wrinkle ridge assemblages on the terrestrial planets the wrinkle ridge assemblage is a group of associated features that have been observed on the moon mars and mercury members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first second or third order ridges on the basis of dimensions the ridge assemblages described in this study are known to occur in mare basalts on the moon and smooth plains units that may be volcanic in origin on mars and mercury the structures that occur in the continental flood basalts of the columbia plateau in the northwestern united states are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets the anticlinal ridges of the columbia plateau and many first order ridges on the moon mars and mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling flexure fracture or buckling as a consequence of reverse or thrust faulting fracture flexure estimates of the unit shortening across first order ridges using simple geometric forms to approximate the ridge morphology are lower than those determined for the anticlines of the columbia plateau where displacements are the results of folding and reverse to thrust faulting evidence of layer parallel extension that often accompanies folds developed by fracture flexure mechanisms i e drape or fault bend folds is not found in the anticlinal ridges of the columbia plateau and has only been observed in one ridge on the moon a kinematic model involving buckling followed by reverse to thrust faulting flexure fracture is favored for the origin of the anticlinal ridges of the columbia plateau and many of the first order ridges on the moon mars and mercury

the topography and morphology of low shields and associated landforms of plains volcanism in the tharsis region of mars abstract the morphology and topography of volcanic landforms provide critical information to the investigation of their tectonic setting and the physical characteristics e g rheology of their eruption products their investigation is also an important prerequisite for studies of comparative planetology e g the comparison between surface features of the earth and other planetary bodies numerous small and low shield volcanoes on mars and associated vents and lava flows have previously been compared to terrestrial plains style volcanism which is defined as being an intermediate style between flood basalts and the hawai ian shields this study investigates the topography and morphology of martian landforms associated with plains volcanism using mola moc themis hrsc and hirise data low shields and other landforms of plains volcanism on mars display similarities with terrestrial basaltic volcanic fields and we do not observe any features that do not have morphologic analogues on earth the most typical landforms of martian plains style volcanism are low shields defined as volcanic shields with diameters of typically less than 50 km heights of a few hundred meters only and extremely shallow flank slopes of less than 0 5 other surface features related to plains style volcanism on mars are craters fissure vents cinder and spatter cones lava flows that are commonly associated with lava channels and tubes lava inflation features and volcanic rift zones our results reconfirm the viking orbiter based conclusion that plains volcanism in the eastern snake river plains is perhaps the best terrestrial morphological analogue for these martian surface features icelandic shields distinct structures in hawai i and other basaltic landforms also show similarities to martian plains volcanism sinuous rilles previously not described in association with plains volcanism on mars are interpreted as evidence for high eruption rates the extremely shallow flank slopes of the low shields suggest the eruption of shield building lavas with low viscosity which might be the result of high eruption temperatures high effusion rates a low si and a high mg content along with a possible high fe content or a combination of these factors the spatial distribution of low shield clusters in tharsis does not show any obvious association with large scale tectonic features plains volcanism might represent a relatively recent type of volcanism on mars which is not related to mantle plumes but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust schumacher and breuer geophys res lett 34 l14202 doi 10 1029 2007gl030083 2007

calderas a planetary perspective caldera bearing volcanoes are the largest volcanic edifices on a planet and are expressions of long lived near surface magma chambers on earth only three morphological classes of calderas are common shield calderas e g mauna loa hawaii stratocone calderas e g crater lake oregon and ash flow calderas e g valles caldera new mexico each of these classes has a characteristic caldera size magma type and tectonic location no calderas have been recognized on the moon or mercury but mars has large shield calderas as well as the unique highland patera calderas the latter may represent an explosive ultramafic style of volcanism unlike any volcanic activity in modern earth history radar images suggest that venus has at least one large caldera bearing shield volcano and possible calderas of other types jupiter s satellite io has abundant large shieldlike calderas but no shield massifs the number and variety of calderas on a planet may be proportional to planet mass consistent with a previous suggestion that volcanic complexity increases with planet size io is an exception but heating in io is generated tidally rather than by the decay of radioactive elements so that its calderas are not related to its size the occurrence of shield volcanoes on four solar system bodies suggests that thermal and tectonic mechanisms i e hot spots giving rise to construction of shield mountains and associated collapsed summits are basic processes in planetary geology hot spot volcanism is expected to be most important on planets lacking plate tectonics on earth large shields have lifetimes of only a few million years but on mars large shields may have been active for 1 5 109 years shield calderas on earth are smaller than 20 km in diameter but known or suspected shield calderas on mars venus and io have considerably larger diameters assuming that caldera diameters are related to magma chamber diameters and depths magma chambers on other planets are significantly larger and or deeper than their counterparts on earth terrestrial magma chambers are small because terrestrial shield volcanoes are small in comparison to shields on mars and venus and terrestrial shields are small because the earth s mobile plates move growing shields away from their deep sources thus distributing magma along a narrow chain rather than piling it up around a central vent finally the long lava flows of mars are also explained if the volume of erupted magma has any simple relation to magma chamber volume

deep magma body beneath the summit and rift zones of kilauea volcano hawaii a magnitude 7 2 earthquake in 1975 caused the south flank of kilauea volcano hawaii to move seaward in response to slippage along a deep fault since then a large part of the volcano s edifice has been adjusting to this perturbation the summit of kilauea extended at a rate of 0 26 meter per year until 1983 the south flank uplifted more than 0 5 meter and the axes of both the volcano s rift zones extended and subsided the summit continues to subside these ground surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones kilauea s magmatic system is therefore probably deeper and more extensive than previously thought the summit and both rift zones may be underlain by a thick near vertical dike like magma system at a depth of 3 to 9 kilometers

focal mechanisms of earthquakes related to the 29 november 1975 kalapana hawaii earthquake the effect of structure models initial focal mechanism determinations for the 29 november 1975 kalapana hawaii earthquake indicated discrepancy between the mechanism determined from teleseismic data by ando and the mechanism determined using data from the local u s geological survey network surrounding the epicenter region the resolution of this difference is crucial to correctly understand this earthquake as well as to understand the tectonics of the south flank of kilauea volcano when a model with a low velocity layer at the base of the crust is used for projection back to the focal sphere for the local network mechanisms the discrepancy vanishes to further investigate this result focal mechanisms were determined using several contrasting models for a set of well recorded earthquakes a large number of these earthquakes have mechanisms identical to the main shock when the low velocity layer model is used dispersion of p and t axes is also minimized by use of this model a low angle slip direction favored for the main shock and typical of most other solutions exhibits remarkable stability normal to the east rift zone of kilauea our results suggest a tectonic model similar in nature to that proposed by ando in which the south flank of kilauea consists of a mobile block of crust which is relatively free to move laterally on a low strength zone at about 10 km depth forceful injection of magma along the rift zones provides the loading stress which is released by catastrophic failure in the weak horizontal layer in a cycle of perhaps 100 yr

caldera subsidence and magma chamber depth of the olympus mons volcano mars observations of the distribution of tectonic features and their relationship to local surface topography indicate that the floor of the oldest and largest crater of the summit caldera of mars olympus mons volcano may have undergone subsidence in response to depressurization of a subsurface magma chamber in this study we construct an axisymmetric finite element model to calculate elastic stresses in a volcanic edifice to investigate the relationship between surface tectonism caldera subsidence and the physical characteristics of olympus mons magmatic reservoir constraints on the model are provided by the stress field within the crater indicated by the distribution of radial ridges and circumferential graben that we hypothesize formed due to a postcollapse and postresurfacing phase of subsidence of the caldera floor model results show that the surface stress state is not strongly sensitive to the aspect ratio or pressure distribution of the magma chamber or to the contrast in stiffness between the magma chamber and surroundings but is strongly dependent on the width and depth of the chamber for a range of plausible model parameters we find the maximum depth to the top of the magma chamber to have been 16 km which indicates that the chamber at the time of crater floor subsidence was positioned within the olympus mons edifice at a level much shallower than the estimated source depth of martian magmas 140 200 km the allowable range of solutions indicates that the vertical position of the magma chamber may have been controlled by the neutral buoyancy level of ascending magma our results suggest a gross similarity between the configurations of the magmatic plumbing systems of olympus mons and several well studied terrestrial volcanoes such as the hawaiian shields

lava lakes on io new perspectives from modeling abstract loki patera 310 w 12 n is io s largest patera at 180 km in diameter its morphology and distinct thermal behavior have led researchers to hypothesize that loki patera may either be an active lava lake that experiences periodic overturn or a shallow depression whose floor is episodically resurfaced with thin flows using results from mathematical models we suggest that a better model for loki s behavior is the terrestrial superfast spreading east pacific rise epr near 17 30 south we propose that like at the southern epr loki patera is underlain by a thin persistent magma lens that feeds thin temporary lava lakes within the patera also like the southern epr overspilling of the volcanic depression is rare with most of the lava volume being emplaced via a subsurface network of lava tubes

topography of the shield volcano olympus mons on mars olympus mons one of the largest known shield volcanoes in the solar system covers an area of 3 2 105 km2and has a diameter of 600 km excluding its vast aureole deposits the structure is five times larger than the largest shield volcano on the earth it is situated on the north west flank of the tharsis volcanic region a broad topographic rise on the martian surface the volcano has three physical subdivisions the summit caldera the terraced upper flanks and the lower flanks which terminate in a scarp 2 10 km high that nearly surrounds the structure a large block of images of the tharsis region including olympus mons was obtained by the viking mission1 here we present a topographic map of olympus mons compiled using various combinations of stereo pairs of these images together with stereoscopic perspective views generated by image processing techniques

the role of magma buoyancy on the eruption of lunar basalts it has long been recognized that mare basalts on the moon are preferentially located both on the earth facing hemisphere and within large impact basins a popular model that accounts for this observation assumes that these magmas were denser than the lunar crust that they accumulated at the crust mantle interface and that eruptions occurred only when this magma chamber became overpressurized in this paper we re evaluate this model and argue that it is not consistent with the available data nor with models of dike propagation as an alternative hypothesis we propose that magma buoyancy is the predominant factor that determines whether mare basalts erupt at the surface or form crustal intrusions instead we have computed the densities of mare basaltic magmas and find that some are in fact less dense than the moon s upper anorthositic crust based on the widely accepted view that the lunar crust becomes more mafic with depth we also show that all mare basaltic magmas should be less dense than the lower portion of the crust thus if the upper anorthositic crust was regionally removed by an impact event then any mare basaltic magma could have risen to the surface there based on buoyancy considerations alone in support of this model we note that mare basalts are indeed found wherever geophysical crustal thickness models predict the upper crust to be absent furthermore many of the basalts that erupted within the anorthositic highlands are predicted to be less dense than the underlying crust based on remote sensing data the high titanium flows within oceanus procellarum are somewhat problematical to our model in that an anorthositic crust is predicted to be present beneath them using results from recent lunar thermal models we suggest that these magmas may have overcome their negative buoyancy in the crust by possessing superliquidus temperatures if magma buoyancy does indeed control whether or not a basaltic eruption will occur then this implies that the quantity of magma produced beneath the south pole aitken basin was about 10 times less than that of the nearside fl 2001 elsevier science b v all rights reserved

linear volcanic features at alba patera mars probable spatter ridges a number of extensive linear constructional features occur on the north and east flanks of alba patera they are found in a 150 km wide zone and comprise aligned ridge elements that are locally associated with short volcanic flows they are among the youngest volcanic features in the region and are believed to be martian analogs of terrestrial spatter ridges on the basis of measurements made on viking images it appears that active volcanic fissures on mars were larger than their terrestrial counterparts

volcanism on mars more than 15 years of planetary exploration of mars have given insight into the geologic processes that have shaped its surface the newly acquired viking data have shown that volcanism is one of the most important geologic processes operating on mars throughout its history in situ chemical analyses of martian soil by the viking lander spacecraft indicate mafic to ultramafic source rocks this is consistent both with available remote sensing data which indicate the presence of mafic minerals such as pyroxene and olivine and with petrologic modeling based on available geophysical data which suggest that martian lavas are probably iron rich and ultramafic these data strongly suggest that basaltic volcanism is widespread on mars and much of the photogeological data may be studied in this context photogeological analysis of the martian surface has shown two main types of volcanic morphologies the first type is central volcanoes which are volcanic landforms developed by continued and prolonged eruption from a point source vent this category includes 1 shields the classic low profile volcanic mountains of which olympus mons is the most spectacular example 2 domes steep sided constructs such as tharsis tholus that may represent lower rates of eruption than the shields or possibly more silicic lava compositions 3 highland patera radially textured low profile volcanoes that occur in the cratered terrain and are interpreted as ash shields 4 alba patera an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree and 5 various small features such as cinder cones the second major category is volcanic plains which are units recognized by several criteria of which the presence of mare ridges and flow lobes are the most useful volcanic plains are subdivided into four main groups 1 simple flows broad smooth to rolling plains that contain numerous mare type ridges but no flow lobes interpreted as being composed of thick single cooling units 2 complex flows displaying multiple overlapping flow lobes interpreted to be indicative of thin multiple cooling units 3 undifierentiated flows plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers and 4 questionable plains volcanic units heavily modified by other processes erosion tectonism etc so that their origins are uncertain when these categories of volcanic morphologies are combined with relative age data provided by crater statistics a volcanic history for mars can be derived as follows early heavy bombardment of mars was accompanied and followed by small scale fluvial channeling extensive flood volcanism the plateau plains and ash shield volcanism in the cratered terrain shortly after this time less extensive flood volcanism continued to resurface the planet during formation of the northern southern hemisphere dichotomy central volcanism became more prominent with the development of the alba patera center as well as the older shields and domes of the northern hemisphere early tharsis and elysium regions the development of the tharsis and elysium uplifts may have triggered the release of large scale catastrophic floods producing large channels continued uplift and lithospheric thinning concentrated volcanic activity in the tharsis region producing large shield volcanoes and extensive lava plains both central vent and plains volcanism have been active throughout martian history but the volumes of extrusion have gradually decreased with time this is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time gradually turning off the volcanism although many questions remain regarding martian volcanism the viking data have provided a remarkable detailed overview of the probable nature of the volcanic history of mars

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

deformation around the creede caldera a consequence of isostatic adjustment following caldera formation the pattern of deformation around the creede caldera 26 5 ma southwest colorado may provide clues to the physical mechanisms of caldera evolution particularly resurgent doming this paper considers the hypothesis that the deformation is due to isostatic relaxation of surface load topography following caldera formation involving uplift of the caldera floor and subsidence of a surrounding tumescent dome this hypothesis is tested by constructing finite difference models of relaxing topography which have as parameters the mechanical properties of rocks surrounding the caldera the goal is to find models that predict spatial and temporal patterns of deformation matching those observed around the caldera as inferred from 1 the present structural form of the region as indicated by the surface distribution of stratigraphic units and 2 past structural forms as indicated by the locations orientations and ages of faults which are presumed to be due to a combination of regional tension and stresses caused by bending at the surface successful models of the deformation have been found using a variable viscosity half space overlain by a thin elastic plate in general the nature of deformation at the surface is strongly dependent on the radial variation of viscosity at shallow depths beneath the elastic plate and is much less sensitive to variation of viscosity with depth models with relatively low viscosities beneath the caldera and relatively high viscosities in the surrounding region accurately predict uplift of the caldera floor which is rapid compared to the rate of marginal subsidence the results indicate that the rate of deformation is governed by the flow of material with viscosities characteristic of wallrock roughly 1019 1024 pa s rather than magma a gap in faulting around the caldera at about one caldera radius outside the wall suggests the existence of a relatively low radial viscosity gradient in that position higher viscosity gradients inside and outside of this low may reflect the margins of two nested magma bodies at depth a central stock within a larger batholith in order to match the observed pattern of faults around the caldera the orientation of the regional tension that is superimposed on the bending stresses must rotate clockwise with time this change in stress direction is consistent in sense but earlier than that proposed by others for the western united states

the rocks of mars from far and near award of the meteoritical society 2001 h y mcsween jr the leonard medal address the rocks of mars from far and near

on the mechanics of caldera resurgence the mechanisms suggested to cause caldera resurgence can be categorized as 1 viscous rebound of magma 2 regional detumescence and 3 pressure against the base of the caldera block due to magmatic vesiculation and convection or addition of new magma the critical observation that resurgence takes of the order of 103 105 years insists that the causative mechanism be regulated by a highly viscous e g 1022 pa s structural member of the crust the relatively low viscosity magma and a purely elastic crust cannot be important in achieving this time scale the most likely mechanism is the coupled effect of regional detumescence and magmatic vesiculation detumescence compresses the magma chamber squeezing magma upward against and through the caldera block vesiculation also compresses the chamber and tends to dome the caldera block the times of both detumescence and doming of the caldera block by magmatic pressure are probably inversely proportional to the size of the magma chamber but the rate of cooling of the chamber is proportional to its size because solidification spreads viscous relaxation over a wide region of the crust and curtails resurgence there is likely to be a minimum volume of magma that can undergo resurgent doming it is difficult to be exact about this critical size until more precise models have been developed

polygenic eruptions on alba patera mars a combination of photogeologic mapping analysis of viking orbiter thermal inertia data and numerical modelling of eruption conditions has permitted us to construct a new model for the evolution of the martian volcano alba patera numerous digitate channel networks on the flanks of the volcano are interpreted to be carved by sapping due to the release of non juvenile water from unconsolidated flank deposits using the thermal inertia measurements we estimate the particle size of these deposits to be 3 10 m which together with theoretical modelling of the disperison of explosively derived volcanic materials leads us to conclude that the flank deposits on alba patera are low relief pyroclastic flows the recognition of numerous late stage summit and sub terminal lava flows thus makes alba patera a unique martian volcano that is transitional between the older pyroclastic dominated highland paterae and the more recent effusive central vent volcanoes such as the tharsis montes

gamma ray constraints on the chemical composition of the martian surface in the tharsis region a signature of partial melting of the mantle the gamma ray spectrometer grs on board mars odyssey is the first instrument to provide quantitative estimates of the composition of the martian surface at the global scale at the present time published concentration maps are available across a broad equatorial swath for ca cl fe h k si and th however it remains to be determined to what extent the observed chemical signatures are the result of primary magmatic processes or secondary alteration at the surface with this question in mind we have considered in detail the grs signature on the western part of the tharsis region including olympus mons this region was chosen as it is clearly of volcanic origin has had a prolonged magmatic history and is of spatial extent sufficiently large that potential contributions of surrounding regions to the grs signature are limited it also has a characteristic composition being notably low in silica relative to the average composition of the martian mid latitudes the major element chemistry of the surface in this region sio2 feo cao k2o is compared with predictions of thermodynamic calculations of liquids produced by partial melting of the martian mantle composition proposed by dreibus and wanke dreibus g wanke h 1985 mars a volatile rich planet meteoritics 20 no 2 267 381 over a range of pressures and temperatures for all four oxides the tharsis compositions are consistent with liquids produced at pressures from 15 to 20 kbar and degrees of partial melting between 3 and 10 this pressure of melting implies a magma source depth in the range 125 170 km consistent with current estimates of the thickness of the lithosphere in this region the idea that the surface chemistry in the tharsis region is dominated by the signature of primary magmatism would therefore appear to be a reasonable working hypothesis although as more data become available more stringent and detailed tests of this simple model will be possible

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

geodetic analysis of dike intrusion and motion of the magma reservoir beneath the summit of kilauea volcano hawaii 1970 1985 we use leveling and trilateration data collected on kilauea volcano to constrain the location of deformation sources caused by magma accumulation intrusion and eruption for the 13 inflationary epochs examined combinations of an expanding point source and one or two opening rectangular dislocations mimic inflation of the summit reservoir and formation of dike s respectively the combined model adequately accounts for the deformation data and is consistent with the seismicity observed during each epoch for 10 deflationary epochs however the data require only a contracting point source confidence in these results is gained by noting that locations of the sources of both inflation and deflation are coincident within the observed uncertainties of the data the function of network geometry and the inversion procedure it appears therefore that magma accumulation at kilauea volcano may be characterized by the growth of dikes during inflation of the summit reservoir drainage of the reservoir on the other hand is not accompanied by significant closure of dikes in contrast to previous studies e g fiske and kinoshita 1969 dvorak et al 1983 that do not include the dislocation or dike growth component of summit magma accumulation and concluded that the source of inflation migrates over a 5 km2 area we find that a single magmatic reservoir source accounts for data collected during all inflationary and deflationary epochs results which compare favorably with those obtained from the point ellipsoid model can be used to estimate the distribution of stresses within the volcano in the near field of the source

crustal structure of mauna loa volcano hawaii from seismic refraction and gravity data abstract in october 1978 the u s geological survey established a 100 km long seismic refraction profile normal to the kona coast of hawaii island analysis of these data along with available gravity data suggest that the oceanic crust dips about 3 landward under the submerged flank of the island increasing to 8 5 under the kona coast maximum vertical deflection of the base of crust from beneath the deep ocean to a point beneath the summit of mauna loa is about 9 km high velocity high density rocks vp about 7 1 km sec density about 2 9 gm cm 3 comprise the bulk of the volcanic edifice and reach to within a few kilometers of the surface beneath the north flank of mauna loa the data also suggest that an elongate high velocity high density body lies parallel to the kona coast just below the surface this body probably represents an extinct buried rift zone

rays and secondary craters of tycho abstract the large fresh crater tycho in the nearside lunar highlands has an extensive system of bright rays covering approximately 560 000 km 2 containing dense clusters of secondary craters examination of crater densities in several clusters shows that tycho produced almost 10 6 secondary craters larger than 63 m diameter this is a lower limit because small crater densities are reduced most likely by mass wasting we estimate a crater erasure rate of 2 6 cm myr varying with crater size and consistent with previous results this process has removed many small craters and it is probable that the original number of secondary craters formed by tycho was higher also we can only identify distant secondaries of tycho where they occur in bright rays craters on mars and europa also formed large numbers of secondaries but under possibly ideal conditions for spallation as a mechanism to produce high velocity ejecta fragments the results from tycho show that large numbers of such fragments can be produced even from impact into a heavily fragmented target on which spallation is expected to be less important

fault type predictions from stress distributions on planetary surfaces importance of fault initiation depth faults on planetary surfaces initiate at depth where stresses due to the weight of overburden are important consequently the prediction of fault type from calculated stresses at depth can yield different faults than those predicted using the surface criteria commonly employed in geophysical models for elastic plate flexure models of mascon loading on the moon stresses calculated at the surface predict that the most likely faulting is strike slip for which there is no direct evidence at a distance from the basin center where grabens are found modifying these plate flexure calculations to account for the weak surface layer of ejecta megaregolith 1 3 km thick that overlies the relatively strong lunar lithosphere results in model stresses in the outer layers of the moon tens of bars that are an order of magnitude lower than in the lithosphere hundreds of bars because faults bounding lunar grabens initiate at the mechanical discontinuity between the megaregolith and lithosphere and because the reverse faults likely responsible for wrinkle ridges probably initiate at the basalt basin floor contact 0 5 2 km deep model stresses must be calculated at these depths superposing nonisotropic stresses due to the weight of the overburden at the depths of these mechanical discontinuities with plate flexure model stresses correctly predicts maximum stress differences in an interior zone of expected wrinkle ridges surrounded by a zone of concentric normal faults or grabens and effectively eliminates the strike slip zone in agreement with observations the absence of strike slip faults from almost all planetary and satellite surfaces may be explainable if lithospheric deformation model stresses are calculated at important shallow crustal mechanical discontinuities where the maximum compressive stress is vertical thereby favoring the formation of normal faults if true mechanical discontinuities in the shallow crusts of planets and satellites play an extremely important and hitherto unrecognized role in the formation of structures observed on their surfaces

observations models and mechanisms of failure of surface rocks surrounding planetary surface loads geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with increased distance from the center of the load of radial thrust faults to strike slip faults to concentric normal faults these model predictions are in conflict with the absence of annular zones of strike slip faults around prominent loads such as lunar maria martian volcanoes and the martian tharsis rise we suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the stress models indications that model stresses are inappropriate for use in fault type prediction include 1 tensile principal stresses larger than realistic values of rock tensile strength and or 2 stress differences significantly larger than those allowed by rock strength criteria predictions of surface faulting that are consistent with observations can be obtained instead by using tensile and shear failure criteria along with calculated stress differences and trajectories with model stress states not greatly in excess of the maximum allowed by rock fracture criteria

evolution of the olympus mons caldera mars synoptic images of the martian volcano olympus mons are of a quality and quantity that are unique for mars and somewhat surprisingly are appreciably better than image data that exist for many volcanoes on earth useful information about the evolution of shield volcanoes on earth can thus be derived from the investigation of this extraterrestrial example we have used shadow length measurements and photoclinometrically derived profiles to supplement and refine the topographic map of the olympus mons caldera as much as 2 5 km of collapse took place within the 80 65 km diameter caldera and the elevation of the caldera rim varies by almost 2 0 km low around the oldest collapse events high around the youngest an eight stage evolutionary sequence for the caldera of olympus mons is identified which shows that caldera subsidence was a longterm process rather than the near instantaneous event that has been interpreted from comparable terrestrial examples tectonic features on the caldera floor indicate a transition from an extensional environment graben formation around the perimeter of the caldera to compression ridge formation towards the caldera center this transition from a compressional to extensional environment is surprisingly sudden occurs at a radial distance of 17 km from the caldera center and is import because it can be used to infer that the magma chamber was relatively shallow thought to be at a depth of 16 km beneath the caldera floor zuber and mouginis mark 1990 ample evidence is also found within the olympus mons caldera for solidified lava lakes more than 30 km in width and for the localzed overturning and or withdrawal of lava within these lakes

state of stress faulting and eruption characteristics of large volcanoes on mars the formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting in turn lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes we develop models of time dependent stress and deformation for the tharsis volcanoes on mars by means of a finite element code we calculate stresses and displacements due to a volcano shaped load emplaced on an elastic plate overlying a viscoelastic mantle models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere the models illustrate the manner in which time dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories and the derived stress fields can be related to tectonic features on and surrounding martian volcanoes as a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting according to the mohr coulomb criterion at the surface of the plate just outward of the volcano near the base of the elastic lithosphere beneath the center of the volcano and on the upper flanks of the volcano early in its growth history normal faulting is the dominant mode of failure predicted for the first region consistent with circumferential graben observed around the tharsis montes and with the scarp at the base of olympus mons interpreted as a large offset listric normal fault normal faulting mostly radially oriented is predicted for the second region failure in the third region is predicted to consist of thrust faulting circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks in models simulating a growing volcano this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail this volume of early failure remains the most highly stressed area in the edifice concentric terraces interpreted by some workers as thrust faults on the upper flanks of olympus mons may correspond to the predicted circumferential thrust features if the most recent increments of volcano growth were relatively large or in the presence of local material property or stress field variations for volcanoes detached from the plate predicted failure in the edifice takes the form of radial normal faulting near the volcano base the addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the tharsis montes including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben this stress state is also consistent with an interpretation of the aureole deposits of olympus mons as the result of gravity sliding along a basal detachment our models also suggest an explanation for the lack of strike slip features predicted by previously published flexural models around the tharsis volcanoes for a given load increment the first mode of near surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation as the volcano grows and the flexural response to the increasing load proceeds the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike slip faulting because normal faulting has been predicted to have taken place earlier however it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike slip faults

origin of planetary wrinkle ridges based on the study of terrestrial analogs wrinkle ridges linear asymmetric topographic highs are common physiographic features on the moon mars and mercury we describe here some terrestrial features of similar morphology and scale that have formed under compressional stress systems similar to planetary wrinkle ridges all of the terrestrial analogs are produced by the anticlinal folding of rocks above reverse faults that shallow and typically break the surface characteristics common to both terrestrial and planetary ridges include 1 a linear asymmetric antiformal shape 2 tension cracks and grabens along the hinges of the antiform 3 overlapping en echelon lobes 4 reversals of the sense of asymmetry along strike 5 occurrence in compressional environments and 6 similarity to features produced during compression of various materials in laboratory scale models these similarities strongly suggest that planetary wrinkle ridges result from deformation associated with shallow thrust faults that either break or come close to the surface kinematic models capable of explaining the development of wrinkle ridges include 1 anticlinal folding of rocks over a buried thrust fault that may subsequently propagate to the surface and 2 fault bend folding of upper plate rocks over the surface flattening bend of a thrust fault

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

three hawaiian calderas an origin through loading by shallow intrusions the calderas of kilauea and mauna loa are highly dynamic structures and in the 200 year historic period have varied in volume by a factor of 2 and gained or lost 1 km3 per century the deeply eroded caldera of the extinct koolau volcano in oahu is wider than active hawaiian calderas and its lavas have a strong centripetal dip and funnel structure not evident at kilauea or mauna loa the differences can be attributed to the different erosion depths and the time integrated subsidence profile of kilauea is also a stepped funnel having its apex at halemaumau koolau caldera is the focus of an extraordinarily intense dike complex and an intriguing feature is the great diminution in dike concentration into the caldera it is thought that dike injection in any part of the complex generally continued until it reached 50 to 65 in outer parts of the caldera the complex was maintained at or rebuilt to this value despite subsidence in the center of the caldera where the positive bouguer anomaly is centered subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex assuming the same dike injection rate as kilauea and mauna loa yields a volumetric subsidence rate in koolau caldera exceeding 1 km3 per century hawaiian calderas are much more dynamic than calderas of silicic volcanoes shaped by frequent small events instead of a few great ones the temporal and volumetric correspondence of historical subsidence events with eruptions is poor and this and the high subsidence rates argue for a caldera forming mechanism that consumes the subsided rocks it is suggested that subsidence is caused by the great localized excess load of intrusive rocks carrying the center of the volcano into the thermally weakened lithosphere above the hawaiian hot spot it is envisaged that under steady state conditions the magma chamber rises as the injection of intrusions causes the level of neutral buoyancy at which the chamber is located to ascend and keeps pace with subsidence

volcanism in the solar system the myriad bodies that occur in the solar system show a wide range of physical properties exploration by spacecraft during the past four decades has shown that volcanism a major mechanism by which internal heat is transported to the surface is common on many of these bodies

a volcanotectonic survey of ascraeus mons mars 1 ascraeus mons is one of the largest volcanoes on mars it is replete with well preserved features that can be used to understand its volcanotectonic evolution previous studies of this volcano focused on specific features and were limited by the quality and coverage of contemporary data our objective is to review and enhance the existing developmental model for ascraeus by considering all endogenic surface features on the volcano we surveyed the volcano s caldera complex flank terraces pit structures sinuous rilles arcuate grabens and small vents we report the spatial and temporal distributions of these features appraise their proposed formation mechanisms in light of our mapping results and propose a detailed geological history for ascraeus mons an initial shield building phase was followed by the formation of a summit caldera complex and small parasitic cones while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks an eruptive hiatus followed ending with the construction of expansive rift aprons to the northeast and southwest against later extensive flank resurfacing in the late amazonian continued flexure formed arcuate grabens concentric to the edifice localized eruption and surface flow of a fluid agent lava and or water from within the volcano then produced a population of rilles on the lower flanks finally in a change of flank tectonic regime from compression to extension pit crater chains and troughs developed on the main shield and rift aprons eventually coalescing to form large embayments at the northeast and southwest base of the volcano

analysis of surface deformation data kilauea volcano hawaii october 1966 to september 1970 a least squares inversion technique has been applied to surface displacement measurements made at kilauea volcano hawaii in an attempt to locate centers of intrusive activity utilizing analytic expressions for the displacement field arising from dilatational sources in an elastic half space this technique allows for the simultaneous inversion of level tilt and horizontal distance measurements in terms of the location and volume of the intrusive body a variety of simple elastic model geometries representing possible shapes for intrusive bodies in the summit region of kilauea were examined none of the model geometries considered significantly improved the quality of the model fit to the available data over that obtained by a point source model calculations based on elastic models to consecutive level surveys were compared to available tilt data and horizontal distance measurements these comparisons indicate occasional departures from ideal elastic behavior the departure of measured tilt patterns from calculated tilt values may be the result of earthquake activity or episodic block movements superimposed on larger elastic displacements the departure of measured and calculated horizontal measurements reveals a gradual north south extensional strain across the summit caldera of kilauea in addition to the elastic uplift and subsidence patterns determined from level surveys a minimum estimate for the magma supply rate to kilauea is provided by the elastic models to the level data the average rate for the period from october 1966 to september 1970 was 0 07 km3 per year

nature of local magma storage zones and geometry of conduit systems below basaltic eruption sites pu u o o kilauea east rift hawaii example the fluid dynamics of the well documented eruptive episodes at pu u o o kilauea wolfe et at 1987 are used to investigate quantitativeely the size and shape of the shallow conduit system beneath the vent cooling calculations are employed to study the long term survival of conduits and the consequences of multiple dike injection events we find that the subvent conduit must have a planar geometry at depths greater than a few tens to at most a few hundreds of meters with a width of less than a few meters a length of the order of 100 m and a height of the order of 1 kin this structure is clearly the residue of the preeruption dikes although such a feature can be widened somewhat by repeated dike emplacement events there is no evidence to suggest that a larger and much more equant magma reservoir should develop at shallow depth the extensive degassing often occurring after single eruptions or between repeated eruptions previously thought to imply the presence of a large equidimensional shallow magma chamber can be readily explained by the volume of magma in a dikelike subvent planar magma storage zone on the basis of these observations which we infer to apply commonly to basaltic eruption sites we suggest that the terms chamber or reservoir be used with caution because they most often connote a relatively large equant magma body distinct from the feeder dike lying just below the vent we find that the subvent region has the same basic geometric characteristics as the parental dike and we propose that it be referred to as a planar magma storage zone

magma reservoir subsidence mechanics theoretical summary and application to kilauea volcano hawaii an analytic model has been developed for the prediction of the three dimensional deformation field generated by the withdrawal of magma from a sill like storage compartment during an intrusion or eruption cycle the model is based on the work of berry and sales 1961 1962 and predicts the vertical displacement components over the areal plane model parameters are the depth of burial h the intrusion half width a the intrusion half length b the thickness of the magmatic interior at the moment of melt withdrawal tm and the planform aspect ratio a b the products of the model include areal deformation maps systematic variation in model parameters within the context of kilauea volcano hawaii have revealed that circular and elliptical deformation patterns result from the collapse of draining rectilinear intrusions at depth moreover the geometric parameters of a storage compartment may interact in complex ways to produce similar deformation patterns the model has been applied to kilauea volcano for three periods of pronounced summit subsidence 1 1921 1927 bracketing the steamblast eruptive phases of 1924 2 june 1972 to december 1972 and 3 december 1972 to may 1973 application of the model requires the simultaneous optimization of five predicted deformation features with respect to field measurements and the derivative deformation maps 1 the vertical displacement maxima 2 the vertical displacement gradients over the areal plane 3 the lateral extent of the deformation field 4 the aspect ratio of the subsidence pattern and 5 the strike of the major axis of the deformation field the constrained geometries and volumes of the inferred collapsed storage cavities for each period are 1 1921 1927 depth 3 km a 1500 m b 4500 m tm 20 m v 540 106 m3 2 june 1972 to december 1972 depth 3 3 km a 600 m b 2000 m tm 1 m v 4 8 106 m3 and 3 december 1972 to may 1973 depth 2 2 km a 500 m b 1612 m tm 1 m v 3 2 106 m3 for 2 and 3 calculated magmatic thicknesses tm happen to be in the range 3 48 0 15 m of measurements for sill like bodies in deeply dissected hawaiian shield volcanoes the fits obtained between calculated and observed deformation patterns allow quantification of the location overall dimensions orientation and volume of the discrete still molten interior of sill like compartments from which magma is tapped during eruption or intrusion

the mars observer laser altimeter investigation the primary objective of the mars observer laser altimeter mola investigation is to determine globally the topography of mars at a level suitable for addressing problems in geology and geophysics secondary objectives are to characterize the 1064 nm wavelength surface reflectivity of mars to contribute to analyses of global surface mineralogy and seasonal albedo changes to assist in addressing problems in atmospheric circulation and to provide geodetic control and topographic context for the assessment of possible future mars landing sites the principal components of mola are a diode pumped neodymium doped yttrium aluminum garnet laser transmitter that emits 1064 nm wavelength laser pulses a 0 5 m diameter telescope a silicon avalanche photodiode detector and a time interval unit with 10 ns resolution mola will provide measurements of the topography of mars within approximately 160 m footprints and a center to center along track foot print spacing of 300 m along the mars observer subspacecraft ground track the elevation measurements will be quantized with 1 5 m vertical resolution before correction for orbit and pointing induced errors mola profiles will be assembled into a global 0 2 deg x 0 2 deg grid that will be referenced to mars center of mass with an absolute accuracy of approximately 30 m other data products will include a global grid of topographic gradients corrected individual profiles and a global 0 2 deg x 0 2 deg grid of 1064 nm surface reflectivity

density and lithospheric thickness of the tharsis province from mex mars and mro gravity data 1 radio science tracking of mars express mars experiment and mars reconnaissance orbiter produced high resolution martian gravity data applying localized spectral analysis on the new gravity data sets we study the surface density and lithospheric elastic thickness in the tharsis province the gravity signal is predicted with geophysical models either including bottom loading top bottom model or taking into account the loading history top top model volcanic shields are mainly composed of high density lava but their construction could have begun with lower density lava with the exception of ascraeus mons which has a top of lower density buoyant bottom loading may have been present in the form of a mantle plume under olympus mons the elastic thickness was much larger at olympus mons than at other volcanoes suggesting large spatial variations of heat flux during the hesperian alternatively small elastic thicknesses could be artifacts reflecting the presence of very localized high density crustal intrusions beneath the volcanoes thaumasia highlands were probably supported by a mantle plume at the time of formation in valles marineris top bottom models predict low densities and large elastic thicknesses in conflict with the basaltic rock composition and hesperian age of the valley dense mafic dikes underlie the western part of the valley the top top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion the elastic thickness increasing in between at the large volcanoes the relation between gravity and topography is anisotropic probably because of density variations

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

the relationship between earthquake swarms and magma transport kilauea volcano hawaii a detailed investigation of earthquake locations and focal mechanisms for swarms associated with intrusive events at kilauea volcano hawaii further illuminates the relationships among stress state faulting and magma transport we determine the earthquake locations and mechanisms using a three dimensional crustal model to improve their accuracy and consistency swarms in kilauea s upper east and southwest rift zones from the years 1980 through 1982 provide clear evidence for the propagation and or dilation of dikes focal mechanisms are predominantly strike slip and the faulting and inferred dike orientations can be interpreted quite consistently in terms of the model ofhill 1977 stresses induced by the summit magma reservoir system strongly control faulting and magma transport in the rift zones close to the summit

late stage intrusive activity at olympus mons mars summit inflation and giant dike formation by mapping the distribution of 351 lava flows at the summit area of olympus mons volcano on mars and correlating these flows with the current topography from the mars orbiter laser altimeter mola we have identified numerous flows which appear to have moved uphill this disparity is most clearly seen to the south of the caldera rim where the elevation increases by 200 m along the apparent path of the flow additional present day topographic anomalies have been identified including the tilting down towards the north of the floors of apollo and hermes paterae within the caldera and an elevation difference of 400 m between the northern and southern portions of the floor of zeus patera we conclude that inflation of the southern flank after the eruption of the youngest lava flows is the most plausible explanation which implies that intrusive activity at olympus mons continued towards the present beyond the age of the youngest paterae 200 300 myr neukum et al 2004 robbins et al 2011 we propose that intrusion of lateral dikes to radial distances 2 000 km is linked to the formation of the individual paterae at olympus mons two specific dikes to the se of the volcano are inferred to have volumes of 4 400 km3 and 6 100 km3 greater than the volumes of individual calderas and implying triggering of both caldera collapse and lateral dike injection by the arrival of large inputs of magma from the mantle a comparable disparity between lava flow direction and current topography together with a tilted part of the caldera floor has been identified at ascraeus mons

flank tectonics of martian volcanoes on the flanks of olympus mons is a series of terraces concentrically distributed around the caldera their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone in an attempt to understand the mechanism of faulting and the possible influences of the interior structure of olympus mons we have constructed a numerical model for elastic stresses within a martian volcano in the absence of internal pressurization the middle slopes of the cone are subjected to compressional stress appropriate to the formation of thrust faults these stresses for olympus mons are 250 mpa if a vacant magma chamber is contained within the cone the region of maximum compressional stress is extended toward the base of the cone if the magma chamber is pressurized extensional stresses occur at the summit and on the upper slopes of the cone for a filled but unpressurized magma chamber the observed positions of the faults agree well with the calculated region of high compressional stress three other volcanoes on mars ascraeus mons arsia mons and pavonis mons possess similar terraces extending our analysis to other martian volcanoes we find that only these three and olympus mons have flank stresses that exceed the compressional failure strength of basalt lending support to the view that the terraces on all four are thrust faults

the geometry of volcano flank terraces on mars abstract flank terraces are subtle expansive structures on the slopes of many large martian shield volcanoes several terrace formation hypotheses including self loading lithospheric flexure magma chamber tumescence volcano spreading and shallow gravitational slumping have been suggested terraces are not readily visible on photogeological data consequently terrace geometry has not yet been comprehensively described terrace provenance therefore is poorly understood we used three dimensional mars orbiter laser altimeter mola data to characterise the geometry of these elusive structures with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on mars terraces have a broad convex upward profile in section and a systematic fish scale imbricate stacking pattern in plan they are visible at all elevations on at least nine disparate martian volcanoes terrace like features also occur on three shield volcanoes on earth an observation not recorded before analysis of a suite of morphometric parameters for flank terraces showed that they are scale invariant with similar proportions to thrust faults on earth we compared predicted formation geometries to our terrace observations and found that only lithospheric flexure can fully account for the morphology distribution and timing of terraces as a volcano flexes into the lithosphere beneath it its upper surface will experience a net reduction in area resulting in the formation of outward verging thrusts we conclude therefore that flank terraces are fundamental volcanotectonic structures that they are the surface expressions of thrust faults probably formed by lithospheric flexure and that they are not restricted to mars

topographic evidence for geologically recent near polar volcanism on mars abstract the mars orbiter laser altimeter mola was used to measure the topography of several putative near polar martian volcanic craterforms we believe they were formed by effusive lava shield building eruptions and were not hydromagmatic events as previously suggested furthermore these craterforms appear to be geologically recent with ages between 1 and 20 million years and may provide new evidence of localized thermal anomalies in the upper martian crustal column

tectonic evolution of bell regio venus regional stress lithospheric flexure and edifice stresses in order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of venus we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in bell regio using magellan synthetic aperture radar sar images and altimeter measurements of topography the major volcanoes of bell regio tepev mons and nyx mons exhibit tectonic characteristics that are unique relative to other volcanic edifices on venus the most prominent distinction is the lack of large rift zones within the overall highland uplift which characterize many other highland rises on venus also previous studies have determined that many large venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on earth and mars tepev and nyx montes exhibit both the radial tectonic features associated with other venusian edifices and numerous concentric graben nyx mons implies a more distributed magmatic system by its broad shape radial chains of pit craters and expansive flow fields whereas tepev mons is a more centralized volcanic system with limited associated long flows we investigate the regional stresses associated with bell regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading modeling both nyx mons and tepev mons as axisymmetric loads with gaussian mass distributions on an elastic plate the relationship between the tectonic features surrounding tepev mons and stresses associated with magma chamber inflation are also examined through finite element analysis using topography data to model the shape of the volcano we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in magellan images these results imply a shift in volcanic style within bell regio from an early phase of broad low shield formation to later steep sided more centralized edifice development such changes are consistent with an increase in the thickness of the lithosphere over time

discrete element simulations of gravitational volcanic deformation 1 deformation structures and geometries 1 we have carried out two dimensional particle dynamics simulations of granular piles subject to frictional coulomb failure criteria to gain a first order understanding of different modes of gravitational deformation within volcanoes under uniform basal and internal strength conditions granular piles grow self similarly developing distinctive stratigraphies morphologies and structures piles constructed upon cohesive substrates exhibit particle avalanching forming outward dipping strata and angle of repose slopes systematic decreases in basal strength lead to progressively deeper and steeper internal detachment faults and slip along a basal decollement landslide forms grade from shallow slumps to deep seated landslide and finally to axial subsidence and outward flank displacements or volcanic spreading surface slopes decrease and develop concave up morphologies with decreasing decollement strength depositional layers tilt progressively inward spatial variations in basal strength cause lateral transitions in pile structure stratigraphy and morphology this approximation of volcanoes as coulomb granular piles reproduces the richness of deformational structures and surface morphologies in many volcanic settings the gentle slopes of hawaiian volcanoes and olympus mons on mars suggest weak basal decollements that enable volcanic spreading high angle normal faults favored above weak decollements are interpreted in both settings and may explain catastrophic sector collapse in hawaii and broad aureole deposits surrounding olympus mons in contrast steeper slopes and shallow detachment faults predominate in the canary islands thought to lack a weak decollement favoring smaller more frequent slope failures than predicted for hawaii the numerical results provide a useful predictive tool for interpreting dynamic behavior and associated geologic hazards of active volcanoes

fissure eruptions in tharsis mars implications for eruption conditions and magma sources the tharsis region has been the focus of many studies of volcanism on mars not only are the largest volcanoes on the planet located here but also many of the youngest volcanic features are found within tharsis comparatively little attention has been given to understanding the origin of the smaller volcanoes within tharsis and these smaller structures may provide important information on both the regional tectonics of the area and the availability of magma from the shallow or deep crust we have identified six separate but physically close vent systems in eastern tharsis just to the east of the volcano jovis tholus these vents are typically linear fissures a few to 20 km in length that have built small shields rising to 50 85 m above the level of the surrounding topography the maximum length of individual lava flows from these fissures is 30 km but more typical lengths are 15 20 km mapping of the vent complexes reveals that smooth material interpreted to be spatter from fire fountaining is located on the rims of some of the fissures this spatter then formed either short 5 km flows or merged close to the fissure to form the longer flows there appears to have been some temporal evolution of the flow field as the oldest parts of the basement at each center are built from a series of compound flows that cannot be subdivided into individual flows we find evidence of frozen lava ponds and channelized flows from central vents photoclinometric profiles are constructed across one of the longer flows to confirm an unusual attribute originally identified by mouginis mark and christensen mouginis mark p j christensen p r 2005 new observations of volcanic features on mars from the themis instrument j geophys res 110 e8 doi 10 1029 2005je002421 of the flows from the central vent complex they are all 5 m thick which is a factor of 8 15 thinner than flows elsewhere on mars to investigate the unusual eruption conditions limited total volume of the construct and of individual flows we model the lava rheology the duration of emplacement the subsurface conditions that may have led to these eruptions and the volatile content of the magma we find lava viscosities and yield strengths to be 100 pa s and 100 pa respectively eruption rates to be 5000 m3 s 1 flow speeds to be 1 2 m s 1 durations of emplacement of individual flow units to be 5 h and the equivalent magma water content to be 0 1 0 2 mass these eruption conditions are consistent with a wide range of possible depths of the magma reservoirs feeding the eruptions small vents such as the ones studied here have also been identified in other parts of eastern tharsis so that the eruptions described here may characterize a common style of volcanism on mars that can only be identified now that image spatial resolution in the range 1 20 m pixel is available it is therefore possible that additional searches of other volcanic areas syrtis planum elysium planitia and hesperia planum may also show greater diversity of activity than is currently accepted