magnetotelluric multiscale 3 d inversion reveals crustal and upper mantle structure beneath the hangai and gobi altai region in mongolia

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

palaeomagnetic results from palaeocene basalts from mongolia reveal no inclination shallowing at 60 ma in central asia summary we present the results of a palaeomagnetic study of 277 cores drilled at 35 sites in 32 basaltic flows from three early palaeocene volcanic regions in the gobi desert mongolia at sumber uul 62 2 ma 42 6 n 104 0 e tulga 62 0 ma 43 2 n 104 1 e and khuts uul 57 1 ma 43 2 n 104 6 e localities samples from sumber uul 62 2 0 9 ma and khuts uul 57 1 0 8 ma localities were dated using the k ar cassignol gillot technique stepwise thermal and alternating field demagnetizations isolated a stable a component of magnetization carried by single domain sd to nearly sd magnetite we interpret this a component to be the primary magnetization of these basaltic lava flows the sumber uul and tulga data were combined and recomputed at the sumber uul locality because of their similar ages the palaeopoles computed from the a components lie at 85 2 n 92 5 e dp dm 3 9 4 9 n 14 flows for sumber uul tulga average age 62 1 5 9 ma and 69 6 n 148 0 e dp dm 6 3 7 3 n 14 flows for khuts uul average age 57 1 0 8 ma the palaeomagnetic inclinations are steeper than expected at the sites and consequently our palaeopoles occupy a near sided position with respect to the 60 ma reference apparent polar wander path apwp pole for europe besse courtillot 2002 however they appear to fully conform to the new high resolution apwp poles for the 65 42 ma period of moreau et al 2007 following these authors we interpret this anomalous near sided position of our poles as arising from a rapid true polar wander tpw event in the palaeocene highlighted by a cusp at anomalies 26 25 61 56 ma inexistent in the european apwp of besse courtillot 2002 we conclude that our new data do not reveal any anomalous shallow inclinations in the central asia palaeocene effusive rocks which is consistent with the late jurassic early cretaceous age of mongol okhotsk ocean closure and amalgamation of amuria and siberia forming a rigid entity since then

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

paleomagnetic evidence for a mid miocene clockwise rotation of about 25 of the guide basin area in ne tibet abstract ten sections of neogene molasse type sediments were sampled in the guide basin of northeastern tibet for magnetostratigraphy x m fang m d yan r van der voo d r rea c song j m pares j gao j nie s dai late cenozoic deformation and uplift of the ne tibetan plateau evidence from high resolution magnetostratigraphy of the guide basin qinghai province china geol soc america bull 107 2005 1208 1225 1 but they also yield seven well dated formation mean directions that reveal changing declinations as rotations occurred in response to crustal deformation north of the india asia collision zone three formations are of early miocene and oligocene age as indicated by fossils and magnetic reversal records whereas four younger formations yield late miocene and pliocene ages the dual polarity magnetizations are typically antipodal but reveal inclinations that are too shallow most likely because of post depositional inclination flattening the late miocene and younger directions show formation mean declinations between 354 and 7 whereas three early miocene and late oligocene mean declinations range from 31 to 44 this indicates that a clockwise rotation of 25 1 4 6 took place during the middle part of the miocene best estimate 11 17 ma no rotations appear to have occurred during that time in the xining lanzhou linxia and jingning basins longzhong basin to the northeast and east of the guide basin however a rotation of similar magnitude has been documented by dupont nivet and colleagues for pre miocene 29 ma time in these areas collectively these results show that the basins in ne tibet have had independently evolving structural histories

new late cretaceous and paleogene paleomagnetic results from south china and their geodynamic implications 1 to better understand the tectonic evolution of south china block scb in response to the india asia collision we present new paleomagnetic results from late cretaceous and paleogene red bed formations of the hengyang basin of hunan province in the interior part of the scb stepwise thermal demagnetization of the rocks isolated a high temperature component the tilt corrected mean direction from the late cretaceous rocks is d 15 6 i 29 9 with a95 5 7 n 26 sites corresponding to a paleopole at 71 9 n 236 3 e with a95 4 7 which passes reversal tests the tilt corrected mean direction d 358 9 i 35 4 with a95 5 0 n 22 sites from the paleogene rock which passes the reversal test gives a pole at 82 6 n 300 6 e with a95 4 4 the low field anisotropy of magnetic susceptibility results suggest that the red beds have not experienced significant strain due to compaction or tectonic stress while the anisotropy of isothermal remanence results suggest that postdepositional compaction in these red beds produced no more than 3 4 of inclination shallowing these paleomagnetic and rock magnetic tests imply that the remanence is primary on the basis of paleomagnetic results obtained from coeval basalts in mongolia and siberia we suggest that no significant latitudinal motion has taken place between the scb and siberia since the late cretaceous the significant latitudinal discrepancy between the scb and european block could be due to nonrigid behavior of the eurasian plate since the late cretaceous a 16 7 5 0 clockwise rotation of the scb during the late cretaceous to the paleogene could be related to the collision of india and asia in the west and the subduction of the circum pacific plate to eastern asia in the east

paleomagnetic and geochronological constraints on the post collisional northward convergence of the southwest tian shan nw china a combined paleomagnetic and geochronological study is reported of paleogene basalt lavas and an intercalated red bed succession comprising a minimum of 14 basalt flows and 10 red bed horizons in the tuoyun basin of the southwest tian shan range china two basalt matrix samples yield 40 ar 39 ar isochron ages of 58 5f1 3 ma 2r mswd 0 9 and 60 4f1 3 ma 2r mswd 1 7 these compare well with a previously published k ar dilution age of 61 7f2 3 ma for comparable paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity demagnetization and component analysis identify a stable characteristic remanence chrm with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250 350 8c or 5 mt rock magnetic analysis identifies pseudo single domain magnetite or titanomagnetite as carriers the stable chrm passes a fold test it was probably acquired at the time of lava emplacement results from the bulk of the collection imply that paleomagnetic data from the upper and lower 115 ma basalt series in the tuoyun basin are not distinguishable at the 95 significance level and indicate that this tectonic domain remained essentially stationary with respect to the earth s spin axis for 50 ma prior to onset of the india asia collision in early eocene times it is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest tian shan prior to collision paleomagnetic data sets from the tuoyun basin also show that little or no paleolatitude difference is present between the tian shan and the reference latitude of eurasia at 60 ma this supports previous evidence suggesting that central asian blocks in the vicinity of the tian shan are unlikely to have experienced appreciable northward convergence relative to eurasia since onset of the india asia collision and initiation of the himalaya d 2005 elsevier b v all rights reserved

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

palaeomagnetism and 40ar 39ar geochronology of upper palaeogene volcanic rocks from central tibet implications for the central asia inclination anomaly the palaeolatitude of tibet and post 50 ma shortening within asia summary palaeomagnetic results from upper palaeogene volcanics from three localities in the qiangtang terrane of central tibet record a maximum likelihood palaeolatitude of 28 7 3 7 n these are the first volcanic based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate this palaeolatitude estimate is significantly lower than the 36 3 n predicted for central tibet by late palaeogene eurasia reference poles consistent with the so called central asia inclination anomaly but it is consistent with values predicted from upper palaeogene reference poles for mongolia the mean inclination of coeval sedimentary rocks from central tibet is 15 5 shallower than our volcanic results similar to the magnitude of sedimentary inclination shallowing estimated from elongation inclination analysis of remanent directions in clastic rocks from the same region we conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable asia and by cenozoic intracontinental shortening within central asia on the order of several hundred not more than 1000 kilometers our compilation of palaeomagnetic data from the qiangtang terrane further suggest that the southern margin of the lhasa terrane the southern margin of pre india asia was located as far south as 20 n throughout eocene time these palaeolatitudes in conjunction with the upper crustal shortening record for asia indicate that only approximately one third of the post 50 ma india asia convergence is partitioned in asian lithosphere low latitudes also imply that much of the proto tibetan plateau occupied southerly tropical to subtropical latitudes which has important implications for the weathering and erosion of the tibetan plateau palaeoaltimetry proxies and early cenozoic climate dynamics

structural and lithological characteristics of the bayankhongor ophiolite zone central mongolia the mechanism of continental growth of central asia is currently debated between models invoking continuous subduction accretion or punctuated accretion due to closure of multiple ocean basins ophiolites in central asia may represent offscraped fragments in an accretionary complex or true collisional sutures the bayankhongor ophiolite a nw se striking sublinear belt 300 km long and 20 km wide is the largest ophiolite in mongolia and possibly central asia we present results of the first detailed structural and lithological study of the ophiolite the study area is divided into four zones baidrag complex burd gol bayankhongor and dzag zones the archaean baidrag complex comprises tonalitic granulites and metasediments the burd gol zone is a metamorphosed sedimentary and igneous melange the bayankhongor zone contains the dismembered ophiolite forming a serpentinite melange the dzag zone consists of asymmetrically folded chlorite mica schists resembling meta turbidites the structure is dominated by steeply dipping ne directed thrusts and ne vergent folds we suggest the bayankhongor ophiolite marks the closure of an ocean separating two microcontinents the baidrag complex with the burd gol accretionary complex to the south and a northern continent that forms the basement for the hangai region subduction was towards the sw with ne directed ophiolite obduction onto a passive margin represented by the dzag zone

shallow bias of paleomagnetic inclinations in the paleozoic and precambrian abstract an updated analysis of the global paleomagnetic database shows that the frequency distributions of paleomagnetic inclinations for the cenozoic and mesozoic eras younger than 250 ma are compatible with a random geographical sampling of a time averaged geomagnetic field that closely resembles that of a geocentric axial dipole in contrast the frequency distributions of paleomagnetic inclinations for the paleozoic and precambrian eras prior to 250 ma are over represented by shallow inclinations after discounting obvious secondary causes for the bias such as from data averaging sedimentary inclination error inhomogeneous lithological distributions and tropical remagnetization we show that the anomalous inclination distributions for the paleozoic and precambrian can be explained by a geomagnetic field source model which includes a relatively modest 25 contribution to the axial dipole from a zonal octupole field and an arbitrary zonal quadrupolar contribution the apparent change by around 250 ma to a much more axial dipolar field geometry might be due to the stabilization of the geodynamo from growth of the inner core to some critical threshold size a gross speculation which would imply that either the threshold size was rather large or the inner core nucleated rather late in earth history alternatively if a geocentric axial dipole model is assumed or can eventually be demonstrated independently the anomalous inclination distributions for the paleozoic and precambrian may reflect a tendency of continental lithosphere to be cycled into the equatorial belt perhaps because geoid highs associated with long term continental aggregates influence true polar wander

magnetic inclination shallowing problem and the issue of eurasia s rigidity insights following a palaeomagnetic study of upper cretaceous basalts and redbeds from se china

apparent polar wander paths of the major chinese blocks since the late paleozoic toward restoring the amalgamation history of east eurasia abstract high quality paleomagnetic poles a k a paleopoles are essential for quantitative lithospheric plate reconstruction however the current paleomagnetic database of the major chinese blocks including the north china block ncb the south china block scb and the tarim block tb since the late paleozoic era contain outdated and low quality data here we update the database by adding recently published high quality paleopoles and rejecting low quality outdated ones the database includes 288 paleopoles published 1980 2014 90 of them published after 2000 following the van der voo selection criteria 75 paleopoles each with a quality factor q smaller than 4 were rejected in the first round of selection we then removed another 59 paleopoles that have been locally rotated or shallowed since the acquisition of stable remanence eventually 154 paleopoles were selected and adopted to calculate the new apparent polar wander paths apwps for the major chinese blocks we found comparable clastic and igneous paleopoles at intervals when a quantitative comparison was available as such comparison was not possible during most periods epochs our conclusions reflect an unclear extent of inclination errors in chinese clastic paleopoles new models of the chinese apwps with and without inclination error corrections were computed from carefully selected paleopoles version 1 running mean v1rm paths were calculated at the period epoch level version 2 running mean v2rm paths were computed in a set sliding time window of 20 or 30 myr spline paths were calculated with the same time windows along with a smoothing parameter of 50 using a recent global reconstruction model and up to date geological observations new models of the chinese apwps allowed us to re evaluate the coalescence history of east eurasia since the late paleozoic era four major tectonic events were confirmed 1 the tb accreted with the kazakhstan orocline during amalgamation of the west altaids during the middle late permian period ca 265 250 ma 2 the suturing of the ncb and the scb likely occurred in a scissor like pattern and had been accomplished no later than the middle jurassic period ca 180 160 ma 3 the amalgamation between the ncb and the tb along with the microblocks between the two might have been achieved during the late jurassic early cretaceous periods ca 160 140 ma 4 the mongol okhotsk ocean should have been closed no later than the early cretaceous period ca 140 120 ma

persistently low asian paleolatitudes implications for the india asia collision history 1 paleomagnetism provides independent paleolatitude constraints on the india asia convergence however implied cenozoic latitudinal convergence within asia thousands of km largely exceeds geologic estimates of tectonic shortening hundreds of km this discrepancy may result from a notoriously low bias in paleomagnetically determined cenozoic paleolatitudes in asia we provide here new paleomagnetic data from cenozoic mongolian volcanic rocks and from chinese paleogene sediments corrected from the depositional bias of inclination shallowing these results combined with similar asian data sets confirm that paleolatitudes are still 5 10 lower than predicted by the paleomagnetic apparent polar wander path apwp for asia between 50 and 20 ma inclination shallowing being excluded from the selected data sets we investigate the likeliness of other proposed mechanisms for this discrepancy 1 more southerly positions of asia than expected by the apwp due to apwp inaccuracies rather than eurasian non rigidity or 2 non dipolar geomagnetic field contributions fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large 10 apwp inaccuracies or unrealistically large octupolar field contributions up to 16 a combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy but their respective contributions cannot be quantified given the still relatively low amount and poor quality of cenozoic paleomagnetic data from stable cratons of asia india and europe by allowing for reasonable time dependant non dipolar contributions and a slight 5 apwp bias the latitudinal discrepancy can be resolved and the excessive amounts of intra asian shortening decrease to values in line with tectonic shortening from structural studies

first palaeogene sedimentary rock palaeomagnetic pole from stable western eurasia and tectonic implications a palaeomagnetic investigation of lower eocene ca 52 ma london clay formation cemented mudstones from sheppey se england has yielded a mean direction of dec 1 1 inc 43 2 where n 9 a95 6 8 and k 58 5 this apparently high quality direction q factor 5 has an associated palaeopole of 178 6 e 63 7 n where a 9 5 6 8 the data represent the first pole from post jurassic stable eurasia rocks outside of the european north atlantic igneous province naip of which most results have been obtained from nw britain and the faroe islands the data can in part be used to constrain the position of palaeogene eurasia in particular the zero offset declination implying negligible rotation of western eurasia since the early cenozoic this is in contrast with data derived from the european naip which imply small to moderate clockwise rotations for this part of the plate the inclination angle may provide less useful information as it appears to be anomalously shallow when compared with that associated with the naip derived poles in an attempt to understand the shallowing we re examined data from palaeocene eocene sediments recovered in several boreholes bathyal sediments in dsdp hole 550 four cores through fluvio delatic to middle shelf sequences in the london area and one borehole sequence from east anglia in all cases the sediments show systematic inclination shallowing similar in magnitude to that reported from sheppey tectonic and geomagnetic explanations can be discounted sediment compaction appears to be the likely cause in light of the current controversy surrounding the stable asia shallow inclination problem the result reinforces the suggestion that tectonic modelling needs to he done carefully when the supporting data are based exclusively on palaeomagnetic studies of sedimentary rocks

new paleomagnetic results from the lhasa block revised estimation of latitudinal shortening across tibet and implications for dating the india asia collision knowing how and when the tibetan plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths previous paleomagnetic work suggested that intracontinental shortening from deformation related to the india asia collision played a major role in building the tibetan plateau however these studies were derived from red bed facies which are often afflicted by inclination shallowing leading to overestimates of continental shortening we report new paleomagnetic data from the lhasa block the southern continental margin of asia before the india asia collision a high temperature characteristic remanent magnetization chrm was isolated from 43 sites of late cretaceous red beds 32 sites of late cretaceous lava flows and nine sites of eocene tuffs all the chrm directions are significantly different from the present day geomagnetic field and the mean directions of red beds lava flows and tuffs pass the fold test the distribution patterns of chrm directions derived from volcanic rocks are consistent with a secular variation model suggesting that secular variation was likely averaged out in contrast the red bed chrm directions exhibit an east west elongated distribution pattern indicative of inclination flattening the mean chrm inclination of the red beds 23 5 degrees 2 5 degrees is consistent with previous results but significantly shallower than that of 41 9 degrees 4 4 degrees carried by the coeval lava flows an elongation inclination analysis of the red bed chrm directions yielded an unflattened mean inclination of 39 7 degrees with 95 confidence limits between 37 7 degrees and 41 9 degrees the paleolatitudes of the late cretaceous volcanic rocks and inclination corrected sediments suggest about 10 degrees less shortening occurred between lhasa and siberia than previous estimates with total shortening lying between 3 3 degrees 2 6 degrees and 7 3 degrees 5 4 degrees since the late cretaceous the mean chrm inclination derived from the eocene tuffs 51 8 degrees 5 2 degrees is indistinguishable from the expected reference direction our new results imply that india asia convergence was accommodated mainly through consumption of the india plate and yield a much younger age ca 43 ma for the initial india asia collision than previously estimated by paleomagnetic data c 2010 elsevier b v all rights reserved

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

positioning paleogene eurasia problem solution for 60 50 ma and broader tectonic implications abstract recently published paleomagnetic data from the faroe islands and se england have enabled a hybrid pole 72 0 n 177 9 e a95 7 9 to be calculated for stable eurasia 55 ma it is somewhat different to previous proposals being a further 8 9 from the present day north pole a strong positive test of the new pole is provided by 2002 published paleomagnetic data from basaltic rocks in the tien shan range in kyrgyzstan the paleolatitude derived from the inclination angle matches the predicted value to within 0 2 an unfortunate drawback with kyrgyzstan pole is its large age error 15 m y for rocks estimated to have formed 50 ma fortuitously an alternative test is now available using paleomagnetic data from paleocene basalts in the tien shan range of western china for which a robust radiometric age date 59 1 ma based on two ar ar results also exists although the locality has experienced a large vertical axis rotation the mean declination being 54 5 the inclination angle appears undisturbed and the derived paleolatitude matches the value predicted by hybrid pole to within 4 0 thus it is contended the faroe sheppey pole provides one of the most reliable means of fixing eurasia s position for the interval 60 50 ma it also impacts on various model proposals for the india asia collision and subsequent crustal shortening and or extrusion between southern tibet and stable eurasia north of the tien shan

relative hotspot motions versus true polar wander abstract the fixity of hotspots and mantle plume locations has long been axiomatic if the assumption of fixed hotspots is granted absolute plate motions and movements of the spin axis with respect to the hotspot framework defined by some as true polar wander tpw can be determined however this assumption can be tested by paleomagnetic data and such tests are gradually raising some doubts about the fixity of hotspots the result is that discrepancies between cretaceous and tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle in the indo atlantic hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr however the pacific hotspots notably hawaii appear to have undergone large scale southward drift with respect to the spin axis during the early tertiary global paleomagnetic data do not indicate that any tpw occurred during the late cretaceous or tertiary although the early cretaceous paleomagnetic and hotspot frames for the indo atlantic realm can be interpreted as slow tpw direct estimates of paleolatitude and hotspot motion in particular the kerguelen hotspot challenge tpw as a global phenomenon at present we consider that the large early cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the atlantic hotspots prior to 95 ma

shallow bias in neogene palaeomagnetic directions from the guide basin ne tibet caused by inclination error summary too shallow inclinations have frequently been observed in cenozoic sedimentary strata in central asia and new palaeomagnetic results obtained by us from the guide basin in ne tibet are no exception we use a statistical analysis technique developed by tauxe and kent tk03 gad which is based on a geomagnetic field model that predicts distributions of palaeomagnetic directions and show that the too shallow neogene mean inclination 44 from 627 sites can be corrected to a value of 58 which closely matches the inclination predicted for the area we conclude that syn to post depositional flattening is the most likely cause for the widely observed inclination bias in central asia

paleomagnetism of paleocene maastrichtian 60 70 ma lava flows from tian shan central asia directional analysis and paleointensities

testing models of the tertiary paleomagnetic field we examine selected paleomagnetic data of tertiary age in order to assess previously suggested models of the non dipole portion of the paleomagnetic field data from three localities on the african plate from dsdp leg 73 odp leg 115 and from gubbio indicate that the axial octupole contribution was near zero in the paleocene and was likely no greater than 5 6 of the axial dipole in the oligocene additional data from north america which can be linked to africa using well known relative plate motion parameters suggest that a positive axial quadrupole field has been present throughout the tertiary to investigate some of the practical consequences of these estimates we apply a non dipole field model 3 to 4 go go 0 g g l to paleomagnetic data from two odp sites on the african plate sites 710 711 and from the deccan traps we show that paleomagnetic directions from these deep sea sediments and from this flood basalt province appear consistent with a fixed hotspot model suggesting in particular that there has been little or no cenozoic drift of the rrunion hotspot 1 the paleomagnetic field configuration the application of paleomagnetic measurements to tectonic studies requires that the paleomagnetic reference field be accurately known usual practice in most tectonic studies invokes the dipole hypothesis to determine paleolatitude from the observed paleomagnetic directions observations from rocks that are only a few million years old generally support the dipole hypothesis the time averaged field configuration largely matches the field of a geocentric axial dipole gad however non dipole components can also be detected in the time averaged field 1 2 3 4 this long term non dipole field amounts to a few percent of the geocentric axial dipole and suggests that in detailed tectonic studies non dipole field effects cannot be neglected without consequence although these non dipole components of the mean field have been relatively small during the past few million years less than 5 of the gad larger non dipole fields may have existed during other intervals in earth history indeed two spherical harmonic studies of the time averaged field available for pre pliocene times 2 5 indicate significantly larger non dipole fields during parts of the tertiary and mesozoic amounting at times 0012 821x 90 03 50 1990 elsevier science publishers b v to 10 15 or more of the gad non dipole components of such magnitudes would lead to substantial systematic errors in paleomagnetic pole positions and thus could complicate tectonic interpretations and reconstructions considerably in this paper we attempt to test the reality of these prior indications of large non dipole contributions

the interaction of fluid processes and progressive deformation during shallow level accretion examples from the shimanto belt of sw japan the interaction of deformation and fluid flow is recorded in the mesostmcture and microstructure of the shimanto belt in the eastern hata peninsula of sw japan large areas of disrupted strata including the okitsu melange have been generated during the progressive deformation associated with shallow level accretion the resulting assemblage of structures and their crosscutting relations reflect the complex interplay between pore fluid pressures lithification states and strain rates evidence for the relative timing of structures diagenesis and modes of fluid migration has been used to constrain a generalized model for the nature of fluid outflow during the accretion and burial of trenchslope lithologies early outflow was diffuse but the flow direction was controlled mainly by a regionally pervasive structural anisotropy and the predominance of independent paniculate flow as the sediments became more lithified fluid flow was focused along faults in which transient microfractures controlled the permeability fractures released pore fluid pressure promoting further deformation by diffusive mass transfer this in turn resulted in the buildup of pore fluid pressures and tectonic stresses and a recurrence of fracturing in some fault zones and in the okitsu melange several such deformation mechanism cycles can be identified the development of a complex array of faults during the late stages of accretionary complex evolution resulted in fluid flow paths becoming multidirectional

discordant paleomagnetic direction in miocene rocks from the central tarim basin evidence for local deformation and inclination shallowing from exposures at the southeastern end of the maza tagh range in the central tarim basin latitude 38 5 n longitude 80 5 e 55 paleomagnetic sites were collected from red mudstones and sandstones of the miocene wuqia formation thermal demagnetization revealed a high unblocking temperature characteristic remanent magnetization chrm five sites collected across a kink fold yield a positive fold test at 99 confidence level the mean directions computed from normal and reversed polarity sites are antipodal suggesting a primary origin for the chrm in stratigraphic coordinates the final set of 30 site mean chrm directions yields a section mean direction inclination i 29 4 declination d 24 7 k95 6 2 when compared to the miocene expected direction at 20 ma the observed direction indicates 30 8 5 5 flattening of inclination and 15 3 6 7 clockwise vertical axis rotation anisotropy of magnetic susceptibility measurements on 155 samples show a strong foliation of 1 092 with a subvertical minimum susceptibility axis these observations indicate a rock magnetic depositional or compaction shallowed origin for the inclination flattening the clockwise deflection of the observed declination can be interpreted as either 1 15 3 6 7 clockwise rotation of the entire tarim basin since the miocene or 2 a local km scale structural deformation it is not a simple matter to discard the interpretation of 15 3 6 7 clockwise rotation of the tarim basin because the fastest rates of rotation determined from global positioning system and slip rate studies of quaternary faults could produce such a rotation if extrapolated to 20 ma nevertheless we argue that local deformation is the preferred interpretation because the map pattern of local structures shows v20 clockwise deflection toward the southeastern end of the maza tagh range where the paleomagnetic samples were collected 2002 elsevier science b v all rights reserved

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

structural geological evidence from odp leg 131 regarding fluid flow in the nankai prism japan abstract the fluid flow regime of accretionary prisms influences their morphology and dynamics and is closely interrelated with deformation for example the lesser antilles prism shows drainage principally along channels provided by tectonic structures such as thrusts and the basal decollement a similar behaviour has been suggested for the nankai prism and this was investigated during odp leg 131 coring at site 808 penetrated the entire prism and underthrust sediments and allowed detailed documentation of structures seen in the cores however neither the frontal thrust nor the decollement show any visible sign of channelised fluid flow no veins mineralised surfaces or clastic dykes were observed the single clastic dyke seen within the prism appears unrelated to a deformation zone on the other hand small scale structures such as faults and deformation bands are unusually numerous in the nankai prism and colour and diagenetic changes within them suggest that they may have acted as micro conduits for drainage further contrasts with other prisms are shown by the sequence of deformation at nankai and the relative paucity of deformation in the underthrust material therefore the tectonic and fluid flow regime at site 808 differs from the expected pattern marked increase in porosity immediately below the decollement hints at overpressuring in this zone but the structures show no record of localised fluid activity the structural geological observations throughout the nankai prism are more consistent with bulk pervasive dewatering perhaps utilising the abundant deformation microstructures

the south siberia central mongolia transect abstract studies of geological and geophysical data on the south siberia central mongolia geoscience transect which runs approximately along 100 e show that the asian continent was formed in the phanerozoic by accretion of terranes most of which were microcontinents with precambrian basement suture zones separating the terranes are deformed and deeply eroded magmatic arcs of different widths likewise classified as specific terranes in these sutures magmatic arc rocks are locally associated with fore and back arc basin series most of the oceanic crust has been subducted the observed ophiolite belts are as a rule attributed to the base of oceanic island arcs the fragment of the asian continent traversed by the transect was formed through early middle and late paleozoic stages of accretion

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

the late cretaceous paleomagnetic field in north america a south american perspective determining the late cretaceous paleomagnetic pole for north america has been difficult because of the lack of suitable rocks of that age in cratonic areas to provide the necessary data as an alternative different studies have appealed to paleomagnetic data from rocks in western north america using paleopoles from stable areas in neighboring continents it is suggested that the available late cretaceous paleomagnetic record in western north america should be analyzed in terms of rigid body deformations rather than be used to represent the cratonic reference field

mongolian plateau evidence for a late cenozoic mantle plume under central asia the 2500 x 700 km mongolian plateau average elevation 2000 m is situated between the altai orogen and the siberian craton and occupies much of mongolia and transbaikalia in russia the plateau is characterized by 1 basin and range topography and two major domes hentai 600 x 300 km and hangai 800 x 550 km where altitudes reach 3905 m 2 lithosphere that is thinner than adjacent areas minimum 50 km 3 elevated heat flow up to 120 mw m2 4 dominantly alkaline basaltic volcanism in the form of cones lava fields and volcanic plateaus mostly of miocene quaternary age and 5 rifts including baikal main evolution in the pliocene quaternary tunka oligocene early miocene and hobsogol pliocene quaternary existing models explain these features in terms of diapiric upwelling of a mantle asthenolith below the main rifts and or as a long distance effect of the india asia collision we propose that the late cenozoic uplift of the whole mongolian plateau and associated rifting magmatism high heat flow and lithospherec thinning are not externally driven by the india asia collision but are the expression of the interaction of a mantle plume with overlying lithosphere some rifts link and interact with major strike slip faults such as the bolnai such faults may be the major expression of the india asia collision in this region

the zhaheba ophiolite complex in eastern junggar nw china long lived supra subduction zone ocean crust formation and its implications for the tectonic evolution in southern altaids abstract the composite zhaheba ophiolite complex exposed in eastern junggar in the southern altaids records an unusually long record of oceanic crust and magmatic arc evolution the zhaheba ophiolite complex consists of ultramafic rocks gabbro diorite basalt and chert intruded by diabase dikes and diorite porphyry these rocks are overlain by a several km thick section of tuffaceous rocks volcaniclastic sedimentary rocks and intermediate volcanic rocks the igneous rocks of the ophiolite complex show negative nb and ta anomalies and lree enrichment relative to hree suggesting the influence of fluids derived from a subducting oceanic slab the la icpms u pb age of zircons from gabbro is 495 1 3 5 ma zircon ages from diorite and basalt are 458 3 7 2 ma and 446 6 6 0 ma respectively the basalt is locally overlain by bedded chert diabase dikes and diorite porphyry yield the u pb ages of 421 5 4 1 ma and 423 7 6 5 ma respectively the age of stratigraphically lower part of the overlying volcanic volcaniclastic section is constrained to be about 410 ma the maximum depositional age of the tuffaceous sandstone from u pb detrital zircon ages late rhyolite at the top of the stratigraphic section yielded a u pb zircon age of 280 3 3 7 ma the age and stratigraphic relationships for the zhaheba ophiolite complex and related rocks suggest that the period of 70 ma of initial supra subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 ma many other ophiolites in the southern altaids appear to record similar relationships and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra subduction zone the occurrence of the zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic tethyan ssz ophiolites but similar to some ophiolites in north america although the zhaheba ophiolite belt is flanked by the dulate arc in the north and yemaquan arc in the south it cannot stand a suture between two arcs it is suggested that devonian carboniferous dulate arc was built on the late cambrian middle ordovician zhaheba supra subduction oceanic crust the late carboniferous rocks and early permian rocks in dulate arc are interpreted to form in the extensional process within zhaheba dulate arc composite system

cenozoic vertical axis rotation of the altyn tagh fault system paleomagnetic declination data were collected from cenozoic strata of the southern rim of the tarim basin to address whether the altyn tagh fault has undergone significant rotation during the indian asian collision results from the eastern and central altyn tagh fault zone suggest that it has undergone no significant rotation since the oligocene this implies that the boundary between the tarim and tibet has remained relatively stationary during most of the cenozoic in contrast declination data from the western kunlun shan on the eastern limb of the pamir orocline where the karakax segment of the altyn tagh fault system terminates suggest that the range has undergone clockwise rotations of between 19 3 8 6 and 27 8 5 8 such rotation is in mirror image with the documented counterclockwise rotation of 20 50 in the western pamir orocline and implies relatively small displacements on the karakorum fault our results also suggest that the karakax fault may have formed as an accommodation zone between the western kunlun shan and the karakorum mountains

refining gondwana and pangea palaeogeography estimates of phanerozoic non dipole octupole fields summary reliable phanerozoic paleopoles have been selected from the stable parts of the gondwana continents and upon appropriate reconstruction have been combined in an apparent polar wander apw path which can be compared with a previously compiled path for laurussia this comparison once again confirms that pangea a reconstructions for late palaeozoic and early mesozoic times cannot be reconciled with the available paleomagnetic data unless these data are corrected for latitudinal errors caused by non dipole octupole field contributions or by inclination shallowing because the discrepancies persist even when only paleopoles from igneous rocks are used inclination shallowing cannot be the sole cause of the problem there is an apparent decrease in the percentage of octupole field contributions needed as a function of time for mesozoic and younger time gondwana laurussia comparisons require on average lower ratios of octupole dipole fields than for palaeozoic time however the gondwana paleopoles for the palaeozoic include a much greater proportion of results derived from sedimentary rocks than do those for the mesozoic so that this apparently diminishing octupole field contribution may be an artefact we have also examined whether the clustering of coeval gondwana poles improves with optimal g3 contributions but found that while there are improvements they are not systematic and not statistically significant a combined apw path has been constructed for pangea for times since the mid carboniferous which accounts for octupole fields or equivalently inclination shallowing we argue that this global path is an improvement over previous constructions as it represents a self consistent plate tectonic model and does not violate widely accepted pangea a reconstructions

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

paleomagnetic evidence for cenozoic block rotations in the tadjik depression central asia this paper presents results of a paleomagnetic study of oligo miocene red beds of the tadjik depression in central asia we sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the pamirs samples were thermally demagnetized and high temperature components appear to predate folding of upper tertiary age throughout the depression paleomagnetic inclinations are consistent with those observed on the stable turan platform at the western margin of the depression however they are shallower by about 30 than the inclination predicted from the reference apparent polar wander path this appears to indicate a 23 difference in latitude which is incompatible with paleogeographic reconstructions for the tertiary a sound interpretation of this anomaly would require a better constrained tertiary paleomagnetic reference for asia inside the tadjik depression paleomagnetic declinations are all significantly rotated counterclockwise with respect to those measured on the turan platform the eastern part of the depression is a domain of large rotation 52 13 to 46 15 whereas smaller amounts of rotation have occurred in the western part 27 14 to 14 15 the similarity between tertiary and cretaceous data available for the area shows that rotations have occurred since the miocene little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression paleomagnetic and structural data suggest that block rotations in the tadjik depression are associated with indentation of the pamirs into stable asia at a larger scale observed rotations are compatible with a model of regional sinistral wrenching along a strip running from the gulf of oman to lake baikal

structural evolution of the bayanhongor region west central mongolia abstract most of previous models suggest that the central asia orogenic belt grew southward in the phanerozoic however in the bayanhongor region in west central mongolia volcanic arc accretionary prism ophiolite and passive margin complexes accreted northeastward away from the baydrag micro continent and hence the region constitutes the southwestern part of a crustal scale syntaxis close to the west the syntaxis should be original because presumably reorientation due to strike slip faulting can be ignored it is reconfirmed that the baydrag eventually collided with another micro continent the hangai to the northeast a thick sedimentary basin developed along the southern passive margin of the hangai micro continent this region is also characterized by an exhumed metamorphosed accretionary complex and a passive margin complex which are both bounded by detachment faults as well as basal reverse faults which formed simultaneously as extrusion wedges this part of the central asia orogenic belt lacks exhumed crystalline rocks as observed in the himalayas and other major collisional orogenic belts in addition we identified two phases of deformation which occurred at each phase of zonal accretion as d1 through cambrian and devonian and a synchronous phase of final micro continental collision of devonian as d2 the pre collisional ocean was wide enough to be characterized by a mid ocean ridge and ocean islands two different structural trends of d1 and d2 are observed in accretionary complexes formed to the southwest of the late cambrian mid ocean ridge that is the relative plate motions on both sides of the mid ocean ridge were different accretionary complexes and passive margin sediments to the northeast of the mid ocean ridge also experienced two periods of deformation but show the same structural trend unmetamorphosed cover sediments on the accretionary prism and on the hangai micro continent experienced only the d2 event due to micro continental collision these unmetamorphosed sediments form the hanging walls of the detachment faults moreover they were at least partly derived from an active volcanic arc formed at the margin of the baydrag micro continent

quantifying paleosecular variation insights from numerical dynamo simulations abstract thanks to advances in dynamo modelling it is now possible to produce numerical sequences of magnetic polarity reversals over periods of time equivalent to several tens of millions of years using a set of five numerical models integrated over the equivalent of 40 50 myr we generated synthetic data analogous to paleomagnetic data derived from volcanic flows we find that the distributions of directions are remarkably similar to those observed in the paleomagnetic database paleosecular variation among the five models is best discriminated by the relative variability in paleointensity e f and the relative dispersion of directions or poles as defined by the precision parameter k whether the geodynamo operated in different regimes in its past can be best tested with these parameters in combination roughly one million years of time with 200 time independent samples is required to achieve convergence of e f and k the quantities e f and k correlate well with the average chron duration chr which suggests that excursions and reversals are an integral part of paleosecular variation if applicable to the geodynamo the linear dependence of k on chr could help to predict chr for the earth during geologic times with no available reversal frequency data it also predicts much higher average k for directions during superchrons k 2500 for the cretaceous normal superchron than during actively reversing times k 35 for the last 80 myr as such high k values are not observed either this family of dynamo models is not applicable to the geodynamo or the geodynamo regime acting during superchrons is independent of that acting during times with frequent reversals

new early cretaceous paleomagnetic data from volcanic and red beds of the eastern qaidam block and its implications for tectonics of central asia abstract in order to better understand anomalously low paleomagnetic inclinations in the cretaceous rocks of central asia a combined geochronologic and paleomagnetic investigation has been performed on cretaceous basalt sequences intercalated with red beds from zeku area in the eastern qaidam block northeastern tibetan plateau potassium argon k ar dating indicates that the basalt sequences are of early cretaceous age 89 9 2 8 to 112 1 2 4 ma isothermal remanent magnetization irm thermal demagnetization of a three component irm and curie point experiment suggest that magnetite and hematite dominate in red beds magnetite in basalt flows stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization chrm from 7 lava flows and 12 red beds sites the chrms of red bed and basalt sites pass fold test and likely represent primary remanent magnetization the tilt corrected mean direction from red bed sites basalts plus baked red bed site is d s 18 4 i s 55 3 s 69 1 95 5 5 n 11 sites d s 16 1 i s 52 0 s 42 6 95 8 6 n 8 corresponding to a pole at 75 2 n 182 3 e with a 95 7 07 76 9 n 194 9 e with a 95 10 0 respectively the consistent inclination recorded in red beds and basalts implies no significant inclination shallowing caused by deposition compaction in red beds from the studied area compared with the early cretaceous poles from the north china block ncb or eurasia insignificant post early cretaceous northward motion may have occurred between the qaidam block and the ncb on the basis of the existing regional paleomagnetic data the qaidam block has not experienced wholescale vertical axis rotation since the early cretaceous with respect to ncb

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

paleomagnetism and tectonics of the southern tarim basin northwestern china we report late carboniferous permian and early tertiary paleomagnetic data from the southern tarim basin prefolding magnetizations were isolated in each case the late carboniferous permian and early tertiary poles lie at 64 6 n 166 5 e a95 6 3 and 58 1 n 202 0 e a95 12 7 respectively the late jurassic to early tertiary j3 e1 paleolatitudes of tarim and several basins throughout central asia are similar yet significantly 10 to 20 shallower than those predicted by the eurasian apparent polar wander path resolving this discrepancy remains a major problem in asian paleomagnetism discordance of the late paleozoic poles from tarim and siberia suggest that tarim has rotated about 30 counterclockwise with respect to siberia since the permian where paleomagnetic samples of both late carboniferous to early triassic c3 t1 and j3 e1 ages were collected from the same area of tarim a great circle passes through the means of the poles and the sampling locality this suggests that 1 only a difference in inclination and not declination distinguishes the two data sets and 2 vertical axis block rotations of the c3 e1 strata occurred after e1 although based on data of lesser quality the mean early to middle jurassic j1 2 pole from tarim differs significantly from the eurasian reference pole requiring radical tectonic solutions to resolve them the tarim j1 2 pole is indistinguishable from both the mean j3 e1 and c3 t1 poles the similarity of all the poles and the analogous tectonic setting of present day central asia to that of the late paleozoic in eastern north america raises the question whether all the data from tarim are overprinted

tectonic evolution of palaeozoic terranes in west junggar xinjiang nw china nine separate cambrian to carboniferous terranes recognized in west junggar northwest china they were amalgamated as part of the central asian orogenic belt which records accretion of continental island arc and oceanic terranes to archaean proterozoic continental nuclei tangbale kekesayi ebinur and mayila terranes cambrian silurian evolved in intra oceanic settings and docked along a series of north dipping subduction zones on to the laba terrane to their south this southern continent was contiguous with lithosphere of the kulumudi ocean to the north devonian subduction on the northern edge of this ocean resulted in formation of a continental arc toli terrane and accretionary complex kulumudi terrane the karamay terrane formed as an accretionary complex during the carboniferous the ophiolitic sartuohai terrane was emplaced as melange between kulumudi and karamay terranes during the late carboniferous subduction migrated southward continuing beneath these terranes resulting in the intrusion of i type granites into the toli kulumudi sartuohai and karamay terranes these granites are closely associated with epithermal and porphyry style gold mineralization composite terranes either side of the kulumudi ocean collided in the late carboniferous marking the final consolidation of central asia collision was accompanied by anorogenic granite and diabase dyke intrusion followed by widespread latest carboniferous to permian extension and subsequently the formation of the junggar basin west junggar has been further disrupted by cenozoic strike slip faulting along junggar and dalabute faults

noyon uul syncline southern mongolia lower mesozoic sedimentary record of the tectonic amalgamation of central asia to explore the hypothesis that a regionally extensive early mesozoic collisional foreland basin existed on strike to the east of documented mesozoic foreland basins in western china we conducted a reconnaissance study of a 5 km section of upper permian lower jurassic nonmarine strata exposed in the noyon uul syncline in southern mongolia this paper documents the depositional settings of these poorly known strata their provenance and their regional tectonic significance in the context of the mesozoic amalgamation of central asia we interpret noyon uul strata to be composed of 1 a coarse grained braided fluvial facies 2 a meandering fluvial facies and 3 a poorly oxygenated lacustrine facies that represent three principal depositional environments sandstone compositions are dominantly volcanic mean compositions qm 20 f 23 lt 57 qp 13 lv 82 ls lm 5 likely reflecting derivation from carboniferous and older volcanic arc sequences increased proportions of alkalic granite pebbles and cobbles above the first coarse grained braided fluvial unit and the first stratigraphic appearance of detrital k feldspar in basal coarse grained braided fluvial strata suggest unroofing of permian alkalic granites the noyon uul deposits likely are syntectonic as suggested by their thickness environments of deposition coarse grained character and accumulation rates overlap of the noyon uul syncline by relatively undeformed upper jurassic and lower cretaceous strata together with the presence of a regional unconformity between middle and upper jurassic rocks requires a phase of middle late jurassic deformation the erosional remnants of a middle upper jurassic north verging thrust belt have been documented just 100 km south of noyon uul suggesting that the noyon uul deposits may be foreland in origin specifically triassic thrusting has not been recognized in southern mongolia but permian jurassic contractile deformation associated with the collisional amalgamation of asia is documented or reasonably inferred for extensive areas of china to the south and west although sedimentary attributes of the noyon strata are most compatible with a foreland interpretation it is possible that the noyon deposits and their deformation may be related to strike slip faulting neither the attributes of the noyon deposits nor their ages are compatible with widespread late jurassic early cretaceous rifting that occurred across southeastern mongolia

evidence for fluid and melt generation in response to an asthenospheric upwelling beneath the hangai dome mongolia abstract the hangai dome mongolia is an unusual high elevation intra continental plateau characterized by dispersed low volume intraplate volcanism its subsurface structure and its origin remains unexplained due in part to a lack of high resolution geophysical data magnetotelluric data along a 610 km profile crossing the hangai dome were used to generate electrical resistivity models of the crust and upper mantle the crust is found to be unexpectedly heterogeneous the upper crust is highly resistive but contains several features interpreted as ancient fluid pathways and fault zones including the south hangai fault system and ophiolite belt that is revealed to be a major crustal boundary south of the hangai dome a clear transition in crustal properties is observed which reflects the rheological differences across accreted terranes the lower crust contains discrete zones of low resistivity material that indicate the presence of fluids and a weakened lower crust the upper mantle contains a large low resistivity zone that is consistent with the presence of partial melt within an asthenospheric upwelling believed to be driving intraplate volcanism and supporting uplift

zircon ages from the baydrag block and the bayankhongor ophiolite zone time constraints on late neoproterozoic to cambrian subduction and accretion related magmatism in central mongolia central mongolia represents a heterogeneous crustal domain of the central asian orogenic belt and is composed of contrasting lithotectonic units with distinct preorogenic histories we report single zircon evaporation and shrimp ages for high grade rocks of the neoarchean paleoproterozoic baydrag block and for metaigneous rocks of the junction between the late neoproterozoic bayankhongor ophiolite zone boz and the baydrag block zircon ages for metamorphic rocks of the baydrag block indicate a major tectonothermal event between 1840 and 1826 ma coeval with the emplacement of granitic rocks at middle crustal level dated at 1839 ma a granite gneiss yielded a much younger crystallization age of 1051 ma the first grenvillian age reported for this region together with predominantly mesoproterozoic detrital zircon ages for a quartzite lens from the burd gol accretionary complex these data attest to the heterogeneity and long precambrian history of the baydrag block crystallization ages for granite gneisses from the northeastern margin of the baydrag block indicate prolonged plutonic activity between 579 and 537 ma probably related to southward subduction of the bayankhongor oceanic crust a syntectonic granite vein yielded a crystallization age of 519 ma probably linked to accretion of the boz onto the northeastern active margin of the baydrag block lastly a felsic metavolcanic rock from the southeastern termination of the boz yielded a crystallization age of 472 ma and suggests that punctuated volcanic centers developed during the early ordovician in response to protracted convergence

paleomagnetism of miocene sediments from the turfan basin northwest china no significant vertical axis rotation during neotectonic compression within the tian shan range central asia abstract paleomagnetic results are reported from 377 samples of miocene red sediments in the turfan basin of east tian shan northwest china thermal demagnetization for most specimens isolates a low temperature component that groups around the present geomagnetic field a high temperature characteristic component is isolated from four localities and interpreted as primary remanent magnetization from apparent pre folding origin and positive reversal test as for previous results obtained from tertiary rocks in central asia the magnetic inclination is significantly shallower than the reference field computed from the eurasian apw path at 10 and 20 ma magnetic susceptibility measured in 324 specimens exhibits low anisotropy of less than 1 05 for 80 of the sample the low anisotropy degrees ams shape and orientations suggest that the studied rocks are unlikely to have undergone significant strain during compaction or tectonism we argue that the shallow inclination is likely to result from an initially shallow detrital remanent magnetization comparison of the paleomagnetic declination from eurasia north china and tarim suggests that the turfan basin has been subjected to little or no vertical axis rotation since the miocene together with paleomagnetic declinations on both sides of the talas ferghana fault we propose that the tian shan to the east of the talas ferghana fault has remained relatively stationary with respect to adjacent blocks since the miocene

inclination shallowing in eocene linzizong sedimentary rocks from southern tibet correction possible causes and implications for reconstructing the india asia collision a systematic bias towards low palaeomagnetic inclination recorded in clastic sediments that is inclination shallowing has been recognized and studied for decades identification understanding and correction of this inclination shallowing are critical for palaeogeographic reconstructions particularly those used in climatemodels and to date collisional events in convergent orogenic systems such as those surrounding the neotethys here we report palaeomagnetic inclinations from the sedimentary eocene upper linzizong group of southern tibet that are 20 lower than conformable underlying volcanic units at face value the palaeomagnetic results from these sedimentary rocks suggest the southern margin of asia was located 10 n which is inconsistent with recent reviews of the palaeolatitude of southern tibet we apply two different correction methods to estimate the magnitude of inclination shallowing independently from the volcanics the mean inclination is corrected from 20 5 to 40 0 within 95 per cent confidence limits between 33 1 and 49 5 by the elongation inclination e i correction method an anisotropy based inclination correction method steepens the mean inclination to 41 3 3 3 after a curve fitting determined particle anisotropy of 1 39 is applied these corrected inclinations are statistically indistinguishable from the well determined 40 3 4 5o mean inclination of the underlying volcanic rocks that provides an independent check on the validity of these correction methods our results show that inclination shallowing in sedimentary rocks can be corrected careful inspection of stratigraphic variations of rock magnetic properties and remanence anisotropy suggests shallowing was caused mainly by a combination of syn and post depositional processes such as particle imbrication and sedimentary compaction that vary in importance throughout the section palaeolatitudes calculated from palaeomagnetic directions from eocene sedimentary rocks of the upper linzizong group that have corrected for inclination shallowing are consistent with palaeolatitude history of the lhasa terrane and suggest that the india asia collision began at 20 n by 45 55 ma

morphometric analysis of predatory drillholes abstract quantitative studies of post jurassic predatory drillholes assume that drill holes were made by either naticid or muricid gastropods however other potential predators can leave similar traces and the traditional method of discrimination based on differences in vertical shape of drillholes cylindrical vs countersunk is not always applicable poor preservation or thin shells may prevent identification a multivariate morphometric approach including principal component analysis pca canonical variate analysis cva and size free discriminant analysis is suggested here as a supplementary method i test the approach by analyzing 98 drillholes made in the valves of the thin shelled bivalve mulinia coloradoensis collected from subfossil deposits in northeastern baja california using traditional criteria 10 10 2 of 98 drillholes were not correctly identified morphometric analyses indicate that in planar view muricid and naticid drillholes do not differ in shape however they perfectly discriminate in size the cylindrical drillholes oichnus simplex attributed to muricid predators are significantly smaller than countersunk ones oichnus paraboloides made most likely by naticid gastropods the results do not necessarily undermine the validity of the traditional ichnotaxonomy muricids tend to produce smaller drillholes than naticids nevertheless the vertical shape of a drillhole may be a function of both the type of predator and the size of hole drilled more than half of the unidentified drillholes 6 out of 10 from an independent group characterized by a more oval shape octopod drilling the traditional method supported by the morphometric methods employed here is a powerful tool for analysis of drillholes it may be valuable not only as a tool for analysis of muricid and naticid drillholes but also as a method for comparative analysis of drillholes of unknown source found throughout the phanerozoic fossil record

an integrated stratigraphic record from the paleocene of the chijiang basin jiangxi province china implications for mammalian turnover and asian block rotations new paleomagnetic and isotopic results from a similar to 1000 meter thick paleocene stratigraphic section in the chijiang basin of china s jiangxi province provide chromostratigraphic constraints on the shanghuan nongshanian asian land mammal age boundary and allow for a more accurate determination of an early paleogene paleomagnetic pole for the south china block paleomagnetic analysis of 121 sites 326 samples reveals that the shanghuan nongshanian boundary lies close to a normal to reverse polarity change stable carbon isotope analysis of dispersed organic matter and paleosol carbonates indicate a secular increase of similar to 1 5 superimposed on higher frequency variations correlation of this magnetochemostratigraphic pattern to the global timescale suggests that the polarity reversal near the shanghuan nongshanian boundary likely represents the chron c27n c26r transition the torrejonian tiffanian north american land mammal age boundary is closely correlated to this same polarity transition indicating nearly synchronous global mammalian turnover at this time because both land mammal age boundaries are thought to record turnover of mostly endemic taxa it is unlikely that the synchroneity of faunal change is due to intercontinental dispersal as documented for other early paleogene faunal transitions e g paleocene eocene boundary instead these mammalian faunal transitions may represent independent ecological or evolutionary responses to environmental changes that have been interpreted from marine records at this time e g top chron 27n event these new paleomagnetic results from the chijiang basin are also used to augment other published data and to calculate a paleocene paleomagnetic pole for the south china block the new pole shows no significant vertical axis rotation compared to the paleocene reference pole for eurasia indicating that much of the clockwise rotation that has been documented for the south china block from cretaceous deposits cannot be the result of extrusion tectonics associated with the early paleogene india asia collision observed cretaceous rotations of the south china block may be the result of late cretaceous early paleocene rifting in the backarc of the kula pacific subduction zone c 2008 elsevier b v all rights reserved

paleomagnetic constraints on the cenozoic kinematic evolution of the pamir plateau from the western kunlun shan foreland abstract thick cenozoic marine and terrestrial sediments are widely distributed along the perimeter of the pamir plateau and provide valuable information on the kinematic evolution of the region here we report new biostratigraphic and paleomagnetic results from the piedmont of the western kunlun shan to constrain the magnitude and timing of vertical axis rotations along the eastern margin of the pamir sampling sites were selected by rock formations and ages which are based on previous field mapping and on litholostratigraphic and biostratigraphic work presented here thermomagnetic analysis step wise thermal demagnetization behavior and positive field tests all suggest that the characteristic remanent magnetization chrm directions most probably have a primary detrital and chemical origin our results indicate variable minor but in some intervals significant vertical axis rotations with respect to a stable asian reference frame this pattern of rotations is similar to paleomagnetic data reported in previously published studies from the eastern pamir foreland in contrast published paleomagnetic data from the western pamir foreland consistently indicate significant ccw rotations within that region collectively these results challenge simple oroclinal bending models for the origin of the pamir salient and instead are more consistent with an asymmetric half orocline kinematic model in which the curvature of the western pamir is the product of a combination of lithospheric bending of an originally quasi linear mountain belt and radial thrusting and the subdued curvature of the eastern edge of the plateau is the result of lateral translation of the pamir plateau northward past tibet and tarim along the kashgar yecheng transfer system our results are consistent with activity on the kashgar yecheng transfer system in the early miocene

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

a new late cretaceous to present apwp for asia and its implications for paleomagnetic shallow inclinations in central asia and cenozoic eurasian plate deformation based on a compilation of 533 cretaceous to present day palaeomagnetic poles obtained from both sedimentary and igneous rocks we present a new analysis of the so called asian inclination anomaly and demonstrated the anomaly to be twofold a 2nd order anomaly characterized by high palaeolatitudes in indochina and low palaeolatitudes over tibet and central asia is superimposed on a 1st order anomaly characterized by cenozoic low palaeolatitudes found all over northeastern asian stable blocks the analysis herein convincingly shows that the europe apparent polar wandering path apwp can no longer be used to interpret palaeomagnetic data east of the urals including interpretation of asian tertiary deformation related to the india asia collision we thus construct a new apwp for east asia based on palaeopoles from blocks assumed to be stable this new apwp is consistent with and reinforces previous analyses of asian tectonics such as the age 55 ma and locus 5 10 n of the indo asian collision the lateral extrusion of se asian continental blocks and the intracontinental shortening in central asia possible origins of the 1st order palaeolatitude anomaly are 1 a geomagnetic origin due to long lasting non dipolar contribution to the magnetic field and 2 a tectonic hypothesis in which a newly defined east asia plate was located 10 farther south than expected from the current europe apwp based on a set of six new reconstructions from 90 ma to present we show that our tectonic model reconciles geophysical geological and tectonic observations throughout eurasia from siberia to europe including kinematics in the arctic ocean up to northwestern arctic alaska beyond possible occurrences of non dipolar field contribution and or local inclination flattening in the sedimentary data our model leads us to conclude that cenozoic tectonics is the dominant contributor to the observed 1st order 10 low palaeolatitude anomaly over asia during the tertiary

reconstructions of the continents around the north atlantic at about the 60th parallel abstract late carboniferous early tertiary apparent polar wander apw paths 300 40 ma for north america and europe have been tested in various reconstructions these paths demonstrate that the 500 fathom bullard et al fit is excellent from late carboniferous to late triassic times but the continental configuration in northern pangea changed systematically between the late triassic ca 214 ma and the mid jurassic ca 170 ma due to pre drift extension best fit north atlantic reconstructions minimize differences in the late carboniferous early jurassic and late cretaceous tertiary segments of the apw paths but an enigmatic difference exists in the paths for most of the jurassic whereas for the early cretaceous the data from europe are nearly non existent greenland s position is problematic in a bullard et al fit because of a late triassic early jurassic regime of compression 300 km that would be inherently required for the norwegian shelf and the barents sea but which is geologically not defensible we suggest a radically new fit for greenland in between europe and north america in the early mesozoic this fit keeps greenland locked to europe for the late paleozoic early mesozoic and maintains a reconstruction that better complies with the offshore geological history of the norwegian shelf and the barents sea pre drift a24 extension amounted to approximately 450 km on the mid norwegian shelf but with peak extension in the late cretaceous

anomalous low palaeomagnetic inclinations from oligocene lower miocene red beds of the south west tien shan central asia summary we studied oligocene to lower miocene red beds from the western margins of the south west ghissar range the westernmost part of the tien shan fold belt central asia a total of 19 sites were sampled on both limbs of a gentle syncline after removal of an unstable component at 200 to 300 c the natural remanent magnetization in these red beds from 18 sites is accounted for by one characteristic component chrm which persists up to 680 c intermediate directions of the geomagnetic field were recorded at five sites after tilt correction the overall mean direction for the remaining 13 sites is d 3 i 30 k 19 95 8 9 both the reversal and the fold tests are positive haematite is found to be the main remanence carrier in these red beds the calculated mean declination fits well with the reference data besse courtillot 1991 and no rotation with respect to eurasia is detected in accord with geological data in contrast the mean inclination is about 30 shallower than expected we argue that this result is not due to large scale northward transport of the area mean inclinations showing a similar amount of shallowing were also reported from the tadzhik and fergana basins and kirghiz tien shan from various rock types thomas et al 1993 1994 and we are of the opinion that neither inclination error nor compaction could have led to such a consistent pattern the analysis of all reliable early tertiary palaeomagnetic results extracted from the global paleomagnetic data base for the northern part of the eurasian plate reveals that inclination anomalies are very common in this data set whatever the rock type the general tendency is a progressive increase in the anomaly amplitude from the atlantic ocean to central asia the size and localization of this tertiary field anomaly is comparable with the present day non dipole field anomaly this suggests that there may have been a long term standing non dipole anomaly in the tertiary earth magnetic field perhaps correlated to major features of the contemporaneous plate tectonics for example the india asia collision

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

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

quantifying arabia eurasia convergence accommodated in the greater caucasus by paleomagnetic reconstruction since the late eocene convergence and subsequent collision between arabia and eurasia was accommodated both in the overriding eurasian plate forming the greater caucasus orogen and the iranian plateau and by subduction and accretion of the neotethys and arabian margin forming the east anatolian plateau and the zagros to quantify how much arabia eurasia convergence was accommodated in the greater caucasus region we here provide new paleomagnetic results from 97 volcanic sites 500 samples in the talysh mountains of nw iran that show 15 net clockwise rotation relative to eurasia since the eocene we apply a first order kinematic restoration of the northward convex orocline that formed to the south of the greater caucasus integrating our new data with previously published constraints on rotations of the eastern pontides and lesser caucasus this suggests that north of the talysh 120 km of convergence must have been accommodated north of the eastern pontides and lesser caucasus this is significantly more 200 280 km our reconstruction independently confirms previous caucasus convergence estimates moreover we show for the first time a sharp contrast of convergence between the lesser caucasus and the talysh this implies that the ancient paleozoic mesozoic transform plate boundary preserved between the iranian and east anatolian plateaus was likely reactivated as a right lateral transform fault since late eocene time

too low magnetic inclinations in central asia an indication of a long term tertiary non dipole field observed tertiary palaeolatitudes in central asia are more southerly by about 1600 km on average than those predicted from the eurasian reference palaeopoles subsequent northward displacements of the central asian terranes are unlikely to have been this large in this study we analyse to what extent non dipole fields especially octupole fields can explain this phenomenon a global zonal octupole field manifests itself in two ways 1 because the reference apwp is based mostly on results from the uk and north america its palaeopoles will be far sided as seen from the north atlantic but near sided as seen from eastern asia giving predicted palaeolatitudes that are too high 2 an octupole field contribution produces observed palaeolatitudes as calculated with the dipole formula for central asia that are too low both effects therefore increase the palaeolatitude anomalies in asia we find that an octupole dipole field ratio g3 of 0 06 or greater will reduce the palaeolatitude discrepancies significantly and is of the same magnitude as the g3 estimate of a recent analysis of early tertiary european and north american data

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

did a large departure from the geocentric axial dipole hypothesis occur during the eocene evidence from the magnetic polar wander path of eurasia abstract low paleolatitudes are very frequent in eocene to oligocene formations particularly in the tethyan mobile belt these anomalies and their possible sources are discussed they are neither due to unreliable results nor to inclination errors in sedimentary rocks neither can large tectonic movements explain these discrepancies the most probable causes having been rejected therefore the best explanation consists in an alternative magnetic pole position for eurasia located at 69 n 215 e it is proposed that the mean magnetic field of the earth was dipolar but that its axis has swung from one stable position to another during the middle tertiary and was therefore sometimes different from the rotation axis

cretaceous tertiary geodynamics a north atlantic exercise summary new reconstructions are presented for the cretaceous early tertiary north atlantic using a combination of palaeomagnetic hotspot and magnetic anomaly data we utilize these reconstructions in an analysis of previously described misfits between the north atlantic plate elements at successive intervals during this time period we are able to achieve reasonable overlap between the hotspot and palaeomagnetic reconstructions between 40 and 95 ma and thus are able to support the idea that the indo atlantic hotspots are relatively stationary small but systematic discrepancies for this time interval can readily be modelled with a long term octopole non dipole field contribution g3 g30 g10 0 08 however hotspot and palaeomagnetic reconstructions for the early cretaceous north atlantic show substantial differences that cannot be explained by constant non dipole fields and we favour an explanation for these discrepancies in terms of true polar wander tpw triggered by mantle instabilities between 125 and 95 ma this constitutes the only identifiable event of significant tpw since the early cretaceous taken in the context of available geochronological and geological data and seismic tomography from the region the 95 40 ma reconstructions and their time consequent geological products are interpreted in terms of specific conditions of mantle crust coupling and global plate motions tectonic activity highlights from these reconstructions show uniform ne movement of the coupled north american greenland and eurasian plates from 95 to 80 ma a marked cusp in the paths for all three elements at 80 ma where the three plates simultaneously change direction and follow a uniform nw directed motion until c 20 ma when eurasia diverges ne away from the still nw moving greenland and north american elements positioning of the iceland plume beneath the spreading ridge at 20 ma may have increased upwelling below the ridge increased the ridge push and caused a ne shift in the absolute direction of eurasia

paleomagnetism and u pb zircon geochronology of lower cretaceous lava flows from the western lhasa terrane new constraints on the india asia collision process and intracontinental deformation within asia to better understand the india asia collisional process and intracontinental deformation within asia a paleomagnetic study has been conducted on the qushenla formation lava flows dated at 132 120 ma from the yanhu area in the western lhasa terrane stepwise thermal demagnetization isolates stable characteristic remanent magnetizations which include dual polarity and pass fold tests at a 99 confidence level indicating primary magnetizations the tilt corrected site mean direction for 51 sites is d 28 2 i 34 5 and k 74 3 with 95 2 3 corresponding to a paleopole at 61 4 n 192 9 e a95 2 1 our new paleomagnetic data combined with previous cretaceous volcanic paleomagnetic data from the lhasa terrane show that the precollisional southern margin of asia was at 16 8 n with a relatively e w alignment and that the lhasa terrane did not experience significantly discrepant north south movement although local vertical axis rotations did take place after the cretaceous comparison with the apparent polar wander paths apwps of india and the cretaceous paleocene paleopoles of the tethyan himalaya shows that the india asia collision was at 54 3 ma and the greater india during the paleocene had a large northern extent of 2000 km 18 1 beyond its present northern margin comparison with the cretaceous eurasian apwp indicates that a latitudinal convergence of 1000 km has taken place between the lhasa terrane and eurasia since the india asia collision the amount of north south shortening deduced from cretaceous paleomagnetic data is consistent with that accommodated by the cenozoic fold and thrust belts between the lhasa terrane and the hexi corridor

resolving the problem of shallow magnetizations of tertiary age in asia insights from paleomagnetic data from the qiangtang kunlun and qaidam blocks tibet china and a new hypothesis we present new paleomagnetic results obtained at 39 sampling sites from five sections of tertiary red bed formations two eocene formations from the qiangtang block of tibet xialaxiu locality 32 8 n 96 6 e and the xining basin of qaidam xining locality 36 5 n 102 0 e and three neogene formations from the xining basin jungong locality 34 7 n 100 7 e and the kunlun block tuoluo lake and west yushu localities 35 3 n 98 6 e and 33 2 n 96 7 e respectively thermal demagnetization of the rocks isolated a high temperature component that we interpret as the primary magnetization in four localities the paleopoles lie at 52 6 n 352 e dp dm 6 0 10 7 for xialaxiu 61 6 n 211 3 e dp dm 9 7 16 1 for xining 66 0 n 228 6 e dp dm 3 6 6 9 for jungong and 53 9 n 205 4 e dp dm 5 6 10 0 for west yushu as in previous studies of tertiary formations from asia the inclinations we obtained are shallower by 18 to 26 than the magnetic field computed from the eurasian apparent polar wander path apwp at 10 and 20 ma for neogene rocks and at 40 and 60 ma for eocene rocks on the basis of a compilation of eocene data from the south china block tibet central asia and kyrgyzstan we conclude that this inclination anomaly reflects erroneous predictions of positions of the siberian craton when based on the apwp of eurasia the main reason for this discrepancy might be nonrigid behavior of the eurasian plate in the tertiary combination of this with intracontinental shortening of asia under the penetration of india provides a full explanation for the anomaly verification of this new interpretation of the inclination anomaly will require new geologic and paleomagnetic data from the northern parts of these remote regions in mongolia and siberia

precambrian crystalline basement in southern mongolia as revealed by shrimp zircon dating single zircon ages determined by ion microprobe shrimp ii for granitoid gneisses from the southern slope of the baga bogd massif gobi altai southern mongolia reveal several episodes of zircon growth ranging from late palaeoproterozoic to late cambrian the oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 11 ma and by a xenocrystic zircon from a dark grey augen gneiss yielding an age of c 1701 ma discrete igneous events are recorded in granite gneisses with protolith emplacement ages of 983 6 956 3 and 954 8 ma these ages provide the first record of early neoproterozoic magmatic activity in this region a much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 ma these late cambrian granitoids of calc alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the baga bogd precambrian crustal fragment was either docked against the southward present day coordinates growing margin of the caob or was a large enough crustal entity to develop an arc along its margin we speculate that the precambrian gneisses of this massif may be part of a crustal fragment rifted off the tarim craton

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

new tertiary paleomagnetic poles from mongolia and siberia at 40 30 20 and 13 ma clues on the inclination shallowing problem in central asia we report results of a paleomagnetic study of 490 cores from 59 sites corresponding to 52 distinct basaltic flows from mongolia and siberia khaton sudal 39 4 ma 44 5 n 101 4 e taatsyn gol 1 31 5 ma 45 4 n 101 3 e 2 28 0 ma 45 5 n 101 1 e ust bokson 19 9 ma 52 1 n 100 3 e and taatsyn gol 3 12 7 ma 45 5 n 101 0 e stepwise thermal and alternating field demagnetizations isolated a stable high temperature component htc of magnetization in most specimens which we interpret as the primary magnetization of these basaltic lava flows the four corresponding paleopoles appear consistent with coeval paleopoles from other asian effusive formations however except for the 12 7 ma paleopole the paleopoles are systematically far sided from the european apparent polar wander path apwp with respect to site locations corresponding to anomalously shallow inclinations in tertiary asian effusive formations in the hypothesis of a dipolar magnetic field in the tertiary this indicates a 1000 1500 km position of the siberia craton and amuria block farther south than expected at 40 and 30 ma tectonically this interpretation implies decoupling and relative rotations between the western and eastern parts of eurasia between the cretaceous and present we show that if siberia were located more to the south the 15 20 paleolatitude anomaly generally observed in sedimentary formations from central asia reduces to a more reasonable average value of 7 which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of asian blocks under the effect of ongoing penetration of india into eurasia in the tertiary

the oyut ulaan volcanic group stratigraphy magmatic evolution and timing of carboniferous arc development in se mongolia abstract the palaeozoic mesozoic tectonic evolution of central asia including the vast terrane collage that makes up mongolia has been a topic of considerable debate the oyut ulaan volcanic group is a sequence of volcanic and sedimentary rocks in se mongolia that forms the southern part of the devonian permian saykhandulaan inlier fieldwork traverses and mapping have established four distinct formations in the oyut ulaan volcanic group that record the nature of arc activity in part of the central asian orogenic belt during the carboniferous physical volcanological and sedimentological characteristics of the four formations suggest three clear eruptive styles 1 periodic andesite volcanism in an actively eroding arc setting that also contained large rivers and swamps 2 highly effusive plateau andesite volcanism 3 explosive rhyolitic effusion geochemical analyses of volcanic lithologies suggest that the group represents subduction related mature continental arc volcanism geochemical results document an evolving magma system to which surface processes of the volcano sedimentary model may be linked magma pulses and replenishments are identified from variations in chemostratigraphy newly obtained zircon ages from the volcanic succession fix its emplacement eruption at 323 0 0 7 ma mid carboniferous or late mississippian a granite cobble from the lower part of the oyut ulaan volcanic group gives a u pb zircon age of 338 9 0 4 ma indicating that arc plutons were emplaced 10 ma prior to the oyut ulaan volcanism and were eroded soon after our work provides timing constraints for final accretion and continental assembly in se mongolia and also sheds light on the petrological development of a magmatic arc system within an evolving accretionary orogen

paleomagnetism of eocene and miocene sediments from the qaidam basin implication for no integral rotation since the eocene and a rigid qaidam block the qaidam basin is the largest topographic depression inside the tibetan plateau and it is a key factor to understanding the cenozoic evolution of the northern tibetan plateau paleomagnetic data was obtained from the middle to late eocene xiaganchaigou formation and the early to middle miocene xiayoushashan formation from seven localities the paleomagnetic results indicate that the qaidam basin has not undergone obvious basin scale vertical axis rotation with respect to the eurasia plate since the eocene local clockwise rotation took place only at a few special locations along the northern margin of the qaidam basin the uniform paleomagnetic results at different localities support that the qaidam basin is a relatively rigid block regional paleomagnetic and geodetic observations also suggest that crust south of the kunlun fault moves eastward faster than crust north of the kunlun fault

lower paleozoic oceanic crust in mongolian caledonides sm nd isotope and trace element data mongolian ophiolites range in age from early proterozoic to middle paleozoic with maximum of ophiolite production in late riphean to early cambrian time one of the best examples is the bayan khongor complex which comprise serpentinized peridotite cumulate sequence sheeted dike unit and metavolcanics sm nd mineral isochron obtained from upper gabbro yields the age of 569 ma and initial nd ratio 11 9 chemical data for gabbros dikes and metavolcanics reveal their oceanic nature both isotopic and geochemical signatures of the bayan khongor ophiolite are consistent with its origin in the plume related spreading ridge segment of the paleoasiatic oceanic basin

position of the lhasa terrane prior to india asia collision derived from palaeomagnetic inclinations of 53 ma old dykes of the linzhou basin constraints on the age of collision and post collisional shortening within the tibetan plateau the lhasa terrane comprises the southern eurasian margin since at least mid cretaceous therefore paleomagnetic data from the lhasa terrane are crucial to reconstruct the pre collisional southern margin of eurasia previous paleomagnetic data from the cretaceous takena formation provide a rough constraint on the pre collisional paleolatitudinal position of the southern eurasian margin however paleogene paleomagnetic data from the lhasa terrane which can provide more direct constraints are very limited in addition reliability of the existing data is ambiguous because of a possible later overprint in our new study 17 sites were drilled from mafic dykes intruded in the t1 unit of the linzizong formation of the linzhou basin near lhasa for paleomagnetic studies figure 1 the dykes were ar 40 ar 39 dated to c 53 ma by yue and ding 2006 from ten sites a higher coercivity component demagnetized between 20 and 100 mt could be isolated detailed rockmagnetic analyses reveal ti rich titanomagnetite as the remanence carrier which suggests that the rock is not much altered and the remanent magnetization is likely of primary origin this is supported by a positive fold test the occurrence of normal and reverse polarity directions within single sites the subvolcanic character of the dykes and the within site scatter as well as the scatter between site mean directions indicate that paleosecular variation is recorded within sites tilt correction was performed using the bedding of overlying fluvial lacustrine sediments and tilt angles of ignimbrite columns postdating the dyke emplacement bedding corrected directions give an overall mean direction of d i 12 3 27 2 95 10 6 k 21 7 n 10 corresponding to a paleolatitude of 13 2 5 8 n at a reference location on the suture zone 20 n 90 e comparison with previous cretaceous data mainly from the takena formation yields a stable position of the lhasa terrane during cretaceous and early eocene this indicates that no large north south crustal shortening occurred along the southern eurasian margin between late cretaceous and early eocene the difference between expected paleolatitudes determined from the apwp of eurasia and observed paleolatitudes reveals relative northward movement of the lhasa block of ca 1847 763 km since early eocene this is attributed to indentation of india into asia and implies a considerable amount of north south crustal shortening two models of greater india were used to determine the extended northern margin of india ali and aitchison 2005 proposed a greater india based on bathymetric features in the eastern indian ocean and patzelt and others 1996 determined the paleolatitude of northern india using paleomagnetic investigations on paleocene sediments of the zongpu formation at gamba and duela intersection of the pre collisional southern eurasian margin and the northern margin of india yields a collision age of the indian and eurasian continents of c 5349 ma an initial collision within this time range is in accordance with previous studies based on paleomagnetic or geologic data e g patriat and achache 1984 klootwijk and others 1992 guillot and others 2003 a significantly later collision age at the eocene oligocene boundary as proposed by aitchison and others 2007 is not in agreement with our new and previous paleomagnetic data from the lhasa block