The Victorian Division of the GSA held its annual Selwyn Symposium in Melbourne on 26th October to a packed house of 120 geologists. Many aspects of plate tectonics were covered by the speakers, including the Selwyn Lecturer, Prof Chris Powell, UWA. The abstracts volume, Geological Society of Australia Abstracts 63 is recommended to all (misha@gsa.org.au).

The symposium started with a historical perspective of tectonics by Homer Le Grand, an earth science historian. Homer, with his North American accent, gave credibility to a review of the initial misgivings of North American geologists about plate scrapings but in the end embracement of the ideas. The science then progressed towards concepts of terranes and plate shufflings. As an outsider and non-geologist, but trained observer, he made the point that the simple theory of subduction and plate tectonics is not easily applied in relation to continental tectonics. Indeed, he maintained that empirical and fine-scale established maps and geological data can be re-interpreted in terms of terranes, and that this is perhaps the way to solve many intractable problems.

Mike Sandiford illustrated his ideas on the use of topographic relief relative to the geoid as a key to understanding aspects of plate tectonics. He reminded us of the driving forces of plate tectonics as being the resultant of the loss of heat from the Earth versus gravitational pull. He told us about flexure topography being a consequence of lithospheric density variations, and of dynamic topography reflecting large-scale convective mantle flows, best seen in a resultant residual field after deducting isostatic contributions. He noted the anomalously deep nature of the Australia–Antarctica sub-sea area, perhaps caused by mantle downturn to our south. During the last 150 My, Australia has moved north across this anomaly and this may have contributed to Mesozoic flooding; Australia is now moving towards a dynamic and geoid high in the western Pacific. A fascinating analysis.

Pat Vickers-Rich brought us all up-to-date on the scientific voyage of discovery and of interpretations of dinosaur and other animal fossils found in southern Victoria. As a non-palaeontologist, I was interested to learn that the Mesozoic dinosaurs were living in a very cold sub-polar climate, 7–10 C, comparable with say Chicago or Fairbanks today. Her work and that of her many collaborators is being reflected in the naming of many species after them and the funding companies, such as Oricasaurus, and Qantasaurus!

Mark Barley reviewed various ore deposit models in relation to plate tectonics in a conventional mode but added some controversial spice that made a lot of sense. His contribution was to query the static model of mineral deposits in relation to simplistic subduction models, and assumptions of uniformitarianism. He tended towards episodic periods, a view that I personally support. He maintains that assumptions of erosion removing geological record as an explanation as to why some deposits are scarce in certain sections of time must be revisited. Analysis suggests that episodic changes in the relative movement of plates and ocean crust can result in epochs of ore deposits. For instance at around 5 Ma in the SW Pacific a widespread mineralising Cu–Au porphyry and epithermal gold episode occurred. This may reflect the reactivation of fertile sub-arc mantle reservoirs with previously integrated subduction components. The complexity of reconstruction of the most recent and most readily observable plate movement geology makes me wonder how we are ever going to be confident of reconstructions of more incomplete records back in the immensity of time that we deal with in geology. As always the exploration geologist's job of forecasting where the next big one might occur, on a theoretical basis, becomes ever harder as more information comes to hand!

Andy Gleadow showed some excellent GPS digital terrain data implying major plate movements between Australia and North Victoria Land in Antarctica and highlighted the complexity of plate boundaries. He led us through the investigation of the low-temperature thermal record of lithospheric plates using apatite fission-track information, and focussed our attention on the margins of the plates as being where action takes place during extension tectonics. Fission-track ages along the east coast of Australia are lower than the age of the host rocks. This anomaly varies from place to place along the coast and shows differences in thermal histories at different points along the continental margin; in turn, these differences reflect variations in denudation/tectonics during continental breakup rather than specific thermal events. He reminded us of the controversial nature of various geomorphological theories of the various processes that are operating along the coast, with perhaps scarp retreat being the preferred model.

The evening Selwyn Lecture, by Chris Powell, was a highlight of the symposium. In his own pleasantly professorial way, Chris walked us through the politics of scientific thinking as ideas on the tectonics of the Earth evolved since Wegener. Those of us in the audience who came from the Uni of Tas, Sam Carey School, enjoyed the recap enormously. Chris held the large audience in the palm of his hand with as smooth a delivery as you would ever enjoy. He tried to engage the younger ones by encouraging boldness of scientific view contrary to conventional wisdom and to not be timid in having views contrary to the ‘establishment’. But he noted that there are practicalities of life that compromise an independent stance; for example, only a small proportion of applications for ARC grants are successful. Chris graphically reminded us of the immense task in hand of attempting plate reconstructions over several periods of major and minor plate breakup and collision during the Earth's history. The Symposium concluded by noting that we are only at the start of the job and so the science of tectonics lives on!

Overall, a very rewarding day, with special congratulations to the organisers. Peter Legge