palaeomagnetic and geochronological evidence for a major middle miocene unconformity in soke basin western anatolia and its tectonic implications for the aegean region cenozoic convergence between the eurasian and african plates and concurrent slab roll back processes have produced a progressive extension in back arc areas such as the aegean region and western anatolia there is still a long standing controversy as to whether this was a continuous or stepwise process to shed light on this controversy and on the driving mechanism of regional extension we present palaeomagnetic and geochronological results from the soke basin located at the southeastern rim of the izmir balikesir transfer zone our improved geochronology shows that volcanic activity in the region occurred between 11 66 and 12 85 ma middle to late miocene palaeomagnetic data for the soke basin show a c 23 clockwise rotation whereas early miocene data show a c 28 counterclockwise rotation the primary nature of the magnetization is indicated by a positive tilt test the resulting c 51 counterclockwise rotations during the middle miocene signify a major tectonic reorganization during a period when an interruption of exhumation of metamorphic massifs has been reported we suggest that the izmir balikesir transfer zone is the main driver of the reorganization in the region the regional fingerprint of this tectonic reorganization coincides with the acceleration of trench retreat and illustrates the surface impact of tearing of the hellenic slab supplementary material details of 40 ar 39 ar analysis including heating steps and the output pmag file including details of paleomagnetic analysis performed in this study are available at https doi org 10 6084 m9 figshare c 3690871

toward constraining the long term reversing behavior of the geodynamo a new maya superchron 1 billion years ago from the magnetostratigraphy of the kartochka formation southwestern siberia abstract we present new magnetostratigraphic data obtained from the late mesoproterozoic 1 ga kartochka formation in the east angara terrane of the yenisey ridge region southwestern siberian platform a 200 m thick section encompassing the carbonate kartochka formation was densely sampled and another more limited outcrop several kilometers away was sampled in order to conduct a paleomagnetic fold test between the two sites paleomagnetic analyses revealed the existence of two magnetization components a low unblocking temperature component which likely has a viscous origin was first isolated below 200 c a characteristic component carried either by magnetite or by a mixture of magnetite and hematite was then isolated in the medium to high temperature range up to 565 585 c or 680 c the hematite bearing component has a shallower inclination than the magnetite component showing that magnetite was less sensitive to flattening a positive paleomagnetic fold test was obtained between the two studied sections indicating that the characteristic magnetization was likely acquired during or very soon after sediment deposition this primary origin was further verified by comparison between the paleomagnetic poles derived for the kartochka formation and other late mesoproterozoic siberian sections all data from the kartochka formation contain a single magnetic polarity assumed to be normal this long normal polarity interval is consistent with the polarity zonation previously determined from magnetostratigraphic and paleomagnetic data from the siberian uchur maya region the new data fortify evidence of a normal polarity superchron spanning the 1000 ma old mesoproterozoic neoproterozoic boundary which we propose to call the maya superchron these data confirm the occurrence of sharp transitions between a frequently reversing regime and a non reversing regime of the geodynamo this behavior also observed in other regions may represent a consistent property of the proterozoic geomagnetic field together with changes in the amount of time spent in the superchron regime they may testify to a different field behavior during the late precambrian than during the phanerozoic this difference could reflect a stronger influence of the heterogeneous heat flux patterns at the core mantle boundary during the precambrian possibly as a consequence of the inner core not yet being nucleated at this time

palaeomagnetism and magnetic anisotropy of late neoproterozoic strata south australia implications for the palaeolatitude of late cryogenian glaciation cap carbonate and the ediacaran system abstract the ediacaran global stratotype section and point gssp is defined near the base of the nuccaleena formation the cap carbonate overlying the terminal cryogenian glaciogenic elatina formation at a basin wide sequence boundary in the adelaide geosyncline south australia previous palaeomagnetic data for 205 specimens from the elatina formation yielded a structurally corrected mean declination d 208 3 and inclination i 12 9 95 4 2 palaeolatitude 6 5 2 2 and pole position at latitude p 47 3 s longitude p 359 3 e dp 2 1 and dm 4 2 new palaeomagnetic data for 141 specimens from the nuccaleena formation yield a structurally corrected mean direction of d 208 3 i 34 9 95 3 4 19 2 2 3 2 2 and pole position at p 32 3 s p 350 8 e dp 2 2 and dm 3 9 there is a substantial difference in inclination between the elatina and nuccaleena formations despite their juxtaposition positive fold tests for both formations seem to rule out remagnetisation of one or both formations fold tests on tepee like structures in the nuccaleena formation accord with the magnetisation having been acquired at or near the time the structures formed during early diagenesis magnetisation therefore variably lags deposition causing the three identified magnetostratigraphic horizons to appear highly bedding transgressive whereas local correlations may seem satisfactory regional correlations are not nevertheless the number and duration of polarity chrons at various localities appear valid implying that the nuccaleena formation accumulated over an interval of 10 5 years the question of compaction related inclination shallowing was addressed by examining the anisotropy of remanence using a superconducting 12 t magnet the effective flattening factors f determined were nuccaleena dolostones n 4 f 0 98 0 02 elatina tidal rhythmites n 6 f 0 97 0 01 elatina arenites n 9 f 0 92 0 02 the mean elatina inclination is corrected to 14 0 palaeolatitude of 7 1 and the mean nuccaleena inclination to 35 4 palaeolatitude of 19 6 inclination shallowing also was investigated using the elongation inclination method the distributions of directions for the elatina were somewhat skewed with respect to bedding possibly invalidating use of this method nevertheless for the elatina the method yields an inclination of 19 palaeolatitude of 10 for the nuccaleena the corrected inclination is 41 9 palaeolatitude of 24 these corrections do not materially alter the near equatorial palaeolatitude for the elatina formation which is supported by data for other neoproterozoic strata in southern australia however the substantial increase in palaeolatitude between the late cryogenian and the early ediacaran is maintained the sequence boundary below the nuccaleena formation is a disconformity to low angle unconformity that in places cuts deeply into the 1500 m thick late cryogenian glaciogenic succession and may represent a significant time gap marked by rapid polar wander such a hiatus may be an important factor in the juxtaposition of glacial deposits and cap carbonates

uplift structure of the southern kapuskasing zone from 2 45 ga dike swarm displacement the ca 2 ga kapuskasing zone a 500 km long belt of uplifted crust cuts across the regional structural trend of the archean superior province in canada and disrupts the continuity of 2 45 ga matachewan dikes that form a major swarm in its vicinity as expressed by positive gravity and magnetic anomalies it appears to end about 100 km east of lake superior until now the nature of this termination has remained obscure regional variations in paleomagnetic polarity and in the intensity of feldspar clouding caused by magnetite exsolution are two physical properties of the dike swarm that depend upon postemplacement erosion level they show that the kapuskasing zone as a narrow fault bounded crustal uplift continues after sinistral offset for a further 60 km to the southwest before terminating in a major cross fault dual magnetic polarity and variable levels of feldspar clouding are features found in many other proterozoic dike swarms worldwide and therefore in principle may be used to document other examples of craton response to tectonism

paleomagnetic constraints from the baoshan area on the deformation of the qiangtang sibumasu terrane around the eastern himalayan syntaxis the sibumasu block in se asia represents the eastward continuation of the qiangtang block here we report a detailed rock magnetic and paleomagnetic study on the middle jurassic and paleocene rocks from northern sibumasu to document the crustal deformation during the india asia collision since the paleocene and reconstruct the overall strike of the qiangtang sibumasu elements before the india asia collision although the fold test is inconclusive based solely on our data a positive reversal test a positive regional fold test with previous paleomagnetic results and a detrital origin of hematite in the red beds as indicated by scanning electron microscopy suggest that the magnetizations obtained from the jurassic and paleocene rocks are most likely primary showing an 80 clockwise rotation since paleocene these results together with previously published paleomagnetic data suggest that the northern sibumasu and northern simao elements experienced a 60 80 clockwise rotation since paleocene this large clockwise rotation is also consistent with the surface gps velocity field and ne sw fault networks suggesting a rotational motion of crustal material from southeastern tibet during late cenozoic we infer that the large clockwise rotation is a sum of rotation in the eocene to middle miocene time associated with indochina extrusion and rotation after the middle miocene associated with the e w extension in central tibet this suggests that the eastward motion of tibetan crustal material along the xianshuihe xiaojiang fault after middle miocene is transmitted to the southwest toward myanmar jurassic and cretaceous paleomagnetic results suggest that the qiangtang northern sibumasu was originally a curved structure with an orientation of n60 w in tibet and changes to n10 w in southern sibumasu

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

palaeomagnetism of archaean rocks of the onverwacht group barberton greenstone belt southern africa evidence for a stable and potentially reversing geomagnetic field at ca 3 5 ga abstract palaeomagnetic data from the palaeoarchaean era 3 2 3 6 ga have the potential to provide us with a great deal of information about early conditions within and processes affecting the earth s core mantle and surface environment here we present new data obtained from some of the oldest palaeomagnetic recorders in the world igneous and sedimentary rocks from the onverwacht group of the barberton greenstone belt kaapvaal craton southern africa our palaeomagnetic measurements strengthen a recently published positive conglomerate test y usui j a tarduno m watkeys a hofmann and r d cottrell 2009 and our new u pb date constrains the conglomerate to older than 3455 8 ma the new palaeomagnetic data from other units are nontrivial to interpret and are of uncertain reliability when taken individually similar we argue to all other published palaeomagnetic data of this age nonetheless four poles two new two derived from published data produced from high temperature components of magnetisation recorded in the komati noisy and hooggenoeg formations exhibit considerably improved clustering when their directions are corrected for differences in attitude resulting from a large fold structure dated at 3 23 ga on the basis of this enhanced consistency in stratigraphic coordinates the positive conglomerate test and the absence of any clear indications of their remagnetisation from comparison with younger poles we argue that these are the most trustworthy palaeomagnetic results yet produced from any rocks of palaeoarchaean age when taken in conjunction with published data the new results present the most compelling evidence to date that the earth had a stable geomagnetic field at ca 3 5 ga in addition to presenting tentative evidence that it was undergoing polarity reversals the data do not appear to support a claim made previously from palaeoarchean palaeomagnetic data from the pilbara craton y suganuma y hamano s niitsuma m hoashi t hisamitsu n niitsuma k kodama and m nedachi 2006 of extremely rapid latitudinal plate motion during this period finally when compared with similarly aged data from the pilbara craton western australia the new data do not rule out the hypothesis that the two cratons were conjoined at this point in their history in the supercraton vaalbara

evidence for a 3 45 billion year old magnetic remanence hints of an ancient geodynamo from conglomerates of south africa 1 paleomagnetic and rock magnetic analyses of 3445 million year old dacite conglomerate clasts and parent body rocks from the barberton greenstone belt south africa define two dominant components of magnetization one component unblocked at low temperature is an overprint acquired 180 million years ago the other component is unblocked at high temperatures and passes a conglomerate test indicating that this component is older than the depositional age of the conglomerate 3416 ma the high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes archean overprinting and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations alternatively this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo including magnetization by an external field related to solar wind interaction with the atmosphere such exotic mechanisms can be tested with the acquisition of paleointensity data while more scattered than paleomagnetic data recording the more recent geomagnetic field the high unblocking temperature component in the dacite parent body shows some consistency and the simplest explanation of the data is that they reflect a geodynamo 3445 million years ago

the tectonic relationship of two early proterozoic dyke swarms to the kapuskasing structural zone a paleomagnetic and petrographic study a tea coloured cloudiness in groundmass feldspars characterizes dykes of two early proterozoic swarms 2 45 ga matachewan 2 04 ga kapuskasing that intrude amphibolite to granulite host rocks with

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

paleomagnetism of the late archean nyanzian system western kenya abstract the nyanzian system lavas of western kenya are believed to be the oldest rocks of the tanzanian craton intrusive age relationships suggest an age 2850 ma although direct attempts at dating the nyanzian have produced disparate results our study involves a suite of samples collected from the nyanzian basalts pillow basalts andesites and rhyolites from sixteen sites in western kenya these rocks yield a tilt corrected paleomagnetic pole at 14 n 150 e k 59 dp 5 dm 7 this pole is constrained to be older than the first d1 deformation 2472 30 ma by positive fold conglomerate and reversal tests analysis of the paleomagnetic data base for three african cratonic nuclei tanzanian kaapvaal zimbabwe and west africa for the time period from 2 0 ga to 3 0 ga demonstrates a paucity of well dated poles although there are several poles from the kaapvaal zimbabwe and tanzanian cratons which allow spot readings of their relative positions we demonstrate based on these data that the kaapvaal zimbabwe and tanzanian cratons were drifting independently at 2875 ma 2700 ma and 2450 ma this independent motion of the tanzanian and kaapvaal zimbabwe cratons indicates that previously proposed models involving african cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 ma

a comparison of detailed equatorial red bed records of secular variation during the permo carboniferous reversed superchron eve basin southern france also deposited at low latitudes additionally we critically review published data from low latitude permian red beds from the same basins we focus on records with a sufficient number of samples and acquire whenever possible the original data or as a second choice parametrically sampled published site means we test the distributions of these directions using a statistical secular variation field model tk03 gad the angular standard deviation asd of the virtual geomagnetic pole vgp distribution is a measure of palaeo secular variation and we compare our values of asd from the permian red beds to those from lavas erupted in the last 5 myr contrary to our previous studies we now conclude that secular variation during the permo carboniferous reversed superchron is slightly but significantly reduced at low latitudes compared with the recent field as based on a latest compilation that supercedes earlier compilations it also agrees with a new study which found that psv at low palaeolatitudes during the cretaceous normal superchron was also significantly lower than in periods with frequent reversals

palaeomagnetism core message archaean and early proterozoic rocks reveal that the earth s magnetic field two billion years ago behaved differently than over most of the past 200 million years do these changes relate to the growth of the inner core

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

response of the changjiang yangtze river water chemistry to the impoundment of three gorges dam during 2010 2011 abstract the environmental impact of three gorges dam tgd in the changjiang yangtze river is an important topic of concern for scientific communities and the public however the changes in river water chemistry in response to the dam construction remain poorly constrained this study presents the seasonal variability of all major cations and some anions in the lower changjiang during a full hydrological year from 2010 to 2011 the concentrations of all ions except for hco 3 are higher after the tgd operation than before p

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

geological constraints on the precambrian history of earth s rotation and the moon s orbit over the past decade the analysis of sedimentary cyclic rhythmites of tidal origin i e stacked thin beds or laminae usually of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided information on earth s paleorotation and the evolving lunar orbit for precambrian time before 540 ma depositional environments of tidal rhythmites range from estuarine to tidal delta with a wave protected distal ebb tidal delta setting being particularly favorable for the deposition and preservation of long detailed rhythmite records the potential sediment load of nearshore tidal currents and the effectiveness of the tide as an agent of sediment entrainment and deposition are related directly to tidal range or maximum tidal height and consequent current speed hence the thickness of successive laminae deposited by tidal currents can be a proxy tidal record with paleotidal and paleorotational values being determined by analysis of measured records of lamina and cycle thickness the validity of the findings can be investigated by testing the primary observed values for internal self consistency through application of the laws of celestial mechanics paleotidal and paleorotational values provided by late neoproterozoic 620 ma tidal rhythmites in south australia are validated by these tests and indicate 13 1 0 1 synodic lunar months yr 400 7 solar days yr a length of day of 21 9 0 4 h and a relative earth moon distance a a0 of 0 965 0 005 the mean rate of lunar recession since that time is 2 17 0 31 cm yr which is little more than half the present rate of lunar recession of 3 82 0 07 cm yr obtained by lunar laser ranging the late neoproterozoic data militate against significant overall change in earth s moment of inertia and radius at least since 620 ma cyclicity displayed by paleoproterozoic 2450 ma banded iron formation in western australia may record tidal influences on the discharge and or dispersal of submarine hydrothermal plumes and suggests 14 5 0 5 synodic months yr and a a0 0 906 0 029 the combined rhythmite data give a mean rate of lunar recession of 1 24 0 71 cm yr during most of the proterozoic 2450 620 ma suggesting that a close approach of the moon did not occur during earlier time concentrated study of precambrian tidal rhythmites promises to illuminate the evolving dynamics of the early earth moon system and may permit the lunar orbit to be traced back to near the time of the moon s origin

magnetodynamo lifetimes for rocky earth mass exoplanets with contrasting mantle convection regimes 1 we used a thermal model of an iron core to calculate magnetodynamo evolution in earth mass rocky planets to determine the sensitivity of dynamo lifetime and intensity to planets with different mantle tectonic regimes surface temperatures and core properties the heat flow at the core mantle boundary cmb is derived from numerical models of mantle convection with a viscous pseudoplastic rheology that captures the phenomenology of plate like tectonics our thermal evolution models predict a long lived 8 gyr field for earth and similar dynamo evolution for earth mass exoplanets with plate tectonics both elevated surface temperature and pressure dependent mantle viscosity reduce the cmb heat flow but produce only slightly longer lived dynamos 8 9 5 gyr single plate stagnant lid planets with relatively low cmb heat flow produce long lived 10 5 gyr dynamos these weaker dynamos can cease for several billions of years and subsequently reactivate due to the additional entropy production associated with inner core growth a possible explanation for the absence of a magnetic field on present day venus we also show that dynamo operation is sensitive to the initial temperature size and solidus of a planet s core these dependencies would severely challenge any attempt to distinguish exoplanets with plate tectonics and stagnant lids based on the presence or absence of a magnetic field

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

on the low inclination bias of the precambrian geomagnetic field abstract a long tradition has emerged in applying the inclination frequency analysis to estimate the functionality of the geocentric axial dipole gad hypothesis in paleomagnetism in the present study the theory has been tested to the database with 3246 records of the precambrian geomagnetic field to find the best fitting inclination distribution different combinations of geocentric axial dipolar gad quadrupolar g2 and octupolar g3 spherical harmonics have been analyzed the influence of various factors on the inclination distribution has been studied including the geologic age rock type magnetic polarity quality of data and their spatiotemporal distribution by two dimensional chi square analysis on crystalline rocks only which avoid problems associated with remanence shallowing among sedimentary rocks the most plausible estimates for the zonal non dipolar contributions of the field have been determined as 2 for g2 and 5 for g3 values much lower than in previous estimates using the same method however the inherent non uniqueness of the inclination frequency analysis and the uneven spatiotemporal sampling of the field around the globe during the precambrian necessitate other independent methods of testing the gad hypothesis

history of earth s magnetic field and possible connections to core mantle boundary processes the core mantle boundary cmb sets boundary conditions for processes occurring within the core thus the history of the geomagnetic field is intimately connected with the history of the cmb information about one can often provide information about the other it is a simple matter to separate sources of external origin from those of internal origin but there is no unique way to specify the location of the internal sources indirect arguments suggest that sources within earth s outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees building on the pioneering studies of roberts and scott and of backus in the 1960s the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data however even with appropriate assumptions about location of the magnetic field source there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines additional assumptions about the flow are therefore required sometimes leading to quite different estimates in the pattern of fluid flow the resulting velocity field estimates appear to be sensitive to conditions e g small lateral variations in temperature at the cmb consideration of mantle dynamics suggests that large changes in the cmb conditions probably occur on a 10 to 100 m y timescale secular variations with periods shorter than a million years but longer than several years almost certainly originate from processes operating in the outer core unfortunately there is not yet consensus as to what those processes are longer period variations in the paleomagnetic record including nonstationarities in the rates of magnetic field reversals in paleointensities and in paleosecular variation may reflect changes in cmb conditions because these boundary conditions are controlled primarily by mantle dynamics there have been several speculations regarding causal links between changes in earth s lithosphere and changes in earth s magnetic field finally the evidence linking lateral variations at the cmb to perceived systematics in polarity transition data is found to be intriguing but insufficient

crustal uplift in the southern superior province canada revealed by paleomagnetism abstract a change in the polarity of magnetization with depth in the 2 45 ga matachewan dyke swarm is used to document vertical crustal movements that occurred at 1 9 2 3 ga along the kapuskasing structural zone a 500 km long fault zone that transects the archean superior province of canada at shallow crustal levels dykes have a primary magnetization dominantly of one polarity but at greater depths 20 km down a polarity change occurs associated with the growth of exsolved magnetite in feldspar due to slow crustal cooling after cessation of matachewan igneous activity regions of the dyke swarm with one dominant polarity are separated from those with opposite polarity by major faults using this polarity distribution and associated variations in the intensity of feldspar clouding and hydrous alteration maps of the southern superior province are produced that display regional crustal tilting on which are superimposed more local fault bounded blocks associated with the kapuskasing zone some of these blocks have been recognized for the first time as a result of this study the paleomagnetic work has also shown that the matachewan swarm is regionally distorted both within and north of the kapuskasing zone and originally had a more radial disposition this widespread distortion suggests that the lower crust was still relatively ductile at the time of deformation perhaps due to high heat flow associated with the waning stages of the matachewan mantle plume beneath

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

paleomagnetism of 2 44 ga mafic dykes in russian karelia eastern fennoscandian shield implications for continental reconstructions abstract results of a paleomagnetic study of paleoproterozoic dykes and archean host rocks in the eastern fennoscandian shield are reported here most of the dykes are related to ca 2440 ma layered intrusions and have isotopic ages between 2349 and 2476 ma the dykes form an extensive nw se and e w trending swarm the majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature the high temperature remanence component d which is thought to represent the primary 2 44 ga magnetization of the dykes has a paleomagnetic pole at 20 s 278 8 e a95 6 0 n 11 sites another d like component d with shallower inclination vgp 9 6 n 256 2 e eight samples at one site from baked archean host rock may also represent the primary remanence of the dykes the commonest remanence component a records an extensive svecofennian partial overprint and is interpreted to be ca 1 84 ga old it yields a paleomagnetic pole at 52 6 n 226 8 e a95 2 7 n 23 the fourth component b has a paleopole at 53 9 n 169 6 e a95 6 3 n 16 and is inferred to be ca 1 75 ga on the basis of isotopic dating although ages of 2 1 1 3 and 0 3 ga are also possible baked contact tests for the 2 44 ga dykes are negative as shown by the occurrence of component d in archean host rocks beyond one dyke width it is proposed that the negative baked contact test indicates regional reheating and remagnetization of the archean basement at ca 2 44 ga ago due to the spatially vast occurrence of coeval 2 44 ga mafic dykes layered intrusions volcanics and granites in the area based on this hypothesis it is suggested that component d in the dykes may be primary and acquired at ca 2 44 ga ago the similarity of pole d with poles from coeval dykes and layered intrusions elsewhere in the karelian province and kola peninsula suggest that within the archean domain of the fennoscandian shield no large scale block movements have taken place since 2 44 ga at 2 44 ga the fennoscandian shield was located at the latitude of ca 30 s which is different to that at 1 84 and 1 75 ga when fennoscandia was located at the latitudes of ca 30 n a continental reconstruction based on pole d and the pole from the 2 45 ga matachewan dykes of the superior craton in laurentia places the kola peninsula adjacent to southern greenland reconstructions based on recent geological interpretations and a reconstruction yielding pole d are also discussed

insights into deposition and deformation of intra caldera ignimbrites central nevada abstract magnetic studies together with structural and 40 ar 39 ar age analyses were used to investigate the development and deformation of ignimbrites of the caetano tuff formation in central north nevada magnetic susceptibility susceptibility anisotropy ams and magnetic remanence approaches were used to decipher events accompanying emplacement and subsequent deformation of that tuff for both intracaldera tuffs and outflow deposits 40 ar 39 ar ages both those presented here and previously published ages indicate about 33 8 ma for the age of caetano ignimbrites approximately at the eocene oligocene boundary although extensive faulting associated with regional extension of the great basin region has dissected these deposits the thickness of the intra caldera ignimbrites has been estimated at 3 5 km or more thermal modeling for cooling of a caldera filling of 3 5 km thickness indicates acquisition of remanent magnetization can persist up to 25 ka sufficient to average secular variation this contrasts with rapid cooling and remanence acquisition in outflow ignimbrites typically only tens of meters thick a 750 m thick reference section was selected for more detailed study about 10 km to the west of the type area this reference section fault bounded at both base and top represents the upper part of the formation its lower member includes the uppermost 250 m of the lower massive resistant unit of ignimbrites a middle member of 375 m of well stratified pyroclastic deposits vitrophyres and sediments and an upper member of about 125 m of recessive ignimbrite and thin bedded clastic and pyroclastic deposits structural and magnetic remanence measurements suggest 13 eastward tilting of the caldera floor occurred early during deposition of the middle member flow directions inferred from ams k max axes in the middle member diverge about 45 relative to those in the underlying member and suggest a change in vent source accompanying the deposition of the middle member correcting for e w extension the outflow ignimbrites reached approximately 20 km to the west and south of the caldera north of the caldera ignimbrites formerly assigned to the caetano tuff but now assigned to the tuff of cove mine exhibit variable flow directions from an indeterminate source significantly however the intracaldera ams magnetic lineations conventionally used as proxies for flow lines trend mainly wnw parallel to a proposed major paleodrainage which stretches from the southern cortez range through 40 km to the tobin range caetano tuff magnetic remanence is entirely reversed and corresponds to magnetization acquired in the younger part of magnetochron 13r the magnetic remanence directions of both the intra caldera and outflow ignimbrites are similar d 151 i 60 significantly different from those predicted by a coeval reference pole for north america raising the possibility of a regional counterclockwise rotation of the sampling area of 15 6 presumably this began during faulting and major regional extension about 16 ma ago during the middle miocene and possibly continued later

evidence for a very long term trend in geomagnetic secular variation reconstructions of palaeosecular variation suggest that the earth s magnetic field reversed less frequently 2 82 to 2 45 billion years ago relative to the cenozoic era this suggests a long term trend of decreasing geodynamo stability since the archaean eon the earth s inner core is believed to inhibit rapid fluctuations in the geomagnetic field from developing into full polarity reversals1 2 consequently during the precambrian the smaller size of the inner core might suggest that polarity reversals could occur more readily it is therefore surprising that there are indications that reversals were rare during this period3 4 here we use new and existing palaeomagnetic data from three continents to examine the stability of the earth s magnetic field from 2 82 to 2 45 billion years ago we show that on average geomagnetic secular variation the field variations produced by normal geodynamo action during the late archaean and early proterozoic was different from that of the past 200 million years specifically the apparent variability of the geomagnetic pole as viewed at low and mid latitudes was reduced relative to the past 200 million years according to both dynamo simulations4 and more recent palaeomagnetic field observations5 the observed pattern of secular variation suggests a lower frequency of polarity reversals 2 5 billion years ago this may imply that the geodynamo is becoming progressively less stable over long timescales consistent with some numerical simulations6 7 possibly as a result of changing outer core geometry that has accompanied inner core growth

analysis of an updated paleointensity database q pi pint for 65 200 ma implications for the long term history of dipole moment through the mesozoic

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

variations in geomagnetic reversal frequency during the earth s middle age 1 we have obtained new magnetostratigraphic results from two precambrian sedimentary sections from eastern siberia and the southern urals dated between 1100 ma and 800 ma sample magnetizations from the uppermost mesoproterozoic talakh khaya section in the siberian uchur maya region appear to be mostly carried by a mixture of magnetite and hematite a sequence of 33 magnetic polarity intervals is recorded within the section all reversals occur in the first 24 m while the upper 160 m are characterized by a single magnetic interval of normal polarity assuming a northern hemisphere position of siberia around 1000 ma the lower neoproterozoic uralian minyar section also possesses an ancient magnetization carried by magnetite and hematite the high temperature magnetization component obtained from the minyar section establishes a sequence of 43 magnetic polarity intervals positive reversal tests obtained from the data sets presented here and from other previously analyzed proterozoic sections provide no convincing evidence for a long standing asymmetric geomagnetic field during the proterozoic that would make the precambrian field markedly less dipolar than during the phanerozoic the new data further reveal the occurrence of sharp transitions and alternations between long periods without any reversal one superchron is observed from the talakh khaya section and periods with high reversal frequencies possibly larger than 5 10 reversals per myr this characteristic may well be an important property of the precambrian field although rather sudden transitions between reversing and nonreversing states of the geodynamo may not be unique to this period

dating basaltic volcanism in the neoarchaean sukumaland greenstone belt of the tanzania craton using the sm nd method implications for the geological evolution of the tanzania craton abstract metabasalts from the sukumaland greenstone belt of north western tanzania yield a whole rock sm nd isochron age of 2823 44 ma initial e nd 2 7 mswd 1 24 this age which is interpreted as dating the eruption of the oldest mafic volcanics in the belt is at the 95 confidence level indistinguishable from 2780 3 and 2808 3 ma single zircon u pb ages previously reported from stratigraphically higher rhyolitic pyroclastic rocks from the southern margin of the belt the age equivalency suggests that the entire 5 7 km thick greenstone sequence that has been traditionally classified into a predominantly mafic lower part overlain by an upper part in which felsic volcanics and bif predominate was emplaced within a relatively short time interval not exceeding 44 ma the sm nd age of the metabasalts is significantly older than a published zircon u pb age of 2680 3 ma obtained from a migmatitic gneiss on the southernmost fringe of the sukumaland greenstone belt this corroborates previous evidence that high grade metamorphism in the tanzania craton postdates emplacement of the greenstones and is most likely associated with the regional emplacement of the large granitic bodies that intrude and flank the greenstones

estimates of heat flow in the deep mantle based on the power requirements for the geodynamo 1 regeneration of the magnetic field by convection in the core places demands on heat flow into the base of the mantle if the heat flow is too low thermal convection is shut off and the rate of generation of compositional buoyancy by the solidification of the core becomes too low to sustain the geodynamo conversely a large heat flow causes rapid growth of the inner core so that convection prior to the appearance of the inner core must be sustained by thermal buoyancy alone the attendant requirements on primordial heat become more severe as the age of the inner core decreases we show that estimates of the present day heat flow satisfy the power requirements for the geodynamo however the resulting thermal history is incompatible with estimates of mantle temperatures prior to 3 ga this discrepancy can be resolved by accumulating radioactive isotopes in d or adding heat sources to the core

broad scale proterozoic deformation of the central superior province revealed by paleomagnetism of the 2 45 ga matachewan dyke swarm an extensive paleomagnetic study of the 2 45 ga matachewan dyke swarm of the north american superior province suggests that the interior of an archean shield can undergo broad scale distortion as a result of later proterozoic orogenic activity around the craton margins data collected from over 300 sites of which 137 are reported here for the first time reveal that the dykes contain a dual polarity primary remanence that varies across the swarm in both inclination and declination these regional variations are statistically significant at the 95 confidence level and cannot be attributed to remagnetization or to magnetic anisotropy inclination variation is probably due to real or apparent polar wander during the emplacement of the swarm and may in part explain the declination variation as well however for dykes within and northwest of the kapuskasing structural zone ksz a positive correlation is found between regionally averaged values of declination and dyke trend here the dykes appear to hav

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

behavior of the paleosecular variation during the permian carboniferous reversed superchron and comparisons to the low reversal frequency intervals since precambrian times we investigated the paleo latitudinal dependence of angular dispersion sets of virtual geomagnetic poles vgps for the permian carboniferous reversed superchron pcrs 262 318 ma in order to analyze the paleosecular variation during this period we prepared different paleomagnetic datasets from scientific databases and recent literature based on selection criteria which provided high degree of refinement model g of mcfadden et al 1988 was fitted to the vgp dispersion data providing the shape parameters a and b which were further compared with similar results for the cretaceous normal superchron cns and other periods of different geomagnetic reversal rates throughout the last 3 ga our results indicate high similarity between the angular vgp dispersion from the pcrs and cns a low vgp dispersion at low paleolatitudes and strong paleolatitudinal dependence in contrasting difference for periods of higher reversal rates the last 5 ma despite the geodynamic differences related to both phanerozoic superchrons such evidence could point out similar heat flux conditions in the cmb which may favor compatible stability conditions throughout these magnetozones notably two additional observations which arose from the shape parameters relations for both phanerozoic superchrons and precambrian datasets demand further investigation i a pattern of increase for the b a ratio throughout the 0 5 1 6 ga interval was observed which could be partially explained by the maya superchron recently proposed by gallet et al 2012 ii the b a relationship for both phanerozoic superchrons and the 2 45 2 82 ga interval pointed to the presence of one or more long term magnetozones throughout the paleoproterozoic

paleomagnetic and 40ar 39ar geochronological results from the linzizong group linzhou basin lhasa terrane tibet implications to paleogene paleolatitude and onset of the india asia collision abstract the linzizong group 64 44 ma of the lhasa terrane in tibet is critically positioned for establishing the paleoposition of the southern leading edge of the asian continent during paleogene times and constraining onset of the india asia collision here we report paleomagnetic results from a collection comprising 384 drill core samples from 34 sites embracing all three formations of this group comprehensive demagnetization and field tests isolate characteristic remanent magnetizations chrm summarized by overall tilt corrected formation mean directions of d 183 6 i 12 4 95 8 1 for the dianzhong 64 60 ma d 1 0 i 18 1 95 8 1 for the nianbo 60 50 ma and d 12 4 i 23 2 95 7 3 for the pana 50 44 ma fold tests are positive in each formation suggesting a pre folding origin and we interpret the magnetizations as quasi primary and acquired at or slightly later than formation of the linzizong group revised paleogene paleopoles with ar ar age constraints for the lhasa terrane indicate that onset of the india asia collision occurred no later than 60 5 1 5 ma at a low paleolatitude of 10 n analysis of 60 site mean observations from a range of studies of the pana formation in the higher part of the succession highlight a large dispersion of chrm directions a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue

detecting the oldest geodynamo and attendant shielding from the solar wind implications for habitability abstract the onset and nature of the earliest geomagnetic field is important for understanding the evolution of the core atmosphere and life on earth a record of the early geodynamo is preserved in ancient silicate crystals containing minute magnetic inclusions these data indicate the presence of a geodynamo during the paleoarchean between 3 4 and 3 45 billion years ago while the magnetic field sheltered earth s atmosphere from erosion at this time standoff of the solar wind was greatly reduced and similar to that during modern extreme solar storms these conditions suggest that intense radiation from the young sun may have modified the atmosphere of the young earth by promoting loss of volatiles including water such effects would have been more pronounced if the field were absent or very weak prior to 3 45 billion years ago as suggested by some models of lower mantle evolution the frontier is thus trying to obtain geomagnetic field records that are 3 45 billion years old as well as constraining solar wind pressure for these times in this review we suggest pathways for constraining these parameters and the attendant history of earth s deep interior hydrosphere and atmosphere in particular we discuss new estimates for solar wind pressure for the first 700 million years of earth history the competing effects of magnetic shielding versus solar ion collection and bounds on the detection level of a geodynamo imposed by the presence of external fields we also discuss the prospects for constraining hadean paleoarchean magnetic field strength using paleointensity analyses of zircons

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

modelling the palaeo evolution of the geodynamo although it is known that the geodynamo has been operating for at least 3 2 ga it remains difficult to infer the intensity dipolarity and stability occurrence of reversals of the precam brian magnetic field of the earth in order to assist the interpretation of palaeomagnetic data we produce models for the long term evolution of the geodynamo by combining core ther modynamics with a systematic scaling analysis of numerical dynamo simulations we update earlier dynamo scaling results by exploring a parameter space which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to earth in the precambrian our analysis highlights the central role of the convective power which is an output of core thermodynamics and the main input of our updated scalings as the thermal evolution of the earth s core is not well known two end member models of heat flow evolution at the core mantle boundary cmb are used respectively terminating at present heat flows of 11 tw high power scenario and 3 tw low power scenario the resulting models predict that until the appearance of the inner core a thermal dynamo driven only by secular cooling and without any need for radioactive heating can produce a dipole moment of strength comparable to that of the present field thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence the observed lack of strong long term trends in palaeointensity data throughout the earth s history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter specifically the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation but the dynamo becomes deeper seated in the core thus largely cancelling the increase at the core and earth surface and diminishing the prospect of observing this event in palaeointensity data our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history in the precambrian we predict a dynamo with similar dipolarity and less frequent reversals than at present times due to conditions of generally lower convective forcing quantifying the typical cmb heat flow variation needed for the geodynamo to cross the transition from a reversing to a non reversing state we find that it is unlikely that such a variation may have caused superchrons in the last 0 5 ga without shutting down dynamo action altogether

new palaeomagnetic results from the oslo graben a permian superchron lava province we have performed an extended palaeomagnetic study of the oslo graben volcanics compared to the study of half a century ago by van everdingen using modern techniques and a four times larger amount of sites plus additional rock magnetic experiments we conclude that the average direction d 204 0 i 37 9 k 46 9 95 2 0 and associated palaeomagnetic pole 48 3 155 5 k 52 2 a95 1 9 of the krokskogen and vestfold volcanics together are statistically identical to those of the earlier study this gives confidence in the fact that older palaeomagnetic studies can be reliable and robust even though methods have improved our larger number of samples and better age constraints enable us to separate the data into two major intervals the younger on average krokskogen area and the older vestfold area the results show firstly that palaeolatitudes are slightly higher than predicted by the latest apparent polar wander path apwp for eurasia by torsvik et al these data support an early permian pangaea a configuration and do not necessitate a pangaea b configuration the larger data set also allows us to assess the distribution of the characteristic remanent magnetization directions of the oslo graben in terms of geomagnetic field behaviour which were acquired during a long period of dominantly single polarity the permo carboniferous reversed superchron pcrs the distributions show a significantly lower virtual geomagnetic pole vgp scatter at the observed low latitudes than expected from a compilation from lavas of the last 5 myr the data do however show excellent agreement with the scatter observed both during the cretaceous normal superchron and the pcrs a comparison of the directional distributions in terms of elongation is less discriminating since the large errors in all cases allow a fit to the predicted elongation inclination behaviour of the tk03 gad model

evolving core conditions ca 2 billion years ago detected by paleosecular variation abstract paleomagnetic data provide one of the few probes available to interrogate early evolution of the core here we apply this probe by examining the latitudinal dependence of paleosecular variation psv data derived from high quality paleomagnetic data collected from proterozoic and neoarchean rocks these data define a neoarchean geomagnetic field that was more dipolar than that during proterozoic times indicating a change in core conditions the signals observed may reflect a change in forcing of the dynamo and an early onset of inner core growth we propose a model that links evolution of the core mantle and crust in three principal phases i before approximately 3 5 ga an entirely liquid core may not have hosted a geodynamo if heat transport was sufficient across the core mantle boundary however a geodynamo could have been generated if so sources in the shallow outer core could have been more important for generating the dynamo relative to deeper convection resulting in a field that was less dipolar than that generated in later times ii cooling of the lower mantle between ca 2 and 3 5 billion years ago was promoted by deep subduction and possibly coincided with inner core growth the geodynamo during this episode was deeply seated producing a highly dipolar surface magnetic field iii after ca 2 billion years ago continued subduction led to large scale core mantle boundary compositional and heat flux heterogeneity with these changes shallow core contributions to the geomagnetic field grew in importance resulting in a less dipolar field

paleointensity record from the 2 7 ga stillwater complex montana 1 the record of geomagnetic intensity captured in the 2 7 ga stillwater complex montana usa provides a statistical description of the archean geodynamo we present results of modified thellier paleointensity experiments on 441 core specimens 114 of which pass strict reliability criteria the specimens are from 53 sites spanning most of the banded series rocks in the stillwater complex on the basis of thermochronologic and petrologic evidence we interpret the highest temperature component of remanence to be a late archean thermoremanence though the possibility remains that it is a thermochemical remanence thermal models indicate that the highest temperature magnetization component at each of the sites averages 20 200 ka of geomagnetic secular variation the suite of sites as distributed through the banded series samples a roughly a 1 ma time interval the average of the most reliable paleointensity measurements uncorrected for the effects of anisotropy or cooling rate is 38 2 11 3 t 1 remanence anisotropy cooling rate and the nonlinear relationship between applied field and thermoremanence have a significant effect on paleointensity results a corrected average of 30 6 8 8 t is likely a more appropriate value earth s average dipole moment during the late archean 5 05 1 46 1022 am2 pmag 44 5 was well within the range of estimates from phanerozoic rocks the distribution of site mean paleointensities around the mean is consistent with that expected from slow cooling over timescales expected from thermal models and with secular variation comparable to that of the phanerozoic field

the signature of inner core nucleation on the geodynamo abstract energy considerations indicate that the power delivered to the present day geodynamo comes mainly from the growth of the solid inner core through light element and latent heat releases the nucleation of the inner core was therefore a major transition for the geodynamo here we use numerical dynamo simulations linked by thermochemical evolution of the core to investigate the effects of inner core nucleation icn on the geodynamo and identify possible icn footprints in the palaeomagnetic field our results suggest that a strong field dipole dominated dynamo prior and after icn the uniformitarian scenario is most likely we predict little footprint of icn on the surface field intensity consistent with the observed lack of a long term trend in paleointensity data instead we find that the best proxy for inner core growth is an increase in axial octupole strength with age from present day to icn which might be resolvable in the palaeomagnetic record we confirm the existence of weak field multipolar dynamos prior to icn suggesting the dynamo might have transiently reached a weak field state the catastrophic scenario the narrow stability domain of these weak field dynamos implies that the catastrophic scenario is less probable than the uniformitarian scenario

an assessment of long duration geodynamo simulations using new paleomagnetic modeling criteria qpm abstract long term temporal variations of the magnetic field timescales 10 myr characterized from paleomagnetic data have been hypothesized to reflect the evolution of earth s deep interior and couplings between the core and mantle by tying observed changes in the paleomagnetic record to mechanisms predicted from numerical geodynamo simulations we have a unique tool for assessing changes in the deep interior back in time however numerical simulations are not run in an earth like parameter regime and assessing how well they reproduce the geomagnetic field is difficult criteria have been proposed to determine the level of spatial and temporal agreement between simulations and observations spanning historical and holocene timescales but no such criteria exist for longer timescales here we present a new set of five criteria quality of paleomagnetic modeling criteria q pm that assess the degree of semblance between a simulated dynamo and the temporal and spatial variations of the long term 10 myr paleomagnetic field these criteria measure inclination anomaly virtual geomagnetic pole dispersion at the equator latitudinal variation in virtual geomagnetic pole dispersion normalized width of virtual dipole moment distribution and dipole field reversals we have assessed 46 geodynamo simulations using the q pm criteria the simulations have each been run for the equivalent of at least 300 kyr span reversing and non reversing regimes and include either homogeneous or heterogeneous heat flux boundary conditions we find that none of our simulations reproduce all salient aspects of the long term paleomagnetic field behavior for the past 10 myr nevertheless our simulations bracket earth values suggesting that an earth like simulation is feasible within the available computationally accessible parameter space this new set of criteria can inform future simulations that aim to reproduce all aspects of earth s long term magnetic field behavior

holocene dynamics of the rhone glacier switzerland deduced from ice flow models and cosmogenic nuclides abstract we describe efforts to model the holocene extent of the rhone glacier switzerland using four paleoclimate records as templates for paleo equilibrium line altitude to identify candidate driving mechanisms of glaciers in the alps we evaluate the success of each paleoclimate template by comparing cosmogenic 10 be and 14 c concentrations in pro glacial bedrock derived from modeled glacier configurations to measured values an adequate fit can be obtained using mean summer insolation for 46 5 n however use of the dongee cave china speleothem record yields the best fit by accounting for both sub millennial e g little ice age and medieval warm period and multi millennial climate variations summer insolation our result indicates that glaciers in the alps primarily responded to changes in insolation during the holocene were smaller than today during the early holocene when insolation was relatively high and became larger during the mid to late holocene superimposed on the first order insolation response were shorter sometimes large amplitude length changes in response to short lived climate events such as the medieval warm period and the lia

do superchrons occur without any palaeomagnetic warning abstract the mainly dipolar geomagnetic field generated by the geodynamo within the earth s liquid core has reversed its polarity many times in the past this succession of intervals of alternate polarity defines the geomagnetic polarity time scale gpts usually interpreted as resulting from a gamma renewal process the rate of which would be controlled by the boundary conditions imposed by the mantle on the core in this interpretation boundary conditions would have occasionally evolved towards being unfavourable to reversals leading the reversal rate of the process to progressively decrease and reach zero at least twice causing the onset of the so called kiaman and cretaceous very long polarity intervals superchrons here we reconsider this causal link analysing the latest and best constrained gpts thanks to the continuous record provided by marine magnetic anomalies describing the past 160 myr we found that contrary to earlier claims no long term behaviour over the 40 myr preceding the cretaceous superchron can be seen in the reversal rate that could explain its onset at 120 ma more generally it turns out that hardly any special behaviour can be identified in the gpts which could have announced this superchron only the occurrence of the longest of all pre superchron intervals cm1n 3 myr before the onset of the superchron could be identified as a precursor such a behaviour could possibly be the consequence of medium term on the 10 myr time scale changes in the boundary conditions imposed by the mantle on the core but we note that the sole analysis of the gpts does not allow this to be tested in fact it appears that a single stationary process could also explain the entire pre superchron gpts with the only possible exception of cm1n this suggests that the occurrence of superchrons does not necessarily require changes in boundary conditions and could simply attest for a sudden non linear transition between a reversing and a non reversing state of the geodynamo

geochronology of a late archaean flood basalt province in the pilbara craton australia constraints on basin evolution volcanic and sedimentary accumulation and continental drift rates abstract eleven high precision 2 5 million years shrimp zircon u pb ages have been obtained from felsic rocks within a single stratigraphic section of late archaean volcanic and sedimentary rocks in the east pilbara of western australia the stratigraphic succession nullagine and mount jope supersequences in sequence stratigraphic terminology fortescue group in lithostratigraphic terminology is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional rift related environment between about 2772 and 2715 ma the geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy field mapping geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs while flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession felsic volcanism is now known to have been active periodically through each geotectonic cycle the succession covers a time period of about 57 million years the lower 1400 m of a thick 1700 m clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years in contrast a younger 150 m thick cogenetic tuff basalt unit accumulated in less than 3 million years and others probably accumulated at similar rates comparable to those of phanerozoic flood basalts unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time gap of more than 10 million years the unconformity based stratigraphic framework the new geochronology and palaeomagnetic studies j geophys res 108 2003 b12 2551 emp 2 1 to 2 21 have been combined to determine a possible late archaean continental drift rate for one part of the succession implying a period of motion as fast as or up to five times faster than any known from the phanerozoic

secular variation of the late archean early proterozoic geodynamo 1 we report new paleomagnetic data from 2 5 ga dikes of the superior province and derive estimates of paleosecular variation when combined with data from coeval dikes of the karelia craton russia and older 2 7 ga volcanics of the pilbara craton these estimates suggest a latitudinal variation of secular variation similar to that of the last 5 million years but with potentially smaller quadrupole family contributions the observations are consistent with some numerical models of the field that suggest a more dipolar field when the earth had a much smaller solid inner core

more stable yet bimodal geodynamo during the cretaceous superchron we report palaeomagnetic and ar 40 ar 39 dating results from two sequences of basaltic lava flows deposited at the same locality in western china yet separated in time by similar to 50 myr one set lies within the cretaceous normal superchron at 112 115 ma and a second at 59 70 ma spanning the cretaceous palaeogene boundary we find that magnetic field directions during the superchron exhibit bimodal populations one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex multipolar field however the time dependent variability in field directions was 50 lower during the superchron than after which implies greater field stability during the superchron our results suggest that episodes of less dipolar field behavior occurred within the cretaceous superchron and raise the question whether a second more multipolar field state is more persistent than previously thought

geomagnetic secular variation and the statistics of palaeomagnetic directions in this study we examine the role of palaeosecular variation psv in the use of statistics for palaeomagnetic studies and we provide new reliability criteria for palaeomagnetic poles or directions we first conclude that fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole vgp distributions instead secondly we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets the latitude dependent properties e s k provide useful guidelines for the reliability of a palaeomagnetic data set a reliable assessment of these properties depends on the sufficient number of palaeomagnetic samples being taken therefore as an additional instrument of assessing data sets we provide a n dependent a95 envelope bounded by an upper limit a95max and a lower limit a95min that helps to ascertain whether or not a distribution has sufficiently well sampled psv and therefore geomagnetic field behaviour applying these criteria is indispensable for studies of geomagnetic behaviour or for studies aiming at using tk03 gad for inclination error correction through the elongation inclination e i method for palaeomagnetic studies aimed at geological reconstructions they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field an analysis of published eastern mediterranean data shows that the vast majority of studies do not conform to the van der voo criteria in particular with respect to n and a95 we have provided criteria that are on the one hand more lenient lower n may still provide relevant information and on the other hand more strict for high n the criterion of a95 16 should be adapted to a requirement of lower a95 e g a95 80

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

late devonian palaeomagnetism of the north tien shan kyrgyzstan can secular variation vary on a short timescale summary we studied more than 80 lava flows from a 600 m thick pile of upper devonian frasnian basalts and andesites of the aral formation in the north tien shan kyrgyzstan central asia with the aid of stepwise thermal demagnetization a high temperature dual polarity component was reliably isolated from most flows the primary origin of the high temperature component is demonstrated by positive reversal conglomerate and fold tests the most prominent and intriguing feature of this late devonian data set involves a clear distinction in angular dispersion between the lower and upper parts of the studied section a rather low concentration parameter k 13 and several directional anomalies characterize the lower section in contrast a much better grouping k 46 and a lack of directional outliers is observed for the dual polarity vectors from the upper flow sequence of the formation we analysed different mechanisms to account for this directional pattern and found that it is possible in just two ways one is to assume that secular variation sv in the upper sequence is strongly underrepresented and it is a coincidence that the mean directions of both polarities are statistically antipodal and the corresponding concentration parameters are statistically equal the other explanation is to hypothesize that the magnitude of sv can vary several fold at the same palaeolatitude and over time intervals estimated as 10 5 10 6 yr this is in sharp contrast with other models of sv where this magnitude has been assumed to be rather time independent for a givenlatitude ourhypothesisaccountsfortheobservedirregularitiesinpalaeomagneticdata but makes attempts to establish a correlation between sv and other parameters geographic latitude reversal frequency age etc more difficult we are aware however that more data are needed to refute or confirm it

mantle plumes link magnetic superchrons to phanerozoic mass depletion events abstract the four most recent large mass extinction events in the phanerozoic the cretaceous tertiary kt the triassic jurassic tj and the permo triassic pt and guadalupian tatarian gt doublet are associated with a major flood basalt eruption with the timing of peak volcanic activity corresponding within measurement uncertainties to the extinction event three magnetic superchrons precede the four largest phanerozoic extinctions the cretaceous long normal superchron duration 35 myr precedes the kt and the permian kiaman long reversed superchron 50 myr precedes the pt gt doublet in addition the newly recognized ordovician moyero long reversed superchron 30 myr precedes the end ordovician extinction event there is a 10 20 myr delay between the end of each superchron and the subsequent mass depletion event both of which represent distant outliers from their respective populations we propose that deep mantle plumes link these seemingly unrelated phenomena long term 200 myr variations in mantle convection possibly associated with the wilson cycle induce temporal and spatial variations in heat flow at the core mantle boundary polarity reversals are frequent when core heat flow is high and infrequent when it is low thermal instabilities in the d layer of the mantle increase core heat flow end the magnetic superchron and generate deep mantle plumes the plumes ascend through the mantle on a 20 myr time scale producing continental flood basalt trap eruptions rapid climatic change and massive faunal depletions

palaeomagnetic field intensity variations suggest mesoproterozoic inner core nucleation the earth s inner core grows by the freezing of liquid iron at its surface the point in history at which this process initiated marks a step change in the thermal evolution of the planet recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the earth s core resulting in estimates of the timing of inner core nucleation ranging from less than half a billion to nearly two billion years ago recent inner core nucleation high thermal conductivity requires high outer core temperatures in the early earth that complicate models of thermal evolution the nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner core nucleation to be estimated directly previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements by the lack of an effective means of assessing their reliability and by shorter timescale geomagnetic variations here we examine results from an expanded precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria our analysis provides intensity based support for the dominant dipolarity of the time averaged precambrian field a crucial requirement for palaeomagnetic reconstructions of continents we also present firm evidence for the existence of very long term variations in geomagnetic strength the most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1 5 billion years ago this observation is most readily explained by the nucleation of the inner core occurring during this interval the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the earth

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