thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

past ice sheet flow east of svalbard inferred from streamlined subglacial landforms the pattern of late weichselian ca 20 ka ice flow in the northern barents sea is not well known due mainly to a lack of marine data east of svalbard several years with little summer sea ice have allowed acquisition of swath bathymetric imagery of well preserved subglacial landforms characterizing late weichselian ice flow directions over 150 000 km2 of the northwestern barents sea we show that a major ice dome was located on easternmost spitsbergen or southern hinlopen strait at least 500 km west of its previously inferred position in the northern barents sea this dome controlled the regional flow pattern ice flowed eastward around kong karls land into franz victoria trough and north through hinlopen strait an ice dome west of kong karls land is required to explain the observed ice flow pattern but does not preclude an additional ice dome to the southeast discrepancies with earlier ice sheet reconstructions reflect the lack of previous seafloor observations with evidence limited mainly to past ice loading and postglacial rebound the new pattern of ice flow directions shows predominantly eastward rather than northward flow with franz victoria trough a major drainage pathway with a full glacial balance flux of 40 km3 yr 1

high resolution landform assemblage along a buried glacio erosive surface in the sw barents sea revealed by p cable 3d seismic data abstract the quaternary sedimentary record in the arctic captures a diverse and evolving range of landscapes reflecting climate changes here we study the geological landform assemblage of the upper regional unconformity uru in the sw barents sea the aims are i to characterize buried geological landforms on a meter scale resolution ii to understand their link with underlying structures and iii to reconstruct paleo ice sheet dynamics and configurations the data consist of a high resolution three dimensional 3d p cable seismic cube with an extent of c 200 km2 and an inline separation of 6 m dominant frequencies of c 150 hz allow to image landforms at uru with a vertical resolution of 1 5 m and a horizontal resolution of 3 6 m we conduct detailed horizon picking and seismic attribute analysis of the buried uru horizon we identified four sets of mega scale glacial lineations and shear band ridges located to the west of a shear margin moraine other characteristic features include hill hole pairs transverse ridges rhombohedral ridges and depressions iceberg ploughmarks and pockmarks polygonal faults below uru and deeper faults have a strong effect on the location of structures observed on uru bedrock packages deformed down to 30 m below uru and up to 5 m high transverse ridges at uru are imprints of glacio tectonic activity deformed strata below uru indicate normal faulting superimposed by glaciotectonic deformation the four sets of mega scale glacial lineations indicate four streaming events with thawed glacial beds with shear band ridges forming in the shearing zone during one of these streaming events hill hole pairs and rhombohedral ridges are frozen bed features which indicate a polythermal regime at the base of the barents sea ice sheet during multiple streaming phases this study therefore shows that paleo ice streams have been temporarily frozen to the ground in the sw barents sea and that landforms evidencing this freezing are associated with underlying faults

elastic finite difference modeling of the 1971 san fernando california earthquake finite difference seismograms calculated for the 1971 san fernando earthquake show strong effects due to lateral variation in sediment thickness in the san fernando valley and the los angeles basin using basin structure derived mostly from well logs and teleseismically determined source parameters two dimensional sh and p sv finite difference calculations reproduce the amplitude and duration of the strong motion velocities recorded across the los angeles and san fernando basins for the period range of 2 to 10 sec the edges of basins nearest the seismic source show ground motion amplification up to a factor of 3 over the case without the basin and tend to convert direct shear waves into love and rayleigh waves that travel within the basins

deglaciation of a marine based ice sheet late weichselian palaeo ice dynamics and retreat in the southern barents sea reconstructed from onshore and offshore glacial geomorphology abstract marine and terrestrial datasets swath bathymetry 2d and 3d seismic and remote sensing data are used to map the megascale glacial geomorphology of the southern barents sea seafloor northern fennoscandia and the kola peninsula giving new insights into the dynamics and behaviour of the barents sea ice sheet during retreat the glacial geomorphology indicates clear spatial and temporal variations in ice dynamics with evidence for both active ice streaming and frozen bed conditions at the maximum and during deglaciation a five stage reconstruction of the late weichselian deglaciation is proposed at the late weichselian maximum ice covered the whole of the barents sea shelf and the configuration and dynamics of the ice sheet were controlled by the bjornoyrenna bear island trough ice stream this was fed by source areas in the central and north eastern barents sea and in the fennoscandian mainland deglaciation was initiated along the western continental margin available dating results show that this occurred after 19 cal ka bp the bjornoyrenna ice stream retreated rapidly ice margin retreat rates of 275 m a 1 but was periodically grounded as evidenced by a well developed series of grounding zone wedges this was followed by an advance of two short lived ice streams from the eastern sector of the ice sheet into the south western barents sea these ice streams advanced and retreated very rapidly with no evidence for ice margin standstills correlation to dated cores suggests an age of around 16 000 cal ka bp for this readvance the period of operation of these two ice streams was coincident with maximum ice extent along the eastern margin in russia the western barents sea ice margin then retreated onshore with deglaciation continuing at a reduced rate the initiation of deglaciation coincided with rising global eustatic sea level acceleration of calving due to grounding line floatation rapidly opened up a major calving bay in bjornoyrenna encouraging rapid loss of the marine based ice the retreat rate stabilised only once the ice sheet retreated onshore and calving ceased we note that the observed pattern of rapid retreat with high variability in ice streaming for the marine based barents sea ice sheet is similar to that of the marine based sectors of the late weichselian british laurentide and west antarctic ice sheets

organic geochemical and petrographic characteristics of the miocene salif organic rich shales in the tihama basin red sea of yemen implications for paleoenvironmental conditions and oil generation potential abstract this study is the first investigation which provides information regarding the organic geochemical and petrographic characteristics of miocene salif organic rich shales from tihama basin in the red sea yemen we evaluate organic matter content type maturity and oil generation potential as well as depositional environmental conditions the total organic carbon toc contents of the miocene salif shales vary between 0 59 and 5 40 indicating fair to very good source rock potential the salif shales have hydrogen index values in the range of 64 576 mg hc g toc the organic matter in the salif shales are dominated by type ii kerogen and mixed ii iii kerogens with a minor contribution of type iii kerogen as supported by kerogen microscopy this is also confirmed by their biomarker and carbon isotope results which indicate that the salif shales were deposited in highly reducing marine conditions and received high contributions of aquatic organic matter e g algal and microbial and terrigenous organic matter consequently the salif formation is likely to be an oil source rock maturity indicators such as vitrinite reflectance and pyrolysis data i e tmax and pi indicate that most of the salif shale samples are generally thermally mature at the early mature to peak oil window stage the new data presented in this paper suggest that early mature oil has been generated from miocene salif organic rich shales so exploration strategies should focus on the known location of miocene salif organic rich shales for predicting the location of the source kitchen

ice stream dynamics of the sw barents sea revealed by high resolution 3d seismic imaging of glacial deposits in the hoop area abstract recent developments in seismic acquisition systems and seismic data visualization have contributed to improve the imaging of seismic geomorphologies over a broad range of topics this study focuses on a new high resolution p cable 3d seismic cube located in the hoop area in the sw barents sea the scientific motivations of the study are 1 to document glacial landforms on a meter scale 2 to study the link of these landforms with the subsurface and 3 to understand ice stream dynamics by detailed interpretation of main horizons including the seabed intra glacial horizons and upper regional unconformity uru the horizons are assessed using the concept of seismic geomorphology and compared with interpretations derived from conventional 3d seismic and state of the art multibeam echosounder technologies to discuss the benefits and limitations of the different geophysical technologies a well defined intra glacial seismic reflection is imaged in the eastern part of the p cable cube this reflection has an overall ne trending ridge shaped morphology which is up to 50 m high and 5 10 km wide and interpreted as a shear margin moraine the top of the moraine displays well defined iceberg ploughmarks and clear slope failure events along its eastern flank the seabed geomorphology which has been directly shaped by glacial processes related to multiple phases of ice streaming appears to be influenced by the buried shear margin moraine mega scale glacial lineations msgls indicative of fast flowing ice streams characterize the seabed west of the moraine infilled msgls and different sets of iceberg ploughmarks with large berms dominate the seabed geomorphology above the moraine the seabed east of the moraine characterized by a smooth morphology and few pockmarks is inferred to reflect more slowly moving ice not resolved by other geophysical techniques seismic attribute maps of p cable 3d allowed the identification of smaller ploughmarks and corrugated ridges related to paleo tides eroded by the larger iceberg ploughmarks these small ploughmarks are suggested to be linked to an earlier rapid disintegration of the barents sea ice stream the new high resolution 3d seismic data have led to the development of a revised ice stream model of the barents sea linking the glacial deposits to different ice stream events

ice stream shear margin moraines the distribution and pattern of subglacial bedforms provides valuable insights into the configuration and behaviour of former ice sheets and recent work has begun to address the landform assemblages of palaeo ice streams in this paper we focus on subglacially formed ice stream shear margin moraines these landforms are thought to form in the shear zone between fast flowing stream ice and slow moving sheet ice using satellite imagery and aerial photographs we identify four ice stream shear margin moraines associated with the former m clintock channel palaeo ice stream victoria island arctic canada the moraines range in length from 11 to 22 km maintain fairly constant widths of around 500 m and range in height from 10 to 50 m above the surrounding terrain they are composed of carbonate drift of a similar composition as the ice stream bedforms and have been laid down irrespective of local and regional topography two of the moraines display a lateral offset and are thought to reflect minor migrations of the ice stream margin we explore their possible mode of formation and suggest that they occur when erosion at the ice stream margin provides a surplus of sediment which is smeared out in the downstream direction the identification of other palaeo ice stream shear margin moraines has probably been hampered by their interrupted continuity which can only be appreciated at the large scale it is hoped that the descriptions presented in this paper will help to identify these landforms and increased detection of palaeo ice streams will reveal other ice stream shear margin moraines copyright 2002 john wiley sons ltd

insights into the emplacement dynamics of volcanic landslides from high resolution 3d seismic data acquired offshore montserrat lesser antilles we present results from the first three dimensional 3d marine seismic dataset ever collected over volcanic landslide deposits acquired offshore of the soufriere hills volcano on the island of montserrat in the lesser antilles the 3d data enable detailed analysis of various features in and around these mass wasting deposits such as surface deformation fabrics the distribution and size of transported blocks change of emplacement direction and erosion into seafloor strata deformational features preserved on the surface of the most recent debris avalanche deposit deposit 1 reveal evidence for spatially variant deceleration as the mass failure came to rest on the seafloor block distributions suggest that the failure spread out very rapidly with no tendency to develop longitudinal ridges an older volcanic flank collapse deposit deposit 2 appears to be intrinsically related to large scale secondary failure of seafloor sediments we observe pronounced erosion directly down slope of a prominent headwall where translational sliding of well stratified sediments was initiated deep reaching faults controlled the form and location of the headwall and stratigraphic relationships suggest that sliding was concurrent with volcanic flank collapse emplacement we also identified a very different mass wasting unit between deposit 1 and deposit 2 that was likely emplaced as a series of particle laden mass flows derived from pyroclastic flows much like the recent since 1995 phase of deposition offshore montserrat but at a much larger scale this study highlights the power of 3d seismic data in understanding landslide emplacement processes offshore of volcanic islands highlights 3d seismic data show new detail of volcanic landslide deposits offshore montserrat volcanic flank collapse material has been diverted around seafloor topographic highs this bending during emplacement has caused pronounced erosion into seafloor strata erosion can destabilize seafloor slopes which then fail as translational slides block distributions and surface deformation give insight into debris avalanche style

geological record of ice shelf break up and grounding line retreat pine island bay west antarctica the catastrophic break ups of the floating larsen a and b ice shelves antarctica in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time this raises a question about the larger west antarctic ice shelves do these shelves with their much greater glacial discharge have a history of collapse here we describe features from the seafloor in pine island bay west antarctica which we interpret as having been formed during a massive ice shelf break up and associated grounding line retreat this evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line the break up occurred prior to ca 12 ka and was likely a response to rapid sea level rise or ocean warming at that time

source rock characterization and oil generating potential of the jurassic madbi formation onshore east shabowah oilfields republic of yemen abstract the hydrocarbon source rock potential of shales from the jurassic madbi formation was evaluated using organic geochemistry rock eval pyrolysis toc and bitumen extraction and organic petrology very good source rock potential is suggested by high values of total organic carbon content toc 2 0 wt and high extractable organic matter content and hydrocarbon yields 11 000 and 6000 ppm respectively the madbi formation samples have high hydrogen index hi and low oxygen index oi values most of the samples in the study area are thermally mature and contain oil prone type i and type ii kerogen vitrinite reflectance ro values for the madbi shale samples were measured from 0 74 to 0 89 tmax values range from 430 to 438 c which are in reasonably good agreement with vitrinite reflectance data reveal that the madbi shales are at the early to peak mature stage on the basis of biomarker maturity parameters these shales are thermally mature shales of the madbi formation contain abundant hydrogen rich kerogen liptinite up to 55 and therefore good oil generating potential is anticipated this is supported by high hydrogen indices hi of up to 400 mg hc g toc and early to peak mature oil window range

engineering ground motion modeling in the near source regime using the specific barrier model for probabilistic seismic hazard analysis the specific barrier model is used for the first time to simulate ground motion accelerations for the purpose of probabilistic seismic hazard analyses at sites near a dominant fault system it incorporates the simulation of fault geometry and the relationship between the stress drop and seismic moment to estimate the number of cracks on the fault for the specific barrier model radiated direct shear waves are established following boore s 1983 procedure the simulated peak ground accelerations pga are then calibrated by strong motion data basically the model is of uniform source and the directivity of the source is not taken into consideration the results show that the calibrated pga values are not sensitive to the relationship between the stress drop and seismic moment however the calibrated pga values may increase about 20 percent for sites near the fault when the cut off frequency fmax is raised from 5 hz to 10 hz the variability of the simulated ground motion is in general smaller than that of the empirical strong motion data shown in the literature this may be improved by adding randomness into the parameter of fmax and uncertainties into the empirical relationships adopted in the model the simulated attenuation curves may be used to judge which types of conventional attenuation equations are better at representing the attenuation of pga for sites near the fault especially for large earthquake events

deglaciation of the central barents sea abstract the marine based barents sea ice sheet covered the polar continental shelf north of norway and western russia during the last glacial maximum initial ice sheet retreat along the western margin is well established while the retreat pattern in the interior parts of the ice sheet remains poorly known here we present new geological data from the central barents sea the results are based on analysis of several marine geophysical datasets including geomorphological mapping of multibeam swath bathymetry data and analysis of seismic and acoustic stratigraphy the new results provide insights into the configuration and dynamics of the ice sheet during its retreat across the central barents sea in particular they show clear changes in the location of the main ice divides and domes with ice flow becoming gradually more topographically controlled as deglaciation progressed major troughs were characterised by episodic retreat and reoccurring cycles of fast and slow ice flow sometimes leading to stagnation and ice shelf formation meanwhile adjacent bank areas were covered by comparatively slowly retreating ice although evidence of streaming ice is also seen locally

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

ground motion on stratified alluvial deposits for incident sh waves a boundary method is applied to study the response of a two dimensional horizontally stratified deposit of arbitrary shape under the incidence of sh waves the procedure combines the line source method with a discrete wavenumber representation for the stratified part in terms of propagator matrix comparison of results for a circular nonhomogeneous deposit with those obtained with the finite element method shows excellent agreement the response of a basin with two strata is analyzed variation of results as a function of incidence angle is presented for several frequencies transfer functions at several points on the surface of the deposit are obtained for vertical incidence and comparison with results from a one dimensional analysis show significant differences time response for an incident ricker wavelet clearly shows important effects due to lateral heterogeneity

seafloor geomorphology of the sw barents sea and its glaci dynamic implications abstract three dimensional 3d reflection seismic imaging techniques provide a unique means of identifying large scale submarine geomorphic features produced by former ice sheets we have used such techniques to image the geomorphology of the sw barents sea but using a dense grid of 2d seismic covering a 240 000 2 km large study area supplemented with 3d seismic from selected areas the study area is located at the convergence of ice flow from the former fennoscandian barents sea and svalbard ice sheets the morphology of the major cross shelf troughs is characterized by elongated ridge groove features interpreted to be glacial bedforms produced by fast flowing grounded glaciers mega scale glacial lineations have been grouped into distinct flow sets related to three specific ice flow events these events document the last glacial maximum lgm and two readvances during deglaciation of specific interest is a detailed and well preserved set of seafloor imprints in the bjornoyrenna trough which acted as the main pathway for ice streams draining the surrounding ice sheets the geomorphic features indicate that bjornoyrenna hosted six separate ice stream lobes that diverged fan like at their margins but were not all active simultaneously an end moraine zone 68 km wide up to 85 m high and 300 km long filled the whole width of this major glacial trough as a result of high sediment flux within sub ice stream deformable beds the imprint of this ice stream system can be followed on our seabed images for 500 km however detailed geomorphologic elements demonstrate that these imprints reflect time transgressive behaviour and do not indicate the length or width of single ice streams the seismic derived seafloor geomorphology demonstrates that the barents sea ice behaved in a much more dynamic way during the last glaciation than previously assumed with major shifts between ice originating from the fennoscandian spitsbergen and barents sea ice sheets

regional difference in scaling laws for large earthquakes and its tectonic implication we compiled 67 large earthquakes which occurred at and around plate boundaries for the last 140 yrs and classified them into four groups interplate strike slip events intraplate strike slip events underthrust events at island arc subduction zones and underthrust events at continental margin subduction zones for each group of earthquakes we examined relations between seismic moment m 0 fault length l fault width w and average fault slip d and found the following scaling laws in the case of interplate strike slip events the well known l cubed dependence of seismic moment breaks down when l exceeds 30 km because the extent of the seismogenic zone is limited in depth 12 km for large events l 30 km d and m 0 increase with l as d overline delta tau l mu left alpha l beta right and m 0 overline delta tau w l 2 left alpha l beta right respectively where the mean fault width bar w is 12 km and the mean stress drop overline delta tau is 1 8 mpa here and are structural parameters for intraplate strike slip events we obtained nearly the same relations except for significantly higher stress drop 3 1 mpa the difference in stress drop between interplate and intraplate events may be ascribed to the difference in stress accumulation rates and thus the recurrence time of earthquakes in the case of underthrust events at island arc subduction zones we also found the saturation of fault width bar w 120km and the breakaway from the l cubed dependence of m 0 for events larger than l 200 km if we consider the average dip angle of plate boundaries at island arc subduction zones to be 20 30 this indicates that the extent of the seismogenic zone in depth is limited to 40 60 km in the case of continental margin subduction zones on the other hand we could not find the saturation of fault width nor the breakaway from the l cubed dependence of m 0 from the analysis of the present data set w 200 km l 1000 km for sufficiently large earthquakes in general the downward rupture growth is limited to a certain depth due to the existence of a ductile unstressed region which extends under the brittle seismogenic zone since the brittle ductile transition occurs at 300 400 c the difference in the lower limit of the seismogenic zones between tectonically different regions may be attributed to the difference in thermal state there

seismic geomorphology an overview seismic geomorphology the extraction of geomorphic insights using predominantly 3d seismic data is a rapidly evolving discipline that facilitates the study of the subsurface using plan view images a variety of analytical techniques is employed to image and visualize depositional elements and other geologically signi cant features this volume presents key technical papers presented at a recent research conference seismic geomorphology conference february 10 11 2005 co sponsored by the society for sedimentary geology and the geological society london these papers cover a broad range of topics from detailed depositional element analysis to big picture regional issues from lithology prediction to diagenetic modi cation of the stratigraphic section this discipline is only in its early stages of development and will henceforth expand rapidly in response to the growing availability to researchers of high quality 3d seismic data

guidelines for assessing the petroleum potential of coaly source rocks using rock eval pyrolysis abstract coaly source rocks are sufficiently different from marine and lacustrine source rocks in their organic matter characteristics to warrant separate guidelines for their assessment using rock eval pyrolysis the rank threshold for oil generation is indicated by the increase in bi s1 toc at rank sr 9 10 tmax 420 430 c ro 0 55 0 6 and the threshold for oil expulsion is indicated by the peak in qi s1 s2 toc at rank sr 11 12 5 tmax 430 440 c ro 0 65 0 85 the pronounced rank related increase in hi s2 toc prior to oil expulsion renders the use of immature samples inappropriate for source rock characterisation a more realistic indication of the petroleum generative potential and oil expulsion efficiency of coaly source rocks can be gained from samples near the onset of expulsion alternatively effective hi values i e his near the onset of expulsion can be estimated by translating the measured his of immature samples along the maturation pathway defined by the new zealand or other defined coal band coaly source rocks comprise a continuum of coaly lithologies including coals shaly coals and coaly mudstones determination of the total genetic potential of coaly source rock sequences is best made using lithology based samples near the onset of expulsion

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

geochemical and biological marker assessment of depositional environments using brazilian offshore oils abstract a combined geochemical and molecular characterization of a wide selection of oils from the major brazilian offshore basins has been undertaken the elemental sulphur nickel and vanadium and bulk oapi and 13c properties of each sample been considered together with its molecular composition determined using liquid and gas chromatography and quantitative biological marker investigations using gas chromatography mass spectrometry for alkanes the results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups the distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils each group is correlated tentatively with source rocks laid down in a specific depositional regime namely lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate or marine deltaic the diagnostic features that allow this classification are the relative abundance and carbon number distributions of n alkanes pristane phytane ratios sulphur nickel and vanadium contents carbon isotope data the absolute concentrations of hopanes and steranes and their abundance relative to 4 methylsteranes and also the occurrence and abundance of several specific biological markers including 18 h oleanane gammacerane carotane tricyclic terpanes higher acyclic isoprenoids 28 30 bisnorhopane and 25 28 30 trisnorhopane this investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils

a groove ploughing theory for the production of mega scale glacial lineations and implications for ice stream mechanics mega scale glacial lineations msgls are longitudinally aligned corrugations ridge groove structures 6 100 km long in sediment produced subglacially they are indicators of fast flow and a common signature of ice stream beds we develop a qualitative theory that accounts for their formation and use numerical modelling and observations of ice stream beds to provide supporting evidence ice in contact with a rough scale of 10 10 3 m bedrock surface will mimic the form of the bed because of flow acceleration and convergence in ice stream onset zones the ice base roughness elements experience transverse strain transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards where such keels slide across a soft sedimentary bed they plough through the sediments carving elongate grooves and deforming material up into intervening ridges this explains msgls and has important implications for ice stream mechanics groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream keels may provide basal drag in the force budget of ice streams thereby playing a role in flow regulation and stability we speculate that groove ploughing permits significant ice stream widening thus facilitating high magnitude ice discharge

centroid single force inversion of seismic waves generated by landslides the centroid moment tensor cmt waveform inversion method of dziewonski et al 1981 is modified to analyze long period seismic waves generated by single force events such as the gigantic landslides associated with the 1980 eruption of mount st helens we refer to the method as centroid single force csf inversion as the result of the inversion we obtain the spatio temporal centroid and three components of a vector we call the csf vector which is the seismic point source representation of a landslide and whose direction is the opposite of the direction of sliding the sealer magnitude of the csf vector measures the overall size of the landslide and is given by mcsf md where m is the mass of the sliding object and d is the sliding distance we apply csf inversion to the long period surface waves generated by the 1980 mount st helens event the 1975 kalapana earthquake and the 1974 mantaro landslide using seismograms from the gdsn ida and hglp networks the csf solution for the st helens event is remarkably consistent with the actual geological observations and proves the efficacy of csf inversion the csf solution for the kalapana event does not fit the overall waveforms better than the cmt solution does however because the cmt solution does not explain the observed love wave radiation pattern it may be necessary to invoke a combination of both types of mechanism for this event although the geometry of the csf solution the direction of the force for the mantaro event is consistent with the actual landslide mcsf is about 5 times smaller than that expected from the mass and travel distance of the landslide estimated by geological observations this discrepancy may suggest either the relatively aseismic nature of this landslide or that the total volume of the slide was overestimated by analogy to single station cmt inversion single station csf inversion also appears to be stable and useful

teleseismic and strong motion source spectra from two earthquakes in eastern taiwan the 20 may and 14 november 1986 hualien earthquakes occurred in a seismically active region of taiwan locally determined focal mechanisms and aftershock patterns from the taiwan telemetered seismographic network indicate that both earthquakes occurred on steeply dipping reverse faults that trend nne this agrees with teleseismic first motion data for the may event but not for the november event this discrepancy is due to a moderate foreshock before the november event surface wave analysis gives a solution for the november event of dip 57 rake 100 and strike 43 which is similar to the locally reported focal mechanism the seismic moment of the november event is m 0 1 7 10 27 dynecm and the magnitudes determined from wwssn data are m b 6 4 m s 7 3 teleseismic source spectra show that the two events also have similar spectral signatures above 0 15 hz reference acceleration spectra are computed from the average teleseismic source spectra and compared to the averaged acceleration spectra computed from strong motion stations for both events correlations between the spectral amplitudes of the strong motion spectra obtained from the main portion of the smart 1 array and the teleseismically estimated reference spectra are poor above 0 2 hz data from the hard rock site situated outside of the basin indicates that amplification of the ground motion between 0 17 1 7 hz is due to the alluvial valley where the smart 1 array is located the amplitude of the observed spectrum is five times the reference spectrum at the hard rock site this is consistent with similar observations from the 1985 michoacan and 1983 akita oki earthquakes the analysis of these and more teleseismic and strong motion records will lead to a better understanding of the relationship between their spectra

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

reconstructing the retreat dynamics of the bjornoyrenna ice stream based on new 3d seismic data from the central barents sea the stability of contemporary ice sheets is influenced by the discharge from ice streams corridors of fast flowing ice bounded by ice flowing an order of magnitude slower reconstructions of palaeo ice stream dynamics contribute to our understanding of ice stream sensitivity to the ocean climate system and can aid in the numerical modelling and prediction of future changes in contemporary ice sheets here we use 3d seismic data covering 13 000 km2 in the central bjornoyrenna bear island trough barents sea to investigate the record of ice streaming preserved on the seafloor and on a buried palaeo seafloor surface the unusually broad coverage and high resolution of the dataset as well as its location in the central area of the trough enables improved reconstruction of dynamics of the former bjornoyrenna ice stream in terms of number of streaming events their trajectory and their relative age sequence during deglaciation our results reveal major changes in the configuration and flow dynamics of the ice stream with up to 10 flow switching events identified for the first time we also document ice streaming sourced from the eastern barents sea around the time of the lgm this high degree of flow variability is suggested to have resulted from climate driven changes in ice sheet geometry and ice divide migration and variations in topography that influenced calving at the ice stream terminus

comparison of strong motion spectra with teleseismic spectra for three magnitude 8 subduction zone earthquakes we studied strong motion spectra observed for three m w 7 8 to 8 0 earthquakes the 1985 michoacan mexico 1985 valparaiso chile and 1983 akita oki japan earthquakes we determined the decay of spectral amplitude with distance from the station considering different measures of distance from a finite fault we compared strong motion spectra fourier acceleration spectra observed for these three earthquakes with those estimated from the source spectrum determined from teleseismic p waves we scaled the teleseismic source spectra to produce reference strong motion spectra at periods from 1 to 10 sec using a simple physical model of far field s body waves from a point source recorded at the surface of a homogeneous half space for all three earthquakes the reference spectral amplitudes at periods of 1 to 5 sec are about half the observed ones at distances of about 50 km the difference increases as the distance increases at distances of 200 to 300 km the reference spectrum is about 1 10 of the observed one the difference between the reference and the observed spectrum is attributed to the contribution of phases other than direct s waves and to site response we applied corrections for the finiteness spatial extent of the source using a simple model of rupture propagation on a dipping two dimensional fault including the source finiteness did not improve the estimate substantially at periods from 1 to 20 sec but it modeled significant changes in the signal duration as a function of azimuth for the 1985 michoacan earthquake our results can be used to establish empirical relations between the observed spectra and the half space responses depending on the distance and the site condition if such empirical relations can be established source spectra determined from teleseismic records may be used to estimate strong motions

seismic cycle stress change in western taiwan over the last 270 years 1 the island of taiwan is affected by intense seismic activity which includes large events as the disastrous 1999 chi chi earthquake to improve seismic hazard assessment in this area we estimate the effect of both interseismic loading and major events since 1736 on the state of stress of major active faults we focus our approach on western taiwan which is the most densely populated part of taiwan we pay a specific attention to faults geometry and to both interseismic and coseismic slip distributions our results suggest that both earthquakes and interseismic loading before 1999 increase the coulomb stress in the north western part of the chelungpu fault a region which experienced the highest coseismic slip during the chi chi earthquake more importantly our results reveal a coulomb stress increase in the southern part of the changhua thrust fault below a densely populated area

grounding line proximal sediment characteristics at a marine based late stage ice stream margin this is the peer reviewed version of the following article ruther d c winsborrow m andreassen k forwick m 2017 grounding line proximal sediment characteristics at a marine based late stage ice stream margin journal of quaternary science 32 4 463 474 https doi org 10 1002 jqs 2939 which has been published in final form at https doi org 10 1002 jqs 2939 this article may be used for non commercial purposes in accordance with wiley terms and conditions for use of self archived versions

organic facies and maturation of jurassic cretaceous rocks and possible oil source rock correlation based on pyrolysis of asphaltenes scotian basin canada abstract this paper establishes source rock characterization and oil oil and oil source rock correlations of selected organic rich shale source rocks and selected light oils and condensates from jurassic and cretaceous formations of the scotian basin multiple source rocks of kerogen types iia oil prone iia iib oil and condensate prone iib condensate and gas prone iii gas prone and iv nonsource are identified in various stratigraphic horizons vitrinite reflectance and t inmax data indicate variable maturation histories in different parts of the basin which are related to differences in lithology related heat conductivity and the presence of older sediments closer to areas of rifting multiple linear regression and cluster analyses of aromatic hydrocarbon ratios reveal two oil families some kerogen type iib rock extracts match many of the light oil and condensates in terms of the aromatic hydrocarbon distributions a second family of oils could not be correlated with any of the analyzed source rock extracts pyrolysis gas chromatography of asphaltenes from the cohasset a 52 and sable island 3h 58 wells are significantly different in n alkane aromatics and phenol content the asphaltene pyrolyzates of samples from venture b 52 and s desbarres o 76 have a high aromatic content low content of n alkanes and high phenol values s desbarres which is typical of a terrestial source they correlate well with one another and also with sable island light oil the n triumph g 43 whole rock and extract asphaltene pyrolyzates are dominated by normal hydrocarbons with aromatic phenolic and thiophenic compounds in low concentrations suggesting an algal source algal type organic matter may have made a contribution to the cohasset petroleum as its asphaltene pyrolyzate is also dominated by normal hydrocarbons

cenozoic erosion and sedimentation in the western barents sea abstract seismic reflection data and well logs are used to interpret cenozoic sedimentary environments in the south western barents sea using volumetric calculations and compaction trend analysis the cenozoic erosion and sedimentation rates have been quantified marine environments ranging from neritic to bathyal water depths dominated the southern barents sea during the late palaeocene at the palaeocene eocene boundary a southwards progradation commenced along the uplifted and eroded loppa and stappen highs during the early eocene to mid miocene sedimentation continued in the intracratonic basins as well as along the newly established continent ocean boundary in the embryonic northern norwegian greenland sea substantial erosion occurred on the westernmost margin particularly on the stappen high during this time interval in the mid miocene during a period of relative lowstand of sea level a system of lowstand fans was deposited west of the stappen high later a pronounced progradation of the shelf edge commenced during the mid miocene to late pliocene the shelf edge prograded between 20 and 40 km during the mid pliocene to early pleistocene the barents sea and adjacent land areas were uplifted a fluvial glaciofluvial drainage system developed on the continental shelf resulting in pronounced erosion there and very high sedimentation rates on the continental margin during the last 0 8 ma several large glaciations on the barents sea shelf proper have resulted in overdeepened troughs and high sedimentation rates at the continental shelfbreak and on the continental slope the overall cenozoic erosion varies spatially as well as temporally in the svalbard area close to 3 km of rocks have been eroded since the oligocene the same amount or more was eroded from the stappen high during the palaeocene and early neogene in the south western areas during the eocene mid miocene erosion occurred along the margin in addition the loppa high and adjacent areas served as sediment source areas during the late pliocene and pleistoncene the average erosion in the south western barents sea amounted to about 600 m erosion was greater in the south east than to the west and north and was more extensive where the former glacial ice streams had flowed

cenozoic margin development and erosion of the barents sea core evidence from southwest of bjornoya abstract five stratigraphic boreholes at the western barents sea margin reveal an evolution from a palaeocene normal shelf setting to a complex late cenozoic offshore marginal marine system influenced by mass flows and finally a glacially influenced shelf environment palaeocene epeiric sea sedimentation was sourced by erosion at the stappen high earliest eocene extension associated with volcanism was followed by rapid subsidence of the vestbakken volcanic province which became filled by sediments from an uplifted stappen high by the middle eocene siliciclastic shelf sediments were deposited and the infill had reached a balance with subsidence tectonic movements at this time may have caused local uplift which restricted the ventilation of local basins west of the stappen high but marine conditions existed in the area until early oligocene times when subaerial exposure was caused by tectonic uplift and probably also a fall in eustatic sea level pronounced oligocene and pliocene unconformities reflect major events of uplift and erosion at the margin and abundant fossil reworking indicates erosion of the uplifted cretaceous and tertiary strata the oligocene erosion had its associated depocentre to the south and west of the study area and was less important regionally than the pliocene pleistocene phase the pliocene uplift first led to local erosion along the margin subsequent subsidence may initially have had a thermal component this was followed by extensive downwarping under the load of largescale deposition sourced by regional erosion in the barents shelf region an eocene volcanic phase was probably related to the initiation of seafloor spreading isotopically dated volcaniclastic intervals in one of the cores give evidence of a late pliocene volcanic phase this event was associated with local tectonic movements which may be part of a regional uplift in the western northwestern part of the barents sea glaciations caused by the neogene climatic deterioration became enhanced by uplift and this led to extensive regional glacial erosion the main depocentre was offset to the south by ice flow curving around the uplifted area

evolution of subglacial bedforms along a paleo ice stream antarctic peninsula continental shelf geophysical data from the antarctic peninsula continental shelf reveal streamlined subglacial bedforms in a cross shelf trough bedforms exhibit progressive elongation with distance along the trough and record flow of a paleo ice stream from the antarctic peninsula ice sheet during the last glacial maximum downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf this in turn is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf although streaming flow operated across both substrates the highest flow velocities occurred over the soft bed spatial variation in the inferred nature of fast flow from sliding to subglacial sediment deformation and or ploughing was also lithologically controlled these data highlight the control of subglacial geology on ice stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo ice streams

model for prediction of organic carbon content in possible source rocks abstract a quantitative model has been developed to improve the prediction of the occurrence of marine siliciclastic source rocks and their likely organic carbon content the input parameters of this quantitative model are organic matter supply in the surface layer of the ocean photic zone water depth calculation of the carbon flux sedimentation rate and preservation conditions presence or absence of oxygen at the sediment water interface to handle the difficulties in the quantification of input parameters a probabilistic approach is applied depending on the available data or the interpretation of the kind of environment different risk distribution functions are used to describe the possible range of each input parameter the calculation is then carried out using the monte carlo simulation technique resulting in a probability distribution of the total organic carbon content which forms the basis of the analysis different scenarios currently under debate e g high primary productivity or the possible effects of enhanced preservation under anoxic bottom water conditions and the influence of varying water depth and sedimentation rate can be tested very quickly using examples of recent peru continental margin and ancient lower toarcian shale germany organic rich marine shales the potential of this tool is demonstrated firstly to quantify souce rock quality in this case total organic carbon content only secondly to assess quantitatively the uncertainties in source rock prediction e g evaluation of organic matter supply water depth and sedimentation rates and thirdly to determine source quality risk values to be used in the appraisal of sedimentary basins and prospects

significance of coniferous rain forests and related organic matter in generating commercial quantities of oil gippsland basin australia contrary to the conventional belief that humic coal generates primarily gas 3 billion bbl of recoverable oil has been discovered in the humic coaly succession of the fluviodeltaic latrobe group upper cretaceous tertiary that serves as both the reservoir and the source for hydrocarbons in the offshore gippsland basin of southeastern australia evidence for generation of liquid hydrocarbons from the coaly succession includes 1 similarity of n alkane distribution in the oil and in the coal extracts 2 high wax content of oil up to 27 by weight 3 high ratio of pristane phytane in oil 5 6 and 4 dominance of c29 steranes in the oil in the gippsland basin coniferous rain forests dominated by kauri vegetation flourished in a raised bog setting present temperate climate and kauri vegetation of new zealand are considered to be the modern analog to the gippsland basin the coniferous vegetation provided large quantities of hydrogen rich exinite macerals such as cutinite and resinite with potential to generate oil high rainfall raised ground water level low oxygen high acidity and low nutrient conditions of a raised bog setting were suitable for preserving organic matter a comparison of gas chromatograms of oils in the gippsland basin with gas chromatograms of oils generated by hydrous pyrolysis in the laboratory from the immature source rocks suggests that the paraffinic fraction of the oil was derived from coal and the naphthenic fraction was derived chiefly from resin

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

source rock characteristics of the lower cretaceous abu gabra formation in the muglad basin sudan and its relevance to oil generation studies organic rich shale and claystone are present within the lower cretaceous abu gabra formation of the muglad basin representing excellent oil source rocks the source rock potential of abu gabra organicrich whole rock samples from two oilfields in the muglad basin was investigated using bulk and quantitative pyrolysis techniques the analytical program included toc pyrolysis sra pyrolysis gc bulk kinetics elemental analysis and kerogen microscopy the results were used to characterize the kerogen composition and type of petroleum generated from these abu gabra organic rich sediments to clarify the structural variability of the organic matter in the abu gabra source rock the investigated abu gabra organic rich sediments proved to contain excellent quality lacustrine kerogens the samples have moderate to high hydrogen index hi values 267e837 mg hc g rock consistent with predominantly oil prone types i and ii kerogen with a minor contribution of type iieiii kerogens this is supported by the abundant liptinitic materials i e lipid rich structured algae structureless amorphous alginite and sporinite and high atomic hydrogen to carbon h c ratio however pyrolysis gc analysis proved that all the investigated abu gabra samples contain homogeneous type i kerogen that produces mainly waxy oil bulk kinetic experiments on these samples indicate that the source rock facies derived from a homogenous organic matter spans an extremely narrow distribution of activation energy ea ranges from 49 kcal mol to 51 kcal mol the magnitude order of frequency factors a also show a narrow distribution range from 10

the vestfjorden traenadjupet palaeo ice stream drainage system mid norwegian continental shelf abstract an ice stream in the vestfjorden traenadjupet depressions off northern norway transported glacial debris to the shelf edge and into the deep sea during the last glaciation our interpretation is based mainly on regional and detailed bathymetry and 2d and 3d seismic data this palaeo ice stream was approximately 400 km long and covered an area of 20 000 km 2 including a complex system of tributary fjords and valleys in nordland county the drainage basin had an area of ca 150 000 km 2 within the fennoscandian ice sheet fis during the last glacial maximum lgm the location of the palaeo ice stream prevented large ice masses from reaching the montainous areas of lofoten and vesteralen a local ice dome was established in these areas the palaeo ice stream eroded extensively into both mesozoic and cenozoic sedimentary rocks and quaternary sediments a pattern of well developed subglacial sedimentary bedforms was produced e g mega scale lineations drumlins ice stream shear margin moraines are located on both sides of traenadjupet defining the width of the palaeo ice stream 90 km at the shelf edge these moraines are commonly 10 20 m high a few kilometres wide and reach several tens of kilometres in length a large recessional moraine the tennholmen ridge has been mapped in vestfjorden it is 80 m high 20 km wide and 60 km long we have interpreted this ridge as a grounding line moraine formed during a halt or a readvance of the ice stream during the last deglaciation the large transport of glacial debris towards the grounding line occurred during this stage northeast of the tennholmen ridge several smaller moraine ridges were developed at still stands during deglaciation

hard rock spectral models for intermediate depth vrancea romania earthquakes the frequency dependent amplification for rock sites as well as apparent source spectra were studied using an earthquake ground motion database collected in romania almost all events occurred in the vrancea focal zone southeast carpathians which is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume the seismic activity ranges within an almost vertical stripe in depths between 60 and 170 km the database used includes several hundred records from more than 100 small magnitude m w 3 5 5 3 earthquakes that occurred in 1996 2004 and six acceleration records obtained during four large m w 7 2 6 9 6 3 and 6 0 earthquakes the characteristics of amplification were evaluated using the well known technique of horizontal to vertical h v fourier spectral ratio of the s wave phase lermo and chavez garcia 1993 the apparent source spectra were obtained from horizontal components using site h v propagation path geometric spreading and anelastic attenuation and near surface attenuation kappa factor corrections seismic radiation within the frequency range from 0 2 0 3 to 15 20 hz during the earthquakes of various magnitudes in the vrancea zone may be described by an omega square single corner frequency spectral model 2 brune spectrum the stress parameter which controls high frequency spectral amplitudes increases with magnitude from 20 30 bars for m w 3 5 up to 200 250 bars for m w 4 8 5 3 and up to 1000 bars for the case of large m w 6 0 6 5 events empirical amplification h v for rock sites showing stability for particular locations reveal high variability from station to station which together with variations of values of kappa reflects influence of local geologic and geomorphologic factors

submarine glacial landforms and rates of ice stream collapse the rate of deglacial ice sheet retreat across polar continental shelves and possible ice stream collapse and sea level rise has been much debated high resolution imagery of seafloor morphology is available for many polar shelves and fjords the rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms produced at ice stream sedimentary beds these landforms exposed by ice retreat across high latitude shelves demonstrate that deglaciation occurs in three main ways rapidly by flotation and breakup episodically by still stands and or grounding events punctuating rapid retreat or by slower retreat of grounded ice submarine landform assemblages imply through the presence of grounding zone wedges overprinting mega scale glacial lineations on many polar shelves that ice stream retreat is more often episodic than catastrophic these observations provide a robust test of the ability of numerical models to predict the varied response of ice sheet basins to environmental changes

ice stream retreat dynamics inferred from an assemblage of landforms in the northern barents sea abstract palaeo records of rapid ice stream retreat are important as they can be used to identify the processes involved and inform investigations of present day ice masses here we document a previously unknown retreat stage in northernmost bjornoyrenna bear island trough in the northern barents sea representing the youngest stage in a stepwise retreat of the bjornoyrenna ice stream we present a descriptive landsystem model for retreat of a marine based ice stream which provides new insights into the glacial dynamics of the episodic retreat stages this model captures the landforms produced during a cycle of 1 fast ice stream flow 2 intense calving of large icebergs locked in a dense matrix of smaller icebergs probably from a collapsed ice shelf 3 ice stream stagnation 4 ice stream floating off forming an ice shelf as the grounding line retreats and eventually 5 ice shelf disintegration we hypothesize that the presented ice stream retreat model reflects glacial surging the formation of corrugated furrows in the study area is consistent with interpretation of similar features in pine island bay west antarctica which are interpreted to have been formed during a massive ice shelf break up and associated grounding line retreat two directions of past ice flow are indicated from streamlined glacial landforms in the study area ice flow from north north east is indicated for the last glacial maximum and an early phase of deglaciation whereas a shift to flow from north north west from an ice dome located over hinlopenstretet svalbard is indicated for the latest deglaciation phases of the bjornoyrenna ice stream

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

large earthquakes and crustal deformation near taiwan source parameters of 15 large mb 6 earthquakes which occurred near taiwan during the last 25 years are determined by synthetic seismogram analysis of teleseismic p and s waveforms source depths range from 13 to 88 km three earthquakes of intermediate depth are located within the southernmost limit of the ryukyu arc and show thrust type mechanisms with t axes subparallel to a northward dipping wadati benioff zone shallow earthquakes 30 km are distributed over a broad region along and off the east coast of taiwan and show both high angle reverse and strike slip mechanisms the p axes of the shallow events located off the east coast of northern taiwan are oriented near horizontal trend south to southwest and may reflect relative motion between the okinawa platelet and taiwan or complex deformation at the southwest limit of the ryukyu trench in contrast p axes of shallow earthquakes both onshore and off the east coast of central and southern taiwan consistently trend northwest and are interpreted to reflect relative convergence between the philippine sea and eurasian plates summation of the seismic moment tensors of eight large shallow earthquakes that occurred along and off the central and southern coast of taiwan shows that relative plate motion between the eurasian and philippine sea plates when averaged over the last 25 years has been accommodated by crustal contraction at a rate of 2 6 5 4 cm yr along an azimuth of about 290 o

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

buried iceberg ploughmarks in the early quaternary sediments of the central north sea a two million year record of glacial influence from 3d seismic data abstract buried linear to curvilinear depressions interpreted as iceberg ploughmarks were identified through most of the 2 ma long and about 700 m thick early quaternary sedimentary record in two 3d seismic cubes from the central north sea basin 56 58 n 2 3 e the mean width of 402 features measured at 6 time slices in cube c08 was between 49 and 63 m and mean length was 2 5 to 3 7 km mapping the

the tectonized central peak of the mjolnir impact crater barents sea abstract the mjolnir structure sw barents sea is one of the best preserved marine impact craters on earth after impact on the paleo seafloor about 142 ma ago this crater experienced an atypical deformation of its central peak which is now elevated 435 m above the crater rims here we investigate the effect of far field tectonic stresses on the central peak uplift based on interpretation of new high resolution p cable and conventional seismic reflection data nearby wells provided stratigraphic control on the interpreted horizons the reconstruction of the crater sedimentary infill supports a subdued original central peak relief with a 5 km wide gentle mound 15 m below the rim our interpretation shows that subvertical outward dipping impact induced faults were reactivated by uplifted segments of the central peak up to 500 m above the platform level during one or several contractional episodes we postulate that post albian tectonic compressional events triggered the structural uplift of the mjolnir central peak differential compaction previously seen as the main deformation process may have increased the original central peak height by only 10 m the mobilization of impact shattered rocks by tectonic compression provides a new and robust explanation for the structural rise of mjolnir s central peak

repeated fluid expulsion through sub seabed chimneys offshore norway in response to glacial cycles abstract focused fluid flow through sub seabed sediments is a common phenomenon on continental margins worldwide however the governing controls and timing of this fluid release have been difficult to understand in particular for fluid flow features buried beneath sub surface sediments a link between fluid flow activity ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated norwegian margin new high resolution p cable 3d seismic data from the nyegga area on the mid norwegian margin reveal at least two more periods of fluid expulsion from sub seabed sediments the 3d seismic data show depositional patterns within chimney features expressed as truncations of seismic horizons against the flanks of the chimneys the truncations are interpreted by analogy with present day observations as evidence for buried carbonate mounds and or sediment wash out during formation of pockmarks in the past the truncations are hence an indicator and a chronological marker for fluid expulsion in the past the classification of chimneys results in three major groups 1 chimneys that have been formed and consecutively reactivated one or two times during the last 200 kyr and that have a fluid flow expression at the present seafloor 2 chimneys that are approx 125 160 ka bp old without any associated fluid flow expression at the present day seafloor 3 chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum 18 25 ka bp the observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region the emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the naust formation at nyegga

the seismic architecture and geometry of grounding zone wedges formed at the marine margins of past ice sheets grounding zone wedges form where halts of decades to centuries during ice sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast flowing ice streams thirty grounding zone wedges were identified on two dimensional seismic reflection profiles across the continental shelves of ne and nw greenland grounding zone wedges close to the seafloor are probably of late weichselian age others are on paleoshelves buried beneath thick prograding glacial sediments several greenland shelf grounding zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water grounding zone wedges are asymmetric in the ice flow direction with steeper ice distal sides typical grounding zone wedges are approximately 5 20 km long and 50 100 m thick with a lateral width of several tens of kilometers forty grounding zone wedges from the greenland norwegian and antarctica margins show considerable variability about these values grounding zone wedge dimensions are controlled by sediment flux the duration of halts in ice retreat subice cavity shape and ice stream width low angle ice shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space preventing formation of high amplitude ridges grounding zone wedges are mainly transparent or chaotic on seismic profiles probably resulting from delivery of diamictic debris offlapping reflectors represent progradation into an ice roofed cavity reflector truncations of grounding zone wedge bases indicate erosion during initiation channels are present within some grounding zone wedges meltwater flow is under high pressure and v shaped incisions suggest high energy flow channels are not ubiquitous within greenland grounding zone wedges and may represent nonsteady meltwater release perhaps through lake drainage where ice sheet mass loss is dominated by meltwater runoff grounding zone wedges probably contain more sorted sediment than those from greenland simple models of grounding zone wedge architecture are presented grounding zone wedges indicate episodic rather than catastrophic ice sheet retreat and are a mechanism for ice shelf stabilization because wedge growth counteracts collapse induced by ice sheet thinning and sea level rise

spatial variations in heat at the base of the antarctic ice sheet from analysis of the thermal regime above subglacial lakes antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure melting point we have used a one dimensional vertical heat transfer equation to determine theoretical temperature values for the ice sheet base above 77 subglacial lakes identified from airborne radio echo sounding data covering 50 of antarctica variations in our temperature results to below the pressure melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from a internal ice deformation and b basal sliding in the thermal model our results indicate that when the geothermal heat flux is set at 54 mw m 2 the ice sheet base above 70 of the known antarctic subglacial lakes is calculated to be at the pressure melting value these lakes are located mainly around dome c ridge b and vostok station for the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide using the same thermal model loss of heat due to vertical advection is calculated to be relatively high in such regions if the ice sheet base is at the pressure melting point heat lost due to vertical advection must be supplemented by heat from other sources for the three lakes beneath terre adelie and george v land for instance the basal thermal gradient calculated to produce pressure melting at the ice sheet base is equivalent to 1 5 2 times the value obtained when 54 mw m 2 of geothermal heat is used as the sole basal thermal component we suggest that as distance from the ice divide increases so too does the amount of heat due to internal ice deformation and basal sliding moreover by considering the ice sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline we demonstrate empirically that the heat due to these horizontal ice motion terms varies pseudo exponentially with distance from the ice divide the location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large scale basal sliding

deterministic frequency wavenumber methods and direct measurements of rupture propagation during earthquakes using a dense array data analysis data recorded by the smart 1 array in taiwan are used to make the first direct measurements of two dimensional earthquake rupture propagation using frequency wavenumber techniques and ray theory we obtain estimates of the spatial extent duration and directions of rupture propagation during two earthquakes located offshore taiwan our estimates include uncertainties due to errors in fault plane location and orientation velocity structure and slowness ray parameter measured at the array we find that the january 29 1981 ml 6 3 taiwan earthquake ruptured unilaterally updip and toward the west on a 60 dipping 109 striking reverse fault using p waves we find its fault length and duration to be 25 18 km and 7 4 3 4 s respectively s waves indicate a fault length and duration of 27 15 km and 9 4 3 6 s respectively we suggest that the november 14 1986 ml 7 0 hualien taiwan earthquake was triggered by a shallow h 14 km foreshock ml 5 4 which occurred on a subparallel or splay fault approximately 5 s before the mainshock based on p waves rupture propagation during the foreshock is unilateral towards the northeast and slightly downdip on a 58 dipping 38 striking reverse fault its spatial extent and duration are approximately 23 km and 5 4 s the mainshock initiated at greater depth h 34 km than the foreshock with its primary direction of rupture unilateral toward the northeast and slightly updip on a 57 dipping 43 striking reverse fault using p waves we found a spatial extent and duration of 68 15 km and 12 4 2 2 s s waves indicate a similar extent duration and direction of propagation for the mainshock however we found it difficult to distinguish the foreshock and mainshock 5 waves we show by example that these results represent an improvement over what can be obtained using long period t 5 s teleseismic methods comparison of our results with those obtained using teleseismic data suggests either a short duration slip time function or that most slip occurs near the rupture front our results indicate that much of the complexity in the seismograms recorded during these events by the smart 1 array is due to spatial and temporal variations of the source this suggests that at least in some cases predictions of both the phase and amplitude of strong ground motion as a function of time would require a priori knowledge of source properties finally we recommend siting small 10 station dense arrays on land above offshore subduction zones which are considered seismic gaps because they can provide more detailed measurements of rupture propagation than is presently possible

signature of ice streaming in bjornoyrenna polar north atlantic through the pleistocene and implications for ice stream dynamics the geomorphology of palaeo ice stream beds and the internal structure of underlying tills can provide important information about the subglacial conditions during periods of fast flow and quiescence this paper presents observations from three dimensional seismic data revealing the geomorphology of buried beds of the bjornoyrenna bear island trough ice stream the main drainage outlet of the former barents sea ice sheet repeated changes in ice dynamics are inferred from the observed successions of geomorphic features megablocks aligned in long chains parallel to inferred ice stream flowlines and forming dipping plates that are thrust one on top of another are taken as evidence for conditions of compressive ice flow mega scale glacial lineations msgl and pull apart of underlying sediment blocks suggest extensional flow the observed pattern of megablocks and rafts overprinted by msgl indicates a change in ice dynamics from a compressional to an extensional flow regime till stiffening due to subglacial freezing is the favoured mechanism for creating switches in sub ice stream conditions the observed pattern of geomorphic features indicates that periods of slowdown or quiescence were commonly followed by reactivation and fast flow during several glaciations suggesting that this may be a common behaviour of marine ice streams

seismic architecture and sedimentology of a major grounding zone system deposited by the bjornoyrenna ice stream during late weichselian deglaciation abstract a 280 km wide sediment wedge in outer bjornoyrenna bear island trough south western barents sea has been investigated using 2d and 3d seismic data sediment gravity cores as well as regional swath and large scale bathymetry data the bathymetry data indicate a division into an up to 35 m high frontal wedge with large depressions and an upstream part characterized by mega scale glacial lineations msgl from seismic sections increasing erosion is demonstrated for the upstream part coinciding with the location of msgl whether the latter are depositional features postdating an extensive erosional event or formed by erosion remains inconclusive based on the distinct morphology and internal structures we infer that the system was deposited during a rapid readvance whereby the ice front pushed and bulldozed predominantly soft diluted proglacial sediments analyses in the eastern part of the sediment system reveal the existence of imbricated thrust sheets in the frontal part of the wedge this is suggested to imply upstream erosion of sedimentary rock and incorporation of thrusted blocks into the moraine forming a composite ridge locally we argue that observed large scale depressions are dead ice features in the marine environment it is envisioned that intense englacial thrusting may have developed into a decollement as the cold glacier snout got overrun by ice masses from the interior thereby enabling the inclusion of slabs of ice in the push moraine mass radiocarbon dates indicate that the sediment wedge was deposited around 17 090 cal yrs bp 14 530 14 c yrs bp and that the ice front probably remained stable until 16 580 cal yrs bp 13 835 14 c yrs bp

late cenozoic erosion of the high latitude southwestern barents sea shelf revisited the southwestern barents sea has experienced profound erosion during the last 2 7 m y that has resulted in the development of a characteristic glacial morphology of the continental shelf and deposition of a several kilometer thick sediment fan along the western margin prograding into the deep sea in the period from ca 2 7 to 1 5 ma proglacial processes including fluvial and glaciofluvial erosion dominated for this period the total average erosion of the shelf was 170 230 m the average erosion rate was 0 15 0 2 mm yr and the average sedimentation rates on the fan were 16 22 cm k y subglacial erosion affected an area of 575 000 km 2 during the period from ca 1 5 to 0 7 ma total average erosion is estimated at 330 420 m for this interval and the average erosion rate was 0 4 0 5 mm yr average sedimentation rates were 50 64 cm k y during the last 0 7 m y glacial erosion mainly has occurred beneath fast flowing paleo ice streams topographically confined to troughs 200 000 km 2 the total average erosion is estimated at 440 530 m average erosion rate is 0 6 0 8 mm yr and average sedimentation rate on the continental slope is 18 22 cm k y the amount of erosion was mainly determined by the duration of the glaciations and the location velocity and basal properties of the ice streams in total glacial erosion of the troughs has been relatively high throughout the last 2 7 m y at 1000 1100 m for the banks erosion is inferred to have increased from ca 2 7 ma to a peak between 1 5 and 0 7 ma subsequently little erosion occurred in these areas which implies a total of 500 650 m of erosion compared with other high latitude areas our rates are among the highest so far reported this comparison also demonstrates that there have been large variations in the rate of sediment delivery to the glaciated continental margins

toward one meter resolution in 3d seismic high resolution 3d hr3d seismic data are important for hydrocarbon exploration of shallow reservoirs site characterization and geohazard assessments the goal of this contribution is to identify and quantify the parameters to increase the resolution of hr3d seismic data to meter scale the main acquisition parameters controlling the resolution of the collected data are the spectrum of the seismic source source receiver offset range and trace density an evolution to one meter scale resolution of 3d seismic will rely on combining a reproducible seismic source with high frequencies up to at least 600 hz a high uniform trace density of more than 4 million traces per square kilometer and an offset range shorter than approximately 200 m the resulting 3d seismic data volume will reach meter scale resolution for water and target depths of less than 600 m the proposed hr3d system will be suitable for 3d and 4d characterization of seabed properties and shallow stratigraphy the identification of geohazards and hydrocarbon leakage and monitoring the environmental impact of offshore activities the p cable 3d system is an excellent starting point for achieving one meter scale resolution due to its flexible and tight meter scale shot and receiver spacing

iterative deconvolution of complex body waves from great earthquakes the tokachi oki earthquake of 1968 abstract a method of body wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone a sequence of point sources each being characterized with the seismic moment the onset time and the location are iteratively derived from observed records at multi stations where the two dimensional extent of the source location is taken into account a modification is made of the iterative method of kikuchi and kanamori on the formulation of inversion procedure to facilitate the computation using this method we analyse long period p waves of the tokachi oki earthquake of 1968 mw 8 2 and obtain several significant subevents with time durations of 10 s their spatio temporal distribution shows that the rupture process consists of three characteristic stages a a stage of introductory rupture b a stage of main rupture and c a stage of aftershocks the main rupture takes place in the form of clustering around a few sites of the fault plane the largest subevent occurs in the northwestern corner the stress drop associated with this event is estimated to be 200 bars one order of magnitude higher than the stress drop averaged over the entire fault plane the sum of the seismic moments of the individual subevents amounts to 2 3 1028 dyn cm which approximately coincides with the one estimated from the analysis of long period surface waves this implies that the source of the tokachi oki earthquake consists of several major subevents with time durations of 10 s in addition to other minor subevents

high resolution 3d seismic study of pockmarks and shallow fluid flow systems at the snohvit hydrocarbon field in the sw barents sea accepted manuscript version published version available at https doi org 10 1016 j margeo 2018 06 012 accepted manuscript version licensed cc by nc nd 4 0

icebergs rework shelf sediments to 500 m off antarctica icebergs and sea ice rework the sediments of high latitude shelves producing modern diamicts ice keel turbates unrelated to glacial proximity off antarctica sidescan sonar data indicate the presence of ice gouge features formed by the physical interaction between ice keels and the sea bed these are recognized as incisions a few metres deep and tens of metres wide in water depths up to 500 m on the submarine bank tops and slopes off wilkes land and in the weddell sea subcircular depressions 30 to 150 m in diameter a washboard pattern and hummocky bed features also represent iceberg resting sites the freshness of sea bed morphology nearby holocene sediment ponding and active hydraulic sedimentary processes indicate that the sea floor is being reworked by iceberg keels tabular iceberg drafts in excess of 330 m have been measured and modeling studies suggest that nontabular iceberg drafts of 500 m are possible we conclude that a modern ice keel turbate deposit in the form of a poorly stratified diamicton is probably widespread on that part 54 of the antarctic shelf less than 500 m deep

the sources and deposition of organic matter in the late quaternary pigmy basin gulf of mexico abstract the concentration and carbon isotopic composition 13c of sedimentary organic carbon corg n c ratios and terrigenous and marine 13corg endmembers form a basis from which to address problems of late quaternary glacial interglacial climatic variability in a 208 7 m hydraulic piston core dsdp 619 from the pigmy basin in the northern gulf of mexico while interpretations of sedimentary 13corg time series records are often not unique paired analyses of 13corg and n c are consistent with the hypothesis that the corg in the pigmy basin is a climatically determined mixture of c3 photosynthetic terrigenous and marine organic matter confirming the earlier 13corg model of sackett 1964 a high resolution 1 4 2 9 ka sample 13corg record shows that sedimentary organic carbon in interglacial oxygen isotope sub stages 1 and 5a b are enriched in 13c average 1 values are 24 2 1 2 o and 22 9 0 7 o relative to pdb respectively while glacial isotope stage values 2 are relatively depleted 25 6 0 5 concentrations of terrigenous and marine sedimentary organic carbon are calculated for the first time using 13corg and corg measurements and empirically determined terrigenous and marine 13corg endmembers the net accumulation rate of terrigenous organic carbon is 4 3 2 6 times higher in isotope stages 2 4 than in sub stages 1 and 5a b recording higher erosion rates of terrigenous organic material in glacial times the concentration and net accumulation rates of marine and terrigenous corg suggest that the nutrient bearing plume of the mississippi river may have advanced and retreated across the pigmy basin as sea level fell and rose in response to glacial interglacial sea level change

glacial geology of outer bjornoyrenna southwestern barents sea the upper cenozoic glacigenic succession in outer bjornoyrenna bear island trough southwestern barents sea is investigated utilizing data from six shallow boreholes 143 m below sea bed and seismic profiling overdeepened troughs in the lowermost part of a large plio pleistocene wedge at the shelf margin are interpreted as glacial erosion forms this suggests that the overlying succession which basinward attains a thickness of several km was deposited during a period with glaciations large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north south trending zones parallel to the present shelf break they probably formed in a continental shelf coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations a core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till most of the other cores are dominated by middle and upper pleistocene muddy massive partly fissile diamictons interpreted to be mainly tills undrained shear strength of these cores ranges from about 20 kn m 2 to more than 300 kn m 2 and both underconsolidated intervals compared to the present burial depth are found an underconsolidated interval comprising eemian interglacial biourbated marine sediments overlain by laminated weichselian glaciomarine sediments on the outher shelf is covered by 50 100 m of muddy diamicton this diamicton is suggested to be mainly till deposited by a late weichselian ice sheet which was grounded to more than 500 m below present sea level earlier weichselian ice expansions did not reach the outer shelf based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca 440 ka the present study suggests that glacial erosion was a major process in the late cenozoic denudation of the barents sea and in large areas perhaps as much as 200 250 m of preexisting strata could have been eroded by glaciers since about 440 ka

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

source spectra of great earthquakes teleseismic constraints on rupture process and strong motion short period body waves recorded at teleseismic distances from great earthquakes provide information about source rupture processes and strong motions first we examine mostly wwssn records of 19 earthquakes of moment magnitude m w of 6 5 to 9 5 four parameters are measured from the short period p wave train the maximum amplitude the period at maximum amplitude the time between the first arrival and when the maximum amplitude is attained and coda length an extension m b of the teleseismic magnitude m b is defined using the maximum amplitude of the entire short period p wave rather than the amplitude achieved in the first few p wave cycles a least squares fit to the data yields the following relationship between m b and m w m b 0 53 m w 2 70 for m w 6 5 to 9 5 the time from the first arrival until the maximum amplitude is achieved and the coda length are roughly proportional to m w but are further interpreted by a simple asperity model of the rupture process these data support that short period waves are on average generated preferentially in the same regions of the fault plane as long period waves with periods of 10 to 50 sec we analyze the spectra of short and intermediate period teleseismic gdsn records for seven earthquakes with m w s of 6 4 to 7 8 and hand digitized short period wwssn records of the 1971 san fernando earthquake significant differences exist between the spectra of different events due partly to variations in tectonic setting or seismic coupling using the digital data we also investigate the relationship between time domain amplitude and spectral amplitude for short period p waves from our empirical relation between spectral amplitude and time domain amplitude we estimate the spectral amplitudes implied by the m b data we compare our results to the 2 and gusev spectral models neither model can completely represent the data nevertheless we consider the 2 model a useful reference model for comparing different events the average source spectrum of six large events with m w 7 4 to 7 8 does not have the spectral structure suggested by gusev an application to strong motion modeling is presented in which a 1971 san fernando teleseismic short period record is summed up to simulate teleseismic records produced by five great earthquakes the summation procedure matches the moment of the event to be simulated and includes rupture propagation fault plane roughness and randomness the m b data provide an important constraint on the summation procedures thus constrained this summation procedure can be more confidently used with near field strong motion records as green s functions to predict strong motions from great earthquakes

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

subglacial roughness of the former barents sea ice sheet 1 the roughness of a glacier bed has high importance for the estimation of the sliding velocity and can also provide valuable insights into the dynamics and history of ice sheets depending on scale measurement of basal properties in present day ice sheets is restricted to ground penetrating radar and seismics with surveys retrieving relatively coarse data sets deglaciated areas like the barents sea can be surveyed by shipborne 2 d and 3 d seismics and multibeam sonar and provide the possibility of studying the basal roughness of former ice sheets and ice streams with high resolution here for the first time we quantify the subglacial roughness of the former barents sea ice sheet by estimating the spectral roughness of the basal topography we also make deductions about the past flow directions by investigating how the roughness varies along a 2 d line as the orientation of the line changes lastly we investigate how the estimated basal roughness is affected by the resolution of the basal topography data set by comparing the spectral roughness along a cross section using various sampling intervals we find that the roughness typically varies on a similar scale as for other previously marine inundated areas in west antarctica with subglacial troughs having very low roughness consistent with fast ice flow and high rates of basal erosion the resolution of the data set seems to be of minor importance when comparing roughness indices calculated with a fixed profile length a strong dependence on track orientation is shown for all wavelengths with profiles having higher roughness across former flow directions than along them

lateral shear moraines and lateral marginal moraines of palaeo ice streams abstract an understanding of the nature of sedimentation at ice stream lateral margins is important in reconstructing the dynamics of former ice sheets and modelling the mechanisms by which sediment is transported beneath contemporary ice streams theories of the formation of ice stream lateral moraines islms have hitherto been based on a relatively limited number of terrestrial and marine examples here an inventory of islms is compiled from available studies together with independent analysis of seismic reflection and bathymetric datasets the locations and dimensions of 70 islms alongside a synthesis of their key architectural and geomorphic characteristics are presented two different types of islms are identified type 1 islms are up to 3 5 km wide and 60 m thick they maintain a constant width thickness and cross sectional shape along their length type 1 islms are interpreted and referred to as ice stream lateral shear moraines that form subglacially in the shear zone between ice streams and slower flowing regions of an ice sheet in contrast type 2 islms are up to 50 km wide and 300 m thick they are only identified close to the shelf break in the marine environment type 2 islms exhibit an increase in width and thickness along their length and their distal slopes become steeper in a seaward direction they contain internal dipping reflections that indicate sediment progradation away from the former ice stream type 2 islms are interpreted and referred to as ice stream lateral marginal moraines that were formed at the lateral boundary between ice streams and seafloor terrain that was free of grounded ice we suggest that using bathymetric images and acoustic profiles it is possible to differentiate between ice stream lateral shear moraines and lateral marginal moraines in the geological record this distinction is important for understanding the mechanisms of sediment transfer beneath ice streams and for making inferences about the conditions that existed beyond the lateral ice stream margin at the time of lateral moraine formation slow moving ice beyond lateral shear moraines and ocean water rather than ice beyond lateral marginal moraines

a conceptual model of the deglaciation of atlantic canada abstract we present a conceptual model for the deglaciation of atlantic canada in which a role is played by ice streams the ice margin lay close to the edge of the continental shelf at the last glacial maximum ice streams occupying shelf troughs comprised 1 a major ice stream in the laurentian channel 2 secondary streams in the bay of fundy gulf of maine trinity trough and notre dame channel and 3 lesser ice streams elsewhere early retreat in the northeast notre dame bay and in the southwest bay of fundy gulf of maine was mainly by calving along embayments by 18 ka radiocarbon years a calving embayment had opened on emerald basin scotian shelf retreat along channels marooned ice on intervening banks a major calving episode beginning just before 14 ka removed ice from the gulf of st lawrence triggering re advances down secondary troughs isolation of a newfoundland ice cap produced radial drainage via fiords and more extensive flow into the st george s bay lowlands on the west coast by 13 ka most ice was on land and was ablating by melting rather than calving shelf ice caps persisted until 11 ka

observed variations of earthquake motion across a sediment filled valley the response to earthquake motion of a site in one sediment filled valley was observed to depend strongly on frequency and the position of the site within the valley and weakly on the input signal s azimuth and incidence angles the observed input motion was weak varying between 10 5 and 10 3 g measurements of ground velocity were made on profiles across and along the valley and simultaneously on two nearby rock sites the valley was 400 m wide and 700 m long with a maximum sediment thickness of 60 m the average seismic impedance contrast between the sediments and underlying basement rock was about 6 1 ratios of fourier spectra from soil sites to spectra from nearby rock sites showed apparent site amplifications of as much as a factor of ten depending strongly on frequency and the distance of the site from the valley edge the scatter in spectral ratios for earthquakes with different azimuths and angles of incidence was about a factor of two thus local site effects that changed the amplitude of incident motion by a factor of two or more could be identified in the records of most earthquakes ground motion at valley edge and mid valley sites separated by less than 100 m differed by as much as a factor of five this difference could cause large differential motion in structures spanning these sites a theoretical flat layer model of sediment response predicts the average behavior of the middle of the valley but not that of the valley edge the lowest frequency responance apparently involves the sediments across the width of the valley not just the vertical column of soil beneath each site

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

massive blow out craters formed by hydrate controlled methane expulsion from the arctic seafloor widespread methane release from thawing arctic gas hydrates is a major concern yet the processes sources and fluxes involved remain unconstrained we present geophysical data documenting a cluster of kilometer wide craters and mounds from the barents sea floor associated with large scale methane expulsion combined with ice sheet gas hydrate modeling our results indicate that during glaciation natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates upon ice sheet retreat methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters we propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets

trough mouth fans palaeoclimate and ice sheet monitors abstract trough mouth fans are fans at the mouth of transverse troughs channels on glaciated continental shelves on the northwest european glaciated continental margin eight trough mouth fans varying in size between 2700 and 215 000 km 2 have been identified the trough mouth fans are depocentres dominated by debris flows accumulated in front of ice streams draining the former large northwest european ice sheets the debris flow units are separated by hemmipelagic interglacial interstadial sediments it is inferred that the number of debris flow units record the number of shelfbreak positions of the ice sheet margins the number found varies between three in the south and eight in the north typical trough mouth fans and related debris flow units seem to have been formed later than the early mid pleistocene thus the north european ice sheets did not form ice streams extending to the shelfbreak in any appreciable length before the mid pleistocene besides being loci for sediment deposition the trough mouth fans were also the main sites of fresh water supply to the ocean in the form of icebergs during the mid late pleistocene ice ages

hydrocarbon source rock generative potential of the sudanese red sea basin abstract to date the sudanese red sea like the red sea as a whole represents a grossly under explored basin recently the basin has seen increased exploration interest with more than 15 wells being drilled some of which have been petroleum discoveries gas and condensate with oil shows a similar exploration trend has been seen in the saudi eriteria and egyptian sectors of the red sea the red sea rift marks the break up of the afro arabian plate in eocene oligocene time the tectonostratigraphic evolution can be subdivided into three major phases pre rift syn rift and post rift phase generally terrestrials and marginal to deep marine sediments were deposited with both good reservoir quality and potential source rocks being present the pre salt rudeis kareem shales are expected to be good petroleum source rocks for the basin moreover shales of base zeit post salt are considered the main source of the known accumulated hydrocarbon gas and condensate in the shallow targets with the condensate charge also expected from intra evaporites shales and pre salt sources along the salt windows the measured amount of toc hi and hydrocarbon generation from pyrolysis of kerogen of the examined samples indicate source rock generative potential of these intervals the toc contents for base zeit and interbeded shales of dungnab formations range from 0 60 to 5 40 wt while the hydrogen index hi values are in the range of 50 600mg hc gtoc suggesting dominantly type iii ii kerogen the high toc 1 5 and hi 300 values of base zeit and dungnab formations are limited to some intervals within these formations consequently the base zeit source rocks are considered very good for condensate and gas generation in comparison to their equivalents burqan and maqna group sources hi 350 600 mg hc gtoc on the saudi red sea sector rudeis kareem shales could perhaps possess good quality type ii kerogen the red sea s high pressure and temperature regimes have a significant impact on hydrocarbon maturation generation and the quality of the preserved accumlations presently the zeit source varies in maturity across the basin and within the oil window in the suakin and bashayer areas and at gas window elsewhere to the east towards offshore on the other hand the pre salt rudies kareem formations are currently in the late oil to wet gas zone and probably out of gas phase distally offshore however the salt tectonics makes for challenging exploration the salt movement is considered recent as it is only the recent sediments that appear to be controlled by salt movement