paleomagnetism of cretaceous rocks in the jiaodong peninsula eastern china insight into block rotations and neotectonic deformation in eastern asia 1 although first order agreement of paleomagnetic directions between south china and siberia shows that terranes between them were sutured by cretaceous times deformation by ongoing tectonism has continued and been especially focused near the continental margin of eastern asia to help evaluate differential rotations and relate them to regional strike slip and extensional tectonic regimes we have conducted paleomagnetic study on cretaceous sediments and volcanic rocks from the jiaodong peninsula south of the bohai sea in eastern china bordering the tan lu fold zone tlfz paleomagnetic results from 73 sites in the early cretaceous laiyang and qingshan groups and late cretaceous wangshi group yield magnetizations of primary or synfolding mid cretaceous origins large differential block rotations of early cretaceous age are identified near the muping jimo fracture zone mjfz dissecting the peninsula although late cretaceous declinations are regionally systematic and show post late cretaceous counterclockwise rotation of 14 in the jiaolai basin relative to the reference north and south china blocks block boundaries are identified as the tlfz to the west and the mjfz in the southeast the eastern jiaodong peninsula is rotated clockwise relative to the north and south china blocks by a comparable amount 20 to the southern korean peninsula to the east and to the liaodong peninsula and yanji basin to the north although these blocks are unlikely to be tightly coupled their collective clockwise rotation mirrors counterclockwise rotations north of the japan sea and probably resulted from regional extension focused above uprise of asthenosphere beneath eastern asia during cretaceous times

inclination variation in the late jurassic to eocene red beds from southeast asia lithological to locality scale approach summary shallowpalaeomagneticinclinationshavebeenfrequentlyreportedfromtheredbedsofcentral and southeast asia to trace the origin of this phenomenon inclination variation in southeast asia is examined on lithological to locality scale lithological aspect of this variation has been studied using the early cretaceous bailong and cangxi formations of the bazhong area 32 1 n 106 7 e the northern sichuan basin samples from 36 sites consisting 18 pairs of successive sandstones and mudstones layers were collected for this purpose from a synclinal structure stepwise thermal demagnetization of most samples revealed the presence of stable characteristic remanent magnetization which is generally unblocked by 680 c positive fold and reversal tests suggest a primary origin for this component yielding the early cretaceous palaeomagnetic direction of declination inclination 20 9 26 5 ks 37 2 95 4 4 n 30 10 pairs of sandstone and mudstone layers show almost identical inclinations isandstones 23 3 3 7 and imudstones 24 7 2 4 but 27 shallower than that expected from the eurasian apparent polar wander paths apwps indicating that no lithological variation in inclination has occurred location wise variation in inclination shallowing is examined through palaeomagnetic data from late jurassic to eocene red beds distributed around southeast asia based on these investigations no inclination shallowing is observed in the eastern part of the south china block scb whereas large degree of shallowing is observed in the sichuan and xining ninhe basins variation in inclination shallowing from one sedimentary basin to another could probably be caused by changes in the depositional environment the eastern part of the scb where no inclination shallowing have been observed is characterized by basin and range type tectono geological setting in contrast the foreland basins are bounded to the north by east west striking high altitude orogenic belt swift developmentofthesebasinsasaresultofflexuralsubsidenceisthemostlikelyreasonforinclination shallowing

palaeomagnetic re investigation of early permian rift basalts from the baoshan block sw china constraints on the site of origin of the gondwana derived eastern cimmerian terranes a palaeomagnetic investigation was carried out on a series of rift basalts woniusi formation that accumulated on the baoshan block sw china in the early permian the aim being to provide quantitative palaeogeographical information on the eastern cimmerian terrane as it detached from eastern gondwana reliable data were obtained from four locations 28 individual cooling units and when combined with the findings of an earlier study three locations 19 sites indicate that breakup occurred at 41 9 s with errors 34 2 51 2 s using this information we fit baoshan against gondwana within a narrow longitudinal belt close to where northeast greater india and northwest australia were once in close proximity furthermore we suggest that sibumasu simao burma malyasia sumatra the largest of the eastern cimmerian blocks lay directly to the east offshore of australia qiangtang and lhasa almost certainly sat to the west off northern greater india se arabia but we are uncertain as to their exact configuration our findings are compared with several rather different models that have been published in recent years the new palaeomagnetic constraint highlights the flexibility authors currently have in reconstructing the region principally because of the overall lack of similar high quality data from the various blocks we explain how new data could resolve these ambiguities thereby offering more robust explanations for eastern gondwana s late palaeozoic development

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

late triassic paleolatitude of the qiangtang block implications for the closure of the paleo tethys ocean abstract to better constrain the late triassic paleolatitude of the qiangtang block and the closure of the paleo tethys ocean a combined paleomagnetic and zircon u pb geochronological study has been conducted on the upper triassic jiapila formation volcanic rocks on the northern edge of the qiangtang block of central tibet 34 1 n 92 4 e these rocks are dated to 204 213 ma progressive thermal or alternating field demagnetization successfully isolated stable characteristic remanent magnetizations chrm that pass both the fold and reversal tests consistent with a primary magnetization these are the first volcanic based paleomagnetic results from pre cretaceous rocks of the qiangtang block that appear to average secular variation well enough to yield a reliable paleolatitude estimate based on our new paleomagnetic data from upper triassic lavas we conclude that the late triassic pole of the qiangtang block was located at 64 0 n 174 7 e with a 95 6 6 n 29 we compile published paleomagnetic data from the qiangtang block to calculate a late triassic latitude for the qiangtang block at 31 7 3 0 n the central paleo tethys ocean basin was located between the north china ncb and tarim blocks to the north and the qiangtang block to the south during late paleozoic early mesozoic a comparison of published early triassic paleopole from the qiangtang block with the coeval paleopoles from the ncb and tarim indicates that the paleo tethys ocean could not have closed during the early triassic and that its width was approximately 32 38 latitude 3500 4200 km however the comparison of our new combined late triassic paleomagnetic result with the late triassic poles of the ncb and tarim as well as numerous geological observations indicates that the closure of the paleo tethys ocean at the longitude of the qiangtang block most likely occurred during the late triassic

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

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

distribution and height of methane bubble plumes on the cascadia margin characterized by acoustic imaging submersible investigations of the cascadia accretionary complex have identified localized venting of methane gas bubbles in association with gas hydrate occurrence acoustic profiles of these bubble plumes in the water column in the vicinity of hydrate ridge offshore oregon provide new constraints on the spatial distribution of these gas vents and the fate of the gas in the water column the gas vent sites remained active over the span of two years but varied dramatically on time scales of a few hours all plumes emanated from local topographic highs near the summit of ridge structures the acoustic images of the bubble plumes in the water column disappear at water depths between 500 to 460 m independent of the seafloor depth this coincides with the predicted depth of the gas hydrate stability boundary of 510 to 490 m suggesting that the presence of a hydrate skin on the bubble surface prevents them from rapid dissolution the upper limit of the acoustic bubble plumes at 460 m suggests that dissolution of the residual bubbles is relatively rapid above the hydrate stability zone

petrogenesis of the triassic bayan ulan alkaline granitic pluton in the north gobi rift of central mongolia implications for the evolution of early mesozoic granitoid magmatism in the central asian orogenic belt abstract the 221 ma bayan ulan bu granitic pluton occurs in the north gobi rift of central mongolia which was superimposed on the paleozoic central asian orogenic belt caob an area characterized by voluminous juvenile crust this large 1000 km 2 pluton is part of an early mesozoic granitoid province in the central part of the caob it consists mainly of alkali feldspar granites containing mesoperthite with interstitial quartz minor ca na and na amphiboles and rare fe rich biotite graphic texture is common the bu granites are fractionated leucocratic alkaline a type rocks that are enriched in incompatible elements particularly rb and th and depleted in ba sr eu and ti most have nd t values of 1 4 to 1 7 and t dm model ages 800 and 1200 ma and lie within the range of other a type granites of the caob granitoid province the granites are interpreted to have formed by the partial melting of underplated neoproterozoic mildly alkaline basalts in the lower crust followed by fractional crystallization dominated by feldspars the formation of the bu pluton and the coeval khentei batholith the largest late triassic intrusion in north central mongolia appears to be related either to a plume or to the closure of the mongol okhotsk ocean north gobi rift magmatism extends across the mongol okhotsk suture so a plume related origin is more consistent with available data this plume which would provide the heat source for magmatism in this area explains the eastward movement of large scale magmatism over time from the tarim traps 300 275 ma through khangai 270 240 ma to khentei 230 195 ma in central mongolia

vertical axis block rotations in southwestern china since the cretaceous new paleomagnetic results from hainan island a paleomagnetic study of early cretaceous redbeds from hainan island south china reveals a pre folding remanence from six sites with a mean direction of d 4 3 i 41 8 k 51 5 95 9 4 the results suggest that hainan island may have been rotated 10 counterclockwise about a vertical axis since the early cretaceous this rotation is likely to have occurred during the mid tertiary when large scale left lateral motion occurred along the red river fault as a result of the indentation of the indian plate into eurasia causing extrusion of the indochina block and opening of the south china sea a comparison of early cretaceous paleomagnetic results from south china indicates that local rotations are widespread in the western part of the south china block such rotations have to be taken into account when examining pre cenozoic tectonic kinematics of south china and constructing apparent polar wander path s for the region

morpho acoustic characterization of natural seepage features near the macondo wellhead ecogig site oc26 gulf of mexico abstract oc26 r v oceanus site 26 is one of three primary sites selected for study by the ecosystem impact of oil and gas inputs to the gulf ecogig consortium to determine the impacts of natural seepage versus large pulse inputs of hydrocarbons into the gulf of mexico gom and to chart the long term effects and mechanisms of ecosystem recovery from the deep water horizon oil spill in 2010 oc26 is located on the western slope of the gloria dome about five kilometers south east of the macondo wellhead in 2011 the site was visited by the noaa research vessel okeanos explorer during this cruise several natural oil gas plumes originating from the seabed were identified in order to establish precise locations for these naturally occurring sources of hydrocarbons and to inform biogeochemical and biological studies of the water column the seafloor and the shallow subseafloor in the wake of the deep water horizon spill we conducted several autonomous underwater vehicle auv surveys the surveys were designed to include swath bathymetry chirp sub bottom profiles and seafloor still photos to augment the okeanos explorer and the 2012 ecogig falkor multibeam bathymetric and water column data these auv high resolution datasets were collected to provide in depth geophysical analyses of seafloor morphology and to link where possible morphological features to the subsurface structure and plumbing system at this site where natural seepage possibly intersects hydrocarbon inputs from the spill the site exhibits many morphological seafloor features depressions elongated erosional structures dome shaped mud volcanoes mounds fault traces and round pockmarks seabed photos show the presence of gas hydrate outcrops and benthic communities in the vicinity of maximum seafloor backscatter intensities and many of the morphological features subsurface profiles show gas related anomalies blanking and acoustic wipeout where pockmarks mud volcanoes mounds and depressions are located in addition to the discovery of novel seafloor features these studies provide definitive links between seafloor seeps and particular morphologies and communities contributing to the fundamental understanding of seeps in the deep sea

evidence of episodic fluid gas and sediment venting on the northern gulf of mexico continental slope surficial sediments and sea floor features of the northern gulf of mexico continental slope record dramatic episodic venting and slower seepage of formation fluids brines crude oil gases and fine grained sediments faults activated by massive sediment input during periods of plio pleistocene falling to low sea levels accompanied by compensating salt movement provide avenues of vertical transport to the continental slope surface many of these faults cut thick sedimentary sequences that frequently contain geopressured zones which provide the driving force for fluid and gas expulsion flux rate and fluid characteristics are interpreted as important determinants of modern sea floor geology and biology qualitatively under conditions of rapid flux of sediment rich fluids mud volcanoes up to 1 km wide and 50 m high and extensive sheets of extruded mud result these sediments contain old and displaced microfaunas muds are frequently extruded with large volumes of crude oil and gas both biogenic and thermogenic in water depths greater than approximately 500 m rapid to moderate vertical flux of hydrocarbon gases and fluids results in the construction of relief features composed of gas hydrates ejected mud and authigenic carbonates areas with near surface hydrates frequently support complex chemosynthetic communities with associated hardgrounds containing calcareous remains of mussels clams and associated gastropods slow flux seeps promotes the formation of 13 c depleted carbonate hardgrounds stacked carbonate slabs and moundlike carbonate buildups frequently 20 m relief barite along with carbonate has been found in areas where rapid flux of sediment carrying fluids has waned in the recent past and mineral rich brines are now being ejected crests chimneys and cones of these minerals are found in such settings both recent numerical simulations of fluid release from geopressured zones and direct observations at the sea floor support a pattern of episodic venting short term episodes of venting are probably regulated by fault movement perhaps controlled by local salt adjustment destabilization of gas hydrates by oceanographic processes also causes short term episodic gas expulsion these events occur with interannual to intra annual frequencies longer gas hydrate stabilization destabilization cycles in continental margin settings are forced by hydrostatic loading and unloading at the frequency of sea level change radiometric dating suggests that other long term episodes of venting and major venting expulsion events are also probably modulated by sea level change time scales of thousands of years in response to low stand sedimentary loading results of this summary of vent seep related phenomena suggest that sediment input from the shelf margin associated with cyclic plio pleistocene falling to low sea level periods followed by major listic fault and salt adjustments to a new sediment load have activated venting seepage throughout this period and perhaps longer

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

further paleomagnetic results from the 155 ma tiaojishan formation yanshan belt north china and their implications for the tectonic evolution of the mongol okhotsk suture abstract a new paleomagnetic study on well dated 155 ma volcanic rocks of the tiaojishan formation fm in the northern margin of the north china block ncb has been carried out a total of 194 samples were collected from 26 sites in the yanshan belt areas of luanping beipiao and shouwangfen all samples were subjected to stepwise thermal demagnetization after removal of a recent geomagnetic field viscous component a stable high temperature component htc was isolated the inclinations of our new data are significantly steeper than those previously published from the tiaojishan fm in the chengde area pei et al 2011 tectonophysics 510 370 380 our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation psv but the psv can be averaged out by combining all the virtual geomagnetic poles vgps from the tiaojishan fm in the region the mean pole at 69 6 n 203 0 e a95 5 6 passes a reversal test and regional tilting test at 95 confidence and is thus considered as a primary paleomagnetic record this newly determined pole of the tiaojishan fm is consistent with available late jurassic poles from red beds in the southern part of the ncb but they are incompatible with coeval poles of siberia and the reference pole of eurasia indicating that convergence between siberia and the ncb had not yet ended by 155 ma our calculation shows a 1600 km latitudinal plate movement and crustal shortening between the siberia and ncb after 155 ma in addition no significant vertical axis rotation was found either between our sampled areas or between the yanshan belt and the major part of the ncb after 155 ma

late triassic paleomagnetic result from the baoshan terrane west yunnan of china implication for orientation of the east paleotethys suture zone and timing of the sibumasu indochina collision abstract in order to better understand the paleogeographic position of the baoshan terrane in the northernmost part of the sibumasu block during formation of the pangea supercontinent a paleomagnetic study has been conducted on late triassic basaltic lavas from the southern part of the baoshan terrane in the west yunnan region of southwest china following detailed rock magnetic investigations and progressive thermal demagnetization stable characteristic remanent magnetizations chrms were successfully isolated from late triassic niuhetang lava flows the chrms are of dual polarity and pass fold and reversal tests with magnetic carriers dominated by magnetite and subordinate oxidation induced hematite we thus interpret them as a primary remanence this new paleomagnetic result indicates that the baoshan terrane was located at low paleolatitudes of 15 n in the northern hemisphere during late triassic times together with available paleomagnetic data from the baoshan terrane and surrounding areas a wider paleomagnetic comparison supports the view that the east paleotethys ocean separated the sibumasu and indochina blocks and closed no later than late triassic times we argue that the currently approximately north to south directed changning menglian suture zone is very likely to have been oriented nearly east to west at the time of the sibumasu indochina collision

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

a preliminary upper triassic paleomagnetic pole for the khorat plateau thailand consequences for the accretion of indochina against eurasia we report new paleomagnetic results from the upper triassic norian huai hin lat formation at the base of the khorat group in central thailand five sites of conglomerate sandstone and limestone reveal a stable component of magnetization d 43 i 43 95 7 5 we argue that this component is of primary origin on the basis of a positive fold test observation of both normal and reversed directions and a reported magnetostratigraphy from middle triassic to cretaceous the new triassic pole yields together with published younger poles a mesozoic apparent polar wander path for indochina this unit is found to have remained at tropical northern latitudes since the upper triassic comparison with eurasian poles reveals 1650 850 km of sw ne convergence and 15 10 of counterclockwise rotation between 205 and 160 ma two mechanisms are discussed to account for this motion the first one implies the closure of an oceanic basin to the north of the south china block the second one involves post welding intracontinental deformation

new mesozoic paleomagnetic results from the northeastern sichuan basin and their implication abstract this paper presents the results of a paleomagnetic study undertaken on samples from lower triassic and lower cretaceous sediments within the northeastern sichuan basin china in an area close to the daba mountains stepwise thermal demagnetization was used to isolate the characteristic higher temperature component htc for these samples these htc t1 d 53 7 i 21 6 95 6 5 k1 d 28 9 i 25 7 95 6 6 passed both fold and reversal tests and indicate that these sediments have undergone sequential clockwise rotations relative to the sichuan basin these detected tectonic rotations were probably caused by the obstruction of the hannan massif during mesozoic thrusting within the southern daba mountains the northeastern sichuan basin and the central yangtze fold belt formed part of a continuous belt of basins the northern yangtze basin and acted as a coherent tectonic domain during the early middle jurassic the daba mountains and the central yangtze fold belt subsequently underwent rapid uplift and cooling around the late jurassic and early cretaceous this suggests that tectonic rotations within both the central yangtze fold belt and the daba mountains were very probably caused by intra continental deformation over a long period of time rather than a single tectonic event

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

jurassic paleomagnetic constraints on the collision of the north and south china blocks this paper reports a paleomagnetic study of jurassic rocks from the ordos basin in the north china block ncb a recent remagnetization and a high temperature and or high coercivity component with dual polarities have been isolated the middle jurassic pole 74 n 233 e a95 5 is roughly consistent with previous results and the lower jurassic pole 82 n 286 e a95 7 is located between the late triassic and middle jurassic poles these results combined with paleomagnetic results from adjacent blocks suggest that 1 the pole for the ncb suffered an angular displacement of 47 8 between the late triassic and middle jurassic 2 the scb underwent final accretion to the ncb in the middle jurassic and 3 accretion of the ncb to siberia was not complete until the late jurassic and possibly even until the early cretaceous

new cretaceous paleomagnetic poles from the tarim basin northwestern china most paleomagnetic studies of the tarim have been focused on the paleozoic with little data published for mesozoic rocks however the ancient configuration of this part of asia has been affected by the indian eurasian collision during the second french chinese karakorum expedition from kashkar to kunjerab in the autumn of 1990 we sampled about 500 cores from 53 sites of upper and lower cretaceous red sandstones from three areas near uytak yingjisha and wuqia 38 5 39 5 n 75 76 5 8 isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components ltc 600 c in most samples the mean direction of ltcs is aligned with that of the present earth field normal and reversed polarities have been found for the htcs of each age group htcs show positive fold tests for at least one sub study where the attitudes of bedding were sufficiently different to allow the test to be performed more important the four sub studies from yingjisha and wuqia give fully compatible results results from the uytak area show a wide dispersion in declinations as could be expected from the rather severe deformation and proximity to major thrust and strike slip faults data obtained in the kuche area from northern tarim by li and co workers and in southern junggar by chen and co workers are statistically identical to the new data from yingjisha and wuqia data from paicheng in northern tarim appear to have been subject to local rotation excluding those sites which appear from paleomagnetic study to have been significantly rotated we calculated a mean cretaceous pole for the tarim basin at 69 1 n 223 88 ass 3 5 n 5 this result indicates that the total post cretaceous rotation of tarim relative to junggar and to eurasia has been negligible 2 6 6 1 and 0 4 4 6 respectively continental shortening and rotation between tarim and junggar within the tien shan are not resolved by paleomagnetism although the values obtained 400 t 600 km 3 6 are compatible with tectonic models of the range a major discrepancy warranting further geological investigation is that between the tarim and north china block poles which would imply some 1400 1060 km of north south relative displacement

paleomagnetism of jurassic rocks from southwestern sichuan and the timing of the closure of the qinling suture a paleomagnetic study has been carried out on the jurassic redbeds from southwestern sichuan province on the south china block thermal demagnetization has isolated a high temperature component from 20 of 24 sites from the upper jurassic which passes the fold and reversal tests the tilt corrected mean direction computed using principal component analysis in combination with great circle intersection techniques is d i 17 9 45 6 with 95 3 7 corresponding to a paleopole at 74 2 n 185 7 e with a95 3 8 most of the mid lower jurassic samples studied are unusually weak in magnetization and failed to give stable directions at high temperatures however a tilt corrected mean direction of d i 30 3 44 6 95 13 6 was determined utilizing the techniques given above from 8 sites of the lower jurassic which yields a paleopole at 63 8 n 188 2 e with a95 16 1 this mean direction marginally passes the fold and reversal tests a comparison with the coeval poles for the north china block and eurasia indicates that during the early jurassic these plates had not coalesced together but that they had sutured together by the late jurassic minor but noticeable differences still exist between the late jurassic poles for these plates probably due to uncertainties associated with the age of magnetization and the apparent polar wander path for eurasia

new late jurassic palaeomagnetic data from the northern sichuan basin implications for the deformation of the yangtze craton summary upper jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the penglaizhen formation this formation is in the north of bazhong county 31 8 n 106 7 e in the sichuan basin which is located in the northern part of the yangtze craton thermal demagnetization isolated a high temperature magnetic component with a maximum unblocking temperature of about 690 c from 45 layers the primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities as well as positive fold and reversal tests at the 95 per cent confidence level the tilt corrected mean direction of 36 layers is d 20 0 i 28 8 with a 95 5 8 a late jurassic palaeomagentic pole at 64 7 n 236 0 e with a 95 7 0 is calculated from the palaeomagnetic directions of 11 localities this pole position agrees with the two other late jurassic poles from the northern part of the yangtze craton a characteristic late jurassic pole is calculated from the three poles 68 6 n 236 0 e with a 95 8 0 for the northern part of the yangtze craton this pole position is significantly diverent from that for the southern part of the yangtze craton this suggests that the southern part of the yangtze craton was subjected to southward extrusion by 1700 1000 km with respect to the northern part intracraton deformation occurred within the yangtze craton

regional deformational features of the south china block inferred from middle triassic palaeomagnetic data summary middle triassic red purple sandstones were sampled at four localities in the northeastern part of the south china block scb to detect regional deformational features most of the samples gave a characteristic remanent magnetization with unblocking temperatures up to 680 c the primary nature of this magnetization was ascertained by a positive fold test and a reversal test the tilt corrected mean direction from the yueshan area 30 6 n 116 8 e which is located at a distance of 25 km from the eastern part of the tan lu fault shows a counter clockwise deflection of 67 3 with respect to north however mean directions from the nanzhang 31 5 n 111 7 e tongling 30 9 n 117 9 e and nanjing 32 0 n 118 9 e areas which are located between 75 and 90 km from the northeastern boundary of the scb indicate a northeasterly deflection in declination such as d 38 2 i 25 1 95 6 4 d 32 1 i 25 2 95 9 7 and d 34 9 i 27 8 95 5 4 corresponding to palaeopoles of 50 5 n 220 5 e dp dm 3 7 6 9 55 6 n 231 8 e dp dm 5 6 10 4 and 53 9 n 229 1 e dp dm 3 2 5 9 respectively comparison with the previously reported permo triassic palaeomagnetic results from the scb suggests that very consistent palaeopoles are observed within a stable belt up to 1800 km along the northern margin of the scb covering the changxing nanjing tongling nanzhang and emeishan areas this type of behaviour implies that an area located between 70 and 225 km from northern margin of the scb forms a stable platform where very little relative rotation has been detected however the inner region of this block especially the mesozoic fold thrust belt experienced a clockwise rotation of 11 5 5 0 to 30 3 9 2 with respect to the stable outer region later than the middle triassic areas close to the tan lu fault were subjected to post triassic counter clockwise rotation by 90 with respect to the stable tectonic belt we thus conclude that the northern stable belt of the scb suffered no oroclinal bending during its suturing with the north china block

middle triassic paleomagnetic results from central hubei province china and their tectonic implications twenty one sites consisting of 4 to 7 samples per site were drilled from the redbeds of the middle triassic badong formation from central hubei province china these samples were subjected to progressive thermal demagnetization which revealed two components of magnetization the low temperature a component was probably acquired at a late stage of folding and is similar in direction to the late mesozoic overprint widely observed in the eastern yangtze block the high temperature b component is prefolding with dual polarity unlike the prefolding paleomagnetic directions observed in eastern sichuan western hubei and northwestern hunan the b component resolved from central hubei indicates no significant rotation with respect to other parts of the yangtze block since one of the two sampled sections is in the strongly deformed foreland fold thrust belt of the qinling fold belt this would seem to negate the possibility that the differential rotations observed in the border area between sichuan hubei hunan and guizhou are associated with the collision of the yangtze block with the north china block

a paleomagnetic study from the late jurassic volcanics 155 ma north china implications for the width of mongol okhotsk ocean abstract a new paleomagnetic study has been carried out on the tiaojishan formation effusives in the chengde area north china block ncb the age of the tiaojishan formation volcanics was recently dated from 153 ma to 161 ma by u pb and 40 ar 39 ar a total of 154 samples from 16 sites within the tiaojishan formation were collected detailed stepwise thermal demagnetization allows us to isolate two components from 77 oriented drill core samples a low temperature component identified in most samples falls close to the present local earth field direction characteristic remanent directions obtained from high temperature pass reversal test yield a pole position at 240 3 e 59 9 n with 95 6 8 combined with coeval paleopoles obtained from the siberia block sib these results show a significant paleolatitude displacement between the ncb and sib which indicates that the mongol okhotsk ocean was still open with 3000 km width in its eastern part at ca 155 ma

paleomagnetism of middle triassic redbeds from hubei and northwestern hunan provinces south china abstract paleomagnetic investigations have been conducted on redbeds from the middle triassic badong formation at badong hubei and sangzhi hunan as well as from the coeval puxi formation from puxi southeastern hubei province south china the characteristic remanent magnetization chrm determined from badong and sangzhi passes the fold test with dual polarity the tilt corrected chrm direction from puxi is congruent with those from badong and sangzhi but is based on a smaller number of samples collected from a monocline these chrm directions are believed to be primary the data indicates that badong has been rotated clockwise by 13 6 8 5 relative to sangzhi which in turn has possibly been rotated in the same sense with respect to other parts of the yangtze block yb except eastern sichuan based on the bend of the sampled fold axis and the lower triassic paleomagnetic data published for the yb the lower and middle triassic paleomagnetic data together appear to indicate that eastern sichuan and the border area between sichuan guizhou hubei and hunan provinces have been affected by differential rotations probably due to oroclinal bending the paleomagnetic data obtained from this study further constrain the timing of the final suturing of the yb with the north china block ncb to be post middle triassic

new paleomagnetic data from the huanan block china and cretaceous tectonics in eastern china abstract a total of 72 grey green fine grained sandstone and siltstone core samples from 10 sites were collected from the early cretaceous jieshan and hekou formations at yongan in the huanan block of southeastern china fujian province after removal of an erratic soft magnetic component in fields of 10 20 mt stable magnetizations were retrieved from both rock units which are carried mainly by single domain pseudo single domain sd psd magnetite or titanomagnetite as suggested by isothermal remanent magnetization coercivity measurements and microscopic observations the hekou formation yields a tilt corrected pole at 66 9 n 221 4 e n 5 sites k 203 3 a 95 5 4 a fold test on this result suggests that the magnetization was acquired before tilting which occurred shortly after deposition between early and late cretaceous this magnetization is related to a primary sedimentary fabric as indicated by the low field anisotropy of magnetic susceptibility ams measurements the positive fold test and occurrence of a primary sedimentary fabric suggest a depositional origin for the stable magnetization the jieshan formation gives a pole at 70 8 n 228 7 e n 5 sites k 47 7 a 95 11 2 in geographic coordinates which is statistically indistinguishable from the tilted pole for the hekou formation the associated stable magnetization corresponds to a secondary fabric these features indicate that the jieshan formation has been remagnetized during cretaceous time the predominance of sd psd magnetite or titanomagnetite and igneous activities in eastern fujian during the cretaceous suggest a viscous thermal remanent magnetization origin for the secondary magnetization the early cretaceous pole from the hekou formation is similar to those for the yangzi block suggesting that the huanan block has been accreted to the yangzi block at least since early cretaceous time a high consistency of cretaceous poles from the sino korean yangzi and huanan blocks as well as hainan island indicates that eastern china has been a coherent tectonic unit since cretaceous time although there might have been a small amount of northward displacement of the sampling area with respect to the yangzi block during the early late cretaceous period as a result of the overthrust of the huanan block onto the yangzi carbonate platform comparison of cretaceous poles for eastern china with the eurasian apparent polar wander path apwp suggests that no paleomagnetically detectable relative motion has taken place since the cretaceous the cretaceous poles of eastern china however display a paleomagnetically significant westward motion about 9 in angular distance which is not observed in the eurasian apwp this difference may be indicative of a younger age corresponding to the cretaceous tertiary boundary for all late cretaceous poles from eastern china or more probably may imply a minor error in the definition of the eurasian apwp

paleomagnetism of cretaceous to lower tertiary rocks from southwestern sichuan a revisit abstract a re study has been carried out on the cretaceous to lower tertiary rocks from southwestern sichuan province on the south china block the characteristics remanent magnetization chrm resolved from the paleocene eocene leidashu formation is of dual polarity and very shallow inclination the anomalouslylow paleolatitude 6 9 6 3 n indicated by the inclination is not supported by geological evidence from this region and is probably due to inclination error related to depositional and or post depositional processes a positive fold test as well as the prevalence of normal polarity suggests that the chrm of the mid upper cretaceous xiaoba formation might be acquired during the cretaceous long normal superchron clns the tilt corrected mean direction of the xiaoba formation d i 8 1 38 8 95 6 6 n 18 sites is not significantly different from a preliminary result d i 357 6 31 6 95 8 6 n 12 samples we reported earlier for the collective xiaoba and leidashu formations the resultant paleopole 81 5 n 220 9 e a95 7 1 is coincident with other cretaceous poles for the chinese blocks comparison with the eurasian apparent polar wander path apwp indicates neither significant rotation nor latitudinal translation of the study area relative to eurasia the newly obtained mean direction for the xiaoba formation is believed to be more reliable than our earlier result since it is based on a larger sample size and is apparently free from effects of the shallow inclination observed in the leidashu formation and should supercede our previous preliminary result the paleopole derived from sevenentes of the lower cretaceous feitianshan formation 69 0 n 204 6 e a95 4 3 deviates from the track from the late jurassic to late cretaceous poles the deviation might be reduced by further sampling

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

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

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

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

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

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

biostratigraphy and palaeobiogeography of lower permian lower kungurian conodonts from the tak fa formation saraburi limestone thailand abstract lower permian lower kungurian conodonts are reported from the indochina block of southeast asia the fauna from the tak fa formation of the saraburi limestone group exposed in limestone hills nnw of khok samrong thailand includes sweetognathus subsymmetricus wang ritter and clark early forms and pseudosweetognathus costatus wang ritter and clark the co occurrence of these species indicates a lower permian age upper half of the lower kungurian for the sampled limestones representative of the mesogondolella siciliensis s subsymmetricus zone of south china sweetognathus and pseudosweetognathus are for the first time reported from the indochina terrane located in the palaeoequatorial belt in the kungurian pseudosweetognathus appears restricted to the kungurian of the south china and indochina terranes thus supporting palaeogeographic reconstructions that isolate these terranes within the palaeo tethys during the kungurian

the pre devonian tectonic framework of xing an mongolia orogenic belt xmob in north china abstract a new tectonic division of the xing an mongolia orogenic belt xmob in north china has been presented according to our research and a lot of new data of tectonics geochronology and geochemistry four blocks and four sutures have been recognized in the xmob including the erguna eb xing an airgin sum xab songliao hunshandake shb and jiamusi jb and xinlin xiguitu xxs xilinhot heihe xhs mudanjiang ms and ondor sum yongji sutures oys the framework of the xmob is characterized by a tectonic collage of the blocks and orogenic belts between them different precambrian basements have been found in the blocks including the neoproterozoic metamorphic rocks and plutons in the eb the neoproterozoic metamorphic rocks in western and eastern of segments of the xab mesoproterozoic and neoproterozoic metamorphic rocks in middle segments of the xab respectively the neoproterozoic metamorphic rocks and mesoproterozoic volcanic rocks and plutons in the shb and neoproterozoic metamorphic rocks in the jb the xxs resulted from a northwestward subduction of the xab beneath the eb during the cambrian which was followed by the forming of the xhs and oys in the northwest and south margins of the shb in the silurian respectively the ms was caused by a westward subduction of the jb beneath the east margin of the shb during the middle devonian the three cambrian silurian and middle devonian events indicate that the xmob belongs to a pre middle devonian multiple orogenic belt in the central asian orogenic belt caob forming of the xmob suggests that the southeast part of the paleo asian ocean closed before the middle devonian

paleomagnetic study of middle cretaceous rocks from yunlong western yunnan china evidence of southward displacement of indochina middle cretaceous red sandstones and siltstones were collected at 20 sites for paleomagnetic study from the nanxin formation around yunlong 25 8 n 99 4 e western part of yunnan province china after stepwise thermal demagnetization high temperature component magnetization with unblocking temperature of about 675 c is isolated a positive fold test at the 99 confidence limit shows that the high temperature component magnetization is primary for the middle cretaceous nanxin formation a tilt corrected formation mean direction is d 40 2 i 49 9 with 95 3 9 corresponding to a paleopole at 54 6 n 171 8 e with a95 4 4 comparison with previously reported cretaceous paleomagnetic directions indicates that the yunlong area has been subjected to southward displacement by 11 9 7 5 corresponding to 1300 800 km with respect to the stable yangtze craton since cretaceous time as well as clockwise rotation through 36 3 13 6 this study elucidates that indochina was squeezed out of the asian continent due to collision of india accompanied with clockwise rotation

new mesozoic apparent polar wander path for south china tectonic consequences a new paleomagnetic study in the south sichuan basin south china block has been carried out on sediments of early triassic to middle jurassic age detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components a low temperature component component a falls close to the local present earth field direction a middle temperature component component b is found both in limestone and red beds of early triassic age in limestones of middle triassic age and finally in red beds of jurassic age the component b directions failed the fold test but show a significant increase of the ks kg ratio at 60 when a progressive stepwise unfolding is applied the directions obtained at 60 unfolding are grouped best and yield a pole position at 76 5 n 174 1 e a95 3 7 that is close to the late cretaceous pole of the south china block scb or the early tertiary pole of the eurasian plate characteristic directions are finally obtained component c for early triassic and early to middle jurassic sediments only both with reversal and fold tests they yield pole positions at 44 1 n and 217 2 e a95 4 9 and 79 n and 227 e a95 4 5 respectively the early triassic result well fits other data of the same age from the scb the revised late permian to cretaceous paleopole data provide a new mesozoic apparent polar wander path for south china the successive pole positions from late permian to early triassic indicate motion of the south china block as a whole rather than local rotations linked to important strain within the block the early to middle jurassic pole is statistically indistinguishable from the compiled north china block ncb poles and indicates that final suturation between the two blocks occurred by the end of the triassic which is coherent with the age of ultra high pressure metamorphism in the dabie sulu areas of eastern china the asian plate including the mongol central asian block ncb and scb however was probably separated from siberia by the mongol okhotsk ocean during jurassic time

acoustic and optical observations of methane gas seeps in the gulf of mexico in 2011 and 2012 measurements of acoustic backscatter from natural methane seeps were made in the northern gulf of mexico in water depths between 1000 2000 m the measurements were made using a calibrated 18 khz echo sounder with an 11 degree beamwidth in order to estimate the depth dependent target strength ts the ts data indicate a wide variation in the rate of gas seepage from the seafloor several of these seeps were revisited with a remotely operated vehicle in order to optically assess the bubble size distribution and to estimate the rate at which gas bubbles were exiting the seafloor the optical data show bubble sizes between 1 10 mm radius and similar rates of gas seepage ranging from a few bubbles per second to several tens of bubbles per second together these data help to suggest the requirements for acoustically estimating gas flux from the seafloor over large regions

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

discrepant cretaceous paleomagnetic poles between eastern china and indochina a consequence of the extrusion of indochina abstract a paleomagnetic study of cretaceous and paleocene red beds has been carried out in the yunlong area yunnan province of china of the northern indochina block a high temperature component with dual polarities was isolated from both lower cretaceous and paleocene rocks and of solely polarity for upper early cretaceous and lower late cretaceous rocks thirteen magnetic polarity zones are found along the pijiang section the primary nature of cretaceous magnetic remanence is ascertained by positive and reversal fold tests these results combined with previous cretaceous results from the yunlong and yongping areas of the lanping basin indicate that these areas behaved as a relatively rigid block and rotated 37 2 17 clockwise relative to eurasia since the paleocene comparing available early cretaceous paleomagnetic results obtained both from indochina and eastern china a statistically significant paleolatitude difference of 9 6 5 7 is inferred this difference is consistent with an estimated 1000 400 km extrusion of indochina relative to south china through a counter clockwise rotation of 14 of indochina around a finite rotation pole inferred from the stage poles of rotation derived from the magnetic isochrons in the south china sea the reconstruction is fully consistent between the cretaceous pole obtained from the khorat basin of the stable core of the indochina block and the coeval pole of eastern china

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

linking the sulu uhp belt to the korean peninsula evidence from eclogite precambrian basement and paleozoic sedimentary basins abstract considerable debate on whether and how the sulu orogenic belt extends eastward to the korean peninsula has remained over the past decade new results reported here include the following 1 an eclogite and retrograded eclogite bearing complex hongseong complex is discovered in south korea in which the eclogite occurs as lenses in circa 810 820 ma granitic gneiss shrimp zircon dating of the eclogite yields 230 ma for the metamorphic age and 880 ma for the protolith age 2 the basement of the rangnim gyeonggi and yeongnam massifs have affinities to the basement of the north china block ncb however the gyeonggi massif encloses a minor amount of large or small slabs of the hongseong complex that are similar to the rocks of the sulu belt 3 two main paleozoic basins within the rangnim and gyeonggi massifs have a similar paleozoic tectono stratigraphy to the ncb 4 the imjingang and ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts based on these facts we propose a crustal detachment and thrust model and suggest that the collision belt between the yangtze block yb and ncb sino korea craton is preserved along the western margin of the korean peninsula the lower part of the uhp metamorphosed lithosphere of the yb was subducted under the korean peninsula and not uplifted to the surface the lower crust of the yb the hongseong complex was detached from the subducted lithosphere and thrust over the korean peninsula and inserted into the basement rocks of the gyeonggi massif the upper crust of the yb possibly was detached from the lower crust and overthrusted along the honam and chugaryong shear zones the imjingang and ogcheon belts possibly represent the detached upper crust of yb and their present occurrences are controlled by a mesozoic strike slip shear structure all these detached lower and upper crustal slabs were strongly deformed during the late jurassic and early cretaceous tectonic event leading to their present geological distribution and characteristics

a new paleogeographic configuration of the eurasian landmass resolves a paleomagnetic paradox of the tarim basin china 1 new paleomagnetic data from permian red beds and middle jurassic limestones from the tarim basin pose a paradox their declinations are similar to upper carboniferous to neogene rocks collected from the same sections and their inclinations parallel present values when assuming that lower than expected inclinations in continental sedimentary rocks arise from inclination shallowing effects then the paleolatitudes of all upper carboniferous to present rocks from tarim are indistinguishable local vertical axis block rotations occurring in the last 20 million years explain why declinations vary at different localities in the basin our middle jurassic data positions tarim 23 6 8 4 farther south than that predicted from the coeval reference pole for eurasia however no geologic argument exists to support the closure of a large ocean basin between tarim and siberia since the middle jurassic thus the paradox are the rocks from tarim totally overprinted or is the middle mesozoic part of the reference eurasian apparent polar wander path erroneous several lines of evidence suggest the tarim rocks are not remagnetized we conclude that tarim has experienced little or no apparent polar wander since the carboniferous moreover our middle mesozoic reconstruction of eurasia using the new middle jurassic pole from tarim results in a more geologically compatible solution for the eastern asian blocks over previous reconstructions

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

early paleozoic paleomagnetism of east kazakhstan implications for paleolatitudinal drift of tectonic elements within the ural mongol belt abstract conflicting paleogeographic reconstructions of the ural mongol fold belt have been developed over the past decade very few of them however have been based on paleomagnetic data which still remain scarce the chingiz range in northeastern kazakhstan provides an opportunity to evaluate the tectonic evolution of the central part of this belt through extensive paleomagnetic studies in this paper we report the results from upper cambrian andesites and lower ordovician tuffaceous sandstones within the chingiz range a high temperature component htc decaying to the origin is isolated from upper cambrian rocks at eight sites with an easterly and low intermediate upward direction this component passes the tilt test and is likely primary a htc is also isolated from lower ordovician rocks at 11 sites these site means exhibit a shallow inclination girdle distribution within the southeast quadrant likely due to net rotations within the complex structure studied the inclination only tilt test is positive and the best grouping of data is observed at 100 untilting also included in our paleolatitudinal analyses are new middle and late paleozoic paleolatitudes from nearby parts of the chingiz range as well as published paleomagnetic data from central and east kazakhstan we favor normal polarity of the early paleozoic results of this paper and conclude that the study area moved northward throughout the paleozoic the observed paleolatitudes generally fit the siberian reference data and hence indicate coherent latitudinal motion of the boshekul chingiz tectonic zone and the siberian plate

paleomagnetic study of upper jurassic rocks from the sichuan basin tectonic aspects for the collision between the yangtze block and the north china block abstract a paleomagnetic study has been carried out on late jurassic redbeds in the sichuan basin at the northern part of the yangtze block upper jurassic brownish red sandstones and brownish red siltstones were collected in the penglaizhen formation at 26 sampling sites around jiangyang city 30 4 n 104 5 e thermal demagnetization isolated a high temperature magnetization component with a maximum unblocking temperature of about 690 c the primary nature of magnetization acquisition is ascertained through a 95 confidence level positive fold test as well as a positive reversal test the tilt corrected overall mean direction of 24 sites is d 25 9 i 26 7 95 4 7 leading to a paleomagnetic pole situated at 61 3 n 222 7 e a95 4 2 this pole position agrees with two other late jurassic poles from the sichuan basin indicating that this basin has behaved as a rigid block since late jurassic these three poles from the sichuan basin therefore provide a characteristic late jurassic pole for the stable part of the yangtze block 64 3 n 231 2 e a95 6 7 comparison of the late jurassic pole of the yangtze block with the contemporaneous pole for the north china block ncb identifies a significant difference implying that the relative movement between the yangtze block and the ncb continued during and probably after the late jurassic paleomagnetic and geological evidence reveals that the yangtze block collided first with the ncb during the early permian and continued to penetrate into it while undergoing a clockwise rotation until the early cretaceous

further paleomagnetic results for lower permian basalts of the baoshan terrane southwestern china and paleogeographic implications abstract the baoshan terrane of southwestern china is considered to have been part of the cimmerian block during the late paleozoic consequently knowledge of its paleoposition and geological evolution can provide constraints on the permian breakup of northern east gondwana therefore we conducted paleomagnetic and rockmagnetic studies on lower permian basalts from four localities in the baoshan terrane the basalts hold a stable characteristic remanent magnetization chrm at high temperatures 300 680 c that is carried by magnetite maghemite and hematite with both pseudo single and multiple domains to test the reliability of data from these volcanic rocks we analyzed the geomagnetic secular variation gsv and reliability of both the present data and previous paleomagnetic data the results from 23 sites yield a single reversed polarity directed downwards to the southwest giving a site mean direction of d g i g 156 7 56 6 k g 8 0 95 11 4 before tilt correction and d s i s 218 3 60 1 k s 14 1 95 8 4 after tilt correction the result passed the fold test but the gsv was able to be averaged out in only two sections all available data were examined section by section using the angular dispersion s b of virtual geomagnetic poles vgps to ensure that the gsv was completely averaged out because the dispersion in declinations is likely to have been affectedby subsequent tectonic deformation the paleosecular variation psv could not be evaluated from all the data amassed from different sections and the psv was able to be removed from only four combined sections a small circle fit of these vgps gives an averaged paleocolatitude of 51 9 3 7 n 31 sites centered on 24 n 99 e the result indicates that the sampled area of the baoshan terrane was located at a latitude of 38 s 3 7 during the late early permian a comparison of this result with early permian data from gondwanan blocks suggests that the baoshan terrane was situated near the junction of northern india and northwestern australia and that it broke away from western australia after the early permian

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

further paleomagnetic results from the silurian of the yangtze block and their implications abstract silurian redbeds were sampled at two localities in northeastern guizhou and yunnan provinces of china for paleomagnetic study progressive thermal demagnetization revealed multiple magnetic components from these samples a low unblocking temperature component a of recent origin a post folding middle to high unblocking temperature component b probably acquired during late mesozoic time and a pre folding high unblocking temperature component c two statistically distinguishable groups of directions were further recognized from the c component with one c1 having more easterly declination than the other c2 and both having a of shallow inclination the paleomagnetic pole from the c1 magnetization coincides with a pre folding pole reported from the coeval rocks in neighboring southern sichuan province its relation to the apparent polar wander path apwp for the yangtze block yb suggests that the c1 magnetization was probably acquired during the silurian by the same argument the c2 magnetization was probably acquired sometime between the silurian and carboniferous these results reconfirm the equatorial position for the yb during mid paleozoic time and clockwise rotation of the yb since at least the mid cambrian

cretaceous paleomagnetism of the eastern south china block establishment of the stable body of scb abstract a paleomagnetic investigation was performed on the cretaceous red sandstones collected at the eastern side of the south china block scb china and attempted to establish the stable part of the scb since the cretaceous paleomagnetic specimens were collected at 39 25 and 14 sites from three independent parts the northern central and southern regions of eastern scb respectively characteristic directions of higher temperature components htcs with an unblocking temperature of 680 c were isolated from 69 78 sites the optimal concentrations of global mean htc directions calculated using the direction correction tilt test were achieved at 79 19 95 27 71 37 and 117 98 untilting for the early cretaceous sites from the northern part late cretaceous sites from the northern central and southern parts respectively this treatment gives positive tilt tests or brings the optimal concentration not far from being indistinguishable from positive tilt tests although this observation can be due to imperfect separation of a primary component htc from a secondary one lower temperature component we adopted completely 100 untilted directions of the htcs as the paleomagnetic field directions during the cretaceous because the mean directions after complete untilting were almost equal to each mean direction after incomplete untilting showing the optimal concentration the mean paleomagnetic poles for three independent parts were located at almost the same positions and were indistinguishable from that for sichuan the western side of scb at the 95 confidence level this observation indicates that there is no relative movement between the eastern and western sides of scb and implies that a large part of the scb excluding a 400 km wide swath along the red river fault has behaved as its coherent stable body since the cretaceous the cretaceous paleomagnetic pole for the stable body of the scb latitude 80 0 n longitude 206 7 e a 95 2 5 is worth using as a reference cretaceous paleomagnetic pole of the scb and agrees strongly with the reference eurasian paleomagnetic pole during the cretaceous period