shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

seismic imaging of a hot upwelling beneath the british isles the iceland plume has had an important influence on vertical motions in the north atlantic the convecting mantle in this region contains a large scale low velocity seismic anomaly which correlates with a long wavelength gravity high and bathymetric feature this suggests that an arm of plume material has extended or is extending from iceland in a direction perpendicular to the mid atlantic ridge here we present the results of a detailed teleseismic traveltime study that reveals the high resolution morphology of this low velocity anomaly beneath the british isles our images provide insights into the nature of plume lithosphere interactions the low velocity anomaly imaged in this study correlates geographically with a region of high gravity anomalies and high topography that was associated with paleogene magmatism and phases of epeirogenic uplift during the cenozoic era there is evidence that the distribution of british earthquakes is also related to the low velocity anomaly the low velocity anomaly is interpreted to represent hot material from the original iceland plume head that became trapped beneath thinned regions of lithosphere ca 60 ma

a model for the deep structure of the moine thrust zone seismic refraction and electrical conductivity profiles across northern scotland together with geological and other geophysical data are interpreted in terms of the deep structure of this part of the orthotectonic caledonides our preferred interpretation is that the moine thrust zone extends to the base of the crust and the caledonian front is steep this contrasts with recently proposed ultra thin skinned solutions the results of the recently completed moist deep seismic reflection experiment should inter alia test our hypothesis in order to test the geometrical feasibility of our interpretation balanced sections have been constructed to illustrate the crustal evolution of the region during the caledonian orogeny these sections permit estimates to be made of crustal shortening thickening and erosion our model requires that the n highlands were once overlain by major exotic nappes which by analogy with nw newfoundland may have been ophiolitic the proposed regional structural pattern comprises a duplex of crustal dimensions we suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins

seismological and field observations from the 1990 november 6 furg hormozgan earthquake a rare case of surface rupture in the zagros mountains of iran we document a unique example of a co seismic surface rupture in the zagros mountains of se iran using p and sh waveform inversion aftershock distributions and field observations we show that the mw 6 4 1990 november 6 furg hormozgan earthquake was associated with 15 km of south facing surface ruptures with an average vertical displacement of 1 m earthquakes of mw 6 7 are common in the zagros mountains of iran however no co seismic surface ruptures associated with thrust faulting have been reported before now and blind thrusting appears to be more common source parameters determined from seismology and measurements from the surface ruptures indicate slip on a fault dipping 30 n although there is likely to be considerable uncertainty in at least some locations the aftershock activity measured in the month following the earthquake appears to be broadly confined to a diffuse area around the northward dipping fault the geomorphology of the epicentral region contains indications of both long term quaternary uplift and cumulative fault scarps from repeated earthquakes the furg earthquake probably occurred on a reactivated section of the high zagros fault a geological suture from the early history of the iran eurasia collision the 1990 earthquake is likely to have ruptured to the surface because of its unusual location in the high zagros zone away from the present day focus of seismic activity in the simply folded zone of the zagros it is possible that other sections of the high zagros sutures still have the potential to generate earthquakes

transdimensional love wave tomography of the british isles and shear velocity structure of the east irish sea basin from ambient noise interferometry we present the first love wave group velocity and shear velocity maps of the british isles obtained from ambient noise interferometry and fully non linear inversion we computed interferometric inter station green s functions by cross correlating the transverse component of ambient noise records retrieved by 61 seismic stations across the uk and ireland group velocity measurements along each possible inter station path were obtained using frequency time analysis and converted into a series of inter station traveltime datasets between 4 and 15 seconds period traveltime uncertainties estimated from the standard deviation of dispersion curves constructed by stacking randomly selected subsets of daily cross correlations were observed to be too low to allow reasonable data fits to be obtained during tomography data uncertainties were therefore estimated again during the inversion as distance dependent functionals we produced love wave group velocity maps within 8 different period bands using a fully non linear tomography method which combines the transdimensional reversible jump markov chain monte carlo rj mcmc algorithm with an eikonal raytracer by modelling exact raypaths at each step of the markov chain we ensured that the non linear character of the inverse problem was fully and correctly accounted for between 4 and 10 seconds period the group velocity maps show remarkable agreement with the known geology of the british isles and correctly identify a number of low velocity sedimentary basins and high velocity features longer period maps in which most sedimentary basins are not visible are instead mainly representative of basement rocks in a second stage of our study we used the results of tomography to produce a series of love wave group velocity dispersion curves across a grid of geographical points focussed around the east irish sea sedimentary basin we then independently inverted each curve using a similar rj mcmc algorithm to obtain a series of one dimensional shear velocity profiles by merging all 1d profiles we created a fully three dimensional model of the crust beneath the east irish sea the depth to basement in this model compares well with that averaged from seismic reflection profiles this result is the first 3 dimensional model in the uk with fully quantified uncertainties it shows basin depths and basement structures and their concomitant uncertainties

small scale variations in seismic anisotropy near kimberley south africa summary we have exploited a new data set of broad band seismic waveforms recorded near the region of kimberley south africa to place new constraints on seismic anisotropy in an area of extensive mantle modification within an archaean cratonic setting the kimberley region is significant in that it represents archaean cratonic mantle that has been studied extensively via petrologic geochemical and seismic means and provides a unique opportunity to meld the results of a broad range of data sets to examine processes of crust and mantle deformation shear wave splitting observations using sks phases recorded by a dense broad band seismic array in the kimberley region exhibit consistent fast polarization directions of ne sw and splitting times that range from 0 15 s in the se to nearly 0 75 s in the nw regions of the array multichannel cross correlation of relative arrival times of teleseismic sks phases across the array reveals clear azimuthal variations and a relative arrival time range comparable to the shear wave splitting delay time range a like analysis of teleseismic pkpdf phases does not exhibit significant differences in relative arrival times across the array the combined relative arrival time and splitting analyses indicate that a significant change in the strength of anisotropic structure is required across the region our results are most consistent with a model in which variations in seismic anisotropy near the kimberley region are constrained to the lithosphere with an average anisotropic strength of 1 8 per cent extend to depths no greater than 150 km and are primarily controlled by a significant change in the strength of anisotropy within the lateral bounds of the seismic array this inferred structural boundary is near the location of the ca 2 9 gyr old colesberg magnetic lineament a prominent aeromagnetic feature interpreted as the primary suture zone joining the eastern meso archaean and western neo archaean blocks of the kaapvaal craton the proposed boundary is also located within the southern exposure of the ca 2 7 gyr old ventersdorp volcanic sedimentary sequence a key continental rifting feature in the region we hypothesize that the observed seismic anisotropy is the result of straininduced lattice preferred orientation of olivine caused by the amalgamation and deformation of the eastern and western blocks of the kaapvaal craton this collisional process mechanically weakened the region to generate the observed mantle fabric which was exploited by significant rifting events that may have subsequently enhanced the regional fabric this model is also consistent with sks splitting results for the whole of southern africa which suggest either weak anisotropic or isotropic mantle structure in the eastern kaapvaal and coherent mantle anisotropy across much of the western kaapvaal our results support the notion that seismic anisotropy beneath archaean continental regions is not created or alternatively not preserved during initial continental formation but is instead generated during subsequent significant mantle deforming events

constraints on the tectonic evolution of the westernmost mediterranean and northwestern africa from shear wave splitting analysis abstract the westernmost mediterranean mantle and lithosphere have evolved into their current configuration due to complex interactions between the african and eurasian plates to help unravel the regional tectonics we use new broadband seismic data across the gibraltar arc and into southern morocco to infer azimuthal seismic anisotropy and flow patterns for the upper mantle based on shear wave splitting analysis a deep 600 km earthquake in april 2010 was recorded by the array and allowed us to compare 31 direct s measurements with 235 teleseismic sk k s events from 3 years of deployment the patterns of apparent fast polarization orientations and delay times suggest three major tectonic domains when interpreted jointly with recent tomographic images of the subducted slab 1 a subducted slab related toroidal flow domain centered upon the alboran sea and southern spain leading to complex splits 2 a region where the west african craton deflects mantle flow in the anti atlas and high plateaux and 3 an intermediate domain across the central high atlas across the axis of the mountain belt a coherent regional maximum of delay times is observed for both s and sks splitting measurements with polarizations predominantly parallel to the strike we interpret this as possible sw ne channeling of mantle flow beneath the region with a thinned lithosphere and slow seismic velocities beneath the central high atlas mountains

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

seismic anisotropy beneath east greenland revealed by shear wave splitting 1 seismic anisotropy was investigated by measuring shear wave splitting at 7 broadband stations in eastern greenland splitting parameters fast polarization and time delay were determined for sks and skks phases fast polarizations are oriented roughly parallel to the east greenland coast at 5 stations while for two they are nearly perpendicular to the coast the anisotropy at 5 is unrelated to the spreading direction of the north atlantic or absolute plate motion the fast polarization direction can be related to tectonic fabric in the caledonides in the north suggesting that anisotropy is frozen in the lithosphere deformed coherently with depth another explanation of the coast parallel fast polarization is asthenospheric flow along the mid atlantic ridge at the time of opening of the ocean inducing fabric in the thickened lithosphere the time delays 0 5 1 3 s do however require that fabric to be strong or distributed over a substantial depth range

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

nature of the moho beneath the scottish highlands from a receiver function perspective abstract the geology of scotland documents a protracted geological history from precambrian basement formation through the caledonian orogeny and to the tertiary opening of the atlantic a temporary deployment of 22 closely spaced broadband seismic stations in scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure with teleseismic p wave receiver functions we use the h method to determine variations in bulk crustal parameters crustal thickness h and v p v s ratio mean crustal thickness is 28 km varying from 23 km in the ne highlands and increasing to 30 km near the highland boundary faultin the southern part of the study area mean v p v s values of 1 76 show no significant variation across the study area an abrupt increase in crustal thickness of 4 5 km nw across the moine thrust is not easily linked to tertiary recent tectonic activity instead it appears that scotland s crust has retained features for hundreds of millions of years since it was first formed despite abundant volcanic activity in tertiary times our work favours the view that the moho is a compositional boundary rather than a mineralogical one defined by a reaction in p t space

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

structural styles of paleozoic intracratonic fault reactivation a case study of the grays point fault zone in southeastern missouri usa abstract a pronounced subparallel set of northeast striking faults occurs in southeastern missouri but little is known about these faults because of poor exposure the commerce fault system is the southernmost exposed fault system in this set and has an ancestry related to reelfoot rift extension recent published work indicates that this fault system has a long history of reactivation the northeast striking grays point fault zone is a segment of the commerce fault system and is well exposed along the southeast rim of an inactive quarry our mapping shows that the grays point fault zone also has a complex history of polyphase reactivation involving three periods of paleozoic reactivation that occurred in late ordovician devonian and post mississippian each period is characterized by divergent right lateral oblique slip faulting petrographic examination of sidewall rip out clasts in calcite filled faults associated with the grays point fault zone supports a minimum of three periods of right lateral oblique slip the reported observations imply that a genetic link exists between intracratonic fault reactivation and strain produced by paleozoic orogenies affecting the eastern margin of laurentia north america interpretations of this link indicate that right lateral oblique slip has occurred on all of the northeast striking faults in southeastern missouri as a result of strain influenced by the convergence directions of the different paleozoic orogenies

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

241pu 239 240pu ratios in polar glaciers abstract the analyses of plutonium isotopes in dated strata of polar ice sheets indicate that the 241 pu 239 240 pu activity ratio produced in the u s dominated nuclear tests during the 1950s was about 26 while that in the u s s r dominated weapons tests in the early 1960s was between 12 and 14 this difference provides time horizons for sedimentary deposits further the 239 pu 240 pu ratio may show a similar difference in fallout values from these two periods of weapons testing and may provide an additional and perhaps more sensitive chronology for sediments

lithospheric deformation inferred from teleseismic shear wave splitting observations in the united kingdom observations of splitting in teleseismic shear waves recorded at stations in the united kingdom uk seismic network operated by the british geological survey are related to the tectonic structures and history in the uk the combination of network density and tectonic variability in the uk permits an assessment to be made of the relationship between splitting and geology fast polarization directions are most commonly parallel to geologic features resulting from compressional or strike slip events and splitting delay times mirror the increasing involvement of basement in deformation as observed through field investigation different orogenic episodes may be distinguished in the splitting orientations a peak in delay times is suggested in the observations at the iapetus suture a site of paleozoic subduction now in the uk interior these observations are consistent with the idea that continental anisotropy arises through coherent deformation of the lithosphere and is preserved long after orogenic deformation ceases

a sub crustal piercing point for north atlantic reconstructions and tectonic implications plate tectonic reconstructions are usually constrained by the correlation of lineaments of surface geology and crustal structures this procedure is however largely dependent on and complicated by assumptions on crustal structure and thinning and the identification of the continent ocean transition we identify two geophysically and geometrically similar upper mantle structures in the north atlantic and suggest that these represent remnants of the same caledonian collision event the identification of this structural lineament provides a sub crustal piercing point and hence a novel opportunity to tie plate tectonic reconstructions further this structure coincides with the location of some major tectonic events of the north atlantic post orogenic evolution such as the occurrence of the iceland melt anomaly and the separation of the jan mayen microcontinent we suggest that this inherited orogenic structure played a major role in the control of north atlantic tectonic processes

crust mantle coupling mechanism in cameroon west africa revealed by 3d s wave velocity and azimuthal anisotropy abstract to understand the depth variation of deformation beneath cameroon west africa we developed a new 3d model of s wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion we found that the cameroon volcanic line cvl is well delineated by slow phase velocities in contrast with the neighboring congo craton in agreement with previous studies apart from the congo craton and the oubanguides belt the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere the direction of fast axis in the upper crust is mostly ne sw but trending approximately n s around mt oku and the southern cvl the observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities a widespread zone of weak to zero azimuthal anisotropy in the mid lower crust shows evidence for vertical mantle flow or isotropic mid lower crust in the uppermost mantle the fast axis direction changed from ne sw to nw se around mt oku and northern cameroon this suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed ne sw fast azimuths are observed beneath the congo craton and are consistent with the absolute motion of the african plate suggesting a mantle origin for the observed azimuthal anisotropy our tomographically derived fast directions are consistent with the local sks splitting results in some locations and depths enabling us to constrain the origin of the observed splitting the different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in cameroon

crustal structure of the british isles and its epeirogenic consequences crustal receiver functions have been calculated for a network of 51 three component broadband seismometers distributed across the british isles and nw europe over 3200 receiver functions were assembled for 1055 events for each station preliminary estimates of crustal thickness and vp vs ratio were obtained from h plots stacked receiver functions were then inverted to determine shear wave velocity as a function of depth each result was checked by guided forward modelling and by monte carlo error analysis in this way the robustness of our final calculated velocity profiles was carefully tested a set of depth migrated profiles was also constructed using an average of 50 events for each station over a range of backazimuths these profiles agree well with legacy wide angle crustal models our results show that crustal thickness varies between 24 and 36 km across the british isles thicker crust is found beneath north wales and beneath central scotland thinner crust occurs beneath northwest scotland and northwest ireland by combining our database with the results of controlled source wide angle experiments and with depth converted reflection profiles we have produced a detailed crustal thickness map for a region encompassing the british isles our synthesis of crustal thickness and structure has important implications for the tectonic and magmatic histories of this region complex moho structure with lower crustal p wave velocities of 7 km s 1 occurs beneath regions of cenozoic magmatism which may be consistent with magmatic underplating thin crust beneath northern britain suggests that present day long wavelength topography is maintained by regional dynamic support originating beneath the lithospheric plate

shear wave splitting around the eifel hotspot evidence for a mantle upwelling summary we present the results of a shear wave splitting analysis across the eifel hotspot in westcentral europe our data set includes 18 permanent european broadband stations and 102 temporary eifel broadband and short period stations we observe variations in splitting at most permanent stations but are not able to model them reliably with unique dipping axis or two layer anisotropy models we prefer instead a single layer model with a horizontal fast axis to approximate the first order anisotropy which varies smoothly between stations we observe a first order parabolic pattern in fast polarization azimuth around the hotspot which suggests that a lattice preferred orientation lpo of olivine fast axes exists in the asthenosphere as a result of the interaction between the slowly wsw moving eurasian plate and a mantle upwelling beneath the eifel volcanic fields the minority of the variation not explained by this model correlates with rapid lateral changes in splitting and is interpreted as a result of additional anisotropy and or the effects of dynamic recrystallization associated with lpo development in the region of corner flow near the conduit our parabolic asthenospheric flow paf interpretation is consistent with eifel geological tomographic receiver function global absolute plate motion electrical conductivity anisotropy and geochemical studies as well as with splitting studies in the great basin and around hawaii we suggest that the eifel upwelling is sporadic and a result of a low excess upwelling temperature and or varying crustal stresses that periodically shift and facilitate eruption the paf pattern we observe neither suggests nor rules out anisotropy in the conduit associated with a wet eifel upwelling we use our optimum pa f model parameters to calculate a eurasian plate speed of 12 km ma 1 w hich is consistent with the recent hs3 nuvel1a speed of 19 14 km ma 1

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

the thrust structures of southern assynt moine thrust zone the moine thrust zone of southern assynt forms part of the northwest margin of the caledonide belt and has aroused controversy concerning amounts and timing of thrust displacement and sequence of thrust development recent mapping shows it to have been a foreland propagating thrust sequence the uppermost ductile moine thrust formed first followed by sequences of imbricates a major thrust the ben more thrust and then several lower duplex zones this sequence is clear from new observations that many of the earlier thrusts were folded and or breached during the development of the underlying structures a displacement of over 54 km has been estimated for the zone as a whole an alkaline igneous complex including the large borrolan syenite was intruded during the development of the thrust zone and much of it was carried some 30 km to the west northwest onto the foreland late extensional structures in southern assynt are an integral part of the caledonide thrust sequence and probably developed from the collapse of the thrust wedge as it climbed from stronger basement rocks on to a weaker cover sequence on the foreland

the great glen fault a major vertical lithospheric boundary two lamprophyre suites are used to constrain sub continental lithospheric mantle domains in late caledonian northern britain at 400 ma a northern highlands and a southern domain are resolved the first has low nd 6 4 to 12 8 the latter has higher 3 9 to 3 4 the boundary between them is coincident with the surface expression of the great glen fault the two mantle domains tightly bracket the fault the lamprophyre magmas were generated at depths of at least 100 km at the end of the caledonian orogeny the great glen fault was a major vertical discontinuity that transected the sub continental lithospheric mantle

the nature of roof thrusts in the moine thrust belt nw scotland implications for the structural evolution of thrust belts buried thrust systems commonly analysed in terms of duplex structures are often interpreted as forming in a particular sequence this model considers that only one thrust is active at a time and the resulting structural geometry is well ordered however a reappraisal of structural geometries from the moine thrust belt in nw scotland has shown that many apparent duplex structures are more complex although the upper roof thrust of some duplexes may be folded and bulged by underlying imbricate slices in accordance with simple duplex models the footwall to these roof thrusts cut up and down stratigraphic section in conflict with the simple duplex models examples include the glencoul thrust with its underlying imbricate system and the foinaven duplex simultaneous slip on an array of imbricate thrusts can bulge duplex roofs relationships classically used to infer piggy back sequences of thrust propagation however activity on more hinterland ward imbricate thrusts can cause the roof onto which they branch to truncate more foreland ward structures in this fashion the duplex roof can cut up and down stratigraphic section mimicking an extensional fault by varying thrust activity in three dimensions within an imbricate system local parts of roof thrusts can generate domains of overstep behaviour that have only local significance existing interpretations of overstep relationships in the moine thrust belt as representing late orogenic low angle extensional faults may need revision the new interpretations of deformation activity where displacements in thrust arrays happen synchronously rather than sequentially are consistent with emergent thrust and fold belts with analogue experiments and with mechanical models of orogenic wedges

precambrian plate tectonics seismic evidence from northern hudson bay canada the canadian shield is one of the largest exposures of precambrian rocks on earth it is a mosaic of several archean terranes that were brought together during a series of paleoproterozoic orogens culminating in the so called trans hudson orogen which is thought to have been similar to the himalayan orogen in scale and nature the tectonic evolution and lithospheric subdivisions of this region are poorly understood but new seismic networks in northern hudson bay provide fresh opportunity to place constraints on the precambrian processes that formed and shaped it we show via a study of seismic anisotropy that the lithosphere of the northern hudson bay region retains a strong signature of archean paleoproterozoic tectonics we map the boundary between the upper churchill and lower superior plates that collided ca 1 8 ga and identify back azimuth dependent splitting parameters t on baffin island that indicate complex anisotropy e g dipping fabric beneath the region our results support the view that significant lithospheric deformation occurred during the paleoproterozoic and that modern day plate tectonic processes were thus in operation by at least ca 1 8 ga

rayleigh wave tomography of the british isles from ambient seismic noise we present the first rayleigh wave group speed maps of the british isles constructed from ambient seismic noise the maps also constitute the first surface wave tomography study of the crust under the british isles at a relatively high resolution we computed interferometric interstation rayleigh waves from vertical component records of ambient seismic noise recorded on 63 broad band and short period stations across the uk and ireland group velocity measurements were made from the resulting surface wave dispersion curves between 5 and 25 s using a multiple phase matched filter method uncertainties in the group velocities were computed by calculating the standard deviation of four dispersion curves constructed by stacking a random selection of daily cross correlations where an uncertainty could not be obtained for a ray path using this method we estimated it as a function of the interreceiver distance group velocity maps were computed for 5 25 s period using the fast marching forward solution of the eikonal equation and iterative linearized inversion at short and intermediate periods the maps show remarkable agreement with the major geological features of the british isles including terrane boundaries in scotland regions of late palaeozoic basement uplift areas of exposed late proterozoic early palaeozoic rocks in southwest scotland northern england and northwest wales and sedimentary basins formed during the mesozoic such as the irish sea basin the chester basin the worcester graben and the wessex basin the maps also show a consistent low velocity anomaly in the region of the midlands platform a proterozoic crustal block in the english midlands at longer periods which are sensitive velocities in the lower crustal upper mantle the maps suggest that the depth of moho beneath the british isles decreases towards the north and west areas of fast velocity in the lower crust also coincide with areas thought to be associated with underplating of the lower crust such as northern ireland the eastern irish sea and northwest wales

shear wave splitting in the rajasthan craton india abstract the shear wave splitting parameters i e fast axes orientation and delay time t are measured at four broadband stations in rajasthan using sks skks core phases from three component broadband waveforms of 30 events recorded during 2014 16 the thicknesses of lithosphere at these four stations are also inferred from the estimated delays the average splitting parameters t at the northern most jai and southern most ban stations are modeled to be 36 44 08 36 and 1 48 0 32 s and 35 72 10 47 and 1 86 0 34 s respectively while these parameters at udi and bun stations are found to be 41 08 08 54 and 1 61 0 26 s and 43 86 08 96 and 1 68 0 27 s respectively thus our modeling reveals a mean fast axes orientation of 39 9 with a mean delay of 1 7 0 3 s for rajasthan which could be attributed to the frozen lithospheric anisotropy in the direction of current absolute plate motion of the indian plate a slight regional variation of observed orientation of fast axes ranging from 35 7 at ban to 43 9 at bun could be attributed to the tectonic control rather than current plate motion in addition the mean orientations of fast axes are measured to be 36 4o at udi and 41 1o at jai respectively thus this observed coherent variation of fast axes orientation across the rajasthan suggests a significant contribution from the asthenospheric flow further estimated mean delay time 1 7 0 3 s does also suggest a contribution from the anisotropy associated with asthenospheric flows the maximum delay 1 86 s is estimated at southernmost ban station suggesting a thick anisotropic layer of 214 km while minimum delay 1 48 s is modeled at northernmost jai station suggesting a 170 km thick anisotropic layer while asthenospheric layer thicknesses at udi and bun are found to be 185 and 193 km respectively this observation suggests a relatively more anisotropic and thick lithosphere in the south in comparison to that in the north the measured deviation of fast axis orientation from the absolute indian plate motion varies from 1 to 4 3 which could be related to the changes in asthenosphric flow pattern due to the presence of topography at the base of lithosphere

deformation and mantle flow beneath the sangihe subduction zone from seismic anisotropy abstract subduction of oceanic lithosphere is the most direct feedback between the earth s surface and deep interior however the detail of its interaction with the broader convecting mantle is still unclear mantle flow around subduction zones can be constrained using seismic anisotropy but despite many such studies a simple global picture is lacking the sangihe subduction zone where the molucca sea microplate is subducting westward beneath the eurasian plate is part of the tectonically complex sulawesi philippine region and an ideal natural laboratory to study complex subduction processes we investigate the anisotropic structure of the sangihe subduction zone with shear wave splitting measurements of local s and sks phases at two stations mni in sulawesi dav in the philippines as well as downgoing s phases at five stations at teleseismic distances combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase the broad depth distribution of local events 60 630 km allows us to observe a change in splitting behaviour at 380 km depth above fast directions are trench parallel and delay times t are 0 34 0 53 s with no increase with depth we suggest this anisotropy is caused by aligned cracks possibly melt filled beneath the volcanic arc and fossil anisotropy in the overriding plate below 380 km is predominantly trench normal and t are slightly higher 0 53 0 65 s as no correlation is observed with inferred distance travelled inside the slab we attribute this anisotropy to shear layers atop the slab which are coherent from 200 to 400 km depth and perhaps extend into the transition zone sks and source side measurements show larger t 1 53 and 1 33 s respectively and trench parallel since these phases predominantly sample sub slab mantle we consider along strike lateral flow associated with the double sided subduction of the molucca sea microplate to be the most likely explanation we thus infer three dominant regions of anisotropy at the sangihe subduction zone one within the overriding lithosphere one along the slab wedge interface and one below the subducting molucca sea slab the mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy this study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems

insights into the structure and dynamics of the upper mantle beneath bass strait southeast australia using shear wave splitting abstract we investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in bass strait and the surrounding region of southeast australia our dataset includes 366 individual splitting measurements from sks and skks phases the pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current n s absolute plate motion in the eastern lachlan fold belt fast shear waves are polarized parallel to the structural trend n25e further south fast shear wave polarization directions trend on average n25 75e from the western tasmania terrane through bass strait to southern victoria which is consistent with the presence of an exotic precambrian microcontinent in this region as previously postulated stations located on and around the neogene quaternary newer volcanics province in southern victoria display sizeable delay times 2 7 s these values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere or a contribution from both the lithospheric and asthenospheric mantle in the eastern tasmania terrane nearly all observed fast directions are approximately nw se although part of our data set strongly favours anisotropy originating from fabric frozen in the lithospheric mantle a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters we suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring and so variations in anisotropy related to mantle flow may be expected alternatively the pattern of fast polarisation orientations observed around bass strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered cosgrove volcanic track however it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs asthenospheric upper mantle due to poor backazimuthal coverage of the data

critical point theory of earthquakes observation of correlated and cooperative behavior on earthquake fault systems 1 the critical point theory for earthquakes was originally proposed to explain the scaling relations observed in earthquakes including the gutenberg richter frequency magnitude relation and the omori s law for aftershocks in this model main shocks their foreshocks and aftershocks are all associated with the formation of a correlated cooperative spatial region with high stress until now only indirect evidence of the existence of these correlated regions has been reported here in this paper we present observations and analyses that allow us to directly map the high stress spatially correlated regions preceding four major earthquakes i e the 1992 landers california 1995 kobe japan 1999 chi chi taiwan and 1999 hector mine california earthquakes we therefore conclude that the locations and extent of large main shocks and their immediate aftershocks can be determined from seismicity data taken prior to the main shocks and provide additional evidence in support of the critical point theory for earthquakes

structural evolution in the moine of northwest scotland a caledonian linked thrust system a model is proposed whereby the caledonian metamorphic basement cover complex of northwest scotland the moine is considered as a linked thrust system this system lies between the moine thrust at its base and the naver sgurr beag slide at its top ductile fold and thrust zones which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies are interpreted as decoupling from a detachment presently situated at relatively shallow depths this model is illustrated by two preliminary balanced cross sections these imply shortening across the northwest scottish caledonides in excess of 130 km and probably over 200 km when these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth the most likely location for the lower crustal wedge is beneath the grampian highlands

spatial and temporal constraints on sources of seismic anisotropy evidence from the scottish highlands 1 routine sks splitting analyses of seismic anisotropy usually suffer from limitations in station density that hinder attempts to place spatial constraints on anisotropic fabrics data from a 20 km station spacing network in scotland show that splitting parameters vary considerably t 0 45 1 5 s 49 128 over short 10 20 km length scales improved spatial constraints then lead to tighter temporal constraints on the anisotropic sources asthenospheric fabrics due to tertiary rifting and present day plate motions do not strongly influence our results that instead correlate with lithospheric scale trends inferred from surface geology splitting observations track scotland s tectonic history from the precambrian emplacement of crustal basement and through the activation of large scale faulting and thrusting during the caledonian orogeny the shallow lithosphere beneath scotland has preserved a fossil anisotropic signature up to hundreds of millions of years after it was formed

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation