THE EARLIEST SUBDUCTION ZONE DEPICTED: A TRIUMPH OF COMPARATIVE GEOLOGY

When the Index and Maps volume of the authorised English translation of Eduard Suess' monumental Das Antlitz der Erde (=The Face of the Earth) appeared very belatedly (because of the Great War) in 1924, the geological world was presented with a series of hand-drawn sketches by Suess himself (drawn before 1914) sent to the English editor William Johnson Sollas upon the latter's request for certain clarifications. Two of these sketches were cross-sections: One across Asia, from the Siberian Table-Land (within the Amphitheatre of Irkutsk) and the other across a deep-sea trench of some 7000 m depth bordering 1-2000 m-high mountains. It is clear that the second sketch is meant to fit to the right end of the first and was drawn with the Kuril-Kamchatka Trench in mind. The section thus shows a table land at left with horizontal strata forming a vertex. Towards it a series of backfolds are seen. Then forefolded high mountains first rise to 2000 m and then slope down to an inbreak forming a broad bay and then the mountains rise again but this time atop a huge thrust fault that dips at a moderate angle below the peninsular mountains from a foredeep forming the trench. The sketch has no scale, but it is clear that the giant thrust goes no less than 1100 km inland beneath the marginal mountains. This is the same scale as the subduction zone in the same place today. In his book Suess had inferred that the same thing was going on below the Andes and there the downthrust sediments were contaminating the volcanics erupted. The significance of the sketch is that it was drawn only on the basis of surface geological and topographical information. Suess' database was immense. His insight was lost later during the Dark Intermezzo of tectonics (1924-1965) when geologists fell under the tyranny of certain mathematical geophysicists. They were rescued by Tuzo Wilson in 1965, but still have not fully grasped Suess' immortal methodology: detailed, comparative geology of the entire planet.