relic magma chamber structures preserved within the mesozoic north atlantic crust the north atlantic transect seismic reflection data collected southwest of bermuda have been reinterpreted following post stack migration and reveal two major intracrustal reflections the shallower of these two events located 1 s below the igneous basement is a subhorizontal undulating surface that in some places is continuous for as much as 10 km on the basis of its position within the section and its laterally discontinuous nature we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro a second set of lower crustal reflections dipping 20 40 eastward is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection moho several lines of evidence argue against these features being either artifacts or out of the plane events instead their presence may be ascribed either to crustal penetrating fault zones or to mafic ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo spreading center because of the laminated character of these events and their typical occurrence within 1 0 to 1 5 s of the reflection moho we prefer a compositional versus a structural interpretation for their origin the gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities significant departures are recognized elsewhere similarly the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the blake spur fracture zone an interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal thinning model of mutter and others

palaeoseismology review of the state of the art abstract historical seismicity provides only a partial indication of the seismotectonic character i e earthquake potential of a given region even in italy where a relatively long historical record exists available catalogues are generally unsatisfactory for this purpose fortunately strong earthquakes often leave geological evidence herein referred to as seismites that are mainly related to surface faulting folding or other deformation and occasionally to secondary ground failures e g landslides liquefaction features etc these features may be preserved and therefore palaeoseismological studies may detect and date them thereby permitting extension of knowledge beyond the local historical threshold up to 2000 years in the most fortunate cases this paper provides a summary of criteria which may aid in the identification of areas suitable for palaeo earthquake investigation historical seismicity and the presence of well developed tectonic geomorphic features are recognized as the most reliable criteria for identifying active faulting however lack of these characteristics cannot be considered conclusive evidence for lack of activity palaeoseismological techniques aim at identifying not only the obvious effects of surface faulting such as offset of young geologic units but also the more subtle tectonic or non tectonic features including liquefaction structures gravity features and coseismic uplift and or subsidence finally we discuss some questions that pertain to palaeoseismicity evaluation techniques among these are the reliability of the quaternary dating techniques the evaluation of the seismic energy released minimum magnitude and principally the unequivocal identification of a seismic origin for the considered geological structures

seismic reflection imaging of the juan de fuca plate from ridge to trench new constraints on the distribution of faulting and evolution of the crust prior to subduction we present prestack time migrated multichannel seismic images along two cross plate transects from the juan de fuca jdf ridge to the cascadia deformation front df offshore oregon and washington from which we characterize crustal structure distribution and extent of faults across the plate interior as the crust ages and near the df in response to subduction bending within the plate interior we observe numerous small offset faults in the sediment section beginning 50 70 km from the ridge axis with sparse fault plane reflections confined to the upper crust plate bending due to sediment loading and subduction initiates at 120 150 km and 65 80 km seaward of the df respectively and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity most bend faulting deformation occurs within 40 km from the df on the oregon transect bright fault plane reflections that extend through the crust and 6 7 km into the mantle are observed if attributed to serpentinization 0 12 0 92 wt water within the uppermost 6 km of the mantle is estimated on the washington transect bending faults are confined to the sediment section and upper middle crust the regional difference in subduction bend faulting and potential hydration of the jdf plate is inconsistent with the spatial distribution of intermediate depth intraslab seismicity at cascadia a series of distinctive ridgeward dipping 20 40 lower crustal reflections are imaged in 6 8 ma crust along both transects and are interpreted as ductile shear zones formed within the ridge s accretionary zone in response to temporal variations in mantle upwelling possibly associated with previously recognized plate reorganizations at 8 5 ma and 5 9 ma

crustal architecture and tectonic evolution of the sergipe alagoas and jacuipe basins offshore northeastern brazil abstract the sergipe alagoas and the jacuipe basins are divergent margin basins located in the south atlantic ocean at the northeastern brazilian margin high quality 18 s two way travel time seismic reflection profiles which extend from the coastline towards the boundary with the oceanic crust were integrated with potential field data and calibrated with the results of several exploratory boreholes when combined these data provide evidence that the deep seismic reflectors in the deep water region are related to the crustal architecture of the basin rather than to sedimentary features arrays of high impedance reflectors previously interpreted as top of basement probably correspond to detachment planes near the base of the crust or to underplated igneous rocks above the moho the extensional processes that were active during rifting resulted in normal crustal faults that apparently detach near the seismic moho in the deep water region these crustal faults control and rotate syn rift sedimentary units along the margin the seaward portion of the rift seems to have been uplifted and highly eroded during a post break up tectonic event the deep seismic reflection profiles also indicate that the deep water province is characterized by wedges of reflectors that dip seawards these features probably correspond to seaward dipping reflectors sdr which are composed of volcanic rocks marking the transition to oceanic crust the sdr wedges are locally associated with volcanic intrusions several volcanic plugs in deep water region are aligned with oceanic fracture zones that apparently penetrate through the whole crust and reach the upper mantle on the other hand there are some diapiric structures located near the boundary between continental and oceanic crust that bear evidence for deep water salt tectonics

oceanic crustal thickness and seismic character along a central pacific transect seismic reflection data along a flow line of crustal generation in the central pacific that spans ages of zero to 85 ma and spreading half rates of 30 to 100 km m y 1 shows a nearly constant travel time of 2 s through igneous crust to reflection moho the highest amplitude and most laterally continuous moho reflections were recorded over 20 30 ma crust that was emplaced at superfast 75 95 km m y 1 spreading rates the superfast spread portion also records the lowest scatter about the 2 s average travel time to reflection moho seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest these dipping reflectors are truncated by the reflection moho

complex structure along a mesozoic sea floor spreading ridge birps deep seismic reflection cape verde abyssal plain summary as part of an intensive study of a small area of oceanic lithosphere the british institutions reflection profiling syndicate birps acquired closely spaced deepseismic reflection profiles over the early cretaceous crust of the cape verde abyssal plain off west africa the survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea floor spreading axis isochron profiles and orthogonal dip lines oriented in the original direction of spreading flow profiles a large capacity well tuned airgun source and very quiet shooting conditions ensured a high signal to noise ratio for deep reflection devising a strategy for mitigating contamination from wrap around multiples arriving from previous shots enabled us to use the minimum possible shot point interval 50 m allowed for collecting long 18 s records data processing was oriented towards a medium with low root mean square velocity steeply dipping structure and pervasive low apparent velocity noise from diffraction at the top of the igneous crust the contrast between the isochron and flow profiles is striking isochron profiles are typically highly reflective throughout the igneous crust consisting of bright bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted moho reflection these reflections do not offset intracrustal or top basement structure and thus are not interpreted as faults an igneous intrusive origin seems more likely flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust continuing down in places to the moho dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust the dominant dip of these reflections is to the west towards the spreading ridge axis reflections from the vicinity of the moho while well developed in some places are not particularly prominent across the survey area moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles on isochron profiles dipping reflections occasionally flatten out into and may merge with the moho reflection on flow profiles as dipping crustal reflections approach the moho reflection they are usually abruptly cut off by it without extending deeper this survey shows how oceanic crustal structure can vary rapidly over relatively small areas provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust and gives a conclusive observation of faults that penetrate the entire oceanic crust

sedimentary structures tentatively attributed to seismic events abstract fingerprints of earthquakes in the sedimentary record are as yet difficult to identify this is particularly true for failure phenomena dewatering slumping fault graded and turbidite structures that could have been triggered also by other kinds of energy input other promising candidates are strange biostratinomic features convex down stacking of shells current orientation in wave ripple accumulations as well as certain preservational features found in geosynclinal pelagic limestones but in every case verification of the seismic origin is still needed

crustal anatomy of a transform continental margin abstract this paper describes new deep seismic reflection results across the major transform margin southwest of the grand banks off eastern canada an interpretation of these results is presented supplemented with previous seismic refraction gravity and magnetic measurements the results show a fairly sharp transition between oceanic and continental regions at the transform margin a narrow zone about 40 km wide separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust faults in this region extend throughout the crust and there is some evidence that a major strike slip fault may extend some tens of kilometres into the upper mantle magmatism may have further modified the crust in this shear zone which is bounded on the oceanic side by a small seamount the continental crust is thinned by a factor of 3 or more in this zone which may be due to erosion or to flow in the lower crust during shearing the oceanic crust does not appear to thin significantly as the transform margin is approached crustal scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two appalachian terranes the meguma and avalon terranes this supports earlier interpretations of a large magnetic lineament in the region as marking this boundary these crustal seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins the picture they provide is different in many respects from that across rifted margins and similar in others in some cases it might be difficult to distinguish between these two classes of atlantic type margins on the basis of crustal structure

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

the seismicity along the dead sea fault during the last 60 000 years abstract evidence for unchanging slip rate and a gutenberg richter relation for earthquake distribution along the dead sea fault during the past 60 000 yr are presented the evidence comes from three different segments approximately 100 km apart and from three different timescales prehistoric paleoseismic historical and modern instrumental records the paleoseismic data are based on two different methods in the southern arava valley and the northern jordan valley segments the amount of normal displacement along several faults is used while in the dead sea basin the appearance of brecciated beds which are considered as seismites is used we found that for the southern arava valley segment a constant dip slip rate of 0 5 mm yr can explain the cumulative normal slip during the past 45 000 yr this suggests that normal faulting is only 10 of the total left lateral strike slip motion we also found that for all three segments the paleoseismic and historical records of strong earthquakes lie on the linear extrapolation of the frequency magnitude relation of the instrumental record the calculated b values for all three segments are between 0 85 and 1 similar to other major strike slip faults in the world it is concluded that the gutenberg richter distribution is a stable mode in the tectonic setting of the dead sea fault during the past 60 000 yr

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

along axis variability in crustal accretion at the mid atlantic ridge results from the ocean study the ocean experiment is an integrated geophysical study of a region of the cape verde abyssal plain that formed at 140 ma deep seismic reflection and ocean bottom hydrophone obh refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey the igneous basement is overlain by about 1 3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the mid atlantic ridge axis we describe the results of a 150 km long profile oriented parallel to magnetic anomalies m15 and m16 along which deep seismic reflection data collected by the british institutions reflection profiling syndicate are complemented by refraction data constrained by four obhs the line spans an entire spreading segment between two fracture zones the northern of which has an offset of 40 km and the other central has an offset of only 10 km away from the fracture zones the mean igneous crustal thickness is 7 2 km near both fracture zones thinning of up to 4 km is observed giving a mean igneous crustal thickness over the whole segment of approximately 6 5 km differences are seen between the two fracture zones in their seismic velocity structure in the associated basement topography and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone the boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity a number of planar reflections up to 10 km in length are present within the middle and lower crust dipping outward from beneath low amplitude basement highs at 15 these appear to be present only within layer 3 the moho has several expressions in the reflection data including isolated reflection events a local increase in reflected amplitudes and a downward decrease in coherent reflections at the center of the segment there is a zone at the base of the crust within which both high and low velocity materials are present this zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge

constructional features of the troodos ophiolite and implications for the distribution of orebodies and the generation of oceanic crust the map pattern of the troodos ophiolite in cyprus is an expression of a simple elongate anticlinal structure the cross sectional form of the anticline can be described by a sinusoidal dip model on the northern flank of the anticline the thickness of various units number of gossans etc were measured along profiles at 2 5 km intervals normal to strike using existing maps supplemented by new observations profiles are approximately perpendicular to the spreading direction during crustal construction and uncertainties in unit thickness are smaller than the range of unit thicknesses data on the profiles were analyzed quantitatively by harmonic analysis using the fourier transform method the harmonic analysis discloses cyclic variation in the thickness of major units number of gossans etc that provides a link with the regular spacing of five groups of cu fe zn orebodies groups of orebodies occur preferentially where sheeted dikes are at a shallow crustal depth sheet and massive flows and dike

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

earthquake magnitude frequency epicentral distance and soft sediment deformation in sedimentary basins abstract empirical equations available for earthquake magnitude frequency and the maximum epicentral distance of liquefied sites versus magnitude are combined with the characteristic instantaneous sediment deposition rate at a station in a sedimentary basin to form a model for the local stratigraphic frequency of earthquake induced soft sediment deformation structures both the style of sedimentation and the characteristics of the earthquake source combine to determine the pattern of incidence of the structures in a basin the model is probably most directly applicable to the geodynamic interpretation of fluvial basins in which substantial volumes of readily liquefied sands accumulate at high local short term rates

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

high seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern canada abstract a deep seismic reflection profile on the continental rise off eastern canada shows strong dipping reflections apparently from within lower oceanic crust over a region of limited lateral extent the origin of these and similar reflections described elsewhere is uncertain this paper reports new seismic refraction results from the region of these reflections they appear to be associated with a layer of p wave velocity of about 7 4 km s 1 or higher while the surrounding crust has a velocity 7 km s 1 the moho with velocity 8 1 km s 1 lies just below the reflectors and the total crustal thickness is around 8 km the crust is interpreted to be oceanic rather than continental and the high velocities of the dipping reflectors are comparable to those of the mafic ultramafic crust mantle transition zone in the bay of islands ophiolite complex

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

lake deposits of moderate salinity as sensitive indicators of lake level fluctuations example from the upper rotliegend saline lake middle late permian northeast germany abstract the rotliegend saline lake periodically covered wide areas of the southern permian basin in northwest europe during the permian the sedimentology mineralogy and geochemistry of lake deposits were studied to document very high frequency lake level fluctuations and to evaluate their triggers increased precipitation and marine ingressions into the basin resulted in lake extension increased run off is documented by intercalated fluvial deposits in low salinity deposits of the lake because lake deposits reflect mainly deposition in relatively wet climate phases they are not correlatable to halite deposits in other basin areas decreased precipitation is followed by shrinkage of the lake desiccation at its margins and higher lake salinity due to concentration of the brine carbonate and anhydrite contents of lake deposits increased considerably before the areas fell dry but halite is not preserved in the study area falling lake level is also reflected by the occurrence of wave ripples reflecting decreased water depth and finally by desiccation and formation of evaporite crusts not only the mineralogical content but also the colour of lake claystones changes with varying salinity anhydrite and carbonate free red claystones were replaced by violet to green and grey anhydritic and calcareous claystone or marls with increasing salinity the amount of boron adsorbed on illite also corresponds to changes in salinity and can therefore be used as a palaeosalinity indicator rare earth element concentrations within lake deposits do not reflect variations in salinity but different palaeogeographical settings marine ingressions into the lake are neither reflected by the amount of boron adsorbed onto illite nor by rare earth element contents of the lake deposits highly saline deposits are partly characterised by intensive deformation which can be interpreted as seismites or dissolution breccias

marine deep seismic reflection profiles off central california a strong reflection horizon at two way travel time of approximately 6 s is observed in a deep seismic profile across the outer continental shelf of central california it is interpreted as the seismic image of subducted oceanic crust emplaced prior to the change from principally convergent to principally transcurrent motion between the pacific and north american plates during the late paleogene the reflector dips landward at a very shallow angle and is at a depth of 14 15 km under santa lucia bank the reflection is not observed or at best is very discontinuous under the inner shelf santa maria basin this suggests that tectonic or other processes have produced significantly different structural styles or compositions on the two sides of the santa lucia bank fault under the outer shelf a prominent apparently deeper later arrival time horizon dips more steeply and diverges from the 6 s reflector the deep horizon is at least partially composed of diffracted energy but is nearly linear after migration possible interpretations are that the horizon indicates crustal imbrication or out of plane diffractions alternatively it is a relict feature imparted to the crust at the now inactive pacific farallon spreading ridge reflective zones at intermediate depths are observed in apparently accreted sediments in parts of the santa lucia and santa maria basins these features could represent technically induced fabrics within the accretionary complex or they could be coherent depositional sequences

crustal structure of endeavour ridge segment juan de fuca ridge from a detailed seismic refraction survey a detailed seismic refraction survey was carried out over the endeavour segment of the juan de fuca ridge a medium rate spreading center which lies off western north america to investigate the creation and evolution of oceanic crust a bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading travel time and amplitude data from 15 in line air gun ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure but they appear to be random rather than distributed symmetrically about the ridge we attribute them to fracturing which causes porosity changes hydrothermal circulation which fills voids and fractures with alteration products and variations in magmatic and or deformational processes at the spreading center layer 2a is found to have low 2 6 2 8 km s velocities to average 0 4 km in thickness with variations up to 0 2 km and to be bounded at its base by a sharp velocity increase to 4 8 km s along the axial ridge velocities 0 4 0 6 km s higher than average are interpreted for layers 2b and 2c but these values are confined to a 2 km wide zone centered below the ridge velocities along ridge parallel lines offset 10 km are normal indicating that maturation to off ridge structure has occurred within at most 0 3 ma layer 3 velocities decrease by 0 1 0 2 km s for arrivals traveling along and under the axial ridge perhaps caused by higher temperatures however we find no anomalously low velocities beneath the ridge indicating that no large crustal magma chamber exists on the basis of this study we conclude that magmatic accretion is a fully three dimensional process within ridge segments such as endeavour ridge

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

active tectonics along the wadi araba jordan valley transform fault a geological study has been carried out along the 200 km long wadi araba following the transform fault that separates the arabian and sinai african plates recent movements along this structure affect upper pleistocene holocene deposits and archaeological sites kinematic indicators show sinistral strike slip and oblique movements in agreement with the relative motion between the two plates other evidence of recent horizontal displacement exists along the jordan valley fault both on the dead sea lake tiberias segment and on the segment north of lake tiberias a minimum horizontal slip rate of 1 cm yr 1 has been estimated for both the southern segment of the wadi araba fault and for the southern jordan river fault the two faults can be roughly subdivided into at least four segments two in the wadi araba 80 km long each one from the dead sea to lake tiberias 130 km and one from lake tiberias to the hula graben 30 km faulting may also occur along shorter subsegments as shown by bends in the wadi araba jordan river fault and by the growth of local compression and extension features instrument recorded seismicity appears to be mainly concentrated along some of these subsegments a comparison between the observed seismic and field determined slip rates across the fault indicates possible strain accumulation during the last 2000 years

dynamics of abundance of the mid to late pridoli conodonts from the eastern part of the silurian baltic basin multifractals state shifts and oscillations

soft sediment deformation within seismogenic slumps of the dead sea basin abstract the late pleistocene lisan formation preserved next to the dead sea provides exceptional 3 d exposures of folds and faults generated during soft sediment slumping and deformation it is possible to generate a range of four different scenarios associated with overprinting in a single slump event the progressive evolution of slump systems may be broadly categorised into initiation translation cessation relaxation and compaction phases thrust packages typically define piggyback sequences during slump translation with back steepening of imbricate faults leading to collapse of folds back up the regional palaeoslope detailed evaluation of slumped horizons may also permit structures to be traced across apparently separate and distinct slumped units the recognition that slumps may be reworked by younger seismically triggered events suggests that in some cases the seismic recurrence interval may be shorter than previously anticipated

crustal structure of northern juan de fuca plate from multichannel reflection data the northern juan de fuca plate forms the basement of a young 8 ma ocean basin off western canada two subparallel multichannel seismic reflection lines totaling 230 km in length and trending obliquely to the spreading direction were recorded across the basin and provide high quality images of crustal structure the processing sequence applied to the data includes a prestack inside trace mute of common midpoint cmp gathers to reduce noise levels on the deep data cmp stack poststack dip filtering f k migration and band pass filtering coherency filtered stacks are helpful in tracing weaker reflectors the stacked sections reveal a horizontally layered sedimentary sequence overlying a rugged and prominent basement reflector which dips slightly landward topography of the basement was formed at or near the spreading ridge there is no evidence for subsequent deformation a strong fairly continuous reflection from the base of the crust oceanic moho at 2 0 2 2 s two way travel time below the basement surface generally mimics the basement topography and shows the characteristic multicyclic waveform seen in other surveys estimates of reflection coefficients for the moho at several locations indicate that in general the crust mantle transition is unlikely to occur as a discrete first order interface although much of the crust appears acoustically transparent a number of discontinuous intracrustal reflectors are observed over 40 km at the eastern end of the northern line some of these are interpreted to arise from the oceanic layer 2 3 and layer 3a 3b boundaries the noncontinuous nature of these reflectors is an indication of the complexity of the processes producing intracrustal reflectivity and the lateral variability of crustal formation however the interpreted crustal thickness varies by only about 10 away from the distal ends of spreading segments indicating that while the process of crustal formation may be locally variable it produces a broadly uniform result

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

seismites in the fluviatile bima sandstones identification of paleoseisms and discussion of their magnitudes in a cretaceous synsedimentary strike slip basin upper benue nigeria abstract the upper benue basin northeastern nigeria is interpreted as a complex strike slip basin which consists of a set of sub basins controlled by synsedimentary n60 c e trending strike slip faults which were active during early cretaceous times the oblique planar cross beds made of the medium to fine grained sandstones of the upper member of the bima sandstone formation are affected by various soft sediment i e early diagenetic but soft sediment deformational structures these are present not only close to the synsedimentary faults but also on a larger regional scale three main types of structures undulated and recumbent folded cross bedded lamination dislocated convolutions and concordant microbreccias discordant fluidised intrusions are described these are related to processes of sediment deformation favored by sand hydroplasticity liquefaction and fluidization the local organization of these soft sediment intrastratal deformational structures their spatial distribution with respect to synsedimentary faults and their independance with respect to marginal paleoslopes suggest that they cannot be explained by slumping or current drag all the deformations are related to pore water expulsion and it is proposed that the required sand reorganization was triggered by seismic waves the spatial and chronologic distribution and intensity of the various types of deformation structures suggest that they were produced around different epicenters repeated by earthquakes of varying magnitudes associated with the synsedimentary strike slip tectonics

complex exposure histories of chert clasts in the late pleistocene shorelines of lake lisan southern israel activities of 26 al and 10 be in five chert clasts sampled from two beach ridges of late pleistocene lake lisan precursor of the dead sea in southern israel indicate low rates of chert bedrock erosion and complex exposure burial and by inference transport histories the chert clasts were derived from the senonian mishash formation a chert bearing chalk which is widely exposed in the nahal zin drainage basin the drainage system that supplied most of the material to the beach ridges simple exposure ages assuming only exposure at the beach ridge sampling sites range from 35 to 354 ky using the ratio 26 al 10 be total clast histories range from 0 46 to 4 3 my unrelated to the clasts current position and exposure period on the late pleistocene beach ridges 160 177 m below sea level optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20 0 s 1 4 ka and 36 1 s 3 3 ka these ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of lake lisan which suggest that the lake reached its highest stand around 27 000 cal years bp if the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50 1300 ky and 390 3130 ky respectively alternatively the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock back calculating such rates of rock erosion suggests values between 0 4 and 12 m my 1 t he relatively long burial periods indicate extended sediment storage as colluvium on slopes and or as alluvial deposits in river terraces some clasts may have been stored for long periods in abandoned pliocene and early pleistocene routes of nahal zin to the mediterranean before being transported again back into the nahal zin drainage system and washed on to the shores of lake lisan during the late pleistocene copyright 2003 john wiley sons ltd

structural characteristics of the ocean continent transition along the rifted continental margin offshore central labrador abstract studies of the structural deformation and the nature of the ocean continent transition zone at the rifted continental margin offshore central labrador and its conjugate off west greenland have been conducted over several decades and helped to define our understanding of the controlling factors in the development of magma poor margins globally new two dimensional seismic reflection data are now available across the labrador margin which can help to modify and refine this understanding in this paper we describe some of these new seismic data and the corresponding potential field data in relation to earlier studies including an existing deep seismic crustal velocity profile these data cross the three major structural and compositional zones below the sedimentary wedge defined in earlier deep seismic experiments as 1 extended zone of continental crust including the hyper extended region partly underlain by serpentinized mantle 2 exhumed and serpentinized continental mantle zone and 3 zone of oceanic crust we are able to extend these zones laterally over 200 km along the margin the rifted continental crust is hyper extended over tens of kilometers landward of the breakup location where many varieties of normal faults and possible low angle detachment surfaces are present some of which are near or at the base of the crust further seaward mantle was serpentinized and likely exhumed and possesses a seismic character very different from the continental crust basement in this zone may locally form prominent ridges within the oceanic zone we define a further sub division into a proto oceanic domain with thin variable oceanic crust of about 70 65 ma in age magnetic chrons 27 to 31 and a steady state seafloor spreading region further seaward thus seafloor spreading starts at about chron 31 time potential field data and models are broadly consistent with these zones a strongly reflective post rift volcanic event is present across much of the region especially over the exhumed mantle zone and parts of the oceanic crust these results are discussed and compared to earlier studies of the west greenland conjugate margin and to other well studied magma poor margins given that this rift occurred in thick cold cratonic lithosphere the results are remarkably similar to those off iberia and newfoundland suggesting that a long rift history and or mantle metasomatism may have weakened the lithosphere prior to the main rifting event

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

seismic reflection images of intraplate deformation central indian ocean and their tectonic significance multichannel seismic reflection profiles collected from the intraplate deformation area in the central indian ocean basin are used to describe brittle structures produced under compressive stress reflectors within oceanic basement are divided into four types north dipping and south dipping reverse faults that are interpreted as reactivations of structures formed at the spreading centre as outward and inward dipping faults respectively lower angle north dipping reflectors that probably represent new faults or faults just initiating and sub horizontal reflectors within the uppermost crust that are interpreted as hydrothermal alteration fronts within the sedimentary cover upwards fault propagation shows the faults steepening from c 40 just above basement to near vertical and is preceded by sediment folding a fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure measurements of north south crustal shortening indicate a shortening rate of 2 5 0 9 mm a 1 which is at the lower end of predictions from plate motions but significant enough to recognize this area as a diffuse plate boundary the formation of long wavelength basement undulations and the reactivation of fracture zones and ridge parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area there is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 ma

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

towards establishing criteria for identifying trigger mechanisms for soft sediment deformation a case study of late pleistocene lacustrine sands and clays onikobe and nakayamadaira basins northeastern japan five main deformation units discrete sheets of deformed sediments that lie between a significant thickness of undeformed sediment were selected for study within late pleistocene lacustrine sands and clays in the onikobe and nakayamadaira basins northeastern japan the deformed units show evidence of deformation by a variety of mechanisms including fluidization liquefaction brittle failure and cohesive flow driving forces are thought to be primarily reverse density gradient systems but also include gravitational body force shear stress and unequal loading the main trigger mechanisms are firstly earthquakes secondly overloading from volcanic sands and thirdly to a lesser extent subaqueous currents consideration is given to criteria that allow the trigger mechanism to be identified this study shows that the following criteria can be used to identify a seismic triggering agent i setting ii the extent of the deformation units iii absence of evidence relating to other potential trigger mechanisms and iv evidence relating to other potential trigger mechanisms is present but can be seen elsewhere in the stratigraphic section associated with undeformed sediment conversely the following criteria while they are important in interpreting the driving force and deformation mechanism have no relevance to the trigger mechanism i sediment composition ii deformation structures being restricted to a single stratigraphic interval 1 m thick not necessarily correlatable over large areas and iii similarity to structures in the literature

soft sediment deformation in a tectonically active area the plio pleistocene zarzal formation in the cauca valley western colombia abstract the plio pleistocene zarzal formation corresponds to fluvio lacustrine sediments deposited in an intramontane depression within the colombian andes associated with the cauca romeral fault system it crops out mainly in the cauca valley where numerous field sections have permitted the mapping of the vertical and lateral lithological variations lacustrine deposits of sands silts clays and diatomites are interbedded with fluvial sand and gravel beds and fluvio volcanic mass flows derived from the volcanic central cordillera numerous soft sediment deformation structures are encountered in this formation particularly in fine to medium grained sands silts and clays load structures load casts flame structures pseudonodules water escape structures water escape cusps dish and pillar and pocket and pillar structures soft sediment intrusions clastic sills and dykes disturbed laminites convolute laminations slumps and synsedimentary faulting deformation mechanisms and driving forces are related essentially to gravitational instabilities dewatering liquidization and brittle deformations field and regional geological data show that most of these deformations are related to seismicity and can be interpreted as seismites this area has a geological and recent seismic history and outcrops show both syn and post depositional faulting related to the transpressional regime of this part of the colombian andes which generates strike slip faults and associated local normal faults the drainage pattern within the zarzal formation shows the signature of neotectonics moreover the fine to coarse grained sands of the zarzal formation are lithologies prone to liquefaction when affected by seismic waves the intercalation of the deformed intervals within undisturbed strata confirms the catastrophic nature of the events finally the large areal extent of the deformations and the type of structures are compatible with seismites consequently the existence of seismites in the zarzal formation represents corroboration of tectonic activity in this area during the pleistocene earthquakes with a magnitude higher than 5 can be postulated based upon the proximity of active faults and the types of deformations

prehistoric earthquake deformations near masada dead sea graben earthquake inducedfluidizations and suspensions of lake sediments associated with syndepositionalfaults formapaleoseismicrecordinthedeadseagraben theassociation offluidized beds with surface faulting supports the recognition of mixed layers as reliable earthquake indicators and provides a tool for the study of very long term 70 kar seismicity along the dead sea transform the faults compose a fault zone that offsets laminated sediments of the late pleistocene lake lisan they exhibit displacements of as much as 2 m layers of massive mixtures of laminated fragments are interpreted as disturbed beds each formed by an earthquake the undisturbed laminated layers between these mixed layers represent the interseismic interval a typical vertical slip of about 0 5 m per event is separated by several hundred years of quiescence the fault zone lies within the dead sea graben 2 km east of masada where archaeology and historical accounts indicate repeated strong earthquake damage the distribution of strikes in the fault zone resembles that of the faults exposed in and around the graben including the seismogenic ones the excellent exposures over hundreds of metres allow an unprecedented temporal and spatial resolution of slip events on faults

effects of strong seismic shaking in lake sediments and earthquake recurrence interval temiscaming quebec the magnitude 6 3 timiskaming earthquake of 1935 resulted in discoloration in small lakes in the epicentral region sonar profiles for lac tee have shown that organic rich gyttja has been removed from the sides and redeposited in the deep parts of the lake lake sediment cores from deep basins contain a 2 3 cm uppermost layer which is interpreted as the normal accumulation of sediment since 1935 this is followed by a 20 cm chaotic zone of black gyttja mixed with partly tabular fragments of a previously formed silt layer underlain by a dense 1 2 cm thick silt horizon the latter is interpreted as having formed by rapid settling of the denser silicate portion of the resuspended sediment this 20 cm zone is insufficient to account for the abnormal thickness of gyttja in the basins so events of this kind would have had to have occurred repeatedly since deglaciation the only major prehistoric event revealed by these 100 cm cores is at a depth equivalent to about 1500 years ago based on the amount of sed

seismic structure of oceanic crust in the western north atlantic we examine the seismic structure of cretaceous aged ocean crust north of the blake spur fracture zone in the western north atlantic by using a combination of multichannel seismic and wide angle reflection refraction data although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure it displays large variations in crustal reflectivity on both ridge normal and ridge parallel profiles the upper crust is highly reflective and contains both subhorizontal or shallow dipping events and more steeply dipping reflectors subhorizontal reflectors are typically present between 1 5 and 2 km depth and are in some cases laterally continuous for distances of 15 to 20 km generally these events do not correlate with the depth of the seismic layer 2 layer 3 boundary determined from refraction data and their origin is still poorly understood the steeply dipping events are generally confined to the upper 2 5 km of the crust we interpret these reflectors as the subsurface expression of ridge parallel extensional faults commonly mapped at mid ocean ridges the lower crust in this area exhibits alternating regions of high and low reflectivity the highly reflective zones are made up of packages of linear or concave upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust some of these dipping reflectors appear to cut through the whole crustal section although most fade out in the acoustically transparent midcrust on ridge normal profiles the majority of these events dip to the east toward the paleo spreading center whereas on ridge parallel profiles the events typically dip south toward the blake spur fracture zone the base of the crust is generally not associated with a strong discrete moho reflection but is a comparatively indistinct boundary usually associated with a 1 to 2 km thick band of diffuse high reflectivity we interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8 10 km with the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge normal and ridge parallel profiles we may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults alternatively these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

active faulting in the dead sea rift abstract manifestations of late quaternary and holocene faulting were studied in a 500 km long segment of the dead sea transform rift most prominent are left slip faults whose characteristic physiographic features are recognizable along most of the studied segment where these faults bend or are stepped to the left rhomb shaped grabens or pull aparts are produced forming depressions in the reverse situation compressional features such as pressure ridges domes and folds form positive topographic features such structures are combined on a variety of scales ranging from a few hundred meters long to tens of kilometers normal faults sub parallel to the left slip faults produce a trough like valley along much of the dead sea transform but are most prominent along the margins of the large rhomb grabens e g the dead sea trough they apparently record a small component of transverse extension generally their motion is slow young slip did not occur along some segments during the last few 10 4 y elsewhere throws of 10 20 m at least occurred in this period the dead sea transform is seismically active the instrumental and historic records indicate a seismic slip rate of 0 15 0 35 cm y during the last 1000 1500 y while estimates of the average pliocene pleistocene rate are 0 7 1 0 cm y either much creep takes place or the slip rate varies over periods of a few 10 3 y

quantitative analysis of seismogenic shear induced turbulence in lake sediments spectacular deformations observed in lake sediments in an earthquake prone region lisan formation pre dead sea lake appear in phases of laminar moderate folds billow like asymmetric folds coherent vortices and turbulent chaotic structures power spectral analysis of the deformation indicates that the geometry robustly obeys a power law of 1 89 similar to the measured value of kelvin helmholtz kh turbulence in other environments numerical simulations are performed using properties of the layer materials based on measurements of the modern dead sea sediments which are a reasonable analogue of lake lisan the simulations show that for a given induced shear the smaller the thickness of the layers the greater is the turbulent deformation this is due to the fact that although the effective viscosity increases the reynolds number decreases the bulk richardson number becomes smaller with decrease in the layer thickness the latter represents the ratio between the gravitational potential energy of the stably stratified sediments and the shear energy generated by the earthquake therefore for thin layers the shear energy density is larger and the kh instability mechanism becomes more efficient the peak ground acceleration pga is related to the seismogenic shear established during the earthquake hence a link is made between the observed thickness and geometry of a deformed layer with its causative earthquake9s pga

holocene earthquakes inferred from a fan delta sequence in the dead sea graben abstract the holocene sequence of the fan delta of nahal darga in israel records deformation associated with earthquakes related to the dead sea transform in general and to the jericho fault in particular the fan delta sequence is well exposed and 20 radiocarbon ages help to date the earthquakes that are inferred from a displacement along faults b liquefaction features associated with 11 separate sandy and silty layers and c slumped allocthonous bodies of sediments located directly above one of the main splays of the jericho fault on average an earthquake larger than m 5 5 has occurred approximately every 600 years this estimate is based on the earthquake record of the complete stratigraphic sequence with erosional hiatuses omitted from the calculations the most recently deformed layer is related to the 1927 jericho m l 6 2 earthquake this layer provides a modern analog for the style of soft sediment deformation associated with earthquakes in the late pleistocene and holocene silty sand beds of the fan delta complexes of the dead sea and its predecessor lake lisan

seismogenic deformation structures in microbialites and mudstones silurian lockport dolomite northwestern ohio u s a syndepositional deformation structures are present at two different stratigraphic levels in the silurian lockport dolomite at a quarry in maumee ohio such structures are absent in this unit at twelve other quarries in northwestern ohio the structures at maumee present within stromatolites thrombolites and variably bedded mudstones include convolute layers homogenization of bedding and breccia clasts clasts of stromatolites boudinage structures normal reverse and thrust faults and discordant and concordant breccias the distribution of deformation structures within beds suggests substantial control from substrate rheology possibly resulting from differences in degrees of lithification and from variations in water content the extensive variety of syndepositional deformation structures juxtaposed evidence for nearly synchronous compression and dilation proximity 4 5 km of the structures to the largest fault in northwestern ohio the bowling green fault and the restricted geographic distribution of the deformation structures suggest seismic shaking as a probable mode of genesis similarities between features produced by seismogenic deformation of siliciclastic sediments and deformation features in the lockport dolomite provide additional evidence that the latter features are seismogenic it is postulated that two earthquakes one with a magnitude 5 caused the formation of the syndepositional deformation structures present in the youngest facies in the lockport dolomite at maumee quarry the bowling green fault 4 5 km west of maumee was a growth fault during formation of the lockport dolomite and is considered to have been the source of earthquakes that created the syndepositonal deformation structures in this unit at maumee quarry evidence from the lockport dolomite in northwestern ohio suggests that seismogenic deformation structures created by two earthquakes along the bowling green fault with magnitudes of 5 did not form in this unit over large areas i e all of northwestern ohio this evidence challenges the suggestion that syndepositional seismogenic deformation structures are always formed over a large area

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

earthquake deformations in the lisan deposits and seismotectonic implications summary earthquake deformations and induced sedimentary structures preserved in quaternary sediments include faults folds fissures slumps sand boils and other effects of liquefaction such deformations and structures are well preserved in the lisan deposits of the dead sea of most importance are the fold type deformations known as decollement structures which are present all along the eastern side of the lisan and seem to decrease gradually westwards to disappear in the middle of the lisan these may indicate that palaeoearthquakes originating along the araba fault have triggered such structures due to shaking of elastoplastic unconsolidated sediments over gentle slopes dipping to the west preliminary results from studies on decollement structures preserved in a section representing some 1733 years of continuous deposition in the uppermost pleistocene in the vicinity of wadi araba indicate that 1 seismic activity has fluctuated with time average recurrence period is about 340 20yr for earthquakes with magnitudes greater than or equal to 6 5 earthquakes with magnitude greater than 7 seem to have occurred along the araba fault 2 deduced earthquake magnitudes conform to the frequency magnitude relationship log n 5 24 0 68m 3 the deduced seismic slip rate along the araba fault seems to be not less than 0 64 0 04 cm yr 1

statistical relations among earthquake magnitude surface rupture length and surface fault displacement abstract in order to refine correlations of surface wave magnitude fault rupture length at the ground surface and fault displacement at the surface by including the uncertainties in these variables the existing data were critically reviewed and a new data base was compiled earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the gutenberg methods and results which make up much of the data base measurement errors were estimated for the three variables for 58 moderate to large shallow focus earthquakes regression analyses were then made utilizing the estimated measurement errors the regression analysis demonstrates that the relations among the variables magnitude length and displacement are stochastic in nature the stochastic variance introduced in part by incomplete surface expression of seismogenic faulting variation in shear modulus and regional factors dominates the estimated measurement errors thus it is appropriate to use ordinary least squares for the regression models rather than regression models based upon an underlying deterministic relation in which the variance results primarily from measurement errors significant differences exist in correlations of certain combinations of length displacement and magnitude when events are grouped by fault type or by region including attenuation regions delineated by evernden and others estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating m s with the logarithms of rupture length fault displacement or the product of length and displacement fault rupture area could be reliably estimated for about 20 of the events in the data set regression of m s on rupture area did not result in a marked improvement over regressions that did not involve rupture area because no subduction zone earthquakes are included in this study the reported results do not apply to such zones

multichannel seismic evidence for variations in crustal thickness across the molokai fracture zone in the mid pacific coincident multichannel seismic reflection and refraction data from a n s transect near oahu hawaii provide evidence for thickening of the pacific crust by 1 2 1 km south of the large offset 16 m y molokai fracture zone fz tau p stacks tau sum inversions and forward modeling of the refraction data indicate that the crustal thickening occurs primarily within the lower portion of seismic layer 2 assuming isostatic balance the differences in crustal thickness predict that seafloor having the same age will have different elevations across the fz observations of sea floor depths across the fz east of the hawaiian islands are consistent with this prediction implying that the processes which have generated the crustal differences have been stable for over 50 m y previous correlations between the chemical composition of ridge crest basalts crustal thickness and ridge crest elevation have been attributed to variations in the thermal regime of the upper mantle under mid ocean spreading centers in accord with this hypothesis we propose that the observed differences in crustal structure across the molokai fz may have been produced by small 25 c differences in the thermal regime of the upper mantle beneath the ancestral east pacific rise discontinuous intracrustal reflections located about 1 6 s below the sediment basement interface are observed in migrated reflection data south of oahu these reflections are similar in character to the lower crustal horizon r event observed in the western north atlantic shallower intracrustal reflections possibly from within seismic layer 2 are also observed the observation of these intracrustal reflections in both the atlantic and pacific oceans suggests that they are a fundamental signature of the crustal accretion process at a variety of spreading rates and that they are mappable using modern seismic reflection refraction methods

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

lake level change climate and the impact of natural events the role of seismic and volcanic events in the formation of the lake patzcuaro basin michoacan mexico in central mexico lake level change is commonly used as a proxy for climatic fluctuation lacustrine sequences are not stable units however and have been dramatically impacted by processes related to the evolution of the mexican volcanic belt mvb and the fragmentation of the southern part of the north american plate including volcanism tectonic action landslides and human activity one notable example is the lake patzcuaro basin mexico which has one of the most intensively studied paleoenvironmental records in latin america geologic processes have altered the lacustrine sequence in the southwest portion of the lake basin during the last 48 000 years these include volcanic activity major uplift landslides and related tsunamis and earthquakes this new record has important implications for interpretations of climatic events in central mexico during the pleistocene and the impact these may have had on human settlement in the region

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

wide angle imaging of reflectors in mesozoic oceanic crust two ship synthetic aperture data from mesozoic oceanic crust in the western north atlantic contain wide angle reflections and refractions from oceanic layer 2 at the longer offsets 6 9 km with appropriate velocity filtering to remove cross cutting reflections from the sediment column the wide angle data may be stacked to produce images of the upper oceanic crust some dipping events are imaged both near normal incidence and at wide angle and are most likely due to zones of anomalously high impedance such zones may be generated by hydrothermal alteration of material in fault zones generated near the spreading center

a deep seismic reflection profile across the extinct mid labrador sea spreading center in 1990 a deep multichannel seismic reflection line was shot along a flow line across the labrador sea the results from the central portion of this line between magnetic anomalies 25 and across the extinct central ridge are described here spreading rates in this part of the labrador sea are very low from 10 mm yr to less than 3 mm yr so that the line provides a unique opportunity to examine the relationship between very slow spreading and crustal structure a clear division is observed between two types of crust the oldest crust between dirons 21 and 25 which formed at a mean half spreading rate of 10 mm yr exhibits smoothly undulating basement with only minor normal faulting this region shares many of the reflection characteristics of north atlantic crust formed at moderate to low spreading rates in contrast the younger central region between chrons 21 and 13 which formed at a mean half spreading rate of 3 mm yr displays evidence of intense normal faulting of the crust giving a total extension of about 70 inward facing normal faults on both sides of the extinct ridge with large offsets many of which extend to lower crust or moho depths dominate the seismic section the axial region is characterized by a deep fault bounded median valley these results suggest that mechanical extension plays a more important role in seafloor spreading at low spreading rates than previously documented integration of the reflection data with previous refraction measurements and with gravity modeling of the region shows variations in crustal thickness which can be correlated with spreading rates the region formed at a mean spreading rate of 10 mm yr where about 15 extension is observed exhibits slightly thinner than normal crust 4 8 km or less versus a normal thickness of about 7 km at a lower spreading rate of 3 mm yr across the axial region where extension is about 70 an average crustal thickness of 3 km is obtained thus lower spreading rates are associated with regions of thinner crust and greater amounts of extension while many studies suggest that thin crust at slow spreading rates may result from a reduced magma supply this study suggests that extension is at least equally important and may be responsible for most of the variations in crustal thickness the increased cooling of young oceanic lithosphere formed at these very low spreading rates approximately 3 mm yr may have amplified brittle failure in response to plate separation however the timing of extension is still uncertain and some of it may be related to postextinction tectonics and not to the spreading process the role of extension as a control on crustal thickness needs to be considered further in studies of crustal generation and magmatism at slow spreading ridges extension will decrease the importance of magmatism in generating thin oceanic crust and will favor models of magmatic processes which produce thicker crust

distribution of holocene sediments and neotectonics in the deep north basin of the dead sea abstract the distribution of holocene sediments and active faults in the central portion of the northern dead sea basin was analyzed from high resolution seismic reflection data the seismic reflection profiles reveal that sediments buried at a shallow depth are flat lying continuous and unfaulted within the central basin floor four prominent subsurface seismic reflectors were identified that based on limited core data represent the contact between rock salt and marl layers isopach and structural maps of recent sediments indicate an asymmetry of active subsidence with thick accumulations located closer to the east margin of the basin sediment thickness increases within a rhomb shaped depression whose long axis is oriented approximately 20 30 counterclockwise to the regional basin bounding faults the morphology and trend of the locus of maximum subsidence suggests tectonic control of the depression however the proximity of the depocenter of recent sediments to the ein gedi diapir and rim depression suggests that local sub basinal subsidence may also be related to movement of rock salt at depth the basinal strata are deformed along the faults that form a bathymetric escarpment along the west east and south margins of the north basin of the dead sea thinning of recent sediments toward the faults indicates syntectonic deposition

old pacific crust near hawaii a seismic view an expanding spread profile esp over 80 ma pacific ocean crust near hawaii was modeled with a one dimensional solution for travel times relative amplitudes and sometimes frequency content the resulting profiles of compressional and shear wave velocities and poisson s ratio are similar to those determined by other ocean crustal studies except that the shear wave q in the sediments is very low 10 and some unusual high frequency high phase velocity arrivals require a laminated structure within the mid crustal section at about 3 km below the base of the sediment the laminations are probably alternating layers of gabbro and ultramafic cumulates the cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation reflections which may correspond to these laminations have been observed in reflection records both near this esp and in the north atlantic comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity lithology model is probably in error for the pillow and dike basalts resulting in an interpretation of overly thick layers in previous studies of refraction lines the preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1 0 and 1 5 km

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

the crustal structure of the canary basin accretion processes at slow spreading centers multichannel seismic reflection and gravity data define the structure of mesozoic ocean crust of the canary basin formed at slow spreading rates single and multichannel seismics show a transition from smooth to rough basement topography from jurassic to cretaceous crust and a coeval change in crustal structure internal reflectivity of the rough basement area comprises upper upper middle or whole crust cutting discrete dipping reflections lower crustal reflectivity is almost absent and reflections from the crust mantle transition are short and discontinuous or absent for several kilometers in contrast crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust mantle boundary the crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust in the smooth basement area fracture zones exhibit neither a clear topographic expression nor crustal thinning we interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from jurassic to cretaceous times in addition the crust imaged across the path of the cape verde hot spot in the canary basin exhibits a widespread lower crustal reflectivity very smooth topography and apparently thick crust our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the mesozoic mid atlantic ridge

quantitative comparison of seismic data sets with waveform inversion testing the age dependent evolution of crustal structure studies of seismic variability play a more important role in the understanding of processes that form oceanic crust than individual velocity depth solutions found in isolation for such variability studies solutions must be contrasted with one another this is normally not straightforward owing to the lack of rigorous uncertainty estimates available for solutions here i present a method for the direct comparison of such solutions by hypothesis testing a null hypothesis is proposed that a single velocity solution is consistent with two sets of data the hypothesis is tested by seeking such a model using waveform inversion and comparing its misfit against the misfit from fully independent solutions the degree of similarity of the velocity structures can be further quantified by solving for the pair of solutions that are as similar as possible while consistent with each set of data the f test is used to evaluate the significance of each additional effective model parameter as it lends independence to the two solutions the method possesses advantages of a hypothetical monte carlo search of all models because the complete forward problem is carried out at each step yet it is tractable because the search direction is well defined the method is applied to two data sets collected near the mid atlantic ridge on 0 and 7 m y crust in order to quantify the differences between the solutions the comparison finds quantifiable differences in both layers 2 and 3 between these two regions suggesting that significant evolution of the entire crustal section has taken place in the first 7 m y

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

reevaluation of the lake sediment chronology in the dead sea basin israel based on new 230th u dates the dead sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes lake samra and lake lisan the sedimentary history of these lakes was recon structed by means of 230th234u ages of 30 samples mostly of argonite laminae from 8 columnar sections up to 110 km apart the general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency agreement with stratigraphic order and concordance with 14c ages in the south only the part of the samra formation older than 170 000 yr is exposed while the aragonite detritus rhythmites found in the central and northern region are generally younger than 120 000 yr the lisan formation started accumulating about 63 000 yr b p with the clay and aragonite beds in the south central area reflecting a rise in water level to at least 280 m the upper part of the lisan formation the aragonite rich white cliff member started accumulating about 36 000 yr b p the lake probably reached its highest level sometime after this based on the ages of lisan sediments preserved in the southernmost reaches of the basin

a detailed geophysical study in the canary basin eastern atlantic implications for the internal structure of 130 ma oceanic crust summary we report the results of a detailed geophysical survey sited on two previously imaged dipping reflectors one dipping towards and the other away from the present day mid atlantic ridge axis within mesozoic aged crust of the east atlantic the new data include gravity magnetic and seismic refraction and reflection profiles collected in a grid that constrain the crustal structure as well as the reflector geometry a second order axial discontinuity which is associated with anomalous crustal structure is located within the survey area the westward dipping reflector can be traced from the moho to the top of the igneous crust where it coincides with a step of 800 m in the basement this reflector is approximately planar with a dip of 28 2 w and occurs 8 km north of the axial discontinuity we interpret this reflector as being a whole crustal normal fault that was generated within the median valley the eastward dipping reflector cannot be traced across seismic layer 2 and does not correlate with any feature of the basement it penetrates 2 3 km into the upper mantle its reflection strength is about twice that of the moho reflection this reflector which is approximately planar strikes parallel to the isochrons has a length of 18 2 km and a dip of 28 2 e we speculate that this reflector is also a fault that was generated on the western wall of the median valley but was captured from the north american plate by a lateral shift in the axis of spreading possibly associated with a major change in plate motion

north anatolian fault in the gulf of izmit turkey rapid vertical motion in response to minor bends of a nonvertical continental transform 1 the catastrophic rupture of the north anatolian fault east of the marmara sea on 17 august 1999 highlighted a need for mapping the underwater extension of that continental transform a new bathymetric map of izmit gulf indicates that the fault follows the axis of the gulf with a few minor bends submerged shorelines and shelf breaks that formed during the last glacial maximum provide markers to quantify vertical deformation variable tilting of these horizons reveals that vertical deformation is highest just south of the fault a correlation between vertical deformation of the southern fault block and distance to fault bends can be accounted for by a fault dipping steeply to the south hence subsidence uplift of the southern hanging wall block would be expected where the fault strikes at a slightly transtensional transpressional orientation to relative plate motion subsidence reaches about 8 mm yr west of the town of golcuk and might be accommodated in 1 2 m subsidence events during large earthquakes that scenario is compatible with the tsunami runups and the coseismic subsidence of the southern shore that occurred in 1999 seafloor morphology also suggests that earthquakes are accompanied by widespread gas and fluid release the periphery of the deepest basin displays a hummocky texture diagnostic of sediment fluidization and mud volcanoes occur west of hersek peninsula that might be activated by earthquakes finally the backscatter imagery reveals a series of lineaments midway through the gulf that are interpreted as products of the 1999 surface rupture the seafloor is undisturbed farther west suggesting that surface slip decreased to an insignificant level beyond hersek possibly the stress shadow from the 10 july 1894 earthquake which was felt strongly along the western izmit gulf contributed to arrest the 1999 surface rupture

fault geometry and spatial clustering of microearthquakes along the dead sea jordan rift fault zone abstract earthquakes 1 0 m l 5 3 in the region around the dead sea jordan rift fault zone recorded by the israel scismograph station network issn from 1982 until the beginning of 1989 have been analyzed and interpreted the scismicity pattern clearly reveals activity along three fault zones the dead sea jordan rift fault zone the carmel faraa fault zone and the central sinai negev shear zone microearthquake occurrence tends to cluster in or near tensional features rhomb shaped grabens such as the hula sea of gallilee and dead sea depressions further eight representative focal mechanisms of individual earthquakes or earthquake clusters have been derived the seismicity and focal mechanism solutions near the branching of the carmel faraa and the dead sea jordan rift fault zones suggest a complicated tectonic breakup in this region microearthquakes in the arava may indicate a more complex pattern of active faults than was previously assumed

the tethys sea and the alpine himalayan orogenic belt mega elements in a new global tectonic system abstract analysis of meso cainozoic palaeomagnetic data for africa india and eurasia has led to the development of a new mobilistic alpine plate tectonic model characterized by a hierarchical system of plates in relative rotation the new model which discounts seafloor spreading implies that there have been no significant palaeogeographic changes in the overall distribution of continental and oceanic regions the mid oceanic ridges are interpreted as transpressive tectonic features caused by rotation of megaplates containing both continental and oceanic crust the isostatic uplift due to crustal lithospheric thickening giving rise to the general ridge topography as well as to the ridge parallel structural grain the new plate tectonic theory gains strong support from a variety of geophysical geological and palaeoclimatological evidence and several observations that have remained enigmatic or awkward within the context of the orthodox model can be readily accounted for in the new tectonic framework the model maintains the tethys as a relatively narrow epicontinental sea which during its maximum extent stretched latitudinally from the caribbean across the central atlantic to se asia the alpine himalayan orogenic belt developed along the boundary of two megaplates in relative rotation which provided a transpressive tectonic regime the location of the plate boundary to the north of the mediterranean has important implications for discussion of mediterranean microplates for example it now seems that italy has been subjected to 10 15 of clockwise microplate rotation previous conclusions in favour of 30 40 of anticlockwise rotation are regarded as artefacts which arise from incorrectly linking the mediterranean region to the european palaeomagnetic frame instead of to the african one the model suggests further that the indo pakistani plate was closely tied to eurasia this challenges the conventional view that the peninsula was part of an alleged gondwanaland the new pre drift configuration implies that the indo pakistani plate rotated 135 clockwise at around the cretaceous tertiary boundary before redocking with asia in approximately its present relative orientation

geometry of extensional faults developed at slow spreading centres from pre stack depth migration of seismic reflection data in the central atlantic canary basin we present depth images from portions of profiles that are close to flow lines of cretaceous oceanic crust in the eastern central atlantic compared with post stack time migrations the images illustrate the improvement resulting from the application of pre stack depth migration the images document the scale and geometry of normal faulting in oceanic crust formed over 25 myr at a half spreading rate of less than 10 mm yr 1 and the variation in extensional style with position within the spreading segment away from major fault zones fzs most faults are subplanar dip more than 35 are associated with moderate basement relief 0 2 1 km relief and may penetrate to deep crustal levels these faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains also observed away from fzs are gently dipping to subhorizontal reflections in the upper crust which resemble detachment faults in contrast the inside corner crust is more rugged with basement highs rising up to 2 km above the intervening basins this larger scale topography is associated with a different style of faulting the depth images reveal gently dipping 35 faults that are rooted in the deep crust and that project to the ridgeward flank of the dome shaped large basement highs 1 2 km vertical relief the faults seem to continue as the ridge facing flank of these highs and some may extend over the crest of the high to breakaways beyond in this case the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes these controlling faults are up to 20 km long and have a heave of 10 km sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks we suggest that similar faults can explain the structure and lithologies found at megamullion structures oceanic core complexes at inside corners near the present day spreading ridge

formation and evolution of the nova scotian rifted margin evidence from deep seismic reflection data a deep marine seismic reflection profile was obtained across the mesozoic rifted continental margin off nova scotia eastern canada this profile crosses the seotian basin one of the deepest basins on the margin of eastern north america and it complements other deep crustal seismic data on this margin the seismic data have been interpreted in conjunction with gravity anomaly and subsidence data they show significant thinning of the continental crust over a zone about 200 km wide the mode of extensional deformation is probably a combination of pure and simple shear there is evidence for simple shear in the crust the continent ocean boundary lies near the seaward edge of synrift salt below the continental rise a 100 km wide zone of very thin approximately 9 km or less continental or transitional crust extends seaward from the outer shelf to this boundary reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction perhaps modified by extensional faulting this margin is nonvolcanic and the transition to the volcanic margin off the eastern united states occurs about 500 km southwest of the seismic line the width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south it seems likely that the prerift fabric of the continental lithosphere controls this width a narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism however significantly narrower zones of crustal extension occur on other nonvolcanic margins so factors in addition to rift width such as the rate of rifting may also be important

recurrent liquefaction induced by the 1989 loma prieta earthquake and 1990 and 1991 aftershocks implications for paleoseismicity studies sandblows formed in man made lake deposits near watsonville california resulted from the october 1989 m 7 1 loma prieta earthquake and its april 1990 m 5 5 aftershock and a march 1991 m 4 6 aftershock excavations in the dry bed of soda lake expose the subsurface structure of sandblows induced by the october and april events we also made surface observations three days after the march 1991 event lateral spread fissures developed in the lake sediment during the mainshock and the host deposits were vertically offset 3 to 6 5 cm liquefied sand erupted through the fissures onto the dry lake bed and formed sandblow cone deposits these deposits range from low angle conical structures 30 to 50 cm thick and 2 to 5 m in diameter to elongate deposits up to 35 m long sandblows associated with the april 1990 aftershock commonly reoccupied preexisting fissures but also formed new solitary vents sandblows developed during the march 1991 aftershock erupted only through preexisting vents sandblow vents reactivated in april 1990 contain sediment clasts of sandblow deposits formed in the october 1989 mainshock clasts of host sediment were also present in the sand dikes lamination in the sandblow cones and sand dikes shows that they were not formed in a single episode surface cones and feeder dikes composed of multiple laminated subunits represent the three generations of sandblows the subunits suggest that the cone deposits were formed by pulses of water and sediment expelled from the vent the pulses result from cyclic phases of local excess pore pressure and its release in the liquefied bed at depth the three generations of structures at soda lake formed six months to a year apart have distinct tonal and textural differences that may mimic liquefaction structures formed decades or centuries apart this study provides modern analogs for comparison to and interpretation of prehistoric liquefaction structures that are frequently partly eroded or poorly exposed

recurrence pattern of holocene earthquakes along the dead sea transform revealed by varve counting and radiocarbon dating of lacustrine sediments abstract a high resolution holocene seismic history of the dead sea transform dst is established from laminated sedimentary cores recovered at the shores of the dead sea radiocarbon dating and annual laminae counting yield excellent agreement between disturbed sedimentary structures identified as seismites and the historical earthquake record all recent and historical strong events of the area were identified including the major earthquakes of a d 1927 1837 1212 1033 749 and 31 b c the total of 53 seismites recognized along the entire holocene profile indicate varying recurrence intervals of seismic activity between a few and 1000 years with a conspicuous minimum rate at 2100 31 b c and a noticeable maximum during the past six to eight centuries most of the epicenters of the correlated earthquakes are situated very close to the dead sea within 150 km or up to 400 km north of it along the dst between 1000 b c and a d 1063 and from a d 1600 to recent time the epicenters are all located on the northern segment of the dst whereas prior to 1000 b c and between a d 1000 and 1600 they appear to scatter along several segments of the dst we establish how the local intensity exerts a control on the formation of seismites at historically estimated intensities greater than vii all well documented earthquakes are correlated whereas at intensities smaller than vi none are matching the periods with enhanced earthquake rate along the dst correlate with those along the north anatolian fault as opposed to the intervening east anatolian fault this may indicate some elastic coupling on plate boundary scale that may also underlie escape and extrusion tectonics typical of continental collision

the mohorovicic discontinuity in ocean basins some observations from seismic data abstract since the late 1970s studies of the oceanic crust using airgun sources and towed hydrophone arrays have been conducted in a wide range of ocean basin settings including mid ocean ridges and old oceanic crust the very earliest studies were performed at the fast spreading east pacific rise epr and revealed a very distinct almost continuous vertical incidence reflection event at a depth corresponding to the crust mantle transition as inferred from seismic refraction studies this suggested that the transition was quite sharp in comparison to the source wavelength that moho was observed very close to or even exactly beneath the ridge crest implied that it was formed at essentially zero age since then many experiments using progressively improving airgun arrays and streamer systems have expanded these observations here we review the literature presenting studies of normal oceanic crust produced at mid ocean ridges with the objective of assessing the age of formation of moho and the nature of variability of moho signature in multi channel seismic data moho is observed as a consistent feature for all spreading rates but appears quite variable being very distinct in some areas complex in form in others and absent in many regions as much as 40 although fast spread crust is associated with the strongest simplest and most laterally continuous moho images we see significant variability at almost all spreading rates and ages where moho is absent from vertical incidence data this cannot be fully explained by the effect of scattering or attenuation in the crustal section above moho can be observed at zero age in only a small number of areas at or near overlapping spreading centers on the epr after three decades of marine seismic studies many fundamental questions remain concerning the nature of the moho that will require targeted experiments to solve

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments

holocene seismic and tectonic activity in the dead sea area abstract the dead sea is a large active graben within the dead sea rift which is bounded by two major strike slip faults the jericho and the arava faults we investigated the young tectonic activity along the jericho fault by excavating trenches up to 3 5 m deep across its trace the trenches penetrate through late pleistocene and holocene sediments we found that a zone up to 15 m wide of disturbed sediments exists along the fault these disturbed sediments provide evidence for two periods of intensive activity or more likely for two major earthquakes that occurred during the last 2000 years the earthquakes are evident in small faults vertical throw of a few layers cracks unconformities and wide fissures we further documented evidence for recent sinistral shear along the jericho fault in deformed sediments and damage to an 8th century palace on a subsidiary fault we suggest that the two earthquakes may be correlated with the 31 b c earthquake and the 748 a d earthquake reported by the ancients