paleointensity of the geomagnetic field from dated lavas of the chaine des puys france 1 7 12 thousand years before present paleointensities were determined by the thellier method for six dated lava flows from the chaine des puys france with ages from 7 to 12 ka ka kiloannum before present the three younger flows overlap the archaeomagnetic period for europe and the older flows extend the record to more ancient times our results support the quasi sinusoidal variation of the geomagnetic field intensity since 10 ka for europe punctuated by a peak virtual dipole moment of about 11 1022 a m2 between 8 and 10 ka the data from 10 to 12 ka show considerably lower virtual dipole moments between 4 and 5 1022 a m2 about 60 of the global average since 10 ka when compared with available worldwide data the maximum in the virtual dipole moment between 8 and 10 ka is supported by measurements from japan and possibly hawaii lower than average virtual dipole moments between 10 and 12 ka have also been observed in north america and hawaii the existing data suggest that from 8 to 10 ka the geomagnetic field supported a dipole moment up to 30 above and from 10 to 12 ka about 30 below the average value since 10 ka

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

secular variation of the earth s magnetic field and application to paleomagnetic dating of historical lava flows in chile abstract the recent geomagnetic secular variation is mainly characterized by the large growth of the south atlantic magnetic anomaly during the last three centuries first documented by the geomagnetic model gufm1 jackson et al 2000 we report new paleomagnetic results directions and paleointensities from several sites in two well dated lava flows in chile the 1835 ad eruption of the osorno volcano and the 1751 ad eruption of the llaima volcano in addition paleointensities were obtained on 14 samples from bricks of shelters built along the main road across the andes from santiago chile to mendoza argentina in 1770 5 ad the results confirm the high reliability of the global geomagnetic model gufm1 for the last three centuries with a large amplitude of the secular variation in inclination 20 and intensity 25 t results from three 14 c dated volcanic units in the time interval 1400 1750 ad indicate that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of existing global geomagnetic models at this stage precise paleomagnetic or archeomagnetic dating in south america using global models should be restricted to the last 3 centuries to illustrate the potential of paleomagnetic dating in region and time interval with very large geomagnetic secular variation we report paleomagnetic data from several sites in historical lava flows 1700 1900 ad from the antuco llaima and villarrica volcanoes that permit to refine the ages of the major historical effusive volcanic events

paleomagnetism from deception island south shetlands archipelago antarctica new insights into the interpretation of the volcanic evolution using a geomagnetic model deception island shows the most recent exposed active volcanism in the northern boundary of the bransfield trough the succession of the volcanic sequence in the island is broadly divided into pre and post caldera collapse units although a well constrained chronological identification of the well defined successive volcanic episodes is still needed a new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of deception island south shetlands archipelago antarctic peninsula region distributed in 1 volcanic breccia 3 sites and lavas 2 sites prior to the caldera collapse 2 lavas emplaced after the caldera collapse 10 sites and 3 dikes cutting pre and the lowermost post caldera collapse units 5 sites the information revealed by paleomagnetism provides new data about the evolution of the multi episodic volcanic edifice of this quaternary volcano suggesting that the present day position of the volcanic materials is close to their original emplacement position the new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model despite the uncertainties in the use of averaged paleomagnetic data per volcanic units the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre caldera units f1 and f2 erupted before 12 000 year bc the caldera collapse took place at about 8300 year bc and post caldera units s1 and s2 are younger than 2000 year bc

palaeomagnetism of the western and central sectors of the trans mexican volcanic belt implications for tectonic rotations and palaeosecular variation in the past 11 ma summary new palaeomagnetic data from the central and western sectors of the trans mexican volcanic belt tmvb are presented and analysed for palaeosecular variation psv and tectonic rotations fifty one sites with geochronological control were collected from selected volcanic lava flows which cover the temporal and spatial activity in the magmatic arc for the past 11 ma rock magnetic experiments reveal that generally ti poor titanomagnetites sometimes maghemitized are the magnetic carriers of the characteristic remanent magnetizations after analysis of detailed progressive demagnetization data 47 mean site directions are determined in addition data from previous regional palaeomagnetic study in the eastern tmvb sector 53 sites are re analysed palaeomagnetic data are grouped according to geographic distribution three structural sectors and according to age late miocene pliocene or quaternary to avoid discarding tectonic effects the less stringent criterion i e a fixed cut off angle to the mean equal to 45 was applied to identify transitional data affecting four sites palaeomagnetic results backed by positive reversal tests indicate no palaeomagnetically detectable vertical axis block rotations in the study areas the nearly geocentric axial dipole gad inclinations suggest no significant tilting effects and negligible quadrupolar contributions since the mean palaeomagnetic directions for the study areas do not differ from reference directions from the north american polar wander path two data sets for 11 5 and 5 0 ma are tested for geomagnetic purposes virtual geomagnetic poles vgps were selected using both fixed 45 and optimum variable cut off angles this resulted in vgp dispersions that increase back on time and are consistent with those obtained from globally distributed observations at the site latitude for their respective age ranges and cut off criteria palaeomagnetic data from late miocene and pliocene tmvb rocks can be considered in the databases for time averaged field taf and palaeosecular variation from lavas psvl analyses and geomagnetic field geometry characterization during the past 11 ma

paleosecular variation record of geomagnetic full vector during late miocene from the nayarit area mexico abstract we have sampled a sequence of 45 late miocene consecutive lava flows in the tepic area nayarit state mexico the age of the volcanic units lies between 8 and 9 million years ma according to available radiometric data all samples were stepwise demagnetized partly with alternating field af partly thermally with very similar results most of the rocks exhibited well defined one component remanent magnetisation with high unblocking temperatures mostly above 525 c and high median destructive fields mdf 40 50 mt rock magnetic experiments combined with microscopy show that in most cases the main magnetic mineral is ti poor titanomagnetite associated with exsoluted ilmenite continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with curie points close to that of magnetite and pseudo single domain characteristics characteristic remanent magnetisations chrm isolated after the first steps of demagnetisation are all normal polarity according to the dispersion of virtual geomagnetic pole vgp directions paleosecular variation was abnormally lower than the one observed in general during miocene considering our paleomagnetic results together with available radiometric data it seems that the volcanic units have been emplaced during a very short time span of about 0 08 million years the mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle miocene thirty one samples from eight individual flows yielded acceptable paleointensity estimates the site mean paleointensities range from 27 8 0 9 to 42 0 7 9 t the virtual dipole moments vdm range from 5 9 to 9 5 1022 am2 this corresponds to a mean value of 7 6 1 4 1022 am2 which is higher than the average vdm value for late miocene altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations these results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core with the inner core tending to stabilise core convection and hence the field when intensity is high some fluctuation of absolute intensity was detected within the same directional group dg indicating that the intensity of the geomagnetic field varies faster than its direction

palaeosecular variation in central mexico over the last 30 000 years the record from lavas summary 13 lava flows of known age ages from i4c dating which have been erupted in the last 30000 years have been studied to determine the palaeosecular variation of the geomagnetic field in central mexico samples were taken from two different monogenetic volcanic fields the michoacan guanajuato volcanic field six sites and the chichinautzin formation seven sites both part of the transmexican volcanic belt the lavas were studied in detail using rock magnetic methods magnetic susceptibility at room temperature low temperature susceptibility behaviour hysteresis loops curie temperatures combined with reflected light microscopy in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals the magnetic carriers are titanomagnetites which show differing degrees of high temperature deuteric oxidation and seem to be predominantly pseudo single domain psd though in many cases are probably a mixture of domain states mean palaeomagnetic directions and palaeointensity values using shaw and thellier techniques were obtained using several specimens from each flow our data seem to indicate a sharp easterly swing in declination about 5000 years ago which is also observed in lake sediments from central mexico the calculated values of the virtual dipole moment vdm range from 3 1 to 14 9 x a m2 our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr with a peak at about 9000 years bp these are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field

geomagnetic field intensity between 70 000 and 130 000 years b p from a volcanic sequence on la reunion indian ocean abstract a detailed paleointensity study was made of a sequence of 70 successive lava flows of the piton des neiges volcano on the island of la reunion indian ocean radiometric dating brackets the age of this sequence between 130 3 ka and 72 3 ka rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the natural remanent magnetisation nrm over 350 samples were used for paleointensity determinations carried out with the thellier method in vacuum or in an argon atmosphere of these 89 samples yielded reliable results with within flow scatter often lower than 20 these results indicate that the geomagnetic field intensity has varied at la reunion between 13 and 65 t during the period of time explored the average value 42 t is higher than the present field at la reunion 35 t the results from the upper part of the section are consistent with previous results obtained for the 82 98 ka period also at la reunion 1 and document a broad low around 95 ka not associated with large directional changes on a larger geographic scale the paleointensity values from la reunion are significantly higher than those obtained from mount etna 2 precise comparison is however difficult because of the lack of detail in the etna results in the lower part of the section a marked intensity low coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the blake event

geomagnetic dipole strength and reversal rate over the past two million years independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1 2 and thus provide information regarding the processes governing the geodynamo so far this procedure has been limited to the past 800 000 years 800 kyr ref 3 which does not include any geomagnetic reversals here we present a composite curve that shows the evolution of the dipole moment during the past two million years this reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas which were used for calibration we show that at least during this period the time averaged field was higher during periods without reversals but the amplitude of the short term oscillations remained the same as a consequence few intervals of very low intensity and thus fewer instabilities are expected during periods with a strong average dipole moment whereas more excursions and reversals are expected during periods of weak field intensity we also observe that the axial dipole begins to decay 60 80 kyr before reversals but rebuilds itself in the opposite direction in only a few thousand years

paleomagnetism and 40ar 39ar chronology of lavas from meseta del lago buenos aires patagonia 1 as part of a larger project investigating paleosecular variation in south america 36 paleomagnetic and geochronology sites were sampled on meseta del lago buenos aires 47 s 289 e 200 km east of the active plate boundary in southern patagonia basaltic lavas sampled range from late miocene to late pleistocene yet most of the individual lavas are younger than 3 3 ma thirty eight isochron ages determined via 40ar 39ar incremental heating experiments frame a detailed stratigraphy the isochron ages range from 66 9 4 1 ka for scoria cones and a youthful lava flow in the rio pinturas valley to 10 12 1 35 ma for thick plateau forming lavas exposed along the se edge of the meseta the last 3 3 myr is characterized by seven episodes of volcanism at ca 3 2 3 0 ma 2 4 ma 1 7 ma 1 35 ma 1 0 ma 750 ka 430 330 ka and 110 ka the bulk of lavas forming the surface of the meseta erupted in the last 1 7 my all sites have stable magnetization and after step demagnetization using either thermal or alternating field techniques yield characteristic directions held by magnetite and or titanomagnetite ten sites have distinct transitional directions defined by pole latitudes 55 and associated ages indicate possible connections to known reversals within the matuyama chron including the onset of the jaramillo subchron 1 016 0 01 ma the cobb mountain subchron 1 25 0 03 ma the ontong java 1 event 1 37 0 03 ma and the termination of the olduvai subchron 1 72 0 02 ma remaining sites are divided into normal 14 sites and reversed polarity 12 sites at 95 confidence mean normal and reversed directions overlap the mean direction for 26 sites is i 63 0 d 3 4 95 5 4 which is indistinguishable from the expected geocentric axial dipole paleosecular variation measured by the dispersion of virtual geomagnetic poles about the rotation axis is higher than expected at 20 0 model g using data for the past 5 myr predicts a dispersion of 17 this discrepancy may be due to true dispersion of the field in southern south america or it may be an artifact of inadequate sampling

plio pleistocene time averaged field in southern patagonia recorded in lava flows 1 paleomagnetic directions were obtained from stepwise alternating field or thermal demagnetization of 53 lava flows from southern patagonia latitudes 49 5 52 1 s that include the pali aike volcanic field and the meseta viscachas plateau lavas in addition to previous miocene late quaternary ages of these flows 40ar 39ar dates spanning from 0 1 to 15 4 ma were obtained for 17 of the sites all except one of the magnetic polarities coincide with the expected polarities of the magnetic polarity timescale cande and kent 1995 for the obtained 40ar 39ar ages the mean direction from 33 sites eliminating sites 4 ma that pass a selection criteria of 95 5 is dec 358 7 inc 68 2 95 3 5 a value that coincides within the statistical uncertainty with the direction of the geocentric axial dipole for that area inc 68 1 likewise the mean virtual geomagnetic pole vgp coincides within the statistical uncertainty with the geographic north pole the secular variation described by the vgp angular standard deviation for these sites is 17 1 a value expected for that latitude according to model g of paleosecular variation mcfadden et al 1988 the characteristics of the data presented are optimum for time averaged field taf studies because of the good age control and good quality of the paleomagnetic data 1 primary components of magnetization were obtained using principal component analysis kirschvink 1980 from at least five points and maximum angular deviation 5 2 site means were calculated with fisher statistics using at least three samples and c 38 of the 53 flows had 95 5 no results five sites or high 95 values 5 were obtained primarily from sites affected by lightning

paleomagnetic secular variation of the honolulu volcanic series 33 700 ka o ahu hawaii abstract thirty years after the precursory study j geophys res 13 1972 2129 19 dated sites with ages between 33 and 700 ka from the honolulu volcanic series hvs southeastern part of the island of o ahu in hawaii have been revisited a minimum of 16 samples per site have been studied using alternating field and thermal demagnetization both techniques yielded similar results coeval flows could be identified by relying on their k ar ages similar rock magnetic parameters and identical directions reliable paleomagnetic directions have been obtained for all sites two site mean directions which deviate significantly from the geocentric axial dipole gad correspond to periods of low dipole intensity and may thus reflect a larger proportion of non dipole components the average value of the site mean declinations is 358 6 while the mean inclination of 34 5 is 3 5 lower than the value of the geocentric dipole at the site the north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles vgp latitude the deviation of 2 3 in latitude is further reduced by 1 if we consider the existence of a standing small quadrupole component that would represent 5 of the dipole the angular dispersion of the virtual geomagnetic poles 12 is only 1 7 smaller than expected at the site latitude and thus not a firm indication of low secular variation thus there is no significant difference with the global time averaged field for the same period other recent results from the same island for an older time period display similar characteristics but with reverse and normal polarities that are not perfectly antipodal it is suggested that this could result from changes in the mean dipole field intensity during these time intervals this points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes these results do not bring any evidence for a persistent field anomaly under hawaii

the growth collapse and quiescence of teno volcano tenerife new constraints from paleomagnetic data tenerife basically consists of three miocene shield volcanoes the anaga the teno and central shield as well as the pliocene canadas volcano the temporal evolution and structural significance of each volcano with respect to the history of tenerife is still a matter of debate we present paleomagnetic results in order to enhance the view of the volcanic history of the teno volcano by means of magnetostratigraphy it is found that the initial subaerial phase shows reverse magnetizations throughout after two major sector collapses dominantly normally magnetized lavas extruded comparisons of observed magnetic polarities with the geomagnetic polarity timescale show that these volcanic activities occurred within 0 4 myr between 6 3 and 5 9 ma significantly younger flows 5 3 myr old according to their radiometric age revealed again normal polarity throughout the absence of inversely magnetized lavas in between the two normal periods indicates a volcanic hiatus or erosional phase the evolutionary sequence and the estimated high production rates for the initial building phase are similar as would be expected for a hotspot volcano the average geomagnetic field for 6 0 0 2 ma is close to an axial dipole field showing a slight far sided right handed effect the field strength determined by thellier type intensity determinations corresponds to a virtual axial dipole moment of 4 9 1022 a m2 this value is approximately half of the present day field strength but similar to values obtained for the mid miocene it also corresponds to the proposed tertiary low field level of the geomagnetic dipole moment

equatorial paleosecular variation of the geomagnetic field from 0 to 3 ma lavas from the galapagos islands abstract complete progressive thermal demagnetization of nearly 400 oriented samples from 58 sites lava flows from the galapagos islands of santa cruz san cristobal and floreana provide data for the statistical characterization of the time averaged geomagnetic field near the equator for the past few million years estimates of vgp dispersion due to paleosecular variation range from 9 2 to 11 8 depending on site selection criteria our preferred estimate based on 64 site vgps 51 accepted from this study and 13 from the 1971 study by cox is 11 4 95 confidence interval 10 2 13 0 consistent with previous estimates from the galapagos islands as well as paleosecular variation model g and confirming that angular dispersion of vgps near the equator is relatively low the mean direction is not significantly different from a geocentric axial dipole field when account is taken of southward plate motion over the galapagos hotspot preliminary paleointensity results from a comparison of the natural remanence with a total thermal remanence produced in a lab field of 15 t on a subset of 321 samples from 48 sites that had relatively small changes in magnetic susceptibility after laboratory heating suggest that the time averaged field was about 21 t or only two thirds the present strength in agreement with some other recent estimates

paleomagnetism paleointensity and geochronology of miocene basalts and baked sediments from velay oriental french massif central paleomagnetic study of miocene basalts and baked sediments from velay oriental france yields a paleomagnetic pole with coordinates 84 1 n 171 2 e a95 8 6 k 29 n 11 the pole is supported by a positive reversal test and it corresponds well with the apparent polar wander path for eurasia for 10 ma paleosecular variation estimated as the angular standard deviation of the vgp distribution 14 9 6 5 3 5 is close to expected suggesting that the paleomagnetic pole represents a time averaged field moreover five new 40ar 39ar plateau ages falling between 9 20 and 13 55 ma were obtained thellier paleointensity experiments were carried out on both basalts and baked sediments almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results in contrast the baked sediments are ideal for thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine grained magnetite and hematite with a large proportion of superparamagnetic grains in addition to the standard partial thermoremanent magnetization ptrm check used in thellier experiments to detect chemical crystalline alteration we introduce a second ptrm tail check designed to test the basic prerequisite of the method namely the equality of unblocking and blocking temperatures for the baked sediments unblocking and blocking temperatures are found to be identical which indicates that the remanence is carried by single domain grains

computer synthesis of geomagnetic palaeosecular variations computer models designed to synthesize palaeosecular variations of the geomagnetic field cast doubt on some widely accepted palaeomagnetic dogmas westward or eastward drift cannot be uniquely deduced from the sense of rotation of the geomagnetic vector the use of unit vectors in statistical analyses leads to systematic errors which render investigation of the global symmetry of the palaeofield more difficult than previously supposed at least some geomagnetic excursions may be regarded as large amplitude secular variations

palaeomagnetic results from the indian heaven volcanic field south central washington summary palaeomagnetic data have been obtained from plio pleistocene rocks of the indian heaven volcanic field ihvf located in the high cascade province in southern washington alternating field demagnetization yielded well defined directions from 56 individual lava flows sites from 23 volcanic units all units sampled were normally magnetized suggesting that volcanic activity within the indian heaven region occurred primarily during the last 0 73 myr and not over the last 3 75 myr as suggested by k ar ages combining palaeomagnetically similar site mean directions from lava flows from the same volcanic unit resulted in n 35 the mean direction calculated from this data set is i 65 2 and d 0 2 095 3 3 k 56 4 while the palaeomagnetic pole calculated using virtual geomagnetic pole vgp data is located at 87 7 n 231 4 e or95 4 7 k 27 2 this pole is not significantly different from the pole determined from all data n 56 or the rotational axis and is slightly near sided with respect to the mean sampling site longitude the between site angular dispersion of the vgps about the mean for n 35 and n 56 although low is not statistically different at the 95 confidence level from the expected value predicted by model f of mcfadden mcelhinny 1984 our data together with other recent data from the western united states do not confirm the presence of a persistent standing non dipole field component in this region during the plio pleistocene as suggested by brown mertzman 1979 additionally these data suggest that vgp angular dispersion associated with the geomagnetic field has been decreasing over the last 5 myr

geomagnetic paleosecular variation at hawaii around 3 ma from a sequence of 107 lava flows at kaena point oahu abstract we have sampled a sequence of 107 lava flows in the waianae series on oahu hawaii in two separate stretches the first 51 flows at kaena point extends from sea level to an altitude of 190 m and the second 56 flows follows the satellite tracking station road from 218 m to 360 m above sea level thermal and af demagnetization yield very similar results and reveal only normal polarities k ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the upper mammoth and the lower kaena polarity transitions 3 22 3 11 ma some of the flows are serially correlated filtering does not however drastically affect the final results inclinations are shallower than those expected from a centered dipole field although large 13 the inclination anomaly is consistent with results from other sites at the same latitude the scatter of the vgps about the geographic pole is consistent with available data from other regions at similar latitudes the scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation therefore these results do not support the hypothesis of a pacific dipole window

paleomagnetism and 40ar 39ar ages from volcanics extruded during the matuyama and brunhes chrons near mcmurdo sound antarctica maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis these lobes persist in field models for the last few millenia the anomalous lobes are associated with observation sites at extreme southerly latitudes to determine whether these features persist for millions of years paleomagnetic vector data from the continent of antarctica are essential we present here new paleomagnetic vector data and ar 40 ar 39 ages from lava flows spanning the brunhes and matuyama chrons from the vicinity of mcmurdo sound antarctica oriented paleomagnetic samples were collected from 50 lava flows by e mankinen and a cox in the 1965 1966 austral summer season preliminary data based largely on the natural remanent magnetization nrm directions were published by mankinen and cox 1988 we have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection of these only one site fails to meet our acceptance criteria for directional data seven sites are reversely magnetized the mean normal and reverse directions are antipodal the combined mean direction has d over bar 12 i over bar 86 alpha 4 kappa 37 and is indistinguishable from that expected from a gad field we obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 zam 2 and a standard deviation of 28 zam 2 ar 40 ar 39 age determinations were successfully carried out on samples from 18 of the flows our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales

reconstructing projectile technology during the pre pottery neolithic b in the levant an integrated approach to large tanged points from halula abstract large tanged points made on bidirectional blades constitute the most characteristic tool type during the pre pottery neolithic b in the levant studies on projectile typology and on bidirectional technology have revealed important stylistic differences reflecting chronological and geographical patterning contributing significantly to the understanding of early farming communities in the near east however the reconstruction of the weapons these large tanged points were part of has not received the same attention this investigation aims to fully characterize stone point production at halula a ppnb settlement in the middle euphrates valley and reconstruct the type of weapons and delivery mechanisms used our study also includes the analysis of various ballistic attributes using a series of recent morpho metric methods and comparison with ethnographic and experimental data about projectiles of known use results indicate that byblos points might have been used as dart points propelled with the help of spear throwers indicating a shift from bow to spear thrower in projectile technology associated with the appearance and expansion of bidirectional blade technology during the ppnb in the levant and synchronous with the consolidation of agricultural systems in the region

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

paleomagnetism of miocene volcanics on sao tome paleosecular variation at the equator and a comparison to its latitudinal dependence over the last 5 myr a collection was made in january 2009 of 10 oriented samples from each of 54 sites in lavas on sao tome island nominal location 0 3o n 6 5o e some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation overall magnetic properties were excellent highly stable magnetizations carried by pseudo single domain magnetite after principal component analysis of progressive alternating field demagnetization data for the samples twenty two sites had normal polarity magnetizations d 0 6o i 8 3o 95 4 3o 53 1 and 20 had reverse magnetizations d 176 0o i 4 2o 95 7 3o 20 8 the directions are within 5 of antiparallel yielding a positive reversal test the combined data set of 42 site mean virtual geomagnetic poles converted to common normal polarity yields a pole position at 86 0 n 211 5oe a95 3 1o ar ar and k ar dating reveals that these rocks are miocene in age 5 11 ma old enough to allow northerly plate motion to help explain the slightly far sided pole position the between site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation sb and equaled 11 4 with 95 confidence interval between 9 9 and 13 4 this article is protected by copyright all rights reserved

persistent anomalous inclinations recorded in the koolau volcanic series on the island of oahu hawaii usa between 1 8 and 2 6 ma abstract ten sites of basaltic lava flows from the koolau series island of oahu hawaii were selected to improve the determination of the matuyama time averaged geomagnetic field between 1 8 and 2 6 ma identical primary directions with reverse polarity carried by magnetite were isolated by alternating field and thermal demagnetization in 13 16 specimens from each site the final mean direction points south while the inclination is 7 lower than the inclination of the geocentric axial dipole gad this offset is reduced after correcting the site latitude for the pacific plate motion which has been neglected so far in most studies the mean corrected direction is almost perfectly antipodal to the brunhes mean direction derived from the honolulu series in the same island despite this correction the inclination remains 3 6 larger than the inclination of the axial dipole and cannot be accounted for by a small axial quadrupole which would reverse with the dipole the mean inclinations of other hawaiian records mostly younger than 1 ma involving long sequences of lava flows exhibit similar deviations from the gad which are too large to be accounted for by a small axial quadropole tilting of all sections is difficult to defend for so many localities and persistant secondary components have not been detected thus these inclination anomalies would reflect the presence of a long term standing component under hawaii but crustal magnetization linked to the volcanic edifice cannot be completely ruled out

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

paleomagnetic directions from mid latitude sites in the southern hemisphere argentina contribution to time averaged field models back arc volcanism located to the east of the andean cordillera was sampled in the argentina provinces of mendoza and neuquen for paleomagnetic time average field and paleosecular investigations the activity ranges from 2 ma to very recent time with a large variety of products from basalts to highly differentiated lavas after removal of sites affected by lightning those with 95 higher than 10 and combining of nearby sites displaying close directions we present new paleomagnetic results from 31 flows units belonging to two volcanic massifs the payun matru and the cerro nevado previous and new k ar age determinations constrain the volcanic activity of these massifs from 300 to 0 ka and from 1 9 to 0 9 ma respectively most paleomagnetic samples have nrm intensities between about 1 and 20 a m and depict progressive removal of magnetization components in a consistent fashion during stepwise af or thermal demagnetization nineteen flows yielded a normal direction declination 354 8 inclination 53 0 95 6 8 and 12 flows a reverse direction declination 181 0 inclination 52 3 95 5 9 the combined data yielded a mean direction declination 357 3 inclination 52 8 95 4 6 which is not statistically different from the axial dipole field g10 expected at this latitude 36 s the angular dispersion of virtual geomagnetic poles calculated from flows with normal directions asd 16 5 compares well with the observed value from global datasets for this site latitude but flows with reverse directions display a surprisingly low dispersion asd 12 5 since most reverse directions were sampled from flows ranging between 1 9 and 0 9 ma this can be interpreted as an interval of low paleomagnetic secular variation additional data also with accurate time constraints are obviously needed to better support this observation finally no convincing evidence for a complex time average field significantly different from the axial dipole can be supported by this study for the last 2 myr

paleomagnetism of the southwestern u s a recorded by 0 5 ma igneous rocks 1 the issue of permanent nondipole contributions to the time averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field in this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern u s a in the age range 0 5 ma and investigate both the time averaged field and its variability about the mean value several decades of work in the southwestern united states have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites as part of a new investigation of the san francisco volcanics we collected paleomagnetic samples from 47 lava flows many of which have been previously dated the new data combined with published data are highly scattered contributions to the scatter were considered and we find that removal of data sets from tectonically active areas and judicious selection according to fisher s 1953 precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal monte carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector average directions for the southwestern u s a show small deviations from a geocentric axial dipole field but these cannot be considered statistically significant virtual geomagnetic pole vgp dispersions are consistent with those from globally distributed observations analyzed by mcelhinny and mcfadden 1997 however a systematic investigation of the effect of imposing a cutoff on vgps with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction such a procedure can result in severe underestimates of the variance in the geomagnetic field a more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution

padm2m a penalized maximum likelihood model of the 0 2 ma palaeomagnetic axial dipole moment summary we present a new time varying model for palaeomagnetic axial dipole moment padm for the past 2 myr and compare it with earlier virtual axial dipole moment vadm reconstructions which have been based on stacking and averaging scaled relative palaeointensity records the padm is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood pml approach to recover a cubic b spline representation of axial dipole field variations on million year timescales the pml method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average vadm reconstructions we apply the pml method to a set of 96 032 published data 1800 palaeointensities from igneous rocks 3300 archaeointensities and 86 relative palaeointensity time series of variable lengths and resolutions two models are discussed padm2mp is a trial model based on a subset of the nine longest available sedimentary records padm2m uses a comprehensive data set 76 records 81 446 data 10 records were eliminated and is our preferred model padm2m has a lower mean than existing vadm reconstructions but shows similarities in long period variability some differences in timing amplitude and resolution of certain features can be attributed to variations in age assignments others result from our more comprehensive data set and a reduction in bias attributable to pml modelling padm2m has an average axial dipole moment over 0 2 ma of 5 3 1022 am2 with a standard deviation of 1 5 1022 am2 the brunhes chron average 6 2 1022 am2 is higher than for earlier epochs of matuyama 4 8 1022 am2 as seen in some previous studies the power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 myr 1 we see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour

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

paleointensity of the earth s magnetic field recorded by two late quaternary volcanic sequences at the island of la reunion indian ocean a paleomagnetic study has been undertaken on two late quaternary volcanic sequences from the piton de la fournaise volcano island of la reunion in order to provide absolute paleointensities of the earth s magnetic field from a site in the southern hemisphere the pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time a detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the thellier method samples with minimum overprint composed by single domain or pseudo single domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during thellier s experiments values of the paleofield strength from 19 to 55 mt have been obtained on 23 flows among the 30 which were sampled few variations 7 5 9 9 1022 a m2 are observed in the virtual dipole moments vdms determined on the youngest sequence 5 12 ka those values are within the range of fluctuations of the worldwide variations of vdms for this period on the other hand data from the oldest sequence 82 98 ka are the first paleointensity values obtained in that time period and the observed vdms extend from 4 1 to 8 8 1022 a m2 these results suggest that for this time interval the field did not depart from its average behavior

absolute paleointensity from hawaiian lavas younger than 35 ka abstract paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3 3 and 28 2 ka from the mauna loa volcano in the big island of hawaii determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in hawaii although the overall average field intensity appears to be lower since the present results were determined at higher temperatures than in the previous studies this discrepancy raises questions regarding the selection of low versus high temperature segments that are usually made for absolute paleointensity the virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows when averaged within finite time intervals the worldwide values match nicely the variations of the sint 200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period the convergence between the existing records at hawaii and the rest of the world does not favour the presence of persistent strong non dipole components beneath hawaii for this period

secular variation of the middle and late miocene geomagnetic field recorded by the columbia river basalt group in oregon idaho and washington usa

revised and updated paleomagnetic results from costa rica 1 paleomagnetic results from globally distributed lava flows have been collected and analyzed under the time averaged field initiative tafi a multi institutional collaboration started in 1996 and designed to improve the geographic and temporal coverage of the 0 5 ma paleomagnetic database for studying both the time averaged field and its very long term secular variations paleomagnetic samples were collected from 35 volcanic units either lava flows or ignimbrites in costa rica in december 1998 and february 2000 from the cordilleras central and guanacaste the underlying canas liberia and bagaces formations and from volcano arenal age estimates range from approximately 40 ka to slightly over 6 ma although initial results from these sites were used in a global synthesis of tafi data by johnson et al 2008 a full description of methodology was not presented this paper documents the definitive collection of results comprising 28 paleomagnetic directions 24 normal 4 reversed with enhanced precision and new geological interpretations adding two paleointensity estimates and 19 correlated 40ar 39ar radiometric ages the average field direction is consistent with that of a geocentric axial dipole and dispersion of virtual geomagnetic poles 17 3 4 6 is in general agreement with predictions from several statistical paleosecular variation models paleointensity estimates from two sites give an average field strength of 26 3 t and a virtual axial dipole moment of 65 zam2 the definitive results provide a useful augmentation of the global database for the longer term goal of developing new statistical descriptions of paleomagnetic field behavior

paleomagnetism of ar ar dated lava flows from the ceboruco san pedro volcanic field western mexico evidence for the matuyama brunhes transition precursor and a fully reversed geomagnetic event in the brunhes chron 1 we report a detailed paleomagnetic and rock magnetic study of 17 independent lava flows belonging to the trans mexican volcanic belt 175 oriented samples were collected in the ceboruco san pedro volcanic field these sites were previously dated by means of a state of the art 40ar 39ar geochronological method and span from 819 to 2 ka rock magnetic experiments which included continuous susceptibility and hysteresis measurements point to simple magnetic mineralogy in most cases the remanence is carried by ti poor titanomagnetite of pseudosingle domain magnetic structure fourteen flows give normal magnetic polarities while two are reversely magnetized only one cooling unit yields intermediate paleodirections the paleodirections of the flow dated at 819 25 ka correspond to a vgp latitude of 18 n this anomalous field behavior apparently recorded prior to the matuyama brunhes m b reversal may coincide with the transitionally magnetized lavas on la palma canary islands 40ar 39ar age of 822 2 8 7 ka and with an event featured in several marine sediment records thus this geomagnetic event defined as m b precursor is probably global in extent two independent lava flows dated at 623 91 and 614 16 ka yield reverse paleodirections age uncertainties make it difficult to claim the discovery of a new geomagnetic event it is possible that these lavas erupted during the worldwide observable big lost event 40ar 39ar age of 580 2 7 8 ka which has probably been longer and more complex than it is generally believed for geomagnetic excursions

paleomagnetic results from the newer volcanics of victoria contribution to the time averaged field initiative 1 the newer volcanics of victoria australia were sampled in 2001 and 2003 as part of the time averaged field initiative which is designed to provide a reliable data set for modeling the earth s magnetic field over the past 5 my the collection consisted of ten cores per site at 42 sites complete demagnetization was carried out on all samples using af or thermal demagnetization and line fitting techniques were employed to determine the resultant vector thirty two of these sites gave results that met the criterion that n 5 and 95 was less than 6 20 sites were reversely magnetized dec 173 4 inc 54 6 95 3 8 and 13 normally magnetized dec 2 1 inc 62 3 95 6 3 the data gave a global mean paleomagnetic pole lat 87 2 long 21 9 and a95 4 4 which is not significantly different from the pole of rotation for the present location of victoria and for its latitude 2 5 million years ago when most of the lavas were erupted

palaeosecular variation recorded by 9 ka to 2 5 ma old lavas from martinique island new evidence for the la palma aborted reversal 617 ka ago

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

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

geomagnetic intensity and inclination variations at hawaii for the past 98 kyr from core soh 4 big island a new study and a comparison with existing contemporary data abstract reliable thellier thellier paleointensity determinations were obtained from 321 out of 766 examined samples from the upper 200 units 675 m of core soh 4 drilled in the kilauea volcano the main trend of the soh 4 record is a marked decrease from values higher than present in the last 4 kyr to values about 40 smaller from about 20 kyr to the bottom of the sampled interval three main lows coinciding with shallow or negative inclinations observed in the record were identified with an excursion previously documented in hawaii and the mono lake and laschamps events for the last 15 kyr the soh 4 intensities are largely in agreement with the worldwide distribution of volcanic data in the 15 75 kyr interval the general lack of volcanic data makes comparison with the soh 4 data difficult the soh 4 record is similar to a composite record of paleointensity largely based on a recent north atlantic sedimentary paleointensity stack napis 75 the soh 4 record is also generally consistent with a record of geomagnetic field intensity obtained from a study of 36 cl in the grip ice core calculated with the assumption that the production rate of 36 cl is controlled only by the geomagnetic field one pronounced discrepancy between the different records is observed around 50 kyr bp when the sedimentary and cosmogenic based records document high intensities which are not observed in soh 4 these high intensities are also absent in a recent record based on the analyses of sea floor magnetic anomalies the histogram of vadms obtained from soh 4 is different from that obtained from the sedimentary or 36 cl based records and is more skewed than the distribution given by kono and tanaka for the last 2 5 million years

a paleomagnetic and paleointensity study on pleistocene and pliocene basaltic flows from the djavakheti highland southern georgia caucasus abstract new paleomagnetic rock magnetic and paleointensity results obtained on samples from 23 basaltic lava flows belonging to four different flow sequences mashavera kvemo orozmani zemo karabulaki and diliska of pleistocene and pliocene age from the eastern djavakheti highland in southern georgia are presented radiometric dating of these sequences yields ages between 1 8 and 2 18 ma for mashavera 2 07 and 2 58 ma for zemo karabulakhi and 2 12 and 3 27 for diliska no radiometric ages are available for the kvemo orozmani sequence which is considered to be coeval to the mashavera sequence rock magnetic experiments including measurement of thermomagnetic hysteresis and irm acquisition curves suggest low ti titanomagnetite as main carrier of remanence although a lower curie temperature component was also observed in several cases reversible and non reversible curves were recorded in thermomagnetic experiments paleomagnetic analysis generally indicated the presence of a single component mainly in the mashavera sequence but also two more or less superimposed components in some other cases in 21 sites a characteristic component could be determined and all except one were characterised by normal polarity directions flows from the mashavera sequence had a rather steep inclination 73 1 nevertheless a mean paleomagnetic direction of all four sequences is obtained d 8 5 i 60 8 n 4 95 11 7 k 62 7 which agrees with the plio quaternary directions obtained in previous studies in georgia the paleomagnetic pole obtained latitude 82 1 longitude 118 2 a 95 8 0 k 240 7 agrees with the pole values of both the 0 ma and the 5 ma windows of the synthetic eurasian polar wander path from besse and courtillot 2002 in order to analyse the behaviour of secular variation the scatter of paleosecular variation of virtual geomagnetic poles of both the mashavera flow and all 18 studied flows of pleistocene age was calculated it could be observed that both data sets seem to fit well the expected scatter at latitude 41 n paleointensity experiments were carried out with the coe modification of the thellier method twenty five out of 84 samples 30 provided reliable paleointensity results these successful results were mainly obtained in the mashavera sequence most flows yielded paleointensity results in the 30 45 t range in accordance with expected pliocene to present day intensities two flows however located near the top of the mashavera sequence yield high paleointensity values around 60 t anomalous paleointensity results in the upper lying mashavera flows together with the steep inclinations observed in that sequence could perhaps signal the near onset of the olduvai matuyama reversal

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

new paleomagnetic and paleointensity data from pliocene lava flows from the lesser caucasus abstract a paleomagnetic rock magnetic and paleointensity study has been carried out on 14 basaltic lava flows from two pliocene k ar age between 3 09 0 10 ma and 4 00 0 15 ma sequences apnia and korxi from the eastern djhavakheti highland in southern georgia caucasus measurement of strong field magnetisation versus temperature curves yielded three types of thermomagnetic curves i reversible curves with magnetite as only remanence carrier type h ii irreversible curves with magnetite as only carrier of remanence type h and iii irreversible curves showing a low curie temperature phase and magnetite type l analysis of hysteresis curves showed that samples were characterised by a mixture of single domain and multi domain grains paleomagnetic experiments allowed determining characteristic components for all flows and normal polarities 6 flows reversed polarities 7 flows and intermediate polarities 1 flow were observed paleomagnetic poles were calculated using only those sites unequivocally showing normal or reversed polarities the paleomagnetic pole obtained from flows of both combined sequences latitude 77 9 n longitude 152 1 e n 13 a 95 11 8 k 13 4 showed a good agreement with the 5 ma window of the european synthetic apparent polar wander path of besse and courtillot 2002 the paleomagnetic direction of the combined apnia korxi flows agrees well with the expected one showing no significant tectonic rotation the latter cannot be however completely excluded in the korxi section in that section analysis of the angular dispersion of virtual geomagnetic poles yields a much higher value than expected paleointensity experiments using the coe method were performed on 31 specimens from 10 flows after application of specific selection criteria 19 samples from 8 flows were observed to provide successful determinations with mean flow values showing a wide scatter if only flows with more than one successful paleointensity determination are taken into account virtual dipole moments vdms vary between 3 5 10 22 a m 2 and 8 3 10 22 a m 2 in intermediate polarity site ap2 no weak transitional paleostrength values were observed

age and magnetism of lavas in jokuldalur area eastern iceland gilsa event revisited abstract we present results of paleomagnetic measurements and k ar age determinations of 38 lava flows collected in five separate sections in the jokuldalur area of eastern iceland including one section previously studied by watkins et al watkins n d kristjansson l mcdougall i 1975 a detailed paleomagnetic survey of the type location for the gilsa geomagnetic polarity event earth planet sci lett 27 436 444 these sites are close to the type locality of the normal gilsa event in the matuyama chron first identified by mcdougall and wensink mcdougall i wensink h 1966 paleomagnetism and geochronology of the pliocene pleistocene lavas in iceland earth planet sci lett 1 232 236 using new experimental results as well as field observations we could correlate the sequence of lava flows in the jokuldalur area spanning the time interval between 1 8 and 0 5 ma the magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for brunhes matuyama ages given by cande and kent cande s c kent d v 1995 revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic j geophys res 100 6093 6095 based on marine magnetic anomalies except that the gilsa event needs to be added existence of olduvai age lavas under gilsa was inferred but not certain our results are fairly in good agreement with the former studies by wensink wensink h 1964a secular variation of earth magnetism in plio pleistocene basalts of eastern iceland geol mijnbouw 43 403 413 wensink h 1964b paleomagnetic stratigraphy of younger basalts and intercalated plio pleistocene tillites in iceland geol rund 54 364 384 and watkins et al watkins n d kristjansson l mcdougall i 1975 a detailed paleomagnetic survey of the type location for the gilsa geomagnetic polarity event earth planet sci lett 27 436 444 but the assignment to the polarity zones is different because of the newly obtained k ar ages based on the present study we propose that the gilsa event is a short normal subchron in the matuyama chron distinct from and above the more well established oldvai subchron

absolute paleointensities and paleodirections of miocene and pliocene lavas from fernando de noronha brazil abstract forty four volcanic units were sampled on the equatorial atlantic island of fernando de noronha brazil twenty seven sites are from the 1 8 3 3 myr old quixaba formation and the others are from the approximately 8 12 myr old remedios formation analyses of paleodirections yielded normal and reverse polarities in both formations with virtual geomagnetic poles vgps and vgp dispersion s f of vgp lat 86 9 vgp long 136 3 s f 11 2 for the quixaba formation and vgp lat 86 8 vgp long 196 7 s f 15 5 for the remedios formation successful paleointensity determinations were obtained for 9 of the 27 sampled units in the quixaba formation a modified thellier method was used that includes checks to monitor alterations and repetition of demagnetization steps to detect multidomain tails of partial thermoremanent magnetizations ptrm and magnetization stabilization processes furthermore the additivity of ptrms was monitored during this modified technique of the thellier experiment these additivity checks provide a quality test for correcting the paleointensity determination for magnetomineralogical changes which occur during laboratory treatment such correction methods were used for analyzing approximately 50 of the determinations and lead to a reduction of the within site scatter the mean paleointensity result of 29 3 t corresponding to a virtual axial dipole moment vadm of 7 5 10 22 a m 2 yields approximately the present day field strength mean values of the paleodirections for volcanic units of reversed and normal polarity are closer to a dipolar field configuration than previously observed today s geomagnetic field in the equatorial atlantic shows a significant dipole anomaly our results indicate that this anomaly was far less pronounced in the geological past

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

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

40ar 39ar chronology and paleomagnetism of quaternary basaltic lavas from the persani mountains east carpathians abstract quaternary volcanism in the persani mountains forms an na alkali basaltic province inside the bend area of the carpathians in the southeastern part of europe previous k ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles which were tentatively associated with the cobb mountain subchron and a brunhes chron excursion we report a new paleomagnetic and rock magnetic study coupled with 40ar 39ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity 19 are normal polarity and 16 have transitional polarity 40ar 39ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142 41 and 800 25 ka four of the normally magnetized lavas erupted at 1060 10 1062 24 684 21 and 683 28 ka and two transitionally magnetized lavas formed at 1221 11 and 799 21 ka both the new 40ar 39ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the jaramillo and brunhes chrons this results shows that the last phases of alkalic and calc alkaline magmatism in the south east carpathians were contemporaneous the age of the older transitionally magnetized lava flow is within error of recent unspiked k ar and astrochronologic ages for the reversal that defines the onset of the cobb mountain normal polarity subchron the age of the younger transitional lava is similar to that of an excursion that preceded the matuyama brunhes polarity reversal and which has come to be known as the matuyama brunhes precursor omitting the excursion data the dispersion of the virtual geomagnetic poles around 19 is larger than the expected value around 45 n from the global compilation but closer to the value obtained only from the time averaged geomagnetic field initiative studies

paleomagnetic investigation of some volcanic rocks from the mcmurdo volcanic province antarctica paleomagnetic data for lava flows from sporadic but long lived eruptions in the mcmurdo sound region are combined with previously published geologic and geochronologic data to determine the general eruptive sequence of the area lava flows in the walcott bay area were erupted during the gauss normal matuyama reversed and brunhes normal polarity chrons the youngest flows on black island probably erupted near the boundary between the gilbert and gauss chrons the most recent activity was concentrated on the volcanic edifices of mounts morning and discovery and on ross island where mount erebus is currently active combining virtual geomagnetic poles calculated for 39 units sampled during this study with those of eight flows that were published previously yields a mean paleomagnetic pole at 87 3 n 317 3 e 95 6 3 the ancient geomagnetic field dispersion about this mean pole is 23 5 with upper and lower limits of 95 confidence equal to 27 4 and 20 5 respectively this value probably is a reasonable estimate of secular variation for the antarctic continent during pliocene and pleistocene time

paleomagnetic secular variation study of ar ar dated lavas flows from tacambaro area central mexico possible evidence of intra jaramillo geomagnetic excursion in volcanic rocks more than 350 oriented paleomagnetic cores were obtained for rock magnetic and paleomagnetic analysis from radiometrically dated 40 ar 39 ar magmatic rocks occurring in the southern segment jorullo and tacambaro areas of the michoacan guanajuato volcanic field in the trans mexican volcanic belt most of the lavas 37 stem from monogenetic volcanoes dated at less than 4 ma two additional sites were sampled from the plutonic basement dated at 33 30 ma primary remanences carried by low ti titanomagnetites allowed to determining 34 reliable site mean directions of mostly normal 27 but also reversed 7 polarities the mean directions of these two populations are antipodal and suggest neither major vertical axis rotations with respect to the north america craton nor tilting in the region for the last 4 ma rotation and flattening of the inclination parameters being less than 5 9 3 8 and 0 1 3 9 respectively the corresponding paleomagnetic pole obtained for pliocene pleistocene times is plat 83 4 plon 2 4 n 32 a95 2 7 virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation psv investigations at low latitudes from lavas for the last 5 ma showing a geomagnetic dispersion value that is in agreement with available psv models when comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the cenozoic geomagnetic polarity time scale gpts a good correlation is observable this finding underscores the suitability of data obtained on lavas in central mexico for contributing to the gpts furthermore the detection of short lived geomagnetic features seems possible since the possible evidence of intra jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks

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

holocene paleosecular variation from dated lava flows on maui hawaii ow inclination and low secular variation seem to be present for at least the past 5 myear in central pacific the period covering the past 10 kyear is crucial to the study of the field variability and to fill the gap between historical field measurements and long term paleomagnetic records we have conducted a paleomagnetic study of 13 sites of basaltic lava flows from maui island with 14c ages between 10 3 and 0 2 ka two other sites dated at 45 and 730 ka were also sampled eight to 10 samples from each site were demagnetized using thermal treatment and companion specimens from the same samples were demagnetized by alternating fields af thermomagnetic and hysteresis measurements indicated that magnetite 575 c in fine grains was the dominant magnetic carrier although in many cases we also observed a low temperature phase which is likely carried by titanomagnetite with low titanium content the existence of relatively high coercivities associated with these two mineralogical phases generated overlapping components which could not be properly isolated using af demagnetization successful results were obtained after thermal demagnetization for 13 sites with a mean inclination of 36 9 4 the mean inclination inc 36 3 of the eleven sites younger than 10 5 ka is also very close to the value 37 of the geocentric axial dipole gad at the site latitude but the angular dispersion of the virtual geomagnetic poles vgps about the spin axis 9 with n 13 and 6 7 with n 11 is significantly lower than the predictions of the models of paleosecular variation at this latitude the inclination variations for the past 10 kyear are in excellent agreement with the very detailed dataset which has previously been obtained from the big island of hawaii the mean inclination of all volcanic records n 132 is 1 8 lower than expected but this is likely caused by the lack of records between 5 and 7 ka b p we note also that the inclinations from lake waiau sediments big island are shallower than those of the volcanic records mauna kea hawaii and were thus probably affected by compaction we infer that there is no striking evidence for an inclination anomaly under hawaii during this period being aware of the need for additional records covering at least the 5 7 ka b p time interval the absence of a systematic deviation going beyond the dispersion of the inclinations obtained for the past millions of years neither pleads for a significant long term persistent anomaly however all studies report a low dispersion of the vgps which must reflect low secular variation

geomagnetic paleosecular variation recorded in plio pleistocene volcanic rocks from possession island crozet archipelago southern indian ocean possession island in the crozet archipelago consists of volcanic units erupted mainly between 5 and 0 5 ma a paleomagnetic sampling was carried out along several sections distributed near the northern eastern and southeastern coasts a total of 45 independent flows were sampled 320 samples for each flow a precisely defined characteristic remanence direction was usually isolated after a careful progressive cleaning in alternating fields however particularly complex remanence behavior is often observed the magnetostratigraphy of the lava pile is quite simple with reversed rocks in the lower part and normal units in the upper part of two sections a third section is of normal polarity throughout its whole thickness including three excursional directions we did not find any intermediate directions between the normal and reverse magnetozones thus we have no evidence for the recording of the matuyama brunhes transition expected from a previous study watkins 1972 the amplitude of paleosecular variation estimated from between flow dispersion from the field of an axial dipole is 11 8 with 95 confidence limits between 9 3 deg and 14 0 deg this value is consistent with the general anisotropic statistical model for paleosecular variation of constable and johnson 1999

paleomagnetic field properties at high southern latitude 1 statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales previous paleomagnetic studies have lacked high latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations these simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude whereas lower latitude observations are expressions of columnar helical flow outside the tangent cylinder this paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the erebus volcanic province evp antarctica and 21 new age determinations by the 40ar 39ar incremental heating method the new evp data are combined with previously published paleomagnetic and geochronological results providing 133 sites 91 having radioisotopic dates modified thellier thellier paleointensity estimates are reported for 47 sites 37 have dates ages for the combined data set span 0 03 to 13 42 ma the 125 high quality evp directional data selected from the merged data set have a non fisherian distribution and a mean direction with an inclination anomaly of 3 but 95 confidence limits include the prediction from a geocentric axial dipole virtual geomagnetic pole vgp dispersions for brunhes matuyama and the combined 0 5 ma data set are consistently high compared with values from middle to low latitude regions regardless of the criterion used to determine transitional fields with vgp latitude cut off at 45 the dispersion 23 9 2 1 for the combined 0 5 ma evp data set is consistent with earlier high latitude data and paleosecular variation psv in model g but not with some more recent statistical psv models mean evp paleointensity of 31 5 2 4 t derived from 41 high quality sites is about half the current value at mcmurdo 63 t the result is essentially independent of data selection criteria high vgp dispersion and low intensity values support the global observation of anticorrelation between directional variability and field strength simulations of time varying dipole strength show that uneven temporal sampling may bias the mean evp intensity estimate but the possibility of persistently anomalous field behavior at high latitude cannot be excluded

paleosecular variation and gad studies of 0 2 ma flow sequences from the aleutian islands alaska 1 a study was undertaken in the late 1960s to investigate paleosecular variations of the geomagnetic field as recorded in volcanic rocks from the aleutian islands the early results were internally consistent but to be wholly credible by today s standards needed more detailed demagnetization complete thermal demagnetization protocols have been applied to the unmeasured archived samples from the six flow sequences used in the initial study and 40ar 39ar techniques have been used to improve the time resolution the flow sequences have ages ranging from about 50 ka to 2 ma and the number of sequential flows in the individual sequences varies from 8 to 21 after strict selection criteria were applied mad 5 95 5 for both demagnetization data from samples and samples within one flow the number of acceptable flows per flow sequence dropped to between 5 and 15 with the exception of a sequence showing transitional field behavior the between flow dispersion and the 95 values for the other sequences were notably low with respect to secular variation models and their mean directions were very close to the gad field since the time represented by the individual sequences is not well determined the low dispersion could represent very short eruption times in contrast the lack of dispersion with respect to the gad field can be taken to indicate good time averaging since the locations of the sampled flows are at roughly the same latitude about 50 n but are spread over about 10 of longitude the dispersion was calculated for both the locality means and the flow means these data represent the whole 2 myr and give a dispersion which is lower than current secular variation models predict a similar data set published for locations in western canada that are at roughly the same latitude and overlap in age with the aleutian sites gives dispersions that are close to the model predictions at face value this can be interpreted as indicating low secular variation for the aleutian sites

localized ductile shear below the seismogenic zone structural analysis of an exhumed strike slip fault austrian alps 1 the miocene salzachtal ennstal mariazell puchberg semp strike slip fault in austria allows study of the internal structure of a fault zone from the near surface to 30 km depth as it enters the tauern window along the rinderkarsee shear zone the semp fault passes from a dominantly brittle to a dominantly ductile structure the shear zone consists of three 1 to 100 m wide zones of brittle ductile and ductile deformation separated by 500 m wide zones of less deformed rocks the southern shear zone is mylonitic with ductile amphibole and plagioclase weak crystal preferred orientations imply that the main deformation mechanism was dislocation accommodated grain boundary sliding the northern and central shear zones are characterized by discrete millimeter wide shear zones with ductile quartz muscovite and biotite and brittle feldspar shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite strain then localized in muscovite rich grain boundary shear zones that linked to form throughgoing shear zones

paleomagnetic results from the snake river plain contribution to the time averaged field global database 1 this study presents paleomagnetic results from the snake river plain srp in southern idaho as a contribution to the time averaged field global database paleomagnetic samples were measured from 26 sites 23 of which 13 normal 10 reverse yielded site mean directions meeting our criteria for acceptable paleomagnetic data flow ages on 21 sites range from 5 ka to 5 6 ma on the basis of 40ar 39ar dating methods the age and polarity for the 21 dated sites are consistent with the geomagnetic reversal time scale except for a single reversely magnetized site dated at 0 39 ma this is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records combining the new data from the srp with data published from the northwest united states between the latitudes of 40 and 50 n there are 183 sites in all that meet minimum acceptability criteria for legacy and new data the overall mean direction of 173 normally magnetized sites has a declination of 2 3 inclination of 61 4 a fisher concentration parameter of 58 and a radius of 95 confidence 95 of 1 4 reverse sites have a mean direction of 182 4 declination 58 6 inclination of 50 and 95 of 6 9 normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95 confidence level virtual geomagnetic pole dispersion was found to be circularly symmetric while the directional data were elongate north south an updated and corrected database for the northwestern u s region has been contributed to the magnetics information consortium magic database at http earthref org

paleosecular variation and time averaged field recorded in late pliocene holocene lava flows from mexico 1 this paper presents results from 13 paleomagnetic sites from an area west of mexico city and 7 sites from an area of dispersed monogenetic volcanism in the state of san luis potosi accompanied by seven 40ar 39ar radiometric dates an analysis of secular variation and time averaged paleomagnetic field in the trans mexican volcanic belt tmvb using compiled data both newly obtained and from the literature is presented interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected the selected data include 187 sites of late pliocene holocene age the mean direction among these sites is dec 358 8 inc 31 6 95 2 0 k 29 this direction does not overlap the expected geocentric axial dipole gad but is consistent with a gad plus a 5 quadrupole the virtual geomagnetic pole scatter of this group of sites 12 7 with lower and upper 95 confidence limits of 11 9 and 14 1 is consistent with the value expected from model g 13 6

no evidence for brunhes age excursions santo antao cape verde abstract we present new 40 ar 39 ar ages and palaeomagnetic directions from two re examined sequences of basaltic lava flows on the island of santo antao cape verde tarrafal and agua nova we find no evidence for the fully reversed brunhes age excursions reported in knudsen et al earth planet sci lett 206 199 214 2003 at tarrafal complex geology including repeated gravitational collapses and or considerable erosion followed by in filling of subsequent valleys by younger lavas coupled with an insufficient number of dated reversely and normally magnetised flows led earlier researchers to suggest a series of brunhes age excursions at agua nova the use of single samples to determine palaeodirections resulted in sampling of anomalous reversed directions creating apparent excursions we report new palaeomagnetic directions from agua nova based on multiple measurements per flow determined by both alternating field and thermal demagnetisation all flows of brunhes age have virtual geomagnetic pole latitudes 60 our findings highlight the need for precise age constraints and a high sampling density of dated flows with multiple directional results per flow when using long lava sequences to reconstruct palaeofield behaviour

paleosecular variation of brunhes age lava flows from british columbia canada 1 brunhes age lava flows have been sampled for paleosecular variation studies from several volcanic fields of southern british columbia silverthrone garibaldi lake mt meager clearwater wells gray park and kelowna area a total of 52 lava flows were sampled and 7 to 10 samples were drilled at each site previous radiometric studies indicate that the ages of these lava flows range from 2 3 to 760 ka stepwise thermal demagnetization 14 to 21 steps was carried out for all the samples in each site and af demagnetization was performed on one sample per site forty five sites were selected based on rigorous criteria 95 5 to calculate a mean direction d 356 9 i 70 2 95 2 8 that is statistically indistinguishable from the direction of the dipole field at the area i 68 3 virtual geomagnetic poles vgps do not show the far sided effect and the angular standard deviation is 17 5 a value in agreement with the paleomagnetic field dispersion for that latitude these high quality results are expected to improve the time averaged field taf and secular variation models

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

time averaged field at the equator results from ecuador seventy sites were collected from pliocene to recent lavas in ecuador between 0 19 and 1 1 s latitude lightning affected many of the sites sampled and the final data set consisted of 51 sites with 95 of 10 or less 21 flows yielded normal directions of magnetization declination 354 6 inclination 5 95 7 2 and 30 yielded reverse declination 183 5 inclination 5 6 95 5 these sites pass a reversal test at a high level the combined data give a direction of declination 359 9 inclination 5 4 and 95 4 2 which is just significantly different from the gad field but is in agreement with a gad field plus 5 quadrupole field mean vgps are not significantly different from the geographic poles the asd for the combined sites is 13 3 which is in agreement with model g paleointensity measurements yield values of vadm which range from 1 39 1022 to 22 6 1022 am2 the mean value for the reverse field is 3 2 1022 am2 while for the normal field it is 10 2 1022 am2 the 40ar 39ar dates on selected sites show that they are 2 6 ma and younger the age dates and magnetic polarity indicate that the chacana structure is brunhes matuyama in age

insights from geodynamo simulations into long term geomagnetic field behaviour abstract detailed knowledge of the long term spatial configuration and temporal variability of the geomagnetic field is lacking because of insufficient data for times prior to 10 ka we use realisations from suitable numerical simulations to investigate three important questions about stability of the geodynamo process is the present field representative of the past field does a time averaged field actually exist and supposing it exists how long is needed to define such a field numerical geodynamo simulations are initially selected to meet existing criteria for morphological similarity to the observed magnetic field a further criterion is introduced to evaluate similarity of long term temporal variations allowing for reasonable uncertainties in the observations observed and synthetic axial dipole moment frequency spectra for time series of order a million years in length should be fit by the same power law model this leads us to identify diffusion time as the appropriate time scaling for such comparisons in almost all simulations intervals considered to have good morphological agreement between synthetic and observed field are shorter than those of poor agreement the time needed to obtain a converged estimate of the time averaged field was found to be comparable to the length of the simulation even in non reversing models suggesting that periods of stable polarity spanning many magnetic diffusion times are needed to obtain robust estimates of the mean dipole field long term field variations are almost entirely attributable to the axial dipole nonzonal components converge to long term average values on relatively short timescales 15 20 kyr in all simulations the time averaged spatial power spectrum is characterised by a zigzag pattern as a function of spherical harmonic degree with relatively higher power in odd degrees than in even degrees we suggest that long term spatial characteristics of the observed field may emerge on averaging times that are within reach for the next generation of global time varying paleomagnetic field models

paleomagnetic study of the ages of lavas on the island of lanai i hawai i abstract a detailed paleomagnetic study of tholeiitic basaltic of eight sites drilled from kaumalapau harbor manele bay and maunalei gulch areas was conducted to investigate the volcanic evolution of the island of lana i stepwise alternating field af demagnetizations accompanied by investigations of rock magnetic properties such as low field susceptibility versus temperature k t and saturation isothermal remanent magnetization sirm indicate that the characteristic remanent magnetization chrm is primarily carried by ti poor magnetite demagnetization experiments have identified eight mean flow directions characterized by negative polarities our paleomagnetic results along with the radiometric age determinations of the lavas in question indicate that the tholeiitic flows that formed the lanai volcano were erupted very rapidly and only during the matuyama polarity chron 0 780 2 58 ma and not during the brunhes chron 0 0 78 ma as reported previously for the k ar dated lavas in the maunalei gulch

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors