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At least 127 records · Page 7

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NASA NTRS1979

Venus tectonics - Another earth or another Mars

The paper reexamines the evidence on the intensity of Venusian tectonic/volcanic activity and suggests alternate hypotheses. Three major questions are discussed: (1) whether the presence of large, presumably primordial craters on Venus requires an intensity of tectonic/volcanic activity significantly less than on earth, (2) what thicknesses of lithosphere are implied for reasonable models of temperature and volatile content of the upper mantle of Venus, and (3) can the recently obtained Ar-40 content of the Venus lower atmosphere help define the relative tectonic/volcanic activities of Venus and earth. It was shown that the abundance of Ar-40 in the Venus atmosphere lies between the earth value and one-tenth of the earth value, and since erosional liberation of Ar-40 on Venus will be inefficient, this range for Ar-40 abundance indicates an active tectonic history. It is concluded that the presence of craters and possible mantle dryness does not restrict Venus tectonics to a Mars-like model, and an earth-like model is equally probable.

Mcgill, G. E.↗

NASA NTRS1981

A global tectonic activity map with orbital photographic supplement

A three part map showing equatorial and polar regions was compiled showing tectonic and volcanic activity of the past one million years, including the present. Features shown include actively spreading ridges, spreading rates, major active faults, subduction zones, well defined plates, and volcanic areas active within the past one million years. Activity within this period was inferred from seismicity (instrumental and historic), physiography, and published literature. The tectonic activity map was used for planning global geodetic programs of satellite laser ranging and very long base line interferometry and for geologic education.

Lowman, P. D., Jr.↗

NASA NTRS1981

Plate tectonics on Venus

The high surface temperature of Venus implies a permanently buoyant lithosphere and a thick basaltic crust. Terrestrial-style tectonics with deep subduction and crustal recycling is not possible. Overthickened basaltic crust partially melts instead of converting to eclogite. Because mantle magmas do not have convenient access to the surface the Ar-40 abundance in the atmosphere should be low. Venus may provide an analog to Archean tectonics on the earth.

Anderson, D. L.↗

NASA NTRS1981

Quantitative tests for plate tectonics on Venus

Quantitative comparisons are made between the characteristics of plate tectonics on the earth and those which are possible on Venus. Considerations of the factors influencing rise height and relating the decrease in rise height to plate velocity indicate that the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock-fluid density contrast and lower temperature differential between the surface and interior. Statistical analyses of Pioneer Venus radar altimetry data and global earth elevation data is used to identify 21,000 km of ridge on Venus and 33,000 km on earth, and reveal Venus ridges to have a less well-defined mode in crest heights and a greater concavity than earth ridges. Comparison of the Venus results with the spreading rates and associated heat flow on earth reveals plate creation rates on Venus to be 0.7 sq km/year or less and indicates that not more than 15% of Venus's energy is delivered to the surface by plate tectonics, in contrast to values of 2.9 sq km a year and 70% for earth.

Kaula, W. M.↗

NASA NTRS1982

The tectonics of Ganymede

Evidence of tectonic activity has been found on the Jupiter moon Ganymede, which is the largest solar system icy object known. The transition of dense ice polymorphs in the deep interior of large icy bodies to either less dense polymorphs or liquid water at shallow depths, as internal differentiation and formation of a silicate core occurs, could increase Ganymede's surface area by as much as 7%. If the tectonic evolution of Ganymede is to be described by a progressive fragmentation of an initially uniform lithosphere in response to planetary expansion, the stress transmitted across different areas of bright terrain must differ significantly. Alternatively, lithospheric thickness variations or such other mechanisms as tidal distortion or convection stresses beneath the lithosphere may have been important.

Parmentier, E. M.↗

NASA NTRS1982

Magsat scalar anomalies and major tectonic boundaries in Asia

The distribution of Magsat scalar anomalies is compared with locations of major tectonic boundaries in Asia. Foldbelts separate large-scale positive and negative anomalies inside and outside the Siberian Platform, and sediment-filled aulacogens within the Platform separate the anomalies into two parts which overlie the Anabar and Aldan Shields. Tectonic boundaries appear to exercise some control over the extent and distribution of the satellite-observed crustal anomalies.

Frey, H.↗

NASA NTRS1982

Tectonic deformation of Galileo Regio and limits to the planetary expansion of Ganymede

Galileo Regio is the largest and most prominent unit of dark, ancient, heavily cratered terrain on Ganymede. Its major tectonic feature, an arcuate system of rimmed furrows, formed very early, as nearly the entire cratering record post-dates it. Galileo Regio has undergone very little subsequent structural alteration, as opposed to the rest of the planet (i.e., replacement by grooved and smooth terrain). Its survival as an intact lithospheric unit during the era of grooved terrain formation constrains the concomitant expansion of Ganymede, if any, to be less than one percent in radius. This limit is derived from an analysis which considers Galileo Regio to be a thin, freely floating elastic spherical shell on an expanding planet. A comparison of tectonic features (both endogenic and impact produced) on Ganymede and Callisto suggests that the ultimate source that powered the creation of grooved terrain lies in the Ganymedean core.

Mckinnon, W. B.↗

NASA NTRS1981

Application of ring tectonic theory to Mercury and other solar system bodies

It is pointed out that multiringed structures, by their presence or absence, provide a powerful tool for deciphering the thermal histories of the solid planets. The theory of ring tectonics considered by Melosh and McKinnon (1978) and Melosh et al. (1980) establishes the framework of that undertaking. The present investigation has the objective to apply this conceptualization in detail to the multiringed basins on Mercury, taking into account also a brief review concerning the current state of understanding of ring tectonics on the moon, Mars, earth, Ganymede, and Callisto. The small, icy satellites of Saturn are also discussed. The mechanics of multiple ring formation are related to the collapse of the transient basin cavity when the excavation depth and lithosphere thickness are comparable. Attention is given to the Caloris Basin on Mercury, the peak ring basins on Mercury, and the Argyre Basin on Mars.

Mckinnon, W. B.↗

NASA NTRS1982

Could plate tectonics on Venus be concealed by volcanic deposits

The present investigation is supplementary to a study reported by Kaula and Phillips (1981). From an analysis of Pioneer Venus altimetry, Kaula and Phillips had inferred that any heat loss from the planet by plate tectonics must be small compared to that from the earth. However, it has been suggested by others that plate tectonic may exist on Venus, but that the expected 'square root of s' dependence of the topographic drop off is not observed because it is concealed by lava flows. The present investigation has the objective to conduct an examination whether this suggestion of concealment by lava flow is correct. On the basis of the performed analysis, it is concluded that the results obtained by Kaula and Phillips appear to be well justified.

Kaula, W. M.↗

NASA NTRS1982

Tectonic history of the Tharsis region, Mars

The faults of the Tharsis region of Mars have been mapped and delineated on the basis of age. The orientation of each group of faults was analyzed to determine if they were radial to a point. The results indicate that there are at least four discrete centers of faulting within the Tharsis area which are of significantly different age. The four centers proposed are, from oldest to youngest, (1) in the Thaumasia highlands, (2) in northern Syria Planum, and finally, (3) and (4) near Pavonis Mons. While episodes 3 and 4 occupy the same geographic position, they are separated in time by the development of the Tharsis lava plains and shields. Each episode of tectonism was separated from the next by a period of major basaltic volcanism. The tectonic features of the Tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting, rather than the result of one episode about a single point.

Plescia, J. B.↗

NASA NTRS1983

Venus topography: Clue to hot-lithosphere tectonics?

The topography of Venus is studied in order to learn about hot lithosphere tectonic processes of Archean Earth. Except for the difference in H2O abundances (Archaen Earth was wet; modern Venus is very dry), linear zones on Venus may be analogous to the tectonic settings for some Archean greenstone belts.

Mcgill, G. E.↗

NASA NTRS1984

Venus banded terrain: Evaluation of tectonic models for the origin of banding

A tectonic orgin for Venus banded terrain is consistent with band spacing. Both compressional (folding) and extensional models for band formation can fit present observations. Band spacing cannot distinguish among scenarios for global heat loss and for the origin of highland terrain. Tectonic models for band formation indicate that the surface brittle layer in the venus highlands is no more than a few kilometers thick.

Solomon, S. C.↗

NASA NTRS1984

A tectonic geomorphological classification of the walls of Valles Marineris

Viking 1 imagery of the Coprates NW quadrangle was used in an attempt to develop a geomorphic classification scheme for the canyon walls of Valles Marineris analogous to that devised to evaluate the relative tectonic activity of terrestrial mountain fronts. The four classes of walls established are described and mapped. Regions where a class cannot be assigned owing to the presence of intra canyon sediments, landslides, or landslide debris; and apparent fault scarps that occur on the canyon floor rather than at the wall base are also shown. The most striking feature is the concentration of active tectonic features within lus Chasma, and to a lesser extent in Tithonium Chasma, as well as along the north walls of Coprates and East Candor.

Spencer, J. R.↗

NASA NTRS1984

Elysium Region, Mars: Characterization of tectonic features

In this paper the tectonic features of the Elysium region are identified and characterized. Identification of features was made using USGS controlled photomosaics (Elysium quadrangle, and portions of Amenthes and Cebrenia quadrangles); Viking Orbiter photographic data were used in individual cases to assist in identification. The positions and orientations of tectonic features can then be used, in conjunction with estimates of the mass of the volcanic load obtained from gravity modelling, to constrain the thickness of the elastic lithosphere in the region.

Hall, J. L.↗

NASA NTRS1984

Ancient Volcano-Tectonic Structures of the Olympus Mons Region, Mars

Two old volcano-tectonic collapse structures are exposed north and northeast of the huge shield volcano, Olympus Mons. They are semicircular and were probably low shield volcanoes similar to Alba Patera, but whose central portions have subsided or collapsed. They form the basement upon which younger volcanic materials of Alba Patera Olympus Mons have been emplaced. The oldest structure, Acheron Fossae is over 700 km across and is north of Olympus Mons. It has a surface of considerable relief broken by graben and extensive en echelon and parallel fractures and faults with varied displacements. Acheron Fossae is the most densely cratered in the Olympus Mons region. The structure must have formed very early in martian time. The second is Halex Fossae, northeast of Olympus Mons. Where exposed, it is cut by a series of arcuate grabens that become closer spaced toward its center. The radii of the fractures indicate that the structure may be at least 250 km across and centered beneath the Olympus Plains. Lava flows appear to have issued from some of the arcuate fractures and to have flowed radially away from the center of Halex Fossae. North of Halex Fossae, material of the same age as Halex Fossae overlaps Acheron Fossae. The density of craters on the Halex Fossae material (3,200-4,400 >1km/10 6 km 2 indicates an old age for its formation but one younger than that of Acheron Fossae. These two ancient volcanic-tectonic structures are evidence that the Olympus Mons region has been the locus of major volcanism over much of martian history.

Elliot C Morris↗

NASA NTRS1984

Euro-african MAGSAT Anomaly-tectonic Observations

Preliminary satellite (MAGSAT) scalar magnetic anomaly data are compiled and differentially reduced to radial polarization by equivalent point source inversion for comparison with tectonic data of Africa, Europe and adjacent marine areas. A number of associations are evident to constrain analyses of the tectonic features and history of the region. The Precambrian shields of Africa and Europe exhibit varied magnetic signatures. All shields are not magnetic highs and, in fact, the Baltic shield is a marked minimum. The reduced-to-the-pole magnetic map shows a marked tendency for northeasterly striking anomalies in the eastern Atlantic and adjacent Africa, which is coincident to the track of several hot spots for the past 100 million years. However, there is little consistency in the sign of the magnetic anomalies and the track of the hot spots. Comparison of the radially polarized anomalies of Africa and Europe with other reduced-to-the-pole magnetic satellite anomaly maps of the Western Hemisphere support the reconstruction of the continents prior to the origin of the present-day Atlantic Ocean in the Mesozoic Era.

Hinze, W. J.↗

NASA NTRS1983

Digital images of combined oceanic and continental data sets and their use in tectonic studies

It is shown how crustal and lithospheric studies can benefit when continental and oceanic data sets are combined. It is also shown how digital imaging techniques provide an effective means for displaying the information contained in these combined data sets. The region of Australia, New Zealand, and the surrounding ocean is chosen for illustrating the advantages of combining continental and oceanic data sets. Here, the tectonic setting of Australia, a relatively stable continent in an intraplate environment, can be contrasted with New Zealand, which is traversed by one of the world's major plate boundaries. Simultaneous display and analysis of complementary data sets make possible a rapid geologic and tectonic interpretation of regional areas. It is shown, by way of example, that the relationship between topography and gravity anomalies in central Australia gives important new information concerning the state of isostasy of thrust terrains and their related sedimentary basins and hence provides a means of understanding the mechanical properties of the continental lithosphere.

Haxby, W. F.↗

NASA NTRS1984

Martian impact basins: Morphology differences and tectonic provinces

Detailed geomorphic and structural mapping of five Martian basins and preliminary study of eleven other basins reveal four characteristic styles of modification that relate to the degree and age of past tectonic activity. Within regions that exhibit no evidence for tectonic activity, the modification style can be used to distinguish areas dominated by different exogenic processes. A framework for understanding these different styles of basin modification is provided.

Stam, M.↗