Paleozoic rocks exposed across the northern flank of the mid-Cretaceous to Late Cretaceous Koolen metamorphic dome make up two structurally superimposed tectonic units: (1) weakly deformed Ordovician to Lower Devonian shallow marine carbonates of the Chegitun unit which formed on a stable shelf and (2) strongly deformed and metamorphosed Devonian to Lower Carboniferous phyllites, limestones, and andesite tuffs of the Tanatap unit. Trace element geochemistry, Nd isotopic data, and textural evidence suggest that the Tanatap tuffs are differentiated calc-alkaline volcanic rocks possibly derived from a magmatic arc. We interpret the associated sedimentary facies as indicative of deposition in a basinal setting, probably a back arc basin. Orthogneisses in the core of the Koolen dome yielded a Devonian (between ~369 and ~375 Ma) U-Pb zircon age which is similar to the ages of the Tanatap tuffs as well as granitic plutons formed within a Devonian active continental margin of northern Alaska. The stratigraphy of the Chegitun unit is similar to that of the Novosibirsk carbonate platform which overlies the Late Precambrian Bennett-Barrovia block. The basement of the block is exposed in Chukotka where ortogneiss in the Chegitun River valley yielded Late Proterozoic (~650 to 550 Ma) U-Pb ages. These two tectonic units form the shelf of the Chukchi and East Siberian Seas and may continue into northern Alaska as the Hammond subterrane. The deep-water Tanatap unit can be traced along the southern boundary of the Bennett-Barrovia block from the Novosibirsk Islands to northern Alaska. This basin was paired with a Devonian magmatic arc that existed farther to the south. The northern margin of the Bennett-Barrovia block collided with North American in the Late Silurian to Early Devonian. In Chukotka, during Middle to Late Carboniferous time the reconstructed Devonian arc-trench system at the southern edge of the Bennett-Barrovia block collided with an unknown continental object, fragments of which now occur to the south of the South Anyui suture. Triassic to Cretaceous deformation strongly modified the Paleozoic units. Our results provide new constraints on the geometry and Paleozoic history of the Chukotka-Arctic Alaska block, the essential element involved in the opening of the Canada basin. ¿ 1999 American Geophysical Union