Global gravity maps and the structure of the Earth
Published:January 01, 1985
It is now possible to view global maps depicting long-wavelength and short-wavelength features of the Earth's gravity potential field, both as geoid anomalies and as free-air anomalies. This progress has resulted from (1) an increased number of surface gravity measurements, (2) orbiting of artificial satellites, (3) instal-lation of radar altimeters in three spacecraft, and (4) improved computing and display capabilities. The ratio of gravity to geoid anomalies (g/N) has been utilized to suggest a decomposition of the Earth's gravity field. The g/N ratio for each individual spherical harmonic degree is independent of the harmonic coefficient values, being determined only by the degree and values for normal gravity and the Earth's radius. This g/W-ratio value has an associated point-mass depth that is more limiting of source depth than that provided by wavelength considerations. Mass anomalies that may exist in the Earth at great depth will be best represented in the coefficients of low-degree spherical harmonics, not only because of their longer wavelength anomalies at the surface but also to provide a better match of the resulting ratio of gravity to geoid at the center of the anomaly feature.
Shallow (less than 600-km-deep), broad masses that might be considered possible sources of the degree 2* and 3* Sri Lanka geoid low and the New Guinea geoid high are considered geologically unlikely, whereas 1 to 2 km of relief on the core-mantle boundary is plausible and can explain the combined degree 2 and 3 geoid and gravity anomalies. The core-mantle boundary may become warped as a result of stresses developed by hydrodynamic motions in the outer part of the core, possibly related to those that produce the Earth's magnetic field. Combined contributions from harmonic degrees 4 through 10 identify a narrow positive mass anomaly beneath convergent-plate zones. These positive anomalies are not due to the anomaly of a downgoing slab because they are much broader and have larger magnitude. The fact that association of this positive mass anomaly with convergent-plate zones is more sharply identified in the selected packet of harmonic coefficients than in the full field is a strong indication that the preliminary decomposition is proceeding in the correct direction.
Cumulative geoid degree contribution curves for the low harmonic terms of the gravity field of Venus have a different pattern than that for the Earth. Maps of the low harmonic coefficients for Venus indicate correspondence with surface topographic features in marked contrast to the Earth. Venus rotates very slowly and has no internal magnetic field. Perhaps coherent motion within the core is negligible, so that there is no magnetic field and no distortion of the core-mantle boundary. These relations support the conclusion that long-wavelength undulations at the Earth's core-mantle boundary may contribute significantly to the lowest degree harmonic coefficients of the Earth's potential field.
Figures & Tables
The Utility of Regional Gravity and Magnetic Anomaly Maps
The first composite magnetic-anomaly map of the conterminous United States and adjacent offshore areas has been published at a color-contour interval of 200 gammas and at the scale and projection of other national geologic and geophysical maps for easy comparison. This map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in providing a regional framework for the interpretation of magnetic studies of limited areas, in selecting areas for more detailed magnetic investigations, and in studying the distribution and character of regional geologic features.
The map has a wide variation of magnetic-anomaly patterns, trends, and types, thus reflecting the diversity of the geologic terranes of the United States. In general, the anomaly pattern east of the Cordillera in the craton and in the Appalachian Mountains consists of more and greater intensity anomalies. The muted nature of the anomalies of much of the Cordillera is a result of several factors but appears to be primarily related to a decreased crustal magnetization caused by an abnormally shallow Curie isotherm. The anomalies of the Appalachian Mountains and the Cordilleran system primarily reflect the major structural patterns of the orogens, but important exceptions occur, such as those associated with rocks underlying thrust sheets in the Appalachian Mountains and westerly-striking anomaly trends in the Cordillera, which are correlated with igneous intrusives, faults, and mineral deposits.
The buried southern and eastern edges of the Pre-cambrian craton are indicated by changes in the magnetic anomalies and their dominant trends. Within the central United States, numerous regional magnetic-anomaly provinces are observed that reflect the long, complex history of the Precambrian basement rocks of the craton. These provinces are transected by conspicuous, intense, long, generally linear anomalies that originate from mafic extrusive or shallow intrusive igneous bodies within failed rifts, such as the Midcontinent rift system, the Southern Oklahoma aulacogen, and the Reelfoot rift buried beneath the Mississippi embayment. These are only a few of the many interesting regional geologic features that are observed on the composite magnetic-anomaly map of the United States.