Old Orogens

Abstract

Since Early Proterozoic times, Wilson cycles have operated, each beginning with the breakup of continents, evolving into the stage with a mature ocean framed by passive continental margins, and concluding with subduction and collision (Chap. 11). Remnants of oceans, present as ophiolite complexes in orogens, are the best indication of plate tectonic processes and mountain building. However, ophiolites older than 800 Ma are uncommon. Older ophiolite complexes do exist that resemble those in young Alpine orogens. They were derived from oceans formed 2500–2000 Ma ago by the break-up of Archean cratons and 1600–600 Ma by the break-up of the supercontinents Panotia and Rodinia. Examples are the Zunhua–Wutaishan ophiolite in northeast China, the Purtuniq ophiolite in northeast Canada, and the Jormua ophiolite in Finland (Fig. 12.1). These ophiolites suggest that oceanic crust at that time was at least as thick as in modern oceans (ca. 6 km). Probably oceanic crust was generally even thicker than today and thus reflects a hotter mantle with a higher percentage of basaltic melts beneath the mid-ocean ridges. In the mentioned examples, the ocean became subducted; this led to collision of the opposing continents and mountain building, similar to the case in younger Earth history. In fact, the Early Proterozoic Wopmay orogen in Canada shows an evolutionary history (including the sedimentary record) that resembles modern orogens in much detail; therefore, it is generally accepted that the plate tectonic processes at that time closely mimicked those of the present.