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University of Toronto - II

Tuzo remained continuously on the faculty of the University of Toronto during his international activities, but when these became less demanding,after 1960, he was able to devote all of his efforts to research. The next seven years were to be the most creative of his career, marked by a series of his most innovative publications. At some point in the late 1950s he abandoned his former belief that the continents were fixed on earth and began to examine the consequences of large-scale displacements of continents and oceans. It is fascinating in retrospect to trace this conversion.  According to his own statement, quoted above, it would be nine years after his first visit, in 1950,to South Africa before he could accept continental drift. It is known that he remained unconvinced at the time of the 1957General Assembly of the IUGG in Toronto, where there was an important discussion of the paleomagnetic evidence for drift.

Tuzo's acceptance of ocean-floor spreading and continental displacement first became apparent in his contribution to discussion (63) of a paper by R. S. Dietz (1961). It was a very perceptive discussion, involving also J. D. Bernal, of Dietz's comprehensive model of uppermantle convection, spreading of the ocean floor from ridges with its subsequent pushing under continents by 'transcurrent faults.' Reading it in hindsight, one is impressed by how close Dietz, Bernal and Tuzo were to the complete model of plate tectonics that would develop in another four years. Indeed, Bernal wrote: 'One gets the impression that with it (i.e. Dietz's model) a stage has been reached like the last but one in fitting together a jigsaw puzzle...' Bernal's objection dealt principally with mantle properties, especially the fact that Dietz's upper-mantle faults did not explain deep-focus earthquakes. Tuzo appeared ready to accept most of the evidence, including that provided by paleomagnetism, but he particularly welcomed the fact that two models of earth behaviour which he had helped to develop need not be discarded: 'Besides admitting of some features of compression in mountains, the new theory admits of continental growth...' Tuzo's principal objection was that Dietz had not fully provided a mechanism for continental crust to thicken under continental shields. He pointed out that if, over time, material from the interior is added to continents, their sialic crust must thicken everywhere for isostasy to maintain continental freeboard. Dietz had visualized 'underplating' of sialic material, but only around continental margins. The objections of both Bernal and Tuzo would eventually be largely answered with the recognition of the full significance of deep subduction zones, and of the presence of relict subduction zones under continental interiors.

By the time of a subsequent discussion on the tectonics of the Canadian Shield (65), Tuzo was obviously prepared to accept large-scale continental displacement:

...continental blocks can join and rift at random. The fact that two provinces of the Canadian Shield have been together during post-Cambrian time does not necessarily mean that they were formed close together or that the sediments lying on one province were derived from the province now beside it.

The transformation was complete.

Tuzo then turned his attention to the study of the Atlantic Ocean, beginning with the study of a major fault system in the Maritime provinces of Canada, the region in which he had carried out field work 25 years before. He proposed (67,68,70) that a system of left-lateral transcurrent faults of eastern North America, designated by him the Cabot fault, could be considered the extension of the Great Glen fault of Scotland, which was known to have a similar sense of displacement. The connection between the two systems was most convincing when the Atlantic Ocean was closed as it would have been in pre-Jurassic time. Thus, Tuzo was forced to acknowledge the possibility of major continental displacement, although his emphasis at this time was clearly on the fault structure itself. In the second paper of the series he wrote: 'If continental drift has taken place, and if this reconstruction is correct, it shows that the Cabot fault and the Great Glen fault in Scotland might have been joined...' Clearly Tuzo's ideas on the fixation of continents and oceans had changed. Furthermore, his detailed examination of the geology along the Cabot and Great Glen fault systems was to be of crucial importance in his later study of the history of the Atlantic Ocean. But first, he was to turn his attention to oceanic islands.

By early in 1963 Tuzo had not only accepted the ideas of ocean-floor spreading and continental drift, he was actively contributing to them. His analysis of the ages of oceanic islands (68) revealed not only their great youth as compared to the continents, but also a trend toward older ages with increasing distance from a mid-ocean ridge, tending to confirm oceanfloor spreading from these ridges. As a driving mechanism he accepted (73) a system of mantle convection currents, rising under and spreading from, the ridges. However, he recognized that some oceanic islands do not fit the pattern. The Hawaiian islands, for example, lie far from a ridge, yet exhibit present-day volcanic activity. According to his own recollections, Tuzo attributed his interest in Hawaii to an ascent of Mauna Loa. In any case, it led to one of his most imaginative and influential papers (72). He proposed (fig. 2) that the source of volcanic rock for the Hawaiian islands is a plume rising from a 'hot spot' within the stable core of a mantle convection cell. As the Pacific lithospheric plate moves across this fixed source, older islands of the chain are carried 'downstream', while the present-day volcanic activity lies over the source. The existence of a chain of older islands north and west from Hawaii lent strong support to the hypothesis. This concept, so elegantly simple when reduced to Tuzo's cartoon (fig. 2), has a consequence which goes far beyond the origin of the Hawaiian islands themselves. Dating of the older islands in the train permits the determination of the velocity of plate motion, relative to the hot spot which is supposedly fixed in the mantle. At about the same time, Vine and Matthews (1963) published their explanation of the magnetic character of the ocean floor, in terms of magnetization of new ocean floor in the earth's reversing magnetic field and its subsequent spreading from a ridge. The papers are complementary: the magnetic snipes providing a relative velocity of plate motion across spreading lines, the island trains an ‘absolute' velocity relative to the mantle. The concept of a global framework of reference points provided by plumes or hot spots was to be greatly extended by others. Incidentally, Tuzo's paper on the Hawaiian islands was rejected by a leading earth-science journal before it was published in theCanadian Journal of Physics.

By the end of 1963 the main concepts of ocean-floor dynamics were recognized and accepted by most earth scientists. These included two types of plate boundaries: accreting boundaries, marked by mid-ocean ridges; and consuming boundaries marked by Benioff zones of deep-focus earthquakes, to become more generally known as subduction zones. But the formulation of a global pattern of lithospheric plates remained unsatisfactory, because not all plate boundaries fitted these types. Tuzo decided to concentrate on the study of the ocean floor when he accepted the invitation of Teddy Bullard to spend a term at the Department of Geodesy and Geophysics, University of Cambridge, in the winter of 1964-1965. It was an ideal environment, enhanced by discussions with Fred Vine and Drummond Matthews and with Harry Hess, visiting from Princeton. The latter's most active period of research was past, but Tuzo often attributed his own acceptance of ocean-floor spreading to Hess's early models.

It was well known that the major ocean ridges show frequent perpendicular offsets, by fractures which can be mapped bathymetric ally and which, in part, are seismically active. Tuzo recognized that the conventional theory of faulting, as applied to a medium that is conserved, must be modified if the ocean floor is characterized by non-conservation. Thus, the shear motion across a fault joining two ridge segments ends abruptly at these segments. For this reason he refeITed to these faults initially as half-shears, but because they can also transform an accreting boundary into a consuming one, or vice versa, he modified this to 'transform faults' (74). In this way was born the most striking example of a new concept in structural geology in many years. Once again the elegance of the concept is best shown by one of Tuzo's own diagrams (fig. 3). As the diagram illustrates, the distinguishing feature of the transform fault as a plate boundary is the fact that the vector velocity difference between the adjacent plates is parallel to the boundary, in direct contrast to accreting or consuming boundaries.  A remarkable feature is shown in fig. 3a, where a transform fault appears to offset a ridge. Because of the plate motions, the sense of shear across the transform is the reverse of what would exist if the fault were a conventional transcurrent fault. Furthermore, as Tuzo pointed out, while traces of the transform may be seen in the bathymetry beyond the ridge segment, they will be seismically inactive because there is no relative motion across them. These properties were confirmed through earthquake mechanism studies by Sykes (1967).

The recognition of the transform fault as a type of plate boundary revolutionized the analysis of the kinetics of global plate motions. Tuzo left to others the assembling of global models of finite numbers of plates and the detailed analysis of the permitted types of intersection of three boundaries at triple junctions. But he was a past master at explaining to a general audience the concepts of the transform fault and the trace of a hot spot. He would demonstrate how a folded-paper model could be made to represent the spreading ocean floor, and then pass a piece of paper across a candle flame, at charring height, to produce a representation of the Hawaiian Islands.

The third major area of Tuzo's research during this fruitful decade concerned the life history of oceans. He returned to the study of the north Atlantic Ocean, in fact to the area of the Cabot-Great Glen fault system, and showed (81) that there was strong evidence for the dosing of an earlier Atlantic and reopening along a similar, but not identical, line. Small areas of rock, including fossil assemblages, were left on the 'wrong' side of the ocean. It is true that for as long as continental drift had been considered, the opening and closing of oceans must have been implied, and even the term 'proto-Atlantic' long predated the era of plate tectonics. But Tuzo's documentation of closing and re-opening of the Atlantic was beautifully detailed. It led (83) to his categorization of the world's oceans in terms of their stage of maturity in a life cycle (Table I). This cycle became known as the Wilson cycle of oceans.

Tuzo had been cross-appointed as Professor of Geology, and could therefore supervise research students in that Department. One occasion on which he took advantage of this arrangement was to propose to student Barry Clark that a careful mapping of the coast of Baffin island, for comparison with the coastal geology of Greenland, could provide evidence as to whether they had ever been joined. In 1964, years after he had supervised a student in geophysics, Tuzo travelled with Clark by small fishing boat along the precipitous Baffin coast and ascended cliffs several hundreds of feet high in order to start the student on his mapping (79).

By 1967 plate tectonics was accepted by the great majority of earth scientists, and Tuzo was an acknowledged leader in the study of the earth. At this very time he was asked by the President of the University of Toronto, Claude Bissell, to be the Principal of a new suburban college of the university, Erindale College. At first glance the request was an unusual one, because Tuzo had never shown great interest in university administration; he had never, for example, been head of a department. Later, he credited Isabel with convincity him to accept the post. The challenges facing the principal were daunting: it was a time of student unrest in many countries, the 'college' had 300 acres of land but only one inadequate building, and the relationships of both students and faculty to departments on the main campus were unclear. Assisted by a very able Dean (Professor E. A. Robinson) Tuzo set out to confront these. He and Isabel moved into the principal's residence, which had been the country house of the original estate acquired by the University. Their hospitality to students, faculty, and visitors to the College became famous: in a period of seven years, 10,000 guests were entertained in their home. The permanent buildings, both academic and residential, were designed with a sense of openness, attractive to students. Fields of study and research were chosen to be slightly different in emphasis from those on the main campus. In his own field, Tuzo prevailed in having a single department of Earth and Planetary Science, combining both geology and geophysics, and he stimulated the development of lines of experimental research, such as rock magnetism, which would benefit from the undisturbed suburban environment.  Altogether, Erindale College was in a very flourishing state when he left it in 1974.

Tuzo continued to publish during his term as Principal. While he produced some papers on tectonics with Erindale colleagues Kevin Burke and William Kidd (101, 103), he also moved into a more philosophical field, the structure of scientific revolutions (88, 99). He compared the change in the study of the earth which had taken place in the 1960s to the effect of quantum mechanics on chemistry and physics, or to genetics on biology. Not specifically trained in the philosophy of science himself, he acknowledged the influence on his thinking of writers such as Kuhn (1967). He urged the acceptance of plate tectonics as the new paradigm, which according to Kuhn was at the heart of every scientific revolution, and which could unify, even simplify, the study of the earth. By the late 1960s there were two outstanding opponents to plate tectonics and continental drift: Sir Harold Jeffreys on physical grounds, and V.V. Beloussov for geological reasons. Beloussov and Tuzo engaged in a debate by exchange of open letters in 1968 Geotimes, 13). Beloussov raised several points of objection, citing problems of geology to which, he claimed, plate tectonics provided no solution; he made no reference to its obvious successes. Tuzo's reply was eloquent: 'The chief difference between us is not about what you say, but arises from what you have omitted to say.' And further: 'If indeed the Earth is, in its own slow way, a very dynamic body and we have regarded it as essentially static, we need to discard most of our old theories and books and start again with a new viewpoint and a new science.'

While still Principal of Erindale College, in 1972-3, Tuzo served as President of the Royal Society of Canada (R.S.C.). He had always been a strong supporter of the Society, publishing through its journals since 1938, and being elected to fellowship in 1948. Tuzo was an effective leader of, and eloquent spokesman for, the R.S.C., but he was limited in what he could accomplish by the one-year term of office which was then the rule.

During those busy years the Wilson family escaped, in the summers, to a venerable cottage which they had acquired on the Precambrian shore of Go Home Bay, Lake Huron.  But it was not all relaxation. Tuzo and Isabel, with the help of friends, enlarged the original building to provide space for a study, where he continued to write. The renovated building was christened' Awry on the Rocks'. Tuzo' s interest in Chinese culture led him to the conclusion that the junk is a very practical water craft. He commissioned one to be built in Hong Kong, had it imported to Toronto, and with the help of his brother and friends sailed it to Go Home Bay. Fortunate were the cottage guests of Tuzo and Isabel, of whom there were many, who were taken on an evening sail on Lake Huron in the lantern festooned junk. Others could see Tuzo at the helm during the television series 'Planet Earth', which he assisted to produce and introduced.