LINKING PENOBSCOT AND TACONIC DEFORMATION EVENTS IN THE NORTHERN APPALACHIANS AND CALEDONIDES

In the development of the Appalachian-Caledonide Orogen, the earliest stages of deformation of peri-Laurentian and peri-Gondwanan rocks took place concurrently from latest Cambrian into Early Ordovician. Deformation close to the Laurentian margin, preserved in the foreland belt of the orogen, is characterized as Taconian (in N. America) or Grampian (in Europe) whereas deformation of the Gondwanan margin, preserved in the hinterland, is characterized as Penobscottian (in N. America) or Monian (in Europe). Some models for the origin of this deformation require that subduction was initiated along passive margins on both sides of the young Iapetus Ocean. However, the record in Mesozoic-Cenozoic oceans suggests that spontaneous inversion of passive margins is tectonically unlikely.

Volcanic arcs were certainly present in the developing ocean system by late Cambrian time, as recorded in numerous parts of the orogen, from Maine and Newfoundland, through Ireland and Great Britain, to Scandinavia. Many show isotopic signatures suggesting that they did not originate on older continental margins. Kinematic records of the direction of thrusting in the early ophiolite emplacement events are rare. However, in coastal Maine, deformation appears to have been northwest-directed, suggesting that mafic and ultramafic rocks were emplaced on Ganderia from a still more outboard position (relative to Laurentia). The subsequent histories of arc-microcontinent systems within Iapetus were complex, involving the opening of back-arc basins within an ocean that was progressively closing. Paleomagnetic data suggest substantial vertical-axis rotations during ocean closure.

Traditional models for the Appalachian-Caledonide system invoke subduction initiation by spontaneous inversion of passive margins. Our observations support an alternative model, in which arc systems entered the Iapetan realm from an external ocean, a process similar to the entry of the modern Caribbean and Scotia plates into the Atlantic. Thus the Taconian and Penobscot deformation episodes may have occurred at different points on a single sinuous subduction system. Closure of the Iapetus eventually incorporated both peri-Gondwanan and peri-Laurentian microcontinents, with their records of early deformation, into a complex orogen.