evidence for postseismic viscoelastic relaxation following the 1959 m 7 5 hebgen lake montana earthquake a 1983 releveling survey conducted by the national geodetic survey combined with previous surveys in 1923 1960 1967 and 1975 provides evidence for ongoing vertical deformation of a broad region diameter of 150 km surrounding the site of the 1959 hebgen lake earthquake deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault maximum observed elevation change during the postseismic period 1960 1983 exceeds 30 cm assuming that the entire uplift i e 1923 1983 occurred following the 1959 earthquake i e uplift measured between 1923 and 1960 was entirely postseismic the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years the spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer thickness of 30 40 km overlying a viscoelastic half space viscosity of 1019 pa s this model may also account for the anomalously large horizontal strain around the hebgen lake region measured by repeated trilateration surveys during the period 1973 1984 the releveling measurements in the hebgen lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the united states

imaging granitic plutons along the iberseis profile abstract the parameters used for the acquisition of the iberseis deep seismic reflection profile in the sw iberian peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface a very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest granitic plutons the three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the iberseis transect in the areas of la bazana la dehesilla feria and villafranca in these high resolution velocity models sampled by 50 50 50 m cells high velocity anomalies indicate the geology and extension of the granitic plutons at depth this directly correlates with the surface outcrops moreover tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume furthermore a piggy back seismic acquisition experiment using additional seismic instrumentation from the university of paris sud 40 channel dmt provided perpendicular offline recordings of the vibroseis sources this additional recording system was deployed perpendicularly to the main iberseis seismic reflection line and provided additional 3d control

model for the evolution of the linear fold belt in the continents example of the urals abstract the following events of the well known wilson cycle are refined and amplified by results obtained mainly for the urals the uplift and diaschysis tectono magmatic activity protoorogenesis of the earth s crust were caused by upwelling of low density material in the upper mantle the diaschysis occurred long before or was occasionally partly contemporaneous with rifting accompanied by depression it is characterized by alkaline magmatites pseudostratified gabbro ultrabasite plutons and carbonatites features of recurrent diaschysis and rifting were recorded in mobile belts during several hundred millions of years they are occasionally terminated by complete lithospheric break up and formation of oceanic crust in other cases which were probably characteristic of the proterozoic the spreading and formation of oceanic crust did not take place due to rheological lithospheric stratification a brittle fracture of the upper ten kilometers of the earth s crust occurs simultaneously along two or more parallel fissures the subsequent separation of newly formed blocks and interblock basaltic magmatism lead to the production of microcontinents i e future median massifs of mobile belts the break up and separation of segments of continental lithospheric plates are followed by plastic extension at lower middle levels within the earth s crust this causes formation of different temperature blastomylonites and schists at about the same time there occur intrusions of granitoids into the extension zone and low moderate pressure zonal metamorphism until recently these phenomena were wrongly considered as indications of post geosynclinal orogenic regimes of inferred earlier geosynclinal cycles it might be suggested that the crust of the silurian ural ocean was almost completely subducted along the zavaritsky benioff zone dipping to the east in paleozoic back arc seas as well as in modern seas secondary spreading zones were present with abundant subvolcanic tholeiite volcanism the products of volcanism have suffered greenstone metamorphism as long ago as in pre orogenic oceanic island arc stage during formation of a mountain structure attended by development of granitoid magmatism there appeared to occur contrasting metamorphisms of sodic plagiogneiss schist complexes of a disthene series and a zone of glaucophane blastomylonites and subsequently potassic gneiss schist complexes of an andalusite series and disthene kalifeldspar blastomylonites geochemical backgrounds this metamorphism probably caused inner differentiation of crustal material repeated impulses of extension and outbursts of rifting occurred during the closure of oceanic structures and later in preorogenic and post orogenic stages

nature of the lithosphere across the variscan orogen of sw iberia dense wide angle seismic reflection data 1 two wide angle seismic transects have been acquired across the sw iberian massif they crossed three major geological zones south portuguese zone ossa morena zone and central iberian zone with their tectonic contacts and the pyrite belt being of greatest interest a total of 690 digital seismic recording instruments 650 texans and 40 reftek 3 component units from the iris passcal instrument pool were used the transects a and b are each approximately 300 km long and consist of 3 and 6 shot points respectively with an approximately 60 km shot point interval the charge sizes range from 1000 kg at the edges to 500 kg at the center these recently acquired experiments were designed to provide velocity constraints on the lithosphere and to complement the previously acquired normal incidence seismic profile iberseis both data sets are part of the sw iberia project which was developed within the europrobe program and designed to address fundamental questions about the nature and dynamics of the variscan lithosphere the acquisition parameters provide closely spaced wide angle seismic images of the lithosphere beneath sw iberia in transect a the station spacing was on average 400 m while along transect b the receiver spacing was approximately 150 m because of this close trace spacing the lateral continuity of the seismic arrivals is greatly improved frequency analysis revealed that the recorded events feature relatively low frequencies 6 25 hz after processing the shot records show high amplitude and well defined arrivals the interpreted pmp arrival located at approximately 11 s normal incidence traveltime is characterized by high amplitude and relatively low frequency 6 12 hz a well defined pn arrival appears at offsets beyond 120 km at far offsets greater than 180 km an upper mantle reflection is observed furthermore within the upper crust the shots records feature a relatively high velocity arrival located at 4 5 s normal incidence traveltime the analysis of this arrival indicates that it probably corresponds to the top of the iberian reflective body identified in the iberseis deep seismic profile the velocity models obtained by forward modeling show a complex crust especially in the middle crust the velocity models are the most detailed ones that have been produced in the area and contain a large amount of new features that are relevant to the understanding of the composition of the crust and upper mantle beneath the zone the velocity depth functions derived from the velocity models have higher middle crustal velocities than the average in other continental areas a comparison between laboratory seismic velocity measurements and the velocities of the models was carried out in order to estimate the crustal and the upper mantle composition results indicate that the high middle crust velocities correspond to rocks of a mafic composition the combined data set reveals new aspects related to the lithospheric evolution of this transpressive orogen and allows us to attempt an interpretative cross section of the upper lithosphere in sw iberia

crustal versus asthenospheric relaxation and post seismic deformation for shallow normal faulting earthquakes the umbria marche central italy case summary following a normal mode approach for a strati ed viscoelastic earth we investigate the eiects induced by shallow normal faulting earthquakes on surface post seismic vertical displacement and velocity at the surface when stress relaxation occurs in the crust or in the asthenosphere the modelled earthquake is a moderate one characteristic of some slowly deforming plate boundaries in the central mediterranean region we focus on the umbria marche central italy region where deep seismic re ection studies crop03 and the 1997 earthquake sequence clearly show a seismogenic layer decoupled from the lower crust by a sizeable transition zone accordingly the crust is subdivided into three layers an elastic upper crust a transition zone and a low viscosity lower crust the fault is embedded in the upper crust the layered viscoelastic structure of the crust and mantle imposes a pattern and scale on the modelled coseismic and post seismic deformation with a major contribution from the transition crustal zone and low viscosity lower crust stress relaxation in the mantle being negligible we show that signi cant vertical deformation rates of the order of 1 mm yr 1 could be expected for a shallow and moderate event such as the recent umbria marche earthquake for viscosity values of 10 19 and 10 18 pa s in the crustal transition zone and lower crust respectively

seismic imaging and modelling of the lithosphere of sw iberia abstract data from a closely spaced wide angle transect has been used to study the middle to lower crust and the moho in sw iberia a low fold wide angle stack image reveals a highly heterogeneous seismic signature at lower crustal levels changing laterally along the profile the lower crust features an irregular distribution of the reflectivity that can be explained by a heterogeneous distribution of physical properties the moho discontinuity also features a high variability in its seismic character that correlates with the different tectonic terranes in the area a 2d finite difference code was used for solving the elastic wave equation and to provide synthetic wide angle shots relatively simple layer cake model derived from conventional refraction interpretation generates the main events of the shot records however these models cannot account for the lateral variability of the seismic signature in order to obtain more realistic simulations the velocity model was modified introducing stochastic lensing at different levels within the crust the moho was modelled as a 3 km thick layered structure the resulting average velocity models include a high velocity layer at mid crustal depth a highly reflective lower crust and a relatively thin horizontal moho this heterogeneous model can be achieved by lensing within the crust a layered mafic intrusion and a strongly laminated lower crust and moho

characterization of a fractured granitic pluton p and s waves seismic tomography and uncertainty analysis abstract a detailed characterization of the physical properties of a granitic pluton including a three dimensional reconstruction of the fractures network and its surrounding are obtained by high resolution seismic tomography of p and s waves the input p and s wave travel times were picked from three component offset and azimuth variable seismic profile ovsp acquired in a 500 m deep vertical borehole the travel time inversion resulted in two three dimensional p and s wave velocity models and the uncertainty robustness and sensitivity are estimated by performing a monte carlo type analysis and checkerboard test this study provides an indicator of the uniqueness of the determined model the 500 m deep borehole was continuously cored and extensively surveyed by geophysical techniques thus the detailed surface geological mapping fracture index logs borehole televiewer images and p and s wave velocities derived from sonic logs provide solid constraints for the interpretation of the tomographic images the three dimensional tomographic images reveal fracture zones mapped at surface that intersect the borehole at different depths and they can be identified in the core descriptions and in the borehole televiewer images the fractures identified on the cores feature relatively low values for the sonic p and s wave velocities and high values for poisson s ratio for single type lithologies the distribution of poisson s ratio differentiates fractured and altered granite from fresh unfractured domains within the rock poisson s ratios close to 0 3 correlate with fractured volumes consistent with fracture index and sonic logs otherwise poisson s ratio values close to 0 25 are related to unaltered and unfractured rock

small scale heterogeneity and large scale velocity structure of the continental crust since conrad 1925 first identified a midcrustal refraction event in central europe seismologists have developed layered models of the continental crust with velocity steps at various levels modern analysis of crustal refraction data including automated inversion relies on ray theoretical techniques or wave theory for plane layered media which generally parameterize the earth as a series of grossly horizontal layers the bias toward layered models is in part based on physical and geologic intuition but it is to a large measure a result of available interpretation methods for example layer based reflectivity and ray tracing methods in contrast reflection seismology has produced a picture of a highly heterogeneous crust with wavelength scale variations in velocity existing at different levels of the crust in different tectonic regimes finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide angle reflections some of the events identified as crustal wide angle reflections in field data may in fact be the result of scattering from wavelength scale heterogeneities and need not result from reflection from a first or second order velocity discontinuity significant changes in the large scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field this observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large scale velocity structure as a consequence the ray theoretical interpretation of such wide angle reflections as first order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail while correctly predicting average crustal velocity and thickness we are not suggesting that crustal velocity does not change with depth on the large scale rather we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength scale velocity fabric rather than specular reflection or refraction from first order discontinuities

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

eclogite facies shear zones deep crustal reflectors strongly foliated eclogite facies rocks in 30 150 m thick shear zones of caledonian age occur within a grenvillian garnet granulite facies gabbro anorthosite terrain in the bergen arcs of norway the predominant eclogite facies mineral assemblages in the shear zones are omphacite garnet zoisite kyanite in gabbroic anorthosite and omphacite garnet in gabbro eclogite facies rocks in shear zones are generally fine grained alternating omphacite garnet and kyanite clinozoisite rich layers define gneissic layering a strong shape preferred orientation of omphacite kyanite and white mica phengitic muscovite and or paragonite define the foliation the anorthositic eclogites show omphacite b axis maxima approximately normal to the foliation and c axis girdles within the foliation plane p wave velocities v determined at confining pressures to 600 mpa for samples from eciogite facies shear zones range from 8 3 to 8 5 km s l and anisotropy ranges from 1 to 7 the few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation the fast propagation direction lies within the c axis girdles parallel to foliation and the slow propagation direction is parallel to the b axis concentration normal to foliation in samples for which omphacite crystallographic preferred orientation was determined v for the granulite facies protoliths average about 7 5 km s high calculated reflection coefficients for these shear zones 0 04 0 14 indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high pressure conditions but escaped thermal reactivation

seismic wide angle constraints on the crust of the southern urals a wide angle seismic reflection refraction data set was acquired during spring 1995 across the southern urals to characterize the lithosphere beneath this paleozoic orogen the wide angle reflectivity features a strong frequency dependence while the lower crustal reflectivity is in the range of 6 15 hz the pmp is characterized by frequencies below 6 hz after detailed frequency filtering the seismic phases constrain a new average p wave velocity crustal model that consists of an upper layer of 5 0 6 0 km s which correlates with the surface geology 5 7 km depths at which the velocities increase to 6 2 6 3 km s 10 30 km depths at which on average the crust is characterized by velocities of 6 6 km s and finally the lower crust from 30 35 km down to the moho which has velocities ranging from 6 8 to 7 4 km s two different s wave velocity models one for the n s and one for the e w were derived from the analysis of the horizontal component recordings crustal sections of poisson s ratio and anisotropy were calculated from the velocity models the poisson s ratio increases in the lower crust at both sides of the root zone a localized 2 3 anisotropy zone is imaged within the lower crust beneath the terranes east of the root this feature is supported by time differences in the sms phase and by the particle motion diagrams which reveal two polarized directions of motion velocities are higher in the central part of the orogen than for the siberian and eastern plates these seismic recordings support a 50 56 km crustal thickness beneath the central part of the orogen in contrast to moho depths of 45 km documented at the edges of the transect the lateral variation of the pmp phase in frequency content and in waveform can be taken as evidence of different genetic origins of the moho in the southern urals

the root of the urals evidence from wide angle reflection seismics abstract the existence of a crustal root beneath the urals which would deflect the position of the moho by some 20 km is still largely controversial a french russian project carried out a wide angle reflection seismic experiment across the middle urals to image the moho topography along a 175 km profile running approximately east west north of ekaterinburg new data show a 6 km moho deflection beneath the central part of the orogen the moho reflectivity is variable along the section with very sharp reflections beneath the russian platform 45 km depth and fainter attenuated signals in the root zone 51 km depth even if this crustal root is not as thick as indicated by some previous speculations it makes the urals the only palaeozoic orogen in the world to show such a peculiarity a major ultramafic overload in the upper crust would partly balance the crustal root in accordance with isostatic equilibrium we finally postulate that sharp wide angle reflections from the moho can be considered indicative of a layered lower crust this would apply to the russian platform which maybe gained this structuring during the ordovician extensional regime

geometric and kinematic evolution of the foreland thrust and fold belt in the southern urals the first balanced and restored cross section through the foreland thrust and fold belt of the southern urals along the belaya river of bashkortostan is presented this cross section is used to discuss the paleozoic structure and kinematic evolution of the area and to provide quantitative estimates of fault displacement and shortening the belaya river thrust stack can be divided into a late imbricate thrust system nygush thrust system the burzyan thrust sheet and an early duplex system timirovo thrust system overprinting relationships indicate two deformation events during which the foreland thrust and fold belt evolved by a forward propagating thrust sequence the first deformation event involved emplacement of the low metamorphic grade timirovo thrust system and the overlying zilair nappe during accretion of the magnitogorsk volcanic arc with the east european craton at the end of the devonian a deformational hiatus followed until the end of the carboniferous at which time a second deformation event related to final closure of the uralian ocean resulted in development of the nygush thrust system the calculated shortening in the belaya river area 16 7 km or 17 is significantly less than that in other foreland thrust and fold belts the basal thrust in the belaya river section is located within the basement and cuts continuously up section with a staircase like geometry instead of following an easy glide horizon

mapping the moho beneath the southern urals with wide angle reflections a stack of the wide angle reflection refraction component of the urseis 95 experiment provides the first well resolved imaged of the moho beneath the southern urals the processing consisted of low pass filter 0 6 hz cmp sorting and a nmo correction without stretch the pmp phase a very narrow band and low frequency up to 6 hz wavelet changes character from west to east along the transect in the depth converted section the moho reaches a maximum depth of 53 2 km beneath the magnitogorsk arc thickness estimates determined from high amplitudes at near critical distances also support a 53 km thick crust a selective offset stack consisting of traces at 150 250 km offset indicate an undulating irregular moho suggesting either strong lateral velocity variations or high topographic relief beneath the magnitogorsk arc

vertical deformation associated with normal fault systems evolved over coseismic postseismic and multiseismic periods vertical deformation of extensional provinces varies significantly and in seemingly contradictory ways sparse but robust geodetic seismic and geologic observations in the basin and range province of the western united states indicate that immediately after an earthquake vertical change primarily occurs as subsidence of the normal fault hanging wall a few decades later a 100 km wide zone is symmetrically uplifted the preserved topography of long term rifting shows bent and tilted footwall flanks rising high above deep basins we develop finite element models subjected to extensional and gravitational forces to study time varying deformation associated with normal faulting we replicate observations with a model that has a weak upper mantle overlain by a stronger lower crust and a breakable elastic upper crust a 60 dipping normal fault cuts through the upper crust and extends through the lower crust to simulate an underlying shear zone stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift under the footwall the breakable upper crust is a necessary model feature to replicate footwall bending over the observed width 10 km which is predicted to take place within 1 2 decades after each large earthquake thus the best preserved topographic signature of rifting is expected to occur early in the postseismic period the relatively stronger lower crust in our models is necessary to replicate broader postseismic uplift that is observed geodetically in subsequent decades

super deep reflection profiling exploring the continental mantle lid abstract reflections from the continental mantle lithosphere have now been reported from common mid point cmp profiles at numerous sites around the world these reflections include 1 distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle 2 kilometer scale discontinuous diffusely distributed or isolated reflections at varying depths 3 continuous sub horizontal reflections tens of kilometers in extent often interpreted as the base of the lithosphere in spite of a wide variation in depth and 4 very deep reflections in the mantle lithosphere 125 km the first category of mantle reflections is most commonly observed in proximity to crustal sutures lending credence to their interpretation as fossil subduction zones isolated discontinuous mantle reflections are by far the most common and though observed in both ancient and modern collisional environments are difficult to interpret due to both their limited extent and a corresponding lack of three dimensional control sub horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide of special note are the various reports of specific reflections at 80 100 km 22 24 s twt in the north sea eastern europe and the urals which suggest a more ubiquitous mantle horizon the deepest events imaged thus far in a controlled source cmp survey are the super deep mantle reflections of the urseis project that have been identified at 135 165 km 35 43 s and 225 km 56 s of the many reported mantle reflections these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo mechanical transition should be reflective at high frequencies 10 20 hz

imaging the crustal structure of the central iberian zone variscan belt the alcudia deep seismic reflection transect alcudia is a 230 km long vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern central iberian zone part of the pre mesozoic gondwana paleocontinent of the variscan orogen of spain the carefully designed acquisition parameters resulted in a 20 s twtt deep 60 90 fold high resolution seismic reflection transect the processed image shows a weakly reflective upper crust the scarce reflectivity matching structures identified at surface a thick highly reflective and laminated lower crust and a flat moho located at 10 s twtt 30 km depth the transect can be divided into three segments with different structural styles in the lower crust in the central segment the lower crust is imaged by regular horizontal and parallel reflectors whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust north and tectonic wedging involving the mantle south the alcudia seismic image shows that in an intracontinental orogenic crust far from the suture zones the upper and lower crust may react differently to shortening in different sectors which is taken as evidence for decoupling the interpreted structures as deduced from surface geology and the seismic image show that deformation was distributed homogeneously in the upper crust whereas it was concentrated in wedge thrust structures at specific sectors in the lower crust the seismic image also shows the location of late variscan faults in spatial association with the lower crustal thickened areas

fault kinematics and surface deformation across a releasing bend during the 2010 mw 7 1 darfield new zealand earthquake revealed by differential lidar and cadastral surveying dextral slip at the western end of the east west striking greendale fault during the 2010 m w 7 1 darfield earthquake transferred onto a northwest trending segment across an apparent transtensional zone here named the waterford releasing bend we used detailed surface mapping differential analysis of pre and postearthquake light detection and ranging lidar and property boundary cadastral resurveying to produce high resolution centimeter scale estimates of coseismic ground surface displacements across the waterford releasing bend our results indicate that the change in orientation on the greendale fault incorporates elements of a large scale releasing bend from the viewpoint of westward motion on the south side of the fault as well as a smaller scale restraining stepover from the viewpoint of southeastward motion on the north side of the fault these factors result in the waterford releasing bend exhibiting a decrease in displacement to near zero at the change in strike and the presence within the overall releasing bend of a nested localized restraining stepover with contractional bulging the exceptional detail of surface deformation and kinematics obtained from this contemporary surface rupture event illustrates the value of multimethod investigations our data provide insights into strike slip fault bend kinematics and into the potentially subtle but important structures that may be present at bends on historic and prehistoric rupture traces

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

aleutian basin oceanic crust abstract we present two dimensional p wave velocity structure along two wide angle ocean bottom seismometer profiles from the aleutian basin in the bering sea the basement here is commonly considered to be trapped oceanic crust yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes line 1 extends 225 km from southwest to northeast while line 2 extends 225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation velocities of the sediment layer increase from 2 0 km s at the seafloor to 3 0 3 4 km s just above basement crustal velocities increase from 5 1 5 6 km s at the top of basement to 7 0 7 1 km s at the base of the crust and upper mantle velocities are 8 1 8 2 km s average sediment thickness is 3 8 3 9 km for both profiles crustal thickness varies from 6 2 to 9 6 km with average thickness of 7 2 km on line 1 and 8 8 km on line 2 there is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features the velocity structure is consistent with that of normal or thickened oceanic crust the observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation

models of crustal deformation this review is principally concerned with theoretical models of crustal deformation developed during this quadrennium observational data as well as topics in lithospheric flexure and isostasy are covered in companion reviews in this volume progress has been made toward constructing both new inelastic and inhomogeneous elastic models of earthquake induced crustal deformation most of the inelastic models involve dislocations or green s functions in an elastic or anelastic lithosphere which overlies a maxwell viscoelastic asthenosphere

effects of lithospheric viscoelastic relaxation on the contemporary deformation following the 1959 mw 7 3 hebgen lake montana earthquake and other areas of the intermountain seismic belt 1 the 1959 mw 7 3 hebgen lake mt normal faulting earthquake occurred in an extensional stress regime near the yellowstone volcanic field time dependent crustal deformation data following this major earthquake were acquired by precise trilateration and gps surveys from 1973 to 2000 around the hebgen lake fault zone modeling the changes of baseline lengths across and near the fault reveals a lateral variation of transient rheology in which the lithosphere is stronger near the hebgen lake fault zone than in the vicinity of the yellowstone volcano system the models also imply that the lower crust is stronger than the upper mantle in agreement with results from studies of postseismic and post lake filling relaxations 100 years in addition evaluations of the postseismic motion produced by the hebgen lake and the 1983 mw 6 9 borah peak id earthquakes indicate that horizontal transient motion of up to 1 mm yr contribute significantly to the contemporary regional crustal deformation near the epicentral areas for the eastern basin and range 500 km south of the hebgen lake fault similar rheologic models were derived from the observed uplift associated with the lake bonneville rebound and were used to evaluate the postseismic deformation associated with six most recent paleoearthquakes of the wasatch fault zone and three m 5 6 historic earthquakes of northern utah the results show 0 1 mm yr of horizontal postseismic motion at present time that are within the horizontal uncertainties of continuous gps velocity from the basin and range and significantly smaller than the contemporary extension of 1 3 mm yr in the wasatch front

the reflectivity of magmatic underplating using the layered mafic intrusion analog abstract the reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates one dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0 18 a seismic profile across the bjerkreim intrusion in southern norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0 05 produces detectable reflections two dimensional models demonstrate that the subhorizontal cumulate layering in typical mid to upper crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image reflection coefficients predicted for granulite facies mineral assemblages are similar to those predicted for the igneous assemblages reflection coefficients predicted for cumulates intruded or re equilibrated at eclogite facies conditions may be either decreased or enhanced with respect to the igneous assemblages depending on the abundance of plagioclase in the cumulate layers and the pressure results suggest that magmatic underplating should not be invoked as an explanation for non reflective crust except in cases where eclogitization has occurred

a new map of crustal terranes in the united states from cocorp deep seismic reflection profiling a new map has been constructed to summarize major systematic variations in seismic reflection character observed by over 10000 km of cocorp profiling in the us among the seismic characteristics represented on this map are crustal scale detachments which can be traced as relatively low angle reflectors over large areas deeply penetrating dipping reflection zones some of which mark major shear zones perhaps even crustal sutures moho reflectivity which appears to vary from craton to phanerozoic periphery seismic transparency which can sometimes be interpreted as geologic homogeneity seismic layering prominent in the lower crust in many areas and now known to dominate the upper basement in other regions and unusually strong reflections or bright spots at mid crustal levels that may mark deep crustal fluids

crustal root beneath the urals wide angle seismic evidence wide angle reflection and refraction data acquired as part of the urseis 995 geophysical experiment across the southern uralide orogen provide evidence for a 12 to 15 kilometer thick crustal root yielding a total crustal thickness of 55 to 58 kilometers strong reflections from the mohorovicic discontinuity moho at relatively small precritical distances suggest that the crust mantle transition beneath the crustal root is a sharp feature the derived p and s wave velocity models constrain key physical properties of the crust including the depth of the mafic rocks of the magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the east uralian zone

an elastic rebound model for normal fault earthquakes a viscoelastic finite element method is used to formulate an elastic rebound model for normal fault earthquake cycles model generated displacements correlate well with geodetic survey data collected before and after the 1954 m 7 1 fairview peak nevada and the 1959 m 7 1 hebgen lake montana earthquakes model coseismic deformation results in an uplift of the footwall block and a depression of the hanging wall block as has been reported by savage and hastie 1966 the interseismic phase of the deformation is characterized by a regional doming of the crust in the vicinity of the modeled normal faults doming observed before and after and adjacent to the faults which ruptured during the hebgen lake earthquake indicate that areas which are experiencing doming may be the future sites of normal fault earthquakes two areas which are presently experiencing crustal doming and are in the vicinity of a normal fault are the socorro new mexico and diablo plateau texas brown et al 1978 regions the interseismic doming will not occur if the modeled fault ruptures through the entire lithosphere if a single normal fault is active then model generated coseismic and interseismic deformations result in sum to a tilt block topography if two normal faults are active one master fault and a minor fault which dips toward the master fault then the resultant deformation is a horst and graben type of topography the rate at which the doming proceeds is primarily a function of the seismic stress drop and the effective viscosity of the asthenosphere doming rates following the hebgen lake earthquake indicate an asthenospheric viscosity of about 2 1021 p model recurrence intervals for the fairview peak and hebgen lake earthquakes are on the order of 103 yr

analysis of tilting near normal faults using calculus of variations implications for upper crustal stress and rheology abstract an initially horizontal surface in the surroundings to a normal fault will after extension tilt towards the fault if in the hanging wall or away from it if in the footwall coseismic deformation of these surroundings can be validly modelled as flexure cumulative deformation on geological time scales has been regarded as flexure also but this has led to the apparent paradox that many observed structures require the effective elastic thickness of the brittle upper crust to be much smaller than its seismogenic thickness this article presents a possible alternative that cumulative deformation of the upper crust near some active normal faults may be regarded instead as partitioned into separation of the blocks that comprise their surroundings through the horizontal component of dip slip on the fault and distributed vertical simple shear in their surroundings which causes them to tilt and takes up the vertical slip on the fault a generalized profile of tilting near a normal fault which approximates a parabola can be derived following two assumptions first the brittle layer retains an elastic term in its rheology on geological time scales such that it resists arbitrary length changes second the brittle layer minimizes its overall vertical deflection thus minimizing the buoyancy force acting on it maximum stress and total strain energy associated with observed profiles of tilting are typically smaller for distributed vertical simple shear favouring this mechanism over flexure

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

deep structure of the continental lithosphere in an unextended orogen an explosive source seismic reflection profile in the urals urals seismic experiment and integrated studies urseis 1995 explosive source near vertical seismic reflection data from the urals seismic experiment and integrated studies urseis profile displays an image of orogenic lithosphere to depths of 225 km the reflective crustal section is characterized by dipping crustal scale shear zones and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late paleozoic collision toward the east the uralian crustal root is composed of a partially subducted volcanic arc while to the west it has affinity with the east european platform the reflection character of the moho varies across tectonic strike and implies that the moho 1 acted as a structural detachment beneath the trans uralian zone 2 forms a tectonic boundary between terranes beneath the east uralian zone 3 consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root 4 is a 3 9 km thick reflective zone beneath the fold and thrust belt and 5 is a boundary 200 m thick in the west mantle reflections at 80 100 km depth 22 24 s may represent a continuation of the main uralian fault into the subcrustal lithosphere alternatively these reflectors may signify paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle deeper mantle reflections at 140 160 km 35 45 s and 225 km 55 s may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle

crustal scale structure and evolution of an arc continent collision zone in the southern urals russia the outcropping geology of the southern urals contains a well preserved accretionary complex related to the paleozoic collision that took place between the magnitogorsk arc and the former east european craton the crustal scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles the reflection profiles show the accretionary complex to be highly reflective allowing direct comparison of many reflections with surface geological features we interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material syncollisional sediments derived from the arc deeply subducted high pressure gneisses that are intercalated with eclogites and blueschist and at the highest structural level ophiolite complexes it is bound at the base by a thrust and at the rear by a highly deformed zone the main uralian fault adjacent to the backstop the magnitogorsk arc deposition of the late devonian volcaniclastic sediments of the zilair formation appears to be related to collision uplift and erosion of the arc possibly following the arrival of the full thickness of the east european craton continental crust at the subduction zone with the arrival of the continental crust at the subduction zone offscraping and underplating of paleozoic slope and platform material took place at the base of the accretionary complex uplift of the arc was followed by its collapse and the unconformable deposition of lower carboniferous shallow water carbonates on top of it a time lag of 10 15 myr occurred between the high pressure metamorphism and the subsequent arrival of the east european craton at the subduction zone

seismic imaging of reflectors in the sg4 borehole middle urals russia the upper crust in the middle urals is known to be highly reflective however it is not clear whether the observed reflections originate from lithological boundaries or from fault zones the sg4 borehole located in the tagil synform of the middle urals drilled to 5354 m as of august 1995 presents one opportunity to study the source of this reflectivity the hole has penetrated several different lithologies as well as numerous tectonic or fracture zones the upper 5070 m consist of island arc rocks which have presumably been thrust on top of basalts to allow correlations between available surface seismic studies and the borehole observations vertical and offset seismic profile data were acquired in the borehole over the interval 520 m to 3940 m two shot points were used a near offset one at 135 m from the wellhead and a far offset one at 1845 m from the wellhead after processing the borehole seismic data show numerous reflecting horizons many reflections correlate with major fracture zones while others have no clear correlation with fracture zones nor lithological contrasts the interval between approximately 4450 m and 5100 m which is imaged below the bottom of the survey shows a layered reflective pattern that correlates with the lower part of a flysch unit which ends at 5070 m the basalts below appear to be relatively transparent a prominent east dipping reflector on a w e cdp line about 700 m north of the wellhead is not clearly imaged on the borehole seismic however in a pilot hole which deviates a few hundred metres in the northerly direction from the main hole a marked low velocity zone is observed at the corresponding depth for this reflector indicating its source to be from a fault zone that the fault zone is not clearly observed on the borehole seismic data is attributed to 3d effects

probing the structure of the deep continental crust old buried deformed crystalline rocks apparently make up most of the 40 kilometer thick continental crust this part of the earth is poorly explored and constitutes a major frontier of modern earth science two techniques seismic reflection profiling and drilling which were developed by industry for other purposes offer special potential for such exploration seismic profiling of the deep crust by cocorp the consortium for continental reflection profiling has already produced important information including evidence for extensive thin skinned thrusting of older rocks over a continental margin as the corresponding ocean basin closed deep drilling of crystalline rocks of the continents for scientific purposes is so far relatively unexploited in the united states but is already being carried on elsewhere in general big science is likely to become more important in basic geology as this frontier is explored

crustal reflections and the nature of regional p coda abstract f k spectra for regional p coda are presented for events recorded at the scandinavian noress finesa and arcess arrays and the new england nynex array the spectra are dominated by on azimuth energy with apparent velocities between pn or faster and lg following this analysis reflection coefficients calculated with an efficient boundary integral scheme are used to study the role irregular interfaces play in the creation of regional p coda we find that observed crustal scattering in these regions is strikingly consistent with p p and p sv scattering from the two dimensional 2d irregular moho and even more consistent with scattering from a 2d irregular near surface interface

effects of the shallow subsurface on upper crustal seismic reflection images abstract seismic imaging of the upper crust is strongly affected by the interactions of the seismic wavefield with the shallow subsurface in order to better understand these effects we have evaluated the seismic responses of a suite of canonical models of the upper crystalline crust and the near surface region our modelling is restricted to two dimensions but otherwise includes the full seismic wavefield notably the effects of attenuation and topographic variations along the free surface we find that s wave scattering is important in the upper crust and contributes significantly to the vertical component of the seismic reflection response the backscattered wavefield undergoes mode conversions when it interacts with the free surface even moderate topographic variations or velocity variations in the near surface region enhance these mode conversions and cause additional scattering much of the earth s surface is covered by thin layers of unconsolidated material or weathered bedrock with high attenuation and high velocity contrasts at the layer boundaries scattered and mode converted seismic energy gets trapped in these layers and thus interacts repeatedly with the free surface and its topography in analogy to large scale seismic resonance effects of sedimentary valley fills the level and duration of source generated noise depends heavily on the degree of attenuation in the shallow layers low attenuation causes high noise levels and vice versa in contrast the level of source generated noise is less sensitive to near surface layer thicknesses and velocity contrasts

a model for the 1908 messina straits italy earthquake by inversion of levelling data abstract the 28 december 1908 messina earthquake was a normal faulting event which produced considerable vertical displacements three levelling surveys having an overall extension of 125 km in calabria and sicily were performed by the igmi italian military geographical institute shortly before the occurrence of the main event and during the first months of 1909 vertical subsidence up to 70 cm was measured at bench marks located close of messina the inversion of the observed displacements has been performed by using a model consisting of uniform dislocation on one or more rectangular faults and the mathematical formalism developed by matsu9ura and hirata 1982 results indicate that the observed displacements are well modeled by a normal fault oriented about ns with a dislocation of 1 5 m dipping 39 e this fault is located beneath the messina straits with the upper end at 1 km depth and crossing to the city of messina this model gives a seismic moment m o 4 9 0 8 10 26 dyne cm and shows a good agreement with the fault plane solution derived from the long period p wave first arrivals

observation of high frequency teleseismic p n on the long range quartz profile across northern eurasia short period three component recordings of the seismic wave field of peaceful nuclear explosions pnes on long range profiles are used to determine the fine structure of the mantle lithosphere by analyzing the frequency content of the recorded phases and applying different band pass filters to the data it is possible to divide the wave field into two distinctly different constituents low frequency body waves traveling along fermat paths first arrivals and the high frequency teleseismic or long range pn phase traveling with a group velocity of 8 1 km s and accompanied by a long incoherent coda this high frequency teleseismic pn phase is observable from about 750 km where it separates from the faster first arrival to the maximum recording distance of 3145 km it is recorded from shots at different locations and appears to be almost unaffected by the major tectonic feature along the profile the urals the frequency spectrum of this pn phase contains more high frequency energy up to 12 hz than first arrivals that penetrate deeper into the upper mantle the teleseismic high frequency pn arrival has a remarkable coda which is incoherent between closely spaced stations the coda duration is dependent on the component of motion being shortest on the vertical and longest on the transverse component we propose a velocity model that is characterized by a zone extending from the crust mantle boundary to a depth of about 100 km this zone has randomly distributed spatially anisotropic velocity fluctuations we propose that these velocity heterogeneities are stretched in the horizontal direction this zone forms a scattering waveguide that confines the high frequency teleseismic pn there are indications that below this pn waveguide either the scale of the velocity fluctuations or the q factor changes this is expressed in a separation of the teleseismic pn phase from the phase diving deeper into the mantle lithosphere

postseismic deformation due to subcrustal viscoelastic relaxation following dip slip earthquakes the deformation of the earth following a dip slip earthquake is calculated by using a three layer rheological model and finite element techniques the three layers are an elastic upper lithosphere a standard linear solid lower lithosphere and a maxwell viscoelastic asthenosphere attention is focused on the magnitude of the postseismic subsidence and the width of the subsidence zone that develops due to the viscoelastic response to coseismic reverse slip detailed analysis for a fault extending from the surface to 15 km with a 45 dip reveals that postseismic subsidence is sensitive to the depth to the asthenosphere but is only weakly dependent on lower lithosphere depth the greatest subsidence occurs when the elastic lithosphere is about 30 km thick and the asthenosphere lies just below this layer asthenosphere depth 2 times the fault depth the extremum in the subsidence pattern occurs at about 5 km from the surface break of the fault and lies over the slip plane at this location the subsidence reaches 60 of the coseismic uplift after about 30 maxwell times unlike the horizontal deformation following a strike slip earthquake significant vertical deformation due to asthenosphere flow persists for many times the maxwell time of the asthenosphere as a consequence the relaxation of the lower lithosphere has only a secondary effect on uplift and subsidence it does however substantially increase horizontal straining the width of the postseismic subsidence zone also depends strongly on the depth to the asthenosphere and shows little sensitivity to the presence of a slowly relaxing lower lithosphere parametric adjustment of the lithosphere thickness and fault dip angle over a considerable range of values in a simple model of an elastic lithosphere and viscoelastic asthenosphere reveals that the postseismic subsidence is greatest when the fault penetrates through about one half the lithosphere previous estimates of the asthenosphere viscosity 1019 pa s and lithosphere thickness 30 40 km for the region of japan affected by the 1896 riku u earthquake are confirmed by model calculations

upper mantle reflectivity beneath the williston basin phase change moho and the origin of intracratonic basins cocorp consortium for continental reflection profiling deep reflection profiles crossing the northern williston basin show 1 dipping reflections within the crust of the underlying trans hudson orogen that extend downward into the upper mantle and 2 no clearly defined reflection moho in contrast lithoprobe canada9s national geoscience project profiles crossing the trans hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defined subhorizontal reflection moho a reflection character typical of the interior of collapsed phanerozoic orogens we suggest that a crustal root produced during hudsonian collision was incompletely removed by normal postorogenic extension in the region now underlain by the williston basin this remnant crustal keel much later underwent eclogite facies metamorphism which overprinted a new nonreflective moho across the root and induced the subsidence of the williston basin

the structural architecture of the footwall to the main uralian fault southern urals abstract the uralide orogen is a linear collisional belt formed during the upper paleozoic as a result of convergence between the east european craton and outboard terranes and accretion of intervening island arcs and obduction of oceanic crust onto the east european craton this paper presents a new structural subdivision of the southern urals to the west of the main suture zone that puts this part of the mountain belt in a context similar to that of other collisional orogenic belts this subdivision consists of a west vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin and a parautochthonous foreland thrust and fold belt flanked to the east by a metamorphic thrust stack the latter units are structurally overlain by allochthonous rocks of an accretionary complex slope sediments oceanic crust and high grade eclogite blueschist bearing gneiss where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

contrasting tectonic history of the arc continent suture in the southern and middle urals implications for the evolution of the orogen the main uralian fault has been considered the original arc continent suture for 2000 km along the uralide orogen the symmetry of the tectonic units across it suggested a consistent east dipping polarity for the palaeosubduction zone which together with its topographic and aeromagnetic signature supported the idea of a single suture however several characteristics vary at different latitudes in the middle urals it is a strike slip fault zone with moderately deformed and metamorphosed volcanic arc fragments in its hanging wall and low grade metamorphic rocks of the east european craton in its footwall here it has a prominent nnw trending magnetic signature which cross cuts north trending anomalies in its hanging wall and a pronounced reflection seismic signature that can be traced to the top of the middle crust at c 5 s twt in the southern urals it is a serpentinite melange zone of ambiguous kinematics with a weakly deformed and metamorphosed volcanic arc in its hanging wall and moderately metamorphosed to high pressure rocks of the east european craton in its footwall in this part of the orogen it has a weak reflection seismic character and a magnetic signature that parallels that of its hanging wall on the basis of an integrated analysis of these different data sets we suggest that the main uralian fault as it is currently defined is not a single entity but rather the original arc continent suture in the south and the western strand of a strike slip fault system that reworked the original suture in the middle urals

generation of new continental crust and terrane accretion in southeastern alaska and western british columbia constraints from p and s wave wide angle seismic data accrete abstract the accrete study addresses the question of continental assemblage in southeastern alaska and western british columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc we present results of wide angle p and s wave seismic interpretation of a 300 km long marine land seismic line across the contacts between accreted terranes and coast mountains additional constraints on the model are obtained from correlation with geologic mapping our results indicate that the coast shear zone csz is a nearly vertical fault zone probably related to a transpressive regime west of the csz the mid cretaceous 90 ma thrust belt is rooted in the deep crust and is truncated by the csz from the interpretation of the imaged sub vertical reflecting zones we infer the positions of the alexander wrangellia terrane boundary awb and of tertiary extensional grabens within dixon entrance near its intersection with the profile the observed values of vp and vp vs in the lower crust of the alexander terrane are similar to those of oceanic crust and distinctly different from the coast mountains batholith cmb to the northeast the crust under the cmb 32 km is thinner than the average continental crust and the moho is sharp 200 m and highly reflective the low velocity mantle 7 9 km s suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust the lower crustal velocity of 6 9 km s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic restite related to batholith generation the crustal section under the cmb is seismically identical to the lower two thirds of normal crust heated and inflated by intrusions of tonalite and gabbro interlayered with restites from batholith generation and uplifted during exhumation

lithosphere scale seismic image of the southern urals from explosion source reflection profiling explosive source deep seismic reflection data from the southern ural mountains of central russia provided a lithosphere scale image of the central eurasian plate that reveals deep reflections 35 to 45 seconds in travel time 130 to 170 kilometers deep from the mantle the data display laterally variable reflectivity at the base of the crust that deepens beneath the central part of the profile documenting a crustal thickness of 55 to 60 kilometers beneath the axis of the orogen these data provide an image of the structure of the crust and underlying mantle lithosphere in a preserved collisional orogen perhaps to the base of the lithosphere

uralian magmatism an overview abstract this paper is an attempt to summarize current knowledge of uralian magmatism focusing on those aspects relevant for understanding its geodynamic evolution the urals consist of three tectonomagmatic domains a suture sector in the west and two n s imbricated island arc continental sectors in the east the suture sector comprises lower palaeozoic mafic ultramafic complexes which show eastward impoverishment in lile thus reflecting the transition of the subcontinental lithospheric mantle of the russian plate to the suboceanic lithospheric mantle of the subducted uralian palaeo ocean the two island arc continental sectors represent the transition from oceanic to continental environments in the middle and south urals collisional magmatism started in the silurian and persisted till the permian migrating progressively eastward and increasing in abundance of lile and 87 sr 86 sr initial magmatic polarity is very similar to that of modern subduction zones and indicates that the subducted slab was dipping eastward during that period the northern and southern island arc continental sectors show many similarities regarding the nature and spatial temporal distribution of magmatism but there are also some important differences which probably indicate somewhat different geodynamic regimes in the northern sector carboniferous tonalite granodiorite batholiths have features compatible with an origin by melting of the oceanic crust in the subducted slab in the southern sector however carboniferous tonalite granodiorite batholiths have features more consistent with a melting event within the lower continental crust above the subduction zone than with melting within the subducted slab upper carboniferous permian granites have high 87 sr 86 sr initial in the north e g 0 7120 in the murzinka batholith but very low 87 sr 86 sr initial in the south e g 0 7045 in the dzhabyk batholith in spite of rocks from both batholiths being equally peraluminous and showing evidence of derivation by anatexis of metasediments the low 87 sr 86 sr initial at dzhabyk might indicate that in the southern sector the accretionary prism grew very rapidly thus allowing involvement in upper palaeozoic melting events

from coseismic offsets to fault block mountains in the basin and range extensional province of the western united states coseismic offsets under the influence of gravity display predominantly subsidence of the basin side fault hanging wall with comparatively little or no uplift of the mountainside fault footwall a few decades later geodetic measurements gps and interferometric synthetic aperture radar insar show broad 100 km aseismic uplift symmetrically spanning the fault zone finally after millions of years and hundreds of fault offsets the mountain blocks display large uplift and tilting over a breadth of only about 10 km these sparse but robust observations pose a problem in that the coesismic uplifts of the footwall are small and inadequate to raise the mountain blocks to address this paradox we develop finite element models subjected to extensional and gravitational forces to study time varying deformation associated with normal faulting stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift which is predicted to take place within one to two decades after each large earthquake thus the best preserved topographic signature of earthquakes is expected to occur early in the postseismic period

modelling and imaging the moho transition the case of the southern urals summary synthetic seismic modelling is used to test different possible structures and forming processes of the lower crust and moho one and two dimensional synthetic seismic modelling of the moho reflection is carried out to constrain the internal structure of a crust mantle transition a reflectivity algorithm was used to calculate the synthetic seismograms for models of the crust mantle transition consisting of gradient zones and layered sequences the synthetic seismograms for models consisting of laterally variable velocity structure were calculated by an explicit finite difference wave equation algorithm the laterally heterogeneous transition models can be achieved by assuming a moho with different degrees of variable topography or laterally discontinuous layering the effects of a low velocity surface cover on the seismic signature of deep signals is also analysed the seismic modelling coupled with stacked images of wide angle seismic reflection data constrain the case history of the moho transition beneath the southern urals a smooth gradational crust mantle transition gradient velocity depth function does not predict the seismic features observed in the shot records the coda of the pmp and the amplitude behaviour of this phase with offset are modelled with a 6 km thick transition zone which consists of approximately 600 m thick laterally variable layers with a bimodal velocity distribution the horizontal component stacks of the wide angle data feature arcuate events suggesting boudin like structures or a boundary with topographic relief high amplitude sub moho reflections support that the structural complexity of the crust mantle transition can be followed to the upper mantle suggesting eastward dipping structures indicating either remnants of an old subduction zone trapped crustal material within the mantle or active crust mantle interactions

wave propagation in a multiple scattering upper mantle observations and modelling summary a striking feature of russian long range seismic refraction data from peaceful nuclear explosions is the observation of a high frequency teleseismic p phase which travels with a group velocity of 8 0 km s l out to distances of several thousands of kilometres modelling using the reflectivity method shows that this phase can be understood as the response of an upper mantle that contains random rms velocity fluctuations of about 44 per cent superimposed on a positive velocity gradient this class of model explains the existence of the teleseismic p its high frequency content and its coda length a teleseismic p can only be generated if velocity fluctuations are strong enough to cause multiple scattering and occur on a subwavelength scale cross correlation properties of p and s wave velocity fluctuations exert a substantial influence on the wavefield a completely unexpected phase can be observed if the fluctuations are imposed on a negative gradient

scales of structure in the lithosphere images of processes advances in deep seismic sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust mantle transition in this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near vertical reflection and wide angle refraction experiments the abrupt termination of near vertical reflections at the moho and the coincident presence of a strong coda accompanying the supercritical pmp reflection phase in record sections of continental seismic wide angle refraction experiments reveal a new property of the crust mantle boundary the moho forms a sandwiched mix of crust mantle material between lower crust and topmost mantle in combination with a step in mean velocity a significant change in scale of the structural dimensions and of velocity variance this view is strongly supported by the recognition of the subcrustal lithosphere as a high frequency 5 10 hz scattering wave guide for long distance propagation of the pn phase over more than 3000 km detected during seismic experiments in russia using peaceful nuclear explosions evidence is brought forward that this high frequency wave guide is likely to be present also in the phanerozoic upper mantle of western europe it is furthermore hypothesized that the global propagation of teleseismic sn or corresponding pn waves observed on continents and oceans combined with the recording of strong pmp codas for wave paths on continents points to a world wide change of scale of structure and velocities near the moho a change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the moho the global presence of the scattering pn wave guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle locations of present day tectonic activity where horizontal decoupling between crust and mantle are likely to occur e g subduction at continent continent collision zones change of tectonic regimes and continental transform faults are discussed

is the moho flat seismic evidence for a rough crust mantle interface beneath the northern basin and range a rough crust mantle boundary can produce amplitude versus offset characteristics similar to those of a second order discontinuity i e a gradient namely a reduction in precritical pmp basin and range reflection and refraction data can be simulated as well with rough moho models as with gradient models a rough moho presents a different view of the interaction between the mantle and crust where tectonic and magmatic structures are preserved and the crust and mantle are distinct the rough moho supports a crustal evolution model in which the crust is injected by mantle derived mafic dikes and sills but not to the extent that the lower crust is a continuous sequence of mafic ultramafic intrusions as commonly inferred from gradient models as such in the basin and range the moho represents a well defined chemical boundary

on the nature of mantle heterogeneities and discontinuities evidence from a very dense wide angle shot record abstract a seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide angle seismic reflection images of the lithosphere this unique data set recorded by an 18 km long spread reveals that at wide angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times this change in the seismic signature is located at approximately 120 150 km depth which correlates with the depth estimates of the lithosphere asthenosphere boundary lab of previous dss studies this seismic signature can be simulated by two layer mantle model both layers with similar average velocities differ in their degree of heterogeneity the shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere respectively studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks lherzolites harzburgites pyroxenites peridotites dunites and small amounts of eclogites laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide angle reflectivity a temperature increase would homogenize the mixture decreasing the seismic reflection properties due to melting processes it is proposed that this would take place below 120 150 km 1200 c the lab

preserved collisional crustal structure of the southern urals revealed by vibroseis profiling the vibroseis reflection profiling component of the urseis 95 experiment provides a high resolution crustal scale image of the unextended southern uralide orogen a marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the east european craton to the west from accreted terranes to the east between these regions is a less reflective zone corresponding to the magnitogorsk volcanic arc and the crustal root zone continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the paleozoic collisional structure has been largely preserved