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

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

the magnetic fraction a tracer of deep water circulation in the north atlantic abstract we report on a mineral magnetic study of a series of holocene marine sedimentary sequences distributed from northern to southern bjorn and gardar drifts 56 n to 61 n and in the charlie gibbs fracture zone about 53 n all the cores are located at water depths bathed by the iceland scotland overflow water isow a precursor of the north atlantic ocean deep water formed in the nordic seas and flowing into northeast north atlantic after passing over the volcanic sills between iceland and scotland the magnetic fraction is composed of low ti content magnetites with a grain size distribution in the pseudo single domain range these grains originate from a unique source in the north through the erosion by the bottom waters of the icelandic basaltic province the magnetic grain size remains fairly constant in each core but on average progressively fines towards the south and downstream likewise the magnetic concentration and mean magnetic flux decrease towards the south the spatial evolution of the magnetic concentration and of the magnetic grain size suggests a transport of these magnetites by bottom currents associated with the overflow water downstream from the source with progressive deposition of this magnetic fraction along the path of the isow this study provides evidence that characterization of the magnetic parameters of sediments deposited in such an oceanographic context of a unique source upstream and a well defined water mass may be used to detect and trace deep oceanic circulation changes

biogeography of neogene calcareous nannofossils in the caribbean and the eastern equatorial pacific floral response to the emergence of the isthmus of panama abstract principal components analysis of nannofossil assemblages in five odp sites in the caribbean and the eastern equatorial pacific was used to monitor neogene surface water circulation changes in 15 83 10 71 ma floral distributions clearly show the existence of the circum tropical current between the caribbean and the eastern equatorial pacific and no surface water communication between the northern and southern caribbean in 10 71 9 36 ma the circum tropical current weakened and the northward intra caribbean current had been initiated northern and southern caribbean surface waters again became separated in 8 35 3 65 ma and the circum tropical current was regenerated after 2 76 ma the northward intra caribbean current has completely been established and the circum tropical current disappeared this suggests that a barrier to the circum tropical current had formed indicating the final closure of the isthmus of panama

cretaceous paleomagnetism of the fergana basin and adjacent ranges central asia tectonic implications abstract paleomagnetic results have been obtained from cretaceous red beds sampled at fifteen localities in the fergana basin and adjacent ranges of the tien shan central asia the natural remanent magnetization of these rocks was accounted for by an late cretaceous overprint of normal polarity at fourteen localities and dual polarity presumably primary component at seven localities no definite correlation was found between the declinations of the two components and structural trends thus implying no oroclinal bending of the arcuate structures in the area studied inclination data are difficult to interpret due to probable tectonic movements inclination errors of various types and ambiguities in the cretaceous reference data all locality mean declinations were found to be deflected westward with respect to the eurasian paleomeridian the proposed counter clockwise rotation of the fergana block during the late cenozoic is shown to account for the main features of the alpine tectonics in particular the 200 km offset along the talas fergana fault is connected with rotation which contradicts the common idea that this displacement is of late paleozoic age the horizontal movements are most probably not older than 10 ma

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

geological evidence against large scale pre holocene offsets along the karakoram fault implications for the limited extrusion of the tibetan plateau two end member models proposed to accommodate the convergence between india and asia north of the himalaya are 1 homogeneous crustal thickening of the tibetan plateau and 2 continental escape or extrusion of tibet and southeast asia away from the indenting indian plate foremost among the arguments supporting the latter would be large scale 1000 km offsets and high present day slip rates along the major strike slip faults bounding the postulated extruding crust notably the altyn tagh fault along the northern margin of tibet and the karakoram fault along the sw margin satellite photographic interpretation and field mapping in the karakoram mountains in pakistan the nubra siachen area of north ladakh and the pamirs in xinjiang show that although the karakoram fault is extremely active today geological offsets along the right lateral fault are probably less than 120 km the 21 0 5 ma baltoro monzogranite leucogranite batholith has been rotated clockwise about a vertical axis 35 40 into nw se alignment parallel with the karakoram fault east of the siachen glacier with a maximum offset of 90 km across the fault the bangong shyok suture zone similarly has a dextral offset of 85 km the course of the indus river which was antecedent to the rise of the ladakh karakoram and himalayan ranges has been offset dextrally by 120 km south of pangong lake if present day slip rates approximately 32 mm yr avouac and tapponnier 1993 are correct only 4 ma are required to obtain a 120 km offset there is no geological evidence for any larger scale pre holocene offsets and it is suggested that the karakoram fault cannot have accommodated major eastward lateral motion of tibetan crust the fault has also exerted little or no influence on surface topographic uplift cutting obliquely across the highest peaks of the karakoram dextral motion along the central part of the karakoram fault has been transferred in the north to the rangkul murghab and karasu transpressional faults in the central pamir north of tashkurgan the karakoram fault shows mainly normal motion around the kongur and mustagh ata gneiss domes metamorphic core complexes and the extensional muji graben minor dextral displacement has occurred along the shiquanhe fault but this motion cannot be linked to the jiale fault of east tibet the southern end of the karakoram fault merges with the indus yarlung suture zone near mount kailas and does not cut across the himalaya the lack of large scale geological offsets along the karakoram fault together with its very recent initiation 5 or 4 ma suggests that it is related to the pliocene quaternary northward indentation of the pamir and not to any long term extrusion of tibetan crust following the indian collision

neogene magnetic polarity stratigraphy of some fluviatile siwalik sections nepal summary the magnetostratigraphy of five new sections through the neogene siwalik group of nepal is presented rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary drm or a pdrm after stepwise thermal demagnetization of specimens from all sections directions of characteristic remanent magnetization were obtained the results show a positive reversal test and exhibit inclination errors of about 20d due to gaps in exposures and extremely variable demagnetization behaviour of the haematite bearing sediments the resulting polarity sequences are poorly defined for some parts of the sections however double sampled parts of some sections yield similar results a correlation with the surai khola section appel rosler corvinus 1991 is proposed based on rock magnetic parameters ams results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the surai khola section

paleomagnetism of cretaceous red beds from tadzhikistan and cenozoic deformation due to india eurasia collision abstract we have carried out structural and paleomagnetic studies in the tadzhik depression in order to evaluate the main features of the alpine tectonics of this area about 340 cores from 43 sites of lower cretaceous red beds were sampled from four different localities in the basin and adjacent ranges a well defined component of magnetization a of normal polarity with high unblocking temperatures up to 650 670 c was isolated from all the sites another component of magnetization b with unblocking temperatures between 650 and 680 c was isolated from only fifteen sites this component is bipolar the fold test is positive for both components we believe that component a was acquired during the cretaceous long interval of normal polarity comparison with eurasian reference data shows significant counterclockwise rotation of a locality close to the pamir wedge r 51 5 and another counterclockwise rotation from the inner part of the basin r 15 5 no significant rotations are observed at the two other localities on the periphery of the tadzhik basin

final closure of panama and the onset of northern hemisphere glaciation abstract the greenland ice sheet is accepted as a key factor controlling the quaternary glacial scenario however the origin and mechanisms of major arctic glaciation starting at 3 15 ma and culminating at 2 74 ma are still controversial for this phase of intense cooling ravelo et al 1 a c ravelo d h andreasen m lyle a o lyle m w wara regional climate shifts caused by gradual global cooling in the pliocene epoch nature 429 2004 263 267 proposed a complex gradual forcing mechanism in contrast our new submillennial scale paleoceanographic records from the pliocene north atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation nhg since it was linked to a 2 3 c surface water warming during warm stages from 2 95 to 2 82 ma these records support previous models g h haug r tiedemann effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation nature 393 1998 673 676 2 claiming that the final closure of the panama isthmus 3 0 2 5 ma j groeneveld s steph r tiedemann d nurnberg d garbe schonberg the final closure of the central american seaway geology in prep 3 induced an increased poleward salt and heat transport associated strengthening of north atlantic thermohaline circulation and in turn an intensified moisture supply to northern high latitudes resulted in the build up of nhg finally culminating in the great irreversible climate crash at marine isotope stage g6 2 74 ma in summary there was a two step threshold mechanism that marked the onset of nhg with glacial to interglacial cycles quasi persistent until today

paleomagnetic study of cenozoic sediments from the zaisan basin se kazakhstan and the chuya depression siberian altai tectonic implications for central asia abstract this paper presents new paleomagnetic results on cenozoic rocks from northern central asia eighteen sites were sampled in pliocene to miocene clays and sandy clays of the zaisan basin southeastern kazakhstan and 12 sites in the upper oligocene to pleistocene clays and sandstones of the chuya depression siberian altai thermal demagnetization of isothermal remanent magnetization irm showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the zaisan basin stepwise thermal demagnetization up to 640 660 c isolated a characteristic chrm component of either normal or reverse polarity at nine sites at two other sites the great circles convergence method yielded a definite direction measurements of the anisotropy of magnetic susceptibility showed that the hematite bearing sediments preserved their depositional fabric these results suggest a primary origin of the chrm and were substantiated by positive fold and reversal tests the mean paleomagnetic direction for the zaisan basin d 9 i 59 k 19 95 11 is close to the expected direction derived from the apw path of eurasia j geophys res 96 1991 4029 and shows that the basin did not rotated relative to stable asia during the tertiary in the upper pliocene pleistocene sandstones of the chuya depression a very stable chrm carried by hematite was found its mean direction d 9 i 46 k 25 95 7 is characterized by declination close to the one excepted for early quaternary whereas inclination is lower in the middle miocene to lower pliocene clays and sandstones a stable chrm of both normal and reverse polarities carried by magnetite was isolated its mean direction d 332 i 63 k 31 95 4 is deviated with respect to the reference direction and implies a neogene 39 8 counterclockwise rotation of the chuya depression relative to stable asia these results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style predominantly compressive in the zaisan basin and transpressive in the siberian altai at a larger scale the pattern of vertical axis rotations deduced from paleomagnetic data in northern central asia is consistent with the hypothesis of a large left lateral shear zone running from the pamirs to the baikal heterogeneous rotations however indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations

a paleomagnetic study of tertiary formations from the kyrgyz tien shan and its tectonic implications as part of a general investigation of cenozoic deformation in central asia we studied the paleomagnetism of tertiary red beds and lava flows from intermontane basins in the tien shan region of kyrgyzstan we collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them thermally or with alternating fields for most sites there are magnetic components with unblocking temperatures higher than 600 c we infer that the magnetic carriers are mainly hematite and magnetite for most localities the high temperature component appears to predate tectonic folding for all localities mean inclinations are shallower than expected from apparent polar wander paths inclination anomalies range from 16 5 for the issyk kul basin to 26 7 for the fergana basin if due to changes in latitude these anomalies imply at least 2000 km of northward displacement of the tien shan during the tertiary for which there is no tectonic evidence we thus consider that the paleomagnetic reference directions cannot be directly compared with our tertiary data we explored other possible reasons for this anomaly which has also been reported from other parts of the alpine belt but we could not find a satisfactory explanation absolute rotations cannot be accurately determined because of problems with the reference direction nevertheless the mean declination for the fergana basin lies counterclockwise by 20 11 from the mean declination of the issyk kul basin this result is consistent with the counterclockwise rotation inferred for the fergana basin from the pattern of cenozoic faults and folds it suggests a cenozoic right lateral displacement of 110 60 km on the talas fergana fault

enhanced circulation during a warm period the early pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today to characterize thermohaline circulation during this warm period 4 4 3 1 ma we combined new benthic foraminiferal isotopic data from the southeast atlantic with published data and reconstructed pacific and atlantic ocean nutrient distributions the data indicate enhanced ventilation of the deep atlantic and intermediate pacific during the early pliocene enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of north pacific and atlantic subtropical gyres was probably also enhanced future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation

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

late cenozoic transpression in southwestern mongolia and the gobi altai tien shan connection the gobi altai region of southwestern mongolia is a natural laboratory for studying processes of active transpressional intracontinental mountain building at different stages of development the region is structurally dominated by several major e w left lateral strike slip fault systems the north gobi altai fault system is a seismically active right stepping left lateral strike slip fault system that can be traced along the surface for over 350 km the eastern two thirds of the fault system ruptured during a major earthquake m 8 3 in 1957 whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the quaternary the highest mountains in the gobi altai are restraining bend uplifts along the length of the fault system detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross sectional geometries with thrust faults rooting into oblique slip and strike slip master faults continued ne directed convergence across the fault system coupled with left lateral strike slip displacements will lead to growth and coalescence of the restraining bends into a continuous sublinear range possibly obscuring the original strike slip fault system this may be a common mountain building process the largely unknown gobi tien shan fault system is a major left lateral strike slip fault system 1200 km long that links the southern ranges of the gobi altai with the barkol tagh and bogda shan of the easternmost tien shan in china active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section indicating quaternary activity the total displacement is unknown but small parallel splays have apparent offsets of 20 km suggesting that the main fault zone has experienced significantly more displacement field investigations conducted at two locations in southwestern mongolia indicate that late cenozoic transpressional uplift is still active along the fault system the spatial relationship between topography and active faults in the barkol tagh and bogda shan strongly suggests that these ranges are large coalescing restraining bends that have accommodated the fault s left lateral motion by thrusting oblique slip displacement and uplift thus from a mongolian perspective the easternmost tien shan formed where it is because it lies at the western termination zone of the gobi tien shan fault system the gobi tien shan fault system is one of the longest fault systems in central asia and together with the north gobi altai and other smaller subparallel fault systems is accommodating the eastward translation of south mongolia relative to the hangay dome and siberia these displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low e w trending corridor between the northern edge of the tibetan plateau and the hangay dome presumably in response to the indo eurasian collision 2500 km to the south

paleomagnetic study of cretaceous rocks from the yangtze block central yunnan china implications for the india asia collision abstract more than 200 samples for this study were collected at 30 sites from the cretaceous matoushan and puchanghe formations around the city of chuxiong 25 n 101 5 e twenty four sites have characteristic directions with a high temperature component above 350 c paleomagnetic directions from 21 sites of the cretaceous formations reveal clockwise deflection in declination d 44 6 i 41 3 and 95 10 7 the reliability of the direction is ascertained through a positive fold test and the presence of normal and reversed polarities comparison with paleomagnetic data from other areas indicates that the chuxiong area was rotated clockwise through26 17 with respect to the eastern part of the yangtze block whereas it was subjected to more than 44 of clockwise rotation with respect to the neighboring blocks of lhasa 56 21 and western sichuan 44 16 since cretaceous time the clockwise rotation of chuxiong is a reflection of rotational motion of the large region including the southeastern qiangtang terrane and the jinsha suture red river fault paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central yunnan province probably during the paleogene this large rotation is a spectacular example of the deformation in the southeastern part of the asian continent due to the collision of india

junggar turfan and alakol basins as late permian to early triassic extensional structures in a sinistral shear zone in the altaid orogenic collage central asia the junggar turfan and alakol basins in northwestern china and kazakhstan formed as late permian to early triassic extensional structures in a broad sinistral shear zone between large strike slip faults that separate two main domains of the altaid orogenic collage this extension was in response to an inferred large l000 km sinistral motion of the east european craton with respect to the angaran craton during this time deformation associated with the formation of the basins was taken up in part by counter clockwise rotations of crustal blocks with respect to the altaid orogenic collage and to the angaran craton this event is the only important phase of extension in a region otherwise dominated by compressional tectonics throughout the phanerozoic the basement rocks of these basins formed by altaid subduction accretion through the latter half of the palaeozoic and the region was subsequently thrown into compression again during the mesozoic cimmeride and cenozoic alpide evolution

chronology of greenland scotland ridge overflow what do we really know abstract as a sill constricting the exchange of deep water masses between the nordic seas and the north atlantic which forms an essential part of the atlantic meridional overturning circulation the dynamic height of the greenland scotland ridge and thus its overflow have an important influence on global climate several dsdp odp and iodp sites have been drilled in the north atlantic to shed light on the overflow and climate development reconstructions of bathymetry and sediment thickness have been put forward as well as calculations of the potential temperature of the conduit feeding the iceland plume the available studies have been screened to construct a conceptual model for the evolution of the palaeo circulation in the north atlantic and identify possible weaknesses in our knowledge details e g timing and location about the onset of the overflow are unknown and especially the paleogene development remains enigmatic the database for this period is inadequate and covers only small areas the discussion centres on the earliest traces of the overflow leading to formation of sediment drifts in the eastern north atlantic more data provide a better base to reconstruct variations for the neogene overflow but also appears insufficient for in depth analyses in time and space sediment drifts in the iceland basin indicate a first iceland faroe ridge overflow for the early miocene denmark strait overflow appears to have started in mid miocene times but evidence for this still is sparse grids of high resolution seismic reflection data across all sediment drifts and all limbs of the greenland scotland ridge combined with deeper drill sites targeting the complete sedimentary column down to basement are needed to fully understand the chronology of the greenland scotland ridge overflow and its detailed impact on climate

scanning squid microscopy of a ferromanganese crust from the northwestern pacific submillimeter scale magnetostratigraphy as a new tool for age determination and mapping of environmental magnetic parameters ferromanganese crusts record long term deep sea environmental changes thus providing their reliable high resolution age models is important we applied a magnetostratigraphic technique to estimate the growth rate of a ferromanganese crust using scanning squid superconducting quantum interference device microscope ssm ssm is designed to map the magnetic field across thins sections with sub millimeter resolution the crust sample was taken from the takuyo daigo seamount northwestern pacific and recorded a limited supply of dust and sediment from continents after drift correction and removal of spike noises the magnetic field values were stacked within the areas of high signal to noise ratios by correlating the obtained profiles with a standard geomagnetic polarity timescale we obtained an average growth rate of 3 37 0 06 mm m y which is consistent with that obtained by 10be 9be geochronology 2 93 0 15 mm m y s ratio mapping shows low values after 3 ma associated with voids between columnar structures

new cretaceous and early tertiary paleomagnetic results from xining lanzhou basin kunlun and qiangtang blocks china implications on the geodynamic evolution of asia we present the results of a paleomagnetic study of 360 cores drilled at three different areas from the tibetan plateau 13 sites from lower cretaceous red beds in the xining lanzhou basin around xining and lanzhou cities southwest of the qilian mountains 36 2 n 103 5 e 13 sites from cretaceous red beds in the kunlun block near maqin 34 5 n 100 1 and 9 sites from paleocene lower eocene red beds in the qiangtang block near fenghuoshan 34 5 n 92 8 e thermal demagnetization of the samples allowed us to isolate a high temperature component which passes both positive reversal and fold tests for all formations the corresponding paleopoles lie at 50 3 n 195 5 e a95 4 6 for xining lanzhou 80 1 n 281 2 e dp 7 8 dm 12 7 for maqin and 62 6 n 210 5 e dp 3 9 dm 6 8 for fenghuoshan we discuss these in the frame of a new paleogeographic reconstruction of the cretaceous paleoposition of the blocks forming the asian mosaic we conclude that the xining lanzhou area could not be part of the north china block but rather was associated with the tarim qaidam assemblage paleomagnetic data argue in favor of a qaidam kunlun tarim junggar assemblage in the cretaceous significantly to the south of its current position with respect to the asian continent siberia mongolia and north china the large n s convergence 800 500 km implied since the cretaceous appears to be far larger than could be absorbed in the altay range to the north and qilian shan to the east of the order of 300 km part of this motion could have occurred along a large left lateral strike slip fault system which may connect with the mongol okhotsk suture to the northeast verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions

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

mid pliocene warmth stronger greenhouse and stronger conveyor abstract three million years ago prior to the onset of northern hemisphere glaciation global mean temperatures may have been as much as 3 5 c warmer than at present we present evidence based on the carbon isotopic composition of marine organic matter that atmospheric co2 levels at this time were on average only about 35 higher than the preindustrial value of 280 ppm we also present carbon isotopic evidence for stronger thermohaline circulation in the atlantic ocean during the warmest intervals and propose that the north atlantic conveyor belt may act as a positive feedback to global warming by enhancing sea ice retreat and decreasing high latitude albedo based on our results it seems unlikely that the mid pliocene warm period was a doubled co2 world

millennial scale iceberg surges after intensification of northern hemisphere glaciation iceberg discharges from continental ice sheets are widely believed to have exerted a great influence on global climate but an iceberg discharge regime in early glacial periods after intensification of northern hemisphere glaciation nhg remains largely unclear here we present high resolution rock magnetic records during the period from 2 1 to 2 75 ma after intensification of nhg reconstructed from the subpolar north atlantic although the establishment of the middle pliocene chronology of north atlantic sediments is often a serious problem we overcame it based on findings concerning the properties of magnetic susceptibility and natural gamma radiation reconstructed rock magnetic records indicate that millennial scale iceberg surges were dominant features in the early glacial periods additionally the millennial scale iceberg surges occurred within glacial stages during intervals when ratios of global oxygen isotope stack from benthic foraminifera lr04 18o stack surpassed approximately 3 5 these are comparable to the climatic and environmental changes in pleistocene glacial periods as represented by last glacial dansgaard oeschger cycles suggesting that continental ice sheets have oscillated and calved icebergs in a similar manner since intensification of nhg

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

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

a paleomagnetic study of mesozoic sediments from the junggar and turfan basins northwestern china mesozoic sedimentary rocks have been collected from the karamay area northwestern junggar block and in the turfan basin northeastern tarim block for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central asia prior to the collision with india the results from the karamay area are disappointing lower cretaceous grey sandstones having apparently been recently remagnetized this remagnetization is carried by magnetite and yields highly clustered normal polarity directions both a medium mtc and a high htc unblocking temperature component are isolated from flat lying or weakly tilted jurassic red sandstones the mtc could be carried by maghemite and the htc is carried by hematite the two components are statistically identical and seem to correspond to an ancient magnetization however there is no fold test and comparison with previous data from junggar and the reference apwp of siberia does not allow us to decide between a middle jurassic primary magnetization and an upper jurassic or cretaceous remagnetization stronger constraints are obtained from upper jurassic to lower eocene fine grained red sandstones from anticlines that have been thrust southwards in the turfan area of south tien shan after the elimination of a recent overprint that is probably related to weathering a high coercivity high unblocking temperature component carried essentially by hematite is isolated all directions from the jurassic to the lower eocene which yield positive fold and reversal tests are statistically indistinguishable this is compatible with a primary origin for these magnetizations similar to what has previously been found in junggar tarim and western tibet the corresponding turfan pole is not statistically distinct from that of tarim for the same period in accordance with the notion of a large quasi rigid tarim block remaining attached to eurasia in a rather constant position and at a constant latitude throughout the second half of the mesozoic this new datum can therefore be included to derive a new mean pole for the tarim that is valid for the period from the middle jurassic to the end of the cretaceous 68 4 n 224 5 e a95 3 1

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

threshold behavior of millennial scale variability in deep water hydrography inferred from a 1 1 ma long record of sediment provenance at the southern gardar drift 1 integrated ocean drilling program iodp site u1314 of the north atlantic is a critical sedimentary archive record of subpolar deep water from the southern gardar drift for which we derived an age model of orbital resolution for the last 1 8 ma this chronology combined with high resolution cm scale x ray fluorescence core scanning measurements of major elements allows tracking changes in terrigenous provenance during the last 1 1 ma low potassium to titanium k ti ratios reflect enhanced transport of basalt derived titanomagnetites during warm climate intervals while high k ti ratios indicate a dominance of acidic sediment sources typical for glacial and stadial events changes in k ti and magnetic concentration at site 1314 are coeval with fluctuations in smectite content and grain size data from nearby piston cores suggesting that the provenance changes are mainly controlled by variable flow of the iceland scotland overflow water an important branch of north atlantic deep water furthermore k ti variations on orbital time scales show a striking similarity to the deep sea 13c record from odp site 607 pervasive features of the k ti time series during and after the mid pleistocene transition are suborbital changes similar to dansgaard oeschger and bond oscillations that appear to be strongly amplified during ice growth phases when global benthic 18o was within the range of 4 1 4 6 per mil the strong increase in variability of sediment provenance and subsequently deep hydrography at benthic 18o values below 4 1 suggests that the extent of glaciations and therefore sea level corresponding to this value constitutes an important physical threshold that was persistent at least for the last 1 1 ma

further palaeomagnetic data from chitral eastern hindukush evidence for an early india asia contact the eastern hindukush forms part of an elongate belt central domain collage of cimmerian microcontinents that encircles the northern part of the indian subcontinent a gondwanan origin is commonly assumed for this belt but a laurasian origin for the chitral region has been argued on palaeontological talent and mawson 1979 and palaeomagnetic klootwijk and conaghan 1979 grounds the laurasian view was based on a pilot study we undertook of upper devonian pisolitic ironstones from a thrust sheet at kur gh spur in chitral preliminary results showed a characteristic magnetization component d 318 i 6 5 n 7 block samples k 14 95 16 5 indicating an equatorial palaeoposition this component was thought to be of primary origin and was interpreted in terms of a late devonian laurasian affinity of the kur gh spur rocks this controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the resh n kur gh b ni region of chitral and the primary origin of the characteristic magnetization component refuted thermal demagnetization of 333 block samples from middle to upper devonian variegated sediments permian quartz flysch permo triassic carbonates and mid cretaceous redbeds showed two interpretable components a softer component of recent origin a and a harder characteristic component b of both normal and reverse polarity whose mean direction d 314 1 i 6 0 n 4 thrust sheets k 198 2 95 6 5 is comparable to the characteristic component observed in our preliminary study however the universal presence of this component throughout the thrust pile proves its overprint origin which we attribute to initial india asia contact palaeomagnetic information pertinent to the controversy of a laurasian versus a gondwanan origin of the chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt hence we retract herewith our original conclusion of a late devonian laurasian affinity of the chitral region on the basis of the palaeomagnetic evidence the secondary component b comprises a suite of secondary magnetizations acquired at equatorial to low northern palaeolatitudes and is attributed to initial contact between greater india and southern asia component b has been observed previously in the himalayan tibetan region both north and south of the indus tsangpo suture zone identification is herein extended to the hindukush region north of the northern kohistan or shyok suture zone which is a western continuation of the indus tsangpo suture comparison of this suite of collision at tributed equatorial palaeolatitude data from the india asia convergence zone with new palaeolatitude constraints from the ninetyeast ridge on the northward movement of the indian plate constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of greater india indicates that initial contact between northwestern greater india and southern asia was established at or before the cretaceous tertiary boundary the overprint origin of component b at about this time is further supported by observations by zeitler 1985 on rocks from the sampled area in chitral of partially reset zircon fission track ages around 68 55 ma the nw se declination axis of component b indicates a 60 70 counterclockwise rotation of the sampled thrust pile with respect to eurasia and a counterclockwise rotation between 10 and 30 with respect to india some of the recent field components a show a comparable rotation and indicate that the tectonic activity that led to the formation of the hindukush pamir karakorum syntaxial zone has continued into recent times

variability of silt grain size and planktonic foraminiferal preservation in plio pleistocene sediments from the western equatorial atlantic and caribbean abstract records of mean sortable silt and planktonic foraminiferal preservation from the ceara rise western equatorial atlantic and from the caribbean are presented to analyze the pliocene 3 5 2 2 ma to pleistocene 1 6 0 3 ma evolution of near bottom current strength and the carbonate corrosiveness of deep water during the mid pleistocene climate transition 1 ma a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered demonstrating a substantial decrease in the glacial contribution of the lower north atlantic deep water lnadw to the atlantic ocean also an increased sensitivity to eccentricity orbital forcing is registered after the mpt by contrast carbonate preservation increases considerably in the deep caribbean in response to a strong and persistent stable contribution of upper north atlantic deep water unadw we found evidence for the strongest and most stable circulation within the lnadw cell during the northern hemisphere cooling period between 3 2 and 2 75 ma this is in agreement with the superconveyor model which postulates that the highest nadw production took place prior to 2 7 ma a considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large scale continental ice sheets in the northern hemisphere this documents the final termination of the superconveyor at ca 2 75 ma however our data do not support a superconveyor in the interval between 3 5 and 3 2 ma when high amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed because of the great sensitivity of nadw production to changes in surface water salinity we assume that the high amplitude fluctuations of lnadw circulation prior to 3 2 ma are linked to changes in the atlantic salinity budget after 2 75 ma they are primarily controlled by ice sheet forcing in contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep atlantic a trend toward better preservation from the pliocene to pleistocene is observed in the deep caribbean this indicates a long term increase in the contribution of unadw to the atlantic ocean

strong and deep atlantic meridional overturning circulation during the last glacial cycle extreme abrupt northern hemisphere climate oscillations during the last glacial cycle 140 000 years ago to present were modulated by changes in ocean circulation and atmospheric forcing1 however the variability of the atlantic meridional overturning circulation amoc which has a role in controlling heat transport from low to high latitudes and in ocean co2 storage is still poorly constrained beyond the last glacial maximum2 3 4 here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle dominating ocean circulation in the atlantic whereas a shallower glacial mode with southern sourced waters filling the deep western north atlantic prevailed during glacial maxima3 5 our results are based on a reconstruction of both the strength and the direction of the amoc during the last glacial cycle from a highly resolved marine sedimentary record in the deep western north atlantic parallel measurements of two independent chemical water tracers the isotope ratios of 231pa 230th and 143nd 144nd 6 7 8 which are not directly affected by changes in the global cycle reveal consistent responses of the amoc during the last two glacial terminations any significant deviations from this configuration resulting in slowdowns of the amoc were restricted to centennial scale excursions during catastrophic iceberg discharges of the heinrich stadials severe and multicentennial weakening of north atlantic deep water formation occurred only during heinrich stadials close to glacial maxima with increased ice coverage probably as a result of increased fresh water input in contrast the amoc was relatively insensitive to submillennial meltwater pulses during warmer climate states and an active amoc prevailed during dansgaard oeschger interstadials greenland warm periods

late neogene history of deepwater ventilation in the southern ocean we compiled carbon isotope records from the north atlantic south atlantic and pacific oceans to estimate changes in vertical and interbasinal 13c gradients for the last 9 myr benthic 13c values of deep water in the south atlantic decreased in a series of steps at 7 2 75 and 1 55 ma away from the north atlantic and toward the pacific the benthic 13c of intermediate water in the south atlantic evolved differently from deep waters resulting in an increase in the intermediate to deep 13c gradient 13cid the 13cid increased in steps at 2 75 and 1 55 ma marking the development and intensification of a chemical divide in the atlantic ocean between well ventilated intermediate water and poorly ventilated deep water we suggest these changes in interbasinal and vertical gradients resulted from decreasing northern component water and reduced ventilation of southern component water scw the latter was caused by several interrelated processes in antarctic sources areas including increased sea ice cover enhanced surface water stratification decreased ekman induced upwelling and reduced vertical mixing across the thermocline because antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric co2 concentration these processes may have acted as a positive feedback that contributed to the major cooling steps in late neogene climate

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

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

abrupt intensification of north atlantic deep water formation at the nordic seas during the late pliocene climate transition knowledge of the evolution of north atlantic deep water nadw is key to understanding the past evolution of the climatic system here we developed a new rock magnetic method to determine the constituent magnetic minerals of sediments and report on the evolution of nadw during 2 2 2 9 ma we measured isothermal remanence acquisition curves of north atlantic deep sea sediments drilled at the gardar drift and decomposed the first derivatives of these curves into high coercivity and low coercivity components residuals of the decomposition were sufficiently small throughout the study interval confirming that the gardar drift sediments represent a mixing of the two end members fractional changes of the high coercivity component represent variation of the iceland scotland overflow water a branch of nadw formed at the nordic seas the high coercivity component increased significantly during an interglacial period just after 2 68 ma which suggests that nadw formation in the nordic seas abruptly intensified at this time

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

a new species of polacanthus ornithischia ankylosauria from the lower cretaceous of sussex england the first specimen of the ankylosaur genus polacanthus from the mainland barremian of southeast england is described as polacanthus rudgwickensis sp nov polacanthus rudgwickensis is larger than polacanthus foxii and there are significant differences in the dermal armour the tibia and caudal vertebrae of the two species polacanthus foxii appears to be restricted at present to the isle of wight with one specimen from southwest england whilst polacanthus rudgwickensis is only known from sussex this geographical distribution the palaeobiological implications and possible sexual dimorphism are discussed for this genus

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

field examples of strike slip fault terminations in mongolia and their tectonic significance deformation at the ends of large intracontinental strike slip faults that do not simply link other major structures often involves rotations about a vertical axis we use earthquake slip vectors surface rupture in earthquakes and geomorphology to examine the ends of three major strike slip faults in mongolia in these places a simple pattern is seen consisting of a thrust fault on one side with a displacement that decreases away from the strike slip fault consistent with local rotational deformation strike slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area for example from dominantly strike slip to dominantly dip slip while still accommodating the overall deformation required by larger scale regional motions such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis which may be detected paleomagnetically the kind of strike slip fault termination described here may have consequences for how large strike slip fxaults evolve and grow and for the variation in displacement along their length