SUBDUCTION INITIATION IN THE IAPETUS OCEAN: WHEN, WHERE AND HOW?

Appalachian–Caledonian orogenic ophiolites occur along 6,500 km of the Laurentian margin, from Alabama to northern Norway. Well preserved examples crop out in Alabama/Georgia, Maryland/Pennsylvania, southern Quebec, Newfoundland, Scotland, the Shetland Islands, and central and northern Norway. These Laurentian margin ophiolites have supra-subduction zone (SSZ) origin, displaying a diversity of magmatic products similar to the Izu–Bonin–Mariana (IBM) Arc. Like the IBM and many Phanerozoic examples of obducted oceanic lithosphere (e.g., the Semail Ophiolite, Oman), the Laurentian margin ophiolites have been interpreted to record subduction initiation.

Existing geochronology suggests the ophiolites that decorate the eastern Laurentian margin all formed during the latest Cambrian to earliest Ordovician (495–480 Ma). These data suggest that subduction initiation could have occurred over 1000s of km of the Iapetus Ocean during a very short time interval, resulting in rapid establishment of a new convergent plate margin. The extensive Appalachian–Caledonian archive of earliest convergence in the Laurentian realm of the Iapetus offers an opportunity to study: (a) when, where and how subduction initiates within an ocean basin, and (b) the kinematics of subduction propagation throughout an ocean basin.

We present new whole-rock geochemistry and high-precision U–Pb zircon geochronology to further constrain the context for and timing of subduction initiation recorded by the Laurentian margin ophiolites. These new constraints will help to understand exact rates of convergent plate margin development and potential implications for the global plate tectonic system, including any potential role of sudden, widespread subduction initiation in driving plate reorganization events.