HOLOCENE DISPLACEMENT HISTORY OF RIDGETOP DEPRESSIONS (SACKUNGEN) IN THE SAN GABRIEL MOUNTAINS, SOUTHERN CALIFORNIA

The San Gabriel Mountains contain anomalous ridgetop depressions that resemble depressions formed or reactivated during large earthquakes elsewhere in California and worldwide. We trenched three ridgetop troughs, two near the San Andreas fault in Pelona Schist, and one northeast of the San Fernando Valley in granitic gneiss. Two of the three trenched ridgecrest troughs were the surface expressions of half-grabens with moderately dipping (45o-65o) normal faults, whereas the third trough was underlain by a more symmetrical graben. Each trench exhibited stratigraphic evidence for 2-4 Holocene displacement events. In the Blue Ridge and Upper Lytle Creek Ridge trenches, the most recent event (MRE) occurred after 790-867 cal yr. BP but before 285-300 cal yr. BP. This time span overlaps the age ranges of three dated paleoearthquakes on the San Andreas fault at the Wrightwood paleoseismic site of Fumal et al. (1993), only a few km NW of the Blue Ridge trench. Dating constraints from the sackung trenches, due to the dispersed nature of soil organics and charcoal, are not tight enough to make a positive correlation with any particular paleoearthquake on the San Andreas fault. The older displacement events at Blue Ridge and Upper Lytle Creek Ridge are generally older than the oldest dated event in the Wrightwood or Pallet Creek chronologies. The MRE at the Kagel Mountain site is relatively well bracketed between 1830 and 2355 cal yr. BP. This age range barely overlaps with the age range of the latest paleoearthquake on the San Gabriel fault, the closest active fault to the site, as dated by Cotton et al. between 907 and 1993 cal. yr BP. We have demonstrated that sackung troughs contain a stratigraphic record of repeated displacement events that can be dated. However, given the type and distribution of carbon in the trough deposits, it will require 2-3 times as many radiocarbon dates as we had budgeted for to bracket event dates well enough to compare them with paleoearthquake chronologies from other trench studies.