reservoir sedimentation i delta and density current deposits in china where rivers draining loess soils have some of the highest sediment concentrations in the world reservoir sedimentation is a serious problem that has been the subject of intensive investigation since 1957 this paper describes patterns of sediment deposition and erosion drawing on field measurements in china at the sanmenxia guanting shanyiujiang and liujiaxia reservoirs as well as the shanshenggorig barrage and xijin hydropower station the longitudinal and lateral geometry of reservoir deltas is described it is shown that a marked change in grain size can occur at the downstream limit of the delta even when this limit is not apparent from geometry density current data for guanting reservoir are presented problems caused by reservoir sedimentation in china are described including aggredation in the backwater region causing increased flood levels elevated ground water levels and navigation impairment deposition from or into a tributary entry of sediments into hydropower turbines a

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

sea level and climatic controls on turbidite occurrence for the past 26 kyr on the flank of the gaoping canyon off sw taiwan submarine canyons are major conduits for delivery of sediments originating from the continental margin into the deep sea here we analyze the effects of changing sea levels and climate e g sediment supply on the frequency of turbidites over the last 26 kyr our study is based on high resolution age controls for turbidites in core md178 10 3291 from a water depth of 2070 m on the flank of the gaoping canyon offshore in sw taiwan unlike most other submarine canyons worldwide the head of the gaoping canyon has remained connected to the river mouth during the recent flooding of the continental shelf our results reveal that turbidity currents are less frequent during periods of sea level lowstand in contrast turbidity currents have been more frequent since 12 kyr bp during the final stage of rising sea level and sea level highstands this may be the result of increased terrestrial sediment delivery due to enhanced rainfall intensity moreover comparing to other source to sink systems the turbidite occurrence in the flank of gaoping canyon through the last glacial cycle might have been more strongly influenced by climatic changes due to short sediment storage response time

experimental study of reservoir turbidity current a series of experiments were conducted in a flume to study the hydraulic characteristics of the turbidity current in a reservoir kaolin was used as the suspended material the plunge points were found to be unstable initially as the experiment went on it moved downstream from the incipient plunge location and finally reached a stable location the densimetric froude number of the incipient and the stable plunge points are approximately equal to 1 0 and 0 6 respectively it is found that the length of the plunge region is 15 times the water depth at the stable plunge point the volumetric flow rates in the plunge region increase 17 during the plunging process about 70 of the water discharge and 80 of the sediment discharge are confined to the denser layer a layer close to the channel bed with approximately constant concentration the thickness of the turbidity current increases while the layer averaged velocity and concentration decrease in the longitudinal direction the layer averaged velocity has the smallest variation rates equations for the dimensionless velocity and concentration profiles are obtained

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

compatibility of reservoir sediment flushing and river protection in this paper we propose a system of numerical models for the compatibility assessment of reservoir sediment flushing and protection of downstream river environments the model system is made up of two simulation models the first model simulates soil erosion in watershed slopes and sediment transport in the tributary of the reservoir by means of a weighted essentially nonoscillatory weno method which is conservative and fourth order accurate in space and time the second model simulates velocity and suspended solid concentration fields in the reservoirs this model is based on the three dimensional 3d numerical integration of motion and concentration equations expressed in contravariant form on a generalized boundary conforming curvilinear coordinate system by using a conservative and higher order accurate numerical scheme the proposed system of models is applied to the pieve di cadore veneto italy reservoir and to its catchment area by comparing suspended solid concentrations that are discha

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

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

sediment transport dynamics near a river inflow in a large alpine lake sediment dynamics were investigated in lake maggiore italy with field observations from october to mid december 2012 three moorings were deployed in pallanza bay a small embayment on the western side of the lake near the toce river inflow to measure temperature and currents throughout the water column and suspended sediment concentration ssc was estimated with acoustic instrumentation river intrusions are shown to dominate observed ssc although a small amount of sediment resuspension was observed at the site of the shallowest mooring during a large wind event that produced strong upwelling of the thermocline followed by downwelling although vertical turbulent sediment flux is typically assumed to indicate resuspension and the upward transport of sediment w c 0 downward turbulent sediment flux was observed w c 0 near the bed during the largest observed intrusion event the downward turbulent sediment flux significantly contributes to net deposition rates which are one order of magnitude larger than rates of erosion measured during the two major events observed horizontal transport of sediment occurs in vertically confined layers due to buoyancy driven intrusions beneath the intrusions sediment settles out of the water column at settling rates that appear to be constant with depth based on acoustic doppler current profiler backscatter measurements the effective settling velocities needed to produce the observed vertical transport of ssc during an inflow intrusion are one order of magnitude larger than those due to the stokes settling velocity ws alone particle flocculation and possible convective instabilities may play a role in generating the large observed effective settling rates

observation and simulation of floodwater intrusion and sedimentation in the shichikashuku reservoir wash load sedimentation during flood events is studied through comprehensive field measurements at the shichikashuku reservoir japan and numerical simulation field data during a flooding event caused by a typhoon in 1996 are used to set the boundary conditions for the model and to verify the simulation in the field experiments continuous monitoring of inflow turbidity is shown to be an effective and reliable means of estimating sediment influx in the simulations the proposed model is shown to provide an accurate reproduction of floodwater currents and sedimentation in the reservoir the model achieves good accuracy and resolution by adopting an orthogonal curvilinear grid to discretize the reservoir and by solving the flow field using a three dimensional standard k e turbulence model the simulation reproduces the observed velocity turbidity and distribution of sedimentation well demonstrating the potential utility of such models in the management of reservoir water quality and sedimentation

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

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

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

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

three dimensional modeling of turbid density currents in imha reservoir south korea abstractthe propagation dynamics of the turbid density currents of the imha reservoir is investigated using the flow 3d computational fluid dynamics code the renormalization group rng e turbulence scheme in a reynolds averaged navier stokes rans framework was applied for the field investigation a new particle dynamics algorithm was developed and coupled with the flow 3d model to simulate the settling of sediment particles and the model was tested with field measurements during typhoon ewiniar the coupled model correctly predicted the spatial and temporal evolution of intrusive density currents with plunging flows at a depth of 20 m and interflows propagating downstream at about 0 2 m s the simulation results with sediment settling demonstrated that the ratio of transported to initial sediment concentration c0 ci varies as a function of particle size ds and sediment concentration density currents can be classified into three regimes 1 a suspended regime ds 10 m where interflows will

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

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

scaling analysis of multipulsed turbidity current evolution with application to turbidite interpretation deposits of submarine turbidity currents turbidites commonly exhibit upward fining grain size profiles reflecting deposition under waning flow conditions however more complex grading patterns such as multiple cycles of inverse to normal grading are also seen and interpreted as recording deposition under cycles of waxing and waning flow such flows are termed multipulsed turbidity currents and their deposits pulsed or multipulsed turbidites pulsing may arise at flow initiation or following downstream flow combination prior work has shown that individual pulses within multipulsed flows are advected forward and merge such that complex longitudinal velocity profiles eventually become monotonically varying although transition length scales in natural settings could not be predicted here we detail the first high frequency spatial vertical streamwise and temporal measurements of flow velocity and density distribution in multipulsed gravity current experiments the data support both a process explanation of pulse merging and a phase space analysis of transition length scales in prototype systems the point of merging corresponds to the transition in any deposit from multipulsed to normally graded turbidites the scaling analysis is limited to quasi horizontal natural settings in which multipulsed flows are generated by sequences of relatively short sediment failures 10 km sequences of breaches or where pulsing arises from combination at confluences of single pulsed flows such flows may be responsible for the pulsing signatures seen in some distal turbidites 100 km from source

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

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

snow mapping and classification from landsat thematic mapper data use of satellite multi spectral remote sensing data to map snow and estimate snow characteristics over remote and inaccessible areas requires that we distinguish snow from other surface cover and from clouds and compensate for the effects of the atmosphere and rugged terrain because our space borne radiometers typically measure reflectance in a few wavelength bands for climate modeling we must use inferences of snow grain size and contaminant amount to estimate snow albedo throughout the solar spectrum although digital elevation data may be used to simulate typical conditions for a satellite image precise registration of an elevation data set with satellite data is usually impossible instead an atmospheric model simulates combinations of thematic mapper tm band radiances for snow of various grain sizes and contaminant amounts these can be recognized in tm images and snow can automatically be distinguished from other surfaces and classified into clean new snow older metamorphosed snow or snow mixed with vegetation

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

interpretation of sar imagery of the greenland ice sheet using coregistered tm imagery abstract landsat thematic mapper tm images are coregistered with a seasat synthetic aperture radar sar image of the margin of the greenland ice sheet and used in the interpretation of features in the sar data a winter tm image is used to provide topographic information while a summer tm image provides information on the pattern of melt and the location of lakes many previous hypotheses are confirmed and some are rejected it is shown that 1 the snow line is easily d detectable in sar imagery 2 areas of higher radar backscatter in the bare ice zone most often correspond to areas full of crevasses too small to resolve in the sar image 3 subsurface scattering causes significantlblurring of features in sar imagery 4 the band of surface lakes occurring above the snow line corresponds to at least a portion of the wet snow zone 5 water is the cause of reduced radar backscatter above the snow line and 6 the fact that water causes reduced radar backscatter above the snow line permits the use of backscatter variations as a proxy indicator of surface topography in snow covered regions experiencing melt

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