Venus Tectonics Research Articles

Implications of Anomalous Crustal Provinces for Venus' Resurfacing History

AbstractCurrent Venus tectonics suggests a stagnant lid mode of mantle convection. However, the planet is debated to enter an episodic regime after long quiescent periods, driven by resurfacing due to rapid subduction and global crustal recycling. Tessera regions that cover approximately 10% of Venus' surface appear to be strongly deformed, which suggests that they have survived at least the latest resurfacing event, although the composition and age of the tesserae are unknown. Based on mantle convection modeling, we studied the effects of anomalous crustal provinces (ACPs) on mantle dynamics and postoverturn lithospheric survival. As a hypothesis, we assume ACPs to be thick, compositionally anomalous, and rheologically strong units, similar to terrestrial cratons. We model Venus with a varying number of preimposed ACP units and differing lithospheric yield stress in 2‐D and 3‐D spherical geometry. The impact of ACPs on mantle dynamics and the survival of lithosphere is investigated by examining the thermal evolution, crustal thickness, and surface age distribution. We find that the number and timing of overturns are highly dependent on the yield stress and, to some degree, on the number and size of the preimposed ACPs. ACPs in particular affect the wavelength of convection and may foster the survival of lithosphere even of those portions not being part of an ACP. However, ACPs do not seem to be a good analog for tessera regions due to their exaggerated age and (likely) thickness, but—with appropriate density contrast—may be more useful representatives of Venus' highland plateaus.

Topographic comparisons of uplift features on Venus and Earth: Implications for Venus tectonics

Venus and Earth display different hypsography. We use topographic profiles to search for well-understood terrestrial analogs to venusian features. Specifically, by using cross-correlation, we correlate average profiles for terrestrial rifts (slow and fast, “ultra-slow,” incipient and inactive) and also hotspots (oceanic and continental) with those for venusian chasmata and regiones, to draw inferences as to the processes responsible for shaping Venus’ surface. Correlations tend to improve with faster spreading rates; Venus’ correlations rank considerably lower than terrestrial ones, suggesting that if chasmata are analogous to terrestrial spreading centers, then spreading on Venus barely attains ultra-slow rates. Individual features’ normalized average profiles are correlated with profiles of other such features to establish the degree of similarity, which in turn allows for the construction of a covariance matrix. Principal component analysis of this covariance matrix shows that Yellowstone more strongly resembles Atla, Beta and W. Eistla regiones than it does the terrestrial oceanic hotspots, and that venusian chasmata, especially Ganis, most closely resemble the ultra-slow spreading Arctic ridge.

Structural Analysis of a Sampling of Tesserae: Implications for Venus Geodynamics

Understanding the nature and timing of tessera formation is fundamental to Venus tectonic and geodynamic models. Tesserae are commonly considered to exhibit complex deformation histories, to represent the oldest global stratigraphic unit, to have formed during a global phase of tesserization, and to require weak lithosphere for their formation. Although these characterizations of tesserae are gaining widespread mention in the literature, they are essentially unsupported by an assemblage of data, yet they figure prominently in the way we frame and address questions about Venus tectonics. Thus they are quickly becoming accepted paradigms despite a lack of scientific foundation. Open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of Venus. We examined tessera terrain in Ishtar Terra, crustal plateaus, and as inliers within the plains using high-resolutionMagellanradar imagery. We describe several types of tessera terrain that record a wide range of structural histories. Fold and S-C terrains are found only in Ishtar Terra; ribbon, lava flow, and basin-and-dome terrains reside within the interior of crustal plateaus, whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus. Inliers are divisible into fracture-dominated and graben-dominated tesserae, although some inliers host early contractional fabrics. The range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit. Tessera deformation records local to regional surface strain patterns, and reflects near-surface rheology at the time of deformation. Progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time. Thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation. Tesserae likely formed in several tectonic environments, including (1) as a result of subsurface flow in Ishtar Terra, (2) as sequences of surface-layer extension and contraction in crustal plateaus, (3) as flooded crustal plateaus, and (4) as densely fractured surface layers—fractured as a result of corona and chasma formation.

Venus reconsidered.

The Magellan imagery shows that Venus has a crater abundance equivalent to a surface age of 300 million to 500 million years and a crater distribution close to random. Hence, the tectonics of Venus must be quiescent compared to those of Earth in the last few 100 million years. The main debate is whether the decline in tectonic activity on Venus is closer to monotonic or episodic, with enhanced tectonism and volcanism yet to come. The former hypothesis implies that most radioactive heat sources have been differentiated upward; the latter, that they have remained at depth. The low level of activity in the last few 100 million years inferred from imagery favors the monotonic hypothesis; some chemical evidence, particularly the low abundance of radiogenic argon, favors the episodic. A problem for both hypotheses is the rapid decline of thermal and tectonic activity some 300 million to 500 million years ago. The nature of the convective instabilities that caused the decline, and their propagation, are unclear.

Correction to “Venus tectonics: An overview of Magellan observations” by Sean C. Solomon et al.

Correction to “Venus tectonics: An overview of Magellan observations” by Sean C. Solomon et al.

Venus tectonics: An overview of Magellan observations

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

Hot-Spot Evolution and the Global Tectonics of Venus

The global tectonics of Venus may be dominated by plumes rising from the mantle and impinging on the lithosphere, giving rise to hot spots. Global sea-floor spreading does not take place, but direct convective coupling of mantle flow fields to the lithosphere leads to regional-scale deformation and may allow lithospheric transport on a limited scale. A hot-spot evolutionary sequence comprises (i) a broad domal uplift resulting from a rising mantle plume, (ii) massive partial melting in the plume head and generation of a thickened crust or crustal plateau, (iii) collapse of dynamic topography, and (iv) creep spreading of the crustal plateau. Crust on Venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea-floor spreading.

Venus Tectonics: Initial Analysis from Magellan

Radar imaging and altimetry data from the Magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the Venus surface. The plains record a superposition of different episodes of deformation and volcanism; strain is both areally distributed and concentrated into zones of extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening; the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression. Venus displays two geometrical patterns of concentrated lithospheric extension: quasi-circular coronae and broad rises with linear rift zones; both are sites of significant volcanism. No long, large-offset strike-slip faults have been observed, although limited local horizontal shear is accommodated across many zones of crustal shortening. In general, tectonic features on Venus are unlike those in Earth's oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide, and separated by stronger and less deformed blocks hundreds of kilometers in width, as in actively deforming continental regions on Earth.

Mantle convection and crustal evolution on Venus

Venus is probably similar to the Earth in that recycling of basaltic crust has been induced by the development of cratons: combinations of siliceous crust with high Mg:Fe mantle. Venus has less remaining energy sources at depth, but still enough to to support the great plateaus, and to deliver appreciable heat close to the surface by convection. The main differences of Venus from the Earth arise from its lack of water, rather than higher temperatures. Lack of water (plus lower stress levels due to lesser energy) makes the upper mantle more viscous, and hence Venus tectonics more driven by bulk mantle, rather than boundary layer, characteristics. Making tectonics difficult to infer from Magellan imagery will be shallower and more widespread layers of weakness in crustal rocks obscuring the mantle‐driven patterns.

Consequences of a hydrous mantle for Venus tectonics: Predictions for Magellan

We will consider the consequences of a Venus mantle enriched in water relative to the Earth. If the Moon‐forming impact hypothesis is correct, Earth may have lost substantial volatiles. Venus, however, experienced no such impact and is therefore likely to be relatively more water‐rich. We demonstrate that a higher abundance of water in the Venus mantle has the effect of inhibiting the rise of crustforming melts and volatiles to the surface, which results in a thin stable crust, a dry, stiff upper mantle, and restricted magmatism. Conduction through a shallow thermal lithosphere dominates the removal of heat from the interior, and flow in the mantle is strongly coupled to the surface deformation. Current observations of Venus are consistent with this model. A review of the consequences of this model allows us to make predictions for the Magellan radar images. These include: 1) No evidence for plate tectonic processes; 2) younger apparent surface ages and enhanced crater relaxation for highland areas; 3) limited quantity of hot‐spot volcanism, and 4) occasional evidence of localized magmatism (diatremes) associated with a volatile‐rich mande source. The model has implications to the Earth as well: in the absence of the Moon‐forming impact, crustal formation in the water‐rich Earth mantle may have been arrested, and plate recycling would have ceased early in the planet's history.

Tectonics of Venus: A review

The article presents a new tectonic scheme of Venus and gives the following interpretation of the planet's main structural units: (1) plains — areas of flood volcanism over stretched crust; (2) dome-like uplifts — areas of uplifting and volcanic activity above the mantle hot-spots; (3) coronae —former dome-like uplifts, partially subsided and diffused by gravity; (4) ridge belts — fold zones; (5) tesserae — fragments of ductile compression and shortening of crust; (6) supercoronae — coronae formed in the course of further evolution and relaxation of Beta-type uplifts. Ishtar Terra is considered to be a fragment of an ancient tessera paleocontinent, on the edge of which the Lakshmi supercorona is superimposed. Aphrodite Terra is considered as a belt of mantle hot-spot structures (dome-like uplifts, coronae, supercoronae, volcanoes, rifts).

Orogeny and large‐scale strike‐slip faulting on Venus: Tectonic evolution of Maxwell Montes

The Ishtar Terra highland region of Venus contains mountain belts morphologically similar to terrestrial orogenic belts. Akna Montes and Freyja Montes are long, linear mountain belts with about 4 km of relief above the 3‐ to 4‐km‐high Lakshmi Planum. The Maxwell Montes mountain belt, however, is more rectangular in plan view and has relief of approximately 8 km. We have investigated the nature of this latter range using complementary radar images obtained by the Arecibo Observatory and the Venera 15/16 spacecraft, as well as allimetry data obtained by the Venera 15/16 and Pioneer Venus spacecraft. Geological and structural maps have been compiled on the basis of these data, the individual features and their stratigraphic relationships interpreted, and a model for the origin and evolution of Maxwell Montes is proposed. The dominant features of this mountain range are long, parallel ridges and valleys that are intepreted to be anticlines and synclines and to have resulted from ENE‐WSW oriented compression. Two major shear zones bound Maxwell Montes on the north and south, converging toward the west. Nine narrow linear parallel features several hundreds of kilometers long (identified as cross‐strike discontinuities, CSDs) cut obliquely across the strike of the ridges and valleys in a NW‐SE direction and disrupt the structural fabric of the mountain range, dividing Maxwell Montes into 10 crustal domains. Among several possible origins, we find that the data are most consistent with their interpretation as right‐lateral strike‐slip faults with offsets of up to 125 km. Retrodeformation and reconstruction of Maxwell Montes using the offsets determined for individual domains produces a long linear mountain range similar to Akna Montes. Using geologic unit maps and topographic maps of the present configuration of Maxwell, similar reconstructions were made; these reconstructions restored several linear tectonic elements, topographic trends, and sinuous unit boundaries to more contiguous positions. On the basis of these data and observations we suggest that Akna Montes may represent the initial form of compressional orogenic belts on Venus, while Maxwell Montes attained its present morphology through a second stage of deformation involving large‐scale strike‐slip faulting. We examine several models for this second stage and favor the interpretation that it occurred as the Akna Montes‐like proto‐Maxwell Montes was transported to the west between two converging shear zones, accompanied by rotation of the entire mountain range. Thirty percent shortening of the mountain range is implied by the geometries of the present and reconstructed Maxwell Montes. On the basis of the deforamation in Maxwell Montes and the evidence for large‐scale transport we conclude that large‐scale horizontal a movement of crustal materials is a significant aspect of the tectonics of Venus.

Tectonics of the Venus and the early Precambrian

Analyzing the tectonics of planets and their satellites we use all the information available from the studies of the Earth and other celestial bodies such as the Moon, Mars and Mercury. An important condition in such analysis is naturally the scale of the phenomena compared. Most surface structures of Venus are known to have no direct analogues on the surface of the present Earth, with its global systems of mid-oceanic ridges, deep trenches and vast lithospheric plates. This might be due to the sharp differences in the present thermal regimes of the Earth and Venus. It has already been suggested in numerous papers that the key to the genesis of the Cytherean surficial structures must be looked for in the geodynamics of the Early Precambrian Earth. Such an approach appears very logical indeed since the rheology of the present Cytherean crust must be closer to that of the Precambrian rigid lithosphere of the Earth which is as if ‘floating’ in the low-viscous asthenosphere. An attempt has therefore been made to evaluate certain elements in the tectonics of Venus through the theological properties of its crust comparing structural formation in the low-viscous layers of the Earth crust in the Early Precambrian with data on the morphology of structures on the surface of Venus.

Support of long‐wavelength loads on Venus and implications for internal structure

A thin‐shell formulation for computing stress distributions on the surfaces of one‐plate planets is developed. The lithospheric model includes laterally varying density anomalies at two depths, corresponding to undulations on a crust‐mantle boundary and variations in upper mantle density. For a given set of model parameters, values for the vertical displacement, crustal thickness anomaly, and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed, along with the resulting stress field. The computed stresses can then be compared to stress directions inferred from observed tectonic features. This formulation is used to study loads on Venus's lithosphere, and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift, with a significant portion of their support probably derived from dynamic processes in the mantle. A possible exception is Ishtar Terra, whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading. Reasonable lithospheric models predict low‐density mantle beneath Atla, Beta, Eisila, Thetis, and Tethus Regiones and thick crust accompanied by higher‐density mantle under Ishtar Terra and Ovda Regio. Thus western Aphrodite Terra appears to be more closely related to Ishtar than to the rest of the equatorial highlands. These results favor a hot spot model of Venus tectonics in which Beta, eastern Aphrodite, Eisila, and Tethus are expected to be relatively young and thermally active, while Ovda and Ishtar should be older and less active.

Volcanoes on Mars and Tectonics of Venus

Volcanoes on Mars and Tectonics of Venus

Tectonics of Venus

Tectonics of Venus

Tectonics of Venus

Tectonics of Venus

Tectonics and Evolution of Venus

The global tectonics of Venus differs significantly from that of Earth, most markedly in that the surface is covered predominately by gently rolling terrain; there apparently are no features like ocean rises; the gravity is positively correlated with topography at all wavelengths; and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers. The surface of Venus appears to be covered mainly by an ancient crust, the high surface temperature making subduction difficult. It seems likely that well over 1 billion years ago water was destabilized at the surface and, soon after, plate tectonics ceased. The highlands appear to be actively supported, presumably as manifestations of long-enduring hot spots.

Venus tectonics: Another Earth or another Mars?

The presence of presumably primordial large craters has led to the suggestion that Venus may have a thick lithosphere like that of Mars despite its similarities to Earth in size and density. Howevercrust and upper mantle temperatures on Venus are very likely higher than on Earth so that a dry Venus could have a lithosphere with a thickness similar to that of Earth. If a trace of volatiles is present in the mantlethe lithosphere of Venus could be thinner. Due to the absence of liquid watererosion and deposition will be much slower on Venus than on Earthfavoring retention of primordial cratered surfaces on portions of the crust that have not been destroyed or buried by tectonic and volcanic activity. Geochemical models of solar system origin and petrological considerations suggest that K is about as abundant in Venus as in Earth. The abundance of 40Ar in the atmosphere of Venus lies somewhere between the Earth value and one‐tenth of the Earth value. Because erosional liberation of 40Ar on Venus will be relatively inefficientthis range for 40Ar abundance at least permits an active tectonic historyand if the 40Ar abundance is towards the high end of the rangeit may well require an active tectonic history. Thus we are not constrained to a Mars‐like model of Venus tectonics by craters and possible mantle dryness; an Earth‐like model is equally probable.