LATE PLEISTOCENE TO HOLOCENE FOLD GROWTH ABOVE THE SERRA FAULT, NORTHERN SAN FRANCISCO PENINSULA, CALIFORNIA

The Serra fault is the northernmost fault in a zone of northwest-striking range front thrust faults adjacent to and northeast of the Peninsula segment of the San Andreas fault (SAF). The thrust faults generally dip southwest towards the SAF and probably merge with it at depth. The Serra fault lies approximately 1.5 to 3 km northeast of the SAF and extends for over 20 km from Hillsborough to near Daly City on the northern San Francisco Peninsula. Recent mapping of uplifted terrestrial/marine stratigraphic units exposed in the sea cliffs at Fort Funston in southwestern San Francisco provides evidence for late Pleistocene to Holocene fold growth above a blind northwestern extension of the Serra fault where it intersects the coastline. Bedding dips measured in the Plio-Pleistocene Merced Formation define a northeast-vergent monoclinal fold, which we interpret as a fault-propagation fold forming above the Serra fault. Three angular unconformities within the Merced and overlying Colma formations have formed on the steeply dipping fold forelimb, and appear to record the timing of onset and development of the fold during the late Pleistocene. Sequential shallowing of bedding dips across the unconformities indicates consistent and progressive northeastward tilting related to the fold development. Most significantly, mid to late Holocene fluvial channel deposits inset into the Merced Formation along the forelimb of the fold appear to be deformed, demonstrating the active nature of the folding. Optically-stimulated luminescence (OSL) dating of the uppermost strata of the Merced Formation and the Colma Formation are pending. However, the Colma Formation may correlate to recently recognized Olema ash-bearing fluvial deposits (55-75 ka) that unconformably overlie the Merced Formation at the north end of the sea cliff exposures. Assuming that the age of the unconformity can be approximated by the age of the overlying Olema ash, the apparent upwarping of the unconformity suggests average late Pleistocene uplift rates of 0.4-0.5 mm/yr at Fort Funston, and up to 0.7-0.9 mm/yr farther south at Thornton Beach. The late Pleistocene to Holocene fold growth recorded at Fort Funston demonstrates that active transpressional tectonics characteristic of the range front thrust faults may be traced as far north as southwestern San Francisco.