paleomagnetism of small basalt exposures in the west puget sound area washington and speculations on the accretionary origin of the olympic mountains twenty two sites from two small exposures of basalt of probable eocene age immediately east of the olympic mountains have the following mean paleomagnetic direction declination 176 5 inclination 66 5 circle of 95 confidence 8 6 this is the only paleomagnetic study of pre miocene rocks anywhere in western washington or oregon that matches the polar wandering curve for north america however a marked fanning of paleomagnetic declinations around the olympic core suggests that concordance in this case is fortuitous and results from oroclinal bending during or immediately after olympic orogenesis creation of the olympic mountains involved accretion of thickened basaltic crust followed by underthrusting of large quantities of oceanic sedimentary debris as the farallon plate pressed northeastward into a recess formed by the thick crystalline blocks of vancouver island and the north cascades

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

has the washington oregon coast range moved northward paleomagnetic studies on middle and early tertiary rocks from the coast range consistently find inclinations that are a few degrees shallower than predicted by reference poles for the craton rocks of very similar age and lithology in the cascade range and elsewhere in washington and oregon do not show this effect this suggests that the coast range has moved northward several hundred kilometres relative to stable north america this movement could have been accomplished in a few million years prior to accretion of the coast range to the continent by passive transport aboard the kula and or farallon plates alternatively northward displacement could reflect dextral strike slip faulting in the north american borderland probably both mechanisms contributed

application of seismic refraction tomography in karst terrane seismic refraction tomography field data were collected on several bridge foundation sites in pennsylvania in close proximity to geotechnical boring locations profiles determined from these field measurements were plotted against drilling data and these comparisons revealed the ability of seismic wave velocities to differentiate overburden soil from rock in addition foundation construction data were collected at each of the sites and compared with refraction test results determined prior to construction in particular top of rock revealed by an excavation and pile tip elevations at driving refusal were compared with refraction test results from these data it appears that seismic wave tomograms can characterize the soil rock interface and that it is possible to predict expected design pile lengths based upon a measured p wave velocity tomogram it can be concluded from these site comparisons that geophysical techniques such as seismic refraction tomography can provide important additional information to site characterization for bridge foundations in karst terrane however these techniques should not be viewed as a replacement but should be conducted during design stage site investigation to aid selection of borehole locations and other testing needs

subcrustal lithosphere around the saxothuringian moldanubian suture zone a model derived from anisotropy of seismic wave velocities abstract we have modelled the thickness and large scale fabric of the subcrustal lithosphere from the variations of p wave delay times and the shear wave splitting observed at seismological observatories and four mobile stations in central europe the saxothuringian s lithosphere is characterized by a total thickness of 90 120 km and by a coherent fabric in its mantle part with olivine a c foliation planes dipping to the north northwest the moldanubian m lithosphere is generally thicker 120 140 km on the average and characterized by a c foliations dipping to the south the divergently dipping anisotropic structures may represent remnants of successive paleosubductions of the ancient oceanic lithosphere frozen in the subcrustal lithosphere of both units a long time before their variscan collision the deep s m contact is marked by a lithosphere thinning to about 80 90 km beneath the s m surface boundary within the subcrustal lithosphere a complex structure of the transition of both units extends to about 150 km toward the south as indicated by the anisotropic characteristics of body waves

shear wave velocity characteristics of geological units throughout tehran city iran abstract one simple way of accounting for site conditions in calculating seismic hazards is to use the shear wave velocity vs in the shallow subsurface to classify materials we used the average shear wave velocity to a depth of 30 m v s 30 to develop site categories for tehran this parameter can further be used for modifying a calculated ground motion to account for site conditions the national earthquake hazard reduction program nehrp classification was used to group sites into different classes our classification was based on v s 30 values measured in 188 seismic profiles and on the assumption of similar values for similar geological materials for each geological unit characteristics of shear wave velocity distribution including histograms of shear wave velocity and variation of vs versus depth were calculated and shown this study reveals that the v s 30 of the mountainous zones fall into categories a and b of nehrp site classification but in the tehran plain the northern alluvium tends to have v s 30 values which fall into range of category c of the nehrp site classification whereas those in the southern alluvium would be categorized as nehrp d

love waves from local traffic noise interferometry surface wave interferometry on local scale usually aims at recovering rayleigh waves this is because of the predominant use of vertical component geophones in exploration seismology and the fact that rayleigh waves occur for any given subsurface structure on the other hand love waves are present only in layered media and require horizontal component geophones for their observation as they depend on shear wave velocity structure and density only the analysis of love waves provides a potentially powerful supplement to rayleigh wave inversion perhaps surprisingly recent studies show that low frequency love waves 0 05 0 1 hz excited by the interaction of ocean waves with the ocean floor the earth s microseism can be recovered by interferometry and that their s n is high compared to rayleigh waves lin et al 2008 on a regional scale jay et al 2012 analyzed the ambient noise field in a volcanic region and found that love waves with frequencies of about 0 3 hz are observed more clearly than c

gravity modelling of the lithosphere in the eastern alpine western carpathian pannonian basin region abstract gravity models illustrate changes in the degree of continental convergence in the eastern alpine western carpathian region and modifications to the lithosphere due to the plate convergence and subsequent pannonian basin extension analysis of the continental collision zone incorporates a kinematic model of ocean basin closure whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening erosion and isostatic rebound thick crust and high topography in the eastern alps along with a broad bouguer anomaly of 140 mgal amplitude are consistent with about 175 km of crustal shortening followed by 10 km of isostatic rebound eastward crustal thicknesses and gravity anomaly widths and amplitudes are less so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the western carpathians preservation of thick flysch deposits and small isostatic rebound are attributable to the high density shallow mantle of the intact continent ocean transition zone seismic delay time studies have suggested that relative to the average thickness of the region the lithosphere thickens by about 70 km beneath the eastern alps and thins by about 60 km under the pannonian basin in both regions gravity anomalies cannot be explained fully without considering this large relief on the lithosphere asthenosphere boundary the eastern alpine crustal root which extends 15 km below the average depth for the region overcompensates the topography and results in gravity anomalies that are 40 mgal lower than those observed the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mgal difference observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the pannonian basin whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle regional cross sections suggest that shallowing of the lithosphere asthenosphere boundary associated with pannonian basin extension has propagated northward beyond the carpathians to within the european platform crustal thinning however appears to be confined to exotic terranes of the carpathian interior so that crustal structure in the eastern alps and outer carpathians is a remnant of the earlier collision orogen

application of near surface seismic refraction tomography and multichannel analysis of surface waves for geotechnical site characterizations a case study in this case study two geophysical methods based on seismic waves were employed to evaluate the geotechnical properties of the soil deposits at a site for a planned wind farm regional geological information shows that the study area is mainly covered by recent alluvium which is classified as a mixture of fine sand and gravel twenty five seismic refraction surveys were carried out in the study area the acquired shot gathers were processed from which primary wave p wave and shear wave s wave velocities were derived the p wave velocities were obtained by using the seismic refraction tomography method srt while the s wave velocities were extracted by utilizing the multichannel analysis of surface waves masw technique which is established based on the dispersion characteristics of rayleigh waves seismic hazard at the site was evaluated according to the iranian code using the measured s wave velocities and the friction angles of the soil deposits were approximated using an empirical method and by making use of jaky s formula for earth pressure at rest which yielded similar values the deformation properties of the foundation soil were also estimated based on the correlations between seismic wave velocities and elastic properties of soil as well as janbu s formula for soil modulus the findings of this study may serve as references for economic and non destructive geotechnical site characterizations using seismic wave methods

paleomagnetic evidence for spatially distributed post miocene rotation of western washington and oregon anomalous paleomagnetic directions have been determined for 17 sites in the frenchmans springs member of the wanapum basalt formation columbia river basalt group these sites are located in the ginkgo flows from near vantage washington to portland oregon a distance of approximately 300 km the average paleomagnetic direction for six of these sites centered around vantage is d 147 i 41 95 4 5 the expected miocene field direction is d 355 i 65 at some localities there are two distinct ginkgo flows in direct stratigraphic succession with statistically identical anomalous directions their anomalous paleomagnetic direction makes these flows a valuable marker horizon in the columbia river basalt group the nondipole field direction of the ginkgo flows correlates well with available results from the miocene cape foulweather basalts of oregon this correlation strongly supports the hypothesis that these coastal basalts of oregon are the distal ends of columbia plateau derived basalt flows the spatial distribution of these anomalous field directions suggests about 14 of clockwise rotation between vantage and portland combining these data with data from the oregon coast basalts allows a maximum declination difference of about 35 the increase in declination can be best explained by clockwise rotation about nearby vertical axes increasing to the southwest across the columbia plateau and oregon coast

a review of seismic geophysical testing in iran for building near surface velocity models abstract it has long been understood that the near surface structure of the earth affects several aspects of civil engineering practice particularly in seismic applications therefore the implementation of a competent subsurface exploration program not only addresses the need for comprehensive site characterization but also reduces the devastating consequences of seismic activities typical geotechnical practice for subsurface exploration has often relied on a combination of drilling in situ testing such as standard penetration testing and cone penetration testing and laboratory testing of field samples however geophysical techniques have been developed over the years to evaluate near surface velocity models as a proxy for subsurface stiffness in seismic applications in this regard seismic geophysical methods such as refraction reflection downhole and crosshole testing spectral analysis of surface waves sasw and multichannel analysis of surface waves masw have continued to receive increase

tillamook volcanic series further evidence for tectonic rotation of the oregon coast range paleomagnetic directions from the eocene tillamook volcanic series of the oregon coast range point 46 o clockwise from the expected eocene field direction potassium argon dating of six dikes and flows from this formation yields a mean age of 44 3 0 6 m y these results establish that the oregon coastal block of simpson and cox 1977 extends north to the oregon washington border and that this block has rotated clockwise 46 o 13 o during the past 44 m y the block has undergone no detectable north south translation a two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the pacific northwest region phase i of the rotation active between 50 and 42 m y b p results from fragmentation of the farallon plate with rotation of a fragment during its accretion to north america phase ii of the rotation active between 20 m y b p and the present occurs in association with extension in the basin and range province

post 12 million year clockwise rotation in the central columbia plateau washington the pomona flow 12 m y b p of the columbia river basalt group was sampled for paleomagnetic analysis at 32 sites in the yakima fold belt of the central columbia plateau to assess the pattern of deformation field directions from anticlinal ridges are found to be rotated clockwise relative to sites from synclinal areas two geographic patterns of rotation are present the primary pattern is one in which the greatest amount of rotation occurs in the crest hinge area of the anticlines and decreases into synclines a secondary pattern in which the amount of rotation is consistent within a geometric segment but varies among them is superimposed on the primary pattern the pattern of clockwise rotation is interpreted to be the result of local tectonic rotation occurring along a closely spaced north northwest trending primary shear system and northeast trending secondary shear system the shear system and concurrent rotation are interpreted to have developed during the growth of the yakima folds the relationship between the paleomagnetic data and the geologic structures can be used to constrain tectonic models for the columbia plateau the data reported in this study indicate that clockwise rotation can not be attributed to rigid block or microplate rotation but support models that predict localized dextral shear furthermore it is postulated that this mechanism may contribute to clockwise rotation observed in other parts of the western united states

spatial variations of p residuals and deep structure of the european lithosphere spatial variations of these normalized residuals as large as 2 s were derived from events in different focal regions an average residual at each station was calculated from 23 selected groups compiled from 181 events evenly distributed as to their azimuths and distances the 3 d inversion of the residuals yielded very much the same picture of the relatively high and low velocity provinces for the upper layer of the mantle as compared with the map of average residuals the map of average residuals mainly reflects variations in the thickness of the lithosphere and its thermal state regions of low velocities include the pannonian basin and central carpathians the northern apennines and the po plain as well as the rhine graben the rhenish massif and the north german polish platform high velocities are observed in the south western margin of the east european platform and in three well defined zones situated in the inner part of the western alps in the eastern part of the austrian alps and in the eastern carpathians besides the effects of inhomogeneities also anisotropic effects are anticipated from the spatial diagrams of the residuals which show a similar pattern within large tectonic provinces r

kinematics of austro alpine cover nappes changing translation path due to transpression upper and middle austro alpine nappes of the eastern alps the structural uppermost subunits of the alps suffered two major superposed deformations d1 d2 during the eo alpine cretaceous orogeny d1 is characterized by nw to w directed thrusting and nappe internal rock flow d2 by large scale folding and imbrication toward the north and northeast structures strain metamorphic history microfabrics and stretching trajectories indicate that the superposition of d2 over d1 structures can be interpreted as a progressive deformation it is concluded that the emplacement of the nappes follows a curved translation path a model which emphasizes non coaxial deformation structures explains the detachment of the nappes the configuration of the separating thrust planes and the distribution of intense deformation zones the overall kinematic history of the austro alpine nappes fits a model in which strike slip shearing horizontal shortening and vertical lengthening act together continuously over a finite deformation zone transpressional model this model emphasizing a gradually changing strain regime eliminates the space problems encountered in models which consider simple south over north imbrication during the alpine orogeny the study stresses two interpretations 1 1 stretching lineations indicate nappe transport directions 2 2 transpression may be a common feature of orogenic belts

ages and stratigraphy of lower and middle tertiary sedimentary and volcanic rocks of the central cascades washington application to the tectonic history of the straight creek fault in the central cascade range of washington three structural blocks of early tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the straight creek fault east of the fault in the teanaway river block the early eocene fluviatile feld spathic sandstone of the swauk formation is interbedded with largely dacitic volcanic rocks of the silver pass volcanic member zircon fission track ages on the silver pass are about 52 m y overlying the tightly folded swauk and silver pass is the teanaway formation a gently dipping accumulation of basalt andesite rare dacite and rhyolite that yields middle eocene about 47 m y whole rock k ar ages conformably overlying the teanaway is the roslyn formation a coal bearing fluviatile feld spathic sandstone of probable middle to late eocene age the late eocene and oligocene naches formation exposed west of the straight creek fault in the cabin creek block is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite the basal guye sedimentary member of the naches formation underlies and is interbedded with the mount catherine rhyolite member in the snoqualmie pass area on the basis of zircon fission track ages on silicic tuffs and whole rock k ar ages on basalt the basal part of the naches is about 40 to 44 m y old the formation is tightly folded and complexly faulted along the straight creek fault the straight creek fault intersects the olympic wallowa lineament in the strongly deformed manastash river block in this block fluviatile coal bearing feldspathic sandstone of the manastash formation is overlain by principally dacitic volcanic rocks of the taneum formation overlying the taneum is the basalt of frost mountain we correlate this threefold sequence feldspathic sandstone dacite basalt with the sequence in the teanaway river block small patches of rhyolitic ash flow tuff dated at 33 m y late oligocene and interbedded feldspathic sandstone overlie the roslyn and teanaway formations in the teanaway river block and are probably correlative with the late oligocene wenatchee formation exposed in the chiwaukum graben east of the teanaway river block as well as with the late oligocene volcanic rocks 30 m y along the cascade crest to the southwest these latter rocks are little deformed and unconformably overlie the naches formation the ages and depositional record in the three blocks indicate a tertiary history of dominantly vertical movement along the straight creek fault and its southeasterly splays that merge with the olympic wallowa lineament the horst and graben structure of the blocks as well as enechelon fold axes in the swauk formation suggests some post early eocene right lateral shear along the fault although there is no direct evidence of lateral offset vertical movement significant since early eocene swauk deposition followed late eocene naches deposition but tapered off by late oligocene time and ceased by miocene time when the fault was intruded by the 25 m y old snoqualmie batholith and other plutons however structures with olympic wallowa lineament trends appear to have influenced folding of the miocene yakima basalt subgroup

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

the deep lithosphere in the alps a model inferred from p residuals abstract the tomographic studies of the alps and their surroundings based on the relative p residuals indicated two lithosphere roots attaining depths over 200 km separated by an upwelling of the lithosphere asthenosphere transition beneath the central part of this mountain range the directional dependence of relative p velocities indicates an anisotropic wave propagation in the subcrustal lithosphere beneath the alps and their surroundings the presented model of the deep lithosphere structure suggests several phases of the tectonic development of this region collision of a tip of the adriatic plate with europe in the eastern alps and dextral strike slip movement of the adriatic ending with underthrusting of europe beneath the western alps and counterclockwise rotation the latter contributes to a steep plunge of both plates beneath the eastern alps and to stress accumulation in the friuli region a model with delamination of the rigid upper crust and its rotation resolves some of the inconsistencies between the near surface and deep tectonics and also explains the distribution of seismicity

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

mapping shallow faults at the evrona playa site using high resolution reflection method a shallow high resolution seismic reflection survey was carried out at the evrona playa site in the southern arava valley israel the aim of the survey was to detect and map faults in the shallow subsurface upper 100 150 m and establish the relationship of the morphological features revealed by aerial photographs and surface geological mapping with the faults detected in the subsurface the survey included three seismic lines shot using the p wave technique and one sh wave line which overlapped one of the p wave lines the seismic energy source on all the lines was a sledge hammer the acquired reflection data were of good quality and did not require special processing efforts the seismic sections along the lines show a sequence of reflected events within the 8 150 m range at several locations continuity of the events is interrupted by a system of faults these faults form flower structures apparently related to strike slip motions typical of the region comparison of the faults mapped on the seismic sections with those expressed by surface morphological features generally show good correspondence the results of the seismic survey provide important information for the study of paleoseismicity and seismic hazards in the investigated area

paleomagnetism of the tertiary clarno formation of central oregon and its significance for the tectonic history of the pacific northwest the clarno formation a mostly eocene and partly early oligocene sequence of andesitic lavas and volcaniclastic rocks is the oldest tertiary formation exposed in north central oregon remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84 n 278 e with a 95 confidence cone of 7 comparison of this pole with the north american reference pole for eocene time indicates that the clarno formation has rotated 16 clockwise with an uncertainty of 10 but has undergone no significant latitudinal displacement further comparison with paleomagnetic data from the miocene columbia river basalt group and steens basalt shows that large scale block rotations in the eastern columbia embayment ceased sometime between 38 m y and 15 m y a paleogeographic reconstruction for 38 m y is offered which is consistent with the observed rotation of the clarno and with the other paleomagnetic data from the pacific northwest which are briefly reviewed this reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre miocene tertiary rocks of the pacific northwest by an extensional tectonic model as a consequence only part of the rotations in the pre tertiary rocks of the eastern columbia embayment need to be the result of accretionary tectonics the initiation of magmatism in the eastern columbia embayment that is represented by the clarno formation itself may have been contemporaneous with the beginning of crustal extension in the pacific northwest

paleomagnetism and the adriatic promontory a reappraisal the geodynamic evolution of the western mediterranean area was influenced strongly by the adriatic promontory of the african plate which can be outlined at present by current seismicity and by facies and ophiolite distributions paleomagnetic results from different parts of the promontory contribute useful information about its past history the evaluation of these data is made difficult by the lack of a well defined apparent polar wander path for africa this has resulted in occasional overinterpretation of some paleomagnetic studies results from umbria and the southern alps cover a long enough period of time to define part of an apw path and support the idea that the promontory moved as an adjunct to the african plate during the late mesozoic and early cenozoic a conservative evaluation of cretaceous paleomagnetic data the most abundant for the promontory as a whole shows no strong evidence for previously proposed counterclockwise rotations of sicily or the southern alps relative to the african plate paleomagnetic data however suggest about 17 degrees counterclockwise rotation of autochthonous gargano and istria and about 25 degrees counterclockwise rotation of allochthonous umbria tectonic rotation of the middle part of the promontory appears to have occurred without significant rotation of its northern and southern extremities

oligo miocene rotation of sardinia kar ages and paleomagnetic data of tertiary volcanics abstract conventional k ar dating was carried out on mineral separates mostly plagioclases from middle tertiary volcanics of sardinia along with paleomagnetic studies the close agreement of dates from the two methods establishes the rotation age of sardinia relative to europe within a rather short time span of 1 5 ma between 20 5 and 19 ma from correlation of our results with the geomagnetic time scale it is suggested that the rotation took place shortly after the magnetic anomaly 6

neogene rotations and quasicontinuous deformation of the pacific northwest continental margin paleomagnetically determined rotations about vertical axes of 15 to 12 ma flows of the miocene columbia river basalt group of oregon and washington decrease smoothly with distance from the plate margin consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid the average rate of northward translation of the continental margin since 15 ma calculated from the rotations using this model is about 15 mm year which suggests that much of the tangential motion between the juan de fuca and north american plates since middle miocene time has been taken up by deformation of north america the fluid like character of the large scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

paleomagnetism and tectonic significance of eocene basalts from the black hills washington coast range the black hills located about 15 km west of olympia constitute one of several large basement uplifts in the washington coast range the oldest rocks exposed in this west dipping homocline consist of early to middle eocene basalt flows and breccias with minor sedimentary interbeds which are correlated with the crescent formation in the northern olympic peninsula k ar geochronology of five basalt samples yields an average age of 53 1 2 0 m y b p or early eocene a total of 263 paleomagnetic cores were collected from 37 sites all sites were cleaned using af demagnetization and 35 sites were corrected for measured and inferred tilt the mean direction of these sites was virtually unchanged by applying tectonic corrections although the precision was slightly improved no flattening or steepening of inclination is apparent although the mean declination of the 35 sites is easterly discordant with respect to the expected early eocene declination for stable north america by 28 7 15 4 this discordance indicates that the black hills have rotated clockwise about 29 since early eocene time the black hills basalts show significantly less rotation than eocene rocks in the oregon coast range but they show nearly the same amount as analogous basalts in the southwestern washington coast range and latest eocene to late oligocene volcanic rocks in the oregon washington cascade range thus the rotations of coastal oregon and washington while identical in direction differ significantly in amount suggesting that the entire coast range block from the olympic peninsula to north of the klamath mountains has not been a coherent terrane since early eocene time several tectonic models are discussed that involve accretion and independent clockwise rotation of two or more coast range blocks or microplates in response to episodic periods of underthrusting and extensive right lateral shear along the farallon north america plate boundary during early and middle tertiary time the easterly discordances of declination that are observed in almost all tertiary rocks sampled in the oregon washington coast and cascade ranges reflect a composite of differential rotation during accretion of the coast range seamount terrane followed soon afterward by large scale rotation of the western margin of the pacific northwest in response to oblique subduction and consequent distributed shear

paleomagnetic results from the central sierra nevada constraints on reconstructions of the western united states paleomagnetic results from three plutons of the central sierra nevada limit the motion of the sierra nevada relative to the north american craton to 5 8 of poleward motion and 7 11 of clockwise rotation since about 90 ma the indicated rotation and translation are not significant at the 95 confidence level a new tectonic model is presented that is consistent with all available paleomagnetic data from the sierra nevada great valley klamaths oregon coast range cascades and western nevada the paleogeographic reconstructions of this model associate the displacement of major crustal blocks with basin and range extension starting at about 30 ma this model accounts for about 15 of clockwise rotation of the klamaths and southern cascades about 2 of clockwise rotation of the sierra nevada and great valley and 26 of clockwise rotation of the oregon coast range since 30 ma the model is consistent with 40 of clockwise rotation of the oregon coast range before 30 ma as proposed by magill and cox 1980 the amount of basin and range extension in this model at latitude 41 n is 225 km directed e w and at latitude 35 n is 245 km directed wsw ene

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

paleomagnetism of ocean layers 2 and 3 evidence from the metchosin complex vancouver island abstract the early eocene metchosin complex is an ophiolitic sequence and is the northernmost part of the crescent terrane it was underthrust beneath wrangellia later in the eocene directions of magnetization in sheeted diabase layer 2 and gabbroic intrusions layer 3 from the complex are internally consistent and both normal and reversed polarities are present mean directions irrespective of sign differ not only from the expected eocene palaeofield but also from one another these divergences have not therefore been caused by tectonic rotation of the metchosin complex as a whole but by variable tilting of bodies within the complex it is argued that these tilts probably were imposed during the emplacement of the metchosin complex adjacent to and beneath wrangellia the systematic clockwise rotations found further south in eocene rocks of the crescent terrane of washington and oregon do not occur the remanent magnetizations of the gabbros of layer 3 are commonly strong and stable these are the only rocks in the metchosin complex that had they been part of the present oceanic crust would be capable of contributing to marine magnetic anomalies the stable magnetization of diabase of layer 2 is too weak the dominant magnetization being a soft viscous remanence imposed by the present geomagnetic field the remanence of basalt lavas of layer 2 as previous work on the metchosin complex has shown is weak and scattered in direction

isotopic provenance of paleogene sandstones from the accretionary core of the olympic mountains washington conventional modal sandstone data from paleogene units of the pacific northwest are not precise enough to pinpoint source areas and constrain displacement histories of accreted sedimentary terranes isotopic provenance study used in conjunction with traditional basin analysis techniques provides a powerful means of identifying source areas analysis of rb sr data in both wholerock and detrital white micas of paleogene sandstones from allochthonous units in the eastern core of the olympic mountains and coeval autochthonous sandstones from coastal pacific northwest shows that the needles gray wolf and grand valley lithic assemblages of the core came from the same source as sandstones of the chuckanut formation and puget group in northern washington the source of these units is isotopically distinct from the source for units farther south such as the tyee formation in oregon chemical compositions conventional k ar age determinations and oxygen and hydrogen isotope compositions of white micas support this conclusion similar analyses of sandstones from the western olympic lithic assemblage and the yakutat terrane of southeastern alaska suggest that these two units have a similar source but that they differ slightly from sandstones of the eastern olympic core and autochthonous washington units the overall composition of sandstones lithic arkosic and the very high initial sr values 0 710 moderately high 18 o values 9 and mesozoic age for detrital white mica of olympic core rocks and sandstone of the yakutat terrane suggest a source in the high rank metamorphic and plutonic rocks from the eastern part of the omineca crystalline belt in southeastern british columbia furthermore rapid uplift of this source area during eocene time is consistent with the depositional age of the olympic rocks sediment derived from this source area was transported westward by major river systems that crossed the low lying north cascade range and supplied the deposits of the autochthonous units of the northern washington coast and the offshore equivalents before the latter were accreted to form the olympic core limited data from the yakutat terrane suggest that it lay offshore of southern british columbia sometime during middle eocene to early oligocene time consistent with paleomagnetic and some paleontologic interpretations and subsequently migrated northward by plate motions

upper mantle heterogeneities beneath central europe summary an inversion of isc travel time data from selected earthquakes in the distance range 30 90 to 53 stations in central europe has been used to model velocity down to 600 km depth the model explains 0 1 0 2s of the residuals as for other array studies leaving 0 5 s unexplained as noise the uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology this may be due to deep seated thermal anomalies or in some areas to delays introduced by passage of the rays through sedimentary cover the deeper anomalies are smaller and unrelated to those in the lithosphere which suggests that the asthenosphere is decoupled from the rigid lithosphere the structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity the models show some suggestion of a high velocity slab trending from east to west beneath the alps

paleomagnetism and geology of eocene volcanic rocks of southwest washington implications for mechanisms of tectonic rotation paleomagnetic and geologic investigations in eocene volcanic rocks of the southwest washington coast range demonstrate a close relationship between tectonic rotations and the local structural geology the allochthonous middle eocene submarine basalt basement of the crescent formation consists of several fault bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation 20 to perhaps as much as 65 when compared to the eocene reference pole for north america structural analysis shows the differential rotations postdate middle eocene folding of the crescent formation against the continental margin and predate the unconformably overlying upper eocene goble volcanics which are rotated about 23 and do not show the same domains of rotation as the underlying crescent formation post goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults major north northwest trending faults with several kilometers of dextral displacement form the boundaries of cross faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west northwest trending sinistral r riedel shears thirty five to 100 of the observed post late eocene rotations could have occurred by this shear rotation mechanism other paleomagnetic study areas in the coast range and western cascades have a similar fault geometry and may also have undergone significant shear rotations long term northward oblique subduction of the farallon plate beneath the coast range throughout most of the tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large rigid plates

near surface downhole shear wave surveys a primer downhole shear wave surveys may be used for a wide variety of applications that include determining the shear wave velocity function with depth for static corrections or time to depth conversions for petroleum surveys involving shear wave reflections however the principal nonpetroleum application is for investigations of engineering sites

a case study of seismic zonation in municipal areas an earthquake of 7 4 magnitude in august 1999 caused major loss of life and severe damage within the municipality of izmit 170 km southeast of istanbul figure 1 a survey of the damaged buildings was made by the municipal authorities shortly after the earthquake the municipal government decided to conduct a pilot seismic zonation project to determine whether the cause of the damage was poor construction materials and methods or weak soil conditions in this project we investigated the soil conditions with two objectives in mind 1 to estimate the seismic model of the soil column at each district to determine the geotechnical earthquake engineering parameters and 2 to map active faults within the municipal area

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

slab geometry of the cascadia subduction zone beneath washington from earthquake hypocenters and teleseismic converted waves using earthquake hypocenters obtained from the washington regional seismograph network and analysis of ps converted phases from teleseisms at special temporary stations located south of the olympic peninsula we have developed a preliminary model of the structure of the subducted slab beneath western washington and southern british columbia the slab is smoothly arched beneath puget sound subducting at a dip of about 10 12 at the top of the arch north and south of the arch crest the slab dips 15 20 subcrustal earthquakes are localized near the top of the arch this deformation from planar slab geometry is caused by the change in azimuth of the trench or hinge line of the slab from almost ns south of 47 north latitude to approximately n30 w north of 48 north the slab appears to be easily flexed probably due to small elastic thickness but its geometry reflects conservation of surface area and minimization of in plane strain crustal seismicity in the puget sound basin may result indirectly from the presence of the arch in the underlying juan de fuca plate

variable dispersion of neogene geomagnetic field directions in iceland abstract analysis of the primary paleomagnetic directions in 4000 lava flows in iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 ma this dispersion which is independent of the polarity of the field is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka

paleomagnetism and tectonics of the crescent formation northern olympic mountains washington a stable prefolding magnetization has been discovered in pillow basalts of the eocene lower crescent formation of the northern olympic mountains the curved outcrop pattern of the crescent formation has been the target of several unsuccessful studies to test for oroclinal bending the success of this study is due in part to the development of a small diameter electric core drill for sampling the fractured rims of basalt pillows thermal demagnetization produced stable endpoints by 580 c in 12 of the 34 sites sampled large within site scatter was common in the remaining sites among the accepted sites within site scatter was small and correction for bedding tilt significantly reduced the scatter between sites the mean paleomagnetic pole for newly sampled pillow basalts 86 4 north latitude 170 0 east longitude a95 16 5 is indistinguishable from the early to middle eocene pole expected for north america including previous results from sites in subaerial basalt of the upper crescent formation in and near the easterm olympic mountains results in a more regional paleomagnetic pole 80 7 north latitude 192 0 east longitude a95 8 0 n 46 that shows no significant rotation 0 8 14 4 or poleward displacement 3 6 8 5 analysis of the magnetic mineralogy suggests that the remanence is early probably primary the pole therefore should be valid for tectonic interpretation of the region a circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern crescent formation experienced deformation due to the rise in a domelike fashion of the sediments of the olympic core terrane erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the crescent formation the lack of significant rotation of the northernmost coast ranges contrasts with the net clockwise rotation seen to the south the difference could be that irrotational northward translation paleomagnetically insignificant yet geologically important driven by oblique convergence was accommodated by the north south trending strike slip faults to the east of the olympic mountains

shear wave measurements of the elasticity of the ground shear waves provide a direct way of determining the dynamic shear modulus of the ground that is independent of poisson s ratio two shear wave methods have been applied to determine in situ properties as a function of depth on three clay sites the first method was that of shear wave refraction using a strong shear wave source and an enhancement seismograph the second method measured the velocity of rayleigh or surface waves generated by vibrators on the surface of the ground in addition compression wave velocities were measured enabling the dynamic poisson s ratio to be calculated on all three sites the moduli were found to increase with depth they were compared with semi static values from plate tests and pressuremeter measurements the comparisons were made by means of a generalized viscoelastic model based on published values of damping for clays as a function of strain les ondes de cisaillement permettent de determiner directement le module de cisaillement dynamique du sol qui est independant du

the crustal structure of the eastern alps from a combination of 3d gravity modelling and isostatic investigations abstract the transition between european and adriatic crust is an important feature related to the plate collision that formed the european alps the diversity of seismic and geological information allows the construction of two alternative 3d density models which both match the observed gravity field different seismic experiments suggest a thickness for the adriatic crust between 30 and 40 km the thick crust model requires an unusually dense lower crust 3050 kg m 3 to reproduce the observed bouguer anomaly to evaluate the two alternative models the isostatic implications of the geometry and density distribution within both 3d models are investigated using local airy and regional vening meinesz isostasy airy isostatic investigations show that the eastern alps are not isostatically compensated and the residuals correlate strongly with exposed geological formations subsequently subsurface loading is an important factor controlling isostatic processes the different geometry and densities in the two 3d models imply different loading at the crust mantle boundary the subsurface loads calculated from the 3d density models were used to estimate regional isostasy by a convolution method in general small rigidity values d 21 nm are determined for the eastern alpine lithosphere in the model with a 40 km thick adriatic crust high flexural rigidities are inferred for the adriatic plate 100 10 21 nm but these values are unusual for an active orogenic region the results point to the interfingering of european and adriatic crust that results in the squeezing of european crust between adriatic crust and mantle with additional contamination by mantle material

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

post folding remagnetization that passes the fold test summary the palaeomagnetism of folded cretaceous redbeds in the methow pasayten belt of washington was studied in an attempt to resolve the tilt versus translation origin of shallow inclinations found in most cretaceous plutonic rocks of the pacific northwest after elimination of results from apparently strained overturned beds correction to palaeohorizontal of the high temperature dominant components produced two distinct directions one from each of two areas treated separately the magnetizations from these two areas appear to pass the fold test both directions are anomalously shallow which is typical of the region but the discrepancy between them casts doubt that both could have been acquired when the strata were horizontal instead we propose that they date from at least two episodes of remagnetization after the planar strata were tilted but before development of the tighter folds observed today our caution is that a positive fold test only establishes that a magnetization likely was acquired when strata in an area were planar other arguments are required to demonstrate that this planar surface was in fact horizontal during magnetization

ongoing oroclinal bending in the cascadia forearc and its relation to concave outboard plate margin geometry author s finley t morell k leonard l regalla c johnston st zhang w abstract 2019 geological society of america the concave inboard concave toward the overriding plate geometry of most convergent margins is considered a natural consequence of the depression of the edge of a thin spherical cap whereas concave outboard margin segments commonly form around indenters on the subducting plate at the cascadia subduction zone the apex of a g 500 km long concave outboard bend in the trench presently shows no obvious subduction of an indenter but does coincide with the axis of an outboard facing concavity in upper plate rocks arched around the olympic peninsula in northwestern washington state usa here we synthesize paleomagnetic and structural data together with new analyses of global navigation satellite system data to show that the upper plate at cascadia has been folded from the miocene to the present into an orocline with an axial trace that bisects the olympic peninsula the processes that accommodate bending which we suggest include 1 folding by flexural slip on the orocline limbs and 2 shortening uplift and escape within the core of the fold at the olympic mountains have the combined result of relative motion of the forearc towards the arc at the core of the orocline and sustained opposing rotations of the upper plate on the orocline limbs we propose that oroclinal bending is promoted and maintained by along strike variations in plate boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of concave outboard margins without the need for a subducting indenter

the european african collision and its effects on the lithosphere asthenosphere system abstract in the european area lateral changes in the shear wave velocities of the lithosphere asthenosphere system are quite pronounced gravity data imply a positive density contrast 0 03 0 07 g cm 3 in the upper mantle related to almost aseismic lithospheric roots which seem to characterize many orogenic belts the analysis of a transect along the longitudinal axis of the alps seems to indicate that relevant lateral variations in density are also present in the lithospheric part of the mantle

3 d velocity structure beneath france in different frequency bands abstract the 3 d velocity structure beneath france is investigated by using a method based upon teleseismic waveform analysis and a block inversion technique in this method the p wave travel time is computed exactly in cach block for a spherical earth model and the p wave arrival times are obtained by cross correlating the waveforms observed in 26 stations of the ldg network one image is obtained directly from the observed signals the other two are obtained from low pass filtered waveforms with 0 5 and 0 25 hz cut off frequencies the results show that the 3 d velocity image depends weakly but significantly on the cut off frequency of the low pass filter in addition to possible changes in the signal to noise ratio this may indicate a dependence of the tomographic images on the seismic wavelength the best fit to the data is obtained from the 0 5 hz image the main features of this image are a clear indication of a lithospheric root beneath switzerland a general large scale w e variation of velocity within the lithosphere and a diffuse low velocity area beneath the french massif central and the western part of the alps in the 100 250 km layer

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

high pressure metamorphism and uplift of the olympic subduction complex the discovery of the critical assemblage lawsonite quartz calcite indicates that a significant part of the cenozoic olympic subduction complex of northwestern washington state formed by underplating at a depth of about 11 km the deep structural level exposed in this area is attributed to the presence of a 10 km high arch in the underlying juan de fuca plate we postulate that this arch was formed when the southern cordilleran coastline swung westward as a result of middle miocene to recent extension in the basin and range province

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

chapter 22 paleomagnetism of continental north america implications for displacement of crustal blocks within the western cordillera baja california to british columbia

lithosphere thickness in the contact zone of the moldanubicum and saxothuringicum in central europe abstract the analysis of the time residuals of p waves and their 3 d inversion show strong lateral velocity variations beneath the grf array situated above the contact zone of two important tectonic units the saxothuringicum and moldanubicum variations of teleseismic p residuals have provided data for constructing a detailed model of the lithosphere asthenosphere boundary in the region assuming the effects of lateral changes of average velocities in the subcrustal lithosphere to be much smaller than the effects of lithosphere thickness variations we relate the changes of representative average residuals in direct proportionality to changes of the lithosphere thickness under this assumption we can explain the variations of average residuals over the grf array by a relatively high increase in the thickness of the lithosphere of about 50 km from the northernmost to the southernmost stations the lithosphere asthenosphere boundary dips steeply to the sse and the isopachyte contours follow the general direction of the variscan fold belt in central europe fitting a general pattern of the lithosphere thickness derived for central europe

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

evidence for a margin truncating transcurrent fault pre late eocene in western washington i postulate that a major dextral transcurrent fault truncated the pre tertiary continental framework of western washington and southern vancouver island during the late cretaceous and early tertiary the fault trace trends north and is covered in the puget lowland it corresponds to the west trending san juan and northwest trending survey mountain faults on southern vancouver island the geometry of the structure has probably been greatly modified by subsequent deformation sedimentary basins that formed marginal to the fault are characterized by rapid fades changes and sediment accumulation rates abrupt stratigraphic thinning and thickening irregular basin margins petrographic mismatches and other features consistent with origin in a zone of strike slip faulting the structure was probably part of a network of intracontinental late cretaceous early tertiary strike slip faults in the northern north american cordillera as an outboard member of this fault network it likely served as a major avenue of northward translation along the northeast margin of the pacific basin

material balance in alpine orogeny recent geophysical data are combined with older information for an updated picture of alpine kinematics using semiquantitative considerations of material balance in the alps a conventional pile of thin peel nappes is disrupted by a central longitudinal system of steep faults and steeply limbed folds both affecting disharmoniously the entire edifice of otherwise flat lying nappes it is particularly from late tertiary phases of deformation that structures of the second type the insubric system with the insubric line and the belt of late alpine windows acquired prominence for the present aspect of the alps these late structures formed as a dextrally transpressive intracontinental branch of the africa europa plate boundary mostly between areas of extension early motions in the late cretaceous to the late eocene were probably responsible for at least half of the 300 km dextral displacement required for the palinspastic restoration of the inner west carpathians and the dinarides during oligocene extension the late alpine batholiths intruded in a belt roughly along the insubric line while deep crustal and lithospheric roots were destroyed in the latest oligocene to middle miocene the adriatic plate moved dextrally to the west along the insubric transfer fault from its frontal insubric indenter a lower crust upper mantle flake probably first obducted in the cretaceous was detached and wedged into the penninic nappes of the western alps bird s head of the ivrea body both the northern and the southern transfer faults of the insubric indenter were disrupted and inactivated by late miocene to later events the giudicarie and neo north apennine events instead of the inactivated insubric fault the windows belt of en echelon folds assumed the role of dextral transpression all of these successive zones of motion are entangled in the ligurian knot largely hidden under the sea or young sediments a large part of the pre alpine crust and practically the whole mantle were subducted only in some cases were high pressure rocks of shallow continental origin re obducted the lithospheric root of the neo alps is largely contained in a vertical slab under the alps but some parts seem to have been removed and others disharmonically displaced with respect to the surface structures

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

metchosin igneous complex southern vancouver island ophiolite stratigraphy developed in an emergent island setting the metchosin igneous complex of southern vancouver island has three components gabbro stocks formerly sooke gabbro a newly recognized sheeted dike complex and a basaltic volcanic sequence formerly the metchosin volcanics the sheeted dikes occupy a position transitional between the gabbro stocks and submarine volcanics which they tie together in an ophiolitic pseudostratigraphy the submarine volcanics shoal upward and pass into subaerial sheet flows the complex formed in an area of regional extension as an emergent island