paleomagnetism of archean rocks from northwestern ontario iii rock magnetism of the shelley lake granite quetico subprovince the shelley lake granite of northwestern ontario contains five magnetic phases deuteric and post crystallization hematites which are relatively abundant but carry only 1 4 of the natural remanent magnetization nrm primary magnetite in coarse 50 500 m grains both optically homogeneous and subdivided by hematite lamellae micrometre size secondary magnetite in chloritized biotites and submicrometre size magnetite whose presence is inferred from low blocking temperatures in thermal decay curves of the nrm the nrm is a composite of type 1 and type 2 remanences which differ in direction by about 90 see companion paleomagnetic paper both nrm components occur in normal n and reverse r polarities type 1 remanences 1n 1r have the hallmarks of multidomain md behaviour high blocking temperatures but low coercivities exponential alternating field af decay curves generally md results of the lowrie fuller test and md to transitional values 0 3 10 of the koenigsberger qn ratio further

the development of divergent margins insights from the north volcanic zone iceland abstract iceland is a natural laboratory where plate separation and mid ocean ridge formation can be studied directly in the field in this setting it is not fully understood how magma interacts with faulting and fissuring and how rift zones propagate in order to improve the understanding of how these processes operate we conducted a detailed field study about the kinematics and propagation of 33 holocene faults along the n s theistareykir rift north volcanic zone and mapped all tension fractures this rift hosts the holocene theistareykir central volcano for which ground deformation data insar and gps indicate an inflating shallow magma chamber throughout 2006 2008 analyses of the cumulative fault slip distribution at 696 sites show two opposite directions of fault rift propagation the first more developed is directed northwards and affects the area located north of theistareykir volcano whereas south of the volcano the rift propagates southwards tension fractures increase in frequency with respect to faults outwards from the volcano along a n s to nne ssw direction the presence of aligned vents dykes and topographic bulging along some of the n s structures and the active magma chamber below theistareykir volcano suggest that faults and tension fractures propagate following repeated dyke intrusions from the magma chamber outward along the plate margin the fact that the rift is more developed north of the volcano than south of it is interpreted as the effect of buttressing from another coterminous magma chamber sitting below a neighbouring central volcano other causes for the general observed pattern such as tectonic stresses mechanical interaction of faults and changes in the rheological characteristics of rocks are discussed as well

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

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

dyke intrusion into a pre existing joint network the aiguablava lamprophyre dyke swarm catalan coastal ranges abstract a structural analysis has been performed in the upper permian lamprophyric dyke swarm of aiguablava costa brava ne iberia emplacement of sub vertical dykes is related to the presence of a widespread joint network mostly developed during the cooling and decompression of the late variscan leucogranitic host rocks the joint pattern consists on multiple sub vertical joint sets among which two orthogonal sets at n23 and n113 are predominant a sub horizontal set is also prominent the sub vertical dykes have a mean n100 n125 trend which corresponds to the trend of one of the main joint sets however dyke segmentation is noticeable at the dm to cm scale and this is inferred to be related to dyke propagation and emplacement along the variably oriented pre existing joints a mean robust sub horizontal nne ssw net dilation direction was measured from matching dyke jogs markers in the host rock and analysis of maximum dyke thicknesses and this is in line with the minimum principal stress axis 3 derived from a three dimensional paleostress analysis from dyke orientations the inferred maximum principal stress axis 1 is sub vertical indicating that dykes intruded under conditions of tectonic extension furthermore a mohr construction allowed calculation of the stress ratio 0 29 close to a prolate stress ellipsoid and a driving pressure ratio r 0 27 which corresponds to a magmatic pressure almost equal to the intermediate principal stress axis 2 it is inferred that many of the pre existing joint sets were exploited by the magma the wnw ese joint set normal to 3 being the most favorable for dyke emplacement the present study highlights the structural control of a pre existing fracture network on emplacement of the aiguablava lamprophyres in the upper crust during late permian nne ssw brittle extension

emplacement and growth of alkaline dikes insights from the shonkinite dikes elchuru alkaline complex se india abstract the mesoproterozoic elchuru alkaline complex in the prakasam alkaline province pap eastern ghats belt se india is intruded by coeval shonkinite dikes magmas parental to both the host nepheline syenite and shonkinite dikes formed during ca 1350 ma continental rift related magmatism the shonkinite dikes show fine to medium grained equigranular to foliated textures with clinopyroxene biotite amphibole k feldspar perthite and nepheline as major mineral phases the dikes grew by inflation and coalescence of dike segments along intrusive steps anastomosis and subsequent coalescence and thermal mechanical erosion of the host rocks a few dikes resulted by filling the fractures created by shearing inflation of intrusive steps between magma segments by mode i opening is the major mechanism of shonkinite dike emplacement and growth however the shonkinite dikes also demonstrate the application of remote shear stress on the dike walls resulting in anastomosis entrapment and segmentation of tabular host rock xenoliths and in extreme cases brecciation of the host rocks under volatile rich and low viscous melt conditions fractal analysis of xenolith bearing dike further illustrates that thermal erosion was facilitated and accentuated by mechanical breakdown of host rock xenoliths due to interactions of tensile and shear forces cross cutting relationships between the shonkinite dikes suggest local deviation in the least compressive stress direction during dike emplacement the shonkinite dikes of elchuru demonstrate that spatially and temporally restricted high volatile low viscous alkaline magmas may have distinctly different styles of emplacement as controlled by interaction of near field magmatic tensile stress and far field tectonic shear stress in an evolving continental rift setting

magnetic felsic intrusions associated with canadian archean gold deposits felsic intrusions spatially and temporally associated with archean au deposits in the superior province of the canadian shield are magnetic due to high magnetite content in comparison to felsic igneous rocks in most of the archean terrain examples are found in au camps of hemlo red lake harker holloway mishibishu lake wawa missanabie geraldton matachewan and bouquet east cadillac these archean intrusions are comparable with phanerozoic magnetite series plutons accompanying metallic mineralization and magnetiterich intrusions hosting au rich porphyry deposits the evidence shows a distinct tectonic setting for the gold mineralization involving the generation of the particular magmas the association of the two indicates either the derivation of the ore constituents from the intrusions or the emplacement of the magmas along dilation zones which also provided fluid flow from depth the identification of magnetite rich felsic intrusions may aid in targeting favorable exploration areas for archean au deposits

geological constraints on the dynamic emplacement of cone sheets the ardnamurchan cone sheet swarm nw scotland cone sheets are a significant constituent of many central volcanoes where they contribute to volcano growth by intrusion and through flank eruptions although the exact emplacement mechanisms are still controversially discussed in particular it is not yet fully resolved whether cone sheets propagate as magma driven opening mode fractures or as shear fractures and to what extent pre existing host rock structures and different stress fields influence cone sheet emplacement to shed further light on the role of these parameters in cone sheet emplacement we use detailed field and remote sensing data of the classic ardnamurchan cone sheet swarm in nw scotland and we show that the cone sheets primarily propagated as opening mode fractures in the 1 2 plane of the volcanic stress field in addition more than one third of the ardnamurchan cone sheet segments are parallel to lineaments that form a conjugate set of nnw and wnw striking fractures and probably reflect the regional nw se orientation of 1 during emplacement in the palaeogene cone sheets exploit these lineaments within the ne and sw sectors of the ardnamurchan central complex which indicates that the local volcanic stress field dominated during sheet propagation and only allowed exploitation of host rock discontinuities that were approximately parallel to the sheet propagation path in addition outcrop scale deflections of cone sheets into sills and back into cone sheets also referred to as staircase geometry are explained by the interaction of stresses at the propagating sheet tip with variations in host rock strength as well as the influence of sheet induced strain as a consequence cone sheets associated with sill like segments propagate as mixed mode i ii fractures hence cone sheet emplacement requires a dynamic model that takes into account stress fields at various scales and the way propagating magma interacts with the host rock and its inherent variations in rock strength

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

new insights into thermal history from single grain 40ar 39ar analysis of biotite single grains of biotite extracted from two hand samples of two granitoid rocks from the red lake greenstone belt of northwestern ontario were examined using a high sensitivity 40ar 39ar laser dating system for variations in age for small grains a striking correlation is seen between the ages and the grain sizes as measured by the cylindrical radii with smaller samples yielding younger ages this effect saturates at a radius of about 225 m above which the integrated age is independent of grain size by combining this grain size effect with biotite diffusion parameters taken from the literature continuous cooling curves were derived for the region these curves suggest the region cooled at a rate of about 0 3 c myr between 2635 and 2570 ma with its ability to define a grain size effect the laser probe mass spectrometer system provides new insight into thermal history

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

palaeomagnetism and geochronology of the proterozoic dolerite dyke from southwest greenland indication of low palaeointensity summary intensity of the geomagnetic field during the archaean can potentially be used to study an evolution of the dynamo activity in the earth s core in order to investigate this issue we present new palaeomagnetic and geochronological results from the dolerite dyke which have been intruded into the archaean gneisses of nuuk area southwest greenland 40ar 39ar dating of the pyroxene grains from dolerite yield an age of 2585 21 ma the high temperature component which has been recognized as a characteristic direction is identified in 24 dolerite samples comparison of the palaeomagnetic directions from dolerite dykes with those from host gneisses suggests a primary origin for this component magnetic mineralogical and grain size investigations revealed a pseudo single domain magnetite as a dominant career of magnetization thellier palaeointensity determinations of 14 dolerite specimens yield a mean field value of 13 9 2 5 t strength of the virtual dipole moment 2 30 0 42 1022 am2 obtained from this study is about one quarter of the present earth s filed value a simulation based numerical model of the thellier experiments suggests that the presence of multidomain grains can play a role in enhancing the strength of palaeointensity even after taking in to account the effects of multidomain grains an intensity value 13 9 t from this study suggests that the strength of geomagnetic field at about 2 6 ga was much lower than that of the present time

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

the lebombo monocline and associated feeder dyke swarm diagnostic of a successful and highly volcanic rifted margin abstract as part of the karoo large igneous province lip the 700 km long and 182 ma old lebombo monocline outcrops inland of a younger mozambique lowland cover that separates it from the indian ocean detailed mapping of a particularly well exposed olifant river traverse across this monocline correlates basaltic and rhyodacitic lava units within the lebombo group to at least three hosted generations of intrusions within the monocline parallel northern lebombo dyke swarm implying that a much denser feeder dyke swarm is covered by younger flows measured attitudes and thicknesses of up to 197 lavas and 791 dykes statistically quantify a structural cross section from which a properly weighted lava log of nearly 5 km basalts inter bedded with an overlying 3 5 km of rhyodacites is constructed the geological section is compatible with a slightly deeper sub lava section through the innermost 30 km s of a relatively narrow east greenland margin to which the lebombo monocline can be rigorously compared in both cases field relationships are consistent with early tectonic extension being overwhelmed by dyke dilation within margin parallel swarm segments that may have been injected laterally from central magma chambers thus it is argued that the lebombo monocline is part of a successfully rifted margin that furthermore belongs to a distinct class of highly volcanic and relatively narrow margins which only form proximal to classical triple rift lip centers where an elevated magma flux sustains a magmatic mode of extension for the karoo lip this conforms with 1 a tight gondwana reconstruction of africa with antarctica 2 successful breakup during intense 182 ma emplacement of the lebombo group 3 triple rift formation together with a short lived 181 178 ma okavango dyke swarm and 4 anomalously thick igneous crust formation during continued 2 5 cm yr separation into a loose 155 ma plate configuration of africa and antarctica

preliminary palaeomagnetic results of an archaean dolerite dyke of west greenland geomagnetic field intensity at 2 8 ga summary the geomagnetic field intensity during archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 ma nuuk gneisses at nuuk 64 2 n 51 7 w west greenland plagioclase from the dolerite dyke yields a mean k ar age of 2752 ma palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked contact test indicating that the high temperature component magnetization of the dyke is primary thellier experiments on 12 dyke specimens yield a palaeointensity value of 13 5 4 4 pt the virtual dipole moment at ca 2 8 ga is 1 9 0 6 x 10 am2 which is about onequarter of the present value the present study and other available data imply that the earth s magnetic field at 2 7 2 8 ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca 2 6 ga

hybrid veins from the southern margin of the bristol channel basin uk sinistral and dextral en echelon hybrid calcite veins are exposed on the southern margin of the bristol channel basin north somerset on a liassic carbonate platform these hybrid veins are transitional between pure extensional mode i and shear fractures mode ii mode iii and experienced both extensional and shear displacements during their formation in the study area they occur in conjugate sets with the conjugate angle ranging between 10 and 50 based on the kinematic analysis of these veins a new model for the opening of the bristol channel is proposed

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

the form distribution and anisotropy of magnetic susceptibility of jurassic dykes in h u sverdrupfjella dronning maud land antarctica implications for dyke swarm emplacement abstract the mesozoic dyke swarms of western dronning maud land antarctica form a minor intrusive component of the karoo large igneous province five hundred and sixty one dykes were recorded intruding neoproterozoic gneisses and middle jurassic syenite plutons 40 ar 39 ar geochronology data reveal two temporally distinct components the 178 175 ma alkaline straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton and the 206 204 ma tholeiitic jutulrora dyke swarm found throughout the study area the straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction interpreted to be the result of high magma pressure equal to the maximum principal stress the jutulrora swarm displays a fan of dyke trends with dyke thickness and spacing increasing away from the inferred point of fan convergence anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the straumsvola area with the southern outer exposures of the jutulrora swarm exhibiting lateral magma transport although associated with a long lived local igneous centre comparison of palaeostress estimates for the straumsvola dyke swarm and contemporaneous dykes in ahlmannryggen and vestfjella indicates the presence of a regional scale radial stress system in western dronning maud land between 178 175 ma supporting a mantle plume origin for the karoo large igneous province

scaling of fault displacements from the badajoz cordoba shear zone sw spain faulting occurs over a large range of scale parts of which are sampled by various techniques e g microscopy outcrop measurement mapping seismic reflection and other forms of remote sensing use of a single technique to measure displacement or strain will not sample faults at all scales and hence will give a biased estimate in order to assess this bias a knowledge of the distribution over all scales is needed many samples of fault displacement appear to follow a power law distribution with departures which can be attributed to sampling effects the number of faults with a displacement u is given by n u cu d the power law distribution of displacement is consistent with similar distributions of other fault parameters and earthquake magnitudes when sampling along a line e g a bedding trace on a map or section a self similar fault population would have d 1 whereas self affine geometries yield d 1 displacement and extension are dominated by small faults when d 1 and by large faults when d 1 when sampling over areas or volumes these critical values are 2 and 3 respectively a set of strike slip faults from the badajoz cordoba shear zone spain were sampled at two different scales using 1 50000 maps and outcrop measurements displacement ranges over 6 orders of magnitude these and other fault populations typically have d ranging from 0 6 to 1 5 the power law relationship may be integrated to yield estimates of the displacement or extension for faults which lie beyond the resolution of the sampling system for example a knowledge of d allows the extension measured on a map or seismic section to be corrected for faults whose displacement is below the resolution of the survey based on an overall estimate of d 0 9 for the badajoz cordoba data only some 40 of the extension would be recorded by map scale faults a corrected extension of 41 along the shear zone is estimated which if typical for the entire 300 km zone represents some 87 km of along strike extension thus work suggests that significant displacement occurs on faults which are too small to be interpreted from conventional seismic profiles and geological maps

regional sand injectite architecture as a record of pore pressure evolution and sand redistribution in the shallow crust insights from the panoche giant injection complex california abstract observations on outcrop of a regionally developed sand injectite are used to infer and estimate the pore pressure conditions in the shallow crust that caused the fluidization and injection of tens of cubic kilometres of sand the estimated pore fluid pressures at the base of the injection complex at 1500 m burial depth below a regionally developed shale dominated seal are from 22 26 to 25 08 mpa which respectively correspond to 0 81 and 0 95 lithostatic pressure a theoretical basis for prediction of sand injection is defined and applied to the prediction of pore pressure at the time of sand injection the depth at which seal failure occurred and the density and granular content of the fluidized flow lateral variations in the style and abundance of sandstone intrusions are described and these all fit into a remarkably uniform tripartite division of parent units an intrusive complex and an extrusive complex a sill zone intrusions are dominated by sills occurs in a restricted stratigraphic interval 200 270 m thick location of the base of the sill zone is directly related to the thickness of the overburden and an isobaric surface at the time of sand injection the lithostatic equilibrium surface is defined at the base of the sill zone when the sills formed an extended period of supra lithostatic pressure occurred within the sill zone

transpressional folding and associated cross fold jointing controlling the geometry of post orogenic vein type w sn mineralization examples from minas da panasqueira portugal the world class w sn panasqueira deposit consists of an extensive subhorizontal vein swarm peripheral to a late orogenic greisen cupola the vein swarm consists of hundreds of co planar quartz veins that are overlapping and connected laterally over large distances various segmentation structures a local zigzag geometry and the occurrence of straight propagation paths indicate that they exploited a regional joint system a detailed orientation analysis of the systematic joints reveals a geometrical relationship with the subvertical f2 fold generation reflecting late variscan transpression the joints are consistently orthogonal to the steeply plunging s0 s2 intersection lineation both on the regional and the outcrop scale and are thus defined as cross fold or ac joints the joint system developed during the waning stages of the variscan orogeny when already uplifted to an upper crustal level veining reactivated these cross fold joints under the conditions of hydraulic overpressures and low differential stress the consistent subperpendicular orientation of the veins relative to the non cylindrical f2 hinge lines also when having an inclined attitude demonstrates that veining did not occur during far field horizontal compression vein orientation is determined by local stress states variable on a meter scale but with the minimum principal stress consistently subparallel to fold hinge lines the conspicuous subhorizontal attitude of the panasqueira vein swarm is thus dictated by the geometry of late orogenic folds which developed synchronous with oroclinal buckling of the ibero armorican arc

restoration of geometry and emplacement mode of the permian mafic dyke swarms in keping and its adjacent areas of the tarim block nw china abstract the permian mafic dyke swarms are widely distributed in the tarim block east tianshan and the western junggar block in northwestern china these mafic dyke swarms together with coeval basalts mafic ultramafic intrusions and a type granitic plutons constitute the permian tarim large igneous province lip which is considered to be mantle plume derived in this study multi source high resolution remote sensing images are used to identify the geometry and emplacement mode of the mafic dykes sills and flood basalts in keping and its adjacent areas including bachu northwestern tarim block beishan and kuluketag northeastern tarim block and kelamyi western junggar block structural analysis and paleomagnetic results are used to restore the primary geometry of the mafic dykes the results indicate that the lengths and thicknesses among a total of 117 extracted mafic dykes in keping follow negative exponent size distributions the mafic dykes have a mean thickness of 3 8 m with a maximum of 21 4 m the length of unconnected mafic dyke segments ranges from 0 127 km to 17 1 km with an arithmetic length of 1 8 km the restoration of geometry of the mafic dykes indicates that the permian mafic dyke swarms in keping may have extended as far as 61 69 km along a primary orientation of about n320 w and the flood basalts of the tarim lip may have extended to keping so that the areal extent of the tarim lip could be enlarged for about 12 000 km 2 compared with that originally reported about 250 000 km 2 the geometric features of the mafic dyke swarms in keping and its adjacent areas are different from those of the giant radiating dyke swarms due to radial fractures associated with domal uplift the mafic dyke swarms and sills in the tarim lip make up the plumbing system of the mantle plume accompanied by the upwelling of mantle plume many eruptive centers and regional dykes are generated in beishan east tianshan and tarim basin the flood basalts in keping are fissure type which may have been fed by magma conduits from the plume center whereas in bachu the central volcanic eruption is dominant

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

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

palaeomagnetic correlation and dating of mineralization and intrusive events abstract in the mining district of santa eulalia chihuahua northern mexico the mineralization consisting of simple massive concentrations of lead zinc iron sulphides and oxides confined in largely unaltered cretaceous limestones has traditionally been related to several intrusive emplacement events that occurred during the tertiary however the relative age relationships of the intrusives volcanics and mineralization have proved difficult to establish in this work we report palaeomagnetic results which permit definition of the mean relative age relationships for the igneous units and their correlation with the mineralizing event the mean directions and pole positions for the intrusives mineralization and volcano sedimentary cover capping series indicate a tertiary age eocene miocene the intrusives show normal intermediate and reverse polarities suggesting several emplacement events or a long period of intrusion the direction and polarity of the remanent magnetization for the mineralization correlates with those for the felsites which occur as a complex set of sills and dykes and they are therefore penecontemporaneous with the mineralization

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

reassembling the paleogene eocene north atlantic igneous province new paleomagnetic constraints from the isle of mull scotland abstract the paleomagnetic data sets from the british tertiary igneous province btip have recently been criticized as being unreliable and discordant with data from elsewhere in the north atlantic igneous province naip riisager et al earth planet sci lett 201 2002 261 276 riisager et al earth planet sci lett 214 2003 409 425 we offer new paleomagnetic data for the extensive lava flow sequence on the isle of mull scotland and can confirm the paleomagnetic pole positions emanating from important earlier studies our new north paleomagnetic pole position for eurasia at 59 0 2 ma has latitude 73 3 n longitude 166 2 e dp dm 5 2 7 0 a re evaluation and an inter comparison of the paleomagnetic database emanating from the naip were carried out to test for sub province consistency we find a general agreement between the eurasian part of naip btip and faeroes and east greenland data however a compilation of west greenland data displays a large and unexplained dispersion we speculate on if this is related to different sense of block rotation of the tertiary west greenland constituents combining all data from the naip constituents give a pole position at 75 0 n 169 9 e n 25 k 84 3 a95 3 2 in eurasian reference frame

pegmatite dyke emplacement and the state of stress during cratonization an example from the dharwar craton south india abstract in this study we measured orientations of 797 pegmatite dykes intruding the 2 5 ga koppal pluton dharwar craton south india and use the data to evaluate the state of stress prevalent during dyke emplacement we infer that the dykes were emplaced synchronously with late d3 regional deformation that was controlled by nw se directed shortening dykes within the koppal pluton kp have a restricted range of orientations which is indicative of fluid pressure pf

the dyke swarm of mount calanna etna italy an example of the uppermost portion of a volcanic plumbing system a new multidisciplinary study combining geology petrography and geochemistry on the rocks of the isolated hill of mount calanna mount etna italy has provided evidence for the existence of a dyke swarm formed by more than 200 dykes distributed over an area of 0 7 km2 with an intensity of intrusion up to 40 all bodies are deeply altered and the geological and mesostructural surveying of 132 dykes revealed that they intruded in e w direction with an average dip of 60 the faults affecting the outcrop have in general an e w strike and dip of 55 these have all normal motion and have been interpreted as coeval with the dykes this interpretation contrasts with the previous hypothesis that considered mount calanna as a thrust resulting from compressive deformation resulting from the gravitational spreading of the volcanic edifice mount calanna is here interpreted as the uppermost portion of a vertically extensive magmatic plexus that fed the eruptive activity of one or more eruptive center s sited in the valle del bove area measurements of the apparent densities on 23 dykes and host rock samples give an average value of 2 420 kg m3 for the entire complex 15 lower than the density expected for hawaiitic magma placing an important constraint on the geophysical identification of similar structures considering that mount etna is not an old eroded edifice but an active and growing volcano the exposure of this subvolcanic structure can be regarded as exceptional its geometry and physical characteristics can be thus regarded as an interesting example of the present day shallow plumbing system of mount etna as well as of other basaltic volcanoes

measurement and characterisation of spatial distributions of fractures abstract a variety of line sample 1 d and map 2 d datasets of faults and joints has been used to investigate the spatial distributions of fractures and to test techniques of fractal analysis the natural fracture datasets have been supplemented by synthetic datasets with known characteristics the fault datasets investigated range in scale from regional to outcrop and the joint datasets are derived from outcrop 1 d datasets were analysed by the spacing population interval counting and fracture number interval counting techniques 2 d datasets were analysed by box counting and fracture number box counting techniques results indicate that fracture spacing can be characterised in line samples using either the 1 d interval counting technique or more simply by measuring the spacing population as the cumulative frequency distribution of spaces between adjacent fractures tectonic faults frequently show a power law spacing population indicating fractal dimensions of between 0 4 and 1 0 but except for outcrop data truncation effects degrade the analysis unrefined joint datasets commonly show negative exponential or lognormal spacing cumulative frequency distributions however single orientation joint sets are characterised by a regular spacing and are therefore non fractal for both fault and joint map data box counting techniques do not yield the power law relationship between box size and the number of boxes that is expected of fractal geometries it is concluded that 2 d box counting techniques are too insensitive to characterise the many different parameters of a fracture array the fracture density technique appears not to discriminate between very different fracture patterns and therefore does not provide useful results a fracture pattern incorporates several attributes e g size distribution orientation linkage density roughness each of which may scale independently and each of which may be fractal with its own fractal dimension full characterisation of fracture patterns will therefore require independent analysis of each of these attributes

partial melting of the appin quartzite driven by fracture controlled h sub 2o infiltration in the aureole of the ballachulish igneous complex scottish highlands the ballachulish igneous complex consists of an outer quartz diorite and an inner granite emplaced at about 300 mpa initially at 1000 to 1050 c the contact aureole 0 5 2 km wide occurs in metapelites and metapsammites plus minor graphitic slates carbonate rocks and metaquartzites a textural examination of the arkosic appin quartzite which was previously believed to have melted only within a few metres of the intrusion demonstrates that partial melting occurred up to 500 m away from the vertical eastern contact coupling petrographic observations with qtz ab or h2o phase relations we determined both the amounts of actual melt and the maximum possible amounts of melt in the samples melting efficiency was everywhere less than 100 and decreased with distance from the intrusion though perhaps not the only possible source of fluid throughout the aureole simple models demonstrate that h2o evolution from the pluton would have been volumetrically sufficient and persisted long enough to account for the observed partial melting a time integrated fluid flux of 7000 kg m2 from the pluton is necessary to account for the observed amounts of partial melt in the appin quartzite from its inefficiency we infer that infiltration of the appin quartzite cannot have occurred along interconnected grain edge channels rather it was controlled by hydraulic fracturing with fracture density determining melting efficiency bulk rock permeability is calculated to be 10 20 m2 an order of magnitude lower than that necessary to permit pervasive flow of all the fluid exsolving from the pluton there is little difference between the calculated time integrated fluid flux through the appin quartzite on the eastern flank and an estimate of the infiltrating flux through the pelitic leven schist on the western flank in the time interval during which both rock types were above their solidus temperature despite differences in their equilibrium quartz h2o dihedral angles at temperatures immediately below the solidus and differences in the attitude of the contact the rates of h2o expulsion from the cooling pluton are consistent with highly efficient fracture dominated flow allowing insufficient time for textural equilibration

paleomagnetic and rock magnetic evidence for inverse zoning in the parguaza batholith southwestern venezuela and its implications about tectonics of the guyana shield abstract we report paleomagnetic and rock magnetic data from the rapakivi granites of the parguaza batholith guyana precambrian shield southwestern venezuela these results suggest that the pluton is inversely zoned with respect to the cooling ages in order to explain such an age pattern tentative structural settings are proposed placing the parguaza intrusions in a plate tectonic context six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith thermomagnetic curves x ray diffraction and fluorescence hysteresis loops thermal and alternating field af intensity plots transmission tem and scanning sem electron microscope analyses and konigsberger ratios qn values were used to identify the different magnetic mineralogies and their distribution of grain sizes magnetite titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations nrms in these rocks magnetic granulometry indicators such as konigsberger ratios qn values suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect the paleomagnetic results reveal the existence of two primary thermoremanent and or thermochemical magnetizations trms tcrms for sites csp 3 pi 2 and pi 4 decl 328 incl 21 k 15 95 11 4 and site csp 2 pi 1 and pi 3 decl 16 incl 87 k 10 95 13 respectively there is also a poorly defined g3r2 magnetization decl 284 incl 86 k 6 95 27 found in sites csp 2 pi 1 and pi 3 the overlap of coercivities and the unblocking temperatures spectra probably resulting from the coexistence of primary single domain magnetic mineralogies with secondary exsolutions of single domain like ti poor fe rich regions almost pure magnetite in multidomain titanomagnetite grains in most cases precludes the complete resolution of hybrid g1 n r or g3 n r the relative ages for these components were determined using a map of rb sr model age chrontours g1 and g3 are the older and the younger trms tcrms respectively and were acquired at two discrete moments of the batholith s geological history the final map resulting from integrating the paleomagnetic and rb sr data shows an age pattern for this batholith and the rest of the intrusions that belong to the parguaza igneous complex that could be explained either as the effect of inverse cooling of a single intrusive body the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of ne trending horsts and grabens cutting through the pluton because of its feasibility and agreement with the most recent theories about the tectonic evolution of the guyana shield we favor the latter hypothesis

structural control on the shape of intrusions in the koktokay ore district chinese altai north western china abstract the koktokay pegmatite type rare metal bearing ore district in the altai orogen is famous for both its large scale and its diversity of rare metals however the emplacement mechanisms of the ore bearing pegmatite intrusions in the koktokay ore district are still unclear based on field observations the emplacement of the ore bearing pegmatite intrusions falls into two types the first type is typical of the formation of dykes and sills whereby they intruded into fan shaped moderate dipping joints within plutonic rocks the second type involves the formation of a punched laccolith that was fed by a pegmatite sill magmatic stoping is the main mechanism of the laccolith emplacement the peripheral faults played an important role in helping the emplacement of the laccolith the trend of dykes and sills indicate two potential prospecting areas which are located in the western and northern regions of the koktokay ore district

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

paleomagnetic evidence for post 2 55 ga tectonic tilting and 1 1 ga reactivation in the southern kapuskasing zone ontario canada we report paleomagnetic results from two traverses in the southern part of the kapuskasing structural zone ksz of north central ontario which is hypothesized to be an oblique cross section through upper and middle archean crust exposed by uplift and tilting anorthosites and granulite grade gneisses were sampled to the east in the shawmere traverse while amphibolite grade tonalitic gneisses were sampled to the west in the racine traverse three distinct remanent magnetization rm components were isolated by alternating field and thermal demagnetization the a rm has mean directions d 50 i 62 k 138 95 5 n 5 sites 23 samples for the shawmere traverse and d 23 i 53 k 49 95 7 n 8 sites 46 samples for the racine traverse because of the high 500 c laboratory unblocking temperatures of the a rm in the shawmere granulite grade rocks we believe this magnetization was acquired in early slow uplift without tilting from about 24 km to about 16 km depths and has survived the later main ksz uplift and tilting event the racine a rm has laboratory unblocking temperatures down to 400 c and was blocked prior to uplift from a shallower crustal level about 8 km with tilt corrections of 30 and 15 respectively about the 35 striking axis of the ksz the shawmere and racine a paleomagnetic poles fall on the laurentian apparent polar wander path around 2 55 ga dating the time of initial uplift crustal tilting in the ksz occurred after 2 55 ga and was greater in the east than in the west the b rm found mainly in shawmere anorthosites has d 115 i 68 k 17 95 17 n 4 sites 14 samples with a paleopole at 144 w 49 n near 1 1 ga keweenawan reverse poles and the normal pole of the nearby 1 0 ga nemegosenda carbonatite the b rm is probably a thermochemical overprint related to reactivation of the ksz at the time of keweenawan rifting and volcanism in the lake superior region a novel c rm with d 64 i 2 k 18 95 13 n 6 sites 10 samples is either an unresolved hybrid of the a and b rms or an earliest keweenawan 1 3 ga overprint

conditions for the arrest of a vertical propagating dyke magma ascent towards the earth s surface occurs through dyke propagation in the vast majority of cases we investigate two purely mechanical effects unrelated to cooling or solidification that lead to the arrest of propagation so that no eruption occurs the first is that the input of magma from the source is not maintained continuously such that a fixed volume of magma is released laboratory experiments show that in this case the dyke stops at a finite distance from the source this behaviour is specific to the fracturing process in 3 d we derive a relationship for the minimum magma volume required for an eruption as a function of magma buoyancy and source depth when large magma volumes are available eruption may also be prevented by a thick low density layer in the upper crust numerical studies of dyke propagation show that the dyke continues to rise even though it is negatively buoyant magma accumulates in a swollen nose region at the interface between the low density layer and the dense basement magma overpressure is largest at this interface and increases with increasing penetration into the upper layer it may become large enough to induce horizontal fractures in the dyke walls and lateral intrusion of a sill which prevents eruption this requires that the thickness of the low density layer exceeds a threshold value that depends on the density contrast between magma and host rock if the magma volume is smaller than a threshold value neither sill intrusion nor eruption are possible and magma gets stored in a horizontal blade shaped dyke straddling the interface scaling laws for variations of ascent rate and for the minimum magma volume allow diagnosis of a failed eruption

an alternative emplacement model for the classic ardnamurchan cone sheet swarm nw scotland involving lateral magma supply via regional dykes abstract sub volcanic intrusive networks of which cone sheets are recognised as a major constituent partially control volcano growth and eruption style the accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip parallel magma flow however the emplacement mechanism of cone sheets remains poorly understood the 58 ma cone sheet swarm of ardnamurchan nw scotland offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets through studying magma flow and their importance in volcanic edifice construction structural measurements and anisotropy of magnetic susceptibility ams analyses have constrained a lateral magma flow pattern consistently oriented nw se in the majority of the ardnamurchan cone sheets field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry our observations suggest cone sheet formation may be linked to laterally propagating nw se trending regional dykes sourced from laterally adjacent magmatic systems likely the palaeogene mull central complex which are deflected around the central complex by stress field interference implicitly edifice construction and potential eruption precursors observed at a volcano may instigate or result from magmatic activity in laterally adjacent volcanic systems

zircon u pb ages from the wabigoon manitou lakes region wabigoon subprovince northwest ontario three distinct stratigraphic units have been recognized in the crow lake savant lake belt these are a basal unit consisting of high magnesium tholeiitic flows a middle unit consisting mostly of tholeiitic to calc alkaline pyroclastic rocks and flows and an upper unit of high iron tholeiitic flows a zircon u pb study has been carried out on seven rocks from the wabigoon manitou lakes region of this belt in the manitou lakes area a minimum age of 2755 ma on a porphyry pluton intruded into the wapageisi volcanics gives a minimum age for the high magnesium tholeiitic sequence a trondhjemite phase of the atikwa batholith near wabigoon lake that is dated at 2732 2 2 9 ma has the same age within experimental error as a rhyolite flow from the lower wabigoon volcanics in the same area that is dated at however a trondhjemite phase of the atikwa batholith at eagle lake that is dated at is distinctly younger than a dacite flow collected nearby from the lower wabigoon volcanics that is dated at both thes

geochronological constraints on the timing of magmatism deformation and gold mineralization in the red lake greenstone belt northwestern ontario the paper presents u pb ages on zircon baddeleyite titanite rutile and monazite from volcanic and plutonic rocks of the red lake greenstone belt in the northern superior province the purpose of the study was to refine a previously outlined time frame for the volcanic evolution of the belt define the time of major plutonism and deformation and constrain the age of formation of gold deposits volcanism spanned at least 270 ma between 3000 and 2730 ma new ages of 2989 3 and 2894 2 ma for units of a lower volcanic sequence and and 2744 1 ma for units of an upper volcanic sequence corroborate and refine the previously established relationships local gabbroic and felsic intrusions cutting through the lower sequence yield ages of 2870 15 and respectively volcanism was succeeded by major plutonism deformation alteration and metamorphism between about 2730 and 2700 ma these processes were interrelated and progressed as a relatively continuous sequence of events accentuated by major activity d

late paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east tennessee evidence for a post ore chemical event thermal demagnetization studies on samples from mineralized and unmineralized rocks in the lower ordovician knox group of east tennessee reveal a mean direction of d i 157 18 95 7 7 k 52 9 after 60 unfolding combining stable endpoint and remagnetization circle data although not significant at the 95 confidence level the fold test is suggestive of a synfolding alleghenian age of the magnetization the magnetization cannot be postfolding i e permian or younger because the in situ directions do not resemble those for post carboniferous times the magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the early permian but unlike poles for pre late carboniferous times characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test the best estimate for the paleopole resulting from this study falls at 40 n 126 e within the late carboniferous segment of the north american paleozoic apparent polar wander path thus the magnetization is interpreted as synfolding and secondary due to widespread kiaman remagnetization blocking temperatures of up to 520 c and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres secondary dolomites have the same magnetic characteristics as the primary limestones and it is inferred that similar magnetite spheres are the carriers of their magnetization because this magnetization and therefore its carrier occurs in sphalerite and secondary dolomite it must be coeval with or younger than the ore which itself is thought to be of middle to late paleozoic age it is argued that the magnetization was acquired during growth of magnetite associated with a late paleozoic chemical event

40ar 39ar step heating ages of the tertiary igneous rocks of mull scotland twenty one samples from throughout the mull tertiary igneous succession were dated of these eight gave a satisfactory age plateau and the dates show a systematic decrease throughout the succession extrusion of the lavas began c 60 ma ago contemporaneous with the bulk of activity in the british tertiary igneous province igneous activity continued until c 57 ma probably the lavas were extruded during the reversely magnetized chron 26r development of the central igneous complex continued into the normal chron 26 while the late dykes were intruded during 25r if this is so then according to the polarity timescale of harland et al 1982 the duration of igneous activity lies between 0 7 and 4 7 ma consistent with the dates the usefulness of the age plateau selection criteria of lanphere dalrymple 1978 is supported by the results

3 d mohr circle analysis of vein opening indarama lode gold deposit zimbabwe implications for exploration the indarama lode gold deposit is hosted by vertically dipping basalt in the late archaean midlands greenstone belt of zimbabwe major deformation events at 2 68 and 2 58 ga established a complex array of fractures a limited range of orientations of this fracture network opened towards the end of the younger deformation event creating a lode pattern where 92 of mineralised veins dip at less than 50 mainly to the e and w and most strike directions are represented a clustered distribution of poles to the quartz carbonate veins indicates a constrictional stress field at the time of vein opening where 1 and 2 were near horizontal directed nnw sse and ene wsw respectively and 3 was near vertical 3 d mohr circle analysis demonstrates that 2 was approximately 67 of 1 the stress ratio and that the driving pressure ratio r was approximately 0 4 reflecting the role of fluid pressure mean stress and the maximum shear stress in controlling conditions of fracture opening

zinc and cadmium isotope evidence for redox driven perturbations to global micronutrient cycles during oceanic anoxic event 2 late cretaceous abstract this study uses organic rich sediments from the tarfaya basin morocco to assess the cd and zn isotope response to dramatic global palaeoenvironmental change during the cenomanian turonian interval late cretaceous these organic rich continental margin deposits include an expression of oceanic anoxic event 2 oae 2 94 ma an interval associated with the spread of low oxygen marine environments and widespread burial of organic rich sediments due to placement of the tarfaya basin in a region of upwelling and relatively constant local environmental conditions the stratigraphic variations in 114 cd and 66 zn values largely reflect changes in the seawater isotopic composition of the sub surface proto north atlantic ocean positive shifts of 0 2 0 3 away from background values in 114 cd and 66 zn are observed during the main phase of the positive carbon isotope excursion associated with oae 2 these isotopic shifts are coeval with decreases in cd toc and zn toc ratios and thus imply that drawdown of isotopically light cd and zn from seawater inventories was a result of extensive burial of these metals in organic rich marine sediments globally low 66 zn values during the plenus cold event a cooler episode during oae 2 are similar in timing and magnitude to variations found in the english chalk eastbourne uk and support the inference of a global control on these isotopic excursions the 66 zn values during the plenus cold event are taken to record global oxygenation possibly including the remobilization of isotopically light zn from continental margin sediments a considerably smaller change in 114 cd values for this interval implies that the cd and zn isotope systems can provide information about slightly different environmental processes with global seawater composition with respect to zn also being influenced by the magnitude of oxic removal sinks and isotopically light zn input fluxes from sediments and hydrothermal fluids

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

paleomagnetic evidence for proterozoic tectonism in the kapuskasing structural zone ontario the kapuskasing structural zone ksz in the superior province of north central ontario is thought to be an oblique cross section through middle archean crust samples from 30 sites in granulite gneisses and anorthosites of the ksz have been analyzed using alternating field and thermal demagnetization methods three remanent magnetization components were found with mean directions of d 23 i 50 k 52 95 11 n 5 sites for the a component d 73 i 68 k 50 95 5 n 18 for the b component and d 332 i 59 k 53 95 17 n 3 for the c component v constanzo alvarez and d j dunlop found similar a and b components in less metamorphosed rocks from the same units to the immediate west they attributed the a component to uplift at 2 55 ga with subsequent 15 to 30 wnw tilt and the b component to thermochemical overprinting at 1 1 ga as the ksz was reactivated during keweenawan rifting and volcanism we also attribute the a component to uplift during the 2 55 ga kenoran orogeny with concomitant 10 5 wnw downward tilting before intrusion of the 2 45 ga matachewan dikes to give its present paleopole of 33 e 67 dp 6 dm 9 n 13 using data from both studies we attribute acquisition of the b component to uplift at about 1 88 ga as suggested by recent thermal modeling and tectonic syntheses to give a paleopole of 124 w 30 n dp 7 dm 8 n 20 the c component is found in only three sites and may be either an hybrid of a 1 1 ga keweenawan crm with the b component or a fault block tilted b component near the ivanhoe lake cataclastic zone

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

paleomagnetism of archean rocks from northwestern ontario iv burchell lake granite wawa shebandowan subprovince the late archean burchell lake granite of the shebandowan greenstone belt in northwestern ontario has a characteristic natural remanent magnetization nrm resembling the type 1 nrm of the nearby shelley lake granite of the quetico gneiss belt of 36 stably magnetized samples 21 had predominantly normal polarity 1n nrm and 15 had reverse polarity 1r the mean direction based on stable end point and vector subtracted directions is d 2 3 i 48 9 k 23 9 95 10 0 n 10 sites intersecting remagnetization circles gave a similar direction the corresponding paleopole bl1 lies at 83 2 e 71 1 n near track 6 of the laurentian apparent polar wander path around 2600 ma although the burchell lake pluton is not dated radiometrically neighbouring granites give k ar biotite ages of 2550 2600 ma and it is reasonable to suppose that the nrm dates from the time of intrusion or shortly thereafter a type 2 nrm like that of the shelley lake granite was isolated in 12 samples its mean direction is d

paleomagntism of archean rocks from northwestern ontario ii shelley lake granite quetico subprovince the 2580 20 ma shelley lake granite of the quetico gneiss belt in northwestern ontario preserves two distinct natural remanent magnetizations nrm s of different ages type 1 nrm of either normal 1n or reverse 1r polarity was isolated in 43 samples using three methods stable end point directions orthogonal vector plots and converging remagnetization circles the mean direction based on both 1n and 1r results is d 5 9 i 56 4 k 95 4 95 4 4 n 12 sites which is significantly different from both the present earth s field pef and axial dipole field directions at the site type 2 nrm either normal 2n or reverse 2r was successfully separated from 1n 1r in 13 samples its mean direction combining both polarities is d 70 3 i 27 8 k 20 1 95 9 4 n 13 samples the paleopole sl1 corresponding to 1n 1r falls at 65 8 e 77 6 n near track 6 of the laurentian apparent polar wander path apwp around 2600 ma this remanence is carried principally by primary multido

variation in the form and distribution of dykes in the mull swarm scotland the dykes of the mull swarm part of the british tertiary igneous province have been examined in a series of well exposed coastal sections traverses through the dyke swarm show systematic changes in the thickness and spacing of dykes both of which are typically log normally distributed both the geometric mean thickness and spacing of dykes increase with distance from the mull centre concordant with a decrease in crustal extension variations in the thickness and spacing result from changes in character of the magma flow it is inferred that the magma flux was fairly evenly distributed near the mull centre but farther away from the source it becomes focused into larger dykes the form orientation and opening direction are also observed to vary with distance away from the volcanic centre on mull the dykes intrude a range of pre existing fractures the range of fractures used decreases with distance from the centre this together with other features of the form of the dykes have been interpreted in terms of the varying magma pressure and regional stress which is expressed as the r ratio analysis of the variable strikes of fracture filled dykes is consistent with decreasing magma pressure with distance from the mull centre assuming that regional stresses were relatively invariant across the same distance

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

structural signatures of igneous sheet intrusion propagation cm acknowledges a junior research fellowship funded by imperial college london jdm acknowledges national science foundation grant ear 1654518 ken mccaffrey paul bons and sandy cruden are thanked for their constructive reviews

igneous sills record far field and near field stress interactions during volcano construction isle of mull scotland abstract sill emplacement is typically associated with horizontally mechanically layered host rocks in a near hydrostatic far field stress state where contrasting mechanical properties across the layers promote transitions from dykes or inclined sheets to sills we used detailed field observations from the loch scridain sill complex isle of mull uk and mechanical models to show that layering is not always the dominant control on sill emplacement the studied sills have consistently shallow dips 1 25 and cut vertically bedded and foliated metamorphic basement rocks and horizontally bedded cover sedimentary rocks and lavas horizontal and shallowly dipping fractures in the host rock were intruded with vertical opening in all cases whilst steeply dipping discontinuities within the sequence i e vertical fractures and foliation in the basement and vertical polygonal joints in the lavas were not intruded during sill emplacement mechanical models of slip tendency dilation tendency and fracture susceptibility for local and overall sill geometry data support a radial horizontal compression during sill emplacement our models show that dykes and sills across mull were emplaced during nw se horizontal shortening related to a far field tectonic stress state the dykes generally accommodated phases of ne sw horizontal tectonic extension whereas the sills record the superposition of the far field stress with a near field stress state imposed by emplacement of the mull central volcano we show that through detailed geometric characterisation coupled with mechanical modelling sills may be used as an indication of fluctuations in the paleostress state

along axis structure and crustal construction processes of spreading segments in iceland implications for magmatic rifts magmatic rift systems are composed of discrete spreading segments defined by morphologic structural and volcanic features that vary systematically along strike in iceland structural features mapped in the glaciated and exhumed miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone integrating information from both the active rift zones and ancient crust provides a three dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments crustal exposures in the skagi region of northern iceland reveal significant along strike variations in geologic structure the upper crust at exhumed magmatic centers segment centers is characterized by a variety of intrusive rocks high temperature hydrothermal alteration and geologic evidence for kilometer scale subsidence in contrast the upper crust along segment limbs which extend along strike from magmatic centers is characterized by thick sections of gently dipping lava flows cut by varying proportions of subvertical dikes this structure implies relatively minor upper crustal subsidence and lateral dike intrusion the differing modes of subsidence beneath segment centers and segment limbs require along axis mass redistribution in the underlying upper middle and lower crust during crustal construction this along axis material transport is accomplished through lateral dike intrusion in the upper crust and by along axis flow of magmatic to high temperature solid state gabbroic material in the middle and lower crust these processes inferred from outcrop evidence in skagi are consistent with processes inferred to be important during active rifting in iceland and at analogous magmatic oceanic and continental rifts

geochemical evolution of the tertiary mull volcano western scotland the early tertiary mull volcano western scotland is one of the most dissected and best exposed igneous complexes of the north atlantic province the new and published geochemical data enable us to chart the magmatic evolution of the mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres to the youngest dykes in this study we identify four successive magma types within the remnant volcano the earliest type the mull plateau group comprises mildly alkaline basaltic rocks with steep chondrite normalized rare earth element ree patterns this type is succeeded within the lava succession and dyke swarm by the coire gorm magma type with essentially flat chondrite normalized ree patterns a third magma type represented within the lava and dykes the central mull tholeiites is more depleted in incompatible trace elements than the preceding types and has flat to lree depleted chondrite normalized patterns the major intrusions and cone sheets of mull centre 1 and early centre 2 belong to this magma type midway through the igneous activity associated with centre 2 the magma type changed to become more alkalic and more enriched in incompatible trace elements this magma type the late mull type is found to persist through the cone sheets and major intrusions of centre 3 to the youngest dykes these changes in magma composition were related to variations in the mantle source and depth of partial melting beneath mull and or differences in the efficiency of melt pooling before ascent through the lithosphere with the exception of the early staffa magma sub type part of the mull plateau group the location of magma chambers in which the bulk of contamination occurred changed with time from deep lower crustal lewisian gneiss to shallow upper crustal moine schist intermediate members of the plateau group and the late mull magma type are enriched in fe ti and p relative to the central mull tholeiites we attribute this difference to the more alkalic nature of these suites lower fo2 and the formation of fe3 p complexes in the magma the intermediate rocks were important in magma mixing processes with two types of mixing identified on mull 1 cryptic mixing between basalts and low fe intermediate magmas typified by lavas and early basic cone sheets of the central mull tholeiite magma type 2 observable mingling between rhyolitic magmas and high fe intermediate magmas of the late mull type shown by the mixed magma bodies of the glen more and loch ba ring dykes the main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas it may safely be maintained that mull includes the most complicated igneous centre as yet accorded detailed examination anywhere in the world bailey et al 1924 memoir of the geological survey of great britain scotland hmso edinburgh 1924

paleomagnetism of archean rocks from northwestern ontario wabigoon gabbro wabigoon subprovince the wabigoon gabbro of the archean wabigoon greenstone belt in northwestern ontario preserves a univectorial natural remanent magnetization nrm with d 246 i 12 k 19 5 95 10 5 n 11 sites the precision is reduced if sample means are averaged however k 9 3 95 9 2 n 29 samples the paleomagnetic pole falls either at 160 w 11 s p 5 3 m 10 6 corresponding to an age of 1300 ma on the laurentian apparent polar wander path or the reverse of this 20 e 11 n corresponding to a late archean age 2800 ma no 1300 ma igneous or metamorphic event is known in the area a major west northwest trending dike about 9 km south of the gabbro yields a virtual geomagnetic pole at 122 w 45 n and seems to be of abitibi age 2150 ma rather than mackenzie age 1250 ma a few gabbro samples and some greenstones from the intrusive baked zone have hybrid remanences in which a higher blocking temperature kenoran age 2600 ma nrm is superimposed on the gabbro characteristic n

semi automatic mapping of dyke and dyke related fractures using uav based photogrammetric data a case study from sijiao island coastal southeastern china abstract mapping dyke and dyke related fractures is important for magma propagation and emplacement mechanisms analysis here we propose a semi automatic workflow for dyke and fracture mapping using unmanned aerial vehicle uav based photogrammetric data our workflow integrates mean shift and grabcut in one cut gcoc algorithms to segment dykes and harnesses the phase symmetry algorithm and a series of follow up actions to detect fractures sijiao island of zhejiang province se china is examined as a case study to validate the approaches with our workflow the generated dyke map attains more than a 97 recall rate when compared with the results of expert interpretation and the ratio of the number of detected false fracture segments to the number of total segments is less than 18 the dyke strike and fracture strike calculation match well with field measurements moreover by combining visual interpretation and the resulting geometric information our results manifest that dyke magma invaded along pre existing fractures to form dykes on sijiao island and the difference between dyke fractures and host rock fractures may be because of the effect of major changes of regional stress this workflow has great potential for high resolution terrestrial mapping

paleomagnetism of the mississippi valley type ores and host rocks in the northern arkansas and tri state districts a paleomagnetic study has been done on 452 specimens from the early ordovician cotter and middle ordovician everton formations in the northern arkansas mississippi valley type mvt lead zinc ore d

review of drones photogrammetry and emerging sensor technology for the study of dykes best practises and future potential abstract dykes play a major role in transporting magma through the crust to subsurface storage reservoirs or eruption sites at the surface the geometry connectivity and internal textures of dykes preserve an important record of dyke emplacement linkage and magma flow dynamics unravelling these facets of dyke evolution is critical to interpreting signals of magma migration in active volcanoes as well as tectonic processes including crust formation and continental break up during hazard assessment unfortunately our ability to develop new insights is limited by the continuity of high precision data that can be extracted from the field the emergence of unmanned aerial vehicles uavs a k a drones photogrammetry and other sensor techniques are set to greatly improve our field capabilities by collecting data at higher density and with greater precision here we present a workflow emphasising best practise in uav survey design the construction of photogrammetric models and standardised reporting of those models emphasis is given to the factors that should be considered in order to resolve the geological features of interest in any study e g wall rock fractures chilled margins magmatic breccias dyke apertures etc a point to raster technique is documented that measures spatial error in the rasters that are derived from photogrammetric models e g orthomosaics and digital elevation models using three case studies we show how dyke swarms can be mapped accurately over areas approaching 1 km2 at sub centimetre resolution the data illustrate how questions about dyke induced deformation host rock controls on segmentation and aperture variation may be addressed and we present a method for the high density high precision measurement of dyke apertures in 3d we also review semi automated mapping tools and emerging uav sensing techniques including hyperspectral and aeromagnetic sensors and the potentially important contributions they will make to the study of intrusive systems and volcanology more broadly

relative age of otto stock and matachewan dykes from paleomagnetism and implications for the precambrian polar wander path north trending diabase dykes within and near the otto syenite stock of the kirkland lake area of the superior province carry a stable paleomagnetic remanence identical to that of 2454 2 ma matachewan dykes far from the stock a positive baked contact test for the remanence of a dyke within the stock demonstrates that the remanence is primary thus 1 these dykes belong to the matachewan swarm 2 the otto stock has not magnetically overprinted matachewan dykes in its contact aureole as previously reported and 3 the stock must be older than 2454 ma the magnetic remanence associated with the otto stock previously thought to be primary must now be considered of uncertain age nevertheless the positive baked contact test for the remanence of the matachewan dyke within the stock demonstrates that the remanence associated with the stock predates intrusion of the matachewan swarm the conventional polar wander path for precambrian north america decreases in age from the matachewan paleopole to the otto

analytical predictions for a natural spacing within dyke swarms dykes often grow next to other dykes evidenced by the widespread occurrence of dyke swarms that comprise many closely spaced dykes in giant dyke swarms dykes are observed to maintain a finite spacing from their neighbors that is tens to hundreds of times smaller than their length to date mechanical models have not been able to clarify whether there exists an optimum or natural spacing between the dykes and yet the existence of a natural spacing is at the heart of why dykes grow in swarms in the first place here we present and examine a mechanical model for the horizontal propagation of multiple closely spaced blade like dykes in order to find energetically optimal dyke spacings associated with both constant pressure and constant influx magma sources we show that the constant pressure source leads to an optimal spacing that is equal to the height of the blade like dykes we also show that the constant influx source leads to two candidates for an optimal spacing one which is expected to be around 0 3 times the dyke height and the other which is expected to be around 2 5 times the dyke height comparison with measurements from dyke swarms in iceland and canada lends initial support to our predictions and we conclude that dyke swarms are indeed expected to have a natural spacing between first generation dykes and that this spacing scales with and is on the order of the height of the blade like dykes that comprise the swarm

a mohr circle construction for the opening of a pre existing fracture abstract the opening of a fracture is dependent on the fluid pressure and the local stress regime around the fracture for a fracture to open the fluid pressure must exceed the normal stress acting on the fracture the mohr circle provides a useful tool allowing the visualization of these relationships this construction determines the range of fracture orientations that are able to open as well as determining the opening direction of these fractures this is then used to produce ratios of the magma pressure and the principal stresses for some dykes from the lizard ophiolite u k

mesozoic hydrothermal alteration associated with gold mineralization in the mercur district utah k ar dates and chemical data show that a mesozoic gold bearing hydrothermal system altered black shales of the mississippian great blue limestone throughout an area encompassing the mercur gold district utah k ar dates of illite veins and illite rich clay sized separates of altered shales that are enriched in au as hg sc and other heavy metals indicate that hydrothermal activity occurred from 193 to 122 ma several ages from within the mercur district cluster near 160 ma and may date the minimum age of gold mineralization

structure of a sheet laccolith system revealing the interplay between tectonic and magma stresses at stardalur volcano iceland abstract this paper provides detailed structural data on the faults and intrusions of the eroded stardalur volcano in the esja peninsula at the sw margin of the icelandic rift and new lithostratigraphic data on the host rocks a swarm of centrally dipping sheets emplaced with a fan shaped geometry in section view depicts an ellipse in plan view with an about e w major axis sheet intensity reaches 80 in one 100 m transect near the central portion of the swarm where sheets strike e w with sub vertical dip a 200 m thick multiply intrusive body forms a laccolith in the middle of the centrally dipping sheet swarm at a higher altitude individual intrusive units within this laccolith are sub vertical and strike e w in the center acquiring a sill like geometry outwards and were emplaced in a stress field locally dominated by a n s directed least principal stress 3 within the inclined sheet swarm the stress tensor rotated in response to an excess magma pressure probably linked to an underlying magma chamber which interacted with the tectonic stresses crosscutting relationships between the various intrusions and with the host rock indicate that the activity in the central zone of the stardalur volcano ended with the emplacement of the laccolith later eruptions if they occurred at all were fed by centrally dipping sheets in the final stage nne striking regional vertical dykes and normal faults cut the previous structures in response to dominant regional plate tectonic stresses with a wnw ese directed 3 and a nne ssw directed intermediate principal stress 2

on the thermal viability of dikes leaving magma chambers dikes beginning to propagate away from a magma source are thin and grow slowly and thus are susceptible to freezing a self similar solution is obtained for a dike propagating down a temperature gradient when the wallrock and magma temperatures are equal at the chamber wall the solution applies only to the special case of a single component magma intruded at the solidus temperature but provides insight into the case in which the magma temperature is above the solidus the dike propagation velocity relative to that in the absence of freezing is determined by a single dimensionless parameter for sufficiently large magma freezing chokes the dike faster than propagation can elastically widen it and the dike cannot grow if the magma intrudes at a temperature above the solidus there is some value of above which the dike will propagate a certain distance and freeze and below which it will propagate until depletion of the source or three dimensional elastic effects promote freezing for reasonable parameters this process can explain why basaltic dikes are found far from obvious source regions while granitic dikes are scarce except in the immediate vicinity of granitic plutons even if a rhyolite is 5 orders of magnitude more viscous than a basalt however this distinction can be offset if the rhyolite has an excess source pressure only 1 order of magnitude greater than that of the basalt

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

a comparison between subaerial and submarine eruptions at kilauea volcano hawaii implications for the thermal viability of lateral feeder dikes eruption styles on the subaerial east rift zone erz of kilauea volcano are reviewed and a classification scheme for the different types of eruption is proposed the various eruption types are produced by differing thermal and driving pressure behaviour in the feeder dikes existing evidence is reviewed and new evidence presented of the types and volumes of eruptions on the puna ridge which is the submarine extension of the erz eruptions on the puna ridge fall into the same five classes as and are of comparable volume to those on the subaerial erz evidence is presented which suggests that feeder dikes for puna ridge eruptions are more thermally viable than those feeding subaerial eruptions and this difference causes long lived large volume eruptions to be more common on the puna ridge than on the subaerial erz this systematic variation in thermal viability may be due to increased dike width for puna ridge dikes or increased pressure gradients driving magma flow lateral dike emplacement is common to many basaltic systems including on other hawaiian volcanoes in iceland and at mid ocean ridges the systematic trend inferred for the erz of kilauea implies that in the other systems large volume eruptions may also be more common at great distances than they are close to the magma centre

edifice and substrata deformation induced by intrusive complexes and gravitational loading in the mull volcano scotland it is likely that the structure of a volcanic edifice can be significantly modified by deformation caused by large shallow intrusions such deformation may interact with that caused by volcano loading we explore such intrusion related and loading related deformation with field evidence and analogue models to do this we have chosen the eroded palaeogene mull volcano scotland that had a major edifice has well exposed intrusions and significant deformation there are thin mesozoic sedimentary rocks forming ductile layers below the volcano but their thickness is insufficient to allow the gravitational spreading of the volcanic edifice especially when considering that a thick lava pile covers them thus intrusive push may have been the driving force for deformation the mull activity migrated toward the northwest forming three successive intrusive complexes centres 1 2 and 3 our detailed fieldwork reveals that deformation due to these was accommodated on three levels along thrust planes in lava sequences along a decollement located in a thin clay rich sediment succession and in basement schists a relative chronology has been established between different groups of structures using dyke and sill cross cutting relationships centre 1 is surrounded by a fold and thrust belt leading to radial expansion in contrast centre 2 and 3 are connected to thrusts located to the south and east bounded by strike slip faults leading to expansion to the southeast the migration of centres and the directed sliding of the edifice may be related to the presence to the southeast of low resistance dalradian basement that failed significantly during growth of centres 2 and 3 to study the observed relationships we have carried out scaled analogue models models are made with fine powder intruded by a viscous magma analogue the models show an intimate relationship between intrusion growth uplift of the volcano and subsequent flank sliding the structures produced can be compared with mull and suggest that the centre 1 thrust belt probably formed following edifice gravitational sliding as a consequence of the uplift associated with centre 1 formation centre 2 and 3 are responsible for the sector sliding of the edifice flank toward the southeast as the magmatic complex became more asymmetric the features observed at mull and in the models are similar to those seen on active volcanoes such as etna providing a structural framework for their deformation and evolution

aeromagnetic patterns in southern uruguay precambrian mesozoic dyke swarms and mesozoic rifting structural and tectonic evolution abstract new high resolution airborne magnetic data of uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms main faults and other magnetic bodies which compose the uruguayan shield we combined geophysical analyses vertical derivatives upward continuation euler deconvolution structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the uruguayan shield and contribute to a better understanding of its tectonic evolution the magnetic maps revealed several outstanding features in the uruguayan shield the paleoproterozoic dyke swarm is larger denser more widespread and complex than originally thought suggesting a possible plume origin in addition a new mesozoic dyke swarm as complex as the previous one was identified crosscutting the paleoproterozoic dyke swarm and the neoproterozoic orogenic structures moreover this swarm is connected to volcanic calderas in the merin basin and shows displacements along neoproterozoic shear zones in the magnetic maps revealing its brittle reactivation during mesozoic times the new observations clarify how proterozoic basement structures controlled the development of the mesozoic rift paleoproterozoic dyke swarms were reactivated as normal faults and neoproterozoic structures hindered the rift growth deflecting the deformation in transcurrent movements meanwhile the mesozoic dyke swarm was developed in a perpendicular direction to the neoproterozoic structures moreover these findings contradict the current rift model for uruguay and rise a new model in which the mesozoic rift developed as two rift basins connected by a central transfer zone generated by the reactivation of dom feliciano belt structures between the sierra ballena and sarandi del yi shear zones

dikes emplaced into fractured basement timna igneous complex israel dikes are usually envisioned as arrays of parallel segments dilated perpendicular to the direction of the least compressive stress we describe here four dikes of highly irregular shape intruded in the fractured basement in the timna igneous complex southern israel the dikes include a doleritic dike 2 3 km long and 1 6 m to 32 m thick and three andesitic dikes up to 1 5 km long and 8 m thick the dikes each display significant variations of dip up to 60 strike up to 160 and thickness the thickness variations correlate better with the segment attitude than with the position along the dikes we show that the irregular shapes of the timna dikes are the result of emplacement into fractured host rock under different paleostress states and driving pressures three dilation styles that differ by the geometry of the initial cracks are analyzed an array of randomly oriented cracks style a a single linear crack style b and an array of interconnected nonparallel cracks style c the analysis of style a provides the stress state during dike emplacement including the orientations of the three principal stresses 1 2 3 the stress ratio 2 3 1 3 and the normalized driving pressure r pm 3 1 3 the stress ratio indicates the shape of the stress ellipsoid and it ranges from 0 for 2 3 prolate ellipsoid to 1 for 1 2 oblate ellipsoid the normalized driving pressure r indicates the relative magnitude of the internal magma pressure pm with respect to the tectonic stresses and it ranges from r 0 for pm 3 to r 1 for pm 1 we found that for three dikes in timna 0 25 indicating small differences between the two horizontal principal stresses and for one dike 0 9 indicating a large difference between the two horizontal principal stresses the normalized driving pressure r is about 0 08 in two horizontally propagating dikes and about 0 25 in two vertically propagating dikes style b predicts an elliptical thickness profile along the dike due to dilation of a linear crack this prediction agrees with the profile of one of the dikes the predicted thicknesses due to dilation of the interconnected array of cracks style c are in good agreement with the thickness variations of the doleritic dike and in fair agreement with two of the andesitic dikes deviations from the ideal geometry suggest separate stages of propagation and dilation in some of the dike segments

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

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

paleomagnetism and the age of copper mineralization in the portage lake volcanics upper peninsula michigan the close association of hematite with native copper in the flow tops of the portage lake volcanics invites the opportunity to establish the relative age of copper mineralization for this purpose

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

polarity stratigraphy and duration of the tertiary igneous activity of mull scotland previous work left unresolved whether the tertiary igneous activity had spanned a magnetic polarity succession r n r n r or if overprinting had occurred only r n r new work is reported that demonstrates that thermal overprinting has occurred in a narrow zone c 600 m wide bordering the outermost major intrusions of centre 2 criteria for the recognition of overprinting are examined the polarity sequence is now believed to be r n r in conjunction with radiometric dates which show that activity occurred from c 60 ma to c 57 ma our data show that the sequence corresponds to chrons 26r 26 25r of the polarity time scale of harland et al 1982 or 25r 25 and 24r of breggren et al 1985 upper and lower limits to the duration of the activity are respectively 4 67 ma and 0 67 ma or 4 07 ma and 0 60 ma a duration of about 3 5 ma is thought probable the magnetic evidence supports the view that the loch scridain sills correspond to the early acid and intermediate cone sheets of centre 1

stability of remanence and paleomagnetic studies of some chromite ores from barramiya and allawi occurrences eastern desert egypt abstract rock magnetic investigations were carried out on natural precambrian chromite ore at two occurrences in the eastern desert egypt a medium high coercivity component of remanence of reversed polarity can be defined the mean direction of the five sites studied is d 198 i 44 with k 55 and 95 10 ore microscopic and magnetic examinations indicate the absence of magnetite and it is shown that the chromite mineral ferrimagnetic phase could be the carriers of the high stable nrm the result of paleopole position of these precambrian chromites are presented and discussed in the context of its age and the apparent polar wander path apwp of africa

paleomagnetism of archean rocks from northwestern ontario v poohbah lake alkaline complex quetico subprovince the poohbah lake alkaline complex is a late synkinematic kenoran pluton in the quetico gneiss belt of the western superior province three units of the complex porphyritic syenite ps malignite