radial upper mantle attenuation structure of inactive back arc basins from differential shear wave measurements we measure differential attenuation between ss s and sscs scs phase pairs to characterize the variation of attenuation with depth in the upper mantle of five inactive back arc basins the kuril basin sea of japan banda sea the celebes and sulu seas and the shikoku basin a spectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency band of 10 to 83 mhz two algorithms are employed to compute the vertically averaged attenuation structure a spectral stacking procedure and a least squares inversion in the spectral stacking method the individual spectra are corrected for the elastic structure at the ss or sscs bounce point and the differential attenuation operator is computed by spectral division the attenuation operators are then normalized and stacked by source depth to obtain more stable spectra and an average t for sources within a restricted depth range is obtained from the slope of the log amplitude spectrum a model for the depth dependent q structure is then calculated from the t measurements assuming q is frequency independent alternatively t measurements for individual phase pairs are made using a similar technique and analyzed by ray tracing and a least squares inversion to obtain the q 1 estimates the q results obtained from the stacking and inversion methods are generally in good agreement the q 1 structures for the various back arc regions are similar to each other within the uncertainties of the derived q models in addition to computing q models for each individual region we partitioned the data into three tectonic provinces based upon bounce point locations the resulting average radial q structure for an inactive basin shows a q of 54 in the uppermost mantle 115 at intermediate depths and 173 in the transition zone we also find a low q zone of 36 beneath active island arcs the q values for inactive back arc basins are lower than the global averages derived from normal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions the most striking feature of this study is the observation of very strong attenuation concentrated at shallow depths 160 km in the upper mantle beneath these basins

twin active spreading ridges in the north fiji basin southwest pacific recent work on the north fiji basin axial spreading ridge and on the active propagating ridge located west of the fiji islands has established the existence of two active spreading ridges that have been functioning synchronously in the basin since at least 1 or 1 5 ma this hypothesis is tested and the kinematics of the twin active ridges are refined further on the basis of analysis of bathymetric and magnetic data

seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

hydrocarbon generation migration alteration entrapment and mixing in the central and northern north sea abstract there is little doubt that sediments of upper jurassic to lowermost lower cretaceous age particularly those of the kimmeridge clay formation and its equivalents are the source of the vast bulk of the oil in the central and northern north sea this is true both of oil in conventional mesozoic plays and in tertiary clastic reservoirs the timing of oil generation and migration ranges from the late cretaceous through to the present the oil productivity of the kimmeridge clay formation in the central graben moray firth and south viking graben areas of the north sea is estimated to be in excess of 250 billion barrels of which about 25 is accounted for in accumulations discovered to date there is strong areal differentiation between oil trapped in mesozoic reservoirs as opposed to upper cretaceous chalk and tertiary reservoirs this paper is particularly concerned with hydrocarbons in tertiary reservoirs which have a wide range of compositions and appear to have complex accumulation and degradation histories migration to chalk and younger tertiary reservoirs occurs often through thick shale sequences the evidence for the timing and mechanism of this process is considered and a preferred model for the vertical migration and subsequent lateral migration mixing and degradation is proposed

simulation of oil expulsion by 1 d and 2 d basin modelling saturation threshold and relative permeabilities of source rocks abstract in order for oil to accumulate in economic quantities it first has to be generated and expelled from source rocks in sufficient quantities in spite of long term efforts the mechanism of oil expulsion from the source rocks is not completely understood for modelling expulsion the adoption of pressure driven multiple phase fluid flow governed by darcy s law is widely accepted however relative permeabilities for fine grained source rocks which is an essential parameter for this model are very difficult to specify the conventional reservoir rock curve is obliged to be used for the modelling simplification of the relative permeability model is generally used for one dimensional basin modelling the 1 d model also requires substantial optimization of the key parameter saturation threshold laboratory measurement of relative permeabilities for fine grained rocks is very difficult therefore we carefully interpreted the analysed laboratory data on various sandstones to establish a relationship between relative permeability curves and pore geometry parameters with a view to extrapolate the relationships developed in sandstones to fine grained source rocks it was found that the relative permeability curves or irreducible water saturation are controlled by two factors grain size and clay content since the total surface area of the pore system becomes greater as the grain size decreases we consider surface water adsorbed on the grain surface as a part of irreducible water since clay contains micro porosity which cannot be displaced by oil due to high capillary pressure we regard this to also play a role prediction of relative permeability curves for fine grained rocks by both processes results in the curve with high irreducible water saturation the new relative permeability curves were tested by both one dimensional and two dimensional basin modelling test results indicate that the new curves can reproduce expulsion efficiency locations of accumulations and leaking through cap rock which is consistent with actual observations

lithofacies relationships in a late quaternary gravel and loess fan delta complex new zealand abstract coastal cliffs up to 70 m high provide a nearly continuous section along depositional strike of a small quaternary fan delta complex south of conway flat north canterbury new zealand a gentle northeasterly dip permits observation of all facies of the complex from the offshore prodelta to an emergent interdeltaic embayment facies showing beds with abundant rhizoliths and in situ roots and stumps of large 8000 yr old podocarp trees coarse gravels derived from the hawkeswood range 650 m high 5 km west of the sea cliffs were transporte largely by debris flow processes into the pacific ocean towards the conway trough 800 m deep 10 km to the east of the cliffs narrow up to 1 km wide gilbert type gravel deltas built elongate lobes into channels created by slumping and sediment gravity flows which deeply incised muddy prodelta deposits between the fan deltas embayment facies are dominated by loessial sediment with sand silt clay ratios of about 7 68 25 thin sand beds are present prodelta sediments also are composed largely of redeposited loess loessial sediment came from the distant glaciated southern alps transported both in air and along major rivers that produced broad plumes of suspended sediment in the ocean which were carried northwards by the prevailing longshore currents laterally adjoining the fan delta lobes on several wave cut benches within them and scattered in the distal embayment facies are winnowed wave sorted bars of fine gravel and sand the proximal embayment facies includes sparse poorly sorted gravelly storm bars gravelly fluvial crevasse deposits and shallow broad tidal channel filled with loessial sediment and fine sand well displayed trace fossils permit close correlation of ichnofacies with lithofacies and extend paleoenvironmental interpretation in this young deposit trace fossil associations and interpretations are described in the following paper in this journal

hydrocarbon generation and migration from jurassic source rocks in the e shetland basin and viking graben of the northern north sea in the e shetland basin oil generation began 65 ma ago peak oil generation maturity occurs today at 3250 m 0 7 r 0 and was first reached 40 50 ma ago the oil generation threshold is at 2500 m highest oil saturations in the kimmeridge clay occur at 0 8 r 0 oil expulsion efficiencies are 20 30 oil phase migration has probably occurred through oil wet kerogen laminae and through interconnected large pores aided by low oil water interfacial tensions oil migrated along strong lateral fluid pressure gradients from overpressured source rocks in half grabens to jurassic reservoirs in tilted fault blocks in the viking graben the kimmeridge clay is at oil floor maturity below 4500 m oil and peak oil generation began 7c 80 and 55 65 ma ago respectively 40 ma ago the kimmeridge clay passed through peak generation and gas generation by cracking of oil had begun peak dry gas generation from brent coals occurs today below 5000 m and began 40 ma ago the frigg field gas probably generated from late jurassic source rocks migrated through microfractures in overpressured mudstones below 3500 m above 3500 m methane probably migrated in aqueous solution and was exsolved in the early tertiary aquifer

sedimentology and reservoir potential of matilija sandstone an eocene sand rich deep sea fan and shallow marine complex california the matilija sandstone of the santa ynez mountains of southern california records a major regressive event in the eocene santa ynez basin in which turbidites were deposited and subsequently covered by shallow marine deposits despite thick favorable source beds and generally good initial reservoir characteristics the matilija sandstone is not a productive unit in the basin low reservoir permeability generally less than 1 md and porosity 1 to 10 are interpreted to be the result of early compaction cementation and diagenesis the lower part of the matilija sandstone is a 1 300 ft 400 m thick sand rich deep sea fan complex which overlies basin plain turbidite deposits of the juncal formation the matilija sandstone consists of anastomosing outer fan lobes lobe fringe and interlobe deposits overlain by channelized middle and inner fan deposits the upper matilija sandstone consists of shallow marine cross bedded sandstone red beds and carbonate evaporite lignite sequences which overlie the turbidites deposition of the matilija sandstone was terminated by a rapid basin deepening event which culminated in deposition of basin plain slope and turbidite deposits of the cozy dell formation the lower matilija sandstone deep sea fan deposits have a high sandstone to shale ratio 4 1 to 10 1 and consist of submature arkoses folk 1980 the depositional lobes consist of facies b c and d turbidites and a channelized sequence of facies a and b turbidites the average sandstone is medium grained moderately sorted subangular to subrounded and massive containing 40 quartz 35 feldspar about equal amounts of plagioclase and potassium feldspars 10 lithic fragments mostly volcanic and metamorphic types and minor amounts of detrital clay illite mica chert and heavy minerals early compaction carbonate cementation and authigenic pore lining and pore filling chlorite albite potassium feldspar quartz and localized laumontite have reduced the initial porosit and permeability the only remaining porosity is secondary resulting from the dissolution of detrital feldspars and minor fracture porosity

depositional facies and progradational sequences in eocene wave dominated deltaic complexes southwestern oregon the tyee and coaledo formations of southwestern oregon contain lateral and vertical transitions of delta to shelf facies within an eocene forearc basin fluvial marsh swamp delta distributary channel delta front prodelta shelf and delta margin facies wave reworked deposits are identified coarse grained plant rich sandstones characterized by large scale contorted cross bedding and northwest trending paleocurrent indicators suggest fluvial contributions during river flood stages within delta distributary channels in marine deposits abundant hummocky cross stratification flat laminations and symmetrical ripples indicate dominance of wave processes two types of vertical shoaling sequences are distinguished by their capping lithology type one sequences formed in moderate wave energy and are capped by delta distributary sandstones type two sequences are capped by delta margin facies and indicate reworking during higher wave energy conditions both successions record variations in shoreline progradation basin subsidence sea level change or a combination of these processes wave dominated deltaic deposition was a persistent style of cenozoic sedimentation in western oregon this study may be useful for predicting facies relationships and geometries in similar eocene systems such as the gas producing cowlitz formation mist field northern oregon and may enable a better assessment of hydrocarbon accumulations in the pacific northwest

systematic variation of bedrock channel gradients in the central oregon coast range implications for rock uplift and shallow landsliding abstract in tectonically active regions bedrock channels play a critical role in dictating the pace of landscape evolution models of fluvial incision into bedrock provide a means of investigating relationships between gradients of bedrock channels and patterns of active deformation variations in lithology orographic precipitation sediment supply and erosional processes serve to complicate tectonic inferences derived from morphologic data yet most tectonically active landscapes are characterized by these complexities in contrast the central oregon coast range ocr which is situated above the cascadia subduction zone has experienced rock uplift for several million years did not experience pleistocene glaciation boasts a relatively uniform lithology and exhibits minor variations in precipitation although numerous process based geomorphic studies suggest that rates of erosion across the ocr are relatively constant it has not been demonstrated that bedrock channel gradients in the region exhibit spatially consistent values analysis of broadly distributed small drainage basins 5 20 km in the central ocr enables us to explore regional variability in bedrock channel gradients resulting from differential rock uplift or other sources consistent with previous studies that have documented local structural control of deformed fluvial terraces in the western portion of our study area our data reveal a roughly 20 km wide band of systematically elevated channel slopes roughly twice the background value roughly coincident with the strike of n s trending mapped folds although many factors could feasibly generate this pattern including variable rock strength precipitation gradients or temporal or spatial variations in forearc deformation the elevated bedrock channel slopes likely reflect differential rock uplift related to activity of local structures importantly our analysis suggests that rock uplift and erosion rates may vary systematically across the ocr although our calculations were focused on the fluvial dominated portion of study basins our results have implications for upstream areas including unchanneled valleys that often serve as source areas for long runout debris flows zero order basins or topographic hollows within the n s trending band of elevated channel slopes tend to be steeper than adjacent areas and may experience more frequent evacuation by shallow landsliding thus this region of the ocr may be highly sensitive to land use practices and high intensity rainstorms

propagating rift west of the fiji archipelago north fiji basin sw pacific a sea beam survey of an area located west of the fiji islands in the north fiji basin between 17 10 s and 18 s and 175 40 e and 176 40 e shows a complex structure characterized by two deep grabens bounding a central plateau in previous papers we interpreted this area as an intraoceanic strike slip deformation zone rejecting the possibility of oceanic spreading in the area by comparison with the structures observed in the galapagos 95 5 w area we now reinterpret the area as a southward propagating spreading system the analysis of structural and magnetic data and earthquake focal mechanisms confirms this new interpretation the full spreading rate for the north fiji basin ridge deduced from magnetic lineations varies from 50 to 80 mm yr for the west fiji spreading ridge the magnetic anomaly analysis gives a spreading rate of about 50 mm yr for the northern segment and slightly slower 40 mm yr for the southeastern segment using a 50 mm yr spreading rate the angles between the pseudofaults allow us to calculate the propagation velocity for an angle of 75 north of 17 35 s the propagation velocity is 33 mm yr the tip of the propagator is characterized by a 20 angle between the pseudofaults which implies a recent increase of propagation velocity to 141 mm yr the initiation of this propagation could be related to the emplacement of the north fiji fracture zone 1 to 1 5 m y ago resulting in the formation of the 16 50 s triple junction the main kinematic implication of the existence of a propagating rift west of the fiji islands is that in the north fiji basin the present day accretion is distributed on two parallel active spreading ridges one in the central part of the basin the other one in its southeastern quadrant such a phenomenon unusual on mid oceanic ridges except for microplates such as galapagos easter and juan fernandez may be more common in marginal basins because of their peculiar geodynamic setting

middle cenozoic depositional tectonic and sea level history of southern san joaquin basin california the eocene to lower miocene fill of the southern san joaquin basin contains three complete depositional sequences the tejon san emigdio and pleito each of which corresponds to a formal formation the tejon sequence lower to middle eocene is marine and incorporates nearshore shelf slope and basinal deposits the san emigdio upper eocene to lower oligocene and pleito upper oligocene sequences intertongue eastward with alluvial fan deposits of the tecuya formation the lower part of the san emigdio sequence was deposited by a westward prograding gilbert type delta the upper part of the san emigdio sequence and lower part of the pleito sequence were deposited by a system of shelf fan deltas that prograded at least 10 km to the west the middle and upper parts of the pleito sequence were deposited by a slope fan delta in relatively deep hundreds of meters water regional transgression during the early eocene initiated deposition in the southern san joaquin basin the lower san emigdio gilbert type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 ma relative highstand deposits in the san emigdio and pleito formations consist of widespread progradational shallow marine and nonmarine facies the eocene to early miocene tectonic history of the southern san joaquin basin included three distinct periods of increasingly intense activity rapid early to middle eocene subsidence of the basin was associated with the emplacement of the salinian block late eocene to early oligocene uplift and subsidence occurred in two discrete pulses and a seismically generated submarine slumping episode triggered the change from shelf fan deltas to slope fan deltas during the late oligocene to early miocene the southern san joaquin basin was disrupted by major uplift proximal volcanism and syndepositional faulting as the mendocino triple junction migrated past the study area and san andreas fault related tectonism was initiated in general local tectonic events rather than eustatic ea level events seem to have exerted the predominant control on middle cenozoic sedimentation in the southern san joaquin basin

paleogeographic reconstruction of the eocene idaho river north american cordillera eocene lake gosiute in southwestern wyo ming was progressively fi lled in by volcani clastic sediment between 49 6 and 47 0 ma the source of this material has long been thought to have been the absaroka volcanic province immediately north of the greater green river basin lead isotope compositions of sandstone from this interval however are consistent with derivation from the challis volcanic fi eld the 40 ar 39 ar ages of single de trital k feldspar crystals from greater green river basin sandstones are nearly identical to 40 ar 39 ar eruptive ages for volcanic rocks of the challis volcanic fi eld 49 8 45 5 ma but we also identify mesozoic and proterozoic crystals that are consistent with cooling ages for rocks that were likely exposed in the idaho segment of the north american cordillera during the eocene most of these rocks are traversed by or are up gradient of a major eocene paleovalley in central idaho the sud den appearance of challis derived sediment in the greater green river basin indicates that a major river here named the idaho river connected the interior of the north american cordillera to the greater green river basin this connection requires 500 km to reach from central idaho to the greater green river ba sin the idaho river probably carried detritus that was stripped from distant uplifted moun tains above the active challis volcanic fi eld as far south as the piceance creek basin suggest ing a total length of at least 1000 km middle eocene metamorphic core complexes in the northern cordillera likely produced a major highland in the headwaters of the idaho river which generated river systems that drained both eastward into the wyoming foreland and westward into the oregon coast ranges

characterizing structural and lithologic controls on deep seated landsliding implications for topographic relief and landscape evolution in the oregon coast range usa in mountainous areas landslides regulate temporal variations in sediment production and may suppress simple linkages between topographic development and tectonic forcing rates and mechanisms of mass wasting depend on lithology bedrock structure and climatic and tectonic setting these factors tend to vary signifi cantly in active tectonic regions thus clouding our ability to predict how landsliding modulates topographic development over human and geological time scales here we use a novel dem based technique to document the distribution of large landslides in the oregon coast range ocr and quantify how they affect topographic relief we developed an automated algorithm that exploits the distinctive topographic signature specifi cally the relationship between curvature and gradient of large landslides to map their distribution within the gently folded tyee formation eocene deltaicsubmarine ramp sediments in contrast to steep and highly dissected terrain frequently identifi ed as characteristic of the ocr which exhibits steep planar sideslopes and highly curved low gradient ridgetops and valleys terrain prone to large landslides tends to have low values of both drainage density and curvature and gradient values that cluster between 0 16 and 0 44 the distribution of failure dominated terrain in our 10 000 km2 study area is infl uenced by systematic variations in sedimentary facies and bedrock structure the fraction of terrain altered by large landslides 0 1 km2 varies from 5 in the sand rich delta slope and proximal ramp facies southern section of our study area to 25 in the north distal ramp facies coincident with an increase in the thickness of siltstone beds and a decrease in the sandstone siltstone ratio local relief declines progressively northward suggesting that deep seated landsliding is sensitive to the thickness and frequency of low shearstrength siltstone beds and may serve to limit topographic development in the ocr structural controls are superimposed on facies related variations as deep seated landslides are frequently found on slopes whose downslope aspect corresponds to the bedrock dip direction for 1516 strike and dip measurements in our study area we calculated the fraction of proximal terrain 2 5 km altered by deep seated landsliding in the sand rich southern region the proportion of proximal slide dominated terrain increases modestly with bedrock dip in the silt rich northern region terrain altered by deep seated landsliding is pervasive and an increase in dip from 0 to 16 corresponds to a change in the fraction of slide prone terrain from 10 to 28 our technique for mapping large landslides has utility for hazard analysis and land management over million year time scales the progradational character of the tyee formation suggests that continued uplift and exhumation of the ocr should result in a southward propagation of slide prone silt rich distal facies as a result deep seated landsliding will become increasingly prominent and topographic relief in the central and southern ocr will progressively decline whereas spatial variability in climatic or tectonic forcing is often invoked to explain systematic variations in topographic development our results emphasize the importance of structural and intraformation lithologic controls on landsliding as such analyses linking surface processes climate tectonics and landscapes should be couched in the context of diverse geologic and topographic data

differential shear wave attenuation and its lateral variation in the north atlantic region because the attenuation of seismic waves is sensitive to variations in temperature and to partial melting mapping seismic q in the upper mantle is an important tool for assessing the processes responsible for lateral heterogeneity utilizing a digital data base of over 150 seismograms and a spectral ratio technique we have measured ss s differential attenuation in the north atlantic region differential attenuation is positively correlated with ss s travel time residual and both differential attenuation and travel time residual decrease with increasing seafloor age in an earlier paper we showed that the age dependence of ss s travel time residual can be explaned entirely by the cooling of the oceanic lithosphere i e contributions from the asthenosphere or from a mantle melt fraction are not required the assumption that plate cooling also dominates the variation of differential attenuation with age permits the derivation of an empirical relation between q 1 and temperature for the oceanic thermal lithosphere i e the depth interval 0 125 km however both the absolute values and the depth distribution of q we obtain under this assumption are at variance with the results of surface wave attenuation studies and the variation of q 1 with temperature that we derive is not as strongly dependent on temperature as indicated by some laboratory measurements we therefore develop models for q in which lateral variations include contributions from the asthenospheric low q zone as well as from lithospheric cooling the q models obtained under this alternative assumption are in good agreement with those obtained from surface wave studies and are therefore preferred over those models with lateral variations confined to the upper 125 km systematic long wavelength 1000 7000 km variations in differential attenuation corrected for seafloor age are evident along the axis of the mid atlantic ridge these variations can be qualitatively correlated with long wavelength variations in ss s differential travel time residuals and are attributed to along axis differences in upper mantle temperature

anelastic structure of the upper mantle beneath the northern philippine sea we investigated the upper mantle anelastic structure beneath the northern philippine sea region including the izu bonin subduction zone and the shikoku basin we used regional waveform data from 69 events in the pacific and the philippine sea slabs recorded on f net and j array network broadband stations in western japan using the s p phase pair method we obtained differential attenuation factors t which represent the relative whole path q we conducted a tomographic inversion using 978 t values to invert for a fine scale 50 100 km three dimensional anelastic structure the results shows two high q regions qp 1000 which are consistent with the locations of the pacific and the philippine sea slabs also there is a low q qp 110 area extending to the deeper parts 350 400 km of the model just beneath the old spreading center and the kinan seamount chain in the shikoku basin a small depth dependence of the laterally averaged qp was found with values of 266 0 250 km 301 250 400 km and 413 400 500 km 2003 elsevier b v all rights reserved

oil source rock correlation in the north viking graben abstract based on evaluation of geochemical properties of nearly 100 oils from the north viking graben we have identified three hydrocarbon populations one population is derived from marine shales of the draupne fm one from marine shales of the heather fm that are rich in land plant detritus and one from a mixture of the two while the draupne fm is the dominant source rock the heather fm lower callovian is an important source on the northern horda platform furthermore on the northern horda platform heather fm shales play a major role in controlling migration oil from the draupne fm migrates only into upper jurassic reservoirs as no large titled fault blocks permit juxtaposition of draupne fm and brent gp sands and the heather fm shales are thick and extensive oil from the heather fm migrates into both upper and middle jurassic reservoirs variations in properties of the hydrocarbons in the north viking graben petroleum system are due to a complex interplay of organic source maturity migration pressure distribution and biodegradation integration of geochemical and geological data on a regional scale has enhanced our ability to evaluate prospects in this important hydrocarbon system

geophysical evidence for post miocene rotation of the island of viti levu fiji and its relationship to the tectonic development of the north fiji basin geophysical studies of the island of viti levu fiji show largeamplitude magnetic anomalies of up to 1500 nt to be located above the tavua volcanic center field sampling of the volcanic rocks from the intrusive and extrusive centers of the volcano show the rock samples to have initial intensities of natural remanent magnetization of up to 380 a m radiometric dating of fifteen oriented samples show an age range of 2 9 to 7 0 m y b p for the suite of rocks collected from the mba basaltic group paleomagnetic analysis of the dated rocksshows declination vectors ranging from 325 9 for an age of 2 9 0 2 m y b p to 290 4 for an age of 7 0 0 4 m y b p as a group the paleomagnetic declination values of the samples systematically decrease with age this decrease is attributed to progressive anticlockwise rotation of viti levu during the process of formation of the north fiji basin a linear least squares analysis of the paleomagnetic data indicates that the island has rotated in an anticlockwise direction through 90 during the past 7 m y at a rate of 13 7 per million years declination values as low as 245 measured for some sites not dated suggest that anticlockwise rotation of viti levu and development of the north fiji basin could have commenced as early as 10 m y b p following the breakup of the ancient solomons new hebrides fiji lau island arc large rotations of microplates may have occurred along other island arcs and marginal basin systems elsewhere in the western pacific and such rotational histories may reflect in detail the evolutionary development of these arcs and basins

marine debris flows late precambrian tillites of the avalonian cadomian orogenic belt abstract correlative late precambrian strata of the avalonian cadomian orogenic cycle deposited after 670 and before 550 ma are now widely dispersed around the margins of the north atlantic these contain several prominent diamictite horizons long interpreted as tillites within thick turbidite successions associated with volcanic horizons the granville formation of the armorican massif in normandy france contains many pebby mud diamicite beds up to 20 m thick that have been interpreted as glaciogenic regionally the granville formation occurs within the lowest part of the upper brioverian supergroup which consists of several kilometres of turbidites and volcanogenic rocks at granville diamictites can be traced along striked into deformed and intermixed graded conglomerates and mudstones diamictites are identified as debris flows generated on a submarine slope by the emplacement of gravel turbidites on a mud substrate and their mixing during downslope slumping a glacial influence cannot be identified in north america the conception group of the avalon peninsula newfoundland canada contains nearly 4 km of turbidites with a conformable sequence of diamictites the gaskiers formation near its base massive and crudely stratified diamicite beds the diamictite successions of the granville and gaskiers formations provide significant data regarding facies types associations and the geometry of resedimented diamictite sequences in deep marine basins along active plate margins similar deposits at the same tectonic stratigraphic horizon occur in new england and africa this paper emphasizes the difficulty of identifying a direct glacial influence on diamictite sedimentation in such settings and the need for a sedimentological reappraisal of other supposed late precambrian tillites preserved in orogenic belts this is a necessary first step prior to modelling late precambrian paleoclimates

forearc basin evolution in the pan african damara belt central namibia the hureb formation of the khomas zone abstract the hureb formation in central namibia is one of the uppermost stratigraphic units of the precambrian pan african damara sequence the hureb formation occurs only in a part of the khomas zone a forearc basin which evolved complexly with time the formation is composed of the well graded upward fining schists of the quartzitic keises member the graphitic and calcitic waterval member and the altered scapolite bearing dagbreek member these units accumulated in an elongate turbidite fan system longitudinally deposited on the northern slopes of a wide forearc basin the active continental margin initiated when the kalahari plate subducted oceanic crust underneath the passive continental margin of the congo craton the development and deposition of the turbiditic hureb formation in the foredeep occurred during subduction of oceanic crust simultaneous with d 1 deformation and tholeiitic to andesitic arc volcanism on the continental margin the basin was bound to the north by the rising volcanically active continental margin and to the south by the accretionary prism of obducted slivers of oceanic crust and pelagic sediments above the subduction zone with continental collision the sedimentation in the forearc and continental margin arc volcanism ceased the forearc basin was uplifted and shortened horizontally by synclinoria and anticlinoria which propagated northwards and died out at the edge of the forearc basin the deformation slightly affected the underlying foliated fahlwater schist from the passive margin setting further uplift of the basin and re adjustment between the oblique sutured cratons simultaneous with the d 3 doming in the congo continental margin arc occurred with the underplating of the thin passive margin of the kalahari craton as no more crustal shortening could be absorbed by the upright folding dk 1 reactivation of the thrusts in the accretionary prism leads to the producing of southeast verging with a right lateral sense of movement thrusts dk 2 in the folded forearc basin the dk 2 thrusts propagated and died out northwards and extended towards the south the stacking of the d k2 thrusts southwards thickened the crust creating a low heatflow environment with higher pressure conditions it leads to the preservation of chlorite and production of sillimanite garnet staurolite and kyanite to the south of the accretionary prism collision related deformation created a peripheral foreland thrust zone outlasting orogenic activities in the overriding congo continental margin where isostatic rebound already had occurred

shallow seismicity and tectonics of the central and northern lau basin abstract the lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time as such crustal earthquakes within the lau basin derive from intrabasin tectonic volcanic and hydrothermal sources because of high mantle attenuation small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity however the 1994 labatts ocean bottom seismograph experiment in the lau basin recorded more than 100 000 local t and body phases allowing for a detailed examination of basin seismotectonics nearly 1000 events are located within the basin proper with another 2000 associated with the forearc and the aseismic front we identify two previously undocumented tectonic features a triple junction in the northwestern basin and a nanoplate in the central lau basin the single most seismically active feature within the lau basin is the lau extensional transform zone letz just north of the central lau spreading center significant seismicity also occurs near the northern extension of the eastern lau spreading center and along a southeast extension of the letz delineating the boundary of a previously unknown nanoplate swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics we see no evidence of deformation along the southern boundary of the niuafo ou microplate where 4 5 cm yr of strain is predicted it is possible that the plate boundary extends eastward from the fonualei rift fr tip towards the trench creating a niuatoputapu tonga plate division rather than westward implied by the currently accepted niuafo ou tonga plate system but more data are necessary from the fr to test that hypothesis instead we suggest that the niuafo ou tonga pole is 1 5 further north than previously proposed reducing the predicted strain in this region to

the use of bisnorhopane as a stratigraphic marker in the oseberg back basin north viking graben norwegian north sea abstract this paper discusses the occurrence of 28 30 dinor 17 18 21 hopane bisnorhopane in stratigraphically fairly well preserved viking group sections from wells in local depressions in the north viking graben area the results show the presence of high relative amounts of bisnorhopane in the syn rift sections whilst the post rift sections contain little or no bisnorhopane since most exploration wells are drilled on structural highs primarily penetrating the post rift draupne this may explain why many analyzed source rock sections in this area contain little bisnorhopane as a correlation of draupne sections using the vertical relative bisnorhopane distributions it is suggested to be a potential stratigraphic marker for the area indicating the presence of syn rift draupne sediments the relative bisnorhopane amounts follow a logarithmic reduction with depth and thermal maturity the bisnorhopane signal is nearly extinguished at 3700 m depth at a maturity of r o 0 9 1 0

seismic constraints on a model of partial melts under ridge axes in a recent global scale seismic study the correlation between s wave velocity under ridge axes and spreading rate was pointed out the correlation is strong for depths to about 70 km but it diminishes below this depth we present the correlation plots at four depths 38 66 90 and 110 km for which correlation is strong at 38 and 66 km but is weak at 90 km and is almost nonexistent at 110 km we present a model to explain this behavior which includes a thermal conduction model for the development of lithosphere and a simple melt percolation thermal effects on s wave velocity are assumed to be accounted for entirely by the plate cooling thermal conduction model we point out that the thermal model under this assumption predicts asymptotically no correlation between s wave velocity and spreading rate specifically for spreading rate larger than about 3 cm yr 1 this contradicts the correlation observed in the data at shallow depths the existence of partial melt is thus required to explain the observed behavior at 38 and 66 km depths we start from four basic equations that govern the distribution of partial melt and derive the relation between the amount of partial melt and the spreading rate we adopt a simple power law relation between permeability k and porosity by k k 0 n where k 0 and n are constants and assume that pores are filled with melt we then set up an integral relation between s wave velocity and spreading rate the final formula indicates that the gradient in the correlation plots is the inverse of the power 1 n in the permeability porosity relation thus enabling us to constrain n as well as k 0 from seismic data the data also have some sensitivity to the depth to solidus we show that 1 the depth to solidus is probably within the range 60 100 km and 2 if the power n is n 2 3 then k 0 10 8 10 10 m 2 these parameters predict that porosity and fluid velocity are 1 2 and about 0 5 m yr 1 respectively the depth to solidus is consistent with previous estimates by petrological data but is perhaps the first and direct seismological evidence of partial melt from surface wave data analytical forms for the dependence on depth and spreading rate of porosity fluid velocity within permeable rocks and ascent times of magma are also obtained

depositional history of the maastrichtian lewis shale in south central wyoming deltaic and interdeltaic marginal marine through deep water marine environments the maastrichtian lewis shale in south central wyoming was deposited during the final major transgression and regression of the late cretaceous north american epicontinental sea information on this formation was obtained from outcrops along the margins of several basins and from well logs and cores within those basins regional correlations were made using biostratigraphic control which also allowed relatively fine scale resolution of the approximately 71 to 69 m y ages of the lewis shale at least two branches of the sand rich sheridan delta system supplied sediments that either were deposited directly within deltaic environments or were reworked and redeposited in laterally equivalent interdeltaic environments facies recognized within the study region are interpret d to represent nearshore shell and basin marine depositional environments throughout the study region facies representing shallow water environments are iocated on the north and those representing deep water deposits are on the south this distribution combined with data on the ages of stratigraphic intervals supports the conclusion that the lewis shale was deposited during north to south progradation the lewis shale contains significant hydrocarbon reserves understanding the distribution of facies and the paleogeography of the formation is crucial for designing hydrocarbon exploration and development strategies for the region

q tomography of the upper mantle using three component long period waveforms summary we present a degree 8 3 d q model qrlw8 of the upper mantle derived from three component surface waveform data in the period range 60 400 s the inversion procedure involves two steps in the first step 3 d whole mantle velocity models are derived separately for elastic sh transverse component and sv vertical and longitudinal component velocity models using both surface and body waveforms and the non linear asymptotic coupling theory nact approach in the second step the surface waveforms thus aligned in phase are inverted to obtain a 3 d q model in the depth range 80 670 km various stability tests are performed to assess the quality of the resulting q model and in particular to assess possible contamination from focusing effects we find that the 3 d patterns obtained are stable but the amplitude of the lateral variations in q is not well constrained because large damping is necessary to extract the weak q signal from data the model obtained agrees with previous results in that a strong correlation of q with tectonics is observed in the first 250 km of the upper mantle with high attenuation under oceanic regions and low attenuation under continental shields it is gradually replaced by a simpler pattern at larger depth at the depths below 400 km the q distribution is generally dominated by two strong minima one under the southern pacific and one under africa yielding a strong degree 2 pattern most hotspots are located above regions of low q at this depth ridges are shallow features in both velocity and q models

seismic attenuation tomography of the tonga fiji region using phase pair methods the anelastic structure of the region surrounding the tonga slab and lau back arc spreading center in the southwest pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs two differential attenuation methods determine t over the frequency band 0 1 to 3 5 hz for earthquakes in the tonga slab the s p method measures the difference in spectral decay between p and s waves arriving at the same station the p p method measures the difference in spectral decay for p waves with different paths through the upper mantle eight hundred sixty phase pairs are used to invert for two dimensional 1 q structure using a nonnegative least squares algorithm a grid search method determines the q q ratio most compatible with both the s p and p p differential measurements the highest attenuation q 90 is found within the upper 100 km beneath the active portions of the lau basin extending westward to the lau ridge these regions probably delineate the source region for the back arc spreading center magmas expected to be within the upper 100 km based on petrological considerations the high attenuation regions also correlate well with zones of low p wave velocity determined by regional velocity tomography somewhat lower attenuation is found beneath the fiji plateau than beneath the lau basin the entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km the slab is imaged as a region of low attenuation q 900 material a q q ratio of 1 75 provides the best fit between the s p and p p data sets upon inversion spectral stacking shows no frequency dependence within the frequency band analyzed

global lateral variations of shear wave attenuation in the upper mantle we analyze several thousand high quality globally recorded ss s differential waveforms to constrain the lateral variation of shear wave attenuation q in the upper mantle we use a multitaper frequency domain technique to measure attenuation parameterized by a t operator and implement a robust estimation technique to compute t and its variance the differential waveform technique minimizes the effect of factors such as finite source duration and structural complexity near the source and receiver so the differential ss s waveforms are mainly sensitive to the shear attenuation in the upper mantle under the ss bounce point we use seismograms recorded at ranges of 45 to 100 and compute the ss s differential t from the broadening of the ss waveform relative to the hilbert transform of the s waveform a careful choice of fitting windows allows us to reduce the biasing effects of interfering phases which can affect t by up to 0 5 s the t residuals with respect to preliminary reference earth model prem vary by 1 5 s with an average of 0 24 s our study suggests an average q value of 112 most of the lateral variations of q are within 30 of this value in the top 400 km of the mantle slightly lower than the prem value of 128 there is a qualitative correlation of t residual with tectonic region with distinctly higher attenuation observed under young oceans compared to platforms and shields also the lateral variations of the residuals are similar in trend to those observed in studies of the attenuation of scs multiples at long wavelengths the qp map shows a modest correlation with shear wave attenuation maps computed from surface wave analyses and with the patterns of lateral variations of shear velocities at certain upper mantle depths predicted by the model s16b30 the correlation with the velocity model is highest at 300 500 km depth indicating that there may be a contribution to long wavelength attenuation from relatively deep regions formal inversion for an upper mantle q model shows that while lateral resolution is quite good depth resolution is poor as might be expected better depth resolution must await combined body wave and surface wave inversions

a global tomographic model of shear attenuation in the upper mantle we present a global three dimensional model of shear attenuation in the upper mantle based on the measurement of amplitudes of low frequency 100 300s rayleigh waves observed at stations of the geoscope and iris networks attenuation coefficients are measured on r1 and r2 paths using a method which minimizes the effects of focussing due to propagation in a three dimensional elastic earth through a series of tests which in particular involve the computation of synthetic models of attenuation and focussing we demonstrate that long wavelength lateral variations in attenuation in the first 400 500 km of the mantle can indeed be resolved the model is obtained in a two step procedure the first step consists in the computation of maps of rayleigh wave attenuation at different periods using an inversion method without a priori parametrisation which involves the introduction of a correlation length chosen here at 3000 km to optimize the trade off between resolution and variance in the model in the second step after corrections for shallow structure an inversion with depth is performed assuming lateral heterogeneity is confined to depths between 80 and 650 km the resulting model presents lateral variations in qp that are correlated with tectonic features in particular ridges and shields in the first 250 km of the upper mantle below that depth the pattern shifts and becomes correlated with the hotspot distribution particularly so if the buoyancy strength of hotspots is taken into account two major low velocity zones appear to be located in the central pacific and beneath northern africa in the depth range 300 500 km this pattern seems to continue at greater depth but resolution becomes insufficient below 500 km to draw definitive conclusions the smooth lateral variations retrieved are on the order of 50 down to 400 km we propose an interpretation in terms of plume lithosphere ridge interaction in the upper mantle arguing for deflection of the bulk of hot upwelling material from plumes towards ridges which may be occurring between 200 and 300 km depth

submarine ramp facies model for delta fed sand rich turbidite systems reply to collinson discussion we appreciate and agree with the points raised by waldron 1986 regarding the recognition of cyclic sedi mentation in turbidite sequences by statistical methods the moving average technique that we used in the appen dix of our paper heller and dickinson 1985 will yield a bias toward increased length of runs because counting a bed twice produces a dependency unrelated to stochastic processes furthermore it is probably inappropriate to test for randomness in the differences between the thick nesses of adjacent beds for reasons stated by waldron 1986 the runs up and down test described by miller and kahn 1962 and summarized by waldron 1986 is cer tainly an improvement over the runs test that we used by using the z statistic figure 12 of heller and dickinson 1985 with waldron s substitution for calculating the mean uu and standard deviation a the null hypothesis hj tested is that the thicknesses of beds in the stratigraphic succession are random the runs test that we formerly pre sented tests the null hypothesis that the differences of thicknesses between adjacent beds is random the runs up and down test is considered rehable if the total number of beds n exceeds 9 miller and kahn 1962 p 331 the results of applying the runs up and down test to the stratigraphic sequences originally analyzed heller and dickin son 1985 our table 2 are shown here in tables 1 and 2 in table 1 the total number of thickening and thinning upward runs is counted by examining each bed in the stratigraphic sequence as with our previous study we assumed that differences in thicknesses of 2 cm or less between adjacent beds are not significant i e could result from measuring errors and so are ignored the resultant z scores and whether the hypothesis of random ness ho was accepted or rejected at a 0 05 and a 0 10 are shown in table 1 when individual beds are used the sequences appear to be statistically random even the cortina formation of the great valley sequence ingersoll and dickinson 1981 which consists of apparent thickening upward sandstone packages separated by thick mudstone intervals heller and dickinson 1985 is accepted as having random bedding thicknesses this

the lau havre taupo back arc basin a southward propagating multi stage evolution from rifting to spreading abstract the lau havre taupo lht system is an active 2000 km long back arc basin associated with pacific australian plate convergence in the sw pacific data from the lht back arc system include odp drillhole swath bathymetric and side scan imagery seismic reflection rock geochemistry magnetic anomaly heat flow shallow seismicity gravity and seismic refraction data the lht forms a longitudinal transect from southward propagating true oceanic spreading in the lau basin near tonga similarly southward propagating arc rifting in the havre trough and southward propagating continental rifting in the taupo volcanic zone tvz in northern new zealand four transitional tectonic settings are identified along the lht north to south between the continuum of spreading to rifting and comprise 1 oceanic spreading of the central lau spreading centre clsc propagating into oceanic crust formed by the eastern lau spreading centre elsc 2 oceanic spreading of the elsc propagating into rifted lau havre arc crust 3 rifting within havre arc crust propagating into rifting new zealand continental crust and 4 rifting of continental crust propagating into as yet non rifted but flexurally subsiding continental crust the enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the lht is used to constrain a model for the evolution of back arc rifting to back arc spreading a sequence of five stages is identified and comprise 1 a combination of incipient simple and pure shear 2 early distributed and spatially heterogeneous half graben rifting with low angle synthetic and antithetic structures associated with rapid graben sediment infilling ephemeral early morb like rift magmatism and the initial trenchward migration of the arc front 3 a latter phase of half graben rifting mostly confined to a contiguous axial zone with continued ephemeral morb like rift magmatism and further trenchward migration of the arc front 4 initial oceanic spreading with evolving steady state morb magmatism along the axial rift graben system and 5 the reconfiguration of ridge geometry in response to changes in opening orientation leading to the development of ridge propagation into newly created oceanic crust rifting stages 2 and 3 can produce at least 120 km wide back arc crust by a balance of crustal extension with an estimated factor of 3 with rift and constructional arc magmatism in the lht system depending on latitude rates of widening during stages 2 and 3 vary between 15 and 45 mm yr 1 whilst stages 4 and 5 have full spreading rates of 90 mm yr 1

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

how well can hillslope evolution models explain topography simulating soil transport and production with high resolution topographic data the morphology of hillslopes is a direct refl ection of tectonic forcing and climatic and biologic processes that drive soil production mobilization and transport soil transport on hillslopes affects river incision by providing tools for channel abrasion and controls the distribution of sediment that infl uences aquatic habitat although numerous hillslope transport relationships have been proposed over the past 60 years a comprehensive analysis of model predictions for a real landscape has not been performed here we use high resolution topographic data obtained via airborne laser swath mapping alsm to simulate the long term evolution of oregon coast range hillslopes and test three published transport models and a new model that accounts for nonlinear depth and slopedependent transport analysis of one dimensional steady state solutions for these four models suggests that plots of gradient curvature may be diagnostic for distinguishing model predictions to evaluate two dimensional model predictions for our fi eld site we assumed local steady state erosion for a 72 000 m 2 sequence of hillslopes and valleys after calibrating each of the four models we imposed constant base level lowering for cells within the valley network simulated 500 000 yr of soil production and transport and determined which transport model best preserved morphologic patterns that describe the current landscape form models for which fl ux varies proportionally with hillslope gradient generated broadly convex hilltops inconsistent with the sharp crested steep sided slopes of our study site whereas the two nonlinear slope dependent models produced convex planar slopes consistent with current hillslope form our proposed nonlinear slope and depth dependent model accounts for how soil thickness controls the magnitude of biogenic disturbances that drive transport this model best preserved the current landscape form particularly the narrow sharply convex hilltops characteristic of the oregon coast range according to our formulation which provides an explicit linkage for relating the distribution of biota to hillslope processes the degree of hilltop convexity varies nonlinearly with the ratio of erosion rate to maximum soil production rate highlighting the profound infl uence of soil depth on hillslope evolution

attenuation structure beneath the lau back arc spreading center from teleseismic s phases teleseismic ss and sscs waveforms provide constraints on the attenuation structure above deep seismic rones we estimate the average q along ss and sscs paths beneath the lau back arc by comparison with s and scs waveforms q is determined using both time and frequency domain analyses of transverse long period records assuming q is frequency independent in the frequency domain technique spectra from reflected ss or sscs and direct s or sscs waveforms are analyzed using least squares the time domain method uses a grid search to determine the attenuation operator that when convolved with the direct phase produces the best fit to the reflected phase in both methods the direct phase is convolved with a crustal response function to simulate the crustal interaction of the reflected phase large events subject to source directivity effects and source station combinations involving ray propagation down the subducting slab are not used the resulting attenuation estimates show low q values which increase with focal depth suggesting high attenuation in the upper mantle and a rapid decrease in attenuation below 200 300 km lateral variations are also indicated with very high attenuation q 20 35 in the upper mantle near the active spreading centers and lower attenuation west of the lau ridge q temperature relationships extrapolated from laboratory experiments suggest that the low q regions represent zones of asthenospheric partial melting

the lithostratigraphy sedimentology and tectonic setting of the monian supergroup western anglesey north wales the monian supergroup of western anglesey comprises three major lithostratigraphic units the basal south stack group the pelitic new harbour group and the overlying gwna melange the south stack group comprises a sequence of deformed turbiditic metasandstones pelites and subordinate quartzites a dominance of t abcde and t bcde turbiditic units high sandstone to mudstone ratio and the presence of coarsening and thickening upward sequences indicate that the south stack group was deposited in a sand rich fan system the quartzites form large scale sand bodies 10 25 m thick and are interpreted as major channel fill sequences that transported mature quartz arenites from relatively shallow marine environments deeper into the basin by passing the inner to mid fan environments whole rock sedimentary geochemistry indicates that the south stack group was deposited in a continental margin setting the new harbour group forms a monotonous sequence of highly deformed pelites semipelites and subordinate volcaniclastic psammites which exhibit geochemical characteristics of sandstones deposited in a continental island arc setting petrographic data indicate that the new harbour group records the dissection and unroofing of a pre existing andesitic arc complex limited palaeocurrent data obtained for the south stack group and facies changes within the new harbour group suggest that the monian supergroup was deposited within a ne sw trending basin no evidence was found to suggest that the monian supergroup represents part of an accretionary prism consequently the deposition of the monian supergroup within the trench of a monian subduction system cannot be justified

shelf and deep sea sedimentation in eocene forearc basin western oregon fan or non fan the lower eocene tyee and flournoy formations in the southern and central coast range of oregon contain unusual associations of shelf and deep sea facies deposited in an elongate portion of a forearc basin the sandstone rich nature and abrupt transition of sedimentary facies suggest that a variation of existing deep sea fan models is present the 6 500 ft 2 000 m thick sequence calls for shelf sandstone virtually cascading into deep water along a line shelf edge rather than from a point source submarine canyon to feed a sand rich system facies are described in terms of a sand rich fan system from south to north these facies include deltaic shelf inner fan mid fan outer fan and basin plain the deltaic facies contains coarse grained cross bedded distributary channel sandstone and interbedded coal some prograding deltaic sand extended to the edge of the narrow eocene shelf and cascaded into the basin many complex nested channels up to 1 200 ft 350 m wide and 130 ft 40 m deep were incised into fine grained shelf and slope sediments at slope depths line source these channel and shelf slope deposits comprise an inner fan facies a single large canyon or feeder channel point source typical of some fan systems was not recognized mid fan facies consist of broad coalescing channels filled with thick am lgamated sandstone beds sandstone shale ratios in the mid fan facies are from 85 to 15 up to 95 to 5 outer fan facies have sandstone shale ratios of 60 to 40 and contain well developed graded turbidite beds about 5 ft 1 5 m or less in thickness interbedded with shale basin plain facies consist of fine grained strata sandstone shale ratios of 30 to 70 deposited at the north end of the basin these thin bedded deposits also contain graded fine grained turbidite sandstones paleogeographic reconstructions based on paleocurrent data facies distribution and sandstone composition indicate sediment derivation from the pre cenozoic klamath mountains terrane and a volcanic arc system to the south and southeast the average sandstone composition is 19 quartz 27 feldspar 19 volcanic rock fragments 7 other rock fragments and 28 accessories and cement paleocurrent data from cross bedding and groove and flute marks indicate transport directions from south to north instead of a radially dispersed fan pattern the stratigraphic succession of the tyee flournoy system indicates a general shoaling trend and a progradation of deltaic facies across a narrow shelf and out over the basin fill during the early eocene the predominance of sandstone and confinemen within a narrow basin account for a turbidite facies differing from more familiar mixed sandstone and shale submarine fan facies models

depositional relations of umpqua and tyee formations eocene southwestern oregon studies of the depositional relations of the umpqua and tyee formations eocene in southwestern oregon indicate a need for reassessing the correlations and currently used terminology the umpqua formation as herein restricted consists of as much as 10 000 ft 3 000 m of mudstone sandstone and conglomerate of nonmarine to deep marine origin a basaltic basement that underlies the sedimentary rocks in most of the area and was formerly included in the umpqua is herein considered a separate unit and assigned to the siletz river volcanics a proposal to subdivide the umpqua into three unconformity bounded formations in ascending order roseburg lookingglass and flournoy formations in the area west of roseburg oregon is not recognized in this report because of quest onable correlations and limited extent of some units foremost of these questionable correlations is that of the flournoy formation with rocks formerly assigned to the tyee formation in areas as far north as the latitude of salem oregon my investigations of depositional facies and lithologic criteria do not substantiate this correlation the tyee formation which conformably overlies the umpqua is a predominantly sandstone unit about 6 000 ft 1 800 m thick deposited in environments ranging from shallow marine and nonmarine deltaic on the south to slope and deep marine basinal to the north deposition across the umpqua tyee boundary contact represents a change in tectonic setting from active plate margin of the umpqua to more stable marginal basin deposition of the tyee this change is also reflected in a change in provenance as indicated by the characteristic arkosic and micaceous aspects of tyee sandstones

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

post fire geomorphic response in steep forested landscapes oregon coast range usa abstract the role of fire in shaping steep forested landscapes depends on a suite of hydrologic biologic and geological characteristics including the propensity for hydrophobic soil layers to promote runoff erosion during subsequent rainfall events in the oregon coast range several studies postulate that fire primarily modulates sediment production via root reinforcement and shallow landslide susceptibility although few studies have documented post fire geomorphic response here we describe field observations and topographic analyses for three sites in the central oregon coast range that burned in 1999 2002 and 2003 the fires generated strongly hydrophobic soil layers that did not promote runoff erosion because the continuity of the layers was interrupted by pervasive discontinuities that facilitated rapid infiltration at each of our sites fire generated significant colluvial transport via dry ravel consistent with other field based studies in the western united states fire driven dry ravel accumulation in low order valleys of our sulphur creek site equated to a slope averaged landscape lowering of 2 5 mm given holocene estimates of fire frequency these results suggest that fire may contribute 10 20 of total denudation across steep dissected portions of the oregon coast range in addition we documented more rapid decline of root strength at our sites than has been observed after timber harvest suggesting that root strength was compromised prior to fire or that intense heat damaged roots in the shallow subsurface given that fire frequencies in the pacific northwest are predicted to increase with continued climate change our findings highlight the importance of fire induced dry ravel and post fire debris flow activity in controlling sediment delivery to channels

body wave attenuation in the tonga subduction zone attenuation p and s waves of in the tonga subduction zone is estimated from locally recorded data using a spectral ratio method the data consist of three component digital seismograms of 77 earthquakes recorded at two stations in fiji and one in tonga the stations in the fiji islands lie above the deep earthquakes in the subduction zone while the tonga station is close to the top of the subducted slab variations in the high frequency spectral decay rate define four attenuation provinces paths from earthquakes deeper than 350 km to stations on viti levu fiji and the lau ridge traverse mantle with normal attenuation qp 570 qs 200 400 the subducting tonga slab has very low attenuation qp 2000 qs 1800 relative to the surrounding mantle the wedge of mantle above the slab at depths shallower than 350 km and east of the lau ridge is extremely attenuating for p waves qp 240 several paths from main thrust zone events to the station in tonga show intermediate attenuation qp 300 500 in the forearc region the observed spectra do not require q to be frequency dependent in the 1 0 hz to 7 6 hz band in this region

source rock heterogeneity of the upper jurassic draupne formation north viking graben and its relevance to petroleum generation studies abstract the upper jurassic draupne formation in the north viking graben north sea has been studied to identify lateral and vertical variations in source rock characteristics and their effect on the timing of petroleum generation fourteen source rock samples from the western flank of the graben and five from the eastern flank were analysed using rock eval analysis microscopy open system pyrolysis gas chromatography and kinetic experiments two major organic facies with different petroleum generation characteristics were identified a stratigraphically higher thermally less stable facies corresponding to a type ii kerogen with hi hydrogen index values of 350 mg hc g toc and a stratigraphically lower more stable type ii iii facies with hi values

submarine fan and slope facies of tonkawa missourian virgilian sandstone in deep anadarko basin the tonkawa sandstone missourian virgilian in the anadarko basin consists of upper middle and lower sandstone units separated by intervening shales the lower tonkawa sandstone consists of coarse to pebbly sandstones deposited in thick coarsening upward sequences the unit is up to 400 ft 120 m thick and the sandstones grade to shale from the southeast to the northwest gross sand isopachs outline northwest southeast oriented linear thicks that extend for tens of miles the middle tonkawa sandstone consists of thin and thick bedded fine grained sandstones displaying wavy and flaser bedding a convolute laminae and some small scale cross laminae these sandstones range in thickness from 100 ft 30 m to 400 ft 120 m the thicks in the middle tonkawa extend nly for a few miles and do not show a preferred directional orientation the upper tonkawa sandstone caps the entire sequence and is overlain by oolitic limestones we interpret the lower tonkawa sandstone to represent a submarine fan complex for the following reasons the regional setting indicates that the sandstones were deposited tens of miles from the shelf edge graded beds convolute laminae and load casts indicate transportation by sediment gravity flows the areal geometry of sandstones indicates channelized flows that coalesced to form an overall fanlike geometry the middle tonkawa sandstones are interpreted as a sequence deposited on the basin slope regional setting indicates that the unit is located immediately basinward of the shelf edge and that it progrades over the lower tonkawa sandstone deposition as sheets or aprons is suggested by lateral continuity at least for several miles of individual beds small scale cross laminae wavy and flaser bedding and sharp bed contacts suggest bottom current activity presence of short numerous channels is suggested by the thick beds of middle tonkawa sandstones that extend for only a few miles although the sedimentology and areal geometry of the upper tonkawa are not discussed in this paper the regional setting and lithologic character suggest shallow water nearshore deposition the entire tonkawa clastic sequence is capped by shallow water marine limestone the lower and middle tonkawa sandstones represent offshelf deposition in an interior basin setting the present study shows that the understanding of the geologic setting and the integration of all available data rather than sedimentologic data alone are required for proper identification the following formations in the anadarko basin also appear to contain fan and or slope facies springer morrow springerian morrowan red fork desmoinesian and cleveland marchand and cottage grove all missourian if the deep water sandstones in interior basins are indeed as common as they appear to be in the anadarko basin then it is crucial for stratigraphic exploration that we recognize and document such deposits in other interior basins as well

diagenetic variation in the oregon coast range implications for rock strength soil production hillslope form and landscape evolution the mechanisms by which lithology modulates geomorphic processes are poorly known in the oregon coast range ocr rhythmically bedded sandstones of the eocene tyee formation underlie steep soil mantled hillslopes with relatively uniform ridge valley spacing these characteristic landforms are perturbed where diagenetic variations manifest as resistant cliffs here we use petrology rock mechanics and lidar to characterize grain scale variations in rock properties and their influence on rock strength hillslope processes and landscape morphology in two adjacent watersheds petrographic analyses suggest that a suite of diagenetic products in the resistant bedrock account for a 2 5 times increase in tensile strength relative to typical tyee bedrock our reference catchment exhibits negligible resistant outcrops and consistent hillslope gradients and longitudinal valley profiles by contrast the adjacent catchment teems with resistant 1 to 10 m thick noncontiguous sandstone beds that form hanging valleys with gentle upstream hillslopes and anomalously narrow valleys mechanical and topographic analyses suggest that the low fracture density characteristic of these resistant beds may render them relatively impervious to comminution by tree root activity the dominant ocr soil production mechanism based on both hillslope gradient and hilltop curvature erosion models we estimate that hillslopes perched above resistant beds erode at approximately half the pace of hillslopes unencumbered by downstream knickpoints the diagenetic variations likely influence relief at the watershed scale depositional position and diagenetic processes appear to control the occurrence of resistant beds providing a framework to quantify how seemingly subtle variations in rock properties can impose first order controls on landscape form and evolution

p wave attenuation structure of the lau back arc basin and implications for mantle wedge processes abstract we image 3 d seismic p wave attenuation structure of the tonga subduction zone and the lau back arc basin using local earthquake waveforms recorded by land and ocean bottom seismographs of the 2009 2010 lau spreading center imaging project the amplitude spectra of p waves from local earthquakes are inverted for the path average attenuation operator t with a variety of frequency dependence exponent values analysis shows that the data are best fit by the assumption of a weak frequency dependence 0 3 consistent with laboratory results modeled as the effects of grain boundary sliding the t measurements are inverted for a 3 d tomographic q p 1 model the results show high p wave attenuation within the upper 100 km of the back arc mantle with a sharp boundary at the tofua arc between high attenuation sub arc mantle and the low attenuation fore arc perhaps the highest p wave attenuation q p 1 0 036 or q p 28 known in the mantle occurs at 20 60 km depth beneath the back arc spreading centers high attenuation anomalies form an inclined zone dipping from the spreading centers to the west away from the slab indicating abnormally high temperature and the existence of hydrous partial melt these observations suggest that hot materials supplied from the australian mantle upwell along with the mantle wedge flow pattern triggering extensive decompression melting near the back arc spreading centers the along strike variation of p wave attenuation is different from that of sv velocity possibly implying the different effects of partial melt on seismic attenuation and velocity

deep sea channel submarine yazoo system of the labrador sea a new deep water facies model the deep sea channel submarine yazoo system in the labrador sea consists of a major basin draining trunk channel the northwest atlantic mid ocean channel namoc with an interconnected basin wide network of tributary and satellite channels that are linked up with slope canyons the 3800 km long and up to 200 m deep namoc has asymmetrical levees rising up to 80 m above the adjacent flood plains and braid plains whereas the channel floor lies up to 100 m below the plains this limits major mid reach channel avulsion and permits the main channel to retain a relatively stable position over long periods of time tributary channels are denied entrance to the main channel by the high natural levees and are forced to flow parallel to the namoc for hundreds of kilometers in a fashion similar to yazoo type rivers on land the satellite channels east of the namoc form a submarine braid plain grain size fractionation into coarse to very coarse grained channel fill deposits and muddy spillover deposits on the levees is extreme for the main channel fractionation is min r for the shallow yazoo type and other tributary channels on the basin plain whose levees tend to be sandy nine depositional facies have been differentiated on the basis of piston cores and echo character in seismic profiles 1 deep channel fill 2 small scale slide slump facies on the walls of deep channels or canyons 3 spillover facies on the levees of the main channel 4 channel fill of shallow second order channels including yazoo type channels 5 levee facies of second order channels 6 submarine braid plain facies 7 debris flow deposits 8 contourite facies and 9 pelagic and hemipelagic sediments in contrast to the centrifugally diverging pattern of distributary channels of classical deep sea fan depositional systems the submarine drainage system of the namoc forms a centripetally converging channel pattern the submarine yazoos and other tributaries join a single trunk channel the system is multisourced by definition progressive proximal to distal merger of tributaries with the main trunk channel focuses the coarse grained facies along the basin axis this contrasts sharply with classical deep sea fans whose sands are being diffused distally in depositional lobes development of deep sea channel submarine yazoo systems is controlled by basin geometry and the number and intensity of sediment sources along the basin margins marginal seas or small ocean basins in the vicinity of waxing and waning major continental ice caps or actively rising mountain belts are ideal hosts for this kind of deep water turbidite system

multiple scs probing of the ontong java plateau abstract we use multiple scs phases recorded at pohnpei from a 1996 deep earthquake under the solomon islands to investigate the shear velocity and attenuation characteristics of the upper mantle under the ontong java plateau in the western equatorial pacific cross correlation results indicate a vertical one way travel time under the plateau 3 s slow with respect to prem in agreement with previously obtained tomographic results which revealed the existence of an anomalously slow root under the plateau additionally the study of stacked spectral ratios suggests an unusually high qscs 366 with a lower limit of qscs 253 this value is considerably higher than elsewhere in the pacific suggesting the absence of an asthenospheric layer under the edifice the combination of slow shear wave speed and lower than average shear attenuation rules out a temperature anomaly as the source of the ontong java plateau ojp root which must then be related to a chemical or mineralogical heterogeneity

mapping of p wave slab anomalies beneath the tonga kermadec and new hebrides arcs abstract p wave velocity anomalies of several per cent fast relative to the jeffreys bullen jb model are seen around subduction zones under the tonga kermadec and new hebrides trenches from inversion of international seismological centre isc travel time residuals from intermediate and deep focus earthquakes mantle heterogeneities outside the model are corrected using previous velocity models ray paths inside the model region are traced allowing two dimensional velocity variations the rays of similar source and receiver locations are combined into summary rays and the hypocenters are relocated as part of the inversion iterations the resulting images are interpreted in conjunction with resolution and error estimations the slab like anomalies are generally continuous in places of high seismicity and decrease in amplitude with depth most coherent slab anomalies are surrounded by large slow patches between the surface and 350 km depth the fast slab beneath the tonga arc appears to end at 550 km depth before the seismicity ends but there is a fast band at 750 1000 km depths below the deepest tongan earthquakes the fast slab like anomaly beneath the kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle and it occurs aseismically near new zealand down to 500 550 km depth interpretations in the lower mantle in many places such as kermadec and new hebrides arcs however are hampered by poor resolution

deep sea fan deposits in the macigno formation middle upper oligocene of the gordana valley northern apennines italy abstract facies characteristics are described for a 2300 m thick turbidite succession of the macigno formation middle upper oligocene which crops out as a continuously exposed section in the gordana valley northern apennines in the study area the macigno formation consists of two sandy members separated by a shaly member the sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes the middle shaly member represents a period of basin starvation three main facies associations are recognized in the macigno fan deposits fan fringe outer fan lobe and braided midfan facies associations the fan fringe deposits consist of monotonous or small scale cyclically arranged thin bedded turbidites the outer fan lobe deposits are composed of medium to very thick bedded classical turbidites internally organized in thickening upward cycles with minor intercalations of thin bedded sandstone beds two distinct types of outer fan lobe deposits distal and proximal are recognized on the basis of rock type internal organization and vertical facies relationships the midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning upward cycles the three main facies associations are superposed t define major thickening and coarsening upward cycles reflecting distinct episodes of fan progradation the relationships between the facies associations of the macigno deposits and the existing models of submarine fan sedimentation are discussed after the deposition of the fan complexes fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes

petroleum geochemistry of the lower and middle jurassic in atlantic margin basins of ireland and the uk potential hydrocarbon source rocks of lower and middle jurassic age have been reported from outcrop shallow boreholes and exploration wells in atlantic margin basins of the uk hebrides west of shetlands and flanking the ne rockall trough and recently in the continuation of this trend offshore ireland slyne erris and porcupine basins previously these organic rich mudrocks were considered to be of little economic importance due largely to their perceived limited areal distribution and low maturity however recent geochemical studies of oils and shales from exploration drilling of these basins shows the lower and middle jurassic to have considerable potential as effective hydrocarbon source rocks supplanting the late jurassic early cretaceous kimmeridge clay formation equivalents as the only viable oil source rock in the region flanking the atlantic margin in the irish and uk sectors rich oil source potential occurs in two transgressive mudrock cycles of lower jurassic age these are the sinemurian pliensbachian interval and the overlying toarcian section present in basins such as the solan minch hebrides slyne north celtic sea st george s channel and central english channel the middle jurassic source rocks have a more limited areal distribution and occur in the faroe shetland solan west lewis west flannan hebrides slyne and north porcupine basins with oil source potential in regressive marginal marine to lacustrine facies mudrocks geochemical studies were undertaken on mudrocks from the lower and middle jurassic sections in atlantic margin basins outcrop shallow borehole core and exploration well cores and cuttings samples and on oils from drill stem test and shows core and cuttings extracts detailed analyses using gc gc ms and carbon isotopes allowed both characterization of the source rocks and oil to source correlation biomarker and carbon isotope studies of oils from the faroe shetland basin foinaven and schiehallion fields the porcupine basin connemara accumulation the wessex basin wytch farm and kimmeridge oil fields and wells in the slyne basin show strong correlations to the various source rock developments in the lower and middle jurassic the mixed biodegraded foinaven and schiehallion oils have a major waxy component and correlate with lacustrine middle jurassic source rocks in the solan and west lewis west flannan basins middle jurassic sourcing of the connemara oils is also suggested while oils in the slyne basin appear to have been largely sourced by the lower jurassic pabba shale formation oils in the wessex basin wytch farm and kimmeridge appear to have been sourced by hettangian sinemurian mudrocks and those in the north celtic sea basin by toarcian source rocks the results from this study in combination with previously published data show that rich effective oil prone source rocks occur in both the lower and middle jurassic of the atlantic margin basins offshore ireland and the uk these source rocks can be correlated with indigenous oils indicating the existence of a previously under evaluated petroleum system

migration of hydrocarbons in the tampen spur area norwegian north sea a reservoir geochemical evaluation abstract this paper describes the application of reservoir geochemical protocols to a study of oil field and regional petroleum compositional variations in the major fields in the tampen spur area the tampen spur high is located in the middle of several oil mature to gas mature source rock kitchens and most of the structures might receive petroleum from several different directions based on the definition of several different petroleum populations within the tampen spur area we define five major petroleum charging systems correlating the petroleum population variations with geological information allows us to derive likely migration fill scenarios for many of the oil fields and requires us to invoke a new upper jurassic carrier system in parts of the area connecting brent group reservoirs across major faults based on this hypothesis two exploration wells were drilled on structures to test this carrier and petroleum was found in one of the structures an understanding of the migration pathways and petroleum drainage efficiency of the basins is of great economic interest because most of the smaller substructures are marginally economic thus it is important to provide an effective risk analysis and ranking of the satellite prospects prior to drilling this study has demonstrated that filling and migration in a geological province are controlled by a series of complex processes and that the filling history of the structures within the tampen spur is more complicated than suggested earlier

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

tomographic study of upper mantle atttenuation in the pacific ocean the quality factor q has been measured in the pacific ocean along many paths using long period seismograms of the geoscope and ida networks the love and rayleigh phase velocity and qr 1 have been regionalised in the period range 60s to 200s and then simultaneously inverted at depth to obtain 3d images of s wave velocity and attenuation q 1 in the depth range 60 300km up to 100s three very attenuating zones appear one under the east pacific rise e p r another one around hawaii a third one east of kermadec by inversion those three anomalies are found stable down to 160km below 200km lateral variations are less intense and differently distributed the maximum attenuation seems to concentrate along a northeast trend in the north central pacific

upper mantle structure of the tonga lau fiji region from rayleigh wave tomography we investigate the upper mantle seismic structure in the tonga lau fiji region by jointly fitting the phase velocities of rayleigh waves from ambient noise and two plane wave tomography the results suggest a wide low velocity zone beneath the lau basin with a minimum sv velocity of about 3 7 0 1 km s indicating upwelling hot asthenosphere with extensive partial melting the variations of velocity anomalies along the central and eastern lau spreading centers suggest varying mantle porosity filled with melt in the north where the spreading centers are distant from the tonga slab the inferred melting commences at about 70 km depth and forms an inclined zone in the mantle dipping to the west away from the arc this pattern suggests a passive decompression melting process supplied by the australian plate mantle from the west in the south as the supply from the australian mantle is impeded by the lau ridge lithosphere flux melting controlled by water from the nearby slab dominates in the back arc this source change results in the rapid transition in geochemistry and axial morphology along the spreading centers the remnant lau ridge and the fiji plateau are characterized by 60 80 km thick lithosphere underlain by a low velocity asthenosphere our results suggest the removal of the lithosphere of the northeastern fiji plateau lau ridge beneath the active taveuni volcano azimuthal anisotropy shows that the mantle flow direction rotates from trench perpendicular beneath fiji to spreading perpendicular beneath the lau basin which provides evidence for the southward flow of the mantle wedge and the samoan plume this article is protected by copyright all rights reserved

fault charged mantle fluid contamination of united kingdom north sea oils insights from re os isotopes 187 os 188 os as expected from a kimmer idge clay formation source however oils from the viking graben with which the brent fi eld is associated and main east shetland basin display unradiogenic 187 os 188 os in this study we seek to demonstrate that these anomalous results are due to interaction with a mantle like fl uid that propagated through the main fault zone of the viking graben and east shetland basin this study enhances our understanding of the re os oil systematics and supports cur rent ideas regarding fault architecture crustal thinning and associated fl uid fl ow and interac tion in the north sea oil system abstract united kingdom north sea oils sourced from the kimmeridge clay formation contain unradiogenic 0 17 to 0 48 and radiogenic 1 04 to 3 34 187 os 188 os values the unra diogenic 187 os 188 os values are spatially associated with the main basin bounding faults of the viking graben and east shetland basin in contrast the radiogenic 187 os 188 os values are associated with north sea basins located farther away from the basin boundary faults we suggest that crustal thinning and strain localization within the viking graben and east shetland basin are suffi cient to have allowed basin bounding faults to propagate to suffi cient depth to act as conduits for mantle derived fl uids to interact with oil this hypothesis is sup ported by previous geochemical data for north sea oil fi elds furthermore we demonstrate the use of os isotopes as an important tracker of crustal scale fl uid dynamics and petroleum migration pathways in extensional basins

late archaean foreland basin deposits belingwe greenstone belt zimbabwe abstract the c 2 65 ga old sedimentary cheshire formation of the belingwe greenstone belt bdb central zimbabwe has been studied in detail for the first time to shed some light on the much debated evolution of this classical belt the cheshire formation rests sharply on a mafic volcanic unit zeederbergs formation and comprises a basal eastward sloping carbonate ramp sequence built of shallowing upward metre scale sedimentary cycles the cycles strongly resemble proterozoic and phanerozoic carbonate cycles and might have formed by small scale eustatic sea level changes the top of the carbonate ramp is represented by a karst surface the carbonates are overlain by and grade laterally to the east into deeper water sub wave base siliciclastic facies conglomerate shale and minor sandstone were deposited by high to low density turbidity currents and were derived from the erosion of zeederbergs like volcanic rocks from the east shortly after deposition the cheshire formation and underlying volcanics were affected by a northwest directed thrusting event thrusting gave rise to the deformation of semi consolidated sediments and resulted in the juxtaposition of a thrust slice of zeederbergs basalts onto cheshire sediments the stratigraphy asymmetric facies and sediment thickness distribution palaeogeographic constraints and evidence for an early horizontal tectonic event suggest that the cheshire formation formed in a foreland type sedimentary basin

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

architecture of shoreface to deep water systems in segmented rift systems evidence from the southern margin of the sogn graben northern north sea the spatial distribution of syn rift deposits and temporal evolution of sandy gravity flows in the context of rift segmentation are poorly constrained based on conventional core description well log analysis and 3d seismic interpretation this study focuses on the upper jurassic successions in the southern margin of the sogn graben northern north sea thirteen facies associations are classified in order to discuss the architecture of shoreface gravity driven submarine fans and basin plain deposits on the scale of centimeters to meters in the eastern part of the study area gamma ray log stacking patterns suggest that northeast southwest oriented shoreface successions underwent progradational to retrogradational deposition during the oxfordian age lithological description of conventional cores indicates that large submarine fan system formed by debris flow dominated channels and turbidity flow dominated lobe complexes covers a broad 22 20 km depositional area the entry of regional drainages and the location of sandy gravity flow deposits were controlled by first order rift segmentation whereas the route of submarine canyon feeder system and the orientation of submarine fan lobes were determined by second order rift segmentation thus both first order and second order rift segmentation should be considered when determining the sedimentary facies and depositional architecture of sandy gravity flow deposits in individual rift segments

absorption of p and s waves above subduction zones we investigate local variations of absorption above the subduction zones of the northwest pacific teleseismic broad band bb recordings at german arrays from 59 earthquakes in this area allow the determination of the quality factor qp and qs the method to determine q is the phase pair method here for the first time p wave bb recordings at dense arrays are used extending the frequency range studied and increasing the reliability of the qp determination for the northwest pacific 1 d earth models have typical qp values of 200 to 800 qs 100 to 400 in the upper 200 km qp under the kuriles the kurile basin and japan is about 100 i e significantly lower than average qs under japan is also only about 1 3 of the value of 1 d models no frequency dependence of qp and qs could be detected for the broad frequency range studied 0 05 to 1 hz

a new approach to bridge the effect of organofacies variations on kinetic modelling and geological extrapolations abstract reliable prediction of petroleum formation is impeded by significant variations in organofacies that result from varying conditions during source rock deposition basin simulation programs can model lateral variations but are limited in their ability to account for vertical variations due to the impact on computation time and lack of representative kinetic parameters here we present a model that merges the petroleum formation characteristics of different organofacies vertically distributed in a heterogeneous source rock prior to kinetic modelling this approach allows the evaluation of one single overall kinetic model that accounts for the different petroleum generation characteristics of a variety of organofacies preserved in a source rock and balance the different petroleum formation processes according to their importance within the different organofacies bulk petroleum generation kinetics were determined on eleven draupne formation samples taken from a well in the tampen spur area using programmed temperature open system pyrolysis at 0 1 0 7 and 5 0 k min the samples covered the entire draupne formation from its bottom to its top in the studied well individual measurements taken at the same heating rate were summed to one pyrolysis yield curve and normalized to the average rock eval s2 value the three summed curves were then used as input to determine a kinetic model that is representative of the draupne formation in this well enhanced kinetic modelling was applied in order to account for the expected variations of frequency factors after geological extrapolation the resulting kinetic model represents the petroleum generation characteristics of the entire draupne formation in the study area accounting for the variability in source potential across organofacies

data calibrated migration modelling in the assessment of brent group prospectivity a case study from the veslefrikk area of the norwegian north viking graben abstract a study of the prospectivity around the veslefrikk oil field in block 30 3 of the norwegian north sea included an evaluation of petroleum migration and trapped gas to oil ratios the study followed a procedure which involved the assessment of source rock distribution and quality source rock maturity petroleum generation and expulsion and secondary migration inside a semi regional framework two prospects are discussed 1 b prospect a down faulted hanging wall trap to the west of the veslefrikk horst where the existing discoveries of veslefrikk oseberg east and zeta exist and 2 c prospect a tilted fault block foot wall trap to the south of the huldra discovery 1d thermal modelling coupled to map based petroleum migration analysis and 2d multi layer migration modelling were used in this multi source multi carrier setting to evaluate the petroleum system with the integration of available well and seismic data basin modelling techniques can deliver more confident assessments especially where multiple geological models may exist in this study area data such as temperatures and source rock properties were used to constrain the model input options thereby reducing parameter uncertainty other data such as geochemical fluid to source correlation and observed petroleum distribution and composition served as a validity check for the simulated results from different geological models the use of these data in this case history are discussed and illustrated

stratigraphic evolution of an outcropping continental slope system tres pasos formation at cerro divisadero chile depositional slope systems along continental margins contain a record of sediment transfer from shallow water to deep water environments and represent an important area for natural resource exploration however well preserved outcrops of large scale depositional slopes with seismic scale exposures and tectonically intact stratigraphy are uncommon outcrop characterization of smaller scale depositional slope systems i e 600 m thick succession units 3 and 4 which is interpreted as reflecting increased gradient conditions as the system accreted basinward

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

geotherms in the pacific ocean from laboratory and seismic attenuation studies seismic methods have been used extensively to measure attenuation coefficients in various regions of the earth attenuation is inversely related to the quality factor q which may be measured directly a low velocity and high attenuation zone in the upper mantle is commonly reported1 3 until now there has been no direct means to derive thermal structures from such seismic investigations recently q values have been determined in the laboratory for a dry peridotite an upper mantle rock for pressures and temperatures of up to 0 73 gpa and 1 280 c respectively4 the results indicate a quantifiable relation between q and temperature here we use these results to derive geotherms from q measurements for a region beneath the pacific ocean the geotherms are consistent with temperatures obtained from heat flow observations5 6 and imply that the oceanic upper mantle is rather dry in asthenos phere older than 40 myr geotherms for this region are sub solidus and no partial melt is expected

post oligocene tectonic rotation of the oregon western cascade range and the klamath mountains paleomagnetic data indicate that the western cascade range from northern california to central washington has undergone about 27 of clockwise rotation since 25 m y b p all the available regional paleo magnetic data fit a two phase rotation model the first phase being an eocene rotation that accompanied the accretion of the oregon coast range to the continent and the second phase being a rotation associated with post early miocene extension in the basin and range province

depth extent of the lau back arc spreading center and its relation to subduction processes seismic tomography and wave form inversion revealed that very slow velocity anomalies 5 to 7 percent beneath the active lau spreading center extend to 100 kilometer depth and are connected to moderately slow anomalies 2 to 4 percent in the mantle wedge to 400 kilometer depth these results indicate that geodynamic systems associated with back arc spreading are related to deep processes such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab the slow regions associated with the tonga arc and the lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers suggesting that slab components of back arc magmas occur through mixing at these depths

magnetic stratigraphy and tectonic rotation of the middle eocene coaledo formation southwestern oregon summary the coaledo formation in coos bay on the southern coast of oregon consists of over 2000 m of deltaic sediments that filled a forearc basin during the middle eocene this unit yields molluscs of the cowlitz coaledo fauna one of the oldest and best studied faunas in the pacific northwest but its precise age has been controversial over 100 magnetic samples spanning the formation were analysed using both thermal and alternating field demagnetization they yielded a stable remanence held mainly in magnetite which passed a reversal test and showed a tectonic rotation of 105 5 this is greater than any other tectonic rotation so far reported from the region it is consistent with the previously observed trends that the tectonic rotations in the coast ranges of oregon and washington increase with increasing age of the rock unit and increase towards the southwest the magnetic polarity pattern in the coaledo formation best correlates with chrons c18r c20r 40 0 44 0 ma which is significantly older than the magnetic pattern in the cowlitz formation 36 5 38 3 ma this suggests that the cowlitz coaledo fauna is very long lived at least 36 5 44 0 ma spanning most of the middle eocene

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient