Description
A trench was excavated, logged, and interpreted across projections of the Rose Canyon Fault Zone in the downtown area of San Diego, California. The trench traversed 930 m (3150 ft), crossing mapped projections of the Coronado, Old Town, Rose Canyon, and San Diego Bay faults. In examining the trench exposures, a normal fault, with an apparent strike-slip component, termed the San Diego Fault, was observed on Broadway between Front Street and First Avenue in downtown San Diego. The San Diego Fault strikes N5°W, dips 60°-70°E and has a vertical separation of 10 m (33 ft) based on faunal and stratigraphic correlations within sediments that have been dated to be 360,000 to 560,000 ± 75,000 years old using amino acid racemization techniques. A paleosol that has subsequently developed on these sediments has been vertically offset a minimum of 60 cm (24 in.) and has been estimated to be 75,000 to 128,000 years old based on its pedogenic development. A 20,000-year old palesol overlies the fault and displays no evidence of offset. The San Diego Fault appears to be contemporary with the extensional stress environment found within the San Diego area. There was no readily discernible evidence of faulting elsewhere in the trench, suggesting that the major north-south faults projected through the downtown area do not offset Middle to Upper Pleistocene sedimentary rocks.
