on the reliability of mesozoic dipole low new absolute paleointensity results from parana flood basalts brazil 1 thellier paleointensity experiments were carried out on early cretaceous parana flood basalts forty two samples from 11 lava flows yielded apparently reliable absolute intensity determinations the mean paleointensity values per flow obtained in this study are ranging from 19 4 4 0 to 46 7 7 0 t and the corresponding virtual dipole moments are ranging from 4 0 0 6 to 10 5 1 1 10 22 am 2 these yield a mean value of 7 2 2 3 x 10 22 am 2 which is about 92 of the present geomagnetic axial dipole our results in conjunction with tarduno et al s 2001 data suggest that the paleostrength during early cretaceous period might be comparable or even much higher than recent field intensity and not anomalousely low as previously suggested thus the mesozoic dipole low mdl may probably be considered as unreliable

geomagnetic paleosecular variation in the brunhes period from the island of el hierro canary islands we have sampled a sequence of 69 flows in the island of el hierro canary islands 46 of them belong to the el golfo formation and 23 to the most recent rift formation radiometric dating brackets this section between 442 ka and 134 ka with six intermediate datings all the samples were stepwise demagnetized partly with alternative field partly thermally with very similar results the large majority of the samples are characterized by a single stable component of magnetization chrm isolated after the first steps of demagnetization are all of normal polarity inclinations are on average shallower than expected from a geocentered dipole field but consistent with results from other areas at the same latitude mean declinations of the rift and el golfo sequences are both easterly deviated about 5 and 15 respectively and significantly different suggesting that the section has undergone a progressive clockwise rotation vgp scatter and directional scatter calculated for the rift sequence are consistent with available data from other geographical areas and also with recent models of paleosecular variation on the other hand a significantly smaller directional scatter is obtained for the lower el golfo sequence suggesting that a period of low secular variation occurred in the canary islands area between approximately 440 and 260 ka

palaeomagnetism and 40ar 39ar age from a cretaceous volcanic sequence inner mongolia china implications for the field variation during the cretaceous normal superchron abstract an integrated palaeomagnetic and 40ar 39ar dating study was carried out on an early cretaceous volcanic lava sequence from the suhongtu section inner mongolia to determine the field behavior within the cretaceous normal superchron cns 40ar 39ar ages were obtained from 12 lava flows indicating that the studied lava was formed around 114 1 0 3 ma for the lower interval and 110 6 0 1 ma for the upper interval rock magnetic experiments and electron microprobe analyses indicate that the primary fe ti oxides are the main magnetic carriers all lava flows carry normal palaeomagnetic directions which can be grouped into 31 units by an f test with a fisher mean characteristic remanent magnetization chrm of d i 12 8 58 6 95 2 3 the corresponding palaeomagnetic pole is located at 80 3 n and 200 3 e a95 3 2 k 64 4 which is indistinguishable at the 95 confidence level from the eurasia pole derived from the apparent polar wander path for the early cretaceous using a modified thellier palaeointensity method with stringent acceptance criteria we obtained two time series of palaeointensity records from 15 independent palaeomagnetic units total 136 samples the virtual dipole moment vdm values varied from 2 53 1022 am2 to 9 92 1022 am2 the mean vdms for the upper and lower intervals are 5 38 2 06 1022 am2 and 4 61 2 67 1022 am2 respectively the observed time series of palaeointensity together with the previously available data for the cns suggest that magnetic field strength during the cns fluctuated significantly with time

new late jurassic to early cretaceous paleomagnetic results from north china and southern mongolia and their implications for the evolution of the mongol okhotsk suture

a palaeomagnetic study of jurassic intrusives from southern new south wales further evidence for a pre cenozoic dipole low the results of rock magnetic thermal demagnetization and thellier palaeointensity studies are presented for two high level intrusions from southern new south wales australia the gingenbullen dolerite gb 34 5 s 150 3 e and the gibraltar microsyenite gs 34 5 s 150 4 e were emplaced during the second of three major phases of igneous activity that affected the southern sydney basin and are k ar dated at 172 and 178 ma respectively the magnetic mineralogy of the two intrusions is different the gb is dominated by single domain sd pseudo single domain grains of magnetite whereas the gs has both sd magnetite and haematite components although the haematite component does not record a stable remanence the gb records two opposed components of magnetization above 100 c occupying distinct parts of the blocking temperature spectrum the lower blocking temperature lbt component resides between 150 and 400 c and is of normal polarity having a mean direction of d i 021 79 95 8 k 63 with a corresponding vgp at lat long 54 s 137 e dp 12 dm 13 this component is believed to be a tcrm or tvrm related to the initial opening of the tasman sea at 90 ma the higher blocking temperature hbt chrm component is recognized between 450 and 580 c and is of reverse polarity with a mean direction of d i 174 81 95 11 k 33 and a corresponding vgp at lat long 52 s 153 e dp 17 dm 18 the chrm for the gs resides in the 100 450 c region and although the scatter is high has a similar mean direction d i 167 79 95 24 k 26 to the hbt component of the gb with a pole position at lat long 54 s 158 e dp 30 dm 31 there is no evidence of the gb lbt component in samples from the gs suggesting that this intrusion was magnetically unaffected by the later event which is also seen for other intrusions in the sydney basin the vgps calculated from the chrm directions both plot on the jurassic segment of the australian apwp dunlop et al 1997 providing further palaeomagnetic confirmation for the k ar ages thellier palaeointensity results were of variable quality as expected for rocks of this age the mean vdm values calculated for the chrms of the gb and gs were 1 3 1022 and 4 1 1022 am2 respectively approximately 16 and 51 per cent of the present value on application of a cooling rate correction estimate the gb result falls to a minimum of 0 87 1022 am2 approximately 11 per cent of the present day value representing an overestimate of up to 56 per cent these values arising from a time window previously lacking in thellier estimates provide further confirmation of the existence of a pre cenozoic dipole low the vdm values derived from the overprint in the gb samples yield an average of 4 8 1022 am2 from a range between 44 and 81 per cent of the present day mean vdm and are tentatively attributed to the cretaceous normal superchron cns these results may document evidence for a previously unrecognized low dipole moment during the bulk of the cns a possibility that requires further investigation by performing palaeointensity work on igneous rocks with radiometric ages between 110 and 90 myr that carry unequivocal primary trms

does the permo triassic geomagnetic dipole low exist studies of absolute paleointensity have been conducted on specimens from permo triassic sills from the eastern areas of the kimberlite pipes sytikanskaya yubileinaya and aikhal and northwestern extrusions near noril sk city parts of the siberian platform a total of 341 specimens have been subjected to a modified thellier thellier technique which included partial thermoremanent magnetization checks and multidomain tail checks the paleomagnetic directions derived from the present set of experiments d 107 5 i 75 5 k 82 8 a95 5 3 n 10 sills for the eastern localities and d 95 1 i 71 8 k 35 2 a95 15 7 n 4 outcrops for the northwestern locality agree with the directions from previous studies the specimens from the northwestern sites noril sk did not provide any suitable result of absolute paleointensity the mean virtual dipole moment obtained from only the eastern localities has a value close to 6 01 1 45 10 22 am 2 which is over 50 higher than the results obtained by heunemann et al 2004 and shcherbakova et al 2005 on northwestern specimens the discrepancy could possibly be explained by the presence of multidomain grains in northwestern specimens where previous studies were undertaken which would reduce the paleointensity estimates at high temperatures however the present results lead us to conclude that 1 the geomagnetic field intensity at the permo triassic boundary was very close to the observed present day values and as a consequence a dipole low is not characteristic during this period of time 2 the mesozoic dipole low did not extend to the permo triassic boundary contrary to previous studies

finite fault inversion of dinsar coseismic displacement of the 2009 l aquila earthquake central italy 1 we define the geometric and kinematic characteristics of the fault activated during the mw 6 3 6 april 2009 l aquila earthquake from the modeling of envisat and cosmo skymed the first ever x band interferogram inverted for a coseismic dislocation study dinsar interferograms our best fit solution for the main shock is represented by a normal fault 16 km long and 12 km wide with a small right lateral component dipping 47 sw with a maximum slip of 90 cm although the seismic dislocation probably ended at 1 km depth the updip projection of the fault plane corresponds to the northern segment of the mapped paganica s demetrio fault where alignment of surface breaks was observed in the field the absence of this fault in existing seismic source catalogues suggests that an improved approach involving detailed surface and subsurface geological and geophysical investigations is needed for a better assessment of the seismic hazard at the local scale

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

paleointensity determinations from the etendeka province namibia support a low magnetic field strength leading up to the cretaceous normal superchron paleointensity estimates provide much needed information on field generation within earth s core and upon the convective processes at work within the mantle we present new paleointensity estimates from the early cretaceous etendeka large igneous province in namibia 135ma which add to the sparse southern hemisphere data set the early cretaceous marks an important change in the earth s magnetic field from a state of rapid polarity reversals to one of long term stability associated with the onset of the cretaceous normal superchron at 121 ma paleointensity determinations using the izzi protocol were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the etendeka province numerous checks of data reliability were considered before results were accepted including partial thermoremanent magnetization ptrm checks and ptrm tail checks hysteresis properties thermomagnetic analyses observations under reflected light and changes to room temperature susceptibility during the experiments following these checks a total of 64 individual samples from five sites were considered to provide reliable paleointensity determinations these results were combined to provide site mean data with an overall average virtual dipole moment vdm for the study of 2 5 1 0 1022 am2 this value equates to approximately 30 of present earth s field and when considered alongside existing studies suggests that earth s field strength was low in the time leading up to the cretaceous normal superchron

characterization of the composition of dust fallout and identification of dust sources in arid and semiarid north china abstract a source analysis to characterize the composition of dust fallout in arid and semiarid north china an important source area of dust for east asia was performed monthly samples were obtained from april 2001 to march 2002 at 29 sites in sandy and gobi deserts steppes and loess areas and the following elements were analyzed ce co cs eu fe hf lu nd rb sc ta tb th as la na sb sm u yb al ca mg mn ti and v although the dust compositions differed it was difficult to determine the potential dust source areas based solely on the elemental fingerprints of the dust principal components analysis showed that at most sites sm sc la ce eu and th were most strongly correlated r 0 7 with other elements whereas crustal elements such as al fe and ti were only strongly correlated with the other elements at a few sites of the 26 elements that we analyzed maybe only sb was representative of anthropogenic pollution but pollutant sources were unclear because at most sites in different months the crustal derived elements and those derived from anthropogenic pollutants always appeared synchronously creating difficulties and high uncertainties in source identification

a whole rock absolute paleointensity determination of dacites from the duffer formation ca 3 467 ga of the pilbara craton australia an impossible task abstract we have conducted a whole rock type magnetic and absolute paleointensity determination of the red dacite of the duffer formation from the pilbara craton australia the age of the dated rock unit is 3467 5 ma 95 confidence vector analyses results of the step wise alternating field demagnetization nrm up to 100 mt and thermal demagnetization from nrm up to 650 c yield three components of magnetization curie point determinations indicate three characteristic temperatures one at 150 200 c a second one at 450 c and a third one at 580 c magnetic grain size experiments were performed on small specimens with a variable field translation balance vftb the coercivity of remanence hcr suggests that the nrm is carried by low coercivity grains that are associated with a magnetite fraction as is shown by the high temperature component with blocking temperatures above 450 c and up to at least 580 c the ratios of the hysteresis parameters plotted as a modified day diagram show that most grain sizes are scattered within the single domain sd and the superparamagnetic and single domain sp sd domain ranges in addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified thellier coe double heating method to determine the paleointensities partial trm p trm checks were performed systematically to document magnetomineralogical changes during heating the temperature was incremented by steps of 50 c between room temperature and 590 c the paleointensity determinations were obtained from the slope of arai diagrams our paleointensity results indicate that the paleofield obtained was 6 4 0 68 n 11 micro teslas with a virtual dipole moment vdm of 1 51 0 81 1022 am2 from a medium to high temperature component ranging from 300 to 590 c that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the duffer formation the absolute paleointensity is relatively low and we interpret this low paleofield bias a result of a thermochemical remanent magnetization tcrm process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the duffer fm

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

absolute paleointensity and reversal records from the waianae sequence oahu hawaii usa abstract paleointensity experiments were performed on samples from four parallel sections of the waianae volcanics of oahu hawaii that document 0 4 myr of geomagnetic field variations the records of the three successive gilbert gauss lower and upper mammoth reversals confirm that large oscillations of directions precede or follow the reversals which are similar to waveforms generated by paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole determinations of absolute paleointensity were attempted on 546 samples we only selected data which were obtained from segments covering a large part of the remanent magnetization more than 70 on the average and without concave up arai diagrams this procedure limited the success rate to 14 but provided consistent and reliable paleointensities in addition to other time intervals the results document the field variations surrounding the lower mammoth transition a weak field period dominated before the reversal then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of the field intensity in reverse polarity a second episode of a very weak field was accompanied by a return to positive inclinations before definitely reaching the reverse polarity a strong and apparent rapid recovery of the dipole following the completion of the reversal culminated at a value of 16 1022 am2 similar to field intensities reported for the other detailed volcanic records of reversals studied so far the asymmetry between the pre and the post reversal phases indicates the importance of field regeneration to initiate a new stable polarity interval

coseismic deformation of the destructive april 6 2009 l aquila earthquake central italy from gps data 1 on april 6 2009 01 32 39 gmt the city of l aquila was struck by a mw 6 3 earthquake that killed 307 people causing severe destruction and ground cracks in a wide area around the epicenter four days before the main shock we augmented the existing permanent gps network with five gps stations of the central apennine geodetic network cageonet bordering the l aquila basin the maximum horizontal and vertical coseismic surface displacements detected at these stations was 10 39 0 45 cm and 15 64 1 55 cm respectively fixing the strike direction according to focal mechanism estimates we estimated the source geometry with a non linear inversion of the geodetic data our best fitting fault model is a 13 15 7 km2 rectangular fault sw dipping at 55 3 1 8 consistent with the position of observed surface ruptures the estimated slip 495 29 mm corresponds to a 6 3 moment magnitude in excellent agreement with seismological data

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

paleointensities from a cretaceous basalt platform in inner mongolia northeastern china abstract thellier s paleointensity experiments were carried out using cretaceous basalts of 90 ma which were taken from a basalt platform in inner mongolia northeastern china hysteresis parameters indicate that bulk grain size is psd in all samples js t curves indicate that the main magnetic mineral is titanomagnetite with curie temperature tc of 200 350 c with minor phases of higher tc which is known from the long tails continuing to the tc of magnetite samples often undergo severe chemical alteration on heating resulting in a decrease of trm capacity over the temperature range of 300 350 c nevertheless successful results were obtained from 47 of the total of 30 samples for the temperature steps lower than this critical point combining two results from possible identical flows four paleointensities were obtained ranging from 10 to 36 t which corresponds to a virtual dipole moment vdm of 1 8 6 0 1022 am2 these results support a low dipole moment during the later period of the cretaceous long normal superchron lns

paleointensity study of the middle cretaceous iritono granite in northeast japan implication for high field intensity of the cretaceous normal superchron abstract paleointensity of the cretaceous normal superchron cns has been studied using the middle cretaceous iritono granite of the abukuma massif in northeast japan our previous study wakabayashi k tsunakawa h mochizuki n yamamoto y takigami y 2006 paleomagnetism of the middle cretaceous iritono granite in the abukuma region northeast japan tectonophysics 421 161 171 indicates that the iritono granite retains a stable and primary component of high blocking temperatures and high coercivities which is characterized by the shallow inclination and carried mainly by single domain magnetite applying coe s version of the thellier method and the ltd dht shaw method we have obtained 16 successful results with an average of 58 4 7 3 t however an effect of long cooling time of the granite on the paleointensity measurement should be taken into account an estimate of cooling time to acquire the primary component ranges in 4 10 4 to 1 4 10 7 years from a thermal diffusion model of the granite body and the difference between 40 ar 39 ar biotite age of 101 9 0 2 1 ma and u pb zircon age of 115 7 1 9 1 ma from single domain theory thermoremanent magnetization trm of the iritono granite samples in nature is estimated to be about 1 5 times as strong as the laboratory trm applying this correction factor the corrected paleointensity is 39 0 4 9 t and the virtual dipole moment vdm is calculated to be 9 1 1 1 10 22 a m 2 this vdm can be interpreted as representative of the middle cns geomagnetic field since the individual granite samples with long cooling time can average out the paleosecular variation the obtained vdm is a few times higher than mean vdms averaged for 0 5 ma 3 6 10 22 a m 2 and 0 160 ma 4 8 10 22 a m 2 except for the cns by previous studies this suggests that the geomagnetic field intensity was high in the middle cns

paleointensity study of early miocene lavas from pingzhuang inner mongolia china 1 a paleomagnetic and geochronologic investigation has been conducted on 17 lava flows from inner mongolia north china radiometric k ar dating experiments give an age between 23 and 22 ma for these volcanic units rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo single domain titanomagnetite stepwise thermal demagnetization isolated a well defined direction of characteristic remanent magnetization chrm in all studied lava flows with a mean direction of d i 359 8 62 2 95 3 8 fifty samples from 11 lavas were pre selected on the basis of thermomagnetic analyses to carry out thellier thellier paleointensity experiments with systematic partial thermoremanent magnetization ptrm checks twenty three samples from 7 lavas yield reliable paleointensity estimates with a mean virtual dipole moment vdm of 3 9 0 9 1022 am2 about half the strength of the modern day field

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

remagnetization in lava flows recording pretransitional directions basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of la palma are characterized by overlapping components with very high unblocking temperatures a medium high temperature mht component with normal polarity is resistant up to 500 550 c while in most cases the high temperature ht primary reverse component cannot be isolated below 520 c the same situation with opposite polarities is observed also in a lava flow associated with the onset of the upper reunion reversal in the gamarri sequence in ethiopia the presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling the direction of mht is either close or similar to the direction of the ht component of the overlying flows which indicates that remagnetization occurred shortly after emplacement rock magnetic studies microscopic observations microprobe analyses and remagnetization experiments have been carried out to investigate the origin of mht the rock magnetic parameters do not exhibit specific anomalies the unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling the most plausible scenario is that baking by the overlying flows was accompanied by low temperature oxidation of titanomagnetite to cation deficient titanomagnetite this description also includes the presence of cracks fractures and other heterogeneities of the lava which induce variability between parallel profiles similarities with characteristics observed at steens mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution

paleointensity of the earth s magnetic field during the laschamp excursion and its geomagnetic implications the reversed paleomagnetic direction of the laschamp and olby flows represents a specific feature of the geomagnetic field this is supported by paleomagnetic evidence showing that the same anomalous direction was recorded at several distinct sites including scoria of the laschamp volcano to examine this anomalous geomagnetic fluctuation we studied the paleointensity of the laschamp and olby flows using the thellier method twenty five samples were selected for the paleointensity experiments and from seven we obtained reliable results because the paleointensity results of the olby and laschamp flows as well as laschamp scoria are very similar they can be represented by a single mean paleointensity f 7 7 t considering that this low paleointensity is less than 1 6 of the present geomagnetic field and is more characteristic of transitional behavior our results suggest that the paleomagnetic directions of the laschamp and olby flows were not acquired during a stable reversed polarity interval a more likely explanation is that the laschamp excursion represents an unsuccessful or aborted reversal

new palaeointensity results from cretaceous basalt of inner mongolia china we present new thellier thellier palaeointensity results from three cooling units 32 samples of inner mongolia lava flows 91 7 ma emplaced during the cretaceous normal superchron cns based on rock magnetic and microscopy observations the magneto mineralogy of all samples is determined to be primary and unaltered high ti titanomagnetite accepted palaeointensity determinations obtained in the 80 200 c temperature interval are of good technical quality with positive standard partial thermoremanent magnetisation ptrm checks and ptrm tail checks obtained palaeointensity estimates range from 14 7 to 28 0t with virtual axial dipole moments vadm of 2 4 to 4 6 10 22 am 2 the data agree well with recently published results from the same region and combining the two datasets we obtain independent estimates from six different cooling units yielding a time averaged vadm of 3 2 1 6 10 22 am 2 these data suggest a relatively low dipole moment towards the end of the cretaceous normal superchron

palaeointensities determined from the middle cretaceous basalt in liaoning province northeastern china abstract a palaeomagnetic and geochronologic investigation has been conducted on a basalt lava sequence at jianguo in liaoning province northeastern china radiometric 40 ar 39 ar dating experiments indicate the volcanism occurred in relatively short time intervals between 105 5 and 102 2 ma thermomagnetic experiments and electron microprobe show that main primary carrier of remanence to be titanomagnetite with curie temperature of 300 c a modified version of the thellier thellier technique with systematic partial thermoremanent magnetization ptrm checks was used for palaeointensity determinations fourteen out of 61 samples from the studied lavas yield reliable palaeointensity estimates with a mean virtual dipole moment vdm of 4 07 0 2 1022 am2 about half the strength of the modern day field

no evidence for strong fields during the r3 n3 icelandic geomagnetic reversal a previous study of the r3 n3 icelandic reversal which corresponds probably to the matuyama reunion reversal provided relatively large virtual dipole moments when the pole is close to equator to check these former results obtained using the shaw paleointensity method we resampled the very same sections and flows and reanalysed the samples using the thellier method 38 lava units and one baked horizon from 4 sections provide intermediate r3 n3 paleofield directions most of them possibly related to a single eruption display similar directions corresponding to the near equator vgp cluster unfortunately most of the intermediate lava flows are thermally unstable as attested by the irreversibility of the susceptibility versus temperature k t curves which precludes their use for paleointensity purpose the rest of the units provide fairly reversible k t curves and a near magnetite tc due to fe rich titanomagnetite resulting from oxyexsolution at high temperature 25 baked units or lava flows of this kind yield reliable thellier paleointensity data average paleostrengths of 44 7 14 9 t and 32 5 22 9 t were found for the reversed r3 and the normal n3 polarity zones respectively combining these non transitional paleostrengths with other thellier data from over the world indicates that the pliocene dipole moment was close to 8 1022 a m2 the average paleostrength is reduced to 11 3 3 0 t during the r3 n3 transition we find no indication for the strong fields 20 4 to 38 6 t previously obtained by the shaw method thus our results do not confirm the existence of large virtual dipole moments when the pole approaches the equator

assessment of the usefulness of lithic clasts from pyroclastic deposits for paleointensity determination 1 paleomagnetic and rock magnetic measurements were carried out on lithic clasts found within pyroclastic deposits to assess their potential for paleointensity determinations the use of multiple lithologies in a single paleointensity determination would provide confidence that the result is not biased by alteration within one lithology lithic clasts were sampled from three historically active volcanoes lascar in the chilean andes mt st helens united states and vesuvius italy at lascar triple heating paleointensity experiments allow development of new selection criteria for lithic clasts found within pyroclastic deposits using these criteria the lascar data yield a mean paleointensity of 24 3 1 3 t 1 n 26 which agrees well with the expected value of 24 0 t this indicates that pyroclastic rocks have promise for paleointensity determinations pyroclastics however still suffer from the range of problems associated with conventional paleointensity experiments on lava flows samples from mt st helens are strongly affected by multidomain md behavior which results in all samples failing to pass the paleointensity selection criteria at vesuvius md grains magnetic interactions and chemical remanent magnetizations contributed to failure of all paleointensity experiments rock magnetic analyses allow identification of the causes of failure of the paleointensity experiments however in this study they have not provided adequate preselection criteria for identifying pyroclastics that are suitable for paleointensity determination

geomagnetic palaeointensity just prior to the cretaceous normal superchron abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on a basalt lava sequence at zhuanchengzi in liaoning province northeastern china the new k ar age obtained from these lava flows is 120 93 0 88 ma marine anomaly m0 detailed investigations were conducted on two samples from each lava flow to determine their rock magnetic properties the temperature dependences of the saturation magnetization j t and the initial susceptibility t show evidence of low titanium magnetite and a curie temperature of 560 585 c with generally good reversibility of heating and cooling curves hysteretic properties ratios of mrs ms and hcr hc indicate that pseudo single domain psd grains of magnetite are dominant in these lavas stepwise thermal demagnetization isolated well defined characteristic magnetization chrm in all the 20 lava flows with a mean direction of d i 174 9 60 1 with 95 2 3 the modified version of the thellier and thellier palaeointensity method coe 1967a with systematic partial thermoremanent magnetization ptrm checks was used forty two of 71 samples 59 yielded reliable palaeointensity results virtual dipole moment vdm values range from 1 71 1022 to 5 27 1022 a m2 with an average of 3 96 0 07 1022 a m2 this low dipole field intensity value is approximately half the present day vdm consistent with older results obtained from the caribbean submarine basalt glasses for the period of 125 1 ma pick and tauxe 1993 this suggested that a weak geomagnetic field probably occurred just prior to the cretaceous normal superchron cns

palaeointensity and palaeomagnetic study of cretaceous and palaeocene rocks from western antarctica summary a combined palaeodirectional and palaeointensity study of a representative collection of plutonic rocks from the antarctic peninsula batholith from the western part of the antarctic peninsula near the ukrainian antarctic base academik vernadsky were carried out petrographically the collection includes gabbros diorites and quartz diorites tonalities granodiorites and granites the ages of igneous complex emplacement vary from 50 to 117 ma with most of the rocks belonging to the cretaceous normal superchron the characteristic remanent magnetizations were isolated by stepwise thermal demagnetization over the temperature interval 440 590 c and their intensities amount to 95 per cent of the nrm the geographic positions of palaeopoles do not contradict the key poles of the antarctic peninsula between 90 and 60 ma a significant part of the collection was subjected to coe modified thellier palaeointensity experiments with the ptrm checks which yielded seven reliable palaeointensity determinations for seven different locations the obtained vdms are relatively low for all sites being on average about half of the present day vdm the analysis of available palaeointensity data for the cretaceous miocene and middle jurassic indicates the existence of strong correlations between the mean vdm and vdm scatter versus the rate of reversals however due to the shortage of data the correlations are not significant at the 5 per cent significance level

changes of the geomagnetic field vector obtained from lava sequences on the island of vulcano aeolian islands sicily abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on five volcanic sections 70 flows on the island of vulcano the sections span the interval from 15 2 to 135 4 ka with large hiatuses rock magnetic investigations indicate that the natural remanent magnetisation nrm is carried by titanomagnetites in the four oldest sections with a contribution from high coercivity magnetic minerals in the youngest section palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion at around 110 ka with virtual geomagnetic poles vgps that lie over eastern siberia these directions may represent a volcanic record of the termination of the blake event only about 15 of the samples from 19 flows proved suitable for palaeointensity thellier and thellier determinations these measurements indicate that the geomagnetic field has varied at vulcano between 16 and 53 t in the explored time interval when combined with previous results obtained at mt etna for the period from 140 to 60 ka tric et al 1994 phys earth planet inter 85 113 129 these results document a broad low at around 115 ka the palaeointensity values from vulcano and etna are consistent with global values suggesting the presence of a dominant dipole field contribution these values on the other hand are significantly smaller than those recently obtained from la reunion in the southern hemisphere this suggests that large long lived non dipolar components of the geomagnetic field may be present at la reunion in the interval from 130 to 90 ka these observations are consistent with the fact that the vgp angular dispersion calculated from non excursional data is significantly smaller at vulcano than at la reunion

palaeointensity just at the onset of the cretaceous normal superchron abstract a combined palaeomagnetic and geochronologic study has been conducted on a basaltic lava sequence of five flows at jianchang in liaoning province northeastern china radiometric 40 ar 39 ar dating indicates that the volcanism occurred at about 119 ma within the marine anomaly c34n in cretaceous normal superchron cns rock magnetic investigations show that pseudo single domain psd or a mixture of single domain and multi domain grain titanium poor titanomagnetite is dominant in the studied lava flows both stepwise thermal and alternating field demagnetizations isolate the well defined normal characteristic remanent magnetization chrm in five independent lava flows with a mean direction of d i 11 1 57 2 95 5 7 palaeointensity was determined using the coe modified thellier method with systematic partial thermoremanent magnetization ptrm checks all together 61 samples from 286 samples with positive ptrm and tail checks yield palaeointensities ranging between 10 1 and 73 6 t to further check the effects of thermal alteration on the palaeointensity results a new ratio m rs 480 c m rs 25 c where m rs 25 c and m rs 480 c are the room temperature saturation isothermal remanent magnetizations for the raw sample and the product after a 480 c thermal treatment respectively is proposed results show that the ratio changes significantly even for those samples with positive ptrm check we further put forward that palaeointensity results are acceptable with ratio between 0 9 and 1 1 with this new criteria 17 samples show consistent palaeointensity results with an average of 20 0 3 2 t this value corresponds to the virtual dipole magnetic moments vdm of 3 5 0 6 10 22 am 2 which is about half of the value of present field this finding combined with other available results suggests that magnetic field of the earth just at the onset of the cns is characterized by a weak palaeointensity

an investigation of the magnetic variation within two recent lava flows summary samples collected in vertical sections from two recent 1169 ad and 1971 ad lava flows show significant variations in both the palaeomagnetic vector and the rock magnetic characteristics the age and the short cooling time of the flows both are less than 3 m thick are such that the observed range of palaeomagnetic variation is more than can reasonably be explained by secular variation the variations in palaeomagnetic and rock magnetic characteristics appear in part to be a response to the relative proportions of two coexisting ferrimagnetic phases one that displays a primary coarsegrained and ti rich composition and a second that has finer grained ti poor characteristics the use of af demagnetization in this study means that the latter phase will have a disproportionate influence on the palaeomagnetic results it has been suggested elsewhere that in such a magnetic system the ti poor phase may represent a crm formed at some temperature below its final curie temperature if so the results of this study imply that not only does it affect the palaeointensity estimate which requires a pure trm but it appears also to influence the palaeomagnetic direction to an extent that far exceeds the normal secular change that might be expected to occur during delayed acquisition such a result has important implications for palaeomagnetic studies that use lavas both in the selection of a suitable sampling strategy and in the geomagnetic interpretation of palaeomagnetic variation within single flow units

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

geomagnetic field strength during late miocene first paleointensity results from baja california 1 thellier paleointensity experiments were carried out on 73 selected samples from the late miocene baja california volcanic flows for the first time the selected samples showed a stable remanent magnetization observed upon thermal treatment and reasonably reversible continuous susceptibility curves compatible with ti poor titanomagnetites judging from the ratios of hysteresis parameters it seems that the samples fall in the small pseudosingle domain grain size region thirty three samples from 10 individual flows yielded acceptable paleointensity estimates the natural remanent magnetization nrm fractions used for paleointensity determination range from 37 to 81 and the quality factor varies between 4 5 and 24 5 being normally 5 the mean paleointensity values per flow obtained in this study range from 18 1 3 1 to 45 6 4 3 t and the corresponding virtual dipole moments vdms range from 3 8 0 6 to 8 8 0 8 1022 a m2 these values correspond to mean value of 6 6 1 7 1022 a m2 which is 85 of the present geomagnetic axial dipole the average vdm value for late miocene according to all available selected data is 6 1 1 5 1022 a m2 which is higher than average paleofield for the period 5 160 ma 4 2 2 3 1022 a m2 after juarez and tauxe 2000 given the large dispersion and the very poor distribution of reliable absolute intensity data it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field more high quality determinations are strongly needed

layered seismogenic source model and probabilistic seismic hazard analyses in central italy we defined a seismogenic model for central italy based on three layers of sources and computed the relative seismic hazard maps one layer is constituted by individual structures liable to generate major earthquakes m 5 5 we defined them as seismogenic boxes by using geological information in terms of plan projec tion of active faults the seismicity rates associated with an individual source are based on the geometry and kinematics of the fault the recurrence model is controlled by the earthquake source association and when possible we defined the occurrence time of the last major event using it in a time dependent approach another layer is given by the instrumental seismicity analysis of the past two decades which allows us to evaluate the background seismicity m 5 5 using a sliding window selec tion of events we defined a model of regular adjacent cells of variable a and b values of the gutenberg richter relation the last layer utilizes all the instrumental earth quakes and the historical events not correlated to known structures 4 5 m 6 by separating them into seismotectonic provinces shaped on a geological structural basis the seismic hazard computations first use this layered model in a traditional probabilistic scheme the results indicate a narrow belt of peak ground acceleration pga higher than 0 30g with standard deviation in attenuation functions in the axial part of the apennine chain with a maximum spot of pga 0 40g southeast of the area damaged by the 1997 1998 umbria marche sequence pga expected not to be exceeded in 50 years at 90 probability level the background seismicity gives a nonnegligible contribution to the hazard at least for first damage levels then a simplified time dependent hypothesis has been introduced for the individual sources alone computing the conditional probability of occurrence of characteristic earth quakes for each source by brownian passage time distributions adopting equivalent fictitious seismicity rates we obtained maps referring to the next 50 years by using traditional codes these results show that the contribution of the recently active sources vanishes and the most hazardous sites are now located south of l aquila and in the sulmona area we consider that the methodology and results obtained are useful for seismic risk reduction strategies

anisotropy of magnetic susceptibility of hannuoba basalt northern china constraints on the vent position of the lava sequences 1 anisotropy of magnetic susceptibility ams was determined for hannuoba basaltic samples from 17 flows 21 4 0 7ma distributed in three sections jianshaba yuershan and manjing around zhangbei northern china the results show normal flow fabrics with horizontal foliation planes the typical ratios of the principal ams axes are 1 01 for the magnetic lineation l and 1 02 for the foliation f respectively magnetic hysteresis loop isothermal remanence thermomagnetic properties and optical observations indicate that pseudo single domain psd low titanium magnetite particles dominate the low field susceptibility of samples thus the ams of the magma at this area is believed to be originated from the lineation of the low titanium magnetite particles driven by the lava flow since the kmax trends coincide well with the observed flow directions the intersection of the mean kmax trends from these three sites perfectly defines the previously known vent position of the lava sequences

geomagnetic field intensity at hawaii for the last 420 kyr from the hawaii scientific drilling project core big island hawaii four hundred twenty five new paleointensity thellier thellier determinations out of 545 analyzed samples have been obtained from core hsdp which penetrates about 1000 meters 208 flows of the mauna loa and mauna kea volcanic series encompassing the last 420 kyr rock magnetic investigations identify pseudo single domain magnetite as the main magnetic mineral inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude the inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events the paleointensity record from the mauna loa sequence is not very detailed and does not allow precise comparison with other data in the 0 50 kyr interval the record from the mauna kea sequence on the contrary is very detailed and documents relatively short lived episodes of low and high field strength from 15 to 60 t the average virtual dipole moment 8 7 3 0 10 22 a m 2 is not significantly different from the value reported by kono and tanaka 1995 for the last 2 5 myr a comparison with other data from hawaii and other geographical regions is described in detail there are no drastic changes in paleointensity with the inclination anomaly in agreement with previous results from hawaii but in contrast with most published results which however consider data from polarity transition spectral analysis of a particularly detailed portion of the record between 420 and 326 kyr documents significant periodicities at 36 8 5 and 4 ka in the inclination record but not in the intensity record suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity

experimental reassessment of the shaw paleointensity method using laboratory induced thermal remanent magnetization 1 artificial thermoremanent magnetization imposed on natural basalt samples was used to check the reliability of shaw s palaeointensity method shaw 1974 kono 1978 rolph and shaw 1985 and to give an indication of the accuracy of paleointensity determinations first thermal remanence was induced in all samples 50 in total in a known field 50 0 1 t this remanence was regarded as the original natural remanent magnetization nrm the samples were divided in to three subsets a b and c and intensity experiments were performed each subset of samples used different thermal remanent magnetization trm acquisition parameters rock magnetic properties on 14 representative subsamples were also measured both before and after the trm acquisition steps to monitor possible magnetochemical alteration it has been demonstrated that intensity estimates by the shaw method give values close to the expected value 50 5 t in all three sets of basalt samples the modified method with rolph and shaw 1985 correction i e using ratios of anhysteretic remanent magnetization given before the trm step arm1 and after arm2 is proved to be a powerful approach to correct the trm capacity changes that result from magnetochemical alteration this is especially so for sample set c where the samples were heated up to 700 c and then were held at this temperature for 40 min during trm acquisition after correction 45 of set c samples gave acceptable results however some samples with strong magnetochemical alteration sometimes invalidated the arm1 arm2 ratio correction and gave wrong intensity estimates monitoring rock magnetic properties is therefore very important when applying the shaw method

absolute paleointensities recorded during the brunhes chron at la guadeloupe island abstract ten absolute paleointensity determinations within the brunhes chron have been obtained on andesitic lava flows from la guadeloupe island french west indies f w i the thellier and thellier method performed on 124 specimens under either ambient or argon atmosphere allows reliable determinations from high temperature steps a satisfactory within flow reproducibility has been observed and at least two samples for each flow have been used for calculation of the weighted mean paleointensity the average virtual axial dipole moment vadm 7 1 1 8 1022 a m2 for normal polarity flows from the brunhes chron recorded at la guadeloupe is compatible with the historical field the 10 vadm values obtained show a relatively good agreement with the deep sea composite record sint800 of relative paleointensity focusing on the 100 75 ka interval a 20 ka oscillation observed from a compilation of available volcanic data and present in high resolution sedimentary records has been tentatively related to the axial dipole moment variations finally a paleointensity decrease of a factor of 10 is observed for flows emitted during the matuyama brunhes m b transition as already observed in other records of this reversal because a large area of the globe around the caribbean islands was previously devoid of data the present dataset fills a gap towards construction of a global paleointensity database for the brunhes chron

thellier palaeointensity experiments on faroes flood basalts technical aspects and geomagnetic implications based on palaeomagnetic and rock magnetic results 108 samples from 27 flows from palaeogene flood basalts of faroe islands were chosen for whole rock thellier palaeointensity experiments altogether 90 samples were rejected due to either chemical alterations or typical multidomain md behaviour evidenced by ptrm tails af pre treatment was used to reduce the effect of md grains on thellier experiments only five flows 18 samples yielded acceptable palaeointensity estimates with flow mean vdms ranging from 3 5 to 7 4 10 22 am 2 modest selection criteria imposed on all published 5 160 ma palaeointensity data left only 15 palaeomagnetic dipole moments eight from whole rock samples six from submarine basaltic glass and one from single plagioclase crystals more data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context 2002 elsevier science b v all rights reserved

absolute geomagnetic paleointensity after the cretaceous normal superchron and just prior to the cretaceous tertiary transition 1 we report a detailed rock magnetic and thellier paleointensity study from autlan volcanic succession in western mexico the whole rock k ar age obtained for these lavas 67 4 1 2 ma together with the magnetostratigraphy results indicates that the lava flows were erupted during chron 30n just prior to cretaceous tertiary transition thirty nine samples from eight cooling units were preselected for paleointensity experiments on the basis of their low viscosity index stable remanent magnetization and close to reversible continuous thermomagnetic curves high quality determinations were obtained for 18 individual samples from four independent lava flows the mean virtual dipole moment vdm obtained in this study is 4 9 0 6 x 10 22 a m 2 which is significantly lower than the present geomagnetic field strength but in reasonably good accord with other comparable quality determinations between 35 and 80 ma our data are not significantly different from almost all published paleointensities between 5 and 90 ma moreover rigorously selected worldwide paleointensity data in the same interval better fit trend 2 of heller et al 2002 a careful examination of high quality paleointensity data from cretaceous indicates that there is no simple relation between magnitude of earth s magnetic field and reversal rate

palaeointensity studies of a late permian lava succession from guizhou province south china implications for post kiaman dipole field behaviour summary palaeomagnetic results are presented from a volcanic sequence in zhijin county guizhou province southern china the lavas and associated volcanic breccias comprising the sequence represent a southern extension of the emeishan volcanic province biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late permian 263 255 ma and is thus somewhat older than the emei stratotype section in sichuan province and close in age to reported estimates of the termination of the permo carboniferous kiaman reverse superchron rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh primary magnetites containing a significant fraction of single domain grains thermal demagnetization behaviour of the breccia units is poor but most lava samples have one or two components of remanence above 250 c the normal polarity characteristic remanence held by the lavas implies a post kiaman age for this succession and suggests that the termination of the kiaman occurred prior to 263 ma supporting recently published estimates according to standard criteria thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron 80 per cent of samples record relative vdm values in the range 42 52 per cent of the present day value supporting recent studies of mid kiaman field intensity this suggests that a low energy dipole existed at least between 300 and 255 ma and does not appear to have been confined to the stable reverse polarity interval

in search of high fidelity geomagnetic paleointensities a comparison of single plagioclase crystal and whole rock thellier thellier analyses the strength of earth s magnetic field on timescales greater than a million years is known primarily from thellier thellier analyses of igneous rocks in the thellier thellier method the natural remanent magnetization nrm is compared with a thermoremanent magnetization trm imparted during heating in a known laboratory field unfortunately the multiple heating steps required by the method often result in alteration of magnetic minerals in clay bearing groundmass of whole rock samples this alteration may result in the creation of new magnetic minerals that could affect trm acquisition analyses of single plagioclase crystals which contain magnetic inclusions that retain paleofield values may offer a way to retrieve paleointensity data that is less affected by alteration we test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid cretaceous 113 115 ma basalt flow of the rajmahal traps northeastern india magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar suggesting a common magnetic mineralogy however hysteresis data from heated whole rock samples indicate that a fine grained magnetic phase has formed this effect was either absent or much less pronounced in the single plagioclase crystals tested paleointensity data from these crystals suggest a paleofield value of 65 1 5 3 t n 15 data from whole rock samples overlap these values but are on average lower we attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples resulting in an enhanced acquisition of trm and shallowing of the slope of nrm lost versus trm gained curves these data suggest that subtle alteration effects may bias thellier thellier results from some whole rock samples toward low paleofield values in these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity

geomagnetic paleointensities from excursion sequences in lavas on oahu hawaii paleomagnetic data demonstrating three late tertiary excursions in the direction of the geomagnetic field recorded in sequences of basaltic lavas on the island of oahu hawaii were published by r r doell and g b dalrymple in 1973 we have determined geomagnetic paleointensities by the thelliers method for 14 lavas from the three sites during these experiments considerable difficulty was encountered because of the presence of titanomaghemite in many lavas and the contamination of natural remanent magnetization by lightning in many others moreover we often observed the production of spurious high temperature chemical remanent magnetization during the thellier experiments an analysis of this particularly troublesome problem is presented two of the sites showed low paleointensities associated with angular departures of the paleomagnetic field direction from that of a geocentric axial dipole which suggests that these excursions represent aborted reversals or fragments of reversals at the third site however the paleointensity did not become low as the field diverged this excursion may reflect the variation of a large nondipole source near hawaii

cretaceous geomagnetic paleointensities thellier experiments on pillow lavas and submarine basaltic glass from the ontong java plateau 1 we present new thellier paleointensity results on pillow lavas and pillow rim submarine basaltic glass sbg from 120 ma basalts erupted on the ontong java plateau all pillow lavas have suffered low temperature alteration and are unsuitable for thellier paleointensity experiments in contrast pillow rim sbg is virtually impermeable to seawater and therefore has suffered little or no low temperature alteration out of 21 selected sbg samples with magnetic moment larger than 10 10 am2 18 samples from 12 chilled sbg margins yielded paleointensity estimates of good technical quality rock magnetic measurements show the primary carrier of remanence is low ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite as suggested by a recent study magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400 440 c which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature we suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength for several pillow margins two to three samples were measured and in such cases the data agree well altogether we obtain reliable estimates from 12 sbg pillow rims yielding a mean paleointensity of 27 1 13 4 t and a corresponding virtual axial dipole moment of 5 8 2 8 1022 am2 this value is slightly higher than whole rock data from similar aged subaerial lava flows the paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the cretaceous normal superchron

paleointensity studies of a lava succession from jilin province northeastern china evidence for the blake event paleomagnetic results are presented from a volcanic sequence in tianchi jilin province northeastern china five basalt lava flows can be identified at the sampling site with ages from 210 4 to 88 15 kyr b p rock magnetic analyses and reflected light microscopy indicate that magnetite with a significant fraction of pseudo single domain grains is dominant in these lava flows a single stable characteristic remanence can be thermally isolated above 150 200 c intermediate direction is preserved in lava 3 at an age of 123 kyr corresponding to the blake event and its virtual geomagnetic pole is located in eastern asia and australia from a total of 121 specimens used for paleointensity determinations by the thellier method in an argon atmosphere 34 specimens yielded reliable results with low within flow scatter the results indicate that a significant intensity low is coincident with the blake event

the intensity of the earth s magnetic field over the past 160 million years in contrast to our detailed knowledge of the directional behaviour of the earth s magnetic field during geological and historical times1 2 data constraining the past intensity of the field remain relatively scarce this is mainly due to the difficulty in obtaining reliable palaeointensity measurements a problem that is intrinsic to the geological materials which record the earth s magnetic field although the palaeointensity database has grown modestly over recent years3 4 5 these data are restricted to a few geographical locations and more than one third of the data record the field over only the past 5 myr the most recent database5 covering the time interval from 5 to 160 myr contains only about 100 palaeointensity measurements here we present 21 new data points from the interval 5 160 myr obtained from submarine basalt glasses collected from locations throughout the world s oceans whereas previous estimates for the average dipole moment were comparable to that of the earth s present field6 the new data suggest an average dipole moment of 4 2 2 3 1022 a m2 or approximately half the present magnetic field intensity this lower average value should provide an important constraint for future efforts to model the convective processes in the earth s core which have been responsible for generating the magnetic field

toward age determination of the m0r barremian aptian boundary of the early cretaceous china most 973 program 2006cb701403 national natural science foundation of china 90714010 40573040 40634024 40221402 chinese academy of sciences kzcx3 sw 142

effects of thermally heterogeneous structure in the lowermost mantle on the geomagnetic field strength abstract we have conducted a study of numerical dynamos in a rapidly rotating spherical shell with prescribed non uniform heat flux patterns at the outer boundary to examine effects of thermal structure at the core mantle boundary cmb on the geodynamo especially on the magnetic field strength large heat flux heterogeneity with equatorial symmetry enhances strength of the dipolar magnetic field for a sufficiently small ekman number whereas that with equatorial anti symmetry does not change the strength but affects the tilt of the magnetic dipole axis the thermal wind induced by heat flux heterogeneity at the cmb has strong influence on the flow structure and on the magnetic field intensity these results suggest that thermally heterogeneous structure of the lowermost mantle might give rise to an anomalously strong geomagnetic field such as that during the cretaceous normal superchron

geomagnetic palaeointensities during the cretaceous normal superchron measured using submarine basaltic glass high quality palaeointensity data have been obtained from thellier thellier experiments on recent and cretaceous submarine basaltic glasses whereas the recent samples faithfully yield today s geomagnetic intensity at the site the palaeointensities for the beginning and end of the cretaceous normal superchron are only 45 and 25 respectively of today s value the data thus extend the mesozoic dipole low into the cretaceous superchron and confirm that submarine basaltic glass is an excellent material for palaeointensity studies

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition

paleomagnetic and paleointensity study of an oligocene miocene lava sequence from the hannuoba basalts in northern china abstract a combined geochronologic paleomagnetic and paleointensity investigation has been performed on a lava sequence from the hannuoba basalts in northern china potassium argon k ar dating indicates that the lava sequence is of late oligocene to middle miocene age 24 15 ma rock magnetic measurements of thermomagnetic curves hysteresis properties saturation isothermal remanent magnetization sirm and back field demagnetization on representative samples from each lava flow in conjunction with reflected light microscopy observations show the primary carriers of remanence to be fine grained pseudo single domain titanomagnetite with various titanium contents stepwise thermal and alternating field demagnetization successfully isolated stable characteristic remanent magnetization from 40 out of the 43 lava flows which record 24 paleomagnetically independent directions data excluding directions regarded as abnormal transitional yield a mean direction of d i 6 8 56 3 with 95 5 2 and 51 6 corresponding to a paleomagnetic pole at 232 3 e 83 7 n with dp 5 4 and dm 7 5 consistent with previously published paleomagnetic poles for the north china block the modified thellier method with systematic partial thermoremanent magnetization ptrm checks and ptrm tail checks was conducted on 72 selected samples reliable paleointensity estimates from four lava flows yield a mean virtual dipole moment of 5 6 1 6 1022 a m2 suggesting moderately low intensity of the geomagnetic field this is in good accord with reliable paleointensity datasets for the late oligocene to middle miocene 30 15 ma but lower than the average for middle miocene to pliocene age 15 2 ma it thus appears that at some point in the mid miocene there was a transition where the field intensity changes between these two states although many more data are needed to confirm this

the 2009 l aquila earthquake central italy a source mechanism and implications for seismic hazard an edited version of this paper was published by agu copyright 2009 american geophysical union

geomagnetic paleointensity and direct age determination of the isea m0r chron abstract a combined paleomagnetic and geochronologic study has been conducted on an andesite lava sequence at jianguo liaoning province northeastern china thermal demagnetization and thermomagnetic analysis revealed that natural remanent magnetization is carried by both magnetite and hematite stepwise thermal demagnetization up to 675 c isolated well defined reverse characteristic remanent magnetization chrm in three time independent lava flows with a mean direction of d i 179 2 59 7 with 95 3 0 it also showed that a high temperature component 585 c has the same chrm direction as that of the low to medium temperature ca 170 300 585 c components a modified version of the thellier thellier paleointensity method coe j geophys res 72 1967 3247 3262 with systematic partial thermoremanent magnetization checks was used for paleointensity determinations twelve out of 39 samples yielded reliable results in the temperature interval of 170 550 c virtual dipole moment values range from 3 9 to 4 7 1022 am2 with an average of 4 2 0 1 1022 am2 40ar 39ar age determination on one lava flow is 116 8 3 0 myr 2 error relative to fish canyon sanidine 28 02 0 28 myr the magnetic reversed polarity with well defined age could correspond to the isea within the older part of the cretaceous normal superchron cns but we cannot rule out the possibility that this reversal interval could also correspond to the m0r due to many ambiguities on previous published ages on isea and m0r intervals as well as the new monitor age correction we applied it also suggests that a weak magnetic field nature already documented before the cns extended at least into the very beginning of the cns

timescale and evolution of the intracontinental tianchi volcanic shield and ignimbrite forming eruption changbaishan northeast china abstract around the tianchi eruption cone the junjianshan formation which builds the tianchi basalt shield can be divided into three units they are from bottom to top the toudao baishan and laofangzixiaoshan units respectively individual basaltic flows of the tianchi shield vary significantly in volume and in spatial distribution from a few to dozens of kilometers in length hundreds to thousands of meters in width and a few to dozens of meters in thickness k ar age data indicate that the tianchi shield was formed within a very short time period between 2 77 0 08 and 0 31 0 002 ma our detailed mapping in combination with the k ar dating shows that the toudao unit erupted at 2 77 1 99 ma the baishan unit at 1 64 1 11 ma and the laofangzixiaoshan at 0 31 0 58 ma the huge millennium eruption from the tianchi volcano sustained a 35 km high column h t and lasted a few days producing the comenditic ignimbrite around the volcano wiggle matching measurements of a sample series of charcoal wood indicate the millennium eruption might have happened around 1024 ad features of eruptions stemming from the same source surrounding the tianchi volcano plus the volcanic evolution from effusive flows to explosive eruption are interpreted to reflect crustal thickening within this intracontinental tectonic environment field observations and lava compositional variations suggest that crystal fractionation and magma mixing or mingling process must have played an important role in the development and evolution of the tianchi volcano

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity