In the Norwegian-Greenland Sea and the Arctic Ocean the present-day North American plate boundary exhibits great variety in morphology and structural style. From the neovolcanic rift zone in Iceland the plate boundary extends into the Norwegian-Greenland Sea and the Eurasian Basin of the Arctic Ocean as a system of mid-oceanic ridge segments and transform faults, but becomes less distinct beneath the Siberian continental margin (Fig. 1). The asymmetry in the location of some of the active spreading axes, as well as the existence of extinct spreading centers, microcontinents, and volcanic plateaus and ridges imply a complex structural evolution of the surrounding ocean basins. It is reasonably well documented that the oldest oceanic crust formed by sea-floor spreading between Greenland, Lomonosov Ridge, and Eurasia dates back to the negative polarity interval between magnetic anomalies 24B and 25 (late Paleocene/early Eocene). In this chapter we focus on a description of the present plate boundary in terms of morphology, relative plate motion, variation in geological and geophysical parameters, and seismicity. For practical purposes we restrict ourselves to the youngest crust and primarily discuss the crust formed during the past 10 m.y. More comprehensive treatments of the ocean basins and their plate tectonic evolution are presented by Eldholm and others (this volume) and by Kristoffersen and others (this volume). Various aspects of the plate boundary north of Iceland have also been discussed in the Western North Atlantic synthesis volume of this series (Einarsson, 1986; Melson and O’Hearn, 1986; Schilling, 1986; Srivastava and Tapscottt, 1986; Vogt, 1986).

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