palaeomagnetic data suggest link between the archaean proterozoic boundary and inner core nucleation it is generally believed that the earth s magnetic field arises from dynamo action in the convecting outer core1 3 thus the geomagnetic field and especially its strength can provide information about core processes recent determinations of the geomagnetic palaeointensity using archaean and early proterozoic samples from the kaapvaal and superior cratons are consistent with a sharp increase in the magnitude of the geomagnetic field between 2 7 and 2 1 gyr analysis of published late archaean and early proterozoic apparent polar wander paths shows that an abrupt change in the rate and style of continental motion occurred at about 2 5 2 6 gyr ago if this change is also interpreted to be related to the growth of an inner core then an intriguing conjecture arises that the archaean proterozoic transition is related to inner core nucleation

phase equilibria testing of a multiple pulse mechanism for origin of mafic ultramafic intrusions a case example of the shiant isles main sill nw scotland in this paper we examine the role of multiple emplacement of sills into partly solidified rocks an intrusive mechanism liquid into solid as a possible explanation for some textural and compositional anomalies of single cyclic mafic intrusions as a case study we used the shiant isles main sill that is widely regarded as a classical example of a multiple picrite picrodolerite crinanite alkaline sill this sill is currently interpreted as having been formed by several olivine phenocryst rich pulses of magma which were successively emplaced into their almost solidified predecessors such an intrusive mechanism is a random process in which many parameters vary independently and unpredictably among them are the number relative volume and bulk composition of magma pulses and their place sequence and timing of emplacement as well as modal abundance phase composition and distribution of intratelluric phenocrysts in magmas upon emplacement in terms of these variables one can envisage countless different profiles through alkaline sills produced from only three randomly intruded magma pulses of picritic picrodoleritic and crinanitic composition such multiple sills can readily be distinguished from simple ones formed from a single pulse of magma by anomalous compositional profiles with several prominent breaks in crystallization and compositional sequences the compositional profile of the shiant isles main sill is remarkably similar to an m shaped profile expected from fractional crystallization of a single pulse of olivine saturated magma along a crystallization path ol sp l picrite ol pl sp l picrodolerite troctolite ol pl cpx l crinanite the probability of the accidental formation of such a compositional profile by multiple intrusion liquid into solid is exceedingly small even for the single case of the shiant isles main sill the probability approaches zero when considering that exactly the same sequence of intrusive events must have been repeated in about 20 neighbouring alkaline sills with similar compositional profiles this can only be achieved by some universally operating differentiation process the best candidate for this is the classical fractional crystallization of magma constrained by liquidus phase equilibria this suggests that the shiant isles main sill can be best interpreted and modelled as a simple sill that crystallized from one large pulse of magma with possible involvement of minor refilling events further progress in our knowledge of intrachamber magma fractionation processes will probably enable us to interpret many anomalous textural and compositional features of mafic ultramafic intrusions in the frame of a single magma pulse model

regional tilt of the mount stuart batholith washington determined using aluminum in hornblende barometry implications for northward translation of baja british columbia we have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum in hornblende ah barometer the method is used to correct previously published paleomagnetic data from the 93 96 ma mount stuart batholith of the cascades mountains washington state for the effects of postemplacement tilting ah barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation high contrastback scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration the success of the ah barometry is indicated by two observations first increases in the al content of the hornblendes are governed almost entirely by a pressure sensitive tschermak type substitution second amphibole plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions 650 c and that temperature has a negligible effect on our pressure estimates ah barometry results indicate that the depth of crystallization across the batholith decreases systematically from 0 3 gpa in the northwest to 0 15 gpa in the southeast consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations using a best fit planar tilt model and bootstrap analysis of uncertainties we estimate that the paleohorizontal plane has a strike of 43 630 4 and dip of 7 62 0 southeast 695 confidence our estimated paleohorizontal allows us to restore the paleomagnetic data of beck et al 1981 and to estimate the original paleolatitude of the mount stuart batholith beck et al found that the southern part of the batholith yielded a number of sites with a well defined high coercivity remanence the carrier of this remanence was not resolved but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith 1 the stable sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our ah results and concordant k ar ages for hornblende biotite pairs 2 the high coercivity component was always normal in polarity which is consistent with emplacement of the mount stuart batholith at the end of the cretaceous long normal 3 sites from the batholith and the contact aureole gave similar directions 4 the directions show no indication of tilt related smearing after restoration the paleomagnetic data indicate 42 6 11 clockwise rotation and 3100 6 600 km of northward offset

paleomagnetism of archean granites and matachewan dikes in the wawa subprovince ontario reevaluation of the archean apparent polar wander path paleomagnetic measurements have been completed on 400 specimens from dated archean granites and matachewan dikes in the michipicoten and gamitagama greenstone belts in the western wawa subprovince

paleomagnetic and rock magnetic results from the twin creek formation middle jurassic wyoming abstract a magnetization which passes the fold test has been observed in 73 limestone samples 10 sites from the middle jurassic twin creek formation the pole calculated from the site mean poles is located at 68 4 n 145 0 e k 31 8 a 95 8 7 this pole lies in a segment of the north american apparent polar wander apw path for which there are only a few reliable poles in the literature the results corroborate earlier studies which conclude that the jurassic segment of the apw path does not include the present north pole however the position of the twin creek pole suggests that significantly more apw took place prior to the late jurassic than previous studies indicated

magnetic felsic intrusions associated with canadian archean gold deposits felsic intrusions spatially and temporally associated with archean au deposits in the superior province of the canadian shield are magnetic due to high magnetite content in comparison to felsic igneous rocks in most of the archean terrain examples are found in au camps of hemlo red lake harker holloway mishibishu lake wawa missanabie geraldton matachewan and bouquet east cadillac these archean intrusions are comparable with phanerozoic magnetite series plutons accompanying metallic mineralization and magnetiterich intrusions hosting au rich porphyry deposits the evidence shows a distinct tectonic setting for the gold mineralization involving the generation of the particular magmas the association of the two indicates either the derivation of the ore constituents from the intrusions or the emplacement of the magmas along dilation zones which also provided fluid flow from depth the identification of magnetite rich felsic intrusions may aid in targeting favorable exploration areas for archean au deposits

conflicting sm nd whole rock and u pb zircon ages for archean lavas from newton township abitibi belt ontario abstract archean komatiitic and tholeiitic lavas from newton township ontario have a sufficient range of sm nd ratios to define a well constrained line on the normal 143 nd 144 nd vs 147 sm 144 nd isochron plot the data give an isochron age of 2826 64ma and an initial e nd of 2 65 0 26 however u pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite tholeiite suite give an age of 2697 1 1ma there is strong evident that the zircon age is the eruption age suggesting that the older sm nd age is incorrect and probably results from mixing between isotopically distinct mantle sources at the time of eruption the sources had e nd values from about 4 2 to 1 6 indicating that the archean mantle in this area was markedly heterogeneous and not uniformly depleted

paleomagnetism of late archean metavolcanics and metasediments abitibi orogen canada tholeiites of the kinojevis group iron rich and magnesium rich basaltic tholeiites of the latest archean kinojevis group central abitibi belt northeastern ontario contain a natural remanent magnetization that is generally multiv

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

the influence of the thick banded series anorthosites on the crystallization of the surrounding rock of the stillwater complex montana the banded series of the archean aged stillwater complex contains three thick anorthosites overlain by olivine bearing rocks with apparently different crystallization sequence in that orthopyroxene appears late if at all as compared with that of the ultramafic and lower banded series where orthopyroxene appears before plagioclase conventional models suggest that the rocks above the anorthosites represent injection of a magma with a different liquid line of descent than the rest of the complex alternatively it has been suggested that the plagioclase pyroxene mush protolith above the anorthosites was hydrated by the degassing of the underlying thick anorthosite units to produce olivine rather than pyroxene by either expansion of the olivine phase field by addition of h2o incongruent melting of pyroxene and or silica loss to the vapor to test for hydration melting and a late silica loss electron microprobe core rim analyses were done on plagioclase grains from troctolite of olivine bearing zone v ob v approximately a dozen grains were analyzed for each of six thin sections in total 69 pairs were reversely zoned and 22 were normal or unzoned the most negative an difference was 8 25 the most positive was 4 32 but the majority of pairs were clustered from 5 to 1 the average was 2 1 core grains had an average an number of 77 8 typical for other plagioclase from the middle banded series a green na rich hornblende is commonly associated with plagioclase where the latter is enclosed in olivine the prevalence of reversely zoned grains is consistent with silica and sodium loss from pre existing plagioclase this evidence as well as the lack of plagioclase core compositions being reset to more primitive an rich compositions the amoeboidal poikilitic habit of the olivine and the presence of amphibole are all consistent with the troctolite and olivine gabbro of the middle banded series and lower parts of the upper banded series having formed as the result of a combination of processes including hydration incongruent melting and element leaching during degassing of the underlying anorthosites

evolution and structure of a cold front in an alpine valley as revealed by a doppler lidar the propagation of a cold front and its interaction with foehn winds is investigated in an alpine valley based on observations collected during the field campaign of the mesoscale alpine programme map on 6 november 1999 the key instrument of this study is a doppler lidar that had been operated in the wipp valley austria the cold front approached the european alps from the northwest became distorted at the mountain barrier and entered the east west aligned inn valley near the town of innsbruck primarily via two passes it continued to propagate towards innsbruck from both valley directions as two separate fronts that eventually collided east of innsbruck after part of the cold air had entered the adjacent north south aligned wipp valley a synthesis of doppler lidar measurements with conventional meteorological data including automatic weather stations and radiosondes leads to the conclusion that the cold front in the wipp valley was an atmospheric density current characterized by an elevated head a front relative feeder flow and a typical propagation speed of 7 m s 1 the foehn flow on top of the density current caused strong wind shear and triggered shear flow instability that led to the formation of a turbulent wake behind the head as the density current propagated towards the brenner pass it slowed down the shape of the frontal surface varied in time its inclination of about 10 20 is steeper than previously reported for the inn valley but is consistent with other observations of atmospheric density currents copyright 2010 royal meteorological society

a paleomagnetic estimate of the age and thermal history of the kentland indiana cryptoexplosion structure the kentland indiana cryptoexplosion structure consists of a centrally uplifted zone of steeply dipping fault blocks surrounded by a ring depression and a low amplitude structural high with a radius of 12 km the age of the deformation is constrained stratigraphically to the interval between mississippian and quaternary paleomagnetic samples were collected from seven carbonate formations and one siltstone unit of these only the middle ordovician quimby s mill limestone contains a stable magnetization with consistent directions stepwise thermal and alternating field demagnetization shows the presence of a northwesterly moderately steep characteristic remanence with a median destructive field of 10 to 30 mt and maximum unblocking temperatures of 300 c the pole position calculated from the mean direction falls near late cretaceous paleomagnetic poles dispersion increases when a bedding correction is applied indicating post deformational magnetization because the magnetization fails the fold test it

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

reply to comment on paleomagnetism of a polarity transition in the lower triassic chugwater formation wyoming by emilio herrero bervera and charles e helsley

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

paleomagnetism of the red lake greenstone belt northwestern ontario possible evidence for the timing of gold mineralization we report paleomagnetic results from 1 largely unaltered batholiths and 2 sheared and altered units in the archean red lake greenstone belt a major gold producing region in the uchi subprovince of northwestern ontario group 1 includes the hammell lake killala baird trout lake and little vermilion lake batholiths which were intruded between 2730 and 2700 ma ago u pb zircon group 2 includes the howey diorite the dome stock and dikes and andesites from the dickenson mine these units from the mineralized deformation zones dzs between the batholiths were sheared and hydrothermally altered in the same event that caused gold mineralization in supracrustal units the batholiths carry a stable reversed remanence rlr withd 159 i 74 k 20 95 16 n 4 units 14 sites rlr has high coercivities unblocking temperatures and remanence intensities and appears to be a primary thermal remanence dating from 2700 ma the altered units from the gold bearing dzs carry a different remanence rlg withd 180 i 57 k 23 95 3 n 3 units 6 sites rlg also has relatively high coercivities and unblocking temperatures but its magnetization intensities are 1 2 orders of magnitude lower than those of rlr opaque minerals and silicates observed in thin section are highly altered rlg is therefore believed to be a chemical or thermochemical remanence acquired 2580 ma ago during the late stages of cooling and gold mineralization seeing as rlg characterizes the shear zone rocks but is absent from unaltered rocks outside the dzs it could be used as a rapid prospecting tool in delineating areas of alteration and possible mineralization the paleopoles for rlr 2700 ma and rlg 2580 ma link with those of the shelley lake granite 2580 ma 40ar 39ar hornblende and biotite and the matachewan hearst dikes 2450 ma u pb zircon and baddeleyite the sense of motion on the late archean early proterozoic apparent polar wander path for laurentia as defined by this sequence of paleopoles is opposite to that previously accepted

finescale vertical structure of a cold front as revealed by an airborne doppler radar abstract in the afternoon of 24 may 2002 a well defined and frontogenetic cold front moved through the texas panhandle detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary this paper primarily uses reflectivity and doppler velocity data from an airborne 95 ghz radar as well as flight level thermodynamic data to describe the vertical structure of the cold front as it intersected with the dryline the prefrontal convective boundary layer was weakly capped weakly sheared and about 2 5 times deeper than the cold frontal density current the radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail 40 m the echo structure and dual doppler inferred airflow in the vertical plane reveal typical features such as a nose a head a rear inflow current and a broad current of rising prefrontal air that feeds the accelerating front to rear current over the head the 2d cross fro

effects of thermal remanent magnetization on the magnetic anomalies of intrusives interpretations of magnetic anomalies often assume negligible remanent magnetization because detailed information about magnetic properties is rarely available this paper attempts to assess the effects of thermal remanent magnetization trm on magnetic anomalies caused by intrusives several three dimensional axisymmetric and two dimensional intrusives of different sizes are emplaced at different depths and at different times and their thermal evolutions are calculated on the basis of these thermal evolutions we determine their trm patterns assuming reversals of the earth s magnetic field have occurred and using typical magnetic blocking temperatures for igneous rocks results indicate that an intrusive with a uniform distribution of magnetic minerals acquires a layered pattern of trm near surface intrusives acquire stronger magnetization than deep seated ones the contribution of trm to magnetic anomalies is quite different from the contribution of induced magnetization especially for near surface intrusives interpretations of magnetic anomalies caused by deep seated intrusives in terms of induced magnetization are more reliable than those of the anomalies which are associated with near surface intrusives

braided peridotite sills and metasomatism in the rum layered suite scotland the rum eastern layered intrusion eli scotland is an open system layered intrusion constructed of 16 macro rhythmic units each of the macro rhythmic units consists of a peridotite base and a troctolite gabbro top previously attributed to the fractional crystallisation of a single magma batch this classic paradigm has been challenged however with evidence presented for the emplacement of peridotite sills in units 9 10 and 14 such as cross cutting relationships upward oriented apophyses and lateral discontinuities to test whether the other major peridotites within the eli represent sills we have carried out new field petrographic and mineral chemical analyses of the peridotites in units 7 8 and 9 the peridotites display large and small scale cross cutting relationships with the overlying troctolite indicative of an intrusive relationship the peridotites also show large scale coalescence and lateral spatial discontinuities such that the eli unit divisions become arbitrary harrisite layers and cr spinel seams found throughout units 7 8 and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma our observations allow for distinct subtypes of peridotite to be defined separated by intrusive contacts allowing for their relative chronology to be determined older poikilitic peridotite rich in clinopyroxene is truncated by younger well layered peridotite containing abundant harrisite layers in addition to the new peridotite subtypes defined in this study we find strong evidence for laterally oriented metasomatism within clinopyroxene rich wehrlites at the top of the unit 8 peridotite the wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions the observations presented in this study from the eli cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre existing feldspathic cumulate gabbro or troctolite the evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism forming many of the unusual often clinopyroxene rich lithologies that surround the sills the broad scale formation of the layered peridotites via incremental sill emplacement suggested by the occurrence of upward oriented apophyses coalescence and lateral discontinuity could be applied to much larger ultramafic intrusions which might have formed by similar mechanisms

paleomagnetism and late diagenesis of jurassic carbonates from the jura mountains switzerland and france a crucial fold test for magnetizations measured in bajocian through kimmeridgian limestones from the jura mountain region shows that rare blue gray limestones found in this sequence preserve a prefolding magnetization in contrast tan limestones which predominate in this sequence contain secondary magnetizations acquired after folding in the late miocene the postfolding magnetizations in tan limestones appear to be carried in goethite and because reversed as well as normal polarity directions are present the magnetization in these limestones is probably a post miocene pre brunhes 700 000 yr chemical remanent magnetization crm field relations between tan and blue gray limestones which are present in some cases in the same rock unit for example the hauptrogenstein suggest that the tan limestones may have formed from alteration of blue limestones whereas goethite pseudomorphs after pyrite are found in tan limestones the occurrence of pyrite in the blue limestones supports this model indicating that goethite could have formed from oxidation of pyrite abundant meteoric water probably was available for such an oxidation process during uplift of these strata subsequent to deformation natural remanent magnetization in blue limestones appears to be carried in magnetite the positive fold test for the characteristic magnetization in these rocks is significant at the 99 confidence level k values for 24 site mean directions before and after tilt correction are 9 3 and 43 2 respectively the simplest interpretation for the origin of observed remanence in magnetite in the blue limestones is a primary magnetization of depositional or biogenic origin characteristic directions in these rocks yield a mean paleopole position of 77 7n 148 4e k 26 0 a95 5 9 this pole is in reasonable agreement with one other upper jurassic pole from western europe and with other middle to upper jurassic poles from the atlantic bordering continents after rotation of these continents to a reconstructed configuration for middle to late jurassic time

paleomagnetism and structural history of the ghost range intrusive complex central abitibi belt ontario further evidence for the late archean geomagnetic field of north america on the basis of their remanence properties the lithologies of the late archean 2710 2703 ma ghost range complex an east west trending layered mafic ultramafic extrusive sequence in the central

comment on the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion by wall et al j petrology 59 153 190 2018

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion a reply to the comment by rais latypov on wall et al j petrology 59 153 190 2018

remagnetizations in late permian and early triassic rocks from southern africa and their implications for pangea reconstructions a paleomagnetic study of late paleozoic and early mesozoic sedimentary rocks from southern africa suggests wide spread remagnetization of these rocks samples of the mofdiahogolo formation in botswana and of the lower beaufort group in south africa were treated using thermal alternating field and chemical demagnetization the mofdiahogolo redbeds show a univectoral decay of the remanence revealing a characteristic direction of

petrophysical constraints on magnetic anomalies associated with metamorphic reactions in northern saskatchewan canada1 the western chipman domain of the east athabasca mylonite triangle in northern saskatchewan canada displays a large positive aeromagnetic anomaly that is the result of retrograde magnetite produc

influence of non dipole field on determination of plio pleistocene true polar wander some studies of global paleomagnetic data have found an offset of the magnetic pole during the plio pleistocene which has been interpreted as indicating a period of rapid true polar wander with a rate of movement comparable to the present day rate of polar motion deduced from astronomical observations we show that much of the polar offset determined from the paleomagnetic data may be due to deviations in pole position caused by persistent non dipole zonal components of the geomagnetic field a correction of paleomagnetic poles for the long term non dipole field reduces the polar offset and thus suggests a s lower or shorter episode of true polar wander over the past 5 million years

a middle late silurian paleomagnetic pole from limestone reefs of the wabash formation indiana u s a a middle late silurian paleomagnetic pole has been obtained from reefal limestones in north central indiana eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization one additional site gave the same characteristic direction after great circle analysis these 12 site level directions were used to calculate the mean paleomagnetic pole reef flank beds dip steeply about 38 away from reef centers field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12 a 12 tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement indicating that the magnetization was acquired early the presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence the mean of the site paleomagnetic poles with sites from steeply inclined flank beds corrected for 12 of tilt is 16 8 n 124 9 e k 74 a95 5 1 in good agreement with the middle silurian rose hill pole for north america

a paleozoic pangaea paleozoic paleogeographies should be consistent with all available reliable data however comparison of three different devonian paleogeographies that are based largely or wholly on the data of remanent magnetism show them to be inconsistent in many regards when these three paleogeographies are provided with possible ocean surface current circulation patterns and have added to them lithofacies and biogeographic data they also are shown to be inconsistent with such data a pangaeic reconstruction positioned in the southern hemisphere permits the lithofacies and biogeographical data to be reconciled in a plausible manner

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

synopsis of paleomagnetic studies in the kapuskasing structural zone this paper summarizes results from paleomagnetic studies sponsored by lithoprobe on the kapuskasing structural zone ksz data from archean rocks outside the ksz indicate that the wawa subprovince has not been significantly rotated or translated 5 relative to the abitibi subprovince results from the granulites and amphibolites indicate that the ksz underwent several kilometres of uplift at ca 2 51 ga and then 10 5 west northwest tilt with several kilometres of further uplift between 2 04 and 1 88 ga from thrust faulting on the ivanhoe lake fault zone localized chemical remagnetization occurred at 1 1 ga along the west side of the shawmere anorthosite paleomagnetic data from the 2 45 ga matachewan diabase dike swarm indicate that it was emplaced within one reversed to normal polarity interval of less than 5 ma their polarity pattern indicates major north trending faults with several kilometres of dip slip displacement their remanence confirms that the superior province was deformed around th

an observational study of atmospheric bore formation from colliding density currents abstract observations from east central florida during the convection and precipitation electrification cape experiment are used to investigate the factors that influence atmospheric bore formation from colliding density currents ten cases involving the collision of a gust front with a sea breeze front are analyzed with doppler radar and sounding and surface mesonet data the gust fronts in these collisions were generally deeper denser and faster propagating than their sea breeze front counterparts seven of the 10 cases produced dual boundaries that moved away from each other in a relative sense after the collision post collision boundaries moving in the same direction and oriented similarly to the pre collision gust front occurred in all 10 cases they transported mass in the manner of a density current in six cases while the others behaved more like bores or bore density current hybrids as they were characterized by little or no mass transport similarity of gust front and sea breeze front prop

baked contact test demonstrates primary nature of dominant n1 magnetisation of nipissing intrusions in southern province canadian shield abstract nipissing diabase sills cone sheets and dykes define a major episode of igneous intrusion in the southern province of the canadian shield at about 2200 ma the dominant n1 remanence of these intrusions has been established in several palaeomagnetic studies and hence is a key tie point for the north american apparent polar wander path yet the age of this magnetisation remains very controversial positive baked contact tests reported for the n1 remanence have often been dismissed as inconclusive and the n1 palaeopole attributed to overprinting during the hudsonian orogeny 1900 1800 ma due to its position on a widely accepted polar wander path for north america previous baked contact studies utilise sediments of the huronian supergroup as host rock the remanence of these sediments is often difficult to interpret in this study a positive baked contact test is described for an n1 nipissing sill or cone sheet using stably magnetized mafic dykes of the matachewan swarm as host thus the n1 remanence is primary and no longer supports the time calibration of polar wander paths which assume that it is a 1900 1800 ma overprint

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

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

aeromagnetic interpretation of the kirkland lake larder lake portion of the abitibi greenstone belt ontario high resolution aeromagnetic data from the larder lake kirkland lake region have been analyzed to develop optimum maps for geologic interpretation the use of the apparent susceptibility technique has led to a map that shows clearly the major lithologies in a major synclinorium in the abitibi greenstone belt calc alkaline volcanics of the blake river group are weakly magnetic but show ring like anomalies associated with intrusives where magnetite has been generated in the country rock alternating bands of mg rich and fe rich tholeiites of the kinojevis group show a strong magnetization associated with the fe rich phases gabbroic intrusions underlain by serpentine and peridotite give strong levels of magnetization these various units have been sampled and it is shown that the susceptibility is not high enough to account for the anomalies rather the anomalies are associated with the mean natural remanent magnetization in the formations sampled these studies have led to a revised geological map of this region

late cretaceous apparent polar wander of the pacific plate evidence for a rapid shift of the pacific hotspots with respect to the spin axis paleomagnetic data from the pacific plate show that the average azimuth of the pacific plate decreased by 12 between 90 ma and 81 ma whereas trends of late cretaceous seamount chains suggest that the pacific plate moved northwestward relative to the hotspots with little change in azimuth during the same time interval this difference is used to infer a 13 4 shift of the spin axis relative to the hotspots in the pacific ocean basin this shift occurred within 5 m y of the occurrence of anomalous behavior of the paleomagnetic field recorded in anomalies 33 and 34 this coincidence in timing suggests there may be a causal link between these three phenomena

paleomagnetic evidence for post 2 55 ga tectonic tilting and 1 1 ga reactivation in the southern kapuskasing zone ontario canada we report paleomagnetic results from two traverses in the southern part of the kapuskasing structural zone ksz of north central ontario which is hypothesized to be an oblique cross section through upper and middle archean crust exposed by uplift and tilting anorthosites and granulite grade gneisses were sampled to the east in the shawmere traverse while amphibolite grade tonalitic gneisses were sampled to the west in the racine traverse three distinct remanent magnetization rm components were isolated by alternating field and thermal demagnetization the a rm has mean directions d 50 i 62 k 138 95 5 n 5 sites 23 samples for the shawmere traverse and d 23 i 53 k 49 95 7 n 8 sites 46 samples for the racine traverse because of the high 500 c laboratory unblocking temperatures of the a rm in the shawmere granulite grade rocks we believe this magnetization was acquired in early slow uplift without tilting from about 24 km to about 16 km depths and has survived the later main ksz uplift and tilting event the racine a rm has laboratory unblocking temperatures down to 400 c and was blocked prior to uplift from a shallower crustal level about 8 km with tilt corrections of 30 and 15 respectively about the 35 striking axis of the ksz the shawmere and racine a paleomagnetic poles fall on the laurentian apparent polar wander path around 2 55 ga dating the time of initial uplift crustal tilting in the ksz occurred after 2 55 ga and was greater in the east than in the west the b rm found mainly in shawmere anorthosites has d 115 i 68 k 17 95 17 n 4 sites 14 samples with a paleopole at 144 w 49 n near 1 1 ga keweenawan reverse poles and the normal pole of the nearby 1 0 ga nemegosenda carbonatite the b rm is probably a thermochemical overprint related to reactivation of the ksz at the time of keweenawan rifting and volcanism in the lake superior region a novel c rm with d 64 i 2 k 18 95 13 n 6 sites 10 samples is either an unresolved hybrid of the a and b rms or an earliest keweenawan 1 3 ga overprint

paleomagnetic evidence for crustal and thin skinned rotations in the european hercynides devonian and carboniferous paleomagnetic data from the european hercynides show a coherent pattern in inclination but significant dispersion in declination the declination anomalies with respect to expected declinations for stable europe indicate a strong correlation with the regional changes in the structural trend of the mountain belt detailed analysis of the paleomagnetic and structural data revealed the highest degree of correlation between changes in strike and declination anomalies for external parts of the orogen where thin skinned nappe emplacement has been most prevalent the correlation between changes in strike and declination anomalies from interior parts of the hercynides is less well defined and displays a greater degree of scatter nevertheless the partly secondary character of the arcuate shape can be demonstrated thus the internal hercynides are at least in part a true orocline with secondary bending in the sense of s w carey we infer that the bending of the european hercynides resulted from the combination of multiple deep reaching deformations of the european lithosphere as well as the effects of thin skinned rotations the decreasing degree of rotations toward the crystalline interior of the belt appears to favor a geodynamic model involving indentations and or buttressing effects the indentation of hercynian europe by a microplate or an african promontory during the hercynian orogeny might be one of the principal causes for the secondary bending and the associated nappe rotations

paleomagnetism of late archean metavolcanics and metasediments abitibi orogen canada volcanics of the blake river group and sediments and volcanics of the timiskaming group a paleomagnetic study 159 samples 30 sites was done on calc alkaline volcanics of the blake river group dated at 2703 2 ma and occupying the central portion of a major synclinorium in the abi

proterozoic magnetic overprinting of archaean rocks in the canadian superior province we have measured the various components of natural remanent magnetization nrm exhibited by mafic extrusive and intrusive rocks from the abitibi subprovince of the canadian precambrian shield in addition to providing what seems to be a primary archaean pole for the region the results testify to three episodes of magnetic overprinting at approximately 2 600 2 100 and 1 800 myr respectively the two earlier episodes coincide with the intrusion of matachewan and abitibi diabase dyke swarms here and elsewhere in the superior province magnetic overprinting in these cases extends far beyond areas of surficial dyke exposure the third episode postdates magmatic activity in the abitibi region but coincides in age with the 1 900 1 700 myr hudsonian orogeny in the churchill and bear provinces and the penokean orogeny in the southern province although magnetic overprints presumed to be of this age were known previously from abitibi dykes1 and nipissing sills2 4 our data are the first evidence of a 1 800 myr magnetic overprinting on a regional scale in the superior province

paleomagnetic evidence for proterozoic tectonism in the kapuskasing structural zone ontario the kapuskasing structural zone ksz in the superior province of north central ontario is thought to be an oblique cross section through middle archean crust samples from 30 sites in granulite gneisses and anorthosites of the ksz have been analyzed using alternating field and thermal demagnetization methods three remanent magnetization components were found with mean directions of d 23 i 50 k 52 95 11 n 5 sites for the a component d 73 i 68 k 50 95 5 n 18 for the b component and d 332 i 59 k 53 95 17 n 3 for the c component v constanzo alvarez and d j dunlop found similar a and b components in less metamorphosed rocks from the same units to the immediate west they attributed the a component to uplift at 2 55 ga with subsequent 15 to 30 wnw tilt and the b component to thermochemical overprinting at 1 1 ga as the ksz was reactivated during keweenawan rifting and volcanism we also attribute the a component to uplift during the 2 55 ga kenoran orogeny with concomitant 10 5 wnw downward tilting before intrusion of the 2 45 ga matachewan dikes to give its present paleopole of 33 e 67 dp 6 dm 9 n 13 using data from both studies we attribute acquisition of the b component to uplift at about 1 88 ga as suggested by recent thermal modeling and tectonic syntheses to give a paleopole of 124 w 30 n dp 7 dm 8 n 20 the c component is found in only three sites and may be either an hybrid of a 1 1 ga keweenawan crm with the b component or a fault block tilted b component near the ivanhoe lake cataclastic zone

paleomagnetism of the north american precambrian a look at the data base more than half of the approximately 650 paleopoles reported from all precambrian cratons have been obtained from north america the observations have spurred a proliferation of apparent polar wander paths to support different hypotheses for the tectonic evolution of the continent many of the reasons for this development can be traced to the inadequacy of the data base with its average of only 1 6 poles 10 ma 1 as compared to 7 poles 10 1 ma for the last 300 ma even when it is assumed that a primary pole has been derived from each rock unit sampled the total number of primary poles excluding the 900 1200 ma interval yields an average of only 0 4 pole 10 ma 1 an inspection of the data base shows a non systematic sampling of a non continuous fragmented paleomagnetic record with gaps as large as tens and even hundreds of ma more importantly a search through the literature reveals that very few poles have an age known to better than 50 ma and for many poles the age bracket could be as large as a few hundreds of ma this is partly owing to the fact that radiometric and paleomagnetic sites rarely originate from the same localities it is shown that this limited data base with all its deficiencies and uncertainties cannot be used to support certain hypotheses or for the drawing of a meaningful apparent polar path for the whole of the precambrian it appears that because of the enormous time scale precambrian paleomagnetism should rather be directed towards the construction of segments of polar paths such as those in the 900 1200 ma interval comparison of well dated contemporaneous segments from different parts or structural provinces could be used most effectively to delineate relative movements if any and reconstruct the mosaic of the past continent for certain time intervals one of the recent developments of precambrian paleomagnetism has been to show that the apparent pole did not remain constrained to the equatorial region but made several excursions into high latitudes

rapid crystallization of precious metal mineralized layers in mafic magmatic systems the solidified remnants of mafic magmatic systems host the greatest concentrations of platinum group metals in the earth s crust our understanding of precious metal mineralization in these intrusive bodies is underpinned by a traditional view of magma chamber processes and crystal mush solidification however considerable uncertainty remains regarding the physical and temporal controls on concentrating these critical metals despite their importance to modern society we present high precision 87sr 86sr analyses of plagioclase and clinopyroxene from within centimetre thick precious metal enriched layers in the palaeogene open system rum layered intrusion northwest scotland isotopic heterogeneity is present between plagioclase crystals between clinopyroxene and plagioclase and within plagioclase crystals throughout the studied section on the basis of these observations we demonstrate that platinum group element mineralization formed by repeated small volume reactive melt percolation events the preservation of strontium isotope heterogeneities at 10 100 m length scales implies cooling of the melts that formed the precious metal rich layers occurred at rates greater than 1 c per year and cooling to diffusive closure within tens to hundreds of years our data highlight the importance of cyclic dissolution recrystallization events within the crystal mush and raise the prospect that precious metal bearing mafic intrusions may form by repeated self intrusion during cooling and solidification mineralization of platinum group elements in mafic intrusions occurs due to repeated self intrusion of magma according to strontium isotope heterogeneities preserved in the rum layered intrusion scotland

details of colliding thunderstorm outflows as observed by doppler lidar abstract three cases of colliding outflow boundaries are examined using data collected from the noaa doppler lidar and a meteorological tower during the summer of 1986 near boulder colorado the data are unique because the lidar and the 300 m tower were colocated providing measurements of both kinematic and thermodynamic properties lidar data reveal small scale vortex roll instabilities within the leading edge of the outflow observations of the post collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km in all cases this forced mechanical lifting was sufficient to produce convection a simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

paleomagnetism of archean rocks from northwestern ontario wabigoon gabbro wabigoon subprovince the wabigoon gabbro of the archean wabigoon greenstone belt in northwestern ontario preserves a univectorial natural remanent magnetization nrm with d 246 i 12 k 19 5 95 10 5 n 11 sites the precision is reduced if sample means are averaged however k 9 3 95 9 2 n 29 samples the paleomagnetic pole falls either at 160 w 11 s p 5 3 m 10 6 corresponding to an age of 1300 ma on the laurentian apparent polar wander path or the reverse of this 20 e 11 n corresponding to a late archean age 2800 ma no 1300 ma igneous or metamorphic event is known in the area a major west northwest trending dike about 9 km south of the gabbro yields a virtual geomagnetic pole at 122 w 45 n and seems to be of abitibi age 2150 ma rather than mackenzie age 1250 ma a few gabbro samples and some greenstones from the intrusive baked zone have hybrid remanences in which a higher blocking temperature kenoran age 2600 ma nrm is superimposed on the gabbro characteristic n

an early proterozoic vgp from an oriented drillcore into the precambrian basement of southern alberta summary we report paleomagnetic results for 25 samples from a fully oriented pancanadian petroleum drillcore which penetrated the precambrian basement of southern alberta at a depth of 2550 m samples were taken at approximately 1 m intervals over a 7 m section of core intensities of natural remanent magnetization nrm were 0 5 14 a m over the section which consists mainly of granodiorite with one short granite interval x ray and curie temperature data indicated magnetite as the only important nrm carrier most of the nrm was an intense steeply inclined drilling remanence with coercivities 20 mt and distributed low to intermediate unblocking temperatures after magnetic cleaning a weak but stable primary nrm with high coercivities and high unblocking temperatures was isolated with d 157 i 52 k 118 mg5 3 n 20 the corresponding virtual geomagnetic pole vgp is 4 s 93 w dp 3 dm 4 which falls on the north american apparent polar wander path around 1850 ma the basement of this part of alberta is probably an extension of the churchill structural province of the laurentian shield

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

cretaceous and early tertiary paleomagnetic results from the antarctic peninsula paleomagnetic results are presented for early tertiary and cretaceous igneous and volcanic units from the northern antarctic peninsula when combined with previously published data they permit the construction of a polar wander path that is very similar to one derived for the east antarctic craton by rotating tertiary and cretaceous poles from india and australia into the antarctic frame of reference when considered in this context the paleomagnetic data from the orville coast which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern antarctic peninsula with respect to the northern part may be considered as an in situ resulte the implications of the new data are as follows 1 the antarctic peninsula down to the orville coast has been a part of the antarctic plate since 100 m y ago 2 movement of the antarctic peninsula required to avoid overlap in reconstructions of gondwanaland must have occurred before 100 m y ago

paleomagnetism of the late precambrian fourchu group cape breton island nova scotia volcanogenic sediments of the fourchu group and a gabbroic intrusion which are found in the avalonian terrane of south eastern cape breton island have been sampled for paleomagnetic analysis upo

low paleointensities recorded in 1 to 2 4 ga proterozoic dykes superior province canada abstract paleointensity studies have been conducted on six mafic dyke swarms from the superior province in canada with u pb ages between 1141 ma and 2473 ma buchan et al tectonophysics 319 2000 167 198 the mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests this primary component was isolated over a very narrow range of high unblocking temperatures usually 550 580 c indicating that it is carried by magnetite or low titanium titanomagnetite magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains taken together the characteristics of this component suggest that it is thermal in origin paleointensity experiments were conducted with a specially designed oven using a revised approach of the thellier coe method eighty five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms doubling the number of existing well constrained precambrian data the paleofield estimates for the dyke swarms vary between 5 and 11 2 t yielding virtual dipole moments between 0 85 0 1 and 2 8 0 87 1022 am2 the overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000 2400 ma period than has been recorded between 0 3 and 400 ma however additional determinations are required particularly for the 400 2000 ma interval in order to establish whether a long term evolution in the time averaged field intensity can be linked to the onset of the growth of the earth s inner core

paleomagnetism and magnetic mineralogy of the nahant gabbro and tonalite eastern massachusetts paleomagnetic analyses of the nahant gabbro and an associated tonalite of eastern massachusetts reveal five separate populations of paleomagnetic directions the gabbro and tonalite exposed in the northernmost portion of the boston basin were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation the tonalite has not been radiometrically dated however an ordovician date has been reported for the gabbro detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration a mean alteration index was calculated for each site in the nahant gabbro and tonalite there is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level with normalized intensity decreasing as magnetite alteration increases this relationship illustrates the interdependence between paleomagnetic pr

paleomagnetism of ordovician alkalic intrusives and host rocks from the pedernal hills new mexico positive contact test in remagnetized rocks jackson m van der voo r and geissman j w 1988 paleoma etism of ordovician alkalic intrusives and host rocks from the pedemal hills new mexico positive contact test in remagnetized rocks tectonophysics 147 313 323 a set of thin dikes from central new mexico dated at 469 7 ma rb sr loring and armstrong 1980 have yielded a virtual geomagnetic pole which lies on the late paleozoic segment of the north american apparent polar wander path the remanence of the dikes appears to be a product of late paleozoic hydrothermal alteration paradoxically however the magnetization of the host rocks is most simply explained in terms of a positive contact test samples collected between 0 2 and 0 5 dike widths from the contact contain a component of remanence parallel to the magnetization in the dikes with unblocking temperatures which decrease with distance from the dikes host rocks from a distance of more than 1 dike width show no evidence of the characteristic dike magnetization there are two possible resolutions of this paradox 1 the magnetization of the host rocks is secondary despite the apparent positive contact test and is a product of hydrothe al fluid migration through the dikes or along the contact zones or 2 the magnetization of the dikes is primary but not representative of the ordovician paleofield for north america possible reasons for inaccurate representation include a incomplete averaging of secular variation b tectonic rotation with respect to the stable craton or c erroneous age determination for the rocks we argue that explanation 1 is the most likely

radar and profiler analysis of colliding boundaries a case study abstract the kinematics of a head on collision between two gust fronts followed by a secondary collision between a third gust front and a bore generated by the initial collision are described using analyses of weather surveillance radar 1988 doppler wsr 88d and mobile integrated profiling system mips data each gust front involved in the initial collision exhibited a nearly north south orientation and an east west movement the eastward moving boundary was 2 c colder and moved 7 m s 1 faster than the westward moving boundary two dimensional wind retrievals reveal contrasting flows within each gravity current one exhibited a typical gravity current flow structure while the other assumed the form of a gravity wave current hybrid with multiple vortices atop the outflow one of the after collision boundaries exhibited multiple radar finelines resembling a solitary wave shortly after the collision about 1 h after the initial collision a vigorous gust front intersected the eastward moving bore severa

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

paleomagnetism of siluro devonian and cambrian granitic rocks from the avalon zone in cape breton island nova scotia as part of a major study of the avalon zone in cape breton island we describe the paleomagnetism of radiometrically well studied granitic rocks of two generations representing the acadian 350 45

dynamics of evolving magma chambers textural and chemical evolution of cumulates at the arrival of new liquidus phases abstract basaltic magma chambers are best exemplified by layered intrusions fossilized natural laboratories that historically constrain most fundamental principles of igneous petrology progressive fractional crystallization of basaltic melts in layered intrusions results in successive appearance of 5 10 liquidus minerals most minerals appear in the stratigraphy of layered intrusions via two distinct stages they first emerge as oikocrysts large irregularly shaped and interstitial crystals that form in a crystal liquid framework and then as primocrysts relatively small idiomorphic and cumulus crystals that crystallize from a main magma body the transition from oikocrysts to primocrysts is commonly marked by the following textural and chemical features a a similarity in chemical composition of the last forming oikocrysts and the first forming primocrysts b slow arrival modal overabundance and finer than normal size of the first primocrysts c a low abundance of trapped liquid in rocks with the first forming primocrysts d a step like increase in mineral dihedral angle in rocks slightly below the level of the primocryst arrival and e the rims on plagioclase primocrysts that are identical in composition to cores of plagioclase primocrysts at the first appearance of new liquidus phases higher up in the section the phase equilibria control on the cumulus stratigraphy of layered intrusions require large long lived and largely molten magma chambers and therefore their origin is not compatible with several recent concepts that deny the existence of such reservoirs in the earth s crust we therefore suggest that the systematic changes associated with the arrival of cumulus phases in layered intrusions are best explained by a traditional concept that considers layered intrusions as large initially crystal free melt bodies that gradually lose heat and crystallize from margins inwards through thin solidification fronts the entire magma body during its internal evolution is thought to be kinetically supercooled i e kept slightly below its liquidus temperature with respect to cumulus phases that crystallize at the margins each new cumulus phase arrives on the liquidus with some kinetic delay and nucleates heterogeneously against pre existing crystals at the floor roof and sidewalls of basaltic magma chambers this implies that internal differentiation of basaltic magma chambers mostly occurs through convective separation of evolved liquid from in situ growing crystals and its mixing with the main magma body

paleomagnetism of early cambrian redbeds on cape breton island nova scotia results are reported from steeply dipping red sandstones collected at nine sites 45 samples of the lower cambrian morrison river and maccodrum formations on southeast cape breton island which forms part of the avalon terrane demagnetization studies indicate minimal overlap of stability spectra in thermal alternating field and chemical treatments a stable component hematite with high unblocking temperatures 680 c is dominant at all sites its mean direction corrected for geologic tilt is d 281 i 32 k 97 giving a paleomagnetic pole pole m at 20 n 146 w dp dm 3 6 the corresponding in situ direction is d 147 i 59 k 97 with a pole m at 2 s 39 w dp dm 6 8 poles m and m were compared with published paleozoic poles for the avalon zone and cratonic north america to test whether the magnetization originated before or after the taconic ordovician folding it could have been acquired during deposition or early diagenesis early to middle cambrian in whic

paleomagnetism of the hearst dike swarm and implications for the tectonic history of the kapuskasing structural zone northern ontario prominent north to northwest trending dikes herein termed the hearst swarm west of the kapuskasing structural zone ksz in northern ontario yield a paleomagnetic pole position of 48 6 n 69 0 e dm 6 9 dp 3 6 60 samples from 12 dikes using paleomagnetic petrographic and structural criteria the hearst swarm is correlated with the 2 6 ga matachewan swarm east of the ksz a positive correlation is observed between the trend and paleomagnetic declination of hearst matachewan dikes and may result from an 15 post 2 6 ga rotation of terranes across the ksz paleomagnetic and structural data from the hearst dikes are consistent with the regional westward tilting of the crust near the southern ksz as first proposed in 1983 by percival and card

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

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time

paleomagnetism of archean rocks from northwestern ontario v poohbah lake alkaline complex quetico subprovince the poohbah lake alkaline complex is a late synkinematic kenoran pluton in the quetico gneiss belt of the western superior province three units of the complex porphyritic syenite ps malignite