Mesozoic rocks of the Baja California Peninsula form a convergent-margin complex that is one of the best-preserved and longest-lived convergent-margin complexes in the world. It shows a three-phase evolutionary trend that we propose is typical of arc systems facing large ocean basins. The trend progresses from phase 1, highly extensional intraoceanic-arc systems, to phase 2, a mildly extensional fringing-arc system, to phase 3, a compressional continental-arc system. This trend is largely due to the progressively decreasing age of lithosphere that is subducted. The modern Earth is strongly biased toward long-lived arc-trench systems, which are compressional, and so evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California.