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A2. UNDERSTANDING THE NATURE AND EVOLUTION OF THE LITHOSPHERE FROM MAGNETIC ANOMALIES

Appendix A2 was largely developed by the following workshop group: Richard Blakely (Group Leader), J. Arkani-Hamed, J. Behrendt, S. Cande, V. Chandler, D. Chapin, W. Dewhurst, R. Frost, V. J. S. Grauch, S. Haggerty, R. Hansen, T. Hildenbrand, W. Hinze, P. Hood, J. MacQueen, R. Pawlowski, J. Phillips, C. Raymond, R. Reynolds, G. Smith, P. Taylor, P. Toft, P. Vogt, P. Wasilewski.

Scientific Framework

Few geophysical methods have had a greater impact on the geological sciences than the magnetic method. The Swedish mining compass was used successfully as early as the 1600s to search for iron ores, and dip needles were used until the middle part of this century. Although limited geological applications of ground-based magnetic surveying were realized prior to World War II, 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. Over oceanic areas, seafloor spreading anomalies revealed by airborne and shipborne surveys were critical for the development of the plate tectonic paradigm. Recent surveys continue to be the primary tool for estimating the age and relative movement of plates. The power of the magnetic method as a geological mapping tool has increased with time; high-resolution surveying in conjunction with modern processing and graphic routines continues to improve our knowledge of the oceanic and continental lithosphere.



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Page 100 A2. UNDERSTANDING THE NATURE AND EVOLUTION OF THE LITHOSPHERE FROM MAGNETIC ANOMALIES Appendix A2 was largely developed by the following workshop group: Richard Blakely (Group Leader), J. Arkani-Hamed, J. Behrendt, S. Cande, V. Chandler, D. Chapin, W. Dewhurst, R. Frost, V. J. S. Grauch, S. Haggerty, R. Hansen, T. Hildenbrand, W. Hinze, P. Hood, J. MacQueen, R. Pawlowski, J. Phillips, C. Raymond, R. Reynolds, G. Smith, P. Taylor, P. Toft, P. Vogt, P. Wasilewski. Scientific Framework Few geophysical methods have had a greater impact on the geological sciences than the magnetic method. The Swedish mining compass was used successfully as early as the 1600s to search for iron ores, and dip needles were used until the middle part of this century. Although limited geological applications of ground-based magnetic surveying were realized prior to World War II, 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. Over oceanic areas, seafloor spreading anomalies revealed by airborne and shipborne surveys were critical for the development of the plate tectonic paradigm. Recent surveys continue to be the primary tool for estimating the age and relative movement of plates. The power of the magnetic method as a geological mapping tool has increased with time; high-resolution surveying in conjunction with modern processing and graphic routines continues to improve our knowledge of the oceanic and continental lithosphere.

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Page 101 Magnetic surveys carried out on the ground and from ships, aircraft, and low-orbit satellites provide key information concerning the geological, tectonic, and thermal state of the Earth's lithosphere through measurements of magnetization, anomaly fabric, and depth to magnetic sources. While traditional research continues to refine our understanding of plate kinematic frameworks and provide regional subsurface structural and lithologic information, new research directions have emerged. Sources of anomalies are being evaluated at very broad (> 100-kilometer [km]) and very fine (< 10-meter [m]) scales as a direct consequence of the availability of satellite data, properly configured airborne, land, and ship surveys, and the large volume of data accumulated from certain areas during recent decades. 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. The large dynamic range of magnetization intensity in rocks (varying as much as 5 orders of magnitude) enables the detection of otherwise subtle variations in lithology, rock properties, and structure. The persistence of magnetization to great depth in the lithosphere makes the magnetic method useful for studying its deeper levels. Perhaps the most unique attribute of the magnetic method is that it can provide the added dimension of time to geophysical analyses. For example, the age of most of the oceanic crust is known from magnetic analyses. Societal Applications In addition to elucidating large-scale geological structures, magnetic anomaly studies can also delineate features associated with mineral or hydrocarbon accumulations; such features include igneous intrusions, fault zones, salt domes, and anticlines. Examples of prospective hydrocarbon

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Page 102 producing basins discovered by their magnetic expression include the Navarin Basin in Alaska and the North Sea basins. Magnetic anomaly maps in areas where the basement is concealed by sedimentary cover have stimulated and focused mineral exploration in many areas of the world. Crustal magnetization is sensitive to metamorphism and hydrothermal alteration; therefore, magnetic contrasts in the crust reflect variations in thermal and geochemical history that may be diagnostic for certain energy and mineral environments. National programs to evaluate natural hazards, to characterize environmentally contaminated areas, and to permit safe disposal of radioactive waste all benefit from magnetic anomaly studies. For example, the active New Madrid Seismic Zone lies within an Eocambrian graben of the Mississippi Embayment. The graben is completely concealed by younger sedimentary rocks and is identified primarily on the basis of magnetic anomalies. Similar magnetic anomaly analysis delineates concealed structures of the San Andreas fault zone, which represent major hazards to urban areas. Evaluation of candidate sites for critical hazards (for example, dams, nuclear reactors) is aided by the subsurface information revealed by magnetic anomaly maps. Changes in magnetic anomalies over historical time scales may indicate magmatic and/or tectonic activity. 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. Despite the obvious scientific and societal benefits of magnetic anomaly studies, the question of who will perform this work in the future remains unclear. The decrease in the ranks of entry-level researchers threatens to produce a critical shortage of trained personnel to perform basic and applied geomagnetic research. This trend appears to be correlated with the decrease in securely funded academic, federal, and industry-related positions. The strongest indicator of the impending shortage of geomagnetists is the decrease in number of graduate students specializing in the field. In order for society to continue to reap the many

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Page 103 and varied benefits that geomagnetism has to offer, this trend must be reversed. Specific Issues and Research Foci Magnetic Anomaly Studies Studies of Oceanic Lithosphere The magnetic source layer in ocean basins constitutes a continuous, high-fidelity record of geomagnetic field history and tectonic motion since the Jurassic. Greater understanding of the recording mechanism and its longevity is fundamental to extracting information on paleofield intensity, true polar wander, and the thermal and chemical evolution of the oceanic lithosphere. Knowledge of paleofield intensity will enable investigation of the relationship between the stability of the core field, generation of large mantle plumes, and climatic variations. 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. Intraplate deformation, defined by nonclosure of plate circuits, has been investigated by detailed aeromagnetic surveying with track spacings on the order of 20 km. Shipborne, satellite, and deep-tow studies are being used to define the systematic differences of magnetization with age in the oceanic crust and thus define characteristics of the recording process. Such differences may reflect variables of the source layer (for example, thickness or chemical composition), paleofield intensity, or processes operative during the evolution of the crust. At the finest scale, anomalies that are barely resolvable at the sea surface occur between major anomalies in seafloor spreading profiles (the so-called tiny wiggles); these tiny wiggles are finally being exploited for information concerning the shortest resolvable

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Page 104 time scales of core-generated geomagnetic field behavior. Magnetization-versus-age studies are complemented by detailed studies of along-strike variations in the magnetization of the ridge crest. Their aim is to resolve the cause of morphologically and geochemically defined segmentation of the ridge axis in relation to heterogeneity in magma composition and supply. Studies of Continental Lithosphere Magnetic surveying of continental regions during the past 40 years has yielded tremendous scientific and economic benefits. Magnetic studies illuminate the nature and location of terrane boundaries, orogenic 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. Deep drilling, crustal transect, and tectonic framework investigations draw heavily on magnetic data to extend geological mapping into the subsurface and to place detailed but sparsely distributed geological and other geophysical information into a regional context. Inversions of airborne and satellite data have been performed to estimate the depth and configuration of the Curie-temperature isotherm. This difficult inverse problem has the potential to advance understanding of the geothermal and tectonic setting of regions and to guide the systematic exploration of geothermal resources, but meaningful results will require an understanding of magnetic mineralogy at depth. In addition to constraining the modern-day tectonic framework of an area, analysis of lithospheric magnetic anomalies can lead to improved understanding of the historical record of tectonic activity. This historical perspective can provide insights into paleogeography and evolving terranes, thereby illuminating paleoclimatic conditions. The continental margin is a particularly fruitful area of investigation. The ocean-continent boundary frequently displays a distinct magnetic anomaly in airborne and shipborne surveys. Substantial controversy

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Page 105 exists, however, concerning its expression in satellite data. This fundamental lithospheric boundary deserves additional study. Global and Regional Studies Satellite magnetic anomaly data and large-scale airborne surveys are being utilized to investigate the regional magnetization of the Earth's lithosphere. These studies show spatial/temporal variations in the thickness, thermal gradients, and composition of the lithosphere. The Polar Orbiting Geomagnetic Observatory (POGO) and Magsat missions have mapped the Earth's magnetic field at a resolution sufficient to reveal previously unknown intermediate- to long-wavelength (400-to-4,000-km) lithospheric magnetic anomalies without complications from secular variation. Many of these anomalies were previously unknown because regional anomalies defined by aeromagnetic data suffer from biases inherent in patching together individual surveys. The most serious problem is our imprecise knowledge of the reference field. 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. Regional magnetic anomalies also can be studied with properly compiled airborne and shipborne data. Upper-crustal sources may produce anomalies in these low-altitude data that coalesce into regional anomalies when observed at satellite heights. Comparison of satellite and near-surface data is essential for proper interpretation of regional anomalies. Likewise, knowledge of the satellite anomalies is required to properly calibrate regional compilations of low-level data. Thus, magnetic studies at both satellite and near-surface heights are critical and will enable interpretation of geological and tectonic evolution of the lithosphere, especially in concert with global gravity and topography data sets. The Magnetic Anomaly Map of North America published in 1987 has proven invaluable for first-order regional interpretation of magnetic structure. Perhaps its greatest contribution has been the promotion of

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Page 106 interchange between geophysicists and geologists in addressing regional geological problems. Yet, in spite of its general application, the U.S. part of the map has many shortcomings caused by disparate survey specifications, nonuniform treatment of regional fields, and inadequate coverage in some regions. Studies of long-wavelength magnetic anomalies of the United States based on this map are difficult or impossible mainly due to problems in datum shifts between merged surveys. Now is clearly an appropriate time to evaluate this map quantitatively and determine how it might be improved with existing data or newly acquired surveys. In addition, the aeromagnetic data base should be extended offshore, at least to the limits of the Exclusive Economic Zone (320 kilometers). Analysis and Interpretation Techniques The ultimate goal of magnetic anomaly analysis is a better understanding of surface and subsurface geology, rock magnetization, plate kinematics, and paleomagnetic field behavior. Analysis begins with a compilation of magnetic anomaly data, usually in the form of maps or digital data bases. The data are then addressed in several ways that commonly interplay: analysis of the characteristics of the data, modeling of crustal magnetization, and interpretation of the magnetization in terms of geological units and structures. Compilation requires accurate knowledge of a geomagnetic reference field. Analysis and interpretation commonly require knowledge of rock magnetic properties, especially remanence, available geology, and data representation techniques. Many of these techniques for compilation, analysis, and interpretation present opportunities for improvement. Further development of interpretation and analysis techniques must proceed hand in hand with improvements in data quality. Factors reducing data quality include position uncertainties, external field contamination, and inaccurate geomagnetic reference fields. Increased utilization of the Global Positioning System (GPS) will greatly reduce navigational uncertainties. Survey strategies that employ parameters inappropriate for the relevant geological or geophysical problem also

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Page 107 introduce errors. In many areas of the United States and the world, surveys that are known to be inadequate provide the only available data. These areas should be resurveyed. Although standard techniques are available to accomplish this task, consideration should be given to applying new technology, such as gradiometry and vector magnetometers, to the problem. 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. For example, the standard digital representation of a magnetic anomaly map required by most analysis techniques is an equi-spaced grid. These grids are inadequate for several reasons: (1) information is discarded along densely sampled flight lines and must be interpolated between flight lines; (2) magnetic features are treated as though they are symmetrical; and (3) differences of elevation between points are not taken into account. Intelligent and efficient gridding techniques are needed that can accommodate asymmetric anomalies, unequally spaced intervals, differences in elevation, and a curved Earth. In addition, we should consider new ways to analyze data directly without gridding. New analysis techniques can take advantage of today's more powerful computers. Highly interactive, easily visualized three-dimensional modeling and inversion algorithms should flourish and become standard practice. Development of techniques that recognize patterns in multiple data sets and associate them with certain geological environments should continue. Previously developed methods that are computer- intensive, such as space-domain filtering and some spectral methods, may now be more useful because of improved computer technology. It is well recognized that synergistic analysis of multiple data sets provides important constraints on interpretation of subsurface geology. The synthesis of multiple data sets has generally been treated in qualitative ways. New computer technology, such as geographic information systems (GIS), can be utilized to handle multiple data sets. Future

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Page 108 developments should improve on GIS techniques by synthesizing information from all the data sets. Future developments in analysis techniques should focus on facilitating the modeling of magnetization distributions and the interpretation of these distributions in geological terms. These techniques will evolve toward greater automation. In order to be successful in this direction, a better understanding is needed of how magnetization is distributed in the lithosphere through a broad range of scales and how magnetization (in terms of rock-magnetic properties) is related to geological units and structures. Ultimately, presentation and interpretation of the magnetic anomaly data must have greater relevance to the broad field of geoscience. 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. The geological interpretation of magnetic anomalies that originate from contrasts in total magnetization depends greatly upon a fundamental understanding of the physical, chemical, and mineralogical factors that control rock magnetism. For example, the magnitudes of induced and remanent magnetization in rocks are largely determined by the abundance, composition, grain size and shape, and microstructure of ferromagnetic minerals; however, diamagnetic and paramagnetic phases may be important in certain settings where ferromagnetic phases are essentially absent. An outstanding problem is the extrapolation of data from laboratory measurements to the pressure-temperature environment of anomaly source regions. As described in the next four subsections, the

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Page 109 lithologic environments of the sources may be divided as follows: exposed and shallow crystalline rocks, exposed and shallow sedimentary rocks, intermediate-depth and deep magnetic sources, and oceanic sources. Exposed and Shallow Crystalline Rocks The petrological factors that control the magnetization of crystalline rocks at shallow depths in the crust are not well understood. In igneous rocks the factors that control the stability and composition of magnetic minerals may include bulk chemistry of the magma, temperatures and oxygen fugacities of formation, and cooling conditions. Factors that influence magnetic properties of metamorphic rocks include bulk chemistry inherited from the original rock, temperature of metamorphism, pressure and composition of attendant fluids, and fabric of the magnetic minerals. 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. Exposed and Shallow Sedimentary Rocks Magnetic contrasts within sedimentary rocks are important aeromagnetic targets; the interpretation of these contrasts must rely heavily on the application of rock magnetism, petrology, and geochemistry. Magnetic surveys over thick sedimentary sections have potential for detailed mapping of (1) faults that offset layers of different magnetization or that focus the flow of fluids; (2) structurally deformed beds having magnetization greater or less than that of enclosing beds; (3) lithologic contrasts associated with diapiric salt domes; and (4) zones affected by inorganic alteration or microbial production of iron oxide and iron sulfide minerals.

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Page 110 Intermediate-depth and Deep Magnetic Sources Long-wavelength magnetic anomalies from the lithosphere can be generally attributed to (1) variations in the thickness of zones of constant magnetization or (2) contrasting magnetizations that reflect different composition and evolution of the rocks. A combination of factors is likely, but the detailed physical and mineralogical basis for changes in deep-crustal and upper-mantle magnetization is not well understood. Indeed, even fundamental magnetic properties, including the Curie temperature, of deep-seated magnetic minerals are a matter of dispute. An especially challenging and perplexing problem is that magnetizations inferred from satellite and regional aeromagnetic anomalies are larger than most magnetizations actually measured in rocks. Better understanding of anomalies from lithospheric sources must come from experimental and theoretical research on equilibrium mineral assemblages and chemistry at high temperature and pressure. In addition, experimental studies of the effects of temperature and pressure on both induced and remanent magnetization of different rock types are needed. The magnetic petrology of rocks from intermediate and deep-crustal depths and the uppermost mantle can be studied in three ways: (1) investigation of xenoliths from kimberlites and from alkalic volcanic rocks; (2) measurement of rocks from exposed crustal sections; and (3) determination of the likelihood of occurrence of stable magnetic minerals in rocks of particular compositions by theoretical and experimental methods. 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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Page 111 tude and anomalous skewness in intermediate- and short-wavelength anomalies. Irregularities in the magnetic anomaly patterns can be attributed to many factors, including the geometry, composition, and cooling history of intrusive and extrusive rocks; tectonic evolution and setting; spreading rate; composition and temperature of hydrothermal fluids; geometry and duration of hydrothermal convection cells; and positions of the convection cells with respect to the spreading ridge or the accreting plate. Clearly, a better understanding of these influences is crucial for mapping and interpreting the geology of the oceanic crust. Such an understanding will come partly from rock magnetic and petrological studies of samples from different tectonic and hydrothermal settings and partly from the paleomagnetic study of unaltered rocks of similar age found in outcrop on land or in cores from the seafloor. A critical question concerns the magnetic mineralogy of the deeper magnetic layers and the associated depth to the Curie-temperature isotherm: where is the base of the magnetic oceanic lithosphere? Experimental work is needed on the magnitudes and stabilities of secondary remanent magnetizations (viscous and chemical remanences) acquired under the physical and chemical conditions of magnetic sources. Programmatic and Operational Considerations Several operational issues are critical to the improved acquisition and analysis of magnetic surveys. First, future aeromagnetic surveys should implement advancing magnetometer technologies (for example, gradiometers and vector magnetometers) and navigational systems (for example, GPS). Second, research on the configuration and placement of base-station magnetometers is needed to improve the ways by which the detrimental effects of external fields are removed. 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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Page 112 Increased collaboration among individual investigators measuring physical and chemical properties is also essential to the complete interpretation of magnetic anomalies. Laboratory investigations of magnetic and petrological properties of rocks and minerals are supported primarily by NSF. Additional support of laboratory and field studies is crucial to a systematic approach under the recommendations outlined below. While some of this work can be performed by individual investigators, no single investigator or institution now supports the necessary range of analytical and experimental facilities. Centralized support for advanced rock magnetic studies is provided by the Institute for Rock Magnetism (IRM, University of Minnesota). The importance of IRM is demonstrated by the research productivity of this laboratory and by the collaboration and communication that it fosters. What is needed, as an essential complement to the IRM, is a comparable facility dedicated to the mineralogical, petrological, and geochemical aspects of magnetic petrology. The issues listed above can be more successfully addressed through greater cooperation and increased collaboration among agencies and individual investigators. Indeed, agency cooperation is essential to the scientific objectives of the proposed initiative. As relevant circumstances change, the U.S. Navy should be encouraged to release classified data no longer critical to national security. NOAA and other agencies should promote the use of “ships of opportunity.” The USGS observatory program is a key element in understanding the temporal and spatial global magnetic field. Moreover, USGS should take the lead in coordinating statewide and local aeromagnetic surveys and marine magnetic surveys in order to promote a consistent national magnetic data base. NASA and other agencies (NOAA and USGS) should be committed to future low-altitude satellite missions specifically designed to study the magnetic lithosphere. International efforts, such as the ARISTOTELES and Magnetic Field Explorer (MFE) Magnolia missions, are particularly advantageous to the scientific community.

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Page 113 Conclusions and Recommendations The following represent the conclusions and recommendations of the Working Group on Lithospheric Magnetic Anomalies. The order of the recommendations does not reflect a priority ranking. New and Improved Regional and Global Data Sets An understanding of the lithosphere is required at a variety of scales covering the entire globe. Consequently, the first four recommendations have equal priority. The remaining recommendations in this subsection are crucial to implementing and complementing the first four. The launch of a low-altitude satellite to map the lithospheric magnetic field of the Earth, as well as to improve main field models, is needed. Orbital altitudes should be as low as practical in order to focus on lithospheric problems. The ARISTOTELES mission, currently being considered as a joint venture between NASA and the European Space Agency, or the alternate MFE Magnolia mission between NASA and the Centre National d'Etudes Spatiales (CNES) 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. External field contamination of low-altitude data will be severe. Therefore, a high degree of interaction between those working on lithospheric problems and those working on external fields is essential to promote innovative solutions to the problem. A second-generation digital magnetic anomaly map should be developed for the United States and its Exclusive Economic Zone (EEZ) (out to 320 kilometers offshore). To achieve this objective, a systematic survey strategy is required that employs state-of-the-art instrumentation (that is,

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Page 114 vector magnetometers with 0.01 nanotesla precision, gradiometers, and GPS navigation) and flightline spacings appropriate for the local geological setting. As a minimum goal, the quality of this second-generation map should be comparable to those of Canada and Zimbabwe, thereby permitting more sophisticated analysis techniques. High-resolution and high-sensitivity magnetic surveys over specific local areas are required to study a number of important Earth processes. Studies of sedimentary basins, midocean ridges, continental margins, volcanoes, faults, and continental rift zones are greatly improved with high-resolution and high-sensitivity magnetic surveys, especially when constrained by high-resolution bathymetry or topography, gravity and altimetry data, seismic reflection and refraction data, and direct sampling. Key topics of investigation include oceanic evolution, intraplate and interplate deformation, hydrothermal activity, and upper-crustal magnetization. Such surveys should be carried out by ships, deep-towed underwater vehicles, aircraft, and wheeled vehicles on land. Studies in remote regions, such as Antarctica and the southern oceans and arctic ice-covered areas, are needed. 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. Aeromagnetic surveys over ice-covered seas and reconnaissance ship surveys are helping to remedy this deficiency. However, additional work remains to be done before definitive tests of driving forces and the fixity of hot spots can be conducted. Additionally, such surveys will help to resolve the timing and character of rifting and basin development in the polar regions and structure and evolution of the antarctic lithosphere.

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Page 115 Support for stationary magnetometer installations in tectonically and magmatically active areas should continue. Innovative research on the relationship between catastrophic geological events and changes in the local magnetic field has critical societal implications. Increased cooperation between industry, academia, and government agencies is critical to future studies of the magnetic lithosphere. A highly successful example of such cooperation is the aeromagnetic survey of the East Coast continental margin, funded jointly by USGS and a private contractor, that took place in the mid 1970s. It is imperative that such data be released in a timely fashion, as it was in the example cited. Many foreign magnetic surveys result from joint government/industry funding and cooperation. Development and implementation of a marine mid-depth-tow magnetometer package would greatly increase the resolution of seafloor anomalies and enhance interpretation of high-resolution swath bathymetry surveys. Magnetic data held by the U.S. Navy should be released to the public if no longer critical to national security. These data would be a great resource in studies of paleofield intensity and chemical alteration of the oceanic crust. Alternatively, analysis could be carried out without compromising the classified status of the original data. 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. There is a need for the development of methods to determine time-varying magnetic fields from multiple or distant base magnetometers and seafloor observatories.

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Page 116 Better Ways to Interpret Data Improved interpretive and graphic methods of interpretation and presentation of magnetic data are needed to better relate magnetic anomalies to geology. Recently developed apparent susceptibility and terracing methods represent steps in this direction. New approaches should include ways to better visualize the data in terms of three-dimensional geological features. Improvements in data quality must proceed hand in hand with development in analysis and interpretation techniques. Improvements in data quality can be effected through better survey design, more accurate models of the Earth's main field, and proper compilation of existing data sets. 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. This type of data representation is inadequate for modern analysis, which demands high data resolution. Gridding as presently practiced must be reexamined and revamped in order to better honor the original resolution of irregularly sampled data. New ways to visualize and model magnetic anomaly data using interactive graphics are required. Modeling and inversion methods should accommodate the three-dimensionality of magnetic sources. Such development has been hindered in the past by computer limitations. Development in this direction can now progress using modern computer power and advanced programming techniques. Techniques to address data characterization and semiautomatic modeling need further development. The relevant computer-intensive methods, such as pattern recognition and neural network analysis, should follow the lines of artificial intelligence.

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Page 117 New techniques are needed to facilitate interpretation of gradiometry, vector magnetic data, and multiple data sets. Standard data sets are needed for comparing the results of analysis techniques. The data sets can include both synthetic and real-Earth data. The real-Earth data should come from a variety of geophysical and geological environments that are fairly well defined. Analysis techniques need wider distribution, and the users of the techniques need better reference materials and training in their 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. This objective may be approached by implementing the following recommendations. Collaboration among anomaly modelers, petrologists, and rock magnetists should be promoted. Models should be developed that describe the abundance and character of magnetic minerals in metamorphic, igneous, and sedimentary rocks. Magnetic and petrological studies of a wide range of rock types that reflect a variety of geological processes should be supported. Such processes include the following: hydrothermal alteration; metamorphic transitions; igneous differentiation and crystallization; and deposition, diagenesis, and alteration of sedimentary rocks.

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Page 118 Such studies should be relevant to and in the context of a full range of lithospheric magnetic anomalies. Rock magnetic experiments at high pressure and high temperature should be supported to improve the understanding of deep-seated lithospheric magnetization. Centers for the study of the mineralogical, petrological, and geochemical aspects of rock magnetism should be strengthened and new ones established.