We begin this lecture at the end Paleozoic. Up to now, the tectonic assemblage of western North America has been dominated by the accretion of terranes over the passive margin. After the period of passive margin sedimentation, the Antler material overthrust the margin, then the Havallah (with a small island arc) overthrust the Antler, then finally the Sonoma Terrane overthrust the Havallah. These events were broadly experienced across the edge of western North America as we find remnants of the tectonic events from southern California to Alaska. At the southern end of this continental margin we recognize that this sequence may have been truncated by what we believe to have been a large left lateral transform fault that offset the southern extension of the continental margin sequence into the region near Caborca, Mexico. Though the sequence is poorly exposed due to extensive cover beds of Cenozoic volcanics, there are exposures of rocks like those found in the western US. This truncation fault is important because it exposed the region the southeastern corner of the US as a continental margin. This continental margin was very different compared to the margin further north where the Passive Margin, Antler, Havallah and Sonoma materials bordered, covered and insulated the continental basement of North America. In the area where the truncation happened, the continental basement was directly exposed at the new raw margin and also was not covered by thick sequences of sediment. At this time, the continental margin also began to switch from a ‘West-Pacific’ style margin (where the tectonics are dominated by irregular collisions of island arcs and continental fragments) to an ‘Andean’ style margin (where the tectonics are dominated by steady subduction of oceanic crust beneath continental curst resulting in arc volcanism and back arc or marginal basins). Courtesy of Prof. Burchfiel.