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Keller, G. R.

Publications and source records attributed to Keller, G. R..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Crustal structure of the Southern Rio Grande rift determined from seismic refraction profiling

As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south-central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area, and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho (32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province, and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province.

Sinno, Y. A.↗
NASA NTRS1986

Structure of the southern Rio Grande rift from gravity interpretation

Regional Bouguer gravity anomalies in southern New Mexico have been analyzed by two-dimensional wave number filtering and poly-nomial trend surface analysis of the observed gravity field. A prominent, regional oval-shaped positive gravity anomaly was found to be associated with the southern Rio Grande rift. Computer modeling of three regional gravity profiles suggests that this anomaly is due to crustal thinning beneath the southern Rio Grande rift. These models indicate a 25 to 26-km minimum crustal thickness within the rift and suggest that the rift is underlain by a broad zone of anomalously low-density upper mantle. The southern terminus of the anomalous zone is approximately 50 km southwest of El Paso, Texas. A thinning of the rifted crust of 2-3 km relative to the adjacent Basin and Range province indicates an extension of about 9 percent during the formation of the modern southern Rio Grande rift. This extension estimate is consistent with estimates from other data sources. The crustal thinning and anomalous mantle is thought to result from magmatic activity related to surface volcanism and high heat flow in this area.

Daggett, P. H.↗
NASA NTRS1984

Application of Magsat lithospheric modeling in South America. Part 1: Processing and interpretation of magnetic and gravity anomaly data

Scalar magnetic anomaly data from MAGSAT, reduced to vertical polarization and long wavelength pass filtered free air gravity anomaly data of South America and the Caribbean are compared to major crustal features. The continental shields generally are more magnetic than adjacent basins, oceans and orogenic belts. In contrast, the major aulacogens are characterized by negative anomalies. Spherical earth magnetic modeling of the Amazon River and Takatu aulacogens in northeastern South America indicates a less magnetic crust associated with the aulacogens. Spherical earth modeling of both positive gravity and negative magnetic anomalies observed over the Mississippi Embayment indicate the presence of a nonmagnetic zone of high density material within the lower crust associated with the aulacogen. The MAGSAT scalar magnetic anomaly data and available free air gravity anomalies over Euro-Africa indicate several similar relationships.

Hinze, W. J.↗
NASA NTRS1984

Application of MAGSAT to Lithospheric Modeling in South America. Part 2: Synthesis of Geologic and Seismic Data for Development of Integrated Crustal Models

Research activities performed on MAGSAT scalar data over South America, Central America, and the adjacent marine areas are summarized. The geologic utility of magnetic anomalies detected by satellite is demonstrated by focusing on the spherical-Earth interpretation of scalar MAGSAT data in combination with ancillary geological and geophysical data to obtain lithospheric models for these regions related to their contemporary crustal dynamics processes, geologic history, current volcanism seismicity and natural resources.

Keller, G. R.↗
NASA NTRS1984

Correlation of Tectonic Provinces of South America and the Caribbean Region with MAGSAT Anomalies

Intensities of MAGSAT scalar magnetic anomaly data correlate with the main tectonic provinces of South America and the Caribbean region. Magnetic anomalies of the continents generally have higher amplitudes than oceanic anomalies. This is particularly evident in Central America and in the shield areas of South America. The Caribbean Sea and Gulf of Mexico are underlain by prominent magnetic minima. Within these oceanic areas, linear magnetic highs correlate with topographic ridges which separate the Gulf of Mexico, the Colombian Basin, and the Venezuelan Basin. The boundaries of the Caribbean plate occur along magnetic gradients which are particularly sharp along the northern and western margins of the plate, but gradational along the southern margin where they merge with the Andean Cordillera. The anomalies along the western margin of the South American plate are also distinct and appear to be separate from those of the adjacent ocean basin. Eastern South America is characterized by magnetic anomalies which commonly extend into the Atlantic Ocean.

Lidiak, E. G.↗
NASA NTRS1984

Satellite Elevation Magnetic and Gravity Models of Major South American Plate Tectonic Features

Some MAGSAT scalar and vector magnetic anomaly data together with regional gravity anomaly data are being used to investigate the regional tectonic features of the South American Plate. An initial step in this analysis is three dimensional modeling of magnetic and gravity anomalies of major structures such as the Andean subduction zone and the Amazon River Aulacogen at satellite elevations over an appropriate range of physical properties using Gaus-Legendre quadrature integration method. In addition, one degree average free-air gravity anomalies of South America and adjacent marine areas are projected to satellite elevations assuming a spherical Earth and available MAGSAT data are processed to obtain compatible data sets for correlation. Correlation of these data sets is enhanced by reduction of the MAGSAT data to radial polarization because of the profound effect of the variation of the magnetic inclination over South America.

Vonfrese, R. R. B.↗
NASA NTRS1984

Relation of MAGSAT Anomalies to the Main Tectonic Provinces of South America

Comparison of MAGSAT scalar magnetic anomaly data to the main tectonic provinces and boundaries of South America reveals a number of geologic correlations. South America is divisible into a broad platform of Precambrian shields and cratons separated by Phanerozoic basins, grabens and aulacogens to the east, the Phanerozoic Patagonian Platform to the south, and the Mesozoic to Cenozoic Andean Fold Belt and Caribbean Mountain System to the west and north. The continental shields are mainly more magnetic then continental basins and orogenic belts. Cratons, mainly covered by younger sedimentary rocks, are generally associated with magnetic gradients. Most of the anomalies associated with the Patagonian Platform are positive and have higher amplitudes eastward away from the Andean Fold Belt. The northern Andes are coincident with positive magnetic anomalies, whereas the central and southern Andes are associated mainly with negative anomalies.

Lidiak, E. G.↗
NASA NTRS1983

Seismic properties of the crust and uppermost mantle of North America

Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies.

Braile, L. W.↗
NASA NTRS1982

Application of MAGSAT to lithospheric modeling in South America

Progress continues on all aspects of the project. The prime emphasis is on the Rayleigh wave study and determination of both group and phase velocity dispersion is almost complete. Existing data sets were prepared for inversion. A paper on the relation of MAGSAT anomalies to the main tectonic provinces of South America was delivered at the annual meeting of the Society of Exploration Geophysicists.

Keller, G. R.↗
NASA NTRS1981

The crustal structure and tectonics of South America

A preliminary crustal structure map and a bibliography of crustal and upper mantle studies for South America are given. Work on the surface wave analysis and tectonic characterizations continues.

Keller, G. R.↗
NASA NTRS1981

Characterization of the structure and tectonic of South America

Geologic studies of the South American plate were undertaken. The Guayana shield is reasonably well studied, and although data are sparce, the central Brazilian shield appears similar. Both the Amazon and Parnaiba basins seem to be related to an aulocogen structure. The collection of crustal structure information and the generation of measurement of surface wave dispersion in the shield areascontinues. Long period seismograms are digitized and analyzed. Exisiting crusted and upper mantle studies were indexed. Both MAGSAT scaler and vector magnetic anomaly data were used with regional gravity anomaly data to investigate the regional tectonic features of the South American plate.

Keller, G. R.↗
NASA NTRS1980

Evaluation of existing knowledge of the tectonic history and lithospheric structure of South America

While data is available on the lithospheric and crustal structure of the Andes region of South America, there is limited knowledge of these aspects of the eastern portion of the continent. For this reason, a surface wave dispersion study of the area was initiated. Long period seismograms were obtained for a tripartite analysis of dispersion. A flow chart of the analysis to be conducted is presented along with a preliminary geologic/tectonic map that was prepared. Efforts to characterize the provinces identified in terms of their geological and geophysical parameters continue.

Keller, G. R.↗
NASA NTRS1979

Crustal structure, geophysical models and contemporary tectonism of the Colorado Plateau

A regional analysis of the crust and upper mantle of the Colorado Plateau is presented, using existing geophysical and geological data combined with new surface wave dispersion and groundwater geothermometry data; the tectonic implications of these models are also investigated. Surface wave and seismic refraction data indicate that the crust of the interior of the Colorado Plateau is 44 + or - 3 km thick, and its crustal structure is typical of stable continental areas. Pn velocities, however, appear to be lower (7.8 km/s) than would be expected in a stable region, while silica geothermometry indicates that the average heat flow for the plateau is 55 mW per sq m (1.3 HFU).

Keller, G. R.↗