QUATERNARY DEFORMATION RATES CONSTRAIN BLIND FAULT ACTIVITY, NORTHERN LOS ANGELES BASIN, CALIFORNIA

A three-dimensional stratigraphic and structural model has been constructed using cored boreholes to evaluate Quaternary growth of a complex south-verging monocline that occurs along the northern margin of the Los Angeles basin central trough. The monocline has been proposed to be undergoing uplift and folding by active “blind” faulting. Our data indicate that growth of the northern shelf monocline continued into the early Pleistocene as evidenced by progressive basinward tilting of chronohorizons, vertical uplift of the northern shelf monocline, and subsidence in the central trough of the Los Angeles basin. However, activity slowed significantly during middle Pleistocene time. Deformation virtually ceased by the oxygen-isotope stage 9 highstand, ~320 ka. The oxygen-isotope stage 9 highstand shoreline angle along the western margin of the northern shelf is at an elevation of 41 to 44 m, constraining vertical uplift to 0.11 to 0.13 mm/yr. This value is within the range of ambient regional uplift.

Recent structural interpretations using various fold models have provided long-term (Pliocene-Recent) vertical slip rate estimates of 0.26 to >1.3 mm/yr for blind thrust fault(s) inferred to underlie the monocline. These rates are a factor of 2 to an order of magnitude greater than our data allow. Deformation of Quaternary sediment instead indicates that rates of more recent compressive fault activity in this portion of the northern Los Angeles basin are considerably slower than previously proposed and may have essentially ceased. Model derived long-term deformation rates overestimate late Quaternary blind-fault activity of the northern shelf, which suggests that long-term rates may not adequately predict blind-fault activity in other areas of the basin as well. Therefore, we urge caution in applying long-term slip rates derived from blind-fault structural models to present-day seismic hazard analysis.