TERRANE FORENSICS: AN HISTORICAL PERSPECTIVE

The terrane concept was embraced with enthusiasm by many Appalachian geologists as a way to deal with the apparently ever-increasing complexity of the orogen during the data boom of the 1980s. The apparently straight-forward division into orogen-parallel and orogen-extensive zones, as developed by Harold Williams and others in the 1970s, became increasingly difficult to manage as more detailed mapping, especially coupled with U-Pb dating, showed that the geological components were more diverse in stratigraphy, age, and magmatic, metamorphic, and deformational history than previously assumed. Plutonic rocks in Maritime Canada showed the story particularly well: plutons assumed to be almost entirely Devonian yielded ages ranging from Mesoproterozoic to Carboniferous and almost everything in between. As a result, terranes proliferated throughout the areas previously assigned to the Avalon, Gander, and Dunnage zones. Only Meguma and Humber seemed to hold on to their integrity. However, it soon became apparent that identifying terranes was the easy part; more difficult proved to be the process of convincing colleagues of the validity of the terranes, and understanding their original relationships to one another and their place in the global continental mosaic, itself a matter of controversy. Then the issue of terrane forensics came to the forefront: what robust criteria can we use to distinguish one terrane from another, and to trace its parentage? We tried them all. To answer the first question, initially it seemed that contrasts in geological history and lack of shared history should be convincing - but was not. Then contrasts in isotopic compositions as indicators of lower crustal composition seemed promising, but if a terrane is large, considerable diversity might, and should, be expected. Paleomagnetism is fraught with uncertainty, and does not seem to work where one needs it most. In terms of tracing parentage, detrital zircon ages looked promising for a few years, but distinctive, unambiguous fingerprints still elude us. Detrital muscovite ages are looking promising, but as in the case of detrital zircon, a definitive signature is needed for the potential parent continents, and such data remain rare. Is there hope for terrane forensics? Maybe not, but the fun of searching for the answers never diminishes.