- Email: [email protected]
Deep seismic sounding of the continental crust and mantle — a long-term view
TECTONOPHYSICS ill
ELSEVIER
ii
Tectonophysics 286 (1998) I-4
Deep seismic sounding of the continental crust and mantle a long-term view Jack Oliver * Department of Geological Sciences and INSTOC, Cornell Universit3; Ithaca, NY 14853, USA
Received 10 February 1997; accepted 5 June 1997
Exciting progress in deep seismic sounding is being made, activity is taking place at many locales in many countries around the world, and numerous, talented, capable, people are involved. ! am going to skip over the near present and remark instead on some activities of the more distant past and try to suggest how the historical approach might be useful or helpful as we move toward, and plan strategically for, the long-term future. Perhaps there is a parallel with the way the subject of earthquake seismology developed so as to play an important role in the plate-tectonics revolution of the 1960s, and perhaps deep reflection profiling may provoke that kind of upheaval in its future. We never know, of course, but with a large part of the earth, the continental crust, waiting to be explored by this powerful technique, our situation resembles historical ones that have often produced major enlightenment. Back in the early 1970s when we were trying to get deep seismic reflection profiling of the continents underway through establishment of the COCORP project, we talked about the buried continental basement as the great, or one of the great, frontiers of modern earth science. We believed that characterization then, and we still do. We drew a parallel between the continental basement and the floor of the deepocean basins which, as history clearly shows, was surely the great frontier of the 40s, 50s and 60s. Dur*Fax: +1 (607) 254-4780: E-mail: [email protected]
ing those decades especially, the floors of the ocean basins were explored, essentially for the first time, by the techniques of seismic refraction and reflection. The resulting observations, along with other kinds of data, were important and sometimes critical in the plate-tectonics revolution, one of the major achievements ever of earth science. Let us look then at how seismic exploration of the ocean basins evolved during the 50s and 60s, at some of the triumphs and missteps along the way, and at some of the tactical and strategic decisions that led to those successes or failures, all the while keeping in mind that we may, or may not, be encountering parallel situations along the way as we explore our frontier, the continental crust and the uppermost mantle beneath it, now and in the future. Let me note that I will be focusing mostly on non-commercial, or academic, activity, but l will have something to say about the importance and relevance of industrial activity a little later. However, 1 will be referring here mostly to the deep-sea floor, not the shelves and margins where most commercial seismic was, and is, done. Let us begin with seismic refraction. The earliest results in this topic by the academic institutions were primitive alright, but they were also very significant and attention-getting. The first seismic refraction work in the deep sea was done in the 30s0 led by Maurice Ewing, but such studies only began to blossom in the post WW II years, the late 40s and the
0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. Pll $0040- 195 l (97)00250-3
2
J. Oliver/Tectonophysics 286 (1998) l-4
50s. Initially the experimental technique was very crude, perhaps utilizing only one or two hydrophones suspended from one receiving ship, certainly no array, and a pitifully small number of widely spaced shots, often of a size smaller than was really needed. The interpretations of the resulting observations were often murky to say the least. Nevertheless in this very primitive stage, they provided a surprising and very important result. Although at first only two layers were reported (one a 'liquid' layer that included the water and all ocean floor sediments, and the other a high speed layer with velocity approaching 8 kin/s), the data demonstrated that the mantle was very shallow beneath the deep sea and hence that no thick submerged or subsided continental crust was there, at least where the experiments were done. This result had important implications at a time when geologists were wrestling with concepts such as land bridges, drifting or subsiding continents, a moon ejected from the Pacific, etc. This crude initial result turned out to be more or less correct and, in conjunction with refraction work on land that showed that the mantle was much deeper there, was an important factor as tectonic hypotheses founded in great contrast between the oceanic and continental crusts developed. In our early efforts to explore the continental crust by reflection profiling, have we found something that is murky or obscure now but that will turn out to be of comparable importance? Perhaps, by revealing, for example, the contrast in reflective character between continental crust and the mantle below, or the details of the Moho, or the reflectors in the mantle, we have hit upon something of more significance than we currently assign to it. I leave those matters for you to resolve, but it seems a major breakthrough or enlightenment is certainly within the realm of possibility at this early stage of deep seismic profiling of the continents, even though we may not have worked it all out yet. As the refraction technique was developed over time, new kinds of information were obtained at sea. The sedimentary layer was resolved by direct observation, a thin oceanic crust above the mantle was delineated by Officer, and Raitt discovered velocity anisotropy in the rock layers. These results all were of value as the sea-floor spreading and the plate-tectonics hypotheses evolved. However, refraction sites were scattered and widely spaced in the
1960s and, in general, did not produce the seismic information of high resolution that would turn out to be important as innovative and more detailed tectonic ideas proliferated in the 60s and early 70s. Reflection profiling hence moved into a position of more prominence and began to provide detailed and significant observations of the sea-floor sediments. In 1949, as a student, I was privileged to go on a cruise on which one of the very first attempts at reflection profiling of the deep-sea floor was made. It mostly failed. Only one hydrophone was used and, although some interesting arrivals were observed, shots were too widely spaced to permit correlation. Gradually, however, the technique was developed by John and Maurice Ewing and others (not me), and also, of course, by industry, and a new and revealing way of viewing the sea floor began to unfold. Interesting discoveries followed. For example, grabens on the seaward wall of deep-sea trenches were observed. They confounded some because they indicated extension in the crust near a feature (the trench-arc subduction zone) that was supposed to be a zone of convergence, but eventually it turned out that they were associated with bending of the plate and fit nicely into the plate tectonics, subduction zone, scheme. Also, in this era, deeply buried deepsea sediments were sometimes dated for the first time by tracing reflectors to outcrop where they could be cored and then dated paleontologically. Note the obvious parallel here with our studies of the continental basement. However, and most unfortunately, and this is a very important point to ponder for those currently trying to interpret seismic reflection studies of the continents, in a very general way the early seismic reflection results at sea completely misled some of those attempting to interpret the observations. The trouble was that the technique worked best on flat-lying layers and so, in the early stages of development of the technique, only flat-lying layers were detected. Thus many were misled into thinking that all sediments on the ocean floors were flat and hence that ocean basins were permanent features, i.e. that continents had not moved or drifted, or they had not otherwise been deformed. Later, as the technique was developed, highly deformed sediments were observed, as in the accretionary wedges at the trenches, and it became clear that ocean floors were indeed
J. Oliver/Tectonophysics 286 (1998) 1-4
not undeformed everywhere and hence not static. Is there some note of caution in this part of history for those who are interpreting modern seismic reflection profiles of the continental crust? Are we seizing too much upon our early observations and generalizing too much to global or continent-wide conclusions'? Will improvements in our techniques, or in the scale and thoroughness of our surveys, make us regret our initial or early interpretations, and the conclusions that, it will turn out, did not really conclude the subject'? Will future historians record a parallel on land with early work at sea that we would rather they did not? Let us be alert to that possibility and that history and try to avoid such missteps. Are we missing something that is currently being detected but that is subtle and that will become obvious to us as our surveys increase in number and are more thoroughly distributed throughout the continents'? As I recall, it was only after the sea-floor spreading model was proposed and supported by magnetic anomaly data that seismologists recognized that seismic data showed the oceanic crust and mantle systematically deepening away from the ridges. Had we more thoroughly surveyed the oceans and made that observation earlier, seismologists might have had, or might have stimulated earlier, the idea of sea-floor spreading. For the ocean basins, and for the continents, it was a major achievement to show that the Moho exists and is at about the same depth everywhere within each province. That important conclusion was reached based mostly on data obtained by the seismic refraction technique. But now seismic reflection profiling of the continents is forcing us to see the Moho in a new light. Sometimes what we designate by the simple term 'Moho' seems able to maintain abrupt offsets throughout much of geologic time. Sometimes it seems to rejuvenate itself into a smooth flat configuration following or accompanying some deformation. We have got something important here but as yet we are not completely sure about what it is. Lurking around in the background of this matter is the possibility that we may have created a psychological confinement or barrier for ourselves by developing and teaching an image of the crustmantle boundary that is far too simple, and too glibly described by the term 'Moho'. Another enigma, and a refreshing one, facing us
3
today, of course, concerns the reflectors that we are observing well down into the upper mantle. We do not understand them and we are hypothesizing and arguing about them now, but it is easy to predict that they will become well-understood once we have a more comprehensive understanding of their configuration and spatial distribution throughout the earth. There is a big task ahead to get that spatial distribution of course, but ultimately we must have it, just as we must have an equally, or still more comprehensive picture of the crust everywhere else. That is our job - - exploring the earth. To give some emphasis to the following point, let me introduce it by saying that it may be the most important point I have to make here. Just now, we have only sampled a small fraction of the crust geographically, or spatially, speaking. Furthermore, our efforts tend now to be concentrated at places where we see a major geoproblem of some kind based on our knowledge of the surface or the near-surface. The style of science of many of us tends to force us that way, i.e. toward the problemsolving or deductive kind of science. So does peer review and so do the mechanisms for funding our science. We tend to operate in the deductive style almost exclusively, in other words. That is somewhat unfortunate because we therefore tend to be limited in what we discover by what we can imagine within the brains of our scientists. However, history shows that many major discoveries, the really big ones, in an observational science like ours, are found in the inductive style in which the earth surprises us in ways beyond our imagination. As we are facing a new and relatively unexplored frontier in the form of the buried continental crust and underlying mantle. I think a significant fraction of our effort should be in the inductive style. We should observe parts of the earth just to see what is there and not because a site has a particular, wellrecognized, geological problem. We do not want to be carried away or misled or handicapped by our egos, in other words. The inductive style calls for a little more humility on the part of the scientist, and the peer, or committee, reviewer, and the funding agency, but history shows it to be an essential part of our mix of scientific styles. We must, in my opinion, strive to explore the entire crust, not just the parts that look interesting to us at the moment.
4
J. Oliver / Tectonophysics 286 (1998) I-4
Another point that may be of some relevance to our future is the manner in which seismic exploration of the sea floor in the 1950s was organized, or to put it slightly differently, left unorganized. Each marine institute had an organized small group within it that did the seismic work, of course, but at a higher level each institute was relatively free to work where and how it chose. There was, in other words, no central Beltway committee or organization completely controlling just how the science was done. Most of the direction instead was left to the leaders of the institute who: (a) were close to the actual work, (b) were generally exceptionally fine leaders, and (c) had to perform outstandingly or see their organization falter or wither away. We might take note of this style of organization and contrast it with some modern methods in which proposals submitted in endless detail are peer reviewed and subjected to discussions by those who bear little or no responsibility if the decisions turn out to be less than ideal. Such methods result in routine science and often inhibit discovery science. Let me turn now briefly to the role of industry in seismic exploration of the deep-sea floor. Industry, of course, has played the prominent and overwhelming role in exploration of shallow-water areas, developing the technique to higher and higher levels and doing the bulk of the seismic exploration, and the accompanying drilling, of course. We all recognize that great advances in the technique of seismic surveying that could be applied to deep-sea surveying were made by the petroleum industry once that industry began to focus on prospects in sedimentary basins in marine areas. In a sense, support flowed indi-
rectly through the geophysical prospecting industry to the study of basic deep-sea geology. Those of us exploring the deep continental basement already know about that. we rely heavily on equipment, and techniques, and skill developed by the petroleum industry, developed essentially because of human need for hydrocarbon resources. We might find a parallel to that situation in the future in the case of deep seismic reflection studies of the continents. Perhaps the search for hydrocarbon resources at greater depths in the future, or something like production of energy or mineral-laden brines from depths, or something else, will stimulate commercial interest in, and need for, knowledge of the basement, and so enhanced understanding of the basement will be sought in both and the academic and the commercial context. If that should happen our activities would, of course, receive a strong boost. In any case, it seems fundamental that, as society advances, humans on this planet will both want and need to know thoroughly and in detail what the earth is like just a few kilometers down. I think the longterm future of the kind of earth exploration discussed at this meeting holds much activity and much excitement for us, and is something all earth scientists can eagerly look forward to. We must lay out plans for the future with the long-term goal of obtaining detailed comprehensive seismic reflection information for every part of the continental crust, recognizing, as we do so of course, that the wonderful things we have accomplished to date and reported at these meetings will be seen by scientists and historians of the future as but a preliminary.
ELSEVIER
ii
Tectonophysics 286 (1998) I-4
Deep seismic sounding of the continental crust and mantle a long-term view Jack Oliver * Department of Geological Sciences and INSTOC, Cornell Universit3; Ithaca, NY 14853, USA
Received 10 February 1997; accepted 5 June 1997
Exciting progress in deep seismic sounding is being made, activity is taking place at many locales in many countries around the world, and numerous, talented, capable, people are involved. ! am going to skip over the near present and remark instead on some activities of the more distant past and try to suggest how the historical approach might be useful or helpful as we move toward, and plan strategically for, the long-term future. Perhaps there is a parallel with the way the subject of earthquake seismology developed so as to play an important role in the plate-tectonics revolution of the 1960s, and perhaps deep reflection profiling may provoke that kind of upheaval in its future. We never know, of course, but with a large part of the earth, the continental crust, waiting to be explored by this powerful technique, our situation resembles historical ones that have often produced major enlightenment. Back in the early 1970s when we were trying to get deep seismic reflection profiling of the continents underway through establishment of the COCORP project, we talked about the buried continental basement as the great, or one of the great, frontiers of modern earth science. We believed that characterization then, and we still do. We drew a parallel between the continental basement and the floor of the deepocean basins which, as history clearly shows, was surely the great frontier of the 40s, 50s and 60s. Dur*Fax: +1 (607) 254-4780: E-mail: [email protected]
ing those decades especially, the floors of the ocean basins were explored, essentially for the first time, by the techniques of seismic refraction and reflection. The resulting observations, along with other kinds of data, were important and sometimes critical in the plate-tectonics revolution, one of the major achievements ever of earth science. Let us look then at how seismic exploration of the ocean basins evolved during the 50s and 60s, at some of the triumphs and missteps along the way, and at some of the tactical and strategic decisions that led to those successes or failures, all the while keeping in mind that we may, or may not, be encountering parallel situations along the way as we explore our frontier, the continental crust and the uppermost mantle beneath it, now and in the future. Let me note that I will be focusing mostly on non-commercial, or academic, activity, but l will have something to say about the importance and relevance of industrial activity a little later. However, 1 will be referring here mostly to the deep-sea floor, not the shelves and margins where most commercial seismic was, and is, done. Let us begin with seismic refraction. The earliest results in this topic by the academic institutions were primitive alright, but they were also very significant and attention-getting. The first seismic refraction work in the deep sea was done in the 30s0 led by Maurice Ewing, but such studies only began to blossom in the post WW II years, the late 40s and the
0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. Pll $0040- 195 l (97)00250-3
2
J. Oliver/Tectonophysics 286 (1998) l-4
50s. Initially the experimental technique was very crude, perhaps utilizing only one or two hydrophones suspended from one receiving ship, certainly no array, and a pitifully small number of widely spaced shots, often of a size smaller than was really needed. The interpretations of the resulting observations were often murky to say the least. Nevertheless in this very primitive stage, they provided a surprising and very important result. Although at first only two layers were reported (one a 'liquid' layer that included the water and all ocean floor sediments, and the other a high speed layer with velocity approaching 8 kin/s), the data demonstrated that the mantle was very shallow beneath the deep sea and hence that no thick submerged or subsided continental crust was there, at least where the experiments were done. This result had important implications at a time when geologists were wrestling with concepts such as land bridges, drifting or subsiding continents, a moon ejected from the Pacific, etc. This crude initial result turned out to be more or less correct and, in conjunction with refraction work on land that showed that the mantle was much deeper there, was an important factor as tectonic hypotheses founded in great contrast between the oceanic and continental crusts developed. In our early efforts to explore the continental crust by reflection profiling, have we found something that is murky or obscure now but that will turn out to be of comparable importance? Perhaps, by revealing, for example, the contrast in reflective character between continental crust and the mantle below, or the details of the Moho, or the reflectors in the mantle, we have hit upon something of more significance than we currently assign to it. I leave those matters for you to resolve, but it seems a major breakthrough or enlightenment is certainly within the realm of possibility at this early stage of deep seismic profiling of the continents, even though we may not have worked it all out yet. As the refraction technique was developed over time, new kinds of information were obtained at sea. The sedimentary layer was resolved by direct observation, a thin oceanic crust above the mantle was delineated by Officer, and Raitt discovered velocity anisotropy in the rock layers. These results all were of value as the sea-floor spreading and the plate-tectonics hypotheses evolved. However, refraction sites were scattered and widely spaced in the
1960s and, in general, did not produce the seismic information of high resolution that would turn out to be important as innovative and more detailed tectonic ideas proliferated in the 60s and early 70s. Reflection profiling hence moved into a position of more prominence and began to provide detailed and significant observations of the sea-floor sediments. In 1949, as a student, I was privileged to go on a cruise on which one of the very first attempts at reflection profiling of the deep-sea floor was made. It mostly failed. Only one hydrophone was used and, although some interesting arrivals were observed, shots were too widely spaced to permit correlation. Gradually, however, the technique was developed by John and Maurice Ewing and others (not me), and also, of course, by industry, and a new and revealing way of viewing the sea floor began to unfold. Interesting discoveries followed. For example, grabens on the seaward wall of deep-sea trenches were observed. They confounded some because they indicated extension in the crust near a feature (the trench-arc subduction zone) that was supposed to be a zone of convergence, but eventually it turned out that they were associated with bending of the plate and fit nicely into the plate tectonics, subduction zone, scheme. Also, in this era, deeply buried deepsea sediments were sometimes dated for the first time by tracing reflectors to outcrop where they could be cored and then dated paleontologically. Note the obvious parallel here with our studies of the continental basement. However, and most unfortunately, and this is a very important point to ponder for those currently trying to interpret seismic reflection studies of the continents, in a very general way the early seismic reflection results at sea completely misled some of those attempting to interpret the observations. The trouble was that the technique worked best on flat-lying layers and so, in the early stages of development of the technique, only flat-lying layers were detected. Thus many were misled into thinking that all sediments on the ocean floors were flat and hence that ocean basins were permanent features, i.e. that continents had not moved or drifted, or they had not otherwise been deformed. Later, as the technique was developed, highly deformed sediments were observed, as in the accretionary wedges at the trenches, and it became clear that ocean floors were indeed
J. Oliver/Tectonophysics 286 (1998) 1-4
not undeformed everywhere and hence not static. Is there some note of caution in this part of history for those who are interpreting modern seismic reflection profiles of the continental crust? Are we seizing too much upon our early observations and generalizing too much to global or continent-wide conclusions'? Will improvements in our techniques, or in the scale and thoroughness of our surveys, make us regret our initial or early interpretations, and the conclusions that, it will turn out, did not really conclude the subject'? Will future historians record a parallel on land with early work at sea that we would rather they did not? Let us be alert to that possibility and that history and try to avoid such missteps. Are we missing something that is currently being detected but that is subtle and that will become obvious to us as our surveys increase in number and are more thoroughly distributed throughout the continents'? As I recall, it was only after the sea-floor spreading model was proposed and supported by magnetic anomaly data that seismologists recognized that seismic data showed the oceanic crust and mantle systematically deepening away from the ridges. Had we more thoroughly surveyed the oceans and made that observation earlier, seismologists might have had, or might have stimulated earlier, the idea of sea-floor spreading. For the ocean basins, and for the continents, it was a major achievement to show that the Moho exists and is at about the same depth everywhere within each province. That important conclusion was reached based mostly on data obtained by the seismic refraction technique. But now seismic reflection profiling of the continents is forcing us to see the Moho in a new light. Sometimes what we designate by the simple term 'Moho' seems able to maintain abrupt offsets throughout much of geologic time. Sometimes it seems to rejuvenate itself into a smooth flat configuration following or accompanying some deformation. We have got something important here but as yet we are not completely sure about what it is. Lurking around in the background of this matter is the possibility that we may have created a psychological confinement or barrier for ourselves by developing and teaching an image of the crustmantle boundary that is far too simple, and too glibly described by the term 'Moho'. Another enigma, and a refreshing one, facing us
3
today, of course, concerns the reflectors that we are observing well down into the upper mantle. We do not understand them and we are hypothesizing and arguing about them now, but it is easy to predict that they will become well-understood once we have a more comprehensive understanding of their configuration and spatial distribution throughout the earth. There is a big task ahead to get that spatial distribution of course, but ultimately we must have it, just as we must have an equally, or still more comprehensive picture of the crust everywhere else. That is our job - - exploring the earth. To give some emphasis to the following point, let me introduce it by saying that it may be the most important point I have to make here. Just now, we have only sampled a small fraction of the crust geographically, or spatially, speaking. Furthermore, our efforts tend now to be concentrated at places where we see a major geoproblem of some kind based on our knowledge of the surface or the near-surface. The style of science of many of us tends to force us that way, i.e. toward the problemsolving or deductive kind of science. So does peer review and so do the mechanisms for funding our science. We tend to operate in the deductive style almost exclusively, in other words. That is somewhat unfortunate because we therefore tend to be limited in what we discover by what we can imagine within the brains of our scientists. However, history shows that many major discoveries, the really big ones, in an observational science like ours, are found in the inductive style in which the earth surprises us in ways beyond our imagination. As we are facing a new and relatively unexplored frontier in the form of the buried continental crust and underlying mantle. I think a significant fraction of our effort should be in the inductive style. We should observe parts of the earth just to see what is there and not because a site has a particular, wellrecognized, geological problem. We do not want to be carried away or misled or handicapped by our egos, in other words. The inductive style calls for a little more humility on the part of the scientist, and the peer, or committee, reviewer, and the funding agency, but history shows it to be an essential part of our mix of scientific styles. We must, in my opinion, strive to explore the entire crust, not just the parts that look interesting to us at the moment.
4
J. Oliver / Tectonophysics 286 (1998) I-4
Another point that may be of some relevance to our future is the manner in which seismic exploration of the sea floor in the 1950s was organized, or to put it slightly differently, left unorganized. Each marine institute had an organized small group within it that did the seismic work, of course, but at a higher level each institute was relatively free to work where and how it chose. There was, in other words, no central Beltway committee or organization completely controlling just how the science was done. Most of the direction instead was left to the leaders of the institute who: (a) were close to the actual work, (b) were generally exceptionally fine leaders, and (c) had to perform outstandingly or see their organization falter or wither away. We might take note of this style of organization and contrast it with some modern methods in which proposals submitted in endless detail are peer reviewed and subjected to discussions by those who bear little or no responsibility if the decisions turn out to be less than ideal. Such methods result in routine science and often inhibit discovery science. Let me turn now briefly to the role of industry in seismic exploration of the deep-sea floor. Industry, of course, has played the prominent and overwhelming role in exploration of shallow-water areas, developing the technique to higher and higher levels and doing the bulk of the seismic exploration, and the accompanying drilling, of course. We all recognize that great advances in the technique of seismic surveying that could be applied to deep-sea surveying were made by the petroleum industry once that industry began to focus on prospects in sedimentary basins in marine areas. In a sense, support flowed indi-
rectly through the geophysical prospecting industry to the study of basic deep-sea geology. Those of us exploring the deep continental basement already know about that. we rely heavily on equipment, and techniques, and skill developed by the petroleum industry, developed essentially because of human need for hydrocarbon resources. We might find a parallel to that situation in the future in the case of deep seismic reflection studies of the continents. Perhaps the search for hydrocarbon resources at greater depths in the future, or something like production of energy or mineral-laden brines from depths, or something else, will stimulate commercial interest in, and need for, knowledge of the basement, and so enhanced understanding of the basement will be sought in both and the academic and the commercial context. If that should happen our activities would, of course, receive a strong boost. In any case, it seems fundamental that, as society advances, humans on this planet will both want and need to know thoroughly and in detail what the earth is like just a few kilometers down. I think the longterm future of the kind of earth exploration discussed at this meeting holds much activity and much excitement for us, and is something all earth scientists can eagerly look forward to. We must lay out plans for the future with the long-term goal of obtaining detailed comprehensive seismic reflection information for every part of the continental crust, recognizing, as we do so of course, that the wonderful things we have accomplished to date and reported at these meetings will be seen by scientists and historians of the future as but a preliminary.