source mechanisms of usu volcano japan earthquakes and their tectonic implications abstract an intensive earthquake swarm and remarkable surface deformation due to dacitic doming have taken place at the usu volcano in southwestern hokkaido japan since august 1977 a major u shaped fault has formed in the summit crater and the amount of upheaval reached 180 m for 5 years in order to reveal the quantitative relation between this doming deformation and the associated earthquakes the mechanisms of the latter have been studied for this purpose broadband and wide dynamic range seismic observations were performed in a near field region of the usu volcano for the period from june 1979 to july 1980 long period seismometers and a digitally recording accelerometer system were used for relatively large earthquakes the data from the digital recording system permit very accurate calculations of ground displacements by employing the recursive filter method source parameters such as seismic moment average dislocation stress drop and fault dimensions for five earthquake families have been determined from a comparison of the observed ground displacements with synthetic ones synthetic displacements were computed using haskell s formulation for relatively deep earthquakes the discrete wavenumber dw method was used for relatively shallow earthquakes the stress drop varies widely among earthquakes in the same earthquake family the cumulative seismic deformation caused by the large earthquakes is almost consistent with the observed surface deformation it is suggested that the large earthquakes are caused by stick slip motions at the two sides of the u shaped fault

a gravity survey of type 2 coronae on venus 1 coronae are roughly circular tectonic features believed to form as a result of small scale mantle upwellings we examine the admittance signature for all the 32 type 2 coronae defined as having less than 50 complete fracture annuli that are both well resolved in the gravity data and have fairly well constrained lithospheric parameters we use top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation both cartesian and spatiospectral methods are used to calculate admittance overall we find reasonable agreement between the two methods the shape of the filter used in the spatiospectral method simons et al 1997 provides a means of isolating the signature of coronae that occur in close proximity to other geologic features the narrow filters used in the cartesian approach are useful for identifying transitions between different types of compensation as well as for estimating error we obtain a wide range of elastic thicknesses from 25 to 80 km larger values are derived from bottom loading models which have not been used in most prior studies of venus fifteen of the coronae have elastic thickness values between 0 and 20 km and are probably isostatically compensated estimates of crustal and elastic thicknesses and apparent depth of compensation obtained for type 2 coronae span the range obtained for venus globally neither the thickness of the elastic lithosphere nor the crust appear to control whether type 1 coronae type 2 coronae or volcanoes form over small scale mantle upwellings

gravity analysis of parga and hecate chasmata implications for rift and corona formation 1 the two largest rift systems on venus parga and hecate chasmata contain one third of all coronae we map variations in elastic thickness and apparent depth of compensation adc in these two regions using the admittance function for gravity and topography we examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism topographic shape fracture pattern diameter and stratigraphic age at hecate chasmata both adc and elastic thickness correlate with two main rift branches the fracture style and corona characteristics hecate chasma thus appears to be dominated by extensional processes parga chasma shows little correlation between lithospheric properties and the location of coronae or rift segments the adc at hecate and parga chasmata is mostly less than 75 100 km implying that there is not large scale upwelling underlying the rift systems the variations in the corona population sizes between parga chasma which has 131 coronae and hecate chasma with 50 coronae suggest differences in the evolution of the two rift systems the much larger population of coronae at parga contains more small coronae many of which have relatively low volcanism and post date the rift further many of the small coronae occur in chains and have similar diameters as predicted by rayleigh taylor type instabilities these characteristics suggest that the smaller coronae may form via a later secondary decompression melting phase thus hecate chasma may be younger than parga chasma and could experience a secondary stage of corona formation in the future

origin and formation of metabreccia in the parkin offset dike sudbury impact structure canada the 1 85 ga sudbury impact structure is considered a remnant of a peak ring or multi ring basin with an estimated original diameter of 150 to 200 km the offset dikes are radial and concentric dike

magellan mission summary magellan started mapping the planet venus on september 15 1990 and after one cycle one venus day or 243 earth days had mapped 84 of the planet s surface this returned an image data volume greater than all past planetary missions combined spacecraft problems were experienced in flight changes in operational procedures and reprogramming of onboard computers minimized the amount of mapping data lost magellan data processing is the largest planetary image processing challenge to date compilation of global maps of tectonic and volcanic features as well as impact craters and related phenomena and surface processes related to wind weathering and mass wasting has begun the magellan project is now in an extended mission phase with plans for additional cycles out to 1995 the magellan project will fill in mapping gaps obtain a global gravity data set between mid september 1992 and may 1993 acquire images at different view angles and look for changes on the surface from one cycle to another caused by surface activity such as volcanism faulting or wind activity

instability of exogenous lava lobes during intense rainfall on many volcanoes there is evidence of a relationship between dome collapse and periods of high precipitation we propose a mechanism for this relationship and investigate the conditions that optimize failure by this process observations of elongate lobes that evolve through exogenous growth of lava domes reveal that they commonly develop tensile fractures perpendicular to the direction of motion these cracks can increase in depth by localized cooling and volumetric contraction during periods of high rainfall water can fill these cracks and the increase in fluid pressure on the base of the lobes and within the crack can trigger the collapse of the hot exogenous lava domes using limit equilibrium analysis it is possible to calculate the water and vapor forces acting on the rear and base of the potentially unstable part of the lobe the model presented is rectangular in cross section with material properties representative of andesitic dome rocks vapor pressures at the base of cracks are sealed by the penetrating rainfall which forms a saturated cap within the lobe this leads to an increase in fluid pressurization both through the underlying gas pressure and the downslope component of the liquid water cap fluid pressurization increases as the penetration depth increases this rainfall penetration depth is dependent on the thermal properties of the rocks antecedent temperature lobe geometry and the intensity and duration of precipitation dominant parameters influencing the stability of the lobe are principally lobe thickness duration and intensity of rainfall and antecedent lobe temperature our modeling reveals that thicker lobes are intrinsically more unstable due to the amplification of downslope forces in comparison to cohesive strength the increase in the duration and intensity of rainfall events also increases the potential for collapse as it leads to deeper liquid penetration deeper penetration depths are also achieved through lower antecedent temperatures since less fluid is lost through vaporization thus the potential for rain triggered collapse increases with time from emplacement

propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields abstract palaeofluid transporting systems observed as networks of mineral filled veins in deeply eroded parts of extinct geothermal fields indicate that hydrofractures commonly supply fluids to geothermal fields here we examine well exposed vein networks that occur at crustal depths of around 1 5 km below the initial surface of the tertiary lava pile in north iceland the veins are located in the damage zone of a major fault zone that dissects basaltic lava flows the most common host rocks of geothermal fields in iceland the lava flows contain numerous weaknesses particularly columnar cooling joints and contacts for hydrofractures to supply fluids to geothermal fields the fractures must be able to propagate and transport fluids to the surface we explore hydrofracture pathway formation using boundary element models of hydrofractures with fluid overpressure varying linearly from 10 mpa at the fracture centre to 0 mpa at the fracture tip or the fluid front the hydrofractures propagate through a vertically jointed and horizontally layered pile of lava flows with a general rock matrix young s modulus of 1 10 10 pa and a poisson s ratio of 0 25 the joints and contacts between layers are modelled as internal springs each with a stiffness strength of 6 mpa m the location and sizes of discontinuities as well as the location of the hydrofracture tip vary between the models the results indicate that tensile stresses generated at the tip of an overpressured hydrofracture can open up horizontal and vertical discontinuities out to a considerable distance from the tip and that these discontinuities eventually link up to form the hydrofracture pathway analytical models indicate that for a hot spring of a given yield associated with a fault the dimensions of the fluid transporting part of the fault are likely to be similar for a typical normal fault and a strike slip fault also a hot spring of yield 180 l s the maximum in the low temperature fields of iceland can be supplied through a hydrofracture of aperture 3 mm and trace length 1 2 m these dimensions are very similar to those of typical veins in the studied networks buoyancy rather than excess pressure in the fluid source appears to be the primary driving force of hydrofractures in the geothermal fields of iceland

uplift deduced from remanent magnetization sudbury area since 1250 ma ago abstract two contacts between sudbury norite and northwest trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement a zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion or sampling surface at the time of magma emplacement remanence direction in one of the dikes and norite contact zones is not typical for the sudbury dike swarm of 1250 ma age and this contact is not further considered an earlier published result for a norite dike contact was reconsidered because of complicated dike geometry and included in this study in one of the four usable contacts the hybrid zone is represented by three samples in another by one sample and in the remaining two only the contact zone width could be used the final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287 c s d 13 and 267 c s d 11 calculated without and with a correction for viscosity of the host remanence respectively using a gradient of 26 c km for 1250 ma ago indicates a burial depth of9 5 2km at that time the fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of precambrian terranes the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages

structural interpretation of the midcontinent rift in eastern lake superior from seismic reflection and potential field studies integrated interpretations of potential field and glimpce and industry seismic reflection data in eastern lake superior reveal the structural and stratigraphic complexity of the midcontinent rift in this region projection of the keweenaw fault into southeastern lake superior suggested by early potential field studies is confirmed by seismic reflection data analysis of seismic data in conjunction with aeromagnetic anomalies and regional gravity data also reveals a continuous section of basalt in the footwall of the keweenaw fault the lateral dimensions of this section vary along the strike of the rift from the center of the basin towards the southern flank spatially extensive anticlinal and synclinal features reverse faults and related drag folds imaged by the reflection and enhanced potential field data attest to the influence of a late stage compressional event in this region east northeast trending gradients and displacements associated with observed potential field anomalies and fault traces mapp

structural characteristics of the sudbury impact structure canada impact induced versus orogenic deformation a review orogenic deformation both preceding and following the impact event at sudbury strongly hinders a straightforward assessment of impact induced geological processes that generated the sudbury impact structure central to understanding these processes is the state of strain of the sudbury igneous complex the solidified impact melt sheet its underlying target rocks overlying impact breccias and post impact sedimentary rocks this review addresses 1 major structural metamorphic and magmatic characteristics of the impact melt sheet and associated dikes 2 attempts that have been made to constrain the primary geometry of the igneous complex 3 modes of impact induced deformation as well as 4 mechanisms of pre and post impact orogenic deformation the latter have important consequences for estimating parameters such as magnitude of structural uplift tilting of pre impact huronian strata and displacement on major discontinuities which collectively have not yet been considered in impact models in this regard a mechanism for the emplacement of offset dikes is suggested that accounts for the geometry of the dikes and magmatic characteristics as well as the occurrence of sulfides in the dikes moreover re interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact uncertainties still exist as to whether the sudbury impact structure was a peak ring or a multi ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification

upper crustal basement involved folding in the east range of the sudbury basin ontario inferred from paleomagnetic data and spatial analysis of mafic dykes tilting of crystalline basement rocks associated with folding strain at uppermost crustal levels is difficult to recognize if basement rocks are devoid of traceable marker planes here we use the spatial variation in strike of paleoproterozoic mafic dyke segments complemented by compiled paleomagnetic data to identify tilting in archean basement rocks associated with kilometre scale folds of the eastern sudbury basin ontario spatial analysis of the strike of dyke segments is consistent with generation of the ne lobe and a newly identified anticline referred to as the west bay anticline in the layered sudbury igneous complex sic this anticline accounts better for the structural characteristics of the eastern sudbury basin than a previously proposed anticline with west plunging hinge line the west bay anticline is characterized by abrupt plan view thickness variations in the lower sic and curved faults displaying significant strike separations of sic contacts these structural characteristics are co

central eistla regio origin and relative age of topographic rise large topographic rises on venus pose a number of fundamental questions concerning the mantle processes responsible for them eistla regio a 7500 km long elevated region extending wnw from the west end of aphrodite terra is a particularly interesting example the sappho patera quadrangle 0 25 n 0 30 e includes all of central eistla regio a region dominated by two interesting apparently hybrid structures irnini and anala montes with characteristics of both shield volcanoes and coronae mapping and tectonic analysis within this region indicate that this part of the rise experienced progressive deformation extending from prior to emplacement of the regional plains to after emplacement of young lavas that is for most of the stratigraphically defined history of venus estimates of the relative roles of uplift and volcanic construction in the evolution of irnini and anala montes are critically dependent on whether they are primarily anomalous coronae that have been uplifted or whether they are true shield volcanoes that have evolved into coronae studies elsewhere as well as most models of corona evolution favor the first alternative the geology of central eistla regio favors the second suggesting that local thinning of the lithosphere allowed shield volcanoes to evolve into coronae because these hybrid edifices are not typical of either most coronae or most shield volcanoes on venus they deserve detailed modeling for now the relative roles of uplift and volcanic construction in central eistla regio cannot be even approximately estimated

fractals in geology and geophysics the definition of a fractal distribution is that the number of objectsn with a characteristic size greater thanr scales with the relationn r d the frequency size distributions for islands earthquakes fragments ore deposits and oil fields often satisfy this relation fractals were originally introduced by mandelbrot to relate the length of a coastline to the length of the measuring stick this application illustrates a fundamental aspect of fractal distributions scale invariance the requirement of an object to define a scale in photographs of many geological features is one indication of the wide applicability of scale invariance to geological problems scale invariance can lead to fractal clustering geophysical spectra can also be related to fractals these are self affine fractals rather than self similar fractals examples include the earth s topography and geoid

an overview of venus geology the magellan spacecraft is producing comprehensive image and altimetry data for the planet venus initial geologic mapping of the planet reveals a surface dominated by volcanic plains and characterized by extensive volcanism and tectonic deformation geologic and geomorphologic units include plains terrains tectonic terrains and surficial material units understanding the origin of these units and the relation between them is an ongoing task of the magellan team

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

extension and uplift at alba patera mars insights from mola observations and loading models high resolution mars orbiter laser altimeter mola profiles of the large shield volcano alba patera call for a critical reexamination of models for the growth and evolution of this volcanic edifice an extensive system of graben cuts across alba patera extending from ceraunius fossae to the south to tantalus fossae to the northeast in the vicinity of the edifice the otherwise generally north south orientations of these graben are deflected to circumferential directions mola topography reveals that the circumferential graben are located well up the flanks of alba patera both the type extensional and location midflank of faulting on alba patera are inconsistent with the state of stress predicted for purely surface loaded flexure models to constrain the conditions governing the evolution of alba patera we employ finite element models for the volcano and lithospheric stress field under a combination of loading mechanisms buoyant sublithospheric loads representing underplated magma low density mantle residuum or dynamic support from mantle convection and intralithospheric displacements representing sills trapped by horizontal compressive stresses in the upper lithosphere can generate the observed midflank slope breaks and circumferential extensional fault zones at radial distances comparable to that of the subsurface load s characteristic radial extent both mechanisms moderate stresses in the upper lithosphere in favor of continued magma ascent however bottom loading requires a disk like load geometry and an elastic lithosphere thickness te sufficiently small to allow the load s shape to be apparent in the surface deformation te 32 km such low te is inconsistent with mars global surveyor mgs gravity and topography data in contrast intralithospheric sill complexes can produce the observed topography and faulting at te values consistent with those inferred from gravity and topography 50 te 100 km for these conditions principal stress orientations on the lower flanks are also consistent with the orientations of lower flank graben provided that faulting was induced by dike intrusion or tensile failure rather than shear failure

hotspot evolution and venusian tectonic style because hotspots represent an important manifestation of heat loss on venus their geological evolution is of fundamental importance for any attempt to understand venusian tectonics eistla regio is a 7500 km long moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots it also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle central eistla regio includes two large volcanoes sappho and anala and several coronae in close proximity detailed mapping in this region results in two conclusions of tectonic significance 1 sappho and anala occur near the intersection of two major extensional deformation zones and 2 the coronae are older than the large volcanoes several of the coronae occur as a chain along guor linea one of the major extensional deformation zones stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials thus central eistla regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes the expected result of such a swarm would be thermal thinning of the elastic lithosphere with time however model results geological observations and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time this thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near global resurfacing the global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes

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

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

the mechanics of harmonic gas pressurization and failure of lava domes summary solutions are developed for the harmonic interior pressurization of idealized hemispherical lava domes to determine the evolution of instability with time integrated gas pressure distributions are evaluated to determine uplift on inclined planes of potential detachment that traverse the dome and to evaluate the limit equilibrium stability of blocks isolated by these planes average pressure distributions show an initial transient rise to peak dynamic gas pressures that corresponds to a harmonic steady state progress towards potential instability mirrors this rise and oscillates with the harmonic steady state the timing of pressurization of the dome lavas is modulated by a diffusive time the magnitude of this pressurization is modulated by dome geometry and the magnitude and non dimensional frequency of the interior inflation two distinct pressurization behaviours are apparent the first at high relative inflation frequencies is where the dynamic steady state corresponds to static pressurization at the mean interior pressure and the second at low relative inflation frequencies is where the dynamic state synchronously responds to the oscillating interior pressure boundary condition and peak uplift is proportional to the mean pressure plus the maximum harmonic overpressure these two conditions are separated by a frequency shift or change in fluid diffusivity of only two orders of magnitude and are significant because the destabilizing uplift force may increase by a factor of two where all other parameters except excitation frequency or diffusivity remain fixed these results are applied to stability evaluations of the lava dome at the soufriere hills volcano to examine failure mechanisms potentially consistent with the observed delay in collapse linear models incorporating time invariant strength and transport parameters are only able to represent collapse for interior gas pressures at the upper limit of observed magnitudes conversely behaviours incorporating fatigue weakening or hysteretic permeability enhancement of the dome rocks are able to approach failure for modest pressurization magnitudes to match observed delays in collapse and potentially to explain why some domes collapse explosively whereas others do not

origin of corona dominated topographic rises on venus abstract both large scale mantle upwellings comparable to terrestrial hotspots on earth and smaller scale mantle upwellings known as coronae occur on venus corona dominated rises have many of the characteristics of large scale mantle upwellings or hotspots such as broad topographic rises greater than 1000 km in diameter and large positive gravity anomalies due to the presence of clusters of three to eight coronae three large volcanic rises or hotspots on venus have been classified as corona dominated rises cdrs themis eastern eistla and central eistla regiones cdrs have been interpreted to result from the break up of a large scale plume comparison of the topographic morphology for individual coronae at themis and eastern eistla region to a model of corona evolution indicate that they are in varying stages of evolution at eastern eistla regio all the coronae have essentially the same topographic morphology consistent with a late stage of evolution and the presence of a depleted mantle layer at depth the complex deformation sequences and stratigraphic relationships both between coronae and with respect to the regional plains observed at all three rises indicate a prolonged origin this observation as well as the varying stages of evolution rule out the previously proposed interpretation of corona dominated rises as a manifestation of the break up of a large scale mantle upwelling which requires essentially simultaneous formation of the coronae instead we suggest that other large topographic rises are the manifestation of deep mantle plumes likely to originate at the core mantle boundary and that cdrs are clusters of coronae that originate at a shallower interface perhaps at an upper lower mantle boundary using top and bottom loading flexural models to fit the gravity topography admittance spectrum for each of the three cdrs yields elastic thickness estimates that are 10 15 km greater for bottom loading at longer wavelengths than top loading at shorter wavelengths estimates of elastic thickness assuming top loading are 10 12 and 22 km and 20 25 and 35 km from bottom loading for eastern eistla central eistla and themis regiones respectively we believe that the bottom loading elastic thickness estimates are more reliable because using a top loading model when both types of loading are present yields an unrealistically low elastic thickness estimate as there is no obvious source of surface loading at either themis or at eastern eistla we interpret the top loading admittance signature to be a result of delamination of the lower lithosphere depressing the surface which is consistent with the observed coronae morphologies

toughness and failure of volcanic edifices abstract major volcanic edifices are of two main types stratovolcanoes composite edifices and basaltic edifices shield volcanoes these types of edifices show several remarkable differences for example while some basaltic edifices erupt very frequently many stratovolcanoes erupt very infrequently also most basaltic edifices have gentle slopes commonly 2 12 whereas stratovolcanoes have steeper slopes particularly in their uppermost parts where the slopes may reach 35 42 there have been various attempts to explain these differences but here the focus is on edifice strength or rather toughness toughness of a material is a measure of its resistance to fracture alternatively for a tough material large amounts of energy are needed to cause failure most quoted material toughness critical energy release rate values for rocks are 20 400 j m 2 for extension fractures such as dykes and 0 01 0 03 mj m 2 for shear fractures such as dip slip faults the related property of critical stress intensity or fracture toughness is for most rocks 0 5 3 mpa m 1 2 these values for material and fracture toughness refer to small essentially homogeneous laboratory rock samples here i present equations for calculating the material toughness in volcanic edifices based on the measured or estimated dimensions of associated large structures such as dykes and faults for typical dykes the calculated material toughness is 1 3 47 mj m 2 with a corresponding stress intensity of 114 690 mpa m 1 2 similarly for a 3 8 km long dip slip fault in an edifice the calculated material toughness is 2 3 mj m 2 and the corresponding stress intensity 150 mpa m 1 2 these results compare well with previous material toughness estimates for large strike slip faults and stress intensity estimates for large dykes generalising these results and using field data as well as materials science results i conclude that stratovolcanoes normally have a much higher material toughness than basaltic edifices this is primarily because of different internal structures a basaltic edifice is made of layers and contacts all of which have essentially similar mechanical properties whereas a stratovolcano is made of layers and contacts many of which have widely different properties accordingly it requires much more energy is more difficult for a fracture to propagate through a stratovolcano than through a basaltic edifice thus contrasting toughnesses may be one principal reason why these edifices differ in slope and in failure frequencies including the number of dyke fed eruptions caldera collapses and landslides

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

modeling volcanic deformation in a regional stress field implications for the formation of graben structures on alba patera mars abundant grabens transect the volcano alba patera their complex geometry and formation mechanisms are still poorly understood tectonic processes and magmatic intrusions are responsible for these long surface features cross cutting relationships of the grabens show radial fractures that were formed during early stages and were progressively overprinted by concentric fractures on the mid and upper flanks of the volcano two modeling methods are used to understand the formation of the observed structures and to evaluate their implications for hidden subvolcanic processes surface deformation and fault arrangements predicted in finite element models are compared to the graben systems observed in viking images the orientation and position of the concentric grabens are found to be best reproduced by local crustal subsidence superimposed on a regional nw se oriented extension with decreasing magnitude from south to north in analogue sandbox models we also simulate surface structures of arrangements that almost perfectly mimic the observed lineaments on alba patera formation of the grabens spans a period on the order of a billion years suggesting long term geodynamic processes to be responsible for the subsidence of the central alba patera area the progressive change toward higher concentricity is likely resultant from an increase in density in the crust by accumulation of intrusive material and cooling thus causing subsidence of the region above this volcanic root

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

subduction initiation by thermal chemical plumes numerical studies abstract prior suggestions for the initiation of subduction have included sediment loading compression and plate reconfiguration as potential triggers here we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume numerical testing of this hypothesis has been conducted in 2d with the i2vis thermo mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal chemical or purely thermal plume we demonstrate that a mantle plume can break the lithosphere and initiate self sustaining subduction provided the plume causes a critical local weakening of the lithospheric material above it the intensity of the required weakening depends on the plume volume plume buoyancy and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes another necessary condition is the presence of high pressure fluids at the slab upper interface reducing the effective friction coefficient there to very low values based on our results we suggest that sheet like instabilities of the archean mantle convection could have initiated subduction on earth where ocean was already present in less stable tectonic settings provided that mantle plumes sheets at that time were rich in water and melt which could drastically reduce the effective friction coefficient in the lithosphere above the plume our numerical models are also in good agreement with suggested concepts for corona formation on venus

filling of flexural moats around large volcanoes on venus implications for volcano structure and global magmatic flux the absence of topographic moats and concentric normal faulting around large volcanoes on venus is attributed to filling of the annular flexural depression by lava flows from the central edifice large volcanoes on venus are characterized by prominent approximately conical edifices surrounded by relatively flat flow aprons the surfaces of both of these constructional components consist dominantly of radially oriented flows from analytic plate flexure models we generate a synthetic stratigraphy for venus volcanoes from which we calculate the volume of material filling the flexural moats to the level of the flow apron the total volume of volcano associated extrusive lavas including moat fill can be an order of magnitude greater than the volume of the edifice alone extended to all large volcanoes on venus this procedure yields estimates for magmatic flux on venus comparable to the present terrestrial intraplate extrusive flux but this scenario is nonetheless consistent with observations of areally limited resurfacing since the most recent global resurfacing event the absence of flexure related tectonic features around most large volcanoes by the arguments advanced here is attributable to masking by apron flows and low stresses in the apron moat fill on venus a volcanic edifice and associated moat filling material constitute a single structurally coherent unit large volcanoes on venus thus appear to be structurally distinct from those on earth and mars

embayed intermediate volcanoes on venus implications for the evolution of the volcanic plains abstract volcanoes on venus are classified according to size with studies on the stratigraphic position of large volcanoes proposing that most of the large volcanoes postdate the regional volcanic materials some studies regarding intermediate volcanoes proposed that some of these volcanic features could be large volcanoes with embayed flow aprons a situation that would alter the previous stratigraphic considerations about large volcanoes on venus in this work i analyze the global population of embayed intermediate size volcanoes and compare their summits with that of other edifices classified as large volcanoes intermediate size volcanoes are considered embayed when 1 flows from another source clearly overlap the volcano slopes and 2 display scarps related to flank failure processes but with the associated collapse deposits being absent i e interpreted as covered as result of the survey 88 embayed intermediate size volcanoes have been catalogued and integrated into a geographic information system these embayed volcanoes have summit sizes and characteristics similar to large volcanoes and therefore could be interpreted as possible large volcanoes with their flow aprons embayed embayment materials for these volcanoes include all the units present in the history of the volcanic plains and would indicate that this type of central volcanic edifice would occur throughout the geologic history recorded in the venusian plains

emplacement of sharp walled sulfide veins during the formation and reactivation of impact related structures at the broken hammer mine sudbury ontario broken hammer is a hybrid cu ni platinum group element pge footwall deposit located in archean basement rocks below the impact induced sudbury igneous complex sic canada the deposit consists

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

preliminary analysis of an expanded corona database for venus we have conducted an extensive survey of the magellan data to reassess the population of coronae we identify a new type of coronae here referred to as type 2 coronae having the same basic morphology as previously identified coronae type 1 but lacking a significant 50 annulus of closely spaced fractures 106 type 2 coronae are included in the new database giving a total of 515 coronae on venus the characteristics of the expanded population of type i coronae are similar to those of the previously described population but the type 2 coronae are smaller tend to have relatively flat interiors surrounded by a topographic rim and are more likely to be found isolated from other features our observations in particular the morphology and setting of the type 2 coronae may provide supporting evidence for the existence of a depleted mantle layer under the venusian plains

formation and evolution of volcanic edifices on the dione regio rise venus mapping of dione regio venus a regional highland 1200 by 2700 km in size with mean elevation of 6052 km located in the southern hemisphere to the nw of lavinia planitia reveals a region characterized by early tectonically dominated structure and later widespread volcanism and centralized edifice construction small patches of complexly deformed terrain tessera and larger regions of plains characterized by a single direction of deformation tectonized plains preserved as inliers are embayed by more recent widespread volcanic plains smaller scale plains resurfacing superposed these plains following a pervasive compressional event that lead to distributed wrinkle ridge formation four major volcanoes dominate the topographically irregular highland each with similar early stage radar dark lava flow deposits but significantly different late stage eruptions these late stage differences are manifested as dark diffuse deposits possibly pyroclastic in origin and construction of a single steep sided dome at ushas mons a small shield volcano field at nepthys mons and bright diffuse deposits possible pyroclastic deposits with summit dome and pit formation at innini and hathor montes the presence at all of these volcanoes of radial lineaments and graben interpreted to be related to near surface lateral dyke emplacement and late stage summit constructional features indicate that shallow magma chambers occur at each edifice variations in the evolutionary history of and eruptions from these chambers may explain the differences observed in the deposits at these major edifices it is also possible however that fundamental differences in the magma source region was a major factor in determining the appearance of the deposits of the several models considered for the formation of the region secondary upwellings from a single large mantle plume or the near contemporaneous upwelling of multiple smaller plumes best explain the observed characteristics at the rise a comparison to other volcanic highlands suggests that there are at least two classes of volcanic rises on venus an areally and topographically large type with significant volcanism and rift zone formation like beta regio and a smaller scale type with extensive volcanism but moderate rifting like western eistla regio these two classes reflect a spectrum in size and vigor of mantle upwellings the dione rise is most like western eistla regio in terms of areal extent topographic elevation and geologic complexity

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

corona formation and heat loss on venus by coupled upwelling and delamination coronae are volcanotectonic features that are unique to venus and are interpreted to be small scale upwellings a model in which upwelling causes delamination at the edge of the plume head along with deformation of a preexisting depleted mantle layer produced the full range of topographic forms of coronae if half of the coronae are active delamination of the lower lithosphere could account for about 10 percent of venus9 heat loss with another 15 percent due to upwelling delamination may occur in other geologic environments and could account for some of venus9 heat loss deficit

the geomorphology of planetary calderas abstract satellite derived observations of the geomorphology of calderas on earth mars and venus can be used to learn more about shield volcanoes examples of terrestrial basaltic volcanoes from the galapagos islands hawaii and the comoro islands show how these volcanoes contrast with examples found on mars and venus caldera structure degree of infilling and the location of vents on the flanks are used to interpret each volcano s recent history the geometry of the caldera floor can be used to infer some of the characteristics of the magma storage system and the orientation of the deep magma conduits the formation of benches within the caldera and the effects of the caldera on the distribution of flank eruptions are considered and it is evident that most calderas on the planets are were dynamic features presently deep calderas with evidence of overflowing lavas and ponded lavas high in the caldera wall show that these calderas were once shallow similarly shallow calderas filled with ponded lavas are evidence that they were once deeper it is probably a mistake therefore to place great significance on caldera depth with regard to the position shape or size of subsurface plumbing

instability and collapse of hazardous gas pressurized lava domes an important type of gravitational lava dome failure involves significant internal gas overpressure that influences the failure process for the first time this paper presents a rigorous analysis of the mechanics of gas pressurized dome failure we develop diffusion models to calculate gas overpressures in a lava dome and embed these pressure data into stability analyses to demonstrate how gas pressurization can initiate instability the results bear on the mechanistic understanding of hazardous dome failures gas pressurization can promote deep seated failure and the timing of conduit pressurization can control the timing of dome collapse theory explains why dome collapse can be delayed for cases of oscillating effusion dome collapses and explosive events linked to collapse of lava domes can be hazardous monitoring criteria are considered to anticipate whether or not such collapses are likely to be explosive

the 1980 pressure response and flank failure of mount st helens usa inferred from seismic scaling exponents the cataclysmic 18 may 1980 eruption at mount st helens was preceded by intense seismic activity marking the mechanical response of the volcanic edifice to interior pressurisation this seismicity is analysed to yield the temporal change in the seismic scaling exponent d inferred from the seismic b value that in turn is related to the seismic moment of an earthquake time evolution of d preceding the eruption onset reveals 1 a major decrease in d occurring over only a few days at the end of march 2 a steady but stepped decrease in d steps 5 10 days occurring from the end of march to early may 3 a sharp decrease in d in early may and 4 steady low values of d occurring 2 3 days before the eruption onset this response is interpreted as major ruptures formed at the end of march arresting and participating in but not triggering the ultimate failure of the flank rather the rate of interior fracturing slowed in the 2 months preceding the 18 may 1980 major blast and the triggering of failure is consistent with interior gas overpressurisation the occurrence of two swarms of low frequency seismic events and the high values of the harmonic tremor indicate the action of interior pressurisation on a cycle of 20 25 days solutions are applied to represent the harmonic interior pressurisation of the edifice by gas exsolving from the volcano core the transient radial migration of overpressured gas may reduce the apparent strength of the edifice and ultimately trigger failure of the flank importantly this mechanism is capable of triggering flank failure both after multiple core pressurisation cycles have been sustained and as core pressures are low and diminishing and may be a minimum these twin attributes are both apparent in the seismic record for mount st helens used as a proxy for the unrecorded timing and magnitude of gas pressurisation at the volcano core d 2004 elsevier b v all rights reserved

a constrained potential field data interpretation of the deep geometry of the sudbury structure two coincident high resolution airborne gravity and magnetic profiles of the sudbury structure were forward modelled to better understand the geology of the structure at depth a north south profile was used to further investigate the deep geological setting of the sudbury igneous complex sic along lithoprobe seismic transect while an east west profile was selected to examine a discontinuity in the magnetic and gravity fields near the centre of the sic in the north range constraints imposed on the best fit model were the location of surface magnetic contacts interpreted seismic and geological sections and petrophysical data acquired from surface and borehole data the constrained model computed for the north south profile elements of which are consistent with known lithoprobe seismic reflectors defines a north verging fold in the deeper portion of the sic it may have developed during the modification of the initial geometry of the sic by either a post sic thick skinned basement shortening event

strain fabric evolution within and near deformed igneous sheets the sudbury igneous complex canada abstract deformation structures differing in style and orientation within a given terrane are often attributed to distinct tectono metamorphic events however mechanical heterogeneities may locally cause strain perturbations that can have a profound effect on the geometry of such structures here we document highly variable orientations of planar structures metre to kilometre scale folds and shortening directions inferred from brittle fault analysis within the synclinal and layered sudbury igneous complex sic and its huronian host rocks nw se shortening during palaeoproterozoic deformation led to the formation of planar structures in the host rocks that are parallel to the ne sw striking contact of the southern sic during deformation local contact parallel shortening became more important than regional nw se shortening and generated contact orthogonal planar mineral fabrics and folds local contact parallel shortening is attributed to the shape change of the southern sic from a convex outward to a concave inward curvi planar geometry contact parallel shortening accounts for the formation of a previously unidentified kilometre scale buckle fold in the sic and respective axial planar mineral fabrics in huronian host rocks this buckle fold shares similar structural characteristics with known higher order buckle folds of the eastern sic we suggest that non cylindrical buckling and associated mineral fabric development accommodated the shape change of the sic this resulted in mutually perpendicular fabric orientations which are compatible with overall nw se shortening the original shape of the sic seems to have had a profound influence particularly on the geometry of metamorphic foliations our structural analysis supports earlier structural studies advocating that highly discordant planar strain fabrics can be generated by local strain perturbations near igneous sheets under uniform regional shortening

effects of lithospheric properties on the formation of type 2 coronae on venus 1 over 500 coronae occur on venus a recent survey defined type 2 coronae as those with 50 complete fracture annuli this paper synthesizes the results from a statistical study of the size topographic morphology and geologic setting of the entire corona population and a study of the gravity and topography of the 32 type 2 coronae resolved in the gravity data using the estimated lithospheric properties loading signature and geologic characteristics we examine the factors controlling corona morphology size and fracture completeness type 1 versus type 2 we find no difference between lithospheric properties in the plains and fractures belts nearly half of the type 2 coronae studied are isostatically compensated and probably inactive this implies that the final stage of a corona may have a wide range of topographic morphologies elastic thickness has no correlation with diameter and therefore does not limit corona size the ratio of crustal thickness to plume diameter does not control morphology as a function of size as predicted by a spreading drop model however rim only coronae which are predicted to form via isostatic rebound of crust thickened by delamination are supported by a density interface isostasy and delamination of the lower lithosphere are likely to have a greater effect on corona formation and possibly in understanding the evolution of the venusian lithosphere than variations in elastic thickness the role of isostasy in the formation of type 2 coronae suggests that surface fractures may be reduced due to low strain rates

admittance survey of type 1 coronae on venus in this study we analyze magellan gravity and topography data for type 1 coronae on venus to estimate crustal thickness z c elastic thickness t e and apparent depth of compensation z l we examine the free air admittance for all 103 type 1 coronae defined as having greater than 50 complete fracture annuli that are resolved in the gravity data a spatio spectral method is used to calculate a localized admittance signature of each corona from a global admittance map this method extracts spectral information from a region in space whose area is varied as a function of degree to make it large enough to yield robust results at every individual wavelength elastic flexure models with either bottom or top loading compensation are used to fit the data the estimated lithospheric flexural parameters span the range obtained for other topographic features on venus suggesting that the lithosphere on which coronae form is variable we find no significant difference in lithospheric properties between type 1 and 2 coronae fifty four percent of all coronae are consistent with local isostasy which may indicate that they are no longer active very few coronae with dome or plateau morphologies have a bottom loading signature or the small t e and large z l expected if a mantle plume were present approximately 71 have z l estimates less than 70 km indicating that compensation probably occurs within the crustal layer we find no systematic relationship between t e or z c and corona diameter as would be expected for the spreading drop model of formation

seismological significance of the 1977 1978 eruptions and the magma intrusion process of usu volcano hokkaido abstract an earthquake swarm and the major pumice eruptions in august 1977 which followed marked the start of the dacitic doming activity of usu volcano in southwestern hokkaido japan the sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data the distribution of the abundant hypocenters shows clearly an earthquake free zone beneath the summit crater the hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations considered due to magma intrusion into this earthquake free zone the earthquake mechanism solutions are mostly of dip slip type and are interpreted in terms of the doming deformations the existence of earthquake families earthquakes with similar waveforms is the main cause of the peculiar occurrence of earthquakes in space time and magnitude the concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm and the observed episodic deformations

extent of grenvillian remanence components in rocks of the southern province early aphebian gowganda sedimentary rocks and intruding nipissing diabase sills were sampled for paleomagnetic study at 88 sites 500 cores 1000 specimens along two 42 km long profiles extending north from the grenville front into the cobalt plate of the southern structural province in the river valley lake temagami area of ontario after af demagnetization a postfolding pre nipissing 2200 ma remanence was found in eight of the 37 gowganda sediment sites that were 2 km north of the front giving a pole at 109 w 63 n dp 10 dm 19 the nippissing diabase from 2 km north of the front retains a stable antiparallel prefolding n1 remanence direction in 22 of 40 sites giving a pole position of 85 w 17 s dp 6 dm 10 these south and down remanence directions found in the southern portion of the plate contrast with the antiparallel north and up directions found in the northern portion thereby indicating the occurrence of either two nearly cogenetic nipissing intrusive events or

grenville age deformation of the sudbury impact structure evidence from magnetic modelling of the sudbury diabase dyke swarm terra nova 24 213 220 2012 abstract the sudbury structure is located 10 km from the grenville front the northern boundary of the grenville metamorphic terrane previous geological studies have suggested that late grenvillian deformation 1000 1070 ma of the sudbury structure was limited to minor brittle deformation olivine diabase dykes which were intruded at 1238 ma before the last pulse of grenville deformation cross much of the sudbury structure any grenville age deformation of the sudbury structure therefore should produce systematic displacements in the geometry of the dykes this study uses information derived from an aeromagnetic survey of the sudbury structure to map and measure fault displacements lateral displacements of dyke segments define a progressive increase in fault rotation from ne to sw across the basin a two dimensional inversion modelling of magnetic anomaly profiles across segments of the dykes outlines a series of fault blocks with common dip the dyke dip detected by the inversion models suggests that displacement on the faults included rotation about an axis perpendicular to the trend of the fault the resulting model of scissor or splay faults is compatible with previously published palaeomagnetic studies of the olivine dykes in the sudbury region

morphometric properties of martian volcanoes 1 mars orbiter laser altimeter mola data have been used to construct digital elevation models dem of the martian volcanoes in order to determine height flank slope caldera depth and volumes summit elevations range from 21 1 km to 0 5 km and relief varies from 1 0 km to almost 22 km average flank slopes are in the range of 1 to 10 consistent with basaltic shield volcanism the very low slopes of highland patera are also consistent with pyroclastic volcanism minimum volumes range from 1012 to 1015 m3 estimates of the time required to build these volcanoes on the basis of long term terrestrial eruption rates range from hundreds of thousands to tens of millions of years

seismic scaling exponents as a tool in detecting stress corrosion crack growth leading to the september october 1989 flank eruption at mt etna volcano the september october 1989 flank eruption at mt etna marked a change in the recent eruptive style of the volcano seismicity at mid order of months and short order of days time preceding the eruption onset has been analysed by means of d scaling exponent inferred from the seismic b value that represents the scaling exponent of fracture length distribution time evolution of d preceding the eruption onset revealed a general increase of d 9 to 4 months prior the eruption onset intermittent rates of d decreases and increases 4 to 1 month steady values of d 1 month and sharp decreases of d marking the eruption onset integration of these results with ground deformation data reveals that at mt etna stress corrosion crack growth enhanced by fresh magma arrival is a plausible mechanism for weakening the country rock by promoting crack growth and the coalescence of major faults

formation of caldera periphery faults an experimental study changing stresses in multi stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism s associated with caldera formation and the corresponding zones of structural weakness we evaluate characteristic structures resulting from doming chamber inflation evacuation collapse chamber deflation and cyclic resurgence inflation and deflation and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation the structural dome so produced is characterised bysteepening in the periphery whereas the broadening apex subsides pure evacuation causes the chamber roof to collapse along adjacent bell shaped reverse faults the distribution of concentric faults is influenced by the initial edifice morphology steep and irregular initial flanks result in a tilted or chaotic caldera floor the third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation following doming caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome the flanks of the edifice relax resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults the latter form locally uplifted and tiltedwedges half grabens that grade into horst and graben structures this superimposed fault pattern also extends inside the caldera we suggest that major pressure deviations in magma chamber s are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first order indication of the processes and mechanisms involved in caldera formation

3d geophysical inversions of the north east amer belt and their relationship to the geologic structure the amer lake area is located within the churchill structural province in the kivalliq region of nunavut approximately 160 km north west of baker lake two distinct geophysical geological entities are structurally intercalated an archean mixed granitoid gneiss metasedimentary metavolcanic basement and the unconformably overlying paleoproterozoic amer group metasediments from east of amer lake stretching toward the south west these two entities form the amer fold and thrust belt at the north east end of this belt high resolution aeromagnetic data define a distinctive oval shape that has been interpreted as a south west trending doubly plunging synform the outcrop within the interior of this structure is sparse resulting in limited structural data and speculative geological interpretations with multiple geometries possible the high resolution aeromagnetic data compiled through an industry government consortium and newly acquired detailed gravity profiles were modelled to provide constraints on the geometry of this synform we document a geophysical geological feedback process whereby the available geological and geophysical data were used to derive constraints on inversion models for the synform starting with available limited litho structural data the presence of a double plunging synform was directly inferred from the aeromagnetic data segments of the aeromagnetic data have 2d morphology and so can be modelled using a simple parametric 2d dipping slab inversion approach models of profiles extracted from the aeromagnetic data were used to provide preliminary dip and magnetic susceptibility constraints for the three lakes mudstone with iron formation and the five mile lake basalt landsat imagery outlined the spatial limits of the stratigraphically underlying non magnetic ayagaq quartzite incorporating these outputs as bounds in the input reference model for a ubc gif 3d magnetic inversion helped to accentuate the geological structure in the output mesh an enhanced inversion that incorporates both geological and geophysical constraints the validity of the resulting inversion model was tested by computing 2d forward models of the gravity profile data the inversion model generated by this study emphasizes the importance of integrating information from as many knowledge sources as one can find more trust can be placed on forward and inversion models where there is agreement among all data sets and a coherency of structural style

source parameters and scaling relationships of small earthquakes in the northeastern caribbean abstract the seismic moments and stress drops of 23 earthquakes 1 1 m 2 4 that occurred during an earthquake swarm in the virgin islands were determined from the analysis of their p waveforms the data consist of digitally recorded seismograms collected by a short period seismic network operating in the northeastern caribbean the events of the swarm are particularly useful for comparing the relative stress drops of small earthquakes because their source to receiver paths and focal mechanisms are very similar the static stress drops calculated for these earthquakes varied from about 0 2 to 2 bars the data clearly illustrate that the static and dynamic stress drops of these earthquakes generally increased with the size moment of the events the fault radii for these shocks increased with seismic moment but only by a factor of 2 for a 100 fold increase in seismic moment the velocity waveforms of the larger events were systematically more impulsive than those of the smaller earthquakes these observations imply that for this set of earthquakes the final fault radius is a function of the stress drop that occurs during the rupture process

extraterrestrial lava lakes abstract active lava lakes are rare on earth with only ten documented examples all formed by lavas of basaltic composition and housed inside summit craters or calderas the existence of lava lakes on other planetary bodies may imply similarities in either composition or volcano tectonic settings and so has important implications for understanding the link between melt production and volcanism we review lava lakes on earth and other planets particularly active caldera like features interpreted to be currently containing active lava lakes on jupiter s moon io and features interpreted as remnants of lava lakes on venus and mars mercury and the moon do not boast the major calderas or shield volcanoes of their larger rocky neighbors partly due to a horizontally compressive tectonic regime arising from global contraction that makes it difficult to move melts through the upper crust we discuss the evidence for active lava lakes on io and show how modeling based on terrestrial lava lakes can reveal how these phenomena differ on both bodies the superficial similarities do not necessarily imply that the plumbing is similar observations of the largest lava lake on io loki patera provide insight into the nature of ionian lava lakes in general which may be more similar to eruptive episodes on the east pacific rise on earth which lead to temporary lava lakes although temporal data for io s lava lake activity are scarce studies of temporal variability of lava lakes on earth are useful for providing ground truth for comparisons future studies of earth by remote sensing and field observations and of io by both ground based observations and future missions are needed to answer many questions including why lava lakes rare on earth appear to be common on io

imaging slow failure in triaxially deformed etna basalt using 3d acoustic emission location and x ray computed tomography we have deformed basalt from mount etna italy in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice 40 mpa and at a constant strain rate of 5 similar to 10 6 s 1 despite containing a high level of pre existing microcrack damage etna basalt retains a high strength of 475 mpa we have monitored the complete deformation cycle through contemporaneous measurements of axial strain pore volume change compressional wave velocity change and acoustic emission ae output we have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation locations of ae events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm s 1 we also find excellent agreement between ae locations and post test images from x ray microtomography scanning that delineates deformation zone architecture

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

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

3 d kinematic restoration of the eastern sudbury igneous complex canada and its importance for cu ni pge sulphide exploration abstract kinematic restorations of deformed igneous terranes have rarely been attempted due to the general absence of 1 planar primary lithological contacts 2 tightly constrained deformation kinematics and 3 geometric constraints on lithologies and structures at depth because of the vast number of exploration drill holes the deformed and layered 1 85 ga sudbury igneous complex sic in ontario canada is reasonably well understood with respect to 1 and 3 we therefore explored the kinematics of folding and faulting in the eastern sic and use the subsurface geometry of faults extracted from a model integrating surface mapping and drill core logs in an attempt to restore the geometry of part of the sic specifically we identified rotation magnitudes and the tilt axis of a kilometer scale plunging fold the west bay anticline which appears to have folded the eastern part of the sic we combined a 3 d model of the faults and lithological contacts of the sic fault kinematics calculated from the inversion of brittle shear faults at surface and limb rotations of the west bay anticline to kinematically restore the eastern part of the sic results of restorations using simple shear and flexural slip algorithms highlight the importance of slip on prominent curved faults that displaced the sic and facilitated its folding flexural slip appears to be the more plausible mechanism of the two as it removed the curvature of and displacement on prominent faults flexural slip restoration helps to estimate primary thickness variations of the sic and the results show that thickened parts of the main mass of the sic correspond spatially with known cu ni pge sulfide deposits this result demonstrates that 3 d restoration can be an important tool for mineral exploration in igneous terranes

geophysical characteristics of dacite volcanism the 1977 1978 eruption of usu volcano abstract the 1977 1978 eruption of usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism the activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes the hypocenters of the earthquakes occurring beneath usu volcano have been located precisely their distribution defines an earthquake free zone which underlies the area of doming within the summit crater this zone is regarded as occupied by viscous magma the domings within the summit crater forming the cryptodomes have amounted to about 160 m in addition to uplift they showed thrusting towards the northeast as a result the northeastern foot of the volcano has contracted by about 150 m the relation between crustal deformation and earthquake occurrence is examined and it is found that the abrupt domings are accompanied by the larger earthquakes m 3 4 3 both the seismic activity and the ground deformation are shown to have a unique and common energy source the energy of activities of usu volcano consists of the explosive type the deformation type and the seismic type the second and the third are in parallel with each other in discharges and both energies are complementary to the explosive energy the explosive energy and the seismic energy have been calculated for an explosion sequence and it is concluded that the deformation energy is about 10 times greater than the seismic energy the discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption except for a small increase at the end of january 1978 eruptions are governed not only by the supply of the energies but also by the depth of the magma which has gradually approached the surface the last eruption occurred in october 1978 however the crustal deformations and the earthquake swarms are still proceeding as of january 1980 albeit at a lower rate of activity

kunhild and ereshkigal an extinct hot spot region on venus analysis of imagery topography and gravity for kunhild and ereshkigal adjacent large volcanoes on venus indicates that they are extinct hot spots that were located over a broad mantle upwelling the volcanoes probably formed contemporaneously but volcanism persisted at kunhild after ereshkigal s extinction a late event in each structure s formation was sagging of the central region to form a broad depression ereshkigal the shorter of the two structures has a broader and deeper central depression than kunhild we suggest that when a long lived plume goes away from beneath a large venusian shield volcano removal of dynamic support from the plume stem produces a central sagging analogous to the collapse phase associated with corona formation on venus

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

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

themis regio venus evidence for recent volcanism from virtis data abstract themis regio is interpreted to be a hotspot rise underlain by one or more mantle plumes many volcanic features in the themis region have high emissivity anomalies in venus express visible and infrared thermal imaging spectrometer virtis data other high emissivity anomalies have been found to correspond to volcanic flows interpreted to be relatively recent smrekar et al 2010a science 328 605 608 similarly at themis the majority of the high emissivity anomalies closely correspond to flows associated with coronae and volcanic edifices consistent with the interpretation that they represent recent volcanism these volcanic features also have negative bouguer gravity anomalies indicating low density at depth consistent with hot mantle upwellings leading to pressure release melting in addition several volcanic features at themis have low emissivity anomalies suggestive of more evolved compositions combining geologic mapping data with gravity and emissivity data provide support that themis is an active hotspot swell with associated long lived volcanoes and coronae that are likely associated with small scale upwellings

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

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

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

laboratory simulations of tensile fracture development in a volcanic conduit via cyclic magma pressurisation during volcanic unrest high magma pressure induces cracking and faulting of the country rock providing conduits for the transport of magma and other fluids these conduits known as dykes are fundamental structures for the transport of magma to the surface in volcanically active regions the mechanics of dyke propagation is not yet fully understood but is crucial to better model dyke emplacement and eruption in volcanoes central to this need is a greater understanding of the mechanical properties of the magma country rock interaction as a function of known magmatic pressure temperature and stress here we report data from a series of experiments in which we cyclically compress viscoelastic rhyolitic magma at 828 c 892 c and 918 c inside a cylindrical conduit like shell of basalt from mt etna italy until fracture occurs the compression is performed under strain rates cyclically varying between 5 10 6 and 5 10 5 s 1 the resultant monitored axial loading and relaxation illustrates how the presence of a visco elastic fluid magma controls the stress induced at the conduit margin boundary this is achieved by analysing the viscoelastic relaxation through time to calculate an apparent modulus which is found to decrease with both increasing temperature and time in the 4 cycles before failure we find that the apparent modulus decreases from 180 to 40 gpa 80 to 20 gpa and 8 to 1 gpa for imposed stress cycles at 828 c 892 c and 918 c respectively we theoretically estimate a tensile strength at failure of approximately 7 11 mpa consistent with recent field data and in agreement with a model derived from the sample geometry and basic material parameters post experimental neutron computed tomography and microscopic analyses further reveal the fragmentation of the melt and generation of tuffisite veins inside the conduit due to spontaneous crack nucleation associated with conduit wall fracture the geometry of the rupture area inside the melt is akin to a mach cone associated with supershear fractures we discuss our findings in terms of magma rock interaction leading to dykes tuffisite veins and magma fragmentation

the geological evolution of atai mons venus a volcano corona hybrid atai mons venus is classified as a large volcano yet displays characteristics that are typical of coronae coronae are widespread volcano tectonic features that are apparently unique to venus detailed analysis of atai mons using photogeological and geophysical techniques illustrates that it has experienced a complex history and may still be active we show that processes found at both coronae and large volcanoes have occurred throughout the history of atai mons with no definite temporal preference it does not have an evolution that can be described simply as the evolution of one feature into another these results suggest that large volcanoes and coronae are more intimately linked than previously thought and that detailed mapping and gravity surveys are reliable ways of establishing the evolution of hybrid features

a trishear model for the deformation of the sudbury igneous complex canada abstract the sudbury igneous complex sic canada is an impact induced layered sheet of crystalline rocks deformed into an asymmetrical fold basin the sudbury basin the basin geometry at depth is largely unknown as few attempts were made to quantify displacement and rotation of layer contacts we propose that the dip of layer contacts and foliation surfaces in the southern sic can be approximated by trishear fault propagation folding trishear deformation accounts for 1 angular discordances between upper and basal sic contacts 2 local overturning of the sic 3 progressive steepening of foliation surfaces from nw to se 4 strain gradient in the sudbury basin sedimentary rocks and 5 thickness variations in sic layers moreover the south range shear zone a zone of moderately strong metamorphic fabrics coincides with the surface manifestation of the proposed trishear zone we demonstrate the use of structural data together with forward kinematic modelling to identify the strain distribution within the sic rotation of sic contacts and thickness changes of sic layers backward kinematic modelling provides information on the initial geometry of the sic and is used to restore the shape of the igneous sheet showing that the sic was shortened by approximately 10 km in nw se diameter most of the shortening was accommodated by 40 reduction in the thickness of the upper sic layer

regional cross section of the southern province adjacent to lake huron ontario implications for the tectonic significance of the murray fault zone the southern province in the lake huron area of ontario and the penokean fold belt of minnesota wisconsin and michigan have the earmarks of a collisional orogen the huronian supergroup a southerly facing passive margin sequence was deposited during early aphebian early proterozoic crustal stretching and ocean basin formation along the southern margin of the superior province archean craton it accumulated as four unconformity bounded northerly tapering onlapping clastic wedges megacycles the thickness and facies variations reflect syndepositional down to the basin south normal faulting that controlled the accumulation and preservation of the lower three huronian megacycles these are overlapped northward by the youngest megacycle an extensive sheet of clastic sediments recording post stretching regional subsidence of the cratonic margin due to cooling and thermal contraction soft sediment folds in the rocks of the youngest megaycycle to the extreme south probably indicate southward slu

new insights into coronae evolution mapping on venus coronae are geologically and geophysically important features on venus because they are thought to contribute to planetary heat loss it is thus necessary to understand their role in space and time in the evolution of venus detailed mapping of five coronae in guinevere and sedna planitia illustrates that although previous models invoked initial uplift not all coronae can be explained in such a simple way we show that the formation of corona annuli can be multistaged and that the position of the annulus does not always coincide with the main topographic ridges and troughs that outline the feature the magnitude and timing of volcanism are not necessarily the same at each corona and coronae can have long and complex histories in contrast with the stratigraphic relations suggested by other workers we demonstrate that coronae do not all have the same relative ages with respect to adjacent units these results suggest that corona formation was is not confined to a single time period in the history of venus and that detailed mapping is a reliable method of establishing the relative timing of corona formation

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated