TECTONIC GEOMORPHOLOGY OF MARINE TERRACES: SANTA BARBARA FOLD BELT, CALIFORNIA

The Santa Barbara Fold Belt (SBFB) is an east-west linear zone of active folds and (mostly) blind faults on the coastal piedmont and in the Santa Barbara Channel. The fold belt is characterized by several flights of emergent late Pleistocene marine terraces uplifted and preserved on the flanks of active, anticlinal folds. At eight locations along the fold belt, the first emergent marine terrace is numerically dated by methods that include uranium-series on terrace corals, 14C ages on terrace shells and detrital charcoal, optically stimulated luminescence of terrace sands, and oxygen isotopic signatures of mollusks. The same terrace may be dated by as many as four different methods and results compared to provide a best estimate of age. Prominent marine terraces apparently correlate with sea level highstands. At several sites additional terraces are present that may represent earthquakes, change in uplift rate or climate change.

The first emergent marine terraces in the western SBFB are ~45 ka (marine oxygen isotope substage 3a) which yields a calculated rate of local surface uplift that is ~2 m/ky. First emergent terraces preserved in the eastern fold belt range from 70 ka to 105 ka (marine oxygen isotope stage 5) and rates of local surface uplift are ~1 m/ky. These rates are several times more rapid than previously thought and implies that the seismic risk for this region is also higher than previously estimated.