neoproterozoic tectonics of australia antarctica and laurentia and the 560 ma birth of the pacific ocean reflect the 400 m y pangean supercycle unequivocal evidence for the proterozoic reconstruction of australia antarctica and laurentia remains elusive although various authors have interpreted sedimentary and igneous events in terms of initial neoproterozoic rifting and final neoproterozoic cambrian drifting the synchronous rifting and drifting reflect the tectonics of a late neoproterozoic pangea east gondwanaland and laurentia that amalgamated along the mozambiquean belt 720 ma and broke up at 560 ma by growth of the paleo pacific and lapetus oceans in this paper these events are interpreted as part of a 400 m y supercycle comparable to the 320 ma amalgamation of pangea a and its 160 ma breakup during supercycle a 320 ma to present events near the end of the cambrian 500 ma in east gondwanaland included epeirogenic uplift of cratons shown by widespread k ar and apatite fission track dates and the inception of quartz turbidite fans intense deformation and intrusion by granite of the antarctic and south eastern australian mar

proterozoic blueschist belt in western china best documented precambrian blueschists in the world a coherent blueschist terrane near aksu northwestern china is unconformably overlain by unmetamorphosed sedimentary rocks of sinian age ca 600 800 ma the pre sinian metamorphic rocks termed the aksu group have maintained stratal continuity and were derived from shale sandstone basaltic volcanic rocks and minor pelagic sediments they have undergone multistage deformation and transitional blueschist greenschist facies metamorphism and are represented by strongly foliated chlorite stilpnomelane graphite schist stilpnomelane phengite psammitic schist greenschist blueschist and minor quartzite metachert and metamorphosed ironstone metamorphic minerals present in blueschists include crossitic amphibole epidote chlorite albite quartz and actinolite mineral parageneses and composition of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and about 300 400 c post tectonic alkali diabase dikes cut the aksu group the basal conglomerate of the overlying sinian to eocambrian sedimentary succession contains clasts of both the metamorphic and dike rocks clearly demonstrating that conditions required for blueschist facies metamorphism were attained at least 800 m y ago

timing the breakup of a proterozoic supercontinent evidence from australian intracratonic basins in the late proterozoic several broad shallow intracratonic depressions appeared across a vast area of central australia the basins which all contain shallow marine to fluvial successions appear to have been tenuously interconnected through much of their history analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension one at about 900 ma and a second at about 600 ma these extensional episodes were probably the result of failed rifting events that almost fragmented the australian continent during the late proterozoic the second period of extension almost certainly relates to the breakup of a proterozoic supercontinent the results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions

tectonics of western tibet between the tarim and the indus a tarim to indus traverse provides insight into the tectonics of western tibet the kunlun was the site of a mid paleozoic collision at least three phases of post paleozoic accretion have thickened the blanket of sediments that covers western tibet sizeable parts of western qiangtang have remained stable however since the mid mesozoic since the neogene deformation and volcanism have been localized near the edges of the plateau strike slip motion along the karakorum and altyn tagh faults has been coeval with overthrusting in the himalayas and kunlun such slip partitioning and the volcanism appear to result simply from northward subduction of india and southward subduction of the tarim as tibet is extruded eastwards by india s penetration into asia

glacial incursion on a neoproterozoic carbonate platform in the kimberley region australia the 80 m thick egan formation preserves sediment deposited during the younger of two episodes of glaciation recorded in the neoproterozoic succession of the kimberley region northwestern australia like many terminal proterozoic glaciations recorded in australia and elsewhere the glacial strata of the egan formation are associated with carbonate rocks of likely warm water affinity but they are sedimentologically distinct from the marker cap carbonate horizons that overlie glacial strata in other neoproterozoic successions the carbonate strata comprise a wide range of facies indicative of shallow water patch reef shoal and lagoonal deposition detailed facies analysis of the egan formation indicates interruption of the carbonate system by glaciation and subsequent resumption of warm water conditions this sedimentological analysis allows a reassessment of the regional stratigraphic correlations proposed for the egan formation which is here considered to record a glacial event younger than the widespread marinoan glaciation of central and south australia and therefore a speculated third global glaciation in terminal proterozoic time

evidence for late proterozoic subduction from 700 myr old blueschists in china in a comprehensive review of the world s blueschists ernst1 noted an apparent absence of these high pressure low temperature rocks in the precambrian and ascribed it to a cooling of the earth in the phanerozoic eon as blueschists are thought to be formed in subduction zones the absence of precambrian blueschists has been adduced as evidence that plate tectonics did not operate on the early earth2 although this conclusion is not inevitable in that ancient mobile belts are more likely to have been obliterated than younger ones3 the presence of paired metamorphic belts of kyanite sillimanite and andalusite sillimanite grade in proterozoic gondwanaland supports the existence of a subducted slab but with ancient geothermal gradients too high to form blueschists along the convergent plate boundary4 5 on the other hand data from inclusions in african diamonds support the opposing view that archaean continental geotherms were similar to those of today6 here we present rb sr and k ar ages of 698 718 myr for phengitic mica from high pressure pelitic schists from the aksu region of china which are interleaved with blueschists we interpret these dates as representing the time of the blueschist facies metatmorphism suggesting that subduction was occurring in the late proterozoic

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

sequence stratigraphy in active extensional basins implications for the interpretation of ancient basin fills abstract systematic variations in sequences their component units and their stacking patterns exist within rift basins these variations can be related to displacement gradients associated with the large scale 12 50 km segmentation of normal fault zones which control accommodation sediment supply and basin physiography sediment supply is also strongly influenced by the nature of pre rift drainage networks and variations in bedrock lithology high rates of hangingwall subsidence close to the centre of normal fault segments may cancel out the effects of glacio eustatic sea level fall so that accommodation development is normally characterized by the continual addition of new space the resulting sequences lack type 1 sequence boundaries and lowstand systems tracts and stack into aggradational sequence sets the adjacent footwall is subject to uplift which may lead to subaerial exposure and incision generating a composite type 1 sequence boundary away from the fault zone and near segment boundaries slip rates are much lower and hence relative sea level change is dominated by eustasy here falls in relative sea level are important resulting in type 1 sequences sequence stacking patterns reflect not only eustasy and local fault controlled subsidence but also the interaction of these with adjacent uplifting footwalls and or regional uplift tectonic influence on sequence development will be more pronounced during greenhouse times than icehouse times

a mechanism to explain rift basin subsidence and stratigraphic patterns through fault array evolution rift basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate the physical basis for this transition is not well understood although an increase in extension rate is commonly implied here a numerical fault growth model is used to investigate the influence of segment linkage on fault displacement rate patterns along an evolving normal fault array the linkage process we describe is controlled by a stress feedback mechanism which leads to enhanced growth of optimally positioned faults model results indicate that even with constant extension rates slow displacement rates prevail during an initial phase of distributed extension followed by an increase in displacement rates as strain becomes localized on linked fault arrays this is due to the dynamics of fault interactions rather than mechanical weakening comparison of model simulations with rift basin subsidence and stratigraphic patterns in the gulf of suez and north sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault zone evolution without invoking a change in regional extension rates

tectono thermal evolution of cambrian ordovician source rocks and implications for hydrocarbon generation in the eastern tarim basin nw china abstract the tectono thermal evolution is an important control on petroliferous basin development however this evolution is difficult to reconstruct in sedimentary basins subjected to multiple tectono thermal events the tarim basin is the largest petroliferous basin in northwestern china and hosts a complex oil gas system comprising several sets of source reservoir and cap rocks that have experienced multiple tectonic thermal and sedimentary events large oil and gas reserves have been discovered in the central and northern tarim basin during the last 30 years but none in the eastern part of the basin this is generally attributed to the high thermal maturity reached by source rocks during the late ordovician and subsequent hydrocarbon loss therefore reconstructing the cambrian ordovician tectono thermal evolution is critical in facilitating successful oil and gas exploration in this region we compiled equivalent vitrinite reflectance data n 178 of source rocks from seven wells in the tarim basin which for cambrian ordovician source rocks are 1 3 beyond the hydrocarbon generation threshold and in the main gas and dry gas generation stages the cambrian ordovician source rocks experienced rapid burial and heating during the caledonian period entered the hydrocarbon generation stage during the middle late ordovician and reached peak hydrocarbon generation at the end caledonian period they might have experienced secondary hydrocarbon generation after the jurassic homogenization temperatures of fluid inclusions n 616 from wells gc4 lx1 md1 and yd2 as well as the modeled thermal evolution reveal a main hydrocarbon generation event during the caledonian period and a secondary event in the yanshanian and himalayan period in other wells lx1 md1 and yd2 hercynian and indosinian uplift and exhumation in the eastern tarim basin controlled the main structures and faulting created favorable conditions for secondary hydrocarbon generation and migration at this time since the yanshanian and himalayan the tarim basin has experienced further sedimentation and heating and the tectono thermal evolution might have been related to the formation of the tibetan plateau in summary our reconstruction of the tectono thermal evolution of cambrian ordovician strata and the hydrocarbon generation history of the tarim basin suggests that the eastern tarim basin may also have hydrocarbon generation potential and can guide future oil and gas exploration

paleozoic oil source rock correlations in the tarim basin nw china we studied a suite of 40 oils and extracts of purported source rocks from the tarim basin in nw china the main group of oils comes from tazhong and tabei wells which sample the largest known petroleum accumulations in the basin these oils can be statistically correlated with extracts of ordovician rocks based upon high relative concentrations of 24 isopropylcholestanes and low relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes in contrast extracts from cambrian rocks have low relative concentrations of 24 isopropylcholestanes with high relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes although some tarim basin cambrian rocks yield high total organic carbon contents we see little evidence in the analyzed oil samples to suggest that they came from cambrian source rocks

an apparent polar wander path from the tarim block china abstract the apparent polar wander apw path from the tarim block consists of devonian 16 n 165 e a95 4 late carboniferous 41 n 160 e a95 4 permian 61 n 177 e a95 9 early triassic 69 n 183 e a95 11 and jurassic cretaceous 65 n 214 e a95 6 poles on the basis of this apw path we conclude that tarim rode on a plate subducting under the kazakhstan plate between devonian and permian times by the end of the permian subduction had almost finished early triassic poles from tarim siberia north china and south china are discordant indicating post early triassic relative motions among them cretaceous paleomagnetic poles from tarim north china and south china however are consistent suggesting that china has been a coherent tectonic unit since cretaceous time the shape of the apw path from tarim is similar to that of siberia except that tarim s apw path is systematically displaced to the east a 26 clockwise rotation of tarim s apw path about the euler pole 60 n and 14 e reconciles these two apw paths tarim has moved northeastward some 2000 km relative to a fixed siberia since the cretaceous this northeastward motion of tarim with respect to siberia agrees well with the overall cenozoic tectonic style of asia most of southern china has been pushed eastward to give way to the penetrating indian continent east west and northeast trending left lateral strike slip faults north of the tarim block have been documented however the exact amounts of displacement along these faults are not well known using paleomagnetic data from tarim we will also discuss the tectonic evolution of terranes in central asia and the amount of continental shortening in the tibetan plateau

ordovician conodonts of the tarim region xinjiang china occurrence and use as paleoenvironment indicators abstract a review of the distribution of different ordovician conodont faunas in eight areas of the tarim region shows that these conodont faunas can be classified into the north china and south china types the north china type is characterized by aurilobodus leptosomatus a aurilobodus a simplex tangshanodus tanshanensis loxodus dissectus parasseratognathus paltodiformis microcoelodus symmetricus belodina compressa b confluens pseudobelodina dispansa yaoxianognathus yaoxianensis and taoqupognathus blandus these were adapted to shallow warm water environments the south china type is represented by the genera amorphognathus baltoniodus cahabagnathus eoplacognathus lenodus microzarkodina oepikodus paroistodus paracordylodus periodon polonodus and pygodus which were adapted to outer shelf deeper and colder water environments using the general pattern of conodont distribution it is possible to interpret the various depositional environments and to reconstruct broad changes in palaeogeography of the tarim region during ordovician time in general during tremadocian to early middle ordovician time most of the tarim region was a shallow semi restricted platform that became deeper towards the north and east with an open platform in kalping and in the northern part of taklimakan desert a slope and deep basin existed in the current tianshan mountains region the tarim sea was shallow during the early ordovician and became deeper during caradocian sandbian and early katian time to become shallow again during ashgillian late katian time with the exception of part of central taklimakan which was a land area during caradocian sandbian and early katian time

basin analysis of the jurassic lower cretaceous southwest tarim basin northwest china the jurassic through cretaceous southwest tarim basin northwest china contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with eurasia of the changtang block during late triassic early jurassic time and with the mega lhasa block during late jurassic early cretaceous time this tectonism is chronologically linked with the creation of a narrow lower middle jurassic transtensional basin with thick sedimentary infill succeeded by a broader upper jurassic lower cretaceous compressional basin with thinner sedimentary infill the older basin formed between a north northwest striking dextral fault on the eastern side of the southwest tarim basin in the tian shan and a postulated strike slip or normal fault on the western margin of the basin along the kunlun shan the former fault is now the talas ferghana fault the latter may be a predecessor to the main pamir thrust subsidence analysis of the thickest sedimentary section suggests thermal subsidence interpreted as the result of transtension between the two basin bounding faults the younger basin extends farther east and west and does not preserve evidence of activity along the talas ferghana fault the change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

compressional wave velocity measurement in seabed materials by use of equipment deployed near but above the bottom for the measurement at midaudio frequencies of the acoustic compressional wave velocity methods are described which make use of either the refraction or the wide angle reflection techniques in ways which do not require station keeping by the survey vessel as when equipment is deployed on the bottom but still minimize the adverse effect of a long water column between the sensing equipment and the seabed some results from areas on the nova scotia shelf are presented ranging from about 3 0 km s in a nearshore sedimentary rock sequence through 1 6 km s on a shelf edge sandbank to 1 25 km s in a deep intrashelf basin

problems in applying preservation facies and sequence models to sinian neoproterozoic glacial sequences in australia and asia abstract the sinian 800 530 ma is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate eustasy and possibly tectonics understanding sinian stratigraphy may help interpret the co evolution of life and environment during the period though sinian successions are poorly dated and difficult to correlate preservation facies and sequence models based on mesozoic recent successions may be used to interpret them preservation models based on basin subsidence permit estimates of how thick and complete successions can be in different tectonic settings and how long it took for them to be deposited facies models for rift craton shelf slope and basin environments help in identification of these different tectonic settings sequence models facilitate interpretation of the interplay of tectonic subsidence eustasy rates of sedimentation and climate such models applied to the sinian successions of australia and asia indicate the following great shallowing upwards megacycles occur in sinian successions and each ends with regional glaciations deep rift basins formed around 850 800 ma preserve the oldest megacycles but these basins soon were filled with rapidly deposited glaciogenic sediments relatively complete sequences are found only in deep basins subject to repeated rifting cratons and shallow shelves preserve only the youngest megacycles older deposits have been removed by younger glacial advances deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types but these sequences are not developed to the same extent in all blocks problems in correlating the four great megacycles recognized elsewhere with those of asia will persist until the requisite facies analysis and u pb dating of interbedded volcanics are done

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

sedimentary record of late neoproterozoic rifting in the nw tarim basin china abstract upper neoproterozoic sediments which have been exhumed in the nw tarim basin china comprise a 600 900 m thick non marine to marine synrift succession that reveals an early phase of extension this study presents new data from two measured and interpreted stratigraphic sections near aksu and wushi the synrift succession unconformably overlies high grade metamorphic basement and consists of the non marine sugaitebulake fm and the marine qegebulake fm the sugaitebulake fm is characterised by a basal alluvial fan conglomerate that passes upwards into fluvial and lacustrine facies sandstones and siltstones the stratigraphic position of the basal unconformity and the thickness of the basal conglomerate is locally variable which reflects the formation of small isolated depocentres during the onset of extension in addition to a pre existing topography the sugaitebulake fm also contains a series of concordant extrusive basalts which record episodic volcanism and together with the fluvial and lacustrine facies indicate the development of the rift system and the growth interaction and linkage of extensional faults the conformable qegebulake fm is characterised by foreshore to intertidal facies dolomites that contain abundant stromatolites representing a basin wide marine transgression and the rift climax stage this synrift sequence is shown to correlate with a similar succession in nw australia and corresponds to at least partial separation of the tarim block from east gondwana during the latest neoproterozoic to early palaeozoic

precambrian geology of china preface abstract the north china south china and tarim cratons compose the tectonic framework of china in the last decade extensive structural metamorphic geochemical geochronological and geophysical investigations have been carried out on these cratonic blocks producing an abundant amount of new data and competing interpretations which have resulted in refinement of and in some cases significant reinterpretations of the precambrian crustal formation and evolution of china this forms a justification for organizing this special issue in which we invited representative experts working in the north china south china and tarim cratons to present overviews or detailed studies on the formation and tectonic evolution of major precambrian blocks in china twenty four contributions collected in this special issue provide current knowledge of the components and tectonic evolution of major precambrian blocks in china and discuss some key issues based on new data and interpretations which will provide important insights into understanding the precambrian geology of china

a possible middle paleozoic suture in the altyn tagh nw china preliminary 40ar 39ar thermochronologic results from two transects across the altyn tagh at range allow new interpretations for the genesis of the range and provide constraints for its subsequent displacement recent reports of middle to upper ordovician pillow lavas in the ne portion of the range as well as the first reports of high pressure metamorphic rocks within the range are interpreted as the remnants of a lower paleozoic ocean subducted within the at muscovite and biotite ages from schist gneiss and granodiorite document a significant metamorphic and intrusive event throughout the range at 435 20 ma this fabric is crosscut by undeformed postorogenic microgranite plutons dated at 383 7 ma by muscovite these data suggest that an ocean basin closed after the early silurian and prior to the middle devonian forming the herein named lapeiquan suture the lack of similary aged plutons to the north and the suggestion of a synchronous arc to the south implies that subduction was south dipping present geography if the altyn tagh contains a middle paleozoic suture lateral continuations should be identifiable possibilities to the west include the kudi suture in the western kunlun shan comparisons of lithologies and radiometric data reveal significant similarities between the kudi suture and the altyn tagh suggesting that the at originated as part of the same middle paleozoic orogenic belt reconstruction of the paleozoic structure of northern tibet is complicated by cenozoic deformation particularly along the altyn tagh fault although this could be used as a measure of the finite post devonian deformation a more thorough understanding of paleozoic geodynamics is a prerequisite to unraveling subsequent displacements

inter laboratory comparison of x ray fluorescence analyses of eruptive products of el chichon volcano chiapas mexico abstract an inter laboratory comparison has been made of x ray fluorescence analyses of 10 samples of lava and pumices from el chichon volcano chiapas mexico some determinations of major element constituents agree within analytical uncertainty whereas others exchibit significant bias analyses carried out at the michigan technological university mtu laboratory are systematically lower in mgo 26 48 fe total 5 18 cao 4 15 and higher in k 2 o 0 15 than analyses made at the u s geological survey usgs denver laboratory these differences are ascribed in part to a complex combination of calibration assumptionsand mineralogical and particle size effects inherent in the use of pressed rock powder pellets in the analytical procedure of the mtu laboratory other but as yet unknown differences in sample preparation and or analytical technique may also be important effects related to natural sample inhomogeneityare believed to be insignificant the inter laboratory differences in the analytical data complicated accurate assessment of whether el chichon magmas have changed composition during the past 300 000 a knowledge of such change is needed for understanding petrogenetic history and for such related studies as evaluation of volcanic hazards

cryptic climatic signatures and tectonic controls on cryogenian diamictites in the nw tarim craton china unravelling the origins of cryogenian diamictites is vital in determining their significance in a snowball earth context and their relationship to environmental processes here we present new sedimentological and geochemical data for the cryogenian sedimentary rocks from the tarim craton nw china the diamictite member of the east qiaoenbrak formation contains 1 granitic pebbles with high sphericity 2 well developed stratification and 3 bed scale grain size changes suggesting cyclicity also 4 ice rafted debris is conspicuously absent rather than deposits of direct glacial origin the succession is instead interpreted as a subaqueous fan complex the major and trace element ratios and ree patterns are consistent throughout 22 sampled intervals in the qiaoenbrak group suggesting a uniformly felsic source area probably from an island arc chemical index of alteration cia analysis indicates low cia values 49 5 54 3 average 51 6 throughout the qiaoenbrak group we suggest that both the qiaoenbrak group and yuermeinak formation were deposited during the marinoan glaciation in a long lasting cold and dry interval it is likely that the qiaoenbrak group was deposited in a fault controlled basin adjacent to an active continental margin although the material may ultimately have derived from a glacial source e g outwash

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

north atlantic sea floor spreading rates implications for the tertiary development of inversion structures of the norwegian greenland sea the tertiary development of the norwegian continental margin was dominated by the opening of the arctic north atlantic ocean the correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins three ocean domains the aegir reykjanes and mohns regions were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break up spreading rates generally slowed down from 2 cm a 1 after early eocene initiation of sea floor spreading to values around 0 5 cm a 1 in oligocene time an increase in spreading rates to around 1 cm a 1 coincided with the positioning of the iceland hotspot under the north atlantic mid ocean ridge at the same time the european plate changed its absolute plate motion from a north directed drift to a motion more towards the east the location of inversion structures in the voring and faeroes basin rather than in the more basin is related to differences in spreading rates the mohns and the reykjanes ridges produced more ocean floor than the aegir kolbeinsey ridges asymmetric ocean floor formation in the aegir ridge led to differential stress at the base of the lithosphere which probably explains the absence of inversion features in the more basin less mantle drag furthermore upper plate margins such as the voring basin and possibly the faeroe basin have a lower compressional strength than lower plate margins such as the more basin and therefore preferentially developed inversion structures along the transform boundaries separating the domains additional stress probably built up along extension of the transform zones into the extended continental crust this additional stress probably also assisted initiation of the inversion structures in the voring basin and the faeroes area the amplification of the inversion structures in the voring basin and the faeroes basin was subsequently caused by a variety of processes related to sedimentation and uplift erosion

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

late neoproterozoic paleomagnetic results from the sugetbrak formation of the aksu area tarim basin nw china and their implications to paleogeographic reconstructions and the snowball earth hypothesis abstract in order to better constrain the neoproterozoic paleogeographic reconstruction of continents and to improve the understanding of the snowball earth hypothesis paleomagnetic investigations were carried out in the aksu area of the northwestern tarim basin forty eight sites of samples were collected from the sugetbrak and chigebrak formations twenty four sites of sandstone and volcanic rock from the sugetbrak formation revealed stable characteristic remanent components chrm isolated between 500 and 680 c the computed magnetic directions from these components are consistent and significantly distinguished from those of younger ages both normal and reverse polarities have been observed and a positive fold test is revealed after bedding corrections at 95 confidence level anisotropy of magnetic susceptibility measurements shows primary sedimentary fabrics with weak anisotropy degrees a paleomagnetic pole is calculated at 19 1 n 149 7 e k 11 2 a95 9 3 with n 24 yielding a paleolatitude of 27 n for the sampling area the chemo stratigraphic correlation of this section with reference ones reveals an average age of 595 ma a new paleogeographic reconstruction has been attempted showing general low paleolatitude positions for the australia south china and tarim blocks at about 600 ma this observation provides new evidences for the snowball earth hypothesis