The Los Angeles basin formed in late Neogene time on a continental margin previously shaped by Cretaceous and early Paleogene subduction, Paleogene terrane accretion, and mid-Miocene rifting and block rotation. During Neogene time, the boundary between the Pacific and North American plates shifted progressively eastward beneath the Los Angeles region, creating the broad San Andreas transform zone. As reviewed in this paper, structures and rocks within the Los Angeles basin document each stage of that Neogene evolution.
The Los Angeles basin began to take its present shape in late Miocene time (ca. 7 Ma) by subsidence between the right-oblique Whittier and Palos Verdes fault zones and the left-oblique Santa Monica fault system. The principal phase of basin opening involved early Pliocene extension in a northwest direction, which accompanied the opening of the Gulf of California and the eastward shift of the southern San Andreas fault to its present position. Most of the structural traps that hold the basin's oil fields began to form during this latest Miocene-early Pliocene deformation.
Since mid-Pliocene time, many of these traps have been altered and enhanced—and a few have been breached—by Pasadenan deformation, involving southward shortening, the uplift of the Transverse Ranges, and the propagation of blind thrusts beneath the northern Los Angeles basin. The rapid transition from early Pliocene extension to late Pliocene contraction was associated temporally with a change in relative plate motion dated at 3.9- 3.4 Ma. In analyzing Pasadenan deformation, the flake-tectonics model is more appropriate than the fold-and-thrust-belt model, although both models incorporate aseismic detachment at midcrustal depths. The flake-tectonics model is valid for all phases of Neogene deformation, both transtensional and transpressive, in the Los Angeles region.
Fields discovered to date in the Los Angeles basin will yield an ultimate 10.4 billion oil-equivalent barrels (GOEB) of petroleum. Of this, approximately 73% is trapped in faulted anticlines, 12% in simple anticlines, 10% in fault traps, and 5% in stratigraphic traps. Folding has been controlled primarily by preexisting structural hingelines and sedimentary wedge belts and secondarily by en echelon folding associated with wrench faults. Oil seeps and Quaternary topographic uplifts led to most of the discoveries prior to 1925 along the Whittier and Newport−Inglewood fault zones and in the Coyote Hills. Most later discoveries, including the 3-billion-barrel Wilmington oil field, were in structures with little or no Quaternary expression.