Abstract

The northern Los Angeles fault system along the southern range front of the Santa Monica Mountains includes potentially seismogenic faults directly beneath the Los Angeles metropolitan area. For a better assessment of seismic hazards, we mapped late Cenozoic faults and folds in the northern Los Angeles basin using an extensive set of oil-well and surface geologic data. The northern Los Angeles fault system developed through early to late Miocene transrotational and transtensional regimes and a Pliocene and Quaternary transpressional regime. The Santa Monica, San Vicente, and Las Cienegas faults are early to late Miocene normal faults that were later reactivated as reverse faults, suggesting that the orientation of reverse faults is largely controlled by Miocene extensional tectonics rather than by the post-Miocene stress field. Tectonic inversion occurred at the beginning of Pliocene time with the reactivation of Miocene normal faults and initiation of reverse faults. Many Pliocene contractile structures became inactive by the middle Pleistocene, and younger deformation is taken up by new active structures, including the West Beverly Hills lineament and an active strand of the Santa Monica fault. The West Beverly Hills lineament is the northernmost segment of the Newport- Inglewood fault zone, which may have propagated northward to the Santa Monica Mountains in Quaternary time. The lineament acts as a segment boundary for the active left-lateral Santa Monica−Hollywood fault system and bounds the Hollywood basin to the west. Uplift of an oxygen- isotope substage 5e marine terrace north of the city of Santa Monica and an assumed dip of >45° for the Santa Monica Mountains thrust fault underlying and uplifting the Santa Monica Mountains suggest that an average dip-slip rate for the fault is <1.3 mm/yr. Crustal shortening across the northern Los Angeles fault system accounts for less than a third of the current rate of shortening between the San Gabriel Mountains and Palos Verdes Hills based on global positioning system observations.

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