Geology of the San Diego margin, Carlsbad to La Jolla, San Diego County, California

Abstract

Geophysical interpretation of nearly 800 line km of various types of seismic reflection data along the San Diego margin between Carlsbad and San Diego was used to correlate the adjacent onshore stratigraphy and structure with offshore seismic stratigraphic units and structures. Bedrock units projected from onshore to the shelf include the Late Cretaceous age Rosario Group and the Eocene age La Jolla Group. South of La Jolla-Scripps submarine canyon, rocks of the Rosario Group crop out in the canyon walls, while north of the canyon, rocks of the La Jolla Group crop out on the shelf. Neogene units unconformably lap onto the Paleogene bedrock units along the slope. A thin veneer of Holocene sediment overlies the bedrock on the shelf. The dominant structural feature along this portion of the San Diego margin is the Rose Canyon Fault zone (RCFZ). This zone of right-lateral shear is 2 to 4 km wide and runs northward along the shelf from La Jolla Bay to Carlsbad. At Carlsbad, it steps basinward 2 km and continues north-northwestward along the outer portion of the shelf and apparently joins the Newport-Inglewood Fault zone. The main portion of the RCFZ consists of north-northwest-trending, subparallel, discontinuous splays representing synthetic strike-slip faults within a zone of compressional right-lateral shear. The eastern side of the main fault zone on the inner shelf area is flanked by west-trending en echelon folds and north-northeast striking normal faults in the Cretaceous and Eocene bedrock. West of the main fault zone on the outer shelf and upper slope are north-trending folds in less competent Neogene and Quaternary marine sediments. Two of these folds are expressed on the upper slope as submarine ridges. Right-lateral separation along the offshore portion of the RCFZ was documented by mapping a series of en echelon faults that show apparent offset of over 300 m at Agua Hedionda, 200 m at Encinitas and as much as 700 m at both Del Mar and Soledad Valley. This compares well with the 1 to 4 km of separation along the main splays of the RCFZ onshore.