The National Geomagnetic Initiative (1993) / Chapter Skim
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A2: Understanding the Nature and Evolution of the Lithosphere from Magnetic Anomolies
A2: Understanding the Nature and Evolution of the Lithosphere from Magnetic Anomolies
Pages 100-118
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From page 100... ...
the postwar development of the aeromagnetic method and the development of the marine-towed magnetometer have had the greatest impact on Earth science. Aeromagnetic data over continental areas, often in conjunction with gravity data, have helped in the preparation of many geological maps and have often provided the only economic means of investigating geology at depth.
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From page 101... ...
New insights into the character and depth of magnetic source regions have aided investigations of the mechanical and thermal structure of the lithosphere, crustal and oceanic accretion and evolution, true polar wander, the variation of field intensity with time, and the process of field reversals. Magnetic anomaly data combined with gravity, electrical conductivity, heat flow, and seismic reflection and refraction data provide interpretations that are superior to those based on only one type of data.
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From page 102... ...
Permanent monitoring of the magnetic field at volcanoes and fault zones could potentially reveal systematic behavior prior to catastrophic events and contribute to prediction efforts. Magnetic anomalies due to cultural sources, such as metal pipes and drums, can help to locate and characterize areas of contamination.
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From page 103... ...
Much work in the ocean basins currently focuses on defining the second-order plate kinematic framework and the character of the magnetic source layer. The response of the lithosphere to major plate reorganizations is recorded in structures, such as propagating rifts, microplates, and migrating transform faults, defined mainly by their magnetic signatures.
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From page 104... ...
Magnetic studies illuminate the nature and location of terrane boundaries, erogenic belts, continental rift zones, and sedimentary basins. High-resolution aeromagnetic surveys, such as the statewide survey recently completed by the Minnesota Geological Survey, provide extremely powerful tools for geological studies of continental lithosphere.
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From page 105... ...
Lack of anomaly resolution in existing satellite data and lack of information on the magnetic properties of the lower crust and upper mantle limit the interpretation of these satellite-derived regional magnetic anomalies. Unresolved external field contamination also degrades the data quality.
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From page 106... ...
Factors reducing data quality include position uncertainties, external field contamination, and inaccurate geomagnetic reference fields. Increased utilization of the Global Positioning System (GPS)
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From page 107... ...
Simultaneous acquisition of other airborne geophysical information, such as gravity, gamma ray, and electromagnetic data, has many advantages; future analysis techniques should strive to synthesize multiple data sets. Analysis and interpretation also suffer from inadequate representation of data during analysis.
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From page 108... ...
Rock Magnetism and Petrology There is a growing awareness among researchers of the need to link studies of lithospheric magnetic fields, rock magnetism, petrology, and geochemistry. When combined with petromagnetic understanding, magnetic surveys will yield the maximum information on key questions, such as lateral variations in magnetization, the character and location of hydrothermal alteration, the degree of metamorphism, the mineralogical constitution of the deep crust and upper mantle, and the chemical aging of oceanic crust.
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From page 109... ...
Although the general way that these factors affect rock magnetism is recognized, the sources of short-wavelength magnetic anomalies are commonly difficult to decipher. A greater and more detailed understanding of how factors influence rock magnetic properties is needed to help unravel the histories of complex igneous and metamorphic regions near the surface.
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From page 110... ...
Oceanic Sources It is well recognized that magnetic anomalies from the oceanic crust provide a record of seafloor spreading, yet the magnetization causing these anomalies remains incompletely understood. Many enigmatic magnetic features provide clues to the evolution of ocean crust, including the decrease in amplitude away from spreading ridges, the enhanced magnetization of the Cretaceous Quiet Zone, and discrepancies in ampli
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From page 111... ...
Third, improved main field models are critical to isolate lithospheric anomalies from the fields of other internal sources. Fourth, magnetic surveys result in large volumes of digital data that must be archived systematically.
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From page 112... ...
Centralized support for advanced rock magnetic studies is provided by the Institute for Rock Magnetism (IRM, University of Minnesota)
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From page 113... ...
of France, will provide a valuable data set for lithospheric magnetic studies. Moreover, the extended high-altitude phase of the mission will permit accurate main field and secular variation models to be derived, thus improving the resolution of the lithospheric field.
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From page 114... ...
Knowledge of global relative plate motions through time, derived from the first-order kinematic framework defined by magnetic anomalies in the ocean basins, forms the foundation of studies aimed at deducing the driving mechanism of plate motions. These studies are hindered by a lack of even first-order plate motion histories in remote areas of the southern oceans.
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From page 115... ...
Average values in 1° bins would be useful in some areas for main field models. · Studies of the effects of ionospheric currents and induced Earth currents are essential to magnetic studies of the lithospheric magnetic field.
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From page 116... ...
Traditional ways of representing the data during analysis need improvement and reexamination. Interpolating data sampled at irregular spacing onto equi-spaced grids or profiles remains a necessary and standard technique for representing magnetic anomaly data during analysis.
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From page 117... ...
use. Rock Magnetic and Petrologic Studies The objective of rock magnetism and petrology applied to magnetic anomalies is to explain the physical and chemical setting, evolution, and magnetic properties of the rocks responsible for the anomalies.
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From page 118... ...
· Centers for the study of the mineralogical, petrological, and geochemical aspects of rock magnetism should be strengthened and new ones established.
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