precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

palaeomagnetism of the ferrar dolerite in the northern prince albert mountains victoria land antarctica summary during the sixth italian expedition to victoria land antarctica jurassic ferrar dolerite sills were sampled at 23 sites in the northern prince albert mountains the rock magnetic properties were similar in all sites saturation remanence remanent coercive force and curie temperature values pointed to titanomagnetite as the main carrier of magnetization thermal and af demagnetizations were both effective in removing weak secondary magnetization components below 250 c and 20 mt peak field further demagnetization resulted in single component stable chrm palaeomagnetic directions were derived after af demagnetization at 20 mt and site mean vgps computed for all sites but one thern a where systematic changes of the chrm inclination have resulted in single sample vgps dispersed astride the equator the site mean vgps distribution is clearly bimodal 15 vgps cluster around a palaeopole lat 48 s long 226 e in good agreement with the literature data whereas seven vgps are displaced toward a palaeopole at higher latitude lat 79 s long 181 e all vgps can be regarded as jurassic because no evidence of substantial secondary components was found and experimental data showed that the ptvrm partial thermal viscous remanent magnetization acquisition rate is too low to account for complete remagnetization of the rocks giving the anomalous vgps the thern a and the southward displaced vgps plot along a great circle through the east antarctica jurassic palaeopole as a high reversal rate in the jurassic is well documented by magnetostratigraphy and the reversal vgp path is often confined to a narrow longitudinal band we suggest that the great circle is a palaeomeridian and the sills characterized by anomalous vgps were intruded during transitional phases of the geomagnetic field similar anomalous vgps previously reported from the ferrar supergroup rocks of northern victoria land also plot along the same palaeomeridian which is only 30 away from the palaeomeridian derived from the jurassic intrusives of the dufek massif in the transantarctic mountains burmester sheriff 1980 the jurassic palaeopole of east antarctica has been recalculated from 103 vgps available in the literature the position is 57 s 221 e when all vgps are taken into account and 51 s 223 e when the high latitude vgps from victoria land are discarded as anomalous

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

the time averaged paleomagnetic field we review indications of persistent deviations from the geocentric axial dipole model of the time averaged geomagnetic field and present a zonal harmonic model derived from 185 deep sea sediment piston cores taken from low to middle latitudes to approximately 45 analysis of the paleomagnetic inclination recorded in these cores for the brunhes normal polarity 0 73 ma and matuyama reverse polarity 0 73 2 47 ma chrons after plate motion correction gives well constrained estimates of the dominant long term nondipole contributions the axial quadrupole and axial octupole and shows no significant deviation from axial symmetry the amplitude of the axial quadrupole is found to vary with polarity 2 6 of the geocentric axial dipole for normal 4 6 for reverse while the axial octupole does not show appreciable change 2 9 for normal 2 1 for reverse these estimates of the quadrupole contribution agree well with prior determinations for the plio pleistocene 0 5 ma however the octupole contribution we find is opposite in sign to previous estimates we suggest that a negative octupole is representative of the actual time averaged paleomagnetic field while prior positive octupole estimates probably reflect spurious inclination shallowing the lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact

thermal core mantle interactions and time averaged paleomagnetic field recent studies of historical geomagnetic secular variation and lower mantle seismic velocity suggest the earth s magnetic field is controlled by lateral variations in mantle temperature in this paper the average expected magnetic field pattern is compared with the time averaged field obtained from paleomagnetic pole positions from the last 5 m y the historical record suggests that the time averaged inclination anomalies are negative far sided in the atlantic region north of 30 s and the west pacific and positive in the south atlantic extreme north and south pacific and east pacific the pole positions give positive inclination anomalies in the south atlantic and extreme north pacific and negative anomalies elsewhere in agreement with the theory except for the east pacific this suggests that the historical record is not long enough to give an accurate picture of long term secular change in the eastern pacific but that elsewhere the time averaged field of paleomagnetism is predicted satisfactorily the preponderance of far sidedness in the paleomagnetic record is a consequence of the present distribution of temperature at the base of the mantle rather than a fundamental property of the geodynamo and it may therefore change on the long time scale of mantle convection in the historical record declinations average to negative values in the atlantic hemisphere and positive values in the pacific which if typical of longer term behavior could explain the preponderance of positive declinations for reversed polarity which are mainly in the atlantic hemisphere

evidence of a partitioned dynamo reversal process from paleomagnetic recordings in tahitian lavas 1 lavas erupted at the society hotspot during the matuyama brunhes m b polarity reversal record transitional field behavior containing two tight paleodirectional groups that when averaged are antipodal at the 95 confidence level and thus correlate to antipodal clustered virtual geomagnetic poles vgps the occurrence of these observations data obtained from two published records of the m b transition from distinct sections of a succession of flows on tahiti is associated with a time when the strength of the axial dipole was significantly reduced one of the clusters was recorded by lavas that were not erupted in succession suggesting that significant time had passed during this volcanic activity time spent during the formation of the antipodal cluster is unknown although it resides in the same location as vgp clusters associated with four other transitional events obtained from society hotspot lavas calculated vgps at the society hotspot for both polarities of the averaged historic field less the axial dipole term are found in the cluster locations these findings offer strong evidence for a two tiered dynamo process in which nearly the entire axial dipole component undergoes both demise and regeneration quasi independently from that of the remainder of the field the pattern of which is tied to long held physical conditions of the lower most mantle

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

polarity transitions and excursions of the geomagnetic field the nature of geomagnetic field behavior during polarity transitions is one of the most hotly debated issues in modern geophysics dynamo action in the earth s fluid outer core is known to be responsible for generating the earth s main magnetic field however the mechanism for polarity transition is still unknown polarity transitions are a fundamentally important property of the geomagnetic field but because they have not taken place in historic time it is necessary to turn to the paleomagnetic record to understand the mechanism by which they occur

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

long lived transitional states of the geomagnetic field and the two dynamo families palaeomagnetic data from lavas produced at the hawaii and society islands hotspots suggest that one or more specific long lived field configurations recur during successive polarity reversals spanning perhaps several million years the apparent quasistationarity of these states and the relative placement of the two recording sites offer a rare opportunity to explore the fundamental geometrical properties of transitional fields

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

simulated geomagnetic reversals and preferred virtual geomagnetic pole paths summary the question of whether virtual geomagnetic poles vgps recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists one possible mechanism for such vgp clustering is the heterogeneity of heat flux at the core mantle boundary cmb we use 3 d convection driven numerical dynamo models with imposed non uniform cmb heat flow that show stochastic reversals of the dipole field we calculate transitional vgps for a large number of token sites at the earth s surface in a model with a simple heat flux variation given by a y22 harmonic the vgp density maps for individual reversals differ substantially from each other but the vgps have a tendency to fall around a longitude of high heat flow the mean vgp density for many reversals and excursions shows a statistically significant correlation with the heat flow in a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean vgp density at american and east asian longitudes roughly consistent with the vgp paths seen in several palaeomagnetic studies we find that low latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated they contribute to the equatorial dipole component and bias its orientation in longitude during reversals the equatorial dipole part is not necessarily dominant at the earth s surface but is strong enough to explain the longitudinal preference of vgps as seen from different sites

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

double seismic zone for deep earthquakes in the izu bonin subduction zone a double seismic zone for deep earthquakes was found in the izu bonin region an analysis of sp converted phases confirms that the deep seismic zone consists of two layers separated by 20 kilometers numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 to 550 c which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high pressure phase in the subducting slab

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

simulations of a time varying non dipole field during geomagnetic reversals and excursions abstract following brown et al 2007 reversals and excursions have been simulated by decreasing the axial dipole field to zero while leaving up a time varying non dipolar field similar to the cal7k model korte m constable c g 2005 continuous geomagnetic models for the past 7 millennia ii cals7k geochem geophys geosyst 6 2 doi 10 1029 2004gc000801 we also envisaged the situation in which the total field changes sign we first investigated the relationship between the non axial dipole structure of the historical and present field snapshots and the virtual geomagnetic poles vgps the accumulation of vgps in the vicinity of large flux concentrations in south atlantic and australia is constrained by the equatorial dipole only a few sites directly above the anomalies are associated with vgps in their vicinity therefore it seems delicate to associate recurring clusters of vgps during reversals with magnetic flux concentrations at the cmb the individual vgps resulting from simulations of reversals show all kinds of trajectories some being very simple with longitudinal confinement while others are complex with alternances of slow and fast changes as well as loopings of vgps preference for a specific longitude frequently happens in individual records but this is not systematic a direct consequence is that records of different reversals from a global array of sites are effectively dominated by preferred longitudinal bands thus a time varying non dipolar transitional field can generate anisotropy of the vgp distribution smearing of the signal by sedimentary records considerably amplifies the longitudinal confinement of the vgps overall these simulations account for most characteristics reported in detailed reversal records and thus strongly support the concept of a non dipolar transitional field we also simulated field excursions by assuming either a vanishing dipole followed by its return to initial polarity or by a short episode with the opposite polarity preceding its restoration the model accounts for the different characteristics inherent to the volcanic and sedimentary records of the recent excursions the simple structure of sedimentary vgp paths dominated by a large loop between northern and southern latitudes partly results from smearing of the signal therefore these records do not fully describe the complexity of the excursional field and remain exclusively sensitive to very low degree harmonics most detailed studies of excursions exhibit reversed directions which cannot exist without a very short dipolar episode of opposite polarity

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

phase transformations earthquakes and the descending lithosphere abstract the energy dissipated from metastable phase transformations is investigated in this paper in addition to the kinetic energy envisaged by randall at least two other major forms of energy latent heat and metastability energy are involved in a metastable phase transformation the seismic energy density for metastable phase transformation is of the order of 10 24 10 25 erg km 3 for most mantle silicates as an example an earthquake of magnitude m 7 5 would require only 0 01 0 03 or 0 04 km 3 of basalt olivine or pyroxene respectively undergoing metastable phase transformation earthquakes caused by both faulting and phase transformation mechanisms have been observed in high pressure laboratory experiments the energy density associated with a faulting mechanism appears to be four to five orders of magnitude less than that associated with metastable phase transformations of mantle silicates confirming theoretical expectations according to all of the thermal models for the descending lithosphere available in the literature metastable phase transformations almost certainly occur in the descending lithosphere enormous energies would be released suddenly in association with these metastable phase transformations thus earthquakes within the descending lithosphere at depths greater than 100 km may be induced by metastable phase transformations however metastable phase transformations in the descending lithosphere have little or an opposing effect on the tectonic engine driving the motion of plates into the mantle

rare palaeomagnetic evidence of long term mantle control of the geodynamo and possible role of the nad field in the reversal process the degree to which the lower most mantle influences behavior of the geodynamo has been debated over the past quarter century our analysis of a comprehensive set of 17 cenozoic paleomagnetic transitional field records obtained from lavas in the southern hemisphere provides robust evidence of stable mantle control since the pliocene the records come from a region where given a significantly weakened axial dipole the magnetic field today would be largely controlled by the non axial dipole nad flux patch currently emanating from earth s outer core beneath western australia the paleomagnetic recording sites from west to east include the south indian ocean eastern downloaded from https academic oup com gji advance article abstract doi 10 1093 gji ggz480 5602599 by guest on 17 november 2019 australia new zealand and french polynesia the analyzed records contain from 2 to 25 sequential transitional virtual geomagnetic poles vgps ten of the 17 records supply at least one vgp within a narrow longitudinal band 10 wide between 60 s and the equator centered along longitude 102 4 e that is transitional data from 59 of the cenozoic recordings are found to reside in a region that encompasses a mere 2 8 of the vgp transitional area on earth s surface a robust monte carlo approach applied to this dataset one that takes into account the number of transitional vgps contained in each record finds this result highly improbable p value 0 0006 the present day pattern of vertical flux at the core mantle boundary shows an anomalously strong thin southern hemisphere longitudinal band off the west coast of australia that strikingly coincides with this unusual paleomagnetic finding we conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 myr seemingly independent of the behavior of the axial dipole our findings indicate it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity

the gilbert gauss sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic record of the gilbert gauss reversal boundary in deep sea marls from southern sicily shows that the remanence characterising the reversal is carried by a high temperature magnetite component a low temperature component is intermediate between the high temperature component and a present day overprint suggesting that it is obscured by weathering the actual reversal record is clearly affected by filtering caused by early diagenetic acquisition of a chemical remanent magnetisation crm intermediate vgps show a general confinement to a longitude over the americas more specifically they are restricted to two clusters a less distinct one off se south america and a very prominent one near ne north america the remarkable coincidence with two hoffmann clusters hoffmann 1992 suggests that even a crm acquisition filter cannot entirely obscure long lived transitional features

plate motions and the geomagnetic field i quaternary and late tertiary summary we have tested the hypothesis that the apparent increase in the northward offset of the axial dipole i e the ratio g gy with age for the last 25 ma wilson mcelhinny is due to the failure to correct for plate motions spherical harmonic analyses were performed on two types of data palaeomagnetic poles from continents islands and seamounts and magnetic inclinations from deep sea cores after returning the sampling sites to their predrift locations in fixed hotspots coordinates the results show that the quadrupole term g maintained a value of about 0 05 gy throughout the last 35 ma and that the axial octupole had a small value of about 0 02 gy for the last 5 ma sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years independent data sets for the past 2 ma and for the period 2 6 ma gave nearly identical solutions showing that the persistence of the axial terms is not a phenomenon of the more densely sampled quaternary field alone nor is it an artifice of predominantly northward motion of the plates returning all 0 5 ma data to a 5 ma reconstruction failed to eliminate the quadrupole the non zonal coefficients for the 0 5 ma field are typically an order of magnitude less than the zonal terms with the exception of h and are probably insignificant suggesting that longitudinal drift is generally effective in averaging out these components the present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 ma but there is evidence that g may have changed little during the last 35 ma in addition there has been very little relative motion between the palaeomagnetic dipole axis as defined by the first degree terms and the axis of the hotspots reference frame

history of earth s magnetic field and possible connections to core mantle boundary processes the core mantle boundary cmb sets boundary conditions for processes occurring within the core thus the history of the geomagnetic field is intimately connected with the history of the cmb information about one can often provide information about the other it is a simple matter to separate sources of external origin from those of internal origin but there is no unique way to specify the location of the internal sources indirect arguments suggest that sources within earth s outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees building on the pioneering studies of roberts and scott and of backus in the 1960s the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data however even with appropriate assumptions about location of the magnetic field source there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines additional assumptions about the flow are therefore required sometimes leading to quite different estimates in the pattern of fluid flow the resulting velocity field estimates appear to be sensitive to conditions e g small lateral variations in temperature at the cmb consideration of mantle dynamics suggests that large changes in the cmb conditions probably occur on a 10 to 100 m y timescale secular variations with periods shorter than a million years but longer than several years almost certainly originate from processes operating in the outer core unfortunately there is not yet consensus as to what those processes are longer period variations in the paleomagnetic record including nonstationarities in the rates of magnetic field reversals in paleointensities and in paleosecular variation may reflect changes in cmb conditions because these boundary conditions are controlled primarily by mantle dynamics there have been several speculations regarding causal links between changes in earth s lithosphere and changes in earth s magnetic field finally the evidence linking lateral variations at the cmb to perceived systematics in polarity transition data is found to be intriguing but insufficient

do virtual geomagnetic poles follow preferred paths during geomagnetic reversals virtual geomagnetic poles vgps recorded in sediments during reversals of the earth s magnetic field show an apparent preference for two antipodal sectors of longitude1 3 not only in records of the same reversal from different sites but also in records of different reversals if preferred bands really have persisted from one reversal to the next this would imply that the mantle exerts a significant control over the reversal process4 here we analyse the available database of reversal records from the past 12 myr using a statistical test specifically designed to test the hypothesis of two preferred antipodal longitudinal bands our analysis shows that the records taken as a group of independent observations do show an overall preference for two antipodal longitudinal bands however the site longitudes are also strongly grouped5 and a comparison of the transitional vgp longitudes with site longitudes shows an unlikely grouping under the hypothesis of a genuine geographical preference for transitional vgps we conclude that it is premature to accept the hypothesis of mantle control over the core during geomagnetic reversals

a model of virtual geomagnetic pole motion during reversals abstract a new model of virtual geomagnetic pole motion during a reversal of the earth s magnetic field is proposed the model is based on the idea that the geomagnetic field is the summation of the dipole field and the field of global magnetic anomalies gmas at the time of a reversal the dipole field changes its sign and the field of the gmas remains non zero the geographic location of the global magnetic anomalies canadian brazilian siberian and antarctic are close to the 90 w and 90 e meridians this defines the paths of the virtual pole drift at the time of a reversal

the upper olduvai geomagnetic field reversal from death valley california a fold test of transitional directions high resolution records of the upper olduvai geomagnetic field reversal were obtained from two localities within steeply dipping lacustrine sediments of the confidence hills southern death valley california the difference in bedding attitude between the two localities allowed us to perform a fold test of both transitional and non transitional paleomagnetic directions this is the first positive fold test obtained for transitional directions from any geomagnetic field reversal in addition variations in lithology allowed a comparison of transitional records from different sedimentary environments alternating field and thermal demagnetization reveals that most samples have either single or simple two component magnetizations the low coercivity low blocking temperature component is most likely held by large multidomain grains of magnetite and aligns with the present day field when uncorrected for bedding orientation the high coercivity component is held primarily by single and pseudo single domain titanomagnetite as determined from standard magnetic studies and electron microscopy this component is interpreted as a primary magnetic signature based on the presence of stratigraphically bound reversals in the tilt corrected data and positive reversal tests short term field variations can be correlated between the two records and differences are readily explained by the recording process inherent to the two sedimentary environments the sampling site is located within one of the preferred bands of transitional virtual geomagnetic poles vgp s 1 3 yet the transitional vgp paths from both localities are clustered within two primary longitudinal bands which are approximately 90 away from the sampling site longitude one lies over the atlantic ocean 10 50 w and the other antipodal to this 130 170 e these bands are also inconsistent with eight previous studies of the upper olduvai transition 4 9 3 10 when all reported studies of this reversal are examined however no global consistency exists instead the transitional vgp s are better grouped in a sampling site reference frame

paleomagnetic volcanic data and geometric regularity of reversals and excursions mostly on the basis of paleomagnetic sedimentary data it has been suggested that maps of virtual geomagnetic poles vgps corresponding to directions of the magnetic field at each site tend to fall along american and asian longitudes during reversals and excursions such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled however studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields in this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 myr volcanic eruptions occur sporadically thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions but we prefer not to use averaging methods of previous investigators who discarded or combined directions when they appeared to be similar we find that volcanic data give intermediate vgps which tend to fall along american and asian longitudes roughly consistent with the sedimentary data this result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites provided the appropriate polarity is assigned to intermediate vgps we find that icelandic vgps tend to fall along asian longitudes other patterns in the data for example latitudinal clustering of vgps or distinguishing longitudinal preferences of excursions from reversals are not resolved however it appears that in general transitional fields are nondipolar

paleomagnetic tomography of the core mantle boundary abstract analysis of updated global paleomagnetic data indicates that over the last 5 million years the normal and reverse polarity states are discernibly different this difference which can be characterized by a higher ratio of the axial quadrupole to dipole field for the reverse polarity relative to the normal polarity is attributed to lateral variations in temperature or composition at or near the core mantle boundary although several mechanisms are considered thermal electric effects associated with core mantle topography and or with chemical variations in d the seismic layer approximately 150 km thick at the base of the mantle are judged to be the most likely sources of the observed polarity asymmetry this asymmetry should not necessarily be expected to give rise to systematic trends in polarity transitions

geomagnetic polarity reversals a connection with secular variation and core mantle interaction geomagnetic polarity reversal remains one of nature s most enigmatic phenomena dynamo theory admits solutions in pairs with reversed magnetic fields b and b but detailed calculations are required to understand how the field can change sign theory also admits separate solutions with different symmetry across the equatorial plane the symmetric es quadrupole and the antisymmetric ea dipole solutions which may be important in the reversal process and which offer a simple framework for interpreting small paleomagnetic data sets ordinary secular variation leads to very large changes in the magnetic field over several centuries and could easily develop into full reversal the theory of secular variation which is relatively well developed may therefore help in understanding reversals other clues to geomagnetic reversals come from the sun whose magnetic field reverses every 11 years paleomagnetic data show the earth s magnetic field reverses every million years or so with each transition taking about a thousand years during which the intensity may fall by as much as 1 order of magnitude reversal frequency undergoes a modulation on the long timescale 107 years of mantle convection and there have been two long intervals in the past with no reversals such behavior is typical of a highly nonlinear dynamical system but the very long timescale of changes in reversal frequency and its close proximity to the overturn time of mantle convection suggests some control of the dynamo by the mantle short term phenomena such as change in the length of day and secular variation have been studied extensively for evidence of core mantle interactions and we may draw on this body of evidence in order to understand long term effects three physical mechanisms have been proposed topographic electromagnetic and thermal with the last two being most significant for long term effects symmetries allow the dynamo to generate an ea field with the major component a dipole but lateral variations on the core mantle boundary may lead to magnetic fields with no symmetry reflecting the structure of the boundary anomalies changes in reversal frequency on the mantle convection timescale could arise either from changes in total heat flux from the core to the mantle or from instabilities associated with lateral variations at the core mantle boundary neither mechanism is well understood but the former involves significant changes to the earth s overall heat budget whereas the latter must always arise as a natural consequence of deep mantle convection recent measurements of transition fields show pole paths that lie close to two preferred longitudes near 90 w and 90 e if substantiated the result would provide the first definitive evidence of long term mantle control of the geomagnetic field further evidence suggests that the geomagnetic pole during stable polarity also lies along these two longitudes and that magnetic flux at the core surface tends to concentrate along the same longitudes as does the present field a simple theory is proposed relating changes in the core field to apparent transition paths measured at the earth s surface the model shows that longitude confinement of the transition paths can occur for quite complicated core fields and that surface intensities can drop by 1 order of magnitude on average simply because of the reduction in length scale of the transitional field simple transition paths may be an indication of some organization of flux at the core surface but not of large scale or small amplitude core fields

analysis of the early jurassic geomagnetic data recorded at the breggia gorge ticino switzerland abstract geomagnetic directions recorded in a lower jurassic stratigraphic sequence that outcrops at the breggia gorge are analyzed a new method to identify directions suitable for calculating a time averaged dipolar geomagnetic direction has been applied to calculate a hypothetical early jurassic geocentric axial dipole the data were separated to identify normal reverse directions corresponding to periods of stable field and intermediate directions from an analysis of the data in the palaeogeographic coordinates of the breggia gorge at the time of the acquisition of the remanence it is suggested that polarity transitions in the early jurassic behaved in a similar manner to that described for late cenozoic reversals with the vgps preferentially following longitudinal paths the preferred paths coincide with a time averaged location of the jurassic subduction area of pangea this agreement could indicate a connection between the geometry of the reversals and the subduction zones an analysis to test whether both n r and intermediate directions have similar preferred distributions suggests that the underlying structure of the early jurassic geomagnetic field was similar to that observed for the late cenozoic the dispersion of directions with inclinations around 0 is higher than those belonging to n r polarity directions suggesting that there was an enhancement of fluctuations of the geomagnetic field during intermediate states in the early jurassic

sampling bias in vgp longitudes i derive probability densities for virtual geomag netic pole vgp longitudes for a general statistical model of local magnetic field variations i show that even for very simple statistically homogeneous models of secular varia tion the distribution of vgp longitudes is peaked 90 o away from the sampling longitude thus when sites are distri buted unevenly a non uniform overall distribution of vgp longitudes is to be expected analysis of the recent geomag netic field indicates that this bias can be significant particu laxly when random errors in paleomagnetic data are allowed for it is possible that some of the deviations from unifor mity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the vgp transformation and the non uniformity of paleomagnetic sampling sites

behaviour of the geomagnetic field during the matuyama brunhes polarity transition abstract the ability to derive gauss coefficients up to and including degree 3 and their variation through a geomagnetic polarity transition is studied using simulated palaeomagnetic data it is concluded that for a specified distribution of palaeomagnetic sites reasonable estimates of the behaviour of the coefficients can be derived even when uncertainties in the data and in the compilation of contemporaneous records are considered published palaeomagnetic records of the matuyama brunhes transition are then used as basis for deriving the variation of the gauss coefficients over a 32 kyear period encompassing the reversal individual records are interpolated to uniform time intervals of 0 5 kyear and put on to a common time scale by correlating between sites the variation in the latitude of vgp s through the reversal relative palaeointensity data are scaled by the geocentric axial dipole field intensity for 2000 at each site and the gauss coefficients derived by a matrix inversion employing singular value decomposition the derived variation with time of the gauss coefficients suggests that over the time span of the data the dipole and non dipole fields have approximately equal intensities plots of the variation of the surface vertical magnetic field through the reversal suggest that immediately prior to the reversal a large patch of reverse flux appears in the southern hemisphere this may subsequently have been responsible for the weakening of the vertical field leading into the reversal a similar patch of reverse flux is observed some 20 15 kyear prior to the actual reversal and may be associated with an observed excursion in vgps at several sites

preferred reversal paths caused by a heterogeneous conducting layer at the base of the mantle 1 time variations in the geomagnetic field induce secondary electric currents in the mantle which contribute to the magnetic field at the surface we explore the possibility that currents in a conducting layer at the base of the mantle disturb the transition field during magnetic reversals and alter the path of the virtual geomagnetic pole vgp we calculate the magnitude of the magnetic perturbation during a reversal and quantify the influence of this perturbation on the vgp reversal path the calculations show that the vgp reversal path depends on the geographic position of the observation site a histogram of reversal paths obtained using a randomly distributed set of sites reveals a preferred path through the americas when the histogram of reversal paths is evaluated with a limited number of sites based on locations in existing lava and sediment databases we find that the paths are confined to asian and american longitudes which is compatible with reversal path distributions inferred from paleomagnetic records

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals according to a recent compilation of sedimentary records from the past 12 myr the reversing geomagnetic field displays a marked long term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle1 however the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the americas or antipodal to this sector2 3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments4 6 here we present a compilation of 400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 myr old sampled at various longitudes and latitudes which does not confirm previous inferences1 and reinforces doubts4 6 about the ability of sediments to properly record a rapidly varying field in contrast to virtual poles from excursions those from reversals are uniformly distributed in longitude which indicates that the reversing field is statistically axisymmetrical thus we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle

evidence for the blake event in volcanic rocks from lipari aeolian archipelago summary palaeomagnetic and geochronological measurements have been carried out on the late pleistocene basaltic andesitic unit of monte chirica costa rasa on the island of lipari aeolian archipelago the lava flow sequence is about 10 m thick and has been sampled in detail magnetic properties are rather uniform curie temperatures of 540 to 580 c and the saturation irm reached at applied values of 0 1 t point to titanomagnetite as the main magnetization carrier thermal and af demagnetization have shown the presence of secondary magnetization components these were removed mostly at 450 500 c or 20 30 mt indicating a highly stable chrm with directions from transitional to reverse where a chrm could not be isolated by application of the demagnetization techniques the converging remagnetization circles method gave a mean chrm value fully comparable with that obtained from other methods 40ar 39ar determinations were performed on two lava flows in the lower and upper parts of the sequence the former shows a transitional chrm direction and a whole rock age of 157 12 ka the latter a reverse direction a whole rock age of 143 17 ka and a ground mass age of 128 23 ka the radiometric data and the reconstructed stratigraphy which indicate ages of 150 10 ka and 104 3 5 ka respectively for the volcanic units at the bottom and top of the monte chirica costa rasa unit suggest that the reverse directions recorded in lipari are related to the blake event

preferred vgp paths during geomagnetic polarity reversals symmetry considerations the reversing geomagnetic field has been said to have virtual geomagnetic poles vgps confined to a pair of longitudes close to where magnetic flux is concentrated at the core surface today at approximately 90 w and 110 e this can be explained if flux remains concentrated on the same longitudes throughout the transition the vgp path then depends on site position reversal sense r n or n r and sense of flux migration pole or equator ward for transitional fields which remain antisymmetric about the equator n r transitions with poleward flux migration give western longitude vgp paths for sites in the ne and sw quadrants and eastern longitude vgp paths for sites in the nw and se quadrants sites on quadrant boundaries record rapid variable vgp positions data from the last matuyama brunhes reversal are broadly consistent with poleward flux migration

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

a simple statistical model for geomagnetic reversals the diversity of paleomagnetic records of geomagnetic reversals now available indicate that the field configuration during transitions cannot be adequtely described by simple zonal or standing field models a new model described here is based on statistical properties inferred from the present field and is capable of simulating field transitions like those observed some insight is obtained into what one can hope to learn from paleomagnetic records in particular it is crucial that the effects of smoothing in the remanence acquisition process be separated from true geomagnetic field behavior this might enable us to determine the time constants associated with the dominant field configuration during a reversal

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

persistent patterns in the geomagnetic field over the past 2 5 myr historical geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations lobes at the surface of the liquid core and regions of rapid change extending from the atlantic to the indian ocean1 palaeomagnetic data define a time average over several thousand years2 which might reflect the stationary parts of the present field but unfortunately the historical record is too short to provide a satisfactory average3 here we model palaeomagnetic directions from the past 2 5 myr using the same methods as for modern data4 and find the two northern lobes in the same position as today over arctic canada and siberia in southern regions by contrast the field appears to have been smoothed out as might be expected from the current rapid secular variation1 we propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years as would occur if the solid mantle controls flow at the top of the core

palaeomagnetism and chronology of the central taupo volcanic zone new zealand summary the central taupo volcanic zone tvz of new zealand is a region of intense quaternary silicic volcanism active since 1 6 ma we report palaeomagnetic measurements from 59 distinct volcanic units sampled at 98 sites in the tvz these are mainly rhyolitic ignimbrites and lava domes with a few basaltic andesitic and dacitic lavas most have new k ar or 4oar 39ar ages the remanent magnetizations are generally stable to both thermal and alternating field demagnetization and well determined mean palaeodirections were obtained for all sites our findings suggest that the taupo whakamaru maroa reporoa rotorua and okataina volcanic centres were magnetized during the brunhes normal chron kapenga is an older volcanic centre where activity commenced around 0 89 ma and extended into the brunhes mangakino volcanic centre is significantly older and was active from 1 6 to 0 95 ma transitional or intermediate palaeodirections were obtained from ahuroa ignimbrite 1 18 ko 02 ma and mamaku ignimbrite 0 22k0 01 ma the former almost certainly corresponds to the cobb mountain event the latter is significantly older than the blake event and probably corresponds to the recently reported pringle falls summer lake magnetic episode multiple sites from the whakamaru ignimbrite have indistinguishable 4oar 39ar ages 0 33 f 0 01 ma and glass composition but divergent palaeomagnetic directions this contrast suggests that either 1 the different sites were formed during a phase of extremely violent activity lasting up to a few hundred years during which geomagnetic secular variation was recorded or 2 that they were formed in a single eruption and rotation during subsequent extensional tectonism has caused divergence of the palaeodirections 40ar 39ar ages of 0 77 0 03 ma for the reversely magnetized rahopeka ignimbrite and 0 71 k 0 06 ma for the overlying normally magnetized waiotapu ignimbrite bracket and constrain the age of the matuyama brunhes transition

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

an attempt at reconstructing the geomagnetic field at the core mantle boundary during the upper olduvai polarity transition 1 66 myear abstract the palaeomagnetic directions of five published records obtained from sedimentary sections from various parts of the world were used to calculate maps of the radial field at the core mantle boundary cmb during the upper olduvai polarity reversal dated at 1 66 myear the five records were synchronized assuming that major changes in field direction occurred simultaneously at the five sites at the beginning and the end of the polarity transition low order coefficients of the field were determined using the hypothesis that the magnetic energy remained constant during the transition maps of the radial field at the cmb were calculated assuming a perfectly insulating mantle the results of this mapping exercise show a non axisymmetric field at the cmb in the first and last stages of the transition with strong flux moving in the longitudinal bands evidenced as preferred ones for transitional virtual geomagnetic poles vgp paths maps calculated at mid reversal do not show a simple geometry more palaeomagnetic records and more refined modelling techniques are desirable however the results are consistent with the idea that the transitional geodynamo is controlled by the thermal structure of the lower mantle

stability of mantle control over dynamo flux since the mid cenozoic studies of paleomagnetic fields during polarity transitions recorded over the past few million years frequently suggest a configurational persistence consistent with long lived mantle control over the pattern of flux emanating from earth s fluid core fundamental to an understanding of the core mantle dynamics involved is the question of the spatial temporal nature of the physical conditions at the base of the mantle that may be responsible our analysis of the field during a mid cenozoic reverse to normal r n polarity change recorded in a continuous sequence of lava flows in southeastern queensland australia provides strong evidence that the time span over which local mantle features retain such an influence is far longer than had thus far been observed specifically the reversal is dominated by two sequential groupings of transitional magnetic vector directions each associated with a clustering of virtual geomagnetic poles vgps at locations common to several more youthful reversal datasets from about the globe our 40

exploring the influence of the non dipole field on magnetic records for field reversals and excursions summary we have used the model cals7k 2 to explore the possible influence of the time varying non dipole components of the geomagnetic field during field reversals and excursions our findings suggest that non dipole components could add significant structure to the field during the reversal and excursion processes globally the main polarity reversal is variable in duration and rapid reversals on subdecadal timescales are seen for a small number of locations the model generates variable reversal paths however there is a longitudinal preference both spatially and more weakly temporally directional reversal features are not globally synchronous some polarity changes finish before they start elsewhere global intensity variations however appear more coherent we also find support for the idea that field intensity changes occur some time before and after the major directional changes of the reversal large excursions appear naturally when the axial dipole has been reduced to 20 per cent for the whole time period however they are not globally synchronous or uniform

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

thermal core mantle interactions maps of the magnetic field at the core mantle boundary for 1715 1980 reveal static features in the field and an absence of westward drift from much of the core mantle boundary the static features suggest that flow in the core is coupled to the mantle we examine different coupling mechanisms and predict lateral inhomogeneity at the core mantle boundary comparison with models of the lower mantle and core mantle boundary derived seismically and from the geoid is encouraging though their resolution is currently inadequate for a full comparison to be made with the much better resolved magnetic field

analysis of nrm directions from the rossello composite implications for tectonic rotations of the caltanissetta basin sicily abstract the paleomagnetic results of the pliocene marine marls from the rossello composite section 30 in the caltanissetta basin of sicily have been analysed to determine the amount and timing of tectonic rotations the analysis shows that there is a consistent inclination error for the nrm component which is removed between 500 and 600 c which may be caused by compaction of the sediment differences in inclination error throughout the composite section are related to differing carbonate content a higher carbonate content implying less compaction and smaller inclination error furthermore consistent differences are found between directions of normal and reversed polarities as well as between directions of the lower and upper part of the composite section these differences are to a large extent related to differences in carbonate content porosity and influence of weathering the caltanissetta basin shows a total clockwise rotation of 34 occurring since the early pliocene the magnetostratigraphy of the rossello composite section shows that from the miocene pliocene boundary 5 2 ma up to the gilbert gauss boundary 3 6 ma there were no tectonic rotations of the caltanissetta basin further we speculate that 10 of this total clockwise rotation occurred during approximately kaena times ca 3 05 ma and that at least 24 of this clockwise rotation is younger than the gauss matuyama boundary 2 6 ma

a direct measurement of the distance between a hypocenter in a benioff wadati zone and the slab asthenosphere contact we made a direct measurement of the distance from a hypocenter in the benioff wadati zone to the boundary between the downgoing slab and overlying asthenosphere this was accomplished by identifying a low amplitude p wave reflection off the slab asthenosphere contact which arrives at teleseismic stations several seconds after the initial p arrival the measured delay time after p of the reflected phase shows a consistent azimuthal variation thus eliminating possible source or receiver effects the relative amplitude polarity and delay time of this observed phase are consistent with a model in which the distance between hypocenter and slab asthenosphere contact at this depth is about 38 5 km the event studied was an intermediate depth 181 km earthquake in the southern kuriles which occurred in the lower layer of a double seismic zone our measured distance when compared to the distance between the two seismic zones indicates that the upper layer of seismicity is within 15 km of the slab asthenosphere contact appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington u c 20009 document jb1 010 1 00 payment must accompany order

sedimentary records of reversal transitions magnetization smoothing artefact or geomagnetic field behaviour sedimentary records of reversals for the past 12 million years exhibit preferred longitudes of their vgp paths this observation may be related to long term aspects of core mantle dynamics however it has been suggested that sedimentary vgp paths could be an artefact due to smoothing which mixes non antipodal pre and post transitional directions we show here that smoothing over unrealistically long time scales is required to generate intermediate directions of magnetization which are purely mixtures of pre and post transitionals directions in sediments whose remanence is primarily depositional drm or post depositional remanence pdrm sedimentary records of transitions showing vgp paths confined to the planes of immediately pre and post transitional vgps have however been observed these can be explained by non axisymmetric bias in the non reversing field gubbins and bloxham 1987 constable 1992 which plays a role in reversals therefore provided that magnetization directions are properly cleaned to give single component directions the preferential longitudes of reversal vgp paths will be due to field behaviour and not to an artefact of the magnetization process if however mixtures of primary magnetization acquired at the time of the formation of the rock and secondary magnetization acquired later are measured then artefacts will be generated

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1