ANALOGY BETWEEN TETHYAN-TYPE AND PACIFIC-TYPE OROGENS

A long standing issue in tectonics has been the contrast between Tethyan-type and Pacific-type orogens. Tethyan (Alpine/Himalayan) orogens involve the convergence and collision of previously separate continents, and island arc collision and/or ophiolite emplacement along one continental edge or another prior to final collision. The role of collisions in Pacific-type orogeny is more controversial. "Exotic" or "suspect" terranes characterize the North American Cordillera, and are present in parts of the Andes and Japan. Seismic (Snyder et al., 2002) and structural (Hansen and Dusel-Bacon, 1998) data indicate that in Canada and Alaska, respectively, many if not most terranes are thin sheets, reminiscent of Alpine nappes. A model for the Mesozoic-early Tertiary Cordilleran orogen of the conterminous US invokes initiation by island arc-continental margin collisions and ophiolite emplacement with subsequent oceanic plateau subduction; this model with modifications may be applicable to the entire North American Cordillera. A similar model for the Andes is possible, based upon the presence along much of the Andes’ length of diachronous onset of deformation, oceanic terranes along the Pacific margin, and a remnant marginal basin. Both the North American Cordilleran and the Andean orogens may have been initiated by arc-continent collisions (reminiscent of active collisions in Taiwan, Sangihe-Halmahera or Indonesia-Australia in the western Pacific), and subsequent reactivation of previously deformed regions through oceanic plateau/aseismic ridge subduction and flat slab development, as well as orogen-subparallel strike-slip faulting. Pacific-type and Tethyan-type orogens may not be so different, after all.