Tectonic geomorphology of marine terraces: Santa Barbara fold belt, California

Abstract

Uplifted marine terraces are common landforms in coastal regions where active tectonics are an important component of landscape evolution, such as along the coastal stretches of southern California. The pattern and elevation of shoreline angles on active folds provide information about rates of uplift and fold growth, which is important for defining tectonic models. A particularly impressive succession of marine terraces are developed across the Santa Barbara fold belt (SBFB) in southern California, which comprises 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 anticlines. At several locations along the fold belt, the first emergent marine terrace is numerically dated by methods that include uranium-series dating on terrace corals, 14 C dating on terrace shells and detrital charcoal, optically stimulated luminescence of marine terrace sands, and oxygen isotopic signatures (δ 18 O) of mollusks. Individual marine terraces have as many as four ages, using up to three different dating methods, providing confidence in terrace chronology. Ages of higher terraces are estimated assuming a constant rate of uplift for a particular flight. Of the 31 terraces, 22 formed during a time of falling sea level, with 9 forming at or near marine oxygen isotope stage (MIS) 3 or 5 highstands. Ages and rates of uplift of the first emergent terrace vary systematically from west (younger and higher) to east (older and lower). The first emergent marine terraces in the westernmost SBFB are approximately 45 ka (MIS 3), and the rate of local surface uplift is ∼2 m/k.y. In the central part of the belt, first emergent terraces date to 60–70 ka (MIS 5), and uplift rates decrease to ∼1.2 m/k.y. First emergent marine terraces preserved in the easternmost fold belt range from 70 ka to 105 ka (MIS 5), with rates of local surface uplift of ∼0.5 m/k.y. Lower rates of uplift in the eastern end of the fold belt result from the MIS 5 terrace being tilted down into the Carpinteria syncline. Rates of vertical uplift in the western end of the fold belt are about six times higher than previously reported, suggesting the seismic hazard is also greater.