finite element study of uplift and strain across vancouver island geological evidence for sudden coastal subsidence along the west coast of southern vancouver island points to the occurrence of great prehistorical subduction earthquakes contemporary uplift and crustal shortening patterns in southern vancouver island appear to indicate that the subduction megathrust fault is currently locked to understand better the dynamics of the observed surface deformation we develop a finite element model of earthquake cycles for the northern cascadia subduction zone across southern vancouver island using a linear viscoelastic rheology the model consists of the continental and oceanic lithospheres the asthenospheric mantle with a viscosity of 5 1019 pa s and a low viscosity 1018 pa s mantle wedge between the subducted oceanic plate and the overlying continental plate the shallow geometry of the subducted juan de fuca plate is well defined by the results of various geophysical surveys and the deep geometry is constrained by the results of seismic tomography the model me

crustal structure of the northwestern basin and range province and its transition to unextended volcanic plateaus 1 the northwestern margin of the basin and range province is characterized by a transition from low magnitude 20 extension in northwestern nevada to relatively unextended volcanic plateaus in northeastern california seismic velocity and potential field modeling provides new control on the mesozoic to present tectonic evolution of this poorly understood portion of the u s cordillera we document 20 crustal thinning associated with basin and range extension from a crustal thickness of 37 km under northeastern california to 31 km under northwestern nevada that is consistent with the amount of extension recorded in the upper crust in northwestern nevada suggesting the crustal response to extension was relatively homogeneous over the entire crustal column our modeling also shows a well defined 80 km wide zone of unusually low upper crustal velocities 5 9 6 1 km s that coincide with the surface location of sparse cretaceous granites locating the elusive northern extension of the sierra nevada batholith through northwestern nevada for the first time in the subsurface combining geological and geophysical data we reconstruct the late cretaceous to present crustal evolution of this region documenting an interplay between magmatic addition to the crust erosional exhumation sedimentation and extension that has reversed the direction of crustal thinning from a west facing continental margin to an east facing interior basin margin over this time interval finally we find no evidence in northwestern nevada for unusually thick crust 40 km prior to basin and range extension

pb zn deposits and salt bearing diapirs in southern europe and north africa many diapirs with triassic evaporite successions in southern europe and north africa are accompanied by strata bound pb zn mineral deposits these are of two types 1 epigenetic mineralization filling cavities exhibiting analogies with mississippi valley type deposits and 2 syndiagenetic mineralization in sediments rich in organic materials this second type is very similar to the so called volcanic exhalative type in most of these ore deposits diapirism seems to be a determining factor in the development of base metal concentrations diapir growth can be divided into four distinct stages initiation ascent piercement and subsequent deformation study of north african and southern european deposits has led to a genetic model of diapir emplacement and associated pb zn mineralization the source of these metals is unknown the transport of the metals is provided or facilitated by the brine from adjacent triassic evaporites deposition of sulfides occurs where the deep fluids meet the upper beds these deposits can be remobilized with each new phase of diapir growth

gold in the volcanogenic mercury rich sulfide deposit langsele skellefte ore district northern sweden the mineralogy of gold from a volcanogenic sulfide deposit in northern sweden langsele was studied the enrichment of gold and silver in massive ore occurs associated with an intrusive metadolerite vein which can be traced across the ore the emplacement of this vein mobilized the precious metals together with galena and pb sb sulfosalts into fractures and low pressure zones a high mercury content at l ngsele average 250 ppm has influenced the gold mineralogy thus gold occurs as au ag amalgam native gold and rarely as aurostibite ausb2 the results of microprobe analyses of amalgam cluster around au0 45ag0 45hg0 10 the composition of ternary amalgam is discussed in connection with the synthetic au ag hg system furthermore native gold was found myrmekitically integrown with stibnite which has been interpreted as a breakdown product of aurostibite at low temperature

broadside wide angle seismic studies and three dimensional structure of the crust in the southeast canadian cordillera broadside or fan recordings of a lithoprobe seismic refraction wide angle reflection experiment in the southeastern canadian cordillera show several features further illuminating the crustal structure beyond that previously derived from score 90 southern cordillera refraction experiment of 1990 in line data analysis of a nearly in line profile centred on castlegar british columbia shows lower velocities in the upper crust associated with the purcell anticlinorium as well as velocity variations that may have some association with the purcell fault zone the depth to moho is almost 38 km somewhat deeper and on trend with the structure that has been established farther north the broadside records show high signal to noise ratio pmp arrivals i e reflections from the bottom of the crust these pmp fan picks were analysed in regions away from in line profiles providing further measurements of the depth to moho in the southeastern cordillera the analysis of the broadside records combined with t

crustal velocity structure in the eastern insular and southernmost coast belts canadian cordillera seismic refraction data recorded along a 330 km cross strike profile through the eastern insular and southernmost coast belts of the canadian cordillera are interpreted using an iterative combination of traveltime inversion and amplitude forward modelling the resultant model is characterized by large lateral variations in velocity the most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the harrison fault which likely represents the transition from crust of the insular superterrane to that of the intermontane superterrane this interpretation is consistent with some present geological models that place the possible probable location of the suture between the two superterranes less than 20 km east of the harrison fault velocities at the base of the upper crust average 6 4 and 6 2 km s west and east of the fault respectively mid crustal velocities average 6 6 6 9 km s to the west and 6 35 6 45 km s to the east of the fault low

continental crust anisotropy measurements from tectonic tremor in cascadia we present new observations of crustal anisotropy in the southern cascadia forearc from tectonic tremor the abundance of tremor activity in oregon and northern california during slow slip events offers an enormous amount of information with which to measure and analyze anisotropy in the upper brittle continental crust to accomplish this we performed analyses of wave polarization and shear wave splitting of tectonic tremor signals using three component broadband seismic stations the splitting times range between 0 11 and 0 32 s and are consistent with typical values observed in the continental crust fast polarization azimuths are in general margin parallel and trend n s which parallels the azimuths of the maximum compressive stresses observed in this region this pattern is likely to be controlled by the stress field comparatively the anisotropic structure of fast directions observed in the northern section of the cascadia margin is oblique with respect to the southern section of cascadia which in general trends e w and is mainly controlled by active faulting and geological structures source distribution analysis using a bivariate normal distribution that expresses the distribution of tremors in a preferred direction shows that in northern california and oregon the population of tremors tends to distribute parallel to fast polarization azimuths and maximum compressive stresses suggesting that both tremor propagation and anisotropy are influenced by the stress field results show that the anisotropy reflects an active tectonic process that involves the northward movement of the oregon block which is rotating as a rigid body in northern cascadia previous results of anisotropy show that the crust is undergoing a shortening process due to velocity differences between the oregon block and the north america plate which is moving more slowly with respect to the oregon block making it clash against vancouver island

inversion of three dimensional wide angle seismic data from the southwestern canadian cordillera seismic refraction wide angle reflection data were recorded on a triangular array in southwestern british columbia centered on the boundary between the coast belt to the southwest and the intermontane belt to the northeast the experiment part of the lithoprobe southern cordillera transect enabled determination of the three dimensional 3 d velocity structure of the crust and upper mantle an algorithm for the inversion of wide angle seismic data to determine 3 d velocity structure and depth to reflecting interfaces is developed the algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times including 1 forward modeling using a 3 d finite difference algorithm and 2 a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations the existing procedure is extended to allow 1 the inversion of reflection times to solve for depth to a reflecting interface and or velocity structure 2 the inversion of first arrival travel times to solve for depth to a refracting interface and 3 layer stripping application of the algorithm to southern cordillera data uses pg to constrain upper crustal velocity structure pmp to constrain lower crustal velocity structure and depth to moho and pn to constrain upper mantle velocities and depth to moho the 3 d velocity model for the southwestern canadian cordillera is characterized by 1 significant lateral velocity variations at all depths that do not in general correlate with surface geological features or gravity data 2 a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower wrangellia an accreted terrane forming the insular belt to the west 3 a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the fraser fault system lending additional support to the view that it is a crustal penetrating fault 4 an average upper mantle velocity of 7 85 km s and 5 a depth to moho of 33 36 km in the intermontane belt and 36 38 km throughout most of the coast belt decreasing in the west to 33 km near the insular coast contact horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region

three dimensional crustal velocity structure beneath the strait of georgia british columbia summary the strait of georgia is a topographic depression straddling the boundary between the insular and coast belts in southwestern british columbia two shallow earthquakes located within the strait m 4 6 in 1997 and m 5 0 in 1975 and felt throughout the vancouver area illustrate the seismic potential of this region as part of the 1998 seismic hazards investigation of puget sound ships experiment seismic instruments were placed in and around the strait of georgia to record shots from a marine source within the strait we apply a tomographic inversion procedure to first arrival traveltime data to derive a minimum structure 3 d p wave velocity model for the upper crust to about 13 km depth we also present a 2 d velocity model for a profile orientated across the strait of georgia derived using a minimum parameter traveltime inversion approach this paper represents the first detailed look at crustal velocity variations within the major cretaceous to cenozoic georgia basin which underlies the strait of georgia the 3 d velocity model clearly delineates the structure of the georgia basin taking the 6 km s 1 isovelocity contour to represent the top of the underlying basement the basin thickens from between 2 and 4 km in the northwestern half of the strait to between 8 and 9 km at the southeastern end of the study region basin velocities in the northeastern half are 4 5 6 km s 1 and primarily represent the upper cretaceous nanaimo group velocities to the south are lower 3 6 km s 1 because of the additional presence of the overlying tertiary huntingdon formation and more recent sediments including glacial and modern fraser river deposits in contrast to the relatively smoothly varying velocity structure of the basin velocities of the basement rocks which comprise primarily palaeozoic to jurassic rocks of the wrangellia terrane and possibly jurassic to mid cretaceous granitic rocks of the coast belt show significantly more structure probably an indication of the varying basement rock lithologies the 2 d velocity model more clearly reveals the velocity layering associated with the recent sediments huntingdon formation and nanaimo group of the southern georgia basin as well as the underlying basement we interpret lateral variation in sub basin velocities of the 2 d model as a transition from wrangellian to coast belt basement rocks the effect of the narrow onshore offshore recording geometry of the seismic experiment on model resolution was tested to allow a critical assessment of the validity of the 3 d velocity model lateral resolution throughout the model to a depth of 3 5 km below the top of the basement is generally 10 20 km

basin architecture and density structure beneath the strait of georgia british columbia georgia basin is located within one of the most seismically active and populated areas on canada s west coast over the last decade geological investigations have resolved important details concerning the basin s shallow structure and composition yet until recently relatively little was known about deeper portions of the basin in this study new seismic velocity information is employed to develop a 3 dimensional density model of the basin comparison of the calculated gravity response of this model with the observed gravity field validates the velocity model at large scales at smaller scales several differences between model and observed gravity fields are recognized analysis of these differences and correlation with independent geoscience data provide new insights into the structure and composition of the basin fill and underlying basement specifically four regions with thick accumulations of unconsolidated pleistocene and younger sediments which were not resolved in the velocity model are id

a comparison of the receiver structure beneath stations of the canadian national seismograph network three component broadband data from the recently deployed canadian national seismograph network provide a new opportunity to examine the structure of the crust and upper mantle beneath the canadian landmass receiver function analysis is an ideal method to use with this data set as it can provide constraints on the s velocity structure beneath each station of this seismograph network this analysis method is particularly useful in that it provides site specific information i e within 5 15 km of the station low velocity layers can be identified and it is possible to examine structure to upper mantle depths in this study receiver functions were computed for each of the 19 stations that made up the seismograph network in june 1994 five stations sampling a variety of tectonic environments including the appalachian orogen the canadian shield the western canadian sedimentary basin and the cascadia subduction zone were chosen for detailed modelling the results presented here are the first estimates of the s velocity structure beneath these five stations for those stations where comparisons can be made with seismic reflection and refraction results there is excellent agreement in eastern canada simple receiver functions and clear moho ps conversions at most stations indicate a relatively transparent crust and a moho depth of 40 45 km in northwestern canada moho ps phases indicate a crustal thickness of 33 38 km beneath inuvik northwest territories the moho is interpreted as two velocity steps separated in depth by 5 km and an upper mantle low velocity zone is near 47 km depth in western canada the data indicate a mid crustal low velocity zone beneath edmonton the moho of the subducting juan de fuca plate is interpreted at 52 krn depth beneath southern vancouver island several stations exhibiting complex receiver functions warrant further study they include stations at scheffewille quebec in the canadian shield deer lake newfoundland on the boundary of the grenville province and the appalachian orogen and yellowknife northwest territories at the intersection of the churchill and slave provinces and the western plains

low frequency earthquakes below southern vancouver island 1 the nature and distribution of low frequency earthquakes lfes in subduction zones provide insight into plate boundary deformation downdip of the locked seismogenic zone we employ network autocorrelation detection to identify lfe families beneath southern vancouver island and environs an initial suite of 5775 lfes detected in 2004 and 2005 at a select set of 7 stations is grouped into 140 families using waveform cluster analysis these families are used as templates within an iterative network cross correlation scheme to detect lfes across different tremor episodes incorporate new stations and improve lfe template signal to noise ratio as in southwest japan representative lfe locations define a relatively tight dipping surface several km above the locus of intraslab seismicity within a prominent dipping low velocity zone lvz lfe polarizations for near vertical source receiver geometries possess a remarkably uniform dipolar signature indicative of point source double couple excitation focal mechanisms determined fromp wave first motions are characterized by a combination of strike slip and thrust faulting we suggest that lfes and regular intraslab seismicity occur in distinct structural and stress regimes the lvz inferred to represent weak overpressured porous and mylonitized metabasalts of oceanic crustal layer 2 separates lfes manifesting deformation within a plate boundary shear zone from intraslab earthquakes generated by tensional stresses and dehydration embrittlement within a more competent lower oceanic crustal layer 3 and underlying mantle

crustal structure of nw british columbia and se alaska from seismic wide angle studies coast plutonic complex to stikinia crustal structure beneath the transition from the coast belt to the intermontane superterrane of the northern canadian cordillera is interpreted from the inversion of refraction and wide angle reflection seismic data the profile traverses an accretionary suture zone coast plutonic complex to continental crust deformed by the transpressive collision stikine terrane using data acquired by the accrete onshore offshore experiment and by a partially overlapping lithoprobe onshore experiment p wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure the model exhibits a well defined transition throughout the crust that distinguishes the coast plutonic complex cpc from stikinia average crustal velocities beneath the cpc 6 45 km s are considerably faster than those beneath stikinia 6 25 km s crustal thickness also changes across the transition thin crust beneath the coast belt 30 32 km thickens beneath stikinia 35 37 km the observations within the coast belt are consistent with a tectonic history most recently dominated by extensional deformation primary structural control could be associated with either neogene extension and or the processes that are responsible for exhuming the coast belt during the early paleogene and have been inferred from geological studies slow mantle velocities 7 8 7 9 km s beneath the entire profile are indicative of high upper mantle temperatures comparison with the southern cordilleran coast belt reveals similar velocity structure within the massive plutonic complexes however substantial differences between the northern and southern coast belts emphasize along strike variations in terranes orogen geometry and postorogenic tectonics

new constraints on subduction zone structure in northern cascadia summary a detailed passive seismic experiment was carried out across southwestern british columbia and northwestern washington to investigate the structure of the subducting juan de fuca plate and mantle wedge in cascadia and its relation to intraslab seismicity as part of the polaris project 31 three component broad band stations were deployed in an approximately linear array spanning southern vancouver island the gulf and san juan islands watcom county and the british columbia lower mainland p wave coda from 41 teleseismic events have been employed in formal inversions for fine scale shear velocity structure our results indicate a structure very similar to that identified across a comparable profile in central oregon the continental moho is evident at the eastern end of the profile near 35 km depth but disappears towards georgia strait puget sound a prominent low s velocity zone is clearly evident below southern vancouver island dipping eastwards through georgia strait puget sound and coincides with the e reflection zone originally identified in lithoprobe studies structure below the e layer is much less prominent and varies intermittently along the array based on the observations and interpretations of similar structures beneath oregon alaska and south america and its projection to mantle depths we suggest that the low velocity e layer represents the dehydrating oceanic crust of the subducting juan de fuca plate this interpretation is consistent with recent seismicity studies that place shallow wadati benioff events within the oceanic mantle and implies that the oceanic crust is 6 8 km shallower beneath vancouver island than previously assumed as in oregon we interpret the diminished signature of oceanic crust below a depth of 45 km to signal the presence of eclogitization which in turn supplies water to serpentinize the overlying forearc mantle

crustal velocity structure in the southern coast belt british columbia we applied an iterative combination of two dimensional traveltime inversion and amplitude forward modelling to seismic refraction data along a 350 km along strike profile in the coast belt of the southern canadian cordillera to determine crust and upper mantle p wave velocity structure the crustal model features a thin 0 5 3 0 km near surface layer with an average velocity of 4 4 km s and upper middle and lower crustal strata which are each approximately 10 km thick and have velocities ranging from 6 2 to 6 7 km s the moho appears as a 2 km thick transitional layer with an average depth of 35 km and overlies an upper mantle with a poorly constrained velocity of over 8 km s other interpretations indicate that this profile lies within a collision zone between the insular superterrane and the ancient north american margin and propose two collision zone models i crustal delamination whereby the insular superterrane was displaced along east vergent faults over the terranes below and ii crustal

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

crustal structure along the west flank of the cascades western washington knowledge of the crustal structure of the washington cascades and adjacent puget lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region we present a model for crustal velocity structure derived from analysis of seismic refraction wide angle reflection data collected in 1991 in western washington the 280 km long north south transect skirts the west flank of the cascades as it crosses three tectonic provinces including the northwest cascades thrust system nwcs the puget lowland and the volcanic arc of the southern cascades within the nwcs upper crustal velocities range from 4 2 to 5 7 km s 1 and are consistent with the presence of a diverse suite of mesozoic and paleozoic metasediments and metavolcanics in the upper 2 3 km of the puget lowland velocities drop to 1 7 3 5 km s 1 and reflect the occurrence of oligocene to recent sediments within the basin in the southern washington cascades upper crustal velocities range from 4 0 to 5 5 km s 1 and are consistent with a large volume of tertiary sediments and volcanics a sharp change in velocity gradient at 5 10 km marks the division between the upper and middle crust from approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from 6 0 to 7 2 km s 1 these high velocities do not support the presence of marine sedimentary rocks at depths of 10 20 km beneath the cascades as previously proposed on the basis of magnetotelluric data crustal thickness ranges from 42 to 47 km along the profile the lowermost crust consists of a 2 to 8 km thick transitional layer with velocities of 7 3 7 4 km s 1 the upper mantle velocity appears to be an unusually low 7 6 7 8 km s 1 when compared to velocity models from other regions this model most closely resembles those found in active continental arcs distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect in the nwcs and puget lowland most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5 10 km depth and continues to 20 30 km depth the region of transition between the nwcs and the puget lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two earthquakes within the cascades are generally shallower 0 20 km and are dominated by events associated with the rainier seismic zone

a holocene coral algal reef at mavra litharia gulf of corinth greece structure history and applications in relative sea level change a holocene coral algal reef at mavra litharia south central coast of gulf of corinth greece is exposed from ca 2 to 9 3 m above sea level on an uplifting footwall associated with the eliki fault the reef lacks sea level critical species but its coralline algal assemblage indicates a ca 10 m water depth reef frame components date from 9280 8730 years bp to 6343 5993 years bp so the reef frame grew between ca 10 000 and 6000 years bp the youngest dated shells 1860 305 years bp from the site are accessory organisms collected from the lowest 2 m of outcrop one of which dendropoma grew at sea level stiros s c and pirazzoli p 1998 late quaternary coastal changes in the gulf of corinth greece tectonics earthquake archaeology guidebook for the gulf of corinth field trip patras university greece patras september 14 16 1998 reef history has four phases a growth and lithification of reef b development of smooth walled dissolution pipes and caves in the reef c colonisation of dissolution surfaces by mn fe crusts that may be bacterially formed barnacles serpulid worms and rock boring bivalves and d uplift to present position where much of the reef is eroded sea level history after 11 500 years bp when rising post glacial sea level overtopped the rio sill and returned the gulf of corinth to a marine environment is reconstructed calculations of interplay between sea level rise and tectonic uplift suggest that between 11 500 and 10 000 years bp sea level rose very quickly associated with deglaciation at the close of the younger dryas mwp 1b at a maximum of 25 6 mm year broadly consistent with other studies then slowed to ca 4 4 mm year until 6000 years bp when sea level was ca 3 m below modern after which sea level rose at ca 0 5 mm year to modern day tectonic uplift rate of maximum 3 mm year slower than sea level rise means that the reef could not catch up to sea level until recent times explaining its fossil assemblage the pipe cave system is interpreted as having formed when the reef top was emerged but much dissolution occurred beneath sea level because of insufficient time to allow uplift and resubmergence intense rainfall during the mid holocene pluvial phase ca 6000 years bp for the eastern mediterranean which affected large areas of the mediterranean is suggested as the mechanism and timing of submarine dissolution

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

fine structure of the continental reflection moho comparisons of deep seismic reflection profiles from a variety of tectonic regions that span a large fraction of the earth s history lead to a threefold categorization of reflection patterns near the crust mantle transition type i patterns have no distinct reflections including no reflection moho at all type ii patterns have one or more subhorizontal reflections and type iii patterns have reflections that project from the crust into the mantle subdivisions within each of these are based upon the geometric relationships between the reflection moho and reflections immediately above and in some cases below hence type i examples may have essentially no reflectivity near the crust mantle transition or they may be characterized by crustal reflectivity that fades downward type ii examples may have a single prominent reflection or they may have several subhorizontal reflections type ii patterns may also have crustal reflections that are either listric downward or that appear to be truncated at or near the base of reflectivity type iii examples may have a single prominent reflection that projects below the moho as delineated by seismic refraction data or they may have a series of subparallel dipping reflections more than one type of reflection moho may be visible along a single profile sometimes changing within distances of only a few tens of kilometers this diversity of both geometry and character leads to the conclusion that the variable appearance of the reflection moho is a consequence of variations in the thermal and structural evolution of the crust mantle transition at many scales from a few tens to many hundreds of kilometers

comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward

shear wave splitting above the subducting juan de fuca plate shear wave splitting observed in three component broadband recordings of local earthquakes near southern vancouver island british columbia provide the first constraints on shear wave anisotropy in the continental crust above the subducting juan de fuca plate waveforms of 22 shallow earthquakes 15 30 km depth and 16 deep earthquakes 40 60 km depth were examined for shear wave splitting the fast polarization direction is well defined at 312 19 for the deep earthquakes and 293 27 for the shallow earthquakes these fast polarization directions are approximately parallel to the continental margin and coincide with the orientation of the pressure axes of small crustal earthquakes in this region interpreting this polarization pattern in terms of the extensive dilatancy anisotropy model indicates a margin parallel compressive stress regime comparing the average time delays between the fast and slow shear waves for the deep earthquakes 0 32 s and the shallow earthquakes 0 20 s suggests that the source of the seismic anisotropy is stronger in the upper 20 km of the continental crust here the average differential shear wave anisotropy is estimated at 2 3 these observations support the theory of weak coupling between the downgoing juan de fuca plate and the overlying north america plate as the principal stress is perpendicular to the direction of subduction and the s wave splitting above the subducting plate is nearly perpendicular to the sks splitting direction in the upper mantle beneath the subducting plate

characterization of active faulting beneath the strait of georgia british columbia southwestern british columbia and northwestern washington state are subject to megathrust earthquakes deep intraslab events and earthquakes in the continental crust of the three types of earthquakes the most poorly understood are the crustal events despite a high level of seismicity there is no obvious correlation between the historical crustal earthquakes and the mapped surface faults of the region on 24 june 1997 a m l 4 6 earthquake occurred 3 4 km beneath the strait of georgia 30 km to the west of vancouver british columbia this well recorded earthquake was preceded by 11 days by a felt foreshock m l 3 4 and was followed by numerous small aftershocks this earthquake sequence occurred in one of the few regions of persistent shallow seismic activity in southwestern british columbia thus providing an ideal opportunity to attempt to characterize an active near surface fault we have computed focal mechanisms and utilized a waveform cross correlation and joint hypocentral determination routine to obtain accurate relative hypocenters of the mainshock foreshock and 53 small aftershocks in an attempt to image the active fault and the extent of rupture associated with this earthquake sequence both p nodal and cmt focal mechanisms show thrust faulting for the mainshock and the foreshock the relocated hypocenters delineate a north dipping plane at 2 4 km depth dipping at 53 in good agreement with the focal mechanism nodal plane dipping to the north at 47 the rupture area is estimated to be a 1 3 km diameter circular area comparable to that estimated using a brune rupture model with the estimated seismic moment of 3 17 10 15 n m and the stress drop of 45 bars the temporal sequence indicates a downdip migration of the seismicity along the fault plane the results of this study provide the first unambiguous evidence for the orientation and sense of motion for active faulting in the georgia strait area of british columbia

a regional study of shear wave splitting above the cascadia subduction zone margin parallel crustal stress recordings of local earthquakes from 16 three component broadband seismic stations in southwestern british columbia washington and northern oregon are used to study regional variations of shear wave anisotropy in the north american plate above the subducting juan de fuca plate there is evidence for shear wave splitting at all sites with good agreement of fast polarization directions and travel time delays at adjacent stations most stations exhibit fast directions parallel to the strike of the margin with anisotropy of 1 2 these fast polarization directions are consistent with earthquake focal mechanisms and borehole stress studies indicating that the observed anisotropy is likely due to crustal stresses i e extensive dilatancy anisotropy theory the margin parallel stresses may be due to oblique subduction of the juan de fuca plate however at the station closest to the coast ozb the fast direction shows a more margin normal orientation that may be associated with the proximity of the locked portion of the underlying subduction thrust fault

les mineraux a cu sb ag hg des minerais de pb zn de sakiet koucha diapir de sakiet sidi youssef tunisie septentrionale la formation a anhydrite du diapir de sakiet sidi youssef contient aux environs de sakiet koucha des mineraux a cu sb ag hg au sein des minerais de type bpg habituels des formations corticales des diapirs de tunisie septentrionale

crustal velocity structure of the omineca and intermontane belts southeastern canadian cordillera we provide information on the physical properties across two important morphogeological regions of the southeastern canadian cordillera the omineca and intermontane belts and tie together two previously interpreted wide angle seismic lines which have separately probed each zone along strike the data analyzed here line 7 of lithoprobe s 1990 southern cordillera refraction experiment in southern british columbia is along a 410 km profile from the fraser river to radium hot springs near the rocky mountain trench iterative forward and inverse modeling of the seismic travel times constrained by seismic amplitude characteristics and earlier cross line interpretations defines a thin crust with relatively low velocities the upper crust has an average thickness of 15 3 km and an average p wave velocity of less than 6 1 km s 1 with velocities at the bottom reaching 6 2 6 3 km s 1 it is further characterized by velocity gradients which decrease markedly with depth and by low velocity trends which correlate with the traces of crustal scale shear zones a high velocity region at the west end of the profile is coincident with a major uplift structure the midcrust has an average thickness of 9 2 km with velocities below 6 1 km s 1 except for the extreme west end of the profile this defines a midcrustal low velocity zone lvz in the omineca belt and partly in the intermontane belt the lower crust from 24 km to 34 km depth has velocities averaging 6 6 6 7 km s 1 higher velocities in the deep crust may mark the westward extent of north american cratonic rocks at the okanagan valley the upper mantle velocity has an average 8 0 km 5 increasing to 8 2 km s 1 at the maximum depth of ray penetration at 48 km the model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles the velocity structure of the crust especially the lvz closely correlates with the regional isotherms while there seems to be no well defined velocity boundary between the intermontane and omineca belts there are distinct velocity differences between the intermontane and neighboring coast plutonic complex to the west the low velocity characteristics of the omineca and much of the intermontane belts may be a consequence of temperature control of the physical parameters of the crust the lithosphere in the southeastern canadian cordillera is clearly anomalous with respect to much of northwest north america and in terms of crustal thickness and velocity structure it is similar only to the basin and range province of the united states

the northern limit of the subducted juan de fuca plate system analysis of data recorded at an array of three component broadband seismograph stations deployed on northern vancouver island and the adjacent british columbia mainland at the northern end of the cascadia subduction zone provides the first constraints on the s wave velocity structure of this region and permits us to define the northern limit of the subducted juan de fuca plate system during a 2 year period more than 80 teleseisms were recorded at our five stations the method of receiver function analysis was used to constrain the s velocity structure to upper mantle depths beneath the northern three stations a relatively simple continental crust is interpreted with a well defined moho near 37 39 km depth an upper crustal s velocity discontinuity at these stations is interpreted as the top of the high velocity rocks of the wrangellia terrane in contrast more complicated structure dominated by pronounced low velocity zones dipping to the ne are interpreted beneath our southern two stations the shallower low velocity zone is 6 8 km thick has an s velocity contrast of 0 6 1 1 km s and lies within the continental crust this feature is similar to a pronounced low velocity layer the e zone imaged beneath southern vancouver island the deeper low velocity zone is interpreted as the subducted oceanic crust we interpret the pronounced change in s velocity structure that we observe as the northern limit of the subducted oceanic plate beneath vancouver island this change coincides with significant changes in topography heat flow gravity and geochemistry

crustal seismic velocity and density structure of the intermontane and coast belts southwestern cordillera seismic refraction wide angle reflection data were recorded along a 450 km profile across the intermontane coast and insular belts of the canadian cordillera crust and upper mantle structure was interpreted from traveltime inversion and forward amplitude modelling and the resultant seismic velocities were used to constrain modelling of the bouguer gravity data along the profile a high velocity high density block in the upper 8 km of crust was interpreted as the subsurface extension of harrison terrane the harrison fault at its eastern boundary may extend to at least 8 km depth and perhaps 20 km throughout the crust both seismic velocities and densities are in general high beneath the insular belt low beneath the coast and western intermontane belts and intermediate beneath the eastern intermontane belt however densities are unusually low in the lower crust beneath the coast belt 2800 kg m3 relative to velocities 6 6 6 8 km s this indicates that coast belt plutonic material is present

temporal and spatial evolution of northern cascade arc magmatism revealed by la icp ms u pb zircon dating we present thirty new laser ablation inductively coupled plasma mass spectrometry u pb zircon dates for intermediate to silicic plutons of the northern cascade arc with emphasis on the chilliwack batholith mount baker magmatic focus located in southwestern british columbia and northern washington chilliwack magmatism commenced at 35 ma in southwestern british columbia and the most voluminous plutons define a cluster at 32 29 ma documenting an early flare up during the same interval the index squire creek and cascade pass intrusions were emplaced south of the chilliwack batholith north of the chilliwack maximum pluton ages become progressively younger northward tracking the northerly migration of the edge of the farallon juan de fuca explorer plate system relative to north america chilliwack magmatism continued from 29 ma to 22 ma at a slightly reduced flux followed by a lull from 22 to 11 ma during which magmatism shifted north to the mount barr batholith 18 ma chilliwack magmatism res

reply to comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look by e irving et al

nacr14 a 3d model for the crustal structure of the north american continent abstract based on the large number of crustal seismic experiments carried out in the last decades we create nacr14 a 3d crustal model of the north american continent at a resolution of 1 1 we present maps of thickness and average velocities of the main layers that comprise the north american crystalline crust obtained from the most recent seismic crustal models within the usgs crustal structure database however the crustal data are unevenly distributed and in some cases discrepancies exist between published models in order to construct a consistent 3d crustal model with three layers in the crystalline crust we refrained from a direct interpolation of the crustal seismic parameters in the database instead we implemented the following sequence of steps 1 definition of the geometry of the main tectonic provinces of north america 2 selection and evaluation of the reliability of seismic crustal models in the database 3 estimation of the p wave seismic velocity and thickness of the upper middle and lower crust for each tectonic province 4 estimation of the interpolated pn velocity distribution the resulting average velocity of the crystalline crust is mostly consistent with that of the seismic points the main variations of the structure of the crystalline crust of north america displayed in the model can be related to its tectonic evolution the model available in a digital form can be used in various geophysical applications such as the correction for the crustal effects in gravity and seismic tomography and models of dynamic topography in order to detect heterogeneities characterizing the underlying upper mantle

crustal velocity and moho structure beneath the gulf of corinth greece summary the gulf of corinth goc greece is a continental rift with high rates of seismicity and extensional strain how this strain is accommodated in the crust and whether there are variations in the mechanism along strike remain open questions in part because of a lack of wide angle reflection refraction studies that constrain crustal velocity structure in 2001 an extensive multichannel seismic survey was conducted within the goc one component of which included the wide angle recording of sources from within the gulf at stations on land surrounding the gulf in this paper we use wide angle data in two separate but allied studies to constrain crustal velocities and depth to the moho a 2 d inversion of refraction and reflection traveltimes along an axial profile through the goc constrains the shallow crustal velocity structure images the moho at 29 km depth in the east dipping to 39 km in the west and images the eastward subducting african slab beneath the western goc at a depth of 74 km the 1 d average of the 2 d velocity model was used in a tomographic inversion of pmp reflection times to solve for depth to the moho throughout the corinth region this model shows generally thick isostatically compensated crust 37 km beneath the hellenide mountains except immediately south of the goc and a singular region of thin crust 30 km beneath the perahora peninsula at the eastern end of the gulf a comparison with moho depths derived from gravity inversion shows a general agreement with crust thickening from east to west but a number of differences in detail the 3 d crustal thickness variations are more complex than those predicted by either pure shear or simple shear models of continental extension and suggest significant pre rift structural variability

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

review receiver function studies in the southern canadian cordillera receiver function analysis has proven to be a powerful yet inexpensive tool for estimating the s wave velocity structure of the crust and upper mantle beneath three component seismograph stations in the southern canadian cordillera receiver function studies using a portable broadband seismograph array across southwestern british columbia provided site specific estimates for the location of the subducting juan de fuca plate the oceanic crust was imaged at 47 53 km beneath central vancouver island and 60 65 km beneath the strait of georgia further these studies revealed a prominent low velocity zone vs 1 0 km s that coincides with the e reflectors imaged 5 10 km above the subducting plate on lithoprobe reflection lines the e low velocity zone was shown to extend into the upper mantle beneath the strait of georgia and the british columbia mainland to depths of 50 60 km combining the receiver function and refraction models revealed a high poisson s ratio 0 27 0 38 for this feature the contin

the southern coast mountains british columbia new interpretations from geological seismic reflection and gravity data the southern coast mountains of british columbia are characterized by voluminous plutonic and gneissic rocks of mainly middle jurassic to eocene age the coast plutonic complex as well as metamorphic rocks folds and thrust and reverse faults that mostly diverge eastward and westward from an axis within the present mountains and by more localized eocene and younger normal faults in the southeastern coast mountains mid cretaceous and younger plutons intrude bridge river cadwallader and methow terranes and overlap middle jurassic through early cretaceous marine clastic rocks of the tyaughton methow basin the combination of geological data with new or reanalyzed geophysical data originating from lithoprobe and related studies enables revised structural interpretations to be made to 20 km depth five seismic profiles show very cut up and chaotic reflectivity that probably represents slices and segments of different deformed and rearranged rock assemblages surface geology seismic interpretations ph