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At least 1,549 records · Page 86
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NASA NTRS1988

Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate - Application to the 95.5 deg W Galapagos propagator

Possible rigid plate models and shear models for the formation of curved seafloor lineaments by rift propagation are investigated and are applied to the Galapagos propagation rift system at 95 deg W. It is shown that the geometry noted at the Galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years. It is postulated that the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate. The data interpreted as requiring a shear zone are found to be equally consistent with two distinct models based on rigid plate tectonics.

Acton, Gary↗
NASA NTRS1988

Somali Basin, Chain Ridge, and origin of the Northern Somali Basin gravity and geoid low

Geophysical data are used to investigate the origin of the Northern Somali Basin and its relationship to surrounding tectonic elements. The results show the Northern Somali Basin to be the third of a series of oceanic basins separated by long transform faults created during movement between East and West Gondwanaland. The flexure resulting from differential subsidence across Chain Ridge along with the difference in lithospheric thermal structure on either side of it can account for the amplitude and shape of the observed geoid step and gravity anomalies across Chain Rige. It is suggested that the geoid and gravity low over the Northern Somali Basin may result from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly.

Cochran, James R.↗
NASA NTRS1988

Geodesy by radio interferometry - Determination of vector motions for sites in the western United States

The use of VBLI to study plate tectonic motion, plate boundary deformation, earth dynamics, and the crustal motions in the western U.S. is discussed. Results from dual frequency (S and X band) mobile VLBI experiments between October 1982 and February 1987 are presented. It is suggested that significant northwest motion is occurring well east of the San Andreas fault.

Gordon, David↗
NASA NTRS1988

Planetary environments and the conditions of life

Geophysical models of the first 600 Ma ofthe earth's history following accretion and core formation point to a period of great environmental disequilibrium. In such an environment, the passage of energy from the earth's interior and from the sun through gas-liquid-solid domains and their boundaries with each other generated a dynamically interacting, complex hierarchy of self-organized structures ranging from bubbles at the sea-air interface to tectonic plates. The ability of a planet to produce such a hierarchy is speculated to be a prerequisite to the origin and sustenance of life. The application of this criterion to Mars argues against the origin of Martian life.

Chang, S.↗
NASA NTRS1989

Global positioning system network analysis with phase ambiguity resolution applied to crustal deformation studies in California

An efficient algorithm is developed for multisession adjustment of GPS data with simultaneous orbit determination and ambiguity resolution. Application of the algorithm to the analysis of data from a five-year campaign in progress in southern and central California to monitor tectonic motions using observations by GPS satellites, demonstrates improvements in estimates of station position and satellite orbits when the phase ambiguities are resolved. Most of the phase ambiguities in the GPS network were resolved, particularly for all the baselines of geophysical interest in California.

Dong, Da-Nan↗
NASA NTRS1989

Fortuna Tessera, Venus - Evidence of horizontal convergence and crustal thickening

Structural and tectonic patterns mapped in Fortuna Tessera are interpreted to reflect a change in the style and intensity of deformation from east to west, beginning with simple tessera terrain at relatively low topographic elevations in the east and progressing through increasingly complex deformation patterns and higher topography to Maxwell Montes in the West. These morphologic and topographic patterns are consistent with east-to-west convergence and compression and the increasing elevations are interpreted to be due to crustal thickening processes associated with the convergent deformational environment. Using an Airy isostatic model, crustal thicknesses of approximately 35 km for the initial tessera terrain, and crustal thicknesses of over 100 km for the Maxwell Montes region are predicted. Detailed mapping with Magellan data will permit the deconvolution of individual components and structures in this terrain.

Vorder Bruegge, R. W.↗
NASA NTRS1989

A continuum model of continental deformation above subduction zones - Application to the Andes and the Aegean

A continuum model of continental deformation above subduction zones was developed that combines the viscous sheet and the corner flow models; the continental lithosphere is described by a two-dimensional sheet model that considers basal drag resulting from the viscous asthenosphere flow underneath, and a corner flow model with a deforming overlying plate and a rigid subducting plate is used to calculate the shear traction that acts on the base of the lithosphere above a subduction zone. The continuum model is applied to the Andes and the Aegean deformations, which represent, respectively, compressional and extensional tectonic environments above subduction zones. The models predict that, in a compressional environment, a broad region of uplifted topography will tend to develop above a more steeply dippping slab, rather than above a shallower slab, in agreement with observations in the various segments of the central Andes. For an extensional environment, the model predicts that a zone of compression can develop near the trench, and that extensional strain rate can increase with distance from the trench, as is observed in the Aegean.

Wdowinski, Shimon↗
NASA NTRS1989

Divergent plate boundary characteristics and crustal spreading in Aphrodite Terra, Venus - A test of some predictions

Predictions concerning the divergence and crustal spreading in the Western Aphrodite Region of Venus are tested using published results from Arecibo, Venera 15/16, and Pioneer Venus data. It is found that the northern middle to high latitudes are characterized by a young average age, that there is a trend in the total number of craters per unit area from high values in the north polar regions to low values toward the equator, and that there is evidence for a latitudinal distribution of tectonic features of different types, with extensional features common in equatorial regions and compressional deformation features common in the northern middle to high latitudes.

Head, James W.↗
NASA NTRS1990

Differential Radar Interferometry Maps Changes In Elevation

Differential radar interferometry uses data from synthetic-aperture radar (SAR). Three passes of SAR yield three amplitude-and-phase images, from which two interferograms (phase-difference images) made. Interferograms used to make third, "double-difference" interferogram indicating vertical motion of terrain between passes. Vertical earthquake motions as small as 1 cm detectable. Used to make extensive, accurate maps of such geophysical phenomena as heaving and buckling in fault zones, motions to tectonic plates, residual displacements from earthquakes, motions from prevolcanic swelling, motions of glaciers, tides, and thermal expansion of mountains from diurnal heating.

Gabriel, Andrew K.↗
NASA NTRS1989

Geology of Southern Guinevere Planitia, Venus, based on analyses of Goldstone radar data

The ensemble of 41 backscatter images of Venus acquired by the S Band (12.6 cm) Goldstone radar system covers approx. 35 million km and includes the equatorial portion of Guinevere Planitia, Navka Planitia, Heng-O Chasma, and Tinatin Planitia, and parts of Devana Chasma and Phoebe Regio. The images and associated altimetry data combine relatively high spatial resolution (1 to 10 km) with small incidence angles (less than 10 deg) for regions not covered by either Venera Orbiter or Arecibo radar data. Systematic analyses of the Goldstone data show that: (1) Volcanic plains dominate, including groups of small volcanic constructs, radar bright flows on a NW-SE arm of Phoebe Regio and on Ushas Mons and circular volcano-tectonic depressions; (2) Some of the regions imaged by Goldstone have high radar cross sections, including the flows on Ushas Mons and the NW-SE arm of Phoebe Regio, and several other unnamed hills, ridged terrains, and plains areas; (3) A 1000 km diameter multiringed structure is observed and appears to have a morphology not observed in Venera data (The northern section corresponds to Heng-O Chasma); (4) A 150 km wide, 2 km deep, 1400 km long rift valley with upturned flanks is located on the western flank of Phoebe Regio and extends into Devana Chasma; (5) A number of structures can be discerned in the Goldstone data, mainly trending NW-SE and NE-SW, directions similar to those discerned in Pioneer-Venus topography throughout the equatorial region; and (6) The abundance of circular and impact features is similar to the plains global average defined from Venera and Arecibo data, implying that the terrain imaged by Goldstone has typical crater retention ages, measured in hundreds of millions of years. The rate of resurfacing is less than or equal to 4 km/Ga.

Arvidson, R. E.↗
NASA NTRS1989

Divergent plate boundaries and crustal spreading on Venus: Evidence from Aphrodite Terra

The modes of lithospheric heat transfer and the tectonic styles may differ between Earth and Venus, depending on how the high surface temperature (700 K = 430 C), dense and opaque atmosphere (approx. 10 MPa = 100 bars), lack of water oceans, and the other known ways in which Venus differs from Earth, influence basic lithospheric processes, thermal gradient, upper mantle temperature, thermal and chemical evolution, and convection. A fundamental question is whether the lithosphere of Venus is horizontally stable, like the other terrestrial planets, or is mobile like that on Earth. The variety of characteristics, their integrated relationships, and their predictable behavior throughout Western Aphrodite Terra are similar to those features known to occur in association with the terrestrial seafloor at spreading centers and divergent plate boundaries. It is concluded that Western Aphrodite Terra represents the site of crustal spreading centers and divergent plate boundaries. The extent of similar characteristics and processes elsewhere on Venus outside of the 13,000 km long Western and Eastern Aphrodite Terra rise is unknown at the present, but their presence in other areas of the equatorial highlands, suggested from recent analysis, may be tested with forthcoming Magellan data.

Crumpler, L. S.↗
NASA NTRS1989

Evidence for volcanism in NW Ishtar Terra, Venus

Venera 15/16 radar data for an area in NW Ishtar Terra, Venus, show an area with moderate radar return and a smooth textured surface which embays low lying areas of the surrounding mountainous terrain. Although this unit may be an extension of the lava plains of Lakshmi Planum to the southeast, detailed study suggests a separate volcanic center in NW Ishtar Terra. Lakshmi Planum, on the Ishtar Terra highland, exhibits major volcanic and tectonic features. On the Venera radar image radar brightness is influenced by slope and roughness; radar-facing slopes (east-facing) and rough surfaces (approx. 8 cm average relief) are bright, while west-facing slopes and smooth surfaces are dark. A series of semi-circular features, apparently topographic depressions, do not conform in orientation to major structural trends in this region of NW Ishtar Terra. The large depression in NW Ishtar Terra is similar to the calderas of Colette and Sacajawea Paterae, as all three structures are large irregular depressions. NW Ishtar Terra appears to be the site of a volcanic center with a complex caldera structure, possibly more than one eruptive vent, and associated lobed flows at lower elevations. The morphologic similarity between this volcanic center and those of Colette and Sacajawea suggests that centralized eruptions have been the dominant form of volcanism in Ishtar. The location of this volcanic center at the intersection of two major compressional mountain belts and the large size of the calders (with an inferred larg/deep magma source) support a crustal thickening/melting rather than a hot-spot origin for these magmas.

Gaddis, L.↗
NASA NTRS1989

Stress distribution and topography of Tellus Regio, Venus

The Tellus Regio area of Venus represents a subset of a narrow latitude band where Pioneer Venus Orbiter (PVO) altimetry data, line-of-sight (LOS) gravity data, and Venera 15/16 radar images have all been obtained with good resolution. Tellus Regio also has a wide variety of surface morphologic features, elevations ranging up to 2.5 km, and a relatively low LOS gravity anomaly. This area was therefore chosen in order to examine the theoretical stress distributions resulting from various models of compensation of the observed topography. These surface stress distributions are then compared with the surface morphology revealed in the Venera 15/16 radar images. Conclusions drawn from these comparisons will enable constraints to be put on various tectonic parameters relevant to Tellus Regio. The stress distribution is calculated as a function of the topography, the equipotential anomaly, and the assumed model parameters. The topography data is obtained from the PVO altimetry. The equipotential anomaly is estimated from the PVO LOS gravity data. The PVO LOS gravity represents the spacecraft accelerations due to mass anomalies within the planet. These accelerations are measured at various altitudes and angles to the local vertical and therefore do not lend themselves to a straightforward conversion. A minimum variance estimator of the LOS gravity data is calculated, taking into account the various spacecraft altitudes and LOS angles and using the measured PVO topography as an a priori constraint. This results in an estimated equivalent surface mass distribution, from which the equipotential anomaly is determined.

Williams, David R.↗
NASA NTRS1989

Summaries of the thematic conferences on remote sensing for exploration geology

The Thematic Conference series was initiated to address the need for concentrated discussion of particular remote sensing applications. The program is primarily concerned with the application of remote sensing to mineral and hydrocarbon exploration, with special emphasis on data integration, methodologies, and practical solutions for geologists. Some fifty invited papers are scheduled for eleven plenary sessions, formulated to address such important topics as basement tectonics and their surface expressions, spectral geology, applications for hydrocarbon exploration, and radar applications and future systems. Other invited presentations will discuss geobotanical remote sensing, mineral exploration, engineering and environmental applications, advanced image processing, and integration and mapping.

Source record↗
NASA NTRS1989

Geodynamics and temporal variations in the gravity field

Just as the Earth's surface deforms tectonically, so too does the gravity field evolve with time. Now that precise geodesy is yielding observations of these deformations it is important that concomitant, temporal changes in the gravity field be monitored. Although these temporal changes are minute they are observable: changes in the J2 component of the gravity field were inferred from satellite (LAGEOS) tracking data; changes in other components of the gravity field would likely be detected by Geopotential Research Mission (GRM), a proposed but unapproved NASA gravity field mission. Satellite gradiometers were also proposed for high-precision gravity field mapping. Using simple models of geodynamic processes such as viscous postglacial rebound of the solid Earth, great subduction zone earthquakes and seasonal glacial mass fluctuations, we predict temporal changes in gravity gradients at spacecraft altitudes. It was found that these proposed gravity gradient satellite missions should have sensitivities equal to or better than 10(exp -4) E in order to reliably detect these changes. It was also found that satellite altimetry yields little promise of useful detection of time variations in gravity.

Mcadoo, D. C.↗
NASA NTRS1990

The Pioneer Venus Orbiter: 11 years of data. A laboratory for atmospheres seminar talk

The Pioneer Venus Orbiter has been in operation since orbit insertion on December 4, 1978. For the past 11 years, it has been acquiring data in the salient features of the planet, its atmosphere, ionosphere, and interaction with the solar wind. A few of the results of this mission are summarized and their contribution to our general understanding of the planet Venus is discussed. Although Earth and Venus are often called twin planets, they are only superficially similar. Possessing no obvious evidence of plate tectonics, lacking water and an intrinsic magnetic field, and having a hot, dense carbon dioxide atmosphere with sulfuric acid clouds makes Venus a unique object of study by the Orbiter's instruments.

Kasprzak, W. T.↗
NASA NTRS1989

Strike-slip faulting of ridged plains near Valles Marineris, Mars

This paper identifies and documents several well-preserved examples of Martian strike-slip faults and examines their relationships to wrinkle-ridges. The strike-slip faulting predates or overlaps periods of wrinkle-ridge growth southeast of Valles Marineris, and some wrinkle ridges may have nucleated and grown as a result of strike-slip displacements along the echelon fault arrays. Lateral displacements of several km inferred along these arrays may be related to tectonism in Tharsis.

Schultz, Richard A.↗
NASA NTRS1989

Constraints on the deep structure and dynamic processes beneath the Alps and adjacent regions from an analysis of gravity anomalies

Gravity anomalies over the Alps and the Molasse Basin are examined, focusing on the relationship between the anomalies and the tectonic processes beneath the region. Bouguer gravity anomalies measured in France, Germany, Italy, and Switzerland are analyzed. No large isostatic anomalies are observed over the Alps and an elastic model is unable to account for gravity anomalies over the Molasse Basin. These results suggest that the dynamic processes that flexed the European plate down, forming the Molasse Basin and building the Alpine chain, have waned. It is proposed that the late Cenozoic uplift of the region may be due to a diminution or termination of downwelling of mantle material.

Lyon-Caen, Helene↗