Cenozoic extension started about 30 million years ago (Ma). There are two phases of extension observed: late Oligocene and middle Miocene. The first period of extension produced broad, shallow basins bounded by low-angle faults. The crust may have been extended as much as 50% during this episode. Widespread magmatism in mid-Cenozoic time suggests that the lithosphere was hot, the brittle-ductile transition was relatively shallow. There is evidence that the second period of extension began earlier in the central and northern Rio Grande rift than in the south. A third period of extension may have begun in the early Pliocene.