a geomagnetic induction anomaly from ims data near hudson bay and its relation to crustal electrical conductivity in central north america summary canadian geomagnetic data collected during the international magnetospheric study are used to investigate the terrestrial electrical conductivity structure of north eastern manitoba and part of the north west territories the computed transfer functions resolve a major conductor trending east west between the communities of gillam and back in manitoba regional trends in the surface geology suggest that this conductor may be linked with the north american central plains electrical conductor twodimensional modelling of the data suggests that the conductor dips to the north from a small depth beneath gillam and may extend to the lowermost cnst

upper mantle structure under western europe from fundamental and higher mode surface waves using the nars array summary a generalized inversion of a new data set consisting of surface wave phase velocities and sn traveltimes has resulted in a new shear velocity and density model for the west european platform wepl 2 the surface wave phase velocity data set consists of fundamental and higher mode mode 1 6 measurements from 25 to 70mhz using the network of autonomously recording stations nars array which was specifically designed for higher mode surface wave measurements the sn traveltime data set consists of isc readings for selected european events major features of wepl 2 are a high velocity lid of 80 140 km depth underlain by a pronounced low velocity zone between 160 and 220 km depth the transition zone 400 650 km depth is characterized by high shear velocities with respect to prem the density shows a positive gradient from 60 to 140 km depth followed by an approximately constant value down to 400 km depth a model for the baltic shield scsh 2 is obtained by combination of nolet s 1977 higher mode phase velocity data set for western europe including the baltic shield and the present phase velocities for the platform this shield model shows an absence of the low velocity layer and low shear velocities in the top of the transition layer with respect to wepl 2 density although less well resolved shows lower values for the shield structure between 200 and 350 km depth

inhomogeneous structure of the senegal lithosphere from deep magnetotelluric soundings data from 23 magnetotelluric sites were used to determine electrical conductivities within the crust and upper mantle in senegal west africa along a profile about 600 km long data have been obtained in a variety of tectonic zones the west african craton stable since 1850 250 m y the west african mobile belt with ages in the range of 350 650 m y and the senegal sedimentary basin an earth model is derived from two dimensional modeling the model shows several heterogeneities extending deep in the lithosphere and perhaps more deeply in the asthenosphere to a depth of about 460 km at the stations in the western part of the basin the average resistivity is anomalously low 20 30 ohm m down to about 10 km the basin anomaly in the upper crust is assumed to mark an initial rift zone it may reveal a pattern of crustal weakness in this area beneath this zone the resisitivity is in the range of 1000 3000 ohm m down to at least 300 km in the eastern basin the resistivity is decreasing in the depth range of 20 30 km and the top of the ultimate mantle conductor is at a depth of 300 km at the sites in the marginal part of the west african craton conductive layers can be recognized in the depth range of 30 40 and 80 100 km the resistivity drops to 10 ohm m at a depth of about 460 km the model in this region is compared with available geological and geophysical information and an attempt is made to explain some of the observed discontinuities by plate tectonics it is suggested that the major discontinuity which separates the basin and the mobile belt and extends to about 460 km depth could be interpreted as marking a precambrian subduction slab dipping eastward in the west african craton a low resistivity has been determined for a depth range of 130 150 km the upper 130 km were found to have the rather high resistivities of 1000 3000 ohm m a sharp rise in conductivity occurs at a depth of about 460 km beneath the craton in contrast a surprising feature in the kedougou region in the craton is that the major portion of the lower crust is conductive the existence of a birrimian geosynclinal pair in eastern senegal consisting of a eugeosynclinal marine andesitic volcanic trough particularly volcanics with ophiolite affinity and a miogeosynclinal sedimentary sequence could characterize a modern plate tectonic phase dominated by widespread ocean opening and continental collision the conductive material in the lower crust might be explained by the buried ocean crust

an audiofrequency magnetotelluric traverse across the mana pools basin northern zimbabwe summary audiofrequency magnetotelluric soundings in the frequency range 0 016 128 hz have been conducted to investigate the structure of the mana pools basin in northern zimbabwe a karoo sedimentary basin in the zambezi valley data from nine tensorial soundings along a 40 km n s traverse from the river zambezi to the zimbabwe craton and a further sounding on the escarpment have been processed giving the sounding curves required for preliminary modelling studies a variety of l d inversion algorithms have subsequently been applied to these processed data sets to yield interpretive models an approximate 2 d geoelectrical section has been derived by collating the 1 d models a marked and rapid contrast in subsurface resistivity distribution between the escarpment and the basin is apparent within the basin the results show the presence of a top sequence of variable thickness up to 10 km overlying a more resistive 0 1 s m 1 unit that may represent the geoelectrical basement these top sequences which have tentatively been interpreted as sedimentary formations with different stratigraphic units consist of highly conducting 0 3 s m 1 to 1 s m 1 materials overlain by a slightly more resistive typically 0 2 s m 1 unit in some cases with another good conductor adjacent to the surface the data also suggest a rapid thinning of the top sequences close to the escarpment in contrast to the more gradual decrease of the sediments in previous geophysical models for the area on the escarpment the resistivity is higher at all depths the mana pools basin thus appears to be a continuation of the good conductor running from namibia into western zimbabwe identified by de beer 1982a and provides a link to another beneath the lower zambezi basin to the east of mana pools discovered magnetotellurically by losecke knodel muller 1988 comparisons between the geoelectrical model and other available results for the region are presented

telluric currents and magnetic anomalies current densities of 2 5 10 4 amp m can be generated by constant groundwater migration in artesian aquifers these dc currents are sufficient to generate magnetic anomalies exceeding 200 nt in both the menindee trough australia and the karoo basin south africa telluric currents associated with ionospheric activity can be detected by variations in magnetic induction but constant offsets associated with streaming potentials and geochemical activity sp anomalies are obscured by noise consequently some regional magnetic anomalies may be wrongly attributed to variations in magnetic susceptiblity with residuals explained by remanence

landscape evolution and slide processes in a glacioisostatic rebound area a combined marine and terrestrial approach thick fine grained nonuniformly stratified deposits along a 20 km stretch coastline have been investigated using a combined marine and terrestrial approach the sediments were deposited rapidly in glaciomarine marine and submarine ice contact environments during the allerod and younger dryas periods rapid glacioisostatic rebound caused extensive modification of the landscape mainly by sliding slides in the area were affected by the bedrock morphology location and sedimentation rate of the glaciomarine sediments isostatic rebound presence of gas fluids in marine sediments and later river incision tides and groundwater flow spatial and temporal distribution of slide deposits in the marine stratigraphy and morphology of submarine and terrestrial features give detailed information of landscape evolution through time this is illustrated by conceptual profiles from three areas that explain the landscape evolution and changing processes during the rebound the most intensive slide activity both in frequencies and volumes and hence the most considerable landscape modification occurred early in the holocene period i e when the relative sea level fell most rapidly

turbiditic clay rich event beds in fjord marine deposits caused by landslides in emerging clay deposits palaeoenvironmental interpretation and role for submarine mass wasting distinct clay rich beds are common in fjord marine deposits in trondheimsfjorden near the outlet of the nidelva river their characteristic light grey colour makes the beds easily distinguishable from the surrounding brownish bioturbated muddy fjord sediments the clay rich beds commonly display a clear stratification in clay silt and very fine sand the beds are interpreted as originating primarily from large quick clay landslides upstream along the nidelva river such events resulted in a sudden increase in the supply of fines to the fjord from disintegrating landslide debris and heavily loaded effluent plumes possibly favouring hyperpycnal flow typical beds can be divided into a clay rich lower section reflecting an initial surge with high concentrations of suspended mud and a sandier upper section reflecting pulses of higher energy this development can be explained for example by a lowering in the supply of mud an increasing activity of deltaic sediment gravity flows due to a higher availability of sandy sediments in the landslide affected river and by flooding and or breaching of landslide dams the typical stratified beds are interpreted as the result of one quick clay landslide whereas exceptionally thick less organized stratified beds are possibly the result of several large and or complex landslides radiocarbon dating of mollusc shells has helped to establish a chronology for major terrestrial landslides in the area the frequency of landslides increases towards the end of the holocene this is explained by a progressively deeper incision of rivers during glacioisostatic rebound possibly combined with a change to a wetter climate the marine core record displays deformation structures and hiati representing submarine mass wasting events and supports the evidence that the clay rich beds are weak layers in the fjord marine stratigraphy the inherent weakness of these layers may be explained by their composition immature texture loose fabric and contrasting permeabilities in the deposits slide prone layers similar to the clay rich beds described here may be found in other comparable fjord marginal settings and are considered to be of importance for geohazard assessments

geophysics of proposed proterozoic sutures in canada abstract sutures have been identified or proposed at several sites in the canadian shield although many authors have suggested that some form of primitive plate tectonic processes have played a major role in the formation of the archean crust of the superior structural province proterozoic sutures resembling phanerozoic examples have been proposed only within the younger churchill and grenville provinces and at or near the boundaries between the older superior and slave provinces and the younger provinces gravity magnetic seismic and paleomagnetic data related to three proposed sutures in the latter category have been interpreted and generally support the view that the shield comprises a mosaic of once separated but now joined cratonic blocks the sutures vary in complexity from wide zones containing distinctive rocks formed at ancient accreting and consuming plate margins to cryptic sutures marked only by abrupt changes in metamorphism and zones of cataclasis paired negative and positive gravity anomalies associated with these sutures have been interpreted in terms of juxtaposed crustal blocks of different density and thickness magnetic anomaly patterns change abruptly at structural province boundaries available seismic results support the concept of thickened crust in the younger province as derived from gravity modelling most authors prefer interpretation of proterozoic pole positions in terms of one plate models but in some cases the interpretation of paleomagnetic data is equivocal and two plate models have been proposed

processing of regional seismic events using data from small aperture arrays abstract a regional on line array processing package ronapp has been developed and tested on data from a small aperture seismic array noress at norsar consisting of 12 sensors spaced with distances up to about 2 km the processing package automatically detects and locates regional seismic events in the distance range from a few kilometers to the maximum propagation range of regional s type waves sn or lg the package contains a conventional sta lta detector which works on a set of filtered beams deployed at typical phase velocities of pn and lg sn 8 0 and 4 5 km sec when a detection is found data within a certain time window around the detection time are used for estimates of refined arrival time and signal frequency and a wavenumber analysis is then performed following a proper prefiltering this gives in addition to arrival time and frequency estimates of phase velocity azimuth and signal to noise ratio for each detection following each processed detection an event location based on azimuth and s p travel times is performed if the last detection is identified as an s type wave phase velocity below 6 km sec and a preceding p wave phase velocity above 6 km sec has been detected with about the same direction of approach appropriate procedures are developed to allow for several detections within the p and s coda to keep track of used and unused detections and to identify epicenter locations that override preliminary ones as the analysis progresses through the s wave train of the event the processing package has been tested off line and on line on a number of data intervals for various experimental noress array configurations the automatic processing package provides detection results for regional events that compare favorably with those from the full norsar array and location results that deviate from locations by the fennoscandian network of seismological stations by typically 30 km which is of the order of the location uncertainty of the network

substrate influences on step pool sequences in the christopher creek drainage arizona abstract step pool sequences formed in alluvial and bedrock channel segments of the christopher creek drainage in central arizona suggest that bedform geometries may reflect subtle differences in relative substrate resistance and in the ratio of driving forces to resisting forces three alluvial channel segments and two bedrock channel segments along a 72 km2 drainage and a 4 km2 drainage facilitated comparison across different substrates and hydraulic environments as substrate resistance increased or the ratio of hydraulic driving forces and boundary resisting forces decreased channel gradient increased the ratio of step height to length normalized by gradient decreased and pool dimensions normalized by gradient decreased these changes in bedform geometry suggest that as the ratio of driving to resisting forces increases proportionally more flow energy is dissipated within the flow as turbulence and shearing as in the alluvial reaches rather than being applied to erosion of the channel boundaries

the nares strait problem a re evaluation of the geological evidence in terms of a diffuse oblique slip plate boundary between greenland and the canadian arctic islands abstract geophysical models of sea floor spreading in baffin bay labrador sea require that greenland was displaced sinistrally relative to the canadian arctic islands by 200 300 km it has usually been assumed that this movement took place along nares strait but onshore geological evidence recently compiled by dawes and kerr appears to rule out any displacement in excess of 50 km twenty different structural and stratigraphic markers cross the strait without significant displacement a new approach is suggested here which spreads the displacement across a diffuse oblique slip plate boundary 1 1 south of bache peninsula displacement of up to about 150 km may have occurred along nares strait bache peninsula arch probably did not cross the strait but is a north south feature extending south from bache peninsula 2 2 revisions of proterozoic stratigraphy show that the arch cannot have marked the limit of thule basin as formerly assumed thule basin isopach data can readily be accommodated to a 150 km sinistral displacement 3 3 cambrian ordovician strata in bache peninsula area may have occupied a north south embayment similar to others along the southern border of franklinian geosyncline 4 4 north of bache peninsula most of the strike slip displacement probably took place along strike faults within central ellesmere and north greenland fold belts paleozoic stratigraphic markers which cross nares strait without strike slip displacement would not be displaced by these faults either because stratigraphic and structural trends are parallel within the fold belts the structural style of the ellesmere fold belt is probably one of transpressive oblique slip with local counterclockwise block rotations 5 5 up to 50 km of displacement may have occurred along northern nares strait itself probably as a late phase of movement a detailed palinspastic reconstruction must await further detailed groundwork this should include the following which would also serve as independent tests of the hypothesis detailed basin analysis of thule basin and cambro ordovician strata immediately to the north analysis of small scale structural features joints cleavage slickenside and map patterns of faults within the central ellesmere and north greenland fold belts deep reflection seismic and paleomagnetic analysis of the fold belts

azimuthal variation of pg velocity in the moldanubian czech republic observations based on a multi azimuthal common shot experiment westudytheazimuthalvelocityvariationofpg wavesinthemoldanubian whichisacrystallinesegmentwithinthebohemian massif in the czech republic we use the data from a multi azimuthal common shot experiment performed as part of the alp 2002 refraction experiment complemented by profile refraction data from the celebration 2000 experiment we analyze the traveltimesofwavesrecordedby72portableseismicstationsdeployedalongtwocircleswithradiiof35and45kmaroundashot the observed travel times display an azimuthal variation indicating anisotropy of 2 the minimum and maximum velocity values are 5 83 and 5 95 km s respectively the direction of the maximum velocity is n508e these values characterize horizontal anisotropy of the uppermost crust down to 3 km the strength and orientation of uppermost crustal anisotropy in the moldanubian is consistent with the overall upper crustal anisotropy in the entire bohemian massif the high velocity direction is roughly perpendicular to the present day maximum compressive stress in the bohemian massif and central europe and coincides with the orientation of structures formed by the main variscan tectonic events in the area this indicates that the anisotropy is caused predominantly by alignment of textural elements and minerals in the rocks which developed in early geological stages

mechanism for postliquefaction water film generation in layered sand soil investigations at two sites demonstrate that layered structure or stratification in sand deposits is prevalent not only in reclaimed ground but also in natural alluvial ground one dimensional liquefaction tests in a lucite tube are then carried out for models of several types of layered sand indicating that water films will develop under most circumstances beneath or within less permeable sublayers a basic mechanism for the water film generation is discussed based on the measurements of soil settlement and excess pore pressure the development of the water film and the associated soil settlement are numerically simulated by a simple sedimentation analysis and a rational explanation of the test results is found thus a significant involvement of water films in liquefied sand deposits and their basic mechanism are clarified it is highly probable that water films are involved as a part of a sliding surface and play a significant role in a seismically induced flow failure in loose and layered sand deposits during liquefaction

a magnetotelluric traverse across the mauritanides orogenic belt west africa summary magnetotelluric measurements with reference magnetometers for study of the anomalous geomagnetic variation field were made in the period range 10 10000 s at 10 sites crossing the main geological zones of eastern senegal west africa the results indicate that the electromagnetic fields are controlled by the dominant structural trends of the mauritanides orogenic belt apparent resistivities reflect the major structural units identified from surface geology data in the principal directions for most of the stations exhibit anisotropic behaviour and at certain sites the magnetotelluric responses are affected by surface distortion effects the study of the geomagnetic variation field indicates the presence of a concentration of telluric current flow beneath the craton a lack of obviously conductive geological structures at the surface suggests that this flow of telluric current may be attributed to increased electrical conductivity in the crust the quantitative interpretation of the magnetotelluric results is given in terms of 1 d and 2 d resistivity models it is shown that the data require the presence of a highly anomalous crustal structure 40 rm dipping to the east from a small depth beneath the outcropping margin of the west african craton in the mauritanides the crust is resistive 3000 rm t o a depth of 30 km and overlies a conducting layer of 10 km thick in the depth range 250 300 km resistivities decrease to less than 50 rm this may coincide with the top of the asthenosphere the geoelectric model is discussed in relation to the general geological background of the region considered

simulation of mw 6 0 earthquakes in the upper rhinegraben using empirical green functions summary in low seismicity areas such as southwestern germany quantitative statements on strong ground motion suffer from the lack of instrumental strong motion data although the historic catalogue indicates that events of moment magnitudes up to mw 6 are conceivable this is the situation in the upper rhinegraben area were moderate earthquakes with low probability but high impact caused by the significant aggregation of population infrastructure and industrial facilities are expected to happen we have developed scenarios of crustal earthquakes with moment magnitudes mw 6 0 for a source region southeast of basel switzerland itself being struck by a strong earthquake in 1356 with an intensity of up to ix ems98 intensity in order to synthesize strong ground motion seismograms we are using an empirical green s function egf approach we utilize records in the frequency range of 0 5 25 hz of permanent stations in the rhinegraben and the adjacent black forest in southwestern germany as egfs to be consistent with the local seismicity we model earthquakes with a left lateral strike slip focal mechanism the applied egf approach allows one to define independently a variety of rupture scenarios on the fault plane by varying the hypocentre location rupture velocity fault plane geometry slip distribution and rise time therefore we can create worst case scenarios for the chosen sites we compare peak horizontal acceleration pha and response spectra in terms of spectral accelerations and phas with attenuation laws proposed for europe our results encourage the application of the egf approach as a supplementary tool for site specific strong ground motion prediction in low seismicity regions such as the upper rhinegraben area

on the prediction of settlement from high resolution shear wave reflection seismic data the trondheim harbour case study mid norway abstract this paper presents a novel approach to predict long term settlement in the trondheim harbour mid norway from high resolution shear wave seismic reflection data complemented with cone penetration test results cptu and index soil properties from nearby boreholes in this reclaimed coastal area the data enables sub surface geo characterisation in high resolution down to the basement at a depth of about 175 m correlations between shear wave velocity and janbu s classical 1d consolidation parameters m m0 m1 c and ocr are used for settlement analysis both the magnitude and rate of time settlement predictions compare well with the measured settlements from gps levelling and psinsar data acquired since the late 1980s and the early 1990s in the area respectively the shear wave reflection data further provide important input to understand the spatial patterns of land subsidence in the trondheim harbour the highest settlement rates occur in areas where depth to bedrock is greatest and where thicker deposits with low stiffness are found in summary the data and workflow presented here are useful tools for predicting settlements based on correlations with shear wave velocity measurements

a new constraint on the composition of the topmost continental mantle anomalously different depth increases of p and s velocity summary compared to clear crustal p and s waves as well as a strong refracted pn wave propagating in the topmost mantle i e 10 to 15 km below the moho an anomalously weak appearance of the corresponding refracted shear wave sn is observed on refraction seismic profiles in sw germany france coastal maine ne united states and fennoscandia it is possible that the observation of pn pmp ratios close to unity combined with small sn sms ratios i e small sn amplitudes is typical for the continental uppermost mantle since this was observed for quite different types of continental crust it is shown that the observed phenomenon is not a source effect several possible causes to explain the observations are studied also with the aid of synthetic seismograms the effects of temperature attentuation and anisotropy have been investigated their influence cannot explain the observations the most likely candidate is a depth increasing vp vs ratio an increase in the order of 0 027 over a depth range of 10 km forcing p and s waves to propagate on different paths in the topmost mantle the observed phenomenon provides new constraints on the change of composition with depth for the crust mantle transition zone with basalt depletion as the most likely explanation

automated analysis of sks splitting to infer upper mantle anisotropy beneath germany using more than 20 yr of grsn and grf data

deformation structures in organic rich shales seismic images of upper jurassic organic rich siliciclastic rocks are studied along the 2000 km 1243 mi norwegian margin these rocks are considered the main source rock for most of the large oil and gas fields in the north sea and norwegian sea we report characteristic seismic expressions of thin skinned gravitational gliding structures that are strata bound to the organic rich formations the most characteristic structures are listric faults that offset and rotate the upper part and sole out near the base of the organic rich zone these may be present in large areas 10 000 km2 3861 mi2 but are usually restricted to tilted areas the strike of the faults was perpendicular to the downdip direction of the movement of mass and fault directions can therefore be used as paleodip indicators several types of contraction structures are observed and all are formed at a maximum of a few hundred meters of burial although they are not limited to organic rich shales such strata bound structures may help identify organic rich intervals in basins where their presence is unknown we suggest that adding organic material to clay leads to reduced permeability compaction related vertical fluid flow may cause fluid overpressure to build up at the base of a less permeable organic rich layer during early burial this high fluid pressure zone becomes a low friction decollement surface on which overlying sediments slide to form characteristic thin skinned deformation structures

occurrence and origin of submarine plunge pools at the base of the us continental slope high resolution bathymetric data from the new jersey and californian continental margins show a marked depression running along parts of the base of the continental slope detailed analysis reveals that the depressions are a series of discrete plunge pools with associated downslope topographic ramparts we have used new bathymetric data to create our own data base of over 150 examples and systematically analyse plunge pool morphology and location previous observations of plunge pools have been sparse plunge pools are up to 1100 m wide and 75 m deep with a mean diameter of 400 m and a mean depth of 21 m plunge pools only occur where there are sharp decreases in slope of more than 4 and are well developed where changes in slope exceed 15 we propose plunge pools can be created by two mechanisms firstly they may be due to reduced bed shear stress downstream of hydraulic jumps in submarine sediment laden density flows that causes the deposition of bedload and the creation of a sediment bar this bar then defines the downslope margin of a pool secondly the impact of high momentum sediment laden density flows can excavate a depression as has been observed for subaerial snow avalanches sediment deposited downslope of these impact pools is very poorly sorted and partly derived from erosion within the pool both mechanisms influence whether turbidity currents are generated from high density sediment laden density flows influence whether depositional flows are channelised and have implications for base of slope facies models

recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern germany this paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern germany the range of petrological models compatible with the observed azimuthal variation of seismic p wave velocity is explored the azimuthal distribution of amplitudes of mantle phases and the observed increase of p velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation even depletion of the upper mantle in basaltic components as suggested by mantle xenoliths from various parts of germany cannot explain the velocity depth and azimuthal amplitude observations without an increase of anisotropy with depth preferred orientation of olivine is the most likely mechanism for the observed phenomena its fast a axis at the moho level is directed towards n22 5 e the b axis is also required to be horizontal i e the b plane one of the preferred glide planes of olivine is vertical with a strike of n22 5 e therefore this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault plane solutions of earthquakes in western germany this is a strong indication that the preferred orientation of olivine is formed in the recent west european crustal stress field leaking into the upper mantle the distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a axis with depth by 10 towards n32 e and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western germany further experiments are needed to substantiate this suggestion which could lead to a better understanding of the interaction of crustal and upper mantle stress strain fields

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

the nares strait gravity anomaly and its implications for crustal structure a negative free air gravity anomaly is associated with nares strait the waterway that separates ellesmere island and greenland two east west gravity profiles that cross ellesmere island and nares strait were collected a low with values in the range of 100 to 120 mgal 1000 to 1200 m s2 was observed and two dimensional crustal models were created to identify the cause of the anomaly the gravity anomaly cannot be attributed wholly to the bathymetry of the strait or to the sedimentary rocks underlying the strait crustal models that reproduce the anomaly have a m discontinuity that slopes under nares strait towards ellesmere island so that the crust beneath ellesmere island is thickened the anomaly is similar to those associated with ancient and modern suture zones regions of collided continental crust plate reconstructions suggest nares strait is a collisional boundary between the north american plate ellesmere island and the greenland plate the gravity anomaly supports this interpretation

nares strait a suture zone geophysical and geological implications abstract the motion of greenland relative to ellesmere island along nares strait is determined from poles of rotation which provide control for the motion independent of local geology and geophysics the plate kinematics around the north atlantic ocean the norwegian and greenland seas and the eurasian basin of the arctic ocean constrain motion along nares strait these motions are checked by examining the stability characteristics of the triple junctions these junctions are found to be stable the motion along nares strait between anomalies 34 and 13 is a combination of strike slip and compression the regional geology is found to support the plate reconstructions the local geology of the nares strait area is reviewed and found not to refute the predicted motions the geophysical and geological data are interpreted in terms of the wilson cycle the opening and closing of an ocean the nares strait area has the characteristics of a cryptic suture a join between regions of collided continental crust

seismic structural and petrological models of the subcrustal lithosphere in southern germany a quantitative revaluation anisotropy in the subcontinental lithosphere becomes increasingly important because it is observed in many seismic studies especially for p n waves typical rocks of the uppermost mantle are peridotites which predominantly exhibit a pronounced elastic anisotropy this anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation here referred to as texture of olivine to evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere specimens of the northern hessian depression germany and the balmuccia ultramafic massif northern italy have been used they comprise four olivine texture types which are characteristic for olivine textures observed worldwide the bulk rock elastic properties have been calculated using olivine and orthopyroxene textures their single crystal elastic constants at ambient pressure temperature conditions and their volume fraction clinopyroxene and spinel are assumed to be randomly distributed the effect of four different orientations of the foliation within the uppermost mantle has been evaluated since this orientation is usually unknown two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed these variations cause significant azimuthal variations of the p wave reflections coefficients at the moho primarily we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15 to 95 possibly these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface the remaining two texture types only manifest a small directional dependence when anisotropy of compressional waves is observed in seismic studies these latter types can only be of subordinate importance however all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed this ambiguity can be substantially reduced when shear waves s waves are considered the directional distribution of s wave velocities and of the s wave splitting exhibits characteristic patterns for the different olivine texture types this could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle a fundamental requirement for a more comprehensive interpretation is the availability of detailed s wave observations the maximum s wave splitting in the peridotites investigated coincides with the maximum of the faster leading s wave this may be of importance to detect s wave splitting in future seismic studies

detection and delineation of a fracture zone with observation of seismic shear wave anisotropy in the upper rhine graben sw germany local seismic shear wave anisotropy is studied with recordings of microearthquakes near landau and insheim in the central upper rhine graben sw germany although the recordings have a low signal to noise level and there is a complex heterogeneous 3 d tectonic structure a time separation t between horizontally polarised sh waves and vertically polarised sv waves can be observed in seismograms and particle motion diagrams the observations can be explained by azimuthal anisotropy in the upper crust with a direction 0 155 east of north for the fast polarisation direction of sv waves a gradient of time separation with distance x of t x 10 ms km can explain the data this model can be interpreted with a classical scenario of fluid filled sub vertical cracks with a preferred nnw sse orientation known faults strike nnw sse around landau and insheim and the seismicity pattern is also oriented in this direction this direction coincides with the regional orientation of the maximum horizontal stress h and fluids apparently exist at depth as known from geothermal water extraction furthermore we find that 3 d seismic velocity heterogeneities have a much larger influence on the precision of the microearthquake location than the anisotropy effect in this complex tectonic region this is obvious from the up to five times larger travel time residuals maximum 0 5 s and 0 35 s which are used as station corrections during the location procedure compared to the anisotropic t observations maximum 0 1 s

magnetotelluric measurements across the beattie magnetic anomaly and the southern cape conductive belt south africa 1 the beattie magnetic anomaly bma and the southern cape conductive belt sccb two of earth s largest continental geophysical anomalies extend across the southern african continent in an east west direction to resolve structural details of the sccb a high resolution magnetotelluric study was conducted in march 2004 along a 150 km long n s profile across the karoo basin in south africa a two dimensional conductivity model at a scale of the entire crust exhibits three distinct zones of high conductivity 1 beneath the surface trace of the axis of the bma we observe a high conductivity anomaly at 5 to 10 km depth however it remains enigmatic whether both geophysical anomalies have the same source 2 a shallow regionally continuous subhorizontal band of high conductivity can be linked to a 50 to 70 m thick pyritic carbonaceous marker horizon intersected in deep boreholes 3 several highly conductive synformal features in the mid crust are newly imaged near the northern extremity of the profile our data generally support the existence of a deeper conductivity belt but allow for a much better definition of crustal conductivity in this region such laterally confined zones of crustal conductivity can only be resolved with a dense site spacing and broadband recordings

microseismic activity and basement controls on an active intraplate strike slip fault ceres tulbagh south africa earthquakes far from plate boundaries are poorly understood because of few well studied examples and uncertainty of what controls the location of these events in 1969 a damaging local magnitude m l 6 3 strike slip earthquake with no surface expression occurred in the ceres tulbagh region in the south african stable continental interior here we present a microseismic study of the ceres tulbagh area conducted over three months in 2012 in which 172 events recorded on at least three stations follow a gutenberg richter relationship for 1 5 m l 0 5 the events delineate a 5 km wide subvertical zone that is microseismically active to a depth of 15 km this fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress we argue that the microseismically active zone is guided by inherited structures in the basement geology this and similar structures may represent significant earthquake hazard in plate interiors online material tables of station and event parameters and p and s wave arrival times

paleomagnetism of the esayoo formation permian of northern ellesmere island possible clue to the solution of the nares strait dilemma abstract the esayoo formation lower permian of northern ellesmere island 81 n 82 w has been sampled at two localities twelve sites 88 cores studied in the early tertiary eurekan orogeny fold and thrust belt of the northern sverdrup basin northern ellesmere island the directions of remanent magnetization at the two localities are in excellent agreement after correction for geological tilt their mean is 126 2 61 8 b sites 12 k 105 95 4 3 and the corresponding paleopole is 48 1 n 159 2 e k 52 a 95 6 1 comparison with early permian reference paleopoles for north america shows that the formation of overthrust sheets has produced no paleomagnetically detectable displacement in a paleolatitudinal sense as would be inferred from the geology but there is a significant anticlockwise rotation of 36 8 this rotation is the first to be observed in the canadian arctic islands geological evidence allows no more than about 50 km of lateral motion along nares strait between ellesmere island and greenland this falls far short of the 300 km of relative motion required by the marine geophysical evidence for the opening of the labrador sea and baffin bay this discrepancy between the geological and geophysical evidence is the nares strait paradox evidently relative motion between greenland and ellesmere island is not restricted to nares strait itself and must be more generally distributed across ellesmere island we suggest that the 36 rotation observed is a manifestation of this distributed strain and therefore provides a clue to the solution of the nares strait paradox

effects of mass wasting on the stratigraphic architecture of a fjord valley fill correlation of onshore shear wave seismic and marine seismic data at trondheim norway abstract a combined record of onshore high resolution shear wave seismic and marine p wave seismic data shows the near shore stratigraphic organisation of a fjord valley fill at trondheim central norway the overall stratigraphy shows a typical fjord valley fill of up to 160 m in thickness that is composed of glaciomarine and fjord marine sediments overlain by deltaic deposits which in the onshore section are draped by an anthropogenic fill an interval characterised by low shear wave velocities below the onshore anthropogenic fills is shown by sampled drillings to comprise part clay and part organic deposits the seismic record reveals evidence of recurrent mass wasting events that include landslide debris turbidite deposits and channel features a chronological framework for the stratigraphy is provided by existing radiocarbon dates for local deglaciation and mass wasting events the combined onshore offshore datasets document the lateglacial to holocene development of an emerging fjord and delta system affected by major mass wasting events the architectural development of the younger part of the stratigraphy is interpreted as particularly affected by two known mass wasting events each involving several million m 3 of debris a thick layer of debris from a 3500 cal yr event probably resulted in a blockage of the subaqueous channels diversion of turbidity currents and deposition of lateral accretion packages whereby local fjord sedimentation temporarily increased a 2200 cal yr event resulted in a sudden reorganisation of the distributary system whereby distal delta sedimentation abruptly shifted north to northeastwards to an area temporarily subjected to marine erosion during fall of relative sea level the study adds to the understanding of landslides and low stiffness zones in the area of importance for near shore geohazard assessment the study also provides insight into the stratigraphic variability of fjord valley fills and highlights the interplay between fjord marine sedimentation marine abrasion delta progradation and mass wasting during late stages of fjord filling

seismicity and structural complexity of the dinkelberg block southern rhine graben summary a detailed analysis of the dinkelberg seismicity is presented the seismic activity of the central dinkelberg block starts at about 10 km and increases with depth to a maximum between 18 and 20 km joint hypocentre determinations clearly prove that no hypocentres are located within the lower crust which starts at a depth of about 20 km according to results from recent reflection seismic surveys the seismic activity is concentrated just above the top of the lower crust fault plane solutions of the dinkelberg events show that the strike of the nodal planes is parallel to the directions of near surface tectonic features a correlation also exists between the main fault directions and the spatial distribution of the hypocentres in the pratteln earthquake cluster at the southern end of the dinkelberg block the occurrence of negative p residuals from local earthquakes for special propagation directions of waves crossing the dinkelberg block is modelled by a westward dipping layer this postulated upper crustal structure together with observations of preferred s wave polarizations are discussed in regard to recent tectonic models and geological findings

short scale variations of tectonic regimes in the western european stress province north of the alps and pyrenees the data of the european part of the world stress map wsm are used to study the western european stress province north of the alps and pyrenees the study area is characterised by an almost homogeneous n 145 degrees e orientation of the maximum horizont

strength development with burial in fine grained sediments from the saguenay fjord quebec following a sedimentological and geotechnical investigation two main types of materials were identified in the saguenay fjord 1 organic rich sediments deposited by continuous sedimentation and 2 unbioturbated sediments deposited rapidly by episodic events the consolidation state of sediments with depth were analysed in all subsurface deposits sediments are overconsolidated irrespective of the depositional processes however in continuously deposited sediments strength develops more rapidly than in turbidites or mass flows in the surficial bioturbated layer strength and liquidity index gradients are at a maximum and the rate of strength variation can reach values as high as 9 kpa kpa results from one dimensional creep tests suggest that in the organic poor turbidites the strength development is controlled at least partly by thixotropic strengthening for bioturbated clays it appears that the viscosity and the aggregating potential of organic matter controls the evolution of the strength

development of the submarine channel in front of the nidelva river trondheimsfjorden norway abstract the nidelva submarine channel is a 4 km long and up to 100 m deep erosional feature seaward of a former outlet of the river nidelva in trondheimsfjorden norway mechanisms of channel evolution and sedimentary processes associated to its development are revealed by analyzing the sea bed morphology and the stratigraphy of the fjord sediments based on high resolution seismic data swath bathymetry imaging and gravity cores seismic data suggest that the submarine channel is eroded into late pleistocene clays filling a bedrock depression a thick holocene deltaic succession overlay the glacio marine clays seismic facies suggest that turbidity currents likely initiated downcutting of the channel in the early holocene these flows were guided by sea bed morphology and bedrock exposures favouring confluence and acceleration the area was subjected to glacio isostatic rebound and rapid fall of relative sea level in the holocene this resulted in a fast delta progradation likely with an enhanced activity of delta derived sediment gravity flows promoting incision in mid to late holocene such flows including hyperpycnal flows were responsible for deep incision of the channel laminated clay rich beds are interpreted as deposited by the overspill of turbiditic flows along the channel and likely originate from large terrestrial quick clay slides in the last 2000 years erosive flows initiated by mass wasting along the coast controlled the evolution of the channel after the mouth of the nidelva river moved to a more easterly position in the embayment present slope failure along the channel flanks seems to be controlled by the distribution of the laminated clay rich layers in the stratigraphy

terrigenous input to a fjord in central norway records the environmental response to the north atlantic oscillation over the past 50 years during the last century both earth surface temperature and moisture transport towards high latitudes have increased rapidly the response of the sub arctic region to these changes in terms of weathering transport and delivery of terrigenous material towards the coastal and deep ocean is both complex and poorly understood sediments accumulating in fjords offer an excellent opportunity for studying such land ocean interactions and may provide ultra high resolution records of environmental response to short term climate variability as a basis for holocene climate change studies modern sources supply and distribution of particular sediment components in the trondheimsfjord have been investigated and imply lithogenic elements as a promising proxy for terrigenous input and river discharge to better understand the impact of atmospheric variability on central norwegian environment we examine instrumental time series and show that the dominant mode of the atmospheric circulation in the north atlantic region the north atlantic oscillation nao has a strong impact on river discharge temperature and precipitation in central norway in addition elemental composition analysis of a short sediment core reveals that from 1959 to 2010 winter precipitation and temperature changes are recorded by changes in the inorganic geochemical composition of trondheimsfjord sediments elemental ratios of al zr and k ni in the sediment core mc99 3 show a close relation to small scale high frequency climate variations and large scale changes in the northern hemisphere climate this implies that terrigenous input and related erosional processes in the fjord hinterland are highly sensitive to atmospheric circulation variability in the north atlantic region by comparing our results with nao records derived from ice accumulation rates of norwegian glaciers western greenland ice sheets and river discharge anomalies in the eurasian arctic we show that it is possible to reconstruct the nao from sedimentary archives in central norwegian fjords

the influence of the san gregorio fault on the morphology of monterey canyon abstract a side scan sonar survey was conducted of monterey canyon and the san gregorio fault zone off shore of monterey bay the acoustic character and morphology of the sonar images enhanced by seabeam bathymetry show the path of the san gregorio fault zone across the shelf upper slope and monterey canyon high backscatter linear features a few kilometers long and 100 to 200 m wide delineate the sea floor expression of the fault zone on the shelf previous studies have shown that brachiopod pavements and carbonate crusts are the source of the lineations backscatter in monterey canyon the fault zone occurs where the path of the canyon makes a sharp bend from wnw to ssw 1800 m here the fault is marked by nw se trending high reflectivity lineations that cross the canyon floor between 1850 m and 1900 m the lineations can be traced to ridges on the northwestern canyon wall where they have 15 m of relief above the low relief ridges bowl shaped features have been excavated on the canyon wall contributing to the widening of the canyon we suggest that shear along the san gregorio fault has led to the formation of the low relief ridges near the canyon wall and that carbonate crusts as along the shelf may be the source of the high backscatter features on the canyon floor the path of the fault zone across the upper slope is marked by elongated tributary canyons with high backscatter floors and u shaped cross sectional profiles linear features and stepped scarps suggestive of recent crustal movement and mass wasting occur on the walls and floors of these canyons three magnitude 4 earthquakes have occurred within the last 30 years in the vicinity of the canyons that may have contributed to the observed features as shown by others motion along the fault zone has juxtaposed diverse lithologies that outcrop on the canyon walls gully morphology and the canyon s drainage patterns have been influenced by the substrate into which the gullies have formed

a three dimensional crustal model of southwest germany derived from seismic refraction data abstract from densely covered seismic refraction data obtained in 1978 urach experiment and 1984 schwarzer zollern wald experiment and from seismic reflection data and results from previous refraction investigations a three dimensional crustal model of southwest germany was derived travel time and amplitude information of seismic refraction data were interpreted with two dimensional forward modeling ray tracing to calculate two crustal cross sections in southwest germany these results fill a gap in the existing data and enabled the construction of a detailed three dimensional crustal model while seismically the upper crust is laterally homogeneous 5 9 6 0 km s throughout the area the middle and lower crust show pronounced lateral variations in thickness velocity and reflectivity the moho is a flat surface at a relatively shallow depth 25 26 km we classify the middle and lower crust of southwest germany into two characteristic crustal types type i consists of a mid crustal low velocity zone 5 4 5 8 km s overlying a thick 10 km high velocity 6 6 6 8 km s lower crust type ii has no prominent mid crustal low velocity zone and a thin

a magnetovariational study of a geothermal anomaly summary during 1980 september and october an array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the rocky mountains foothills of alberta the purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly examples of magnetograms and fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients a detailed transfer function analysis of the data is presented both single station transfer functions and a solution of the induction matrix are calculated the latter involves the representation of the normal fields by least squares fitted planes and a justification of the validity of this method is given contour plots of the response of the anomalous vertical field to horizontal hypothetical events and pseudosections of iza hni for traverses across the array are shown these indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity by treating the observed values of the in phase part of iza ha i along a traverse across the anomaly as due to vertical dipoles it is shown that the top of the conductive body probably lies at 10 20km depth the pseudosections suggest that the body extends at least to lower crustal depths the correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust

magnetotelluric soundings and the geological structure and tectonics of the senegalo mauritanian basin in northern senegal west africa the results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore senegalo mauritanian basin in northern senegal west africa electrical resistivities can be correlated with the major geological units in the upper 1000 m resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement the resistivity of the sediments is dominated by their porosity and the resistivity of the contained fluids the resistivity of the cretaceous sediments is unexpectedly low especially in the western part of the basin the top of the metamorphic granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious compacted aptian jurassic limestones and the metamorphic crystalline rocks the magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west based on the magnetotelluric interpretation and other independent geophysical and geological information a cross section through the senegalo mauritanian basin along the magnetotelluric traverse is presented a zone of high electrical conductivity is observed in the crust at depths of 4 6 km on the eastern margin of the deep basin this conductive structure may be the extension of one further south that was revealed by an earlier investigation the entire conductive zone which runs north south between longitudes 15 and 16 w may be a major fracture zone related to the opening of the atlantic ocean however its origin remains to be established

the southern cape conductive belt south africa its composition origin and tectonic significance abstract magnetometer array studies have led to the discovery and mapping of the southern cape conductive belt sccb crossing the southern tip of africa from west to southeast coasts the sccb lies just south of the namaqua natal belt of cratonic rocks remobilized about 1000 m y b p it is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m y relationships between the sccb and the basement geochronology geology and tectonics are considered in detail these relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a proterozoic subduction which stopped about 1000 to 800 m y b p associated with this subduction we propose a proterozoic range of andean mountains whose roots are now exposed in the namaqua natal belt later subduction further south near the present south coast is proposed to account for the intrusion between the south coast and the sccb of the cape granites in the time interval 600 500 m y b p there is some evidence for a third yet more distant subduction episode off permian gondwanaland after outlining this tectonic history the paper turns to a closer examination of the hypothesis that the southern cape conductive belt consists of partly serpentinized basalt accumulated at the top of a proterozoic subduction a large static magnetic anomaly which correlates with the sccb over most of its length is well fitted by a model which strongly supports this hypothesis bouguer gravity anomalies along western and central profiles likewise support the hypothesis a discussion follows of the process of formation of the proposed block of serpentinized marine rocks beginning with serpentinization of the crust near oceanic ridges by reaction of warm porous newly extruded basalt with seawater convecting through it the serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction where it accumulates because of its low density examples of such accumulations are cited finally it is shown that any iron in the olivine and pyroxene in the original basalt precipitates when these silicates are hydrated to serpentine as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites in particular the beattie static magnetic anomaly requires in our model calculation a very high susceptibility readily attainable in basalt at 15 20 serpentinization the authors know of no other rock able to provide this high susceptibility a similar percentage of serpentinization provides the density required to model the gravity anomalies

geoelectrical structure of the northern part of the senegal basin from joint interpretation of magnetotelluric and geomagnetic data magnetotelluric mt and differential geomagnetic sounding dgs surveys were made in the senegal basin west africa to investigate the conductivity structure of the crust and upper mantle magnetic and electric variations were recorded at 11 sites along a east west profile 220 km in length the period range of the measurements was 10 10 000 s for the western stations the mt response displays low skew with moderate anisotropy the dgs data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area a telluric current concentration in the thick conductive sediments of the deep basin might cause this anomaly both mt and dgs data are then compared with two dimensional model results to explain the observed features along the profile results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone 2 ohm m at depths in the range 4 23 km within the deep basin the results of the two dimensional modeling are not conclusive acceptable fits between observed and calculated data have been obtained with alternate models a continuous conductive zone 0 7 2 ohm m may exist beneath the uppermost layers at a depth of approximatively 1 km but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover this ambiguity is inherent in the two dimensional models themselves both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast it is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and or by variation of the sedimentary thickness the models also indicate the existence of two decreased resistivity layers in the eastern part of the profile one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m no such layers were detected at these depths below the deep basin at some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile this may indicate the presence of a small degree of partial melt

seismic anisotropy within the uppermost mantle of southern germany summary this paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern germany the dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the p phase that could be effectively used in a time term analysis to determine horizontal velocity distribution immediately below the moho for 12 crossing profiles amplitude ratios of the p phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth a p wave anisotropy of 3 4 per cent in a horizontal plane immediately below the moho at a depth of 30 km increasing to 11 per cent at a depth of 40 km was determined for the axis of the highest velocity of about 8 03 kms at a depth of 30 km a direction of n31 e was obtained the azimuthal dependence of the observed p amplitude is explained by an azimuth dependent sub moho velocity gradient decreasing from 0 06 s in the fast direction to 0 s in the slow direction of horizontal p wave velocity from the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived

investigations of sn and pn phases in the area of the upper rhine graben and northern switzerland summary 22 regional earthquakes in sw germany and northern switzerland recorded with the southern black forest digital seismic network are studied for pn and sn phases in contrast to the results of some seismic refraction studies a large number of these earthquake recordings show clearly correlatable sn phases so far there are two remarkable trends concerning the appearance or disappearance of sn 1 earthquakes with large focal depths z 20 30 km preferentially show sn phases 2 earthquakes from the sw with rays crossing the southern rhine graben show no sn energy regardless of focal depth to investigate the amplitude behaviour of the sn phase synthetic seismograms for varying focal parameters as well as for different structural parameters submoho velocity gradient and attenuation of s waves are calculated using the reflectivity method the synthetics show that the sn amplitude is sensitive especially to the fault plane orientation and the submoho velocity gradient after correction for the radiation pattern the missing of the sn phase from earthquakes in the south western part of the studied area seems to be associated with structural effects in the uppermost mantle a small submoho s wave gradient or a high upper mantle attenuation are the most likely explanations another very interesting observation relates to the pn velocities the azimuthal variation of the velocity as well as the high maximum pn velocity v 8 6 8 8 km s 1 itself point to an anisotropic uppermost mantle the velocity variation of 0 6km s 1 and the directions of minimum and maximum velocities are in accordance with earlier results from seismic refraction experiments and their petrological interpretation the derived sn velocities have a mean value of 4 71 km s 1 up to now the quality and number of the sn recordings do not allow us to resolve a possible s wave anisotropy of the uppermost mantle

a fission track transect across nares strait canada greenland further evidence that the wegener fault is a myth many workers continue to model nares strait between greenland and ellesmere island canada as a plate boundary and locus of major cenozoic compression and strike slip marked by the postulated wegener transform fault this despite continuity of precambrian paleozoic provinces across the seaway including an undeformed neoproterozoic mafic dyke swarm that crosses at a high angle as a test of the speculative plate kinematic modelling apatite fission track aft dating was undertaken along a transect across the paleoproterozoic shield of smith sound at the south end of nares strait running from sea level to ca 600 m elevation and as much as 70 km inland on both sides the shield is overlain by three sedimentary basins separated by major hiatuses the aft ages fall into two groups 282 34 and 650 51 ma they indicate maximum depth was 4 km in the lower paleozoic from both coasts to 50 km inland and at 50 km inland temperatures were below the annealing interval throughout phanerozoic time durin

channelled erosion through a marine dump site of dredge spoils at the mouth of the puyallup river washington state usa abstract channels are relatively common on river mouth deltas but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant hyperpycnal flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in commencement bay at the mouth of the puyallup river shallow channels on the seaward upper surface of the dump site away from any flows that could have been produced by delta front failures suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform the form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile it suggests that the channel evolved by its steep front retreating rather than by simple vertical entrenchment or diffusive like evolution of the profile a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom travelling flows model calculations based on shear strengths back calculated from the geometry of channel wall failures suggest that if the main channel were eroded solely by hyperpycnal flows their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms

unusual rates and patterns of holocene emergence ellesmere island arctic canada substantial postglacial emergence up to 150 m characterizes the canadian high arctic following the last glacial maximum across northern ellesmere island rapid emergence is not observed during the early stages of deglaciation as it is in other parts of arctic canada rather the curves show an interval of slow emergence c 1 m per century during initial ice retreat this is attributed to a slow rate of glacial rebound that was countered by a similar rate of eustatic sea level rise normally isobases drawn on postglacial shorelines define broad cells of uplift that mimic centres of former maximum ice thickness however isobases on the 8 ka bp shoreline surveyed throughout northern ellesmere island indicate more complex patterns of crustal uplift that seem in congruent with reported ice thicknesses on west central ellesmere island the isobases 110 150 m asl trace a plunging ridge aligned with geological structure across an area considered to have had minimal ice cover during the last glacial maximum the ridge may record an important structural influence associated with the geologically youthful sverdrup basin collectively this evidence cautions against the view that postglacial emergence has a universally predictable signature solely portraying former ice loads

shear waves by an explosive point source the earth surface as a generator of converted p s waves the most common source of seismic energy is an explosion at some depth in a borehole the radiated waves are reflected not only at the subsurface layers but also at the free surface the earth s surface acts as a generator of both p and s waves if the source depth is much less than the dominant wavelength the reflected waves resemble closely the waves generated by a single force theoretical seismograms were computed with different methods to look for the relevance of the surface reflected waves the numerical experiments show reflected shear waves even for small shotpoint receiver distances due to their polarization these waves can be detected most easily on in line horizontal geophones the existence of these waves was examined during a conventional survey in northern germany conventional data analysis shows a large variability in the p s ratio the method used here produced a shear wave section with a rather good signal to noise ratio down to 4 s s wave reflection time

two dimensional anisotropic tomography of lithosphere beneath france using regional arrival times this paper presents an anisotropic tomography study beneath france travel times of pn waves were inverted in order to simultaneously retrieve lateral variations of both seismic velocity and seismic anisotropy within the subcrustal lithosphere the parameterization scheme uses a classical approach the subcrustal lithosphere is modeled using a two dimensional horizontal grid of nonoverlapping cells 0 5 0 5 each cell being characterized by an average slowness the isotropic term of slowness and an azimuthal anisotropy term the velocity and anisotropy perturbation amplitudes do not exceed 2 in the paris basin and in the aquitaine basin more significant perturbations can be observed in the tectonically active regions the images of the modeled subcrustal lithosphere emphasize the not recently reactivated areas of the hercynian range by high velocities 2 to 5 from the armorican massif to the west part of the massif central and from the northeast massif central to the vosges mountains and black forest mountains regions these areas are also characterized by an anisotropy of 2 5 with the fast directions parallel to the hercynian structures low velocities 2 to 4 are observed in the neogene volcanic areas of the massif central which confirm the results obtained from a short wavelength teleseismic tomography these low velocities are most probably caused by high temperatures another low velocity region 3 to 4 roughly follows the alpine belt and is interpreted as the image of the crustal root of the range in the inner part of the alps fast pn directions lie perpendicular to the belt with an anisotropy amplitude of 3 while in the outer part they are parallel to the range with amplitudes ranging from 3 to 5 to the east and from 2 to 3 to the west the anisotropy feature of the pyrenean range follows a simple pattern one observes high amplitudes up to 5 and a fast direction which is parallel with the strike of the range the observations show seismic anisotropy for most of the tectonically active zones across france recently formed structural units and old hercynian structures as well and the amplitudes reach the same order as the amplitudes of the velocity perturbations

electromagnetic geophysics and global tectonics lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics the methods of investigation include classical four electrode resistivity depth probing which is of special value in continental shields magnetotellurics long used on land and recently in the pacific to show that in electrical terms the lithosphere is thicker near an oceanic ridge than had been expected and magnetometer arrays which are useful primarily in mapping highly conductive structures examples of the use of all three methods will be reviewed with emphasis on the last in conjunction with other geophysical geological and geochemical data magnetometer arrays have been useful in mapping partial melt under the western united states in discovering an extension of the southern end of the african rift system to the atlantic coast in detecting a major fracture zone in central north america which may be a proterozoic interplate collision feature and in discovering a conductive belt under southernmost africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under proterozoic gondwanaland inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two and three dimensional conductive structures this remains valuable however particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures recent array studies now in progress in western canada have defined two new conductivity anomalies one of these may represent a local partial melt concentration under the rocky mountains trench the other a precambrian rift valley in the lower crust previously suggested by others on the basis of seismological gravity and static magnetic data both are being investigated by more concentrated arrays

water film in liquefied sand and its effect on lateral spread a one dimensional 1d saturated sand layer of 2 m in thickness in which a silt seam is sandwiched is liquefied by an instant shock it is found that a water film is easily formed beneath the silt seam with a thickness as thin as a few millimeters just after liquefaction in loose sand and that the film lasts longer than the postliquefaction settlement the effect of the water film on pore pressure distribution and sand settlement is intensively studied one gram shake table tests are then carried out for 2d models with or without seams of silt within a saturated sand layer in the former case water films formed beneath silt seams just after liquefaction enable the soil mass above them to glide because of an unbalanced force along the water films not only during but also after shaking in the latter case the soil deforms continuously mostly during shaking and stops afterward thus a significnat effect of water films formed beneath thin low permeability sublayers in a liquefied loose sand on the failure mode and timing in lateral spread is clearly demonstrated by these simple model tests

earthquake and flood induced turbidites in the saguenay fjord quebec a holocene paleoseismicity record abstract a 38 m long piston core from the deep basin of the saguenay fjord quebec recorded rapidly deposited layers rdl interpreted to represent major floods and earthquakes over the past 7200 years high resolution physical magnetic and sedimentological analyses revealed at least 14 rdl generally with a sandy base and a light gray color interbedded with hemipelagic sediments digital x radiography and grain size analyses at

an integrated geophysical study of the beattie magnetic anomaly south africa abstract the source of the beattie magnetic anomaly bma still remains unclear with several competing hypotheses here we add a piece to the puzzle by investigating available potential field data over the anomaly filtered magnetic data show the bma as part of a group of linear magnetic anomalies as the linear anomaly north of the bma is associated with exposed supracrustals migmatites and shear zones within the natal thrust terranes we assume a similar source for the bma this source geometry constrained by seismic and mt data fits potential field data over the bma and other magnetic linear anomalies in the south central and south western karoo in these models the bodies deepen from 5 km towards the south with horizontal extents of 20 60 km and vertical extents of 10 15 km densities range from 2800 to 2940 kg m 3 and magnetic susceptibilities from 10 to 100 10 3 si these magnetic susceptibilities are higher than field values from supracrustal rocks 10 60 10 3 si but could be due to the fact that no remanent magnetisation was included in the model the lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes the strong signal of the bma is linked to high magnetic susceptibility granulite facies supracrustals 10 50 10 3 si as seen in the antarctic where the mobile belt continued during gondwana times