The northern termination of the Philippine archipelago is remarkably abrupt. The distinctive, almost rectangular, shape of northern Luzon is a consequence of the almost simultaneous disappearance near 18¿N of the Central Cordillera, the much narrower Sierra Madre, and the broad floodplain of the Cagayan River. The floodplain is underlain by a deep sedimentary basin that is closed off just inland of the coast by an ENE-WSW structural high known as the Sicalao Ridge. Gravity surveys show that the ridge has steep flanks that are almost certainly defined by faulting. In the same region, there is also a dramatic change in the Luzon Arc, which is the volcanic expression of subduction of the South China Sea. The volcanic centers in the Central Cordillera (the North Luzon Segment of the arc) are all now inactive, but further north eruptions are frequent on the small eastern islands of the Bashi Segment. The differences between the North Luzon and Bashi segments suggest that they had very different geological histories until the late Neogene, but the evolution of northern Luzon is so poorly understood that it is not even known on which plate it originated. New paleomagnetic data and the existence of the Sicalao Ridge provide important constraints on reconstructions and, when combined with geological data, favor the possibility that in the Paleogene the Central Cordillera and Sierra Madre were combined as parts of an arc at the southern margin of the Philippine Sea. The Sicalao Ridge can be interpreted as a rift margin feature created during the detachment of Luzon from continental Sundaland.