identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

tomographic vp and vs structure of the california central coast ranges in the vicinity of safod from controlled source seismic data summary a seismic reflection refraction survey across the san andreas fault near parkfield california has refined our knowledge of the upper crustal structure of the central california coast ranges at the san andreas fault observatory at depth safod the survey consisted of a 46 km long line of seismographs 25 50 m spacing and 63 explosions 25 200 kg nominal spacing of 500 m with some gaps the traveltimes of refracted p and s waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce p and s wave velocity models vp vs along the profile extending to as much as 5 km depth the vp and vs models show a prominent lateral drop in velocities a few hundred metres northeast of safod between the drill hole and the san andreas fault the vp model shows particularly well a southwest dipping velocity inversion beneath safod the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below in addition to vp and vs models a vp vs model was derived a vp vs ratio lower than 1 73 is seen only at depth in a narrow zone beginning at the target earthquakes for safod and extending downward and northeastward into the north america plate clusters in the parameter space spanned by vp vs ratios and vp can be identified by two different methods one more intuitive analytical method and one more abstract method based on neural network techniques these clusters are correlated to different rock types based on laboratory and in situ data these clusters are remapped back into x z plane along the profile prominent features mapped this way include salinian granitic rocks beneath and west of safod and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the buzzard canyon fault bcf system these sedimentary rocks extend from this fault to the san andreas fault system unfortunately our cluster analysis shows no significant discontinuity at the san andreas fault owing presumably to the fact that the san andreas fault is located within sedimentary rocks having similar elastic properties this paper is an attempt to downward continue a geological map by geophysical means based on elastic properties of rock samples from the region

global anisotropy in the upper mantle inferred from the regionalization of phase velocities a data set of 2600 paths for rayleigh waves and 2170 paths for love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70 250 s the epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of earth a synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path to process such a large data set a new tomographic technique is designed a good correlation is found with surface tectonics in the whole period range for love waves and at shorter periods for rayleigh waves the directions of maximum velocities are also well correlated with plate tectonics motions for the 2 azimuthal term of rayleigh waves when the power spectrum of heterogeneities is computed it is found that for love waves the even orders are usually larger than the odd orders for rayleigh waves the degrees 2 and 6 dominate only at long periods suggesting a deep origin for these components

upper mantle structure at walvis ridge from pn tomography abstract passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up within the lispwal lithospheric structure of the namibian continental passive margin at the intersection with the walvis ridge from amphibious seismic investigations project combined on and offshore seismic experiments were designed to characterize the southern african passive margin at the walvis ridge in northern namibia in addition to extensive analysis of the crustal structures we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the walvis ridge with the namibian coast upper mantle pn travel time tomography from controlled source amphibious seismic data was used to investigate the sub moho upper mantle seismic velocity we succeeded in imaging upper mantle structures potentially associated with continental break up and or the tristan da cunha hotspot track we found mostly coast parallel sub moho velocity anomalies interpreted as structures which were created during gondwana break up

seismic imaging of the waltham canyon fault california comparison of ray theoretical and fresnel volume prestack depth migration abstract near vertical faults can be imaged using reflected refractions identified in controlled source seismic data often theses phases are observed on a few neighboring shot or receiver gathers resulting in a low fold data set imaging can be carried out with kirchhoff prestack depth migration in which migration noise is suppressed by constructive stacking of large amounts of multifold data fresnel volume migration can be used for low fold data without severe migration noise as the smearing along isochrones is limited to the first fresnel zone around the reflection point we developed a modified fresnel volume migration technique to enhance imaging of steep faults and to suppress noise and undesired coherent phases the modifications include target oriented filters to separate reflected refractions from steep dipping faults and reflections with hyperbolic moveout undesired phases like multiple reflections mode conversions direct p and s waves and surface waves are suppressed by these filters as an alternative approach we developed a new prestack line drawing migration method which can be considered as a proxy to an infinite frequency approximation of the fresnel volume migration the line drawing migration is not considering waveform information but requires significantly shorter computational time target oriented filters were extended by dip filters in the line drawing migration method the migration methods were tested with synthetic data and applied to real data from the waltham canyon fault california the two techniques are applied best in combination to design filters and to generate complementary images of steep faults

the paleocene of antarctica dinoflagellate cyst biostratigraphy chronostratigraphy and implications for the palaeo pacific margin of gondwana abstract the paleocene 66 56 ma was a critical time interval for understanding recovery from mass extinction in high palaeolatitudes when global climate was warmer than today a unique sedimentary succession from seymour island antarctic peninsula provides key reference material from this important phase of the early cenozoic dinoflagellate cyst data from a 376 m thick stratigraphical section including the cretaceous paleogene boundary is correlated with biozones from new zealand the east tasman plateau and southeastern australia a detailed age model is suggested for the lopez de bertodano ldbf and sobral sf formations based on dinoflagellate cyst biostratigraphy and u pb dating of zircons supported by correlated magnetostratigraphy and strontium isotope values from macrofossils the top of the ldbf is confirmed as latest maastrichtian to earliest danian 66 2 65 65 ma in age the overlying sf is mostly danian in age with an inferred hiatus near the top overlain by sediments dated as late thanetian rare apectodinium homomorphum first appear in the uppermost sf the earliest in situ record from antarctica the distribution of marine and terrestrial fossils from uppermost cretaceous to eocene sediments in patagonia antarctica new zealand and australia required both sea and land connections between these fragments of gondwana fossil evidence and reconstructions of antarctic palaeogeography and palaeotopography reveal evidence for persistent embayments in the proto weddell and ross sea regions at this time we conclude that a coastal dispersal route along the palaeo pacific margin of gondwana could explain the fossil distribution without requiring a transantarctic strait or closely spaced archipelago a region in the west to east antarctic boundary zone elevated until the early paleogene perhaps acted as a site for high elevation ice caps this supports fossil geochemical and sedimentological evidence for cold climate intervals and significant sea level falls during the maastrichtian and paleocene

rayleigh and love wave anisotropy in southern california using seismic noise we study surface wave anisotropy using three component frequency wave number analysis of 1 year 2012 of ambient seismic noise measured by the southern california seismic network significant 2 and 4 rayleigh wave anisotropy is observed over most of the frequency range 15 to 100 mhz millihertz the wide rayleigh wave illumination and large data volume allow for relatively high precision and sensitivity the estimation variability above 35 mhz as well as the magnitude of the weakest significant detections is about 0 1 the estimates are consistent with previous anisotropy studies of the region we also show preliminary results for love waves but ambient love wave illumination in southern california may not be sufficient for the approach

subsurface geometry of the san andreas fault in southern california results from the salton seismic imaging project ssip and strong ground motion expectations the san andreas fault saf is one of the most studied strike slip faults in the world yet its subsurface geometry is still uncertain in most locations the salton seismic imaging project ssip was undertaken to image the structure surrounding the saf and also its subsurface geometry we present ssip studies at two locations in the coachella valley of the northern salton trough on our line 4 a fault crossing profile just north of the salton sea sedimentary basin depth reaches 4 km southwest of the saf on our line 6 a fault crossing profile at the north end of the coachella valley sedimentary basin depth is 2 3 km and centered on the central most active trace of the saf subsurface geometry of the saf and nearby faults along these two lines is determined using a new method of seismic reflection imaging combined with potential field studies and earthquakes below a 6 9 km depth range the saf dips 50 60 ne and above this depth range it dips more steeply nearby faults are also imaged in the upper 10 km many of which dip steeply and project to mapped surface fault traces these secondary faults may join the saf at depths below about 10 km to form a flower like structure in appendix d we show that rupture on a northeast dipping saf using a single plane that approximates the two dips seen in our study produces shaking that differs from shaking calculated for the great california shakeout for which the southern saf was modeled as vertical in most places shorter period t 1 s shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall compared to shaking calculated for a vertical fault

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

no moho uplift below the baikal rift zone evidence from a seismic refraction profile across southern lake baikal 1 the late cenozoic baikal rift zone brz in southern siberia is composed of several individual topographic depressions and half grabens with the deep lake baikal at its center we have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the baikal explosion seismic transects best project across the rift zone in the southern part of lake baikal the seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2 d ray tracing of first arrivals and reflections the velocity model shows a gently deepening moho from the siberian platform 41 km depth into the sayan baikal fold belt 46 km depth we can exclude the presence of any moho uplift around the 10 km deep sedimentary graben structure of southern lake baikal the lower crust includes a distinct 50 80 km wide high velocity anomaly 7 4 7 6 0 2 km s slightly offset to the northeast from the rift axis we interpret this feature as resulting from mafic intrusions their presence may explain the flat moho by compensation of lower crustal thinning by intrusion of mafic melts the pn wave velocities 8 15 8 2 km s are normal for the area and do not show any sign of decompression melting in the sub moho mantle such that possible lithosphere thinning has not reached the base of the crust on the basis of the results of the best project we suggest that the brz is formed by passive rifting in the rheologically weak suture between the siberian platform and the amurian plate

major influence of plume ridge interaction lithosphere thickness variations and global mantle flow on hotspot volcanism the example of tristan hotspot tracks are thought to originate when mantle plumes impinge moving plates however many observed cases close to mid ocean ridges do not form a single age progressive line but vary in width are separated into several volcanic chains or are distributed over different plates here we study plume ridge interaction at the example of the tristan plume which features all of these complexities additionally the south atlantic formed close to where plume volcanism began opening from the south and progressing northward with a notable decrease in magmatism across the florianopolis fracture zone we study the full evolution of the tristan plume in a series of three dimensional regional models created with the convection code aspect we then compute crustal thickness maps and compare them to seismic profiles and the topography of the south atlantic we find that the separation of volcanism into the tristan and gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field plume material below the off ridge track can flow toward the ridge and regions of thinner lithosphere where decompression melting leads to the development of a second volcanic chain resembling the tristan and gough hotspot tracks agreement with the observations is best for a small plume buoyancy flux of 500 kg s or a low excess temperature of 150 k the model explains the distribution of syn rift magmatism by hot plume material that flows into the rift and increases melt generation

crustal structure of northwest namibia evidence for plume rift continent interaction the causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute seismic tomography and other studies suggest that hot mantle material rising from the core mantle boundary cmb might play a significant role in the formation of such hotspot trails an important area to verify this concept is the south atlantic region with hotspot trails that spatially coincide with one of the largest low velocity regions at the cmb the african large low shear wave velocity province the walvis ridge started to form during the separation of the south american and african continents at ca 130 ma as a consequence of gondwana breakup here we present the first deep seismic sounding images of the crustal structure from the landfall area of the walvis ridge at the namibian coast to constrain processes of plume lithosphere interaction and the formation of continental flood basalts parana and etendeka continental flood basalts and associated intrusive rocks our study identified a narrow region 100 km of high seismic velocity anomalies in the middle and lower crust which we interpret as a massive mafic intrusion into the northern namibian continental crust seismic crustal reflection imaging shows a flat moho as well as reflectors connecting the high velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the etendeka continental flood basalt we suggest that the observed massive but localized mafic intrusion into the lower crust results from similar sized variations in the lithosphere i e lithosphere thickness or preexisting structures

linking rift propagation barriers to excess magmatism at volcanic rifted margins break up related extrusive magmatism imaged in reflection seismic data as seaward dipping reflectors sdrs extends symmetrically along the volcanic margins of the atlantic ocean recent research found distinct along margin variations in the distribution of sdrs and abundance of volcanic material was found to be spatially linked to transfer fault systems these segmented the propagating rift that later developed into the ocean and are interpreted as rift propagation barriers based on these observations we develop a numerical model which shows that rift parallel mantle flow and locally enhanced rates of volcanism are the result of delays in rift propagation and segmented opening our model suggests that segmentation is one of the major factors in the distribution and localization of riftrelated extrusive magmatism we conclude that in addition to mantle temperature and inherited crustal structures e g weaknesses from previous rift episodes rift propagation delay plays an important role in the distribution of extrusive volcanism at volcanic passive margins by controlling the mantle flow beneath the rift axis

surface wave tomography of the gulf of california 1 we measured interstation fundamental mode rayleigh wave phase velocities using data from the nars baja seismic network located around the gulf of california a region average shear velocity model and a set of azimuthally anisotropic phase velocity maps are obtained from these data the average shear velocity structure shows a strong low velocity zone underlying a thin lid and the data are suggestive of low velocities down into the transition zone the phase velocity maps display signatures of sedimentary layers crustal thickness variations upwelling under the plate boundary and the presence of the subducted farallon microplate remnants beneath the gulf the upper mantle features inferred from this study provide new seismic evidence on the tectonic evolution of the region

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

azimuthal anisotropy from array analysis of rayleigh waves in southern california summary direct detection of rayleigh wave azimuthal anisotropy is reported by applying an array analysis to broad band seismic data in southern california usa our approach has excellent resolution for frequencies between 30 and 60 mhz and good resolution between 10 and 30 mhz limitation from array size limits accuracy below 10 mhz and complicated wave propagation effects lead to difficulty above 60 mhz between 30 and 60 mhz azimuthal anisotropy of rayleigh wave phase velocity is detected cleanly phase velocity results show that the 2 components dominate the 4 components strength of anisotropy hovers around 1 per cent for the 2 components with standard error 1 of about 0 4 per cent fast axes orientations show stable orientation in the direction 110 clockwise from north and its opposite direction 290 depth sensitivity kernels for p wave and s wave anisotropy indicate that anisotropy is confined to depths in the upper 100 km and may even be to shallower depths we speculate that the fast axes alignment may be associated with a strong shearing process over the thickness of lithosphere due to obliquely collisional motions between the pacific plate and the north america plate in the region

azimuthal anisotropy of rayleigh waves in the crust in southern tohoku area japan to reveal seismic anisotropy within the crust in the southern tohoku area japan we analyze the azimuthal anisotropy of rayleigh waves in the frequency range of 40 65 mhz rayleigh wave anisotropy is estimated by array analysis of 102 teleseismic events that occurred between 2007 and 2014 the frequency variation of the estimated azimuthal anisotropy indicates depth variation of the seismic anisotropy within the crust 1 north south fast velocity direction around the upper crust and 2 trench normal fast velocity direction in the deeper crust we also conduct a time lapse analysis of rayleigh wave anisotropy to demonstrate the stability of the estimated anisotropy in time and to estimate possible changes in seismic anisotropy associated with the 2011 tohoku oki earthquake we demonstrate that the azimuthal location of teleseismic events influences the accuracy of time lapse analysis moreover a decrease of azimuthal anisotropic ratio in time lapse results might be related to changes in the velocity structure associated with release of crustal stress during the tohoku oki earthquake

seismic structure of the lithosphere beneath nw namibia impact of the tristan da cunha mantle plume northwestern namibia at the landfall of the walvis ridge was affected by the tristan da cunha mantle plume during continental rupture between africa and south america as evidenced by the presence of the etendeka continental flood basalts here we use data from a passive source seismological network to investigate the upper mantle structure and to elucidate the cretaceous mantle plume lithosphere interaction receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km we interpret this interface as the relic of the lithosphereasthenosphere boundary lab formed during the mesozoic by interaction of the tristan da cunha plume head with the pre existing lithosphere the velocity contrast might be explained by stagnated and frozen melts beneath an intensively depleted and dehydrated peridotitic mantle the present day lab is poorly visible with converted waves indicating a gradual impedance contrast beneath much of the study area converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1 5 s earlier than in the landward plume unaffected continental interior suggesting high velocities in the upper mantle caused by a thick lithosphere this indicates that after lithospheric thinning during continental breakup the lithosphere has increased in thickness during the last 132 myr thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here we propose that the remnant plume material which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration significantly contributed to the thickening of the mantle lithosphere

stable isotope and minor element proxies for eocene climate of seymour island antarctica 1 abundant and well preserved marine macrofossils on seymour island antarctica provide a valuable resource to establish paleoenvironmental conditions at high southern latitudes during the warm paleogene stable isotope minor element and 8 7 sr 8 6 sr compositions have been measured for the aragonite bivalve cucullaea from the eocene la meseta formation the 8 7 sr 8 6 sr stratigraphy suggests an early to late eocene age for the la meseta formation sea surface temperature estimates based on 1 8 o values range from 14 5 to 15 5 c in the early and middle eocene to 10 5 c in the late eocene high resolution sampling along growth structures provides estimates of seasonality in temperature which decrease considerably up section to address the relative contribution of temperature and ice volume or salinity to the 1 8 o record sr ca was measured and evaluated as a potential independent paleothermometer for cucullaea

sedimentological evidence for the formation of an east antarctic ice sheet in eocene oligocene time middle eocene to late oligocene sediments recovered at ocean drilling program sites 689 and 690 on maud rise in the southernmost atlantic ocean and at sites 738 and 744 on kerguellen plateau in the southernmost indian ocean were analysed in order to study the depositional environment and the palaeoclimate special emphasis was placed on the reconstruction of the formation of a cryosphere on the antarctic continent the investigations include quantifications of carbonate and opal contents grain size analyses and studies of clay mineral assemblages the sedimentary sequence at all sites under investigation is highly pelagic with nannofossil oozes and chalks dominant the first indication of probable glacierization at sea level is in the form of isolated gravel and terrigenous sand grains which indicate ice rafting from middle eocene time at c 45 5 ma this is supported by enhanced concentrations of detrital chlorite and reworked kaolinite probably some glaciers reached the sea while most of the continent remained under the influence of a humid and warm to temperate climate the growth of the inland ice resulted in enhanced physical weathering and in increased contents of detrital chlorite and kaolinite from about 40 ma a strengthening of the glacial conditions and the onset of continental east antarctic glaciation is recorded in early oligocene sediments with an age of 36 ma all major sediment parameters document this event the clay mineralogy changed between c 36 3 ma and c 35 5 ma from smectite dominated assemblages to illite and chlorite dominated assemblages the latter being indicative of physical weathering under cooler climates large quantities of ice rafted gravel and sand accumulated on the kerguelen plateau between 36 0 ma and 35 8 ma at the same time an increase in opal content occurred as well as a decrease in carbonate the sediment parameters imply that the east antarctic continent was more or less totally buried beneath the ice during oligocene time the ice sheet however probably did not have a polar but a temperate character as indicated by the occurrence of nothofagus and relatively warm surface water temperatures prohibiting the long distance transport of debris by icebergs

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

segmentation and volcano tectonic characteristics along the sw african continental margin south atlantic as derived from multichannel seismic and potential field data abstract regional seismic reflection and potential field data document the south atlantic s break up history between 39 s and 19 s from the early cretaceous onwards previous maps of distribution of volcanics along the margin showed volcanics along the whole african margin based on extrapolation of data based on previously unpublished marine geophysical data we found the southernmost 460 km long margin segment to be lacking huge volumes of break up related volcanic effusives northwards break up was accompanied by the emplacement of huge volumes of volcanic material prominently featured in seismic sections as huge wedge shaped seaward dipping reflectors sdrs detailed mapping of offsets left and right stepping and variations in structural character of the volcanics reveal the segmentation along and the break up history of the margin several superimposed sdr sequences suggesting episodicity of volcanic emplacement divided by periods of erosion and sedimentation are distinct along southerly lines losing prominence northwards a main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non volcanic to a volcanic margin occurs abruptly we define four distinct first order segments along the 2400 km section of the southwestern african margin covered by our seismic data from south to north these first order segments are magma poor segment i segment ii with enormous sdrs volumes decreasing sdrs volumes in segment iii segment iv again with enormous volcanic output likely influenced by walvis ridge volcanism most important is that there is no systematic increase in the volumes of the effusives towards the tristan da cunha hot spot rather there is an alternating pattern in the sdrs volumes and widths the boundary between the volcanic and magma poor margin segments in the southernmost study area is sharp 10s of km which we propose is reflected in magnetic anomaly data as well we suggest that this variability along the margin is mainly due to a change in stretching rifting character from oblique during the early stages of breakup to conventional seafloor spreading from chron m4 130 ma onwards

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

3 d magnetotelluric image of offshore magmatism at the walvis ridge and rift basin highlights we report on marine 3d magnetotelluric study on walvis ridge derived 3d electrical resistivity model shows a large scale resistive zone which we link to crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting smaller scale resistive region is attributed to magma ascent during rifting rift basin is identified by low resistivity region the namibian continental margin marks the starting point of the tristan da cunha hotspot trail the walvis ridge this section of the volcanic southwestern african margin is therefore ideal to study the interaction of hotspot volcanism and rifting which occurred in the late jurassic early cretaceous offshore magnetotelluric data image electromagnetically the landfall of walvis ridge two large scale high resistivity anomalies in the 3 d resistivity model indicate old magmatic intrusions related to hot spot volcanism and rifting the large scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2 d offshore seismic profiles one of the high resistivity anomalies above 500 m has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust one of the arms stretches underneath walvis ridge the shape is suggestive of crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting a second smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting we attribute a low resistivity anomaly east of the continent ocean boundary and south of walvis ridge to the presence of a rift basin that formed prior to the rifting

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

uppermost mantle and crustal structure at tristan da cunha derived from ambient seismic noise abstract according to classical plume theory the tristan da cunha hotspot located 400 km east off the mid atlantic ridge is thought to have played a major role in the rifting while creating an aseismic walvis ridge during and after the breakup of the south atlantic margins volcanic activity on the tristan da cunha island shows that the hotspot might still be there influencing the upper mantle and crustal structure in this study we present ambient noise data from 24 broadband obs around tristan da cunha and a seismic station on nightingale island which provide first constraints on the crustal and uppermost mantle structure around the island by combining ambient noise techniques dispersion curve analysis of rayleigh waves 2d tomographic inversion of travel times and 3d depth inversion of dispersion data we derived a 3d v s velocity model around the archipelago of tristan da cunha the model shows an isolated vertically sharp bounded thickened and modified crust beneath the islands surrounded by thin oceanic crust

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

the relationship between upper mantle anisotropic structures beneath california transpression and absolute plate motions we calculated sks splitting parameters for the california integrated seismic network in southern california we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves the inferred splitting from surface waves in the mantle lithosphere is small on average 0 2 s compared with sks splitting 1 5 s and obtains a maximum value 0 5 s in the transpressive region of the big bend south of and aligned with the san andreas fault saf in contrast the sks splitting is approximately e w and is relatively uniform spatially either side of the big bend of the saf these differences suggest that most of the sks splitting is generated much deeper down to 300 400 km than previously thought probably in the asthenosphere fast directions align with absolute plate motions apm in northern and southeastern california but not in southwestern california we interpret the parallelism with apm as indicating the sks anisotropy is caused by cumulative drag of the asthenosphere by the overlying plates the discrepancy in southwestern california arises from the diffuse boundary there compared to the north where relative plate motion has concentrated near the saf system in southern california the relative motion originated offshore in the borderlands and gradually transitioned onshore to the saf system this has given rise to smaller displacement across the saf 160 180 km compared with central and northern california 400 500 km thus in southwestern california the inherited anisotropy from prior north american apm has not yet been overprinted by pacific apm

seismic evidence for widespread western us deep crustal deformation caused by extension morgan moschetti and colleagues use ambient noise tomography data from the western united states to show that strong radial seismic anisotropy exists in the deep crust confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since about 65 million years ago the coincidence of crustal radial anisotropy with these extensional provinces supports the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension here a method of seismic wave imaging known as ambient noise tomography is used to generate high resolution images of seismic wave speeds in the crust and uppermost mantle the observations reveal strong and uniform anisotropy where waves travel through rock at different speeds depending on direction in the deep crust in areas of the western united states that have undergone significant extension during the past 65 million years laboratory experiments have established that many of the materials comprising the earth are strongly anisotropic in terms of seismic wave speeds1 observations of azimuthal2 3 and radial4 5 anisotropy in the upper mantle are attributed to the lattice preferred orientation of olivine caused by the shear strains associated with deformation and provide some of the most direct evidence for deformation and flow within the earth s interior although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes large scale observations have been limited to regions of particularly thick crust6 here we show that observations from ambient noise tomography in the western united states reveal strong deep middle to lower crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since 65 myr ago 7 8 the coincidence of crustal radial anisotropy with the extensional provinces of the western united states suggests that the radial anisotropy results from the lattice preferred orientation of anisotropic crustal minerals caused by extensional deformation these observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension9 10 11

layered azimuthal anisotropy of rayleigh wave phase velocities in the european alpine lithosphere inferred from ambient noise abstract passive seismic imaging of the earth is a rapidly developing field of study recent advances in noise cross correlation techniques allow imaging of isotropic surface wave and shear wave velocities in areas where earthquake numbers and distributions are insufficient to implement traditional earthquake based tomography furthermore advances in the theory underpinning surface wave inversion have led to depth dependent mapping of seismic anisotropy in the lithosphere and upper asthenosphere we show that by merging these two rapidly advancing fields we can invert noise based phase velocity measurements for azimuthally anisotropic phase speed thereby providing a highly resolved image of layered azimuthal anisotropy in continental crust we apply this new algorithm in the western alps an area of complex lithospheric structure alpine crustal thickening results from continental collision indentation of the ivrea mantle into the middle crust of the european plate rollback of the european lithospheric mantle and crustal slicing we find that anisotropy beneath the central alps is stratified in two layers one with an orogen parallel fast direction above 30 km depth and another with a strong orogen perpendicular fast direction at greater depth although our resolution is reduced outside the central alps we map orogen parallel anisotropy in the crust of the northern alpine foreland we interpret the results in the central alps as first order evidence for a model of azimuthal anisotropy in which 1 near vertical emplacement of crustal slices following detachment of the lithospheric mantle from the crust gives rise to orogen parallel fast directions of wave propagation in the crust and 2 dominantly horizontal tectonics in the thickened crustal root and uppermost mantle yield orogen perpendicular fast directions at greater depth

south atlantic opening a plume induced breakup upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup the formation of the walvis ridge during the opening of the south atlantic is related to a putative plume induced breakup we investigated the crustal structure of the walvis ridge southeast atlantic ocean at its intersection with the continental margin and searched for anomalies related to the possible plume head the overall structure we identify suggests that no broad plume head existed during opening of the south atlantic and anomalous mantle melting occurred only locally we therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting leaving a track of kimberlites in the african craton

intra annual isotopic variation in venericardia bivalves implications for early eocene temperature seasonality and salinity on the u s gulf coast abstract stable isotope compositions were determined for 166 microsamples of aragonite from the bivalve venericardia hatcheplata collected from the early eocene hatchetigbee formation in southwestern alabama these were milled from two transects one across the left valve hinge plate and one along the growth axis amplitude and spacing of about nine years of seasonal variation is very regular with peaks and troughs occurring at equal intervals and reaching consistent compositions except for some attenuation in both parameters during the last three years of life assuming an ice free world with an ocean 18o value of about 1 0 and using the aragonite water fractionation equation of grossman and ku 1986 isotope derived paleotemperatures are somewhat warmer 31 c than temperatures presently recorded anywhere on the earth s surface and exhibit significantly greater seasonality 12 c than regions with the warmest mean temperatures it is therefore likely that these very negative and extremely seasonal 18o values reflect the influence of seasonal discharge of 18o depleted river water into the eocene gulf of mexico comparison of these data with 1 mean annual temperature versus seasonal temperature variation from modern climates 2 isotopic compositions reported from coeval molluscs and 3 isotopic compositions of global meteoric precipitation and united states rivers suggest that mean annual temperature along this portion of the eocene gulf coast was in fact about 26 c and varied seasonally from a winter low of about 25 c to a summer high of 27 c differences between these temperatures and those derived from direct conversion of oxygen isotope values reflect the influence of seasonal influx of fresh 18o value of 4 7 water during deposition of this portion of the hatchetigbee formation ambient water 18o values varied between winter values of about 1 0 to less than 3 5 during the summer freshwater discharge served to seasonally depress salinity to 10 to 16 ppt along this part of the gulf coast the early eocene was perhaps the warmest time interval of the past 65 million years primarily manifest as lower equator to pole temperature gradients in this context an eocene mean annual temperature of 26 c is not much higher than modern surface water temperatures of about 24 c at 29 30 n immediately off the modern alabama coast seasonal temperature variation of 2 to 3 c about a mean of 26 c however is significantly lower than that off coastal alabama 19 to 30 c and occurs only along modern equatorial coasts at latitudes less than 19 this implies that early eocene tropical conditions extended well beyond modern latitudinal limits

off rift volcanism in rift zones determined by crustal unloading when continents are stretched over a long period of time deep elongated rift valleys form at earth s surface and zones of ponded magma centred beneath the rift form at the crust mantle boundary1 2 ascending magma sometimes erupts within the rift valley3 4 or counterintuitively at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth5 6 7 8 the controls on the distribution of this off rift volcanism are unclear here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley whereas others are confined to the valley we find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure caused by crustal thinning and faulting along the rift borders when gravitational unloading dominates over tectonic stretching forces dykes ascending from the ponded magma are steered towards the rift sides eventually causing off rift eruptions our model also predicts the formation of stacked magma sills in the lower crust above the magma ponding zone as well as the along rift propagation of shallow dykes during rifting events consistent with observations of magmatism and volcanism in rift zones globally we conclude that rift topography induced stress changes provide a fundamental control on the transfer of magma from depth to the surface

possible role of oceanic heat transport in early eocene climate increased oceanic heat transport has often been cited as a means of maintaining warm high latitude surface temperatures in many intervals of the geologic past including the early eocene although the excess amount of oceanic heat transport required by warm high latitude sea surface temperatures can be calculated empirically determining how additional oceanic heat transport would take place has yet to be accomplished that the mechanisms of enhanced poleward oceanic heat transport remain undefined in paleoclimate reconstructions is an important point that is often overlooked using early eocene climate as an example we consider various ways to produce enhanced poleward heat transport and latitudinal energy redistribution of the sign and magnitude required by interpreted early eocene conditions our interpolation of early eocene paleotemperature data indicate that an approximately 30 increase in poleward heat transport would be required to maintain eocene high latitude temperatures this increased heat transport appears difficult to accomplish by any means of ocean circulation if we use present ocean circulation characteristics to evaluate early eocene rates either oceanic processes were very different from those of the present to produce the early eocene climate conditions or oceanic heat transport was not the primary cause of that climate we believe that atmospheric processes with contributions from other factors such as clouds were the most likely primary cause of early eocene climate

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

the wide angle seismic image of a complex rifted margin offshore north namibia implications for the tectonics of continental breakup highlights up to 33 km thick dominantly gabbroic crust beneath walvis ridge massive gabbro addition with subjacent cumulates to the cob south of walvis ridge slow upper mantle at 35 km depth beneath etendeka plateau 4 6 km thick oceanic crust in the angola basin north of florianopolis transform velocity models suggest a dominant tectonic control on the location of magmatism abstract voluminous magmatism during the south atlantic opening has been considered as a classical example for plume related continental breakup we present a study of the crustal structure around walvis ridge near the intersection with the african margin two wide angle seismic profiles were acquired one is oriented nnw sse following the continent ocean transition and crossing walvis ridge a second amphibious profile runs nw se from the angola basin into continental namibia at the continent ocean boundary cob the mafic crust beneath walvis ridge is up to 33 km thick with a pronounced high velocity lower crustal body towards the south there is a smooth transition to 20 25 km thick crust underlying the cob in the walvis basin with a similar velocity structure indicating a gabbroic lower crust with associated cumulates at the base the northern boundary of walvis ridge towards the angola basin shows a sudden change to oceanic crust only 4 6 km thick coincident with the projection of the florianopolis fracture zone one of the most prominent tectonic features of the south atlantic ocean basin in the amphibious profile the cob is defined by a sharp transition from oceanic to rifted continental crust with a magmatic overprint landward of the intersection of walvis ridge with the namibian margin the continental crust beneath the congo craton is 40 km thick shoaling to 35 km further se the velocity models show that massive high velocity gabbroic intrusives are restricted to a narrow zone directly underneath walvis ridge and the cob in the south this distribution of rift related magmatism is not easily reconciled with models of continental breakup following the establishment of a large axially symmetric plume in the earth s mantle rift related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism dividing the margin in a magma dominated southern and an essentially amagmatic northern segment

the onset of walvis ridge plume influence at the continental margin highlights crustal structure of walvis ridge reveals high seismic velocities in the lower crust intruding the african continent this modified crust is localized to approx 100 100 km within the continent no indication for a large plume head observed the opening of the south atlantic is a classical example for a plume related continental breakup flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at tristan da cunha to determine the effect of the proposed plume head on the continental crust we acquired wide angle seismic data at the junction of the walvis ridge with the african continent and modelled the p wave velocity structure in a forward approach the profile extends 430 km along the ridge and continues onshore to a length of 720 km crustal velocities beneath the walvis ridge vary between 5 5 km s and 7 0 km s a typical range for oceanic crust the crustal thickness of 22 km however is approximately three times larger than of normal oceanic crust the continent ocean transition is characterized by 30 km thick crust with strong lateral velocity variations in the upper crust and a high velocity lower crust hvlc where velocities reach up to 7 5 km s the hvlc is 100 to 130 km wider at the walvis ridge than it is farther south and impinges onto the continental crust of the kaoko fold belt such high seismic velocities indicate mg rich igneous material intruded into the continental crust during the initial rifting stage however the remaining continental crust seems unaffected by intrusions and the root of the 40 km thick crust of the kaoko belt is not thermally abraded we conclude that the plume head did not modify the continental crust on a large scale but caused rather local effects thus it seems unlikely that a plume drove or initiated the breakup process we further propose that the plume already existed underneath the continent prior to the breakup and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts

crustal anisotropy beneath hudson bay from ambient noise tomography evidence for post orogenic lower crustal flow the crust underlying hudson bay canada records a long and complex tectonic history in this study we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle hudson bay the ambient noise data were processed to obtain group velocity dispersion measurements from 10 35 s period which were inverted using an algorithm that incorporates the effects of anisotropy this work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy the inversion method uses smoothing and damping to regularize the solution due to the significantly increased number of model parameters relative to the isotropic case we performed a careful analysis for parameter selection to determine whether leakage occurs between isotropic and anisotropic model parameters we observe a robust pattern of anisotropic fast directions in the mid crust that are consistent with large scale tectonic trends based on magnetic anomaly patterns in particular a distinctive double indentor shape for the superior craton is clearly expressed in both data sets this pattern breaks down deeper in the crust suggesting that some degree of lithospheric decoupling in the lower crust such as channel flow occurred during orogenesis given regional evidence for vertically coherent deformation in the crust and underlying mantle we interpret this pattern in the lower crust as a tectonic overprint that post dates the main phase of trans hudson deformation at most levels in the crust we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath hudson bay