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

the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily van hoof a a m van os b j h and langereis c g 1993 the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily phys earth planet inter 77 297 313 the detailed paleomagnetic records of the upper and lower nunivak polarity transitions have been determined from pliocene marine marls in southern sicily magnetites are the most important carrier of the remanence in both records the transitions are recorded in two components a low temperature and a high temperature component the two components do not represent the geomagnetic field because the changes in these components take place at lithological boundaries in addition the directional changes do not completely match directional changes of the same transitions recorded in calabria some 250 km away the character of the virtual geomagnetic poles paths is probably caused by smoothing of the stable directions before and after the transitions the directional changes as well as the smoothing mechanism can be explained by the diagenetic magnetite formation model van hoof et al 1992 in which shortly after burial the remanence carried by newly formed secondary magnetites is superposed on the initial remanence carried by primary magnetite

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

a reversal of the earth s magnetic field recorded in mid miocene lava flows of gran canaria paleointensities 1 an extensive paleointensity study was carried out on an approximately 14 1 myr old reverse to normal transition of the geomagnetic field a total of 188 samples from a volcanic sequence on gran canaria canary islands were subjected to thellier type paleointensity determinations the paleodirectional results of this sequence are reported by leonhardt et al 2002 a modified thellier technique which facilitates the recognition of md tails and the formation of new magnetic remanences with higher blocking temperatures than the actual heating step was used on the majority of the samples we obtained reliable paleointensity results for 35 of the 87 sampled lava flows in general the obtained field intensities are lower than expected for the mid miocene this observation is very likely related to a long term reduction of the field close to transitions very low paleointensities with values 5 t were obtained during an excursion preceding the actual transition and also close to significant changes of the local field directions these are interpreted as nondipolar components becoming dominant for short periods and provoking a rapid change of field directions during the transition 15 successive lava flows recorded similar local field directions corresponding to a cluster of virtual geomagnetic poles close to south america chronologically within this cluster the paleointensity increases from about 9 to 28 t followed by a decrease back to approximately 9 t this variation of paleointensity between lava flows with similar direction indicates independent records of the geomagnetic field and gives strong evidence for a stable transitional state that lasted a significant period

time averaged paleomagnetic field and secular variation predictions from dynamo solutions based on lower mantle seismic tomography abstract we compare three dynamo solutions incorporating laterally varying boundary heat flux with paleomagnetic models and data the boundary condition is defined by the d seismic shear wave velocity and the three solutions have boundary anomalies with different amplitudes the generated fields appear to divide into a stationary boundary locked part and a time varying part with persistent centres of activity both parts contribute to the time average a very long averaging time can be needed for nearly locked solutions but a rough time average that remains within the threshold set by the accuracy of paleomagnetic data is achieved in a few diffusion times the locked part dominates for larger amplitude boundary anomalies in previous work the locked field was shown to have strong similarities with the modern geomagnetic field previous dynamo solutions that were not locked to the boundary show similarities with our solutions with weak boundary forcing the axisymmetric time average has small g 2 0 and larger g 3 0 components and peaks in inclination anomaly in high latitudes associated with the locked field and low latitudes associated with the time average of the time varying fields the non axisymmetric time average displays a striking longitudinal variation in inclination anomaly with a large negative anomaly in the pacific region in agreement with observations none of the dominant geomagnetic coefficients are axisymmetric and g 2 0 negligible in all three models secular variation is concentrated in equatorial latitudes as in some recent paleomagnetic models the locked field agrees with the inclination difference found between hawai i and reunion in agreement with paleomagnetic averages the locked field agrees with the paleomagnetic time average rather better than the fields with less boundary variations we conclude that because the locked field agrees with the modern field as well as some aspects of the long term time average the geomagnetic field spends a considerable time in the present four lobe configuration it is not a coincidence that the present field resembles the time average longitudinal variations are likely to be at least as important as latitudinal variations in the paleomagnetic time average this presents a challenge for dynamo theory since a move to more geophysically realistic parameters would appear to destroy the locked solutions

structure and organization of the porma melange progressive denudation of a submarine nappe toe by gravitational collapse the porma melange is a unit up to 1 km thick underlying the forcada nappe in the variscan foreland of the iberian peninsula most of the porma melange consists of a highly deformed boudinaged sequence with extension values over 300 percent the boudins or melange blocks are the result of bed parallel extension in two perpendicular directions giving rise to equidimensional boudins in plan view choco late tablet structure in cross section view the boudins show different shapes rectan gular rhomboid triangular trapezoid or with arrowhead edges due to various combinations of extension and shear fractures both the provenance of the melange blocks mostly derived from the upper part of the bodon nappe that overlies the forcada nappe and flattening type strain ellipsoid shapes support the interpretation that hanging wall gravitational collapse of these nappes was the cause of the exten sional deformation the current distribution of the melange blocks from different nappe formations implies progressive unroofing of the nappe stack through subse quent submarine slumps the internal structure of the melange blocks depends on the degree of sediment lithification during slumping and illustrates the significance of the so called native and exotic blocks the source of most of the melange material shaly matrix and native blocks or boudins was the semilithified syntectonic sediments of the upper part of the bodon nappe which underwent brittle ductile deformation boudinage during slumping however a few and usually larger exotic blocks also came into the basin as individual slip blocks from competent well lithified formations originally located at the lower part of the nappe stack with higher shear resistance and less suitable for spreading thus the arrangement of major nappe fragments traditionally named klippes and showing sizes that may reach up to several kilometers does not agree with the geometric rules of thrusting and are reinterpreted as megablocks resulting from gravitational sliding during denudation of advancing nappes

the upper and lower thvera sedimentary geomagnetic reversal records from southern sicily detailed paleomagnetic records of the upper and lower thvera polarity transitions have been determined from pliocenc marine marls in southern sicily the dominant magnetic mineral is fine grained magnetite the two transitions have vgp paths following a great circle passing south america and the west coast of north america there are strong indications that the vgp paths result from smoothing of the non antipodal stable directions before and after the transitions the upper thvera transitional record is preceded by two excursions and followed by a third one the first excursion is a sedimentary artefact caused by post depositional migration of magnetic minerals and the third one by is caused by weathering of the sediment the upper thvera transition from sicily is compared with the record from calabria about 250 km away the two records show similarities as well as differences both transitions have identical vgp paths but in calabria the transition is recorded lower in the sediment the first and third excursions have different characters and the second one is not present apparently the registration of transitions of the earth s magnetic field in sediments is strongly influenced by smoothing and diagenetic processes after deposition

kinematic analysis of melange fabrics examples and applications from the mchugh complex kenai peninsula alaska permian to cretaceous melange of the mchugh complex on the kenai peninsula south central alaska includes blocks and belts of graywacke argillite limestone chert basalt gabbro and ultramafic rocks intruded by a variety of igneous rocks an oceanic plate stratigraphy is repeated hundreds of times across the map area but most structures at the outcrop scale extend lithological layering strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation forming broken formation and melange deformation was noncoaxial and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing generally thinly sedimented oceanic plate soft sediment deformation processes do not appear to have played a major role in the formation of the melange a model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to s1 are contracted in both the brittle and ductile regimes layers at 30 458 to s1 are extended in the brittle regime and contracted in the ductile regime and layers at angles greater than 458 to s1 are extended in both the brittle and ductile regimes imbrication in thrust duplexes occurs at deeper levels within the wedge many structures within melange of the mchugh complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge a displacement field for the mchugh complex on the lower kenai peninsula includes three belts an inboard belt of late triassic rocks records west to east directed slip of hanging walls a central belt of predominantly early jurassic rocks records north south directed displacements and early cretaceous rocks in an outboard belt preserve southwest northeast directed slip vectors although precise ages of accretion are unknown slip directions are compatible with inferred plate motions during the general time frame of accretion of the mchugh complex the slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates which became more oblique with time they are not considered indicative of strain partitioning into belts of orogen parallel and orogen perpendicular displacements because the kinematic data are derived from the earliest preserved structures whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion related fabrics 1999 elsevier science ltd all rights reserved

electromagnetic core mantle coupling and paleomagnetic reversal paths calculations of electromagnetic torques between the fluid outer core and the heterogeneous lower mantle demonstrate that the geomagnetic field can lock with the mantle during polarity transitions we show that the virtual geomagnetic pole vgp is attracted toward mantle regions with low electrical conductivity and repelled from mantle regions with high conductivity using seismically determined structure of the lower mantle to infer the pattern of conductivity variations at the base of the mantle we find two preferred reversal routes for the vgp one beneath the americas and another beneath asia these paths particularly the one beneath the americas lie close to the vgp paths seen in some sedimentary and lava records of polarity reversals suggesting that the core and mantle are electromagnetically coupled during reversals

palaeomagnetic records of the brunhes matuyama polarity transition from odp leg 124 celebes and sulu seas summary palaeomagnetic records of the brunhes matuyama geomagnetic polarity transition were obtained from deep sea sediments of odp leg 124 in the celebes and sulu seas advanced piston core apc samples with high magnetization intensities 2 200 ma m1 and high sedimentation rates 8 4 10 cm kyr1 were recovered from this cruise rock magnetic measurements revealed the carrier of the remanence to be nearly pure magnetite in the pseudo single domain range pass through measurements at intervals of 5 mm on apc cores across the brunhes matuyama polarity transition for holes 767b 769a and 769b were deconvolved with the magnetometer sensor response using the abic minimizing method discrete samples were also taken from the polarity transition zones and subjected to either thermal or alternating field stepwise demagnetization the results were generally consistent with the pass through data after the deconvolution results from the three holes are in good agreement particularly those from the two holes 100 m apart at site 769 the transitional vgp paths from these two holes show two small loops near new zealand before the equator is crossed the vgps continue to swing eastwards to the north atlantic and then move to the northeastern margin of the pacific ocean the positions of the vgp loops obtained from site 769 are dierent from the vgp clusters obtained from both the known volcanic records and the sediment records at the boso peninsula and the north atlantic sites with high sedimentation rates such a dierence may imply the predominance of the non dipole field during the transition relatively stable mid high northern latitude vgps are recognized on all three cores just after the reversal lasting about 4000 years the plot of relative intensity versus vgp latitude for the sediment records from the north atlantic dsdp hole 609b shows a remarkable similarity with our records similar patterns were also obtained for the plot of the palaeointensity versus vgp latitude for the la palma volcanic lava records these results may suggest the existence of a metastable state of the geodynamo producing a zonal component just after the brunhes matuyama boundary which may have played a role in the change of the field intensity

paleomagnetic directions and 40ar 39ar ages from the tatara san pedro volcanic complex chilean andes lava record of a matuyama brunhes precursor the same volcanic sequence overlain by flows with normal polarity the 40 ar 39 ar incremental heating experiments on lavas within the two sections provide nine independent age determinations and yield a weighted mean of 791 7 3 0 ka 2s for the paleomagnetic transition the sections are linked by geological mapping the precise radioisotopic dating and geochemical correlations alternating field and thermal demagnetization studies rock magnetic analyses and petrographic observations indicate that the magnetization is primary and carried by titanomagnetite the polarity change is characterized by a jump from reverse poles to a quasi stationary cluster of virtual geomagnetic poles over australia followed by a jump to normal polarity latitudes magnetization of these lavas is thus consistent with either a brief period when the field was dominated by a subequatorial dipole or a more complex nondipolar field that may reflect the influence of a long lived regional lower mantle control over a weakened dynamo the quebrada turbia lavas are circa 16 kyr older than those dated by exactly the same methods which record a later more complex portion of the reversal at haleakala volcano maui moreover the 792 ka radioisotopic age of these chilean lavas is older than most astronomical estimates for the matuyama brunhes reversal suggesting that this section may in fact record a precursor to the actual field reversal that is expressed by low paleointensities in more than a dozen well studied marine sediment cores index terms 1535 geomagnetism and paleomagnetism reversals process timescale magnetostratigraphy 1030 geochemistry geochemical cycles 0330 9360 information related to geographic region south america 9604 information related to geologic time cenozoic keywords paleomagnetism reversal

is earth s magnetic field reversing abstract earth s dipole field has been diminishing in strength since the first systematic observations of field intensity were made in the mid nineteenth century this has led to speculation that the geomagnetic field might now be in the early stages of a reversal in the longer term context of paleomagnetic observations it is found that for the current reversal rate and expected statistical variability in polarity interval length an interval as long as the ongoing 0 78 myr brunhes polarity interval is to be expected with a probability of less than 0 15 and the preferred probability estimates range from 0 06 to 0 08 these rather low odds might be used to infer that the next reversal is overdue but the assessment is limited by the statistical treatment of reversals as point processes recent paleofield observations combined with insights derived from field modeling and numerical geodynamo simulations suggest that a reversal is not imminent the current value of the dipole moment remains high compared with the average throughout the ongoing 0 78 myr brunhes polarity interval the present rate of change in earth s dipole strength is not anomalous compared with rates of change for the past 7 kyr furthermore there is evidence that the field has been stronger on average during the brunhes than for the past 160 ma and that high average field values are associated with longer polarity chrons there is no evidence from recent millennial scale time varying paleofield models to indicate that the field is entering a polarity transition nevertheless it remains a reasonable supposition that the magnetic field will eventually reverse even though the time scale is unpredictable a more immediate concern is that ongoing secular variation in the magnetic field may be expected to moderate the current high dipole strength on centennial to millennial time scales it would not be surprising if it dropped substantially returning closer to the average without necessarily reversing this could have important consequences for space weather and also highlights the need for improved understanding of the impact of geomagnetic field strength on the production rates of cosmogenic isotopes that are used to estimate past solar variability

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

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

assessing the role of precursor cyclones on the formation of extreme greenland blocking episodes and their impact on summer melting across the greenland ice sheet a 30 year climatology of north atlantic cyclones from 1979 to 2008 is examined within the context of extreme greenland blocking and accelerated surface melting across the greenland ice sheet gris a distinct class of north atlantic cyclones known as precursor cyclones was identified as any extratropical cyclones originating to the west of greenland blocks prior to the peak of blocking intensity composite map analysis reveals that precursor cyclones contributed to a significant intensification of extreme greenland blocking episodes gbes through the process of upper level wave amplification across all seasons most extreme gbes are associated with multiple precursor cyclones prior to peaking in intensity and a majority of these cyclones have continental rather than oceanic origins over both the western and eastern sectors of greenland daily meltwater production simulated by the modele atmospherique regional regional climate model is greater during extreme gbes accompanied by precursor cyclones compared to extreme gbes lacking a precursor cyclone based on an analysis of air parcel trajectories and north atlantic sst anomalies enhanced surface melting during the summer particularly over southern and western greenland is strongly linked to the combination of vigorous adiabatic warming generated by subsiding air within the blocking anticyclones and warm air advection supplied by the precursor cyclones with the increased frequency of extreme gbes accompanied by precursor cyclones observed during a strong positive phase of the atlantic multidecadal oscillation recent long term increases in gris surface melting can be partially attributed to the interaction of these atmospheric and oceanic processes

cryptochron c2r 2r 1 recorded 2 51 ma in the koolau volcano at halawa oahu hawaii usa paleomagnetic and 40ar 39ar evidence abstract new paleomagnetic measurements coupled with argon argon 40 ar 39 ar radioisotopic dating are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short term behavior of the paleomagnetic field as part of an investigation of the koolau volcano oahu and the short term behavior of the geomagnetic field we have sampled a long volcanic section located on the volcano s buttressed flank within halawa valley prior paleomagnetic and potassium argon k ar investigations of the koolau volcanic series revealed excursional directions the alkaline composition of lava flows easy access and close geographical proximity to k ar dated lava flows made this newly studied 120 m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis at least eight samples collected from each of 28 successive flow sites were stepwise demagnetized by both alternating field 5 mt to 100 mt and thermal from 28 c to 575 650 c methods mean directions were obtained by principal component analysis all samples yielded a strong and stable chrm trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps with thermal and af results differing insignificantly low field susceptibility vs temperature k t analysis conducted on individual lava flows indicated approximately half with reversible curves curie point determinations from these analyses revealed a temperature close to or equal to 580 c indicative of almost pure magnetite ranging from single domain sd to pseudosingle domain psd grain sizes for most of the flows the mean directions of magnetization of the entire section sampled indicate a reversed polarity with 10 m of the section characterized by excursional directions 5 lava flows the corresponding vgps are located off the southeastern part of africa close to madagascar 40 ar 39 ar incremental heating experiments on groundmass from nine flow sites at different stratigraphic levels yields isochrons between 2 64 0 23 to 2 37 0 17 ma with a weighted mean age of 2 514 0 039 ma which combined with the overall reversed polarity and the absence of polarity reversals strongly suggests that the excursion corresponds to cryptochron c2r 2r 1 s candle and d v kent a new geomagnetic polarity time scale for the late cretaceous and cenozoic jour geophys res 100 1995 6093 6095 current dating is insufficiently precise to unambiguously define this excursion as a microchron

a magnetic reversal record periodically the earth s magnetic field reverses and has done so hundreds of times in the past what happens to the direction of the magnetic field during reversals remains a matter of debate as do what constitute the best sources of palaeomagnetic information on the subject records from north atlantic sedimentary cores now add fresh data on the two most recent well documented events they are especially notable because the sediments concerned were deposited at comparatively high rates and thus provide a high resolution record

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

revisiting the jurassic geomagnetic reversal recorded in the lesotho basalt southern africa summary we carried out a detailed and continuous palaeomagnetic sampling of the reversed to normal geomagnetic transition recorded by some 60 consecutive flow units near the base of the lesotho basalt 183 1 ma after alternating field or thermal cleaning the directions of remanence are generally well clustered within flow units in contrast the thermal instability of the samples did not allow us to obtain reliable palaeointensity determinations the geomagnetic transition is incompletely recorded due to a gap in volcanic activity attested to both by eolian deposits and a large angular distance between the field directions of the flows underlying or overlying these deposits the transition path is noticeably different from that reported in a pioneering work carried out in 1962 the most transitional virtual geomagnetic poles vgps are observed after the volcanic hiatus once continents are replaced in their relative position 180 myr the post hiatus vgps cluster over russia however two successive rebounds from that cluster are found with vgps reaching repeatedly the eastern asia coast thus the vgp path is not narrowly constrained in palaeolongitude the decrease in intensity of magnetization as the field deviates from the normal or reversed direction suggests that the decrease in field magnitude during the reversal reached 80 90 per cent we conclude that although the reversal is of a dipole of much weaker moment than that which existed on average during cenozoic time the characteristics of the reversing geodynamo seem to be basically similar

a southern hemisphere record of the matuyama brunhes polarity reversal the authors present a record of the matuyama brunhes 0 73 ma polarity transition from a southern hemisphere deep sea sediment core v16 58 46 degree s 30 degree e the transition is recorded across at least 30 sm of section and is defined by a nearly 180 degree change in directions which occurs during a low in the relative intensities an increase in the sup 10 be sup 9 be ratios associated with the intensity low suggests that the relative intensities may document a decrease in the strength of the geomagnetic field the virtual geomagnetic pole vgp path is neither clearly near or far sided but tracks approximately 120 degree west of the site longitude considered with other matuyama brunhes transition records this southern hemisphere record supports previous interpretations that the matuyama brunhes transitional field was not dipolar instead the v16 58 transition together with other records of this reversal recently obtained from this longitudinal sector suggests that nonzonal terms which are symmetric about the equator were important during this transitional field

an estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks it has recently been proposed that geomagnetic jerks observed at the earth s surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1 5 when wavelet analyzed such a signal should have suffered some distortion when diffusing from the core mantle boundary cmb through the conducting mantle assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle we compute the distortion that a pure singularity introduced at the cmb suffers as it traverses the mantle we compute this distortion through its effects on the so called ridge functions extracted from the wavelet transform of the signal this distortion is very similar to the small but significant one that we observe in real data we therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones assuming for example a thickness of 2000 km for a uniform lower conducting mantle we find an electrical conductivity smaller than 10 s m 1 this value is in reasonable agreement with values derived from high pressure experiments for a silicate mantle

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

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

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

matuyama brunhes reversal and kamikatsura event on maui paleomagnetic directions 40 ar 39 ar ages and implications abstract eighty nine basaltic lava flows from the northwest wall of haleakala caldera preserve a concatenated paleomagnetic record of portions of the matuyama brunhes m b reversal and the preceding kamikatsura event as well as secular variation of the full polarity reversed and normal geomagnetic field they provide the most detailed volcanic record to date of the m b transition the 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles vgps that move from reverse to normal along the americas concluding with an oscillation in the pacific ocean to a cluster of vgps east of new zealand and back finally to stable polarity in the north polar region all but one of the 16 kamikatsura vgps cluster in central south america the full polarity flows with 40ar 39ar ages spanning a total of 680 kyr pass a reversal test and give an average vgp insignificantly different from the rotation axis with standard deviation consistent with that for other 0 5 ma lava flows of similar latitude precise 40ar 39ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775 6 1 9 and 900 3 4 7 ka for the m b and kamikatsura transitional flows respectively this matuyama brunhes age is 16 kyr younger than ages for m b flows from the canary islands tahiti and chile that were dated using exactly the same techniques and standards suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals

the geomagnetic field over the past 5 million years summary the modern geomagnetic field is usually expressed as a spherical harmonic expansion although the palaeomagnetic record is very incomplete in both space and time sufficient data are available from a span of ages to generate time averaged spherical harmonic field models with many degrees of freedom here three data sets are considered directional measurements from lavas inclination measurements from ocean sediments and intensity measurements from lavas individual data are analysed as well as siteaverages using the same methods that have been developed for the modern field to give models for the past 5 myr the normal polarity field model has an axial dipole intensity similar to that of the modern day field whilst the equatorial dipole component is very much smaller the field is not axisymmetric but shows flux concentrations at the core s surface under canada and siberia similar to those observed in the field over historical timescales tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples the reverse polarity field model does not show such obvious features but this may be due to the sparsity of the data the patterns observed in the normal polarity field with persistent features in the northern hemisphere and a smooth southern hemisphere could be explained if the present pattern of secular variation is typical of the past several million years this would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation with relatively little change over quieter regions however we have been unable to find any evidence for a geographical pattern of secular variation in the data

core flow instabilities and geomagnetic storms during reversals the steens mountain impulsive field variations revisited abstract the origin of the large variations in magnetization direction recorded in two different lava flows at steens mountain oregon is discussed the time constant for lava flow cooling and the amplitude of the variations imply extremely large variation rates for the ambient field four orders of magnitude above the present day secular variation two different mechanisms in the liquid outer core namely turbulence and inertial waves that may produce such variations are investigated energetic considerations indicate that the observed impulsive variations are unlikely to be of internal core origin on the other hand it is shown that during reversals when the dipolar field is reduced by a factor ten magnetic storm perturbations at the earth s surface are enhanced by a factor two with respect to their present amplitude it follows that intense magnetic storms can significantly contribute to ambient field changes during reversals the amplitude of the impulsive variations recorded at steens mountain is shown to be consistent with the effect of a strong magnetic storm occurring during cooling of a flow the direction of the variations is also shown to be consistent with this mechanism if the dipolar field contributes significantly to the internal field during the impulsive change

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

detailed paleomagnetic study of two volcanic polarity transitions recorded in eastern iceland abstract two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern iceland the two sections range in age from 12 09 to 10 21 ma as reported by watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 and are labelled as profiles c and d stepwise alternating field af and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite demagnetization experiments have identified eight and six transitional lavas respectively for these profiles the transitional virtual geomagnetic poles vgps of the r n profile c reversal are located mainly between patagonia and antarctica with a tendency of the virtual poles to move towards west coast of south america subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of south america then to west africa and on to central asia before the poles settle into normal polarity the second and younger vgp path corresponding to profile d is a path characterized by a reverse to normal to reverse r n r motion of the virtual poles this path is characterized by poles located in west antarctica patagonia and the western and eastern part of south america the passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern pacific followed by a rapid motion to the northern siberian region continuing to the western equatorial part of south america before moving on to the central region of asia followed by a motion to the western pacific prior to the final move to the eastern part of antarctica the data from eastern iceland based on highly reliable transitional results as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages seem to indicate that the eastern icelandic high latitude 66 n profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes

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

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

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

incompatible ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis abstract paleomagnetic results obtained from rocks of ediacaran age in several localities in laurentia and baltica persistently display co existence of two magnetization components one shallowly and the other steeply inclined both components pass criteria for a primary magnetization while geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age the conventional interpretation of these results translating the inclination into paleolatitudes using the geocentric axial dipole hypothesis would imply that rocks acquired magnetizations in positions switching back and forth between equatorial and near polar latitudes in a geographic reference frame such large scale and fast 45 cm yr migrations of a continent have been rejected as dynamically implausible neither plate tectonics nor true polar wander are thought to be able to attain the required velocities a highly irregular behavior of the geomagnetic field during the ediacaran possibly an alternation of the geomagnetic dipole axis between a co axial and an equatorial alignment remains the only viable explanation for the paleomagnetic data such a behavior entails specific outer core conditions which in turn impose strong constraints on the possible models for the thermal evolution of the earth s interior

fidelity and time resolution of the magnetostratigraphic record in siwalik sediments high resolution study of a complete polarity transition and evidence for cryptochrons in a miocene fluviatile section summary the surai khola section in southwest nepal a 5000 m continuously exposed record of fluvial sedimentation since middle miocene was revisited for high resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column almost antiparallel palaeoremanence directions residing in diverent haematite phases in the same specimens could generally not be separated properly by thermal demagnetization divering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization drm with considerable inclination errors and scattered directions for individual specimens

fluid flow near the surface of earth s outer core maps of the fluid flow at the core surface are important for a number of reasons foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models from the flow the force balance at the top of the core can at least in part be deduced the flow can provide short term predictions of the secular variation the flow is important in understanding changes in the length of day and constraints on lateral temperature variations and topography at the core mantle boundary may be derived unlike the case of mantle convection only very small lateral variations in core density are required to drive the flow these density variations are too small by several orders of magnitude to be imaged seismically so instead we use the geomagnetic secular variation to infer the flow despite considerable recent progress in mapping the core flow substantial differences exist between maps produced by different researchers here we examine the possible underlying reasons for these differences paying particular attention to the inherent problems of nonuniqueness we focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications

palaeointensity of west greenland palaeocene basalts asymmetric intensity around the c27n c26r transition abstract palaeomagnetic sampling of the lower part of the palaeocene flood basalts of west greenland covering a composite succession of 1 6 km and close to 350 individual lava flows revealed two polarity zones and a 200 m zone of intermediate directions 40 ar 39 ar ages indicate that the transition is c27n c26r following palaeodirectional and rock magnetic analysis 102 samples from 64 flows from the pre transitional normal polarity zone the zone of intermediate directions and the post transitional reverse polarity zone were chosen for thellier palaeointensity experiments altogether 83 samples yielded reliable palaeointensity estimates the mean intensity of the pre transitional c27n field is 16 1 6 9 t 17 flows which is only twice the mean transitional field strength 9 9 3 7 t 10 flows the post transitional c26r field intensity is generally high with a mean 43 3 15 0 t 21 flows the asymmetric palaeointensity around the c27n c26r transition with a high post transitional field resembles the only other detailed absolute palaeointensity studies of geomagnetic transitions recorded in volcanic sequences in hawaii 4 ma and steens mountain 15 5 ma these new data therefore suggest that a strong post transitional field might be a systematic feature of geomagnetic reversals probably indicating a prolonged period of an unusual state of the geodynamo following polarity transitions

extensional tectonics and gravitational collapse in an ordovician passive margin the western argentine precordillera abstract the paper describes ubiquitous extensional structures developed in a passive margin of ordovician age in the argentine precordillera these extensional structures include normal faults and boudinaged sequences in some places the boudinage reaches very high extension values giving rise to block in matrix formations most of these extensional structures developed when sediments were not well lithified as recorded by hydroplastic fractures slump folds and pinch and swell structures the presence of slump folds coeval with the extensional deformation the variable extension directions obtained from the kinematic analysis and a weak cleavage recording layer perpendicular shortening support the interpretation that gravitational collapse related to submarine sliding was the cause for extensional deformation well consolidated rocks located at the lower part of the stratigraphic sequence also display scarce extensional faults these extensional faults predate folding because they were breached by flexural slip faults and as a result of their passive rotation in fold limbs these initial normal faults may now appear as reverse faults particularly in steep and overturned limbs the truncation of extensional faults by flexural slip faults produces typical bed thickness changes across the extensional faults giving rise to apparent synsedimentary faults these normal faults can be attributed to the crustal extension that generated the passive continental margin or may represent deep parts of faults related to gravitational collapse

paleomagnetic full vector record of four consecutive mid miocene geomagnetic reversals abstract seventy mid miocene lava flows from flood basalt piles near neskaupstadur east iceland were sampled which provide a quasi continuous record of geomagnetic field variations samples were collected along the profile b of watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 which was extended about 250 m farther down in a neighboring stream bed published radiometric age determinations harrison c mcdougall i watkins n 1979 a geomagnetic field reversal time scale back to 13 0 million years before present earth planet sci lett 42 143 152 range from 12 2 to 12 8 ma for the sampled sequence four reversals were recorded in this profile with 18 transitional lavas found within or between 17 normal and 30 reversed polarity flows the large amount of transitional lavas and the large virtual geomagnetic pole dispersion for stable field directions are noteworthy as such features are commonly observed in icelandic lavas and manifest in a far sidedness of the average vgp the reason for this characteristic which could be related to an anomaly beneath iceland a global field phenomenon local tectonics and or non horizontal flow emplacement is scrutinized non horizontal flow emplacement is likely in volcanic environments particularly if the sampled lavas are located on the paleoslopes of a central volcano from the difference of the observed paleomagnetic mean directions to the expected directions assuming a geocentric axial dipole gad a paleoslope which would explain the observed difference was calculated numerically the obtained dip and dip direction point consistently to a possible volcanic extrusion center of the lavas the determined paleodip however proved to be significantly too high compared to the usual slope of a central volcano suggesting further reasons for deviations from the gad other datasets of this age from europe also show enhanced vgp dispersion suggesting further contributions of geomagnetic origin for this observation basically all reversal paths move across the pacific transitions were identified as belonging to c5an 1r c5ar 3r based on the astronomically tuned neogene timescale lourens l hilgen f j laskar j shackleton n j wilson d 2004 a geological time scale cambridge university press we selected 122 samples for paleointensity measurements using a modified thellier method including tests for alteration and multidomain bias 85 of the measured samples yielded data of sufficient quality to calculate paleointensities for 26 lava flows the average paleointensity for stable field directions was 23 3 t whereas the intensity drops to a minimum of 5 8 t during field transitions the stable field intensities represent only about half of the present day field the saw tooth pattern of intensities which is characterized by a sharp increase of intensity directly after a reversal and then followed by a gradual decrease towards the next reversal was not found in this study

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

paleomagnetic evidence from cape verde islands basalts for fully reversed excursions in the brunhes chron abstract in this study we present paleomagnetic data from two lava sequences on santo antao cape verde islands the tarrafal and agua nova profiles from which 63 and 43 lava flows were sampled respectively previous 40ar 39ar ages have constrained the two profiles to the brunhes chron which is in accordance with the normal polarity of the majority of the flows some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45 from the geographic north pole are interpreted to represent geomagnetic excursions based on interpretation of the directional data three excursions are recorded in the tarrafal profile t i t ii and t iii and four in the agua nova profile ag i ag ii ag iii and ag iv both 40ar 39ar results and paleomagnetic directional data indicate that an excursion defined by eight flows in the tarrafal profile t i is the same as one recorded in six flows in the agua nova profile ag i this excursion which passes the reversal test with classification c is the first geomagnetic excursion in the brunhes chron to pass the reversal test the 40ar 39ar ages suggest that this excursion could represent chryptochron c1n 1 0 493 0 504 ma the only chryptochron or tiny wiggle identified in the marine magnetic anomaly record for the brunhes chron the remaining excursions may not yet be unambiguously correlated between the two profiles another excursion recorded by two flows in the top of the tarrafal profile t iii also displays completely reversed polarity

anisotropic paleosecular variation models implications for geomagnetic field observables abstract we present a family of statistical models for paleosecular variation psv of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 ma and explore what paleomagnetic observables might be used to discriminate among the various family members we distinguish statistical models with axial anisotropy which provide a suitable description for an earth with homogeneous boundary conditions at the core mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions the models revise and extend earlier ones which are themselves descendants of cp88 devised by constable and parker constable c g parker r l 1988 statistics of the geomagnetic secular variation for the past 5 m y j geophys res 93 11569 11581 in cp88 secular variation is described by statistical variability of each gauss coefficient in a spherical harmonic description of the geomagnetic field with each coefficient treated as a normally distributed random variable the gauss coefficients of the non dipole part of the field exhibit isotropic variability and the variances are derived from the present field spatial power spectrum the dipole terms have a special status in cp88 with a non zero mean for the axial dipole and lower variance than predicted from the spatial power spectrum all non dipole terms have zero mean except the axial quadrupole cp88 is untenable for two reasons it fails to predict the observed geographic dependence of directional variability in the magnetic field and it grossly underpredicts the variance in paleointensity data the new models incorporate large variance in the axial dipole and in the non axial quadrupole gauss coefficients g21 and h21 the resulting variance in paleomagnetic observables depends only on latitude zonal models unless the variance in h21 is different from that in g21 non zonal models non zonal longitudinal variations in psv such as the flux lobes seen in the historical magnetic field are simulated using the non zonal models both the zonal and non zonal models fit summary statistics of the present dataset we investigate the influence of persistent non zonal influences in psv on various paleomagnetic observables it is shown that virtual geomagnetic pole vgp dispersion is rather insensitive to longitudinal variations in structure of psv and that inclination dispersion has the potential to be more informative given the right site distribution there is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate psv models

electromagnetic core mantle coupling iii laterally varying mantle conductance abstract we consider the implications of lateral variations in deep mantle conductance for electromagnetic core mantle coupling and the consequent decadal variations in length of day large scale variations in conductance can alter long term trends in the torque but do not on average enhance its magnitude or alter its decadal variation many previous estimates of the torque predict an unobserved negative trend during this century this trend may result from modelling assuming an incorrect conductance structure by suitable choice of this structure the trend can be removed but there is no correlation between such a structure and seismological observations of lower mantle heterogeneity runcorn argued that a highly conducting layer in the deep mantle beneath the pacific could account for features of the observed secular variation and also preferred palaeomagnetic vgp reversal paths we argue that the conductance customarily advocated for this mechanism is inconsistent with core mantle angular momentum exchange on decadal time scales

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

geomagnetic field behavior before and after the kauai reverse normal polarity transition new paleomagnetic results from 4 m y old lava flows from kauai hawaii suggest that strong poloidal field is associated with an unusual state of the geodynamo that follows attempts at polarity reversal successful or not the new data comprise 50 paleomagnetic sites from superposed lava flows occurring just below and above the kauai reverse normal polarity transition a composite record of 45 distinct field determinations was constructed by combining sites that record similar ancient field and correlating them to previously published results from kauai of the 45 data 25 include paleointensity estimates derived from double heating experiments a comparison of the composite record from kauai with two similar data sets from volcanic sequences shows that field variability in direction intensity or both can change substantially across a polarity transition these changes however do not appear to be systematic in sign or magnitude the only feature common to all three records is high field strength in the posttransitional interval interpreted here as a transient phenomenon associated with the reversal process

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

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

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

lateral variations at the base of the mantle from profiles of digital s diff data average long wavelength shear velocity structure in several regions at the base of the mantle is determined using the profiles of core diffracted sh waves shdiff there is significant lateral variation in the sdiff apparent ray parameters suggesting considerable lateral shear velocity anomalies in d the region just above the core mantle boundary cmb apparent ray parameters are determined by least squares fits through the pulse maxima of the instrument deconvolved ground displacements and heterogeneities are quantified through comparisons with reflectivity synthetic ray parameters corrections are applied to the data to account for the effects of the earth s ellipticity and of mantle heterogeneities along the shdiff upswing paths a total of 161 shdiff profiles were obtained greatly expanding the number of ray parameter measurements reported in previous studies most of the observed slownesses fall in the range between 8 2 and 8 8 s deg these correspond to slowness anomalies of 3 5 the most robust and extensive feature resolved is under the northern and northeastern pacific ocean the slowest values p 4 occur toward the southeast and there is a trend toward fast velocities moving to the northern and eastern rims of the pacific a model of d shear velocities is obtained by converting the slownesses to velocity anomalies superposing the d path profiles onto the cmb and applying a weighted moving cap spatial average this model agrees well with many current tomographic models both at large 5000 km and intermediate 1000 km scales the fact that this occurs with a different type of data and technique of analysis suggests that we are now able to recognize some of the smaller scale lateral variations at the base of the mantle locations of fast and slow velocity anomalies at the cmb are consistent with the model of cold paleoslabs ponding at the cmb and forcing d rock laterally to form hot aggregates that give rise to plumes in the mantle and hot spots at the surface

the c27n c26r geomagnetic polarity reversal recorded in the west greenland flood basalt province how complex is the transitional field 1 we present a paleomagnetic record of the c27n c26r 60 9 ma geomagnetic polarity reversal gpr sampled in two separate lava sequences in the vaigat formation west greenland flood basalt province the two profiles distanced 16 km apart consist of 45 and 35 sampled lava flows of which 31 and 23 respectively record transitional field directions a prominent feature of each profile is the presence of successive flows with similar paleomagnetic directions which based on geological geochemical and paleomagnetic evidences are interpreted to represent flow fields i e lavas erupted in a short period of time rather than periods when the geomagnetic field stood still multimodel photogrammetry shows that the typical flow length of vaigat formation picritic lavas is 2 km which explains why flow to flow correlation of paleomagnetic directions between the two profiles 16 km apart is not possible the two profiles therefore record different and independent time series of the same gpr allowing us to cast light on some of the questions that have arisen from single records particularly noteworthy is the fact that the two high resolution paleomagnetic records share no general trends clearly indicating that the c27n c26r transitional field was more complex and faster changing than can be resolved by either record in addition we obtained stable polarity paleomagnetic data from both pretransitional c27n and posttransitional c26r yielding a paleomagnetic pole latitude 64 8 n longitude 141 5 e n 14 k 19 8 a95 9 2 that passes reversal test

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

progressive development of block in matrix fabric in a shale dominated shear zone insights from the bobbio tectonic window northern apennines italy 1 block in matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary tectonic and mud diapiric processes in many exposed orogenic belts a true melange originates when this fabric is associated with mixing of rocks of different ages and provenance as that characterizing the main decollement shear zone developed between the highly allochthonous ligurian nappe and its substratum of foredeep deposits at the margins of the bobbio tectonic window trebbia valley northern apennines mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement evidence of polyphased deformations spanning from liquefaction related and hydroplastic structures to pseudo hydrofracturing features and mineralized veining have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure a novel evolutionary scheme for block in matrix fabric development is here proposed involving the progressive dismembering of already highly deformed units originally developed in non to poorly lithified conditions through a generalized simple shearing achieved during the evolution of the shear zone coupled with strain hardening due to tectonic compaction synkinematic diagenesis and fluid expulsion

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

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals

comment on the lau basin cobb mountain records by abrahamsen and sager abstract the cobb mountain subchron is one of the few short polarity intervals for which transition records have been obtained both from a wide geographical distribution and from different types of paleomagnetic recorders these records exhibit vgp paths that are remarkably similar given the wide geographic distribution of these sites the similarity in vgp paths is most simply interpreted as the presence of very large scale if not dipolar symmetries in the transitional fields only the records from western california and the lau basin exhibit vgp paths that are markedly different from the others new dates for the punaruu lavas suggest that they recorded a new geomagnetic cryptochron rather than the cobb mountain singer b s hoffman k a chauvin a coe r s 1999 dating transitionally magnetized lavas of the late matuyama chron toward a new 40 ar 39 ar timescale of reversals and events j geophys res 104 679 693 this new age indicates a revised correlation of some of the other california records whose ages agree better with the punaruu cryptochron a re examination of the lau basin records reveals that much of the difference between these and the other records results from a bias introduced when the paleomagnetic data were smoothed reanalysis of these records shows that the lau basin vgp paths are very similar to those obtained from western pacific and the north atlantic

the gilsa excursion and the matuyama brunhes transition recorded in 40ar 39ar dated lavas from lanai and maui hawaiian islands summary directional results thellier type palaeointensity determinations and ar ar ages are presented from volcanic units of pleistocene age from the hawaiian islands of maui and lanai twenty nine non contiguous lava flows were sampled on lanai the lava flows formed during the late matuyama polarity chron about 1 6 ma and recorded the gilsa geomagnetic polarity excursion the obtained ages of 1 6 ma for the lanai lavas are significantly older than ages previously obtained for this island these new ages are however in agreement with the age progression of the hawaiian hotspot the palaeomagnetic record over the sampled succession is characterized by reversed and intermediate directions palaeointensities are generally very low about half the present day field intensity dropping to values of 5 t during transitional field states transitional virtual geomagnetic poles vgps from lanai are situated first near the west coast of south america and then switch to offshore western australia normal polarity is never reached in the sampled succession transitional vgps located close to australia accompanied by relatively low palaeointensities are found in most records of short geomagnetic excursions during the matuyama polarity chron e g the cobb mountain subchron the punaru event and the matuyama brunhes precursor suggesting very similar physical processes likely related to dipolar dominance during the onset of these excursions and influences of lower mantle heterogeneities approximately 800 kyr after the gilsa excursion lava flows on maui recorded the last reversal the matuyama brunhes transition fourteen lava flows were sampled which recorded the pre and post transitional behaviour of the earth s magnetic field the geomagnetic field intensity is low prior to the reversal approximately 8 t and increases strongly afterwards up to 63 t these values of about twice the present field intensity of hawaii after the transition emphasize a strong asymmetry between pre and post transitional fields which is particularly strong in the region of hawaii

mass transport deposits overprinted by contractional tectonics a case study from the southern apennines of italy mass transport deposits mtds created by gravity driven deformation of unlithified sediments and tectonic melanges produced by contractional deformation are characterized by a similar chaotic appearance it follows that distinguishing structures formed by soft sediment deformation during mass transport from those produced by contractional tectonics can be problematic in fact deformation occurring along detachment levels may completely obliterate the original sedimentary fabric although a number of advances have been made during recent decades field criteria for discriminating structures within mtds that are overprinted by later regional contraction are not readily applicable to all the exposed examples we address some of these general issues through a detailed case study of the monte facito formation in italy this triassic unit was formed during the africa europe continental separation and since the miocene has been involved in contractional deformation during the construction of the apennines the monte facito formation consists of a series of stratigraphically coherent units separated by chaotic and often deformed intervals whose origin has been previously attributed to either tectonic or sedimentary processes an example is provided by a characteristic pebbly mudstone or paraconglomerate which has been interpreted as either a triassic gravity flow deposit or alternatively as a product of shearing along regional contractional detachments during the miocene this detailed field based study allows us to recognize structures related to the depositional processes that created these chaotic intervals and which can therefore be interpreted as mtds we also discriminate structures connected to later contractional tectonics that locally produced intense reworking of the mtds