subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

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

intermediate wavelength magnetic anomalies over ocean basins we have examined three very long magnetic field profiles taken over ocean basins for the presence of intermediate wavelength magnetic anomalies one profile was from the atlantic ocean in the transatlantic geotraverse area one ran along latitude 35 sup 0 s in the se pacific and one ran along 150 sup 0 w in the pacific all three profiles show the presence of intermediate wavelength 65 1500 km magnetic anomalies generated in the crust or upper mantle the analysis of magnetic field power spectra shows that the core field becomes unimportant at about a wavelength of 1500 km sea floor spreading anomalies should produce a maximum in power at about a wavelength of 65 km between these two wavelengths there should be a minimum in power which is not seen on observed records inverting the anomalous field to obtain some idea of the magnetization necessary to explain these intermediate wavelength magnetic anomalies shows that values of magnetization in excess of 1 a m sup 1 are needed if the magnetized layer is as thick as the ocean crust alternatively rather large thicknesses of upper mantle material with lower intensities of magnetization need to be used the reason why such magnetization variations exist is not known itmore can be shown that upward continuation of the magnetic anomaly signature to an altitude of 350 km about the perihelion altitude of magsat will produce anomalies up to 10 nt in amplitude these should be capable of being seen by magsat and thus allow us to determine the spatial arrangement of the intermediate wavelength anomalies and hence hopefully a clue as to their origin less

why are marine magnetic anomalies suppressed over sedimented spreading centers the absence of lineated marine magnetic anomalies over sedimented recent spreading centers has been observed in several areas we propose that magnetic anomalies can be suppressed as a result of pervasive hydrothermal reactions underneath thick blankets of sediment the relatively impermeable sediment cover produces comparatively closed hydrothermal systems which increase the residence time of the hot fluids in basaltic layer 2 causing more thorough leaching of the remanence carrying iron titanium oxides and diminishing the marine magnetic anomalies this contrasts with lesser alteration at sparsely or unsedimented zones of extension with more open circulation conversely spreading ridges and oceanic crust characterized by absent or subdued lineated anomalies may signal sites of extensive hydrothermal mineralization this process might also provide an explanation for some magnetic quiet zones such as in the gulf of california the gulf of aden and the northern red sea

an experimental study of magnetic viscosity in synthetic monodomain titanomaghemites implications for the magnetization of the ocean crust a study of magnetic viscosity in well characterized synthetic monodomain titanomaghemites of composition fe2 4ti0 6o4 is described it is found that the samples are capable of acquiring a viscous remanent magnetization vrm only when they are highly oxidized the same assembly does not show significant magnetic viscosity for 0 1 z 0 6 where z is the oxidation degree this is a result of their being in the thermally stable monodomain state with high anisotropies the observed increase in vrm for the highly oxidized material may be entirely due to the decrease in magnetic anisotropy a decrease in the total number of fe2 as a functin of oxidation results in a decrease in coercive force i e magnetic anisotropy of the initial monodomain titanomaghemites it is thought that the dramatic decrease in magnetic anisotropy at room temperature may be the main cause of the observed increase in vrm for the highly oxidized material finally we conclude that the ability of the magnetic layer of the ocean crust to acquire a vrm increases as the maghemitization degree increases

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

paleomagnetism of archean volcanics and proterozoic dikes in la grande riviere area of nouveau quebec paleomagnetic results are reported for six archean volcanics and six proterozoic diabase dike sites in the la grande riviere basin using af and thermal demagnetization the area is now flooded so that future work is not possible the limited geological and statistical data base makes interpretation of the results necessarily tentative the dikes yield an a component pole at 84 w 74 n from magnetization residing in higher stability domains which likely reflects emplacement either at 2150 or 1825 or 1650 ma ago and a b component pole at 33 w 4 s residing in lower stability domains which likely reflects metamorphism during the hudsonian orogeny at 1725 ma or less likely during grenvillian uplift at 975 ma the volcanics appear to yield a prefolding c component pole at 28 w 49 s that dates extrusion at 2900 ma and a postfolding d component pole at 306 w 37 n that dates metamorphism by the kenoran orogeny at 2600 ma

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

vertical distribution and alteration of dikes in a profile through the troodos ophiolite in situ oceanic crust at intermediate depths of from one to five km is presently largely inaccessible to continuous examination and sampling except in the possibly special circumstances of major transform faults the study of ophiolites over this interval where thin sub vertical dikes are usually important may provide guidance to the nature of in situ oceanic crust at these depths the likelihood that similarities exist between in situ and ophiolitic oceanic crust has recently been strengthened by the possible identification of a dike complex below 0 8 km depth in dsdp hole 504b1 sub vertical basic dikes occur over a 4 6 km depth interval from 0 2 to 4 8 km crustal depth in a profile through the troodos cyprus ophiolite exponential growth and arctangent decay in dike density occur over 2 3 km and 1 0 km intervals respectively secondary magnetite is the dominant dike magnetic material of the dikes and associated screens these results have implications for the construction alteration history and magnetization of in situ oceanic crust at intermediate depths

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 wintering lake pluton and the early proterozoic tectonic motion of the superior boundary zone manitoba this lithoprobe funded paleomagnetic study of the early proterozoic wintering lake granitoid body supports tectonic models that suggest continental accretion of the trans hudson orogen with the superior craton occurred at 1822 ma thermal demagnetization data for the granitoid specimens suggest that the magnetic remanence carriers are coarse grained magnetite or titanomagnetite and saturation isothermal remanence tests suggest that the magnetite is mostly multidomain six of seven paleomagnetic contact tests were negative indicating that the host rocks have been remagnetized and that the granitoid body may have been partially remagnetized near its margins acceptable site mean remanence directions for 20 of 21 granitic sites yield a paleopole at 46 8 n 102 2 w with semi axes of the 95 ellipse of confidence about the paleopole of dp 11 and dm 11 the paleopole fits on the extrapolated apparent polar wander path apwp for the superior craton at 1822 ma which is the interpreted emplacement ag

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

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

the galapagos spreading center at 86 w a detailed geothermal field study appendix is available with entire article on microfiche orderfrom american geophysical union 2000 florida avenue n w washington d c 20009 document j80 013 01 00 payment mustaccompany order we report here measurements of the heat flow field of the galapagos spreading center on crust of age less than 1 0 m y the 443 measurements in an area of about 570 km2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near axis conductive heat flux the intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern with circulation apparently continuing beyond the limits of our survey the areal average of the conductive heat flux is 7 1 0 8 hfu 295 33 mw m2 about one third the heat flux predicted by plate models the remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis the pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography sharp lateral gradients in the heat flow greater than 10 hfu km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys may indicate a local concentration of the circulation by surface fault systems and or variable sediment thickness

a comparison of the magnetic properties of synthetic titanomaghemites and some oceanic basalts the variation of some magnetic properties of 1 synthetic titanomaghemites and 2 oceanic basalts from dsdp site 417 d as a function of grain size and oxidation is presented the synthesis of the initial titanomagnetite x 0 6 was carried out using controlled fugacity and self buffering techniques single phase titanomaghemites were produced by heating the titanomagnetite in air at low temperatures 350 c the magnetic properties studied on the synthetic titanomagnetite grain size 1 m show the following variations with increasing degrees of oxidation 0 z 0 6 1 curie temperature tc increases 2 saturation magnetization js at room temperature increases 3 js measured at 77 k decreases for z 0 3 and then increases for 0 3 z 0 6 4 bulk and remanent coercivities hc and hr increase slightly initially z 0 2 and then decrease at both room temperature and 77 k 5 susceptibility 0 decreases for z 0 3 and then increases 6 the magnetic viscosity acquisition and decay coefficients sa and sd increase and 7 the median destructive fields for an anhystretic remanent magnetization and a viscous remanent magnetization increase for z 0 3 and then decrease these magnetic data also suggest that the synthetic titanomaghemites are pseudo single domained psd and with increasing degrees of oxidation these psd grains become more sd like the basalt samples from site 417d are divided into fine grained pillow basalts and coarse grained massive flows most of the magnetic properties measured are distinctly different between these two groups the magnetic data indicate that the domain states of these basalts are probably psd although it is difficult to separate the effects of grain size and oxidation on the magnetic properties of these basalts it appears that grain size effects predominate a preliminary investigation of the magnetic viscosity of the synthetic and oceanic titanomaghemites suggests that a combination of thermally activated and diffusive magnetic viscosity is responsible for their viscous behavior finally we will discuss the roles of grain size and oxidation variation in oceanic basalts and their separate and combined effects on their magnetic behavior

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

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

rb sr and u pb ages of volcanism and granite emplacement in the michipicoten belt wawa ontario the michipicoten greenstone belt at wawa ontario is typical of archean volcanic belts in the superior province the supracrustal rocks are divisible into lower middle and upper metavolcanic sequences which are separated by iron formation and clastic metasedimentary rocks these are intruded by granitic stocks and embayed by granitic batholiths this study reports whole rock rb sr and zircon u pb ages for the lower and upper metavolcanics for the granitic rocks that are physically within the greenstone belt internal granites and for the granitic rocks that embay the greenstone belt external granites the apparent rb sr ages for the lower metavolcanics are 2530 90 2285 70 and 2680 490 ma the u pb ages are 2749 2 and 2744 10 ma the internal granites give an rb sr age of 2560 270 ma and a u pb age of 2737 6 ma the external granite at hawk lake indicates an rb sr age of 2550 175 ma and a u pb age of 2747 7 ma it is possible that this unit contains elements older than 2812 ma as

a model of ocean basin crustal magnetization appropriate for satellite elevation anomalies a model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust the model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes the exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization

correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the mulcahy gabbro north west ontario summary the mulcahy gabbro is an archean layered intrusion in the wabigoon subprovince of north western ontario alternating field and thermal demagnetization of specimens from it have provided consistent within site characteristic remanence chrm directions in eight of 19 sites antipodal polarities of remanence are recovered and the overall in situ formation mean is d 240 i 51 k 21 this direction is unlike any reported archean direction from the superior province of north america the degree of anisotropy of low field magnetic susceptibility is high with k k3 values up to 1 3 cooling selected specimens in weak fields indicate that trm is deflected towards the magnetic fabric plane accordingly the in situ chrm was corrected for the effect of magnetic anisotropy two methods of correction give similar results however the resultant formation mean remains essentially unchanged as a consequence of sufficient variability in the attitudes of k k planes although uncertainty in the initial dip renders the tilt correction problematical partial tilt correction 36 per cent results in a formational mean with better precision than the in situ formation mean furthermore partial untilting of remanence directions results in a positive reversal test with a better classification than that obtained using in situ directions this partially tilt corrected result d 202 i 49 k 35 yields a pole latitude 62 n longitude 42 e which is archean in aspect

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

magnetization of the pacific ocean lithosphere deduced from magsat data magsat anomalies over the pacific ocean are related to volcanic plateaus and the edge effect of the pacific cretaceous quiet zone pcqz four prominent plateaus shatsky rise the mid pacific mountains hess rise and manihiki plateau are considered in this study the major lateral magnetization contrasts associated with these plateaus are considered to arise from the topographic swells and the crustal roots with root thicknesses estimated from topography using an airy isostatic model shatsky rise and the mid pacific mountains are older than the cretaceous quiet period and their natural remanent magnetization nrm is not a significant component of the magnetization contrasts that cause the magsat anomalies associated with these plateaus their magnetic properties are characterized by effective magnetic susceptibility contrasts of 0 07 0 01 s i per unit volume averaged over their topographic swells and crustal roots equivalent to 0 04 if averaged over the entire crustal thicknesses swell plus root plus intervening layer this value agrees well with the estimates of previous investigators for these plateaus and for similar plateaus in other oceans two plateaus of cretaceous quiet period age within the pcqz hess rise and manihiki plateau have origins similar to those of shatsky rise and the mid pacific mountains and they probably have similar effective susceptibility contrasts but they may also carry cretaceous quiet period nrm magnetic anomalies due to hess rise and manihiki plateau calculated from the effective susceptibility contrast deduced for shatsky rise and the mid pacific mountains are substantially larger than the observed magsat anomalies of these plateaus there are also magsat anomalies over parts of the pacific where no plateaus exist some of these anomalies are demonstrated as probably due to a contrast in nrm between the pcqz and its surroundings the total remanent magnetization contrast magnetization contrast in amperes per meter times thickness in meters equivalent to the dipole moment per unit area of the pcqz is 14 000 20 000 amperes 30 which cannot be simply explained by the absolute values of remanent magnetization predicted by conventional models of oceanic crustal magnetic properties modelling of the cretaceous quiet zones in the northern atlantic indicates a similar total remanent magnetization contrast these results may be explained by including uppermost mantle remanent magnetization with an intensity of about 1 2 a m 1 if magnetization persists to a depth of 7 below the seismic moho i e down to the depth of the crustal roots of the plateaus including nrm in the uppermost mantle of the pcqz also serves to reduce the calculated amplitude of the hess rise and manihiki plateau magsat anomalies if the nrm of the crustal roots is less than the nrm of the surrounding uppermost mantle on this basis the estimated intensity of nrm in the uppermost 7 km of the pcqz mantle is 1 3 a m 1 in very good agreement with the intensity estimated from the pcqz edge effect anomalies these estimates of uppermost mantle nrm are probably a maximum limit recognizing that other sources may also contribute to the total remanent magnetization contrasts of the cretaceous quiet zones

the cretaceous pyonghae sequence southeast korea terminal fan facies abstract the cretaceous pyonghae sequence southeast korea comprises a nonmarine clastic sequence of dominant purplish siltstone with subordinate gravelstone and sandstone the sequence consists of eight sedimentary facies which can be organized into three facies associations facies association i occurs along the basin margins and is represented by disorganized gravelstone facies g1b and g2 and stratified gravelly sandstone facies gs the gravelstone and sandstone facies are interpreted as debris flow deposits and braided stream deposits respectively formed in alluvial fans n facies association ii mainly occurs in the central part of the basin and comprises sheet like sandstone bodies facies gs s1 and s2 intercalated with purplish siltstone beds facies m1 the sheet like sandstone beds show a fining upward trend with sharp base and lateral margins and are often horizontally stratified these features most likely result from frequent diversion of ephemeral shallow stream channels on floodplain and subsequent infilling with coarse grained sediments facies association iii is characterized by thick siltstone beds facies m1 with lenticular sandstone and gravelstone beds the siltstone beds contain calcareous nodules the facies association iii is well developed in the southern part of the basin where it represents floodplain environments alluvial fans facies association i and adjacent floodplain facies association iii which are connected by ephemeral stream channel networks facies association ii can be referred to as terminal fan system autocyclic processes such as spasmodic floods under semi arid climatic conditions are important for the development of terminal fan systems

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

constraints on the nature of the kapuskasing structural zone from the study of proterozoic dyke swarms petrochemical paleomagnetic and structural studies on early proterozoic dyke swarms show that the rocks of the kapuskasing structural zone ksz central superior province were upthrust about 10 15 km along the ivanhoe lake fault after intrusion of 2 04 ga kapuskasing dykes this uplift was part of a more widespread deformation in flanking terranes that involved sinistral motion along north north northwest trending faults dextral displacement along east northeast northeast trending faults and a distortion of the 2 45 ga matachewan swarm to the west and northwest of the ivanhoe lake fault the most spectacular demonstration that the ksz is largely a product of proterozoic deformation is that matachewan dykes change their magnetic polarity on crossing the zone a consequence of remanence acquisition at deep crustal levels being delayed until after a reversal of the earth s magnetic field and prior to uplift of the dykes matachewan and kapuskasing dykes within the amphibolite to granulite grade rocks o

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

the cretaceous pyonghae basin southeast korea sequential development of crevasse splay and avulsion in a terminal alluvial fan abstract the pyonghae basin early cretaceous southeast korea comprises a thick sequence of purplish siltstone with subordinate gravelstone and gravelly sandstone deposited in a terminal alluvial fan environment based on bed geometry and facies assemblage the gravelly sandstone bodies are divided into four types type a deposit consists of disorganized gravel patches and massive sandstone lenticules a few decimetres thick and a few metres wide most likely deposited from overflows type 2 deposit is characterized by crudely stratified and massive sandstone beds up to 2 m thick 20 m wide and comprises lower disorganized gravel layers and upper stratified gravelly sandstone beds forming a fining upward trend it probably represents crevasse channel distributary channel fills type 4 deposit 3 5 m thick 20 m wide consists of stratified cross stratified gravelly sandstone beds which are interpreted as main braided channel fills aggradation of the terminal fan was associated with frequent shift of channels which range from crevasse splay channel type 2 through crevasse channel distributary channel type 3 to main channel type 4 the crevassing or avulsion was preceded by overflows type 1 the avulsion processes in the terminal fan were achieved through stepwise enlargement of crevasse channels which reflect different channel responses to floods of different magnitude and frequence

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

archean plate tectonics what can be learned from continental geology some basic questions about archean plate tectonics can be addressed by examining accretionary archean margins in particular fault zones with significant strike slip components on the canadian shield 1 were the oceanic plates typically rigid like modern plates yes significant lateral viscosity contrasts in the lithosphere between plates and plate boundaries are required for major strike slip faults to exist conversely strike slip faults are a kinematic consequence of rigid plates 2 did large oceanic plates exist in the archean probably first the length and offset of the longest preserved segments of archean faults are similar to modern examples such as in alaska less directly the duration of a period with a consistent sense of strike slip at a point on the continental side of an accretionary margin should be related to the time that a typical oceanic plate remains outboard of the margin this time varies proportionally with size of typical ocean plates and inversely with their velocity the

effective magnetic susceptibility of the oceanic upper mantle derived from magsat data passive ocean continent boundaries have little magnetic signatures at magsat altitude 400 km whereas subducting oceanic lithosphere has well defined magnetic anomalies over some of the subduction zones based on this observation and a thermal evolution model of the oceanic lithosphere subducted along the kuril kamchatka trench we determined an effective magnetic susceptibility of 0 0085 si for the upper 35 km of the oceanic upper mantle

precise u pb zircon ages from the gamitagama greenstone belt southern superior province volcanism in the gamitagama lake greenstone belt 50 km south of wawa ontario on the east shore of lake superior took place ago at the same time as similar volcanism in the much larger abitibi greenstone belt to the east two post orogenic intrusions in the metavolcanic metasedimentary terrain are coeval at and and could very well be comagmatic as suggested by ayres a granitic sill in a metasedimentary granitic migmatite terrain that borders the greenstone belt to the southwest has an age of 2615 10 ma and is much younger than the post orogenic plutons to the northeast this migmatite region represents either an uplifted deep zone that remained hot long after volcanism or a region that accumulated heat as a result of its sedimentary cover

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

elevated temperature viscous remanent magnetization of natural and synthetic multidomain magnetite abstract the time temperature relationship of magnetization is a subject of much interest and debate by paleomagnetists rock magnetists and magnetic anomaly modellers we have investigated this relationship by studying the viscous remanent magnetization vrm of coarse grained multidomain md magnetite our experiments covered the temperature range from 22 to 400 c times from minutes to months and included both australian granulite samples and multidomain magnetite samples synthesized by the glass ceramic method vrm acquisition was found to generally increase with temperature but not always at the rate predicted from classical thermal fluctuation theories thermal cycling between room temperature at which the measurements were made and the vrm acquisition temperature sharply decreased the temperature dependence of the vrm room temperature vrm acquisition accelerates with time when plotted on a semilog plot whereas at elevated temperature the curves are quasilinear against log time for both the natural and synthetic samples this change in behavior may suggest a variation in the vrm acquisition mechanism as a function of temperature for md magnetite the granulites have a nearly linear increase in vrm acquisition rate with temperature whereas the glass ceramics display little change in the acquisition rate between 22 and 200 c but increase by nearly a factor of 3 by 400 c the increase in vrm of the glass ceramics between 200 and 400 c is in general qualitative agreement with thermal fluctuation theory there was no systematic change in the rate of vrm acquisition with grain size for the multidomain magnetites used in this study elevated temperature e g 400 c vrm acquisition by the deep crustal granulites if extrapolated over the brunhes chron would produce a magnetization of several a m which if true is of the order required by models for the source of long wavelength magnetic anomalies

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

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

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

geothermal regimes of the costa rica rift east pacific investigated by drilling dsdp ipod legs 68 69 and 70 drilling durin deep sea drilling project legs 68 69 and 70 on the southern limb of the costa rica rift was used to study geothermal processes in the ocean crust two areas were drilled one was a geothermally hot site on 6 2 m y old crust where topography is smooth heat flow is close to that predicted by conductive cooling of the lithosphere 200 mwm 2 and hydrothermal circulation may be sealed within the crust the other was on 3 9 m y old crust where rough topography is associated with low heat flow 15 to 50 mwm 2 and possible open convection of sea water at both sites about 250 m of siliceous calcareous sediments overlies igneous basement in the hot area it blankets the topography whereas in the cold area basement outcrops still occur operations included numerous down hole experiments in both areas and hydraulic piston coring of a 230 m sediment section in the hot area diagenesis of the sediments appears closely related to temperature at the hot site chert was found near basement and the chemistry of pore fluids sampled from both sediments and basement is strongly influenced by reactions within the basement strong lateral gradients in the composition of pore fluids occur in the sediments at the cold site no chert was found and bacterial processes within the sediment dominated the chemistry of the pore fluids basaltic basement in both areas consists mainly of pillow lavas and thin flows with occasional more massive units the basalt is relatively magnesian the degree of alteration is very small in the cold area but much more extensive in the hot area ease of drilling also shows a strong contrast basement penetration reached 562 m in the hot area and was halted because of lack of time at the cold site 43 m of basement was cored only with difficulty the most intensive in hole experiments were conducted in the hot area successful runs with the borehole televiewer allowed basement lithology to be determined and showed the presence of more and less fractured zones pulse tests using a single borehole packer gave values of basement permeability of about 2 to 40 millidarcies numerous temperature logs established a broadly conductive in situ temperature gradient with temperatures reaching 120 c at 562 m into the basement however anomalously low temperatures in the upper part of the hole which persisted after drilling disturbance had decayed away showed that cold ocean water was flowing down the hole and into the basement at about 90 m below the base of the sediments at rates of about 80 to 100 m hr the packer records indicate a pressure at this depth of 10 bars below hydrostatic costa rica rift united scientific team made up of shipboard participants on legs 68 501 69 and 70 who were in reverse alphabetical order

magnetization of the oceanic crust beneath the labrador sea the aeromagnetic anomalies of the oceanic crust northeast of newfoundland canada are strongly skewed with an anomalous skewness of 33 52 with respect to the anomalies calculated using the standard sea floor spreading model this paper examines the possibility that the anomalous skewness is on account of an oceanic crust with a thermo viscous remanent magnetization where unlike the standard model the lower crust is magnetic the thermo viscous remanent magnetization of the lower crust beneath the labrador sea has dominant contributions to broad scale marine magnetic anomalies and to the negative magsat anomaly of the sea

statistical averaging of marine magnetic anomalies and the aging of oceanic crust visual comparison of mesozoic and cenozoic magnetic anomalies in the north pacific suggests that older anomalies contain less short wavelength information than younger anomalies in this area to test this observation magnetic profiles from the north pacific are examined from crust of three ages 0 2 1 29 3 33 1 and 64 9 70 3 m y b p for each time period at least nine profiles were analyzed by 1 calculating the power density spectrum of each profile 2 averaging the spectra together and 3 computing a recording filter for each time period by assuming a hypothetical seafloor model the model assumes that the top of the source is acoustic basement the source thickness is 0 5 km and the time scale of geomagnetic reversals is according to ness et al 1980 the calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short wavelength information as the crust ages these results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer corresponding to pillow basalt of seismic layer 2a acts as a source of noise to the magnetic anomalies as the ocean crust ages this noisy contribution by the pillow basalts becomes less significant to the anomalies consequently magnetic sources below layer 2a must be faithful recorders of geomagnetic reversals

paleomagnetism and orientation of precambrian dykes eastern lake superior region and their use in estimates of crustal tilting archean rocks form the eastern margin of the 1 1 ga old central north american rift along the eastern shore of lake superior and have been tilted westwards in response to rifting paleomagnetic and structural data from 2 6 ga old matachewan dykes suggest a westward crustal tilt of about 60 which agrees well with dips recorded in nearby keweenawan volcanics that rest directly on basement rocks the matachewan dyke swarm occurs throughout the east shore region of lake superior whereas keweenawan supracrustal sequences which give a more precise estimate of tilt are restricted to a few isolated shoreline patches estimates of crustal tilt can be obtained from the dykes on a regional basis thus generating a more complete picture of basement deformation adjacent to a major intracratonic rift

remanent magnetization of the oceanic upper mantle skewnesses of the marine and magsat magnetic anomalies of the atlantic ocean are explained in terms of the remanent magnetization gradually acquired by the oceanic lithosphere during its cooling magnetizations of the lower crust and the upper mantle are about 20 50 and 10 30 percent of the magnetization of the oceanic basaltic layer 2a respectively the oceanic upper mantle controls the skewness of the magsat anomalies whereas the skewness of the marine anomalies arise primarily from the oceanic lower crust

alluvial fans and their tectonic controls the study of alluvial fan sequences the most proximal components of the alluvial spectrum can be one of the most profitable avenues of research in basin analysis because fans are particularly sensitive to external influences and have a relatively high preservation potential tectonism has long been invoked to explain many variations in fan sediments and increasingly the presence and nature of fan sequences in the rock record is being used to denote proximity to active faults and to document movement on these faults there are however many difficulties involved with the interpretation of alluvial fan sequences not the least of which is deciding whether the sediments are truly components of an alluvial fan or the products of a different depositional system such as an alluvial braidplain many of the problems arise from a lack of knowledge about the relative roles in fan construction of allocyclic processes such as climatic change and tectonic activity and of autocyclic processes such as channel avulsion and fanhead entrenchment whilst researching alluvial fan sediments in a variety of tectonic settings and ages it has become increasingly clear to the authors that there are many inadequacies in our understanding of alluvial fans and hence weaknesses in the conclusions drawn from these sediments this prompted the one day meeting organized to promote discussion of the problems of distinguishing tectonic from other influences on the evolution of modern and ancient fans 110 industrial and academic geologists gathered at the university of bristol on 17th october 1987 to hear 16

comment on tectonic rotations in extensional regimes and their paleomagnetic consequences for ocean basalts by kenneth l verosub and eldridge m moores deviate significantly from expected values vetosub and moores 1981 sought to account for what appear to be sys tematic departures of mean inclination at several dsdp base ment sites in terms of tectonic rotations along listtic normal faults such rotations about horizontal axes perpendicular to the extension direction and typically amounting to 30o 50 o

magnetization of the oceanic crust thermoremanent magnetization of chemical remanent magnetization we propose a model in which chemical remanent magnetization crm acquired within the first 20 m y of crustal evolution may account for 80 of the bulk natural remanent magnetization of older basalts the crm of the crust is acquired as the original thermoremanent magnetization trm is lost through low temperature alteration the crm intensity and direction are controlled by the posternplacement polarity history this model explains several independent observations concerning the magnetization of the oceanic crust the model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern it also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the cretaceous quiet zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust the model also explains discrepancies in the anomaly skewness parameter observed for anomalies of cretaceous age further our study indicates varying rates of trm decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer

seafloor spreading anomalies in the magsat field of the north atlantic we show that a simple magnetization model for the crust of the north atlantic basin will explain the major anomalies of the magsat intermediate wavelength magnetic field over the basin our seafloor spreading model incorporates anomalous skewness and enhanced magnetization near the spreading axis and at the cretaceous quiet zones the anomalies over the young oceanic crust indicate a decrease in susceptibility away from the spreading axis the enhanced magnetization over the cretaceous quiet zone could be due to an increased main field intensity or a change in the spreading center processes during the cretaceous though similar observations are not readily apparent for other basins the north atlantic correlation indicates that an investigation of the remanent magnetization of the ocean crust could provide a standard for the evaluation of the intermediate wavelength magnetic field the models presented in this paper demonstrate that a sizable percentage of the intermediate wavelength energy in the magsat field is derived from sources within the earth s crust it also demonstrates that remanent magnetization is an important source of the magsat field over oceanic areas we seek to demonstrate in this paper that although components of the main field are likely to contribute to the intermediate wavelength scalar field we can correlate the anomaly field over the north atlantic to the well studied thermal remanent magnetization of the oceanic crust with a minor component of induced magnetization associated with younger oceanic crust

skewness of marine magnetic anomalies created between 85 and 40 ma in the indian ocean we have performed a detailed analysis of the skewness of marine magnetic anomalies in indian ocean basins created between 85 and 40 ma as a result of the northward motion of india visual and semiautomated methods of skewness determination were applied to the data both provide consistent results but the visual method is prefered for its ability to deal with noisy data plots of apparent effective remanent inclination or skewness corrected for present geomagnetic field inclination versus time for conjugate basins display the combination of three effects a gradual increase with time related to the northward motion of the ridges attached to india in the geomagnetic reference frame a gap between conjugate curves which represents anomalous skewness senso stricto and short period fluctuations which represent the sequence effect i e the effect of neighboring magnetic sources on the skewness of a given anomaly the anomalous skewness decreases with faster spreading rate and completely disappears above 50 km m y an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness

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

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

thermoremanent magnetization of the oceanic lithosphere inferred from a thermal evolution model implications for the source of marine magnetic anomalies abstract thermoremanent magnetization models of the oceanic crust and the uppermost part of the oceanic mantle are derived on the basis of a typical thermal evolution model of the oceanic lithosphere using different magnetic blocking temperature ranges the magnetization of the basaltic layer 2a has a similar pattern but with a lower intensity to that of the standard sea floor spreading magnetic model however the magnetization patterns of the lower crust and upper mantle are skewed so that in a given polarity period the total magnetization of a lithospheric column which is created early in the period is greater than that of a column which is created later in the period magnetic anomalies of the magnetization models are calculated at sea level and at magsat elevation for slow moderate and fast sea floor spreading velocities although the magnetization models are derived based on purely physical reasoning without using any a priori information about the observed marine and magsat magnetic anomalies they do yield magnetic anomalies which are similar to the observed anomalies the models explain several features of the marine and magsat anomalies the anomalous skewness of marine magnetic anomalies is explained in terms of the magnetization of the oceanic lower crust the magnetization of the oceanic upper mantle may account for the anomalous skewness observed over the cretaceous quiet zones in the atlantic ocean the skewness of the magnetic anomalies calculated at sea level decreases with the increase in the sea floor spreading velocity

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define