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

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

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

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

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

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

the utility of desert sand dunes as quaternary chronostratigraphic archives evidence from the northeast rub al khali abstract the distributions of optically stimulated luminescence osl ages from dunes are affected by palaeoenvironmental changes complex dune dynamics and sampling strategy extracting the relative importance of these factors when interpreting discontinuous osl chronologies from sand dunes has proven difficult and is particularly hindered in contexts where the internal sedimentary structures of dunes are not visible in this study samples for osl dating were taken from three major dune exposures in the rub al khali united arab emirates each showing clear internal structure with the aim of addressing these problems specific objectives were to assess how ages of individual sedimentary units represent dune accumulation as a whole and how the dune record in turn reflects known past environmental changes final ages were calculated using the oxcal software package by incorporating the known relative stratigraphy through the application of bayesian methods the results show that stratigraphy alone is not sufficient at these sites to guide osl sampling that is chrono and lithostratigraphic boundaries do not necessarily coincide where chronological hiatuses are present internal sediment stratigraphy can be a useful tool in identifying potential problems of under sampling the full dune record the implications of these findings for reconstructing quaternary climates from dune chronologies are considered

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

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

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

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

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

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

implications for focused fluid transport at the northern cascadia accretionary prism from a correlation between bsr occurrence a d near sea floor reflectivity anomalies imaged in a multi frequency seismic data set a high resolution seismic survey was carried out at the accretionary prism on the continental slope off vancouver island canada two gi gun data sets with different source frequency ranges of 50 150 and 100 500 hz were combined with 4 khz narrow beam echosounding data parasound the data allow spatial correlation between a gas hydrate bottom simulating reflector bsr and distinct areas of high near sea floor reflectivity an integrated interpretation of the multi frequency data set provides insight into the regional distribution of tectonically induced fluid migration and gas hydrate formation in the vicinity of odp leg 146 sites 889 and 890 the bsr at the base of the gas hydrate stability field is observed within accreted and deformed sediments but appears to be absent within bedded slope basin deposits it is suggested that these basin deposits inhibit vertical fluid flow and prevent the formation of a bsr whereas the hydraulic conductivity of the accreted sediments is sufficiently high to allow for pervasive gas migration an elevation of the bsr beneath the flanks of a topographic high is interpreted as an indicator for local upflow of warm fluids along permeable pathways within outcropping accreted sediments parasound data reveal discontinuous zones of high reflectivity at or directly beneath the sea floor which may indicate local cementation of surface sediments in combination with gi gun data the occurrence of these reflective areas can be related to the location of slope sedimentary basins acting as hydraulic seals it is proposed that the seals sometimes fail along faults extending beneath the bsr leading to focused upflow of methane bearing fluid and the formation of carbonate pavements at the sea floor

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

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

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

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