A Decade of Plate Tectonics

478 Nature Vol. 279 7 June, 1979 search could reveal information on versity of Oxford) was consistent with A decade of variations in intensity of the field m this interpretation. Moorbath argued plate tectonics space and time, which can del,imit strongly that isotopic data pointed to changes in the strength of the dipole continental ,growth by irreversible dif from A. Hallam and non-dipole field components with ferentiation of the upper mantle J. Tuzo WILSON is widely acknowledged time. throughout geologica,I time, rather as one of the founding fathers of the Seismological data originally pro than by reworking and regeneration .-,f new global tectonics. Because he has vided strong evidence for the delimita much older crust. Intriguingly enough, been successively Professor of Geophy tion and character of plate boundaries, Tuzo Wi.lson was once most widely sics and Principal of Erindale College and attention is now being directed known for his notiion of continental in tJhe University of Toronto, before more to seismicity within plates. L. R. accretion, albeit at a tune when he was taking up his present position of Direc Sykes (Lamont-Doherty Geological a staunch opponent of continental tor of the Ontario Science Centre, it is Observatory, New York) indicated drift. appropriate that the university recently that intraplate earthquakes tend to Other aspects of subduction were held a conference* to honour his multi be ,concentrated along ,pre-existing dealt with by W. S. Fyfe (University farious contributions to earth science. zones of weakness affected by the of Western Onta11io), who argued for Although various topics were discussed, youngest orogeny that predates the substantial subduction of sediments, a including the early thermal history of opening of the present oceans. Many view contested by Moorbath, and A. the Earth, electromagnetic studies of such zones, marked by fault and suture Miyashiro (State University of New the crust and the origfo of ore deposits, J.ines and failed rifts, were reactivated York, Albany). Miyashiro concerned the dominant theme was plate tec dur,ing the early stages of continental himself with metamorphism and esti tonics. A,pproximately a decade has separation. In contrast, intraplate mated that W. G. Ernst's successive passed since plate tectonics became shocks rarely occur within the older models of the high pressure-low tem generally accepted, and the meeting oceanic lithosphere or within ancient perature zone had greatly improved provided a valuable means of finding continental cratons. our detailed understanding, whereas out how the subject was now being The seismic reflection profiling serfous problems remained with the low studied in a range of disciplines and method, developed by the oil industry pressure-hi~h temperature zone, not of taking stock of the current state of for the study of sedimentary basins, is ably the origin of the magmas gen knowledge. now being a.pplied to the study of the erated there. It seems that the old, Much of the critical research lead structure of the continental basement. widely accepted model assuming mag ing to the earth sciences revolution was The most interesting result so far, as ma generation by simple partial melting in the fields of rock magnetism and described by J. Oliver (Cornell Uni of •the oceanic crust in a descending seismology, and several leading workers versity) is the discovery that the Blue slab is no longer tenable. the were present to outline recent advances. Ridge and Piedmont provinces of A reassessment of Panagaea recon Both A. Cox (Stanford University) and A,ppalachians are allochthonous and structions and time of Mesozoic break E. Irving (Earth Physics Branch, have been thrust at least 275 km up was undertak.en by A. Hallam Energy, Mines and Resources, Ottawa) westwards. (University of Birmingham). The considerable indicated how detailed palaeomagnetic Appropriate,Jy enough, familiar 1-km isobath computer recon studies in alliance with geological attention was paid to the Wilson Cycle structions were inaccurate in detail the research demonstrate that extensive of ocean opening and closure, because it is now known for many being the sectors of the Western CordiMera of principal preoccupation regions that the boundary of conti North America had either rotated or character of subduction zones. nental and oceanic crust must occur been translated northward for sub S. Uyeda (Earthquake Research Insti beneath 2-4 km or even greater depths stantial distances in the late Mesozoic tute, Tokyo) presented a model of two of ocean, breakup having been pre and early Tertiary. While Irving was contrasted t}'lpes of subduction zone ceded by extensive stretching and sub concerned essentially with northward exem,plified by the eastern and western sidence. Furthermore, there may be a movement of two landmasses, Wran sides of the Pacific, which was long time interval between initial gellia and Stikina, Cox dealt mainly developed in more detail by J. F. taphrogenic activity ('rifting') and with inferred rotations. To apply sail Dewey (State University of New York, creation of ocean floor by spreading ing terms, the structural geologist Albany). Flat subduction trajectories ('drifting'). A. J. Boucot (Oregon State studies pitch and role but only the produced by flat mantle flow and/or University) expressed scepticism about palaeomagnetist can determine the the subduction of young lithosphere reconstructions of changing continental yaw. D. York (University of Toronto) produces pervasive compressional positions in the Palaeozoic and showed how a combination of palaeo strain in the overriding slab at high attempted to show by plotting fossil magnetic and isotope studies could slip-rates. Steep subduction trajec and climatically significant sediment help to unravel complex events in the tories, subduction of old lithosphere distributions that an unchanging Pan Precambrian and confirm, for example, and .low slii{rrates induce back-arc gaea was as satisfactory a model as the reality of the polar wandering event spreading. Dewey went on to argue any. On the other hand, H. Williams about 10• years ago known as the that plate tectonics in its present form (Memorial University, Newfoundland) Grenville Loop. has been in operation for about argued that geological evidence in the A quite different approach to rock 2 X 10• yr, a view Slllpported by Appalachians supported the view that of magnetism was adopted by F. J. Vine P. Hoffman (Geological lSurvey the Iapetus was a major ocean. (University of East Anglia) who con Canada) in his description of an early Problems indeed remain in reconstruct cycle in northern cerned himself with the record of Proterozoic Wilson ing continental positions in the reversals of the geomagnetic field re Canada. Mes,ozoic and Cainozoic, but they accord between vealed by ocean floor anomalies, and Similarly, there was seem to be child's play in comparison and W. R. Dickinson (Stanford demonstrated how further detailed re- Dewey to earlier periods of time. Our best University) that a tectonic mosaic in hope would seem to rest with the * The symposium was held at the Uni volv,ing horizontal motions was already palaeomagnetists. D versity of Toronto on 14-16 May 1979 in existence in Archaean times, and and the proceedings will be published as a special paper of the Geological Associa the model of early continental evolu A . Hallam is Lapworth Professor of tion of Canada. tion put forward by S. Moorbath (Uni- Geology at the University of Birmingham. 002~-0836/79/0279-0376$01.00 ,C Macmillan Journals Ltd 1979 .

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