Five well-expressed marine abrasion platforms, and up to 10 additional poorly expressed ones, are preserved from Point Conception to Gaviota in the western Transverse Ranges, southern California. The terraces, which range in elevation from 10 to over 300 m, and in age from 80 ka to over 1 Ma, record the long-term history of uplift for this region of rapid tectonic convergence. The lowest four terraces have direct age control and correlate to oxygen isotope stages 5a, 5c, 5e, and 7. The fifth terrace presumably correlates to stage 9. Age control is based on: (1) amino-acid stereochemistry on mollusk shells collected from the lower four terraces; (2) uranium-series analyses of bone and teeth from the first and fourth terraces and their alluvial covers; (3) the assemblages of molluscan fauna associated with the first, second, and third terraces; and (4) inferred long-term rates of uplift based on the ages and elevations of the terraces.

The terraces are folded across the Government Point syncline near Point Conception. The shoreline angle for the 80-ka terrace decreases in elevation by 8 to 10 m across the hinge of the syncline, with respect to its elevation to the east and west. Furthermore, platform gradients are steeper on the north limb, indicating asymmetric synclinal deformation. Deformation of the lowest three terraces indicates average folding rates in the range of 2 to 9° per million years, consistent with the total post-Miocene deformation. Along with the changes in slope and elevation for the terrace platforms and shorelines, the presence of several flexural-slip bedding-plane faults cutting the platforms and their overlying deposits also supports active folding of the syncline.

The South Branch of the Santa Ynez fault displaces all of the terraces that cross it. The vertical component of slip, based on displacement of the shoreline angle, indicates a vertical rate of 0.05 mm/yr. The horizontal component of the slip rate was not determined but must be low due to the lack of significant deflection of the shorelines beyond that of the modem shoreline.

Our data clearly indicate that the westernmost Santa Ynez Mountains are actively folding in association with north-south crustal shortening across the Transverse Ranges. The rates of folding estimated from the rates of terrace deformation, however, suggest that only a small fraction of the total regional shortening is accommodated onshore: the balance is apparently accommodated offshore to the south in the Santa Barbara Channel.