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Overview and Recommenciations Erosional processes driven by the oceans and atmosphere would eventually reduce the continents to sea level if the earth's surface were static. But the earth's surface is not static; tectonic processes, driven by the earth's internal heat, have continually produced and elevated the continental crust. These processes are manifested by earthquakes, volcanoes, uplifts, anc} depressions. The diverse and intricately distributed rocks ofthe continents yield most of our essential mineral resources. Knowledge of how, `~hen, and why mineral and energy resources are concentrated in the continental crust should enhance the efficiency of exploration for and management of such resources. The complicated architecture of continental finest is the protract of dynamic processes that may e`,nstit~te geological hazards. Earthquakes and ~oicanic, eruptions are obvious hazards. Lore subtle hazards (e.g.. contamination of the environment) may hare I`'n~ time constants grip to many thousands off years); they represent prime c`'n- cems in selecting stable sites for nuclear power plants and other large structures. in the disposal of radioactive waste, or in ensnaring the integrity of supplies of ~~ndergro~nd water. As we increase our demands on the earth and seek to in- crease `~r ability to predict or mitigate the instabilities we moist improve over knowledge ofthe basic tectonic framework and processes involved in the forrna- tion and modification of continental angst. Tectonics is a branch of the earth sciences dealing with the origin, en oblation. 3

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Oren;ietv and Recommendations structure. and internal relations of regional features ofthe earth's crust. Tectonics is closely related to geodynamics, which is that branch of the earth sciences that deals with the forces and processes of the earth s interior. Only during the past few decades has it been possible to study geodynamic forces and processes as a global system. The revolution in the earth sciences that began in the 1960's has provided a major new concept plate tectonics that has been successful in explaining dynamic relations among major features of the earth. This concept, developed largely from data gathered from oceanic areas during the past 20 years, has had greater success in explaining the tectonics of the oceanic crust than of the conti- nental crust. This comparative success was facilitated by the relative youth ofthe oceanic crust approximately 200 million years (m.y.~. The near-surface record- albeit incomplete~f the first 95 percent of earth history (approximately 4000 m.y.) resides solely in the continents. Stuclies of continental geology suggest that much of the continental crust was formed by complex transformation of oceanic crust into continental crust. Plate tectonics offers a framework within which many of the processes of more recent continental evolution can be understood; but as we deal with increasingly an- cient portions of the continents, it is increasingly difficult to test the applicability of the plate-tectonic concepts. Continental tectonics, however, must be explain- able as part of a global geodynamic system, whose component processes may have changed through geological time. 'Scum of the earth" is used as a derogatory teen, but it effectively describes the continents. They are like raBcs in the global dynamic system, built of low~ensity materials physically and chemically extracted from the earth's interior dunng its long history, and floating on a higher~ens~ty interior. Continents have accumu- lated by the aggregation of fragments of earlier continental crust or by the acl~li- tions of transformed oceanic crust, have refragmented, and have drifted apart once again. They are exposed to the atmosphere, weathered and eroded to form soils and sediments, and uplifted or depressed. Persistence of continents is long, and the processes acting on them are complex. Although the oceanic areas lend themselves well to a rather simple thenn~ model, a comparable mocle} has not yet been established for the continents. While we have appreciable knowledge about the surface of continents, we know little about their nature at dept~part~cul~rly the crystalline basement rocks. Major efforts and innovative techniques will be required to explore the continents and their margins in three dimensions. Because evidence indicates that differences between continents and oceans may extend to depths of hundreds of kilometers, we cannot limit our exploration to the crust alone. The resulting fundamental knowledge from the scientific exploration of the conti- nents may well result in a comprehensive synthesis that will rival and comple- ment the concept of plate tectonics. Although the challenge is scientific, we can anticipate practical consequences because the improved unclerstanding should enlarge and strengthen the framework in which we view resources, resource exploration, waste storage, and geological hazard prediction. The time scale for policy decisions that clepend on a geophysical, geological, and geochemical knowledge base is far shorterthan the time scale for developing that knowleclge base. A decision may be required in a few weeks, while the infonnation required to make it intelligently might require a decade to produce. The resultant cost of pursuing an objective from a base of ignorance is likely to be far greater than the cost of proceeding from a solid knowledge base. We have entered a period of an increasing number of important policy decisions that have a significant depenclency on understanding continental tectonics. It is important

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Otiert;ieu, arid Recommendations that ale proceed to develop this knowledge base of continental tectonics promptly and, to the extent possible, well in advance of the need to use it in policy clecisions. Four decades ago our societal demands on the earth were smaller than they are now. Two decades ago we (lid not have the framework to consider the proper Questions about the continents. A decade ago we could ask the oue.stionc hilt ~ . ~ . 1 1 1 1 ~, . _ _ _ many ot the tOOlS were unavallat~le. 1 here is nou, a con?:ergence oJ societal need for an understanding of continental structure and processes' and a developing scientific theory and available technology that will permit US to begin to under- stand the basic framework and processes tnt;olt;ed with the structure and origin of the continents. THE PLATE-TECTONIC FRAM EWORK The general concepts of plate tectonics are simple. The rigid outer shell of the earth is broken into a small number of large plates moving relative to each other. Oceanic plates are generated at and move away from ocean ridges where the plates are separating. The plates plunge into the mantle at subduction zones where plates converge. Along transformer faults such as the San Andreas Fault system in Califomia, relative horizontal motion occurs between adjacent plates. These three types of plate motion (separation, convergence, and lateral sliding) are marked by structural deformation and seismicity that trace a network of activity on the earth's surface. Most of the earthquakes, volcanoes, and deforrna- tion are concentrated along! this network of plate boundaries. The outer part of the earth behaves in a more right manner than does the interior. Nevertheless, the outer portion is subjected to various forces that can result in nonrigid defor- mation and motion. The rigid behavior extends from the surface to depths rang- ing from ~100 km beneath oceans and perhaps 2~250 km beneath continents. The rigid portion of the earth, the lithosphere, which includes the crust ant} the upper portion of the upper mantle, overlies a more ductile portion of the mantle, the asthenosphere, capable of adjusting for the effects of varying surface loads. The lithosphenc plates ride as passengers on the convecting asthenosphere. A large variety of contemporaneous activity is associated with plate bounda- ries, such as volcanism, deformation, seismicity, high heat flow, sedimentation, and ore deposition. Although plate motions have not yet been routinely mea- surecl, geodetic techniques may soon permit such measurements. However, the results of plate motions over long time periods have led to estimates of average rates and to workable models of mountain building, birth and destruction of oceans, distribution of faunal provinces, paleoclimate, evolution of the crust and mantle, and a great variety of other phenomena that reach into every field of earth and related sciences. The understanding of plate tectonics is dependent on a multidisciplinary approach to develop broad concepts further, to establish their limitations, and to identify geological phenomena that require supplementary or complementary models. The development of plate tectonics is mainly a result of the study of oceanic areas. The generation of oceanic lithosphere is a rapid geological process; we know of no crust Dooring the modern oceans older than about 200 million years. This relative youth together with a limited number of geological processes results in a system that lends itself well to simple physical modeling The conti- nents do not appear amenable to simple modeling, but there are clear correla- tions with the plate-tectonic model. For example, modem plate-boundary acti`- ity develops a characteristic suite of geological features, and similar features can - . ~ ~ ~ . . . . 5

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Overuieu, and Recommendations be recognized in the earlier geological record. Thus, from a scientific point of view, the existence of a geological framework makes a major effort to study the continents timely. The revolution in the earth sciences that resulted from the study of the ocean crust should] spread to the continents. THE CONTINENTS AND PLATE TECTONICS The concept of plate tectonics has had a unifying effect on the earth sciences. Field geologists. laboratory geochemists, structural geologists, geophysicists geodesists, stratigraphers. and other specialists can view their ideas and results in a common framework. In joining together they have produced results of greater significance than could the simple sum of their individual efforts. The papers in this book represent a sampling ofthe kind of information that has been accumulated to date about the continental crust, the tools that are available to improve our knowledge, and some of the challenges for the near future. These papers not only illustrate how plate tectonics can be applied to the study of the continents but also iclentify the uncertainties in the application of the plate- tectonic mode} to oicler parts of the continents ant} in deducing the nature of the continents at depth. The papers represent a broad range of disciplines. Advances come from many specialties; excellent communication among the disciplines must continue. Continents clearly are dynamic environments and are the accumulated prod- ucts of long-term dynamic processes. Studies of comparative planetology, made possible through recent space programs, are teaching us that planetary evolution does not involve simple, constant, and predictable processes. The processes may be expected to vary with planetary character and to change with time; only the physical laws are invariant. Because the continents contain most of the geo- logical record, it is to them that we must turn to detennine the variations in geological processes through time. Reliable scientific answers to societal problems related to the earth will de- pend on an improved unclerstanding of continental tectonics. The study of con- tinents should proceed from an understanding of modern plate-boundary processes to efforts to apply this understanding to the geological record! of conti- nents. By such a procedure, we can determine the extent to which modem plate-tectonic processes can explain the development of continents, where in space and when in time tectonic processes in an evolving earth were different from modern analogs, and which features of continents cannot readily be related to plate-boundary processes. Geological and geochemical examinations of rocks that form the underpin- Dings of continents leac! us to the conclusion that many of these origin. oceanic settings through plate-tectonic processes or earlier variants OCR for page 3
Oven;ie?~ and Recommendations demonstrates that no continental interior is safe against catastrophic rifting ancl a renewed cycle of continental drib. The task of understanding the continents requires the determination of evolu- tionary paKems of the dynamic systems that have led to the formation and _, _ _ _, _ _ _ . ~ ~~ ~~~ TV ~~~a.~v~~ ~~ modification of the continents This means (1) understanding plate-boundary systems as we know them today, so that they can be related to contemporaneous geological features and events; (2) projecting this understanding as far as is feasible into the past; (3) Staining the nature and causes of systems that have no Moslem analog; and (4) understanding intracontinental processes that are not explained by plate tectonics in its present form. The dynamic processes that lead to the formation and modification of conti- nental lithosphere involve additions to continents from oceanic and mantle sources, recycling of oceanic ant] perhaps some continental lithosphere, rework- ing and remobilization of continental lithosphere by dynamic systems, and the vertical transfer of material from the deeper mantle to the lithosphere or vice versa. All these processes lead to lateral and vertical inhomogeneities within the outer shell of the earth. The character of the inhomogeneities is mostly beyond our direct observation. We see only the surficial expression of processes and their consequences. No matter how "deeply', we are permitted to see into continental crust as a result of uplift and erosion, there is always more continental crust ant! lithosphere beneath us. Only by using geophysical techniques and direct sampling by drill- ing can we cleterrnine the nature ofthe continents at depth. The goal will be one of learning about, ant! understancling, the evolution of the continents in time and space, at the surface, and at depth. No single continental terrane contains com- plete geological and geophysical information on plate-boundary ant} non-plate- boundary systems, both modern and ancient. The older the rock, the longer it has been subjected to processes that can destroy or bury it; as a result, accessible exposures of older rocks are correspondingly scarce. Thus, as we search for the older geological recorcl, intemationa] cooperation becomes especially important because the relatively rare, accessible occurrences of ancient rocks are widely distnbutecI. CURRENT CAPABILITIES Recent progress in four broad categories of technology (instrumentation, trans- portation, communication, and computation) has had a tremendous impact on advances within the various earth-science disciplines. The importance of tech- nology for the earth sciences is reviewed in detail in an earlier report of the Geophysics Study Committee, Impact of Technology on Geophysics. This report also reviews many examples of the recent innovations in the four broad cate- gories of technology and their application within the earth sciences. Among the most important are solid-state electronics and microminiaturization, which have profoundly influenced design of field! ant} laboratory instruments, transportation and communication systems, and computers. All four categories of technology are crucial to the use of earth-oriented remote-sensing satellite systems, which can provide valuable geological, geophysical, geodetic, and geochemical data. Computers permit a large expansion in utilization of diverse kinds of earth- science data' in automation of instruments' and in modeling. Many uses oftechnology in the earth sciences reflect adaptation from develop- ments outs~cle the earth sciences~specially materials and information sciences. 7

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O?;erv~eu~ anc] Recommendations There are also transfers of discipline-specific technologies among the earth- science disciplines. There have been important transfers of technologies from academic and government laboratories to industry and from industry to both academia and govemment. The adaptation of petroleum exploration techniques to study the deep structure of the continents is one example; the transfer of stable isotope and geochronology techniques from academia to industry is another. Future advances within the earth sciences that can contribute to a multidisci- plinary study of the continents depend on modern technological capabilities. Availability of up-to-date scientific equipment is a basic requirement for the achievement of an improved unclerstanding of the continents and a more effec- tive application of that knowledge. The current generation of U.S. earth scientists is not only stimulated by recent conceptual aclvances, but is also trained to utilize the most sophisticated tech- niques in exploring the parameters of space, time, temperature, pressure, and chemistry by which the continents must be characterized. There now exists in universities, government laboratories, and industry an increasing pool of diverse and able scientists capable of integrating observation and concept into an im- proved understanding of the continents. CONTINENTAL TECTONICS IN PROPOSED GEOSCIENCE PROGRAMS National and international plans and programs in earth science are calling for increased scientific attention to the continents. Almost all of them reflect in- creasecl societal concerns related to resources and natural hazards and concomi- tant demands on a basic unclerstanding of the behavior of continents. The U.S. Geodynamics Committee (USGC) has prepared a report, Geodynamics in the 1980's, that emphasizes crustal dynamics, particularly the dynamics ofthe continents as a framework for unclerstancling resource systems and natural haz- ards. The used prepared a more specific report, Continental Scientific Drilling Program, that is particularly relevant to understanding continental tectonics; the report recommends a national effort that focuses primarily on obtaining greater scientific return from the large amount of ongoing drilling for specific mission purposes in federal agencies, as well as a limited amount of Uniting for broader scientific purposes. The scientific topics in the drilling program are (1) basement structures in deep continental basins, (2) the rrnal regimes of the cmst, (3) pros cesses of mineral resource concentration, and (4) the unclerstancling of earth- quakes and faulting mechanisms. There are several reports of other committees in the National Research Coun- ci! that are related to continental tectonics, for example, Continental ,Uargins (Ocean Sciences Board), which deals with a critical region for understanding many continental tectonic processes, and Geodesy Trends and Prospects (Committee on Geodesy), which deals with the measurement of horizontal plate motions, vertical clisplacements, and gravity. Several other relevant reports ofthe Committee on Geodesy, Committee on Seismology, the U.S. National Commit- tee for Rock Mechanics, and the U.S. National Committee for Geochemistry are included in the bibliography. In response to societal demands, federal agencies are increasing their em- phasis on the structure, behavior, and evolution of the continents. The I-T.S. Geological Survey (USGS) held a workshop and prepared the report, Dynamics of the Continental Crust, to help coordinate existing activities within the USGS in continental tectonics and geodynamics to assist in providing assessments of the 8

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O?:ert;iew and Recommendations _ nation s mineral and energy resources and natural hazards. This workshop was largely motivated by the symposium on Continental Tectonics organized by the Geophysics Study Committee at the American Geophysical Union meeting in April 1978. The National Aeronautics and Space Administration's (NASA'S) in- creasing emphasis on the solid earth is indicated in various reports, especially Application of Space Technology to Crustal Dynamics and Earthquake Re- search. The National Oceanic and Atmospheric Administration (NOAA) iS increas- ing emphasis on crustal movements and gravity. The Department of Energy (DOE) has a developing program in geosciences and a diversity of programs that yield important data regarding the continents. Vanous components of the Department of Defense (DOD) are actively concerned with continental structures and behavior. The National Science Foundation (NSF) has long supported a wide range of research devoted to basic and applied problems in earth science. In recent years, the NSF has indicated the need for increased attention to the conti- nents, including the offshore continental margins. The Nuclear Regulatory Com- mission is sponsoring research dealing with basic questions of earthquakes, faulting pattems, ant! other aspects of earth sciences related to the disposal of radioactive wastes ant! the siting of nuclear power plants. The broad interest of federal agencies in geodynamics led to the fortnation by NONE NASA, USGS, NSF, ant] DOE of an Interagency Coordinating Committee for Application of Space Technology to Geodynamics. The challenge of understanding continental evolution has also led the Intema- tional Union of Geodesy and Geophysics ant! the Intemationa] Union of Geo- logical Sciences to design an international, interdisciplinary program in the solicI-earth sciences. This program is currently under development, but it seems clear that the central focus will be on the outer shell of the earth ant! that the dynamics of the continental crust will be an important element of the program. . .. . . . RECOMMENDATION To remove a major gap in man's understanding of his environment, and to provide an adequate scientific basis for geological hazard and waste-disposal evaluation andfor exploration, assessment, and appropriate utilization of earth resources, a broad multidisciplinary effort based on modern technology should be directed toward the exploration and understanding of the dynamics, struc- ture, evolution, and genesis of the continents. This effort should be a major component in programs for geodynamics in the 1980's. The following are four main targets of this effort: Three-dimensional structure of the continents. Our knowledge of the structure and composition of the continents as a function of depth is particularly limited Moreover, geological mapping' the basic too] in evaluating the surficial portions of the crust' is still incomplete for both the United States and the rest ofthe world. A complete aeromagnetic survey of the United States is necessary to explore the shallower depths (up to 1~20 km) of the crust. Satellite magnetometers show promise of being able to map the depth to the Curie temperature. There should also be a continuation of cletailed gravity surveys' reflection and refraction seismic profiling, and other geophysical investigations to explore the three-dimensional variations within the continental crust and mantle. Where feasible, these data should be corroborated with drill- hole clata. Additional inflation on the structure and composition of the litho- sphere and asthenosphere shouic! also be sought from petrological and geo- 9

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O?~er?;ietc and Recommendations chemical studies of magrnas and xenoliths Data from all these sources will allow a greater understanding of the continents and their processes. Timing of the et;olution of continental crust and associates! mantle. The times of initiation, the rates, and the durations of major tectonic pro- cesses are essential temporal parameters. They need to be known for the prep- aration of effective genetic models of earlier continental development for com- parison with our observations and models of contemporary plate tectonics. Understanding of the changing tectonic forces In relation to the driving energy with geological time can then be achieved. Nature and origin of the stress fields within the continents. The stress field within an area is related to its potential seismic risk and its physical state. Little is known about the stress fields near plate margins' where the majority of earthquakes and deformations occur. Even less is known about the intraplate stress fields and the less-frequent (although equally destructive) intraplate earthquakes. Relating observed strains to the existing stress field should result in methods and models for determining paleostress fields from paleostrain measurements. Knowledge of the nature and origin of the stress fields and their relationship to geological structures of all ages would enhance earthquake predictive capabilities. Thermal processes and thermal structure of the continents and underlying mantle. The importance of thennal processes and thermal structure has long been recognized as fundamental in connection with the evolution of the earth's crust and upper mantle. The driving energy for plate tectonics is heat; ore deposits are in many instances produced as a consequence of thermal processes, and many are indirect results of these; maturation of petroleum and upgrading of coal are functions of temperature; geothermal areas reflect concentrations of heat at depth. Nevertheless, the thermal charactenstics of the crust and upper mantle are poorly understood. Heat flow near the surface can be measured directly. All other determinations of thennal structure depend on indirect geophysical, geo- lo~ical. and Biochemical techniques. Through use of ~eochemical tools it is ~ ~ . ~ ~ . ~ ~ . possible to examine the thermal structure in the past in selected areas and to compare these data with present thermal conditions to gain insight into the e vo l ution o f the rmal proc e s se s . Integrated multidisciplinary studies will be necessary for effective investiga- tions of continental tectonics. Integration can and should be achieved through cooperative programs and through proper communication. These investigations should be aimed at a basic understanding of the processes of continental forrna- tion and modification. Anned with this un~derst:anding, the earth-science com- munity will be much better equipped to aid in the assessment and solution of current and unforeseen problems related to the solid earth. BIBLIOGRAPHY Orientations in Ceochem~stry, (J.S. National Committee for Geochemistry, National Research Coun- cil. National Academy of Sciences, Washington, D.C., 122 pp., 1973. U.S. Program for the Ceodynamics Project: Scope and Objectives, U.S. Geodynamics Committee Geophysics Research Board, National Research Council. National Academy of Sciences. ~vashing- ton, D.C.. 235 pp., 1973. The Geology of Continental Margins (C. A. Burk and C. L. Drake, eds.), Springer-Verlag, New York. 1009 pp. 1974. Ceodynamics: Progress and Prospects (C. L. Drake, ed.), American Geophysical Union, Washington. D.C. _38 pp.. 197~. 0

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Otuer?;ieu~ and Recommendations Predicting Earthquakes. Committee on Seismology National Research Council. ~'at~ona1 .~ademv ot Sciences, Washington, D.C., 62 pp.. 1976. The Earth's Cmst: Its-Vature and Physical Properties (J. G. Heacock, ea.) Geophysical .\lonograph 20, American Geophysical L'nion, Washington. O.C.. 7~4 pp.. 1977. Trends and Opportunities in Seismology, Committee on Seismology, National Research Council. National Academy of Sciences. Washington D.C., 158 pp.. 1977. Geodesy: Trends and Prospects, Committee on Geodesy. National Research Co~mcil. National .\cad- emy f Sciences, Washington D.C.. 36 pp.. 1978. Geophysical Predictions, Geophysics Sturdy Committee. Geophysics Research Board National Re- search Council. .N'ational Academy `>t Sciences. 'Washington. D.C. 91o pp. 19 ~ 8. Limitations of R'ck.VIechanics in Ener~,y-Resource Recovery and Development US. National Com- miKee [or Rock Mechanics .N'ational Research Council. National Academy of Sciences Washing- ton, D.C., 67 pp., 1978. Applications of a Dedicated Crat~itational Satellite .Uission. Committee on Geodesy, National Re- search Council. National Academy of Sciences, Washington, D.C., o3 pp., 1979. Application of Space Technology to Crustal Dynamics and Earthquake Research, .~'ASA Technics Paper 1464. National Aeronautics and Space Administration, Washington, D.C., 25`' pp., 1979. Cr~ntinental.~ar~ins. Panel on Continental Margins, Ocean Sciences Board National Research Ceil. National Academy of Sciences, Washington, D.C., 302 pp., 1979. Continental Scientific Drilling Program, IJ'.S. Geodynamics Committee, Geophysics Research Board. 'national Research Council. National Academy of Sciences, Washington, D.C., 192 pp. 1979. Dynamics of tht Contintnt(tl Crust: Propt'.s`tl.s fierce L'.S Geological Surrey Pro OCR for page 3