surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

possible magma intrusion revealed by temporal gravity ground deformation and ground temperature observations at mount komagatake hokkaido during the 1996 1998 crisis summary mount komagatake 1131 m above sea level is a subduction related active volcano in hokkaido japan where two phreatic eruptions occurred in 1996 march and 1998 october after 54 years of dormancy we analyse four sessions of geodetic microgravity and ground temperature observations made over a two year period from november 1996 to may 1997 neither significant gravity nor significant elevation changes gps or levelling were observed from may 1997 to november 1997 we observed a slight subsidence 1 2 cm and contraction within 1 cm of the edifice a gravity increase 15 30 gal at the benchmarks inside the 2 km wide summit crater and a local temperature increase of 15 20 c at the summit crater from november 1997 to may 1998 we observed an edifice wide gravity increase of about 15 gal with no significant elevation change for benchmarks outside the summit crater besides this edifice scale variation we recorded a subsidence of about 5 6 cm and apparently no change of gravity inside the summit crater we inverted our data using models of increasing complexity elastic models are able to explain our deformation observations but they cannot explain either the gravity data or the temperature observations satisfactorily we introduced both an isothermal and a non isothermal porous medium filled with fluids to model deformation and gravity variations and we used a fissure model to invert the temperature anomaly our observations are consistent with a model of underground shallow water evaporation occurring as a result of heat rising from a hot and dense body density contrast of 200 kg m 3 of about 1011 kg at 4 5 km depth this interpretation suggests a possible intrusion of a magmatic body within the edifice

large topographic rises on venus implications for mantle upwelling topographic rises on venus have been identified that are interpreted to be the surface manifestation of mantle upwellings these features are classified into groups based on their dominant morphology atla and beta regiones are classified as rift dominated dione western eistla bell and imdr regiones as volcano dominated and themis eastern eistla and central eistla regiones as corona dominated at several topographic rises geologic indicators were identified that may provide evidence of uplifted topography e g volcanic flow features trending upslope we assessed the minimum contribution of volcanic construction to the topography of each rise which in general represents less than 5 of the volume of the rise similar to the volumes of edifices at terrestrial hotspot swells the total melt volume at each rise is approximated to be 104 106 km3 the variations in morphology topography and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling instead the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure plume characteristics and regional tectonic environment within each class variations in topography gravity and amount of volcanism may be indicative of differing stages of evolution the similarity between swell and volcanic volumes for terrestrial and venusian hotspots implies comparable time integrated plume strengths for individual upwellings on the two planets

control of relative sea level and climate on coal character in the westphalian c atokan four corners formation central appalachian basin u s a abstract peat successions preserved as coal seams preserve high resolution records of ancient terrestrial water table base level fluctuations that are driven by changes in relative sea level and or paleoclimatic change the aim of this study is to establish the roles of relative sea level and climate in controlling water table fluctuations within coal seams from the fluviodeltaic westphalian c bolsovian atokan four corners formation of the central appalachian basin usa comparison of the characteristics of 10 coals in the four corners formation with accommodation trends identified from analysis of intra coal clastic strata indicates that their thickness and vitrinite inertinite ratio correlate readily with the accommodation setting of the depositional sequence in which they occur coals that accumulated in relatively high accommodation fourth order sequences are high in vitrinite thin and overlain by or intercalated with marine and lacustrine sediments the coals have simple internal organization suggesting that they span parts of single high resolution accommodation cycles coals that accumulated in lower accommodation fourth order sequences are high in inertinite thick and overlain by or intercalated with terrestrial sediments the coals have composite internal organizations typified by multiple high resolution accommodation cycles and characteristic abrupt discontinuities of coal facies representing periods of depositional hiatus the correlations between the composition and thickness of the coals and the magnitude of associated marine transgression imply that relative sea level change was the principal control governing bulk accommodation in these mires landward of the paleoshoreline high frequency accommodation cycles within the coals reflect water table fluctuations in the original mires and were driven by high frequency sea level and or climate change the results of this study provide considerable insight into the manner in which coal composition and stratigraphy at a single locality vary over time in response to changing accommodation and provide a means to predict coal seam quality in an established sequence stratigraphic framework

deformation of grimsvotn volcano iceland 1998 eruption and subsequent inflation 1 grimsvotn is a subglacial volcano under the vatnajokull ice cap at the center of the icelandic hotspot this highly active volcano erupted in december 1998 gps measurements at a single station on a nunatak at the caldera rim were made 7 times during 1992 2001 the measurements prior to the 1998 eruption reveal pre eruption inflation but subsidence of more than 15 cm was measured during the eruption following the eruption re inflation occurred initially at a rate of 20 mm month then declined to 5 mm month measurements were fitted to a mogi model assuming that the source was located under the center of the grimsvotn caldera complex results indicate a source depth of at least 1 6 km the calculated amount of magma outflow during the eruption is comparable to field estimates of the erupted volume grimsvotn continues to inflate but has not reached its 1998 pre eruption level

novae on venus geology classification and evolution 1 novae are radially fractured centers 100 300 km in diameter and 64 have been identified on venus dense radial fracturing and upraised topography are common and massive amounts of volcanism are seen in some novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement we analyzed the topography and character of deformational and morphological structures of all 64 novae leading to a better understanding of their evolution and relation to diapir dynamics and their age we subdivided novae into four topographic classes 1 upraised 2 annular 3 flat and negative and 4 plateau like novae are located mostly in areas of regional rises and rift zones with a small number in the lowlands about one half the novae studied show association with rifts in some of these the rift troughs surround nova rises and form plateau like novae plateau like novae may predate postdate or form simultaneously with rifts our analysis generally confirms and updates previous models of novae evolution with the following geological factors being important 1 depth of the neutral buoyancy level of the rising mantle diapir 2 rheological characteristics of the overlying lithosphere 3 whether or not the visco plastic material above the diapir undergoes spreading and 4 the nature and influence of regional stress and rift structures these combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova such as topography tectonics and volcanism we found clear evidence for 1 the important role of dike swarm emplacement in the formation of radial patterns of novae and 2 the formation of concentric tectonic features during the relaxation stage of novae novae have multiple evolutionary stages and are long lived structures stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units we found that 40 3 of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11 3 completed their activity before this time 88 7 of the population of novae was active after regional plains formation in contrast to novae coronae activity was greatest before formation of regional plains which may be due to thickening of the lithosphere with time detailed structural analysis shows that novae evolution does not always lead to the formation of corona like features

extension across tempe terra mars from measurements of fault scarp widths and deformed craters two independent methods with no common assumptions have been used to estimate the extension across the heavily deformed tempe terra province of the tharsis region of mars one method uses measurements of normal fault scarp width with average scarp slope data for simple grabens and rifts on mars to estimate the fault throw which combined with sparse fault dip data can be used to estimate extension formal uncertainties in this method are only slightly greater than those in other methods given that the total uncertainty is dominated by the likely uncertainty in the fault dip assumed to be 60 15 measurement of normal fault scarp widths along two n25 50 w directed traverses across tempe terra both yield about 22 16 km of extension or 2 strain across the northern traverse and nearly 3 across the southern one about three quarters of the extension has occurred during the two main phases of tharsis related deformation from middle late noachian to early hesperian and from late hesperian to early amazonian with more extension closer to the center of tharsis during the first phase extension across the region was also determined by measuring the elongation and elongation direction of all ancient noachian impact craters without ejecta blankets which predate most of the deformation results have been corrected for initial non circularity of craters established from similar measurements of young post deformation impact craters yielding a statistically significant mean strain of 1 96 0 35 in a n38 10 w direction across tempe terra extension of 20 4 comparable in magnitude and direction to the average result from the scarp measurement method both methods indicate an average extension for single normal fault scarps and shortening across wrinkle ridges for the crater method of 100 m the agreement between the results of the two independent methods in overall extension and average single normal fault extension argues that the average scarp slope and fault dip data in the fault scarp width method accurately represent the actual extension across the observed structures this conclusion supports existing geometric and kinematic models for structural features on mars a preliminary estimate of the total circumferential extension around tharsis at a radius of 2500 km is roughly 60 42 km total hoop strain is about 0 4 distributed heterogeneously tempe terra is the most highly strained region on mars

holocene excursions of the northwest european geoid abstract computer assisted analyses of more than 600 radiocarbon dated sea level indicators from northwestern europe concentrated around the north sea margin indicate that vertical crustal movements are more important that eustatic sea level change in determining the locus of holocene shoreline positions through time for the past 14 000 radiocarbon years the divergence of sea level data for the northwest european sectors exceeds the maximum estimated sea level rise by a factor of two or more projecting these data to a single meridian demonstrates the remarkable variety of vertical crustal movement in northwestern europe accumulating radiocarbon dated sea level indicators into millennial cohorts we generate isobase maps which begin to specify areas of notable vertical crustal mobility these isobase maps appear to confirm that eustatic sea level rise is subordinate to postglacial geoidal excursions in determining the locus of contemporary northwestern european shorelines

the ascent of felsic magmas in dykes abstract the results of a thermal and fluid dynamical analysis are presented that allow geologists to estimate the critical dyke or fracture width w c necessary for the transport of felsic magma through the crust without control by solidification the principal geological variables required to estimate w c are the magmatic viscosity the density and temperature contrasts and t between magma and country rock and the height of ascent h the results show that for typical values of and and for transport distances of 20 to 30 km the critical dyke width is constrained to lie between about 2 and 20 m

determining heights and slopes of fault scarps and other surfaces on venus using magellan stereo radar this report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces such as normal fault scarps on the surface of venus from measurements of their widths in magellan stereo synthetic aperture radar sar images these surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat lying areas in the 120 to 280 m resolution magellan sar images but are not generally imaged in the 10 km resolution magellan altimetry our methods take into account radar distortion effects and allow one to determine whether a slope is foreshortened laid over elongated or in radar shadow the techniques make use of graphs constructed for the magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner but the techniques are applicable to any stereo radar data set if the radar incidence angles are known additionally the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow using these techniques we show that there are normal fault scarps on venus with heights in the 700 m range that have remarkable topographic slopes of close to 60 something unheard of on earth and which suggests a high effective cohesive strength of the venusian crust

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

modelling plume related uplift gravity and melting on venus abstract three roughly circular regions on venus about 1000 km across are identified as potential hawaii scale plume sites on the basis of their gravity 70 90 mgal and topography 1 6 2 0 km anomalies and signs of melt generation and rifting axisymmetric isoviscous convection models are used to reproduce gravity profiles across these plumes and the line of sight acceleration of the magellan spacecraft as it passes over them the best fitting models have a conductive lid thickness of less than 150 km mantle viscosity of 8 9 1019 to 9 6 1020 pa s and a basal heat flux of 15 25 m w m 2 the lid thickness is constrained by requiring a modest amount of melt generation and a potential temperature of about 1300 c the high mantle viscosity relative to that of earth is probably a consequence of the absence of water in the mantle and may help to explain the current absence of plate tectonics on venus the low heat flux suggests that the thermal evolution of venus has differed from that of the earth

long term accretion rates on tidal salt marshes abstract long term variations in salt marsh accretion rates were estimated using data collected on the marshes of the north norfolk coast england fourteen discrete marsh areas were sampled all lying within a 20 km stretch of coast the age of each marsh had previously been determined using radiocarbon or archival evidence for the date of vegetation inception on each marsh surface marsh ages ranged from 10 years to 2000 years the elevation of each marsh surface was determined in the field using levelling techniques the relationship between age and surface elevation was assumed to be a function of the rate of marsh accretion within this area of coast statistical tests showed this to be a highly significant relationship while the accretion rates calculated from the age height curve agree closely with field measurements on these marshes accretion rates were found to vary from 1 7 cm yr on 10 year old marshes to less than 0 002 cm on marshes older than 500 years the age height relationship describes an asymptotic curve with the asymptote lying 0 8 m below the level of the highest spring tides and appears to be controlled by the frequency of tidal maxima which peaks at 0 8 m below the maximum value and declines sharply above that height

global distribution and characteristics of coronae and related features on venus implications for origin and relation to mantle processes coronae on venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies the annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both topographically coronae are domes plateaus plateaus with interior lows and rimmed depressions a subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large scale calderas a number of features have been identified with many of the basic characteristics of coronae similar interior deformation associations with volcanism high topography but lacking a distinct tectonic annulus these features tend to be somewhat smaller than coronae and may represent failed coronae or coronae in an early stage of evolution the size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form n d kd where n is the number of coronae with maximum widths greater than d km and 3 05 the spatial distribution of coronae is not random the features are concentrated in a few groups and along several chains coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as western eistla regio the largest corona artemis is actually larger than several volcanic rises on venus coronae and volcanic rises appear to be surface manifestations of mantle plumes there is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on earth instead a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume the morphology and size distribution of coronae highlands and major shields suggest that mantle upwelling on venus operates either on several spatial scales with coronae representing smaller scale upflows and major volcanic rises representing larger convective upwellings or on several temporal scales with coronae representing shorter duration upflows and major volcanic rises representing long term upwellings

development of large volcanoes on venus constraints from sif gula and kunapipi montes abstract sif gula and kunapipi montes are large volcanoes on venus they have differing morphologies reflecting their different eruptive histories and plumbing systems all have a steeper upper cone surrounded by more shallowly sloping flanks interpreted as resulting from histories of more frequent shorter lower effusion rate summit eruptions accompanied by infrequent more voluminous flank eruptions all three volcanoes have multiple flow field types digitate sheet and fan which probably reflect variations in effusion rate eruption duration and vent geometry sif mons summit has a large caldera gula mons has summit calderas at either end of a summit rift and the summit of kunapipi mons is composed of two volcanic centers on a summit plateau the summit morphologies of sif gula and kunapipi montes are interpreted to reflect differences in magma chamber location over time and differing internal plumbing we find no positive evidence that neutral buoyancy has governed magma chamber position instead magma chambers at the three volcanoes appear to have changed position over time with lithospheric stresses as well as neutral buoyancy affecting chamber location all three volcanoes have had episodes of diking at their summits followed by episodes of extrusion we interpret this to be caused by variations in either magma supply rate or magma volatile content or both over time fractures on the flows of the volcano indicate that dike intrusion has occurred throughout the visible history of the volcanoes indicating that the local stress field within the volcano favored intrusion as well as extrusion over much of its history the overall shape and history of the venusian volcanoes is similar to the histories of terrestrial volcanoes however some aspects of these volcanoes including the rift at gula the large size and very long flows differ significantly from terrestrial volcanoes

constraints on magnitudes of extension on venus from slope measurements slopes related to normal fault scarps on venus appear in magellan synthetic aperture radar sar images as tonal bands of i reased or decreased radar backscatter relative to adjacent regions calculations through measurements on magellan stereo sar of 170 of these surfaces in plains regions show that their mean topographic slope is 36 4 1 2 regardless of height because there is little to no erosion on venus we infer from this mean slope that most faults have collapsed to talus slopes approximately at an angle of repose and thus the effective cohesive strength of the crust near most fault surfaces is low probably because of secondary faulting and jointing in areas well imaged by magellan stereo sar we show that simple balanced cross sections through individual grabens on venus can be made assuming a coulomb shear angle for fault orientation thus constraining slip on causitive faults and extension across the grabens in rifted areas imaged by one magellan sar viewing geometry constraints on extension across rift zones can be made by measuring the tonal bands representing slopes related to individual normal faults and using the mean topographic slope calculated for similar slopes imaged in stereo sar with this technique we show that crustal extension due to faulting and folding for the most recent deformation in the beta rift zone is 20 km these modest strains across one of the best developed rift zones on venus put severe limits on tectonic models of venus that invoke significant lateral displacements of the venusian crust

corona structures on venus models of origin coronae on venus are circular to elongate structures with maximum widths of 150 1000 km characterized by annuli of concentric ridges surrounding complex interiors the features have raised topography relative to the surroundings they are associated with volcanic activity and most are partially surrounded by a peripheral trough variations in morphology between individual coronae are due to differences in their stage of evolution and or differences in the relative significance of the geologic processes that occur in each stage we examine models for three processes that may be involved in corona origin and evolution 1 a hotspot or rising mantle diapir model 2 a sinking mantle diapir model and 3 gravitational relaxation of topography rising mantle diapirs are caused by heating at depth e g hotspot while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere the hotspot model is most consistent with the major characteristics of coronae with gravitational relaxation occurring as a modificational process the sinking mantle diapir would produce dominant central compression that has not been observed at coronae however higher resolution image and altimetry data from magellan can be used to distinguish more fully between the two models coronae in various states of formation and degradation can be identified in the venera 15 16 data suggesting that the process may be continuing today

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

reconstructing relative sea level change using uk salt marsh foraminifera abstract the position of past sea levels can be reconstructed using a series of sea level index points that possess information on age location altitude and a quantified vertical relationship with a former tidal frame the indicative meaning whilst these points fix the altitude of relative sea level at one instant in time they provide no information on its variation between them here a foraminiferal transfer function is used to reconstruct changes in water depth from fossil assemblages preserved within salt marsh sediments the transfer function performs reliably in the high marsh zone where agglutinated foraminiferal species are dominant in lower marsh environments where calcareous species are prevalent postmortem test dissolution alters the fossil assemblages and leaves them without modern analogues to circumvent this problem a new transfer function based on agglutinated foraminifera and utilising preserved test linings is developed this new transfer function significantly reduces the number of samples without modern analogues but at the expense of diminished sensitivity in the low marsh environment additionally the transfer function is used to establish a series of sea level index points that place observed lateral shifts in depositional environment into a vertical and temporal context the combination of age altitude and high resolution water level reconstructions offers the potential to obtain more detailed and reliable records of relative sea level change from salt marsh sediments

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

gravity height correlations for unrest at calderas abstract calderas represent the sites of the world s most serious volcanic hazards although eruptions are not frequent at such structures on the scale of human lifetimes there are nevertheless often physical changes at calderas that are measurable over periods of years or decades such calderas are said to be in a state of unrest and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors here we review combined gravity and elevation data from several restless calderas and present new data on their characteristic signatures during periods of inflation and deflation we find that unless the bouguer gravity anomaly at a caldera is extremely small the free air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient and not the theoretical gradient use of which may introduce significant errors in general there are two models that fit most of the available data the first involves a mogi type point source and the second is a bouguer type infinite horizontal plane source the density of the deforming material usually a magma chamber is calculated from the gravity and ground deformation data and the best fitting model is to a first approximation the one producing the most realistic density no realistic density is obtained where there are real density changes or where the data do not fit the point source or slab model we find that a point source model fits most of the available data and that most data are for periods of caldera inflation the limited examples of deflation from large silicic calderas indicate that the amount of mass loss or magma drainage is usually much less than the mass gain during the preceding magma intrusion in contrast deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms

dike intrusion as a possible cause of linear martian magnetic anomalies a model in which the ancient 100 200 km wide magnetic lineations in the southern hemisphere of mars are formed by repeated dike intrusion is proposed on the basis of magnetizations observed at terrestrial mid ocean ridges a likely upper bound on the martian crustal magnetization is about 15 a m 1 this upper bound gives a minimum magnetized layer thickness of 35 60 km because the magnetic material is likely to be magnetite the long term stability of the anomalies probably requires ilmenite lamellae to have exsolved from the magnetite this exsolution process provides an upper bound on the cooling time available of 102 105 yr these bounds can be satisfied if the magnetic anomalies are created by successive intrusion of dikes from 20 to 2000 m wide production of a martian crust 35 60 km thick by rifting requires either mantle potential temperatures in excess of 1430 c or stretching factors greater than 5

neutrally buoyant diapirs a model for venus coronae coronae are typically circular features 100 600 km in diameter characterized by a deformed annular ring that is often topographically high the central region may be raised or depressed relative to the ambient elevation previous studies have proposed an evolutionary progression beginning with dome shaped features which have radiating extensional rifts followed by plateau shaped features which have both concentric deformation and radial extension ending with the classic corona as described above previous dynamical calculations of a rising and spreading diapir simulated the evolution from dome to plateau shaped topography but could not account for the raised rim or depressed central topography of some coronae we demonstrate that these features can be modeled by a diapir that spreads laterally at a depth of neutral buoyancy

a sea level rise curve from guilford connecticut usa abstract high resolution stratigraphic studies based on sediment chemistry lithology macroflora and benthic foraminiferal assemblages in three peat cores from coastal salt marshes at guilford connecticut show that coastal marshes are ephemeral environments marsh wide environmental variations were common and century long episodes of relative submergence alternated with emergence despite about 2 m of relative sea level rise rslr in connecticut over the last 1500 years the marshes have expanded both landwards and seawards and marsh accretion has been outpaced only marginally by rslr we used radiocarbon dating and the level of anthropogenic markers metal pollution as dated with 210 pb in the cores for age control for most of the last 1000 years the rate of rslr was between 1 3 and 1 8 mm yr but over the last 300 400 years it increased to 2 9 3 3 mm yr and has been faster than the accretion rate especially in the middle marsh the net submergence rate or submergence index ratio of the rate of rslr and marsh accretion rate averaged about 1 15 over the last 1000 years and increased to about 1 5 over the last 200 years the rate of rslr was very sluggish during the early part of the little ice age but we found a slightly higher rate during the little climate optimum this excursion is close to the noise level however the most significant observation is that rslr increased strongly around a d 1650 the onset of this acceleration falls in the middle to end of the little ice age and thus preceded the period of modern global warming that started late last century and that has been tentatively correlated with anthropogenic emissions of greenhouse gases

a coupled thermal mechanical model for corona formation on venus we develop a model of the combined thermal and mechanical evolution of diapirs in the mantle of venus the diapir is treated as an oblate spheroid rising through a viscous fluid and ultimately impinging on a rigid overlaying lid drag forces imposed by the lid cause the diapir to spread and flatten as it rises we parameterize the heat loss from the rising diapir using a nusselt number formulation and treat the resulting loss in buoyancy in our flow calculation the model predicts the evolution timescale and degree of flattening of a diapir as it rises as well as the stresses exerted on the underside of the lithosphere in order to explore diapir behavior further we check our analytical model against the predictions of the finite element code mantle from the combined results of these models and observations of venusian coronae we are able to make a number of inferences about venusian diapirism and mantle properties we find that the diapirs responsible for formation of venusian coronae have an initial size distribution extending from about 30 to 100 km in radius typical evolution timescales for these diapirs are 30 50 myr if venus global average resurfacing age is 300 500 myr then our results are consistent with a rate of diapirism that has been roughly constant over this period and with an effective mantle viscosity of 1021 pa s

sedimentology and sequence stratigraphy of the lopingian late permian coal measures in southwestern china the lopingian coal measures of southwestern china were deposited within a range of facies associations spanning a spectrum of settings from fluvial to marine carbonate platform the transitional to terrestrial coal measures are dominated by siliciclastics but they also contain fifteen laterally extensive marine bands limestone beds and mudstone these bands act as marker horizons that enable correlation between fully marine and terrestrial facies examination of this range of facies and their sedimentology has enabled the development of a high resolution sequence stratigraphic framework set against the established backdrop of second order lopingian transgression sixteen fourth order sequences and three composite sequences third order are recognized results show that in the composite sequences peat accumulation in the seaward parts of the study area predominantly correlates with early transgressive sequence sets tss while in more landward areas it correlates with the middle tss to late highstand sequence sets hss differences in peat accumulation regimes within the sequence stratigraphic framework are attributed to variations in subsidence and background siliciclastic input rates in different depositional settings with these combining to produce differences in the rate of accommodation change the preservation of coal resources in the middle to late hss in this area was most likely related to the rise of the regional base level throughout the lopingian

late devensian and holocene relative sea level changes on the isle of skye scotland uk detailed litho and biostratigraphical analyses from three coastal sites in contrasting coastal settings on the isle of skye scotland uk reveal evidence for several changes in relative sea level during the late devensian and holocene at the start of the record relative sea level in the area was high at ca 12 500 14c ca 14 800 cal yr bp but then fell reaching a low point during the younger dryas at ca 11 000 10 000 14c ca 13 000 11 600 cal yr bp when a rock platform correlated with the main rock platform was formed in the early middle holocene relative sea level was rising by ca 8000 14c ca 8800 cal yr bp and in northeast skye a lagoonal surface correlated with the main postglacial shoreline was formed at ca 6600 14c ca 7500 cal yr bp by the late holocene relative sea level was again falling but a rise registered at at least two sites began probably before ca 4000 14c ca 4500 cal yr bp and a second lagoonal surface in northeast skye correlated with the blairdrummond shoreline was formed although by ca 3000 14c ca 3200 cal yr bp relative sea level in the area had resumed its downward trend the pattern of relative sea level changes disclosed is compared with evidence elsewhere in scotland

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

the holocene sea level story since 7500 bp lessons from the eastern mediterranean the black and the azov seas abstract this paper addresses the obvious controversy between the so far published sea level curves of the black sea and the mediterranean it starts with a discussion of the methods of reconstructing sea level curves the evaluation of sea level indicators and the application of the radiocarbon dating method at least since 7500 bp when the black sea and the mediterranean were connected both water bodies must have reacted synchronously on glacio eustatic changes it is documented that none of the mediterranean sea level curves shows the major wiggles postulated for the black sea which are supposed to reflect trans and regression cycles the very shallow bathymetric condition of the azov sea and the northern black sea should have led to considerable and traceable shoreline displacements there is neither archaeological nor historical evidence of mid and late holocene regressions of several meters the tectonic setting of the black and azov seas implicates that the tectonic signal often overrides the eustatic one therefore only local sea level curves can be established in this paper based on vibracores a locally valid sea level curve for the taman peninsula is demonstrated layers of paralic peat were used as sea level indicators and for 14 c dating the shape of this curve follows the one known from the mediterranean this study also revealed that the present peninsula of taman evolved out of a former archipelago

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

large scale textural patterns and sedimentary processes on tidal salt marshes in the severn estuary southwest britain abstract approximately 840 km2 of largely reclaimed estuarine alluvium border the severn estuary and inner bristol channel today using a direct measure of grain size and the proxy evidence of sediment controlled early land use medieval early modern it is shown that the grain size of the tidal mud deposited on the marshes prior to reclamation declined outward from the main and larger tidal channels toward the bedrock limit of the alluvium variations in the altitude of the marsh surfaces as they survive after reclamation suggest that the deposition rate of this sediment also decreased outward from these sources at elmore a representative narrow ca 1 km marsh grain size change is steep and well defined in contrast at hill in a representative sector from a large and broad ca 5 km marsh the measured change is gradual and disorderly and disturbed by an extensive arcuate belt of relatively coarse sediments based on a review of processes and sedimentation and appealing to a broad geomorphological and hydraulic analogy between tidal wetlands and river floodplains it is suggested that marshes of the elmore type may be supplied with sediment largely by transverse diffusion from the tidal channel whereas hill type marshes may receive considerable mud by convection as well as diffusion

degradation of the plaquemines sub delta and relative sea level in eastern mississippi deltaic coast during late holocene abstract the stratigraphic architecture of plaquemines sub delta lobe is identified from sedimentary cores and sub bottom seismic data collected at lower breton sound louisiana usa four lithofacies were identified in cores from bottom to top 1 organic poor mud 2 organic poor silty sand 3 massive mud 4 organic rich peat and root rich soft mud overall the lithofacies displays a typical progradational deltaic deposit at bottom with upward coarsening feature and then an upward fining deposit overlying on top stratigraphic position of the organic rich peat facies indicates that it is formed after the fluvial sediment source moves away from this region and it is recognized as a termination of progradation of the plaquemines sub delta dated peat samples show that sub delta progradation stops between 876 and 1384 yr ad during which a 20 70 cm relative sea level rise is detected the geological framework reported in this study provides critical information for future modeling on river diversions the stratigraphic architecture underlying lower breton sound implicates that top three facies of root rich soft mud organic rich peat and massive mud are likely to be eroded away before the development of new crevasse splay or sub delta when sediment diversion is in operation the deeper organic poor silty sand lithofacies however can serve as a firm foundation to build new land

strain at radially fractured centers on venus 1 radially fractured centers rfcs on venus are distinctive 100 km long radiating systems of graben fissures and fractures arranged around a central topographic high we use cycle 1 and cycle 3 magellan images to determine the depths and wall dip angles of several large radial graben using two different methods at four rfcs becuma mons didilia corona h uraru corona and pavlova corona we record graben depths of 0 3 0 9 km and mean wall dip angles of about 36 which we interpret as a primary talus slope rather than a fault plane using the calculated depths and a more realistic fault dip of 60 we find extension of between 0 5 and 1 km on graben between about 5 and 10 km wide we use this extension to estimate the hoop strain at each rfc maximum measured strains on the flanks are typically between 1 and 4 and may be up to 7 toward the center we conclude that this strain is too large to be explained by current plume related uplift models and also by magma chamber inflation we conclude that dikes are probably responsible for the majority of the strain at the large radial graben

does livestock grazing affect sediment deposition and accretion rates in salt marshes accretion rates defined as the vertical growth of salt marshes measured in mm per year may be influenced by grazing livestock in two ways directly by increasing soil compaction through trampling and indirectly by reducing aboveground biomass and thus decreasing sediment deposition rates measured in g m per year although accretion rates and the resulting surface elevation change largely determine the resilience of salt marshes to sea level rise slr the effect of livestock grazing on accretion rates has been little studied therefore this study aimed to investigate the effect of livestock grazing on salt marsh accretion rates we hypothesise that accretion will be lower in grazed compared to ungrazed salt marshes in four study sites along the mainland coast of the wadden sea in the south eastern north sea accretion rates sediment deposition rates and soil compaction of grazed and ungrazed marshes were analysed using the 137cs radionuclide dating method accretion rates were on average 11 6 mm yr 1 during recent decades and thus higher than current and projected rates of slr neither accretion nor sediment deposition rates were significantly different between grazing treatments meanwhile soil compaction was clearly affected by grazing with significantly higher dry bulk density on grazed compared to ungrazed parts based on these results we conclude that other factors influence whether grazing has an effect on accretion and sediment deposition rates and that the effect of grazing on marsh growth does not follow a direct causal chain it may have a great importance when interacting with other biotic and abiotic processes on the marsh

radially fractured domes a comparison of venus and the earth radially fractured domes are large tectonic and topographic features discovered on the surface of venus by the magellan spacecraft they are thought to be due to uplift over mantle diapirism and to date are known to occur only on venus since venus and the earth are grossly similar in size composition and structure we seek to understand why these features have not been seen on the earth we model the uplift and fracturing over a mantle diapir as functions of lithospheric thickness and diapir size and depth we find that lithospheres of the same thickness on the earth and venus should respond similarly to the same sized diapir and that radially fractured domes should form most readily in thin oceanic lithospheres on earth if diapiric activity is similar on the two planets however our current knowledge of the earth s oceanic floors is insufficient to confirm or deny the presence of radially fractured domes we compute the expected dimensions for these features on the earth and suggest a search for them to determine whether mantle diapirism operates similarly on the earth and venus

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

on the postglacial isostatic adjustment of the british isles and the shallow viscoelastic structure of the earth summary observations of postglacial relative sea level rsl history at sites beyond the margins of the main accumulations of wu rm wisconsin devensian ice may be invoked to test the accuracy of global models of the glacial isostatic adjustment process the existing database of rsl time series for the british isles forms a continuous network of sites that sample the coasts of scotland england and wales fairly uniformly and which may be especially important in this regard not only are these 14c data for the holocene epoch of very high quality in terms of our knowledge of the coastal environment and relationship to palaeo sea level of each age measurement but they are especially numerous and relate to an island that was itself partly glaciated in the north essentially ice free in the south and located well outboard of the massive ice sheet that covered fennoscandia at the last glacial maximum lgm taken together the time series from 55 distinct locations allow us to perform a fairly stringent test of the recently formulated ice 4g vm2 global model of the isostatic adjustment process even without modification this model has been shown to fit most of the time series in the uk data set rather well in order to satisfy very recently published a priori constraints on the maximum thickness of the scottish ice sheet however we find that the lithospheric thickness the feature of the radial viscoelastic structure to which the rebound data from scotland are most sensitive must be somewhat reduced whereas the assumed thickness of the lithosphere in ice 4g vm2 was near to 120 km best fits to the scottish data require a lithospheric thickness near to 90 km this reduced value for lithospheric thickness agrees exceedingly well with the latest estimates of this earth property based on the inversion of oceanic geoid and topography data as the asymptotic thickness characteristic of the oldest ocean floor this would appear to be the best possible a priori estimate for the thickness of the cratonic lithospheres upon which the largest ice sheets of the late pleistocene are known to have rested we also demonstrate explicitly that inferences of the shallow viscoelastic structure beneath the british isles are strongly influenced by the details of the history of deglaciation at remote locations at antarctica in particular

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

a 17 yr record of sediment accretion in the salt marshes of maine usa abstract marker horizons of brick dust were established in 1986 in 26 salt marshes along the coast of maine usa in 2003 we measured the sediment accumulation rates for the 17 year period in 11 relocated sites and obtained results varying from 1 4 to 4 2 mm yr the mean accretion rate for all marshes was 2 8 mm yr accretion rates measured from 1986 to 2003 were comparable to those measured at the sites in 1987 and 1990 indicating that marsh accretion rates are similar on annual and decadal time scales inorganic material was found to be the primary constituent of maine salt marsh sediments several marshes show clear evidence of transgression since 1986 in the form of landward moving plant zones or encroachment of mudflat onto salt marsh indications for regressive changes were not observed marsh sediment accumulation rates are generally higher than mean rates of relative sea level rise indicating that most salt marshes are currently keeping pace with sea level rise however if relative sea level rise were to exceed rates of 4 mm yr many salt marshes in maine may not survive

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

vertical ground deformation on mount etna 1975 1980 we present the results of repeated precise leveling between the years 1975 and 1980 of an 11 km traverse over the summit of mount etna together with data from a network of dry tilt stations measured between 1976 and 1980 both methods demonstrate that little vertical movement occurs except on fresh lava flows which show a consistent pattern of downward movement interpreted as compaction and downhill sliding of lava after it has solidified other small vertical movements of a few centimetres recorded on the leveling traverse appear to be related to activity particularly the eruptions from a new vent area on the southeast side of the summit in 1978 and 1979 models of the recorded movement indicate that a small amount of magma was already stored in a dike below this eruption site as much as two years before it was erupted although the bulk of the magma for these eruptions was not stored in the upper few kilometres of the volcano the latter conclusion is supported by the dry tilt data which are dominated by local movements of 10 to 20 microradians with no consistent pattern of movement over the whole volcano

dike emplacement on venus and on earth many long linear troughs are visible in synthetic aperture radar images of the surface of venus their geometry closely resembles that of dike swarms on earth such troughs will be formed if dikes fail to reach the surface or if the magma in them is partially drained dikes that form closed ellipses in plan view can be produced by intrusions near a depression and intersecting dikes can result if the stress field changes with time because of the absence of erosion and sedimentation all these effects are better displayed on venus than they are on earth the magnitude of the regional stress on venus can be estimated from the dike patterns to be about 3 mpa or similar to that of earth dikes consisting of en echelon segments whose total length is 1500 km or more occur on both earth and venus on earth their widths are generally greater than 30 m and are sometimes as great as 250 m and they are likely to be similar on venus the melt velocity during their emplacement is predicted to be a few meters per second and the calculated rate of heat loss is sufficiently slow that they can remain molten as they propagate for distances of more than 1000 km the same calculations apply to sills though less is known about their geometry the results therefore suggest that magma intrusion in thin sheets extending over 1000 km or more could produce regional crustal thickening and uplift on both earth and venus

constraints on measurement of sea level movements from salt marsh accretion rates the use of sediment accretion rates measured on coastal and estuarine salt marshes as a surrogate for rates of sea level change rests on the usually implicit assumption that marsh maturity is unchanged over the measurement period this assumption is inherently difficult to test empirically but a simple mathematical analysis shows that it is true only in a severely restricted case under most practical conditions measured accretion rates either over estimate or under estimate the rate of sea level change under certain circumstances they may indicate an upward trend when the tendency is actually downward a knowledge of marsh history is critical to the evaluation of such measurements sea level movements estimated from accretion rates are theoretically most reliable in the case of relatively old marshes and comparatively short measurement periods

glacial rebound and relative sea levels in europe from tide gauge records abstract relative sea levels recorded by tide gauges during the past century in northern europe are dominated by isostatic readjustment of the land following the latest deglaciation of scandinavia and scotland maximum relative uplift of the land is centered near the northern gulf of bothnia at a rate of 6 7 mm yr with a smaller secondary maximum over scotland also at a rate of 6 7 mm yr although there probably is a relaxing peripheral bulge surrounding the regions of maximum uplift such a former bulge is poorly defined by coastal tide gauges in the north sea evidence for sinking of a former peripheral bulge of glacial origin is complicated by post carboniferous basin deepening with sediment loading and possible rejuvenation associated with glaciation other data gravity radiocarbon geomorphology support the interpretation that glacial isostasy controls the structure of relative sea level change included in this pattern of relative rise of land is a eustatic signal that biases the estimates of glacial rebound such a eustatic signal could not be isolated from the isostatic signal using the present data but glacial isostasy clearly is a major control for relative sea levels of the region absence of significant higher frequency 2 50 yr cycles in mean annual sea levels of northern europe reflects the complex hydrologic oceanographic forces to which sea levels must respond whereas other coastal regions show significant higher frequency peaks in the energy spectra of relative sea levels the many marginal seas in northern europe preclude a clear relation between hydrologic oceanographic forcing and relative sea levels although this relation must exist on a more local scale

growth of large volcanoes on venus mechanical models and implications for structural evolution the structure tectonics and evolution of large volcanoes on venus as revealed by data from the magellan mission appear to be distinct from those on earth and mars to determine the conditions and processes that account for these differences we model the evolution of stress and deformation in growing volcanic edifices on venus using the finite element method large volcanoes on venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons the surfaces of both edifice and apron consist dominantly of radially oriented flows tectonic features usually the surface expression of shallowly intruded dikes also have predominantly radial orientations features similar to hawaiian style linear rift zones or large scale flank failure common on large volcanoes on earth and mars are absent by comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features we determine that a welded condition is more likely horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness te contravening stress increments from magma chamber expansion differential thermal contraction or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored incremental volcano growth results in stress distributions that decrease with height in the edifice dike intrusion is easiest in the uppermost youngest layers consistent with the observed shallow radial dikes on venus volcanoes small values of te greatly inhibit the formation of large shields in that conical edifice topography cannot be supported and large contravening stresses are required for further growth large volcano formation is much more likely at large te where more moderate contravening stresses are sufficient for growth the near surface manifestation of mantle upwelling on venus may thus depend on te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile leading to the formation of coronae but a thick elastic lithosphere is required to support the growth of the largest volcanoes the transitional value of te between these modes of evolution likely depends on the horizontal scale of the upwelling

formation of beta regio venus results from measuring strain beta regio is an area of rifting and volcanism on venus constituting a topographic rise a shield volcano theia mons lies near the center of the region and is surrounded by several radially oriented rifts we use magellan altimetry gravity and synthetic aperture radar data of the area to constrain some subsurface parameters first we derive hoop strain using altimetry data and a fault dip angle derived from the split crater somerville we determine the extension in the rifts surrounding beta regio we then derive the hoop strain accommodated by the rifts from the extension in these rifts except near theia mons the hoop strain follows the shape expected from a mantle upwelling the difference near the volcano we believe is due to volcanic infilling we then model three observable quantities the newly derived strain along with gravity and uplift using two separate modeling techniques one for the strain and uplift and another for the gravity the model results show that the data are consistent with the view that a relatively low density contrast region now exists below beta and has caused the uplift and rifting in the region

the formation of coastal peat marshes under an upward tendency of relative sea level the upward growth of a tidal mudflat marsh within the tidal frame is determined theoretically by the rates of minerogenic sediment supply organogenic sediment supply long range sediment compaction and movement of relative sea level where the movement of relative sea level is upward a marsh can achieve a state of dynamic equilibrium in which the composition of the sediment accumulating on its surface is governed by the balance between the rates of organogenic supply and sea level change a peat organogenic marsh should arise where the former rate roughly equals or exceeds the latter a minerogenic marsh should form where the rate of sea level rise is dominant even though the absolute availability of mineral fine sediment may be so high as to appear to suggest that build up to extreme tidal levels where vegetation would dominate should have occurred

mapping and modeling of radial fracture patterns on venus synthetic aperture radar images indicate numerous radial fracture systems on venus the formation of these systems has been attributed either to surface deformation related to subsurface dike emplacement or to fracturing caused by domical uplift above an ascending magma plume we evaluate the dike emplacement hypothesis by investigating the relationship between the process of subsurface dike emplacement and the geometric attributes of radial fracture patterns assuming the surface fracture patterns correspond to the patterns of dikes about a magma chamber below the volcanic summit we select a radial fracture system for mapping and model its formation using a two dimensional analysis which idealizes the chamber as a pressurized hole in an elastic plate subject to a remote biaxial stress field radial fractures correspond to maximum principal stress trajectories and the pattern of these trajectories is controlled primarily by four variables the chamber radius the magma pressure the remote differential stress and the remote principal stress direction we find that stress trajectories also depend on the remote mean stress and that the location of isotropic points in the trajectory pattern can be used to interpret the mean stress state these models are used to estimate the remote loading conditions under which dikes were emplaced the mapped fracture pattern is consistent with dikes emplaced under remote differential stress conditions ranging from 2 to 8 mpa calculated over a range of possible emplacement depths from 1 to 9 km this is consistent with regional stress states on earth

vertical salt marsh accretion since the roman period in the severn estuary southwest britain abstract using three methods of measurement and averaging over the whole estuary the vertical sediment accretion on mature clastic salt marshes amounts to 1 22 m since the roman period ca a d 150 1 05 m since the medieval period ca a d 1250 0 54 m since ca a d 1845 and 0 21 m since ca a d 1945 these data are cautiously interpreted to mean that the trend of relative sea level in the severn estuary is continuing upward shorter term negative tendencies are not excluded and that the current rate of rise could be a few millimetres per year the measured sediment thicknesses cannot be equated directly with relative movements of sea level however because of effects related mainly to 1 changes in the tidal range with general deepening 2 the possible transgression up the severn vale of the estuary as a whole and 3 the possible raising of tidal levels due to wetland reclamation after estimating for all these factors the true relative rise since the roman period could have amounted to 1 6 1 7 m

prompt transgression and gradual salinisation of the black sea during the early holocene constrained by amino acid racemization and radiocarbon dating abstract the restricted environment of the black sea is particularly sensitive to climatic and oceanographic fluctuations owing to its connection with the mediterranean sea via the narrow bosphorus strait the exact mechanism and timing of the most recent connection between these water bodies is controversial with debate on the post glacial history of the black sea being dependent on radiocarbon dating for numerical ages here we present new 23 accelerator mass spectrometer ams radiocarbon ages on peat and bivalve molluscs supported by the first amino acid racemization aar dating of bivalve molluscs n 66 in the black sea these data indicate infilling of the black sea during the early holocene from an initial depth 107 m below sea level and 72 m below that of the bosphorus sill these data combined with a review of previous radiocarbon ages has enabled a unique perspective on the post glacial black sea a sea level curve based on conventional and ams radiocarbon ages on peat and ams based ages on dreissena sp shells indicate the water level in the earlier lake phase continued until the early holocene to be lower than the bosphorus sill after the younger dryas ended however the absence of ams dated mollusc ages from the shelves of this basin older than the younger dryas is suggestive of sub aerial exposure of the shelves and comparatively lower water levels when the younger dryas began thus post glacial outflow from the black sea occurred through a lowered or open bosphorus seaway basin wide radiocarbon ages on peat indicate a prompt increase in water level from that of the pre existing and unconnected palaeo lake during the earliest holocene 9600 9200 cal a bp mass colonisation of the black sea by mediterranean taxa did not occur until salinity had risen sufficiently a process which took 1000 a or more from the initial transgressive event this gradual change in salinity contrasts with the prompt transgression which would have taken 400 a to occur

tectonic patterns and regional stresses near venusian coronae abstract many venusian coronae exhibit distinctive patterns of faults in particular we find the tectonic patterns of one such corona similar to the striking patterns of the grabens surrounding alba patera on mars pursuing this similarity we perform a stress analysis analogous to one that successfully explained the alba patera pattern and in so doing determined a value for the local regional stress local stress fields of venusian coronae are assumed to be created by either a gaussian axially symmetric upwarp or a cylindrical load due to accumulation of volcanic materials on an elastic lithosphere the local coronal stresses are combined with varying amounts of a uniform regional stress field for different elastic lithospheric thicknesses from the magnitudes and orientations of the resultant principal stresses the predicted patterns of surface tectonic features i e graben and thrust fault chains are plotted these patterns are compared to those of coronae on magellan images matching the patterns thus determines the regional stress and elastic lithospheric thickness about the coronae comparison of predicted to observed tectonic patterns for three specific coronae leads to estimated regional stresses of approximately 0 1 0 6 kbar and effective elastic lithospheric thicknesses consistent with 10 5 km although one area may have had a lithosphere as thin as 2 km during corona formation

the timing of giant radiating dike swarm emplacement on venus implications for resurfacing of the planet and its subsequent evolution stratigraphic study of a distributed population of 118 giant radiating dike swarms on venus reveals that within each dike intruded region emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera regional plains and most wrinkle ridges the density of impact craters superimposed on the swarm population 1 80 0 57 craters 10 6 km 2 when compared with the average global density 2 01 0 14 craters 10 6 km 2 and the densities reported for other geologic units is consistent with the observed stratigraphy on the basis of these data we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing the stresses recorded by the dike swarm population when combined with its age and compared with the predictions of several proposed resurfacing models best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long wavelength gravity and topography in addition the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long wavelength characteristics we interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago

surface deformation and shallow dike intrusion processes at inyo craters long valley california the inyo craters are the two largest of four phreatic craters that lie within a 2 5 km long 500 to 700 m wide n s trend of faults and fissures at the south end of the inyo volcanic chain in eastern california the alignment of these features with dike fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650 550 yr b p e w extension south of south inyo crater is ten to several tens of meters suggesting that the dike is at least that thick to understand how the faults and fissures developed we mapped and studied the fault and fissure pattern used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium and conducted physical model experiments of fault and fissure growth results of the field studies and experiments indicate that deformation develops according to the following sequence 1 extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike 2 fissures form along two parallel trends at the surface on opposite sides of the dike plane leaving a relatively unfractured region in between 3 dip slip movement on subsurface fractures and linkage of these fractures with surface fissures produces inward facing normal fault scarps that bound a nested graben above the dike experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium this fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface if the ratio between the depth to the dike top and the distance between the outermost surface fissures at inyo craters is within the range of ratios measured experimentally then the depth to the dike top is between 250 m and several times that a slant hole drilled in the summer of 1987 by the department of energy office of basic energy sciences intersected three breccia bodies of possibly intrusive origin directly below the center of south inyo crater between 550 m and 610 m below the crater rim no intact juvenile igneous rock was intersected suggesting that the hole passed above the magma filled dike top perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results

flandrian sea level changes in the fenland ii tendencies of sea level movement altitudinal changes and local and regional factors stratigraphic micropalaeontologic and radiocarbon data show that since c 6500 bp the fenland has been influenced by 7 periods of positive sea level tendencies and by 6 periods of negative sea level tendencies despite the numerous problems associated with the reconstruction of past altitudes of sea level the periods of positive sea level tendencies were clearly characterised by a rise in sea level the development of transgressive overlaps and a landward movement of the coastline the periods of negative sea level tendencies were characterised by the development of regressive overlaps a seaward movement of the coastline and a reduced or negative rate of sea level rise the various altitudinal errors do not permit the incontrovertible distinction of periods of falling sea levels all changes within the fenland were not synchronous and local factors influenced the exact nature of each sea level indicator dominant regional and local factors have been identified for different areas and different time periods the chronological and spatial characteristics of the sequences within the fenland are best explained by a palaeocoastline without supratidal barriers controlling sedimentation the data indicate an average crustal subsidence in the fenland of 0 9m 1000 years since c 6500 bp and the pattern of positive and negative tendencies of sea level movement is also seen in the chronologies for north west england and north east scotland

holocene coastal environmental changes and human occupation of the lower herault river southern france abstract sea level rise human impacts and climate change have deeply affected coastal environments during the holocene these forcing factors are studied using the lower herault valley which constitutes a very representative mediterranean case study because of i its very early intense and continuous land use since neolithic times and ii its sensitivity to sea level rise and mediterranean climate changes over a relatively small watershed 34 cores and 61 ams radiocarbon dates associated with biological and geochemical analyses have allowed us to precisely reconstruct the holocene evolution of the lower valley until 6500 cal yr bp a wave dominated morphology and retrogradational dynamics were reconstructed during this phase ephemeral channels and successive river mouths formed and were rapidly submerged by sea level rise the progradational phase began after 6500 cal yr bp and the alluvial plain gradually built seawards with the formation of a beachridge system outside the valley growth of the fertile alluvial plain was coeval with the development of neolithic agriculture this alluvial progradation gradually filled the estuary with advances of the mouths several shallow lagoons and sandbar the high density of information collected allows us to recognize for the first time a pronounced fluvial dominated deltaic morphology especially 3000 years ago during the bronze age lagoonal and coastal shores were continually inhabited human land use continually adapted to geomorphological and environmental changes around 300 years ago the delta shifted to a wave dominated system