calcified microbial reefs in cambrian series 2 north china platform implications for the evolution of cambrian calcified microbes abstract this study focuses on the microbial reefs of the zhushadong formation cambrian series 2 in shandong province china in order to understand the evolution of calcified microbes in the north china platform during the cambrian series 2 and 3 the microbial reefs occur in a thin unit ca 3 m thick over an area of 1 km 2 they consist of three types of thrombolite based on their mesostructures rimmed grainstone patch and dendritic the thrombolites mainly occur in various coarse grained carbonate facies including crudely stratified oolitic grainstone stromatolitic grainstone and disorganized limestone conglomerate calcified microbes in the thrombolites include epiphyton kordephyton a tubiform microbe bija tarthinia renalcis amgaina and razumovskia the zhushadong thrombolites were formed within a grainstone shoal and experienced repeated burial and exposure the rimmed thrombolite and grainstone patch thrombolite experienced abundant input of carbonate grains forming grainstone patches in contrast the dendritic thrombolite formed solely by calcification of microbes that mainly include epiphyton tarthinia and the tubiform microbe the outer crusts of the rimmed thrombolite were formed by amgaina under high energy conditions the diverse calcified microbes of the zhushadong formation form the earliest assemblage of their type in the north china platform their descendants mostly epiphyton subsequently thrived forming a ca 180 m thick microbialite oolite dominated succession during the cambrian series 3 zhangxia formation although the reefs in the zhushadong formation are much smaller than those of the overlying zhangxia formation their calcified microbes are more diverse this most likely reflects changes in depositional environments e g abundant siliciclastic input and tidal effects vs those of a stable carbonate platform and or global changes within reef environments e g end cambrian series 2 extinction of archaeocyaths and calcified microbes a decrease in diversity of calcified microbes in the north china platform where archaeocyaths were absent may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence

an outline of the plate tectonics of china china is principally a part of the eurasian plate but the margins of the indian and philippine sea plates are involved in the himalayas and in the coastal range of taiwan respectively within the eurasian plate the cathaysian paleoplate is separated from the angaraian paleoplate by the junggar hegen suture which contains paleozoic ophiolites and rare blueschists the three microplates of the cathaysian paleoplate consist of precambrian cratons and or phanerozoic accretionary fold belts these coalesced precambrian cratons record at least six stages of intense orogeny before cratonization the paleozoic to cenozoic accretionary fold belts of china can be correlated with similar events now found in west pacific andean and atlantic type active continental margins ophiolites occupying many of these tectonic zones provide evidence for the age and igneous history of oceanic crust formed during the paleozoic to cenozoic the presence of blueschist in some of these chinese sutures reveals evidence of large scale subduction and tectonic exhumation during consolidation of the eurasian plate cenozoic collision of the eurasian and indian plates produced deformation and uplift of the himalayas strongly influencing the tectonics of western china in contrast mesozoic tertiary evolution of eastern china is typical basin range geology similar to that of the western united states which included development of deep sedimentary basins along with calc alkaline plutonic and volcanic activity associated with crustal thinning and high heat flow the complicated tectonic evolution of china is greatly illuminated by the presence of ophiolites and blueschists in proterozoic to tertiary convergent boundaries these petrotectonic assemblages provide evidence of an extremely mobile history of plate movement in china

lower permian paleomagnetism of the tarim block northwestern china we collected 262 samples at 39 sites in three thick lower permian sections along the northwestern margin of the tarim block the lower permian sections are basalts interbedded with limestones or sandstones or shallow marine limestones and sandstones alone all with biostratigraphic age control alternating field af and thermal demagnetization experiments remove small secondary components and reveal characteristic magnetizations reversed directions consistent with the kiaman reversed interval similar directions between basalts sandstones and limestones a positive fold test and apparent polar wander and polarity changes recorded in underlying devonian sediments all attest to the primary nature of the remanence two paleomagnetic poles one from the limestone section 65 1 n 162 5 e k 128 a95 4 1 n 11 sites and the other from the two basalt sections 54 5 n 172 3 e k 66 a95 4 0 n 21 sites differ primarily in colatitude 8 1 6 5 suggesting a difference in magnetization age permian paleomagnetic poles from tarim siberia north china and south china all differ significantly suggesting substantial post permian relative motions

the alashan terrane did not amalgamate with north china block by the late permian evidence from carboniferous and permian paleomagnetic results abstract rock magnetic and paleomagnetic studies have been carried out on the early carboniferous limestones and the late permian purple sandstones sampled in the eastern alashan terrane alt northwest of china two components were isolated from the early carboniferous limestone by thermal progressive demagnetisation a low unblocking temperature component ltc of recent origin a pre folding medium temperature component mtc the paleomagnetic pole is 13 1 n 11 0 e a 95 7 0 that is probably the result of the hydrothermal fluids from the qilian orgenic belt acquired during the late carboniferous early permian also two components were separated from the late permian purple sandstone by thermal progressive demagnetisation the ltc with the recent viscous remanent magnetisation and the higher temperature component htc revealed from three sections which has passed a regional fold test at the 95 probability level and reversal test suggesting a primary characteristic magnetisation the corresponding paleomagnetic pole is 27 2 n 18 8 e a 95 12 0 the apparent polar wander path including early carboniferous late carboniferous early permian late permian and early middle triassic poles of the alt is significantly different with those of the ncb comparison of the apwps between the alt and ncb shows a strong similarity if the apwp of hexi corridor alashan rotated counterclockwise around an euler pole at 44 n 84 e by 32 then the coeval apw path of the alt overlaps to that of the ncb this result indicates that the alt migrated to the ncb after the early middle triassic along a tectonic boundary located between helanshan mountain and zhuozishan mountain and finally amalgamated to the ncb before the early cretaceous

new silurian and devonian palaeomagnetic results from the hexi corridor terrane northwest china and their tectonic implications summary a total of 239 orientated drill core samples from 23 sites were collected for palaeomagnetic study from silurian and devonian red beds marlaceous sandstone and limestone rocks in the eastern part of the hexi corridor southwest ningxia north china the characteristic high temperature component resides in both haematite and magnetite it clusters around a northwesterly and shallow to moderate downward direction and its antipode after tilt correction the primary origin of this characteristic remanent magnetization chrm is ascertained by positive fold and reversal tests at the 95 per cent confidence level the corresponding palaeopoles at 339 0 e 60 1 n with a 95 11 2 silurian and 336 0 e 56 0 n with a 95 9 2 devonian imply that the north china block ncb had a low palaeolatitude of around 15 n in the northern hemisphere during the silurian devonian period comparison with the early middle ordovician palaeopole of the ncb suggests that the ncb moved rapidly northwards by 30 8 10 9 to cross the palaeo equator during the early middle ordovician to silurian in combination with the palaeobiogeographical data from ningxia our palaeomagnetic results suggest that the ncb was located close to australia during the late devonian

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

namurian uplift in australia and south america triggered the main gondwanan glaciation the permo carboniferous glacigenic sediment at the base of the gondwanan basins reflects the culmination1 2 of the glacial episode that started in australia and southern south america in the namurian after two local episodes in northern south america and adjacent africa in the visean and latest devonian3 fig 1 here we show that the start of each episode was associated with orogenic uplift of the bordering regions northern south america and adjacent africa in the latest devonian and visean and southern south america and eastern and central australia in the namurian all areas except southern south america in the namurian were at mid and not high latitudes thus we show that whereas the first order control of glaciation was the configuration of the continents and oceans during the closing of palaeo tethys orogenesis provided a second order control by the accumulation of ice about bordering uplifts during the three glacial episodes

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

widespread late mesozoic to recent remagnetization of paleozoic and lower triassic sedimentary rocks from south china kent d v zeng x zhang w y and opdyke n d 1987 widespread late mesozoic to recent remagnetization of paleozoic and lower triassic sedimentary rocks from south china in d v kent and m krs editors laurasian paleomagnetism and tectonics tectonophysics 139 133 143 paleomagnetic results are described from eighteen formations sampled in the nanjing jiangsu province and yichang hubei province areas of south china the marine platform carbonates and elastics range in age from early cambrian to early triassic progressive alternating field and or thermal demagnetization analyses reveal predominantly single component magnetizations sometimes of both normal and reverse polarity these magnetizations isolated in 457 of the 549 samples studied typically have directions in geographic coordinates coincident with late mesozoic to recent fields a secondary origin for the magnetizations is supported by negative fold tests in several units a lower carboniferous and possibly a lower triassic limestone may have escaped the pervasive remagnetization if the remanences isolated are indeed prefolding a large counterclockwise tectonic rotation of the nanjing area is implied by comparison with available carboniferous and triassic paleopoles reported from elsewhere in south china this rotation may have been associated with mesozoic left lateral motion on the tancheng lujiang fault zone

paleomagnetic constraints on the tectonic relationship between the alashan hexi corridor terrane and the north china block we report paleomagnetic results for early middle ordovician limestone rocks from the eastern part of the hexi corridor northwest china the characteristic high temperature component b isolated from 74 out of 114 samples exhibited dual polarity and passed the fold and reversal tests at the 95 confidence level the corresponding paleopole at 326 5 e 31 8 n with a95 9 5 is in agreement with the coeval poles from the stable north china block ncb it implies that the alashan hexi corridor ahc terrane was part of the ncb since the early middle ordovician which is consistent with geological evidence the silurian and devonian paleomagnetic results from the ahc terrane may therefore be used to discuss the middle paleozoic tectonic displacement and paleogeographic reconstruction of the ncb

silurian and devonian paleomagnetic poles from north china and implications for gondwana abstract we have obtained paleomagnetic results from silurian and devonian rocks on the western margin of the north china block ncb ages which previously were thought to be unrepresented throughout north china high unblocking temperature components of magnetization isolated from both the silurian and devonian strata pass fold and reversal tests suggesting that they are primary remanence the corresponding paleopoles at 228 4 e 26 2 n silurian and 228 7 e 34 2 n devonian imply low paleolatitude positions of north china during the middle paleozoic probably adjacent to east gondwana

speculations on the mesozoic plate tectonic evolution of eastern china several orogenic belts transecting eastern china are the sites of former convergent plate margins although there have been varying views on the collisional framework of individual continental blocks styles of convergence at these zones and the timing of respective collisions a tectonic study of eastern china mongolia and the southern soviet far east indicates the collision of the south china block with a combined north china northeast china fold zone block in the late triassic early jurassic their collective suturing to eurasia in the late jurassic early cretaceous followed by the sikhote alin japan block in the mid to late cretaceous the evidence is as follows 1 a linear belt of late triassic early cretaceous granites and granodiorites trends east from the qinlingshan through the dabieshan to the huaiyang massif ophiolites flysch subduction zone melange a paired metamorphic belt indicating north dipping subduction and marine strata of carboniferous to late triassic age from the qinlingshan define the suture between the north and south china blocks 2 a sinuous belt of ultramafics blueschists silicic to intermediate magmatism and west and north vergent folds and thrusts trend from the west margin of the ordos basin through central inner mongolia and along the east great khingan range to the amur river coupled with a mid jurassic early creataceous unconformity a suturing of eastern chinese blocks to eurasia along this zone is suggested 3 a fold and thrust belt with ultramafics flysch blueschists and subduction zone melange along the ussuri river in northeast china indicates the suturing of the sikhote alin japan block to eurasia along a west dipping subduction zone in the mid to late cretaceous similarly a tectonic study of southern china and southeast asia has revealed a complex regional mosaic of suture bounded terrains which nucleated about the eastern western and southern margins of the yangtze craton during the late triassic and early jurassic the evidence is as follows 4 a north south trending belt of ophiolites blueschists calc alkaline volcanics and subduction zone melange including granites granodiorites and strongly deformed marine strata all of late triassic age exposed in the longmenshan of sichuan merge with the kekexilishan ophiolite zone into the ailaoshan tengtiaohe ophiolite and blueschist belt in central yunnan along which the songban ganzi complex and the shan thai malaya block join the craton and 5 a southeastern prolongation of the ailaoshan tengtiaohe belt bifurcates into the southeast trending konvoi zone of northern vietnam and the north south trending pak lay luang prabang zone of laos and eastern thailand zones of ophiolite calc alkaline volcanics and strong late triassic deformation they separate the indosinia and shan thai malaya blocks from the craton respectively these findings differ significantly from previous interpretations of a late paleozoic consolidation of south eastern asia as well as disputing the existence of a true pangea

fish from the upper devonian of the shan thai terrane indicate proximity to east gondwana and south china terranes we report the first discovery of abundant devonian vertebrates from thailand an unusual form of coronodontid shark tooth with six equal sized cusps occurs only in the upper devonian of thailand shan thai terrane and south china and a new species of phoebodus is reported that occurs elsewhere only in south china and australia east gondwana terrane the occurrence of the chondrichthyan harpagodens thrinacodus ferox turner 1982 in the late devonian of thailand australia and south china predates the appearance of this species in euramerica early carboniferous these data suggest late devonian proximity of the shan thai south china and east gondwana terranes and is in accord with recent paleomagnetic data

late paleozoic polar wander path for the tarim platform and its tectonic significance abstract stepwise thermal and a f demagnetizations have been carried out on 518 samples 47 sites from the sishichang section across the akesu area in the tarim basin the results lead to a discussion on some problems concerning the tectonic history of the tarim platform and adjacent blocks during the late paleozoic from the late carboniferous until the early permian the tarim platform collided with the kazakhstan block siberian platform and russian platform no large displacements have occurred between these blocks since the mesozoic however the permian pole position of the tarim platform obviously differs from that for the sino korea platform the difference in paleolatitude between the tarim and sino korea platforms in the permian was about 10 their orientations were also different the paleomagnetic result is in contrast to the geologically accepted idea that the two platforms in question occur as a whole in the late paleozoic

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

paleomagnetic results from the triassic of the yangtze platform two widely separated localities have been sampled from triassic formations of the yangtze platforms the first is from the border area between sichuan and guizhou provinces where the early triassic yelang formation was sampled at 15 sites located on both flanks of an anticline characteristic high temperature components were isolated from nine sites the remainder were severely overprinted by a recent field two polarities are present one directed toward the northeast with shallow positive inclinations and the other to the southwest with almost horizontal inclinations the northeast group passes the fold test at the 95 level the second area of study was from the city of nanjing jiangsu province where 19 sites were drilled from three formations ranging in age from the lower to upper triassic the samples are severely overprinted with a component that is almost vertical and whose origin is unclear however in 23 of the samples a high temperature component was isolated directed to the northeast and positive and to the southwest and negative the fold test is indeterminate the pole positions from the two localities are sichuan

palaeomagnetism of neoproterozoic glacial rocks of the huabei shield the north china block in gondwana abstract a palaeomagnetic study is reported of reddened facies fengtai formation of neoproterozoic glacial rocks which underlie early cambrian rocks with disconformity in the huabei north china sino korean block the diamictite preferred age 620 600 ma carries a dual polarity remanence residing in hematite of the red matrix the mean direction derived from 62 samples is d i 205 9 32 4 95 3 9 yielding a pole position at 233 e 62 n tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction although clasts near the base of the formation are largely overprinted overlying early cambrian sediments of the houjiashan and yutaishan formations have a similar remanence also of dual polarity d i 205 5 32 1 95 3 9 32 samples the magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and or the overlying cambrian succession red shales from the liulaobei formation 890 840 ma near the base of the neoproterozoic succession in the huabei block yield a contrasting remanence of d i 59 4 75 3 95 7 1 19 samples equivalent to a pole position at 150 e 43 n pole positions from this study accord with lower middle cambrian poles from the australian and south china blocks with north china sited adjacent to northeastern australia in accordance with recent biogeographic and palaeogeographic models although the palaeolatitude derived from the fengtai diamictite 17 is cambrian in age and probably later than the glaciation the correlation with australia implies that glaciation in north china took place in low palaeolatitudes 20 and reinforces the view that late neoproterozoic glaciation at least at this perimeter of gondwana occurred at low latitudes neoproterozoic cambrian poles from north china south china and australia fail to conform to the popular rodinia reconstruction with the latter blocks adjacent to western north america and also fail to support the view that gondwana formed by the fusion of widely separated eastern and western segments during pan african orogeny instead they accord to poles from africa and south america in a precursor of gondwana protogondwana the subsequent palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike slip motion during pan african deformation following extinction of the afro arabian arc

paleomagnetism of late mesozoic granitic intrusions in hong kong implications for upper cretaceous reference pole of south china two late jurassic early cretaceous granite plutons in hong kong were sampled at 16 sites for paleomagnetic determination the two granites show differences in magnetic properties and mineralogy but similar natural remanent magnetization nrm directions multiple magnetizations and dual polarity have been detected in the samples isothermal remanent magnetism magnetic susceptibility and coercivity measurements reveal multidomain titanomagnetite as the principal magnetic carrier a rating of paleomagnetic results from individual sites was based on the number of specimens efficacy of the demagnetization procedures and within site variance of nrm directions the pole position from 12 high quality sample sites is 171 9 e 78 2 n a95 10 6 the dual polarity and cooling ages of the granites suggest a late cretaceous acquisition of the remanent magnetism based on four different studies the upper cretaceous reference pole of south china is located at 194 3 longitude 73 5 n latitude a95 10 4 paleomagnetic results from north china south china and eurasia show no large scale cenozoic relative movements among the three blocks but the small separation between the paleomagnetic poles of south china and eurasia probably implies a minor relative movement since the late cretaceous

the apparent polar wander path for the north china block since the jurassic summary a palaeomagnetic study was carried out on sedimentary rocks from datong shanxi province and on volcanic rocks from chifeng inner mongolia and zhangjiakou hebei province china all of these sites belong to the stable part of the north china block ncb after detailed demagnetization experiments reliable palaeomagnetic directions were obtained for middle jurassic late cretaceous and late tertiary these data define the apparent polar wander path for the ncb since the jurassic free from the possible effect of the movement along tanlu fault since the mesozoic comparisons with reliable palaeomagnetic poles obtained in siberia and south china indicate that the middle jurassic poles for north and south china are almost the same while the siberian pole is displaced from them to the west by about 18 13 on the other hand the cretaceous poles for the three areas are not significantly different from each other consequently it is suggested that the amalgamation of north china and south china was almost complete in the middle jurassic and that their accretion to siberia continued until the late cretaceous

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

palaeomagnetism palaeogeography and the multicomponent analysis of middle and upper cambrian rocks of the barrandian in the bohemian massif abstract with a view to the importance of the barrandian in the bohemian massif for global palaeogeographic reconstructions considerable attention has been devoted to palaeomagnetic stability tests and multicomponent analysis of rocks of the pre variscan formations using substantially improved laboratory techniques this study is devoted to samples of upper cambrian andesite and middle cambrian sediments alternating and thermal fields were used for magnetic cleaning a thermal demagnetizer fitted with a newly developed inductor of high sensitivity the rocoma rotating coil magnetometer with a sensitivity of 0 1 to 0 2 nt was used the whole newly built and fully tested mavacs system magnetic vacuum control system guaranteed a high degree of compensation of the magnetic field in a volume of 5 1 to a value not exceeding 2 nt as numerous laboratory experiments have shown the high magnetic vacuum was necessary to derive the palaeomagnetic directions of rocks of the older formations affected significantly by the variscan orogenic phase the position of the upper cambrian pole 28 6 s 131 2 e should be considered tentative because it was derived using samples from only three andesite outcrops two other andesite outcrops preserved carboniferous or permo carboniferous palaeomagnetic directions the position of the middle cambrian pole 11 3 s 154 0 e was determined using samples of sediments mainly greywackes and shales from ten outcrops some of which represent extensive rock cross sections by combining multicomponent analysis with fold tests the mean direction of the pre variscan magnetization could be derived the very low values of the moduli of the natural remanent magnetization of the middle cambrian sediments and the considerable amount of magnetically soft components required special care to be taken in the laboratory procedures the palaeozoogeographic zones of the barrandian are related to palaeogeographic latitudes of about 40 s derived from palaeomagnetic data cold water is characteristic for the bohemian fauna but arctic conditions can be ruled out due to the amount of primary red sediments mostly of continental origin

influence of paleo trade winds on facies patterns of the cambrian shanganning carbonate platform north china abstract the shanganning carbonate platform scp of the north china craton ncc was located in the trade winds belt during the cambrian which influenced its facies development in recent years numerous drillcores have been recovered from these units but the depositional environments and facies architecture of this large 2 0 105 km2 carbonate platform system have not yet been reported in detail in this study we integrate sedimentological and magnetic fabric data collected from outcrops and drillcores in order to document variation in microfacies and facies associations within the scp to infer paleowind directions and constrain the paleogeographic location of the scp and to evaluate eolian controls on platform development this analysis documented numerous carbonate microfacies belonging to six fabric types mudstone wackestone packstone grainstone framestone and dolostone along with rarer siliciclastic sediments these microfacies belong to facies associations representing tidal flat open platform platform margin slope and deep basinal depositional environments anisotropy of magnetic susceptibility ams measurements yielded mean paleocurrent directions for the early middle and late cambrian of 114 63 and 83 30 from paleo north respectively that were relatively consistent for 12 out of 13 platform margin sites all vectors being corrected for a post cambrian 150 clockwise rotation of the ncc these wind orientations are consistent with the scp shifting from south of the paleo equator in the early cambrian to north of it by the middle cambrian paleo trade winds influenced facies patterns across the platform producing a distinct windward leeward zonation the windward margin is characterized by well sorted oolitic grainstone as well as favositids coral and dasycladacean algal sponge framestone and the leeward margin by poorly sorted oolitic wackestone grainstone and fine bioclastic sediments the present study is innovative in 1 integrating microfacies and ams analyses in order to establish paleowind patterns and influences on an ancient carbonate platform 2 documenting windward leeward variation in quantifiable sediment properties across the scp and 3 constraining the paleo hemispheric location of the ncc during the cambrian

paleomagnetic results from some permian triassic rocks from southwestern china the ailao shan fault zone in west yunnan represents a suture boundary between south china and indosinian cratons preliminary paleomagnetic results from three sites in sichuan guichou and yunnan provinces reveal a 14 northward migration of south china craton between late permian and middle triassic the two cratons were separated by about 20 latitudes during the early triassic the tectonic setting of east asia during the triassic was probably analogous to the present indonesian region

en echelon volcanic elongate ridges connecting intraplate foundation chain volcanism to the pacific antarctic spreading center abstract the development of volcanic elongate ridges vers could be controlled in many cases by complex processes related to interaction between mantle plumes and spreading centers we report here measured ages for the foundation chain se pacific showing that volcanism along morphologically distinct vers can develop occasionally as rapidly formed continuous lines of coeval volcanism extending from a region of intraplate volcanism to the pacific antarctic spreading center but a significantly more dominant trend is for coeval yet structurally disconnected segments of foundation chain vers to develop in a series of en echelon ne sw elongate zones of coeval hotspot volcanism these elongate zones developed at intervals of approximately 1 myr while maintaining a basically steady state orientation and size as the pacific antarctic spreading center migrated continually closer to the foundation plume hotspot we infer from this new information that foundation chain ver development was controlled by an interplay between short lived local factors e g location of nearest spreading center segment lithospheric stress and long lived attributes of the foundation plume hotspot e g size orientation periodicity

palaeomagnetism and palaeogeography of mongolia in the cretaceous permian and carboniferous preliminary data abstract the paper reports on the results of palaeomagnetic stability tests of oriented samples of lithologically different types of cretaceous permian and carboniferous rocks from northern and southern mongolia the permian andesites and tuffites from three localities barun buren tel tin gol and chulut tsagan del have yielded interesting results groups of normally and reversely polarized andesite tuffs were found even within a single outcrop by means of thermal field demagnetization and multicomponent analysis of the magnetization with a view to the number of samples 177 suitable for analysis and the number of localities 9 the reported palaeomagnetic data should be considered as preliminary the interpreted results indicate a different palaeogeographic evolution of northern and southern mongolia relative to the siberian platform and north china in the permian and carboniferous on the other hand similar apparent polar wander paths for north china lin et al 1985 and the studied regions of northern and southern mongolia indicate similar palaeogeographic evolution in the permian to carboniferous

non hotspot formation of volcanic chains control of tectonic and flexural stresses on magma transport the south pacific in the vicinity of both the superswell and the east pacific rise is repleat with volcanic chains that unlike the emperor hawaiian chain defy the hypothesis of formation via the relative motion of plates and hotspots we propose two nearly identical models for the origin of near axis and superswell chains assuming that both regions are underlain by significant quantities of more or less uniformly distributed partial melt given an initial volcanic load or a local anomaly in the melt source region volcanic chains form by magmatic hydrofracture at local tensile maxima of flexural and membrane stresses fracture wall erosion by magma flow provides a feedback which results in discrete edifices within the chains the model predicts island chains aligned with a deviatorically tensile tectonic stress near the ridge the elastic lithosphere is thin and observations and theoretical considerations indicate a strong deviatorically tensile stress field perpendicular to the ridge axis under such conditions the model results in parallel lines of volcanoes perpendicular to the spreading ridge later interstitial volcanism within the individual chains reduces the average spacing and results in nearly continuous ridges on the thicker lithosphere of the superswell membrane stresses are negligible and the model produces chains of much more widely spaced volcanoes a more isotropic stress field may result in broader chain like patterns of volcanoes in both cases the chains represent self propagating disturbances the resulting age progressions are thus independent of plate velocity but depend only on the dynamics of volcano formation and evolution

intraplate versus ridge volcanism on the pacific antarctic ridge near 37 s 111 w exploration of the foundation volcanic chain 33 s 131 w 37 s 111 w revealed the existence of different magmatic provinces with relation to their geological settings 1 the pacific antarctic ridge par is made up of several en echelon segments where both glassy midocean ridge basalts morbs with low incompatible elements k2o 150 ppm and ce 48 ppm at about 306 1300 km from the par axis 4 the old pacific seamounts built on a crust older than 23 m y located west of longitude 124 w 1300 km from the par axis consist of t and emorb on the par axis extensive crystal fractionation 65 produced the silicic lavas on the basis of pacific plate reconstruction using a half spreading rate of about 50 mm yr and integrating the observed compositional changes with respect to the structural settings it is inferred that the last volcanic events giving rise to the fs took place at about 110 km from the par axis about 5 m y ago the oblique ridges built between 5 m y and 23 m y with morb volcanics comparable to those of the the oblique ridge par provinces could also have been formed by an interaction between the foundation seamount dredge site 28 hotspot magmatism and that of an ancient accreting ridge magmatism precursor of the par

the foundation seamounts tectonic setting of a newly discovered seamount chain in the south pacific abstract large areas of the south pacific remain unsurveyed because they are so remote a major volcanic chain the foundation seamounts lies between 33 s and 39 s and between 111 w and 125 w poorly defined on conventional bathymetric charts it becomes very apparent when combining an analysis of conventional geophysical data with the satellite altimeter data the length of the seamount chain is 1350 km and the volcanoes are clustered within a 180 km wide band trending approximately n70w which is the direction of absolute motion of the pacific plate over a fixed reference system some of these volcanoes are so shallow that they may have emerged above the sea surface at an earlier time no dates are known for the seamounts surrounding the foundation seamounts chain the tectonic setting is that of the pacific plate at the time when the strike of the spreading ridge changed from a northwesterly direction to the present northeasterly one this episode of spreading reorganization involved the existence of a short lived microplate west of the pacific antarctic ridge a captured segment of nazca plate accounts for the large discrepancies in the magnetic anomalies offsets along the agassiz fracture zone large propagators like the adventure and kurchatov troughs are described the newly discovered western segment of the chile fracture zone is the pacific plate match of the chile fracture zone on the nazca plate

effective elastic thickness of the lithosphere along the easter seamount chain bathymetry and gravity data collected during legs 5 6 and 7 of the 1993 gloria expedition and the recently released 2 min altimetry derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the easter seamount chain esc forward modeling admittance and coherence methods yield consistent restfits with the exception of the eastern and western ends of the esc the effective elastic thickness along the chain is 1 4 km the thin elastic thickness for the majority of the esc seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the nazca plate seafloor age the elastic thickness southeast of the nazca fracture zone is 6 km apparently because of the seafloor age discontinuity across the fracture zone the elastic thickness near the san felix island at the eastern end of the esc is even greater 11 km which is compatible with the estimated seafloor age at the time of loading a slight increase in the effective elastic thickness of the far western part of the esc suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center these findings combined with published geochemistry support a hotspot origin for the esc complicated by large scale plate boundary reorganizations and channeling of plume material to the east pacific rise

ridge hotspot interaction the pacific antarctic ridge and the foundation seamounts the study of different magmatic provinces between the resolution fracture zone 33 s 131 w and the pacific antarctic ridge par axis 37 s 111 w suggests that similar processes of interaction between hotspot and spreading axial magmatism occurred 20 25 ma and 0 5 ma ago there is evidence of this process from the changes in composition observed in the lavas erupted near 400 300 km between the present day par axis 37 s 111 w and the eastern tip of the foundation seamount fs hotspot near 36 20 s 114 w where the last alkali enriched volcanics k ti 0 30 zr y 6 and ce yb n 4 have erupted this transitional province between the par and the fs consists of volcanic cones built on several volcanic ridges 20 ma also built on volcanic ridges are identified west 1200 km from the par axis of a failed rift propagator frp forming the eastern boundary of the ancient selkirk microplate some of these seamounts made of alkali basalts were built during the initiation of the fs hotspot 20 23 ma ago the interaction and the influence thermal of mantle plume magmatism with the ancient spreading ridge of the farallon pacific plates was responsible for the eruptions of the t morbs and andesitic lavas this situation is comparable to that presently observed on the par axis where silicic lavas are also erupted in association with t morbs

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

age geochemistry and tectonic setting of buqingshan ophiolites north qinghai tibet plateau china abstract the buqingshan ophiolite complex is a sector of the a nyemaqen ophiolite belt in the east kunlun southern marginal suture zone in the northern part of the qinghai tibet plateau the wnw trending ophiolite complex consists of metaperidotite gabbro diabase pillow basalt massive basalt and pelagic sedimentary rocks including radiolarian chert we have determined that early paleozoic and early carboniferous early permian ophiolites are present in this ophiolite complex that were previously thought of permian middle triassic age a zircon u pb age of 467 2 0 9 ma for gabbro and a rb sr isochron age of 481 130 ma for diabase and gabbro have been obtained from the early paleozoic ophiolite slice and middle late ordovician acritarchs have been found in the melange matrix that envelopes the sliver we interpret the first two ages to represent the formation age of the ophiolite granodiorite tonalite plutons with a zircon u pb age of 402 24 ma intrude the slice early carboniferous early permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle most mafic rocks in the early paleozoic ophiolite are of n morb type with small amount of t morb and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination the basalts in the early carboniferous early permian ophiolite have similar characteristics to the mafic rocks and also show dupal anomaly and a mixed source of dmm and em ii the original formation environments of the two stage ophiolites are both mid ocean ridges these lines of evidence suggest that a mature early paleozoic kunlun qilian qinling ocean basin and a mature paleotethyan ocean basin once existed in the study area

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

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

the seismic architecture and geometry of grounding zone wedges formed at the marine margins of past ice sheets grounding zone wedges form where halts of decades to centuries during ice sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast flowing ice streams thirty grounding zone wedges were identified on two dimensional seismic reflection profiles across the continental shelves of ne and nw greenland grounding zone wedges close to the seafloor are probably of late weichselian age others are on paleoshelves buried beneath thick prograding glacial sediments several greenland shelf grounding zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water grounding zone wedges are asymmetric in the ice flow direction with steeper ice distal sides typical grounding zone wedges are approximately 5 20 km long and 50 100 m thick with a lateral width of several tens of kilometers forty grounding zone wedges from the greenland norwegian and antarctica margins show considerable variability about these values grounding zone wedge dimensions are controlled by sediment flux the duration of halts in ice retreat subice cavity shape and ice stream width low angle ice shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space preventing formation of high amplitude ridges grounding zone wedges are mainly transparent or chaotic on seismic profiles probably resulting from delivery of diamictic debris offlapping reflectors represent progradation into an ice roofed cavity reflector truncations of grounding zone wedge bases indicate erosion during initiation channels are present within some grounding zone wedges meltwater flow is under high pressure and v shaped incisions suggest high energy flow channels are not ubiquitous within greenland grounding zone wedges and may represent nonsteady meltwater release perhaps through lake drainage where ice sheet mass loss is dominated by meltwater runoff grounding zone wedges probably contain more sorted sediment than those from greenland simple models of grounding zone wedge architecture are presented grounding zone wedges indicate episodic rather than catastrophic ice sheet retreat and are a mechanism for ice shelf stabilization because wedge growth counteracts collapse induced by ice sheet thinning and sea level rise

paleomagnetic insights into the cambrian biogeographic conundrum did the north china craton link laurentia and east gondwana

late permian paleomagnetic pole from dikes of the tarim craton china we have obtained a late permian paleomagnetic pole from dikes in the tarim craton of china directions of magnetization from 21 mafic and ultramafic dikes at three localities pass a fold test at the 95 confidence level the new paleomagnetic pole lies at lat 66 n long 181 e n 21 k 61 and 95 3 9 on the basis of a comparison with paleomagnetic data from adjacent tectonic units we conclude that 1 the late permian paleolatitude of the tarim craton differs from that of the sino korean craton suggesting that the two cratons were distinct tectonic units in late permian time 2 there was relative motion between the tarim craton and the siberian craton after late permian time and 3 negligible relative motions have occured between different thrust sheets in the keping bachu area of the north western tarim craton

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

tertiary paleomagnetism of north and south china and a reappraisal of late mesozoic paleomagnetic data from eurasia implications for the cenozoic tectonic history of asia abstract to contribute to the understanding of the cenozoic tectonic history of china and eastern eurasia in general we collected paleomagnetic samples from tertiary rocks in both north and south china our miocene paleomagnetic data for northern china are from four widely separated basalt sections in the inner mongolia autonomous region and shandong province both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence and the mean normal and reversed directions are closely antiparallel to each other the fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto oxidation processes during primary cooling of the basalts the paleomagnetic data from these four areas also pass a regional generalized fold test the corresponding mean miocene paleomagnetic pole for northern china is longitude 261 6 e latitude 88 3 n a 95 6 4 our early tertiary paleomagnetic results for south china are from two sandstone sections around the city of nanning 22 7 n 108 3 e in guangxi zhuang autonomous region high temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests the paleomagnetic pole position determined from the five sites lies at longitude 228 0 e latitude 83 6 n a 95 8 5 the tertiary paleomagnetic poles for north and south china are statistically indistinguishable from each other and thus confirm that north and south china were fully assembled at least by the tertiary our new tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of north and south china were in the same position as today during the tertiary a critical evaluation and selection of paleomagnetic results from the late jurassic onward for siberia europe and north and south china further corroborate this hypothesis most previously existing jurassic and cretaceous paleomagnetic poles from europe siberia are of uncertain reliability and should not be used for tectonic reconstruction using poles derived from transferring reliable data from other continents we show that the previous paleomagnetic inferences of large eastward displacements between these chinese blocks during the cenozoic are no longer warranted the available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for asian tectonics additional mesozoic and cenozoic paleomagnetic data from siberia and north and south china are still needed to resolve the smaller second order displacements that appear to characterize the cenozoic tectonic history of this part of asia

preliminary early cretaceous paleomagnetic results from the gansu corridor china we report results from our paleomagnetic study of lower cretaceous redbeds from the gansu corridor northwestern china the characteristic remanent magnetization chrm resides in hematite often at very high unblocking temperatures 660 c the directions associated with this component exhibit only reversed polarities from locality a sunan area but the samples from locality b lanzhou area 480 km to the southeast show roughly antipodal normal and reversed polarities the combined sample directional data from both localities pass a fold test at the 99 confidence level the mean paleomagnetic pole is located at 48 7 n 199 7 e with a 95 4 1 which is discordant with poles of similar age elsewhere from neighboring regions in china although represented by relatively few samples n 21 this pole suggests that significant post cretaceous motion may have occurred between the gansu corridor and adjacent blocks relative to eurasia or north china the discordance corresponds to 28 1 5 2 or 35 6 9 7 clockwise rotation and 9 5 4 5 or 9 8 8 2 northward displacement respectively the rotations support but do not yet distinguish between several neotectonic models assumed to have acted over the past 15 40 m y the displacement is not predicted by any of these models if real it may have occurred early in the history of the india asia collision or even before

magnetostratigraphy of the permo triassic boundary section at shangsi guangyuan sichuan province china continous marine sedimentation characterizes many late permian to early triassic sections on the yangtze terrane in south china the permo triassic p tr boundary section at shangsi sichuan province consists of limestones intercalated with clays and mudstones which belong to the wuchiapingian and changxingian upper permian and the griesbachian and dienerian lower triassic stages the p tr boundary is formed by a clay horizon with an unusually high iridium concentration the intensity of natural remanent magnetization is very low with a mean of 0 15 ma m 1 about 40 of the samples contain secondary or unstable magnetization components only whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones the p tr boundary being situated very close to the transition from a reversed to a normal polarity zone the shangsi polarity sequence represents part of the illawarra interval of mixed polarity the exact beginning of which has still to be determined

permo triassic magnetostratigraphy in china northern tarim abstract the upper boundary of the permo carboniferous reversed polarity superchron has been identified in a palaeomagnetic study of the permo triassic of the northern part of the tarim basin china this boundary serves as an important marker horizon for correlation with other permo triassic sequences both in china and world wide a permo triassic palaeomagnetic pole for the tarim block is estimated to be at 71 8 n 187 6 e comparison with similar age poles from the adjacent blocks of china and asia suggests that the tarim was widely separated from the sino korean block in permo triassic times but was not yet sutured to kazakhstan

palaeomagnetic investigation on early middle triassic sediments of the north china block a new early triassic palaeopole and its tectonic implications summary palaeomagnetic and rock magnetic study is reported of early middle triassic terrestrial sediments from the jiaocheng and yushe areas shanxi province thermomagnetic analysis indicates that magnetite and haematite are the main magnetic carriers thermal demagnetization shows that the majority of early triassic specimens from the jiaocheng area contain an intermediate temperature component itc between 500 250 c and a high temperature component htc between 500 680 c the itc has a negative fold test and might be a cretaceous or recent overprint the htc contains both normal and reversed polarities with positive fold and c level reversal tests we interpret the htc as primary magnetization unfortunately we failed to isolate a meaningful mean direction from middle triassic rocks of the yushe area new early triassic palaeopole from the jiaocheng area confirms an 10 counter clockwise displacement of the taihang early triassic palaeopoles relative to their ordos counterparts this study thus suggests that the north china block ncb has generally behaved as rigid block since the early triassic but there might be an 9 counter clockwise rotation of the taihang terrane with respect to stable ncb in the late triassic the slight rotation of the taihang terrane does not bear significantly on what has been previously concluded concerning the mesozoic history of the ncb south china block scb collision however triassic palaeomagnetic data from stable ncb indicate that the east end of the ncb and scb did not undergo large convergence or significant opposite latitudinal displacement after the initial contact at the end of the late permian

ordovician paleomagnetism of eastern yunnan china three magnetic components have been isolated in ordovician formations of the yangtze paraplatform south china block two of these daqing a and hongshiya b components yield paleopoles that conform to the carboniferous to triassic segment of the apparent polar wander path for south china and are therefore interpreted as remagnetizations the third component declination inclination 301 degree 66 degree n 5 sites k 21 4 alpha sub 95 17 degree passes the fold test and is interpreted as primary late early ordovician the paleopole at 39 degree s 236 degree e and the paleolatitude of 48 degree s support an ordovician position of south china adjacent to gondwana

paleomagnetic constraints on the paleogeography of the east asian blocks during late paleozoic and early mesozoic times abstract although the axial and dominant geocentric character of the earth s magnetic field means that paleolongitude is indeterminate paleomagnetism is otherwise the only truly quantitative method available to the earth scientist for constraining paleogeography and reconstructing the kinematic evolution of continental blocks during the past half century numerous paleomagnetic results have provided substantial quantitative constraints on the origin and tectonic evolution of the major tectonic divisions now comprising eastern asian in this paper we first assess the available early paleozoic to early cretaceous paleomagnetic results from the south china north china and tarim blocks using internationally recognized reliability criteria we then construct a running mean through a 20 ma window by weighting the poles according to their q factors and fit a spherical spline with smoothing factor of 300 to derive apparent polar wander apw paths for these three major blocks during this early paleozoic to late mesozoic time interval together with late paleozoic to early mesozoic paleomagnetic poles from the qaidam qiangtang lhasa sibumasu indochina and some other smaller blocks terranes of east asia we produce a series of paleogeographic reconstructions for these major blocks and lesser terranes of east asia between mid ordovician and late jurassic times 460 160 ma which although based primarily upon paleomagnetic evidence aim to embrace the geological constraints finally we discuss the current evidence for closure times of the paleo asian mianlue and east paleo tethyan oceans which incorporate fundamental issues concerning the formation of the east asian continental collage and collision with the northern main body of the pangean supercontinent we use the collective evidence to argue that these major paleo oceans had closed by the late triassic and that the east asian sector of the supercontinent had united to become an integral part of pangea by that time 220 ma

new cambrian and ordovician paleomagnetic poles for the north china block and their paleogeographic implications multicomponent magnetization has been revealed in several cambrian and ordovician sedimentary units from localities in hebei and shandong provinces of the north china block ncb a well defined component with lower unblocking temperature is characterized by northerly declination and moderate to steep inclination with both polarities and postdates the late cretaceous folding the pole position derived from the overprinted directions is close to both the cretaceous pole for the ncb as well as the present north pole the overprint is not a simple thermal remanent magnetization but is probably a chemical remanent magnetization imposed at moderately elevated temperature during the major episode of regional deformation in the late mesozoic the higher unblocking temperature characteristic components pass reversal baked contact and fold tests and are internally consistent with rocks of different lithologies from widely distributed localities moreover the data exhibit an overall similarity in the magnetic polarity pattern in the stratigraphic sequence at sampled localities hence the high unblocking temperature characteristic component was probably acquired at or near the time of deposition the cambrian and ordovician paleopoles are somewhat different from the other coeval poles for the ncb which are based on much less data and do not pass as many reliability tests but all imply that the ncb was in the equatorial zone in the early paleozoic

the earliest phanerozoic carbonate hardground cambrian stage 5 series 3 implications to the paleoseawater chemistry and early adaptation of hardground fauna abstract carbonate hardgrounds are lithified seafloors formed by synsedimentary cementation of carbonate sediments which dominantly occur during the period of calcite seas the earliest typical phanerozoic hardground known until now was reported from the furongian of usa which was suggested to indicate onset of the early paleozoic calcite sea period in this study we report hardgrounds from the early and middle parts of the cambrian series 3 stage 5 and drumian of the north china platform which predate previously reported hardgrounds the hardground surfaces developed on oolitic grainstone oncolitic wackestone and microbialite thrombolite and dendrolite which sharply truncate the underlying deposits the radial fibrous calcite cements between the carbonate grains below the hardground surfaces indicate that the cements formed by early marine cementation epma analysis reveals that the fibrous cements typically consist of low mg calcite the hardgrounds are sometimes encrusted by microbialites and coated by hematite suggesting long exposure to the open seawater after formation of the surface in addition detailed review on the sedimentological studies of cambrian series 3 to furongian deposits throughout the world reveals that there may be several other hardgrounds during these times which could have been overlooked the abundant occurrence of hardgrounds in cambrian series 3 deposits suggests that the general paleoseawater chemistry was suitable to induce synsedimentary cementation of low mg calcite implying that seawater chemistry would have changed from the aragonite to calcite seas during the cambrian series 3 or even earlier period metazoan encrustors and macroborers possibly could not have adapted to the newly appeared substrate condition yet until the latest cambrian series 3

palaeomagnetism of red beds of the late devonian worange point formation se australia summary gently folded strata of the late devonian merrimbula group along the south coast of new south wales are similar to numerous deposits of late devonian to early carboniferous subaerial to shallow marine quartzose sandstone that are known as the lambie facies of se australia because the lambie sands overlap the early palaeozoic tectonic elements of the lachlan fold belt large displacement since the late devonian of any lachlan terranes with respect to interior australia is precluded we collected oriented core samples from 37 sites primarily in reddish quartzose litharenites of the worange point formation the remanent magnetization is carried by haematite incremental thermal demagnetization reveals a dominant well defined steep upward north component of magnetization that post dates the mid carboniferous folding the south pole position 146 4 e 68 6 s ag5 3 1 derived from the overprinted specimens is close to both the late carboniferous and mid cretaceous reference poles as well as the spin axis of today the overprint is attributed to both viscous partial thermoremanent and chemical remanent magnetization vptrm and crm its exclusively normal polarity is consistent with a mid cretaceous acquisition perhaps related to the rifting of the se australian margin a characteristic component of magnetization is isolated between 660 and 680 c in 30 per cent of the samples although a regional fold test of the overall result is inconclusive because the directions of characteristic magnetization are generally close to the axes of folding two sets of samples from opposite limbs of a local syncline pass a fold test at 90 per cent confidence indicating a pre folding origin of the remanence the presence of both polarities in thin horizons some of which are palaeosols and diagenetic haematite associated with incipient cleavage formed in an early stage of deformation before folding indicates that the acquisition of remanent magnetization occurred over a protracted period an analysis that combines great circle demagnetization paths and set points facilitates the incorporation of more data but gives a biassed result although normal and reverse polarity subsets are roughly antipodal residual post folding components cause the normal subset to fail a fold test a south pole position 19 7 e7 70 8 s ag5 7 1 is derived from the specimens with characteristic component of reverse polarity in which the isolation of a characteristic component is clear the pole position is close to that of a previous palaeomagnetic study from a widely separated occurrence of lambie facies sediments both results are applicable not only to the lachlan fold belt but also to the rest of australia and gondwana as a whole the position of the two poles derived from the lambie facies overlap assemblage in the lachlan fold belt supports the interpretation that the progression

early paleozoic paleomagnetic poles from the western part of the north china block and their implications abstract we collected 917 oriented drill core samples 124 sites from 22 units spanning the early cambrian to middle ordovician formations of the north china block ncb to construct an apparent polar wander path apwp of the early paleozoic for the ncb sampling areas are located in the helanshan tongchuan hancheng and yuncheng luliang regions along the margin of the ordos basin characteristic remanent magnetizations for five periods between the cambrian and ordovician are isolated from 281 samples by careful demagnetization directions cluster around northwesterly direction with upward shallow inclination and its antipode after tilt correction reliability of these five directions is ascertained through positive fold and reversal tests the five cambrian and ordovician paleopoles plot in the present atlantic ocean which is consistent with previously reported paleomagnetic poles a newly constructed apwp between the cambrian and ordovician indicates that the ncb was located in the southern hemisphere around 15 s during cambro ordovician time motion of the ncb between the early and middle cambrian comprised a counterclockwise rotation of 22 3 7 0 and a northward latitudinal displacement of 7 2 6 8 combining the paleobiogeographic and paleoclimatic data with the paleomagnetic implies that the ncb was probably located in east gondwana in the early cambrian and may have separated from gondwana in post early cambrian time the ncb is also inferred to have been located close to siberia and north america during the middle ordovician

palaeomagnetism of lower cambrian sediments from the olenek river section northern siberia palaeopoles and the problem of magnetic polarity in the early cambrian summary new palaeomagnetic data from the lower and middle cambrian sedimentary rocks of northern siberia are presented during stepwise thermal demagnetization the stable characteristic remanence chrm directions have been isolated for three cambrian formations both polarities have been observed and mean chrm directions for normal polarity are kessyusa formation lower cambrian d 145 i 4o n 12 ug5 12 8 pole position cd 38 s a 165 e erkeket formation lower cambrian stratigraphically highly d 152 i 47 n 23 ug5 6 8 pole position cd 45 s a 159 e yunkyulyabit yuryakh formation middle cambrian d 166 i 33 n 38 ctg5 4 6 pole position 0 36 s l 140 e these poles are in good agreement with the apparent polar wander path based on the bulk of existing cambrian palaeomagnetic data from the siberian platform in cambrian times the siberian platform probably occupied southerly latitudes stretching from about 35 to o and was oriented reversely with respect to its present position siberia moved northwards during the cambrian by about 10 of latitude this movement was accompanied by anticlockwise rotation of about 30 the magnetostratigraphic results show the predominance of reversed polarity in the early cambrian and an approximately equal occurrence of both polarities in the part of the middle cambrian studied these results are in good agreement with the palaeomagnetic polarity timescale for the cambrian of the siberian platform constructed previously by khramov et al 1987

migration rate of volcanism along the foundation chain se pacific abstract volcanism has migrated along the nw se trending foundation chain se pacific at a rate of 91 2 mm y for the past 21 million years my the foundation chain is at least 1900 km long and was erupted mostly in the interior of the pacific plate the linear migration of intraplate often geochemically enriched volcanism along the foundation chain can be explained by the theory of tectonic plate drift over a narrow stationary plume of hot mantle material upwelling from depth the present hotspot or at least its eastern extent generated by this foundation plume is now located on or close to the pacific antarctic spreading axis local spreading microplate boundaries have migrated systematically towards the foundation plume for at least the past 21 my a process that was however interrupted by the creation of the selkirk microplate the foundation chain has erupted therefore onto ocean crust ranging in age from 17 ma to 0 ma with distances between the foundation plume and local spreading microplate boundaries varying from 900 km to 0 km such interplay between the foundation plume and local seafloor spreading and microplate boundaries probably in combination with changing stresses in the lithosphere apparently induced major changes in the fabric and composition of the foundation chain e g the switch from individual seamounts to elongated en echelon ridges at 5 ma it can also explain why ages for a few foundation seamounts located at the edges of the selkirk microplate do not lie along the overall linear trend of migrating volcanism the linear velocity of the pacific plate over the foundation plume calculated on the basis of the migration rate of volcanism along the foundation chain is predicted by a number of rotation poles for the motion of the pacific plate in a fixed hotspot reference frame the fact that the measured migration rates of volcanism along the foundation and hawaiian chains are predicted by these rotation poles 1 confirms that the distribution of foundation chain ages records the absolute motion of the pacific plate and 2 supports the fundamental assumption of stationary mantle plumes in the case of hawaii versus foundation for at least the past 21 my the foundation chain was created by a weak plume primarily as relatively small probably rapidly erupted 1 my individual seamounts or seamount clusters at a rate of approximately one every my linear migration rates of volcanism are most likely defined unequivocally along such types of hotspot chain on the basis of age distance data it should also be easier to identify and characterize volcanism created as a result of plume interactions with local spreading microplate boundaries rotation poles for pacific plate drift show that the initial stages of volcanism along the southern austral chain 34 ma could have been created by the foundation plume suggesting that the path of pacific plate drift over the stationary foundation plume could be at least 3000 km long

the pacific antarctic ridge foundation hotspot interaction a case study of a ridge approaching a hotspot the foundation hotspot pacific antarctic ridge par system is the best documented case of a fast spreading ridge approaching a hotspot and interacting with it the morphology crustal structure inferred from gravity anomalies and the chemical composition of the lavas of the axial area of the par show evidence of the influence of the hotspot that is presently located roughly 35 km west of the spreading ridge axis along axis variation in the mantle bouguer anomaly is about 28 mgal corresponding to a crustal thickening of 1 5 km where the hotspot is nearer to the par anomalous ridge elevation is 650 m and the along axis width of the chemical anomaly is 200 km a comparison of these axial parameters with those derived for other ridge hotspot systems suggests that the amount of plume material reaching the ridge axis is smaller for the foundation par system this implies a weaker connection between the plume and the ridge cumulative effects of a fast spreading rate and of a fast ridge hotspot relative motion can be responsible for this weak plume ridge flow the flow from the hotspot may be less efficiently channelled towards the ridge axis when a fast ridge is rapidly moving towards a hotspot

magnetostratigraphic constraints on the gondwanan origin of north china cambrian ordovician boundary results summary a significant gap in the middle palaeozoic apparent polar wander apw path precludes polarity definition of the early palaeozoic palaeopole for north china this problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals a magnetostratigraphic study was carried out on upper cambrian to lower ordovician sediments near zhaogezhuang long 118 5 e lat 39 7 n north china after stepwise thermal or thermal and alternating field demagnetizations a characteristic magnetic component with normal and antipodal reversed directions was identified these data drawn from 49 samples yield a north palaeopole at long 294 6 e lat 32 9 n dp 3 0 dm 5 3 a concordant magnetic polarity pattern around the cambrian ordovician boundary and lowest ordovician obtained from different continents favours a southern hemisphere origin 17 formation site using the cambrian ordovician apw paths between north china and gondwana we suggest that the north china block ncb was part of gondwana during the cambro lowest ordovician and started breaking away from gondwana in the lower ordovician this finding is contrary to some palaeomagnetic models where the ncb was quite separate from gondwana in the late proterozoic and was attached to the pacific side of antarctica

preliminary phanerozoic polar wander paths for the north and south china blocks polar wander paths for the north and south china blocks suggest that 1 both were parts of gondwana in the palaeozoic 2 the north china block accreted to siberia in the late permian and 3 the south china block accreted to the north china block in the middle triassic to the early jurassic comparison of the polar wander path for the south china block with that for northern eurasia suggests that relative motion of over 4 000 km has occurred between them

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we first re examined the apparent polar wander curves for stable eurasia and africa since the triassic these curves were then combined together with curves of north and south america according to the kinematics of the atlantic ocean and a synthetic polar wander curve was given then most of the paleomagnetic results from the tethys mobile belt from the atlantic to the pamirs were analysed several groups of plates microplates and blocks can be seen first relatively stable regions like maghreb and sicily which have not moved much then we have a group formed by iberia sardinia italy and to a lesser extent corsica and the western and central alps for these blocks movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of africa but they are sometimes stronger to the east a major change takes place the north of the aegean sea and the ionian zone are clockwise rotated and these rotations are recent oligocene miocene for the first part pliocene to the present for the second part a major problem arises in turkey caucasus and iran paleomagnetic results indicate a position far to the south of eurasia and at the same time geological evidence is in favour of a position close to eurasia we discuss these discrepancies

new paleomagnetic results for ordovician and silurian rocks of the tarim block northwest china and their paleogeographic implications abstract the kinematics of the tarim block is poorly understood prior to the late paleozoic and this study aims to clarify motions during ordovician and silurian times we report the paleomagnetic record from 12 sites in ordovician marine sediments and 21 sites in silurian fine grained sandstones and mudstones from the aksu and kalpin areas of northwestern tarim together with our previously reported preliminary results the ordovician sediments isolate a sporadically preserved pre cenozoic characteristic remanence with dual polarity although a positive fold test can only define the remanence as pre cenozoic the corresponding paleomagnetic pole 33 7 s 185 0 e dp dm 2 7 4 0 is removed from post silurian paleopoles from tarim and combined with evidence from magnetic fabrics indicating a quasi primary origin a characteristic remanence isolated at 17 sites in silurian sediments from the aksu and kalpin areas includes 7 sites with suspected contamination from regional permian igneous activity the 10 remaining sites all from kalpin yield a positive fold test and a paleopole at 19 1 n 172 9 e a95 5 5 significantly removed from younger paleopoles from tarim indicating a late llandovery to wenlock remanence age the new mid late ordovician paleopole places the tarim block at 32 s probably in the vicinity of the arabian margin of gondwana rapid apw between 460 and 433 ma records large differential dextral motions of tarim together with other continental blocks along the paleo asian ocean margin which brought the china continental blocks into proximity with australia by mid silurian times in low 9 n northerly paleolatitudes

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

paleomagnetism of upper cretaceous rocks from south china paleomagnetic study of upper cretaceous redbeds from two areas in south china was undertaken in an effort to constrain better the history of tectonic movements in southeastern asia from the nanjing area 32 n l19 e high temperature characteristic directions which pass a fold test were obtained from ten sites 43 samples in the upper cretaceous puko and yanzijing formations the paleomagnetic pole position 76 3 n 172 6 e a95 10 3 agrees well with a late cretaceous reference pole for eurasia and thus confirms that this part of south china was fully sutured to eurasia by the end of the cretaceous with no paleomagnetically resolvable subsequent relative rotation in western sichuan 26 5 n 102 3 e high temperature characteristic directions of normal and reverse polarity were obtained from twelve samples 48 specimens in the upper cretaceous to lower paleocene xiaoba and leidashu formations a provisional paleomagnetic pole position 80 9 n 296 8 e a95 7 7 differs significantly from a late cretaceous eurasia reference pole in terms of apparent tectonic movement the difference in paleopole positions would indicate a 13 10 3 northward translation and 22 11 2 counterclockwise rotation of western sichuan relative to eurasia confirmation of this reconnaisance result which would suggest that western sichuan was pushed ahead of an impinging india is needed

north pacific plate convergence a quantitative record of the past 140 m y a quantitative record of the rate and direction of plate convergence around the north pacific ocean can be derived by combining information on northern hemisphere plate motion as related to hot spots with the details of north pacific plate evolution such a record permits reconstruction of both north south and east west components of convergence and shows that the north and east pacific margins were characterized by convergence rates in excess of 20 cm yr during the time period between about 80 and 45 ma the west pacific was characterized by north northwesterly convergence at 15 to 20 cm yr prior to 80 ma and by moderate westerly convergence since then

tectonic and paleogeographic controls on development of the early middle ordovician shanganning carbonate platform ordos basin north china lower middle ordovician platform carbonates are important oil and gas reservoir rocks in the ordos basin of the north china craton ncc in recent years numerous cores have been recovered from these units but the depositional environments and facies architecture of this large carbonate platform system have not yet been reported in detail this study integrates sedimentological and magnetic fabric data collected from outcrops and cores to document variation in microfacies and facies associations within the shanganning carbonate platform scp infer paleowind directions and the paleogeographic location of the scp and evaluate tectonic and paleogeographic controls on platform development this analysis documented 25 microfacies belonging to 7 microfacies fabric and sediment types carbonate mudstone wackestone packstone grainstone evaporite bindstone framestone siliciclastics and dolostone microfacies can be grouped into facies associations that represent tidal flat open platform restricted platform evaporite platform platform margin slope and deep basinal depositional environments these environments are mainly distributed zonally around the l shaped qingyang paleo uplift on the western and southern margins of the scp anisotropy of magnetic susceptibility measurements at five platform margin locales yielded paleocurrent directions of all approximately 318 348 51 81 42 72 80 110 and 54 84 from paleo north after correction for a 120 150 clockwise rotation of the ncc since the ordovician the depositional architecture of the scp depended on a combination of tectonic and paleogeographic wind influences local tectonic movements controlled the spatial distribution of facies associations around the qingyang paleo uplift which was characterized by open restricted and evaporite platform facies in the platform interior and mainly slope and basinal facies on the outer platform margin to the west and south of the qingyang paleo uplift vertical tectonic movements resulted in cyclic submergence and emergence of the platform over its approximately 20 m y long formation history paleo trade winds influenced aspects of facies zonation across the platform including growth of metazoan reefs on the windward platform margin and formation of oolitic grainstone and microbial reefs on the leeward platform margin the facies model for the scp developed in this study should provide a useful reference framework for future carbonate oil and gas exploration in lower middle ordovician units of the ordos basin furthermore this study makes innovative use of magnetic anisotropy data in combination with standard petrographic core and field data to discern facies patterns and controls on a paleo carbonate platform which is an integrated methodology that may serve as a model for future studies of other carbonate platforms

geochronology of galapagos seamounts lavas from seamounts of the central galapagos platform and the carnegie ridge increase in age with distance from the western edge of the platform consistent with a hotspot model for the generation of these features the areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 ma age distance relationships are consistent with a decrease in the velocity of the nazca plate relative to the galapagos hotspot during the past 9 m y most likely at 5 ma when there was a change in pacific hotspot motion alternatively the plate velocity remained constant and volcanism at a single seamount endured up to 7 m y lavas from seamounts along the wolf darwin lineament to the northwest of the galapagos platform are young 1 ma with the youngest measured ages closest to the platform

devonian paleomagnetism of yunnan province across the shan thai south china suture a paleomagnetic study of devonian sedimentary sequences from western and eastern yunnan china yields contrasting paleolatitudes for these two areas they indicate an equatorial position for eastern yunnan as part of the yangtze paraplatform or south china block and a paleolatitude of about 40 for western yunnan as part of the shan state or shan thai block which probably was part of the gondwana supercontinent the majority of samples is strongly overprinted by a present day field magnetization but thermal demagnetization has been successful in isolating ancient dual polarity characteristic directions of magnetization in rocks from eastern and western yunnan with results from the latter passing the fold test the paleopole from eastern yunnan is located at 8 9 n 10 4 e declination inclination 279 2 2 7 k 13 alpha 95 6 9 n 33 samples from 7 sites the paleopole from western yunnan is located at 66 5 n 133 8 e declination inclination 18 0 62 0 k 21 alpha 95 5 8 n 31 samples from 7 sites yielding a paleolatitude of 42 the two blocks studied are separated by the red yuan river and lancang mekong river fault zones one of which is inferred to be an ancient suture

paleomagnetic results from the silurian of the yangtze paraplatform gpdyke n d huang k xu g zhang w y and kent d v 1987 paleomagnetic results from the silurian of the yangtze paraplatform in d v kent and m km editors laurasian paleoma etism and tectonics teetonophysics 139 123 132 samples have been collected from sites from three localities located in sichuan and in yunnan provinces the samples were taken from limestones and red siltstones covering most of the silurian of the south china block the results from the limestone sites were disappointing and all the sites that yielded successful results were from red siltstones characteristic high temperature components were successfully isolated from fifteen of these sites the sites from xiushan county sichuan 28 48o n 108 99 0 e successfully pass the fold test at the 99 level indicating that the magnetization was acquired before folding in the mesozoic the direction obtained from this locality is directed toward the east with shallow positive inclinations d 85 6 o i 12 1 cuss 5 33 the results obtained from a single red bed locality from qijiang county sichuan 28 58 n 106 85 e yield a similar direction the results from the upper silurian of yunnan 25 5 n 103 73o e were obtained from only two sites and the isolated directions were towards the west with shallow inclinations this direction is opposed to the characteristic directions obtained from sichuan and are interpreted as a reversed silurian direction the paleomagnetic pole position determined from all sites given equal weight n 15 falls at 4 9 n lat 194 7 e long with ass 5 61 o

geomorphology and structural segmentation of the crest of the southern pacific antarctic east pacific rise geomorphology of the boundary between pacific and antarctic plates was mapped with a sea beam multibeam echosounder and a seamarc ii bathymetric side scan sonar from the southern end of juan fernandez microplate at 35 s to heezen transform at 56 s there are six spreading center systems separated by two large left stepping nontransform offsets at 36 5 s and 41 5 s two right stepping transform fault systems and a left stepping hybrid structure with equally long strike slip and nontransform offsets axial rift zones spreading axes are generally within 1 of normal to the relative motion predicted by current plate rotation models most axial rift zones crop out along the crests of typical east pacific rise epr axial ridges which commonly have flat rather than humped long profiles about 2 of this fast spreading 84 100 mm yr rise crest has an axial rift valley instead of an axial ridge the longest rift valley segment is midway between menard and vacquier transforms near 51 s where an anomalously deep rise crest may mark a zone of below average mantle upwelling nontransform offsets structurally similar to those on the tropical epr but mostly formed by differential asymmetric spreading subdivide the four longest spreading center systems on the northern transform free two thirds of the rise net rift propagation at migrating offsets has been into the faster pacific plate this causes right steps to migrate north left steps to migrate south and crust to be transferred to the east flank this pattern has prevailed for several million years judging from the oblique fracture zones pseudofaults mapped on the rise flanks with older marine data and geosat altimetry on this rise crest the difference in stress on two plates with very different velocities may control the direction of offset migration

the stationary cretaceous paleomagnetic pole of sichuan south china block upper jurassic to lower tertiary sites have been sampled from three localities separated by up to 150 km in the sichuan basin 30 n 103 e of the south china block each locality has at least one stability test but none alone gives a fully reliable result when taken together however identical directions are found with a global average d 12 7 i 34 2 k 42 9 95 3 3 n 44 sites and corresponding pole 73 9 n 234 4 e dp 2 2 dm 3 8 paleolatitude 18 8 there is no significant apparent polar wander apw with age which is consistent with an acquisition of magnetization during the period of the stable tip of the eurasian apw loop 130 70 ma as defined with poles from other continents transferred onto eurasia this age range is compatible with the available paleontological and magnetostratigraphic age constraints but is shorter than that suggested by the assigned formation ages the paleolatitude of sichuan is significantly lower 8 0 4 0 at 90 ma than that predicted by the reference apw path either because of northward shortening during the tertiary or because of errors in the apw path cretaceous poles from other chinese blocks are consistent with the sichuan pole but there is evidence that these blocks have suffered local rotations up to 10 during the tertiary probably as a result of the india eurasia collision

eastern asia in the cretaceous new paleomagnetic data from south korea and a new look at chinese and japanese data lower cretaceous continental sediments and volcanic rocks have been sampled in the gyeongsang basin south korea a detailed analysis of 160 specimens reveals two stable components of magnetization a postfolding low temperature lt component with normal polarity and a stable high temperature ht component with both normal and reversed polarities the blocking temperature of the ht component ranges from 540 c up to the neel temperature of hematite this component predates the folding of the series and yields a pole at 68 n 205 e a95 6 the prefolding magnetization indicates that south korea stood at a latitude nearly identical to its present day latitude in the lower cretaceous comparison of korean data with revised paleomagnetic data from north and south china and japan shows that all these blocks of eastern asia occupied the same positions in term of latitude and were probably parts of a single craton since at least the lower cretaceous the paleomagnetic directions of the sino korean block show large discrepancies with respect to the expected directions of the stable eurasian siberian block an interpretation involving significant n s motion 1000 km between either north and south china or between north china and siberia appears to contradict geological data a preferred interpretation is that the siberian data are in serious error and that the combined china korea lower cretaceous pole provides a valid estimate for the eurasian plate as a whole

fragmentation of asia in the permian asia is a composite continent formed by the accretion of crustal blocks1 3 a likely sequence of events is that siberia collided with a kazakhstan block which together then collided with europe along the urals in permo triassic times2 this was followed by the sucessive accretion of various blocks of south and east asia to siberia culminating in the collision of india with asia during the cenozoic palaeomagnetic data have already demonstrated that the kolyma and sikhote alin blocks of eastern asia collided with the main continent along the late mesozoic fold belts of verkhoyansk and sikhote alin4 furthermore results from the malay peninsula5 and japan6 suggest that south east asia and japan were situated near the equator in permian times and were therefore separated from the asian continent we report here some palaeomagnetic results from the permian of the sino korean and yangtze blocks of china that show they were also situated near the equator were not in their present relationship to one another and were widely separated from siberia

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

the foundation seamount chain a first survey and sampling the foundation seamounts form a 1400 km long chain on the pacific plate from 32 s 127 w to the pacific antarctic spreading axis at 38 s 111 w previously only known from sparse single beam echosoundings and satellite altimetry we present here the first multibeam bathymetric survey and geological sampling results we confirm that the submarine topography correlates with the altimetry and that the chain is volcanic rather than tectonic or microcontinental in origin the chain can be divided up morphologically and geochemically into three section 1 west of 125 w large flat topped volcanoes composed of incompatible element depleted lavas 1 of a near ridge origin with little or no plume influence 2 between 125 and 115 w true intraplate volcanoes with incompatible element enrichment 1 9 generated over the foundation plume 3 east of 115 w e w trending volcanic ridges with compositions 2 0 0 3 suggestive of interaction between the plume and the pacific antarctic spreading axis on the spreading axis moderate incompatible element enrichments 0 8 cf 0 3 outside the foundation area also suggest plume influence it appears that the activity of the foundation plume in the last few million years has 1 significantly waned and 2 become wholly channeled towards the spreading axis the foundation plume may be in the process of dying

observations of ridge hotspot interactions in the southern ocean the evolution of ridge hotspot systems is not well understood in this investigation satellite derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge hotspot interaction at four sparsely explored systems in the southern ocean these systems illustrate three different stages of ridge hotspot interaction in which a migrating spreading center approaches a hotspot pacific antarctic louisville passes over or is captured by the hotspot mid atlantic shona discovery and ultimately migrates away from the hotspot southeast indian kerguelen all of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading interestingly these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge hotspot systems a simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump the presence of previously uncharted obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered the azimuths of the aseismic ridges cannot be explained by plate kinematics alone they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure the features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets

australian palaeozoic palaeomagnetism and tectonics ii a revised apparent polar wander path and palaeogeography abstract new palaeomagnetic data from mid to late palaeozoic rocks in australia have enabled us to revise the palaeozoic apparent polar wander path apwp this modified australian apwp is supported by data from other parts of gondwanaland the palaeomagnetic poles indicate that during the early and mid palaeozoic australia underwent rapid rotation first clockwise during the mid ordovician to the early silurian then counterclockwise from the mid silurian until the end of the devonian while it remained at low to equatorial latitudes this was succeeded by a rapid southward movement during mid carboniferous times the implications of the palaeomagnetic data for the tectonic relationship between the lachlan fold belt lfb and cratonic australia are consistent with the tectonic evidence that the lfb has been in place since the mid devonian

occurrence of secondary magnetite within biodegraded oil samples of solid bitumen from the thornton quarry illinois and the cynthia quarry mississippi were found to be strongly magnetic and to have rock magnetic properties suggesting that the magnetizable grains present are magnetite studies of magnetic isolates revealed that magnetite is present primarily as spherical crystal aggregates that appear identical to magnetite spherules isolated from re magnetized paleozoic carbonate units from other localities organic geochemical analyses of the solid bitumen suggest an origin by microbial attack on what once was liquid crude oil the occurrence of secondary magnetite as inclusions within solid bitumen suggests a relationship between crude oil biodegradation and development of that mineral in our samples we infer that secondary magnetite in other geologic environments may be related to the presence of hydrocarbons the discovery of a natural association of secondary magnetite and hydrocarbons has important implications for paleomagnetism and for petroleum exploration

paleomagnetism of proterozoic and cambrian strata adrar de mauritanie cratonic west africa four paleomagnetic poles are reported from red beds of the adrar de mauritanie cratonic west africa thirteen localities were occupied and 224 samples were collected in seven units from a middle proterozoic to early paleozoic sedimentary section two poles are reported from middle and late proterozoic strata a third pole is reported from cambrian strata the magnetizations for these reside in stable high temperature components revealed by progressive thermal demagnetization analysis to 675 c the fourth pole is interpreted to reflect a permian overprinting in the cambrian strata two cambrian ordovician units sampled contained directions too scattered to allow the calculation of any pole the pole from middle proterozoic strata 998 32 ma rb sr date on argillite derived from four localities 52 samples single polarity plots at 48 7 n 206 9 e dp 1 1 dm 2 9 the pole from late proterozoic strata 775 52 ma rb sr date derived from two localities 15 samples mixed polarity plots at 34 0 n 246 9 e dp 4 6 dm 8 6 neither has correlatives in age in the paleomagnetic data set for cratonic west africa nor are there obvious correlatives with the paleomagnetic data set from the remainder of africa the pole that we consider to reflect a cambrian direction comes from three localities 23 samples mixed polarity plotting at 0 3 s 249 3 e dp 5 7 dm 11 5 its antiparallel pole corresponds with one group of cambrian poles reported from africa but differs markedly from another group of cambrian poles the late paleozoic overprint direction obtained from three units 50 samples from four localities single polarity provides a reversed polarity pole that plots at 47 2 s 42 6 e dp 1 5 dm 3 0 which is in good agreement with permian poles reported from elsewhere in africa in spite of the large number of poles already reported from africa additional data are needed before a reliable apparent polar wandering path can be constructed for the paleozoic and proterozoic of africa