geodynamo solar wind and magnetopause 3 4 to 3 45 billion years ago stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss although the early earth retained its water and atmosphere and thus evolved as a habitable planet little is known about earth s magnetic field strength during that time we report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3 4 to 3 45 billion years ago the measured field strength is 50 to 70 that of the present day field when combined with a greater paleoarchean solar wind pressure the paleofield strength data suggest steady state magnetopause standoff distances of 5 earth radii similar to values observed during recent coronal mass ejection events the data also suggest lower latitude aurora and increases in polar cap area as well as heating expansion and volatile loss from the exosphere that would have affected long term atmospheric composition

paleomagnetism of 1 09 ga lake shore traps keweenaw peninsula michigan new results and implications we report paleomagnetic data from a new section of the 1 09 ga lake shore traps exposed on silver island 10 flows and on the adjacent mainland two flows along the northwestern coastline of the

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

using palaeomagnetism to determine late mesoproterozoic palaeogeographic history and tectonic relations of the sinclair terrane namaqua orogen namibia

human influence and the changing geomorphology of mediterranean deltas and coasts over the last 6000 years from progradation to destruction phase the present geomorphology of the mediterranean s coasts is largely a product of an intricate long term relationship between nature and human societies a cradle of ancient civilisations the mediterranean has seen its shores occupied by humans since prehistory and is therefore a particularly pertinent unit of analysis the morphotectonic context and other forcing agents e g climate shaped out a highly diversified coastal morphology and generated a sediment supply regime potentially favourable to the formation of numerous open coast deltas and bay head deltas in infilled rias as sea level stabilised during the mid holocene this supply of riverine sediment has also been the key agent in mediating human occupation of the mediterranean s clastic coasts expressions of this relationship have been extensively archived in clastic coastal deposits including base level deltaic and estuarine sedimentary sinks which comprise records to explore the interactions between geosystems and the human environment the stratigraphic sequences in these coastal sedimentary archives comprise in many places a clearly identified anthropogenic signature notably in ancient harbours some of which underwent extremely rapid silting up due to massive sediment sourcing generated by new agricultural practices from the neolithic onwards increasing human influence especially over the last 3000 years has been in turn an important driver of changes in sediment supply strongly modulating deltaic development pulses of sediment supply from catchments rendered vulnerable by human perturbations during the roman period resulted in a new cycle of inception of many other deltas and in rapid delta growth e g the ebro the po the arno and the ombrone another progradation dynamic during the little ice age at a time of strong rural population growth river discharge increases technological developments and urbanisation further consolidated delta growth understanding the life cycle of these deltas since their initial formation is in turn key to unravelling the relative role of natural and anthropogenic forcing agents rapid climate changes are deemed to have contributed through both the stripping of landscapes rendered fragile by human activities and active fluvial sediment transport to the coast but disentangling climate change effects from human impacts in the mediterranean remains a challenge the patterns of subsequent deltaic growth and delta morphodynamics reflect adaptations to pulsed sediment supply river discharge variations the microtidal fetch limited context of the mediterranean and direct engineering interventions the progradation dynamic of the roman period and little ice age contrasts markedly with the situation of common coastal destabilisation over the last two centuries particularly well documented for the last 50 years this period has been characterised by reduced sediment flux to base level geosystems due to catchment reforestation retenion within reservoirs fluvial regulation and dredging resulting in the erosion of deltas and barrier lagoon and beach dune systems large stretches of shoreline and narrow coastal plains have been massively engineered for coastal defence and protection against erosion but also for the construction of marinas leisure harbours and artificial beaches resulting in the emergence of veritable artificial seafronts these interventions have collectively and progressively raised societies to a pervasive and overarching position in the geomorphic stability instability of the mediterranean s coasts a situation that will be exacerbated by pressures from sea level rise paving the way for rampant coastal erosion and delta destruction

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

restoring proterozoic deformation within the superior craton abstract geometrical patterns of paleoproterozoic dyke swarms in the superior craton north america and paleomagnetic studies of those dykes both indicate relative motion across the kapuskasing structural zone ksz that divides the craton into eastern and western sectors previous work has optimized the amount of vertical axis rotation necessary to bring the dyke trends and paleomagnetic remanence declinations into alignment yet such calculations are not kinematically viable in a plate tectonic framework here we subdivide the superior craton into two internally rigid subplates and calculate euler parameters that optimally group the paleomagnetic remanence data from six dyke swarms with ages between 2470 and 2070 ma our dataset includes 59 sites from the matachewan dykes for which directional results are reported for the first time our preferred restoration of the eastern superior subprovince relative to the western subprovince is around an euler pole at 51 n 85 w with a rotation angle of 14 ccw although we do not include data from the ksz in our rigid subplate calculations we can align its dyke strikes by applying a 23 ccw distributed shear that preserves line length of all dykes pinned to the western margin our model predicts approximately 90 km of dextral transpressional displacement at ca 1900 ma about half of which is accommodated by distributed strain within the ksz and the other half by oblique lateral thrusting with ne vergence across the ivanhoe lake shear zone we produce a combined apparent polar wander path for the early paleoproterozoic superior craton that incorporates data from both western and eastern subplates and that can be rotated to either of the subplates reference frames for the purposes of archean paleoproterozoic supercraton reconstructions

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

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

geomagnetic field strength 3 2 billion years ago recorded by single silicate crystals analysis of silicate rocks from the archaean kaapvaal craton is used to obtain palaeodirections and intensities of the earth s magnetic field some 3 2 billion years ago and finds that the field strength was within 50 per cent of the present day value indicating that a viable magnetosphere sheltered the early earth s atmosphere from solar wind erosion

palaeomagnetism of the neoarchaean pongola and ventersdorp supergroups and an appraisal of the 3 0 1 9 ga apparent polar wander path of the kaapvaal craton southern africa the style of tectonic processes on the young earth is a topic of intense debate palaeomagnetism provides the only robust tests with which to settle theoretical disputes based on the rock record that infer large scale horizontal motions between palaeocontinents a possible apparent polar wander path apwp of the kaapvaal craton shows significant distances between adjacent palaeomagnetic poles that can be interpreted as evidence for such horizontal displacements during the archaean palaeoproterozoic there are however substantial uncertainties build into this apwp currently the published apwp for the kaapvaal craton comprises eight palaeopoles between 3 0 and 1 9 ga including several between 2 8 and 2 7 ma that indicate possible horizontal motions exceeding 6500 km the aim of this contribution is to increase the pole density within this interval and in doing so to test the robustness of the kaapvaal apwp within this time frame more than 250 samples of basalt diamictite and dolerite of the neoarchaean pongola ca 2 95 2 85 ga and ventersdorp ca 2 71 2 70 ga supergroups of the kaapvaal craton were sampled both at the surface and in an underground mine thermal demagnetisation revealed two magnetic components a high temperature component is present in three pongola formations and in two ventersdorp formations these may be recorders of the primary natural remanent magnetisation because of lack of precise geochronology and positive field tests this magnetisation can only be constrained to be older than ca 2 05 ga a medium temperature overprint in many samples probably records remagnetisation at ca 180 ma during the emplacement of basalts and dolerites of the mesozoic karoo large igneous province this remagnetisation appears to have affected a vast area of the kaapvaal craton our new palaeopoles together with those published previously demonstrate that the neoarchaean apwp of the kaapvaal craton remains poorly defined direct comparisons between the apwp of the kaapvaal craton with apwps of other archaean cratons to determine relative motions between archaean continents therefore need more reliable data

early proterozoic geomagnetic field in western laurentia implications for paleolatitudes local rotations and stratigraphy abstract the east west trending post orogenic subvertical cleaver dykes from the great bear magmatic arc are dated by the u pb baddeleyite method to be 1740 5 4 ma they have a mean direction of magnetization of d i 136 5 and 57 4 k 64 95 4 5 based on samples from 17 diabase dykes a mean location 65 7 n 118 03 w and a corresponding paleopole at 19 4 n 83 3 w a95 6 1 studies of baked contacts country rocks distant from dykes and later intrusions that cut them show that the dykes became magnetized soon after intrusion the paleopole is not significantly different from that obtained from the 1759 1 ma post orogenic jan lake granite from the hudsonian orogen in southeast saskatchewan these paleopoles fall among a cluster previously determined from post orogenic overprints in a wide variety of rocks and geological settings across laurentia their mean is 20 8 n 94 5 w a95 5 2 collectively they record a widespread remagnetization episode following the hudsonian orogeny during which the polarity of the field remained constant this single polarity interval the cleaver superchron has a mean age of approximately 1747 ma and its duration is at least 13 million years but is probably much longer with further refinement it could become a global statigraphical marker there are 18 paleopoles from red beds and igneous rocks which record the paleofield in western laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 ma during the hudsonian orogeny over this interval western laurentia remained at a low and essentially constant latitude reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency there are numerous magnetochrons which could in future assist in regional stratigraphic correlations on the basis of six studies we estimate that the reference paleopole for this interval is at 1 1 s 71 7 w a95 12 1 n 6 paleopoles from the other 12 studies are spread over a 100 arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering archean slave and superior cratons during the hudsonian orogeny these reference paleopoles show that during the hudsonian orogeny western laurentia was in paleolatitudes of 15 30 during post hudsonian time ca 1750 ma paleolatitudes were about 20 higher paleowind directions inferred by others indicate that laurentia during these intervals was in the northern hemisphere

primary curvature in the mid continent rift paleomagnetism of the portage lake volcanics northern michigan usa abstract rocks of the north american mid continent rift mcr in the keweenaw peninsula of northern michigan display a change in structural trend from east to west varying in strike from 100 in the east to 35 farther west before returning to a more east west trend of 80 near wisconsin in general curvature can be described either as of primary origin meaning that the arc developed in its present curved state or as of secondary origin when the arc formed from an initially straighter geometry a powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism where coincident change in declination and strike is evidence for secondary curvature several paleomagnetic studies have been completed on rift related sedimentary rocks as well as lava flows from the mcr in the lake superior region paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination possibly reflecting a vertical axis rotation if secondary rotation has affected the sedimentary rocks then underlying volcanic rocks should also demonstrate a similar rotation thirty one sites were collected from the portage lake volcanics in the most highly curved part of the keweenaw peninsula in the upper peninsula of michigan sites were chosen to maximize the variation in structural trend thermal demagnetization results showed two components in samples from most sites a lower temperature a component 580 c both components were tested for primary remanence using a conglomerate test the a component carried by magnetite passes the conglomerate test at a 95 confidence level and is therefore considered a primary remanence the b component carried by hematite fails at the 95 level and is considered a secondary magnetization most importantly declination of the primary a magnetization between sites shows no correlation with strike demonstrating that vertical axis rotations cannot explain the curvature of the mid continent rift we conclude that curvature in the lake superior region is a primary feature likely reflecting a pre existing zone of weakness that was exploited during rifting

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

paleomagnetism and u pb geochronology of the 2 17 ga biscotasing dyke swarm ontario canada evidence for vertical axis crustal rotation across the kapuskasing zone u pb dates on baddeleyite yield ages of 2167 8 2 2 and 2171 6 1 2 ma on two northeast trending dykes west of the kapuskasing zone in ontario canada these ages identify the dykes as belonging to the biscotasing dyke swarm east of the kapuskasing zone which was previously dated at 2166 7 1 4 ma by u pb on baddeleyite and zircon the new dates show that the biscotasing swarm was emplaced over an area of at least 300 000 km2 much larger than hitherto suspected and in a geologically short period of time of about 5 million years a comparison of paleomagnetic data from biscotasing and 2 45 ga matachewan dykes on either side of the kapuskasing zone suggests that the western half of the superior province has rotated about 10 20 counterclockwise relative to the eastern half across the kapuskasing zone this movement may have been accompanied by rifting farther north which ultimately led to the paleoproterozoic embayment underlying hudson bay that gives the superior province its characteristic butter

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

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

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

paleomagnetism of the 1 1 ga umkondo large igneous province in southern africa 1 the umkondo dolerites are present over a wide area in the kalahari craton southern africa thirty nine sampling sites in botswana and south africa yielded tightly grouped paleomagnetic directions due south with shallow inclinations and three sites of opposite polarity the dolerites have u pb single crystal baddeleyite or zircon crystallization ages of 1112 0 5 to 1108 0 9 ma these results can be combined with published data from 39 additional umkondo sites and 33 sites in the grunehogna province of antarctica after restoring east antarctica to its position next to southern africa grouping the sites geographically yields 10 site mean poles with mean at 64 0 n 38 8 e a95 3 7 this umkondo pole can be correlated with keweenawan poles from laurentia because both sets of poles are precisely of the same age as well as predominantly of one polarity the relative orientation of the two cratons within the rodinia supercontinent is fixed this implies that the namaqua natal maud belt which rims the southern part of the kalahari craton faced away from laurentia the umkondo pole combined with published poles suggest that the kalahari craton remained distinctly south of the laurentian craton between 1 1 and 1 0 ga making it highly unlikely that the two cratons collided

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

the stratigraphic and structural setting of the mt lyell volcanic hosted sulfide deposits volcanic hosted sulfide deposits at mt lyell are confined to a complex silicic volcanic unit mine sequence which is up to 800 m thick and includes laterally impersistent lavas ash flow and ash fall deposits as well as abundant open framework volcanic mass flow breccias pervasive hydrothermal alteration involving na depletion is associated with the sulfide mineralization and persists with minor disseminated pyrite mineralization over a strike length of 8 km within the mine sequence stratigraphic relations together with lithofacies and thickness variations in volcanic units suggest that the sulfide mineralization and hydrothermal alteration occurred at a major eruptive center and that ore deposition occurred over an extended time interval during the accumulation of the mine sequence two orientation groups of middle devonian folds are present in the area first generation d 1 folds are closed to tight and upright north northwest trending structures which largely control the distribution and orientation of rock units in the volcanic sequence but have not been associated with significant microstructural modification of the ores or host rocks open west to west northwest trending upright second generation d 2 folds are well developed in sedimentary sequences overlying the volcanic sequence but are poorly developed in the volcanic sequence itself d 2 strain has been accommodated in the volcanic sequence largely by the development of a mesoscopically penetrative west to west northwest trending upright foliation s 2 and associated downdip lineation l 2 shortenings of up to 60 percent perpendicular to s 2 and elongations of up to 150 percent parallel to l 2 have modified primary sulfide microstructures and are responsible for many of the sulfide deposits in the area being elongate in the l 2 direction

a palaeomagnetic investigation of the mashonaland dolerites north east zimbabwe summary earlier palaeomagnetic work on the mashonaland dolerite sills and dykes 1830 230 ma showed that they record a geomagnetic field reversal 28 new sites have been sampled in sills 26 and dykes 2 in north east zimbabwe detailed alternating field demagnetization afd is more effective in removing secondary components of remanence than thermal demagnetization td and enables an assumed primary remanence to be isolated at 16 sill sites combined with earlier results there are 11 sill sites with a positive inclination p d 287 i 53 k 40 and 18 with a negative inclination n d 114 i 47 k 35 the data lead to a positive class b reversal test field tests suggest that the n remanence is the younger and that the period of mashonaland igneous activity spanned a reversal of the earth s magnetic field from p to n the areal distribution of polarity is not random sills with only p remanence are found in the more elevated central region of the mashonaland igneous province whilst predominantly n bearing sills occupy the lower surrounding terrain

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

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

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

paleomagnetism u pb geochronology and geochemistry of marathon dykes superior province and comparison with the fort frances swarm we report the first detailed study of the paleomagnetism u pb geochronology and major element geochemistry of paleoproterozoic north trending marathon dykes north of lake superior the paleomagnetic and geochemical results demonstrate that marathon dykes can be divided into two subsets one of normal magnetic polarity the other of reverse polarity normal and reverse marathon paleomagnetic poles at 43 n 196 e dm 9 dp 7 number of dykes n 16 and 51 n 175 e dm 9 dp 6 n 12 respectively are statistically distinct and may indicate different ages of normal and reverse dyke emplacement a u pb baddeleyite age of ma has been obtained at a normally magnetized marathon paleomagnetic site the reversely magnetized marathon dykes are undated but have a paleopole rather close to that of the reversely magnetized ma fort frances dykes and major element geochemical signatures as portrayed on jensen plots that are identical to those of the fort frances swarm therefore reverse marathon an

mesoarchean subduction processes 2 87 ga eclogites from the kola peninsula russia the nature of tectonic processes on the early earth is still controversial the scarcity of high pressure metamorphic rocks such as eclogite the high pressure equivalent of basalt in archean cratons has been used to argue that plate tectonics did not operate until earth had cooled to a critical point perhaps around the 2 5 ga archean proterozoic transition however eclogites occur as meter to kilometer sized lenses enclosed in archean gneisses of the belomorian province of the fennoscandian shield geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros troctolites and fe ti oxide gabbros greenschist facies mineral parageneses are enclosed in prograde zoned eclogite garnets and peak metamorphic conditions define an apparent thermal gradient 12 15 c km consistent with metamorphism in a warm archean subduction zone we show here that these eclogites represent the oldest known high pressure metamorphic rocks u pb dating and hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2 87 ga for the uzkaya salma eclogite a 2 70 ga age for the shirokaya salma eclogite is interpreted as the age of a granulite facies overprint thermal overprinting and growth of new zircon also occurred during the svecofennian 1 9 1 8 ga orogeny these new data imply that plate tectonic processes operated at least locally in late mesoarchean time the adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce archean tonalite trondhjemite granodiorite type magmas

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

the paleoproterozoic marathon large igneous province new evidence for a 2 1 ga long lived mantle plume event along the southern margin of the north american superior province abstract three paleoproterozoic diabase dyke swarms the marathon kapuskasing and fort frances which outcrop around the northern margin of lake superior radiate from a focal region south of the lake u pb dating shows that the marathon and kapuskasing swarms range in age between 2126 and 2101 ma and the fort frances between 2076 and 2067 ma the convergence of the dyke swarms to a common focal area and the lack of apparent polar wander during the time interval 2125 2067 ma suggest that a single periodically active plume was responsible this 60 m y period of magmatism manifest over an area of at least 400 000 km2 is designated as the marathon large igneous province only a single magnetic field reversal has been recognized throughout the 60 m y of plume activity this observation together with u pb geochronology has helped to place dykes within each swarm in a chronological order based on their magnetic polarity for the dyke population as a whole older dykes tend to show more enriched and more fractionated incompatible trace element patterns compared to younger ones a result which suggests that the marathon kapuskasing and fort frances swarms are genetically related and underwent a secular reduction both in the degree of crustal contamination and or depth of mantle melting over the course of plume activity a similar geochemical relationship has also been found for the 2171 2166 ma biscotasing swarm but determining whether it is a common feature of plume development must await further study

paleomagnetism of 1 79 ga para de minas mafic dykes testing a sao francisco congo north china rio de la plata connection in columbia abstract paleogeographic connections between sao francisco congo north china and rio de la plata cratons through the paleoproterozoic have been proposed on the basis of geological and paleomagnetic data we conducted new paleomagnetic analyses for mafic dykes of the 1790 ma para de minas swarm occurring in the southern sao francisco craton the data revealed south southwestern low inclination magnetic directions after alternating field af and thermal demagnetization providing a paleomagnetic pole at 39 8 s 196 8 e a95 17 0 the characteristic remanent magnetization is interpreted as a thermo remanent magnetization carried by stable pseudo single domain magnetite grains being acquired during the cooling of the dykes at 1790 ma as attested by a positive baked contact test performed on one of the dykes the new pole in conjunction with 1790 1750 ma paleomagnetic poles available for other blocks allowed an improved reconstruction of columbia nuna supercontinent at this time frame laurentia baltica siberia proto amazonia west africa and proto australia were positioned as in previously proposed models the main difference from our reconstruction concerns the link of the sao francisco congo north china and rio de la plata cratons implying that much of the columbia supercontinent had already agglutinated around 1850 1800 ma ago this configuration is consistent with the onset of eventual taphrogenic episodes over most blocks accompanied by the emplacement of mafic dyke swarms and granitic intrusions at 1790 1750 ma

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

no asymmetry in geomagnetic reversals recorded by 1 1 billion year old keweenawan basalts 1 1 billion year old volcanic rocks in north america are thought to record asymmetric geomagnetic reversals indicating non axial dipolar behaviour of the magnetic field high resolution data from ontario suggest that the reversals were instead symmetric and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of north america

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