neogene rotations in the jiuquan basin hexi corridor china vertical axis rotations of blocks in around the tibetan plateau can be attributed to the india asia collision study of the vertical axis rotations of these blocks will increase our understanding of the mechanisms and kinematics of continent continent collisions we report here a new palaeomagnetic study of rotations using data from four localities five magnetostratigraphy sections in the jiuquan basin our study indicates that the mean declinations of each section are different from each other similar to what has been observed in the other localities in the ne tibetan plateau however using the mean directions of every 100 m of section we observe that the four localities have similar sequential patterns of rotations during the last 13 ma significant continuous counterclockwise before c 8 0 ma insignificant rotations between 8 0 4 0 ma and slight clockwise rotation after 4 0 ma this indicates that rather than being a record of spatially varying declinations it is a temporal variation in the occurrence of regional rotations combined with other geological evidence the rotation patterns may suggest two major tectonic activity phases of the northeastern tibetan plateau during the last 13 ma an eastward extrusion and strike slip dominant phase before 8 0 ma a significant shortening and a rapid uplift dominant phase after 8 0 ma

e i corrected paleolatitudes for the sedimentary rocks of the baja british columbia hypothesis paleomagnetic inclinations from sediments of the western terranes of canada are consistently too shallow for their reconstructed paleogeographic positions two contradicting explanations for these discrepancies are 1 terranes have been displaced northward with respect to the stable american craton by several thousands of kilometres between the late cretaceous 75 ma and the eocene 50 ma and 2 sedimentary inclination error has caused a shallow bias in the paleomagnetic directions here we apply the elongation inclination e i method to paleomagnetic data sets from sedimentary rocks of supposedly allochtonous terranes of western north america to correct for inclination flattening our results indicate that the paleomagnetic directions from the continental silverquick sediments 95 92 ma of southern british colombia are not seriously affected by inclination error because the magnetic signal most likely concerns a chemical remanent magnetisation crm in contrast the marine sediments of the nanaimo group 84 72 ma of vancouver island region appear seriously affected by inclination flattening f 0 7 and the e i corrected mean inclinations are about 98 steeper than the original data we arrive at corrected inclinations paleolatitudes of i 578 k 388n for the silverquick and i 558 k 368n for the nanaimo sediments our corrected paleolatitudes indicate that the canadian terranes were indeed located adjacent to the baja californian margin during the late cretaceous thus supporting the baja bc hypothesis d 2005 elsevier b v all rights reserved

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

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

the time averaged paleomagnetic field we review indications of persistent deviations from the geocentric axial dipole model of the time averaged geomagnetic field and present a zonal harmonic model derived from 185 deep sea sediment piston cores taken from low to middle latitudes to approximately 45 analysis of the paleomagnetic inclination recorded in these cores for the brunhes normal polarity 0 73 ma and matuyama reverse polarity 0 73 2 47 ma chrons after plate motion correction gives well constrained estimates of the dominant long term nondipole contributions the axial quadrupole and axial octupole and shows no significant deviation from axial symmetry the amplitude of the axial quadrupole is found to vary with polarity 2 6 of the geocentric axial dipole for normal 4 6 for reverse while the axial octupole does not show appreciable change 2 9 for normal 2 1 for reverse these estimates of the quadrupole contribution agree well with prior determinations for the plio pleistocene 0 5 ma however the octupole contribution we find is opposite in sign to previous estimates we suggest that a negative octupole is representative of the actual time averaged paleomagnetic field while prior positive octupole estimates probably reflect spurious inclination shallowing the lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact

a millennium of geomagnetism 1 the history of geomagnetism began around the year 1000 with the discovery in china of the magnetic compass methodical studies of the earth s field started in 1600 with william gilbert s de magnete gilbert 1600 and continued with the work of among others edmond halley charles augustin de coulomb carl friedrich gauss and edward sabine the discovery of electromagnetism by hans christian oersted and andre marie ampere led michael faraday to the notion of fluid dynamos and the observation of sunspot magnetism by george ellery hale led sir joseph larmor in 1919 to the idea that such dynamos could sustain themselves naturally in convecting conducting fluids from that came modern dynamo theory of both the solar and terrestrial magnetic fields paleomagnetic studies revealed that the earth s dipole had undergone reversals in the distant past and these became the critical evidence in establishing plate tectonics finally the recent availability of scientific spacecraft has demonstrated the intricacy of the earth s distant magnetic field as well as the existence of magnetic fields associated with other planets and with satellites in our solar system

paleomagnetism of early paleogene marine sediments in southern tibet china implications to onset of the india asia collision and size of greater india article i nfo we report a paleomagnetic study of paleocene marine sediments in the gamba area of the tethyan himalayan terrane southern tibet which aims to accurately locate the position of the northern margin of greater india and further constrain timing of initial contact between india and asia following detailed rock magnetic and paleomagnetic experiments on a total of 675 drill core samples collected from the zongpu and upper jidula formations characteristic remanent magnetizations chrms were successfully isolated from most samples following alternating field af or integrated thermal and af demagnetization the chrms are of dual polarity and pass fold and reversal tests indicating a pre folding origin together with detailed biostratigraphic investigation of the sampling section a magnetic polarity sequence is constructed from data at 167 sampled horizons and satisfactorily correlates with polarity chrons c24r to c27r dating the zongpu formation between 56 2 and 61 8 ma on the geomagnetic polarity time scale the chrm directions from the zongpu formation are grouped stratigraphically into 33 sites and yield two paleopoles of 71 6 n 277 8 e a95 2 5 and 67 3 n 266 3 e a95 3 5 for the time intervals comprising 56 to 59 ma and 59 to 62 ma respectively these new paleopoles imply that the tethyan himalayan terrane was sited at low latitudes in the northern hemisphere during the interval 62 56 ma suggestingthatinitial contact between thetethyanhimalayaand lhasa terranes was established before60 5 1 5 ma and verylikelyoccurred nearthecretaceous tertiary boundary at least in the central partof the suturezone the results alsoindicate that atleast 1500 480 kmof post collisionalcrustal shortening occurred within the himalayas from the analysis of available paleomagnetic data obtained on both sides of the suture zone we propose a conceptional collision model for the india asia collision

compaction corrected inclinations from southern california cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the peninsular ranges terrane paleomagnetic data have been used extensively to delineate the terrane displacement and accretion history of the western margin of north america however the anomalously shallow paleomagnetic inclinations used to indicate large scale northward translation might be alternatively interpreted as due to postmagnetization tilting of batholithic rocks and compaction of marine sediments to understand the magnitude of burial compaction effects on the post cretaceous motion history of the peninsular ranges baja borderlands terrane a rock magnetic compaction and paleomagnetic study of the ladd formation and the point loma formation from southern california was conducted the anhysteretic anisotropy of remanence of the characteristic remanence carrying grains and individual magnetic grain anisotropy were used to correct the inclinations of each formation individual magnetic grain anisotropy was determined by both compaction experiments and redeposition of a magnetic separate in a dc magnetic field standard paleomagnetic studies of the units indicated that previous ladd formation results could be reproduced and a correction was made at the sample level we were unable to adequately reproduce earlier results of the point loma formation so the average remanence anisotropy was used to correct the previously reported mean direction the mean inclination of the ladd formation was conected from 46 95 8 to 58 95 4 and the mean inclination of the point loma formation was conected from 39 5 5 4 normal and 36 4 16 6 reversed to 56 0 5 1 normal and 53 0 16 7 reversed these results suggest that the peninsular ranges baja borderland terrane has been part of the western north america since the late cretaceous and that clay containing sedimentary rocks may typically experience from 10 to 15 of inclination shallowing due to burial compaction

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

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

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

non antipodal directions in magnetostratigraphy an overprint bias s u m m a r y paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal as one would expect for an axial geocentric dipole model specifically in magnetostratigraphic studies of tertiary sediments which include large data sets of paleomagnetic directions of both polarities it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions we investigate the conventional explanation for such asymmetry which involves the presence of a persistent partially unremoved present day field magnetization in the studied rocks both plate motion history and inclination shallowing which we evaluate in detail can play an important role in the paleomagnetic record our analysis shows that alternatively it is plausible that the inclination anomaly is due to a contribution of an axial octupole in accordance with observations of the recent 0 5 ma geomagnetic field recorded in lava flows

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

processes governing arsenic retardation on pleistocene sediments adsorption experiments and model based analysis in many countries of south south east asia reliance on pleistocene aquifers for the supply of low arsenic groundwater has created the risk of inducing migration of high arsenic groundwater from adjacent holocene aquifers adsorption of arsenic onto mineral surfaces of pleistocene sediments is an effective attenuation mechanism however little is known about the sorption under anoxic conditions in particular the behavior of arsenite we report the results of anoxic batch experiments investigating arsenite 1 25 mol l adsorption onto pleistocene sediments under a range of field relevant conditions the sorption of arsenite was non linear and decreased with increasing phosphate concentrations 3 60 mol l while ph range 6 8 had no effect on total arsenic sorption to simulate the sorption experiments we developed surface complexation models of varying complexity the simulated concentrations of arsenite arsenate and phosphate were in good agreement for the isotherm and phosphate experiments while secondary geochemical processes affected the ph experiments for the latter the model based analysis suggests that the formation of solution complexes between organic buffers and mn ii ions promoted the oxidation of arsenite involving naturally occurring mn oxides upscaling the batch experiment model to a reactive transport model for pleistocene aquifers demonstrates strong arsenic retardation and could have useful implications in the management of arsenic free pleistocene aquifers

a compaction correction for the paleomagnetism of the nanaimo group sedimentary rocks implications for the baja british columbia hypothesis 1 a paleomagnetic and magnetic anisotropy study of the late cretaceous northumberland formation on hornby island british columbia was conducted to determine if burial compaction could have caused its anomalously shallow inclinations the shallow nanaimo group inclinations have been used to support the baja british columbia baja bc model of continental dynamics in which superterranes were transported thousands of kilometers northward along the active north american continental margin in the cretaceous a mean of the site means from the northumberland formation is d 5 8 i 50 9 95 7 9 n 8 anisotropy of magnetic susceptibility ams and anisotropy of anhysteretic remanence aar were measured to identify and correct for any inclination shallowing caused by depositional compactional processes both concretion and fine grained rock samples had typical depositional compactional ams and aar fabrics with bedding perpendicular minimum principal axes a revised correction equation was used to account for the effects of either triaxial magnetic fabrics or small angular deviations of principal axes from the bedding plane the average inclination was increased by 9 6 after the compaction correction d 6 1 i 60 5 95 7 1 n 8 and the mean inclination of concretion and fine grained rock sites was significantly steeper than ward et al s 1997 result i 42 5 placing hornby island and the insular superterrane at a paleolatitude of 41 n in the late cretaceous this paleolatitude indicates that baja bc did translate northward since the late cretaceous by about 1600 km but not by the 3500 km previously indicated

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

compaction induced inclination shallowing of the post depositional remanent magnetization in a synthetic sediment summary a synthetic sediment comprised of kaolinite distilled water and either equidimensional or acicular magnetite was given a post depositional remanent magnetization pdrm by being stirred in a magnetic field this sediment was compacted under pressures which varied continuously from 0 to 0 14 mpa in a water tank consolidometer and to higher pressure steps 2 53 mpa in a standard soil consolidometer compaction took place in the same magnetic field in which the sample was given its pdrm the compaction caused shallowing of the sample s magnetic inclination this shallowing was found to be a function of the sample s initial magnetic inclination and the degree of sample compaction tan i 1 aav tan lo where ir is the remanent inclination after compaction av is the volume change i is the initial magnetic inclination and a is an empirically derived constant the data show a maximum inclination shallowing of 12 for an initial inclination of 54 in good agreement with the maximum inclination shallowing predicted by the above equation we propose an electrostatic mechanism to be the cause of the inclination shallowing in this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain s surface as the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post compaction nrms implying that the smaller higher coercivity grains are most affected by the compaction process these data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected

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

on the shape of paleomagnetic data sets a quick and easy method to analyze the shape of paleomagnetic data sets is described and illustrated the elongation e of a set of directions of virtual geomagnetic poles vgp is defined as e k2 k3 where k2 and k3 are the second and third eigenvalues of the bingham distribution it is shown that e 3 or so indicates that the distribution is significantly elongate although the size of the sample and the paleolatitude both affect this conclusion because the azimuth of k2 gives the direction of elongation it can be used to help determine why a particular distribution is elongate it is shown that for most units from the craton both directions and vgp are nearly circular except for units magnetized at very low latitudes in general poles are more circular than directions some units formed in plate margin locations also are circular but many are highly elongate many units from the western cordillera of north america have highly elongate distributions that are compatible with origin within a shear zone others are obviously underformed internally units formed during extension are less commonly elongate but some show the probably effects of normal faulting it is hoped that shape analysis will be part of any complete paleomagnetic study in the future

plate motions and the geomagnetic field i quaternary and late tertiary summary we have tested the hypothesis that the apparent increase in the northward offset of the axial dipole i e the ratio g gy with age for the last 25 ma wilson mcelhinny is due to the failure to correct for plate motions spherical harmonic analyses were performed on two types of data palaeomagnetic poles from continents islands and seamounts and magnetic inclinations from deep sea cores after returning the sampling sites to their predrift locations in fixed hotspots coordinates the results show that the quadrupole term g maintained a value of about 0 05 gy throughout the last 35 ma and that the axial octupole had a small value of about 0 02 gy for the last 5 ma sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years independent data sets for the past 2 ma and for the period 2 6 ma gave nearly identical solutions showing that the persistence of the axial terms is not a phenomenon of the more densely sampled quaternary field alone nor is it an artifice of predominantly northward motion of the plates returning all 0 5 ma data to a 5 ma reconstruction failed to eliminate the quadrupole the non zonal coefficients for the 0 5 ma field are typically an order of magnitude less than the zonal terms with the exception of h and are probably insignificant suggesting that longitudinal drift is generally effective in averaging out these components the present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 ma but there is evidence that g may have changed little during the last 35 ma in addition there has been very little relative motion between the palaeomagnetic dipole axis as defined by the first degree terms and the axis of the hotspots reference frame

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

paleomagnetism and dating of a thick lava pile in the permian bakaly formation of eastern kazakhstan regularities and singularities of the paleomagnetic record in thick lava series abstract paleomagnetic results on thick lava series are among the most important sources of information on the characteristics of ancient geomagnetic fields most paleo secular variation data from lavas psvl are of late cenozoic age there are far fewer results from lavas older than 5 ma the central asia orogenic belt that occupies several million square kilometers in asia is probably the world s largest area of paleozoic volcanism and is thus an attractive target for psvl studies we studied a ca 1700 m thick lava pile in eastern kazakhstan of early permian age magmatic zircons successfully separated from an acid flow in this predominantly basaltic sequence yielded an early permian age of 286 3 3 5 ma oriented samples were collected from 125 flows resulting in 88 acceptable quality flow means n 4 samples radius of confidence circle 95 15 of the high temperature magnetization component the uniformly reversed component is pre tilting and arguably of a primary origin the overall mean direction has a declination 242 0 and an inclination 56 2 k 71 5 95 1 8 n 88 sites pole at 44 1 n 160 6 e a95 2 2 our pole agrees well with the early permian reference data for baltica in accord with the radiometric age of the lava pile and geological views on evolution of the western part of the central asia orogenic belt the new early permian result indicates a comparatively low level of secular variation especially when compared to psvl data from intervals with frequent reversals still the overall scatter of dispersion estimates that are used as proxies for sv magnitudes elongation values and elongation orientations for psvl data is high and cannot be fitted into any particular field model with fixed parameters both observed values and numerical simulations indicate that the main cause for the scatter of form parameters elongation values and elongation orientations is the too small size of collections dispersion estimates concentration parameter and standard angular deviation are more robust and their scatter stems from other sources which may include non stochastic features of datasets like clusters loops etc or non stationary behavior of secular variation magnitude over time intervals of many million years

a quasi linear structure of the southern margin of eurasia prior to the india asia collision first paleomagnetic constraints from upper cretaceous volcanic rocks near the western syntaxis of tibet we report the first combined geochronologic and paleomagnetic study of volcanic rocks from the shiquanhe and yare basins at the westernmost lhasa terrane which aims to provide an accurate constraint on the shape and paleoposition of the southern margin of asia prior to the india asia collision three new 40ar 39ar ages of 92 5 2 9 ma 92 4 0 9 ma and 79 6 0 7 ma determined by fresh matrix or feldspar from lava flows suggest a late cretaceous age for the investigated units characteristic remanent magnetizations have been successfully isolated from 38 sites which pass positive fold and or reversal conglomerate tests and are hence interpreted as primary in origin the two paleopoles obtained from yare and shiquanhe yield consistent paleolatitudes of 13 6 n 9 6 n and 14 2 n 2 7 n respectively for a reference site of 31 5 n 80 e indicating that the southern margin of asia near the western syntaxis was located far south during the late cretaceous time a reconstruction of the lhasa terrane in the frame of eurasia with paleomagnetic data obtained from its western and eastern parts indicates that the southern margin of eurasia probably had a quasi linear orientation prior to the collision formerly trending approximately 315 e this is compatible with the shape of the neo tethys slab observed from seismic tomographic studies our findings provide a solid basis for evaluating cenozoic crustal shortening in the asian interior and the size of greater india near the western syntaxis

paleomagnetic results from the early cretaceous zenong group volcanic rocks cuoqin tibet and their paleogeographic implications article i nfo article history a paleomagnetic study has been carried out on the early cretaceous zenong group volcanic rocks ca 110 130 ma near cuoqin town in the central lhasa terrane stepwise thermal demagnetization up to 680 c isolated a stable high temperature component htc including antipodal dual polarities the htc directions passed both a fold test at 99 confidence and a reversal test at 95 confidence suggesting a primary origin the tilt corrected average direction for 18 sites is d 327 0 i 35 7 59 3 95 4 5 which corresponds to a paleopole at 58 2 n 341 9 e a95 4 6 yielding a paleolatitude of 19 8 4 6 n for the study area our results combined with previous paleomagnetic data suggest that southern tibet was at a mean paleolatitude of 20 n during the cretaceous and paleogene when compared with the apparent polar wander paths of asia the difference suggests that only ca 870 km north south crustal shortening has accumulated between the lhasa terrane and stable asia since ca 110 ma when comparing the paleolatitudes of the lhasa terrane and india the results indicate that the width of the neotethys was 6720 690 km during the early cretaceous

paleomagnetism of eocene red beds in the eastern part of the qiangtang terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the tibetan plateau abstract a primary magnetic component with the acquisition time of 56 0 43 2 ma was isolated between 580 c and 685 c from the eocene gonjo and ranmugou formations in the eastern part of the qiangtang terrane tibetan plateau the tilt corrected site mean direction is d s 35 5 i s 29 3 k 45 9 95 3 2 the site mean inclination increased from 29 3 to 41 6 after multiple inclination shallowing corrections giving a paleopole of 57 9 n 192 1 e a 95 2 9 comparison of the paleogene paleomagnetic data for the qiangtang terrane and lhasa terrane reveals that both terranes experienced latitudinal crustal shortening before 54 43 ma indicating the uplift of southern and central tibet in the early eocene subsequently since 35 4 2 4 ma the north of the qiangtang terrane experienced 1300 410 km of crustal shortening indicating the uplift of the northern tibetan plateau the lhasa terrane and qiangtang terrane have not experienced further crustal shortening since the late eocene and the southeastern part of tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the tibetan plateau the crust of the tengchong terrane and shan thai block did not experience significant southward extrusion since the late oligocene early miocene the indochina block was situated in the north of the qiangtang terrane before the oligocene and since the early oligocene the indochina block began to experience southward extrusion from the north of the qiangtang terrane which absorbed part of the crustal shortening in the north of the qiangtang terrane

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

magnetic anisotropy as a correction for compaction caused palaeomagnetic inclination shallowing summary a theoretical model presented by jackson et al 1991 suggests that the anisotropy of anhysteretic remanence aar can be used to recognize and correct for compaction caused inclination shallowing in sediments and sedimentary rocks however we have conducted experiments in which inclination shallowing is more strongly dependent on magnetic anisotropy during compaction of synthetic clay rich sediments containing 0 45 m acicular magnetite than predicted by the jackson et al model this discrepancy can be explained if randomization or disturbance of subvertical magnetite particles is the cause of inclination shallowing early in the compaction process the principal experimental evidence for this effect is a decrease in magnetization intensity coincident with inclination shallowing an intensity decrease during inclination shallowing is also observed in some dsdp cores suggesting that this randomization process may also occur in nature after randomization magnetite particles become firmly attached to clay particles and both magnetic anisotropy and inclination shallowing show a regular increase during compaction consistent with the jackson et al model we suggest that models like that of jackson et al can be used to correct for inclination shallowing if they are used in conjunction with an experimentally derived relationship between inclination shallowing and magnetic anisotropy for a particular sediment

morphological changes of the active cone of galeras volcano colombia during the last century abstract the morphology of the active cone of galeras volcano is a consequence of the complex interplay between effusive and explosive processes and erosion the main crater several secondary craters and zones of fumarolic activity were formed on the active cone as a result of these processes analysis of detailed descriptions from observations that began in 1989 indicates that volcanic activity led to the formation of many fissures in the main crater and on the active cone which appear to show an important relationship with the volcano s structure the fractures and fissures are associated with a tangential system generally aligned with the crater rim and a radial system that tends to intersect the crater rim at high angles the intersections of the tangential and radial systems have generated zones of weakness some of which have developed into eruptive centers thus certain fumaroles have evolved into small craters and these craters have contributed to the growth of the main crater on the active cone

correcting distorted paleosecular variation in late glacial lacustrine clay abstract an undisturbed horizontal chronostratigraphic marker horizon of laminated red glacio lacustrine clay crops out over 25 000 km 2 in northern ontario canada the primary clastic hematite laminae possess two stable vector components of magnetization we sampled a 1 6 m vertical section at overlapping 2 cm intervals in cubic specimens 8 cm 3 n 106 precisely oriented in geographic coordinates which permitted measurement of inclination and declination alternating field demagnetization 12 17 steps per specimen isolated a characteristic chrm primary component coercivity 40 mt approximately 30 shallower than the mean geomagnetic field inclination at this latitude with distorted paleosecular secular variation psv a lower coercivity overprint 20 40 mt is 6 shallower than the present geomagnetic field and similarly inclined to the depositional remanence deflection when the clay was re sedimented in the laboratory we believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition caused by the magnetic anisotropy of the clay using this as a proxy for the initial inclination the chrm hard component inclinations were restored to their original values assuming vertical compaction which averages to 51 the restored inclinations are compatible with site latitude and the restored secular variation loops centre reasonably on the geographic north pole the structural correction based on homogeneous vertical shortening over simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape size and packing and effectively combines influences of compaction and depositional settlement nevertheless this correction makes the psv data more useful and interpretable in terms of paleopole migration the more logical correction technique using anisotropy of anhysteretic remanence aarm jackson m j banerjee s k marvin j a lu r gruber w 1991 detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results geophys j int 104 95 103 is foiled here due to the high coercivity of the remanence bearing grains and their unknown orientation distribution and shape distribution

palaeomagnetic study of a subaerial volcanic ridge sao jorge island azores for the past 1 3 myr evidence for the cobb mountain subchron volcano flank instability and tectonomagmatic implications we present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1 3 myr on s jorge island azores archipelagonorth atlantic ocean the sections sampled in the southeastern and central western parts of the island record reversed and normal polarities respectively they indicate a mean palaeomagnetic pole 81 3 degrees n 160 7 degrees e k 33 and a95 3 4 degrees with a latitude shallower than that expected from geocentric axial dipole assumption suggesting an effect of non dipolar components of the earth magnetic field virtual geomagnetic poles of eight flows and two dykes closely follow the contemporaneous records of the cobb mountain subchron odp dsdp programs and constrain the age transition from reversed to normal polarity at ca 1 207 0 017 ma volcano flank instabilities probably related to dyke emplacement along an nnwsse direction led to southwestward tilting of the lava pile towards the sea two spatially and temporally distinct dyke systems have been recognized on the island the eastern is dominated by nnwsse trending dykes emplaced before the end of the matuyama chron whereas in the central western parts the eruptive fissures oriented wnwese controlled the westward growth of the s jorge island during the brunhes chron both directions are consistent with the present day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre existing fractures the distinct timing and location of each dyke system likely results from a slight shift of the magmatic source

hydrothermal fluid controlled remagnetization of sedimentary rocks in korea tectonic importance of pervasive tertiary remagnetization abstract in order to better understand the tectonic environment of the korean peninsula since cretaceous a paleomagnetic study was carried out on the jinan basin located in southwestern korea from the demagnetization of 597 samples we found that remanence is carried by pyrrhotite and or magnetite black siltstone and hematite red siltstone which is a common magnetic mineral assemblage of the rocks remagnetized in early tertiary in korea all the rock units recorded identical ancient geomagnetic field directions in particular paleodirections were better clustered in geographic coordinates indicating a secondary origin the presence of abundant silicates as inclusions within fe oxides and fe sulfides confirms the authigenic origin of the magnetic carriers precipitated by fluid mediated chemical processes compilation of the korean cretaceous and tertiary poles indicates that the jinan basin was remagnetized in the early tertiary age in addition comparison of the korean cretaceous and tertiary poles with those from adjacent major terrains including china and siberia reveals that a dominant feature of tectonic motions in korea was a vertical axis rotation sense of vertical axis rotation was time dependent with clear distinction between clockwise rotation in cretaceous and counter clockwise rotation in early tertiary such differences in the mode of vertical axis rotation might be caused by the activities of basin bounding strike slip faults in korea possibly driven by the changes of subduction regime of the kula pacific plate in early tertiary

widespread inclination shallowing in permian and triassic paleomagnetic data from laurentia support from new paleomagnetic data from middle permian shallow intrusions in southern illinois usa and virtual geomagnetic pole distributions abstract recent paleomagnetic work has highlighted a common and shallow inclination bias in continental redbeds the permian and triassic paleomagnetic records from laurentia are almost entirely derived from such sedimentary rocks so a pervasive inclination error will expectedly bias the apparent polar wander path of laurentia in a significant way the long standing discrepancy between the apparent polar wander paths of laurentia and gondwana in permian and triassic time may be a consequence of such a widespread data pathology here we present new middle permian paleomagnetic data from igneous rocks and a contact metamorphosed limestone from cratonic laurentia the exclusively reversed middle permian magnetization is hosted by low ti titanomagnetite and pyrrhotite and yields a paleomagnetic pole at 56 3 s 302 9 e a95 3 8 n 6 this pole which is unaffected by inclination shallowing suggests that a shallow inclination bias may indeed be present in the laurentian records to further consider this hypothesis we conduct a virtual geomagnetic pole distribution analysis comparing theoretical expectations of a statistical field model tk03 gad against published data sets this exercise provides independent evidence that the laurentian paleomagnetic data is widely biased likely because of sedimentary inclination shallowing we estimate the magnitude of this error from our model results and present and discuss several alternative corrections

are hydrodynamic shape effects important when modelling the formation of depositional remanent magnetization recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached making remanence acquisition an inefficient process alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel reducing the overall magnetic torque existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other in the case of non spherical flocs the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column calculations will be presented that show the torque exerted on a non spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque non spherical flocs will therefore align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence

palaeolatitude and age of the indo asia collision palaeomagnetic constraints summary ongoing controversies on the timing and kinematics of the indo asia collision can be solved by palaeomagnetically determined palaeolatitudes of terranes bounding the indo asia suture zone we show here based on new palaeomagnetic data from the linzizong volcanic rocks 54 47 ma near the city of lhasa that the latitude of the southern margin of asia was 22 8 4 2 n when these rocks were deposited this result combined with revised palaeomagnetic results from the northernmost sedimentary units of greater india and with apparent polar wander paths of india and eurasia palaeomagnetically constrain the collision to have occurred at 46 8 ma 95 per cent confidence interval these palaeomagnetic results are consistent with tomographic anomalies at 15 25 n that are interpreted to locate the tethyan oceanic slab that detached following collision and with independent 56 46 ma collision age estimates inferred from the timing of slowing down of india high pressure metamorphism the end of marine sedimentation and the first occurrence of suture zone and arc detritus on the greater indian margin when compared with apparent polar wander paths of india and eurasia the 46 ma onset of collision at 22 8 4 2 n implies 2900 600 km subsequent latitudinal convergence between india and asia divided into 1100 500 km within asia and 1800 700 km within india

anisotropic paleosecular variation models implications for geomagnetic field observables abstract we present a family of statistical models for paleosecular variation psv of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 ma and explore what paleomagnetic observables might be used to discriminate among the various family members we distinguish statistical models with axial anisotropy which provide a suitable description for an earth with homogeneous boundary conditions at the core mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions the models revise and extend earlier ones which are themselves descendants of cp88 devised by constable and parker constable c g parker r l 1988 statistics of the geomagnetic secular variation for the past 5 m y j geophys res 93 11569 11581 in cp88 secular variation is described by statistical variability of each gauss coefficient in a spherical harmonic description of the geomagnetic field with each coefficient treated as a normally distributed random variable the gauss coefficients of the non dipole part of the field exhibit isotropic variability and the variances are derived from the present field spatial power spectrum the dipole terms have a special status in cp88 with a non zero mean for the axial dipole and lower variance than predicted from the spatial power spectrum all non dipole terms have zero mean except the axial quadrupole cp88 is untenable for two reasons it fails to predict the observed geographic dependence of directional variability in the magnetic field and it grossly underpredicts the variance in paleointensity data the new models incorporate large variance in the axial dipole and in the non axial quadrupole gauss coefficients g21 and h21 the resulting variance in paleomagnetic observables depends only on latitude zonal models unless the variance in h21 is different from that in g21 non zonal models non zonal longitudinal variations in psv such as the flux lobes seen in the historical magnetic field are simulated using the non zonal models both the zonal and non zonal models fit summary statistics of the present dataset we investigate the influence of persistent non zonal influences in psv on various paleomagnetic observables it is shown that virtual geomagnetic pole vgp dispersion is rather insensitive to longitudinal variations in structure of psv and that inclination dispersion has the potential to be more informative given the right site distribution there is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate psv models

detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results summary we suggest that inclination errors in detrital remanent magnetization drm may be recognized and corrected by measurement of the anisotropy of anhysteretic remanent magnetization arm arm anisotropy reflects directional variations in the remanence capacity of relatively fine grained magnetic particles in rocks or sediments generally the same particles that carry the stable component of drm relative vertical and horizontal drm magnitudes are controlled by this directional remanence capacity as well as by the alignment efficiency of the particle magnetic moments which in turn is governed by the relative intensities of the vertical and horizontal components of the ambient magnetic field thus the respective vertical and horizontal components of palaeointensity and therefore palaeofield inclination may be obtained by normalizing the measured drm components by parallel arm intensities synthetic sediments containing silica kaolin and sized magnetite powders have previously been found to acquire experimental drm with an inclination error that is a direct function of kaolin concentration further experiments have now shown that arm anisotropy in the synthetic sediments is also a direct function of kaolin content and thus a correlation exists between drm inclination errors and arm anisotropy multiplying the measured drm vector by the inverse of the arm anisotropy matrix yields an improved estimate of the actual laboratory depositional field in all cases for samples with acicular 0 5 m magnetite this procedure reduces a mean inclination error of over 5 to less than 0 5 samples with subequant magnetite of slightly larger grain size 0 75 m exhibit similar arm anisotropy 10 per cent but somewhat larger inclination errors 10 we interpret this in terms of stronger horizontal alignment but weaker particle anisotropies for the larger more equant magnetites

high resolution paleomagnetic secular variations and relative paleointensity since the late pleistocene in southern south america abstract paleomagnetic inclination declination and relative paleointensity were reconstructed from the sediments of laguna potrok aike in the framework of the international continental scientific drilling program icdp potrok aike maar lake sediment archive drilling project pasado here we present the u channel based full vector paleomagnetic field reconstruction since 51 2 ka cal bp the relative paleointensity proxy rpi was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the chrm interval nrm arm 10 40 mt a grain size influence on the rpi was removed using a correction based on the linear relationship between the rpi and the median destructive field of the natural remanent magnetisation mdf nrm the new record is compared with other lacustrine and marine records and stacks from the mid to high latitudes of the southern hemisphere revealing consistent millennial scale variability the identification of the laschamp and possibly the mono lake geomagnetic excursions and a direction swing possibly associated to the hilina pali excursion at 20 ka cal bp nonetheless a global scale comparison with other high resolution records located on the opposite side of the earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern south america at 46 ka cal bp

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

persistent patterns in the geomagnetic field over the past 2 5 myr historical geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations lobes at the surface of the liquid core and regions of rapid change extending from the atlantic to the indian ocean1 palaeomagnetic data define a time average over several thousand years2 which might reflect the stationary parts of the present field but unfortunately the historical record is too short to provide a satisfactory average3 here we model palaeomagnetic directions from the past 2 5 myr using the same methods as for modern data4 and find the two northern lobes in the same position as today over arctic canada and siberia in southern regions by contrast the field appears to have been smoothed out as might be expected from the current rapid secular variation1 we propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years as would occur if the solid mantle controls flow at the top of the core

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

late permian palaeomagnetic data east and west of the urals summary we studied upper permian redbeds from two areas one between the urals and the volga river in the southeastern part of baltica and the other in north kazakhstan within the ural mongol belt which are about 900 km apart a limited collection of lower middle triassic volcanics from north kazakhstan was also studied a high temperature component that shows rectilinear decay to the origin was isolated from most samples of all three collections for the late permian of north kazakhstan the area mean direction of this component is d 224 3 i 56 8 k 161 95 2 7 n 18 sites palaeopole at 53 4 n 161 3 e the fold test is positive the triassic result d 55 9 i 69 1 k 208 95 4 2 n 7 sites pole at 57 0 n 134 1 e is confirmed by a positive reversal test the corresponding palaeomagnetic poles from north kazakhstan show good agreement with the apwp for baltica thus indicating no substantial motion between the two areas that are separated by the urals our new mean late permian direction for se baltica d 42 2 i 39 2 k 94 95 3 5 n 17 sites palaeopole at 45 6 n 170 2 e is confirmed as near primary by a positive tilt test and the presence of dual polarity directions the corresponding pole also falls on the apwp of baltica but is far sided with respect to the coeval reference poles as the observed mean inclination is shallower than expected by 13 4 in principle lower than expected inclinations may be attributed to one or more of the following causes relative tectonic displacements quadrupole and octupole terms in the geomagnetic field higher order harmonics incl secular variation of the same field random scatter non removed overprints or inclination error during remanence acquisition and or diagenetic compaction our analysis shows that most mechanisms from the above list cannot explain the observed pattern leaving as the most likely option that it must be accounted for by inclination shallowing comparison with selected coeval results from eastern baltica all within russia shows that all of them are biased in the same way this implies that they cannot be used for analysis of geomagnetic field characteristics such as non dipole contributions without a more adequate knowledge of the required correction for inclination shallowing

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

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

a time framework based on magnetostratigraphy for the siwalik sediments of the khaur area northern pakistan the magnetostratigraphy of eleven new sections in the khaur area of northern pakistan is presented all specimens have been subjected to thermal demagnetization the sections taken in adjacent ravines incising the siwalik formations nearly penpendicular to strike form a temporal framework in which to interpret the biological and lithological information a composite section is constructed from several long sections and is correlated to the magnetic polarity time scale chron 6 to chron 15 providing absolute age information for the biostratigraphic zonations of barry et al 1982 detailed sections along the strike of major stratigraphic marker beds document an isochronous horizon and form the basis for paleoenvironmental reconstructions the preservation of a polarity transition in fluvial sediments suggests continuity of sedimentation on a time scale of 103 104 years

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

tectonics and paleomagnetic rotation pattern of yunnan 24 n 25 n china gaoligong fault shear versus megablock drift

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

mid cretaceous polar standstill of south america motion of the atlantic hotspots and the birth of the andean cordillera article i nfo a paleomagnetic study on mid cretaceous rocks from the san bernardo foldbelt patagonia yields high unblocking temperature and high coercivity magnetizations the results indicate absence of relative vertical axis rotations during development of the foldbelt with the associated pole position being highly concordant with coeval poles from brazil and patagonia taken together mid cretaceous poles derived from studies in widely distributed localities provide supportive evidence that south america was essentially motionless with respect to the paleomagnetic axis from ca 125 to at least 100 ma the paleolatitudes of south america are not consistent with the occurrence of mid cretaceous true polar wander suggesting that the previously observed discrepancy between the paleomagnetic and the fixed indo atlantic hotspot reference frames be related to motion of the atlantic hotspots in agreement with this the discrepancy is diminished by half when the cretaceous poles of the americas are observed in a moving hotpots reference frame with the residual offset being comparable to that seen for younger time intervals the south american paleopoles and the moving hotspot framework provide a kinematic scenario that allows relating the extensional tectonics in the early stages of andean evolution with episodic divergence between the trench and the continental interior likewise the beginning of contractional events correlates with model predicted westward acceleration of south america in the late cretaceous suggesting that the continent episodically overrode the andean trench by those times we argue that this change in andean tectonic regime is associated to major plate reorganization at ca 95 ma

archeomagnetic secular variation from korea implication for the occurrence of global archeomagnetic jerks abstract an archeomagnetic paleosecular variation psv was first defined in korea using baked materials collected from 26 kilns or hearths with ages ranging from 1100 bc to ad 1790 variations of geomagnetic declination and inclination from the korean peninsula are distinctively different from the prediction of a global model cals3k 3 or cals7k 2 for the past 3500 yr in particular a distinctive offset in magnitude and phase is noticeable between the observations and predictions at 745 bc ad 300 and ad 1400 1700 a bi plot of magnetic declination versus inclination displays three cusps at the corresponding time intervals these time intervals are nearly identical to or at least overlap with three of the four potential archeomagnetic jerks suggested by gallet et al 2003 from the european archeomagnetic data a comparison of the psv curves for neighboring countries regions revealed that european archeomagnetic jerks at 800 bc ad 200 ad 800 and ad 1400 were all preserved in east asia suggesting that the archeomagnetic jerks were global or at least northern hemispheric features

paleomagnetism of the upper cretaceous red beds from the eastern edge of the lhasa terrane new constraints on the onset of the india eurasia collision and latitudinal crustal shortening in southern eurasia abstract the late cretaceous location of the lhasa terrane is important for constraining the onset of india eurasia collision however the late cretaceous paleolatitude of the lhasa terrane is controversial a primary magnetic component was isolated between 580 c and 695 c from upper cretaceous jingzhushan formation red beds in the dingqing area in the northeastern edge of the lhasa terrane tibetan plateau the tilt corrected site mean direction is d s i s 0 9 24 3 k 46 8 95 5 6 corresponding to a pole of plat plon 71 4 273 1 with a 95 5 2 the anisotropy based inclination shallowing test of hodych and buchan 1994 demonstrates that inclination bias is not present in the jingzhushan formation the cretaceous and paleogene poles of the lhasa terrane were filtered strictly based on the inclination shallowing test of red beds and potential remagnetization of volcanic rocks the summarized poles show that the lhasa terrane was situated at a paleolatitude of 13 2 8 6 n in the early cretaceous 10 8 6 7 n in the late cretaceous and 15 2 5 0 n in the paleogene reference point 29 0 n 87 5 e the late cretaceous paleolatitude of the lhasa terrane 10 8 6 7 n represented the southern margin of eurasia prior to the collision of india eurasia comparisons with the late cretaceous to paleogene poles of the tethyan himalaya and the 60 ma reference pole of east asia indicate that the initial collision of india eurasia occurred at the paleolatitude of 10 8 6 7 n since 60 5 1 5 ma reference point 29 0 n 87 5 e and subsequently 1300 910 km post collision latitudinal crustal convergence occurred across the tibet the vast majority of post collision crustal convergence was accommodated by the cenozoic folding and thrust faulting across south eurasia