palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks a case study from the palaeozoic ballantrae ophiolite scotland abstract palaeomagnetic studies of the ballantrae ophiolite southwest scotland are assessed in the light of palaeomagnetic and rock magnetic reliability criteria the remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction rock magnetic reliability criteria are found to be ambiguous when treated in isolation original grain size effects are detected in magnetic properties across three sections through pillow lavas we propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability thermomagnetic analyses and microscopic observations suggest three remanence carrying magnetic phases exist within the extrusive layer of the ophiolite these are almost pure magnetite hematite and titanomaghemite palaeomagnetic rock magnetic and geological observations are best satisfied by a pre obduction magnetisation age this study links the remagnetisation of an ultra oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested

palaeomagnetism and rb sr ages of the ratagan and comrie intrusions the newer igneous rocks in the scottish highland region which post date the main grampian metamorphism and tectonism range in composition from basic to granitic two members of this suite were chosen for palaeomagnetic study in an attempt to refine the apparent polar wander path for scotland in the palaeozoic this paper describes the results of the detailed palaeomagnetic analysis which has been carried out thermal and alternating field af demagnetization was supplemented by rock magnetic experiments on a limited selection of specimens from each sampling site to help characterize the remanence carriers these consisted of thin section observations and the measurement of thermomagnetic and hysteresis properties each complex provides an example of the widespread variability in remanence behaviour which may be encountered in a single intrusion for an accurate interpretation of palaeomagnetic data from any formation an estimate of the age of the rock is required brown miller grasty 1968 give a poorly defined k ar now at bp research centre str epd chertsey road sunbury on thames middlesex tw16 7ln

palaeomagnetism of nw scotland syenites in relation to local and regional tectonics summary a detailed thermal demagnetization study has been carried out on a number of syenitic intrusions from north of the great glen fault in scotland multicomponent remanences of normal and reversed polarity have been recognized these define a series of palaeomagnetic poles from which an ordovician to silurian apparent polar wander path for northern scotland is deduced comparison of this apparent polar wander path with the established path for britain throws new light on the sequence of thrusting and magmatic activity in the nw highlands and precludes large scale 1500 km movement along the great glen fault later than the ordovician the data emphasize previously published discrepancies between the apparent polar wander paths of britain baltica and laurentia and are consistent with the suggestion that britain was among a set of intracratonic microplates which underwent complicated distinct movements throughout the caledonian orogeny

paleomagnetism of rocks from burin peninsula newfoundland hypothesis of late paleozoic displacement of acadia criticized the posttectonic st lawrence granite 360 5 ma and associated subvertical dikes have two magnetizations n and h n points north with an intermediate upward inclination declination 005 inclination 50 alpha 95 10 paleopole 12 n 120 e we consider n to have been acquired close to the time of initial cooling the h magnetization points south with low inclination h is also the dominant magnetization in the nearby permo carboniferous spanish room formation h is considered to be an overprint acquired chemically during the kiaman reversed superchron late carboniferous and permian the n magnetization corresponds to a paleolatitude of 31 7 s very different from the near equatorial values expected from the commonly accepted late devonian and earliest carboniferous tournaisian reference paleofields for cratonic north america this is generally consistent with the hypothesis of a southerly displaced acadia proposed by kent and opdyke although the displacement is much greater 30 rather than 15 however we show that all inclinations observed from middle to upper devonian and tournaisian rocks from the craton are not significantly different either from the kiaman paleofield or from the h overprints observed from newfoundland some of these cratonic studies are not very detailed but it is noteworthy that the declination observed in the most comprehensive of them catskill red beds also agrees with the kiaman paleofield and with the h overprints from newfoundland declinations in some other cratonic studies show small departures of only a few degrees with one exception based on limited sampling and two from the colorado plateau which may have been rotated which could be caused by experimental inaccuracies moreover the directions obtained from cratonic studies are 97 reversed as they are in rocks with kiaman magnetizations we suggest therefore that the commonly accepted late devonian to earliest carboniferous reference paleofield for north america is either entirely kiaman in age or has been seriously affected by kiaman overprinting consequently the 30 of southerly displacement which the n magnetization of the st lawrence granite apears to indicate and the 15 invoked by kent and opdyke we consider to be illusory three late precambrian rock units 623 to 606 ma have scattered declinations but similar inclinations yielding paleolatitudes of about 35 the scattered declinations presumable were caused by local pre carboniferous relative rotations there is little or no evidence of kiaman overprinting in precambrian rocks indicating that overprinting is comparatively superficial and is probably related to late paleozoic groundwater circulation

synfolding and prefolding magnetizations in the upper devonian catskill formation of eastern pennsylvania implications for the tectonic history of acadia the upper devonian catskill formation was sampled for paleomagnetic study in east central pennsylvania 41on 76ow in one area the dominant component of magnetization se is revealed over a broad spectrum of demagnetization temperatures ranging to at least 660oc a conventional fold test is positive at the 99 confidence level however statistical analysis of dispersion with incremental bedding tilt correction shows a significant peak in the precision parameter after about 3 4 unfolding the magnetization is therefore secondary with a mean direction of d 166 6 o i 1 8 o the

multicomponent magnetization in the helmsdale granite north scotland geotectonic implication abstract a palaeomagnetic study of the helmsdale granite u pb 420 m y k ar 400 m y northeast scotland has revealed a multicomponent remanence dominated by two characteristic axes of magnetization the suggested oldest of these magnetizations the direction of which is nearly horizontal and directed n s is thought to have been acquired in upper silurian lower devonian times the existence of this shallow direction of magnetization discounts a recent hypothesis of a ca 2000 km sinistral offset along the great glen fault the second component of magnetization appears to be partly carried by haematite that apparently formed through disintegration of biotite and or plagioclase this secondary magnetization has a direction that can be associated with a permian early mesozoic age similar overprinted magnetizations are characteristic features also in the devonian sedimentary sequences north of helmsdale

paleomagnetic study of devonian rocks from ste cecile st sebastien region quebec appalachians three devonian rock units were collected for paleomagnetic study from the ste cecile st sebastien region located in the gaspe connecticut valley synclinorium of quebec appalachians 15 sites 48 samples from the metasediments subgreenschist facies siltstones sandstones of the compton formation early middle to late early devonian age based on fossil plants which were tightly folded during middle devonian acadian orogeny 14 sites 61 samples from the ste cecile st sebastien granitic body 362 m y k ar age which intruded the metasediments 14 sites 62 samples from the hornfels created subsequent to the emplacement of the intrusive detailed thermal cleaning of the metasediments yielded a mean remanence direction characterized by linear vector trajectories after tilt correction at d 209 i 20 k 30 95 7 2 with unblocking temperature spectrum 300 550 c carried probably by magnetite the predeformational origin of this remanence is tested at 99 confidence level as well as antipodal normal and reverse polarity directions the corresponding paleomagnetic pole ms falls at 28 n 77 e dp dm 6 11 in agreement with the recently reported poles which are mostly early devonian in age and derived from the southeastern margin of the presumed proto atlantic ocean af demagnetization of granites resulted in a univectorial mean remanence at 1 d 332 i 39 k 31 for n 5 sites in fine grained granite fg unit 2 d 313 i 25 k 19 for n 9 sites in medium grained granite mg unit and 3 d 319 i 30 k 17 for all the total n 14 granite g sites af demagnetization of hornfels revealed a post intrusive mean remanence direction hf at d 346 i 22 k 8 for n 14 sites the individual site mean remanence directions with coercivity spectra 10 60 mt from both granites and hornfels suffer from a large declination spread of about 60 but the confidence ovals for the fg and hf remanences overlap this indicates that the fg and hf remanences are indistinguishable from each other and therefore are proximal in time and probably originated during middle to late devonian period interpretation of this kind is speculative in the absence of a convincing evidence for a two fg younger than mg or three phase intrusive history of the pluton since the pole positions of g 18 n 150 e mg 18 n 158 e fg 18 n 137 e and hf 32 n 125 e spread over 40 in longitude these data cannot confirm or negate the possibility of a mid paleozoic transcurrent motion proposed for northern appalachians we regard only the pole ms with probable late early or early middle devonian age as reliable for the apparent polar path construction

transtensional deformation in the lake tahoe region california and nevada usa dextral transtensional deformation is occurring along the sierra nevada great basin boundary zone sngbbz at the eastern edge of the sierra nevada microplate in the lake tahoe region of the sngbbz transtension is partitioned spatially and temporally into domains of north south striking normal faults and transitional domains with conjugate strike slip faults the normal fault domains which have had large holocene earthquakes but account only for background seismicity in the historic period primarily accommodate east west extension while the transitional domains which have had moderate holocene and historic earthquakes and are currently seismically active primarily record north south shortening through partitioned slip the upper crust in this region undergoes overall constrictional strain major fault zones within the lake tahoe basin include two normal fault zones the northwest trending tahoe sierra frontal fault zone tsffz and the north trending west tahoe dollar point fault zone most faults in these zones show eastside down displacements both of these fault zones show evidence of holocene earthquakes but are relatively quiet seismically through the historic record the northeast trending north tahoe incline village fault zone is a major normal to sinistral oblique fault zone this fault zone shows evidence for large holocene earthquakes and based on the historic record is seismically active at the microearthquake level the zone forms the boundary between the lake tahoe normal fault domain to the south and the truckee transition zone to the north several lines of evidence including both geology and historic seismicity indicate that the seismically active truckee and gardnerville transition zones north and southeast of lake tahoe basin respectively are undergoing north south shortening in addition the central carson range a major north trending range block between two large normal fault zones shows internal fault patterns that suggest the range is undergoing north south shortening in addition to east west extension a model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes one mode characterized by an east west minimum principal stress and a north south maximum principal stress as at present in this mode seismicity and small scale faulting reflecting north south shortening concentrate in mechanically weak transition zones with primarily strike slip faulting in relatively small magnitude events and domains with major normal faults are relatively quiet a second mode occurs after sufficient north south shortening reduces the north south shmax in magnitude until it is less than sv at which point sv becomes the maximum principal stress this second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains which dominate the overall moment release in the region producing significant east west extension

a review of paleomagnetic results from the carboniferous of north america the concept of carboniferous geomagnetic field horizon markers results from the east coast carboniferous basin contain an important time stratigraphic horizon of possible global significance late carboniferous units are found to have mostly reversed magnetizations while older carboniferous units may have normal reverse or most frequently dual polarity magnetizations this feature records the change in geomagnetic field from a period of common polarity changes to a period of mainly reversed polarity the kiaman magnetic interval recognition of the stratigraphic position of this polarity feature provides a time line for correlation of units within and among carboniferous basins in the east coast carboniferous basin of atlantic canada this horizon has now been located at the riversdale group level early pennsylvanian namurian westphalian boundary since remanence acquisition of red sediments is often a lengthy process possibly spanning field reversal time scales red bed units pre dating and post dating this time line can in most instances be readily distinguished the former generally recording both polarities and the latter just one reverse polarity a polar displacement of at least 16 is recorded in the results from the east coast carboniferous basin paleolatitude maps show that the area was located near the equator and was displaced northwards by about 8 during the pennsylvanian it cannot be ascertained if those observations are representative of the whole of north america or merely the atlantic seaboard poles derived from other areas of north america do not all fit the regular pattern observed on the atlantic coast using the horizon marker as a tool it is suggested that those poles are post time line in age so that none of them is representative of the position of north america prior to the time line

statistical features of foreshocks in comparison with other earthquake clusters summary when earthquakes occur in a cluster the probability that they will be foreshocks of a forthcoming significantly larger earthquake appears dependent on the magnitude differences origin time spans and distances between several of the earliest events in a cluster the earthquake catalogue of the japan meteorological agency jma 1926 91 mj 2 4 is decomposed into numbers of clusters in time and space to compare statistical features of foreshocks with those of swarms and main shockaftershock sequences since the number of foreshocks in individual earthquake clusters is usually small two types of data stacking are considered first we consider the spatial and temporal distribution of events relative to the location and time of the coming main shock for the foreshocks a relative doughnut shaped pattern which converges in time to the main shock s epicentre is seen while this pattern is weak for swarms next we consider the temporal spatial and magnitude distributions of events relative to the time location and magnitude of earlier events in every sequence then a statistical discrimination of foreshocks from earthquakes of other types of clusters is explored the search for significant trends of relative frequency of foreshocks in comparison with other cluster types revealed the following features of foreshocks 1 their relative frequency is high when the inter event time span is less than several days but the maximum is attained at about several hours span 2 their relative frequency is high at short distances between epicentres especially within 10 km and then decreases as the distance gets larger and 3 their relative frequency increases as the magnitude increases among the earliest shocks in a cluster the stability of the results is confirmed by finding similar trends using different cluster identification methods and different threshold magnitudes further microearthquakes in the japan university network data file 1983 87 m 2 2 and moderate to large earthquakes in the international seismological centre world catalogue isc 1979 90 mh 2 5 are examined to see some effects of the magnitude thresholds of these catalogues similar features to those obtained in the jma catalogue are also found for these catalogues

a shallow detachment beneath the north greenland fold belt implications for sedimentation and tectonics in northern greenland in early palaeozoic time a turbidite trough the eastward extension of the hazen trough of arctic canada was flanked to the south by a carbonate platform the trough was deformed during the mid palaeozoic ellesmerian orogeny to form the e w trending north greenland fold belt this fold belt was deformed further by eurekan tertiary structures important among which is a major fault complex the harder fjord fault zone hffz the suggestion has been made that this fault zone controlled early cambrian sedimentation even though the fault trace does not coincide with the trough platform facies transition in sediments of that age this has led to some controversy we report new information from a mapping programme by the geological survey of greenland which has established the thin skinned nature of ellesmerian deformation at the trough platform transition and implies that much of the fold belt is underlain by a shallow detachment this in turn implies that the hffz exists in the hanging wall of the detachment while the early cambrian trough platform transition is located autochthonously in the foot wall we adduce evidence to show that the latter was probably controlled by syndepositional faulting with actively eroding fault scarps and suggest that these basement structures were reactivated in a dextral strike slip mode in early tertiary time to form the hffz as now observed

palaeomagnetism of caledonian intrusive suites in the northern highlands of scotland constraints to tectonic movements within the caledonian orogenic belt abstract to evaluate the scale of tectonic movements within the northern sector of the 500 400 ma caledonian orogenic belt and its precambrian foreland zone between the great glen fault ggf zone to the southeast and the laurentian block to the northwest we have studied the palaeomagnetism of minor intrusive rocks within the northern highlands terrain these rocks include 1 1 amphibolites and other metamorphic rocks predating deformation 2 2 microdiorities dolentes and related suites emplaced and probably magnetised between 450 and 420 ma and 3 3 lower middle devonian lamprophyres a range of predominantly nne negative and ssw positive components are resolved by cleaning treatment with a dispersion of declinations towards a minority wnw ese axis isolated southerly negative directed hematite held components suggests limited but no widespread remagnetisation in devonian carboniferous times comparison is made with data from other tectonic divisions in the caledonian orogenic belt and the bordering forelands palaeopoles from the northern highlands closely conform in part with north american ordovician poles and in part with the post ordovician palaeopoles from britain south of the ggf the definitive motions of the british caledonides to emerge from the palaeomagnetic analysis are an anticlockwise rotation of the caledonian terrain in early ordovician times small relative motions during the remainder of ordovician times followed by large clockwise and then anticlockwise rotations during late ordovician to early silurian times contemporary with the last major movements on the moine thrust ca 430 ma late silurian devonian movements along the ggf were probably below the limits of palaeomagnetic detectability the collective data require that apparent polar wander movements and concomitant continental movements have currently been incompletely recovered by north american studies and the path for lower palaeozoic times is more complex than recognised hitherto

a paleomagnetic reinvestigation of the upper devonian perry formation evidence for late paleozoic remagnetization america we undertook a reinvestigation of the upper devonian perry formation of coastal maine and adjacent new brunswick thermal demagnetization of samples from the redbeds yielded a characteristic direction d 166 l 4 that fails a fold test comparison of the corresponding paleopole 312 e 41 s with previously published paleozoic poles for north america suggests that the sediments were remagnetized in the late carboniferous after the removal of a steep northerly component the volcanics also reveal a shallow and southerly direction d 171 1 25 without tilt correction no stability test is available to date the magnetization of the volcanics however similarity of several of the directions to those seen in the sediments raises the suspicion that the volcanics are also remagnetized although the paleopole without tilt correction 303 e 32 s could be taken to indicate an early carboniferous age for the remagnetization scatter in the data suggests that the directions are contaminated by the incomplete removal of a steeper component due to present day field thus it is more likely that the volcanics were remagnetized at the same time as the sediments isothermal remanent magnetization irm acquisition curves blocking temperatures coercivities and reflected light microscopy indicate that the magnetization is carried by hematite in the sediments and by both magnetite and hematite in the volcanics it is therefore likely that the remagnetization of the perry formation involved both thermal and chemical processes related to the variscan alleghe nian orogeny our results indicate that previously published directions for the perry formation were based on the incomplete resolution of two magnetic components these earlier results can no longer be considered as representative of the devonian geomagnetic field

palaeomagnetism of the foyers and strontian granites scotland abstract palaeomagnetic data from the newer granites of foyers and strontian show similar directions of magnetization with southerly declinations and horizontal to intermediate mostly downward dipping inclinations these palaeomagnetic directions which accord with previous data from the helmsdale granite are thought to be of siluro devonian age the present results are not sufficiently precise to justify discussion of possible lateral displacements along the great glen fault of the order of a few hundred kilometres but it is clear that the recent idea of a c 15 20 latitudinal offset along the fault in the carboniferous has to be disregarded

paleomagnetism of the upper devonian catskill formation from the southern limb of the pennsylvania salient possible evidence of oroclinal rotation multiple components of magnetization were isolated in the natural remanent magnetiza tion of samples of the upper devonian catskill formation red beds taken from the southern limb of the pennsylvania salient the dominant ther mally distributed component sf previously thought to predate folding is demonstrably syn folding in origins the mean direction for sf based on data from the current study and a pre vious study is declination inclination 161 6o 7 9 o a95 3 9 o pole position 127 3oe 43 1on a95 3 1 o n 14 sites although the remagnetization is clearly synfolding in most areas the relative ages of folding and remagne tization vary locally a subordinate high unblocking temperature component ht has a mean tilt corrected direction of 160o 36 o a95 16 o pole position 123 5oe 26 ion a95 15 4 o s n m 7 samples comparison of ht with the prefolding magnetization isolated in the northern limb of the salient suggests that the paleolatitude of this part of north america was about 16os and that part of the curvature of the salient was acquired during orogenesis

palaeomagnetism of the middle upper devonian esha ness ignimbrite w shetland abstract the ignimbrite division of the esha ness volcanics of w shetland has been studied palaeomagnetically the major blocking temperature spectrum ranges between 650 and 680 c implying that haematite is the principal remanence carrier the magnetization build up is simple a minor low stability magnetization aligned along the direction of the present earth s field is superimposed on a well defined and highly stable palaeomagnetic component with dec 220 and inc 3 the esha ness volcanic sequence is folded into a gentle syncline trending nne the rock collection concerned covers 8 sites 7 from the western flank and 1 from the eastern flank of the syncline the 7 sites on the west flank are consistently reversely magnetized while the site on the east flank has a normally directed magnetization after structural unfolding the normal and reverse directions are almost perfectly antiparallel i e the stable magnetization most likely predates the tilting which is probably of devonian age esha ness is located some 12 km to the west of the walls boundary fault wbf which is considered as the northward continuation of the great glen fault ggf the shallow palaeomagnetic inclination found in old red sandstone rocks to the west of ggf wbf corresponds to that encountered in most other european old red formations discounting therefore megascale 1000 km late post devonian movements along this dislocation

implications of early devonian poles from the canadian arctic archipelago for the north american apparent polar wander path lower devonian red beds from the upper member of the peel sound formation at prince of wales island canadian arctic yield two different paleopoles at 25n 099e and at 01n 091e the first one being

early proterozoic oceanic crust and the evolution of subcontinental mantle eclogites and related rocks from southern africa seven eclogitic nodules from kimberlites in southern africa have been studied in detail for whole rock major and trace element geochemistry petrography and mineral chemistry by electron microprobe high purity mineral separates from six of these samples have been analyzed for trace elements and for the isotopic composition of sr nd and oxygen three eclogite groups are recognized group a eclogites have very high mg fe low na in pyroxene moderate 18o 4 7 to 5 3 and low 87sr 86sr and 143 144nd olivine and enstatite may also be present as accessory phases in this group group b eclogites have moderate to high mg fe ratios high na in pyroxene low 18o 3 0 to 3 4 high 87sr 86sr and 143nd 144nd ratios and extremely lree depleted hree enriched garnets with no eu anomalies group c eclogites have low mg fe ratios high na in pyroxene and variable sr nd and o isotope compositions accessory feldspar is present in one sample that may be of crustal origin mineral separates of garnet and pyroxene have positive eu anomalies the group a eclogites are too refractory to represent magma compositions and must have formed as cumulate dike rocks in the upper mantle that contain a minor trapped liquid component this is supported by the presence of accessory olivine and enstatite and by their ksm and knd which are similar to empirical pyroxene garnet partition coefficients the group b eclogites are extremely depleted in incompatible elements and have ndvalues 10x to 20x morb the high 87sr 86sr of these eclogites is not consistent with their strongly depleted lree and high nd and cannot be primary it must have been imposed on the protolith the low 18o of these eclogites cannot form by mantle fractionation processes and must also be inherited from the protolith both the sr and oxygen isotope data are consistent with high temperature hydrothermal alteration of a basaltic protolith followed by partial melting to form the refractory compositions observed now the hydrothermal fluid may have been sea water but secondary enrichment of the protolith in rb is required to generate the observed high sr ratios the high na content of the pyroxenes supports a spilitic alteration event the major and trace element characteristics of the group c eclogites are consistent with recrystallization of a cumulate gabbro protolith one group c rock is probably a garnet granulite derived from the lower crust the other may represent the plutonic portion of oceanic crust a reconstructed whole rock isochron for the three bellsbank eclogites yields an age of 2 1 0 1 b y this implies that plate tectonic processes involving the generation and subduction of oceanic crust have been active since the early proterozoic the early proterozoic ocean crust consisted of two components a plutonic section of gabbro cumulates group c eclogites and a volcanic section of basalt group b eclogites the volcanic component has undergone substantial hydro thermal alteration and subsequent partial melting to form a refractory residue the plutonic component is less modified

magnetostratigraphy of the plio pleistocene kap kobenhavn formation eastern north greenland abstract the newly discovered late cenozoic kap kobenhavn formation in peary land eastern north greenland 82 30 n 21 w which consists of at least 100 m unconsolidated sands silts and clays deposited in marine shallow waters covers an area of some 500 km2 the time of deposition spanned by the sequence is estimated by comparisons with typical rates of isostatic rebound in greenland to be roughly of the order of 3 104 years abundant content of organic remains indicates a warmer climate than at present a number of 185 oriented palaeomagnetic specimens from two parallel composite profiles indicate after af cleaning that the sampled part of the formation was deposited during a period of reversed geomagnetic polarity combining the polarity information with information from biostratigraphy and amino acid ratios the most likely age of the kap kobenhavn formation is estimated to be constrained to the older part of the matuyama reversed polarity chron i e with an age of between 1 87 ma pre olduvai and 2 48 ma post gauss the most reliable least scattered parts of the magnetic record give cleaned mean inclinations of 77 and 59 respectively indicating a systematic departure from the axial dipole in the near sided sense in contrast to most other late tertiary palaeomagnetic results

palaeomagnetic re examination of the basal caithness old red sandstone aspects of local and regional tectonics abstract a palaeomagnetic re examination of the basal strata of the caithness old red sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region after structural correction the dominant remanence component has d 205 i 3 95 6 4 n 27 the existence of this shallow inclined magnetization in the middle devonian strata of caithness invalidates the model proposed by van der voo and scotese 1981 involving a ca 2000 km sinistral offset along the great glen fault in the carboniferous however the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone however the possibility of there having been a much larger transcurrent shift between europe and north america in late post devonian times accumulated along various fracture zones within the caledonian fold belt is discussed on the basis of an inferred overprinted magnetization it is tentatively concluded that the tectonic deformation of the old red sandstone of caithness has a mid jurassic or younger age

paleomagnetism of the lower devonian traveler felsite and the acadian orogeny in the new england appalachians a suite of felsic extrusive rocks from the lower devonian traveler felsite north central maine 46 1 n lat 68 9 w long was collected for paleomagnetic study 28 samples from 5 sites in the lower pogy member 56 samples from 10 sites in the upper black cat member and 13 samples from 10 cobbles in the basal conglomerate of the overlying trout valley formation characteristic magnetizations based on af demagnetization analyses and supported by thermal studies give a formation mean direction after simple correction for bedding tilt of d 25 2 i 20 2 95 10 6 for 13 sites in the traveler felsite the occurrence of normal and reversed magnetozones under stratigraphic control and the antiparallel directions of the polarity units following the tilt correction indicate an acquisition of remanence near the time of formation of the rock units the mean direction corresponds to a south paleopole position of 29 s lat 262 e long p m 6 11 for the traveler felsite this pole position is in close agreement with poles from two other rock units of similar age from the same lithostratigraphic belt in the northern appalachians the mean of these three poles is distinct from late silurian and middle devonian poles from cratonic north america laurentia and from early devonian results from the acadia displaced terrane suggesting that this area constituted a separate tectonic unit during the early devonian we propose a tectonic model in which a traveler terrane is rotated 20 to 30 while converging with laurentia and undergoing deformation during the early to middle devonian acadian orogeny

evidence against large scale carboniferous strike slip faulting in the appalachian caledonian orogen kent and opdyke1 have argued that during the late devonian and early carboniferous the eastern part of the northern appalachians acadia was situated 1 500 km further south relative to cratonic north america than it is at present and that it moved to its present position sometime later in the carboniferous fig 1 van der voo and scotese2 proposed an even greater displacement of 2 000 km which they attribute to motion between cratonic north america and europe acadia being attached to europe fig 1 the argument is that palaeolatitudes p derived palaeomagnetically from upper devonian and lower carboniferous rocks of cratonic north america are 15 20 more northerly than those of acadia and europe the proposed shear zone passes through central newfoundland using palaeomagnetic results from eastern and western newfoundland we show here that no such motion occurred we also show that kiaman late carboniferous and permian overprinting is widespread in newfoundland and that these secondary magnetizations agree with two exceptions attributed to dextral rotation of the colorado pleateau with observations from late devonian and early carboniferous rocks of the north american craton confirming the proposal3 that magnetizations of the cratonic early carboniferous rocks are kiaman not early carboniferous in age our results also enable us to extend this proposal3 to late devonian rocks of the craton hence the palaeolatitudinal offset of fig 1 is almost certainly not tectonic but is an artefact of the wrong assumption of the equivalence of rock and magnetization ages

palaeomagnetism of silurian sediments in w ireland evidence for block rotation in the caledonides summary a palaeomagnetic study of mid silurian wenlock sediments of the salrock formation in nw galway yields a reliable silurian palaeomagnetic pole position for w ireland at latitude 1 6 s longitude 287 7 e dp 2 5 dm 4 6 n 27 sites a further 12 sampling sites were located in silurian redbeds on clare island s mayo where a palaeomagnetic remanence of silurian age was also identified both remanences pre date local late caledonian folding however the clare island result has an anomalous azimuth when compared with the salrock result and with published silurian results for britain the two irish results can be reconciled by anticlockwise block rotation of the clare island region through 40 50 the rotation brings both palaeomagnetic and structural trends within the two sampling regions into alignment strike slip displacement across the clew bay fault zone producing a sinistral shear couple might have enabled such a rotation the timing and geographic extent of the proposed block rotation is as yet poorly defined however a younger age limit for rotation is provided by a late devonian or carboniferous partial magnetic overprint observed throughout both study areas and from geological evidence it seems likely that rotation is confined to the clew bay fault zone the dalradian rocks of the connemara massif located to the south of the salrock district have an e w structural grain some workers have proposed clockwise rotation of the connemara massif of ca 30 at between 500 and 390 ma which would bring the structural trends in w ireland into parallelism with caledonian trends in scotland because the silurian salrock succession which rests unconformably on the dalradian basement of connemara shows no such anomaly it provides a younger age limit for the possible rotation of connemara

palaeomagnetic studies within the ballantrae ophiolite southwest scotland magnetotectonic and regional tectonic implications abstract the early arenig slockenray formation within the ballantrae ophiolite southwest scotland displays a multivectorial magnetisation structure two components s and m are identified delineated by differing blocking temperature coercivity spectra component s is removed around 200 c 10 mt and is regarded to be of recent viscous origin component m forms the characteristic formation magnetisation and resides in both magnetite and haematite extensive sampling of all exposed lithologies reveals an in situ non fisherian distribution of the characteristic magnetisation defining an envelope from se moderate positive to sw shallow negative directions a negative infra formation conglomerate test identifies this component as a pervasive overprint a second conglomerate test performed in the overlying benan conglomerate of llandeilo age reveals dispersely directed magnetisation with a stability range equivalent to that of component m this field test therefore defines a maximum remagnetisation window of 30 million years for the characteristic remanence hard viscous magnetisations are identified both in the benan conglomerate and at some sites within the slockenray formation structurally corrected site mean results from the slockenray formation define a non fisherian distribution and form a small circle partial arc centred on a vertical axis nw moderate positive to sw moderate positive directions a combined palaeomagnetic fold and fault test suggests that acquisition of component m pre dates both folding and faulting the resulting palaeolatitude of remanence acquisition 28 8 s implies a tectonic position close to the southern laurentian margin for the ballantrae ophiolite in arenig times

magnetism of igneous rocks from the tourmakeady and glensaul inliers w ireland mode of emplacement and aspects of the ordovician field pattern summary results of palaeomagnetic and rock magnetic studies are reported mainly for quartz porphyry collected at 18 sites from an ordovician igneous sedimentary succession at tourmakeady and glensaul in co mayo eire both magnetite and haematite are shown to be prominent magnetic constituents the magnetite is usually in the form of large multidomain grains these are held responsible for a soft remanence component called the b magnetization with d i 000 75 95 3 7 i e practically identical to the direction of the present axial geocentric dipole field in the area hence it is inferred that the b magnetization which is dominant at most sites is of recent origin haematite which has a more patchy occurrence apparently formed through high temperature oxidation of magnetite haematite is the main carrier of the palaeomagnetically significant component the a magnetization which gave well grouped in situ directions 29 samples from 10 sites with a mean value of d i 182 6 corresponding to a north pole at 31 n 169 e dp dm 2 1 4 2 correction for folding along the glensaul tourmakeady are shows that the a magnetization is of post folding age an earlier suggestion that the rocks represent a lower ordovician extrusive suite is discussed and discounted it is inferred that the quartz porphyries constitute syntectonic intrusions having formed during the taconic orogeny these rocks as well as their characteristic magnetization are therefore regarded as late ordovician in age it is concluded that a c 50 relative clockwise shift between the mid ordovician and late ordovician silurian geomagnetic field axes previously demonstrated for scotland has now been recognized also in ireland

palaeomagnetic signature of slow post orogenic cooling of the north east highlands of scotland recorded in the newer gabbros of aberdeenshire summary consideration of previous work and new data show a systematic distribution of the directions of high coercivity and high blocking temperature magnetization in the newer gabbros of aberdeenshire the site mean directions show a non fisherian distribution with the corresponding pole positions distributed along a published early palaeozoic apparent polar wander path for great britain the mean of the distribution closely agrees with the mean direction previously reported and widely quoted there is a close correlation of the site mean directions with the position of the sampling sites with respect to the contours chrontours of the k ar coolingage sites where stable nrm has se declinations and shallow 20 inclinations have the oldest chrontour ages 468 myr the mean palaeomagnetic direction progressively shifts and the sites with the youngest chrontour ages 428 myr have sw declinations with moderately steeper 45 inclination the relative age of magnetization determined from the position of the corresponding pole on the apparent polar wander path is consistent with the relative chrontour age the gabbros have therefore recorded the shift of the north east highlands with respect to the palaeomagnetic pole during the slow cooling of the terrain the slow cooling is presumably related to uplift and erosion and the resulting downward migration of isothermal surfaces a previously published suggestion that all magnetizations with southerly declinations and moderately downward inclination are the resultant of normal and reversed magnetizations with shallow inclination is shown to be incorrect

paleomagnetism of the silurian devonian andreas redbeds evidence for an early devonian supercontinent two components of magnetization were isolated in the silurian devonian andreas redbeds of the central appalachians of pennsylvania lat 40 75 n long 75 78 w a thermally distributed synfolding b component and a thermally discrete pre alleghenian age folding c component the c component mean direction and associated pole position correspond to a silurian devonian paleolatitude for the andreas location of about 35 s which in conjunction with early devonian results from gondwana is consistent with an early devonian supercontinent configuration

a multicomponent dual polarity palaeomagnetic regional overprint from the moine of northwest scotland abstract palaeomagnetic remanence may be reset by orogeny in a manner analogous to isotopic systems the heavy mineral seams and associated psammite in the deformed and metamorphosed lower morar division of the moine assemblage of northwest scotland have a palaeomagnetic signature that is characteristic over a wide area the various components of the magnetization are resolved by detailed thermal demagnetization these components are post tectonic as they are not affected by any structural fabric they may be classified according to blocking temperature and when statistically distinguishable show a systematic change of direction from high to lower temperature possibly associated with apparent polar wander comparison to the european palaeomagnetic record suggests a late silurian to early devonian age of magnetization for the localities south of the strath conon fault the distribution of remanance directions and blocking temperatures is not consistent with the pattern of potassium argon ages in this part of the british caledonides if the magnetization is purely of thermal origin the palaeomagnetic record indicates higher temperatures at later time than does the potassium argon record this remagnetization event is related to the closing stages of the caledonian orogeny

palaeomagnetism of middle proterozoic c 1 25 ga dykes from central north greenland summary from a nunatak in central north greenland 81 son 44 7 w nine sites of middle proterozoic basic dykes cutting archaean basement were palaeomagnetically investigated after af and thermal cleaning the nine dyke sites and three adjacently baked gneiss sites give a stable characteristic remanent mean direction of d 2654 i 21 5 n 12 ags 5 6 the direction being confirmed by a detaded and positive baked contact test the polarity of the dykes in the nunatak area is opposite to that of the zig zag dal basalts and the midsommers dolerites in eastern north greenland some 200 300 km away the volcanics of which are assumed to be of similar age about 1 25 ga the remanent directions of the two sets of data are antiparallel within the 95 per cent significance level of confidence when rotating greenland 18 clockwise back to north america by the bullard fit the pole of the central north greenland dolerites ndl falls at 14 3 n 144 3 w the reversed pole 14 3 s 35 7 e fits well on to the loop between 1 2 and 1 4 ma on the apparent polar wander swath of berger york for cratonic north america the palaeomagnetic results from the middle proterozoic basic dykes from central north greenland thus strengthen previous palaeomagnetic results from the midsommersa dolerites and zig zag dal basalts from the peary land region in eastern north greenland suggesting that greenland was part of the north american craton at least for the period between c 1 3 and 1 ma and probably up to the end of cretaceous time the major geographical meridian of greenland was orientated approximately e w and the palaeolatitude of greenland was about 10 150

a new siluro devonian paleopole from early paleozoic rocks of the franklinian basin north greenland fold belt demagnetization of dolomitic shales dolomites and carbonate breccias of the cambro ordovician kap stanton formation within the franklinian basin sequence of the north greenland fold belt yields a characteristic direction declination inclination is 16 16 tilt corrected from 14 of the 28 sites a negative breccia test and positive local and regional fold tests bracket the age of the magnetization as postdepositional but pre ellesmerian pre carboniferous after rotating the corresponding paleopole 1 n 302 e to account for the cretaceous tertiary opening of the labrador sea the greenland pole remains removed by approximately 20 from the existing north american apparent polar wander path apwp however the rotated paleopole 2 s 284 e in north american coordinates is similar to british silurian and devonian paleopoles in a bullard et al 1965 reconstruction pole is 2 s 322 e in british coordinates we interpret our new paleopole as late silurian to early devonian in age requiring a modification of previously published apwps for laurentia if the age assignment is correct the new paleopath for north america greenland and that for great britain can be brought into good agreement by closing the atlantic ocean thus indicating that north america greenland and great britain were assembled in a supercontinent in late silurian to early devonian time

paleomagnetic results from the peterhead granite scotland implication for regional late caledonian magnetic overprinting abstract palaeomagnetic results from the peterhead granite silurian ne grampian highlands define a characteristic magnetization with overall parameters of dec 180 4 inc 22 3 and a 95 5 2 these results agree with other newer granite data from north scotland an east west trending dyke cutting the peterhead granite shows an extremely well defined paleomagnetic direction with dec 192 5 inc 19 1 and a 95 1 3 suggesting a permo carboniferous age the peterhead granite results are seen in conjunction with other palaeozoic palaeomagnetic data from scotland it is concluded that the observed directional spread of characteristic remanence directions previously reported from the aberdeenshire gabbros are not related to slow post orogenic cooling watts and briden but is rather seen as the result of late caledonian magnetic overprinting

palaeomagnetic resetting in the barrovian zones of scotland and its relationship to the late structural history the haematite schists in the lowest kyanite and the staurolite barrovian zones exposed in glen lethnot and glen esk angus scotland have a characteristic magnetization with blocking temperatures generally greater than 600 c and less than 695 c and coercivity greater than 200 mt the mean palaeomagnetic direction derived from 54 samples collected from n 10 sites isd 159 i 45 k 47 95 7 1 with the associated south pole position oflat 3 long 22 e dp 5 7 anddm 9 0 this magnetization is younger than folds of d3 generation and older than folds of d4 generation the pole position falls on the aberdeenshire polar wander track in a place consistent with an average kar biotite chrontour age of 455 m y a comparison of the blocking temperatures of magnetization to the estimated peak temperature of metamorphism suggest the magnetization is a chemical remanent magnetization crm possibly acquired during the metamorphic chemical reactions

evolution of the early paleozoic deep water basin of north greenland aulacogen or narrow ocean a major lower paleozoic deep water sequence is exposed for more than 800 km along the coast of north greenland the maximum preserved width of the basin is about 200 km and the thickness of the sedimentary sequence may reach 8 km nine evolutionary stages in basin evolution are recognized each characterized by distinct sedimentary facies and structural style of the shelf basin boundary 1 the oldest sequence of unknown age is so deformed that inferences about the depositional environment are precluded 2 late precambrian early cambrian incipient basin 3 early cambrian narrow turbidite basin 4 middle cambrian ordovician basin expansion and initial starvation 5 latest ordovician early silurian longitudinally filled turbidite basin 6 middle silurian basin expansion and starvation 7 middle late silurian wide turbidite basin 8 middle late silurian caledonian thrusting and conglomerate deposition and 9 probably latest silurian megasliding and advance of the caledonian front the basin evolution that came to an end during the mid paleozoic ellesmerian orogeny was punctuated by events related to the closure of the iapetus ocean and formation of the caledonian mountain belt to the east the basin may have been fully ensialic and formed during the early rifting stages preceding back arc spreading or it may have reached a narrow ocean stage at present however we are inclined to favor an interpretation of the basin as being an aulacogen extending deeply into an old continent at a right angle to the caledonian orogenic front to the east

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

magnetic properties of the lower old red sandstone lavas in the midland valley scotland palaeomagnetic and tectonic considerations abstract thermal demagnetization studies of lavas in the strathmore area of the midland valley scotland support overall palaeomagnetic data found in previous studies of these rocks reduced directional scatter as compared to some earlier studies is attributed to more effective demagnetization resolving some of the directional complexity of previous studies combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism the existence of almost antiparallel directional groups and field tests give supporting evidence for a primary deuteric origin of the main magnetization of these rocks additionally a second remanence component having shallow reverse directions of magnetization is attributed to later remagnetization in old red sandstone time the midland valley results are seen in conjunction with other palaeozoic palaeomagnetic results and possible geodynamic implications are discussed