The 7th International Symposium on "Deep Seismic Profiling of the Continents" continued the series of biennial meetings that involve specialists in the acquisition, processing and interpretation of crustal-scale seismic reflection data, and geoscientists interested in the application of deep seismic reflection results to geological problems throughout the world. 197 delegates from 23 countries registered for the Symposium, held September 15-20 at the Asilomar Conference Center, on a dramatic granite headland jutting into the Pacific Ocean at Monterey Bay, California. 61 oral and 142 poster contributions were presented exclusively in plenary sessions, and all delegates were hosted on the Asilomar campus to encourage continual interaction. The scientific themes of the meeting around which the presentations were organised, and which the eleven invited speakers were encouraged to discuss, were:
In addition to the formal scientific presentations, evening discussion sections were organised on several topics:
Beyond the exchange of ideas, one most important result of these discussions was to publicize the free availability of all controlled-source and earthquake-source seismic data acquired with participation of the IRIS/PASSCAL consortium (visit their website), and the efforts of COCORP to develop and maintain a meta-library of deep seismic groups and their data archives, worldwide (visit their website).
The 7th International Symposium was held in the 20th anniversary year of the publication of COCORP's first deep reflection profile from Hardeman County, Texas (Oliver et al., 1976). In those two decades, reflection studies have dramatically advanced our understanding of the continents, and the techniques utilized and the regions of geological coverage have broadened. Nonetheless, it remains cogent to quote from the introduction to the proceedings of the 1st International Symposium (Barazangi and Brown, 1986a):
"First, there is nearly universal awareness and acceptance of deep seismic reflection profiling as a necesary tool for exploring the deep basement of continents. The rapid and widespread increase in its use ... by numerous countries is a clear manifestation of such awareness and acceptance.
"Second, a new kind of earth scientist has emerged as the traditional fields of geology and geophysics are of necessity unified to achieve the best interpretation of deep reflection data.
"Third, results presented during the symposium demonstrated how much we still have to learn about the deep basement of continents.
"Fourth, because of the global nature of the deep reflection profiling programs it is already possible to decipher similar seismic "signatures" for different parts of the continental crust with similar geological environments. With the rapid growth of available deep reflection data, it is not unreasonable to expect that within our lifetime a clear understanding of the structure and evolution of the continental lithosphere may be achieved."
Following the symposium, 63 papers were submitted for publication, and, following rigorous review, 42 papers were accepted for publication in two special issues of Tectonophysics, volume 286: "Deep Seismic Profiling of the Continents: General Results and New Methods" and volume 287: "Deep Seismic Profiling of the Continents: A Global Survey". The first volume contains papers that are of general interest to the field (beyond any geographical focus), including papers describing new interpretational and processing methodologies. The second volume contains papers that are best described by their regional/geological focus, though of course all also contain general messages for the study of comparable regions worldwide.
The scientific results of the symposium are well summarised by the following quoted from the Preface to these volumes (Klemperer and Mooney, 1998 a, b):
"Deep Seismic Profiling of the Continents: General Results and New Methods" is fittingly introduced by a "long-term view", a forward-looking prognostication for our discipline. This is followed by a series of papers ("Upper crust, lower crust, upper mantle") that show the state-of-the-art in imaging and interpreting the shallow crystalline basement, the lower crust and the mantle part of the lithosphere. A decade previously our scientific focus was often on the lower crust, which had been almost terra incognita before the advent of deep reflection profiling. In contrast, the last ten years have seen an increasing emphasis on applying the technique to image at higher resolution in the crystalline upper crust (often in commercially important hard-rock mining areas) and to image to greater depths into the upper mantle, even arguably to the point of observing the lithosphere-asthenosphere boundary.
"The power of reflection seismology is continuously increased by new technical advances, some of which are reviewed in the section on "Acquisition and Processing methods, and Scattering". Two papers in this section relate to an important development in the way we look at reflection profiles, the attempt to treat the crust as a scattering medium describable by purely statistical parameters. This approach has value for geologically characterising the crust (though has not yet reached the fruition of providing descriptions useful to a "field" geologist) and also for understanding the effects of the scattering medium on the propagating wavefield, thereby setting ultimate limitations on our deterministic interpretations.
"The power of reflection seismology to study the lithosphere is also greatly increased by the collocated application of other methodologies ("Incorporating VSP, wide-angle, earthquake, rock physics and electromagnetic data"). Some of these methods are highly synergistic: wide-angle methods, and VSPs where a deep drill-hole exists, can utilise different parts of the seismic wavefield already generated for the seismic reflection profiles, and earthquake imaging with local earthquakes can utilise other seismic sources recorded on the same sensors already deployed for seismic profiling. In contrast, teleseismic methods typically require deployment of different (lower frequency) seismometers, and electromagnetic and rock-physics measurements require entirely different experiments. In addition to examples of datasets acquired with the various techniques, this section includes papers describing synthetic tests of the possibilities of near-vertical to wide-angle imaging using denser observation than is yet possible with our limited instrument inventory, thereby looking forward to improved experiments in the future.
"The second volume of this pair, "Deep Seismic Profiling of the Continents: A Global Survey", groups together papers by the type of tectonic setting that is profiled. The first section, "Continental interiors: Precambrian tectonics", contains a fair geographic coverage with papers showing new data from the Baltic Shield, from India, Australia and North America. Regrettably, Symposium papers on the other shields of the FSU could not be published here; and no papers on the African or South American shields were presented at the Symposium. The second section, "Continental interiors: Phanerozoic tectonics", can not cover more than a fraction of the earth, but it is particularly welcome to us to include papers from both Russia and China, areally significant but as yet relatively poorly known (at least in the English-language literature) by reflection profiling. Papers on modern continental margins are divided into sections on "Continental margins: passive margins" and "Continental margins: active margins", containing papers from a range of Atlantic, Pacific, and Australian margins.
"In this volume it would be impossible to select any one of these profiles, or any one of these areas, as more significant than any other. Instead it is perhaps more instructive to note the trends towards looking deeper (eg. into the mantle beneath the Uralian Phanerozoic orogen); using more advanced processing and interpretational techniques (eg. migrating data in complex velocity structures on the Cascadia compressional margin, and incorporating data from the Proterozoic Mt. Isa inlier into 2.5D Geographic Information System); and acquiring ever-more multi-disciplinary data-sets (eg. across the Betic Phanerozoic orogen, and across the New Zealand transpressional plate boundary). It is heartening also to see data acquired for strictly commercial purposes also fulfill academic interests (eg. 16 s data across the Brazilian extensional margin). Finally, it is remarkable to observe the increasing sophistication of geological interpretations of reflection profiles (eg. in the claims to distinguish multiple styles of mass transfer in orogenic wedges, and in the efforts to characterise the tectonic architecture of the entire plate boundary from Alaska to southern California). Reflection seismologists are becoming ever more bold in extending surface mapping into crustal cross-sections using their inferences from reflection profiles, and earth scientists in general are becoming ever more accepting that these reflection-constrained cross-sections represent a truth about the earth's crust.
"These trends, and indeed the fact that the contributions of Symposium attendees fill not one but two volumes of Tectonophysics, speak to the continuing vitality, and continuing necessity, of the discipline of crustal-scale continental reflection profiling. We are proud to have continued the tradition of excellence set by the previous meetings in this series, hosted by COCORP (Consortium for Continental Reflection Profiling) at Cornell University in 1984 (Barazangi and Brown, 1986a, 1986b); by BIRPS (British Institutions' Reflection Profiling Syndicate) at Cambridge University in 1986 (Matthews and Smith, 1987); by the Australian National University and Bureau of Mineral Resources in Canberra in 1988 (Leven et al., 1990); by DEKORP (Deutsches Kontinentales Reflexionsseismisches Program) at Bayreuth University in 1990 (Meissner et al., 1991); by Lithoprobe at the Banff Conference Center in 1992 (Clowes and Green, 1994); and by the Eötvös-Lórand Institute in Budapest in 1994 (White et al., 1996). The 8th international meeting will be held in Barcelona from 20-25 September 1998 (contact Ramon Carbonell,firstname.lastname@example.org ), and the 9th in Oslo in June 2000 (contact Eystein Husebye, email@example.com )."
The 7th International Symposium was organised by members of the Crustal Geophysics Program of the Stanford University Department of Geophysics, and by members of the U.S. Geological Survey Branches of Seismology and of Pacific Marine Geology. Additional financial support was received from the National Science Foundation Continental Dynamics Program, IASPEI, ILP, SEG, AAPG and Reftek Inc. Scientific sponsorship was accorded to the Symposium by IASPEI, ILP, AAPG, AGU, GSA and SEG. In addition to the Guest Editors, the Organizing Committee comprised George Thompson (Stanford University) and Jill McCarthy (U.S. Geological Survey). Invaluable assistance during the organisation of the meeting was provided by Robin Wilson (U.S. Geological Survey).
Barazangi, M., and L. Brown, editors, 1986a. Reflection seismology: a global perspective. Am. Geophys. Un., Geodynamics Series, 13, pp. 311.
Barazangi, M., and L. Brown, editors, 1986b. Reflection seismology: the continental crust. Am. Geophys. Un., Geodynamics Series, 14, pp. 339.
Clowes, R.M., and A.G. Green, editors, 1994. Seismic reflection probing of the continents and their margins. Tectonophys., v. 232, 1-450.
Klemperer, S.L., and Mooney, W.D., editors, 1998a. Deep seismic profiling of the continents: general results and new methods. Tectonophys., v. 286, 1-xxx.
Klemperer, S.L., and Mooney, W.D., editors, 1998b. Deep seismic profiling of the continents: a global survey. Tectonophys., v. 287, 1-xxx.
Leven, J.H., D.M. Finlayson, C. Wright, J.C. Dooley, and B.L.N. Kennett, editors, 1990. Seismic probing of continents and their margins. Tectonophys., v. 173, 1-641.
Matthews, D.H., and C. Smith, editors, 1987. Deep seismic reflection profiling of the continental lithosphere, Geophys. J. R. astr. Soc., v. 89, 1-447.
Meissner, R., L. Brown, H.-J. Dürbaum, W. Franke, K. Fuchs and F. Siefert, editors, 1991. Continental lithosphere: deep seismic reflections. Am. Geophys. Un., Geodynamics Series, 22, pp. 450.
Oliver, J.E., M. Dobrin, S. Kaufman, R. Meyer and R. Phinney, 1976. Continuous seismic reflection profiling of the deep basement, Hardeman County, Texas. Geol. Soc. Am. Bull., 87, 1537-1546.
White, D.J., J. Ansorge, T.J. Bodoky and Z. Hajnal, editors, 1996. Seismic reflection probing of the continents and their margins. Tectonophys., v. 264, 1-392.
Tectonophysics - Volume 286 - Nos. 1-4 Edited by S.L. Klemperer and W.D. Mooney
Crustal structure of China from deep seismic sounding profiles S.-L. Li and W.D. Mooney