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The Los Angeles basin is a polyphase Neogene basin within the San Andreas transform system between the Pacific and North American plates. The basin was initiated in the mid-Miocene by widespread extension associated with significant strike slip and rotation of the Transverse Ranges of southern California. Late Miocene to early Pliocene extension, which accompanied the opening of the Gulf of California, led to the principal phase of basin opening. The early Pliocene to Recent deformational history of the basin is characterized by shortening associated with the active North Los Angeles fold and thrust system.

The Los Angeles basin is the richest basin in the world in terms of hydrocarbons per volume of sedimentary fill. Each phase of basin evolution has contributed to the basin's productivity. Some aspects of the basin's history that have affected the occurrence of oil and gas are related to basin-forming mechanisms and can be used to guide thinking in similar settings. Other first-order controls on hydrocarbon occurrence stem from processes that operated on a much larger scale and are not related to basin type. Deposition of thick, high-quality source rock within the Los Angeles basin is the result of such regional controls.

The polyphase history of the Los Angeles basin demonstrates the complexity that can occur along active transform margins. Such complexity can be expected in basins that have formed in similar settings.

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