| Abstract |
- The northern Los Angeles basin is influenced by two structural styles: the west-trending compressional Transverse Ranges to the north, and the strike-slip Peninsular Ranges to the south. The interaction of these two structural styles has resulted in a complex fold fault belt at the northern margin of the Los Angeles basin, which deforms a variable sequence of late Miocene through Quaternary marine strata. Subsurface mapping of Quaternary marine gravels by electric-log correlation documents the latest phase of deformation in the northern Los Angeles basin. The Quaternary marine gravels are folded at the Wilshire arch, the Hollywood basin, the central trough, the Newport-Inglewood fault, and the Santa Monica fault. The west-plunging Wilshire arch, which follows Wilshire Boulevard east of the Newport- Inglewood fault, is a broad fold identified and named in this study. Deformation of the Wilshire arch, which is underlain and caused by the potentially-seismogenic Wilshire fault, began around 0.8 - 1.0 Ma. A fault-bend fold model, based on the shape of the Wilshire arch, indicates a dip-slip rate of 1.5 - 1.9 mm yr for the Wilshire fault, whereas a three-dimensional elastic dislocation model indicates a right-reverse slip rate of 2.6 - 3.2 mm per year for the Wilshire fault. The finer-grained marine Pliocene strata include the late Pliocene to early Pleistocene Pico member, and the early Pliocene Repetto member, of the Fernando Formation. Thickness and lithology variations in the Pico and Repetto strata, which were influenced by syndepositional structures, indicate that the entire Pliocene and the latest Miocene were characterized by compression. The primary structure present throughout the Pliocene is a south-dipping monocline, which was underlain and caused by a deep reverse fault, dipping ~55 - 60° to the north, referred to here as the Monocline fault. Relative subsidence of the central trough resulted in deposition of up to 7000 ft (2135 m) of Pico strata, and up to 5000 ft (1525 m) of Repetto strata, compared to zero deposition on the monoclinal high. In the western part of the study area, the south-dipping monocline is interrupted by the secondary East Beverly Hills fold, which may be a rabbit-ear fold that accommodates excess volume by bedding-parallel slip. The East Beverly Hills fold was active in the latest Miocene through Pliocene, and was most active during early Pliocene Repetto deposition. In the eastern part of the study area, the monocline is interrupted by the Las Cienegas fold, which formed in the hangingwall of the Las Cienegas fault. The Las Cienegas fault was a normal fault in the late Miocene, and was reactivated in the Pliocene as a steep reverse fault. Folding and uplift on the Las Cienegas anticline occurred throughout the Pliocene, with the greatest amount occurring during lower and lower-middle Pico deposition.
- The northern Los Angeles basin is influenced by two structural styles: the west-trending compressional Transverse Ranges to the north
|
|---|