the structural control of venusian polygonal impact craters pre impact inhomogeneities of the target material sometimes cause the rim of an impact crater to be composed of several straight segments instead of being circular the venusian surface hosts 121 such polygonal impact craters pics 12 km in diameter their straight rim segments are often parallel to the orientations of the surrounding tectonic structures particularly those in tessera terrain and young rift zones as well as the concentric components of coronae this match is notably more distinct in distances less than two crater diameters between the pics and the tectonic structures than further away surrounding wrinkle ridges lineaments or radial components of volcano tectonic features seem to have very little influence on the orientations of pic rims these results imply that the formation of straight segments of venusian pic rims is controlled by pre existing tectonic structures of the crust but not by the apparently most surficial ones thus pics could be used to provide further constraints on the distribution and orientations of zones of weakness in the venusian crust

an analytical study of the sea breeze abstract in this paper we present theoretical results concerning the sea breeze intensity and its inland penetration as a function of latitude and friction we produce solutions for the spatial structure of the streamfunction and momentum components and for their line behavior in the case of a step function forcing of finite duration and in the case of periodic forcing results show that the sea breeze initially has an aspect ratio equal to unity and that earth rotation and friction aged its intensity but they are unimportant in determining its early geometry the sea breeze has a characteristic time scale which is a combination of the inertial period and the e folding time due to friction for a time larger than this characteristic time scale the inland penetration of the sea breeze is confined by a rossby deformation radius which includes a frictional effect in fact friction and inertia reduce not only the intensity but also the horizontal scale of motion at the equator the controlling parameter

the influence of land surface heterogeneities on cloud size development abstractthis study analyzes the effects of land surface heterogeneities at various horizontal scales on the transition from shallow to deep convection and on the cloud size distribution an idealized case of midlatitude summertime convection is simulated by means of large eddy simulations coupled to an interactive land surface the transition is accelerated over heterogeneous surfaces the simulation with an intermediate patch size of 12 8 km exhibits the fastest transition with a transition time two thirds that over a homogeneous surface a similar timing is observed for the precipitation onset whereas the total accumulated rainfall tends to increase with patch size the cloud size distribution can be approximated by a power law with a scale break the exponent of the power law is independent of the heterogeneity scale implying a similar cloud cover between the simulations in contrast the scale break varies with patch size the size of the largest clouds does not scale with the boundary layer height

polar margin dunes and winds on mars the approximately concentric arrangement of layered deposits and dune fields at the two martian poles may reflect a nearly steady state dispersal of material from the polar deposits data on effective surface winds from high resolution viking images combined with theory of local winds suggest that the northern dunes are in part confined to a latitude band by winds generated by their own low albedo dispersal of the dark sand from the southern polar region is not subject to this kind of feedback because the irregular topography prevents areal accumulations sufficiently extensive to produce winds

a linear theory of three dimensional land sea breezes abstractland sea breezes lsbs induced by diurnal differential heating are examined using a three dimensional linear model employing fast fourier transform with emphasis on the complex coastline shape and geometry the earth s rotation and background wind effects it has been demonstrated that the low level vertical motion associated with lsb can be significantly enhanced over a bay peninsula because of convergence of perturbations induced by differential heating along a seaward concave convex coastline the dependence of surface winds and vertical motion patterns and their evolutions on the coastline geometries such as the width and the aspect ratio of the bay the earth s rotation and the background winds are investigated the lsb induced by an isolated tropical island is characterized by onshore flow and ascent over the island in the afternoon to early evening with a reversal of direction from midnight to early morning the diurnal heating induced vertical motion is greatly enhanced over the isl

a study of landscape generated deep moist convection abstract a two dimensional version of a cloud resolving model was used to study the generation of deep moist convection over heterogeneous landscapes alternating patches of dry and wet soil were simulated for various profiles of background wind results suggested a significant systematic impact of patch length and background wind on moist convection rainfall occurred most intensely along sea breeze like fronts which formed at patch boundaries total accumulated rainfall as the average over simulations with the same patch size but with different background wind profiles was largest for a patch length of 128 km this patch length was similar in size to a local radius of deformation ro hn the deposition of rainfall generated a much different distribution of soil moisture after one day of model simulation this new distribution however was far from equilibrium as the landscape still consisted of a number of wet and dry soil patches the cloud structure of moist convection was also examined using

age of rifting and associated volcanism in atla regio venus an estimation of the age of rifting and associated volcanism in atla regio is carried out based on photogeologic analyses of the stratigraphic relationships between rift associated features and impact craters which have associated radar dark paraboloids the paraboloids are believed to be thin mantles of debris derived from the fallout of crater ejecta and their parent craters are considered to be among the youngest 10 of venus crater population campbell et al 1992 in the area under study there are three impact craters with associated dark paraboloids 1 von schuurman which is clearly superimposed on the dali chasma rift 2 sitwell which is superimposed on ganis chasma and may have been followed by subsequent rift associated fracturing and 3 luxemburg which is superimposed on lavas from ozza mons and dissected by dali chasma fractures which in turn are covered by lavas from maat mons this means that at least the last stages of tectonic and volcanic rift associated activity in atla regio are contemporaneous with the formation of impact craters which are young enough to preserve the associated dark paraboloids if these craters represent the youngest 10 of the venus crater population and the average age of the venusian surface is about 500 m y schaber et al 1992 this means that rifting and rift associated volcanism in atla regio were active as recently as during the last 50 m y in the current debate over models of venus resurfacing the results of our study favor the equilibrium resurfacing model phillips et al 1992

reply to allen et al s comments on on magma fragmentation by conduit shear stress evidence from the kos plateau tuff aegean volcanic arc by palladino simei and kyriakopoulos jvgr 2008 178 807 817

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

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

the global distribution of giant radiating dike swarms on venus implications for the global stress state magellan radar data of venus reveal 163 large radial lineament systems composed of graben fissure and fracture elements on the basis of their structure plan view geometry and volcanic associations at least 72 are interpreted to have formed primarily through subsurface dike swarm eraplacement the remainder through uplift or a combination of these two mechanisms the population of swarms is used to determine regional and global stress orientation the stress configura tion recorded from 330 210oe aphrodite terra is best ex plained by isostatic compensation of existing long wave length topography or coupling between mantle flow and the lithosphere the rest are correlated with concentrations of rifting and volcanism in the beta atla themis region the global stress field on venus is different than that of earth where plate boundary forces dominate from their surroundings where domical topography occurs the ra dius of the associated lineament system is on average 2 5 times greater than that of the dome some 53 of the radiat ing systems are associated with concentric structures ranging from central depression bounding scarps 25 km wide to tec tonic rings 575 km in diameter in 51 of these cases the ra dial pattern originates within but also extends significantly beyond the annulus only 9 of the radiating systems are confined completely leaving 40 located outside but focused upon a central annular structure finally the radial geometry is usually quite pronounced near the center of each structure 52 of the time radial lineaments occupy 270 o of azimuth and 80 exceed 180 o figure 2c in their distal regions how ever only 72 of the radiating systems retain a purely radial geometry as the remaining 28 gradually develop a non radial unidirectional lineament configuration all but seven of the radiating systems exhibit volcanism figure 2d lobate flows emerging from individual linea ments or many such flows strongly correlated with multiple radial lineaments occur for 45 of the systems clusters of small shields occur 72 of the time and in 75 of these in stances distinctive alignment of small shields and or pits along radiating lineaments is seen finally 65 are associ ated with some other form of centralized volcanism including multiple forms of edifice construction limited extrusions and sheet like flows which extend into the surrounding terrain

resurfacing history of venus the stratigraphy impact cratering record and geoid of venus indicate that a major change in surface geologic activity has occurred before 800 ma venus was dominated by horizontal movement and tectonic deformation of the crust a brief period of global volcanism subsequently filled in all low lying areas and left much of the surface at a nearly uniform elevation current geologic activity is dominated by large volcanoes and limited horizontal movement of the crust this geologic history may have resulted from mantle cooling causing the lithosphere to become positively buoyant globally

corona classification by evolutionary stage abstract the formation of coronae abundant volcano tectonic features on the surface of venus has been modeled as the interaction of mantle diapirs with the lithosphere however the applicability of this model to features of widely different sizes may not be evident from sar images due to the lack of a common scale we use the radius as a scaling parameter for both the distance from the center and relief obviating this problem and mapping the effects from coronae of different radii onto a single scale normalizing profiles for 394 features we find many features classified by e r stoffanet al 1992 j geophys res 97 13 347 13 378 as different types have very similar profiles and relief coronae distribute into three shapes domal circular calderic two shapes not related to coronae radial and volcanic remain variation between corona shapes is gradational and mirrors the evolution of diapirs from initial domal upwellings to calderas we test the implied age progression using impact crater populations within three corona radii despite the inherent uncertainties of this method crater statistics for a size restricted 275 325 km corona set suggest an age progression from domal through circular to calderic the calderic coronae have an associated impact crater population very similar to the global mean though with slightly too few tectonized craters implying that they approximate the average surface age and are no longer active the circular coronae have a low impact crater density and a slightly elevated proportion of tectonized craters and thus are currently active the domal coronae have fewer than the expected number of impact craters of which more are tectonized than expected suggesting that surface modification has begun

numerical simulation of the interaction between the sea breeze front and horizontal convective rolls part i offshore ambient flow abstract a three dimensional cloud resolving model is used to investigate the interaction between the sea breeze circulation and boundary layer roll convection horizontal convective rolls hcrs develop over land in response to strong daytime surface heating and tend to become aligned parallel to the vertical wind shear vector whereas the land sea heating contrast causes the formation of the sea breeze front sbf along the coastline the ability of hcrs to modulate the along frontal structure of the sbf is examined complementing and extending previous observational and numerical studies three simulations are discussed the first two demonstrating that the model can produce both phenomena independently the third is initialized with offshore mean flow and vertical shear perpendicular to the coastline and results in a sharply defined inland propagating sbf that encounters hcrs aligned perpendicular to it before the interaction takes place the sbf is nearly two dimensional and devoid of along front

syneruptive and intereruptive lithofacies in lacustrine environments the cretaceous beolkeum member wido island korea abstract the cretaceous beolkeum member was deposited in a lacustrine environment affected by explosive volcanism and a number of syneruptive lithofacies are intercalated with lacustrine mudstones the syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features which can be grouped into four syneruptive lithofacies assemblages sla 1 to sla 4 from bottom to top primary syneruptive lithofacies are represented by welded massive lapilli tuff in sla 1 and a normally graded lapilli tuff in sla 2 in sla 1 sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water resulting in the formation of a welded texture and columnar joints in sla 2 unsteady pyroclastic density currents rapidly disintegrated mixed with ambient water and transformed into high density turbidity currents depositing thick normally graded lapilli tuff resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions when volcaniclastic sediments were continuously supplied into the lake by debris flows a volcaniclastic fan can be formed sla 1 resulting in a coarsening upward trend and progradational geometry in case of relatively small amounts of volcaniclastic sediment supply turbidity currents would be a main depositional process depositing a series of normally graded tuff on the primary syneruptive lithofacies sla 2 showing a fining upward trend in sla 3 and sla 4 there are only a few cm thick normally graded tuff reflecting minor volcaniclastic sediment supply however overlying normally graded sandstones in sla 3 showing a coarsening upward trend and a thick normally graded sandstone in sla 4 suggest favorable conditions for the generation of the sediment gravity flows probably due to an increase in volcaniclastic sediment supply after eruptions

modeling and analysis of ageostrophic circulation over the azores oceanic front during the semaphore experiment abstract in the conventional quasigeostrophic qg form of the equation developed by hoskins et al the unique forcing of vertical velocity is the geostrophic deformation as the qg or even the semigeostrophic sg hypotheses are not adapted to study the frontal dynamics in the atmospheric boundary layer this paper proposes a generalized expression of the hoskins et al form of the vertical velocity two thermal and three dynamical sources of the vertical velocity are identified these forcings allow for identification of each of the physical processes acting simultaneously on the ageostrophic circulation in the boundary layer this new form of the equation is used to explain wind increase in the atmospheric boundary layer over the warm waters of the sea surface temperature sst front observed during a fair anticyclonic day of the semaphore experiment 1993 and simulated with a nonhydrostatic mesoscale atmospheric model since the sst gradients are weak of the order of 1 5 c 100 km 1 the surfac

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

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

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

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

constraints on the source of lunar cataclysm impactors multiple impact basins formed on the moon about 3 8 gyr ago in what is known as the lunar cataclysm or late heavy bombardment many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main belt asteroids destabilized by delayed planetary migration we show that morphologically fresh class 1 craters on the lunar highlands were mostly formed during the brief tail of the cataclysm as they have absolute crater number density similar to that of the orientale basin and ejecta blanket the connection between class 1 craters and the cataclysm is supported by the similarity of their size frequency distribution to that of stratigraphically identified imbrian craters majority of lunar craters younger than the imbrium basin including class 1 craters thus record the size frequency distribution of the lunar cataclysm impactors this distribution is much steeper than that of main belt asteroids we argue that the projectiles bombarding the moon at the time of the cataclysm could not have been main belt asteroids ejected by purely gravitational means

effects of onshore and offshore topography on sea breeze circulation an observational study at eastern attica greece field measurements of wind air temperature and humidity were taken at the eastern part of the attika district in june 1991 to examine the topographic influences exerted on the local sea breezes these influences are due either to the elongated evia island faced by the northern half of attica coastline some tens of kilometers offshore or to the coast parallel range of hymettos mountain rising steeply 12 km onshore the instrumentation consisted mainly of three tethered meteorological balloons released at characteristic sires i e the coast a location between shoreline and mountain foot and the mountain top and three autographic ground based anemometers operating at selected locations data from the ground based and upper air stations of the hellenic national meteorological service as well as the diurnal weather maps were also obtained and analyzed observations were made under different synoptic wind and the latter was found to determine remarkably the significance of the topographic effects a preliminary two dimensional numerical approach was also made concerning the sea breeze capability to reach the hymettos mountain top in the case of a weak opposing geostrophic flow

mechanisms for diurnal boundary layer circulations in the great basin desert abstract the purpose of this observation and model based study of the great basin desert boundary layer is to illustrate the variety of locally forced circulations that can affect such an area during a diurnal cycle the area of the great basin desert or great salt lake desert that is studied is located to the southwest of salt lake city utah it is characteristic of the arid basin and range province of north america in that it contains complex terrain varied vegetation and substrates and high water tables associated with salt encrusted basin flats playas the study area is especially well instrumented with surface meteorological stations operated by the u s army s west desert test center and a collection of cooperating mesonets in northeastern utah the study period was chosen based on the availability of special radiosonde data in this area one of the processes that is documented here that is unique to desert environments is the salt breeze that forms around the edge of playas as a result of

plume channeling as one possible mechanism for the origin of multiple coronae on venus a case study from the helen planitia quadrangle v52 1 coronae are large volcanotectonic features formed in response to plume diapir activity that have played an important role in the geodynamical evolution of venus coronae are classified in terms of their morphology one of the types is the so called multiple coronae models that account for multiple coronae formation include the following emplacement of several diapirs the movement of the lithosphere over a stationary plume and migration of a plume under a stationary lithosphere this work presents the study of a multiple corona located in the helen planitia quadrangle v52 oanuava corona and another corona unnamed formed in the vicinity of oanuava in order to test the viability of these models detailed structural mapping shows a migration of magmatic activity along the structure during corona formation and the development of a set of fractures in response to a local stress field the latter is used as a time marker for the evolution of oanuava and the unnamed coronae we propose that this migration could be explained as the result of a sublithospheric lateral flow and ponding of a mantle plume or diapir into a zone of uplifted and thinned lithosphere plume channeling induced by the interaction of both coronae

sedimentary characteristics and interactions among volcanic terrigenous and marine processes in the late permian kuishan member eastern block of the north china craton abstract understanding the variability of depositional environment and depositional processes of the late permian kuishan member north china craton is crucial for unraveling the evolution of sedimentary basin and epicontinental sea in shandong tectonic domain the kuishan member in the eastern block of the north china craton formed in association with general regression of an epicontinental sea is composed of abundant volcanic deposits and nearshore sedimentary deposits three lithofacies assemblages together with eleven lithofacies are identified in the study area including volcanic deposits of the turbidity lithofacies assemblage epiclastic deposits of the nearshore lithofacies assemblage and mixed volcaniclastic and epiclastic sedimentary deposits of the terrestrial lithofacies assemblage these alternating volcanic epiclastic lithofacies assemblages illustrate the depositional environments induced by volcanic activities and nearshore wave influence syn eruptive lithofacies in the shallow water area were formed by high density turbidity currents and ash fall into water introduced by rising sea levels and high rainfall due to volcanic eruptions to the north whereas thin bedded shallow marine silty mud developed in the distal area inter eruptive lithofacies were represented by tuffaceous mudstone deposited in an exposed and oxidized setting due to falling sea level whereas epiclastic nearshore deposits accumulated by wave or current flow regimes paleocurrent measurements show that the predominant regression direction changes from uniform southwest to divergent southwest and southeast during volcanic inter eruptive periods the kuishan succession therefore can be regarded as a volcano related sequence in which the input of volcaniclastic and epiclastic deposits were significantly modulated by controls of volcanism with frequently changing coastline position of the epicontinental sea and alternating depositional environments

volcaniclastic debris flows at la fossa volcano vulcano island southern italy insights for erosion behaviour of loose pyroclastic material on steep slopes abstract this paper describes a qualitative model of erosion of loose volcaniclastic material on steep slopes of la fossa cone vulcano island italy that generates small volume debris flows the loose volcaniclastic cover rests on an almost impermeable substratum which comprises a fine grained hydromagmatic tuff morphologically three sectors were distinguished 1 a water supply zone slope 32 where rain water is enriched in mud and concentrated in the initiation zone 2 an initiation zone or source area sensu stricto slope with 30 where debris flows form and 3 a transport and deposition zone where debris flows deposited levees and terminal lobes slope 30 main episodes of erosion of loose volcaniclastic cover and debris flow generation occur in response to thunderstorms the erosion of tuff from superficial water added a significant amount of fines mud fraction to the initiating flows debris flow stoppage on steep slopes was interpreted as the result of a loss of mass estimated between 35 and 53 of total mass to form levees from field and laboratory data the density of debris flows was estimated at about 1950 kg m3 morphological data matrix texture mass number nm and clay content of debris flow deposits suggest that they were deposited by saturated granular slurries

linear theory of the sea breeze in a thermal wind this article investigates the linear dynamics of the sea breeze in an along shore thermal wind shear the present analysis shows that the sea breeze circulation is tilted towards the slanted isentropes associated with the thermal wind at a critical value of the thermal wind shear the tilt of the sea breeze circulation becomes equal to the slope of the background isentropes the present analysis also shows a spatial shift between the heating pattern and the sea breeze circulation the present linear theory is then applied to interpret measurements made in the vicinity of new york city where there is a warm season synoptic southwesterly jet it is compared with observations and past numerical simulations agreement is found with respect to the enhanced along coast wind that follows the tilted isentropes the order of magnitude of the isentrope tilt and the clockwise rotating wind hodograph showing the jet maximum peaking at 1800 solar time there is a disagreement between theory and observations on the phase lag between the jet maximum and the cross shore pressure gradient maximum however this disagreement can reasonably be attributed to either the angle made by the synoptic jet to the coastline and or the presence of friction the inland spatial shift of the breeze indicated by the theory might also be indirectly confirmed by the coastal inlan dw ind observations of al arger diurnal amplitude for a stronger synoptic jet copyright c 2011 royal meteorological society

symmetries of volcanic distributions on mars and earth and their mantle plume dynamics the symmetries in the distribution of major volcanic centers on mars have been analyzed by employing a technique involving the angular self similarity of point fields the distribution of all central vent volcanoes exhibits an axisymmetry along with a reflecting symmetry about a plane whose normal is the axis of axisymmetry the latter axis is skewed by about 30 from the rotational axis and is suggested to represent the early axis of symmetry of the interior dynamics the high level of symmetry of mars volcanic distribution may be the result of a focusing of seismic energy from a major impact in the hellas area that formed the hellas basin and may have triggered the onset of volcanic activity in the greater tharsis area if the highland paterae near the hellas region are not taken into account in the analysis an asymmetry with respect to the reflecting plane appears this is probably representative of the late hesperian and amazonian when the once global volcanic activity had retreated to the tharsis province these angular symmetries suggest a simple pattern of long wavelength axisymmetric martian mantle convection involving a large stationary plume as shown by our three dimensional 3 d spherical shell simulations on the basis of these calculations we venture to propose that the volcanic history of mars reflects the transformation of a bipolar mantle plume structure into a single megaplume by the process of plume plume collision acting in concert with the deep martian phase transitions this transformation might have overcome the original pattern induced by a major impact forming the hellas basin we have repeated the procedure for the earth s hotspot distribution which revealed both axisymmetry and a certain level of reflecting symmetry this similarity between the two planets is striking the symmetries of the earth s hotspot distribution are probably due to the interaction of the lower mantle megaplumes with the endothermic perovskite to spinel phase transition

topographic effects on the tropical land and sea breeze abstractthe effect of an inland plateau on the tropical sea breeze is considered in terms of idealized numerical experiments with a particular emphasis on offshore effects the sea breeze is modeled as the response to an oscillating interior heat source over land the parameter space for the calculations is defined by a nondimensional wind speed a scaled plateau height and the nondimensional heating amplitude the experiments show that the inland plateau tends to significantly strengthen the land breeze part of the circulation as compared to the case without terrain the strengthening of the land breeze is tied to blocking of the sea breeze density current during the warm phase of the cycle the blocked sea breeze produces a pool of relatively cold stagnant air at the base of the plateau which in turn produces a stronger land breeze density current the following morning experiments show that the strength of the land breeze increases with the terrain height at least for moderate values of the height

an analytical solution for three dimensional sea land breeze abstractbased on the hydrostatic incompressible boussinesq equations in the planetary boundary layer pbl the three dimensional sea land breeze slb circulation has been elegantly expressed as functions of the surface temperature distribution the horizontal distribution of the horizontal or vertical motion is determined by the first or second derivative of the surface temperature distribution for symmetric land sea and temperature distribution the full strength of the sea breeze occurs inland but not at the coastline and the maximum updraft associates with the heating center setting the temperature difference between land and sea tdls which varies with the island size there would exist an optimal island size corresponding to the strongest slb circulation that weakens with both a larger and smaller island size each velocity component approaches a peak at a certain vertical level both the peak value and the corresponding vertical level link with the vertical scale of the surface temperature

venus resurfacing by coronae implications from impact craters coronae common volcano tectonic features on venus may be actively modifying the planet s surface to evaluate the extent of their effects we compare impact crater densities and proportions of modified craters out to four radii for monte carlo simulations of corona location with those for two corona classes we study the radial concentric and concentric caldera coronae the two classes believed to represent the youthful and senescent stages of formation respectively a high proportion of tectonized craters and low crater density near radial concentric coronae indicate recent activity based on the average crater density and proportion of tectonized craters we interpret concentric caldera coronae as old these results show that tectonism and volcanism is not limited to the corona interior and can extend up to four radii from the center

volcaniclastic aggradation in a semiarid environment northwestern vulcano island italy most studies of volcaniclastic facies and aggradation cycles have been conducted in regions with humid climates and on regional scales here we document a volcaniclastic succession that formed in a semiarid climate characterized by rare heavy rains onto a relatively barren volcanic landscape on vulcano island italy the deposits which we informally call the cuesta succession occur in a small valley between two rhyolite domes and consist of a sequence of pyroclastic surge and fallout deposits interbedded with their reworked equivalents a phase of eruptive activity characterized by sporadic hydrovolcanic explosions supplied ash to the valley and its flanks runoff events during the eruptive phase continually washed ash down to the valley floor in the form of hyperconcentrated flows the inferred transport mechanism and depositional facies of these beds reflect the control of primary volcanic fragmentation processes on sedimentation the abundant medium to coarse ash supplied by eruption was incorporated sufficiently easily into the runoff to hyperconcentrate the flows but not sufficiently fine grained to make the flows cohesive these flows moved out onto the floor merged and transported ash down the valley axis depositing the ash as a monotonous succession of massive to faintly laminated beds the combination of primary deposits and the deposits washed off the valley flanks led to aggradation of the valley floor as eruptions waned ash was eroded off the flanks faster than it was replenished leading to a stratigraphic upward increase in lithic clasts derived from the underlying lavas and a wider range of sedimentary facies as the ash load became more variable after eruptions ceased and all remaining ash was removed from the flanks aggradation gave way to degradation to form small canyons that expose the succession

venus coronae craters and chasmata the distributions of venus coronae and craters are related to chasmata which are thought to be extensional zones coronae are almost twice as dense near the chasmata as a random set of the same size of the various types of coronae the radial concentric and multiple are even more highly concentrated near chasmata whereas the concentric caldera type are absent near the chasmata craters to the first order are randomly distributed although when distributions are compared with random sets there is a deficit of about 15 20 craters close to the chasmata the tectonized and embayed craters tend to be near the rift zones and their distribution closely resembles that of the coronae the higher proportion of tectonized and especially embayed craters within 5 10 of the arcs is about the fraction that would be expected from modification of the full set of craters over a width of 100 200 km kolmogorov smirnov statistics are used to compare cumulative distributions for craters and coronae with a large random distribution and to compute probabilities craters and coronae fill disjoint regions that are more connected than regions of randomly assigned points this suggests that the volcano tectonic process creating coronae may be the same one destroying craters

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

relation of major volcanic center concentration on venus to global tectonic patterns global analysis of nasa magellan image data indicates that a major concentration of volcanic centers covering 40 percent of the surface of venus occurs between the beta atla and themis regiones associated with this enhanced concentration are geological characteristics commonly interpreted as rifting and mantle upwelling interconnected low plains in an annulus around this concentration are characterized by crustal shortening and infrequent volcanic centers that may represent sites of mantle return flow and net down welling together these observations suggest the existence of relatively simple largescale patterns of mantle circulation similar to those associated with concentrations of intraplate volcanism on earth

convective circulations induced by surface heterogeneities a simple theory for convective circulations induced by surface heterogeneities is proposed the theory is based on the thermodynamics of heat engines and provides a simple physical explanation for the general characteristics of circulations forced by surface inhomogeneities in sloping terrains in particular the theory is applied to a mesoscale circulation induced by deforestation it predicts that the intensity of the mesoscale convective circulation forced by deforestation depends on the difference of the near surface nonadiabatic temperature and humidity between the forest and cleared regions and on the depth of the convective boundary layer the theory was successfully tested against observations made during a field experiment in the amazon forest and a nearby clearing

correlation of venus surface features and geoid abstract the distributions of pairs of features on venus are compared using simple statistical tests we consider coronae types crater types and chasmata some of these are obviously associated and some are possibly associated coronae are grouped to represent evolutionary stages tectonic and volcanic modification of craters is considered and the chasmata are represented by five great circle arcs these arcs fit rift or extension zones at the 90 level totaling 55 000 km comparable to the length of spreading ridges on earth comparing these sets with venus geoid field we find that all the chasmata correlate positively with geoid highs and negatively with lows with the peaks in the geoid field occurring at the intersections of the arcs modified craters also exhibit this relation to the geoid the domal coronae the youngest and currently active may be sites of upwelling these correlate strongly with geoid highs and negatively with geoid lows whereas later stage coronae the majority do not show this correlation strong association of the youngest coronae with themis atla suggests that the most recent activity occurs along this arc

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

sea breeze circulation over jakarta indonesia a climatology based on boundary layer radar observations abstract characteristics of sea breeze circulation over the tropical site of jakarta indonesia have been documented based on analyses of satellite images and data from long term l band boundary layer radar measurements carried out at serpong 6 4 s 106 7 e inspection of satellite imagery reveals that a sea breeze front develops well along the northern coastal plain of west java and propagates inland until its structure is deformed over complex topography it is found that the sea breeze signal detected by the boundary layer radar is most well defined during the dry season months of july october in all of these months radar observations indicate a late afternoon intensification of sea breeze flow in the 0 5 0 8 km height range between 1700 and 1800 lt which is not elucidated upon by surface measurements the effect of weather conditions on the sea breeze pattern is investigated by using a cloudiness index derived from data of incoming solar radiation the results show that sea breeze intrusion over

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

mesoscale climatology and variation of surface winds over the chukchi beaufort coastal areas abstractthe detailed mesoscale climatology of surface winds in the chukchi beaufort seas and adjacent arctic slope region is analyzed using the recently developed chukchi beaufort high resolution atmospheric reanalysis cbhar within the study area surface winds are mainly driven by the prevailing synoptic weather patterns of the beaufort high and aleutian low and are further modulated by local geographic features through thermodynamic and dynamic processes sea breezes up or downslope winds and the mountain barrier jets are all clearly captured by cbhar sea breezes emerge in june september and last most of the day with a maximum spatial extent 100 km inland and 50 km offshore and maximum speed around 1 3 m s 1 in the late afternoon 1500 alaska standard time akst thermodynamic impacts of mountains on the surface winds vary from time to time drainage flows begin to build at the mountaintop in september and reach the strongest during november february occupying the entire slope upslope winds

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

models for the hotspot distribution published hotspot catalogues all show a hemispheric concentration beyond what can be expected by chance cumulative distributions about the center of concentration are described by a power law with a fractal dimension closer to 1 than 2 random sets of the corresponding sizes do not show this effect a simple shift of the random sets away from a point would produce distributions similar to those of hotspot sets the possible relation of the hotspots to the locations of ridges and subduction zones is tested using large sets of randomly generated points to estimate areas within given distances of the plate boundaries the probability of finding the observed number of hotspots within 10 of the ridges is about what is expected

equilibrium resurfacing of venus results from new monte carlo modeling and implications for venus surface histories abstract venus impact crater population imposes two observational constraints that must be met by possible model surface histories 1 near random spatial distribution of 975 craters and 2 few obviously modified impact craters catastrophic resurfacing obviously meets these constraints but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2 whereas those with large incremental resurfacing areas meet constraint 2 but not 1 results of monte carlo modeling of equilibrium resurfacing strom et al 1994 is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing however the monte carlo models did not consider intermediate size incremental resurfacing areas nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing we construct three suites of monte carlo experiments that examine incremental resurfacing areas not previously considered 5 1 0 7 and 0 1 and that vary the duration of resurfacing relative to impact crater formation time 1 1 suite a 5 6 suite b and 2 3 suite c we test the model results against the two impact crater constraints several experiments met both constraints the shorter the time period of equilibrium resurfacing or the longer the time of crater formation following the cessation of equilibrium resurfacing the larger the possible areas of incremental resurfacing that satisfy both constraints equilibrium resurfacing is statistically viable for suite a at 0 1 suite b at 0 1 and suite c for 1 0 7 and 0 1 areas of incremental resurfacing

the effect of moist convection on thermally induced mesoscale circulations a basic understanding of the mechanisms controlling the characteristics of thermally induced mesoscale circulations rests primarily on observations and model studies of dry convection whereas the influence of moist convection on these characteristics is not well understood large eddy simulations are used to investigate the effect of moist convection on an idealized mesoscale circulation sensitivity studies show that moist convection has a significant influence on the characteristics of the mesoscale circulation we identify three distinct convective phases that influence the mesoscale circulation within the diurnal cycle firstly dry convective onset with a weak circulation and a breeze front that propagates slowly from the cold region into the warmer fluid as a result of the surface discontinuity secondly a deep convective phase where the circulation intensifies and the breeze front propagates faster and finally a precipitating phase where strong cold pools develop at the breeze front and accelerate the propagation speed further classical density current theory fails to represent the second phase and is extended using the cloud base mass flux to account for the observed effects of moist non precipitating convection on the propagation speed we demonstrate the applicability of this theory to the results from large eddy simulations identify the subtle role of cold pools on density current propagation and highlight implications for numerical weather prediction

spatial and temporal relations between coronae and extensional belts northern lada terra venus preliminary studies of the distribution of coronae and volcanic rises on venus show that many of these features tend to cluster along zones of rifting and extension the plains north of lada terra are crossed by two such extensional belts each belt is composed of grabens ridges faults volcanic flows coronae and coronalike features the longer and more prominent belt is the nw trending alpha lada extensional belt which is over 6000 km long and 50 200 km wide and includes the coronae eve tamfana carpo selu derceto otygen and an unnamed corona south of otygen the second belt is the nne trending derceto quetzalpetlatl extensional belt which is about 2000 km long and in places over 300 km wide and includes the coronae sarpanitum eithinoha and quetzalpetlatl the two belts intersect at the 1600 600 km wide derceto volcanic plateau it is apparent that deformation along the two belts overlapped in time though deformation along the alpha lada extensional belt probably continued after the deformation along the derceto quetzalpetlatl extensional belt terminated in certain areas volcanism originated in grabens within the extensional belts whereas in other areas such as in eve selu derceto and quetzalpetlatl volcanism originated in the coronae and flowed into the lower parts of the extensional belts regional extension has affected the evolution of all the coronae at some stage of their development regional deformation occurred before the initiation of derceto and eithinoha and after the initiation of carpo tamfana otygen and sarpanitum it is thus unlikely that coronae formation along the belts is solely a consequence of the regional extension and it is also unlikely that regional extension has been caused solely by the coronae no corona along the belts was formed subsequent to the cessation of the regional extension we therefore suggest that the regional extension and the coronae are interrelated some of the coronae may have determined the location of the surface expression of the regional extension whereas the locations of other coronae may have been influenced by the concentration of regional extensional stresses

numerical simulation of nonlinear mesoscale circulations induced by the thermal heterogeneities of land surface mesoscale circulations forced by a random distribution of surface sensible heat flux have been investigated using a three dimensional numerical model the complex land surface is modeled as a homogeneous random field characterized by a spectral distribution standard deviation and length scale of the sensible heat flux at the surface have been identified as the important parameters that describe the thermal variability of land surface the form of the covariance of the random surface forcing is not critical in driving the mesoscale circulation the thermally induced mesoscale circulation is significant and extends up to about 5 km when the atmosphere is neutral it becomes weak and is suppressed when the atmosphere is stable the mesoscale momentum flux is much stronger than the corresponding turbulent momentum flux in the neutral atmosphere while the two are comparable in the stable atmosphere the mesoscale heat flux has a different vertical profile than turbulent heat flux and may provide a major heat transport mechanism beyond the planetary boundary layer the impact of synoptic wind on the mesoscale circulations is relatively weak nonlinear advection terms are responsible for momentum flux in the absence of synoptic wind

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

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

geologic mapping of tectonic planets geological analysis of planets typically begins with the construction of a geologic map of the planets surfaces using remote data sets geologic maps provide the basis for interpretations of geologic histories which in turn provide critical relations for understanding the range of processes that contributed to the evolution because geologic mapping should ultimately lead to the discovery of the types of operative processes that have shaped a planet surface geologic mapping must be undertaken in such a way as to allow such discovery i argue for modifications in current planetary geologic mapping methodology that admit that tectonic processes may have contributed to the formation of a planet surface and i emphasize a goal of constraining geologic history rather than determining global stratigraphy to this end it is imperative that secondary tectonic structures be clearly delineated from material units each record different aspects of planet surface evolution neglecting such delineation can result in geologic maps and interpreted geohistories in which both the spatial limits and the relative ages of material units and suites of secondary structures are incorrect determination of absolute time is fundamentally difficult to constrain in planetary studies in planetary geology the only means to estimate absolute age is the density of impact craters on a surface crater density surface ages are more akin to terrestrial and average mantle model ages which reflect the average time at which all of a rock s components were extracted from the earth s mantle rocks or planetary surfaces with very different geohistories could yield the same average model age dating tectonic events or determining rates of tectonic events is even more difficult fl 2000 elsevier science b v all rights reserved

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

thermal mesoscale circulations on the baltic coast 2 perturbation of surface parameters a three dimensional numerical study of an observed seabreeze sb is presented in a companion paper part 1 this paper part 2 addresses the influence of various forcing parameters emanating from the physical surface and perturbing the control run the control run in part 1 reveals that the sb stage is preceded by a coastal jet cj stage and that even moderate terrain seems to have a strong influence on the coastal mesoscale circulation the parameter sensitivity analysis attempts to identify the hierarchy of surface effects on the previously observed and modeled dynamics while the background airflow parameters remain unchanged seven perturbation runs are presented two topographic three land surface and two sea surface temperature perturbations it is shown that the terrain elevations h 0 206m have profound impact on the flow and its time history the main offshore island has a notable impact but does not change the main flow structure surface temperature perturbations modify the cj pattern dictated by the mesoscale thermal wind they also influence the sb shape more but the sb front propagation less the sb at one coast is locked up in phase with another sb at an almost perpendicular coast i e the first sb waits until the other sb removes the opposing ambient flow this is governed by the specific distribution of land and water and by the direction of the ambient flow against the coastline introduced sst gradients have minor to moderate localized influence on the dynamics for this case

the spatial distribution of coronae and related features on venus coronae are large quasi circular geologic features that are common on venus they appear to be the surface tectonic and volcanic expressions of mantle diapirs that have impinged on the underside of the venusian lithosphere we have investigated the spatial distribution of 335 coronae and related features identified in magellan radar data it is more clustered than a poisson distribution with a statistical certainty of more than 99 it is dominated by a single large cluster centered near the equator at about 245 longitude the features are preferentially found at elevation and geoid values close to the planetary mean with a paucity at both the highest and lowest levels of topography and geoid some coronae appear aligned in quasi linear chains we attribute the clustering of coronae and related features to preferential formation of these features above regions of broad scale mantle upwelling and suggest that a major mantle upwelling underlies the one large cluster we suggest that coronae are rare at the lowest elevations because these may be regions of mantle downwelling the shortage of coronae at the highest elevations may result both from obscuration by other intense tectonism there and from suppression there of their formation by an unusually thick crust corona chains may be produced by enhanced passive mantle uplift below failed or incipient rifts

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

when land breezes collide converging diurnal winds over small bodies of water over enclosed and semi enclosed bodies of water the land breeze sea breeze circulation is expected to be modified by the presence of opposing coastlines these effects are studied using satellite scatterometer surface wind observations from the quikscat and adeos 2 tandem mission from april october 2003 winds are studied for six bodies of water the red sea the gulf of california the mediterranean the adriatic sea the black sea and the caspian sea these bodies of water are large enough for the geographic orientation of the diurnal winds relative to the coastline to match the expected orientation for a straight coastline land breezes from opposite coastlines converge in the middle of these bodies of water and in some cases the convergence line is shifted substantially away from the midpoint between opposite coastlines displacements in the convergence line appear likely to be explained by differences in the strength of the diurnal winds emanating from opposite coastlines associated with differential heating or island peninsula effects and by geographic displacements associated with large scale mean wind patterns

thermal mesoscale circulations on the baltic coast 1 numerical case study a three dimensional mesoscale numerical model is utilized to investigate a well documented seabreeze event taken from experimental data the simulations are not set up as a model validation instea

implications of a global survey of venusian impact craters abstract we present a global survey of the areal distribution size frequency distribution and morphometric properties of the venusian impact cratering record we explore the resurfacing history of venus crater degradation ejecta emplacement and cratering mechanics the number of volcanically embayed and tectonically deformed craters from 0 5 to 1 0 km above mean planetary radius is disproportionately high for an otherwise crater deficient elevation range more resurfacing occurred in this range an elevation range dominated by volcanic rises rifts and coronae than elsewhere on venus although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets some craters appear particularly fresh because they have radar bright floors a radar dark halo surrounding the ejecta blanket and a west facing parabola of low radar return 20 35 and 8 respectively of craters with diameters 22 6 km have these features statistical correlations support the idea that such craters are younger than the remaining crater population characteristics of ejecta deposits for venusian craters change substantially with size particularly at 20 km crater diameter which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens we suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches a diameter independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape distribution and dispersion of impacting fragments secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters the large distances secondary craters extend from the crater rim imply that the secondary forming projectiles traveled in an atmosphere significantly altered by the impact process comparison of complex craters found on venus with those of other planets gave results that were consistent with the idea that inter planetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength the interplanetary comparison indicates that venus the moon and mercury appear to have stronger crusts than do mars and ganymede callisto

dynamics of reversed urban breeze circulation abstractthe urban breeze circulation ubc is a thermally forced mesoscale circulation that is characterized by low level inward flow toward the urban center updrafts near the urban center upper level outward flows and weak downdrafts outside the urban area previous numerical modeling studies indicate that in the early morning the direction of the ubc can be reversed here the dynamics of a reversed ubc is studied in the context of the response of the atmosphere to specified thermal forcing which represents diurnally varying urban heating for this a linearized two dimensional hydrostatic boussinesq airflow system in a rotating frame with specified thermal forcing is solved using the fourier transform method the occurrence of a reversed ubc in the early morning is confirmed the coriolis parameter affects the strength and vertical structure of the ubc whose role is similar to that of the coefficient of rayleigh friction and newtonian cooling the occurrence condition strength and vertical st

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

palaeomagnetism of welded pyroclastic fall scoriae at vulcano aeolian archipelago abstract this paper deals with the magnetic fabric and remanent magnetization of welded scoriae fall deposits four scoriae blankets were emplaced on vulcano aeolian islands from about 50 to 8 ka their detailed chronostratigraphic position has been debated in the literature and they have therefore been given a different interpretation in the reconstruction of the vulcano s eruptive history the aims of the study were a better understanding of the magnetic features of the scoriae fall deposits which have so far been given little attention and to set the chronostratigraphic relationships of the sampled deposits on the ground of their palaeomagnetic directions overall the magnetic properties of the four deposits are alike investigation of isothermal remanence and susceptibility versus temperature measurements point to low ti titanomagnetite as the main ferromagnetic mineral magnetic foliation is well developed moderately dispersed and in most cases close to horizontal lineation is either clustered or dispersed within the foliation plane stepwise thermal and alternating field demagnetization shows that secondary components are negligible in two units in the other two they are always completely removed in the first steps below 20 mt or 400 c thereafter the characteristic component chrm is clearly isolated and the within site dispersion of its direction is small 95 95 the palaeomagnetic results corroborate the hypothesis that the investigated scoriae deposits correspond to different volcanic phases related to recurrent arrival of mafic magma and show that those of spiaggia lunga and quadrara were produced during two distinct periods of volcanic activity with little time separation between them

observations and modeling of the sea breeze with the return current abstract data from a wind profiler are used to investigate the vertical structure of the sea breeze with the accompanying return current at the north sea coast in the netherlands for three sea breeze cases in one of the cases with a very weak and constant background flow the return current mass flux approximately compensates for the sea breeze mass flux to study the sea breeze with the accompanying return current a simple two dimensional mesoscale model is constructed it is found that the sea breeze as well as the return current of the three cases in this study are simulated well by the model in the case mentioned above the model indicates an overcompensation of the sea breeze by the return current it is found that the latter is a function of the initial vertical temperature profile and the depth of the boundary layer and that it can be as much as 30 the overcompensation is balanced by a mass flow in the sea breeze direction at a larger height return return current as such a three laye

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

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

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

large scale volcanism associated with coronae on venus implications for formation and evolution large scale volcanism in the form of areally extensive flow fields is a previously unrecognized important aspect of the evolution of at least 41 of all coronae on venus the timing and scale of many coronae flow fields is consistent with an origin due to the arrival and pressure release melting of material in the head of a mantle plume or diapir the production of voluminous amounts of volcanism at some coronae is proposed to be the result of larger plume size and or the intersection of mantle upwellings with regions of lithospheric extension and rifting