geology geochemistry and paleomagnetism of the cheneaux metagabbro helikian of southern quebec and eastern ontario abstract the cheneaux metagabbro helikian is a slightly to moderately metamorphosed intrusive body in the southern grenville province the paleomagnetic study undertaken on it yielded 3 main components of magnetization the first component w d 249 i 45 with a corresponding paleopole of 44 e 6 s may represent the time of emplacement or an early phase of mechanical deformation the second component w d 258 i 34 with a corresponding paleopole of 8 e 21 n represents a main metasomatic event related to the initial phase of the grenville orogeny the third component se d 135 i 30 with a corresponding paleopole of 32 w 16 s is related to the final phase of the grenville orogeny the estimated age of acquisition of magnetisation for these 3 components are 1300 1075 and 950 ma respectively the significance of the new set of data in relation to the established grenville apparent polar wandering curve is discussed finally a fourth component ne d 37 i 41 with a corresponding paleopole of 37 e 53 n represents a much younger event its pole may be located on the late hadrynian track 750 ma

paleomagnetism of the cambrian moore hollow group texas evidence for a primary magnetization carried by detrital magnetite petrographic and paleomagnetic studies of the cambrian moore hollow group texas establish the presence of three magnetic minerals and several components of magnetization hematite is the dominant carrier of the magnetic remanence in most samples and is primarily a diagenetic replacement of preexisting fe bearing phases hematite dominant samples have characteristic directions which range from shallow easterly to shallow southerly this distribution suggests multiple components of magnetization acquired at various times in the paleozoic the paleopole obtained for an unmineralized breccia 135 7 e 15 4 n represents either a chemical remanent magnetization of middle ordovician to early silurian age or a vector addition of two or more cambrian through silurian components mineralized breccias at the silver creek mississippi valley type deposit yield a permian direction 117 0 e 51 6 n because hematite in this deposit predates mineralization the permian age represents a maximum age for the mineralization at silver creek many samples contain goethite as a replacement of fe dolomite glauconite and pyrite the direction of magnetization carried by goethite is near the present geomagnetic field or in a minority of samples antipodal to it rounded ti magnetite was identified by sem in magnetic extracts from siliciclastic poor facies irm acquisition and thermal demagnetization confirms the presence of magnetite in these samples some samples contain an apparent modern viscous remanent magnetization the dominant component 72 samples 10 sites has a site mean declination of 93 8 and inclination of 14 5 with an 95 8 5 corresponding to a pole position at 163 0 e 0 6 s the presence of detrital magnetite polarity changes lack of evidence for prolonged burial and the proximity of this pole position to other late precambrian cambrian poles suggest that this component of magnetization indeed reflects the late cambrian paleopole for cratonic north america

reply to discussion on dissolution channels in quartz and the role of pressure changes in gold and sulfide deposition in the archean greenstone hosted hutti gold deposit karnataka india by kolb j rogers a and meyer fm the discussion on our paper pandalai et al 2003 by kolb et al 2003 doi 10 1007 s00126 002 0345 9 raises important issues related to the genesis of the hutti gold deposit the main points raised by kolb et al 2003 relate to the estimation of p and t of ore formation and the chronological and genetic order of vein formation in the shear zones at hutti as identified by them rogers et al 2002 and kolb et al 2003 recognize five stages of deformation d1 d5 one prograde m1 and two retrograde m2 m3 stages of metamorphism and four stages of quartz vein formation with attendant alteration rogers et al 2002 and kolb et al 2001 2003 do not refer to previously published literature on structural studies of the hutti maski schist belt nevertheless the deformational stages identified by kolb et al 2003 need to be placed in the framework of the regional tectonic history of the hutti maski belt structural studies on this belt are reported in roy 1979 1991 and biswas 1990 these studies identify three phases of deformation and metamorphism that are codified here as d1 d2 d3 and m1 m2 m3 respectively to distinguish them from those of kolb et al 2003 a brief summary of the structural framework is given below before discussing the points raised in kolb et al 2003 the first deformational stage d1 led to the development of tight isoclinal f1 folds penetrative schistosity s1 synchronous prograde metamorphism m1 and syntectonic emplacement of tonalite and tronjhemite around the hutti maski belt additionally mylonite and mylonitic foliation sm developed in narrow shear zones on account of the parallelism of sm with s1 it is inferred that they developed during the phase of d1 deformation the prograde m1 phase of metamorphism is low grade greenschist facies but rises to medium grade amphibolite facies with development of garnet and cordierite in the north eastern peripheral zone of the greenstone belt the d2 deformation stage gave rise to small and large scale f2 folds with subvertical axial planes s2 and axes that plunge 50 to 60 nw f2 folds bring f1 isoclinal folds into parallelism with f2 limbs and are responsible for the map pattern of the exposed rocks s2

middle cambrian paleomagnetism of the avalon terrane in cape breton island nova scotia interbedded volcanics and sediments of the bourinot group mid cambrian in central cape breton island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the avalon terrane with respect to the north american craton both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations dual polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of d 293 i 66 n 9 k 41 8 95 8 1 the sediments reveal upon detailed thermal demagnetization complex multivectorial remanences however a few samples show characteristic components and a large number of samples 38 have intersecting great circle remagnetization trajectories when combined this yields a mean direction of d 267 i 65 k 16 mean radius of error ellipse is 8 5 the mean directions for the bourinot rocks correspond to a paleolatitude of 49 s 11 which is different from the range of possible values for the predicted mid cambrian paleolatitudes for nova scotia 29 s to 18 n inferred from the north american apparent polar wander path this discrepancy between predicted paleolatitudes and the observed paleolatitude implies that nova scotia was not part of north america during the mid cambrian but rather they were separated by an ocean with a minimum width of 1000 km 20 10 this interpretation of the relative positions of nova scotia and north america based on paleomagnetism is consistent with the geological record in the northern appalachians and supports previous interpretations of the dissimilar early paleozoic faunas in avalon and north america it is paleomagnetically quite plausible that the avalon terrane formed part of a larger continent comprising africa gondwana and armorica during the cambrian

radiation induced defects in kaolinites indirect assessment of radionuclide migration in the geosphere abstract defect centres are demonstrated in natural kaolinites by means of electron paramagnetic resonance the irradiation origin of these paramagnetic defect centres pdc is demonstrated by artificial irradiation four types have been used x rays rays he and pb2 ion beam implantation of kaolinites representing different formation conditions hydrothermal weathering synthesis several types of pdc designated a a and b centres are clearly identified and can be separated with respect to their relative stability one a centre is stable over geological periods moreover it is shown that defect centres in kaolinites reflect effectively the cumulative radiation dose the potential use of tracing radionuclide migration through pdc in kaolinite has been tested on three low temperature alteration systems some associated with u concentrations 1 a hydrothermal alteration system nopal i chihuahua mexico and 2 two weathering systems in tropical areas latentes from brazil and cameroon the kaolinites investigated show a significant concentration of pdc which corresponds with background natural irradiation the different types of pdc detected may form a record of successive irradiations of kaolinite linked directly to geochemical conditions prevailing during and after kaolinite growth because of their different thermal stability these different centres can be considered as being formed at different periods this makes a unique tool for tracing the dynamics of the transfer of radionuclides and allows the use of kaolinite as a very sensitive in situ dosimeter because some of the geological settings chosen for nuclear wastes are located in environments containing kaolinites pdc dosimetry could be directly useful to a safety assessment of these wastes

automatic grain boundary detection and grain size analysis using polarization micrographs or orientation images abstract a simple procedure for creating grain boundary maps from petrographic thin sections termed the lazy grain boundary lgb method has been devised it is based on a set of macro commands programmed for nih image a public domain image processing software the grain boundary detection is based on gradient filtering the procedure makes use of multiple input images sets of regular polarized micrographs or where possible orientation misorientation images on each image of a given input set only the most significant grain boundaries are detected and by combining those a single grain boundary map is obtained image models and criteria for grain boundary identification are discussed briefly the principal aim of the lgb method is to facilitate the production of large grain boundary maps containing several thousand cross sectional areas to provide statistically valid samples for grain size analysis a section of black hills quartzite was used to test the lgb method five different input sets were prepared and used for automatic lgb analysis one set was additionally used for an interactive analysis and for a manual tracing from all seven grain boundary maps the 2 and 3 d grain size distributions were calculated the results are compared and the different image sets and procedures are rated for precision over and undersegmentation and speed of preparation

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

dome formation and extension in the tethyan himalaya leo pargil northwest india metamorphic dome complexes occur within the internal structures of the northern himalaya and southern tibet their origin deformation and fault displacement patterns are poorly constrained we report new fi eld mapping structural data and cooling ages from the western fl ank of the leo pargil dome in the northwestern himalaya in an attempt to characterize its post middle miocene structural development the western fl ank of the dome is characterized by shallow west dipping pervasive foliation and wnw ese mineral lineation shear sense indicators demonstrate that it is affected by east west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting sustained top to northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation garnet and kyanite indicate that the leo pargil dome was exhumed from the mid crust 40 ar 39 ar mica and apatite fi ssion track aft ages constrain cooling and exhumation pathways from 350 to 60 c and suggest that the dome cooled in three stages since the middle miocene 40 ar 39 ar white mica ages of 16 14 ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation aft ages between 10 and 8 ma suggest that ductile fault displacement had ceased by then and aft track length data from high elevation samples indicate that the rate of cooling had decreased signifi cantly we interpret this to indicate decreased fault displacement along the leo pargil shear zone and possibly a transition to the kaurik chango normal fault system between 10 and 6 ma aft ages from lower elevations indicate accelerated cooling since the pliocene that cannot be related to pure fault displacement and therefore may refl ect more pronounced regionally distributed and erosion driven exhumation

paleomagnetism of lower cambrian volcanics and a cross cutting cambro ordovician diabase dyke from buckingham quebec the buckingham area quebec is characterized by cambrian volcanic flows 573 32 ma cut by a cambro ordovician diabase dyke which is dated at 497 14 ma by k ar method the volcanics yield multicomponent magnetizations a hard characteristic magnetization and a relatively soft one these components could be separated easily and the intensity decay curves of the different components could also be obtained separately using zijderveld s method three different groups of site mean characteristic magnetizations can be distinguished each representing a different stratigraphic level the paleopoles derived from these magnetizations are from oldest to youngest 186 e 10 s 173 e 01 n and 154 e 06 n this is interpreted as a rapid motion of the paleopole for the north american craton during the early cambrian the paleopole derived from the hard characteristic dyke direction is 123 e 03 s this magnetization direction was also found at one of the sites in the volcanics this means that the contact test

oroclinal bending in the caledonides of western ireland new palaeomagnetic data from the caledonides of western ireland indicate that the silurian rocks of south mayo underwent oroclinal bending following folding in siluro devonian time bending was accommodated on faults cutting the silurian sequence and was driven by strike slip motion across the antrim galway line a recently recognized major curvilinear lineament we propose that the silurian rocks in the east of the region suffered up to 30 clockwise rotation increasing towards the west to approximately 70 this newly inferred differential rotation of the silurian rocks about a sub vertical axis implies that the underlying dalradian rocks of the connemara massif to the south must also have undergone clockwise rotation in late silurian early devonian time this is consistent with published palaeomagnetic data from the connemara gabbro and accounts for the swing in strike of the caledonian orogen in this part of western ireland from this we deduce that the swing in strike of the caledonian rocks elsewhere on the west coast of ireland might have a similar explanation

the problem of strain marker centers and the fry method abstract the fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale despite homogeneous behavior at a population scale this difference results from the post deformation centers not coinciding with the original pre deformation centers the amount of error may rapidly approach 50 for small strains in ideal cases but is likely to be in the range of 20 30

primary fabric ellipsoids in sandstones implications for depositional processes and strain analysis abstract when measuring strains in deformed clastic rocks geologists usually assume one or more of the following 1 objects were initially circular 2 objects were initially non circular but had uniform orientations 3 object populations initially had a fabric but that this fabric had a symmetrical relationship to bedding or that 4 initial fabrics are recognizable even after straining to evaluate these assumptions we measured 43 fabric ellipsoids in non strained poorly sorted sandstones from four depositional settings dsdp core 174 fan and abyssal plain deposits off the coast of oregon the cretaceous great valley sequence california fore arc deposits the cretaceous pigeon point formation california accreted and slumped turbidites and crossbedded sandstones from intracontinental basins in california and australia our results indicate the following 1 in two dimensional cuts individual grains have variable but usually small axial ratios these data and microstructures in grains and matrix cements also argue against significant compaction during burial in these clast supported sand rich units and instead suggest that the primary fabrics largely reflect grain packing processes during deposition and or slumping and packing in the case of the pigeon point samples

the fry method applied to an augen orthogneiss problems and results the application of the fry method to measure strain in deformed porphyritic granites is discussed this method requires that the distribution of markers has to satisfy at least two conditions it has to be homogeneous and isotropic statistics on point distribution with the help of a morishita diagram can easily test homogeneity isotropy can be checked with a cumulative histogram of angles between points application of these tests to undeformed mte capanne granite elba and to deformed randa orthogneiss alps of switzerland porphyritic granite reveals that their k feldspars phenocrysts both satisfy these conditions and can be used as strain markers with the fry method other problems are also examined one is the possible distribution of deformation on discrete shear bands providing several tests are met we conclude that the fry method can be used to estimate strain in deformed porphyritic granites

paleomagnetism of the catoctin volcanic province a new vendian cambrian apparent polar wander path for north america the vendian cambrian segment of the lauretian apparent polar wander path apwp has been poorly constrained and the subject of some controversy the catoctin volcanic province in central virginia is well dated at 570 35 ma rb sr and 597 18 ma u pb and therefore presented an excellent paleomagnetic target for resolving the laurentian vendian cambrian apwp a total of 206 samples from 32 sites were collected from the catoctin basalts feeder dikes and sills the study revealed three ancient directions of magnetization the youngest c component fails the fold test and yielded a characteristic in situ direction of d 147 i 44 k 21 95 9 the corresponding paleopole falls along the middle ordovician segment of the laurentian apwp and we consider this component to be the result of a taconic remagnetization the second component the b component is carried by hematite exhibits dual polarities and passes a fold test the tilt corrected b component characteristic direction is d 92 i 17 k 16 95 13 the corresponding paleopole at 4 s 193 e falls near a well established late cambrian 505 ma pole for laurentia and we consider this component to be a remagnetization during a late cambrian tectonic event in the central appalachians the third component isolated in the catoctin basalts the a component yields a tilt corrected mean of d 68 i 84 k 59 95 9 this component passes a fold and reversal test a suite of samples was collected from two catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test the a pole at 43 s 128 e falls significantly away from previously proposed vendian poles for laurentia a reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new apwp this new apw track indicates that laurentia was located near the pole during the interval 615 580 ma and drifted rapidly 16 cm yr 1 toward its late cambrian equatorial position

a spatial statistics approach to the quantification of finite strain variation in penetratively deformed thrust sheets an example from the sheeprock thrust sheet sevier fold and thrust belt utah abstract strain is an important component of the total displacement field in the emplacement of a thrust sheet the finite strain tensor in a penetratively deformed thrust sheet is a spatial variable i describe a method for quantitative estimation of the finite strain variation in thrust sheets by applying spatial statistics analysis on strain data collected from a part of the sheeprock thrust sheet in the southern sheeprock mountains and the west tintic mountains north central utah strain was measured in the quartzites of the sheeprock thrust sheet and the spatial statistics method is illustrated using the x z strain axial ratios the sheeprock thrust sheet was penetratively deformed during sevier age fault propagation folding i quantified finite strain from quartzites using the modified normalized fry method and calculated the three dimensional strain ellipsoid from the quartzites using three orthogonal thin sections from each oriented field sample the variation of finite strain in the sheeprock thrust sheet was best represented by an exponential semivariogram model which i used to predict values of strain from unsampled locations by ordinary kriging cross validation showed that in general the predicted and measured values show good agreement within 1 of each other the sampled space was contoured using the measured and predicted strain values to obtain a detailed finite strain variation pattern in a part of the sheeprock thrust sheet

fabric and kinematic study of the bites orthogneiss southwestern moravia result of large scale northeastward shearing parallel to the moldanubian moravian boundary abstract the body of the bites orthogneiss bo in southwestern moravia was affected by ductile shear deformation of probably late variscan age north to northeast directed shearing is indicated by the orientation of sheath fold axes s c fabric strong asymmetrical c axis preferred orientation and ne dipping shear bands deformational structures stretching lineation mylonitic foliation and sheath folds associated with thrust wrench tectonics are connected with lower amphibolite to green schist facies metamorphism a strain study shows complex fabric development from the core to the boundaries of the bo thrust sheet fabrics in the core of the bo body are characterized by oblate strain ellipsoids low strain intensity and quartz c axis fabrics that can be approximated to small circle girdles centred around the z axis of finite strain strain ellipsoids near the boundaries of the bo thrust sheet are mostly prolate with high strain intensity and quartz diagrams show single or incomplete crossed girdles highly asymmetrical with respect to the structural framework the coaxial strain history in the core and the noncoaxial history which prevails at the boundaries of the bo thrust sheet are interpreted as a result of different strain paths in the framework of a crustal scale shear zone

study of two alteration systems as natural analogues for radionuclide release and migration abstract u deposit hosted in hydrothermally altered tuffs in mexico together with weathering profiles from cameroon were studied as natural analogues of radionuclide release and migration using petrological and spectroscopic methods infrared and electron paramagnetic resonance we have distinguished successive secondary mineral parageneses and the behaviour of radionuclides in the u deposit the mineral parageneses show that uranium migration is mainly controlled by the redox potential and silica activity of the altering solutions the high silica content of the solutions is caused by the intense alteration of volcanic rocks two types of secondary clay mineral parageneses are evidenced a kaolinization intense where uranium is accumulated in the welded tuffs and a smectitization mainly developed in the underlying weakly welded tuffs several types of kaolinite have been defined according to their genesis fillings in fissures and feldspar pseudomorphs their location relative to a breccia pipe where uranium has accumulated core and rim of the pipe surrounding rhyolitic tuffs and particle morphology structural order and substitutional fecontent it is shown that the variations of the concentration of paramagnetic defect centres always more than ten times as important than those measured in weathering kaolinites are only correlated to the location of the kaolinites the highest values correspond to the breccia pipe kaolinites e g kaolinites located in the uranium accumulation zones moreover one or two main defects centres are detected depending on the intimate association of kaolinites with uranium bearing minerals besides in weathering kaolinites from u depleted laterites defect centre concentrations are correlated to the total fe203 content in bulk samples this means that the defect centre acts as a memory of the travel of uranium when this element was sorbed onto iron gels in the first stage of weathering it is concluded that paramagnetic defect centres in kaolinites might allow an efficient fingerprint of successive irradiations in the natural analogues under study and could be an useful tool to control radionuclides migration through kaolinite containing clayey materials such as those used for waste repository a better understanding of radiation efficiency as well as accurate dose ratekaolinite containing clayey materials such as those used for waste reposit estimation are needed for a quantitative tracing of the migration ofa better understanding of radiation efficiency as well as accurate dose ratekaolin radionuclide elements with this aim a simulation has been undertaken withestimation are needed for a quantitative tracing of the migration of various radiations sources we have determined for each irradiation the parameters of the paramagnetic centres created in order to understand the way they are forming the knowledge of the parameters governing the formation and the stability of the radiation centres in kaolinites allow to use this mineral as a natural dosimeter

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

some guidelines for the practical application of fry s method of strain analysis abstract a systematic analysis of patterns in artificial and natural point distributions is presented in order to demonstrate how the nature and degree of anticlustering of point distributions affect the character of fry diagrams derived from them ideally anticlustered artificial point distributions illustrate the nature of the dependence between the degree of anticlustering and the minimum number of points required in such a distribution if it is to reveal strain accurately by fry s method in addition these distributions provide straightforward results which aid in the interpretation of less ideally anticlustered natural point distributions the results from the artificial distributions may be used to estimate the minimum number of points required for strain determination in natural distributions of varying degrees of anticlustering as long as the anticlustering is sufficiently strong however application of the results from the artificial distributions to less strongly anticlustered natural point distributions shows not only that the existence of a vacancy field is an insufficient criterion for determining whether or not fry s method is appropriate for strain determination but also that the nature of the anticlustering in these natural distributions limits the effectiveness of enhancing strain definition by increasing the number of points composing the distribution

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

palaeozoic palaeogeography of laurentia and its margins a reassessment of palaeomagnetic data summary the recognition of wide spread remagnetization events during the palaeozoic has greatly added to the problems of reconstructing the palaeozoic drift history of laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data in an attempt to overcome some of these difficulties in the light of these developments a revised apparent polar wander apw path has been constructed following a rigorous evaluation of all published palaeozoic palaeomagnetic data from laurentia and its palaeozoic margins i e north america northern britain northern ireland and greenland the use of data from the laurentian margins when corrected for the opening of the north atlantic has resulted in a revision of previously published palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for laurentia are in good agreement with faunal and lithological data a comparison of this new apw path with other apw paths from avalonia and baltica reveals a very close agreement between all three paths from mid silurian times onwards indicating that the closure of the iapetus ocean and the tornquist sea was essentially complete by this time laurentia occupied equatorial latitudes from cambrian to early silurian times and its drift history was dominated by anti clockwise rotation up to 0 7 ma 1 and low latitudinal drift rates up to 3 cm yr 1 late silurian and early devonian times were characterized by increased rates of anti clockwise rotation up to 1 3 ma 1 and southerly directed latitudinal drift rates up to 13 cm yr 1 of the continent which place laurentia in moderate southerly latitudes 30 40 s by the early devonian these palaeolatitudes are comparable with those from avalonia and baltica for this time indicating that the amalgamation of eurmerica had already taken place by late devonian time laurentia had begun to drift north rates up to 5 cm yr 1 towards equatorial latitudes by mid carboniferous time

studies of strain and rheology of conglomerates abstract conglomerates can show significant strain variations among their component rock types previous studies have exploited this feature as a method of quantifying the effective viscosity ratios among different rock types using inclusion matrix models that we review here we further investigate the rheology of conglomerates via models of multiphase mixtures of spherical clasts with varying phase fractions using idealised models with two to four phases we can quantify the influence of phase viscosities and volume fractions on the bulk viscosity and reveal how this controls the strain partitioning among the different phases we consider practical methods of analysing conglomerates and choose r f analysis for measuring clast strain and fry analysis for whole rock strain the methods are illustrated through case studies of two very different conglomerates the port askaig tillite scotland and cesson conglomerate brittany we define the important rock phases in each and assess their strain variations although providing strain measurements of regional interest our main purpose is to quantify the viscosity ratios among the component rocks we find that among the most common rock types semipelite quartzite psammite volcanics granite the viscosity ratios span about one order of magnitude agreeing with previous studies these small numerical values for viscosity ratios among many rock types in many conglomerates lead us to conclude that these rocks each deformed at approximately constant viscosity i e as linearly viscous rather than according to a power law

ability of the fry method to characterize pressure solution deformation abstract the applicability of the fry method to marker distributions deformed by pressure solution is examined four types of pressure solution deformation were modeled they differ by the spacing of the dissolution surfaces relative to the distance between marker centers the amount of volume loss and whether or not markers in contact with dissolution surfaces are dissolved completely the results show that the fry method correctly characterizes each type of deformation but without additional information such as the amount of volume loss it can be used only where the deformation is constant volume and where dissolution surfaces are more closely spaced than marker centers

deformation within a strike slip fault network at westward ho devon u k domino vs conjugate faulting abstract a system of ne trending left lateral faults and nw trending right lateral faults at westward ho north devon u k cut steeply dipping 60 strata faults were accurately mapped in the field and from aerial photography and lateral separations of marker beds measured along the fault traces these data are used to examine the displacements within the network of interacting faults and to calculate variations in the density and relative proportions of the fault sets the displacements are also used in a tensor analysis of the strain and together with block rotations used to restore the deformation the results show a range of heterogeneity within the fault network both in terms of the fault patterns and strain some sub areas show a dominance of one fault set with regularly spaced larger displacements separating relatively weakly deformed blocks with smaller antithetic faults within these areas up to 20 rotation of the faults and bedding produces a domino style deformation that accommodates up to 15 extension the domino regions are separated by areas of conjugate faulting in which both sets of faults are equally developed and have similar displacement ranges conjugate areas have little or no rotation of the bedding and generally lower strains than domino regions

estimating uncertainty in normalized fry plots using a bootstrap approach abstract the normalized fry method is a powerful and commonly used tool for measuring fabric in aggregates of packed grains the significance of these parameters is often unclear because the associated uncertainty is unknown basic statistical hypotheses such as deciding if a sample has a fabric or if the fabrics of two samples are significantly different requires knowledge of associated uncertainty for this study a bootstrap version of the normalized fry method was developed this program randomly selects normalized center to center distances with replacement from the population of all possible center to center distances for each sample 100 bootstrap normalized fry plots were constructed using different combinations of center to center distances the variation of fabric parameters for these 100 analyses is used to estimate the uncertainty associated with the sample results of bootstrap analyses of sandstones oolitic limestones and synthetic data sets show considerable variation in fabric parameter uncertainty apparently related to both lithology and the degree of fabric development fabrics with axial ratios less than 2 0 appear only to be significant to one decimal place the variation of fabric uncertainty makes it important to determine the uncertainty associated with individual samples the deformation model for a field example developed based on conventional normalized fry plots needed to be more complicated than deformation models that allowed for variation of fabric parameters

circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from laurentia abstract the paleogeography of the precambrian cambrian transition is still poorly known but is fundamental for understanding late neoproterozoic climate extremes and the spatial associations of the rapidly evolving ediacaran and cambrian fauna this period has also been proposed to host unusual geodynamic events such as rapid plate motions and or the bulk tumbling of the earth with respect to its spin axis called true polar wander tpw new paleomagnetic and 40ar 39ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western quebec the mont rigaud and the chatham grenville stocks are related to a failed rift arm of the iapetus ocean both intrusions bear hornblendes which provide overlapping 40ar 39ar plateau ages of 533 2 1 1 ma mont rigaud and 531 4 3 4 ma chatham grenville corresponding to early cambrian nemakit daldynian time characteristic paleomagnetic reversed directions from both intrusions are easterly and shallow d 89 3 i 30 1 95 6 2 k 38 8 n 15 sites and the mont rigaud intrusion also carries antipodal normal polarity directions the combined result yields a north paleopole at location 11 9 s 184 6 e dp 3 8 dm 6 9 placing the montreal region at 16 4 s paleolatitude in the early cambrian in good agreement with late ediacaran and middle to late cambrian paleomagnetic results from laurentia laurentia likely resided at low southerly paleolatitudes throughout the cambrian ruling out a major inertial interchange tpw event for early to middle cambrian time a comparison of late ediacaran to middle cambrian paleomagnetic results from laurentia baltica siberia and elements of west gondwana implies that by early cambrian time a large paleolatitudinal gap existed between laurentia and its presumed conjugate rift margins thus placing a wide iapetus ocean between them the position of baltica within the circum iapetus paleogeography is contentious due to differing interpretations of the few paleomagnetic results during ediacaran time laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 ma to the south polar region at 590 575 ma and then rapidly back to low latitude by 565 550 ma the large and perhaps rapid apparent polar wander of laurentia and other continents in the ediacaran remains enigmatic and controversial

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp

multiple phases of deformation in the southern helanshan tectonic belt northern china abstract the southern helanshan tectonic belt shtb is surrounded by the alxa massif the ordos basin and the tibetan plateau and its phanerozoic structures records crucial phanerozoic deformation characteristics for studying the multi phased intracontinental deformation processes among these neighboring units combined with regional geology our structural analysis reveals that the shtb has experienced five phases of shortening deformation during the phanerozoic the first phase d1 is represented by e w trending overturned folds in the cambrian and ordovician strata these folds resulted from two episodes of n s shortening during the late cambrian early ordovician and late ordovician early devonian respectively which were attributed to the closure of the qilian ocean the second phase d2 is characterized by e w trending open folds involving the upper devonian strata which was the products of n s shortening caused by the assembly of the qilian ocean and the north china craton ncc at the end of devonian the third phase d3 is indicated by some wnw trending jura type folds in the carboniferous strata formed by nne ssw shortening during the late triassic early jurassic related to the collision between the ncc and the south china block the fourth phase d4 is inferred by nne trending shortening structures in the jurassic and its underlying strata generated by wnw ese shortening in the late jurassic echoing the paleo pacific subduction the fifth stage d5 is characterized by the ne sw shortening in the late miocene leading to the formation of many nw se striking thrust faults in the miocene and its underlying strata because the outward expansion of the tibetan plateau

paleomagnetism of latest cambrian early ordovician and latest cretaceous early tertiary rocks of the florida mountains southwest new mexico paleomagnetic investigations have been carried out on latest cambrian to early ordovician alkaline intrusions their contact intermediate to mafic composition metaigneous rocks and overlying lower paleozoic and early tertiary strata in the florida mountains southwestern new mexico a well defined predominantly normal polarity magnetization of high median destructive induction and high usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late cenozoic uplift combinations of stable endpoint and planar data are used in data analysis at the site level d 262 i 30 95 10 k 12 n 22 site means in addition to the presence of reverse polarity magnetizations at two sites the dispersion of site mean directions in these rocks vgp angular standard deviation of 19 6 suggests that considerable variation of the geomagnetic field has been averaged after correction for a combination of early tertiary laramide deformation and late tertiary basin range block tilting this magnetization gives a paleomagnetic pole at 6 s 169 e and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503 10 ma based on published u pb zircon isotopic age determinations a lack of well defined coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks as interpreted with rb sr isotopic data hematitic sandstones of the earliest ordovician bliss formation which directly overlie the crystalline rocks in portions of the range give dual polarity magnetizations whose directions suggest that these rocks have probably been remagnetized this remagnetization possibly occurred during deformation in late cretaceous tertiary time ordovician carbonate rocks of the el paso and montoya formations give only a holocene magnetization paleocene to earliest eocene hematite cemented clastic rocks of the lobo formation possess dual polarity magnetizations the origin of their magnetization is difficult to assess with the available data in total however paleomagnetic data from the bliss and lobo formations argue against any significant vertical axis rotation of the area since laramide deformation ending in early tertiary time the intermediate to mafic composition metaigneous rocks from the florida mountains provide a latest cambrian early ordovician paleomagnetic pole that compares favorably with other north american paleopoles satisfying several acceptance criteria the florida mountains data further support recent constructions of apparent polar wander paths for north america and thus the hypothesis that significant counterclockwise rotation of north america occurred during the ordovician