an automated time window selection algorithm for seismic tomography we present flexwin an open source algorithm for the automated selection of time windows on pairs of observed and synthetic seismograms the algorithm was designed specifically to accommodate synthetic seismograms produced from 3 d wavefield simulations which capture complex phases that do not necessarily exist in 1 d simulations or traditional traveltime curves relying on signal processing tools and several user tuned parameters the algorithm is able to include these new phases and to maximize the number of measurements made on each seismic record while avoiding seismic noise our motivation is to use the algorithm for iterative tomographic inversions in which the synthetic seismograms change from one iteration to the next hence automation is needed to handle the volume of measurements and to allow for an increasing number of windows at each model iteration the algorithm is sufficiently flexible to be adapted to many tomographic applications and seismological scenarios including those based on synthetics generated from 1 d models we illustrate the algorithm using data sets from three distinct regions the entire globe the japan subduction zone and southern california

extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

imaging alpine crust using ambient noise wave equation tomography we present an improved crustal vs model and moho depth map using ambient noise wave equation tomography the so called ambient noise wave equation tomography is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation which accounts for 3 d and finite frequency effects we use cross correlations of up to four years of continuous vertical component ambient seismic noise recordings from 304 high quality broadband stations in the alpine region we use model lsp eucrust1 0 obtained from traditional ambient noise tomography as initial model and we iteratively improve the initial model by minimizing frequency dependent phase traveltime differences between the observed and synthetic waveforms of rayleigh waves in the period range 10 50 s we obtain the final model after 15 iterations with 65 total misfit reduction compared to the initial model at crustal depth the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies at moho and upper mantle depth the final model corrects an obvious systematic velocity shift of the initial model the resulting iso velocity moho map confirms a moho step along the external side of the external crystalline massifs of the northwestern alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern alps ambient noise wave equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

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

late quaternary geomagnetic secular variation from historical and 14c dated lava flows on hawaii a paleomagnetic record of geomagnetic paleosecular variation psv is constructed for the last 4400 years based on 191 sites in historical and 14c dated lava flows from mauna loa kilauea and hualalai volcanoes on the island of hawaii the features of this new record are similar to those recorded by sediments from lake waiau near the summit of mauna kea volcano but overall mean inclinations for the lava flows 31 to 33 depending on window size are nearer the expected dipole field value 35 than is that for the sediments 27 divergence of the inclination records with increasing age suggests that the lake waiau values at depths below 2 m have been affected by compaction related inclination shallowing although magnetic terrain effects cannot be ruled out the rate of psv indicated by the record presented here is highly variable 20 century and a pronounced shift in inclination from 25 to 40 occurred between 1030 and 975 years b p paleomagnetic directions from undated materials can be correlated with our calibrated curve but the resolution is largely dependent on the psv rate and data densities for both the reference and unknown directions the upper part of the puna basalt 18 lava flows previously sampled for paleomagnetism along the northern wall of kilauea s caldera uwekahuna bluff was likely deposited sometime between 1030 and 750 years b p but the lowest two flows beneath the uwekahuna ash 2100 years b p are correlated with an age of 3034 years b p paleomagnetic data for 54 lava flows of the ka u basalt exposed in the northwest wall of mauna loa s summit caldera mokuaweoweo indicate that they probably accumulated over a relatively short time interval 200 years and are assigned to a 1000 to 1199 year b p time window the mean of ages within this window is 1030 years b p but mapping and other 14c dates indicate that these summit overflows are probably closer to 1200 years b p in age

identification of the short lived santa rosa geomagnetic excursion in lavas on floreana island galapagos by 40ar 39ar geochronology a set of closely related basaltic lava flows supersite ga x on floreana island in the galapagos archipelago has a published record of an excursional or transitional direction virtual geomagnetic pole located at 153 1 e 54 2 s with 95 5 0 and a geomagnetic field strength 1 1 10 22 am 2 that is only 14 of the strength of the modern magnetic field 7 8 10 22 am 2 the very large age uncertainty of previous dating of a lava flow g43 from this set however has prevented placing this event in the geomagnetic polarity time scale here we report highly reproducible and precise 40 ar 39 ar ages on the lava flow that indicate that the distinct geomagnetic excursion is 925 7 4 6 ka 2 n 6 mean square of weighted deviates 1 23 this shows that this dramatic weakening of the geomagnetic field is associated with the santa rosa excursion instead of the matuyama brunhes polarity reversal our high precision 40 ar 39 ar ages for floreana provide evidence for the global significance of the santa rosa excursion

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

structure of the crust beneath the southeastern tibetan plateau from teleseismic receiver functions abstract southeastern tibet marks the site of presumed clockwise rotation of the crust due to the india eurasian collision and abutment against the stable sichuan basin and south china block knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region here we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method we find that the crustal thickness decreases gradually from the tibetan plateau proper to the sichuan basin and yangtze platform but that significant intra crustal heterogeneity exists on shorter lateral scales 0 30 are detected beneath the southeastern margin of tibet but in the sichuan basin and southeastern yangtze platform the values are close to the global average combined with high regional heat flow and independent evidence for mid crustal layers of high electric conductivity the large intra crustal s wave velocity reduction 12 19 and the intermediate to high average crustal poisson s ratios are consistent with partial melt in the crust beneath parts of southeastern tibet these results could be used in support of deformation models involving intra crustal flow with the caveat that significant lateral variation in location and strength of this flow may occur

broad trends in geomagnetic paleointensity on hawaii during holocene time paleointensity determinations have been obtained from 22 basaltic lava flows on the island of hawaii using the thelliers method radiocarbon dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years and results of the experiments provide an estimate of broad trends in geomagnetic paleointensity during holocene time in the vicinity of hawaii most of the samples were obtained from quickly cooled flow margins and as a consequence typically contain two titanomagnetite populations only two of the 79 samples analyzed failed to yield an estimate of the paleofield demonstrating that paleointensities can be obtained from such samples if carefully selected virtual dipole moments calculated for the flows are compared with a published curve of dipole field intensity that was constructed using worldwide archeomagnetic data the large nondipole fields that were previously postulated for the vicinity of hawaii are confirmed and the present data indicate that they were present from about 5000 years b p to perhaps as recently as the past 200 years the data indicate however that these nondipole sources must have been virtually absent between about 12 000 and 5000 years b p as they are at the present time

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

migration of widespread long lived volcanism across the galapagos volcanic province evidence for a broad hotspot melting anomaly abstract the well studied galapagos archipelago is a small part of the much larger galapagos volcanic province gvp consisting of the cocos carnegie coiba and malpelo aseismic ridges and related seamount provinces although these aseismic ridges and seamounts dominate the morphology of the region little is known about their origin due to a lack of direct age and geochemical information in order to establish how well the gvp fits with the predictions of the standard fixed hotspot and mantle plume hypotheses we conducted a first reconnaissance dredge grab sampling of submerged regions of the gvp we present here 40ar 39ar ages for many of these new basement samples and evaluate their implications for the various models put forward to explain the origin of the gvp correlating new and published sample site ages with distance from the western side of the galapagos islands show that volcanism has not progressed in narrow time progressive lines of seamounts and ridges as predicted by the conventional fixed hotspot and mantle plume hypothesis rather volcanism apparently migrated time progressively across the gvp in broad regions of long lived and possibly concurrent volcanism we propose that the most viable explanation for these observations is that the gvp is the product of cocos and nazca plate motions across a broad hotspot melt anomaly the complex spreading history of the cocos nazca spreading centre likely controlled the relative distribution of gvp volcanism between the cocos and nazca plates while creating lithosphere of variable age thickness across the region while the notion of a broad galapagos hotspot melting anomaly linked to a complex regional tectonic history requires significant testing it nevertheless highlights the need to test alternative mantle upwelling shapes and sizes compared to the widely accepted notion of a narrow continuous long lived galapagos mantle plume conduit defined by the size and location of a galapagos island

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

two crustal flowing channels and volcanic magma migration underneath the se margin of the tibetan plateau as revealed by surface wave tomography abstract the se margin of the tibetan plateau is an important area to develop a better understanding of the plateau uplift and the indian eurasian continental collision dynamics previous studies have reported widespread low velocity anomalies beneath this region particularly in the tengchong volcanic field tcvf however the spatial distribution and dynamic processes of these low velocity anomalies are not well constrained in this study a 3 d s wave velocity structure model of the crust and upper mantle 10 120 km in the region is constructed by the inversion of surface wave dispersion data a two step inversion procedure is adopted to generate the s wave velocity structure images the measured phase velocities and inverted s wave velocities jointly show a large scale low velocity anomaly distributed in the crust consistent with the view that the region is the passageway of the eastward migration of tibetan plateau material two crustal flowing channels are clearly observed at depths of 20 km and 30 km which connect and rotate clockwise around the eastern himalaya syntaxis beneath the tcvf there are two prominent low velocity anomaly zones at depths of 15 25 km and 50 80 km which indicate the existence of magma chambers one of the crustal flowing channels is connected with the magma chamber of the tcvf and the other has a short branch north of kunming toward the mile shizong fault at a depth of 20 km based on the distribution of the s wave velocities under the tcvf a dynamic model of the tengchong volcano magma system is proposed to explain the migration patterns of the volcanic material

lithospheric structure of the southeastern margin of the tibetan plateau from rayleigh wave tomography lithospheric shear wave velocity beneath the southeastern margin of the tibetan plateau is obtained from rayleigh wave tomography using earthquake data recorded by the temporary chinarray and permanent china digital seismic array fundamental mode rayleigh wave phase velocities at periods of 20 100 s are determined and used to construct the 3 d shear wave velocity model low velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust both of which play important roles in accommodating the collision between the indian and eurasian plates a vertical low velocity column beneath the tengchong volcano is observed which could be caused by upwelling of warm mantle due to the lithosphere extension in the thailand rift basin to the south or by fluid induced partial melting due to the subduction of the burma slab the western yangtze craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere indicating possible thermal erosion at the western craton edge resulted from the extrusion of the tibetan plateau a low velocity zone is imaged at the depths of 70 150 km beneath the eastern part of the yangtze craton which could be caused by small scale mantle convection associated with the subduction of the burma microplate and or the opening of the south china sea

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

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

low wave speed zones in the crust beneath se tibet revealed by ambient noise adjoint tomography we present a refined 3d crustal model beneath se tibet from ambient noise adjoint tomography different from ray theory based tomography adjoint tomography in this study incorporates a spectral element method sem and takes empirical green s functions egfs of rayleigh waves from ambient noise interferometry as the direct observation the frequency dependent traveltime misfits between sem synthetic green s functions and egfs are minimized with a preconditioned conjugate gradient method meanwhile the 3d model gets improved iteratively utilizing 3d finite frequency kernels the new model shows 3 6 shear wave speed increasing beneath the western sichuan basin scb depth 15 km and the central chuan dian block cdb and 6 12 shear wave speed reduction in the mid lower crust beneath the northern and the southern cdb the inferred spatial pattern of low wave speed zones consistent with possible partial melt suggests more complex and disconnected geometry than the pervasive narrow zone from the channel flow models

radial anisotropy in the crust of se tibet and sw china from ambient noise interferometry 1 we use rayleigh and love wave green s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the sichuan basin and around the eastern himalaya syntaxis in agreement with previous studies low velocity zones are ubiquitous in the mid lower crust with substantial variations both laterally and vertically discrepancies between 3 ds hear velocity from either rayleigh vsv o r love vsh waves are examined both in view of non uniqueness of tomographic solutions and radial anisotropy low shear wave speed and radial anisotropy with vsh vsv are most prominent in mid lower crust in area northwest to the lijiang muli fault and around the red river and xiaojiang faults this anisotropy could be caused by sub horizontal mica fabric and its association with low velocity zones suggests mica alignment due to flow in deep crustal zones of relatively low mechanical strength citation huang h h yao andr d vanderhilst 2010 radialanisotropyinthecrust of se tibet and sw china from ambient noise interferometry geophys res lett 37 l21310 doi 10 1029 2010gl044981

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

on the shape of paleomagnetic data sets a quick and easy method to analyze the shape of paleomagnetic data sets is described and illustrated the elongation e of a set of directions of virtual geomagnetic poles vgp is defined as e k2 k3 where k2 and k3 are the second and third eigenvalues of the bingham distribution it is shown that e 3 or so indicates that the distribution is significantly elongate although the size of the sample and the paleolatitude both affect this conclusion because the azimuth of k2 gives the direction of elongation it can be used to help determine why a particular distribution is elongate it is shown that for most units from the craton both directions and vgp are nearly circular except for units magnetized at very low latitudes in general poles are more circular than directions some units formed in plate margin locations also are circular but many are highly elongate many units from the western cordillera of north america have highly elongate distributions that are compatible with origin within a shear zone others are obviously underformed internally units formed during extension are less commonly elongate but some show the probably effects of normal faulting it is hoped that shape analysis will be part of any complete paleomagnetic study in the future

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

paleomagnetism of miocene volcanics on sao tome paleosecular variation at the equator and a comparison to its latitudinal dependence over the last 5 myr a collection was made in january 2009 of 10 oriented samples from each of 54 sites in lavas on sao tome island nominal location 0 3o n 6 5o e some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation overall magnetic properties were excellent highly stable magnetizations carried by pseudo single domain magnetite after principal component analysis of progressive alternating field demagnetization data for the samples twenty two sites had normal polarity magnetizations d 0 6o i 8 3o 95 4 3o 53 1 and 20 had reverse magnetizations d 176 0o i 4 2o 95 7 3o 20 8 the directions are within 5 of antiparallel yielding a positive reversal test the combined data set of 42 site mean virtual geomagnetic poles converted to common normal polarity yields a pole position at 86 0 n 211 5oe a95 3 1o ar ar and k ar dating reveals that these rocks are miocene in age 5 11 ma old enough to allow northerly plate motion to help explain the slightly far sided pole position the between site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation sb and equaled 11 4 with 95 confidence interval between 9 9 and 13 4 this article is protected by copyright all rights reserved

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

soil phytoliths as indicators of initial human impact on san cristobal island galapagos abstract i analyzed phytoliths from soil profiles on san cristobal island in the galapagos islands to determine whether historical human impact and recent vegetation dynamics are visible in the phytolith record i extracted phytoliths from soil samples taken from four test pits located at the village an abandoned field a forest and an active agricultural field all located within the former el progreso plantation ad 1860 1920 changes in the ratio of tree to grass phytoliths with depth in all four samples suggest changes in vegetation composition from forest to open vegetation dominated by grasses which reflects the removal of forest with colonization and the first permanent human occupation of the archipelago in the middle 19th century this paper shows that the changes in vegetation brought about by colonization and land clearing for plantations are documented in the soil phytoliths record and suggests that phytoliths can be used as an indicator of past vegetation change in future work on the historical ecology of the galapagos archipelago

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

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

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

geology and petrology of floreana island galapagos archipelago ecuador abstract floreana is the sixth largest of the galapagos islands and lies at the extreme southern margin of the galapagos platform in sharp contrast to the other major islands floreana lacks a well developed volcanic center but instead is dominated by pyroclastic vents and derivative ash and cinder deposits the majority of exposed lavas are xenolith bearing alkali olivine basalts that constitute a coherent suite referred to as the main series the flank series comprises a group of isolated vents on the southern flank of the island which erupted xenolith poor basaltic lavas that are generally poorer in incompatible trace elements and richer in plagioclase paleomagnetic measurements supported by k ar age determinations record a relatively limited timespan for subaerial volcanism shield development reached maximum dimensions by 1 0 ma with the final phases of pyroclastic activity at an end by 0 3 ma main series rocks are phenocryst poor and typically contain clusters of xenocrysts in an extremely fine grained granular matrix petrogenetic modelling indicates they represent products of partial melting of a lree enriched spinel lherzolite source little modified by subsequent fractional crystallization porphyritic flank series lavas experienced rather different ascent histories and may have resided in shallow level magma chambers prior to eruption isotopic geochemical and petrographic data demonstrate that flank series lavas are not cogenetic in any simple way with main series flows floreana lavas contain a diverse suite of ultramafic xenoliths al augite group megacrysts are likely to be cognate and comagmatic with host alkali olivine basalts transition metal contents of megacrysts and host lavas however preclude clinopyroxene fractionation as a mechanism of magmatic differentiation it is proposed that megacrysts were products of passive crystallization and were never effectively separated from host magmas cr diopside group nodules may have crystallized at very high temperatures from advanced partial melts and are less likely to bear any direct genetic connection to host basalts

kinematics and dynamics of the southeastern margin of the tibetan plateau summary on the southeastern margin of the tibetan plateau lies a large region which seismicity and gps data show to be actively deforming this paper describes the active faulting in the region and how it relates to the velocity field observed with gps in places the velocity field is accommodated by rotations about vertical axes and most or all of the strain at the surface in the region appears to be released seismically gps velocities are then compared to velocities calculated using a model for deformation driven by gravitational driving forces using rheologies estimated from experimentally derived mineral flow laws the model provides velocities that are in good agreement with observed gps velocities it is not possible to uniquely determine the rheology or flow velocity at depth and there are two forms of model solution which match the observed horizontal surface velocities in one of these vertical planes deform by pure shear and in the other vertical gradients of horizontal velocity are present within the crust two distinct regions of normal faulting earthquakes are present in the region and have mechanisms which are most easily explained by gravity driven deformation

origin of espanola island and the age of terrestrial life on the galapagos islands geological studies of espanola hood island galapagos ecuador indicate that the island had a subaerial rather than a submarine origin because the younger lava flows are dated at 3 million years espanola has apparently existed as an island for at least that long thus terrestrial life may have existed or arrived on the galapagos islands at least 3 million years ago more than twice as long as had been assumed

paleomagnetic study of late miocene through pleistocene igneous rocks from the southwestern usa results from the historic collections of the u s geological survey menlo park laboratory 1 seventy sites from the southwestern united states provide paleomagnetic results that meet certain minimum criteria and can be considered for the time averaged field initiative tafi the virtual geomagnetic poles for these 70 units are circularly distributed and their mean is nearly coincident with the rotational axis when other published data for the southwestern united states are included n 146 the virtual geomagnetic poles are again circularly distributed but their mean is significantly displaced from the rotational axis whichever of these data sets is used the mean poles for normal and reversed polarity data differ by 170 and are not antipodal at greater than 95 confidence when the data are separated into specific age groups the 95 confidence limits about the mean poles for the brunhes matuyama combined gauss gilbert and late miocene intervals all include the rotational axis angular dispersion about these four mean poles increases systematically with increasing age and is consistent with paleosecular variation model g

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

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

stochastic simulation of strong motion records from the 26 september 1997 mw 6 umbria marche central italy earthquake we simulated the strong motion time histories recorded during the main shock m w 6 0 of the umbria marche seismic sequence central italy of september october 1997 ground motion waveforms were computed using the stochastic modeling technique proposed by beresnev and atkinson 1997 1998 for finite faults in this approach the high frequency amplitudes are simulated as a summation of stochastic point sources we used the finsim code beresnev and atkinson 1998 which incorporates regional attenuation and frequency dependent site amplification factors we divided the fault plane into 60 elements whose length and width are 1 2 km and 1 5 km respectively the resulting subfault corner frequency and rise time are 0 91 hz and 0 5 sec we found that the site amplification functions play an important role in the simulation process improving the fit to the observed time histories and spectra the strong motion waveforms recorded at the nocera noc station located at the northern end of the causative fault show an important directivity effect thus to fit the observed ground motions we used an inhomogeneous slip distribution weighting the slip on the fault heavier toward the north we tested two models one that simulates a fault rupture with two main slip patches and a second model that ruptures northward with a less heterogeneous slip distribution where slip is mostly concentrated near the rupture nucleation at the southern edge of the fault plane the simulated low frequency amplitudes at noc however require an additional frequency dependent directivity correction e g bernard et al 1996 in conclusion we found that stochastic finite fault simulations calculated using adequate site amplification functions and crustal attenuation reproduce reasonably well the ground motions from the m w 6 0 umbria marche earthquake

geology and petrogenesis of lavas from san cristobal island galapagos archipelago volcanism became emergent on san cristobal island 2 3 m y b p in the eruption of floods of alkaline basalt volcanism then coalesced in a central shield volcano which erupted continually until 0 6 m y b p at 0 9 m y b p activity began to jump to the northeast and the style of volcanism changed to fissure fed eruptions that have continued almost to historical time three distinct magma series occur on san cristobal an alkaline series a series of morb like tholeiites and a series of incompatible element enriched tholeiites that are typical of oceanic islands each series is represented by very primitive basalts that have been little modified since segregation from their sources each series also has more evolved lavas which range from low mg basalts to hawaiites and icelandites these evolved lavas are the result of simple fractional crystallization of magmas which produces major and compatible element compositions similar to those of observed primitive lavas the isotope and trace element data suggest that the different series are generated by different degrees of partial melting and mixing of both a depleted and an enriched source the first source is thought to be the depleted asthenosphere which generates the morb9s of the galapagos spreading center and the enriched source is the mantle plume associated with the galapagos hotspot

geology of santa fe island the oldest galapagos volcano abstract santa fe island was a volcanic center when it emerged 3 9 0 6 m y ago later upfaulting of a horst along the central axis of the island dominates its present morphology santa fe is made up of evolved transitional lavas that are not related by fractional crystallization alone source heterogeneties differing degrees of melting or open system magma chambers may explain the observed trace element variations santa fe baltra and espanola make up a geologic subprovince in the central galapagos they are older than the other islands and their lavas are compositionally similar at the time of their emergence the three islands were in a tectonic setting similar to that of the young western and central galapagos island

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

sh wave structure of the crust and upper mantle in southeastern margin of the tibetan plateau from teleseismic love wave tomography abstract the deep structure of southeastern tibet is important for determining lateral plateau expansion mechanisms such as movement of rigid crustal blocks along large strike slip faults continuous deformation or the eastward crustal channel flow we invert for 3 d isotropic sh wave velocity model of the crust and upper mantle to the depth of 110 km from love wave phase velocity data using a best fitting average model as the starting model the 3 d sh velocity model presented here is the first sh wave velocity structure in the study area in the model the tibetan plateau is characterized by prominent slow sh wave velocity with channel like geometry along strike slip faults in the upper crust and as broad zones in the lower crust indicating block like and distributed deformation at different depth positive radial anisotropy vsh vsv is suggested by a high sh wave and low sv wave anomaly at the depths of 70 110 km beneath the northern indochina block this positive radial anisotropy could result from the horizontal alignment of anisotropic minerals caused by lithospheric extensional deformation due to the slab rollback of the australian plate beneath the sumatra trench

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

geochronology of galapagos seamounts lavas from seamounts of the central galapagos platform and the carnegie ridge increase in age with distance from the western edge of the platform consistent with a hotspot model for the generation of these features the areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 ma age distance relationships are consistent with a decrease in the velocity of the nazca plate relative to the galapagos hotspot during the past 9 m y most likely at 5 ma when there was a change in pacific hotspot motion alternatively the plate velocity remained constant and volcanism at a single seamount endured up to 7 m y lavas from seamounts along the wolf darwin lineament to the northwest of the galapagos platform are young 1 ma with the youngest measured ages closest to the platform

statistical assessment of preferred transitional vgp longitudes based on palaeomagnetic lava data summary we examine the statistical significance of a recent report indicating that palaeomagnetic lava data recording reversals and excursions over the past 20 myr yield transitional virtual geomagnetic poles vgps tending to fall along american and asian longitudes using a bootstrap analysis of spherical densitygrams we find that this result is neither a sensitive function of nor heavily dependent upon particular transitional records previous analyses used 2 and kuiper tests of longitudinal uniformity of transitional vgps however the bootstrap analysis presented here takes such statistical tests one step further showing specifically that american and asian longitudes are preferred with significance at about the 95 per cent confidence level furthermore provided the appropriate polarity convention is used we confirm that icelandic transitional vgps tend to fall along asian longitudes these results are indicative of non dipolar transitional fields whose structure is governed by some form of core mantle coupling

time averaged paleomagnetic field at the equator complete data and results from the galapagos islands ecuador 1 we present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the galapagos islands ecuador collected by allan cox in 1964 1965 but nearly all previously unpublished the islands are located in the eastern pacific ocean within 1 4 of latitude from the equator and range in age from historically erupted to 3 ma mostly determined by published k ar and 3he isotopic dating the number of sites collected on each island ranges from 1 to 28 for a total of 186 after combining duplicate site mean directions 149 are used for an overall mean direction and 8 represent excursions and one reversal path divided by geomagnetic polarity chron 110 site means are brunhes or jaramillo normal polarity 27 are matuyama reversed polarity and 12 are gauss both polarities we have completed the magnetic cleaning that was commenced in the late 1960s secondary mostly viscous magnetizations were nearly all removed by alternating field demagnetization at 10 mt we have used the so called blanket cleaning method generally at 10 mt all sites were in basalt flows and gave good paleomagnetic results none was rejected in toto and only a few core specimens were magnetically unsatisfactory nearly all sites had eight independently oriented cores and within site angular standard deviations of directions range from 1 to 8 we used both fisher and bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian while the corresponding virtual pole vgp populations are essentially circularly distributed the paleomagnetic poles calculated as the means of vgps are as follows brunhes and jaramillo north latitude 86 9 east longitude 245 1 and 95 confidence radius a95 1 9 matuyama latitude 87 2 longitude 158 2 and a95 3 8 gauss latitude 83 0 longitude 204 7 and a95 7 0 these paleomagnetic poles do not differ significantly from one another but the brunhes and jaramillo combined pole is significantly near sided with respect to the galapagos as is the overall mean pole which is at latitude 87 1 and longitude 227 6 with a95 1 7 omitting the excursion and reversal path data the overall angular standard deviation of vgps is 11 7 with lower and upper 95 confidence limits of 10 8 and 12 7 respectively in good agreement with previously published values for near equatorial sampling latitudes

varied thermo rheological structure mechanical anisotropy and lithospheric deformation of the southeastern tibetan plateau abstract to investigate the lithospheric structure and deformation of the tibetan plateau we present thermo rheological models for two transects across the southeastern tibetan plateau se tibet showing strong lateral heterogeneity the geodynamic implications are also investigated in relation to gps motions seismic anisotropy seismicity distribution and crustal isostatic state the unmatched pattern between pms and sks splittings which even intersect at right angles to the south of 26 n suggests the crust mantle decoupling constrained by a brittle load bearing layer at the uppermost mantle beneath the indochina block the decoupling may occur below the uppermost mantle the strong crust beneath the south china plate and indochina block implied by the seismicity distribution has two thick brittle load bearing layers indicating a coupled system the crust beneath the emeishan large igneous province elip also has two brittle load bearing layers but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust only one brittle load bearing layer resides in the upper crust in the other part of the chuandian block combining with gps and pms splitting we can infer that the weak crust with seismicity restricted into the depth of

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

the volcanic geology and petrology of isla pinta galapagos archipelago pinta one of the northern islands in the galapagos archipelago is the top of a shield volcano which became emergent 0 7 m y ago a regional north north west trending fracture system influenced the locus of volcanism pinta9s geologic development can be divided into an older shield stage and a younger fissure stage each stage produced a similar yet distinct series of evolved olivine tholeiites magnesium number 52 the shield series underwent some additional shallow level fractionation in a periodically replenished chamber rare earth element data indicate that the mantle source for both of the series has residual garnet higher concentrations of incompatible trace elements iri the fissure series imply a lower degree of partial melting 143 nd 144 nd and 87 sr 86 sr ratios suggest that both series have an enriched hotspot type source despite close proximity to the galapagos spreading ridge pinta9s lavas lack features characteristic of mid ocean ridge basalts the isotopic and rare earth element data from pinta and the other northern islands are consistent with previous models for the galapagos spreading ridge which require north south axially directed transfer of enriched asthenosphere and or melts from the galapagos hotspot to the galapagos ridge

the geology and geochemistry of isla marchena galapagos archipelago an ocean island adjacent to a mid ocean ridge abstract isla marchena is the subaerial exposure of a shield volcano located between the galapagos platform and the galapagos spreading center the geologic evolution of the island can be divided into two lava series separated by formation of a cadera and a period of explosive activity both lava series contain evolved alkali olivine and tholeiitic basalts average mg 55 2 and are virtually aphyric with the exception of a plagioclase rich horizon in the pre caldera section although the major element compositions of the two series are similar the abundances of incompatible elements and ratios of highly moderately incompatible elements increase systematically with decreasing stratigraphic age these compositional differences together with textural and mineral chemical evidence for disequilibrium are not easily explained by any single petrologic mechanism the combined effects of a periodically replenished magma chamber assimilation fractionation and in situ crystallization of a solidification zone are possible mechanisms that may be responsible for the temporal changes in lava composition morb like abundances of trace elements in addition to unusually depleted 87 sr 86 sr and 143 nd 144 nd ratios for ocean island basalts suggest that magmas associated with the galapagos plume interacted with those of the galapagos spreading center

time averaged field at the equator results from ecuador seventy sites were collected from pliocene to recent lavas in ecuador between 0 19 and 1 1 s latitude lightning affected many of the sites sampled and the final data set consisted of 51 sites with 95 of 10 or less 21 flows yielded normal directions of magnetization declination 354 6 inclination 5 95 7 2 and 30 yielded reverse declination 183 5 inclination 5 6 95 5 these sites pass a reversal test at a high level the combined data give a direction of declination 359 9 inclination 5 4 and 95 4 2 which is just significantly different from the gad field but is in agreement with a gad field plus 5 quadrupole field mean vgps are not significantly different from the geographic poles the asd for the combined sites is 13 3 which is in agreement with model g paleointensity measurements yield values of vadm which range from 1 39 1022 to 22 6 1022 am2 the mean value for the reverse field is 3 2 1022 am2 while for the normal field it is 10 2 1022 am2 the 40ar 39ar dates on selected sites show that they are 2 6 ma and younger the age dates and magnetic polarity indicate that the chacana structure is brunhes matuyama in age

structure of the upper mantle and transition zone beneath southeast asia from traveltime tomography 1 tomographic images of the mantle beneath east asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the tibetan plateau our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large scale tectonic framework of the continental india asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east in the realm of continental collision west of 100 e a relatively slow p wave speed characterizes the upper mantle beneath much of the tibetan plateau but the wave speed is high beneath cratonic india the southern and western part of the tibetan plateau hindu kush and the tian shan in the subduction realm east of 110 e the main structures are i pronounced low wave speed anomalies at a depth of between 100 and 400 km beneath asia s southeastern seaboard and the back arc regions of ongoing subduction ii narrow fast anomalies in the upper mantle beneath major subduction zones and iii widespread fast anomalies at a depth of 500 700 km beneath the sea of japan the northern part of the philippine sea plate and southeastern china if the latter anomalies represent stagnant slabs their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western pacific island arcs along the old se margin of asia or during the mesozoic collision of cratonic units in southeast asia attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for at least several tens of millions of years shallow slow anomalies beneath the red river fault region connect to deep anomalies beneath the south china fold belt and south china sea suggesting a causal relationship between the evolution of the continental lithosphere of sw china and deeper mantle processes between the collision and the subduction realms tomography reveals high wave speed continental roots beneath the western part of the north china craton ordos block and the south china or yangtze craton sichuan basin to a depth of 300 km

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors

the volcanic history of volcan alcedo galapagos archipelago a case study of rhyolitic oceanic volcanism volcan alcedo is one of the seven western galapagos shields and is the only active galapagos volcano known to have erupted rhyolite as well as basalt the volcano stands 4 km above the sea floor and has a subaerial volume of 200 km3 nearly all of which is basalt as volcan alcedo grew it built an elongate domal shield which was partly truncated during repeated caldera collapse and partial filling episodes an outward dipping sequence of basalt flows at least 250 m thick forms the steepest to 33 flanks of the volcano and is not tilted thus a constructional origin for the steep upper flanks is favored about 1 km3 of rhyolite erupted late in the volcano s history from at least three vents and in 2 5 episodes the most explosive of these produced a tephra blanket that covers the eastern half of the volcano homogeneous rhyolitic pumice is overlain by dacite rhyolite commingled pumice with no stratigraphic break the tephra is notable for its low density and coarse grain size the calculated height of the eruption plume is 23 30 km and the intensity is estimated to have been 1 2x108 kg s rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase the age of the rhyolitic eruptions is 120 ka on the basis of k ar ages between ten and 20 basaltic lava flows are younger than the rhyolites recent faulting resulted in a moat around part of the caldera floor alcedo most resently erupted sometime between 1946 and 1960 from its southern flank alcedo maintains an active transient hydrothermal system acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional scale earthquake