late pleistocene fluvial dynamics in the hochrhein valley and in the upper rhine graben chronological frame during the pleistocene the rhine glacier system acted as a major south north erosion and transport medium from the swiss alps into the upper rhine graben which has been the main sediment sink forming low angle debris fans only some aggradation resulted in the formation of terraces optically stimulated luminescence osl and radiocarbon dating have been applied to set up a more reliable chronological frame of late pleistocene and holocene fluvial activity in the western hochrhein valley and in the southern part of the upper rhine graben the stratigraphically oldest deposits exposed a braided river facies yielded osl age estimates ranging from 59 6 6 2 to 33 1 3 0 ka the data set does not enable to distinguish between a linear age increase triggered by a continuous autocyclical aggradation or two or more age clusters for example around 35 ka and around 55 ka triggered by climate change including stadial and interstadial periods sensu dansgaard oeschger cycles the braided river facies is discontinuously hiatus covered by coarse grained gravel rich sediments deposited most likely during a single event or short time period of major melt water discharge postdating the last glacial maximum osl age estimates of fluvial and aeolian sediments from the above coarse grained sediment layer are between 16 4 0 8 and 10 6 0 5 ka and make a correlation with the late glacial period very likely the youngest fluvial aggradation period correlates to the beginning of the little ice age as confirmed by osl and radiocarbon ages

irregular trends of physical properties in homogeneous clay rich sediments of dsdp leg 87 hole 584 midslope terrace in the japan trench abstract compressional velocity v p attenuation in terms of its inverse the compressional quality factor q p electrical resistivity bulk b and grain g densities porosity o and bulk mineralogy were measured on diatomaceous hemipelagic sediments from dsdp leg 87 hole 584 although the sediment lithology and the bulk mineralogy determined by x ray diffraction are relatively homogeneous the physical acoustic and electrical properties of the sediments show irregular trends with depth with variations in the magnitude and sign of property gradients between subbottom depths of 500 and 800 m v p b g and all sharply increase and then sharply decrease while porosity sharply decreases and then sharply increases the changes in gradients are due to abrupt changes in the opal a content of the sediment as determined from smear slide data q p decreases with depth to about 650 m and then increases such a trend has not been previously documented in laboratory measurements however it is assumed that the attenuation maximum observed here is due to the increased grain density and decreased porosity caused by the reduction in opal a content around this depth the anisotropies for v p and q p undergo sign inversions with depth the v p anisotropies change from positive v ph v pv at the top to negative at the bottom of the hole whereas q p anisotropies change from negative q pv q ph to positive the change in sign with depth is caused by a change in sediment bedding dip from near 0 at the top of the hole to about 65 toward the bottom

luminescence dating of glacial deposits near the eastern himalayan syntaxis using different grain size fractions abstract numerical dating of glacial deposits is important for understanding quaternary glacial evolution optically stimulated luminescence osl dating is one of the techniques widely used on such sediments owing to the short distances traveled before deposition the incomplete bleaching of luminescence signals in glacial sediments may introduce serious dating problems vis a vis glacial and any associated sediments here we report a comparison of osl ages obtained from the fine 4 11 m and medium 38 63 m grain size fractions of quartz extracted from glaciofluvial sediments and from glacial tills in the basongcuo catchment near the eastern himalayan syntaxis initially four glacial stages were identified based on field observations of moraine distribution and geomorpho stratigraphic relations a total of 39 osl samples were then collected from glaciofluvial sand layers or lenses and from till quartz grains in the fine 4 11 m and medium 38 63 m size fractions were extracted from each sample and dated using the single aliquot regeneration sar protocol the modern supraglacial sediment sample was dated to 0 2 fine grain to 0 7 medium grain ka suggesting that the sediment was not completely bleached on deposition contrary to previous experience suggesting that coarse grains are usually better bleached than fine grains prior to deposition our results show that estimated osl ages for fine grains are generally younger than those for medium grains this suggests that the two fractions may have come from different sources and thus have different bleaching histories and that fine grained quartz may be more suitable for osl dating of these materials applying the minimum age model to data from medium grained quartz yields ages close to those obtained from fine grained quartz suggesting that both can be used for dating glacial advances the osl dates suggest that glaciers in the studied area advanced at 0 1 1 3 ka 7 5 ka and 11 13 ka and were furthest advanced between 16 and 30 ka the timing of the local last glacial maximum lgm is broadly consistent throughout the eastern and central himalaya and frequent glacial advances after the lgm all suggest that glaciers in the humid eastern himalayan syntaxis are sensitive to global cooling

optical dating using infrared diodes young samples abstract ages are reported for unburnt sediment samples from 9 archaeological sites in england spanning 1000 5000 years samples from two modern contexts have also been measured

tremor genic slow slip regions may be deeper and warmer and may slip slower than non tremor genic regions slow slip events sses are observed worldwide and often coincide with tectonic tremor notable examples of sses lacking observed tectonic tremor however occur beneath k lauea volcano hawaii the boso peninsula japan near san juan bautista on the san andreas fault california and recently in central ecuador these sses are similar to other worldwide sses in many ways e g size or duration but lack the concurrent tectonic tremor observed elsewhere instead they trigger swarms of regular earthquakes we investigate the physical conditions that may distinguish these non tremor genic sses from those associated with tectonic tremor including slip velocity pressure temperature fluids and fault asperities although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions slip velocities of sses at k lauea volcano 10 6 m s and boso peninsula 10 7 m s are among the fastest sses worldwide k lauea volcano the boso peninsula and central ecuador are also among the shallowest sses worldwide and thus have lower confining pressures and cooler temperatures in their respective slow slip zones fluids also likely contribute to tremor generation and no corresponding zone of high vp vs has been noted at k lauea or boso we suggest that the relatively faster slip velocities at k lauea volcano and the boso peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip while different conditions produce slower sse velocities elsewhere and trigger tectonic tremor

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

observations on attenuation and shear wave velocity in fine grained marine sediments compressional p wave attenuation shear s wave attenuation and shear s wave velocity measurements compiled for fine grained unconsolidated sediments show trends that do not support empirical relationships hamilton j acoust soc am 68 1313 1340 1980 in acoustics and ocean bottom f a s e specialize conf madrid 1987 commonly used for geoacoustic modeling the exception is compressional wave attenuation data from 10 hz to 500 khz that essentially follow a frequency dependence of f1 2 standard deviations p wave attenuation with depth is difficult if not impossible to predict given the variability in the data examination of several attenuation depth profiles indicates that those of mitchell and focke j acoust soc am 67 1582 1589 1980 may be the best choice for predicting p wave attenuation with depth at the present time the assumption that s wave and p wave attenuations are proportional hamilton j acoust soc am 60 334 338 1976c in acoustics and ocean bottom

crust and upper mantle q from seismic refraction data peace river region crustal refraction data from the peace river region of alberta canada have been analyzed using the spectral ratio method to obtain q a total of 1205 first and later arrivals corresponding to turning and reflected p waves within the crust and upper mantle were studied source spectra were estimated from near offset traces assuming typical sedimentary q values the large scatter of measured spectral ratios restricted the resolution to a three layer model of the crust and upper mantle with q constant in each layer this model was obtained using a linear inverse method since the measured spectral ratios and known traveltimes in each layer are linearly related through the attentuation q 1 in each layer a weighted l1 norm was minimized using linear programming the weights being a measure of the certainty of each spectral ratio the inversion was performed using the 25 most certain spectral ratios regardless of magnitude or sign model bounds taking account of the scattered data were estimated the resu

the first direct measurements of upper oceanic crustal compressional wave attenuation the first direct measurement of compressional wave attenuation of the uppermost 650 m of oceanic crust was performed using data recorded by seafloor hydrophones and large 56 116 kg deep explosive sources the site was 13 km east of the southernmost juan de fuca ridge on crust 0 4 m y old spectral ratios were performed between bottom refracting waves and direct water waves adjusted for spreading losses and transmission coefficient losses several tests of the data were performed demonstrating that attenuation is linearly related to frequency between 15 and 140 hz but frequency independent components of attenuation are also evident values of compressional wave q cluster between 20 and 50 and do not show any systematic variation with depth over 650 m the attenuation results also indicate the presence of heterogeneities within the crust as the solutions for each receiver s data set are significantly different no evidence for azimuthal variations of attenuation are supported by the data although the data do not optimally sample a wide variation of azimuths our attenuation values are judged to be normal to higher than expected for the whole oceanic crust based upon comparisons to results from synthetic seismogram modeling by others and by modeling signal to noise ratios of typical seismic refraction profiles the results are consistent with recent laboratory measurements at ultrasonic frequencies for dry and saturated basalts at seafloor pressures and temperatures

margin architecture and seismic attenuation in the central costa rican forearc seismic attenuation across the central costa rican margin wedge is determined fromamplitude analysis ofwideangle seismic data travel time and amplitude modeling are applied to ocean bottom hydrophones along two trench parallel profiles located 30 km p21 and 35 km p18 landward of the deformation front northeast of quepos plateau tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench a 1 1 5 km thick decollement zone with seismic velocities of 3 5 4 5 km s is sandwiched between the marginwedge and the subducting cocos plate for strike line p21 amplitude modeling indicates a qp value of 50 150 for the upper margin wedge with seismic velocities ranging from 3 9 km s to 4 9 km s along strike line p18 qp values of 50 150 are determined with velocities of 4 3 5 0 km s in the upper margin wedge increasing to 5 1 5 4 km s in the lower margin wedge quantitative amplitude decay curves support the observed upper plate qp values in conjunction with earlier results from offshore nicoya peninsula our study documents landward decreasing attenuation across the margin wedge consistent with a change in lithology from the sediment dominated frontal prism to the igneous composition of the forearc middle prism

gravity field of kuwait and its relevance to major geological structures regional gravity surveys were done in the state of kuwait during 1986 1988 the new gravity maps show a good correlation with geomorphological features as well as with deeper geological structures the free air anomaly map clearly reflects the topography of the jal az zor and ahmadi ridges and wadi al batin the bouguer anomaly map is dominated by two prominent gravity highs correlatable with subsurface structural arches the north trending gravity high in eastern kuwait represents the effect of a major structure the kuwait arch along which many important oil fields are located a smaller northwest trending high mapped in western kuwait indicates the presence of a second subsurface arch which in this paper is named the dibdibba arch the two gravity highs are separate by a wedge shaped gravity low presumably caused by thicker sediments of the dibdibba basin magnetic measurements along selected profiles show the two arches to be associated with 100 200 nt nannotesla anomalies apparently reflecting the positive subsurface relief of the crystalline basement along the length of the kuwait arch magnetic data also indicate lateral susceptibility variations possibly related to lithological variations within the basement

aftershock slip behavior of the 1989 loma prieta california earthquake an analysis of 745 aftershocks of the m7 1 loma prieta earthquake of 17 october 1989 reveals a wide variety of focal mechanisms at the northwestern end of the aftershock zone earthquakes that apparently occurred off the main rupture plane exhibit mechanisms with predominantly reverse slip on planes nearly parallel to the san andreas fault at the southeastern end the mechanisms exhibit right lateral motion on near vertical planes suggesting that these aftershocks involve slip on the san andreas fault few of the aftershock mechanisms in the central zone resemble the main shock mechanism strike n130 degree e dip 70 degree sw rake 104 degree but instead exhibit reverse right lateral left lateral and normal motion on planes subparallel to the main shock rupture plane the dip of the aftershock zone is parallel to the main shock slip plane and includes the main shock hypocenter however the lack of agreement between the main shock and the aftershock mechanisms suggests that few of the aftershocks occurred on the main shock slip plane this behavior is consistent with observations of aftershock sequences for other dip slip events and also with studies indicating that main shock rupture zones are at all other times mostly aseismic if the stress drop for the main shock relievedmore most of the tectonic stress the mechanisms could reflect the heterogeneity of the near field stress redistribution alternatively the variety of the aftershock mechanisms may reflect deformation by block motion within a narrow zone adjacent to the main shock rupture plane less

attenuation measurement with sonobuoys sonobuoy data in a number of shelf areas have yielded travel time curves which are relatively smooth and which result in velocity depth profiles that are nearly straight lines to depths of several kilometers moreover many of the records contain several multiples of the first refracted arrival which are well separated from the first arrival and free of excessive noise such data are ideal for comparing the amplitude of the first arrival and the amplitude at the corresponding point on the multiple at double distance and double time by taking the ratio of these amplitudes the effects of geometrical spreading and the angular dependence of the source are removed so that estimates of total attenuation as a function of depth and frequency are much more easily studied in this paper we present an example of such a study using sonobuoy data from the sarawak basin which is located in the south china sea 4 n 110 e measurements made in the frequency range of 10 to 60 hz suggest that the logarithmic decrement or the quality factor q are not constants but depend rather strongly on frequency in the diving waves that were studied

shortening within underplated oceanic crust beneath the central california margin structures within a tectonically underplated layer of early miocene age oceanic crust indicate that the lower crust of offshore central california has been deformed under compression prior to the inception of the san andreas transform system along the continental margin shortening in the lower crust is inferred from the geometry of two structures the santa lucia monocline and the santa maria antiform delineated by an integrated interpretation of multichannel and wide angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions the 700 km of multichannel seismic data and 300 km of wide angle seismic data from the 1986 pacific gas and electric edge seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the wnw trend of the antiform in cross sectional view the santa lucia monocline is a 30 kink in the oceanic crust while the santa maria antiform is characterized by local thickening of the oceanic crust these structures are probably the result of convergence between the monterey and arguello microplates that took place as subduction along the margin waned the multichannel seismic reflection data image two miocene age shelf basins the santa lucia and offshore santa maria basins that formed within a transtensional stress regime the offshore santa maria basin directly overlies the santa maria antiform suggesting that either the upper and lower crustal stress regimes were completely decoupled in miocene time or that the two features formed in different places differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic continental transform zone

luminescence chronology of late pleistocene glacial deposits in north westland new zealand abstract the chronology of the late pleistocene glacial history of north westland south island of new zealand is refined by luminescence dating of sediments it is demonstrated by two case studies that ages determined by different luminescence techniques agree well with the results of radiocarbon dating luminescence ages determined for sediments from two key sites of the loopline formation confirm the previous correlation of this unit with marine isotope stage mis 4 this indicates that late pleistocene glaciers reached their last maximum extent in north westland during the early part of the otira last glaciation furthermore initial dating evidence on coarse alluvial deposits in the lowlands indicates their formation during mis 5b and mis 5d it is probable that these coarse gravel deposits are outwash related to glacial advances if so both advances were probably much less extensive than the glaciations during mis 4 and mis 2

a comparison of velocity and attenuation between the nicobar and bengal deep sea fans the compressional wave velocity and attenuation measurements from a deep source deep receiver seismic refraction experiment from the nicobar fan indian ocean are presented and compared to a similar set of measurements in the bengal fan the two deep sea fans consist of thick sections of turbidites derived from the same sediment source but have slightly different depositional histories travel time inversion of the data into a velocity depth model uses the tau zeta linear inversion scheme we present a revised reparameterization of the travel time data and improved error analysis for the inversion process although the velocity structures with depth are morphologically similar there exist discrete differences between the two stations in the nicobar fan the initial velocity assumed is 1 513 km s with an initial gradient of 2 32 s 1 the velocity increases smoothly with depth while the velocity gradient decreases rapidly to a constant 0 81 s 1 the bengal fan station showed the same trend although the velocity gradient at depth was only 0 67 s 1 the attenuation profiles are similar only in gross structure with the nicobar fan values of q 1 being a factor of 3 to 4 less than the bengal station the differences in velocity and attenuation structure can be attributed to a number of causes including porosity and sedimentation rates which are related to the distance from the sediment source and the presence of a normal fault near the nicobar fan site this fault may act as a conduit for water removal in the presence of overburden pressure it is believed that porosity changes due to whatever cause dominate the lateral and vertical variations of velocity velocity gradients and together with intrabed multiples attenuation in thick sedimentary sections

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

timing of late pleistocene climate change in lowland switzerland abstract a chronology of climate change in lowland switzerland for the time between the last interglacial and the last glaciation maximum lgm is provided by complementary radiocarbon 230 th u and luminescence dating of the gossau key section the gossau interstadial complex is consistently dated to 50 000 30 000 yr bp and hence correlated with marine oxygen isotope stage mis 3 palaeotemperature readings reconstructed from coleoptera analyses and pollen assemblages evidence oscillating environmental conditions from tundra to open woodland and reflect changing climate during the middle wurmian delta deposits from the lower part of the gossau section indicate a cold period prior to the interstadial complex this severe climatic deterioration is correlated with mis 5d based on luminescence dating sedimentological evidence implies the presence of glaciers at the northern margin of the swiss alps during this time

crustal structure along the edge transect beneath the kodiak shelf off alaska derived from obh seismic refraction data summary along the edge alaska transect over the kodiak shelf off alaska a deep crustal seismic refraction experiment was carried out in 1994 using an airgun source with a high shot density and closely spaced ocean bottom hydrophones obhs these data clearly define a gently dipping plate boundary that gradually increases from 2 5d at the trench to 5d 250 km landwards of the trench beneath the shelf a landward increase of the velocities in the subducting oceanic crust was found the sedimentary upper plate beneath the continental slope can be divided into a young neogene accretionary prism beneath the lower slope and a more layered structure below the middle and upper slope where a body with elevated velocities 4 2 5 1 km s 1 in the lower part is interpreted as well consolidated sediment of possibly late eocene age acting as the backstop across the inner shelf between kodiak island and kenai peninsula near surface high velocities 6 0 km s 1 document a strong uplift 10 km of the mesozoic and palaeocene accretionary complex and furthermore suggest its seaward extension beneath the outer shelf the most striking feature along the edge transect is the thick low velocity zone lvz that coincides with the arched reflectors in the lower crust beneath the kodiak island kenai peninsula axis on the edge seismic reflection data this lvz is interpreted as indicating underplated low velocity rocks which contribute the same volume of new material as the frontal accretion to the continent the growth of which is rapid totalling 4200 km2 across the kodiak shelf since eocene times a calculation of the material balance shows that an underplating of underthrust sediment alone may not account for the huge volume of the underplated low velocity rocks we speculate that the underplating of continental fragment or more likely seamounts or plateau may be involved in continental growth

luminescence dating of sediments from the luthern valley central switzerland and implications for the chronology of the last glacial cycle abstract the advancing glaciers of the last glacial maximum either eroded or deeply buried older sediments in the swiss alpine foreland however part of the swiss plateau was not covered by ice and is therefore an excellent area for investigating climate and environmental change during the upper pleistocene repeated fluvial sequences can be studied in several pits along the luthern valley the chronological framework is based on lithostratigraphy pollen analysis u th dating and recently heavy mineral analysis and luminescence dating the oldest unit the untere zeller schotter braided river deposit represents cold climate conditions and presumably a glaciation prior to the eemian interglaciation the last interglacial period and the very beginning of the last glacial cycle is represented by the mittlere zeller schotter sediments of a meandering fluvial system younger braided river sediments the obere zeller schotter seem to correlate with the cold climate of oxygen isotope stage ois 4 weathering of the top of the obere zeller schotter is likely to represent the ois 3 the advancing reuss glacier caused erosion of the recent luthern valley cutting into older sediments with local loess accumulation during the last glacial maximum as indicated by cover sediments on top of the fluvial sequence

velocity and attenuation profiles in the monterey deep sea fan data obtained during a marine refraction experiment are used to estimate velocity and attenuation profiles in an area of thick sediments 2 5 3 km on the monterey deep sea fan a 20 element vertical hydrophone array was deployed at mid depth in 2800 m of water explosive sources set to detonate at 1830 m were launched at ranges between 3 5 and 37 km from the array estimates of velocity and attenuation as a function of depth are obtained from an analysis of the pressure time series generated by the explosive charges and received at the array the sediment is modeled as a horizontally layered laterally homogeneous medium a least squares solution is found for linear velocity gradients in each layer of the model velocity as a function of depth is obtained from these gradients attenuation coefficients in each layer of the model are found using a method of spectral ratios the results presented here valid for frequencies between 10 and 100 hz show an increase in attenuation to a depth of about 500 m

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

comment on first evidence of in situ eemian sediments on the high plateau of evian northern alps france implications for the chronology of the last glaciation by f guiter a triganon v andrieu ponel p ponel j p hebrard g nicoud j l de beaulieu s brewer f guibal

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

thermoluminescence dating of eemian and early weichselian deposits in denmark abstract eemian and early weichselian deposits from a number of localities in denmark have been dated by thermoluminescence tl the tl ages obtained for eemian deposits are considerably younger than the accepted age of the eemian 120 130 ka possible explanations of the discrepancies are discussed including the possibility that eemian climatic conditions prevailed for a longer time than the 10 ka generally assumed the tl dates for the weichselian deposits together with stratigraphic and morphological evidence indicate the existence of an early weichselian ice advance covering the south eastern part of denmark

three dimensional seismic imaging of the costa rica accretionary prism structural diversity in a small volume of the lower slope conventional two dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms we undertook a three dimensional seismic reflection survey of a small part of a prism arcward of the middle america trench off costa rica to more correctly image structure and to use the improved structural information to examine the processes of accretion this survey reveals small features with dimensions of hundreds of meters while also defining features thousands of meters in lateral extent both of which were underappreciated in conventional two dimensional data from the same area we have imaged active off scraping at the trench and both duplexing and out of sequence faulting a few kilometers arcward of the trench fault spacing and reflector geometry vary dramatically over a space of several hundred meters some of these variations are related to visible changes in morphology of the underlying oceanic basement but others are not so easily documented fault surface reflections define an architecture which may control gross fluid motion through the prism this architecture is apparently formed by duplexing and out of sequence faulting and has been maintained by periodic motion on some of the out of sequence faults the slope sediment apron records multiple phases of deformation abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism a primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism

geological evolution of the afro arabian dome abstract the afro arabian dome includes the elevated continental regions enclosing the red sea gulf of aden and the ethiopian rift system and extends northwards as far as jordan it is more than an order of magnitude larger than other african uplifts both the structures and the igneous rocks of the dome appear to be products of the superimposition of two perhaps three semi independent generating systems initiated at different times but all still active a strain pattern dominated by nw trending basins and rifts first became established early in the cretaceous by the end of the oligocene much of the extensional strain had been taken up along the red sea and gulf of aden axes which subsequently developed into an ocean palaeogene trap volcanism of mildly alkaline to transitional character was related to this horizontal extension rather than to doming further west the east sahara swell has a history of intermittent alkaline volcanicity which began in the mesozoic and was independent of magmatism in the afro arabian dome volcanicity specifically related to doming began in the miocene along a n s zone of uplift extending from ethiopia to syria this elongated swell forms the northern termination of the east african system of domes and rifts characterized by episodic vertical uplift but very little extension superimposition of epeirogenic uplift upon structures formed by horizontal extension took place in the neogene volcanicity related to vertical tectonics is mildly alkaline in character whereas transitional and tholeiitic magmas are found along the spreading axes

structural and photogeological evidence for the boundaries to an east arabian block computer enhanced landsat images reveal a 65 km arcuate lineament linking the concealed fault zone beneath wadi al batin to the central arabian graben and trough system detection of the link allows a continuous 1000 km arc of faults and troughs cutting the mesozoic cenozoic rocks of the arabian shelf to be recognized the arc here called the east arabian fault zone is interpreted as forming the western and southern boundaries of the east arabian block a tectonic unit underlying at least 200 000 km 2 of the arabian peninsula stratigraphical relationships within the 560 km central arabian graben and trough system demonstrate that its faults were active during the late cretaceous early palaeogene movements ceasing when the red sea began to open and the arabian subplate was established the principal hydrocarbon fields of the arabian peninsula lie within the boundaries of the east arabian block

tectonic structure of the convergent pacific margin offshore costa rica from multichannel seismic reflection data the middle america trench between the cocos ridge and a well studied corridor off the nicoya peninsula has a more varied morphology and structure than previously reported the morphological positive features on the lower plate significantly affect the upper plate structure the cocos ridge has uplifted the margin opposite the osa peninsula northwest of cocos ridge numerous seamounts on the oceanic crust sculptured the margin as they subducted a seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted the greater portion of the trench sediment is subducted beneath a lower slope accretionary mass beneath the middle and upper slope is a margin wedge consisting of a high velocity rock with few internal reflections its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area overlying this surface is a low velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred eocene section beneath the shelf the shelf basement is composed of nicoya complex ophiolite with the same acoustic velocity similar structure and no apparent dividing geologic boundary with the margin wedge we favor a seaward continuation of the nicoya complex to the middle slope and emphasize the evidence for a non steady state tertiary tectonic history

luminescence dating of the niederweningen mammoth site switzerland abstract luminescence dating is used to further constrain the so far controversial age of the mammoth site at niederweningen switzerland gamma spectrometric analyses reveal the presence of radioactive disequilibria in the uranium decay chain in most of the samples but these cause only minor uncertainties in the age estimates the major uncertainty is actually related to the problem of estimating the average water content for the whole burial time nevertheless even considering these problems involved with dose rate determination the dating results clearly indicate that the peat is significantly younger than early wurmian in contrast to some previous assumptions together with the results of radiocarbon dating and pollen analyses the luminescence ages point to a correlation of the mammoth peat with interstadial conditions during the early part of the middle wurmian at around 45 ka ago

lithospheric behavior with triple junction migration an example based on the mendocino triple junction abstract the northward migration of the mendocino triple junction along the western margin of the north american plate produces a region of thin lithosphere in association with the growth of the san andreas transform numerical modeling of the resultant thermal regime in conjunction with modeling of the mechanical response of the region indicate the role of lithospheric thinning in the tectonism along the western margin of north america the geometry of the region of thin lithosphere is inherited from the previous subduction regime with resulting implications for the mechanical response the elevation of the california coast ranges is maintained primarily by flexure from buoyant loading from below additionally sporadic volcanism associated with the passage of the triple junction is a likely consequence of super solidus mantle emplaced at shallow depths after the removal of the subducting slab

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

new seismic images of the cascadia subduction zone from cruise so108 orwell abstract in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was conducted aboard the german r v sonne this cooperative experiment by geomar and the usgs acquired wide angle reflection and refraction seismic data using ocean bottom seismometers obs and hydrophones obh and multichannel seismic reflection mcs data the main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the cascadia subduction zone and to image the downgoing plate coincident mcs and wide angle profiles along two tracks are presented here the plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca 13 km depth thus the downgoing plate dips more shallowly than previously assumed the dip of the plate changes from 2 to 4 at the eastern boundary of the wedge on the northern profile where approximately 3 km of sediment is entering the subduction zone on the southern profile where the incoming sedimentary section is about 2 2 km thick the plate dips about 0 5 to 1 5 near the deformation front and increases to 3 5 further landwards on both profiles the deformation of the accretionary wedge has produced six ridges on the seafloor three of which represent active faulting as indicated by growth folding the ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement thus the entire incoming sediment package is being accreted at least two phases of accretion are evident and the rocks of the older accretionary phase s forms the backstop for the younger phase which started around 1 5 ma ago this documents that the 30 to 50 km wide frontal part of the accretionary wedge which is characterized by landward vergent thrusts is a pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods velocities increase quite rapidly within the wedge both landward and downward at the toe of the deformation front velocities are higher than 4 0 km s indicating extensive dewatering of deep oceanic sediment further landward considerable velocity variation is found which indicates major breaks throughout the accretionary history

optical dating studies of quartz and feldspar sediment extracts abstract continuing developmental work into optical dating has led to some clarification of the optical behaviour of quartz and feldspar sediment extracts we have verified the key assumption intrinsic to the method that the optical signal is zeroed completely upon exposure to daylight we also show that the rate of zeroing in optical dating is much more rapid than it is in tl dating studies concerned with the extraction of the thermally stable signal from laboratory irradiated samples showed that problems may arise from medium to high temperature preheating of young samples optical alternatives to thermal stabilization were tested using dye and krypton lasers using a krypton laser we have succeeded in obtaining an equivalent dose of 0 0 0 7 gy for the quartz from a modern intertidal sand and an average dose of 120 22 gy for the feldspar extract of a 70 ka sand unit both were obtained using six wavelengths from the infrared to the violet and no dependence on wavelength was found we found that the photon energies of infrared and deep red light can stimulate luminescence in unirradiated quartz and feldspar extracts of sediments this was not expected and suggests that the process of trap emptying is more complex than we had envisioned but implies a far greater than expected degree of technical flexibility for the optical dating method

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

regional joint sets in the arabian platform as indicators of intraplate processes the eastern part of the saudi arabian craton is a platform underlain by gently tilted and horizontal mesozoic cenozoic sedimentary rocks everywhere more than 250 km distant from the zagros deformation front despite its tectonic simplicity the platform contains several laterally extensive but subdued structures and lineaments belonging to more than one system of fault zones and flexures new microtectonic observations of dominant sets of extension and conjugate hybrid joints permit regionally significant extension directions to be inferred and the origin of the macrostructures to be assessed the central arabian arch the e w crest of which coincides with an axis of mesozoic subsidence developed during the late cretaceous to eocene when the basin inverted as a consequence of the emplacement of ophiolite bearing nappes on the zagros and oman margins during arching there was strike parallel elongation of beds at the same time as the arch amplified the central arabian graben system evolved as a result of the northward displacement of an east arabian block or sheet that became detached across an arcuate separation zone which in the south is coincident with the crest of the arch an important neogene response in the arabian platform to ne sw shortening in the zagros ranges was the formation of a swarm of ne striking master joints and lineaments these structures reflect the diffuse and peripheral expansion of the foreland beyond the zagros deformation front

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

crustal structure of the central costa rica subduction zone implications for basal erosion from seismic wide angle data on the pacific margin off central costa rica an anomalous lens shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front this feature was previously recognized in reflection seismic data when it was termed megalens its origin and seismic velocity structure however could not unambiguously be derived from earlier studies therefore during rv sonne cruise so163 seismic wide angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the megalens intersecting on the middle slope the p wave velocities and structure of the subducting oceanic cocos plate and overriding caribbean plate were determined by modelling the wide angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms the margin wedge is defined by high seismic velocities 4 3 6 1 km s 1 identified within a wedge shaped body covered by a slope sediment drape it is divided into two layers with different velocity gradients the lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the megalens seismic velocities of the megalens are lower 3 8 4 3 km s 1 relative to the margin wedge we propose that the megalens represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate upward migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing thus causing material transfer from the upper plate to the lower plate results from amplitude modelling support that the megalens observed off central costa rica is bound by a low velocity zone documenting fluid drainage from the plate boundary to the upper plate

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

an investigation into the fundamental relationships between attenuation phase dispersion and frequency using seismic refraction profiles over sedimentary structures despite many attenuation measurements which indicate a linear functional frequency dependence of absorption or constant q 1 in sediments several theories predict no such linear dependence the primary justification for rejecting a first power frequency dependence of attenuation is that it implies that seismic waves cannot propagate causally seismic waves must also travel with some velocity dispersion to satisfy causality yet there is a lack of velocity dispersion measurements in sediments in situ attenuation is caused by two distinct mechanisms anelastic heating and scattering due to interbed multiples apparent or scattering attenuation can produce both frequency dependent and non frequency dependent effects accurate measurements of attenuation and velocity dispersion are difficult it is not surprising that a systematic investigation into the frequency dependence of absorption and velocity has not been made a reinvestigation into two seismic refraction data sets collected over thickly stratifi

comparison of deep structure along three transects of the western north american continental margin similarities in geology and potential field data that have in the past been noted among the regions of southern alaska southern vancouver island and central california are now seen to be accompanied by similarities in deep crustal structure a number of tectonic elements have been identified in the deep structure along transects in these three regions although not all elements are present along each transect these elements are a an actively subducting oceanic plate and b an overriding continental plate that consists of 1 a cenozoic accretionary prism 2 a mesozoic accretionary prism 3 a backstop to the mesozoic prism 4 a tectonically underplated body of oceanic rocks and 5 a crustal root the mesozoic prism is in some cases an underthrust body type 2a but in other cases forms the principal component of a landward verging tectonic wedge type 2b the technically underplated body of oceanic rocks extends landward from the fault contact between the cenozoic and mesozoic prisms to a point beneath the backstop the crustal root lies beneath the backstop and landward of the underplated body all of these elements are interpreted to be present along the alaskan and vancouver island transects in alaska the underplated body is interpreted to be fragments of the kula plate the same may be true at vancouver island these two transects appear to differ in that in alaska the mesozoic prism in one interpretation is the principal component of a tectonic wedge type 2b whereas at vancouver island it is an underthrust body type 2a along the central california transect active subduction is no longer taking place and the san andreas fault has removed the cenozoic prism from this region of the north american plate on the north american plate i e east of the san andreas fault the mesozoic prism interpreted as the main component of a tectonic wedge type 2b and the backstop to the mesozoic prism are present there is however no clear evidence of tectonically underplated oceanic rocks and the crust is thin no root in both alaska and vancouver island the mesozoic prisms above the underplated bodies experienced low pressure high temperature metamorphism at about the time of tectonic underplating no such metamorphism is currently exposed in california the metamorphism may have been caused by the underplating of young hot oceanic crust or alternatively by subduction of an oceanic ridge the presence of a tectonic wedge type 2b in alaska and california and the absence of such a wedge at vancouver island could arise either from the fact that in the former two locations the mesozoic prisms were more voluminous owing to either more rapid trench sedimentation or more rapid convergence or to the possibility that at the latter location the mesozoic prism was juxtaposed with the backstop primarily by strike slip faulting

the shallow attenuation structure of the fast spreading east pacific rise near 9 30 n direct measurements of q values in the shallow crust are difficult to obtain due to the lack of energy turning in this region we analyze a unique data set consisting of seven refraction experiments carried out with both the source and receiver near the seafloor this experimental geometry allows us to resolve the shallow compressional wave attenuation structure of young crust at the east pacific rise all models depict a low q layer 2a apparent q 11 22 and several models resolve a layer 2b with a much higher q apparent q 70 this structure is consistent with a layer 2a composed of extrusives overlying a layer 2b composed of sheeted dikes

seismic attenuation in the costa rica margin wedge amplitude modeling of ocean bottom hydrophone data abstract travel time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles located 10 20 km landward of the middle america trench offshore costa rica these two profiles bracket a region of rapid change in physical properties and structural styles the travel time modeling indicates little structural change along the profiles and therefore the data are suitable for amplitude modeling with the reflectivity method for the profile 10 km from the trench amplitude modeling indicates a margin wedge with velocities of 4 4 4 km s and qp 25 50 for the profile 20 km from the trench the amplitude modeling indicates a margin wedge with velocities of 4 4 4 8 km s and qp 50 75 the primary new result of this analysis is the constraint that qp 25 75 within the margin wedge at both locations these high attenuation values are consistent with fracturing of the material making up the margin wedge either ophiolitic nicoya complex rocks dewatered and cemented sediments or a tectonic mixture of both

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

regional extent of great valley basement west of the great valley california implications for extensive tectonic wedging in the california coast ranges interpretation and modeling of the magnetic field of central california indicate that the magnetic basement of the forearc deposits of the great valley sequence extends westward beneath the coeval subduction related rocks of the franciscan complex the basement surface slopes gently to the west reaching midcrustal depths 15 19 km at distances of 50 100 km west of the great valley this magnetic basement is disrupted by the hayward rodgers creek fault system and is cut by the san andreas fault at the south end of the great valley and possibly throughout much of central california the widespread presence of the great valley basement beneath rocks of the franciscan complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the franciscan complex great valley sequence contact this result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones the eastern boundary fault of the franciscan complex coast range fault is not and never was a subduction zone thrust fault but rather was originally a roof thrust wedge roof fault formed above the eastward wedging mass of franciscan complex intruded along the top of the basement beneath the great valley deposits this tectonic interpretation offers a solution for the question of how high pressure metamorphic rocks of the franciscan complex were juxtaposed at the coast range fault against low pressure metamorphic rocks of the great valley sequence this interpretation also implies an older flat lying thrust fault wedge floor fault that forms the top of magnetic basement between the active san andreas and hayward faults at depths of 15 17 km this older thrust fault may today transfer strain between the two young strike slip faults possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults the former east dipping subduction zone along which the rocks of the franciscan complex accumulated must lie west of the western limit of the great valley magnetic basement

upper crustal structure of the western colombian basin caribbean sea multichannel seismic reflection profiles show that most of the western colombian basin is underlain by a large oceanic plateau the crust of this plateau has a smooth upper surface an oceanic velocity structure and a thickness at least twice that of typical oceanic crust locally well defined reflections within the plateau crust are related to layered basalt flows and interbedded sediments a mounded reflection pattern within acoustic basement at the crest of the plateau mono rise may represent volcanic knolls smooth surfaced basement of the western colombian basin is probably equivalent to the upper cretaceous basement complex below seismic reflection horizon b in the central venezuelan basin and resembles the basements of some cretaceous oceanic plateaus and aseismic rises in the western pacific an area of apparently typical oceanic crust lies adjacent to the colombian basin plateau and may represent relatively unmodified oceanic crust upon which the plateau was constructed reflections within the flank of the colombian basin plateau appear to overlap this crust indicating that the plateau is younger than the oceanic crust oceanic plateau crust extends to panama and costa rica without apparent structural break and may be continuous with the basement complex of southern central america

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

integrated crustal structure across the south central california margin santa lucia escarpment to the san andreas fault the continental margin of south central california has undergone different styles of deformation over the past 30 m y as the north america pacific plate boundary has evolved from subduction to transform tectonics results presented here identify strike slip and compressional structures in the lower crust that probably developed during this period of tectonic transition an integrated model of crustal structure along deep seismic line pg e 3 crosses the entire continental margin west of the san andreas fault near the latitude of san luis obispo a compressional wave velocity model was developed using coincident marine multi channel seismic and onshore offshore wide angle reflection refraction seismic data and was tested independently with gravity modeling a high velocity layer 6 8 7 0 km s about 6 km thick in the lower crust that extends east possibly as far as the san andreas fault is interpreted as tectonically underplated oceanic crust the upper and middle crust of the patton and sur obispo franciscan terranes extends to depths of 14 to 20 km the velocity and density modeling indicate lower average velocities and densities for the patton terrane supporting earlier interpretations that it is a distinct terrane within the franciscan complex abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the santa lucia escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 ma following the subduction of the pacific arguello ridge an imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust convergence between the monterey and arguello microplates during the final stages of farallon subduction about 24 to 16 ma probably caused the imbrication

late glacial early holocene slope deposits on the swiss plateau genesis and palaeo environment abstract on the swiss plateau glacial and glaciofluvial sediments deposited during mis 2 are widespread they are generally overlain by younger sediments of different origin mainly related to periglacial and slope wash processes depending on their genesis and composition 3 types of slope deposits can be distinguished which are classified as units 1 2a and 2b unit 1 is characterized by its wide spatial occurrence and a constant thickness of 40 60 cm high silt contents and a sharp lithological discontinuity to the lying sediments generally forming the parent material for the ah and al horizons of the luvisols unit 2 represents erosion rills filled up with loess loam and some gravel based on stratigraphic assumptions the formation of unit 1 has been tentatively attributed to the younger dryas optical stimulated luminescence and radiocarbon dating of unit 2 indicate morphodynamic activity during the early holocene up to 7 5 ka suggesting an open landscape this scenario is in conflict with the established notion that reforestation in middle europe took place during the bolling allerod warm period in order to explain this apparent contradiction for the younger dryas to early holocene environment we suggest a continental climate with dry summers and drought stress on the vegetation in combination with very cold winters and deep soil frost at the pleistocene holocene transition caused by the orbital precession maximum after 7 5 ka with the spread of oceanic woodland and reduced seasonal contrasts the landscape stabilized and soils developed

seismic q stratigraphy or dissipation the basic problems encountered in extracting estimates of seismic dissipation from data recorded on vertical seismic profiles are analyzed because anomalous dissipation in the subsurface is likely to be associated with conditions or lithologies of limited vertical extent a knowledge of the factors which influence the spatial resolution of an attenuation measurement is of considerable importance by introducing a statistical perspective it is possible to simulate multiple measurements in an inhomogeneous interval and to draw conclusions which apply to an entire class of impedance structures theoretical seismograms are analyzed to demonstrate that for small receiver separations neither a single measurement nor the mean value determined from multiple measurements is likely to give a good estimate of the attenuation for an inhomogeneous depth interval for small receiver separations the attenuation computed from the amplitude ratios method is much more strongly influenced by the local stratigraphy in the

benthic foraminiferal distribution and sedimentary structures suggest tectonic erosion at the costa rica convergent plate margin distribution patterns of benthic foraminiferal faunas from odp leg 170 sites 1041 and 1042 show that the costa rican convergent margin subsided from coastal to abyssal depth from middle miocene to present this favours the model of a margin undergoing active subduction erosion we propose that subduction erosion leads to the removal of material from the base of the forearc wedge and as a consequence to progressive subsidence of the forearc a mean subsidence rate is estimated to be approximately 0 4 mm yr 1

seismic structure of the upper oceanic crust revealed by in situ q logs in situ seismic attenuation is computed through 2 1 km of the upper oceanic crust in the vicinity of odp hole 504b the results strongly tie crustal properties to seismic measurables and observed geological structures we find that the attenuation can be used to define seismic layer boundaries and is closely related to the intensity of vertical heterogeneity the in situ attenuation q 1 consists of both intrinsic and scattering contributions but is dominated by the scattering attenuation unless porosities are near zero when it approaches typical estimates from seismic refraction studies the attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic vp log and is found to decrease step wise in relatively homogeneous layers from q 25 to q 300 between the top of seismic layer 2a and a sharp discontinuity at 1 3 km depth these changes correspond with heterogeneities at 1 0 1 3 m and at 5 6 10 0 m wavelengths that we interpret to be associated with fracturing and structure of pillow basalts and lava flows in seismic layers 2a and 2b although seismic velocity studies suggest that the layer 2 3 boundary also occurs at about 1 3 km the large variation in q 140 to 460 below this depth indicates that a seismically homogeneous and uniform layer 3 has not been reached in hole 504b we derive an empirical relationship between attenuation and porosity q 1 q0 1 e where q0 1 0 004 and 25 that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models

luminescence dating of fluvial sediments and overbank deposits from gossau switzerland fine grain dating abstract last glacial fluvial sediments and overbank deposits from gossau switzerland have been investigated applying infrared stimulated luminescence irsl and thermoluminescence tl to polymineral fine grains age estimates have been determined using irsl and tl broad band blue yellow and uv emissions the irsl dates are mainly consistent with independent dating results while most of the tl measurements result in age overestimations caused by insufficient bleaching before deposition

q along the fennolora profile summary the fennoscandian long range project fennolora recorded refraction data on a profile from northern norway through sweden to the baltic sea the data have been used to interpret complex crust and upper mantle structure to 450 km depth and the data are unique in scandinavia for their extension and quality in this study crustal q has been determined for p waves along the profile using the spectral ratio method q was found to be frequency dependent of the form q q0fm where q0 and m are constants and f is the frequency in order to simplify the presentation and interpretation of the results and to allow comparison to other studies q was also determined assuming independence of frequency q varied by up to a factor of 3 along the profile with generally increasing values going from north to south this might correlate with more complex structure at the northern segment than at the southern segment average crustal q for the whole profile using all data was q 29f0 96 and the corresponding constant q was 460 since most of the energy in the spectra was from 3 to 30 hz it seems that the frequency independent q is comparable to q f at the average frequency at 16 hz q f 415 the sensitivity of q with depth was investigated using pairs of stations with average distance to shot points of 50 150 and 250 km the average frequency independent q values thus obtained were 430 470 and 480 respectively with a somewhat larger variation for the frequency dependent q values however using the present method and data set there is no indication of significant variation of crustal q with depth this is in contrast to many coda q studies which give increased q with depth

west margin of north america a synthesis of recent seismic transects abstract a comparison of the deep structure along nine recent transects of the west margin of north america shows many important similarities and differences common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust accreted oceanic arc or oceanic like lithosphere of mesozoic through cenozoic ages cenozoic accretionary prisms mesozoic accretionary prisms backstops to the mesozoic prisms and undivided lower crust not all of these elements are present along all transects in this study nine transects including four crossing subduction zones and five crossing transform faults are plotted at the same scale and vertical exaggeration v e 1 1 using the above scheme for identifying tectonic elements the four subduction zone transects contain actively subducting oceanic crust cenozoic accretionary prisms and bodies of basaltic rocks accreted in the cenozoic including remnants of a large oceanic plateau in the oregon and vancouver island transects rocks of age and composition eocene basalt similar to the oceanic plateau are currently subducting in southern alaska where they are doubled up on top of pacific oceanic crust and have apparently created a giant asperity or impediment to subduction most of the subduction zone transects also contain mesozoic accretionary prisms and two of them vancouver island and alaska also contain thick tectonically underplated bodies of late mesozoic early cenozoic oceanic lithosphere interpreted as fragments of the extinct kula plate in the upper crust most of the five transform fault transects all in california reflect 1 tectonic wedging of a mesozoic accretionary prism into a backstop which includes mesozoic early cenozoic forearc rocks and mesozoic ophiolitic arc basement rocks and 2 shuffling of the subduction margin of california by strike slip faulting in the lower crust they may reflect migration of the mendocino triple junction northward seen in rocks east of the san andreas fault and cessation of farallon plate subduction seen in rocks west of the san andreas fault in northern california lower crustal rocks east of the san andreas fault have oceanic crustal velocity and thickness and contain patches of high reflectivity they may represent basaltic rocks magmatically underplated in the wake of the migration of the mendocino triple junction or they may represent stalled subducted fragments of the farallon gorda plate the latter alternative does not fit the accepted slabless window model for the migration of the triple junction this lower crustal layer and the moho are offset at the san andreas and maacama faults in central california a similar lower crustal layer is observed west of the san andreas fault west of the continental slope it is pacific oceanic crust but beneath the continent it may represent either pacific oceanic crust stalled subducted fragments microplates of the farallon plate or basaltic rocks magmatically underplated during subduction of the pacific farallon ridge or during breakup of the subducted farallon plate the transect in southern california is only partly representative of regional structure as the structure here is 3 dimensional in the upper crust a mesozoic prism has been thrust beneath crystalline basement rocks of the san gabriel mountains and mojave desert in the mid crust a bright reflective zone is interpreted as a possible master decollement that can be traced from the fold and thrust belt of the los angeles basin northward to at least the san andreas fault a moho depression beneath the san gabriel mountains is consistent with downwelling of lithospheric mantle beneath the transverse ranges that appears to be driving the compression across the transverse ranges and los angeles basin

shallow structure of oceanic crust in the western north atlantic from seismic waveform inversion and modeling seismic reflection data from mesozoic oceanic crust in the western north atlantic in the vicinity of the blake spur fracture zone have imaged a number of short subhorizontal events in the upper 1 km of the crystalline crust wide angle expanding spread profile esp data from the same region indicate the presence of first order velocity discontinuities in this depth interval using esp data reprocessed to enhance precritical reflection events we investigate the relationship between the precritical and postcritical reflections with three different approaches amplitude analysis forward modeling and waveform inversion the precritical reflections have high apparent reflection coefficients 0 1 and may be divided into two types those which can be traced out to critical ranges 5 6 km representing velocity steps and those which fade out at ranges of 3 4 km representing shorter wavelength velocity variations synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval while the shorter wavelength features may correspond to high or low velocity zones with a thickness of 20 m 10 of the dominant seismic wavelength or less we investigate the velocity structure in detail by full waveform inversion of a selected portion of esp data transformed into the intercept time slowness domain the inversion method minimizes the sample by sample misfit between the data and reflectivity synthetics in a least squares sense a conjugate gradient algorithm was used starting from a velocity structure derived from forward modeling of wide angle esp data the inversion tightly constrains short wavelength components of the velocity structure and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity synthetic seismograms generated from sonic log data from deep sea drilling project hole 418a show reflections of similar amplitude and character to those seen in the esp data these reflections probably originate from rather subtle features of the velocity depth profile related primarily to changes in porosity in the pillow lava sequence a deeper reflector 800 ms below oceanic basement in the blake spur fracture zone may correspond to the top of a zone of serpentinized upper mantle

detailed subduction structure across the eastern nankai trough obtained from ocean bottom seismographic profiles to investigate the deep crustal structure of the philippine sea plate at its northern margin we performed a seismic refraction and wide angle reflection survey around the eastern nankai trough by using an ocean bottom seismograph array with an air gun source we derived a crustal structure model across the trough from the shikoku basin to the continental slope that explains not only our seismic data but also previously published gravity data the origin of the zenisu ridge a conspicuous topographic high along the oceanward slope of the nankai trough is still a matter of argument there has been some controversy as to whether the igneous activity of the izu ogasawara arc or the seafloor spreading of the shikoku basin is responsible for the formation of the ridge although the crustal thickness beneath the ridge is between 8 and 11 km slightly thicker than typical oceanic crust our structure model clearly indicates that the zenisu ridge has an oceanic crust and its structure is very similar to that of the shikoku basin beneath the south flank of the zenisu ridge the moho shows an offset of 5 km in depth this offset may correlate with the recently proposed nascent subduction boundary and subduction collision tectonics of this area the velocity structure beneath the continental slope appears characteristic for a well developed accretionary wedge bounded by the continental upper crust of the japan island arc to the northwest and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

seismic attenuation in the crust at the mid atlantic plate boundary in south west iceland summary very little seismic attenuation occurs in the mid to lower crust of south west iceland the lowest path averaged quality factor for a wave turning in the mid to lower crust 12 20 km is qp 110 for p waves and q5 250 for s waves with most of the data having higher values typically qp 200 300 and qr 400 600 attenuation estimates based on a 1 d layered inversion give correspondingly high values qp 800 and q 800 2000 these q values are inconsistent with thermal models that predict a broad 100 km wide region of above solidus temperatures centred on the volcanic zones the observed attenuation implies an upper limit for mid to lower crustal temperature in the 700 775 c range assuming a gabbroic lithology much higher attenuation qp 60 qr 100 occurs in the uppermost 4 km of crust this is most likely apparent attenuation caused by strong near surface seismic heterogeneity resulting from fissures faults and extreme changes in porosity up to 20 30 per cent the quality factor of the near surface layer varies regionally and is lower in volcanic zones than in either the reykjanes peninsula or the south iceland lowland

luminescence dating using feldspars a test case from southern north island new zealand abstract luminescence dating using coarse grain potassium rich feldspars is applied to a suite of 12 dune sands collected from southern north island new zealand the expected ages of the samples range from modern to 350 ka luminescence ages were calculated using thermoluminescence and infra red stimulated luminescence measurements to determine total bleach and regeneration equivalent doses for each sample no systematic difference was observed between any of these different methods withstanding the uncertainties in the independent ages of some samples comparison of the luminescence ages with those suggested by the independent age control suggests that reliable luminescence ages can be obtained for samples younger than 130 ka luminescence ages for older samples appear to underestimate the true age and it is suggested that this may be due to long term fading of the luminescence signal as suggested by mejdahl 1989