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PLATE TECTONICS ON VENUS.
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Discoveries relating Venus' geology with Earth's crust, plates, rifts, etc.... More...
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Paper Abstract:
Discoveries relating Venus' geology with Earth's crust, plates, rifts, etc.

Paper Introduction:
PLATE TECTONICS ON VENUS? The plate tectonic theory is now widely accepted as a unifying basis for the physical geology of the earth as a whole. Not only does it explain most of the tectonic phenomena on Earth, but a number of key points have been well substantiated by paleomagnetic and seismologic studies (4). Venus, Earth's nearest neighbor, and similar in size, density, and age (8) might be expected to have similar tectonic activity. Indeed Soviet and U.S. explorations of Venus during the 60's and 70's revealed several features that could be interpreted as past or ongoing tectonism (Fig. 1). These included large calderas and arachnoids suggesting volcanism, dome uplifts, low ridge belts on planes, high mountain belts, heavily deformed areas called tesserae, and the circular coronae unique to Venus (9). Now that the

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The "Ovida type" show evidence of a large amount of strainthat has to be accounted for, and furthermore, volcanoes are absent inthose thought to represent late-stage hot spots. Numerous studies have shown that along the ridge hot mantlewells up forming new crust which, as it cools, spreads on either side ofthe ridge, while a central extensional rift develops down the center of theridge. S. Theearliest events involved formation of compressional ridges and troughs,arranged 1 -2 km apart; while on top of these is laid out more narrowlyspaced graben and other extensional features. Physical geography. McKenzie, D.; Ford, P. H. Not only does it explainmost of the tectonic phenomena on Earth, but a number of key points havebeen well substantiated by paleomagnetic and seismologic studies (4).Venus, Earth's nearest neighbor, and similar in size, density, and age (8)might be expected to have similar tectonic activity. J. J. Solomon, S. Briefly, the plate tectonic theory states that Earth's lithosphere isnot one solid crust, but is broken up into eight large inert plates andseveral dozen smaller plates, with their boundaries marked by intensegeologic activity (4). W. G.; Stofan, E. This SAR (synthetic aperture radar) system mapsstrips of the surface, 2 km wide, to the left of the spacecraft, usingboth time for the echo to be received and frequency change of the echo(Doppler shifts) to distinguish altitudes. Science 252:651-658; 1991.7. S.; Schubert, G.; Squyres, S. 94(B3):2779-2785; 1989.----------------------- 9 Thus McKenzie et al. R. Furthermore, they find evidence for only slight horizontalmovement of the crust; nothing to match that found on earth. (11), the tesserae may be blocks of thickened crust whichwould be more susceptible to deformation due to that thickness, andtherefore show the effects of various stages of large-scale, often exogenicstress. Baddington; Geol. There are examples of CRT at variousstages of evolution, from the "linear-ridged terrain" to the "disruptedterrain" (pre-Magellan classifications). Concurrently therim and outer moat developed with extensional faulting and possiblycompressional deformation, in a narrow annulus at the edge of the corona.Volcanism occurred during both stages. At this stage, no suggestionscan be summarily dismissed, and every piece of evidence has to be closelyexamined for its value in narrowing down the possibilities. LITERATURE CITED1. Mercury (September/October) 131-144; 1991.9. Features on Venus generated by plate boundary processes. History of ocean basins. Magellan is equipped with a mapping radar system to "visualize" the planetsince dense, high clouds of sulfuric acid obscure any view of the surface.The high resolving power of this radar is due to "aperture synthesis", acomputerized process which transforms data so that it appears to come froman antenna hundreds of meters long when in fact the antenna on Magellan isonly 3.7 meters (3). In trying to understand thesedifferences, we will gain deeper insight into the causes of plate tectonicson this Earth. Schubert, G. E.; Malin, M. L. The coronae, which are unique to Venus, are considered to haveoriginated at the point where a quasi-spherical plume head reached the baseof the lithosphere. E.; Phillips, R. Continental rift valleys are considered to be younger versions ofsea-floor spreading centers, and have the same characteristics. A second,small, fan-beam horn antenna is used as an altimeter, sending radio signalsvertically below the spacecraft to measure altitude expressed as Venusradius (in km). A heat pipe mechanism for volcanixm and tectonics on Venus. C.; Head, J. It is interesting that Sean Solomon, himself, and Steve Saunders fromJPL are authors on both this paper and the McKenzie et al. S.; Arvidson, R. Thus the sequence of eventsappears to be an early phase of compression and crustal shortening,followed by a more recent phase of lateral extension, probably bygravitational spreading of the elevated terrain. W.; Kaula, W. It is marked by a dipping suture zone,thrust belts, and subsiding basins on both sides of the now inactivemagmatic arc, as well as a broad belt of shallow focus earthquakes (with afew deep-focus earthquakes beneath the mountains). The composition corresponds to that expected from adiabaticdecompression such as occurs at spreading ridges on earth. J. Indeed Soviet andU.S. R. have distinguished three types ofhighlands based on tectonic styles, regional morphology (topography), andgravity characteristics (ADC)(Table 1). J.; and Izenberg, N. Solomon et al. McGeary, D.; Plummer, C. EOS 73(15):161, 168-169; 1992.2. Arvidson, R. Oncloser inspection, they consist of several layers of deformation. 599-62 1962.3. 97(E8):13,533-13544; 1992.6. These images, which McKenzie et al. It is apparent from the Magellan data that the highlands aretectonically more complex and varied than can be explained by the "simple",pre-Magellan models. Only the "Beta type" highlands conform to the hot spot modelpredictions. New Scientist 42-47; 1989.4. The models that have been proposed are based either on the upwellingof mantle in plumes to form "hot spots", or on the downwelling oflithosphere ("cold spots"), caused either by the downward flow of mantlewhich is strongly coupled to lithosphere (#5 above), or by the "sucking"effect of mantle downwelling. C. explorations of Venus during the 6 's and 7 's revealed severalfeatures that could be interpreted as past or ongoing tectonism (Fig. The second stagein corona evolution is considered to have involved flattening and wideningof the plume head against the lithosphere, so that it became a plateau.Then, as the plume activity decreased, the corona surface cooled causinggravitational relaxation and subsidence of the interior. By contrast, trenches and fold belts, elements associated withconverging plates, are common on Venus (the trenches having only become"visible" with the latest data from Magellan). L.; Grimm, R. C. GLORIA images of similar areas on Earth are comparable. This begs the question of what causes the rather differentmorphology and scale of deformation of the two kinds of areas (13,1 ).Alternatively, coronae might be subduction areas (7). point out, the models that havebeen put forward are based on a number of suppositions that in themselvesmay not be very concrete (12:13,222). 6 Ga and continental surface age of 2. Furthermore, these plates are in continual motion,"floating" on the more plastic asthenosphere. The onlydifference is in wavelength and the angle of incidence of the beam; forMagellan it is almost constant across the width of the swathe, whereas forGLORIA it varied across the swathe because of the proximity of the ship tothe sea floor. Another problem is thelack of data to support the changes in gravity that would accompany themassive elevational events predicted by both models, for extended periodsof geological time. Global views of Venus from Magellan. PLATE TECTONICS ON VENUS? Regarding the chasm systems that are so abundant on Venus, Solomonet al. Solomon, S. Venus surface age of .45 Ga lies between Earth's mean oceanfloor age of . W.; Stofan, E. A similar pattern was found for another section within theArtemis Corona on Venus. The authors suggest that thespreading centers may be quickly covered by lavas or tuff falls so thatthey might underlie the relatively smooth planes found extensively on theplanet. The actual trough (rift) was not visible on GLORIAimages, and so it is not surprising that it was also not identifiable bySAR on Venus in the areas considered to represent spreading centers. T.; Schubert, G. the latter may be arranged in broad belts ofvarious types. Associated with ocean-continent subduction zones, are young mountain belts with magmatic volcanicarcs, batholith formation (seen where older mountains have eroded away),and back-arc thrust belts. A recent issue of the Journal of Geophysical Research (Planets (E),August, 1992) is devoted to studies analyzing the enormous amount of datathat have been collected for the two cycles (global mappings) that havebeen completed so far. The resolution of the SAR is about 75 m, ascompared with a previous resolution of 2 km at best for the Venera 15 and16 Soviet missions (3). Earth revealed. Lunary and Planetary Scinece Conference Reports 23:1227-1228; 1992.11. McKenzie's group decided that the only valid means of comparison wasto use the same scale and similar types of sensors for comparing featureson Earth with those on Venus (5). L.; Grimm, R. themid-Atlantic ridge, spreading at 12 mm/year half rate), the ridge does notappear as a continuous line, but has short, staggered discontinuities, andGLORIA images of this area show that the bright lineations are short anddiscontinuous. Nowthat the spacecraft Magellan is yielding images at a resolution about tentimes higher than before (3), it has become apparent that previously theresolution was just not good enough to decide whether structures wereequivalent to tectonic features on Earth (5). E.; Kaula, W. M.; McGill, G. M.; McGill, G. Science 252:297-312; 1991.12. Sandwell, D. Thus there is a continuous movementacross ocean floors of "young" rock aging as it spreads across the oceanfloor on either side to the subduction zones, at a rate of 1 to 6 cm peryear. An example is presented ofshort, curved troughs arranged in a complex pattern in eastern Aphrodite,clearly similar to the system of troughs in the East Indies, between NewGuinea and Fiji. On the other hand, at very slow spreading ridges on Earth (e.g. E.; Phillips, R. C. Also back-arc basins may form and may actuallyhave regional back-arc spreading due to a mantle plume (diapir) in thatregion. III; Schaber, G. The resulting images, that arebeing processed at the Jet Propulsion Laboratory in Pasadena, areequivalent to visual images, with the slopes facing the spacecraft beingbrighter, and those facing away being darker, as if a beam of light werebeing shone at an angle on the surfaces to the left. Associated with spreading centers are rift valleys atop ridges,volcanoes with basalt eruptions, and shallow focus earthquakes (related totransform faults which cross the spreading center so that the ridge is notcontinuous, but offset at intervals). In summary, Solomon et al. Associated with oceanic subductionzones, are deep oceanic trenches, island arcs of volcanic origin, magmaticarcs of andesitic volcanoes, Benioft-zones of earthquakes (shallow todeep), a subduction complex, and back-arc basin. However,Solomon et al. Additionally roughsurfaces or surfaces containing electrically conductive minerals likepyrite or magnetite are more reflective and appear brighter (8). Most models of both types do not include theeffects of magma generation (volcanoes and crustal thickening) which mightobscure the features characteristic of upwelling. E.; Phillips, R. 97(E8):13,199- 13,255; 1992.13. Res. E.; Kaula, W. R.; Solomon, S. In conclusion, it seems that similarities in the basic composition ofVenus and Earth have given rise to comparable tectonic events. Corona development, like thehighland areas, have been attributed to plumes of upwelling magma(hotspots). they have lower relief with more symmetricaltopographical profiles, and they are linear (e.g. Is the Venusian lithosphere subducting? Furthermore, it is argued that the ridges on Venuswould be lower and narrower than on earth because of the high surfacetemperature on Venus (47 C) which would slow cooling of the upwellingmagma. Major resurfacing ofthe planet occurred about 5 m.y.a. W.; Stofan, E. Mantle dynamics and tectonics on Venus. M.; McKenzie, D.; Parsons, R.; Phillips, J.; Schubert, G.; Talwani, M. C. Individual linearitiesextend only for 1 km at most. Ga. The resulting dome uplifting is characterized byradial extensional rifts and is termed the "Nova" stage. The unveiling of Venus. With at least two more cycles still to go, moredata will be available (exceeding three times the volume of any otherplanetary mission so far); so that these papers are considered onlypreliminary. Lunary and Planetary Science Conference Reports 23:12 9- 121 ; 1992.8. The latter method is able to distinguish variations ofheight as small as 3 m (3). 1).These included large calderas and arachnoids suggesting volcanism, domeuplifts, low ridge belts on planes, high mountain belts, heavily deformedareas called tesserae, and the circular coronae unique to Venus (9). Cold-spot models are notsupported because of the lack of evidence of extensional strain in thelowlands (see #1 above), a requirement for secondary crustal flow in orderto create elevated terrain. At the heart of thismovement is the concept of sea-floor spreading (2) which was firstdescribed to account for the world-wide distribution of a continuous mid-oceanic ridge. Phillips, R. As Solomon et al. Whether these events are ongoing is unknown because, witha lack of water on the planet's surface, erosion is minimal, so that onecannot distinguish past events from ongoing events. may or may not be interpretingcorrectly as transform faults associated with spreading centers, have beenfound elsewhere on Venus but not abundantly. Solomon, S. Suchareas have been identified for example within the Artemis Corona wherethere are bands of rolling hills, similar to the so-called abyssal hills onearth, created by the transform faults associated with spreading ridges.GLORIA images of parts of the East Pacific Rise are remarkably similar tothe SAR images, consisting of wavering, bright lineations reflected fromthe contours of the hills, where there are areas of small-scale roughnessproduced by faulting. Both long troughs (1 km or less wide) and linear,sinuous ridges are present. G.; Johnson, C.; Parsons, B.; Sandwell, D.; Saunders, S.; Solomon, S. It is argued that crustal thickness alone could not sustainsuch high elevations. The topography ofthe outer rim of coronae, as determined by altimetry from Magellan, is verysimilar to that of oceanic trenches (Fig. Whetherthese tectonic events are associated with plate boundary interactions isthe subject for discussion in this paper. The lowland plains covering more than 8 % of Venus, are also for themost part composed of tectonic features which are overlaid or underlaidwith signs of volcanism. These are listed as follows:(12:13,223-13,224, 6): (1) internal heat production occurs as on Earth; (2)the high surface temperature of around 47 C (Mason) has resulted in athinner, elastic lithosphere, 1 -3 km thick; (3) there are two elasticlithospheres, one in the upper crust, and one in the upper mantle; (4) theweak lower crust can detach along a ductile decollement in response to thepressure of elevated regions; (5) mantle flow is directly coupled with thelithosphere causing some of the tectonic deformation; (6) large ADCs(apparent depths of isostatic compensation, or gravitational anomalies)imply dynamic support of topography; (7) crust can change in response tostresses within a "geologically reasonable" time scale (about 5 m.y.);(8) compressional events are due to downwellings, while extensional eventsimply upwellings; (9) there is no low-viscosity zone in the mantle; (1 )subduction zones, driven by a large negative buoyancy, are not present on aglobal scale. conclude that there is no system of large, inert plates withnarrow active zones as we know it on Earth. Res. Until we can consider such close-range experiments onVenus, we can argue only by analogy, comparing features on the two planets. according to the abundance of meteorimpact craters. It is thedisparity in certain conditions on the two planets which has resulted indifferences in distribution and proportions. Of particular interest is the origin of the highland regions thatrise higher than 2, m above the mean level on Venus (8% of the surfacearea) (3). As suggested earlier bySolomon et al. In: Petrologic studies: A volume in honor of A. 2). believe that they have found evidence of spreadingcenters on Venus. F. Am. It is not really surprising then, that the two papers dealingwith the possibility of plate tectonics have widely differing viewpoints.The reason seems to originate in the approach that each group took in theiranalysis. feel that all three types of plate boundaryexist on Venus. The only conclusive evidence for plate tectonics on Venus, would belarge-scale magnetometer and seismologic studies such as have been carriedout on Earth (5). A sinking plume is thought to produce coldspots, resulting in secondary crustal flow and compressional elevation.Yet another mechanism for releasing mantle heat has been proposed, wherebyvolcanic outflow of magma could serve as "heat pipes" (14). In contrast, at subduction zones, old crust sinks under an adjacentcrust at a converging plate boundary. Venus tectonics: An overview of Magellan observations. McKenzie et al. Venus tectonics: Initial analysis from Magellan. Continent-continent convergence may occur where there is only asubduction zone between continents. suggest that there are three main featureson Venus contributing to its distinctive styles of tectonics: (1) theabsence of surface water means less erosion and hydrologic transport, andtherefore preservation of more episodes of deformation which cannot easilybe historically placed, and therefore may confuse the overall picture; (2)the higher surface temperature would lead to a shallower level ofplasticity, and therefore the crust would be susceptible to a variety ofsmaller scale deformations; (3) crustal field stresses over largerdistances may be due to the lack of a low-viscosity zone (asthenosphere) inthe upper mantle, so that the lithosphere would be "coupled" to mantlecurrents. Res. Mason, J. E.; Head, J. Saunders, R. J.; Grimm, R. They used data from GLORIA, a sonar scansystem that has been used to map parts of the sea floor, also in 2 -kmswathes and at a similar resolution to Magellan's SAR system. point out that although some are typical of oceanic trenches onearth, having arcuate planforms and asymmetric topographical profiles (e.g.the Dali and Diana chasmata), others are clearly extensional rifts ratherthan compressional, i.e. Perhaps we will discover that, although large lithosphericplates are absent on Venus, there are more basic concepts that underlietectonics on both of these planets. TheMagellan mappings have provided evidence for a diversity of innumerabletectonic events that are widely scattered over the planet. They may be diverging,converging, or moving horizontally past each other. These lower ridges may therefore be difficult to distinguish fromlarger topographic features supported by crustal thickness on Venus, exceptby a linearity and symmetry of appearance. Hess, H. Turcotte, D. An overview of Venus geology. Geophys. S.; Schubert, G.; Squyres, S. The plate tectonic theory is now widely accepted as a unifying basisfor the physical geology of the earth as a whole. Saunders, R. The recent data shows thatthere are indeed many similarities, and this suggests an interactionbetween a hot mantle and a hard, cooler lithosphere as on Earth. Furthermore, the complexity of tectonics in an areamay be difficult to sort out due to the lack of erosion (page 6); there maybe separate components of local as well as large-scale tectonic episodes,and the temporal sequence of events may be difficult to establish. Soc. Dubuque, IA; Wm. Geophys. Lunary and Planetary Science Conference Reports 23:1333-1334; 1992.14. Supportive evidence for spreading centers comes from analysis ofthe composition of basalt rock collected by Venera 13 and 14 and Vega 2missions. The exploration of Venus: a Magellan progress report. Devana Chasma). Datastill to come from Magellan with higher SAR resolution and gravitymeasurements will give a clearer idea of which is involved at eachlocation. Venus tectonics: An overview of Magellan observations. This neatly emphasizes thehighly speculative nature of the explanations and conclusions that arebeing presented by the experts in the field. C. paper ofcontrasting viewpoint discussed above (5). C.; Smerekar, S.; Bindschadler, D. Hot spot evolution and the global tectonics of Venus. On Venus the zones of tectonicactivity are broad (1 1-1 2 km wide), compared to Earth's 1-1 1-km wideboundary zones; while the inert, relatively undeformed areas are muchsmaller than on earth - 1 2 km compared to Earth's plates that are 1 3-1 4km wide. Terrestrial ridges could not be distinguished from the abyssal hillson GLORIA, so again it is not surprising that they could not be identifiedon the SAR images. Science 252:249-251; 1991.1 . These authors conclude that large scale plate movement would beimpossible due to a less cohesive upper crust and a lesser horizontalmobility. Geophys. Brown; 65-98; 1992.5. But there is no consistent pattern in the density of thesecraters, to indicate that the surface is being renewed on a continuousbasis (1). The other JGR paper on the Magellan data takes a global view of thetectonic features on Venus in a lengthy, comprehensive overview (12). J.; Saunders, R. CRT (complex ridged terrain), or tesserae, are the most commontectonic unit in the highlands, and the latest Magellan data have revealedthat they cover three times more area than was formerly realized. C.; Smerekar, S.; Bindschadler, D. J.; Saunders, R.

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