting the well lc1 andesitic tuffs well l8 dacites well wl1 dacitic tuffs and well l5 andesites belong to calc alkaline and metaluminous to peraluminous a cnk 0 8 1 7 series and display low mg values 35 46 and variably positive e nd t 4 5 8 5 and e hf t 10 2 16 8 values as well as young isotopic model ages these devonian carboniferous intermediate felsic volcanics are interpreted as the products of partial melting of a juvenile lower crust with some contributions from mantle components in an evolved island arc setting from immature to mature island arc the basin filling pattern and the distribution of arc volcanics and their zircon hf model ages with the eruptive time suggest that the wulungu depression represents an island arc basin system with the development of a carboniferous retro arc basin in combination with previous work we propose that the northern junggar area comprises three arc basin belts from south to north the darbut luliang karamaili wulungu yemaquan and saur fuhai dulate such tectonic subdivisions are consistent with the regional gravity and magnetic anomaly data the recognition of the wulungu arc basin system demonstrates that the junggar basin is likely underlain by juvenile continental crust rather than ancient precambrian basement and also implies that the caob was built by amalgamation of multiple linear arcs and accretionary complexes

an overview of oceanic island basalts in accretionary complexes and seamounts accretion in the western central asian orogenic belt abstract the central asian orogenic belt caob is one of the largest and long lived accretionary collages worldwide and provides an ideal opportunity to address the fundamental geological processes and continental growth of accretionary orogens however the tectonic history of the caob is still controversial previous studies mainly focused on the process and model of accretion and subduction but did not pay much attention to oceanic intraplate volcanoes in this paper we provide an overview of the salient features of twelve oib hosting accretionary complexes with ages from late neoproterozoic to early carboniferous in western the caob and distinguish rock association of typical seamounts from accretionary complexes and shear zones including pillow basalt volcanic breccia limestone olistostromes and terrestrial clastic rocks combined with unique structural features of seamount the stratigraphic framework of seamount was preliminarily reconstructed in paleo asian ocean but the distribution and scale of seamounts is still not clear geochemically volcanic rocks from seamounts are mainly alkaline basalts with litte tholeiitic basalts the alkaline basalts are characterized by lree enrichment and hree depletion and no obvious nb ta and ti negative anomalies suggesting typical oib affinity the nd sr and pb isotopic features indicate that magma sources have enriched components and show fluid metasomatism in short this is consistent with geochemical features of louisville ontong java and hawaiian seamounts which are related to mantle plumes but do not contain high mg picrite and komatitic basalt based on our observations and in combination with previous work we present an innovative model in which the caob was involved multiple intra oceanic arc with seamounts subduction and accretion which is similar to the present day pacific and can better explain the tectonic evolution of the caob however further studies are needed to document the distribution scale and mantle dynamics of seamounts in the caob

geochemistry and nd isotopic composition of the early paleozoic flysch sequence in the chinese altai central asia evidence for a northward derived mafic source and insight into nd model ages in accretionary orogen abstract although nd isotopic composition of sedimentary rocks is considered as a useful tool to trace the provenance their nd model age tdm is usually misused to discuss the timing of crustal formation in accretionary orogen in the central asian orogenic belt an early paleozoic flysch sequence extensively occurs in the chinese altai and adjacent western mongolian terranes sedimentary rocks in the northwestern chinese altai were collected from the flysch sequence for geochemical and nd isotopic studies although these rocks show different geochemical characteristics e g arenaceous and argillaceous they all have relatively low cia chemical index of alteration values 49 75 indicating a relatively weak weathering source or the absence of compositionally mature alumina rich minerals produced by chemical weathering processes their relatively high icv index of compositional variability values 0 95 1 35 reveal an immature source and indicate that these rocks were probably deposited in an active tectonic setting the sedimentary rocks from the flysch sequence have high mgo sc co and low y nb which suggest significant addition of mafic rocks into the source modeling based on nb cr and zr mgo contents displays nearly 20 to 50 addition of mafic materials in combination with the age and distribution of mafic rocks in the chinese altai and adjacent terranes a northward derived mafic source is recommended for the sedimentary sequence although the sedimentary rocks have nd model ages tdm 1 44 1 57 ga older than devonian granites tdm 1 6 1 1 ga young detrital zircons from the sediments and inherited zircons from the granites 450 500 ma suggest a mixed source consisting of both old crust components and juvenile or less evolved crustal components this reveals that nd model ages tdm of metasedimentary rocks and igneous rocks can hardly provide close constrains on the timing of crustal formation in accretionary orogen with the progresses in adjacent western mongolian terranes a tectonic scenario dominated by subduction accretionary process is suggested to interpret the formation of the early paleozoic flysch sequence in the chinese altai and adjacent terranes

zircon u pb geochronology and hf isotopic composition of granitiods in russian altai mountain central asian orogenic belt the central asian orogenic belt caob consists of many tectonic terranes with distinct origin and complicated evolutionary history understanding of individual block is crucial for the reconstruction of the geodynamic history of the gigantic accetionary collage this study presents zircon u pb ages and hf isotopes for the granitoid rocks in the russian altai mountain range including gorny altai altai mongolian terrane and ctus suture zone between them in order to clarify the timing of granitic magmatism source nature continental crustal growth and tectonic evolution our dating results suggest that granitic magmatism of the russian altai mountain range occurred in three major episodes including 445 429 ma 410 360 ma and 241 ma zircons from these granitoids yield comparable positive ehf t values and neoproterozoic crustal model ages which favor the interpretation that the juvenile crustal materials produced in the early stage of the caob were probably dominant sources for the paleozoic magmatism in the region the inference is also supported by widespread occurrence of short lived juvenile materials including ophiolites seamount relics and arc assemblages in the northern caob consequently the paleozoic massive granitic rocks maybe do not represent continental crustal growth at the time when they were emplaced but rather record reworking of relatively juvenile proterozoic crustal rocks although mantle derived mafic magma was possibly involved to serve as heat engine during granitic magma generation the early triassic granitic intrusion may be a product in an intra plate environment as the case of same type rocks in the adjacent areas the positive ehf t values 1 81 7 47 and corresponding hf model ages 0 80 1 16 ga together with evidence of petrology are consistent with the interpretation that the magma source of the triassic granitic intrusion was derived from enriched mantle and melted under an usually high temperature condition likely due to basaltic magma that underplated the lower crust our data combined with evidence of the regional geology enable us to conclude that the gorny altai and altai mongolian terranes possibly have similar tectonic natures but represent two separate accretionary systems before devonian collision the accretion and amalgamation processes resulted in the paleozoic granitoid magmatism and caused the two terranes to merge as a composite tectonic domain at the siberian continental margin

thermal state under the tohoko arc with consideration of crustal heat generation abstract the thermal structure beneath subduction zones plays a key role in aiding understanding of tectonic activities such as seismicity volcanism arc formation and genesis of back arc basins around subduction zones many simulations have been made to estimate the thermal process in the mantle wedge which causes the surface activities in the tohoku arc many heat flow measurements have been made and the heat flow profile from the trench to the back arc has been constructed in this study the thermal structure in the crust of northeast japan is investigated to account for the surface heat flow data to estimate the contribution of the heat generation in the crust to the surface heat flow the heat production rate was obtained on granitic and metamorphic rocks collected from the hidaka metamorphic zone hokkaido japan where the vertical crustal section of an island arc is understood to be exposed at the surface instrumentation neutron activation analysis and gamma ray spectrometry were adopted to analyze the concentration of heat producing elements the obtained rate of heat generation and its decrease with increasing depth for the upper part of the crust were found to be similar to those estimated for stable continental areas but the heat generation rate was almost constant in the lower part of the crust the vertical distribution of the heat production rate in the crust of the northeastern japan arc was estimated from the present results and previous estimates of heat production rates in various rocks using the crustal structure determined by explosion seismology the total heat generation in the crust thus estimated is about 30 mw m 2 with the surface heat flow data and estimated heat generation distribution in the crust the thermal structure in the crust was simulated numerically modeling calculations were made on a cross section along the line from the japan trench to the japan sea a direction parallel to the direction of convergence of the pacific and eurasian plates the models were two dimensional and included frictional heating at the upper surface of the slab the calculated thermal structure in the crust shows that the temperature at the moho is about 850 c and almost about 250 c lower than previous estimates the magnitude of the interplate stress was derived by considering the rate of frictional heating at the interface between the plates in order to explain the surface heat flow between the trench and aseismic front the interplate stress should be less than 20 mpa at depth in the crust

sondrestrom incoherent scatter radar observations during the lower thermosphere coupling study september 21 26 1987 the incoherent scatter radar located at soendre stroemfjord greenland obtained e and f region measurements during the first lower thermosphere coupling study ltcs 1 september 21 26 1987 lower thermospheric neutral winds deduced from these measurements show that the neutral dynamics are influenced by both tidal oscillations and magnetospheric forcing during an interval which was relatively quiet geomagnetically september 23 24 a semidiurnal oscillation dominated the neutral motion the model equinox tidal amplitudes and phases of forbes 1982 for the diurnal tide are roughly in agreement with the observed diurnal oscillation for the first four days of the experiment vertical variations in the observed diurnal phases are consistent with the results of forbes and hagan 1988 and may provide evidence of dissipation of the propagating 1 1 tidal mode the semidiurnal motion observed during this period is not well represented by the recent theoretical results for the amplitude and phase of the semidiurnal tide forbes and vial this issue neutral winds obtained during a geomagnetically active interval september 25 26 displayed a flow pattern that was significantly distorted from that observed during the preceding relatively quiet interval although the changes in the zonal winds throughout this active interval were consistent with the directionmore of the ion drag force at 115 km and above the variations in the meridional winds suggest that other forces such as pressure gradients driven by joule heating need to be considered to explain the observations less

experimental evidence bearing on the stability of monazite during crustal anaatexis a major host for lree and the heat producing elements th and u in peraluminous granites is monazite and the stability of monazite will affect the redistribution of these elements within the crust during anatexis the compositional stability of monazite in granitic melts ranging from peraluminous to metaluminous compostions has been investigated under water saturated conditions between 850 1000 c at 8 kb monazite solubility concentration of lree s in saturated melts increases with the values of m 1 3 2ca na k alxsi in the melt and is constant for m 1 3 the temperature dependence of monazite saturation has been determined by rapp and watson 1986 for a melt of m 1 3 and model should be applicable to all peraluminous magmas application of the monazite solubility model to crustal anatexis indicates that for fusion temperatures at or below 800 c anatexis producing peraluminous magmas may lead to an enrichment of monazite in the restitic lower crust depending upon the initial lree concentration in the source at temperatures approaching 850 c enrichment of monazite in the lower crust by anatexis is unlikely due to the high monazite saturation concentrations

remnants of a late triassic ocean island in the gufeng area northern tibet implications for the opening and early evolution of the bangong nujiang tethyan ocean abstract in this paper we present new major and trace element compositions of basaltic rocks in the gufeng ocean island gfoi area in the western segment of the bangong nujiang suture zone northern tibet our aim was to assess the genesis of these rocks and discuss the implications of this new dataset for the evolution of the bangong nujiang tethyan ocean an ocean island type double layer structure comprising a basaltic basement and an oceanic sedimentary cover sequence found within the gfoi provides direct evidence for the interpretation that the assemblage is a typical ocean island the basalts in the gfoi can be divided into three types named g1 g2 and g3 basalts and these basalts range in composition from morb to oib types which is typical of ocean islands the g1 basalts have morb type affinities possibly indicating the existence of morb oceanic crust under the gfoi the g2 basalts represent the early stage of formation of the gfoi and are produced by the interaction of rising oib type basaltic magma and the existing morb oceanic crust the g3 basalts are typical oib basalts and they are the products of the direct eruption of oib type basaltic magmas the g3 basalts have high la yb n 12 3 14 4 ce yb n 10 8 11 8 la sm n 2 39 2 76 and sm yb n 4 89 5 23 ratios indicating the presence of oceanic lithosphere below the gfoi with a thickness of 50 60 km geochemical analyses of the gfoi cherts show that they contain terrigenous material indicating the gfoi formed close to a continental margin norian conodont fossils within the gfoi limestones indicate the gfoi formed during the late triassic these data combined with geological evidence and a half space model of lithosphere cooling where the thickness of the oceanic lithosphere is determined from the age of the lithosphere indicate that the western segment of the bangong nujiang tethyan ocean opened initially in the late permian expanded rapidly during the early middle triassic and was a mature ocean during the late triassic

mesosphere thermosphere tidal coupling during the september 21 25 1987 ltcs 1 campaign the thermosphere ge 90 km is dynamically coupled to the mesophere and lower atmospheric regions by gravity waves tides and planetary waves which propagate upward from below and often undergo dissipation in the 80 to 150 km region this paper focuses on the mesosphere thermosphere coupling due to the solar semidiurnal atmospheric tide during the first lower thermosphere coupling study ltcs campaign interval of september 21 25 1987 during this period determinations of semidiurnal eastward u and northward v velocity components and temperature delta t are available within the 80 to 150 km region from a total of 13 incoherent scatter meteor and spaced antenna radars a new method is developed and reported here for assimilating such data sets into a self consistent global empirical description of the tidal dynamics between 80 and 150 km using a set of numerically generated hough mode extension hme functions for u v and delta t corresponding to the 2 2 2 3 2 4 and 2 5 semidiurnal tidal modes a least squares fit to the radar data is performed the fit results in a single complex normalizing factor for each hme from which globally continuous and internally consistent semidiurnal oscillations in u v and delta t as well as vertical velocity and density canmore be derived the quality of fit to these data from various height regions including both northern and southern hemispheres establishes the internal consistency between these measurements as well as the applicability of the method to future data sets the resulting empirical model reproduces the data significantly better than recent theoretical models suggesting use of the method for deriving tidal boundary conditions for thermospheric general circulation models less

characteristics of semidiurnal tides observed by the kyoto meteor radar and saskatoon medium frequency radar we have compared seasonal variations of semidiurnal tides in the upper middle atmosphere observed by the kyoto meteor radar and the saskatoon medium frequency partial reflection radar in 1983 1985 amplitudes of semidiurnal tides are generally larger at saskatoon than at kyoto seasonal variations of phases at 90 km agree fairly well for both zonal and meridional components between kyoto and saskatoon the seasonal variation is mainly composed of two fundamental states appearing in summer centered april may september october and winter centered months november february march which are characterized by long and relatively short vertical wavelengths respectively clear transitions between the two states occur in equinoxes february april and october november using determinations at kyoto and saskatoon with 4 to 5 day observation intervals we have studied the behavior of semidiurnal tides during the transitions in october november in 1983 1985 and february april in 1985 precise comparisons have shown that the transitions are slightly delayed at saskatoon in both vernal and autumnal equinoxes abrupt changes in phases of semidiurnal tides coincide with reduction and pulsating fluctuations of their amplitudes suggesting the existence of interference between fundamental and higher modes which are dominant in summerlike and winterlike months respectively

thermospheric extensions of the classical expansion functions for semidiurnal tides solutions to the thermospheric tidal equations are described that define the extensions into the thermosphere of normalized wind and temperature structures associated with the 2 2 2 3 2 4 and 2 5 semidiurnal propagating tidal modes sample figures are presented that illustrate the degree of alteration of vertical structures with latitude and the change in horizontal shapes with height corresponding to semidiurnal oscillations in northerly westerly and vertical velocity and temperature in the thermosphere additional figures and tables covering the 80 to 400 km altitude region for five levels of solar activity at 6 latitude increments are available on microfiche for potential users the structures can be used to extend meteor wind 80 100 km and partial reflection drift 60 100 km measurements to heights above 100 km for consistency checks with tidal winds and temperatures from thomson scatter measurements at possibly different latitudes or to simultaneously fit data covering these height regions for modeling purposes supplementary figures and tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20002 document j82 003 1 00 payment must accompany order

on the coupling between the lower and the upper thermosphere during the first lower thermosphere coupling study simultaneous observations of the neutral winds and temperatures and electron densities in the upper and lower thermosphere are presented the measurements were made at arecibo and millstone hill during the first lower thermosphere coupling study ltcs 1 campaign in september 1987 the observations show a great deal of day to day variability possibly due to the geomagnetic activity which occurred during the period storm effects are discernible in the data at low altitudes near 120 km even at low latitudes near 18 n theoretical simulations of the period were attempted by the national center for atmospheric research thermosphere ionosphere general circulation model a novel development was the use in the model of lower boundary conditions representative of the upward propagating semidiurnal tides based on the ltcs 1 mesosphere lower thermosphere observations the upward propagating diurnal 1 1 mode was also included in the model simulations of the low and mid latitudes did not substantially improve with the use of lower boundary conditions derived from the measurements the model was also unable to reproduce the observations in both the lower and upper thermosphere this may be partially due to the effects of geomagnetic activity on the atmospheric fields since the observational period included two intervals of geomagnetic disturbances in the lower thermosphere resolution of the discrepancies between the data and model predictions may require the extension of the model to lower altitudes at present the model predictions in the lower thermosphere are strongly dependent on the lower boundary conditions the model predicts that diurnal waves are not negligible in analysis of lower thermosphere data indeed near 100 km at the model location of arecibo the diurnal wave amplitudes of the meridional winds were predicted to be greater than the semidiurnal investigation of the model predictions awaits the development of techniques to obtain observations of at least 24 hours duration in the lower thermosphere and the extension of the theoretical models to lower altitudes

coordinated incoherent scatter radar observations of the semidiurnal tide in the lower thermosphere the incoherent scatter radars at arecibo 18 3 degree n millstone hill 42 6 degree n and sondrestrom 67 degree n simultaneously observed the thermal structure and dynamics of the earth s lower thermosphere during the period september 21 25 1987 as part of the lower thermosphere coupling study plasma spectral measurements were used to derive ion temperatures and horizontal neutral wind velocities in the altitude region 90 140 km and semidiurnal harmonics were determined from the data sets the daily mean temperatures were found to be within 40 degree k at the three stations and the largest semidiurnal temperature amplitudes exceeding 70 degree k above 115 km were recorded at arecibo mean winds were about plus minus 30 m s at most altitudes shifting from eastward to westward directions at 115 km and the semidiurnal amplitudes were in the range 20 70 m s phase variations with height reveal reasonable consistency among the three stations within 3 4 hours and indicate an average vertical wavelength of 40 70 km however a strong variation is seen in the vertical structure in the 110 to 130 km region suggesting the presence of different tidal modes comparison with the most current tidal propagation model by forbes and vial this issue indicates that the model tends to underestimate the amplitudes for this time period particularlymore above 110 km by virtue of the latitudinal distribution of the three radars their coordinated observations of the lower thermosphere serve to constrain numerical models and improve their understanding of this important yet complex region of the atmosphere less

satellite observations of mean winds and tides in the lower thermosphere 2 wind imaging interferometer monthly winds for 1992 and 1993 1 satellite observations of monthly i e 60 d averages centered on the 15th of each month zonal mean and migrating tide winds in the region of 90 120 km and 40 s 40 n from the wind imaging interferometer are presented the zonal mean zonal winds are characterized by annually varying eastward winds at midlatitudes with a maximum in summer and semiannually varying westward winds in the tropics with maxima at equinox below 105 km the zonal mean meridional winds are characterized by a summer to winter flow below 100 km at solstice and by a cell like structure at equinox the latter is the first such global observation of this tidally driven structure two distinct types of diurnal tide are observed an upward propagating one and an evanescent one of comparable magnitude the propagating tide which is dominant in the subtropics below about 105 km exhibits a semiannual variation with a primary maximum in march april and a secondary maximum in september october the evanescent tide which is dominant poleward of 20 n s and above 100 km has an annual variation with a maximum in the summer months the coupling of the two produces a double peaked structure in the vertical profile of the diurnal amplitude the semidiurnal tide is generally weaker than the diurnal tide the maximum semidiurnal winds are at 30 40 n s and at 105 110 km both zonal and meridional components are stronger in april september than in october march in both hemispheres but in the equatorial region in april september the meridional component has a maximum whereas the zonal component has a minimum

solar tides as revealed by measurements of mesosphere temperature by the mls experiment on uars abstract temperatures between 25 and 86 km measured by the microwave limb sounder mls experiment on the upper atmosphere research satellite uars are analyzed to delineate diurnal semidiurnal and terdiurnal tidal structures and stationary planetary waves these fourier components are determined from temperatures averaged in bins covering 5 latitude 30 longitude and 1 h in local time this study confirms the presence of diurnal nonmigrating tides with zonal wavenumbers s 0 2 3 s 0 s 0 implying westward eastward propagation and semidiurnal tides with s 1 and 3 and some components of lesser importance that were previously determined from uars wind measurements near 95 km the seasonal latitudinal and height structures of these components are now revealed and utilized to aid in interpreting their behaviors and ascertaining their origins new discoveries include the terdiurnal s 2 and s 4 components and trapped nonmigrating diurnal tides with s 0 and s 2 the former are like

late paleozoic to early triassic multiple roll back and oroclinal bending of the mongolia collage in central asia abstract the architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components but the fundamental paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined because of unavailable data the central asian orogenic belt caob provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage in this paper we review several lines of available evidence which enable us to propose a new tectonic model of huge roll back in the formation of the accretionary tectonics of the mongolian collage in central asia in the early paleozoic the mongolia collage comprised the southern siberian and the tuva mongol oroclines the siberia craton and the mongolia collage jointly formed a giant tadpole shape within the paleo asian and panthanlassic oceans its head siberia was to the south and the tail tuva mongol to the northwest the structures and tectonic zonation of the mongolia collage are characteristic of multiple arcs which have been separately described in detail in different segments southwards from the southern siberia east sayan west sayan gorny altai chara via the lake zone junggar tianshan gobi altai beishan alxa to the manlay hegenshan baolidao solonker segments almost all segments underwent early paleozoic to permian or even triassic frontal subduction and accretion while rifting in the late carboniferous to permian or triassic occurred in the outward oceanward westward advancing mongolian collage therefore we suggest that a huge complex roll back active from the carboniferous to permian or even to late triassic facilitated the formation of the mongolian collage the outward multiple roll back process was compatible and almost coeval with the start of the tuva mongol orocline and rotation of the siberian craton as confirmed by paleomagnetic and structural data during the roll back processes an archipelago paleogeography was formed behind the frontal subduction and accretion in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times the roll back process was affected by the collision of the kazakhstan collage along the chara and karamay zones in the early permian the collision of the tarim craton along the south tianshan zone in the early permian the collision of the dunhuang block along the liuyuan zone in the early permian triassic the collision of the alxa block along the qugan qulu zone in the permian and the collision of the north china craton along the solonker zone in the middle late triassic the tectonic styles and architecture of accretionary orogenic belts like the caob are characterized both by the amalgamation of multiple terranes and by oroclinal bending the systematic anatomy of the multiple roll back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens

coordinated global radar observations of tidal and planetary waves in the mesosphere and lower thermosphere during january 20 30 1993 a multi instrument global campaign involving incoherent scatter medium frequency and meteor wind radars was conducted during the period of january 20 30 1993 to study the dynamics of the mesosphere and lower thermosphere data obtained from 15 radar stations covering a wide latitude range have been used to determine the global distribution of planetary and tidal waves during this 10 day campaign spectral analysis of the neutral winds measured by the radars in the altitude range from 80 to 130 km indicates the existence of a strong 48 hour wave near 90 km at latitudes between 40 n and 40 s that is present up to 108 km at 18 n the semidiurnal tide is large at middle and high latitudes near 90 km and is predominant above 110 km while the diurnal tide is observed to be particularly important in the upper mesosphere near 40 latitude a least squares fit to the radar data is performed to obtain the amplitudes and phases of the tidal and 48 hour waves comparison with national center for atmospheric research thermosphere ionosphere mesosphere general circulation model shows that the predictions from the model agree reasonably well with the observed global morphology of tidal wave amplitudes

late paleozoic tectonics of the solonker zone in the wuliji area inner mongolia china insights from stratigraphic sequence chronology and sandstone geochemistry abstract the geology in the wuliji area including the enger us and quagan qulu areas is important for understanding the late paleozoic tectonics of the solonker zone ultramafic mafic rocks in the enger us area previously interpreted as an ophiolitic suture are actually lava flows and sills in a permian turbiditic sequence and a small body of fault breccia containing serpentinite subduction zone features such as accretionary complexes magmatic arc volcanics or lp hp metamorphism are absent early permian n morb mafic rocks and late permian radiolarian cherts accompanied by turbidites and tuffeous rocks indicate a deep water setting in the quagan qulu area outcrops of the late carboniferous to permian amushan formation are composed of volcano sedimenary rocks and guyot like reef limestone along with a late permian volcano sedimentary unit a dacite lava in the late permian volcano sedimentary unit yields a zircon u pb age of 254 ma the gabbros in the quagan qulu area are intruded into the amushan formation and caused contact metamorphism of country rocks sandstones in the upper member of the amushan formation contain detrital clasts of volcanic fragments and mineral clasts of crystalline basement rocks i e biotite muscovite and garnet geochemical analysis of volcaniclastic sandstones shows a magmatic affinity to both continental island arc cia and active continental margin acm tectonic settings a late permian incipient rift setting is suggested by analyzing the lithostratigraphic sequence and related magmatism in the wuliji area the volcano sedimentary rocks in the wuliji area experienced a nearly n s shortening that was probably related to the early mesozoic nearly n s compression well developed in other areas close to the wuliji area

distribution of radiogenic heat generation in the arc s crust of the hokkaido island japan radiogenic heat generation was measured for rocks collected in the vertical section of the crust of the arc exposed in the hokkaido island japan measured heat generation value of metasediments is nearly constant of 1 0 w m and independent of the metamorphic grade using the measured heat generation value together with the published heat generation data of a granitic body emplaced in this section vertical profile of heat generation in the crust is constructed the estimated heat generation profile shows steep decrease with depth but is not lower than a value of 1 0 w m in the upper crust in the lower crust metabasites are dominant and heat generation value is one order of magnitude lower than that in the upper crust surface heat flow derived from the crustal heat generation is estimated to be 25 mw m when the crustal heat generation is considered temperature at the bottom of the crust estimated from surface heat flow becomes 300 c lower this temperature difference corresponds to the viscosity variation of more than three orders of magnitude which is important for understanding lithosphere dynamics

geochronological and hf isotopic variability of detrital zircons in paleozoic strata across the accretionary collision zone between the north china craton and mongolian arcs and tectonic implications the central asian orogenic belt formed by accretion subsequent to the contraction of the paleo asian ocean and its southeastern segment terminated along the solonker suture zone amalgamating the mongolian arcs and the north china craton by the end of the early triassic since typical regional collisional features are absent its tectonic evolution remains speculative an investigation into the variability of detrital zircon u pb ages and hf ratios in permian strata along a southeast northwest transect from the mongolian arcs to the north china craton reveals distinct differences northern basins carry a broad mesoproterozoic to latest precambrian age signature and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin in contrast southern basins contain detritus from the north china craton and their sources are of dominantly crustal contaminated magmatic origin provenance analysis suggests that in the early paleozoic ca 429 ma the paleo asian ocean was consumed along the uliastai arc and the north china craton initiating the formation of the northern and southern accretionary orogens respectively by the end of the middle carboniferous the mongolian arcs were consolidated after the accretion of the uliastai arc in the late carboniferous ca 314 ma the hegenshan back arc basin opened detaching the northern accretionary orogen while subduction continued there it may have temporarily ceased in the south after the accretion of a microcontinent ca 300 ma by the end of the middle permian ca 269 ma back arc basin closure led to the formation and obduction of the suprasubduction zone hegenshan ophiolite eventually the paleo asian ocean closed by double sided subduction such accretionary wedge wedge collision would not involve continental deep subduction unlike other continental collisions the early stages of the sequence likely resembled a pacific type scenario dominated by the subduction of oceanic lithosphere including a japanese sea type back arc basin opening the late stages involved the accretion of large scale continental blocks ultimately leading to final ocean closure which may be similar to the archipelago type setting of present day southeast asia

reconstructing in space and time the closure of the middle and western segments of the bangong nujiang tethyan ocean in the tibetan plateau when and how the bangong nujiang tethyan ocean closed is a highly controversial subject in this paper we present a detailed study and review of the cretaceous ophiolites ocean islands and flysch deposits in the middle and western segments of the bangong nujiang suture zone bnsz and the cretaceous volcanic rocks late mesozoic sediments and unconformities within the bnsz and surrounding areas our aim was to reconstruct the spatial temporal patterns of the closing of the middle and western segments of the bangong nujiang tethyan ocean our conclusion is that the closure of the ocean started during the late jurassic and was mainly complete by the end of the early cretaceous the closure of the ocean involved both longitudinal diachronous closure from north to south and transverse diachronous closure from east to west the spatial temporal patterns of the closure process can be summarized as follows the development of the bangong nujiang tethyan oceanic lithosphere and its subduction started before the late jurassic after the late jurassic the ocean began to close because of the compressional regime surrounding the bnsz along the northern margin of the bangong nujiang tethyan ocean collisions involving the arcs back arc basins and marginal basins of a multi arc basin system first took place during the late jurassic early early cretaceous resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the southern qiangtang terrane on the northern side of the ocean however the closure of the bangong nujiang tethyan ocean cannot be attributed to these arc arc and arc continent collisions because subduction and the development of the bangong nujiang tethyan oceanic lithosphere continued until the late early cretaceous the gradual closure of the middle and western segments of bangong nujiang tethyan ocean was diachronous from east to west starting in the east in the middle early cretaceous and being mainly complete by the end of the early cretaceous the bnsz and its surrounding areas underwent orogenic uplift during the late cretaceous

early paleozoic sedimentary record of the chinese altai implications for its tectonic evolution abstract the chinese altai is a typical part of the central asian orogenic belt caob but its tectonic evolution is not well understood geochemical study of early paleozoic metasedimentary rocks was conducted to better constrain their provenance and tectonic environment these rocks have relatively low cia values 49 to 79 with icv values 0 73 1 36 higher than that of post archean australian average shale paas suggesting that they experienced relatively weak chemical weathering and were mainly derived from an immature source compared to paas and upper crust the samples mostly have relatively lower high field strength elements hfse e g zr hf nb ta and large ion lithophile element lile contents e g rb sr ba th u pb however their relatively high rb concentrations 40 ppm low rb sr 0 04 3 24 and th u th u 3 8 ratios are indicative of an acidic intermediate igneous source that had undergone weak chemical weathering all of the samples are moderately enriched in lree light ree and show relatively flat chondrite normalized hree heavy ree patterns la n yb n 3 2 12 6 with various negative eu anomalies eu eu 0 4 0 9 meta arenaceous samples have moderate sio 2 average 72 9 wt al 2 o 3 average 11 5 wt fe 2 o 3 t mgo average 7 16 wt contents and moderate al 2 o 3 sio 2 average 0 16 k 2 o na 2 o average 0 93 ratios similar to those of graywackes from continental island arcs or active continental margins however meta argillaceous samples have high sio 2 average 60 8 wt al 2 o 3 average 17 2 wt fe 2 o 3 t mgo average 12 2 wt contents and high k 2 o na 2 o average 3 45 al 2 o 3 sio 2 average 0 28 ratios similar to mudstones from continental arc basins this kind of tectonic environment is also manifested in tectonic discrimination diagrams including th sc zr 10 and la th sc these geochemical characteristics suggest that the metasedimentary rocks were deposited in an active continental margin or continental arc setting this does not favor the previously proposed passive continental margin model therefore this study supports an arc accretion model for the chinese altai

simultaneous mesosphere lower thermosphere and thermospheric f region observations during geomagnetic storms 1 simultaneous mesosphere lower thermosphere mlt and thermospheric f region observations made during geomagnetic storms are presented using the mu radar 35 n 136 e in alternate meteor and incoherent scatter is modes mf radar 45 n 136 e optical mesosphere thermosphere imagers omti 35 n 136 e and over 1000 gps receivers in japan the observations were made during one of the mtec s mesosphere thermosphere experiments for coupling studies campaigns conducted during 23 march to 2 april 2001 when a major storm with dst reaching 358 nt and a moderate storm with dst reaching 112 nt occurred and solar activity varied from high to very high f10 7 220 to 270 the campaign has provided zonal and meridional wind velocities at mlt altitudes 80 95 km over the mu and mf radars meridional wind velocity in the upper thermosphere 220 450 km and ionospheric electron density 150 600 km peak height and plasma drift velocity over the mu radar all sky airglow intensities at 557 7 nm 630 0 nm and 777 4 nm wavelengths over the mu radar location and vertical gps tec all over japan study of the data indicates that following the major storm the meridional wind in the upper thermosphere over the mu radar 35 n becomes weak less poleward during daytime and its diurnal amplitude reduces the winds and tides at mlt altitudes show changes during both nonstorm times and storm times but this study is not able to confirm any statistically significant changes related to the storms in the ionosphere following the onset of the major morning storm the electron density increases at altitudes near and above the ionospheric peak which after about 5 hours is followed by large increase in density at all altitudes above about 200 km on the following night though the density is generally low there is a large increase in density in the bottomside for about 3 hours centered at 0230 lt during this time a rare intense optical activity has occurred at all wavelengths 557 7 nm 630 nm and 777 4 nm over the low midlatitude location 35 n and an associated large scale traveling ionospheric disturbance lstid has propagated from north to south at f region altitudes

comparison of observed diurnal and semidiurnal tropospheric winds at christmas island with tidal theory four years of hourly averaged atmospheric wind measurements from the christmas island vhf wind profiler were processed into typical days to show the vertical phase structure of the diurnal and semidiurnal tropospheric zonal winds the winds from the lowest resolvable height were compared with both the surface winds calculated from a three year record of surface pressure using the equations of motion on a rotating frictionless sphere and the winds from a tidal model it is concluded that at the lowest height the observed diurnal and semidiurnal zonal winds have tidal 1 1 and 2 2 mode structures respectively further analysis is necessary to evaluate the tidal mode s of the zonal wind throughout the troposphere

lunar tidal winds at adelaide and saskatoon at 80 to 100 km heights 1985 1990 winds data from the two sites were analyzed for lunar tides in 2 year groups the variation with height of the phase and amplitude of the tide showed considerable variability from year to year yet in general seasonal variations at particular heights were similar to those determined previously phases of the tide at the two sites were compared in order to estimate whether the tide was symmetric or antisymmetric for this purpose it was found necessary to use only 1 year data sets the predominance of symmetric tides during solstice months and antisymmetric tides at the equinox came as a surprise yet the result is in broad agreement with phase relations of the lunar geomagnetic tide

long term oscillations of the wind field in the tropical mesosphere and lower thermosphere from hawaii mf radar measurements 1 wind measurements in the mesosphere and lower thermosphere obtained with a medium frequency mf radar in hawaii 22 n 160 w spanning 16 years are employed to examine the intra annual and interannual variability of the mean and tidal motions at altitudes between 84 and 94 km intra annual periodicities range from 3 to 12 months with significant coherence in altitude and between the zonal and meridional components of each motion field interannual variations confirm the dominant periodicities identified previously at this site and elsewhere in particular the significant diurnal and less significant semidiurnal tidal responses at periods of 28 and 48 months amplitudes of these long period oscillations of the diurnal tide increase with altitude below 92 km and are larger than the amplitudes of the 12 month oscillations above 90 km phases of the 28 and 48 month oscillations show a downward progression with a slightly larger altitude variation in the meridional diurnal tide for the 28 month oscillation and a significantly larger altitude variation in the zonal diurnal tide for the 48 month oscillation the long and nearly continuous hawaii data set also enables characterization of the responses of the wind fields to the 11 year solar cycle both wind and tidal fields exhibit this periodicity but these responses display interesting and different relations to the phase of the solar cycle the 11 year oscillation of the meridional wind is nearly in phase with the solar cycle while the 11 year oscillation of the zonal wind is in approximate quadrature with the solar cycle the 11 year oscillations of the semidiurnal tidal amplitudes and the meridional diurnal amplitude are all in approximate quadrature with the solar cycle with the tidal amplitudes leading by 29 to 37 months while the 11 year oscillation of the zonal diurnal amplitude is somewhat nearer an antiphase than a quadrature relation to the solar cycle leading by 54 months

factors controlling the distribution of heat production in selected uk granites the variation of heat production with respect to magma evolution is examined in four uk granites in which fractional crystallization accounts for the range of rock compositions present accessory minerals are the dominant hosts of u and th in these granites and their role in controlling variations of these heat producing elements is assessed melt compositional parameters determine which accessory minerals precipitate solubility control and influence the partitioning of radioelements between early crystallizing fractionating minerals and residual liquid as a result parental magma compositions and the effect of critical properties such as alkalinity aluminosity and volatile contents particularly f complexing on magma evolution are considered important in controlling the primary distribution of radioelements in granites

semidiurnal tidal climatology of the e region theoretical predictions of global monthly and seasonally averaged semidiurnal temperatures and winds between 80 and 150 km are presented and compared with seasonal means from measurements at the arecibo chatanika millstone hill and sondrestrom incoherent scatter radars the model values above 100 km are obtained by utilizing a set of tabulated hough mode extension functions to extrapolate the 2 2 2 3 2 4 and 2 5 propagating tidal modes at 100 km from a spectral model not valid in the e region due to the way molecular dissipation is parameterized to higher altitudes and by superimposing a component driven by in situ excitation important mainly above 140 km the monthly simulations demonstrate the close coupling that exists between seasonal and latitudinal structures and argue for an observational program emphasizing adequate seasonal and latitudinal coverage despite constraints imposed by the distributions of existing data with respect to month and year comparisons with the model reveal several trends that appear to be real 1 the model amplitudes are usually 20 50 smaller than those observed 2 vertical wavelengths are well predicted by the model but model phases generally lead the observations at middle and low latitudes by approximately 2 3 hours at all heights for both winds and temperatures 3 semidiurnal temperatures at middle and low latitudes in both the observations and model indicate significant seasonal changes in the phase of the temperature oscillation but not in the winds and 4 the seasonal trends in the model and observations are consistent with summer leading winter at millstone hill while the reverse is true at arecibo the sources of the absolute amplitude and phase discrepancies in the model which are probably connected with the adopted mean wind and thermal excitation specifications need to be investigated in addition a concerted effort must be made within the coupling energetics and dynamics of atmospheric regions cedar program to better delineate the seasonal latitudinal thermal structure and dynamics of the e region following the example of the extensive f region observational data base which has been accumulated over the last decade

how was the carboniferous balkhash west junggar remnant ocean filled and closed insights from the well tacan 1 strata in the tacheng basin nw china abstract the balkhash west junggar remnant ocean is a conspicuous tectonic feature that had an important effect on the paleozoic tectonic evolution of the central asian orogenic belt how the carboniferous balkhash west junggar remnant ocean was filled and closed is poorly understood in this study we focus on the integrated tectonic sedimentary evolution of the tacheng basin which is located in the eastern part of the balkhash west junggar remnant ocean using high resolution seismic reflection well line geochronological and geochemical data to establish the filling of the basin by sediments and its evolution the carboniferous balkhash west junggar remnant ocean basin was filled with early carboniferous deep marine mudstone and interbedded sandstone with minor andesite basalt and tuff and late carboniferous basalt andesite breccia and tuff and shallow marine mudstone and sandstone the carboniferous igneous assemblage mainly consists of basalt with high nb content and nb la pm values and concomitant magnesian andesites with adakite geochemical characteristics of high sr and low y and yb suggesting a subduction related tectonic setting zircons in the tuff that underlies the basalts and magnesian andesites were dated to ca 315 ma by la icp ms u pb the identified unconformity between the carboniferous and lower permian from seismic reflection profile in well tacan 1 subdivided the carboniferous early permian strata into two tectonostratigraphic units the lower permian sequences overlap the carboniferous strata and the facies transition from the submarine carboniferous to the terrigenous lower permian indicates that the balkhash west junggar remnant ocean closed during the early permian these new data suggest that the sedimentary filling of the basin was in response to subduction and accretionary processes and that oceanic subduction was an important mechanism that led to the shrinking filling and closure of the balkhash west junggar remnant ocean because of compression therefore lateral crust growth through subduction accretionary processes still played an important role in the carboniferous construction of western junggar

atmospheric tide 2 the solar and lunar semidiurnal components solar and lunar semidiurnal wind and temperature oscillations from the surface to 400 km are simulated for average solar activity conditions utilizing the numerical tidal model described in part 1 hough mode decomposition of the solstitial solar semidiurnal tide excited by h2o and o3 insolation absorption below 80 km indicates that it is characterized by a predominance of 2 2 below 60 km about equal contributions of 2 2 2 3 and 2 4 between 70 and 90 km mostly 2 4 with some 2 5 and 2 2 between 90 and 120 km and a predominance of 2 2 above 140 km with secondary contributions from 2 3 and 2 5 the solstitial lunar semidiurnal tide is characterized by a predominance of 2 2 below 70 km 2 4 with secondary contributions from 2 2 2 3 and 2 5 between 80 and 110 km and by 2 2 with a fairly strong secondary contribution from 2 3 above 120 km similar descriptions for both the solar and lunar oscillations apply at equinox except that the asymmetric 2 3 and 2 5 modes are absent relative to the solar component lunar temperature and wind amplitudes are sufficiently large 20 in the lower and upper thermosphere to account for some of the observed variability previously attributed to the solar tide amplitudes of the 2 4 and 2 5 modes above 100 km are significantly affected by mesospheric eddy diffusion coefficients in excess of 2 106 cm s 1 the total solar semidiurnal exospheric temperature oscillation for average solar conditions ranges from 10 20 k poleward of 40 n to 40 60 k equatorward of 30 n and has its origin in three sources of excitation all of comparable importance 1 euv solar radiation absorption in the lower thermosphere 100 200 km 2 in situ momentum coupling due to the interaction of diurnal winds and diurnally varying ion drag and 3 tides propagating upwards from below 100 km in addition the calculations are compared with rocket meteor radar partial reflection thomson scatter and satellite data to evaluate the consistency of the model with observations

tidal oscillations of the arctic upper mesosphere and lower thermosphere in winter the upper mesosphere and lower thermosphere of the northern polar cap was investigated using fabry perot interferometer measurements of the horizontal wind velocity in the lower thermosphere meridian scanning photometer observations of atomic oxygen green line brightness in the lower thermosphere and michelson interferometer records of the infrared meinel oh bands brightness and rotational temperature in the upper mesosphere tidal airglow oscillations in the absence of local solar heating were studied using a superposed epoch analysis of observations obtained around three new moon intervals november 9 22 1993 december 7 20 1993 and january 4 17 1994 in eureka 80 n tidal harmonics were uncovered with an iterated least chi squared fit and their existence in the data tested using a goodness of fit probability against a null hypothesis of no oscillations no single dominant tide was found at all times and altitudes of the winter arctic upper mesosphere and lower thermosphere the horizontal wind velocity of the lower thermosphere exhibited all the first three harmonics of the tide with the largest amplitude oscillations shown by the diurnal component of the meridional wave speed the atmospheric layer near the mesopause that contributes to the oh airglow emission experienced terdiurnal oscillations throughout the winter season theoretical polarization relations for evanescent tides considerations of energy density propagation with altitude and a day by day analysis of the zonal and meridional wind speed and temperature all indicate that the terdiurnal tide observed on november was an evanescent zonally symmetric tide other data indicate propagating tides migrating tides or a mixture of both

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 1 sea beam morphology and shallow structure sea beam bathymetric data closely spaced single channel seismic reflection sections and geopotential field data were collected aboard the r v j charcot over the collision zone between the north d entrecasteaux ridge ndr and the new hebrides island arc the ndr trends east at a small angle 14 to the plate convergence direction so that the ridge creeps northward along the trench at about 2 5 cm yr requiring a continual structural adjustment of the accretionary complex in this report we study the shallow structure tectonic erosion and consequent mass wasting of the accretionary complex all of which were produced by the collision of the ndr in a complex deformational environment the accretionary complex in this collision zone can be divided into northern central and southern parts the northern part of the collision zone which lies north of the leading flank of the ridge shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and n60 w trending strike slip faults a middle accretionary complex that bulges seaward and appears to be rotated 45 e from the regional arc trend and an upper accretionary complex that is shaped by slumps and a canyon network the central part of the collision zone which directly overlies the crest of the ridge forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps rocks forming this protrusion have been uplifted possibly by as much as 1500 2500 m and tilted to the north causing northward block sliding along an extensional detachment surface the southern part of the collision zone which lies in the wake of the ridge is deformed by large slumps and normal faults that trend parallel to the ridge axis rocks forming this southern part collapse causing widespread mass wasting these geophysical data indicate that normal trench convergence has apparently produced only a few trench parallel structures confined to the toe of the accretionary complex whereas the along trench motion of the ndr has resulted in well developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern the area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the ndr and the time required to heal the disturbance may be about 0 8 m y

satellite observations of thermospheric tides results from the wind imaging interferometer on uars thermospheric winds measured by the wind imaging interferometer windii on the upper atmosphere research satellite are analyzed for migrating solar tides the data cover a 2 year period commencing february 1992 and are obtained from the atomic oxygen o 1s 557 7 nm emission which provides observations of the 90 to 200 km altitude range during daytime and the 90 to 110 km range at night the subtropical lower thermosphere is dominated by the diurnal propagating tide which exhibits a vertical wavelength of approximately 22 km grows in amplitude up to 95 km and decays rapidly above where molecular diffusion greatly reduces the vertical shears although the phase remains fairly uniform throughout the year a pronounced semiannual oscillation is observed in the diurnal tide amplitude at both 20 n and 20 s the meridional and zonal wind components attain their maximum values at equinox of approximately 70 and 40 m s respectively while the solstitial minima are nearly a factor of 2 smaller at 35 n the diurnal tide semiannual amplitude oscillation is still present in the lower thermosphere but above 100 km it is replaced by an annual cycle with a maximum in july and august this contrasts with 35 s where the july august peak is absent and the semiannual oscillation extends to 110 km at midlatitudes the zonal and meridional winds are of similar magnitude and no significant hemispheric asymmetries in amplitudes are observed in the lower thermosphere the semidiurnal tide amplitude exhibits an annual oscillation with maximum values of 30 to 40 m s occurring in june july near 100 km at 35 n 35 s and the equator a bimodal structure in the seasonal variation of the semidiurnal phase is observed this feature is characterized by rapid equinoctial transitions and is particularly well defined at the equator examination of the equatorial middle thermosphere indicates that the semidiurnal tide attains its maximum amplitude at 140 km and exhibits a vertical wavelength of approximately 60 km these findings indicate the predominance of the antisymmetric 2 3 hough mode in the tropics

a vendian cambrian island arc system of the siberian continent in gorny altai russia central asia an extended vendian cambrian island arc system similar to the izu bonin mariana type is described in the gorny altai terrane at the margin of the siberian continent three different tectonic stages in the terrane are recognized 1 a set of ensimatic active margins including subducted oceanic crust of the paleo asian ocean the uimen lebed primitive island arc oceanic islands and seamounts the set of rocks is assumed to be formed in the vendian 2 a more evolved island arc comprising calc alkaline volcanics and granites a fore arc trough in middle late cambrian time was filled with disrupted products of pre middle cambrian accretionary wedges and island arcs 3 collision of the more evolved island arc with the siberian continent folding metamorphism and intrusion of granites occurred in late cambrian early ordovician time in the late paleozoic the above mentioned caledonian accretion collision structure of the siberian continent was broken by large scale strike slip faults into several segments this resulted in the formation of a typical mosaic block structure

radar wind profiler observations of solar semidiurnal atmospheric tides semidiurnal solar tides in the mid latitude tropo sphere are investigated using harmonic analysis of 404 mhz radar profiler wind data obtained from a wide longitude zone in the u s the tides are apparent above a 1000 m deep surface layer and increase in amplitude with height attaining speeds of 0 5 0 7 rn s at 5 7 km observed wind characteristics agree well with tidal characteristics obtained with a dynamical model driven by observed global semidiurnal horizontal pressure gradients

controls on carbonate cementation in early syn rift terrestrial siliciclastics the lower cretaceous of the bayindulan sag in er lian basin china abstract carbonate cementation of sandstones at channel bases is commonly interpreted to be a result of solutes generated from surrounding rocks in a burial diagenetic regime however in rapidly buried and under compacted conglomerates presented in this study we interpret carbonate cements formed during early diagenesis 1000 1600 m to be precipitated directly from ca and co2 rich saline formation water derived from hypersaline lake water and driven by gravity and compaction in the bayindulan sag of the er lian basin which is a continental rift basin the sandstones and conglomerates of the a ershan formation k1ba were deposited in fan delta front settings during the syn rift phase within these tightly non ferroan calcite cemented conglomerates are present at the bases of distributary channels in addition some fe dolomite cements are present in sandstones with high present day porosity which are located in the upper parts of distributary channels a petrographic analysis and fluid inclusion microthermometry study show that the calcite cement predates the fe dolomite stable isotopic values indicate that both calcite and fe dolomite are characterized by enrichment of 13c and depletion of 18o 18ov pdb 15 to 8 and the value of 13c v pdb in calcite 1 6 to 3 8 is less positive than that in fe dolomite 3 1 to 8 4 integrated analyses imply that the studied carbonate cements were probably formed within an open diagenetic system and were influenced by multiple diagenetic fluids during deposition of the a ershan formation k1ba the lake water was brackish to hypersaline sr ba 1 and sr content 300 ppm during deposition of the tenggeer formation k1bt the k1ba progressed rapidly into a deep burial phase 90 m ma diagenetic fluids were controlled by hypersaline lake water incursion of meteoric water occurred along faults and methanogenic fermentation calcium was probably derived from the saline formation water and the transformation of smectite to illite in associated mudstones leading to the precipitation of calcite 54 78 c in the saihantala period the k1ba experienced a slight uplift the fault activity ceased and the diagenetic fluids were derived from the saline formation water and were influenced by methanogenic fermentation and fe dolomite began to precipitate in deeper burial detrital feldspar grains and some calcite cement were probably dissolved by organic acid discharged from mudstones in conclusion the early diagenetic characteristics in the continental rift basins were controlled by the composition of the depositional waters and tectonics that controlled the opening and closing the diagenetic system thus influencing fluid composition

seasonal variations of lower thermospheric winds from the millstone hill incoherent scatter radar from july 1976 to november 1977 regular incoherent scatter observations of e and f region ion drifts were conducted at millstone hill 42 6 n with the steerable l band and fixed uhf radars data were collected on a total of 46 days and analyzed for ionospheric electric fields and daytime neutral winds in the lower thermosphere at 105 115 125 and 135 km the wind measurements characteristically exhibited the dominance of the semidiurnal oscillation with both wind components showing a 12 hour variation and an eastward wind maximum 3 hours ahead of a southward wind maximum a model fit to the full set of wind measurements also allowed the smaller annual mean diurnal winds to be determined the winds measured on 38 geomagnetically quiet days were grouped by day number to determine the seasonal dependence of the semidiurnal tide as well as the prevailing wind the semidiurnal winds exhibited the following general characteristics they typically grew in amplitude reaching a maximum in the vicinity of 125 km with larger values at the equinoxes than the solstices the wind field at 105 km showed a significant polarization with southward wind amplitudes larger than eastward amplitudes seasonal variations in tidal phase were generally less than one hour the amplitude and phase of the measured diurnal wind were in reasonable agreement with theoretical expectations for the in situ tidal component and there was no indication of the presence of an upward propagating diurnal component on the other hand the measured semidiurnal winds were consistent with being the result of the upward propagation of tidal energy from below 100 km rather than in situ tidal excitation attempts to match the measured vertical wind structure with that expected for the different semidiurnal hough mode extensions in the lower thermosphere lead to no definite mode identification the measured vertical wavelengths 70 100 km and the altitude of maximum amplitude were in best agreement with the s2 2 mode while the temperature wind ratio suggested the s2 4 a mode there is overall agreement of the millstone hill southward wind measurements with results from the french incoherent scatter radar at saint santin but differences are noted as to the seasonal variation the altitude of maximum wind and the vertical wavelength some of these differences might be explained by a solar cycle dependence of tidal dissipation in the lower thermosphere

geochronology and geochemistry of early permian mafic ultramafic complexes in the beishan area xinjiang nw china implications for late paleozoic tectonic evolution of the southern altaids abstract the hongshishan and pobei mafic ultramafic complexes outcrop in the beishan area in the southernmost altaids they consist of olivine gabbros dunites and pyroxenites zircons from olivine gabbros from the hongshishan complex yield a u pb age for emplacement of 281 8 2 6 ma the olivine gabbros have low sio 2 47 08 48 66 tio 2 0 14 0 29 mno 0 07 0 09 and k 2 o 0 06 0 09 but high mg values 81 t fe 2 o 3 4 93 6 29 the hongshishan and pobei mafic ultramafic complexes are characterized by negative anomalies in high field strength elements zr hf nb ta which we interpret as arc related signatures they also have spikes in large ion lithophile fluid mobile elements ba u pb sr and high mobile immobile element ratios i e primitive mantle normalized sr nd ba la 1 which are attributed to an aqueous fluid component in the source zircon hf isotopes indicate that the source magma was derived either from a depleted mantle with some crustal contamination or from an enriched mantle these data are interpreted to indicate that the hongshishan and pobei mafic ultramafic complexes were emplaced in the early permian subduction related environment which suggests that the paleo asian ocean in the southernmost part of altaids did not close until after early permian

early middle paleozoic granitoids in gorny altai russia implications for continental crust history and magma sources abstract major element and trace element compositions as well as nd isotope ratios and protolith model ages of early to middle paleozoic granitoids from the russian gorny altai are summarized and discussed in terms of the compositions and tectonic settings the observed geographic variations of compositions in the granitoid assemblages correlate with three groups of late neoproterozoic to middle paleozoic terranes corresponding to oceanic island arc and turbidite basin settings plutonic rocks in the oceanic and island arc terranes are commonly low alkali metaluminous tonalite and trondhjemite that formed by melting of oceanic crust the island arc terranes also contain tonalite trondjemite granodioritic and tonalite granodiorite granitic rocks with higher alkali and potassium contents which result from anatectic melting of volcanics and sediments granitoids in the turbidite basins are potassic peraluminous monzogranite produced by anatectic melting of upper crustal turbidites as well as subalkaline tonalite granodiorite and granite derived from upper crustal metasediments and metamorphosed mafic oceanic crust underlying the turbidite basins alkaline granitoids emplaced during two major tectonomogamatic events early and late devonian and are present in terranes of all three settings their source is of crustal origin like in other groups of granitoids but it was a mixture of crustal and mantle material thus the compositional trends of early and middle paleozoic granitoids in the gorny altai record the history of continental crust

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

carboniferous mantle derived felsic intrusion in the chinese altai nw china implications for geodynamic change of the accretionary orogenic belt abstract this paper reports whole rock sr nd and zircon u pb and hf isotopic compositions as well as major and trace element data for the granodiorites and their mafic microgranular enclaves mmes in the tuokesalei area northwestern chinese altai the granodiorites are characterized by peraluminous compositions a cnk 1 01 1 45 with relatively high sio 2 64 4 to 70 9 wt and low mgo 0 75 to 1 97 wt contents these rocks possess low total rare earth element ree contents 67 187 ppm with weakly negative eu anomalies and mantle like whole rock nd sr isotopic compositions e nd t 2 5 to 4 5 87 sr 86 sr i 0 7038 to 0 7048 and zircon e hf t values 5 93 to 12 9 the mmes contain abundant magmatic amphiboles and needle like apatites have low sio 2 51 7 to 54 3 wt and high mgo 2 86 to 3 75 wt and ree contents 162 298 ppm and are characterized by moderately negative eu anomalies eu eu 0 40 0 60 and slightly depleted to enriched lree la nd n 0 74 1 41 and flat hree gd yb n 1 16 1 39 patterns the mmes have whole rock nd sr isotopic compositions e nd t 2 6 to 2 7 87 sr 86 sr i 0 7034 0 7050 similar to those of the host granodiorites their similarities in crystallization age mineral assemblage and isotopic composition document that the mmes may represent an early crystallized phase which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement their precursor magmas were probably derived from an oceanic lithospheric mantle which was characterized by pronounced nd ta depletions mineral compositions of mmes suggest that amphibole was a major early fractional mineral which effectively differentiated the mantle derived basaltic liquids to silica rich melts zircon u pb dating yielded an age of 313 5 ma for the host granodiorites and 313 13 ma for their mmes the geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction related regime to a post tectonic extension results of this study suggest that the accretionary orogenic process did not completely terminate until the late carboniferous to permian in this region

latitude and seasonal dependence of the semidiurnal tide observed by the high resolution doppler imager the high resolution doppler imager hrdi on the upper atmosphere research satellite uars has provided measurements of the horizontal wind field in the mesosphere and lower thermosphere mlt since november 1991 the observed dynamical patterns are dominated by the 1 1 diurnal tide but semidiurnal perturbations are also very prominent in the hrdi data particularly at latitudes greater than 40 this paper presents a preliminary study of the semidiurnal tide and discusses latitudinal and seasonal variations in the phenomenon regular seasonal changes are clearly detected and significant interhemispheric asymmetries have been identified in general the observed features are consistent with previous measurements and model results but some discrepancies have been found this new technique which brings a global perspective to the observation of mlt dynamics is complementary to the very detailed but localized radar data sets currently available and should provide important constraints for numerical models

accretion of oceanic plateaus at continental margins numerical modeling abstract the accretion of oceanic plateaus has played a significant role in continental growth during earth s history which is evidenced by the presence of oceanic island basalts oib and plume type ophiolites in many modern orogens however oceanic plateaus can also be subducted into the deeper mantle as revealed by seismic tomography the controlling factors of accretion versus subduction of oceanic plateaus remain unclear here we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo mechanical numerical model three major factors for the accretion of oceanic plateaus are studied 1 a thinned continental margin of the overriding plate 2 weak layers in the oceanic lithosphere and 3 a young oceanic plateau for a large oceanic plateau the modes of oceanic plateau accretion can be classified into one sided and two sided subduction collisional regimes which mainly depend on the geometry of the continental margin normal or thinned for smaller sized seamounts accretion occurs only if all three factors are satisfied of which a thinned continental margin is the most critical possible geological analogues for the two sided subduction collisional mode include the taiwan orogenic belt and subduction of the ontong java plateau the accretion model for small oceanic plateaus applies to the nadanhada terrane in northeast china