constraints from 40ar 39ar geochronology on the tectonothermal history of the kapuskasing uplift in the canadian superior province the kapuskasing uplift ku in the superior province of the canadian shield has been interpreted as an oblique cross section through the archean mid crust however the time of juxtaposition of the granulites of the ku against the lower grade rocks of the abitibi greenstone belt agb along the ivanhoe lake fault zone is problematic to constrain the postmetamorphic tectonothermal history of the ku we have conducted 57 40ar 39ar step heating analyses on mineral and rock samples collected in a transect across the southern ku and adjacent agb the age spectra record a complex thermal history amphiboles from the agb in the footwall of the ivanhoe lake fault zone have ca 2 66 ga dates similar to closure ages for amphiboles from farther east in the agb amphibole dates of 2 46 2 52 ga from the deepest structural levels of the ku place an upper limit on the time of major uplift of the granulites and their juxtaposition with the agb biotite and muscovite dates from the transect cluster into three age groups

paleomagnetism of the 1 1 ga lackner lake complex and tectonics of the kapuskasing structural zone the middle proterozoic lackner lake complex is a circular alkalic syenite carbonatite stock with a diameter of about 5 5 km it intrudes granulite rank archean gneisses in the kapuskasing structural zone of the wawa subprovince in the superior province of the canadian shield it adjoins the ivanhoe lake fault zone which forms the boundary with the abitibi subprovince and is the probable locus of maximum motion between the subprovinces specimens from 18 sites in the complex were analyzed paleomagnetically by alternating field and thermal demagnetization and by saturation isothermal remanence tests large recent viscous remanence components required removal before a stable remanence with a mean direction of 305 4 64 1 95 5 2 was isolated its pole of 53 7 n 156 5 w dp 6 7 dm 8 3 indicates emplacement at 1108 10 ma during a brief normal interval in a predominantly reversed polarity time this study indicates that there has been no postintrusion tilting of the kapuskasing structural zo

a note on the relationship between ice core methane concentrations and insolation 1 we re examine the link between july 30 n insolation and methane in the vostok ice core based on this link ruddiman 2003 suggested that an anthropogenic source of methane must have been present after 5 kyr bp in order to prevent concentrations from declining as insolation decreased through the holocene we conclude however that since precessional forcing does not explain the large glacial interglacial excursions the component of methane variability associated with precession is significantly smaller than assumed by ruddiman 2003 the implied decrease from 10 kyr bp to the present associated with precession alone is 60 ppbv or less we argue that increased emissions controlled by northern wetlands and river delta development likely contributed to the observed increase consistent with similar stable ch4 levels at mis 11 in the vostok and dome c records therefore a significant anthropogenic input is not obviously required to explain the late holocene record

site specific modeling of sh and p sv waves for microzonation study of kolkata metropolitan city india kolkata one of the oldest cities of india is situated over the thick alluvium of the bengal basin where it lies at the boundary of the zone iii and zone iv of the seismic zonation map of india an example of the study of site effects of the metropolitan kolkata is presented based on theoretical modeling full synthetic strong motion waveforms have been computed using a hybrid method that combines the modal summation and finite difference techniques the 1964 calcutta earthquake which was located at the southern part of kolkata is taken as the source region with the focal mechanism parameters of dip 32 strike 232 and rake 56 four profiles are considered for the computation of the synthetic seismograms from which the maximum ground acceleration a max is obtained response spectra ratios rsr are then computed using a bedrock reference model to estimate local amplifications effects the a max varies from 0 05 to 0 17 g and the comparison of the a max with the different intensity scales mm msk rf and mcs shows that the expected intensity is in the range from vii to x mcs for an earthquake of magnitude 6 5 at an epicentral distance of about 100 km this theoretical result matches with the empirical historical and recent intensity observations in kolkata the rsr as a function of frequency reaches the largest values largest amplification in the frequency range from 1 0 to 2 0 hz the largest site amplification is observed at the top of loose soil

magnetic properties of granitic rocks from the southern half of the idaho batholith influences of hydrothermal alteration and implications for aeromagnetic interpretation the measured magnetic susceptibilities of rocks from more than 600 outcrops in the southern idaho batholith have a good spatial correspondence with the observed intensities of magnetic anomalies koenigsberger ratios are low 0 3 in 75 of cored samples indicating that remanent magnetizations are unimportant relative to induced magnetizations which are related to susceptibilities in aeromagnetic anomaly interpretations in this region extremely low susceptibilities 25 10 5 within the eastern and western parts of the batholith are the sources of positive aeromagnetic anomalies the associated lithologies include mesozoic tonalite and granodiorite as well as a diverse tertiary group of epizonal stocks and batholiths intense meteoric hydrothermal activity associated with the emplacement of these tertiary plutons caused a reduction in in some rocks although coarse igneous magnetite is surprisingly resistant to hydrothermal corrosion and oxidation however under conditions of lower water rock ratios a significant increase in commonly occurred in the leucocratic plutons generally reflecting growth of hydrothermal magnetite because the measured susceptibilities do not necessarily represent more than the uppermost part of the 14 to 20 km thick crust that gives rise to magnetic anomalies an apparent discordance between susceptibility measurements and susceptibilities inferred from aeromagnetic data can occur such discordance patterns occur near the margins of the batholith where granitic rocks are thin and also in hydrothermally altered regions where subsurface tertiary plutons are present magnetic models suggest that the idaho batholith thickens to 20 km in its east central part which is underlain by enormous tertiary intrusive bodies

paleomagnetism of dykes from the groundhog river block northern ontario implications for the uplift history of the kapuskasing structural zone the groundhog river block gb forms the central segment of the kapuskasing structural zone a fault bounded belt of proterozoic crustal uplift in the southern superior province dykes adjacent to t

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

extension of godavari gondwana sediments underneath trap covered region of satpura basin as evidenced from seismic studies in deccan syneclise india abstract we present a shallow p wave velocity structure of the eastern part of deccan syneclise central india from the refraction and wide angle reflection data recorded along two 100 km long seismic refraction profiles in east west e w and north south n s directions using travel time inversion method in addition to the p wave velocity model we have derived s wave velocity model and estimated vp vs and poisson s ratios along the east west profile travel time analysis and inversion of the data revealed three layers above the basement the first layer with a p wave velocity of 1 4 1 5 km s 1 represents alluvium and the thickness of this layer is varying between 100 400 m in the e w and 30 100 m in the n s profile the deccan trap thickness is observed to be varying between 850 and 1250 m constitutes the second layer p wave velocity of the deccan traps is 4 8 5 2 km s 1 amplitude decay and travel time skips in the first arrival refraction data in some of the record sections indicate the presence of a velocity inversion i e low velocity zone lvz beneath the traps wide angle reflection phases from top and bottom of the lvz top of basement together with basement refraction phase are identified and modeled to derive lvz thickness the lvz has been interpreted to be the gondwana sediments sandwiched between deccan traps and basement these sediments are about 200 450 m thick with a p wave velocity of 3 6 km s 1 the basement lies at about 1 60 km depth near khandala in the west and rajola in the north shows an upward trend towards south east the p wave velocity of the basement is about 6 0 6 2 km s 1 the inferred subtrappean sediments could be the possible extension of gondwana sediments of the godavari graben towards the satpura basin

paleomagnetism of the powder mill group michigan and wisconsin a reassessment of the logan loop the apex and the western arm of the 1 1 ga logan loop a prominent feature of the north american precambrian apparent polar wander path are defined by a large body of data from keweenawan rocks of the lake superior basin until now the eastern arm of this paleomagnetic feature has been largely defined by two data points from the powder mill group of michigan a reexamination of this sequence reveals a primary reversed magnetization which plots close to the apex of the logan loop data from the mellen complex in wisconsin together with our data from the powder mill group suggest that for steeply dipping volcanic units the true direction of a prefolding reversed magnetization is not obtained by simple structural rotation about presently observed strike reversely magnetized sites from localities where the powder mill volcanics have shallow dips yield a pole 146 w 46 n a95 9 2 on the apex of the loop a well defined stratigraphically continuous normal to reversed polarity change near the base of the powder mill group is not confirmed normally magnetized flows have been found at two localities only one of which is at the base of the lava sequence the higher metamorphic grade of normally magnetized sites suggests that normal magnetizations within the powder mill group are secondary and should not be used in definition of the eastern arm of the logan loop

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

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

orbital tuning chronology for the vostok climate record supported by trapped gas composition abstract we present data on the o2 n2 ratios of trapped gas samples for the entire length of the vostok climate record as in other cores o2 n2 ratios in these samples are less than the atmospheric ratio by a small and variable amount because o2 is selectively excluded during the gas trapping process and because o2 is also preferentially lost in poorly preserved core samples samples younger than 150 ka have large and variable o2 depletions samples older than 200 ka have o2 n2 ratios that replicate well and vary smoothly with depth we plot o2 n2 ratios of well replicated samples older than 160 ka using a chronology derived by matching the 18o of paleoatmospheric o2 18oatm to northern hemisphere june insolation on this timescale o2 n2 varies coherently with local 78 s summertime insolation based on time series analysis of the o2 n2 record and the dynamics of snow metamorphism at the surface we conclude that summertime insolation influences physical properties of ice grains that control the degree of o2 exclusion during bubble closeoff o2 n2 in vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core we show that the 18oatm chronology supported by the coincidence of o2 n2 ratios with insolation is also compatible with recent radiometric dating of corals from high sea stands we further successfully test the 18oatm tuning chronology by showing that it predicts a chronology for the gisp2 core which is essentially indistinguishable from the standard gisp2 chronology and therefore in excellent agreement with the radiometric chronology of hulu cave china an accurate chronology for the vostok ice core is now in place

detrital remanent magnetization in hematite heavy mineral placer layers within channel sandstones of the morrison formation western united states have been shown to contain a very stable accurate record of the jurassic geomagnetic field these laminae are composed almost entirely of hematite grains in particular they are largely the hematite pseudomorph of magnetite martite the laminae record the correct geomagnetic field direction during a period of rapid apparent polar wander which suggests an early acquisition of the remanence several other remanence characteristics imply that the remanance is probably a detrital remanent magnetization drm 1 there is large directional dispersion in one lamina which had been heavily burrowed by organisms measurement of individual burrows shows that the burrowing disrupted and randomized the magnetization held in the martite grains 2 directions of laminae that were deposited on sloping surfaces are significantly deflected from the directions of the flat lying laminae 3 more than 50 of the martite grains have high temperature oxidation features suggesting that they may have been martite at the time of deposition these characteristics in particular the burrowing disturbance strongly suggest that the martite was deposited as martite and that the remanence is a drm as opposed to deposition of magnetite grains which were altered to hematite after deposition examination of recent weathering debris on igneous outcrops shows a substantial amount of martite on these outcrops this observation provides further evidence in support of the idea that hematite and in particular martite can be a deposited mineral in sedimentary rocks and thus can carry a drm

velocity structure of the west bengal sedimentary basin india along the palashi kandi profile using a travel time inversion of wide angle seismic data and gravity modeling an update the 2 d shallow velocity structure along the north south palashi kandi profile in the west bengal sedimentary basin has been updated by travel time inversion of seismic refraction wide angle reflection and gravity data a six layer shallow model up to a depth of about 7 km has been derived the first layer which has an average velocity of 2 0 kms 1 represents the alluvium deposit which rests over the shale formation with average velocity of 3 0 kms 1 the thin 200 m sylhet limestone observed at a nearby palashi well remains hidden in the present data set hence a 200 m thin layer with a velocity of 3 7 kms 1 corresponding to the sylhet limestone has been assumed to be present throughout the profile the fourth layer with a velocity of 4 5 4 7 kms 1 at a depth of 1 7 2 4 km represents the rajmahal traps the skip phenomenon and rapid amplitude decay of first arrivals indicate a low velocity zone lvz in the study area using the skip phenomena and wide angle reflection data identified on seismograms the lvz with a velocity of 4 0 kms 1 indicating the gondwana sediments has been delineated below the rajmahal traps the next layer with a velocity 5 4 5 6 kms 1 overlying the crystalline basement 5 8 6 25 kms 1 may be associated with the singhbhum group of meta volcanic rock that has been exposed in the western part of the basin the basement lies at a variable depth of 4 9 to 6 8 km the overall uncertainties of various velocity and boundary nodes are 0 15 kms 1 and 0 5 km respectively the elevated basement feature in the north might have acted as a structural barrier for the deposition of sylhet limestone during the eocene epoch the seismically derived shallow structure correctly explains the observed bouguer gravity anomaly along the profile the addition of reflections in the present analysis provides a stronger control on the depths and velocities of basement and overlying sedimentary formations compared to the earlier model derived mainly by the first arrival seismic data

apparent polar wander paths and the close of late archean crustal transpression northern ontario 1 lamprophyre dikes of the southern superior province of the canadian shield crosscut archean structures including a late archean unconformity and its overlying younger archean timiskaming sedimentary and volcanic rocks the dikes also cut the single schistosity that formed in these rocks nevertheless the lamprophyre dikes show a weak internal tectonic fabric that is approximately parallel to the schistosity in the bedrock and oblique to dike walls the consistent orientation of this internal tectonic fabric has been recognized in every lamprophyre dike using anisotropy of magnetic susceptibility ams thus the lamprophyre dikes provide a useful chronological marker trapping the last pulse of archean tectonism unfortunately our previous attempts to compare the lamprophyre paleopoles with those of the age calibrated apparent polar wander path apwp failed because it was not possible to isolate separate young and old components of magnetization that failure occurred due to the overlap of blocking temperatures or coercivities for the different characteristic vector components borradaile et al 1994a using new cores from the same specimens here we distinguish successfully between consistently oriented vector components by using low temperature demagnetization ltd before thermal demagnetization although the paleopoles appear well defined comparisons with published apwps for the superior province from late archean irving 1979 dunlop 1983 symons et al 1994 to early proterozoic buchan et al 2000 are difficult moreover the earlier parts of the paths are progressively less precise and complete nevertheless a component remanences for the dikes give a paleopole near the path at 2650 ma for a timiskaming volcanic breccia with a u pb age of 2696 ma the a component paleopole may lie between 2600 and 2650 ma b component remanences yield paleopoles in the nw pacific region incompatible with any proposed archean path but perhaps representing some much younger proterozoic remagnetization event e g keweenawan 1100 ma

paleomagnetism of the late precambrian coldwell complex ontario canada abstract the coldwell complex is a large multiphase alkaline intrusion on the north shore of lake superior it was emplaced into archean metavolcanics of the superior province in at least three distinct magmatic episodes during the late precambrian giving isotopic ages ranging from 1010 to 1188 ma detailed af and thermal demagnetization was done on 416 specimens from 33 sites representing all petrologic phases additional data are included from an unpublished 1972 study by w a robertson the oldest magnetic component pca was found in 19 episode i sites with a reversed mean direction of d 121 0 i 70 9 104 95 3 3 its pole position falls on the keweenawan apwp at an age of 1109 5 ma the second component pcb was found in 11 sites with a normal mean of d 301 8 i 60 1 49 95 5 8 that indicates an age of 1103 5 ma the youngest component pcc was found in nine episode iii sites and gives a reversed mean of d 119 1 i 54 3 101 95 5 1 that indicates an age of 1095 5 ma the assigned ages agree closely with recent u pb zircon ages but are distinctly older than the k ar and rb sr ages these paleomagnetic results require some changes in existing models for the petrologic evolution of the complex and indicate a much more rapid three stage cooling history than previously postulated they also provide strong evidence against the proposed concept of an asymmetrical geomagnetic field in middle keweenawan time

magnetic properties of surface sediments as proxies of recent anthropogenic pollution in the anllons riverbed nw spain a study of surface sediments from the anllons riverbed northwestern spain was conducted to assess the use of their magnetic properties as proxies of recent trace element contamination i e as zn pb cr ni and cu thermomagnetic curves hysteresis parameters specific magnetic susceptibility frequency dependent magnetic susceptibility fd magnetic remanence and saturation isothermal remanent magnetization ratios i e s and sirm reveal the presence of mostly fine grained magnetite and maghemite in these samples the tomlinson pollution load index pli as well as the magnetic parameters fd sirm and s ratios vary more or less in agreement to each other when they are sequentially arranged throughout a 30 km long transect that goes from the town of carballo to the river mouth in ponteceso from that profile it is possible to identify an anomaly of that lies between two small peaks of the pli and seems to define a contaminated area probably linked to a diffuse low impact source of pollution there is also a conspicuous peak of the pli that is likely associated to a nearby gold mine the variations of the fd s along the transect are the ones that best reflect the changes in the pli values regardless of the level of contamination this result suggests that adsorption is the main mechanism of trace elements removal by fe oxides in these samples direct signal analyses used to decompose the isothermal remanent magnetization curves reveal the presence of four magnetic fractions with varying relative concentrations all over the profile namely single domain magnetite close to the superparamagnetic threshold sd sp and or maghemite log b 1 2 ca 1 2 single domain magnetite log b 1 2 ca 2 hematite log b 1 2 ca 2 5 and goethite log b 1 2 ca 3 5 integrating all the available information it was sketched out a likely sequence of events that included magnetic minerals neoformation in parent soils their subsequent chemical transformations and the contamination episodes that affected the region

seismic structure of the crust and upper mantle in the peace river arch region canada four intersecting 300 km long reversed refraction lines within northwestern alberta and northeastern british columbia have been interpreted for crustal and upper mantle seismic velocity structure in the peace river arch pra region of the western canada sedimentary basin the data have been modeled using a two dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals in addition an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles the major features of the structural models are 1 weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low velocity zones within the crust 2 an average sub basement rms crustal velocity of 6 6 km s average upper mantle velocity of 8 25 km s and average crustal thickness of 40 km 3 regional variations in structure which appear related to the dominant n s trending cratonic structure including crustal thickness upper crustal and upper mantle velocities and pmp character and 4 subtle variations in structure that may be associated with the e w trending devonian axis of the pra including a shallowing of high lower crustal velocities thickening of the crust and an anisotropic pmp character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the pra region evidence from the refraction and other geophysical data for the presence of a local crustal expression of the pra is weak but suggests a thermal as opposed to flexural origin for its anomalous vertical movements

differences in paleomagnetic interpretations due to the choice of statistical demagnetization and correction techniques kapuskasing structural zone northern ontario canada abstract the kapuskasing structural zone ksz reveals a section through the archean lower crustal granoblastic gneisses our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical demagnetization and field correction techniques first where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular the commonly used statistical model is invalid fisher r a proc r soc a217 1953 295 any tendency to form an elliptical distribution indicates that the sample is drawn from a bingham type population bingham c 1964 distributions on the sphere and on the projective plane phd thesis yale university fisher and bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic bingham distribution it seems prudent to apply both models wherever a non fisher population is suspected and that may be appropriate in any tectonized rocks non fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks more dispersed sampling across tectonic structures may be more productive second from the same specimens mean vectors isolated by thermal and alternating field af demagnetization differ which treatment gives more meaningful results is difficult to decipher especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events in this study thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles third of more parochial significance tilt corrections may be unnecessary in the ksz because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off at approximately 15 km depth with archean geothermal gradients primary remanences were blocked after the foliation was tilted to rise on the thrust ramp therefore the rocks were probably magnetized in their present orientation tilting largely or entirely predates magnetization

ramp flat geometry within the central kapuskasing uplift evidence from potential field modeling results potential field data constrained by seismic reflection seismic refraction geologic and physical properties data have been used to study the deep crustal structure of the central kapuskasing uplift the val rita block is interpreted as a thrust sheet of granulites detached at mid crustal depths and uplifted along a ramp producing an arched double granulite layer a thin thrust sheet of southward thickening granulites best describes the groundhog river block whereas simple thrusting along a ramp characterizes the chapleau block from this interpretation a sequential evolution of the kapuskasing uplift is proposed at about 2450 ma northwest southeast compression 1 resulted in variable thrusting along the ivanhoe lake fault zone this was followed by a period of normal faulting resulting in the juxtaposition of high and low pressure rocks across the lepage and saganash lake faults with possible truncation of the val rita block arch further rotation of 1 to an east west direction resulted in the

reversal asymmetry in mesoproterozoic overprinting of the 1 88 ga gunflint formation ontario canada non dipole effects or apparent polar wander abstract eighty two palaeomagnetic samples of calcareous and jaspilitic grainstones iron formation or taconite and chert carbonate were collected from the 1 88 ga gunflint formation at 22 sites in the thunder bay area ontario twenty clasts of gunflint taconite also were sampled from the basal conglomerate of the overlying mesoproterozoic sibley group anisotropy of magnetic susceptibility measurements indicate the gunflint formation in the sampling area has not experienced regional dynamic metamorphism analyses by variable field translation balance and x ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples altogether 213 gunflint specimens and 59 sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 gunflint specimens to stepwise alternating frequency demagnetisation the following components were isolated for the taconites gunflint magnetite normal declination d 293 4 inclination i 30 8 95 7 2 n 21 reverse d 86 7 i 54 6 95 5 8 n 29 gunflint hematite normal d 243 6 i 23 6 95 6 0 n 11 reverse d 70 3 i 51 4 95 3 2 n 79 sibley clasts magnetite normal d 282 7 i 33 4 95 7 6 n 20 sibley clasts hematite normal d 254 5 i 56 2 95 8 4 n 13 reverse d 110 6 i 55 7 95 8 3 n 11 none of these sets passed the reversal test with the normal component generally being the shallower fold tests were negative or inconclusive and the conglomerate test also was negative chert carbonate at one other site appears to have acquired a remanence carried by magnetite d 97 3 i 78 2 95 6 3 n 18 prior to folding related to keweenawan 1 1 ga logan diabase sill emplacement most of the components we identified match components for keweenawan sills volcanic rocks intrusions and baked contact rocks in the thunder bay area indicating that keweenawan magmatism caused widespread chemical remagnetisation of the proterozoic country rock in our sampling area although others have argued that asymmetry was a feature of the keweenawan geomagnetic field the declinations of our gunflint and sibley hematite and magnetite components are different suggesting that the components were acquired at significantly different times we conclude that the reversal asymmetry shown by our gunflint and sibley data may best be ascribed to apparent polar wander during keweenawan times

cold climate during the closest stage 11 analog to recent millennia abstract the early part of marine isotopic stage 11 near 400 000 years ago provides the closest analog to holocene insolation levels of any interglaciation during the era of strong 100 000 year climatic cycles the ch 4 concentration measured in vostok ice fell to 450 ppb and co 2 values to 250 ppm these natural decreases contrast with the increases in recent millennia and support the early anthropogenic hypothesis of major gas emissions from late holocene farming during the same interval d values fell from typical interglacial to nearly glacial values indicating a major cooling in antarctica early in stage 11 other evidence suggests that new ice was accumulating during the closest insolation analog to the present day a major increase in 18 o atm at vostok a similar increase in marine 18 o values and re initiation of ice rafting in the nordic sea the evidence permits extended 20 000 year intervals of stage 11 interglacial warmth in the antarctic and north atlantic yet it also requires that this warmth ended and a new glacial era began when insolation was most similar to recent millennia the holocene co 2 anomaly was produced only in part by direct anthropogenic emissions over half of the anomaly resulted from the failure of co 2 values to fall as they had during previous interglaciations because of natural responses including a sea ice advance in the antarctic and ice sheet growth in the northern hemisphere

a comparative 40ar 39ar study of the kapuskasing structural zone and the wawa gneiss terrane thermal and tectonic implications 40ar 39ar studies were performed on rocks from two traverses within the southern superior province of the canadian shield the racine traverse across the wawa gneiss terrane and the shawmere traver

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

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

palaeomagnetic results from remagnetized mid cretaceous albian cenomanian strata of northeastern venezuela summary we report palaeomagnetic and rock magnetic results of a sedimentary sequence pertigalete cement quarry located in northeastern venezuela sampling was restricted to the vicinity of the contact between the upper cretaceous chimana and querecual formations biostratigraphic evidence reveals an upper albian age for this formational transition profiles of site averaged nrm intensity of the high coercivity over 30 mt and high temperature over 400 c components appear to be related to the contact and distance from the contact we interpret this profile as the probable outcome of overlapping thermochemical remagnetization events resulting from hydrothermal activity that was focused along the two formations direct spectral analyses performed on the site averaged stable nrm intensity profile allow the separation of at least two of these remagnetization events on the other hand palaeomagnetic results show a considerable streaking of site mean declinations suggesting that tectonic or structural horizontal movements around a vertical axis have occurred after nrm acquisitions horizontal rotation angles plotted against stratigraphic levels for bedding corrected data show some features that seem to coincide with the alteration peaks isolated in the profile of site averaged stable nrm intensities thus it appears that repeated thermochemical remagnetizations with overlapping unblocking spectra and horizontal movements around a vertical axis could have been responsible for much of the within site dispersion a simple three stage reconstruction of the possible chain of thermochemical and tectonic occurrences that could lead to the present day palaeomagnetic and rock magnetic evidence is proposed these events including clockwise horizontal rotations around a vertical axis are tentatively placed in a geological time framework between middle miocene and pliocene times according to the main geological and geochemical evidence available

interpretation of first arrival travel times in seismic refraction work the necessary condition for the seismic refraction method to succeed is that the refracted first arrivals from each layer in a multilayered earth system should be detected on a seismogram as first arrivals and this is possible only when velocities of all underlying layers are successively greater the usual procedure to interpret the refraction travel times is to fit such a data set with several intersecting straight lines by employing a visual technique which may lead to errors of subjective judgment as the velocity model depends on the selection of various line segments through the data to remove the visual fit we propose here a layer stripping method based on minimum intercept time apparent velocity rms residual and maximum data points by least squares fitting to yield several intersecting straight lines once data are segmented out the conventional equations can be used to determine the velocity structure

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

a rock magnetic study of the lower massive sandstone moenkopi formation triassic gray mountain area arizona this paper describes the results of rock magnetic and petrographic studies of the lower massive sandstone of the moenkopi formation north central arizona which heretofore has been considered to be a reliable magnetostratigraphic unit the rock magnetic studies included stepwise thermal demagnetization of samples of intraformational mudstone clasts and a deformed shale lens both of which occur within the lower part of the sandstone and stepwise thermal and selective destructive demagnetization sdd of samples of the sandstone containing black sand laminae composed largely of opaque grains in samples both from the claystone clasts and deformed shale lens two secondary components one of modern normal polarity and one of triassic normal polarity were unambiguously identified and a third crm component of triassic reversed polarity was tentatively identified there is no evidence in samples of either the claystone clasts or deformed shale lens of a remanence acquired prior to deposition or to deformation respectively sdd by means of which the small scale homogeneity of remanence within samples of the lower massive sandstone containing black sand laminae could be assessed shows that each sample primarily contains a heterogeneous three dimensional mixture of triassic normal and reversed components the triassic normal direction that is characteristic of the lower massive sandstone is a vector resultant of almost balanced proportions of the two nearly antiparallel components averaging about 55 to 60 normal and 45 to 40 reversed the local variability in both direction and intensity of remanence within and between individual black sand laminae confirms that acquisition of the remanence took place over a geologic time interval that spanned at least two and probably several polarity intervals and that on a small scale the remanence was sporadically acquired more or less randomly probably in response to local conditions in the interstitial physicochemical environment that affected the growth of authigenic hematite no primary component of remanence acquired at or soon after deposition could be identified the remanence in the black sand bearing lower massive sandstone is largely carried by the black sand grains which as determined from reflected light petrographic studies are primarily martite 67 and altered ilmenite grains 20 composed of complex intergrowths of hematite and iron titanium minerals these data strongly suggest that these grains deposited initially as unaltered to only moderately altered magnetite and ilmenite subsequently underwent oxidation during which they acquired their remanence

crustal structure of the west bengal basin india from deep seismic sounding investigations summary basement and sub basement crustal configuration of parts of the west bengal basin has been delineated from inverse and forward modelling of refraction and wide angle reflection data obtained along two east west profiles a beliatorburdwan bangaon profile 1 about 180 km long and b gopali tamluk port canning profile 2 about 140 km long the study has revealed the existence of four groups of prominent refractors velocities 1 7 2 1 km s 2 7 3 2 km s 3 74 2 km s and 4 6 5 3 km s overlying the crystalline basement with a velocity of 5 8 6 2 km s rajmahal traps velocity 4 6 5 3 km s averaging 4 8 km s l occur beneath the base of the tertiary sediments in the middle parts of the profiles the presence of sub trappean gondwanas is also indicated from the wide angle reflection modelling supported by deep well lithological information in general depth to the basement is found to increase from west to east with a maximum depth of around 10 to 11 km at the extreme eastern parts of the profiles a basement upwarp with a sharp flexure west of bishnupur on profile 2 in combination with the fault west of shantipur on profile 1 possibly brings into focus the regionally extending nne ssw hinge zone a low velocity layer lvl of about 2 to 3 km thickness velocity 5 6 5 8 km s has been inferred in the upper crust in this area in general the thickness of the upper including the sedimentary column and the middle 6 4 6 6 km s velocity parts of the crust is found to be larger thickness varying between 22 and 27 km than the lower crustal column interval velocity 6 8 7 0 km s l with a thickness of 5 to 10 km the moho lies at varying depths of 36 to 26 km along profile 1 with a prominent domal feature of about 40 km width between satgachia and west of shantipur in the middle part of profile 2 the moho occurs at depths of 32 to 34 km with a mild flexure east of tamluk configuration of the moho indicates that the crust is more akin to continental in nature in the western and middle parts of the basin whereas significantly thinned continental crust is indicated in the eastern and southeastern parts

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

paleomagnetism of the clay howells carbonatite complex constraints on proterozoic motion in the kapuskasing structural zone superior province canada abstract the clay howells complex is located approximately 130 km east of hearst ontario at 49 50 n 82 05 w near the north end of the kapuskasing structural zone ksz of the superior province in the canadian shield the pluton is a large oviform pluton of about 16 km2 that is composed dominantly of syenite with minor carbonatite it was emplaced into an archean gneissic terrain of amphibolite to granulite facies with middle precambrian diabase dikes the complex is unmetamorphosed and late precambrian in age rb sr 1075 15 ma multistep alternating field af and thermal demagnetization was carried out on 194 specimens from 18 sites in the intrusion and three sites in the host rock dikes and gneisses the 18 syenite sites give a unit mean direction of 294 2 27 1 n 18 k 26 95 7 0 which yields a pole position of 178 8 e 26 5 n p 4 1 m 7 6 for the clay howells complex a crude contact test using the three host rock sites proved inconclusive the pole for the intrusion is concordant falling at about 1080 10 ma on the keweenawan apparent polar wander path which agrees with the rb sr age thus the complex has an untilted primary remanence that indicates that there has been no significant uplift

a methane based time scale for vostok ice abstract tuning the vostok methane signal to mid july 30 n insolation yields a new ice core gas time scale this exercise has two rationales 1 evidence supporting kutzbach s theory that low latitude summer insolation in the northern hemisphere controls the strength of tropical monsoons and 2 interhemispheric ch 4 gradients showing that the main control of orbital scale ch 4 variations is tropical monsoonal sources the immediate basis for tuning ch 4 to mid july insolation is the coincident timing of the most recent pre anthropogenic ch 4 maximum at 11 000 10 500 calendar years ago and the most recent july 30 n insolation maximum all ages in this paper are in calendar years unless specified as 14 c years the resulting ch 4 gas time scale diverges by as much as 15 000 years from the gt4 gas time scale petit et al nature 399 1999 429 prior to 250 000 years ago but it matches fairly closely a time scale derived by tuning ice core 18 o atm to a lagged insolation signal shackleton science 289 2000 1897 most offsets between the ch 4 and 18 o atm time scales can be explained by assuming that tropical monsoons and ice sheets alternate in controlling the phase of the 18 o atm signal the ch 4 time scale provides an estimate of the timing of the vostok co 2 signal against specmap marine 18 o often used as an index of global ice volume on the ch 4 time scale all co 2 responses are highly coherent with specmap 18 o at the orbital periods co 2 leads 18 o by 5000 years at 100 000 years eccentricity but the two signals are nearly in phase at 41 000 years obliquity and 23 000 years precession the actual phasing between co 2 and ice volume is difficult to infer because of likely sst overprints on the specmap 18 o signal co 2 could lead or be in phase with ice volume but is unlikely to lag behind the ice response

seismic delineation of upthrust archaean crust in kapuskasing northern ontario the kapuskasing structure fig 1 is a 500 km long high grade metamorphic zone which cuts obliquely across the predominantly east west structural grain of the superior province of the canadian shield results from surface geological mapping and gravity profile analysis suggest that the structure is a partial cross section of archaean crust exposed by thrust faulting along the ivanhoe lake cataclastic zone during the early proterozoic1 2 similar exposures of the crust have been observed elsewhere3 4 but no seismic investigation has been conducted over a contiguous exposed cross section of archaean crust here we report results from a seismic refraction profile across the kapuskasing structural zone carried out to investigate the deep structure of this greenstone gneiss belt modelling of travel times and amplitudes yields a high velocity anomaly of 6 6 6 7 km s 1 extending from 20 km depth to close to the surface under the kapuskasing zone with a suggested dip of 15 2 to the west this anomaly is in agreement with a structure comprising upthrusting middle to lower crust

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

an analysis of geomagnetic field reversals supports the validity of the geocentric axial dipole gad hypothesis in the precambrian abstract the geocentric axial dipole gad hypothesis has been regarded as the cornerstone of paleomagnetism for decades and disputes on its applicability have mostly dealt with the precambrian data one way to analyze the zonal harmonics of the geomagnetic field and the validity of gad hypothesis is based on the angular difference between the normal and reversed polarities also referred to as reversal asymmetry for this purpose a comprehensive compilation of the reversals of archaean and proterozoic geomagnetic field has been done using a novel paleomagnetic database with 3246 entries globally given the uncertainties prevailing in this type of analysis results from 106 reversal records show no dependence of the reversal asymmetry on paleolatitude thus supporting the outcome of our inclination frequency analysis of the precambrian field in addition to the inclination asymmetry the non antiparallelity between n and r also involves a declination asymmetry which is more difficult to determine and deals with the non zonal features of the field based on our dual polarity data we propose that the geocentric axial dipole gad is a viable approximation of the geomagnetic field throughout the precambrian

paleomagnetism and geochemistry of 1144 ma lamprophyre dikes northwestern ontario implications for the north american polar wander and plate velocities

changes in the atmospheric ch4 gradient between greenland and antarctica during the holocene high resolution records of atmospheric methane over the last 11 500 years have been obtained from two antarctic ice cores d47 and byrd and a greenland core greenland ice core project these cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution this together with a good relative chronology provided by unequivocal ch 4 features allows a direct comparison of the synchronized greenland and antarctic records and it reveals significant changes in the interpolar difference of ch 4 mixing ratio with time on the average over the full holocene records we find an interpolar difference of 44 7 ppbv a minimum difference of 33 7 ppbv is observed from 7 to 5 kyr b p whereas the maximum gradient 50 3 ppbv took place from 5 to 2 5 kyr b p a gradient of 44 4 ppbv is observed during the early holocene 11 5 to 9 5 kyr b p we use a three box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere the model results support the previous interpretation that past natural ch 4 sources mainly lay in tropical regions but it also suggests that boreal regions provided a significant contribution to the ch 4 hudget especially at the start of the holocene the growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the holocene finally our model results suggest a large source increase in tropical regions from the late holocene to the last millennium which may partly be caused by anthropogenic emissions

paleomagnetism of the keweenawan chipman lake and seabrook lake carbonatite complexes ontario the chipman lake complex crops out as a series of carbonatite and related alkalic mafic dikes in the wabigoon subprovince of the superior province whereas the seabrook lake complex crops out as an

paleomagnetism of alkaline complexes and remagnetization in the kapuskasing structural zone ontario canada the shenango and nemegosenda alkaline complexes of the southern kapuskasing structural zone ksz an uplifted crustal block in northern ontario preserve primary thermoremanent magnetizations with mean directions d 112 i 46 k 13 95 13 n 9 sites and d 304 i 51 k 31 95 10 n 7 sites for shenango and d 124 i 57 k 13 9520 n 4 sites and d 312 i 54 k 17 95 13 n 7 sites for nemegosenda their average paleomagnetic pole is 45 n 189 e dp 6 dm 8 falling on the keweenawan track of the north american apparent polar wander path apwp between 1098 1107 ma reverse polarity r and 1087 1098 ma normal polarity n poles of lake superior lavas and dikes unlike the keweenawan paleopoles which have a reversal asymmetry of 30 the shenango and nemegosenda r and n poles are almost 180 reversed which was found also by lewchuk and symons 1990b this symmetry of r and n poles casts doubt on the previously accepted predominance of asymmetric reversals of the earth s field in keweenawan time the archean shawmere anorthosite is overprinted by a b magnetization with mean direction d 97 i 68 k 12 95 10 n 18 sites the shawmere b paleopole at 38 n 226 e dp 13 dm l6 falls between the carbonatite poles and 1850 ma cratonic poles and the b magnetization seems to be a composite of middle and late proterozoic overprints sites in the western part of the shawmere body close to the carbonatite line of faults linking the various alkaline complexes are severely altered with an abundance of secondary magnetite and hematite virtual geomagnetic poles for these sites fall close to the keweenawan track and have an average pole of 189 53 n dp 15 dm 20 suggesting that the b nrm at these sites is an 1100 ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion more easterly sites are less altered their vgps resemble 1850 ma cratonic poles and average to 235 e 30 n dp 15 dm 17 these b natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle proterozoic hydrothermal activity associated with intrusion of early alkaline bodies fault movement on the ivanhoe lake fault zone or late stage uplift of the ksz

structure of the crystalline basement in the west bengal basin india as determined from dss studies summary deep seismic sounding data were acquired in the west bengal basin india along two profiles i bishnupur palashi kandi along a line about 227 km long in the northsouth direction and ii taki arambagh along a line about 120 km long in the east west direction seismic refraction and wide angle reflection data were recorded by continuous profiling using two 60 channel digital seismic units dfs v with an 80 m geophone group interval and 4 ms sampling rate these data were interpreted in order to delineate the basement configuration the 2 d models of the seismic data both indicate a five layer velocity structure above the archaean crystalline basement 5 9 6 2 km s i a low velocity layer 4 0 km s is inferred immediately above the basement in the shelf region of the basin corresponding to the gondwana sediments upper carboniferous to lower triassic below the rajmahal traps upper jurassic to lower cretaceous of 4 6 to 4 8 km s velocity which is also confirmed from the nearby well data the results along the taki arambagh profile and the drilling results at the jaguli j 1 well are used to investigate whether gondwana sediments and the rajmahal traps exist in the deep part of the bengal basin an additional layer of velocity 5 2 5 3 km s delineated in the palashi kandi profile overlying the basement may correspond to the singhbhum group of rocks of the proterozoic a structural contour map of the basement prepared from the present results indicates a south easterly dip of the basement in general the depth of the basement on the stable shelf of the basin gently increases to about 8 km and dips steeply plunging to a maximum depth of 14 km in the deep basin no structural high that can be related to the calcutta gravity high is found in the basement around the hooghly river

geochemistry of late 1 1 ga fluid inclusions in rocks of the kapuskasing archean crustal section three outcrops well constrained by geochronological and structural studies and representing a traverse running from tonalite dominated outcrops in the eastern wawa gneiss terrane to high grade granulites of the kapuskasing structural zone were mapped and sampled in detail in order to study the trapped fluids all fluid inclusions in quartz are secondary and consist mostly of co2 dominated type ii and saline aqueous type iiia fluids usually occurring on separate healed fractures but also coexisting on some fractures healed fractures in quartz contain fluid inclusions but are associated with carbonate sericite alteration where they pass into adjacent mineral grains homogeneous h2o co2 salt fluid inclusions type ia in carbonate rich veins of probable keweenawan 1 1 ga age were trapped at 400 550 c and ambient pressures of 1 5 2 kbar 1 kbar 100 mpa as these fluids cooled on penetration into cool 200 c country rocks along fractures they underwent open system h2o co2 phase separation f

thin thrust sheet formation of the kapuskasing structural zone revealed by lithoprobe seismic reflection data regional and high resolution seismic reflection data across the kapuskasing structural zone in ontario canada image at least three significant thrust faults that are low angle merge into a flat detachment on the west and together were responsible for the uplift of amphibolite and granulite facies rocks their geometry resembles a ramp and flat style of deformation that results in a thin upper plate above the 10 12 km about 4 0 s detachment northwest southeast horizontal shortening is estimated to be at least 55 km this large amount of shortening implies that much of the superior province was detached during the formation of the kapuskasing structural zone

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

magnetization age from paleomagnetism of the copper harbor red beds northern michigan usa and its keweenawan geologic consequences the copper harbor formation on lake superior s keweenaw peninsula records the transition from volcanic to sedimentary infilling of north america s 1 1 ga keweenawan rift radiometric dating shows t

two types of chemical remanent magnetization during the oxidation of magnetite abstract the chemical remanent magnetization crm m crm resulting from oxidation of equidimensional magnetite to a mixture of 90 hematite and 10 cation deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization m arm of the parent magnetite and the field h crm applied during oxidation m crm lies in the plane defined by h crm and m arm which were perpendicular in our experiments in an intermediate orientation determined by the relative strengths of h crm 0 05 0 1 and 0 2 mt and m arm 0 002 0 12 saturation remanence the crms are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization similar experiments in which acicular single domain cation deficient magnetite was oxidized to maghemite gave quite different results m crm was always parallel to m arm its direction was unaffected by field strengths as high as h crm 1 5 mt the essential factors controlling crm direction seem to be the number and crystallographic structure of oxidation products for single phase oxidation without change in crystal lattice crm remains parallel to the initial remanence of the parent mineral for multiphase oxidation to a mixture of rhombohedral and spinel phases both initial remanence and the field acting during oxidation influence the crm direction which therefore has an intermediate orientation that parallels neither m arm nor h crm

paleomagnetism of middle proterozoic diabase sheets from central arizona paleomagnetic results from 1090 ma diabase sheets of the southwestern u s a diabase province exposed in central arizona yield two distinct remanent magnetizations herein termed adn and adr in accordance with the findings of previous investigations magnetization adn is well defined and has an in situ mean direction of d 283 3 i 45 1 k 17 7 95 8 7 n 17 independent observations a mean pole after correction of paleomagnetic site means for a net 5 clockwise rotation of the colorado plateau and transition zone is located at 22 7 n 179 3 e k 21 9 a95 7 8 the second magnetization adr gives an in situ mean direction of d 161 1 i 87 5 k 22 2 95 19 9 n 4 independent observations with a poorly defined pole at 37 6 n 247 6 e k 6 5 a95 38 9 rock magnetic and alternating field and thermal demagnetization characteristics indicate the adn and adr magnetizations are both carried by low ti titanomagnetite both magnetizations are interpreted to be primary

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

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

magnetic properties of rocks of the kapuskasing uplift ontario canada and origin of long wavelength magnetic anomalies summary the sources of long wavelength magnetic anomalies lwa in the crust are poorly understood we have measured remanent and induced magnetizations of 210 samples of anorthosite tonalite and mafic gneiss from the kapuskasing uplift an exposed partial crustal crosssection in northwestern ontario canada anorthosites have generally high q ratios of remanent induced magnetization in the range 0 3 60 and their natural remanent magnetization nrm is resistant to both thermal and alternating field af demagnetization however anorthositenrms 0 001 0 3am 1 aretooweaktoexplainlwaamplitudes maficgneisses also have relatively high q ratios peaking in the range 1 10 and the nrm is resistant to thermal and af demagnetization nrm and induced magnetization im intensities are in the ranges 0 01 2 and 0 01 0 6 a m 1 respectively tonalites have a bimodal distribution of magnetization the more strongly magnetic group has both nrm and im intensities in the range 0 1 5 a m 1 and wide ranging q values from 0 1 to 10 approximately some tonalites could be an lwa source although the long term stability of their nrms at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 c in general q values measured at surface temperatures overestimate remanence at depth in the deep crust im remains more or less constant but remanence decreases both reversibly and irreversibly leading to q ratios of 0 2 0 3 for multidomain grains and 1 3 for single domain grains near their blocking temperatures thermoviscous magnetization over the brunhes chron could add substantially to the effective induced magnetization typically induced thermoviscous magnetization in the direction of the present earth s field will outweigh remanence in the direction of an ancient field as a source of lwa originating in the deepest crust remanence may play a larger role for mid crustal sources where single domain grains are well below their blocking temperatures

age of the firesand river carbonatite complex from paleomagnetism the 2 3 km diameter firesand river complex intrudes archean volcanics and granites of the wawa subprovince in the superior province about 8 km east of wawa ontario it has given differing middle proterozoic k ar biotite ages of 1018 50 and 1097 ma alternating field and thermal step demagnetization of specimens from three calcific carbonatite sites five ferruginous dolomitic carbonatite sites and one lamprophyre dike site isolated a stable mean direction of 290 33 95 12 isothermal remanent magnetization tests indicate the remanence is held by single to pseudosingle domain magnetite and hematite in the carbonatite the dike remanence is keweenawan in age thereby confirming its genetic relationship to the complex and it gives a positive partial contact test with its host rock indicating no postintrusive remagnetization the blocking temperature spectra indicate that postintrusive uplift has not exceeded about 4 km the pole position for the complex is 183 e 27 n dp 8 dm 13 this

interpretation of magnetic susceptibility a new approach to geophysical evaluation of the degree of rock alteration magnetic susceptibility measurements on cores from boreholes in the eye dashwa lakes pluton an archean granitic pluton near atikokan ontario give a quantitative evaluation of the alteration leve

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

deep crustal structure and tectonic history of the northern kapuskasing uplift of ontario an integrated petrological geophysical study the northeast trending kapuskasing uplift transects the east west belts of the central superior province over a distance of some 500 km granulite to upper amphibolite facies rocks of the uplift form three distinct geological geophysical entities from south to north the chapleau groundhog river and fraserdale moosonee blocks uplift of the granulites along a moderately northwest dipping crustal scale thrust fault is attributed to an early proterozoic compressional event major northeast striking faults that bound the kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet orthopyroxene plagioclase quartz assemblages to determine vertical displacement granulites in the kapuskasing zone have 7 to 9 kbar signatures whereas those in the quetico belt to the west indicate metamorphic pressure of 4 6 kbar individual calibrations of the barometer yield consistent pressure differences of 2 3 kbar suggesting 7 10 km of west side down movement on the faults modelling of gravity and aeromagnetic gradients indicates westerly dips of 60 65 with west side down offset of up to 14 km these major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage differences in the configuration of individual blocks of the kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults thus the groundhog river and southern fraserdale moosonee blocks are perched thrust tips analogous to the sangre de cristo range of the laramide uplift province whereas the southern chapleau block is a tilted slab with similar configuration to the laramide wind river range pop up geometry deduced for the northern fraserdale moosonee block resembles the structure of the laramide uinta mountains a normal fault crosses the surface trace of the basal thrust fault between the groundhog river and fraserdale moosonee blocks and causes a 65 km gap without granulites this article contains supplementary material