Modeling of gravity and magnetic data shows that areally extensive mafic and ultramafic rocks of the western Brooks Range, Alaska, are at least 8 km thick, and that gabbro and ultramafic rocks underlie basalt in several places. The basalt, gabbro, and ultramafic rocks have been considered parts of a far-traveled ophiolite assemblage. These rocks are the highest structural elements in the Brooks Range thrust belt and are thought to be hundreds of kilometers north of their origin. This requires these rocks to be thin klippen without geologic ties to the continental shelf sedimentary rocks that now surround them. The geophysically determined, thick and interleaved subsurface character of the basalt, gabbro, and ultramafic rocks is inconsistent with this interpretation. An origin within an extensional setting on the continental shelf could produce the required subsurface geometries and explain other perplexing characteristics of these rocks. Early Mesozoic Alaska, from the North Slope southward to the interior, may have had many irregular extensional basins on a broad, distal continental shelf. This original tectonic setting may apply elsewhere in Cordilleran-type margins where appropriate mafic and ultramafic analogs are present.