STYLES AND RATES OF QUATERNARY DEFORMATION IN THE SAN LUIS AREA, EASTERN DIABLO RANGE, CALIFORNIA

Seismic hazard studies in the San Luis area began in the late 1970's and focused on the 100-km-long Ortigalita fault. These early studies documented evidence of latest Quaternary dextral-shear associated with the fault and considered this form of deformation to represent the primary style of deformation in the area. A re-assessment of seismic hazards in the San Luis area began in the late 1990's. The results of these recent studies indicate the region has higher rates of activity and is more structurally complex than previously recognized, and is characterized by both dextral shear and oblique low-angle convergence, at considerably different rates.

The Ortigalita fault, the tectonic contact between the Franciscan Complex core of the Diablo Range and the Great Valley Sequence at the eastern margin, consists of a 40-km-long northern and a 60-km-long southern segment, separated at San Luis Reservoir by a 5-km-wide, right-stepping, pull-apart basin. Paleoseismic studies on the northern segment in the Cottonwood Arm of San Luis Reservoir in 1999 documented right-lateral offset of a late Pleistocene channel deposit by about 16 m. Although a numerical age for the channel deposits was not obtained, a horizontal slip rate of 1-3 mm/yr for this segment is suggested. At this latitude (37.2°N), and only 10 km to the east, the Coast Range-Great Valley (CR-GV) boundary is marked by the 21-km-long, west-dipping Quinto thrust. Uplift rates of about 0.1 mm/yr and a dip of 35° indicate a slip rate < 0.25 mm/yr.

Definitive evidence of recurrent Holocene has not been documented for the southern segment of the Ortigalita fault but we surmise a late Quaternary slip rate of about 0.5 mm/yr. Conversely, at this latitude (36.7°N) the CR-GV boundary is marked by the 37-km-long Laguna Seca thrust. Uplift rates of 0.3-0.6 mm/yr and a dip of 25-30° indicate a late Quaternary slip rate of 0.6-1.2 mm/yr for this structure. The abrupt changes in rates and styles of deformation in the eastern Diablo Range appear to be due to both inherited crustal structure and small changes in structural orientation with respect to the current stress field.