zircon u pb geochronology of the nyainqentanglha group from the lhasa terrane new constraints on the triassic orogeny of the south tibet abstract the studied amphibolite facies metamorphic rocks typical of the nyainqentanglha group occur as a tectonic block in the central lhasa terrane south tibet and consist mainly of gneiss amphibolite quartz schist and marble the amphibolite has a similar mineral paragenesis of plagioclase garnet biotite amphibole quartz muscovite and were metamorphosed under p t conditions of 0 63 0 66 gpa and 588 622 c zircons from three dated samples include two types first type of zircons has an inherited magmatic core and a metamorphic rim whereas the other type of zircons is of metamorphic origin la icp ms in situ u pb analysis indicates that the inherited magmatic cores of zircons yield widely variable 206 pb 238 u ages ranging from 1857 ma to 743 ma recording the magmatic thermal events of the material source of the metamorphic rock protoliths in contrast the metamorphic zircons yield concordant 206 pb 238 u ages of 225 213 ma the results indicate that the nyainqentanglha group has been subjected to triassic metamorphism instead of the precambrian metamorphic basement of the lhasa terrane as considered traditionally these rocks together with the coeval sumdo eclogite and pana blueschist form a permian to triassic metamorphic belt with an e w extension of up to 300 km therefore this work provides new constraints on the late paleozoic to early mesozoic orogeny in the central lhasa terrane south tibet

paleomagnetic evidence for block rotations in central hokkaido south sakhalin northeast asia abstract the eocene to lower miocene strata in the bykov region of south sakhalin island far east russia show a mean paleomagnetic direction of dec 28 7 inc 55 7 95 6 7 and n 22 and a paleomagnetic pole at 66 3 n 245 7 e and a95 8 8 which shows that south sakhalin rotated 25 15 clockwise with respect to northeast asia since the early miocene this pole agrees with that of the eocene to middle miocene sedimentary rocks in central hokkaido japan it suggests that both south sakhalin and central hokkaido have rotated clockwise with respect to northeast asia since the early miocene these clockwise rotations can be interpreted as a vertical axis rotation of the blocks distributed in the north south direction more than 1000 km along the dextral hokkaido sakhalin shear zone considering the development of an en echelon structure of the basins in the shear zone we recognized five blocks each of which had rotated since the early miocene we conclude that this dextral shear is responsible not only for these block rotations but also for the opening of the kuril basin and the japan sea and that the shear motion was caused by oblique convergence between the eurasian and the okhotsk plates since the early miocene

provenance of lower cretaceous wolong volcaniclastics in the tibetan tethyan himalaya implications for the final breakup of eastern gondwana abstract lower cretaceous volcanic lithic arenites widely distributed in the tethyan himalaya provide insights into the continental breakup of eastern gondwana in southern tibet they are represented by the wolong volcaniclastics the volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts whereas rhyolitic dacitic volcanism becomes the predominant source of the upper strata geochemical analyses of basaltic grains and of detrital cr spinels from the wolong volcaniclastics demonstrate the alkaline character of the volcanism and suggest within plate tectonic setting for the volcanism zircon u pb ages confirm that this volcanism continued from 140 ma to 119 ma hf isotope data on these early cretaceous zircons indicate that their parental magmas were mantle derived but in the later stage of magmatic activity mantle derived magmas were mixed with partial melts derived from the continental crust the lower cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of greater india the onset of volcaniclastic deposition in the himalayas appears to become progressively younger toward the west but it ended synchronously during the late albian 102 ma the low volume of volcanic rocks and their intra plate tectonic setting suggest that they are the result of decompressional melting along extensional deep seated fractures cross cutting the continental crust and reflect changes in regional intra plate tectonic stresses when greater india began to separate from the australia antarctica supercontinent

refined chronostratigraphy of the late mesozoic terrestrial strata in south china and its tectono stratigraphic evolution abstract the late mesozoic terrestrial strata in numerous basins in s south china provide important sedimentary archives to understand the tectonic evolution of s china and east asia however establishing regional stratigraphic framework within a basin and precisely correlating strata among basins remain challenging due to limited chronological constraints in this study we report zircon u pb ages of 21 samples and a compilation of 132 reliable age data from 15 type sections of the volcanic sedimentary basins in s china the synthesized geochronology allows us to establish a refined chronostratigraphy for the late mesozoic terrestrial strata new calibrated results indicate that most of the lithostratigraphic units are diachronic laterally stacking and or interfingering six stacking styles are classified for the relationship of lithostratigraphic units analysis of the refined chronostratigraphy and the stratal stacking styles together with lithological composition reveals three episodes of tectono stratigraphic evolution episode i 145 125 ma is characterized by intense volcanism as evidenced by widespread occurrence of volcanic strata and 137 120 ma a type granites and was probably related to the rollback of the subducting paleo pacific plate the strata deposited during episode ii 125 100 ma are composed of variegated sediments associated with without volcanic intercalations in sedimentary faulted depression basins indicating the waning of volcanism and tectonism attributed to the ending of the izanagi kula plate subduction in episode iii

geology petrology and tectonic setting of the late jurassic ophiolite in hokkaido japan abstract the gokurakudaira formation which has a n s zonal distribution within a latest jurassic greenstone belt in hokkaido island japan constitutes the uppermost ultramafic mafic unit of the horokanai ophiolite the following three hypotheses for the origin of the ophiolite have been proposed 1 a mid oceanic ridge 2 an oceanic plateau and 3 an island arc the gokurakudaira formation can be subdivided into four zones extending nnw to sse from east zone i to west zone iv based on lithofacies and areal distribution zones i and iii consist of aphyric tholeiite resembling back arc basin basalt babb while zone ii is characterized by the coexistence of babb like tholeiite along with high mg andesite zone iv has a different lithology from the other zones and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite iat type andesitic subaqueous pyroclastic deposits and terrigenous sediments these stratigraphic and petrological characteristics of the gokurakudaira formation cannot be explained by the oceanic plateau or mid oceanic ridge models but they could correspond to the marginal sea model as in the lau basin therefore we conclude that the horokanai ophiolite was formed in the late jurassic in a marginal basin above a supra subduction zone on the margin of the asian continent

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

lhasa terrane in southern tibet came from australia the u pb age and hf isotope data on detrital zircons from paleozoic metasedimentary rocks in the lhasa terrane tibet defi ne a distinctive age population of ca 1170 ma with e hf t values identical to the coeval detrital zircons from western australia but those from the western qiangtang and tethyan himalaya terranes defi ne an age population of ca 950 ma with a similar e hf t range the ca 1170 ma detrital zircons in the lhasa terrane were most likely derived from the albany fraser belt in southwest australia whereas the ca 950 ma detrital zircons from both the western qiangtang and tethyan himalaya terranes might have been sourced from the high himalaya to the south such detrital zircon connections enable us to propose that the lhasa terrane is exotic to the tibetan plateau system and should no longer be considered as part of the qiangtang greater india tethyan himalaya continental margin system in the paleozoic reconstruction of the indian plate as current models show rather it should be placed at the northwestern margin of australia these results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern tibet and indicate that the lhasa terrane evolved as part of the late precambrian early paleozoic evolution as part of australia in a different paleogeographical setting than that of the qiangtang greater india tethyan himalaya system

provenance and tectonic setting of upper triassic turbidites in the eastern tethyan himalaya implications for early stage evolution of the neo tethys abstract upper triassic turbidites in the eastern tethyan himalaya preserve archives for early stage evolution of the neo tethys however their provenance and tectonic setting remain controversial here we report petrographic detrital zircon u pb and trace element data of newly discovered sandstones interbedded with layered basalts and whole rock geochemical data of basalts from the yumen melange within upper triassic turbidites and the same types of published data from associated terranes in the eastern gondwana are compiled for comparison the interbedded sandstones are dominated by quartz lithics fragments and plagioclase grains resembling those of the langjiexue group all samples display similar ranges of detrital zircon u pb ages with significant age populations at 280 210 ma 650 450 ma and 1200 800 ma comparable to those of the langjiexue group and coeval turbidites from western myanmar and partly from central tethyan himalaya implying they should share identical provenance trace elements of 300 200 ma aged zircons indicate they were derived from a continental magmatic arc geochemical data from interbedded basalts display typical within plate basalt affinity synthesized with regional geological setting the above observations suggest upper triassic turbidites from eastern and part of central tethyan himalaya and western myanmar were deposited in a marine rift basin of indian passive continental margin with their primary provenance from lhasa however other contemporaneous turbidites from part of central tethyan himalaya and australia had distinct primary sources from indian continent and west papua respectively indicating they may have separate deposystems

methane related carbonate cements in pockmarks of the north sea abstract some pockmarks on the norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms detailed examination of a sandstone sample from a pockmark in norwegian block 25 7 in the north sea has revealed the presence of mg calcite and aragonite cements some of the latter forming botryoids all cement types are characterized by extremely light carbon isotopic compositions with a mean 13c value of 56 1 pdb which shows that the cements contain carbonate produced by oxidation of biogenic methane oxidation occurred in both the oxic and anoxic diagenetic zones trace amounts of interstitial methane mean 13c 40 8 and higher hydrocarbon gases up to c5 with a c1 cn ratio of 0 855 0 874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas

carboniferous submarine volcanism at herbornseelbach lahn dill area germany a 100 m thick sequence of carboniferous submarine dominantly basaltic volcanic rocks at herbornseelbach lahn dill area is subdivided into six units two pillow volcanoes show a characteristic change from basal megapillows up to 5 m across through normal pillows 0 5 to 0 8 m across to a top facies of small pillows 0 5 m across and or breccias of scoriaceous clasts and discontinuous lava stringers sheet flows 0 5 to 4 m thick can be distinguished from sills by asymmetric cooling structures and textures vesicle concentrations near the top basal flow wrinkles and ponding of sediments on irregular flow tops

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

provenance of late triassic sediments in central lhasa terrane tibet and its implication abstract in southern tibet late triassic sequences are especially important to understanding the assembly of the lhasa terrane prior to indo asian collision we report new data relevant to the provenance of a late triassic clastic sequence from the mailonggang formation in the central lhasa terrane tibet petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance including volcanic sedimentary and some ultramafic and metamorphic rocks in situ detrital zircon hf and u pb isotope data are consistent with derivation of these rocks from nearby triassic magmatic rocks and basement that comprise part of the newly recognized late permian triassic sumdo cuoqen orogenic belt the new data suggests correlation with the upper triassic langjiexue group which lies on the opposing southern side of indus yarlung ophiolite sediments from both the mailonggang formation and langjiexue group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the sumdo cuoqen belt

a cretaceous pole from south china and the mesozoic hairpin turn of the eurasian apparent polar wander path to contribute to the apparent polar wander path apwp of the south china block and eurasia in general we collected paleomagnetic samples from mesozoic red beds around the city of ya an 30 n 103 e in the western tip of the sichuan basin in this paper we present the results from 373 oriented cores taken from one section representing 3 km of sedimentary rocks the section is dated with continental ostracods and with a magnetostratigraphic correlation between a densely sampled 272 m sequence and the polarity time scale giving an upper jurassic to upper cretaceous or lower tertiary age the remanent direction is remarkably stable throughout the section d 2 0 i 34 2 k 63 1 95 3 6 n 26 28 sites while this fact might suggest that the section has been remagnetized paleomagnetic and rock magnetic tests indicate that the remanence is primary the pole position 78 6 n 273 4 e dp 2 4 dm 4 1 corresponds to a rather low paleolatitude 18 8 2 4 but is consistent with other cretaceous poles from china if one accepts the eurasian apwp of irving and irving 1982 this result would imply that more than 1000 km of shortening took place between south china and eurasia following the acquisition of the remanence however there is no geological evidence for this large shortening we propose that the remanence was acquired within the time corresponding to the tip of the hairpin turn 150 50 ma in the revised apwp of besse and courtillot this issue the local geology suggests that the syncline from which the samples were taken has been rotated by 15 5 counterclockwise which is reflected in a similar discrepancy between the measured paleodeclination and that predicted by the besse and courtillot this issue eurasian apwp after correcting for this rotation the pole position is 70 9 n 225 2 e dp 2 4 dm 6 5 we conclude that eurasia was fully assembled by the end of the jurassic and that the mesozoic eurasian hairpin turn is a real feature

plate reconstruction based upon age data of japanese accretionary complexes the ages of the oceanic plates for the japanese accretionary complexes are plotted against time of accretion the plots show that early cretaceous transform faulting modified the arrangement of the pre middle cretaceous accretionary complexes and that three or four mid ocean ridges have collided with the japan arc since permian time the plate configuration for the western north pacific is reconstructed

the petrology of the lower palaeozoic fishguard volcanic group and associated rocks e of fishguard n pembrokeshire dyfed south wales the petrology of the fishguard volcanic group and associated rocks is described from a remapped area of cambrian and ordovician rocks between fishguard and newport n pembrokeshire dyfed the volcanic rocks are mainly rhyolitic in composition and consist of pyroclastic flows falls lavas epiclastic deposits and alloclastic intrusions welded pyroclastic flows comprise the major component a small area of basaltic pillow lavas and hyaloclastites is associated with the rhyolitic rocks the volcanic rocks are accompanied by numerous sheets of dolerite and gabbro together with diorites and microgranitic intrusions the sequence at lower fishguard harbour shows an intimate association of welded and non welded pyroclastic flows with graptolitic shales and other marine sediments which suggests that welding may have occurred subaqueously the cooling rate of submarine pyroclastic flows and lavas is thought to be significantly reduced by the formation of a steam jacket and a quenched marginal crust both providing low thermal conductivity the hydrostatic pressure of seawater maintains cohesion and retains heat and volatiles within the flow in addition water saturation of the rhyolitic melt throughout its cooling maintains mobility and is conducive to subsequent welding

cathodoluminescence and composition of calcite cement in the taum sauk limestone upper cambrian southeast missouri abstract study of calcite cements in the taum sauk limestone upper cambrian southeast missouri using cathodoluminescence petrography and microprobe analysis has shown that 1 distinct luminescent zones correspond to distinct compositional zones 2 the degree of luminescent intensity of a particular calcite zone appears to be controlled by the fe mn ratio of the zone not by the absolute concentrations of either cation 3 manganese may induce luminescence in calcite at concentrations as low as 300 ppm and 4 there may be no critical concentration of ferrous iron in calcite above which there can be no luminescene

detrital zircon provenance of the late triassic songpan ganzi complex sedimentary record of collision of the north and south china blocks the majority of the songpan ganzi triassic flysch sequence is believed to have derived from denudation of the dabie and sulu ultrahigh pressure uhp metamorphic belt in eastern china e g nie et al 1994 1 zhou and graham 1996 2 however intense debate still exists on the sources of

u pb zircon dating of paleozoic volcanic rocks from the rheno hercynian zone new age constraints for the steinkopf formation lahn dill area germany small and volumetrically subordinate outcrops of alkali rhyolitic to trachytic metavolcanic rocks are widespread in the lahn dill area of the rheinisches schiefergebirge germany these occurrences are either related to late early devonian or to middle devonian episodes of volcanic activity an unambiguous age assignment for the time of eruption could not be established so far most volcanic rocks of the study area are not suitable for application of u pb geochronology a notable exception is the steinkopf rock suite in the sw lahn dill area but a straightforward interpretation of the la icp ms u pb data is hampered by complex zircon populations recording various processes including crystallization from newly formed melt inheritance assimilation of country rocks variable degrees of pb loss and fluid assisted zr mobility a plausible evaluation of u pb zircon data of two metatrachytes and a metavolcaniclastic rock document a coherent age group at ca 390 384 ma and a probable eruption age at ca 385 ma suggesting a relationship to middle devonian givetian igneous processes zircon bearing veinlets in a metatrachyte indicate post eruptive infiltration of external hydrothermal fluids and or internal zr redistribution in late devonian to early carboniferous time 364 10 ma but this process is of rather limited importance

chemical alteration of the oceanic crust the alteration of the upper oceanic crust through sea water basement rock interaction produces different and distinct alteration zones with increasing depth the zonation generally shows a consistent worldwide pattern

plume related sedimentary basins in east asia during the cretaceous abstract many cretaceous sedimentary basins of various sizes occur in east asia which are classified into four geotectonic settings forearc intra arc back arc and intra continental basins among these intra arc and back arc basins are characterized by rifting and pull apart origins and intra continental basins by rifting related to large scale magmatism or plume the pull apart basins in southwest japan are closely related to plate subduction magmatism the continental region rift basins in the early cretaceous however seem to have no direct relation to subduction controlled magmatisms because almost no accretionary prism was developed along the continental margin at that time this means that plume related magmatism played an important role in the eastern part of the asian continent due to return flow effects from the 670 km thermal boundary layer such a plume magmatism explains systematically the origin and development of varieties of rift basins in east asia in cretaceous time

hydrothermal silica chimney fields in the galapagos spreading center at 86 w silica chimneys were discovered in 1985 at 86 w in the rift valley of the galapagos spreading center at 2600 m depth cauliflower garden the inactive chimneys lack any sulfides and consist almost entirely of amorphous silica up to 96 wt sio2 opal a fe and mn oxides are minor constituents oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32 c 29 9 18o and 42 c 27 8 18o th udating reveals a maximum age of 1440 300y amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid seawater mixture assuming equilibrium with quartz at 500 bars initial fluid temperatures of more than 175 c i e a concentration of 182 ppm sio2 were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40 c and 260 bars if the silica chimneys originate from the same or a similar fluid as higher temperature 300 c sulfide silica precipitates found nearby i e 2 5 km away then subsurface deposition of sulfides may have occurred

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

geochemistry of coeval mesozoic plutonic and volcanic suites in hong kong middle jurassic to lower cretaceous intermediate to silicic plutonic and volcanic rocks of hong kong record a transition from calc alkaline through high k calc alkaline to transitional shoshonitic compositions with time close spatial and temporal associations among comagmatic volcanic plutonic pairs indicate that magmatism occurred in discrete episodes mostly of less than one million years duration synchronous high k calc alkaline and transitional shoshonitic magmatic activity during at least one pulse suggests a relatively rapid transition from a subduction related to an extension related tectonic setting geochemical signatures indicate that the magmatic suites have a mantle origin with a decreasing crustal contribution from two distinct sources the earliest mantle derived magmas interacted strongly with a dominantly archaean crustal protolith younger magmas show evidence for interaction with a dominantly proterozoic crustal protolith the strongest mantle influence is shown by magmas which were intruded along the boundary between the two dominant crustal sources this interface marks a deep crustal discontinuity which promoted the passage of magmas to the surface

multiple sources of the upper triassic flysch in the eastern himalaya orogen tibet china implications to palaeogeography and palaeotectonic evolution abstract the upper triassic flysch langjiexue group tentatively named the shannan terrane of the eastern himalaya orogen has been tectonically assigned either to the tethys himalaya or the yarlung zangbo suture zone yzsz in this work geochronology of detrital zircon u pb isotope shows that the shannan terrane is characterized by the population of 260 200 ma peak ca 240 ma strongly supporting the view of no affinity to the tethys himalaya the detrital zircons dated as 400 290 ma display relatively positive ehf t values of 5 0 to 15 0 with t dm c ages of 2 6 1 3 ga for the shannan terrane whereas highly negative are of 20 0 to 5 0 for the lhasa terrane indicating that the two terranes have different devonian carboniferous sources numerous cr spinels found in the shannan terrane but not in the lhasa terrane exhibit contents in cr 2 o 3 and cr of mainly 44 100 and 48 95 in tio 2 of 0 01 1 0 and in al 2 o 3 of 5 257 respectively denoting several parent lithologies these differences suggested that the shannan terrane has multiple sources not only from the lhasa terrane but also from oceanic island arc seamount and mid ocean ridge areas as well as likely from greater india and australia considering the early cretaceous diabase dykes within the upper triassic flysch defined to the comei bunbury large igneous province we propose that the langjiexue group could have been deposited on the ocean between india and australia and would have not stopped till the lhasa terrane was separated from australia during the terminal triassic according to the cenozoic deformation and metamorphic history and palaeogeography of the langjiexue group we postulate that the shannan terrane could have been loaded onto the greater india during the middle early cretaceous and subsequently drifted northward to the collision zone of india and asia implying that it does not represent an accretionary prism within the yzsz

composition and sediment dispersal pattern of the upper triassic flysch in the eastern himalayas china significance to provenance and basin analysis the paleogeography and basin type of upper triassic flysch langjiexue group in the eastern himalayan orogen are disputed in order to shed new light on the flysch s origin we applied different sedimentological methods assemblages of heavy minerals and clastic components of sandstones were utilized to determine the primary depositional composition heavy mineral indices s m ratios thickness of sandstone siltstone s versus slate mudrock m and paleocurrent data were combined to reveal the sediment dispersal pattern and the location of the source areas in the analyzed sandstones heavy minerals such as zircon rutile tourmaline apatite and anatase are most common and zircon is predominant most over 60 ztr values range from 60 to 98 and systematically increase southward as a provenance sensitive parameter ruzi values vary in large magnitude and are significantly higher in both the east and west 20 than in the center the majority of s m ratios decrease from north to south suggesting an overall decrease in grain size to the south paleocurrent directions vary between 120 and 270 main vector 205 and 185 after 20 counterclockwise correction displaying a radial curved pattern variable heavy mineral assemblages indicate different sources and the sandstones fall in the recycled and mixed arc orogeny fields of dickinson triplots together supporting the view of multiple sources results of the ztr values s m ratios and paleocurrent directions illustrate a dispersal pattern corresponding to a submarine fan system the provenance and submarine fan dispersal pattern along with the basin configuration deep basin with oceanic affinities suggest that the langjiexue group accumulated in a remnant basin between lhasa greater india and australia where the sediments dispersed into the basin toward the developing orogen suture zone and not away from the orogen challenging the provenance direction for the traditional remnant basin model

late jurassic early cretaceous intra arc sedimentation and volcanism linked to plate motion change in northern japan the sorachi group composed of upper jurassic ophiolite and lower cretaceous island arc volcano sedimentary cover provides a record of late jurassic early cretaceous sedimentation and volcanism in an island arc setting off the eastern margin of the asian continent stratigraphic changes in the nature and volume of the sorachi group volcanic and volcaniclastic rocks reveal four tectonic stages these stages resulted from changes in the subduction direction of the pacific oceanic plate stage i in the late jurassic was characterized by extensive submarine eruptions of tholeiitic basalt from the back arc basin slab roll back caused rifting and sea floor spreading in the supra subduction zone along the active asian continental margin stage ii corresponded to the berriasian and featured localized trachyandesitic volcanism that formed volcanic islands with typical island arc chemical compositions at the beginning of this stage movement of the pacific oceanic plate shifted from northeastward to northwestward during stage iii in the valanginian submarine basaltic volcanism was followed by subsidence the pacific oceanic plate motion turned clockwise and the plate boundary between the asian continent and the pacific oceanic plate changed from convergent to transform during stage iv in the hauterivian barremian in situ volcanism ceased in the sorachi yezo basin and the volcanic front migrated west of the sorachi yezo basin

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

tectonic and sedimentary basin evolution of the eastern bangong nujiang zone tibet a reading cycle the ophiolite bearing bangong nujiang zone bnz traversing central tibet from east to west separates the qiangtang block in the north from the lhasa block in the south their stratigraphic development indicates that both blocks once formed a continuous continental platform until the late triassic following late paleozoic triassic rifting ocean crust formed between both blocks during the late triassic creating the dongqiao naqu basin dnb among other basins yu et al 1991 the analysis of the rift flank sequences reveals that rifting was dominated by transtension the basin was shortened by post mid cretaceous transpression thus the overall basin evolution represents a reading cycle despite some active margin processes which gave this cycle a special imprint major basin parts were preserved despite transpressional shortening suggesting that the eastern bnz represents a remnant basin our understanding of the dnb solves the prior problem of viewing the bnz as a mid late jurassic collisional suture although typical collision related deformation thickening mountain building as well as related molasse formation are lacking our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting

sedimentologic and stratigraphic constraints on the orientation of the late triassic northern indian passive continental margin abstract the orientation and distribution of the northern indian passive continental margin basin during different epochs constrained the geometric characteristics of the hyperextended northern indian edge greater india traditional paleogeographic reconstructions envisage the late triassic northern indian passive continental margin as e w trending whereas paleomagnetic data support an 55 counterclockwise rotation during northward drifting this case study of upper triassic units from the northern indian margin based on facies analysis sandstone petrography heavy mineral assemblages u pb detrital zircon geochronology and stratigraphic correlation from the zanskar range to southern tibet provides new constraints and suggests that the northern indian passive continental margin might have been n s trending in the late triassic given the dominance of quartz sandstones with zircon tourmaline and rutile indicating a provenance from continental blocks and the recycling of older quartz rich sandstones and detrital zircon age spectra characterized by a neoproterozoic detrital zircon u pb age peak 921 13 ma the upper triassic sandstones widespread along the tethyan himalayas may have been partly derived from the eastern ghats mobile belt in eastern india and the rayner orogen in eastern antarctica the upper triassic quartzite series comprises feldspatho quartzose to pure quartzose sandstones with intercalated bioclastic oosparites deposited in backshore foreshore and shoreface coastal environments without considering the subsequent counterclockwise rotation of india the consistent pattern of the upper triassic bidirectional e w paleocurrent observed throughout the himalaya suggests that the paleoshoreline was striking near n s approximately perpendicular to the regional orientation of the northern indian margin progressive eastward thickening of the sandstone layers and eastward younging of the quartzite series suggest diachronous sedimentation such stratal patterns and the strike of the late triassic paleoshoreline were attributed to the neo tethys transgression from the zanskar range to southern tibet

the embryonic himalayan foreland basin revealed in the eastern yarlung zangbo suture zone southeastern tibet abstract understanding of early foreland basin development is key to reconstruct the timing and nature of early collisional processes though the nature of the early himalayan foreland basin system is well established in the central himalaya range it lacks analysis in the eastern yarlung zangbo suture zone in this article we investigate the region s lithological and structural evolution and analyze the geochemistry and geochronology of detrital zircons and bulk rocks in the jiacha jindong region southeastern tibet we show that 1 the previously named cretaceous langxian melange comprises two separate stratigraphic units the newly revised early eocene 56 ma langxian unit and the triassic langjiexue group 2 the langxian unit overthrusts the oligocene to miocene dazhuka formation and is overlain and or underlain by the highly deformed langjiexue group 3 shale normalized ree y patterns of marbles with light ree depletion la enrichment and positive y anomaly plus sr isotope ratios 0 707481 0 707897 together demonstrate the langxian unit was originally deposited in a marine environment 4 1270 concordant detrital zircons from 20 rock samples indicate a bilateral provenance from both the gangdese arc and or recycled forearc strata in the north and the langjiexue group in the south the evidence illustrates that the langxian unit records the early india asia collision and its protoliths could have been deposited in a distal foredeep basin to the embryonic eastern himalaya orogeny the simplest explanation is that it reflects an underfilled marine basin with sources from the early mountain range to the north and a region of forebulge erosion to the south

volcanism and reef development in the devonian a case study from the lahn syncline rheinisches schiefergebirge germany abstract new data from the rheinisches schiefergebirge in germany offer insights into a complex marine basinal facies setting on the southern shelf area of the old red continent this interdisciplinary approach has the aim of providing a reconstruction of depositional and palaeoecological conditions of volcanic island induced reef growth during the middle devonian time devonian volcanic activity culminated in a givetian frasnian phase producing mainly alkali basaltic to basanitic melts some volcanic buildups reached sea level and gave rise to the development of reefs during times of reduced volcanic activity reef communities in the lahn syncline were dominated by corals and stromatoporoids in terms of conodont stratigraphy they began to flourish during the middle varcus subzone in the givetian and lasted until the late falsiovalis subzone in the frasnian a facies model is presented for the entire area that clarifies the association of volcanism and reef development

late mesozoic cenozoic sedimentary basins of active continental margin of southeast russia paleogeography tectonics and coal oil gas presence abstract various settings took place during the late mesozoic divergent convergent collisional and transform after mid jurassic collision of the siberian and chinese cratons a latitudinal system of post collision troughs developed along the mongol okhotsk suture the uda torom basins and others filled with terrigenous coal bearing molasse the dispersion of pangea creation of oceans during the late jurassic are correlated to the emergence of the east asian submeridional rift system with volcano terrigenous coal bearing deposits the amur zeya basin at that time to the east there existed an andean type continental margin foreland upper bureya partizansk and razdolny and flexural sangjiang middle amur basins were formed along the margin of the rigid massifs during the late jurassic to berriasian during the valanginian mid albian an oblique subduction of the izanagi plate beneath the asian continent occurred producing a transform margin type considerable sinistral strike slip displacements and formation of pull apart basins filled with turbidites the sangjiang middle amur basin the aptian is characterized by plate reorganization and formation of epioceanic island arcs fore arc and back arc basins in sakhalin and the sikhote alin the alchan and sangjiang middle amur basins filled with volcanoclastics during the mid albian a series of terranes accreted to the asian continental margin by the end of the albian the east asian marginal volcanic belt began to form due to the subduction of the kula plate beneath the asian continent during the cenomanian coniacian shallow marine coarse clastics accumulated in the fore arc basins which were followed by continental deposits in the santonian campanian from the coniacian to the maastrichtian a thermal subsidence started in rift basins and continental oil bearing clastics accumulated the amur zeya basin widespread elevation and denudation were dominant during the maastrichtian this is evidenced by thick sediments accumulated in the western sakhalin fore arc basin during the cenozoic an extensive rift belt rmade up of a system of grabens which were filled with lacustrine alluvial coal and oil bearing deposits developed along the east asian margin

mapping chromite deposits with audio magnetotellurics in the luobusa ophiolite of southern tibet the exploration of podiform chromites in the indus yarlong zangbo suture zone of southern tibet has proved difficult in recent decades because most known deposits pinch out and then reappear in the same direction a number of ground based geophysical approaches including gravity magnetic and controlled source audio frequency magnetotelluric methods have been applied to explore for these chromite deposits but have mostly failed to delineate prospective areas we present a successful podiform chromite exploration case history that is based on audio magnetotellurics amt more than 8 000 amt stations were utilized in this study within a 10 km2 area of the ophiolite belt line separations were 80 m or 40 m and station separation was 20 m we implemented bostick conversion and non linear conjugate gradient inversions were implemented for data interpretation while two dimensional 2d resistivity sections and three dimensional 3d resistivity imaging were used to elucidate the inner structure and distribu

himalayan detrital chromian spinels and timing of indus yarlung ophiolite erosion abstract the geochemistry of detrital chromian spinels is commonly used to discriminate provenance from different tectonic settings of mafic and ultramafic igneous rocks detrital spinels in cenozoic foreland basin successions fed from the himalaya orogen were assertively interpreted as sourced from the ophiolitic rocks of the indus yarlung suture zone this study compares the geochemistry of detrital cr spinels from the tethys himalaya passive margin and cretaceous xigaze forearc successions with those from the indus yarlung ophiolites cr spinels in the indus yarlung ophiolites have low tio2 mostly 0 2 and low al2o3 mainly 6 23 indicating a rift related basaltic origin detrital cr spinels from the xigaze forearc basin have either low tio2 mostly 1 0 indicating intra plate basaltic origin compositional fingerprints of detrital cr spinels from lower eocene foreland basin strata in the central eastern himalaya indicate provenance from the lhasa block without input from the indus yarlung ophiolites only cr spinels from the lower eocene foreland basin strata in the north western himalaya and the upper eocene lower miocene remnant ocean turbidites of the bengal basin are mostly ophiolite derived the indus yarlung ophiolites were thus emplaced and exposed to erosion since the early eocene 50 ma in the nw himalaya but only subsequently 50 38 ma in the eastern himalaya

an alternative plate tectonic model for the palaeozoic early mesozoic palaeotethyan evolution of southeast asia northern thailand burma abstract an alternative model for the geodynamic evolution of southeast asia is proposed and inserted in a modern plate tectonic model the reconstruction methodology is based on dynamic plate boundaries constrained by data such as spreading rates and subduction velocities in this way it differs from classical continental drift models proposed so far the different interpretations about the location of the palaeotethys suture in thailand are revised the tertiary mae yuam fault is seen as the emplacement of the suture east of the suture we identify an indochina derived terrane for which we keep the name shan thai formerly used to identify the cimmerian block present in southeast asia now called sibumasu this nomenclatural choice was made on the basis of the geographic location of the terrane eastern shan states in burma and central thailand and in order not to introduce new confusing terminology the closure of the eastern palaeotethys is related to a southward subduction of the ocean that triggered the eastern neotethys to open as a back arc due to the presence of late carboniferous early permian arc magmatism in mergui burma and in the lhasa block south tibet and to the absence of arc magmatism of the same age east of the suture in order to explain the presence of carboniferous early permian and permo triassic volcanic arcs in cambodia upper triassic magmatism in eastern vietnam and lower permian middle permian arc volcanites in western sumatra we introduce the orang laut terranes concept these terranes were detached from indochina and south china during back arc opening of the poko song ma system due to the westward subduction of the palaeopacific this also explains the location of the cathaysian west sumatra block to the west of the cimmerian sibumasu block

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

tectonic evolution of the sibumasu indochina terrane collision zone in thailand and malaysia constraints from new u pb zircon chronology of se asian tin granitoids three principal granite provinces are defined across se asia as follows 1 the western thailand myanmar burma province consists of hornblende biotite i type granodiorite granites and felsic biotite k feldspar garnet tourmaline granites associated with abundant tin mineralization in greisen type veins new ion microprobe u pb dating results from phuket island show zircon core ages of 212 2 and 214 2 ma and a thermal overprint with rims of 81 2 1 2 and 85 75 ma 2 the north thailand west malaya main range province has mainly s type biotite granites and abundant tin mineralization resulting from crustal thickening following collision of the sibumasu plate with indochina during the mid triassic biotite granites around kuala lumpur contain extremely u rich zircons up to 38000 ppm that yield ages of 215 7 and 210 7 ma 3 the east malaya province consists of dominantly permian triassic i type hornblende biotite granites but with subordinate s type plutons and a type syenite gabbros biotite k feldspar granites from tioman island off the east coast of malaysia also yield a zircon age of 80 1 ma showing cretaceous magmatism in common with province 1 geological and u pb geochronological data suggest that two east dipping in present day coordinates subduction zones are required during the triassic one along the bentong raub palaeo tethyan suture and the other west of the phuket burma province 1 belt supplementary material a full description of u pb analytical methods used and data tables are available at www geolsoc org uk sup18523

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

kinematic evolution of the eastern tethyan himalaya constraints from magnetic fabric and structural properties of the triassic flysch in se tibet anisotropy of magnetic susceptibility ams combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the triassic flysch within the eastern tethyan himalaya thrust belt in se tibet the attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron bearing silicates two different tectonic domains can be defined a the southern domain is controlled by the eohimalayan tectonic foliation s1 recorded in the magnetic foliation which trends east west and dips to the north b the northern domain is dominated by the neohimalayan magnetic foliation with wnw ese strike and dips to the south opposite to the vergence of the main structures a slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of s1 and the subtle development of the s2 tectonic foliation hinterland propagation of the deformation lead to the great counter backthrust generation pointed out by the ssw steeply plunging magnetic lineation furthermore different orientations of magnetic foliation may indicate an early miocene c 208 clockwise vertical

zircon u pb dating and in situ hf isotopic analysis of permian peraluminous granite in the lhasa terrane southern tibet implications for permian collisional orogeny and paleogeography article i nfo the lhasa terrane has long been interpreted as a simple tectonic block rifted from gondwana during the late paleozoic and then drifted northward before finally amalgamating with the qiangtang terrane during the early cretaceous in this paper we document permian peraluminous granites near pikang in the southern margin of the central lhasa terrane close to the recently documented songdo eclogite of permian age zircon shrimp and la icpms u pb dating for a pikang granite sample gives an identical crystallization age of about 263 ma and a wide age range of inherited zircons 283 2141 ma in situ hf isotopic analyses for 20 zircons of 263 ma yielded ehf t values of 4 5 to 1 9 the pikang granites have high a cnk values 1 08 and high normative corundum 1 3 2 0 indicative of peraluminous s type granite they are characterized by moderately negative eu anomalies eu eu 0 48 0 61 and strongly negative ba nb sr p and ti anomalies the granites have high end t values 6 4 to 6 0 and low initial 87 sr 86 sr ratios 0 7082 0 7096 relative to melts derived from mature continental crust these rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials we interpret the pikang peraluminous granite magmatism the regional angular unconformity between the middle and the upper permian and the eclogite of the same metamorphic age 262 ma from the same geotectonic location to represent different products of a common event in time and space we tentatively term this common event as syncollisional orogeny i e the permian gangdese orogeny we further hypothesize that the orogeny may be genetically associated with the collision between the lhasa terrane and the northern margin of australia following the closure of the paleo tethyan ocean south of the lhasa terrane

deposystem architectures and lithofacies of a submarine fan dominated deep sea succession in an orogen a case study from the upper triassic langjiexue group of southern tibet abstract over thirty stratigraphic sections of the himalaya orogen upper triassic langjiexue group in southern tibet china were studied to interpret the environments and lithofacies the facies associations channel a lobe b levee interchannel c and basin plain d with nine facies a1 3 b1 3 and c1 3 were distinguished they form six architectural elements channel interchannel overbank levee crevasse splay outer fan lobe fan fringe and basin plain taking into account the facies analysis sub deposystem correlation paleocurrent dispersal pattern and restoration of primary stratal width the langjiexue group displays the architecture of a coalescing submarine fan dominated deep sea deposystem measuring about 400 500 km 600 700 km in size or even more one of the largest pre cenozoic submarine fans ever reported subdivisionally four fans lacking inner fans could have coalesced laterally within the submarine fan deposystem and at least six submarine fan developments were vertically succeeded by mid to outer fan deposits with progradational to retrogradational successions according to the range of 30 70 of sandstone content the fan deposystem is mud and sand rich suggesting a medium far over 400 600 km transport of sediment from the source area

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

upper triassic turbidites of the northern tethyan himalaya langjiexue group the terminal of a sediment routing system sourced in the gondwanide orogen abstract the upper triassic langjiexue group exposed south of the yarlung zangbo suture zone in south tibet shows sedimentary features different from typical tethyan himalayan successions and its origin is controversial in this article we combine field observations with paleocurrent petrologic geochronological and isotopic data to determine the provenance of langjiexue sandstones these middle to distal deep sea fan turbidites are crosscut by lower cretaceous diabase sills and dikes generated during rifting of india from gondwana indicating that the langjiexue group was originally deposited along or adjacent to the northern passive continental margin of india flute casts at the base of turbidite beds indicate mostly wnw ward paleocurrents pointing to provenance from a source located east of the depositional area common volcanic fragments and plagioclase grains together with a cluster of 400 200 ma aged magmatic zircons with uniform e hf t values from 5 to 10 are incompatible with any nearby sources including the qiantang block the lhasa block or the india subcontinent and indicate instead supply from a long lived magmatic arc terrane considering what is known about late triassic paleogeography a plausible source for langjiexue sediments is represented by the gondwanide orogen generated during subduction of the pan pacific oceanic lithosphere beneath southeastern gondwana this scenario is supported by the age range and hf isotopic signatures of late paleozoic early mesozoic zircons contained in langjiexue turbidites as in coeval turbidites exposed in western myanmar new data are needed to confirm falsify the existence of a thousand km long sediment routing system similar to the modern amazon which sourced in a cordillera type orogen rising along the southeastern margin of gondwana crossed an entire continent to feed turbiditic fans now exposed from western myanmar to the northern tethys himalaya

what drove late mesozoic extension of the northern china mongolia tract the northern china mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late mesozoic time late middle to early late jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated north china mongolia block and the siberian plate but widespread late late jurassic early cretaceous extension has not been satisfactorily explained by existing models some prominent features of the extensional tectonics of the northern china mongolia tract are 1 late jurassic voluminous volcanism prior to early cretaceous large magnitude rapid extension 2 overlapping in time of contractional deformation in the yinshan yanshan belt with development of extensionrelated basins in the interior of the northern china mongolia tract and 3 widespread occurrence of alkali granitic plutonism extensional basins and metamorphic core complexes in the early cretaceous a new explanation is advanced in this study for this sequence of events the collision of amalgamated north china mongolia with siberia led to crustal overthickening of the northern china mongolia tract and formation of a high standing plateau subsequent breakoff at depth of the north dipping mongol okhotsk oceanic slab is suggested as the main trigger for late mesozoic lithospheric extension of that tract slab breakoff resulted in mantle lithospheric stretching of the adjacent northern china mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust collectively these phenomena triggered gravitational collapse of the previously thickened crust leading to late late jurassic early cretaceous crustal extension and importantly coeval contraction along the southern margin of the plateau in the yinshan yanshan belt the proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena such as the synchronous extension of northerly terranes during major contraction in the neighboring yanshan yinshan belt d 2003 elsevier b v all rights reserved

late triassic paleogeographic reconstruction along the neo tethyan ocean margins southern tibet abstract sandstone petrographic and u pb detrital zircon analyses of upper triassic sedimentary rocks from the northern margin of india tethyan himalaya sequence and southern margin of eurasia lhasa terrane provide new constraints on the mesozoic paleogeography of neo tethyan ocean basins the upper triassic nieru formation of the tethyan himalaya sequence ths near lazi city 29 n 87 5 e is dominated by indian affinity precambrian detrital zircons which are typical of the majority of the ths however the upper triassic langjiexue formation of the ths exposed to the east at 90 93 e longitude includes significant populations of permian to early jurassic 291 184 ma detrital zircons for which there is no known indian source in addition the upper triassic nieru formation near kangma town 28 5 n 90 e located 200 km to the southeast of lazi city yielded detrital zircon age spectra that are similar to those of langjiexue formation based on detrital zircon age spectra comparisons we propose that both the langjiexue and nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of australia furthermore we suggest that these ths units and age equivalent strata in northwest australia west sulawesi timor and west papua comprised a late triassic submarine fan along the northern australian shelf the upper triassic mailonggang formation in the southern lhasa terrane 35 km northeast of lhasa city 30 n 91 5 e is dominated by permian detrital zircons which were likely derived from proximal lhasa terrane sources the mailonggang formation differs from all age equivalent strata in the tethyan himalaya therefore we interpret that it was separated from greater india by the neo tethyan ocean

lead isotope systematics of vein type antimony mineralization rheinisches schiefergebirge germany a case history of complex reaction and remobilization processes mesothermal vein type sb mineralization in the rheinisches schiefergebirge germany is characterized by two different mineralization styles which are 1 extensional quartz stibnite vein systems and 2 cu pb sb sulphosalt assemblages in overprinted pre existing pb zn veins a detailed pb isotope study of 52 representative samples from both mineralization types indicates distinct compositional patterns cu pb sb sulphosalts meneghinite boulangerite bournonite formed by reaction remobilization are characterized by pb isotope compositions 206pb 204pb 18 179 18 223 which are identical to the precursor galena 206pb 204pb 18 168 18 223 the pb isotope composition of sulphosalt minerals in these vein systems was controlled by lead inherited from pre existing galena stibnite and pb sulphosalts zinkenite semseyite plagionite formed in quartz stibnite vein systems display pb isotope ratios 206pb 204pb 18 250 18 354 which are more radiogenic than galenas from variscan pb zn ore veins 206pb 204pb 18 162 18 303 detailed small scale investigation of thrust zones hosting pb zn ores and crosscutting quartz ankerite fissure veins ramsbeck deposit indicates that the pb isotope compositions of recrystallized galena and remobilized phases boulangerite semseyite bournonite are arranged along a linear trend this is interpreted as mixing between primary galena with 206pb 204pb 18 206 and overprinting hydrothermal fluids with a more radiogenic composition 206pb 204pb 18 354 expressed by intermediate compositions 206pb 204pb 18 256 18 334 of newly formed sulphosalts the pb isotope systematics of the vein type sb mineralization is in accordance with a model of pb extraction from similar crustal sources palaeozoic sedimentary sequences at different times

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

cretaceous cold seep communities and methane derived carbonates in the canadian arctic lower cretaceous cold seep fossil assemblages have been found in the canadian arctic archipelago serpulid worm tubes and bivalves are most abundant in these communities in contrast fossils are scarce in the surrounding strata the fossils are contained in an isotopically light delta 13 c 25 to 50 per mil carbonate rock groundmass that is interpreted to have formed from bacterial oxidation of methane the rocks were deposited at intermediate depth 400 meters in a cold marine environment nearby normal faults may have provided a conduit for seeping methane and hydrogen sulfide needed to fuel chemosynthetic bacteria and in turn the higher life forms

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

cretaceous tectonics and geological environments in east russia abstract by the late jurassic the northern part of the east asian continental margin was diversified principal structural constituents were the siberian or north asian craton bounded on the east by a miogeosynclinal fold belt a system of smaller ancient blocks and a collage of terranes that had been attached to the craton from the east and southeast at different times such a diverse structural environment caused variability of cretaceous landscapes and related sedimentary systems global processes of lithospheric plate interaction and related regional tectonic processes played a leading role in cretaceous environmental changes in the continental margin of east russia during the early cretaceous the oblique plate convergence produced a transform continental margin over a long period of time during the middle albian a collage of terranes were attached to the continental margin of east russia then during the late albian period the angle of convergence increased subduction resumed and a giant east asian volcanic belt formed along the continental margin this belt was morphologically represented by a chain of mountain ridges up to 3000 m thus creating a sublongitudinal tectonic and climatic zonation the active continental margin with a typical environmental arrangement of marginal seas island arcs the open sea persisted until the end of the cretaceous the late maastrichtian tectonics was mostly characterized by overall uplift of the region

new perspective on aptian carbon isotope stratigraphy data from δ13c records of terrestrial organic matter carbon isotope analyses were performed on detrital woody materials in aptian mid cretaceous marine sediments of central hokkaido northern japan a positive 13cwood excursion from 25 4 to 21 8 following a remarkable negative isotope shift is recognized in the early aptian and a small positive anomaly is also found in the latest aptian this 13cwood profile is exactly in phase with the 13ccarbonate curve from a pacific guyot when the age of the guyot carbonates is revised using published sr isotope stratigraphies the highly conformable 13c profiles of pacific and hokkaido sections suggest that 13c compositions of aptian marine and terrestrial carbon reservoirs changed simultaneously by the same amplitude within the ocean atmosphere biosphere system fluctuation patterns of tethyan 13ccarbonate curves are slightly different from those of pacific and hokkaido sections such differences in 13c profiles may be attributed to the local paleoceanographic setting of the aptian mediterranean tethys

tectonic evolution of the himalayan thrust belt in western nepal implications for channel flow models we present a new geologic map of western nepal and three balanced regional cross sections in the himalayan thrust belt the minimum shortening between the south tibetan detachment and the main frontal thrust is 485 743 km and suggests that total himalayan shortening may exceed 900 km all rocks involved in the thrust belt are of upper crustal affinity implying that a comparable length of indian lower crust and mantle lithosphere was subducted beneath tibet major structural features are the subhimalayan thrust system lesser himalayan imbricate zone dadeldhura thrust sheet lesser himalayan duplex ramgarh thrust sheet main central thrust sheet and a north dipping normal sense shear zone possibly related to the south tibetan detachment these structures are continuous along the entire nepalese portion of the himalayan thrust belt new 40 ar 39 ar ages from the ramgarh thrust zone greater himalayan rocks and the lower part of the tethyan sequence support a kinematic model in which major thrust systems in nepal propagated southward from early miocene time onward the geometry and kinematic history of the thrust belt in western nepal are not compatible with recent models for southward ductile extrusion of greater himalayan rocks in a mid crustal channel instead the thrust belt in western nepal behaved like a typical forward propagating thrust system involving unmetamorphosed brittlely deformed rocks in its frontal part and ductilely deformed higher grade metamorphic rocks in its hinterland region although our results do not support published versions of the channel flow model they provide additional geological and geo chronological data that will assist future attempts to develop geodynamic models for the himalayan tibetan orogenic system

jurassic intraplate magmatism in southern hunan eastern guangxi 40ar 39ar dating geochemistry sr nd isotopes and implications for the tectonic evolution of se china the mesozoic geology of se china is characterized by intensive and widespread magmatism however the tectonic regime that accounted for the mesozoic magmatism has been an issue with little consensus a comprehensive study of ar 40 ar 39 dating geochemistry and sr nd isotopes has been conducted on basalts from southern hunan and syenite intrusions from eastern guangxi three episodes of jurassic magmatism i e alkaline basalts of c 175 ma in age syenitic intrusions of c 160 ma and high mg basalts of c 150 ma are identified the older c 175 ma alkaline basalts are characterized by low sr i sr 0 7035 0 7040 and high nd epsilon nd t 5 to 6 isotopic compositions and oib like trace element patterns e g nb la 1 in contrast the younger c 150 ma high mg basalts have high sr i sr c 0 7054 and low nd epsilon nd t c 2 isotopic compositions and incompatible trace element patterns of arc affinity the c 160 ma syenitic intrusions display a relatively large range of sr and nd isotopic compositions i sr 0 7032 0 7082 epsilon nd t 5 5 to 4 1 with the qinghu syenites having the lowest i sr highest epsilon nd t and oib type incompatible trace element patterns analogous to the c 175 ma alkaline basalts such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources which are inferred to have resulted from the post indosinian orogenic lithosphere extension and thinning the c 175 ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the astheno sphere or an enriched lithospheric mantle source accreted by asthenosphere derived melts during the initial extension the c 160 ma syenitic and c 150 ma high mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning this interpretation is at odds with the active continental margin related to the subduction of palaeo pacific plate but consistent with continental rifting and extension for the mesozoic of se china

iron ore of the lahn dill type formed by diagenetic seeping of pyroclastic sequences a case study on the schalstein section at gansberg weilburg investigations on the reconstruction of processes and facies relationships from submarine volcanics of devonian age in the lahn syncline rhenish mountains western germany reveal a complex development of secondary alteration this is well illustrated by a mafic pyroclastic sequence schalstein at the gansberg near weilburg where alteration processes are visible by petrographic and geochemical means and can be further classified by cathodoluminescence iron ore formation of lahn dill type is recognized as part of this alteration process resulting from diagenetic seeping until recently a direct magmatic source for the lahn dill type iron ore has been the generally accepted model these bodies have therefore been viewed as a rather unique stratiform deposit whose occurrence was virtually confined to this type area however it is here considered that the formation of the iron ore corresponds well with existing models of alteration processes within recent oceanic environments which are of a more universal occurrence

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved