fold origin of the ne lobe of the sudbury basin canada evidence from heterogeneous fabric development in the onaping formation and the sudbury igneous complex structural analysis of the onaping formation a heterolithic impact melt breccia and the granophyre in the ne lobe of the 1 85 ga sudbury igneous complex sic assist in understanding the formation of the sudbury basin previously the lack of mesoscopic strain fabrics in the sic in contrast to pervasive fabrics in the onaping formation of the ne lobe led to interpretations of the shape of the ne lobe as primary we demonstrate that the structures of the onaping formation are consistent with deformation in a fold core controlled by the mechanically stronger granophyre of the ne lobe evidence for this interpretation includes the 1 presence of open structural domes and basins 2 geometry of planar mineral shape fabrics 3 variation in shape fabric intensity and 4 kinematics of prominent faults folding of the onaping formation affected the sic because fold geometry is based on dip data in the sic and fold related faults cut both the onaping formation and granophyre microstructural observations point to different temperatures during deformation in both units that are consistent with initiation of folding during cooling of the sic upon reaching middle greenschist facies metamorphic conditions

integrated paleomagnetic and upb geochronologic studies of mafic intrusions in the southern canadian shield implications for the early proterozoic polar wander path abstract integrated paleomagnetic and u pb geochronologic studies are critical to establishing well dated apparent polar wander paths in the precambrian by collecting samples for both paleomagnetism and u pb geochronology at the same sampling sites and conducting rigorous paleomagnetic field tests to ensure that magnetizations are primary it is now possible to establish the age of paleomagnetic poles with uncertainties of no more than a few million years we have conducted such integrated studies on mafic intrusions of the southern superior province and adjacent portions of the southern province of the canadian shield these studies yield paleomagnetic tiepoints with high precision ages for use in polar wander path construction in the 2220 to 2110 ma period they indicate a direction of polar wander contrary to that of the most widely used paths for the superior province and for the canadian shield as a whole thus current interpretations of the movement of the superior province and the relative location and movement of the superior province and other structural elements of the canadian shield during this period are no longer valid

precise u pb zircon ages for the molson dyke swarm and the fox river sill constraints for early proterozoic crustal evolution in northeastern manitoba canada precise u pb zircon ages have been obtained for samples from the molson dyke swarm and the fox river sill in ne manitoba canada the ages determined for the cross lake and cuthbert lake dykes are 1 883 7 1 5 1 7and 1 883 2 ma respectively and are in excellent agreement with the 1 882 9 1 4 1 5ma age obtained for the fox river sill these results support the contention that the emplacement of the fox river sill and the molson dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities the timing of early proterozoic mafic magmatism in the western superior province appears to be synchronous with igneous activity in other parts of the circum superior belt and in the trans hudson orogen to the west the emplacement of the molson dyke swarm at 1 883 ma indicates a 700 ma interval of quiescence between the final igneous activity that is recorded in the archean basement and dyke intrusion the presence of deformed equivalents of molson dykes in the thompson nickel belt indicates that the intense deformation in this belt occurred sometime after 1 883 ma

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

uplift deduced from remanent magnetization sudbury area since 1250 ma ago abstract two contacts between sudbury norite and northwest trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement a zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion or sampling surface at the time of magma emplacement remanence direction in one of the dikes and norite contact zones is not typical for the sudbury dike swarm of 1250 ma age and this contact is not further considered an earlier published result for a norite dike contact was reconsidered because of complicated dike geometry and included in this study in one of the four usable contacts the hybrid zone is represented by three samples in another by one sample and in the remaining two only the contact zone width could be used the final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287 c s d 13 and 267 c s d 11 calculated without and with a correction for viscosity of the host remanence respectively using a gradient of 26 c km for 1250 ma ago indicates a burial depth of9 5 2km at that time the fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of precambrian terranes the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages

effect of anisotropy on the paleomagnetic contact test for a grenville dike we report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the precambrian grenville province of canada directions of natural remanent magnetization nrm in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric using the measured anisotropy of laboratory induced thermoremanent magnetization trm the trm ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility ams ellipsoids but the anisotropy of remanence is more pronounced than the ams anisotropy correction moves nrm directions in the unbaked zone almost 20 bringing them close to typical grenvillian paleofield directions the anisotropy seems to be controlled by multidomain rather than single domain magnetic minerals nrm directions from the baked gneiss near the dike contact after anisotropy correction agree well with the nrm directions from the chilled margin of the early cambrian 570 3 ma dike this positive baked contact test is augmented by the presence of a hybrid zone in which country rock nrms are increasingly overprinted by the dike nrm as one moves from the unbaked toward the baked zone the dike nrm is therefore primary however the virtual geomagnetic pole vgp for the dike nrm 51 n 131 w dp 10 dm 12 n 24 samples or 41 n 153 w dp 13 dm 18 n 4 chilled margin samples does not fall near most established cambrian paleopoles post emplacement tectonic movement of the dike is unlikely it may be that the dike cooled and acquired trm during a geomagnetic field excursion because of its geologically short cooling blocking period of a few tens of years

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

crustal uplift in the southern superior province canada revealed by paleomagnetism abstract a change in the polarity of magnetization with depth in the 2 45 ga matachewan dyke swarm is used to document vertical crustal movements that occurred at 1 9 2 3 ga along the kapuskasing structural zone a 500 km long fault zone that transects the archean superior province of canada at shallow crustal levels dykes have a primary magnetization dominantly of one polarity but at greater depths 20 km down a polarity change occurs associated with the growth of exsolved magnetite in feldspar due to slow crustal cooling after cessation of matachewan igneous activity regions of the dyke swarm with one dominant polarity are separated from those with opposite polarity by major faults using this polarity distribution and associated variations in the intensity of feldspar clouding and hydrous alteration maps of the southern superior province are produced that display regional crustal tilting on which are superimposed more local fault bounded blocks associated with the kapuskasing zone some of these blocks have been recognized for the first time as a result of this study the paleomagnetic work has also shown that the matachewan swarm is regionally distorted both within and north of the kapuskasing zone and originally had a more radial disposition this widespread distortion suggests that the lower crust was still relatively ductile at the time of deformation perhaps due to high heat flow associated with the waning stages of the matachewan mantle plume beneath

deformation of the sudbury structure paleomagnetic evidence from the sudbury breccia abstract paleomagnetic results from fine grained samples of sudbury breccia collected from 28 sites around the northern half of the 1 85 ga sudbury igneous complex sic reveal the existence of two major components of magnetization a and b a carried by magnetite was formed close to the time of the sic and has a direction that is related to the dip of the norite unit within the different ranges of the sic component b mainly carried by pyrrhotite is a younger secondary magnetization of more problematical origin in part related to heating during the 1 24 ga sudbury magmatic event in the northwest part of the sic a is found in breccia clasts intermediate norite sublayer levack gneiss and matachewan diabase indicating that it is due to baking by the sic tilt correcting the west range by 36 the north range by 22 and the northern east range by 36 about axes parallel to local strike of the norite minimises the dispersion of the paleomagnetic data to give a pre folding direction of 199 4 78 8 the paleomagnetic data are similar to those obtained by morris morris w a 1984 paleomagnetic constraints on the magmatic tectonic and metamorphic history of the sudbury basin region in pye e g naldrett a j giblin p e eds the geology and ore deposits of the sudbury structure special vol 1 ontario geological survey pp 411 427 on the sic suggesting for the first time that the sic and the surrounding basement were deformed probably during the 1 9 1 7 ga penokean orogen as a single unit the un folding of the paleomagnetic directions indicates that sic limbs were already inclined a result of the original intrusion configuration or a syn folding remanence it is possible that the sic was originally flatter if the distribution of paleomagnetic directions is the result of flexural flow folding this explanation could also produce a similar distribution of paleomagnetic directions across the basin but cannot be quantified due to the lack of measurable fold related strain in breccia and sic units around the northern half of the basin

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

archeomagnetic results from southwest united states and mesoamerica and comparison with some other areas abstract archeomagnetic am results from southwestern united states can be represented by an archeomagnetic apparent polar curve aapc that is distributed near the axis of rotation and contained within 75 n latitude the aapc for the past 1600 yr is located mostly on the near side with mainly easterly declinations it is characterized by loops extending outward from the axis of rotation about 10 15 every 250 300 yr at 1350 1100 800 500 and 200 ad inclination of the magnetic field varied from a minimum of 40 to a maximum of 62 with maximum values occurring at 600 1100 1350 ad and modern minimum values occurred at 800 1250 and 1500 ad declination ranged from 18 e to 14 w with maximum easterly values occurring at 500 ad and during the nineteenth century the most westerly declination occurred about 1100 ad the maximum and minimum values of declination and inclination correspond in age with loop like extension of the aapc archeomagnetic paleointensity results suggest changes from 1 5 to 0 8 times the present field with variations occurring generally in unison with the changes in direction comparison of archeomagnetic data from mesoamerica england france japan bulgaria and china with the data from the southwest united states suggests some similarities and dissimilarities of the spatial and temporal position of the aapc early age archeomagnetic results to 8500 bc from the united states confirm a similar pattern of secular variation and limit the time of the most recent reversal some available global data suggest that 200 ad may have been a time of small divergence from an axial field inclined by 7 5 from the rotational axis

age of the grenville dyke swarm ontario quebec implications for the timing of lapetan rifting u pb baddeleyite and zircon ages for three diabase dykes from widely spaced localities within the grenville dyke swarm indicate a single age of emplacement at ma the 700 km long grenville dyke swarm located in the southeastern part of the canadian shield was emplaced syntectonically with the development of the ottawa graben this graben may represent a plume generated lapetan failed arm that developed at the onset of the breakup of laurentia other precisely dated lapetan rift related units such as the callander alkaline complex and the tibbit hill formation volcanic rocks indicate a protracted 36 ma period of rifting and magmatism prior to volcanism along this segment of the lapetan margin the age of the grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 ma franklin dykes and sills to the 615 ma long range dykes along the northern and northeastern margins of laurentia respectively thus the age for these dykes represents a key time marker for

broad scale proterozoic deformation of the central superior province revealed by paleomagnetism of the 2 45 ga matachewan dyke swarm an extensive paleomagnetic study of the 2 45 ga matachewan dyke swarm of the north american superior province suggests that the interior of an archean shield can undergo broad scale distortion as a result of later proterozoic orogenic activity around the craton margins data collected from over 300 sites of which 137 are reported here for the first time reveal that the dykes contain a dual polarity primary remanence that varies across the swarm in both inclination and declination these regional variations are statistically significant at the 95 confidence level and cannot be attributed to remagnetization or to magnetic anisotropy inclination variation is probably due to real or apparent polar wander during the emplacement of the swarm and may in part explain the declination variation as well however for dykes within and northwest of the kapuskasing structural zone ksz a positive correlation is found between regionally averaged values of declination and dyke trend here the dykes appear to hav

structural analysis of shatter cones from the slate islands northern lake superior shatter cones an unusual type of fracturing produced by intense shock are found widespread on the slate islands in northern lake superior the islands have been interpreted as the central uplift of an eroded meteorite impact crater about 30 km in diameter the cones are best developed in certain rock types keweenawan basalt flows and chilled margins of associated feeder dikes and archean diorite and foliated feldspar porphyry at certain sites cones show an elongate cross section and anomalous orientation caused by foliation induced elastic anisotropy in the host rock in general the cones point upward and inward toward the centre of the slate islands group after structural correction of some sites according to paleomagnetic data there is increased convergence of cone axes to a central focus the inferred impact point that occurs at a height of about 1 km above the present land surface

icesat elevations in antarctica along the 2007 09 norway usa traverse validation with ground based gps the 2007 09 norway usa traverse of east antarctica collected dual frequency global positioning system gps data at 5 s intervals on two of the traverse vehicles the traverse covered 2400 km from the coast to the vicinity of the amundsen scott south pole station in 2007 08 and a 2600 km route from the south pole to the coast in 2008 09 side traverses were also conducted in 2008 09 for a total of over 10 000 km of gps data between the two vehicles we use precise point positioning to post process our single receiver kinematic gps data analysis of data obtained while the vehicles were stationary shows individual solutions are accurate to ca 1 cm horizontally and 3 cm vertically we compare our gps elevations with those determined by the national aeronautics and space administration s ice cloud and land elevation satellite icesat a space based altimeter designed to measure ice elevation icesat accuracy is evaluated by cross over analysis mean differences calculated between dh dt corrected icesat data and gps derived surface elevations for two vehicles and two traverse seasons range from 12 to 2 cm within icesat s stated goal of 15 cm while 1 values of the same data imply that icesat s precision is ca 15 8 cm

palaeoenvironments and palaeoecology of the early pennsylvanian lancaster formation fern ledges of saint john new brunswick canada the pennsylvanian lancaster formation fern ledges of saint john new brunswick canada contains a world famous fossil biota largely unstudied since the works of hartt dawson matthew and stopes in the mid nineteenth to early twentieth century we present new data concerning biostratigraphy taxonomy palaeoenvironments and palaeoecology megafloral assemblages suggest a mid to late langsettian age for the succession making it approximately coeval with the classic joggins formation of nearby nova scotia facies analysis suggests deposition on a tectonically influenced coastal plain whose braided channels drained into a shallow brackish gulf most of the historical fossil collections are derived from flooding surfaces formed by abrupt subsidence events along the coastline three communities are recognized phoronids crustaceans and xiphosurans lived in brackish coastal waters a lowland community of gastropods insects arachnids and myriapods inhabited coastal forests coastal vegetation was dominated by shrubby cordaitaleans and pteridosperms whereas ferns sphenopsids and lycopsids were rare an upland or dryland community discernible from allochthonous assemblages comprised forests of giant cordaitaleans archaic pteridosperms and plants of uncertain affinity

the matachewan dyke swarm canada an early proterozoic magnetic field reversal abstract consistent age relationships between oppositely magnetized dykes of the 2 45 ga matachewan dyke swarm suggest that only a single magnetic field reversal occurred during the period of igneous activity the magnetic field throughout most of this time was characterized by a ssw declination and shallow negative inclination but reversed toward the waning stages of magmatism the new paleomagnetic data provide the oldest known magnetic reversal for which the relative reversal sense is known

generation of fragment rich pseudotachylite bodies during central uplift formation in the vredefort impact structure south africa abstract target rocks underlying the central portions of large terrestrial impact structures are characterized by the pervasive presence of fragment rich pseudotachylite bodies debates regarding the formation of these bodies include the origin of pseudotachylitic melts i e friction versus shock induced melting melt mobility causes of target rock fragmentation and timing of fragmentation and melt emplacement with respect to stages of cratering comprehensive structural analysis of pre impact mineral fabrics and properties of fragment rich pseudotachylite in the vredefort dome suggests that melt is allochthonous and was emplaced at an advanced stage of cratering into tensional fracture zones within the crater floor both concentration of bending strains imparted on target rocks during central uplift formation and thermal stresses induced by the emplacement of allochthonous melt led to fragmentation of target rock tensional fracture zones formed in an overall dilational strain field towards the end of cratering likely during collapse of the central uplift and constitute low pressure zones into which melt was forcefully drawn melt may have been drained from the overlying impact melt sheet or from sites within the crater floor and transported into fragment rich dilation zones our field based analysis failed to identify the presence of bona fide shear faults that could potentially have generated in situ frictional melts and fragments rather target rock fragmentation and melt generation that resulted in fragment rich pseudotachylite bodies are processes separated in space and time during cratering

a 100 km long paleomagnetic traverse radial to the sudbury structure canada and its bearing on proterozoic deformation and metamorphism of the surrounding basement abstract the 1850 ma sudbury igneous complex sic considered to be a composite melt sheet of a major meteorite impact has been deformed into an oval shaped basin known as the sudbury structure this paper explores to what extent this deformation has been communicated to the surrounding archean basement around the northern margin of the sic paleomagnetism of 2450 ma matachewan dykes and 1850 ma impact breccia along a traverse about 100 km long and normal to strike of the contact between the sic and the basement suggests that the basement beneath the nw corner of the sudbury structure has been tilted to the se within about 10 km of the contact at this distance a possible fault separates the tilted region from one that shows no evidence of tilting petrographically the dykes out to a distance of about 50 km distant from the sic are altered to upper greenschist facies of metamorphism with a fibrous amphibole replacing pyroxene and with loss of primary texture that characterizes less altered matachewan dykes at distances greater than 50 km the direction of magnetization found in the altered matachewan dykes is an overprint which is probably associated with regional metamorphism related to orogenesis or possibly with thermo chemical alteration associated with sic emplacement the direction of the component is compatible with an age of about 1 8 to 1 9 ga suggesting that the penokean orogen is the most likely cause if not the impact event the paleomagnetism of the breccias together with shatter cone orientation data suggests that within 10 km of the sic basement contact basement tilting to the southeast increases towards the sic

northeast trending early proterozoic dykes of southern superior province multiple episodes of emplacement recognized from integrated paleomagnetism and u pb geochronology integrated paleomagnetic and u pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of early proterozoic tholeiitic dykes of northeast trend in the southern superior province previously referred to collectively as preissac dykes it is demonstrated that they are readily separated on the basis of paleomagnetism into subsets referred to as the biscotasing and senneterre swarms in addition a pair of unnamed dykes may be associated with the north and northwest trending matachewan swarm farther west biscotasing dykes have a down west magnetization of single polarity with a corresponding paleopole at 27 8 n 136 7 w dm 12 3 and dp 9 4 senneterre dykes carry an up north or occasionally down south direction with corresponding paleopole at 15 3 s 75 7 w dm 7 0 dp 4 4 the senneterre direction is indistinguishable from the primary n1 remanence direction that dominates the magnetization of nipissing sills of the southern province paleomagnetic fiel

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

the paleomagnetic significance of new u pb age data from the molson dyke swarm cauchon lake area manitoba u pb geochronology paleomagnetism and petrography indicate that the molson dyke swarm along the western margin of the superior craton is a composite of at least two ages of intrusion the more extensive younger dyke set the molson swarm sensu stricto generally has a 030 trend is mainly pyroxenitic to noritic with subordinate diabase and has been related to rifting in a back arc environment during closure of the manikewan ocean at about 1920 1800 ma which culminated in the trans hudson orogen a u pb zircon age from one of these dykes located at cauchon lake indicates emplacement at 1877 7 150 4 similar to two previous u pb age determinations on molson dykes another dyke from cauchon lake yields a baddeleyite zircon u pb date of 2091 2 ma and appears to be part of an older mainly diabasic suite of east northeast trending dykes that may represent a continental rifting episode that preceded the opening of the manikewan ocean the new u pb age data require a revision to the interpretation of