Three-stage evolution of the Los Angeles basin, southern California

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, whic h acted as the primary transform boundary in southern California during thi s episode. Such an interpretation implies that significant transform motion did not occur an 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-sta ge model for evolution of the Los Angeles basin and vicinity within the evo lving transform-fault system: transrotation (18-12 Ma), transtension (12-6 Ma), and transpression (6-0 Ma). Timing of these stages correlates with mic roplate-capture events, which occurred during conversion from a convergent margin to a transform margin.