CONTINENT-PROXIMAL BACKARC BASIN DEVELOPMENT AND CLOSURE IN THE NORWEGIAN CALEDONIDES, NORTHERN APPALACHIANS AND ANATOLIAN TETHYSIDES

The late Ordovician (ca. 443 Ma) Solund-Stavfjord ophiolite complex (SSOC) in the western Norwegian Caledonides represents an Iapetan, early Paleozoic backarc oceanic crust. N-MORB Fe-Ti basalts of the SSOC and their trace-element patterns indicate a weak subduction influence in their melt evolution, and the Nd isotope signatures suggest derivation of their magmas from an isotopically homogeneous melt source. The occurrence of quartz-rich, continentally derived sediments in its sedimentary cover indicate that the SSOC’s tectonic setting was proximal to a continental margin. We posit that the SSOC backarc basin evolved above a subduction zone dipping westward beneath today’s Greenland. As the subducting Iapetus oceanic lithosphere and the associated trench system retreated eastward due to slab rollback, the upper-plate extension led to seafloor spreading tectonics and the opening of the SSOC backarc basin. We show that this type of backarc basin development adjacent to an active continental margin was also common in the Northern Appalachians (e.g. ~473 Ma Lloyds River ophiolite) and in the Anatolian segment of the early Mesozoic Tethyan orogenic belt (e.g. late Triassic-Jurassic Küre ophiolite); it is also reminiscent of the geodynamic evolution of the modern Lau Basin and the Andaman Sea. However, the closure mechanisms of these continent–proximal backarc basins in the Norwegian Caledonides, Northern Appalachians and Anatolian Tethysides appear to have varied significantly. The SSOC and early Ordovician ophiolites were telescoped into the opposite Baltica margin during the Baltica–Laurentia (Greenland) continental collision. The BAB ophiolites in the Northern Appalachians and Anatolian Tethysides were collapsed into the continental margins along which they developed initially, prior to the continent-continent collisions in these orogenic belts.