stacking paleointensity and oxygen isotope data for the last 1 5 myr piso 1500 coupled relative geomagnetic paleointensity rpi and oxygen isotope records are used to construct rpi and oxygen isotope stacks for the last 1 5 myr the coupled correlations are accomplished using the match algorithm lisiecki l e and lisiecki p a 2002 application of dynamic programming to the correlation of paleoclimate records paleoceanography 17 1049 doi 10 1029 2001pa000733 to simultaneously correlate isotope and rpi records the simultaneous match reduces the degree of freedom associated with correlations using rpi or oxygen isotope records alone the overall compatibility of rpi and oxygen isotopes indicates a dominant global but independent component in both signals the local wavelet power spectrum lwps for the rpi stack indicates little significant orbital power although the accompanying oxygen isotope stack has all the lwps characteristics expected of a high resolution oxygen isotope record containing orbital frequencies the piso 1500 stack represents a new stratigraphic template that can be used to correlate among marine sediment records and link them to polar ice cores via variations in cosmogenic nuclide production scaling the stack to values for virtual axial dipole moment vadm indicate maxima at 15 1022 am2 and minima that imply a threshold of 2 5 1022 am2 below which values are associated with either polarity reversals or magnetic excursions

investigating the role of the itoigawa shizuoka tectonic line towards the evolution of the northern fossa magna rift basin abstract the itoigawa shizuoka tectonic line istl fault system is considered to have one of the highest probabilities for a major inland earthquake occurrence in the whole of japan it is a complex fault system with the dip directions of the local fault segments changing from north to south between an east dipping low angle thrust fault a strike slip fault and a west dipping thrust fault the tectonic relations between the different parts of the fault system and the surrounding geological units are yet to be fully explained this study aims to reveal the juncture of the northern and central parts of the istl and investigate its contribution towards the shaping of the northern fossa magna rift basin we conducted 3 deployments of 1 or 2 linear arrays of seismic stations across the central and northern istl regions and observed local micro earthquakes for a period of 3 years each deployment recorded continuous waveform data for approximately 3 months using arrival times of 1193 local earthquakes we jointly determined earthquake locations and a 3d velocity model applying the tomography method we were able to image the regional crustal structures from the surface to a depth of 20 km with a spatial resolution of 5 km subsequently we used the obtained 3d velocity model to relocate the background local seismicity from 2003 to 2009 the juncture of the northern and central parts of the istl was well constrained by our results the depth extension of the northern parts of the istl fault segments follows the bottom of the miocene northern fossa magna rift basin nfm and forms an east dipping low angle fault in contrast the central parts of the istl fault segments are estimated to lie along the eastern boundary of the matsumoto basin forming an oblique strike slip fault fig 1

a 2 14 myr astronomically tuned record of relative geomagnetic paleointensity from the western philippine sea 1 we present a 2 14 myr astronomically tuned relative geomagnetic paleointensity record from the western philippine sea pseudosingle domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations no significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization this suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at earth orbital periods the paleointensity record is highly coherent with the sint 800 global paleointensity stack for the last 800 kyr and with a coeval record from the west caroline basin our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction we do not observe an asymmetrical sawtooth form in our paleointensity record which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts also we do not observe a persistent 100 kyr inclination periodicity in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity good agreement between our paleointensity record and the coeval west caroline basin record provides the beginning of a detailed view of geomagnetic field behavior between 0 8 and 2 14 ma

repeating earthquake activities associated with the philippine sea plate subduction in the kanto district central japan a new plate configuration revealed by interplate aseismic slips abstract we detect repeating earthquakes associated with the philippine sea plate subduction to reveal the plate configuration in the kanto district we find 140 repeating earthquake groups with 428 events by waveform similarity analysis most repeating earthquakes in the eastern part of the kanto district occur with a regular time interval they have thrust type focal mechanisms and are distributed near the upper surface of the philippine sea plate these observations indicate that the repeating earthquakes there occur as a repetition of ruptures on the isolated patches distributed on the plate boundary owing to the concentration of stress caused by aseismic slips in the surrounding areas this shows that the distributions of repeating earthquakes suggest the aseismic slips in the surrounding areas of small patches we determine spatial distributions of repeating earthquakes in the eastern part of the kanto district and find that they correspond to the upper boundary of the philippine sea plate that is the upper boundary of the oceanic crust layer of the philippine sea plate the plate geometry around choshi is newly constrained by repeating earthquake data and a rather flat geometry in the eastern part of the kanto district is revealed the obtained geometry suggests uplift of the philippine sea plate due to the collision with the pacific plate beneath choshi repeating earthquakes in the western part of the kanto district have extremely shorter recurrence times and their focal mechanisms are not of the thrust types these repeating earthquakes are classified as burst type activity and likely to occur on the preexistent fault planes which are distributed around the collision zone between the philippine sea plate and the inland plate the variation among the repeating earthquake activities in the kanto district indicates that regular repetition of repeating earthquakes is possible only on the plate boundary with a smooth and simple geometry

the time dependence of intense archeomagnetic flux patches 1 the long term temporal behavior of intense geomagnetic flux patches at the core mantle boundary and the relation with lower mantle lateral heterogeneity are under debate we apply an algorithm to detect centers of intense flux patches and track their time evolution in a recent archeomagnetic field model in order to study the kinematics of such intense magnetic flux patches on millennial timescale we find that most intense flux patches appear near the edge of the tangent cylinder quasi stationary periods with small oscillations of patches occur more than drifts detailed comparison of the archeomagnetic patches behavior with that seen in numerical dynamos with tomographic heat flux boundary conditions suggests that core mantle thermal coupling could be the cause of a statistical preference for some longitudes on the long term which does not exclude significant time spent away from the preferred longitudes this could explain the roughly coincident locations of high latitude patches in the historical geomagnetic field with that of the time average paleomagnetic field together with the much weaker patches intensity in the latter alternating eastward and westward drifts are also observed the drifts are more westward than eastward especially in the southern hemisphere indicating that the time average zonal core flow may also be driven by core mantle thermal coupling an average patch lifetime of 300 years is found which we hypothesize may indicate the vortex lifetime in the outer core

magnetostratigraphy and relative palaeointensity of late neogene sediments at odp leg 167 site 1010 off baja california sediments recovered from the most seaward site of the baja transect of the california margin odp leg 167 yielded a high fidelity magnetostratigraphic record spanning the last 6 myr using a long core cryogenic magnetometer we made nrm measurements of 59 u channel samples which cover the interval from 0 to 78 5 mcd metres composite depth of the spliced section of holes 1010c and 1010e stepwise af demagnetization between 20 and 60 mt showed that the remanence is essentially composed of a single stable component which was not affected by a coring induced magnetization the investigated section can be divided into at least 25 polarity intervals which are assigned to the chrons from c3a n2 to the brunhes c1n the sedimentation rate was nearly constant at about 13 m myr 1 except for a reduction to 9 m myr 1 between 4 and 5 ma arm and irm measurements of the u channels revealed that the sediments below 23 mcd exhibit sharp drops in magnetic concentration at several horizons but that the upper part has a relatively uniform magnetic concentration containing magnetite of pseudo single domain size the upper interval yielded consistent relative intensity records when normalized with arm irm and shipboard susceptibility data several peaks in the relative intensity variations correspond to drops in the s ratio in particular at 0 1 t or to anomalous peaks in the irm and magnetic susceptibility profiles suggesting that the normalized intensity record is partly influenced by episodic changes in the flux of magnetic grains the normalized intensity for the intervals younger than 1 2 ma shows similarities with published palaeointensity records from the equatorial pacific including results from odp leg 138 in particular the intensity minima at 0 05 0 11 0 18 and 0 29 ma and those associated with the onset and termination of the jaramillo subchron are well correlated

geochemical records of taiwan sourced sediments in the south china sea linked to holocene climate changes we present a multi proxy geochemical analysis of core kng5 retrieved from the northern south china sea slope spanning the last 13 0 ka bp to assess the response of weathering and erosion processes in the southwestern taiwan river basin to climate change a provenance analysis indicates that the core sediments were primarily derived from the southwestern taiwan river and can be used to reconstruct the erosion and or weathering history of this river basin the chemical index of alteration the plagioclase index of alteration rb sr al ti principle component 1 and k al combined with terrigenous mass accumulation rates and the proportion of clay exhibit sharp excursions from 12 0 to 5 5 ka peaking at 10 0 ka which suggests strong chemical weathering and physical erosion during the early mid holocene the period of the maximum summer monsoon abrupt falling weathering trends during 5 2 and 3 5 ka bp correlate with a weaker monsoon after 3 5 ka bp a stable lower weathering trend to the present day was evident indicating the deposition of less weathered sediments in the slope when the monsoon rainfall was weakening a good correlation between our proxy records of chemical weathering and variations of the monsoon intensity in south china emphasizes that variation in monsoon intensity is most likely the primary drive of the intensity of weathering and erosion in southwestern taiwan river sediments over the last 13 0 ka bp finally we concluded that geochemical parameters can be used as reliable proxies for climate variation on a millennial centennial time scale in such areas as taiwan where weathering limited regimes exist c 2015 elsevier b v all rights reserved

an unusual zone of seismic coupling in the bonin arc the 1972 hachijo oki earthquakes and related seismicity the 1972 february and december hachijo oki earthquakes ms 7 3 and 7 4 in the northernmost part of the izu bonin subduction zone are the only major events ms 7 0 in the bonin arc for the past 80 years relocation of the hypocenters using one smaller event having a wellconstrained focal depth as a master event shows that the depth of the february event is 10 km shallower than that of the december event we have determined the rupture process for both events by minimizing the error in waveform between observed and synthetic seismograms although the number of available stations are limited the depth range of the major energy release for the december event extends deeper than for the february one the rupture propagated up dip for both events it is likely that the rupture zone of the two events overlapped and that the december event ruptured the deeper part this suggestion is consistent with the observation that the aftershock zones of both events overlap with that of the december event shifted landward the waveforms of the december event have a smaller high frequency component than those of the february event suggesting that the stress at the thrust zone became more uniform or reduced after the february event

thickness variation of the descending philippine sea slab and its relationship to volcanism beneath the kanto tokai district central japan 1 we use an iterative nonlinear travel time tomography technique to determine three dimensional p wave and s wave seismic velocity models for the region beneath the kanto tokai district of central japan salient points of the models are the following 1 the subducting philippine sea slab has thickness variation with a stepwise offset east of the izu peninsula the eastern beneath the kanto district and western beneath the area north of izu peninsula and the tokai district regions have respective thicknesses of 60 and 25 km the slab clearly subducts continuously to a depth of 100 km from the sagami trough beneath the kanto district although the tokai slab is indistinct at depths of 50 70 km 2 the oceanic crust at the top of the descending philippine sea slab transforms into eclogite at a depth of about 70 km beneath the kanto district 3 low velocity anomalies are associated with present and past volcanism they are located on the west of a 110 km depth contour of the wadati benioff zone rather than only on the backarc side of the volcanic front deflection of the volcanic front might result from the subducting philippine sea slab which shifts the dehydration reaction deeper as it subducts deeper

three dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the kanto tokai district japan we determine the three dimensional p wave velocity structure beneath the kanto tokai district central japan and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the national research center for disaster prevention the variance of the travel time data is reduced by 75 as compared to the variance for the routinely used laterally homogeneous model in general our velocity model is consistent with tectonic models of the kanto tokai district our velocity model is also in good agreement with seismic explosion data and bouguer gravity anomalies the uppermost layer from 0 to 16 km depth is characterized by lateral velocity differences of more than 20 we find low velocity zones beneath tokyo bay the middle of the boso peninsula and omae zaki where there are thick sedimentary layers a marked 30 km thick high velocity zone extends to the northeast and the west of the izu peninsula this zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the philippine sea phs plate which is being subducted at the sagami and the suruga troughs the data unquestionably require a high velocity zone beneath mount tsukuba that cannot be explained in terms of present tectonic models we use our three dimensional velocity model to relocate about 23 000 earthquakes that took place from 1983 through 1985 the accuracy of the hypocentral parameters improved considerably as a result the double seismic zone within the subducting pacific plate is more clearly delineated than in previous studies the inclined seismic zone for the subducting phs plate coincides fairly well with the high velocity zone of our three dimensional velocity model

low velocity oceanic crust at the top of the philippine sea and pacific plates beneath the kanto region central japan imaged by seismic tomography 1 we construct fine scale three dimensional p and s wave velocity structures beneath the kanto region central japan by seismic tomography with a spatial correlation of velocities the philippine sea and pacific plates subduct beneath the eurasian plate in this area and are imaged with high velocities oceanic crust at the uppermost part of these subducting plates is a low velocity layer low velocity oceanic crust of the philippine sea plate subducts to a depth of approximately 80 km there are also two low velocity bodies with relatively high v p v s ratios of 1 80 1 90 in the mantle wedge above the oceanic crust of the philippine sea plate we speculate that the westernmost of these two low velocity bodies consists of 20 partially serpentinized peridotite continuous with gabbro in the oceanic crust of the uppermost philippine sea plate while the eastern body is composed of 30 partially serpentinized peridotite we trace the subducting oceanic crust of the pacific plate to a depth of 120 km the estimated v p v s ratio of this layer is 1 85 1 90 which indicates a low probability of molten rock the gabbroic oceanic crust may have been metamorphosed to garnet granulite

strike slip tectonics within the northernmost philippine sea plate in an arc continent collisional setting abstract the geological processes in the northernmost philippine sea plate which is bounded by the suruga and sagami troughs are a typical example of an active collision zone we attempt to illustrate the stress field through seismic estimations and geodetic analysis and propose the kinematic mode of the northernmost tip of the philippine sea plate seven events m 4 0 are chosen for waveform inversion by the isola software to distinguish the stress field in particular six of the chosen events which exhibit strike slip motion are distributed in the eastern area where few focal mechanisms have been reported by previous studies according to the available focal mechanisms strike slip faults with similar p and t axes are widely distributed in the study area the stress inversion suggests that the northern area is characterized by a nw se compression and a ne sw extension stress regime although some spatial differences exist as indicated by an analysis of the geodesy epicenters focal mechanisms gravity anomalies and velocity structure the deformation in the northernmost tip is mainly accommodated by several conjugate strike slip fault systems with steep dips that center on the izu volcanic line generally the maximum principal stress of the kinematics is derived from the collision between the philippine sea plate and central japan because of the different subduction angles rates and directions of the down going plate diverging slab pull forces along the suruga and sagami troughs may be causing the ne nne extension in most of the areas that are bounded by the two troughs the extension propagates southwards along the izu volcanic line and reaches the area adjacent to miyake jima

short polarity intervals within the matuyama transitional field records from hydraulic piston cored sediments from the north atlantic abstract detailed sampling of two short magnetozones within the matuyama chronozone recorded at dsdp site 609 49 86 n 335 77 e confirms that one the cobb mountain subchronozone 1 12 ma is a very short full normal polarity interval and that the other the older interval is a record of a geomagnetic excursion which occurred at approximately 1 55 ma the cobb mountain subchron lasted approximately 25 000 years one third the duration of the jaramillo subchron the normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions we interpret the antisymmetry as reflecting a dependence upon the sense of the reversal without significant changes in the relative contributions of non dipole terms the polarity interval recorded at 1 55 ma lasted only 8 800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section this feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the gilsa subchron similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth s outer core

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

a new configuration and an aseismic slab of the descending philippine sea plate revealed by seismic tomography abstract we obtained the three dimensional seismic velocity structure of the uppermost mantle beneath the kanto tokai and chubu regions central japan where the philippine sea plate descends by means of travel time tomography using 18 years of p phase data recorded by a regional seismic network a distinct low velocity zone which was the oceanic crust of the descending philippine sea plate was found beneath the kanto region the low velocity zone revealed a new configuration of the philippine sea plate different from any models obtained through the analysis of hypocenter distribution and focal mechanisms we also found a high velocity area in the deep aseismic region below the volcanic front between the kanto and chubu regions it extends to the philippine sea plate in both regions it was likely to be an aseismic extended section of the philippine sea plate the existence of the aseismic philippine sea plate beneath the chubu region was confirmed there was another low velocity area beneath the active volcanoes high poisson s ratio areas were found both beneath the volcanoes and in the low velocity zone of the philippine sea plate near the volcanic front under the kanto region

crustal structure of the izu collision zone in central japan from seismic refraction data 1 the izu bonin arc iba has been colliding with the more northerly honshu arc since the middle miocene forming a globally unique active izu arc arc collision zone icz in central japan to determine crustal structure of the icz we constrain p and s wave velocities from active source refraction data the velocity structures reveal that the style of collisions of the misaka tanzawa and izu blocks which were derived from the iba has been controlled by subduction of the iba high velocity rocks vp 6 2 km s observed in the icz continuously extend to deeper crust suggesting that plutonic rocks formed by partial melting of the subducted iba crust intruded into the honshu arc crust and the accreted crustal blocks of the iba the northernmost major fault in the icz dips southeastward which is in clear contrast with other major northwestward dipping faults farther southeast in the northernmost part of the icz the upper 5 km crust of the misaka block was delaminated from its lower part and obducted onto the honshu arc the tanzawa block southeast of the misaka block is bounded by northwestward dipping faults and is characterized by crustal stacking a low velocity sedimentary layer between the tanzawa and izu blocks dips northwestward along the collisional boundary and reaches 5 km depth suggesting that the izu block underthrust beneath the tanzawa block the overall geometry of the collisional boundaries exhibits a doubly vergent system which is characterized by inward dipping thrust faults on both sides

sedimentary responses to the east asian monsoon and sea level variations recorded in the northern south china sea over the past 36 kyr abstract sediment grain size is an important proxy to detect sedimentary process and record paleoenvironmental variations in continental slopes grain size measurements were performed for 298 samples of the 5 96 m long core pc338 spanning the past 36 kyr recovered from the northern south china sea scs results show that sea level and climatic influences on grain size distributions are crucial a turbidite formed during the last glacial maximum lgm is ascribed to the favorable topography close to the red river estuary induced by the low sea level four end member em components for the normal sediment layers were unmixed using a lognormal parametric end member modeling emm with the sea level decreasing more em4 15 100 m and less em1 0 2 m supplied the scs slope because em2 2 10 m consisted of fluvial suspension is dominated by the east asian summer monsoon easm precipitation it has the potential to substitute for the easm intensity em2 varied in phase with many south china paleoclimate records and the 30 n july insolation over the past 36 kyr and cooling climate events including the lgm heinrich events h1 and h3 younger dryas 8 2 ka and 10 3 ka lowered the em2 content our results further confirm the synchronous easm evolution in the south china continent mainly forced by the northern hemisphere summer insolation

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

regional variation in surface wave group velocities in the philippine sea abstract group velocities of rayleigh and love waves have been measured within the period range 30 80 s to study the regional variation of seismic wave velocity structures of the crust and upper mantle beneath the philippine sea these group velocity measurements were made by applying the moving window method to long period seismograms recorded at six wwssn stations which are located around the philippine sea plate we divided the philippine sea region into seven small areas on the basis of tectonic features age and topography of ocean floor and determined the group velocities of the rayleigh and love waves in each region as a function of period the surface wave group velocities in the philippine sea plate are higher than those in tectonic regions such as the mariana arc the ryukyu japan arc and the philippine trench the low group velocities in tectonically active regions are attributable to the presence of an extremely low velocity layer in the upper mantle in the philippine sea plate the group velocities in the western region the west philippine basin are higher than those in the eastern region the shikoku parece vela basin this result is interpreted as being due to the difference in the lithospheric thicknesses of the two regions the lithospheric thickness estimated from the group velocity data of the rayleigh wave is about 55 km in the western region and 40 km in the eastern region the northern part of the west philippine basin covering the amami plateau the daito ridge and the oki daito ridge has a thick crust which is needed to explain the fact that the short period group velocities are lower than those in the western and eastern regions of the philippine sea plate these features are commonly found in the seismic velocity structures inferred from the group velocity data of rayleigh and love waves

crustal structure across the coseismic rupture zone of the 1944 tonankai earthquake the central nankai trough seismogenic zone 1 differences in the coseismic rupture process between the 1944 tonankai and the 1946 nankaido earthquakes have been studied by many fault models to understand what factors control coseismic rupture zones it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes the previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the neogene quaternary accretionary prism however little is known about the structure of the rupture zone of the 1944 earthquake to obtain a complete image of the seismogenic zone of the 1944 earthquake a wide angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land our model for the crustal structure is based on ocean bottom seismographic data the crustal structure appears characteristic for subducting oceanic crust and a neogene quaternary accretionary prism bounded by an island arc crust the neogene quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front the subducting oceanic crust can be traced down to 35 km the subduction angle becomes steeper landward reaching up to 11 beneath the island arc crust the depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km while the updip limit is located beneath the island arc upper crust similar structures of the updip and downdip limits are also published for several other subduction zones

deconvolution of u channel paleomagnetic data near geomagnetic reversals and short events 1 the deconvolution scheme of oda and shibuya 1996 designed for half core data from the pass through magnetometer aboard the joides resolution has been adapted for treatment of u channel data the program was tested using u channels from ocean drilling program odp site 1090 a significant increase in resolution of the u channel data can be achieved by deconvolution resulting in data comparable with that obtained by continuous discrete sampling large amplitude changes of the magnetization vector revealed by the deconvolution treatment are in good agreement with the results obtained on discrete samples extracted from the same u channels application of this procedure to u channel data from odp sites 983 and 984 over a short time interval around 1 255 ma permitted enhanced definition of the so called bjorn geomagnetic event

seismotectonics beneath the tokyo metropolitan area japan effect of slab slab contact and overlap on seismicity 1 we first determine the configuration of the upper surface of the pacific pac slab beneath kanto japan from the distribution of interplate earthquakes relocated by an appropriate 1 d velocity model then traveltime tomography is carried out to estimate three dimensional seismic velocity structures around kanto using 735 520 p wave and 444 049 s wave arrival times from 6508 local earthquakes the obtained results suggest that the philippine sea phs slab is subducting to depths of 130 140 km without a gap even to the northwest of the izu collision zone we subsequently define the lateral extent of the contact zone between the bottom of the phs slab and the upper surface of the pac slab phs pac interface and reveal that the slab contact zone underlies a wider area beneath kanto in harmony with the kanto plain the downdip limit of interplate thrust type earthquakes on the pac slab is deepened by 30 km locally under the slab contact zone this deepening is probably caused by a lower temperature environment in the pac slab resulting from the overlap with the phs slab subducting above and consequent thermal shielding by the phs slab from the hot mantle wedge we detect an extremely low velocity anomaly in the easternmost portion of the phs slab which is probably attributable to serpentinization of mantle peridotite interplate earthquakes are almost absent along the phs pac interface overlain by the serpentinized mantle in the phs slab suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

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

earthquake source fault beneath tokyo devastating earthquakes occur on a megathrust fault that underlies the tokyo metropolitan region we identify this fault with use of deep seismic reflection profiling to be the upper surface of the philippine sea plate the depth to the top of this plate 4 to 26 kilometers is much shallower than previous estimates based on the distribution of seismicity this shallower plate geometry changes the location of maximum finite slip of the 1923 kanto earthquake and will affect estimations of strong ground motion for seismic hazards analysis within the tokyo region

deep subduction of h2o and deflection of volcanic chain towards backarc near triple junction due to lower temperature in central japan near the triple junction of the pacific philippine sea and north american or ohotsuku plates the volcanic chain deflects towards backarc compared to the adjacent northeast japan and izu bonin arcs and lies 200 300 km above the wadati benioff zone numerical modeling shows that thermal recovery of the subducting pacific plate is slow due to the overlapping philippine sea plate which shifts the dehydration reactions to greater depths along the pacific plate and causes the magmatism above the deep wadati benioff zone the low geothermal gradient along the subducting pacific plate also implies that a considerable amount of h2o is carried further down by phase a which is formed just above the subducting plate without being released for magmatism central japan can be regarded as an entrance for h2o into the deep mantle

interaction between two subducting plates under tokyo and its possible effects on seismic hazards 1 underneath metropolitan tokyo the philippine sea plate phs subducts to the north on top of the westward subducting pacific plate pac new relatively high resolution tomography images the phs as a well defined subduction zone under western kanto plain as pac shoals under eastern kanto the phs lithosphere is being thrusted into an increasingly tighter space of the pac eurasian mantle wedge as a result zones of enhanced seismicity appear under eastern kanto at the top of phs internal to phs and also at its contact with pac these zones are located at depths greater than the causative fault of the disastrous 1923 great tokyo megathrust earthquake in the vicinity of several well located historical damaging m6 and m7 earthquakes thus a rather unique interaction between subducting plates under tokyo may account for additional seismic hazards in metropolitan tokyo

source to sink transport processes of fluvial sediments in the south china sea the south china sea offers an excellent case for studying source to sink transport processes of fluvial sediments among the global marginal seas this study synthesizes existing clay mineralogical and geochemical data from similar to 1500 samples from the seafloor and surrounding rivers deepwater mooring observation results and high resolution glacial cyclic clay mineralogy records from six high quality sediment cores source to sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales modern process and late quaternary glacial cycles the results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the south china sea e g dominant illite and chlorite in taiwan overwhelming majority of smectite in luzon dominant kaolinite in south china and similar amounts of the four clay mineral species in red and mekong river systems the formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the east asian monsoon climate with warm temperature and high precipitation and subordinately by tectonic activity and specific lithological character the basin wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects the differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the south china sea through combining clay mineralogical distributions on the seafloor with observed oceanic current systems the modern transport pathways can be well established e g in the northern south china sea illite and chlorite from taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies high resolution sediment dynamic analysis for the late quaternary reveals different sediment transport patterns in the northern western and southern south china sea when the land sea configuration dramatically changed during glacial conditions in the north the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport in the west the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds in the south the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods the transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern south china sea the relative contributions from three provenances south china luzon and taiwan highlight their distinct variability over the last 28 ka implying that the southward shift of the inter tropical convergence zone itcz at 16 ka bp caused an increased sediment contribution from south china the intensified influence of the kuroshio current intrusion controls more efficient westward transport of luzon sediments and the stronger deepwater current transports taiwan sourced sediments further westward from the last glaciation to the holocene c 2015 elsevier b v all rights reserved

recent progress of geophysical and geological studies of mt fuji volcano japan abstract mt fuji located in central japan near the triple junction of the philippine sea eurasia or amurian and north american or okhotsk plates is one of the arc volcanoes associated with the subduction of the pacific plate mt fuji has unique features including an average eruption rate in the last 100 000 years of 4 6 km3 ka which is much higher than that of other volcanoes along the same arc 0 01 and 0 1 km3 ka basaltic rocks dominate in mt fuji while other arc volcanoes are dominated by intermediate and felsic compositions and mt fuji has erupted both explosively and effusively with the two largest eruptions in the last 2000 years having different styles the 864 866 ce jogan eruption was effusive while the 1707 hoei eruption the most recent eruption was explosive mt fuji is not only interesting from a scientific point of view but also important from a societal point of view because it is only 100 km from the tokyo metropolitan region one of the biggest cities in the world hosting more than 30 million people the hoei eruption resulted in more than a few tens of millimeters of ashfall in tokyo by more than 150 mm with this background mt fuji has been studied intensively using geological and geophysical methods to understand the evolution magma plumbing system and current activity of the volcano this article synthesizes the current knowledge about mt fuji in particular we provide background information required to understand the jogan and hoei eruptions and discuss how to assess the size timing and style of the next eruption statistical insights suggest that the next eruption of mt fuji is more likely to be voluminous with a long precursor given a repose time of more than 300 years

records of the cobb mountain subchron from the bermuda rise odp leg 172 abstract duplicate records of the cobb mountain subchron were obtained from two holes holes 1063b at 33 41 204 n 57 36 w and 1063c at 33 41 181 n 57 36 w drilled in a sediment drift at ocean drilling program odp site 1063 on the northeast bermuda rise we sub sampled the cores from both holes using u channels up to 1 5 m long samples with 2 2 cm cross section focusing on continuous sampling through the late matuyama interval containing the cobb mountain subchron results of standard progressive alternating field demagnetization experiments reveal that these sediments generally have a stable characteristic component after removal of a low coercivity drilling overprint additional discrete samples 7 cm 3 oriented cubes used in progressive thermal demagnetization studies display similar behavior by taking duplicate sets of u channels we were able to identify and remove several measurement artifacts such as edge effects that affect measurements made near the ends of the u channel samples the upper normal to reverse polarity transition was not obtained because of a coring gap or an interval of coring deformation after removing measurement artifacts the records from the two holes were stacked to obtain a composite record virtual geomagnetic poles vgps computed from the composite record follow a path through the central pacific during the lower polarity transition very similar to vgp paths from cobb mountain subchron records from other sites around the world furthermore the vgps cluster near southern africa early in the reversal process and then group in a nearly antipodal patch in the north pacific late in the reversal taken together these records indicate that the transitional field remained nearly dipolar through the polarity reversal or at least during the beginning and end of the reversal when the field was dominated by an axial dipole tilted about 20 40 to the spin axis if a tilted dipole is an inherent part of the initiation and ending of a reversal it may help explain why many sediment recorders which are thought to smooth the field record antipodal vgp paths through many reversals

dual subduction tectonics and plate dynamics of central japan shown by three dimensional p wave anisotropic structure abstract the central japanese subduction zone is characterized by a complex tectonic setting affected by the dual subduction of oceanic plates and collisions between the island arcs to better understand of the subduction system we performed an anisotropic tomography analysis using p wave arrival times from local earthquakes to determine the three dimensional structure of p wave azimuthal anisotropy in the overriding plate and the pacific and philippine sea phs slabs the principal characteristics of anisotropy in the subducted and subducting plates are 1 in the overriding plate the distribution pattern of fast direction of crustal anisotropy coincides with that of the strike of geological structure 2 in the two oceanic plates fast propagation directions of p wave were sub parallel to the directions of seafloor spreading additionally our tomographic images demonstrate that 1 the bottom of the median tectonic line the longest fault zone in japan reaches to the lower crust and seems to link to the source region of an inter plate earthquake along the phs slab 2 the segmentation of the phs slab the izu islands arc the nishi shichito ridge and the shikoku basin due to the formation history is reflected in the regional variation of anisotropy the tomographic study further implies that there might be a fragment of the pacific slab suggested by a previous study beneath the tokyo metropolitan area the overall findings strongly indicate that seismic anisotropy analysis provide potentially useful information to understand a subduction zone

izu bonin arc subduction under the honshu island japan evidence from geological and seismological aspect abstract the izu bonin intra oceanic arc with 20 35 km thick continental crust is being subducted under the honshu presumably since 17 ma tomographic image clearly demonstrates that the whole izu bonin arc is subducting under the honshu arc geologic cross section and the thickness of continental crust do not support the accretion of thick crust in spite of the continued subduction over 17 ma

low sea level emplacement of a very large late pleistocene megaturbidite in the western mediterranean sea large volume turbidites termed megaturbidites or megabeds result from catastrophic slope failures and the associated downslope transport of enormous quantities of sediment from continental margins to the deep sea such large sediment failures can generate tsunamis and in terrains underlain by gas hydrates clathrates may be associated with the release of substantial amounts of the greenhouse gas methane it has been proposed that the megaturbidite events may be triggered by seismic activity or may result from gas hydrate release itself caused by a lowering of hydrostatic pressure on clathrates as a result of low sea level previous conclusions on the significance of sea level change however have been conditional because of the lack of absolute times of turbidite emplacement here we use accelerator mass spectrometry radiocarbon dating in five widely spaced cores to constrain the date of emplacement of a large volume 500 km3 bed in the balearic basin of the western mediterranean this turbidite is exceptional in its magnitude and represents the main sedimentation event in the balearic basin over the past 100 kyr our data provide an estimate of 22 000 calendar years before present for emplacement of the megabed a time when sea level stood at its lowest level during the last glacial maximum the coincidence of these dates is consistent with emplacement due to clathrate destabilization caused by low sea level although other triggering mechanisms such as seismic shock cannot be ruled out

seismic waves guided by untransformed oceanic crust subducting into the mantle the case of the kanto district central japan a model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the kanto district central japan a distinct pair of later p and s phases xp and xs commonly observed at specific stations for these events in a depth range of 40 60 km and is considered to take place in association with the subducting motion of the philippine sea plate the xp phase is found from its particle motions to be a longitudinal seismic wave a principal component analysis of wavelets including the initial p and the xp phases shows that the latter arrives at the earth s surface with a slightly shallower angle relative to the former close agreement between the relative arrival time ratio of xs s xp p and the vp vs ratio of crustal or mantle rocks suggests that both xp and xs travel along the same path as longitudinal and tangential waves respectively one can clearly observe these later phases at stations located where the upper surface of the philippine sea plate is apparently in direct contact with the bottom of the crust of the honshu island apparent velocities of the xp and the xs phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous travel time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region the hypocentral distribution of the events related to the subducting motion of the philippine sea plate does not form such a thin seismic zone as observed in southwest japan but extends over the entire thickness of the philippine sea plate the earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone the deeper portion does not generate such events these characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the philippine sea plate taking the result of our earlier work into account we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the philippine sea plate

three dimensional q structure beneath the kanto tokai district japan the q structure of the upper mantle beneath the kanto tokai district central japan where the eurasian pacific and philippine sea plates are converging is investigated from seismic p and s wave spectral ratios by means of a three dimensional 3 d inversion method it is found that the subducting philippine sea and pacific plates are characterized by high q zones a high q zone also exists between the philippine sea plate and the pacific plate to the east of the volcanic front on the other hand a low q zone in the uppermost mantle extends westward horizontally from the volcanic front under the philippine sea plate and appears to exist over the wedge mantle beneath the western tokai district a very low q value obtained here is roughly equal to the average qvalue in the low velocity layer derived from surface wave and free oscillation data a low q region that suggests a magma chamber under active volcanoes is not detected in the crust

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

cause of m 7 intraslab earthquakes beneath the tokyo metropolitan area japan possible evidence for a vertical tear at the easternmost portion of the philippine sea slab 1 we perform travel time tomography beneath kanto japan to obtain detailed structures of the wedge shaped serpentinized mantle previously observed in the philippine sea slab the results suggest that the western boundary of the serpentinized mantle serpentine boundary is subvertical and p and s wave velocities vary by 15 20 across it over the short distance of 10 km two intraslab earthquakes in 1921 m7 0 and 1987 east off chiba earthquake m6 7 are inferred to have occurred along the serpentine boundary accompanied by right lateral movement based on analyses of focal mechanisms and aftershock distribution a subvertical earthquake cluster penetrating the entire philippine sea slab is observed along the serpentine boundary and four earthquakes in the cluster have strike slip focal mechanisms similar to that of the 1987 earthquake focal mechanisms obtained for past large earthquakes and present day microearthquakes suggest a concentration of right lateral deformation along the mechanically weak serpentine boundary the philippine sea slab may have been torn in two along this boundary with the eastern portion being left behind relative to subduction of the western portion assuming that one of the large aftershocks m7 1 of the 1923 kanto earthquake m7 9 which occurred off the boso peninsula ruptured the same asperity as did the 1987 earthquake the slip deficit accumulated along the serpentine boundary during the 64 year interval is consistent with the fault slip of the 1987 earthquake interaction between the seismic slip along the plate interface and the serpentine boundary can explain the series of m7 class earthquakes before and after the 1923 kanto earthquake

a sedimentary paleomagnetic record of the matuyama chron from the western antarctic margin odp site 1101 a high resolution paleomagnetic record for part of the matuyama chron 0 7 2 1 ma is reported for ocean drilling program site 1101 leg 178 off the antarctic peninsula pacific margin a rock magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization nrm is carried by pseudo single domain magnetite progressive alternating field demagnetization of 83 m of u channels provided a polarity stratigraphy down to the olduvai subchron two geomagnetic events preceding the jaramillo subchron were identified including the cobb mountain polarity interval the bulk magnetic parameters vary by more than a factor of 20 over the entire time interval but by less than a factor of 6 over the 0 7 1 1 ma interval this latter interval was selected for paleointensity determinations which were carried out by normalizing the nrm by the anhysteretic remanent magnetization arm direct comparison of the site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record a more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0 95 1 1 ma interval this test yielded no outlier for the period considered confirming the geomagnetic character of the records we have constructed a lowresolution stack revealing some of the characteristic paleointensity features of the jaramillo subchron fl 2001 elsevier science b v all rights reserved

fall in earth s magnetic field is erratic earth s magnetic field has decayed by about 5 per century since measurements began in 1840 directional measurements predate those of intensity by more than 250 years and we combined the global model of directions with paleomagnetic intensity measurements to estimate the fall in strength for this earlier period 1590 to 1840 a d we found that magnetic field strength was nearly constant throughout this time in contrast to the later period extrapolating to the core surface showed that the fall in strength originated in patches of reverse magnetic flux in the southern hemisphere these patches were detectable by directional data alone the pre 1840 model showed little or no evidence of them supporting the conclusion of a steady dipole up to 1840

paleomagnetic transition records of the cobb mountain event from sediments of the celebes and sulu seas the cobb mountain cm event is a short normal polarity interval within the matuyama chron just before the jaramillo subchron this event was found in the sediments of site 767 in the celebes sea and site 768 in the sulu sea during odp leg 124 reversal depth and age relation analysis gave ages of 1 11 and 1 10 m y ago for the onset and termination of this event we used discrete samples and high density pass through measurements to study the behavior of the geomagnetic field crossing this event although the two sites are very close to each other the records of the transitional fields were found to be quite different they are also very different from another cm event record from the north atlantic ocean based on the transition data we conclude that the transitional fields for the cm event were non axisymmetric one brief reverse in direction called rebound was found within the onset reversal at site 767 the absence of this feature at site 768 raises an interesting question concerning the source for rebounds

a slab fragment wedged under tokyo and its tectonic and seismic implications a three dimensional evaluation of earthquake hypocentres beneath the kanto basin in japan reveals the presence of a distinct 25 km thick and 100 km wide body its fast seismic velocity and the presence of a double seismic zone suggest that it is a fragment of the pacific slab rather than an extension of the philippine sea slab this implies that the penetration of the philippine sea slab is much shallower beneath the kanto basin than was previously thought the philippine sea and pacific plate slabs both subduct beneath tokyo and so their configuration and seismic potential have been subject to intensive study1 2 3 4 5 6 7 8 previous work suggests that the philippine sea slab extends up to 100 km northwest of tokyo and subducts to a depth of 90 km beneath the kanto basin where it is folded against the underlying pacific slab here we evaluate seismic data in three dimensions and delineate a distinct 25 km thick and 100 km wide body beneath the kanto basin that has hitherto been considered to be part of the philippine sea slab we find that several of its characteristics such as its high seismic velocity and the presence of a double seismic zone point instead to an affinity with the pacific slab implying that the philippine sea slab penetrates to depths of only 35 40 km beneath the kanto basin we propose that the body is a fragment of pacific lithosphere that formed by the collision of two intersecting seamount chains with the japan trench 2 3 million years ago we suggest that such slab fragments may not be uncommon especially where seamount subduction deforms trenches and near triple junctions

the south fossa magna japan revealed by high resolution p and s wave travel time tomography abstract detailed tomographic images of the collision zone between the northern edge of the philippine sea and the eurasian plates reveal a high correlation between tectonic features inferred from seismicity and p and s wave velocity structures the linear tomographic inversion covered a 150 150 60 km region in the south fossa magna centered on the northern izu peninsula thirty three three component stations were located in the target region and 3823 high quality earthquakes were selected from the catalogues of the nriesdp giving rise to 53 593 p and 50 059 s wave phase arrivals used in the inversion the model was parameterized by 60 60 10 rectilinear blocks measuring 2 5 km per side horizontally and 5 10 km varying thicknesses in depth three dimensional perturbations from the one dimensional nriesdp layered model were derived by minimizing the squared misfit of the travel time residuals regularization was employed to reduce the effects of noisy data by constraining the two dimensional laplacian of the model within horizontal layers to be small the large sparse matrix was solved using the conjugate gradient algorithm lsqr reduction of misfit was 50 for the p wave inversion and 57 for the s wave inversion there is a high correlation between the p and s wave inversions between 5 15 km depth both 3d images indicate low velocity anomalies trending northward in the west along the fujikawa from suruga bay to northwest of mt fuji north of izu peninsula there is a broad high velocity zone these results correlate with bouguer gravity anomalies previously observed starting at 5 km depth a high velocity northeast dipping anomaly beneath sagami bay is inferred to be the shallow subducting phs plate to the north a high velocity zone in the vicinity of the east yamanashi seismic swarm 15 32 km depth is clearly observed on the p wave inversion and to a lesser extent on the s wave inversion because of the high seismicity in this region we speculate this is the zone where the phs plate is grazing the eur plate to the northwest and the high velocity anomaly represents a large scale asperity to the west below thmt fuji area a broad low velocity is observed between 15 and 32 km depth we interpret the deep low velocity structure which exhibits little seismicity to be either the subduction of the volcanic arc or a region of elevated temperature associated with hot rising magma near the swarm area on the eastern edge of izu peninsula low velocity anomalies are observed near the surface on both p and s wave inversions along the eastern part of izu peninsula a prominent low velocity anomaly is observed between 10 and 20 km on the s wave image but is less pronounced on the p wave inversion

paleointensity record from pleistocene sediments 1 4 0 ma off the california margin high resolution magnetic measurements of 45 m of sediment from the delgada fan area at ocean drilling program site 1021 39 5 n 127 46 w were performed in order to extract a reliable signal of relative paleointensity a detailed chronology was established by correlation of the magnetic susceptibility variations to the insolation curve for the past 1 4 myr covered by the record the remanence carrier is pseudo single domain sized magnetite that varies in concentration by a factor 3 stepwise alternating field demagnetization isolated a very stable natural remanent magnetization nrm which showed the succession of the cobb and jaramillo subchrons and directional changes associated with the punaruu event 1 105 ma two techniques have been used to extract the signal of relative paleointensity a significant climatic component remained present in the curve obtained from the slopes of the nrm versus the anhysteretic remanent magnetization arm in contrast the nrm and the susceptibility k lf both being driven by orbital forcing normalization by susceptibility is not contaminated by climatically driven changes and is thus taken as our best estimate of relative paleointensity comparison with other high resolution records and compilations reinforces the actual geomagnetic origin of the variations displayed by the nrm k signal overall this record confirms the succession of distinct highs and lows which are concomitant with full field reversals and short term geomagnetic events present during the matuyama chron the presence of large oscillations of the field with no periodicity confirms that the geomagnetic field at least his dipolar part is highly unstable and may have remained in this mode for a long period of time

last glacial maximum in china comparison between land and sea abstract as a result of the sea level drop at the last glacial maximum about 100 120 m in the south china sea and 130 150 m in the east china sea the size and configurations of the china seas changed greatly the east china sea with the yellow sea and bohai gulf was reduced into an elongated trough while the south china sea became a semi enclosed gulf due to the southern shift of the polar front in the north pacific and the reorganization of the surface current system the winter temperature in the south china sea was 6 10 c colder and the seasonality was much stronger than it is now the reduced sea area and thus the increased distance to the coast together with the declined sea surface temperature led to intensified aridity on the land the enhanced eolian processes related to the intensification of the winter monsoon caused an expansion of the loess deposition area reaching the southeastern part of china to the south of the yangtze river and off the modern sea coast vegetation zones greatly shifted to the southeast in north china deserts and steppes were much more extensive than they are now and the modern areas of warm temperate broad leaved forests were occupied by steppes or woodland in south china the deciduous broadleaved forests were developed in areas now covered by evergreen forests and the monsoon rain forest completely disappeared the changes in the climate on land and the configurations of seas altered the sediment supply and sedimentation rate in the china seas giving rise to the atlantic type late quaternary caco3 curve there

detection of the subducting crust of oceanic plates beneath the kanto district japan abstract low velocity crustal layers on the top of the subducting slabs beneath the kanto district central japan were detected by a travel time inversion using 40 763 p wave arrival times from 3038 local earthquakes recorded by a local network of 41 seismic stations the inversion covered 188 228 110 km volume of the target region and the model was parameterized by 47 57 18 rectilinear blocks measuring 4 0 km per side horizontally and 4 10 km varying thickness in depth a prominent e w directed low velocity band of about 25 km width appeared in the central kanto region at a depth of 30 km moving north or northwestward gradually with depth this band can be traced to the depth of about 60 km at a depth of 60 km a slightly lower amplitude nearly n s directed low velocity band was observed at the east coast of the kanto region moving westward with depth these low velocity bands are thin layers attached to the top of the inclined thick layers with relatively high velocity in the kanto district the philippine sea phs plate subducts under the eurasian eur plate and the pacific pac plate also underthrusts beneath both the eur and phs plates some authors already suggested low velocity anomalies near the surface of those descending plates in this region mainly from the analyses of later phases and focal mechanisms of local earthquakes the shapes of the low velocity bands are similar to the depth distribution of the upper boundary of the phs and pac plates proposed by other authors the thrust earthquakes that occur at the boundaries between the eur and phs plates and between the phs and pac plates are almost located in the low velocity layers described above we conclude that these low velocity layers are the crusts that exist at the top of the subducting phs and pac plates while high velocity layers are subducting slabs themselves for the phs plate low density crust of the izu bonin arc is probably stacking at the range from 139 25 e to 139 55 e at 35 55 n which is observed as a thick low velocity region to the south of the range and in a steep inclination to the north of it this is also recognized from the gravity anomaly

relative paleointensity rpi and oxygen isotope stratigraphy at iodp site u1308 north atlantic rpi stack for 1 2 2 2 ma narpi 2200 and age of the olduvai subchron 2015 elsevier ltd integrated ocean drilling program iodp site u1308 49 53 n 24 14 w water depth 3871 m provides a record of relative paleointensity rpi and benthic stable isotope stratigraphy back to 3 2 ma the record since 1 5 ma was published previously and here we present the interval from 1 5 ma to 3 2 ma early pleistocene late pliocene the benthic oxygen isotope record in this interval is correlated to marine isotope stage mis 51 to km2 with an apparent hiatus that removed part of the interval spanning mis 104 g2 2 6 2 65 ma at the gauss matuyama boundary the mean sedimentation rate for the 1 5 3 2 ma period is 8 5 cm kyr the age model was built by correlation of the benthic oxygen isotope record to a global stack lr04 apart from the expected polarity reversals three magnetic excursions are recorded punaruu in mis 31 32 at 1092 ka gilsa in mis 54 55 at 1584 ka and a newly recognized excursion labeled porcupine after the nearby porcupine abyssal plain in mis g6 g7 at 2737 ka the ages of polarity reversals at site u1308 on the lr04 time scale are consistent with the current geomagnetic polarity timescale gpts with the exception of the base of the olduvai subchron that occurred in mis 73 corresponding to 1925 ka on the lr04 time scale 25 kyr younger than in the current gpts the rpi record at site u1308 is calibrated using the oxygen isotope age model and combined with four other north atlantic records to obtain a north atlantic rpi stack for 1 2 2 2 ma narpi 2200 that is compared with published rpi stacks epapis sint 2000 and padm2m for 2 2 3 2 ma the site u1308 rpi record is compared with a rpi record from north atlantic iodp site u1314 and with the pacific epapis stack the mean sedimentation rates of the north atlantic sites in narpi 2200 are greater by about an order of magnitude than most of the records incorporated in other stacks the comparison of epapis and narpi 2200 yields an apparent lag for narpi 2200 relative to epapis attributed the epapis age model constructed by correlation of magnetic concentration parameters a proxy for carbonate percentage to a calibrated oxygen isotope record the long rpi record from site u1308 yields a very similar mean value for the brunhes and matuyama virtual axial dipole moments 7 05 1022 am2 implying no polarity bias in the strength of the main geomagnetic dipole in contrast to interpretations from sint 2000 and padm2m the results strengthen the case that rpi can be used to improve global stratigraphic correlation for sites with mean sedimentation rates up to several decimeters kyr

early interglacial carbonate dilution events in the south china sea implications for strengthened typhoon activities over subtropical east asia abstract a compilation of many late quaternary marine sediment records from the northern south china sea scs continental slope confirms 15 50 reductions in sedimentary calcium carbonate concentrations between 11 0 and 8 5 ka bp in the early holocene this low carbonate event occurred at a time when the regional sea level rose from 50 m to 10 m which drowned large areas of continental shelves especially those near and within the taiwan strait this event is associated with a significant increase in bulk sedimentation rates on the upper continental slope and the relative abundance of fine grained detritus sediment provenance analyses suggest a dominant terrigenous input from taiwan and a minor contribution from luzon during the low carbonate event similar to the background terrigenous deposition in other periods of the holocene two comparable low carbonate events respectively from the beginning of marine isotope stages 5 5 and 7 3 have also been recognized pointing to similar causal factors while carbonate dissolution and carbonate accumulation rate should not have been responsible increased terrigenous input and dilution is considered as the main cause for the recurrent low carbonate events in early interglacials we further hypothesize that during early interglacials fluvial sediment discharge from taiwan and luzon intensified due to stronger typhoon activities and massive fine grained sediments from these two end members may have been transported to the northern scs continental slope via surface and deep ocean currents the conjecture of strengthened typhoon activities over east asia during the early holocene is supported by high ocean heat contents in the west pacific warm pool area with the prevailing la nina like conditions

40ar 39ar chronology and paleomagnetism of quaternary basaltic lavas from the persani mountains east carpathians abstract quaternary volcanism in the persani mountains forms an na alkali basaltic province inside the bend area of the carpathians in the southeastern part of europe previous k ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles which were tentatively associated with the cobb mountain subchron and a brunhes chron excursion we report a new paleomagnetic and rock magnetic study coupled with 40ar 39ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity 19 are normal polarity and 16 have transitional polarity 40ar 39ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142 41 and 800 25 ka four of the normally magnetized lavas erupted at 1060 10 1062 24 684 21 and 683 28 ka and two transitionally magnetized lavas formed at 1221 11 and 799 21 ka both the new 40ar 39ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the jaramillo and brunhes chrons this results shows that the last phases of alkalic and calc alkaline magmatism in the south east carpathians were contemporaneous the age of the older transitionally magnetized lava flow is within error of recent unspiked k ar and astrochronologic ages for the reversal that defines the onset of the cobb mountain normal polarity subchron the age of the younger transitional lava is similar to that of an excursion that preceded the matuyama brunhes polarity reversal and which has come to be known as the matuyama brunhes precursor omitting the excursion data the dispersion of the virtual geomagnetic poles around 19 is larger than the expected value around 45 n from the global compilation but closer to the value obtained only from the time averaged geomagnetic field initiative studies

diagenetic alteration of magnetic minerals in labrador sea sediments iodp sites u1305 u1306 and u1307 1 in order to reveal the potential effects of early diagenesis on magnetic minerals in deep sea sediments we studied early diagenetic zones and magnetic mineral characteristics of lower pliocene hemipelagic sediment samples from iodp sites u1305 u1306 and u1307 on eirik drift labrador sea all samples analyzed were unlithified silty clay sediments recovered by a piston corer from depths down to 200 meters composite depth mcd based on shipboard interstitial water geochemistry we divided the sediment column from each site into six early diagenetic zones magnetite fe3o4 was present at all analyzed depths whereas maghemite fe2o3 was found only above the iron reduction zone we attribute this to associated changes in interstitial redox conditions which induced preferential dissolution of maghemitized surfaces on magnetite grains mineral magnetic results indicate a general down hole change in mean grain size of magnetic minerals at site u1307 which has relatively low organic carbon contents the diagenetic zones occur at greater depths than at the other studies sites this suggests that interstitial oxygen levels at this site remained high enough to degrade organic matter through oxic bacterial activity and that detrital magnetic minerals have been preserved even at depth

age through tandem correlation of quaternary relative paleointensity rpi and oxygen isotope data at iodp site u1306 eirik drift sw greenland abstract planktic oxygen isotope 18 o and relative paleointensity rpi data are used in tandem to generate an age model for the last 1 myr from integrated ocean drilling program iodp site u1306 drilled on the crest of the eirik drift sw greenland in 2272 m water depth for the 1 1 5 ma interval the age model is based on rpi alone due to insufficient foraminifera for isotope analyses utilizing rpi and 18 o in tandem allows recognition of low 18 o events prior to glacial terminations i iii iv v vii viii ix and x that are independently supported by radiocarbon dates through the last deglaciation and are attributed to local or regional surface water effects at site u1306 quaternary sedimentation rates mean 15 cm ka are elevated during peak glacials and glacial onsets and are reduced during interglacials in contrast to the pattern at site u1305 in 3460 m water depth at the distal toe of the drift 191 km sw of site u1306 the contrasting sedimentation rate pattern appears to hold for the entire 1 5 myr record the slackening and or shoaling due to lowered salinity of the deep western boundary current dwbc during glacial intervals coincided with greater sediment supply to site u1306 whereas the deepening and possibly increased vigor of the dwbc during interglacial intervals boosted sediment supply to site u1305

quaternary time scales for the pontocaspian domain interbasinal connectivity and faunal evolution the pontocaspian black sea caspian sea region has a very dynamic history of basin development and biotic evolution the region is the remnant of a once vast paratethys sea it contains some of the best eurasian geological records of tectonic climatic and paleoenvironmental change the pliocene quaternary co evolution of the black sea caspian sea is dominated by major changes in water lake and sea levels resulting in a pulsating system of connected and isolated basins understanding the history of the region including the drivers of lake level and faunal evolution is hampered by indistinct stratigraphic nomenclature and contradicting time constraints for regional sedimentary successions in this paper we review and update the late pliocene to quaternary stratigraphic framework of the pontocaspian domain focusing on the black sea basin caspian basin marmara sea and the terrestrial environments surrounding these large mostly endorheic lake sea systems

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

40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited new 40 ar 39 ar age determinations on sanidine from the rhyolite of alder creek california indicate a 1 186 0 006 ma age for the cobb mountain normal polarity subchron the new age is statistically older 0 05 than the previously reported k ar age 1 12 0 02 ma and agrees with the age suggested by the astronomical polarity time scale incomplete extraction of radiogenic 40 ar 40 ar from the sanidine is the most likely reason for the disparity between the 40 ar 39 ar and k ar ages because the cobb mountain subchron is a worldwide short duration event and because no widely used interlaboratory 40 ar 39 ar standard younger than 27 ma exists we propose that sanidine from the rhyolite of alder creek be considered for use as a new quaternary 40 ar 39 ar mineral standard

paleomagnetic record at odp site 980 feni drift rockall for the past 1 2 myrs 1 a detailed record of variations in the direction and intensity of the geomagnetic field over the past 1 2 myrs has been obtained from an 120 m thick sedimentary section cored at odp site 980 on the feni drift north atlantic the record has high resolution due to high mean sedimentation rates 11 3 cm kyr for the brunhes chron and 5 5 cm kyr for the matuyama chron paleomagnetic measurements every 1 cm along u channel samples and high quality isotopic age control the iceland basin event is manifest by virtual geomagnetic poles crossing the equator at 190 ka the base of the recovered section lies immediately below the cobb mountain subchronozone that occurs within marine isotope stage 35 normalized remanence data can be correlated to paleointensity records from odp sites 983 and 984 700 km to the nw from site 980 and with lower resolution paleointensity data from the pacific ocean differences between the site 980 paleointensity record and the pacific records are attributed to variable sedimentation rates variable quality age control and inadequacies in the normalization procedure used to derive the paleointensity proxies

seismic activity associated with the subducting motion of the philippine sea plate beneath the kanto district japan abstract the pacific plate and the philippine sea plate overlap and subduct underneath the kanto region central japan causing complex seismic activities in the upper mantle in this research we used a map selection tool with a graphic display to create a data set for earthquakes caused by the subducting motion of the philippine sea plate that are easily determined as a result we determined that there are at least four earthquake groups present in the upper mantle above the pacific plate major seismic activity group 1 has been observed throughout the kanto region and is considered to originate in the uppermost part of mantle in the subducted philippine sea plate judging from the formation of the focal region and comparison with the 3d structure of seismic velocity the focal mechanism of these earthquakes is characterized by the down dip compression a second earthquake layer characterized by down dip extension group 2 below the earthquakes in this group is also noted the focal region for those earthquakes is considered to be located at the lower part of the slab mantle and the pacific plate located directly below is considered to influence the activity earthquakes located at the shallowest part group 3 form a few clusters distributed directly above the group 1 focal region judging from the characteristics of later phases in these earthquakes and comparing against the 3d structure of seismic velocity the focal regions for the earthquakes are considered to be located near the upper surface of the slab another earthquake group group 4 originates further below group 2 it is difficult to consider these earthquakes within a single slab the seismic activities representing the upper area of the philippine sea plate are group 3 this paper proposes a slab geometry model that is substantially different from conventional models by strictly differentiating the groups

a quantitative comparison of two paleomagnetic records of the cobb mountain subchron from north atlantic deep sea sediments we present a new paleomagnetic record of the cobb mountain subchron obtained from deep sea sediments cored at ocean drilling program site 647 in the southern labrador sea the details of the transitional field behavior documented by this record appear to be very similar to those recorded in a previously published record of this subchron obtained at deep sea drilling project dsdp site 609 in the north atlantic clement and kent 1987 we used a quantitative correlation technique martinson et al 1982 to establish statistically the degree of similarity between these two records and thereby constrain the spatial variability in these transitional fields the error in the alignments is reduced significantly by aligning records of virtual geomagnetic pole positions rather than directions indicating that these records document such large scale changes in the fields that we can not distinguish them from dipolar changes given the proximity of these two sites these replicate records of the cobb mountain subchron provide evidence that deep sea sediments are capable of providing high resolution records of geomagnetic field behavior a reexamination of the sequence of polarity transitions recorded at dsdp site 609 in light of these results suggests the presence of two preferred transitional field configurations the field appears to change from one configuration to the other for several reversals and then back to the original configuration suggesting that a geographical influence on the reversal process persists through this sequence the variability in these reversal records provides insights into the response of the geodynamo to this geographical influence

geometry and relative motion of the philippine sea plate and pacific plate beneath the kanto tokai district japan the recent acquisition of high density and improved seismic data by the kanto tokai k t observational network of the national research institute of earth science and disaster prevention requires the revision of conventional plate configuration models for the k t district we propose a new interpretation of the configuration and relative motion of the philippine sea phs plate pacific pac plate and eurasian eur plate on the basis of the distribution of hypocenters velocity structure and focal mechanisms in the k t district the new model for the configuration of the phs and pac slabs clearly delineates the phs slab subducting northwestward beneath the k t district from the sagami and suruga troughs and the pac slab subducting westward beneath the phs slab from the japan trench the thickness of the descending phs slab is estimated to be 30 5 km the proposed configuration of the phs slab is significantly different from those obtained by other researchers including our own previous study the new model is fully consistent with global plate motion data the new model leads to an interpretation of the tectonic processes taking place beneath the k t district for the sagami trough wing of the phs slab the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the pac slab the southwestern portion carrying the izu peninsula has been bent upward because of its buoyancy producing the observed contorted shape for the suruga trough wing the dip direction of the phs slab gradually changes from northwestward at the eastern portion to northward at the western portion the new model also indicates the presence of offset within the phs slab between the sagami and suruga trough wings this area is characterized by a low level of seismic activity the absence of a high velocity structure and the presence of quaternary volcanoes it seems reasonable to speculate that the distribution of quaternary volcanic edifice construction is controlled by the location of the offset within the phs slab in view of this new model the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined

quaternary magnetic and oxygen isotope stratigraphy in diatom rich sediments of the southern gardar drift iodp site u1304 north atlantic abstract the sediment sequence from integrated ocean drilling program iodp site u1304 53 03 40 n 33 31 78 w water depth 3024 m on the southern gardar drift north atlantic ocean covers the last 1 8 myr with a mean sedimentation rate of 15 cm kyr at site u1304 paleomagnetic directional and relative paleointensity rpi records have been generated for the last 1 5 myr and benthic oxygen isotope data for the last 1 myr the age model for site u1304 was established by matching 18o and rpi data to calibrated reference records prominent intervals of diatom mats at site u1304 are associated with the latter stages of interglacial marine isotope stages mis 9 11 13 15 21 27 and 35 and with the weak glacial of mis 14 the deposition of diatom mats and associated diluted magnetic mineral concentrations leads to weak magnetizations and susceptibilities in these intervals comparison of histograms of paleomagnetic directions and rpi from diatom rich sediments and from surrounding silty clays indicates that although results from diatom rich sediments are more scattered the occurrence of diatom mats does not appreciably distort the paleomagnetic directional and rpi records site u1304 sediments record the matuyama brunhes boundary 772 ka the jaramillo subchron 993 1071 ka and the cobb mountain subchron 1193 1219 ka as well as three apparent geomagnetic excursions in the matuyama chron interpreted as the kamikatsura santa rosa punaruu and gardar excursions at 888 ka 1124 ka and 1463 ka respectively the site u1304 rpi record can be correlated with the piso 1500 rpi stack and with other high resolution rpi records from the north atlantic ocean wavelet analyses performed on the site u1304 rpi record and a new high resolution north atlantic paleointensity stack for the last 1500 kyr hinapis 1500 comprising ocean drilling program odp sites 983 984 and iodp sites u1306 and u1304 did not reveal significant orbital power

evidence for dipolar fields during the cobb mountain geomagnetic polarity reversals recent palaeomagnetic studies1 4 of geomagnetic polarity reversals have presented conflicting interpretations of the behaviour of the earth s magnetic field during reversals some have argued that the grouping of transitional virtual geomagnetic poles vgps suggests the presence of large scale perhaps even dipolar symmetries during some reversals1 3 whereas others have claimed that the available data do not support the interpretation of simple transitional field geometries4 an earlier comparison5of two north atlantic sedimentary records of reversals bounding the cobb mountain subchron 1 1 myr ago with a volcanic record from tahiti6 revealed striking similarities in the sequences of transitional vgps suggesting the presence of large scale sym metries in these fields here i augment this comparison with two new records of the cobb mountain subchron from the western pacific7 which greatly extend the geographical and temporal coverage these records exhibit sequences of vgp positions that are very similar to those observed in the north atlantic and tahiti providing evidence of dipolar transitional fields during the cobb mountain reversals

climatic and tectonic controls on weathering in south china and indochina peninsula clay mineralogical and geochemical investigations from the pearl red and mekong drainage basins 1 results of clay mineralogy major element geochemistry and sr and nd isotopes in 93 argillaceous samples collected from drainage basins of the pearl red and mekong rivers reveal different degrees of chemical weathering in southeast asia despite similar climate conditions across these regions the kaolinite illite ratio illite chemistry index and illite crystallinity can be used as indicators of chemical weathering intensity these mineralogical proxies combined with the k2o na2o cao molar ratio chemical index of alteration cia and weathering trends observed from major element results indicate intensive silicate weathering in the pearl river basin moderate to intensive in the mekong river basin and moderate in the red river basin although a significant modification of epsilon nd 0 values in our riverine sediments during chemical weathering and transport is unlikely sr 87 sr 86 ratios are controlled by various states of chemical weathering of high sr minerals such as plagioclase rich in na and ca with a linear decrease trend from the pearl mekong to red river basins our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south china and indochina peninsula strong physical erosion caused by tectonic activities and river incision along the eastern margin of the tibetan plateau and along the red river fault system is responsible for high contents of primary minerals in the lowlands of red and mekong river basins

age calibrated relative paleointensity for the last 1 5 myr at iodp site u1308 north atlantic integrated ocean drilling program iodp site u1308 central north atlantic records paleomagnetic directional and relative paleointensity rpi variations for the last 1 5 myr in 110 m of the sediment sequence at a mean sedimentation rate of 7 3 cm kyr a detailed benthic oxygen isotope record was combined with rpi to produce an integrated high resolution magneto isotopic stratigraphy for site u1308 apart from the well known polarity reversals in this interval the punaruu excursion is recorded at 1092 ka and the cobb mountain subchron in the 1182 1208 ka interval the paleointensity proxies are determined as slopes of nrm versus arm and nrm versus armaq arm acquisition with linear correlation coefficients to monitor the quality of the linear fit the rpi record for site u1308 is compared with the three other paleointensity records one from the western equatorial pacific and two from the north atlantic that cover the same time interval and have accompanying oxygen isotope records the match protocol of lisiecki and lisiecki 2002 is used to optimize the correlation of paleointensity records beginning with the original published age models for each record the match routine is used to optimize the rpi correlations to site u1308 with checks to ensure compatibility with oxygen isotope records squared wavelet coherence wtc indicates significant improvement in rpi and oxygen isotope correlations after matching each rpi record to site u1308 particularly for periods 10 kyr the level of coherence for the atlantic rpi records and the lower resolution pacific record implies synchronous global variability at scales 10 kyr that can be attributed to the axial dipole geomagnetic field

millennial scale climate instability during the early pleistocene epoch climate proxy records of the past 100 000 years show that the earth s climate has varied significantly and continuously on timescales as short as a few thousand years 1 7 similar variability has also recently been observed for the interval 340 500 thousand years ago8 these dramatic climate shifts expressed most strongly in the north atlantic region may be linked to and possibly amplified by alterations in the mode of ocean thermohaline circulation4 5 6 7 8 9 here we use sediment records of past iceberg discharge and deep water chemistry to show that such millennial scale oscillations in climate occurred over one million years ago this was a time of significantly different climate boundary conditions not only was the early pleistocene epoch generally warmer but global climate variations were governed largely by changes in earth s orbital obliguity our results suggest that such millennial scale climate instability may be a pervasive and long term characteristic of earth s climate rather than just a feature of the strong glacial interglacial cycles of the past 800 000 years

along arc variation in seismic velocity structure related to variable growth of arc crust in northern izu bonin intraoceanic arc the izu bonin arc is an intraoceanic island arc where the pacific plate subducts beneath the philippine sea plate along arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the izu bonin arc we have imaged three dimensional 3 d seismic velocity structures in the northern izu bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations our 3 d velocity model indicates heterogeneous structure in the mantle wedge along the arc low velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front low velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of aoga shima and south of sumisu jima this coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust along topographic highs in the forearc low velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc providing further evidence of this view of arc crust formation

seismological evidence for the existence of serpentinized wedge mantle serpentinized peridotite is detected seismologically by mapping poisson s ratio beneath the kanto district central japan because serpentinite has a higher poisson s ratio than other rocks in the crust and upper mantle we determine three dimensional p and s wave velocity models using travel time tomography and then construct a three dimensional map of poisson s ratio the models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km in the low velocity region there is an area with poisson s ratio greater than 0 3 at depths of 20 45 km which we attribute to serpentinized peridotite the rest of the low velocity region is interpreted as descending oceanic crust with a poisson s ratio of about 0 25 the high poisson s ratio area is associated with low seismicity and weak interplate coupling which is consistent with the ductility of serpentinite

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

southern ocean temperature records and ice sheet models demonstrate rapid antarctic ice sheet retreat under low atmospheric co2 during marine isotope stage 31 abstract over the last 5 million years the earth s climate has oscillated between warm interglacial and cold glacial states some particularly warm interglacial periods i e super interglacials occurred under low atmospheric co2 and may have featured extensive antarctic ice sheet collapse here we focus on an extreme super interglacial known as marine isotope stage 31 mis31 between 1 085 and 1 055 million years ago and is the subject of intense discussion we reconstructed the first southern ocean and antarctic margin sea surface temperatures ssts from organic biomarkers and used them to constrain numerical ice sheet shelf simulations our ssts indicate that the ocean was on average 5 c 1 2 c warmer in summer than today between 50 s and the antarctic ice margin our most conservative ice sheet simulation indicates a complete collapse of the west antarctic ice sheet wais with additional deflation of the east antarctic ice sheet we suggest the wais retreated because of anomalously high southern hemisphere insolation coupled with the intrusion of circumpolar deep water onto the continental shelf under poleward intensified winds leading to a shorter sea ice season and ocean warming at the continental margin in this scenario the extreme warming we observed likely reflects the extensively modified oceanic and hydrologic system following ice sheet collapse our work highlights the sensitivity of the antarctic ice sheets to minor oceanic perturbations that could also be at play for future changes

three dimensional attenuation qs structure beneath the kanto district japan as inferred from strong motion records 1 the three dimensional 3d s wave attenuation qs structure beneath the kanto plain where the pacific and philippine sea plates subduct was estimated using strong motion records of the k net and kik net in addition to several low qs zones corresponding to the volcanic front a large nw se trending low qs zone is estimated under the non volcanic kanto area at depths of 20 60 km this zone corresponds to a previously identified low velocity both vp and vs zone with a high poisson s ratio at 40 km depth it further indicates a wedge immediately above the subducting philippine sea plate predicted seismic ground motions using the estimated 3d attenuation structure is very similar to the actually recorded by recent earthquake on july 2005 m 6

magneto biostratigraphic age constraints on the palaeoenvironmental evolution of the south caspian basin during the early middle pleistocene kura basin azerbaijan abstract the sedimentary record of the caspian basin is an exceptional archive for the palaeoenvironmental palaeoclimatic and biodiversity changes of continental eurasia during the pliocene pleistocene the caspian basin was mostly isolated but experienced large lake level fluctuations and short episodes of connection with the open ocean as well as the black sea basin a series of turnover events shaped a faunal record that forms the backbone of the caspian geological time scale the precise ages of these events are still highly debated mostly due to the lack of well dated sections here we provide an integrated magneto biostratigraphic age model for two sections from the kura basin goychay and hajigabul our results reveal several major intervals with elevated salinity associated with mesohaline faunas and propose the following age constraints 1 the productive series akchagylian boundary is marked by a marine transgression from the open ocean that occurred around the gauss matuyama reversal 2 7 2 6 ma 2 the akchagylian apsheronian transition is characterized by a regression event and introduction of a new pontocaspian mollusc assemblage dated within the reunion subchron 2 13 ma the ostracod assemblages of the two sections do not show a major faunal turnover here 3 the early bakunian transgression occurs after the upper apsheronian lowstand 0 85 0 83 ma we conclude that major transitions in the age indicative mollusc groups sometimes occur at different time intervals i e diachronic and are highly depended on the local depositional environments a high resolution interdisciplinary approach on sections outside the kura basin is required to better understand the potential diachroneity of these turnover events in the entire caspian basin

anomalous low velocity zone and linear alignment of seismicity along it in the subducted pacific slab beneath kanto japan reactivation of subducted fracture zone 1 a detailed investigation of the hypocenter distribution beneath kanto japan reveals a nw se trending linear alignment of seismicity within the subducted pacific slab we estimate the 3d seismic velocity structure in the pacific slab to understand the factors controlling the genesis of such intraslab earthquakes a narrow low velocity zone is imaged within the subducted slab over a length of 150 km which partly penetrates into the mantle portion of the slab the low velocity zone correlates in space with the nw se trending earthquake cluster a reactivation of hydrated fracture zone formed prior to subduction is probably related to the low velocity anomaly dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone accompanied by intraslab earthquakes these observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers