surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

stratigraphy and paleomagnetism of a 3 km thick miocene lava pile in the mjoifjordur area eastern iceland we have carried out stratigraphic mapping in the upper miocene basalt lava pile around the fjords mjoifjordur and seydisfjordur eastern iceland the mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups these units are also used to provide correlations with the nordfjordur area south of mjoifjordur we present a 3 km composite stratigraphic column for the area between mjoifjordur and seydisfjordur the geology of this area shows some differences from the classical model of walker for the structure of eastern iceland partly due to the fact that most of mjoifjordur is not in the vicinity of central volcanoes detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles the local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria over 20 geomagnetic reversals are recorded in the eastern iceland lava pile in a period approximately 13 10 ma ago the geomagnetic field during this period averages to a central axial dipole field and its overall statistical properties resemble those obtained in earlier surveys in iceland

palaeointensity determinations from historical and holocene basalt lavas in iceland summary palaeomagnetic measurements were made for 11 post glacial basalt lavas in iceland three pre 2 ka lavas seven historical lavas and one present day lavas palaeodirections were easy to determine at most sites but three of the sites seem to have suffered from some block movement at the lava front making determination of palaeodirections difficult reliable site mean palaeodirections were successfully obtained at two of these sites by analysing the remanence components with low blocking temperature but one site was rejected due to low convergence of remanence directions indicating a possible post cooling block movement a total of 45 samples 52 specimens from nine lava flows were selected for palaeointensity experiments using the thellier method based on the thermal stability of remanence and other rock magnetic features together with hysteresis parameters to show single domain to pseudo single domain natures 26 of the 45 samples 29 of the 52 specimens produced successful results subsidiary experiments using shaw s method were also carried out on one specimen from each of eight lava flows and all of these produced successful results combining these thellier and shaw results eight site mean palaeointensities were obtained including one 1984 ad lava giving a specimen level success rate of 56 per cent for the experiments using the thellier method and 100 per cent for those using shaw s method our results together with those of previous studies indicate that variations in palaeointensity and palaeodirection in iceland are basically similar to those in nw europe for the last 2000 yr variations in palaeointensity converted to virtual dipole moment or virtual axial dipole moment are in good agreement with both nw europe and global models some differences in inclination and declination are recognized between iceland and nw europe but overall variation curves are similar and good agreement with the global models is also observed

new palaeomagnetic results from icelandic neogene lavas summary in this paper some statistical properties of new collections of palaeomagnetic direction and intensity data are presented and compared with results from older collections the new data come mostly from three composite surveys in the basalt lava pile of iceland totalling about 1000 lavas of ages 7 14 myr the palaeomagnetism of all these lavas converges on a reproducible long term pattern in several important features of the neogene field behaviour previous conclusions which are confirmed in this paper include 1 the distribution of intermediate virtual geomagnetic poles vgps is not confined to preferred longitude intervals 2 normal and reverse polarity states of the geomagnetic field are essentially equivalent as regards the intensity of the field mean field directions and secular variation 3 the virtual geomagnetic dipole moment fdls off with decreasing vgp latitude by a factor of 3 5 4 between polar and equatorial latitudes 4 a significant reduction in the between flow angular dispersion of vgps may have occurrcd during the last 14 myr and 5 palaeomagnetic directions from icelandic basalt lavas are stable and reliable

extension across tempe terra mars from measurements of fault scarp widths and deformed craters two independent methods with no common assumptions have been used to estimate the extension across the heavily deformed tempe terra province of the tharsis region of mars one method uses measurements of normal fault scarp width with average scarp slope data for simple grabens and rifts on mars to estimate the fault throw which combined with sparse fault dip data can be used to estimate extension formal uncertainties in this method are only slightly greater than those in other methods given that the total uncertainty is dominated by the likely uncertainty in the fault dip assumed to be 60 15 measurement of normal fault scarp widths along two n25 50 w directed traverses across tempe terra both yield about 22 16 km of extension or 2 strain across the northern traverse and nearly 3 across the southern one about three quarters of the extension has occurred during the two main phases of tharsis related deformation from middle late noachian to early hesperian and from late hesperian to early amazonian with more extension closer to the center of tharsis during the first phase extension across the region was also determined by measuring the elongation and elongation direction of all ancient noachian impact craters without ejecta blankets which predate most of the deformation results have been corrected for initial non circularity of craters established from similar measurements of young post deformation impact craters yielding a statistically significant mean strain of 1 96 0 35 in a n38 10 w direction across tempe terra extension of 20 4 comparable in magnitude and direction to the average result from the scarp measurement method both methods indicate an average extension for single normal fault scarps and shortening across wrinkle ridges for the crater method of 100 m the agreement between the results of the two independent methods in overall extension and average single normal fault extension argues that the average scarp slope and fault dip data in the fault scarp width method accurately represent the actual extension across the observed structures this conclusion supports existing geometric and kinematic models for structural features on mars a preliminary estimate of the total circumferential extension around tharsis at a radius of 2500 km is roughly 60 42 km total hoop strain is about 0 4 distributed heterogeneously tempe terra is the most highly strained region on mars

determining heights and slopes of fault scarps and other surfaces on venus using magellan stereo radar this report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces such as normal fault scarps on the surface of venus from measurements of their widths in magellan stereo synthetic aperture radar sar images these surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat lying areas in the 120 to 280 m resolution magellan sar images but are not generally imaged in the 10 km resolution magellan altimetry our methods take into account radar distortion effects and allow one to determine whether a slope is foreshortened laid over elongated or in radar shadow the techniques make use of graphs constructed for the magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner but the techniques are applicable to any stereo radar data set if the radar incidence angles are known additionally the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow using these techniques we show that there are normal fault scarps on venus with heights in the 700 m range that have remarkable topographic slopes of close to 60 something unheard of on earth and which suggests a high effective cohesive strength of the venusian crust

the santa rosa event 40ar 39ar and paleomagnetic results from the valles rhyolite near jaramillo creek jemez mountains new mexico the jaramillo event was originally defined by doell and dalrymple in 1966 on the basis of k ar ages from sanidine in the normally transitionally and reversely magnetized rhyolite domes named cerro del abrigo cerro santa rosa i and cerro santa rosa ii respectively that erupted following collapse of the archetypal valles caldera new mexico we have collected new paleomagnetic data from the three domes and new 40 ar 39 ar laser fusion and furnace incremental heating experiments on sanidine crystals from the cerro santa rosa i rhyolite step wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of doell and dalrymple science 152 1966 1060 1061 are valid cerro del abrigo is normally magnetized whereas the santa rosa i dome is transitional with an inclination of 363 toward the east and the santa rosa ii dome is of reversed polarity twenty five laser fusion experiments on sanidine crystals from the the cerro santa rosa i dome together with the saddle shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption our preferred age of 936 8 ka 2c for the santa rosa i rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages this age is significantly older than was inferred on the basis of earlier laser fusion 40 ar 39 ar results that suggested a trapped component characterized by a 40 ar 36 ar ratio lower than the atmospheric value of 295 5 our new age distinguishes the santa rosa i dome by 65 kyr from the termination of the jaramillo normal subchron 1001 10 ka and by 37 kyr from basaltic lavas at haleakala and tahiti which record the kamikatsura event 899 6 ka moreover this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period the cerro santa rosa i rhyolite dome once intimately linked with the end of the jaramillo event and the acceptance of plate tectonic theory now defines a highly resolved feature the santa rosa event in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments fl 2002 elsevier science b v all rights reserved

the growth collapse and quiescence of teno volcano tenerife new constraints from paleomagnetic data tenerife basically consists of three miocene shield volcanoes the anaga the teno and central shield as well as the pliocene canadas volcano the temporal evolution and structural significance of each volcano with respect to the history of tenerife is still a matter of debate we present paleomagnetic results in order to enhance the view of the volcanic history of the teno volcano by means of magnetostratigraphy it is found that the initial subaerial phase shows reverse magnetizations throughout after two major sector collapses dominantly normally magnetized lavas extruded comparisons of observed magnetic polarities with the geomagnetic polarity timescale show that these volcanic activities occurred within 0 4 myr between 6 3 and 5 9 ma significantly younger flows 5 3 myr old according to their radiometric age revealed again normal polarity throughout the absence of inversely magnetized lavas in between the two normal periods indicates a volcanic hiatus or erosional phase the evolutionary sequence and the estimated high production rates for the initial building phase are similar as would be expected for a hotspot volcano the average geomagnetic field for 6 0 0 2 ma is close to an axial dipole field showing a slight far sided right handed effect the field strength determined by thellier type intensity determinations corresponds to a virtual axial dipole moment of 4 9 1022 a m2 this value is approximately half of the present day field strength but similar to values obtained for the mid miocene it also corresponds to the proposed tertiary low field level of the geomagnetic dipole moment

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

magnetic properties of olivine basalt application to mars abstract the natural remanent magnetisation nrm of basalt lava containing oxidised magnetic phases is usually assumed to be proportional to the weight percent of magnetic phases it is shown here that olivine basalt has a different behaviour the nrm intensity increases at an increasing rate with the amount of magnetic phases this is attributed to the oxidation of olivine during cooling of the basalt that leads to the exsolution of magnetite in a single domain state in this way olivine basalt is found to become an order of magnitude more magnetic than basalt that does not contain olivine a simple explanation for the magnetic anomalies on mars is offered based upon these findings and mossbauer spectroscopy data from gusev crater on mars

some statistical properties of palaeomagnetic directions in icelandic lava flows summary this paper extends the efforts of kristjansson mcdougall to obtain a semi quantitative description of various long term 0 1 10 myr properties of the geomagnetic field in iceland due to the long time intervals between eruptions of the lavas recording the field local average and stochastic properties are emphasized rather than detailed aspects such as pole paths these properties can be represented by smooth functions of virtual pole latitude it is possible to describe both the frequency distribution of virtual poles in latitude and the estimated frequency of geomagnetic reversals by a simple process of random walk of the pole this is also in agreement with the observed long term symmetry of the reversal process and demonstrates that memory effects are unnecessary to account for the behaviour of the field the average transition time for the field isofthe order of 5 6000yr right handedness is a pervasive property of the field in iceland apparently increasing with age it is demonstrated that the size of palaeomagnetic collections in lava sequences has to be much larger than 20 units in order to yield consistent values of statistical parameters for mean palaeofield directions

a coupled thermal mechanical model for corona formation on venus we develop a model of the combined thermal and mechanical evolution of diapirs in the mantle of venus the diapir is treated as an oblate spheroid rising through a viscous fluid and ultimately impinging on a rigid overlaying lid drag forces imposed by the lid cause the diapir to spread and flatten as it rises we parameterize the heat loss from the rising diapir using a nusselt number formulation and treat the resulting loss in buoyancy in our flow calculation the model predicts the evolution timescale and degree of flattening of a diapir as it rises as well as the stresses exerted on the underside of the lithosphere in order to explore diapir behavior further we check our analytical model against the predictions of the finite element code mantle from the combined results of these models and observations of venusian coronae we are able to make a number of inferences about venusian diapirism and mantle properties we find that the diapirs responsible for formation of venusian coronae have an initial size distribution extending from about 30 to 100 km in radius typical evolution timescales for these diapirs are 30 50 myr if venus global average resurfacing age is 300 500 myr then our results are consistent with a rate of diapirism that has been roughly constant over this period and with an effective mantle viscosity of 1021 pa s

investigations on geomagnetic reversals in icelandic lavas 1953 78 recent reviews of the development of palaeomagnetic techniques in geology have shown that palaeomagnetism has contributed significantly to the acceptance of new concepts in global tectonics the present paper extends frankel s 1987 review of palaeomagnetic work on the basalt lava sequences of iceland in the early 1950s these lavas provide some of the most complete series of reliable spot readings of the geomagnetic field available it is shown that research on icelandic basalts has continued to be of importance in improving our understanding of palaeomagnetism and its applications

variable dispersion of neogene geomagnetic field directions in iceland abstract analysis of the primary paleomagnetic directions in 4000 lava flows in iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 ma this dispersion which is independent of the polarity of the field is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka

magnetic surveys of iceland abstract data from a 1968 1980 aeromagnetic survey of iceland and a 1972 1973 marine magnetic survey of a part of the shelf around iceland have been digitized and upgraded additional aeromagnetic survey lines were flown in 1985 1986 after appropriate low pass filtering and regridding a new multicolor map of geomagnetic field intensity anomalies over the island and its surroundings was produced the main features of the anomaly field are of two types the first 1 a comprises broad lineations subparallel to currently active or extinct spreading zones these are believed to be mostly due to tilled basalt lava flows of alternating remanence polarity in quaternary regions other extrusives of subglacial origin also contribute to the lineations the second 2 b comprises localized anomalies of the order of 10 km or less at individual volcan c centers we describe some of these features in terms of current knowledge of the age structure and magnetic properties of the uppermost crust in iceland

strain at radially fractured centers on venus 1 radially fractured centers rfcs on venus are distinctive 100 km long radiating systems of graben fissures and fractures arranged around a central topographic high we use cycle 1 and cycle 3 magellan images to determine the depths and wall dip angles of several large radial graben using two different methods at four rfcs becuma mons didilia corona h uraru corona and pavlova corona we record graben depths of 0 3 0 9 km and mean wall dip angles of about 36 which we interpret as a primary talus slope rather than a fault plane using the calculated depths and a more realistic fault dip of 60 we find extension of between 0 5 and 1 km on graben between about 5 and 10 km wide we use this extension to estimate the hoop strain at each rfc maximum measured strains on the flanks are typically between 1 and 4 and may be up to 7 toward the center we conclude that this strain is too large to be explained by current plume related uplift models and also by magma chamber inflation we conclude that dikes are probably responsible for the majority of the strain at the large radial graben

crustal drift in iceland summary there are indications that tertiary and post tertiary volcanism in iceland has been confined to the present main belt of active volcanism the geological structure of the tertiary lava piles in the east and west of iceland and results of geophysical studies may be explained by invoking an appreciable amount of crustal drift since the beginning of the tertiary the rocks in the extreme east and west of the island may have been carried apart by 400 km or more the crustal drift is believed to result mainly from crustal extension through the injection of dykes the structure of iceland and possibly that of other flood basalt areas is believed to be closely related to the world wide rift system

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

stratigraphy and paleomagnetism of a 2 9 km composite lava section in eyjafjordur northern iceland a reconnaissance study eight mountainside profiles in lava flows south of the fjord eyjafjordur northern iceland were sampled for paleomagnetic studies the sampling was concurrent with pilot stratigraphic mapping of the lava sequences in these and several supplementary profiles the eight profiles are correlated with minor overlaps so that they form a composite section of 2 9 km thickness estimated to cover the age range between about 9 and 5 million years ago hardarson et al 1999 paleomagnetic measurements made on 319 lavas generally yield primary remanence directions of high stability and within unit consistency evidence for at least 17 reversals of the geomagnetic field is seen as well as numerous field excursions frequent clustering of directions in successive lavas indicates that the volcanism was episodic

on low latitude virtual geomagnetic poles in icelandic basalt lava sequences abstract in two paleomagnetic studies from the neogene lava sequences of iceland published in 1977 of the order of 10 of the reliably determined remanence directions corresponded to virtual pole positions below 40 latitude this was a larger proportion than found in most other volcanic locations to explain it harrison harrison c g a 1980 secular variation and excursions of the earth s magnetic field j geophys res 85 3511 3522 suggested that some 10 of the observed virtual geomagnetic poles vgp from iceland might be distributed at random over the globe these random poles can be envisaged as dating from periods when the geomagnetic field is weak and dominated by variable non dipole terms much additional data on remanence directions and intensities has been collected since then and the present study analyses results from over 3500 lava flows in iceland the relative frequency and strength of virtual dipoles in this collection vary with the pole latitude in a similar way as in the collection studied by harrison normalizing of remanence intensity data by the use of anhysteretic remanence arm may improve the resolution of this type of analysis

some aspects of the late tertiary geomagnetic field in iceland summary in 1972 78 the late n d watkins and others carried out ajoint field programme of geological mapping in the mio pliocene flood basalts of iceland including sampling for k ar dating and palaeomagnetic research the major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile this paper deals with various statistical properties of this data set it is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean floor polarity time scale there is no evidence for significant asymmetries between normal and reverse polarity states of the field neither as regards chron lengths secular variation or virtual dipole moment magnitude intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution the latitude distribution of virtual magnetic poles can be fitted with a bingham function having k 4 5 and low latitude poles do not occur preferentially in any particular longitude interval

formation of beta regio venus results from measuring strain beta regio is an area of rifting and volcanism on venus constituting a topographic rise a shield volcano theia mons lies near the center of the region and is surrounded by several radially oriented rifts we use magellan altimetry gravity and synthetic aperture radar data of the area to constrain some subsurface parameters first we derive hoop strain using altimetry data and a fault dip angle derived from the split crater somerville we determine the extension in the rifts surrounding beta regio we then derive the hoop strain accommodated by the rifts from the extension in these rifts except near theia mons the hoop strain follows the shape expected from a mantle upwelling the difference near the volcano we believe is due to volcanic infilling we then model three observable quantities the newly derived strain along with gravity and uplift using two separate modeling techniques one for the strain and uplift and another for the gravity the model results show that the data are consistent with the view that a relatively low density contrast region now exists below beta and has caused the uplift and rifting in the region

mapping and modeling of radial fracture patterns on venus synthetic aperture radar images indicate numerous radial fracture systems on venus the formation of these systems has been attributed either to surface deformation related to subsurface dike emplacement or to fracturing caused by domical uplift above an ascending magma plume we evaluate the dike emplacement hypothesis by investigating the relationship between the process of subsurface dike emplacement and the geometric attributes of radial fracture patterns assuming the surface fracture patterns correspond to the patterns of dikes about a magma chamber below the volcanic summit we select a radial fracture system for mapping and model its formation using a two dimensional analysis which idealizes the chamber as a pressurized hole in an elastic plate subject to a remote biaxial stress field radial fractures correspond to maximum principal stress trajectories and the pattern of these trajectories is controlled primarily by four variables the chamber radius the magma pressure the remote differential stress and the remote principal stress direction we find that stress trajectories also depend on the remote mean stress and that the location of isotropic points in the trajectory pattern can be used to interpret the mean stress state these models are used to estimate the remote loading conditions under which dikes were emplaced the mapped fracture pattern is consistent with dikes emplaced under remote differential stress conditions ranging from 2 to 8 mpa calculated over a range of possible emplacement depths from 1 to 9 km this is consistent with regional stress states on earth

a detailed palaeomagnetic study of the oldest 15 myr lava sequences in northwest iceland summary we have determined the directions of primary remanence in over 400 lava flows in the northwestern peninsula of iceland near 66 1 n 23 3 w these lavas were sampled in 16 profiles spread across 75 km at the stratigraphic level of the oldest approximately 15 myr in age of several distinctive lignite bearing sediments in the peninsula we find that the pattern of polarity reversals in the lava pile is broadly similar in these profiles but it is generally not possible to trace short polarity zones and excursions over more than a few kilometres laterally our results which also include measurements on the lignite sediments at three sites are consistent with previous suggestions that these sediment beds represent a time gap of the order of 0 2 myr between the lavas above and below the mean virtual geomagnetic pole of this collection is far sided by approximately 7 and the scatter of individual poles is greater than that in palaeomagnetic surveys on younger lava series in iceland