tectonic decapitation of a pliocene mega delta on isla del carmen in the gulf of california mexico and a river ran through it abstractthe tiombo conglomerate on isla del carmen in baja california sur represents a mega delta deposited mainly during the middle pliocene early piacenzian age a cross section with the characteristically arched profile of a fan delta is exposed in sea cliffs 60 m high that extend for 2 km along the east side of the fourth largest island in mexico s gulf of california a tidewater aspect to the delta is proven by the inclusion of fossil rich lenses dominated by the pecten patinopecten bakeri which also occurs abundantly together with many other middle pliocene fossils in the arroyo blanco limestone juxtaposed against the south flank of the delta construction inland the conglomerate fills a broad cross island channel approximately 3 km2 in area that ends with cliff exposures of a lesser thickness overlooking the carmen passage between isla del carmen and the peninsular mainland of baja california based on exposures that fail to reveal the full basal contact the combined fluvial and deltaic parts

high frequency cyclicity in a vertical alternation of gilbert type deltas and carbonate bioconstructions in the late tortonian tabernas basin southern spain abstract the deltaic succession in the tabernas basin represents the first deposition of marine sediments deposited during the late tortonian transgression that linked the mediterranean sea to the atlantic ocean through the basins of the betic cordillera the 100 m thick succession consists of seven 5 to 20 m thick vertically stacked wave dominated gilbert type deltas towards the top of each unit coral reef patches overlie the delta plain and are in turn overlain by calcarenites formed by accumulation of branching red algae colonies locally the calcarenite deposits fill microkarst depressions developed on the reefal carbonates during emergence unlike the corals the red branching algae colonies grew at a few metres depth under low energy conditions below wave base the calcarenites derived from branching red algae colonies formed when the delta plain was drowned creating the new accommodation space needed for the progradation of the delta forming of the overlying unit the repeated alternation of coral reef carbonates tropical carbonates and carbonates with bryomol skeletal assemblages temperate carbonates in the deltaic succession is linked to relative sea level changes reflecting the sum of glacio eustatic sea level fluctuations and tectonic subsidence each delta unit is represented by a transgressive systems tract built by the stacking of distributary mouth bars capped by reef patches a maximum flooding surface represented by branching red algal calcarenites and a highstand systems tract consisting of progradational wave dominated gilbert type delta because of the general increase in accommodation space controlled by glacio eustatic sea level rise and tectonic subsidence beach topset strata and distributary mouth bars were preserved in the cases studied subsidence rates exceeded the glacio eustatic sea level lows preventing the development of normal lowstand systems tracts in the delta stack

seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

facies analysis of a pliocene river dominated deltaic succession siena basin italy implications for the formation and infilling of terminal distributary channels the recent recognition and description of terminal distributary channels in ancient delta front deposits have led to important progress when it comes to understanding delta construction processes the plan view shapes of both modern and ancient river dominated deltas and sediment dispersal within deltas deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops approximately up to 300 m wide and 50 to 70 m thick in the central sector of the pliocene siena basin in tuscany italy stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings was not affected by tidal effects and was influenced only locally by wave action as a consequence these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex which was built up almost exclusively as the result of river related processes the studied deltaic succession is composed of vertically stacked parasequence like units which are invariably characterized by the lack of coarse grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition these deposits were interpreted previously as deep water turbidites this study provides new elements for the correct distinction between hyperpycnal dominated delta fronts and basinal turbidite deposits the terminal distributary channel facies documented in this study are more complex than those investigated in previous research the channels are often passively filled during abandonment phases and not just due to the aggradation and lateral upstream migration of mouth bars as classically proposed moreover the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision and before its infilling with subsequent implications for the formational and infilling mechanisms of terminal distributary channels furthermore the channel fills are often the result of alternating depositional and erosional phases leading to a complex stratigraphic arrangement

clinoforms and clinoform systems review and dynamic classification scheme for shorelines subaqueous deltas shelf edges and continental margins abstract clinoforms are inclined and normally basinward dipping horizons developed over a range of spatial and temporal scales in both siliciclastic and carbonatic systems the study of clinoform successions underpins sequence stratigraphy and all efforts to reconstruct the relative partitioning of reservoir seal and source rocks along shoreline to basin floor profiles here we review clinoform research and propose a more systematic description and classification of clinoforms this is a crucial step to improve predictions of facies and lithology distribution within shoreline to continental shelf and abyssal plain successions together with the genesis drivers and dynamics of their constituent sedimentary units four basic clinoform types are here distinguished in delta shorelines lacustrine and marine environments on the basis of their overall spatial and temporal scale morphology outbuilding dynamic and geodynamic and depositional setting 1 2 delta scale clinoforms which in turns are sub divided into shoreline and delta scale subaqueous clinoforms 3 shelf edge clinoforms and 4 continental margin clinoforms delta scale clinoform sets are tens of metres high and typically represent 1 103 kyr with progradation rates ranging from 1 000 100 000 m kyr for shorelines and subaerial deltas to 100 20 000 m kyr for subaqueous deltas shelf edge clinoform sets are hundreds of metres high and are nucleated and accreted in 0 1 20 myr usual progradation rates of 1 100 m kyr by successive cross shelf transits of delta scale clinoforms continental margin clinoform sets are thousands of metres high hallmark key geodynamic crustal boundaries e g continent ocean transition and slowly prograde basinwards in ca 5 100 myr with typical rates of 0 1 10 m kyr as a consequence of the very different progradation rates and of the difficulty of large scale clinothems to backstep during transgressions shorelines are the most dynamic clinoforms with regards to position continental margins the least and shelf edges are intermediate shortly after a transgression therefore the four clinoform types may prograde synchronously along shoreline to abyssal plain transects forming compound clinoform systems during the subsequent regressive cycle however due to the dissimilarity in progradation rates different clinoform types will normally merge progressively with each other giving rise to hybrid clinoforms e g shelf edge deltas and fewer depositional breaks in slope are distinguished along a single shoreline to abyssal plain transect overall all clinoform systems are the result of the dynamic evolution of compound and hybrid clinoforms along a temporal and spatial continuum regulated by the cyclical backstepping of the smaller scale system within natural progradational retrogradational cycles of larger scale clinothem outbuilding all clinothem types may show either an accretionary active or draping passive style depending on the proximity to the sediment source draping clinothems are nearly always composed of condensed fine grained sediments while actively accreting clinothems can be composed of predominantly coarse grained i e reservoir forming or predominantly fine grained i e non reservoir lithotypes a novel hierarchical classification scheme for both recent and ancient clinoforms is here proposed consisting of 12 classes the four basic clinoform types delta scale shoreline delta scale subaqueous shelf edge and continental margin are sub divided into eight accretionary active and draping passive sub types 8 division each accretionary sub type is then sub divided into a sandstone prone and mudstone prone variant 12 division which can be at least tentatively predicted on the basis of the clinoform morphology even in the absence of direct stratigraphic logs

shelf construction in a foreland basin storm beds shelf sandbodies and shelf slope depositional sequences in the upper cretaceous mesaverde group book cliffs utah the upper cretaceous campanian kenilworth member of the blackhawk formation mesaverde group is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of north america analysis of this section at a combination of small sedimentological and large stratigraphical scales reveals the dynamics of progradation of a shelf slope sequence into a subsiding foreland basin four major lithofacies are present in the upper mancos and kenilworth beds of the book cliffs a lag sandstone and channel fill shale lithofacies constitutes the thin basal transgressive sequence which rests on a marine erosion surface it was deposited in an outer shelf environment shale interbedded sandstone and shale and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave dominated delta and its flanking strand plains prograded seaward analysis of grain size and primary structures in kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies the fine to very fine grained graded strata of the interbedded facies are tempestites deposited out of suspension by alongshelf storm flows geostrophic flows there is no need to call on cross shelf turbidity currents density underflows to explain their presence very fine to fine grained hummocky strata are likewise suspension deposits created by waning storm flows but were deposited under conditions of more intense wave agitation on the middle shoreface cross strata sets in this region are bed load deposits that accumulated on the upper shore face in the surf zone lag strata are multi event bed load deposits that are the product of prolonged storm winnowing they occur on transgressive surfaces while the graded beds are tempestites in the strict sense all four classes of strata are storm deposits the distribution of strata types and their palaeocurrent orientations suggests a model of the kenilworth transport system driven by downwelling coastal storm flows and probably by a northeasterly alongshore pressure gradient the stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth but also because of progressive sorting which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments the kenilworth member and an isolated outlier the hatch mesa lentil constitute a delta prodelta shelf depositional system their rhythmically bedded lenticular sandstone and shale successions are a prodelta shelf facies and may be prodelta plume deposits major upper cretaceous sandstone tongues in the book cliffs are underlain by erosional surfaces like that beneath the blackhawk formation which extend for many tens of kilometres into the mancos shale these surfaces are the boundaries of upper cretaceous depositional sequences the sequences are large scale genetic stratigraphic units they result from the arranging of facies into depositional systems the depositional systems are in turn stacked in repeating arrays which constitute the depositional sequences the anatomy of these foreland basin sequences differs

composition and tectonic evolution of the chinese continental crust constrained by poisson s ratio article i nfo we have measured both p and s wave velocities vp and vs and poisson s ratios of 60 typical ultrahigh pressure uhp metamorphic rock samples from the chinese continental scientific drilling ccsd main and pre pilot holes and surface outcrops in the sulu dabie orogenic belt at hydrostatic confining pressures up to 850 mpa the experimental results together with those compiled in handbook of seismic properties of minerals rocks and ores ji s c wang q xia b 2002 handbook of seismic properties of minerals rocks and ores polytechnic international press montreal 630 pp reveal that except for monomineralic rocks such as quartzite serpentinite anorthosite limestone and marble the rest of the rock types have poisson s ratios falling along an upward convex curve determined from the correlations between elastic moduli and density poisson s ratios increase with density as the lithology changes from granite felsic gneiss and schist through diorite syenite intermediate gneiss and metasediment to gabbro diabase amphibolite and mafic gneiss and then decrease as the rocks become ultramafic in composition eclogite has a higher density but a lower poisson s ratio than peridotite the results were applied to constrain the crustal composition and tectonic evolution of the chinese continental crust based on crustal thickness h and poisson s ratio from 248 broadband seismic stations measured using teleseismic receiver function techniques the north china yangtze south china and northeast china blocks and songpan ganzi terrane are dominated by low b0 26 and moderate 0 26 b0 28 values n70 suggesting the dominance of felsic composition in the crust the lhasa terrane qiangtang terrane and indochina block are characterized by high proportions 33 42 of measurements with very high values 0 30 and h is found for the south china block northeast block lhasa block qiangtang terrane and indochina block indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

rapid incision of a small coarse and steep fan delta in response to base level fall the case of nahal qedem the dead sea israel the dead sea has been continuously dropping 0 4 1 0 m yr 1 since the middle of the 20th century and thus provides a unique field laboratory for studying in real time the response of drainage systems to a non tectonic base level fall the aim of this work is to study the short term ongoing erosive response to a rapid base level drop in a small steep fronted erodible fan delta setting the work explores the controls of the steep qedem fan delta guided by its clinoform structure on its incision longitudinal profiles of the fan delta and of its entrenched channel were measured in the field sedimentary facies changes fluviatile shallow lacustrine and beach were followed along exposures the existence of large boulders provided an opportunity to examine the uncertain role of armouring and boulder flux on incision the field study was combined with digital elevation models dems that were extracted from pairs of overlapping aerial photos maps of erosion and deposition were prepared using a change detection algorithm the longitudinal profile of the entrenched channel was found to be steep and linear the outlet temporarily hangs elevated and ungraded above the retreating lake level indicating years without incision flow events which cause lags in response to the rapid lake level drop in spite of the large boulders the small drainage basin and precipitation volume over the basin of the qedem the recorded vertical incision rates in the unconsolidated sediments are as high as 0 8 m yr 1 i e similar to those of the largest wadis draining to the dead sea the steep front of the fan delta is suggested to be a main factor controlling the efficient incision a unique transport mechanism of rolling boulders following undercutting contributed to the entrenchment efficiency copyright 2010 john wiley sons ltd

dehydration induced damage and deformation in gypsum and implications for subduction zone processes 1 experimental heating tests were performed on volterra gypsum to study the micromechanical consequences of the dehydration reaction the experimental conditions were drained at 5 mpa fluid pressure and confining pressures ranging from 15 to 55 mpa one test was performed with a constantly applied differential stress of 30 mpa the reaction is marked by 1 a porosity increase and homogeneous compaction 2 a swarm of acoustic emissions 3 a large decrease in p and s wave velocities and 4 a decrease in vp vs ratio wave velocity data are interpreted in terms of crack density and pore aspect ratio which modeling pores as spheroids is estimated at around 0 05 crack like spheroid complementary tests performed in an environmental scanning electron microscope indicate that cracks first form inside the gypsum grains and are oriented preferentially along the crystal structure of gypsum most of the visible porosity appears at later stages when grains shrink and grain boundaries open extrapolation of our data to serpentinites in subduction zones suggest that the signature of dehydrating rocks in seismic tomography could be a low apparent poisson s ratio although this interpretation may be masked by anisotropy development due to preexisting crystal preferred orientation and or deformation induced cracking the large compaction and the absence of strain localization in the deformation test suggests that dehydrating rocks maybe seen as soft inclusions and could thus induce ruptures in the surrounding nonreacting rocks

a sand gravel gilbert delta subject to base level change laboratory experiments were conducted on a sand gravel gilbert delta to gain insight on its dynamics under varying base level base level rise results in intensified aggradation over the topset as well as a decrease in topset slope and topset surface coarsening the signals of which migrate in an upstream direction preferential deposition of coarse sediment in the topset results in a finer load at the topset foreset break which creates a fine signature in the foreset deposit base level fall has the opposite effects entrainment of the topset mobile armor causes a coarsening of the load at the topset foreset break and so a coarse signature in the foreset deposit the entrainment of the topset substrate and fine top part of the foreset may follow which causes a fining of the load and a fine signature in the foreset deposit the fact that the upstream sediment supply requires a certain slope and bed surface texture to be transported downstream under quasi equilibrium conditions counteracts the effects of base level change this information travels in the downstream direction in nature base level change is likely so slow that the upstream sediment load maintains the topset slope and bed surface texture and so keeps the topset in a quasi equilibrium state base level change is therefore not expected to leave a clear signal in a mixed sediment gilbert delta other than a change in elevation of the topset foreset interface

micro crack enhanced permeability in tight rocks an experimental and microstructural study abstract the elastic and hydraulic response of a rock and its stress sensitivity are strongly affected by the presence of micro cracks therefore a full characterization and quantification of cracks at the micro scale is essential for understanding the physical and transport properties of rocks under stress as yet there is no uniquely accepted method to precisely quantify the density and geometrical characteristics of such microstructural features in this contribution we present results of quantitative analyses of 2d scanning electron microscopy sem images and 3d x ray microtomograms acquired on three samples of carrara marble artificially cracked by thermal shock new semi automatic workflows have been developed to perform these 2d and 3d analyses the main outcome is the quantification of average length aspect ratio and density per unit surface 2d or volume 3d of micro cracks observed the thermal treatment only opens grain boundaries and does not result in the creation of new intragranular cracks the results are consistent with the degree of thermal cracking artificially induced on the rock sample prior to the imaging analysis procedure i e more and wider micro cracks are measured on samples heated to higher temperatures the results of these quantitative microstructural analyses are also consistent with nuclear magnetic resonance nmr data independently acquired on the same samples saturated with water

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

tectonically induced adjustment of passive margin accommodation space influence on the hydrocarbon potential of the orange basin south africa a seismic stratigraphic investigation integrated with the structural modeling of the southern part of the orange basin passive margin south africa demonstrates that a single tectonic event resulted in a significant alteration to both the location and style of sediment accumulation during its postrift evolution this alteration to the margin has a significant effect on the hydrocarbon system of the area we predict that it increased the hydrocarbon potential of the area the evolution of the margin can be divided into two principal phases the first comprised an overall aggradational shelf margin with little or no deformation during the cretaceous deposition during the late cretaceous was punctuated by an episode of margin tilting that resulted in significant erosion of the inner margin and alteration of the margin architecture the second phase of deposition during the tertiary occurred to the west of the cretaceous shelf margin and was characterized by significant margin instability and the development of a coupled growth fault and toe thrust system this change in passive margin configuration and the associated switch in the location of overburden accumulation is likely to have increased the petroleum prospectivity of the deep water part of the margin we predict that the rapid western seaward migration of sediment accumulation resulted in the maturation of the high quality distal source interval whereas the resulting toe thrust geometry provides suitable structural traps for the hydrocarbons

dating volcanism and rifting on venus using impact crater densities although the distribution of impact craters on venus is indistinguishable from a completely spatially random population on the basis of crater data alone the addition of geologic information indicates that the craters are not random with respect to geology areas of low crater density correlate with concentrations of tectonic and volcanic features as well as high proportions of faulted and embayed craters high resolution mapping of large volcanoes flood type lava flow fields rifts and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance because these features also appear young based on stratigraphic data difference in age is the most likely reason for differences in crater density between geologically defined terrains crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters as long as the mapping criteria are independent of impact crater locations this result provides a tool to begin identifying and dating major geologic provinces and to begin developing a geologic history of venus using a simple data model and calculating crater ages relative to an assumed global mean age of 300 ma we show that the ages of large volcanoes 72 45 ma flow fields 128 91 ma rifts 130 145 ma and coronae 120 115 ma are all substantially younger than the mean plains age and probably represent ongoing volcanic and tectonic activity rather than the end of a brief global resurfacing event with a mean age of 300 ma

the effects of fluctuating base level on the structure of alluvial fan and associated fan delta deposits an example from the tertiary of the betic cordillera spain the villanueva complex is a lacustrine alluvial fan fan delta system which accumulated on the northern margin of the guadix basin during the pliocene five transitional zones can be distinguished from proximal to distal areas proximal alluvial fan mid fan fan fringe transition zone proximal fan delta and distal fan delta lacustrine this paper focuses on sedimentation in the fan fringe and the proximal fan delta areas where the effects of fluctuations in the base level lacustrine level are more easily observed the stratigraphical succession is here characterized by an alternation of fine lacustrine sediments mudstones and siltstones and gravels the gravels appear as isolated channels stacked channels lenses and sheets the isolated channels which have a v shaped profile and better developed wings towards the top appear in the fan fringe zone and in the fan delta the stacked channels originate in the fan fringe zone and evolve laterally and distally to isolated channels the lenticular gravel bodies lenses are well represented throughout the fan delta and present small channels at the base indicating a radial flow pattern finally the sheets are characteristic of the fan fringe zone the first episodes of channel incision occurred at lowstand lake level the channels and lenses developed in rising lake level conditions and sheet deposit took place at highstand lake level although the final form of the lithosome was heavily controlled by the fluctuations in lake level it depended on the relative proportions of gravel and silt sedimentation the gravel siltstone interface therefore represents an equilibrium surface between the proportions of sedimentation of these lithologies the bodies mentioned above are organized in coarsening and thickening upwards sequences tens of metres thick in which a distal to proximal evolution can be observed from isolated channels to lenses or from stacked channels to sheets the building and thickness of these sequences were controlled by fluctuations in the lake level an initial fall in base level caused lengthening of the channels and entrenchment from distal to proximal areas and a continuous increase in supply due to erosion in the drainage basin installed on the subaerial fan when the lake level rose lobes were formed at the channel endings and overbank processes were made possible thus generating radial channels whose levees were formed by amalgamation with the levees of the main lens channels at the same time sheets were formed in proximal areas where the available amount of sediment was greater

localized seismic anisotropy associated with long term slow slip events beneath southern mexico 1 slow slip events sses are recently discovered in the transition zone downdip of the locked zone where megathrust earthquakes repeatedly occur at subduction zone interface structural changes from the locked zone to the transition zone and the nature of in situ deformation in the transition zone however remain unclear we model teleseismic converted p to s waves to investigate seismic structural transition and anisotropy associated with shear deformation near the deep locked zone and the transition zone beneath southern mexico the deep locked zone appears to be consistent with normal oceanic crust velocity in the transition zone we find that seismic anisotropy larger than 5 is associated with the ultra slow velocity layer usl near the top of the subducted oceanic crust the foliation plane is plunging 20 10 degrees steeper than the plate interface and it is consistent with crystallographic preferred orientation developed in s c mylonites we suggest that long term sses likely coincide with a near lithostatic semiductile talc rich shear zone

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

amplitude frequency and drivers of caspian sea lake level variations during the early pleistocene and their impact on a protected wave dominated coastline the caspian sea the largest isolated lake in the world witnessed drastic lake level variations during the quaternary this restricted basin appears very sensitive to lake level variations due to important variations in regional evaporation precipitation and runoff the amplitude frequency and drivers of these lake level changes are still poorly documented and understood studying geological records of the caspian sea might be the key to better comprehend the complexity of these oscillations the hajigabul section documents sediment deposited on the northern margin of the kura basin a former embayment of the caspian sea the 2035 m thick well exposed section was previously dated by magneto biostratigraphic techniques and provides an excellent record of early pleistocene environmental lake level and climate changes within this succession the 1050 m thick apsheronian regional stage between ca 2 1 ma and 0 85 ma represents a particular time interval with 20 regressive sequences documented by sedimentary and palaeontological changes sequences are regressing from offshore to coastal lagoonal or terrestrial settings and are bounded by abrupt flooding events sediment reveals a low energy wave dominated reflective beach system wave baselines delimiting each facies association appear to be located at shallower bathymetries compared to the open ocean water depth estimations of the wave baselines allow reconstruction of a lake level curve recording oscillations of ca 40 m amplitude cyclostratigraphic analyses display lake level frequency close to 41 kyr pointing to allogenic forcing dominated by obliquity cycles and suggesting a direct or indirect link with high latitude climates and environments this study provides a detailed lake level curve for the early pleistocene caspian sea and constitutes a first step towards a better comprehension of the magnitude occurrence and forcing mechanisms of caspian sea lake level changes facies models developed in this study regarding sedimentary architectures of palaeocoastlines affected by repeated lake level fluctuations may form good analogues for other semi isolated basins worldwide

geometry and seismic properties of the subducting cocos plate in central mexico the geometry and properties of the interface of the cocos plate beneath central mexico are determined from the receiver functions rfs utilizing data from the meso america subduction experiment mase the rf image shows that the subducting oceanic crust is shallowly dipping to the north at 15 for 80 km from acapulco and then horizontally underplates the continental crust for approximately 200 km to the trans mexican volcanic belt tmvb the crustal image also shows that there is no continental root associated with the tmvb the migrated image of the rfs shows that the slab is steeply dipping into the mantle at about 75 beneath the tmvb both the continental and oceanic moho are clearly seen in both images and modeling of the rf conversion amplitudes and timings of the underplated features reveals a thin low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab by inverting rf amplitudes of the converted phases and their time separations we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting cocos plate and its thickness high poisson s and vp vs ratios due to anomalously low s wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure the evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates this may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust

accurate vp vs relationship for dry consolidated sandstones 1 we use a finite element method to study the elastic properties of dry sandstone morphologies we show that the dry poisson s vp vs ratio becomes independent of the poisson s ratio of the solid phase at a critical porosity and that this limiting behaviour holds for any number of solid phases and is insensitive to the manner in which the phases are distributed we represent the behaviour of the poisson s ratio as a simple flow diagram and present an empirical structure property relationship that holds for all sandstone morphologies independent of the number of phases and the poisson s ratio of the solid phase

depositional record of an avulsive fluvial system controlled by peat compaction neogene most basin czech republic this paper illustrates the response of a fluvial depositional system to the interplay between peat compaction and clastic sediment supply at a range of spatial and temporal scales as documented by extensive exposures in an open cast mine in the most basin part of the oligo miocene oh e rift eger graben basin system in the czech republic the most basin is characterized by the occurrence of a number of phenomena resulting from syn and post depositional interactions between clastic sedimentary systems and the underlying accumulation of organic material that was the precursor of the main lignite seam of up to 45 m thickness the studied strata are interpreted as deposits of an avulsive mixed load fluvial system the large scale depositional architecture documents an existence of at least five stratal packages up to 1500 m wide and up to several tens of metres thick representing a record of long term evolution of a clastic floodplain bordered by accumulating peat within each of the packages several small scale channel belts were documented individual packages are separated by carbonaceous mudstones indicating a period of reduced clastic input and interpreted as due to avulsion of the fluvial channels out of the floodplain limit two main mutually linked processes controlled the evolution of the studied fluvial system i syndepositional compaction of the underlying peat and ii avulsions of the channels away from the original floodplain resulting in formation of a new floodplain the processes which caused the channels of the hrabak fluvial system to reach the avulsion threshold were i decrease of rate of creation of accommodation leading to increased sinuosity and thus to a decreased channel slope and ii cross floodplain tilting of the channel belt caused by differential compaction of underlying organic rich substratum

magellan observations of extended impact crater related features on the surface of venus a total of 57 parabolic shaped and 9 approximately circular extended impact crater related features have been found in magellan synthetic aperture radar sar and thermal emissivity data covering 92 of the surface of venus the parabolic features are with seven exceptions oriented e w with the apex to the east and the impact crater located just west of the apex they were first identified in the surface emissivity data derived from magellan radiometry measurements but the great majority are only clearly visible in the sar imagery the overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the parent craters the floors of almost all these craters have high specific radar backscatter cross sections i e they are bright in the sar imagery relative to their surroundings and tend to have low emissivities approximately one third of the impact craters with diameters 15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the sar or emissivity data no features have been found which overlie the parabolic features indicating that they are among the youngest features on the surface of the planet this suggests that radar bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature a model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the e w zonal winds fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0 6 cm on the wavelength scale 12 6 cm surface roughness and hence of 1 to 2 cm on the largest particle sizes of interest fallout times from 50 km in the venus atmosphere for particles of this size are about 2 hours allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s 1 measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits are consistent with deposit depths of a few centimeters to 1 or 2m

investigation of elastic weakening in limestone and sandstone samples from moisture adsorption summary elastic and mechanical weakening from water saturation are widely known to occur in sedimentary rocks and particularly in carbonate rocks to improve our understanding of the physics underlying this phenomenon ultrasonic f 0 5 mhz elastic properties are measured on a large suite of clean limestones and sandstones at very low saturations from relative humidity rh variations at ambient conditions measurements clearly highlight an elastic weakening i e decrease in elastic wave velocity from moisture adsorption p and s wave velocities are similarly affected by adsorption but in a different way for limestones and sandstone samples while the elastic properties of limestone samples show almost no rh dependence a large weakening is observed for samples of fontainebleau sandstone that increases with the samples porosity the main elastic weakening effect is likely to result from adsorption of fluid at grain contacts it thus affects particularly granular rocks such as sandstones while well cemented limestones are not affected the granular model from murphy et al accounting for surface energy effects proves to be appropriate applying this model it is shown that i p and s wave velocities have the same dependence on surface energy which is consistent with the measurements and ii surface energy values obtained from the ultrasonic data using this model correlate with rh and are consistent with the expected value for quartz crystals at vapour pressure yet porosity which relates to degree of cementation in the particular case of fontainebleau sandstone appears to be an additional parameter a modified model is thus derived using the cementation model from digby accounting for a bonding radius at grain contact it proves to apply well to the measured data the fundamental difference between limestones and sandstones dependence to rh appears to be related to a microstructural difference saturation variations from rh increase depend on specific surface area which is particularly low in fontainebleau sandstones and large in microporous limestones however elastic weakening from rh is more important in sandstones owing to their granular microstructure

common evolution of mechanical and transport properties in thermally cracked westerly granite at elevated hydrostatic pressure increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties including mechanical strength elastic and mechanical anisotropy and the enhancement of transport properties using a non interactive crack effective medium nic theory as a fundamental tool we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density further we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness kic of rocks as measured via a suite of tensile failure experiments chevron cracked notch brazilian disk finally we include fluid flow in our analysis via the gueguen and dienes crack porosity permeability model using the crack density and aspect ratio recovered from the elastic wave velocity inversion we successfully compare permeability evolution with pressure with the laboratory measurements of permeability

impact craters of venus analysis of venera 15 and 16 data from description of impact craters on venus and crater size and depth distributions models are developed that analyze the size distribution curve and radar bright haloes around fresh craters the dense venusian atmosphere has a strong influence on cratering the estimated age of the surface of the area under study is from 0 5 to 1 0 109 years the estimated rate of resurfacing during this time interval is about 0 04 to 0 8 cm 106 years the lithosphere of venus seems to be sufficiently rigid to sustain craters of 100 140 km diameter and 0 8 2 5 km depth for about 108 years

depositional systems and sequence stratigraphy of coarse grained deltas in a shallow marine strike slip setting the bohemian cretaceous basin czech republic deposits of coarse grained gilbert type deltas showing varying degrees of reworking of foresets by basinal currents were identified in middle turonian to early coniacian sandstones of the bohemian cretaceous basin the progradation of the deltaic packages earlier interpreted as large scale subaqueous dunes shelf ridges or subaqueous fault scarp accumulation terraces was controlled by high and low frequency relative sea level changes in a relatively slowly subsiding intracontinental strike slip basin end member types of the bohemian cretaceous coarse grained deltas are deep water deltas characterized by thick 50 80 m foreset packages with steep 10 30 foresets and shallow water deltas which deposited thin 15 m packages with foresets typically between 4 and 10 the differences in thickness and foreset slope angle were controlled predominantly by the accommodation available during progradation the depositional regime of the deltas was governed by i the fluvial input of abundant sand bedload with a minor proportion of gravel ii gravity flows most probably caused by liquefaction of the upper part of the unstable foreset slope and iii migration of sandy bedforms on the foreset slopes the bedform migration was driven by unidirectional currents of possible tidal origin individual foreset packages represent systems tracts or parts of systems tracts of depositional sequences a variety of stacking patterns of high frequency sequences exists in the basin caused by low frequency relative sea level changes as well as by local changes in sediment input because of generally low subsidence rates fluvial or beach topset strata were not preserved in the cases studied the absence of preserved fluvial facies which has been one of the main arguments against the fluvio deltaic origin of the sandstone bodies is explained by erosion of the topsets during transgression and their reworking into coarse grained lags of regional extent covering ravinement surfaces

impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft additional studies of venusian impact craters have been made based on an analysis of venera 15 16 radar imagery and altimetry the crater population on venus has been subdivided into groups representing different morphological classes and types the craters display the size dependent variations in morphology which are well known from other planets assuming a crater production rate based on estimates by hartmann et al 1981 their areal density indicates an age for the total population of approximately 1 b y alternative estimates based on the assumption that the recent cratering rates on the earth and venus were similar would give a significantly lower age the areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b y in age a suggested model of the effect of the atmosphere on the crater diameter frequency curve indicates that cometary impacts played only a minor role in forming the contemporary venus environment a comparative study of craters in the area where the arecibo earth based radar and venera 15 16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle arecibo images correspond to the flat bottoms of the craters seen in the venera images

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

surface formation rates and impact crater densities on venus impact crater density has been used to estimate relative ages of major venus geologic units typically based on the assumption of short globally synchronous formation periods the total number density of craters is the only parameter which may be quantified for most venus subregions in contrast to the size frequency analysis often used for other planetary studies this paper examines the statistical limits on such crater counts and the predicted crater density for various possible time varying surface formation rate functions these results show that 1 the true mean crater density is poorly constrained due to the limited sample size provided by aggregate geologic units and 2 any given crater density can be accommodated by a very short pulse of unit formation at some age t gaussian distributed formation functions of mean age t exponential functions with initiation ages between t and 2t or a host of other more complicated rate functions these results when coupled with the unknown rate of crater removal by tectonic or volcanic activity indicate that the crater counts for specific landforms on venus offer little constraint on the relative timing or duration of their development the hypothesis of globally synchronous formation of landforms such as volcanoes or coronae cannot be validated with the available crater data

comment on impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft by a t basilevsky et al

frequency and fluid effects on elastic properties of basalt experimental investigations 1 in order to investigate the effects of fluid and frequency on the elastic properties we performed hydrostatic experiments on an icelandic basalt specimen under both dry and saturated conditions this basalt is characterized by a bimodal porosity i e cracks and equant pores the elastic properties bulk moduli in our case were investigated under high pressure through two experimental methods 1 a classical one using ultrasonic p and s waves velocities frequency 106 hz 2 and a new one using oscillation tests frequency 10 2 hz in dry condition experimental data show no significant difference between high hf and low lf frequency bulk moduli however in saturated conditions two effects are highlighted a physico chemical effect emphasized by a difference between drained and dry moduli and a squirt flow effect evidenced by a difference between hf and lf undrained moduli

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

eocene post rift tectonostratigraphy of the rockall plateau atlantic margin of nw britain linking early spreading tectonics and passive margin response a regional study of the eocene succession in the uk sector of the rockall plateau has yielded new insights into the early opening history of the ne atlantic continental margin data acquired from british geological survey borehole 94 3 on the rockall high provides a high resolution record of post rift early to mid eocene subaqueous fan delta development and sporadic volcanic activity represented by pillow lavas tuffs and subaerial lavas this sequence correlates with the east rockall wedge which is one of several prograding sediment wedges identified across the rockall plateau whose development was largely terminated in the mid lutetian linking the biostratigraphical data with the magnetic anomaly pattern in the adjacent ocean basin indicates that this switch off in fan delta sedimentation and volcanism was coincident with the change from a segmented transform margin to a continuously spreading margin during chron c21 however late stage easterly prograding sediment wedges developed on the hatton high during late mid to late eocene times these can only have been sourced from the hatton high which was developing as an anticline during this interval this deformation occurred in response to mid to late eocene compression along the ocean margin possibly associated with the reorganisation to oblique spreading in the iceland basin which culminated at the end of the eocene with the formation of the north hatton anticline and the deformation tilting of these wedges a series of intra eocene unconformities of which the mid lutetian unconformity is the best example has been traced from the rockall plateau to the faroe shetland region and onto the greenland conjugate margin bordering the early ocean basin whilst there appears to be some correlation with 3rd order changes in eustatic sea level it is clear from this study that tectonomagmatic processes related to changes in spreading directions between greenland and eurasia and or mantle thermal perturbations cannot be discounted

morphology and sedimentology of two contemporary fan deltas on the southeastern baja california peninsula mexico abstract the san juan de los planes drainage basin is located on the southeastern margin of the baja california peninsula approximately 20 km southeast of the city of la paz pleistocene alluvial fans have been built into the basin from the surrounding areas of high relief and are presently being eroded at the fan apex the dissection depth ranges from 20 to 40 m and is 6 to 15 m at the coast the sediments have been redeposited in several active modern fan deltas built out into the narrow shelf of la ventana bay shelf width ranges from 0 to 5 km average 3 km wave energy is delivered from southeasterly storms and from the local sea breeze winds the respective wave heights average 2 and 1 m the agua caliente and las canoas fan deltas the subjects of this paper are located on the active el sargento fault zone and are fed by channels that deeply dissect the older fan surfaces the agua caliente fan delta is a classic gilbert type delta with a steeply inclined delta front wave reworking and redistribution of the sediments at the delta face are important and a wave cut terrace has developed at about 6 m depth the las canoas fan delta is a shoal water profile hjulstrom type with a gently inclined delta front wave energy is less important the alignments of the coasts of both fans are controlled by the fault line sediment supply is largest for the las canoas fan delta the fan deltas are examples of this depositional form as it has developed in an arid tectonically active high relief and narrow neo trailing edge margin

architectural stacking patterns of the ebro delta controlled by holocene high frequency eustatic fluctuations delta lobe switching and subsidence processes abstract during the late pleistocene and holocene 125 ka b p to present a type 1 4th order depositional sequence comprising regressive lowstand transgressive and highstand system tracts formed worldwide the holocene 10 ka b p to present part of this contains the latest and present highstand systems tract hst within this and the underlying transgressive systems tract tst of the ebro delta in northeastern spain higher frequency 5th to 6th order sea level fluctuations are recognised these form retrogradational and progradational high frequency depositional sequence sets within the tst and hst respectively each high frequency sequence comprises 1 a basal highly reflective shell lag associated with an erosional transgressive surface 2 aggradational deposits which seawards consist of a wedge of marine clays with transparent seismic facies but inland are represented by thick peats 3 progradational deposits composed of sandy delta front facies displaying slope clinoforms these pass seawards into prodeltaic grey silts and landwards into red silts and pebbly sands of delta plain facies the progradational deposits downlap towards their bases the aggradational deposits formed in response to a period of rising sea level and a rapid increase in accommodation space progradation began when sediment supply to the delta exceeded accommodation space as a result of relative sea level fall the relative sea level curve for the delta has a stepped character caused by the punctuation of the 4th order sea level trend by higher frequency eustatic fluctuations as well as by high subsidence rates of about 1 75 mm per year the tst comprises retrogradational parasequences that onlap the underlying pleistocene gravels the maximum flooding surface separating the tst from the hst was formed about 6900 yrs b p on the basis of14c dating of peats the hst comprises progradational deltaic and aggradational units stacked as progradational high frequency parasequence sets the hst of the ebro delta is compared with other deltaic sequences around the world in particular with that of the mississippi delta a tentative chronology of the high frequency climatic and eustatic oscillations influencing deltaic sedimentation globally over the last 7000 years is presented

teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs a review abstract more than a decade after the discovery of deep episodic slow slip and tremor or slow earthquakes at subduction zones much research has been carried out to investigate the structural and seismic properties of the environment in which they occur slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner where three major structural elements are in contact the subducting oceanic crust the overriding forearc crust and the continental mantle in this region thermo petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions and their consumption by hydrating the mantle wedge these fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore fluid pressure and or reducing friction in fault gouges resolving the fine scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging and most advances have been made using teleseismic scattering techniques e g receiver functions in this paper we review the teleseismic structure of six well studied subduction zones three hot i e cascadia southwest japan central mexico and three cool i e costa rica alaska and hikurangi that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior we conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior and that near lithostatic pore fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

experimental delta formation in crater lakes and implications for interpretation of martian deltas 1 the morphology of delta deposits in crater lakes on mars is indicative of upstream e g flow discharge and sediment properties and downstream e g basin characteristics parameters from which the hydrological conditions at the time of formation can be inferred to investigate the influences of these parameters on delta morphology we experimentally created deltas in crater shaped basins by feeding a range of constant flow discharges over a feeder channel of various sand textures we reproduced three categorically different types of deltas including most of the types that have been identified on mars our most striking observation is that water level behavior itself a unique function of flow discharge basin size and hypsometry and basin floor permeability can explain most delta morphologies observed on mars stepped retrograding deltas formed during water level rise prograding deltas formed during water level still stand and during water level fall deltas were partially destroyed by erosion on mars numerous retrograding and prograding deltas were preserved most of them without indications of channel incisions or other fluvial modification we conclude that the main difference between a single foreset prograding delta and a multiple foreset retrograding delta is the behavior of the water level in the basin these simple delta morphologies cannot be reconciled with long duration hydrological activity because that would imply crater lake fluctuations due to inherent complex water level histories along with complex sediment delivery histories our experiments and numerical verifications demonstrate that such deltas preferentially form during one aqueous event which parsimoniously argues for short duration hydrological activity

high resolution subduction zone seismicity and velocity structure beneath ibaraki prefecture japan 1 we use double difference tomography and waveform derived cross correlation differential times to estimate earthquake locations and p and s wave velocity structure in the subduction zone under ibaraki and neighboring prefectures of north central honshu we find evidence in both earthquake hypocenters and the velocity structure that the philippine sea plate or perhaps a microplate fragment may be caught between the subducting pacific plate and overriding okhotsk plate in this region in the southern part of the study area we find a zone of interplate events extending as deep as 60 km forming a distinct lineation in cross section focal mechanisms support the interpretation that these are low angle subduction interface events we infer that these events probably occur on the interface between the pacific and philippine sea plates rather than between the pacific and okhotsk plates in the upper part of the downgoing pacific plate we find a zone of high vp vs ratio 1 95 which may be explained by high pore fluid pressures within the subducting crust at a depth of 60 80 km this region of high vp vs appears to diffuse into the overlying mantle wedge possibly indicating the upward release of fluids from the slab

effects of the readvance of an ice margin on the seismic character of the underlying sediment abstract seismic stratigraphy from a 65 km long transect of the lake melville fjord basin southeast labrador canada has revealed seven cycles of ice front advance retreat from a stratigraphic interval which regionally is correlated to 10 9 ka bp a maximum thickness of 400 m of glacimarine sediments ice contact submarine fans and associated finegrained debris totally 9 8 km 3 was deposited the thick ice contact deposits terminal moraines provide evidence that the lake melville basin was occupied by a grounded temperate tidewater glacier during this early holocene period of deglaciation detailed interpretation of the acoustic architecture of four associated ice contact fans shows that they were formed mainly during advance of the ice front only thinner sediment packages were emplaced during ice front stillstand episodes a subglacial unit equivalent to these foreset subunits mapped on the ice proximal side of the fans is relatively uniform in thickness the subglacial unit is interpreted as a basal deformation layer formed during the ice front advance seismic data from lake melville support the idea of asymmetric ice front advance retreat cycles the individual cycle however shows a more complex pattern than the simple slow advance stillstand rapid retreat model as suggested from many modern tidewater glaciers if the regional chronology is correct these advance retreat cycles have an average duration of 150 yrs they apparently occurred during a period of drastic ablation of the laurentide ice sheet as a result of an unknown combination of extrabasinal and intrabasinal factors

sea level related architectural trends in coarse grained delta complexes distinct stratigraphical styles of delta development can be related to basically different patterns of relative sea level base level change 1 an overall rise with periods of standstill and without significant fall rs 2 an overall rise with superimposed falls rf 3 an overall fall with superimposed rises fr and 4 an overall fall with periods of standstill and no significant rises fs the related stratigraphical models contain either preserved elements of prototype deltas or show characteristic trends in development from one prototype delta into another each stratigraphical model is characterized by lateral and vertical lithofacies features which can be reasonably well established from outcrop in coarse clastic basin margins where age control is poor sedimentological and stratigraphical delta models as presented here may help refining reconstructions of basin fill patterns and basin subsidence kinematics

sequence stratigraphy of the mesozoic domeyko basin northern chile the domeyko basin of northern chile records late triassic to early cretaceous mixed carbonate and siliciclastic marine deposition along the western margin of gondwana sequence stratigraphical analysis has identified five long term sequences of 11 17 ma duration comparison of the relative sea level fluctuations interpreted from the domeyko basin succession with those documented from other similar aged south american marginal basins and northern hemisphere basins allows the distinction of regional from global events relative sea level fall in the early pliensbachian early aalenian and early valanginian and rises in the early hettangian early and late toarcian early and late bajocian late bathonian and early oxfordian of the domeyko basin appear time equivalent to similar events in other southern and northern hemisphere basins and are thus interpreted to be products of global sea level cycles long term relative sea level falls in the early bathonian late oxfordian early valanginian and rise in the late kimmeridgian are interpreted to be tectonically driven continental scale changes in sediment accommodation space the domeyko basin succession thus appears to have been dominantly controlled by global sea level fluctuations during the early to mid jurassic and by continental scale but not global fluctuations during the mid jurassic to mid cretaceous interpreted to have been driven by the fragmentation of gondwana

nass river on the move radar facies analysis of glaciofluvial sedimentation and its response to sea level change in northwestern british columbia the nass valley of northwestern british columbia is a glacial fiord containing extensive glaciomarine and glaciofluvial sediments two parallel braidplains separated by a bedrock ridge were deposited within the fiord mapping of these deposits led to the hypothesis that the braidplains must have terminated at deltas however a lack of surface exposures meant that ground penetrating radar was needed to investigate these deposits radar facies analysis aided in the identification of braidplain braid delta and glaciomarine depositional environments as well as underlying bedrock several deltas graded to different sea levels were discovered allowing inferences to be made about the relationship of falling sea level to sediment architecture the upper section of the western braidplain is graded to a sea level of 185 m above sea level asl indicating that the proto nass river flowed on the western side of the bedrock ridge when the sea was at that level however the river moved to the east side of the ridge as sea level fell depositing the extensive aiyansh braidplain braid delta which is graded to a 152 m sea level stand several other deltas also formed at this sea level stand avulsion occurred and the river flowed on the west side of the ridge again when sea level fell to 134 m asl the river remained in this position throughout late glacial time and eventually evolved into the modern nass river the coarse grained deposits are indicative of forced regression with both stepped top attached and detached stratal architecture present

controls on stratigraphic architecture in contemporaneous delta systems from the eocene roda sandstone tremp graus basin northern spain abstract the eocene roda sandstone of the spanish pyrenees is comprised of two coeval coarse grained delta systems which share correlatable surfaces but show a markedly different internal facies architecture the two deltas developed contemporaneously with the differences in internal architecture attributed to basin morphology and the role of two distinct sediment sources while the correlatability of the larger scale packages indicates an external allocyclic control as such the roda sandstone provides the possibility to address the relative roles of different parameters known to interact and control stratal architecture the roda sandstone crops out at the ne margin of the tremp graus basin a piggyback basin initiated in palaeogene times in the foreland of the southern pyrenees the study area 30 km2 is composed of the well understood and well exposed parts of the roda sandstone cropping out along the isabena valley and the less worked and less exposed outcrops to the north along barranco de ricans and barranco de codoneras data for this study include a combination of conventional sedimentology data coupled with analyses of 3d virtual outcrop models based on these methods 8 different facies associations have been distinguished their distribution suggests that the roda sandstone is composed of two different but coeval delta systems and their linked subenvironments a gilbert type delta outcropping along isabena valley and a shoal water fan delta along barranco de ricans and barranco de codoneras along the isabena valley the outcrops of the gilbert type deltaic system are composed of at least 6 distinct packages roda u z characterized by texturally mature delta front sandstones separated by offshore mudstones the stacked basinward stepping architecture of the roda u y units here represents a minimum of 3 major phases of sw directed progradation capped by hardground horizons formed during the subsequent flooding phases tidally modified delta front deposits found in the distal exposures of roda y represent the redistribution of the delta toeset by w nw directed ebb tidal currents active during advanced stages of progradation of the gilbert type delta the more gentle sloping shoal water fan delta system found in barranco de ricans and barranco de codoneras is characterized by progradational to aggradational stacking of texturally immature wave modified mouth bar sandstone packages separated by offshore mudstones the zone of interference between the gilbert type and shoal water delta is dominated by wave reworked sediments deposited as shoreface barrier and spit deposits biostratigraphic dating suggests that the roda sandstone was deposited during a 3rd order relative sea level cycle as a shelf margin systems tract however the stacking of the individual roda u y units demarcated by offshore mudstones both within the gilbert type and the shoal water deltaic systems suggest that 4th order fluctuations occurred these 4th order units are physically correlatable between the two deltaic systems which implies that both deltas responded synchronously to a combination of regional subsidence and eustatic sea level fluctuations the differing internal architecture such as the steeply dipping textural mature foreset beds of the gilbert type delta versus the more gentle sloping textural immature mouth bars of the shoal water fan delta are controlled by different hinterland characteristics and intrabasinal topography

distributary channels and their impact on sediment dispersal a global analysis of world deltas with details from a natural and an anthropogenic influenced delta demonstrates how distributary channels control the flux of sediment into the coastal ocean the study addresses the range in the number of distributary channels across world deltas using remote sensing techniques a power law relationship is found between the number of distributary channels and the length of river and separately the delta gradient these relationships hold for all types of deltas whether controlled or strongly influenced by waves river discharge tides or ice permafrost or sea ice nature controlled deltas such as the klinaklini delta have distributary channels that act as overflow conduits that become active during flooding events anthropogenic controlled deltas like the po delta have distributary channels that are controlled for flood mitigation or low flow maintenance anthropogenic influences greatly impact the natural rate of delta progradation through changes in sediment supply controlling the position of distributary channels and impacting subsidence from gas and groundwater extraction even with flood controls the po delta traps 16 of the sediment load in its channels that are becoming super elevated at rates of 4 to 10 cm yr with respect to the delta plain a new model is formulated and shown to predict accurately the sediment flux through each channel along with their hydraulic properties deltas with high numbers of distributary channels produce hypopycnal plumes with reduced transport capacity as a result sediment diffuses out of the multi channel deltas as a buoyantly driven plume rather than as a momentum driven jet

development of pleistocene glaciomarginal lake in the foreland of the sudetes sw poland abstract numerous glaciomarginal lakes existed during the pleistocene in the sudetes and their foreland sw poland because of damming off by an ice sheet in the mountain area they were formed in the valleys or intermountain basins during the phase of maximum ice extent in the mountain foreland they were developed at the front of the more active ice margin in the period preceding the maximum ice sheet advance the latter lakes were less confined by the topography and their contact with the ice sheet was often considerably larger this caused their different development the sedimentological development of a small lake of this type located near mokrzeszow was studied the lake was characterized by high variability in water level and high energy depositional processes because of its small size the lake responded very quickly to all changes in external factors the lake became filled mainly by a coarse grained delta this happened in several phases because the water level fluctuated considerably owing to oscillations in the ice sheet extent during lowstands the delta became incised locally subaqueous fans were formed the lake basin eventually became overridden by the ice sheet

evolution in basin fill style during the mesozoic gondwana continental break up in the godavari triple junction se india abstract consistent changes are observed in the mesozoic sedimentary lithofacies and palaeocurrents during the transition from the continental gondwana nw se trending chintalpudi graben cg to the coastal gondwana ne sw trending krishna godavari coastal basin kgcb in the godavari triple junction gtj the mesozoic continental gondwana sediments include the kamthi early triassic kota jurassic and gangapur early cretaceous formations coastal gondwana succession comprising the raghavapuram early cretaceous and the tirupati middle to late cretaceous formations transversely superimpose the continental gondwana the continental gondwana is mainly cross stratified sandstone and fossiliferous glossopteris and ptilophyllum flora clay assemblage of alluvial environment palaeocurrent is towards nw in the kamthi formation wnw in the kota formation and centripetal in the gangapur formation the raghavapuram formation kgcb comprises of grey carbonaceous clay sandstone association of se flowing delta plain distributories with inter distributory swamps and fossiliferous white clay a mix of ptilophyllum flora and invertebrate fauna of prodelta shallow marine environment the succeeding sandstone conglomerate facies of the tirupati formation middle to late cretaceous with se palaeocurrent indicates delta progradation whereas the infratrappean clay limestone beds mainly invertebrate fauna of maastrichtian age point to delta abandonment prior to the k t boundary volcanism tectono sedimentary model presented in this communication incorporates that the change from the continental gondwana to coastal gondwana had occurred in four stages during stage i late carboniferous to early jurassic the sediments were deposited by a north westerly flowing alluvial system whose distal source was located in southeast beyond the east coast of india stage ii middle to late jurassic is marked by westerly shift in palaeocurrents attributed to an obstruction caused by an upliftment of the mailaram high parallel to the ne sw eastern ghat trend stage iii early cretaceous is heralded by the onset of deltaic sedimentation in se due to separation of landmass that supplied detritus to the cg until the end of the jurassic during stage iv the delta progradation is accentuated by the basement uplift eastern ghat mobile belt it is suggested that the change over from continental gondwana to coastal gondwana had occurred over a time span of about 70 my between late jurassic to cretaceous and controlled by phase wise upliftment of the basement as a corollary to the gondwana continental break up

reply to comment on impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft by a t basilevsky et al

reconstructing low levels of lake lisan by correlating fan delta and lacustrine deposits abstract reconstruction of paleo shorelines of lake lisan the late pleistocene precursor of the dead sea is based on sequence stratigraphy of fan delta and lacustrine deposits that are exposed at the perazim valley southwest of the dead sea the shoreline sediments are physically correlated with lacustrine aragonites their ages are determined by u series dating to establish a lake level curve for the time interval between 55 and 35 kyr during most of this period lake lisan was at a relatively low level which fluctuated between 280 and 290 m below sea level m bsl the minimum lake stand 330 m bsl was reached at 47 kyr and lasted for 3 4 kyr this episode is marked by a depositional hiatus in the section and formation of an erosional channel the studied period correlates with the global marine isotope stage 3 the low stand of lake lisan during most of this period indicates relative dry climatic conditions in the region

pore pressure and oceanic crustal seismic structure summary marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust in many localities compressional and shear wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth it is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability this is supported by compressional and shear wave velocity measurements as functions of confining pressure and pore pressure for basalt from the juan de fuca ridge and dolerite from the samail ophiolite oman within the oceanic crust regions of overpressure and underpressure will possess anomalous velocities the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic the influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low of significance poisson s ratio is shown to be dependent on pore pressure as well as confining pressure at constant confining pressure poisson s ratio increases with increasing pore pressure thus overpressured regions within the upper oceanic crust are likely to have relatively high poisson s ratios as well as low compressional and shear wave velocities

the wandering of the volga delta a response to rapid caspian sea level change due to its very low gradient and absence of tide and surf the volga delta is an even more extreme example of the fluvially dominated type than the mississippi delta however it differs from all other large delta systems in that it borders a closed basin the caspian sea now at 26 m below global sea level caspian sea level is much more dynamic than that of the world oceans and rises at present about 1 5 cm yr a hundred time the eustatic rate within the quatemary sea level oscillations of at least 5 orders of magnitudes have been distinguished which seem grossly out of phase with eustatic sea level between the weichselian early khvalyn highstand of 50 m and the early holocene mangyshlak lowstand at 80 m the apex of the volga delta has wandered over 700 km alongstream the present day volga delta is not a highstand deposit but probably represents a minor transgression in a major regressional stage the delta does not show a coarsening upwards sedimentary sequence but consists of a weichselian transgressional fining upwards sequence topped by eolian deposits in which the delta distributary channels have been incised present day sedimentation is limited to a narrow fringe along the delta front and to deeper waters over 200 km offshore sea level changes outpaced aggradation to such an extent that volga sediment is spread over the whole north caspian plain sequence stratigraphical principles are difficult to apply because sea level cycles of five orders of magnitude are superimposed and because there is not enough sediment loading or tectonic subsidence to create sufficient accommodation space

physical properties and brittle strength of thermally cracked granite under confinement 1 effects of thermal crack damage on the rupture processes of a fine grained granite were investigated under triaxial stress under water wet and argon gas dry saturated conditions and at room temperature thermal cracking was introduced by slowly heating and cooling two samples of la peyratte granite up to 700 c which were compared to two intact specimens for each rock sample a hydrostatic test was first carried up to 90 mpa effective pressure 5 mpa constant pore pressure the samples were then deformed to failure at a constant strain rate of 2 10 6s 1 at 30 mpa effective pressure our results show that 1 permeability of heat treated specimens was 4 5 orders of magnitude larger than that of intact specimens at low effective mean pressure 2 nevertheless at our experimental conditions 2 10 6 s 1 thermal cracking had no significant influence on the brittle strength 3 similarly no obvious water weakening effect was observed 4 however with increasing stress elastic anisotropy appeared at lower differential stress in heat treated specimens than in intact ones but close to failure the magnitude of p wave anisotropy was approximately the same for both types of specimens 5 acoustic emission hypocenter locations and p wave velocity anisotropy in the basal plane demonstrate that strain localization started right at the onset of dilatancy for heat treated specimens later in the intact specimens and 6 inverting wave velocities for crack density we show that failure was reached for vertical crack densities of 0 35 for dry specimens and possibly 0 5 for water saturated specimens

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

formation of glaciolacustrine late pleistocene end moraines in the tasman valley new zealand abstract detailed examination of the tekapo formation in the tasman valley new zealand has identified 20 facies and five facies associations these associations are delta foresets and bottomsets sediment density flows ice contact lake sediments with ice rafted debris and resedimentation deposits and outwash gravels interpretation of the sediment landform associations informed by observations at modern glacier termini suggests that the late pleistocene tekapo formation moraines have been formed by downwasting of a more expanded tasman glacier during the early stages of glacier retreat ponds on the glacier surface develop into thermokarst lakes which enlarge and coalesce to form a large supraglacial lake continued downwasting causes the lake outlet river to entrench into the impounding latero frontal ice cored moraine lowering the lake level this exposes lake bottom sediments and forms shorelines on the proximal slopes of the ice cored moraine as the ice cored moraine melts these lake sediments are deformed and deposited against the mt john moraine the observations and interpretations reported here suggest the late pleistocene end moraine is a constructional feature not a structural glaciotectonic feature as suggested by previous studies

topography and significance of a basinwide sequence bounding erosion surface in the cretaceous cardium formation alberta canada abstract the cardium formation turonian coniacian upper cretaceous of the western interior seaway alberta contains six marine sandier upward successions bounded by seven basin wide erosion surfaces in the subsurface erosion surface e5 is associated with major oil and gas fields and is penetrated by thousands of wells a topographic map of e5 based on 6126 well logs and over 1100 measured cores shows a series of subparallel elongate highs steps and lows which trend northwest southeast parallel to tectonic trends and to the actively rising cordillera to the southwest the steps are about 10 km apart and the relative relief step to adjacent low is up to 25 m the steps are asymmetrical with a steeper face to the northeast the e5 erosion surface cuts into offshore marine hummock cross stratified sandstones and interbedded mudstones and it records a lowering of relative sea level our interpretation suggests that erosion was in neither a fully subaqueous nor a fully subaerial environment instead it involved relative sea level lowering and subaerial erosion followed by major modification of the landscape during transgressive shoreface erosion at times of relative stillstand during overall transgression gravel was supplied to the shoreface and was reworked a short distance alongshore by waves these conglomerates now form important oil and gas reservoirs the asymmetry of the steps and lows indicates that the originally flat sea floor was tilted northeastward immediately before erosion giving a gradient of about 1 1000 during transgression shorefaces were incised into the tilted surface to form a stepped morphology also during transgression the sea floor was gradually restored to its pre tilting position each of the six marine sandier upward sequences along with the erosion surface at the top occupies an average of 470 000 years and we estimate that transgression took at least 100 000 to 200 000 years

fluvial response to an historic lowstand of the great salt lake utah understanding fluvial adjustments to base level changes benefits the fields of sequence stratigraphy geomorphology and petroleum geology this investigation is a modern case study of the channel dynamics of lee creek and the goggin drain two streams that are part of the jordan river drainage into the endorheic great salt lake of northern utah a lacustrine system that has experienced multiple decadal scale base level changes since 1965 the lake level has fluctuated in elevation more than 6 m transitioning from an historic lowstand 1284 m a s l and in 2009 2010 approaching an historic lowstand this study uses detailed aerial images fieldwork and lidar data to link the modern geomorphology and channel hydraulics to specific variations in sediment transport channel form and avulsion behavior although lee creek and the goggin drain are situated only a few kilometers apart and share similar shore zone gradients substrates and vegetation patterns and have been subjected to the same changes in lake level their channel forms have evolved very differently differences in discharge patterns are likely the most influential factor causing the meandering form of lee creek and the braiding channel of the goggin drain despite the differences in discharge total sediment eroded from the two streams is comparable and can be attributed to similar stream power unit stream width in the two streams although lee creek has not recently been avulsive three major avulsions of the goggin drain have taken place since 1965 two possible styles of avulsion are interpreted an allogenic response to changing base level and an autogenic response dictated by channel morphology and hydraulics despite a wealth of available information avulsions cannot be unequivocally attributed to one style or another caution should be used when attempting to link the complex process of avulsion to causal mechanisms copyright 2011 john wiley sons ltd

characterization and formation of polygonal fractures on venus 1 fracture theory predicts that polygonal cracks will form in the presence of an isotropic extensional stress field on venus polygonal fractures are observed on scales several orders of magnitude larger than on earth with an average diameter of 1 8 0 9 km proposed formation mechanisms include cooling following lava flow emplacement lithospheric heating and climate change here we examine the characteristics and geologic setting of 204 regions of polygons some regions display two spatially overlapping size ranges with the larger spacing typically 10 25 km most polygonal fractures appear to be extensional but some have the morphology of compressional ridges polygons are confined to plains regions and occur in association with shield fields 49 coronae and coronae like features 21 3 tessera 17 5 and wrinkle ridges 20 in locations where polygons occur with shield fields coronae or both they appear to have formed contemporaneously formation in conjunction with local heating events is consistent with the lithospheric cooling hypothesis however there is almost never the predicted decrease in size away from the center of coronae or shield fields only a small percentage of coronae and shield fields contain polygons indicating that they are not typical of the formation process the climate change induced scenario is consistent with many characteristics of the polygons including the small and large size ranges the compressional ridges and their occurrence with and without evidence of local heating although polygons may have diverse origins including formation by multiple deformation events overall polygon characteristics support the climate change hypothesis

the table rocks sandstone a fluvial friction dominated lobate mouth bar sandbody in the westphalian b coal measures ne england abstract westphalian b duckmantian alluvial coal measures along the northumberland coast ne england comprise coal capped coarsening upward crevasse splay sequences of shale siltstone and sandstone interbedded with a number of major distributary channel sandbodies including the table rocks sandstone lithofacies architectural analysis and outcrop geometries divide the table rocks sandstone into flaggy sandstone massive sandstone heterolithic and mudstone facies associations each comprising up to 7 lithofacies types the three sandy facies associations are characterised by lenticular bed geometries on different scales producing a hierarchy of lensoid packages and associated bounding surfaces all showing typical offset stacking patterns 1 lenses represent individual lenticular cross bed sets bounded by 1st order surfaces 2 packages of lenses called lens clusters are bounded by 2nd order surfaces and are the basic architectural building block of the sandy facies associations and 3 vertically stacked lens clusters called amalgamated lens clusters bounded by 3rd order surfaces the table rocks sandbody has a laterally extensive irregular lobate subsurface plan geometry it displays a radial palaeocurrent pattern with 180 dispersion and it forms part of a 14 m thick coarsening upward regressive sequence it is interpreted as a composite lobate crevasse splay delta system that prograded into a shallow interdistributary fresh to brackish water lake up to 14 m deep the shallow lake water fluvial input and extensive development of traction structures such as cross bedding and ripple cross lamination suggests a friction dominated delta in which the four facies associations can be interpreted in terms of discrete elements of the mouth bar environment the flaggy sandstone facies association represents the main axial part of the mouth bar system the erosively based massive sandstone facies association major subaqeous distributary channels the lithologically more variable heterolithic facies association the medial mouth bar and the mudstone facies association the distal mouth bar fringe and prodelta within this environmental setting amalgamated lens clusters are interpreted as small discrete mouth bar sand lobes whose offset imbricate stacking pattern reflects channel spacing and bifurcation the rate of channel shifting or shallow depths and lack of accommodation space thus lens clusters are interpreted as discrete growth elements of the mouth bar sand lobes and lenses as individual bedforms making up these growth elements because of the high rate of channel shifting lack of extensive erosion of the mouth bar lobes and deposition of low discharge fines the lobes retained much of their original depositional geometry thereby providing advantageous gradients for offset deposition and stacking of adjacent sand lobes although the delta complex was maintained by frequent crevassing from the feeder channel and by subsidence due to contemporaneous compaction and or local tectonism it was deeply incised on two occasions by subaqeous channels in response to high magnitude floods or falling lake level

sequence analysis of early saalian glacial lake deposits nw germany evidence of local ice margin retreat and associated calving processes abstract this paper presents a detailed analysis of high resolution facies architecture of glaciolacustrine ice margin deposits which formed at the southern margin of the scandinavian ice shield the ice margin depositional systems are characterised by coarse grained deltas and subaqueous fans which are formed by stacked transgressive regressive sequences recording an overall lake transgression interrupted by minor short term lake level falls the delta complexes at the northern margin of glacial lake rinteln are thought to reflect a relatively stable position of the ice margin in front of the weser chains mountain ridges the onset of delta progradation probably represents a halt in ice advance and a related high sediment supply period the sedimentary facies and depositional architecture resemble those of nonglacial gilbert type deltas except for the deposition of glacial debris the main delta progradation is recorded from the highstand systems tract when high meltwater and sediment discharge occurred during the melt season a forced regression during opening of outlets led to the formation of subaerially exposed sequence boundaries and the erosion of the highstand systems tract deeply incised channels incised valleys were filled during the subsequent transgression at the eastern lake margin subaqueous fan deposits reflect an unstable ice front that was rapidly retreating and subject to periodic calving which resulted in the generation of floating icebergs dumping ice rafted debris and ploughing into subaqueous fan deposits the rapid retreat of this eastern ice margin is interpreted to result from the overall lake level rise the fan stacking pattern and bulk geometry were determined by the ice front fluctuations the shifting of meltwater outlets and the short term lake level fluctuations depositional processes reflect the character of sediment supply and distance from the ice margin ice proximal upper fan deposits are characterised by coarse grained gravel deposited by debris flows that document continuous discharge slump and slide deposits are related to steep depositional slopes the mid fan contains basinward fining and thinning deposits of quasi steady and surge type high and low density turbidity currents indicating more ice distal and periodic deposition the outer fan deposits mainly consist of surge type low density turbidites and glacial debris dumped by icebergs the forcing parameters governing the development of depositional sequences in both delta and fan settings were lake level fluctuations sediment yield rates and physiography the depositional sequences were deposited on a time scale of seventh and eighth order high frequency cycles 101 102 years in this short time span lake level fluctuations were on the order of 120 m therefore accommodation space was largely controlled by lake level and subsidence can be ignored

interplay between tectonics and compaction in a rift margin lacustrine delta system miocene of the eger graben czech republic the early miocene bilina palaeodelta consists of fluvio deltaic and lacustrine clastics deposited along the south eastern margin of the extensional most basin part of the eger graben in north bohemia czech republic the bilina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault exposures up to ca 4 5 km long in the bilina open cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small scale and basin scale stratal geometries to be evaluated the bilina palaeodelta deposits include sand dominated fluvial channel fills and heterolithic sheets interpreted as delta plain strata sand dominated mouth bar wedges and heterolithic sheets of prodeltaic deposits passing distally into lacustrine clays the depositional environment is interpreted as a fluvial dominated mixed load lacustrine delta with a high degree of grain size segregation at the feeder channel mouths on the largest temporal and spatial scales variable tectonic subsidence controlled the overall advance and retreat of the delta system the medium term transgressive regressive history was probably driven by episodes of increased subsidence rate however at this temporal scale the architecture of the deltaic sequences deltaic lobes and correlative lacustrine deposits was strongly affected by i compaction of underlying peat and clay which drove lateral offset stacking of medium term sequences and ii growth of a fault propagation fold close to the active bilina fault at the smallest scale the geometries of individual mouth bars and groups of mouth bars short term sequences reflect the interaction among sediment loading compaction and growth faulting that produced high frequency relative lake level fluctuations and created local accommodation at the delta front

duration of tessera deformation on venus the density and distribution of impact craters superposed on the highly deformed tessera terrain on venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains eighty percent of tesserae craters are undeformed no existing craters exhibit evidence of contractional deformation suggesting that the early compressional stage of tessera deformation ended abruptly the small number of craters fractured by late stage tessera extension constrains the duration of this phase to less than 20 of the average crater retention age of the tesserae or approximately 30 60 ma these results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains

dispersion of elastic moduli in a porous cracked rock theoretical predictions for squirt flow abstract crustal rocks contain variable amount of both cracks and equant pores depending on tectonic and thermal stresses but also on their geological origin crack damage and porosity change result in effects on elastic waves velocities when rocks are fluid saturated dispersion of the p and s waves should be taken into account this paper deals with frequency dispersion of elastic moduli in a fluid saturated porous and cracked rock with the assumption that squirt flow is the dominant process we develop a theoretical approach to calculate both high hf and low lf frequency bulk and shear moduli the hf moduli are derived from a new effective medium model called cpem with an isotropic distribution of pores or cracks with idealized geometry respectively spheres and ellipsoids lf moduli are obtained by taking hf dry moduli from the cpem and substituting into gassmann s equations in the case of a porosity only supported by equant pores the calculated dispersion in elastic moduli is equal to zero in the case of a crack porosity no bulk dispersion is predicted but a shear dispersion appears finally in the general case of a mixed porosity pores and cracks dispersion in bulk and in shear is predicted our results show that the maximum dispersion is predicted for a mixture of pores and spheroidal cracks with a very small aspect ratio 10 3 our theoretical predictions are compared to experimental data obtained during hydrostatic experiment performed on a basaltic rock and a good agreement is observed we also used our theoretical model to predict elastic waves velocities and vp vs ratio dispersion we show that the p waves dispersion can reach almost 20 and the vp vs dispersion a maximum value of 9 for a crack porosity of about 1 since laboratory data are ultrasonic measurements and field data are obtained at much lower frequencies these results are useful for geophysicists to interpret seismic data in terms of fluid and rock interactions