sedimentology diagenesis and oil habitat of lower cretaceous qamchuqa group northern iraq the zagros basin iraq constitutes a rich petroleum province the lower cretaceous qamchuqa group comprises one of its major reservoirs data from about 30 wells drilled in a limited sector corresponding to a northwest southeast anticlinal structure situated in the kirkuk region permit analysis of several sedimentological and diagenetic events that led to the formation of this reservoir facies changes took place and divided the structure into three parts the northwestern part in which neritic facies dominate the central part in which basinal influence is considerable and the southeastern part that shows basinal mudstone type facies the lower cretaceous carbonate platform in the northwestern part of the study area displays good primary porosity during the course of burial high secondary porosity related to dolomitization appeared however a major part of the porosity was produced when the reservoir was fractured during the priabonian after the collision between the arabian and eurasian plates lopatin s method suggests that organic matter maturation started during the turonian around 90 ma whereas most of the maturation developed during the miocene due to the rapid accumulation of foreland basin sediments containing evaporite facies lower fars formation followed by the accumulation of thick upper molasse type sediments from the erosion of the zagros mountains the accumulation of sediments enhanced the total tectonic subsidence during this period the relatively brief time spent by the source rock in a given temperature interval was compensated for by a rapid rise in temperature this late thermal maturation period controlled most of the transformation of the organic matter into hydrocarbons

sedimentologic and stratigraphic properties of early cretaceous neo tethys shelf margin of arabia the qamchuqa formation of the zagros folded zone kurdistan region of iraq abstract the qamchuqa formation is well exposed in the high folded zone hfz of northeast iraq the formation is highly dolomitized and contains thick reservoir units in the subsurface region south of the hfz deposition of the formation took place at the shelf margin of early cretaceous neo tethys ocean it merges south and southeastward to the shallow marine shelf of arabia and to deep basin accumulations farther north in the study area the formation is divided into three members lower member 250m thick of interbedded bioclastic calcimudstone wackestone mw bioclastic packstone bp bioclastic floatstone bf and medium to coarse crystalline dolorudstone dolograinstone cd these strata accumulated in low to medium energy subtidal fore shoal setting the middle member consists of thick succession 325m of massively bedded cd lithofacies interpreted as high energy shoal facies which represents barremian aptian main shelf margin succession the upper member 155m thick consists of three lithofacies of mw cd and fine crystalline dolomudstone fd in ascending order it is interpreted as lagoon mw through subtidal carbonate sand shoal cd to supratidal penecontemporaneous dolomudstone fd the lower two members represent a long term barremian to aptian shallowing upward megacycle capped by late aptian unconformity incurred by relative sea level drop this temporal gap is coeval with an erosional vacuity recognized throughout the arabian peninsula sea level rise in early albian heralded deposition of the upper member which also represents a shallowing upward cycle surmounted by regionally extensive subaerial unconformity recognized in the arabian peninsula correlation and stratigraphic position of the formation under regional context has been discussed

a hypothesis on the heat flow similarity between oceans and continents abstract a simple physical model the thickening plate model gives a possible explanation of the fact that the averaged terrestrial heat flow over oceans and continents is roughly equal in the model latent heat released by solidification of asthenospheric material plays an important role faster development thickening of oceanic lithosphere releases a larger amount of latent heat than does continental lithosphere and the heat production becomes comparable with radiogenic heat production within continental crust the model also suggests that the rough equality of the heat flows is not an essential feature of the earth but merely coincidental under the conditions of the present plate motion a more important feature may be the similarity of the terrestrial heat flow over the oldest regions of both continents and oceans terrestrial heat flow over an oceanic area may be a function of the spreading rate of the lithosphere if the spreading rate of the organic lithosphere is slower than the present rate the averaged terrestrial heat flow over oceans should be lower

a palaeomagnetic study of jurassic intrusives from southern new south wales further evidence for a pre cenozoic dipole low the results of rock magnetic thermal demagnetization and thellier palaeointensity studies are presented for two high level intrusions from southern new south wales australia the gingenbullen dolerite gb 34 5 s 150 3 e and the gibraltar microsyenite gs 34 5 s 150 4 e were emplaced during the second of three major phases of igneous activity that affected the southern sydney basin and are k ar dated at 172 and 178 ma respectively the magnetic mineralogy of the two intrusions is different the gb is dominated by single domain sd pseudo single domain grains of magnetite whereas the gs has both sd magnetite and haematite components although the haematite component does not record a stable remanence the gb records two opposed components of magnetization above 100 c occupying distinct parts of the blocking temperature spectrum the lower blocking temperature lbt component resides between 150 and 400 c and is of normal polarity having a mean direction of d i 021 79 95 8 k 63 with a corresponding vgp at lat long 54 s 137 e dp 12 dm 13 this component is believed to be a tcrm or tvrm related to the initial opening of the tasman sea at 90 ma the higher blocking temperature hbt chrm component is recognized between 450 and 580 c and is of reverse polarity with a mean direction of d i 174 81 95 11 k 33 and a corresponding vgp at lat long 52 s 153 e dp 17 dm 18 the chrm for the gs resides in the 100 450 c region and although the scatter is high has a similar mean direction d i 167 79 95 24 k 26 to the hbt component of the gb with a pole position at lat long 54 s 158 e dp 30 dm 31 there is no evidence of the gb lbt component in samples from the gs suggesting that this intrusion was magnetically unaffected by the later event which is also seen for other intrusions in the sydney basin the vgps calculated from the chrm directions both plot on the jurassic segment of the australian apwp dunlop et al 1997 providing further palaeomagnetic confirmation for the k ar ages thellier palaeointensity results were of variable quality as expected for rocks of this age the mean vdm values calculated for the chrms of the gb and gs were 1 3 1022 and 4 1 1022 am2 respectively approximately 16 and 51 per cent of the present value on application of a cooling rate correction estimate the gb result falls to a minimum of 0 87 1022 am2 approximately 11 per cent of the present day value representing an overestimate of up to 56 per cent these values arising from a time window previously lacking in thellier estimates provide further confirmation of the existence of a pre cenozoic dipole low the vdm values derived from the overprint in the gb samples yield an average of 4 8 1022 am2 from a range between 44 and 81 per cent of the present day mean vdm and are tentatively attributed to the cretaceous normal superchron cns these results may document evidence for a previously unrecognized low dipole moment during the bulk of the cns a possibility that requires further investigation by performing palaeointensity work on igneous rocks with radiometric ages between 110 and 90 myr that carry unequivocal primary trms

neotectonic deformation in the western sector of tectonic escape in anatolia palaeomagnetic study of the afyon region central turkey abstract following final closure of the neotethyan ocean during the late miocene deformation in central turkey has led to crustal thickening and uplift to produce the anatolian plateau followed by westward extrusion of terranes by strike slip widespread volcanism has accompanied this latter neotectonic phase and palaeomagnetic study of the volcanism shows a coherent record of differential block rotations indicating that the anatolian region is not a plate or platelet sensu stricto but is undergoing distributed internal deformation to evaluate the scale of neotectonic rotations in the transition zone near the western limit of tectonic escape and the border of the extensional domain in central west turkey we have studied the palaeomagnetism at 82 sites in volcanic suites distributed along a 140 km lineament with north south trend and ranging in age from 18 to 8 ma comparable deflection of magnetic remanence from the present field direction is identified along the full length of the lineament a mean clockwise rotation of 12 3 4 2 is determined for this western sector of the anatolian strike slip province since similar rotations are observed in the youngest and oldest units this cumulative rotation occurred after the late miocene when interpreted together with results elsewhere in anatolia it is inferred that the rotation is later than crustal thickening and uplift of the anatolian plateau and entirely a facet of the tectonic escape inclinations are mostly 10 shallower than the predicted miocene field and are considered to reflect the presence of a persistent inclination anomaly in the mediterranean region larger rotations departing from the regional trend are also observed within the study region but are confined to the vicinity of major faults notably those bounding the afyon aksehir graben the pattern of neotectonic declinations across anatolia identifies strong anticlockwise rotation in the east near the arabian pincer with progressive reduction in the amount of rotation towards the west it becomes zero or slightly clockwise at the western extremity of the accreted terrane collage rotations also appear to become generally younger towards the south crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the tauride arc the westward radial expansion of the extruded terranes is inferred to combine with backroll on the hellenic arc to produce the contemporary extensional province in western turkey

paleoproterozoic geomagnetic field strength from the avanavero mafic sills amazonian craton brazil a recent hypothesis has suggested that earth s inner core nucleated during the mesoproterozoic as evidenced by a rapid increase in the paleointensity ancient geomagnetic field intensity record however paleointensity data during the paleoproterozoic and mesoproterozoic period are limited to address this problem we have determined paleointensity from samples from three paleoproterozoic avanavero mafic sills amazonian craton brazil cotingo 1782 ma puiua 1788 and pedra preta 1795 ma we adopted a multi protocol approach for paleointensity estimates combining thellier type izzi and ltd izzi methods and the non heating preisach protocol we obtained an average vdm value of 1 3 0 7 1022am2 cotingo of 2 0 0 4 1022am2 puiua and 6 4 1022am2 pedra preta it is argued that the cotingo estimate is the most robust our results are the first data from the upper paleoproterozoic for south america and are comparable to data available from other regions and similar periods the new data do not invalidate the hypothesis of that earth s inner core nucleated during the mesoproterozoic

high field intensity results from recent and historic lavas a total of 45 recent and historic lava flows of mount etna sicily were sampled to investigate changes in the strength and orientation of the geomagnetic field in the region of the flows sampled 11 yielded one or more samples which provided anomalously high field intensity results a rock magnetic study has shown that a high resistance to alternating field demagnetization and group 2 low temperature susceptibility behaviour are the two factors common to these samples

a palaeointensity study of lower carboniferous transitional geomagnetic field directions the cockermouth lavas northern england palaeomagnetic and palaeointensity results are reported from a thin succession of lavas erupted in lower carboniferous times ca 355 ma in northern england characteristic magnetisations were apparently acquired at the time of eruption but the stable normal field direction is recovered from only two sites out of seven the remainder are not compatible with younger overprinting and are interpreted as thermo remanent magnetisations trms recording transitional fields palaeointensity studies employing the shaw method identify main field intensities averaging 20 8 t within the range of estimates of the lower carboniferous field intensity from studies elsewhere in contrast the lavas recording transitional field directions yield palaeointensity estimates which fall mostly between 10 and 20 of this value hence it is inferred that palaeozoic field inversions were like recent examples characterised by a general decline in the intensity of the field

uplift deduced from remanent magnetization of a proterozoic basic dyke and the baked country rock in the hoting area central sweden a palaeomagnetic and 40ar 39ar study remanent magnetization in dyke contact zones in the hoting area of the fennoscandian shield in the central part of sweden has been studied in order to establish the ambient temperature of the host

paleointensity determination on a 1 786 ga old gabbro from hoting central sweden abstract paleointensities from precambrian rocks are rare and might be biased by remagnetization processes here we present new analyses of samples from a 1 786 ga gabbro near hoting central sweden rock magnetic and mineralogical analyses indicate that one of the sites site 5 may be pristine whereas the others exhibit evidence of alteration characteristic remanent magnetization was determined using principal component analysis for each sample and was compared with results obtained in a previous study of elming et al 2009 intensity measurements from site 5 show higher values compared to those of the other sites suggesting that alteration processes may lead to underestimation of the field intensity after cooling rate and anisotropy correction the field moment at 1 786 ga was estimated to be 25 6 3 3 zam 2 and 15 2 6 1 zam 2 from site 5 only and from all sites respectively we consider the result from site 5 to be more accurate owing to the lack of evidence for alteration our estimates agree well with the proterozoic vdm values suggested by biggin et al 2009

absolute paleointensity at 1 27 ga from the mackenzie dyke swarm canada 1 paleointensity studies have been conducted on 6 mafic dykes from the 1270 ma mackenzie swarm in the slave province canada the mean direction of the characteristic magnetization coincides with results of an earlier study in which the primary origin of the magnetization was established on the basis of a contact test high unblocking temperatures magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains of magnetite paleointensity experiments were conducted with a specially designed oven using a revised version of the thellier coe method thirteen successful determinations of paleointensity were obtained for 4 dykes the paleofield estimates vary between 4 3 and 22 1 t yielding virtual dipole moments vdms between 1 3 0 2 and 4 5 0 9 1022 am2 these new results increase the number of low field determinations during the precambrian which largely dominate the database with an averaged field of 3 1 2 5 1022 am2 they also emphasize the importance of additional studies to understand the differences with the strong paleointensities obtained using new techniques

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

analysis of long term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics summary the 1167 published cooling unit cu palaeointensity estimates contained in the 400 10 ma portion of the pint global database were rigorously filtered according to accurate age determinations palaeodirectional reliability recognition of polarity and the method of palaeointensity acquisition the remaining 865 estimates group 1 were further filtered to ensure self consistency reducing the data set to 425 estimates group 2 group 1 and 2 data were clustered into temporally and or spatially distinct rock suites rs enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation psv the record was segmented according to the distribution of the data rather than using arbitrary time windows to ensure quasi consistent behaviour within each segment differences between these segments clearly indicate that a significant long timescale 10 7 and 10 8 yr variation of the mean geomagnetic poloidal field intensity gpfi occurred during the 400 10 ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field both the mean dipole moment and its standard deviation appear to be a function of the range of values each cu may adopt at one particular time this range is itself controlled by the variation of the maximum limit of dipole moment while the value of the minimum limit remains relatively constant tentative support is provided for the recent suggestion that psv may have been reduced during the cretaceous normal superchron cns though more data are needed in the range 120 60 ma to confirm this no conclusive evidence was found to support the suggestion that the gpfi record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt ptrm checks indeed offsets caused by unreliable data in well represented parts of the record are likely to be random and cancel one another out when gpfi variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency rf it is not possible to confirm whether the two parameters are anticorrelated decoupled or related in some more complex way however it is clear that gpfi and rf are definitely not positively correlated as has been previously suggested the present database documents sharp increases in gpfi around the onset times of the two recognized superchrons itself implying an anticorrelation the implications for geodynamo and mantle modelling of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly a generic geodynamic model is proposed to explain the relationship between observed longterm changes in gpfi and global geodynamic processes the model predicts that changes in gpfi result from a chain of geodynamic processes extending from crust to core beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model four time stages are proposed to describe the major long term changes in gpfi since the early devonian 400 350 350 250 250 175

palaeointensity of the 1 3 billion yr old gardar basalts southern greenland revisited no evidence for onset of inner core growth

long term evolution of the geomagnetic dipole moment abstract the geomagnetic field intensity measured at the surface of the planet is a potential indicator of the dynamo activity in the conducting liquid core of the earth rapid field variations must be generated by the rapid fluid motions within the core whereas long term changes could be associated with other processes such as changes in boundary conditions at the core mantle interface and or at the outer liquid inner solid core boundary for about 50 years paleomagnetists gathered records of absolute paleointensity using the magnetization of lava flows distributed over the globe in order to reconstruct the field intensity changes during the past 3000 ma despite recent acquisition of paleointensity records the temporal distribution of the results remains limited except within a few time intervals to extract coherent features of the time averaged field intensity over periods of a few million years however significant informations can be gained from the recent updated paleointensity database by averaging the virtual dipole moment over very long time intervals the results suggest the existence of a long term evolution of the dipole field intensity during the past 3 billion years from 3 1022 a m2 at 1000 2000 ma to 8 1022 a m2 at present times

a new quality check for absolute palaeointensities of the earth magnetic field absolute palaeointensity determinations of recent lavas from mt etna have been carried out using the koenigsberger thellier thellier ktt method but with two different data presentations the first which is the standard method compares the natural remanent magnetization nrm demagnetized and the thermoremanent magnetization trm acquired in successive temperature intervals cumulatively the second compares the nrm and trm in the successive intervals in an incremental manner the latter method appears to distinguish aberrant behaviour better than the classical method such aberrant behaviour can be related to changes in hysteresis ratio parameters saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force which are reflected by displacements in the day et al 1977 plot these changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling the changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations

hysteresis and susceptibility characteristics of magnetite and titanomagnetites interpretation of results from basaltic rocks abstract the hysteresis and susceptibility characteristics down to liquid nitrogen temperatures have been investigated in over one thousand basalt samples of tertiary age from the society islands norfolk island reunion mauritius france and australia the variation in these properties with temperature is similar to that observed by radhakrishnamurty synthetic samples covering grain sizes from single to multidomain grains of pure magnetite titanomagnetite and cation deficient titanomagnetites of varying composition have been investigated to determine their low temperature characteristics comparison of the results from these synthetic samples with those of natural basalts shows that the composition and domain state predominating in any given sample of basalt cannot be determined uniquely from low temperature characteristics alone however optical electron microprobe and af demagnetization characteristics can be combined with the low temperature properties to determine the predominant grain size and magnetite composition the results show that the simple classification scheme put forward by radhakrishnamurty is questionable samples showing increase in susceptibility and no change in coercivity down to liquid nitrogen temperature are not of the cation deficient type these properties appear to be those associated with titanomagnetites containing exsolved ilmenite lamellae formed during initial cooling this unique characteristic may be useful for the quick selection of samples most suitable for palaeointensity studies

thermal history of sedimentary basins maturation indices and kinetics of oil and gas generation temperature is the most sensitive parameter in hydrocarbon generation thus reconstruction of temperature history is essential when evaluating petroleum prospects no measurable parameter can be directly converted to paleotemperature maturation indices such as vitrinite reflectance tmax from rock eval pyrolysis spore coloration thermal alteration index tai or concentration of biological markers offer an indirect approach all these indices are a function of the thermal history through rather complex kinetics frequently influenced by the type of organic matter their significance and validity are reviewed besides the problems of identification e g vitrinite and interlaboratory calibration it is important to simultaneously interpret kerogen type and maturation and to avoid difficult conversions from one index to another geodynamic models where structural and thermal histories are connected are another approach to temper ture reconstruction which could be calibrated against the present distribution of temperature and the present value of maturation indices kinetics of kerogen decomposition controls the amount and composition of hydrocarbons generated an empirical time temperature index tti originally introduced by lopatin does not allow such a quantitative evaluation due to several limitations no provision for different types of kerogen and different rates of reactions poor calibration on vitrinite reflectance it is of limited interest unless one has no access to a desk top computer kinetic models based on a specific calibration made on actual source rock samples can simulate the evolution of all types of organic matter and can provide a quantitative evaluation of oil and gas generated examples from the jurassic source rocks of the paris basin monterey formation of california green river shales of utah paleozoic source eds of the algerian sahara and miocene rocks of the mahakam delta indonesia illustrate various aspects of the discussion geological geochemical models are the most efficient way to integrate geological seismic and geochemical data and they should greatly help to reduce the risk in exploration

middle cretaceous carbonate reservoirs fahud field and northwestern oman reservoir facies in fahud field and throughout northwestern oman are in shallow shelf carbonates of the middle cretaceous mishrif and mauddud formations interparticle porosity formed in the mishrif as sand aprons of lithoclast and skeletal grainstones surrounding fault block islands and less commonly in the mauddud as biostromes of rudist packstones moldic porosity after fine rudist debris is more common than interparticle porosity and occurs in thicker stratigraphic units interpreted to have formed locally in meteoric water lenses associated with islands and regionally during subaerial exposure associated with sea level lows

palaeomagnetism of the loch doon granite complex southern uplands of scotland the late caledonian palaeomagnetic record and an early devonian episode of true polar wander abstract the southern uplands terrane is an ordovician silurian back arc foreland basin emplaced at the northern margin of the iapetus ocean and intruded by granite complexes including loch doon 408 3 1 5 ma during early devonian times protracted cooling of this 130 km 3 intrusion recorded magnetic remanence comprising a predominant a magnetisation linked to initial cooling with dual polarity and mean direction d i 237 64 95 4 palaeopole at 316 e 21 n subsidiary magnetisations include mesozoic remanence correlating with extensional tectonism in the adjoining irish sea basin b d i 234 59 and minority populations c d i 106 2 and d d i 199 1 recording emplacement of younger 395 ma granites in adjoining terranes and the variscan orogenic event the a directions have an arcuate distribution identifying anticlockwise rotation during cooling a comparable rotation is identified in the orthotectonic caledonides to the north and the paratectonic caledonides to the south following closure of iapetus continental motion from midsoutherly latitudes 40 s at 408 ma to equatorial palaeolatitudes by 395 ma is identified and implies minimum rates of continental movement between 430 and 390 ma of 30 70 cm year more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander silurian devonian palaeomagnetic data from the british scandinavian caledonides define a 430 385 ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from gondwana they reconcile pre 430 ma and post 380 ma apw from this supercontinent and show that laurentia baltica avalonia lay to the west of south america with a relict rheic ocean opening to the north which closed to produce variscan orogeny by a combination of pivotal closure and right lateral transpression

palaeomagnetism of precambrian dyke swarms in the north china shield the 1 8 ga lip event and crustal consolidation in late palaeoproterozoic times abstract the north china shield ncs is cut by a laterally extensive dyke swarm emplaced at 1 78 1 76 ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by 1 85 ga palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with ne to n declinations and rare opposite polarity dykes with ne shallow magnetic declination a1 d i 36 1 recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations a2 d i 8 2 in lower grade metamorphic terranes to the south contact tests indicate a primary cooling related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at 1 85 ga a temporal migration a2 a1 is implied by the higher crustal level and earlier acquisition of the former and the deeper source and later acquisition of the latter a third population of dyke magnetisations a3 d i 18 43 is distributed towards steeper inclinations and close to the mesozoic recent palaeofield these are either partial or complete overprints of a1 a2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra or are much younger dykes of mesozoic tertiary age a minority fourth later precambrian but presently undated dual polarity population has a magnetisation 11 dykes d i 108 7 with contact tests indicating a primary cooling related origin the 1 78 1 76 ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major large igneous province lip related events the new definition of a 1 83 1 76 ga apw swathe from the north china shield permits a comparison with other shields and yields a constraint to continental configurations during the late palaeoproterozoic a quasi integral reconstruction of palaeopangaea is tested here and supported by conformity of predominantly of uplift related palaeopoles from the 1 90 1 70 ga tectono thermal belts and from sw ne trending apw implied by the distribution of poles from the 1 80 ga igneous suites including the lip events this trend incorporates the a2 a1 migration and the granulite terrane cooling polar swathe from north china the reconstruction indicates that continental crust consolidated in palaeoproterozoic times by accretion of 2 3 1 7 ga orogenic belts around a hemispheric and crescent shape core already established by late archaean times the north china shield is interpreted to have bordered the western cratonic margin of the indian shield in a proximity supported by correlation of geological features and suggested by a number of previous workers the central orogenic zone of the north china shield characterised by tectono thermal activity prior to 1 85 ga was then contiguous with a comparable zone running through the centre of the indian shield and continuing into the capricorn belt of western australia the 1 78 1 76 ga dykes in north china continue into dyke swarms in the south india shield and may have been sourced in a plume related lip focussed near the continental margin in the xiong er aulacogen

an investigation of the magnetic variation within two recent lava flows summary samples collected in vertical sections from two recent 1169 ad and 1971 ad lava flows show significant variations in both the palaeomagnetic vector and the rock magnetic characteristics the age and the short cooling time of the flows both are less than 3 m thick are such that the observed range of palaeomagnetic variation is more than can reasonably be explained by secular variation the variations in palaeomagnetic and rock magnetic characteristics appear in part to be a response to the relative proportions of two coexisting ferrimagnetic phases one that displays a primary coarsegrained and ti rich composition and a second that has finer grained ti poor characteristics the use of af demagnetization in this study means that the latter phase will have a disproportionate influence on the palaeomagnetic results it has been suggested elsewhere that in such a magnetic system the ti poor phase may represent a crm formed at some temperature below its final curie temperature if so the results of this study imply that not only does it affect the palaeointensity estimate which requires a pure trm but it appears also to influence the palaeomagnetic direction to an extent that far exceeds the normal secular change that might be expected to occur during delayed acquisition such a result has important implications for palaeomagnetic studies that use lavas both in the selection of a suitable sampling strategy and in the geomagnetic interpretation of palaeomagnetic variation within single flow units

palaeomagnetism of the cappadocian volcanic succession central turkey major ignimbrite emplacement during two short miocene episodes and neogene tectonics of the anatolian collage abstract the central anatolian volcanic province in cappadocia includes 13 high volume calc alkaline ignimbrite sheets emplaced by plinian eruptions within a succession the urgup formation after 10 ma recording the last phase of neotethyan subduction and accompanying emplacement of the tauride orogen in southern turkey to evaluate magnetostratigraphy in the context of recent revisions of the chronostratigraphy we have extended palaeomagnetic investigation to 32 new sites yielding significant chrm directions integrated rock magnetic and palaeomagnetic investigations identify magnetic remanence residing predominantly in ti poor titanomagnetites although secondary processes within the ignimbrite sheets notably post emplacement oxidation have locally produced hematisation expressed by composite irm spectra and variable reduction in intensity of magnetisation the ignimbrite sheets possess weak anisotropies of magnetic susceptibility ams mostly 3 of pyroclastic deposits during polarity chron c4r 1n between 9 31 and 9 43 ma subsequent activity from the acigol centre further to the south west cemilkoy gelveri gordeles and kizilkaya produced in excess of 620 km 3 of pyroclastic deposits during polarity chrons between 5 3 and 7 1 ma the younger incesu ignimbrite was sourced in the sultansazligi pull apart basin to the east during the gauss chron 2 58 3 60 ma all pre incesu ignimbrites are rotated uniformly anticlockwise and the overall reversed group mean direction of magnetisation is d i 170 8 52 4 n 9 r 8 91 95 5 4 k 91 the implied tectonic rotation in this sector of central anatolia 16 4 relative to eurasia is young and postdates the 5 3 7 1 ma acigol centre whilst largely predating emplacement of the incesu ignimbrite whilst rotational deformation within anatolia is young it proves to be distributed with a temporal variation from block to block it is embraced by a complex post miocene tectonic regime of strike slip and extension during tectonic escape and suction towards the hellenic arc to the west

geomagnetic field intensity in early jurassic investigation of the newark supergroup eastern north america abstract a collection of 220 samples from early jurassic newark supergroup volcanic units watchung talcott holyoke hampden and deerfield basalts and intrusive units manitook and palisades sills was studied with the aim of testing the previously suggested occurrence of a low average dipole moment of the earth s magnetic field some 200 ma ago after viscosity tests alternating field demagnetization of all samples and thermal variation studies of saturation magnetization of 74 selected samples we find that only 8 of the entire collection was a priori suitable for paleointensity experiments using the original thellier and thellier method we have obtained 17 reliable paleointensity values of average quality from the palisades sill and the holyoke basalt although these two units recorded substantially different field directions they provide similar estimates of the virtual dipole moment 9 4 1 2 and 11 3 2 2 10 22 a m 2 respectively these values are within the range of secular variation of the recent geomagnetic field

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

stratigraphic sequence microfacies and petroleum prospects of the yamama formation lower cretaceous southern iraq the lower cretaceous yamama formation basin was deposited over two tectonic regimes the northeastern flank of the stable arabian platform and the unstable zone of the mesopotamian foredeep this situation and the probable syntectonic deposition of the formation over growing structures created complex carbonate lithologies from this tectonic configuration and the facies distribution it seems that the yamama formation was deposited in a setting that changed from an inner to an outer ramp along the hinge line separating those two tectonic units oolite shoals developed the crestal areas of the growing structures were occupied by stromatoporoid sponge coral reefs and oolitic facies the areas between reefs were the sites of carbonate mud accumulation for the purposes of regional correlation and reserve estimation the yamama formation is divided into five lithologic units three reservoir units designated from top as yr a yr b and yr c separated by two permeability barrier units yb 1 and yb 2 these reservoir units are thought to be at least partially isolated from each other the best oil prospects are within the oolite shoals and the patch reef buildups in the crestal parts of the structures

comparison of thermal and microwave paleointensity estimates in specimens displaying non ideal behavior in thellier style paleointensity experiments determining the strength of the ancient geomagnetic field is vital to our understanding of the core and geodynamo but obtaining reliable measurements of the paleointensity is fraught with difficulties over a quarter of magnetic field strength estimates within the global paleointensity database from 0 5 ma come from hawai i two previous studies on the soh1 drill core gave inconsistent apparently method dependent paleointensity estimates with an average difference of 30 the paleointensity methods employed in the two studies differed both in demagnetization mechanism thermal or microwave radiation and thellier style protocol perpendicular and original thellier protocols both variables that could cause the strong differences in the estimates obtained paleointensity experiments have therefore been conducted on 79 specimens using the previously untested combinations of thermal perpendicular and microwave original thellier methods to analyze the effects of demagnetization mechanism and protocol in isolation we find that individually neither demagnetization mechanism nor protocol entirely explains the differences in paleointensity estimates specifically we found that non ideal multi domain like effects are enhanced using the original thellier protocol independent of demagnetization mechanism often resulting in paleointensity overestimation however we also find evidence supporting recent findings from the 1960 kilauea lava flow that microwave perpendicular experiments performed without ptrm checks can produce underestimates of the paleointensity due to unaccounted for sample alteration at higher microwave powers together these findings support that the true paleointensities fall between the estimates previously published and emphasize the need for future studies thermal or microwave to use protocols with both ptrm checks and a means of detecting non ideal grain effects

palaeomagnetic study of the karacadag volcanic complex se turkey monitoring neogene anticlockwise rotation of the arabian plate abstract the karacadag volcanic centre in south east turkey is a major basaltic complex sited at the northern margin of the arabian plate and emplaced in several pulses between 11 ma and late quaternary times we have sampled 71 sites in lavas of this complex to constrain the palaeomagnetic record and hence the chronology of magmatic activity and regional rotation palaeomagnetic study at sixty two sites yields significant component definition with mixed normal and reversed polarities from age dating and morphologic criteria we identify three major episodes of lava emplacement and site mean directions of magnetisation for these divisions resolve migration of the palaeofield direction for the northern sector of the arabian plate since mid miocene times successive reversed polarity group mean directions are d i 175 50 5 n 37 r 33 13 95 4 1 for the oldest siverek division mean age estimate 11 1 6 7 ma d i 173 4 46 0 n 16 r 15 67 95 5 5 for the middle karacadag division mean age 3 3 ma and d i 167 7 47 6 n 6 r 5 93 95 7 9 for the youngest ovabag division 1 9 ma present the first two results merit a tectonic interpretation and consistent anticlockwise rotation of 9 is recognised in the karacadag volcanic centre between 11 and 3 3 ma the mean directions conform to palaeomagnetic results from other undeformed neogene igneous complexes of comparable age range further to the west along the northern perimeter of the arabian plate the amount of tectonic rotation observed in late pliocene and older volcanic units from this plate is found to be statistically constant tectonism responsible for rotation therefore appears to have been temporally confined to the last 2 myr which is long after collision with the anatolides and closure of the bitlis suture this conclusion conforms to the young distributed block rotations recognised in the anatolides and aegean and correlates with the regime of tectonic escape of terranes to the west

neotectonic deformation linking the east anatolian and karatas osmaniye intracontinental transform fault zones in the gulf of iskenderun southern turkey deduced from paleomagnetic study of the ceyhan osmaniye volcanics 1 left lateral strike slip along the dead sea fault zone dsfz between the african and arabian plates is partitioned into a complex set of motions at its northern extension where arabia impinges into the anatolian collage as a result the nature of the contemporary link between the east anatolian and dead sea transforms is unclear to quantify strike slip motion expressed as tectonic rotations along the inferred southwest continuation of the east anatolian fault zone eafz we have investigated the paleomagnetism of young 1 ma volcanics in the adana basin gulf of iskenderun region comprising the ceyhan osmaniye volcanic suite this volcanism is sited close to the intersection between the sinistral karatas osmaniye fault zone kofz and the neotethyian suture nine sites from a basaltic lava field in the vicinity of osmaniye are entirely of normal polarity and in the context of their very young morphologies are assigned to the brunhes chron declinations are mostly clockwise from the present field this age assignment identifies rapid recent 780 ka uplift at the head of the gulf of iskenderun and is linked to a diversion of the ceyhan river drainage and ongoing regional tilting eight sites yield a mean declination of 11 3 10 implying clockwise block rotation on riedel type fractures on the northwest side of the kofz seven lavas sites from a volcanic succession near botas at the southwestern extension of the volcanic lineament are of reversed polarity and rotated counterclockwise by 43 7 with respect to the present field polarity k ar and morphological evidence indicate an assignment toward the latter part of the matuyama chron and identify rapid neogene block rotation of a flake within the kofz an inferred maximum slip rate of 0 6 cm year on this fault zone is comparable with lower estimates of slip on the eafz and suggests that most strike slip on the dsfz is now being partitioned to the west of the gulf of iskenderun on a developing strike slip zone linking with the cyprus arc

a mineral magnetic study of some pottery samples possible implications for sample selection in archaeointensity studies abstract we have related the mineral magnetic properties of some archaeological samples to the quality of the paleointensity data that they produce we find that samples which have broad unblocking temperature distributions and most of their remanences unblocked at intermediate temperatures produce high quality paleointensity data this conclusion is based on a comparative study of pottery samples which had previously produced reliable and unreliable paleointensity data for each sample we determined the magnetic mineralogy and the magnetic domain state in all cases magnetite or titanomagnetite is the primary magnetic carrier with domain states that extend from superparamagnetic to single domain and maybe even to pseudo single domain we believe that the magnetic grain size distribution of these samples produces a broad unblocking temperature distribution the advantage of having such a distribution is that high quality data can be obtained before the onset of the mineralogical changes that can occur at high temperatures with the thellier and thellier method the frequency dependent susceptibility appears to be useful as a non destructive estimator of the unblocking temperature distribution for samples having only magnetite as the magnetic mineral

the effects of heating on low temperature susceptibility and hysteresis properties of basalts abstract the variation of the low field susceptibility of basalts down to liquid nitrogen temperature always falls into one of three types that depend on the composition and grain size of the titanomagnetite grains present group 1 basalts contain predominantly unoxidised multidomain homogeneous titanomagnetites having x 0 3 group 2 basalts contain predominantly titanomagnetite grains with many exsolved ilmenite lamallae that subdivide the grains so that they act similarly to single domains group 3 basalts contain predominantly multidomain magnetite or magnetite rich titanomagnetite having x 0 15 after repeated heating to 615 c the group 1 basalts gradually oxidise above 300 c to produce the characteristics of group 2 basalts owing to the exsolution of ilmenite on the other hand both group 2 and 3 basalts are stable to oxidation until at least 500 c they are therefore the most useful material for palaeointensity studies

differential neotectonic rotations in anatolia and the tauride arc palaeomagnetic investigation of the erenlerdaǧ volcanic complex and isparta volcanic district south central turkey abstract in the anatolian sector of the alpine himalayan collisional belt a palaeotectonic phase of terrane accretion has been succeeded by a neotectonic phase of intracontinental deformation as the afro arabian plate has continued to impinge differentially into the accreted collage the resulting tectonic escape has during the last few million years extruded and rotated blocks in a southwesterly direction this paper reports a palaeomagnetic investigation of the erenlerda volcanic complex and isparta alkaline volcanic district undertaken to extend analysis of crustal rotation into the southwestern part of the anatolian region and investigate the interaction with the isparta angle and the extensional province of western turkey the erenlerda volcanism comprises three phases of volcanism the oldest sille volcanics 11 7 11 4 ma yield a mean d 150 i 52 rotated anticlockwise during early stages of crustal thickening and deformation before a later miocene episode c 10 9 8 9 ma d 183 i 47 and a mio pliocene episode d 179 i 51 the latter two episodes indicate that no significant rotation has resulted during the neotectonic crustal extrusion in this southwestern sector of anatolia further to the west within the isparta angle a 4 7 4 0 ma alkaline episode yields a mean of d 186 i 53 rotated slightly clockwise the pattern of palaeomagnetic declinations across anatolia shows rotations that are strongly anticlockwise in the east near the arabian pincer and diminish towards the west to become zero or slightly clockwise at the western extremity of the collage the timing of rotation also appears to become younger towards the south crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the tauride arc and by inference the cyprian arc where good age control exists in cappadocia and the sivas basin rotations are found to be concentrated within the last few million years and are up to an order higher than rates deduced from global positioning system the palaeomagnetic data imply that the neotectonic deformation following collision initially produced crustal thickening resulting in uplift of the anatolian plateau and was only subsequently accommodated by major differential block rotation during tectonic escape

palaeointensity studies of a late permian lava succession from guizhou province south china implications for post kiaman dipole field behaviour summary palaeomagnetic results are presented from a volcanic sequence in zhijin county guizhou province southern china the lavas and associated volcanic breccias comprising the sequence represent a southern extension of the emeishan volcanic province biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late permian 263 255 ma and is thus somewhat older than the emei stratotype section in sichuan province and close in age to reported estimates of the termination of the permo carboniferous kiaman reverse superchron rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh primary magnetites containing a significant fraction of single domain grains thermal demagnetization behaviour of the breccia units is poor but most lava samples have one or two components of remanence above 250 c the normal polarity characteristic remanence held by the lavas implies a post kiaman age for this succession and suggests that the termination of the kiaman occurred prior to 263 ma supporting recently published estimates according to standard criteria thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron 80 per cent of samples record relative vdm values in the range 42 52 per cent of the present day value supporting recent studies of mid kiaman field intensity this suggests that a low energy dipole existed at least between 300 and 255 ma and does not appear to have been confined to the stable reverse polarity interval

neotectonic deformation in the transition zone between the dead sea transform and the east anatolian fault zone southern turkey a palaeomagnetic study of the karasu rift volcanism in southern turkey ongoing differential impingement of arabia into the weak anatolian collisional collage resulting from subduction of the neotethyan ocean has produced one of the most complex crustal interactions along the alpine himalayan orogen several major transforms with disputed motions including the northward extension of the dead sea fault zone dsfz meet in this region to evaluate neotectonic motion on the amanos and east hatay fault zones considered to be northward extensions of the dsfz the palaeomagnetism of volcanic fields in the karasu rift between these faults has been studied remanence carriers are low ti magnetites and all except 5 of 51 basalt lavas have normal polarity morphological polarity and k ar evidence show that rift formation occurred largely during the brunhes chron with volcanism concentrated at 0 66 0 35 ma and a subsidiary episode at 0 25 0 05 forty four units of normal polarity yield a mean of d i 8 8 54 7 with inclination identical to the present day field and declination rotated clockwise by 8 8 4 0 within the 15 km wide hassa sector of the karasu rift the volcanic activity is concentrated between the amanos and east hatay faults both with left lateral motions which have rotated blocks bounded by nw se cross faults in a clockwise sense as the arabian block has moved northwestwards an average lava age of 0 5 ma yields a minimum cumulative slip rate on the system bounding faults of 0 46 cm year according with the rate deduced from the africa arabia euler vector and reduced rates of slip on the southern extension of the dsfz during plio quaternary times estimates deduced from offsets of dated lavas flows and morphological features on the amanos fault zone tectonophysics 344 2002 207 are lower 0 09 0 18 cm year probably because they are limited to surface fault breaks and do not embrace the seismogenic crust results of this study suggest that most strike slip on the dsfz is taken up by the amanos east hatay afrin fault array in southern turkey comparable estimates of quaternary slip rate are identified on other faults meeting at an unstable fff junction dsfz east anatolian fault zone karatas fault zone a deceleration in slip rate across the dsfz and its northward continuation during plio quaternary times correlates with reorganization of the tectonic regime during the last 1 3 ma including tectonic escape within anatolia establishment of the north and east anatolian fault zones bounding the anatolian collage in mid late pliocene times a contemporaneous transition from transpression to transtension and concentration of all basaltic magmatism in this region within the last 1 ma

uplift and cooling magnetisation record in the bamble and telemark terranes sveconorwegian orogenic belt se norway and the grenville sveconorwegian loop article i nfo the 1100 ma sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against and onto the south western margin of the fennoscandian shield to resolve the magnetic signature acquired during post orogenic uplift and cooling and evaluate wider correlations with the contemporaneous grenville belt of north america this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium high metamorphic grade bamble terrane 48 sites and 390 cores and the adjoining medium low grade telemark terrane 33 and 240 cores juxtaposed by an orogen parallel porsgrunn kristiansand fault zone with major vertical displacement magnetite and ilmeno hematites are magnetic carriers with the latter more significant in the higher metamorphic grades magnetic intensities are stronger in the higher grade terrane presumably due to the growth of metamorphic ferromagnets but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust anisotropy of magnetic susceptibility ams largely reflects the ne sw tectonic grain of the last stage of sveconorwegian ductile deformation the magnetisation record is filtered by excluding magnetisations possibly acquired during regional mesozoic dyke emplacement development of the permo carboniferous oslo rift and late proterozoic magmatism the remaining record is a dual polarity signature summarised by mean poles at 31 9 n 50 9 e n 191 components in the bamble terrane and at 34 2 n 58 9 e n 151 components in the telemark terrane however these means are non fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best defined parts

effect of basement tectonics on hydrocarbon generation migration and accumulation in northern iraq 1 stratigraphic geophysical and remote sensing evidence shows that the basement of northern iraq is fragmented at various scales two large blocks have been recognized the northern mosul block south of the east west to west northwest east southeast trending taurus suture zone and the kirkuk block southwest of the northwest southeast trending zagros suture zone these blocks are in turn subdivided into smaller longitudinal blocks relative to the fold trends the mosul block was less stable and more uplifted relative to the kirkuk block and therefore its stratigraphic record is characterized by wider and more extensive erosion and or periods of nondeposition this led to a relatively poor development of the middle jurassic upper cretaceous potential source rocks ove the mosul block the difference in hydrocarbon resources between the mosul and kirkuk blocks is apparent from the estimated reserves of oil and gas in each development of grabens or half grabens enhanced the already favorable stratigraphic circumstances for the deposition and preservation of potential source rocks in the kirkuk block and locally improved the otherwise inhibiting environment in the mosul block these features show significant difference in their plan shape and relative arrangement in the two blocks in addition within each of the large blocks the synorogenic differential movement on longitudinal basement faults fault stacking had a great effect on hydrocarbon generation migration and entrapment the difference in basement tectonics between the mosul and kirkuk blocks reflects a regional difference in the afro arabian plate margin history along the taurus and zagros belts

an integrated rock magnetic approach to the selection or rejection of ancient basalt samples for palaeointensity experiments abstract the rock magnetic properties of basalts are often useful indicators of suitability for use in palaeointensity experiments the normal approach to sample selection is based on the use of a limited amount of rock magnetic data possibly from only a single rock magnetic technique this approach may result in the exclusion of otherwise suitable samples or the inclusion of unsuitable samples this study stresses the importance of analysing results from a number of rock magnetic techniques before selecting or rejecting samples for use in palaeointensity experiments a new integrated rock magnetic method of sample selection or rejection is proposed which uses the combined results from thermal thermomagnetic and high temperature susceptibility and non thermal low temperature susceptibility hysteresis and alternating field demagnetisation rock magnetic techniques to define sets of rock magnetic behaviour for precambrian basalts with a mixed grain size of pure magnetite the behaviour of all samples in each set during both thellier and shaw palaeointensity experiments is categorised allowing a set of selection or rejection criteria to be defined this new approach to sample selection or rejection for palaeointensity experiments has value for basalts of all ages provided the sample collection is large enough and all available rock magnetic techniques are used

palaeointensity results from the permo carboniferous kiaman reversed superchron the great whin and midland valley sills of the northern united kingdom summary fifty thellier palaeointensity determinations were made from the great whin sill gws of northern england and the midland valley sill mvs of central scotland which form the major part of a late carboniferous quartz dolerite complex palaeodirectional analysis confirms that the reversed polarity characteristic remanence carried by samples from both sills is consistent with the permo carboniferous reversed superchron palaeomagnetic results suggest that the two sills may not be exactly contemporaneous the mvs was intruded rapidly some time during the emplacement of the gws which was itself emplaced over a considerable time period the mvs may well represent the northernmost expression of the instrusive activity a mean palaeointensity of 22 9 2 6 t was calculated for the great whin sill with a corresponding vdm value of 5 9 1022 a m2 the palaeointensity value calculated from the midland valley sill was much lower 13 0 0 5 t with a corresponding vdm value of 3 3 1022 a m2 detailed rock magnetic analyses and stepwise thermal demagnetization studies reveal that the remanence carrier in both sills is magnetite of mixed domain state multidomain grains are less important in the midland valley sill and thus greater confidence is attached to palaeointensity results from this sill vdm values from this and other studies of the permo carboniferous reversed superchron p crs indicate that the dipole strength showed similar variations to that witnessed for the past 5 ma there is no conclusive support in favour of either the weak or strong field models that have been suggested for superchrons more palaeointensity results are needed from the termination of the p crs and the late permian early triassic in order to understand how the dipole field evolved during the p crs and its relationship to the so called mesozoic dipole low

evidence for the existence of a transitional geomagnetic field recorded in a proterozoic lava succession summary an anomalous palaeomagnetic field direction is recognized in a sequence of rapidly extruded subaerial basaltic lava flows within the mid proterozoic c 1300 ma gardar lava succession south greenland thermal demagnetization and component analysis link this direction to a thermoremanent magnetization trm acquired during primary cooling the direction which bears no resemblance to any other palaeomagnetic direction witnessed within the group is recorded by seven successive flows which appear to have been extruded and magnetized over a short period of time during a geomagnetic polarity transition palaeointensity experiments employing the modified thellier technique indicate that this transitional direction was acquired in a field with only 25 per cent of the magnitude of the stable polarity field the behaviour of the field direction during and after this transition reflects that witnessed in studies of much younger transitional sequences whereas that for the field intensity differs fundamentally from consistent observations from younger lava successions these results provide a significant contribution to our knowledge of the ancient geodynamo and may well document the oldest transitional field identified by a combined palaeomagnetic and palaeointensity study and the only one reported from the precambrian era

palaeomagnetic evidence for the age of the cumbrian and manx hematite ore deposits implications for the origin of hematite mineralization at the margins of the east irish sea basin uk examples of epigenetic hematite mineralization located at the margins of the east irish sea basin have been sampled for palaeomagnetic investigation to establish the timing of ore formation samples collected from 10 sites in cumbria are dominated by hematite of near pure composition and characterized by dual polarity and in part dual component magnetizations that yield palaeopoles nine sites between 50 n 130 e and 50 n 150 e in contrast samples recovered from seven sites on the isle of man have more varied mineralogical compositions that include ferromagnets with lower unblocking temperatures manx hematite mineralization also exhibits dual polarity but magnetizations yield steeper inclinations with palaeopoles lying between 55 n 130 e and 70 n 150 e steeper directions probably reflect minor tilting of the manx block following hematite emplacement correlation of poles with the european apparent polar wander path indicates that these ore deposits formed during the middle triassic results show that hematite mineralization sited at the margins of the east irish sea basin is essentially contemporaneous and establishes for the first time an underlying temporal relationship between these geographically dispersed ore bodies dating places the mineralization within a broader extensional geotectonic regime associated with the early rifting of pangaea in the north atlantic the context implies a causal link between crustal extension development of ore forming fluids within the evolving east irish sea basin and subsequent migration of fluids to basin margins where iron was precipitated as hematite supplementary materials locations and details of palaeomagnetic sample sites and a tabulation of selected reference palaeomagnetic poles defining the triassic and early jurassic segment of the apparent polar wander path for the european sector of the eurasian plate are available at www geolsoc org uk sup18691

palaeomagnetism and magnetic properties of the cappadocian ignimbrite succession central turkey and neogene tectonics of the anatolian collage abstract the cappadocian ignimbrite succession of central southern anatolia comprises at least nine major and two minor calc alkaline rhyolitic sheets emplaced at 1 2 ma intervals between 11 2 and 1 1 ma it records the last phase of neotethyan subduction during final emplacement of the tauride orogen in southern turkey this study reports magnetostratigraphy and describes associated rock magnetic properties remanence resides in ti poor titanomagnetites haematisation is locally produced by post emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane directions of kmax with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing older ignimbrites upper and lower goreme akdag zelve from the cardak centre are all of normal polarity later ignimbrites partly erupted from the derinkuyu centre comprise the sarimaden r cemilkoy r tahar r kizilkaya r incesu n and valibaba sofular r ignimbrites the overall reversed group mean is d i 174 51 n 10 units r 9 84 95 6 6 k 55 and all magnetisation directions from the upper goreme 9 0 ma onwards are rotated anticlockwise with respect to eurasian and african palaeofields this sense of rotation characterises most of central anatolia and averages 9 5 in this sector the rotation rate from 8 to 1 ma bp was 1 25 ma but it appears to have accelerated during the latter part of the quaternary to about an order higher than rates determined from gps rotation has resulted from extrusion of fault blocks during tectonic escape of the anatolian collage to the southwest and followed crustal thickening as the afro arabia plate has continued to impinge differentially into eurasia cenozoic magnetic inclinations are systematically shallower across anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates only part of this difference 400 km can be accommodated by northward movement and crustal thickening in central anatolia since mid miocene times shallow inclinations observed in the aegean region extend into turkey they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly the pattern of neotectonic declinations across anatolia shows rotations that are strongly anticlockwise rotated in the east near the arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage rotations also seem to become generally younger towards the south crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the tauride arc and by inference the cyprus arc

palaeomagnetic field intensity variations suggest mesoproterozoic inner core nucleation the earth s inner core grows by the freezing of liquid iron at its surface the point in history at which this process initiated marks a step change in the thermal evolution of the planet recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the earth s core resulting in estimates of the timing of inner core nucleation ranging from less than half a billion to nearly two billion years ago recent inner core nucleation high thermal conductivity requires high outer core temperatures in the early earth that complicate models of thermal evolution the nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner core nucleation to be estimated directly previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements by the lack of an effective means of assessing their reliability and by shorter timescale geomagnetic variations here we examine results from an expanded precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria our analysis provides intensity based support for the dominant dipolarity of the time averaged precambrian field a crucial requirement for palaeomagnetic reconstructions of continents we also present firm evidence for the existence of very long term variations in geomagnetic strength the most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1 5 billion years ago this observation is most readily explained by the nucleation of the inner core occurring during this interval the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the earth

a palaeomagnetic and palaeointensity study of the mauchline lavas south west scotland the mauchline lavas of sw scotland erupted close to the carboniferous permian boundary and near the middle of the permo carboniferous reversed superchron pcrs rock magnetic studies have identified mixed magnetite hematite assemblages both minerals record reversed components of magnetisation a fold test is not diagnostic but the mean remanence recovered in 23 stable samples at five sites di 192 5 26 1 95 14 3 accords with the regional field at the time of eruption and the pole position 346 8 e 47 2 s is coincident with late carboniferous early permian poles a palaeointensity study of 15 samples employing the modified thellier thellier method yields nine acceptable estimates of ancient field intensity with a mean value of 13 6 4 5 t from four lava flows the corresponding virtual dipole moment of 3 3 1022 am2 is approximately 40 of the present day value this result supports recent studies from western europe which suggest that the geomagnetic field strength was low during the pcrs but is in conflict with russian data which suggest that the field strength was close to or slightly higher than the present day value