In the Los Angeles basin, the Mesozoic geologic boundary between Peninsular Ranges basement on the east and Catalina Schist (Franciscan) basement on the west is now covered by middle and late Cenozoic strata. Most earlier workers have assumed that the basement boundary is a fault, and that its expression in the younger strata is the Newport-Inglewood zone of faults and folds. However, the basement rocks of the Newport-Inglewood zone, the Alondra oil field west of it, and the Brea-Olinda oil field east of it contain actinolite-bearing greenschist and serpentine, as assemblage unlike either the Catalina Schist or Peninsular Ranges basement. The closest Catalina blueschists are 5 km southwest of the zone. Retrograde plutonic rocks found beneath the Long Beach, Inglewood, and Las Cienegas oil fields are petrographically unlike Peninsular Ranges basement, but are similar to amphibolite-facies tectonic blocks associated with the Franciscan in northern California and are so interpreted here. Retrograde metamorphism and cataclastic textures in Los Angeles basin basement rocks may be related to thrusting accompanying Mesozoic subduction. The distribution of these rocks suggests that the Peninsular Ranges-Franciscan basement boundary does not follow the Newport-Inglewood zone in the Los Angeles basin, but instead departs from it north of Sunset Beach oil field, trending northerly between the Anaheim nose and Las Cienegas oil field.

Cenozoic structural patterns in the Newport-Inglewood zone are quite diverse. Sunset Beach, Huntington Beach, and West Newport oil fields within the zone and Wilmington oil field west of it are characterized by north-trending normal faults with no movement younger than early Pliocene. Dominguez, Rosecrans, and Howard Townsite oil fields are characterized by west-trending reverse faults of the same age as the normal faults. Northwest-trending faults, with no more than 3 km of total right-lateral slip, cut across these diversely oriented structures and affect beds as young as Pleistocene.

The western Los Angeles basin is visualized as a sedimentary blanket which, except for bedding, is structurally isotropic, overlying a basement with diversely oriented structural anisotropies. As the area was subjected to simple right-lateral shear in late Miocene and early Pliocene time, these basement anisotropies propagated upward into the sedimentary blanket as fault systems on which the displacements were controlled by their orientation. West-trending faults were reverse, north-trending faults normal, and northwest-trending faults right-lateral. As distortion continued, the high-ductility-contrast Peninsular Ranges-Catalina Schist basement boundary southeast of the Los Angeles basin propagated itself as the right-lateral slip Newport-Inglewood fault zone northwestward across the ower ductility-contrast Franciscan and greenschist basement of the Los Angeles basin. Localization of shear within this zone caused the older, diversely oriented normal and reverse faults to become inactive and produced the shear system of today.

The older, diversely oriented fault systems are in part parallel with adjacent outcropping fault systems of middle Miocene age in the San Joaquin Hills. Both systems may have been controlled originally by the breakup of the northern end of the Peninsular Ranges as the East Pacific Rise reached the edge of the continent. This caused the Channel Islands-San Nicolas Island block and the Santa Monica Mountains block to move west from the Peninsular Ranges, leaving behind a rift which became the Los Angeles basin.