geochemical variations in the cocos plate subducting beneath central america implications for the composition of arc volcanism and the extent of the galapagos hotspot influence on the cocos oceanic crust new geochemical data from the cocos plate constrain the composition of the input into the central american subduction zone and demonstrate the extent of influence of the galapagos hotspot on the cocos plate samples include sediments and basalts from ocean drilling program odp site 1256 outboard of nicaragua gabbroic sills from odp sites 1039 and 1040 tholeiitic glasses from the fisher ridge off northwest costa rica and basalts from the galapagos hotspot track outboard of central costa rica site 1256 basalts range from normal to enriched morb in incompatible elements and have pb and nd isotopic compositions within the east pacific rise morb field the sediments have similar 206pb 204pb and only slightly more radiogenic 207pb 204pb and 208pb 204pb isotope ratios than the basalts altered samples from the subducting galapagos hotspot track have similar nd and pb isotopic compositions to fresh galapagos samples but have significantly higher sr isotopic composition indicating that the subduction input will have a distinct geochemical signature from galapagos type mantle material that may be present in the wedge beneath costa rica gabbroic sills from sites 1039 and 1040 in east pacific rise epr crust show evidence for influence of the galapagos hotspot 100 km beyond the morphological hotspot track

magnetite authigenesis and diagenetic paleotemperatures across the northern appalachian basin the concentration of magnetite in the lower and middle devonian helderberg and onondaga carbonate formations varies in a distinct pattern along an east west profile across new york state magnetite concentrations are uniformly low in the western half of the profile increase sharply eastward to a peak value near syracuse and decrease farther east the pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile previous paleomagnetic studies have documented that the magnetite carries a well defined magnetization of pennsylvanian permian age and magnetite authigenesis is therefore no younger than late paleozoic we attribute the correlation between high magnetite concentration and high illite content to temperature dependent diagenesis triggered by orogenic fluids the large proportion of secondary magnetite indicates that over most of the area the mechanism of late paleozoic remagnetization was principally chemical thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence

palaeomagnetic properties of a precambrian metamorphic terrane the lewisian complex of the outer hebrides nw scotland abstract the palaeomagnetism of a large area of precambrian metamorphic basement comprising the outer hebrides archipelago in nw scotland is described a stable magnetite held imprint characterises much of the complex dispersed easterly steep a1 and ne intermediate c components are present in the northern sector but are subordinate to nw positive to nw negative a2 a5 b components nw shallow a4 components predominate in the south of the outcrop area a1 a2 and c components are confined largely to areas which have escaped tectonism since scourian 2650 ma times and are probably a relict record the a4 components are most prominent in areas profoundly affected by the laxfordian episode and record the peak of a later magnetising event by correlation with apw from the laurentian and fennoscandian shields relict components are interpreted as a possible acquisition at 1820 1750 ma close to the peak of laxfordian tectonism the younger components are linked to retrogressive amphibolite facies metamorphism and rapid crustal uplift at 1700 1630 ma anisotropy of susceptibility is variable in magnitude and predominantly foliated it appears to be controlled mainly by laxfordian strain although the effect of the earlier scourian episode is also recorded a younger lamprophyre dyke suite yields a palaeomagnetic pole from nine sites 349 e 47 s dated as mid permian by reference to the eurasian apw path the spatial distribution of magnetisations in the outer hebrides basement mirrors the distribution observed in the mainland outcrop comprising the nw foreland to the caledonian orogen but is displaced laterally the two distributions can be matched by adjustment for ca 95 km of sinistral strike slip on the site of the minches basin between the two outcrops this fault zone is the nw member of a suite of ne sw trending transcurrent faults active in late caledonian times and possesses the largest strike slip offset

dolomitic breccia veins as evidence for extension and fluid flow in the dalradian of argyll veins of red dolomite occur extensively in the dalradian rocks of argyll scotland and adjacent areas the veins represent brittle extensional deformation preferentially reactivating caledonian quartz veins the dolomite is associated with reddening of the adjacent dalradian country rock which it partially replaced dolomite was also precipitated in overlying old red sandstone and probably dates to late carboniferous early permian fluid inclusion studies show that the veining involved moderate temperature 75 to 115 c fluids stable isotope data suggest that these fluids were basinal brines traces of chalcopyrite paragenetically late in the veins may reflect the mineralization which occurs more widely in the dalradian rocks of argyll the red colour of the dolomite is due to abundant haematite crystallites that grew in the dolomite crystal fabric palaeomagnetic analysis yields a consistent late permian early triassic age for the haematite growth in the dolomite veins and the reddened dalradian country rocks this age represents the time of haematite precipitation from iron rich dolomite that may have been related to deep oxidizing weathering gold anomalies associated with reddened basement rock must be of this age or younger

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

post laxfordian magnetic imprint in the lewisian metamorphic complex and strike slip motion in the minches nw scotland the palaeomagnetic imprint in the lewisian metamorphic basement was acquired during early post laxfordian times and resides in magnetite filled microfractures an early magnetizing event dated c 1820 1750 ma was largely overprinted by a second event between 1700 and 1500 ma which was probably linked to retrogressive amphibolite fades metamorphism it reaches its greatest intensity in the south of the outcrop while the relict remanence survives most prominently in scourian terranes populations of magnetic components are offset by c 95 km of presumed caledonian sinistral strike slip in the minches intensities and susceptibilities of magnetization are enhanced in zones of laxfordian reworking relative to the relict scourian zones their distributions in the outer hebrides are reproduced in the mainland outcrop and can be matched following correction for the same sinistral strike slip a third line of evidence suggesting this motion is the offset of the deep crustal contribution to the aeromagnetic anomalies the fault zone accommodating this motion may now be represented by one or more of the low angle detachments bordering the western margin of the minches basin post laxfordian remanences in the outer hebrides are rotated by an average of 4 with respect to equivalent vectors on the mainland and may record later block rotations a mid proterozoic reconstruction of the lewisian complex derived from the magnetic evidence is presented

a palaeomagnetic study of a caledonian ophiolite summary a total of 89 sites have been sampled for palaeomagnetic studies from the caledonian ophiolite complex of northeast shetland 60 7dn 359 2de and a post ophiolite emplacement suite of lamprophyre dykes of these only 35 sites from the ultramafite facies of the complex yielded statistically significant results these sites define three distinct populations of magnetization directions which correspond to south palaeomagnetic poles at 47 2ds 244 2de a 18 0ds 323 4de b and 35 0ds 351 1de c of these only the c remanence can be assigned an unambiguous age of mid carboniferous the ages of the other two poles remain ambiguous due to the uncertainty in the apw path and the complexity of the ophiolite history nevertheless palaeomagnetism is shown to be successful in detecting low temperature low pressure alteration events that have affected the complex and which hitherto have gone undetected

the influence of correlations in computing crustal strains from trigonometric network results abstract a variance covariance analysis is made concerning the crustal strain parameters deduced from triangulation results the coordinate difference method is treated the algorithm of the analysis being valid for either free or constrained network adjustments the importance of the correlations entering into the calculation of the standard deviations of strains is emphasized showing how even poor geodetic results can be sometimes useful in crustal strain analyses

palaeomagnetic study of the late mesoproterozoic torridon group nw scotland age magnetostratigraphy tectonic setting and partial magnetic overprinting by caledonian orogeny abstract the late mesoproterozoic torridonian supergroup is a predominantly fluviatile 7 km thick succession of red sediments deposited onto palaeoproterozoic lewisian metamorphic terrane of the hebridean craton in nw scotland although a subject of investigation since the earliest days of palaeomagnetism the age and constitution of the remanent magnetisation in the upper torridonian torridon group have remained unclear in this study we undertake detailed statigraphically controlled sampling through the succession and conduct progressive thermal demagnetisation to resolve component structures rock magnetic analysis shows that magnetite although volumetrically small is a significant component of the ferromagnetism in these sediments in addition to specularite and pigmentary hematite unlike the lower torridonian stoer group which was fully lithified prior to late grenville deformation 1100 ma the torridon group was not influenced by tectonism until early palaeozoic times and records a complex history of partial and sometimes total later overprinting the a remanence is extensively overprinted by a b remanence with e to s directed intermediate to steep positive directions and subordinate reversed representatives and less commonly by a c remanence with ne direction b is resolved mainly in the magnetite spectrum and incorporates a swathe of directions coincident with mid ordovician palaeofields predicted from baltica and adjoining caledonian terranes it is therefore interpreted as a remanence residing in small amounts of authigenic magnetite acquired during burial of this margin of the hebridean craton by nappe stacking during the grampian orogeny 470 430 ma following final closure of the iapetus ocean due to the merging of the b population with the a direction a has not been defined as an antiparallel axis by former studies and can be inseparable by cleaning however defining a to include components with i d i 115 31 95 2 9 for a and d i 293 25 95 3 5 for a the poles at 238 e 1 s dp dm 2 0 3 8 and 235 e 1 n dp dm 1 8 3 3 are close to the laurentian apwp at 1039 1025 ma on the pre drift reconstruction and constrain sedimentation within the 1060 to 994 977 ma range of previous evidence only minor polar movement is detected through the entire thickness of the torridon group and suggests a sedimentation rate of the order of 0 5 m 1000 years for this clastic succession a magnetic stratigraphy in a suggested by early work is confirmed in at least 13 polarity zones and identifies a rapid reversal rate preserved in specular hematite fixed by post depositional mechanisms postdating slumping and sand volcano formation in wet sediment the palaeomagnetic age of the a remanence links torridonian sedimentation to rifting and subsidence accompanying break up of the laurentian and fennoscandian shields at 1050 950 ma following the grenville orogeny rifting between the two shields was followed by rotation and rewelding into a neoproterozoic configuration these events correlate with transition in torridon group palaeoenvironments from enclosed basin to open basin with river systems feeding onto a passive margin to marine transgression and finally return to enclosed basin conditions diagenesis continued in post palaeozoic times and a d remanence with n s shallow axis is sporadically preserved in authigenic hematite and interpreted as mainly a variscan acquisition

short paper the depositional age of the dalradian supergroup u pb and sm nd isotopic studies of the tayvallich volcanics scotland zircons from a keratophyre associated with the tayvallich volcanics in the dalradian rocks of the sw scottish highlands have been dated by u pb methods yielding an age of 595 4 ma this age indicates that most or all of the dalradian is precambrian in age and that dalradian sedimentation may have lasted for about 200 million years the age also constrains the time interval between cessation of dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the grampian orogeny sm nd isotopic data for the tayvallich volcanins and related metadolerite sills yield initial ond values of 2 to 4 which are thought to reffect the effects of melting of lithospheric mantle

palaeomagnetism of palaeozoic peel sandstones and langness conglomerate formation isle of man implications for the age and regional diagenesis of manx red beds

a rock magnetic study of the lower massive sandstone moenkopi formation triassic gray mountain area arizona this paper describes the results of rock magnetic and petrographic studies of the lower massive sandstone of the moenkopi formation north central arizona which heretofore has been considered to be a reliable magnetostratigraphic unit the rock magnetic studies included stepwise thermal demagnetization of samples of intraformational mudstone clasts and a deformed shale lens both of which occur within the lower part of the sandstone and stepwise thermal and selective destructive demagnetization sdd of samples of the sandstone containing black sand laminae composed largely of opaque grains in samples both from the claystone clasts and deformed shale lens two secondary components one of modern normal polarity and one of triassic normal polarity were unambiguously identified and a third crm component of triassic reversed polarity was tentatively identified there is no evidence in samples of either the claystone clasts or deformed shale lens of a remanence acquired prior to deposition or to deformation respectively sdd by means of which the small scale homogeneity of remanence within samples of the lower massive sandstone containing black sand laminae could be assessed shows that each sample primarily contains a heterogeneous three dimensional mixture of triassic normal and reversed components the triassic normal direction that is characteristic of the lower massive sandstone is a vector resultant of almost balanced proportions of the two nearly antiparallel components averaging about 55 to 60 normal and 45 to 40 reversed the local variability in both direction and intensity of remanence within and between individual black sand laminae confirms that acquisition of the remanence took place over a geologic time interval that spanned at least two and probably several polarity intervals and that on a small scale the remanence was sporadically acquired more or less randomly probably in response to local conditions in the interstitial physicochemical environment that affected the growth of authigenic hematite no primary component of remanence acquired at or soon after deposition could be identified the remanence in the black sand bearing lower massive sandstone is largely carried by the black sand grains which as determined from reflected light petrographic studies are primarily martite 67 and altered ilmenite grains 20 composed of complex intergrowths of hematite and iron titanium minerals these data strongly suggest that these grains deposited initially as unaltered to only moderately altered magnetite and ilmenite subsequently underwent oxidation during which they acquired their remanence

some aspects of the geochemistry provenance and palaeoclimatology of the torridonian of nw scotland geochemical data mostly from mudstones are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the torridonian succession in nw scotland the basal part of the stoer group which has been interpreted as glaciogenic is thought to have formed in a warm arid setting the presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice rafted debris some aspects of the major element geochemistry of the stoer group mudstones suggest deposition in an environment with little weathering but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter the stoer group mudstones are enriched in mg ca and na relative to the torridon group samples which are much closer in composition to an average shale estimate paas the cr and ni contents of the stoer mudstones are significantly higher than those of the torridon group samples there is however no evidence that enrichment in these elements was due to contamination with volcanic material like that of the stac fada member the scourian basement appears to be an adequate source of the ni and cr in the stoer group sediments th sc ratios are higher in the torridon than in the stoer mudstones this difference is interpreted as indicating that the stoer mudstones were largely derived from local archaean sources but that the torridon group mudstones incorporated materials from a much wider provenance including younger and recycled materials evidence from rare earth elements also provides evidence for different source terrains for the stoer and torridon groups the torridon group mudstones are more weathered have a higher ree content and display a more prominent eu anomaly lower value for eu eu than do the stoer samples these results support evidence from th sc ratios that the torridon group was derived from a more varied recycled and granite rich terrain than the stoer much of which appears to have relatively local provenance the geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors

fault creep and kinematics of the eastern segment of the pernicana fault mt etna italy derived from geodetic observations and their tectonic significance abstract aseismic fault displacements related to numerous fast slipping active faults are frequently observed on mt etna but the characteristics and the rate of displacements along with the behaviour of individual fault segments are poorly constrained to resolve this we installed two local geodetic networks in 1997 along the eastern segment of the pernicana fault one of the most active structures in the geodynamic framework of the volcano the networks are based on the electroptical distance measurements edm and global positioning system gps techniques and consist of four and six benchmarks respectively geodetic data confirm that the tectonic activity of the eastern pernicana fault is controlled by fault creep i e a near continuous stable sliding at a mean slip rate of 2 8 cm yr punctuated by aseismic displacement episodes the analysis of the strain tensor calculated from the edm and gps data indicates that deformation in the fault zone is related to a clear sinistral strike slip movement a more complete picture is obtained from the gps measurements for the rocca campana area where the main structure branches out eastward into a splay fault the inversion of 3d displacement vectors suggests a deformation model consisting of two principal sinistral strike slip faults with an oblique component of movement the maximum observed displacement 6 1 cm measured in the period april 1997 february 1999 is accommodated by the splay fault this finding corroborates the hypothesis that as proposed by the tectonic models in the literature the movement of the eastern segment of the pernicana fault is transferred easterly towards structures at a lower altitude along the same strike e g the fiumefreddo fault as a consequence the pernicana fault may constitute part of a continuous left lateral trans current zone which displaces the whole north eastern flank of mt etna

a revised magnetostratigraphy for the mid proterozoic gardar lava succession south greenland abstract new palaeomagnetic results from the ca 1300 ma gardar lava succession within the eriksfjord formation of south greenland are presented this study incorporates stepwise thermal demagnetisation of 256 samples from 66 lava flows from all three lava groups in this formation accompanied by principal component analysis the results identify two complete reversals n r n in the upper lava group and two r n and n r in the lower group with a possible palaeoexcursion in the lower group a minimum of two further reversals are predicted in the time interval represented by the sandstone units intercalated with the lava flows both complete reversals in the upper group exhibit inclination asymmetries of 30 this degree of asymmetry is satisfied by a geomagnetic field model equivalent to a dipole axially offset by 570 km however a more complex source is not precluded by the present evidence four new pole positions are calculated for the gardar lavas two for the upper lava group and one each for the middle and lower groups the middle lava group shows an anomalous mean direction and pole position which is unrepresentative of the remainder of the lava succession and is thought to record a virtual geomagnetic pole representative of a transitional field direction the presence of a non axial geocentric dipole field means that the use of the laurentian apparent polar wander path for dating the new poles is not valid

gps monitoring and earthquake prediction a success story towards a useful integration abstract we describe a success story at the junction between south eastern alps and external dinarides that has led to an early deployment of gps stations prior to the predicted july 12th 2004 moderate size slovenia krn mountain earthquake the success story consisted in a straightforward integration between a long lasting lithosphere scale rock mechanics experiment along with gps monitoring leading to a physical model of stress evolution and tested earthquake prediction experiment using m8s cn and rtp algorithms to point out the area of the impending earthquake within the alarmed area by the prediction algorithms the lithosphere scale rock mechanics experiment revealed that the location of the 2004 event falls within an area of stress shadow due to the recent 1998 bovec earthquake but is also very close to an area of increased stress due to the long lasting effect of the 1511 event the pre and post 2004 earthquake gps data provided the following results 1 the krn mountain earthquake magnitude has to be increased from mw 5 2 to 5 5 therefore doubling the fault slip in order to provide a better fit to the near field displacements accordingly the rtp 2004 alarm in northern dinarides can be considered a successful prediction now that the magnitude is inside the prediction range 2 the existence of an important amount of aseismic deformation related to such a moderate size earthquake and the feasibility of monitoring these transients 3 the evidence of a resolved acceleration of the strain rates one year prior to the earthquake 4 the robustness of the bayesian approach in detecting discontinuities in the times series their magnitude and statistical significance the discontinuities or jumps in the time series can correspond to coseismic deformation or time dependent deformation such as creeping slow motion strain acceleration and transients in general 5 when integrated with tested earthquake prediction algorithms the capability to forecast earthquakes can be extended to the scale of the active fault systems

palaeomagnetism of the southern zone of the lewisian precambrian foreland nw scotland summary palaeomagnetic results are reported from 11 1 localities in metamorphic rocks from the southern zone of the lewisian complex and 12 sites from similar terrain on the island of lewis and harris these rocks were magnetized during slow uplift following the ca 1800 ma laxfordian tectonic magmatic episode the southern zone experienced diachronous uplift and there is a transition from predominantly positive nw remanence directions in the north to shallow negative nw directions in the south more prolonged metamorphism in the south correlates with a transition from magnetite sulphide to magnetite hematite assemblages the relict ruadh mheallan zone relatively unaffected by laxfordian tectonism preserves a al remanence d 82 i 65 which is sporadically recovered as high blocking temperature component in areas bordering this zone elsewhere the migration of field directions is summarized as mean directions a3 d 323 i 44 a4 d 3 14 i 14 and as d 3 13 i 11 which appear to follow on sequentially from the predominant a2 direction d 286 i 55 observed in the central zone of the lewisian complex the remanence directions are linked to a first approximation to the k ar hornblende ages and imply a migration of the ambient field direction through ca 110 during an interval between 1 x lo7 and 2 x lo8 x yr at about 1600ma the weight of the evidence suggests that the interval represented is between 0 5 and 1 x 10 x yr the lewisian a2 as directions yield palaeopoles which follow on from 1800 1700 ma magnetizations from the hudsonian terrains of greenland and north america and overlap with the youngest record from these regions on the pre drift reconstruction collectively the data define part of a large apw loop

palaeomagnetism of the torridonian of rhum scotland evidence for limited uplift of the central intrusive complex abstract the tertiary central intrusive of rhum was emplaced into the torridonian sandstone causing a heat influx into the sediment detailed progressive demagnetisation of sandstones from the thermal aureole of the intrusion show that they reached maximum temperatures which decrease systematically with distance in a manner indicative of conduction dominated cooling temperatures reached near to the intrusion are higher than would be predicted theoretically probably as a result of the presence of smaller plugs of magma at depth which are only rarely exposed at the surface the shape and magnitude of the thermal aureole suggest that upward movement of the central intrusive along the bounding fault was limited primary remanence in torridonian sandstones unaffected by the intrusion is carried by both magnetite and haematite often with both occurring in the same specimen it is suspected that only magnetite carries a truly primary detrital remanence the haematite probably formed diagenetically largely by oxidation of detrital magnetite at some time shortly after deposition

the loch quoich line a limit of early palaeozoic crustal reworking in the moine of the northern highlands of scotland the loch quoich line a major structural line at least 60 km in length within the moine of the northern highlands of scotland has hitherto been variously interpreted as a major tectonic dislocation the root zone of a nappe and an unconformity the work reported here was carried out at the type locality of the line and was directed at resolving these ambiguities the lithostratigraphic succession established by sedimentary structures shows that the rocks of the glenfinnan division are older than but are stratigraphically continuous into those of the loch eil division rocks of the loch eil division crop out extensively to the w of the loch quoich line and are not restricted in outcrop to the area e of the line as was previously thought the early d 1 d 2 structures were initially sub horizontal steeply inclined nne trending d 3 structures are weakly developed e of the axial trace of a major fold the beinn beag synform in this area the metasediments remain essentially in their post d 2 gently inclined attitude w of the axial trace of the synform the earlier folds and fabrics were severely reworked into steep attitudes during d 3 producing nne trending zones of 9platy9 rocks which mark regions of severe strain on d 3 fold limbs earlier d 1 d 2 structures are clearly preserved in d 3 hinge zones which mark zones of low d 3 strain it is concluded that the loch quoich line coincides with the axial trace of the beinn beag synform and that it is the eastern limit of severe tectonic reworking probably caledonian over printing early probably precambrian structures recognition of structural and stratigraphic continuity between the glenfinnan and loch eil divisions implies that the loch quoich line cannot be correlated across the great glen fault with the putative unconformity separating the grampian cover and central highland basement divisions in the moine rocks of the central highlands

palaeomagnetism of a caledonian ophiolite complex karmoy southwest norway abstract the palaeomagnetism of the karmoy ophiolite suite emplaced at the southwest extremity of the scandinavian caledonides in middle upper ordovician times is described low intensities of magnetisation and low q n values in the bulk of the constituent facies are linked to the effects of low grade metamorphism but ultramafic pods that have experienced serpentinisation yield well defined single component magnetisations a predominant a component of magnetisation isolated in 35 samples yields a mean of d i 51 58 95 5 this population is discrete from a subsidiary b component recognised in 17 samples with a mean direction of 145 63 95 8 a minor c component d i 330 60 95 8 is also identified and small numbers of anti parallel components are identified in each case an upper ordovician ignimbrite from kattnakken northeast of karmoy yields an in situ component of d i 270 38 together with similar components recognised elsewhere this mean d direction is most closely compatible with directions recognised in upper silurian lower devonian rocks from the scottish sector of the caledonides it is interpreted as an overprint of this age but may include a component of local rotation two interpretations are considered for the remanence record in the karmoy complex the a component could record the influence of a thermal event linked to uplift of the continental margin and development of the adjoining north sea basin in mesozoic times if this margin has been tilted seawards by 20 since magnetisation no plausible tectonic model can bring the b component into conformity with the post 300 ma apparent polar wander apw path and it is alternatively proposed that both the a and b components record upper ordovician post emplacement remanences with this interpretation the data complement a range of palaeomagnetic results from middle upper ordovician igneous and metamorphic rocks from north atlantic bordering terranes yielding intermediate to high palaeolatitudes and contrasting with older and younger data collectively they imply that the iapetus ocean had effectively closed by these times and that large apw movements took place between mid ordovician and mid silurian times

a palaeomagnetic study of recumbently folded and thermally metamorphosed torridon group sediments eishort anticline skye scotland two geologically significant palaeomagnetic components have been isolated in samples of torridon group sandstones from the kishorn nappe of nw scotland the samples were collected from the eishort anticline a large recumbent fold of caledonian age the softer component which is demagnetized by low temperatures and weak magnetic fields when in situ displays a steep upward direction typical of reversed tertiary magnetizations both polarities of the harder component which is demagnetized by high temperatures only are present and when dip corrected display either northwesterly upward or southeasterly downward directions characteristic of torridonian remanences it is suggested that the tertiary component is of chemical origin and the result of alteration of magnetite titanomagnetite grains the geographical distribution of these grains is irregular and as a result the distribution of the tertiary overprint is irregular the torridonian palaeomagnetic components from both limbs of the fold can be restored by rotation about the fold axis to internally consistent directions which lie close to previously published data from the caledonian foreland no correction is necessary for finite strain which suggests that the eishort anticline formed by a flexural slip mechanism however an anticlockwise rotation of 26 is required to reconcile the torridonian directions from the kishorn nappe with those of the caledonian foreland which suggests that during emplacement the thrust sheet rotated on the underlying fault surface

structural evolution in the moine of northwest scotland a caledonian linked thrust system a model is proposed whereby the caledonian metamorphic basement cover complex of northwest scotland the moine is considered as a linked thrust system this system lies between the moine thrust at its base and the naver sgurr beag slide at its top ductile fold and thrust zones which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies are interpreted as decoupling from a detachment presently situated at relatively shallow depths this model is illustrated by two preliminary balanced cross sections these imply shortening across the northwest scottish caledonides in excess of 130 km and probably over 200 km when these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth the most likely location for the lower crustal wedge is beneath the grampian highlands

post jotnian basic intrusions in the fennoscandian shield and the break up of baltica from laurentia a palaeomagnetic and ams study abstract a palaeomagnetic and anisotropy of magnetic susceptibility ams study has been performed on dolerite sills of the central scandinavian dolerite group csdg in the fennoscandian shield the dolerites occur in four previously known complexes in central sweden and finland and from the results of this palaeomagnetic study another complex has been identified in northern sweden these complexes cover an area of at least 100 000 km2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites the measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform ca nw or se directed magma flows the dolerites of the csdg are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings in a palaeomagnetic reconstruction of baltica versus laurentia at ca 1 27 ga the two continents were joined with ne greenland attached to nw baltica ams data from a few dolerites and a basalt in ne greenland indicate magma flow directions that in the tectonic reconstruction are more or less parallel to the flow of the dolerites in sweden this may suggest a common magma source located at the reconstructed contact between baltica and laurentia both the dolerites in greenland and those in sweden are of tholeitic composition indicating an intraplate origin which supports the interpretation of joined continents at that time the tensional regime that is reflected by the huge sill complexes is in our interpretation related to the break up of baltica from laurentia at ca 1 27 ga ago

paleomagnetic chronology fluvial processes and tectonic implications of the siwalik deposits near chinji village pakistan a 2800 m section of siwalik strata containing the stratotypes for both the chinji and nagri formations has been dated by magnetic polarity stratigraphy and the observed polarity zonation has securely been correlated with the chron 17 7 segment of the time scale the base of the section is the base of the kamlial formation which occurs near the top of chron 17 18 3 m y the kamlial chinji formation boundary occurs in the middle of chron 15 14 3 m y the chinji nagri boundary near the bottom of chron 10 10 8 m y and the nagri dhok pathan boundary at the chron 8 9 boundary 8 5 m y the siwalik deposits near chinji village consist of four distinct classes of fluvial cycles each with a characteristic periodicity a first order of 10 7 years a second order of 10 6 years a third order of 10 4 10 5 years and a fourth order of 10 0 10 4 years the river system responsible for siwalik sedimentation flowed from west to east and had properties much like the modern ganges river syste

precambrian tidal and glacial clastic deposits implications for precambrian earth moon dynamics and palaeoclimate abstract over the past decade the study of precambrian clastic tidal rhythmites stacked laminae of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided accurate palaeotidal and palaeorotational data palaeotidal records obtained from tidal rhythmites may be systematically abbreviated however and derived periods and frequencies can be misleading the validity of such values including past length of day can be assessed by testing for internal self consistency through application of the laws of celestial mechanics such a test supports the estimated length of day of 21 9 0 4 h derived from the late neoproterozoic 620 ma elatina reynella rhythmites in south australia and the indicated mean rate of lunar retreat of 2 16 0 31 cm year since 620 ma the validity of estimated lengths of day obtained from other precambrian tidal rhythmites remain unverified because the data sets contain only one primary value directly determined from the rhythmites the elatina reynella data militate against significant earth expansion at least since 620 ma and suggest that the free nutation or tipping of the earth s fluid core has undergone a resonance with the earth s annual forced nutation since the neoproterozoic glaciogenic deposits are readily distinguishable from ejecta resulting from impacts with earth crossing bodies palaeomagnetic data based on the geocentric axial dipole model for the geomagnetic field indicate that neoproterozoic and palaeoproterozoic glaciation and cold climate near sea level occurred in low palaeolatitudes this enigmatic finding may imply global glaciation or an increased obliquity of the ecliptic and is relevant to modelling the effect of ice sheet formation on the earth s obliquity history by obliquity oblateness feedback mechanisms through multidisciplinary studies clastic sedimentology and geophysics together can make substantial contributions to understanding precambrian earth moon dynamics and global palaeoenvironments

finite element analysis of strain patterns from geodetic observations across a plate margin abstract repeated trilateration measurements of local or regional networks are frequently the basis of geodetic strain analyses as a rule the strain field is considered homogeneous for the whole area under investigation or at least for parts of it however the treatment of strain as homogeneous over a large area means averaging strains and is often not desired to get an insight into the local variety of strain patterns the area under investigation can be dissected into finite elements i e triangles the individual analyses of these elements lead to meaningful results which can serve as a basis for refined models of interpretation emphasis is put on a strict analysis considering all correlations rigorously this concerns 1 1 point displacements and their variance covariance matrix as derived from the original geodetic observations through network adjustments 2 2 distortions of observables i e distances and angles as derived from the point displacements and their variance covariance matrix 3 3 global regional and local strain parameters and their variance covariance matrix as derived from point displacements or from distorted observables the method of strain analysis using finite elements is illustrated by a network spanning the san andreas and calaveras faults near hollister california

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials