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Venus tectonics: an overview of Magellan observations
Abstract
The nearly global rader imaging and altimetry measurements of the surface of Venus obtained by the Magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet. Many areas of Venus record a superposition of different episodes of deformation and volcanism. The common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes. The styles and scales of tectonic deformation on Venus may be consequences of three differences from the Earth: 1) The absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well-preserved. 2) A high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller-scale deformational features. 3) A strong coupling of mantle convection to the upper mantle portion of the lithosphere leads to crustal stress fields that are coherent over large distances. -from Authors
... The Venus surface can be divided physiographically into broad highlands, mountain belts, and extensive lowlands, with the latter dominant. Evidence for crustal extension, shortening, and strikeslip motion is widespread across Venus (10,11,21). In places, tectonic structures are pervasively distributed, whereas in other areas strain is concentrated into narrow curvilinear zones. ...
... The extensional counterparts of the ridge belts have variously been labeled "fracture belts" or "groove belts" (23) and comprise arrays of graben and half graben. Some ridge and groove belts reveal transtensional and/or transpressional geometries; that is, these systems have simultaneously accommodated lateral shear in addition to extension or shortening (21,(24)(25)(26). Ridge and groove belts within at least two areas, Lavinia Planitia ...
... Such efforts are challenged, however, by the relatively poor resolution and spatial coverage of image and particularly topographic data currently available for Venus. The acquisition of radar and altimetric data at resolutions surpassing those of the Magellan mission (21), as well as improved measurements of the planet's shape, interior structure, and gravity/topography admittance-as proposed, for example, by the EnVision (48) and Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) (49) spacecraft mission concepts-would substantially enhance our ability to characterize the history of deformation of Venus' lithosphere (cf. ref. 12). ...
Significance
We have identified a pattern of tectonic deformation on Venus that suggests that many of the planet’s lowlands have fragmented into discrete crustal blocks, and that these blocks have moved relative to each other in the geologically recent past. These motions may be the result of mantle convection and, if so, constitute a style of interior–surface coupling not seen elsewhere in the inner Solar System except for continental interiors on Earth. Venus’ fragmented, mobile lithosphere may offer a framework for understanding how tectonics on Earth operated in the Archean.
... This effect is enhanced if the ridges are oriented perpendicular to the radar beam, and becomes progressively weaker with smaller angles between ridge orientation and radar direction. Ridges are commonly interpreted as folds [e.g., Campbell et al., 1983;Crumpler et al., 1986;Head, 1990;Solomon et al., 1991Solomon et al., , 1992 Error bars of standard error are given for the eastern and western areas. Topography has 35 X vertical exaggeration. ...
... Topography has 35 X vertical exaggeration. Solomon, 1992]. Graben floors are generally smooth, but, like the northwest-trending grabens, they preserve variations in radar brightness, indicating that they are not lava filled. ...
... graben sets occur only in areas of steep topographic slope, suggesting that extension is related to the abrupt decrease in topography, with extension directions both parallel and perpendicular to slope. Solomon, 1992] (Figure 5). The grabens average 60 km in length, 7 km in width, and at their closest they are spaced -10 km apart. ...
Four sets of structures were mapped in the western and southern portions of Maxwell Montes. An early north-trending set of penetrative lineaments is cut by dominant, spaced ridges and paired valleys that trend northwest. To the south the ridges and valleys splay and graben form in the valleys. The spaced ridges and graben are cut by northeast-trending graben. The northwest-trending graben formed synchronously with or slightly later than the spaced ridges. Formation of the northeast-trending graben may have overlapped with that of the northwest-trending graben, but occurred in a spatially distinct area (regions of 2 deg slope). Graben formation, with northwest-southeast extension, may be related to gravity-sliding. Individually and collectively these structures are too small to support the immense topography of Maxwell, and are interpreted as parasitic features above a larger mass that supports the mountain belt.
... Rift zones of Venus were for the first time distin guished from the radar data of the Pioneer Venus spacecraft (Masursky et al., 1980). Later, a wider occurrence of rift zones on the surface of the planet was ascertained from the groundbased radar survey (Senske et al., 1991) and the data of the Venera 15, 16, and Magellan missions (Stofan et al., 1989;Solomon et al., 1992;Senske et al., 1992;Crumpler et al., 1993). The photogeologic analysis of the images obtained in the course of the Venera 15, and 16 missions yielded the first description of rift zones (Nikishin, 1990;Nikishin et al., 1992). ...
... The considered unit of rift zones (Rz) occupies approximately 22.6 × 10 6 km 2 that amounts to around 5% of the surface of Venus (Ivanov and Head, 2011). Rift zones are widespread predominately in the equa torial region of the planet (Solomon et al., 1992;Sen ske et al., 1992;Crumpler et al., 1993;Ivanov and Head, 2011). They form a global rift system of Venus extending to approximately 40000-55000 km (Masursky et al., 1980;McGill et al., 1981;Shaber, 1982;Jurdy and Stefanick, 1999). ...
The spatial distribution of rift zones of Venus, their topographic configuration, morphometric parameters, and the type of volcanism associating with rifts were analyzed. This allowed the main characteristic features of rifts to be revealed and two different types of rift-forming structures, serving for classification of rift zones as rift valleys and graben belts, to be isolated. These structural types (facies) of rift zones are differently expressed in the relief: rift valleys are individual deep (several kilometers) W-shaped canyons, while graben belts are clusters of multiple V-shaped and rather shallow (hundreds of meters) depressions. Graben belts are longer and wider, as compared to rift valleys. Rift valleys are spatially associated with dome-shaped volcanic rises and large volcanos (concentrated volcanic sources), while graben belts do not exhibit such associations. Volcanic activity in the graben belts are presented by spacious lava fields with no apparent sources of volcanism. Graben belts and rift valleys were formed during the Atlian Period of geologic history of Venus, and they characterized the tectonic style of the planet at the late stages of its geologic evolution. Formation of this or that structural facies of the rift zones of Venus were probably governed by the thickness of the lithosphere, its rheological properties, and the development degree of the mantle diapirs associating with rift zones.
... The dramatic differences between Earth and Venus have long been deemed enigmatic especially considering their similar size, density, and orbit. Other than life, Earth has at least three critical features that Venus presently lacks: (1) Earth has a massive water reservoir while Venus' atmosphere is dry (Donahue and Hodges, 1993 30 ppm H 2 O); (2) Earth has had active surface tectonics for billions of years, possibly as far back as 4.3 Ga (Mojzsis et al., 2001), while Venus has no evidence of plate tectonics other than a possible resurfacing event around 500 Ma (Solomon et al., 1992); and (3) Earth has maintained an internally generated magnetic field since at least 3.4 Ga (Tarduno et al., 2010) while Venus has no measurable magnetic field (Russell et al., 1980). In addition to being dry, the D/H ratio on Venus (2.5 Â 10 À2 ) is much larger than Earth's (1.6 Â 10 À4 ) indicating that at some point in Venus' history H was preferentially lost from the atmosphere (Donahue, 1999). ...
... Also, observations of Mars' surface indicate that surface tectonics and the core dynamo extinguished around the same time (Nimmo and Stevenson, 2000). Unfortunately, Venus has been completely resurfaced over the last 500-1000 Ma (Solomon et al., 1992), likely erasing any record of early tectonic or magnetic activity. However, it remains a possibility that nascent Venus could have been water rich (Donahue et al., 1982;Kasting, 1988), and maintained active surface tectonics and a core dynamo (Nimmo and McKenzie, 1998;Nimmo, 2002). ...
... At present, Venus's surface is tectonically and volcanically quiescent, showing no clear evidence of Earth-like plate tectonic activity 9 . Yet, determining whether Venus once had active tectonics, as previously suggested 10 , is critical to untangling Venus's tectonic and geologic history. ...
Venus is the least understood of the terrestrial planets. Despite broad similarities to the Earth in mass and size, Venus has no evidence of plate tectonics recorded on its young surface, and Venus’s atmosphere is strikingly different. Numerical experiments of long-term planetary evolution have sought to understand Venus’s thermal–tectonic history with indeterminate results. However, Venus’s atmosphere is linked to interior evolution and can be used as a diagnostic to constrain planetary evolution. Here we compare the present-day Venusian atmosphere to atmospheres generated by long-term thermal–chemical–tectonic evolution models. We find that a continuous single-plate stagnant lid regime operating since antiquity (magma ocean solidification) explains neither the present-day observed atmospheric abundances of N2 and CO2, nor the surface pressure. Instead, the Venusian atmosphere requires volcanic outgassing in an early phase of plate-tectonic-like activity. Our findings indicate that Venus’s atmosphere results from a great climatic–tectonic transition, from an early phase of active lid tectonics that lasted for at least 1 Gyr, followed by the current stagnant lid-like mode of reduced outgassing rates.
... Plate tectonics, our primary example of an active lid tectonic regime, describing when a planet's lithosphere is subducted into the mantle and new crust is formed at mid-ocean ridges 6 , is rare in our Solar System; Earth is the only confirmed example of a planet with active lid tectonics, although evidence suggests that Europa may exhibit an icy version of plate tectonics 7 . On Venus, however, the crust is primarily basaltic, and although arcuate regions associated with coronae such as Artemis and Quetzalpetlatl may be analogous to Earth's subduction zones 8,9 , Venus does not at present seem to be in the plate tectonic regime 10,11 . ...
Despite their critical roles in Venus’s geological evolution, neither heat flow through the Venusian lithosphere nor the corresponding tectonic regime in its geological past is well constrained. However, because impact basin formation is sensitive to thermal conditions at depth, studying large basin development can provide crucial insights into the past geological conditions of a planet. Here we model the formation of Mead Basin, the largest impact structure on Venus, and its two ring faults at approximately 190 km and 270 km diameter, to determine the thermal conditions in Venus’s crust and upper mantle at the time of impact. For present-day surface temperatures, we find that lithospheric thermal gradients no higher than 14 K km⁻¹, corresponding to surface heat fluxes of 28 mW m⁻², are required to reproduce the morphology of Mead Basin. These values are less than half of what is expected for an active lid planet, implying that Venus may have had a stagnant lid when Mead Basin formed, between 0.3 and 1 billion years ago.
... Perhaps due to a water-depleted interior 1,2 and high surface temperatures 3,4 , Venus does not feature Earth-like plate tectonics at present. Instead, its global-scale topographic characteristics and variety of tectonic and volcanic features 5 can be attributed to underlying mantle convective and plume-lithosphere-interaction processes [6][7][8] . Whether Venus is geologically active today, and to what extent surface tectonics reflect the current state of the planet's interior, remains in question. ...
In the absence of global plate tectonics, mantle convection and plume-lithosphere interaction are the main drivers of surface deformation on Venus. Among documented tectonic structures, circular volcano-tectonic features known as coronae may be the clearest surface manifestations of mantle plumes and hold clues to the global Venusian tectonic regime. Yet, the exact processes underlying coronae formation and the reasons for their diverse morphologies remain controversial. Here, we use 3D thermomechanical numerical simulations of impingement of a thermal mantle plume upon the Venusian lithosphere to assess the origin and diversity of large Venusian coronae. The ability of the mantle plume to penetrate into the Venusian lithosphere results in four main outcomes: lithospheric dripping, short-lived subduction, embedded plume and plume underplating. During the first three scenarios, plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy-driven topographic inversion from circular trenches surrounding elevated interiors to raised rims surrounding inner depressions, as observed on many Venusian coronae. Different corona structures may represent not only different styles of plume-lithosphere interactions, but also different stages in evolution. A morphological analysis of large existing coronae leads to the conclusion that at least 37 large coronae (including the largest Artemis corona) are active, providing evidence for widespread ongoing plume activity on Venus.
... Coronae are often found in association with groove belts and rift zones (Ivanov and Head, 2015a), which probably indicates a genetic relation between the coronae and linear extension zones. Such a relation has been suggested by many authors (Solomon et al., 1992;Baer et al., 1994;Stefanick and Jurdy, 1996;Hamilton and Stofan, 1996;Stofan et al., 1997;Roberts and Head, 1993;Condi, 2001;Krassilnikov and Head, 2003;Krassilnikov et al., 2012). ...
... Atla and Beta are rift-dominated rises. Both Regiones are cut by 50 to 100 km wide and up to 2 km deep rift valley, extending for thousand kilometers (i.e., Solomon et al., 1992; Smrekar et al., 1997). The rift-dominated are the topographically highest topographic rises and are characterized by large apparent depth of compensation. ...
The present thesis exposes methods and results obtained exploiting information provided by visible and near-infrared spectra over Mercury and Venus. We use information from different datasets to perform a geologically supervised investigation, in order to identify the presence of compositional heterogeneities and reconstruct the stratigraphy in the shallow crust of the two Terrestrial planets. We name these procedures “Datasets Fusion Techniques” (DFTs). We combine the MASCS dataset with the MDIS dataset from the MESSENGER mission to analyze the local and global crustal properties of Mercury. We select all MASCS observations contained within geologic units previously mapped using high-resolution MDIS NAC images. Similarly, we combine the Venus Express VIRTIS dataset with Magellan SAR imaging dataset for identifying location and extent of the recently active lava flows possibly responsible for the relatively high 1 μm emissivity anomalies observed by the VIRTIS instrument on Idunn Mons on Venus. Using a forward modeling-like procedure, we mapped a number of lava flows on Magellan SAR images, then assigning them a different value of simulated emissivity at each iteration. We found one non-unique solution which well approximates VIRTIS observations. This thesis is a comprehensive study which embraces three main research works performed during the current PhD project. The results show the occurrence of vertical and horizontal heterogeneities in the composition of the shallow crust of Mercury, providing as well important indications for the presence of N-S dichotomy. We also found that the recently active lava flows on Idunn Mons are most likely flank flows located on the eastern flank of the volcanic structure. We could finally reconstruct the stratigraphy beneath the local scale study areas of Mercury and Venus. This PhD thesis demonstrates that DFTs can be used as a powerful expedient for improving the quality of information we can achieve from remote sensing analyses.
... Our geochemical comparison demonstrated that the material analyzed by Venera 10 is close in K, U, and Th contents to the rocks of the IOI and ensimatic OIA tholeiitic series. The formation of island arcs on the Earth is related to subduction processes, whereas such structures are absent on the Venus [115] . In contrast, shield volcanoes characteristic of the intraplate oceanic volcanism (e.g., Mauna Loa, Hawaii) are typical features of the Venusian sur- face [116, 117]. ...
A Database is compiled of analyses of volcanic rocks from intraplate oceanic islands, which were analyzed for for major elements, K, U, and Th, to compare them with Venusian rocks. The rocks from more complicated geodynamic settings, i.e., from places with a combination of hot spot activity and other geodynamic processes, or from oceanic islands underlain by continental crustal blocks, were excluded from the database. Multidimensional discriminant analysis of compositions of these rocks helped to separate petrographically unaltered rocks from the rocks having undergone secondary alteration. This data sample was used together with the sample of unaltered rocks of mid-oceanic ridges and ensimatic island arcs for comparison with Venusian rocks. It was found that the material at the landing sites of Venera 10 and Vega 1 is closer to tholeiites of intraplate oceanic islands, the material analyzed by Venera 8, Venera 9, Venera 13, and Vega 2 is close to the alkaline rocks of intraplate oceanic islands, and the younger material taken by Venera 14 is close to tholeiites of mid-oceanic ridges. Geochemical similarity of most of the Venusian rocks studied to terrestrial volcanics of intraplate oceanic islands indicates an undepleted nature of their mantle sources. These rocks have roughly similar ages and compose regional volcanic plains dominating the Venusian surface. The plains are believed to form due to gravitational instability accumulated in the upper mantle of Venus, which resulted in overturn with rapid subsidence of the upper depleted layer and its replacement with ascending undepleted material and was accompanied by extensive basaltic eruptions. The similarity of younger Venusian material (collected by Venera 14) with terrestrial mid-oceanic-ridge tholeiites may indicate that the Venusian mantle material was depleted at that time in zones of basaltic magma formation.
... Despite these similarities, Venus's tectonics and dynamic evolution are very different from those of the Earth. The venusian lithosphere is stagnant and shows no evidence for present-day global plate tectonics (e.g., Solomon and Head, 1982;Solomon et al., 1992). Recent data provided from the Venus Express Mission show evidence of geologically young, and even ongoing, volcanism on the venusian surface (Smrekar et al., 2010). ...
There are many fundamental and unanswered questions on the structure and evolution of the venusian lithosphere, which are key issues for understanding Venus in the context of the origin and evolution of the terrestrial planets. Here we investigate the lithospheric structure of Venus by calculating its crustal and effective elastic thicknesses (Tc and Te, respectively) from an analysis of gravity and topography, in order to improve our knowledge of the large scale and long-term mechanical behaviour of its lithosphere. We find that the venusian crust is usually 20–25 km thick with thicker crust under the highlands. Our effective elastic thickness values range between 14 km (corresponding to the minimum resolvable Te value) and 94 km, but are dominated by low to moderate values. Te variations deduced from our model could represent regional variations in the cooling history of the lithosphere and/or mantle processes with limited surface manifestation. The crustal plateaus are near-isostatically compensated, consistent with a thin elastic lithosphere, showing a thickened crust beneath them, whereas the lowlands exhibit higher Te values, maybe indicating a cooler lithosphere than that when the venusian highlands were emplaced. The large volcanic rises show a complex signature, with a broad range of Te and internal load fraction (F) values. Finally, our results also reveal a significant contribution of the upper mantle to the strength of the lithosphere in many regions.
... see summary papers in the Venus II volume (Bougher et al 1997)]. It has become evident that even though tectonic deformation is pervasive, there are no signs of an active or relict plate tectonic system (e.g.Solomon et al 1992, Hansen et al 1997, and it is equally clear that the current surface has been subjected to only minimal amounts of weathering and erosion—typical weathering rates are estimated at < 10-3 mm/yr (Campbell et al 1997). While estimates for the age of the Venusian surface continue to vary widely, even in the most extreme cases these rates imply that very little weathering has occurred. ...
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.
... Similar systems are used in the NASA Pathfinder project for global change research on a planetary scale. Radarsat quicklook data should be useful for studying large-scale geological structures, in the same way that Magellan data of Venus was invaluable for studying the geology of Venus (Solomon, 1992). The imagery is useful in these cases only once it has been geocoded and assembled into a large area mosaic so that it may be tied to a particular location on the planet. ...
BIOGRAPHICAL SUMMARY Michael Adair has a M.Sc. in physics from the University of Ottawa (1987) and a B.Sc. (Hon.) in physics from the University of Western Ontario (1984). He has been working at the Canada Centre for Remote Sensing since 1987 and is currently working in the User Systems Development Section of the Data Acquisition Division. This section is responsible for remote sensing systems development directed at the user. His current area of endeavour is in user-interface development for web based mapping. ABSTRACT This paper presents the argument that there is an existing remote sensing data source that is being under-utilized in the form of quicklook browse data. This represents a large, on-line low-cost data source that could be used for scientific and non-scientific applications. A new user interface has been developed to query the browse imagery archive at CCRS that presents the imagery as a full swath containing all data collected in a satellite acquisition segment. Given easy access to this data, it can then be geocoded and combined to create continental or planetary scale mosaics and time-series.
... Nor can the implications of the absence of plate tectonics on Venus (cf. Solomon et al. 1992) presently be evaluated. Some unexpected characteristics of our numerical experiments may provide insight into the conditions required for plate tectonics. ...
The structure and time dependence of 3-D thermal convection in a volumetrically heated, infinite Prandtl number fluid is examined for high values of the Rayleigh number. The methods employed allow the numerical experiments to proceed for long-enough times to derive good estimates of mean and fluctuating parts of the structure. An iterative multirigid method to solve for the buoyant, incompressible viscous flow at each time step of the energy equation is a novel aspect of the methodology. A simple explicit time step of the energy equation is utilized that vectorizes well on serial computers and which is ideally suited to massively parallel computers. Numerical experiments were carried out for Rayleigh numbers from 3 × 106 to 3 × 107 in a cartesian region with a prescribed temperature at the top boundary and an adiabatic bottom boundary. Over this complete range of Rayleigh number, the flow structure consists dominantly of cold, nearly axisymmetric plumes that migrate horizontally sweeping off the cold thermal-boundary layer that forms at the top of the convecting fluid. Plumes disappear by coalescing with other plumes; new plumes are created by thermal-boundary-layer instability. Sheet plumes form only occasionally and do not penetrate to significant depths in the fluid. Plumes have sizes comparable to the thickness of the thermal-boundary layer and an average spacing comparable to the fluid depth. No persistent large-scale motion in the fluid can be identified. Its absence may reflect the large subadiabatic stratification that develops beneath the thermal-boundary layer as cold plumes penetrate to the bottom boundary without thermally equilibrating with surrounding fluid. We consider the possible implications for convection in planetary mantles and for the existence of plate tectonics.
... Plate tectonics require not only sufficient vigorous mantle convection, but also a strong lithosphere with weak, localized, deformation zones (Bercovici et al., 2000;Bercovici, 2003;O'Neill et al., 2007). Venus is likely active today (Smrekar et al., 2010) but its tectonic style is markedly different from Earth's (Solomon et al., 1992). Only rifts, which accommodate limited plate divergence, resemble terrestrial rifts (Schaber, 1982;Campbell et al., 1984;Stofan et al., 1989;Senske et al., 1991;Foster and Nimmo, 1996). ...
... (Also, Kaula and Phillips (1981) derived L ¼ 94 km directly from thermal-boundary layer theory using terrestrial heat flux.) Yet there is no evidence for contemporary lithospheric recycling in the geological record of Venus (Solomon et al., 1992). If indeed L ¼ 200-300 km and Venus has a chondritic or terrestrial complement of radionuclides, then ''Venus cannot presently be in an approximate thermal steady state'' . ...
Understand Venus geodynamics from 180,000 ft.
The Great Dyke of Atla Regio (GDAR) is traced for ~3700 km on Venus, as a surface graben (narrow trough) interpreted to overlie a continuous laterally-emplaced underlying mafic dyke (vertical magma-filled crack). The GDAR belongs to a giant radiating dyke swarm associated with Ozza Mons (volcano), Atla Regio plume, and was fed from a magma reservoir ~600 km south of the Ozza Mons centre. A 50-degree counter-clockwise swing of the GDAR at 1200 km from the centre is consistent with a 1200 km radius for the underlying Ozza Mons plume head, and a stress link to the 10,000 km long Parga Chasmata rift system. Our discovery of the GDAR, should spur the search for additional long continuous single dykes on Venus (and Earth), with implications for estimating plume head size, locating buffered magma reservoirs, mapping regional stress variation at a geological instant, and revealing relative ages (through cross-cutting relationships) over regional-scale distances.
Atla Regio, Venus, is interpreted as a young major mantle plume centre, and we address whether it is at plume head or plume tail stage. Our approach uses graben-fissure lineaments, interpreted as the surface expression of dykes. Mapping > 40,000 such lineaments reveals giant radiating dyke swarms associated with major volcanic centres of Maat (>1500 km dyke swarm radius), Ozza (>2000 km), Ongwuti (>1100 km) and Unnamed montes (>1100 km), indicating that each is due to plume head magmatism rather than plume tail magmatism (maximum swarm length ~ 100 km). The size of an underlying flattened plume head is estimated by the radius where the swarm transitions from a radiating to linear pattern. All four centres and their plume heads group within the 1200 km radius of the Ozza Mons plume head, consistent with a single event. Atla Regio is at the plume head stage with coeval triple-junction rifting, which on Earth would typically precede attempted continental breakup.
It has recently been suggested that deformed crustal plateaus on Venus may be composed of felsic (silica-rich) rocks, possibly supporting the idea of an ancient ocean there. However, these plateaus have a tendency to collapse owing to flow of the viscous lower crust. Felsic minerals, especially water-bearing ones, are much weaker and thus lead to more rapid collapse, than more mafic minerals. We model plateau topographic evolution using a non-Newtonian viscous relaxation code. Despite uncertainties in the likely crustal thickness and surface heat flux, we find that quartz-dominated rheologies relax too rapidly to be plausible plateau-forming material. For plateaus dominated by a dry anorthite rheology, survival is possible only if the background crustal thickness is less than 29 km, unless the heat flux on Venus is less than the radiogenic lower bound of 34 [Formula: see text]. Future spacecraft determinations of plateau crustal thickness and mineralogy will place firmer constraints on Venus's heat flux.
This book describes the tectonic landforms resulting from major internal and external forces acting on the outer layers of solid bodies throughout the Solar System. It presents a detailed survey of tectonic structures at a range of length scales found on Mercury, Venus, the Moon, Mars, the outer planet satellites, and asteroids. A diverse range of models for the sources of tectonic stresses acting on silicate and icy crusts is outlined, comparing processes acting throughout the Solar System. Rheological and mechanical properties of planetary crusts and lithospheres are discussed to understand how and why tectonic stresses manifest themselves differently on various bodies. Results from fault population data are assessed in detail. The book provides methods for mapping and analysing planetary tectonic features, and is illustrated with diagrams and spectacular images returned by manned and robotic spacecraft. It forms an essential reference for researchers and students in planetary geology and tectonics.
In the absence of global plate tectonics, mantle convection and plume-lithosphere interaction are the main drivers of surface deformation on Venus. Among documented tectonic structures, circular volcano-tectonic features known as coronae may be the clearest surface manifestations of mantle plumes and hold clues to the global Venusian tectonic regime. Yet, the exact processes underlying coronae formation and the reasons for their diverse morphologies remain controversial. Here, we use 3D thermomechanical numerical simulations of impingement of a thermal mantle plume upon the Venusian lithosphere to assess the origin and diversity of large Venusian coronae. The ability of the mantle plume to penetrate into the Venusian lithosphere results in four main outcomes: lithospheric dripping, short-lived subduction, embedded plume and plume underplating. During the first three scenarios, plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy-driven topographic inversion from circular trenches surrounding elevated interiors to raised rims surrounding inner depressions, as observed on many Venusian coronae. Different corona structures may represent not only different styles of plume-lithosphere interactions, but also different stages in evolution. A morphological analysis of large existing coronae leads to the conclusion that least 37 large coronae (including the largest Artemis corona) are active, providing evidence for widespread ongoing plume activity on Venus.
Venus provides a rich arena in which to stretch one's tectonic imagination with respect to non-plate tectonic processes of heat transfer on an Earth-like planet. Venus is similar to Earth in density, size, inferred composition and heat budget. However, Venus' lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non-plate tectonomagmatic processes. In this paper, I explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time, divided respectively into temporal eras. Impactors played a prominent role in the ancient era, characterized by thin global lithosphere. The Artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume. The fracture zone complex era, marked by broad zones of tectonomagmatic activity, witnessed coupled spreading and underthrusting, since arrested. These three tectonic regimes provide possible analogue models for terrestrial Archaean craton formation, continent formation without plate tectonics, and mechanisms underlying the emergence of plate tectonics. A bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of Archaean crust, and recycling of significant Archaean crust to the mantle.
This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.
Plate tectonics is one mode of mantle convection that occurs when the surface layer (the lithosphere) is relatively weak. When plate tectonics operates on a terrestrial planet, substantial exchange of materials occurs between planetary interior and its surface. This is likely a key in maintaining the habitable environment on a planet. Therefore it is essential to understand under which conditions plate tectonics operates on a terrestrial planet. One of the puzzling observations in this context is the fact that plate tectonics occurs on Earth but not on Venus despite their similar size and composition. Factors such as the difference in water content or in grain-size have been invoked, but these models cannot easily explain the contrasting tectonic styles between Earth and Venus. We propose that strong dynamic weakening in friction is a key factor. Fast unstable fault motion is found in cool Earth, while slow and stable fault motion characterizes hot Venus, leading to substantial dynamic weakening on Earth but not on Venus. Consequently, the tectonic plates are weak on Earth allowing for their subduction, while the strong plates on Venus promote the stagnant lid regime of mantle convection.
A new analysis of the spatial relationships between volcanic features and rifts on Venus provides new constraints on models of planetary evolution. We developed a new database of volcanic features for the Beta-Atla-Themis (BAT) region, and used nearest neighbour measurements to determine relationships between different types of volcanic features and the rifts. Nearest neighbour analysis shows that all the dome-type and corona-type sub-populations tend to cluster. Rift associations were inferred from the deviation of a feature's population distribution (as a function of distance from rift) from that of a random population. Dome-type features in general have no discernible relationship with rifts. Most corona type features have a strong association with rifts, with intermediate and large volcanoes also tending to occur close to or on rifts. Shield fields, on the other hand, tend to occur away from rifts. Our new evidence supports classifications of rifts on Venus into different types, possibly by age, with a shift from globally dispersed (more uniform) volcanism towards the more rift-focused distribution, which suggests a shift in tectonic regime. Our observations are consistent with recent models proposing the evolution of Venus from a stagnant lid regime to a subcrustal spreading regime. We also present evidence for a failed rift on Venus and note that this process may be analogous, albeit on a larger scale, to a proposed model for the evolution of the East African Rift system.
Statistical methods of multi-dimensional analysis (discriminant functions and factor analysis) were applied to compare the chemical analyses obtained by Venera-13, -14, and Vega-2 landers (contents of major oxides except for sodium) with petrochemical data compiled into the data base on terrestrial ocean. It is shown that the distribution of major petrogenic elements in the terrestrial rocks ascribed to different geodynamic settings (spreading zones, hot spots, and subduction zones) is determined by crystallization differentiation. This process is best manifested in hot spot volcanics (volcanic islands). In spite of the difficulties related to the poor precision of chemical determinations of Venusian rocks, obtained data indicate that the rocks from the Venera-13 and Vega-2 landing sites have no petrochemical analogues among terrestrial oceanic volcanic rocks. Rocks analyzed in the Venera-14 landing site may resemble the mid-ocean ridge volcanic rocks, although geological setting in the Venera-14 landing site ellipse strongly differs from terrestrial spreading zones.
Venus is known to have been volcanically resurfaced in the last third of solar system history and to have undergone a significant decrease in volcanic activity a few hundred million years ago. However, fundamental questions remain: Is Venus still volcanically active today, and if so, where and in what geological and geodynamic environment? Here we show evidence from the Venus Express Venus Monitoring Camera for transient bright spots that are consistent with the extrusion of lava flows that locally cause significantly elevated surface temperatures. The very strong spatial correlation of the transient bright spots with the extremely young Ganiki Chasma, their similarity to locations of rift-associated volcanism on Earth, provide strong evidence for their volcanic origin and suggests that Venus is currently geodynamically active.
The paper addresses the morphology and age relations of plain terrains on Venus. The stratigraphically oldest and strongly tectonized complexes of this planet are remnants of plains made up of tessera-forming tectonic structures. This suggests that tessera terrains were formed mostly by eruptions of low-viscosity (likely basaltic) lavas. The stratigraphically oldest units, in which volcanic activity played a leading role, are shield plains, whose morphology testifies that they were produced by numerous but relatively weak eruptions of basaltic material from likely shallow-sitting chambers. The only Venusian landforms whose shapes suggest eruptions of high-viscosity lavas are festoons and steep-sided domes. The great majority of the domes is spatially and stratigraphically related to the shield plains, and this testifies that the type of volcanism during the origin of the plains was unusual and favorable for the local derivation of highly viscous lavas. The mechanisms that could possibly form the steep-sided domes suggest that the domes were most likely produced by eruptions of silica-enriched (and hence, more viscous) melts that had been generated by crystallization differentiation in chambers. The surface topography of the younger regional plains suggests that these plains were produced by large-volume but brief eruptions of basaltic (or perhaps, also komatiitic) lavas (lava flooding). The surface areas and possible volumes of the regional plains are more consistent with their generation via melting portions of mantle material that had replaced delaminated blocks of lithospheric roots. The morphology, character of spatial distribution, and typical associations of the lobate plains suggest that they were formed during melting of the head portions of mantle diapirs. The morphologic parameters of the dominant volcanic complexes on Venus (shield, regional, and lobate plains) and their unvarying stratigraphic relations highlight a clearly pronounced character of the evolution of volcanic activity in the geological history of Venus.
A comparison of the global geological and topographic maps of Venus allows analysis of the topo-graphic configuration of the major stratigraphic units forming the planet's surface. The main positive relief features on Venus are manifested as prominences of tectonized units, which usually pertain to the initial episodes in the observable portion of its geological history (the beginning of the Guineverian period). The main morphostructural unit in this category is tessera, the main massifs of which constitute a special class of plateau-like highlands up to several kilometers high and tens to hundreds or thousands of kilometers wide. The younger shield and regional plains are confined to the regional slopes of the highlands formed by ancient tectonized units and are concentrated in regional lowlands. This indicates that regional lowlands (hundreds to thousands of kilometers across) as well as the highlands of ancient tectonized units had been formed in the early stages of the geological history of Venus. The estimates for the absolute model age of regional plains, which fill the lowlands, indicate that the formation of this material unit marks the first third of the observable geological history. Thus, the long-wavelength topographic pattern of Venus had formed, in general, in the early stages of the geological history, during the first third of it. In the final stages, which cover the next two thirds of the observable geological history of Venus (the Atlian period), the formation of long-wavelength topographic features is confined to dome-shaped highs, which make a minor contribution (about 15%) to the overall long-wavelength topographic pattern of Venus. This is evidence of a sharp fall in the level of endogenous activity after the emplacement of regional plains, with the style of the activity having changed from intensive crustal dislocations to extensive yet isolated uplifts of the thick thermal lithosphere.
The surface of Venus displays several tectonized terrains in which the morphologic characteristics of the original materials are almost completely erased by superposed tectonic structures whose large dimensions (»100 km) suggest formation related to mantle convection. The characteristics of these tectonized terrains are in contrast to volcanic units in which tectonic structures are less significant or absent and thus do not obscure the volcanic character of the units. We describe the temporal distribution of tectonized terrains, their stratigraphic relationships with volcanic units, and how these outline the major episodes in the geological evolution of Venus. Five major tectonized units make up ~20% of the planet: 1) tessera (t, 7.3%), 2) densely lineated plains (pdl, 1.6%), 3) ridged plains/Ridge belts (pr/rb, 2.4%), 4) groove belts (gb, 8.1%), and 5) rift zones (rz, 5.0%). Clear relationships of relative age are often seen among the tectonic and volcanic units at the global scale and define three contrasting regimes of volcanic and tectonic resurfacing. The majority of tectonized terrains (t through gb) are the products of tectonic resurfacing and are embayed by the vast volcanic plains and, thus, are older. There are no units with either mildly- or non-tectonized surfaces that interleave the tectonic terrains, which would be expected if the tectonic resurfacing operated only during specific repetitive phases in discrete regions. These tectonized terrains (t through gb) thus define a tectonically dominated regime of resurfacing that occurred at a global-scale near the beginning of the observable geological history of Venus. This ancient tectonic regime began with formation of tessera and was followed by formation of pdl and pr/rb. Groove belts formed near the end of this regime. Branches of groove belts compose the tectonic components of many coronae, suggesting that these features are genetically related (e.g., mutual development of mantle diapirs and zones of extension) and that coronae may have punctuated the final stages of the ancient tectonic regime. This regime was followed by emplacement of the vast volcanic plains, such as shield and regional plains, the surfaces of which are extensively deformed by the global network of wrinkle ridges. Emplacement of the plains defines the second, volcanically dominated regime, representing a time when surface tectonic deformation related to the mantle convection waned. Rift zones are the stratigraphically youngest manifestations of regional-scale tectonic deformation on Venus. Rifts are spatially and temporarily associated with the youngest lava flows and often cut the crest areas of large, but isolated, dome-shaped rises. Structures of rift zones always cut the surface of the vast plains, which means that rifts are separated in time from the ancient tectonic regime, post-date the regional plains, and represent a new phase of tectonism that was contemporaneous with the late volcanism of lobate plains. Rift zones and lobate plains define the third, network rifting-volcanism regime, of resurfacing that was related to late stages of evolution of the dome-shaped rises.
The gravity and topography of Venus obtained from observations of the Magellan mission, as well as the gravity and topography from our numerical mantle convection model, are discussed in this paper. We used the hypothesis that the geoid of degrees 2-40 is produced by sublithospheric mantle density anomalies that are associated with dynamical process within the mantle. We obtained the model dynamical admittance (the geoid topography ratio based on a convection model) by a numerical simulation of the Venusian mantle convection, and used it to correct the dynamical effect in the calculation of crustal thickness. After deducting the dynamical effect, the thickness of the Venusian crust is presented. The results show that the gravity and topography are strongly correlated with the Venusian mantle convection and the Venusian crust has a significant influence on the topography. The Venusian crustal thickness varies from 28 to 70 km. Ishtar Terra, and Ovda Regio and Thetis Regio in western Aphrodite Terra have the highest crustal thickness (larger than 50 km). The high topography of these areas is thought to be supported by crustal compensation and our results are consistent with the hypothesis that these areas are remnants of ancient continents. The crustal thickness in the Beta, Themis, Dione, Eistla, Bell, and Lada regiones is thin and shows less correlation with the topography, especially in the Atla and Imdr regiones in the eastern part of Aphrodite Terra. This is consistent with the hypothesis that these highlands are mainly supported by mantle plumes. Compared with the crustal thickness calculated with the dynamical effect, our results are more consistent with the crust evolution and internal dynamical process of Venus.
Volcanism and tectonism are the dominant endogenic means by which planetary surfaces change. This book, in general, and this overview, in particular, aim to encompass the broad range in character of volcanism, tectonism, faulting and associated interactions observed on planetary bodies across the inner solar system – a region that includes Mercury, Venus, Earth, the Moon, Mars and asteroids. The diversity and breadth of landforms produced by volcanic and tectonic processes are enormous, and vary across the inventory of inner solar system bodies. As a result, the selection of prevailing landforms and their underlying formational processes that are described and highlighted in this review are but a primer to the expansive field of planetary volcanism and tectonism. In addition to this extended introductory contribution, this Special Publication features 21 dedicated research articles about volcanic and tectonic processes manifest across the inner solar system. Those articles are summarized at the end of this review.
There are many fundamental and unanswered questions on the structure and evolution of the Venusian lithosphere, which are key issues for understanding Venus in the context of the terrestrial planets. Here we investigate the lithospheric structure of Venus by calculating its crustal and effective elastic thicknesses (Tc and Te, respectively) from an analysis of gravity and topography, in order to improve our knowledge of the large scale and long-term mechanical behaviour of its lithosphere.
A series of numerical models of magmatism and mantle convection with a stagnant lithosphere are developed to understand the mantle evolution in Venus. Magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting, and the solid-state convection of mantle materials with temperature-dependent Newtonian rheology is affected by the garnet-perovskite transition and the post-spinel transition. In our preferred models, the mantle evolves in two stages: The earlier stage is characterized by a layered mantle convection punctuated by repeated bursts of hot materials from the deep mantle to the surface. Mantle bursts induce vigorous magmatism, and also cause the basaltic crust, enriched in heat-producing elements (HPEs), to recycle into the mantle. A part of the recycled basaltic crusts accumulates along the post-spinel boundary to form a barrier, and this basalt barrier causes mantle convection to become layered. At a later stage, when the HPEs have already decayed, in contrast, the basalt barrier disappears and whole-mantle convection occurs more steadily. Mild magmatism is induced by small-scale partial melting at the base of the crust and hot plumes from the deep mantle. The internal heating by the HPEs that recycled into the mantle in the earlier stage allows the magmatism of the later stage to continue throughout the calculated history of mantle evolution. The two stages arise when the barrier effect of the post-spinel transition is weak and the lithosphere is mechanically strong enough. The two-stage evolution model meshes with the observed history of magmatism and the lithosphere on Venus.
This chapter contains short descriptions of material units and
structures observed on the surface of Venus as well as an abbreviated
history of discoveries, that led to the current knowledge of this
planet's geology. It is shown that observed units and structures are
broadly similar and commonly exhibit similar age sequences in different
regions of the planet, although there is debate as to what degree these
sequences can be integrated into any single global stratigraphic model.
There is a broad consensus concerning the recent general geodynamic
style of Venus (no plate tectonics), on the dominating role of basaltic
volcanism in the observed crust-forming processes, and on the
significant roles of both compressional and extensional tectonic
deformation. Also not under debate is that we see the morphologic record
of only the last ˜1 b.y. (or less) of the history of this planet
and that during this time period the role of exogenic resurfacing was
very minor. Several important unresolved questions of Venus geology are
formulated and suggestions for future missions, that could lead to
resolving them are given.
Laufey Regio is a 0.5 km high, elongate topographic rise, measuring 1000
by 2000 km, located within northern Navka Planitia, Venus. It is
dominated by two large volcanoes, Var and Atanua Montes, and numerous
coronae with associated flow deposits. The area has been geologically
mapped, showing that large-scale edifice formation focused at Var and
Atanua Montes and overlapped in time with the formation of smaller
coronae. Several coronae and volcanoes have had complex and protracted
histories with multiple episodes of volcanism and tectonism, supporting
a nondirectional history for this region. The broad topography of the
rise is surrounded by a concentric set of wrinkle ridges interpreted to
have formed after the onset of centralized volcanism. Admittance studies
indicate both bottom and top loading, but lithospheric parameters are
not well constrained. Top-loading models give a range of 0-48 km for
elastic thickness (Te) and 35-115 km for crustal thickness.
We interpret the Te range as most likely indicating isostatic
compensation. Bottom-loading models yield lower bounds of 30-75 km for
Te and 32-74 km for the apparent depth of compensation (ADC).
The combination of the large Te and low topography indicates
that the plume is no longer active. Bottom loading and a small ADC may
indicate that a layer of low-density residuum, produced by pressure
release melting in the now extinct plume, is partially supporting the
topography. On the basis of its broad, regional topography, abundant
volcanism, and gravity signature, we interpret Laufey to be a
volcano-dominated rise in a late stage of evolution.
Magellan started mapping the planet Venus on September 15, 1990, and after one cycle (one Venus day or 243 Earth days) had mapped 84% of the planet's surface. This returned an image data volume greater than all past planetary missions combined. Spacecraft problems were experienced in flight. Changes in operational procedures and reprogramming of onboard computers minimized the amount of mapping data lost. Magellan data processing is the largest planetary image-processing challenge to date. Compilation of global maps of tectonic and volcanic features, as well as impact craters and related phenomena and surface processes related to wind, weathering, and mass wasting, has begun. The Magellan project is now in an extended mission phase, with plans for additional cycles out to 1995. The Magellan project will fill in mapping gaps, obtain a global gravity data set between mid-September 1992 and May 1993, acquire images at different view angles, and look for changes on the surface from one cycle to another caused by surface activity such as volcanism, faulting, or wind activity.
Wrinkle ridges constitute one of the most abundant tectonic features on terrestrial planetary surfaces. On Venus, evidence suggests a connection between wrinkle ridges and the climatic evolution of the planet. However, like other planets and moons that experience more active surface geological processes, such as Earth, Mars, Europa, Io and Titan, visible impact craters on the Venusian surface are less common because they are eroded, buried or transformed by tectonics or other geological processes over time. It is of great importance to identify and understand some characteristics of those surface morphologies, such as orientation, length, spacing, original dimension and topography. Nevertheless, these parameters can only be computed on remotely sensed images after their segmentation. Until now, the manual identification of these features has been focused on those of major geological significance, leaving many more to be identified, mapped and studied. The main aim of this paper is to provide a method for automatic detection of wrinkle ridges from Magellan Synthetic Aperture Radar (SAR) imagery at different scales. The proposed algorithm, based on a combination of fractal dimension and morphological operators, identifies regions of interest to this study, namely those of anisotropic behaviour, but also impact craters and their ejecta blankets. The high performances achieved in a variety of situations demonstrate that its robustness can be
applied to an automated procedure.
A preliminary assessment of mass movements and their geomorphic
characteristics as determined from visual inspection of Magellan cycle 1
synthetic aperture radar images is described. The primary data set was a
catalog of over 200 ten-inch square photographic prints of
full-resolution mosaic image data records. Venus exhibits unambiguous
evidence of mass movements at a variety of scales. Mass movements appear
mostly in the form of block and rock movements; there is little evidence
of regolith and sediment movements. Unique Venusian conditions may play
a role in the creation of some mass movement features. Dark (smooth)
surfaces surrounding many rockslide avalanches are probably fine
materials emplaced as part of the mass movement process, as airfall,
surface-hugging density flows, or coarse-depleted debris flows. The size
and efficiency of emplacement of landslide deposits on Venus are
comparable to those seen on Mars, which in turn generally resemble
terrestrial occurrences.
The topography of a terrestrial planet can be supported by several
mechanisms: (1) crustal thickness variations, (2) density variations in
the crust and mantle, (3) dynamic support, and (4) lithospheric
stresses. Each of these mechanisms could play a role in compensating
topography on Venus, and we distinguish between these mechanisms in part
by calculating geoid-to-topography ratios and apparent depths of
compensation. By simultaneously inverting for mass anomalies at two
depths, we solve for the spatial distribution of crustal thickness and a
similar map of mass anomalies in the mantle, thus separating the effects
of shallow and deep compensation mechanisms on the geoid. The roughly
circular regions of mantle mass deficit coincide with the locations of
what are commonly interpreted to be buoyant mantle plumes. Additionally,
there is a significant geographic correlation between patches of
thickened crust and mass deficits in the mantle, especially for
spherical harmonic degree l < 40. These mass deficits may be
interpreted either as lateral thermal variations or as Mg-rich melt
residuum. The magnitudes of mass deficits under the crustal highlands
are roughly consistent with a paradigm in which highland crust is
produced by melting of upwelling plumes. The mean thickness of the crust
is constrained to a range of 8-25 km, somewhat lower than previous
estimates. The best two-layered inversion of gravity incorporates a
dynamic mantle load at a depth of 250 km.
Venus is similar to Earth in size and bulk composition. The dramatic
differences between the two planets indicate that planetary size alone
does not control geologic evolution. Earth's geology is dominated by
plate tectonics, or active lid convection. The crater retention age of
Venus demonstrates that the planet has been very geologically active in
the last b.y., but there is no evidence of active plate tectonics.
Instead the surface is dominated by diverse volcanism and tectonism, and
highlands interpreted as mantle upwellings and downwellings. Venus'
surface is much hotter and drier than Earth's, suggesting that the
interior may be drier as well. The volatile content has a major effect
on both the rheology of the interior and predicted melt products. The
low water content of the interior may create a stronger lithosphere and
preclude asthenosphere formation, leading to the development of a
stagnant rather than an active lid convective regime. A stable, stagnant
lid fosters development of phase changes and other density anomalies
that can lead to delamination and associated opportunities for melting.
Additionally, the thicker boundary layer formed in a stagnant lid regime
may increase the number of expected plumes. Mars also is in a stagnant
lid regime, probably due to early heat loss rather than volatile loss.
Here we discuss what is known about interior volatile content and
explore the implications of a dry interior for volcanism, tectonism, and
interior convection as a working hypothesis to explain the profound
differences in the evolution of Venus, Earth, and Mars.
The topographic midlands on Venus comprise about 80% of the surface and an understanding of their mode of formation is essential to unraveling the geologic and geodynamic history of the planet. We explore this question by undertaking a comprehensive geological mapping of the Fredegonde Quadrangle (V-57, 50–75°S, 60–120°E, 1:5M scale) that represents the transition zone from the midlands to the lowlands at the edge of Lada Terra. We report on the geologic units and structures and the sequence of events and, thus, the major stages in the evolution of this region of the midlands. At earlier stages of evolution of the long-wavelength topography, broad (hundreds of kilometers wide) and relatively low (1–1.5 km high) topographic ridges formed due to sequential development of deformation zones, first of contractional ridge belts (NW orientation) and then crosscut by extensional groove belts (NE orientation). Arcuate swarms of graben within groove belts often form the rims of coronae and represent their tectonic component. This suggests that groove belts and coronae within the quadrangle formed simultaneously. Intersections of these deformation zones caused separation of the topography of the region into a series of broad, shallow equidimensional basins many hundreds of kilometers across and currently hundreds of meters up to a kilometer deep. Thus, the principal topographic features within the quadrangle were established near the beginning of its observable geological record. The basins then remained sites of accumulation of successive volcanic plains units such as shield plains (psh) and the lower unit of regional plains (rp1). The flows of the younger plains, such as upper unit of regional plains (rp2) and lobate plains (pl), are less voluminous, and flow down the current topographic gradients. This implies that the major topographic pattern of the Fredegonde quadrangle has been stable since its establishment. Further evidence for this is that the vast volcanic plains units (psh and rp1) that postdate the heavily tectonized units of the deformation zones are only mildly deformed. This suggests that since the emplacement of shield plains, volcanism has been the primary geologic process and that the time of formation of unit psh corresponds to a major change from the earlier regime dominated by tectonics to the later volcanically dominated regime. Consistent age relationships among the main volcanic units within the quadrangle from older shield plains, through regional plains, to lobate plains, documents an evolution in volcanic style. Shield plains were formed from small eruptions from ubiquitous small shield volcanoes and are interpreted to be derived from broadly distributed and shallow magmatic sources. The lower unit of regional plains is widely distributed but vents and flow fronts are rare; this unit is interpreted to represent massive and probably short-lived flood basalts-like eruptions that filled in the lowlands basins. The upper unit of regional plains (rp2) and lobate plains (pl) are associated with localized and distinctive sources, such as late-stage volcanic activity at coronae. Thus, the tectonic stage of evolution of coronae (formation of the rims) and the volcanic stage when coronae served as magmatic centers and sourced lava flows, were separated in time by the emplacement of the shield and lower regional plains. How and when did the major components of Venus midland topography form? Clearly, in the Fredegonde quadrangle, regional deformation produced the deformation belts and groove belts/coronae in the earliest phases, and this topography formed the basis for the next, volcanic stage of emplacement (filling of the basins), with coronae-associated volcanism following this phase. The broad topography resulting from this early phase has persisted until the present. We compare this tectonic-volcanic sequence and history of topography in the Fredegonde quadrangle with other areas on Venus and find that the sequence has widespread application globally, and that the history of topography may be similar planet-wide.
We report on attempts to find the ongoing volcanic activity from near-infrared night-time observations with the Venus Monitoring Camera (VMC) onboard of Venus Express. Here we consider VMC images of the areas of Maat Mons volcano and its vicinities, which, as it follows from analysis of the Magellan data, show evidence of geologically very recent volcanism. Analysis of VMC images taken in 12 observation sessions during the time period from 31 October 2007 to 15 June 2009 did not reveal any suspicious high-emission spots which could be signatures of the presently ongoing volcanic eruptions. We compare this time sequence of observations with the history of eruptions of volcano Mauna Loa, Hawaii, in the 20th century. This comparison shows that if Maat Mons volcano had the eruption history similar to that of Mauna Loa, the probability to observe an eruption in this VMC observation sequence would be about 8%, meaning that the absence of detection does not mean that Maat is not active in the present epoch. These estimates do not consider the effect of absorption and blurring of the thermal radiation coming from Venus surface by the planet atmosphere and clouds, which decreases detectability of thermal signature of fresh lavas. To assess the role of this effect we simulated near-infrared images of the study area with artificially added circular and rectangular (with different aspect ratios) lava flows having surface temperature 1000 K and various areas. These simulations showed that 1 km2 lava flows should be marginally seen by VMC. An increase of the lava surface area to 2–3 km2 makes them visible on the plains and increase of the area to 4–5 km2 makes them visible even in deep rift zones. Typical individual lava flows on Mauna Loa are a few km2, however, they often have been formed during weeks to months and the instantaneous size of the hot flow surface was usually much smaller. Thus the detection probability is significantly lower than 8%, but it is far from negligible. Our consideration suggests that further search of Maat Mons area and other areas including young rift zones makes sense and should be continued. More effective search could be done if observations simultaneously cover most part of the night side of Venus for relatively long (years) time of continuous observations.
Regional tectonism and volcanism affect crater modification and crater loss on Venus, but a comparison of Venusian craters to lunar floor-fractured craters suggest that a third style of more localized, crater-controlled magmatism also may occur on Venus. Based on lunar models for such magmatism, Venusian crustal conditions should generally favor crater-filling volcanism over crater-centered floor fracturing. Nevertheless, three craters on Venus strongly resemble extensively modified craters on the Moon where deformation can be attributed to failure over large crater-centered intrusions. Models for crater modification over igneous intrusions indicate typical magmatic pressure beneath these three craters of approximately 200-300 bars and intrusion depths of the order of 1-6 km. All three craters also share common settings and low elevations, whereas craters embayed by regional volcanism preferentially occur at much higher elevations on Venus. We suggest that the style of igneous crater modification on Venus thus may be elevation dependent, with crater-centered intrusions primarily occurring at low elevations on Venus. This interpretation is consistent with theoretically predicted variations in magmatic neutral buoyancy depth as a function of atmospheric pressure suggested by other authors.
It is suggested that episodic plate tectonics occurs on Venus; episodes of rapid plate tectonics are separated by periods of surface quiescence. For the last 500+/-200 m.y. it is postulated that the surface of Venus has been a single rigid plate that has been thickening due to conductive cooling. A near-uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows. A lithosphere that has conductively thickened for some 500 m.y. has a thickness of about 300 km, nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss. Such a thick lithosphere can support the high topography and associated gravity anomalies on Venus as well as the unrelaxed craters; studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere. Incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics. On the Earth, 75-90% of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges. This process is not operative on Venus. This paper suggests that episodic plate tectonics on Venus constitutes the primary mechanism for mantle heat transport on that planet.
Venus is without water in its outer parts; not only in the atmosphere, but in the upper mantle, to account for the high ratio of correlated gravity to topography. The upper mantle stiffness required by this high ratio leads to a regionalization of magmatism, and thus a lower rate of crustal formation. There is still sufficient heat in places, however, for secondary differentiations of the crust. From several indicators, Venus appears to have retained in its outer parts appreciable carbon dioxide and sulfur. But all hypotheses proposed to date have their difficulties; more needs to be understood about the physics and chemistry of several magma types.
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