Seismic stratigraphy and fault activity of the inner San Pedro shelf, California

As part of the southern California Continental Borderland, the San Pedro Shelf represents the southwestern extension of the Los Angeles Basin. The wide configuration of the shelf is predominantly a function of tectonic activity and the resulting sedimentation patterns. The major structural features of the San Pedro Shelf are the Palos Verdes uplift and Palos Verdes fault, the Wilmington Graben, and the offshore extension of the Newport Inglewood Zone of Deformation. Pliocene sediments of the Wilmington Graben consist of the Repetto and Pico Formations. Cumulative thickness of these formations ranges from tens of meters in the inshore area to hundreds of meters in the axis of the graben. Quaternary sediments consist of the lower Pleistocene San Pedro Formation, a series of upper Pleistocene deposits designated LP IV through LP II, and Holocene sediments. The upper Plesitocene units represent stratigraphic sequences and they have been correleted to published sea level fluctuation curves and corresponding oxygen isotope stages. A series of faults corresponding to faults mapped by Junger and Wagner ( 1977) have been identified on the shelf. These faults, which bound the landward edge of the Wilmington Graben, do not disrupt sediments younger than lower Pleistocene. It is suggested that the faults became inactive when the right-lateral shear regime of the present-day Newport Inglewood fault took effect in early Pleistocene time. It is possible that these faults represent an old branch of the Newport Inglewood fault. Two active (?) faults offshore of Newport Beach are believed to represent offshore extensions of the Newport Inglewood Zone of Deformation. Recent activity on this zone is also evidenced by recent structural growth on the Huntington Beach anticline. Sedimentation patterns of the Pleistocene units imply continued uplift of the anticlinal structure during the depositional periods. Several minor faults exhibiting upper Pleistocene deformation are mapped on the southern flank of the anticline. These faults trend northeast-southwest and show evidence of movement in late Pleistocene time.