knickzone propagation in the black hills and northern high plains a different perspective on the late cenozoic exhumation of the laramide rocky mountains geomorphic research in the black hills and northern high plains poses an intriguing hypothesis for the cenozoic evolution of this salient of the laramide rockies most recently geologists have appealed to late cenozoic epeirogenic uplift or climate change to explain the post laramide unroofing of the rockies on the basis of field mapping and the interpretation of long valley profiles we conclude that the propagation of knickzones is the primary mechanism for exhumation in the black hills long profiles of major drainages show discrete breaks in the slope of the channel gradient that are not coincident with changes in rock type we use the term knickzones to describe these features because their profiles are broadly convex over tens of kilometers at and below the knickzone the channel is incising into bedrock abandoning a flood plain and forming a terrace above the knickzone the channel is much less incised resulting in a broad valley bottom numerous examples of stream piracy are documented and in each case the capture is recorded in the same terrace level these observations are consistent with migrating knickzones that have swept through black hills streams rearranging drainages in their wake we demonstrate there are two knickzone fronts associated with mapped terraces preliminary field evidence of soil development shows that these terraces are time transgressive in nature our data strongly suggest that knickzone propagation must be considered a viable mechanism driving late cenozoic fluvial incision and exhumation of the northern high plains and adjacent northern rocky mountains

character and distribution of borehole breakouts and their relationship to in situ stresses in deep columbia river basalts the character and distribution of borehole breakouts in deeply buried basalts at the hanford site in south central washington state are examined in light of stress indicator data and hydraulic fracturing stress data by means of acoustic televiewer and acoustic waveform logging systems a series of boreholes penetrating the grande ronde basalt of the columbia river basalt group were logged to examine the extent of breakouts at depths near 1000 m breakouts occur discontinuously throughout the interiors of most flows in some boreholes the distribution of borehole wall breakouts closely correlates with the incidence of core disking differences in the distribution of breakouts and disking are attributed to differences in failure mechanisms a thin interval of breakout free basalt occurs near the upper and lower limits of flow interiors with many intervals of breakouts terminating at the intersection of oblique fractures with the borehole hydraulic fracturing stress measurement results obtained from four deep boreholes indicate anisotropic horizontal principal stresses with maximum principal stress along an approximate north trending axis consistent with the east trending orientation of breakouts acoustic waveform logs indicate that there is no measurable difference between the seismic properties of breakout free flow tops and flow interiors the highly coherent waveforms obtained in almost all flow interiors indicate that damage to the borehole wall associated with breakout formation remains confined to the thin annulus of stress concentration

resistivity structure of the upper crust in iceland the results of numerous resistivity soundings from all parts of iceland are analyzed the resistivity and the p wave velocity of the uppermost kilometer in the crust show a strong correlation with the geological structure of iceland by estimating porosity from p wave velocity archie s law is found not to be valid in general the temperature dependence of nine water saturated basaltic rock samples was investigated in a laboratory showing stronger temperature dependence than expected from pore fluid conduction these two facts suggest that interface conduction is the most important conduction mechanism a relation between bulk resistivity fracture porosity pore fluid resistivity and temperature is established comparison of regional bulk resistivity with different zeolite zones shows that the resistivity is mainly a function of the maximum depth of burial of the lava pile emphasizing the importance of clay minerals and zeolites for the measured bulk resistivity

the structure of the lower crust at the argentine continental margin south atlantic at 44 s abstract it is well established that the argentine passive margin is of the rifted volcanic margin type this classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope manifested by seismic data as a seaward dipping reflector sequence sdrs here we investigate the deep structure of the argentine volcanic margin at 44 s over 200 km from the shelf to the deep oceanic argentine basin we use wide angle reflection refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times the resulting p wave velocity depth model confirms the typical volcanic margin structure an underplated body is resolved as distinctive high seismic velocity vp up to 7 5 km s feature in the lower crust in the prolongation of a seaward dipping reflector sequence a remarkable result is that a second isolated body of high seismic velocity vp up to 7 3 km s exists landward of the first high velocity feature the centres of both bodies are 60 km apart the high velocity lower crustal bodies likely were emplaced during transient magmatic volcanic events accompanying the late rifting and initial drifting stages the lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the south atlantic rift evolved by instantaneous breakup of longer continental margin segments these segments are confined by transfer zones that acted as rift propagation barriers a lower crustal reflector was detected at 3 to 5 km above the modern moho and probably represents the lower boundary of stretched continental crust with this finding we suggest that the continent ocean boundary is situated 70 km more seaward than in previous interpretations

palaeogeographical evolution of the rattray volcanic province central north sea the rattray volcanics member at the triple junction of the north sea rift is here subdivided into two informal sub members based on analysis of core wireline and seismic data the lower and upper rattray volcanics were emplaced during two distinct phases of volcanism separated by a sustained volcanic hiatus the presence of hyaloclastite and abundant freshwater algae at the base of the lower rattray indicates that large lakes were present in the area prior to the volcanism possibly indicating that collapse of the regional jurassic central north sea dome began prior to volcanism pulsed subsidence probably occurred through the duration of the volcanism with lacustrine conditions becoming re established during the mid volcanic hiatus sediments were deposited across the rattray volcanic province in fluvial systems and floodplain coal swamps after the final cessation of volcanism with later marine transgression leading to drowning of the area in the callovian to oxfordian in terms of hydrocarbon prospectivity no evidence is currently found to confirm the presence of an intra basaltic play analogous to the rosebank field in the faroe shetland basin although post volcanic pentland formation sedimentary sequences in the fisher bank basin area indicate the possibility of supra basaltic prospectivity in the triple junction supplementary material a table containing the complete palynological record is available at https doi org 10 6084 m9 figshare c 4857195

the iceland research drilling project this issue of the journal of geophysical research contains the collected results of the initial studies of a 3 km vertical section of icelandic crust the material described in these papers was largely presented at a meeting of the investigators working in this project held in reykjavik iceland from may 13 to may 15 1980 iceland presents a very well exposed example of crust formed at an accretional plate margin the crest of the mid atlantic ridge the desirability of a detailed study of a long vertical section of icelandic crust by deep drilling has been expressed in proposals and recommendations of scientists and international scientific committees since the early 1960 s a formal proposal with descriptions of several alternative drill sites was presented in 1975 working group on deep crustal drilling in iceland 1975 but difficulties in financing a drill hole that would penetrate into crustal layer 3 vp 6 5 km s delayed implementation of the recommendations

the use of potential field data in revealing the basement structure in sub basaltic settings an example from the more margin offshore norway this study investigates the utility of the potential fields gravity and magnetics in volcanic settings as observed on the more margin synthetic models are used to investigate the effect of volcanics on the gravity and magnetic fields the focus is on detecting sub basaltic basement structures the methods applied to the models are euler deconvolution on magnetic data gravity gradients and integrated 3d gravity and magnetic forward modelling the same methods are used on the more margin and the results compared to the synthetic models the euler deconvolution on the magnetic signal does provide limited depth solutions in the volcanic environment and the use of different observation levels does not enhance the results forward gravity and magnetic models provide a valuable tool to estimate both the basalt and sub basaltic sedimentary thickness but are limited by the ambiguity inherent in potential field methods the use of gravity gradients significantly decreases the available model solutions and provides boundary detection even in sub basaltic settings

elastic wave velocities within oceanic layer 2 from sonic full waveform logs in deep sea drilling project holes 395a 418a and 504b multichannel full waveform acoustic logs were recorded in deep sea drilling project dsdp hole 418a during ocean drilling program odp leg 102 dsdp hole 395a during odp leg 109 and dsdp hole 504b during odp leg 111 to provide nearly continuous measurements of elastic wave velocities as a function of depth within oceanic layer 2 for different spreading rates and crustal ages the velocities depend primarily on the morphology of the basalt massive units have vp above 5 km s vs above 2 8 km s and vp vs below 1 9 their velocities increase with depth as expected from crack closure due to confining stress and are lower where fractures are present vp and vs within pillows are quite variable but in general are lower than in massive units and vp vs is higher velocities within breccias are similar to or lower than those of pillows within a given morphology the elastic properties do not depend on spreading rate within pillows velocity variations depend primarily on the degree of alteration infilling and the mineralogy of the infilling material and secondarily on conditions at emplacement coherent shear arrivals are sometimes not found within shallow pillows and in extreme cases the compressional arrival is also incoherent because of scattering resulting from the similarity between the sonic wavelength and the pillow size this sampling bias in the sonic log is the most likely cause of somewhat higher average sonic than seismic velocities measured within extrusive basalts at shallow depths other causes include the fact that laterally finite massive units may be oversampled by borehole measurements and that voids between pillows and fractures whose size is within an order of magnitude of the sonic wavelength have less effect on sonic than on seismic velocities the crustal section can be separated only roughly into seismic layers on the basis of the sonic velocities changes in velocity with depth within the extrusive section at a single site giving rise to previously defined seismic layers 2a and 2b are due to a change with depth in the properties of the pillows associated with differences in the amount and type of infilling materials rather than to a change with depth in the relative proportion of pillows and massive units because alteration occurs early and its history is different at different depths depth dependent variations in pillow properties are 1 established at young age and 2 persist even in old crust within the intrusive section of dsdp hole 504b velocities are similar to or higher than those measured in saturated minicores at atmospheric pressure and seismic and sonic velocities are approximately equal suggesting that little large scale porosity is present

seismic attenuation of atlantic margin basalts observations and modeling paleogene basalts are present over much of the northeastern atlantic european margin in regions containing significant thicknesses of layered basalt flows conducting seismic imaging within and beneath the volcanic section has proven difficult largely because the basalts severely attenuate and scatter seismic energy we use data from a vertical seismic profile vsp from well 164 07 1 that penetrated 1 2 km of basalt in the northern rockall trough west of britain to measure the seismic attenuation caused by the in situ basalts the effective quality factor qeff of the basalt layer is found from the vsp to be 15 35 which is considerably lower more attenuative than the intrinsic attenuation measured on basalt samples in the laboratory we then run synthetic seismogram models to investigate the likely cause of the attenuation full waveform 1d modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the vsp can b

the development of volcanic sequences at rifted margins new insights from the structure and morphology of the voring escarpment mid norwegian margin on the voring margin offshore mid norway paleogene continental breakup was characterized by the extrusion of large volumes of flood basalts erupted in different depositional environments the transition from subaerial to submarine emplacement environment is marked by the formation of the voring escarpment which records the early encroachment of flood basalt into the basin and the buildup of a lava delta system the increased availability of new and reprocessed high quality seismic data allows a more detailed characterization of the along strike and across strike continuity and variability of the different volcanic seismic facies units detailed seismic interpretation shows that the 350 km long ne sw trending voring escarpment is a prominent feature along the voring margin with a height ranging between 200 and 1600 m structurally the voring escarpment is segmented along strike into five segments e1 e5 with different controlling factors leading to variation in accommodation space relative sea level change and magma supply are the major controlling factors for segments e2 and e4 which are characterized by a well developed lava delta system and significant escarpment height tectonic movements along the jan mayen fracture zone resulted in second order segmentation of the e1 segment into pseudoescarpments with a very thin lava delta system and limited escarpment height segments e3 and e5 situated along the flanks of cretaceous paleocene highs are controlled by the structural highs which were possibly reactivated during breakup time our mapping results provide crucial information about the paleogeography and yield important information regarding the paleo water depth and depocenter locations prior to and during the breakup of the voring margin

microcrack studies of basalts from the iceland research drilling project microcavities in basaltic core from the irdp borehole in reydarfjordur iceland can be characterized by using direct observations with the scanning electron microscope sem and inversion of velocity data in the iceland basalts neither method provides an accurate complete picture of the crack aspect ratio spectrum but by using both sem and inversion data a complete spectrum may be obtained sem observations provide information on high aspect ratio pores and cracks 0 005 that cannot be separated by velocity inversion theory while velocity inversion theory provides information on low aspect ratio cracks 0 005 that are too narrow to be seen with the sem alteration rims along cracks blunt ends and aspect ratios which imply closure pressures greater than the vertical stress all provide evidence that cracks are open in situ cracks coincident with grain boundaries are abundant in the basalt flows and suggest an origin early in the rock s history probably during cooling crack healing and alteration are observed to increase with depth causing a shift in crack shape distribution toward higher aspect ratios

density and porosity logging in the irdp hole iceland the results from the gamma gamma density log and the neutron neutron porosity log in the continuously cored irdp hole are presented as the core recovery was 99 7 there is an unique opportunity to compare the response to logging tools in basaltic environment to laboratory measurements on the core samples special calibration pits for density calibration were constructed and the logging data for densities were found to be in an agreement with laboratory measurements on core samples for porosity the laboratory data were found to be lower than those obtained from the logs the reason for this was found to be that effective porosity as measured in the laboratory is generally significantly lower in basalt than the total porosity measured by the logs the mean deviation in the repeatability of the density is 160 kg m3 and for the apparent limestone porosity index 3 a correlation coefficient of 0 71 is found between the matrix density and the concentration of sio2 in the rocks and a correlation coefficient of 0 76 is found between the matrix density and the gamma ray intensity of the rocks it is proposed that the presence of zeolites and other secondary minerals cause the observed systematic scatter in the density porosity relation within individual units in the hole owing to a lower porosity the mean bulk density of the intrusions is higher than the mean bulk density of the flows in contrast the matrix densities of flows and dikes are shown to be equal within limits of error the average density of the 1900 m thick section logged is 2 85 0 27 103 kg m3 for the bulk density and 3 03 0 20 103 kg m3 for the matrix density the average porosity is 9 significant variations in the matrix density with depth are shown to be associated with three phases in the volcanic activity building up to basaltic pile in reydarfjordur the matrix density obtained from the logs in the irdp hole is consistent with other published determinations of the matrix density of the oceanic crust

effect of compositional and structural variations on log responses of igneous and metamorphic rocks i mafic rocks well logging data provide continuous information about the physical in situ properties of the borehole wall it has become a standard method in oil industry for the investigation of subsurface geology interpretation techniques thus have been mainly developed for the use in sedimentary rocks and log responses of sediments are well known and calibrated to core data however this is not the case for igneous and metamorphic rocks we present a compilation of log responses of mafic rocks from different drill holes into oceanic and continental basement including holes drilled by deep sea drilling project dsdp and ocean drilling program odp and boreholes on land german continental deep drilling project ktb germany sancerre couy france hsdp usa these holes cover a variety of mafic rocks mid ocean ridge basalt morb flood basalt from large igneous provinces lip basalt and andesitic basalt from back arc basins gabbro metagabbro meta ultramafitites and amphibolites metamorphic rocks from continental holes are interpreted to represent ancient oceanic basement rocks

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

on the use of fractal surfaces to understand seismic wave propagation in layered basalt sequences the aim of this study is to better understand how a layered basalt sequence affects the propagation of a seismic wave which has implications for sub basalt seismic imaging this is achieved by the construction of detailed realistic models of basalt sequences using data derived directly from outcrop analogues field data on the surface roughness of basaltic lava flows were captured using terrestrial laser scanning and satellite remote sensing the fractal properties of the surface roughness were derived and it can be shown that the lava flow surface is fractal over length scales up to approximately 2 km the fractal properties were then used to construct synthetic lava flow surfaces using a von karman power spectrum and the resulting surfaces were then stacked to create a synthetic lava flow sequence p wave velocity data were then added and the resulting model was used to generate synthetic seismic data the resulting stacked section shows that the ability to resolve the internal structure of the lava flows is quickly lost due to scattering and attenuation by the basalt pile a further result from generating wide angle data is that the appearance of a lower velocity layer below the basalt sequence may be caused by destructive interference within the basalt itself

deep crustal reflectors under reydarfjordur eastern iceland crustal accretion above the iceland mantle plume summary deep seismic reflection profiling on the faroe iceland ridge experiment fire imaged a sequence of dipping events in the upper to crust beneath the tertiary lava pile in reydarfjordur eastern iceland these events dip towards the spreading centre at which the crust was formed and continue down to 6 s two way time corresponding to depths down to 15 18 km similar crustal reflections have been observed previously within the crust in other areas of iceland although the reflective events located by previous surveys have not been interpreted rigorously in this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava le the synthesized lava pile sections are based on field measurements made in the reydarfjordur area the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions and some intercalated tuffaceous clastic horizons we compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas we conclude that the upper and mid crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface with the main reflections resulting from packets of flows of similar thicknesses the reflectors that we have imaged beneath iceland are similar to those defining the seaward dipping reflectors on the volcanic continental margins and we therefore suggest that the seaward dipping reflections commonly result from packets of lavas with similar distributions of thicknesses the occurrence of lavas down to 15 18 km beneath reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust although the volcanic processes forming the crust in iceland generally lead to advection of extrusive material to mid crustal levels the production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity the advection of frozen lava flows to mid crustal levels contributes to the maintenance of the subsolidus crust that is observed on iceland and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques

the coastal flexure of disko west greenland onshore expression of the oblique reflectors there is a clear analogy between the coastal flexured basalts of the disko island w greenland and the offshore oceanward dipping reflectors otherwise observed in the volcanic margins of the thulean province our tectonic analysis suggests that the coastal flexure is of both syn magmatic and tectonic origin it is contemporaneous with dyke injection parallel to the axis of the flexure and to an increase in the extension across the area during the palaeocene the proposition that the coastal flexure of disko island is syn tectonic and syn magmatic throws new light on its origin we propose that this flexure and by extrapolation part of the plume related seaward dipping reflectors elsewhere may correspond to volcanic layers forming a roll over anticline overlying a continentward dipping detachment

using formation micro imaging wireline logs and onshore analogues to distinguish volcanic lithofacies in boreholes examples from palaeogene successions in the faroe shetland basin ne atlantic

interface scattering versus body scattering in subbasalt imaging and application of prestack wave equation datuming the presence of high velocity highly heterogeneous layers such as basalt have a detrimental effect on imaging below those structures internal characteristics of the basalt itself can also degrade the subbasalt image quality in many cases the boundary surfaces of a basalt sequence are not smooth but contain some roughness at a scale similar to the seismic wavelength interface scattering resulting from interface roughness can be a serious problem in this paper body and interface scattering problems are addressed through acoustic finite difference modeling interface scattering seems to have the most detrimental effect on imaging at depth a wave equation prestack datuming procedure is applied to synthetic acoustic data to remove the effects of rough interfaces this computational process transforms seismic data to a new datum plane in the subsurface eliminating propagation effects between the surface and the new datum plane the technique leads to a much improved image of below the basalt with the g

evidence for textural and alteration changes in basaltic lava flows using variations in rock magnetic properties odp leg 183 the different effects of cooling and alteration on magnetic properties in single thick flows from subaerial and submarine eruptions cored and logged during ocean drilling program odp leg 183 at sites 1137 and 1140 kerguelen plateau are examined downhole logging data from both sites is supplemented by petrology and geochemistry of 32 samples from three subaerial lava flows at site 1137 and two flows units at site 1140 covering transects from fresh to highly altered basalts changes in magnetic properties have previously been observed in several odp drill holes which penetrate basaltic basement in subaerial basalts a typical trend is that of high magnetic susceptibility and natural remanent magnetization nrm values in the altered flow top and lower values in the less altered massive flow interior in contrast submarine lava flows display the opposite behavior in their magnetic properties altered pillow rims have lower susceptibility and nrm values than the fresh pillow interiors it is concluded that rate of cooling and degree of alteration are the main factors influencing the magnetization and hence the distribution of iron oxides the effects of low temperature alteration are most noticeable in the distribution of more mobile elements such as k consequently the spectral gamma ray sgr log which in basaltic basement is largely controlled by k concentration is an excellent proxy to the downhole identification of alteration the strong positive correlation observed for the subaerial basalts between the downhole total magnetic field ftot and sgr suggest a potential link with alteration in the drilled sections the alteration of the submarine basalts is not as pronounced and therefore no correlation is evident

imaging igneous rocks on the north atlantic rifted continental margin summary extruded basalt flows with thicknesses of several kilometres occur ubiquitously along the rifted continental margins of the northern north atlantic their total volume exceeds 1 million km 3 and may reach several million cubic kilometres intruded igneous rock comprising the complementary melt fraction to that extruded at the surface should exist in the lower crust with a volume at least as large as that of the extrusive basalts to image the extrusive and intrusive igneous rocks on the continental margin near the faroe islands in 2002 we shot a 375 km long deep penetration seismic profile across the margin using 85 ocean bottom seismometers for wide angle acquisition and shot a separate reflection profile using a 12 km streamer for optimal imaging by using large airgun sources tuned to produce low frequency energy we were able to constrain the seismic velocity structure of the whole crust we imaged not only layering and structure within the extrusive basalts but also lower crustal intrusions on the continent ocean transition cot combination of good velocity control on the lower crust together with direct imaging of the extrusive basalts enables us to constrain the volumes of extrusive and intrusive igneous rocks produced near the faroe islands to be 340 420 and 560 780 km 3 per kilometre along strike respectively the cot marked both by high velocity intrusions and lower crustal layering is surprisingly narrow with only 50 km separating unstretched continental crust from fully oceanic crust in contrast the extruded basalts flow up to 150 km landward at the paleo surface

seismic characterization of basalt flows from the faroes margin and the faroe shetland basin high quality seismic profiles across the faroes volcanic continental margin and the adjacent continental shelf show contrasting styles of basalt emplacement across two major structural boundaries at the rifted continental margin the basalts form prominent seaward dipping reflector sequences up to 5 km thick the same basalts after they had travelled in a landward direction across the elevated subaerial faroes shelf produced prominent landward dipping foresets where they flowed across the paleo coastline into the deep water of the faroe shetland basin in both cases we are able to image details of the internal structure of the basalts as a result of using a low frequency airgun source capable of penetrating through the basalt sequence together with long offset recordings from ocean bottom seismometers and a 12 km long deep towed hydrophone streamer

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

failure to predict igneous rocks encountered during exploration of sedimentary basins a case study of the bass basin southeastern australia abstract the bass basin a mesozoic cenozoic intra continental rift basin along the southern australian continental shelf offers an excellent natural laboratory for examining igneous rocks in the subsurface igneous material within the basin is manifested as a mixture of predominately mafic extrusive and intrusive rocks mainly cretaceous palaeocene and oligocene miocene in age igneous rocks have been encountered in 20 out of 36 55 6 exploration wells drilled within the bass basin but the presence of these has historically been poorly predicted of the first 11 exploration wells to penetrate igneous rocks their presence was not predicted in pre drill interpretations we present a series of case studies from wells that unexpectedly encountered igneous rocks the first of these wells bass 1 targeted a carbonate reef structure which instead penetrated the flank of a submarine volcano in another notable example flinders 1 well a relatively discontinuous high amplitude seismic reflection thought to be a clastic reservoir was found to be an igneous intrusion with a relatively unusual composition a number of these incidents where igneous rocks were unexpectedly encountered can be accounted for by human factors such as a sparsity of good quality data and a lack of knowledge transfer as companies entered and left basin during different phases of exploration however a number of examples particularly the unexpected occurrence of igneous intrusions appear to have been caused by anomalously low acoustic impedance contrasts between igneous rocks and surrounding sedimentary sequences our findings have generic implications for other sedimentary basins impacted by magmatic activity such as the importance of integrating available outcrop data in the absence of nearby well control and the value of fully appraising previous exploration results

influence of cooling on lava flow dynamics experiments have been carried out to determine the effects of cooling on the flow of fluids with strongly temperature dependent viscosity radial viscous gravity currents of warm glucose syrup were erupted at constant rate into a flat tank filled with a cold aqueous solution cold viscous fluid accumulates at the leading edge altering the flow shape and thickness and slowing the spreading the flows attain constant internal temperature distributions and bulk viscosities the value of the bulk viscosity depends on the peclet number which reflects the advective and diffusive heat transport properties of the flow the flow skin viscosity which reflects cooling and the eruption viscosity our results explain why most lava flows have bulk viscosities much higher than the lava eruption viscosity the results can be applied to understanding dynamic lava features such as flow front thickening front avalanches and welded basal breccias

the 3d facies architecture of flood basalt provinces and their internal heterogeneity examples from the palaeogene skye lava field quantifying the facies architecture of flood basalt provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the lava sequence the 3d facies architecture and internal heterogeneity of the skye lava field for example provides important insights into the evolution of the british tertiary igneous province and valuable information to aid in the exploration of potential offshore reservoirs underlying significant flood lavas along the north atlantic margin the volcanic stratigraphy of the talisker bay area of skye comprises 1 lower compound braided lavas flow lobes 3 the transitional sequence 7 4 km 3 and the upper sequence 17 0 km 3 the lower sequence lavas formed on the flanks of a shield volcano and were sourced from the ne volcanological features such as lava feeder tubes pahoehoe textures and lobes indicate a scale of volcanism similar to that of present day hawaii the within flow heterogeneity of the basalts is characterized using an intrafacies scheme allowing comparison of variations in lithofacies with characteristic geophysical rock properties of compressional wave velocity and density

sub surface geology and velocity structure of the krafla high temperature geothermal field iceland integrated ditch cuttings wireline and zero offset vertical seismic profile analysis abstract the krafla geothermal area in northern iceland comprises one of the best studied examples of a high temperature geothermal field associated with an active volcanic rift zone of key importance to improved resource exploration and development in volcanic areas such as krafla is the interpretation and understanding of the subsurface geology within this study we present results from an integrated analysis of the downhole volcanic stratigraphy from the k 18 borehole within the krafla caldera based on combined wireline ditch cuttings and zero offset vsp vertical seismic profile analyses this study presents the first published sonic log velocity data from a high temperature geothermal borehole in iceland and clearly demonstrates the importance of borehole velocity data for improving volcanic facies interpretations the shallow subsurface geology of the k 18 site from c 0 380 m comprises an inter layered sequence of sub aerial basaltic lavas hyaloclastites and tuffaceous lithologies of both felsic and basic composition which are progressively replaced by basaltic sheet intrusions with increasing depth an interval of variably basic to more evolved mixed tuffaceous facies is identified based on cuttings analysis gamma and sonic velocities between c 790 1120 m depth discrete high sonic vp units cut the lower c 100 m of this interval and are interpreted as either sheet intrusions or lava interiors at c 1800 m a sharp increase in p wave velocity from c 4 5 to c 5 5 km s represents the transition from a mixed lava and sheet intrusion dominated sequence into a dense basic intrusion forming the local basement that persists to the bottom of the borehole at c 2215 m reduced travel time analysis of processed zero offset vsp results reveal good correspondence with the major stratigraphic boundaries in the borehole supporting the potential for vsp studies to robustly characterize complex volcanic stratigraphy in high temperature geothermal fields finally the application of percentage based ditch cuttings analyses methods for volcanic facies analysis in geothermal boreholes is tested and assessed to have future potential

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

three dimensional lithofacies variations in hyaloclastite deposits analysis of the spatial lithofacies variability within lava fed delta formations in southern iceland has revealed complex three dimensional volcanic architectures in hyaloclastite deposits in non glacial settings two depositional environments are studied a lava entering a marine embayment stori nupur and b lava advancing into a body of water of the flanks of a surtseyan cone hjorleifshof i interaction between environmental factors such as shoreline geomorphology water depth wave energy levels the nature of the lava transport system lava supply rate all affect the resulting lava deltas creating complex lithofacies arrangements and stacking patterns recognised here are two types of hyaloclastite deltas one of syn sedimentary origin hjorleifshof i and one derived from primary fragmentation processes stori nupur syn sedimentary systems are dominated by destabilisation of the hyaloclastite pile leading to reworking downslope and share similarities to alluvial delta systems conversely primary fragmented systems are controlled by waxing and waning cycles in volcanic effusivity whereby the hyaloclastite unit recorded is not the product of one lava flow rather than one eruptive event

lithology and structure of the volcanic sequence in eastern iceland the reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99 7 subaerial lava flows comprise 54 of the core dikes 41 and clastic rocks about 5 in addition a 1 3 km thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled the lava flows are chiefly low magnesia basalts and ferrobasalts with lesser amounts of icelandite basaltic andesite and olivine tholeiite in the drill core complete flows range from 1 2 to 19 7 m in vertical thickness and average 7 1 m in the exposed section flow thicknesses range from 1 to 30 m and average 5 2 m most of the flows are characterized by an upper scoriaceous zone 1 3 meters thick grading downward through a zone of fractured basalt into a massive flow interior some flows also have a thin basal breccia olivine tholeiites and some low magnesia basalts are commonly mottled whereas ferrobasalts and icelandites have well developed flow banding most of the flows are fine to medium grained aphyric to sparsely phyric and moderately vesicular a few are moderately plagioclase phyric others have scattered phenocrysts of clinopyroxene and olivine over 100 intrusive units are recognized in the core and they span approximately the same compositional range as the flows they are characterized by having steeply inclined chilled contacts a uniform massive aspect and an absence of vesicles or brecciation most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures many of the dikes are also aphyric but some contain notable plagioclase or olivine phenocrysts groundmass textures range from fine grained intersertal to coarse grained ophitic clastic units occur typically as thin varicolored layers between lava flows rarely exceeding 0 5 m in thickness these are chiefly fine to coarse grained poorly to well bedded lithic tuffs showing varying degrees of reworking most are silicic to intermediate in composition but basaltic units are also present two rhyolitic ash flow tuffs occur in the section one of which is 30 m thick we interpret the lavas to be the flanking portions of broad low angled central volcanoes centered on silicic complexes some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms most of the dikes were probably intruded laterally into the lava pile thus they do not represent direct vertical feeders from deep magma bodies

geochemistry of rare elements in waters and sediments of alkaline lakes in the sasykkul depression east pamirs abstract the saline lakes tuzkul chukurkul sasykkul lakes of the sasykkul depression located in the semidesert part of the pamirs provide a prominent example of the evaporative concentration of alkaline waters in a closed basin under arid climate conditions chemical evolution of the lake waters has resulted in the development of residual na cl co3 so4 and na cl co3 brines the contents of a number of rare elements were determined in waters and sediments of the sasykkul depression lakes speciation modelling suggests that the main aqueous species of ag are chloride complexes cu pb zn mn are carbonate complexes be bi nb are hydroxocomplexes mo w as v are oxyanions it was established that the salt lake sediments are significantly enriched in b ba mo w u th high contents of trace elements are considered to be due to chemical weathering of rocks within the catchment basin significant contribution from thermal springs derived from the residual fluids associated with acid igneous and volcanic rocks and following evapoconcentration in a closed basin the processes of evaporative concentration solution complexation chemical precipitation and water sediment interaction lead to the partitioning of trace metals in the salt sediments

seismic properties density and composition of the icelandic crust near reydarfjordur compressional and shear wave velocities densities and porosities have been measured to confining pressures of 6 kbar for 135 samples from a deep borehole in eastern iceland the measurements are from a random sampling of dikes and sills through the 2 km hole velocities within the flow units are relatively low at the tops reach maximum values within central portions and decrease near the bases this systematic variation correlates well with variations in porosity and bulk density the dike units on the other hand have relatively uniform velocities which are approximately equivalent to maximum velocities measured within flow units below 700 m the dike and flow velocities show systematic increases with depth which are related to changes in mineralogy and independent of porosity the velocity gradient dvp dz t within this region is well defined by the laboratory velocities and approximates 0 6 s 1 in good agreement with field refraction measurements high velocities near the base of the hole are related to increasing alteration with depth and in particular an increase in epidote content the abundance of epidote in the rocks recovered from the lower portion of the drillhole supports a metamorphic origin for the layer 2 layer 3 boundary in this region

the rattray volcanics mid jurassic fissure volcanism in the uk central north sea the middle jurassic rattray volcanic province is located at the triple junction of the north sea continental rift system it has previously been thought to be sourced from three large central volcanoes the glenn fisher bank and ivanhoe volcanic centres re interpretation using 3d seismic and well data shows that no volcanic centres are present and the rattray volcanics were instead sourced in fissure eruptions from linear vents including the buchan glenn fissure system a c 25 km long zone of wsw ene striking linear fissure vents and associated small volcanic edifices across the buchan glenn horst the orientation of the fissures is broadly parallel to the highland boundary fault which intersects the rattray volcanics at the buchan glenn fissure system implying that mid jurassic magmatism exploited pre existing crustal structural anisotropies established during the caledonian orogeny the lack of large intrusive complexes beneath the rattray volcanics indicates that the pre middle jurassic sedimentary sequences e g the devonian carboniferous old red sandstone group the permian rotliegend and zechstein groups and the triassic skagerrak formation extend further than previously supposed and therefore the presence of possible subvolcanic reservoir and source rock units within the triple junction of the central north sea may have been overlooked

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

eocene volcanism in offshore southern baffin bay new data are presented on the offshore part of the west greenland volcanic succession part of the north atlantic igneous province located in southern baffin bay the data were obtained from the delta 1 hydrocarbon exploration well drilled by cairn energy in 2011 wireline log data were collected over the drilled section providing information on the physical volcanology of the volcanic succession and 23 sidewall cores were analysed for their geochemical composition the well encountered 65 lava flows with a well developed core and crust signature typical for sub aerial eruptions major and trace element data showed a good correlation with parts of the succession onshore west greenland the xrf major and trace element data allowed careful picking of samples for 40ar 39ar dating and the main part of the sequence was found to be early eocene in age this coincides with the second main phase of volcanism in onshore west greenland and is likely to be linked to the change in the tectonic regime in baffin bay at this time this finding substantially expands the area of west greenland affected by volcanism during the eocene

effect of alteration formation absorption and standoff on the response of the thermal neutron porosity log in gabbros and basalts examples from deep sea drilling project ocean drilling program sites this study focuses on the effects of hydrous alteration minerals formation absorption and standoff on the response of the thermal neutron porosity log in the basaltic and gabbroic rocks logged at sites 395 418 504 642 and 735 during the ocean drilling program the concentration of hydrogen present in the rocks in the form of free water pore space and bound water hydrous minerals is the primary factor controlling the neutron elastic scattering process while the presence of other elements such as chlorine gadolinium boron lithium and samarium in the fluids and in the rock matrix can largely affect the thermal diffusion phase these neutron absorbers cause an increase of the capture cross section and in turn of the apparent thermal porosity further perturbations occur when the recording conditions depart from those under which the tool has been calibrated a large and irregular hole diameter and a lack of eccentralization both produce erroneous porosity readings the effect of hydrous alteration minerals on the thermal neutron porosity log has been estimated from 922 core oxide measurements using an analysis program that calculates the slowing down length and converts it into apparent porosity the results show that the computed apparent porosity ranges from less than 1 in fresh basalts and gabbros to about 30 in highly altered units depending on the alteration mineral assemblage natural gamma ray capture cross section or hydrogen logs have been used to continuously predict the effect of bound hydrogen at each site corrected porosities generally show excellent agreement with core data for massive units whereas they are higher for pillow basalts and fractured zones the discrepancy is interpreted as the result of 1 difference in the volume of rock investigated core specimens do not sample large vugs and fractures and 2 frequent variations in the hole size and lack of tool contact with the borehole wall standoff not completely accounted for in the corrections high capture cross section values measured on selected basalt samples seem to be associated with more altered basalts suggesting a larger concentration of strong neutron absorbers in secondary minerals the concentrations of elements with large capture cross section measured at three of the five sites studied yield a maximum increase of 1 in the apparent neutron porosity this is too low to explain the porosity values some greater than 7 observed in some of the fresh and unfractured massive units a 1 to 2 inch standoff due to the lack of eccentralization of the tool string used in the ocean drilling program is considered the most probable cause of such discrepancy this interpretation is confirmed by the neutron porosity values recorded at site 735 where the nuclear string was for the first time fully eccentralized and only a correction for borehole size and the presence of hydrous minerals has been applied

crustal thickening in an extensional regime application to the mid norwegian voring margin abstract the structure of the mid norwegian volcanic voring margin at the onset of the maastrichtian paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between norway and greenland the most important imprints moho depth thermal structure of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase we focus on a possible mechanism of formation of the voring marginal high and address the relationship between spatial and temporal evolution of crustal thinning and thickening uplift of the surface and strength of the lithosphere it is found that the voring basin formed the strongest part of the margin which explains why the maastrichtian paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations the modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone convergence of material at the particular part of the margin is mainly taken up by the lower crust the model shows that during the final phase of extension the crust in the voring marginal high area was thickened and the surface uplifted it is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the marginal high

crustal structure of the outer voring plateau offshore norway from ocean bottom seismic and gravity data four ocean bottom seismograph obs profiles acquired across the continent ocean transition zone of the outer voring plateau have been modeled by inversion and forward modeling of the obs vertical component traces p waves the thickness of the sedimentary layers deposited since the early eocene continental breakup varies from up to 2 2 km above the basaltic inner flows if to a minimum of 0 2 km about 20 km seaward of the voring escarpment ve with a velocity of 1 6 2 55 km s the thickness of the uppermost layer of basalt varies from 0 5 to 4 5 km and its velocity ranges from 3 4 to 5 km s the modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend 40 km seaward of the ve farther seaward another layer of extrusives intrusives is found beneath the uppermost layer of basalt the lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent ocean transition extends over a 30 50 km wide zone in between strike profile 3 oceanic 6 6 7 35 km s and strike profile 4 continental 6 1 7 0 km s an up to 7 km thick lower crustal high velocity body 7 1 7 4 km s is interpreted in terms of intrusions in the lower crust beneath and landward of the continent ocean transition the total thickness of the normal oceanic crust is estimated to be 7 9 km the anomalously thick oceanic crust reaches a maximum thickness of about 25 km and the crustal thickness is estimated to be 17 22 km beneath the if gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

seismic wave attenuation in volcanic rocks from vsp experiments seismic wave attenuation in the columbia plateau basalts and snake river plain volcanics was analyzed using vertical seismic profiling vsp data the computation of attenuation coefficients is based on fitting a straight line to the logarithm of amplitude ratios computed for fixed values of frequency and variable depth this approach does not require any assumptions on the dependence of q on frequency for the columbia plateau basalts the attenuation coefficients obtained from the field data are smaller than those computed from the synthetic vsp generated using the sonic and density logs indicating that the observed attenuation is related to scattering effects and is substantially larger than the intrinsic attenuation of basalt therefore it is concluded that only a lower bound for q can be established in agreement with recent findings by other authors the effective attenuation of seismic energy in basalts about 1 1 10 4dbs m for the peak frequency is comparable to the effective attenuation observ

u s mid atlantic margin structure and early thermal evolution marine seismic experiments have revealed that the u s east coast rifted margin crustal structure is dominated by voluminous volcanics emplaced during continental breakup we present results from seismic data recorded onshore during the 1990 edge mid atlantic onshore offshore seismic experiment that extend the margin crustal profile westward 180 km across the coastal plain and place the offshore results into a broader tectonic context the onshore offshore crustal model defines the crustal thinning profile across the margin enabling calculation of total extension based on direct measurements of crustal thickness we find that the crust beneath the coastal plain is largely unextended has a uniform thickness of 35 km shows no evidence for magmatic additions of sufficient volume to affect seismic velocity and that the total half extension of the crust is 30 to 40 km less than previous estimates by a factor of 3 to 4 we also present results of depth migration of the offshore vertical incidence and wide angle seismic data that yield the most complete and accurate image to date of reflectivity across the u s east coast margin we use this image to analyze margin subsidence by exploiting the stratigraphy of the subaerially extruded basalts beneath the sediments we find that 1 margin subsidence is consistent with a petrogenetic model for initial seafloor spreading magmatism involving high average melting pressures 3 5 4 0 gpa and potential temperatures 1500 c for a present day thermal lid thickness of 225 km 2 it is difficult to explain margin subsidence with a present day lid thickness of much less than 150 km and 3 rapid subsidence following breakup suggests the initially hot mantle underlying the rifting margin cooled very quickly implying a limited and finite initial distribution of hot mantle rather than a continuous upwelling or a ubiquitously warm upper mantle

seismic volcanostratigraphy of the norwegian margin constraints on tectonomagmatic break up processes voluminous volcanism characterized early tertiary continental break up on the mid norwegian continental margin the distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the more voring and lofoten vesteralen margins into five segments the central more margin and the northern voring margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins the lofoten vesteralen margin the southern voring margin and the area near the jan mayen fracture zone show volcanic seismic facies units that are related to small volume submarine volcanism the distribution of subaerial and submarine deposits indicates variations of subsidence along the margin vertical movements on the mid norwegian margin were primarily controlled by the amount of magmatic crustal thickening because both the amount of dynamic uplift by the icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin thus subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence whereas submarine deposits indicate little magmatic thickening and earlier subsidence from the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening 1 a general northward decrease of magmatism due to increasing distance from the hot spot and 2 subdued volcanism near the jan mayen fracture zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region furthermore we interpret small lateral variations in the distribution of volcanic seismic facies units such as two sets of inner seaward dipping reflectors on the central voring margin as indications of crustal fragmentation

sub basalt seismic imaging using optical to acoustic model building and wave equation datuming processing the presence of highly heterogeneous basalt layers has a detrimental effect on seismic imaging of underlying structures due to the prevalence of basalt the challenge of imaging below basalts impinges on a broad spectrum of geological problems from hydrocarbon exploration to volcano seismology both internal basalt characteristics and rough interfaces can degrade the sub basalt image quality in this paper we investigate the effects of both internal and surface heterogeneity on the wavefield using highly heterogeneous models of basalts derived from optical imagery and borehole sonic logs combined with wave simulations to generate synthetic data we explore the causes of poor sub basalt imagery we establish the significant role that surface roughness plays in the imagery problem and demonstrate through a novel application of wave equation datuming how to suppress its effect the methodology demonstrated herein may be applicable in any environment where heterogeneous surface structures interfaces severely degrade seismic image quality

the morphology of lava flows in planetary environments predictions from analog experiments the rates of surface cooling and lateral spreading are evaluated for lava flows on the surface of earth venus mars the moon and io differences between the flow morphologies expected in these environments are then predicted under the assumption that the results of recent laboratory simulations of lavas using wax extruded beneath cold water fink and griffiths 1990 can be carried over these experiments involved the spreading of viscous liquid under gravity in the presence of a solidifying surface crust and revealed a set of four distinct surface morphologies transitions from one morphology to the next occurred in a well defined sequence when the relative rates of surface solidification and lateral spreading were varied in comparison with subaerial flows on earth the surface of lavas solidifies faster on venus where the dense atmosphere provides additional convective heat transfer lateral flow of lava is much slower under the smaller gravity of the moon hence for a given extrusion rate and viscosity solid crust is predicted to form closer to the vent on both the moon and venus than on earth equivalently faster extrusion rates than those on earth are required on the moon io and venus in order to produce a given surface morphology a comparison of observed structure flow rates and estimated viscosities for some well documented terrestrial lava flows shows reasonable agreement with the predictions of the model extrapolation to extraterrestrial lavas is then illustrated by constraining the effusion rate for a martian lava flow whose composition is assumed spectral information on the chemical composition of martian flows to be sought by the thermal emission spectrometer on the mars observer mission in conjunction with detailed morphologic observations from the mars observer camera could allow much more accurate estimates of effusion rates for martian lavas

intrabasement reflections of the ontong java plateau implications for plateau construction 1 the early cretaceous ontong java plateau ojp in the western equatorial pacific earth s most voluminous large igneous province lip is characterized by intrabasement seismic reflections to investigate the nature of such reflections we have analyzed new and all older multichannel seismic mcs reflection data from the ojp using an instantaneous phase velocity analysis technique and synthetic seismograms intrabasement reflections are most prevalent on the main ojp especially on its crest on mcs data the reflections 1 are semicontinuous and subparallel to the top of igneous basement 2 in places have the opposite phase of seafloor reflections and 3 have an average frequency of 20 hz we calculate synthetic seismograms using impedance contrasts between massive lava and pillow lava flows obtained from downhole logs at ojp scientific drill sites and show that these lithologies can produce intrabasement reflections similar to those observed in the mcs data alternatively to evaluate the possibility of sediment sedimentary rock interbeds causing the intrabasement reflections we use published early cretaceous sedimentation rates determined from ojp drill sites and published estimates for the duration of massive ojp volcanism to calculate a range of possible sedimentary interbed thicknesses the range proves to be below the detection limit of the mcs data therefore we conclude that the intrabasement reflections arise from alternating relatively thin lava flows produced at low effusion rates and relatively thick massive flows generated at high effusion rates both originating from vents and fissures on the ojp probably on its crest

introduction to special section on logging and downhole measurements in deep sea drilling project ocean drilling program deep crustal holes one of the most important achievements during the first two years of the ocean drilling program odp was the concerted effort to conduct major programs of logging and downhole measurements upon revisiting three of the deepest holes drilled into the oceanic crust by the deep sea drilling project dsdp this effort established holes 395a 418a and 504b table 1 as our best reference sites to ground truth the structure and physical properties of the upper oceanic crust the results of logging and downhole measurements at these and other odp holes and their relationship to regional geophysical studies were the subject of a poster symposium at the agu 1988 fall meeting from which this special section developed although it was not the intent to restrict this special section to results from holes 395a 418a and 504b all of the papers here deal with at least one of these holes this emphasizes their importance as reference sites which arises not only because of the extensive downhole measurements but simply because they are among the four deepest dsdp odp penetrations into genuine upper oceanic crust i e crust formed at a mid ocean ridge figure 1 holes 395a and 418a were drilled in north atlantic crust formed at a slow spreading rate and 504b was drilled in eastern pacific crust formed at a moderate spreading rate thus the set of reference sites lacks deep holes in crust formed at fast spreading rates which is a serious shortcoming that will hopefully be addressed by upcoming odp drilling in the pacific ocean the set of deep reference holes has already been complemented by the drilling and logging success in 500 m of tectonically exposed gabbros in hole 735b during leg 118 to the southwest indian ocean in 1987 robinson et al 1989 however interpretation of the logging results from hole 735b had not progressed enough to be included here and will appear first in the work by robinson et al 1990

lithospheric structure of the costa rican isthmus effects of subduction zone magmatism on an oceanic plateau we present the results of a multidisciplinary geophysical study conducted to investigate the lithospheric structure of the costa rican isthmus the physical properties of the lithosphere are resolved by three dimensional 3 d simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2 d forward modeling of onshore and offshore seismic refraction and gravity data according to our results the northern half of the costa rican isthmus is constituted by a 40 km thick crust with a 6 to 7 km thick oceanic crust subducting under it the uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities in good agreement with those described for flood basalts the midlevel shows velocities and densities representative of oceanic crust the bottommost level 20 40 km shows high velocities and densities typical of mafic rocks and the upper mantle displays anomalously low densities and velocities intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies these results are consistent with a basement beneath the costa rican isthmus being part of the caribbean plateau originated at 85 90 ma with the onset of the galapagos hotspot the upper level corresponds to the flood basalts extruded during this phase and it includes most of the marginal wedge the second level represents the preexisting oceanic crust the mafic lower crust intracrustal heterogeneities and anomalous upper mantle are interpreted to be built up by underplating intrusion and crystallization of basaltic melts formed under the influence of subducting lithosphere dehydration

high velocity breakup related sills in the voring basin off norway multichannel seismic reflection profiles in the hel graben v0ring basin reveal a sill complex at approximately 5 km depth it is associated with exceptionally high 7 4 km s 1 seismic wide angle velocities the existence of observable wide angle arrivals shows that the sills act as efficient waveguides seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m sonic logs from sills of similar thickness on the nearby utgard high show an average velocity of 7 0 km s 1 such high velocities require an olivine gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes a possible reason for such an emplacement environment is the hei graben s role as an intrusion center during breakup volcanism this would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex affecting depth conversion and gravity modeling furthermore the results question the concept of lower crustal bodies as large scale homogeneous accumulations of mafic melt