high resolution sequence stratigraphy of lower paleozoic sheet sandstones in central north america the role of special conditions of cratonic interiors in development of stratal architecture well known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental scale tectonism oceanographic dynamics of epeiric seas and eustasy we used a multi disciplinary approach to construct a high resolution stratigraphic framework for lower paleozoic strata in the cratonic interior of north america within this framework these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins settings markedly different from the cratonic interior parasequences parasequence stacking patterns systems tracts maximum flooding intervals and sequence bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification the similarity of cratonic interior and foreland basin successions in size geometry constituent facies and local stacking patterns of nearshore parasequences is especially striking this similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same despite marked differences in tectonism physiography and bathymetry between the two settings our results do not support the widespread perception that paleozoic cratonic interior successions are so anomalous in stratal geometries and constitute such a poor record of time that they are poorly suited for modern sequence stratigraphic analyses the particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another thus paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography special conditions of exceptionally slow subsidence rate shallow bathymetry and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings these results suggest that if other cratonic interior successions are as anomalous as reported a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture the results also lead us to suggest that a nonvegetated lower paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones particularly their widespread yet thin geometry and a scarcity of shale and siltstone

modelling facies belt distribution in fan deltas coupling sequence stratigraphy and geostatistics the eocene sant llorenc del munt example ebro foreland basin ne spain abstract the eocene sant llorenc del munt fan delta complex developed in the ebro foreland basin the stratigraphy of this succession about 1000 m thick has been described as made up of several transgressive regressive composite sequences each composed of a stack of fundamental sequences in turn made up of a transgressive systems tract and an overlying regressive systems tract systems tracts are composed of several facies belts proximal alluvial fan distal alluvial fan fan delta front carbonate platform and fan delta slope and prodelta a 3d facies model of a 130 m thick stratigraphic succession was built using outcrop data from one of the composite sequences the modelling followed a hierarchical approach a deterministic method was used to reconstruct the surfaces bounding fundamental sequences and their constituent systems tracts data input was outcrop traces digitized on photorealistic terrain models the stacking trend and interfingering of adjacent facies belts within each systems tract was added by means of a geostatistical algorithm and was conditioned to measured stratigraphic logs the analysis of the resultant 3d facies distributions allows revising the architectural interpretation of the studied interval leading to the conclusion that the observed arrangements are accommodation not supply driven furthermore the 3d models were flow simulated in two injector producer scenarios up depositional and down depositional dip waterflooding flow simulation results show that the depositional architecture of the studied succession controls potential recovery efficiencies in that a considerable proportion of moveable oil is trapped at the dead ends in depositional pinch outs of the main potential reservoir fan delta front deposits also that up dip waterflooding predicts better recovery efficiencies than down dip the dataset methods and results herein make the sant llorenc del munt complex a ready for application analogue for marine transitional reservoirs

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

rupture characterization of a low magnitude earthquake of central apennines italy abstract this paper describes a detailed study of the rupture mechanism of a m l 3 aftershock of the 1984 abruzzo central italy earthquake m s 5 8 to study the rupture extension shape and directivity the time durations of p and s far field pulses were analyzed in different directions corresponding to six recording stations the path corrected far field displacement p and s pulses at six seismic stations were determined using the empirical green function method the fault plane solution has been estimated by joint inversion of p polarities and s polarizations measured on three component records finally we used a method based on the plot of isochrons associated with the observed pulse widths and projected on each nodal plane the comparison of isochron plots for the two nodal planes suggests a rupture fault plane striking along the apenninic direction a weak source directivity up and west ward on the fault plane was observed a maximum ruptured surface of 0 4 km 2 was estimated assuming a sub shear rupture process the estimate of the seismic moment from spectral measurements gives the static stress drop ranging from 1 to 66 bar for the rupture velocity ranging between 0 2 and where is the shear wave velocity at the source

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

evidence for active subduction below western panama widespread arc tholeiitic and calc alkaline volcanism characterized the geologic evolution of western panama and southeastern costa rica in middle to late miocene and quaternary time volcanism was and is clearly associated with subduction of nazca lithosphere geochemical data suggest that formation of mantle wedge derived melts and their differentiated products predominated during miocene time whereas slab derived melts were generated in the quaternary this difference in magma chemistry is attributed to a change in the thermal regime of the nazca lithosphere which may be related to emplacement of the galapagos plume

wave ravinement surfaces classification and key characteristics abstract the wave ravinement surface wrs is a surface of sequence stratigraphic significance that forms in shallow water settings during transgression it develops progressively landward during shoreface retreat due to wave action which erodes the substrate and leads to sediment bypass in the offshore direction the wrs may rework previously formed surfaces such as the maximum regressive surface also known as the transgressive surface and the subaerial unconformity becoming a systems tract and or sequence boundary due to the associated lithological contrast between underlying and overlying deposits and the presence of lags and or shell pavements the wrs has a very good appearance in both outcrop and cores and is often used as the boundary of shallow marine stratigraphic sequences the wrs therefore represents an important lithological and paleo environmental break and its recognition is significant for reconstructing stratigraphic architectures and the history of relative sea level change as well as for reservoir characterization several controlling factors govern the development of the wrs and of the associated deposits they include the accommodation to sedimentation a s ratio environmental energy topographic gradient roughness and lithology of the substrate relatively low rates of relative sea level rise and sediment supply high wave energy steep topographies and easily erodible substrates favor the development of the wrs these factors interact leading to a high variability in the amount of erosion shape length along depositional dip and deposits associated with these surfaces wrss may be classified based on either underlying or overlying deposits or a combination of both a classification based on the deposits that underlie the wrs helps to determine the location of the studied outcrop along depositional dip whereas a classification based on the deposits that overlie the wrs afford insights into the physical conditions that accompanied its formation however the best description of the wrs in terms of location of a considered outcrop physical conditions under which the wrs developed and their lateral variability type of substrate as well as its practical significance for reservoir modeling is achieved with a combination of the two criteria

high resolution sequence stratigraphy of clastic shelves vii 3d variability of stacking patterns abstract in the geological record high frequency sequences at the scale of fourth order and lower rank stratigraphic frameworks are usually stacked to compose higher rank lower frequency systems tracts i e sequence sets along dip oriented transects higher rank transgressive normal regressive highstand and or lowstand and falling stage systems tracts are defined by retrogradational upstepping progradational and downstepping progradational stacking patterns of component high frequency sequences respectively however these stacking patterns may vary markedly along depositional strike due to lateral changes in the rates of accommodation and sedimentation resulting in the coeval deposition of different types of higher rank systems tracts in this case surfaces bounding such systems tracts are diachronous along depositional strike moreover if the high frequency sequences that compose the lower frequency systems tracts are laterally continuous the higher rank bounding surfaces are discontinuous along depositional strike consisting of a series of stepped surfaces that can be observed in high resolution studies these aspects have important implications for petroleum exploration and need to be considered in high resolution sequence stratigraphic analysis

architecture and trace fossil characteristics of a 10 000 20 000 year fluvial to marine sequence se ebro basin spain abstract the eocene sant llorenc del munt fan delta complex lies on the southeastern edge of the ebro basin in ne spain alluvial fans and fan deltas prograded northwestwards from the catalan coastal range to form a series of clastic wedges that were interrupted by marine transgressions of period varying from 60 000 to a few hundred years resulting in a highly cyclic succession the cyclicity of the succession was controlled by a combination of syndepositional basin margin tectonics high sediment supply and eustatic sea level changes analysis of one of the component 25 40 m thick transgressive to regressive sequences 10 000 20 000 year duration shows that within its transgressive part the alluvial and coastal plain deposits are extensive and thickly developed whereas shoreline lithosomes are narrow and thin in the overlying regressive part the opposite relationship is documented the transgressive and regressive tracts thus have an overall landward expanding and seaward expanding geometry respectively six ichnofabrics have been documented in the succession ranging from nonmarine to offshore transition in origin these ichnofabrics are used to help define sequence boundaries flooding surfaces and maximum flooding surfaces as well as to increase the resolution of the stratigraphic analysis within the transgressive tract of the main sequence nonmarine deposits are separated from marine deposits by a steep landward inclined complex of wave ravinement surfaces successive ravinement surfaces have a vertical spacing of about a meter are offset landwards from each other by less than 100 m and diverge slightly from each other as they climb through the section these same erosion surfaces merge with each other basinwards causing the transgressive tract to thin significantly five high frequency sequences are recognized within the main sequence the middle one is nearly symmetrical the lower two are asymmetric and dominated by their transgressive tract and the upper two are asymmetric but dominated by their regressive tract

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

pn velocity anisotropy in southern california we analyze p n propagation as a function of azimuth across a 28 station 150 km aperture subarray of the scarlet network centered near the central transverse ranges california we selected signals from 81 earthquakes and explosions with epicentral distances ranging from 150 to 400 km covering all azimuths except a 40 gap from the southwest and a lesser gap from the northeast direction for each source the apparent velocity of p n was determined using a one norm measure of misfit the apparent p n velocity does not show any systematic variation with epicentral distance but exhibits a strong azimuthal dependence our preferred interpretation calls for a slightly dipping 2 to n40w planar moho with 3 to 4 per cent anisotropy of subcrustal material transverse isotropy with a nearly horizontal symmetry axis is sufficient to explain the data the direction of sagittal symmetry is n50w the isotropic velocity of p n is 7 8 km sec in contrast a higher 8 1 km sec p n velocity is found in the mojave block with no indication of anisotropy these observations are consistent with a subcrustal model of the pacific north america plate boundary where ductile flow is characterized by simple shear in a vertical plane with strike parallel to the direction of relative plate motion

effects of the free surface on shear wavetrains summary the behaviour of shear waves is of great importance in identifying and investigating seismic anisotropy in the earth however shear wavetrains recorded at the earth s surface do not always reflect the motion at depth introducing practical problems of interpretation shear wavetrains incident on the surface of an isotropic half space at angles less than critical about 35 are broadly preserved but at greater angles substantial distortions can occur for stations situated close to the source as in local earthquake studies the local sp phase a radially polarized precursor to s may occur the behaviour at the surface of an anisotropic half space is further complicated by the divergence of phase and energy propagation vectors all of these complications suggest that detailed seismogram modelling is essential to any study of shear wave propagation in the earth and in particular to investigations of anisotropy induced shear wave splitting

analysis of bioerosion in clasts from a miocene rocky shore concepcion formation veracruz mexico abstract study of bioerosion traces in the geologic record of rocky shores is can establish the location of ancient shorelines and aid understanding of the evolution of marine communities in marginal environments as an example the ichnologic record associated with cobbles of the conglomerate that crops out along the east side of neovolcanic ridge near plan del rio village in veracruz state south central mexico is described and explained the conglomerate corresponds to the upper part of the concepcion formation deposited during the miocene the bivalves and gastropods included in the sandy matrix and the presence of oysters and bryozoans on the surfaces of clasts allowed interpretation of this conglomerate as an ancient rocky shore the present work deals with bioerosion recorded in the clasts six ichnogenera and ten ichnospecies were determined corresponding to the activity of polychaetes and or sipunculids caulostrepsis taeniola maeandropolydora sulcans and trypanites weisei echinoids circolites isp clionaid sponges entobia cateniformis e geometrica and e ovula and boring bivalves gastrochaenolites cluniformis g lapidicus and g torpedo the relative abundance of boring organisms allows these biota to be placed within the entobia ichnofacies which is characteristic of hard substrates with deep bioerosion the abundance and diversity of gastrochaenolites and entobia in the ichnoassociations studied indicate a shallow marine environment with a low sedimentation rate including episodes of removal or rolling of clasts this process is inferred from clast roundness and the presence of bioerosion traces on most clast surfaces on the other hand the presence of complete and eroded gastrochaenolites borings in the same clast shows different episodes of bioerosion results of this work allow for an understanding of the genetic history of the conglomerate confirming its deposition in a rocky shore palaeoenvironment and indicating the location of an ancient shoreline on the west margin of the gulf of mexico during the miocene

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

modeling of three component lg waves in anisotropic crustal models abstract lg wave synthetic seismograms are presented in order to analyze the influence of anisotropy in the continental crust on their three component waveforms the effect of dry and liquid filled open cracks in the upper crust and petrological anisotropy in the lower crust on waves produced by surface explosions and crustal earthquakes is examined the polarization and especially the amplitude of the transverse component proves to be very sensitive to introduction of weak anisotropy in the crust comparisons of the modeling with lg wave data produced by explosions in southern scandinavia show that a realistic crustal anisotropy can explain observations that the transverse components of these waves are as large as their vertical ones a 10 km thick surficial layer with liquid filled open cracks is our perferred model for southern scandinavia

low accommodation detrital apron alongside a basement uplift pennsylvanian of midcontinent north america abstract the northern end of the 650 km long nemaha uplift nebraska and kansas usa is an important example of basin margin sedimentation in the north american midcontinent an apron of coarse basal pennsylvanian arkosic clastic sediments bpc was deposited on the flanks of the uplift while marine cyclothems were encroaching from the east small scale fining upward intervals many with demonstrably erosional bases dominate the bpc and are interpreted as overridingly fluvial in origin weak paleosols desiccation cracks and reddened intervals in the bpc record episodic subaerial exposure multiple burrowed horizons and heterolithic strata of probable tidal origin and rare marine fossils also indicate episodic marine influence the bpc appear to have been deposited as a thin apron of coalesced alluvial fans and fan deltas deposition of the bpc occurred during the waning of uplift and subsequent quiescence the comparative thinness and large scale packaging of the bpc are compatible with the controlling effects of relict relief regional subsidence and eustasy rather than ongoing major vertical displacements along active faults a strong autocyclic influence on sedimentation is evidenced by stacked fining upward intervals of poorly sorted conglomerates sandstones and sandy mudstones correlations demonstrate that the accumulation of the bpc took place over more than seven major sea level cycles beginning in cherokee group times middle moscovian middle pennsylvanian and ending only when the eroded uplift was inundated and buried by marine cyclothems on the basis of local correlations with marine cyclothems and using black phosphatic shales so called core shales of heckel p h 1986 sea level surve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america geology 14 330 334 heckel p h 1994 evaluation of evidence for glacio eustatic control over marine pennsylvanian cyclothems in north america and correlation of possible tectonic effects in dennison j m ettensohn f r eds tectonic and eustatic controls on sedimentary cycles sepm concepts in sedimentology and paleontology no 4 pp 5 87 as marker beds we speculate that the bpc exist in backstepping sequences and or parasequences alongside the flanks of the nemaha uplift the bpc are lithologically comparable to contemporaneous deposits alongside the ancestral rockies amarillo wichita uplift and other buried basement highs in north america nonetheless the bpc are an order of magnitude thinner are dominantly retrogradational rather than progradational and their occurrence was not associated with major displacements along basin bounding faults in this manner the bpc are a useful example of low accommodation eustasy dominated coarse grained terrigenous clastic deposition around an uplifted basement block

anatomy of regressive tracts in a regressive sequence set vilomara unit sant llorenc del munt ebro basin ne spain abstract the sant llorenc del munt fan delta succession forms a clastic wedge ca 3 my duration that built out into the ne ebro basin in response to transpression and uplift along the paleogene catalan coastal range this fan delta succession is arranged in a series of transgressive regressive sequences of different scales a large scale composite megasequence the milany megasequence includes the whole fan delta succession this megasequence is subdivided in seven composite sequences that in turn can be subdivided into fundamental sequences the vilomara composite sequence the vilomara unit is located in the regressive part of the milany megasequence the regressive part of the vilomara composite sequence consisting of a seaward stepping set of fundamental sequences the vilomara unit has been studied to understand the facies relations geometries and the characteristics of key surfaces in progradational settings the vilomara regressive sequence set is formed by stacked alluvial fan fan delta plain proximal and distal fan delta front fan delta slope within the regressive part of fundamental sequences and carbonate platform usually within the transgressive part of fundamental sequences deposits the vilomara regressive unit shows a sigmoidal geometry in contrast to the underlying el marcet transgressive unit which thickens landwards this sigmoidal geometry for the regressive unit and landward thickening for the transgressive unit is common for all scales of sequence composite megasequences composite sequences and fundamental sequences there are evident facies arrangement differences between transgressive and regressive deposits within a fundamental sequence key surfaces have been recognised from vertical trend changes from transgressive to regressive and vice versa the facies successions immediately above and below the surfaces bounding fundamental sequences may indicate at different stratigraphic levels a basinward or a landward shift of facies belts

evidence of shear wave anisotropy in the upper crust of central italy abstract shear wave polarization analysis has been performed on data from twelve earthquakes recorded in central italy these data are part of a set of high quality seismograms recorded by a three component digital network installed in abruzzo region after a m s 5 8 earthquake on 7 may 1984 analysis performed on 2 to 6 hz bandpass filtered seismograms reveals shear wave splitting in order to determine the direction for which the time separation between the two split s waves is maximum the horizontal traces are rotated in the range 10 to 90 with steps of 10 at each step the cross correlation between the two shear waves and the time separation is estimated the azimuth of 50 n yields the highest correlation coefficient and the maximum time separation of about 0 09 sec this direction and the orthogonal to it represent axes of minimum and maximum shear wave velocity respectively if the time delay is distributed over the whole hypocenter seismic station path the maximum variation in velocity is at least 1 4 per cent the theoretical linear polarization of particle motion at the source is verified after correcting the seismograms for the anisotropic propagation effect a distribution of vertical cracks aligned in the direction 140 n may explain the observed anisotropy the stress field deduced from the focal mechanism of the abruzzo earthquake is compatible with this hypothetical crack distribution

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

stress directions and shear wave anisotropy observations from local earthquakes in southeastern sicily italy the spatial distribution of 414 earthquakes 1 0 m l 4 6 recorded from 1994 to 2002 in southeastern sicily italy has been analyzed the seismicity generally coincides with mapped plio quaternary faults including the north north west south southeast striking offshore fault system which is the most important tectonic structure of the area for the best located events we computed 70 focal mechanisms by combining p wave polarities with s wave polarizations a predom inance of strike slip and normal faults was observed focal mechanisms were then inverted for stress tensor parameters by using the algorithm of gephart and forsyth the results highlighted a region governed mainly by a north northwest south south east to northwest southeast compressional stress regime moreover anisotropy anal ysis of shear waves showed a polarization of fast s waves coherently aligned with this stress direction a finer scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis r1 the eastern sector displays a nearly horizontal r1 trending northwest southeast the central sector is affected by a low dip north northwest south southeast r1 whereas in the western sector a north northwest south southeast oriented r1 with a higher dip angle was detected finally the comparison of the spatial distribution of seismicity occurring during 1994 2002 with locations of previous instrumental earthquakes and larger m 5 0 historical events showed that the seismicity patterns are persistent

unusual features of sediment supply dominated transgressive regressive sequences paleogene clastic wedges se pyrenean foreland basin spain abstract the paleogene marine to nonmarine succession in the southeastern pyrenean foreland basin consists of a series of 3rd order 3 5 m y transgressive to regressive sequences that can be related directly to the stacking and southward displacement of successive basement cover thrust sheets the regressive limbs of the 3rd order clastic wedges are coeval with thrust movement with maximum subsidence developing towards peak regression this relationship between sedimentation and tectonics is sediment supply driven one of these 3rd order wedges which was produced by the montserrat and sant llorenc del munt fan delta systems demonstrates how such steep sloped coarse grained high sediment supply systems generate a hierarchy of transgressive regressive architectures striking features of this rapidly subsiding supply dominated basin margin in addition to the tectonics wedge architecture relationship include the nonperiodic character of the contained high frequency sequences the unusual thickness and landward thickening geometry of transgressive systems tracts a tendency for misfit between alluvial fan and shorezone sectors of sequences a lack of marked incision associated with the tops of coarse grained regressive phases of sequences most of the features are symptomatic of high subsidence rates and very high sediment supply the last feature suggestive of few major relative falls of sea level is also consistent with continuous rapid subsidence however it should be remembered that lack of incision may be a poor indicator for lack of base level fall in a setting where the steepness of alluvial slopes exceeds the slope of the coeval shelf

style contrast between forced regressive and lowstand transgressive wedges in the campanian of south central wyoming hatfield member of the haystack mountains formation

analysis of strong motion s wave polarization of the 15 october 1979 imperial valley earthquake abstract this article illustrates the use of near source s wave polarization analyses for constraining some rupture parameters of large earthquakes in the particular case of the 1979 imperial valley mainshock the ten sma1 strong motion records closest to the source have been studied in the 1 to 3 hz frequency range the records were first corrected following the analysis of zollo and bernard 1989 from some anisotropy that produced strong s splitting at a few sites the rotation of the s polarization with time at most of the sites was then interpreted in terms of the northwestward propagation of the rupture we assumed a constant focal mechanism on the fault plane vertical strike slip the polarization data and the absolute times at some stations allowed the identification of a set of subsources their location in space and time assuming a depth of 8 km provided a mean rupture velocity of 3 1 km sec 0 25 km sec i e 90 of the shear velocity the high frequency radiation 3 hz during most of the rupture thus is very likely to have been generated about 1 sec before the beginning of the low frequency radiation 0 5 to 1 hz studied by archuleta 1984 the final rupture model fits the s polarization history well at most sites it also constrains the anisotropy characteristics of the sediments under some sites in particular em0 where it is consistent with the alignment of vertical cracks in the ns stress field applying such a method to other earthquakes requires a careful selection of the distance and frequency range in particular the effect of perhaps pervasive ubiquitous anisotropy in the upper crust must be taken into account which may require the analysis of aftershock records at the same sites

nearshore and alluvial facies in the sant llorenc del munt depositional system recognition and development abstract the eocene sant llorenc del munt fan delta complex forms a part of the coarse grained foreland basin fill of the southeastern margin of the ebro basin it was sourced from the rising catalan coastal range in the se and shows an overall northwesterly progradation during a time period of approximately six million years coarse clastic deposits coarse sands and gravels dominated this fan delta whereas finer grained sands silts and muds were deposited laterally off active sedimentation sites or in the more marine area depositional processes were clearly influenced by the high sediment input giving a dominance of sediment gravity flow deposits the sediments studied have been grouped into four distinct facies belts including coastal plain proximal fan delta front distal fan delta front and fan delta slope sediments respectively the facies belts combine to form the nearshore and coastal alluvial reaches of the fan delta system developed during a transgressive phase lasting some 50 000 years

evidence of shear wave splitting in the eastern corinthian gulf greece abstract the analysis of local earthquakes recorded by the cornet network and located in the gulf of corinth greece has revealed the existence of shear wave splitting the visual inspection technique is used to estimate the polarization direction of the fast shear wave and the time delay between the two split shear waves the selected earthquakes are located close to one of the cornet stations most of these earthquakes are recorded by only one station and their azimuth and angle of incidence are estimated using the covariance matrix decomposition method combined with the p wave polarization direction polarigrams plotted for the horizontal components present a clear linear and almost constant polarization for each station independent of the azimuth of the earthquake except for one station where different sfast polarization directions are observed the mean direction of the fast shear wave polarization at paradeisi station is 146 n at sofiko station 104 n and at villia station 142 n at desfina station two different main sfast polarization directions were observed one 143 n and the other 55 n the calculated time delays between the two split shear waves are higher for the stations located in the eastern part of the gulf comparing the mean sfast polarization direction with the direction of local faulting we observe that they are approximately parallel at sofiko station while at villia station they are almost perpendicular in general the obtained mean sfast polarization directions at the cornet stations are perpendicular to the direction of the extension of the gulf which is nne ssw and consistent with the extensive dilatancy anisotropy eda model

a review of wave dominated clastic shoreline deposits abstract barrier islands beaches wave dominated deltas and chenier plains probably form more than one fifth of present coasts and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record the geometry vertical sequences and composition of their deposits are the result of 1 shoreline behaviour whether transgressive regressive or stable 2 physical processes particularly the relative intensities of waves and tides 3 climate 4 global tectonic setting and 5 topography of underlying deposits published interpretations indicate that equivalent deposits are abundant in the geological record sandstones are mostly quartzose cemented by carbonates and quartz and have sheet or linear geometries ancient deposits can provide evidence of palaeogeography and some are economically significant particularly as hydrocarbon reservoir rocks four end member sedimentation models are proposed for the interpretation of ancient deposits 1 transgressive sheet sands associated with deltas 2 non deltaic transgressive sheet sands 3 regressive sheet sands and 4 linear sand bodies probable depositional settings geometries characteristics of deposits and palaeocurrent patterns are discussed

sequence stratigraphy and architectural variability in growth fault bounded basin fills a review of plio pleistocene stratal units of the crotone basin southern italy field examples from the pliocene to pleistocene succession of the crotone basin document the variability of stratal architecture in syntectonic units deposited in normal fault bounded basins relatively thick trangressive intervals are common within these successions as shown in the zinga 2 zinga 3 and strongoli stratal units transgressive packages are formed by lagoonal mudstones that are abruptly overlain by shoreface sandstones aggradational highstand shallow marine packages are typical of the lower pliocene succession in half graben basins these deposits resulted from balanced conditions between the rate of sediment supply and the rate of creation of accommodation relatively thick forced regressive deposits are common within basins during phases where the accommodation supply ratio is low e g the middle pleistocene san mauro sandstone these deposits influenced by growth faulting and folding recorded base level lowering linked to glacio eustasy a complex stratal architecture may be the consequence of the alternation between tectonic subsidence and uplift e g the serra piani stratal unit such variations are thought to be related to the onset of an earliest mid pliocene transpressional tectonic phase the studied deposits therefore show a marked tectonic control that strongly influenced their stratal architecture these examples represent interesting cases of sequence stratigraphic analysis in growth faulted contexts

polarities of p and s waves and shear wave splitting observed from the bucaramanga nest colombia the bucaramanga nest colombia is an intense seismic source centered at 6 48 n 73 10 w and 161 km depth beneath the eastern andes during a 3 week period 161 microearthquakes mb 4 3 were recorded by a 260 by 140 km array of 12 three component university of wisconsin uw seismographs centered above the nest eighty eight percent of the events recorded were from the nest providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths shear wave splitting analysis shows that the average polarization direction of the first arriving shear waves from the nest events at nine of the seismographs is n12 e 18 after corrections based on the amount of discrepancy between the observed average p wave azimuth and the expected event to station azimuth average s wave polarization is n6 e 15 variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system shear wave splitting observed from 17 events along the wadati benioff zone outside the nest shows that the time delays increase with increasing depth and defines a wedge shaped anisotropic volume above the subducting plate the anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip the anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the wadati benioff zone a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab

how does an asperity break new elements from the waveform inversion of accelerograms for the 2319 ut october 15 1979 imperial valley aftershock the 16 accelerograms obtained at less than 30 km during the 2319 ut october 15 1979 imperial valley aftershock were analyzed after having been corrected for the crack induced anisotropy in the shallow sediments both p and s waveforms are used to study the earthquake source process and to infer an estimate of attenuation and site amplification effects on the waveforms at frequencies 0 6 1 hz and 2 4 hz the comparison between the 0 6 1 hz p and s wave seismic moment measures and their dependence on distance has provided an estimate for this parameter 1 9 40 1023 dyn cm and the bound limits for attenuation qs 0 6 1hz 20 25 and near site amplification of shear waves at low frequency the inversions of the 2 4 hz s velocity waveforms for different values of rupture velocities and attenuation parameters have been preliminarily performed to investigate the variability of static slip and static stress drop estimates a circular growth of the rupture at a constant velocity and a final elliptical shape are assumed the bipolar character of the velocity waveform excludes the asperity models consisting of a small highly stressed area inbedded in a large stress free crack therefore only the slip distribution of a quasidynamic crack with healing phases has been considered the dynamic stress release has been independently measured from the amplitude level of the first acceleration pulses for various rupture velocities and attenuation the constraint that the static stress drop does not significantly exceed the dynamic stress drop suggests relatively high qs values 200 at 2 4 hz and small rupture velocities ur 0 5 0 7 is the shear wave velocity the elliptical source is about 1 2 km long and 0 7 km wide 0 2 km elongated in the nw upward direction the nucleation point location indicates a dominant propagation toward the nw top fault direction for the inferred values of attenuation parameter and rupture velocity the minimum values of the dynamic and static stress drop are 300 700 bars and are associated with about l m of average slip on the fault the nucleation of the rupture is explained as an effect of the high loading stress on a larger size barrier left unbroken by the mainshock in which the aftershock fault area is embedded the hetereogeneous stress and rock strength distribution on the barrier region would therefore have controlled the rupture development and arrest an estimate of the s amplification in the considered frequency bands has been obtained through the seismic moment computed from the p wave records which are less sensitive to site amplification and attenuation effects for the sites under study at frequencies below 5 hz the results show a differential amplification of s waves 3 at 1 hz and 2 at 2 4 hz which is attributed to the propagation in the very shallow low velocity sediments of the imperial valley

positively correlated nd and sr isotope ratios of lavas from the central american volcanic front twenty four volcanic rocks from the central american arc have been analyzed for their sr and nd isotopic compositions on a nd sr isotope correlation diagram these lavas define an unusual positive trend that correlates strongly with crustal thickness nicaraguan lavas situated on the thinnest crust have morb like nd but elevated sr isotopes and provide strong evidence for incorporation of a subducted slab component lavas from costa rica and guatemala have lower values of both isotopes and are underlain by crust 30 thicker than that beneath nicaragua isotope correlations with trace elements support the idea that the central american volcanics are related by two component mixing of mantle above the subducted slab including a high sr sr fraction derived from altered oceanic crust and lower crust with isotopic values that lie along the mantle array the correlation of chemical parameters with crustal thickness can be explained if the lower crust acts as a density filter thus the least fractionated lavas nicaragua have the highest sr and nd isotopic ratios they have the highest mgo and lowest si02 and are the most isotopically similar to the melted mantle wedge if fractionation is coupled with assimilation and the extent of both is controlled by crustal thickness then lavas above thick crust will have isotopic values more similar to lower crust because they are the most fractionated

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

sclerobionts on biogenic substrates from the monte leon formation lower miocene in santa cruz province argentina taphonomic and paleoenvironmental considerations abstract study of hard substrate communities in a paleoenvironmental and taphonomic context contributes towards increasing knowledge on the depositional environments and on the history of the substrate colonized before final burial in this work we study traces produced by sclerobionts on biogenic substrates from two shell concentrations in the monte leon formation lower miocene exposed at cabeza de leon southern patagonia argentina the aims of this study are to describe the bioerosion traces and to infer the taphonomic history of the hard substrates mollusk shells based on ichnodiversity and sclerobiont distribution together with other preservational features fifteen ichnotaxa were identified at different taxonomic ranks one ichnofamily three ichnogenera and eleven ichnospecies traces produced by bryozoans were highly predominant followed by those produced by sponges foraminifera and polychaetes the biogenic substrates on which the sclerobionts settled were gastropod and bivalve shells showing different taphonomic signatures shells with high fragmentation and surface alteration are dominant suggesting that they were reworked by wave action before deposition and final burial on the other hand well preserved shells reflect less reworking and shorter transport distance the mixture of these elements could be the result of short term high energy events such as storms yet these taphonomic signatures were modified later by biologic factors mainly bryozoan bioerosion in a quiet and shallow environment with a low sedimentation rate and abundant nutrient availability bioerosion valuation in conjunction with other taphonomic attributes allowed identifying the concentrations as of mixed origin biogenic sedimentologic and including elements that underwent different taphonomic histories the results contribute to improve the knowledge about marine hard substrate communities in middle to high latitudes of the southern hemisphere in addition they highlight the importance of conducting studies on these communities in conjunction with other taphonomic data when elucidating the history of colonized substrates

late quaternary depositional sequences and landforms in relation to sea level changes in the osaka intra arc basin japan a borehole database analysis abstract development of late quaternary depositional sequences in the osaka intra arc basin was reconstructed based on the analysis of sequence stratigraphy and new shazam stratigraphy using numerous and high density borehole data the latest pleistocene to holocene sequence i and the lower late pleistocene sequence ii underwent several significant phases 1 formation of alluvial fans and terraces during regression from marine isotope stage mis 5 to mis 2 2 formation of a boundary between the two sequences during rapid sea level fall of ca 30 20 ka 3 development of ravinement surfaces by wave and tidal erosion during the transgression from mis 2 to mis 1 4 development of barrier systems and coastal cliffs during the transgression to the maximum high stand of 6 5 ka and 5 progradation of alluvial fans deltas and strand plains during the high stand since 5 ka depositional facies and systems of the two sequences are different among the western study area around kobe the central area with the nishinomiya amagasaki lowland and the eastern area with the osaka plain due to differences in sediment flux wave tide shore current basement and landforms distribution such as plateau and hills development of the sequence ii could be also affected by tectonic differences which fault fold structure and flexure three regions with different landforms and depositional systems were identified within the osaka basin the systems of the three regions correspond to the three major types of the latest pleistocene to holocene transgressive regressive depositional sequences and alluvial plains recognized in japan

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

extinct spreading on the cocos ridge using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with odp leg 170 age determinations we present a revised plate tectonic model for the southern cocos and northern nazca plate area according to this model the formation of the southern cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern in the cocos and malpelo ridge area we have identified two precursors of the recently active cocos nacza spreading system which were active from 22 8 to 14 7 ma with a change in spreading direction from nw se to ene wsw at 19 5 ma the oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the cocos and malpelo ridges the centre of this hotspot volcanism is about 500 km away from but most probably related to the galapagos hotspot

composite sequence stratigraphic patterns in alluvial to shallow marine successions examples from the piacenzian of the valdelsa basin central italy abstract the sequence stratigraphic analysis of continental and shallow marine successions is a helpful tool for investigating the cyclicity of the sedimentary record a similar approach is adopted in the present study to illustrate the composite cyclothemic facies architecture that characterizes the piacenzian strata of the southern valdelsa basin central italy specifically we integrated facies analysis paleopedology paleobiology of marine mollusk assemblages and biostratigraphy to assess the response of geomorphic sedimentary and biotic systems to piacenzian multifrequency relative sea level fluctuations this integrated approach resulted in the definition of a composite sequence stratigraphic framework that includes a hierarchy of units ranging from the elementary to the composite depositional sequences referred to the 5th to 3rd order cycles of sea level fluctuations respectively a correlation with the piacenzian oxygen isotopic signature of glacial and interglacial stages and related sea level fluctuations associates the cyclic development of fluvial coastal and shallow marine depositional systems to the mid piacenzian warm period and to the subsequent transition to the gelasian thus the glacio eustatic control prevailed on the development of the described multi scale cyclical pattern of sedimentation compared to the role played by the deformation of the northern apennines orogen

the co occurence of trypanites and glossifungites substrate controlled trace fossil assemblages in a triassic mixed siliciclastic carbonate depositional system baldonnel formation williston lake b c canada abstract detailed outcrop analysis of an upper triassic shallow marine mixed siliciclastic carbonate ramp succession in northwestern canada reveals numerous disconformities that are characterized by substrate controlled trace fossil assemblages although glossifungites assemblages and to a lesser extent trypanites assemblages are common in the upper carnian baldonnel formation in several horizons a pattern of paired trypanites glossifungites demarcated discontinuity surfaces occurs these paired surfaces occur in the same overall environmental setting and are interpreted to have the same general stratigraphic implications in each case the trypanites surfaces invariably occur lower in the section they penetrate into sandy bioclastic limestone and are infilled and overlain by a coarse grained detrital lag their development indicates early cementation in a coastal setting the trypanites surfaces form coplanar maximum regressive surfaces and transgressive surfaces of erosion and are correlatable on a regional scale the lags are overlain in turn by laminated calcareous siltstone denoting offshore deposition and bioclastic sandstone a return to shoreface depositional conditions the siltstone sandstone contacts are characterized by low diversity glossifungites assemblages which are interpreted as an amalgamated succession of storm induced wave ravinement surfaces emplaced in front of a rapidly prograding shoreface paired trypanites glossifungites surfaces in the baldonnel formation reflect fluctuations in relative sea level and sediment input in an overall low accommodation mixed siliciclastic carbonate ramp setting

bored pebbles and ravinement surface clusters in a transgressive systems tract sant llorenc del munt fan delta complex se ebro basin spain abstract a 25 m thick transgressive systems tract in the sant llorenc del munt wave influenced fan delta system eocene se ebro basin has an internal framework consisting of a cluster of transgressive erosion surfaces each of which has minor relief and which are collectively stacked vertically no more than two metres apart each erosion surface bounds a cycle containing a conglomeratic lag up to 0 5 m thick followed by a coarsening upward sandstone to conglomeratic unit the uppermost levels of which can be nonmarine individual cycles become entirely nonmarine landwards of the termination of the basal bounding erosion surface whereas they thin and eventually become entirely marine basinwards the individual erosion surfaces within the transgressive tract some 16 of them within a 20 m thick lithosome are interpreted as wave ravinement surfaces that repeatedly eroded into the conglomeratic shoreface during transgression this interpretation rather than one invoking nonmarine flooding or other marine erosion surface types is consistent with the arrangement of bivalve and sponge borings on the top surfaces of clasts and with the associated lag pavements multiphase boring around the entire surface of clasts as well as erosion of the clasts at some horizons particularly in reaches of the tract where the ravinement trajectory is sub horizontal suggest repeated reworking of previously generated lag pavements in zones of minimal aggradation during transgression where the transgressive shoreline trajectory rises more steeply and there has been more rapid aggradation during transgression the lag pavements show only single phase borings with the borings on the upper surface of the pebble pavement only the close spacing of erosion surfaces within the transgressive systems tract together with estimates of time span in the tract suggest that transgressive erosion occurred with a frequency of less than 500 years

s wave polarization inversion of the 15 october 1979 23 19 imperial valley aftershock evidence for anisotropy and a simple source mechanism s polarizations from the 16 near source strong motion stations of the 15 october 1979 23 19 imperial valley aftershock have been analyzed for constraining the source mechanism they show a significant delay of the east component relative to the north component at most stations assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments where the rays are dominantly vertical the measured fast s polarizations are mainly oriented between n0 e and n25 e and show a mean delay of 0 25 s between slow and fast s this is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it with a mean fluid filled crack density of 0 1 to 0 2 the records once corrected for the splitting effect show s polarizations stable in time which have been used to estimate the fault mechanism parameters this is done by applying a new inversion method which is based on a model probability estimate the error function is related to the polarization misfits and the associated joint probability function of the data and parameter is computed in the whole model space the inversion of the corrected s polarizations provides a unique and well constrained solution for the imperial valley aftershock mechanism strike 145 5 dip 85 10 slip 155 15 which is compatible with the available p polarities and is quite similar to the main shock mechanism

azimuth dependent amplification of weak and strong ground motions within a fault zone nocera umbra central italy 1 during three moderate magnitude earthquakes occurred in september october 1997 in the central apennines italy accelerations larger than 0 5 g were recorded in the town of nocera umbra 10 to 15 km n nw of the epicenters the accelerograph is sited in a fault zone close to a n30 e tectonic contact six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones the colfiorito sellano area s se of the array and the gualdo tadino area to the north the array data reveal large variations in terms of both peak ground motions and spectral amplitudes within the fault zone amplifications show a strong dependence on the source azimuth at the accelerograph site the effects are particularly large for events from s se peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 hz along the n30 e direction of motion parallel to the strike of the fault nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to m 0 1 2 x 10 25 dyn cm a particle motion analysis shows that the directional effect is also present in the strongest motions even though the amplification of peak ground motion decreases when m 0 increases results from stochastic simulations indicate that such a behavior is not due to nonlinearity applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily

a detailed analysis of microearthquakes in western crete from digital three component seismograms summary seismological data collected during the summer of 1988 are presented and analysed to discuss the microearthquake deformation in western crete greece the experiment consisted in a dense network of 34 stations 5 km interstation spacing among them 13 three component digital stations the seismicity is essentially localized between 0 and 15 km and suggests a deformation of crete decoupled from the subducting plate which shows a very low seismicity the fault plane solutions are constrained by combining the information from p wave polarities with that from horizontal directions of s wave polarizations using the probabilistic approach developed by zollo bernard 1991 among the 16 fault plane solutions calculated there are essentially two families normal faulting on planes oriented ns and strike slip faulting both have a horizontal se to ne oriented tension axis the mean direction of the t axis confirms the e w extension of this part of the hellenic arc where observations at larger scale are available active and holocene fault scarps and fault plane solution of one teleseismic event observations of elliptical polarizations of s waves at two stations suggest the presence of a shallow low velocity anisotropic layer assuming a model of vertical cracks this anisotropy would imply that the cracks be oriented n145 170 this direction of aligned cracks is consistent with the regional e w oriented extension as determined by the fault plane solutions

carbonate sedimentation on inactive fan delta lobes response to sea level changes sant llorenc del munt fan delta complex ne spain abstract the tertiary ebro basin is a part of the south pyrenean foreland basin and is bounded to the southeast by the catalan coastal range this feature developed as a major sinistral strike slip system during alpine deformation which led to uplift of the southeastern basement the eocene sant llorenc del munt fan delta complex with its complex transgressive regressive stratigraphy evolved on the down thrown side of this fault system this work presents an analysis of a ca 20 m thick sedimentary succession occurring across the transition between the transgressive and the regressive parts of a 50 200 000 year sequence during the study time interval there was low siliciclastic influx and relatively extensive carbonate platform covering an area of approximately 12 km2 developed over the middle and distal portions of the sant llorenc del munt fan delta complex the carbonate production was dominated by benthic foraminifera mainly nummulites but also discocyclina and coralline algae the nummulites formed platform shoals mixed with sand sized siliciclastics and buildups with sediments rich in discocyclinae deposited in the depressions between them the coralline algae grew mainly on top of coarse siliciclastic deposits forming algal ridges and bioherms limestone units have been identified on five different stratigraphic levels within the studied interval and the limestones are separated by marls and sandy marls these marl deposits probably represent partial suppression of carbonate production due to an increase in the influx of sand sized siliciclastics this interbedding between limestone and marls makes it possible to subdivide the carbonate succession into five high order transgressive regressive sequences the limestone units dominate the transgressive phases and the marls the regressive phases of these sequences the base of each sequence is found in a progressively more landward position indicating the transgressive regime during early deposition in the topmost sequence however the transition from the transgressive into the regressive part is represented by a basinward shift of patch reef development representing the onset of an overall regressive depositional regime the guruguru regressive unit

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

anatomy of high sediment supply transgressive tracts in the vilomara composite sequence sant llorenc del munt ebro basin ne spain abstract the sant llorenc del munt fan delta succession forms a clastic wedge ca 3 m y duration that built out into the ne ebro basin in response to transpression and uplift along the catalan coastal range the wedge contains a series of composite transgressive regressive sequences the transgressive part of one of these sequences the vilomara composite sequence has been examined to highlight the character of transgressive successions in a highly aggradational setting the analysis focuses on the nature of the component high frequency fundamental sequences there is a marked difference in how alluvial and marine sediment volumes are partitioned within the transgressive and regressive tracts of the three fundamental sequences within this overall landward stepping succession in transgressive tracts the marine shoreline clastics are well developed but only in narrow 250 m wide strike parallel zones which are closely associated with ravinement clusters the time equivalent alluvial strata are thick and extensive in regressive tracts the marine clastics are extensively developed along both dip and strike as prograding shoreline sheets whereas the interfingering alluvial deposits are only thinly developed or eroded out completely by their overlying sequence boundary in regressive intervals there was significant sediment bypass in the alluvial reaches with feeding to the shorezone and shelf in transgressive intervals in contrast great volumes of sediment were stored in the coastal plains this partitioning of sediment volumes commonly led to wedge shaped seaward thickening regressive geometries and wedge shaped landward thickening transgressive geometries the unusual thickness of transgressive strata the aggradationally stacked nature of successive ravinement surfaces and the thick development of nonmarine strata during times of transgression are symptomatic of an unusually high supply of sediment to the system during high rates of accommodation creation and a near continuous rise of relative sea level an unusual type of parasequence in the transgressive tracts showing well developed transgressive and regressive components is probably also symptomatic of the high sediment supply setting

signatures of storms oceanic floods and forearc tectonism in marine shelf strata of the quinault formation pliocene washington usa marine shelf strata of the quinault formation reflect the influences of storm flood processes and convergent margin tectonism on sedimentation and palaeocommunity distributions in an active forearc basin of early pliocene age western washington usa the sedimentologic ichnologic and invertebrate megafaunal character of coastal sea cliff exposures in the pratt cliff duck creek area quinault indian nation reveal five different sedimentary facies scoured rosselia bioturbated mixed and acharax these facies document the shifting interplay and intensities among storms waves and river flood plumes during transgression in inner to mid shelf settings storm sedimentation on the inner shelf is recorded north of pratt cliff by amalgamated proximal tempestites of the scoured facies which grade up section to thick deposits of hummocky cross stratified sandstone indicative of strong wave influences these hummocky beds alternate in metre scale packages with banded mudstone and siltstone that have distinctive sedimentologic and ichnofaunal characteristics rosselia facies in particular the mudstone and siltstone occur as 1 15 cm thick rhythmic parallel beds that are laterally continuous internally homogeneous to faintly laminated and thus similar in nature to fine grained oceanic flood deposits reported from shelf settings offshore the modern eel river northern california the quinault flood deposits are dominated by the ubiquitous trace fossil rosselia socialis comprising vertical mud packed flaring burrows with a sand filled central shaft which has been inferred as the feeding dwelling structure of a vermiform invertebrate adapted to high sedimentation rates in inner shelf settings fairweather conditions in between the higher energy periods of storms waves and floods are recorded north of pratt cliff by the mixed facies which is interpreted as representing the sand and mud zone of the inner to mid shelf transition quieter deeper mid shelf fairweather settings are typified by the bioturbated facies south of pratt cliff where lower sedimentation rates and lower physical energies produced extensively bioturbated deposits of sandy siltstone punctuated in places by isolated sandy beds of distal tempestites quinault strata also chronicle stratigraphic signatures of subduction of the juan de fuca plate beneath western washington during the pliocene for example the imprint of geochemically unusual authigenic carbonates and a chemosynthetic palaeocommunity acharax facies have been interpreted as a methane seep on the quinault seafloor furthermore a mobile rockground epifauna of pholadid bivalves became established on abundant dark mudstone cobbles and pebbles sourced from the hoh assemblage a miocene accretionary prism that was actively deforming as well as interacting with quinault forearc sediments during the pliocene hoh mudstone clasts were supplied to the quinault shelf via seafloor piercing diapirs and eroding melange shear zones exposures of which today occur in fault contact with quinault strata along the coast from taholah to the raft river

one year along beach variation in the maximum depth of erosion resulting from irregular shoreline morphology abstract beach erosion and accretion occur across multiple time scales over long time scales decades to millennia the shoreface ravinement surface which is recognized as a coarse lag deposit forms at the shoreface toe as a result of wave and current induced erosion during shoreline transgression over short time scales hours to days the depth of sediment disturbance which is recognized as coarse lamina and measured at the foreshore by devices and monitoring tracer beds forms as a result of wave and current induced reworking during a tidal cycle the maximum depth of erosion mdoe quantified here over 1 year is modulated by processes that operate over a time scale that is between the drivers of short term e g tides and waves and long term e g sea level rise beach erosion the mdoe integrates the erosion that occurs over a discrete time interval and records the maximum depth of erosion that is likely principally induced by storms which is difficult to quantify by other methods that rely on discrete observations e g changes in elevation or movement of the mean high water line a novel technique for quantifying the mdoe based on comparing the bedding and stratigraphy between cores collected at the same locations over a discrete time interval is presented here and applied at onslow beach nc usa this 12 km long barrier island has irregular shoreline morphology characterized by two embayments separated by a central headland this shape is largely the result of variations in the depth of underlying rock strata and produces a steeper beachface at the headland than at the embayments at each of the six sites examined along the barrier the mdoe is found to increase from the backshore to the middle intertidal zone and is higher at the sites closer to the headland these variations in the mdoe are likely due to the increase in average wave energy impacting the beachface from an offshore direction and steeper beaches intermediate beach state at the headland where the mdoe is within the beach facies it is not associated with a coarsening which is due to the heterolithic nature of the onslow beach strata where the mdoe is the contact between back barrier and beach facies it is always associated with a coarsening and a gravel rich lag deposit because in this case the mdoe is an amalgamation of multiple erosional events which is similar to the shoreface ravinement surface along beach variation in the mdoe does not correspond with discrete observations of beach change over the same period and is likely a better indicator of erosion potential than long term discrete observations such as changes in surface elevation or the position of the mean high water line

transgressive wave ravinement on an epicontinental shelf as recorded by an upper pennsylvanian soil nodule conglomerate sandstone unit kansas and oklahoma u s a abstract a thin 3 25 cm persistent conglomerate sandstone unit occurs at the base of the transgressive systems tract of an upper pennsylvanian cyclothem below a transgressive limestone in outcrop and subsurface in se kansas and ne oklahoma it has an erosional base and a sharp top and is composed of calcitic clasts some shell fragments and quartz sand and rare coal fragments clasts are rounded equant to elongate coarse sand to pebble sized and moderately to well sorted they are micritic to microsparitic or radial fibrous calcite grains and pisoids micritic and microsparitic clasts are heterogeneous and contain rounded to elongate moulds and radiating or concentric spar or micrite filled cracks pisoids have micrite cores and superficial calcite or clay rims the clasts have similar texture and composition to the calcitic nodules and rhizoconcretions in underlying paleosols and thus were probably derived from these soil nodules upper shoreface erosion during shoreline transgression excavated the underlying paleosols and eroded coeval transgressive deposits landwards of the shoreline the eroded sediments were then reworked and transported to the lower shoreface and inner shelf by storm return flows to be deposited as a soil nodule conglomerate sandstone unit its base is an amalgamated surface of wave ravinement and initial transgression as well as the lower sequence boundary the widespread conglomerate sandstone unit suggests extensive transgressive wave ravinement on an epicontinental shelf the transgressive record composed of upward deepening dominantly fine grained lithofacies defines a new type t c4 of transgressive record which is composed of mixed carbonate and siliciclastic lithofacies with a simple transgressive lag on a low relief shelf

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

seismic anisotropy around the gulf of corinth greece deduced from three component seismograms of local earthquakes and its relationship with crustal strain several thousand three component seismograms from local earthquakes recorded during two field experiments in august 1991 and november 1992 in the gulf of corinth have been analyzed to detect shear wave splitting after a first selection of the events located in the s window of the considered stations a second very strict selection of the records is applied in order to avoid the effect of scattered or converted phases which can mimic the behavior of shear wave splitting two main directions of fast s wave polarization have been detected one oriented n105 e n120 e the other n55 e n75 e the first one is perpendicular to the main direction of extension of the gulf provided by focal mechanism global positioning system measurements and tectonic studies and is thus consistent with the extensive dilatancy anisotropy eda model the second direction is subparallel to the direction of the active normal fault closest to the sites this suggests a local control of the anisotropy by these active faults either by a local rotation of the total stress field in which case the eda model may still explain the anisotropy or by the existence of a specific microstructure or macrostructure generated by the long term fault activity set of secondary fault planes parallel to the major one in which case the anisotropy direction would be significantly rotated from the stress direction about 50 the anisotropic signature does not seem to be affected by the geology of the site pre tertiary limestone and pleistocene sediments except for a station located on the thick plio quaternary deposits of a delta where the time delay is significantly larger

interplay between shoreline migration paths architecture and pinchout distance for siliciclastic shoreline tongues evidence from the rock record facies geometry and key internal stratigraphic surfaces from eight cretaceous and eocene clastic shoreline tongues have been documented the regressive parts of all the studied tongues represent storm wave influenced strandplains deltas or fan deltas and the regressive shoreline trajectories varied from descending to ascending the transgressive parts of the tongues are dominated by either estuarine or coastal plain deposits the distance from the coeval up dip non marine deposits to the basinward pinchout of amalgamated shoreface sandstones measured along depositional dip is here termed the sand pinchout distance the study shows that the angle of regressive to transgressive turnaround defined by the angle between the regressive and subsequent transgressive shoreline trajectories and the process regime during turnaround largely control the sand pinchout distance the amount of transgressive erosion can also partly control the pinchout distance but this parameter was comparable for the different examples presented here if the type of depositional system at turnaround and the depth of transgressive erosion are constant small angles of turnaround are associated with large pinchout distances whereas larger angles of turnaround result in smaller pinchout distances the model developed allows sand pinchout distance to be predicted using data for the landward parts of shoreline tongues the dataset also shows that steeply rising aggrading shoreline trajectories tend to produce more heterolithic sandstone tongues than those formed by lower angle trajectories

seismic anisotropy of the shallow crust in the umbria marche italy region abstract small magnitude earthquake data recorded during the 1997 umbria marche italy seismic crisis have been used to study the shear wave splitting in the central apennine using the method of diagonalization of the covariance matrix a set of 1234 s waves impinging the earth at an angle less than the critical angle has been selected the cross correlation method was used to estimate the splitting time between the two horizontal s components and the direction of the symmetry axis of the anisotropic structure a further analysis based on visual inspection of corrected traces allowed us to estimate the anisotropic parameters for 884 s waves an average splitting time 32 3 4 ms has been estimated splitting times do not generally show a clear pattern with distance which is an indication of possible spatial variations of seismic anisotropy a minimum 2 km depth of the anisotropic layer is estimated the trend of normalized splitting time vs depth roughly indicates a maximum 4 5 km depth of the anisotropic layer the distribution of the fast symmetry axis indicates a main symmetry axis oriented along the direction of the maximum compressive stress in the area and a secondary symmetry axis almost orthogonal to the first one the stereographic plots of fast axis direction seem exclude a depth dependence of anisotropic parameters a clear spatial correlation between the directions of the fast symmetry axes at the considered stations and the orientations of the nearest fault systems and thrust fronts is found which confirms the spatial variability in the anisotropic properties of the area as inferred by the analysis of splitting times based also on previous studies results are interpreted in terms of the extensive dilatancy anisotropy eda model the estimated crack density is always smaller than the critical coalescence value enforcing the hypothesis that fluid filled separated crack systems control the polarization of s waves

lithium isotopic composition of central american volcanic arc lavas implications for modification of subarc mantle by slab derived fluids abstract li contents and isotopic compositions were determined for a suite of well characterized basaltic lavas from the central american volcanic arc cava variable li y 0 2 0 5 li sc 0 1 0 4 and 6 li values 2 6 to 7 7 attest to significant compositional heterogeneity in the subarc mantle within specific arc segments these parameters correlate strongly with each other and with a number of other constituents e g k rb ba b la 10 be 9 be 87 sr 86 sr u ce and 230 th 232 th among others these correlations are particularly strong for nicaragua samples coupling of this particular set of constituents is best explained in terms of addition of subduction components to the subarc mantle moreover their selective enrichment with respect to relatively fluid immobile incompatible elements signifies the dominance of fluid vs silicate melt transport of slab components to the subarc mantle several interesting nuances are revealed by the li data first although li and b are strongly correlated in both costa rica and nicaragua there are systematic along strike variations in li b that are consistent with these elements having different fluid release patterns from subducted slab segments for example li b is highest in costa rica where auxiliary evidence indicates higher subduction zone temperatures apparently b is preferentially depleted and li retained in the slab under warmer conditions the same relations are reflected in li 10 be and other subduction tracer systematics all of which point to larger subduction contributions below nicaragua yet even nicaragua lavas vary widely in levels of subduction enrichment high ti basalts from nejapa are the least enriched and have the highest 6 li 1 4 to 2 6 these values are greater than in fresh morb ca 4 and are not easily explained by additions of subducted li because most oceanic crustal rocks and marine sediments have lower 6 li than morb with typical values between 8 and 20 thus it appears the nejapa data may be representative of isotopically light mantle domains relatively light 6 li values in an undepleted spinel lherzolite 11 3 from zabargad is red sea and in primitive backarc basalts 1 6 to 0 5 from lau basin support this conclusion considering representative fluid and mantle endmember compositions the cava data are consistent with limited up to a few percent additions of slab derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources in our view the subarc mantle is heterogeneous on a small scale but some arc sectors clearly received greater slab inputs than others

symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front abstract the regional variation of physical and geochemical characteristics of central american volcanoes occurs in two fundamentally different patterns the first pattern is symmetrical about nicaragua crustal thickness silica contents of mafic lavas and volcanic edifice heights are lowest in nicaragua and increase smoothly toward costa rica to the south and guatemala to the north magma density is maximum in nicaragua and decreases smoothly outward the regional variation in crustal thickness is just enough so that magma densities calculated at appropriate moho pressures are the same at the base of the crust throughout the region this is consistent with magma ponding at the base of the crust the bulk compositions of central american basalts show the same symmetrical variation suites of nicaraguan basalts plotted in pseudo ternary cmas projections indicate large olivine and plagioclase primary phase volumes toward costa rica and guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller which is consistent with fractionation at increasing depth the second pattern is the segmentation of the volcanic front and the plate margin in general the segmentation strongly affects the spacing and size of volcanic centers at segment boundaries volcanic centers are generally small and unusually widely spaced toward segment interiors volcano spacing and size increase systematically the lil element contents of lavas strongly reflect this pattern for lavas with similar silica contents the larger the volcano the higher the lil element contents the relationships between segmentation volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab the relationship between volcano volume and lil element content is qualitatively in agreement with an open system fractionation model

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

shear wave polarizations near the north anatolian fault i evidence for anisotropy induced shear wave splitting summary the turkish dilatancy projects tdp1 in 1979 and tdp2 in 1980 recorded small earthquakes near the north anatolian fault with closely spaced networks of three component seismometers in order to investigate the possibility of diagnosing dilatancy from its effects of shear wave propagation this paper examines the polarizations of shear wavetrains recorded in the shear wave window immediately above the earthquake foci abrupt changes in the orientation and or ellipticity of the shear wave polarizations are almost always observed during the first few cycles following the initial shear wave arrival on each seismogram the horizontal projections of the polarizations of the first shear wave arrivals at any given station show nearly parallel alignments with approximately the same orientations at each of the recording sites with one exception it is difficult to explain this uniform alignment over a wide area in terms of scattering at the irregular surface topography or by earthquake focal mechanisms we demonstrate that the shear wave splitting is likely to be the result of anisotropy in the region above the earthquake foci which could produce polarizations displaying the observed alignments the temporal change of the azimuth of alignment observed at one locality between 1979 and 1980 may be due to the release of a local stress anomaly by a very near earthquake

shear wave polarizations in the peter the first range indicating crack induced anisotropy in a thrust fault regime summary three component seismograms of small local earthquakes recorded in the peter the first range of mountains near garm tadzhikistan ssr display shear wave splitting similar to that previously observed near the north anatolian fault in turkey the peter the first range is in a region of compressional tectonics whereas the north anatolian fault is a comparatively simple strike slip fault detailed analysis of the turkish records suggests that the splitting is diagnostic of crack induced anisotropy caused by vertical microcracks aligned parallel to the direction of maximum compression preliminary examination of paper records from garm shows that most shear waves arriving within the shear wave window display shear wave splitting and that the polarizations of leading shear waves are consistently aligned in a ne sw direction the area is complicated and the tectonics are not well understood but the ne sw direction is approximately perpendicular to the compressional axis in many of the fault plane mechanisms of the earthquakes these earthquakes are usually at depths between 5 and 12 km although there are some deeper events nearby parallel shear wave polarizations such as those observed are expected to indicate the strike of nearly vertical parallel microcracks which would be aligned parallel to the direction of maximum compression thus the shear wave polarizations in the peter the first range indicate that the directions of principal stress are reversed in the rock above the earthquake foci where thrust faulting is taking place

sedimentology and stratigraphy of cenozoic deposits in the kagizman tuzluca basin northeastern turkey the kagizman tuzluca basin is located in the northeastern anatolia to the east of the intersection point near karliova of the major north and east anatolian fault systems this intermontane basin displays a thick sequence over 2000 m of mostly terrestrial deposits represented by repetitive alternations of the lake and fluvial environments ranging from late eocene oligocene to middle late miocene a marine incursion only mappable in the southeastern margin of the basin deposited limestones and sandy limestones rich in marine mollusks and nummulites in particular n fichteli that constrain an early oligocene age for this marine unit kagan fm the terrestrial basin fill deposits show different thicknesses throughout the basin due to irregular bottom topography and tectonic configuration of the basin margins the thickest deposits were accumulated along the different margins of the basin which received high quantities of siliciclastics from meandering river alluvial and coastal fans fan delta gilbert type delta and wave worked fluvial delta climate changes have also driven the development of lake environments during distinct depositional periods siliciclastic dominated overfilled lakes halikislak and kizilkaya formations and chemical dominated underfilled lakes turabi and tuzluca formations were formed they have been also classified as newark type and fundy type lakes respectively fluvial systems evolved from high energy meandering rivers deposited under humid climate gungoren formation to low energy meandering rivers resulted from arid and semiarid climates cincavat formation the transitional intervals from ephemeral river dry mudflat cincavat formation to saline pan lake tuzluca formation indicate wadi sand flat playa fluvial systems the chronostratigraphic constrains of the entire sequence remain poor and so far solely based on vertebrate fossil assemblages the evaporitic tuzluca formation would be middle miocene in age due to its stratigraphic position over the cincavat formation