We propose that an episode of transtension dominated development of the Los Angeles basin area from 12 to 6 Ma, following middle Miocene transrotation and prior to the modern transpressional regime. Transtension resulted from the releasing bend of the San Gabriel–Chino Hills–Cristianitos fault, which acted as the primary transform boundary in southern California during this episode. Such an interpretation implies that significant transform motion did not occur on the southern San Andreas fault prior to 6 Ma and that the Gulf of California has opened primarily since 6 Ma. We propose a three-stage model for evolution of the Los Angeles basin and vicinity within the evolving transform-fault system: transrotation (18–12 Ma), transtension (12–6 Ma), and transpression (6–0 Ma). Timing of these stages correlates with microplate-capture events, which occurred during conversion from a convergent margin to a transform margin.

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