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The northern Main Ethiopian Rift (MER) in East Africa is considered a region of incipient oceanic spreading, with Miocene border faulting now largely abandoned at the expense of magmatic extension in the Wonji Fault Belt (WFB). However, whether magmatic extension began when the Ethiopian lithosphere was still-thick, or heavily stretched, is unknown. The Galema range, a linear Pliocene dike swarm parallel to the eastern margin of the present-day central MER, is an ideal study locale to constrain melting depths, and by inference the thickness of the lithosphere, during early magmatic rifting. To address this issue, we present whole-rock, trace element data on 77 samples of Galema range magmas. We interpret contrasting results between two modeling approaches as evidence for magma ponding subsequent to melt generation. Trace element models of melt generation reveal melting conditions of TP = 1418–1450 °C at 2.9–3.2 GPa, some ~68–100 °C above ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re-equilibrated with the mantle, yielded broadly similar temperatures (1435–1474 °C) but at lower pressures (2.1–2.6 ± 0.2 GPa: 78–89 km depth); these results are broadly parallel to contemporaneous magmatism on the western rift margin in the Akaki Magmatic Zone. We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent: the lithosphere-asthenosphere boundary. The Ethiopian continental lithosphere has therefore remained relatively thick late into the rifting process, with important potential implications for late-stage decompression melting prior to the onset of seafloor spreading.

Polygonal ridge networks, also known as boxwork or reticulate ridges, are found in numerous locations and geological contexts across Mars. Distinguishing the morphologies and geological context of the ridge networks sheds light on their potential as astrobiological and mineral resource sites of interest. The most widespread type of ridge morphology is characteristic of the Nili Fossae and Nilosyrtis region and consists of thin, criss-crossing ridges with a variety of heights, widths, and intersection angles. They are found in ancient Noachian terrains at a variety of altitudes (between -2500 and 2200 m) and geographic locations and are likely to be chemically altered fracture planes or mineral veins. They occur in the same general areas as valley networks and ancient lake basins, but they are not more numerous where these water-related features are concentrated, and can appear in places where th morphologies are absent. Similarly, some of the ridge networks are located near hydrated mineral detections, but there is not a one-to-one correlation. Smaller, light-toned ridges of variable widths have been found in Gale Crater and other rover sites and are interpreted to be smaller versions of the Nili-like ridges, mostly formed by the mineralization of fractures. This type of ridge is likely to be found in many other places on Mars as more high-resolution data become available. Sinus Meridiani contains many flat-topped ridges arranged into quasi-circular patterns. The ridges are eroding from a clay-rich unit, and could be formed by a similar process as the Nili-type ridges, but at a much larger scale and controlled by fractures made through a different process. Hellas Basin is host to a fourth type of ridge morphology consisting of large, thick, light-toned ridges forming regular polygons at several superimposed scales. While still enigmatic, these are most likely to be the result of sediment-filled fractures. The Eastern Medusae Fossae Formation contains large swaths of a fifth, previously undocumented, ridge network type. The dark ridges, reaching up to 50 m in height, enclose regular polygons and erode into dark boulders. These ridge networks are interpreted to form as a result of lava flow embayment of deeply fractured Medusae Fossae Formation outcrops.

Recent analytical investigations have suggested that magma buoyancy is critical for triggering catastrophic caldera forming eruptions. Through detailed assessment of these approaches, we illustrate how analytical models have been misapplied for investigating buoyancy and are, therefore, incorrect and inconclusive. Nevertheless, the hypothesis that buoyancy is the critical trigger for larger eruptions warrants further investigation. As such, we utilize viscoelastic finite element models that incorporate buoyancy to test overpressure evolution and mechanical failure in the roof due to the coalescence of large buoyant magma bodies for two model cases. In the first case, we mimic empirical approaches and include buoyancy as an explicit boundary condition. In the second set of models, buoyancy is calculated implicitly due to the density contrast between the magma in the reservoir and the host rock. Results from these numerical experiments indicate that buoyancy promotes only minimal overpressurization of large silicic magma reservoirs ( 100 km3) they surpass a rheological threshold where their subsequent evolution is controlled by host rock mechanics. Consequently, this results in a thermomechanical division between small systems that are triggered “internally” by magmatic processes and large systems that are triggered “externally” by faulting related to roof uplift or tectonism. Finally, critical assessment of recent analytical approaches illustrates that care must be used when applying previously derived analytical solutions to ensure that assumptions used in the original formulation are not violated during application to new geologic problems.

Understanding the connection between shallow subsurface magmatism and related surface expressions provides first-order insight into the volcanic and tectonic processes that shape a planet's evolution. When assessing the role of flexure, previous investigations assumed homogeneous host rock, but planetary lithospheres typically include crust and mantle material, and the mechanical response of a layered lithosphere subjected to flexure may influence both shallow magma reservoir failure and intrusion propagation. To assess the formation of giant radial dike systems, such as those observed on Venus, we create axisymmetric elastic finite element models of a spherical reservoir centered at the contact between stiff, dense mantle overlain by softer, lighter crust. We analyze magma chamber stability, overpressure at rupture, and resulting intrusion types for three distinct environments: lithostatic, upward flexure, and downward flexure. In the lithostatic case, reservoir failure at the crust-mantle contact favors lateral sill injection. In the flexure cases, we observe that failure location depends upon the crust/lithosphere thickness ratio and, at times, will favor radial dike intrusion. Specifically, upward flexure can promote the formation of giant radiating dike swarms, a scenario consistent with a plume-derived origin. Our results present a mechanical explanation for giant radial dike swarm formation, showing that both the stability of magma chambers on Venus and the type of intrusions that form are influenced by lithospheric layering. Furthermore, where dike swarms occur, our approach provides a powerful new way to constrain local crust/mantle layering characteristics within the lithosphere at the time the swarm was forming.

The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

In this study, we utilize thermomechanical models to investigate how magma chambers overpressurize as the result of either magmatic recharge or volatile exsolution. By implementing an adaptive reservoir boundary condition we are able to track how overpressure dissipates as the magma chamber expands to accommodate internal volume changes. We find that the size of the reservoir greatly impacts the resultant magma chamber overpressure. In particular, overpressure estimates for small to moderate-sized reservoirs (1–10 km 3 ) are up to 70% lower than previous analytical predictions. We apply our models to Santorini volcano in Greece where recent seismic activity and ground deformation observations suggested the potential for eruption. The incorporation of an adaptive boundary condition reproduces Mogi flux estimates and suggests that the magma reservoir present at Santorini may be quite large. Furthermore, model results suggest that if the magma chamber is >100 km 3 , overpressures generated due to the high magma flux may not exceed the strength of the host rock, thus requiring an additional triggering mechanism for eruption. Although the adaptive boundary condition approach does not calculate the internal evolution of the magma reservoir, it represents a fundamental step forward from elastic Mogi models and fixed boundary solutions on which future investigations of the evolution of the magma can be built.

[1] Using parameterized finite element models, we investigate the emplacement of both radial and circumferential intrusions in the configuration observed at Fernandina volcano in the Galapagos Islands. When situated within the edifice at depths consistent with petrological and surface displacement data, inflation of a mildly oblate magma reservoir to the point of rupture can initiate either radial or circumferential intrusions in response to minor, volcanologically plausible variations in reservoir geometry (i.e., aspect ratio). In addition, more oblate reservoirs inject lateral sills into an inflation-derived stress field consistent with rotation about their propagation axis to form gently dipping radial dikes, a mode of behavior recently inferred from InSAR data at Fernandina. All three styles of intrusion occur in near-surface configurations consistent with field observations.

Giant radiating dike swarms on Venus are geomorphic manifestations of deep-seated volcano-tectonic processes. Through an initial series of experiments we assess if these features can be explained as a product of the relationship between magmatism and the flexural stress state caused by lithospheric uplift (due to a rising mantle diapir, plume or underplating processes) using gravitationally loaded, axisymmetric, elastic finite element models. Magma chambers situated in the upper (extensional) lithosphere fail at or near the crest, redirecting magma vertically into radially aligned dikes, but these are expected to feed surface eruptions; conditions promoting lateral propagation of dikes do not occur. Magma chambers located in the lower (compressional) lithosphere are predicted to generate horizontal intrusions (sills) upon midsection failure. When both uplift (from basal loading) and magmatic surface loading (from edifice growth) are introduced in sequence at discrete times, however, conditions that promote shallow lateral propagation of magma within radial dikes are created: (1) as magma from shallow reservoirs ascends and erupts, surface-loading volcanic edifices can form; (2) as the vertical loads increase, flexing the lithosphere downward, high compressional stresses produced within the edifice create a shallow stress barrier that makes it increasingly difficult for magma to complete the process of ascent and eruption; and then (3) stresses acting on dikes ascending from a reservoir toward the surface will thus increasingly favor fracture along their lateral margins, redirecting magma through radial dikes toward distal regions at shallow depth. Our results are consistent with observed patterns of laterally extensive dike systems at volcanic centers such as Mbokomu Mons and Becuma Mons. The models thus provide a robust mechanical explanation that for the first time links lithosphere flexure and reservoir pressurization to the ascent and lateral spreading of magma through radial dike systems on Venus, thereby providing key new spatial and temporal insights into an important magmatic process operating on multiple terrestrial planets.

Composed of a series of circular to elliptical bowl-shaped depressions, pit crater chains are common on the surface of many of our solar system’s terrestrial planets and moons. Using Magellan synthetic aperture radar (SAR) images, four areas of Venus are examined in which a total of 354 pit crater chains are found: Ganiki Planitia (180°E–210°E, 25°N–50°N), Ulfrun Regio (200°E–240°E, 0°N–25°N), Themis Regio (270°E–300°E, 25°S–40°S), and Idunn Mons (205°E–225°E, 35°S–55°S). A study of the distribution of these pit crater chains at regional and local scales reveals hierarchical clustering. On a regional scale, pit crater chain clusters are associated with graben–fissure systems that are radiating (associated with volcano-tectonic features), circumferential (associated with coronae), and linear (with uncertain volcano-tectonic genesis). At a local scale, pit crater chains are found with marked restriction to particular portions of graben–fissure systems. We conclude that this hierarchical clustering is an indication that both an extensional process and a lithological control contribute to the formation of pit crater chains. Specifically, we propose that pit crater chain formation on Venus occurs in poorly welded volcaniclastic material (e.g., shield plains material unit) that has been crosscut by graben–fissure system(s). Only portions of the shield plains material unit may have sufficient thickness of volcaniclastic material, thus explaining the lack of a co-extensive relationship. Additionally, pit crater chains in other map units may be explained by shallow burial of the volcaniclastic material.

Approximately every 100,000 years the Earth experiences catastrophic caldera-forming “supereruptions” that are considered to be one of the most hazardous natural events on Earth. Utilizing new temperature-dependent, viscoelastic numerical models that incorporate a Mohr-Coulomb failure criterion, we find that eruptive failure of the largest magma chambers is a function of the geometry of the overlying roof and the location of the brittle-ductile transition. In particular, the ductile halo created around the hot magma chamber buffers increasing overpressures and prevents pressure relief via magmatic injection from the magma chamber. The numerical results indicate that as chamber volume increases, the higher temperatures in the host rock and the decrease in the roof aspect ratio cause a shift from reservoir-triggered eruption to an external roof-triggered mechanism. Specifically, as overpressure increases within the largest magma chambers, extensive uplift in the overlying roof promotes the development of through-going faults that may trigger eruption and caldera collapse from above. We find that for magma chamber volumes > 103 km3, and roof aspect ratios (depth/width)

Eruptions fed from subsurface reservoirs commonly construct volcanic edifices at the surface, and the growth of an edifice will in turn modify the subsurface stress state that dictates the conditions under which subsequent rupture of the inflating reservoir can occur. We re-examine this problem using axisymmetric finite element models of ellipsoidal reservoirs beneath conical edifices, explicitly incorporating factors (e.g., full gravitational loading conditions, an elastic edifice instead of a surface load, reservoir pressures sufficient to induce tensile rupture) that compromise previous solutions to illustrate why variations in rupture behavior can occur. Relative to half-space model results, the presence of an edifice generally rotates rupture toward the crest of a spherical reservoir, with increasing flank slope (for an edifice of constant volume) and larger edifices (or greater reservoir scaled depths) normally serving to enhance this trend. When non-spherical reservoirs are considered, the presence of an edifice amplifies previously identified half-space failure characteristics, shifting rupture to the crest more rapidly for prolate reservoirs while forcing rupture closer to the midpoint of oblate reservoirs. Rupture is always observed to occur in the σ t orientation, and depending on where initial failure occurs rupture favors the initial emplacement of either lateral sills, circumferential intrusions or vertically ascending dikes. Ultimately, integration of our numerical model results with other information, for instance the sequence of intrusion/eruption events observed at a given volcano, can provide useful new insight into how a volcano's subsurface magma plumbing system evolved. We demonstrate this process through application of our model to Summer Coon, a well-studied stratocone on Earth, and Ilithyia Mons, a large conical shield volcano on Venus.

Elastic, gravitationally loaded finite element models of an ellipsoidal magma reservoir in mechanically layered host rock yield new insight into the effect of layer stiffness, thickness, and spatial configuration on the resulting surface displacement curve and tensile rupture of the reservoir wall. Decreasing layer stiffness enhances surface and subsurface deformation, focusing pressure across a narrow area, and generally decreases the likelihood of vertical magma intrusion and surface eruption. In contrast, increasing layer stiffness impedes uplift and distributes strain across a broad area, enhancing the likelihood of vertical magma intrusion. Application of the model to deformation that occurred between 1985 and 1999 at the Long Valley caldera, California shows that the predicted reservoir radius and depth change by 16% and 12% respectively when mechanical layers inferred to be present are included in the model, highlighting the importance of explicitly incorporating mechanical layering. Analysis of rupture conditions for the reservoirs modeled in the present paper indicates that vertical intrusions are likely. In addition, analysis of rupture conditions for Long Valley magma reservoirs recently inferred from surface displacement inversions indicates that they will fail before achieving sufficient pressure to reproduce measured surface displacement.

We introduce a weighted method of clustering the individual units of a segmented image. Specifically, we analyze geologic maps generated from experts’ analysis of remote sensing images and provide geologists with a powerful method to numerically test the consistency of a mapping with the entire multidimensional dataset of that region. Our weighted model-based clustering method (WMBC) employs a weighted likelihood and assigns fixed weights to each unit corresponding to the number of pixels located within the unit. WMBC characterizes each unit by the means and standard deviations of the pixels within that unit and uses the expectation-maximization algorithm with a weighted likelihood function to cluster the units. With both simulated and real data sets, we show that WMBC is more accurate than standard model-based clustering. Specifically, we analyze Magellan data from a large, geologically complex region of Venus to validate the mapping efforts of planetary geologists.

Results from a finite element model characterizing tensile rupture of an internally pressurized ellipsoidal magma reservoir within an axisymmetric elastic half space illustrate that gravity plays a critical role in this process. Failure to incorporate gravitational loading correctly, which is the case for most published models, affects for example: (a) application of corrections designed to account for the presence of the free surface in analytical models; (b) inferences about the internal pressure that a reservoir can sustain prior to rupture; (c) conclusions about the importance of neutral buoyancy, i.e. the relative host rock and magma density structures; and, (d) predictions about the location at which rupture of the reservoir wall will occur and the style of intrusion which will be favored. Analyses that reduce magma reservoirs to a cavity within an unloaded elastic medium, inflated by only an excess pressure component, sacrifice important information and should not be used to interpret reservoir activity or to calibrate more advanced models of volcanic regions and phenomena; an exception to this rule occurs, however, when constraining the pressure that can be inferred from surface displacements for a reservoir of known geometry. In a gravitationally loaded model, the characteristics of the failure process are insensitive to geologically plausible variations in the tensile strength, shear modulus, density structure and gravitational acceleration. As a result the half-space analysis presented here, which will benefit from future expansion to include topography and other factors, can yield insight into not only magma reservoirs on Earth but those thought to have formed within the crusts of Mars, Venus and other solar system bodies as well.

Detailed mapping in a 14,000,000 km 2 area of northwestern Guinevere Planitia and northern Beta Regio bounded by 264 ◦ –312 ◦ E, 24 ◦ – 60 ◦ N has revealed thousands of long extensional lineaments (graben, fissures and related fractures). These can be grouped into radiating, circumferential and linear systems. Thirty four radiating systems have been identified, of which 16 have radii greater than 300 km and eight have radii greater than 1000 km. Twenty six linear (straight) systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km. Linear systems are generally associated with rifts, although some may represent distal portions of radiating systems. In addition, 19 circumferential systems, some associated with coronae, have been identified. The distribution of each system is compared with the host geology in order to place the graben–fissure systems in a regional stratigraphic framework. The majority of systems are: (1) younger than tesserae, ridge belts and densely fractured plains, (2) coeval with, and in many cases, define fracture belts, (3) partially flooded by wrinkle-ridged plains units, and (4) older than smooth and lobate plains units and young rifts. The inventory of radiating graben–fissure systems that we catalogue represents a database of tectono-magmatic centers that complements the centers defined using other criteria, e.g., large volcanoes, coronae, and shield fields. We have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes. At least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms.

Seismic refraction profiling and gravity measurements along the length of the northern Devils Lane graben in Canyonlands National Park reveal that the bedrock floor of the graben is buried beneath a sedimentary cover more than 90 m thick, at least four to ten times more fill than previously measured. This sediment thickness, when combined with exposed fault topography, yields a throw of ≥145 m on the master fault. We estimate that horizontal extensional strain across the graben is on the order of 2–3%, and if this is typical of grabens elsewhere in Canyonlands the maximum average strain rates across the region are about 1×10−14 s−1, or ∼2 mm year−1. Our results indicate that published length to displacement relationships underestimate fault offset at Devils Lane by a factor of at least 1.5. This suggests that careful geophysical evaluation of sediment thickness elsewhere within the Canyonlands graben system, an important ‘type area’ for investigating the mechanics which characterize the formation of kilometer-scale extensional faults, may be required before these structures can be used to help define accurate fault length to displacement scaling laws.

▪ Abstract Earth, Venus, and Mars all exhibit populations of giant (radiating, linear, and arcuate) mafic dike swarms hundreds to >2000 km in length. On Earth the dikes are exposed by erosion, while on Venus and Mars their presence is mainly inferred from associated volcanic morphology and surface deformation. The apparent absence of plate tectonics in the geologic record of Venus and Mars means that the observed population of swarms remains geometrically intact, while on Earth plate tectonics has fragmented swarms. About 30 giant radiating swarms have so far been identified on Earth, but with further study the number is expected to rise and may eventually coincide with the hundreds of mantle plume head events now being proposed. On Venus, at least 118 radiating swarms are distributed across the planet, and new high resolution mapping is revealing additional swarms. On Mars, up to 16 giant dike swarms are observed, most associated with the Tharsis region.

Stratigraphic study of a distributed population of 118 giant radiating dike swarms on Venus reveals that within each dike-intruded region, emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera, regional plains, and most wrinkle ridges. The density of impact craters superimposed on the swarm population (1.80±0.57 craters/10 6 km 2 ), when compared with the average global density (2.01±0.14 craters/10 6 km 2 ) and the densities reported for other geologic units, is consistent with the observed stratigraphy. On the basis of these data, we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing. The stresses recorded by the dike swarm population, when combined with its age and compared with the predictions of several proposed resurfacing models, best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer. Formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long-wavelength gravity and topography. In addition, the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long-wavelength characteristics. We interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long-wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago.

Theoretical calculations of extrusive volcanic degassing on Venus yield atmospheric pressure-related rock density profiles consistent with the formation of magma neutral buoyancy zones and magma reservoirs at different depths as a function of altitude (Head and Wilson, J. geophys. Res. 97, 3877, 1992). Global analysis of radiating dike swarms interpreted to originate at magma reservoirs show that their distribution matches these predictions across approximately 90% of the planet's surface; only those highland regions whose elevations exceed 6053 km appear anomalous. The distribution of the large volcano population (extrusive reservoir products) (Keddie and Head, Planet. Space Sci. 42, 455, 1994) has yielded similar results. Comparison between the dike swarm (intrusive) and large volcano (extrusive) populations suggests that neutral buoyancy plays an important role in governing volcanic processes near the venusian surface and that the depth to the level of neutral buoyancy increases systematically at altitudes above 6051 km.

Concentrations of dykes of basic composition emplaced in the same igneous episode or along similar trends are known as mafic dyke swarms and they occur in a wide variety of environments and over a wide range of scales on Earth. Recent radar mapping of Venus has revealed families of linear features interpreted to be the surface expression of near-surface dyke swarms. The lack of significant erosion on Venus provides a view of the surface manifestation of dyke swarm emplacement, one which complements the terrestrial perspective of erosion to deeper levels. The goal of this review is to synthesize the information available on both planets in order to use the complementary and synergistic record of mafic dyke swarm emplacement to build toward a better understanding of this important phenomenon in planetary history. We focus on the formation and evolution of giant dyke swarms which cover tens to hundreds of thousands of square kilometres on both Earth and Venus. Mafic dyke swarms on Earth occur in a wide range of modes and are observed in environments ranging from volcanic edifices (e.g., Hawaii), to central complexes (e.g., Spanish Peaks Complex, USA; Ramon Swarm, Israel), spreading centres and ophiolite complexes, compressional plate boundaries in back-arc settings (Columbia River Basalts, USA) and in continent-continent collisions. One of the most impressive modes of occurrence is that linked to the formation and evolution of mantle plumes. Terrestrial examples include a giant radiating swarm covering 100° of azimuth (the Mackenzie swarm, Canada), a 360° giant radiating swarm (the Central Atlantic reconstructed swarm), deformed giant radiating swarms (the Matachewan swarm, Canada), rift-arm associated swarms (e.g., Grenville swarm, Canada; Yakutsk swarm, Siberia), and one consisting of widely separated dykes (e.g., the Abitibi swarm, Canada). We summarize the geometric, chemical and isotopic characteristics of terrestrial dyke swarms, including their size and geometry, ages, presence and absence of subswarms, and the relation between swarms of different ages. We also summarize the characteristics of individual dykes, examining dyke length and continuity, en echelon offsets, dyke bifurcation, dyke height, width and depth, dyke intrusion and cooling history, and evidence for flow directions. On Venus at least 163 large radiating lineament systems (radius generally > 100 km) composed of graben, fissure and fracture elements have been identified. On the basis of their structure, plan view geometry and volcanic associations, the radial elements of more than 70% of these are interpreted to have formed primarily through subsurface dyke swarm emplacement, with the remainder forming through uplift or some combination of these two mechanisms. These systems are essentially uneroded and provide a view of the surface characteristics of giant radial swarms prior to the erosion which commonly occurs on Earth. The individual graben, fissures and fractures of which the systems are composed are typically less than several kilometres in width and cluster near the centre, with fissures grading smoothly into fractures at greater distances to define the overall radial pattern. While the largest systems, like those on Earth, are thousands of kilometres in radius, the population average is about 325 km, and they generally do not extend to equal lengths in all directions. In their distal regions, however, the elements in 72% of the systems continue along a purely radial trend, while distal elements in the remaining 28% curve gradually into unidirectional, sub-parallel geometries, generally interpreted to be related to regional stress patterns. The radial systems have a strong association with volcanism; all but seven display some form of volcanic signature. A review of models of the emplacement of lateral dykes from magma chambers under constant (buffered) driving pressure conditions and declining (unbuffered) driving pressure conditions indicates that the two pressure scenarios lead to distinctly different styles of dyke emplacement. Emplacement of lateral dykes in the constant driving pressure (buffered) case, however, can produce dykes which have sizes and widths which are very large and independent of chamber size. On Earth, the characteristics of giant mafic dyke swarms such as the Mackenzie dyke swarm in Canada strongly suggest that they were emplaced in buffered conditions. On Earth, giant radiating dyke swarms are usually preserved as fan-shaped fragments which have been dismembered and distorted by subsequent plate tectonic rifting events. The abundant intact giant radiating swarms on Venus provide criteria by which fragmented terrestrial swarms can be reconstructed.

Magellan radar data of Venus reveal 163 large radial lineament systems composed of graben, fissure, and fracture elements. On the basis of their structure, plan view geometry, and volcanic associations, at least 72% are interpreted to have formed primarily through subsurface dike swarm emplacement, the remainder through uplift or a combination of these two mechanisms. The population of swarms is used to determine regional and global stress orientation. The stress configuration recorded from 330-210 deg E (Aphrodite Terra) is best explained by isostatic compensation of existing long wavelength topography or coupling between mantle flow and the lithosphere. The rest are correlated with concentrations of rifting and volcanism in the Beta-Atla-Themis region. The global stress field on Venus is different than that of Earth, where plate boundary forces dominate.

Magellan radar data from western Vinmara Planitia on Venus reveal a system of radiating lineaments extending 450 km from a small central annulus. Spatial variations in lineament density, orientation, and morphology, as well as structural and volcanic correlations, provide strong evidence that formation of the lineaments was related to subsurface dike emplacement. We infer from the observed surface deformation that the dikes were emplaced laterally, at shallow depth, from a large central magma reservoir. This configuration is analogous to that of radiating dike swarms found on Earth. Because dikes inject normal to the least compressive stress direction, swarm plan view geometry will reveal the greatest horizontal compressive stress trajectories. We interpret strongly radial orientations near the swarm center to represent radial stresses linked to pressurization of the magma reservoir. Increasingly non-radial behavior dominating at greater distances is interpreted to reflect a N60E±20° regional maximum horizontal compressive stress. Contrary to previous inferences that a persistent E–W compressive stress dominated throughout, analysis of the arachnoid indicates that a N60E compressive stress must have existed across western Vinmara Planitia during a portion of its deformation. This and the absence of distributed shear within the adjacent deformation belts indicates that the regional maximum horizontal compression orientation has varied over time. Comparison between the regional stress orientations inferred from the arachnoid and several nearby ridge belts illustrates that stress orientations may potentially be useful for determining relative belt ages in areas where the timing of ridge belt formation is difficult to assess by more direct means. This demonstrates one way that identification and analysis of giant radiating dike swarms can provide new information critical for regional stress interpretations on Venus.

[1] The growth and evolution of large volcanic edifices on Venus should reflect interactions between local magma reservoir-induced stresses and broader-scale stresses resulting from flexure of the lithosphere beneath the edifice load. Here, we explore the relationship between magma movement in the lithosphere and the flexural stress state via static, gravitationally loaded, axisymmetric finite element models. We find that reservoirs situated in the lower (extensional) lithosphere fail at the bottom and are therefore not viable long-term conduits for upward magma transport. Furthermore, for high-stress conditions (e.g., large edifices or thin lithospheres), chambers in the lowermost lithosphere exceed the failure criterion even before pressurization and are therefore unstable. In contrast, magma chambers located in the upper (compressional) lithosphere fail at or somewhat above the reservoir midsection, promoting lateral sill injection; continued failure in this mode would tend to produce oblate magma chambers with zones of intrusion at their margins. Reservoirs near the flexural neutral plane require the greatest overpressure to reach failure, emplacing cone sheets that transition to sills further from the chamber. The out-of-plane orientation of principal extensional stresses in the flexed lower lithosphere predicts the presence of radial dikes that are likely the main conduits for any subsequent magma ascent from the mantle melt source region. Our results also explain how the evolving stress state in the lithosphere tends to redirect magma passage over time: magma ascending into the lithosphere beneath the edifice is diverted to lateral sills in the upper lithosphere, inhibiting summit eruptions and possibly shifting eruption locations to the lower flanks at and beyond the distal margins of an oblate chamber or sill complex. We apply these results to interpret the observed structure and tectonism of Sapas Mons, Venus, in terms of flexurally controlled intrusive processes.

[1] The total volume of extrusive volcanism on the Moon provides a basic thermal and geologic constraint, and accurate volume assessments are contingent upon constraining lava flow depths. Here, employing UV-VIS data from the Clementine mission, we estimate mare thickness values in the Imbrium Basin by analyzing ejecta from large (>10 km diameter) impact craters that penetrate (or failed to penetrate) through the mare. Mare thickness values are found to range from at least ∼2 km at the basin center to 1.5–2.0 km in the buried ring shelf annulus. This corresponds to a basalt volume of ∼1.3 × 106 km3, almost a factor of two greater than the volume estimated from partially filled craters alone. Our results indicate that thickness measurements from penetrating craters, combined with minimum estimates from partially filled and non-penetrating craters, provide a more complete picture of the spatial variation of basalt thickness values than could previously be obtained.

[1] Radar bright lava flows in the Radunitsa Labyrinthus region of the Carson quadrangle on Venus traveled up to 200 km within a network of orthogonal grabens. Using a cooling model for basalt to evaluate the graben-channelized lava flow emplacement process, we demonstrate that the lava flows would need to either be very thin (

The surface of Venus, obscured by dense cloud cover, is similar in many ways to the seafloor that lies hidden beneath the deep waters of Earth’s oceans. Although both are difficult to observe, decades of research indicate that each surface is dominated primarily by basaltic volcanism. This is not surprising as Earth and Venus are similar in size, bulk density, and position in the solar system, and the probability of similar elemental abundances and internal heat sources implies corresponding similarity between their interior melting, magma production, and surface volcanism. Even though Earth’s seafloor and Venus are dissimilar in many ways, both environments are characterized by significantly elevated pressure at the surface resulting, respectively, from the burden imposed by the overlying ocean water and the weight of the dense atmosphere. This provides volcanologists with an excellent opportunity to examine how elevated surface pressure affects the development and behavior of volcanic systems.