the effect of low temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites a case for acquisition of chemical remanent magnetization the effect of hydrothermal alteration on the thermoremanent magnetization trm of synthetic titanomagnetite tm40 fe2 6ti0 4o4 has been studied to simulate the alteration that occurs in the oceanic crust pseudo single domain titanomagnetite grains similar in size to those often found in oceanic basalts were dispersed in a permeable but rigid glass matrix this resulted in a trm in the sample which was subsequently oxidized in acidic solutions while a magnetic field 0 1 mt was applied perpendicular to the trm direction the experiments were conducted in a non magnetic stainless steel pressure vessel at 150 c in solutions of acidity varying from ph 2 to ph 7 in addition to being time and temperature dependent the acquired chemical remanent magnetization crm was also found to be very ph dependent the degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions long term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating it was found that during the alteration of tm40 to titanomaghemite the crm is along the ambient field direction from the onset and not partly or wholly along the trm direction as has been found in previous air oxidation experiments this has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples

a reduced order successive linear estimator for geostatistical inversion and its application in hydraulic tomography national natural science foundation of china 51779179 51609173 51479144 51522904 crdf daa2 15 61224 1 tianjin normal university from the thousand talents plan of tianjin city special fund for public industry research from ministry of land and resources of china 201511047

chemical remagnetizations in the illizi basin saharan craton algeria and their acquisition process summary during remagnetization chemical changes limited to moderate grain growth from pre existing single domain sd grains do not modify the palaeomagnetic direction carried by these grains palaeomagnetic direction from new sd grains on the contrary is that of the magnetic field during remagnetization the resulting direction becomes intermediate between the directions carried by previous and new sd grains as it appears often in case of a partial magnetic overprint if the growth of pre existing grains is more important these grains become large multidomain md and loose the primary magnetization stable magnetization is then only related to the new sd grains which carry the total remagnetization in the illizi basin saharan platform these different cases of partial or total magnetic overprint have been observed resulting from palaeomagnetic studies of different palaeozoic and mesozoic formations

river stage tomography a new approach for characterizing groundwater basins received 19 june 2008 revised 16 february 2009 accepted 12 march 2009 published 9 may 2009 1 data from tomographic surveys make an inverse problem better posed in comparison to the data from a single excitation source a tomographic survey provides different coverages and perspectives of subsurface heterogeneity nonfully redundant information of the subsurface fusion of these pieces of information expands and enhances the capability of a conventional survey provides cross validation of inverse solutions and constrains inherently ill posed field scale inverse problems basin scale tomography requires energy sources of great strengths spatially and temporally varying natural stimuli are ideal energy sources for this purpose in this study we explore the possibility of using river stage variations for basin scale subsurface tomographic surveys specifically we use numerical models to simulate groundwater level changes in response to temporal and spatial variations of the river stage in a hypothetical groundwater basin we then exploit the relation between temporal and spatial variations of well hydrographs and river stage to image subsurface heterogeneity of the basin results of the numerical exercises are encouraging and provide insights into the proposed river stage tomography using naturally recurrent stimuli such as river stage variations for characterizing groundwater basins could be the future of geohydrology however it calls for implementation of sensor networks that provide long term and spatially distributed monitoring of excitation as well as response signals on the land surface and in the subsurface

what does hydraulic tomography tell us about fractured geological media a field study and synthetic experiments abstract between 2005 and 2010 japan atomic energy agency conducted four long term independent pumping tests in a fractured granite formation at the mizunami underground research laboratory miu site in mizunami city central japan during these tests drawdowns were monitored at different depths along several deep boreholes these tests become one of the few if not the only hydraulic tomographic survey conducted in the world over thousands of meters in a fractured geologic medium with several fault zones we analyzed the drawdown time data set associated with each pumping test independently and then the data sets from all pumping tests jointly to derive the spatial distributions of hydraulic conductivity k and specific storage s s of the medium these estimated distributions revealed some large scale high k and low k zones while the low k zones corroborated well with known low permeable layers and fault based on geological investigations there were no clear geological features that can be related to the large scale high k zones in order to understand and substantiate these high and low k zones we simulated a hydraulic tomographic survey in a synthetic fractured aquifer which bears similar geologic features i e formations fractures and faults at the miu site with exception that the hydraulic properties fracture and fault distributions were known exactly results of the simulation show that not only are the identified high k zones related to fracture networks connected with pumping and observation locations of each pumping test but also their values reflect the degree of connectivity of the network afterward we investigated the extent of the improvement of characterization of the fault and fractures through the use of deploying dense monitoring intervals and late time flux measurements

do lower crustal rocks record reversals of the earth s magnetic field magnetic petrology of oceanic gabbros from ocean drilling program hole 735b ocean drilling program odp leg 118 at the southwest indian ridge recovered the first vertically oriented continuously cored gabbros from in situ ocean crust magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the earth s magnetic field primary igneous magnetite is common only in evolved fe ti oxide rich gabbros and averages by volume 2 of the rock although substantially higher contents are common secondary magnetite produced by high temperature exsolution and hydrous alteration of olivine and pyroxene is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros cumulate olivine gabbros because of the high temperatures associated with this alteration it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization or a very high temperature chemical remanent magnetization acquired very near the ridge crest hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant stable remanent magnetization finally previously published drilling results and field studies from odp site 735b indicate that although this crustal section has been tectonically uplifted it formed at a mid ocean ridge spreading center similar to much of the oceanic crust therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section in general and argue strongly that layer 3 crustal rocks are capable of recording reversals of the earth s magnetic field

magma flow exsolution processes and rock metasomatism in the great messejana plasencia dyke iberian peninsula magma flow in dykes is still not well understood some reported magnetic fabrics are contradictory and the potential effects of exsolution and metasomatism processes on the magnetic properties are issues open to debate therefore a long dyke made of segments with different thickness which record distinct degrees of metasomatism the messejana plasencia dyke mpd was studied oriented dolerite samples were collected along several cross sections and characterized by means of microscopy and magnetic analyses the results obtained show that the effects of metasomatism on rock mineralogy are important and that the metasomatic processes can greatly influence anisotropy degree and mean susceptibility only when rocks are strongly affected by metasomatism petrography scanning electron microscopy sem and bulk magnetic analyses show a high temperature oxidation exsolution event experienced by the very early ti spinels during the early stages of magma cooling which was mostly observed in central domains of the thick dyke segments exsolution reduced the grain size of the magnetic carrier multidomain to single domain transformation thus producing composite fabrics involving inverse fabrics these are likely responsible for a significant number of the abnormal fabrics which make the interpretation of magma flow much more complex by choosing to use only the normal fabric for magma flow determination we have reduced by 50 per cent the number of relevant sites in these sites the imbrication angle of the magnetic foliation relative to dyke wall strongly suggests flow with end members indicating vertical dominated flow seven sites and horizontal dominated flow three sites

triple junctions of bay of biscay and north atlantic new constraints on the kinematic evolution recently surveyed triple junction trajectories in the bay of biscay and newly available magnetic maps constrain the early oceanic evolution of the north atlantic and bay of biscay from chrons m0 to a33o 118 to 80 ma the opening of the bay of biscay is of the scissors opening type with a pole of rotation located near bordeaux lat 44 6 n long 0 3 w and predicted simultaneous compression in the central and eastern pyrenees structural directions west of iberia do not fit the north america iberia motion therefore a second triple junction of the ridge ridge fault type is postulated just south of the first one which propagated north ward from a more southerly position

the relative importance of head flux and prior information in hydraulic tomography analysis using cross correlation analysis we demonstrate that flux measurements at observation locations during hydraulic tomography ht surveys carry nonredundant information about heterogeneity that are complementary to head measurements at the same locations we then hypothesize that a joint interpretation of head and flux data even when the same observation network as head has been used can enhance the resolution of ht estimates subsequently we use numerical experiments to test this hypothesis and investigate the impact of flux conditioning and prior information such as correlation lengths and initial mean models i e uniform mean or distributed means on the ht estimates of a nonstationary layered medium we find that the addition of flux conditioning to ht analysis improves the estimates in all of the prior models tested while prior information on geologic structures could be useful its influence on the estimates reduces as more nonredundant data i e flux are used in the ht analysis lastly recommendations for conducting ht surveys and analysis are presented

aquifer heterogeneity characterization with oscillatory pumping sensitivity analysis and imaging potential 1 periodic pumping tests in which a fluid is extracted during half a period then reinjected have been used historically to estimate effective aquifer properties in this work we suggest a modified approach to periodic pumping test analysis in which one uses several periodic pumping signals of different frequencies as stimulation and responses are analyzed through inverse modeling using a steady periodic model formulation we refer to this strategy as multifrequency oscillatory hydraulic imaging oscillating pumping tests have several advantages that have been noted including no net water extraction during testing and robust signal measurement through signal processing through numerical experiments we demonstrate additional distinct advantages that multifrequency stimulations have including 1 drastically reduced computational cost through use of a steady periodic numerical model and 2 full utilization of the aquifer heterogeneity information provided by responses at different frequencies we first perform fully transient numerical modeling for heterogeneous aquifers and show that equivalent results are obtained using a faster steady periodic heterogeneous numerical model of the wave phasor the sensitivities of observed signal response to aquifer heterogeneities are derived using an adjoint state based approach which shows that different frequency stimulations provide complementary information finally we present an example 2 d application in which sinusoidal signals at multiple frequencies are used as a data source and are inverted to obtain estimates of aquifer heterogeneity these analyses show the different heterogeneity information that can be obtained from different stimulation frequencies and that data from several sinusoidal pumping tests can be rapidly inverted using the steady periodic framework

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

geophysical evidence for a magmatic intrusion in the ocean continent transition of the sw iberia margin abstract a prominent positive gravimetric and magnetic anomaly stands out in the region of the guadalquivir portimao banks gulf of cadiz off sw iberia the bathymetric and gravimetric highs have been interpreted as resulting from basement uplift occurred during mesozoic rifting followed by overthrusting during the alpine orogeny however we show that basement uplift alone does not account for the observed magnetic anomaly implying that an additional magnetic source must be considered for an integrated model that fits with all the data we present a new magnetic study that calls for the existence of a large magmatic body intruded beneath the guadalquivir and portimao banks the guadalquivir portimao intrusion we perform 3d inversion of magnetic data to obtain a susceptibility model that estimates the shape and extension of the intrusion the modeled volume is constrained by estimation of regional curie depth and the susceptibility model is limited to a realistic range allowing interpretation in terms of rock nature to clarify the relationship between the intrusion and the local lithospheric structure we build 2d cross sections integrating the modeled intrusion with recent crustal models and use 2d forward modeling to ensure that both the predicted magnetic and gravimetric fields fit the observed data these sections show the guadalquivir portimao intrusion emplaced along a lithospheric decollement within the necking domain of the algarve rifted margin across the lower and upper crust it is suggested that is crosscuts jurassic sedimentary sequences suggesting that it intruded after continental rifting given the similarities with other onshore magnetic anomalies corresponding to late cretaceous alkaline intrusions it is likely that the guadalquivir portimao intrusion represents the southernmost expression of the late cretaceous alkaline magmatic event that affected both onshore and offshore west iberia becoming the fourth element of the enigmatic nnw sse trending alignment of sintra sines monchique plutons whose significance remains to be fully understood

validation of hydraulic tomography in an unconfined aquifer a controlled sandbox study in this study we demonstrate the effectiveness of hydraulic tomography ht that considers variably saturated flow processes in mapping the heterogeneity of both the saturated and unsaturated zones in a laboratory unconfined aquifer the successive linear estimator sle developed by mao et al 2013c for interpreting ht in unconfined aquifers is utilized to obtain tomograms of hydraulic conductivity k specific storage ss and the unsaturated zone parameters pore size parameter and saturated water content s for the gardner russo s model the estimated tomograms are first evaluated by visually comparing them with stratigraphy visible in the sandbox results reveal that the ht analysis is able to accurately capture the location and extent of heterogeneity including high and low k layers within the saturated and unsaturated zones as well as reasonable distribution patterns of and s for the gardner russo s model we then validate the estimated tomograms through predictions of drawdown responses of pumping tests not used during the inverse modeling effort the strong agreement between simulated and observed drawdown curves obtained by pressure transducers and tensiometers demonstrates the robust performance of ht that considers variably saturated flow processes in unconfined aquifers and the unsaturated zone above it in addition compared to the case using the homogeneous assumption ht results as expected yield significantly better predictions of drawdowns in both the saturated and unsaturated zones this comparison further substantiates the unbiased and minimal variance of ht analysis with the sle algorithm

variation of magnetic properties in sedimentary rocks hosting the foum zguid dyke southern morocco combined effects of re crystallization and fe metasomatism the effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood therefore we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the foum zguid dyke in southern morocco the study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments the present study shows that 1 close to foum zguid dykes the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re crystallization and fe metasomatism intensity 2 the thermal experiments on ams of samples collected farther from the dyke and thus less affected by heating during dyke emplacement indicate that 300 400 c is the minimum experimental temperature necessary to trigger appreciable transformations of the pre existing magnetic fabrics for temperatures higher than ca 580 c the magnetic fabric transformations are fully realized with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke therefore measured variations of the magnetic fabric can be used to evaluate re crystallization temperatures experienced by the host sedimentary rock during dyke emplacement the distinct magnetic behaviour observed along the cross sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 c after dyke emplacement 3 ams data shows a gradual variation of the magnetic fabric with distance from the dyke margin from sub horizontal k3 away from the dyke to vertical k3 close to the dyke experimental heating shows that heat alone can be responsible for this strong variation therefore such orientation changes should not be unequivocally interpreted as the result of a stress field resulting from the emplacement of the dyke for instance 4 magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations thermally and chemically induced by dyke intrusion in hosting sediments

the matuyama brunhes boundary interval 500 900 ka in north atlantic drift sediments summary ocean drilling program odp sites 984 and 983 located on north atlantic sediment drifts provide high resolution records across the matuyama brunhes boundary at odp site 984 bjorn drift iceland basin the mean sedimentation rate in the 500 900 ka interval is 12 cm kyr 1 based on an age model derived by matching the planktonic and benthic oxygen isotope records to an ice volume model the matuyama brunhes polarity transition at site 984 as defined by virtual geomagnetic polar vgp latitudes 50 has an apparent duration of 7 kyr with a mid point at 773 5 ka compared with 772 5 ka at neighbouring site 983 outside the polarity transition at both sites 984 and 983 excursions in vgp latitudes to values 20 at 540 590 and 670 ka correspond to troughs in the palaeointensity record new u channel palaeomagnetic data across the matuyama brunhes boundary for working and archive halves of core sections from three holes at both sites 984 and 983 augment data published by channell lehman 1997 and are compared with back to back 1 cm 3 discrete samples clusters of vgps in the south atlantic and northeast asia in both u channel and discrete sample records imply that polarity transition fields have characteristics similar to the modern non axial dipole nad field

new paleomagnetic mesozoic data from the algarve portugal fast rotation of iberia between the hauterivian and the aptian abstract we sampled nine sites of jurassic and cretaceous limestones in the algarve region portugal our study yields a paleopole for the portlandian valanginian 256 58 a95 5 and a paleopole for the aptian albian 232 75 a95 8 these results allow better constraint of the relative rotation of iberia with respect to europe this rotation of 22 14 is a rather fast event occurring during the barremian between 132 and 124 ma confinement of iberia between africa and eurasia would explain the rapid rotation of this microplate the iberian plate was probably attached to europe until 132 ma then detached from europe and acted as an independent plate during the time of fast rotation preceding the oceanic spreading in the bay of biscay

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

remagnetization of lower cretaceous limestones from the southern pyrenees and relation to the iberian plate geodynamic evolution paleomagnetic study 34 sites of thick cretaceous 5000 m marine carbonate strata from the organya basin ob in the southern pyrenees reveals a uniformly normal polarity characteristic remanence with varying declination d berriasian barremian d 295 o aptian albian d 348 o upper cretaceous d 354 o that resides in magnetite and predates late cretaceous tertiary folding the lack of reverse polarity magnetizations in the berriasian barremian series together with their distinct hysteresis properties that are typical of remagnetized carbonates elsewhere are taken as evidence for a secondary overprint in the lower part of the succession a hypothesized age of remagnetization near the barremian aptian boundary fits with a major tectosedimentary event in the basin that changed deposition from platform to basinal conditions and increased subsidence during the aptian the angular difference 53 o between declination from the remagnetized strata and the mean for aptian albian strata indicates that a counterclockwise rotation occurred between the two acquisition times part of this rotation is interpreted to be local and due to slip variation along strike of a normal fault that bounded the basin and tilted the pre aptian series this cretaceous extensive phase is framed in the rift evolution of the northern margin of the iberian plate the inferred remagnetization age within the ob falls within the range of previously inferred widespread cretaceous remagnetization events affecting the iberian plate in the ob remagnetization is speculated to have taken place through chemical burial diagenetic origin rather than simple thermoviscous resetting

palaeomagnetism in the sines massif sw iberia revisited evidences for late cretaceous hydrothermal alteration and associated partial remagnetization summary this study revisits the palaeomagnetism of the sines massif 76 ma in the southwestern iberian margin portugal the palaeomagnetic analysis was complemented by a comprehensive study of the magnetic mineralogy by means of rock magnetic measurements and petrographicobservations theoveralldispersionofpalaeomagneticdirections declinationranging between n0 and n50 and their migration observed during stepwise demagnetizations have revealed the superposition of remanence components we interpret this complex palaeomagnetic behaviour as related to the regional hydrothermalism associated with the last stages of late cretaceous magmatic activity this environment favoured mineralogical alteration and a partial chemical remagnetization giving in most samples a composite magnetization which has been erroneously interpreted as the primary one in a previous study then leading to a questionable model for cretaceous iberia rotation nonetheless for some samples a single component has been isolated interesting rock magnetic properties and microscopic observations point to a well preserved magnetic mineralogy for these samples with magnetite clearly of primary origin the associated chrm mean direction d i 3 9 46 5 95 1 7 n 31 samples then represents the true primary magnetization of the sines massif this new palaeomagnetic direction and the corresponding palaeomagnetic pole long 332 0 l at 79 5 a95 1 7 agrees with those from the other palaeomagnetic works for the same period and region e g the sintra and monchique massifs yielding a lack of significant rotation of iberia relative to stable europe since the uppermost late cretaceous campanian maastrichtian

correction to apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr 1 in the paper apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr by jean besse and vincent courtillot journal of geophysical research 107 b11 2300 doi 10 1029 2000jb000050 2002 the data in the top part of table 4 were incorrect the correct table 4 is given here journal of geophysical research vol 108 no b10 2469 doi 10 1029 2003jb002684 2003

intrusion of lamprophyre dyke and related deformation effects in the host rock salt a case study from the loule diapir portugal abstract a rock salt lamprophyre dyke contact zone sub vertical ne sw strike was investigated for its petrographic mechanic and physical properties by means of anisotropy of magnetic susceptibility ams and rock magnetic properties coupled with quantitative microstructural analysis and thermal mathematical modelling the quantitative microstructural analysis of halite texture and solid inclusions revealed good spatial correlation with ams and halite fabrics the fabrics of both lamprophyre and rock salt record the magmatic intrusion plastic flow and regional deformation characterized by a nw se trending steep foliation ams and microstructural analysis revealed two deformation fabrics in the rock salt 1 the deformation fabrics in rock salt on the nw side of the dyke are associated with high temperature and high fluid activity attributed to the dyke emplacement 2 on the opposite side of the dyke the emplacement related fabric is reworked by localized tectonic deformation the paleomagnetic results suggest significant rotation of the whole dyke probably during the diapir ascent and or the regional tertiary to quaternary deformation

correcting distorted paleosecular variation in late glacial lacustrine clay abstract an undisturbed horizontal chronostratigraphic marker horizon of laminated red glacio lacustrine clay crops out over 25 000 km 2 in northern ontario canada the primary clastic hematite laminae possess two stable vector components of magnetization we sampled a 1 6 m vertical section at overlapping 2 cm intervals in cubic specimens 8 cm 3 n 106 precisely oriented in geographic coordinates which permitted measurement of inclination and declination alternating field demagnetization 12 17 steps per specimen isolated a characteristic chrm primary component coercivity 40 mt approximately 30 shallower than the mean geomagnetic field inclination at this latitude with distorted paleosecular secular variation psv a lower coercivity overprint 20 40 mt is 6 shallower than the present geomagnetic field and similarly inclined to the depositional remanence deflection when the clay was re sedimented in the laboratory we believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition caused by the magnetic anisotropy of the clay using this as a proxy for the initial inclination the chrm hard component inclinations were restored to their original values assuming vertical compaction which averages to 51 the restored inclinations are compatible with site latitude and the restored secular variation loops centre reasonably on the geographic north pole the structural correction based on homogeneous vertical shortening over simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape size and packing and effectively combines influences of compaction and depositional settlement nevertheless this correction makes the psv data more useful and interpretable in terms of paleopole migration the more logical correction technique using anisotropy of anhysteretic remanence aarm jackson m j banerjee s k marvin j a lu r gruber w 1991 detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results geophys j int 104 95 103 is foiled here due to the high coercivity of the remanence bearing grains and their unknown orientation distribution and shape distribution

palaeomagnetic study of a subaerial volcanic ridge sao jorge island azores for the past 1 3 myr evidence for the cobb mountain subchron volcano flank instability and tectonomagmatic implications we present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1 3 myr on s jorge island azores archipelagonorth atlantic ocean the sections sampled in the southeastern and central western parts of the island record reversed and normal polarities respectively they indicate a mean palaeomagnetic pole 81 3 degrees n 160 7 degrees e k 33 and a95 3 4 degrees with a latitude shallower than that expected from geocentric axial dipole assumption suggesting an effect of non dipolar components of the earth magnetic field virtual geomagnetic poles of eight flows and two dykes closely follow the contemporaneous records of the cobb mountain subchron odp dsdp programs and constrain the age transition from reversed to normal polarity at ca 1 207 0 017 ma volcano flank instabilities probably related to dyke emplacement along an nnwsse direction led to southwestward tilting of the lava pile towards the sea two spatially and temporally distinct dyke systems have been recognized on the island the eastern is dominated by nnwsse trending dykes emplaced before the end of the matuyama chron whereas in the central western parts the eruptive fissures oriented wnwese controlled the westward growth of the s jorge island during the brunhes chron both directions are consistent with the present day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre existing fractures the distinct timing and location of each dyke system likely results from a slight shift of the magmatic source

cretaceous slab break off in the pyrenees iberian plate kinematics in paleomagnetic and mantle reference frames the pyrenees at the iberia europe collision zone contain sediments showing albian cenomanian high temperature metamorphism and coeval alkaline magmatic rocks stemming from different views on jurassic cretaceous iberian microplate kinematics two schools of thought exist on the trigger of this thermal pulse one invoking hyperextension of the iberian and eurasian margins the other suggesting slab break off competing scenarios for mesozoic iberian motion compatible with pyrenean geology comprise 1 transtensional eastward motion of iberia versus eurasia or 2 strike slip motion followed by orthogonal extension both favoring hyperextension related heating and 3 scissor style opening of the bay of biscay coupled with subduction in the pyrenean realm favoring the slab break off hypothesis we test these kinematic scenarios for iberia against a newly compiled paleomagnetic dataset and conclude that the scissor type scenario is the only one consistent with a well defined 35 counterclockwise rotation of iberia during the early aptian we proceed to show that when taking absolute plate motions into account aptian oceanic subduction in the pyrenees followed by late aptian early albian slab break off should leave a slab remnant in the present day mid mantle below nw africa mantle tomography shows the reggane anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below southern algeria mantle tomography is therefore consistent with the scissor type opening of the bay of biscay coupled with subduction in the pyrenean realm slab break off may thus explain high temperature metamorphism and alkaline magmatism during the albian cenomanian in the pyrenees whereas hyperextension that exhumed pyrenean mantle bodies occurred much earlier in the jurassic

comparison of soil nonlinearity in situ stress strain relation and g gmax reduction observed in strong motion databases and modeled in ground motion prediction equations earthquake ground motions are strongly affected by the upper tens of meters of the earth s crust and consequently local site effects need to be included in any ground motion prediction it is increasingly common in ground motion prediction equations gmpes to account for possible non linear behavior of near surface materials soil these non linear site terms adjust observations made on soft soil sites to the ground motion expected on bedrock and hence allow these abundant soil records to be used within the regression analysis for the derivation of empirical gmpes these nonlinear site terms also allow rapid predictions of the expected ground motions on soil rather than requiring a site response analysis to be conducted in this study we compare the signature on observed peak ground acceleration as a function of a strain proxy of non linear soil behavior within four large strong motion databases to the predicted signature from four recent gmpes three of which explicitly include non linear site terms we find that observed non linearity in the databases interpreted in terms of strain stress relationships and reduction of shear modulus is limited but that even this limited effect is underestimated by the non linear site terms of the considered gmpes which suggests that predictions from these gmpes could be biased for soft soil sites but also on bedrock some of this mismatch could be explained by the use of the average shear wave velocity in the top 30m vs30 to characterize sites as well as errors in these values

a temporal sampling strategy for hydraulic tomography analysis 1 this paper investigates optimal sampling times of drawdowns for the analysis of hydraulic tomography ht survey the investigation was carried out by analyzing the spatial and temporal evolution of cross correlations between the head responses at an observation well and transmissivity t and storage coefficient s properties during a pumping test in homogeneous and heterogeneous aquifers the analysis shows that the cross correlation between the head and s values is limited to the region between the observation and the pumping well in the aquifers it reaches the highest value near the early time tm and decays to zero afterwards the time tm is approximately equal to the time t0 at which the extrapolated drawdown from the first straight line portion of an observed drawdown log time plot becomes zero at early times the high cross correlation between the head and t is confined to the region between the observation and the pumping well this region then evolves into two humps one on each side of the circular region encompassing the observation well and the pumping well the size of the two humps expands and their values reach the maximum as flow reach steady state as a consequence we hypothesize that pairs of head data at t0 and those at either the steady state or a late time during an ht survey could yield the best estimates of the heterogeneous t and s fields results from numerical experiments have verified this hypothesis and demonstrated that this sampling strategy is generally applicable even when the boundary condition is unknown we therefore recommend in principle that 1 carrying out pumping tests of ht surveys for sufficiently long period of time such that drawdown reaches the entire area of interest and 2 using a constant head or zero drawdown for all boundaries during the inverse modeling analysis

magnetic properties of synthetic analogues of the altered olivines of igneous rocks summary a laboratory simulation has been made of the high temperature deuteric alteration of olivine such as occurs in slowly cooled basic intrusions three compositions in the forsterite fayalite solid solution series were synthesized by a three firing method using self buffering and controlled atmosphere to obtain material containing no detectable magnetite fe o as a contaminant 0 005 per cent the prepared olivines were oxidized between 850 and 1280 c at oxygen fugacities of between lo and lop5 atmospheres the composition and concentration of the multiphase oxidation products were studied by magnetic optical electron optical and x ray methods the magnetic constituent of the intergrowth was magnetite below about 1150 c the microstructure is similar to the dendritic intergrowths found for example in the altered olivine of the central layered intrusion of the isle of rhum above about 1150 c the form may be similar to symplectite intergrowths the magnetization process parameters coercive force etc vary systematically with temperature of oxidation to which evidently controls the microstructure of the magnetite the magnetite produced is magnetically hard samples altered at 900 c and 950 c having higher coercive force hc than any magnetite reported in the literature the room temperature properties appear to bridge the published trends found in grown crystal magnetites and crushed grains inferred effective grain sizes of the complex intergrowths range from 0 1 to 3 pm as to increases from 900 to 1200 c the temperature dependence of magnetization process parameters principally h measured between room temperature and the curie point are modelled in terms of i monodomain grains subject to thermal fluctuations ii monodomain grains not subject to thermal fluctuations and iii as multidomain grains model i yields plausible values for effective grain dimensions and length diameter ratios higher to producing larger more equidimensional particles model ii suggests that the relative importance of cubic and uniaxial anisotropies may be influenced by a microstructure dependent uniaxial particle particle interaction term model iii implies that if domain walls are pinned by both surface and volume defects magnetostriction plays a greater role in volume pinning than in surface pinning plausible values of demagnetizing factors appropriate to grains with few domains are produced by model iii

palaeomagnetism and isotopic age data from upper cretaceous igneous rocks of w portugal geological correlation and plate tectonic aspects summary palaeomagnetic results and k ar age data for the sintra and sines intrusive complexes w portugal and further details on the palaeomagnetic structure of the lisbon volcanics are reported the sintra complex consists of two main intrusive phases having been emplaced in the upper cretaceous at around 90 ma and 75 ma respectively the radiometric results show that the sines complex formed concurrently with the second sintra magmatism the early main sintra pluton has a characteristic magnetization of d 358 i 27 95 3 3 this remanence direction is defined by gabbros and diorites as the granitic rocks constituting the bulk of the complex are shown to possess stable secondary magnetization imposed during quaternary weathering the characteristic magnetization of the younger intrusive event as defined by the sines rocks has a mean direction of d 041 i 41 95 3 3 these two upper cretaceous palaeomagnetic directions are significantly different at the 95 per cent probability level the lisbon volcanics show the presence of the 75 ma magnetization suggesting that also this volcanic complex dates from the upper cretaceous in addition the palaeomagnetic results from the lisbon complex have given further substance to a previously reported magnetization component with shallow inclination and north northwest declination now defined by d 333 i 14 95 7 4 it is inferred that this magnetization which has a dual polarity structure formed through low temperature oxidation in late cretaceous to lower tertiary time the declination difference between the 90 ma and 75 ma magnetizations may be interpreted in terms of a two phase rotation of iberia in the upper cretaceous prior to the assumed late cretaceous early tertiary secondary magnetic imprint implying a counterclockwise rotation of minimum 40 followed by a clockwise rotation of similar magnitude however if the relevant palaeomagnetic reference frame for iberia before and after its independent tectonic movements in the late cretaceous is the african plate then the clockwise rotation is about 20 larger than the counterclockwise one apparently accounting for the compressive tectonism in the bay of biscay

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

a reply to the comment on assessing discrepancies between previous plate kinematic models of mesozoic iberia and their constraints by barnett moore et al we welcome the comments of van hinsbergen et al 2017 on the recent efforts of barnett moore et al 2016 specifically van hinsbergen et al 2017 raise concerns about two of the major conclusions made by barnett moore et al 2016 firstly barnett moore et al 2016 choose to negate the cretaceous iberian paleomagnetic database as a viable plate kinematic constraint on the plate motions of mesozoic iberia this conclusion criticized by van hinsbergen et al 2017 was based on citing the previous efforts of neres et al 2012 2013 which exposed several shortcomings elaborated on below within this dataset secondly van hinsbergen et al 2017 criticize barnett moore et al 2016 for dismissing mantle tomographic interpretations in support of a preserved cretaceous pyrenean subducted slab beneath northern africa below we have addressed each of these major criticisms from van hinsbergen et al 2017 in a two section layout similar to their comment above

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

paleomagnetic study of the great foum zguid dyke southern morocco a positive contact test related to metasomatic processes 1 when a paleomagnetic pole is sought for in an igneous body the host rocks should be subjected to a contact test to assure that the determined paleopole has the age of the intrusion if the contact test is positive it precludes the possibility that the measured magnetization is a later effect therefore we investigated the variations of the remanent magnetization along cross sections of rocks hosting the foum zguid dyke southern morocco and the dyke itself a positive contact test was obtained but it is mainly related with chemical crystalline remanent magnetization due to metasomatic processes in the host rocks during magma intrusion and cooling and not only with thermo remanent magnetization as ordinarily assumed in standard studies paleomagnetic data obtained within the dyke then reflect the earth magnetic field during emplacement of this well dated 196 9 1 8 ma intrusion

detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results summary we suggest that inclination errors in detrital remanent magnetization drm may be recognized and corrected by measurement of the anisotropy of anhysteretic remanent magnetization arm arm anisotropy reflects directional variations in the remanence capacity of relatively fine grained magnetic particles in rocks or sediments generally the same particles that carry the stable component of drm relative vertical and horizontal drm magnitudes are controlled by this directional remanence capacity as well as by the alignment efficiency of the particle magnetic moments which in turn is governed by the relative intensities of the vertical and horizontal components of the ambient magnetic field thus the respective vertical and horizontal components of palaeointensity and therefore palaeofield inclination may be obtained by normalizing the measured drm components by parallel arm intensities synthetic sediments containing silica kaolin and sized magnetite powders have previously been found to acquire experimental drm with an inclination error that is a direct function of kaolin concentration further experiments have now shown that arm anisotropy in the synthetic sediments is also a direct function of kaolin content and thus a correlation exists between drm inclination errors and arm anisotropy multiplying the measured drm vector by the inverse of the arm anisotropy matrix yields an improved estimate of the actual laboratory depositional field in all cases for samples with acicular 0 5 m magnetite this procedure reduces a mean inclination error of over 5 to less than 0 5 samples with subequant magnetite of slightly larger grain size 0 75 m exhibit similar arm anisotropy 10 per cent but somewhat larger inclination errors 10 we interpret this in terms of stronger horizontal alignment but weaker particle anisotropies for the larger more equant magnetites

the rotation of iberia during the aptian and the opening of the bay of biscay abstract the cretaceous rotation of the iberian plate in a geodynamical active setting between africa and europe is contemporaneous with the opening of the northern atlantic ocean and provides fundamental temporal and kinematical constraints on the evolution of the bay of biscay and the pyrenees here we report new paleomagnetic data from 50 sites 1109 drilled cores from marine sediments of the cretaceous organya basin in the southern pyrenees to constrain the timing and magnitude of the rotation of iberia berriasian to barremian directions in exclusively limestone lithology are remagnetised in agreement with previous organya basin studies the aptian to cenomanian marls and limestones in the organya basin however provide primary magnetisations asserted by rock magnetic experiments and positive fold tests our results constrain the rotation of iberia to be finished just before the albian paleomagnetic data cannot well constrain the onset of the rotation but sea floor anomaly studies restrict the onset of the rotation to anomaly m0 or the barremian aptian boundary this implies that the entire rotation of iberia happened in one single phase and is confined to the aptian this is in contrast to earlier studies that suggested a two phase rotation of iberia but we argue that the large errors in published ages are not in disagreement with our results in summary we constrain the 35 rotation of iberia to the aptian period which implies a rotation rate of 2 7 5 1 myr depending on the controversial duration of the aptian

a new kinematic model for iberia further palaeomagnetic and isotopic age evidence abstract palaeomagnetic results and k ar age data from the algarve igneous province have uncovered three ancient relative field components it is inferred that the principal magnetization of the monchique complex with a mean direction of d 42 i 181 95 4 5 was acquired at around the time of the pluton s emplacement some 72 2 ma ago a second magmatic pulse affected the region at 68 2 ma leaving behind a relative field component of d 42 i 341 95 1 7 furthermore an overprinted magnetization characterized by shallow inclination and somewhat smeared northwesterly southeasterly declination most likely originates from the lower tertiary the new palaeomagnetic data are seen in conjunction with previously published upper cretaceous results from the sintra and sines intrusive complexes the resulting database gives entirely new perspectives for evaluating the plate tectonic history of the peninsula in upper cretaceous lower tertiary time a two phase rotation of the iberian micro plate an anticlockwise one of 40 followed by a clockwise one of 70 is clearly in evidence the net clockwise rotation provided a major impetus for the compressive deformation in the pyrenees and the bay of biscay and led to extensive remagnetization of older rock formations in the northern regions of iberia previous palaeomagnetic conclusions in favour of a single anticlockwise rotation of 35 supposed to be directly connected with the formation of the bay of biscay appear to be artefacts which stem from an incorrectly adopted palaeomagnetic reference frame

new paleomagnetic results from cretaceous sediments near lisboa portugal and implications for the rotation of iberia abstract paleomagnetic study of outer platform sediments of late jurassic early cretaceous age from portugal lisboa area leads to characteristic remanent magnetization directions for the hauterivian barremian d 317 i 58 and the aptian d 344 i 47 the absolute rotation of iberia between 125 and 110 ma is consequently of about 27 in an anticlockwise sense a later rotation of about 14 follows the first one in the same sense from 110 ma the total absolute rotation of iberia from the hauterivian age determined by paleomagnetism is thus about 41 the comparison of declinations measured with paleodeclinations calculated in lisboa from the eurasian and african poles of similar age indicates that the main phase of relative rotation of iberia with respect to eurasia occurs during the first stage of relative motion i e hauterivian to aptian the tectonic history of the bay of biscay is in good agreement with these two stages of rotation the total relative rotation with respect to europe is 34 this value is consistent with results previously established by van der voo 2 3 25 but we have established that the main phase of rotation 30 occurs in the hauterivian aptian interval the movement of iberia is clearly linked to the movement of africa in direction and age but the amount of rotation of iberia during the early cretaceous is much larger and implies that iberia was not a real part of the african plate

a note on the geochronology of the iberian alkaline province abstract a critical assessment of previously published ages and new k ar results for two complexes in the iberian alkaline province monchique and sintra lead to the conclusion that two separate magmatic episodes occurred around 72 and 82 ma since emplacement of these alkaline rocks is undoubtedly related to rifting and subsequent changes in the atlantic ocean floor spreading geometry accurate timing of magmatic events within the iberian alkaline province is of fundamental significance

reconciling cretaceous paleomagnetic and marine magnetic data for iberia new iberian paleomagnetic poles 1 the cretaceous paleogeography and the kinematic evolution of the iberian plate are poorly constrained especially problematic is to reconcile iberian paleomagnetic data with paleomagnetic data of the neighboring plates and with euler poles derived from seafloor magnetic anomalies the first limitation arises from the cretaceous normal polarity superchron where paleogeographic reconstruction using marine magnetic anomalies is handicapped the second arises from the paucity of reliable paleomagnetic poles with satisfactory statistical criteria and age in order to address these shortcomings and provide new high quality paleomagnetic poles for iberia we conducted a detailed rock magnetic and paleomagnetic study of two cretaceous magmatic sills the paco de ilhas pi and foz da fonte ff sills from the lusitanian basin portugal recently dated at about 88 and 94 ma respectively our results show that the magnetic mineralogy of the sills is primary i e acquired during magma cooling and essentially represented by titanomagnetite the corresponding paleomagnetic poles match the synthetic apwp from the african plate at 80 and 100 ma on the basis of a rigorous selection of iberian cretaceous poles we then calculated mean paleomagnetic poles for different time intervals and found that iberian paleomagnetic data fit well the global apwp between 70 and 120 ma but move far away from the apwp at pre rift times our approach shows that new and better constrained paleomagnetic poles can aide in solving part of the contradiction between iberian and african apwps

relations between the emplacement and fabric forming conditions of the kapitan dimitrievo pluton and the maritsa shear zone central bulgaria magnetic and visible fabrics analysis the kapitan dimitrievo pluton was emplaced within the 15 km wide maritsa shear zone during the late cretaceous it has well known u pb zircon age 78 54 0 13 ma and appears as a late syntectonic intrusion that marked the last ductile deformation in the maritsa shear zone magnetite is believed to be the main carrier of the magnetic fabric in this pluton and crystallized mainly late after the main rock forming minerals two fabrics are recorded a visible syn magmatic fabric due to magma flow and magnetic late magmatic fabric related to regional stresses although different both are mainly related to the shearing along this shear zone these results constrain in age the dextral strike slip controlled emplacement and evolution of the late cretaceous plutons from central bulgaria

the late cretaceous alkaline igneous province in the iberian peninsula and its tectonic significance abstract the iberian province consists of the following the three subvolcanic syenitic major intrusive complexes of monchique sines and sintra in w and sw portugal together with their basanitic lamprophyric minor intrusive suites basanitic volcanic complexes around lisbon at least some of a widespread suite of basanitic to theralitic minor intrusives in west central portugal about 80 small basanitic lamprophyric to nepheline syenitic intrusions scattered through the pyrenees ne spain the french corbieres and off the coast of nw spain and the ormonde seamount of the gorringe bank off the sw coast of portugal most of these occurrences have been dated isotopically or from field evidence as late cretaceous geological and petrological details of the various occurrences are compiled and reviewed primary basanitic magmas were probably parental to the entire province and generated syenitic magmas by differentiation processes oversaturated rocks were produced by alkali loss and perhaps also by crustal involvement the iberian province is related to the opening of the n atlantic specifically that of the bay of biscay

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

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

pyrenean orogeny and plate kinematics 1 the evolution of the pyrenees a mountain range between iberia and eurasia has remained the subject of many debates between geologists and geophysicists for a long time by combining the identification of seafloor spreading anomalies a33o to m0 in the bay of biscay with those in the north atlantic we have derived a position of a mean pole of rotation for the entire opening of the bay of biscay four hundred kilometers of shortening took place between the iberian and eurasian plates in the pyrenean domain during the opening of the bay of biscay from chrons m0 to a33o time 118 to 80 ma the deep seismic etude continentale et oceanique par reflexion et refraction sismique ecors profile shot across the pyrenees and teleseismic data show the presence of two distinct slabs which dip to the north the southern slab is linked to the subduction of the neo tethys ocean which was created from late jurassic to early aptian simultaneously elongated back arc basins formed along the future pyrenean domain this slab was active from at least 118 ma early aptian to 100 ma late albian the northern slab active since 85 ma is linked to the subduction of the lower continental crust located south of the pyrenean domain in the upper crust normal faults as well as the north pyrenean fault became reverse faults and former back arc basins were inverted giving rise to the uplift of the pyrenees as a double asymmetrical wedge