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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

On the relationship between isostatic elevation and the wavelengths of tectonic surface features on Venus

Venus lithospheric structure models are presently formulated in which regional isostatic elevation, d, and the spacing wavelength, lambda, of tectonic features formed due to horizontal extension and compression are functions of both surface thermal gradient and crustal thickness c. It is shown that, in areas of Venus where the upper mantle is stronger than the upper crust, the spacings of short-wavelength features should increase with increasing d, if that change in turn is due to increasing c, but should decrease with increasing d, if this change is in turn due to increasing surface thermal gradient.

Zuber, M. T.↗
NASA NTRS1990

Tectonics of Lakshmi Planum, Venus - Tests for Magellan

The origin of Lakshmi Planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of Venus. From the perspective of gravity signature and potential driving forces, the mantle upwelling model is the simplest, and its principal failure, that it cannot produce radial shortening on the uplift periphery, may be resolved if the lithosphere is laterally heterogeneous. The preferred model consists of a hot mantle plume rising beneath a preexisting block of tessera. The lithosphere is weakened at this hotter and presumably thicker crust, and the outward near-surface flow is attenuated at the peripheral discontinuity in lithospheric strength. Crustal thickening and mountain belt formation occur there. Several criteria are proposed to test this 'tessera-plume' model together with its competitors at the higher resolution in both imaging and gravity afforded by the Magellan mission.

Grimm, Robert E.↗
NASA NTRS1990

Mars tectonics and volcanism

The focus of this research was on three broad areas: (1) the relation between lithospheric stress in the vicinity of a growing volcano and the evolution of eruption characteristics and tectonic faulting; (2) the relation between elastic lithosphere thickness and thermal structure; and (3) a synthesis of constraints on heat flow and internal dynamics on Mars. The two reports presented are: (1) Heterogeneities in the Thickness of the Elastic Lithosphere of Mars--Constraints on Heat Flow and Internal Dynamics; and (2) State of Stress, Faulting, and Eruption Characteristics of Large Volcanoes on Mars.

Solomon, Sean C.↗
NASA NTRS1990

The evolution of Tharsis: Implications of gravity, topography, and tectonics

Dominating the Western Hemisphere of Mars, the Tharsis rise is an elongate area centered on Syria Planum that ascends as much as 8 to 10 km above the datum. It is intensely fractured by long, narrow grabens that extend radially hundreds of kilometers beyond the rise and is ringed by mostly concentric wrinkle ridges that formed over 2,000 km from the center of the rise. Its size, involving a full hemisphere of Mars, gives it a central role in the thermo-tectonic evolution of the planet and has stimulated a number of studies attempting to determine the sequence of events responsible for this feature. The constraints that gravity and topography data place on the current structure of Tharsis, along with insights into its development derived from comparisons of detailed regional mapping of faulting with theoretical deformation models are reviewed. Finally, a self-consistent model for the structure of Tharsis is proposed.

Banerdt, W. B.↗
NASA NTRS1990

MEVTV study: Early tectonic evolution of Mars: Crustal dichotomy to Valles Marineris

Several fundamental problems were addressed in the early impact, tectonic, and volcanic evolution of the martian lithosphere: (1) origin and evolution of the fundamental crustal dichotomy, including development of the highland/lowland transition zone; (2) growth and evolution of the Valles Marineris; and (3) nature and role of major resurfacing events in early martian history. The results in these areas are briefly summarized.

Frey, Herbert V.↗
NASA NTRS1990

Mafic magmatism and associated tectonism of the central High Cascade Range, Oregon

Results are presented on chemical analyses of basaltic core samples obtained from holes drilled at three locations in the mafic platform within the central region of the High Cascade Range (Oregon), as well as of some surface samples to enhance the original data base. Analyses were obtained using a variety of procedures; the most appropriate data commensurate with the technique were selected. The data illustrate the development of the mafic platform in terms of geochemical variations and tectonic configurations required to satisfy petrochemical associations.

Hughes, Scott S.↗
NASA NTRS1990

Mantle flow tectonics - The influence of a ductile lower crust and implications for the formation of topographic uplands on Venus

The crust and mantle of Venus can be represented by a model of a layered structure stratified in both density and viscosity. This structure consists of a brittle-elastic upper crustal layer; a ductile weaker crustal layer; a strong upper mantle layer, about 10 percent denser than the crust; and a weaker substrate, representing the portion of the mantle in which convective flow occurs which is a primary source of large-scale topographic and tectonic features. This paper examines the interactions between these four layers and the mantle flow driven by thermal or compositional variations. Solutions are found for a flow driven by a buoyancy-force distribution within the mantle and by relief at the surface and crust-mantle boundary. It is shown that changes in crustal thickness are driven by vertical normal stresses due to mantle flow and by shear coupling of horizontal mantle flow into the crust.

Bindschadler, Duane L.↗
NASA NTRS1990

Tectonic motion and deformation from satellite laser ranging to Lageos

Data on satellite laser ranging (SLR) to Lageos aquired during the period 1978-1988 are analyzed on the basis of the precise modeling of the orbit dynamics of Lageos, producing estimates of tectonic motion for 22 sites located on seven major plates. It was estimated that intraplate motion within northern Europe is below the 2 mm/yr level in absolute rate, in agreement with conclusions of Zoback et al. (1989) regarding the stress across the region. A comparison of SLR geodesic rates with those from NUVEL-1 and AMO-2 models showed high correlations between tracking sites that are well within plate interiors, but displayed small but significant departures from unity in slope which are attributed to the possibility of recent changes in relative velocities or geologic time scale uncertainties. For lines crossing the Nnorth Atlantic, the San Andreas fault, and within the Basin and Range province, the geodesic rates determined by SLR are in good agreement with those determined by VLBI.

Smith, David E.↗
NASA NTRS1990

Tectonic evolution of Eastern Ishtar Terra, Venus

Tectonic deformation is cited as the cause for the complex morphology and topography of Eastern Ishtar Terra. A number of models proposed to account for this deformation are reviewed, and their predictions for the geology of Fortuna Tessera are examined. The detailed geology, topography, and structural fabric of Eastern Ishtar are reviewed, and these observations are compared to the predictions of the existing models for the formation and evolution of Fortuna Tessera. On the basis of the analysis, a number of conclusions are drawn regarding the evolution of Eastern Ishtar Terra. These include the recognition of primary morphological and topographical patterns which allow Eastern Ishtar Terra to be divided into four major physiographic regions named Maxwell Montes, Eastern Fortuna Tessera, Central Fortuna Tessera, and Northern Fortuna Tessera. A model is suggested that accounts for the morphology and high topography of Eastern Ishtar Terra.

Vorder Bruegge, R. W.↗
NASA NTRS1991

Late Cenozoic tectonic evolution of southwestern California

Geologic and geophysical data from southern California and adjoining areas are used to reconstruct the tectonic evolution of the southern Coast Ranges, western Transverse Ranges, and borderland regions since 30 Myr ago. Premises include specified relative plate motions for times prior to 10.5 Myr ago and after 3 Myr ago, a mid-Tertiary bight in the continental margin, midcrustal detachment faults, rotation of the western Transverse Ranges about an eastern pivot, and specified fault displacement histories. Prior to 18 Myr ago, about 90 percent of the tangential component of Pacific-North America relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope. From 18 to 5.5 Myr ago, dextral slip was accommodated predominantly on the offshore system but also on a second, inboard system that included the San Andreas fault.

Sedlock, Richard L.↗
NASA NTRS1991

Venus tectonics - Initial analysis from Magellan

The styles of lithospheric deformation, the inferred mechanical properties of the lithosphere, and their implications for the tectonic history of Venus are discussed on the basis of radar imaging and altimetry data from Magellan. Observations of the planet plains reveal a superposition of different episodes of deformation and volcanism, strain both distributed and concentrated into zones of extension and shortening, and features reflecting a crustal response to mantle dynamic processes. Lithospheric shortening and crustal thickening are represented by ridge belts and mountain belts. The latter show the evidence for extension and collapse both during and following crustal compression. Venus displays quasi-circular coronae and broad rises with linear rift zones, associated with significant volcanism. Large-offset strike-slip faults have not been observed, although horizontal shear is accommodated across broad zones of crustal shortening. On Venus strain is distributed across 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.

Solomon, Sean C.↗
NASA NTRS1991

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 (1) a broad domal uplift resulting from a rising mantle plume, (2) massive partial melting in the plume head and generation of a thickened crust or crustal plateau, (3) collapse of dynamic topography, and (4) 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.

Phillips, Roger J.↗
NASA NTRS1991

Tectonic setting of Martian volcanoes and deep-seated intrusives

More than 50 volcanoes have been mapped on Mars, and recent geologic studies indicate structural evidence of deep seated intrusive bodies. Most volcanoes in the Tharsis region are volcanotectonic features; they have been associated with large scale tectonic and volcanic processes. They occur along complex systems of faults and grabens having a dominant northwest to southwest trend closely coincident with a great circle, which extends along 90 deg of arc from Tempe Patera to probable volcanic mountains near lat. 40 deg S, long. 150 deg. Deep seated intrusive bodies are also concentrated in the Tharsis region and are recognized mostly where faults have been deflected around their cores. The Elysium Mons-Amphitrites Patera volcanic alignment is subparallel to that of Tharsis but is longer, extending through about 120 deg of arc; it transects the dichotomy boundary and is radial to the Hellas basin. Volcanoes in the Tharsis region have the widest age range of all volcanoes on Mars, as determined by the size-frequency distribution of their craters having diameters of 2, 5, and 16 km.

Scott, David H.↗
NASA NTRS1991

Valles Marineris, Mars: Are pit chains formed by erosion and troughs by tectonism?

The origin of the Valles Marineris remains controversial. Erosional, tectonic, and hybrid processes have been proposed. To clarify these contradictions, the widths and depths of pit chains and troughs were compared and it was found that the features do not form a continuum. Rather, results are consistent with the hypothesis that pit chains formed by surficial collapse and troughs by deeper seated and coherent failure. All pit chains and linear depressions in the Valles Marineris region were classified by inspection into six morphologic categories: pit chains (linear arrays of small pits); floored chains (arrays of pit chains having flat or hummocky floors); scalloped troughs (wider linear depressions with scalloped wall segments); narrow troughs (depressions of intermediate width with straight wall segments); wide troughs (broad, linear depressions); and chaotic troughs (more irregular depressions displaying some channel morphologies). Topographic profiles were drawn across the classified depressions and the erosional width was determined between plateau margins along with the depth from the surrounding plateau level to the deepest part.

Lucchitta, Baerbel K.↗
NASA NTRS1991

Tethys geology and tectonics revisited

Tethys, a medium sized icy satellite of Saturn, was imaged by both Voyager 1 and 2 spacecraft at sufficiently high resolution to allow some geologic analysis. One fairly complete and several brief descriptions of Tethys' geology have been given. Partial results are given herein of a new analysis of Tethys' geology done as part of a comparative tectonic and cryovolcanic study of the saturnian satellites. A new geologic sketch map of Tethys' north polar area is given. This map is based on a sequence of images transformed to a polar stereographic projection at the same scale. The images present the same area under different illuminations, each of which brings out different features. A new global map is in progress.

Croft, Steven K.↗
NASA NTRS1992

Models of convection-driven tectonic plates - A comparison of methods and results

Recent numerical studies of convection in the earth's mantle have included various features of plate tectonics. This paper describes three methods of modeling plates: through material properties, through force balance, and through a thin power-law sheet approximation. The results obtained are compared using each method on a series of simple calculations. From these results, scaling relations between the different parameterizations are developed. While each method produces different degrees of deformation within the surface plate, the surface heat flux and average plate velocity agree to within a few percent. The main results are not dependent upon the plate modeling method and herefore are representative of the physical system modeled.

King, Scott D.↗
NASA NTRS1992

Western Aphrodite Terra, tectonics, geology, and line-of-sight gravity

Aphrodite Terra is the largest area of high-standing topography on Venus, and isostatic considerations strongly suggest that this high topography is supported at least in part by thickened crust. Previous studies of line-of-sight gravity data from the Pioneer Venus Orbiter indicate rapidly changing apparent depths of compensation across Aphrodite Terra. Magellan imaging data provide the first detailed images of this region, and we are mapping the region along Pioneer Venus orbit 440 to investigate whether the changing apparent depths of compensation correlate with changes in surficial tectonics. Preliminary mapping of geological features on Magellan images along the path of Pioneer Venus orbit 440 do not indicate a first-order correlation among surface features and changes in the apparent depth of compensation of line-of-sight gravity data. The apparent depth of compensation appears to be most variable in regions dominated by tessera, but not all areas of tessera have distinct gravity signatures. There is a weak correlation among areas in which impact craters are relatively common and areas in which the observed and predicted gravity anomalies are poorly correlated.

Hays, John E.↗
NASA NTRS1992

Structural characteristics and tectonics of northeastern Tellus Regio and Meni Tessera

The Tellus Regio-Meni Tessera region is an interesting highland area characterized by large areas of complex ridged terrain or tessera terrain. The area was previously studied from the Venera 15/16 data, typical characteristics of complex tessera terrain of Tellus Regio were analyzed, and a formation mechanism was proposed. Apparent depths of compensation of approximately 30-50 km were calculated from Pioneer Venus gravity and topography data. These values indicate predominant Airy compensation for the area. Regional stresses and lithospheric structures were defined from analysis of surface structures, topography, and gravity data. In this work we concentrate on northeastern Tellus Regio and Meni Tessera, which are situated north and west of Tellus Regio. Structural features and relationships are analyzed in order to interpret tectonic history of the area. Study area was divided into three subareas: northeastern Tellus Regio, Meni Tessera, and the deformed plain between them.

Toermaenen, T.↗