D. L. Bindschadler

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.

Global distribution and characteristics of coronae and related features on Venus: Implications for origin and relation to mantle processes

Coronae on Venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies. The annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both. Topographically, coronae are domes, plateaus, plateaus with interior lows, and rimmed depressions. A subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large-scale calderas. A number of features have been identified with many of the basic characteristics of coronae (similar interior deformation, associations with volcanism, high topography) but lacking a distinct tectonic annulus. These features tend to be somewhat smaller than coronae and may represent “failed” coronae or coronae in an early stage of evolution. The size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form N(D) = kD−α, where N is the number of coronae with maximum widths greater than D (km) and α = 3.05. The spatial distribution of coronae is not random; the features are concentrated in a few groups and along several chains. Coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as Western Eistla Regio. The largest corona, Artemis, is actually larger than several volcanic rises on Venus. Coronae and volcanic rises appear to be surface manifestations of mantle plumes. There is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on Earth. Instead, a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume. The morphology and size distribution of coronae, highlands, and major shields suggest that mantle upwelling on Venus operates either on several spatial scales, with coronae representing smaller-scale upflows and major volcanic rises representing larger convective upwellings, or on several temporal scales, with coronae representing shorter duration upflows and major volcanic rises representing long-term upwellings.

The morphology and evolution of coronae on Venus

Coronae on Venus are large, circular to ovoidal surface features that have distinctive tectonic, volcanic, and topographic expressions. They range in diameter from less than 200 km to at least 1000 km. New data from the Magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail. Typical coronae are dominated by concentric tectonic features and have a raised rim, a central region higher than the surounding plains but in many instances lower than the rim, and, commonly, a peripheral depression or “moat”. Some coronae also show significant amounts of radial tectonic structure, and in most cases this predates the concentric features. In addition, there are other features on Venus, recognized for the first time in Magellan data, that consist of domical rises with intense radial tectonic patterns and little or no concentric structure. All of these features commonly are associated with moderate to large quantities of volcanism. In fact, some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography. We explore a model of corona formation that links these forms into a genetic sequence. The model begins with the ascent of a mantle diapir. Upward mantle flow driven by its ascent forces the lithosphere above the diapir upward, producing a gentle dome with a radiating pattern of extensional fractures. As the diapir impinges on the underside of the lithosphere it flattens and spreads, transforming the uplift to a more flat-topped shape. In this flattened, near-surface configuration the diapir can cool rapidly. With the resultant loss of buoyancy the raised plateau can relax to form a central sag, a raised rim, and a depressed moat. Concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae. Volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs. Our analyses to date suggest that this scenario is broadly consistent with many of the coronae on Venus. However, there is enormous diversity in corona morphology, and features are present that require substantial deviations from this simple model. In particular, some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas. Some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the Venusian lithosphere.

GEOPHYSICAL MODELS FOR THE FORMATION AND EVOLUTION OF CORONAE ON VENUS

Coronae are large circular features on Venus characterized by an annulus of concentric tectonic features, interior fracturing, volcanism, and generally upraised topography. They are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation. We examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the Magellan altimetry data and radar images. Our results show that mantle diapirism can produce the domical topography of novae, which may be coronae in the earliest stage of formation. The model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae. The dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick. Diapirs that have reached the top of the mantle are expected to spread and flatten, producing plateaulike rather than domical topography. We model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift. The volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae. We model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae.

Corona Structures on Venus' Models of Origin

Coronae on Venus are circular to elongate structures with maximum widths of 150–1000 km characterized by annuli of concentric ridges surrounding complex interiors. The features have raised topography relative to the surroundings, they are associated with volcanic activity, and most are partially surrounded by a peripheral trough. Variations in morphology between individual coronae are due to differences in their stage of evolution and/or differences in the relative significance of the geologic processes that occur in each stage. We examine models for three processes that may be involved in corona origin and evolution: (1) a hotspot or rising mantle diapir model, (2) a sinking mantle diapir model, and (3) gravitational relaxation of topography. Rising mantle diapirs are caused by heating at depth (e.g., hotspot), while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere. The hotspot model is most consistent with the major characteristics of coronae, with gravitational relaxation occurring as a modificational process. The sinking mantle diapir would produce dominant central compression that has not been observed at coronae; however, higher-resolution image and altimetry data from Magellan can be used to distinguish more fully between the two models. Coronae in various states of formation and degradation can be identified in the Venera 15/16 data, suggesting that the process may be continuing today.

Magellan observations of Alpha Regio: Implications for formation of complex ridged terrains on Venus

Magellan images of Alpha Regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain. We examine the complex ridged terrain of Alpha Regio, using morphology and crosscutting relationships between structures to derive a sequence of tectonic events. Structures include broad (∼10–20 km wide) linear and arcuate ridges, fine-scale (<3 km wide) ridges, linear disruption zones (LDZs) up to several kilometers wide, and numerous grabens (∼5 km wide) and associated scarps and troughs. Based on their morphology, we interpret the broad and fine-scale ridges as compressional structures, possibly folds. LDZs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric. Graben and associated structures are interpreted as extensional features. They crosscut ridges and LDZs and thus appear to be the youngest structures in Alpha Regio. This sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain (and highlands on Venus in general): a hotspot model and a coldspot model. The presence of compressional features along and parallel to the margins of Alpha Regio and the lack of any high-elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling. Mantle downwelling appears to be the most likely mode of formation of the upland of Alpha Regio and is likely to be important in other highland regions, such as Ovda and Thetis regiones, which are also dominated by complex ridged terrain.

Large topographic rises on Venus: Implications for mantle upwelling

Topographic rises on Venus have been identified that are interpreted to be the surface manifestation of mantle upwellings. These features are classified into groups based on their dominant morphology. Atla and Beta Regiones are classified as rift-dominated, Dione, western Eistla, Bell, and Imdr Regiones as volcano-dominated, and Themis, eastern Eistla, and central Eistla Regiones as corona-dominated. At several topographic rises, geologic indicators were identified that may provide evidence of uplifted topography (e.g., volcanic flow features trending upslope). We assessed the minimum contribution of volcanic construction to the topography of each rise, which in general represents less than 5% of the volume of the rise, similar to the volumes of edifices at terrestrial hotspot swells. The total melt volume at each rise is approximated to be 104-106 km3. The variations in morphology, topography, and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling. Instead, the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure, plume characteristics, and regional tectonic environment. Within each class, variations in topography, gravity, and amount of volcanism may be indicative of differing stages of evolution. The similarity between swell and volcanic volumes for terrestrial and Venusian hotspots implies comparable time-integrated plume strengths for individual upwellings on the two planets.

Spatial and temporal relations between coronae and extensional belts, northern Lada Terra, Venus

Preliminary studies of the distribution of coronae and volcanic rises on Venus show that many of these features tend to cluster along zones of rifting and extension. The plains north of Lada Terra are crossed by two such extensional belts. Each belt is composed of grabens, ridges, faults, volcanic flows, coronae, and coronalike features. The longer and more prominent belt is the NW trending Alpha-Lada extensional belt, which is over 6000 km long and 50–200 km wide, and includes the coronae Eve, Tamfana, Carpo, Selu, Derceto, Otygen, and an unnamed corona south of Otygen. The second belt is the NNE trending Derceto-Quetzalpetlatl extensional belt, which is about 2000 km long and in places over 300 km wide, and includes the coronae Sarpanitum, Eithinoha, and Quetzalpetlatl. The two belts intersect at the 1600 × 600 km wide Derceto volcanic plateau. It is apparent that deformation along the two belts overlapped in time, though deformation along the Alpha-Lada extensional belt probably continued after the deformation along the Derceto-Quetzalpetlatl extensional belt terminated. In certain areas, volcanism originated in grabens within the extensional belts, whereas in other areas, such as in Eve, Selu, Derceto, and Quetzalpetlatl, volcanism originated in the coronae and flowed into the lower parts of the extensional belts. Regional extension has affected the evolution of all the coronae at some stage of their development. Regional deformation occurred before the initiation of Derceto and Eithinoha and after the initiation of Carpo, Tamfana, Otygen, and Sarpanitum. It is thus unlikely that coronae formation along the belts is solely a consequence of the regional extension, and it is also unlikely that regional extension has been caused solely by the coronae. No corona along the belts was formed subsequent to the cessation of the regional extension. We therefore suggest that the regional extension and the coronae are interrelated. Some of the coronae may have determined the location of the surface expression of the regional extension, whereas the locations of other coronae may have been influenced by the concentration of regional extensional stresses.

Styles of deformation in Ishtar Terra and their implications

Ishtar Terra, the highest region on Venus, appears to have characteristics of both plume uplifts and convergent belts. Magellan imagery over longitudes 330°–30°E indicates a great variety of tectonic and volcanic activity, with large variations within distances of only a few 100 km. The most prominent terrain types are the volcanic plains of Lakshmi and the mountain belts of Maxwell, Freyja, and Danu. The belts appear to have marked variations in age. There are also extensive regions of tessera in both the upland and outboard plateaus, some rather featureless smooth scarps, flanking basins of complex extensional tectonics, and regions of gravitational or impact modification. Parts of Ishtar are the locations of contemporary vigorous tectonics and past extensive volcanism. Ishtar appears to be the consequence of a history of several 100 m.y., in which there have been marked changes in kinematic patterns and in which activity at any stage has been strongly influenced by the past. Ishtar demonstrates three general properties of Venus: (1) erosional degradation is absent, leading to preservation of patterns resulting from past activity; (2) many surface features are the responses of a competent layer less than 10 km thick to flows of 100 km or broader scale; and (3) these broader scale flows are controlled mainly by heterogeneities in the mantle. Ishtar Terra does not appear to be the result of a compression conveyed by an Earthlike lithosphere. But there is still doubt as to whether Ishtar is predominantly the consequence of a mantle upflow or downflow. Upflow is favored by the extensive volcanic plain of Lakshmi and the high geoid: topography ratio; downflow is favored by the intense deformation of the mountain belts and the absence of major rifts. Both could be occurring, or have recently occurred, with Lakshmi the most likely locus of upflow and Maxwell the main locus of downflow. But doubts about the causes of Ishtar will probably never be resolved without circularization of the Magellan orbit to obtain a more detailed gravity field.

Characterization of Venera 15/16 Geologic Units from Pioneer Venus Reflectivity and Roughness Data

Abstract Geologic units have been defined for the surface of Venus from Venera 15/16 image data. A characterization of these geologic units is carried out using information on surface properties derived from Pioneer Venus (PV) reflectivity and rms slope data. The geologic context provided by Venera 15/16 units allows additional, more specific interpretations of surface radar properties to be made. Characterization of Venera units results in the definition of four groups of Venera units: (I) smooth rocky units, (II) rough rocky units, (III) rough high dielectric units, and (IV) diffusely scattering units. On the basis of correlations of surface morphology to spatial and statistical distributions in rms slope and reflectivity data, we test models for the origin of the surface properties of some units. We conclude that plains and tectonic units can be contrasted in terms of the average roughness of the surface and that tectonic deformation appears to roughen the surface at 0.5- to 10-m and 5- to 50-cm scales. This tectonic weathering process appears to dominate the erosional regime of Venus. Unlike Earth or Mars, production and transport of soils dominates only a small portion (≤5) of the surface. Some of the Venera units display distinctive spatial and statistical distributions of PV radar data. In particular, apparent low reflectivity in the tesserae appears to be caused by small (5–50 cm) rock fragments on the surface which cause diffuse scattering at Pioneer Venus wavelenghts. Analysis of models for the formation of these fragments suggests that they are due to the pervasive deformation undergone by the tesserae. Finally, aspects of this study have been used to extend results of Venera image data analysis southward of 30°N lat, resulting in a prediction of the distribution of tessera. Such results can aid in Magellan investigations.