the unpredictable nature of internal tides on continental shelves abstractpackets of nonlinear internal waves nliws in a small area of the mid atlantic bight were 10 times more energetic during a local neap tide than during the preceding spring tide this counterintuitive result cannot be explained if the waves are generated near the shelf break by the local barotropic tide since changes in shelfbreak stratification explain only a small fraction of the variability in barotropic to baroclinic conversion instead this study suggests that the occurrence of strong nliws was caused by the shoaling of distantly generated internal tides with amplitudes that are uncorrelated with the local spring neap cycle an extensive set of moored observations show that nliws are correlated with the internal tide but uncorrelated with barotropic tide using harmonic analysis of a 40 day record this study associates steady phase motions at the shelf break with waves generated by the local barotropic tide and variable phase motions with the shoaling of distantly generated internal tides

reflection of linear internal tides from realistic topography the tasman continental slope abstractthe reflection of a low mode internal tide on the tasman continental slope is investigated using simulations of realistic and simplified topographies the slope is supercritical to the internal tide which should predict a large fraction of the energy reflected however the response to the slope is complicated by a number of factors the incoming beam is confined laterally it impacts the slope at an angle there is a roughly cylindrical rise directly offshore of the slope and a leaky slope mode wave is excited these effects are isolated in simulations that simplify the topography to separate the incident from the reflected signal a response without the reflector is subtracted from the total response to arrive at a reflected signal the real slope reflects approximately 65 of the mode 1 internal tide as mode 1 less than two dimensional linear calculations predict because of the three dimensional concavity of the topography it is also less than recent glider estimates likely as a result o

modeling semidiurnal internal tide variability in the southern california bight abstractthe regional oceanic modeling system roms is applied in a nested configuration with realistic forcing to the southern california bight scb to analyze the variability in semidiurnal internal wave generation and propagation the scb has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency the model predicts the observed barotropic and baroclinic tides reasonably well although the observed baroclinic tides feature slightly larger amplitudes the strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900 m deep santa cruz basin this causes a forced near resonant semidiurnal poincare wave that rotates clockwise in the basin and is of the first mode along the radial azimuthal and vertical directions the associated tidal mean depth integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kw m 1 and 100 m and are strongly modulated by the sprin

impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

the geography of semidiurnal mode 1 internal tide energy loss 1 the semidiurnal mode 1 internal tide receives 0 1 0 3 tw from the surface tide and is capable of propagating across ocean basins the ultimate fate of mode 1 energy after long distance propagation is poorly constrained by existing observations and numerical simulations here global results from a two dimensional semi analytical model indicate that topographic scattering is inefficient at most locations deeper than 2500 m next results from a one dimensional linear model with realistic topography and stratification create a map of mode 1 scattering coefficients along the continental margins on average mode 1 internal tides lose about 60 of their energy upon impacting the continental margins 20 transmits onto the continental shelf 40 scatters to higher modes and 40 reflects back to the ocean interior these analyses indicate that the majority of mode 1 energy is likely lost at large topographic features e g continental slopes seamounts and mid ocean ridges where it may drive elevated turbulent mixing

the timing of martian valley network activity constraints from buffered crater counting abstract valley networks concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity have been cited as indicators that the climate of mars differed significantly in the past from the present hyperarid cold desert conditions some researchers suggest that the change in climate was abrupt while others favor a much more gradual transition thus the precise timing of valley network formation is critical to understanding the climate history on mars we examine thirty valley network incised regions on mars including both cratered upland valley networks and those outside the uplands and apply a buffered crater counting technique to directly constrain when valley network formation occurred the crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units from these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age 1 valley network activity in the cratered highlands has an average cessation age at the noachian hesperian boundary and all valleys that we crater counted are early hesperian or older no evidence is found for valley networks in the cratered uplands of late hesperian or amazonian age the timing of the cessation of cratered upland valley network activity at the noachian hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate type weathering 2 a few valley network incised regions formed outside of the cratered uplands on volcanic edifices in association with younger impact craters and on the rim of valles marineris we applied our buffered crater counting technique to four such valleys on the volcanoes ceraunius tholus hecates tholus and alba patera and on the rim of echus chasma and find that each has distinctive and different late hesperian or early amazonian ages indicating that valley networks formed from time to time in the post noachian period unlike the cratered upland valley networks these isolated occurrences are very local and have been interpreted to represent local conditions e g snowpack melted during periods of intrusive volcanic activity in contrast to a gradual cessation in the formation of valley networks proposed by some workers our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the noachian followed only by episodic and very localized valley network formation in later mars history very likely due to specific conditions e g local magmatic heating these valley network ages and correlations are thus consistent with a major change in the near surface aqueous environment on mars at approximately the noachian hesperian boundary the noachian environment supported surface running water and fluvial erosion across mars in the cratered uplands enhanced crater degradation and a weathering environment favoring the formation of phyllosilicates the hesperian amazonian environment was more similar to the hyperarid cold desert of today with valley networks forming only extremely rarely and confined to localized special conditions sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin axis orbital parameters and to periodic catastrophic emergence of groundwater

a study of colour changes in purple green slate by petrological and rock magnetic methods abstract the purple cambrian slates of north wales show colour changes to green in bedding parallel patches and ellipsoidal spots these are referred to as reduction spots but the process is more complex the colour change is not simply associated with the reduction of iron the green discoloration is accompanied by diffusion of fe 3 away from and possibly by mg and ca towards the green material the actual colour change is brought about by the replacement of the fine metamorphic hematite pigment with magnetite however coarser clastic hematite persists in the green slate the green slate has twice as much chlorite and traces of magnetite despite having less total fe the additional chlorite is chiefly responsible for the doubling of the magnetic susceptibility of the green slate with respect to the purple slate chlorite and epidote have the same compositions in both green and purple slates despite the difference in whole rock fe content both purple and green slates behave predominantly as paramagnets between 194 c and 700 c the anisotropy of low field susceptibility magnetic fabric is increased by heating and therefore made easier to measure and the principal directions of anisotropy are preserved heating to 700 c increases the saturation isothermal remanence dramatically and increases its anisotropy due to the growth of single domain magnetite however heating is not universally recommended as a means of enhancing otherwise feeble magnetic fabrics some other slates and schists show regressive changes in magnetic fabric on heating with erratic changes in principal directions

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

east west asymmetry in nonlinear internal waves from luzon strait the nonhydrostatic regional ocean modeling system is applied to study the effects of thermocline shoaling deepening bathymetry and asymmetric modulated tides on the soliton growth to the west and east of luzon strait in the south china sea and western pacific ocean luzon strait comprises a shallow east ridge and a deep west ridge and its interaction with barotropic tidal currents yields strong westward internal tides that disperse into solitons satellite imagery indicates that the westward solitons are more numerous and better defined than the eastward solitons the model results show that the eastward solitons are 45 39 28 and 23 smaller than the westward solitons due to asymmetric modulated barotropic tides at the east ridge a deeper pacific ocean westward thermocline shoaling related to the kuroshio current and internal tide resonance in a double ridge configuration respectively due to the westward location of the kuroshio little thermocline deepening occurs east of the east ridge hence the influence of thermocline deepening on counteracting eastward soliton growth is small the kuroshio mainly enhances westward soliton growth the dispersion of internal tides into solitons is governed by the balance between the nonlinearity parameter on the one hand and the nonhydrostatic and coriolis dispersions on the other it is shown that this balance favors soliton growth for thermocline shoaling while it counters it for a deeper ocean a series of double ridge experiments is performed in which the distance between the ridges and the height of the west ridge are varied for a semidiurnal tidal forcing and two gaussian ridges separated by 100 km barotropic to baroclinic energy conversion is enhanced at both ridges causing larger westward internal tides and solitons the combination of coriolis forcing thermocline shoaling and a double ridge configuration enhances the distinctiveness of the so called type a and b solitons when a modulated tide occurs

meteorological and internal wave forcing of seiches along the sri lanka coast regularly observed seiches from tide gauge records around the sri lanka coast have been analysed to determine the forcing mechanisms the seiche periods range from a few tens of minutes to 2 hr fortnightly and seasonal variations of seiche amplitudes are clearly visible on the east coast these seiche amplitudes are particularly large approximately 6 8 days after spring tides with maximum seiche amplitudes observed during march april and october november suggesting that the seiche variation could be related to stratification and internal wave activity there is no visible fortnightly and seasonal variation of seiche amplitudes in west coast records instead they show a daily pattern with relatively larger seiches around 10 00 hrs local time suggesting that the seiches on the west coast could result from diurnal atmospheric forcing barotropic and 2 layer models have been developed to investigate the influence of atmospheric forcing and internal wave activity on the seiches a barotropic model applied to the west coast shows that the daily seiche amplitude variation could be simulated with cyclic diurnal meteorological forcing two layer model runs for trincomalee bay and the adjacent east coast shelf suggest that seiche amplitudes are proportional to vertical stratification and mixed layer depths therefore the observed seiches there could be excited by internal waves which originate as far away as the andaman sea during spring tides and have a travel time of 6 8 days to the sri lankan east coast however further studies including direct measurements of internal wave activity within the region are required to confirm this hypothesis

energetics of m2 barotropic to baroclinic tidal conversion at the hawaiian islands abstract a high resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide m2 over a region of the hawaiian ridge from niihau to maui this region includes the kaena ridge one of the three main internal tide generation sites along the hawaiian ridge and the main study site of the hawaii ocean mixing experiment the 0 01 horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations moored adcp currents and notably reasonable agreement with microstructure data barotropic and baroclinic energy equations are derived from the model s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget the m2 barotropic tide loses 2 7 gw of energy over the study region of this 163 mw 6 is dissipated by bottom friction and 2 3 gw 85 is converted into internal tides internal tide generation primarily occurs along the flanks of the kaena ridge and

three dimensional double ridge internal tide resonance in luzon strait abstractthe three dimensional 3d double ridge internal tide interference in the luzon strait in the south china sea is examined by comparing 3d and two dimensional 2d realistic simulations both the 3d simulations and observations indicate the presence of 3d first mode semi diurnal standing waves in the 3 6 km deep trench in the strait as in an earlier 2d study barotropic to baroclinic energy conversion flux divergence and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides the resonance in the 3d simulation is several times stronger than in the 2d simulations for the central strait idealized experiments indicate that in addition to ridge height the resonance is only a function of separation distance and not of the along ridge length that is the enhanced resonance in 3d is not caused by 3d standing waves or basin modes instead the difference in resonance between the 2d and 3d simulations is attributed to the topographic blocking

evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the main central thrust zone garhwal himalaya india inferences from finite strain and ams analyses abstract the footwall of the main central thrust mct zone along the bhagirathi valley comprises a wide zone of mylonitic quartzite and deep level tectonites the systematic variation of finite strain parameters es k and v in the mylonites indicates heterogeneous deformation which is determined to vary between simple shear and non coaxial flattening type in such a strain regime the outer boundary of the quartz clasts are no longer preserved thus leading to an error in finite strain measurement in order to supplement the finite strain studies anisotropy of magnetic susceptibility ams analyses were carried out on the mylonitic quartzites a systematic variation in degree of anisotropy p with distance from the mct is documented and is interpreted to be tectonic in origin based on these results it is concluded that p can be used as a strain intensity gauge at least on an outcrop scale where a systematic variation in p values from one part of the outcrop to the other can be established however the quantitative relation between principal axes of finite strain ellipsoid and ams axes magnitude of principal susceptibility difference k1 and k3 and finite strain magnitude e1 ln 1 e1 and e3 ln 1 e3 were related by a logarithmic relationship with a correlation coefficient of 0 844

crater associated dark diffuse features on venus properties of surficial deposits and their evolution 1 in order to assess the nature of crater associated radar dark diffuse features ddfs on venus and to understand their formation and evolution we analyzed magellan radar roughness emissivity and reflectivity data in the vicinity of craters accompanied by these features following others we assumed that ddfs are deposits mantles of ejected material emplaced during formation of the impact crater the majority of radar dark parabolas the youngest ddfs are characterized by a smooth mantle atmosphere interface having low root mean square rms slopes on scales of 1 100 m as derived from magellan altimeter data older ddfs also often have areas with low rms slopes suggesting that the mantle rms slopes can be preserved for geologically long periods of time some parabolas and older ddfs have asymmetric small scale decimeter scale relief that is interpreted to be dunes that formed as a result of eolian processes this implies that the mantle material is mobile and can saltate under the influence of wind action on average aging of these features is accompanied by a decrease of mantle material dielectric permittivity the most efficient mechanism for parabola degradation seems to be the removal of mantle material from the site of initial deposition by subsequent winds we found a few examples of features that could be interpreted to be the result of in situ modification of the primary mantle

venus crater distributions at low northern latitudes and in the southern hemisphere from new arecibo observations arecibo high resolution radar data covering approximately 23 of the surface of venus was analyzed for circular features with crater like appearances overlapping venera 15 16 radar data with its more normal to the surface viewing geometry was used to verity crater diameters measured from the arecibo data a comparison of the diameter frequency distribution of the possible impact features in the arecibo data for northern latitudes south of 40 n and for the southern hemisphere with the distribution from the venera 15 16 data for the northernmost quarter of the planet indicated similar distributions for large crater like features with diameters approximately 30 km but a deficiency of smaller craters in the arecibo data the average crater density measured for all craters in the northernmost quarter of the planet from the venera 15 16 data was 1 27 per 106 km2 while the average for the 106 l06km2 covered by the arecibo data was 0 95 per 106 km2 for craters with diameters less than 22km the densities were 0 59 per million square kilometers for the venera data compared with 0 33 per 106 km2 for the average of the two arecibo data regions all circular features in the arecibo data with a crater like appearance that could not be excluded as being impact features were used in this study so the result represents an upper limit on the crater densities in the size interval considered one possible process responsible for the density differences of small craters is a difference in the rate of volcanic resurfacing an increase in the resurfacing rate of approximately 50 would explain the lower density of small craters in the area covered by the arecibo data compared with the venera data but would not account for the differences being confined to the 30 km crater diameter range

phyllosilicate fabric characterization by low temperature anisotropy of magnetic susceptibility lt ams 1 this paper examines the use and applicability of anisotropy of magnetic susceptibility at low temperature 77 k called lt ams paramagnetic susceptibility increases significantly with low temperature which permits high precision determination of the anisotropy of paramagnetic susceptibility of a rock from measurements at liquid nitrogen temperature we describe a new procedure that measures directional susceptibility in an oriented rock specimen while immersed in liquid nitrogen our procedure overcomes the problem of temperature heterogeneity within a rock sample while its susceptibility is being measured offering the opportunity to measure low temperature susceptibility in multiple orientations this non destructive method provides a way of characterizing the principal paramagnetic minerals present in a rock based on temperature induced changes of the anisotropy degree moreover the method offers a better resolved relationship between anisotropy degree and grain alignment which is used to quantify rock fabrics

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

internal tidal mixing as a control on continental margin ecosystems we show that a breaking internal tide at a shelf edge is a fundamental control on the structural and functional properties of ecosystems contrasts in vertical mixing of nitrate between the shelf and the open ocean correspond with horizontal and vertical changes in phytoplankton communities with largest cells found in surface waters at the shelf edge intense fishing activity is commonly seen at continental shelf edges targeting spawning fish stocks we suggest that the internal tide a globally ubiquitous physical process at steep shelf edge bathymetry supports shelf edge fisheries by providing large celled phytoplankton for first feeding fish larvae the repeatability of the internal tide removes fish from the need to time spawning with a spring bloom also with large phytoplankton cells dominating particulate organic carbon export the internal tides could be an important influence on spatial and temporal variability in patterns of global carbon sequestration in deep water and sediments citation sharples j c m moore a e hickman p m holligan j f tweddle m r palmer and j h simpson 2009 internal tidal mixing as a control on continental margin ecosystems geophys res lett 36 l23603 doi 10 1029 2009gl040683

a new view of the stratigraphic history of venus abstract studies of venus using magellan data have so far generated two views about the way venus has evolved on the one hand venus has been suggested to have had a history in which there was a series of epochs each represented by a different volcanic or tectonic process on a global scale basilevsky and head 1995 j w head et al 1996 lunar planet sci 27th 525 526 p t basilevsky et al 1997 in venus ii univ of arizona press tucson this we define as a directional history on the other hand there is evidence to suggest that coronae rifts wrinkle ridges small and large edifices and large flow fields have each formed throughout the portion of venus history revealed by presently exposed rock units we propose that the plains have been built up by lavas erupted in a number of different styles each occurring throughout the history represented by the exposed stratigraphy of the planet dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae e g m h price et al 1996 j geophys res 101 4657 4672 must be interpreted with care as the method is based upon the assumption that features of like morphology have the same age detailed studies from full resolution magellan data indicate that venus has had a complex history in which most geologic processes have operated in a nondirectional fashion to a greater or lesser extent throughout the planet s history

variation of magnetic properties in sedimentary rocks hosting the foum zguid dyke southern morocco combined effects of re crystallization and fe metasomatism the effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood therefore we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the foum zguid dyke in southern morocco the study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments the present study shows that 1 close to foum zguid dykes the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re crystallization and fe metasomatism intensity 2 the thermal experiments on ams of samples collected farther from the dyke and thus less affected by heating during dyke emplacement indicate that 300 400 c is the minimum experimental temperature necessary to trigger appreciable transformations of the pre existing magnetic fabrics for temperatures higher than ca 580 c the magnetic fabric transformations are fully realized with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke therefore measured variations of the magnetic fabric can be used to evaluate re crystallization temperatures experienced by the host sedimentary rock during dyke emplacement the distinct magnetic behaviour observed along the cross sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 c after dyke emplacement 3 ams data shows a gradual variation of the magnetic fabric with distance from the dyke margin from sub horizontal k3 away from the dyke to vertical k3 close to the dyke experimental heating shows that heat alone can be responsible for this strong variation therefore such orientation changes should not be unequivocally interpreted as the result of a stress field resulting from the emplacement of the dyke for instance 4 magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations thermally and chemically induced by dyke intrusion in hosting sediments

catastrophic resurfacing and episodic subduction on venus abstract there is clear evidence that a catastrophic resurfacing event occurred on venus in the relatively recent past a primary data source for this resurfacing event is the spatial distribution of impact craters in this paper we apply the pair correlation technique to the observed crater distribution and find that the result is identical to that for a random distribution in order to test the sensitivity of the technique we also apply it to the spatial distribution of coronae on venus for the coronae we find substantial deviations from a random distribution one explanation for the catastrophic resurfacing is the episodic subduction hypothesis we model episodic subduction using a thermal boundary layer stability analysis we find that episodic subduction events with intervals of 500 to 700 myr can transport only 15 25 of the radiogenic heat produced within the planet we suggest that the remainder of the heat must be lost to the surface during a period of vigorous tectonic activity following the subduction event but prior to the subsequent stabilization of a global lithosphere

evidence for rapid regional plains emplacement on venus from the population of volcanically embayed impact craters we have reexamined the population of craters on venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater presenting nonvolcanic processes which may produce similar crater morphologies we conclude that 29 craters have been embayed by lava flows from the exterior and an additional 27 craters are ambiguous cases 3 6 of the total crater population all of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges through geologic mapping of an area covering over 8 of the surface we examined the spatial relationships between wrinkle ridged plains and stratigraphically older plains units and estimate that a large portion of the wrinkle ridged plains are thinner than 500 m which is thin enough for the rims of the largest half of preexisting craters to be preserved there are not enough craters embayed by the wrinkle ridged plains to support a model in which the plains were erupted directly onto an ancient surface given the number of large craters embayed by thin deposits of wrinkle ridged plains we develop a simple statistical model to overestimate the number of craters on the preplains surface this model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years the short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle ridged plains

investigation of the generation and propagation of low frequency internal waves a case study for the east coast of india sar synthetic aperture radar images in the northwest bay of bengal indicate the existence of internal waves and their occurrence and intensity is topography dependent as indicated by in situ data and satellite information to complement and comprehend the observations a three dimensional princeton ocean model is applied to study the generation and propagation of internal waves the model domain is configured with a variable curvilinear grid and the input fields comprise bathymetry initial temperature and salinity wind stress air sea heat flux and tidal information the numerical investigation indicated a predominant activity of internal waves in the north and the rationale is three fold the first one could be the stable stratification due to fresh water discharge from head bay major river system secondly the significant magnitude range of the tides and finally the bathymetry in the coastal waters off paradip is about 12 shallower compared to that of visakhapatnam and further south the cumulative effect of these causes the predominance of internal waves in the north the core of the energy is essentially in the low frequency range and the model is able to simulate semi diurnal and diurnal components reasonably well up to 6 h frequency 0 162 cph

global patterns of low mode internal wave propagation part i energy and energy flux abstract extending an earlier attempt to understand long range propagation of the global internal wave field the energy e and horizontal energy flux f are computed for the two gravest baroclinic modes at 80 historical moorings around the globe with bandpass filtering the calculation is performed for the semidiurnal band emphasizing m2 internal tides generated by flow over sloping topography and for the near inertial band emphasizing wind generated waves near the coriolis frequency the time dependence of semidiurnal e and f is first examined at six locations north of the hawaiian ridge e and f typically rise and fall together and can vary by over an order of magnitude at each site this variability typically has a strong spring neap component in addition to longer time scales the observed spring tides at sites northwest of the hawaiian ridge are coherent with barotropic forcing at the ridge but lagged by times consistent with travel at the theoretical mode 1 group speed from the ridge phase c

systematic bias in baroclinic energy estimates in shelf seas abstracta simple model of an internal wave advected by an oscillating barotropic flow suggests flaws in standard approaches to estimating properties of the internal tide when the m2 barotropic tidal current amplitude is of similar size to the phase speed of the m2 baroclinic tide spectral and harmonic analysis techniques lead to erroneous estimates of the amplitude phase and energy in the m2 internal tide in general harmonic fits and bandpass or low pass filters that attempt to isolate the lowest m2 harmonic significantly underestimate the strength of m2 baroclinic energy fluxes in shelf seas baroclinic energy flux estimates may show artificial spatial variability giving the illusion of sources and sinks of energy where none are actually present analysis of previously published estimates of baroclinic energy fluxes in the celtic sea suggests this mechanism may lead to values being 25 60 too low

tides in the bay of bengal the amplitudes and phases of the tidal constituents m2 s2 k1 and o1 in the bay of bengal are deduced from tide gauge records at 112 locations with the aid of numerical models based on the vertically integrated equations of motion and continuity tidal elevations along the southern open boundary of the bay were obtained by interpolation from observations the results are presented in the form of cotidal charts these show that the constituents examined have no amphidromic points in the bay of bengal apart from a degenerate amphidrome for m2 in the northwest part of the bay

structure of the baroclinic tide generated at kaena ridge hawaii repeat transects of full depth density and velocity are used to quantify generation and radiation of the semidiurnal internal tide from kaena ridge hawaii a 20 km long transect was sampled every 3 h using expendable current profilers and the absolute velocity profiler phase and amplitude of the baroclinic velocity pressure and vertical displacement were computed as was the energy flux large barotropically induced isopycnal heaving and strong baroclinic energy flux divergence are observed on the steep flanks of the ridge where upward and downward beams radiate off ridge directly above kaena ridge strong kinetic energy density and weak net energy flux are argued to be a horizontally standing wave the phasing of velocity and vertical displacements is consistent with this interpretation results compare favorably with the merrifield and holloway model

mapping the nonstationary internal tide with satellite altimetry temporal variability of the internal tide has been inferred from the 23 year long combined records of the topex poseidon jason 1 and jason 2 satellite altimeters by combining harmonic analysis with an analysis of along track wavenumber spectra of sea surface height ssh conventional harmonic analysis is first applied to estimate and remove the stationary components of the tide at each point along the reference ground tracks the wavenumber spectrum of the residual ssh is then computed and the variance in a neighborhood around the wavenumber of the mode 1 baroclinic m2 tide is interpreted as the sum of noise broadband nontidal processes and the nonstationary tide at many sites a bump in the spectrum associated with the internal tide is noted and an empirical model for the noise and nontidal processes is used to estimate the nonstationary semidiurnal tidal variance the results indicate a spatially inhomogeneous pattern of tidal variability nonstationary tides are larger than stationary tides throughout much of the equatorial pacific and indian oceans

ejecta correlations with spatial crater density and venus resurfacing history we examine the alternative hypotheses that on venus i the spatial randomness of the crater population reflects predominantly the spatially stochastic nature of impact crater formation or ii a significant amount of spatial variation in crater density is due to variations in the geological rate of crater removal three endmember models of resurfacing are formulated case i assumes that a single age production surface accounts for the spatial distribution of craters case ii assumes that variations in spatial crater density reflect local production surface ages while case iii assumes that individual craters are removed regionally so that variations in spatial crater density reflect local retention ages these endmember models are tested by examining correlations of crater properties with crater spatial density we analyze the distribution of embayed and tectonized craters and the relative abundances of extended ejecta deposits termed halos the three endmembers provide specific predictions as to the occurrence of halos as a function of for 1 5 craters per 106 km case iii provides the best explanation for the deficiency of halos as well as the relative abundances of modified craters for the interval 1 5 2 5 case ii is applicable but with only a modest spread in production ages for densities 2 5 case ii is the only endmember model that matches the global observations suggesting that regions with these high densities represent a series of distinct production surfaces with ages increasing from the planetary mean retention age

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

the role of internal tides in mixing the deep ocean abstract internal wave theory is used to examine the generation radiation and energy dissipation of internal tides in the deep ocean estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth varying stratification and two dimensional topography specific estimates of energy flux are made for midocean ridge topography weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km the richardson number of the radiated internal tide typically exceeds unity for these motions and so direct shear instability of the generated waves is not the dominant energy transfer mechanism it also seems that wave wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux instead it appears that most of

three dimensional numerical simulation of m2 internal tides in the east china sea the east china sea and adjacent seas are one of the most significant generation regions of the m 2 internal tide in the world s oceans in the present study we investigate the distribution and energetics of the m 2 internal tide around the continental shelf edge in the east china sea using a three dimensional numerical model the numerical experiment shows that m 2 internal tides are effectively generated over prominent topographic features such as the subsurface ridges in the bashi luzon and tokara straits the ridges along the ryukyu island chain and the continental shelf slope in the east china sea the former particularly so all of these topographic features are characterized by steep slopes at the depth of the thermocline onto which the m 2 barotropic tide is almost normally incident the m 2 internal tides propagating away from these multiple source regions interfere with each other to create a complicated wave pattern it is found that the calculated pattern of the m 2 internal tide agrees well with topex poseidon altimeter observations the conversion rate from m 2 barotropic to baroclinic energy over the whole analyzed model domain is estimated to be 35 gw roughly 10 of the energy in the m 2 surface tide incident on the prominent topographic features is converted to the m 2 internal tide although about half of the m 2 internal tidal energy is subject to local dissipation in close proximity to the generation sites

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

impact crater air fall deposits on the surface of venus areal distribution estimated thickness recognition in surface panoramas and implications for provenance of sampled surface materials 1 during their formation impact craters on venus larger than 11 km in diameter acquired parabolic features composed of air fall deposits that are radar dark in magellan synthetic aperture radar images a model of the parabola planimetric shape and size the latter being a function of crater diameter was constructed from the data of campbell et al 1992 reconstructing model parabolas around all impact craters with d 11 km in the regions of the venera vega sites revealed that parabolas cover 80 90 of the surface these regions are representative of the majority of the venus surface in terms of general geology and impact crater abundance this implies that a very significant part of the surface of venus was covered at least once with parabola deposits and thus it is logical to look for them in the tv panoramas of the venus surface taken by the venera landers these panoramas show the presence of soil and layered rocks with the rocks shown by the data of several instruments consisting of porous mechanically weak material implying an altered or sedimentary in a broad sense including volcanic tuffs and impact deposits nature we propose the hypothesis that 1 the layered rocks may be the partly lithified and then partially eroded parabola deposits while 2 the soil may partly be unlithified crater deposits and partly a product of the degradation of layered rocks if this hypothesis is correct then the interpretations related to the provenance of the materials studied geochemically by the landers may need revision in particular it is very possible that deep seated 1 3 km rocks the lower units of the regional plains complex rocks of the plains basement and plutonic rocks are a part perhaps significant of the material analyzed by the landers the possible presence of this deeply derived material should also be taken into account in selecting landing sites for future missions to venus

semidiurnal internal tide incoherence in the equatorial pacific the jets in the equatorial pacific ocean of a realistically forced global circulation model with a horizontal resolution of 1 12 5 cause a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the french polynesian islands and hawaii this loss of coherence is quantified with a baroclinic energy analysis in which the semidiurnal band terms are separated into coherent incoherent and cross terms for time scales longer than a year the coherent energy flux approaches zero values at the equator while the total flux is 500 w m the time variability of the incoherent energy flux is compared with the internal tide travel time variability which is based on along beam integrated phase speeds computed with the taylor goldstein equation the variability of monthly mean taylor goldstein phase speeds agrees well with the phase speed variability inferred from steric sea surface height phases extracted with a plane wave fit technique on monthly time scales the loss of phase coherence in the equatorward beams from the french polynesian islands is attributed to the time variability in the vertically sheared background flow associated with the jets and tropical instability waves on an annual time scale the effect of stratification variability is of equal or greater importance than the shear variability is to the loss of coherence in the model simulations low frequency equatorial jets do not noticeably enhance the dissipation of the internal tide but merely decohere and scatter it

impact craters on regional plains on venus age relations with wrinkle ridges and implications for the geological evolution of venus 1 a global assessment of impact craters on venus 30 km in diameter shows that 146 craters are completely or partially superposed on the regional plains that dominate the surface and typically are deformed by wrinkle ridges in this study we found that 10 of these craters are deformed by wrinkle ridges pre wr 93 craters are superposed on wrinkle ridges post wr and 43 craters were found to be ambiguous cases we compared our findings with the results of a similar study by mcgill 2004 done for craters of all sizes but for 1 3 of the surface of venus we found that the percentage of pre wr craters found by mcgill 2004 among the unambiguous cases 15 is close to our percentage 10 but we are in disagreement on the classification of a significant number of specific craters and in this analysis we describe the major reason for the disagreements we then summarize published data on the crater densities for different plains on venus we assess monte carlo modeling of volcanic resurfacing which competed with the ongoing formation of impact craters on the basis of these data and analyses we conclude that the mean age of the regional plains is close to the mean global surface age of venus and that the potential time span of regional plains emplacement was relatively short we interpret our results as evidence that wrinkle ridging on venus was a distinct tectonic phase that occurred rather early in post regional plains time probably within the first 10 although the exact duration remains uncertain regional and global stratigraphic relationships strongly support the hypothesis that wrinkle ridging on venus predominantly occurred during a specific phase of history the geologic record of wrinkle ridging and post plains tectonic evolution of volcanic plains on the moon and mars show interesting similarities to those of venus

the resurfacing history of venus constraints from buffered crater densities because of atmospheric shielding and endogenic resurfacing the population of impact craters on venus is small about a thousand and consists of large craters this population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of venus but the nearly spatially random nature of the crater population has complicated efforts to understand this history here we utilize the recent 1 15 m scale global geological map of venus ivanov m a head j w 2011 planet space sci 59 1559 1600 to help address this problem the global geological map provides a stratigraphic sequence of units and known areas where each unit is exposed on the planet for each crater on venus we identify the specific geological units predating and postdating the crater we perform a statistical analysis of this set of observations with a buffered crater density approach which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and or postdate more than one unit in this analysis we consider crater emplacement as random and resurfacing history as determined although unknown we obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries we find that 1 size frequency distributions of craters superposed on each unit are consistent with each other 2 regional plains and stratigraphically older units have similar crater retention ages 3 stratigraphically younger units have a mean crater retention age significantly younger than the regional plains these findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10 of the mean surface age tens of millions of years the termination of intensive resurfacing may or may not be synchronous over the planet our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images but obscure radar contrasts between predating lava flows we do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch

barotropic and baroclinic tidal currents in the eastern basins of the north atlantic data from a large scale moored array in the iberian and canary basins are used to determine the energies of barotropic and baroclinic m2 and s2 tides an analysis of time varying dynamical modes is performed the results for barotropic modes confirm the global surface tide model results of schwiderski 1980 for this region the barotropic modes dominate in the deep basins but increased baroclinic contributions are usually found over rough topography at three locations near the continental slope in the southern canary basin the baroclinic modes dominate the barotropic mode results from an array of three moorings at the northern part of the cape verde rise show an inverse behavior of barotropic and baroclinic energies such that the baroclinic energy is steadily enhanced while the barotropic energy is reduced towards the continental margin the increase in baroclinic energy is consistent with a generation of internal tides close to the shelf by surface tidal forcing over topography further evidence for this process is provided by the 2 week periodicity of the first order baroclinic mode at the slope corresponding to the spring neap cycle of the barotropic tide

thermally enhanced mimetic fabric of magnetite in a biotite granite the thermally induced growth of mimetic magnetite after biotite is here demonstrated by the equivalence of anisotropies of magnetic susceptibility ams and anhysteretic remanence aar after heating of biotite single crystals and whole rock specimens from a magnetite free granite elba island italy it is concluded that between 500 and 725 c magnetite nucleates and grows within and parallel to the biotite cleavages yielding a magnetic fabric mimetic of the biotite fabric at higher temperatures magnetite undergoes partial haematization accounting for the decrease in remanence susceptibility and anisotropy degree this experiment may be used as a proxy for fabric study of granites having composite tourmaline biotite paramagnetic fabrics in which the fabric of biotite can be selectively thermally enhanced

diagnosing a partly standing internal wave in mamala bay oahu 1 an internal partly standing wave in mamala bay hawaii is studied using new observations and the princeton ocean model pom previous work suggested a convergence in the bay of east and westbound waves emanating from kaena ridge and makapuu point respectively new energy flux measurements with shipboard adcp ctd confirm that makapuu point is the eastern source after validating the pom results against observations the model output is modally decomposed and compared with the expected theoretical patterns of kinetic and available potential energy energy flux and group velocity for a partly standing wave agreement is seen for the first baroclinic mode which also contains most of the energy the results confirm previous suggestions of standing wave dynamics in mamala bay

mean age of rifting and volcanism on venus deduced from impact crater densities unlike the extensively cratered highlands of the moon and mars the surface of venus does not preserve a record of heavy bombard ment from the early history of the solar system1 3 those craters that are found on venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds faults and lava flows1 3 although the volcanic and tec tonic history of venus is still much debated2 5 there is mounting evidence for near global resurfacing 300 500 myr ago1 2 6 moreover it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet suggestive of significant activity after the resurfacing event7 it is not clear however whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet to address this question we have used the regional variations in crater density to date volcan oes rifts and coronae which based on stratigraphic evidence clearly post date the main resurfacing event8 11 the calculated mean ages of 70 125 myr exclude the possibility that the majority of these features represent the final stages of the global event

internal tide generation and destruction by shoaling internal tides 1 internal tide generation is usually predicted from local topography surface tides and stratification however internal tides are often observed to be unrelated to local spring neap forcing appearing intermittently in 3 5 day bursts here we suggest a source of this intermittency by illustrating how remotely generated shoaling internal tides induce first order changes in local internal tide generation theory numerical simulations and observations show that pressure perturbations associated with shoaling internal tides can correlate with surface tide velocities to generate or destroy internal tides where shoaling internal tides have random phase such as on the new jersey slope time averaged internal tide generation is unaffected but instantaneous internal tide generation varies rapidly altering internal tide energy and possibly affecting nonlinear internal waves across shelf transport and mixing where shoaling internal tides are phase locked to the local surface tide such as in double ridge systems time averaged internal tide generation is affected and may result in resonance

long range propagation of the semidiurnal internal tide from the hawaiian ridge abstract the northeastward progression of the semidiurnal internal tide from french frigate shoals ffs hawaii is studied with an array of six simultaneous profiling moorings spanning 25 5 37 1 n 1400 km and 13 yr long ocean topography experiment topex poseidon t p altimeter data processed by a new technique the moorings have excellent temporal and vertical resolutions while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide s coherent portion together these two approaches provide a unique view of the internal tide s long range propagation in a complex ocean environment the moored observations reveal a rich time variable and multimodal internal tide field with higher mode motions contributing significantly to velocity displacement and energy in spite of these contributions the coherent mode 1 internal tide dominates the northeastward energy flux and is detectable in both moored and altimetric data over the entire array phase and group prop

internal tidal currents and solitons in the southern bay of bengal abstract tidal currents and higher frequency internal solitons are observed from current and hydrographic fields that were recently measured at six deep water moorings in the southern bay of bengal by the naval research laboratory current temperature and salinity were measured in the upper 500 m of the water column between december 2013 and august 2015 harmonically fitted coherent tidal currents in the upper 500 m showed maximum amplitudes for the m2 constituent with velocities of the order of 3 cm s 1 m2 tidal ellipses were generally oriented towards the north northeast incoherent internal tides accounted for at least 60 of the total semidiurnal tide energy solitons were generated at multiple source regions along the andaman islands and contribute significantly to the high frequency energy greater than 1 cph solitons were present throughout the measurement period but their activity was reduced during the 2014 2015 winter monsoon soliton and internal tide amplitudes show a fortnightly modulation consistent with the spring neap cycle of the semidiurnal tide the interaction of tides and the associated high frequency waves 20 40 min periods with mesoscale eddies produced regions of enhanced shear and strain in the thermocline high strain regions were found within cyclonic eddies and high shear regions were found within anticyclonic eddies shear strain based dissipation models indicate that mixing rates can be enhanced by a factor of 3 5 by tidal and higher frequencies during interactions with eddies this study further suggests that tidal harmonics and internal tides have an important impact on the near surface currents away from coastal boundaries in the interior of the bay of bengal while the effects of the depth averaged tides are more important deeper in the water column

venus estimation of age of impact craters on the basis of degree of preservation of associated radar dark deposits 1 venus impact craters larger than 5 km in diameter 90 of all craters have been classified based on the presence and prominence of associated radar dark deposits approximate estimates of the absolute age of craters of different morphologic classes are made analysis of age relations of dated craters and volcanic and tectonic structures permits the estimation of the age and duration of different volcanic and tectonic events

the breaking and scattering of the internal tide on a continental slope abstracta strong internal tide is generated in the luzon strait that radiates westward to impact the continental shelf of the south china sea mooring data in 1500 m depth on the continental slope show a fortnightly averaged incoming tidal flux of 12 kw m 1 and a mooring on a broad plateau on the slope finds a similar flux as an upper bound of this 5 5 kw m 1 is in the diurnal tide and 3 5 kw m 1 is in the semidiurnal tide with the remainder in higher frequency motions turbulence dissipation may be as high as 3 kw m 1 local generation is estimated from a linear model to be less than 1 kw m 1 the continental slope is supercritical with respect to the diurnal tide implying that there may be significant back reflection into the basin comparing the low mode energy of a horizontal standing wave at the mooring to the energy flux indicates that perhaps one third of the incoming diurnal tidal energy is reflected conversely the slope is subcritical with respect to the semidiurnal tide and the observed

volumetric changes in weathered profiles iso element mass balance method questioned by magnetic fabric abstract a major issue in understanding weathering processes is to determine to what extent fabrics structures and volumes from the parent rock are preserved through the weathering profile iso element mass balance methods imb are often used to estimate volume changes during progressive weathering based on the assumption that a given element either ti zr or th is not mobile the petrofabric of the weathered material is often characterized by a mimetic replacement of primary minerals that fully preserve the primary fabric this suggests an isovolume weathering as volume change must be associated with shape change due to boundary conditions not allowing horizontal strains so collapse or dilation should induce vertical compaction or constriction respectively the weak petrofabric of weathered materials may be precisely quantified using the anisotropy of low field magnetic susceptibility technique ams this paper reports for the first time a combined imb and ams study of two different environments a complex lateritic sequence on precambrian metamorphic rocks in cameroon and a weathering profile on a pleistocene basaltic flow in morocco the lateritic profile divided into a lower saprolite zone 12 m and an upper nodular iron rich unit 4 m is characterized by neoformed magnetic minerals goethite hematite spinels and a weak but rather consistent magnetic fabric saprolitization which induces low susceptibility values 50 10 9 m3 kg preserves the linear tectonic fabric of the parent gneiss ams evidence for isovolumetric weathering agrees with th based imb while ti and zr partly indicate apparent collapse and dilation up to 50 respectively ferralitization in the nodular iron crust which enhances susceptibilities 500 to 700 10 9 m3 kg due to spinel phases induces drastic reductions in anisotropy due to multiple generation of neoformed minerals destroying primary fabric however a weak horizontal planar magnetic fabric suggests compaction in agreement with th and ti based imb while zr would indicate isovolume weathering conversely in the one meter thick weathering profile on basalt the strong susceptibility 1 5 to 3 10 6 m3 kg is dominated by inherited titanomaghemite grains which totally preserve the weak magmatic fabric of the fresh basalt this again suggests that volume change is minor while ti and zr based imb indicate 20 to 50 of dilation in both examples large volume changes inferred from the imb in units showing preserved primary ams fabric appear unrealistic our ams study together with the large discrepancies between the imb results produced by various elements indicate that the assumption of immobility of a given element is probably not fulfilled all along these profiles isovolumetric weathering in the saprolitic zone may be more widespread than suggested by imb results

the tectono magnetic signature of the old red sandstone and pre devonian strata in the hasteinen area western norway and implications for the later stages of the caledonian orogeny the structural fabrics recorded from devonian rocks and substrate in the hasteinen area western norway show near e w trends in the devonian rocks these fabrics are a consequence of post depositional regional folding at a considerable depth presumably attended by the aquisition of an thermoviscous tvrm or thermochemical tcrm remanence group a post or possibly late syntectonic group a remanences declination 210 inclination 18 95 9 constitute the principal remanence in both devonian rocks and their substrate whereas a younger group b magnetization declination 047 inclination 59 95 7 relates to early mesozoic shearing the palaeomagnetic group a and structural data from the hasteinen and kvamshesten old red sandstone massifs favor a regional compressive n s shortening event svalbardian solundian which probably occurred during latest devonian times some recent extensional basin models appealing to listric normal faulting to account for both the sedimentation and the subsequent deformation of the western norwegian devonian rocks are rejected on the basis of inadequate structural information

paleomagnetic results from the southeastern margin of the ebro basin northeastern spain evidence for a tertiary clockwise rotation abstract triassic rocks underlying the paleogene deposits of the ebro basin are exposed along its se margin about 200 samples of middle triassic and paleogene rocks from this margin have been demagnetized and analysed the middle triassic red beds contain one or two secondary components of magnetization the high unblocking temperature component has a mean direction of d 1 and i 6 the eocene red beds display a very stable magnetization nevertheless some samples have a high temperature component with the present day field direction the eocene mean direction is d 22 and i 51 the paleomagnetic results show a systematic 20 clockwise rotation relative to the data from the pyrenean and cantabrian ranges and the western and sw part of iberia to explain these results two hypotheses are presented one implies the rotation of the ne part of iberia whereas the other refers to a rigid block rotation mechanism along the catalan coastal range

morphometry of wrinkle ridges on venus comparison with other planets the widths of wrinkle ridges on venus and the spacing of their network were measured at 30 randomly selected sites on the venusian plains at one of the sites heights of ridges were also estimated results of our measurements indicate that 1 wrinkle ridges on venus form a quasi periodic rather than chaotic network 2 there are regional variations of the typical spacing within the network with the mean for all 30 sites being 12 9 km and the standard deviation of the means of all sites being 5 8 km 3 there are regional variations of typical ridge width with the mean width being 1 25 km and the standard deviation of the means of all sites being 0 66 km 4 there is no site to site correlation between the mean ridge widths and spacings comparison with other terrestrial planets shows that wrinkle ridges on venus are generally narrower than on the other planets and the spacing of the ridge network on venus is smaller than on tharsis plateau mars global surface shortening due to formation of the wrinkle ridge network is estimated to be of order of 0 1

thermal enhancement of magnetic fabrics in high grade gneisses the orientation distributions of ams and aarm ellipsoids are non coaxial in the kapuskasing zone granulites aarm is due to the preferred orientation of magnetite which mostly postdates the silicate fabric the ams orientation distribution combines susceptibility anisotropies of magnetite and the silicates whereas heating does not increase the bulk susceptibility it does improve the definition of the orientation distributions of both ams and aarm however the orientation distribution of aarm is most enhanced and therefore post heated samples emphasize the magnetite sub fabric thus heating does not clarify the interpretation of the silicate fabric in this study however it does change the samples ams fabric in such a way as to reveal the magnetite subfabric which can only otherwise be detected by more tedious aarm measurements

low temperature ams and the quantification of subfabrics in deformed rocks abstract we evaluate the application and significance of low temperature anisotropy of magnetic susceptibility lt ams measurements in deformed mudrocks originally conceived as a way to enhance paramagnetic relative to ferromagnetic susceptibility lt ams studies offer significant potential in constraining the coexistence of subfabrics that are due to phyllosilicate grains with different preferred orientations in this study we report a detailed procedure to obtain such directional susceptibilities measuring samples in multiple orientations at liquid nitrogen temperatures in order to determine the lt ams due to unequal changes of magnetic susceptibility in micas at low temperature the enhancement of standard ams at low temperature better separates interacting fabrics in natural rocks particularly depositional fabrics versus deformational fabrics lt ams is a non destructive technique that readily offers an ability to separate ferromagnetic and paramagnetic fabrics and allows the characterization and quantification of multiple fabrics in natural rocks

energetics and spatio temporal variability of semidiurnal internal tides in the bay of bengal and andaman sea abstract energetics of semidiurnal barotropic and internal tides in the bay of bengal bob and andaman sea as are studied using the simulations from a high resolution ocean general circulation model barotropic m 2 tide which is the largest tidal constituent in this region loses about 65 5 gw of energy in the bob and as most of this barotropic energy 46 5 gw is dissipated by means of bottom friction in the shallow regions such as the head of the bay and the gulf of martaban due to the interaction of barotropic tides with bottom topography about one third of the barotropic m 2 energy 19 1 gw gets converted into internal tides and most of this conversion 84 occurs over the andaman nicobar an ridge on the contrary to the estimates in earlier studies our results show that a large part 59 of this internal tide energy in the bob and as gets dissipated far away from the generation sites compared to the bob energy dissipation rates are found to be large over the entire as and it could result in enhanced vertical mixing in this region further analysis of the temporal variability of conversion within the model domain shows that energy conversion in the semidiurnal band varies between 11 to 27 gw over a spring neap cycle seasonal variability in the stratification due to river runoff significantly changes the conversion of tidal energy at the generation sites in the northern bob and as our study revealed that westward radiating internal tides from an ridge undergo path alteration refraction due to the time varying mesoscale circulation and such path alterations cause large intraseasonal variability in internal tide activity and redistribution of internal tide energy available for mixing in the remote areas such as the continental margins of the western bob

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

impact craters on geologic units of northern venus implications for the duration of the transition from tessera to regional plains using magellan sar images and the schaber et al 1998 crater data base we examined impact craters in the area north of 35 n and determined the geologic units on which they are superposed the crater density of the regional plains with wrinkle ridges pwr was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet t about 80 to 97 of the craters superposed on a composite unit that includes materials of tessera terrain tt densely fractured plains pdf fractured and ridged plains pfr and fracture belts fb also postdate the regional plains thus the time interval between the formation of these older units and emplacement of the regional plains t should be geologically short from a few percent to about 20 of t or approximately 40 to 150 m y this means that in the area under study volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of venus about the last 750 m y was much more active than in the subsequent time

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

venus resurfacing rates constraints provided by 3 d monte carlo simulations the range of surface ages on venus and the rate of recent volcanism are two major questions that can be addressed through an analysis of magellan images we report on a 3 d monte carlo resurfacing model that seeks to explain the resurfacing history of venus by simulating the evolving surface under the competing processes of impact cratering and volcanism by incorporating the observed size distribution of volcanic forms on venus and the assumptions of constant volcanic and impactor fluxes we seek a solution that is consistent with the observed impact crater population in the monte carlo simulations a volcanic flux of 0 37 km3 yr operating for 550 m y on initially craterless surfaces yields surfaces that are statistically similar to venus it is shown that the statistics of the impact cratering record are inconsistent with an equilibrium resurfacing model

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

internal tide dynamics in a topographically complex region browse basin australian north west shelf 1 field observations and numerical circulation modeling revealed the spatial variability of the tidally driven dynamics in the topographically complex continuously stratified macrotidal environment of the browse basin on the australian north west shelf internal wave generation occurred at a number of discrete topographic features and the resultant interaction of multiple waves led to a spatially variable internal wave climate the generation of baroclinic energy was most intense in regions where the barotropic tide was aligned with steep topography generation of low mode internal waves occurred at the inner shelf break where the ratio of tidal excursion distance to topographic length scale was large in contrast generation of high mode beam like internal waves occurred at the outer shelf break where the ratio of tidal excursion distance to topographic length scale was very small the most efficient conversion from barotropic to baroclinic energy occurred at the outer shelf break the internal waves generated at the inner and outer shelf breaks interacted to produce a partly standing internal wave resulting in a large along shelf energy flux and discrete locations with small ratios of horizontal kinetic energy to available potential energy this phenomenon is likely to occur in other regions with semienclosed topography and multiple generation sites

propagation of low mode internal waves through the ocean abstract the baroclinic tides play a significant role in the energy budget of the abyssal ocean although the basic principles of generation and propagation are known a clear understanding of these phenomena in the complex ocean environment is only now emerging to advance this effort a ray model is developed that quantifies the effects of spatially variable topography stratification and planetary vorticity on the horizontal propagation of internal gravity modes the objective is to identify baroclinic shoals where wave energy is spatially concentrated and enhanced dissipation might be expected the model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field the refraction of tidally generated internal waves at the hawaiian ridge is examined using an ensemble of mesoscale background realizations derived from weekly ocean topography experiment topex poseidon altimetric measurements the path of mode 1 is only slightly affected by typical cur

interpretation of anisotropy of magnetic susceptibility fabric of ignimbrites in terms of kinematic and sedimentological mechanisms an anatolian case study abstract a study of the neogene low aspect ratio kizilkaya ignimbrite in the calc alkaline central anatolian volcanic province turkey has been carried out using the anisotropy of magnetic susceptibility ams technique on 46 sites irm hysteresis loops and curie temperature analysis indicate that magnetite is the main carrier of the magnetic signal in the ignimbrite the shape of the ams ellipsoids varies widely from site to site and the anisotropy is commonly low we review the data processing used in the literature on the ams of ignimbrites to calculate principal directions or to evaluate the quality of the results and we develop various simple techniques data filtering and contouring procedures in order to more accurately define the axis or symmetry of the ams fabric there is no simple relationship between the shape of the ams ellipsoids and the shape of the petrofabric ellipsoids similarly there is no direct relationship between magnetic and kinematic axes our results show that the source of the measured magnetic signal in these rocks is a complex result of various contributions including shape anisotropy of free and fixed magnetic crystals shape ams crystallization of magnetic grains on all surfaces and discontinuities during cooling of the ignimbrite distribution anisotropy and alteration by hydrothermal and meteoric water particular attention is paid to the interpretation of the ams stereoplots departing from the standard fabric of sedimentary rocks formed under unidirectional flow in order to explain these non standard ams fabrics we discuss their possible origin 1 in analytical and mineralogical terms instrumental artifact pollution by xenoclasts mineralogical inversion of the susceptibility axes secondary mimetic fabric and 2 in terms of sedimentological mechanisms rheology depositional processes in the present state of knowledge we conclude that due to the physico chemical complexity of the magnetic source the ams signal should be used and interpreted with care when attempting to decipher the sedimentological or rheological processes occurring in moving pyroclastic flows on the other hand ams has it greatest potential when used in conjunction with other techniques and when a large number of sampling sites and specimens is used

internal tide reflection and turbulent mixing on the continental slope abstract observations of turbulence internal waves and subinertial flow were made over a steep corrugated continental slope off virginia during may june 1998 at semidiurnal frequencies a convergence of low mode onshore energy flux is approximately balanced by a divergence of high wavenumber offshore energy flux this conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide it is suggested that elevated near bottom mixing k 10 3 m2 s 1 observed offshore of the supercritical continental slope arises from the reflection of a remotely generated low mode m2 internal tide based on the observed turbulent kinetic energy dissipation rate the high wavenumber internal tide decays on time scales o 1 day no evidence for internal lee wave generation by flow over the slope s corrugations or internal tide generation at the shelf break was found at this site

from qualitative to quantitative magnetic anisotropy analysis the prospect of finite strain calibration abstract a method of separating low field susceptibility anisotropies due to ferrimagnetics and the matrix proposed hitherto in cases where the principal axis of these two components are merged is generalized in cases where the axes are distinct the method consists of analysing in specimens close in the same sample anisotropies the differences of which are due to the variation of respective percentages of the two components although the strain rate remains constant one thus uses the cause which itself renders global anisotropy unusable a first application is presented where a control using the anisotropy of isothermal remanent magnetization allows the validity of the method to be tested a discussion of the choice of the best ellipsoid relative either to the matrix or to the ferrimagnetics to give the best indication of relative finite strain rate is opened

coherence of internal tide modulations along the hawaiian ridge long time series of sea level from tide gauges along the north side of the hawaiian ridge and shorter series of dynamic heights inferred from inverted echo sounders moored just north of the main hawaiian islands are examined for evidence of internal tides at the m2 frequency we find that the amplitudes and phases of the m2 tidal components have low frequency variability which is consistent with a superposition of an internal tide with the larger barotropic tide further the low frequency variability is correlated with low frequency changes in the depth of the pycnocline which suggests a simple physical mechanism to account for the low frequency modulations in the internal tidal amplitude these modulations are coherent for long distances along the hawaiian ridge indicating a coherent generation of the internal tide that is consistent with acoustic observations in the north pacific and with recent analyses of sea surface heights from satellite altimetry

internal tide radiation from the luzon strait the m2 k1 and o1 internal tides originating in the luzon strait are investigated using the sea surface height measurements by multiple satellites ers 2 envisat topex poseidon jason 1 2 and geosat follow on a plane wave fit method is used to resolve multiple internal tides in arbitrary horizontal directions the luzon strait is an energetic internal tide generation site and radiates internal tides both westward into the south china sea scs and eastward into the western pacific wp in the scs the k1 and o1 internal tides propagate over 1600 km reaching the vietnam coast in the wp they propagate over 2500 km and arrive to the mariana ridge and guam the k1 and o1 internal tides refract toward the equator during propagation the m2 internal tides in the scs bifurcate into two beams the northwestward beam is coincident with the frequent occurrence of internal solitary waves in this region implying their causative relation the phase speeds inferred from the altimetric along beam propagation agree with the theoretical values due to the influence of the earth s rotation the k1 and o1 phase speeds decrease remarkably from high to low latitudes for the diurnal internal tides the eastward radiation is about 50 greater than the westward radiation for m2 the westward radiation is about two times the eastward radiation the altimetric energy fluxes are about 50 of those in numerical model simulations

the lifecycle of semidiurnal internal tides over the northern mid atlantic ridge abstractthe life cycle of semidiurnal internal tides over the mid atlantic ridge mar sector south of the azores is investigated using in situ a high resolution mooring and microstructure profiler and satellite data in combination with a theoretical model of barotropic to baroclinic tidal energy conversion the mooring analysis reveals that the internal tide horizontal energy flux is dominated by mode 1 and that energy density is more distributed among modes 1 10 most modes are compatible with an interpretation in terms of standing internal tides suggesting that they result from interactions between waves generated over the mar internal tide energy is thus concentrated above the ridge and is eventually available for local diapycnal mixing as endorsed by the elevated rates of turbulent energy dissipation e estimated from microstructure measurements a spring neap modulation of energy density on the mar is found to originate from the remote generation and radiation of strong mode 1 internal tides fr

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

standing internal tides in the tasman sea observed by gliders abstractlow mode internal tides are generated at tall submarine ridges propagate across the open ocean with little attenuation and reach distant continental slopes a semidiurnal internal tide beam identified in previous altimetric observations and modeling emanates from the macquarie ridge crosses the tasman sea and impinges on the tasmanian slope spatial surveys covering within 150 km of the slope by two autonomous underwater gliders with maximum profile depths of 500 and 1000 m show the steepest slope near 43 s reflects almost all of the incident energy flux to form a standing wave starting from the slope and moving offshore by one wavelength 150 km potential energy density displays an antinode node antinode node structure while kinetic energy density shows the opposite mission mean mode 1 incident and reflected flux magnitudes are distinguished by treating each glider s survey as an internal wave antenna for measuring amplitude wavelength and direction incident fluxes are 1 4 and 2 3 k

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

thermal enhancement of magnetic fabric in cretaceous sandstones heating of samples of the san juan cretaceous sandstones ne venezuela results in large increases in their low field susceptibility mostly above 650 c which allows measurement of their low field susceptibility ellipsoids where measurements are available before and after heating the same fabric is almost always enhanced samples previously too weak for measurement showed thermally induced fabrics which are consistent within each site but vary between different localities the susceptibility changes are all largely attributed to magnetite formation probably from vermiculite at 450 c in the proximal sediments and from illite at 850 c in all areas the induced magnetic fabrics can be interpreted in terms of either sedimentary structures or low grade metamorphism related to cleavage parallel to the fold axes it is considered that this thermal enhancement technique offers a method for measuring low field magnetic fabrics in a wider range of geological conditions and lithologies than reviously although the processes involved need considerable study

effects of remote generation sites on model estimates ofm2internal tides in the philippine sea abstractthis study investigates the impact of remotely generated internal tides on model estimates of barotropic to baroclinic tidal conversion for two generation sites bounding the philippine sea the luzon strait and the mariana island arc a primitive equation model is used to characterize the internal tides generated by the principal semidiurnal tide m2 over a domain encompassing the two generation sites energetic internal tides are generated at the luzon strait where nearly 17 gw of barotropic tide energy is converted to baroclinic energy of which 44 4 78 gw is radiated eastward into the philippine sea from the mariana arc baroclinic energy propagates westward into the philippine sea as a result of 3 82 gw of barotropic to baroclinic energy conversion simulations that focus on each generation site without influence of the other are performed and comparisons show that remotely generated internal tides have a significant effect on local conversion at the two sites total conversion is greate

geology of venus mapping of a global geotraverse at 30 n latitude we mapped a swath of terrain on venus that extends circumferentially completely around the globe at 30 n latitude representing about 11 of the surface of the planet in order to address the following questions do presently defined stratigraphic units prove useful for broad application to regional and global mapping do units described in one area occur in other regions of the planet and how laterally continuous are they if similar sequences of units are found in different parts of the planet do they correlate temporally e g time correlative or do they represent similar sequences formed at different times e g time transgressive or independently locally repetitive we found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of venus at this latitude providing evidence that these units are not only global in extent but also laterally contiguous this gives confidence that the stratigraphic sequence can be extended to other parts of venus for further testing and refinement on the basis of this mapping we find that several areas including tessera massifs and beta regio show a concentration of older units and appear to have been topographically high over much of the visible history of venus tessera terrain is not uniformly distributed in some areas where these older units are concentrated ridge belts and fracture belts are not randomly distributed throughout the area suggesting that they represent regional patterns of deformation the altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest with older units tessera and earlier plains at intermediate to high elevations consistent with regional plains emplacement following the formation of the most significant regional topography sixty corona and corona like features were found in the geotraverse and the vast majority show evidence of being initiated in the earliest posttessera time early guineverian in an extensional tectonic environment few show evidence of postregional plains volcanism and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of venus regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges early part of the middle guineverian perhaps overlapping somewhat with its lower member evidence is found for changes in styles of volcanism with time and tectonic activity changed in time in terms of pervasiveness style and areal distribution on the basis of mapping of this globally continuous swath representing about 11 of the planet we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time rather than the nondirectional history proposed by guest and stofan 1999