high velocity anomaly adjacent to the western edge of the pacific low velocity province koki idehara 1 satoru tanaka2 and nozomu takeuchi3 1department of earth and planetary science university of tokyo 7 3 1 hongo bunkyo ku tokyo 113 0033 japan e mail idehara eps s u tokyo ac jp 2institute for research on earth evolution japan agency for marine earth science and technology 2 15 natsushima cho yokosuka 237 0061 japan 3earthquake research institute university of tokyo 1 1 1 yayoi bunkyo ku tokyo 113 0032 japan

numerical modeling of propagation and overlap of thrust faults with application to the thrust fold belt of central alberta abstract we use two computer programs to analyze the propagation of multiple thrust faults and their influence on the geometry of a thrust belt they use algorithms based on current equations of fault propagation to generate graphical simulations the simulations are used to demonstrate a model of thrust propagation and thrust belt development that fits current knowledge of fault propagation an alternative to the thrust transfer zone model is proposed called thrust overlap zones the computer simulations provide useful information about the generation and behavior of multiple faults the number of faults the positions of their nucleation points and hence their density the rock properties and rate of fault propagation govern the uniformity of the shortening along the thrust belt thus shortening of a thrust belt can be distributed more evenly by means of propagation of a large number of small faults than by a few large faults the final geometry of a large thrust fault on a geological map provides few clues about its origin i e whether it was produced by the coalescence of a series of en echelon thrust faults or from only a single fault a simple kinematic model based on map length and position of known thrust faults imitates the curved fault map patterns of a segment of the thrust fold belt of central alberta

structural features and shear velocity structure of the pacific anomaly 1 we constrain structural features and shear velocity structure of a low velocity anomaly in the lower mantle beneath the pacific we term it the pacific anomaly on the basis of forward travel time and waveform modeling of the observed direct s sdiff scs sks and skks phases sampling a great arc across the anomaly from eastern eurasia to southern south america after correction for the effects of earthquake mislocation and the seismic heterogeneities outside the pacific anomaly seismic observations suggest that the pacific anomaly along the great arc consists of at least two separated portions with a 740 km wide gap between them the western portion of the anomaly is about 1050 km wide extends at least 740 km above the core mantle boundary cmb and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth the velocity structure of the western portion varies from 3 0 at the top 740 km above the cmb to 3 5 at 100 km above the cmb and an average shear velocity reduction of 5 in the bottom 100 km of the mantle the eastern portion of the anomaly reaches at least 340 km above the cmb beneath the mid pacific with an 1800 km wide base and has a uniform velocity reduction of 3 waveform modeling further suggests a very low velocity layer with a shear velocity reduction of 10 located at the edge of the western portion of the anomaly combining the latest results from others we present a general picture of structural and velocity structures of the pacific anomaly the structural and velocity features suggest that the anomaly represents a cluster of metastable thermo chemical piles

thrust faults and back thrusts in madison range of southwestern montana foreland in the rocky mountain foreland of southwestern montana a zone of late cretaceous thrust faults named the hilgard fault system extends along the west side of the madison range from hebgen lake northward for about 50 mi 80 km the thrust faults are steep at their leading edges but flatten westward beneath the associated plates where they commonly dip 25 30 structural lows and highs are apparent beneath the beaver creek plate the major thrust sheet of the system and correlate with salients and reentrants of the plate the beaver creek plate consists primarily of archean metamorphic rocks but phanerozoic strata are preserved along the northern part of the plate s leading edge only the forward part of the plate is preserved in the madison range because c nozoic normal faults of the madison range fault system dropped much of the plate beneath the madison valley on the west the kirkwood plate lies east of and beneath the beaver creek plate and contains structures of the eastern part of the hilgard system the central segment of the leading edge of the kirkwood plate is not completely detached from underlying strata the leading edge of the northern part of the plate defined by the cache creek fault is flanked on the west by an associated anticline the southern end of the cache creek fault is an eastward dipping back thrust which abruptly steepens on the west adjacent to the anticline both the fault and the anticline are believed to have formed above a concealed detachment fault the southern part of the kirkwood plate displays structures interpreted to represent displacement transfer from the beaver creek plate back thrusts are associated with both thrust plates and played a key role in accommodating strain associated with folding the strata at the leading edges of the plates the back thrusts project upward from the main thrust fault beneath each plate they dip eastward at moderate angles in their lower segments but steepen at structurally higher levels they become overturned locally and dip westward in their uppermost segments the northern part of the study area contains the northwest trending buck creek reverse fault which dips steeply to the northeast movement on this structure ceased before the north trending thrust faults of the hilgard system were emplaced

evaluation of 1 d and 3 d seismic models of the pacific lower mantle with s sks and skks traveltimes and amplitudes 1 in this study we analyzed the seismic phases s sks and skks from 31 deep focus earthquakes in the tonga fiji region recorded in north america between epicentral distances of 85 and 120 the differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one dimensional 1 d reference earth models the increase of the s sks amplitude ratio up to a factor of 10 is accompanied by an increase of the s sks differential traveltime of up to 10 s skks sks differential traveltimes of 2 3 s and skks sks amplitude ratios of a factor of 2 4 across the epicentral range have maxima near 107 we examined these observations using full 1 d and 3 d waveforms for three 1 d seismic velocity profiles for the central pacific region and for the tomographic model s40rts including modifications different regularization parameters great circle path azimuthal variation strength of s wave velocity perturbations s wave velocity gradients in the lower mantle and ultra low velocity zones to explain these data we constructed a hybrid model that combines both features of s40rts and short wavelength features from the 1 d profiles the large scale seismic structure is represented by s40rts embedded within s40rts are a 20 km thick ultra low velocity zone at the core mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths our analysis demonstrates that the s wave velocity structure of the pacific large low shear velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

evidence for a thick and localized ultra low shear velocity zone at the base of the mantle beneath the central pacific abstract prominent postcursors to s sdiff waves with delays as large as 26 s are observed in northern america for papua new guinea events these waves sample the northern side of the pacific large low shear velocity province revealed by global shear velocity vs tomographic models the emergence of the postcursors strongly depends on the epicenter to station azimuth indicating that the waveforms are in general strongly affected by 3 dimensional 3d heterogeneities we limit our focus to an azimuthal range around 60 measured clockwise from north at the epicenter where the records show a relatively small azimuthal variation suggesting a relatively small 3d effect there in this azimuthal range we attempt 2d structural modelling along the great circle plane towards stations in southern us first we use a 2d ray theory to search for a range of models which generate a postcursor to the main sdiff phase with a delay time consistent with the observations then for some typical models we calculated waveforms at periods down to 5 s using the spectral element method we obtained several models that provide synthetic waveforms in a fair agreement with the observations the result shows that two types of low vs regions are required to explain the data one is a broad and weak anomaly region with a vs reduction of 5 or so constituting a part of the pacific large low shear velocity province at the base of the mantle the other is a laterally localized strong anomaly region with a more than 25 reduction of vs within a thickness of at least 80 km and a width on the order of 500 km

geographic boundary and shear wave velocity structure of the pacific anomaly near the core mantle boundary beneath western pacific we determine the geographical boundary and shear velocity structure of a very low velocity province at the base of the earth s mantle beneath western pacific we term it the pacific anomaly based on the waveform modeling and travel time analysis of scsh sh phases our seismic data are from the china national digital seismographic network the f net in japan the global seismographic network and several passcal arrays the observed scs sh differential travel time residuals allow the northwestern geographic boundary of the anomaly to be clearly defined the seismic data also suggest that the average shearvelocity reduction inside the anomaly reaches 5 in the lowermost 300km of the mantle waveform modeling of the seismic data sampling the edge of the anomaly suggests that the northwestern boundary is best characterized by a shear velocity model with a velocity jump of about 2 at about 100 145km above the core mantle boundary and a thin 30 km thick basal layer with a shear wave velocity reduction of 13 stacked seismic data sampling the middle of the anomaly however show no evidence for any internal discontinuity with a velocity decrease greater than 2 in the middle of the anomaly overall the seismic data sampling the base of the pacific anomaly can be explained by a negative shear velocity gradient from 0 to 1 top to 13 bottom in the lowermost 220 km of the mantle similar to those of a very low velocity province beneath the south atlantic ocean and the indian ocean such a strong negative shear velocity gradient can be explained by partial melting of a compositional anomaly produced early in the earth s history located within a bottom thermal boundary layer our travel time data also exhibit small scale variations inside the anomaly indicating existence of internal small scale seismic heterogeneities inside the pacific anomaly

extent of the low velocity region in the lowermost mantle beneath the western pacific detected by the vietnamese broadband seismograph array 1 we present evidence showing the extent of the low velocity region in the lowermost mantle beneath the western pacific we analyzed s ss scs s and sscs ss travel times observed by the vietnamese broadband seismograph array deployed as part of the ocean hemisphere project the abrupt changes in scs s and sscs ss travel times suggest that the western geographical boundary of the low velocity region is located around 140 e and is sharp more than 4 velocity contrast within 200 km the dependency of s and ss travel time anomalies of epicentral distances suggests that the strong low velocity region is confined to within 400 km from the cmb core mantle boundary the existence of lateral heterogeneities with a 100 km scale inside the low velocity region is also suggested by the abrupt changes in s and scs waveforms

high resolution 3 d s velocity structure in the d region at the western margin of the pacific llsvp evidence for small scale plumes and paleoslabs abstract although previous tomographic studies found a large low s velocity province llsvp in the lowermost mantle beneath the pacific due to a lack of resolution it remains unclear whether the llsvp consists of clusters of small scale low velocity anomalies or large scale anomalies we recently deployed a seismic array in thailand which provides a dataset with wide azimuthal coverage at the western pacific llsvp we analyze the new dataset using waveform inversion and find high velocity anomalies extending vertically to a height of 400 km above the core mantle boundary cmb beneath the philippine sea and small scale low velocity patches with a diameter of 300 km at the cmb beneath new guinea the locations of the high velocity anomalies are consistent with the past izanagi plate subduction boundary and the low velocity anomalies can be interpreted as a small scale plume cluster hence we conclude that vertical flow upwelling plumes and downwelling of slabs is dominant in the lowermost mantle beneath the western pacific region

topological constraints on imbricate thrust networks examples from the mountain city window tennessee u s a abstract the geometry of thrust belts is best described in three dimensions as a network of linear geometric elements trailing branch lines where two thrusts diverge leading branch lines where two thrusts converge tip lines where thrust displacement vanishes and cutoff lines where a stratigraphic horizon intersects a thrust plane there are a number of topological constraints in three dimensions on the interrelationship of these linear elements these constraints provide a method of deciphering the three dimensional structure of an area from the geologic map pattern alone if topography or structural culminations and depressions create significant structural relief the mountain city window tennessee is an example of an imbricate thrust network which reveals its topology on the map pattern because of the presence of culminations the analysis of linear elements in this area allows the regional structural geometry to be described and also two kinematically distinct types of imbricate thrust systems to be characterized namely hinterland and foreland dipping duplexes the doe ridges culmination is a hinterland dipping duplex characterized by leading branch lines in a common roof thrust with the branch lines arranged in the same order hinterland to foreland as in the undeformed state the limestone cove culmination is a foreland dipping duplex characterized by trailing branch lines in a common roof thrust with the branch lines reversed in order hinterland dipping duplexes are the common and well known type foreland dipping duplexes are formed by successive trailing branch lines moving up off the lower flat horizon onto the ramp at which the imbricate slices are formed

detection of ridge like structures in the pacific large low shear velocity province abstract waveform tomography is conducted for sh velocity structures of the entire mantle using approximately 3 5 times the data used for obtaining the previous model sh18ce the resultant new model sh18cex exhibits a cluster of ridge like low velocity anomalies in the western part of the pacific large low shear velocity province llsvp the location of the ridge like anomalies is in good agreement with the location of the abrupt change in the topography of the d discontinuity these results suggest that the llsvp is associated with a cluster of ridge like piles rather than a single large pile spread over the entire region the piles probably consist of intrinsically dense material however either their volume or density contrast may not be sufficiently large to develop large scale domes

whole mantle sh velocity model constrained by waveform inversion based on three dimensional born kernels summary a whole mantle sh velocity model is obtained by using a unique data set and techniques body and surface waveforms including major and multi orbit phases are used as a data set and are inverted by using 3 d born kernels the resultant model sh18ce reveals the different natures of the two major upwelling systems the strong low velocity anomalies beneath africa extend for more than 1000 km from the core mantle boundary cmb whereas those beneath the pacific are restricted to 300 400 km from the cmb the results also show the variable natures of stagnant slabs on the 670 discontinuity around japan the depths of the strongest high velocity anomalies within the stagnant slabs are different region by region which is consistent with the detailed delay time tomography model in this area

on the vertical extent of the large low shear velocity province beneath the south pacific superswell the three dimensional s wave velocity structure beneath the south pacific superswell is obtained from joint broadband seismic experiments on the ocean floor and islands we collected only approximately 800 relative times of long period teleseismic sh waves by using a waveform cross correlation from 76 events occurring from january 2003 to may 2005 we conducted relative time tomography to obtain a 3d structure to depths of 1600 km in the resultant image we find a characteristic distribution of low velocity regions the most prominent features are a large doughnut shaped low velocity region at 800 km depth and an elongated large low velocity region beneath the society to pitcairn hotspots at 1200 km depth our model suggests that a large low shear velocity province rooted in the d00 extends upwards and culminates near the top of the lower mantle beneath the central part of the south pacific superswell although its perfect continuity is not still confirmed

lower mantle structure from paleogeographically constrained dynamic earth models seismic tomography reveals two large low shear velocity provinces llsvps beneath africa and the pacific ocean these structures may have existed for several 100 myr and are likely compositionally distinct based on observed seismic characteristics interpreted in light of geodynamic models and mineral physics constraints we investigate the dynamics of the llsvps through the use of evolutionary models of thermochemical convection from 250 ma to present day we use a spherical convection model in which the anomalous structures have a high bulk modulus consistent with seismic interpretation a new progressive assimilation method incorporates constraints from paleogeography using a refined plate history model with 1 myr time spacing to guide the thermal structure of the lithosphere and steer the thermal evolution of slabs in the uppermost mantle the thermochemical structures deform and migrate along the core mantle boundary cmb through coupling to plate motions and in response to slab stresses the models produce a ridge like anomaly beneath africa and a rounded pile beneath the pacific ocean which at present day agrees with tomography waveform modeling and other geodynamic studies plumes emanate from the margins of the domes and ridges of thickened boundary layer between the domes dense and viscous slabs can undermine the stability of high bulk modulus structures at the cmb high bulk modulus structures are not necessarily required to satisfy dynamic constraints on the llsvps

mapping the lowermost mantle using core reflected shear waves a map of laterally varying d velocities is obtained for the region from 50 s to 50 n in latitude and 70 e to 190 e in longitude velocities are found using an analysis of the differential travel time residuals from 481 scs s and 266 sscs ss phase pairs the long period data are taken from the global digital seismograph network digital waveform catalog for the time period of january 1980 to march 1987 each differential travel time is found by a cross correlation of the s phase ground displacement corrected to simulate differential attenuation with all following phases travel times are corrected for ellipticity and mantle heterogeneity outside of their d paths and the remaining residuals are interpreted as the result of d heterogeneity ray tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies the resulting map reveals significant long wavelength d structure including a 3 low velocity region beneath northeastern indonesia surrounded by three identified high velocity zones beneath northwestern pacifica 4 southeast asia 3 and australia 3 5 this structure is of continent ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle the low velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature d rock which has been heated by the core has become gravitationally unstable and begun to rise a chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site much the way continents are dragged toward subduction zones the high velocity zones possibly result from the downward convection of cold lower mantle plumes which pond at the core mantle boundary these seismic anomalies may also contain a chemical signature from faster iron poor materials brought down through the lower mantle or the additional presence of sio2 stishovite perhaps in its higher pressure polymorph

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

elasticity of subducted basaltic crust at the lower mantle pressures insights on the nature of deep mantle heterogeneity abstract the origin of the large low velocity structures observed in the lowermost mantle is still unclear to make new insights into deep mantle heterogeneity we performed ab initio modeling of the elasticity of representative petrology with the pyrolitic and basaltic compositions based on the elasticities of the following mineral phases magnesium silicate perovskite post perovskite ferropericlase calcium silicate perovskite stishovite cacl 2 and calcium ferrite titanate type mgal 2 o 4 calculations elucidated that the basalt composite is 2 denser than the pyrolitic one through all of the pressures in the lower mantle in addition the shear v s and bulk sound v velocities were found to be 2 slower and faster in the basalt composition in particular at 30 gpa and 120 gpa respectively while both the pyrolitic and basaltic composites have almost the same compressional wave speed v p thermochemically distinct piles which are thought to be related to accumulated hot and dense subducted crusts would therefore be expected to produce as much as a 5 to 6 low velocity anomaly in the s wave but only 1 in the p wave these suggest that the basaltic pile seems less compatible to the seismological signatures of the large low velocity structures if accompanied by thermal anomalies reaching 1000 to 1500 k

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios