Most tectonic models attribute the opening of the Los Angeles basin to lithospheric extension produced by breakaway of the Western Transverse Ranges from the Peninsular Ranges and more than 90 degrees of clockwise rotation from ca. 18-8 Ma. Evidence of this extension includes crustal thinning on tomographic profiles between the Santa Ana Mountains and the Santa Monica Mountains and the presence in the Los Angeles basin of Middle Miocene volcanic rocks and normal faults. However, to account for the amount of extension produced by rotation of the Western Transverse Ranges, strata of the Middle Miocene Topanga Formation (ca. 16-14 Ma) should be distributed on isolated structural blocks that are widely separated by normal faults. This distribution does not appear to be demonstrable from well data. Across the northern part of the Los Angeles basin, Topanga strata can be correlated as an almost unbroken unit from the Santa Ana Mountains to the Santa Monica Mountains. In the southern Los Angeles basin, Topanga strata also exist relatively intact over a wide area. These observations suggest that the rift boundary of the Los Angeles basin is not yet well known and, at least in the northern Los Angeles basin, significant crustal thinning has not taken place since ca. 14 Ma.

The closing of the Los Angeles basin in response to N-S contraction, which began at ca. 8 Ma and continues today, has resulted in a system of faults that are a significant seismic hazard to the greater Los Angeles region. The distribution of these faults may have been strongly influenced by crustal heterogeneities that developed during the previous extension phase. Some Miocene normal faults have been reactivated as reverse faults to form inverted half grabens. Spatial relationships between regions of high tomographic P-wave velocities in the middle crust (possibly Middle Miocene plutons) and active faults suggest that the “plutons” have influenced the geometries of the faults and may act as barriers that limit the potential for an earthquake rupture to propagate from one segment to another. These observations raise the intriguing possibility that better knowledge of the distribution of middle crustal heterogeneities may prove to be of critical importance in the assessment of the seismic hazard of the Los Angeles basin, especially by providing constraints for active fault models.